Lowest Energy Triplet Research Articles

Luminescent properties of terbium(III) and europium(III) complexes with six 4-acetyl-bispyrazolones

The lowest triplet energy levels of the six ligands(T) were determined to be 22989 cm−1[1,3-bis-(1′-phenyl-3′-methyl-5′-pyrazolon-4′)-1,3-propanedione, BPMPTD], 23148 cm−1[1,4-bis-(1′-phenyl-3′-methyl-5′-pyrazolon-4′)-1,4-butanedione, BPMPBD], 23419 cm−1[1,5-bis-(1′-phenyl-3′-methyl-5′-pyrazolon-4′)-1,5-pentane-dione, BPMPPD], 23310 cm−1[1,6-bis-(1′-phenyl-3′-methyl-5′-pyrazolon-4′)-1,6-hexanedione, BPMPHD], 21978 cm−1[1,9-bis-(1′-phenyl-3′-methyl-5′-pyrazolon-4′)-1,9-nonanedione, BPMPND] and 21930 cm−1[1,10-bis-(1′-phenyl-3′-methyl-5′-pyrazolon-4′)-1,10-decanedione, BPMPDD], respectively. It was explained satisfactorily that the six ligands are more efficient for sensitizing the luminescence of Tb3+ than that of Eu3+ at room temperature, and the order of the luminescent intensities for the Tb3+ complexes is explained by the relative energy gap between T and 5DJ of Tb3+ or Eu3+. As a conclusion, when 2700 cm−1<ΔE(T-5D4)<3000 cm−1, the luminescent intensity of the Tb3+ complex is the strongest. This means that the lowest triplet energy level of the ligand is a chief factor to dominate RE3+ luminescence.

A density functional theory study of paramagnetic cyclopentadienylcobalt(III) derivatives: fluoride versus cyanide

The cobalt(III) complexes Cp₂Co₂F₄ and Cp₂Co₂(CN)₄ have been studied by density functional theory methods as representatives of the experimentally known Cp₂Co₂X₄ species with the weak-field fluoride ligand and the strong-field cyanide ligand. Both complexes were found to have relatively complicated energy surfaces with low-energy triplet and quintet spin state structures as well as the expected singlet-state structures for Co(III) complexes. This existence of singlet-, triplet-, and quintet-state structures of similar energies complicates the study of these complexes by density functional theory. The B3LYP* method of Reiher et al. was chosen in an effort to provide the most reliable estimates of the relative energies of the singlet, triplet, and quintet spin states. The lowest-energy Cp₂Co₂F₄ structure was found to be a doubly bridged quintet spin state structure, with similar triplet and singlet structures lying within ∼4 kcal mol⁻¹ of this quintet structure. The lowest-energy Cp₂Co₂(CN)₄ structure was found to be a triplet spin state structure, with a singlet structure lying within ∼1 kcal mol⁻¹ of this triplet structure. Almost all of the Cp₂Co₂X₄ structures were found to have nonbonding Co···Co distances in excess of 2.9 Å, as expected for Co(III) complexes. In general, structures with trans stereochemistry of the Cp and other terminal ligands were found to be of lower energy than the corresponding structures with cis stereochemistry.

Quantitative Bimolecular Recombination in Organic Photovoltaics through Triplet Exciton Formation

The nanoscale morphology and high charge densities in organic photovoltaics (OPVs) lead to a high rate of bimolecular encounters between spin-uncorrelated electrons and holes. This process can lead to the formation of low-energy triplet excitons on the donor polymer that decay nonradiatively and limit the device performance. We use time-resolved optical spectroscopy to characterize the effect of morphology through the use of solvent additives such as 1,8-octanedithiol (ODT) on triplet dynamics and charge recombination in blends of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] and [6,6]-phenyl-C71-butyric acid methyl ester. This is an attractive OPV system since the extended absorption of the polymer into the near-infrared gives good coverage of the solar spectrum, but nevertheless, the internal quantum efficiency (IQE) has not been reported to be higher than ~65% under short circuit conditions. We find that, without ODT, the IQE is 48% and 16% of excitations decay via bimolecular triplet formation. With ODT treatment, which improves crystallinity and carrier mobility, the IQE increases to 65%, but bimolecular triplet formation significantly increases and now accounts for all of the recombination (35% of charges).

The diversity of structural features in binuclear cyclobutadiene manganese carbonyls: Relationship to homoleptic manganese carbonyls and cyclopentadienyl chromium carbonyls

The structures and energetics of the cyclobutadiene manganese carbonyls (C4H4)2Mn2(CO)n (n=6, 5, 4, 3) have been investigated using density functional theory. In this connection the lowest energy (C4H4)2Mn2(CO)6 structure consists of two C4H4Mn(CO)3 units coupled through an Mn–Mn bond of length ∼3.0Å. This rather fragile dimer is predicted to dissociate readily into C4H4Mn(CO)3 radicals. A higher energy (C4H4)2Mn2(CO)6 structure has an agostic hydrogen atom and an Mn⋯Mn distance of ∼4.3Å, too long for a direct bond. The unsaturated pentacarbonyl (C4H4)2Mn2(CO)5 system resembles its (C5H5)2Cr2(CO)5 counterpart by having low energy triplet spin state structures and being disfavored relative to (C4H4)2Mn2(CO)6+(C4H4)2Mn2(CO)4. A singlet tetracarbonyl (C4H4)2Mn2(CO)4 structure is found with a short MnMn distance of ∼2.2Å suggesting a formal triple bond analogous to the known (C5H5)2Cr2(CO)4 structure. However, the lowest energy (C4H4)2Mn2(CO)4 state is a novel triplet spin state octahedral Mn2C4 cluster with Mn(CO)3 and (η4-C4H4)Mn(CO) vertices. The lowest energy (C4H4)2Mn2(CO)3 structure by ∼13kcal/mol is an unsymmetrical triplet spin state structure with a C4H4 ligand bridging a dative formal MnMn triple bond connecting an Mn(CO)3 group to a (η4-C4H4)Mn group.

Zethrene biradicals: How pro-aromaticity is expressed in the ground electronic state and in the lowest energy singlet, triplet, and ionic states

A analysis of the electronic and molecular structures of new molecular materials based on zethrene is presented with particular attention to those systems having a central benzo-quinoidal core able to generate Kekulé biradicals whose stability is provided by the aromaticity recovery in this central unit. These Kekulé biradicals display singlet ground electronic states thanks to double spin polarization and have low-energy lying triplet excited states also featured by the aromaticity gain. Pro-aromatization is also the driving force for the stabilization of the ionized species. Moreover, the low energy lying singlet excited states also display a profound biradical fingerprint allowing to singlet exciton fission. These properties are discussed in the context of the size of the zethrene core and of its substitution. The work encompasses all known long zethrenes and makes use of a variety of experimental techniques, such as Raman, UV-Vis-NIR absorption, transient absorption, in situ spectroelectrochemistry and quantum chemical calculations. This study reveals how the insertion of suitable molecular modules (i.e., quinoidal) opens the door to new intriguing molecular properties exploitable in organic electronics.

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

AbstractTo 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,C2′)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.

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.

Computational Investigation on the Spectroscopic Properties of Thiophene Based Europium β-Diketonate Complexes.

The adiabatic transition energies from the lowest triplet states of four Europium tris β-diketonate/phenantroline complexes have been determined in vacuo and in dicholomethane solution by the ΔSCF approach at the density functional theory level, using the PBE1PBE and the CAM-B3LYP hybrid functionals. The calculated adiabatic transition energies have been compared with the experimental 0-0 transitions of each complex determined from phosphorescence spectra of the corresponding Gd(3+) complexes and followed by direct comparison between simulated and experimental spectra line shapes. For compound 1, the Eu(TTA)3Phen system, triplet states other than the lowest one and conformational isomers other than the one present in the crystallographic structure have been considered. In the crystallographic structure, this compound presents three quasi-degenerate low energy triplet states, differing for the TTA ligand where the two unpaired electrons are localized and showing close adiabatic transition energies. For compound 1, the lowest triplet states of the four investigated conformational isomers show similar characteristics and close adiabatic transition energies. On the basis of these results, an investigation of compounds 2-4 (Eu(Br-TTA)3Phen, Eu(DTDK)3Phen, and Eu(MeT-TTA)3) has been performed by considering only the isomer present in the crystallographic structure and only the lowest triplet state of each compound. For compounds 1-3, the energies of the lowest triplet states calculated by both functionals in solution including zero-point energy corrections well reproduce the experimental trends as well as the values of the adiabatic transition energies: CAM-B3LYP, the best performing functional, provides energies of the lowest triplet state with deviations from experiments lower than 1200 cm(-1). Also, the calculated vibrationally resolved phosphorescence spectra and UV-vis absorptions well reproduce the main features of their experimental counterparts. Significant differences between calculated and experimental results are observed for compound 4, for which difficulties in the experimental determination of the triplet state energy were encountered: our results show that the negligible photoluminescence quantum yield of this compound is due to the fact that the energy of the most stable triplet state is significantly lower than that of the resonance level of the Europium ion, and thus the energy transfer process is prevented. These results confirm the reliability of the adopted computational approach in calculating the energy of the lowest triplet state energy of these systems, a key parameter in the design of new ligands for lanthanide complexes presenting large photoluminescence quantum yields.

Electronic structure and reactivity in water splitting of the iron oxide dimers and their hexacarbonyls: A density functional study

The iron oxide dimers (FeO)2 and their peroxide isomers are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. Among the bare clusters the planar four-member ring structures are more stable than the non-planar ones and the rhombic dioxide Fe2O2 with antiferromagnetically ordered electrons on iron centers is the global minimum. Water adsorption on the bare diiron dioxide is exothermic, but dissociation does not occur. Carbonylation favors a non-planar Fe2O2 ring for both the dioxides and the peroxides and high electron density at the Fe centers is induced, evidenced by the natural charge distribution, the high proton affinity, and the values of global electronegativity and hardness. The iron dioxide hexacarbonyl Fe2O2(CO)6 is diamagnetic in the state of the global minimum. It is separated from the next low-lying triplet state by a small energy gap of 0.22 eV. Time-dependent density functional theory methods were applied to examine electron excitations from the ground state to the low-lying triplet states in the hexacarbonyls and their adsorption complexes with water. Singlet-to-triplet state excitations occur via ligand-to-metal charge transfer in the hexacarbonyls; in the adsorption complexes excitations from the oxygen lone pairs to the adsorption center also occur and they appear in the IR-visible region. The lowest energy singlet and triplet state reaction paths for water splitting were followed. On the singlet potential energy surface (PES), water splitting is spontaneous, while for the triplet PES an activation barrier of 14.1 kJ mol(-1) was determined.

Thienylpyridine-based cyclometallated iridium(iii) complexes and their use in solid state light-emitting electrochemical cells

The synthesis and characterization of four iridium(iii) complexes [Ir(thpy)2(N^N)][PF6] where Hthpy = 2-(2'-thienyl)pyridine and N^N are 6-phenyl-2,2'-bipyridine (1), 4,4'-di-(t)butyl-2,2'-bipyridine (2), 4,4'-di-(t)butyl-6-phenyl-2,2'-bipyridine (3) or 4,4'-dimethylthio-2,2'-bipyridine (4) are described. The single crystal structures of ligand 4 and the complexes containing the [Ir(thpy)2(1)](+) and [Ir(thpy)2(4)](+) cations have been determined. In [Ir(thpy)2(1)](+), the pendant phenyl ring engages in an intra-cation π-stacking interaction with one of the thienyl rings in the solid state, and undergoes hindered rotation on the NMR timescale in [Ir(thpy)2(1)](+) and [Ir(thpy)2(3)](+). The solution spectra of [Ir(thpy)2(1)][PF6] and [Ir(thpy)2(4)][PF6] show emission maxima around 640 nm and are significantly red-shifted compared with [Ir(thpy)2(2)][PF6] and [Ir(thpy)2(3)][PF6] which have structured emission bands with maxima around 550 and 590 nm. In thin films, the emission spectra of the four complexes are similar with emission peaks around 550 and 590 nm and a shoulder around 640 nm that are reminiscent of the features observed in solution. In solution, quantum yields are low, but in thin films, values range from 29% for [Ir(thpy)2(1)][PF6] to 51% for [Ir(thpy)2(4)][PF6]. Density functional theory calculations rationalize the structured emission observed for the four complexes in terms of the (3)LC nature predicted for the lowest-energy triplet states that mainly involve the cyclometallated [thpy](-) ligands. Support for this theoretical result comes from the observed features of the low temperature (in frozen MeCN) photoluminescence spectra of the complexes. Photoluminescence and electroluminescence spectra of the complexes in a light-emitting electrochemical cell (LEC) device configuration have been investigated. The electroluminescence spectra are similar for all [Ir(thpy)2(N^N)][PF6] complexes with emission maxima at ≈600 nm, but device performances are relatively poor probably due to the poor charge-transporting properties of the complexes.

How disorder controls the kinetics of triplet charge recombination in semiconducting organic polymer photovoltaics.

Recent experiments by Rao et al. (Nature, 2013, 500, 435-439) indicate that recombination of triplet charge-separated states is suppressed in organic polymer-fullerene based bulk-heterojunction (BHJ) photovoltaic cells exhibiting a high degree of crystallinity in the fullerene phase relative to systems with more disorder. In this paper, we use a series of Frenkel-exciton lattice models to rationalize these results in terms of wave-function localization, interface geometry, and density of states. In one-dimensional co-linear and co-facial models of the interface, increasing local energetic disorder in one phase localizes the interfacial triplet charge-transfer ((3)CT) states and increases the rate at which these states relax to form lower-energy triplet excitons. In two dimensional BHJ models, energetic disorder within the fullerene phase plays little role in further localizing states pinned to the interface. However, inhomogeneous broadening introduces strong coupling between the interfacial (3)CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher degrees of energetic disorder.

Non-Born–Oppenheimer molecular dynamics of the spin-forbidden reaction O(3P) + CO(X 1Σ+) → CO2($\tilde X{}^1\Sigma _g^ +$X̃Σg+1)

The lowest-energy singlet (1 (1)A') and two lowest-energy triplet (1 (3)A' and 1 (3)A") electronic states of CO2 are characterized using dynamically weighted multireference configuration interaction (dw-MRCI+Q) electronic structure theory calculations extrapolated to the complete basis set (CBS) limit. Global analytic representations of the dw-MRCI+Q∕CBS singlet and triplet surfaces and of their CASSCF∕aug-cc-pVQZ spin-orbit coupling surfaces are obtained via the interpolated moving least squares (IMLS) semiautomated surface fitting method. The spin-forbidden kinetics of the title reaction is calculated using the coupled IMLS surfaces and coherent switches with decay of mixing non-Born-Oppenheimer molecular dynamics. The calculated spin-forbidden association rate coefficient (corresponding to the high pressure limit of the rate coefficient) is 7-35 times larger at 1000-5000 K than the rate coefficient used in many detailed chemical models of combustion. A dynamical analysis of the multistate trajectories is presented. The trajectory calculations reveal direct (nonstatistical) and indirect (statistical) spin-forbidden reaction mechanisms and may be used to test the suitability of transition-state-theory-like statistical methods for spin-forbidden kinetics. Specifically, we consider the appropriateness of the "double passage" approximation, of assuming statistical distributions of seam crossings, and of applications of the unified statistical model for spin-forbidden reactions.

Matrix Isolation and Computational Study of the [H, C, N, Se] Isomers

Nine minima on the ground-state singlet and ten minima on the lowest-energy triplet potential energy surfaces of the [H, C, N, Se] system were located at the B3LYP/aug-cc-pVTZ level of theory. The singlet isomers were further investigated by the higher-level CCSD(T) method. Besides their structure and relative energies, isomerization barriers and the dissociation energies of the most important fragmentation channels were determined. Anharmonic vibrational wavenumbers, infrared intensities, relative Raman intensities, and UV excitation energies were also computed to assist the detection of these species. Two of the singlet isomers were generated and investigated by IR and UV spectroscopic methods. First, HNCSe and its deuterated isotopomer, DNCSe, were prepared by the reaction of HBr/DBr with AgNCSe and deposited in an 8 K Ar matrix. Photolysis of (H/D)NCSe at 254 nm led to the formation of the novel (H/D)SeNC isomer, which decomposed upon broad-band UV irradiation.

Iron–iron bonding versus iron–phosphorus bonding in binuclear diphosphacyclobutadiene iron carbonyl complexes

The diphosphacyclobutadiene sandwich compounds (η4-But2C2P2)2Mz (M=Ni, z=0; M=Co, z=−1, 0; M=Fe, z=−1, 0) as well as the iron carbonyl derivative (η4-But2C2P2)Fe(CO)3 have recently (2008) been synthesized by Lammertsma and co-workers using the dimerization of the phosphaalkyne ButCP: on suitable reactive transition metal sites. The structures and energetics of the closely related dimethyl derivatives (Me2C2P2)Fe(CO)n (n=3, 2, 1) and (Me2C2P2)2Fe2(CO)n (n=5, 4, 3, 2) have now been investigated by density functional theory. For (Me2C2P2)2Fe2(CO)5 a structure with a bridging η1,η4-Me2C2P2 ring and no iron–iron bond is energetically preferred by more than 24kcal/mol over an alternative structure with only terminal η4-Me2C2P2 rings and an iron–iron single bond. The lowest energy singlet and triplet (Me2C2P2)2Fe2(CO)4 structures have at least one bridging η1,η4 ligand, which in one case is in the form of an (η4-Me2C2P2)2Fe(CO) sandwich bidentate ligand chelating to an Fe(CO)3 unit through two ring phosphorus atoms. In contrast to (Me2C2P2)2Fe2(CO)n (n=5, 4), the lowest energy structures for (Me2C2P2)2Fe2(CO)3 have only terminal η4-Me2C2P2 ligands, three carbonyl groups, and an FeFe triple bond. Interesting structures for the dicarbonyl (Me2C2P2)2Fe2(CO)2 include a structure with a (η4-Me2C2P2)2Fe sandwich ligand chelating to an Fe(CO)2 group and a structure with exclusively terminal η4-Me2C2P2 ligands and a short Fe–Fe distance of ∼2.16Å, suggesting a formal quadruple bond.

The role of spin in the kinetic control of recombination in organic photovoltaics

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

High efficient white organic light-emitting diodes with single emissive layer using phosphorescent red, green, and blue dopants

Phosphorescent white organic light-emitting diodes (PHWOLEDs) with single emissive layer were fabricated by co-doping phosphorescent blue, green, and red emitters with different concentrations. WOLEDs using Ir(piq)3 and Ir(ppy)3 as red and green dopants along with 8% of Firpic as blue dopant with host materials of 4CzPBP in the emissive layer were compared under various doping ratio between Ir(piq)3 and Ir(ppy)3. Triplet-triplet Dexter energy transfer in single emissive PHWOLEDs including three primary colors was saturated from higher triplet energy levels to lower triplet energy levels directly.

Generation of Powerful Tungsten Reductants by Visible Light Excitation

The homoleptic arylisocyanide tungsten complexes, W(CNXy)6 and W(CNIph)6 (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400-550 nm). MLCT emission (λ(max) ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)6 and W(CNIph)6 with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)6 (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bu(n)4N][PF6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)6](+) and [anth](•-). ET from *W to benzophenone and cobalticenium also is observed in [Bu(n)4N][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)6](+)/*W couple is -2.8 V vs Cp2Fe(+/0), establishing W(CNIph)6 as one of the most powerful photoreductants that has been generated with visible light.

Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene‐Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence

Two new molecules, CzFCBI and CzFNBI, have been tailor-made to serve as bipolar host materials to realize high-efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole-transporting block and N-phenylbenzimidazole as the electron-transporting block hybridized through the saturated bridge center (C9) and meta-conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N-phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy-level alignment allows CzFCBI to outperform its N-connected counterpart CzFNBI while employing other long-wavelength-emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue-to-red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low-roll-off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish-green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish-green with a sky-blue emitter and a red emitter, a CzFNBI-hosted single-emitting-layer all-phosphor three-color-based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A(-1) , 28.3 lm W(-1) ) and highly stable chromaticity (CIE x=0.43-0.46 and CIE y=0.43) at an applied voltage of 8 to 12 V, and a high color-rendering index of 91.6.

Coordination of N,N-Chelated Re(CO)3Cl Units Across a Mo2 Quadruple Bond: Synthesis, Characterization, and Photophysical Properties of a Re–Mo2–Re Triad and Its Component Pieces

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2'-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand. Electronic structure calculations on model compounds II' and III', where formate ligands substitute for T(i)PB, show that the highest occupied molecular orbital (HOMO) in II is Mo2δ. When the Re(CO)3Cl unit is attached to the PMT ligand to form III, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d(6), t2g type orbitals. Oxidation of III is shown to be Mo2-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo2δ-to-PMT π* transition. The S1 states in both II and III are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T1 is (3)MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds I (T1) and III (S1 and T1) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of I parallel those for Re(bpy)(CO)3Cl, with the lowest-energy T1 state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For III, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T1 state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state.

Comparison of the difluoromethylene and carbonyl ligands in binuclear iron complexes

The binuclear difluoromethylene iron carbonyl Fe2(CF2)(CO)8 has been isolated as a stable compound from the reaction of Na2Fe2(CO)8 with CF2Br2. The structures and energetics of its decarbonylation products Fe2(CF2)(CO)n (n=7, 6, 5) have now been investigated using density functional theory. These are possible products from the photolysis of Fe2(CF2)(CO)8 under suitable conditions. In the lowest energy Fe2(CF2)(CO)n (n=7, 6, 5) structures the CF2 group is always a bridging rather than terminal group and iron–iron triple and quadruple bonds are avoided in the highly unsaturated structures. The lowest energy Fe2(CF2)(CO)7 structure has a bridging CF2 group and two semibridging CO groups. However, a singly bridged Fe2(CF2)(CO)7 structure lies only ~2kcal/mol in energy above this global minimum. The lowest energy Fe2(CF2)(CO)6 structure is a doubly bridged structure with a four-electron donor bridging η2-μ-CO group, the usual bridging CF2 group, and an FeFe distance of ~2.44Å suggesting a formal double bond. An Fe2(CF2)(CO)6 structure with an FeFe distance of ~2.23Å suggesting a formal triple bond, analogous to the lowest energy Fe2(CO)7 structure, lies ~13kcal/mol in energy above this structure. The lowest energy Fe2(CF2)(CO)5 structures are doubly bridged triplet spin state structures having iron–iron single or double bonds. The lowest energy singlet Fe2(CF2)(CO)5 structure, lying ~12kcal/mol above the lowest energy triplet structure, has an unprecedented kind of four-electron donor η2-μ-CF2 group, which is bonded to the central Fe2 unit though both its carbon atom and one of its fluorine atoms.