Effect of terminal groups, polyene chain length, and solvent on the first excited singlet states of carotenoids

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 C=C 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.