Aggregation of an <i>n</i>–π* Molecule Induces Fluorescence Turn-on

Although <i>n</i>–π* molecules can serve as electroluminescent materials because of the harvesting of singlet and triplet excitons through El-Sayed-rule-allowed reverse intersystem crossing, the weak fluorescence emissions of such molecules have prevented applications into devices. We have discovered a 1 order of magnitude enhancement of the fluorescence of a prototypical <i>n</i>–π* fluorophore, 7-phenylcoumarin (PC), upon aggregation. We performed a mechanistic study consisting of structural, photophysical, and quantum chemical investigations, and found that the aggregation positioned the fluorescent electronic state below the nonemissive triplet <i>n</i>–π* transition state. Our studies, for the first time, demonstrate intramolecular geometry and intermolecular arrangements in the solid state to be significant factors in the photoluminescence quantum yields of <i>n</i>–π* fluorophores