Crystallization induced enhanced emission in conformational polymorphs of a rotationally flexible molecule†
Abstract
The crystallization of weakly fluorescent 4-amino-2,2â˛-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 2,2â˛-bipyridine could be attributed to the âbuttressing effectâ offered by the amino substituent at the meta position. A smaller yet significant difference in energy (0.1â2.6 kJ molâ1) with respect to the global minima facilitates the isolation of AMBPY-IâIII polymorphs. A unique nitrogenânitrogen interaction is observed in two of the polymorphs, namely, AMBPY-I and AMBPY-III, promoted by cooperative CâŻH and NâŻH interactions. A crystallization-induced enhancement (ca. 5â10 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.
- This article is part of the themed collection: Shape-Responsive Fluorophores