The two-step nucleation model for crystal nuclei formation explains several experimental and theoretical results better than the classical nucleation theory. We report here direct visualization of the two-step nucleation model for organic molecular crystallization. Evaporative crystallization from a solution of a dibenzoylmethane boron complex that displays mechanofluorochromism, a fluorescence color change induced by mechanical perturbation, was probed by fluorescence change. The dependence of fluorescence change on dispersion concentration of the complex in a polymer matrix was also investigated. We detected transitional emission from the amorphous cluster state prior to crystallization. This is the first demonstration of the two-step nucleation model based on fluorescence color changes.
Organic light emitting diodes (OLEDs) displaying a wide range of emission colors with emission peaks from 450 to 665 nm using a single emitting material, avobenzone boron difluoride (AVB-BF 2 ), are This article is protected by copyright. All rights reserved. 2 reported. Color tuning is achieved by controlling the aggregation of AVB-BF 2 and the formation of a "triadic" exciplex of an AVB-BF 2 dimer and a host molecule. Various electroluminescent devices containing AVB-BF 2 cover the whole visible light spectrum and a white-emitting device with CIE coordinates of (0.35, 0.37) is obtained with a single emitting material in a single emissive layer.Furthermore, an exceptionally high external quantum efficiency of nearly 13% is achieved for a greenemitting OLED because AVB-BF 2 exhibits thermally activated delayed fluorescence by forming the exciplex.
Kinetics of the thermal back-reaction of β-diketonate boron difluoride complexes after mechanical perturbation were evaluated by fluorescence intensity changes for the first time, suggesting that the activation parameters of the reaction intermediates governed intermolecular interactions such as 10 hydrogen bonding assisted by substituent groups. we studied and quantitatively evaluated for the first time the thermodynamic parameters for thermal back-reaction after mechanically induced fluorescence change. The molecular structure of boron difluoride β-diketonate complexes is shown in Chart 1. In this study we used 1-(4-tert-55 butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione (ab), 1,3-bis-(4-methoxyphenyl)-1,3-propanedione (2a), and 1,3-bis-(4-tert-butylphenyl)-1,3-propanedione (2b) as ligands. The boronation was performed by BF 3 ·OEt 2 in CH 2 Cl 2 according to a previous report. 12 The purification was carried out with column 60 chromatography. A 2.0×10 -3 mol·dm -3 dichloromethane solution was dropped on paraffin-coated weighing paper, and then the solvent-evaporated sample was used for spectroscopic studies. Fluorescence spectra were recorded on a Shimadzu RF-5300PC fluorescence spectrophotometer. In order to apply a mechanical 65 perturbation to the boron difluoride β-diketonate complexes, the samples were rubbed with a spatula. The temperature around the sample was controlled by a home-made system using a rubber heater (Hakko) and digital temperature controller (Omron E5CN-QT).
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.
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