Solvent Effects on the Resonance Raman and Electronic Absorption Spectra of Bacteriochlorophyll a Cation Radical

Resonance Raman and electronic absorption spectra of bacteriochlorophyll a cation radical (BChl a •+ ) were recorded in 14 different kinds of 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 pentacoordinated state in neutral bacteriochlorophyll a (BChl a ), while it was in the region of 1584−1588 cm -1 in solvents forming the hexacoordinated state. BChl a •+ exhibited a key absorption band in the regions 546−554 and 557−563 nm in the above penta- and hexacoordinating solvents. Therefore, it has been concluded that the penta- and hexacoordinated 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 hexacoordinated state upon one-electron oxidation in this particular solution. The coordination states of BChl a •+ could be correlated with the donor number (DN) and the Taft parameters, β and π*, of the solvent: The hexacoordinated state was formed in solvents with DN ≥ 18 or β > 0.5 showing higher electron-donating power, while the pentacoordinated state was formed in solvents with π* > 0.65 showing higher dielectric stabilization. Preferential solvation of 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 solvation was increased approximately 3 times by one-electron oxidation.