In situ synthesis and assembly of gold nanoparticles embedded in glass-forming liquid crystals.

It is of tremendous interest to use nanoparticles (NPs) as building blocks for the fabrication of multifunctional mesoscale assemblies by organizing them into twoand threedimensional structures with precise size and shape control for applications based on their collective properties. 2] Several ways have been reported to create ordered assemblies of NPs by using templates that have sites with specific binding to NPs, for example, self-assembled monolayers, surfacemodified polymers, electrophoretic assembly onto suitable substrates, electrostatic attachment to Langmuir monolayers at the air–water interface and air–organic solvent interface, hydrogels, and by diffusion into ionizable fatty lipid films, DNA, and so on. However, in those instances the synthesis and assembly of NPs are done in multiple steps. Therefore, it would be extremely useful if one could develop a protocol in which the templates used for the assembly can also provide active sites for reducing the nanoparticle precursors so that the formation and assembly of the nanoparticles are achieved in one single step. An ideal candidate template for NP assembly would be liquid crystal (LC) materials that have direct function in display applications, ferroelectric materials, and photonics, among others, and could be tailored/tuned easily in any twoor three-dimensional structures by using external stimuli such as light and electric or magnetic fields. LCs are self-organizing materials characterized by their uniaxial, lamellar, helical, or columnar arrangement in nematic, smectic, cholesteric, and discotic phases, respectively. With their molecular arrangements frozen, glassy LCs (GLCs) represent a class of materials that combine properties intrinsic to LCs with those common to polymers. The advantage of GLCs is that they can be processed like conventional LCs but maintain their optical property and molecular alignment when cooled into a solid phase. Thus, GLCs would be suitable hosts for synthesizing and aligning the NPs. LC-embedded metal nanoparticles (MNPs) and inorganic porous networks have been studied in recent years. However, most of these processes involve multiple steps that involve separate synthesis of metal nanoparticles, their functionalization, and doping of particles in LC domains. Another major problem is destabilization of LC domains during incorporation of MNPs owing to chemisorption of LC mesogens on the NPs. Hence, we reasoned that the development is necessary of a suitable procedure in situ to generate GLC–MNP conjugates whereby chemisorption is reduced and where well-aligned NP arrays produce novel hybrid materials. Herein, we demonstrate a novel approach to obtain the MNPs embedded in LCs based on the synthesis in situ of MNPs by using glass-forming mesogens without any external reducing and stabilizing agents. New low-molecular-weight mesogens were synthesized that could reduce metal salts to nanoparticles as well as form glassy LC phases. These thermotropic liquid-crystalline systems were used for synthesis in situ and spontaneous assembly of MNPs in LC phases into mesostructures. We also successfully demonstrate to some extent the ability to control shapes of NPs obtained in different LC phases owing to the templating effect of the inherent LC domains. The major advantage of this approach is that we could achieve assembly of MNPs directly from metal ions without a separate synthesis of MNPs and their later derivatization. In the present case, MNPs embedded in LCs were highly stable and showed LC properties at room temperature. We synthesized new amphiphilic low-molecular-weight mesogens, A11, A7, and A5, containing cholesterol (mesogenic core) and aniline (known to reduce HAuCl4 ) groups with different methylene chains (Scheme 1). A detailed synthetic scheme of amphiphiles is discussed in the Supporting Information. The LC properties of newly synthesized mesogens (e.g. A11 to A5) were examined using a polarized optical microscope (POM) and differential scanning calorimetry (DSC) studies (Figures 1 and 2, respectively). Mesogen A5 melts to a chiral smectic A (SmA*) phase at 67.9 8C before changing into an isotropic phase at 79.8 8C. The SmA* phase was identified from its characteristic homeo[*] Dr. V. A. Mallia, Dr. P. K. Vemula, Prof. G. John Department of Chemistry The City College and the City University of New York New York, New York 10031 (USA) Fax: (+1)212-650-6107 E-mail: [email protected] Homepage: http://www.sci.ccny.cuny.edu/chemistry/faculty/ john.html