Role of Excited Singlet State in the Photooxidation of Carotenoids:  A Time-Resolved Q-Band EPR Study

Spin-polarized 35 GHz time-resolved EPR (TREPR) spectra were obtained for the first time of the cation radicals of β-carotene (I), 15,15‘-didehydro-β-carotene (II), 7‘,7‘-dicyano-7‘-apo-β-carotene (III), and 7‘-cyano-7‘-ethoxycarbonyl-7‘-apo-β-carotene (IV) and the anion radical of the solvent which were formed by 308 nm photoexcitation of the carotenoids in carbon tetrachloride solution. Although the EPR spectra are weak in intensity due to the small dimensions of the 35 GHz quartz flat cell and have very broad line widths, it was possible to positively determine from the polarization pattern that the electron transfer to the solvent occurs from the excited singlet state of the carotenoids. The 35 GHz EPR spectra consist of two resolved EPR lines (one absorption and one emission) from which it has been possible to measure the g factor and ΔHpp of the solvent-separated radical ion pair that was formed; measurements were not possible at 9 GHz