Research On Crystal Growth,Structure And Optical Properties Of Solvated Crystalline Tris(8-hydroxyquinoline)Aluminum(Ⅲ)(Alq3)

Tris(8-hydroxyquinoline)aluminum(Ⅲ) (Alq3) was a kind of small molecules semiconductor materials with excellent luminescent properties. Since Tang and van Slyke demonstrated the possibility to design high-luminance low-voltage driven devices based on Alq3 in 1987, an intense effort has been dedicated to develop and improve organic electroluminescent diodes (OLEDs). According to the reports, Alq3 was a green emitter material with a good electron transporting property. Moreover, due to the rigid ball like geometry, high glass transition temperature (Tg～175℃) and polymorphic nature, Alq3 is readily sublimed to form amorphous thin films, which is beneficial to the fabrication and operation lifetime of OLEDs. Now, the OLED display devices and products based on Alq3 has used in TV, computer monitors and smartphone. Moreover, the solar cells, organic field-effect transistors and organic light pumped laser based on Alq3 are marching to the industrialization. Alq3 has two different isomeric forms, meridional and facial. The fac-Alq3 presents blue emission, instead of the typical green emission of mer-Alq3. It was mer-Alq3 amorphous thin film in the multilayer OLEDs used as the electron transport layer and also as the host for the emitter layer. But research shows that the mer-Alq3 amorphous thin film contains fac-Alq3 forms and with different preparation conditions, the content of fac-Alq3 was changed. Since the mixed isomeric structures of facial and meridional may involved, the amorphous films exhibit inconsistent fluorescence maxima and less repeatability in illuminant color, which are generally caused by the different preparation conditions.Compared to the amorphous film, however, single crystal possesses higher carrier mobility and excellent light-emitting properties due to the high chromophore density and ordered orientation of the molecular packing. The research for the crystallographic of Alq3 was begin in 1991, Mazhar-UI-Haque and his colleagues reported the structures of Alq3(C6H10O2)1/4, after that, several kinds of solvent phase structure been reported. But it was until to 2000 that Brinkmann obtained the crystal ofα-Alq3,β-Alq3 and γ-Alq3 prepared by vacuum deposition. Then, a lot of studies have been dedicated to the crystalline growth, polymorphic transformations and photoelectric property of crystalline Alq3. So far, crystallographic studies and spectroscopic characterization have shown the existence of different stable crystalline phases of Alq3:α, β,γ,δ and δ. Besides, eight solvated polymorphisms were obtained from crystallization in various organic solvents. These single crystals provide abundant information about the molecular structure, stereoisomers, as well as the intermolecular interactions, which enable people to gain in-depth knowledge of the structure-PL properties relationship of the crystalline Alq3. Compared to the amorphous film, however, single crystal has more excellent characteristics due to the high chromophore density and ordered orientation of the molecular packing. The theoretical calculation for β-Alq3 shown that it’s carrier mobility was 0.23±0.03 cm2 V-1 s-1, higher five orders of magnitude than the amorphous form. But the crystal size stoped the researce for Alq3 crystal. So, to grown bulk crystal Alq3 with high optical quality is very important for the next work. However, so far, few efforts were made on the crystal growth of high quality Alq3 single crystals. From the report data for Alq3 we can see that Alq3 decompose in 414℃, didn’t have any melting point at atmospheric pressure. Under the high temperature and vacuum condition, Alq3 was very tended to sublimation. Therefore, Alq3 was not suitable for the method such as the melt method. While, by the method of vapor deposition, the crystal was in mm magnitude and existed a lot of flaw. Compared all the condition, solution method was the best way to grown Alq3 crystal.In this article, the crystallization habits of Alq3 in several model solvents N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), methanol, ethanol, acetonitrile, ethylene glycol (EG), acetone and mixed solvents of DMF/chloroform have been studied. We found distinct growth behaviors and profiles of Alq3 in above solutions. High quality bulk single crystals or crystalline microstructures are obtained, which depends on the varied conditions of crystallization. Moreover, we obtained three new solvated phase of Alq3 in EG, chloroform and DMF respectively. This paper innovation points are followed as:1 By the cooling method and organic solvent volatilization, we obtained three new solvated phase of Alq3 in EG, chloroform and DMF respectively:Alq3((CH2OH)2), Alq3((CHCl3)2.5), Alq3(HCON(CH3)2). The experimental result and crystal structure of Alq3((CHCl3)2.5) and Alq3(HCON(CH3)2) were different with the literature report.2 X-ray crystal structural determination shows that the two new solvated phases of Alq3(HCON(CH3)2) and Alq3((CH2OH)2) belong to a couple of new crystal phases and isomers, the lattice parameters of which are different to any other polymorphisms reported in literatures. While Alq3((CHCl3)2.5) belong to monoclinic system C2/c space groups, different to all the crystal structural of Alq3 been reported.3 Form mixed solvents of chloroform and acetone, we obtained a new crystal with porous structure and X-ray crystal structural determination shows that it was crystalline in trigonal system R-3 space groups.