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

Abstract

The first excited singlet state and the lower energetic triplet states of carotenoids are considered to be involved in the light-harvesting as well as in the photochemical protection of cells, respectively. For this reason, the symmetry-forbidden S 0–S 1 (1 1A g–2 1A g) transitions and the multiplicity-forbidden S 0–T 2 (1 1A g–2 3A g) transition of the model carotenoid 8,13-dimethyl-2,2,19,19-tetramethoxy-icosa-4,6,8,10,12,14,16-heptaene-3,18-dione were investigated by intracavity absorption spectroscopy from low-concentrated ethanolic solutions. Both transitions are shaped by promoting modes caused by Herzberg–Teller coupling and the sequence of these modes allows the precise determination of 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 crystalline samples by directly exciting vibronic triplet states. The vibronic coupling is promoted by C–H bending vibrations of the chain and mainly by deformation modes of the terminating groups of the carotenoid.