TIME-RESOLVED RESONANCE RAMAN-SPECTRA OF MESOSUBSTITUTED COPPER PORPHYRINS IN THE T-1 STATE - IMPLICATION FOR THE SOLVENT-DEPENDENT RED SHIFT OF EMISSION-SPECTRA

Time-resolved resonance Raman (TR(3)) spectra were obtained with a pump/probe technique using two 7 ns pulsed lasers for Cu(II) complexes of four meso-substituted porphyrins, including tetraphenylporphin (TPP), tetrakis(3,4,5-trimethoxyphenyl)porphin (T3,4,5OMePP), tetramesitylporphin (TMP), and tetrakis(pentafluorophenyl)porphin (TF5PP) to investigate why emission spectra of (TPP)Cu, and (T3,4,5OMePP)CU are significantly red-shifted in fluid media compared with in rigid media, but those of (TMP)Cu and (TFSPP)Cu are not. Raman bands of T-1 spectra were assigned on the basis of deuteration shifts for TPP-d(8) and TPP-d(20), All porphyrin skeletal bands are similarly broader in the TI state except for a phenyl internal mode. It was unexpected from the empirical rule on the a(1u) and a(2u) cation radicals that both the upsilon(2) and upsilon(11) bands were shifted to lower frequencies in the T-1 state than those of the S-0 state irrespective of the symmetry property, a(1u) or a(2u) of the HOMO. The upsilon(27) mode (C-m-phenyl out-of-phase stretching) was resonance enhanced and shifted to higher frequencies in the T-1 state. The magnitudes of frequency shifts of upsilon(2), upsilon(1), and upsilon(27) bands upon excitation to the T-1 state changed in the same order as that of the red-shift of the emission spectra in the fluid solution, that is, (T3,4,5OMePP)Cu approximate to (TPP)Cu > (TMP)Cu > (TF5PP)Cu. Since these vibrations contain C-a-C-m or C-m-phenyl stretching character, the present observation suggests that the red-shift in the fluid solvent is associated with the increase of their coupling term and thus structural distortions at the methine bridges, The phenyl ve band was strongly enhanced in the T-1 state for (TPP)Cu but nor for (TMP)Cu and (T3,4,5OMePP)Cu, although their frequencies remain unaltered, suggesting that the T-n <-- T-l excitation involves distortion of the phenyl ring along upsilon(8a) coordinate for (TPP)Cu but not for others.