Synthesis and Characterization of Spiro-Triphenylamine Configured Polyfluorene Derivatives with Improved Hole Injection Doojin Vak, ² Jang Jo, ² Jieun Ghim, ² Chaemin Chun, ² Bogyu Lim, ² Alan J. Heeger, ²,‡ and Dong-Yu Kim* ,² Heeger Center for AdVanced Materials, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Gwangju, Korea, and Physics Department and Materials Department, UniVersity of California, Santa Barbara, California 93106 ReceiVed March 26, 2006; ReVised Manuscript ReceiVed July 13, 2006 ABSTRACT: We report on the synthesis of a spiro-fluorene derivative with bulky tert-butyl-substituted triphenylamine structure. The compound was synthesized in an attempt to enhance both hole injection and physical properties. A series of copolymers of the compound and a spiro-anthracenefluorene structure were synthesized, and their physical, optical, and electrochemical characteristics were investigated. The polymers showed good thermal stability with decomposition temperatures in excess of 389 °C and high glass transition temperatures in the range of 112-207 °C. The emission characteristics of the polymers were similar to that of dialkyl polyfluorenes. Cyclic voltametry studies revealed that the energy level of polymers can be tuned by adjusting the spiro-triphenylamine content with a tuning range of about 0.5 eV. Organic light-emitting diodes were fabricated using the polymers. All of the devices showed an emission in the deep blue region, and the copolymers showed improved hole injecting/transporting characteristics. Conjugated polymers have attracted considerable research interest in the past decade due to their potential applications in the areas of large area plat panel display and other optoelectronic devices. 1,2 Organic light-emitting diodes (OLEDs) are promising devices for use in full color plat panel displays and have a number of advantages over conventional devices such as a low driving voltage, wide viewing angle, thin film structure, and a simpler manufacturing process. 3 Polymer-based OLEDs have a considerable potential for use in flexible displays. 4 Various conjugated polymers have been developed for such applications. Among them, polyfluorene derivatives (PFs) have generated considerable interest as blue-emitting materials due to their high photoluminescence (PL) efficiency, wide band gap for blue emission, and thermal stability. 5-9 In addition, the ability to readily functionalize the C-9 position of the fluorene unit is one of the advantages of these types of compounds. Various solublizing alkyl chains or side groups have been introduced at this position. However, the C-9 position of the fluorene unit recently has been regarded as a problem. List et al. reported that the degradation of blue emission of PFs was due to oxidation of the C-9 position to form fluorenone, the emission of which is similar to the broad band emission of degraded PFs, in conjugated backbones the so-called keto defect. 10 Keto defects can be formed not only by photoirradiation or heat treatment but during the operation of device via electrooxidation. The appearance of a broad band emission of PFs in the green region, the so-called g-band, constitutes an important issue in terms of enhancing OLED lifetime and has been attributed to a physical defect, aggregate formation followed by excimer emission. 11-15 The results of more recent studies suggest that this can be attributed to chemical degradation. 16-19 Although keto defects are currently believed to be the most probable source of long wavelength emission, the chemical defect model does not completely explain some phenomena such as the enhanced stability by cross-linking, 20,21 copolymerization with bulky groups, 22 blending, 23 introduction of complex backbone struc- tures, 24 the dependency on the length of alkyl groups, 25 and the increase in long wavelength emission in the solution state by enhanced molecular interaction. 26 Therefore, both models are needed to completely explain all of the experimental observa- tions. Although the major reason for the appearance of g-bands is still not clear, most previous reports have concluded that the introduction of bulky phenyl side groups instead of long alkyl chains can effectively suppress the degradation phenomenon. 27-35 The reason for this is that bulky phenyl side groups not only enhance physical properties in a fully amorphous state and enhance glass transition temperature of polymers followed by reduced aggregate formation but also enhance the chemical stability toward oxidative degradation due to the more stable C(sp 2 )-C(sp 2 ) bonds at the C-9 position of fluorene units compared to the C(sp 2 )-C(sp 3 ) bonds of conventional alkyl- substituted PFs. One of the useful bulky phenyl side groups is a fluorene unit itself connected by a spiro-linkage, referred to as a spiro- bifluorene. A spiro-bifluorene contains two biphenyl units connected by a tetrahedrally bonded carbon atom, in which the planes of the biphenyl units lie perpendicular to each other. Once incorporated into PFs, this three-dimensional structure should prevent the approach of other polymer backbones, and therefore aggregate formation of the conjugated polymer backbone would be minimized efficiently. Although the introduction of a spiro- bifluorene into PF has been shown to alleviate the spectral stability problem, 32 its incorporation into the polymer structure greatly reduces its solubility in common organic solvents. As a result, no homopolymer of spiro-bifluorene has been reported. We recently reported on poly[10,10-bis(2-ethylhexyl)-10H- spiro(anthracene-9,9′-fluorene)-2′,7′-diyl] (PEHSAF), which contains a spiro-anthracenefluorene (SAF) rather than a spiro- bifluorene. 33 The SAF structure still contains the useful spiro- structure and, at the same time, permits the facile functional- ² Gwangju Institute of Science and Technology. ‡ University of California, Santa Barbara. * Corresponding author: Tel +82-62-970-2319, Fax +82-62-970-2304, e-mail kimdy@gist.ac.kr. 6433 Macromolecules 2006, 39, 6433-6439 10.1021/ma060683o CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006