Explaining the Temperature Dependence of Spirilloxanthin’s S* Signal by an Inhomogeneous Ground State Model

Abstract

We investigate the nature of the S* excited state in carotenoids by performing a series of pump–probe experiments with sub-20 fs time resolution on spirilloxanthin in a polymethyl-methacrylate matrix varying the sample temperature. Following photoexcitation, we observe sub-200 fs internal conversion of the bright S2 state into the lower-lying S1 and S* states, which in turn relax to the ground state on a picosecond time scale. Upon cooling down the sample to 77 K, we observe a systematic decrease of the S*/S1 ratio. This result can be explained by assuming two thermally populated ground state isomers. The higher lying one generates the S* state, which can then be effectively frozen out by cooling. These findings are supported by quantum chemical modeling and provide strong evidence for the existence and importance of ground state isomers in the photophysics of carotenoids.

Ultra-broadband 2D electronic spectroscopy of carotenoid-bacteriochlorophyll interactions in the LH1 complex of a purple bacterium

“…4 shows-also Q x . This dynamics should be compared to the one of isolated Spx in solution, which has been characterized by pump-probe spectroscopy; 65 in that case, the build-up time constant of hot S1 was found to be ≈100 fs. The significant shortening, by a factor of more than 2, within the LH complex is consistent with the opening of an additional relaxation channel for S 2 /S x , besides IC to S 1 : EET to the BChl.…”

“…The significant shortening, by a factor of more than 2, within the LH complex is consistent with the opening of an additional relaxation channel for S 2 /S x , besides IC to S 1 : EET to the BChl. The transition from 2D-EAS3 to 2D-EAS4 (196 fs time constant) essentially reflects hot S 1 thermalization, 65 while the transition from 2D-EAS4 to 2D-EAS5 (1.36 ps time constant 65 ) refers to loss of S 1 . The final 2D-EAS5 show S * as dominant photoexcitation.…”

Collisional relaxation of apocarotenals: identifying the S* state with vibrationally excited molecules in the ground electronic state S0*

“…2,6,7,13 Others and we identify this S* signal as the vibrationally hot ground electronic state (S 0 **/S 0 *) discussed above, 6,7,[14][15][16][17][18] Alternatively, S* has been either described as an electronically excited state, [18][19][20][21][22][23][24] different S 0 conformers, 25,26 or the vibrationally hot S 1 state. For instance, the S* signal was observed which is still under debate.…”

Effects of tunable excitation in carotenoids explained by the vibrational energy relaxation approach

“…As can be seen, the model accurately describes the first two peaks, 0-0 and 0-1, but fails at higher frequencies, where the experimental intensity is larger. It was argued previously that the high energy spectrum of carotenoids might be influenced by a distribution of geometrical conformers (Lukeš et al, 2011;Papagiannakis et al, 2006;Hauer et al, 2013;Chábera et al, 2009), which would not be captured by our model.…”