Near infrared electroluminescence in light-emitting electrochemical cells from the binuclear copper (I) complexes bearing π-extended benzimidazole and benzothiazole ligands

The design of compounds that efficiently emit into the deep-red to near-infrared (NIR) region is highly challenging, yet they can play pivotal roles in optoelectronic devices for phototherapy, encryption and telecommunication technology. To date, examples of NIR-emissive 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 chemical and time-resolved optical spectroscopies as well as single-crystal X-ray diffraction analysis. The optical and electronic properties are further elucidated with time-dependent density functional theory (TD-DFT) computations that confirm the nature of the transitions and excited states involved. It is shown that the introduction of sulphur and nitrogen heteroatoms in the peripheral π-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 profiles centred at λem = 734–776 nm and 644–757 nm, respectively, attributable to an emissive excited state with admixed 3MLCT/3LLCT character. Finally, the derivatives Cu1 and Cu4 are tested as electroluminescent materials in light-emitting electrochemical cells (LECs). The former displays deep-red electroluminescence (EL), with λEL = 686–697 nm and an external quantum efficiency (EQE) up to 0.5%, while the latter remarkably achieves NIR EL with λEL > 770 nm in combination with high spectral stability. Overall, the presented results demonstrate that Cu(i) complexes represent a valid alternative to precious metals for NIR EL devices.