Effects of cyano groups on the properties of thiazole-based β-ketoiminate boron complexes: aggregation-induced emission and mechanofluorochromism

Abstract

A series of new thiazole-based β-ketoiminate boron complexes BF₂-TT-CN, BPh₂-TT-CN, BF₂-TT and BPh₂-TT, have been designed and successfully synthesized. These compounds show clear intramolecular charge transfer (ICT) from the electron donor (triphenylamine unit) to the electron acceptor (the β-iminoenolate boron moiety). It is particularly noteworthy that these compounds exhibit strong cyano-dependent aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties. In the cyano-substituted derivatives, BF₂-TT-CN and BPh₂-TT-CN, the high electron affinity of the cyano group confers stronger ICT properties relative to BF₂-TT and BPh₂-TT. In addition, the internal steric hindrance arising from the cyano group results in more twisted conformations, endowing BF₂-TT-CN and BPh₂-TT-CN with evident AIE characteristics and reversible mechanofluorochromism. The MFC behavior of BF₂-TT-CN (from bright yellow to orange emission, λ_em from 542 to 599 nm) and BPh₂-TT-CN (from bright yellow to orange emission, λ_em from 549 to 566 nm) is reversible by repeating the grinding-fuming process. However, BF₂-TT and BPh₂-TT, which lack cyano groups, exhibit no AIE and mechanofluorochromism, partly because of their relatively weak twisted configurations. Therefore, the introduction of the cyano group plays an important role in the strong ICT behavior and excellent mechanofluorochromic AIE properties of BF₂-TT-CN and BPh₂-TT-CN. These results will be of great help in understanding the AIE and MFC mechanisms and designing new MFC AIE materials.