ELECTRONIC RELAXATION IN LONG POLYENES

A series of carotenols with from 7 to 11 conjugated double bonds have been synthesized and purified by using HPLC techniques. Absorption, fluorescence, and fluorescence excitation spectra have been obtained in 77 K glasses. The shorter members of this series exhibit the Stokes-shifted, S1 → S0 emissions seen in previous studies of model polyenes. For carotenols with more than eight conjugated double bonds, however, the fluorescence is dominated by "anti-Kasha", S2 → S0 fluorescence. These findings in part can be attributed to a larger S2-S1 energy difference and the resultant decrease in S2 → S1 radiationless decay rates in longer polyenes. The precipitous crossover from S1 → S0 to S2 → S0 emission, however, cannot be fully accounted for by the energy gap law, which predicts only modest changes in radiative and nonradiative decay rates with increasing polyene length. The lack of S1 fluorescence prohibits the direct observation of the S1 state in the longer carotenols. Nevertheless, trends noted in the shorter polyenes indicate a 5500-6500-cm-1 S2-S1 energy difference for polyenes such as β-carotene (11 conjugated double bonds). The implications of large S2-S1 energy gaps for the spectroscopy and photochemistry of β-carotene and other long polyenes also are discussed. © 1990 American Chemical Society.