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

Abstract

Carotenoids are fundamental building blocks of natural light harvesters with convoluted and ultrafast energy deactivation networks. In order to disentangle such complex relaxation dynamics, several studies focused on transient absorption measurements and their dependence on the pump wavelength. However, such findings are inconclusive and sometimes contradictory. In this study, we compare internal conversion dynamics in [Formula: see text]-carotene, pumped at the first, second, and third vibronic progression peak. Instead of employing data fitting algorithms based on global analysis of the transient absorption spectra, we apply a fully quantum mechanical model to treat the high-frequency symmetric carbon-carbon (C=C and C-C) stretching modes explicitly. This model successfully describes observed population dynamics as well as spectral line shapes in their time-dependence and allows us to reach two conclusions: Firstly, the broadening of the induced absorption upon excess excitation is an effect of vibrational cooling in the first excited state ([Formula: see text]). Secondly, the internal conversion rate between the second excited state ([Formula: see text]) and [Formula: see text] crucially depends on the relative curve displacement. The latter point serves as a new perspective on solvent- and excitation wavelength-dependent experiments and lifts contradictions between several studies found in literature.

Trivial Excitation Energy Transfer to Carotenoids Is an Unlikely Mechanism for Non-photochemical Quenching in LHCII

“…For Lut we adopt the Vibrational Energy Relaxation Approach (VERA) (Balevičius et al, 2018 ; Balevicius et al, 2019 ) to reproduce several independent spectral measurements. The details are discussed in Methods (and section C of the Supporting Information ) but essentially the four singlet electronic states (| S 0 〉, | S 1 〉, | S 2 〉, | S n 〉) are replaced by sets of vibronic states, ( ), where i is the electronic index and a 1 and a 2 are the vibrational quantum numbers associated with the high-frequency, optically-coupled C − C and C = C modes, respectively (Balevičius et al, 2018 ). For example, the state |0 00 〉 corresponds to the absolute ground state and |1 10 〉 to the first excited vibrational state of the C-C stretching mode on the first excited electronic state | S 1 〉.…”

“…For Lut we adopt the Vibrational Energy Relaxation Approach (VERA) (Balevičius et al, 2018;Balevicius et al, 2019) to reproduce several independent spectral measurements.…”

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

“…Here, we employ an intermediate scheme suitable for modeling optical response of strongly-pronounced vibronic transitions, termed the vibrational energy relaxation approach 42. It treats population dynamics via quantum mechanical master equations and models the TA spectra in a semi-phenomenological fashion, and it has been shown to reproduce numerous TA features of carotenoids in particular with high accuracy 43,44…”

Trivial Excitation Energy Transfer to Carotenoids is an Unlikely Mechanism for Non-Photochemical Quenching in LHCII

“…For Lut we adopt the Vibrational Energy Relaxation Approach (VERA) [53,54] to reproduce several independent spectral measurements. The details are discussed in Methods (and Section C of the Supporting Information) but essentially the four singlet electronic states (|S 0 ⟩, |S 1 ⟩, |S 2 ⟩, |S n ⟩) are replaced by sets of vibronic states, (|i a 1 a 2 ⟩ = |i⟩ |a 1 ⟩ i |a 2 ⟩ i ), where i is the electronic index and a 1 and a 2 are the vibrational quantum numbers associated with the high-frequency, optically-coupled C − C and C = C modes respectively [53]. The LA is given by the sum of all vibronic transitions belonging to |S 0 ⟩ → |S 2 ⟩ (weighted by the Franck-Condon overlaps and the initial populations on |S 0 ⟩) and is shown in Fig.…”

“…We use the VERA approach developed by Balevičius et al [53] the basic assumptions of which are discussed in the main text. The time-evolution of the vibronic populations are given by, (25) where the three terms correspond to the vibrational relaxation on the electronic states (IVR), interconversion (IC) between electronic states, and the initial resonant excitation by the pump pulse.…”