Heteroleptic Cu(I) complexes with aromatic diimines and phosphines: Synthesis, structure, photophysical properties and THz time domain spectroscopy

Photoactive Copper Complexes: Properties and Applications

Photocatalyzed and photosensitized chemical processes have seen growing interest recently and have become among the most active areas of chemical research, notably due to their applications in fields such as medicine, chemical synthesis, material science or environmental chemistry. Among all homogeneous catalytic systems reported to date, photoactive copper(I) complexes have been shown to be especially attractive, not only as alternative to noble metal complexes, and have been extensively studied and utilized recently. They are at the core of this review article which is divided into two main sections. The first one focuses on an exhaustive and comprehensive overview of the structural, photophysical and electrochemical properties of mononuclear copper(I) complexes, typical examples highlighting the most critical structural parameters and their impact on the properties being presented to enlighten future design of photoactive copper(I) complexes. The second section is devoted to their main areas of application (photoredox catalysis of organic reactions and polymerization, hydrogen production, photoreduction of carbon dioxide and dye-sensitized solar cells), illustrating their progression from early systems to the current state-of-the-art and showcasing how some limitations of photoactive copper(I) complexes can be overcome with their high versatility.

A counterion study of a series of [Cu(P^P)(N^N)][A] compounds with bis(phosphane) and 6-methyl and 6,6'-dimethyl-substituted 2,2'-bipyridine ligands for light-emitting electrochemical cells

The syntheses and characterisations of a series of heteroleptic copper(I) compounds [Cu(POP)(Mebpy)][A], [Cu(POP)(Me2bpy)][A], [Cu(xantphos)(Mebpy)][A] and [Cu(xantphos)(Me2bpy)][A] in which [A]- is [BF4]-, [PF6]-, [BPh4]- and [BArF4]- (Mebpy = 6-methyl-2,2'-bipyridine, Me2bpy = 6,6'-dimethyl-2,2'-bipyridine, POP = oxydi(2,1-phenylene)bis(diphenylphosphane), xantphos = (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane), [BArF4]- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) are reported. Nine of the compounds have been characterised by single crystal X-ray crystallography, and the consequences of the different anions on the packing interactions in the solid state are discussed. The effects of the counterion on the photophysical properties of [Cu(POP)(N^N)][A] and [Cu(xantphos)(N^N)][A] (N^N = Mebpy and Me2bpy) have been investigated. In the solid-state emission spectra, the highest energy emission maxima are for [Cu(xantphos)(Mebpy)][BPh4] and [Cu(xantphos)(Me2bpy)][BPh4] (λemmax = 520 nm) whereas the lowest energy λemmax values occur for [Cu(POP)(Mebpy)][PF6] and [Cu(POP)(Mebpy)][BPh4] (565 nm and 563 nm, respectively). Photoluminescence quantum yields (PLQYs) are noticeably affected by the counterion; in the [Cu(xantphos)(Me2bpy)][A] series, solid-state PLQY values decrease from 62% for [PF6]-, to 44%, 35% and 27% for [BF4]-, [BPh4]- and [BArF4]-, respectively. This latter series of compounds was used as active electroluminescent materials on light-emitting electrochemical cells (LECs). The luminophores were mixed with ionic liquids (ILs) [EMIM][A] ([EMIM]+ = [1-ethyl-3-methylimidazolium]+) containing the same or different counterions than the copper(I) complex. LECs containing [Cu(xantphos)(Me2bpy)][BPh4] and [Cu(xantphos)(Me2bpy)][BArF4] failed to turn on under the LEC operating conditions, whereas those with the smaller [PF6]- or [BF4]- counterions had rapid turn-on times and exhibited maximum luminances of 173 and 137 cd m-2 and current efficiencies of 3.5 and 2.6 cd A-1, respectively, when the IL contained the same counterion as the luminophore. Mixing the counterions ([PF6]- and [BF4]-) of the active complex and the IL led to a reduction in all the figures of merit of the LECs.

Molecular-Level Insight of Cu(I) Complexes with the 7,8-Bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate Ligand as a Thermally Activated Delayed Fluorescence Emitter: Luminescent Mechanism and Design Strategy

Investigation of the clear structure-property relationship and microscopic mechanism of thermally activated delayed fluorescence (TADF) emitters with high emission quantum yield is a direction worthy of continuous efforts. The instructive theoretical principle of TADF material design is critical and challenging. Here, we carried out theoretical calculation on two experimental Cu(I) complexes with the same 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate (dppnc) but different N^N ligands [dmbpy = 6,6'-dimethyl-2,2'-bipyridine (1) or dmp = 2,9-dimethyl-1,10-phenanthroline (2)] to briefly elaborate the structure-TADF performance relationship and luminescence mechanism. It was found that enhanced rigidity by the fused benzene ring between two pyridyl units in complex 2 leads to (i) higher allowedness of S₁ → S₀, (ii) more effective reverse intersystem crossing (RISC), and (iii) better relative stability of the T₁ state, which could be responsible for its excellent TADF behavior. Thus, a strategy of extending π conjugation in the N^N ligand could be deduced to further enhance the quantum yield. We validated it and have succeeded in designing analogue complex 4 by extending π conjugation with an electron-withdrawing pyrazinyl. Benefiting from the smaller energy gap (ΔE_ST) and plunged reorganization energy between the S₁ and T₁ states, the rate of RISC in complex 4 (1.05 × 10⁸ s^-1) increased 2 orders of magnitude relative to that of 2 (5.80 × 10⁶ s^-1), showing more superiority of the TADF behavior through a better balance of RISC, fluorescence, and phosphorescence decay. Meanwhile, the thermally activated temperature of 4 is only 165 K, implying that there is a low-energy barrier. All of these indicate that the designed complex 4 may be a potential TADF candidate.

The Role of Percent Volume Buried in the Characterization of Copper(I) Complexes for Lighting Purposes

The usefulness of percent volume buried (%V_bur) as a readily quantifiable property is investigated with regard to [Cu(NN)(PP)]⁺ complexes of interest for lighting purposes. Photoluminescence quantum yields (PLQYs) and single crystal X-ray structures of 100 reported compounds were assembled, %V_bur of the ligand systems were calculated and analyzed for correlations. We found that increased shielding of the central Cu(I) cation relying on shared contributions of both (NN) and (PP) ligand systems led to increased PLQYs. These findings are of relevance for future characterizations of Cu(I)-based complexes and their photophysical behavior in the solid-state.