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β-Iminoenolate boron complexes bearing non-planar phenothiazine (P2B and P16B) and phenothiazine-S,S-dioxide (PO2B and PO16B) have been synthesized. It was found that P2B and P16B gave ICT emission due to the strong electron-donating ability of phenothiazine, and no ICT emission was detected for PO2B and PO16B because of the weak electron-donating ability of phenothiazine-S,S-dioxide. In particular, P2B and P16B exhibited high-contrast MFC behaviors compared with that of PO2B. On account of the formation of π-aggregates in the as-synthesized crystals of P2B, several channels for the changes in emitting colors in different solid states were provided. The as-synthesized crystals of P2B emitting weak brown light could be changed into ground powders emitting deep orange-red light after grinding. When the ground powders were fumed with DCM for 5 s or heated at 160 °C for 3 s, the samples emitted yellow and yellowish green light, respectively. Although the π-conjugated skeleton of P16B was the same as that of P2B, the introduction of a long carbon chain led to different MFC properties. The as-synthesized crystals of P16B emitting yellow light could be changed into ground powders emitting orange light upon grinding. Particularly, the amorphous state of P16B could be self-recovered to the crystalline state when placed at room temperature for 5 min because the π–π interactions might be restrained by the long carbon chain, leading to low stability of the amorphous state. In the case of PO2B, low-contrast and reversible MFC properties were observed under grinding/fuming or heating treatment. No mechanofluorochromism was observed for PO16B since a long carbon chain was involved and no ICT emission happened. Therefore, we suggested that the introduction of a non-planar conjugated unit and the construction of a D–π–A system with ICT emission were favorable for designing MFC dyes with high-contrast changes in emitting colors in different solid states, and the introduction of a long carbon chain could improve the MFC reversibility so as to gain self-recovering MFC materials. [ABSTRACT FROM AUTHOR]

D–π–A type phenothiazine modified triphenylacrylonitrile derivatives PVTPAN, P3TPAN and P10TPAN have been synthesized. The three compounds exhibited AIE behaviors, and the emission enhancement of PVTPAN and P3TPAN, in which 3-position in phenothiazine was functionalized, was more significant than P10TPAN (10-position of phenothiazine was modified) with more twisted conformation. The single crystal structures revealed that the multiple intermolecular interactions, including π–π interactions and hydrogen bonds of C–H…π and C–H…N, would lock the molecular conformation and reduce the energy loss via non-radiative channels, leading to enhanced emission. Interestingly, the as-synthesized crystals of PVTPAN, P3TPAN and P10TPAN emitting strong yellow, yellowish-orange and yellowish-green light under UV irradiation could be transferred into powders emitting red, orange-red and orange light, respectively, after grinding. Such mechanochromism was reversible upon the treatment of grinding and heating/fuming with DCM. The XRD patterns suggested that the mechanochromism originated from the transition between the crystalline and amorphous states. It should be noted that the recovery time of the emission of the ground powders to that of crystals for P10TPAN was much shorter than those for PVTPAN and P3TPAN at the same temperature. Extremely, the emission of the ground powders of P10TPAN could recover at room temperature, but that of PVTPAN and P3TPAN could not. We suggested that such different mechanochromic behaviors of P10TPAN from PVTPAN and P3TPAN were coming from its more twisted conformation, which could lead to the exothermic peak appearing at lower temperatures and the easier molecular rearrangement of the amorphous state into crystals. Therefore, we could prepare the dyes with stable or reversible mechanochromism by tuning the molecular conformation. [ABSTRACT FROM AUTHOR]

Carbazole and tert-butylcarbazole functionalized β-iminoenolate boron complexes CB and TCB with different mechanofluorochromic (MFC) properties have been synthesized. It was found that the as-synthesized crystal of CB emitted orange light under UV illumination because of the appearance of the emission from the monomers and the excimers. After grinding for a while, the ground powder 1 of CB emitted bright yellow light since the ratio of the emission intensity from excimers/monomers decreased. On further grinding for a long time, the excimers disappeared and the obtained amorphous ground powder 2 emitted dark green light. However, TCB emitted sky blue light in the as-synthesized crystal because no excimer was formed due to the steric hindrance of tert-butyl. After grinding, the amorphous ground powder of TCB emitted bright green light derived from excimers. It should be noted that the fluorescence quantum yield of TCB in an amorphous solid state reached 0.53, which was the highest one for the β-iminoenolate boron complexes that have ever been reported. In addition, the emission changes of CB and TCB in different solid states were reversible upon repeating the treatment of mechanical grinding and fuming with CH2Cl2. Therefore, the obtained β-iminoenolate boron complexes might be used as sensors and memory chips on the basis of the solid fluorescence response to external mechanical forces and organic solvents. [ABSTRACT FROM AUTHOR]