NHC Copper(I) Complexes Bearing Dipyridylamine Ligands: Synthesis, Structural, and Photoluminescent Studies

[Ag(IPr)(bpy)][PF6]: brightness and darkness playing with aggregation induced phosphorescence for light-emitting electrochemical cells

Heteroleptic silver(I) complexes have recently started to attract attention in thin-film lighting technologies as an alternative to copper(I) analogues due to the lack of flattening distortion upon excitation. However, the interpretation of their photophysical behavior is challenging going from traditional fluorescence/phosphorescence to a temperature-dependent dual emission mechanism and ligand-lock assisted thermally activated delayed fluorescence. Herein, we unveil the photoluminescence behavior of a three-coordinated Ag(I) complex with the N-heterocyclic carbene (NHC) ligand and 2,2'-bipyridine (bpy) as the N^N ligand. In contrast to its low-emissive Cu(I) complex structural analogues, a strong greenish emission was attributed to the presence of aggregates formed by π-π intermolecular interactions as revealed by the X-ray structure and aggregation induced emission (AIE) studies in solution. In addition, the temperature-dependent time-resolved spectroscopic and computational studies demonstrated that the emission mechanism is related to a phosphorescence emission mechanism of two very close lying (ΔE = 0.08 eV) excited triplet states, exhibiting a similar delocalized nature over the bipyridine ligands. Unfortunately, this favourable AIE is lost upon forming homogeneous thin films suitable for lighting devices. Though the films showed very poor emission, the electrochemical stability under device operation conditions is remarkable compared to the prior-art, highlighting the potential of [Ag(NHC)(N^N)][X] complexes in thin-film lighting.

NHC-Cu Three-Coordinate Complex as a Promising Photocatalyst for Energy and Electron Transfer Reactions

Herein, we describe a simple three-coordinate complex of Cu(I) with an NHC and 1,10-phenanthroline ligands as an effective photocatalyst for energy (e.g., olefin E/Z isomerization) and electron transfer (e.g., aryl halide dehalogenation) reactions under blue-light irradiation. This complex can be obtained in a one-pot procedure starting from commercially available reagents and green solvents (EtOH, water). We hereby present a study of its activity and mechanistic insight into its mode of operation.

Synthesis and photophysical properties of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes and their application to photoluminescent light-emitting diodes and anti-counterfeiting

Here, a series of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes having 3,3'-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-1H-imidazolylidene as bis-NHC ligand and bis[(2-diphenylphosphino)phenyl]ether (POP) as auxiliary ligand have been successfully prepared, and their photophysical properites were investigaged experimentally and theocitcally. The resulting complexes all exhibited intense green to yellow emission that originated from the thermally activated delayed fluorescence (TADF) with a high photoluminescence quantum yield of up to 0.67 and longer excited-state lifetimes on the microsecond time scale in the solid state. Green and yellow light-emitting diode (LED) devices based on Cu(I) complexes have successfully achieved good color rendering indices. Moreover, the anti-counterfeiting patterns and QR codes made of Cu(I) complexes have been applied to clothing, banknotes, books and glass plates with excellent anti-counterfeiting effects.

Polymeric copper(I)-NHC complexes with bulky bidentate (N^C) ligands: synthesis and solid-state luminescence

Starting from imidazolium chlorides bearing bulky nitrogen donors, a series of four complexes, mainly [Cu(C^N)Cl]n coordination polymers were obtained directly as luminescent species by simple filtration from the aqueous reaction medium, highlighting a simple, eco-friendly, robust and reproducible synthetic procedure. Additionally, we have shown on the most efficient example that chloride could be exchanged very easily by other halides/pseudohalides (Br-, I-, NCS-, N3-) allowing to slightly modulate the emitted colour while conserving the polymeric structure, except for azide for which a dimer was obtained. The combination of chemical analyses, of photoluminescence studies in the solid state including quantum yield measurement and X-ray diffraction on single crystals and as-synthesized microcrystalline powders highlighted that the polymeric luminescent species was indeed obtained directly by simple filtration and that no major alteration of the structure was observed upon recrystallisation. Samples of all polymeric complexes displayed remarkable stability towards air oxidation remaining unchanged upon storage for several months and partially retaining their photoluminescence properties even after a thermal treatment at 100 °C for 24 h.

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.

Helicenic N-heterocyclic carbene copper(I) complex displaying circularly polarized blue fluorescence

Enantiopure copper(I) chloride complexes bearing a monodentate N-(carbo[6]helicenyl)-NHC ligand have been prepared and characterized experimentally and computationally. Their high stability enables the stereochemistry to be probed by X-ray crystallography and NMR spectroscopy. The resolved enantiomeric complexes emit circularly polarized blue fluorescence with g_lum ∼1.3 × 10^-3 in solution. The photophysical and chiroptical properties of these systems, with their helicene-centred origin, are similar to those of the organic helicene-benzimidazole precursor proligand, although the reverse axial chirality configuration is preferentially observed for the complex compared to the ligand.

Bulky and Stable Copper(I)-Phenanthroline Complex: Impact of Steric Strain and Symmetry on the Excited-State Properties

The steric strain around copper(I) in typical [Cu(NN_R)₂]⁺ complexes, where NN_R is a diimine ligand substituted in α-positions of the nitrogen atoms by R, is known to strongly impact the excited-state properties. Generally speaking, the larger the R, the longer the emission lifetime and the higher the quantum yield. However, the stability of the coordination scaffold can be at stake if the steric strain imposed by R is too large. In this work, we explore a way of fine-tuning the steric strain around Cu(I) to reach a balance between high emission quantum yield and stability in a highly bulky copper(I) complex. Taking stable [Cu(dipp)₂]⁺ and unstable [Cu(dtbp)₂]⁺ (where dipp and dtbp are, respectively, 2,9-diisopropyl-1,10-phenanthroline and 2,9-di-tert-butyl-1,10-phenanthroline) as the boundary of two least and most sterically strained structures, we designed and characterized the nonsymmetrical ligand 2-isopropyl-9-tert-butyl-1,10-phenanthroline (L1) and corresponding complex [Cu(L1)₂]⁺ (Cu1). The key experimental findings are that Cu1 exhibits a rigid tetrahedral geometry in the ground state, close to that of [Cu(dtbp)₂]⁺ and with an intermediate stability between that of [Cu(dipp)₂]⁺ and [Cu(dtbp)₂]⁺. Conversely, the nonsymmetrical nature of ligand L1 leads to a shorter emission lifetime and smaller quantum yield than those of either [Cu(dipp)₂]⁺ or [Cu(dtbp)₂]⁺. This peculiar behavior is rationalized through the in depth analysis of the ultrafast dynamics of the excited state measured with optical transient absorption spectroscopy and theoretical calculations performed on the ground and excited state of Cu1. Our main findings are that the obtained complex is significantly more stable than [Cu(dtbp)₂]⁺ despite the sterically strained coordination sphere. The nonsymmetrical nature of the ligand translates into a strongly distorted structure in the excited state. The distortion can be described as a rocking motion of one ligand, entailing the premature extinction of the excited state via several deactivation channels.

Copper(I) Bis(diimine) Complexes with High Photooxidation Power: Reductive Quenching of the Excited State with a Benzimidazoline Sacrificial Donor

The reductive quenching of photoexcited photosensitizers is a very efficient way to achieve challenging reduction reactions. In this process, the excited photosensitizer is reduced by a sacrificial electron donor. This mechanism is rarely observed with copper(I) bis(diimine) complexes, which are nevertheless acknowledged as very promising photosensitizers. This is due to the fact that they are very poor photooxidants and prove unable to react with common donors once promoted in their excited state. In this article, we evidence the rare reductive quenching cycle with two specially designed copper(I) complexes. These complexes exhibit improved photooxidation power thanks to an optimized coordination sphere made of strongly π-accepting ligands. Reductive quenching of the excited state of the latter complexes with a classical benzimidazoline sacrificial donor is monitored, and reduced complexes are accumulated during prolonged photolysis. Trials to utilize the photogenerated reductive power are presented.

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S1) and excited triplet (T1) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]+ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)2(N^N)]+ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N3)]+, [Cu(P)(N^N)(N)]+ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]+ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]+ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]+ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]+ and two-coordinate [Cu(N)(NHC)]+ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

Ruthenium-based PACT agents based on bisquinoline chelates: synthesis, photochemistry, and cytotoxicity

The known ruthenium complex [Ru(tpy)(bpy)(Hmte)](PF6)2 ([1](PF6)2, where tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine, Hmte = 2-(methylthio)ethanol) is photosubstitutionally active but non-toxic to cancer cells even upon light irradiation. In this work, the two analogs complexes [Ru(tpy)(NN)(Hmte)](PF6)2, where NN = 3,3'-biisoquinoline (i-biq, [2](PF6)2) and di(isoquinolin-3-yl)amine (i-Hdiqa, [3](PF6)2), were synthesized and their photochemistry and phototoxicity evaluated to assess their suitability as photoactivated chemotherapy (PACT) agents. The increase of the aromatic surface of [2](PF6)2 and [3](PF6)2, compared to [1](PF6)2, leads to higher lipophilicity and higher cellular uptake for the former complexes. Such improved uptake is directly correlated to the cytotoxicity of these compounds in the dark: while [2](PF6)2 and [3](PF6)2 showed low EC50 values in human cancer cells, [1](PF6)2 is not cytotoxic due to poor cellular uptake. While stable in the dark, all complexes substituted the protecting thioether ligand upon light irradiation (520 nm), with the highest photosubstitution quantum yield found for [3](PF6)2 (Φ[3] = 0.070). Compounds [2](PF6)2 and [3](PF6)2 were found both more cytotoxic after light activation than in the dark, with a photo index of 4. Considering the very low singlet oxygen quantum yields of these compounds, and the lack of cytotoxicity of the photoreleased Hmte thioether ligand, it can be concluded that the toxicity observed after light activation is due to the photoreleased aqua complexes [Ru(tpy)(NN)(OH2)]2+, and thus that [2](PF6)2 and [3](PF6)2 are promising PACT candidates.

Towards rainbow photo/electro-luminescence in copper(i) complexes with the versatile bridged bis-pyridyl ancillary ligand

The synthesis and characterization of a family of copper(i) complexes bearing a bridged bis-pyridyl ancillary ligand is reported, highlighting how the bridge nature impacts the photo- and electro-luminescent behaviours within the family. In particular, the phosphonium bridge led to copper(i) complexes featuring good electrochemical stability and high ionic conductivity, as well as a stark blue-to-orange luminescence shift compared to the others. This resulted in high performance light-emitting electrochemical cells reaching stabilities of 10 mJ at ca. 40 cd m^-2 that are one order of magnitude higher than those of the other complexes. Overall, this work sheds light onto the crucial role of the bridge nature of the bis-pyridyl ancillary ligand on the photophysical features, film forming and, in turn, on the final device performances.

Wide-Bite-Angle Diphosphine Ligands in Thermally Activated Delayed Fluorescent Copper(I) Complexes: Impact on the Performance of Electroluminescence Applications

We report a series of seven cationic heteroleptic copper(I) complexes of the form [Cu(P^P)(dmphen)]BF₄, where dmphen is 2,9-dimethyl-1,10-phenanthroline and P^P is a diphosphine chelate, in which the effect of the bite angle of the diphosphine ligand on the photophysical properties of the complexes was studied. Several of the complexes exhibit moderately high photoluminescence quantum yields in the solid state, with Φ_PL of up to 35%, and in solution, with Φ_PL of up to 98%. We were able to correlate the powder photoluminescence quantum yields with the % V_bur of the P^P ligand. The most emissive complexes were used to fabricate both organic light-emitting diodes and light-emitting electrochemical cells (LECs), both of which showed moderate performance. Compared to the benchmark copper(I)-based LECs, [Cu(dnbp)(DPEPhos)]⁺ (maximum external quantum efficiency, EQE_max = 16%), complex 3 (EQE_max = 1.85%) showed a much longer device lifetime (t_1/2 = 1.25 h and >16.5 h for [Cu(dnbp)(DPEPhos)]⁺ and complex 3, respectively). The electrochemiluminescence (ECL) properties of several complexes were also studied, which, to the best of our knowledge, constitutes the first ECL study for heteroleptic copper(I) complexes. Notably, complexes exhibiting more reversible electrochemistry were associated with higher annihilation ECL as well as better performance in a LEC.

Stereochemical Control of Tricoordinate Copper(I) Complexes Based on N-(9-Alkyl-9-fluorenyl)-Substituted Heterocyclic Carbenes

A series of tricoordinate copper(I) complexes of general formula [Cu(Et F-NHC)(2,2′-dipyridylamine)][BF4], in which Et F-NHC represents an imidazol-2-ylidene ligand bearing a 9-ethyl-9-fluorenyl N-substituent have been synthesised stepwise from appropriate N-arylimidazoles. All complexes are remarkably air-stable, both in solution and in the solid state. X-ray diffraction studies revealed that in three of the complexes the fluorenylidene plane and the dipyridylamine (dpa) unit undergo intramolecular π–π stacking. The resulting bending of the fluorenilydene plane towards the metal atom is likely to contribute to maintain the trigonal planar geometry of the [Cu–C,N,N] unit upon binding of exogenous substrates, thereby considerably increasing complex stability.

Synthesis and Photophysical Properties of T-Shaped Coinage-Metal Complexes

The photophysical properties of a series of T‐shaped coinage d¹⁰ metal complexes, supported by a bis(mesoionic carbene)carbazolide (CNC) pincer ligand, are explored. The series includes a rare new example of a tridentate T‐shaped Ag^I complex. Post‐complexation modification of the Au^I complex provides access to a linear cationic Au^I complex following ligand alkylation, or the first example of a cationic square planar Au^III−F complex from electrophilic attack on the metal centre. Emissions ranging from blue (Cu^I) to orange (Ag^I) are obtained, with variable contributions of thermally‐dependent fluorescence and phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutral T‐shaped Au^I, cationic linear Au^I and square planar cationic Au^III). The higher quantum yield and longer decay lifetime of the linear gold(I) complex are indicative of increased phosphorescence contribution.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

The synthesis and full characterization of new air-stable AgI and CuI complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag2 O afforded a collection of AgI complexes, and their first use as ligand transfer reagents led to novel isostructural CuI or AuI complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new CuI complexes. The solid-state structures feature large N-CNHC -N angles (118-128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%Vbur ) values span easily in the 50-57 % range, and that one of (9-Dipp)CuBr complex (%Vbur =57.5) overcomes to other known erNHC-metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76-93 % product at the 0.5-2.5 mol % catalyst loading, proving the stability of CuI erNHC complexes at elevated temperatures (100 °C).

2,2'-Dipyridylamines: more than just sister members of the bipyridine family. Applications and achievements in homogeneous catalysis and photoluminescent materials

2,2'-Dipyridylamines (dpa) and related compounds belong to the family of polydentate nitrogen ligands. More than a century has passed since their first report but new complexes and applications have been emerging in recent years owing to the versatility of dpa-based architectures. This review aims to present and highlight the main achievements attained with dpa-containing metal complexes in the domains of homogeneous catalysis and luminescent materials.

Synthesis, Structures, and Photophysical Properties of Novel Four-Coordinate Cu(I) Complexes Supported by Chelating N-Heterocyclic Carbene Ligands

Luminescent Cu(I) complexes are promising emitting materials for electroluminescent devices due to their low cost and abundant resources, as well as high emission efficiency. It is well-known that N-heterocyclic carbenes (NHCs) are excellent ligands for transition metal complexes. To investigate the photophysical properties of Cu(I)-NHC complexes, a series of new mononuclear four-coordinate Cu(I) complexes supported by the diphosphine ligand bis[2-(diphenylphosphino)phenyl]ether (POP) and the NHC ligands, consisting of imidazolylidene and pyrimidine units, were synthesized and fully characterized. To tune the photophysical properties of these Cu(I)-NHC complexes, the NHC ligands were attached with electron-withdrawing/donating groups (fluorine, chlorine, methyl and methoxyl) at the pyrimidine unit. All of these Cu(I)-NHC complexes adopt the typical distorted tetrahedral configuration. The electron-donating groups can lead to shorter Cu–N bond distances and longer Cu–C bond distances compared to the electron-withdrawing groups. Theoretical calculation results show that the highest occupied molecular orbitals are mainly distributed on the Cu(I) ion, POP, and carbene unit, while the lowest unoccupied molecular orbitals are mostly located on the pyrimidine unit of NHC ligands. The lowest energy electronic transitions of these Cu(I)-NHC complexes are mainly the metal-to-ligand charge transfer transition and ligand-to-ligand charge transfer transition. These Cu(I)-NHC complexes in solid state show tunable emissions from 530 to 618 nm with efficiencies of 0.5–38.1% at room temperature. The photophysical behaviors of these complexes at 298 and 50 K match well with the thermally activated delayed fluorescence (TADF) characteristics.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I) Complexes Featuring Nonconventional N-Heterocyclic Carbenes

A series of six luminescent two-coordinate Cu(I) complexes were investigated bearing nonconventional N-heterocyclic carbene ligands, monoamido-aminocarbene (MAC*) and diamidocarbene (DAC*), along with carbazolyl (Cz) as well as mono- and dicyano-substituted Cz derivatives. The emission color can be systematically varied over 270 nm, from violet to red, through proper choice of the acceptor (carbene) and donor (carbazolyl) groups. The compounds exhibit photoluminescent quantum efficiencies up to 100% in fluid solution and polystyrene films with short decay lifetimes (τ ≈ 1 μs). The radiative rate constants for the Cu(I) complexes ( k_r = 10⁵-10⁶ s^-1) are comparable to state of the art phosphorescent emitters with noble metals such as Ir and Pt. All complexes show strong solvatochromism due to the large dipole moment of the ground states and the transition dipole moment that is in the opposite direction. Temperature-dependent studies of (MAC*)Cu(Cz) reveal a small energy separation between the lowest singlet and triplet states (Δ E_S1-T1 = 500 cm^-1) and an exceptionally large zero-field splitting (ZFS = 85 cm^-1). Organic light-emitting diodes (OLEDs) fabricated with (MAC*)Cu(Cz) as a green emissive dopant have high external quantum efficiencies (EQE = 19.4%) and brightness of 54 000 cd/m² with modest roll-off at high currents. The complex can also serve as a neat emissive layer to make highly efficient OLEDs (EQE = 16.3%).

Synthesis of copper(i) cyclic (alkyl)(amino)carbene complexes with potentially bidentate N^N, N^S and S^S ligands for efficient white photoluminescence

Copper(i) complexes of cyclic (alkyl)(amino)carbenes coordinated to monodentate or hemilabile guanidinato and formamidinato complexes show intense photoluminescence including white emissions.

Synthesis, characterization, and photoluminescent studies of three-coordinate Cu(i)–NHC complexes bearing unsymmetrically-substituted dipyridylamine ligands

A series of heteroleptic three-coordinate Cu(i) complexes bearing monodentate N-heterocyclic carbene (NHC) ligands of the type 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), and bidentate N-donor ligands of the type unsymmetrically-substituted dimethyl dipyridylamine (Me₂Hdpa) and bis(mesityl)biazanaphthenequinone (mesBIAN) have been synthesized. The complexes [Cu(IPr)(3,4′-Me₂Hdpa)]PF₆, 1; [Cu(IPr)(3,5′-Me₂Hdpa)]PF₆, 2; [Cu(IPr)(3,6′-Me₂Hdpa)]PF₆, 3; [Cu(IPr)(mesBIAN)]PF₆, 6; [Cu(SIPr)(3,4′-Me₂Hdpa)]PF₆, 7; [Cu(SIPr)(3,5′-Me₂Hdpa)]PF₆, 8; and [Cu(SIPr)(3,3′-Me₂Hdpa)]PF₆, 11 have been characterized by ¹H and ¹³C NMR spectroscopies, elemental analysis, cyclic voltammetry, and photophysical studies in solid and solution phase. Single crystal X-ray structures were obtained for all complexes except 11. The crystallographic data reveal a mononuclear structure for all complexes with the copper atom ligated by one C and two N atoms. The UV-Vis absorption spectra of all dipyridylamine complexes in CH₂Cl₂ show a strong ligand-centered absorption band around 250 nm and a strong metal-to-ligand charge transfer (MLCT) band around 300 nm. When irradiated with UV light, the complexes exhibit strong emission maxima at 453–482 nm with photoluminescence quantum yields (PLQY) ranging from 0.21 to 0.87 in solid state. While the PLQY values are comparable to those of the symmetrical [Cu(IPr)(Me₂Hdpa)]PF₆ complexes, a stabilizing CH–π interaction has been reduced in the current systems. In particular, complex 3 lacks any strong CH–π interaction, but emits more efficiently than 1 and 2 wherein the interactions exist. Structural data analysis was performed to clarify the role of ligands' plane angle and the NH/CH⋯F interactions to the observed light interaction of unsymmetrical [Cu(NHC)(Me₂Hdpa)]PF₆ complexes. DFT calculations were performed to assist in the assignment of the electronic structure and excited state behavior of the complexes.

The photoluminescent Cu(i)–NHC complexes bearing unsymmetrical dipyridylamine ligands have been synthesized and characterized. The structure–light reactivity has been elucidated.

Photoredox catalysts based on earth-abundant metal complexes

Visible light photoredox catalysis has exploded into the consciousness of the synthetic chemist. We critically review Earth-abundant metal complexes photocatalysts including Cu(i), Zn(ii), Ni(0), V(v), Zr(iv), W(0), W(vi), Mo(0), Cr(iii), Co(iii) and Fe(ii).

A Cu-NHC based phosphorescent binuclear iridium(iii)/copper(i) complex with an unpredictable near-linear two-coordination mode

A novel Cu-NHC based phosphorescent binuclear iridium(iii)/copper(i) complex has been prepared. The copper(i) center adopts an unpredictable near-linear two-coordination mode with short Cu-N and Cu-C bond lengths and two obvious CH-π interactions, which support its stability in the solid state. It also shows better optical properties (higher luminescence efficiency and shorter emission lifetime) in both solution and solid states relative to the model binuclear copper(i)/copper(i) complex, and was successfully applied in solution-processed organic light-emitting diodes.

Photoactive Complexes with Earth-Abundant Metals

In this invited Perspective, recent developments and possible future directions of research on photoactive coordination compounds made from nonprecious transition metal elements will be discussed. The focus is on conceptually new, structurally well-characterized complexes with excited-state lifetimes between 10 ps and 1 ms in fluid solution for possible applications in photosensitizing, light-harvesting, luminescence and catalysis. The key metal elements considered herein are Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, W and Ce in various oxidation states equipped with diverse ligands, giving access to long-lived excited states via a range of fundamentally different types of electronic transitions. Research performed in this area over the past five years demonstrated that a much broader spectrum of metal complexes than what was long considered relevant exhibits useful photophysics and photochemistry.

Interplay between the Conformational Flexibility and Photoluminescent Properties of Mononuclear Pyridinophanecopper(I) Complexes

The macrocyclic ligand conformational behavior in solution, solid-state structures and the photophysical properties of copper(I) cationic and neutral mononuclear complexes supported by tetradentate N, N'-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane ligands ^RN4 (R = H, Me, ⁱBu, ^secBu, ^neoPent, ⁱPr, Ts) were investigated in detail. Steric properties of the alkyl group at the axial amine in the ^RN4 ligand were found to strongly affect the conformational preferences and dynamic behavior in solution. Several types of conformational exchange processes were revealed by variable-temperature NMR and 2D exchange spectroscopy, including degenerative exchange in a pseudotetrahedral species as well as exchange between two isomers with different conformers of tri- and tetracoordinate ^RN4 ligands. These exchange processes are slower for the complexes containing bulky alkyl groups at the amine compared to less sterically demanding analogues. A clear correlation is also observed between the steric bulk of the alkyl substituents and the photoluminescent properties of the derived complexes, with less dynamic complexes bearing bulkier alkyl substituents exhibiting higher absolute photoluminescence quantum yield (PLQY) in solution and the solid state: PLQY in solution increases in the order Me < ^neoPent < ⁱBu < ^secBu ≈ ⁱPr < ^tBu. The electrochemical properties of the cationic complexes [(^RN4)Cu^I(MeCN)]X (X = BF₄, PF₆) were also dependent on the steric properties of the amine substituent.