# Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells
## 3 Results and Discussion
### Time Resolved Spectroscopy in Solution and on ZrO\({}_{2}\) Thin Films  
The 532 nm laser excitation (FWHM of 7 ns) of the coordination compounds herein reported led to the population of the lowest triplet charge transfer state within the instrumental time response (Figure 5). We note that excitation was in the CT absorption manifold of both the **1** (Ru(TMAM)) and **Z907** related moieties. Species **3** (Z2907-Ag\({}^{+}\)) exhibited transient spectroscopic features that are summarized by the bleaching of the ground state absorption bands at 450 and 540 nm, followed by a broad triplet absorption with a maximum at ca. 680 nm. A weak negative band followed in the NIR region caused by spontaneous emission, consistent with the previously discussed steady state emissive properties. **1** (Ru(TMAM)) and **2** (Ru(TMAM)-Ag\({}^{+}\)) displayed nearly identical triplet features with an intense ground state bleaching at ca. 490 nm followed by the spontaneous emission in the 600-800 nm interval. A high energy absorption rose for \(\lambda\) < 430 nm. The transient spectrum of the **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM))) compound was almost identical to that of the **2** (Ru(TMAM)-Ag\({}^{+}\)) since, at the same laser excitation energy, **2** featured \(\Delta\)A amplitudes that were about ten times larger (up to \(-\)500 m\(\Delta\)OD vs. \(-\)60 m\(\Delta\)OD) than those of the **Z907** fragment. In addition, the residual spontaneous emission of photodissociated **1** (Ru(TMAM)) or **2** (Ru(TMAM)-Ag\({}^{+}\)) fragments, resulting in a bleaching band at \(\lambda\) > 650 nm, offset the much weaker absorption of the excited state centered on the Z907 units. This corroborates the indication of limited photostability of the tetranuclear adduct **4** in solution under visible light excitation. For the complexes under investigation, the absorption and scattering properties of the ZrO\({}_{2}\) thin films prevented a fruitful application of the time resolved absorption spectroscopy, however, the emission properties could be studied with some detail. The transient emission spectra, taken on sensitized ZrO\({}_{2}\), revealed the distinct emission features of the complexes **2** (Ru(TMAM)-Ag\({}^{+}\)) (Figure 6a) and **3** (2907-Ag\({}^{+}\)) (Figure 6b). For the former. the emission maximum was found at 640 nm, with an average lifetime of 41 ns. The shortening of the excited state lifetime, with respect to the solution kinetics, was due to aggregation of the chromophores on the semiconductor surface, which enhanced the non-radiative deactivation pathways of the triplet state, whereas the multiexponential decay stemmed from the heterogeneity of surface adsorption sites, which caused a dispersion in excited state energetics and a distribution in decay rates. Compound **3** (2907-Ag\({}^{+}\)) exhibited an emission centered at 750 nm, substantially convolved with the time response of the laser pulse. Sampling of the emission signal allowed us to well define the shape of the band  
Figure 4: Emission spectra (\(\lambda_{\rm exc}\) = 450 nm) of **(a)** **1** Ru(TMAM) (green line), **2** (Ru(TMAM)-Ag\({}^{+}\)) (purple line), **3** (2Z907-Ag\({}^{+}\)) (red line), and **4** (2Z907-Ag\({}^{+}\)-Ru(TMAM)) (blue line) in ethanolic solutions and of **(b) Z907** (black line), **2** (Ru(TMAM)-Ag\({}^{+}\)) (purple line), **3** (2Z907-Ag\({}^{+}\)) (red line), and **4** (2Z907-Ag\({}^{+}\)-Ru(TMAM)) (blue line) supported on ZrO\({}_{2}\). up to a delay of 26 ns after the laser pulse. The shape of emission spectrum of complex **4** (2Z907-Ag\({}^{+}\)-(Ru(TMAM)))), Figure 6c, was nearly identical to that of species **3** (2907-Ag\({}^{+}\)), except for a very small shoulder observed around 640 nm and assigned to a residual unquenched **2** (Ru(TMAM)-Ag\({}^{+}\)) emission. This residual intensity, upon normalization by the light harvesting of the film at the excitation wavelength, provided a quenching efficiency of 94%.