Resonance Raman and absorption spectroscopy of 2- and 3-methyl-1,3,5-hexatriene in the T1 state. Steric control of excited state molecular structure

Resonance Raman (RR) spectra of (E)-2-methyl-1,3,5-hexatriene [(E)-2-MHT], (E)-4-deuterio-2-methyl-1,3,5-hexatriene, (Z)-2-methyl-1,3,5-hexatriene [(Z)-2-MHT], (E)-3-methyl-1,3,5-hexatriene [(E)-3-MHT], and (Z)-3-methyl-1,3,5-hexatriene [(Z)-3-MHT] in the lowest triplet state T1 are reported and discussed. QCFF/PI calculations are performed to determine energies and optimized geometries in the S0, T1, and Tn electronic states. For each T1 species, T1 Æ Tn transitions, vibrational frequencies, and RR intensities are calculated and compared with the observed frequencies and intensities. The ground-state distribution of rotamers is shown to be preserved upon excitation to T1 and during the T1 lifetime (NEER principle). In contrast, the E and Z isomers associated with the torsion around the central CC bond are found to equilibrate in T1 according to the Boltzmann rule. In 2-MHT, the T1 RR spectrum is attributed to both the E and Z forms, the former being dominant. For (E)-2-MHT, only the tt conformer is shown to be present, while for (Z)-2-MHT also the contribution of the ct conformer is demonstrated. The T1 RR spectrum of 3-MHT receives a larger contribution from the Z than from the E isomer. The spectra obtained from (E)-3-MHT and (Z)-3-MHT are identical, since in both isomers only the tt conformer is present. Measurements of time-resolved triplet-triplet absorption as a function of temperature yield activation energies and frequency factors for the decay of the T1 state of (E)-2-MHT and (Z)-3-MHT. The implications of these results on the shape of the T1 potential energy curves are discussed.