Synthesis and properties of a meso- tris–ferrocene appended zinc(ii) porphyrin and a critical evaluation of its dye sensitised solar cell (DSSC) performance

Abstract

A zinc(II) porphyrin derivative, F3P, was prepared containing a single ferrocene group appended at three of the meso positions. The final meso position contains a benzoic acid unit, and was designed to be an anchoring moiety to a semiconductor surface. The cyclic voltammogram for the porphyrin derivative reveals that the three ferrocene units are oxidised and reduced at E_1/2 = +0.04 V vs. Fc⁺/Fc. Other redox responses are porphyrin-based at +1.1 V vs. Fc⁺/Fc (irreversible) and −1.87 V and −2.20 V vs. Fc⁺/Fc (both quasi-reversible). The room temperature Mössbauer spectrum for F3P comprises a doublet (δ Fe 0.44 mm s⁻¹; ΔE_Q 2.35 mm s⁻¹; peak width 0.27 mm s⁻¹). The data are consistent with the ferrocene groups non-interacting with the porphyrin moiety in the ground state. Excitation of the compound in THF with an ultra-short laser pulse produces in less than 1 ps the charge separated state which comprises Fc⁺˙–porp⁻˙ and decays back to the ground state in 24 ps. Roughly the same behaviour is observed for the dye absorbed on TiO₂, the only slight difference is the appearance of a long-lived component in the decay records. The electron for the porphyrin radical anion does not inject into the conduction band of the TiO₂, only the porphyrin excited state participates in any electron injection process. Partly because of this factor the performance of the dye attached to TiO₂ in a DSSC device is rather limited (J_SC = 0.068 mA cm⁻², V_OC = 283 mV, FF = 0.42, η = 0.008%).