Ricardo Pino-Rios

Singlet fission requires a delicate energetic alignment between the lowest singlet and triplet excited states, typically expressed as E(S₁)/E(T₁) > 2. Achieving this condition in compact organic chromophores remains challenging because small π-conjugated systems often exhibit strongly coupled singlet and triplet energetics. Here, we present a resonance-structure-based strategy for tuning excited-state energies in fused heteroaromatic systems. The approach relies on heteroatom placement as a chemical handle to select specific resonance structures according to the Glidewell–Lloyd rule, which favours the localisation of 4n + 2 π-electron circuits in the smallest available ring. Previous studies on indole and benzoborepine isomers have shown that heteroatom connectivity controls local aromaticity patterns and relative ground-state stability. We extend this concept to the design of singlet-fission chromophores using isoindole-type frameworks. In these topologies, heteroatom-induced resonance selection reduces ground-state aromatic stabilisation in the six-membered ring, while triplet excitation restores a degree of local Hückel aromaticity in this ring despite the globally Baird-antiaromatic 10π-electron framework. This topology-driven redistribution of aromaticity localises the triplet spin density mainly on the five-membered ring and selectively stabilises T₁. In contrast, S₁, governed by the same frontier-orbital topology, remains sufficiently high to approach the 2E(T₁) energetic condition required for singlet fission. As a result, isoindole and chalcogen-containing isoindole analogues display enhanced E(S₁)/E(T₁) ratios, approaching or exceeding the singlet-fission threshold. These results establish heteroatom-guided resonance selection, interpreted through the Glidewell–Lloyd rule, as a transparent design principle for compact singlet-fission chromophores.