Effects of α-Substitution on Mechanochromic Luminescence and Aggregation-Induced Emission of Difluoroboron β-Diketonate Dyes

Though the effects of arene substituents on difluoroboron β-diketonate (BF2bdk) mechanochromic luminescence (ML) are known, substitution at the α carbon of the dioxaborine ring has yet to be carefully investigated. Substitution at the α position can force these dyes out of a planar conformation, causing materials to take on unique aggregation-induced enhanced emission (AIEE) character. Furthermore, if ML arises from the formation of H-aggregates upon smearing, as postulated, substitution at the α position could hinder this process and influence ML. To test this hypothesis, luminescent BF2bdk derivatives containing substituents at the α position were synthesized and their optical and morphological properties were characterized by fluorescence spectroscopy, UV/vis spectroscopy, atomic force microscopy (AFM), differential scanning calorimetry (DSC), and density functional theory (DFT) calculations. Specifically, a difluoroboron dibenzoylethane derivative with a methyl group at the α position exhibits bright blue emission in the solid state, but it is practically nonemissive in solution (ΦF < 1% in CH2Cl2). Additionally, it shows clear AIEE behavior in varying water fractions (fw) with either dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF) as the solvent. In comparison, a difluoroboron 2-benzoyl-1-tetralone derivative is much brighter in solution (ΦF = 61% in CH2Cl2) and only a decrease of emission intensity was observed upon inducing aggregation. Furthermore, a methoxyphenyl-substituted dye exhibited bright, observable emission from a twisted intramolecular charge transfer (TICT) state in solution (ΦF = 16% in CH2Cl2) and unique attenuation of emission intensity in low fw and AIEE at higher fw with DMSO as the solvent. We demonstrate that stereoelectronic perturbations of this kind can be used to tune the stimuli-responsive emission properties of these dyes.