Photophysics of ^tzAdenine and ^tzGuanine fluorescent nucleobases embedded into DNA and RNA

UV–VIS photoinduced events of ^tzA and ^tzG embedded into DNA and RNA are described by combining the Extended Multi-State Second-Order Perturbation Theory (XMS-CASPT2) and electrostatic embedding molecular mechanics methods (QM/MM). Our results point out that the S₁¹(ππ* L_a) state is the bright state in both environments. After the photoexcitation to the S₁¹(ππ* L_a) state, the electronic population evolves barrierless towards its minimum, from where the excess of energy can be dissipated by fluorescence. As the minimum energy crossing point structure between the ground and first bright states lies in a high-energy region, the direct internal conversion to the ground state is an unviable mechanism. Other spectroscopic properties (for instance, absorption and Stokes shifts) and comparisons with photochemical properties of canonical nucleobases are also provided.