Self‐Assembly, Disassembly, and Reassembly of Gold Nanorods Mediated by Bis(terpyridine)–Metal Connectivity

Abstract

Since the advent of nanotechnology, a primary challenge has been to assemble materials from predesigned building blocks for the fabrication of nanometer-sized materials. Metal nanocrystals (NCs) have attracted significant attention due to their unique shape and size, as well as their tuneable electrical, optical and catalytic properties. A bottomup approach has generally been used for constructing the desired multi-unit composites from individual components. Although extensive studies have been performed on spherically shaped NCs, research on anisotropic nanoparticles, for example, nanorods (NRs) with a distinct 1D or linear structure, is limited. Since the properties of NCs are mostly sizeand shape-dependent, NRs have the potential to show novel as well as improved properties, such as in surface-enhanced Raman scattering (SERS) effects or optical and fluorescent properties, when compared to isotropic nanospheres (NSs). c,3] Geometrically, the aspect ratio (the length of major axis divided by the width of minor axis) of NRs is >1 and their cofacial assembly produces more angularly complex structures than that of the NSs. Side-to-side assemblies of NRs have been achieved by introducing active sites on the sides of the rods and subsequently connecting via DNA, electrostatic interaction, and more recently with click chemistry; however, arrangement of NRs in an endto-end orientation, for example, forming chains or branched structures, requires a regiospecific functionalization. Toward this goal, the tips of NRs have been modified with different metals, which were subsequently termed nanodumbbells, for participation in direct end-to-end assembly. It is well-known that Au NRs have different crystallographic facets, which comprise the ends and side surfaces. Recently, it was found that these side facets have a higher surface energy, and in turn, adsorb more bilayer surfactant, for example, cetyltrimethylammonium bromide (CTAB); notably, the yield of these surface-modified nanorods has been shown to be supplier (CTAB) dependent. Current progress on direct end-to-end Au NR assembly has focused on using biocompatible connectors, a,w-dithiols, and synthetic polymers. As well, organometallic connections, such as bis(terpyridine)-transition metal complexes, are also of interest due to their potential to impart unique electrical, optical, and photovoltaic properties. For example, hybrid Au NCs and bis(terpyridine)–Ru complexes have been fabricated and shown to exhibit enhanced electrochemical properties. Herein, we report the first example of a bottom-up approach for the end-to-end linear and branched assembly of Au NRs into multicomponent structures using [(disulfide-modified terpyridine)2–M ] (M=Fe or Cd) interconnectors and their facile disassembly and reassembly. The synthesis of disulfide-modified monoterpyridine (SStpy) 2, the [(SS-tpy)2–M ] complexes 3 (M=Fe), and 4 (M= Cd) is shown in Scheme 1. In a typical Gabriel synthesis, 4’(4-hydroxyphenyl)-2,2’:6’2’’-terpyridine was treated with N-(3-bromopropyl)phthalimide to afford the intermediate protected amine, which was subsequently deprotected (H2NNH2·H2O) to give the amine-modified monoterpyridine 1. The structure was supported (H NMR) by signals for the new OCH2 (4.17 ppm) and CH2NH2 (3.20 ppm) moieties; a dominate peak (ESI-MS) at m/z 383.3 [M+H] further confirms the structure. Disulfide-modified monoterpyridine 2 [a] Y.-T. Chan, Prof. Dr. G. R. Newkome Departments of Polymer Science and Chemistry The University of Akron, Akron, OH 44325-4717 (USA) Fax: (+1)330-972-2413 E-mail : newkome@uakron.edu [b] Dr. S. Li Excel Polymers, 14330 Kinsman Rd. Burton, OH 44325 (USA) [c] Dr. C. N. Moorefield Maurice Morton Institute for Polymer Science The University of Akron, Akron, OH 44325 (USA) [d] Dr. P. Wang ChemNano Materials, 2220 High St. Suite 605 Cuyahoga Falls, OH 44221 (USA) [e] Prof. Dr. C. D. Shreiner Chemistry Department, Hiram College, P.O. Box 67 Hiram, OH 44234 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000040.