Ultrafast bimolecular electron transfer dynamics in micellar media

Ultrafast photoinduced bimolecular electron transfer (ET) dynamics between 7-aminocoumarin derivatives and N,N-dimethylaniline (DMAN) has been studied in neutral (TX100), cationic (DTAB) and anionic (SDS) micellar media. A very fast decay time constant (tau(fast)) shorter than approximately 10 ps has been observed for the coumarins in the presence of DMAN in all of the three micellar media. In this time scale, reactants in the micellar phase undergo ET interactions without involving diffusion or reorientation of the reactants and thus can be envisaged as equivalent to nondiffusive bimolecular ET reaction. The fastest ET rates estimated as the inverse of the shortest lifetime components of the fluorescence decay (k(et) congruent with tau(fast)(-1)) nicely follow the predicted Marcus inversion behavior with reaction exergonicity (-DeltaG degrees), irrespective of the nature of micelles considered. Onset of inversion in ET rates occur at approximately 0.61 eV lower exergonicity in SDS and TX100 micelles compared with that in DTAB micelle and are rationalized following two-dimensional ET (2DET) theory. These differences suggest the possibility of tuning Marcus inversion by proper selection of micelles. Interestingly, ET rates (k'(et)) obtained from the conventional Stern-Volmer analysis of the relatively longer time constants of the fluorescence decays also exhibit similar Marcus correlation with DeltaG degrees, showing clear inversion behavior. Fitting of Marcus correlation curves for k(et) and k'(et) indicate two largely different values for the electronic coupling parameters. In micellar media, as the interacting donor-acceptor molecules are on an average expected to be separated by an intervening surfactant chain and the reorientation rate of the reactants is quite slow, it is predicted that the ultrafast ET (k(et)) component arises because of the surfactant separated donor-acceptor pairs that are orientated perfectly to give the maximum electronic coupling. The slower ET (k'(et)) is predicted to arise because of those pairs where the donor-acceptor orientations are not very suitable but good enough to give a sizable electronic coupling.     

Bimolecular hole transfer from the trimethoxybenzene radical cation in the excited state

Bimolecular hole transfer quenching of the 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) by hole quenchers (Q) such as biphenyl (Bp), naphthalene (Np), anisole (An), and benzene (Bz) with higher oxidation potentials than that of TMB was directly observed during the two-color two-laser flash photolysis at room temperature. From the linear relationships between the inverse of the transient absorption changes of TMB*+ during the second 532-nm laser excitation versus the inverse of the concentration of Q, the rate constant of the hole transfer from TMB*+* to Q was estimated to be (8.5 +/- 0.4) x 10(10), (1.4 +/- 0.7) x 10(11), (1.3 +/- 0.6) x 10(11), and (6.4 +/- 0.3) x 10(10) M(-1)s(-1) for Bp, An, Np, and Bz, respectively, in acetonitrile based on the lifetime of TMB*+*. The estimated rate constants are larger than the diffusion-controlled rate constant in acetonitrile. Short lifetime, high energy, and high oxidation potential of TMB*+* cause the lifetime-dependent quenching process or static quenching process as the major process during the quenching of TMB*+* by Q as indicated by the Ware's theoretical model. The subsequent hole transfer from Q*+ to TMB, giving TMB*+, was found to occur at the diffusion-controlled rate for Bp and An as Q. For Q such as Np and Bz, the dimerization of Q*+ with Q to give dimer radical cation (Q2*+) occurred competitively with the hole transfer from Q*+ to TMB.     

Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.     

Ultrafast interfacial electron transfer from the excited state of anchored molecules into a semiconductor

Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the large organic chromophore perylene into the inorganic semiconductor rutile TiO₂ was investigated with femtosecond time-resolved two-photon photoemission (TR-2PPE). The strength of the electronic interaction between the chromophore and the semiconductor was varied by inserting different anchor/bridge groups that functioned either as electronic wire or electronic tunnelling barrier. Both anchor groups, i.e. carboxylic and phosphonic acid, formed strong chemical bonds at the TiO₂ surface. The perylene chromophore with the different anchor/bridge groups was adsorbed from solution in a dedicated ultra-high-vacuum (UHV) chamber. The adsorption geometry of the chromophore perylene was determined from angle and polarization dependent two-photon photoemission (2PPE) signals and was found to be very different for the two different anchor/bridge groups. The measured adsorption geometries are compatible with recent DFT (density functional theory) calculations by P. Persson and co-workers [M. Nilsing, S. Lunell, P. Persson, L. Ojamäe, Phosphonic acid adsorption at the TiO₂ anatase (1 0 1) surface investigated by periodic hybrid HF-DFT computations, Surf. Sci. 582 (2005) 49–60]. Two different processes contributed to the TR-2PPE transients, firstly electron transfer from the chromophore to the electronic acceptor states on the surface and secondly escape of the electrons from the surface into the bulk of the semiconductor. The latter escape process was measured separately by making the interfacial electron injection process instantaneous when the chromophore catechol was employed in place of the perylene compounds. The thus measured electron escape behavior was governed by the same time constants that have recently been predicted by Prezhdo and coworkers from time dependent DFT calculations [W.R. Duncan, W.M. Stier, O.V. Prezhdo, Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the Alizarin-TiO₂ interface, J. Am. Chem. Soc. 127 (2005) 7941–7951]. The HET times derived from the 2PPE transients showed very good agreement with HET times measured via transient absorption (TA) on anatase TiO₂ layers. The measured energy distribution of the 2PPE signals for the injected electrons suggests that a high density of electronic acceptor states is operative in both systems and is spread over an at least 1 eV wide energy range. The acceptor states are tentatively identified with surface states created through the formation of chemical bonds between the anchor groups of the organic molecules and surface atoms of the semiconductor.     

Reaction of triarylphosphine radical cations generated from photoinduced electron transfer in the presence of oxygen

[reaction: see text] The 9,10-dicyanoanthracene (DCA)-sensitized photoreaction of triarylphosphines (1) was carried out in acetonitrile under aerobic conditions. Phosphine 1 was oxidized to the corresponding phosphine oxide with no appreciable side reactions. Product analysis and laser flash photolysis experiments suggest that the radical cation of 1 formed by the electron transfer from 1 to DCA in the singlet excited state ((1)DCA) reacts with O(2) to eventually afford the phosphine oxide.     

Ultrafast photoinduced electron transfer in directly linked porphyrin-ferrocene dyads

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.     

Spurious observation of the Marcus inverted region in bimolecular photoinduced electron transfer

The effect of viscosity on the bimolecular electron transfer quenching of a series of coumarins by N,N-dimethylaniline was investigated using steady-state and time-resolved fluorescence spectroscopy. The data reveal that the static and transient stages of the quenching become dominant as viscosity increases. When extracting the quenching rate constants using a simple Stern-Volmer analysis, a decrease of the rate constant with increasing driving force is observed above ~2 cP. However, this apparent Marcus inverted region, already reported several times with the same system in micelles and room temperature ionic liquids, totally vanishes when analyzing the data with a model accounting for the static and transient stages of the quenching. It appears that the apparent Marcus inverted region arises from the neglect of these quenching regimes together with the use of fluorophores with different excited-state lifetimes.     

Ultrafast twisting dynamics in the excited state of auramine

Relaxation dynamics of the excited state of bis-[4-(dimethylamino)-phenyl] methaniminium chloride (Auramine) has been investigated using subpicosecond time-resolved absorption spectroscopic technique in both aprotic and alcoholic solvents. The locally excited (LE) state, formed following photoexcitation of Auramine using 400 nm light, undergoes intramolecular charge transfer (ICT) process, which is accompanied by the twisting of the dimethylanilino groups. Time evolution of the transient absorption-stimulated emission spectra as well as the wavelength dependence of the temporal dynamics investigated in each kind of solvents suggest that the relaxation process proceeds via the formation of at least two transient states (TS I and TS II), which are geometrical conformers and consecutively formed following the decay of the LE state. Twisting of the dimethylaniline groups are nearly barrierless processes, the rates of which show linear correlation both with the macroscopic or shear viscosities as well as the solvation times of the solvents. Time-dependent and fractional viscosity dependence of the relaxation rates of the LE and the TS I states in aprotic solvents suggest the multidimensionality of the reaction coordinate as well as reveal the viscoelastic property of the solvents. However, in normal alcohols, in addition to these two factors, activation energy of the solvent viscosity may be another important factor for the slower twisting dynamics of Auramine in alcohols. To explain the viscosity dependence of the decay time of the TS II state, which undergoes an efficient internal conversion process to the ground state, the possibility of occurrence of different mechanisms, such as, energy gap law, involvement of intramolecular high frequency modes, as well as the phenyl group twisting motion on a potential energy surface having a photochemical funnel, have been discussed. TDDFT method has been applied to obtain the optimized electronic structures of the transient states but it has been possible to obtain only that for the TS II state.     

Ultrafast photoinduced electron transfer within a self-assembled donor-acceptor system

A photoactive supramolecular assembly that is based on the hydrogen-bonded system H1.G2, consisting of a methyl viologen-functionalized barbiturate host (H1) (1-(N-(3,5-bis[[(6-tert-butylacetylamino-2-pyridyl)amino]carbonyl])-phenylacetamide)-1'-methyl-4,4'-bipyridium) and a [Re(Br)(CO)3(barbi-bpy)] (barbi-bpy = 5-[4-(4'-methyl)-2,2'-bipyridyl]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione) complex as the guest (G2) is described. The host molecule contains a well-known electron accepting group (methyl viologen), whereas the guest system can act as an efficient electron donor in the excited state. Upon self-assembly, the resulting adduct (H1.G2) represents an interesting noncovalently linked donor-acceptor system. The H1.G2 complex has been characterized in acetonitrile-d3 using 1H NMR and diffusion-ordered NMR spectroscopy (DOSY). The photophysical properties of the components and of the assembly have been studied in dichloromethane, in which the assembly has a high binding constant (Kass > or = 2 x 10(5) M(-1)), using time-resolved fluorescence and transient absorption spectroscopy. A detailed investigation of the hydrogen-bonded complex H1.G2 revealed that, upon excitation of the rhenium compound G2, an ultrafast electron-transfer process occurs from the metal-based component to the acceptor unit. The kinetics of the forward and back electron-transfer processes have been determined.     

Cationic dye-sensitized degradation of sodium hyaluronate through photoinduced electron transfer in the upper excited state

The formation of ground-state complexes of methylene blue (MB) and thionine (TN) with sodium hyaluronate (NaHA) was clearly observed by means of absorption spectra in aqueous solution. Irradiation of the complexes using 313 nm light caused significant degradation of NaHA under oxygen and argon. However, the use of visible light over 400 nm, which gives the lowest excited singlet state of the cationic dyes, caused no degradation. MB and TN were more efficient sensitizers for the degradation of NaHA than rose bengal (RB), although RB is a more efficient singlet oxygen (1O2) sensitizer than the cationic dyes. Under similar conditions the polysaccharides with carboxyl groups, such as alginic acid and polygalacturonic acid, also photodecomposed. However, the polysaccharides without carboxyl groups, such as pullulan and methyl cellulose, did not. The irradiation of the polysaccharides in the presence of powdered titanium dioxide as a photocatalyst to generate the hydroxyl radical (.OH) in aerated aqueous solution caused the fragmentation of all the polymers. It was confirmed that methyl viologen, an electron-accepting sensitizer, formed a charge-transfer complex with NaHA, the irradiation of which caused the efficient degradation of NaHA. In the presence of beta- and gamma-cyclodextrins the MB- and TN-sensitized photodegradation of NaHA was markedly suppressed. This was probably due to the formation of the inclusion complexes comprising the cationic dyes and the cyclodextrins. On the basis of the results obtained we propose that the cationic dye-sensitized degradation of NaHA involves a photoinduced electron-transfer process between the upper excited dyes and the ground-state NaHA and that .OH and 1O2 do not participate in the degradation.     

Ultrafast intermolecular hydrogen bond dynamics in the excited state of fluorenone

Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.     

In situ generated tris(p-bromophenyl)amine radical cation promoted electron transfer reaction of cyclopropyl silyl ethers

Tris(p-bromophenyl)aminium hexachloroantimonate and perchlorate were utilized to promote the oxidative ring-opening reaction of cyclopropyl silyl ethers giving ring-expanded ketones. Exploration of salt quantity effect on the reaction allowed us to hypothesize that amine radical cation is regenerated through the oxidation of neutral amine by hexachloroantimonate anion. Based on this hypothesis, amine radical cation was initially generated by the treatment of parent amine with either antimony pentachloride or the mixture of silver perchlorate and molecular iodine, and subsequently reacted with same substrates. The in situ generated amine radical cation was found to promote the reaction, and the expected products were obtained in better yields than via use of the corresponding salt reagents.     

四氰基乙烯—四甲基乙烯体系光诱导电子转移的溶剂效应及电荷分离激发态研究

用从头算方法,在HF/6-31 G^**和CASSCF（8,8）/6-31G^*基组水平上对四氰基乙烯与四甲基乙烯间电子转移的溶剂效应及电荷分离激发态进行了理论计算与研究。通过对给、受体各种几何构型的优化,计算了孤立给、受体之间的电荷分离反应热。在假定碰撞络合物形成过程中给、受体内部结构不发生变化的前提下,通过优化给、受体中心间距的方法,找出了络合物的稳定构型。计算了水溶剂及二氯甲烷溶剂中两种稳定构型络合物的电荷分离激发态,计算结果表明光激发可以直接导致体系的电荷分离。     

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。     

ESR spectroscopy of the C60 cation produced by photoinduced electron transfer

In this study the existence of the C60 cation produced by photochemically induced electron transfer in the presence of different sensitising molecules is proved for the first time by using ESR spectroscopy. It is demonstrated for triphenylpyryliumtetrafluoroborate (TPP) by this spectroscopic method that the electron transfer from C60 to TPP occurs without an application of a cosensitiser. Furthermore it is shown that the addition of alcohols causes a new radical in the system C60/TPP. The stationary concentration of the C60 cation diminishes even in the presence of a cosensitiser to such a low concentration that it is not detectable by ESR spectrosopy. The spectroscopic study of the sensitiser/C60 system is also extended to the reaction products.     

Ultrafast excited state dynamics in protonated GWG and GYG tripeptides

The excited state dynamics of two protonated tripeptides GWG and GYG has been investigated by pump/probe femtosecond measurements on photofragments, to explore the behavior of peptides where the terminal protonated amino group is not directly linked to the aromatic residue. The dynamics observed are short and surprisingly similar to the dynamics observed on the free protonated tryptophan and tyrosine aromatic amino acids. Specific photofragments observed for protonated GWG are related to the formation of a radical species WG degrees (+) after cleavage of the C(alpha)-N bond near the tryptophan residue.     

Effect of micellar environment on Marcus correlation curves for photoinduced bimolecular electron transfer reactions

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amine has been investigated in two cationic micelles, namely, cetyltrimethyl ammonium bromide (CTAB) and dodecyltrimethyl ammonium bromide (DTAB), and the results have been compared with those observed earlier in sodium dodecyl sulphate (SDS) and triton-X-100 (TX-100) micelles for similar donor-acceptor pairs. Due to a reasonably high effective concentration of the amines in the micellar Stern layer, the steady-state fluorescence results show significant static quenching. In the time-resolved (TR) measurements with subnanosecond time resolution, contribution from static quenching is avoided. Correlations of the dynamic quenching constants (k(q) (TR)), as estimated from the TR measurements, show the typical bell-shaped curves with the free-energy changes (DeltaG(0)) of the ET reactions, as predicted by the Marcus outersphere ET theory. Comparing present results with those obtained earlier for similar coumarin-amine systems in SDS and TX-100 micelles, it is seen that the inversion in the present micelles occurs at an exergonicity (-DeltaG(0)> approximately 1.2-1.3 eV) much higher than that observed in SDS and TX-100 micelles (-DeltaG(0)> approximately 0.7 eV), which has been rationalized based on the relative propensities of the ET and solvation rates in different micelles. In CTAB and DTAB micelles, the k(q) (TR) values are lower than the solvation rates, which result in the full contribution of the solvent reorganization energy (lambda(s)) towards the activation barrier for the ET reaction. Contrary to this, in SDS and TX-100 micelles, k(q) (TR) values are either higher or comparable with the solvation rates, causing only a partial contribution of lambda(s) in these cases. Thus, Marcus inversion in present cationic micelles is inferred to be the true inversion, whereas that in the anionic SDS and neutral TX-100 micelles are understood to be the apparent inversion, as envisaged from two-dimensional ET theory.     

Intermolecular electron transfer from excited benzophenone ketyl radical

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.