Solvation Structures and Deactivation Pathways of Luminescent Isothiazole-Derived Nucleobases: ^tzA, ^tzG, and ^tzI

The photophysical relaxation pathways of ^tzA, ^tzG, and ^tzI luminescent nucleobases were investigated with the MS-CASPT2 quantum-chemical method and double-ζ basis sets (cc-pVDZ) in gas and condensed phases (1,4-dioxane and water) with the sequential Monte Carlo/CASPT2 and free energy gradient (FEG) methods. Solvation shell structures, in the ground and excited states, were examined with the pairwise radial distribution function (G(r)) and solute-solvent hydrogen-bond networks. Site-specific hydrogen bonding analysis evidenced relevant changes between both electronic states. The three luminescent nucleobases share a common photophysical pattern, summarized as the lowest-lying ¹(ππ*) bright state that is populated directly after the absorption of radiation and evolves barrierless to the minimum energy structure, from where the excess of energy is released by fluorescence. From the ¹(ππ*)_min region, the conical intersection with the ground state ((ππ*/GS)_CI) is not accessible due to the presence of high energetic barriers. By combining the present results with those reported earlier by us for the pyrimidine fluorescent nucleobases, we present a comprehensive description of the photophysical properties of this important class of new fluorescent nucleosides.