Solvent effects on the resonance Raman spectra of bacteriochlorophyll a cation radical

Resonance Raman (RR) spectra were measured for the cation radical of bacteriochlorophyll a in acetone, methanol, dichloromethane and mixed solvents of acetone and methanol. The ring-breathing (Ca-Cm stretching) frequency of the radical (abbreviated as vr+) was observed at 1601 cm−1 in acetone (forming a penta-coordinated monomer), at 1587 cm−1 in a methanol (forming a hexa-coordinated monomer) and at 1600 cm−1 in dichloromethane (forming a penta-coordinated aggregate). The RR spectrum of the radical in dichloromethane is almost identical to the transient RR spectrum ascribed to ‘the aggregated T1 species of Bchl a’ formed in the particular solvent by Nishizawa, Limantara, Nanjou, Nagae, Kakuno and Koyama, indicating that their interpretation needs to be revised.