Effect of a conjugated carbonyl group on the photophysical properties of carotenoids

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Abstract
Effects of introducing a carbonyl group into the conjugation system of carotenoids were studied for four naturally occurring carotenoids: peridinin, fucoxanthin, siphonaxanthin and spheroidenone. The conjugated carbonyl group affects energetics and dynamics of all these carotenoids in a similar way, although the magnitude of the changes depends strongly on the carotenoid structure. Firstly, presence of a carbonyl group considerably narrows the S1/ICT–S2 gap, and this effect does not depend on polarity. The S1/ICT energies of carotenoids were measured by recording S1/ICT–S2 spectral profiles in the near-infrared region and the resulting energies were 16100 cm−1 for peridinin, 16520 cm−1 for fucoxanthin and 16610 cm−1 for siphonaxanthin. Narrowing of the S1/ICT–S2 gap has important consequences for functionality of these carotenoids in light-harvesting systems of oceanic organisms, since while the S2 state is red-shifted to capture green light, the S1/ICT state is still high enough to transfer energy to chlorophyll. The S1/ICT energy of spheroidenone was determined to be 13000 cm−1. Secondly the carbonyl group introduces some polarity-dependent effects: (1) polarity-induced change of the S1/ICT lifetime. When changing from nonpolar to polar solvent, the S1/ICT lifetime is changed from 160 to 8.5 ps for peridinin, from 60 to 30 ps for fucoxanthin, from 60 to 20 ps for fucoxanthin, while for the longer carotenoid spheroidenone the S1/ICT lifetime remains 6 ps regardless of solvent polarity. This effect is explained in terms of stabilization of charge-transfer character of both ground and excited states. (2) stabilization of the charge-transfer character in polar solvents is also demonstrated by appearance of new polarity-induced bands in the transient absorption spectra. (3) polarity-induced changes of the ground state are manifested by asymmetric broadening of the absorption spectrum accompanied by a loss of vibrational structure.

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