Thermally activated delayed fluorescence tetradentate ligand-containing gold(III) complexes with preferential molecular orientation and their application in organic light-emitting devices.

A new class of thermally activated delayed fluorescence (TADF) pyridine-/pyrazine-containing tetradentate C^C^N^N gold(III) complexes have been designed and synthesized. Displaying photoluminescence quantum yields (PLQYs) of up to 0.77 in solid-state thin films, these complexes showed at-least a six-fold increase in the radiative decay rate constant (kr) in toluene upon increasing temperature from 210 to 360 K. Using variable-temperature (VT) ultrafast transient absorption (TA) spectroscopy, the reverse intersystem crossing (RISC) processes were directly observed and the activation parameters were determined, in line with the results of the Boltzmann two-level model fittings, in which the energy separation values between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔE(S1-T1), of these complexes were estimated to be in the range of 0.16-0.18 eV. Through strategic modification of the position of the electron-donating -tBu substituent in the cyclometalating ligand, the permanent dipole moments (PDMs) of these tetradentate gold(III) emitters could be manipulated to enhance their horizontal alignment in the emitting layer of organic light-emitting devices (OLEDs). Consequently, the resulting vacuum-deposited OLEDs demonstrated a 30% increase in the theoretical out-coupling efficiency (ηout), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.

Tetradentate C∧C∧N∧N Ligand-Containing Gold(III) Complexes with Orange to Deep-Red Thermally Activated Delayed Fluorescence (TADF) and Their Application in Organic Light-Emitting Devices.

A new class of thermally activated delayed fluorescence (TADF) tetradentate C∧C∧N∧N ligand-containing gold(III) complexes containing acridinyl moieties has been designed and synthesized. These complexes exhibit orange-red to deep-red emission with photoluminescence quantum yields (PLQYs) of up to 0.76 in solid-state thin films. Short excited-state lifetimes of ≤2.0 µs and large radiative decay rate constants (kr) in the order of 105 s-1 have also been found in the complexes. High-performance solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on these complexes have been fabricated, demonstrating high maximum external quantum efficiencies (EQEs) of 12.2 and 12.7%, respectively, which are among the best values ever reported for red-emitting gold(III)-based OLEDs. In addition, satisfactory operational half-lifetime (LT50) values of up to 34,058 h have been attained in these red-emitting devices. It is found that the operational stability is strongly dependent on the choice of functional groups on the acridinyl moieties, of which the incorporation of -O- and -S- linkers can effectively prolong the LT50 value by an order of magnitude. The TADF properties of the complexes are substantiated by the hypsochromic shift in emission energies and the remarkable enhancement in the emission intensity upon increasing temperature. The TADF properties have also been supported by temperature-dependent ultrafast transient absorption studies, with the direct observation of reverse intersystem crossing (RISC) and the determination of the activation parameters for the very first time, together with their excited-state dynamics.

Realization of Long Operational Lifetimes in Vacuum-Deposited Organic Light-Emitting Devices Based on para-Substituted Pyridine Carbazolylgold(III) C

A new series of robust C^C^N carbazolylgold(III) complexes is designed and synthesized through the introduction of inert and sterically bulky oligophenyl substituents on the pyridyl moiety of the cyclometalating ligand. High photoluminescence quantum yields of up to 96% are recorded with these complexes doped in solid-state thin films, and short excited-state lifetimes of 0.3 µs or less in the solid state at room temperature are found. Promising electroluminescence (EL) performances are shown by the vacuum-deposited organic light-emitting devices (OLEDs) based on this series of gold(III) complexes. High external quantum efficiencies of up to 19.5% with efficiency roll-offs of down to 10% at a practical luminance brightness level of 1000 cd m-2 are achieved. More importantly, record-long operational lifetimes (LT50) of up to 470,700 h at 100 cd m-2 are realized, which is currently the highest value among all classes of gold(III) complexes with tridentate pincer ligands. Particularly, by introducing a sterically bulky terphenyl moiety on the reactive site of the pyridine ring, the LT50 value is shown to attain ∼7 times longer half-lifetime than that based on the unsubstituted complex. These unprecedented EL performances and the simple synthetic route in a mercury-free fashion make them promising emitting materials for practical OLEDs toward commercialization.

Carbazolylgold(iii) complexes with thermally activated delayed fluorescence switched on by ligand manipulation as high efficiency organic light-emitting devices with small efficiency roll-offs.

A series of carbazolyl ligands has been designed and synthesized through the integration of various electron-donating and electron-accepting motifs, including electron-donating 4-(diphenylamino)aryl and electron-accepting cyano and diphenylphosphine oxide moieties, for the development of a new class of gold(iii) complexes, where the energies of their triplet intraligand and ligand-to-ligand charge transfer excited states can be manipulated for the activation of thermally activated delayed fluorescence (TADF). Upon excitation, these complexes show high photoluminescence quantum yields of up to 80% in solid-state thin films, with short excited state lifetimes down to 1 µs. Vacuum-deposited and solution-processed organic light-emitting devices based on these complexes demonstrate promising electroluminescence (EL) performance with maximum external quantum efficiencies of 15.0% and 11.7%, respectively, and notably small efficiency roll-off values of less than 1% at the practical luminance brightness level of 1000 cd m-2. These distinct EL performances are believed to be due to the occurrence of multichannel radiative decay pathways via both phosphorescence and TADF that significantly shorten the emission lifetimes and hence reduce the occurrence of the detrimental triplet-triplet annihilation in the gold(iii) complexes.

Design and synthesis of yellow- to red-emitting gold(III) complexes containing isomeric thienopyridine and thienoquinoline moieties and their applications in operationally stable organic light-emitting devices.

A new class of yellow- to red-emitting carbazolylgold(III) complexes containing isomeric thienopyridine or thienoquinoline moieties in the cyclometalating ligand has been designed and synthesized, which showed high photoluminescence quantum yields of over 80% in solid-state thin films. The isomeric effect and extended π-conjugation of the N-heterocycles have been found to remarkably perturb the photophysical, electrochemical and electroluminescence properties of the gold(III) complexes. In particular, the operational lifetimes of organic light-emitting devices based on that incorporated with thieno[2,3-c]pyridine are almost three orders of magnitude longer than that incorporated with thieno[3,2-c]pyridine. This has led to long device operational stability with a LT70 value of up to 63 200 h at a luminance of 100 cd m-2 and a long half-lifetime of 206 800 h, as well as maximum external quantum efficiencies of up to 8.6% and 14.5% in the solution-processed and vacuum-deposited devices, respectively. This work provides insights into the development of robust and highly luminescent gold(III) complexes and the identification of stable molecular motifs for designing efficient emitters.

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices.

A new class of C^C^N ligand-containing carbazolylgold(iii) dendrimers has been designed and synthesized. High photoluminescence quantum yields of up to 82% in solid-state thin films and large radiative decay rate constants in the order of 105 s-1 are observed. These gold(iii) dendrimers are found to exhibit thermally activated delayed fluorescence (TADF), as supported by variable-temperature emission spectroscopy, time-resolved photoluminescence decay and computational studies. Solution-processed organic light-emitting diodes (OLEDs) based on these gold(iii) dendrimers have been fabricated, which exhibit a maximum current efficiency of 52.6 cd A-1, maximum external quantum efficiency of 15.8% and high power efficiency of 41.3 lm W-1. The operational stability of these OLEDs has also been recorded, with the devices based on zero- and second-generation dendrimers showing maximum half-lifetimes of 1305 and 322 h at 100 cd m-2, respectively, representing the first demonstration of operationally stable solution-processed OLEDs based on gold(iii) dendrimers.

Incorporation of Fluorene and Its Heterocyclic Spiro Derivatives To Realize High-Performance and Stable Sky-Blue-Emitting Arylgold(III) Complexes.

A series of arylgold(III) complexes of tridentate diphenylpyridine ligand incorporated with fluorene and its heterocyclic spiro derivatives, spiro[fluorene-9,9'-xanthene] and spiro[acridine-9,9'-fluorene], as auxiliary ligands has been prepared. This class of complexes exhibits high decomposition temperatures of up to 387 °C, excellent film morphologies in solid-state thin films with a root-mean-square roughness smaller than 0.20 nm, as well as high photoluminescence quantum yields of up to 0.72 in solid-state thin films. Solution-processed organic light-emitting devices (OLEDs) fabricated from this series of complexes as dopants show intense electroluminescence in the sky-blue region with maximum external quantum efficiencies of 10.0%. Taking advantage of their high thermal stability, vacuum-deposited OLEDs have also been fabricated and satisfactory operational lifetimes of ∼300 h have been recorded.

Cyclometalated Platinum(II) Complexes with Donor-Acceptor-Containing Bidentate Ligands and Their Application Studies as Organic Resistive Memories.

A series of heteroleptic cyclometalated platinum(II) complexes, [Pt(C^N)(O^O)], (1-10) with various donors and acceptors has been synthesized and characterized by 1 H NMR spectroscopy, elemental analyses, infrared spectroscopy and mass spectrometry. The X-ray structure of 2 has also been determined. The electrochemical and photophysical properties of the platinum(II) complexes were studied. These experimental results have been supported by computational studies. Furthermore, two of the complexes have been employed as the active material in the fabrication of resistive memory devices, exhibiting stable binary memory performance with low operating voltage, high ON/OFF ratio and long retention time.

Molecular design of efficient yellow- to red-emissive alkynylgold(iii) complexes for the realization of thermally activated delayed fluorescence (TADF) and their applications in solution-processed organic light-emitting devices.

Here, we report the design and synthesis of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which show tunable emission colors spanning from the yellow to red region in the solid state and exhibit thermally activated delayed fluorescence (TADF) properties. These complexes display high photoluminescence quantum yields of up to 0.87 and short excited-state lifetimes in sub-microsecond timescales, yielding high radiative decay rate constants on the order of up to 106 s-1. The observation of the drastic enhancement in the emission intensity of the complexes with insignificant change in the excited-state lifetime upon increasing the temperature from 200 to 360 K indicates an increasing radiative decay rate. The experimentally estimated energy splitting between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔE S1-T1 , is found to be as small as ∼0.03 eV (250 cm-1), comparable to the value of ∼0.05 eV (435 cm-1) obtained from computational studies. The delicate choice of the cyclometalating ligand and the fused heterocyclic ligand is deemed the key to induce TADF through the control of the energy levels of the intraligand and the ligand-to-ligand charge transfer excited states. This work represents the realization of highly emissive yellow- to red-emitting gold(iii) TADF complexes incorporated with fused heterocyclic alkynyl ligands and their applications in organic light-emitting devices.

Design Strategy Towards Horizontally Oriented Luminescent Tetradentate-Ligand-Containing Gold(III) Systems.

Phosphorescent dopants are promising candidates for organic light-emitting diodes (OLEDs). Although it has been established that the out-coupling efficiency and overall performances of vacuum-deposited OLEDs can be significantly improved by a horizontal orientation of the dopants, no horizontally oriented gold(III) complexes have been reported to date. Herein, a novel class of tetradentate C^C^N^N ligand-containing gold(III) complexes with a preferential horizontal orientation successfully generated through a one-pot reaction is reported. These complexes demonstrate high photoluminescence quantum yields of 70 % and a high horizontal dipole ratio of 0.87 in solid-state thin films. Green-emitting OLEDs based on these complexes operate with a maximum external quantum efficiency of 20.6 % with an estimated out-coupling efficiency of around 30 %. A promising device stability has been achieved in the vacuum-deposited OLEDs, with operational half-lifetimes of around 37 500 h at 100 cd m-2 .

Judicious Choice of N-Heterocycles for the Realization of Sky-Blue- to Green-Emitting Carbazolylgold(III) C

A new class of sky-blue- to green-emitting carbazolylgold(III) C^C^N complexes containing pyrazole or benzimidazole moieties has been successfully designed and synthesized. Through the judicious choice of the N-heterocycles in the cyclometalating ligand and the tailor-made carbazole moieties, maximum photoluminescence quantum yields of 0.52 and 0.39 have been realized in the green- and sky-blue-emitting complexes, respectively. Solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on the benzimidazole-containing complexes have been prepared. The sky-blue-emitting device shows an emission peaking at 484 nm with a narrow full-width at half-maximum of 57 nm (2244 cm-1 ), demonstrating the potential of this class of complexes in the application of OLEDs with high color purity. In addition, high maximum external quantum efficiencies of 12.3 % and a long operational half-lifetime of over 5300 h at 100 cd m-2 have been achieved in the vacuum-deposited green-emitting devices.

Three-Dimensional Spirothienoquinoline-Based Small Molecules for Organic Photovoltaic and Organic Resistive Memory Applications.

A new electron-rich spirothienoquinoline unit, tBuSAF-Th, has been developed via incorporation of a thienyl unit instead of a phenyl unit into the six-membered ring of the spiroacridine (SAF) and utilized for the first time as a building block for constructing small-molecule electron donors in organic solar cells (OSCs) and as active layers in organic resistive memory devices. The resulting three-dimensional spirothienoquinoline-containing 1-4 exhibit high-lying highest occupied molecular orbital (HOMO) energy levels. By the introduction of electron-deficient benzochalcogenodiazole linkers, with the chalcogen atoms being varied from O to S and Se, a progressively lower lowest unoccupied molecular orbital (LUMO) energy level has been achieved while keeping the HOMO energy levels similar. This strategy has allowed an enhanced light-harvesting ability without compromising open-circuit voltage (Voc) in vacuum-deposited bulk heterojunction OSCs using 1-4 as donors and C70 as the acceptor. Good photovoltaic performances with power conversion efficiencies (PCEs) of up to 3.86% and high short-circuit current densities (Jsc) of up to 10.84 mA cm-2 have been achieved. In addition, organic resistive memory devices fabricated with these donor-acceptor small molecules exhibit binary logic memory behaviors with long retention times and high on/off current ratios. This work indicates that the spirothienoquinoline moiety is a potential building block for constructing multifunctional organic electronic materials.

Thermally Stimulated Delayed Phosphorescence (TSDP)-Based Gold(III) Complexes of Tridentate Pyrazine-Containing Pincer Ligand with Wide Emission Color Tunability and Their Application in Organic Light-Emitting Devices.

A new class of pyrazine-based carbazole-containing gold(III) complexes featuring thermally stimulated delayed phosphorescence (TSDP) properties has been designed and synthesized. The emission colors are found to be sensitive to the coordinating atom of the carbazolyl ligands at the gold(III) center, with emission energies spanning from green to red. The efficiency of TSDP can be enhanced by lowering the polarity of the solvent, as supported by the variable-temperature emission and computational studies. Interestingly, a significant spectral shift in electroluminescence with the change of Commission Internationale de L'Eclairage (CIE) coordinates from (0.35, 0.60) to (0.44, 0.54) has been achieved by simply changing the host material from CBP to TmPyPB. Solution-processable organic light-emitting devices (OLEDs) have also been fabricated, with maximum current efficiencies of up to 22.4 cd A-1 and maximum external quantum efficiencies (EQEs) approaching 7.0%. A higher current efficiency of 35.1 cd A-1 and EQE of 10.7% can be achieved for the vacuum-deposited device based on 1, representing the first demonstration of pyrazine-based tridentate ligand-containing gold(III) complexes as phosphorescent material for OLED application.

Green-emitting dendritic alkynylgold(iii) complexes with excellent film morphologies for applications in solution-processable organic light-emitting devices.

Bipolar cyclometalated dendritic alkynylgold(iii) complexes that exhibit excellent film morphologies in solid-state thin films have been designed and synthesized. Together with their high luminescence quantum yields, high performance solution-processable organic light-emitting devices have been realized, maintaining high external quantum efficiencies of >12% for dendrimers up to the second generation.

Ligand Mediated Luminescence Enhancement in Cyclometalated Rhodium(III) Complexes and Their Applications in Efficient Organic Light-Emitting Devices.

A series of luminescent cyclometalated rhodium(III) complexes have been designed and prepared. The improved luminescence property is realized by the judicious choice of a strong σ-donor cyclometalating ligand with a lower-lying intraligand (IL) state that would raise the d-d excited state and introduction of a lower-lying emissive IL excited state. These complexes exhibit high thermal stability and considerable luminescence quantum yields as high as up to 0.65 in thin film, offering themselves as promising light-emitting materials in OLEDs. Respectable external quantum efficiencies of up to 12.2% and operational half-lifetimes of over 3000 h at 100 cd m-2 have been achieved. This work demonstrates a breakthrough as the first example of an efficient rhodium(III) emitter for OLED application and opens up a new avenue for diversifying the development of OLED materials with rhodium metal being utilized as phosphors.

Four-Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices.

A new class of four-coordinate donor-acceptor fluoroboron-containing thermally activated delayed fluorescence (TADF) compounds bearing a tridentate 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligand has been successfully designed and synthesized. Upon varying the donor moieties from carbazole to 10H-spiro[acridine-9,9'-fluorene] to 9,9-dimethyl-9,10-dihydroacridine, these boron derivatives exhibit a wide range of emission colors spanning from blue to yellow with a large spectral shift of 2746 cm-1 , with high PLQYs of up to 96 % in the doped thin film. Notably, vacuum-deposited organic light-emitting devices (OLEDs) made with these boron compounds demonstrate high performances with the best current efficiencies of 55.7 cd A-1 , power efficiencies of 58.4 lm W-1 and external quantum efficiencies of 18.0 %. More importantly, long operational stabilities of the green-emitting OLEDs based on 2 with half-lifetimes of up to 12 733 hours at an initial luminance of 100 cd m-2 have been realized. This work represents for the first time the design and synthesis of tridentate dppy-chelating four-coordinate boron TADF compounds for long operational stabilities, suggesting great promises for the development of stable boron-containing TADF emitters.

Rational molecular design for realizing high performance sky-blue-emitting gold(iii) complexes with monoaryl auxiliary ligands and their applications for both solution-processable and vacuum-deposited organic light-emitting devices.

A new class of sky-blue-emitting arylgold(iii) complexes containing tridentate bis-cyclometalating ligands derived from 2,6-diphenylpyridine (C^N^C) has been successfully designed and synthesized. By systematically varying the electron-withdrawing groups from cyano, fluoro, and trifluoromethyl to trifluoromethoxy groups on the phenyl ring of the tridentate C^N^C ligands, the emission maxima of these complexes have been significantly blue-shifted from 492 nm to 466 nm in dichloromethane solution. In addition, the higher excited state distortion with respect to the ground state associated with the multiple fluoro substitutions at the tridentate ligand has been successfully reduced by the employment of trifluoromethyl and trifluoromethoxy groups, as revealed by the Huang-Rhys factor. Taking advantage of their high photoluminescence quantum yields of up to 43% in the solid-state MCP thin-films, high performance solution-processable and vacuum-deposited organic light-emitting devices with external quantum efficiencies of up to 5.3% and 11.3%, respectively, have been realized. This work represents the first demonstration of sky-blue-emitting gold(iii) complexes with an x chromaticity coordinate of <0.2.

Realization of Thermally Stimulated Delayed Phosphorescence in Arylgold(III) Complexes and Efficient Gold(III) Based Blue-Emitting Organic Light-Emitting Devices.

To address and overcome the difficulties associated with the increased reactivity and susceptibility of blue emitters to deactivation pathways arising from the high-lying triplet excited states, we have successfully demonstrated an innovative strategy of harvesting triplet emission via the "thermally stimulated delayed phosphorescence" mechanism, where thermal up-conversion of excitons from the lower-energy triplet excited states (T1) to higher-energy triplet excited states (T1') are observed to generate blue emission. The lower-lying T1 excited state could serve as a mediator to populate the emissive T1' state by up-conversion via reverse internal conversion, which could enhance the photoluminescence quantum yield by over 20-folds. Organic light-emitting devices with respectable external quantum efficiencies of up to 7.7% and sky-blue emission with CIE coordinates of (0.17, 0.37) have been realized. The operational stability for the device based on complex 1 has also been explored, and the device is found to show fairly respectable lifetime. This work opens up a new avenue to the design and synthesis of blue phosphorescent emitters.

Highly luminescent phosphine oxide-containing bipolar alkynylgold(iii) complexes for solution-processable organic light-emitting devices with small efficiency roll-offs.

We report the synthesis of alkynylgold(iii) complexes with an electron-transporting phosphine oxide moiety in the tridentate ligand and hole-transporting triarylamine moieties as auxiliary ligands to generate a new class of phosphine oxide-containing bipolar gold(iii) complexes for the first time. Such gold(iii) complexes feature high photoluminescence quantum yields of over 70% in 1,3-bis(N-carbazolyl)benzene thin films with relatively short excited-state lifetimes of less than 3.9 µs at a 20 wt% dopant concentration. Highly efficient solution-processable organic light-emitting devices have been prepared with superior current efficiencies of up to 51.6 cd A-1 and external quantum efficiencies of up to 15.3%. Notably, triplet-triplet annihilation has been significantly reduced, as exemplified by a very small efficiency roll-off of ∼1% at a practical brightness of 500 cd m-2.

Versatile Design Strategy for Highly Luminescent Vacuum-Evaporable and Solution-Processable Tridentate Gold(III) Complexes with Monoaryl Auxiliary Ligands and Their Applications for Phosphorescent Organic Light Emitting Devices.

A new class of brightly blue-green-emitting arylgold(III) complexes has been synthesized, characterized, and applied as phosphorescent dopants in the fabrication of solution-processable and vacuum-deposited organic light-emitting devices (OLEDs). These arylgold(III) complexes can be readily synthesized by reacting the corresponding arylboronic acids with the gold(III) precursor complexes in a one-pot Suzuki-Miyaura coupling reaction. When compared to the structurally related alkynylgold(III) complex, arylgold(III) complexes 1 and 2 exhibit much higher photoluminescence quantum yields in solution state. High photoluminescence quantum yields are also observed in solid-state thin films. More importantly, the solid-state emission spectra show strong resemblance to those in solution, irrespective of the dopant concentration, leading to significant improvement in the color purity of the OLEDs by suppressing any excimer emission resulting from the π-stacking of the tridentate ligand. High performance solution-processable and vacuum-deposited blue-green-emitting OLEDs have also been realized, with maximum external quantum efficiencies of 7.3% and 14.7%, respectively, representing the first demonstration of efficient blue-green-emitting OLEDs based on cyclometalated arylgold(III) complexes.