# Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells
## 3 Results and Discussion
### Spectroscopic Properties
#### 3.1.2 Multinuclear Systems
The IR signature of the NCS and CN groups bridged by Ag\({}^{+}\) is also the result of the individual contribution of the composing fragments **2** (Ru(TMAM)-Ag\({}^{+}\)) and **3** (2Z907-Ag\({}^{+}\)), corroborating the identity and the proposed stoichiometry of such compound. The complex **2** (Ru(TMAM))-Ag\({}^{+}\)) emits in ethanolic solution due to radiative deactivation of the lowest triplet state (T\({}_{1}\)) with an E\({}^{00}\) of 2.18 eV and a monoexponential lifetime of 222 ns, which was increased with respect to the parent compound **1** (Ru(TMAM)) (189 ns). The increased emission lifetime is consistent with the energy gap law [26,27]. The complex **3** (2Z907-Ag\({}^{+}\)) also showed an increased spectroscopic energy (E\({}^{00}\) 2.03 eV) with respect to the parent **Z907** compound (1.71 eV) and an increased emission lifetime of 23 ns with respect to less than 10 ns for the latter (Table 1). The 450 nm excitation of an isoabsorbing (with the respect to **2** (Ru(TMAM)-Ag\({}^{+}\)) solution of compound **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM))) revealed a considerable, but incomplete quenching of the emission centered on the Ru(TMAM) antenna due to energy transfer to the Z907 units. The incomplete quenching is evidenced by the broad emission band (Figure 4a, blue line) of the complex **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM)))), which bears both the contribution from the fragments **3** (2Z907-Ag\({}^{+}\)) and **2** (Ru(TMAM)-Ag\({}^{+}\)), resulting in an emission maximum (ca. 720 nm) which is an intermediate between that of **3** (2Z907-Ag\({}^{+}\)) (790 nm) (Figure 4a, red line) and that of **2** (Ru(TMAM)-Ag\({}^{+}\)) (665 nm) (Figure 4a, purple line). We have, however, observed partial photodissociation of complex **4** upon 450 nm excitation, which also resulted in the formation of a silver colloid in solution. In order to stabilize **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM))), we resorted to its immobilization on a ZrO\({}_{2}\) thin film, which is an inert semiconductor whose conduction band is not energetically accessible to the oxidative quenching of the excited state by electron transfer [28, 29, 30]. Actually, partial injection dynamics in sub-band gap states of ZrO\({}_{2}\) have been reported in the case of strongly reducing excited states of organic chromophores [31,32]. However, the excited state oxidation potential of those species was from ca. \(-\)400 to \(-\)600 meV higher than the Ru(II) species investigated in this work, from which we can thus expect a very marginal quenching by injection on zirconia substrates. When **4** was loaded on ZrO\({}_{2}\), we observed that no residual emission of fragment **2** (Ru(TMAM)-Ag\({}^{+}\)) was present in the emission spectrum of species **4** (Figure 4b, blue line), whose emissive features coincided with those of fragment **3** (2Z907-Ag\({}^{+}\)) (Figure 4b, red line). This confirms that the energy funneling from the antenna to the injecting units is, within the sensitivity of this technique, complete. We note that the improved stability of **4**, once adsorbed on semiconductor thin films can be explained by the lack of translational degrees of freedom of the coordinated metal fragments, which, being blocked together by the interaction with the surface can re-associate after a possible photodissociation event. On the other hand, in solution, dissociated fragments can diffuse away before re-association can occur, resulting  
Figure 3: Comparison between absorption spectra of: **(a) Z907** (black line) and **3** (2Z907-Ag\({}^{+}\)) (red line), **(b) 1** (Ru(TMAM)) (green line) and **2** (Ru(TMAM)-Ag\({}^{+}\)) (purple line) and **(c)** between **2** (purple line), **3** (red line) and **4** (2Z907-Ag\({}^{+}\)-Ru(TMAM)) (blue line). in the irreversible photochemical reduction of Ag\({}^{+}\), which leads to the formation of the observed silver colloids.