Luminescent zinc(II), copper(I) and gold(I) complexes : structures, photophysical properties and OLED applications

Abstract

Luminescent metal complexes have important applications in diverse areas, including organic light-emitting diodes (OLEDs). This thesis focuses on luminescent zinc(II), copper(I) and gold(I) complexes bearing bidentate N-donor ligands and/or diphosphine ligands, particularly the 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate (dppnc) ligand. Luminescent tetranuclear zinc(II) complexes of substituted 7-azaindolates (AID) and a series of luminescent copper(I) complexes containing dppnc ligands were synthesized. All these complexes are stable against air and moisture. Thin film samples of the Zn(II) complexes in poly(methyl methacrylate) (PMMA) and Cu(I) complexes in 2,6-dicarbazolo-1,5-pyridine (PYD2) show emission quantum yields of up to 0.96 (for [Zn4O(4-phenyl-AID)6] (Zn-1)) and 0.60 (for [Cu(bathocuproine)(dppnc)] (Cu-3)). The complexes’ photophysical properties were examined by ultrafast time-resolved emission (TRE) spectroscopy, temperature-dependent emission lifetime measurements and density functional theory (DFT) calculations. Monochromic blue and orange solution-processed OLEDs, with these Zn(II) and Cu(I) complexes as light-emitting dopants, were respectively fabricated. A maximum external quantum efficiency (EQE) of 5.55% and Commission Internationale de l´Eclairage (CIE) coordinates of (0.16, 0.19) were accomplished with the optimized Zn-1-OLED while these values were, respectively, 15.64% and (0.48, 0.51) for the optimized Cu-3-OLED. Solution-processed white OLEDs having a maximum EQE of 6.88%, CIE coordinates of (0.42, 0.44) and a color rendering index (CRI) of 81 were fabricated using these luminescent Zn(II) and Cu(I) complexes as blue and orange light-emitting dopant materials, respectively. Several series of luminescent copper(I) complexes with diphosphine, functionalized 1,10-phenanthroline (phen) and/or diisocyanide ligands, including those bearing both dppnc and 2,9-(R1)2-4,7-(R2)2-phen, were developed. These complexes display high emission quantum yields of up to 0.55 in thin film samples. Their emission colors were tuned from green to deep red, via rational modification of the phen ligand. The molecular structures of the emissive Cu(I) complexes were characterized, their electronic structures and related transition properties were elucidated by photophysical and computational (DFT) studies. The calculations suggest that thermally activated delayed fluorescence (TADF) is the emission mechanism for these Cu(I) complexes. Efficient solution-processed green-, yellow- and red-emitting OLEDs were fabricated based on the emissive Cu(I) complexes as dopants. A high EQE of 15.20% and a current efficiency (CE) of 48.15 cd A-1 at 1000 cd m-2 were achieved in the green device with [Cu(2,9-Me2-4,7-R2-phen)(dppnc)] (R = 2-ethylhexyloxy, Cu-2.4). A maximum EQE of 10.17%, CIE coordinates of (0.61, 0.38) and a maximum electroluminescent (EL) peak of 631 nm were achieved in the red device based on [Cu(2,9-Me2-4,7-R2-phen)(dppnc)] (R = 4-N-carbazolylphenylacetylene, Cu-2.8). A number of luminescent three- and four-coordinate gold(I) and gold(III) complexes with dppnc or 1,2-bis(diphenylphosphino)benzene (dppbz) were prepared. The photophysical and electrochemical properties of these air-stable Au(I) and Au(III) complexes were investigated. The excellent optical properties of these complexes are afforded by their rich excited-state properties, including triplet intra-ligand (3IL), metal-perturbed 3IL and metal-perturbed ligand-to-ligand charge-transfer (3LLCT) states. An emission quantum yield of up to 0.174 in degassed CH2Cl2 solution was accomplished with the four-coordinate Au(I) complex, [Au(xantphos)(dppnc)] (Au-13). The appreciably less common three- and four-coordination modes of the Au(I) complexes were revealed by detailed single crystal X-ray diffraction studies. Some of these luminescent Au(I) complexes are potential candidates as triplet emitters for OLED fabrication.