Ruthenium complexes bearing N-heterocyclic carbene based CNC and CN^CH2C’ pincer ligands: Photophysics, electrochemistry, and solar energy conversion

Abstract

A combination of N-heterocyclic carbene (NHC) based CNC, or CN^CH2C′ pincer ligands and carboxy-phenyl terpyridine donors is used to prepare ruthenium complexes. The unsymmetrical coordination of CN^CH2C′ pincer ligand via the CN donor atoms gave a rigid five-membered ring, and binding through N^CH2C donor side rendered a six-membered chelate ring in [Ru(CN^CH2C′)(tpy4′-Ph-COOH)](PF6)2. The latter binding gives the ruthenium center a near-ideal octahedral configuration with a more potent ligand field that could destabilize the thermally accessible-d-d states. The ambient condition excited-state lifetimes became consequently more prolonged than in the small-bite angle [Ru(CNC)(tpy4′-Ph-COOH)](PF6)2 congener. Enhancement of lifetimes could be essential for better electron injection into the TiO2 conduction band. Photophysical attributes of these complexes are studied to evaluate their potential use in dye-sensitized solar cells (DSSCs). The electrochemical and computational study also suggests a favorable regeneration of [Ru(CN^CH2C′)(tpy4′-Ph-COOH)](PF6)2 bearing I3−/I−electrolyte in a DSSC set-up. We herein report the syntheses, photo-functional attributes, redox behavior, and the preliminary device characteristics. Computed geometries of complexes in different electronic states and the bonding attributes of NHC ligands in different pincer-motifs of CNC vs. CN^CH2C’ are also reported.