Optical Properties

UV-visible spectroscopy

The UV-visible spectrum of Ru-PPyBBIM is shown in Figure 4.1 overlaid with the uncoordinated polymer for comparison. It is evident that the coordination compound retains many features of the parent polymer. The intense polymer $\pi$ $\rightarrow$ $\pi^{*}_{}$ transition is shifted only slightly, from 400 to 410 nm, although this could be in part attributed to solvent effects. New absorptions at 292 and 342 nm are attributed to bipyridine-based $\pi$ $\rightarrow$ $\pi^{*}_{}$ and MLCT transitions respectively [124]. A broad Ru(II) $\rightarrow$polymer d $\rightarrow$ $\pi^{*}_{}$ [125] MLCT band appears on the shoulder of the $\pi$ $\rightarrow$ $\pi^{*}_{}$ peak at ca. 510 nm. This is red-shifted considerably from its position in model compound 42 (ca. 450 nm), indicating that the Ru centre is indeed incorporated into the conjugated polymer backbone [83].

Figure 4.1: UV-Vis spectra of Ru-PPyBBIM in wet nitromethane (solid line) and PPyBBIM in dimethyl acetamide (dashed line)

The spectral characteristics of Ru-PPyBBIM vary as a function of pH. In Figure 4.2, this is illustrated. A stock solution of the polymer complex was diluted quantitatively in a series of phosphate buffers whose pH had been measured. The resulting spectra were normalized to the local minimum at 264 nm to compensate for minor variations due to dilution and spectrometer errors.

Figure 4.2: Spectral changes in Ru-PPyBBIM. Arrows indicate the direction of change with increasing pH

As the pH is raised, the intensity of the polymer and bipyridine $\pi$ $\rightarrow$ $\pi^{*}_{}$ transitions decrease while the intensity of the MLCT band increases. The changes are fairly gradual except in the pH$\simeq$5-6 region. This behaviour strongly parallels that of the reported [62] dinuclear ruthenium complex 42 whose pKa₁ was estimated to be 5.8. The isosbestic points at 360 and 450 nm suggest that protonation and deprotonation do not inflict irreversible change in the polymer complex.

Electrochromism

As is typical of pyridine-type complexes of these metals, the ruthenium and osmium metallopolymers are characterized by intense coloration due to charge transfer. The previous section detailed the spectral changes resulting from the modification of the polymer ligand electron density by protonation. Changes in the metal centres' oxidation states by electrochemical means produces the spectacular colour changes summarized in table 4.2.

Table 4.2: Electrochromism in the polymer complexes

Polymer	neutral colour	oxidized colour	reduced colour
Ru-PPyBBIM	red	green-yellow	brown
Ru-PPyBDIM	red	green-yellow
Ru-PPzBBIM	green	red-brown (1)	brown
		purple (2)
Os-PPyBBIM	red-brown	green	dark brown
Os-PPzBBIM	green	red (1)
		purple (2)

Two colour changes are observed for Ru-PPzBBIM and for Os-PPzBBIM, one for each oxidation. After being reduced, Ru-PPzBBIM does not revert to its original green colour, suggesting the polymer complex is damaged by exposure to negative potentials.

Figure 4.3: Differential spectroelectrochemical response of Ru-PPyBBIM as deposited on an ITO electrode. Electrolyte solution is CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄. Spectra are referenced against that acquired at 0 V, and are shown in 0.2 V intervals

Figure 4.4: Differential spectroelectrochemical response of Ru-PPyBBIM as deposited on an ITO electrode. Electrolyte solution is CH₃CN containing 0.1 mol dm^-3 Et₄NClO₄. Spectra are referenced against that acquired at 0 V, and are shown in 0.2 V intervals

The differential spectroelectrochemistry of a film of Ru-PPyBBIM deposited on an optically transparent ITO electrode is shown in Figure 4.3 for positive potentials only, and in Figure 4.4 over a wider voltage range. Each spectrum is presented as the change in transmission with respect to a reference spectrum taken at 0 V. Bleaching of the Ru-polymer d$\pi$-to-L$\pi^{*}_{}$ MLCT transition at 497 nm is observed as Ru(II) is oxidized to Ru(III). Concurrently, a broad new band due to polymer-Ru(III) LMCT [126,85] arises with $\lambda_{{\mbox{\scriptsize\it max}}}^{}$ = 775 nm. This behaviour is similar to the solution spectroelectrochemistry of the dinuclear Ru complex bridged by model compound 34 [61]. It should be pointed out that the bleaching is due entirely to the change of oxidation state at the metal centre, not to dissociation of the complex.

Reducing the polymer film down to -1.4 V results in the intensification of the Ru-polymer d$\pi$-to-L$\pi^{*}_{}$ MLCT, indicating that the introduced electrons are indeed located on the bipyridine ligands; concentration of electron density on the polymer would discourage this transition. Further reduction to -2 V leads to the splitting of this band into two new absorptions at 442 and 557 nm. It is also interesting to note that slight spectral changes are detected in potential regions where the polymer complex is electrochemically inactive.