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 pum p/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), tetramesitylpo rphin (TMP), and tetrakis(pentafluorophenyl)porphin (TF5PP) to investi gate why emission spectra of (TPP)Cu, and (T3,4,5OMePP)CU are signific antly red-shifted in fluid media compared with in rigid media, but tho se of (TMP)Cu and (TFSPP)Cu are not. Raman bands of T-1 spectra were a ssigned 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 ex cept 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) a nd upsilon(11) bands were shifted to lower frequencies in the T-1 stat e 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-pha se stretching) was resonance enhanced and shifted to higher frequencie s 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 cha nged in the same order as that of the red-shift of the emission spectr a 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 t he red-shift in the fluid solvent is associated with the increase of t heir coupling term and thus structural distortions at the methine brid ges, The phenyl ve band was strongly enhanced in the T-1 state for (TP P)Cu but nor for (TMP)Cu and (T3,4,5OMePP)Cu, although their frequenci es remain unaltered, suggesting that the T-n <-- T-l excitation involv es distortion of the phenyl ring along upsilon(8a) coordinate for (TPP )Cu but not for others.