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.

Concentration-Dependent Fluorescence Color Tuning of the Difluoroboron Avobenzone Complex in Polymer Films

“…BF 2 AVB was synthesized according to a previous report. 23,29 Perylene was purified by repeated zone refining. PMMA with a number-average molecular weight of 350 000 was purchased from Sigma-Aldrich (Schnelldorf, Germany) and it was used as-received.…”

“…Dibenzoylmethanatoboron difluoride (BF 2 DBM) derivatives have excellent optical properties, for example, two-photon absorption cross-sections, 20,21 high fluorescence quantum yield in the solid state, 22 multiple fluorescence colors, [23][24][25]26,27 and reversible mechanofluorochromic properties. 28,29 In particular, BF 2 DBM based on the 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone) boron difluoride complex (BF 2 AVB) exhibits different emission depending on the crystal phase (polymorph). 30 BF 2 AVB also has excellent fatigue resistance by photoirradiation and a high fluorescence quantum yield even in the solid state (∼0.5), which is advantageous for photonics applications.…”

Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals

“…[14][15][16][17][18] These compounds often assume several crystalline polymorphs with distinct fluorescence properties and, in most cases, their mechanofluorochromic properties have been attributed to a phase transition between crystalline and amorphous phases. [19,20] Photophysical studies are used to uncover prevalent emitting species which, may occur from single molecules (monomers) or from two or more molecular units (excimers or aggregates). [21] Additionally, emission from either of these species may be predicated by long-or short-range energy transfer between adjacent molecules.…”

β-Diketone derivatives: influence of the chelating group on the photophysical and mechanofluorochromic properties

“…4 Since the first description of their mechanofluorochromism by Fraser et al in 2010, 5 difluoroboron ßdiketonate (DFB) complexes have been widely studied since they possess appealing properties such as versatile synthesis, high fluorescence quantum yields in the solid state, large fluorescence emission shifts upon mechanical stimulation, and a substituent-dependent backward reaction to the original fluorescence emission after mechanical stimulation. 6 As a consequence, numerous derivatives belonging to this family have been described, such as dibenzoylmethane systems with different substituents on the phenyl rings, [7][8][9] or diketones substituted with larger aromatic rings 10,11 or heterocycles. 12 However, the studies aiming at deciphering the influence of the boron chelating group on the photophysical properties of these complexes are more scarce.…”