Luminescent dinuclear d⁸ gold(III) and platinum(II) complexes : impact of metal-metal interaction on photo-physical properties, excited state dynamics and OLED application

Abstract

Inspired by the profound applications of IrIII phosphors, the development of luminescent d8 metal complexes, such as those of PtII and AuIII described in this thesis, has generated a surge of interest in recent years. To improve the phosphorescent transition metal complex radiative decay rate constant (kr), which is crucial for practical application in OLED devices, the emissive excited state is deemed to have high metal character for effective spin-orbit coupling (SOC). This thesis comprises studies on luminescent PtII complexes with 3MLCT and 3MMLCT emissive excited states, AuIII complexes with TADF, 1LLCT and/or 1ILCT emission, and the effect of the closed shell metal-metal interaction on these excited states. A series of multinuclear AuIII complexes with various bridging ligands was synthesized. The complexes’ X-ray crystal structures, and the relationship between AuIII–AuIII distances and photo-physical properties, were studied. The crystal structures of new tri- and tetranuclear AuIII complexes, which are rare in the literature, are also described. One of the dinuclear AuIII complexes shows a remarkably short AuIII–AuIII distance of 2.92 Å. These dinuclear AuIII complexes are emissive in degassed solutions at room temperature, with emission quantum yields and emission lifetimes in the range of < 0.001–0.14 and 0.67–85.9 µs, respectively. The complex with the shortest AuIII–AuIII distance (2.92 Å) shows a bathochromic shift in emission energy, the excited state of which has 3LMMCT character, based on TD-DFT calculations. A class of phosphorescent dinuclear PtII complexes were synthesized. Bulky N-heterocyclic carbene and tethered bridging ligand are employed to suppress photo-induced structural change and to improve thermal stability of the complexes. These complexes show mixed 3IL/3MLCT and 3MMLCT emission with emission quantum yields of up to 0.95, emission lifetimes down to 0.9 µs and kr of up to 1.0 × 106 s-1 in 4 wt% doped PMMA films. The large kr of these blue emitting complexes are attributed to the high metal character of the 1MLCT excited state (leading to large SOC) and a large transition dipole moment, based on DFT calculations. This gives rise to blue OLEDs with simultaneously high efficiency (EQEmax > 20%) and markedly suppressed efficiency roll-off of less than 7% at 1000 cd m-2. The employment of a tethered bridging ligand further increases the stability of the complexes, giving rise to substantial lengthening of device lifetime (LT50) to over 100 h at an L0 of 1000 cd m-2, further suppressing the efficiency roll-off to less than 1% at 1000 cd m-2 (EQEmax > 22%). Phosphor-sensitized blue hyper-OLEDs with Commission Internationale de L’Eclairage (CIE) coordinates of (0.13, 0.12) exhibit EQEmax of 23.4% with full-width-at-half-maximum of 18 nm and LT50 > 250 h at an L0 of 1000 cd m-2. A series of pincer type AuIII C^N^C complexes incorporating amine donors was also prepared. The effects of the position of the amine donor and the donor strength on the excited state and photophysical properties were studied. These complexes have high emission quantum yields of up to 0.68 and short emission lifetimes down to 0.5 µs in degassed toluene solution. The incorporation of a fused benzofuran moiety on the pyridine ring of the C^N^C ligand enhances the kr of the complex in the solution state. Based on the results of solvatochromic studies, thermochromic studies and TD-DFT calculations, it was deduced the AuIII complexes emit from 1LLCT or 1ILCT excited states through TADF. The large kr values of up to 1.15 × 106 s-1 in both degassed toluene solution and 4 wt% PMMA film is one of the largest reported among AuIII complexes. In contrast to the conventional wisdom that TADF complexes require a large kr, results from the TD-DFT calculations reveal that TADF AuIII complexes can possess kr as low as 2.5 × 103 s-1, with EST as large as 1838 cm-1.