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Abstract Three rigid and structurally simple heterocyclic stilbene derivatives, (
E )‐3H ,3′H ‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, (E )‐3‐(3‐oxobenzo[c] thiophen‐1(3H )‐ylidene)isobenzofuran‐1(3H )‐one, and (E )‐3H ,3′H ‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S2) and lowest (S1) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S2and S1states (T2and T1states) 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 S2excited states (phosphorescence from T2excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S2states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule. -
more » « less
Abstract Three rigid and structurally simple heterocyclic stilbene derivatives, (
E )‐3H ,3′H ‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, (E )‐3‐(3‐oxobenzo[c] thiophen‐1(3H )‐ylidene)isobenzofuran‐1(3H )‐one, and (E )‐3H ,3′H ‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S2) and lowest (S1) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S2and S1states (T2and T1states) 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 S2excited states (phosphorescence from T2excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S2states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule.
