Spectroscopic and photochemical properties of open-chain carotenoids

The spectroscopic properties of open-chain, all-trans-C-30 carotenoids having seven, eight and nine pi-electron conjugated carbon-carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation and time-resolved absorption spectroscopy. These diapocarotenes were purified by high performance liquid chromatography (HPLC) prior to the spectroscopic experiments. The fluorescence data show a systematic crossover from dominant S-1 --> S-0 (2(1)Ag --> 1(1)A(g)) emission to dominant S-2 --> S-0 (1(1)Bu --> 1(1)Ag) with increasing extent of conjugation. The low temperatures facilitated the determination of the spectral origins of the S-1 --> S-0 (2(1)Ag - 1(1)A(g)) emissions, which were assigned by Gaussian deconvolution of the experimental line shapes. The lifetimes of the S-1 states of the molecules were measured by transient absorption spectroscopy and were found to decrease as the conjugated chain length increases. The energy gap law for radiationless transitions is used to correlate the S-1 energies with the dynamics. These molecules provide a systematic series for understanding the structural features that control the photochemical properties of open-chain, diapocarotenoids. The implications of these results on the roles of carotenoids in photosynthetic organisms are discussed.