New aspects concerning the energy transfer in carotenoids by measuring intracavity absorption spectra and delayed fluorescence

The first excited singlet state and the lower energetic triplet states of c arotenoids are considered to be involved in the light-harvesting as well as in the photochemical protection of cells, respectively. For this reason, t he symmetry-forbidden S-0-S-1 (1(1)A(g)-2(1)A(g)) transitions and the multi plicity-forbidden S-0-T-2 (1(1)A(g)-2(3)A(g)) transition of the model carot enoid 8,13-dimethyl-2,2,19,9-tetramethoxy-icosa-4,6,8,10,12,14,16-heptaene- 3,18-dione were investigated by intracavity absorption spectroscopy from lo w-concentrated ethanolic solutions. Both transitions are shaped by promotin g modes caused by Herzberg-Teller coupling and the sequence of these modes allows the precise determination off the non-visible S-0-S-1 (0-0)- and S-0 -T-2 (0-0)-transitions. The assignments of the singlet-triplet transitions were additionally supported by measuring delayed fluorescence from crystall ine samples by directly exciting vibronic triplet states. The vibronic coup ling is promoted by C-H bending vibrations of the chain and mainly by defor mation modes of the terminating groups of the carotenoid. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.