Photophysical Characterization of Natural cis-Carotenoids¶

Abstract

By means of steady-state fluorescence spectroscopy we explore the photophysics of two lowest lying singlet excited states in two natural 15-cis-carotenoids, namely phytoene and phytofluene, possessing three and five conjugated double bonds (N), respectively. The results are interpreted in relation to the photophysics of all-transcarotenoids with varying N. The fluorescence of phytofluene is more Stokes-shifted relative to that of phytoene, and is ascribed to the forbidden S 1 → S 0 transition, with its first excited singlet state (S 1 ) lying 3340 cm Ϫ1 below the dipole allowed second excited singlet state (S 2 ), at 77 K. For phytoene the S 2 and S 1 potential surfaces are closer in energy, probably giving rise to the mixed S 2 and S 1 fluorescence characteristics. The origin of phytoene fluorescence is discussed and is suggested to be due to the S 1 → S 0 transition; with the S 1 state located 1100 cm Ϫ1 below S 2 at 77 K. The dependence of the fluorescence quantum yield on temperature and viscosity shows that large amplitude molecular motions are involved in the radiationless relaxation process of phytoene. The transition dipole moment of absorption and emission are parallel in phytoene and nonparallel in phytofluene.

Effects of Molecular Symmetry on the Electronic Transitions in Carotenoids

“…16 Thus, in photosynthetic light harvesting alltrans isomers are exclusively involved, whereas in the photoprotection of reaction centers, mainly the 15 cis isomers are involved. 16−18 The reason for this distinction is a matter for debate 19 and there are indications that light-induced cis−trans isomerization of the LHCII-bound Crts may occur in situ. 20 Considerations of molecular symmetry based on group theory have largely contributed to the understanding of electronic structures and photophysical features of important biological chromophores.…”

“…48,62 A significant change in molecular geometry in the S 1 state gives rise to a CC stretching mode at ∼1800 cm −1 in time-resolved Raman spectra of all-trans β-Car and its derivatives. 55,63 Other indications of a large deviation from equilibrium geometry in the S 1 state are strong temperature and solvent viscosity effects on the fluorescence quantum yield 19 and the ease of isomerization in this excited state. 64 Most likely, this deformation is the reason why this formally allowed transition remains extremely weak or nonexistent.…”

Effect of Isomerization on Excited-State Dynamics of Carotenoid Fucoxanthin

“…Many carotenoids readily form various cis-isomers, either spontaneously or upon chemical treatment. 21,22 The excited-state properties of cis-carotenoids are very similar to those of trans-carotenoids; despite the perturbed symmetry, the S 1 state remains dark 8,23,24 and the main absorption band is due to the S 0 −S 2 transition. The characteristic spectral feature distinguishing cis-and trans-carotenoids is the presence of the "cis-peak", a pronounced spectral band of cis-isomers occurring in the 300−400 nm spectral region.…”

Use of time‐resolved spectroscopy as a method to monitor carotenoids present in tomato extract obtained using ultrasound treatment

“…The shorter wavelength emission is consistent with the presence of phytofluene, with the excitation coinciding with the absorption peaks (Fig. ) and there are reports of its emission at this wavelength, although the spectral shape can be affected by solvent and temperature (Gillbro et al ., ; Andersson et al ., ). The longer wavelength emission can be assigned to the presence of chlorophyll (Franck et al ., ; Egea et al ., ; Cho et al ., ).…”

“…Optical spectroscopy, therefore, potentially offers a good, sensitive, non‐destructive and remote method by which to monitor systems containing these compounds. Techniques such as fluorescence and Raman have been used to study carotenoids (Christensen, ; Andersson et al ., ; Ermakov et al ., ), with the sensitivity of fluorescence and Raman used to illicit structural changes. As these two processes occur on different timescales (Raman sub picoseconds; fluorescence several picoseconds to many nanoseconds) it can sometimes be possible to separate them temporally and recently this has been demonstrated in a complementary metal‐oxide‐semiconductor (CMOS) device (Kostamovaara et al ., ).…”