The T1 and S1 raman spectra of 15N- and 2H-enriched bacteriochlorophyll a: changes in bond order upon triplet and singlet excitation

The Raman spectra of bacteriochlorophyll a with natural abundance and 15N- and 2H-enriched isotope compositions were recorded in the S0, T1 and S1 states. On the basis of isotope shifts, Raman lines were characterized in each state. Comparison of the characterized Raman lines showed the following. (1) Upon triplet excitation of the macrocycle, a decrease in bond order takes place in the Ca′Cm bond having the highest bond order, whereas an increase in bond order takes place in the CaN bond having the lowest bond order. (2) The trend is enhanced further in the S1 state.