β-Iminoenolate boron complex with terminal triphenylamine exhibiting polymorphism and mechanofluorochromism

Abstract

A new β-iminoenolate boron complex with a terminal triphenylamine (TP) was synthesized, and it could emit strong fluorescence in solutions and in solid states. Two types of crystals were obtained via evaporation of the solutions of TP in different solvents. A yellow ribbon-like crystal (Y-crystal) giving yellowish green emission centered at 518 nm was obtained from tetrahydrofuran (THF) solution, and a green needle-like crystal (G-crystal) emitting green light with maximum emission at 495 nm was yielded from dichloromethane (DCM)/petroleum ether solution. It was found that the molecules adopted a stacking mode of J-aggregation in the Y-crystal, and the π–π interaction in the G-crystal was weaker than that in the Y-crystal. Besides the polymorphism feature, TP exhibited mechanofluorochromic (MFC) properties. Upon being ground, the Y-crystal and G-crystal could transform into similar ground powders with yellow emission centered at 533 nm. Interestingly, when the ground powder formed from the Y-crystal or G-crystal was fumed with DCM or heated, the XRD pattern and the fluorescence emission spectrum of the fumed sample would be recovered to those of each of their own initial crystal states. Such memory ability of the molecular packing mode during reversible MFC processes might be useful for our well understanding the MFC mechanism.