Gold Nanoparticles Linked by Pyrrole- and Thiophene-Based Thiols. Electrochemical, Optical, and Conductive Properties G. Zotti* and B. Vercelli Istituto CNR per l′ Energetica e le Interfasi, c.o Stati Uniti 4, 35127 PadoVa, Italy A. Berlin* Istituto CNR di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133 Milano, Italy ReceiVed June 26, 2007. ReVised Manuscript ReceiVed October 8, 2007 A series of new alkyl-substituted pyrrole, bithiophene, and terthiophene thiols, terthiophene and sexithiophene dithiols, and polythiophene polythiol have been synthesized. The compounds form self- assembled monolayers on gold with high surface coverages generally in the range (2–4) × 10 -10 mol cm -2 as indicated by cyclic voltammetry and UV–vis spectroscopy. The thiols were reacted with 5 nm gold nanoparticles in toluene to form monodisperse, stable, and soluble thiol-capped gold clusters with the same gold core diameter, which were oxidatively coupled electrochemically (in solution or as films) and chemically (with iodine) to polymeric gold clusters. The dithiols (including ethanedithiol) and the polythiol formed analogous polymeric structures via layer-by-layer alternation with gold nanoparticles on gold-modified ITO and glass surfaces. The new materials were investigated by cyclic voltammetry, UV–vis and FTIR spectroscopy, and conductivity. The capped gold clusters display conductivities in the range 10 -7 –10 -2 S cm -1 and give solvoconductive responses fast and stable, which parallel the degree of swelling measured by QCM. The conductivities of the polymeric clusters are in the range 2 × 10 -2 -10 -1 S cm -1 . Comparison with the literature indicates 10 -1 S cm -1 as a practical limit to the conductivity in such systems. 1. Introduction Hybrid systems consisting of metal nanoparticles and organic compounds became an interesting research topic in recent years. The integration of metal nanoparticles into polymer matrixes attracts substantial research efforts directed to the development of hybrid materials for new catalytic, electronic, and optoelectronic applications. In particular, hybrid materials containing metal nanopar- ticles and π-conjugated polymers (CPs) 1 have unique proper- ties, and application of these materials in optoelectronic devices (such as solar cells, light-emitting diodes, electronic memories, etc.) is being explored intensively. 2 This combi- nation is of high interest because of strong electronic interactions between the nanoparticles and the polymer matrixes. The conductivity of the hybrid systems is improved in the presence of metal nanoparticles embedded into the polymers. Thus, the electroswitchable conductivity of the nanoparticles/polymer system may be also applied to control the sensing properties of the hybrid system. Among metal nanoparticles, gold nanoparticles (AuNPs) are particularly investigated, 3 and thiols are the linker moieties mostly used for them. Using R,ω-dithiols as spacer units between AuNPs, layer-by-layer (LBL) films attached to a solid substrate have been prepared. 4 Such materials show conductivities that mimic the behavior of semiconductors and that depend markedly on the length of the dithiol used to link the AuNPs together. Recently, a self-assembled poly- meric monolayer, chemisorbed onto a gold surface via multiple thiol or disulfide groups grafted on the polymer backbone, has been reported. 5 As compared to organothiol SAMs, this self-assembled polymeric monolayer shows improved stability because of cooperative binding via multiple thiol–gold or disulfide–gold bonds. Considering the combination of CPs and AuNPs, poly- aniline (emeraldine polycation) has been multilayered with AuNPs, 6 and close-packed planar arrays of gold nanoclusters were covalently linked to each other by the rigid, double- ended organic molecules aryl R,ω-dithiols. 7 Polypyrrole, an excellent material to be used as a substrate or matrix for deposition of metal nanoparticles, has been used as template * To whom correspondence should be addressed: tel (+39)49-8295868; fax (+39)49-8295853; e-mail g.zotti@ieni.cnr.it. (1) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (2) (a) See e.g.: Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (b) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (c) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077. (3) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (4) Brust, M.; Bethell, D.; Kiely, C.; Schiffrin, D. J. 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