Published: November 23, 2011 r2011 American Chemical Society 14828 dx.doi.org/10.1021/la201308g | Langmuir 2011, 27, 14828–14833 ARTICLE pubs.acs.org/Langmuir Self-Assembly of Insoluble Porphyrins on Au(111) under Aqueous Electrochemical Control Sedigheh Sadegh Hassani, †,‡ Youn-Geun Kim, † and Eric Borguet* ,† † Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States ‡ Nanotechnology Research Center, Research Institute of Petroleum Industry, Tehran, Iran ’ INTRODUCTION Porphyrins are important components of many organic com- pounds found in biological systems. 1,2 For example, hemoglobin is a metalloprotein with a porphyrin structure at its core which is responsible for oxygen transport in the blood. Porphyrins have a flat, robust architecture. 3 They play an important role in the design of extended self-assembled adlayers of controlled size and shape. Porphyrins are of interest because of the versatile electro- nic and photonic properties, 4 a consequence of their structural flexibility, 5 and interesting redox properties. 6 The large number of fundamental surface science studies of ordered monolayers of porphyrins on single-crystal surfaces are driven in part by potential applications as active elements in optical devices, 7 chemical sensors, 8À10 solar cells, 11,12 functional supramolecular materials, 13 artificial photosynthesis, information storage, 14 and catalytic processes. 12 In recent years, extensive studies have been performed on the self-assembly of organic molecules on surfaces. 15,16 Several techniques have been used to investigate porphyrins on surfaces: ultraviolet/visible absorption spectroscopy (UV/vis), 17,18 X-ray photoelectron spectroscopy (XPS), 19,20 cyclic voltammetry, 17,19 and scanning tunneling microscopy (STM). 12,13,21À23 These techniques make use of different physical principles to probe the monolayer and thus provide independent methods for the characterization of porphyrins on surfaces. STM, because it can provide atomic-scale images of adlayer molecules, has been used to characterize porphyrin derivatives in air, 22,24,25 under ultrahigh vacuum (UHV), 1,12,13,26À28 and under liquid 29À32 after deposi- tion on different metal single-crystal surfaces 23,33À35 and highly oriented pyrolytic graphite (HOPG). 1,13 STM images can be employed to determine the size and conformation of molecules on the surface as well as the lattice parameters of the ordered structures that they form. Under electrochemical conditions, the electrode potential con- trols the excess charge on the electrode surface. This can alter the interactions between the surface and the electrolyte and solutes. Thus, the surface potential can play an important role in the ordering of adsorbates, in the adsorption or desorption of molecules on the electrode surface, and in redox reactions. 16,29À31 Electro- chemical scanning tunneling microscopy (EC-STM) is a powerful technique for the study of electrified interfaces. 36 EC-STM allows the characterization of the topography of an electrode surface under potential control with atomic resolution enabling, for example, the investigation of electrochemical oxidationÀreduction reactions at the single-molecule level at metal Àelectrolyte interfaces. 14,36À38 An important goal of this report is to understand the adsorp- tion of one of the simplest porphyrins, tetraphenylporphyrin (TPP) Figure 1, at the single-molecule level under electrochem- ical control. All previous publications related to STM studies of TPP monolayers were performed in UHV. 1,12,13,27,39,40 Although there has been tremendous progress in studying porphyrin self- assembly at solidÀliquid interfaces with STM, there has been no detailed report of the metal-free tetraphenyl porphyrin (TPP). In this article, we present the first detailed investigation of TPP Received: April 10, 2011 Revised: July 19, 2011 ABSTRACT: Self-assembled monolayers of a water-insoluble porphyrin, tetraphenyl porphyrin (TPP), in the presence of an aqueous electrolyte were characterized in situ with electrochem- ical scanning tunneling microscopy (EC-STM) at working electrode potentials of between 0.5 and À0.2 V. Isolated domains of TPP monolayers with differing orientation were observed on Au(111) in 0.1 M HClO 4 over this entire potential window. Individual TPP molecules could be resolved over a range of 700 mV, from open circuit potential (OCP) to near the hydrogen evolution potential. The unit cell is square, and the distance between neighboring molecules is about 1.4 ( 0.1 nm. High-resolution images allow the internal molecular structure to be discerned. No changes in the STM contrast of individual molecules were observed as the potential was changed. In a neutral electrolyte (0.1 M KClO 4 , pH ∼6), the potential range of stability of ordered structures is reduced. On HOPG, TPP forms ordered hexagonal structures with a lattice constant of about 2.6 nm in the double-layer potential region in 0.1 M HClO 4 .