Langmuir and Langmuir-Blodgett Films of a Viologen Derivative Pilar Cea, Carlos Lafuente, Jose ´ S. Urieta, Marı ´a C. Lo ´pez, and Fe ´lix M. Royo* Departamento de Quı ´mica Orga ´ nica-Quı ´mica Fı ´sica, Facultad de Ciencias, Plaza de San Francisco, Ciudad Universitaria, 50009 Zaragoza, Spain Received July 13, 1998. In Final Form: September 28, 1998 We present here the study of a symmetrically substituted viologen derivative, 1,1′-dioctadecyl 4,4′- bipyridinium salt, at the air-water interface and arranged in Langmuir-Blodgett (LB) films which were characterized using UV-vis spectroscopy. Detailed research of the electrochemical behavior of this molecule in LB films has been performed in terms of the nature of the counterion present in the water subphase, the number of layers, the transference pressure, and the nature of the electrolyte. Introduction Over the past two decades much interest has been centered on the application of the Langmuir-Blodgett (LB) technique to achieve organic films due to the potential of this method to buildup mono- or multilayers in which the order at the molecular level can be controlled. Such control is specially important in applications such as the formation of ultrathin modified electrodes with high density of electroactive sites and regularity in molecular organization, as well as sensors, electro- and photoelec- trochemical devices, and photoelectrochemical storage devices. The LB technique also has the advantage of creating films with large-scale order, providing new insights on electron-transfer reactions at interfaces. 1,1′-Disubstituted 4,4′-bipyridyls, so-called “viologens”, are of great electrochemical interest because of their three oxidation states and the high reversibility of their redox reactions 1 (specially the first one), their use in solar energy storage devices and electron mediators in herbicides, 2 preparation of chemically modified electrodes, 3 studies of the rates of electron transfer at electrode substrate-film interface, 4 their potential electrochromic applications, 5 etc. Several research groups, such as Bard’s, 6-8 Buttry’s, 9-11 and Cotton’s, 12,13 have performed interesting experiments with viologen derivatives arranged in LB films. From the bibliographic study we have done, it can be concluded that the reported experiments are not conclusive and sometimes even contradictory. Some of the cited papers reported a first reduction peak splitting in the cyclic voltammetry experiments 6 meanwhile this phenomenon has not been observed in other cases 12 where the experi- mental conditions were apparently the same or extremely similar. Several reasons have been proposed to explain the mentioned splitting, such as the existence of two different conformations of the viologens in the LB film, interactions between the parent and the reduced species, the formation of radical dimers, as well as the influence of several factors such as the effects of the specific anion species associated with the viologen dication, the influence of surface coverage, the number and length of the alkyl chains, presence of fatty acids, etc. The aim of this work is to provide some insight about the electrochemical behavior of these kind of molecules arranged in LB films, given their both theoretical and practical interests. We present here the results of a detailed research into the electrochemical properties of LB films constituted by a symmetrically substituted viologen, 1,1′-dioctadecyl 4,4′-bipyridyl, shown in Figure 1. We have studied the influence of several factors (e.g., the compression speed, the composition of the subphase, the rate and kind of transference of the monolayer, the nature of the electrolyte in the electrochemical cell, etc.) on the behavior of the viologen derivative. The deter- mination of the architecture and degree of aggregation of the molecules in the LB film has been performed using UV-vis spectroscopy. Experimental Section The 1,1′-dioctadecyl 4,4′-bipyridinium dibromide was pur- chased from Aldrich, and no further treatment was carried out. Solutions of the viologen derivative were prepared in chloroform/ absolute ethanol in the rate 9:1. The chloroform was HPLC grade (99.9%) purchased from Aldrich, and the ethanol was HPLC grade (99.0%) purchased from Normasolv. The solutions were kept in dark bottles wrapped with aluminum foil in a refrigerator. Despite this, the solutions were used as fresh as possible. This study has been performed with a Teflon trough (460 × 210 mm 2 ) designed by us and whose details have been reported before. 14 The water used for the subphase was purified by * To whom correspondence should be addressed. (1) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (2) Summers, L. A. The Bypyridinium Herbicides; Academic Press: New York, 1980. (3) Murray, R. W. Molecular Design of Electrode Surfaces; John Wiley & Sons: New York, 1992. (4) Goss, C. A.; Miller, C. J.; Majda, M. J. Phys. Chem. 1988, 92, 1937. (5) Bruinink, J.; Kregting, C. G. A.; Ponjee, J. J. J. Electrochem. Soc. 1977, 124, 1854. (6) Lee, C.; Bard, A. J. Electroanal. Chem. 1988, 239, 441. (7) Obeng, Y. S.; Founta, A.; Bard, A. J. New J. Chem. 1992, 16, 121. (8) Lee, C. W.; Bard, A. J. Chem. Phys. Lett. 1990, 170, 57. (9) De Long, H. C.; Butty, D. A. Langmuir 1990, 6, 1319. (10) De Long, H. C.; Butty, D. A. Langmuir 1992, 8, 2491. (11) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921. (12) Ye, S.; Kim, J. H.; Uphaus, R. A.; Cotton, T. M.; Lu, T.; Dong, S. Thin Solid Films 1992, 210/211, 822. (13) Kim, J. H.; Bunding, K. A.; Uphaus, R. A.; Cotton, T. M. Thin Solid Films 1992, 210/211, 825. (14) Royo, F. M.; Lo ´ pez, M. C.; Ruiz, B.; Camacho, A.; Lozano, J. M.; Urieta, J. Rev. Acad. Cienc. Exactas, Fis., Quim. Nat. Zaragoza. 1993, 48, 177. Figure 1. 1,1′-Dioctadecyl 4,4′-bipyridyl. 7306 Langmuir 1998, 14, 7306-7312 10.1021/la980865e CCC: $15.00 © 1998 American Chemical Society Published on Web 11/10/1998