FREQUENCIES OF THE FRANCK-CONDON ACTIVE AG C=C STRETCHING MODE IN THE 21AG- EXCITED-STATE OF CAROTENOIDS

Picosecond time-resolved resonance Raman spectra (CC stretching region) of the 2 1A-g excited state (S1) have been measured for β-carotene, three derivatives of β-carotene (β-apo-8′-carotenal, ethyl β-apo-8′-carotenoate and canthaxanthin), and five carotenoids extracted from photosynthetic bacteria (neurosporene, spheroidene, spheroidenone, rhodopin and spirilloxanthin). In all of these carotenoids, the frequency of the Franck-Condon active ag CC stretching mode in S1 is much larger than that found in the 1A-g ground state (S0). The frequencies in both S0 and S1 correlate well with the absorption maxima for all carotenoids examined except β-apo-8′-carotenal and ethyl β-apo-8′-carotenoate, in which cases the S1 frequencies are smaller than expected. These results are discussed with respect to the previous studies of the shorter polyenes and a vibronic coupling mechanism. © 1990.