Photoimaging through in-Situ Photopolymerization of Heterobifunctional Mesogenic Compounds in Liquid Crystalline State Suk Hoon Kang, † Ki Suk Jang, † Patrick Theato, ‡ Rudolf Zentel, ‡ and Ji Young Chang* ,† Department of Materials Science and Engineering, and Hyperstructured Organic Materials Research Center, College of Engineering, Seoul National UniVersity, Seoul 151-744, Korea and Institute of Organic Chemistry, Johannes Gutenberg-UniVersity Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany ReceiVed June 2, 2007; ReVised Manuscript ReceiVed September 3, 2007 ABSTRACT: A series of heterobifunctional mesogenic biphenyl esters having two different polymerizable groups, i.e., acryl and diacetylene groups, were synthesized and their thermal behaviors and polymerization investigated. All compounds showed enantiotropic transitions. Under POM, highly birefringent focal-conic fan textures appeared on heating and cooling from the isotropic melt. Compounds 6-8 having a butyl spacer between a biphenyl and a diacetylene group exhibited LC phases even at room temperature. The X-ray diffractograms of compounds 6-8 showed a set of reflections in the small-angle region. They consisted of more than three sharp diffraction peaks with d spacings in the ratio of 1:1/2:1/3, showing that the compounds had well-defined smectic A structures. For the photoimaging a mixture of 6 and a photoinitiator (2,2-dimethoxy-2-phenylacetophenone, 4 wt %) was cast on a glass plate and sheared with a cover glass at room temperature to result in an LC monodomain. The acryl group was then selectively polymerized by irradiation with low-intensity 365 nm UV light to yield a polymer film. Subsequent UV irradiation at 254 nm using a 100 W high-pressure mercury arc lamp through a photomask produced conjugated polyacetylene chains in the irradiated area. The polydiacetylene chains were fluorescent, and the patterned image was directly visualized by fluorescence microscopy. Introduction Polymerization of aligned monomers is an attractive way to obtain anisotropic materials having thermal and mechanical stability since the molecular ordering of monomers offers advantages such as the fact that it proceeds much faster and easier than that of macromolecules. In particular, photopoly- merizable liquid crystals (LCs) have attracted considerable attention because of their unique phase properties showing both fluidity and molecular ordering, which allow better control over their alignment. 1-3 LC molecules are macroscopically oriented by electric or magnetic fields, shearing, and on mechanically rubbed substrates, 4-9 and they have been polymerized in nematic, smectic, and discotic phases in order to maintain their aligned structures over a wide range of temperatures. 10-21 The photopolymerization of oriented polymerizable LC molecules also changes their optical properties and thus has potential applications in imaging. We previously reported photopolymerizable LC molecules having two diacetylene units 22-24 or chalcone groups. 25,26 The LC molecules were aligned macroscopically and polymerized by UV irradiation through a photomask to produce excellent patterned images. 24-26 This result is ascribed to the fact that the photoreaction disrupted ordered structures in an irradiated part. Aligned molecules remained in a masked part. Here we present the synthesis of novel heterobifunctional mesogenic compounds having two different polymerizable groups, i.e., acryl and diacetylene groups, and selective polym- erization in the LC state to produce an image on an anisotropic polymer film. We took advantage of the photoreactivity differ- ence between two groups in the selective polymerization. The acryl groups are known to undergo rapid polymerization upon irradiation even with low-intensity UV light in the presence of a photoinitiator. 27 Photopolymerization of diacetylenes requires relatively high energy for the initiation and proceeds topochemi- cally via 1,4-addition. 28 We were able to polymerize an acryl group first by irradiation with 365 nm UV light in the LC state to form an anisotropic film. A patterned image on the film was obtained by the subsequent polymerization of a diacetylene group through a photomask with 254 nm UV light (Scheme 1). Some of the compounds showed smectic phases even at room temperature, which allowed us to carry out the imaging process without any additional heating step. Experimental Section Measurements. 1 H and 13 C NMR spectra were recorded on a Bruker Avance DPX-300 (300 MHz) and Avance 500 (125 MHz) spectrometer. Fourier transform infrared (FT-IR) spectra were obtained with a Perkin Elmer Spectrum GX I using a KBr pellet. Elemental analysis was performed using a CE instrument EA 1110 analyzer. The differential scanning calorimetry (DSC) measurements were performed with a TA modulated DSC Q10 at a scanning rate of 10 °C/min. The optical microscopy study was performed using a Leica DM LP equipped with a Mettler Toledo FP 82HT heating stage and a Mettler Toledo FP 90 central process controller. X-ray diffraction (XRD) patterns were recorded by a Bruker NANOSTAR (Cu KR radiation, λ ) 1.54 Å). UV-vis spectra were obtained with the use of a Sinco 3150 spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-5301PC spectrofluoro- meter. Fluorescence images were obtained using Carl Zeiss- LSM510 confocal laser scanning microscope. Materials. 6-Bromo-1-hexanol (97%), 4′-hydroxy-4-biphenyl- carboxylic acid (99%), 1-hexyne (97%), 1-octyne (97%), 1-decyne (98%), 1-dodecyne (98%), 5-hexyn-1-ol (96%), 3-butyn-1-ol (97%), acryloyl chloride (98%), copper(II) acetate (98%), potassium iodide (99%), 2,6-di-tert-butylphenol (99%), N,N-diethylaniline (99%), N,N-dicyclohexylcarbodiimide (DCC, 99%), and N,N-(dimethyl- amino)pyridine (DMAP, 99%) were purchased from Aldrich and * To whom correspondence should be addressed. Phone: +82-2-880- 7190. Fax: +82-2-885-1748. E-mail: jichang@snu.ac.kr. † Seoul National University. ‡ Johannes Gutenberg-University Mainz. 8349 Macromolecules 2007, 40, 8349-8354 10.1021/ma0712293 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/20/2007