Cationic Iridium(III) Complexes with Benzothiophene-Quinoline Ligands for Deep-Red Light-Emitting Electrochemical Cells

Abstract

Three new cationic cyclometalated iridium(III) complexes equipped with differently substituted benzo[b]thiophen-2-ylquinoline cyclometalating ligands and with a sterically demanding tert-butylsubstituted 2,2′-bipyridine ancillary ligand were synthesized and structurally characterized by NMR and X-ray diffraction techniques. To tune the electronic properties of such complexes, the quinoline moiety of the cyclometalating ligands was kept pristine or equipped with electronwithdrawing phenyl and −CF 3 substituents, leading to complexes 1, 2, and 3, respectively. A complete electrochemical and photophysical investigation, supported by density functional theory calculations, permits a deep understanding of their electronic properties. The emission of all complexes arises from ligand-centered triplet states in the spectral range between 625 and 950 nm, with excited-state lifetimes between 2.10 and 6.32 μs at 298 K. The unsubstituted complex (1) exhibits the most blue-shifted emission in polymeric matrix at 298 K (λ max = 667 nm, photoluminescence quantum yield (PLQY) = 0.25 and τ = 5.32 μs). The phenyl-substituted complex (2) displays the highest photoluminescent quantum yields (up to 0.30 in polymeric matrix), while the CF 3 -substituted counterpart (3) shows the most redshifted emission, peaking at approx. 720 nm, but with lower quantum yields (e.g., 0.10 in polymeric matrix at 298 K). Complexes 1 and 2 were tested in single-layer nondoped light-emitting electrochemical cells (LEECs), using a nozzle-printing technique; both devices display deep-red electroluminescence with an external quantum efficiency close to 20%.

Recent advances of organic emitters in deep‐red light‐emitting electrochemical cells

“…Since the first ruthenium complex-based LEC was made in 1996 [42], researchers have adjusted the electronic properties of complexes by adding donor or acceptor groups [43], employing new ligands [44] or new metal cores [45] (zinc [46] and copper [47]) to build new complexes. With these efforts, metal complex materials thiophen-2-ylquinoline as cyclo-metalating ligands, and a sterically demanding tert-butyl-substituted 2,2 0 -bipyridine as ancillary ligands [34]. The equipped ligands and groups could suppress nonradiative decay while the polaron-triplet and triplet-triplet annihilation processes, leading to better device efficiency and improving the performance at elevated current densities (Figure 2, complexes 1-3).…”

“…With these efforts, metal complex materials have obtained great achievements in deep‐red LECs. In 2023, Bejoymohandas et al prepared iridium complexes equipped with phenyl and trifluoromethyl substituents on the quinoline ring of the benzo[ b ]thiophen‐2‐ylquinoline as cyclo‐metalating ligands, and a sterically demanding tert‐ butyl‐substituted 2,2′‐bipyridine as ancillary ligands [34]. The equipped ligands and groups could suppress non‐radiative decay while the polaron‐triplet and triplet‐triplet annihilation processes, leading to better device efficiency and improving the performance at elevated current densities (Figure 2, complexes 1 – 3 ).…”

Efficient 1-(thiophen-2-yl)isoquinoline-based ionic iridophosphors with bulky counterions for solution-processed deep-red electroluminescence

“…19 According to the energy-gap law, the rate of non-radiative transitions of the red/deep-red emitters usually increase with the decrease of the energy gap between the states, resulting in low efficiency in photoluminescence. 20 Thus, it is necessary to further explore the new pure-red/deep-red ionic iridophosphors with high efficiency for electroluminescence.…”