Bergman cyclopolymerization within the channels of functional hybrid nanocomposites formed by co-assembly of silica and polymerizable surfactant monomer { Chetan Jagdish Bhongale, Chung-He Yang and Chain-Shu Hsu* Received (in Cambridge, UK) 3rd March 2006, Accepted 12th April 2006 First published as an Advance Article on the web 26th April 2006 DOI: 10.1039/b603195a The Bergman cyclopolymerization of polymerizable surfactant monomer was carried out within the hexagonal channels of functional hybrid nanocomposite formed by co-assembly with silica. Ordered periodic mesoscopic materials allow the construction of composites with many guest types like organic molecules or polymers. Inclusion of dye molecules such as Coumarin 40, Rhodamine BE50, Oxazine 1 inside the nanopores has been demonstrated. 1–3 Nanocomposites that contain conjugated poly- mers confined within a silica matrix show enhanced conductivity, mechanical strength, processability, environmental stability, and other unique properties 4 that allow for potential use in light emitting diodes, information storage devices, optical signal processors, and sensors. To name a few, nanocomposite formation of polymers such as poly(phenylene vinylene), 5 polyaniline, 6 polydiacetylene, 7 poly(2,5-thienylene ethynylene), 4 polythiophene, polypyrrole, and polyacetylene 8,9 have been reported. Several synthetic efforts to obtain such nanocomposites used mainly slow procedures like monomer or polymer infiltration of inorganic nanostructures 5,10–13 or sequential deposition. 14,15 Such nanocom- posites are heterogeneous, exhibiting two distinct conjugated polymer environments, that is, polymers inside and outside the hexagonally arranged pore channels of the silica particles. However, self-assembly, one of the few practical strategies for making ensembles of nanostructures provides one solution to the fabrication of ordered aggregates from components with sizes from nanometers to micrometers. 16 It typically employs asym- metric molecules that are pre-programmed to organize into well- defined supramolecular assemblies. The use of polymerizable surfactants as both structure-directing agents and monomers in various evaporation-driven self-assembly schemes represents a general, efficient route to the formation of robust and functional nanocomposites. 7 In this research we utilize this self-assembly 4 route to form mesostructured polynaphthalene/ silica nanocomposites. One of the many approaches to form the polynaphthalenes (PN) is through Bergman cycloaromatiza- tion, 17,18 a remarkable isomerization in which an endiyne forms an arene 1,4-diradical. Here, we make use of this approach with a materials synthesis view-point. Amphiphilic polymerizable surfac- tant monomer was synthesized (Chart 1). It has two long alkyl chains with hydrophilic hydroxyl head groups at one end and the phenyl ring with two terminal acetylene groups ortho to each other at the tail end. (see Supplementary Information{ for synthetic details). Beginning with a homogeneous solution of soluble silica— tetraethoxy orthosilicate (TEOS), acid catalyst, and surfactant monomer in ethanol–water solvent, thin films were drawn by dip- coating or spin-coating. Solvent evaporation during the coating process enriches nonvolatile components and induces their co-assembly into liquid crystalline mesophases. 4 Polymerizaion of silica during the coating process freezes the mesophases and spatially organizes the monomer surfactant into mesostructures. These films were vacuum-dried overnight and kept immersed in benzene in thick-walled screw cap glass tubes which were capped in the glove box under nitrogen atmosphere prior to heating (see Supplementary Information for experimental details{). Fig. 1 A and B show scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the nanocom- posite formed after the evaporative self-assembly from the gel solution mounted on the substrate, respectively. The highly ordered nanocomposite shows a center-to-center spacing of 53 A ˚ . X-ray diffraction (XRD) patterns of the nanocomposites before (solid lines) and after (dashed lines) the Bergman cyclopolymeriza- tion are shown in Fig. 1D. Along with an intense diffraction peak at around 2h = 1.93, additional higher order diffraction peaks are ‘clearly’ observed at 2h = 3.3, 3.8, and 5. The intense one is attributed to the [100] orientation of the hexagonal mesophase. There was no change in the XRD peak position for the heat- treated (polymerized) sample, except a decrease in the intensity of the spectrum which indicates that the nanocomposite is intact even after heat-treatment (polymerization). Energy dispersive spectro- scopy (EDS) elemental analysis identified carbon, silicon and oxygen as expected for the nanocomposite prepared and is shown in Fig. 1C. The topochemicity generated during the co-assembly favors the Bergman cyclopolymerization. Formation of polymer within the hexagonal channels of the nanocomposite was verified by UV-vis Department of Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050. E-mail: chetan.ac90g@nctu.edu.tw; cshsu@mail.nctu.edu.tw; Tel: +86-5712121 { Electronic supplementary information (ESI) available: Scheme, synthetic and experimental details. 1 H NMR and EI-MS data of surfactant monomer (S1–S7). See DOI: 10.1039/b603195a Chart 1 Chemical structure of amphiphilic surfactant monomer. COMMUNICATION www.rsc.org/chemcomm | ChemComm 2274 | Chem. Commun., 2006, 2274–2276 This journal is ß The Royal Society of Chemistry 2006 Published on 26 April 2006. Downloaded by National Chiao Tung University on 26/04/2014 11:40:26. View Article Online / Journal Homepage / Table of Contents for this issue