Luminescent tridentate and tetradentate cyclometalated gold(III) complexes : synthesis, photophysical properties and material applications

Abstract

Compared to the most efficient phosphorescent emitters, such as Ir(III) and Pt(II) complexes with kr of ~ 10^5–10^6 /s that show 3MLCT or mixed 3IL–3MLCT emission with significant metal character, gold(III) emitters commonly display 3IL phosphorescence with small kr of ~ 10^3 /s due to the electrophilic nature and low-lying d orbitals of the gold(III) ion. An effective approach to boost the kr of gold(III) emitters is to develop gold(III) complexes with TADF properties. This thesis focuses on the design and preparation of cyclometallated gold(III) TADF emitters as well as investigation of their photophysical properties by experimental and theoretical studies. As considered hydrides as strong σ-donors, the luminescent properties of gold(III) hydrides are examined.

Cyclometalated gold(III) aryl complexes exhibiting highly efficient luminescence with Φ of up to 93% and kr of ~ 10^5 /s were prepared. The spatial separation and favourable twisted angles between donors and acceptors resulted in efficient TADF in the majority of the complexes. These Au(III)-TADF emitters showed excellent performance in solution-processed OLEDs with maximum EQEs of up to 23.8% and EQEs of up to 16.5% at 1000 cd m-2. However, their use in vacuum-deposited OLEDs was hampered by the thermal stability of the Au−C(aryl) bond. To develop thermally stable gold(III) TADF emitters, a series of Au(III)-acetylide TADF complexes was synthesized. The incorporation of a chelating ligand with suitably high 3IL excited state energy allows the excited state of these complexes to decay efficiently via TADF. These gold(III) TADF complexes showed Φ of up to 88% and kr of ~ 10^5 /s. Vacuum-deposited OLEDs based on these thermally stable gold(III) emitters displayed outstanding performance with EQEs of up to 23.4% and an EQE of 22.1% was maintained at 1000 cd m-2.

A class of tetradentate gold(III) complexes with trianionic C^C^N^C ligands was synthesized with the assistance of microwave energy. By introducing substituents with different donor strengths, these tetradentate AuIII complexes not only showed emission spanning from blue-green to orange-red but also exhibited different emission origins (3IL, 3ILCT and TADF). Some of the complexes with TADF properties had kr of ~ 10^5 /s and Φ of up to 94%, which was the highest value observed among luminescent gold(III) complexes. Ultrafast time-resolved spectroscopic measurements conducted on the tetradentate Au(III) complexes revealed ISC time constants of 0.4−56 ps and estimated k(ISC) of (0.2–25) × 10^11 /s. Vacuum-deposited OLEDs fabricated with these tetradentate AuIII-TADF emitters showed excellent performance with maximum EQEs of up to 25% and EQEs of up to 22% at a luminance of 1000 cd m-2.

A series of cyclometallated gold(III) hydrides supported by C^N^C ligands was prepared and well characterized. These gold(III) hydrides exhibited 3IL phosphorescence with Φ and τ of up to 9.3% and 89 μs, respectively. Large Stokes shift, vibronic emission bands, small kr (~ 10^3 /s) and the investigation of nanosecond time-resolved absorption lend support to the emission assignment of 3IL excited states localized on the C^N^C ligand for these hydrides. The emission color of the gold(III) hydrides could be tuned from blue-green to red by modulating the substituents on the chelating ligand. Photo-induced Au–H and Au–C bond cleavage observed in one of these hydrides led to the formation of a new gold(III) aryl complex.