Dual-Phosphorescent Heteroleptic Silver(I) Complex in Long-Lasting Red Light-Emitting Electrochemical Cells

The design of red-emitting silver(I) complexes and their implementation in thin-film lighting are still challenging as (i) their high ligand-field splitting energy leads to high-energy emissions with a controversial mechanism (thermally activated delayed fluorescence vs fluorescence/phosphorescence), and (ii) their low electrochemical stability leads to the formation of silver nanoclusters, limiting device stability to a few seconds. Herein, a thoughtful complex design [Ag(xantphos)(deebq)]PF₆ combining a large-bite angle diphosphine ligand (xantphos), a rigid, sterically hindered, π-extended biquinolin (deebq) is reported. In contrast to prior-art, this complex possesses (i) efficient red-emission (λ_em = 660 nm; photoluminescence quantum yield of 42%) assigned to a thermally equilibrated dual-phosphorescent emission based on spectroscopic/theoretical studies and (ii) stable reduction behavior without forming silver nanoclusters. This results in the first red light-emitting electrochemical cells featuring (i) improved stability of two orders of magnitude compared to prior-art (from seconds to hours) at irradiances of 20 µW cm⁻², and (ii) a new degradation mechanism exclusively related to p-doping as confirmed by electrochemical impedance spectroscopy analysis. Indeed, a multi-layered architecture to decouple hole injection/transport and exciton formation enables a further 2-fold enhanced irradiance/stability. Overall, this work illustrates that deciphering the rules for silver(I) complex design for lighting is tricky, but worthwhile.