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Resonance-Structure Collapse Enables Aromaticity-Controlled Design of Small Singlet-Fission Chromophore Candidates Luis Leyva-Parra (10520129) Ricardo Pino-Rios (1368354) Biophysics Biochemistry Genetics Neuroscience Physiology Evolutionary Biology Chemical Sciences not elsewhere classified te series reveals like aromatic character fully adiabatic 2 fission chromophore candidates fission chromophore 1 comparative screening tools chemically transparent strategy type derivatives satisfying structure selection provides results establish resonance competing resonance structures membered heteroarenes across heteroatom connectivity governs fully adiabatic calculations experimentally established singlet density analyses show type benzannulated five selected substituted iso controlled aromaticity partitioning 1 </ sub controlled resonance membered ring compact heteroarenes adjacent five structure principle structure control controlled design >< sub vertical td unpaired electrons tuning excited tunable singlet triplet excited trend reproduced state energies state energetics stabilization arises small singlet preferentially localized identifying singlet identifying compact functionalization preserve chalcogen substitution 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 <i>T</i>₁ and increases <i>E</i>(<i>S</i>₁)/<i>E</i>(<i>T</i>₁), 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 <i>T</i>₁ 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 2<i>E</i>(<i>T</i>₁) < <i>E</i>(<i>S</i>₁) criterion. These results establish resonance-structure control as an electronic-structure principle for identifying compact, tunable singlet-fission chromophore candidates. 2026-10-02T00:00:00Z Dataset Dataset 10.1021/acs.joc.6c01083.s001 https://figshare.com/articles/dataset/Resonance-Structure_Collapse_Enables_Aromaticity-Controlled_Design_of_Small_Singlet-Fission_Chromophore_Candidates/34057292 CC BY-NC 4.0