2824
Chem. Mater. 2001, 13, 2824-2832
Polymer Films on Electrodes. 30. Electrochemistry and Scanning Electrochemical Microscopy Characterization of Benzimidazolebenzophenanthroline-Type Ladder (BBL) and Semiladder (BBB) Polymer Films Maurizio Quinto,†,‡ Samson A. Jenekhe,§,¶ and Allen J. Bard*,† Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 and Department of Chemical Engineering, University of Rochester, Rochester, New York 14627 Received December 29, 2000. Revised Manuscript Received June 11, 2001
Polymer-modified electrodes were obtained by spin-coating solutions of poly(benzobisimidazobenzophenanthroline) ladder (BBL) and semiladder (BBB) polymers on indium-tin oxide electrodes. An electrochemical study of these films showed reproducible and stable cyclic voltammograms for reduction of the polymer (in the absence of oxygen), the absence of waves for oxidation of the polymer, and brilliant changes in color during the potential scan. Measurements with the scanning electrochemical microscope showed changes in the film conductivity, with the films being more conductive in the reduced form. The potential dependence of the film resistance and atomic force microscopic characterization are also reported. These results confirm the electron transport and electron-accepting nature of both BBL and BBB, which can accept up to two electrons per monomer repeat unit. Although the electrochromic features of BBL and BBB are similar, a significant difference in the film morphologies was observed by atomic force microscopy.
Introduction In the class of organic polymers as a whole, laddertype polymers exhibit many remarkable properties,1-9 and a systematic study of their electronic and spectroscopic behavior is of interest. The poly[6,9-dihydro-6,9dioxobisbenzimidazo[2,1-b:1′,2′- j]benzo[1mn]phenanthroline-2,12-diyl] (BBB) and poly[17-oxo-7,10-benz[de]imidazo[4′,5′:5,6]-benzimidazo[2,1-a]isoquinoline-3,4: 10,11-tetrayl)-10-carbonyl]) (BBL) materials (Figure 1) are members of a broad class of thermally stable rigidchain polymers.2-9 They generally contain aromatic or heterocyclic rings together with other groups that promote strong primary and van der Waals bonding †
The University of Texas at Austin. Current address: Universita` di Foggia, Facolta` di Agraria, via Napoli 25, 71100 Foggia, Italy. § University of Rochester. ¶ Current address: Department of Chemical Engineering, University of Washington, Seattle, WA 98195-1750. (1) Schluter, A. D. Adv. Mater. 1991, 3, 282. (2) Van Deusen, R. L. J. Polym. Sci., Polym. Lett. 1966, 4, 211. (3) (a) Van Deusen, R. L.; Goins, O. K.; Sicree, A. J. J. Polym. Sci. A-1 1968, 6, 1777. (b) Arnold, F. E.; Van Deusen, R. L. Macromolecules 1969, 2, 497. (4) Frazer, A. H. High Temperature Resistant Polymers; WileyInterscience: New York, 1968. (5) Cassidy, P. E. Thermally Stable Polymers; Marcel Dekker: New York, 1980. (6) Critchley, J. P.; Knight, G. J.; Wright, W. W. Heat-Resistant Polymers; Plenum: New York, 1983. (7) Hergenrother, P. M. In Encyclopedia of Polymer Science and Engineering; Korschwitz, J. I., Ed.; Wiley: New York, 1985; Vol. 7, pp 639-665. (8) (a) Jenekhe, S. A.; Johnson, P. O. Macromolecules 1990, 23, 4419. (b) Roberts, M. F.; Jenekhe, S. A Polymer 1994, 35, 4313. (9) Roberts, M. F.; Jenekhe, S. A J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 577. ‡
forces, resonance stabilization, and molecular symmetry, which make them very robust materials.2-9 BBB and BBL polymers, which are the primary focus here, are made by polycondensations of 1,4,5,8-naphthalenetetracarboxylic acid with 3,3-diaminobenzidine and 1,2,4,5-tetraaminobenzene, respectively.2,3 In general, these rigid-chain polymers, which show desirable physicochemical properties such as hightemperature resistance, are difficult to process because of their insolubility in organic solvents and high glass transition, softening, or melting temperatures (>250300 °C).2-9 Jenekhe and Johnson8a used a reversible electron donor-acceptor complex formation between heterocyclic polymers and Lewis acids to solubilize these polymers in aprotic organic solvents. In this work, a metal halide Lewis acid, GaCl3, was used to solubilize and process rigid-chain polymers, including ladder polymers such as BBL and semiladders such as BBB.8,9 This complexation involves coordination of the Lewis acid to the electron-rich carbonyl oxygen and the imine nitrogen in the polymer backbone. The resulting solutions of the BBB or BBL complex can then be processed into films by methods such as solvent casting or spin coating.8,9 BBL probably has a delocalized electronic structure and, upon doping, forms an electronically conducting polymer.10-13 It can be chemically10 or electrochemi(10) (a) Kim, O.-K. J. Polym. Sci., Polym. Lett. Ed. 1982, 663. (b) Kim, O.-K. Mol. Cryst. Liq. Cryst. 1984, 105, 161. (11) (a) Narayan, K. S.; Alagiriswamy, A. A.; Spry, R. J. Phys. Rev. B. 1999, 59, 10054. (b) Antoniadis, H.; Abkowitz, M. A.; Osaheni, J. A.; Jenekhe, S. A.; Stolka, M. Chem. Mater. 1994, 6, 63.
10.1021/cm001426f CCC: $20.00 © 2001 American Chemical Society Published on Web 08/10/2001
SECM of Ladder Polymer Films
Chem. Mater., Vol. 13, No. 9, 2001 2825
Figure 1. Structures of rigid-chain conjugated polymers BBB (left) and BBL (right).
cally11 doped. Pristine BBL and BBB are both insulators with conductivities in the range of 10-14 S cm-1.11 The direct current conductivity of their films can be varied across a wide range by either ion implantation (g102 S cm-1) or thermal treatment.11,12 This polymer also shows interesting mechanical properties and thermal stability14 because of its ladder structure. BBL forms liquid crystalline solutions owing to its rodlike extended chain structure.8b BBB, in contrast to BBL, is a linear polymer that does not form liquid crystalline mesophases in solution.8 A scanning tunneling microscopy (STM) study of BBB films shows structural features that include helical fibers coiled into strands, with the strands coiled into strings.15 Recent interest in BBL films has focused on its intrinsic semiconducting,11,16 photoconductive,17 and nonlinear optical18 properties together with optoelectronic device applications.19 This conjugated ladder polymer has an optically determined semiconductor band gap of 1.8 eV. Photovoltaic cells based on BBL thin films were recently demonstrated,19a showing very high charge collection efficiencies (49%) and power conversion efficiencies of up to 2%. Photodetectors with ultralow dark currents (
