Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application

Abstract

Since the emergence of the first green light emission from a fluorescent thinfilm organic light emitting diode (OLED) in the mid-1980s, a global consumer market for OLED displays has flourished over the past few decades. This growth can primarily be attributed to the development of noble metal phosphorescent emitters that facilitated remarkable gains in electrical conversion efficiency, a broadened color gamut, and vibrant image quality for OLED displays. Despite these achievements, the limited abundance of noble metals in the Earth's crust has spurred ongoing efforts to discover cost-effective electroluminescent materials. One particularly promising avenue is the exploration of thermally activated delayed fluorescence (TADF), a mechanism with the potential to fully harness excitons in OLEDs. Recently, investigations have unveiled TADF in a series of twocoordinate coinage metal (Cu, Ag, and Au) complexes. These organometallic TADF materials exhibit distinctive behavior in comparison to their organic counterparts. They offer benefits such as tunable emissive colors, short TADF emission lifetimes, high luminescent quantum yields, and reasonable stability. Impressively, both vacuum-deposited and solution-processed OLEDs incorporating these materials have achieved outstanding performance. This review encompasses various facets on two-coordinate TADF coinage metal complexes, including molecular design, photophysical characterizations, elucidation of structure−property relationships, and OLED applications.

Simultaneous Thermally Stimulated Delayed Phosphorescence (TSDP) and Thermally Activated Delayed Fluorescence (TADF) in a Two-Coordinated Au(I) Bimetallic Complex Featuring a Tandem Carbene Structure

“…TADF emitters have attracted great attention because of their capability to make use of the otherwise wasted 75% triplet excitons in the electroluminescent process, resulting in a 100% internal quantum yield theoretically. , TADF is usually observed in compounds with a general donor–acceptor structure for both organic and organometallic materials. Recently, a series of two-coordinated d 10 coinage metal complexes featuring a carbene-metal-amine (cMa) structure have shown promising TADF properties, including competitively high Φ PL and short emission decay lifetime (τ) compared to the noble metal phosphorescent complexes. − In comparison with organic TADF emitters, the involvement of metal character accelerates the ISC rate significantly . As a consequence, in a TADF cMa complex, the prompt fluorescence is almost vanished, and the delayed fluorescence can be recorded even on the submicrosecond time scale …”

Luminescent carbene–copper(i)–amide polymers for efficient host-free solution-processed OLEDs

“…33,34 With these unique emission features, two-coordinate Cu( i )–CMA complexes become the mainstream focus towards the OLED application. 35–37 Despite the promising development, Cu( i )–CMA polymers, that show great potential in solution-processed OLEDs, have not been reported to date.…”

Enhanced Thermally Activated Delayed Fluorescence by Sole Coordination: From an Organic Molecule to Its Zinc Complex

“…Most thermally activated delayed fluorescence (TADF) emitters belong to pure organic compounds that have made remarkable progress in display and lighting applications, because Adachi’s group proposed the unambiguous constructed rule possessing at least a twisted donor/acceptor structural unit in one organic molecule. − In contrast to the fast development of the organic TADF emitters benefiting from more easily transforming in the molecular structure, the TADF complexes lag far behind, although they are earlier used as the emitters in organic light-emitting diodes (OLEDs). For example, Cu­(I) complexes as electroluminescent (EL) materials were reported as early as 1999 and exhibit comparable EL efficiencies with phosphorescent materials. − Other metal complexes, including those of Ag­(I), Au­(I), Au­(III), Sn­(IV), W­(VI), Zr­(IV), Pd­(II), Zn­(II), and Pt­(II), also have TADF characteristics and partly give highly efficient EL performance. − In the TADF complexes, metal ions can play the following positive roles: (1) the spin orbit coupling effect to increase the reverse intersystem crossing (RISC) rate ( k RISC ) and to decrease the delayed decay time (τ d ), (2) metallization to improve thermal stability, and (3) multiple radiation transitions such as intraligand charge transfer (ILCT), ligand–ligand charge transfer (LLCT), and metal–ligand charge transfer (MLCT) . Therefore, metal complexes should have the potential to be types of efficient TADF emitters and gain ground in EL materials.…”