Synthesis and crystallographic structure of a novel photoresponsive azobenzene-containing organosilane† Nanguo Liu, a Darren R. Dunphy, b Mark A. Rodriguez, b Sarany Singer b and Jeffrey Brinker* ab a Department of Chemical and Nuclear Engineering and Center for Micro-Engineered Materials, the University of New Mexico, Albuquerque, NM, 87131, USA b Sandia National Laboratories, MS 1349, Albuquerque, NM, 87106, USA. E-mail: cjbrink@sandia.gov; Fax: 01-505-272-7336; Tel: 01-505-272-7627 Received (in Corvallis, OR, USA) 10th February 2003, Accepted 5th March 2003 First published as an Advance Article on the web 10th April 2003 A novel photoresponsive azobenzene-containing organosi- lane was synthesized via an isocyanato-amino coupling reaction, and its crystal structure was determined by X-ray crystallography. Materials containing photochromic azobenzene moieties have been of great interest for applications in photomemories, light- driven displays, optical data storage, optical switching, opto- mechanical actuation, and optoelectronics. 1–3 Photoirradiation may cause reversible changes in physical properties, such as surface energy and wettability, color, structure, morphology, dielectric constant, and polarity, as a result of the photo- isomerization of the azobenzene moieties. 4–9 Photoisomeriza- tion may also induce phase transitions and result in chiroptical behavior. 10,11 The synthesis of azobenzene-containing organo- silanes is of interest for their potential use in functionalizing mesoporous silica materials and sol–gel materials to produce photoresponsive composites in which material properties are controlled by light. In an effort to make photoresponsive sol–gel hybrid materials, Ueda et al. synthesized an azobenzene- containing organosilane, 4-methoxy-4A-(N-(3-triethoxysilylpro- pyl)-carbamoylmethoxy)azobenzene (MTAB). 12 However, the yield of MTAB was low (ca. 27% in the report), potentially limiting its further application. Here we report the synthesis of a novel azobenzene-containing organosilane, 4-(3-triethox- ysilylpropyl-ureido)azobenzene (TSUA) with high yield and high purity through an isocyanato-amino coupling reaction which has been reported previously to form urea or bisurea groups. 13–15 As shown in Scheme 1, TSUA was synthesized from triethoxysilylpropyl isocyanate (TESPIC) and 4-phenylazoani- line (PAA) in anhydrous tetrahydrofuran (THF). After refluxing for 24 h, hexane was added to facilitate the crystalization of TSUA at a low temperature (220 °C). Shiny needle-like orange crystals were precipitated out from the solution after 24 h. The yield was ca. 80%, discounting the loss of TSUA product dissolved in the solution.‡ The purity of the prepared TSUA crystals is high based on NMR spectral data and elemental analysis.§ Using thermogravi- metric analysis (TGA) and differential thermal analysis (DTA), we determined the melting point and decomposition tem- perature of TSUA crystals in argon to be 127.5 °C and 194.3 °C, respectively. Large single crystals were grown by dissolving TSUA in acetone (0.1 g ml 21 ) at room temperature and cooling to 25 °C. After 2 days, orange single crystals (ca. 2 mm 3 3 mm 3 14 mm) were observed. UV/visible spectroscopy was used to investigate the reversi- ble photo and thermal isomerization behavior of TSUA in a dilute ethanol solution (28 μg ml 21 ) (Fig. 1). From the absorption band at 362 nm (the p–p* transition of the trans isomer), we determined that e max = 2.59 3 10 4 M 21 cm 21 using the assumption of 100% trans isomer in (1a), which was supported with 1 H NMR data.§ UV irradiation with a Hg arc lamp (l max = 350 nm, 30 W) decreased the intensity of the 362 nm band and slightly increased the intensity of the 450 nm band attributed to the n–p* transition of cis isomer; a photostationary state (spectrum 1c, ca. 70% cis isomer based on Lambert’s law) was reached within ca. 10 min. Exposure to room light caused the reverse isomerization (cis ? trans). For example, from the photostationary state (spectrum 1c), room light exposure increased the intensity of the 362 nm absorption band (spectra 1d–h) progressively. Exposure of (1c) to room light for a long time (12 h) increased the trans isomer to ca. 96%. The Cis <–> trans isomerization process was observed to be reversible over three cycles. These results demonstrate the facile photo- isomerization characteristics of the TSUA compound in solution. The crystallographic structure of TSUA crystals was resolved using single crystal X-ray diffraction.¶ As shown in Fig. 2, there are 4 molecules in the unit cell (M1–M4, from left to right in Fig. 2). The distances between the two urea hydrogen atoms and the neighboring carbonyl oxygen atom are 2.03 and 2.08 Å, respectively, sufficiently close to form hydrogen bonds (N– H…O). The stable 6-membered ring formed by two H-bonds between two neighboring urea groups is energetically favorable, thus providing the main intermolecular interaction during the crystallization process. This is direct evidence of the H-bonding capability of urea ligands. 13–16 The azobenzene ligands are all in the trans conformation. Azobenzene planes in M1 and M2 are † Electronic supplementary information (ESI) available: crystallographic data of TSUA compound, including crystal data, atomic coordinates, bond length and bond angles, anisotropic displacement parameters, observed and calculated structure factors. See http://www.rsc.org/suppdata/cc/b3/ b301569f/ Scheme 1 Synthesis of TSUA compound. Fig. 1 UV/visible absorption spectra of: a) TSUA in EtOH (28 μg ml 21 ); b, c) after UV irradiation of (a) for 2 and 10 min; d, e, f, g, h) after room light exposure of (c) for 3, 12, 40, 60 min, and 12 h, respectively. This journal is © The Royal Society of Chemistry 2003 1144 CHEM. COMMUN. , 2003, 1144–1145 DOI: 10.1039/b301569f