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.

Remote Modification of Bidentate Phosphane Ligands Controlling the Photonic Properties in Their Complexes: Enhanced Performance of [Cu(RN‐xantphos)(N^N)][PF6] in Light‐Emitting Electrochemical Cells

“…The first oxidation of [Cu(BnN‐xantphos)(N ^ N)][PF 6 ] is significantly lower in potential than previously reported for analogous xantphos‐containing complexes. [ 30 ] This is consistent with the former being BnN‐xantphos ligand‐centered while the latter are metal‐centered. The absorption spectra of the [Cu(BnN‐xantphos)(N ^ N)][PF 6 ] compounds have been interpreted with the aid of TD‐DFT calculations.…”

“…This effect was already observed for similar series of Cu complexes. [ 29,30 ] Therefore, more negative reduction potentials are expected along both series of complexes. Regarding the highest‐occupied molecular orbital (HOMO), it is calculated at ‐6.00 eV for [Cu(xantphos)(bpy)] + and mainly resides over the metal atom with some extension to the neighboring phosphorus atoms (Figure 3).…”

“…This is consistent with the description of the first triplet state previously discussed for complexes bearing the xantphos ligand. [ 10,29–31 ]…”

“…In good agreement with the theoretical predictions, the E 1/2 ox values of +0.58, +0.62, and +0.61 V recorded for the BnN‐xantphos‐containing complexes (Table 1) are significantly lower than the values of +0.76, +0.85, and +0.90 V previously reported for the xantphos complexes [Cu(xantphos)(bpy)][PF 6 ], [Cu(xantphos)(Mebpy)][PF 6 ] and [Cu(xantphos)(Me 2 bpy)][PF 6 ], respectively. [ 30 ] As discussed above, in the xantphos complexes the HOMO is located on the copper atom and its phosphorus environment and the oxidation process mainly arises from a Cu + /Cu 2+ process. In contrast, for the BnN‐xantphos‐containing compounds, the HOMO fully resides on the phenoxazine moiety at higher energies (Figure 3) and the first oxidation is expected to be ligand‐centered.…”

White Light‐Emitting Electrochemical Cells Based on Deep‐Red Cu(I) Complexes

“…However, the same authors proposed to use another family of Cu(I) complexes bearing different N ‐heterocyclic carbenes and dipyridylamine ligands–, i.e., di‐iso‐propylphenyl)imidazole‐2‐ylidene and 2,2ʹ‐bis‐(3‐methylpyridyl)amine, showing an effective TADF emission that led to the first blue‐emitting Cu(I)‐iTMC LECs . Other studies by Zhang et al, Bolink and co‐workers, and Costa and co‐workers showed green‐ and yellow‐emitting Cu(I)‐iTMC based devices by changing the pattern substitution of heteroleptic diamine and diphosphine ligands.…”