title: Synthesis, photo-, and electrochemistry of ruthenium bis(bipyridine) complexes comprising a N-heterocyclic carbene ligand.
abstract: Analogues of [Ru(bpy)3](2+) were prepared in which one pyridine ligand site is substituted by a N-heterocyclic carbene (NHC) ligand, that is, either by an imidazolylidene with a variable wingtip group R (R = Me, 3a; R = Et, 3b; R = iPr, 3c), or by a benzimidazolylidene (Me wingtip group, 3d), or by a 1,2,3-triazolylidene (Me wingtip group, 3e). All complexes were characterized spectroscopically, photophysically, and electrochemically. An increase of the size of the wingtip groups from Me to Et or iPr groups distorts the octahedral geometry (NMR spectroscopy) and curtails the reversibility of the ruthenium oxidation. NHC ligands with methyl wingtip groups display reversible ruthenium oxidation at a potential that reflects the donor properties of the NHC ligand (triazolylidene > imidazolylidene > benzimidazolylidene). The most attractive properties were measured for the triazolylidene ruthenium complex 3e, featuring the smallest gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the series (2.41 eV), a slightly red-shifted absorption profile, and reasonable excited-state lifetime (188 ns) when compared to [Ru(bpy)3](2+). These features demonstrate the potential utility of triazolylidene ruthenium complexes as photosensitizers for solar energy conversion.
authors.affiliation: School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: Vivienne
authors.initials: V
authors.lastname: Leigh
authors.forename: Wadih
authors.initials: W
authors.lastname: Ghattas
authors.forename: Ralte
authors.initials: R
authors.lastname: Lalrempuia
authors.forename: Helge
authors.initials: H
authors.lastname: Müller-Bunz
authors.forename: Mary T
authors.initials: MT
authors.lastname: Pryce
authors.forename: Martin
authors.initials: M
authors.lastname: Albrecht