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 re sonance Raman spectra of the ground and triplet states of these molecu les are reported. The radical cations were generated following electro n transfer quenching of triplet 1-nitronaphthalene in methanol and Tri ton X-100 micelles. The quenching of triplet 1-nitronaphthalene by the se carotenoids involves solvent-dependent competition between energy t ransfer and electron transfer, and for both carotenoids, estimates are given for the efficiencies of these two processes in methanol and hex ane. The resonance Raman spectra of septapreno-beta-carotene ground an d triplet states are consistent with spectra reported previously for o ther 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 r esonance Raman spectrum of the radical cation of 7,7'-dihydro-beta-car otene is quite distinct from that of septapreno-beta-carotene. These o bservations are attributed to differences in electronic structure aris ing from septapreno-beta-carotene having an odd number of conjugated d ouble bonds while 7,7'-dihydro-beta-carotene, unusually, has an even n umber.