Vibrationally resolved optical spectra of chlorophyll derivatives in different solid media

The site-selection vibronic fluorescence and excitation spectra of chlorophyll a (Chl a), protochlorophyll (PChl), and some of their derivatives in a number of organic solvent matrices are recorded at 5 K. The strength of electron-phonon coupling, estimated as a relative line-to-background intensity in the excitation spectra, was found to be dependent mainly on the state (microcrystallic or glassy) of the matrix. The resolution is better in vitreous samples than in snowy ones. For PChl and 4-vinyl-PChl marked differences were found in both fluorescence and excitation spectra. Chl a, Chl a', and pyro-Chl a could be distinguished on the basis of some characteristics S/sub 1/-state vibrational frequencies, while the spectra of Chl a and ethyl-chlorophyllide a (where the phytyl ester group is replaced by ethyl) are practically identical. The coordination number of the Chl a and PChl central magnesium atom (with one or two axial extra ligands) is found to influence specifically the vibrational modes and the S/sub 1/-S/sub 2/ splitting in the excitation spectra. It has been established that a bisligated Chl a species prevails in sterically uncrowded alcohols, e.g., 1-butanol.