Excited-State Vibrational Coherence and Anisotropy Decay in the BacteriochlorophyllaDimer Protein B820

Abstract

We have employed dynamic absorption spectroscopy to monitor coherent wave packet dynamics and anisotropy decays following impulsive excitation of the B820 subunit of the LH1 light-harvesting complex, which was isolated from Rhodospirillum rubrum G9. When the lower exciton-state transition of the bacteriochlorophyll a dimer is pumped, the time-resolved pump−probe spectrum exhibits contributions from a fully Stokes shifted stimulated-emission spectrum and a nonstationary vibrational character within 40 fs of excitation. Coherent wave packet motion in both the ground state and the excited state is observed via modulations of single-wavelength transients. The photobleaching portion of the spectrum exhibits strong components only at low frequencies, 20−60 and 180 cm-1, and a weaker component is observed at 400 cm-1. The stimulated-emission portion of the spectrum exhibits weak modulation components at 20−60 and 180 cm-1, but strong components are observed at fairly high frequencies: 360, 400, 470, 600, and 730 cm-1. An anisotropy decay observed in the stimulated-emission region reports a prompt >20° tilt of the photoselected transition-dipole moment. A possible explanation for these results is that an intradimer charge-transfer event occurs on a very short time scale following optical preparation of the lower π → π* exciton state of the bacteriochlorophyll a dimer at room temperature.