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

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.