Modular synthesis of simple cycloruthenated complexes with state of the art performance in p type DSCs
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 (JSC) of 3.38 and 3.34 mA cm 2 and photoconversion efficiencies (e) of 0.116 and 0.109% respectively, compared to values of JSC 1.84 and 1.96 mA cm 2 and e 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.