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 i...
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