Jia-Ling Liao

A New Class of Sky-Blue-Emitting Ir(III) Phosphors Assembled Using Fluorine-Free Pyridyl Pyrimidine Cyclometalates: Application toward High-Performance Sky-Blue- and White-Emitting OLEDs

Two pyrimidine chelates with the pyridin-2-yl group residing at either the 5- or 4-positions are synthesized. These chelates are then utilized in synthesizing of a new class of heteroleptic Ir(III) metal complexes, namely [Ir(b5ppm)2(fppz)] (1), [Ir(b5bpm)2(fppz)] (2), [Ir(b4bpm)2(fppz)] (3), and [Ir(b5bpm)(fppz)2] (4), for which the abbreviations b5ppm, b5bpm, b4bpm, and fppz represent chelates derived from 2-t-butyl-5-(pyridin-2-yl)pyrimidine, 2-t-butyl-5-(4-t-butylpyridin-2-yl)pyrimidine, 2-t-butyl-4-(4-t-butylpyridin-2-yl)pyrimidine, and 3-trifluoromethyl-5-(pyridin-2-yl) pyrazole, respectively. The single crystal X-ray structural analyses were executed on 1 to reveal their coordination arrangement around the Ir(III) metal element. The 5-substituted pyrimidine complexes 1, 2, and 4 exhibited the first emission peak wavelength (λmax) located in the range 452-457 nm with high quantum yields, whereas the emission of 3 with 4-substituted pyrimidine was red-shifted substantially to longer wavelength with λmax = 535 nm. These photophysical properties were discussed under the basis of computational approaches, particularly the relationship between emission color and the relative position of nitrogen atoms of pyrimidine fragment. For application, organic light-emitting diodes (OLEDs) were also fabricated using 2 and 4 as dopants, attaining the peak external quantum, luminance, and power efficiencies of 17.9% (38.0 cd/A and 35.8 lm/W) and 15.8% (30.6 cd/A and 24.8 lm/W), respectively. Combining sky blue-emitting 2 and red-emitting [Os(bpftz)2(PPh2Me)2] (5), the phosphorescent white OLEDs were demonstrated with stable pure-white emission at CIE coordinate of (0.33, 0.34), and peak luminance efficiency of 35.3 cd/A, power efficiency of 30.4 lm/W, and external quantum efficiency up to 17.3%.

Blue-emitting Ir(III) phosphors with 2-pyridyl triazolate chromophores and fabrication of sky blue- and white- emitting OLEDs

Heteroleptic Ir(III) complexes with 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazolate chromophore (bptz) and cyclometalating benzyldiphenylphosphine (bdp) or phenyl diphenylphosphinite (pdpit) ancillary (i.e. [Ir(bptz)2(bdp)] (1) and [Ir(bptz)2(pdpit)] (2)) are synthesized upon treatment of [IrCl3(tht)3] (tht = tetrahydrothiophene) with the relevant phosphine, followed by the addition of 2 equiv. of bptz chelate at elevated temperature. Their photophysical properties in solution were measured, along with the characteristics detected as dopants in thin solid films. For application, organic light emitting diodes (OLEDs) were also fabricated using 1 and 2 as dopants, achieving respective maximum efficiencies of 17.8% (44.8 cd A−1 and 46.3 lm W−1) and 9.1% (22.8 cd A−1 and 23.6 lm W−1). In addition, sky blue iridium complex 1 was used with red osmium complex [Os(bpftz)2(PPhMe2)2] (3) to fabricate phosphorescent OLEDs with a sophisticated red/blue/red emitting layer architecture, attaining a stable warm white color with CIE coordinates of (0.397, 0.411). This white OLED attained an electroluminescence efficiency of up to 18.1%, 39.6 cd A−1, and 35.7 lm W−1 for the forward direction.

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.

Os(II) metal phosphors bearing tridentate 2,6-di(pyrazol-3-yl)pyridine chelate: synthetic design, characterization and application in OLED fabrication

Treatment of 2,6-di(5-trifluoromethylpyrazol-3-yl)pyridine (pz2py)H2 with Os3(CO)12 affords a mononuclear Os(II) complex [Os(pz2py)(CO)2(H2O)] (1) in excellent yield. Ligand substitution reactions were next executed to identify products with good photoluminescence at both fluid and solid states at RT. Therefore, substitutions with phosphorus donors such as PPh2Me and 2,6-bis(diphenylphosphinomethyl) pyridine (P2N), and nitrogen donors such as pyridine, 2,2′-bipyridine (bpy) and 2,2′:6′,2′′-terpyridine (tpy), afforded products with formula [Os(pz2py)(PPh2Me)2(CO)] (2), [Os(pz2py)(P2N)] (3), [Os(pz2py)(CO)2(py)] (4), [Os(pz2py)(CO)(bpy)] (5) and [Os(pz2py)(CO)(tpy)] (6). The single crystal X-ray structural analyses were executed on 1, 2, 3 and 6 to reveal the bonding of pz2py chelate as well as the structural effect imposed by the phosphorus and/or nitrogen donor groups. The photophysical properties were studied and discussed using the results of DFT and TDDFT calculations. For application, fabrication and analysis of organic light emitting diodes (OLEDs) were also carried out. OLEDs using 2 as a dopant exhibited an intense yellow emission with a maximum efficiency of 18.3%, 61.0 cd A−1, and 53.8 lm W−1, which are higher than those of most reported devices with greenish yellow/yellow emitters. Moreover, dopant 2 was combined with a red emitting dopant Os(bpftz)2(PPhMe2)2 (7) and two different sky-blue phosphors FIrpic and Ir(bptz)2(bdp) (8) to fabricate white OLEDs (WOLEDs). Device W1 achieved the highest efficiency of 18.0%, 33.9 cd A−1, and 31.2 lm W−1 while the maximized efficiency of device W2 was 15.3%, 29.3 cd A−1, and 27.0 lm W−1. Both devices showed stable warm-white emissions with a wide luminance range. In addition, device W2 exhibited a higher CRI of 84.2 with a low CCT of 2675 K at 103 cd m−2, making it a potential candidate for domestic lighting.

Near infrared-emitting tris-bidentate Os(II) phosphors: control of excited state characteristics and fabrication of OLEDs

A series of four Os(II) complexes bearing (i) chromophoric diimine ligands (N^N), such as 2,2′-bipyridine (bpy) and substituted 1,10-phenanthrolines, (ii) dianionic bipz chelate ligands derived from 5,5′-di(trifluoromethyl)-2H,2′H-3,3′-bipyrazole (bipzH2), and (iii) bis(phospholano)benzene (pp2b) as the third ancillary ligand completing the coordination sphere were synthesized. X-ray diffraction studies confirm the heteroleptic tris-bidentate coordination mode. These Os(II) complexes [Os(N^N)(bipz)(pp2b)], N^N = bpy (3), phenanthroline (4), 3,4,7,8-tetramethyl-1,10-phenanthroline (5) and 4,7-diphenyl-1,10-phenanthroline (6), display near infrared (NIR) emission between 717 nm and 779 nm in the solid state at RT. On the basis of hybrid-DFT and TD-DFT calculations, the emissions are assigned to metal-to-ligand charge transfer transitions (3MLCT) admixed with small ligand-to-ligand charge transfer (3LLCT) contributions. Successful fabrication of organic light emitting diodes (OLEDs) using Os(II) complex 5 as the dopant and either tris(8-hydroxyquinoline) aluminum (Alq3) or 3,3′,5,5′-tetra[(m-pyridyl)-phen-3-yl]-biphenyl (BP4mPy) as the host is reported. These OLEDs were measured with emission maxima at 690 nm and extending into the NIR, with peak power efficiencies of up to 0.13 lm W−1 and external quantum efficiencies of up to 2.27%.

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.

Luminescent Pt(II) complexes featuring imidazolylidene–pyridylidene and dianionic bipyrazolate: from fundamentals to OLED fabrications

Pt(II) complexes bearing imidazolylidene–pyridylidene (impy) and dianionic biazolate chelates were synthesized, for which the end products depend on the alkyl substituents of the impy chelate. Treatment of Pt(DMSO)2Cl2 with dimethyl substituted imidazolium–pyridinium pro-ligand Me2impy(PF6)2, followed by addition of 5,5′-(1-methylethylidene)-bis-(3-trifluoromethyl-1H-pyrazole) (mepzH2), 5,5′-di(trifluoromethyl)-3,3′-bis-pyrazole (bipzH2), and 5,5′-di(pentafluoroethyl)-3,3′-bis-pyrazole (biepzH2), afforded Pt(II) complexes [Pt(Me2impy)(mepz)] (1), [Pt(Me2impy)(bipz)] (2) and [Pt(Me2impy)(biepz)] (3), respectively. In contrast, reactions with ethyl and isopropyl substituted Et2impy(PF6)2 and Pr2impy(PF6)2 and with bipzH2 gave [Pt(EtimHpy)(bipz)] (4) and [Pt(PrimHpy)(bipz)] (5) respectively, where notable alkyl-to-hydrogen transformations on the pyridylidene fragment took place. The reaction of Pt(DMSO)2Cl2 with Et2impy(PF6)2 followed by addition of (biepzH2) gave two products [Pt(Et2impy)(biepz)] (6) and [Pt(EtimHpy)(biepz)] (7). Single crystal X-ray diffraction analyses of 1, 2 and 5 revealed negligible intermolecular Pt⋯Pt interactions. Hybrid-DFT and TD-DFT computations were carried out on 1, 2 and 5 to model the observed crystal structures and explain the photophysical data successfully. Organic light emitting diodes (OLEDs) were fabricated from complexes 4 or 5 using a multiple layered device architecture. The associated OLED performances (i.e. ηmax = 12.5%, 11.2%, ηL = 44.0 cd A−1, 40.6 cd A−1, and ηP = 28.0 lm W−1, 25.8 lm W−1 for 4 and 5) confirmed their suitability in serving as potential OLED phosphors.

Sky Blue-Emitting Iridium(III) Complexes Bearing Nonplanar Tetradentate Chromophore and Bidentate Ancillary

Tetradentate chelates bearing tripodal arranged terpyridine and a functional pyrazole unit (i.e., L1-H and L2-H) were employed in preparation of Ir(III) complexes [Ir(L1)Cl2] (1) and [Ir(L2)Cl2] (2); subsequent chloride-to-bipyrazolate substitution gave [Ir(L1)(bipz)] (3) and [Ir(L2)(bipz)] (4). Single-crystal X-ray structural studies on 1 and 3 showed the possession of a tetradentate chelate, whereas the remaining cis-sites are occupied by either dual chlorides or the bipz chelate, respectively. Sky blue organic light-emitting diode with peak efficiencies (10.1%, 19.8 cd·A-1, and 20.4 lm·W-1) was successfully fabricated using 3 (or 4) as dopant emitter, highlighting the potential application of this class of Ir(III) phosphor.

Isomeric spiro-[acridine-9,9′-fluorene]-2,6-dipyridylpyrimidine based TADF emitters: insights into photophysical behaviors and OLED performances

TADF molecules with a higher horizontal–dipole ratio have recently been realized to show a large conversion efficiency in organic light-emitting diodes (OLEDs) and hence great promise for their application in display and solid state lighting sources. The current work focuses on fine-tuning the structure of the parent 2,6-diphenylpyrimidinyl compound PhPMAF and its derivatives with a series of 2,6-dipyridylpyrimidine acceptors; namely: 2NPMAF, 3NPMAF and 4NPMAF, among which the intramolecular CH⋯N bonding interaction was confirmed for 2NPMAF, showing good agreement with the NMR results. The solvatochromism and TADF behaviors, together with TD-DFT computations, of the titled compounds were explored to gain insights into the structure–photophysics relationship. Eventually, these functional 2,6-dipyridylpyrimidine acceptor based TADF compounds exhibit electroluminescence ranging from deep-blue to sky-blue, among which an external quantum efficiency (EQE) of 24.9% was achieved for the derivative 3NPMAF. The high horizontal–dipole ratios (∼0.86–0.91) obtained for this class of compounds elucidates the superior OLED performance. Moreover, the non-doped OLED architecture achieves an EQE of 14.1% for 3NPMAF, which is superior to that of PhPMAF with an EQE of 5.1%, demonstrating the significance of dipyridylpyrimidine as an acceptor for the future structural design of TADF emitters.

Electroluminescence Stability of Organic Light-Emitting Devices Utilizing a Nondoped Pt-Based Emission Layer

We study the effects of using an emitting material (Pt(II) bis(3-(trifluoromethyl)-5-(2-pyridyl)pyrazolate—Pt(fppz)2) characterized by a preferred horizontal dipole alignment and a nearly unitary quantum yield regardless of concentration on the lifetime of organic light-emitting devices (OLEDs). Using such a material as a dopant in increasingly higher concentrations is found to lead to an increase in device stability, a trend that is different from that commonly observed with conventional OLED guests. The results are consistent with the newly discovered exciton–polaron-induced aggregation degradation mechanism of OLED materials. When this emitter is used as a neat emission layer, the material is already in a highly aggregated state, and the device is no longer affected by exciton–polaron interactions. The results demonstrate the potential stability benefits of using such materials in OLEDs.