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Langmuir 1993,9, 786-791
Mixed Monolayers Formed by the Self-Assembly on Gold of Thiol-Functionalized Anthraquinones and 1-Alkanethiols Litao Zhang,Tianbao Lu, George W. Gokel, and Angel E. Kaifer' Department of Chemistry, University of Miami, Coral Gables, Florida 33124 Received September 14,1992. In Final Form: December 8, 1992
The self-assembly and electrochemical properties of a series of thiol-functionalized anthraquinone derivativeswere studied on gold by cyclic voltammetry. The compounds were 1,&bis(lJ7-dithia-4-oxahepty1)anthracene-9,lO-dione (1),1,8-bis(4,7-dioxa-l,10-dithiadecyl)anthracene-9,l0-dione (21, and 1-(4,7-dioxa1,lO-dithiadecyl)anthracene-9,10-dione (3). Compounds 1-3 formed self-assembled monolayers on gold with surface coverages of 2.7 X 10-lo,2.5 X 10-'0, and 2.9 X 10-10 mol/cm2,respectively. All monolayers showed voltammetric responses in 0.1 M KOH that corresponded to the two-electron reduction of the anthraquinone subunit. However, the voltammetric response was not reversible, exhibiting large peaktu-peak potential splitting5 (6Cb80 mV at a scan rate of 0.1 VIS) and other distortions. The voltammetric reversibility increased substantially in mixed monolayers prepared by the competitive self-assembly of one of the anthraquinone derivatives and a l-alkanethiol (Clo to C d . Interestingly, the anthraquinone derivativescompetedquite effectivelywith alkanethiols for adsorptionsites on the gold surface. Compound 2 was found to be optimum in this regard because it has two thiol-terminated chains that may serve as anchoring points to the gold surface.
Introduction The self-assembly of organosulfur compounds on gold surfaces has been extensively studied during the last few years.' Self-assembled monolayers (SAMs as they were dubbed by Whitesides) have engendered considerable interest among electrochemists since these systems afford the possibility of controlling the molecular architecture at the electrode-solution interface. Initially, numerous studies focused on the investigation of the blocking properties of l-alkanethiol monolayers.2 Although Porter and co-workers have recently shown that the adsorbed l-alkanethiolates are electroactive? it is generally accepted that monomer functionalization is necessary to prepare monolayers exhibiting properties other than simple blocking of the electrode surface. For most applications in electrochemistry functionalization requires the incorporation of electroactive subunits into the structure of the organosulfurmonomer. Usually,the electroactivesubunits are sterically quite bulky and tend to disrupt the organized packing arrangement of monomers at the electrode surface.4 Therefore, it is a common practice to dilute the electroactive monomer with an excess of a different monomer having a large tendency to organize the interfacial structure. The first monomer, possessing the electroactive subunit, affords the sought-after electrochemical properties. The second one, usually a linear alkanethiol, provides the packing properties required to produce an organized monolayer assembly. However, the preparation of mixed self-assembledmonolayers is far from (1) (a) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J.Phys. Chem. 1992,96,5097. (b) Doblhofer, K.; Figura, J.; Fuhrhop, J. H. Langmuir 1992,8,1811. (c) Finklea, H. 0.;Hanshew, D. D. J.Am. Chem. SOC. 1992, 114, 3173. (d) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (e) Steinberg, S.; Tor,Y.; Sabatani, E.; Rubinstein, I. J. Am. Chem. SOC. 1991,113,5176. (0Obeng, Y.S.; Bard, A. J. Langmuir 1991,7,195. (g) Chidsey, C. E. D. Science 1991, 251, 919. (2) (a) Finklea, H. 0.;Snider, D. A.; Fedyk, J. Langmuir 1990,6, 371. (b) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332,426. (3) (a) Weieshaar, D. E.; Lamp, B. D.; Porter, M. D. J.Am. Chem. SOC. 1992,114,5860. (b) Widrig,C.A.;Chung,C.;Porter,M.D. J.Electroanal. Chem. 1991,310, 335. (4) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC. 1990,112, 4301. (b) Collard, D. M.; Fox,M. A. Langmuir 1991,7,1192. (c) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307.
trivial since kinetic and thermodynamic factors may affect the final composition of the m~nolayer.~ For instance, Whitesides and co-workers have observed that the monomer ratio in the self-assembled monolayer sometimes differs widely from the concentration ratio in the solution used to prepare the m~nolayer.~ Furthermore, ideal mixing of monomers in the monolayer might be difficult to attain, especially if their polarities are quite different, because this would tend to fwor contacts between identical monomer molecules over contacts between dissimilar molecules. This situation may result in the onset of microdomains or islands of one of the monomers in the sea formed by the second monomer. In this work we describe the voltammetric behavior of self-assembled monolayers prepared from a series of thiolfunctionalized anthraquinone derivatives (see structures below). We have investigated the electrochemicalbehavior of monolayers made from each one of these derivatives as well as monolayers prepared from mixtures of the anthraquinone derivatives and several l-alkanethiols.
**m 0
0
