Luminescence Properties of 1,8-Naphthalimide Derivatives in Solution, in Their Crystals, and in Co-crystals: Toward Room-Temperature Phosphorescence from Organic Materials

Crystalline 1,8-naphthalimide derivatives bearing a bromine atom at the 4-position and a 2-, 3-, or 4-methylpyridine at the imidic N-position have been synthesized, and their co-crystals with the coformer 1,4-diiodotetrafluorbenzene have been obtained via mechanochemistry. The structure of crystals and co-crystals has been characterized by means of X-ray diffraction and Raman and IR analysis. The luminescence properties of the derivatives have been explored both in solution and in their solid crystals and co-crystals. All of the compounds exhibit weak fluorescence in air-equilibrated solutions at room temperature and both fluorescence and phosphorescence at low temperature. In air-free solvent, all of the derivatives show phosphorescence at room temperature, at variance with the unsubstituted 1,8-naphthalimide model. Solid crystals display a red-shifted fluorescence with an increased emission quantum yield as compared to solution, whereas co-crystals show different behaviors. For all of the solid compounds, phosphorescence could be observed at room temperature by means of a gated detection. The dependence of the luminescence features of the solid materials on the intermolecular interactions that occur in the lattice is discussed.