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.

An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand

“…Both cyclometalating carbon atoms are trans to the pyridine rings of the d t Bubpy, and the pyridyl unit of the bnpy-type ligands is trans to the chloride. This contrasts with the configuration of the Ir–Cl bond in previously reported Ir(III) complexes, 29,31,32,36,37,40,41 where an Ir–C bond is trans to the chloride ligand. For 1 – 3 , the Ir–Cl bond [2.375(3) Å for 1 , 2.3612(8) Å for 2 , and 2.369(2) Å for 3 ] is in the same range as that found for [Ir(tpy)(dmbpy)Cl] 2+ (2.357 Å, where tpy = 2,2′:6′,2″-terpyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine) 42 but is significantly shorter (by ca.…”

“…0.1 Å) than the Ir–Cl bond in other cyclometalated tridentate Ir(III) complexes. 29,31,32,36,37,40,41 Given the short Ir–C C^N^C bonds [2.048(13) and 2.064(6) Å for 1 , 2.028(4) and 2.031(3) Å for 2 , and 2.017(7) and 2.027 Å for 3 ], this leads also to a correspondingly shorter Ir–N C^N^C bond [2.055(11) Å for 1 , 2.044(3) for 2 , and 2.032(7) Å for 3 ] compared to the Ir–N d t Bubpy bonds [2.158(10) and 2.159(11) Å for 1 , 2.127(3) and 2.140(3) Å for 2 , and 2.122(6) and 2.133(5) Å for 3 ]. The bite angle of the N^N ligand is unremarkable at 75.60(4)° for 1 , 75.85(12)° for 2 , and 76.1(2)° for 3 and in line with cationic Ir(III) complexes of the form [Ir(C^N) 2 (N^N)] + .…”

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

“…As a consequence, fast intersystem crossing (ISC) to the lowest triplet state of several tens of fs can occur and relatively high phosphorescence rates from the T 1 state to the electronic ground state S 0 are induced. These latter rates can become as high as ≈10 6 s −1 . Therefore, these phosphorescent compounds are frequently denoted as triplet emitters.…”

Synthesis, Diastereomer Separation, and Optoelectronic and Structural Properties of Dinuclear Cyclometalated Iridium(III) Complexes with Bridging Diarylhydrazide Ligands

“…[44][45][46] In diiridium complexes, which incorporate conjugated bridging ligands, theoretical calculations often predict that a degree of frontier orbital character is localized on the bridging unit. 23,26,29,32,35,[37][38][39][40]42 In these cases a highly flexible bridge is expected to promote nonradiative processes and weaken emission. Consequently, the majority of the reported diiridium complexes that possess high solution state PLQYs incorporate rigid bridging units (such as compounds 4 and 6).…”

“…These examples demonstrate that correctly designed diiridium complexes can overcome the limitation of poor luminescence and present a feasible alternative to typical monoiridium systems. 38 An important feature of the bridging ligands within diiridium complexes is that they provide an increased potential for structural variation compared to monoiridium systems. 37,[41][42][43] Bimetallic complexes also present a number of other potential advantages, such as increased spin-orbit coupling due to the presence of multiple metal centers (which may lead to an increase in the radiative rate constant (k r )), easier access to efficient red emitters due to conjugated bridging units, 36,38 higher stability due to the improved chelating effect of a bridging ligand, and the possibility of dual emission.…”