Photoinduced Electron Transfer Dynamics Research Articles

Photoinduced Electron Transfer Processes of C60-Doped Poly(N-vinylcarbazole) Films As Revealed by Picosecond Laser Photolysis

Photoinduced electron transfer dynamics of poly(N-vinylcarbazole) (PVCz) films doped with a small amount of C60 was investigated by using picosecond and nanosecond laser flash photolysis techniques under various experimental conditions. From the time evolution of transient absorption spectra in the 600−1180 nm wavelength region, it was revealed for 0.7 mol % C60-doped PVCz films that the charge-separated state (PVCz+−C60-) between PVCz and C60 was produced immediately after excitation with a picosecond 532 nm laser pulse. The decay of the initial charge-separated state with a time constant of 1.2 ns comprised the following three channels: the charge-recombination process, the hole-migration reaction to the neighboring carbazolyl chromophores, and the formation of the local triplet state of C60 (3C60*). The reaction yield of the hole migration was estimated to be 30%. The present results were quite different from those reported previously for densely C60-doped PVCz films excited with a picosecond 355 nm l...

Classical molecular dynamics simulation of the photoinduced electron transfer dynamics of plastocyanin

Classical molecular dynamics simulations are used to investigate the nuclear motions associated with photoinduced electron transfer in plastocyanin. The blue copper protein is modeled using a molecular mechanics potential; potential parameters for the copper-protein interactions are determined using an x-ray crystallographic structure and absorption and resonance Raman spectra. Molecular dynamics simulations yield a variety of information about the ground (oxidized) and optically excited (charge-transfer) states: 1) The probability distribution of the potential difference between the states, which is used to determine the coordinate and energy displacements, places the states well within the Marcus inverted region. 2) The two-time autocorrelation function of the difference potential in the ground state and the average of the difference potential after instantaneous excitation to the excited state are very similar (confirming linear response in this system); their decay indicates that vibrational relaxation occurs in about 1 ps in both states. 3) The spectral densities of various internal coordinates begin to identify the vibrations that affect the optical transition; the spectral density of the difference potential correlation function should also prove useful in quantum simulations of the back electron transfer. 4) Correlation functions of the protein atomic motions with the difference potential show that the nuclear motions are correlated over a distance of more than 20 A, especially along proposed electron transport paths.

Electron transfer of betaine-30 in the inverted region

We use a density matrix theory to describe the photoinduced electron transfer dynamics of betaine-30 in solution, including environmental effects which may lead to relaxation and dephasing. We restrict ourselves to only one reaction coordinate. The remaining degrees of freedom of the molecule and the solvent form the environment which is bilinearly coupled to the relevant system. We investigate the S1→S0 reverse electron transfer of betaine-30 in solution, which occurs in the Marcus inverted region. We compare the temperature dependence of our theoretical results with experimental data and other theoretical predictions.

Experimental and Theoretical Analysis of Photoinduced Electron Transfer: Including the Role of Liquid Structure

Experimental determinations of the dynamics of photoinduced electron transfer from rubrene to duroquinone in three solvents, dibutyl phthalate, diethyl sebacate, and cyclohexanone are presented. Measurements of the donor (rubrene) fluorescence decays were made with time-correlated single-photon counting. The data are analyzed using recent theoretical developments that include important features of the solvent, i.e., the effects of finite molecular volume on local solvent structure and on the mutual donor−acceptor diffusion rates. Inclusion of the liquid radial distribution function (rdf) in the theory accounts for the significant variation of the acceptor concentration near a donor. Because the concentration of acceptors near a donor is substantially greater than the average concentration used in a featureless continuum liquid model, incorporating the rdf is necessary to properly analyze experimental data. Hydrodynamic effects, which slow the rate of donor−acceptor approach at short distance, are important and are also included in the theoretical analysis of the data. The data analysis depends on a reasonable model of the rdf. A hard-sphere liquid rdf is shown to be sufficiently accurate by comparing model electron-transfer calculations using a hard-sphere rdf and an rdf from neutron-scattering experiments reported in the literature. A method is presented to obtain the hard-sphere parameters needed to calculate the rdf. The method uses a self-consistent determination of the hard-sphere radius and diffusion constant and the solvent self-diffusion constant calculated from the Spernol and Wirtz equation. The Marcus form of the distance-dependent transfer rate is used. For the highest viscosity solvent (dibutyl phthalate), a unique set of the Marcus transfer parameters is obtained. For lower viscosity solvents, the transfer parameters are less well defined, but information on the distance and time dependence of charge separation is still acquired. These experiments, combined with the theoretical analysis, yield the first realistic description of through-solvent photoinduced electron transfer.

Direct Detection of Hole Migration along the Polymer Chain: Poly(N-vinylcarbazole) in 1,2-Dichloroethane Solution As Revealed by Picosecond Transient Absorption and Dichroism Measurements

Picosecond transient absorption spectroscopy and transient dichroism measurements were applied for the direct elucidation of photoinduced electron-transfer dynamics (charge separation, charge recombination, and hole-transfer reactions) in poly(N-vinylcarbazole) (PVCz)−electron acceptors as well as the corresponding monomer model system N-ethylcarbazole−electron acceptors in 1,2-dichloroethane solution. The measurement of the dichroism of the ion-pair absorption in the monomer model system demonstrated that the time constants of the rotational relaxation of the anion and the cation were identical with each other, indicating that the mutual geometry of the ions in the pair was maintained at least during the rotational relaxation process. In the polymer systems, the decay of the dichroism signal of the cation was much faster than that of the anion, indicating that the rapid hole-migration process diminished the dichroism signal of the cation of the carbazolyl (Cz) groups in the PVCz chain. The electron-transfer dynamics in PVCz in 1,2-dichloroethane solution was well-described by the simple scheme that the cation continuously migrates along pendant Cz moieties in a PVCz chain with the charge recombination at the initial position of the charge separation. The time constant for the hole transfer from the cation state of Cz moiety (Cz+) to neighboring Cz's was obtained to be ca. 500 ps, although the activation energy for the hole-transfer reaction from Cz+ in the initial ion pair (which was produced by the excitation of the ground state charge transfer complex) to the neighboring Cz's was estimated to be ≫10kBT by the usual theories of electron transfer. By integrating the present results with those in other solutions and in other aromatic vinyl polymers, the factors regulating the rapid hole-transfer process in PVCz were discussed.

Ultrafast spectroscopic studies of photoinduced electron transfer from semiconducting polymers to C60.

Using femtosecond time-resolved photoinduced absorption spectroscopy, we have studied the dynamics of photoinduced electron transfer from semiconducting polymers to ${\mathrm{C}}_{60}$. We find that with ${\mathrm{C}}_{60}$ in poly[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene)], poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), and poly(3-octylthiophene) (P3OT), ultrafast photoinduced electron transfer occurs from the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band of the semiconducting polymer to the lowest unoccupied molecular-orbital level of ${\mathrm{C}}_{60}$. Time-resolved measurements of the dichroic ratio place a 300-fs upper limit on the electron transfer time. The charge-separated state is rendered metastable due to the spatial separation of the electron and hole and to polaron formation associated with the electron on the ${\mathrm{C}}_{60}$ and the hole on the semiconducting polymer. Increasing the ${\mathrm{C}}_{60}$ concentration in P3OT increases the lifetime of the charge-separated state. The two p-phenylene vinylene derivatives also displayed metastable charge separation.

Picosecond dynamics of intramolecular singlet excitation energy transfer and photoinduced electron transfer in covalently-linked carotenoid—porphyrin and carotenoid—porphyrin—pyromellitimide molecules

Intramolecular singlet excitation energy transfer from the carotenoid to the free base porphyrin takes place in a 1,4-phenylenebridged carotenoid-free base porphyrin in competition with rapid internal conversion (<10 ps) of the singlet excited state of carotenoid, while efficient excitation transfer from the zinc porphyrin to the carotenoid has been observed in the corresponding zinc complex. Photoexcitation of a carotenoid-free base porphyrin—pyromellitimide triad molecule (CH 2PI) leads to the formation of a charge separated state ((C) +H 2P(I) −) with a lifetime of 15 ns. Rapid accumulation of this charge separated state with 40–50 ps time constant indicates the long-distance electron transfer from the singlet excited state of carotenoid to the pyromellitimide.

Photoinduced electron transfer and spin dynamics in mixed porphyrin-quinone solutions studied by time-resolved EPR

From time-resolved direct detection cw EPR with pulsed laser excitation, the photoinduced electron transfer and spin dynamics (CIDEP) in mixed zinc-tetraphenylporphyrin (ZnTPP)/benzo-1,4-quinone (BQ) ethanol solutions were determined as functions of temperature and BQ concentration. At lower temperatures the EPR spectra reveal that mixing of the S and T−1 states in the charge separated radical pair gains in importance relative to the ST0 mixing. Furthermore, at lower temperatures, the EPR spectra of the spin-correlated radical pairs of ZnTPP+ and BQ7 could also be observed. From the temperature/viscosity dependence of the electron transfer rates and of the polarization contributions from the triplet and radical pair mechanisms, deviations from a macroscopic diffusion behaviour are inferred at lower temperatures.