Understanding Carotenoid Dynamics via the Vibronic Energy Relaxation Approach

Abstract

Carotenoids are an integral part of natural photosynthetic complexes, with tasks ranging from light harvesting to photoprotection. Their underlying energy deactivation network of optically dark and bright excited states is extremely efficient: after excitation of light with up to 2.5 eV of photon energy, the system relaxes back to ground state on a time scale of a few picoseconds. In this article, we summarize how a model based on the vibrational energy relaxation approach (VERA) explains the main characteristics of relaxation dynamics after one-photon excitation with special emphasis on the so-called S* state. Lineshapes after two-photon excitation are beyond the current model of VERA. We outline this future line of research in our article. In terms of experimental method development, we discuss which techniques are needed to better describe energy dissipation effects in carotenoids and within the first solvation shell.

Figure 1

(A) Energy level scheme for carotenoids. Electronic states S₀, S₁, and S₂ are drawn as parabolas. Higher lying states S_n (S_m) explain the ESA-signals from S₁ (and S₂). Vibrational levels, which are an explicit part of the model within VERA, are drawn as horizontal lines. Colored arrows denote optical transitions. The orange arrows indicate the origin of the debate S*-signal, which is here explained as transitions of vibrational nonequilibrium states on both S₁ and S₀. (B) Transient absorption spectrum of lutein in acetone at 7 ps delay time. The main excited state absorption features for S₁ and S* are indicated.

Figure 2

(A) Transient absorption spectra of lutein in acetone at 1.4 ps after 1210, 1250, and 1300 nm (shades of red) two-photon excitation. Transient absorption spectrum obtained after one photon excitation of the S₂ state at 480 nm is shown for comparison. (B) Steady state absorption spectrum of acetone in the NIR spectral region. Arrows represent two-photon energies used to excite the samples in the 2PE experiment and the corresponding absorbance in 2 mm cuvette, used for the experiments. (C) Energy level scheme describing electronic (dashed horizontal lines) and vibrational (wavy vertical lines) energy relaxation in lutein along reaction coordinate q after two-photon (reddish arrows) excitation. Colored vertical arrows indicate allowed two-photon transitions.

Figure 3

(A) Evolution of the experimental transient absorption signal of lutein in acetone in the S₁ ESA region of lutein in acetone following one-photon excitation of S₂ (solid lines). The fits obtained from VERA are shown as dotted lines, where we note the failure to fit the enhanced shoulder on blue edge of S₁ at early times. (B) Experimental transient absorption signal following direct, two-photon excitation of S₁ at 1210 nm (blue line). While VERA (with parameters taken from the one-photon fits except for λ_α¹, which had to be reduced by a factor of approximately ¹/₂) reproduces the main ESA peak reasonably well, it fails to reproduce the broad positive feature between 575 and 675 nm at early times. Raising the solvent temperature in the VERA model (here 10 000 K to exaggerate features) results in line-broadening and slowing of both internal conversion and vibrational relaxation (dotted red line). The vibronic peaks resulting from the latter do not align with the 575–675 nm feature. Reducing the reorganization energy associated with vibrational relaxation on S₁, λ_α¹, to zero does enhance the vibronic features but again does not capture the 575–675 nm feature.