The following STSMs have been approved to receive support from the Horizon 2020 Framework Programme of the European Union, through COST Action CA18222 funds (for a wider description, follow the links below or take a look to the News section ‘STSM results’).

Theoretical assignment of the electronic spectra of aqueous uracil and state specific analysis of solvation effects

Mr Branislav Milovanović, Faculty of Physical Chemistry, University of Belgrade, Belgrade, RS.
Host Institution: Institute Rudjer Bošković (HR)

Due to their biological relevance, DNA nucleobases excited state dynamics is extensively examined both experimentally and theoretically. Understanding competing photophysical mechanisms that can lead to and prevent light-inducted damage of the DNA nucleobases is of interest because of their widespread potential biological and/or technological applications. Determination of the character of large number of the excited electronic states is a challenging problem for nucleobases and other polyatomic molecules in general. Thus, resolving this problem is one step forward in understanding physical consequences underlying electronic excitation for these systems. Using previously developed procedure (Phys. Chem. Chem. Phys. 2019, 21, 22782) we are able to automatically determine character of the excited electronic states of nucleobase uracil both in gas phase and aqueous solution.

This procedure informs us about: theoretical assignment of the electronic spectra; state specific analysis of solvation effects; effects of vibrational averaging, temperature and solvent-induced structural changes; state specific solvent shifts for the solvent sensitive nπ* electronic states. Electronic absorption spectra profiles are calculated and general remarks can be drawn: (1) vibrational averaging and temperature increase produce comparable effects on all examined excited states energetically stabilizing them; (2) bright electronic states (ππ* states) contributing to spectral profile are stabilized by the introduction of electrostatic effects while dark (nπ* states) are strongly destabilized; (3) apparent stabilization of the bright electronic states is present when introducing solvent effects while this effects are rather complex in the case of dark and delocalized states (Rydberg transitions).

Acquired data should provide more convenient insights into absorption spectra of the uracil and therefore foundation for studying photodynamics of uracil in aqueous solution and the possibility of the excited state driven N-H bond fission. Hopefully, detailed characterization of the electronic states involved in the relaxation of gas phase and aqueous uracil structures will shed new light on the active role of the chemical environment in the exited state deactivation events of nucleobases.

Total effect of solvation on the computed UV absorption spectra for uracil. Gas phase spectra at 0 K (black) and 298 K (red), spectra with electrostatic COSMO environmental effect (blue) and COSMO solution/optimized COSMO ground state geometries (green). Vertical bars represent reference geometry vertical excitation energies. Insets represents influence of the solvation on the energy of the selected excited states and chemical structure of uracil.