Excited-state reorganization in minimal carbon-based photoactive models: effects of edge oxidation and guest adsorption from TD-DFT and NTO analysis

Carbon-based photoactive nanomaterials often exhibit complex excited-state behavior that is difficult to rationalize with realistic structural models. Here, minimal coronene-based systems were used to isolate the effects of edge functionalization, substitution topology, and π-stacking using DFT, TD-DFT, and natural transition orbital (NTO) analysis. Single OH and COOH substitution partially activates the otherwise forbidden S1 transition of pristine coronene, whereas doubly functionalized isomers show a marked positional dependence: C12 and C14 retain appreciable S1 intensity, while C17 is nearly dark. NTO analysis further distinguishes the more dominant single-pair character of C14 from the nearly balanced two-pair character of C17. In the C0···Pen complex, the lowest pentacene-like excitation is preserved but red-shifted and attenuated, while higher-energy states show stronger oscillator-strength redistribution and increased multi-pair character. Overall, local edge chemistry, substitution topology, and π-contact provide complementary routes for tuning excited-state organization.