Resonance Raman study of the T-1 excited state of Zinc(II) complexes of beta-substituted chlorins

Nanosecond time-resolved transient absorption (TA) and time-resolved resonance Raman (TR(3)) spectroscopies have been used to study the first triplet excited state, T-1, of Zn(II) octaethylchlorin (ZnOEC). Additionally, TR(3) spectra are reported for Zn(II) etiochlorin I (ZnEtI) and the meso-d(4) and N-15 isotopomers of ZnOEC. Formation of the T-1 state in ZnOEC results in a red shift of the Soret absorption band from 400 to 421 nm. Features in the resonance Raman spectra of the ground and T-1 states are assigned on the basis of the isotope shift patterns and of normal coordinate analyses of MOECs. Bands at 1464 and 1597 cm(-1) in the T-1 spectrum are assigned to CalphaCm stretching modes, nu(3) and nu(10) respectively, downshifted by 21 and 19 cm(-1) from their ground state frequencies. The only observed CbetaCbeta stretching mode seen in the T-1 spectrum is at 1542 cm(-1) and is assigned to nu(2), downshifted from 1571 cm(-1) in the ground state. The CalphaN and CalphaCbeta stretching modes located in the 1350-1400 cm(-1) region of the ground state downshift in the triplet excited state to an unresolved set of peaks in the 1325-1380 cm(-1) region. The frequency downshifts indicate that the observed peaks arise from vibrational modes which are localized on bonds that are weakened in the excited state. Peaks corresponding to substituent vibrational modes are not shifted in the excited state spectrum, implying a lack of hyperconjugation in the T-1 state.