Fayer Lab: Dynamics of Photoinduced Electron Transfer in Complex Systems and Nanostructured Materials

Photoinduced Electron Transfer and Geminate Recombination in Liquids and Micelles
Ksenija D. Glusac and Alexei A. Goun

Photoinduced electron transfer is important in a variety of biological and chemical systems such as photosynthetic centers in plants and photovoltaic cells. In systems, the crucial step determining the efficiency is the conversion of the solar energy into the long lived charge separated species.

The number of radicals that are available is established by the competition between the forward electron transfer and geminate recombination processes. The objective of this project is to study the effects that determine the rates of these processes. We are currently studying Rhodamine 3B (R3B) or octadecyl-Rholdamine B (ODRB) and N,N-dimethylaniline (DMA) as donor/acceptor model system in various environments, such as liquids and micelles (Figure 1).

Upon excitation of R3B, an electron is transferred from DMA to R3B producing radicals (Figure2). The process is controlled by diffusion of donors and acceptors in solution or in the head group regions of micelles. Once the radicals are produced, the process of geminate recombination is initiated. Again, the competition between the geminate recombination and diffusion will determine the number of radicals that escape recombination and can be used in a chemical processes.
In order to study the dynamics of these processes, we employ time-resolved absorption (pump-probe) experiments. The transient absorption spectra of R3B solutions in the absence and presence of DMA are shown in Figure 3. As can be seen from the spectrum represented by a solid line, the transient absorption of R3B is governed by 3 distinct features: R3B excited state absorption appears as a positive signal in the 400-500 nm range, while the negative signals correspond to R3B bleach due to ground state absorption (500-600 nm range) and ODRB stimulated emission (600-700 nm range). The addition of DMA molecules enables electron transfer and the production of radicals. The dashed line in Figure 3 shows the spectrum of R3B in ethanol in the presence of DMA.

The intensities of all 3 absorption features decrease, since the R3B excited state population is depleted by a fast electron transfer. The transient absorption spectrum in the 500-600 nm range shows the appearance of additional absorbing species buried under the ODRB excited state absorption. This shoulder in the excited state absorption was assigned to ODRB radical absorption. The R3B radical spectrum shown in Figure 3 (dotted line) was obtained by properly scaled subtraction of the other spectra. The ODRB radical exhibits an absorption maximum at 425 nm, which is in accordance with the previous pulse radiolysis studies.
The determination of the charge transfer state signal is complicated by the fact that the small radical ion signal has to be found as the difference of two large signals (optical bleach and scaled stimulated emission).

In order to carry out the measurement with sufficient signal/noise ratio we employ a novel pump-two color probe system (Figure 4). The 565 nm pump beam is followed by 565 nm and 620 nm probes that are co-aligned. Following the sample, the two probe beams diffract from a grating after the sample and are separately detected by two photodiodes. The signals from the photodiodes were collected using gated integrators, fed directly to the computer and analyzed. Using this experimental approach the radical ion signal is measured directly as a difference of the bleach/stimulated emission signals.
Once the experimental kinetic curves are obtained, a statistical mechanical model is used to describe them.

This model is based on a pair of partial differential equations that describe the relative motions of the donor/acceptors in the process of the forward electron transfer as well as in the process of the geminate recombination. The model takes into the account the realistic description of the processes. It includes the spatial distribution of the donor/acceptors which is influenced by the radial distribution function of the structured solvent. Second, the diffusion of the donor/acceptors is modeled in such a way to account for the hydrodynamic effect. The third component of the model is the realistic description of the electron transfer rate, as a function of the distance, energy of the process, and solvent parameters. Experimental results for the forward electron transfer together with theoretical calculations for the experiments a micelle system are shown in Figure 5. Similar data is obtained for other micelles and a wide variety of liquids. In addition data showing the time dependence of the radical populations formed by forward electron transfer have been measured and analyzed using the statistical mechanics theory.
Work on micelles and liquids is continuing but future work will focus on electron transfer and geminate recombination in nanoscopically confined environments. Experiments on the dynamics of water confined on nanometer length scales (see Water Dynamics in Nanoscopic Environments on this web site) show substantial changes in dynamics. The influence of confinement and solvent dynamics on electron transfer will be investigated.

“Photoinduced Electron Transfer and Geminate Recombination for Photoexcited Acceptors in a Pure Donor Solvent,” V. O. Saik, A. A. Goun and M. D. Fayer, J. Chem. Phys. 120, 9601-9611 (2004). pdf

“Photoinduced Intermolecular Electron Transfer in Liquid Solutions,” V. O. Saik, A. A. Goun, J. Nanda, Koichiro Shirota, H. L. Tavernier, and M. D. Fayer, J. Phys. Chem. 108, 6696-6703 (2004). pdf

“Photoinduced Intermolecular Electron Transfer in Micelles: Dielectric and Structural Properties of Micelle Head Group Regions,” H. L. Tavernier, Florence Laine, and M. D. Fayer J. Phys. Chem. A 105, 8944-8957 (2001). pdf

“Photoinduced Intermolecular Electron Transfer in Complex Liquids: Experiment and Theory,” H. L. Tavernier, M. M. Kalashnikov, and M. D. Fayer J. Chem. Phys. 113, 10191-10201 (2000). pdf