Understanding the aggregation induced emission enhancement for a compound with excited state intramolecular proton transfer character

A few of excited state intramolecular proton transfer (ESIPT) compounds have been discovered for their aggregation induced emission enhancement (AIEE). To understand the AIEE mechanism, an ESIPT compound BTHPB (N-(4-(benzod]thiazol-2-yl)-3-hydroxyphenyl)benzamide) with simple structure was designed and synthesized. BTHPB showed apparent AIEE property and the emission efficiency was observed as high as 0.27 in the aggregates. On the basis of viscochromism experiments and calculations employing the linear coupling model, the restriction of the rotation between the two subunits taken place in ESIPT was considered as the main factor for the AIEE. The micro- and femtosecond transient absorption experiments offered evidence for the considerations. Additionally, we also observed a negative effect of aggregation on the fluorescence emission in the system. So the AIEE of ESIPT compound BTHPB originated from the combination effects of positive and negative factors induced by the aggregation.