Ultrafast intramolecular relaxation dynamics of Mg- and Zn-bacteriochlorophyll a

Ultrafast excited-state dynamics of the photosynthetic pigment (Mg-)bacteriochlorophyll a and its Zn-substituted form were investigated by steady-state absorption/fluorescence and femtosecond pump-probe spectroscopic measurements. The obtained steady-state absorption and fluorescence spectra of bacteriochlorophyll a in solution showed that the central metal compound significantly affects the energy of the Q(x) state, but has almost no effect on the Q(y) state. Photo-induced absorption spectra were recorded upon excitation of Mg- and Zn-bacteriochlorophyll a into either their Q(x) or Q(y) state. By comparing the kinetic traces of transient absorption, ground-state beaching, and stimulated emission after excitation to the Q(x) or Q(y) state, we showed that the Q(x) state was substantially incorporated in the ultrafast excited-state dynamics of bacteriochlorophyll a. Based on these observations, the lifetime of the Q(x) state was determined to be 50 and 70 fs for Mg- and Zn-bacteriochlorophyll a, respectively, indicating that the lifetime was influenced by the central metal atom due to the change of the energy gap between the Q(x) and Q(y) states. (C) 2013 AIP Publishing LLC.