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Chemical Approaches to Artificial Photosynthesis. 2
Abstract
The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.
... While there has been an increased focus on decarbonization and on sustainable H 2 production as a fuel (amongst others), mechanisms to bridge the cost of production and market value are needed. [1][2] By coupling proton reduction to high value-added oxidative processes, it is possible to overcome this cost barrier, while also producing new streams of chemical feedstocks. [3] Dye-sensitized photoelectrochemical cells (DS-PECs) mimic the natural process of photosynthesis by storing solar energy in chemical bonds. ...
... [12,[23][24][25][26] On global analysis, four separate transitions are proposed. Initial features that form rapidly (τ 1 = 1 ps, Figure 3C) correspond to pseudo-Jahn-Teller (pJT) distortions where the coordination sphere converts from tetrahedral to a square planar geometry [as copper(I) becomes copper(II) in the initial 1 MLCT state], [22,[27][28] with spectral features that are qualitatively similar to related [Cu(Xantphos)(dppz)] + [Xantphos = (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)] complexes. [26] The negative signal observed is due to the growth of the following observed species, as is demonstrated by the mirrored ESA in τ 2 . ...
... TEMPO sensitization -Photocatalysis: In a representative procedure, TEMPO (4.9 mg, 0.025 mmol, 5 mM), furfuryl alcohol (FA, 12.3 mg, 0.125 mmol, 25 mM) or benzyl alcohol (BA, 13.5 mg, 0.125 mmol), and pyridine (39.5 mg, 0.50 mmol, 100 mM), were dissolved in CH 3 CN (5 mL) in a 20 mL scintillation vial. 1 Subsequently, a ZnO j A-C-D device was placed in the vial, and the vial was sealed and degassed with argon, bubbled with O 2 or left open under ambient conditions and irradiated with white light in the above described photoreactor for 4 hours. A fan was placed on top of the reactor to maintain the reaction temperature below 25°C. ...
Dye‐sensitized photoelectrochemical cells can enable the production of molecules currently accessible through energetically demanding syntheses. Copper(I)‐based dyes represent electronically tunable charge transfer and separation systems. Herein, we report a Cu(I)‐bisdiimine donor‐chromophore‐acceptor dye with an absorbance in the visible part of the solar spectrum composed of a phenothiazine electron donor, and dipyrido[3,2‐a:2′,3′‐c]phenazine electron acceptor. This complex is incorporated onto a zinc oxide nanowire semiconductor surface effectively forming a photoanode that is characterized spectroscopically and electrochemically. We investigate the photo‐oxidation of hydroquinone, and the photosensitization of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl and N‐hydroxyphthalimide for the oxidation of furfuryl alcohol to furfuraldehyde, resulting in near quantitative conversions, with poor selectivity to the alcohol.
... water splitting into molecular oxygen and hydrogen has become one of the most attractive strategies. [1] Thew hole water splitting process consists of two half reactions:t he proton reduction and the water oxidation. Thef ormer half reaction is less energy demanding compared with the latter one,w hich is the key step and is always considered as abottleneck because it requires the transfer of four electrons and formation of the O À Ob ond. ...
... The solvent was removed and the residual solid was purified by column chromatography on silica gel using DCM-methanol (3/2, v/v) as eluents, 2 was obtained as red solid (44 mg,6 2.3 %). 1 At ypical proceduref or ac atalysis measurement using the method of displacement of water is as follows:1.0 gCe(NH 4 ) 2 (NO 3 ) 6 was dissolved in 3.5 mL water and then an aqueous solution of catalyst (0.5 mL, 1.6 10 À3 mol L À1 )w as immediately injected to the CANs olution under vigorous stirring.T he generatedO 2 was measured through the numerical reading variation of the upsidedown burette,w hich was connected to the sealed reaction bottle by av ery thin pipe. ...
The oxidation of water to molecular oxygen is the key step to realize water splitting from both biological and chemical perspective.I na ne ffort to understand howw ater oxidation occurs on am olecular level, al arge number of molecular catalysts have been synthesized to find an easy access to higher oxidation states as well as their capacity to make O À Obond. However,most of them function in amixture of organic solvent and water and the OÀOb ond formation pathway is still as ubject of intense debate.H erein, we design the first amphiphilic Ru-bda (H 2 bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water oxidation catalysts (WOCs) of formula [Ru II (bda)(4-OTEG-pyridine) 2 ](1,O TEG = OCH 2 CH 2 OCH 2 CH 2 OCH 3)a nd [Ru II (bda)(PySO 3 Na) 2 ](2, PySO 3 À = pyridine-3-sulfonate), whichpossess good solubility in water.D ynamic light scattering (DLS), scanning electron microscope (SEM), critical aggregation concentration (CAC) experiments and product analysis demonstrate that they enable to self-assemble in water and form the O À Ob ond through different routes even though they have the same bda 2À backbone .This work illustrates for the first time that the O À Obond formation pathwayc an be regulated by the interaction of ancillary ligands at supramolecular level. Tomeetthesustainableproductionofcleanenergydemands, water splitting into molecular oxygen and hydrogen has become one of the most attractive strategies. [1] Thew hole water splitting process consists of two half reactions:t he proton reduction and the water oxidation. Thef ormer half reaction is less energy demanding compared with the latter one,w hich is the key step and is always considered as abottleneck because it requires the transfer of four electrons and formation of the O À Ob ond. [2,3] Over the past decades, ag reat quantity of molecular catalysts in relation to mono-, di-, and polynuclear Ru-, [4-27] Ir-, [28, 29] Mn-, [30-32] Co-, [33-36] Fe-[37-39] and Cu-based [40-44] metal complexes have been developed. Among them, Ru-based WOCs are the most representative.P articularly,S un and co-workers reported in 2012 aset of quite efficient catalysts bearing the tetradentate bda 2À as equatorial ligand and having amaximal TOF value of 303.0 s À1 (complex 3,s ee Scheme 1), [14] which was even comparable with that of oxygen evolution complex (OEC) in photosystem II (PSII). [45] Despite this progress,t he comprehensive understanding of the reaction pathways for OÀO bond formation, together with the full spectroscopic characterization of reaction intermediates,isahuge challenge. It is generally accepted two different pathways with regard to the O À Ob ond formation, namely water nucleo-philic attack (WNA), and interaction of two higher oxidation state of M À Ou nits (I2M). [19] But there is still no final evidence in favor of WNAorI2M no matter for natural PSII or the synthetic transition-metal complexes.T ill now,most of the synthetic Ru-based water oxidation systems adopt WNA except Ru-bda type catalysts reported by Sun and co-workers [14, 18] and several other binuclear WOCs that undergo intramolecular I2M pathway. [6, 16] Furthermore,almost all the reported catalysts function in the presence of organic solvent, which might lead to undesired deactivation pathways and increase the complexity of an already very complex reaction since the high thermodynamic redox potential needed for water oxidation permits the catalyst to oxidize abroad range of organic and inorganic substrates. In 2014 Llobet and co-workers revealed that WNAa nd I2M pathways had relatively close activation energy barriers and the OÀOb ond formation mechanism would be influenced by changing equatorial ligand constrains. [19] Such Scheme 1. Subtle supramolecular interactions regulate the OÀObond formation pathwaysf or amphiphilic WOCs of 1 and 2 in water.
... The knowledge generated over the years about natural photosynthesis has provided a comprehensive picture of the process leading to water oxidation and CO 2 reduction. However, the key to understanding the photosynthetic model lies in separating this complex photosynthetic puzzle into individual pieces and analyzing them separately (Barber, 2009;Gust et al., 2012;El-Khouly et al., 2017;Symes et al., 2013;Alstrum-Acevedo et al., 2005;Gust et al., 2001;Meyer, 1989;Kalyanasundaram and Graetzel, 2010;Concepcion et al., 2012). This strategy dramatically reduces the complex natural mechanism to its primary functional units and allows faster progress in the design of integrated artificial photosynthetic systems (Brinkert, 2018;Meyer, 1989). ...
... Although this complexity is clearly beyond the scope of synthetic chemists due to the living nature of this process, i.e., it is evolving nature, the understanding of natural photosynthesis has gradually allowed us to move toward the design of artificial systems (Bozal-Ginesta and Durrant, Balzani et al., 2008;Collings and Critchley, 2007). As noted above, this progress is based on the specific design of individual photosynthetic devices and on employing molecular engineering to assemble these systems homogeneously and functionally into an integrated device by exploiting covalent and non-covalent interactions (Alstrum-Acevedo et al., 2005;Keijer et al., 2021;Bozal-Ginesta and Durrant, 2019). Although significant progress has been made in this direction, it has yet to be possible to unite the various components into an integrated functional system; this is undoubtedly one of the current challenges of modern science. ...
The design of molecular systems with capabilities to carry out the water oxidation reaction and thereby overcome the bottleneck of artificial photosynthesis is one of the scientific fields of most significant interest and urgency due to its potential to address energy demand and climate change. Nevertheless, the search for efficient and robust catalysts has been limited by the degradation of carbon-based ligands under oxidative conditions, leading to the search for fully inorganic catalysts. Polyoxometalates (POMs), an emerging class of carbon-free ligands with oxygen-enriched surfaces, offer a unique alternative as inorganic scaffolds to self-assemble and stabilize transition-metal clusters with unique redox properties. Under catalytic working conditions, POMs can undergo electron transfer reactions coupled to O2 formation without modifying their parental structure. As a result, these materials have recently entered the scene as catalytic players in designing new artificial photosynthetic platforms for water oxidation. We focus on the methods used to create these compounds, their unique structural characteristics, and how effectively they function as catalysts. We also explore the proposed mechanisms behind their ability to produce O2 and their potential use in designing photosynthetic devices.
... Innovation of new materials having the ability of photo- [1][2][3][4][5][6][7][8] and/or electrochemical [9][10][11][12][13] carbon dioxide (CO 2 ) reduction to yield molecules such as CO, HCOOH, alcohols, etc., getting significant research interest recently because these may simultaneously provide a solution for the global energy crisis and would decrease its associated environmental risk. Among various materials, ruthenium complexes may be chosen as suitable candidates to achieve these goals, since they are broadly used as catalysts in photo- [2,5,[14][15][16] and electrochemical [9][10][11][12][13][17][18][19][20][21][22][23] reduction of CO 2 and other organic syntheses as well. ...
A new ruthenium(II) NAD (NAD=Nicotinamide Adenine Dinucleotide) type bis‐carbonyl complex, [Ru(bpy)(pbn)(CO)2]²⁺ [pbn = 2‐(pyridin‐2‐yl)benzo[b]‐1,5‐naphthyridine; bpy = 2,2′‐bipyridine] [1]²⁺, was successfully synthesized and fully characterized by single‐crystal X‐ray structural analysis, ESI‐MS, IR and NMR spectroscopy. Complex [1]²⁺ together with [Ru(bpy)(pbn)(CO)(COOH)]⁺ and/or [Ru(bpy)(pbn)(CO)(CO2)]⁰ complexes exist as equilibrium mixtures in aqueous solutions as evident from spectroscopic study. Chemical reduction of [1]²⁺ resulted the formation corresponding NADH form i. e., [Ru(bpy)(pbnHH)(CO)2]²⁺ (pbnHH = 2‐(pyridin‐2‐yl)‐5,10‐dihydrobenzo[b][1,5]naphthyridine) [1.HH]²⁺ as a two‐electron‐reduced species. The electrochemical behavior of complex [1]²⁺ in the presence of acid was investigated based on cyclic voltammetry analysis. A control potential electrolysis of [1]²⁺ afforded formate (HCOO⁻) as the major product with a lesser amount of CO and H2, whereas that of [Ru(bpy)2(CO)2]²⁺ complex produced CO as the main product with a lesser amount of HCOO⁻ and H2. The experimental results suggest that the selectivity of HCOO⁻ over CO should be due to catalytic hydride transfer from the NADH‐type ligands of [1]²⁺ to CO2.
... Even though there are numerous production methods, water electrolysis has been the subject of many studies for energy applications, particularly renewable energy technologies (Undertaking et al., 2016;Urf Manoo et al., 2024). Photosynthesis reactions can also be used to produce hydrogen (Alstrum-Acevedo et al., 2005). According to a growing number of reports, hydrogen has the potential to play a role in almost every aspect of the energy system, including electricity generation and transportation (Hart et al., 2016), and energy system-level audits show hydrogen to be a technically and economically legitimate option for decarbonizing heat (Sadler et al., 2016;Gastec and Association, 2016). ...
The increase in the feasibility of hydrogen-based generation makes it a promising addition to the realm of renewable energies that are being employed to address the issue of electric vehicle charging. This paper presents technical and an economical approach to evaluate a newer off-grid hybrid PV-hydrogen energy-based recharging station in the city of Jamshoro, Pakistan to meet the everyday charging needs of plug-in electric vehicles. The concept is designed and simulated by employing HOMER software. Hybrid PV-hydrogen and PV-hydrogen-battery are the two different scenarios that are carried out and compared based on their both technical as well as financial standpoints. The simulation results are evident that the hybrid PV-hydrogen-battery energy system has much more financial and economic benefits as compared with the PV-hydrogen energy system. Moreover, it is also seen that costs of energy from earlier from hybrid PV-hydrogen-battery is more appealing i.e. 0.358 /kWh cost of energy from hybrid PV-hydrogen. The power produced by the hybrid PV-hydrogen-battery energy for the daily load demand of 1700 kWh /day, consists of two powers produced independently by the PV and fuel cells of 87.4 % and 12.6 %, respectively.
... Various comprehensive reviews of the techniques for hydrogen production can be found in [11], [12], [13], and [14]. ...
The accurate knowledge of the thermophysical and thermodynamic properties of pure hydrogen and hydrogen mixtures plays an important role in the design and operation of many processes involved in hydrogen production, transport, storage, and use. These data are needed for the development of theoretical models necessary for the introduction of hydrogen as a promising energy carrier in the near future. A literature survey on both the available experimental data and the theoretical models associated with the thermodynamic properties of hydrogen mixtures, within the operational ranges of industrial interest for composition, temperature, and pressure, is presented in this work. Considering the available experimental data and the requirements for the design and operation of hydrogen systems, the most relevant gaps in temperature, pressure and composition are identified.
... Central to the efficiency and selectivity of these processes is the strategic employment of metal complexes, which can be precisely engineered at the molecular level to optimize their performance. [1,2] Typically, these functional metal complexes are incorporated into electrochemical cells, where they are immobilized on the electrode surface via semiconductor particles, with a carboxylic acid [À C(=O)OH] or phosphonic acid [À P(=O)(OH) 2 ] group serving as functional linkers. [3,4] While this approach offers advantages such as robust surface immobilization and fine-tuning of the steric and electronic properties, it also presents challenges including a more complex synthetic process, increased production costs, and potential alteration of the metal complex's intrinsic properties. ...
Aluminosilicate (Al‐SiO2) thin films with vertically aligned mesochannels were successfully synthesized on Indium Tin Oxide (ITO) electrodes and employed for the immobilization of a cationic Ru(II) water oxidation catalyst without requiring linker groups. Optimal synthesis conditions yielded uniform mesoporous Al‐SiO2 films with tunable Al content, high surface area (568 m²/g), 3.94 nm pore size, and 155 nm thickness. Electrochemical studies confirmed the presence of the immobilized Ru complex undergoing diffusion‐controlled Ru(III/II) and Ru(IV/III) electron transfer. The Ru loading reached 4.71 nmol/cm² at Si/Al=9.6, with higher Al content enhancing loading amounts via cation exchange. The Ru‐modified electrode exhibited high electrocatalytic water oxidation activity, achieving 75.3 % Faradaic efficiency and a turnover number of 298.6 for O2 evolution for 1 hour. This work provides a new approach to construct porous environments on an electrode surface to immobilize positively charged transition‐metal complexes as catalysts, offering potential applications in the development of electrocatalytic systems for energy conversion.
... In order to realize the sustainable development goals of the United Nations, artificial photosynthesis, which mimics the enzyme in natural biochemical photosynthesis, has been developed to convert CO 2 and H 2 O (typically) into other chemical feedstocks [3,4]. Overall, the vast majority of artificial photosynthesis research, especially for mimicking photosynthesis, relies on biomass-based catalytic methods, such as gasification and pyrolysis [5,6]. While these systems are widely studied and have become important hydrogen production and CO 2 conversion alternatives, the detailed mechanisms and the cost are still challenging because the catalysts are often based on biochemical materials with different optimal conditions. ...
Artificial photosynthesis, which could utilize renewable solar energy to produce fuels, e.g., hydrogen, syngas, methanol, and so on, is regarded as a prospective alternative to the global energy and environment crisis. Semiconductor-based photocatalysts play a key role in such solar-to-fuel conversions. In this chapter, we will review the recent demonstrations of advanced semiconductors used in solar water splitting and carbon dioxide reduction. The requirements for the physical and chemical properties of photocatalytic semiconductors are discussed to distinguish the potential candidates. The demonstrations of such advanced semiconductors successfully driving solar water splitting with hydrogen production and carbon dioxide reduction with hydrocarbon fuels are summarized. Moreover, current bottleneck issues, possible strategies, and perspectives are proposed at the end. This chapter will be an important reference for the research in artificial photosynthesis using emerging semiconductor materials.
... Protonated and hydroxide-rich water clusters that are thought to catalyze atmospheric reactions and are also of fundamental interest in condensed phase chemistry [5][6][7], have spectral features influenced by the quantum mechanical nature of the hydrated proton [8][9][10][11]. Several problems in materials chemistry including the study of nitrogen fixation [12][13][14] and artificial photosynthesis [15] are strongly controlled by such hydrogen transfer processes [1]. In all these cases, there is a critical interplay between the electronic and nuclear degrees of freedom that influences chemical properties including reactivity. ...
Quantum nuclear dynamics with wavepacket time-evolution is classically intractable and viewed as a promising avenue for quantum information processing. Here, we use an IonQ 11-qubit trapped-ion quantum computer, Harmony, to study the quantum wavepacket dynamics of a shared-proton within a short-strong hydrogen-bonded system. We also provide the first application of distributed quantum computing for chemical dynamics problems, where the distributed set of quantum processes is constructed using a tensor network formalism. For a range of initial states, we experimentally drive the ion-trap system to emulate the quantum nuclear wavepacket as it evolves along the potential surface generated from electronic structure. Following the experimental creation of the nuclear wavepacket, we extract measurement observables such as its time-dependent spatial projection and its characteristic vibrational frequencies to good agreement with classical results. Vibrational eigenenergies obtained from quantum computational are in agreement with those obtained from classical simulations to within a fraction of a kcal/mol, thus suggesting chemical accuracy. Our approach opens a new paradigm for studying the quantum chemical dynamics and vibrational spectra of molecules and also provides the first demonstration for parallel quantum computation on a distributed set of ion-trap quantum computers.
... It provides the multiple protons/electrons required for subsequent CO 2 or H + reduction to produce valuable fuels and green hydrogen. [1][2][3][4] Molecular catalysts, with their well-defined and tunable structures, offer a molecular-level platform to understand the catalyst's dynamic evolution during the catalytic process. However, it is important to note that molecular catalysts are not permanent under operating conditions. ...
... Solid-state 13 C cross-polarization magic angle spinning nuclear reaction kinetics. Nevertheless, accomplishing these objectives collectively is challenging 6,7 . ...
Charge transfer and mass transport to catalytic sites are critical factors in photocatalysis. However, achieving both simultaneously is challenging due to inherent trade-offs and interdependencies. Here we develop a microporous covalent organic framework featuring dense donor–acceptor lattices with engineered linkages. The donor–acceptor columnar π-arrays function as charge supply chains and as abundant water oxidation and oxygen reduction centres, while the one-dimensional microporous channels lined with rationally integrated oxygen atoms function as aligned conduits for instant water and oxygen delivery to the catalytic sites. This porous catalyst promotes photosynthesis with water and air to produce H2O2, combining a high production rate, efficiency and turnover frequency. This framework operates under visible light without the need of metal co-catalysts and sacrificial reagents, exhibits an apparent quantum efficiency of 17.5% at 420 nm in batch reactors and enables continuous, stable and clean H2O2 production in flow reactors.
... Its lowest-lying excited state is the 3 MLCT one, thus there is a virtual charge separation on the compound. The redox potential of this excited state was verified that it is thermodynamically favorable to perform both oxidation and reduction processes then there were several papers aiming to develop water-splitting systems [5,19,50,68,77]. ...
... This has been the subject of extensive research in the past decades, and significant progress has been achieved toward the design and construction of chemical assemblies that function as artificial reaction centers [4,5]. Though artificial photosynthesis is a promising way to store renewable energy in chemical bonds, it is still very challenging to integrate multiple chemical functions in a stable chemical architecture and, simultaneously, optimize the activity, stability, selectivity, and energetic efficiency of the catalysts [6]. Moreover, most research has only focused on the functional imitation of photosynthesis, and has neglected the structural effect. ...
Plant leaf ashes were obtained via the high temperature calcination of the leaves of various plants, such as sugarcane, couchgrass, bracteata, garlic sprout, and the yellowish leek. Although the photosynthesis systems in plant leaves cannot exist after calcination, minerals in these ashes were found to exhibit photochemical activities. The samples showed solar light photocatalytic oxidation activities sufficient to degrade methylene blue dye. They were also shown to possess intrinsic dehydrogenase-like activities in reducing the colorless electron acceptor 2,3,5-triphenyltetrazolium chloride to a red formazan precipitate under solar light irradiation. The possible reasons behind these two unreported phenomena were also investigated. These ashes were characterized using a combination of physicochemical techniques. Moreover, our findings exemplify how the soluble and insoluble minerals in plant leaf ashes can be synergistically designed to yield next-generation photocatalysts. It may also lead to advances in artificial photosynthesis and photocatalytic dehydrogenase.
... Carbon neutrality is broadly recognized as a vehicle for climate action and sustainable development due to the markedly increased carbon dioxide (CO 2 ) concentration caused by the utilization of fossil fuels and human activities (1)(2)(3). While some outstanding progress referring to the catalytic conversion of CO 2 into simple C1 and C2 products driven by renewable electricity or solar energy has been completed (4,5), sustainable conversion of CO 2 into high value-added longer-chain products has important technical and social implications (6)(7)(8)(9). Recently, a hybrid chemobiological pathway has been demonstrated to synthesize starch from CO 2 and H 2 through 11 core reactions relying on expensive enzyme catalysts and stringent carbon conversion conditions (10). ...
Developing artificial symbionts beyond natural synthesis limitations would bring revolutionary contributions to agriculture, medicine, environment, etc. Here, we initiated a solar-driven multi-organism symbiont, which was assembled by the CO2 fixation module of Synechocystis sp., N2 fixation module of Rhodopseudomonas palustris, biofunctional polypeptides synthesis module of Bacillus licheniformis, and the electron transfer module of conductive cationic poly(fluorene-co-phenylene) derivative. The modular design broke the pathway to synthesize γ-polyglutamic acid (γ-PGA) using CO2 and N2, attributing to the artificially constructed direct interspecific substance and electron transfer. So, the intracellular ATP and NADPH were enhanced by 69 and 30%, respectively, and the produced γ-PGA was enhanced by 104%. The strategy was further extended to produce a commercial antibiotic of bacitracin A. These achievements improve the selectivity and yield of functional polypeptides with one click by CO2 and N2, and also provide an innovative strategy for creating photosynthetic systems on demand.
... [1][2][3][4][5][6][7][8][9][10] However, for the production of solar fuels from small molecules such as H 2 O or CO 2 , the temporary accumulation and storage of more than one redox equivalent seems indispensable, yet proves very challenging until now. [11][12][13][14][15][16] To gain insight into the basic principles of photoinduced accumulation of redox equivalents, covalent [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] as well as noncovalent [36][37][38] molecular compounds were investigated from a fundamental mechanistic perspective. In most of the previously studied molecular multi-electron acceptors, the second reduction step is thermodynamically more challenging compared to the first one, due to electrostatic repulsion between the accumulating electrons. ...
The activation of N2, CO2 or H2O to energy‐rich products relies on multi‐electron transfer reactions, and consequently it seems desirable to understand the basics of light‐driven accumulation of multiple redox equivalents. Most of the previously reported molecular acceptors merely allow the storage of up to two electrons. We report on a terphenyl compound including two disulfide bridges, which undergoes four‐electron reduction in two separate electrochemical steps, aided by a combination of potential compression and inversion. Under visible‐light irradiation using the organic super‐electron donor tetrakis(dimethylamino)ethylene, a cascade of light‐induced reaction steps is observed, leading to the cleavage of both disulfide bonds. Whereas one of them undergoes extrusion of sulfur to result in a thiophene, the other disulfide is converted to a dithiolate. These insights seem relevant to enhance the current fundamental understanding of photochemical energy storage.
Metalloporphyrins have been widely utilized as building blocks for molecular self-assembly in organic solvents, but their application in water is less common due to competition from water molecules for the metal center. However, Co(III) metalloporphyrins are notable for their strong binding to two aromatic amine ligands in aqueous buffers. In this study, we present a comprehensive investigation of the binding behavior of Co(III) tetraphenyl sulfonic acid porphyrin with selected aromatic and aliphatic amines in aqueous solution. Our findings reveal that the ligand affinity is influenced by the pKa values of both the ligand and the porphyrin, as well as the hybridization state of the nitrogen atom, with binding to sp³-hybridized nitrogen being significantly weaker than to sp²-hybridized nitrogen. DFT calculations further suggest that the variations in binding affinities are due to differences in the electrostatic potential at the nitrogen atoms, with aromatic ligands generally exhibiting stronger Co–N coordination due to greater electrostatic attraction. Moreover, our study and the binding model we developed demonstrate that changes in pH affect the affinity for each ligand to varying degrees, sometimes resulting in an allosteric cooperative effect. This effect is linked to electronic changes introduced by the binding of the first ligand. Our model provides a predictive tool for understanding the assembly behavior of these porphyrins in aqueous buffers, with potential applications in developing more efficient catalysts and in the creation of smart materials for fields ranging from catalysis to nanomedicine and optoelectronics
A simple and efficient method for the synthesis of symmetric 2,3,5,6-tetrasubstituted pyridines from enaminones and triethyl orthoformate, catalyzed by pyridinium p-toluenesulfonate, has been established in which triethyl orthoformate was applied as a carbon source. The procedure was smoothly executed, culminating in the synthesis of symmetrical 2,3,5,6-tetrasubstituted pyridines with moderate to exceptional yields across a diverse array of substrates.
Photocatalysis powered by unlimited solar energy is an effective strategy to resolve energy and environmental issues. To achieve an efficient photocatalytic system, photocatalysts need to be highly crystalline and porous...
Here, the use of high‐performance microfluidic techniques (HPMT) is reportedto afford conjugated porous polymer (CPPs) with smaller particle size and narrower particle size dispersion than the obtained by miniemulsion methodology. Specifically, polymers based on BODIPY or BOPHY dyes are synthesized by HPMT conditions and conventional miniemulsion conditions. The polymer textural properties from HPMT are notably improved, giving rise to high‐quality thin films that are photoelectrochemically characterized. Furthermore, hybrid materials of CPPs synthesized by HPMT and TiO2 inorganic semiconductor revealed enhanced photocatalytic activity in the hydrogen evolution reaction (HER). The most active hybrid UN_IEP‐7@T‐10 photocatalyst, containing 10 wt% polymer loading, achieved a hydrogen evolution rate of 3.10 mmol g⁻¹ h⁻¹ (ƺ = 1.13%), which is threefold higher than that of its non‐nanostructured from bulk synthesis, two‐times greater than its nanostructured by conventional miniemulsion techniques and even surpassed by 39‐times the performance of bare TiO2. It is noteworthy that both photoluminescence lifetime (τPL) and transient lifetime (τT) are not affected by the nanoestructuration of CPPs, which agrees with the preservation of the chemical structure by both synthetic methodologies. The employment of HPMT as nanostructuration strategy clearly supports the obtaining of more processable polymers for a wide range of energy applications.
The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D–2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.
Continued increase in green house gas emissions will lead to irreversible and unpredictable damage to the environment and biosphere. CO2 alone contributes to about two-thirds of the energy imbalance causing climate change. The bulk of these CO2 emissions can be traced to fossil fuels, which presently account for 82% of this green house gas. Developing effective renewable–alternative fuel sources is crucial to mitigate the harmful effects of fossil fuels. One such alternative fuel source is molecular hydrogen, which can be produced by the photocatalytic splitting of water using metal-based catalysts. This study investigates the thermodynamics and kinetics associated with single-step protonations and reductions of Ni[(S2C2H2)(N2C2H4)], Ni[(Se2C2H2)(N2C2H4)], and Ni[(Te2C2H2)(N2C2H4)] complexes using density functional theory. Results found diselenolene to be the least endergonic for the formation of the triply reduced, doubly protonated species with a Gibbs protonation energy of 79.6 kJ mol⁻¹. Ditellurolene and dithiolene were slightly more endergonic with Gibbs protonation energies of 87.7 and 91.6 kJ mol⁻¹, respectively. The most thermodynamically favorable pathways were found to be the ECCEE pathway for dithiolene and diselenolene, whereas the CECEE mechanism was found to be most favorable for ditellurolene. Kinetically, it was found that there were two feasible pathways for all three complexes. The first was a CCEEE pathway and the second was a CECEE pathway. All complexes were found to have similar Gibbs activation energies.
In this study, we highlight the impact of catalyst geometry on the formation of O−O bonds in Cu2 and Fe2 catalysts. A series of Cu2 complexes with diverse linkers were designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu2 complexes is enhanced as their molecular skeleton become more rigid, which contrast with the behavior observed in our previous investigation with Fe2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O−O bond formation in Cu2 and Fe2 catalysts. In Cu2 systems, the coupling takes place between a terminal CuIII−OH and a bridging μ−O• radical. Whereas in Fe2 systems, it involves the coupling of two terminal Fe−oxo entities. Furthermore, an in‐depth structure‐activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ−O• radical, ultimately leading to the O−O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O−O bonding pathway.
In this study, we highlight the impact of catalyst geometry on the formation of O−O bonds in Cu2 and Fe2 catalysts. A series of Cu2 complexes with diverse linkers are designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu2 complexes is enhanced as their molecular skeletons become more rigid, which contrasts with the behavior observed in our previous investigation with Fe2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O−O bond formation in Cu2 and Fe2 catalysts. In Cu2 systems, the coupling takes place between a terminal CuIII−OH and a bridging μ−O⋅ radical. Whereas in Fe2 systems, it involves the coupling of two terminal Fe–oxo entities. Furthermore, an in‐depth structure–activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ−O⋅ radical, ultimately leading to the O−O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O−O bonding pathway.
This manuscript describes the syntheses, structures and magnetism of MnIII-CaII/SrII complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions CaII or SrII were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the CaII/SrII ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes.
The conversion of sunlight into chemical energy to produce solar hydrogen, opens the door to one promising solution towards a sustainable alternative in the current energy crisis. Herein, we describe...
Stimuli-responsive ruthenium complexes proximal- and distal-[Ru(C10tpy)(C10pyqu) OH2]²⁺ (proximal-1 and distal-1; C10tpy = 4′-decyloxy-2,2′:6′,2″-terpyridine and C10pyqu = 2-[2′-(6′-decyloxy)-pyridyl]quinoline) were experimentally studied for adduct formation with a model DNA base. At 303 K, proximal-1 exhibited 1:1 adduct formation with 9-ethylguanine (9-EtG) to yield proximal-[Ru(C10tpy)(C10pyqu)(9-EtG)]²⁺ (proximal-RuEtG). Rotation of the guanine ligand on the ruthenium center was sterically hindered by the presence of an adjacent quinoline moiety at 303 K. Results from ¹H NMR measurements indicated that photoirradiation of a proximal-RuEtG solution caused photoisomerization to distal-RuEtG, whereas heating of proximal-RuEtG caused ligand substitution to proximal-1. The distal isomer of the aqua complex, distal-1, was observed to slowly revert to proximal-1 at 303 K. In the presence of 9-EtG, distal-1 underwent thermal back-isomerization to proximal-1 and adduct formation to distal-RuEtG. Kinetic analysis of ¹H NMR measurements showed that adduct formation between proximal-1 and 9-EtG was 8-fold faster than that between distal-1 and 9-EtG. This difference may be attributed to intramolecular hydrogen bonding and steric repulsion between the aqua ligand and the pendant moiety of the bidentate ligand..
Calculating observable properties of chemical systems is often classically intractable and widely viewed as a promising application of quantum information processing. Here, we introduce a new framework for solving generic quantum chemical dynamics problems using quantum logic. We experimentally demonstrate a proof-of-principle instance of our method using the QSCOUT ion-trap quantum computer, where we experimentally drive the ion-trap system to emulate the quantum wavepacket dynamics corresponding to the shared-proton within an anharmonic hydrogen bonded system. Following the experimental creation and propagation of the shared-proton wavepacket on the ion-trap, we extract measurement observables such as its time-dependent spatial projection and its characteristic vibrational frequencies to spectroscopic accuracy (3.3 cm-1 wavenumbers, corresponding to >99.9% fidelity). Our approach introduces a new paradigm for studying the chemical dynamics and vibrational spectra of molecules and opens the possibility to describe the behavior of complex molecular processes with unprecedented accuracy.
We synthesized three new dyads composed of a Zn porphyrin and fac-Re(bpy)(CO)3Br (bpy = 2,2′-bipyridine) units, ZnP-nBpy 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 ReBr (n = 4, 5, and 6), in which the porphyrin is directly connected at the meso-position through the 4-, 5-, or 6-position of the bpy. We investigated the relationships between the connecting positions and the photophysical properties as well as catalytic activity in the CO2 reduction reaction. The dyad connected through the 6-position, ZnP-6BpyReBr, showed obvious phosphorescence with a lifetime of 280 μs at room temperature, in N,N-dimethylacetamide (DMA), whereas the other two dyads showed almost no phosphorescence under the same conditions. The photocatalytic CO2 reduction reactions in DMA using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the electron donor and the three dyads ZnP-nBpyReBr selectively produced CO with similar initial rates, but the durabilities were low. The addition of triethanolamine (TEOA) suppressed the decomposition of dyads, improving their durabilities and reaction efficiencies. In particular, ZnP-5BpyReBr was remarkably improved—it gave the highest durability and reaction efficiency among the three dyads; the reaction quantum yield reached 24%. The reason for this significant activity is no accumulation of electrons on the Zn porphyrin in ZnP-5BpyReBr, which would be caused by dual interactions of TEOA with the Re and Zn ions in the dyad. As the highest catalytic activity was observed in ZnP-5BpyReBr among the three dyads, which had no room-temperature phosphorescence (RTP), the catalytic activities and RTP properties are considered independent, but they are greatly influenced by the connecting positions on the bpy ligand in ZnP-nBpyReBr.
Photosynthesis is biochemical cascade of reactions in plants to produce glucose, the form of energy, using the raw materials water and carbon dioxide in presence of sunlight. Artificial photosynthesis is considered as a process that mimics natural photosynthesis. In this process, the solar energy is trapped into chemical fuels which are high energy compounds. This process utilises hydrogen released by splitting of water, in combination with carbon monoxide from carbon dioxide reduction. This method of converting solar light into high-energy fuel aids in the transition from a dependency on fossil and gaseous fuels to a carbon-neutral, or more specifically, a carbon-negative approach. Along with splitting water molecules producing oxygen, hydrogen released gets trapped. This makes artificial photosynthesis an ideal and possible way-out for the energy predicament, the world is facing.Even though various methods are there to provide alternate energy, the attractive feature of artificial photosynthesis is that, it can produce methanol also as an alternate fuel along with hydrogen. Along with ability to produce liquid hydrogen fuel free of greenhouse gas emission; it also utilises carbon dioxide and water in presence of solar energy. These features protect our environment also. Cost aspect considered reveals that it is least expensive. Since this technology is amenable to modifications various strategies are on trial.Nanoparticle techniques mimicking natural processes can be exploited in artificial photosynthesis. Because the major challenge here is lack of energy efficient catalytic systems. In addition to the existing cobalt oxide and titanium dioxide particles, recent advances in this area explore electron-rich gold nanoparticles as catalysts. Another approach to this mechanism is use of semiconducting microwires with flexible polymeric membranes. Hybrid nanoparticles, like Ag-decorated reduced titanium oxide, are another promising suggestion. Plasmonic metal nanoparticles enhance light absorption by these catalysts providing chemical reactive pathways, improving solar energy conversion. Another area that has to be looked upon to enhance artificial photosynthesis is standardisation of controlled size and form.This chapter attempts to understand and analyse the artificial synthetic nanoparticle-biotic interface of artificial photosynthesis. The goal is to achieve high catalytic performance in hydrogen release along with oxygen evolution by creating a biomimetic system with nano techniques. The system is expected to provide cleaner environment with maximum energy production.KeywordsPhotosynthesisSemiconductorsNanoparticlesFuel generation
Non-conjugated pendant electroactive polymers (NCPEPs) are an emerging class of polymers that offer the potential of combining the desirable optoelectronic properties of conjugated polymers with the superior synthetic methodologies and stability of traditional non-conjugated polymers. Despite an increasing number of studies focused on NCPEPs, particularly on understanding fundamental structure-property relationships, no attempts have been made to provide an overview on established relationships to date. This review showcases selected reports on NCPEP homopolymers and copolymers that demonstrate how optical, electronic and physical properties of the polymers are affected by tuning of key structural variables such as the chemical structure of the polymer backbone, molecular weight, tacticity, spacer length, the nature of the pendant group and in the case of copolymers the ratios between different comonomers and between individual polymer blocks. Correlation of structural features with improved π-stacking and enhanced charge carrier mobility serve as the primary figures of merit in evaluating impact on NCPEP properties. While this review is not intended to serve as a comprehensive summary of all reports on tuning of structural parameters in NCPEPs, it highlights relevant established structure-property relationships that can serve as a guideline for more targeted design of novel NCPEPs in the future. This article is protected by copyright. All rights reserved.
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We analyzed the enormous scale of global human needs, their carbon footprint, and how they are connected to energy availability. We established that most challenges related to resource security and sustainability can be solved by providing distributed, affordable, and clean energy. Catalyzed chemical transformations powered by renewable electricity are emerging successor technologies that have the potential to replace fossil fuels without sacrificing the wellbeing of humans. We highlighted the technical, economic, and societal advantages and drawbacks of short- to medium-term decarbonization solutions to gauge their practicability, economic feasibility, and likelihood for widespread acceptance on a global scale. We detailed catalysis solutions that enhance sustainability, along with strategies for catalyst and process development, frontiers, challenges, and limitations, and emphasized the need for planetary stewardship. Electrocatalytic processes enable the production of solar fuels and commodity chemicals that address universal issues of the water, energy and food security nexus, clothing, the building sector, heating and cooling, transportation, information and communication technology, chemicals, consumer goods and services, and healthcare, toward providing global resource security and sustainability and enhancing environmental and social justice.
Supplementary information:
The online version contains supplementary material available at 10.1007/s11244-023-01799-3.
The theory of electron transfer reactions establishes the conceptual foundation for redox solution chemistry, electrochemistry, and bioenergetics. Electron and proton transfer across the cellular membrane provide all energy of life gained through natural photosynthesis and mitochondrial respiration. Rates of biological charge transfer set kinetic bottlenecks for biological energy storage. The main system-specific parameter determining the activation barrier for a single electron-transfer hop is the reorganization energy of the medium. Both harvesting of light energy in natural and artificial photosynthesis and efficient electron transport in biological energy chains require reduction of the reorganization energy to allow fast transitions. This review article discusses mechanisms by which small values of the reorganization energy are achieved in protein electron transfer and how similar mechanisms can operate in other media, such as nonpolar and ionic liquids. One of the major mechanisms of reorganization energy reduction is through non-Gibbsian (nonergodic) sampling of the medium configurations on the reaction time. A number of alternative mechanisms, such as electrowetting of active sites of proteins, give rise to non-parabolic free energy surfaces of electron transfer. These mechanisms, and nonequilibrium population of donor-acceptor vibrations, lead to a universal phenomenology of separation between the Stokes shift and variance reorganization energies of electron transfer.
Molecular ruthenium (Ru) complexes derived from the Ru blue dimer complex have been extensively studied for water oxidation. For example, monomeric Ru catalysts of polypyridyl-type ligands, such as 2,2-bipyridine-6,6-dicarboxylate (bda),...
The catalytic organic‐hydride transfer to CO2 was first achieved through the photoinduced two‐electron reduction of the [Ru(bpy)2(pbn)]²⁺/[Ru(bpy)2(pbnHH)]²⁺ (bpy=2,2’‐bipyridine, pbn=2‐(pyridin‐2‐yl)benzo[b]‐1,5‐naphthyridine, and pbnHH=2‐(pyridin‐2‐yl)‐5,10‐dihydrobenzo[b]‐1,5‐naphthyridine) redox couple in the presence of 1,3‐dimethyl‐2‐phenyl‐2,3‐dihydro‐1H‐benzo[d]imidazole (BIH). The active species for the catalytic hydride transfer to carbon dioxide giving formate is [Ru(bpy)(bpy⋅⁻)(pbnHH)]⁺ formed by one‐electron reduction of [Ru(bpy)2(pbnHH)]²⁺ with BI⋅.
Using solar energy through green and simple artificial photosynthesis systems are considered as a promising way to solve the energy and environmental crisis. However, one of the important primary steps of photosynthesis, i.e., energy transfer, is long being ignored especially in inorganic semiconducting systems due to the small exciton binding energies. Herein, the simultaneous interrogation of the charge transfer and energy transfer steps in a photoexcitation process is proposed by utilizing few‐layered nanosheet‐assembled hierarchical BiOBr nanotubes with rich oxygen vacancies (OVs) as efficient multifunctional photocatalysts. Benefiting from the integrated 1D/2D structure and abundant OV defects, the excitonic effect strikes a delicate balance in the optimized BiOBr photocatalyst, showing not only improved charge carrier separation and transfer but also enhanced exciton generation. As a result, the hierarchical BiOBr nanotubes exhibit high efficiency toward photocatalytic CO2 reduction with an impressive CO evolution rate of 135.6 µmol g⁻¹ h⁻¹ without cocatalyst or photosensitizer. The dominant reactive oxygen species of singlet oxygen (¹O2) are discriminated for the first time, which originated from an energy transfer process, with electrophilic character, whereas the minor effect of superoxide anion radical (•O2⁻) with a nucleophilic rate‐determining step in the photocatalytic aerobic oxidation of sulfides.
A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2′-bipyridine) complexes ([Ru(bpy)2(πAr-CM)]⁺) in which πAr-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)2(πAr-CM)]⁺ complexes had remarkably high phosphorescence rate constants, kRAD(p), and the intrinsic phosphorescence efficiencies (ιem(p) = kRAD(p)/(νem(p))³) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)2(sc-CM)]⁺), where νem(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)2(CM)]⁺ complexes indicated numerous singlet metal-to-ligand charge transfers for ¹MLCT-(Ru-bpy) and ¹MLCT-(Ru-CM), excited states in the low-energy absorption band and ¹ππ*-(aromatic ligand) (¹ππ*-LAr) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (Te) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ιem(p) for the spin-forbidden Te (³MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin–orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy ¹MLCT states and second-order intensity perturbation from the significant configuration between the low-energy ¹MLCT and high-energy intense ¹ππ*-LAr states. In addition, the observation of unusually high ιem(p) magnitudes for these [Ru(bpy)2(πAr-CM)]⁺ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)2(sc-CM)]⁺ ions.
Natural photosynthesis uses an array of molecular structures in a multiphoton Z-scheme for the conversion of light energy into chemical bonds (i.e., solar fuels). Here, we show that upon excitation of both a molecular photocatalyst (PC) and a substituted naphthol (ROH) in the presence of a sacrificial electron donor and proton source, we achieve photocatalytic synthesis of H2. Data support a multiphoton mechanism that is catalytic with respect to both PC and ROH. The use of a naphthol molecule as both a light absorber and H2 producing catalyst is a unique motif for Z-scheme systems. This molecular Z-scheme can drive a reaction that is uphill by 511 kJ mol-1 and circumvents the high-energy constraints associated with the reduction of weak acids in their ground state, thus offering a new paradigm for the production of solar fuels.
For public health and environmental protection reasons, there is an urgent need to replace traditional fossil fuels with clean and renewable sources. Electrochemical splitting of water has the potential to produce clean hydrogen as a possible solution to global energy problems. This review article introduces the rational design of molecular metal complexes based on earth-abundant metals for electrocatalytic hydrogen production in water or water-organic media. Emphasis is placed on providing insight into structure-function relationships in catalytic properties for future ligand and catalyst design.
Recent magnetic-resonance work on YŻ suggests that this species exhibits considerable motional flexibility in its functional site and that its phenol oxygen is not involved in a well-ordered hydrogen-bond interaction (Tang et al., submitted; Tommos et al., in press). Both of these observations are inconsistent with a simple electron-transfer function for this radical in photosynthetic water oxidation. By considering the roles of catalytically active amino acid radicals in other enzymes and recent data on the water-oxidation process in Photosystem II, we rationalize these observations by suggesting that YŻ functions to abstract hydrogen atoms from aquo- and hydroxy-bound managanese ions in the (Mn)4 cluster on each S-state transition. The hydrogen-atom abstraction process may occur either by sequential or concerted kinetic pathways. Within this model, the (Mn)4/YZ center forms a single catalytic center that comprises the Oxygen Evolving Complex in Photosystem II.
The present paper is a review of current situation in space solar cell engineering. The comparison of the Si and III-V solar cell performances, as well as their parameter variation with temperature rise, radiation treatments and improving design were analyzed. The modern directions of the space solar cell development and international space projects, applied new types of solar cells, were discussed as well.
In this article we present a multistate continuum theory for multiple charge transfer reactions such as proton-coupled electron transfer and multiple proton transfer reactions. The solute is described with a multistate valence bond model, the solvent is represented as a dielectric continuum, and the transferring protons are treated quantum mechanically. This theory provides adiabatic free energy surfaces that depend on a set of scalar solvent variables corresponding to the individual charge transfer reactions. Thus this theory is a multidimensional analog of standard Marcus theory for single charge transfer reactions. For processes involving significant inner-sphere (i.e., solute) reorganization, the effects of solute intramolecular vibrations can be incorporated into the adiabatic free energy surfaces. The input quantities required for this theory are gas phase valence bond matrix elements fit to standard quantum chemistry calculations and solvent reorganization energy matrix elements calculated with standard continuum electrostatic methods. The goal of this theory is to provide insight into the underlying fundamental physical principles dictating the mechanisms and rates of multiple charge transfer reactions.
This paper considers electron transfer between biological molecules in terms of a nonadiabatic multiphonon nonradiative decay process in a dense medium. This theoretical approach is analogous to an extended quantum mechanical theory of outer sphere electron transfer processes, incorporating the effects of both low-frequency medium phonon modes and the high-frequency molecular modes. An explicit, compact and useful expression for the electron transfer probability is derived, which is valid throughout the entire temperature range, exhibiting a continuous transition from temperature independent tunneling between nuclear potential surfaces at low temperatures to an activated rate expression at high temperatures. This result drastically differs at low temperatures from the common, semiclassical, Gaussian approximation for the transition probability. The experimental data of De Vault and Chance [Biophys. J. 6, 825 (1966)] on the temperature dependence of the rate of electron transfer from cytochrome to the chlorophyll reaction center in the photosynthetic bacterium Chromatium are properly accounted for in terms of the present theory.
This book contains the proceedings of the 1991 Nobel Symposium devoted to the study of carbon dioxide (CO2) fixation and reduction in biological and model systems. With chapters authored by leading experts from around the world, the book covers subjects as diverse as photosynthetic carbon dioxide fixation and electrochemical reduction of CO2. Others topics include the organometallic chemistry of CO2 pertinent to catalysis, how enzymes deal with carbon dioxide and bicarbonate, photochemical electron transfer applied to the reduction of CO2, and the molecular biology and biochemistry of CO2, among many others. Students and researchers in biochemistry and the chemistry of CO2 fixation will welcome this timely survey of the field.
The intellectual and utilitarian opportunities that lie at the frontiers of chemistry have been recently emphasized by the Pimentel Report. Such report recommends that in the field of chemical research priority should be given to "understanding chemical reactivity" and proposes initiatives aimed at the clarification of factors that control the rates of reaction and the development of new synthetic pathways for chemical change. In the broad field of chemical reactivity, a discipline that has grown with an extraordinary rate is photochemistry. Since the knowledge of the photochemical properties at the molecular level has made a substantial progress in the last few years, there is currently a trend to study more and more complex photochemical systems. In particular, an emerging and rapidly expanding branch of photochemistry is that concerning studies of assemblies of molecular components properly combined so as to obtain light-induced functions (supramolecular photochemistry). Although much of the current work in supramolecular photochemistry is fundamental in nature, it is clear that progress in this field will be most rewarding for several applications concerning the interaction of light with matter. In particular, it will allow us to pursue research aimed at the photochemical conversion of solar energy by means of artificial systems and to make progress towards futuristic branches of science called "photonics" (photo-generated electron migration processes on a molecular basis) and "chemionics" (design of components, circuitry, and information treatment at the molecular level).
Peptides are a very important biologically active class of naturally occurring compounds, and the number of newly discovered peptides has grown rapidly. The need for a large number of synthetic peptides and peptide analogs has grown even faster, and the most effective preparative methods are essential. This chapter describes solid-phase peptide synthesis. In the original and simplest form, solid-phase peptide synthesis involves a heterogeneous reaction mixture composed of an insoluble resin-bound peptide chain, a soluble activated amino acid derivative, and a solvent. The solid-phase principle for peptide synthesis was conceived and developed for providing a rapid, simplified, and effective way to prepare peptides and small proteins. This chapter focuses on solid supports, methods for attachment and removal of the peptide chain, protecting group schemes, and coupling methods. It also discusses difficult couplings and peptide ligation methods. The chapter also describes special synthetic applications including protein–nucleic acid compounds and the de novo design and synthesis of peptides and proteins.
The results of photophysical studies on chromophore-quencher complexes, ligand-bridged metal complexes, and soluble polymers which contain combinations of chromophores and quenchers are described. The results begin to show how light induced electron and energy transfer processes can be controlled by chemical design in intramolecular chemical systems.
Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing presents the physical and chemical principles of the sol-gel process. The book emphasizes the science behind sol-gel processing with a chapter devoted to applications. The first chapter introduces basic terminology, provides a brief historical sketch, and identifies some excellent texts for background reading. Chapters 2 and 3 discuss the mechanisms of hydrolysis and condensation for nonsilicate and silicate systems. Chapter 4 deals with stabilization and gelation of sols. Chapter 5 reviews theories of gelation and examines the predicted and observed changes in the properties of a sol in the vicinity of the gel point. Chapter 6 describes the changes in structure and properties that occur during aging of a gel in its pore liquor (or some other liquid). The discussion of drying is divided into two parts, with the theory concentrated in Chapter 7 and the phenomenology in Chapter 8. The structure of dried gels is explored in Chapter 9. Chapter 10 shows the possibility of using the gel as a substrate for chemical reactions or of modifying the bulk composition of the resulting ceramic by performing a surface reaction (such as nitridation) on the gel. Chapter 11 reviews the theory and practice of sintering, describing the mechanisms that govern densification of amorphous and crystalline materials, and showing the advantages of avoiding crystallization before sintering is complete. The properties of gel-derived and conventional ceramics are discussed in Chapter 12. The preparation of films is such an important aspect of sol-gel technology that the fundamentals of film formation are treated at length in Chapter 13. Films and other applications are briefly reviewed in Chapter 14. Materials scientists and researchers in the field of sol-gel processing will find the book invaluable.
Highly resolved emission spectra of isolated [Ru(LL)n(LL′)3-n]2+ complexes doped into [Zn(bpy)3] (ClO4)2 with LL and LL′ representing the ligands bpy-hg, bpy-dg, and bpz are presented. The information displayed in the electronic origins and the vibrational satellite structures combined with the emission decay behavior is used to provide a clear distinction between localized and delocalized MLCT transitions to the lowest excited states. In particular, the appearance or nonappearance of ligand-centered vibrational modes belonging to different types of ligands provides crucial evidence for delocalization or localization. Thus, for a localized excitation in ]Ru(bpy)2(bpz)]2+ only the ligand-centered modes of bpz can be found in the emission spectra. In contrast, for [Ru(bpy-hg)2(bpy-dg)]2+ the ligand-centered vibrations of bpy-hg and bpy-dg accompany the same electronic origin. This is strong evidence for the delocalization of the excited electron in mixed-ligand [Ru(bpy-hg)2(bpy-dg)[2+. Consequently, the lowest MLCT excited states in homoleptic complexes like [Ru(bpy-hg)3]2+ and [Ru(bpy-dg)3]2+ are also delocalized over the metal and the different ligands.
The traditional development of photovoltaics has been based on crystalline-silicon-wafer technology. In the early 1970's, however, a new approach arises based on the possibility to grow silicon in the form of a thin film onto a given substrate. Several techniques are used for such a deposition, among which plasma-enhanced chemical vapour deposition (PECVD) is clearly outstanding given its widespread use and success. More recently, very-high-frequency (VHP PECVD) and hot-wire CVD have appeared as very promising and fast developing alternatives with important potential and actual advantages. Thin-film technology introduces completely novel concepts and challenges in silicon photovoltaics. Low-temperature processes particularly adequate for large-area devices open up not only very important cost-reduction potential, but also new possibilities such as making semi-transparent or flexible modules. Additional important features are a highly automated production system, an enormous potential for building integration, a good performance at realistic working temperatures (around 40°C) and an excellent durability in outdoor conditions among others. Photovoltaics are facing important challenges for the near future. Silicon-wafer technology is evolving towards making thinner, cheaper, multicrystalline silicon. Thin-film-silicon researchers are in turn striving to make thicker, better, more crystalline films. Both ways seem to converge to new-generation photovoltaics in which wafer and thin-film technologies may be used in a synergistic rather than competing manner. Silicon heterojunction cells (made up of a crystalline silicon absorber onto which one or more thin-film silicon layers are deposited), such as the well-known HIT cell, are in the forefront of photovoltaics and may represent a breakthrough in the next few years.
Many of the features of electron transfer reactions involving excited states can be understood based on electron transfer theory.
This is an overview of some of the important, challenging, and problematic issues in contemporary electron transfer research. After a qualitative discussion of electron transfer, its time and distance scales, energy curves, and basic parabolic energy models are introduced to define the electron transfer process. Application of transition state theory leads to the standard Marcus formulation of electron transfer rate constants. Electron transfer in solution is coupled to solvent polarization effects, and relaxation processes can contribute to and even control electron transfer. The inverted region, in which electron transfer rate constants decrease with increasing exoergicity, is one of the most striking phenomena in electron transfer chemistry. It is predicted by both semiclassical and quantum mechanical models, with the latter appropriate if there are coupled high-or medium-frequency vibrations. The intramolecular reorganizational energy has different contributions from different vibrational modes, which, in favorable cases, can be measured on a mode-by-mode basis by resonance Raman spectroscopy. Alternatively, mode-averaging procedures are available for including multimode contributions based on absorption or emission spectra. Rate constants for intramolecular electron transfer depend on electronic coupling and orbital overlap and, therefore, on distance. Mixed-valence systems have provided an important experimental platform for investigating solvent and structural effects and the transition between localized and delocalized behavior. One of the important developments in electron transfer is the use of absorption and emission measurements to calculate electron transfer rate constants. UItrafast electron transfer measurements have been used to uncover nonequilibrium relaxation effects, an area that presents special challenges to the understanding of the dynamics and relaxation of the coupled processes. Electron transfer in the gas phase offers substantial insights into the nature of the electron transfer process. Similarly, electron transport in conductive polymers and synthetic metals depends on the basic principles of electron transfer, with some special nuances of their own.
We adopt the theory of intramolecular dynamics to explore charge separation and recombination in two classes of ‘isolated’ solvent-free molecular systems: (A) Supermolecules containing bridged electron donor and electron acceptor, where electron transfer occurs on a spatial scale of ≈ 10 Å. (B) Ultrahigh molecular Rydberg excitations with a principle quantum number n ≃ 50−300 and spatial dimensions of ≈ 104 Å, where relaxation processes, e.g., electron-core recombination via internal conversion or predissociation, and charge separation by autoionization, are manifested. The molecular limit for photoinduced long-range ET in isolated supermolecules [class (A)] is treated on the basis of the statistical limit for interstate radiationless transitions, which involve either a direct or a mode-selective mediated coupling. The level structure, optical excitation modes and dynamics of high molecular Rydbergs [class (B)] interrogated by time-resolved ZEKE (zero-electron kinetic energy) spectroscopy, are treated by the effective Hamiltonian formalism. We pursue the formal analogy between the coupling, accessibility and decay of ultrahigh Rydbergs in an external weak (F = 0.01−1.0 V/cm) electric field and intramolecular (interstate and intrastate) relaxation in a bound level structure. Model calculations for the field-induced (l) mixing reveal that the Rydberg dynamics is characterized by two distinct (≈ ns and ≈μs) time scales. Up to date, long time-resolved (10 μs − 100 ns time scales) nonexponential decay of ZEKE Rydbergs was experimentally documented, in accord with our analysis. The predicted existence of the short decay times (1 − 10 ns) was not yet subjected to an experimental test.
Untersucht werden die Komplexe (I)-(IV), wobei Fe(H2O)ä+ und N,N′-Dimethyl-4,4′-bipyridinium als Quencher verwendet werden.
Systematic variations appear in the photophysical and photochemical properties of metal ligand charge transfer excited states which can be accounted for qualitatively or even quantitatively, based on the properties of the molecules and of the surrounding medium. The successful utilization of Ru(bpy)//3**2** plus and related complexes in energy conversion schemes relies on establishing a basis for understanding the light absorptivity and photophysical and photochemical properties of the chromophores involved. The goal of this account is to describe those properties and to provide a basis for understanding them at the microscopic level.
The behaviour of (i) the luminescence spectrum, (ii) the (relative) luminescence quantum yield and (iii) the luminescence lifetime of [Os(phen)3]2+(phen = 1,10-phenanthroline) and [with respect to (ii) and (iii)] of [Os(bipy)3]2+(bipy = 2,2′-bipyridine), has been examined in the temperature range 77–300 K, principally in 9 mol dm–3 LiCl + H2O and LiCl + D2O, but also in cellulose acetate film. Two principal routes provide the deactivation of the excited state: (i) a non-radiative pathway of high energy (≈ 13 kJ mol–1 in LiCl + H2O and LiCl + D2O, and 6–7 kJ mol–1 in cellulose acetate) and (ii) what is probably a radiative pathway of energy ≈ 0.7 kJ mol–1(60 cm–1) in all media.
Fluorescence quenching rate constants, k q , ranging from 10 ⁶ to 2 × 10 ¹⁰ M ⁻¹ sec ⁻¹ , of more than 60 typical electron donor‐acceptor systems have been measured in de‐oxygenated acetonitrile and are shown to be correlated with the free enthalpy change, Δ G 23 , involved in the actual electron transfer process
magnified image
in the encounter complex and varying between + 5 and −60 kcal/mole. The correlation which is based on the mechanism of adiabatic outer‐sphere electron transfer requires Δ G ≠ 23 , the activation free enthalpy of this process to be a monotonous function of Δ G 23 and allows the calculation of rate constants of electron transfer quenching from spectroscopic and electrochemical data.
A detailed study of some systems where the calculated quenching constants differ from the experimental ones by several orders of magnitude revealed that the quenching mechanism operative in these cases was hydrogen‐atom rather than electron transfer.
The conditions under which these different mechanisms apply and their consequences are discussed.
This paper reviews recent work on the development of electrocatalysts for CO2 reduction. Comparison of our electrocatalysts based on polypyridine complexes of the second and third row transition metals is made with previous work, and both areas are set in the framework of the known chemistry and electrochemistry of both uncoordinated CO2 and CO2-transition metal complexes. The emphasis of our work has been on mechanistic questions. For example, the family complexes fac-[ReI(bpy)(CO)3L]n+ (where bpy is 2, 2′-bipyridine and L is Br-, Cl- or CH3CN) are facile stoichiometric or catalytic reagents that reduce CO2 to CO, formate, or oxalate depending the external conditions. Synthesis, electrochemical, and kinetic studies implicate the involvement of a minimum of five different pathways for this unusual system. A newly discovered electrocatalyt is the reactive metal hydride, [Os(bpy)2(CO)H]-, that has been found to reduce CO2 by an associative mechanism yielding either CO or formate from a common intermediate. Related kinetic studies of fundamental steps CO2 activation or reduction have been conducted and their relationship to electrocatalytic CO2 reduction has been highlighted. Examples include CO2 insertion into a metal-alkoxide or metal-hydride bond. Finally, chemically modified electrodes have been prepared which allow the transposition of solution electrocatalytic chemistry to electrode surfaces. Although these studies are in their infancy it appears that new products (e.g., oxalate), and therefore new mechanistic pathways, have been found for some of the surface immoblized electrocatalsyts.
Properties of the excited states of tris(2,2'-bipyridine) and
tris(1,10-phenanthroline) complexes of chromium(3), iron(2),
ruthenium(2), osmium(2), rhodium(3), and iridium(3) are described. The
electron transfer reactions of the ground and excited states are
discussed and interpreted in terms of the driving force for the reaction
and the distortions of the excited states relative to the corresponding
ground states. General considerations relevant to the conversion of
light into chemical energy are presented and progress in the use of
polypyridine complexes to effect the light decomposition of water into
hydrogen and oxygen is reviewed.
The effect of vibrations in the inner-sphere of reacting molecules on the outer-sphere electron-transfer is investigated. We use a classical model of the solvent and a quantum two-state electron subsystem. In addition, we include a quantum mechanical treatment of the vibrational modes of the inner-sphere. A general derivation of the rate constant is given. Using a continuous model for the solvent a usable form of the rate constant is obtained. The various effects of the vibrations on the rate constant are investigated. Considerable effects are noticed for very exothermic and endothermic reactions. Some relevant experimental evidence is discussed.
This article describes examples of the application of time-resolved infrared Spectroscopy (TRIR) to the probing of the excited states of transition metal species. Other “direct” methods for excited states include time-resolved absorption Spectroscopy, and, more importantly, time-resolved resonance Raman Spectroscopy (TR3). Both of these techniques have limitations and hence TRIR is of value in complementing them. The common “indirect” methods are absorption, emission, and excitation spectroscopies, to which has been more recently added, resonance Raman, particularly in the time-dependent formulation. The relevance of these methods is also discussed.
Photophysical properties of Ru(bpy)32+, Ru(bpy)2(biq)2+, and Os(bpy)32+ (bpy=2,2′-bipyridine; biq=2,2′-biquinoline) in poly(ethyleneoxide) matrices (PEO) constituted by (CH2CH2O) repeating units, with average molecular weight 400 (PEO-400, a highly viscous fluid) and 600000 dalton (PEO-600000, a semicrystalline solid) have been studied at room temperature and 77 K. Comparison with similar systems is made. The absorption spectra, luminescence spectra and lifetimes at room temperature of the three complexes in both matrices are in agreement with the typical features reported for the same complexes in fluid solutions, and indicate that fast excited state relaxation via solvent reorganization occurs in both PEO matrices at room temperature. Such behaviour is not usual for solid matrices and is attributed to the microheterogeneous nature of PEO-600000 and to the ability of the solid PEO amorphous region to stabilize polar species within the timescale of radiative relaxation. The results suggest that PEO-600000 is a promising medium for studying electron and energy transfer processes having mild driving forces in the solid state at room temperature.
The complex cis-[(bpy)2Ru2]4+ (bpy is 2,2′-bipyridine) has been prepared by methylation of (bpy)2Ru2]2+. Electrochemical studies show that introduction of the bound pyridinium group creates a chemically attached electron acceptor site (E1/2 = −0.76 V in 0.1 M [N(n-C4H9)4]PF6-acetonitrile versus the SSCE). Evidence for a low-lying dπ — π* charge transfer (CT) state has been obtained by the appearance of a low energy emission at λmax 680 nm in ecetonitrile (τ0 = 104 ns) and for an upper dπ — π* (bpy) state by a higher energy emission at 580 nm in a methanol glass at 77 K (τ0 = 7.59 μs). Both emissions appear in a water—ethylene glycol solution containing 5% by weight polyvinyl alcohol at room temperature.
In artificial photosynthesis, the goal is to mimic the ability of green plants and other photosynthetic organisms in their use of sunlight to make high-energy chemicals. This is a difficult problem chemically, which accounts for much of the complexity of the natural photosynthetic apparatus. Nonetheless, a number of promising approaches have appeared in recent years based on semiconductors, membranes, vesicles, and molecular systems. In this Account the author will describe some of the approaches that my own group has taken to the design of chemical systems for artificial photosynthesis.
New luminescent complexes of Os(II) that contain either 2,2{prime}-bipyridine (bpy) or 1,10-phenanthroline (phen) as the chromophorib acceptor ligand have been prepared by a combination of established and new synthetic methods. Extensive use of Os(IV) and Os(III) precursors, e.g., Os{sup IV}(bpy)Cl{sub 4} and mer-Os{sup III}(PMe{sub 2}Ph){sub 3}Cl{sub 3} has led to the preparation of materials with ancillary ligands such as tertiary phosphines as preparative intermediates, including Os{sup III}(bpy)(PMe{sub 2}Ph)Cl{sub 3} and cis-Os{sup II}(phen)(diphosphine)Cl{sub 2}. Further substitution of chloro ligands into complexes such as these results in the formation of emissive complexes of Os(II). Another new synthetic route utilizes the versatile Os(II) precursor Os(bpy){sub 2}CO{sub 3}, which allows the facile preparation of dicationic, disubstituted species such as (Os(bpy){sub 2}(norbornadiene)){sup 2+}. Another general procedure, based on the control of solvent and temperature in the substitution chemistry of cis-Os(bpy){sub 2}Cl{sub 2}, has been further developed to produce a variety of new complexes of the types cis-(Os(bpy){sub 2}(L)Cl){sup +} and cis(Os(bpy){sub 2}(L){sub 2}){sup 2+}, where L is a phosphine, arsine, nitrogen, or olefin donor ligand. In a few cases, phosphine entering groups cause the cis geometry to be unfavorable and new trans-(Os(bpy){sub 2}(L){sub 2}){sup 2+} complexes have also been isolated. The resultant complexes comprise the largest family of transition-metal-based excited-state reagents with tunable photophysical and redox properties available. When possible, the new complexes have been characterized by UV-visible spectroscopy, emission spectroscopy, cyclic voltammetry, and {sup 31}P and/or {sup 1}H NMR spectroscopy.
Rate constants for both radical ion pair formation and recombination in porphyrin - quinone donor-acceptor molecules are reported. The ion pair formation and recombination depend on the exothermicity of the respective electron-transfer reaction in a manner previously reported. The question as to what fraction of energy change is due to relaxation processes involving the solvent vs. the porphyrin - quinone molecules has not been answered by these studies. However, the measured reorganization energy was found to be approximately solvent independent. It was observed that in these molecules the donor-acceptor distance is restricted, and the highly exothermic charge recombination reactions do not produce electronically excited states to the donor or acceptor. 11 references, 2 figures.
The time dependence of MLCT excited-state emission for (Os(phen)(das)/sub 2/)/sup 2 +/ in the glass-to-fluid transition region in 4:1 ethanol-methanol is analogous to that observed earlier for (Ru(bpy)/sub 3/)/sup 2 +/. This observation calls into question the earlier suggestion that the origin of the time dependence in the emission spectrum of Ru(bpy)/sub 3//sup 2 +/ is a delocalized, (Ru(bpy/sup -1//3)/sub 3/)/sup 2 +/*, to localized, ((bpy)/sub 2/Ru(bpy/sup -/))/sup 2 +/I, transition. In the osmium complex the time-dependent shifts to lower energy are proposed to arise from a dynamic solvent effect involving solvent dipole reorientations.
The temperature dependences of nonradiative decay for the metal-to-ligand charge transfer excited states of [Re(bpy)(CO)[sub 3](4-Etpy)](PF[sub 6]) and [Os(bpy)[sub 2](CO)(py)](PF[sub 6])[sub 2] (bpy = 2,2[prime]-bipyridine; 4-Etpy = 4-ethylpyridine; py = pyridine) follow the behavior predicted by the energy gap law. The variations with temperature can be calculated to within a factor of 2-3 by using parameters derived from a Frank-Condon analysis of emission spectral profiles. 17 refs., 4 figs., 1 tab.
The photophysical and photochemical properties of the series of tris-chelate complexes Ru(bpy)/sub n/(bpyz)/sub 3-n//sup 2 +/, Ru(bpy)/sub n/(bpym)/sub 3-n//sup 2 +/, Ru(bpym)/sub n/(bpyz)/sub 3-n//sup 2 +/, and Ru(bpy)(bpym)(bpyz)/sup 2 +/ (n = 0, 1, 2, 3; bpy = 2,2'-bipyridine, bpyz = 2,2'-bipyrazine, bpym = 2,2'-bipyrimidine) are described. From the results of temperature-dependent lifetime (210-345 K) and room-temperature emission quantum yield measurements have been obtained: (1) k/sub r/ and k/sub nr/, the radiative and nonradiative decay rate constants for the emitting MLCT manifold and (2) kinetic parameters which suggest the intervention of additional excited states. The significant points of the study are the following: (1) trends in k/sub nr/ properties are understandable based on the energy gap law, (2) low-lying dd states strongly influence lifetimes and photochemical instabilities for the complexes Ru(bpyz)/sub 3//sup 2 +/, Ru(bpym)/sub 3//sup 2 +/, Ru(bpy)(bpyz)/sub 2//sup 2 +/, Ru(bpy)(bpym)/sub 2//sup 2 +/, Ru(bpym)(bpyz)/sub 2//sup 2 +/, Ru(bpym)/sub 2/(bpyz)/sup 2 +/, and Ru(bpy)(bpym)(bpyz)/sup 2 +/ at room temperature, and (3) for the complexes Ru(bpy)/sub 2/(bpyz)/sup 2 +/ and Ru (bpy)/sub 2/(bpym)/sup 2 +/ there is no evidence for low-lying dd states and these and/or related mixed-ligand complexes may provide a basis for a new series of photochemically stable Ru-polypyridyl chromophores.
The crux of the problem is the fact that the equilibrium configuration of a species changes when it loses an electron. Configuration changes of organometallic metal complexes involve the metal-ligand and intra-ligand bond lengths and angles as well as changes in vibrations and rotation of surrounding solvent dipoles. Discussion indicates that rate constants can be expressed as a product of a nuclear, an electronic, and a frequency factor. Good agreement with measured rate constants is obtained in the normal free-energy region. Understanding of electron transfer rates in highly exothermic regions remains uncertain. 75 references, 2 figures, 2 tables.
The two primary intermediates that play a major role in determining the efficiencies of bimolecular photoinduced electron-transfer reactions are the contact (A{sup {sm bullet}{minus}}D{sup {sm bullet}+}) and the solvent-separated (A{sup {sm bullet}{minus}}(S)D{sup {sm bullet}+}) radical ion pairs, CRIP and SSRIP, respectively. These two species are distinguished by differences in electronic coupling, which is much smaller for the SSRIP compared to the CRIP, and solvation, which is much larger for the SSRIP compared to the CRIP. The present work addresses the quantitative aspects of these and other factors that influence the rates of energy-wasting return electron transfer within the ion-pair intermediates. The electron acceptor tetracyanoanthracene (TCA) forms ground-state charge-transfer complexes with alkyl-substituted benzene donors. By a change of the excitation wavelength and/or donor concentration, either the free TCA or the CT complex can be excited. Quenching of free {sup 1}TCA{sup *} by the alkylbenzene donors that have low oxidation potentials, such as pentamethylbenzene and hexamethylbenzene, in acetonitrile solution leads to the direct formation of geminate SSRIP. Excitation of the corresponding charge-transfer complexes leads to the formation of geminate CRIP. Rates of return electron transfer within the two types of ion pair are determined from quantum yields for formation of free radical ions pairs depend upon the reaction exothermicity in a manner consistent with the Marcus inverted region.
Two new methods for the photochemical synthesis of complexes of the type Ru(bpy)/sub 2/LX/sup +/ and Ru(bpy)/sub 2/X/sub 2/ (L = pyridine, CH/sub 3/CN, etc.; X = ClO/sub 4/, NO/sub 3/, NCS, Br, etc.) have been developed. The photochemistry is based on photosubstitution reactions of the parent complexes Ru(bpy)/sub 2/L/sub 2//sup 2 +/ most notably where L is pyridine in solvents of low polarity like dichloromethane. The photochemical methods are versatile and give products in good yield and purity. Mechanistically, the reactions appear to involve a dissociative step at the metal; a quantum yield of 0.18 is found for monosubstitution in Ru(bpy)/sub 2/(py)/sub 2//sup 2 +/, which is independent of the chemical identity and concentration of the entering ligand, X/sup -/. Some deductions about the photochemical behavior of the related complex Ru(bpy)/sub 3//sup 2 +/ based on our observations with Ru(bpy)/sub 2/L/sub 2//sup 2 +/ are also presented, and the possibility of a special medium effect in low-polarity solvents like dichloromethane is discussed.
Electropolymerized thin films of of poly[Ru(vbpy){sub 2}(py){sub 2}]{sup 2+} (vbpy is 4-vinyl-4{prime}-methyl-2,2{prime}-methyl-2,2{prime}-bipyridine; py is pyridine) were prepared. Photolysis of the films in the presence of chloride ion leads to photochemical substitution of Cl{sup -} for pyridine ligands, the structures of which were confirmed by small spot XPS. The absorption spectra and redox potentials of the ruthenium complexes were altered upon substitution of chloride for the pyridine ligands, suggesting the potential for using these materials to fabricate electrochromic film assemblies on optically transparent electrodes. The spectroelectrochemical response of the films was measured. 47 refs., 14 figs., 4 tabs.
Deuteron spin–lattice T1 and spin–spin T2 relaxation times have been measured at 10 MHz in aqueous (D2O) solutions of lithium chloride as a function of concentration (R= 3.5 to 6.3 mol D2O per mol LiCl) and temperature (175–320 K). These measurements are analysed using an empirical Cole–Davidson distribution of reorientational correlation times τ2 for D2O. The mean values of τ2 are found to be represented by 2=(1.24 ± 0.27)× 10–13 s exp {(701 ± 21) K/(T–T0)} with T0 values of 137 K (R= 3.7), 129 K (R= 4.8) and 125 K (R= 6.3). At low temperatures, the 2 are similar to the corresponding shear relaxation times s, measured between 149–173 K, while at 298 K, 2, the dielectric relaxation time D and the neutron diffusional correlation time τ0 have similar values. It is, therefore, concluded that the reorientation of the D2O molecules in these solutions is governed by the structural fluctuations associated with the glass transition. T1 measurements at 298 K, in the range R= 3 to 10, are interpreted to show that these solutions have similar structures to the corresponding glasses at 100 K : small clusters of Li+(H2O)4Cl– with excess water incorporated interstitially and having dynamic properties remarkably similar to those of bulk water. Previous T1 measurements for water (D2O) are reanalysed and the 2 shown to be dominated by a thermodynamic singularity at Ts= 228 K.
The non-adiabatic multiphonon theory of electron transfer provides a complete description of nuclear tunnelling and final-state excitation phenomena, which modify the classical Marcus theory. Ultrafast electron-transfer processes, where the electronic process competes with medium-induced vibrational relaxation, cannot be handled by the conventional theory, being amenable to description in terms of quantum-mechanical models.
Systematic variation of the ligand environment has allowed design of the absorbance characteristics of polypyridyl complexes of ruthenium(II) to produce “black absorbers” which absorb throughout the visible region. The presence of acceptor ligands with low-lying π* levels red shift the energies of the lowest energy MLCT bands, while MLCT and π
→
π* bands originating on other ligands can be used to fill in the higher-energy regions of the spectrum. Incorporation of anionic ligands or other electron-donating ligands causes a red shift in MLCT band energies compared to bpy by manipulation of dπ energy levels. Attention to these design principles has led to the synthesis of complexes which absorb appreciably in the near IR, and are free from complications caused by thermally accessible dd states. Although their emission energies (and energy gaps) are at low energy in the near IR, the use of lowest lying, delocalised acceptor ligands provides lifetime enhancements (compared to bpy) that can be dramatic.
The coordination of appropriate organic molecules containing
multiple
metal-binding domains to metal ions provides a versatile alternative to
carbon–carbon or carbon–heteroatom bond formation for the
assembly of dendrimers.
Highly resolved emission spectra of isolated [Ru(LL)[sub n](LL[prime])[sub 3[minus]n]][sup 2+] complexes doped into [Zn(bpy)[sub 3]] (ClO[sub 4])[sub 2] with LL and LL[prime] representing the ligands bpy-h[sub g], bpy-d[sub g], and bpz are presented. The information displayed in the electronic origins and the vibrational satellite structures combined with the emission decay behavior is used to provide a clear distinction between localized and delocalized MLCT transitions to the lowest excited states. In particular, the appearance or nonappearance of ligand-centered vibrational modes belonging to different types of ligands provides crucial evidence for delocalization or localization. Thus, for a localized excitation in [Ru(bpy)[sub 2](bpz)][sup 2+] only the ligand-centered modes of bpz can be found in the emission spectra. In contrast, for [Ru(bpy-h[sub g])[sub 2](bpy-d[sub g])][sup 2+] the ligand-centered vibrations of bpy-h[sub g] and bpy-d[sub g] accompany the same electronic origin. This is strong evidence for the delocalization of the excited electron in mixed-ligand [Ru(bpy-h[sub g])[sub 2](bpy-d[sub g])][sup 2+]. Consequently, the lowest MLCT excited states in homoleptic complexes like [Ru(bpy-h[sub g])[sub 3]][sup 2+] and [Ru(bpy-d[sub g])[sub 3]][sup 2+] are also delocalized over the metal and the different ligands. 39 refs., 4 figs., 1 tab.
Selective luminescence and excitation spectroscopy and Stark experiments of the title systems are reported. Dramatic differences of the x=1 and x=2 spectra, in comparison with the x=0 and x=3 spectra are observed. The lowest-excited state in the x=1 and x=2 spectra is a charge-transfer excitation localized on the protonated ligand(s). A superposition of ‘‘deuterated’’ and ‘‘protonated’’ transitions is observed for the x=1 and x=2 systems in the excitation spectra. The deuteration shift (≊32 cm−1) is much larger than the excitation exchange interaction β between the metal–ligand subunits. ‖β‖ is ≊2 cm−1, as directly determined from the observed splittings (5–7 cm−1) of I, the lowest-energy electronic origin. Stark shifts confirm the charge transfer character of the lowest-excited states. The splittings and relative intensities of the origins and their Stark effect can be quantitatively described by an exciton formalism.
Nanocrystallites of antimony-doped tin dioxide have been prepared hydrothermally by treating colloids of tin antimony oxide in an autoclave. At a synthesis temperature of 270 °C stable colloidal solutions of blue-colored SnO2:Sb nanocrystallites have been obtained. High resolution transmission electron microscopy (TEM) images show highly crystalline particles in the 4 to 9 nm size regime. X-ray powder diffraction patterns of the nanocrystals indicate the same rutile lattice structure as known from bulk SnO2. Powders of the antimony-doped nanocrystals exhibit an up to 105 fold increase in electrical conductivity as compared to the corresponding undoped systems. The blue color of the doped colloids corresponds to a broad absorption peak in the red and the IR region. The IR-absorption spectrum of doped nanoparticles deposited onto sapphire substrates has been fit to a model based on the Drude theory of a free electron gas in SnO2:Sb and an effective medium approximation. The model indicates that the IR absorption corresponds to a plasmon polariton excitation of weakly interacting n-doped nanoparticles.
A general mathematical treatment of vibronic coupling of two electronic states in molecules and dimers is presented. The 2×2 matrix Hamiltonian which is derived is shown to reduce to two one-dimensional Hamiltonians provided a certain minimum amount of symmetry is present. Some general selection rules governing electronic transitions to these states from the ground state are obtained, showing that the spectral distribution in hot bands may differ considerably from that in normal bands when vibronic coupling occurs. A special model which considers the coupling to arise from a single mode of vibration is presented. Two limiting cases which can be treated approximately by perturbation theory are considered in detail. These are shown to give rise to a ``pseudo Jahn-Teller effect'' and to vibrational borrowing in the two different limits.
Observation of benzophenone triplet emission in fluid solution by an electrogenerated chemiluminescence (ECL) technique from the benzophenone(−)/thianthrene(+) system and related mixed systems is reported. The triplet character of the ECL was confirmed by experiments involving energy transfer from the benzophenone triplet to naphthalene.
Fast time-resolved IR (TRIR) spectroscopy is used to record ν(C–O) bands of the lowest lying excited state of [ClRe(CO)3(4,4′-bipyridyl)2](half-life of approximately 1 µs in CH2Cl2 solution); there is a shift of the ν(C–O) bands to higher frequency relative to the ground state, which is consistent with oxidation of the metal centre via electron transfer to the bipyridyl ligands.