Photophysical study of solute-solvent energy transfer

After photoexcitation a series of processes occur in the solute immersed in a given solvent, such as internal conversion and possible photochemical reactions. The relaxation mechanisms of the solute in the presence of the solvent can be simplified by considering two steps1.

After the initial excitation, an internal vibrational redistribution (IVR) process occurs, followed by vibrational cooling (VC). The first step corresponds to an electronic energy redistribution in the vibrational modes. In the second step one has an energy transfer from the solute to the solvent, resulting in a cooling of the former and heating of the latter. Described in these terms, the process is restricted to a simple scenario of thermal equilibrium via vibrational relaxation of the solute and consequent redistribution of energy to the solvent. This process is of fundamental importance in Chemistry, as the reactivity of a given molecule depends on its energy. It is also crucial to understand the light to heat conversion process.

In this work solvated cytosine was studied as a prototypical system for solute to solvent energy transfer. Cytosine is a well-studied system and has a fast S1/S0 internal conversion.2 The process of energy transfer from cytosine to the environment (after an electronic excitation) can be monitored by computing the kinetic energy of the subsystems. This process depends on the solute-solvent interactions. We performed nonadiabatic mixed quantum-classical (NA-MQC) dynamics simulations of electronically excited cytosine system in surrounding media of increasing interaction strength with cytosine: argon, benzene, water and deoxyribose. A QM/MM protocol was used for the cytosine-solvent systems. Kinetic energy of cytosine in different solvents was fitted by a rate model based on the population of its first four singlet states. For each solvent the temporal dependence of the kinetic energy is tri-exponential (see Figure 1). According to this model, the maximum solute-solvent energy transfer rate occurs at very different times for each solvent, being 4400fs, 1463fs and 373fs for argon, benzene and water, respectively. 1 ps after reaching its maximum temperature, cytosine transfers 6.5%, 33% and 59% of its excess kinetic energy to argon, benzene and water, respectively. A model for describing the heat transfer to the first solvation shell and to the bulk solvent is under development.

1- Balevičius Jr, V.; Wei, T.; Di Tommaso, D.; Abramavicius, D.; Hauer, J.; Polívka, T.; Duffy, C. D. P., 2019 Chem
Sci 10: 4792–4804.
2- Barbatti, M.; Aquino, A. J. A.; Szymczak, J. J.; Nachtigallová, D.; Lischka, H. 2011 Phys Chem Chem Phys 13,
6145-6155.