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

Abstract

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 AE 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.