Triphenylamine-substituted 2-pyridyl-1,2,3-triazole copper(I) complexes: an experimental and computational investigation

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.

The surprising effects of sulfur: achieving long excited-state lifetimes in heteroleptic copper(i) emitters

A series of heteroleptic [Cu(N^N)(P^P)][PF₆] complexes is reported in which N^N is a di(methylsulfanyl)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeS)₂phen) or di(methoxy)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeO)₂phen) and P^P is bis(2-(diphenylphosphano)phenyl)ether (POP) or 4,5-bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos). The effects of the different substituents are investigated through structural, electrochemical and photophysical studies and by using DFT and TD-DFT calculations. Introducing methylsulfanyl groups in the 2,9-, 3,8- or 4,7-positions of the phen domain alters the composition of the frontier molecular orbitals of the [Cu(N^N)(P^P)]⁺ complexes significantly, so that ligand-centred (LC) transitions become photophysically relevant with respect to metal-to-ligand charge transfer (MLCT). Within this series, [Cu(2,9-(MeS)₂phen)(POP)][PF₆] exhibits the highest photoluminescence quantum yield of 15% and the longest excited-state lifetime of 8.3 μs in solution. In the solid state and in frozen matrices at 77 K, the electronic effects of the methylsulfanyl or methoxy substituents are highlighted, thus resulting in luminescence lifetimes of 2 to 4.2 ms at 77 K with predominantly LC character for both the 3,8- and 4,7-(MeS)₂phen containing complexes. The results of the investigation give new guidelines on how to influence the luminescence properties in [Cu(N^N)(P^P)]⁺ complexes which will aid in the development of new sustainable and efficient copper(i) emitters.

Can 2-Pyridyl-1,2,3-triazole "Click" Ligands be Used to Develop Cu(I)/Cu(II) Molecular Switches?

Molecular switching processes are important in a range of areas including the development of molecular machines. While there are numerous organic switching systems available, there are far less examples that exploit inorganic materials. The most common inorganic switching system remains the copper(I)/copper(II) switch developed by Sauvage and co-workers over 20 years ago. Herein, we examine if bidentate 2-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pytri) and tridentate 2,6-bis[(4-phenyl-1H-1,2,3-triazol-1-yl)methyl]pyridine (tripy) moieties can be used to replace the more commonly exploited polypyridyl ligands 2,2'-bypyridine (bpy)/1,10-phenanthroline (phen) and 2,2';6',2″-terpyridine (terpy) in a copper(I)/(II) switching system. Two new ditopic ligands that feature bidentate (pytri, L1 or bpytri, L2) and tridentate tripy metal binding pockets were synthesized and used to generate a family of heteroleptic copper(I) and copper(II) 6,6'-dimesityl-2,2'-bipyridine (diMesbpy) complexes. Additionally, we synthesized a series of model copper(I) and copper(II) diMesbpy complexes. A combination of techniques including nuclear magnetic resonance (NMR) and UV-vis spectroscopies, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography was used to examine the behavior of the compounds. It was found that L1 and L2 formed [(diMesbpy)Cu(L1 or L2)]²⁺ complexes where the copper(II) diMesbpy unit was coordinated exclusively in the tridenate tripy binding site. However, when the ligands (L1 and L2) were complexed with copper(I) diMesbpy units, a complex mixture was obtained. NMR and MS data indicated that a 1:1 stoichiometry of [Cu(diMesbpy)]⁺ and either L1 or L2 generated three complexes in solution, the dimetallic [(diMesbpy)₂Cu₂(L1 or L2)]²⁺ and the monometallic [(diMesbpy)Cu(L1 or L2)]⁺ isomers where the [Cu(diMesbpy)]⁺ unit is coordinated to either the bidentate or tridentate tripy binding sites of the ditopic ligands. The dimetallic [(diMesbpy)₂Cu₂(L1 or L2)](PF₆)₂ complexes were structurally characterized using X-ray crystallography. Both complexes feature a [Cu(diMesbpy)]⁺ coordinated to the bidentate (pytri or bpytri) pocket of the ditopic ligands (L1 or L2), as expected. They also feature a second [Cu(diMesbpy)]⁺ coordinated to the nominally tridentate tripy binding site in a four-coordinate hypodentate κ²-fashion. Competition experiments with model complexes showed that the binding strength of the bidentate pytri is similar to that of the κ²-tripy ligand, leading to the lack of selectivity. The results suggest that the pytri/tripy and bpytri/tripy ligand pairs cannot be used as replacements for the more common bpy/phen-terpy partners due to the lack of selectivity in the copper(I) state.

Time-Resolved Spectroscopy and Electronic Structure of Mono-and Dinuclear Pyridyl-Triazole/DPEPhos-Based Cu(I) Complexes

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ (CuL, CuLH) and their dinuclear analogues (Cu2 L', Cu2 L'H2 ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu2 L' and Cu2 L'H2 . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices.

6,6'-Ditriphenylamine-2,2'-bipyridine: Coordination Chemistry and Electrochemical and Photophysical Properties

A 2,2'-bipyridine with bulky triphenylamine substituents in the 6 and 6' positions of the ligand (6,6'-ditriphenylamine-2,2'-bipyridine, 6,6'-diTPAbpy) was generated. Despite the steric bulk, the ligand readily formed bis(homoleptic) complexes with copper(I) and silver(I) ions. Unfortunately, efforts to use the 6,6'-diTPAbpy system to generate heteroleptic [Cu(6,6'-diTPAbpy)(bpy)]⁺ complexes were unsuccessful with only the [Cu(6,6'-diTPAbpy)₂](PF₆) complex observed. The 6,6'-diTPAbpy ligand could also be reacted with 6-coordinate metal ions that featured small ancillary ligands, namely, the [Re(CO)₃Cl] and [Ru(CO)₂Cl₂] fragments. While the complexes could be formed in good yields, the steric bulk of the TPA units does alter the coordination geometry. This is most readily seen in the [(6,6'-diTPAbpy)Re(CO)₃Cl] complex where the Re(I) ion is forced to sit 23° out of the plane formed by the bpy unit. The electrochemical and photophysical properties of the family of compounds were also examined. 6,6'-diTPAbpy exhibits a strong ILCT absorption band (356 nm, 50 mM^-1 cm^-1) which displays a small increase in intensity for the homoleptic complexes ([Cu(6,6'-diTPAbpy)₂]⁺; 353 nm, 72 mM^-1 cm^-1, [Ag(6,6'-diTPAbpy)₂]⁺; 353 nm, 75 mM^-1 cm^-1), despite containing 2 equiv of the ligand, attributed to an increased dihedral angle between the TPA and bpy moieties. For the 6-coordinate complexes the ILCT band is further decreased in intensity and overlaps with MLCT bands, consistent with a further increased TPA-bpy dihedral angle. Emission from the ¹ILCT state is observed at 436 nm (τ = 4.4 ns) for 6,6'-diTPAbpy and does not shift for the Cu, Ag, and Re complexes, although an additional ³MLCT emission is observed for [Re(6,6'-diTPAbpy)(CO)₃Cl] (640 nm, τ = 13.8 ns). No emission was observed for [Ru(6,6'-diTPAbpy)(CO)₂Cl₂]. Transient absorption measurements revealed the population of a ³ILCT state for the Cu and Ag complexes (τ = 80 ns). All assignments were supported by TD-DFT calculations and resonance Raman spectroscopic measurements.

Excited-State Switching in Rhenium(I) Bipyridyl Complexes with Donor-Donor and Donor-Acceptor Substituents

The optical properties of two Re(CO)₃(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA₂) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA₂ the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH₂Cl₂ to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin-orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states.

Insights into the charge-transfer character of electronic transitions in RCp2Ti(C2Fc)2 complexes using solvatochromism, resonance Raman spectroscopy, and TDDFT

A series of complexes with low-energy FeII to TiIV metal-to-metal charge-transfer (MMCT) transitions, Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and MeOOCCp2Ti(C2Fc)2, was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters - solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) - was performed. The effect of SP indicates a greater degree of electron delocalization in the excited state (ES) than the ground state (GS). The small negative solvatochromism with respect to SdP indicates a smaller dipole moment in the ES than the GS. The effect of SB is consistent with charge-transfer to Ti. Upon excitation into the MMCT absorption band, the RRS data show enhancement of the alkyne stretching modes and of the out-of-plane bending modes of the cyclopentadienyl ring connected to Fe and the alkyne bridge. This is consistent with changes in the oxidation states of Ti and Fe, respectively. The higher-energy transitions (350-450 nm) show enhancement of vibrational modes consistent with ethnylcyclopentadienyl to Ti ligand-to-metal charge transfer (LMCT). The RRS data is consistent with the TDDFT predicted character of these transitions. TDDFT suggests that the lowest-energy transition in Cp2Ti(C2Fc)2CuI, where CuI is coordinated between the alkynes, retains its FeII to TiIV MMCT character, in agreement with the RRS data, but that the lowest-energy transitions have significant CuI to Ti character. For Cp2Ti(C2Fc)2CuI, excitation into the low-energy MMCT absorption band results in selective enhancement of the symmetric alkynyl stretching mode.