Spectroscopic and Photochemical Properties of Open-Chain Carotenoids

Abstract

The spectroscopic properties of open-chain, all-trans-C30 carotenoids having seven, eight and nine π-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 S1 → S0 (21Ag → 11Ag) emission to dominant S2 → S0 (11Bu → 11Ag) with increasing extent of conjugation. The low temperatures facilitated the determination of the spectral origins of the S1 → S0 (21Ag → 11Ag) emissions, which were assigned by Gaussian deconvolution of the experimental line shapes. The lifetimes of the S1 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 S1 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.