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J. Phys. Chem. B 2002, 106, 4658-4662
Electron Transport in Bithiophene-Bithiazole Based Metallopolymers Brian J. MacLean and Peter G. Pickup* Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7 ReceiVed: NoVember 14, 2001; In Final Form: March 5, 2002
Electron transport in electrochemically polymerized films of Ru(5,5′-bis(2-thienyl)-2,2′-bithiazole)(bpy)22+ and Os(5,5′-bis(2-thienyl)-2,2′-bithiazole)(bpy)22+ (bpy ) 2,2′-bipyridine) has been investigated by electrochemical impedance spectroscopy. Electron diffusion coefficients for the M(III/II) mixed valence states are significantly higher than for similar nonconjugated polymers, indicating that the conjugated backbone facilitates electron transport between metal sites. The operation of a hole-type superexchange mechanism is proposed on the basis of the observations that electron transport is faster for the Ru polymer than for the Os polymer and is faster than for the electron-deficient polybenzimidazole based polymers previously investigated.
Introduction Polymers in which metals are coordinated directly to a longrange π network have attracted significant attention in recent years because of their intrinsic scientific interest and their potential value in the development of nanoelectronics, electrocatalysts, sensors, and optical devices.1,2 Electronic interactions between the metal-based dπ orbitals and the polymer π or π* orbital can provide additional mechanisms for electron transport between metal centers and facilitate the movement of electrons necessary for high electrocatalyst performances or rapid switching in devices.3 Electron transport in redox polymers has been shown to occur by at least three mechanisms.2 In saturated systems, outer-sphere electron exchange between redox sites (outer-sphere mechanism) provides the only significant contribution to electron transport. In unsaturated systems, and particularly in highly conjugated systems, electron transport can also occur through the polymer backbone by mediated or superexchange mechanisms. These are distinguished by the availability of redox states of suitable energy on the polymer to mediate electron transport. If such states are available, the electron can hop between a localized metal-based redox site, a polymer-based site, and a second metal site in two steps (mediated mechanism). If such states are not available, then electron transfer through the backbone must result from a mixing of appropriate orbitals of both metals with the HOMO or LUMO of the backbone (superexchange). Previously, we have demonstrated the existence of superexchange mechanisms for electron transport in several benzimidazole-based metallopolymers (Structure 1).3-6 Comparisons of
Ru and Os polymers and analysis of the effects of deprotonation of the polymer backbone led to the conclusion that both hole* To whom correspondence should be addressed. E-mail: ppickup@ mun.ca. Fax: 709-737-3702.
type (via an interaction with the polymer HOMO) and electrontype (via the polymer LUMO) mechanisms could occur.3 It was predicted that higher rates of electron transport could be achieved by better matching of orbital energies. To test this hypothesis, we have prepared a series of Ru and Os metallopolymers containing a poly(bithiophene-co-bithiazole) backbone7 and report here on the electron-transport properties of two of these materials (poly-[Os(2)(bpy)22+] and poly-[Ru(2)(bpy)22+]; 2 ) 5,5′-bis(2-thienyl)-2,2′-bithiazole, bpy ) 2,2′bipyridine). The use of the electron rich poly(bithiophene-co-
bithiazole) backbone should lower the energy of the HOMO of the polymer backbone and promote electron transfer by holetype superexchange or mediation. Electrochemical impedance spectroscopy has been used here to measure electron-transport rates because it was the most precise technique in a previous study which compared its use with rotating disk voltammetry and dual electrode voltammetry.5 Experimental Section Chemicals. Ru(5,5′-bis(2-thienyl)-2,2′-bithiazole)(bpy)2(ClO4)2 and Os(5,5′-bis(2-thienyl)-2,2′-bithiazole)(bpy)2(ClO4)2 were prepared as previously reported.7 CH3CN (spectroscopic grade) was distilled over CaH2 under Ar before use. Et4NClO4 (prepared from Et4NBr and HClO4) was recrystallized thrice from water and dried under vacuum at 110 °C for 12 h. All other chemicals were used as received. Electrochemistry. Electrochemical experiments were conducted in conventional cells under a nitrogen atmosphere, with Pt disk working electrodes (5.2 × 10-3 cm2) and an SSCE reference electrode. Impedance experiments were performed using a Solartron (Schlumberger) 1268 electrochemical interface and 1250 frequency response analyzer. Preparation of Metallopolymer Films. Metallopolymer films were deposited on Pt disk electrodes by anodic polym-
10.1021/jp0141934 CCC: $22.00 © 2002 American Chemical Society Published on Web 00/00/0000
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Electron Transport
Figure 1. Cyclic voltammograms (100 mV s-1) of a poly-[Os(2)(bpy)22+] coated Pt electrode in CH3CN containing 0.1 M Et4NClO4.
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Figure 3. Complex plane impedance plots for a poly-[Os(2)(bpy)22+] coated Pt electrode at selected potentials (0.75 V (4), 0.80 V (2), 0.89 V (9), 1.00 V (O)) in CH3CN containing 0.1 M Et4NClO4. ΓM ) 2.2 × 10-8 mol cm-2.
redox wave is due to the M(III/II) couple, while the higher potential, irreversible wave is due to oxidation of the polymer backbone.7 The irreversibility of the backbone oxidation and its absence after the first scan indicate that the oxidized backbone is unstable. As will be seen below, this deactivation of the conjugated backbone by “overoxidation” provides a valuable means of probing the mechanism of the charge transport between metal centers in these polymers. Overoxidation is ubiquitous in the electrochemistry of conjugated polymers and is known to result in a loss of conjugation.8 Although the exact mechanism is not clear in most cases, it generally results from nucleophilic attack of some species on the oxidized polymer backbone. In the present case, it is presumably due to attack by trace water in the electrolyte solution. This would result (initially) in addition of hydroxy groups to the polymer backbone (e.g., Structure 3), and loss of conjugation through tautomerisation to the ketone (Structure 4).8,9 Figure 2. Cyclic voltammograms (100 mV s-1) of a poly-[Ru(2)(bpy)22+] coated Pt electrode in CH3CN containing 0.1 M Et4NClO4.
erization of the monomer metal complex ([Os(2)(bpy)22+] or [Ru(2)(bpy)22+]) from a ca. 2 mM solution of the complex in BF3‚Et2O (Aldrich).7 No supporting electrolyte was used in the polymerization solution. For 2.2 mM Ru(2)(bpy)22+, a constant current density of 0.15 mA cm-2 was found to produce high quality films while maintaining the potential during polymerization below levels that would cause oxidative degradation of the film. During polymerization under these conditions, the potential slowly increased to ca. 1.5 V. Poly-[Ru(2)(bpy)22+] films were also grown at a constant potential of 1.45 V to investigate whether the polymerization method (constant current or constant potential) influences the polymer’s transport properties. All poly-[Os(2)(bpy)22+] films were prepared from 2.5 mM Os(2)(bpy)22+ at a constant potential of +1.5 V. Polymerization times up to 45 min were used (see Figure 8). Results Cyclic Voltammetry. Figures 1 and 2 show cyclic voltammograms of poly-[Os(2)(bpy)22+] and poly-[Ru(2)(bpy)22+] films, respectively, under the conditions used for the electrontransport studies. In both cases, the lower potential, reversible
Impedance of Poly-[Os(2)(bpy)22+] Films. Figure 3 shows complex plane impedance (Nyquist) plots for a poly-[Os(2)(bpy)22+] coated Pt electrode at various potentials in acetonitrile containing 0.1 M Et4NClO4. The potential range used for these experiments was chosen to cover the range of the Os(III/II) voltammetric wave but not to cause overoxidation of the polymer backbone. The impedance plots shown in Figure 3 all approximate the behavior of a finite transmission line (a ca. 45° high frequency region gives way to a ca. 90° limiting capacitance at low frequency) and can be analyzed according to a generalized electroactive film model.10 The invariance of the high-frequency intercept (Figure 4) with potential indicates that the ionic resistance of the film is negligible relative to the uncompensated solution resistance.11 The potential dependent resistance associated with the ca. 45° (charge transport) region of the impedance plot can therefore be attributed solely to electron transport within
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MacLean and Pickup
TABLE 1: DeCM2 Values and Standard Deviations for Various Ru and Os Metallopolymers polymer
DeCM2/10-14 mol2 cm-4 s-1
ref
poly-[Ru(2)(bpy)23+/2+] overoxidized poly-[Ru(2)(bpy)23+/2+] poly-[Os(2)(bpy)23+/2+] overoxidized poly-[Os(2)(bpy)23+/2+] poly-[Ru((6,6-bibenzimidazole-2,2-diyl)2,5-pyridine)(bpy)23+/2+] (1; M ) Ru) poly-[Os((6,6-bibenzimidazole-2,2-diyl)2,5-pyridine)(bpy)23+/2+] (1; M ) Os) poly-[Ru(4-vinylpyridine)2(bpy)23+/2+] poly-[Os(4-vinylpyridine)2(bpy)23+/2+]
32 ((13) 4.6 ((1.6) 5.8 ((4.1) 0.8 ((0.2) 1.5 ((0.8) 0.71 ((0.41) 0.18 ((0.09) 1.2 ((0.4)
this work this work this work this work 3 3 14 14
Figure 4. Rhf (O) and Re (9) vs potential from impedance data for the poly-[Os(2)(bpy)22+] coated Pt electrode in Figure 3.
the metallopolymer film. This resistance (Re), determined here from the difference between the real axis intercepts of the ca. 45° (Rhf) and ca. 90° (Rlf) regions (i.e., Re ) 3(Rlf - Rhf); see Figure 3), is given by12
Re ) d2/DeClf ) ΓM2/DeCM2Clf
Figure 5. Clf vs potential from impedance data for the poly-[Os(2)(bpy)22+] coated Pt electrode in Figure 3.
(1)
where d is the polymer film thickness, De is the diffusion coefficient for electron hopping between metal sites, Clf is the low-frequency limiting capacitance (obtained as the slope of a plot of imaginary impedance vs 1/frequency (in rad)), ΓM is the surface coverage of metal sites (obtained by cyclic voltammetry), and CM is the concentration of metal sites in the film. The electron diffusion coefficient (De) provides a quantitative measure of electron transport through metallopolymers, and there is a large body of data in the literature for comparison with new systems.13 However, determination of De (by any of the available methods) requires knowledge of the film thickness, which is extremely difficult to determine accurately. It is therefore more appropriate to make comparisons of DeCM2 values, which can be more reliably determined (eq 1). Figures 4, 5, and 6 show plots of electronic resistance, lowfrequency capacitance, and DeCM2 versus potential, determined from the impedance data shown in Figure 3. As expected for a charge-transport process involving electron hopping between discrete (metal based) redox sites, Re goes through a minimum at the Os(III/II) formal potential while the faradaic pseudocapacitance (Clf) goes through a maximum, and DeCM2 is not significantly dependent upon potential. These results verify the validity of the above analysis of the impedance data and show that the polymer backbone is stable over the potential range and time scale (ca. 15 min) used here. Following the measurements shown in Figure 3, the conjugation of the polymer backbone was disrupted by overoxidation (2 voltammetric scans to +2.0 V at 100 mV s-1). The impedance measurement at 0.89 V was then repeated, yielding a DeCM2 of
Figure 6. DeCM2 vs potential from impedance data for the poly-[Os(2)(bpy)22+] coated Pt electrode in Figure 3.
5.8 × 10-15 mol2 cm-4 s-1. Thus, overoxidation of the polymer backbone resulted in a ca. 7-fold decrease of DeCM2 from the initial value at 0.89 V of 4.4 × 10-14 mol2 cm-4 s-1. Averages and standard deviations for DeCM2 values determined for eight different films before and after overoxidation are presented in Table 1. Impedance of Poly-[Ru(2)(bpy)22+] Films. The impedance of poly-[Ru(2)(bpy)22+] is more complex than that of poly-[Os(2)(bpy)22+] because of the closer proximity of the M(III/II) and backbone overoxidation waves (cf. Figures 1 and 2). Thus, impedance measurements conducted at potentials on the Ru(III/ II) wave can lead to partial loss of conjugation in the backbone and an increase in the electronic resistance (decrease in DeCM2) with time. This is illustrated in Figure 7, which shows DeCM2
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J. Phys. Chem. B, Vol. 106, No. 18, 2002 4661 Impedance measurements at 1.32 V on poly-[Ru(2)(bpy)22+] prepared at constant current (0.15 mA cm-2) yielded an average DeCM2 value of (3.4 ( 1.3) × 10-13 mol2 cm-4 s-1 (Figure 8), while measurements on four films prepared at constant potential (1.45 V) yielded an average value of (2.6 ( 1.2) × 10-13 mol2 cm-4 s-1. Following overoxidation at +1.8 V for ca. 1 min, DeCM2 values for both types of film had dropped to ca. 5 × 10-14 mol2 cm-4 s-1 (Table 1). Clearly, the polymerization method does not significantly influence the polymer’s chargetransport properties. Discussion
Figure 7. DeCM2 vs potential from impedance data for a poly-[Ru(2)(bpy)22+] coated Pt electrode in CH3CN containing 0.1 M Et4NClO4. ΓM ) 4.2 × 10-8 mol cm-2. The order of experiments was from low (1.22 V) to high (1.5 V) potential, followed by a final experiment at 1.22 V. The duration of each impedance experiment was ca. 60 s.
Figure 8. Surface coverage (ΓM, 4) and DeCM2 (b) vs polymerization time for a series of poly-[Ru(2)(bpy)22+] films prepared at a constant current density of 0.15 mA cm-2.
values obtained from a series of impedance measurements run at increasingly positive potentials. DeCM2 drops off sharply as the potential is increased above 1.25 V, and the value obtained at 1.22 V had dropped by an order of magnitude following the measurements at higher potentials. Fortunately, this decay of DeCM2 is slow relative to the time (ca. 60 s) required to collect impedance data at a single potential, and so reasonably accurate DeCM2 values can be obtained from single potential experiments on each film. The results that follow were obtained by averaging DeCM2 values from two consecutive impedance experiments at 1.32 V. On average, DeCM2 decreased by ca. 20% between measurements, although it did increase in several cases. Figure 8 shows surface coverages (ΓM) and DeCM2 values determined at 1.32 V for a series of poly-[Ru(2)(bpy)22+] films grown at constant current for various times. It can be seen that the surface coverage obtained increases linearly with time to about 10 min, and then the rate begins to decrease slightly. DeCM2 values are not significantly dependent on film thickness. The slightly lower values obtained for the thinnest films are probably due to the greater uncertainty involved in determining their resistances, which were low relative to the uncompensated solution resistance.
Table 1 summarizes the DeCM2 results obtained in this work together with values from the literature for comparison. The average DeCM2 of 3.2 × 10-13 mol2 cm-4 s-1 for poly-[Ru(2)(bpy)22+] (before overoxidation) stands out as being particularly high. It is a factor of 5 higher than for the analogous Os metallopolymer (poly-[Os(2)(bpy)22+]), a factor of 21 higher than for the conjugated Ru polybenzimidazole polymer, poly[Ru((6,6′-bibenzimidazole-2,2′-diyl)-2,5-pyridine)(bpy)23+/2+] (1), and a factor of 180 higher than for the nonconjugated Ru polymer, poly-[Ru(4-vinylpyridine)2(bpy)23+/2+]. It is clear from these comparisons that electron transport in poly-[Ru(2)(bpy)22+] must be enhanced by some mechanism other than simple outersphere electron exchange between metal centers. The comparison with poly-[Os(2)(bpy)22+] is particularly instructive since outer-sphere electron exchange should be faster, and hence DeCM2 should be higher, for the Os polymer,14 as observed in Table 1 for the nonconjugated polymers, poly-[Ru(4-vinylpyridine)2(bpy)23+/2+] and poly-[Os(4-vinylpyridine)2(bpy)23+/2+]. The decrease in DeCM2 for both poly-[Ru(2)(bpy)22+] and poly-[Os(2)(bpy)22+] when they are overoxidized shows that their enhanced electron-transport rates are due to the involvement of the conjugated backbone. The higher DeCM2 for the Ru polymer is then an obvious consequence of its smaller energy gap between the M(III/II) and backbone oxidation redox processes. Since this gap corresponds approximately to the energy gap between the metal d orbitals and the polymer HOMO, this implies that the HOMO of the polymer facilitates electron transfer between metal sites via a hole-type superexchange mechanism or by mediation. The higher DeCM2 values observed for poly-[Ru(2)(bpy)22+] and poly-[Os(2)(bpy)22+] relative to Ru and Os polybenzimidazoles (1), which also show enhanced electron transport due to superexchange, can be explained by the better matching of the M(III/II) and HOMO energy levels (from the electronic spectra in ref 5 the d-HOMO gap for the Ru polybenzimidazole can be estimated to be ca. 0.6 eV, while those estimated for poly-[Ru(2)(bpy)22+] and poly-[Os(2)(bpy)22+] from the voltammograms in Figures 1 and 2 are ca. 0.3 and 0.7 eV, respectively). This therefore vindicates the strategy of using a more electron-rich backbone to bring the HOMO energy level of the polymer backbone closer to the M(III/II) level. However, the poly(bithiophene-co-bithiazole) backbone may be more flexible than the polybenzimidazole backbone, and this may also be a contributing factor the to high electron-transport rates in the former materials. The question of whether the dominant electron-transport mechanism in poly-[Ru(2)(bpy)22+] and poly-[Os(2)(bpy)22+] is superexchange or mediation is more difficult to answer. In poly-[Os(2)(bpy)22+], the Os(III/II) and polymer oxidation waves are so far apart (Figure 1) that mediation would appear to be unlikely. The fact that DeCM2 does not increase with increasing potential (Figure 6) also suggests the absence of a significant
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4662 J. Phys. Chem. B, Vol. 106, No. 18, 2002 mediation mechanism, although it is possible for a conjugated backbone to mediate electron transport between metal centers without causing DeCM2 to be potential dependent.15 For poly-[Ru(2)(bpy)22+], a mediation mechanism is more likely because of the closer proximity of the M(III/II) and polymer oxidation waves (Figure 2). However, the decay of DeCM2 with increasing potential (Figure 7) suggests that oxidized polymer backone sites are not sufficiently stable to contribute either directly or indirectly through mediation to electron transport. Finally, it is curious that overoxidation of poly-[Ru(2)(bpy)22+] and poly-[Os(2)(bpy)22+] does not lead to a reversal of their relative electron-transport rates. This was anticipated because outer-sphere electron transfer should be the only mechanism available in the overoxidized materials, and this should be faster in the Os polymer.14 Indeed, DeCM2 for overoxidized poly-[Os(2)(bpy)22+] is very close to that for the nonconjugated poly-[Os(4-vinylpyridine)2(bpy)23+/2+] (Table 1). Why then is DeCM2 for overoxidized poly-[Ru(2)(bpy)22+] (4.6 × 10-14 mol2 cm-4 cm-1) so much higher than for overoxidized poly-[Os(2)(bpy)22+] (0.8 × 10-14 mol2 cm-4 cm-1) and the nonconjugated poly-[Ru(4-vinylpyridine)2(bpy)23+/2+] (0.2 × 10-14 mol2 cm-4 cm-1)? Presumably, there is still some conjugation in the overoxidized backbone of poly-[Ru(2)(bpy)22+]. If structure 4 is accurate for the overoxidized backbone, then this residual conjugation could be due to the presence of a small amount of the enol form (Structure 3). Structure 3 would then also be expected to be present in overoxidized [Os(2)(bpy)22+], but its low concentration and the low potential of the Os(III/II) couple presumably make its contribution to electron transport negligible. Implicit in the above discussion is the assumption that all of the polymers listed in Table 1 have similar concentrations of metal sites. It must therefore be noted that some of the differences in DeCM2 values may be due to differences in concentrations. Such effects, however, will be relatively minor and cannot explain the large differences in DeCM2 that have been observed. For example, a more than 2-fold difference in metal concentration would be needed to explain the difference in DeCM2 values between poly-[Ru(2)(bpy)22+] and poly-[Os(2)(bpy)22+], and an unreasonably large 2-fold increase in film thickness would be required to explain the decreases in DeCM2 accompanying overoxidation of the polymer backbones. The
MacLean and Pickup concentration of Ru in poly-[Ru(2)(bpy)22+] would have to be 13 times higher than in poly-[Ru(4-vinylpridine)2(bpy)23+/2+]. Conclusions DeCM2 values from impedance spectroscopy on films of electrochemically polymerized Ru(2)(bpy)22+ and Os(2)(bpy)22+ clearly show that electron transport in these materials involves the HOMO of the polymer backbone via superexchange or mediation. Closer matching of the energies of the metal couple and the polymer HOMO than in previously studied materials has resulted in a substantial increase in electron-transport rates. Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council of Canada and Memorial University. Note Added after Print Publication. Due to a production error, the chemical structures of 1-4 were not included in the version of this article published on the Web 4/17/2002 (ASAP) and in the May 9, 2002, issue (Vol. 106, No. 18, pp 46584662). The correct electronic version of the paper was published on 5/23/2002 and an Addition and Correction appears in the June 20, 2002, issue (Vol. 106, No. 24). References and Notes (1) Pickup, P. G. J. Mater. Chem. 1999, 8, 1641. (2) Kingsborough, R. P.; Swager, T. M. Progr. Inorg. Chem. 1999, 48, 123. (3) Cameron, C. G.; Pittman, T. J.; Pickup, P. G. J. Phys. Chem. B 2001, 105, 8838. (4) Cameron, C. G.; Pickup, P. G. Chem. Commun. 1997, 303. (5) Cameron, C. G.; Pickup, P. G. J. Am. Chem. Soc. 1999, 121, 11773. (6) Cameron, C. G.; Pickup, P. G. J. Am. Chem. Soc. 1999, 121, 7710. (7) MacLean, B. J.; Pickup, P. G. J. Mater. Chem. 2001, 11, 1357. (8) Pud, A. A. Synth. Met. 1994, 66, 1. (9) Qi, Z.; Rees, N. G.; Pickup, P. G. Chem. Mater. 1996, 8, 701. (10) Vorotyntsev, M. A.; Badiali, J. P.; Vieil, E. Electrochim. Acta 1996, 41, 1375. (11) Albery, W. J.; Elliott, C. M.; Mount, A. R. J. Electroanal. Chem. 1990, 288, 15. (12) Mathias, M. F.; Haas, O. J. Phys. Chem. 1992, 96, 3174. (13) Oyama, N.; Ohsaka, T. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992; pp 333-402. (14) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1991. (15) Ochmanska, J.; Pickup, P. G. J. Electroanal. Chem. 1991, 297, 197.
