Aggregation-induced Emission And Mechanofluorochromic Properties Of D-A Type Organoboron Complexes

Mechanofluorochromic（MFC）materials refer to a new type of functional materials whose fluorescence emission is offset or whose fluorescence intensity is obviously changed under external forces.These materials have a huge application in mechanosensors,optical information storage,memory chips.Recent research results show that some solid fluorescent dyes with aggregation-induced emission（AIE）properties usually exhibit MFC properties.In addition,β-diketone boron andβ-ketoimine boron complexs have attracted wide attention duing to their potential applications in organic solid laser,photosensitive materials and OLEDs.Therefore,we have taken the synthesis of D-A type MFC molecules with AIE properties as the starting point to prepar a series ofβ-diketone boron andβ-ketoimide boron complexes conjugated fluorescent dyes modified by tetraphenylethene and tetraphenylbutadiene,and their intramolecular charge transfer（ICT）,AIE and MFC properties were studied,and the main results are outlined as following:（1）A tetraphenylethene functionalized D-A typeβ-diketonate boron complex fluorescent dye BF₂-TPE has been designed and synthesized,and its AIE characteristic and MFC behavior were investigated.The results showed that BF₂-TPE could emit intense yellow fluorescence in the solid state（544 nm）and exhibited obvious AIE characteristics(α_AIE=231).After grinding,the fluorescence color of the solid powder changed into orange-red（606 nm）.And the MFC process has good reversibility.（2）Two D-A type tetraphenylethene functionalizedβ-diketonate boron complexes C2TPE-CAR and C16TPE-CAR that were bridged by carbazole have been designed and synthesized.Results showed that the two compounds exhibited alkyl-dependent AIE,self-assembly and MFC properties.C16TPE-CAR with a long hexadecyl chain formed gels in certain solvents.By contrast,complex C2TPE-CAR,which possessed the short ethyl group,exhibited no self-assembly behavior.Simple mechanical force could change the emission color of C2TPE-CAR and C16TPE-CAR solid powders from initial bright yellow（571 and 558 nm）to final red（617 and 610 nm）,and the MFC process has good reversibility.（3）β-ketoiminate boron complexes TTPE-H and TTPE-CN modified by tetraphenylethene have been designed and synthesized.The two compounds exhibited typical ICT,evident AIE characteristics and reversible MFC properties.Upon grinding treatment,the emission color of TTPE-H and TTPE-CN could changed from bright green（497 nm）and bright yellow-green（525 nm）into yellow-green（515nm）and yellow（565 nm）,respectively.In addition,the compound TTPE-CN showed more contrast MFC behavior relative to TTPE-H.The reason is that the steric hindrance of the cyano group in the boron chelating ring makes TTPE-CN possess more distorted spatial conformation.（4）β-Ketoiminate boron complexes BF₂-TBO and BF₂-TBT modified by tetraphenylbutadiene have been designed and synthesized.The results showed that these two compounds have significant AIE characteristics and high solid-state fluorescence quantum yields.Moreover,BF₂-TBO and BF₂-TBT showed the reversible MFC behavior.Under external force,the fluorescence color of the solid powder of the two complexes could changed from yellowish green（located at 528 and530 nm）to yellow and orange（located at 552 and 572 nm）,respectively.In addition,the sulfur atom in BF₂-TBT facilitates the delocalization ofπ-electrons,as a result,BF₂-TBT shows more obvious MFC behavior than that of BF₂-TBO.