Octatetraene m 1A g states: Two-photon fluorescence excitation spectrum from 28 000 to 50 000 cm−1

Abstract

The 2-photon fluorescence excitation spectra of all-trans octatetraene in n-hexane and n-octane host crystals at 77 and 1.8 K have been recorded for the region 28 000 to 50 000 cm−1. Measurements of the dependence of emission signal on excitation intensity and fluorescence lifetimes contributed to the assignment of the observed features. On the basis of their frequencies, intensities, and solvent shift behaviors all of the bands are seen in the region 28 000 to 44 000 cm−1 are assigned to vibronic bands of the 1 1Ag–2 1Ag transition. The unusual temperature dependence of one excitation feature is consistent with a near degeneracy of singlet and triplet levels. Above 44 000 cm−1 there are four broadbands, two of which must be assigned to 0–0 transitions from the 1 1Ag ground state to higher lying m 1Ag states at 45 030 and 46 710 cm−1, respectively. The other two bands at 48 000 and 49 860 cm−1 could be either vibronic additions to the 0–0 band at 46 710 cm−1 or 0–0 bands for new m 1Ag states.