Synthesis And Mechanofluorochromic Properties Of New Difluoroboron β-carbonyl Cyclic Ketonate Complexes

Mechanofluorochromic（MFC）materials are a kind of smart materials that changes the color of solid state luminescence under external force.In recent years,it has attracted great attention in the application of stress sensors,date storage,security ink,bioimagingandlightemittingdevices.Atpresent,typical mechanofluorochromic materials include tetraphenylethene,9,10-divinyl-anthracene and oligo（p-phenylene vinylene）.Therefore,the synthesis of mechanofluorochromic materials with high luminescence efficiency and long wavelength luminescence is the focus of research.As we all know,difluoroboronβ-diketonate（BF₂bdk）derivatives as a kind of important fluorescence dyes,they exhibited excellent properties,such as large molar extinction coefficients,high fluorescence quantum yield and its emission spectrum can be adjusted by the structure of ligands and the change of external environment,so that it has an attractive application value in the field of optoelectronic materials.On the one hand,there are few reports about theβ-carbonyl cyclic ketones boron complexes.On the other hand,it is considered that if the aliphatic unit was introduced into the system,its molecular conformation is easy to change under external force,further affect the properties of MFC materials.Therefore,based on the designed of a new type ofβ-carbonyl cyclic ketonate difluoroboron complexes and investigated the influence of the structure of ligand and substituent on the properties of MFC materials.In addition,mechanofluorochromic materials with near infrared luminescence have been prepared.Some creative results have been obtained,and outline below:（1）The D–π–A type difluoroboronβ-carbonyl cyclic ketonate complexes that contain tert-butyl carbazole units（TCCH and TCCP）and carbazole units（CCH and CCP）were synthesized.These complexes were highly emissive in the solid state,and their solid fluorescence yields were more than 0.50.In addition,these complexes exhibited reversible MFC behavior under grinding/fuming or heating treatment.Taking TCCH as an example,the as-synthesized sample of TCCH exhibited an emission band at 518 nm,and emitted yellowish-green light under UV irradiation.Grinding of the as-synthesized sample resulted in the color of the emission turning orange-red,accompanied by a red-shift of the emission to 567 nm.When the ground powder was exposed to CH₂Cl₂ vapor or heated,the emission color recovered yellowish-green fluorescence.During the MFC process of TCCH,the peak shift of the maximum fluorescence spectrum is 49 nm.However,for the carbazole derivative CCH with similar structure,the maximum emission peak only red-shift 20 nm under grinding,and theβ-carbonyl cyclopentone difluoroboron complexes（TCCP and CCP）also gived similar MFC behavior.Therefore,the derivatives which contained a tert-butyl carbazole group,exhibited high-contrast mechanofluorochromism.The XRD results showed that the change of the color of the luminescence is due to the transition from the crystalline to the amorphous.The solid-state UV-vis absorption spectra showed that the steric hindrance effect of tert-butyl group led to loose arranged in crystal and existed as isolated molecules in the as-synthesized crystals,and J-aggregates were formed upon grinding.However,π-aggregates were formed in the as-synthesized crystals of carbazole derivatives CCH and CCP,and grinding only weakened the strength of theπ–πinteractions,thereby resulting in low-contrast mechanofluorochromism.Therefore,this work provides a useful strategy for the design of MFC dyes with high performance.（2）Difluoroboronβ-carbonyl cyclic ketonate complexes CnB（n=2,16）,in which carbazole was linked to difluoroboronβ-diketonate directly,and DCnB（n=2,16）,where two terminal carbazoles were bridged by vinyl groups to link to difluoroboronβ-diketonate core were synthesized.The results showed that all of these complexes have the mechanofluorochromism properties.Compared with C2B,C16B showed high-contrast mechanofluorochromism,because of the long alky chain in C16B might decrease the strength ofπ-pinteractions in the as-synthesized crystals,so that its emission bands appeared at high-energy region compared with C2B,for instance,the emission of C16B in crystals was located at 483 nm.Upon grinding,the disassembling of parts ofπ-aggregates and the improvement of the molecular planarity led to the red-shift of the emission of C16B to 509 nm.The high-contrast mechanofluorochromism of C16B was mainly originated from the high-lying excited state in as-synthesized crystals.In the case of C2B,single crystal datas showed that C2B had J-aggregates,and they could not be destroyed completely upon grinding,so that only 15 nm of shift in emission during MFC process was observed.Notably,the emission of DC2B and DC16B,which emerged in near infrared region in solid states,and they showed red and rose-red luminescence in the as-synthesized crystals and in ground powders.The XRD results showed that the change of the color of the luminescence is due to the transition from the crystalline to the amorphous.At present,there are few reports about the mechanofluorochromism dyes with near infrared luminescence.The near infrared（NIR）light materials may have potential applications in bioimaging and security inks.