TADF: Enabling luminescent copper() coordination compounds for light-emitting electrochemical cells

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2022-03, Vol.1 (12), p.4456-4482
• Main Authors: Housecroft, Catherine E, Constable, Edwin C
• Format: Article
• Language: English
• Subjects: Chelation; Chemistry; Computer architecture; Coordination compounds; Copper; Electrochemical cells; Fluorescence; Heavy metals; Iridium; Ligands; Photoluminescence; Room temperature; Spin-orbit interactions; Transition metal compounds
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/d1tc04028f

The last decade has seen a surge of interest in the emissive behaviour of copper( i ) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper( i ) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S 1 ) and excited triplet (T 1 ) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper( i ) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)] + (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P) 2 (N^N)] + (P = monodentate phosphane ligand), [Cu(P)(tripodal-N 3 )] + , [Cu(P)(N^N)(N)] + (N = monodentate N-donor ligand), [Cu(P^P)(N^S)] + (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)] + (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)] + (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)] + and two-coordinate [Cu(N)(NHC)] + complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations. In this review, we illustrate how the field of luminescent copper( i ) compounds has developed with a focus on ionic copper( i ) complexes and those exhibiting thermally-activated delayed fluorescence (TADF).