Triplet state photosensitization of nanocrystalline metal oxide electrodes by zinc-substituted cytochrome c: application to hydrogen evolution

The interfacing of nanostructured semiconductor photoelectrodes with redox proteins is an innovative approach to the development of artificial photosynthetic systems. In this paper, we have investigated the photoinduced electron-transfer reactions of zinc-substituted cytochrome c, ZnCyt-c, immobilized on mesoporous, nanocrystalline metal oxide electrodes. Efficient electron injection from the triplet state of ZnCyt-c is observed into TiO(2) electrodes (t(50%) approximately 100 micros) resulting in a long-lived charge-separated state (lifetime of up to 0.4 s). Further studies were undertaken as a function of electrolyte pH and metal oxide employed. Optimum yield of a long-lived charge-separated state was observed employing TiO(2) electrodes at pH 5, consistent with our previous studies of analogous dye-sensitized metal oxide electrodes. The addition of EDTA as a sacrificial electron donor to the electrolyte resulted in efficient photogeneration of molecular hydrogen, with a quantum yield per one absorbed photon of 10 +/- 5%.     

Supramolecular control of charge-transfer dynamics on dye-sensitized nanocrystalline TiO2 films

A [Ru(dcbpy)(2)(NCS)(2)] dye has been chemically modified by the addition of a secondary electron donor moiety, N,N-(di-p-anisylamino)phenoxymethyl. Optical excitation of the modified dye adsorbed to nanocrystalline TiO(2) films shows a remarkably long-lived charge-separated state, with a decay half time of 0.7 s. Semiempirical calculations confirm that the HOMO of the modified dye molecule is localised on the electron donor group. The retardation of the recombination dynamics relative to the unmodified control dye is caused by the increase in the spatial separation of the HOMO orbital from the TiO(2) surface. The magnitude of the retardation is shown to be in agreement with that predicted from the non-adiabatic electron-tunnelling theory.     

Ultrafast excitation transfer and charge stabilization in a newly assembled photosynthetic antenna-reaction center mimic composed of boron dipyrrin, zinc porphyrin and fullerene

A self-assembled supramolecular triad as a model to mimic the light-induced events of the photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron transfer ultimately generating a long-lived charge-separated state, has been accomplished. Boron dipyrrin (BDP), zinc porphyrin (ZnP) and fullerene (C(60)), respectively, constitute the energy donor, electron donor and electron acceptor segments of the antenna-reaction center imitation. Unlike in the previous models, the BDP entity was placed between the electron donor, ZnP and electron acceptor, C(60) entities. For the construction, benzo-18-crown-6 functionalized BDP was synthesized and subsequently reacted with 3,4-dihydroxyphenyl functionalized ZnP through the central boron atom to form the crown-BDP-ZnP dyad. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity of the dyad via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational and electrochemical methods. Steady-state fluorescence and excitation studies revealed the occurrence of energy transfer upon selective excitation of the BDP in the dyad. Further studies involving the pump-probe technique revealed excitation transfer from the (1)BDP* to ZnP to occur in about 7 ps, much faster than that reported for other systems in this series of triads, as a consequence of shorter distance between the entities. Upon forming the supramolecular triad by self-assembling fullerene, the (1)ZnP(*) produced by direct excitation or by energy transfer mechanism resulted in an initial electron transfer to the BDP entity. The charge recombination resulted in the population of the triplet excited state of C(60), from where additional electron transfer occurred to produce C(60)(?-):crown-BDP-ZnP(?+) ion pair as the final charge-separated species. Nanosecond transient absorption studies revealed the lifetime of the charge-separated state to be ~100 μs, the longest ever reported for this type of antenna-reaction center mimics, indicating better charge stabilization as a result of the different disposition of the entities of the supramolecular triad.     

Nanosecond time-resolved studies of long-lived photoinduced charge separation in the dyad fluorescein-anthraquinone-methyl ester adsorbed on TiO2 colloids

A dyad composed of fluorescein and 2-methyl-anthraquinone (FL-AQ) was synthesized and its photophysical properties were examined by absorption, fluorescence spectroscopy, and fluorescence lifetime. The charge-separated state formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy for the first time. When FL is excited in solution, the photoinduced electron transfer from FL to AQ proceeds efficiently. The rate constant and the efficiency of intramolecular electron transfer are 3.95 x 10(9) s(-1) and 95%, respectively. Its charge-separated state lifetime is too short to detect by transient absorption spectroscopy. Adding nanometer colloidal TiO(2) to an FL-AQ ethanol solution prolongs the lifetime of the charge-separated state, so its transient absorption signal is recorded significantly. The lifetimes of FL(+). at 480 nm and AQ(-). at 560 nm in the FL-AQ/TiO(2) colloidal system are 11.1 and 8.93 mivros, respectively.     

荧光素蒽醌甲酯分子内光诱导电子转移及电荷分离态的检测

合成了以荧光素为光敏剂的电子给体-受体二元化合物荧光素蒽醌甲酯(FL-AQ),用吸收光谱、荧光光谱、荧光寿命研究了该化合物在乙醇溶液中的光物理性质,并首次用纳秒级瞬态吸收光谱检测了此化合物分子内光诱导电子转移所形成的电荷分离态.在溶液中激发FL,电子可从FL有效地转移到AQ,其速率常数为3.95×10⁹s^-1,效率为95%.但由于电荷分离态寿命较短,瞬态吸收信号弱,若在此溶液中加入二氧化钛(TiO₂)纳米胶体,使FL-AQ吸附在胶体上,电荷分离态信号明显增强.480nm处FL^+·的寿命为11.1μs;560nm处AQ^-·的寿命为8.93μs.     

Ligand-localized electron trapping at sensitized semiconductor interfaces

Nanocrystalline (anatase), mesoporous TiO2 thin films were derivatized with [Ru(bpy)2(deebq)](PF6)2 or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine and deebq is 4,4'-diethylester-2,2'-biquinoline. Both compounds bind to the nanocrystalline TiO2 films with typical limiting surface coverages of 7 (+/-2) x 10-8 mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.60 V vs SCE) occurs prior to TiO2 reduction. Steady-state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced compound, MII(deebq-)(bpy)2+/TiO2. Neither the photoluminescent excited states or the reduced forms of these compounds inject electrons efficiently into TiO2. Transient absorption measurements after a approximately 10-ns laser pulse, reveal greater than 80% MLCT excited states and a smaller fraction of extremely long-lived charge-separated state intermediates assigned to equal concentrations of MII(deebq-)(bpy)2+/TiO2 and MIII(deebq)(bpy)23+/TiO2. The results are consistent with a mechanism of ultrafast electron injection followed by ligand-localized trapping on a second compound. The quantum yield for formation of the charge-separated states (phiCSS) is excitation wavelength dependent. With 417 nm excitation, phiCSS(417) = 0.14 +/- 0.03, and this decreases with 532.5 nm excitation, phiCSS(532.5) = 0.08 +/- 0.03, and 683 nm excitation for M = Os, phiCSS(683) = 0.05 +/- 0.01. Electron transfer to yield ground-state products, MII(deebq-)(bpy)2+/TiO2 + MIII(deebq)(bpy)23+/TiO2 --> 2 MII(deebq)(bpy)22+/TiO2, occurs with a driving force of 2.05 eV for Ru/TiO2 and 1.64 eV for Os/TiO2. The dynamics of this process were quantified on a millisecond time scale and were found to follow second-order kinetics. The intermediates are sufficiently long-lived that continued pulsed excitation at 10 Hz leads to high concentrations and the formation of transient images on the semiconductor surface that are easily observed by the naked eye.     

A semiempirical study for the ground and excited states of free-base and zinc porphyrin-fullerene dyads

The ground and excited states of a covalently linked porphyrin-fullerene dyad in both its free-base and zinc forms (D. Kuciauskas et al., J. Phys. Chem. 100 (1996) 15926) have been investigated by semiempirical methods. The excited-state properties are discussed by investigation of the character of the molecular orbitals. All frontier MOs are mainly localized on either the donor or the acceptor subunit. Thus, the absorption spectra of both systems are best described as the sum of the spectra of the single components. The experimentally observed spectra are well reproduced by the theoretical computations. Both molecules undergo efficient electron transfer in polar but not in apolar solvents. This experimental finding is explained theoretically by explicitly considering solvent effects. The tenth excited state in the gas phase is of charge-separated character where an electron is transferred from the porphyrin donor to the fullerene acceptor subunit. This state is stabilized in energy in polar solvents due to its large formal dipole moment. The stabilization energy for an apolar environment such as benzene is not sufficient to lower this state to become the first excited singlet state. Thus, no electron transfer is observed, in agreement with experiment. In a polar environment such as acetonitrile, the charge-separated state becomes the S, state and electron transfer takes place, as observed experimentally. The flexible single bond connecting both the donor and acceptor subunits allows free rotation by ca. +/- 30 degrees about the optimized ground-state conformation. For the charge-separated state this optimized geometry has a maximum dipole moment. The geometry of the charge-separated state thus does not change relatively to the ground-state conformation. The electron-donating properties of porphyrin are enhanced in the zinc derivative due to a reduced porphyrin HOMO-LUMO energy gap. This yields a lower energy for the charge-separated state compared to the free-base dyad.     

Photochemical charge separation in closely positioned donor-boron dipyrrin-fullerene triads

A series of molecular triads, composed of closely positioned boron dipyrrin-fullerene units, covalently linked to either an electron donor (donor(1)-acceptor(1)-acceptor(2)-type triads) or an energy donor (antenna-donor(1)-acceptor(1)-type triads) was synthesized and photoinduced energy/electron transfer leading to stabilization of the charge-separated state was demonstrated by using femtosecond and nanosecond transient spectroscopic techniques. The structures of the newly synthesized triads were visualized by DFT calculations, whereas the energies of the excited states were determined from spectral and electrochemical studies. In the case of the antenna-donor(1)-acceptor(1)-type triads, excitation of the antenna moiety results in efficient energy transfer to the boron dipyrrin entity. The singlet-excited boron dipyrrin thus generated, undergoes subsequent energy and electron transfer to fullerene to produce a boron dipyrrin radical cation and a fullerene radical anion as charge-separated species. Stabilization of the charge-separated state in these molecular triads was observed to some extent.     

Carotenoid and pheophytin on semiconductor surface: self-assembly and photoinduced electron transfer

Self-assembling of a carotenoid and pheophytin a into a supramolecular system was observed on the surface of nanocrystalline TiO2, and the photoinduced electron-transfer reactions within the system were studied by means of femtosecond transient absorption and laser flash photolysis techniques. Excitation of the pheophytin moiety results in ultrafast electron transfer from carotenoid to the excited pheophytin, creating a long-lived charge-separated state. Two decay pathways of the formed pheophytin a anion radical are proposed. The first is a direct back electron recombination forming a carotenoid triplet state on the nanosecond time scale, while the other is suggested to occur via electron injection to the TiO2 nanoparticle. These results demonstrate that a self-assembled carotenoid-pheophytin system leads to an efficient reductive quenching of the pheophytin moiety, suggesting that a similar mechanism can operate also in natural photosynthetic systems. Moreover, the formation of a long-lived charge-separated state indicates that such self-assembling strategy may be also considered for novel dye-sensitized solar cell constructions and other artificial systems aiming to mimic the electron-transfer chain in natural photosynthesis.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

Combining Zinc Phthalocyanines,Oligo(p-Phenylenevinylenes), and Fullerenes to Impact Reorganization Energies and Attenuation Factors

A study on electron transfer in three electron donor-acceptor complexes is reported. These architectures consist of a zinc phthalocyanine (ZnPc) as the excited-state electron donor and a fullerene (C₆₀) as the ground-state electron acceptor. These complexes are brought together by axial coordination at ZnPc. The key variable in our design is the length of the molecular spacer, namely, oligo-p-phenylenevinylenes. The lack of appreciable ground-state interactions is in accordance with strong excited-state interactions, as inferred from the quenching of ZnPc centered fluorescence and the presence of a short-lived fluorescence component. Full-fledged femtosecond and nanosecond transient absorption spectroscopy assays corroborated that the ZnPc ⋅ ⁺-C₆₀ ⋅ ⁻ charge-separated state formation comes at the expense of excited-state interactions following ZnPc photoexcitation. At a first glance, the ZnPc ⋅ ⁺-C₆₀ ⋅ ⁻ charge-separated state lifetime increased from 0.4 to 86.6 ns as the electron donor-acceptor separation increased from 8.8 to 29.1 Å. A closer look at the kinetics revealed that the changes in charge-separated state lifetime are tied to a decrease in the electronic coupling element from 132 to 1.2 cm⁻¹, an increase in the reorganization energy of charge transfer from 0.43 to 0.63 eV, and a large attenuation factor of 0.27 Å⁻¹.     

Long-Lived Multiple Charge Separation by Proton-Coupled Electron Transfer

Multiple charge separation has been successfully realized by a proton-coupled electron transfer reaction in an organic cocrystal. Benefiting from the adjustable electronic energy level of the electron donor and acceptor through thermal-induced proton migration, distinct optical absorption behaviors combined with color changes to blue or green are observed in these charge-separated states. It is of interest to note that such charge-separated states exhibit a longer lifetime of over a month as a result of the excellent coplanarity and π-π interaction of the electron acceptors. Moreover, the enhanced absorption toward longer wavelengths endows the charge-separated state with near-infrared (808 nm) photothermal conversion for imaging and bacterial inhibition, whereby the conversion performance can be controlled by the degree of proton migration.     

Hydrogen-bonding effects on the formation and lifetimes of charge-separated States in molecular triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d(6) metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ～750 ns in dichloromethane to ～3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ～50 to ～2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET.     

Solid-state photochromic device based on nanocrystalline TiO2 functionalized with electron donor-acceptor species

In this Communication, we report on a new type of solid-state photochromic device, S-TiO(2)-A, which is based on electron-transfer processes involving a molecular sensitizer (S) and an electron acceptor (A) coadsorbed onto the surface of nanocrystalline TiO(2). The electron-transfer process is mediated by the semiconductor conduction band and leads to a long-lived charge-separated state S(+)/A(-). The lifetime of this state can be controlled by oxygen diffusion through a polymeric coating deposited on the device.     

A triad based on an iridium(III) bisterpyridine complex leading to a charge-separated state with a 120-micros lifetime at room temperature

A triad D-Ir-A, where Ir is an Ir(III) bisterpyridine complex connected through an amidophenyl spacer to D, a triphenylamine electron donor, and to A, a naphthalene bisimide electron acceptor, has been synthesized and electrochemically investigated. The photoinduced processes in the triad, which is more than 4-nm long, have been characterized by steady-state and time-resolved optical spectroscopy by comparison with the model dyads D-Ir, Ir-A, and the reference monomers D, Ir, and A. A sequential electron transfer occurs upon excitation of the D and Ir units, leading to the charge-separated state D+-Ir- -A in 100 % yield and subsequently to D+-Ir-A- in about 10 % yield. The final charge-separated state has a lifetime at room temperature of 120 micros in air-free acetonitrile and of 100 micros in air-equilibrated solvent. Excitation of the A units does not yield intramolecular reactivity, but the resulting triplet-excited state localized on the acceptor, D-Ir-3A, displays intermolecular reactivity.     

Conformationally constrained macrocyclic diporphyrin-fullerene artificial photosynthetic reaction center

Photosynthetic reaction centers convert excitation energy from absorbed sunlight into chemical potential energy in the form of a charge-separated state. The rates of the electron transfer reactions necessary to achieve long-lived, high-energy charge-separated states with high quantum yields are determined in part by precise control of the electronic coupling among the chromophores, donors, and acceptors and of the reaction energetics. Successful artificial photosynthetic reaction centers for solar energy conversion have similar requirements. Control of electronic coupling in particular necessitates chemical linkages between active component moieties that both mediate coupling and restrict conformational mobility so that only spatial arrangements that promote favorable coupling are populated. Toward this end, we report the synthesis, structure, and photochemical properties of an artificial reaction center containing two porphyrin electron donor moieties and a fullerene electron acceptor in a macrocyclic arrangement involving a ring of 42 atoms. The two porphyrins are closely spaced, in an arrangement reminiscent of that of the special pair in bacterial reaction centers. The molecule is produced by an unusual cyclization reaction that yields mainly a product with C(2) symmetry and trans-2 disubstitution at the fullerene. The macrocycle maintains a rigid, highly constrained structure that was determined by UV-vis spectroscopy, NMR, mass spectrometry, and molecular modeling at the semiempirical PM6 and DFT (B3LYP/6-31G**) levels. Transient absorption results for the macrocycle in 2-methyltetrahydrofuran reveal photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene to form a P(?+)-C(60)(?-)-P charge separated state with a time constant of 1.1 ps. Photoinduced electron transfer to the fullerene excited singlet state to form the same charge-separated state has a time constant of 15 ps. The charge-separated state is formed with a quantum yield of essentially unity and has a lifetime of 2.7 ns. The ultrafast charge separation coupled with charge recombination that is over 2000 times slower is consistent with a very rigid molecular structure having a small reorganization energy for electron transfer, relative to related porphyrin-fullerene molecules.     

Light intensity effects on photoinduced charge separation parameters in a molecular triad based on an Iridium(III) bis(terpyridine) unit.

The effect of photon flux on the yield and lifetime of charge separation over the extreme components of a D-Ir-A triad, where D is a triphenyl amine electron donor, A is a naphthalene bis(imide) electron acceptor and Ir is an Ir(III) bis(terpyridine) complex, has been investigated. In usual laboratory conditions, with nanosecond and picosecond laser pulses in the 4-8 mJ range, biphotonic processes take place. Biphotonic products and their evolution can introduce complications in reaction mechanisms and their interpretation but can also drastically reduce the yield and the lifetime of the charge-separated state. In the present case, after discussion of several possible mechanisms, the process detrimental to charge separation is ascribed to absorption of a photon by the photogenerated charge-separated state D(+)-Ir-A(-).     

Effect of the addition of a fused donor-acceptor ligand on a Ru(II) complex: synthesis, characterization, and photoinduced electron transfer reactions of [Ru(TTF-dppz)2(Aqphen)]2+

The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)(2)(Aqphen)](2+) (TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2',3'-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal-ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy.     

Ultrafast charge-transfer dynamics of donor-substituted truxenones

The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.