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.

Electrochemical oxidation of all-trans-canthaxanthin and β-carotene in dichloromethane leads to significant trans-to-cis isomerization, with cis isomers accounting for about 40% of the products formed. The electrochemically generated isomers were separated by reverse-phase high-performance liquid chromatography and identified as 9-cis, 13-cis, 15-cis, and 9,13-di-cis isomers of the carotenoids by 1H-NMR spectroscopy and optical spectroscopy (Q ratio). The results of simultaneous bulk electrolysis and optical absorption spectroscopy indicate the following isomerization mechanism:  the all-trans cation radicals and/or dications formed by electrochemical oxidation of all-trans-carotenoids can easily undergo geometrical isomerization to form cis cation radicals and/or dications. The latter are converted by the comproportionation equilibrium to cation radicals which are then transformed to neutral cis-carotenoids by exchanging one electron with neutral carotenoids. AM1 molecular orbital calculations, which show that the energy barriers of configurational transformation from trans to cis are much lower in the cation radical and dication species than in the neutral molecule, strongly support the first step of this mechanism.

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When canthaxanthin (I) and β-carotene (II) dichloromethane solutions are treated with small amounts of ferric chloride (≤0.26 mol equiv), extensive photodegradation of the remaining unreacted neutral carotenoid occurs upon subsequent irradiation with near-UV to visible light. The rate of this photodegradation is independent of the neutral carotenoid concentration at a given initial FeCl3 concentration and first-order in initial FeCl3 (k 1 = 1.43 and 3.30 min-1 for I and II, respectively). The data are consistent with a mechanism in which Fe(II) is photochemically converted in the presence of CH2Cl2 to Fe(III), which then oxidizes unreacted neutral carotenoids. We also report that canthaxanthin radical cations in dichloromethane form cation dehydrodimers at low temperature or upon irradiation with near-UV to visible light, which have an absorption maximum near 770 nm and a reduction peak at 20 mV in cyclic voltammetry. It is proposed that radical cations associate with parent molecules and then undergo two subsequent steps to form the final cation dehydrodimers. In the presence of supporting electrolytes, canthaxanthin radical cations form ion pairs with the anions PF6 -, resulting in a 10 nm blue shift of the absorption maximum of the radical cations.