Efficient photodeactivation mechanism in an unnatural nucleic acid base

5-aza-7-deazaguanine (5N7C-G), also known as P, is an unnatural nucleic acid base (NAB) closely related in structure to the natural NAB guanine. It forms a Watson-Crick base pair with the unnatural NAB Z synthesized by Benner and co-workers. We study the ultrafast decay pathways for this modified NAB with high-level multireference methods. We observe from both static and dynamic studies that there are multiple deactivation channels with significant differences in energetics and, therefore, timescales. The nonradiative deactivation mechanisms at sub-picosecond timescales are remarkably similar to those of the natural NAB guanine. These findings explain the experimental observations of Krul <i>et al.</i> [Krul <i>et al.</i>, <i>Photochem. Photobiol.</i>, 2023, <b>99</b>, 693-705].