Naphthalimide/benzimide-based excited-state intramolecular proton transfer active luminogens: aggregation-induced enhanced emission and potential for chemical modification

Fluorescent organic particles are important in a number of areas, including medical imaging; hence, the development of organic materials that exhibit aggregation-induced emissions is an important objective. To that end, we report the synthesis of naphthalimide- and benzimide-based 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives (HNIBT and HPIBT, respectively) that exhibit aggregation-induced emission enhancement (AIEE), in contrast to most naphthalimide- or benzimide-based derivatives that are prone to aggregation-induced quenching. Experimental studies like single-crystal X-ray diffraction analysis and theoretical calculations demonstrate that the ability to undergo excited-state intramolecular proton transfer is pivotal for AIEE. Further studies revealed that a terminal alkynyl chain at the N-imide site of HPIBT has little impact on the emission behavior of the resultant compound (HPIBT-yl). HPIBT-Pe, an amphiphilic molecule obtained through the click reaction of HPIBT-yl and a tetraethylene-glycol-derived azide, self-assembled to form highly photostable particles that have long-term fluorescence imaging potential in cellular environments.