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

The photochemical reaction path approach, the MS-CASPT2 quantum-chemical method, and double-ζ basis sets (cc-pVDZ) were used to study 9 H-8-azaguanine and 8 H-8-azaguanine relaxation pathways. Several potential energy hypersurfaces were characterized by means of equilibrium geometries, surface crossings (conical intersections and singlet-triplet intersystem crossings), minimum energy paths, and linear interpolation in internal coordinates. The 9 H-8-azaguanine main photochemical event begins with the direct population of the 1(ππ* La) state, which evolves toward a conical intersection with the ground state after surmounting a small energy barrier, explaining why it is nonfluorescent. For 8 H-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(nOπ*) 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, favoring 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 8 H-8-azaguanine tautomer, while the 9 H-8-azaguanine major tautomer is nonfluorescent.