Multiple‐State Emissions from Neat, Single‐Component Molecular Solids: Suppression of Kasha's Rule

Abstract Three rigid and structurally simple heterocyclic stilbene derivatives, ( E )‐3 H ,3′ H ‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, ( E )‐3‐(3‐oxobenzo[c] thiophen‐1(3 H )‐ylidene)isobenzofuran‐1(3 H )‐one, and ( E )‐3 H ,3′ H ‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S 2 ) and lowest (S 1 ) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S 2 and S 1 states (T 2 and T 1 states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S 2 excited states (phosphorescence from T 2 excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S 2 states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule.