Dye­sensitized solar cells (DSSC) in which photon-­to-­electron conversion is carried out by a dye molecule adsorbed on nanocrystalline TiO₂, have long been considered a potential alternative to silicon­based PV devices,^[1]  but their industrial development has been so far hampered by their lower efficiency compared to other technologies, as well their limited lifetime, often due to partial dye desorption from the semiconductor layer.^[2] The aim of this study was to design, synthesize and characterize a small library of new DSSC sensitizers bearing innovative, pyridine-­based anchoring groups, and to compare their  performances (cell efficiency and stability) with those of similar dyes having a traditional cyanoacrylic acceptor unit. During the work, we focused on purely organic compounds characterized by the well-­known donor­-acceptor architecture (D­-π-­A),^[3] aiming to improve the anchoring moiety first. The common D­-π scaffold of the new dyes was built using palladium­-catalyzed cross-­coupling reactions as key synthetic steps. After suitable optimization, another Pd-catalyzed reaction, namely a Stille coupling, was used to introduce the novel anchoring units. Following such strategy, we managed to prepare the target dyes having tree regioisomeric carboxypyridines as acceptor groups. The new compounds underwent full spectroscopic, electrochemical and computational characterization, and their properties where compared with those of a reference compound endowed with a classic cyanoacrylic acid acceptor. Test devices prepared with these new dyes as sensitizers, provided power conversion efficiencies corresponding to 54-63% of those obtained with the reference compound. More significantly, device stability tests carried out on transparent, larger   area  cells  and  determination  of desorption  pseudo-­first  order  rate  constants  showed  that  some  of these  new compounds were removed from TiO₂  more slowly than the reference dye, suggesting, in the case of isomers having the nitrogen atom close to the carboxylic moiety, a possible cooperative effect of the two functional groups on semiconductor binding.^[4] At this point, we wondered if a higher photovoltaic efficiency could be obtained by increasing the electron-withdrawing character of the terminal pyridine rings. To this end, we selected two of the three isomers and prepared the corresponding N­-methylpyridinium salts. Despite more favorable photophysical properties, however, the cells built with the cationic sensitizers provided lower efficiencies than their neutral counterparts. Further investigations concerning the anchoring stability of the new N-­methylpyridinium salts on TiO₂   substrates are currently underway to confirm the "double binding" theory initially proposed for the original carboxypyridine compounds.

[1] Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595-6663. [2] Uam, H.-S.; Jung, Y.-S.; Jun, Y.; Kim, K.-J. Relation of Ru(II) dye desorption from TiO2 film during illumination with photocurrent decrease of dye-sensitized solar cells. J. Photoch. Photobio. A 2010, 212, 122–128. [3] Ooyama, Y.; Harima, Y. Photophysical and Electrochemical Properties, and Molecular Structures of Organic Dyes for Dye-Sensitized Solar Cells. ChemPhysChem. 2012, 13, 4032–4080. [4] Franchi, D.; Calamante, M.; Reginato, G.; Zani, L.; Peruzzini, M.; Taddei, M.; Fabrizi de Biani, F.; Basosi, R.; Sinicropi, A.; Colonna, D.; Di Carlo, A.; Mordini, A. A Comparison of Carboxypyridine Isomers as Sensitizers for Dye-Sensitized Solar Cells: Assessment of Device Efficiency and Stability. Tetrahedron 2014, 70, 6285-6295.