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

Abstract

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 S₂(ππ*) → S₁(nπ*) → T₁(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 S₂ 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 on-the-fly excited-state molecular dynamics simulations. We find that in sulfur-substitution the S₂ state predominantly decays to the S₁ state, while upon selenium-substitution the S₂ state deactivation leads to simultaneous population of the S₁ and T_2,3 states in the same time scale and multi-state quasi-degeneracy region S₂/S₁/T_2,3. Interestingly, the ultrafast deactivation of the spectroscopic S₃ state of both studied molecules to the S₁ state occurs through a successive S₃ → S₂ → S₁ path involving a multi-state quasi-degeneracy S₃/S₂/S₁. The populated S₁ and T₂ states will cross the lowest triplet state, and the S₁ → T intersystem crossing happens in a multi-state quasi-degeneracy region S₁/T_2,3/T₁ 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.