Modular synthesis of simple cycloruthenated complexes with state-of-the-art performance in p-type DSCs

Abstract

A modular approach based on Suzuki–Miyaura cross coupling and Miyaura borylation has been used to prepare two cyclometallated [Ru(N⁁N)₂(C⁁N)]⁺ complexes which possess either a carboxylic or phosphonic acid group attached via a phenylene spacer to the 4-position of the pyridine ring in the C⁁N ligand. The key intermediate in the synthetic pathway is [Ru(bpy)₂(1)]⁺ where bpy = 2,2′-bipyridine and H1 is 4-chloro-2-phenylpyridine. The crystal structure of [Ru(bpy)₂(1)][PF₆] is presented. Reaction of [Ru(bpy)₂(1)][PF₆] with 4-carboxyphenylboronic acid leads to [Ru(bpy)₂(H6)][PF₆], while the phosphonic acid analogue is isolated as the zwitterion [Ru(bpy)₂(H5)]. The cyclometallated complexes have been characterized by mass spectrometry, multinuclear NMR spectroscopy, absorption spectroscopy and electrochemistry. [Ru(bpy)₂(5)] adsorbs onto NiO FTO/NiO electrodes (confirmed by solid-state absorption spectroscopy) and its performance in p-type dye-sensitized solar cells (DSCs) has been compared to that of the standard dye P1; two-screen printed layers of NiO give better DSC performances than one layer. Duplicate DSCs containing [Ru(bpy)₂(H5)] achieve short-circuit current densities (J_SC) of 3.38 and 3.34 mA cm⁻² and photoconversion efficiencies (η) of 0.116 and 0.109%, respectively, compared to values of J_SC = 1.84 and 1.96 mA cm⁻² and η = 0.057 and 0.051% for P1. Despite its simple dye structure, the performance of [Ru(bpy)₂(H5)] parallels the best-performing cyclometallated ruthenium(II) dye in p-type DSCs reported previously (He et al., J. Phys. Chem. C, 2014, 118, 16518) and confirms the effectiveness of a phosphonic acid anchor in the dye and the attachment of the anchoring unit to the pyridine (rather than phenyl) ring of the cyclometallating ligand.