Modulating Mechanochromic Luminescence Quenching of Alkylated Iodo Difluoroboron Dibenzoylmethane Materials

Difluoroboron β-diketonate compounds exhibit solid-state luminescence phenomena. Among these are reversible mechanochromic luminescence (ML), aggregation induced emission (AIE), and mechanochromic luminescence quenching (MLQ). These properties can be tuned by alterations to the molecular structure. Dyes with varying halide substituents exhibit tunable ML, MLQ, and solid-state emission with high quantum yields. A series of difluoroboron dibenzoylmethane (BF₂dbm) dyes with iodide and alkoxyl substituents (BF₂dbm(I)OR) were synthesized where R = CH₃ (C1), C₅H₁₁ (C5), C₆H₁₃ (C6), C₁₂H₂₅ (C12), and C₁₈H₃₇ (C18)). The 4-iodo parent compound BF₂dbm(I) (H) was made for comparison. By keeping the heavy atom static, the dependence of ML properties on alkyl chain length was probed. The hydrogen derivative is only weakly emissive as a solid and exhibited minimal mechanoresponsive behavior. In contrast, alkoxy dyes exhibited tunable ML and MLQ properties depending on the length of the alkyl chain. Longer chain dyes corresponded to smaller singlet triplet energy gaps, greater triplet emission enhancement (77 K), and longer recovery times after smearing under ambient conditions. Shorter chain dyes have a much greater affinity for the ordered emissive state, as confirmed by atomic force microscopy (AFM). Powder X-ray diffraction (XRD) was performed on pristine dye powders as well as drop-cast films to gauge crystallinity in various forms. Single crystal XRD analysis of the H and C5 dyes revealed significant differences in crystal packing and π-π stacked dimers for dyes bearing alkyl chains and dyes without alkyl chains. Unique I-I and F-π close interactions were discovered in the dye crystals.