Laser flash photolytic generation of radical ions of carotenoids in organic solvents. Studies of their subsequent fates, including formation of stable carotenoid sigma dimer radical anion (CAR)2

In this study, time-resolved laser flash photolysis technique (LFP) was used to generate and study extraordinary stable transients formed following the decay of radical anions of canthaxanthin (CAN) and astaxanthin (ASTA). The radical anions were obtained via one-electron reduction of CAN and ASTA by acetone ketyl radical (AC center dot- ) in various solvents. At room temperature, the decay of CAN center dot- (lambda max = 870 nm) and ASTA center dot- (lambda max = 1000 nm), in acetonitrile (ACN), led to the formation of indefinitely stable colored transients (blue-green for CAN and navy blue for ASTA) absorbing at shorter wavelengths (lambda max = 690 nm and 720 nm, respectively) relative to their corresponding radical anions. Rapid quenching by oxygen was observed indicating the radical nature of these transients, which was further confirmed by EPR spectroscopy. By employing EPR spectroscopy, LFP technique, and UV-visible absorption measurements, these extraordinary stable transients were identified as sigma-dimer radical anions ((CAR)2 center dot- ) formed via the dimerization of the parent carotenoid (CAR) with its radical anion (CAR center dot- ). The suggested structures of these dimers were postulated using DFT calculations, which indicated that the unpaired electron is delocalized over the entire molecule. This result is extremely important because of the paucity of reports on dimer radical anions in solution.