Substituent-dependent backward reaction in mechanofluorochromism of dibenzoylmethanatoboron difluoride derivatives

Abstract

The thermally backward reaction involved in the mechanofluorochromism of dibenzoylmethanatoboron difluoride (BF₂DBM) derivatives, accompanied by an amorphous–crystalline phase transition, was quantitatively evaluated based on kinetics and thermodynamics. The kinetics was discussed by evaluation of the effect of temperature on the time-dependent changes of the fluorescence intensity for amorphous samples obtained by mechanical grinding. The thermodynamics was discussed based on data for the amorphous–crystalline phase transition obtained by differential scanning calorimetry. The enthalpy of activation (ΔH^‡) of BF₂DBM derivatives with MeO groups (2aBF₂) was larger than that of derivatives with alkyl groups (2b-dBF₂), whereas the entropy of activation (ΔS^‡) was smaller than that of the derivatives with alkyl groups. It is proposed that the reaction dynamics of 2aBF₂ will be governed by rotational motion around the C(methyl)–O bond. Interestingly, the Gibbs energies of activation (ΔG^‡) were comparable for the reactions of all members of the BF₂DBM series, though ΔH^‡ and ΔS^‡ were strongly dependent on the identity of the substituent. It is proposed that the substituent-dependent ΔS^‡ term is one of the key parameters for understanding the mechanofluorochromism of BF₂DBM derivatives associated with the amorphous–crystalline phase transition. These findings will also provide important insights into the process of formation of crystal nuclei in moving from the melted to the crystalline state.