Mechanochromic Luminescent Properties of Difluoroboron β-Diketonate Dyes

Abstract

Difluoroboron β-diketonates (BF2bdks) exhibit unique solid-state photoluminescent properties. Among these is reversible mechanochromic luminescence (ML), where the color of emission can be changed by shear force. Halide-substituted BF2bdks also show mechanochromic luminescent quenching (MLQ) where mechanical force quenches emission by generating low energy singlet excited states closer in energy to the dark triplet excited state that is quenched at room temperature under air. Crossover to the excited triplet state is responsible for this quenching and halides make this effect more pronounced. These phenomena and other solid-state emission properties can be tuned by changes to the molecular structure. The effects of varying halides, alkoxy chain lengths, heterocycles, and substitution at the α-position of the dioxaborine core were explored. Both halide and alkoxy chain substitution have a significant impact on ML and MLQ properties, with heavier halides and longer chain lengths correlating to stronger MLQ. Substitution with a furan heterocycle brings about emission switchable by different methods of heating while thiophene heterocycle substitution creates high-contrast, highly reversible ML behavior. Solid state emission color, dye morphology, and reversibility of ML are also modulated by halide substituents. Furthermore, density functional theory (DFT) calculations were performed to understand structure/property relationships associated with luminescent properties of these dyes both as solvated molecules and polymeric materials and to begin modeling aggregate emission similar to what would be observed in the solid state. Intermolecular distance between BF2dbm(OMe)2 monomers was found to affect the position of a dark singlet excited state relatively close in energy to the strongest observed singlet excited state.