Tuning the Mechanical and Thermal Properties of Emissive ß-diketones, Modulating Electronic and Molecular Packing Effects

New stimuli responsive luminescent materials are incorporated into emerging technologies such as optical memory storage devices and force sensors. In particular, difluoroboron coordinated β-diketones (BF2bdk), which are easily synthesized from commercially available starting materials, exhibit impressive solid-state optical properties and show emission changes in response to thermal and mechanical perturbations. In some cases, mechanically responsive BF2bdk dyes even self-erase at room temperature, a rare property in other materials. Prior studies with BF2bdk dyes explored halide and alkyl chain substituent effects on mechanoresponsive properties, however, clear design strategies for tuning optical properties had yet to be elucidated.

While exploring methods for modulating the properties of BF2bdk materials, it was discovered that boron-free β-diketones (bdk) also possess thermally and mechanically responsive emission. Substitution of bdk dyes with donor and acceptor groups resulted in tunable emission wavelengths. In combination with BF2 coordination, a series of mechano-responsive dyes were synthesized that encompassed the entire visible spectrum. Additionally, rapid self-erasure was observed for a trimethoxy-substituted bdk derivative, which was used to evaluate processing effects (i.e. substrate and film thickness) on smeared state recovery. Diketone dyes incorporating donor-acceptor motifs also exhibited stable supercooled liquid phases upon cooling after melting. In some cases, shear-induced crystallization was observed, a new property for this class of dyes. Given rich thermal and mechanical properties, responsive bdk and BF2bdk materials show great promise for practical applications.