Excited-State Properties of Bacteriochlorophyll-a and of Bacterial Photosynthetic Reaction Centers as Revealed by Picosecond Absorption Studies

Abstract

Picosecond and femtosecond spectroscopic techniques have been used to probe excited-state dynamic processes in a variety of dye molecules. These studies indicate that intramolecular vibrational relaxation generally occurs on a time-scale of 1 to 2 ps, or less, whereas relaxations due to solvent reorganization at room temperature can occur from femtoseconds to hundreds of picoseconds. Electron transfer in bacterial photosynthetic reaction centers (RC) proceeds from an excited state of a pair of strongly interacting pair (P) of bacteriochlorophyll molecules (BChl) in a few picoseconds. It has been suggested that vibrational relaxations may influence the rate of this electron transfer reaction (1). To investigate this question, transient absorption changes resulting from excitation of monomeric BChl-a in methanol and in 1-propanol were measured with approx. 1 ps time-resolution. In addition, stimulated emission spectra of RCs of the purple non-sulfur bacterium Rhodobacter sphaeroides were monitored as a function of time.