Synthesis of High Refractive Index Polyimides Derived from 1,6-Bis(p-aminophenylsulfanyl)-3,4,8,9-tetrahydro-2,5,7,10-tetrathiaanthracene and Aromatic Dianhydrides Nam-Ho You, Yasuo Suzuki, Daisuke Yorifuji, Shinji Ando, and Mitsuru Ueda* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, H-120, O-okayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed May 1, 2008; ReVised Manuscript ReceiVed June 13, 2008 ABSTRACT: Highly refractive and transparent polyimides (PIs) containing a 3,4,8,9-tetrahydro-2,5,7,10- tetrathiaanthracene moiety in their main chains have been developed. These PIs were prepared from several dianhydrides such as 4,4′-[p-thiobis(phenylsulfanyl)]diphthalic anhydride (3SDEA), 4,4′-[(9H-fluorene-9- ylidene)bis(p-phenylsulfanyl)]diphthalic anhydride (FPSP), 4,4′-oxidiphthalic anhydride (ODPA), and a new sulfur- containing aromatic diamine, 1,6-bis(p-aminophenylsulfanyl)-3,4,8,9-tetrahydro-2,5,7,10-tetrathiaanthracene (BTTA), by a two-step polycondensation procedure. The PIs exhibit good thermal and optical properties such as glass transition temperatures higher than 213 °C, thermal decomposition temperatures (T 10% ) in the range of 390-443 °C, and optical transparency higher than 80% at 500 nm for a thickness of ca. 10 μm. Because of the very high sulfur content (28.4%) in the polymer main chain, the PI derived from BTTA and 3SDEA exhibits the highest refractive index, i.e., 1.769 at 633 nm. Introduction A high refractive index (high-n) and low birefringence (∆n) combined with good thermal stability and high optical transpar- ency are the basic concerns in designing optical polymer coatings for advanced display devices, such as organic light- emitting diodes (OLEDs), 1 microlens components for charge coupled devices (CCD), and high-performance CMOS image sensors (CISs), etc. 2-4 Recently, many conventional polymers combined with sulfur atoms have been developed to increase the refractive index of the polymers. 5 In fact, sulfur-containing polymers, including poly(methacrylates), 6 epoxies, 7 polyure- thanes, 8 and polyimides, have been used for advanced integrated optical applications. 9-11 Among them, polyimides (PIs) are good candidates for optical application owing to their excellent thermal stability, high chemical resistance, and high mechanical properties. Quite recently, we developed transparent and sulfur-contain- ing new polyimides derived from various aromatic dianhydrides and aromatic diamines in our laboratory. 12-17 All of them exhibited excellent thermal stability, a high refractive index, good transparency, and low birefringence. In particular, the polyimide derived from 2,7-bis(4-aminophenylsulfanyl)thian- threne (APTT) and 3SDEA exhibited the highest refractive index (up to 1.76) at a wavelength of 633 nm and birefringence lower than 0.01. To obtain further improved refractive indices while maintaining high thermal and optical properties, we designed and synthesized a novel diamine containing tetrathiaanthracene, since the tetrathiaanthracene unit containing four sulfur atoms in its molecular structure increases the sulfur content in the repeating unit. According to the Lorentz-Lorenz equation, 18 two alicyclic units in the tetrathiaanthracene moiety may yield low molar volumes, which are required for high refractive indices. The high polarizability and high percent of the sulfur atoms in the tetrathiaanthracene moiety may increase refractive indices of polymers. Moreover, the bent structures of the alicyclic and thioether units may prevent intermolecular packing between the polymer chains, which is required for high transparency and low birefringence. In this study, we report the synthesis of 1,6-bis(p-aminophe- nylsulfanyl)-3,4,8,9-tetrahydro-2,5,7,10-tetrathiaanthracene (BTTA) as a novel diamine and PIs derived from BTTA and several dianhydrides (3SDEA), 4,4′-[(9H-fluorene-9-ylidene)bis(p-phe- nylsulfanyl)]diphthalic anhydride (FPSP), and 4,4′-oxidiphthalic anhydride (ODPA). In particular, the PI derived from BTTA and 3SDEA exhibited the highest refractive index (1.7692) with a high glass transition temperature (>212 °C), high transparency (>500 nm), and low birefringence (0.0093). The structure-property relationships of the PIs, such as the thermal and optical properties, and the refractive indices of the PIs are investigated in detail. Experimental Part Materials. 1,2,4,5-Tetrachlorobenzene, 2-propanethiol, 4,4′-thio- bis(benzenethiol), and 1,2-dibromoethane were purchased from Wako, Japan. Pyridine, N,N-dimethylformamide (DMF), and N-meth- yl-2-pyrrolidone dehydrated (NMP) were used as received. All other chemicals were purchased from TCI Japan. ODPA was dried in vacuum at 120 °C for 4 h prior to use. 3SDEA and FPSP were synthesized in our laboratory according to our previous works. 15,16 Measurements. The NMR spectra were recorded on a Bruker DPX-300S spectrometer at the resonant frequencies at 300 MHz for 1 H and at 75 MHz for 13 C nuclei using CDCl 3 or DMSO-d 6 as solvent and tetramethylsilane as the reference. The FT-IR spectra were measured by a Horiba FT-120 Fourier transform spectropho- tometer. The UV-vis optical transmission spectra were recorded on a Hitachi U-3210 spectrophotometer at room temperature. The transmittance of PI films peeled from substrates was evaluated in the wavelengths range of 250 and 800 nm. Elemental analyses were performed on a Yanaco MT-6 CHN recorder elemental analysis instrument. Inherent viscosity was measured using an Ubbelohde viscometer with a 0.5 g dL -1 NMP solution at 30 °C. The thermal properties were estimated from a Seiko TG/DTA 6300 thermal analysis system (TGA) and a Seiko DSC 6300 differential scanning calorimetry (DSC) under a nitrogen atmosphere at a heating rate of 10 °C min -1 . Melting points of all monomers were measured using DSC analysis unless otherwise indicated. Dynamic mechanical thermal analyses (DMA) were carried out on PI films (30 mm long, 10 mm wide, and 50-80 μm thick) on a Seiko DMS 6300 instrument at a heating rate of 2 °C min -1 with a load frequency of 1 Hz in air. The glass transition temperatures values (T g ) were * To whom correspondence should be addressed: Tel +81-3-5734-2127, Fax +81-3-5734-2127, e-mail ueda.m.ad@m.titech.ac.jp. 6361 Macromolecules 2008, 41, 6361-6366 10.1021/ma800982x CCC: $40.75  2008 American Chemical Society Published on Web 08/16/2008