Two-colour pulsed Raman studies of the lowest excited singlet state of tetraphenylporphyrin: band assignments and electronic structure

Abstract

The resonance Raman spectra of the lowest lying singlet (S1) state of free-base tetraphenylporphyrin and seven of its isotopomers were recorded under pump-and-probe conditions with a time delay of −2 ns between pump and probe laser pulses. In the S1 spectra of the isotopomers, as in the ground state, there are dramatic splittings of what appear to be single bands in the natural isotopic abundance spectrum. The most structurally significant bands of the S1 state were assigned on the basis of the isotope data. In some cases it was necessary to curve fit unresolved bands in the excited-state spectra in order to account for observed intensity ratios and to rationalize isotope shifts. The changes in band positions on excitation to the S1 state were compared with those from earlier studies on the T1 state. The changes in band positions were found to be similar for both excited states. Most notable was the similar shift in ν2, the most widely used marker band for orbital character. The data are interpreted as implying that the lowest lying singlet state is a configuration interaction admixture of b1ub2g + aub3g configurations with the coefficients weighted heavily in favour of b1ub2g, which is the configuration of the T1 state. Copyright © 2000 John Wiley & Sons, Ltd.