Synthesis,Assembly And Mechanofluorochromic Properties Of Donor-Receptor Type Fluorescent Dyes Containing Carbazole

Recently,organicπ-conjugated luminescent materials have received increasing attention in recent years due to their potential for applications in organic light emitting diodes（OLEDs）,photovoltaic device,sensors and switches,bioimaging and so on.It is worth noting thatnitrogen-containing heterocyclic compounds as an important class of fluorescent dyes has the advantages of simple structure,many reaction sites,easy modification of molecular structure,stable properties and strong electron donating ability.Introducing it into aπ-conjugated system,we will get a new class ofπ-conjugated luminescent materials.In this paper,the relationship between the self-assembly properties of molecules and molecular structure was investigated by designing D-π-A nitrogen-containing heterocyclic compounds.The stimulating response of materials to acid and mechanical forces was studied.The following innovative research results have been achieved:New carbazole-modified pyrazole derivatives were synthesized.It was found that tert-butylcarbazole derivatives of TCPDP,TCPHP,TCPPM and TCPPF exhibited reversible mechanofluorochromic（MFC）behavior under the treatment of grinding/fuming with CH₂Cl₂.Particularly,the emission of the as-synthesized crystals of TCPDP was weak and emerged in ultraviolet region,while the grinding led to significant red-shift of the emission to visible-light region,accompanied with the significant fluorescence enhancement on account of the formation of J-aggregates.Thus,TCPDP showed intriguing fluorescence switching in visible-light region during MFC processes,which could be observed by naked-eyes easily.It provided a new idea in the design of MFC dyes with high-contrast performance via a fluorescence off-on mode.Besides,TCPDP,TCPHP,TCPPM and TCPPF could form stable organogels in some organic solvents.Notably,the solutions of TCPPM and TCPPF were almost non-emissive,but gelation-induced blue emission was achieved.Thus,the fluorescence could be also turned on due to the AIE（aggregation-induced emission）during the gel formation.New D-π-A type N,O-chelated BF₂ complexes（TCBOB,TCBTB,TCQB and TCPB）are synthesized.It was found that TCQB and TCPB could self-assemble into nanofibers in diameters of 60-300 nm via organogelation directed byπ-πinteractions.Interestingly,the emission of spin-coating films of the BF₂ complexe could be quenched by TFA（trifluoroacetic acid）due to the protonation of the N atom in amide group.In particular,the xerogel-based film of TCPB in the detection of gaseous TFA showed higher performance（the decay time and the detection limit are 0.8 s and 1.31ppm,respectively）than the spin-coating films of TCBOB,TCBTB and TCQB.In addition,the BF₂ complexes exhibited reversible mechanofluorochromic behavior under the treatment of grinding/fuming or heating,and TCPB gave high-contrast mechanofluorochromism compared with others.XRD patterns in different solid states demonstrated that the mechanofluorochromism was accompanied with the transformation between the crystalline and amorphous states.Here,we report a new difluoroboronβ-carbonyl nitrogen heterocycles ketonate complexes CPZOB and TCPZOB.Solvent-dependent fluorescence emission spectra indicate that the emission peaks of CPZOB and TCPZOB are derived from ICT transitions.CPZOB and TCPZOB emit strong fluorescence under ultraviolet light irradiation,and the solid-state fluorescence quantum yields are 53%and 11%,respectively.Interestingly,the synthesized difluoroboron complexes exhibited reversible mechanofluorochromism.The synthesized TCPZOB sample emitted green fluorescence with a maximum emission wavelength at 518 nm.After grinding,it turned orange and the maximum emission peak was red shifted to 593 nm.It should be noted that the shift of 75 nm for the emission band for the dye during MFC process was seldom reported in metallic-free organic compounds.Similarly,the grinding caused the CPZOB emission peak to red-shift from the 550 nm sample to 577 nm,and the fluorescence changed from yellow to orange.The UV-visible absorption spectra of different solid states showed that the high contrast mechanofluorochromism of TCPZOB are derived from the change between single molecule emission and J-aggregate emission,while CPZOB has low contrast mechanofluorochromism.It is derived from the change in the intensity ofπ-πinteraction between molecules.XRD and DSC spectra illustrated that the transformation between crystalline and amorphous states was reasonable for the mechanofluorochromism.These MFC emitters based on difluoroboronβ-diketonate complexes may have potential applications in mechanical sensors,security printing and data-storage.