Photophysical characterization of natural cis-carotenoids

By means of steady-state fluorescence spectroscopy we explore the photophys ics of two lowest lying singlet excited states in two natural 15-cis-carote noids, namely phytoene and phytofluene, possessing three and five conjugate d double bonds (N), respectively. The results are interpreted in relation t o the photophysics of all-trans-carotenoids with varying N. The fluorescenc e of phytofluene is more Stokes-shifted relative to that of phytoene, and i s ascribed to the forbidden S-1 --> S-0 transition, with its first excited singlet state (S-1) lying 3340 cm(-1) below the dipole allowed second excit ed singlet state (S-2), at 77 K. For phytoene the S-2 and S-1 potential sur faces are closer in energy, probably giving rise to the mixed S-2 and S-1 f luorescence characteristics. The origin of phytoene fluorescence is discuss ed and is suggested to be due to the S-1 --> S-0 transition; with the S-1 s tate located 1100 cm(-1) below S-2 at 77 K. The dependence of the fluoresce nce quantum yield on temperature and viscosity shows that large amplitude m olecular motions are involved in the radiationless relaxation process of ph ytoene. The transition dipole moment of absorption and emission are paralle l in phytoene and nonparallel in phytofluene.