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The tetradentate ligand, merging a carbazole unit with high triplet energy and dimethoxy bipyridine, renowned for its exceptional quantum efficiency in coordination with metals like Pt, is expected to demonstrate remarkable luminescent properties. However, instances of tetradentate ligands such as bipyridine-based pyridylcarbazole derivatives remain exceptionally scarce in the current literature. In this study, we developed a tetradentate ligand based on carbazole and 2,3′-bipyridine and successfully complexed it with Pt(II) ions. This novel compound (1) serves as a sky-blue phosphorescent material for use in light-emitting diodes. Based on single-crystal X-ray analysis, compound 1 has a distorted square-planar geometry with a 5/6/6 backbone around the Pt(II) core. Bright sky-blue emissions were observed at 488 and 516 nm with photoluminescent quantum yields of 34% and a luminescent lifetime of 2.6 μs. TD-DFT calculations for 1 revealed that the electronic transition was mostly attributed to the ligand-centered (LC) charge transfer transition with a small contribution from the metal-to-ligand charge transfer transition (MLCT, ~14%). A phosphorescent organic light-emitting device was successfully fabricated using this material as a dopant, along with 3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) and 9-(3′-carbazol-9-yl-5-cyano-biphenyl-3-yl)-9H-carbazole-3-carbonitrile (CNmCBPCN) as mixed hosts. A maximum quantum efficiency of 5.2% and a current efficiency of 15.5 cd/A were obtained at a doping level of 5%. [ABSTRACT FROM AUTHOR]

Abstract: Two groups of cationic macrocycles with contrasting behaviors have been investigated. The boron group has singlet states well below triplet states. This is the standard behavior for most macrocycles without transition metals. The aluminum group has triplet states stabilized through metal-centered and macrocycle-centered separation of unpaired electrons. The cationic boron subphthalocyanine (1B) has a triplet state whose unpaired electrons are delocalized in the conjugated ring. This is not surprising because a conjugated macrocycle can act as an electron buffer for multiple electrons. On the other hand, coexistence of metal-centered and ligand-centered distributions of unpaired electrons leads to the stabilized triplet state of 1Al. Results from the spin density, natural bond orbital (NBO) charges, and electron localization function (ELF) indicate distinguishable electron distributions between 1B and 1Al. The metal-centered unpaired electron in the triplet state of 1Al leads to the longer Al–N bond length and smaller N–Al–N bond angle compared with these in its singlet state. Consistent with this, the TDDFT study suggests a significant energy relaxation after the first vertical singlet-to-triplet transition. The ligand-centered unpaired electron in the triplet state of 1Al leads to the odd number of π electrons in the macrocycle. NICS values of this macrocycle are distinguishable from these of an aromatic or an antiaromatic macrocycle. The two highest singly occupied orbitals in the triplet state of 1Al are almost degenerate, spatially separated, and have little overlap. This rationalizes its stabilized triplet state. The stabilization mechanism of an unpaired electron at the tetrahedral corner above the central aluminum can also be rationalized by considering the spatial requirement in the valence shell electron pair repulsion (VSEPR) model. The metal-centered and ligand-centered separation of unpaired electrons also leads to stabilized triplet states in some other cationic macrocycles. It is hoped that this theoretical study will stimulate more synthetic attempts to access the experimentally unknown 1Al and more interest in the experimentally known 1B. [Copyright &y& Elsevier]

Abstract: The [Ir(ppy-F2)2Me4phen]+1 complex, where ppy-F2 is 2-(2′,4′-fluorophenyl)pyridine and Me4phen is 3,4,7,8-tetramethyl-1,10-phenanthroline, has been theoretically investigated by means of DFT calculations. The molecular and electronic properties calculated for [Ir(ppy-F2)2Me4phen]+1 are compared with those obtained for the simpler [Ir(ppy)(bpy)]+1 complex. The introduction of fluorine substituents in the ppy ligands and the use of phenanthroline instead of 2,2′-bipyridine as the diimine ligand increase the HOMO–LUMO energy gap and blue-shift the emission colour. The phenanthroline ligand causes the appearance of two nearly-degenerate LUMO orbitals of different symmetry in [Ir(ppy-F2)2Me4phen]+1 and determines that two almost isoenergetic doublet states are obtained for the reduced complex. Calculations predict three lowest-energy triplet excited states of different nature in an energy difference of only 0.10eV. Emission from the 3LC π–π* state takes place at higher energies and accounts for the structured blue emission observed in solution. Emission from the lowest-energy 3MLCT state occurs in the green region and explains the structureless broad band observed in solid films. [Copyright &y& Elsevier]