Conjugated Redox Polymers: Hybrid
Materials

The rate of electron exchange between the electrode and the catalyst sites is of obvious importance in the context of electrocatalysis. Facilitated electron transfer will translate to higher turnover rates. It was shown in the preceding sections that redox polymers have a somewhat slow rate of electron transport by self-exchange through a potential gradient, and that electronically conducting polymers exhibit rapid electron transport through a delocalized electronic structure. A polymer which somehow combines these properties would clearly be desirable; the use of an electronically conducting polymer as an electron pipeline is intuitively sensible. Early attempts to combine redox-active centres with conjugated backbones [42], such as the copolymer 3 [43], showed little improvement. In these cases the redox active group was held at some distance from the conjugated backbone by a saturated link and was therefore effectively isolated electronically from the backbone.

This thesis introduces the concept of coordinating a metal atom directly into the long-range $\pi$ system of a conjugated polymer as a means of enhancing electron transport. We have come to call these systems ``conjugated/redox polymer hybrids'' or ``conjugated metallopolymers''. At the onset of this project, only a few examples of this class of material existed in the literature, but they demonstrated many novel properties. Since then a few more examples have been reported including several from this research group [44,45,46,47]. Polymer hybrids have also been featured in two recent review articles [48,49]. It should be noted that a closely related type of polymer where a metal atom forms a bridge between two conjugated segments, for instance poly-ferrocenes [50,51], salen-based polymers [52,53,54], and the meta-linked bpy system of Petersen [55], has also been of interest. However, since these polymers do not possess a quasi-infinite conjugated backbone they will be considered outside the scope of this work.

The premise is that the hybrid system will facilitate self-exchange in much the same way that conjugated bridging ligands lead to more rapid self-exchange in dinuclear complexes [56]. One means of assessing the relative strength of the interactions between the bridged pair is through the comproportionation constant K_com. For the hypothetical system

$$\rightleftharpoons$$

$${\frac{{\left[\mbox{[M---L---M]}^{n+}\right]^2}}{{\left[\mbox{[M---L---M]}^{(n-1)+}\right] \left[\mbox{[M---L---M]}^{(n+1)+}\right]}}}$$ K_com = (1.2)

$$\left[\vphantom{\frac{ (E_1^\circ-E_2^\circ)F}{RT} }\right. {\frac{{ (E_1^\circ-E_2^\circ)F}}{{RT}}} \left.\vphantom{\frac{ (E_1^\circ-E_2^\circ)F}{RT} }\right]$$ = (1.3)

The strength of the interaction through the bridge is indicated by the separation of the voltammetric waves. [57]

The stability of the mixed valence state is a reflection on the strength of the electronic interactions between metal atoms, and this can be assessed voltammetrically if the interaction is strong enough. Materials that possess these strong interactions are being examined for use in advanced applications such as energy storage and molecular electronics [58].

Figure 1.9: Superexchange mechanisms. BL=bridging ligand

Conjugated bridging ligands are advantageous as they allow the metal atoms to interact through the adjoining $\pi$ segment, influencing the metal-metal communication. It has been recognized [59] in dinuclear Ru(II) complexes bridged by a conjugated ligand that K_com is primarily related to the orbital overlap between the metal atoms and the bridging ligand. The distance between metal centres is apparently less important. In a series of dinuclear Ru(II) complexes bridged with benzimidazole- and benzothiazole-based ligands [60,61,62,63,64] a superexchange mechanism is believed to be at play. As indicated in Figure 1.9, two pathways are possible. In the hole-type superexchange pathway, an electron is promoted from the bridging ligand (BL) HOMO to the M(III) site, giving the symmetric transition state with both metals in the reduced form. The missing electron from the BL HOMO is subsequently replaced by an electron from the other metal atom, resulting in a system where the valences have been swapped. Conversely, electron-type superexchange involves the transition of an electron from the M(II) state metal to the BL LUMO, leaving a symmetric symmetric transition state with the metals in the oxidized form. Exchange is completed by the electron's transfer to the opposing M(III) atom. The preferred pathway and the ease with which it is taken depend on the relative energies of the metal orbitals and the BL $\pi$ or $\pi^{*}_{}$ orbitals; metal-d and BL-$\pi$ orbital mixing leads to hole superexchange and metal-d and BL-$\pi^{*}_{}$ orbital mixing leads to electron superexchange. In either route the valences are interchanged, enhancing stability of the mixed valence system.

We propose that the concept of the bridging ligand superexchange mechanism can be extended to describe long distance charge migration in a hybrid polymer, with infinite propagation being achieved through the sequential superexchanges between neighbouring redox sites