Synthesis and Characterization of Neutral Luminescent Diphosphine Pyrrole- and Indole-Aldimine Copper(I) Complexes

Syntheses, characterization, crystal structures and applications as sensitizers in solar cells of novel heteroleptic Cu(I) complexes containing nitrile-substituted 2,2'-bipyridyl ligands

Two novel Cu(I) tetradentate heteroleptic complexes, including nitrile-substituted bipyridines that can be anchored to semiconductor surfaces to be assembled in DSSCs, were synthesized and characterized by spectroscopic and electrochemical techniques. The crystal structures of both species were determined by X-ray diffraction. Results from DFT and TD-DFT calculations were found to be consistent with the experimental data. Emission at room temperature was observed for both complexes in the solid state, making them promising alternatives for the development of light-emitting diodes. We report for the first time the experimental evidence of photovoltaic conversion devices formed by Cu(I) complexes anchored to a TiO2 surface by means of nitrile groups present in substituted bipyridines, and subsequently tested as sensitizers for DSSCs, obtaining efficiency values for light to electrical energy conversion similar to those previously reported for analogous complexes with anchoring carboxylic groups.

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.

Comprehensive Picture of the Excited State Dynamics of Cu(I)- and Ru(II)-Based Photosensitizers with Long-Lived Triplet States

Ru(II)- and Cu(I)-based photosensitizers featuring the recently developed biipo ligand (16H-benzo-[4',5']-isoquinolino-[2',1',:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one) were comprehensively investigated by X-ray crystallography, electrochemistry, and especially several time-resolved spectroscopic methods covering all time scales from femto- to milliseconds. The analysis of the experimental results is supported by density functional theory (DFT) calculations. The biipo ligand consists of a coordinating 1,10-phenanthroline moiety fused with a 1,8-naphthalimide unit, which results in an extended π-system with an incorporated electron acceptor moiety. In a previous study, it was shown that this ligand enabled a Ru(II) complex that is an efficient singlet oxygen producer and of potential use for other light-driven applications due to its long emission lifetime. The goal of our here presented research is to provide a full spectroscopic picture of the processes that follow optical excitation. Interestingly, the Ru(II) and Cu(I) complexes differ in their characteristics even though the lowest electronically excited states involve in both cases the biipo ligand. The combined spectroscopic results indicate that an emissive ³MLCT state and a rather dark ³LC state are populated, each to some extent. For the Cu(I) complex, most of the excited population ends up in the ³LC state with an extraordinary lifetime of 439 μs in the solid state at 20 K, while a significant population of the ³MLCT state causes luminescence for the Ru(II) complex. Hence, there is a balance between these two states, which can be tuned by altering the metal center or even by thermal energy, as suggested by the temperature-dependent experiments.

Closo- or Nido-Carborane Diphosphane as Responsible for Strong Thermochromism or Time Activated Delayed Fluorescence (TADF) in [Cu(N^N)(P^P)]0/

Ortho-closo or ortho-nido-carborane-diphosphanes have been selected to prepare the heteroleptic cationic or neutral [Cu(N^N){(PPh2)2C2B10H10}]PF6 (1) and [Cu(N^N){(PPh2)2C2B9H10}] (2) [N^N = 2-(4-thiazolyl)benzimidazole], respectively. Complexes 1 and 2 display very different emissive behavior. Neutral complex 2 exhibits TADF (time activated delayed fluorescence) which has been studied both as powder and PMMA composite with similar ΔE(S1 - T1), τ(T1), and τ(S1) in both phases. Cationic complex 1 displays a much lower quantum yield than 2 and does not show TADF, but it exhibits a significant thermochromic luminescence, and its emission is very dependent on the medium. Theoretical studies show that metal-ligand (M-diphosphane) to ligand (L', diimine) transitions, MLL'CT, are responsible of the transitions which originate the emissive properties, but with very different contribution of the copper center, carborane cluster, and diphosphane phenyl rings for 1 and 2.

Titanium complexes of pyrrolylaldiminate ligands and their exploitation for the ring-opening polymerization of cyclic esters

A series of six-coordinate titanium complexes 1-6 supported by pyrrolylaldiminate ligands were prepared via the reaction of 2 equivalents of ligands and Ti(OⁱPr)₄ in toluene at 70 °C. The X-ray structure of 2 revealed that the two ligands were κ²-coordinated to the titanium center with the two pyrrole nitrogen atoms in trans positions and the two imine nitrogen atoms in cis positions. All complexes were active initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), ε-caprolactone (ε-CL), and three substituted ε-caprolactones (γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL)). Polymerizations of all monomers were well controlled, affording predetermined molar masses and narrow dispersity values. Complex 5 exhibited the highest polymerization activities with rac-LA and ε-CL and its performance was comparable to other highly active six-coordinate titanium complexes reported thus far. Kinetic results revealed a first-order dependency on the monomer concentration, and the rate of polymerization was greatly influenced by the substituent on the imine nitrogen. End-group analysis of the isolated PLA and PCL suggested a coordination-insertion mechanism.

A new heteroleptic phosphorescent cuprous complex supported by a BINAP ligand: synthesis, structure, luminescence properties and theoretical analyses

Luminescent cuprous complexes are an important class of coordination compounds due to their relative abundance, low cost and ability to display excellent luminescence. The heteroleptic cuprous complex solvate rac-(acetonitrile-κN)(3-aminopyridine-κN)[2,2'-bis(diphenylphosphanyl)-1,1'-binaphthyl-κ²P,P']copper(I) hexafluoridophosphate dichloromethane monosolvate, [Cu(C₅H₆N₂)(C₂H₃N)(C₄₄H₃₂P₂)]PF₆·CH₂Cl₂, conventionally abbreviated as [Cu(3-PyNH₂)(CH₃CN)(BINAP)]PF₆·CH₂Cl₂, (I), where BINAP and 3-PyNH₂ represent 2,2'-bis(diphenylphosphanyl)-1,1'-binaphthyl and 3-aminopyridine, respectively, is described. In this complex solvate, the asymmetric unit consists of a cocrystallized dichloromethane molecule, a hexafluoridophosphate anion and a complete racemic heteroleptic cuprous complex cation in which the cuprous centre, in a tetrahedral CuP₂N₂ coordination, is coordinated by two P atoms from the BINAP ligand, one N atom from the 3-PyNH₂ ligand and another N atom from a coordinated acetonitrile molecule. The UV-Vis absorption and photoluminescence properties of this heteroleptic cuprous complex have been studied on polycrystalline powder samples, which had been verified by powder X-ray diffraction before recording the spectra. Time-dependent density functional theory (TD-DFT) calculations and a wavefunction analysis reveal that the orange-yellow phosphorescence emission should originate from intra-ligand (BINAP) charge transfer mixed with a little of the metal-to-ligand charge transfer ³(IL+ML)CT excited state.

Synthesis and Characterization of Three New Emissive Mononuclear CuI Heteroleptic Complexes with Functionalized 6-Cyano-2,2′-bipyridine Chelating Ligands

A new series of luminescent mononuclear CuI complexes with functionalized 6-cyano-2,2′-bipyridine chelating ligands, [Cu(xantphos)(cbpy)]ClO4 (1), [Cu(xantphos)(4,4’-Me2cbpy)]ClO4·CH2Cl2·H2O (2), and [Cu(POP)(cbpy)]ClO4·CH2Cl2 (3) (xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; POP=bis(2-diphenylphosphinophenyl)ether; 4,4’-Me2cbpy=4,4’-dimethyl-6-cyano-2,2′-bipyridine; cbpy=6-cyano-2,2′-bipyridine) have been successfully prepared, and their structures and photophysical properties are investigated. Single crystal structures of the three complexes reveal a distorted tetrahedral coordination geometry around the CuI centres with the P atoms of diphosphane ligands and N donors of 2,2′-bipyridine ring. Luminescence measurements indicate that these CuI complexes display good emission properties both in the solution and solid states at room temperature, which can be well modulated through modifying the structure of 6-cyano-2,2′-bipyridine. It is shown that the introduction of two electron-donating methyl groups at the 4,4’-positions of the 6-cyano-2,2′-bipyridine is favourable to enhance the luminescence properties of the CuI complexes.

Boron complexes of aromatic 5-substituted iminopyrrolyl ligands: synthesis, structure, and luminescence properties

A group of new mononuclear boron chelate compounds [BPh₂{κ²N,N'-5-R-NC₄H₂-2-C(H)[double bond, length as m-dash]N-Ar}] (R = Ar = C₆H₅7; R = C₆H₅, Ar = 2,6-iPr₂C₆H₃8; R = Anthracen-9-yl (Anthr), Ar = C₆H₅9; R = Anthr, Ar = 2,6-iPr₂C₆H₃10) were synthesized via the reaction of B(C₆H₅)₃ with the corresponding 5-substituted 2-(N-arylformimino)pyrrole ligand precursors 3-6. These complexes were prepared in order to evaluate the luminescence potential derived from the substitution of the position 5 of the pyrrolyl ring with an aromatic group. Compounds 7-10 were photophysically characterized in solution and in the solid state. The 5-phenyl-2-iminopyrrolyl-BPh₂ complexes 7 and 8 are blue emitters and have enhanced photoluminescence quantum yields in the solid state (Φ_PL) up to 0.95, whereas the 5-anthracenyl derivatives 9 and 10 have green-bluish fluorescence and a Φ_PL of 0.49 and 0.24, respectively. DFT and TDDFT studies were performed, considering the effect of solvent and dispersion, in order to show how the geometries of compounds 7-10 changed from the ground to the excited state, to assign electronic transitions, and to rationalize the observed luminescence. These materials were applied in organic light-emitting diodes (OLEDs), with various device structures, the best showing an external quantum efficiency of 2.75% together with a high luminance of 23 530 cd m^-2.

Syntheses and photoluminescence of copper(i) halide complexes containing dimethylthiophene bidentate phosphine ligands

Neutral dinuclear four-coordinate Cu(i) halide complexes containing thiophene exhibit intense bluish green to yellow green emission in the solid state at room temperature.

Heterobimetallic copper(i) complexes bearing both 1,1′-bis(diphenylphosphino)ferrocene and functionalized 3-(2′-pyridyl)-1,2,4-triazole

Copper(i) 1,1′-bis(diphenylphosphino)ferrocene complexes are well regulated by functional 3-(2′-pyridyl)-1,2,4-triazole exhibiting mono-anionic η²(N1,N2) and neutral η²(N1,N2) and η²(N1,N4) chelating modes.

Variable oxidation state sulfur-bridged bithiazole ligands tune the electronic properties of ruthenium(ii) and copper(i) complexes

The synthesis of homoleptic and heteroleptic ruthenium(ii) and copper(i) complexes containing sulfur-bridged bithiazole ligands of varying oxidation states are reported.

Near-saturated red emitters: four-coordinate copper(i) halide complexes containing 8-(diphenylphosphino)quinoline and 1-(diphenylphosphino)naphthalene ligands

Cu(i) complexes exhibit near-saturated red emission.

Synthesis, structures and luminescence properties of amine-bis(N-heterocyclic carbene) copper(i) and silver(i) complexes

Mononuclear Ag(i), Cu(i) and heterometallic Cu(i)/Ag(i) complexes with the tridentate amine-bis(N-heterocyclic carbene) were prepared, among which Cu(i)- and Cu/Ag complexes show luminescence properties.

A new phosphorescent heteroleptic cuprous complex with a neutral 2-methylquinolin-8-ol ligand: synthesis, structure characterization, properties and TD-DFT calculations

Luminescent Cu^I complexes have emerged as promising substitutes for phosphorescent emitters based on Ir, Pt and Os due to their abundance and low cost. The title heteroleptic cuprous complex, [9,9-dimethyl-4,5-bis(diphenylphosphanyl)-9H-xanthene-κ²P,P](2-methylquinolin-8-ol-κ²N,O)copper(I) hexafluorophosphate, [Cu(C₁₀H₉NO)(C₃₉H₃₂OP₂)]PF₆, conventionally abbreviated as [Cu(Xantphos)(8-HOXQ)]PF₆, where Xantphos is the chelating diphosphine ligand 9,9-dimethyl-4,5-bis(diphenylphosphanyl)-9H-xanthene and 8-HOXQ is the N,O-chelating ligand 2-methylquinolin-8-ol that remains protonated at the hydroxy O atom, is described. In this complex, the asymmetric unit consists of a hexafluorophosphate anion and a whole mononuclear cation, where the Cu^I atom is coordinated by two P atoms from the Xantphos ligand and by the N and O atoms from the 8-HOXQ ligand, giving rise to a tetrahedral CuP₂NO coordination geometry. The electronic absorption and photoluminescence properties of this complex have been studied on as-synthesized samples, whose purity had been determined by powder X-ray diffraction. In the detailed TD-DFT (time-dependent density functional theory) studies, the yellow emission appears to be derived from the inter-ligand charge transfer and metal-to-ligand charge transfer (M+L')→LCT excited state (LCT is ligand charge transfer).

Influences of the protonic state of an imidazole-phenanthroline ligand on the luminescence properties of copper(i) complexes: experimental and theoretical research

Deprotonation of an imidazole–phenanthroline ligand leads to entirely different luminescence properties of neutral Cu(i) complexes compared with ionic Cu(i) complexes.

Boron complexes of aromatic ring fused iminopyrrolyl ligands: synthesis, structure, and luminescence properties

New fluorescent aromatic ring-fused 2-iminopyrrolyl diphenyl boron complexes are emitters in the range blue to orange.

Luminescent dinuclear copper(i) complexes bearing 1,4-bis(diphenylphosphino)butane and functionalized 3-(2′-pyridyl)pyrazole mixed ligands

The luminescence properties of 3-(2′-pyridyl)pyrazole-based dinuclear Cu(i) complexes can be well modulated by altering the substituent on the pyrazolyl ring.

Synthesis of new phosphorescent imidoyl-indazol and phosphine mixed ligand Cu(i) complexes – structural characterization and photophysical properties

New Cu(i) complexes were prepared, [Cu(N,N)₂]⁺ (1) and [Cu(N,N)(P,P)]⁺ (2–4), and characterized by spectroscopic methods and DFT calculations. It highlights the influence of the different ligands on the photophysical properties of the complexes.

Neutral copper(i) complexes featuring phosphinesulfonate chelates

The reaction of diphenylphosphinobenzenesulfonic acid with copper(i) oxide resulted in the formation of the new neutral dimeric copper(i) complex {Cu₂(DPPBS)₂·(MeOH)₂}.

Alkynyl triphosphine copper complexes: synthesis and photophysical studies

A chelating triphosphine was used to synthesize luminescent mono-, di- and trinuclear copper(i) alkynyl complexes, the photophysical properties of which are determined by the nature of alkynyl groups.

Highly emissive copper(i) complexes bearing diimine and bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane

Highly emissive copper(i) complexes bearing 4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane and 1,10-phenanthroline derivatives were newly synthesized. The quantum yields of the photoluminescence are up to 0.38.