Solvent Dependence of the Ultrafast S₂−S₁ Internal Conversion Rate of β-Carotene

Abstract

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S2 (1Bu) state of all- trans-β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S2 (11Bu) state. The lifetime of the S2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the rate of internal conversion from the S2 state and the previously reported S1 (21Ag) state Raman shifts of the CC stretching mode supports the contention that the S2 and S1 states are vibronically coupled. The implications of these results for light harvesting in photosynthesis are also discussed.

Femtosecond dynamics of the forbidden carotenoid S 1 state in light-harvesting complexes of purple bacteria observed after two-photon excitation

“…ET2, Time constant for Car S2 3 BChl EET [Ϸ50 fs (30)]. 21, Time constant for Car S2 3 S1 IC [Ϸ135 fs (30)]. ET1, Time constant for Car S 1 3 BChl EET.…”

“…The overall EET efficiency is the sum of both efficiencies, weighted by their population probabilities: OA ϭ ET2 ϩ ET1 21 , where 21 is the quantum efficiency for Car S 2 3 Car S 1 IC, 21 ϭ 21 Ϫ1 ͞( ET2 Ϫ1 ϩ 21 Ϫ1 ) ϭ 1 Ϫ ET2 (see Fig. 1).…”

“…We now assume that OA differs from 100% entirely owing to Car S 1 3 Car S 0 IC. The quantum yield for Car S 1 3 Car S 0 IC is 10 ϭ (1 Ϫ ET1 ) 21 . Therefore OA also can be written as The results of these quantum efficiency calculations are summarized in Table 2.…”

“…The electronic states of Cars often are described by analogy to polyenes ( Only the S 0 7 S 2 transition is optically dipole-allowed. Excitation of S 2 is followed by very rapid ( 21 Ϸ100-200 fs) internal conversion (IC) to S 1 , which means that a significant fraction of the excitation energy flows through this state (21). However, because the S 1 3 S 0 transition is dipole-forbidden it remains a challenge to elucidate the mechanism that promotes Car S 1 to Chl EET.…”

Vibrational Relaxation in β-Carotene Probed by Picosecond Stokes and Anti-Stokes Resonance Raman Spectroscopy

“…This value of τ 1 is consistent with the 144 ± 10 fs S 2 → S 1 IC time constant determined in previous fluorescence upconversion experiments in toluene. 9 To determine Δt 0 , both the S 1 kinetics, with τ 1 fixed at 0.2 ps, and the S 0 recovery kinetics (Figure 5a,b) were fit. The weighted mean of all five fits gave Δt 0 = 0.3 ± 0.1 ps, and this zero of time was used in all subsequent analyses.…”