Femtosecond dynamic-absorption studies of vibrational coherence in carbocyanine dyes and bacteriochlorophyll proteins

Femtosecond dynamic-absorption spectroscopy has been employed to assess the dynamics associated with coherent wave packet motions in the carbocyanine dyes IR144 and DTTCI and in the bacteriochlorophyll a containing proteins B820 and B777. Vibrational coherences were created and observed in both the ground-state depletion region and the stimulated-emission region of all systems studied. In both DTTCI and B820 there was an enhancement of higher frequency vibrational modes in the red portion of the stimulated emission that may be associated with fast surface crossing events from the initially prepared Franck-Condon excited-state surface to a product state surface.; A contour representation of the dynamic-absorption spectra allows us to assign the wave packet motions to arising from either ground state motions or excited state motions. In IR144, the ground-state depletion region and the stimulated-emission region are modulated by the same set of oscillatory components suggesting a bound excited-state potential energy surface. In DTTCI, the stimulated emission region to the red of 830 nm is modulated by higher frequency components in the 300–800 cm−1 range that are weak or absent in the ground-state depletion region and in the Franck-Condon excited state. This observation along with the results of solvatochromic experiments strongly suggests that a surface crossing event occurs from the DTTCI Franck-Condon surface to a product-state surface that exhibits at least partial charge-transfer character.; A comparison of the vibrational coherences observed in B777 and B820 shows a pronounced lack of activity observed in the 0–100 cm −1 region for B777 as opposed to B820 where there is substantial low-frequency character observed. An anisotropy decay observed in the B820 stimulated-emission region reports a prompt >20° tilt of the photo-selected transition-dipole moment. A possible explanation of 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. Our results suggest the possibility that B820 provides a model system for the earliest events in the charge-separation process that occur in the photosynthetic reaction center.