Photoluminescent binuclear platinum(II) and palladium(II) complexes : syntheses, photophysical properties, and applications in OLEDs and photocatalysis

Abstract

Binuclear d8-d8 metal complexes play an important role in the realms of photophysics and photochemistry. The intramolecular non-covalent interactions, in particular metal-metal and metal-π interactions, can be controlled using appropriate bridging ligands and have significant impact on the excited-state properties. This thesis focuses on the syntheses, photophysics, photochemistry, and applications of different classes of luminescent binuclear Pt(II) and Pd(II) complexes containing various cyclometalated and bridging ligands and that exhibit intramolecular metal-metal or metal-π interactions. A series of binuclear Pt(II) complexes bearing bidentate pyridine/pyrazine fused N-heterocyclic carbene (NHC) or arylpyridine cyclometalated ligands, and ditopic bis-μ2-formamidinate bridging ligands, were synthesized and characterized. These complexes with short intramolecular Pt–Pt distances (2.85–2.87 Å) are thermally stable up to 446 °C and show strong red to near-infrared 3MMLCT emission (604–928 nm) with emission quantum yields up to near unity. Efficient red and deep-red organic light-emitting diodes (OLEDs) using these complexes as emitting dopants have been achieved and showed high peak external quantum efficiencies (EQEs) up to 21.3%. More importantly, the employment of ditopic bis-μ2-formamidinate ligands increases the structural rigidity of binuclear Pt(II) complexes, affording OLEDs with prolonged operational lifetimes (LT97 up to 2446 hrs at an initial luminance of 1000 cd m-2) relative to those using complexes with mono-μ2-formamidinate ligands. A panel of binuclear Pd(II) complexes bearing bidentate arylpyridine, NHC cyclometalated ligands, pyrazolate, and N,N’-diphenylformamidinate bridging ligands were synthesized and characterized. These complexes show strong phosphorescence with Фem and kr values up to 0.70 and 2×105 s-1, respectively. By DFT/TDDFT calculations and comparison of the photophysical properties of the binuclear Pd(II) complexes and their Pt(II) counterparts, emissions of the binuclear Pd(II) complexes with short intramolecular Pd–Pd distances (2.79–2.89 Å) are assigned to the 3MMLCT excited state. Modulation of the excited state of binuclear Pd(II) complexes from 3IL to 3MMLCT can be achieved by increasing the steric bulk of the pyrazolate ligands and the superior photophysical attributes of the 3MMLCT excited state result from the combination of strongly σ-donating NHCs and structurally robust formamidinate ligands. These binuclear Pd(II) complexes have been used as photocatalysts or photosensitizers for C-C coupling reactions, and as emitting dopants for OLEDs where high max. luminance and max. EQE up to 104,000 cd m-2 and 22.9% were achieved, respectively. Two classes of binuclear Pt(II), Pd(II), and heteronuclear Pt(II)-Pd(II) complexes bearing C-deprotonated RC^N^NR’ (3-(4-(3,5-di-tert-butylphenyl)-6-phenylpyridin-2-yl)isoquinoline) and C^N^N (6-phenyl-2,2’-bipyridine) cyclometalated ligands were prepared. The complexes bearing diacetylide bridging ligands have intramolecular metal-π interactions as revealed by X-ray crystallography. The emission energies of these complexes in dilute solutions follow the order Pt2 < PtPd < Pd2, and the emissions are tentatively assigned to originate from an intramolecular excimer induced by intramolecular Pt/Pd-π interactions. Another class of complexes incorporating N,N’-diphenylformamidinate bridging ligands shows intramolecular metal-metal interactions and 3MMLCT emission; the latter is responsive to organic vapours and mechanical force.