Generating and Trapping Metalla‐N‐Heterocyclic Carbenes

By means of a combined experimental and theoretical approach, the electronic features and chemical behavior of metalla‐N‐heterocyclic carbenes (MNHCs, N‐heterocyclic carbenes containing a metal atom within the heterocyclic skeleton) have been established and compared with those of classical NHCs. MNHCs are strongly basic (proton affinity and pK_a values around 290 kcal mol^?1 and 36, respectively) with a narrow singlet–triplet gap (around 23 kcal mol^?1). MNHCs can be generated from the corresponding metalla‐imidazolium salts and trapped by addition of transition‐metal complexes affording the corresponding heterodimetallic dicarbene derivatives, which can serve as carbene transfer agents.     

Long‐Lived Room‐Temperature Deep‐Red‐Emissive Intraligand Triplet Excited State of Naphthalimide in Cyclometalated IrIII Complexes and its Application in Triplet‐Triplet Annihilation‐Based Upconversion

Cyclometalated Ir^III complexes with acetylide ppy and bpy ligands were prepared (ppy=2‐phenylpyridine, bpy=2,2′‐bipyridine) in which naphthal ( Ir‐2 ) and naphthalimide (NI) were attached onto the ppy ( Ir‐3 ) and bpy ligands ( Ir‐4 ) through acetylide bonds. [Ir(ppy)₃] ( Ir‐1 ) was also prepared as a model complex. Room‐temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir‐3 and Ir‐4 showed strong absorption in the visible range (ε=39600 M ^?1 cm^?1 at 402 nm and ε=25100 M ^?1 cm^?1 at 404 nm, respectively), long‐lived triplet excited states (τ_T=9.30 μs and 16.45 μs) and room‐temperature red emission (λ_em=640 nm, Φ_p=1.4 % and λ_em=627 nm, Φ_p=0.3 %; cf. Ir‐1 : ε=16600 M ^?1 cm^?1 at 382 nm, τ_em=1.16 μs, Φ_p=72.6 %). Ir‐3 was strongly phosphorescent in non‐polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir‐4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non‐polar solvents. Emission of Ir‐1 and Ir‐2 was not solvent‐polarity‐dependent. The T₁ excited states of Ir‐2 , Ir‐3 , and Ir‐4 were identified as mainly intraligand triplet excited states (³IL) by their small thermally induced Stokes shifts (ΔE_s), nanosecond time‐resolved transient difference absorption spectroscopy, and spin‐density analysis. The complexes were used as triplet photosensitizers for triplet‐triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir‐2 and Ir‐3 , respectively, whereas the upconversion was negligible for Ir‐1 and Ir‐4 . These results will be useful for designing visible‐light‐harvesting transition‐metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.     

Reaction of Aromatic Peroxyl Radicals with Alkynes: A Mass Spectrometric and Computational Study Using the Distonic Radical Ion Approach

The reaction of the aromatic distonic peroxyl radical cations N‐methyl pyridinium‐4‐peroxyl (PyrOO^.+) and 4‐(N,N,N‐trimethyl ammonium)‐phenyl peroxyl (AnOO^.+), with symmetrical dialkyl alkynes 10a – c was studied in the gas phase by mass spectrometry. PyrOO^.+ and AnOO^.+ were produced through reaction of the respective distonic aryl radical cations Pyr^.+ and An^.+ with oxygen, O₂. For the reaction of Pyr^.+ with O₂ an absolute rate coefficient of k₁=7.1×10^?12 cm³ molecule^?1 s^?1 and a collision efficiency of 1.2 % was determined at 298 K. The strongly electrophilic PyrOO^.+ reacts with 3‐hexyne and 4‐octyne with absolute rate coefficients of k_hexyne=1.5×10^?10 cm³ molecule^?1 s^?1 and k_octyne=2.8×10^?10 cm³ molecule^?1 s^?1, respectively, at 298 K. The reaction of both PyrOO^.+ and AnOO^.+ proceeds by radical addition to the alkyne, whereas propargylic hydrogen abstraction was observed as a very minor pathway only in the reactions involving PyrOO^.+. A major reaction pathway of the vinyl radicals 11 formed upon PyrOO^.+ addition to the alkynes involves γ‐fragmentation of the peroxy O? O bond and formation of PyrO^.+. The PyrO^.+ is rapidly trapped by intermolecular hydrogen abstraction, presumably from a propargylic methylene group in the alkyne. The reaction of the less electrophilic AnOO^.+ with alkynes is considerably slower and resulted in formation of AnO^.+ as the only charged product. These findings suggest that electrophilic aromatic peroxyl radicals act as oxygen atom donors, which can be used to generate α‐oxo carbenes 13 (or isomeric species) from alkynes in a single step. Besides γ‐fragmentation, a number of competing unimolecular dissociative reactions also occur in vinyl radicals 11 . The potential energy diagrams of these reactions were explored with density functional theory and ab initio methods, which enabled identification of the chemical structures of the most important products.     

Structure–Property Relationships and 1O2 Photosensitisation in Sterically Encumbered Diimine PtII Acetylide Complexes

A series of sterically encumbered [Pt( L )(σ‐acetylide)₂] complexes were prepared in which L , a dendritic polyaromatic diimine ligand, was held constant ( L =1‐(2,2′‐bipyrid‐6‐yl)‐2,3,4,5‐tetrakis(4‐tert‐butylphenyl)benzene) and the cis ethynyl co‐ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron‐withdrawing phenyl‐CF₃ substituents ( 4 ) with electron‐donating pyrenes ( 5 ) resulted in a red shift of both the lowest‐energy absorption (ΔE=3300 cm^?1, 61 nm) and emission bands (ΔE=1930 cm^?1, 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited‐state lifetimes, switched from being ³MMLL′CT‐derived (mixed metal–ligand‐to‐ligand charge transfer) when phenyl/polyphenylene substituents ( 3 , 4 , 6 ) were present, to ligand‐centred ³ππ* when the substituents were more conjugated aromatic platforms [pyrene ( 5 ) or hexa‐peri‐hexabenzocoronene ( 7 )]. The novel Pt^II acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited‐state lifetimes suggested they would be good candidates for use as singlet‐oxygen photosensitisers. Determined by in situ photooxidation of 1,5‐dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl‐ 5 as sensitiser (k_obs=39.3×10^?3 min^?1) was over half that of the known ¹O₂ sensitiser tetraphenylporphyrin (k_obs=78.6×10^?3 min^?1) under the same conditions. Measured ¹O₂ quantum yields of complexes 5 and 7 were half and one‐third, respectively, of that of TPP, and thus reveal an efficient triplet–triplet energy‐transfer process in both cases.     

Substituent Effects on the Low-Lying Singlet and Triplet States of Methylene

The relative energies of the three lower-lying singlet states (here called S_a, S_b, and S_c for the sake of generality) and the lowest triplet state of CHX and CX₂ carbenes (in which X = Li, BeH, BH₂, NH₂, OH, or F) are evaluated by means of the semiempirical MNDO method as well as, for some species, by means of ab initio calculations at the 6-31G, MP3/6-31G, and MP3/6-31G* levels. Calculations for CH(CN) and C(CN)₂ are also reported. In spite of the known MNDO overestimation of the stability of the σ¹π¹ configurations of methylene, this method turns out to be satisfactory for most carbenes reported here. Emphasis is put on the appearance of the plots of the ΔH values vs. the carbene bond angles for the different states and on the seldom considered S_b states (¹B₁ for C_2v carbenes). A carbene classification is proposed on the basis of the form of these plots. For carbenes with π-acceptor substituents such as those of “type IA”, open-shell, diradical configurations are predicted for the lowest singlet states, so that no significant structural differences should be expected between their lowest singlet and triplet states. On the other hand, for carbenes with strong π-donor substituents, either “type ID” or “IID”, the closed-shell singlets appear to be the ground states, and the singlet and triplet behaviors should be much more clearly distinguishable.     

Highly Efficient Triplet Photosensitizers: A Systematic Approach to the Application of IrIII Complexes containing Extended Phenanthrolines

A series of Ir^III complexes, based on 1,10‐phenanthroline featuring aryl acetylene chromophores, were prepared and investigated as triplet photosensitizers. The complexes were synthesized by Sonogashira cross‐coupling reactions using a “chemistry‐on‐the‐complex” method. The absorption properties and luminescence lifetimes were successfully tuned by controlling the number and type of light‐harvesting group. Intense UV/Vis absorption was observed for the Ir^III complexes with two light‐harvesting groups at the 3‐ and 8‐positions of the phenanthroline. The asymmetric Ir^III complex (with a triphenylamine (TPA) and a pyrene moiety attached) exhibited the longest lifetime. Red emission was observed for all the complexes in deaerated solutions at room temperature. Their emission at low temperature (77 K) and nanosecond time‐resolved transient difference absorption spectra revealed the origin of their triplet excited states. The singlet‐oxygen (¹O₂) sensitization and triplet‐triplet annihilation (TTA)‐based upconversion were explored. Highly efficient TTA upconversion (Φ_UC=28.1 %) and ¹O₂ sensitization (Φ_Δ=97.0 %) were achieved for the asymmetric Ir^III complex, which showed intense absorption in the visible region (λ_abs=482 nm, ?=50900 m ^?1 cm^?1) and had a long‐lived triplet excited state (53.3 μs at RT).     

Synthesis of main chain polymeric benzophenone photoinitiator via thiol‐ene click chemistry and Its use in free radical polymerization

Main chain polymeric benzophenone photoinitiator (PBP) was synthesized by using “Thiol‐ene Click Chemistry” and characterized with ¹H NMR, FTIR, UV, and phosphorescence spectroscopies. PBP as a polymeric photoinitiator presented excellent absorption properties (ε₂₉₄ = 28,300 mol^?1L^?1cm^?1) compared to the molecular initiator BP (ε₂₅₂ = 16,600 mol^?1L^?1cm^?1). The triplet energy of PBP was obtained from the phosphorescence measurement in 2‐methyl tetrahydrofurane at 77 K as 298.3 kJ/mol and according to phosphorescence lifetime, the lowest triplet state of PBP has an n‐π* nature. Triplet–triplet absorption spectrum of PBP at 550 nm following laser excitation (355 nm) were recorded and triplet lifetime of PBP was found as 250 ns. The photoinitiation efficiency of PBP was determined for the polymerization of Hexanedioldiacrylate (HDDA) with PBP and BP in the presence of a coinitiator namely, N‐methyldiethanolamine (MDEA) by Photo‐DSC. The initiation efficiency of PBP for polymerization of HDDA is much higher than for the formulation consisting of BP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Elevating the Triplet Energy Levels of Dibenzofuran‐Based Ambipolar Phosphine Oxide Hosts for Ultralow‐Voltage‐Driven Efficient Blue Electrophosphorescence: From D?A to D?π?A Systems

A series of donor (D)–π–acceptor (A)‐type phosphine‐oxide hosts ( DBF _x POPhCz _n ), which were composed of phenylcarbazole, dibenzofuran ( DBF ), and diphenylphosphine‐oxide (DPPO) moieties, were designed and synthesized. Phenyl π‐spacer groups were inserted between the carbazolyl and DBF groups, which effectively weakened the charge transfer and triplet‐excited‐state extension. As the result, the first triplet energy levels (T₁) of DBF _x POPhCz _n are elevated to about 3.0 eV, 0.1 eV higher than their D? A‐type analogues. Nevertheless, the electrochemical analysis and DFT calculations demonstrated the ambipolar characteristics of DBF _x POPhCz _n . The phenyl π spacers hardly influenced the frontier molecular orbital (FMO) energy levels and the carrier‐transporting ability of the materials. Therefore, these D? π? A systems are endowed with higher T₁ states, as well as comparable electrical properties to D? A systems. Phosphorescent blue‐light‐emitting diodes (PHOLEDs) that were based on DBF _x POPhCz _n not only inherited the ultralow driving voltages (2.4 V for onset, about 2.8 V at 200 cd m^?2, and <3.4 V at 1000 cd m^?2) but also had much‐improved efficiencies, including about 26 cd A^?1 for current efficiency, 30 Lm W^?1 for power efficiency, and 13 % for external quantum efficiency, which were more than twice the values of devices that are based on conventional unipolar host materials. This performance makes DBFDPOPhCz _n among the best hosts for ultralow‐voltage‐driven blue PHOLEDs reported so far.     

A computational quest for the effects of fused rings on the stability of Hammick carbenes type remote N‐heterocyclic carbenes

Substituent effects of fused six, and five‐membered aromatic rings are investigated on the stability, aromaticity, charge distribution, nucleophilic (N), and electrophilic (ω) characters of 20 singlet (s) and triplet (t) Hammick carbenes, at B3LYP/AUG‐cc‐pVTZ and M06‐2X/AUG‐cc‐pVTZ. Results display: (a) The higher thermodynamic and kinetic stability is revealed by carbenes situated between two nitrogen and/or two oxygen heteroatoms of two substituted rings, in a “W” arrangement toward the carbenic center; (b) Regardless of the arrangement, the order of thermodynamical and kinetic stabilization for fused rings is pyrrole > furan > thiophene > phosphole. (c) The substituted Hammick carbenes with two fused heterocyclics, in a given arrangement, show more stability than unsubstituted Hammick carbene; (d) While two five‐membered heterocyclic rings stabilize their corresponding substituted carbenes, two benzene rings destabilize Hammick carbene; (e) In all structures, s species emerges as ground state, exhibiting more stability than its t state; (f) The scrutinized s carbenes show higher N and lower ω than their corresponding t states.     

1,1‐Dilithioethylene: Toward Spectroscopic Identification of the Definitive Singlet Ground Electronic State of a Peculiar Structure

1,1‐Dilithioethylene is a prototypical carbon–lithium compound that is not known experimentally. All low‐lying singlet and triplet structures of interest were investigated by using high‐level theoretical methods with correlation‐consistent basis sets up to pentuple ζ. The coupled cluster methods adopted included up to full triple excitations and perturbative quadruples. In contrast to earlier studies that predicted the twisted C_2v triplet to be the ground state, we found a peculiar planar C_s singlet ground state in the present research. The lowest excited electronic state of 1,1‐dilithioethylene, the twisted C_s triplet, was found to lie 9.0 kcal mol^?1 above the ground state by using energy extrapolation to the complete basis set limit. For the planar C_s singlet and twisted C_s triplet states of 1,1‐dilithioethylene, anharmonic vibrational frequencies were reported on the basis of second‐order vibrational perturbation theory. The remarkably low (2050 cm^?1) C?H stretching fundamental (the C?H bond near the bridging lithium) of the singlet state was found to have very strong infrared intensity. These highly reliable theoretical findings may assist in the long‐sought experimental identification of 1,1‐dilithioethylene. Using natural bond orbital analysis, we found that lithium bridging structures were strongly influenced by electrostatic effects. All carbon–carbon linkages corresponded to conventional double bonds.     

Alkyne gem‐Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

Parahydrogen (p‐H₂) induced polarization (PHIP) NMR spectroscopy showed that [Cp^XRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem‐hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich Cp^X rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT‐assisted analysis of the chemical shift tensors determined by solid‐state ¹³C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis‐active Grubbs‐type catalysts.     

Iridium(III) Complexes Bearing Pyrene‐Functionalized 1,10‐Phenanthroline Ligands as Highly Efficient Sensitizers for Triplet–Triplet Annihilation Upconversion

“Chemistry‐on‐the‐complex” synthetic methods have allowed the selective addition of 1‐ethynylpyrene appendages to the 3‐, 5‐, 3,8‐ and 5,6‐positions of Ir^III‐coordinated 1,10‐phenanthroline via Sonogashira cross‐coupling. The resulting suite of complexes has given rise to the first rationalization of their absorption and emission properties as a function of the number and position of the pyrene moieties. Strong absorption in the visible region (e.g. 3,8‐substituted Ir‐3 : λ_abs=481 nm, ?=52 400 m ^?1 cm^?1) and long‐lived triplet excited states (e.g. 5‐substituted Ir‐2 : τ_T=367.7 μs) were observed for the complexes in deaerated CH₂Cl₂. On testing the series as triplet sensitizers for triplet–triplet annihilation upconversion, those Ir^III complexes bearing pyrenyl appendages at the 3‐ and 3,8‐positions ( Ir‐1 , Ir‐3 ) were found to give optimal upconversion quantum yields (30.2 % and 31.6 % respectively).     

Excitonic Coupling Strength and Coherence Length in the Singlet and Triplet Excited States of meso–meso Directly Linked Zn(II)porphyrin Arrays

Excitation‐energy transport (EET) phenomena in meso–meso directly linked Zn(II )porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady‐state and time‐resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II )porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm^?1, and that of the Q bands is about 570 cm^?1. The coherence length in the S₁ state of the Zn(II )porphyrin arrays was determined to be 4–5 porphyrin units, which is comparable to that of the well‐ordered two‐dimensional circular structure B850 in the peripheral light‐harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II )porphyrin arrays are well suited for mimicking natural light‐harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 μs^?1, and the very weak coupling between the triplet states (ca. 0.003 cm^?1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.     

Kinetic Study of Peroxidase‐Catalyzed Coupling of Benzene‐1,4‐diamine and N‐(2‐Aminoethyl)naphthalen‐1‐amine: Development of Micromolar Hydrogen Peroxide Reaction System

A novel chromogenic method to measure the peroxidase activity using para‐phenylenediamine dihydrochloride (=benzene‐1,4‐diamine hydrochloride; PPDD) and N‐(1‐naphthyl)ethylenediamine dihydrochloride (=N‐(2‐aminoethyl)naphthalen‐1‐amine; NEDA) is presented. The PPDD entraps the free radical and gets oxidized to electrophilic diimine, which couples with NEDA to give an intense red‐colored chromogenic species with maximum absorbance at 490 nm. This assay was adopted for the quantification of H₂O₂ between 20 and 160 μM . Catalytic efficiency and catalytic power of the commercial peroxidase were found to be 4.47×10⁴ M ^?1 min^?1 and 3.38×10^?4 min^?1, respectively. The catalytic constant (k_cat) and specificity constant (k_cat/K_m) at saturated concentration of the co‐substrates were 0.0245×10³ min^?1 and 0.0445 μM ^?1 min^?1, respectively. The chromogenic coupling reaction has a minimum interference from the reducing substances such as ascorbic acid, L ‐cystein, citric acid, and oxalic acid. The method being simple, rapid, precise, and sensitive, its applicability has been tested in the crude vegetable extracts that showed peroxidase activity.     

Photophysical Properties and OLED Applications of Phosphorescent Platinum(II) Schiff Base Complexes

The syntheses, crystal structures, and detailed investigations of the photophysical properties of phosphorescent platinum(II) Schiff base complexes are presented. All of these complexes exhibit intense absorption bands with λ_max in the range 417–546 nm, which are assigned to states of metal‐to‐ligand charge‐transfer (¹MLCT) ¹[Pt(5d)→π*(Schiff base)] character mixed with ¹[lone pair(phenoxide)→π*(imine)] charge‐transfer character. The platinum(II) Schiff base complexes are thermally stable, with decomposition temperatures up to 495 °C, and show emission λ_max at 541–649 nm in acetonitrile, with emission quantum yields up to 0.27. Measurements of the emission decay times in the temperature range from 130 to 1.5 K give total zero‐field splitting parameters of the emitting triplet state of 14–28 cm^?1. High‐performance yellow to red organic light‐emitting devices (OLEDs) using these platinum(II) Schiff base complexes have been fabricated with the best efficiency up to 31 cd A^?1 and a device lifetime up to 77 000 h at 500 cd m^?2.     

Cyclometalation of Aryl‐Substituted Phosphinines through C?H‐Bond Activation: A Mechanistic Investigation

A series of 2,4,6‐triarylphosphinines were prepared and investigated in the base‐assisted cyclometalation reaction using [Cp*IrCl₂]₂ (Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) as the metal precursor. Insight in the mechanism of the C? H bond activation of phosphinines as well as in the regioselectivity of the reaction was obtained by time‐dependent ³¹P{¹H} NMR spectroscopy. At room temperature, 2,4,6‐triarylphosphinines instantaneously open the Ir‐dimer and coordinate in an η¹‐fashion to the metal center. Upon heating, a dissociation step towards free ligand and an Ir‐acetate species is observed and proven to be a first‐order reaction with an activation energy of ΔE_A=56.6 kJ mol^?1 found for 2,4,6‐triphenylphosphinine. Electron‐donating substituents on the ortho‐phenyl groups of the phosphorus heterocycle facilitate the subsequent cyclometalation reaction, indicating an electrophilic C? H activation mechanism. The cyclometalation reaction turned out to be very sensitive to steric effects as even small substituents can have a large effect on the regioselectivity of the reaction. The cyclometalated products were characterized by means of NMR spectroscopy and in several cases by single‐crystal X‐ray diffraction. Based on the observed trends during the mechanistic investigation, a concerted base‐assisted metalation–deprotonation (CMD) mechanism, which is electrophilic in nature, is proposed.     

Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low‐Dose Photodynamic Therapy

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co‐existence of efficient ISC and long triplet excited lifetime in a heavy atom‐free bodipy helicene molecule. Via theoretical computation and time‐resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (?=1.76×10⁵ m ^?1 cm^?1 at 630 nm), satisfactory triplet quantum yield (Φ_T=52 %), and long‐lived triplet state (τ_T=492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT‐mediated antitumor immunity amplification with an ultra‐low dose (0.25 μg kg^?1), which is several hundred times lower than that of the existing PDT reagents.     

Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes

A series of tridentate benzimidazole‐substituted pyridine‐2‐carboxylic acids have been prepared with a halogen, methyl or alkoxy group in the 6‐position of the benzimidazole ring, which additionally contains a solubilising N‐alkyl chain. The ligands form neutral homoleptic nine‐coordinate lanthanum, europium and terbium complexes as established from X‐ray crystallographic analysis of eight structures. The coordination polyhedron around the lanthanide ion is close to a tricapped trigonal prism with ligands arranged in an up–up–down fashion. The coordinated ligands serve as light‐harvesting chromophores in the complexes with absorption maxima in the range 321–341 nm (ε=(4.9–6.0)×10⁴ M ^?1 cm^?1) and triplet‐state energies between 21 300 and 18 800 cm^?1; the largest redshifts occur for bromine and electron‐donor alkoxy substituents. The ligands efficiently sensitise europium luminescence with overall quantum yields ( ) and observed lifetimes (τ_obs) reaching 71 % and 3.00 ms, respectively, in the solid state and 52 % and 2.81 ms, respectively, in CH₂Cl₂ at room temperature. The radiative lifetimes of the Eu(⁵D₀) level amount to τ_rad=3.6–4.6 ms and the sensitisation efficiency η_sens= (τ_rad/τ_obs) is close to unity for most of the complexes in the solid state and equal to approximately 80 % in solution. The photophysical parameters of the complexes correlate with the triplet energy of the ligands, which in turn is determined by the nature of the benzimidazole substituent. Facile modification of the ligands makes them promising for the development of brightly emissive europium‐containing materials.     

Robust Tris‐Cyclometalated Iridium(III) Phosphors with Ligands for Effective Charge Carrier Injection/Transport: Synthesis,Redox, Photophysical,and Electrophosphorescent Behavior

With the target to design and develop new functionalized green triplet light emitters that possess distinctive electronic properties for robust and highly efficient phosphorescent organic light‐emitting diodes (PHOLEDs), a series of bluish–green to yellow–green phosphorescent tris‐cyclometalated homoleptic iridium(III) complexes [Ir(ppy‐X)₃] (X=SiPh₃, GePh₃, NPh₂, POPh₂, OPh, SPh, SO₂Ph, Hppy=2‐phenylpyridine) have been synthesized and fully characterized by spectroscopic, redox, and photophysical methods. By chemically manipulating the lowest triplet‐state character of Ir(ppy)₃ with some functional main‐group 14–16 moieties on the phenyl ring of ppy, a new family of metallophosphors with high‐emission quantum yields, short triplet‐state lifetimes, and good hole‐injection/hole‐transporting or electron‐injection/electron‐transporting properties can be obtained. Remarkably, all of these Ir^III complexes show outstanding electrophosphorescent performance in multilayer doped devices that surpass that of the state‐of‐the‐art green‐emitting dopant Ir(ppy)₃. The devices described herein can reach the maximum external quantum efficiency (η_ext) of 12.3 %, luminance efficiency (η_L) of 50.8 cd A^?1, power efficiency (η_p) of 36.9 Lm W^?1 for [Ir(ppy‐SiPh₃)₃], 13.9 %, 60.8 cd A^?1, 49.1 Lm W^?1 for [Ir(ppy‐NPh₂)₃], and 10.1 %, 37.6 cd A^?1, 26.1 Lm W^?1 for [Ir(ppy‐SO₂Ph)₃]. These results provide a completely new and effective strategy for carrier injection into the electrophosphor to afford high‐performance PHOLEDs suitable for various display applications.     

Using an Organic Molecule with Low Triplet Energy as a Host in a Highly Efficient Blue Electrophosphorescent Device

To achieve high efficiencies in blue phosphorescent organic light‐emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6‐difluorophenylpyridinato‐N,C^2′)iridium(III)] tetrakis(1‐pyrazolyl)borate (FIr6; T₁=2.73 eV). In both the photo‐ and electro‐excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic‐layer device, the maximum current efficiency of 37 cd A^?1 and power efficiency of 40 Lm W^?1 were achieved for the FIr6‐based blue PhOLEDs.