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-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

Dynamic Characterization of Crystalline Supramolecular Rotors Assembled through Halogen Bonding

A modular molecular kit for the preparation of crystalline molecular rotors was devised from a set of stators and rotators to gain simple access to a large number of structures with different dynamic performance and physical properties. In this work, we have accomplished this with crystalline molecular rotors self-assembled by halogen bonding of diaza­bicyclo­[2.2.2]­octane, acting as a rotator, and a set of five fluorine-substituted iodo­benzenes that take the role of the stator. Using variable-temperature ¹H <i>T</i>₁ spin–lattice relaxation measurements, we have shown that all structures display ultrafast Brownian rotation with activation energies of 2.4–4.9 kcal/mol and pre-exponential factors of the order of (1–9) × 10¹² s^–1. Line shape analysis of quadrupolar echo ²H NMR measurements in selected examples indicated rotational trajectories consistent with the 3-fold or 6-fold symmetric potential of the rotator