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 )₂ bpy, 6-CF₃ bpy, 5,5'-(CF₃ )₂ bpy, 4,4'-(CF₃ )₂ bpy, 6,6'-Me₂ -4,4'-(CF₃ )₂ bpy)) 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 )₂ bpy}][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 )₂ bpy}][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-CF₃ bpy)][PF₆ ], [Cu(xantphos)(6-CF₃ bpy)][PF₆ ], or [Cu(xantphos){6,6'-Me₂ -4,4'-(CF₃ )₂ bpy}][PF₆ ] emit yellow electroluminescence. The LEC with [Cu(xantphos){6,6'-Me₂ -4,4'-(CF₃ )₂ bpy}][PF₆ ] had the fastest turn-on time (8 min), and the LEC with the longest lifetime (t_1/2 =31 h) contained [Cu(xantphos)(6-CF₃ bpy)][PF₆ ]; these LECs reached maximum luminances of 131 and 109 cd m^-2 , respectively.