Electrical and Optical Properties of Fullerenol Langmuir-Blodgett Films Deposited on Polyaniline Substrates M. E. Rinco ´ n,* ,† H. Hu, † J. Campos, † and J. Ruiz-Garcı ´a ‡ Centro de InVestigacio ´ n en Energı ´asUNAM, Apartado Postal 34, Temixco, Mor. 62580, Mexico, and Instituto de Fı ´sicasUASLP, A Ä lVaro Obrego ´ n 64, San Luis Potosı ´, SLP 78000, Mexico ReceiVed: September 26, 2002; In Final Form: January 31, 2003 The synthesis of fullerenol with various degrees of hydroxylation and the use of a low hydroxylated product to form stable Langmuir films are reported in this work, along with the optical and electrical properties of Langmuir-Blodgett (LB) films obtained on glass and polyaniline (PANI) substrates. The data suggest that an average of 9-12 hydroxyl groups are bound preferentially on one side of the C 60 cage and this allows the formation of stable two-layer films at the air/water interface. The large anisotropy of the hydroxylated molecule provides organized LB films of the type substrate-(D-C 60 -D-C 60 -) n , with D representing the hydroxyl groups. The electrical conductivity of LB films deposited on glass is equivalent to that reported for highly conductive polymeric C 60 but several orders of magnitude higher than that for disordered fullerenol pellets. UV-vis absorptions provide evidence that fullerenol layers cause the deprotonation of PANI, rendering a polymer with low conductivity. The loss of conductivity disagrees with the behavior expected for a donor (PANI)/acceptor (fullerenol) interface, even though current-voltage (I-V) curves of fullerenol LB /PANI junctions indicate some degree of electrical rectification. Additionally, transients observed at large bias on the I-V curves agree with the reported proton conductivity of fullerenol. Introduction The chemistry of fullerene C 60 and its derivatives has attracted a great deal of interest due to the outstanding physical and chemical properties of these compounds. Among the great number of derivatives that have been synthesized, water-soluble fullerene derivatives have been investigated to date mostly within the framework of biological and medical applications. 1-3 Fullerenol, synthesized for the first time by Chiang et al., 4-9 is a good example of a water-soluble fullerene derivative; however, this compound has been used not only in medical applications 10-17 but also as a piezoelectric 18 and proton conducting material. 19 Our research interest in these compounds is based upon their potential use as new materials for solar energy conversion and storage. These applications of fullerenes have attracted much interest since the discovery of photoinduced electron-transfer processes in composites of conductive polymers and C 60 . 20 Since then, most of the research efforts have been focused on understanding the optoelectronic properties of the donor/acceptor interface and developing new ways to avoid bulk recombination of the photogenerated carriers. Currently, it is well accepted that the design of new composites based on polymers and fullerene derivatives, particularly the choice of heterojunction, will be limited by our ability to impose some structural ordering through miscibility control of the various phases. Recently, we have found that deposition of commercial fullerenol (Aldrich: C 60 (OH) 24-28 ) (unpublished results) by the method of drop and evaporation produces discontinuous and disordered films on glass substrates. The disordered film degrades the electrical properties of polymeric substrates, such as polyaniline (emeraldine salt) and polypyrrole, by several orders of magnitude. This raises the interesting possibility that the electrical properties of the conductive polymer/fullerenol junction can be improved by reducing the number of hydroxyl groups and by attempting highly organized structures at the interface. The conductive properties of fullerenol have been reported to be more ionic than electronic and a function of the amount of hydroxyl groups. 19 Highly hydroxylated molecules typically are associated with a dielectric material, while low levels of hydroxylation indicate a proton conductive material with ionic conductivity of 7 × 10 -6 Ω -1 cm -1 . Although extensive work on the formation of Langmuir and Langmuir-Blodgett films has been reported recently on several fullerene derivatives, 21,22 the formation of monolayers at the air/water interface has been difficult to achieve due to the high water solubility of the fullerenols. The work of Chiang et al. 9,23 indicates that fullerenol water solubility depends on the number of OH’s attached to C 60 . These authors were able to obtain Langmuir films of C 60 (OH) 12 and reported a minimum area per molecule of 190 Å 2 . The thickness of a two-layer C 60 (OH) 12 LB film deposited on mica was measured to be 13 Å/layer (approximately the diameter of a molecule), supporting their evidence of monolayer formation at the air/water interface. 23 In this work we also attempted monolayer formation of fullerenol. To do so, we modified the synthesis of highly soluble fullerenol to obtain different degrees of hydroxylation and used the water-insoluble fraction to form LB films on various substrates. IR analysis and MALDI-MS experiments were run to identify the approximate number of hydroxyl groups in the selected fraction, while UV-vis spectroscopy and electrical characterization studies gave information on the LB films deposited on glass and on polyaniline substrates. For pur- * Corresponding author. E-mail: merg@cie.unam.mx. † Centro de Investigacio ´n en Energı ´asUNAM. ‡ Instituto de Fı ´sicasUASLP. 4111 J. Phys. Chem. B 2003, 107, 4111-4117 10.1021/jp022159z CCC: $25.00 © 2003 American Chemical Society Published on Web 04/08/2003