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Theoretical Investigation of a Novel One-Dimensional Infinite Monatomic Zinc Wire with Excellent Electronic Properties
ChemistrySelect
Abstract
Herein, a novel and economical one‐dimensional infinite monatomic zinc wire (1D‐IMZW) is designed, and its structural and electronic properties are investigated by using density functional theory. It is found that this zinc nanowire can be stabilized by the Zn−Zn bonding interaction between the adjacent Zn atoms. From the band structure and the density of states, it is discovered that the 1D‐IMZW is a typical semiconductor material. Moreover, the 1D‐IMZW exhibits strong absorption across the entire visible range.
Mixed-valence metal-organic nanostructures show unusual electronic properties. In our pervious investigation, we have designed and predicted a unique one-dimensional infinite monatomic gold wire (1D-IMGW) with excellent conductivity and the interesting characteristic of mixed valency (Auc³⁺ and Au0i). For further exploring its conduction properties and stability in conducting state, here we select one electron as a probe to explore the electron transport channel and investigate its electronic structure in conducting state. Density functional theory (DFT) calculations show the 1D-IMGW maintains its original structure in conducting state illustrating its excellent stability. Moreover, while adding an electron, 1D-IMGW is transformed from a semiconductor to a conductor with the energy band mixed with Auc (5d) and Aui (6s) through the Fermi level. Thus 1D-IMGW will conduct along its gold atom chain demonstrating good application prospect in nanodevices.
One-dimensional (1D) semiconductor ZnO nanowires have been successfully synthesized by a novel soft-chemical hydrothermal method with allylpolyethoxy amino carboxylate (AA-APEA) at low temperature. Their structure and properties have been characterized by a series of techniques, including X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and transmission electron microscopy (TEM). It was found that ZnO nanowires with diameters around 50 nm and lengths up to about several micrometers are well-distributed. The photocatalytic activity toward degradation of methylene blue (MB) aqueous solution under ultraviolet (UV) was investigated and the results showed that the ZnO nanowires exhibit a markedly higher photoactivity compared to the ZnO nanoparticles which were obtained without AA-APEA polymer assistant, and it can be ascribed to the special 1D morphology of the ZnO nanowires. In particular, the rate of degradation of the ZnO nanowires was 11 times faster than that of ZnO nanoparticles. In addition, the ZnO nanowires could be easily recycled in UV photocatalytic activity. These observations could promote new applications of photocatalyst for wastewater treatment utilizing oxide semiconductor nanostructures.
Comprehension and modulation of optical activity at nanoscale have attracted tremendous interest in the past decades due to its potential application in many fields including chemical/biological sensing, artificial metamaterials, asymmetric catalysis, etc. As for the conventional molecular materials, magnetic field is among the most effective routes in inducing and manipulating their optical activity; whereas the magnetic optical activity at nanoscale calls for deeper understanding, especially for anisotropic noble metal nanoparticles. In this work, distinctly different magnetic circular dichroism (MCD) responses are demonstrated in gold nanorods (GNRs) with a derivative-shaped MCD signal corresponding to the transverse surface plasmon resonance (TSPR) band and a Gaussian-shaped signal at the position of the longitudinal surface plasmon resonance (LSPR) band. Furthermore, changing the aspect ratio of GNRs easily regulates such magnetoplasmonic CD response. More interestingly, GNR assemblies with different geometric configuration (end-to-end and side-by-side) show structure-sensitive magnetoplasmonic CD response. Armed with theoretical calculation, we clearly elucidate the intrinsic relationship of the resultant magnetoplasmonic CD response with the optical symmetry and geometry factor inside one-dimensional GNRs. This work not only greatly benefits our understanding towards the nature of SPR mode in anisotropic plasmonic nanostructures but also opens the way to achieve tunable magnetoplasmonic response, which will significantly advance the design and application of optical nanodevices.
This paper addresses the synthesis and a detailed electrical analysis of individual copper nanowires (CuNWs). One dimensional CuNWs are chemically grown using Bromide ions (Br-) as a co-capping agent. By partially replacing alkyl amines with Br-, the isotropic growth on Cu seeds was suppressed during the synthesis. To study the electrical properties of individual CuNWs, a fabrication method is developed which does not require any e-beam lithography process. Chemically grown CuNWs have an ampacity of about 30 million amps per cm², which is more than one order of magnitude larger than the bulk Cu. These good qualities easy to synthesize CuNWs are an excellent candidate for creating high ampacity wires and flexible printable electronics.
One-dimensional growth of copper (Cu) is observed when Cu is deposited on cuprous iodide (CuI) films at elevated temperatures by thermal evaporation in high vacuum. The morphology, crystallinity, and chemical composition were examined for the samples using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Straight single-crystalline Cu nanowires can be formed on polycrystalline CuI films at 200 °C. Anisotropic growth of Cu in one-dimension was promoted by enhancing grain growth and continuity of CuI films. At a lower temperature of 100 °C, fuzzy branching nanowires were grown on the surface of CuI films, enabling to simply fabricate a self-supporting, highly porous film of Cu. It is revealed that a CuI film serves as a functional substrate to synthesize nanostructured Cu at low temperatures. Possible growth processes are proposed to explain the observed morphology evolution of Cu with respect to synthesis conditions. The characterization of water wettability showed that the hydrophobicity was remarkably enhanced due to the formation of nanostructured Cu on the CuI films.
We revealed symmetry-breaking charge transfer (SBCT) processes in the excited state of directly linked push-pull porphyrin dyad (AD) and triad (ADA) by spectroscopic measurements including steady-state absorption and fluorescence, time-resolved fluorescence (TRF), femtosecond transient absorption (fs-TA) and time-resolved infrared (TRIR) measurements. Unprecedented broad fluorescence spectra in porphyrin arrays were observed in polar solvents, which were attributed to the existence of a charge transfer state as shown by TRF measurements. TA also revealed rising features of a CT state for AD and ADA in polar solvents. These dynamics were also confirmed by TRIR measurements, which provided further information on solvation and structural relaxation processes of the SBCT state. This is the first observation of SBCT process in porphyrin arrays providing fundamental understanding of strongly coupled porphyrin arrays. New opportunities for porphyrin arrays will come forward in relevant applications where SBCT is needed.
Because of their unique physical and chemical properties, one-dimensional (1-D) metal-oxide nanostructures have been extensively applied in the areas of gas sensors, electrochromic devices, nanogenerators, and so on. Solar water-splitting has attracted extensive research interest because hydrogen generated from solar-driven water splitting is a clean, sustainable, and abundant energy source that not only solves the energy crisis, but also protects the environment. In this comprehensive review, the main synthesis methods, properties, and especially prominent applications in solar water splitting of 1-D metal-oxides, including titanium dioxide (TiO2), zinc oxide (ZnO), tungsten trioxide (WO3), iron oxide (Fe2O3), and copper oxide (CuO) are fully discussed.
The aim of this review is to introduce mercaptoacetic acid (MAA) as a potential capping agent and sulfur source in the synthesis of one-dimensional (1D) nanostructured metal sulfides such as CdS, ZnS and PbS nanostructures. The effects of reaction temperature, reaction time, the concentration of MAA and source of metal (conventional salt or complex salt) on the phase and morphology evolutions of the product have been reviewed. The general strategies and formation mechanism of 1D growth, well as the optical properties of the products, have been discussed using this new capping agent.
It is well-known that the inner electric field formed between the counter electrode and the semiconductor surface has a substantial effect on the efficiencies of dye-sensitized solar cells (DSSCs). To reveal the function of the inner electric field for different types of porphyrin sensitizers in DSSCs, the properties of three types of porphyrin sensitizers ([small alpha], [small beta], and center axial positions) under different electric fields were calculated by using density functional theory (DFT) and time dependent density functional theory (TD-DFT). The electronic structures and optical properties of these studied dyes in tetrahydrofuran (THF) solution were also investigated correspondingly. Key parameters of the short-circuit current density (Jsc) including light harvesting efficiency (LHE), electron injection driving force ([capital Delta]Ginject) and intramolecular charge transfer (ICT) were detailedly discussed. The results show that the a-position type porphyrin sensitizer can be used as a potential sensitizer for DSSCs under an enhanced electric field. We expect that the present study would deepen the understanding of the function of an inner electric field and may be helpful in DSSC design in the future.
Gold nanowires have attracted considerable attention owing to their potential applications in mesoscopic research and nanodevices. However, monatomic Au long chains are naturally metastable, making it difficult to use them directly in these applications. Herein, a unique one-dimensional (1D) infinite monatomic gold wire (1D-IMGW) was designed, and its electronic and optical properties were characterized by density functional theory (DFT) calculations. The 1D-IMGW was stabilized by corrole rings, and there were strong interactions between the dz(2) orbitals of the Auc atoms in the centers of the corrole rings and the 6s orbitals of the Aui atoms in the middle of adjacent corrole rings. The excellent conductivity of one-dimensional metal nanowires was observed along the Au wire perpendicular to the corrole rings. Moreover, the 1D-IMGW demonstrated the unique characteristic of mixed valences (Au(3+) and Au(0)) and exhibited strong absorption across the entire visible range.
The severe consequences of fossil fuel consumption have resulted in a need for alternative sustainable sources of energy. Conversion and storage of solar energy via a renewable method, such as photocatalysis, holds great promise as such an alternative. One-dimensional (1D) nanostructures have gained attention in solar energy conversion because they have a long axis to absorb incident sunlight yet a short radial distance for separation of photogenerated charge carriers. In particular, well-ordered spatially high dimensional architectures based on 1D nanostructures with well-defined facets or anisotropic shapes offer an exciting opportunity for bridging the gap between 1D nanostructures and the micro and macro world, providing a platform for integration of nanostructures on a larger and more manageable scale into high-performance solar energy conversion applications. In this review, we focus on the progress of photocatalytic solar energy conversion over controlled one-dimension-based spatially ordered architecture hybrids. Assembly and classification of these novel architectures are summarized, and we discuss the opportunity and future direction of integration of 1D materials into high-dimensional, spatially organized architectures, with a perspective toward improved collective performance in various artificial photoredox applications.
One-dimensional (1D) nanostructures can efficiently scatter incident light, resulting in improved absorption or complete absorption for solar energy conversion and storage. However, 1D nanostructures often lack good conductivity for fast charge transfer and/or transport. A thin-layer coating of graphene gives superior conductivity for improving the charge transport ability while its highly transparency does not deteriorate the light absorption. Thus, construction of 1D nanostructured materials on graphene as an electrode to synergistically boost high-efficiency energy conversion and storage have attracted great attention in recent years. In this feature review, starting with general concepts of 1D nanostructures on a substrate, various advanced methods for the design, fabrication and characterization of different 1D nanostructures comprising inorganic, organic and hybrid materials built on graphene are systematically surveyed. In particular, the significant progress in fabrication strategies, superior nanostructures and unique architectures is discussed, while the excellent electrical, optical, mechanical, and electrochemical properties of the nanostructured composites as well as their important applications in lithium-ion batteries, supercapacitors, solar cells, light-emitting diodes and nanogenerators are also presented. The enhancement mechanisms for the efficient energy conversion and storage are highlighted to elicit scientific insights. The challenges and prospects are also deliberated to spark our future researches. This review provides critical and updated knowledge for researchers to further explore new 1D-structured materials on graphene and their important applications in energy conversion and storage.
Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (ηrel) are affected by the molecular spacer connecting the porphyrin core to the TiO2 surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO2 surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO2 surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, ηrel, and the concentration of long-lived electrons in the conduction band of TiO2 was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO2 surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO2 occurs “through-space” rather than “through the molecular spacer”. It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties.
We present the synthesis of four perylene–porphyrin building blocks for use in Glaser, Sonogashira, or Suzuki polymerizations. The building blocks bear synthetic handles (4-ethynylphenyl, 4-iodophenyl, bromo) at the trans
(5,15)
meso-positions of a zinc porphyrin and contain two or four perylene-monoimide dyes attached at the 3,5-positions of the non-linking meso-aryl rings of the porphyrin. Each perylene-monoimide bears three 4-tert-butylphenoxy substituents (at the 1-, 6-, and 9-positions) and two isopropyl groups (on the N-aryl unit) for increased solubility. In each case the intervening linker is a diarylethyne unit that bridges the N-imide position of the perylene and the meso-position of the porphyrin. The perylene–porphyrin building blocks were prepared by (1) reaction of a diperylene-dipyrromethane with an aldehyde yielding a trans-A2B2-porphyrin, (2) reaction of a diperylene-aldehyde with a dipyrromethane yielding a trans-A2B2-porphyrin, and (3) reaction of a diperylene-dipyrromethane with a dipyrromethane-dicarbinol yielding a trans-AB2C-porphyrin or ABCD-porphyrin. The building blocks were subjected to Glaser, Sonogashira, or Suzuki coupling conditions in an effort to prepare oligomers containing porphyrins joined via 4,4′-diphenylbutadiyne (dpb), 4,4′-diphenylethyne (dpe), or 1,4-phenylene linkers (p), respectively. Each porphyrin in the backbone bears two or four pendant perylene-monoimide dyes. The Glaser and Sonogashira reactions afforded a distribution of oligomers, whereas the Suzuki reaction was unsuccessful. The oligomers were soluble in solvents such as toluene, THF, or CHCl3 enabling routine handling. The use of perylenes results in (1) increased light-harvesting efficiency particularly in the green spectral region where porphyrins are relatively transparent and (2) greater solubility than is achieved with the use of porphyrins alone. The soluble perylene–porphyrin oligomers are attractive for use as light-harvesting materials in molecular-based solar cells.
The first examples of zinc compounds with bridging fluorine
and hydrogen atoms
[{HC(CMeNAr)2}Zn(μ-X)]2 (Ar =
2,6-Me2C6H3, X = F, 8; X = H, 9) have been
synthesised and structurally characterised.
Time-dependent density functional theory (TDDFT) calculations were performed on the excitation energies and oscillator strengths of fully π-conjugated zinc oligoporphyrins. The TDDFT calculated results for the Q bands of the triply meso-meso-, β-β-, and β-β-linked zinc oligoporphyrins can well reproduce the significantly increasing redshifted and intensified absorption maximum for the increasing number of porphyrins in the experimental data, which originates from the greater stabilized lowest unoccupied molecular orbital (LUMO) and the increasing contribution of the highest occupied molecular orbital (HOMO)→LUMO excitation to the bands. In order to estimate the minimum value of the HOMO−LUMO energy gap (Eg) of the infinite size oligoporphyrins, the DFT solid-state calculations for the one-dimensional tapelike and two-dimensional sheetlike π-conjugated zinc porphyrins were carried out using periodic boundary conditions. The results show that there is a metallic energy gap, Eg ≈ 0.0 eV for the tape, and a semiconducting one, Eg ≈ 0.5 eV for the sheet, which is caused by the difference in the local periodic structures at the minimum energy gap between them, namely the directly and indirectly connected zinc porphyrins for the former and the latter, respectively. Similarly, the corresponding infinite fused free-base porphyrins were also examined for comparison, which showed that a metallic behavior is also expected for them. The present fused porphyrin arrays may be used as conducting molecular wires or films. © 2002 American Institute of Physics.
We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N-atoms(2) scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special 'preconditioning' optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package), The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
A computational method for partitioning a charge density grid into Bader volumes is presented which is efficient, robust, and scales linearly with the number of grid points. The partitioning algorithm follows the steepest ascent paths along the charge density gradient from grid point to grid point until a charge density maximum is reached. In this paper, we describe how accurate off-lattice ascent paths can be represented with respect to the grid points. This improvement maintains the efficient linear scaling of an earlier version of the algorithm, and eliminates a tendency for the Bader surfaces to be aligned along the grid directions. As the algorithm assigns grid points to charge density maxima, subsequent paths are terminated when they reach previously assigned grid points. It is this grid-based approach which gives the algorithm its efficiency, and allows for the analysis of the large grids generated from plane-wave-based density functional theory calculations.
Among the porphyrin molecules with different metal insertion sites and functional ligands, Zn-porphyrin most efficiently regenerates NADH through photo-induced electron transfer in the presence of [Cp*Rh(bpy)H(2)O](2+), a rhodium-based electron mediator. Photochemical regeneration of NADH by Zn-porphyrin is successfully coupled with redox enzymatic synthesis under dark state conditions.
Directly meso-meso, beta-beta, beta-beta triply linked porphyrin arrays are exceptional pi-conjugated molecules exhibiting remarkably red-shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12-mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso-meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso-meso linked porphyrin arrays BOn up to 24-mer, which have two different aryl groups, a 2,4,6-tris(3,5-di-tert-butylphenoxy) phenyl group (Ar(1)) and a 3,5-dioctyloxy phenyl group (Ar(2)). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ-Sc(OTf)(3). Importantly, the low energy Q-band-like absorption bands of TBOn are progressively red-shifted with an increase in the number of porphyrins n until 16 but the red-shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14-16. The regularly introduced meso-aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of pi-pi interactions as well as improvement of the chemical stabilities of TBOn.
We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.
Molecules designed to carry out photochemical energy conversion typically employ several sequential electron transfers, as do photosynthetic proteins. Yet, these molecules typically do not achieve the extensive charge transport characteristic of semiconductor devices. We have prepared a large molecule in which four perylene-3,4:9,10-tetracarboxydiimide (PDI) molecules that both collect photons and accept electrons are attached to a central zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP) electron donor. This molecule self-assembles into ordered nanoparticles both in solution and in the solid-state, driven by van der Waals stacking of the PDI molecules. Photoexcitation of the nanoparticles results in quantitative charge separation in 3.2 ps to form ZnTPP(+)PDI(-) radical ion pairs, in which the radical anion rapidly migrates to PDI molecules that are, on average, 21 A away, as evidenced by magnetic field effects on the yield of the PDI triplet state that results from radical ion pair recombination. These nanoparticles exhibit charge transport properties that combine important features from both photosynthetic and semiconductor photoconversion systems.
Unlike mercury, which has an extensive +1 oxidation state chemistry, zinc usually adopts the +2 oxidation state. Decamethyldizincocene,
Zn2(η5-C5Me5)2, an organometallic compound of Zn(I) formally derived from the dimetallic [Zn–Zn]2+ unit, has been isolated from the low-temperature (–10°C) reaction of Zn(C5Me5)2 and Zn(C2H5)2 in diethyl ether. X-ray studies show that it contains two eclipsed Zn(η5-C5Me5) fragments with a Zn–Zn distance (± standard deviation) of 2.305(±3) angstroms, indicative of a metal-metal bonding interaction.
More than 1 g of the dizinc compound [Zn2(η5- C5Me5)2] can be readily obtained in a single preparation. Density functional calculations reveal that the Zn-Zn bond is relatively strong and derives mainly from interaction between the Zn 4 s orbitals (see picture).
Potassium reduction of RZn(mu-I)2Li(OEt2)2 (R = [{(2,6-Pri2C6H3)N(Me)C}2CH]) affords the second compound with a Zn-Zn bond, RZn-ZnR. The air- and moisture-sensitive title compound was characterized by 1H NMR, elemental analyses, and single-crystal X-ray diffraction. The Zn-Zn bond was determined to be 2.3586(7) A; this value is only about 0.05 A longer than the Zn-Zn bond reported for Cp*Zn-ZnCp* (Cp* = C5Me5), the first reported compound with a Zn-Zn bond. In addition, density functional theory (DFT) computations on related model RZn-ZnR compounds provide insight into the intriguing Zn-Zn bond.
We have comparatively investigated the photophysical properties of a series of meso-meso directly linked orthogonal porphyrin arrays (Zn, n = 1, 2, 3, 4, 6, 8, 9, 12, 16, 32, 48, 64, and 96) by ensemble average and single molecule fluorescence spectroscopy. In single molecule fluorescence study, we have recorded the fluorescence intensity trajectories of Zn arrays as the number of porphyrin molecules in the array increases. Up to Z8 in porphyrin arrays, each single array exhibits multiple stepwise photobleaching behaviors in fluorescence intensity trajectories, indicating that each porphyrin unit in the array acts as an individual fluorescent unit due to a maintenance of linear rigid structure of the array. On the other hand, porphyrin arrays longer than Z8 such as Z16, Z32, Z48, and Z64 show complicated photobleaching behaviors in fluorescence intensity trajectories. The origin of complex photobleaching behaviors is believed to be increasing nonradiative decay channels contributed by the enhanced structural nonlinearity in longer arrays. The fluorescence measurements of Zn arrays on single molecule level show a mismatch in the maximum fluorescence intensity level as compared to the solution measurements, which is attributable to the difference in local environment surrounding the porphyrin array. In this work, we have demonstrated the presence of conformational heterogeneity in longer porphyrin arrays by analyzing average survival times and fluorescence spectra of single arrays as the number of porphyrin molecules in the array increases. We believe that the fluorescence properties of porphyrin arrays on single molecule level will provide a platform for further applications as molecular photonic wires.
The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.
We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.
Selective metal-ligand interactions have been used to prepare supramolecular bidentate ligands by mixing monodentate ligands with a suitable template. For these assemblies pyridine phosphorus ligands and a zinc(II) porphyrin dimer were used. In the rhodium-catalysed hydroformylation of 1-octene and styrene improved selectivities have been obtained for some of the assembled bidentate ligand systems. In the palladium catalysed asymmetric allylic alkylation similar effects were observed; the enantioselectivity increased by using a bisporphyrin template. The preparation of supramolecular catalyst systems was also explored using tin-oxygen interactions. Dihydroxotin(IV) porphyrin and carboxylic phosphorus ligands assemble into supramolecular ligands and the phosphorus donor atom coordinates to transition metals. The stronger oxygen-tin bond, compared to pyridine-zinc does not result in a better performance of the catalyst.
- Xia
- Zhu