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-diiodotetrafluorobenzene 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.

A series of 4-pyperidinyl-1,8-naphtalimide derivatives containing at the N-position an n-methylpyridine (n = 2, 3, or 4) have been synthesized and isolated as crystalline materials. These isomers were further reacted, in the solid state, with the coformer 1,4-diiodotetrafluorobenzene (I2F4) to give three new co-crystals of general formula n2•I2F4. All crystalline materials have been thoroughly characterized in the solid state via single-crystal and powder X-ray diffraction, infrared spectroscopy and thermal methods; in addition, the photophysical properties of all compounds have been investigated in solution and in the solid state. The fluorescence features of the crystalline solids depend on the different arrangement of the molecules in the lattice and co-crystallization leads to important changes in the emission properties of the crystalline compounds. Emission quantum yields of the solids vary between 0.11 and 0.50. ‡ Results and discussion Synthesis and crystal growthThe synthetic strategy employed for isomers 2-4 is based on two high yielding steps, namely (i) direct reaction between the 4-bromo-1,8-naphthalic anhydride and piperidine in the high boiling solvent 2-methoxyethanol, as reported in the literature, 46, 47 followed by (ii) condensation with 2, 3 or 4-picolylamine in a mixture of water and ethanol in basic ambient (Scheme 1). Graphical AbstractFluorescent crystals were obtained by solid-state co-crystallization of 4-pyperidinyl-1,8-naphtalimide derivatives with a halogenated co-former.