Crystallization induced enhanced emission in conformational polymorphs of a rotationally flexible molecule

The crystallization of weakly fluorescent 4-amino-2,2'-bipyridine (AMBPY) in solution phase under ambient conditions afforded three fluorescent conformational polymorphs. The marginal increase in the barrier to rotation observed in AMBPY as compared to unsubstituted 2,2'-bipyridine could be attributed to the buttressing effect'' offered by the amino substituent at the meta position. A smaller yet significant difference in energy (0.1-2.6 kJ mol(-1)) with respect to the global minima facilitates the isolation of AMBPY-I-III polymorphs. A unique nitrogen-nitrogen interaction is observed in two of the polymorphs, namely, AMBPY-I and AMBPY-III, promoted by cooperative C center dot center dot center dot H and N center dot center dot center dot H interactions. A crystallization-induced enhancement (ca. 5-10 fold) in the fluorescence quantum yield of AMBPY polymorphs is observed relative to the solution/amorphous state. Controlling the luminescence properties of molecular solids by tuning their packing arrangements via various interactions is an integral aspect in the construction of novel photo-functional materials.