Cyclic Voltammetry

The cyclic voltammetry of thin films of Ru and Os complexes of the polymers exhibit several interesting features. For all but very thick films, thin layer cyclic voltammetric behaviour was exhibited, even for sweep rates up to 1 V/sec. Small $\Delta$E_p and i $\propto$ $\nu$ were the rule.

For Ru(bpy)₂ complexes, a redox wave due to the Ru(III/II) couple appears in the region of ca. 0.8 to 1.3 V, while for the corresponding osmium complexes the Os(III/II) waves are found in the range of 0.4 to 1.1 V.

The potential of the M(III/II) wave is influenced by the degree of protonation of the polymer backbone. The two extremes are presented in Figure 4.5, where a thin film of Ru-PPyBBIM on Pt was cycled in CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄ which had been treated with a few drops of either concentrated HClO₄ or MeOH containing 1 mol dm^-3 Bu₄NOH. In the basic medium, E_1/2 [Ru(III/II)] is at ca. 0.8 V vs. SSCE, while in acidified electrolyte the wave occurs at ca. 1.2 V vs. SSCE. These potentials vary slightly between films.This behaviour opens up some intriguing possibilities of using this sort of polymer complex system as a proton-switched molecular wire or valve [127].

Figure 4.5: Cyclic voltammetry of a thin ( $\Gamma_{{Ru}}^{}$ = 3.6×10^-8 mol ^. cm^-2) film of Ru-PPyBBIM on a 0.0052 cm² Pt disc electrode. 0.1 M NEt₄ClO₄ in MeCN containing ca. 50 mM HClO₄ (solid line) or ca. 5 mM NBu₄OH (dashed line)

Figure 4.6: Changes in E_1/2 of a film of Ru-PPyBBIM exposed to a series of aqueous phosphate buffers. The indicated line has slope -63 mV/pH

The acid-base behaviour of the Ru(III/II) wave is predictable. In Figure 4.6, the relationship between E_1/2 and pH is plotted for a series of Ru-PPyBBIM cyclic voltammograms acquired in aqueous phosphate buffer electrolyte. It is evident that there is a linear relationship between these two parameters, with a slope of -63 mV/pH in the pH 2-4 region. This is consistent with the anticipated one-electron one-proton process. The relationship between E_1/2 and pH is an important one since it clearly indicates that the charge distribution along the polymer backbone, controlled by adding or removing the imidazole proton, has a direct influence over the electron density on the metal centre.

The steady state cyclic voltammetry of a thin film of Ru-PPyBBIM on Pt in neutral CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄ over a broad potential window typically resembles that presented in Figure 4.7. A pair of bipyridine-based reductions [125,128] appear at -1.50 and -1.76 V vs. SSCE. The Ru(III/II) couple is present near its previously established lower extreme of ca. 0.8 V, and varies somewhat between films.

Figure 4.7: Steady state cyclic voltammetry for a thin film of Ru-PPyBBIM on a 0.078 cm^-2 Pt disc electrode in CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄. $\nu$=100 mV/sec

An interesting development in the present voltammogram is the appearance of a pair of sharp pre-peaks, an oxidation before the Ru(III/II) wave, and a reduction before the first bipyridine wave. The pre-peaks vary in position and magnitude from film to film, and exist only as a pair; one peak will not appear for more than one or two cycles unless the sweep potential window is sufficient such that the other has been accessed. The evidence suggests that the pre-peaks result from the oxidation and reduction of water trapped within the polymer complex film: exposing the film to potentials below ca. -1.2 V reduces the water to OH^-, while sweeping the potential past ca. 0.6 V results in the oxidation of OH^- to O₂ and H⁺. This is supported by the following observations:

1. Either peak exists for any extended duration only when the other is present
2. The influence of the reduction pre-peak on the Ru(III/II) redox wave

The influence of hydroxide on this wave was detailed earlier. If the reduction pre-peak is in fact due to some hydroxide producing process, this can account for the observed position of the Ru(III/II) wave in Figure 4.7, near the limit exhibited in basic electrolyte. If the potential limits of cyclic voltammetry are set so that the reduction pre-peak is accessed, and then the lower limit changed so that the switching potential is made more positive than the pre-peak, a gradual drift of the Ru(III/II) wave towards the acidic limit is seen. This is demonstrated in Figure 4.8 (note that in this CV the oxidation pre-peak overlaps the Ru(III/II) wave).

Figure 4.8: Gradual migration of the Ru-PPyBBIM Ru(III/II) wave following change of the lower switching potential to more positive than the reduction pre-peak

The addition of a small quantity of water to the electrolyte solution has no effect on the position or magnitude of these pre-peaks. It appears then that the water involved in this process is trapped within the polymer matrix and has its origin either before or during the film casting process. Benzimidazole polymers are extremely hygroscopic [2], and on the basis of elemental analyses (see section 2.7) this trait is shared to some extent by their complexes.

The behaviour of the osmium analogue Os-PPyBBIM is similar with acid-mediated redox waves clearly evident. A cyclic voltammogram of Os-PPyBBIM in neutral CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄ is shown in Figure 4.9. Similar to the ruthenium analogue, the migration of the Os(III/II) wave to lower potentials is observed following exposure to negative potentials. It is interesting to note that the first potential sweep exhibits two distinct waves, suggesting localized protonated and deprotonated regions exist along the backbone. In Figure 4.10 the behaviour of the polymer film in acidic and basic electrolyte is presented, indicating the potential extremes of the Os(III/II) wave. In the acidified electrolyte, there appears to be some small response due to deprotonated backbone, suggesting the acid did not fully penetrate the film. Reasons for this discrepancy are not clear.

Figure 4.9: Cyclic voltammetry of a thin Os-PPyBBIM film in CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄

Figure 4.10: Cyclic voltammetry of two Os-PPyBBIM films in CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄ containing either a small quantity of HClO₄ (solid line) or Bu₄NOH (dashed line)

The cyclic voltammograms of Ru-PPyBDIM and Ru-PPyBBIM are very similar, and so the former will not be discussed in any detail. A list of M(II)-M(III) redox potentials is summarized in table 4.3

Table 4.3: Estimated E_1/2 for the M(III/II) couple for ruthenium and osmium polymer complexes. Potentials are given in volts vs. SSCE

	E1/2
Polymer complex	acid	base
Ru-PPyBBIM	1.24	0.79
Ru-PPyBDIM	1.23
Ru-PPzBBIM	1.13
	1.29
Os-PPyBBIM	0.78	0.40
Os-PPzBBIM	0.75
	1.075

Figure 4.11: Cyclic voltammetry of a thin film of Ru-PPzBBIM on a Pt disc electrode in 2:1 CH₂Cl₂:MeCN containing 0.1 mol dm^-3 Et₄NClO₄ and two drops HClO₄

Like its model compound 47, the ruthenium and osmium complexes of the pyrazine polymer, PPzBBIM, exhibit the unusual property of two distinct M(III/II) based redox waves, indicating a high degree of coupling through the pyrazine ring. In Figure 4.11 the pair of ruthenium waves are clearly evident at 1.13 and 1.29 V vs. SSCE. The large separation between the Ru(III),Ru(III)/Ru(III),Ru(II) and the Ru(III),Ru(II)/Ru(II),Ru(II) waves is a well known characteristic [122,129] of the famous Taube-Creutz compound [(NH₃)₅Ru(pz)Ru(NH₃)₅]⁵⁺ [130] and many pyrazine-bridged bis(ruthenium) variants such as this. Applying equation 1.4, K_com for this system is 5.0 ×10². Interestingly, the redox wave of the metal in a singly coordinated pyrazine is coincident with the first wave in a doubly coordinated ring (as opposed to occurring somewhere between the two waves). This was shown by a sample of Ru-PPzBBIM which had been prepared with one-half loading. The cyclic voltammograms in Figure 4.12 are for films of the half loaded and nearly fully loaded polymers. With the majority of Ru in singly occupied pyrazine units, the lower potential wave is much larger in magnitude than the second. This provides a quick and convenient means of ascertaining the fraction of metal centres that exist in dinuclear pyrazine sites in the polymer.

Figure 4.12: Cyclic voltammetry of films of fully loaded and half loaded Ru-PPzBBIM on a Pt disc electrode in 2:1 CH₂Cl₂:MeCN containing 0.1 mol dm^-3 Et₄NClO₄ and two drops HClO₄

The osmium polymer complex exhibits a pair of redox waves with an even larger $\Delta$E_1/2, ca. 0.33 V, as shown in Figure 4.13. K_com for the Os complex turns out to be 3.1 ×10⁵, indicative of the enhanced stability of the mixed valence unit in this system.

Figure 4.13: Cyclic voltammetry of a thin film of Os-PPzBBIM on a Pt disc electrode in 2:1 CH₂Cl₂:MeCN containing 0.1 mol dm^-3 Et₄NClO₄ and two drops HClO₄

Ru-PPzBBIM is not stable towards reduction as indicated in Figure 4.14. One sweep through the negative potential region results in the immediate disruption of the film's electrochemistry. Only in the first sweep are the two Ru(III/II) waves clearly evident in addition to a pair of irreversible reductions at -1.70 and -1.87 V vs. SSCE. The current magnitude of these reductions seems disproportionately large, indicating there is more involved than the simple bipyridine reductions observed in Ru-PPyBBIM, probably some process which degrades the backbone. This is further supported by the failure of the polymer to return to its neutral colour after being reduced (see section 4.2.3.2).

Figure 4.14: Three consecutive cyclic voltammograms of a thin film of Ru-PPzBBIM on a Pt disc electrode in 2:1 CH₂Cl₂:MeCN containing 0.1 mol dm^-3 Et₄NClO₄.

Attempts to coordinate Ru(bpy)₂²⁺ with PHyBDIM and PPyBDT were unsuccessful.