Carbazole-Based Cyano-Stilbene Highly Fluorescent Microcrystals

Aggregation-induced enhanced emission (AIEE) is reported for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 mu m sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue-shift in emission and enhances the intensity. X-ray cryatallographic analysis revealed that key factors causing high luminescence efficiency in the crystal are a lack of strong cofacial pi-pi alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seem lobe responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation, thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for the unusual emission blue-shift in the aggregate.