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

Author(s)

Sanches de Araujo, Adalberto Vasconcelos ^[1] ; Borin, Antonio Carlos ^[1]

Affiliation

^[1] Univ Sao Paulo, Inst Chem, Dept Fundamental Chem, Ave Prof Lineu Prestes 748, BR-05508000 Sao Paulo, SP - Brazil

• Document type: Journal article
• Source: Journal of Physical Chemistry A; v. 123, n. 14, p. 3109-3120, APR 11 2019.
• Web of Science Citations: 0

Abstract

The photochemical reaction path approach, the MS-CASPT2 quantum-chemical method, and double-zeta basis sets (cc-pVDZ) were used to study 9H-8-azaguanine and 8H-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 9H-8-azaguanine main photochemical event begins with the direct population of the (1)(pi pi{*} L-a) state, which evolves toward a conical intersection with the ground state after surmounting a small energy barrier, explaining why it is nonfluorescent. For 8H-8-azaguanine, two relaxation mechanisms are possible, depending on the excitation energy. If the S-1 (1)(pi pi{*}) state is initially populated (lower-energy region), the system evolves barrierless to the S-1 (1)(pi pi{*})(min )region, from where the excess energy is released. If the (1)(pi pi{*} L-a) state is populated (higher-energy radiation), the system will encounter conical intersections with the S-2 (1)(n(o)pi{*}) and S-1( 1)(pi pi{*}) states before evolving to the (1)(pi pi{*} L-a)(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)(pi pi{*} L-a) to the S-1( 1)(pi pi{*}) state, emission from the S-1( 1)(pi pi{*})(min) region is also expected, although weaker than it would be if the S-1( 1)(pi pi{*}) 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 nonfluorescent. (AU)

• FAPESP's process: 18/19454-5 - Structure and Photochemistry of Fluorescent Nucleobaseas and Metal Mediated Base Pairs
• Grantee: Antonio Carlos Borin
• Support Opportunities: Regular Research Grants