Synthesis And Applications Of Multifunctional Organic Luminescent Materials Based On Carbazole And Triphenylphosphine Oxide Derivatives With Different Topologies

Organic light-emitting diode(OLED)has attracted extensive attention from the industry because of its rapid response,high contrast,low power consumption,flexible technology and other advantages.As an integral part of OLED,OLED materials have a profound influence on OLED device performance.In order to improve the device performance,more and more organic light-emitting materials with different luminescent pathways have been designed and synthesized.The thermal activation delay fluorescence materials,aggregation-induced emission materials and room temperature phosphorescence materials are main research hotspots in recent years.It is a new and important research direction for organic light-emitting materials to achieve multifunctional luminescence in a single molecule.Meanwhile,the electroluminescence performance of OLED devices is often affected by the imbalance of electron injection and transporting in the emitting layer.Bipolar materials,which can balance the carriers and expand the recombination area of exciton,are expected to be the preferred materials for fluorescence chromophore.To the best of our knowledge,the intramolecular charge-transfer and unique molecular orbit of heteroatom have shown to be very effective to modulate the excited properties of organic light-emitting materials.The most common way to construct bipolar materials is to combine electron donor(D)and electron acceptor(A)into a single molecule to form D-A or D-A-D* compounds.In typical molecular design,triphenylphosphine oxygen has attracted much attention due to its special structure and properties.The multifunctional properties of phosphorus atoms make it very easy to connect with other groups to form derivatives with phosphine as the core.On the other hand,the electronegativity of oxygen atom makes the structure of phosphine oxide highly polarized and has strong electron-withdrawing properties.Therefore,triphenylphosphine oxygen is often used as A-group of D-A-type compounds due to its excellent electron transport capacity,high triplet state energy level and unique tetrahedral space configuration.Similarly,carbazole derivatives are often used as D-group of D-A-type compounds due to their excellent luminescence efficiency and wide energy gap.In this paper,seven compounds based on these two groups were synthesized and the effects of different substituent groups and positions on the general optoelectronic properties of the compounds were compared.First,two topology-varied blue light-emitting materials of bis(3-(9-ethyl-9H-carbazol-3-yl)(phenyl)phosphine oxide(M-2Et Cz)and bis(4-(9-ethyl-9H-carbazol-3-yl)(phenyl)phosphine oxide(P-2Et Cz)were synthesized,and the effects of triphenylphosphine oxide and its position on the general optoelectronic properties of the obtained materials were investigated.Experimental results showed that the decomposition temperatures at 5% weight loss of compounds M-2Et Cz and P-2Et Cz in solid powder states were 437 and 495 °C,and their photoluminescent quantum yield in solution were 53.93 and 75.78%,which were significantly improved relative to those of parent 9-ethyl-3-phenylcarbazole(Et Cz-Ph)in the same state.Similarly,the introduction of triphenylphosphine oxide into Et Cz-Ph can lower the highest occupied molecular orbital levels from-5.36 e V to-5.54 and-5.58 e V for compounds M-2Et Cz and P-2Et Cz,while keeping their lowest unoccupied molecular orbital levels practically unchanged.All the compounds exhibited blue light emission with wavelengths of 393 nm for Et Cz-Ph,395 nm for M-2Et Cz and P-2Et Cz in dilute solution,and their blue light emissions were aggregation-induced emission active in a mixture solvent of water and tetrahydrofuran with water volume fractions from 60% to 90%.To further correlate the electroluminescent performance with the chemical structure of materials,deep-blue non-doped organic light-emitting diodes with highest external quantum efficiencies of 2.68% for M-2Et Cz and 2.45% for P-2Et Cz were obtained,and the color purity of electroluminescent blue light emission from P-2Et Cz was much better than that of M-2Et Cz.Moreover,it was noticeable that the doped device with P-2Et Cz as the host and 4-di-[4-(N,N-diphenyl)amino] styryl-benzene as the dopant achieved a maximum brightness of 10891 cd/m2,maximum external quantum efficiency of 2.04% and maximum current efficiency of 5.65 cd/A,which were much better than those of the same device hosted by M-2EtCz.Subsequently,in order to further study the effects of different groups and their substituent positions on the optoelectronic properties of the compounds,phenothiazine and phenoxazine were used to synthesize four D-A-D* compounds,M-Et Cz-PTZ,P-Et Cz-PTZ,M-Et Cz-PXZ and P-Et Cz-PXZ.The results showed that the thermal decomposition temperature of the M-Et Cz-PTZ,P-Et Cz-PTZ,M-Et Cz-PXZ and P-Et Cz-PXZ were 263,284,278 and 286 °C and the glass transition temperature were 142,90,153 and 98 °C,respectively.The optical band gaps of compounds M-Et Cz-PTZ,P-Et Cz-PTZ,M-Et Cz-PXZ and P-Et Cz-PXZ in toluene solution were 3.25,3.27,3.06 and 3.28 e V and their triplet energy levels were 2.51,2.53,2.74 and 2.74 e V,respectively.The HOMO and LUMO levels of the four compounds were calculated to be-5.29/-2.07,-5.38/-2.13,-5.31/-2.10 and-5.29/-2.17 e V.All the four compounds have the aggregation-induced emission-active properties and the fluorescence spectra in dilute solution appear red shift with the increase of solvent polarity.Interestingly,the fluorescence emission spectra of the four compounds presented different emission behavior in toluene solutions at different concentrations.As the concentration of the solvent decreased,the fluorescence spectra of the compounds showed double-peak emission.Among them,the emission of P-Et Cz-PTZ with the chromaticity coordinate at(0.29,0.25)was realized in dilute toluene solution at a concertation of 10-6 mol/L,which was the closest one to the white light emission.Through the study of seven compounds mentioned in the thesis,the following conclusions can be drawn: first,the introducing of triphenylphosphine oxide group into the parent molecule Et Cz-Ph can increase the rigidity of the compounds,so that the compounds have better thermodynamic stability.Moreover,compounds M-2Et Cz and P-2Et Cz,formed by the introduction of triphenylphosphine oxide groups,are D-A-type intramolecular charge transfer compounds,which are superior to the parent molecule Et Cz-Ph in terms of photophysics,electrochemistry and device performance.Secondly,four other D-A-D*-type compounds,M-Et Cz-PTZ,P-Et Cz-PTZ,M-Et Cz-PXZ and P-Et Cz-PXZ,were synthesized on this basis.Due to the difference in the electron-donating characteristics of substituent groups,the photophysical and electrochemical properties of obtained compounds are also different,which makes it feasible to tune the optoelectronic properties of the compounds in a wide range.To sum up,the organic blue luminescent materials based on carbazole and triphenylphosphine oxide derivatives mentioned in this paper illustrate excellent thermal stability,intramolecular charge transfer characteristics,wide energy gap and aggregation induced luminescence characteristics,which have broad application prospects in the field of multifunctional OLED.