Carotenoid cation radical electronic absorption

A characteristic feature of carotenoid cation radicals is their electronic absorption in the NIR region, e.g. [36,40,55,103,116,117], in contrast to... 

Electronic Absorption Spectroscopy. Absorption spectra have been obtained for radical cations and anions generated from a number of carotenoids [phytoene (7,8,ll,12,7, 8, ll, 12 -octahydro-i//,i/ -carotene) (135) and canthaxanthin ()3,/3-carotene-4,4 -dione) (130)] and related polyenes [7,7 -dihydro- -carotene (131),... 

In the carotenoid radicals, the unpaired electron is highly delocalized over the conjugated polyene chromophore. This has a stabilizing effect and also allows subsequent reactions. The cation and anion radicals can be detected by their characteristic spectral properties, with intense absorption in the near-infrared region. 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

A closely related tetrad featuring two porphyrin moieties and a single naphthoquinone acceptor has also been reported [13]. Excitation of either porphyrin moiety of C-P-P-Q in anisole solution is followed by rapid (>10 s ) singlet-singlet energy transfer between the two porphyrins, whose absorption and emission spectra are essentially identical. C-P- P-Q decays by photoinduced electron transfer to the quinone with a rate constant of 2.4 x 10 s. Sequential transfer of the radical cation hole to the second porphyrin, and then to the carotenoid yields a final C +-P-P-Q state with a quantum yield of 0.25 and a lifetime of 2.9 ps. 

Information regarding the solution conformation of 13 was derived from the pyropheophorbide ring current induced shifts in the resonance positions of the carotenoid and quinone moieties. These two species were found to be extended away from the tetrapyrrole, rather than folded back across it. The absorption spectrum of 13 was essentially identical to the sum of the spectra of model compounds. The pyropheophorbide fluorescence, however, was strongly quenched by the addition of the quinone. This implies the formation of a C-Phe -Q state via photoinitiated electron transfer from the pyropheophorbide singlet state, as was observed for C-P-Q triads (see Figure 4). Excitation of the molecule in dichloromethane solution at 207 K with a 590 nm laser pulse led to the observation of a carotenoid radical cation transient absorption. Thus, the C-Phe -Q " state can go on via an electron transfer step analogous to step 4 in Figure 4 to yield a final C -Phe-Q state. This state had a lifetime of 120 ns. The quantum yield at 207 K was 0.04. At ambient temperatures, the lifetime of the carotenoid radical cation dropped to about SO ns, and the quantum yield could not be determined accurately because of the convolution of the decay into the instrument response function. 

The absorption spectra of a wide range of carotenoid radical cations and anions were first established by nanosecond pulse radiolysis under conditions of mono-electronic processes (Dawe and Land, 1975 Laffertyetal., 1977). This workreported the spectra in hexane for the radical cations and in hexane and methanol for the radical anions. Subsequently, such studies for the radical cations have been extended to other solvents (Hill et al., 1995 Edge et al., 1998). Table 1 gives a selection of A, values for carotenoid radical cations in four... 

Some oxy-radicals lead to both electron transfer reactions and addition to the carotenoid. The most well studied is the trichloromethylperoxyl radical CCI3OO-, a peroxyl radical which is known to cause hepatotoxity and other types of tissue injury (Packer et al, 1981 Hill et al, 1995). Packer et al. (1981) showed that, in the presence of/3-carotene, there was a fast bleaching of the carotene ground state absorption with a rate constant of 1.5 x lO M s". The loss of absorption at 450 nm was accompanied by an increase in absorption in the near infrared region (950-1000 nm), indicating that the reaction produces the /3-carotene radical cation. 

Carotenoids, in particular P-carotene, are well-known free-radical quenchers [78]. It has been shown that peroxy radicals can oxidise P-carotene to the corresponding radical cations [78]. Absorption spectra demonstrate the appearance of broad bands in the range 600-1000 nm with = 910 nm (e = 9.4 x lOVM/cm) during pulse radiolysis of solutions of P-carotene in tert-butyl alcohol - water mixtures containing nitrate [79]. These bands were attributed to P-carotene radical cations generated by electron abstraction from the substrate by NO radicals ... 

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