Enhancement of Optical and Electrochemical Properties via Bottom- Up Assembly of Binary Oligomer System Prakash Chandra Mondal,* ,†,‡,⊥ Megha Chhatwal, † Yekkoni Lakshmanan Jeyachandran, § and Michael Zharnikov* ,§ † Department of Chemistry, University of Delhi, Delhi, 110007, India ‡ Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel § Applied Physical Chemistry, Heidelberg University, Heidelberg, 69120, Germany * S Supporting Information ABSTRACT: Heterometallic, coordination-based, binary oligomer films were fabricated on SiO x - based solid substrates using successive layer-by-layer assembly of optically rich and redox-active polypyridyl complexes, Ru(pytpy) 2 ·2PF 6 (Ru-PT) and Os(pytpy) 2 ·2PF 6 (Os-PT) (where pytpy = 4′-pyridyl-2,2′:6′,2″-terpyridyl). The individual oligomer chains comprised alternating Ru-PT and Os-PT units connected via Cu 2+ , Pd 2+ , Ag + , Fe 2+ , Co 2+ , or Zn 2+ metallo-linkers. The growth and properties of the oligomer films were monitored in detail by UV-vis spectroscopy and cyclic voltammetry. The films exhibited a linear growth upon addition of the successive building blocks, with a joint grafting density of 3.9-5.0 × 10 14 metallo-ligands/cm 2 for the final oligomer films (10 layers), corresponding to a characteristic area of 2.0-2.5 nm 2 /oligomer. The only exception was the Pd 2+ -linked film on glass that showed an exponential growth, which, however, could also be changed to the linear mode by the introduction of a conductive substrate. The combination of two different functional molecular units in the oligomer chains resulted in enhancement of the optical window and in an increase in the number of the available redox states as compared to the analogous single component assemblies. ■ INTRODUCTION A paradigm shift from traditional disciplinary research to interdisciplinary fields motivates chemists to get engaged at the interface between chemistry and other disciplines, such as physics, material science, biology, nanofabrication, and engineering. 1-4 In particular, recent advances in developing molecular-based thin films are very impressive, leading to potential applications in catalysis, 5 sensors, 6,7 photovoltaics, 8,9 nanoscale electronic devices, 10 and electrochromic materi- als, 11,12 to name a few. An important issue in this context is modification and functionalization of solid substrates, such as glass, quartz, indium-tin oxide (ITO)-coated glass, silicon, and gold, with optically rich and redox-active molecular building blocks, accompanied by molecular level design of the respective systems. 13 Several successful methodologies within the general “bottom-up” and “top-down” frameworks were developed to fabricate such molecular assemblies. 14 Among them, the bottom-up, layer-by-layer (LBL) technique, described also as the stepwise coordination method, emerged as a facile, inexpensive, environment friendly, and well-controllable approach. 15-18 This approach is widely used to grow metal complex wires composed either of metal ions and bridging metal ions or of organo-metal complexes and bridging ligands or bridging metal ions. 16,17 In addition, LBL attachment of functional molecular units to preorganized molecular templates makes possible the preparation of surface-confined inorganic- organic hybrid materials (SURIOHMs) with layered architec- tures as well as with enhanced optical and electrochemical properties. 19 The oligomer films prepared from metallo-ligand/metal ions have some advantages as compared to the films composed of multidentate ligand/metal ion combination; e.g., they are believed to be more robust. Among different metallo-ligands, polypyridyl-based transition-metal complexes have attracted a particular attention owing to their unique and tunable optical, photochemical, and redox properties. 20,21 Such complexes are well-known for their versatility in supramolecular chemistry, applications in nanoscale materials, etc. 22,23 They have been immobilized on various substrates for different purposes. For instance, van der Boom et al., 24-26 Forster et al., 27-29 and others 30,31 fabricated a variety of robust monomolecular layers composed of such complexes. Recently, we have also assembled several representative heterometallo-ligand-terminated homo- oligomer films and heterogeneous molecular dyads comprising functionalized terpyridyl complexes on SiO x substrates and studied their optical, electrochemical, molecular recognition, and data storage properties. 32-34 The results for the heterogeneous dyads were especially promising since a combination of two different metallo-organic complexes at a single platform can lead to significant enlargement of the optical window, which can be of interest for potential Received: March 3, 2014 Revised: April 10, 2014 Published: April 14, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 9578 dx.doi.org/10.1021/jp502166k | J. Phys. Chem. C 2014, 118, 9578-9587