Mei-Yee Chan

Thermally activated delayed fluorescence (TADF) and the very recently established thermally stimulated delayed phosphorescence (TSDP) are two promising approaches for enhancing the performance of organic light-emitting devices (OLEDs). Here, we have developed a new class of through-space charge transfer (TSCT) carbazolylgold(III) C^C^N complexes with unique TADF-TSDP properties by introducing a rigid arylamine on the carbazolyl auxiliary ligand. The highly twisted conformation between the C^C^N and carbazolyl ligands induces strong through-bond ligand-to-ligand charge transfer (TB-LLCT) character in their lowest singlet and triplet excited states, with small singlet-triplet energy gaps for efficient TADF. Moreover, the close spatial proximity between the cyclometalating ligand and the lateral arylamine enables appreciable intramolecular through-space electronic coupling that allows the generation of relatively low-lying triplet through-space ligand-to-ligand charge transfer (³TS-LLCT) excited states. The TADF-TSDP properties are verified by temperature-dependent emission, lifetimes, and ultrafast transient absorption studies. Interestingly, through better alignment with extended planarity and the strengthening of the electron-donating ability of the lateral arylamine, the enhanced through-space electronic coupling can effectively perturb the energies of ³TBCT, ³TSCT, and intraligand (³IL) excited states and thus manipulates the TSDP efficiency. Orange-emitting vacuum-deposited OLEDs made with these gold(III) complexes demonstrate respectable maximum external quantum efficiencies of >10% and long operational half-lifetimes of up to 65,314 h at a luminance of 100 cd m^-2. This work not only demonstrates the realization of interesting TADF-TSDP and TSCT properties in the gold(III) C^C^N cyclometalated system but also enriches the diversity of molecular design for high-performance TSDP and TSCT emitters.

A unique class of tridentate diaryltriazine ligand-containing gold(III) complexes with thermally activated delayed fluorescence (TADF) and/or thermally stimulated delayed phosphorescence (TSDP) properties has been designed and synthesized. With a simple structural modification on the coordination of carbazole moiety in the monodentate ligand, a large spectral shift of ∼160 nm (ca. 4900 cm^-1) spanning from sky blue to red emissions has been demonstrated in solid-state thin films. Three-state or four-state models have been employed in fitting the emission lifetimes of the gold(III) complexes at various temperatures. The findings clearly indicate the presence of three emitting states, S₁, T₁, and T₁', suggesting the coexistence of TADF, phosphorescence, and TSDP. Notably, a minor structural change in the donor moiety between phenylcarbazolyl and diphenylaminoaryl has been demonstrated to turn on/off the TSDP, resulting in TADF-TSDP-phosphorescence or TADF-phosphorescence emitters. The TADF and/or TSDP properties have also been supported by temperature-dependent ultrafast transient absorption studies, with the direct observation of reverse intersystem crossing (RISC) and reverse internal conversion (RIC) and the determination of the activation parameters and excited state dynamics. Solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) have been prepared, in which sky blue emitting devices based on 5 exhibit an operational lifetime LT₇₀ ∼ 5 times longer than the previously reported sky blue emitting analogue that shows only TSDP property. These results have provided valuable insights into the manipulation of the excited states via rational molecular design toward the realization of gold(III)-based TSDP and/or TADF materials with multiple radiative decay pathways that show enhanced radiative decay rate constants (k_r) for practical OLED applications.

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor–acceptor (D–A) type TADF materials featuring a four-coordinated boron with 2,2′-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1–8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (³CT) and the triplet local excited (³LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ_EL) at 460–492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT₅₀) of 9113 hours for 8 than that for 7 as well as satisfactory LT₅₀ reaching 26 643 hours for 6 at an initial luminance of 100 cd m⁻² have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

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 (k_r) 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 (η_out), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.

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 (k_r) 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.

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.

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.

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.

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.

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.