Coordination complexes exhibiting room-temperature phosphorescence: Evaluation of their suitability as triplet emitters in organic light emitting diodes

ion and completion of the cyclometalation reaction are feasible and produce the cyclometalated complex 205. Notably, addition of a strong acid reverts the cyclometalation and regenerates the agostic species 204. Small changes in the rhodium precursor significantly modify the product outcome and lead at fast rates to the cyclometalated complex 203, that is, the final oxidative addition product S. Similarly, alkene C-H bond activation has been observed to occur readily in a PCP ligand setting.348 Milstein and co-workers developed a procedure for preparing the cyclometalated PCP, PCN, or PCO pincer complexes 209 by rhodium-mediated cleavage of a strong Caryl-Calkyl bond (Scheme 87).32,349,350 Kinetically, Calkyl-H bond activation is competitive and complex 208 is formed as a second product at essentially similar rates. However, C-C bond cleavage appears to be the thermodynamically more favored process. Obviously, the higher energy required for Caryl-Calkyl bond activation (bond dissociation energy 427 kJ mol-1) compared to C-H bond breaking (368 kJ mol-1) is compensated by the stronger Rh-Caryl bond and the formation of two fiverather than six-membered metallacycles. Because rhodium-mediated Caryl-Calkyl and Calkyl-H bond activations in 206 are thermodynamically and kinetically very similar processes,351 subtle changes in the ligand parameters strongly affect the reaction outcome. For example, PPh2 donors induce C-H bond cleavage to afford 208d exclusively at ambient temperatures. C-C activation and formation of 209d is only induced upon heating under H2 atmosphere,352 presumably because these conditions allow the product selectivity to be controlled thermodynamically. In contrast, bulky PtBu2 donors as in 206a promote C-C bond activation already at room temperature, and the aryl complex 209a is the only product observed. Similarly, replacing one phosphine donor by a harder NEt2 group or the use of less basic phosphinite donors results in selective C-C bond cleavage at room temperature. These preferences suggest steric factors to play a more dominant role than electronic differences in determining the C-C vs C-H selectivity. Steric congestion induced by the donor substituents has been proposed to arrange the orbitals of the metal center to be directed toward the C-C bond rather than toward the C-H bond.353 Kinetic investigations have identified the coordination compound 207 as a common intermediate of Calkyl-H and the Caryl-Calkyl bond activation. On the basis of the similar rates for both cleavage processes and because of the absence of any detectable intermediate, formation of 207 and not C-H or C-C bond activation appears to be rate-limiting. While displacement of a weakly bound olefin ligand in the metal precursor may be fast, formation of the 8-membered metallacycle in 207 may be less favored, especially due to the potential for forming diand polymeric coordination compounds. It is worth noting that, upon substituting the PCP pincer ligand to a PCN donor array, the coordination complex 207c becomes stable at low temperature and has been spectroscopically characterized.354 As a consequence, the rate-determining step is shifted to a later stage on the reaction coordinate. Notably, slight modifications in the metal precursor, that is, utilization of [Rh(C2H4)(CO)(solv)2]BF4 instead of [RhCl(C2H4)2]2, alters the stability of the intermediates and produces complex 210 (Scheme 88).355 This complex may be represented by an agostic interaction of the C-C bond with the rhodium center, or alternatively by an arenium structure including an sp3-hybridized ipso carbon. Because of only weak Cipso-Rh interactions in solution (e.g., the absence of any 103Rh coupling), the agostic description is favored. The different product outcome has been presumed to originate from the reduced electron density at the cationic rhodium(I) center due to the strongly withdrawing CO ligand and the relatively weak Rh-N bond. This configuration Scheme 87 Scheme 86 Cyclometalation Using d-Block Transition Metals Chemical Reviews, 2010, Vol. 110, No. 2 603

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Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

Novel tricarbazole compounds are provided. By appropriately selecting the nature of the tricarbazole substituents, compounds with appropriate HOMO and LUMO energies can be obtained for use as materials in a secondary hole transport layer.

Organic Electroluminescent Materials and Devices

The synthesis, structures, photophysics, electrochemistry and electrophosphorescent properties of new red phosphorescent cyclometalated iridium(III) isoquinoline complexes, bearing 9-arylcarbazolyl chromophores, are reported. The functional properties of these red phosphors correlate well with the results of density functional theory calculations. The highest occupied molecular orbital levels of these complexes are raised by the integration of a carbazole unit to the iridium isoquinoline core so that the hole-transporting ability is improved in the resulting complexes relative to those with I-phenylisoquinoline ligands. All of the complexes are highly thermally stable and emit an intense red light at room temperature with relatively short lifetimes that are beneficial for highly efficient organic light-emitting diodes (OLEDs).

Red-Light-Emitting Iridium Complexes with Hole-Transporting 9-Arylcarbazole Moieties for Electrophosphorescence Efficiency/Color Purity Trade-off Optimization

Azaindoles (also called pyrrolopyridines) constitute essential subunits in many pharmaceutically important compounds. The synthesis of azaindoles has been a great synthetic challenge for chemists. Many classical methods for indole synthesis (such as Fischer and Madelung cyclizations) often cannot be effectively applied to the synthesis of the corresponding azaindoles. In recent years, advances in organometallic chemistry have enabled a number of novel and efficient methodologies for azaindole formation as well as for the further functionalization of azaindole templates. In this tutorial review, we have surveyed the recent development of organometallic chemistry-based methods for azaindole synthesis.

Organometallic methods for the synthesis and functionalization of azaindoles

An organometallic compound comprising a quinoxaline structure, and having a structure represented by the following general formula (2), wherein M represents a monovalent to trivalent metal, L represents a ligand, Ar1 and Ar2 represent an aryl group in which a part of hydrogens may be substituted, and the same or different, m represents an integer of 1 to 3, n represents an integer of 0 to 2, and m-n is an integer of 1 to 3.

Organometallic compound containing quinoxaline structure and light emitting element

Newly designed cyclometalated iridium(III) complexes bearing 2,3-diphenylquinoxalines, which were shown to be highly efficient and pure-red emitting materials for electrophosphorescent organic ligh...

Iridium(III) Complexes Bearing Quinoxaline Ligands with Efficient Red Luminescence Properties

The ultimately pure red OLED using new electrophosphorescent compound, bis(2,3-diphenylquinoxaline)iridium(acetylace-tonate) [(QH)2 Ir(acac)] is described. Electrophosphorescence peak wavelength and bandwidth were advanced for the purest red color stimulus to 680nm with full width at half maximum of 89nm by doping (QH)2 Ir(acac) into a light emitting electron transport layer composed of 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole. The most vivid red electrophosphorescence with CIE chromaticity coordinates better than x=0.70 and y=0.28, which are the closest to the ultimate limit of pure red among reported OLEDs, was attained and maintained even in luminance brighter than 600cd/m2. A very high maximum external electrophosphorescent quantum efficiency of 10.2% was also realized.

Kazuyasu Tani

Papers

Organometallic compound containing quinoxaline structure and light emitting element

Iridium(III) Complexes Bearing Quinoxaline Ligands with Efficient Red Luminescence Properties

Bright and ultimately pure red electrophosphorescent diode bearing diphenylquinoxaline

Synthesis of Re(I) complexes bearing tridentate 2,6-bis(7′-azaindolyl)phenyl ligand with green emission properties

Highly Efficient and Vivid-Red Phosphors Bearing 2,3-Diphenylquinoxaline Units and Their Application to Organic Light-Emitting Devices