# Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells
## 4 Conclusions  
A multinuclear compound **4** comprising a Ru(TMAM)\({}_{2}\)(CN\({}_{2}\))\({}^{1+}\) (TMAM = (bis-4,4'-bis(trimethylaminomethyl)2,2'bipyridine) and 2 **Z907** units was obtained by bridging the ancillary ligands with Ag\({}^{+}\) ions. **4** displayed antenna effect revealing >95% energy transfer from the Ru(TMAM) unit to the Z907 species interacting with the titania surface (Figure 11). As a result, a 0.1 uM/cm\({}^{2}\) surface concentration of 4 allows for a 95% harvesting efficiency of green photons, which are converted to electrons with an external quantum yield of ca. 65-70%. Compared to Z907, the efficiency of the multinuclear adduct in a transparent DSSC was approximately doubled, however, the improvement, with respect to the multinuclear assembly (3) encompassing 2 Z907 complexes bridged by Ag\({}^{+}\), was quite marginal because of the 50% higher surface concentration achieved by the latter. This led to nearly identical harvesting efficiency within the same spectra region. When a fast redox couple like the [Co(bpy)\({}_{3}\)]\({}^{3+/2+}\) uses a multinuclear assembly, it provides the possibility of obtaining a superior shielding with respect to the parent mononuclear compound, which brings to a substantial reduction of the recombination current arising from electron recapture by Co(III) species. This shielding was achieved by both steric and electrostatic effects.  
Figure 10: (**a**) Open circuit photovoltage decay of **Z907** (black), **3** (red), and **4** (blue) sensitized solar cells; (**b**) -Z” vs. frequency plot of the same cells held at open circuit voltage.  
**Supplementary Materials:** The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/6/2769/s1, Figure S1: Structure of the synthe-sized species, Figure S2: \({}^{1}\)H-NMR spectrum of the TMAM ligand, Figures S3 and S4: \({}^{1}\)H and \({}^{13}\)C NMR spectra of the complex 1 (Ru(TMAM)), Figure S5: SI-MS spectrum of the complex 1 (Ru(TMAM)), Figures S6 and S7: \({}^{1}\)H and \({}^{13}\)C NMR spectra of the complex 2 ((Ru(TMAM))-Ag\({}^{+}\)), Figures S8 and S9: \({}^{1}\)H and \({}^{13}\)C NMR spectra of the complex 3 (ZZ907-Ag\({}^{+}\)), Figure S10: \({}^{1}\)H NMR (400 MHz, DMSO-d6, \(\delta\) ppm) of the complex 4 (ZZ907-Ag\({}^{+}\)-Ru(TMAM)), Figure S11: \({}^{13}\)C NMR (400 MHz, DMSO-d6, \(\delta\) ppm) of the complex 4 (ZZ907-Ag\({}^{+}\)-Ru(TMAM)), Figure S12: Spectroscopic features of the complexes, Figure S13: Comparison between calculated and experimental absorption spectra of the complex 4 (Z2907-Ag\({}^{+}\)-Ru(TMAM)), Figure S14: TCSPC decay on ZrO\({}_{2}\), Figure S15: Cyclic voltammetry of the complexes loaded on TiO\({}_{2}\), Figure S16: Charge separated state decay of 3 (ZZ907-Ag\({}^{+}\)), Figure S17: Integrated current density.  
**Author Contributions:** Conceptualization, C.A.B. and S.C.; Methodology, R.B., C.A.B. and S.C.; Investigation, E.M., R.B., V.C., R.A. and A.N.; Resources, S.C.; Data curation, E.M., V.C. and R.A.; Writing--Original draft preparation E.M. and C.A.B.; Writing--Review and editing, S.C. and E.M. All authors have read and agreed to the published version of the manuscript.  
**Funding:** E.M. thanks the Fondo Sociale Europeo, region Emilia Romagna for a grant.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** We gratefully thank [PERSON] (N.C. State University) and [PERSON] (UNC) for their fruitful discussions and the generous loan of 4-4\({}^{\prime}\)-bis(bromomethyl)-2-2\({}^{\prime}\)-bipyridine.  
**Conflicts of Interest:** The authors declare no conflict of interest.