Mechanofluorochromic properties of β-iminoenolate boron complexes tuned by the electronic effects of terminal phenothiazine and phenothiazine-S,S-dioxide

Abstract

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.