The design strategy for an aggregation-and crystallization-induced emission-active molecule based on the introduction of skeletal distortion by boron complexation with a tridentate ligand

Abstract

We describe here a new design strategy for obtaining boron complexes with aggregationandcrystallization-inducedemission(AIEandCIE,respectively)propertiesbasedontheintroduction of skeletal distortion. According to our recent results, despite the fact that an almost planar structure and robust conjugation were obtained, the boron azomethine complex provided a slight emission in solution and an enhanced emission in aggregation and crystal. Quantum calculation results propose that unexpected emission annihilation in solution could be caused through intramolecular bending in the excited state. Herein, to realize this unique molecular motion and obtain AIE and CIE molecules, the phenyl quinoline-based boron complexes BPhQ and BPhQm with distorted and planar structures were designed and synthesized, respectively. BPhQm showed emission in solution and aggregation-caused quenching (ACQ, BPhQm: ΦF,sol. = 0.21, ΦF,agg. = 0.072, ΦF,cryst. = 0.051), while BPhQ exhibited a typical AIE and CIE (BPhQ: ΦF,sol. = 0.008, ΦF,agg. = 0.014, ΦF,cryst. = 0.017). The optical data suggest that a large degree of molecular motion should occur in BPhQ after photo-excitation because of the intrinsic skeletal distortion. Furthermore, single-crystal X-ray diffraction data indicate that the distorted π-conjugated system plays a positive role in presenting solid-stateemissionbyinhibitingconsecutive π–π interactions. Wedemonstrateinthispaperthatthe introduction of the distorted structure by boron complexation should be a new strategy for realizing AIE and CIE properties.