Photophysics of Emissive C-tz[Isothiazolo-Cytidine] and U-tz[Isothiazolo-Uridine] Pyrimidine Analogues

The C-tz and U-tz emissive properties were investigated with the MS-CASPT2 method and CASSCF/CASPT2 protocol, with inclusion of solvent effects with the aid of the PCM and Sequential QM/MM, the Free Energy Gradient methods. Theoretical vertical emission energies and Stokes shifts, using the ASEC-FEG method, are in a good agreement with the experimental data, within the expected accuracy of CASPT2 method. Their emissive properties can be understood in analogy to the photophysical behaviour of the canonical uracil and cytosine pyrimidine nucleobases, for which the twisting of the C5C6 bond is the key for understanding their ultrashort radiationless decay times. For both C-tz and U-tz, the computed photophysical deactivation pathway, shows that the bright state 1(pi pi*) goes directly to a minimum structure on its potential energy hypersurface (1(pi pi*)min), from where no conical intersection with the ground state can be reached, due to the steric hindrance of the C5C6 position, which forms the bridge between the rings in both molecules. Solvent effects place the conical intersection in higher energy region, reinforcing their emissive nature.