The 2 1<i>A</i> <i>g</i> state in the linear polyene 2,4,6,8,10,12,14,16-octadecaoctaene

Abstract

High resolution fluorescence and fluorescence excitation spectra have been obtained for the all-trans polyene 2,4,6,8,10,12,14,16-octadecaoctaene in an n-hexadecane matrix at 4.2 K. This is the longest discrete linear polyene for which such data are known. The 1 1Ag to 2 1Ag 0–0 band at 17 871 cm−1 is 4900 cm−1 below 1 1Ag to 1 1Bu 0–0 band at 22 770 cm−1. Both of these excitation energies are approximately what would have been predicted by fitting the 0–0 excitation energies of shorter polyenes with N double bonds by a 1/N curve and extrapolating to N=8. These spectra provide further information on the vibronic levels of the ground (1 1Ag) and first excited (2 1Ag ) singlet states and provide insight into the structure of the conjugated polymer polyacetylene. As has been found in the case of shorter polyenes, the strongest vibronic feature (assigned as a totally symmetric C=C stretch) increases 1554 cm−1 in the 1 1Ag state to 1799 cm−1 in the 2 1Ag state) also for the octaene. Possible reasons for this increase are discussed.