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

• Published in: Physical chemistry chemical physics : PCCP 2023-10, Vol.25 (4), p.27756-27765
• Main Authors: Xie, Min, Ren, Shuang-xiao, Hu, Die, Zhong, Ji-meng, Luo, Jie, Tan, Yin, Li, Yan-ping, Si, Li-ping, Cao, Jun
• Format: Article
• Language: English
• Subjects: Atomic energy levels; Carbonyl groups; Carbonyls; Deactivation; Excitation spectra; Guanines; Molecular dynamics; Quantum chemistry; Quantum efficiency; Selenium; Substitutes; Sulfur
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d3cp03730d

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 2 (ππ*) → S 1 (nπ*) → T 1 (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 2 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 S 2 state predominantly decays to the S 1 state, while upon selenium-substitution the S 2 state deactivation leads to simultaneous population of the S 1 and T 2,3 states in the same time scale and multi-state quasi-degeneracy region S 2 /S 1 /T 2,3 . Interestingly, the ultrafast deactivation of the spectroscopic S 3 state of both studied molecules to the S 1 state occurs through a successive S 3 → S 2 → S 1 path involving a multi-state quasi-degeneracy S 3 /S 2 /S 1 . The populated S 1 and T 2 states will cross the lowest triplet state, and the S 1 → T intersystem crossing happens in a multi-state quasi-degeneracy region S 1 /T 2,3 /T 1 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. Both chalcogen-substitution element and initial spectroscopic state influence excited-state relaxation mechanism of nucleobase photosensitizers, and multi-state quasi-degeneracy mediates the complex relaxation processes.