Vibronic Origin of the Q Absorption Tail of Bacteriochlorophyll a Verified by Fluorescence Excitation Spectroscopy and Quantum Chemical Simulations.

The long-wavelength tail of the lowest-energy Q singlet absorption band of bacteriochlorophyll a in triethylamine peaking at 768.6 nm was examined by means of fluorescence excitation spectroscopy at ambient temperature of 22 ± 1 °C. The tail, usually considered a Gaussian, does in fact weaken quasi-exponentially, being clearly evident as far as 940 nm, nearly 2400 cm (∼12 kT) away from the absorption peak. Quantum chemical simulations identified vibronic transitions from the thermally populated normal modes and their overtones in the ground electronic state as the origin of this tail. Because energy transfer and relaxation processes generally depend on vibronic overlap integrals, these findings may have important implications on the interpretation of numerous photoinduced phenomena that involve bacteriochlorophyll and similar molecules, including photosynthesis.