Spontaneous Solution-Phase Redox Deposition of a Dense Cobalt(II) Phthalocyanine Monolayer on Gold Ursula Mazur, Maya Leonetti, William A. English, and K. W. Hipps* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: May 25, 2004; In Final Form: June 4, 2004 A dense monolayer of cobalt(II) phthalocyanine, CoPc, can be formed on a gold surface by spontaneous redox deposition followed by vacuum annealing at about 110 °C. CsCoPc(CN) 2 or KCoPc(CN) 2 in dilute (∼10 -6 M) ethanol solution rapidly forms a dense adlayer that, when washed with ethanol, can be imaged by scanning tunneling microscopy, STM, in air. This adlayer is converted to a monolayer of CoPc by vacuum annealing as confirmed by STM and X-ray photoelectron spectroscopy (XPS). This spontaneous surface redox adsorption process represents a novel method for depositing metallorganic complexes, which are ultimately only physisorbed on gold. Introduction Phthalocyanine (Pc) and porphyrin-based (P) electrical con- ductors are used extensively as building blocks for constructing stable 2D supermolecular electronic assemblies. The architec- tural forms of these structures include simple clusters, wires, arrays, and even designer surfaces. 1-7 Understanding the principles of architecture and the ability to control Pc assembly on surfaces continues to be of critical importance in the rational design and construction of functional optoelectronic nanoscale devices derived from phthalocyanines and porphyrins. The most basic monolayer architectures arise from vapor deposition. Simple complexes deposited on conductive surfaces in vacuum or from solution exhibit a close-packed arrangement that results from van der Waal’s interactions between the molecules. 2-4 Hipps and co-workers vapor deposited different 2D metal phthalocyanine and porphyrin assemblies on gold and imaged them by UHV scanning tunneling microscopy (STM) with remarkable submolecular resolution. 2,3,6 Bai demonstrated that monolayer molecular arrays of alkane-derivitized phthalocyanine can be assembled from solution on straight-chain alkane templates deposited on graphite. 4 Recently, Itaya formed both a pure MPc and a mixed CoPc and copper tetraphenylporphyrin (CuTPP) adlayer by immersing a Au single crystal into a benzene solution containing the parent compounds. By imaging the adsorbate system in aqueous solution, he demonstrated a structural dependence of the adlayer on the crystallographic orientation of the gold surface. 5 The 2D self-assembly of adsorbed Pc and P molecules also can be directed by selectively controlling noncovalent intermo- lecular interactions such as van der Waals attraction and hydrogen bonding. 6-8 The best-known examples of hydrogen bonding-driven surface assembly include data reported by Hipps, Yokoyama, and Griessl. Hipps and co-workers found the formation of a new well-ordered 2D structure with 1:1 stoichi- ometry of F 16 CoPc and NiTPP on gold by vapor-phase deposi- tion in UHV. 6 Yokoyama observed small clusters and chains of CN-substituted porphyrins vacuum deposited on Au(111) in a low-temperature STM environment. 7 Griessl and co-workers studied the extensive hydrogen bonding networks formed by trimesic acid on graphite. 8 All of the above methods for monolayer formation share a common failingsthey produce monolayers that are not substrate location-selective. Because the final layer is physisorbed rather than chemisorbed, almost any surface the adsorbate hits will support it (snow on a field). Given a complex device structure with exposed surfaces of various metals and semiconductors, all of the surfaces will be coated. A pathway to the 2D architecture of physisorbed complex molecules that has not been explored is the formation of self- organized molecular monolayers by spontaneous redox processes at the substrate surface. In this report, we present a generalized approach where a soluble species having a redox reaction induced by a metal electrode spontaneously decomposes to a well ordered physisorbed layer of insoluble product upon electrical contact with that metal. The chemistry described here is very different from that of the conventional thiol or thiolate precursor SAM films. In those systems, the final monolayer is chemisorbed to the substrate surface. 9 The procedure presented here does not require complex vacuum technology. Rather, it relies on solubility partitioning between solution and adsorbed phases, and it can create very reproducible well-ordered submonolayers, monolayers, and multilayers. We note that we previously observed the conversion of soluble FePcCl to insoluble FePc by spontaneous reduction on an aluminum surface. 10 In that work, however, we did not determine the surface structure of the adsorbed layer. Here we demonstrate the formation of a stable 2D assembly of CoPc from solution via spontaneous redox reactions between a biaxially substituted dicyano cobalt phthalocyanine salt, MCoPc(CN) 2 (M ) K, Cs), and a gold substrate. We selected the CoPc(CN) 2 -1 ion (shown in Figure 1) because of its chemical stability and solubility in simple organic solvents. Its X-ray structure has been reported. 11 The process by which the CoPc adlayer is formed on the gold electrode can be elucidated in part from the cyclic voltammetry (CV) studies of MCoPc(CN) 2 reported by Hanack and co-workers. 12 On the basis of their voltammetric data, Hanack proposed that in acetone MCoPc(CN) 2 is oxidized to a * Corresponding author. E-mail: hipps@wsu.edu. 17003 J. Phys. Chem. B 2004, 108, 17003-17006 10.1021/jp047752q CCC: $27.50 © 2004 American Chemical Society Published on Web 07/13/2004