Luminescent platinum(II), palladium(II) and gold(III) complexes containing isocyanide, alkynyl and N-heterocyclic carbene ligands : synthesis, photophysical properties and material applications

Abstract

Several cyclometalated Pt(II) isocyanide complexes containing C-deprotonated C^N^C ligands (C^N^C = 2,6-diphenylpyridine derivatives) were synthesized. These complexes display orange-red emissions with max at 582–619 nm and quantum yields of up to 26% in CH2Cl2 at room temperature. The incorporation of carbazole/fluorene/thiophene unit(s) to C^N^C ligands leads to minimized structural distortion of complexes in their excited states and thereby suppresses non-radiative decay pathways. The high thermal stability (Td >300 °C) renders these complexes good candidates as phosphorescent dopants in organic light-emitting diodes (OLEDs). Red-emitting OLEDs with CIE coordinates of (0.650.01, 0.350.01) were fabricated by vacuum deposition, showing a maximum external efficiency of 12%. In addition, well-defined nano/microstructures were obtained from self-assembly of these complexes driven by π∙∙∙π, C–H∙∙∙π and C–H∙∙∙H–C interactions as observed in the crystal structures.

Two series of organopalladium(II) alkynyl complexes containing a terpy (terpy = 2,2’:6’,2’’-terpyridine) or C^N^C pincer carbene ligand (C^N^C = 2,6-bis(1-butylimidazol-2-ylidenyl)pyridine) were prepared. These complexes are non-emissive in solution at room temperature except that the one containing both C^N^C and pyrenylacetylide ligands shows phosphorescence (Φ = 0.3%) originating from intraligand state of the acetylide ligand. This could be attributed to the strong -donating N-heterocyclic carbene (NHC) in the pincer ligand that strongly destabilizes d-d state, the population of which provides an efficient non-radiative decay channel. To make comparison between Pd(II) and Pt(II) complexes with the two ligand systems, Pt(II) C^N^C alkynyl complexes were also prepared. They are emissive in solution and some display excimer emissions at high concentration (〖10〗^(-4)–〖10〗^(-3) mol 〖10dm〗^(-3)). The X-ray crystal structures of [Pd(L)(CCPh)](〖PF〗_6) (L = terpy and C^N^C) revealed one-dimensional chain stacking of complex cations with alternating Pd(II)∙∙∙Pd(II) contacts of about 3.29–3.35 Å and π-π interactions of about 3.4 Å . Well-defined submicron/nanostructures were obtained from self-assembly of Pd(II) and Pt(II) alkynyl complexes driven by π-π interactions between aromatic moieties and/or metal∙∙∙metal interactions. DFT calculations on the optimized structures of [M(L)(CCPh)]+ (M = Pd(II) and Pt(II)) revealed the existence of metal∙∙∙metal closed-shell interactions. In addition, the complex containing the C^N^C ligand exhibits slightly enhanced metal∙∙∙metal interactions and larger “bonding” energy upon dimerization. Furthermore, spin-orbit coupling between singlet and triplet excited states is more effective which promotes rapid intersystem crossing.

A new class of cyclometalated Au(III) complexes containing C-deprotonated C^N ligands (C^N = 2-phenylpyridine and its derivatives) and cis-chelating bis-NHC ligands was synthesized. These are the first examples of Au(III) complexes supported by cis-chelating bis-NHC ligands. They display emissions in solution under degassed condition at room temperature with λmax at 498–633 nm and quantum yields of up to 10.1%. Some exhibit dual emissions which are assigned to prompt fluorescence and phosphorescence. With a sulfonate-functionalized bis-NHC ligand, water-soluble luminescent Au(III) complexes were prepared. They show similar photophysical properties in water when compared with their counterparts. The long phosphorescence lifetime renders a dual emissive complex able to function as ratiometric sensor for oxygen. Moreover, one of the water-soluble complexes displayed a significant inhibitory activity towards deubiquitinase UCHL3 with IC50 value of 0.15 μM.