Strongly luminescent tetradentate platinum(II) and palladium(II) complexes : structures, photophysics and material applications

Abstract

Cyclometallated platinum(II) and palladium(II) complexes possess rich photophysical properties and are promising candidates in materials science. Three types of platinum(II) complexes supported by tetradentate (N^C^C^N and two types of O^N^C^N) ligands, and palladium(II) complexes supported by tetradentate (N^C^C^N) ligands, are described in this thesis. These complexes exhibit blue to orange-red emission over the spectral range and their syntheses together with characterizations are discussed. The complexes have also been applied as emitters in organic light-emitting diodes (OLEDs), catalysts in photochemical reactions and probes for bio-imaging. Structurally robust platinum(II) complexes supported by two types of dianionic tetradentate O^N^C^N ligands, [Pt(O^N^C^N)], have been synthesized and characterized. The complexes of both types possess good thermal stabilities (Td up to 420 oC) and show green to orange emission (em = 503−583 nm) with high quantum efficiency (em up to 80%). The complexes are featured by incorporation of spiro-compound on the ligand framework to suppress intermolecular interactions; their crystal structures show highly distorted square planar geometry compared with the reported [Pt(O^N^C^N)] complexes without short contact between Pt centers. DFT/TDDFT calculations and fs-TRF experiment revealed that there are minimal excited state distortion and efficient intersystem crossing (ISC) dominants at rate of 6.7 × 1012 s-1. Green OLED based on [Pt(O^N^C^N)] complex has been achieved (PEmax = 126.0 lm W-1; CEmax = 104.2 cd A-1; EQEmax = 26.2%) with performance comparable to that of best reported Ir(III) emitter. A new class of platinum(II) complexes with emission spanning 429613 nm have been synthesized by using tetradentate N^C^C^N ligands with oxygen bridge and spiro-linkage. These strongly phosphorescent complexes (em up to 81%) are applied as dopant in R/G/B-emitting OLEDs. Particularly, OLEDs based on the green emitting [Pt(N^C^C^N)] complex showed PAL standard green emission with luminous efficacy of 63 lm W-1 and EQE of up to 20.6%. The complexes are also active in converting aryl boronic acid to aryl alcohol in the presence of oxygen with conversions and product yields of up to 100% and 87%, respectively. Upon irradiation with Xenon lamp ( > 370 nm), Pt(IV) complex is formed via oxidative addition with methyl iodide. The cytoxtoxicity of the Pt(IV) complex against two human cancer cell lines (HCT-116 colorectal carcinoma and NCl-H460 non-small cell lung carcinoma) were examined using MTT assay. Interestingly, the Pt(IV) complex is non-cytotoxic relative to cisplatin and able to label cytosolic region after incubation in both cell lines for 24–48 hours. Palladium(II) complexes containing the tetradentate N^C^C^N ligands have been prepared. The rigid ligand scaffolds afford complexes possessing high thermal stability (Td up to 430 oC) and high emission quantum yield (em up to 47%). DFT/TDDFT calculations suggest essentially ligand-centered emissive state of the Pd(II) complexes. Sky-blue emitting OLED based on such complex has been achieved with EQE up to ca. 17% at low dopant concentrations (2 wt%). These complexes having high triplet energy and long-lived excited state are also good candidates for photocatalysis as found to be active in visible-light driven reductive C−C bond formation of alkyl bromide and [2+2] cycloaddition of styrene.