Aggregation Promotes Excited-State Intramolecular Proton Transfer for Benzothiazole-Substituted Tetraphenylethylene Compound

Excited-state intramolecular proton transfer (ESIPT) coupled to aggregation-induced enhanced emission (AIEE) offers a better route to design more efficient highly emissive materials. These materials usually present potential abilities in biochemical sensing fields. In this work, we mainly focus on ESIPT and AIEE mechanism for the benzothiazole-substituted tetraphenylethylene compound (BTZ-TPE-1a) in liquid and solid phases. It is worth mentioning that BTZ-TPE-1a is biocompatible in nature and plays important roles in cell imaging and cell viability. Given its nontoxicity and biocompatiblility properties, BTZ-TPE-1a is an excellent imaging agent in cancer biomedical imaging. We first explore hydrogen-bonding interactions for the BTZ-TPE-1a system. The strengthening dual hydrogen bonds and intramolecular charge transfer (ICT) resulting from photoexcitation reveals an ESIPT tendency. Simulated electronic spectra and potential energy surfaces (PESs) indicate the excited-state intramolecular single-proton-transfer (ESISPT) mechanism for BTZ-TPE-1a molecule. Related experimental reports confirm our results. By performing Born-Oppenheimer molecular dynamics (BOMD) simulations starting from a transition state (TS) structure, we can further verify the ESISPT mechanism. In the solid phase, quantum mechanics and molecular mechanics (QM/MM) simulation is realized in the ONIOM model, on the basis of which the AIEE mechanism of BTZ-TPE-1a is elaborated. We not only illustrate the specific ESISPT mechanism for BTZ-TPE-1a and compensate for the inadequacy of the experiment but also present direct insights into ESIPT and AIEE in the aggregation state (i.e., aggregation promotes ESIPT for BTZ-TPE-1a). We hope this work promotes further development of benzothiazole-substituted tetraphenylethylene compounds in biomedical and optoelectronic applications.