Min-Wen Chung

Systematic Investigation of the Metal-Structure–Photophysics Relationship of Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light Emitting Devices

A series of new emissive group 11 transition metal d(10)-complexes 1-8 bearing functionalized 2-pyridyl pyrrolide together with phosphine ancillary such as bis[2-(diphenylphosphino)phenyl] ether (POP) or PPh(3) are reported. The titled complexes are categorized into three classes, i.e. Cu(I) complexes (1-3), Ag(I) complexes (4 and 5), and Au(I) metal complexes (6-8). Via combination of experimental and theoretical approaches, the group 11 d(10)-metal ions versus their structural variation, stability, and corresponding photophysical properties have been investigated in a systematic and comprehensive manner. The results conclude that, along the same family, how much a metal d-orbital is involved in the electronic transition plays a more important role than how heavy the metal atom is, i.e. the atomic number, in enhancing the spin-orbit coupling. The metal ions with and without involvement of a d orbital in the lowest lying electronic transition are thus classified into internal and external heavy atoms, respectively. Cu(I) complexes 1-3 show an appreciable metal d contribution (i.e., MLCT) in the lowest lying transition, so that Cu(I) acts as an internal heavy atom. Despite its smallest atomic number among group 11 elements, Cu(I) complexes 1-3 exhibit a substantially larger rate of intersystem crossing (ISC) and phosphorescence radiative decay rate constant (k(r)(p)) than those of Ag(I) (4 and 5) and Au(I) (6-8) complexes possessing pure π → π* character in the lowest transition. Since Ag(I) and Au(I) act only as external heavy atoms in the titled complexes, the spin-orbit coupling is mainly governed by the atomic number, such that complexes associated with the heavier Au(I) (6-8) show faster ISC and larger k(r)(p) than the Ag(I) complexes (4 and 5). This trend of correlation should be universal and has been firmly supported by experimental data in combination with empirical derivation. Along this line, Cu(I) complex 1 exhibits intensive phosphorescence (Φ(p) = 0.35 in solid state) and has been successfully utilized for fabrication of OLEDs, attaining peak EL efficiencies of 6.6%, 20.0 cd/A, and 14.9 lm/W for the forward directions.

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs†

Organic light-emitting diodes (OLEDs) based on heavy transition-metal complexes are playing a pivotal role in next generation of, for example, flat panel displays and solid-state lighting. The readily available, Os-, Pt-, and in particular Ir-based phosphorescence complexes grant superior advantage over fluorescent materials. This is mainly due to heavyatom-induced spin–orbit coupling, giving effective harvesting of both singlet and triplet excitons. However, tuning of phosphorescence over the entire visible spectrum still remains a challenge. Particularly, designing new materials to show higher energy, such as deep-blue emission—with an ideal CIEx,y coordinate (CIE = Commission Internationale de L Eclairage) of (0.14, 0.09)—encounters more obstacle than the progress made for obtaining green and red colors. Representative blue phosphors are a class of Ir complexes possessing at least one cyclometalated 4,6-difluorophenyl pyridine {(dfppy)H} ligand, known as FIrpic, FIr6, FIrtaz, and others. The majority of blue phosphors showed inferior color chromaticity with a sum of CIEx+y values being much greater than 0.3 or with single CIEy coordinate higher than 0.25. Such inferior chromaticity, in part, has been improved upon adoption of carbene-, triazolyl-, and fluorine-substituted bipyridine (dfpypy) based chelates. The above urgency prompted us to search for better and new blue phosphors. We produced a class of 2-pyridylazolate chelates possessing very large ligand-centered p–p* energy gap, as evidenced by the blue-emitting Os complexes. Subsequently, room-temperature blue phosphorescence was also visualized for the respective heteroleptic Ir complexes, particularly for those dubbed “nonconjugated” ancillary chelate(s). The nonconjugated ligands so far comprise a benzyl substituted pyrazole, an N-heterocyclic carbene, phosphines, and other ingenious molecular designs. Herein, we report the preparation of a novel class of heteroleptic Ir complexes by incorporation of tripodal, facially coordinated phosphite (or phosphonite), denoted as the P^C2 chelate, for serving as the ancillary, together with the employment of 2-pyridyltriazolate acting as blue chromophore. The reaction intermediate, which possesses an acetate chelate, was isolated and characterized to establish the synthetic pathway. The tridentate P^C2 ancillary chelate offers several advantages: 1) Good stabilization of complex and necessary long-term stability in application of for example, emitting devices. 2) The strong bonding of phosphorous donors is expected to destabilize the ligand field d–d excited state, thus minimizing its interference to the radiative process from the lower lying excited state. 3) P^C2 inherits profound and versatile functionality (see below) capable of fine-tuning the electronic character. As a result, highly efficient blue phosphorescence is attained with good OLED performance. Treatment of a mixture of [IrCl3(tht)3] (tht = tetrahydrothiophene) with an equimolar amount of triphenylphosphine (PPh3), triphenylphosphite {P(OPh)3}, and an excess of sodium acetate resulted in a high yield conversion (> 80%) into [Ir(P^C2)(PPh3)(OAc)] (1a); P^C2 = tripodal dicyclometalated phosphite (Scheme 1). Subsequent replacement of acetate in 1a with chelating 3-tert-butyl-5-(2-pyridyl)triazo-

Comprehensive Studies on an Overall Proton Transfer Cycle of the ortho-Green Fluorescent Protein Chromophore

Initiated by excited-state intramolecular proton transfer (ESIPT) reaction, an overall reaction cycle of 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), an analogue of the core chromophore of the green fluorescent protein (GFP), has been investigated. In contrast to the native GFP core, 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (p-HBDI), which requires hydrogen-bonding relay to accomplish proton transfer in vivo, o-HBDI possesses a seven-membered-ring intramolecular hydrogen bond and thus provides an ideal system for mimicking an intrinsic proton-transfer reaction. Upon excitation, ESIPT takes place in o-HBDI, resulting in a ∼600 nm proton-transfer tautomer emission. The o-HBDI tautomer emission, resolved by fluorescence upconversion, is comprised of an instantaneous rise to a few hundred femtosecond oscillation in the early relaxation stage. Frequency analysis derived from ultrashort pulse gives two low-frequency vibrations at 115 and 236 cm(-1), corresponding to skeletal deformation motions associated with the hydrogen bond. The results further conclude that ESIPT in o-HBDI is essentially triggered by low-frequency motions and may be barrierless along the reaction coordinate. Femtosecond UV/vis transient absorption spectra also provide supplementary evidence for the structural evolution during the reaction. In CH(3)CN, an instant rise of a 530 nm transient is resolved, which then undergoes 7.8 ps decay, accompanied by the growth of a rather long-lived 580 nm transient species. It is thus concluded that following ESIPT the cis-proton transfer isomer undergoes cis-trans-isomerization. The results of viscosity-dependent dynamics are in favor of the one-bond-flip mechanism, which is in contrast to the volume-conserving isomerization behavior for cis-stilbene and p-HBDI. Further confirmation is given by the picosecond-femtosecond transient IR absorption spectra, where several new and long-lived IR bands in the range of 1400-1500 cm(-1) are assigned to the phenyl in-plane breathing motions of the trans-proton transfer tautomer. Monitored by the nanosecond transient absorption, the 580 nm transient undergoes a ∼7.7 μs decay constant, accompanied by the growth of a new ∼500 nm band. The latter is assigned to a deprotonated tautomer species, which then undergoes the ground-state reverse proton recombination to the original o-HBDI in ∼50 μs, achieving an overall, reversible proton transfer cycle. This assignment is unambiguously supported by pump-probe laser induced fluorescence studies. On these standpoints, a comparison of photophysical properties among o-HBDI, p-HBDI, and wild-type GFP is discussed in detail.

Phosphorescent Ir(III) complexes bearing double benzyldiphenylphosphine cyclometalates; strategic synthesis, fundamental and integration for white OLED fabrication

A new Ir(III) complex [Ir(bdp)2(OAc)] (1) was prepared by the treatment of IrCl3(tht)3 with approx. two equivalent of benzyldiphenylphosphine in refluxing decalin solution, bdpH = benzyldiphenylphosphine and tht = tetrahydrothiophene. Complex 1 proves to be a versatile precursor, which could further react with various triazolate chelates such as 5-pyridyl-3-trifluoromethyl-1,2,4-triazole (fptzH), 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazole (bptzH), 5-(1-isoquinolyl)-3-tert-butyl-1,2,4-triazole (iqbtzH) and 5-(1-phenanthridinyl)-3-tert-butyl-1,2,4-triazole (pbtzH) to afford the emissive complexes [Ir(bdp)2(fptz)] (2), [Ir(bdp)2(bptz)] (3), [Ir(bdp)2(iqbtz)] (4), and [Ir(bdp)2(phbtz)] (5), respectively. Single crystal X-ray diffraction studies of 1 and 5 revealed a distorted octahedral Ir(III) metal core, both possess two mutually orthogonal bdp cyclometalates, and the respective PPh2 donors reside at the cis-orientation. Formation of complexes 2–5 can be envisioned as simple replacement of acetate with the incoming N-heterocyclic triazolate chelates. As for photophysical properties, the structural variation leads to salient difference in emission features among complexes 2–5. Combining theoretical approaches, the results are rationalized by the contribution from the degree of ligand π-conjugation, together with the occurrence of ligand-to-ligand charge transfer (LLCT) and intra-ligand ππ* transition in the lowest lying excited state. The orange-red and white light-emitting OLEDs were then fabricated using 5 as dopant, for which the respective devices gave peak efficiencies of 13.6% photons/electron, 33.3 cd A−1, 29.8 lm/W and with CIEx,y = 0.530, 0.467 at 100 cd m−2, and peak efficiencies of 13.0% photons/electron, 28.0 cd A−1, 22.8 lm/W, and with CIEx,y = 0.356, 0.348 at 1000 cd m−2.

Authentic-Blue Phosphorescent Iridium(III) Complexes Bearing Both Hydride and Benzyl Diphenylphosphine; Control of the Emission Efficiency by Ligand Coordination Geometry

Sequential treatment of IrCl(3) x nH(2)O with 2 equiv of benzyl diphenylphosphine (bdpH) and then 1 equiv of 3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppzH) in 2-methoxyethanol gave formation to three isomeric complexes with formula [Ir(bdp)(fppz)(bdpH)H] (1-3). Their molecular structures were established by single crystal X-ray diffraction studies, showing existence of one monodentate phosphine bdpH, one terminal hydride, a cyclometalated bdp chelate, and a fppz chelate. Variation of the metal-ligand bond distances showed good agreement with those predicted by the trans effect. Raman spectroscopic analyses and the corresponding photophysical data are also recorded and compared. Among all isomers complex 1 showed the worst emission efficiency, while complexes 2 and 3 exhibited the greatest luminescent efficiency in solid state and in degassed CH(2)Cl(2) solution at room temperature, respectively. This structural relationship could be due to the simultaneously weakened hydride and the monodentate bdpH bonding that are destabilized by the trans-pyrazolate anion and cyclometalated benzyl group, respectively.

Excited-State Intramolecular Proton Transfer (ESIPT) Fine Tuned by Quinoline−Pyrazole Isomerism: π-Conjugation Effect on ESIPT

A series of quinoline/isoquinoline-pyrazole isomers (I-III), in which the pyrazole moiety is in a different substitution position, was strategically designed and synthesized, showing a system with five-membered intramolecular hydrogen bonding. Despite the similarity in molecular structure, however, only I undergoes excited-state intramolecular proton transfer, as evidenced by the distinct 560 nm proton-transfer emission and its associated relaxation dynamics. The experimental results support a recent theoretical approach regarding the conjugation effect on a proton (or hydrogen atom) transfer reaction (J. Phys. Chem. A 2009, 113, 4862-4867). The concept simply predicts that more extended pi conjugation, i.e., resonance, for proton-transfer tautomer species could allow for efficient delocalization of excess charge in the reaction center, resulting in a larger thermodynamic driving force for proton transfer.

Phosphorescent OLEDs assembled using Os(II) phosphors and a bipolar host material consisting of both carbazole and dibenzophosphole oxide

We report on the synthesis of a new series of Os(II) complexes (1–3) functionalized with 2-pyridyl (or 2-isoquinolyl) pyrazole chelates, together with a new diphosphine, 1,2-bis(phospholano)benzene chelate (pp2b). The resulting Os(II) complexes are fully characterized and their structural versus spectroscopic properties have been comprehended by absorption/emission together with computational approaches. The inherent electron richness, restricted rotational barrier and good steric hindrance of pp2b lead to the production of both orange and red phosphorescence with high quantum efficiency. For exploring these Os(II) based OLEDs, we also synthesized a bipolar material 5-[4-(carbazo-9-yl)phenyl] dibenzophosphole-5-oxide (CzPhO), possessing both carbazole donor and dibenzophosphole oxide acceptor. Successful fabrication of OLEDs using complexes 1 and 3 as the dopant and either 4,4′-N,N′-dicarbazolebiphenyl (CBP) or CzPhO as host is reported. For comparison, the CBP and CzPhO devices with 1 as the emitter showed peak efficiencies EQE of 10.9%, ηL of 21.7 cd A−1, and ηp of 11.9 lm W−1, and EQE of 14.3%, ηL of 34.8 cd A−1, and ηp of 45.2 lm W−1, respectively.

Bis(diphenylamino)-9,9 '-spirobifluorene functionalized Ir(III) complex: a conceptual design en route to a three-in-one system possessing emitting core and electron and hole transport peripherals

Conceptual design of a three-in-one (luminescence chromophore with electron and hole transports) system was demonstrated by a functionalized Ir(III) complex 3, in which 4,5-diazafluorene and bis(diphenylamino) serve as electron and hole transporting sites, respectively. The poor emission quantum yield of 3 was systematically examined via a series of photophysical studies in combination with theoretical approaches. The far lifting of the π-electron from -NPh2 renders virtually no 3MLCT contribution to the lowest transition in the triplet manifold as compared with that of the parent model 2 without amino substituents. With an empirical approach, we conclude that an energy gap law may account for the major deactivation process. A light-emitting electrochemical cell (LEC) device based on 3 shows peak EQE, peak current efficiency and peak power efficiency at 2.4 V of 0.020%, 0.013 cd A−1 and 0.017 lm/W, respectively. The low device efficiencies are in accordance with the low PL quantum yield, stemming from the ligand-centered radiationless deactivation. The conceptual design presented here should provide valuable information for future progress en route to an ideal three-in-one system suited for OLEDs.

A new coordination polymer exhibiting unique 2D hydrogen-bonded (H2O)16 ring formation and water-dependent luminescence properties.

A new coordination polymer, [Zn(dpe)(bdc)]·4H(2)O (ZndB; dpe=1,2-bis(4-pyridyl)ethane, bdc(2-)=dianion of benzenedicarboxylic acid), which possesses a 3D metal-organic framework (MOF) has been synthesized and structurally characterized. This 3D MOF is constructed by the assembly of helical channels filled with guest water molecules in both inner and outer regions of the channel. The resulting network also creates a 2D water layer containing hydrogen-bonded (H(2)O)(16) rings as the basic building units. Thermogravimetric and powder X-ray diffraction measurements of ZndB revealed a two-step weight loss of water molecules with a reversible water adsorption/desorption process in the inner channel for the first stage, and irreversible water desorption in the outer channel for the second stage. This spongelike property is manifested by the excimer emission originating from interaction between dpe (π*) and the other dpe (π) of the proximal helical channel, which is highly sensitive to the environmental perturbation. Powder X-ray analyses reveal that the dehydration process induces the readjustment of dpe π-π stacking distance/orientation, which results in dramatic luminescence changes from dim pale blue (λ(em)≈470 nm) upon hydration to bright white-light generation (broad, λ(em)≈500-550 nm) upon water depletion, accompanied by a ≈100-fold increase in the emission intensity.

Structural tuning intra- versus inter-molecular proton transfer reaction in the excited state

A series of 2-pyridyl-pyrazole derivatives 1-4 possessing five-membered ring hydrogen bonding configuration are synthesized, the structural flexibility of which is strategically tuned to be in the order of 1 > 2 > 3 > 4. This system then serves as an ideal chemical model to investigate the correlation between excited-state intramolecular proton transfer (ESIPT) reaction and molecular skeleton motion associated with hydrogen bonds. The resulting luminescence data reveal that the rate of ESIPT decreases upon increasing the structural constraint. At sufficiently low concentration where negligible dimerization is observed, ESIPT takes place in 1 and 2 but is prohibited in 3 and 4, for which high geometry constraint is imposed. The results imply that certain structural bending motions associated with hydrogen bonding angle/distance play a key role in ESIPT. This trend is also well supported by the DFT computational approach, in which the barrier associated with ESIPT is in the order of 1 < 2 < 3 < 4. Upon increasing the concentration in cyclohexane, except for 2, the rest of the title compounds undergo ground-state dimerization, from which the double proton transfer takes place in the excited state, resulting in a relatively blue shifted dimeric tautomer emission (cf. the monomer tautomer emission). The lack of dimerization in 2 is rationalized by substantial energy required to adjust the angle of hydrogen bond via twisting the propylene bridge prior to dimerization.