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5-(Hetero)aryl-Substituted 9-Hydroxyphenalenones: Synthesis and Electronic Properties of Multifunctional Donor-Acceptor Conjugates
Chemistry - A European Journal
Abstract
The universe of donor and acceptor building blocks is often nonluminescent. However, if linked together by our protection-group-free Suzuki coupling as an ignition we can efficiently launch electronically diverse functional donor–acceptor molecules. A colorful “glow in the dark” property can be introduced from the unique electron acceptor 9-hydroxyphenalenone and suitable π-donors. This results in tunable luminophores with interesting functionalities and electronic properties. More information can be found in the Full Paper by T. J. J. Müller et al. (DOI: 10.1002/chem.201700553).
Comprehensive Summary
Phenalenyl derivatives have been of tremendous interest due to their reversible dimerization in response to the external stimuli such as temperature. However, most of PLY derivatives only show the reversibility dependent on temperature. Herein, we report a unique PLY‐based radical anionic salt with reversible σ‐dimerization controlled by solvent rather than temperature. The structure and reversibility of this PLY anion system have been spectroscopically, crystallographically, and theoretically investigated. The molecule exists mainly as the σ‐dimer form of K2[σ‐12] in toluene, while it can be broken into the monomeric radical anion 1•– by re‐dissolving in THF. Moreover, when THF is replaced with toluene, radical 1•– could be σ‐dimerized back again. Theoretical calculations reveal that the potassium ions are necessary for the formation of the σ‐dimer and the solvent polarity controls the σ‐dimerization by regulating the presence of K⁺.
Multicomponent reactions are a valuable tool for the synthesis of functional pi-electron systems. Two different approaches can be taken into account for accessing the target structures. In the more conventional scaffold approach an already existing chromophore is coupled with other components to give the complex functional pi-system. Here, also electronically monotonous components can be introduced, which may exert synergistic electronic effects within the novel compound. The more demanding chromophore concept generates the complete pi-electron system and the scaffold concurrently. The latter approach is particularly stimulating for methodologists since pi-systems might be accessible from simple starting materials. This review encompasses the advances in the preparation of functional pi-electron systems via multicomponent processes during the past years, based both on the scaffold and chromophore concepts. Besides the synthetic strategies the most important properties, i.e. redox potentials, absorption and emission maxima or fluorescence quantum yields, of the synthesized molecules are highlighted.
The present paper reports the synthesis of the acetate of 9-hydroxyphenalenone and related compounds and some of their properties. The crystal structure of 9-butoxyphenalenone has been determined and its geometry is compared with that of 9-hydroxyphenalenone.
The new compounds [Ru(pap)2(L)](ClO4), [Ru(pap)(L)2], and [Ru(acac)2(L)], pap = 2-phenylazopyridine, L- = 9-oxidophenalenone, acac = 2,4-pentanedionate, have been prepared and studied regarding their electron transfer behavior both experimentally and by DFT calculations. [Ru(pap)2(L)](ClO4) and [Ru(acac)2(L)] were characterized by crystal structure analysis. Spectroelectrochemistry (EPR, UV-VIS-NIR) in conjunction with cyclic voltammetry showed a wide range of about 2 V for the potentials of the RuIII/II couple, in agreement with the very different characteristics of strongly π-accepting pap and σ-donating acac. At the rather high potential of +1.35 V versus SCE the oxidation of L- to L• could be deduced from the near-IR absorption of [RuIII(pap)(L•)(L-)]2+. Other intense long-wavelength transitions include LMCT (L- → RuIII) and LL/ CT (pap•- → L-) processes, confirmed by TD-DFT results. DFT Calculations and EPR data for paramagnetic intermediates allowed for an assessment of spin densities which revealed two cases with considerable contributions from L-radical involving forms, i.e. [RuIII(pap0)2(L-)]2+ ↔ [RuII(pap0)2(L•)]2+ and [RuIII(pap0)(L-)2]+ ↔ [RuII(pap0)(L•)(L-)]+. The electrogenerated complex [RuII(pap•-)(pap0)(L-)] was calculated to display considerable negative spin density (-0.188) at the bridging metal.
Continuum solvation models are appealing because of the simplified yet accurate description they provide of the solvent effect on a solute, described either by quantum mechanical or classical methods. The polarizable continuum model (PCM) family of solvation models is among the most widely used, although their application has been hampered by discontinuities and singularities arising from the discretization of the integral equations at the solute-solvent interface. In this contribution we introduce a continuous surface charge (CSC) approach that leads to a smooth and robust formalism for the PCM models. We start from the scheme proposed over ten years ago by York and Karplus and we generalize it in various ways, including the extension to analytic second derivatives with respect to atomic positions. We propose an optimal discrete representation of the integral operators required for the determination of the apparent surface charge. We achieve a clear separation between "model" and "cavity" which, together with simple generalizations of modern integral codes, is all that is required for an extensible and efficient implementation of the PCM models. Following this approach we are now able to introduce solvent effects on energies, structures, and vibrational frequencies (analytical first and second derivatives with respect to atomic coordinates), magnetic properties (derivatives with respect of magnetic field using GIAOs), and in the calculation more complex properties like frequency-dependent Raman activities, vibrational circular dichroism, and Raman optical activity.
For the practical use of a photo-bioreactor for artificial photosynthesis, efficient visible light-absorbing materials have to link reduction and oxidation catalysts for an efficient energy flow. As a step toward this goal of an NADH regeneration system and enzymatic production of solar fuels from CO2, we report the synthesis of a new polydiacetylene compound that is covalently connected with [Ru(phen-NH2)(bpy)2]2+ (bpy = 2,2’-bipyridine, phen = 1,10-phenonthroline-5-amine). The [(bpy)2Ru(phen)]-polydiacetylene absorbed a wide range of visible light because of the presence of two chromophores, the Ru complex and polydiacetylene. The polyacetylene backbone was converted from blue to red by conformational changes under the catalytic reaction conditions in a buffer solution. The electron transfer from the photoexcited [Ru(phen)(bpy)2]2+ to the polydiacetylene backbone was observed. In a visible light-driven photocatalytic NAD+ reduction by (cyclopentadienyl)Rh(bpy)(H2O)2+ with [(bpy)2Ru(phen)]-polydiacetylene, NADH was regenerated, and the reactivity using Ru(bpy)2(phen)-polydiacetylene was enhanced relative to control experiments using only [Ru(phen)(bpy)2]2+ or polydiacetylene. The consecutive carbon dioxide reduction coupled with formate dehydrogenase was carried out to utilize the in-situ photoregenerated NADH catalytically. The catalytic condition using [(bpy)2Ru(phen)]-polydiacetylene also showed much higher reactivity than the controls.
Recently, the use of transition metal based chromophores as photo-induced single-electron transfer reagents in synthetic organic chemistry has opened up a wealth of possibilities for reinventing known reactions as well as creating new pathways to previously unattainable products. The workhorses for these efforts have been polypyridyl complexes of Ru(ii) and Ir(iii), compounds whose photophysics have been studied for decades within the inorganic community but never extensively applied to problems of interest to organic chemists. While the nexus of synthetic organic and physical-inorganic chemistries holds promise for tremendous new opportunities in both areas, a deeper appreciation of the underlying principles governing the excited-state reactivity of these charge-transfer chromophores is needed. In this Tutorial Review, we present a basic overview of the photophysics of this class of compounds with the goal of explaining the concepts, ground- and excited-state properties, as well as experimental protocols necessary to probe the kinetics and mechanisms of photo-induced electron and/or energy transfer processes.
Herein, we report the synthesis and characterization of 9-hydroxophenalenone based alkaline earth and zinc complexes. The reaction of 9-hydroxophenalenone (HO,O-PLY (1)) with one equivalent of KN(SiMe3)2 and MI2 in THF yields heteroleptic complexes [(O,O-PLY)M(THF)n]I [M = Mg (2), Ca (3), Sr (4), Ba (5); n = 1–4], while use of two equivalents of KN(SiMe3)2 in THF (with respect to PLY) produces homoleptic complex (O,O-PLY)2Mg(THF)2 (6). Moreover, reaction between two equivalents of 1 with one equivalent of ZnMe2 in THF produces complex (O,O-PLY)2Zn(THF)2 (7). All these complexes were characterized by NMR spectroscopy and elemental analyses. The solid state structures of complexes 2, 6 and 7 were established by single crystal X-ray diffraction analysis.
Graphical Abstract
A number of alkaline earth and zinc metal complexes have been synthesized using phenalenyl as a ligand backbone. All of these complexes were characterized by NMR spectroscopy and elemental analysis and a few representative compounds were characterized by single crystal X-ray measurements.
The synthesis of organics 9-hydroxyphenalenone (HPO), 2-methyl-9-hydroxyphenalenone (MHPO), 6-hydroxybenz[de] anthracen-7-one (HBAO), 5-amino-1,10-phenanthroline (phen-NH2) and its functionalized mesoporous MCM-41 by covalent band are reported, as well as those of Europium (III) hybrid mesoporous materials, EuL(3)phen-MCM-41 (L = HPO, MHPO, HBAO). The photoluminescence and microstructural properties are characterized, and the XRD and BET results revealed that all of these hybrid materials have uniformity in the mesostructure. It is worth noting that the excitation spectra of these hybrid materials have a broad absorption, which occupies from UV to visible region (250-475 nm). Upon ligand-mediated excitation with the visible light, the low efficient energy transfer exhibit between the organic ligands and europium(III) ion under visible excitation, but the maximum wavelength (456 nm/457 nm/451 nm) located at blue light region, which is in consistent with the blue LED light, then might be a feasible alternative in producing time-resolved luminescence under LED-excitation.
Open-shell phenalenyl chemistry started more than half a century back, and the first solid-state phenalenyl radical was realized only 15 years ago highlighting the synthetic challenges associated in stabilizing carbon-based radical chemistry, though it has great promise as building blocks for molecular electronics and multifunctional materials. Alternatively, stable closed-shell phenalenyl has tremendous potential as it can be utilized to create an in situ open-shell state by external spin injection. In the present study, we have designed a closed-shell phenalenyl-based iron(III) complex, Fe(III)(PLY)3 (PLY-H = 9-hydroxyphenalenone) displaying an excellent electrocatalytic property as cathode material for one compartment membraneless H2O2 fuel cell. The power density output of Fe(III)(PLY)3 is nearly 15-fold higher than the structurally related model compound Fe(III)(acac)3 (acac = acetylacetonate) and nearly 140-fold higher than an earlier reported mononuclear Fe(III) complex, Fe(III)(Pc)Cl (Pc = pthalocyaninate), highlighting the role of switchable closed-shell phenalenyl moiety for electron-transfer process in designing electroactive materials.
The unpolarized absorption and circular dichroism spectra of the
fundamental vibrational transitions of the chiral molecule,
4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields
are obtained using Density Functional Theory (DFT), MP2, and SCF
methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use
the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP)
density functionals. Mid-IR spectra predicted using LSDA, BLYP, and
B3LYP force fields are of significantly different quality, the B3LYP
force field yielding spectra in clearly superior, and overall excellent,
agreement with experiment. The MP2 force field yields spectra in
slightly worse agreement with experiment than the B3LYP force field. The
SCF force field yields spectra in poor agreement with experiment.The
basis set dependence of B3LYP force fields is also explored: the 6-31G
and TZ2P basis sets give very similar results while the 3-21G basis set
yields spectra in substantially worse agreements with experiment.
Novel photosynthetic reaction center model compounds of the type donor2–donor1–acceptor, composed of phenothiazine, BF2-chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X-ray structures of three of the phenothiazine-BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N-substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C-substituted analogue. The geometry and electronic structures were obtained by B3LYP/6-31G(dp) calculations (for H, B, N, and O) and B3LYP/6-31G(df) calculations (for S) in vacuum, followed by a single-point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO−1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy-level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ–BODIPY.+–C60.−; subsequent hole shift resulted in PTZ.+–BODIPY–C60.− charge-separated species. The return of the charge-separated species was found to be solvent dependent. In nonpolar solvents the PTZ.+–BODIPY–C60.− species populated the 3C60* prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The 1BODIPY* generated radical ion-pair in these triads persisted for few nanoseconds due to electron transfer/hole-shift mechanism.
A thin film of polystyrene containing 4c was spin-coated on a CaF2 plate and irradiated with 365 nm light. The absorption band arising from the carboxylate of 4c at 1372 cm-1 in the FTIR spectrum decreased after UV irradiation. Radical UV curing materials that are well established in the marketplace have drawbacks because of high volume shrinkage and oxygen inhibition. The anionically cured film showed high transparency and no volume shrinkage, in contrast to a conventional radical UV curing system, which showed large volume shrinkage. This is probably due to relatively low quantum yields for photobase generation and weaker basicity of photo-generated bases, leading to low photosensitivity of photoreactive materials sensitized with photobase generators. Furthermore, many of the photobase generators reported are generally prepared via several synthetic steps.
A contracted Gaussian basis set (6‐311G∗∗) is developed by optimizing exponents and coefficients at the Møller–Plesset (MP) second‐order level for the ground states of first‐row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple molecules at various levels of MP theory and comparing with experiment.
Easy as pi: The Ugi four-component reaction (4CR) gives access to a functional π system consisting of an electron donor and an electron acceptor (see figure; PT=phenothiazine, AQ=9,10-anthraquinone). Intramolecular photoinduced electron transfer generates charge-separated diradical ion pairs originating from the singlet and triplet excited states. Charge separation in the triplet species persists for more than 2 ns.
The synthesis of hitherto unknown phenaleno[1,9-bc]furan (1), in 11% overall yield from 2-naphthol, has been achieved. Its 1H and 13C n.m.r. spectra have been fully assigned by means of heteronuclear 2D correlations. The furanoid moiety of (1) displays Diels–Alder diene reactivity in its reactions with benzyne, dimethyl acetylenedicarboxylate, and singlet oxygen. The furan ring of (1) also suffers oxidation by m-chloroperbenzoic acid and undergoes catalytic hydrogenation. This chemistry appears to be more characteristic of naphtho[1,2-c]furan than of naphtho[1,2-b]furan although both structural units co-exist in structure (1).
The strong intramolecular hydrogen bonding in 9-hydroxyphenalen-1-one (1) has been investigated by means of quantum chemical calculations and vibrational spectroscopy. Both ab initio molecular orbital (MP2) and density functional theory (B3-LYP, B3-P86) calculations predict a double-minimum potential energy surface with a low (ca. 10 kJ/mol) barrier. The hydrogen-bonding energy, estimated by comparison with the non-hydrogen-bonded anti conformer, is ca. 60 kJ/mol. Our comparative study of the three theoretical levels revealed the very good performance of the B3-LYP density functional in conjunction with a diffuse polarized valence triple-ζ basis set, while the B3-P86 functional tends to overestimate considerably the strong hydrogen-bonding interaction. Based on a joint analysis of the energetics and molecular geometry, the interaction in 1 can be classified as a border case between traditional and short-strong hydrogen bonds. The charge distribution refers to a strong ionic character of the hydrogen-bonding interaction. The vibrational properties of the molecule have been investigated by a combined experimental (FT-IR, FT-Raman) and theoretical analysis. The deficiencies of the computed harmonic force field were corrected by the scaled quantum mechanical (SQM) method of Pulay et al. As a result of our SQM analysis, 45 from a total of 63 fundamentals of the molecule were assigned with an rms deviation of 6.9 cm-1 between the experimental and scaled frequencies. The most characteristic effect of hydrogen bonding on the vibrational properties of 1 is the enhanced mixing of the CO and OH vibrations with each other and with the skeletal modes.
The 1,9-dithiophenalenylium cation has been prepared as a stable hexafluorophosphate salt, by reaction of 9-ethoxyphenalenone with phosphorus pentasulfide. The compound has been fully characterized and a MINDO/3 SCF MO calculation of the ground-state structure is reported. The compound is shown to undergo three one-electron reduction steps (without decomposition) the first two of which are reversible. Moderately highly conducting charge transfer salts are obtained with TCNQ derivatives. The 1,9-dithiophenalenyl radical has been prepared in solution by electrochemical and chemical reduction of 1,9-dithiophenalenylium hexafluorophosphate. The compound has been subjected to ESR, mass, and electronic spectral examination. The radical is found to be monomeric in solution in the temperature range between 18 and -95°C. The observed ESR hyperfine splittings are consistent with the removal of spin density from the phenalenyl nucleus by disulfide substitution.
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Phenalenyl in action: The cationic state of the phenalenyl system can be used to design Lewis acidic catalysts. The phenalenyl backbone can tune the energy of the lowest unoccupied molecular orbital (LUMO) of the molecule, which in turn controls the catalytic activity in the ring-opening polymerization of cyclic esters.
A series of organic D-pi-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length.
We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.
Shedding new light on the matter: Rather than the conventional approach of utilizing the cascade effect, charge separation can be stabilized in artificial photosynthetic systems simply by the geometry. In these latter systems, light-induced charge separation (CS) in a closely spaced molecular dyad is followed by exceptionally slow charge recombination (CR, see picture) to give a charge-separated state with a lifetime of up to 120 s.
Compounds [RuII(bipy)(terpy)L](PF6)2 with bipy = 2,2'-bipyridine, terpy = 2,2':6',2"-terpyridine, L = H2O, imidazole (imi), 4-methylimidazole, 2-methylimidazole, benzimidazole, 4,5-diphenylimidazole, indazole, pyrazole, 3-methylpyrazole have been synthesized and characterized by 1H NMR, ESI-MS and UV/Vis (in CH3CN and H2O). For L = H2O, imidazole, 4,5-diphenylimidazole and indazole the X-ray structures of the complexes have been determined with the crystal packing featuring only few intermolecular C-H...pi or pi-pi interactions due to the separating action of the PF6-anions. Complexes with L = imidazole and 4-methylimidazole exhibit a fluorescence emission with a maximum at 662 and 667 nm, respectively (lambdaexc= 475 nm, solvent CH3CN or H2O). The substitution of the aqua ligand in [Ru(bipy)(terpy)(H2O)]2+ in aqueous solution by imidazole to give [Ru(bipy)(terpy)(imi)]2+ is fastest at a pH of 8.5 (as followed by the increase in emission intensity). Coupling of the [Ru(bipy)(terpy)]2+ fragment to cytochrome c(Yeast iso-1) starting from the Ru-aqua complex was successful at 35 degrees C and pH 7.0 after 5 d under argon in the dark. The [Ru(bipy)(terpy)(cyt c)]-product was characterized by UV/Vis, emission and mass spectrometry. The location where the [Ru(bipy)(terpy)] complex was coupled to the protein was identified as His44 (corresponding to His39 in other numbering schemes) using digestion of the Ru-coupled protein by trypsin and analysis of the tryptic peptides by HPLC-high resolution MS.
We report the preparation, crystallization, and solid-state characterization of the first two members of a new family of spiro-bis(1,9-disubstituted phenalenyl)boron neutral radicals based solely on oxygen functionalization, and we show that this strategy significantly lowers the electrochemical disproportionation potentials (DeltaE), in comparison with other spiro-bis(1,9-disubstituted phenalenyl)boron salts. In the solid state, these radicals pack in a continuous array of pi-pi-stacked phenalenyl units with very short intermolecular carbon...carbon contacts. These two radicals are among the most highly conducting neutral organic solids, with room temperature conductivities reaching 0.3 S/cm. Magnetic susceptibility measurements show that the radicals do not exist as isolated free radicals, and there is significant spin-spin interaction between the molecules in the solid state as expected from the crystal structures and the calculated band structures; the solid-state properties are best rationalized in terms of the resonating valence bond model.
A series of fluorophores with single-exponential fluorescence decays in liquid solution at 20 degrees C were measured independently by nine laboratories using single-photon timing and multifrequency phase and modulation fluorometry instruments with lasers as excitation source. The dyes that can serve as fluorescence lifetime standards for time-domain and frequency-domain measurements are all commercially available, are photostable under the conditions of the measurements, and are soluble in solvents of spectroscopic quality (methanol, cyclohexane, water). These lifetime standards are anthracene, 9-cyanoanthracene, 9,10-diphenylanthracene, N-methylcarbazole, coumarin 153, erythrosin B, N-acetyl-l-tryptophanamide, 1,4-bis(5-phenyloxazol-2-yl)benzene, 2,5-diphenyloxazole, rhodamine B, rubrene, N-(3-sulfopropyl)acridinium, and 1,4-diphenylbenzene. At 20 degrees C, the fluorescence lifetimes vary from 89 ps to 31.2 ns, depending on fluorescent dye and solvent, which is a useful range for modern pico- and nanosecond time-domain or mega- to gigahertz frequency-domain instrumentation. The decay times are independent of the excitation and emission wavelengths. Dependent on the structure of the dye and the solvent, the excitation wavelengths used range from 284 to 575 nm, the emission from 330 to 630 nm. These lifetime standards may be used to either calibrate or test the resolution of time- and frequency-domain instrumentation or as reference compounds to eliminate the color effect in photomultiplier tubes. Statistical analyses by means of two-sample charts indicate that there is no laboratory bias in the lifetime determinations. Moreover, statistical tests show that there is an excellent correlation between the lifetimes estimated by the time-domain and frequency-domain fluorometries. Comprehensive tables compiling the results for 20 (fluorescence lifetime standard/solvent) combinations are given.
Novel organic dyes based on the phenothiazine (PTZ) chromophore were designed and synthesized for dye-sensitized solar cells, which give solar energy-to-electricity conversion efficiency (eta) of up to 5.5% in comparison with the reference Ru-complex (N3 dye) with an eta value of 6.2% under similar experimental conditions.
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Version of record online
Version of record online:J une 26, 2017