Photochemical Relaxation Pathways of 9H-8-Azaguanine and 8H-8-Azaguanine.

The photochemical reaction path approach, the MS-CASPT2 quantum-chemical method, and double-ζ basis sets (cc-pVDZ) were used to study 9H-8-azaguanine and 8H8-azaguanine relaxation pathways. Several potential energy hypersurfaces were characterized by means of equilibrium geometries, surface crossings (conical intersections and intersystem crossings), minimum energy paths, and linear interpolation in internal coordinates. The 9H-8-azaguanine main photochemical event begins with the direct population of the 1 (ππ∗ La ) state, which evolves towards a conical intersection with the ground state after surmounting a small energy barrier, explaining why it is non fluorescent. For 8H-8-azaguanine, two relaxation mechanisms are possible, depending on the excitation energy. If the S1 1 (ππ∗ ) state is initially populated (lower energy region), the system evolves barrierless to the S1 1 (ππ∗ )min region, from where the excess energy is released. If the 1 (ππ* La ) state is populated (higher energy radiation), the system will encounter conical intersections with the S2 1 (nO π∗ ) and S1 1 (ππ∗ ) states before evolving to the 1 (ππ* La )min region, from where a conical intersection with the ground state is accessible, favouring radiationless deactivation to the ground state. However, because a fraction of the population can be transferred from 1 (ππ* La ) to the S1 1 (ππ∗ ) state, emission from the S1 1 (ππ∗ )min region is also expected, although weaker than it would be if the S1 1 (ππ∗ ) state were populated directly. Irrespective of the excitation energy, the emissive state is the same and a single fluorescence band is observed, with the strongest emission occurring upon excitation in the lower energy region, as observed experimentally. Therefore, our computational study corroborates experimental results, attributing the emission of the neutral form of 8-azaguanine in solution to the presence of the minor 8H-8-azaguanine tautomer, while the 9H-8-azaguanine major tautomer is non fluorescent.