title: When two are better than one: bright phosphorescence from non-stereogenic dinuclear iridium(III) complexes.
abstract: A new family of eight dinuclear iridium(iii) complexes has been prepared, featuring 4,6-diarylpyrimidines L(y) as bis-N^C-coordinating bridging ligands. The metal ions are also coordinated by a terminal N^C^N-cyclometallating ligand L(X) based on 1,3-di(2-pyridyl)benzene, and by a monodentate chloride or cyanide. The general formula of the compounds is {IrL(X)Z}2L(y) (Z = Cl or CN). The family comprises examples with three different L(X) ligands and five different diarylpyrimidines L(y), of which four are diphenylpyrimidines and one is a dithienylpyrimidine. The requisite proligands have been synthesised via standard cross-coupling methodology. The synthesis of the complexes involves a two-step procedure, in which L(X)H is reacted with IrCl3·3H2O to form dinuclear complexes of the form [IrL(X)Cl(μ-Cl)]2, followed by treatment with the diarylpyrimidine L(y)H2. Crucially, each complex is formed as a single compound only: the strong trans influence of the metallated rings dictates the relative disposition of the ligands, whilst the use of symmetrically substituted tridentate ligands eliminates the possibility of Λ and Δ enantiomers that are obtained when bis-bidentate units are linked through bridging ligands. The crystal structure of one member of the family has been obtained using a synchrotron X-ray source. All of the complexes are very brightly luminescent, with emission maxima in solution varying over the range 517-572 nm, according to the identity of the ligands. The highest-energy emitter is the cyanide derivative whilst the lowest is the complex with the dithienylpyrimidine. The trends in both the absorption and emission energies as a function of ligand substituent have been rationalised accurately with the aid of TD-DFT calculations. The lowest-excited singlet and triplet levels correlate with the trend in the HOMO-LUMO gap. All the complexes have quantum yields that are close to unity and phosphorescence lifetimes - of the order of 500 ns - that are unusually short for complexes of such brightness. These impressive properties stem from an unusually high rate of radiative decay, possibly due to spin-orbit coupling pathways being facilitated by the second metal ion, and to low non-radiative decay rates that may be related to the rigidity of the dinuclear scaffold.
authors.affiliation: Department of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. valery.kozhevnikov@northumbria.ac.uk.
authors.forename: Ruth E
authors.initials: RE
authors.lastname: Daniels
authors.affiliation: Department of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. valery.kozhevnikov@northumbria.ac.uk.
authors.forename: Stacey
authors.initials: S
authors.lastname: Culham
authors.affiliation: Department of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. valery.kozhevnikov@northumbria.ac.uk.
authors.forename: Michael
authors.initials: M
authors.lastname: Hunter
authors.affiliation: Department of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. valery.kozhevnikov@northumbria.ac.uk.
authors.forename: Marcus C
authors.initials: MC
authors.lastname: Durrant
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Probert
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
authors.forename: William
authors.initials: W
authors.lastname: Clegg
authors.affiliation: Department of Chemistry, Durham University, Durham, DH1 3LE, UK. j.a.g.williams@durham.ac.uk.
authors.forename: J A Gareth
authors.initials: JA
authors.lastname: Williams
authors.affiliation: Department of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. valery.kozhevnikov@northumbria.ac.uk.
authors.forename: Valery N
authors.initials: VN
authors.lastname: Kozhevnikov