SOLVENT EFFECTS ON THE RESONANCE RAMAN AND ELECTRONIC ABSORPTION-SPECTRA OF BACTERIOCHLOROPHYLL-ALPHA CATION-RADICAL

Resonance Raman and electronic absorption spectra of bacteriochlorophy ll a cation radical (BChl a(.+)) were recorded in 14 different kinds o f solvents. The frequency of the ring-breathing Raman band of BChl a(. +) was in the region of 1596-1599 cm(-1) in solvents forming the penta coordinated state in neutral bacteriochlorophyll a (BChl a), while it was in the region of 1584-1588 cm(-1) in solvents forming the hexacoor dinated state. BChl a(.+) exhibited a key absorption band in the regio ns 546-554 and 557-563 nm in the above penta- and hexacoordinating sol vents. Therefore, it has been concluded that the penta- and hexacoordi nated states are retained even after conversion of BChl a into BChl a( .+) (one-electron oxidation). Application of this rule to the case of 2-propanol solution showed transformation from the penta- to the hexac oordinated state upon one-electron oxidation in this particular soluti on. The coordination states of BChl a(.+) could be correlated with the donor number (DN) and the Taft parameters, beta and pi, of the solve nt: The hexacoordinated state was formed in solvents with DN greater t han or equal to 18 or beta > 0.5 showing higher electron-donating powe r, while the pentacoordinated state was formed in solvents with pi > 0.65 showing higher dielectric stabilization. Preferential solvation o f a hexacoordinating solvent over a pentacoordinating solvent in their mixture (methanol/acetone or tetrahydrofuran/methylene chloride) was seen in both BChl a and BChl a(.+), but the constant of preferential s olvation was increased approximately 3 times by one-electron oxidation .