The impact of the chalcogen-substitution element and initial spectroscopic state on excited-state relaxation pathways in nucleobase photosensitizers: a combination of static and dynamic studies.

The substitution of oxygen with chalcogen in carbonyl group(s) of canonical nucleobases gives an impressive triplet generation, enabling their promising applications in medicine and other emerging techniques. The excited-state relaxation S(ππ*) → S(nπ*) → T(ππ*) has been considered the preferred path for triplet generation in these nucleobase derivatives. Here, we demonstrate enhanced quantum efficiency of direct intersystem crossing from S to triplet manifold upon substitution with heavier chalcogen elements. The excited-state relaxation dynamics of sulfur/selenium substituted guanines in a vacuum is investigated using a combination of static quantum chemical calculations and excited-state molecular dynamics simulations. We find that in sulfur-substitution the S state predominantly decays to the S state, while upon selenium-substitution the S state deactivation leads to simultaneous population of the S and T states in the same time scale and multi-state quasi-degeneracy region S/S/T. Interestingly, the ultrafast deactivation of the spectroscopic S state of both studied molecules to the S state occurs through a successive S → S → S path involving a multi-state quasi-degeneracy S/S/S. The populated S and T states will cross the lowest triplet state, and the S → T intersystem crossing happens in a multi-state quasi-degeneracy region S/T/T and is accelerated by selenium-substitution. The present study reveals the influence of both the chalcogen substitution element and initial spectroscopic state on the excited-state relaxation mechanism of nucleobase photosensitizers and also highlights the important role of multi-state quasi-degeneracy in mediating the complex relaxation process. These theoretical results provide additional insights into the intrinsic photophysics of nucleobase-based photosensitizers and are helpful for designing novel photo-sensitizers for real applications.