Intermolecular Electron Transfer Reactions Research Articles

Molecular Basis for Directional Electron Transfer

Biological macromolecules involved in electron transfer reactions display chains of closely packed redox cofactors when long distances must be bridged. This is a consequence of the need to maintain a rate of transfer compatible with metabolic activity in the framework of the exponential decay of electron tunneling with distance. In this work intermolecular electron transfer was studied in kinetic experiments performed with the small tetraheme cytochrome from Shewanella oneidensis MR-1 and from Shewanella frigidimarina NCIMB400 using non-physiological redox partners. This choice allowed the effect of specific recognition and docking to be eliminated from the measured rates. The results were analyzed with a kinetic model that uses the extensive thermodynamic characterization of these proteins reported in the literature to discriminate the kinetic contribution of each heme to the overall rate of electron transfer. This analysis shows that, in this redox chain that spans 23 A, the kinetic properties of the individual hemes establish a functional specificity for each redox center. This functional specificity combined with the thermodynamic properties of these soluble proteins ensures directional electron flow within the cytochrome even outside of the context of a functioning respiratory chain.

Effect of donor orientation on ultrafast intermolecular electron transfer in coumarin-amine systems

Effect of donor amine orientation on nondiffusive ultrafast intermolecular electron transfer (ET) reactions in coumarin-amine systems has been investigated using femtosecond fluorescence upconversion measurements. Intermolecular ET from different aromatic and aliphatic amines used as donor solvents to the excited coumarin-151 (C151) acceptor occurs with ultrafast rates such that the shortest fluorescence lifetime component (tau(1)) is the measure of the fastest ET rate (tau(1)=tau(ET) (fast)=(k(ET) (fast))(-1)), assigned to the C151-amine contact pairs in which amine donors are properly oriented with respect to C151 to maximize the acceptor-donor electronic coupling (V(el)). It is interestingly observed that as the amine solvents are diluted by suitable diluents (either keeping solvent dielectric constant similar or with increasing dielectric constant), the tau(1) remains almost in the similar range as long as the amine dilution does not cross a certain critical limit, which in terms of the amine mole fraction (x(A)) is found to be approximately 0.4 for aromatic amines and approximately 0.8 for aliphatic amines. Beyond these dilutions in the two respective cases of the amine systems, the tau(1) values are seen to increase very sharply. The large difference in the critical x(A) values involving aromatic and aliphatic amine donors has been rationalized in terms of the largely different orientational restrictions for the ET reactions as imposed by the aliphatic (n-type) and aromatic (pi-type) nature of the amine donors [A. K. Satpati et al., J. Mol. Struct. 878, 84 (2008)]. Since the highest occupied molecular orbital (HOMO) of the n-type aliphatic amines is mostly centralized at the amino nitrogen, only some specific orientations of these amines with respect to the close-contact acceptor dye [also of pi-character; A. K. Satpati et al., J. Mol. Struct. 878, 84 (2008) and E. W. Castner et al., J. Phys. Chem. A 104, 2869 (2000)] can give suitable V(el) and thus ultrafast ET reaction. In contrary, the HOMO of the pi-type aromatic amines is largely distributed throughout the whole molecule and thus most of the orientations of these amines can give significant V(el) for ultrafast ET reactions with close-contact C151 dyes.

Ultrafast dynamics and anionic active states of the flavin cofactor in cryptochrome and photolyase.

We report here our systematic studies of the dynamics of four redox states of the flavin cofactor in both photolyases and insect type 1 cryptochromes. With femtosecond resolution, we observed ultrafast photoreduction of oxidized state flavin adenine dinucleotide (FAD) in subpicosecond and of neutral radical semiquinone (FADH(*)) in tens of picoseconds through intraprotein electron transfer mainly with a neighboring conserved tryptophan triad. Such ultrafast dynamics make these forms of flavin unlikely to be the functional states of the photolyase/cryptochrome family. In contrast, we find that upon excitation the anionic semiquinone (FAD(*-)) and hydroquinone (FADH(-)) have longer lifetimes that are compatible with high-efficiency intermolecular electron transfer reactions. In photolyases, the excited active state (FADH(-)*) has a long (nanosecond) lifetime optimal for DNA-repair function. In insect type 1 cryptochromes known to be blue-light photoreceptors the excited active form (FAD(*-)*) has complex deactivation dynamics on the time scale from a few to hundreds of picoseconds, which is believed to occur through conical intersection(s) with a flexible bending motion to modulate the functional channel. These unique properties of anionic flavins suggest a universal mechanism of electron transfer for the initial functional steps of the photolyase/cryptochrome blue-light photoreceptor family.

Site-Selective Photoinduced Electron Transfer from Alcoholic Solvents to the Chromophore Facilitated by Hydrogen Bonding: A New Fluorescence Quenching Mechanism

Solute-solvent intermolecular photoinduced electron transfer (ET) reaction was proposed to account for the drastic fluorescence quenching behaviors of oxazine 750 (OX750) chromophore in protic alcoholic solvents. According to our theoretical calculations for the hydrogen-bonded OX750-(alcohol)(n) complexes using the time-dependent density functional theory (TDDFT) method, we demonstrated that the ET reaction takes place from the alcoholic solvents to the chromophore and the intermolecular ET passing through the site-specific intermolecular hydrogen bonds exhibits an unambiguous site selectivity. In our motivated experiments of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD), it could be noted that the ultrafast ET reaction takes place as fast as 200 fs. This ultrafast intermolecular photoinduced ET is much faster than the diffusive solvation process, and even significantly faster than the intramolecular vibrational redistribution (IVR) process of the OX750 chromophore. Therefore, the ultrafast intermolecular ET should be coupled with the hydrogen-bonding dynamics occurring in the sub-picosecond time domain. We theoretically demonstrated for the first time that the selected hydrogen bonds are transiently strengthened in the excited states for facilitating the ultrafast solute-solvent intermolecular ET reaction.

Incorporating Electron-Transfer Functionality into Synthetic Metalloproteins from the Bottom-up

The alpha-helical coiled-coil motif serves as a robust scaffold for incorporating electron-transfer (ET) functionality into synthetic metalloproteins. These structures consist of a supercoiling of two or more aplha helices that are formed by the self-assembly of individual polypeptide chains whose sequences contain a repeating pattern of hydrophobic and hydrophilic residues. Early work from our group attached abiotic Ru-based redox sites to the most surface-exposed positions of two stranded coiled-coils and used electron-pulse radiolysis to study both intra- and intermolecular ET reactions in these systems. Later work used smaller metallopeptides to investigate the effects of conformational gating within electrostatic peptide-protein complexes. We have recently designed the C16C19-GGY peptide, which contains Cys residues located at both the "a" and "d" positions of its third heptad repeat in order to construct a nativelike metal-binding domain within its hydrophobic core. It was shown that the binding of both Cd(II) and Cu(I) ions induces the peptide to undergo a conformational change from a disordered random coil to a metal-bridged coiled-coil. However, whereas the Cd(II)-protein exists as a two-stranded coiled-coil, the Cu(I) derivative exists as a four-stranded coiled-coil. Upon the incorporation of other metal ions, metal-bridged peptide dimers, tetramers, and hexamers are formed. The Cu(I)-protein is of particular interest because it exhibits a long-lived (microsecond) room-temperature luminescence at 600 nm. The luminophore in this protein is thought to be a multinuclear CuI4Cys4(N/O)4 cage complex, which can be quenched by exogenous electron acceptors in solution, as shown by emission-lifetime and transient-absorption experiments. It is anticipated that further investigation into these systems will contribute to the expanding effort of bioinorganic chemists to prepare new kinds of functionally active synthetic metalloproteins.

Direct observation for photophysical and photochemical processes of folic acid in DMSO solution

Using the fast optical technique of laser flash photolysis by a kinetic absorption detector, the photophysical and photochemical processes of folic acid have been observed. By comparison of transient absorptions from photolysis between folic acid and its components, pteroic acid, pterin and 6-carboxypterin, it has been demonstrated that electron-transfer reactions from 4-aminobenzoyl group to the triplet pterin moiety occur through intra- and inter-molecular processes of folic acid. The intermolecular electron-transfer reaction is suggested to be mediated by two tautomers of triplet states of folic acid, lactim and lactam, in different rate constants, 1.4 × 10 9 and 1.6 × 10 8 M −1 s −1, respectively.

Intermolecular electron-transfer reactions in soluble methane monooxygenase: a role for hysteresis in protein function.

Electron transfer from reduced nicotinamide adenine dinucleotide (NADH) to the hydroxylase component (MMOH) of soluble methane monooxygenase (sMMO) primes its non-heme diiron centers for reaction with dioxygen to generate high-valent iron intermediates that convert methane to methanol. This intermolecular electron-transfer step is facilitated by a reductase (MMOR), which contains [2Fe-2S] and flavin adenine dinucleotide (FAD) prosthetic groups. To investigate interprotein electron transfer, chemically reduced MMOR was mixed rapidly with oxidized MMOH in a stopped-flow apparatus, and optical changes associated with reductase oxidation were recorded. The reaction proceeds via four discrete kinetic phases corresponding to the transfer of four electrons into the two dinuclear iron sites of MMOH. Pre-equilibrating the hydroxylase with sMMO auxiliary proteins MMOB or MMOD severely diminishes electron-transfer throughput from MMOR, primarily by shifting the bulk of electron transfer to the slowest pathway. The biphasic reactions for electron transfer to MMOH from several MMOR ferredoxin analogues are also inhibited by MMOB and MMOD. These results, in conjunction with the previous finding that MMOB enhances electron-transfer rates from MMOR to MMOH when preformed MMOR-MMOH-MMOB complexes are allowed to react with NADH [Gassner, G. T.; Lippard, S. J. Biochemistry 1999, 38, 12768-12785], suggest that isomerization of the initial ternary complex is required for maximal electron-transfer rates. To account for the slow electron transfer observed for the ternary precomplex in this work, a model is proposed in which conformational changes imparted to the hydroxylase by MMOR are retained throughout the catalytic cycle. Several electron-transfer schemes are discussed with emphasis on those that invoke multiple interconverting MMOH populations.

Electron transfer in quinoproteins

Soluble quinoprotein dehydrogenases oxidize a wide range of sugar, alcohol, amine, and aldehyde substrates. The physiological electron acceptors for these enzymes are not pyridine nucleotides but are other soluble redox proteins. This makes these enzymes and their electron acceptors excellent systems with which to study mechanisms of long-range interprotein electron transfer reactions. The tryptophan tryptophylquinone (TTQ)-dependent methylamine dehydrogenase (MADH) transfers electrons to a blue copper protein, amicyanin. It has been possible to alter the rate of electron transfer by using different redox forms of MADH, varying reaction conditions, and performing site-directed mutagenesis on these proteins. From kinetic and thermodynamic analyses of the reaction rates, it was possible to determine whether a change in rate is due a change in Δ G 0, electronic coupling, reorganization energy or kinetic mechanism. Examples of each of these cases are discussed in the context of the known crystal structures of the electron transfer protein complexes. The pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenase transfers electrons to a c-type cytochrome. Kinetic and thermodynamic analyses of this reaction indicated that this electron transfer reaction was conformationally coupled. Quinohemoproteins possess a quinone cofactor as well as one or more c-type hemes within the same protein. The structures of a PQQ-dependent quinohemoprotein alcohol dehydrogenase and a TTQ-dependent quinohemoprotein amine dehydrogenase are described with respect to their roles in intramolecular and intermolecular protein electron transfer reactions.

The Significance of the Flexible Loop in the Azurin (Az-iso2) from the Obligate Methylotroph Methylomonas sp. Strain J

The obligate methylotroph Methylomonas sp. strain J produces two azurins (Az-iso1 and Az-iso2) as candidates for electron acceptor from methylamine dehydrogenase (MADH) in the electron-transfer process involving the oxidation of methylamine to formaldehyde and ammonia. The X-ray crystallographic study indicated that Az-iso2 gives two types of crystals (form I and form II) with polyethylene glycol (PEG4000) and ammonium sulfate as the precipitants, respectively. Comparison between the two Az-iso2 structures in forms I and II reveals the remarkable structural changes at the top surface of the molecule around the copper atom. Az-iso2 possesses Gly43 instead of Val43 or Ala43, which is unique among all other azurins around the copper ligand His46, inducing the remarkable structural change in the loop region from Gly37 to Gly43. When the structure of Az-iso2 is superimposed on that of amicyanin in the ternary complex composed of MADH, amicyanin, and cytochrome c 551, the loop of Az-iso2 deeply overlaps with the light subunit of MADH. However, the Az-iso2 molecule is probably able to avoid any steric hindrance with the cognate MADH to form the complex for intermolecular electron-transfer reaction, since the loop containing Gly43 is flexible. We discuss why the electron-transfer activity of Az-iso2 is fivefold higher than that of Az-iso1.

Conformational Dynamics and Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Coproporphyrin:Cytochrome c Complexes

The focus of the present study is to better understand the complex factors influencing intermolecular electron transfer (ET) in biological molecules using a model system involving free-base coproporphyrin (COP) complexed with horse heart cytochrome c (Cc). Coproporphyrin exhibits bathochromic shifts in both the Soret and visible absorption bands in the presence of Cc and an absorption difference titration reveals a 1:1 complex with an association constant of 2.63 ± 0.05 × 10 5 M −1. At 20°C, analysis of time-resolved fluorescence data reveals two lifetime components consisting of a discrete lifetime at 15.0 ns (free COP) and a Gaussian distribution of lifetimes centered at 2.8 ns (representing 1COP → Cc ET). Temperature-dependent, time-resolved fluorescence data demonstrate a shift in singlet lifetime as well as changes in the distribution width (associated with the complex). By fitting these data to semiclassical Marcus theory, the reorganizational energy ( λ) of the singlet state electron transfer was calculated to be 0.89 eV, consistent with values for other porphyrin/Cc intermolecular ET reactions. Using nanosecond transient absorption spectroscopy the temperature dependences of the forward and thermal back ET originating from triplet state were examined ( 3COP → Cc ET). Fits of the temperature dependence of the rate constants to semiclassical Marcus theory gave λ of 0.39 eV and 0.11 eV for the forward and back triplet ET, respectively ( k f = (7.6 ± 0.3) × 10 6 s −1, k b = (2.4 ± 0.3) × 10 5 s −1). The differing values of λ for the forward and back triplet ET demonstrate that these ET reactions do not occur within a static complex. Comparing these results with previous studies of the uroporphyrin:Cc and tetrakis (4-carboxyphenyl)porphyrin:Cc complexes suggests that side-chain flexibility gives rise to the conformational distributions in the 1COP → Cc ET whereas differences in overall porphyrin charge regulates gating of the back ET reaction (reduced Cc → COP +).

Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations.

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)(3)(3+) (bpy = 2,2'-bipyridine) and Ru(bpy)(3)(3+). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d(2)) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH(*)(+) were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)(3)(3+), the chemiluminescence due to Ru(bpy)(3)(2+) was observed in the second electron-transfer step from BNA(*), produced by the deprotonation of the keto form of BNAH(*)(+), to Ru(bpy)(3)(3+). The observation of chemiluminescence due to Ru(bpy)(3)(2+) provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)(3)(2+) has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)(3)(3+). This is ascribed to the facile C-C bond cleavage in 4-t-BuBNAH(*)(+). In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr(3), the transient absorption spectrum of the enol form of BNAH(*)(+) was detected instead of the keto form of BNAH(*)(+), and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH(*)(+) was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.

EPR Study of the Surface Basicity of Calcium Oxide. 2: The Interaction with Alkanes

The present paper reports experimental evidence of the heterolytic dissociation of some saturated hydrocarbons (methane, ethane, propane, and cyclohexane) at the surface of high surface area CaO activated at high temperature. The reaction takes place at RT on particularly basic O2- sites at the surface of CaO. The reaction has been followed via EPR spectroscopy in two distinct ways. The former one consists of monitoring the formation of superoxide radical anions (O2-) which occurs when the hydrocarbon and molecular oxygen are coadsorbed at the surface via surface intermolecular electron transfer reaction. The latter one is based on ionization of the negative hydrocarbon fragment upon exposure to UV light with consequent formation of a surface trapped electron (a surface color center known as Fs(H)+) in which the electron is stabilized nearby an hydroxyl OH- group produced by the hydrocarbon splitting. This latter OH- group undergoes isotopic exchange transforming in OD- when contacted with D2 or CD4.

Reaction of Human Myoglobin and H2O2: ELECTRON TRANSFER BETWEEN TYROSINE 103 PHENOXYL RADICAL AND CYSTEINE 110 YIELDS A PROTEIN-THIYL RADICAL

The sequence of human myoglobin (Mb) is similar to that of other species except for a unique cysteine at position 110 (Cys(110)). Adding hydrogen peroxide (H(2)O(2)) to human Mb affords Trp(14)-peroxyl, Tyr(103)-phenoxyl, and Cys(110)-thiyl radicals and coupling of Cys(110)-thiyl radicals yields a homodimer through intermolecular disulfide bond formation (Witting, P. K., Douglas, D. J., and Mauk, A. G. (2000) J. Biol. Chem. 275, 20391-20398). Treating a solution of wild type Mb and H(2)O(2) with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) at DMPO:protein </= 10 mol/mol yields DMPO-Cys(110) adducts as determined by EPR. At DMPO:protein ratios (25-50 mol/mol), both DMPO-Tyr(103) and DMPO-Cys(110) adducts were detected, whereas at DMPO:protein >/= 100 mol/mol only DMPO-Tyr(103) radicals were present. The DMPO-dependent decrease in DMPO-Cys(110) was matched by a near 1:1 stoichiometric increase in DMPO-Tyr(103). In contrast, reaction of the Y103F human Mb with H(2)O(2) gave no DMPO-Cys(110) at DMPO:protein </= 10 mol/mol, and only trace DMPO-Cys(110) at DMPO:protein >/= 100 mol/mol (i.e. conditions that consistently gave DMPO-Tyr(103) in the case of wild type Mb). No detectable homodimer was formed by incubation of the Y103F variant with H(2)O(2). However, the homodimer was detected in a mixture of both the Y103F and C110A variants of human Mb upon treatment with H(2)O(2) (C110A:Y103F:H(2)O(2) 2:1:5 mol/mol/mol); the yield of this homodimer increased with increasing ratios of C110A:Y103F. Together, these data suggest that addition of H(2)O(2) to human Mb can produce Cys(110)-thiyl radicals through an intermolecular electron transfer reaction from Cys(110) to a Tyr(103)-phenoxyl radical.

Synthesis of Sulfur-Containing Tricyclic Ring Systems by Means of Photoinduced Decarboxylative Cyclizations

The intramolecular and intermolecular photoinduced electron transfer reactions of a series of mercaptoacetic acid and mercaptopropionic acid derivatives were investigated. In the intermolecular series, the phthalimidoalkylsulfanylalkylcarboxylates 1a-j and 2 were transformed into the tricyclic ring systems 3a-j and 4, respectively, with high regioselectivities. The mercaptoacetic acid and 2-mercaptopropionic acid derived substrates 1a-g and 2 readily cyclized in good to excellent yields (60-98%) but with low diastereoselectivities (except for 1d), whereas the corresponding 3-mercaptopropionic acid derived substrates 1h-j gave the corresponding tricyclic products 3h-j after prolonged irradiation, but with poor yields (11-20%). The intermolecular version - i.e., photodecarboxylative addition to N-methylphthalimide (5) as electron acceptor - was successful with mercaptoacetic acid, and 2-mercaptopropionic acid substrates 6a-c and the addition products 7a-c were obtained in high yields (57-90%). No ad-dition, however, was observed with 3-(methylsulfanyl)propionic acid (6d). The regioselectivity of decarboxylation proceeded in a controlled manner for the mercaptosuccinic acid derivatives in both the intramolecular (with 8a-c) and the intermolecular (with 9) versions. Comparison between sulfuractivated and nonactivated species (13, 15) or irradiation of 1a under nonactivating conditions showed that the carboxylate anion in the position alpha to the electron-donating sulfur atom acts as a superior leaving group. This efficiency is drastically reduced for carboxylate anions in the alpha position. With the former substrates, the photochemical cyclization proceeds with high product yields. Quantum yield measurements for decomposition (as a measure for cyclization) supported these observations. CV measurements indicated preorientation prior to electron transfer in the intramolecular pathway.

ChemInform Abstract: Synthesis of Medium and Large Ring Heterocycles by Photoinduced Intermolecular and Intramolecular Electron Transfer Reactions of Tetrachlorophthalimides with Alkenes.

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Synthesis of medium and large ring heterocycles by photoinduced intermolecular and intramolecular electron transfer reactions of tetrachlorophthalimides with alkenes

Photoinduced intermolecular and intramolecular electron transfer reactions of N-(ω-hydroxyalkyl)-tetrachlorophthalimide with alkenes led to the formation of medium to large sized heterocyclic rings.

Structural Effects on Intermolecular Electron Transfer Reactivity

Rate constants (kij) measured by stopped flow are reported for 50 additional intermolecular electron transfer reactions between 0 and 1+ oxidation states of various compounds, enlarging our data set to 141 reactions between 45 couples in acetonitrile containing 0.1 M tetrabutylammonium perchlorate at 25 °C. Hydrazines with both saturated and unsaturated substituents, ferrocene derivatives, and heteroatom-substituted aromatic compounds are included in the couples studied. Least-squares fit of all the reactions to simple Marcus cross-reaction theory provides an internally consistent set of best fit intrinsic barriers ΔG‡ii(fit) (for self-electron transfer of each couple) covering a range of over 19 kcal/mol (rate constant range 2 × 1014) that predicts the kij rather accurately. All reactions have ratios of calculated to observed kij in the range 0.3−3.3 and 95% fall in the range 0.5−2.0. These results require that the preexponential factor for a cross reaction is close to the geometric mean of those for the self-reactions, which is not expected. Changes in internal reorganization energy (λv) have major effects on ΔG‡ii(fit), and changes in electronic overlap (Hab) have easily detectable ones, but the reactions studied are clearly not strongly nonadiabatic, even though in many cases the only electronic overlap at the transition state is between nonbonded alkyl groups. It is argued that these reactions occur in the “elbow region” between nonadiabatic and adiabatic electron transfer.

Electron transfer in poly( p-phenylene) oligomers: effect of external electric field and application of Koopmans theorem

For the purpose of investigating the interchain electron transfer mechanism in poly( p-phenylene) (PPP), intermolecular electron transfer (ET) reactions between the parallel benzene cation radical and neutral benzene have been studied at the levels of 4-31G and DZP basis sets. The dependence of the ET matrix element V rp upon d, the distance between the two parallel donor and acceptor, has been elucidated. Considering the motion of charge carrier in external electric field, we have studied the influence of the uniform electric field on the electron transfer matrix element and the location of the avoided crossing. We have found that the one-electron Koopmans theorem can be applied to the estimation of the electron transfer matrix element, and that the position of the avoided crossing can be determined by pursuing the minimum MO energy-splitting factor. Conformations of PPP oligomers, up to 10 phenyl rings, have been optimized with AM1 semiempirical method. The torsion angle between two vicinity phenyl rings, and the ionization potential have been calculated and discussed.

Time-resolved Raman study on the photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols

Time-resolved Raman spectroscopy has uncovered a new type of photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols. The Raman bands of the first excited singlet (S1) state, the cation radical and the anion radical of biphenyl were simultaneously observed in the time range 0–200 ns after photoexcitation. The two ion radicals exhibit distinct rise and decay behaviors from each other. A kinetic analysis shows that the electron–transfer reaction consists of the following three elementary processes; (i) monophotonic formation of a biphenyl cation radical and a solvated electron via the S1 state, (ii) migration of the electron in the solvent medium, and (iii) anion radical formation resulting from the capture of the electron by a biphenyl molecule in the ground state. The formation of the cation radical from the S1 state is a slow process with a rate not greater than 10−7 s−1.

Effect of laser intensity on the determination of intermolecular electron transfer rate constants—Observation of Marcus inverted region in photoinduced back electron transfer reactions

The light intensity and concentration dependence of the photoproduct yield are investigated in a monophotonic process. The relationship of the photoproduct yield with the laser intensity and the complex concentration for a monophotonic process is derived under laser flash photolysis. The relationship is confirmed experimentally in a monophotonic process, i.e., triplet–triplet transition for a Cu(I) complex Cu6(DMNSN′)6 (DMNSN′=4,6-dimethylpyrimidine-2-thiolate). At low light intensity, the relationship can be approximated by a linear inverse square root dependence on the light intensity. Based on this equation, a method is proposed to determine the intrinsic back electron transfer rate constant kETb in photoinduced intermolecular electron transfer reactions, precluding the effect from the diffusional encounter pairs. The Marcus “inverted region” is observed by using the method in photoinduced back electron transfer reactions of [Au2(dppm)2](ClO4)2 (dppm=bis(diphenylphosphino)methane) with a series of substituted pyridinium acceptors.