Chalcogen-Controlled Triplet Harvesting in Benzophenone-Derived Chromophores: A Theoretical Investigation of Excited-State Relaxation Pathways

Efficient triplet-state harvesting in organic chromophores is governed by a delicate interplay between excited-state energetics, orbital character, spin–orbit coupling, and vibronic coupling. Here, we present a systematic theoretical investigation of chalcogen substitution in benzophenone-derived chromophores, in which the carbonyl oxygen is progressively replaced by sulfur and selenium. Three molecular families were examined: benzophenone, Michler’s ketone, and 4,4′-dinitrobenzophenone, together with their thio- and seleno-analogues. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to characterize excited-state energetics, absorption and emission properties, natural transition orbitals (NTOs), spin–orbit coupling (SOC) matrix elements, quantitative hole–electron descriptors, interfragment charge-transfer character (IFCT), and state-specific intersystem-crossing (ISC) rates. Chalcogen substitution systematically stabilizes the low-lying excited states, produces pronounced red shifts in the lowest absorption bands, and reorganizes the singlet–triplet manifold. Simultaneously, the heavier chalcogen atoms markedly enhance SOC. For the strongest retained S1 → T2 SOC channels, the calculated SOC increases from 35.96 to 134.19 and 645.36 cm–1 across BP, TBP, and SBP, respectively, with analogous O → S → Se enhancements in the substituted series. Quantitative NTO, hole–electron, and IFCT analyses show that these SOC trends are coupled to excitation localization, hole–electron overlap, and charge-transfer character. NTO analysis reveals that the lowest singlet excited states are predominantly of (nπ*) character, whereas the higher triplet states relevant to strong SOC frequently possess substantial (ππ*) character, providing favorable El-Sayed-type spin-conversion pathways. The results therefore show that chalcogen identity, orbital-character change, and the spatial distribution of the participating excited states act cooperatively in controlling singlet–triplet coupling in these benzophenone-derived chromophores.