Diphenylthienylamine-Based Star-Shaped Molecules for Electroluminescence Applications Iuan-Yuan Wu, Jiann T. Lin,* Yu-Tai Tao,* E. Balasubramaniam, Yi Zhen Su, and Chung-Wen Ko Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China, Taiwan Received January 23, 2001. Revised Manuscript Received May 30, 2001 Star-shaped compounds containing a triphenylamine as the central core and three diphenylthienylamines (NAr 1 Ar 2 (th): 3a, Ar 1 ) Ar 2 ) Ph; 3b, Ar 1 ) Ph and Ar 2 ) 3-tolyl; 3c, Ar 1 ) Ph and Ar 2 ) 1-naphthyl; 3d, NAr 1 Ar 2 ) carbazolyl) as the peripheral functional groups have been synthesized and characterized. These compounds exhibit four successive reversible one-electron redox processes except for 3d in which only two one-electron reversible oxidation waves are observed. The compounds 3a-d can be used as hole transport materials, and electroluminescent devices ITO/3/Alq [tris(8-quinolinolato)aluminum]/Mg:Ag emit green light characteristic of Alq. The device ITO/3d/BCP (bathocuproine)/Alq/Mg:Ag is blue emitting, in which 3d is the luminophore. Introduction Organic light-emitting diodes (OLED) have recently received considerable attention because of their poten- tial application to full-color flat-panel displays. 1 A number of low molecular weight organic materials and polymers have been studied for use as materials in organic electroluminescent (EL) devices. There have been extensive studies on layered organic EL devices with the aim of achieving high brightness and multicolor emission. 2 One major concern of organic EL materials is their durability, i.e., thermal and morphological stability. Amorphous materials may exhibit isotropic properties as well as homogeneous properties due to the absence of grain boundaries. Therefore, amorphous molecular materials with high glass transition temper- ature (T g ) are highly desirable. Furthermore, strain- driven failure 3 resulting from expansion of the charge transport layer in a thermally stressed OLED can be eased if materials of high T g are used. A very simple concept for the formation of amorphous glass is a nonplanar molecular structure because easy packing of molecules and hence ready crystallization can be avoided. Shirota have synthesized several novel families of organic π-electron starburst molecules which readily form amorphous glasses above room tempera- ture because of an increase in the number of conformers together with nonplanar molecular structures. 4 Among these, derivatives of triarylamine, which form amor- phous films on cooling from the melt or on casting appear to be interesting. 5 Recently, we have successfully synthesized star- shaped compounds hexakis[(diarylamino)thienyl]ben- zene containing a hexathienylbenzene core with which was attached six peripheral diarylthienylamine sub- stituents. 6 These compounds are promising hole trans- port materials because of their high thermal stability, high glass transition temperature, and low oxidation potential in comparison with commonly used hole transport materials, 1,4-bis[(1-naphthylphenyl)amino]- biphenyl (R-NPD; T g ) 100 °C) and 1,4-bis[(phenyl-m- tolyl)amino]biphenyl (TPD; T g ) 60 °C). 6 The lower oxidation potential of these compounds than those of R-NPD and TPD apparently are due to incorporation of an electron-rich thienyl ring 7 next to the nitrogen atom. Although Forrest found no clear correlation between the highest occupied molecular orbital (HOMO) energy and device quantum efficiency or turn-on voltage in devices fabricated from a series of triaryldiamines, 8 hole trans- port materials with different HOMO energies are po- tentially useful for multilayer OLED devices containing double hole transport layers. 9 Accordingly, we have set out to synthesize tris{[(diarylamino)thienyl]phenyl}- amine for potential hole transport materials. Herein, we report the synthesis, characterization, and fabrica- tion of LED devices using these materials. Experimental Section All reactions and manipulations were carried out under N2 with the use of a standard inert atmosphere and Schlenk * Corresponding author. Fax: Int. code + (2)27831237. E-mail: jtlin@chem.sinica.edu.tw. (1) Miyata, S., Nalwa, H. S., Eds. Organic Electroluminescent Materials and Derivatives; Gordon and Breach: New York, 1997. (2) Chen, C. H.; Shi, J. Coord. Chem. 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(9) (a) Shirota, Y.; Kuwabara, Y.; Okuda, D.; Okuda, R.; Ogawa, H.; Inada, H.; Wakimoto, T.; Nakada, H.; Yonemoto, Y.; Kawami, S.; Imai, K. J. Lumin. 1997, 72-74, 985. (b) Giebeler, C.; Antoniadis, H.; Bradley, D. D. C.; Shirota, Y. J. Appl. Phys. 1999, 85, 608. 2626 Chem. Mater. 2001, 13, 2626-2631 10.1021/cm0100568 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/29/2001