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 (f w) 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 f w and AIEE at higher f w 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.