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)][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.