Absorption spectra of radical ions of polyenes of biological interest

Facile method for spectroscopic examination of radical ions of hydrophilic carotenoids

“…As has been found to be the case with other carotenoids, the values of e AE max are appreciably larger than the corresponding value of e 0 max . A more interesting feature is the red-shift of S a with respect to S c , which was also found to be the case for crocin (data not shown); apart from crocin and C, the only other example of this ordering is canthaxanthin, 11 also a symmetric molecule with a conjugated carbonyl group at each end.…”

“…At delays longer than about 30 ms in argon-saturated solutions, only S n was present; this was also the case at shorter delays in solutions saturated with O 2 or N 2 O, which are well known scavengers of e À aq . 11,14 In the spectral region where absorption by TOH is negligible, the shapes of the spectra S n and S e did not change with pH. However, the yields of the two photoproducts were found to be larger under more alkaline conditions; since the yield of e À aq determines the amplitude of the signal contributed by C À , and since the shapes of S a and S c were also found to be insensitive to pH, all the transient spectra reported here were recorded at pH 11.9.…”

“…11, 14 In alcohols 11 or alcohol-water mixtures, 12 where the positive hole does not survive for long, only Car À is observed. 11 It is possible to form Car + in polar solvents, 15 including water, 16 by using the triplet state of 1-nitronaphthalene as an electron acceptor.…”

“…The most successful strategy for spectroscopic investigations of singly charged radical ions of carotenoids relies on the production, through pulse radiolysis, of solvated electrons and solvent-derived radical cations; [11][12][13] the latter may be viewed, in the present context, as carriers of positive holes. Carotenoid (Car) molecules subsequently capture the solvated electrons as well as the positive holes, and the resulting radical ions, which have strongly allowed absorption transitions in the visible and near infrared regions, can be easily detected with the help of kinetic spectroscopy.…”

“…11 It is possible to form Car + in polar solvents, 15 including water, 16 by using the triplet state of 1-nitronaphthalene as an electron acceptor. A strategy for producing both Car + and Car À in protic solvents, using a single pump-probe technique, is still lacking.…”