Investigation of carotenoid radical cations and triplet states by laser flash photolysis and time-resolved resonance Raman spectroscopy: Observation of competitive energy and electron transfer

The first nanosecond time-resolved resonance Raman study of carotenoid radical cations is reported for the polyenes septapreno-beta-carotene and 7,7'-dihydro-beta-carotene. In addition, previously unreported resonance Raman spectra of the ground and triplet states of these molecules are reported. The radical cations were generated following electron transfer quenching of triplet 1-nitronaphthalene in methanol and Triton X-100 micelles. The quenching of triplet 1-nitronaphthalene by these carotenoids involves solvent-dependent competition between energy transfer and electron transfer, and for both carotenoids, estimates are given for the efficiencies of these two processes in methanol and hexane. The resonance Raman spectra of septapreno-beta-carotene ground and triplet states are consistent with spectra reported previously for other carotenoids. However, the resonance Raman spectrum of the triplet state of 7,7'-dihydro-beta-carotene displays an intensity profile not found in the triplet spectra of other carotenoids. In addition, the resonance Raman spectrum of the radical cation of 7,7'-dihydro-beta-carotene is quite distinct from that of septapreno-beta-carotene. These observations are attributed to differences in electronic structure arising from septapreno-beta-carotene having an odd number of conjugated double bonds while 7,7'-dihydro-beta-carotene, unusually, has an even number.