Journal of Photochemistry and Photobiology A: Chemistry
Preparation and photoelectrochemical characterization of a red sensitive osmium complex containing 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine and cyanide ligands
Introduction
In the past 15 years there has been much interest to increase the stability and performances of the dye sensitized solar cells (DSSCs) developed by the research group of Hagfeldt and Graetzel [1]. Transition metal charge transfer complexes have been successfully employed as sensitizers of photoanodes based on wide band-gap semiconductors, which represent a key component of the DSSC. Despite the fact that these dyes can exhibit photochemical instability or irreversible electrochemistry, with ligand substitution reactions taking place from the one-electron oxidized form [2], a remarkable stability was observed in long term experiments on sealed DSSCs [3]. This unique feature is due to a peculiar series of electron transfer processes which occur in the DSSCs, and in particular to ultrafast charge injection from the electronically excited dye [4], [5], [6] and to fast reduction of the nascent oxidized dye by an electron transfer mediator [7], [8], [9], normally dissolved in the electrolyte solution. Recently, attention has been paid to the preparation of solid state hole transfer mediators [10], [11] which are obviously of great interest, since allow an easy fabrication of the DSSC, but may have as drawback the fact that have a high resistance and do not provide a homogeneous and intimate contact with the dye molecule adsorbed on the mesoporous semiconductor. These features can be responsible for the slow reduction of the oxidized form of the dye, with consequent decomposition. It can be foreseen, therefore, that a long term stability of solid state DSSCs will require the use of dye molecules exhibiting a good stability in their oxidized forms.
Previous studies by the Lewis group [12], [13] have shown that osmium complexes containing the 4,4′-dicarboxylic acid 2,2′-bipyridine ligand display a reversible electrochemistry and allow at the same time to extend the spectral response of TiO2 photoanodes, providing photocurrents and external quantum yields comparable to those of analogous ruthenium complexes. In this paper, we wish to report the preparation and characterization of the [OsII(H3tcterpy)(CN)3](TBA) complex (Scheme 1) (H3tcterpy=4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine, TBA=tetrabutylammonium), and of its differently protonated species which were found to exhibit a reversible electrochemical behavior as well as photochemical stability and a considerable red sensitivity.
Section snippets
Materials
The following chemicals were purchased and used without further purification: (NH4)2OsIVCl6 (Alfa Aesar, Johnson Matthey), TBAOH (40 wt.% solution in water, Aldrich), TBACN (Fluka). The ligand 4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine (H3tcterpy) was available from previous studies. (TBA)2OsIVCl6 was precipitated by addition of TBACl (Fluka) to an aqueous solution of (NH4)2OsIVCl6. The complex [RuII(H3tcterpy)(NCS)3]− was either available from previous studies or prepared by the synthetic
Results and discussion
The formulation of the complex [OsII(H3tcterpy)(CN)2(CNH)] as a neutral species with one protonated cyanide has been confirmed by elemental analysis, NMR and FT-IR. The NMR spectrum is consistent with C2v symmetry and shows the characteristic signals of the coordinated H3tcterpy ligand in the aromatic region and the absence of any TBA cation in the aliphatic region, according to the elemental analysis findings. The presence of one protonated cyanide is directly demonstrated by the comparison
Conclusion
A simple synthetic procedure has been devised for the preparation of the complex [OsII(H3tcterpy)(CN)3]− and more in general for complexes of formula: [MII(H3tcterpy)(X)3]− (MRu, Os; XCN−, NCS−, Cl−, etc.). The Os complex shows an interesting structured UV-Vis spectrum, covering the whole visible region and part of near infrared, and a perfectly reversible electrochemistry. The presence of carboxylic functions on the chromophoric ligand H3tcterpy allow for fine tuning of the electronic
Acknowledgements
This work was supported by IMRA Europe and by MIUR FIRB Project No. RBNE019H9K.
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