Photoluminescent platinum(II) complexes containing pincer-type N-heterocyclic carbene (NHC) ligands : synthesis, photophysical properties and their applications in photo-catalysis and materials science

Abstract

Platinum(II) complexes exhibiting excellent photoluminescent properties are of high importance in the development of molecular functional materials. This thesis focuses on the use of pincer-type N-heterocyclic carbene (NHC) ligands, including tridentate bis-NHC ligands RC^C^C and C^N^C, for syntheses of novel types of photoluminescent Pt(II) complexes, together with characterization of these complexes by spectroscopic means and X-ray crystallography and exploration of their applications in areas including organic light-emitting diodes (OLEDs) cytotoxicity/bio-imaging, and photo-catalysis. A new class of cyclometalated Pt(II) complexes [Pt(RC^C^C)(L)] (R = H, CF3; L = CN, CNBPh3, CNB(C6F5)3) bearing RC^C^C ligand and isocyanoborate ligands have been synthesized. These complexes are blue emissive both in solution and in the solid state with max at ~ 440 480 nm, and with high quantum yields ( ) of up to unity (doped in PMMA; in degassed CH2Cl2: < 0.2). Because of their excellent photoluminescent properties, these Pt(II) complexes have been utilized as phosphorescent dopants in fabrication of OLEDs via both solution process and vacuum deposition. High external quantum efficiency (EQE) of up to 15.33% and CIE coordinates of (0.17, 0.32) were achieved in the solution-processed device with 4 wt% [Pt(C^C^C)(CNB(C6F5)3)] while the vacuum-deposited device with 4 wt% [Pt(CF3C^C^C)(CNB(C6F5)3)] exhibited a maximum EQE of 23.6% with CIE coordinates of (0.14, 0.16). A white solution-processed OLED with CIE coordinates of (0.32, 0.39) and a maximum EQE of 16.86% was achieved when [Pt(CF3C^C^C)(CNBPh3)] was used as the single emitter with dopant concentration of 10 wt%. Complexes [Pt(C^C^C)(L)] (L = CN, CNBPh3, CNB(C6F5)3) have also been utilized as photocatalysts for photo-induced reductive cyclization of alkyl and aryl halides with conversion and yield up to 99% and 86%, respectively. A number of trinuclear d8-d10-d8 complexes [{Pt(C^N^C)(Pz)}2M]X3 (Pz = pyrazolate; M = Ag, Au; X = PF6-, OTf-) and [{Pt(C^C^C)(Pz)}2M]PF6 (M = Ag, Au) have been prepared. All the trinuclear Pt-Au-Pt complexes adopt a sandwich-type conformation leading by intramolecular metal-metal interaction of Pt Au Pt. For the silver counterparts, two types of conformations are observed, which are probably influenced by the tridentate ligand and/or the counter-anion. These trinuclear complexes are highly emissive in the solid state but weakly emissive in solution. Their biological cytotoxicity and application in bio-imaging have been examined. Complex [{Pt(C^N^C)(Pz)}2Ag](OTf)3 is selectively cytotoxic for cervical epithelia cancer (HeLa) and breast cancer (MCF-7) cell lines with IC50 values of 9.6 M and 3.8 M, respectively. The gold counterpart [{Pt(C^N^C)(Pz)}2Au](OTf)3 shows no cytotoxicity to all tested cell lines, while [{Pt(C^C^C)(Pz)}2Au]PF6 is cytotoxic to all four cancer cell lines including two normal cell lines CCD-19Lu and NCM460 with IC50 values of 0.7 3.3 M. Six dinuclear Pt(II) complexes, [(C^C^C)Pt-C≡C-R-C≡C-Pt(C^C^C)] and [(C^N^C)Pt-C≡C-R-C≡C-Pt(C^N^C)](OTf)2 (R = 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4,5-diyl; o-C6H4C≡CC6H4-o; o-C6H4C≡C-o-C6H4C≡CC6H4-o), have also been synthesized. Due to their absorption in visible light region, these dinuclear complexes have been studied for photocatalytic CO2 reduction as photo-sensitizers by using [CoII(TPA)Cl]Cl as catalyst. With dinuclear Pt(II) complex [(C^C^C)Pt-C≡C-R-C≡C-Pt(C^C^C)] (R = 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4,5-diyl) as photo-sensitizer, a CO production turnover frequency (TOF) of 31.8 min-1 within the first 20 minutes was observed along with a reasonable high turnover number (TON) of 650.