Effect of Terminal Groups, Polyene Chain Length, and Solvent on the First Excited Singlet States of Carotenoids

Abstract

The effect of terminal groups, polyene chain length, and solvent on the first excited singlet states (S1) of carotenoids was studied by steady-state and transient optical absorption spectroscopy, and AM1 semiempirical molecular orbital calculations. The carotenoids studied were ethyl 8‘-apo-β-caroten-8‘-oate (I), ethyl 6‘-apo-β-caroten-6‘-oate (II), ethyl 4‘-apo-β-caroten-4‘-oate (III), 8‘-apo-β-caroten-8‘-nitrile (IV), 6‘-apo-β-caroten-6‘-nitrile (V), 4‘-apo-β-caroten-4‘-nitrile (VI), 8‘-apo-β-caroten-8‘-al (VII), and 6‘-apo-β-caroten-6‘-al (VIII). Solvents were 3-methylpentane (3-MP) and MeCN. The effect of solvent on the S1 absorption maxima is similar to that on the ground state (S0) absorption maxima, which suggests that both effects stem from the same type of interaction, i.e., the dispersive interaction between carotenoids and solvents. Carotenoids with terminal CHO groups have S1 absorption maxima at longer wavelenths than those with terminal CN or CO2Et groups. The S1 absorption maxima are red-shifted with increasing polyene chain length. In the nonpolar solvent 3-MP, the S1 lifetimes of carotenoids depend mainly on the polyene chain length. With a one CC bond increase, the S1 lifetime decreases by a factor of ca. 2 (ca. 24 ps for I, IV, and VII; 12 ps for II, V, and VIII; and 7 ps for III and VI). Terminal groups have little effect on the S1 lifetimes in 3-MP. However, in the polar solvent MeCN, carotenoids with terminal CHO groups have decreased S1 lifetimes (ca. 8 ps for VII and 6 ps for VIII), while carotenoids with terminal CN and CO2Et groups have essentially unchanged S1 lifetimes. This observation, along with the data of β-carotene and 7‘-apo-7‘,7‘-dicyano-β-carotene, suggests that polar solvents could decrease the S1 lifetimes of carotenoids, when, and only when, there is considerable charge transfer character in their excited states.