Tetraphenylethene modified β-ketoiminate boron complexes bearing aggregation-induced emission and mechanofluorochromism

Abstract

Two new tetraphenylethene modified β-ketoiminate boron complexes (TTPE-H and TTPE-CN) have been rationally designed and successfully synthesized. The two D–π–A type compounds exhibit typical twisted intramolecular charge-transfer (TICT) emission and evident aggregation-induced emission (AIE) characteristics, remarkable luminescence mechanochromism, and high solid state efficiency (up to 0.785 and 0.672). The mechanofluorochromic (MFC) behaviors of TTPE-H (from bright green to yellow-green emission, λ_em from 497 to 515 nm) and TTPE-CN (from bright yellow-green to yellow emission, λ_em from 525 to 565 nm) are reversible upon grinding and fuming treatments. From the X-ray diffraction and differential scanning calorimetry analyses, it was confirmed that the MFC properties of TTPE-H and TTPE-CN should be derived from a phase transition between crystalline and amorphous states. More interestingly, the compound TTPE-CN, which possesses a cyano group in the boron chelating ring, showed more contrast MFC behavior relative to TTPE-H. The reason is that the introduction of the cyano group can enhance the distortion degree of the molecule TTPE-CN because of steric hindrance.