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%.

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

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.

Microscopic, Spectroscopic, and Electrochemical Characterization of Novel Semicrystalline Poly(3-hexylthiophene)-Based Dendritic Star Copolymer

In this study, electron-donating semicrystalline generation 1 poly(propylene thiophenoimine)-co-poly(3-hexylthiophene) star copolymer, G1PPT-co-P3HT was chemically prepared for the first time. Copolymerization was achieved with high molecular weight via facile green oxidative reaction. ¹H NMR analyses of the star copolymer demonstrated the presence of 84% regioregular (rr) head-to-tail (HT) P3HT, which accounts for the molecular ordering in some grain regions in the macromolecule’s morphology, as revealed by the high-resolution scanning electron microscopy (HRSEM) and Selected Area Electron Diffraction (SAED) images, and X-ray diffraction spectroscopy (XRD) measurements. The star copolymer also exhibited good absorption properties in the ultraviolet-visible (UV-Vis) and the near infrared (NIR) spectral regions, which give rise to an optical energy bandgap value as low as 1.43 eV. A HOMO energy level at −5.53 eV, which is below the air-oxidation threshold, was obtained by cyclic voltammetry (CV). Electrochemical impedance spectroscopy (EIS) ascertained the semiconducting properties of the macromolecule, which is characterized by a charge transfer resistance, R_ct, value of 3.57 kΩ and a Bode plot-phase angle value of 75°. The combination of the EIS properties of G1PPT-co-P3HT and its highly electron-donating capability in bulk heterojunction (BHJ) active layer containing a perylene derivative, as demonstrated by photoluminescence quenching coupled to the observed Förster Resonance charge transfer, suggests its suitability as an electron-donor material for optoelectronic and photovoltaic devices.

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.

Switch on of TADF can be achieved by tuning the excited state energy levels via ligand manipulation of the carbazolylgold(iii) C^C^N complexes. The resulting OLEDs show maximum EQEs of over 11% and efficiency roll-offs of down to less than 1%.

Isomeric Tetradentate Ligand-Containing Cyclometalated Gold(III) Complexes

A simple one-pot two bond-forming reaction for the rapid construction of cyclometalated gold(III) complexes with fully π-conjugated tetradentate ligand is reported. The coupling of the bifunctional gold(III) precursor with the bifunctional aromatic compound has led to the formation of two regioisomers with either C- or N-coordination. Through monitoring by high-throughput high performance liquid chromatography, the regioselectivity of the reaction has been effectively tuned toward the formation of a single isomer, allowing easy separation of the metal complexes. The structures of the complexes have been determined by X-ray crystallography, and the photophysical, electrochemical, and electroluminescence (EL) studies have been carried out. Computational study has been performed to provide insights into the nature of the excited states. Isomeric effect has been shown to have a significant influence on the EL behavior of the organic light-emitting devices.

Photoinduced electron and hole transfers in carbazole dendrimers with heteroleptic Ir-complex cores

We synthesised carbazole (Cz) dendrimers with heteroleptic Ir-complex cores. Upon excitation of the carbazole (Cz) dendrons, the phosphorescence of the core Ir(iii) complex was quenched due to the photoinduced electron transfer (PET) process. The PET dynamics of the excited Cz-dendrons were investigated using the femtosecond time-resolved transient absorption technique. A broad transient absorption (TA) band attributed to the S1-Sn transition of the 1Cz*-dendron was observed at around 630 nm in the first generation Cz-dendrimer (G1). This TA band in the second-generation dendrimer (G2) decayed with a longer lifetime of 55.5 ps compared to that of G1 (9.8 ps), because G2 has a larger distance between the Cz-dendron and Ir-complex core than that of G1. The decay time of the free 1Cz*-dendron was 6.3 ns, and thus, the reduced decay time in Gn corresponds to the PET dynamics. As a result of the PET process, the Cz cationic radical species (Cz˙+) was observed at around 780 nm. Interestingly, when the core Ir-complex in the dendrimer was excited with a 400 nm pulse selectively, the TA band of Cz˙+ was also detected at around 780 nm. This may be due to the photoinduced hole transfer (PHT) from the highest occupied molecular orbital (HOMO) energy state of Cz to the lowest singly occupied molecular orbital (LSOMO) energy state of the excited Ir-complex. The oxidation potential of Cz is lower than that of the Ir-complex, indicating that the HOMO of the Cz-dendron is located at a higher energy state than that of the Ir-complex. To investigate the relative order of the energy states and their orbital shapes, we performed theoretical calculations using density functional theory. The TA spectra were globally deconvoluted to generate the decay-associated spectra (DAS), from which the species-associated spectra (SAS) were calculated. The SAS can distinguish the individual intermediate species participating in the PET and PHT processes. The analysed rate constants of SAS were consistent with the results determined by the TA decays.