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 fir st excited singlet states (SI) of carotenoids was studied by steady-state a nd transient optical absorption spectroscopy, and AM1 semiempirical molecul ar orbital calculations. The carotenoids studied were ethyl 8'-apo-beta-car oten-8'-oate (I), ethyl 6'-apo-beta-caroten-6'-oate (II), ethyl 4'-apa-beta -caroten-4'-oate (III), s'-apo-beta-caroten-8'-nitrile (IV), 6'-apo-beta-ca roten-6'-nitrile (V), 4'-apo-beta-caroten-4'-nitrile (VI), 8'-apo-beta-caro ten-8'-al (VII), and 6'-apo-beta-caroten-6'-al (VIII). Solvents were 3-meth ylpentane (3-MP) and MeCN. The effect of solvent on the SI absorption maxim a is similar to that on the ground state (So) absorption maxima, which sugg ests that both effects stem from the same type of interaction, i.e., the di spersive interaction between carotenoids and solvents. Carotenoids with ter minal CHO groups have S-1 absorption maxima at longer wavelenths than those with terminal CN or CO2-Et groups. The S1 absorption maxima are red-shifte d with increasing polyene chain length. In the nonpolar solvent 3-MP, the S 1 lifetimes of carotenoids depend mainly on the polyene chain length. With a one C=C bond increase, the SI 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 Si lifetimes in 3-MP. Howev er, in the polar solvent MeCN, carotenoids with terminal CHO groups have de creased S1 lifetimes (ca. 8 ps for VII and 6 ps for VIII), while carotenoid s with terminal CN and CO2Et groups have essentially unchanged Si lifetimes . This observation, along with the data of beta-carotene and 7'-apo-7',7'-d icyano-beta-carotene, suggests that polar solvents could decrease the S-1 l ifetimes of carotenoids, when, and only when, there is considerable charge transfer character in their excited states.