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Notes Electrochemistry of Alkylpyrrole Monolayers Assembled by Self-Assembly and Langmuir-Blodgett Techniques Young-Ha Kim and Yeon-Taik Kim* Department of Chemistry and Center for Artificial Lipid Membranes, Yonsei University, Seoul, Korea Received January 23, 1998. In Final Form: December 10, 1998
We prepared bis(ω-(N-pyrrolyl)-n-undecyl) disulfide (BPUS) self-assembled (SA) monolayers and octadecylpyrrole (ODP) Langmuir-Blodgett (LB) monolayers in order to understand electrochemical properties of surfaceconfined pyrroles and their molecular structure. The electrochemistry of BPUS SA monolayers shows a remarkable difference in the presence and absence of trace amounts of oxygen and/or water indicating the formation of degradation products upon electrochemical oxidation of the surface confined pyrrole. The transfer efficiency of the ODP LB monolayer was approximately 7-10% of the SA BPUS monolayer, which suggests that the ODP molecules lies flat on the surface. Electrochemistry at two-dimensional surfaces has been an important subject in order to understand fundamental charge-transfer kinetics.1-4 The self-assembly (SA)5 and the Langmuir-Blodgett (LB)6 techniques provide a unique opportunity to build a dielectric layer between a metal electrode and an electrochemically active species. Furthermore, a simple synthetic approach can control the dielectric layer thickness to vary the charge-transfer barrier. Langmuir monolayers of alkyl-substituted monomers such as pyrrole, aniline, thiophene have been extensively studied in order to understand the packing structures and electronic properties after they were transferred to solid substrates.7 To date, however, there were no direct * To whom correspondence should be addressed. E-mail ytkim@ alchemy.yonsei.ac.kr. (1) Chidsey, C. E. D. Science 1991, 251, 919. (2) Weber, K.; Creager, S. E. Anal. Chem. 1994, 66, 3164. (3) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (4) (a) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (b) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453. (5) Ulman A. in Introduction to Organic Thin Films from LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (6) Roberts, G. G. Ed. In Langmuir-Blodgett Films; Plenum Press: New York, 1990. (7) (a) Hong, K.; Rosner, R. B.; Rubner, M. F. Chem. Mater. 1990, 2, 82. (b) Hong, K.; Rubner, M. F. Thin Solid Films 1989, 179, 215. (c) Rikukawa, M.; Rubner, M. F. Langmuir 1994, 10, 519. (d) Bodalia, R. R.; Duran, R. S. J. Am. Chem. Soc. 1993, 115, 11467. (e) Sigmund, W. M.; Duran, R. S. Macromolecules 1993, 26, 2616. (f) Sigmund, W. M.; Bailey, T. S.; Hara, M.; Sasabe, H.; Knoll, W.; Duran, R. S. Langmuir 1995, 11, 3153. (g) Schmelzer, M.; Roth, S.; Bauerle, P.; Li, R. Thin Solid Films 1993, 229, 255. (h) Sagisaka, S.; Ando, M.; Iyoda, T.; Shimidzu, T. Thin Solid Films 1993, 230, 65. (i) Iyoda, T.; Ando, M.; Kaneko, T.; Ohtani, A.; Shimidzu,; Honda, K. Tetrahedron Lett. 1986, 27, 5633. (k) Shimidzu, T.; Iyoda, T.; Ando, M., Ohtani, A.; Kaneko, T.; Honda, K. Thin Solid Films 1988, 160, 67.
comparisons between the SA and LB monolayers that have structural similarities. The objective of this study was as follows. What are the similarities and differences in the electrochemical properties of monolayers assembled by the SA and LB techniques? We used the SA and LB technique to build surfaceconfined pyrrole monolayers with a dielectric layer between an electrode (gold and highly oriented pyrolytic graphite) and the electrochemically active species. A schematic representation of the monolayers of bis(ω-(Npyrrolyl)-n-undecyl) disulfide (BPUS) and N-octadecylpyrrole (ODP) is seen in Figure 1. Experimental Section Chemicals. BPUS and ODP were synthesized, and the synthetic method was previously reported.8 Freshly distilled CH3CN and hexane were used for solvents. Tetrabutylammoniumhexafluorophosphate (Bu4N(PF6)) was purchased from Aldrich and stored in a desiccator until its use. Instruments. A Princeton Applied Research 273A potentiostat equipped with software provided by the company was used to produce voltammograms. A three-electrode, singlecompartment glass electrochemical cell was used with a silver wire quasireference electrode and platinum gauze counter electrode in the nonaqueous electrochemical experiment. On the other hand, an Ag/AgCl reference electrode was used in the aqueous electrochemical experiment. Thin gold film working electrodes were prepared by vapor deposition of 100 Å of Cr undercoat followed by 1000 Å of Au onto glass microscope slides at a pressure of 2 × 10-6 Torr. For inert atmosphere experiments, a Vacuum Atmospheres Dri-Lab drybox equipped with a VAC Dri-Train model HE-493 was used. Preparation of Self-Assembled BPUS. The gold film working electrodes were removed from the vacuum chamber and immediately placed in 1 mM solutions of BPUS in hexane for 12-24 h. After completion of the monolayer formation, the solution was replaced with a 0.1 M Bu4N(PF6) electrolyte solution for oxidation of surface-confined pyrroles. Preparation of ODP LB Film. The LB technique was used to prepare the monolayer of ODP. A commercially available NIMA 622D2 type trough (NIMA Technology, Coventry, England) was used. The monolayer was transferred by dipping down a highly oriented pyrolytic graphite (HOPG) substrate. The trough temperature was maintained at 20 ( 0.1 °C using a thermostat bath circulator (model RBC 10, JEIO Tech. Seoul, Korea).
Results and Discussion Voltammetry of BPUS on Gold. The formation of BPUS monolayers by self-assembly was confirmed by electrochemical quartz crystal microbalance as we reported.9,10 After the monolayer formation, the solution was replaced with a 0.1 M pure electrolyte solution. The surface-confined pyrroles were oxidized by cycling the potential from 0.0 to 1.4 V vs Ag quasireference electrode as seen in Figure 2. A distinct oxidation peak at 1.15 V is identified in the first scan but disappears in the (8) Zong, K.; Brittain, S. T.; Wurm, D. B.; Kim, Y.-T. Synth. Commun. 1997, 27, 157. (9) Shin, M.; Kim, E.-Y.; Kwak, J.; Jeon, I. C. J. Electroanal. Chem. 1995, 34, 87. (10) Wurm, D. B.; Zong, K.; Kim, Y.-T.; Shin, M.; Jeon, I. C. J. Electrochem. Soc. 1998, 145, 1483.
10.1021/la980098j CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999
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a
Figure 1. Schematic representation of BPUS on Au and ODP on HOPG.
Figure 2. A cyclic voltammogram of the BPUS monolayer oxidation. The scan rate is 50 mV/s. The electrode area is 0.2 cm2. The pyrrole oxidation peak is shaded.
subsequent scans. This tells that the oxidized compound is not electrochemically active. This type of behavior was well documented in the literature11-14 although the oxidation peak was interpreted in different ways. Willicut and McCarley claimed that the pyrrole monomers at the surface converted to a conducting polymer or oligomer film in propylene carbonate.11a On the other hand, the same authors reported the pyrrole monolayers at the surface did not show apparent polymerization in CH3CN.11b Sayre and Collard proposed the formation of an electrochemically inactive species of the surface-confined pyrrole by a nucleophillic attack of water.12 The voltammetric behavior of BPUS, however, is very different in an air and a glovebox environment, where little oxygen and water exist. Figure 3 shows two cyclic voltammograms for the oxidation of BPUS. Contrary to Figure 2, the first cyclic voltammogram obtained in the glovebox shows the oxidation peak shifted to a higher anodic potential (1.3 V) with a shoulder peak around 1.15 V as seen in Figure 3a. After a small amount of air was introduced to the glovebox, the second cyclic voltammogram was obtained and is shown in Figure 3b. The cyclic voltammogram shows distinct two oxidation peaks. One is at 1.15 V and the other is at 1.3 V. The total charge consumed during the oxidation is identical regardless of the experimental condition (i.e., inside or outside the glovebox). The oxidation charges in Figures 2 and 3 are integrated to be about 60 ( 5 µC/cm2. This means the electrochemical reactions undergo the same number of (11) (a) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10824. (b) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (c) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759. (d) Willicut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269. (12) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (13) Wurm, D. B.; Brittain, S. T.; Kim, Y.-T. Langmuir 1996, 12, 3756. (14) Smela, E.; Kariis, H.; Yang, Z.; Mecklenburg, M.; Liedberg, B. Langmuir 1998, 14, 2984.
b
Figure 3. (a) A cyclic voltammogram of BPUS in a glovebox. (b). A cyclic voltammogram of BPUS in a glovebox contaminated with a small amount of ambient air. The scan rates are 50 mV/s. The pyrrole oxidation peaks are shaded.
electron transfer in the both cases and some fraction of the monolayers was converted to electrochemically inactive material at the first oxidation potential (1.15 V). Thus, the peaks at 1.15 and 1.3 V are different electrochemical processes. We propose that the first peak at 1.15 V indicates the formation of a ketone type degradation product by reacting between the oxidized pyrrole and a trace amount of oxygen and/or water. The origin of the second peak is currently under investigation. Voltammetry of ODP on HOPG. A typical surface pressure (π) vs area (A) per molecule for octadecylpyrrole (ODP) on a pure water phase at 20 °C is shown in Figure 4a. We can detect a surface pressure onset at 24 Å2/ molecule. The limiting area per molecules was obtained by extrapolating the linear portion over 18 Å2/molecule to zero surface pressure as shown in Figure 4a. An average value of approximately 23 Å2/molecule in 15 different runs was obtained. At the surface pressure of approximately 8 mN/m, it shows a transition and the film can be further compressed even to 10 Å2/molecule, which indicates the formation of amorphous multilayers. This behavior tells that the ODP Langmuir monolayers form an ordered twodimensional structure before a pressure of 8 mN/m is reached. The structure becomes amorphous multilayers at a surface pressure higher than 8 mN/m. Thus, the LB monolayers were transferred at the surface pressure in the range of 3-8 mN/m by dipping HOPG substrates downward so that the pyrrole group was exposed to air. The electrochemical oxidation of the ODP LB monolayers did not occur in a CH3CN solution. The cyclic voltammogram was exactly the same as that of a bare HOPG electrode in the blank electrolyte solution. This tells that the LB monolayer was completely dissolved in the CH3CN solution. Thus, we ran the experiment in a 0.1 M H2SO4 solution and obtained a cyclic voltammogram with the ODP LB monolayer electrode transferred at 3.0
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fore the pyrrole oxidation is comparable to that of a bare HOPG electrode in the 0.1 M H2SO4 solution. This indicates that there is almost no dielectric barrier for the electron transfer. Thus, the ODP molecule should be almost flat at the surface. This might be caused by a strong hydrophobic interaction between HOPG and the methylene group of ODP suggesting that an HOPG substrate is not appropriate to have a perpendicular orientation of alkylpyrrole at the surface. Sigmund and co-workers reported a rodlike image of 3-hexadecylpyrrole and 3decylpyrrole LB monolayers obtained by scanning tunneling microscopy. The images suggest that the molecules lie flat on the surface, which agrees with our results.7f The flat geometry can be responsible for the fast electron transfer as observed in Figure 4 although we cannot ignore the possibility of the electron transfer through defects in the monolayers. Conclusions
Figure 4. (a) A π-A diagram of ODP. (b) A cyclic voltammogram of the ODP LB monolayer. The scan rate is 100 mV/s. The pyrrole oxidation peak is shaded.
mN/m as seen in Figure 4b. As observed for the BPUS oxidation, there is a typical oxidation peak in the first scan, but no further oxidation peak was shown in the subsequent scan. The total oxidation charge density of the LB film is determined to be approximately 4 µC/cm2. The same experiments at the surface pressure range of 3-8 mN/m reveal the oxidation charges of 4-6 µC/cm2. The oxidation charges scattered with the experiments and did not show a correlation with the pressure. On the other hand, the oxidation charge of the BPUS SA monolayer is approximately 60 µC/cm2. This tells that the transfer efficiency or packing density of the ODP LB monolayers is approximately 7-10% of the BPUS SA monolayer, which suggests that the structure of ODP at HOPG cannot retain the geometry shown in Figure 1. As we notice in Figure 4b, the overpotential of the pyrrole oxidation is very small and the charging current be-
Characterizations of the BPUS SA monolayer by cyclic voltammetry revealed that the surface-confined pyrrole monolayer forms a compact monolayer and undergoes electrochemical oxidation. The oxidation process shows a remarkable difference in the cyclic voltammograms in the presence and absence of oxygen and/or water. A trace amount of oxygen and/or water is believed to participate in the chemical reaction upon electrochemical oxidation of the surface-confined pyrrole. Characterizations of the ODP LB monolayer with electrochemistry showed that the transfer efficiency was approximately 7-10% of the BPUS SA monolayers. This indicates that the ODP molecule reorients when it is transferred to an HOPG substrate. The ODP LB monolayer was completely dissolved in an acetonitrile solution. As a result, there were no electrochemical oxidation currents of pyrrole. The LB monolayer, however, was retained in an aqueous solution. Further studies are in progress to identify the electrochemical product upon oxidation of the surface-confined pyrrole by infrared spectroscopy and the molecular geometry of the monolayer by scanning probe microscopy. Acknowledgment. This work was supported by KOSEF (971-0305-041-2), Ministry of Education (BSRI97-3425) in Korea, and the Creative Research Initiatives Program of the Korea Ministry of Science and Technology. LA980098J
