The Overtones of the Carotenoid Vibrational Modes Significantly Improve the Quality of Optical Response Modeling

Carotenoids (CAR) are a class of organic pigments that are actively involved in the processes of light absorption and non-photochemical quenching in photosynthetic species. Despite the relatively simple chemical structure, CARs exhibit a specific variety of optical properties, which, in general, can be explained within the framework of the multimode Brownian oscillator model (MBOM). However, being a typical semiclassical theory, MBOM operates with a set of effective parameters necessary to model measured spectra. These parameters can be found only by fitting the experiments and for most organic molecules the determination of these parameters is not always unambiguous. CARs are a good example to apply MBOM, since they have only four active vibrational modes, which strongly affect the lineshape of its absorption spectra. Considering the S0 → S2 optically allowed all-trans electronic transition of CARs, we performed the fitting of the lutein and zeaxanthin absorption in different solvents at room temperature. To assess the uniqueness of the quantum models for each CAR, the spectral density function was extended with vibrational modes whose frequencies are overtones of the two main vibrational modes of carotenoids. After evolutionary optimization of the fitting routine the experimental data were fitted with high precision and statistically significant parameters of the model were obtained.