Effect of the excitation pulse frequency on the ultrafast photoinduced electron transfer dynamics

The dependence of the ultrafast photoinduced electron transfer dynamics in donor-acceptor complexes on the excitation pulse carrier frequency (spectral effect) has been investigated in the framework of a model involving three electronic state. The spectral effect has been shown to strongly depend on the angle theta between the reaction coordinate directions corresponding to optical and charge transfer transitions. Describing the solvent as a linear homogenous polar medium and accounting for Coulombic interaction of the transferred charge with the medium polarization fluctuations, the angle theta has been found out to be typically in the area 40 degrees -85 degrees. Exactly in this area of theta the spectral effect is predicted to be most pronounced.     

光敏电子转移异构反应中的盐效应

本文利用四正丁基四氟硼酸铵为探针, 研究了顺式芪和四环烷的9,10-二氰基蒽敏化光异构化的反应机理, 加入四正丁基四氟硼酸铵明显加快顺式芪的异构反应而减慢四环烷的反应。荧光猝灭及激光闪光光解实验证明四正丁基四氟硼酸铵能促进电荷分离过程而生成离子自由基对。从而证实顺式芪的异构化反应是经由离子自由基的历程, 而四环烷则是通过激基复合物机理。     

Excited states of the high-frequency vibrational modes and kinetics of ultrafast photoinduced electron transfer

The effect of the carrier frequency of the exciting laser pulse on the kinetics of intramolecular photoinduced charge transfer in the multi-channel stochastic model is studied. It is shown that the population of different states of high-frequency intramolecular modes upon varying the frequency of the excitation pulse can considerably alter the rate constant of ultrafast charge transfer. It is found that a negative vibrational spectral effect is expected in the vicinity of a barrier-free area (the rate constant of photoinduced charge transfer decreases along with the carrier frequency of the excitation pulse), while a positive effect is predicted in areas of high and low exergonicity (an inverse dependence). It is concluded that the value of the spectral effect falls along with the time of vibrational relaxation. For ultrafast photo-induced charge transfer, however, it remains considerable up to relaxation times of 100 fs.     

A novel photoinduced electron transfer reaction of N-alkylphenothiazines in carbon tetrachloride

U.V. irradiation of N-alkylphenothiazines (1a—c) in carbon tetrachloride produced 3-(phenothiazin-N'-yl-carbonyl)-N′-alkylphenothiazine (2a—b) and/or 3-(N′-alkylphenothiazin-3′-yl-carbonyl)-N-alkylphenothiazine (3b—c) together with the radical cation carbon tetrachloride salt 1~( ·)CCl_4~(-·)(4a—c), presumably via a photoinduced electron transfer mechanism.     

Theory of photoinduced heterogeneous electron transfer

We consider electron injection into the conduction band of a semiconductor, from an electronically excited state of a dye molecule, adsorbed on its surface. For arbitrary width of the conduction band, the survival probability of the excited state can be calculated using a Green's-function approach. We show that the existence of a split-off state can play an important role in the total injection probability. In the wide band limit, the survival probability decays exponentially, but for finite band widths it does not. We further investigate the effect of vibrations on the process. A Green's operator technique may be used to solve this too exactly. We show that the problem may be reduced to a non-Hermitian eigenvalue problem for the vibrational states alone. Exact results can be obtained for arbitrary bandwidth and for a few vibrational degrees of freedom. In the wide band limit, the dynamics is particularly simple and we find that (1) the survival probability of the excited state is unchanged by the inclusion of vibrational motion, but (2) each vibrational state now has a finite lifetime. Numerical results are presented for the effects of reorganization energy, energy of the injecting level, and the variation of the matrix element for the electron injection, on the survival probability of the electron in the excited state. As an illustration of the approach, we also present results of numerical calculation of the absorption spectrum of perylene adsorbed on TiO(2) and compare it with experimental results.     

Photophysical behavior of lipophilic xanthene dyes without the involvement of photoinduced electron transfer mechanism

The correlation of dibutyl-ether-ester of xanthene dye structures with their photophysical properties is discussed with respect to their capability as fluorescent probes based on ultraviolet–visible absorption, fluorescence spectra and fluorescence lifetimes measured in different solvents. It was found that the dibutyl-ether-ester of fluorescein is very weakly emissive in aprotic solvents, but fairly strong fluorescent in alcohols. The dependence of fluorescence quantum yield (Φ_f) and lifetime (τ_f) on solvent polarity suggests non-involvement of the intra-molecular photoinduced electron transfer (PeT) mechanism, suggested previously to account for the emission efficiency of fluorescein derivatives. The xanthene dyes intend to self-assemble in aprotic solvents, less polar solvents facilitate the aggregation while hydrogen bonding disfavor it. The formation of non-emissive H-aggregates is proposed to be responsible for their fluorescent behavior. The esterification showed stronger influences on the photophysics than the etherification, i.e. the former caused larger reduction of Φ_f owing to the internal conversion. The halogenation decreases the fluorescence quantum yield and lifetime of the xanthene dyes, owing to the enhancement of inter-system crossing process.     

The mechanism of photoinduced hydrogen transfer during photoreduction of carbonyl compounds

A model reaction of hydrogen transfer has been proposed that describes well the results of kinetic studies of the photoreduction of o- and p-quinones and fluorenone in the presence of para-substituted N,N-dimethylanilines and polyalkylbenzenes in various solvents.     

Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer

2-(dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d(2)-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene, 1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosensitizer gives rise to two transients with lambda(max) at 500 and 350 nm assigned to the dianisyl-substituted largely twisted trimethylenemethane cation radical (6.+) and the corresponding diradical (6..), respectively. These intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d(2)-1 thus proceeds via these two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible methylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.     

Synthesis, structures, and photoinduced electron transfer reaction in the 9,9'-spirobifluorene-bridged bipolar systems

[reaction: see text] A series of 9,9'-spirobifluorene-bridged bipolar compounds DnAm bearing various n:m ratios for triarylamine (D) versus 1,3,4-oxadiazole-conjugated oligoaryl moiety (A) have been synthesized to investigate the corresponding photoinduced electron transfer (PET) property. The excitation behaviors were probed by steady-state absorption, emission, fluorescence solvatochromism, and femtosecond fluorescence up-conversion spectroscopy. The overall reaction dynamics can be rationalized by the rate of PET, in combination with solvent relaxation dynamics. It was found that the rate of PET is dependent on the anchored D/A ratio. The rate of D1A1 and D2A1 was resolved to be approximately 2.44 x 10(12) and 2.32 x 10(12) s(-)(1), respectively, while it is irresolvable in D1A2 and D2A2 (>6.6 x 10(12) s(-)(1)). In another approach, based on the comprehensive X-ray data, cyclic voltammetry, and absorption/emission spectra, the rate of photoinduced electron transfer was also qualitatively estimated. Fair comparisons were made between experimental and theoretical approaches to gain detailed insight into the PET for the titled systems.     

Quinone methide generation via photoinduced electron transfer

Photochemical activation of water-soluble 1,8-naphthalimide derivatives (NIs) as alkylating agents has been achieved by irradiation at 310 and 355 nm in aqueous acetonitrile. Reactivity in aqueous and neat acetonitrile has been extensively investigated by laser flash photolysis (LFP) at 355 nm, as well as by steady-state preparative irradiation at 310 nm in the presence of water, amines, thiols, and ethyl vinyl ether. Product distribution analysis revealed fairly efficient benzylation of the amines, hydration reaction, and 2-ethoxychromane generation, in the presence of ethyl vinyl ether, resulting from a [4 + 2] cycloaddition onto a transient quinone methide. Remarkably, we found that the reactivity was dramatically suppressed under the presence of oxygen and radical scavengers, such as thiols, which was usually associated with side product formation. In order to unravel the mechanism responsible for the photoreactivity of these NI-based molecules, a detailed LFP study has been carried out with the aim to characterize the transient species involved. LFP data suggest a photoinduced electron transfer (PET) involving the NI triplet excited state (λ(max) 470 nm) of the NI core and the tethered quinone methide precursor (QMP) generating a radical ions pair NI(?-) (λ(max) 410 nm) and QMP(?+). The latter underwent fast deprotonation to generate a detectable phenoxyl radical (λ(max) 390 and 700 nm), which was efficiently reduced by the radical anion NI(?-), generating detectable QM. The mechanism proposed has been validated through a LFP investigation at 355 nm exploiting an intermolecular reaction between the photo-oxidant N-pentylnaphthalimide (NI-P) and a quaternary ammonium salt of a Mannich base as QMP (2a), in both neat and aqueous acetonitrile. Remarkably, these experiments revealed the generation of the model o-QM (λ(max) 400 nm) as a long living transient mediated by the same reactivity pathway. Negligible QM generation has been observed under the very same conditions by irradiation of the QMP in the absence of the NI. Owing to the NIs redox and recognition properties, these results represent the first step toward new molecular devices capable of both biological target recognition and photoreleasing of QMs as alkylating species, under physiological conditions.     

Life uncertainty broadening in photoinduced electron transfer

Two types of energy-level broadening, one caused by electron exchange between radicals and ground-state molecules, the other by selective sampling at short times, are investigated. Both lead, in accordance with the uncertainty principle, to an increase in the B_1/2 value.     

Salt effects in photoinduced electron transfer reactions

Metal salts and oxygen react synergistically to inhibit back-electron-transfer in photoinduced reactions.     

Direct observation of the transition state of ultrafast electron transfer reaction of a radiosensitizing drug bromodeoxyuridine

Replacement of thymidine in DNA by bromodeoxyuridine (BrdU) has long been known to enhance DNA damage and cell death induced by ionizing/UV radiation, but the mechanism of action of BrdU at the molecular level is poor understood. Using time-resolved femtosecond laser spectroscopy, we obtain the real-time observation of the transition state of the ultrafast electron transfer (ET) reaction of BrdU with the precursor to the hydrated electron, which is a general product in ionizing/UV radiation. The results show that the ET reaction is completed within 0.2 picosecond (ps) after the electronic excitation, leading to the formation of a transition state BrdU*- with a lifetime of approximately 1.5 ps that then dissociates into Br- and a high reactive radical dU*. The present results can greatly enhance our understanding not only of the mechanism of BrdU as a radio-/photosensitizer but of the role of prehydrated electrons in electron-initiated processes in biological and environmental systems.     

Ultrafast excited state dynamics of the perylene radical cation generated upon bimolecular photoinduced electron transfer reaction

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.     

Peculiarities and paradoxes of photoinduced electron transfer reactions

Many chemical reactions involve the electron transfer stage. The kinetics of photoinduced electron transfer reactions is commonly considered in terms of either the transition state theory as preliminary thermally activated reorganization of the medium and reactants (necessary for degeneracy of the electronic levels of the reactants and the products) or nonradiative quantum transitions, which do not require preliminary activation and are observed in the exoergic region. A new approach to the kinetics of such reactions that has been proposed recently considers a substantial reduction of the barrier in the contact reactant pair due to strong electronic interaction and takes into account the intermediate formation of a charge transfer complex. This approach has explained many well-known important features of electron transfer reactions that are inconsistent with the first two theories.     

Initiation of cationic polymerization by photoinduced electron transfer

The objective of this paper is to discuss: (i) the general approaches to the initiation of cationic polymerization by photinduced electron transfer reactions (ii) the use of photoinduced electron transfer reactions for block copolymer synthesis. For the first, it is concluded that three general methods are currently available which involve reduction of onium salts by (a) photogenerated radicals, (b) photoexcited sensitizers or (c) electron donor compounds in charge transfer complexes. According to this view, a variety of initiating systems are discussed. For the second, recent developments on the application of photoinduced electron transfer reactions to the synthesis of block copolymer of monomers polymerizable with different mechanisms are presented.     

Low-field CIDNP study of photoinduced electron transfer reactions

Chemically induced dynamic nuclear polarization in low magnetic field (low-field CIDNP) has been detected and studied in photoinduced electron transfer reactions in the polar solvent acetonitrile. For the radical-ion reactions two different approaches to interpret the low-field CIDNP are demonstrated: interpretation of the low-field CIDNP sign on the basis of quality relationships, and numerical calculations of the CIDNP field dependence. Analysis shows that low-field CIDNP in these reactions is sensitive to the value of the electron exchange interactions in radical-ion pairs.     

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y_xSc_3−xN@C₈₀ (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁–T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Photoexcitation mechanism of Y_xSc_3−xN@C₈₀ metallofullerenes is studied by variable-temperature photoluminescence, advanced EPR techniques, and DFT calculations, revealing photoinduced rotation of the endohedral cluster.