Perylene Monolayer Protected Gold Nanorods: Unique Optical, Electronic Properties and Self-Assemblies

Here we report the synthesis and characterization of organosoluble perylene monolayer protected gold nanorods. From H NMR, FT-IR, and differential scanning calorimetry experiments, the successful thiol exchange and stacking of perylene molecules on gold nanorods were confirmed. The resulting gold nanorods encapsulated with perylene thiol molecules via strong covalent Au−S linkages showed unique optical and electronic properties compared to the initial free perylene molecules and gold nanorods, indicating there were strong interactions between perylene chromophores and gold nanorods. When attached on gold nanorods, the perylene chromophores did not exhibit any typical UV−vis absorption or fluorescence emission signal, originating from the charge transfer from gold nanorods to perylene chromophores. However, the missing signals reappeared upon the addition of iodine, which detached the perylene molecules from gold nanorods. For the hybrid gold nanorods, particular electronic properties were also investigated by cyclic voltammetry and electron diffraction. Furthermore, with strong π−π intermolecular interactions, the perylene thiol monolayer protected gold nanorods were able to aggregate. When drying from highly diluted solution, gold nanorods formed well-organized side-by-side self-assembly arrays. ■ INTRODUCTION Nowadays, gold nanoparticles appear to enjoy a golden age owing to their diverse and distinct properties. Compared to isotropic spherical gold nanoparticles (GNPs), anisotropic gold nanoparticles such as gold nanorods (GNRs) are even more fascinating, for their unusual shapeand size-dependent behaviors. For example, quite different from the widely investigated spherical GNPs, GNRs exhibit distinguishing surface plasmon resonance (SPR), which is tunable in the visible and near-IR region. GNRs provide tremendous opportunities as well as challenges for insights into fundamental science which opens the doors to various applications in optics, sensors, and biological imaging devices, etc. It is well established that modifying the chemical composition of the GNR surface offers a versatile means to tune their properties. To reach this purpose, surface chemistry is the key. Until now, GNRs are mainly prepared in an aqueous medium by the seedmediated growth method, in which cetyltrimethylammonium bromide (CTAB) is used as a shape-directing surfactant. Although this bilayer CTAB structure on GNRs makes GNRs soluble in water, it is not very stable due to the dynamic ionic interactions: CTAB molecules can leave the GNR surface, resulting in irreversible aggregation of GNRs. To avoid this and introduce GNRs into various media such as organic solvents, it calls for the thiol molecules to form strong covalent Au−S bonds between surface molecules and GNRs. So far only a few examples have been developed for organosoluble thiol monolayer protected GNRs, and many other organosoluble GNRs were achieved mainly by electrostatic binding, polymer adsorption, or forming a thin organosilane shell over the CTAB layer, as the seemingly trivial exchanging of hydrophilic CTAB with hydrophobic organic thiol molecules is difficult. With organic thiol monolayer, the hybrid GNRs exhibit superior stability, good compatibility with organic media, and accessibility for further surface functionalization. For generating novel organosoluble monolayer protected GNRs, the perylene diimide (PDI) group is an attractive candidate which has never been used. PDIs with a large π-conjugated system are a well-known class of aromatic materials which have been widely utilized for field-effect transistors, electron luminescent displays, and photovoltaic devices. It is noteworthy to point out that as fluorescence-quenching and -enhancing effects were discovered when chromophores stood in appropriate distances to spherical GNPs, the newly developed aromatic dye-capped anisotropic Received: February 23, 2012 Revised: April 10, 2012 Published: April 12, 2012 Article