Stable Pincer Gold(III)-TADF Emitters with Extended Donor-Acceptor Separation for Efficient Vacuum-Deposited OLEDs with Operational Lifetime (LT95) up to 3831 h at 1000 cd m-2

Abstract

Although gold-TADF (thermally activated delayed fluorescence) emitters have attractive prospects as next-generation practical OLED emitters, the performance of OLEDs utilizing gold(I)- and gold(III)-TADF emitters lags behind the requirements of practical applications, and device lifetime has become a bottleneck. Here, novel pincer gold(III)-TADF emitters that are easily fabricated with tunable donor and acceptor ligands are presented. These pincer gold(III)-TADF emitters exhibit an extended molecular π-distance along the transition dipole moment, resulting in a significant reduction in the electron exchange energy between the S₁ and T₁ excited states, thus narrowing the singlet-triplet energy gap (ΔE_ST). The combination of small ΔE_ST and heavy-atom (Au, S) effect greatly enhances spin-flip dynamics and produces efficient TADF (photoluminescence quantum yields up to 90%) with high radiative decay rate constants (k_r up to 10⁶ s^-1), and short lifetimes (τ less than 1.2 µs) in thin films at room temperature. Vacuum-deposited OLEDs based on these gold(III)-TADF emitters demonstrate impressive stability, achieving i) a high maximum external quantum efficiency (EQE_max) of up to 22.2%, and ii) a record- long operational lifetime (LT₉₅) of 3831 h at an initial luminance of 1000 cd m^-2. This excellent durability makes the pincer gold(III)-TADF emitter a promising and competitive alternative to iridium and platinum emitters for practical OLED applications.

Keywords: OLEDs; gold; operational lifetime; singlet–triplet energy gap; thermally activated delayed fluorescence.

Scheme 1

a) Selected examples of Au^I‐TADF, tetradentate, and pincer Au^III‐TADF emitters. b) Molecular design concepts and schematic diagram of pincer gold(III)‐TADF emitters in this work (The structure of 3 indicates its transition density upon excitation, and the red arrow illustrates the direction of the transition dipole moment).

Scheme 2

Molecular structures of pincer gold(III)‐TADF complexes 1–5.

Figure 1

a) UV–vis absorption and b) emission spectra of 1–5 in toluene at room temperature.

Figure 2

a) Temperature‐dependent plot of emission lifetime of 5 in 2 wt% PMMA film; Inset: Arrhenius fit of the k _TADF value versus temperature. b) Normalized temperature‐dependent PL spectra of 5 in 2 wt% PMMA film.

Figure 3

fs‐TA difference spectra of a) 1, b) 2, and c) 3 in toluene solution (3.0 × 10⁻⁴ mol dm⁻³) under 370 nm pulse excitation. fs‐TA contour map of d) 2 and e) 3 in toluene solution under 370 nm pulse‐excitation. f) fs‐TRF spectra of 3 in toluene solution (2.0 × 10⁻³ mol dm⁻³) under 400 nm pulse excitation.

Figure 4

HOMO and LUMO of pincer gold(III) complexes 2 and 3 calculated using semi‐coplanar geometries in the ground states.

Figure 5

a) Calculated excited state adiabatic potential energy surfaces (relative energy versus torsional angle) of 2. The relative potential energy was calibrated by adjusting the energy of the coplanar T₁ structure to 0 eV for comparison. (b) Calculated ΔE _ST value at different torsional angles (θ_{C1‐Au‐C2‐C3}) of 2. (c) Calculated transition density t(r) upon excitation and the direction of transition dipole moment (red arrow) of semi‐coplanar structures of 3 and 3′. Density increase and decrease are represented by the red and blue isosurfaces, respectively. (d) Calculated hole and electron distribution, the distance between centroids of hole and electron (Δr), and the overlap between hole and electron wavefunctions (O _h,e) of semi‐coplanar structures of 3 and 3′. Hole and electron distributions are represented by the blue and green isosurfaces, respectively.

Figure 6

a) Normalized EL spectra and b) EQE‐luminance characteristics of 1 (2 wt%), 2 (2 wt%), 3 (6 wt%), 4 (12 wt%), 5 (2 wt%). c) Relative luminance‐operational lifetime of OLEDs, with L₀/cd m⁻² = 10 000 (1, 2 wt%), 12 000 (2, 2 wt%), 11 000 (3, 4 wt%), 6500 (4, 12 wt%), 7500 (5, 4 wt%). d) Three‐dimensional diagram summarizing the relative luminance‐operational lifetime of OLEDs using TADF emitters as single dopants, Ir^III‐sensitized hyperphosphorescent emitters, and Pt^II/Ir^III‐doped emitters, with λ _EL in the range of 515–620 nm at L₀ of 1000 cd m⁻² (The blue Arabic numerals 3 and 5 denote the emitters investigated in this study, the estimated operational lifetime (LT₉₀) at 1000 cd m⁻² for the reference^[22b] is calculated with an acceleration coefficient of 1.7).