Effects of Substitution Position and Crystal Polymorphism on Mechanochromic Luminescence in Dibenzoylmethanatoboron Difluoride Derivatives

Mechanochromic luminescence (MCL), defined as a fluorescence color change induced by mechanical stimulation, has recently attracted considerable attention due to its sensitivity to molecular packing and intermolecular interactions. In this study, we synthesized three regioisomers of dibenzoylmethanatoboron difluoride (BF2DBM) with a methoxy substituent at the ortho, meta, or para position of one phenyl ring (o-, m-, and p-a2bBF2), while the other ring was substituted with di-tert-butyl groups. In addition, two crystal polymorphs of p-a2bBF2 (Form I and Form II) were investigated. Clear MCL behavior was observed for o-a2bBF2 and for Form I of p-a2bBF2. In contrast, m-a2bBF2 exhibited only a minor spectral change without an apparent fluorescent color change, and Form II showed virtually no color change. Spectroscopic and crystallographic analyses revealed that these contrasting responses originate from differences in molecular packing. In Form I and o-a2bBF2, smearing induces a transition from monomer-like to excimer-like emission, whereas Form II and m-a2bBF2 retain excimer-like emission sites that are largely insensitive to mechanical perturbation. These findings demonstrate that both substitution position and crystal polymorphism critically govern MCL activity by modulating the balance between weak CH−π and strong π–π intermolecular interactions.