Vibrational model of carotenoids based on explicit treatment of high-frequency carbon-carbon stretching modes

Vytautas Balevičius Jr.a, Jürgen Hauerb, Darius Abramaviciusc
aQueen Mary University of London, Mile End Road, London E1 4NS, U; bPhotonics Institute, Vienna University of Technology, Gusshausstrasse 27, 1040 Vienna, Austria; cVilnius University, Faculty of Physics, Sauletekio av. 9, 10222 Vilnius, Lithuania

Carotenoids are essential for normal functioning of photosynthetic organisms in which they ensure photoprotection and participate in the light-harvesting along with the performance of some auxiliary functions [1, 2]. Such important constituents of the photosynthetic systems have been extensively studied for decades, however, the finest details of their electronic structure remain elusive and highly debated [2]. This is largely a result of a complex photophysical and photochemical deactivation pathway involving short-lived states, some of which are optically dark.

A considerable part of the knowledge of carotenoids comes from the transient absorption (TA) spectroscopy. The main analytical tool for understanding the data is usually the so-called global-target analysis, which is largely a model testing approach based on connections between abstract compartments. By contrast, we have proposed an explicit physical model that takes into account four lowest-lying electronic singlet states and the vibrational levels of the high-frequency C–C and C=C stretching modes [3]. Firstly, the model allows for a unified treatment of the internal conversion and vibrational relaxation dynamics, thereby providing an energy relaxation scheme parametrized by a limited number of externally available (at least in principle) parameters. Secondly, some of the widely discussed features of the TA spectra, such as the vibrational cooling or the so-called S* signal [2] (conf., Fig. 1), emerge naturally in this scheme of quantized vibrations on the manifold of electronic levels.