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.