# Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells
## 3 Results and Discussion
### TiO\({}_{2}\) Sensitization  
Both the adducts **3** and **4** incorporating **Z907** complexes adsorb spontaneously on TiO\({}_{2}\). The adsorption of **1** (Ru(TMAM)) and of its binuclear derivative **2** (Ru(TMAM)-Ag\({}^{+}\)) is quite weak, and occurs by a mixture of electrostatic interactions or via acid-base interaction of the CN lone pair with Ti(IV). While this interaction is sufficiently strong to allow some spectroscopic studies, like the ZrO\({}_{2}\) emission discussed before, it is far too weak to guarantee a sizable light harvesting for energy conversion application. We thus focused mainly on the behavior of the **Z907** containing multinuclear complexes. Absorption spectra of the sensitized titania films are reported in Figure 7a, showing that both complexes **4** and **3** exhibited a very strong spectral similarity in the visible region. With respect to **Z907** alone, a blue shifted absorption was obtained, with a main visible band peaking at 525 nm, with an optical density ca. 40% higher than the **Z907** sensitized thin film. A further confirmation of the assigned nuclearity of the complexes loaded on the TiO\({}_{2}\) film can be gained by cyclic voltammetry, where, given the insulating nature of TiO\({}_{2}\) at anodic potentials, one relies on the oxidation of species electronically coupled with the FTO collector and subsequent charge hopping between adjacent surface adsorbed species. Through cyclic voltammetry (Figure S15), we observed that in species **3** (2Z907-Ag\({}^{+}\)), the intensity of the anodic wave associated to the Ru(II)/(III) oxidation in **Z907** (ca.0.7 V vs. SCE) was positively shifted (by ca. 100 mV) and nearly doubled in intensity, given that the **2 Z907** units bridged by Ag\({}^{+}\), being weakly coupled, underwent simultaneous oxidation. In compound **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM))), the bielectronic anodic wave corresponding to the oxidation of the **2 Z907** fragments was accompanied by a second more positive wave at ca. 1.1 V vs. SCE, having ca. half intensity with respect to the former and is  
Figure 6: Time resolved emission profiles on ZrO\({}_{2}\) thin film: (**a**) **2** (Ru(TMAM)-Ag\({}^{+}\)), (**b**) **3** (2907-Ag\({}^{+}\)), (**c**) **4** (ZZ907-Ag\({}^{+}\)-Ru(TMAM)), and (**d**) Comparison between emission spectra of complex **3** and of complex **4** at 0 delay. Thin films are in contact with 0.1 M Li\({}^{+}\) in ACN. consistent, by comparison with the redox behavior of the mononuclear analog in solution, with the Ru(II)/Ru(III) oxidation localized on the unit **1** (Ru(TMAM)). The surface loading, computed from A(\(\lambda\)) = 1000\(\Gamma\)\(\varepsilon\)(\(\lambda\)) was for the significant cases (**Z907** containing species) of the order of 10\({}^{-7}\) mol/cm\({}^{2}\) (Figure 7b), with the highest chromophore concentration found for species **3** (ZZ907-Ag\({}^{+}\)) (1.5 \(\times\) 10\({}^{-7}\) mol/cm\({}^{2}\)), which nearly completely offset the intrinsically superior light harvesting properties of **4** (ZZ907-Ag\({}^{+}\)-Ru(TMAM)), which achieved a 33% lower concentration, probably due to a combination of higher steric hindrance and electrostatic repulsion between the adsorbed species. The 532 nm excitation of **3** (ZZ907-Ag\({}^{+}\)) and of **4** (ZZ907-Ag\({}^{+}\)-Ru(TMAM)) resulted in both cases, within the instrumental time resolution of our spectrometer (10 KOHm pre-amplifier), in the generation of a very long lived charge separated state corresponding to Z907\({}^{+}\)/e\({}^{-}\)(TiO\({}_{2}\)) (Figure 8). Such a charge separated state does not recover on a time scale of 1 ms (Figure S16), leaving plenty of time for regeneration from the redox electrolyte to occur. It should be noted that, since the oxidation of the unit **1** (Ru(TMAM)) is more positive than that of **Z907**, the hole remains confined within such a unit.