Abstract
In this work, hybrid polymeric bis-tridentate iridium(III) complexes bearing derivatives of terpyridine (tpy) and 2,6-di(phenyl) pyridine as ligands were successfully synthesized and evaluated as red-light emitters. At first, the synthesis of small molecular bis-tridendate Ir(III) complexes bearing alkoxy-, methyl-, or hydroxy-functionalized terpyridines and a dihydroxyphenyl-pyridine moiety was accomplished. Molecular complexes bearing two polymerizable end-hydroxyl groups and methyl- or alkoxy-decorated terpyridines were copolymerized with difluorodiphenyl-sulphone under high temperature polyetherification conditions. Alternatively, the post-polymerization complexation of the terpyridine-iridium(III) monocomplexes onto the biphenyl-pyridine main chain homopolymer was explored. Both cases afforded solution-processable metallocomplex-polymers possessing the advantages of phosphorescent emitters in addition to high molecular weights and excellent film-forming ability via solution casting. The structural, optical, and electrochemical properties of the monomeric and polymeric heteroleptic iridium complexes were thoroughly investigated. The polymeric metallocomplexes were found to emit in the orange–red region (550–600 nm) with appropriate HOMO and LUMO levels to be used in conjunction with blue-emitting hosts. By varying the metal loading on the polymeric backbone, the emitter’s specific emission maxima could be successfully tuned.
Highlights
Complexes involving iridium have shown great promise as organic light-emitting diode (OLED)emitters due to their triplet harvesting properties, increasing the theoretical limit of the external quantum efficiency (EQE) of the final device to 100% through the utilization of triplet excitons [1,2,3].iridium(III) complexes have been intensively studied as emitters in light-emitting electrochemical cells (LEECs) [4,5], as imaging or sensing agents [6], in photo-catalysis [7], and even as light absorbers in dye-sensitized solar cells [8]
According to the procedure of Scheme 1, iridium was first reacted with the terpyridine ligand with a hydroxylphenyl-(HOtpy), a dodecyloxyphenyl-(C12 Otpy), or a methylphenyl-(CH3 tpy) moiety in a mixture of ethanol and tetrahydrofuran at reflux to obtain the crude substituted monocomplexes [16]
Without further purification [16,18], the monocomplexes reacted with the second ligand, namely the 2,6-dihydroxylphenyl-pyridine (HOpy) [47], in refluxing ethyleneglycol to obtain the bis-tridentate complexes HOtpy-Ir-HOpy, C12 Otpy-Ir-HOpy, and CH3 tpy-Ir-HOpy
Summary
Complexes involving iridium have shown great promise as organic light-emitting diode (OLED). Regarding the use of bis-tridentate iridium complexes as light emitters, a series of bis-tridentate Ir(III) complexes with a (NNN)-(NNN) mode consisting of a series of terpyridine derivatives modified at the fourth position of the central pyridine unit [16] have been reported The preparation of those red-emitting complexes (600 nm) involved the stepwise introduction of the two ligands in consecutive steps, with the second ligand requiring high reaction temperatures of 180 ◦ C and above. The preparation of the polymer metallocomplexes followed two routes: (i) the direct polymerization of a dihydroxy-phenyl monomeric Ir(III) complex that was polymerized under high temperature polyetherification conditions and (ii) the post polymerization complexation of the Ir(III) monocomplexes of either dodecyloxyphenyl terpyridine or methylphenyl terpyridine with the free diphenylpyridines along the backbone of the diphenylpyridine–diphenylsulfone homopolymer This latter case led to copolymers combining pure organic and organic–metallocomplex repeating units, allowing for different loads of iridium(III) and ensuring processability, high molecular weights, and film formation. All tridentate complexes showed red–orange emissions, with emission maxima in the range of 565–610 nm depending on the detailed chemical structure of the complexes, the complexation degree of the metallopolymers, and the processing conditions
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