Substituent-dependent backward reaction in mechanofluorochromism of dibenzoylmethanatoboron difluoride derivatives

摘要

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 (Delta H-double dagger) of BF2DBM derivatives with MeO groups (2aBF(2)) was larger than that of derivatives with alkyl groups (2b-dBF(2)), whereas the entropy of activation (Delta S-double dagger) was smaller than that of the derivatives with alkyl groups. It is proposed that the reaction dynamics of 2aBF(2) will be governed by rotational motion around the C(methyl)-O bond. Interestingly, the Gibbs energies of activation (Delta G(double dagger)) were comparable for the reactions of all members of the BF2DBM series, though Delta H-double dagger and Delta S-double dagger were strongly dependent on the identity of the substituent. It is proposed that the substituent-dependent Delta S-double dagger 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.