Synthesis,aggregation Behavior And Optical Properties Of Novel Luminescent Materials Based On Carbazole/Arylamine

Luminescent materials based on carbazole / arylamine have been widely applied in the field of organic optoelectronics due to their excellent optical properties and holetransporting capabilities.Among them,carbazole derivatives with long-lived room temperature phosphorescence(RTP)and arylamine derivatives with highly efficient fluorescence have attracted tremendous attention in recent decades.It was found that both RTP and fluorescent properties are closely related to the molecular packing modes in aggregations besides the molecular structure.For example,most large ?-conjugated plane molecules have a dramatically decreased fluorescence quantum efficiency in aggregated state compared with that in dilute solution.Therefore,it is of great significance to study the influence of aggregation behaviors on the optical properties of materials.The large charge transfer interactions in donor-acceptor(D-A)structured carbazole / arylamine derivatives could effectively control the crystallinity and packing modes of materials,which thus have a great impact on the RTP and fluorescence performance.However,there is still a lack of systematic research on the relationship between the D-A structures with varied dipole strength and the aggregation behaviors as well as luminescent properties.Two important issues are included:(1)how the molecules with different electron push-pull abilities,as well as the molecules with symmetric(D-A-D)or asymmetric(D-A-D')structures,are arranged into the crystal lattice to show different luminescence;(2)how to balance the dipole-dipole interactions with other non-covalent interactions(such as Van der Waals forces and hydrogen bonds)between D-A molecules to control the arrangement of molecules and luminescence of materials.These researches are of great significance for understanding the aggregation behaviors and optical properties of luminescent materials based on carbazole / arylamine.Based on the above problems,we have done the following research work in this paper:1.The impact of crystallization ability and intermolecular coupling intensity on the lifetime of RTP were studied by adjusting the D-A dipole strength.firstly,two kinds of D-A-D structured carbazole derivatives NBCM and PBCM with different D-A strength were designed and synthesized.By studying optical properties of NBCM and PBCM,in single molecular and aggregated modes,we found that the two compounds exhibited RTP with long lifetimes of 558 and 482 ms,respectively,in crystal states.However,no RTP was observed in their solution and films.So both materials show crystal-induced dual emissions of fluorescence and RTP.Compared with PBCM,NBCM displayed a superior RTP performance owing to its stronger charge transfer effect and better crystallization capacity.Besides,the incorporation of pyridine group endows NBCM with a special response to trifluoroacetate acid and a reversible on/off conversion of emissions.These features of NBCM pave the way to its promising applications in antifake security,encryption,fingerprint imaging and sensors.2.The relationship between the molecular packing structures and the intensity of RTP were studied by adjusting the intermolecular interactions.two kinds of D-A-D' structured carbazole derivatives OYCZ and COOCZ with different length of alkoxy chains were designed and synthesized.It was found that both OYCZ and COOCZ showed crystal-induced dual emissions of fluorescence and RTP as well.The RTP lifetimes of the two compounds were 427.6 ms and 473.4 ms,respectively.Notably,OYCZ and COOCZ crystals exhibited excitation-dependent luminescent properties,that is to say,the relative intensities of their dual emission peaks varied along with the changed excitation wavelength,resulting in altered luminescent colors.However,the RTP and chromism of the two compounds disappeared in their film and powder states,which indicate that the packing modes and intermolecular forces have a significant role on the optical performance.The single crystal analysis proved that the introduction of long alkoxy chains could induce large numbers of hydrogen bonds,which lead to close packing between molecules.3.Finally,highly luminescent organic gelator T16 with D-A-D' structure were obtained by introducing a long alkyl chain into the excimer precursor ?-CN-TPA based on arylamine.By studying the optical properties of T16 in dilute solution,it was found that the long alkyl chain endowed T16 with dual channels of twisted charge transfer emissions,the intensity ratio of which was dependent on the irradiation wavelength and solvent polarity.Besides,different degrees of aggregation induced enhanced emission(AIEE)and gelation induced enhanced emission(GIEE)effects were also observed from T16 due to the incorporation of a long alkyl chain.The fluorescence quantum efficiency of T16 was increased more than 380 times in the xerogel,and even almost 500 times in the crystal compared with that of ?-CN-TPA crystal.The model of multilayer self-assembly with antiparallel H-stacking of T16 was proposed according to the results of SEM,XRD and theoretical calculations.The research in this chapter shows that the introduction of long alkyl chains into the D-A structured excimer precursors can effectively transform the forbidden transitions into the allowed fluorescent ones,which provides the possibility to control the luminescent property via regulating the radiation channel of materials.