Resonance-Structure Collapse Enables Aromaticity-Controlled Design of Small Singlet-Fission Chromophore Candidates

Precise control of singlet and triplet excited-state energies is essential for identifying singlet-fission chromophore candidates, particularly in compact heteroarenes with competing resonance structures. Here, we show that heteroatom-controlled resonance-structure selection provides a chemically transparent strategy for tuning excited-state energetics. Comparison of normal-type and iso-type benzannulated five-membered heteroarenes across the NH, O, S, Se, and Te series reveals that heteroatom connectivity governs the redistribution of π-electron delocalization upon excitation. Iso-type connectivity consistently stabilizes T1 and increases E(S1)/E(T1), a trend reproduced by vertical TD-DFT, the Zeng-Hoffmann-Ananth scheme, fully adiabatic calculations, and DLPNO-STEOM-CCSD. Calculations on the experimentally established singlet-fission chromophore 1,3-diphenylisobenzofuran further support these protocols as comparative screening tools. Aromaticity and spin-density analyses show that T1 stabilization arises from topology-controlled aromaticity partitioning, whereby the six-membered ring retains local Hückel-like aromatic character while the unpaired electrons are preferentially localized on the adjacent five-membered ring. Chalcogen substitution and functionalization preserve this mechanism, with selected substituted iso-type derivatives satisfying the fully adiabatic 2E(T1) < E(S1) criterion. These results establish resonance-structure control as an electronic-structure principle for identifying compact, tunable singlet-fission chromophore candidates.