Transition-metal phosphors with cyclometalating ligands: fundamentals and applications

Abstract

One goal of this critical review is to provide advanced methodologies for systematic preparation of transition-metal based phosphors that show latent applications in the field of organic light emitting diodes (OLEDs). We are therefore reviewing various types of cyclometalating chelates for which the favorable metal-chelate bonding interaction, on the one hand, makes the resulting phosphorescent complexes highly emissive in both fluid and solid states at room temperature. On the other hand, fine adjustment of ligand-centered pi-pi* electronic transitions allows tuning of emission wavelength across the whole visible spectrum. The cyclometalating chelates are then classified according to types of cyclometalating groups, i.e. either aromatic C-H or azolic N-H fragment, and the adjacent donor fragment involved in the formation of metallacycles; the latter is an N-containing heterocycle, N-heterocyclic (NHC) carbene fragment or even diphenylphosphino group. These cyclometalating ligands are capable to react with heavy transition-metal elements, namely: Ru(II), Os(II), Ir(III) and Pt(II), to afford a variety of highly emissive phosphors, for which the photophysical properties as a function of chelate or metal characteristics are systematically discussed. Using Ir(III) complexes as examples, the C--N chelates possessing both C-H site and N-heterocyclic donor group are essential for obtaining phosphors with emission ranging from sky-blue to saturated red, while the N--N chelates such as 2-pyridyl-C-linked azolates are found useful for serving as true-blue chromophores due to their increased ligand-centered pi-pi* energy gap. Lastly, the remaining NHC carbene and benzyl phosphine chelates are highly desirable to serve as ancillary chelates in localizing the electronic transition between the metal and remaining lower energy chromophoric chelates. As for the potential opto-electronic applications, many of them exhibit remarkable performance data, which are convincing to pave a broad avenue for further development of all types of phosphorescent displays and illumination devices (94 references).

Synthesis, Diastereomer Separation, and Optoelectronic and Structural Properties of Dinuclear Cyclometalated Iridium(III) Complexes with Bridging Diarylhydrazide Ligands

“…[18][19][20][21][22][23][24][25][26][27][28][29] Consequently, for luminescence applications research has primarily concentrated on monoiridium complexes. 30,31 Nevertheless, the study of phosphorescent diiridium complexes is a rapidly expanding topic and a range of complexes with various conjugated bridging ligands have been reported to possess efficient photo-(PL) and electroluminescence (EL). [32][33][34][35][36][37] Impressive solution photoluminescence quantum yields (PLQYs) of up to 100% have been reported by Williams, Kozhevnikov and coworkers (compound 4), 36,38 while solution-processed OLEDs with external quantum efficiencies (EQEs) of >10% have been reported by our group (complex 5) 39 and also the groups of Zhou and Wong (complex 6).…”

Synthesis and Crystal Structure of Blue Phosphorescent mer-Tris(2',6'-difluoro-2,3'-bipyridinato-N,C4') Iridium(III)

“…1 In general, homoleptic trisIr(III) complexes with this asymmetric chelated ligand (C^N) have two isomers; facial and meridional configurations. 2 There are a number of reports regarding effective synthetic method, photophysical and electrochemical behaviors of Ir(III)(C^N) 3 derivatives with facial configuration due to their long-term stability in device operation.…”

Recent progress of pyrimidine derivatives for high-performance organic light-emitting devices

“…So far, phenylpyridine derivatives have been intensively investigated for the ligands in phosphorescent emitters based on iridium. [37][38][39] The representative examples are a green emitter, fac-tris(2-phenylpyridine) iridium(III) [IrðppyÞ 3 ], 9 and sky-blue emitter, iridium(III)bis(4,6-(difluorophenyl)pyridinato-N, C 2 0 )picolinate (FIrpic). 40 In this series of ligands, long-term stability in blue OLEDs remains a big challenge, and replacement of pyridine with pyrimidine unexpectedly provides great improvements of stability in blue OLEDs.…”