Syntheses And Luminescent Properties Of Quinacridone Derivatives With Carbazole Substituent Groups

Ouinacridone and its derivatives have been attracting attention as dopants inOrganic light emitting devices (OLEDs). High brightness and luminescentefficiency combined with good electrochemical stability have allowed them agood prospect in this field. However, the molecular aggregation and consequentemission-quenching phenomena, caused by the aggregation and the hydrogenbond and π-π interaction between quinacridone molecules, have effected the ELperformance of materials badly. Otherwise, the dopant concentration must beaccurately controlled within a narrow range, resulting that the device fabricationprocess becomes very hard and precise fabrication technologies must be employed.Thus, how to suppress the aggregation and increase the doping concentrationshave been a crucial problem for OLEDs' development.This thesis is designed at that background. We have attached carbazolegroups and 3,6-di-tbutyl-carbazole groups respectively to quinacridone molecules,resulting in 7 compounds in two series (see following figure). We expect theintroduction of bulky groups could weaken the intermolecular interaction andreduce the formation of aggregate. In addition, carbazole groups are introducedalso for enhancing the ability of carrier-transport of the materials.We have recorded the 1H NMR spectra of t-butyl-carbazole-quinacridonecompounds in CDCl3 over a range of concentrations. Increasing theconcentrations lead to the upshift of aromatic protons. The changes in thechemical shifts are attributed to the formation of face-to-face intermolecular π-πstacking aggregates in solution. (Chapter 2)We have obtained crystals of DCBQA and DCOQA. From X-ray structuralanalysis, we found the different lengths of alkyl chains could result in differentpacking structure. In DCBQA crystal, due to intermolecular π-π interactions,quinacridone core is sandwiched between two parallel carbazole groups of twoadjacent DCBQA molecules. In molecule layers, every quinacridone core issurrounded by four carbazole groups. So that the quinacridone cores are isolatedfrom each other and there are no π···π or hydrogen bonding interactions betweenNNOONR RNRR(CH2)n(CH2)nR=Hn=4 DCBQAn=6 DCHQAn=8 DCOQAR=t-butyln=4 DtCBQAn=6 DtCHQAn=8 DtCOQAn=10 DtCDQAquinacridone cores. That structure hinder the aggregation of the quinacridonecores, and suppress the quenching problems. Whereas every DCOQA molecule issurrounded by two alkyl chains and two quinacridone cores in crystal. Thequinacridone core is sandwiched by two parallel octyl chains from two adjacentDCOQA molecules through C-H···π interactions and contacts with two adjacentquinacridone cores through weak C-H···O=C hydrogen bonding interactions. Thisstructures completely avoid the π···π interactions between quinacridone groups,and consequently decrease the aggregation of quinacridone cores(Chapter 3).We studied on the luminescent properties of compounds (Chapter 4). Fromthe results, all the compounds display good PL quantum yields in solutions. theyields beyond 80% even at a high concentration of 1.0×10-4 M.The spectra insolution and doping films show concentration-dependent characterisitics. With theincrease of concentrations, the PL intensity first increases and then decreases.That phenomena indicates PL spectra are governed by the monomer fluorescenceof molecules and the emission intensity increases with the increase of freemonomer of molecules in dilute solutions. When concentrations beyond somelevel, PL spectra gradually display the molecular aggregate emission and spetraare governed by molecular aggregates. Otherwise, we studied on the thermalproperties of compounds. From the DSC curves, these compounds display high Tg,Tc and Tm. The extension of the alkyl chain length render molecules amorphousin nature.We fabricated the devices based on DtCzBQA with a configuration of[ITO/NPB/Alq3:DtCzBQA/Alq3/LiF/Al] which displays a good electroluminesc-ent property. Turn on voltage is 4V, and the luminance efficiency is 11.0 cd/A ata current density of 20mA/cm2. The brightness could reach a level of20000cd/m2 .From above results, the introduction of bulky groups could actually suppressthe aggregation and quenching problems. It is feasible to control the materials'properties through predominating the structure of molecules.