CF3 Substitution of [Cu(P^P)(bpy)][PF6] complexes: Effects on Photophysical Properties and Light-emitting Electrochemical Cell Performance

We report [Cu(P^P)(N^N)][PF 6 ] complexes with 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). We present the effects of CF 3 substitution on structures, and electrochemical and photophysical properties. The HOMO–LUMO gap is tuned by the N^N ligand; the largest redshift in the MLCT band is 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. For powders, [Cu(xantphos)(4,4'-(CF 3 ) 2 bpy)][PF 6 ] has the highest PLQY (50.3%) with an emission lifetime of 12 µs. Compared to 298 K solution behaviour, excited state lifetimes lengthen in frozen Me-THF (77 K) indicating thermally activated delayed fluorescence (TADF). TD-DFT calculations show that the energy gap between the lowest-energy singlet and triplet excited states (0.12–0.20 eV) permits TADF. 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. A LEC with [Cu(xantphos)(6,6'-Me 2 -4,4'-(CF 3 ) 2 bpy)][PF 6 ] had the fastest turn-on time (8 min); 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