Spectroscopic properties of polyenes. III. 1,3,5,7-Octatetraene

Abstract

Absorption and emission spectra of all‐trans 1, 3, 5, 7‐octatetraene are presented along with fluorescence quantum yields and lifetimes. In solution, a gap of about 3000 cm⁻¹ is found between the first band of the 1 ¹A_g→1¹B_u transition and the onset of the emission spectrum. Excitation spectra of concentrated solutions at 77 K show low‐lying bands in this gap, the lowest energy band being almost coincident with the highest energy fluorescence band. On the other hand, gas phase fluorescence spectra show no gap between the lowest energy 1¹A_g→1¹B_u absorption band and the first fluorescence band. The radiative lifetime in hexane is 220 ns at room temperature and 190 ns at 77 K. The radiative lifetime for the gas phase fluorescence is estimated to be longer than 150 ns. The solvent dependence of the absorption and emission spectra, the fluorescence lifetimes, and the vibrational frequencies observed in solution imply support for the conjecture of Karplus et al. that the lowest excited singlet state is of ¹A_g symmetry. The solution data imply that the low‐lying state is about 6400 cm⁻¹ below the 1¹B_u level. On the other hand, the lack of a gap between absorption and emission and the long lifetime found for the gas phase are not compatible with this model.