A first detection of singlet to triplet conversion from the 11Bu− to the 13Ag state and triplet internal conversion from the 13Ag to the 13Bu state in carotenoids: dependence on the conjugation length

Subpicosecond time-resolved absorption spectra were recorded in the visible region for a set of photosynthetic carotenoids having different numbers of conjugated double bonds (n), which include neurosporene (n=9), spheroidene (n=10), lycopene (n=11), anhydrorhodovibrin (n=12) and spirilloxanthin (n=13). Singular-value decomposition and global fitting of the spectral-data matrices lead us to a branched relaxation scheme including both (1) the singlet internal conversion in the sequence of 11Bu+ → 11Bu− → 21Ag− → 11Ag−(ground), and (2) the singlet-to-triplet conversion of 11Bu− → 13Ag followed by triplet internal conversion of 13Ag → 13Bu.