Abstract
Time-resolved resonance Raman spectra are reported for the free-base octaethylporphyrin ((OEP)H-2) and its meso-deuterated ((OEP-d(4))H-2), pyrrole-N-15-substituted ((OEP-N-15(4))H-2), ethyl-deuterated ((OEP-d(28))H-2), and their imino-d(2) isotopomers ((OEP)D-2) in the lowest excited singlet (S-1, (1)(pi pi*)) and tripler states (T-1, (3)(pi pi*)) as well as for those in the ground state (S-0). Both the totally symmetric and nonsymmetric modes were resonance-enhanced in Raman intensity for the S-1 and T-1 states, and their vibrational modes are interpreted on the basis of the observed polarization properties, isotopic frequency shifts, and theoretical assignments reported for (OEP)H-2 and (OEP)Ni in the ground electronic state. As a,common feature for the S-1 and T-1 states, a few polarized Raman bands sensitive to the peripheral ethyl-deuteration were enhanced around 1200 cm(-1), similar to the S-0 state, suggesting the existence of the sigma pi-pi. hyperconjugation between the macrocycle and ethyl groups in the S-1 and T-1 states, too. In the S-1 state, the CaCm, antisymmetric stretching mode with ap (anomalously polarized) polarization, which corresponds to nu(19) of the S-0 state, was resonance-enhanced at the frequency (1534 cm(-1)) downshifted by 54 cm(-1) from that in the ground state (1588 cm(-1)). This indicates the presence of S-1-S-2 vibronic coupling. In the T-1 state, however, no ap band was observed in the 1600-1500 cm(-1) region, but instead, a depolarized band with complicated and unresolved structures was noticeably enhanced around 1440-1400 cm(-1) and dp (depolarized) bands were generally broad. This suggests the presence of another vibronic mechanism, presumably pseudo Jahn-Teller effects between B-3(3u) and B-3(2u) states. As a possible mechanism of band broadening in the T-1 state, unresolved tunnel splitting between N-H tautomers is suggested.