Vibrational relaxation and redistribution in the 2Ag- state of all-trans-lycopene as revealed by picosecond time-resolved absorption spectroscopy

Vibrational relaxation in the 2A(g)(-) (S-1) state of all-trans-lycopene, which takes place in a time scale slower than that of internal conversion, has been found by picosecond time-resolved absorption spectroscopy: The time-resolved S-n (2B(u)(+)) <--S-1 (2A(g)(-)) absorption spectra of this carotenoid in quinoline and CS2 solutions can be explained in terms of vibrational structures originating from the C=C (v(1)) and C-C (v(2)) stretching modes, and the time scales of vibrational relaxation (k(v1) and k(v2) defined for the lowest vibrational transition) relative to that of internal conversion (k) are estimated by spectral simulation; the values of k(v1)/k and k(v2)/k are around 0.6 and 0.3 in the quinoline solution and 0.3 and 1.6 in the CS2 solution. The slower vibrational relaxations are discussed in light of the light-harvesting function of carotenoids in photosynthesis.