3642
J . Phys. Chem. 1988, 92, 3642-3648
Direct Current Redox versus Electronic Conductivity of the Ladder Polymer Poly(benzimidazobenzophenanthroline) K. Wilbourn and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 (Received: December 7 , 1987)
Direct current electron conduction in films of electrochemically doped poly(benzimidazobenzophenanthroline)(BBL)is explored to show that this polymer, depending on the choice of environment, can satisfy experimental current-voltage criteria commonly associated with both redox conductivityand electronic conductivity, showing that such conductivity properties are not necessarily polymer specific. The electron conductivity of BBL is maximized when it is doped in a 1 : 1 mixed-valent state, at the formal potential for its electrochemical reduction. Thus doped BBL films, when wetted by electrolyte solution, are ionically conductive and exhibit non-Ohmic redox (electron) conductivity characteristics associated with concentration polarization of electronically localized states, albeit with electron diffusion coefficients De that are extraordinarily large, up to 6 X cm2/s. Identically doped BBL films that are dried and scrupulously electrolyte free, on the other hand, are not ionically conductive and conduct electrons Ohmically with a 9-fold smaller thermal activation barrier (0.07 eV) and as much as lo4 times larger conductivities.
Electron conduction in polymeric materials is an active research topic especially in connection with the highly conducting materials polyacetylene, polypyrrole, polythiophene, and p ~ l y a n i l i n e . ' ~ ~ Conducting polymers have also been derived from polymeric transition-metal macrocycles3 and heteroaromatic ladder polym e r ~ . ~ -These ~ conducting polymers are highly conjugated molecules and are postulated to have spatially delocalized, bandlike electronic structures,] but chain defects and termini may play a major role in their actual conductivities. Electronically conducting polymers are characterized by Ohmic (linear) current-voltage curves that are conventionally measured by impressing a voltage bias across a dry sample in some designated state of doping. S/cm and generally Reported conductivities range from IO3 to involve small thermal activation barriers. I Another form of electron conduction,9-" "redox conductivity," occurs in concentration-polarized polymers where the mobile electrons are localized on identifiable molecular sites in the polymer and are transported under the impetus of concentration gradients by thermally activated hopping or self-exchanges between occupied and unoccupied sites. These materials include transition-metal complexes that are linked as pendant groups along a nonconjugated backbone] or by electropolymerized functionalities on the ligand Electron hopping between localized states is important in varied other materials, for example, those relevant t o photocopying processes.14 Our reference to redox conductivity is in the electrochemical context, where the polymer sample is usually a film coated on an electrode and wetted by an electrolyte solution. During or as a consequence of electrochemical charging, (1) Skotheim, T. A,, Ed. Handbook of Conducting Polymers; Marcel Dekker; New York, 1986; Vol. 1, 2 . ( 2 ) Frommer, J . E.; Chance, R. R. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1986; Vol. 5 , 462-507. (3) Marks, T. J. Science (Washington, DC) 1985, 227, 88 1 , (4) Ruan, J. Z.; Litt, M. H. J . Polym. Sci., Parr A : Polym. Chem. 1987, 25, 285. (5) Iqbal, 2.; Makysson, C.; Banghinan, R. H. Synrh. Mer. 1986, 15, 161. (6) Chien, J. C. W.; Carling, C. J . Polym. Sci., Polym. Chem. Ed. 1985, 23, 1383. (7) Young, C. L.; Whitney, D.; Vistnes, A. I.; Dalton, L. Annu. Rec. Phys. Chem. 1986, 37, 459. (8) Kim, 0.-K. J . Polym. Sci., Polym. Lett. Ed. 1985, 23, 137. (9) Chidsey, C. E. D.; Murray, R. W. J . Phys. Chem. 1986, 90, 1479. (IO) Pickup, P. G., Kutner, W.; Leidner, C. R.; Murray, R. W. J . A m . Chem. S o t . 1984, 106, 1991. (11) Pickup, P. G.; Murray, R. W. J . A m . Chem. S o t . 1983, 105, 4510. (12) Oyama, N.; Anson, F. C. J . Electrochem. S o t . 1980, 127, 640. (13) White, H.S.; Kittlesen, G. P.:Wrighton, M. S. J . A m . Chem. Soc. 1984, 106, 5375. (14) Gutmann, F.; Lyons, L. E.; Keyzer, H. Organic Semiconductors, Parr B Krieger: Huntington, NY, 1983.
0022-3654/88/2092-3642$01 SO/O
gradients of concentrations of occupied (reduced) and empty (oxidized) electron sites can be developed in the polymer. Such concentration polarization can by itself provide the driving force for the intersite electron hopping, whose rate is measured as an electron diffusion coefficient De. Dc current-voltage curves for concentration-polarizable redox conductors, are non-Ohmic as illustrated in Figures 1 and 2 for the case of a very thin redox polymer film sandwiched between two metal contacts. A relatively large dc potential bias (Figure 1C) produces dc redox conduction currents that reach voltageindependent plateaus when the polymer's electron occupancy sites become completely polarized (maximized d(concn)/dx; Figure 1C). When measured as a function of initial mixed-valent film composition, a small potential bias (Figure 2 ) produces a peaked redox conductivity (curves A-I) centered (Figure 2E) on the formal potential of the polymer redox couple, at which there are equal populations of occupied and unoccupied sites. It is important to note that the situations in Figures 1 and 2 are revealed on reasonable experimental time scales only when the polymer film sample is very thin, micrometers or less, because the migration of charge-compensating ions that is involved in attaining the steady-state concentration-distance profiles is an intrinsically slow process. Another why of saying this is that redox conductors exhibit large capacitances and correspondingly slow time constants as compared to most electronic conductors. This research deals with the problem of classifying doped polymers according to their dc conductivity characteristics, as (Ohmic) electronic conductors or as (non-Ohmic) redox conductors. The previous practice has been to classify a given polymer as one or the other, on the basis of the conductivity criteria described above. The material studied is the ladder polymer poly(benzimidazobenzophenanthroline) (BBL)
r
1
L
which has been described as an electronically conducting polymer,* with cre = 10-2-102 S/cm depending on the dopant. BBL has been presumed to have a delocalized electronic and to fall into the category of an Ohmic, electronically conducting doped. polymer. It can be ~ h e m i c a l l y ' ~or~electrochemi~ally'~ '~ ( 1 5 ) Kim, 0.-K. Mol. Crysr. Liq. Cryst. 1984, 105, 161. (16) Kim, 0.-K. J . Polym. Sci., Polym. Lett. Ed. 1982, 20, 663. (17) Jenehke, S. A. Chem. Eng. News 1987, 65, 27.
0 1988 American Chemical Society
Redox vs Electronic Conductivity of BBL
c
B.
A.
The Journal of Physical Chemistry, Vol. 92, No. 12. 1988
3643
CFS Electrode
Figure I. Concentration polarization and redox conduction in a mixedvalent polymer film containing q u a i concentrations of two oxidation states labeled as neutral and ionic sites and large voltage bias dEldx applied across the film: (A) no bias voltage. film unpolarized: (B) bias applied, as polarization occurs; (C) bias applied, fully developed polarization. steady-state limiting current.
+
aclax
aciax
0.
A.
3
G. iiiiii:::: ..... ..............
-b
”
\
C.
a
3,. H :::: 8BL
=
0
..-
88L’
Flyre 3. Configurationsof BBL-ted sandwich electrodes to measure current responses for conduction processes in the polymer: (A) a closed-face-sandwich(CFS) electrode with BEL sandwiched between Pt and porous gold electrodes; (B) BBL-coated open-face-sandwich (OFS) electrode where the left-hand diagram represents a cross section across the parallel, rectangular Pt electrodes to show d, the spacing between the two platinum eleclrodes. d is typically 70-140 &m.
al-dependent phenomena hut can depend on choice of the environment in which the conductivity measurements are made. Specifically, under electrolyte solution wetted conditions, very thin films of BBL exhibit current-voltage behavior characteristic of wncentration polarizations and large thermal activation barriers normally associated with strongly localized state, redox conductors. On the other hand, identically doped, but dry, BBL films behave Ohmically and exhibit both much larger conductivities and relatively energetically shallow carrier states. Both kinds of conductivity are maximized when the polymer is doped a t the electrochemical formal potential of the polymer so as to give a I:I mixed-valent state. Experimental Seetion Chemicals. Synthesis of BBL was by a literature method;*O
POTENTIAL
Figure 2. Concentration polarization and redox conduction in a model BELo/- film with small voltage bias applied across the film and steadystate current measured as a function of the initial mixed-valent composition of the film as fixed by the average bias voltage, which has potentials A-I relative to a reference electrode. The electrochemical doping reactions of BBL are examined in detail in a separate paper.l9 The results for electrochemically doped BBL show that currentvoltage characteristics conventionally associated with electronic and redox conductivity are strictly speaking not materi(18) Polyak. L.: Rolison. D.R.: Karlcr, R. 1.;Nowak. R. J. Absrrocrs of Popen. 163rd Meeting of the Electrochemical Society. May 1983; Extended Abstran 547: The Electrochemical Society: Princeton. NJ. (19) Wilbourn. K. 0.; Murray, R. W. Mmmmolecules 1988. 21, 89.
characterization of the product is given el~ewhere.’~The dry, undoped polymer had a very low wnductivity (ca. S/cm). The BBL films were dropcoated as methanesulfonic acid solutions onto electrodes; after solvent removal the films were neutralized hy washing with IO% triethanolamine/ethanol solutions. Typical film thicknesses were 4-8 rem, by surface profil~metry.’~Undoped films were deep purple. Acetonitrile (Burdick and Jackson) was stored over 4A molecular sieves. Supporting electrolytes were recrystallized 3 times and dried in vacuo. Sulfuric acid (MCB) was used as received. Experiments a t low pH (aqueous) were conducted with 10% volume added acetonitrile to poison interfering hydrogen evolution at the Pt electrode surface.” Aqueous solutions of selected DH were prepared by adding 40% aqueous Bu,NOH solution (Aldri’ch) to 0.1 M H,S04. Electrach~micaland Conductivity Experiments. Conductivities of BBL films were measured by depositing them on either open-face-sandwich (OFS) or closed-face-sandwich (CFS) ar(20) Arnold, F. E.: VanDcusen. R. L. Macromolecules 1969, 2. 221. (21) Lewir,T. J.: White. H.S.;Wrightan. M.S. J. Am. Chem. Soe. 1984.
106.6941.
3644 The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
rangements, Figure 3. In the closed-face sandwich9-" (Figure 3A) a thin, porous Au film is evaporated on top of a BBL film deposited on the end of a Pt wire sealed in glass. The evaporated Au film overlaps to another, naked Pt wire tip for contact purposes. Sealing 12 Pt wires in one glass tube allows fabrication of six sandwich devices simultaneously (Figure 3A). The thickness of the BBL film, and consequently the interelectrode spacing, in the CFS was typically 4-6 ym. The Au film is porous, to allow access of solvent and counterions necessary for electrochemical doping of the film. The open-face sandwich (OFS; Figure 3B) consists of the edges of two parallel Pt foils (0.4 X 0.05 cm) sealed in glass and separated by 70-140 ym. The polymer film is deposited so as to cover the rectangular electrode tips and the glass gap in between. The interelectrode gap again determines the dimension of the polymer lying between the two electrodes in the OFS and is much larger than in the CFS. For measuring the conductivity of BBL films, three different procedures were used. In each, the film was electrolyzed, in the electrolyte solution, to some designated state of doping by holding the potential of one of the sandwich electrode terminals (either OFS or CFS) at a selected voltage relative to a reference electrode in the solution. In one procedure, the potential of the other sandwich electrode terminal was simultaneously controlled at a potential 25 mV different from the first, so that a AE = 25-mV bias exists between the electrodes. The steady-state current flowing between the electrodes was measured after an equilibration period, and then the electrolysis potential was moved to a new value so that conductivity was measured as a function of the doping potential. This experiment is like that in Figure 2 and produced the data in Figures 5A and 10. In the second procedure, the potential of one sandwich electrode terminal was held at an oxidizing potential (relative to the reference electrode in the electrolyte solution) and the potential of the other electrode was slowly scanned to more negative or reducing values. This experiment is like that of Figure 1 and produced the steady-state current pattern in Figure 1 1. In the third procedure, the film was electrolyzed by holding both sandwich electrode terminals at the same doping potential during electrolysis. The terminals are then disconnected and the sandwich is raised from the solution, and the terminals are reconnected to apply a scanned voltage bias across the film. The film can be either still solvent wetted, or rinsed and dried in this procedure, which produced the current-voltage results in Figure 6-9. A PAR 273 potentiostat and Pine Instruments RDE3 or RDE4 bipotentiostats were used in the preceding manipulations and for cyclic voltammetry, and data were recorded on either an X-Y recorder or a Nicolet digital oscilloscope. A NaC1-saturated calomel electrode (SSCE) served as reference electrode.
Results and Discussion Cyclic Voltammetry of BBL Films in Acidic Aqueous and Acetonitrile Media. Figure 4 shows a set of cyclic voltammograms of BBL films in aqueous acid (curves A and B) and in acetonitrile (curve C). Details of these electrochemical reactions have been worked out elsewhereI9 and are summarized here for reference. I n aqueous acid, the electrochemical potentials for reducing BBL films are strongly pH dependent since undoped BBL participates in an acid-base equilibrium with pKa = 2.2: BBL
+ H+ s HBBL'
(1)
and different potentials are required to drive the one-electron/ one-proton reductions of the two states: BBL
+ H + + e- F? HBBL
(-0.03 V vs SSCE at pH 2.2) (2)
HBBL+ + H + + e- F? H2BBL+ (-0.20 V vs SSCE at pH 2.2) (3) A BBL film in equilibrium with a low-pH electrolyte (pH
