FLUORESCENCE OF GASEOUS TETRAENES AND PENTAENES

Fluorescence, fluorescence excitation, and absorption spectra of room temperature vapors of 1,3,5,7-octatetraene, 1,3,5,7-nonatetraene, 2,4,6,8-decatetraene, and 2,4,6,8,10-dodecapentaene have been obtained. All four polyenes show dual (S1 → S0 and S2 → S0) fluorescences with the ratios of the two emissions (21Ag → 11Ag and 11Bu → 11Ag) being sensitive to the extent of substitution. For example, the (S1 → S0)/(S2 → S0) fluorescence yield ratio increases from 0.06 to 0.7 when terminal methyl groups are added to octatetraene. The gas-phase S2-S1 energy gaps are almost identical for these two molecules (6600 cm-1 for octatetraene versus 6300 cm-1 for decatetraene), suggesting that the apparent difference in internal conversion efficiencies is due to the larger density of vibronic states in the methyl-substituted compounds. This is supported by an even larger (S1 → S0)/(S2 → S0) yield ratio (1.4) in dodecapentaene, for which the gas-phase S2-S1 energy difference is 7200 cm-1. The fluorescence excitation spectrum of decatetraene has been measured in the 34 600-37 450-cm-1 region (S0 → S2) in a supersonic expansion. Vibronic line widths have been used to estimate a lifetime of 0.25 ps for the zero-point level of 11Bu and indicate coupling between the 11Bu and 21Ag manifolds in the statistical limit. These results are discussed in relation to the previous gas-phase spectroscopy of octatetraene and shorter polyenes. © 1990 American Chemical Society.