Improved light absorbance does not lead to better DSC performance: studies on a ruthenium porphyrin–terpyridine conjugate

Abstract

The preparation and characterization of 7-(4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)-5,10,15,20-tetraphenylporphyrinatozinc(II), 3, are reported, and the structure of 3 has been confirmed by a single crystal structure determination. Reaction of RuCl₃·3H₂O with diethyl (4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)phosphonate, 4, followed by 3 in reducing conditions gives [Ru(3)(4)][PF₆]₂. In solution, 3 and [Ru(3)(4)][PF₆]₂ undergo two, reversible porphyrin-centred oxidation processes at lower potential than the Ru²⁺/Ru³⁺ process in [Ru(3)(4)][PF₆]₂. In the solution absorption spectra, the Soret and Q bands in 3 are little perturbed upon complex formation; the MLCT band in [Ru(3)(4)][PF₆]₂ has λ_max = 492 nm. Spectroelectrochemical data for 3 and [Ru(3)(4)][PF₆]₂ are presented. [Ru(3)(4)]²⁺ binds to nanoparticulate TiO₂ and the solid-state absorption spectrum confirms enhanced light absorption with respect to the standard dye-sensitized solar cell (DSC) dye N719. However, the photoconversion efficiencies of DSCs sensitized with [Ru(3)(4)]²⁺ are disappointingly low. Transient absorption spectroscopic studies on this series of compounds indicate that triplet–triplet energy transfer processes are likely to be responsible for this poor performance.