Molecular Photophysics and Photochemistry

Our research group studies photophysical and photochemical events in a variety of molecular systems and materials. We use conventional and laser spectroscopic techniques to elucidate excited state energetics and kinetics, charge transfer rates and mechanisms, energy migration, non-linear optical properties, and photochemistry in condensed media. We employ photochemical hole burning spectroscopy, picosecond time-resolved absorption and emission spectroscopy, femtosecond degenerate four-wave mixing, and fluorescence, fluorescence anisotropy, absorption and excitation spectroscopy in these studies.

Energy and Character of Aromatic Photodimer Excited States: We are investigating low-temperature two-photon, triplet-state photochemistry; photochemical hole burning spectroscopy; and emission spectroscopy of photodimers and their matrix-trapped photodecomposition products. We have determined intermolecular interactions of photodecomposition products to explore two-level barrier distributions in polymer hosts. We are determining excited-state coupling amongst multiple chromophores within photodimers.

Linear and Nonlinear Properties of Charge Transfer Exited States: We explore the photophysics of donor-acceptor-donor molecules with excited CT states by conventional spectroscopy and measurement of nonlinear optical susceptibilities. We are studying a series of squaraine dyes. Using femtosecond degenerate four-wave mixing, we have determined large third-order optical nonlinearities in these molecules, verifying a negative ?(3) value. We are preparing ogliomeric and polymeric squaraines with enhanced values of ?(3).

Self-regulation Effects in Charge Transfer Systems: We are pursuing experimental and theoretical studies of the relationship between charge transfer processes and conformational dynamics in macromolecular, electron transfer (ET) systems. We focus on a recently discovered novel phenomenon--the self-regulation of ET systems under non-equilibrium conditions, an effect that we believe plays an extremely important role in natural photosynthetic systems. A strong correlation of the ET process with the surrounding medium conformation results in a highly efficient self-regulation of the charge separation process. We are explaining this phenomenon in photosynthetic reaction centers and other biomacromolecules. We are analyzing the role of the self-regulatory mechanism on energy conversion as a function of the macromolecular conformation and flexibility. We expect to be able to apply the observed principles of self-regulation to the engineering of artificial charge transfer systems.

Selected Publications

Goushcha, A. O., Scott, G. W., Kharkyanen, V. N., and Holwarth, A. R. “Self-Regulation Effects in Bacterial Reaction Centers. 1. General Theory,” Biophys. J., 2000, 79, 1237-1252.

Berg, O., Chronister, E. L., Yamashita, T., Sweet, R. M., Calabrese, J., and Scott, G. W. “s-Pentacene: Structure, Spectroscopy, and Temperature- and Pressure-Dependent Photochemistry,” J. Phys. Chem., 1999, 103, 2451-2459.

Goushcha, A. O.; Scott, G. W.; Holzwarth, A. R.; Kharkyanen, V. N. “Self-Regulation Effects in QB- Active Bacterial Reaction Centers,” in Biological Physics, Third International Symposium, H. Frauenfelder, G. Hauser, and R. Garcia, eds., AIP Conf. Proc. 1999, 487, 201-211.

Goushcha, A. O.; Scott, G. W.; Barabush, Yu.; Kapoustina, M. T.; Kharkyanen, V. N.; “Experimental and Theoretical Studies of Temperature Dependent Hysteresis with Actinic Light Intensity in the Primary Donor Optical Absorption Band: Reaction Centers from Rhodobacter Sphaeroides,” in Biological Physics, Third International Symposium, H. Frauenfelder, G. Hauser, and R. Garcia, eds., AIP Conf. Proc. 1999, 487, 212-219.

Goushcha, A. O.; Scott, G. W.; Barabush, Yu.; Kharkyanen, V. N.; “Temperature Dependence of Absorbance Hysteresis in Reaction Centers from Purple Bacteria Rb. Sphaeroides” in Photosynthesis: Mechanisms and Effects II, G. Garab, ed., 811-814 (Kluwer Academic Publishers, Dordrecht, The Netherlands, 1999).

Tran, K.; Scott, G.W.; Funk, D.J.; Moore, D.S. “Resonantly Enhanced, Degenerate Four-Wave Mixing Measurement of the Cubic Molecular Hyperpolarizability of Squaraine Dyes at 700 nm,” J. Phys. Chem. 1996, 100, 11863-11869.

Tran, K.; Stiegman, A.E.; Scott, G.W. “Primary Photophysical Processes of Discrete Pseudotetrahedral Oxovanadium Centers Dispersed in a Silica Xerogel Matrix,” Inorg. Chim. Acta 1996, 243, 185-191.

Tran, K.; Hanning-Lee, M.A.; Biswas, A.; Stiegman, A.E.; Scott, G.W. “Electronic Structure of Discrete Pseudotetrahedral Oxovanadium Centers Dispersed in a Silica Xerogel Matrix: Implications for Catalysis and Photocatalysis,” J. Am. Chem. Soc. 1995, 117, 2618-2626.