Resonance Raman Spectra of the Triplet State of Free-Base Tetraphenylporpyrin and Six of Its Isotopomers

Article abstract of DOI:10.1021/j100012a015

The resonance Raman spectra of the ground state and the lowest excited triplet state of free-base tetraphenylporphyrin and six of its isotopomers have been obtained using two-color time-resolved techniques.Ground-state spectra were recorded using low-energy 447 nm probe laser pulses, and triplet-state spectra were probed, with similar pulses, 30 ns after high-energy excitation with 532 nm pump pulses.Polarization data on both the ground and triplet states are also reported.The resonance Raman spectrum of the triplet is very different from that of the ground state but the combination of extensive isotope substitution with polarization data allows bands in the ground state to be assigned and corresponding bands in the triplet state to be located.Isotope shifts of the same bands in the S₀ and T₁ states are similar, implying that the compositions of the vibrational modes do not change significantly on excitation.Two of the strongest bands in the T₁ spectra are associated with phenyl ring substituents; these are shifted less than 5 cm^-1 between the S₀ and T₁ states so that bonding in the phenyl substituents is barely affected by excitation to the T₁ state.The changes in position of the porphyrin ring bands are larger, but still only tens of cm^-1 or less, the main changes in the spectra being due to differences in relative band intensities in the two states.The relatively small shifts in the porphyrin ring band positions which are observed show that the excitation energy is not localized on a single small region of the molecule but is delocalized over the entire porphyrin skeleton.This picture of an excited species with high chemical reactivity, but with an individual bonds only slightly perturbed from the ground state, is contrasted with molecules, such as benzophenone, where excitation causes a lerge perturbation in the bonding within a single functional group.