CF3 Substitution of [Cu(P^P)(bpy)][PF6] Complexes: Effects on Photophysical Properties and Light‐Emitting Electrochemical Cell Performance

Abstract Herein, [Cu(P^P)(N^N)][PF 6 ] complexes (P^P=bis[2‐(diphenylphosphino)phenyl]ether (POP) or 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene (xantphos); N^N=CF 3 ‐substituted 2,2′‐bipyridines (6,6′‐(CF 3 ) 2 bpy, 6‐CF 3 bpy, 5,5′‐(CF 3 ) 2 bpy, 4,4′‐(CF 3 ) 2 bpy, 6,6′‐Me 2 ‐4,4′‐(CF 3 ) 2 bpy)) are reported. The effects of CF 3 substitution on their structure as well as their electrochemical and photophysical properties are also presented. The HOMO–LUMO gap was tuned by the N^N ligand; the largest redshift in the metal‐to‐ligand charge transfer (MLCT) band was for [Cu(P^P){5,5′‐(CF 3 ) 2 bpy}][PF 6 ]. In solution, the compounds are weak yellow to red emitters. The emission properties depend on the substitution pattern, but this cannot be explained by simple electronic arguments. Among powders, [Cu(xantphos){4,4′‐(CF 3 ) 2 bpy}][PF 6 ] has the highest photoluminescence quantum yield (PLQY; 50.3 %) with an emission lifetime of 12 μs. Compared to 298 K solution behavior, excited‐state lifetimes became longer in frozen Me‐THF (77 K; THF=tetrahydrofuran), thus indicating thermally activated delayed fluorescence (TADF). Time‐dependent (TD)‐DFT calculations show that the energy gap between the lowest‐energy singlet and triplet excited states (0.12–0.20 eV) permits TADF. Light‐emitting electrochemical cells (LECs) with [Cu(POP)+(6‐CF 3 bpy)][PF 6 ], [Cu(xantphos)(6‐CF 3 bpy)][PF 6 ], or [Cu(xantphos){6,6′‐Me 2 ‐4,4′‐(CF 3 ) 2 bpy}][PF 6 ] emit yellow electroluminescence. The LEC with [Cu(xantphos){6,6′‐Me 2 ‐4,4′‐(CF 3 ) 2 bpy}][PF 6 ] had the fastest turn‐on time (8 min), and the LEC with the longest lifetime ( t 1/2 =31 h) contained [Cu(xantphos)(6‐CF 3 bpy)][PF 6 ]; these LECs reached maximum luminances of 131 and 109 cd m −2 , respectively.