Emission Properties Control Of A-π-D-π-A Type Carbazole Derivative-based Red/Near-infrared Fluorescent Dyes In Aggregates

In recent years,the red/near-infrared fluorescent dyes have wide applications in the fields of photoelectric materials,fluorescence probes,information encryption and forgeyroof and biological imaging.The organic fluorescent dyes have attracted much attention due to their advantages such as flexible structure,good biocompatibility and so on.In this work,a series of A-π-D-π-A typed carbazole derivatives has been designed and synthesized.We using carbazole as the donor which has the planar structure and multiple modifiers,phenyl or thiophynyl asπbridges,1,3-indandione and TCF as the acceptor,respectively.The band gap and the fluorescence property in the aggregation were adjusted by the dipole moment of the molecule.Their solvatochromism effect,aggregration-induced emission（AIE）or aggregration caused quenching（ACQ）and thermoresponsive luminescence（TRL）property as well as fluorescence quantum yields were fully investigated by UV-vis absorption and photoluminescence spectra,XRD,transient fluorescence specture and DSC characterization.The results indicate that:As the molecule with phenylπ-bridge and 1,3-indandion acceptor the molecular dipole moment was tuned by the molecular configuaration througth changing the substitution position of theπbridge on the carbazole moiety.Six molecules with hexyl side chain（2,7-h-CPDI,2,6-h-CPDI and 3,5-h-CPDI）or ioctyl side chain（2,7-i-CPDI,2,6-i-CPDI and 3,5-i-CPDI）were obtained.Due to the intramolecular change transfer（ICT）from the donor to the acceptor,they exhibited remarkable solvatochromism effect with emission color tuning from green to red as the increase of the solvent polarity.In THF/H₂O mixture,the molecules demonstrated diversified fluorescent behaviors in the aggregration states due to the synergistic effect of the molecular configuration and the alkyl side chains.For the derivatives with hexly side chains,the emission property of the aggregrates was gradually changed from AIE-active（2,7-h-CPDI,2,6-h-CPDI）to ACQ（3,5-h-CPDI）with the increase of the molecular dipole mement.However,for those molecules with ioctyl side chains,all of them exhibited AIE characteristics,which may be attributable to the inhibited intermolecular interactions due to the steric hindrance of the substitution.All of the solids presented thermoresponsive luminescence（TRL）upon disturbing their metastable aggregrates.Interestingly,2,7-h-CPDI and 3,5-h-CPDI realized adverse TRL with high constrast:2,7-h-CPDI achieved ON-OFF switching withΦ_F decreadsed from 20.7%to 0.97%,whereas 3,5-h-CPDI exhibited OFF-ON switching withΦ_F increased from 0.23%to 5.33%upon heating.In order to tune the band gap 2,7-h-CTDI,the counterpart of 2,7-h-CTDI was synthesized using thiophenyl as theπ-bridge.The band gap of 2,7-h-CTDI was effectively decreased in comparison with of 2,7-h-CPDI due to the lowered LUMO energy leved.Similarly,2,7-h-CTDI displayed remarkable solvatochromism with emission color from orange to dark red in solvents from benzene to DMSO.Notabely,aggregration of the thermoresponsive luminescence 2,7-h-CTDI was fluorescence quenching,which achieved TRL turn-on with high constrast.Finally,the band gap of the molecule was further reduced by using TCF as the acceptor on the basis 2,7-h-CPDI,2,7-h-CTDI were developed-3,6-CPBT and 3,6-CTBT,which emission color at near infrared.