Design Rule Hidden from The Eye in S/N‐Bridged Ancillary Ligands for Copper(I) Complexes Applied to Light‐Emitting Electrochemical Cells

Abstract Enhancing low‐energy emitting Cu(I)‐ionic transition metal complexes (iTMCs) light‐emitting electrochemical cells (LECs) is of utmost importance towards Cu(I)‐iTMC‐based white‐emitting LECs. Here, the ancillary ligand design includes (i) extension of π‐systems and (ii) insertion of S‐bridge between heteroaromatics rings. This led to two novel heteroleptic Cu(I)‐iTMCs: 2‐(pyridin‐2‐yl‐l2‐azanyl)quinoline ( CuN2 ) and 2‐(naphthalen‐2‐ylthio)quinoline ( CuS2 ) as N^N and bis[(2‐diphenylphosphino)phenyl] ether as P^P, exhibiting improved photoluminescence quantum yields ( ϕ ) and thermally activated delayed fluorescence processes compared to their reference Cu(I)‐iTMCs: di(pyridin‐2‐yl)‐l2‐azane ( CuN1 ) and di(pyridin‐2‐yl)sulfane ( CuS1 ). Despite CuS2 stands out with the highest ϕ (38% vs 17 / 14 / 1% for CuN1 / CuN2 / CuS1 ), only CuN2 ‐LECs show the expected enhanced performance (0.35 cd A −1 at luminance of 117 cd m −2 ) compared to CuN1 ‐LECs (0.02 cd A −1 at6 cd m −2 ), while CuS2 ‐LECs feature low performances (0.04 cd A −1 at 10 cd m −2 ). This suggests that conventional chemical design rules are not effective towards enhancing device performance. Herein, nonconventional multivariate statistical analysis and electrochemical impedance spectroscopy studies allow to rationalize the mismatch between chemical design and device performance bringing to light a hidden design rule: polarizability of the ancillary ligand is key for an efficient Cu(I)‐iTMC‐LECs. All‐in‐all, this study provides fresh insights for the design of Cu‐iTMCs fueling research on sustainable ion‐based lighting sources.