Near Infrared Electroluminescence in Light-emitting Electrochemical Cells from Binuclear Copper(I) Complexes bearing π-extended Benzimidazole and Benzothiazole Ligands

Abstract

The design of compounds that efficiently emits into the deep-red to near-infrared (NIR) region is still highly challenging, yet they can play pivotal role in optoelectronic devices for phototherapy, encryption and telecommunication technology. To date, examples of NIRemissive earth-abundant Cu(I) complexes are still very rare in literature. Herein, a series of binuclear heteroleptic Cu(I) complexes, namely Cu1-Cu4, is presented and thoroughly characterized by means of chemical and (time-resolved) optical spectroscopies as well as single-crystal X-ray diffractometric analysis. The optical and electronic properties are further elucidated with the help of time-dependent density functional theory (TD-DFT) computations that confirms the nature of the transitions and excited states involved. It is shown that introduction of sulphur and nitrogen heteroatoms in the peripheral p-accepting coordinating scaffolds, such as (substituted) benzimidazoles and benzothiazole, along with the thiazolo[5,4-d]thiazole bridging unit results in deep-red to NIR emissive complexes both in CH2Cl2 solution and in the solid state with long-lived emission with profile centred at lem = 734-776 nm and 644-757 nm, respectively, attributable to an emissive excited state with admixed 3MLCT/ 3LLCT character. Finally, derivative Cu1 and Cu4 are tested as electroluminescent materials in light-emitting electrochemical cells (LECs). The former displays deep-red electroluminescence (EL), λEL = 686-697 nm with external quantum efficiency (EQE) up to 0.5%; whereas the latter achieves NIR EL with λEL > 770 nm in combination with of high spectral stability, remarkably. Overall, the presented results demonstrate that Cu(I) complexes represents a valid alternative precious metals for NIR EL devices.