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41865-B4,6
Linear Polyenes: Models for the Spectroscopy and Photochemistry of Carotenoids
2,4,6,8,10,12,14-hexadecaheptaene was synthesized via a Wittig reaction between 2,4,6,8,10,12-dodecapentaenal and crotyltriphenylphosphonium bromide. The hexadecaheptaene products were isolated using silica gel chromatography and then photolyzed to convert the predominantly cis mixture into the all-trans isomer. HPLC (C18- reverse phase) was used to isolate the cis and trans isomers for spectroscopic analysis. Mass spectrometry (MS/APCI+) and NMR spectroscopy (2D 1H-1H COSY and NOESY) were used to identify the major product of the Wittig reaction as 4-cis hexadecaheptaene. The main photolysis product is the all-trans isomer, as summarized in the following reaction scheme: 
The room temperature absorption and emission spectra of the 4-cis and all-trans isomers of 2,4,6,8,10,12,14-hexadecaheptaene are almost identical, exhibiting the characteristic dual emissions S1→→S0 (21Ag- → 11Ag-) and S2→S0 (11Bu+ → 11Ag-) noted in previous studies of intermediate length polyenes and carotenoids. The ratio of the S1→S0 and S2→S0 emission yields for the cis isomer increases by a factor of ~15 upon cooling to 77 K in n-pentadecane. In contrast, for the trans isomer this ratio shows a two-fold decrease with decreasing temperature. These results suggest a low barrier for conversion between the 4-cis and all-trans isomers in the S1 state. At 77 K, the cis isomer cannot convert to the more stable all-trans isomer in the 21Ag- state, resulting in the striking increase in its S1→S0 fluorescence. These experiments imply that the S1 states of longer polyenes have local energy minima, corresponding to a range of conformations and isomers, separated by relatively low (2-3 kcal) barriers. Steady state and time-resolved optical measurements on the S1 states in solution thus sample a distribution of conformers and geometric isomers, even for samples represented by a single, dominant ground state structure. Complex S1 potential energy surfaces help explain the complicated S2→S1 relaxation kinetics of many carotenoids. The finding that fluorescence from linear polyenes is so strongly dependent on molecular symmetry requires a reevaluation of the literature on the radiative properties of all-trans polyenes and carotenoids.
