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

Herein, [Cu(P^P)(N^N)][PF6] complexes (P^P=bis[2‐(diphenylphosphino)phenyl]ether (POP) or 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene (xantphos); N^N=CF3‐substituted 2,2′‐bipyridines (6,6′‐(CF3)2bpy, 6‐CF3bpy, 5,5′‐(CF3)2bpy, 4,4′‐(CF3)2bpy, 6,6′‐Me2‐4,4′‐(CF3)2bpy)) are reported. The effects of CF3 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′‐(CF3)2bpy}][PF6]. 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′‐(CF3)2bpy}][PF6] 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‐CF3bpy)][PF6], [Cu(xantphos)(6‐CF3bpy)][PF6], or [Cu(xantphos){6,6′‐Me2‐4,4′‐(CF3)2bpy}][PF6] emit yellow electroluminescence. The LEC with [Cu(xantphos){6,6′‐Me2‐4,4′‐(CF3)2bpy}][PF6] had the fastest turn‐on time (8 min), and the LEC with the longest lifetime (t1/2=31 h) contained [Cu(xantphos)(6‐CF3bpy)][PF6]; these LECs reached maximum luminances of 131 and 109 cd m−2, respectively. A light touch: Emissive [Cu(P^P)(N^N)][PF6] complexes with P^P=bis[2‐(diphenylphosphino)phenyl]ether or xantphos and N^N=CF3‐substituted 2,2′‐bipyridines are reported (see figure). On going from solution to frozen 2‐methyltetrahydrofuran, the excited‐state lifetimes increased, thus indicating thermally activated delayed fluorescence. Three compounds were incorporated into light‐emitting electrochemical cells. The influence of the ligand substitution pattern on their performance is discussed.