Tailoring mixed-valence CoIII/FeII complexes for their potential use as sensitizers in dye sensitized solar cells

Abstract

Dinuclear class II Co^III/Fe^II mixed-valence complexes of type [L_nCo^III(µ-NC)Fe^II(CN)₃L₂]^m− (where L_n represents a pentadentate macrocycle and L₂ a bpy ligand or two cyanides) have electronic properties that make them possible sensitizers in DSSC (dye sensitized solar cells). For this purpose the new complex Na₂[{trans-L_14COOCo^III(µ-NC)}Fe^II(CN)₅] has been prepared and characterized by the usual methods and its sensitizer properties compared with those of the already known [{trans-L₁₅Co^III(µ-NC)}Fe^II(CN)₃(bipy)_eq,ax](ClO₄). The complex [{trans-L_14COOCo^III(µ-NC)}Fe^II(CN)₅]²⁻ has been designed for the actuation of an electron injection from the cobalt centre on MMCT, while the [{trans-L_nCo^III(µ-NC)}Fe^II(CN)₃(bipy)]⁺ structure can produce a tuned injection from the iron centre via an MLCT electronic state, as described for similar systems. The characterization on solid oxide semiconductor supports has been carried out for these two complexes and the results have been compared with the behaviour observed in aqueous solution and in solvents of varying polarities. Their use in DSSC has been checked and, while a sensitizer response is observed for [{trans-L_14COOCo^III(µ-NC)}Fe^II(CN)₅]²⁻, complex [{trans-L₁₅Co^III(µ-NC)}Fe^II(CN)₃(bipy)_eq,ax]⁺ does not produce any electrical current on illumination. The low efficiency of the built DSSC can be easily related both with the very low value of the extinction coefficient of the MMCT band responsible for the electron injection, and with the small driving force for the reduction of the complex with the standard I₂/I₃⁻ system used after electron injection.