Topological and Steric Constraints to Stabilize Heteroleptic Copper(I) Complexes Combining Phenanthroline Ligands and Phosphines

Abstract

Heteroleptic copper(I) complexes combining phenanthroline derivatives (NN) and chelating bisphosphine ligands (PP) are an important class of luminescent materials for various applications. Although thermodynamically stable, [Cu(NN)(PP)]+ derivatives are also kinetically unstable. As a result, a dynamic ligand‐exchange reaction is often observed in solution, leading to a dynamic mixture of heteroleptic and homoleptic complexes. To prevent the formation of the homoleptic species, macrocyclic phenanthroline ligands have been used for the preparation of [Cu(NN)(PP)]+ pseudorotaxanes. The topological constraint resulting from the macrocyclic structure of the NN ligand drives the thermodynamic equilibrium towards the exclusive formation of the heteroleptic complex as long as the macrocycle is large and flexible enough to allow for the threading of the PP ligand. Conversely, when the threading is prevented by steric constraints, unprecedented copper(I) complexes with a trigonal coordination geometry are obtained. These results are summarized in the present concept article.

Dinuclear Copper Complexes Triply Bridged by a Tetraphosphane, rac‐Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2

“…Therefore, the solid state emission spectra of powdered samples 3–11 were measured to exhibit broad phosphorescence bands centered around 582–608 nm except for 10 and 12 though the reasons are not clear (Figure c, Table ). While the shapes of the emission bands are unsymmetrically broad, suggesting that very complicated and delicate mechanisms already proposed are mixed in the present case, the emission energy falls within a narrow range and may be comparable to those of the [Cu(N ^ N)(P ^ P)] + complexes possessing large P–Cu–P angles, e.g. [Cu 2 (2,9‐Me 2 phen)(dppb) 2 ] 2+ (λ em = 590 nm, P–Cu–P = 126.4°) and [Cu 2 (2,9‐Me 2 ‐4,7‐Ph 2 phen)(dppb) 2 ] 2+ (λ em = 585 nm, P–Cu–P = 119.6°),, where dppb is 1,4‐bis(diphenylphosphanyl)butane.…”

“…Heteroleptic mononuclear Cu I complexes with diphosphane (P ^ P) and diimine (N ^ N) ligands, having a general formula of [Cu(P ^ P)(N ^ N)] + , have extensively been studied from a view point of developing photo‐functional materials by using non‐noble metals, applying for light‐emitting devices, photochemical sensors and solar cells, photosensitized catalytic reactions including water splitting, and bio‐imaging reagents, since they often exhibit intense phosphorescence with long lifetime and high quantum yields from 3 MLCT states and/or thermally activated delayed fluorescence from 1 MLCT states. Furthermore, analogous dicopper systems have recently been developed to increase their efficiency and add aggregation‐induced property .…”

“…From these, the emission could be assigned to rapid fluorescence from 1 MLCT state of loosely coordinated N ^ N ligands or ligand‐centered emission 1 LC perturbed by weak coordination to Cu I center, and not assignable to fluorescence of free uncoordinated phen molecule , . It is well known that the heteroleptic [Cu(N ^ N)(P ^ P)] + complexes often emit intense phosphorescence with µs order long lifetime from 3 MLCT state and/or thermally activated delayed fluorescence (TADF) from 1 MLCT state, since their 1 MLCT and 3 MLCT states are near in energy and thermally equilibrated, and however, the present dicopper complexes 3–12 did not show any phosphorescence in the solution states, which might result from rather incomplete and semi‐chelating fluxional behaviors of N ^ N ligands as suggested by VT NMR studies, preventing intersystem transfer to long‐lived 3 MLCT state from the rigid 1 MLCT state. When the temperature decreased from 293 K to 193 K, the intensity of the fluorescence band of 3 gradually decreased without energy shift, but in contrast, at 77 K, an intense broad emission band appeared around 590 nm, which may arise from phosphorescence from 3 MLCT state (Figure ).…”

Dinuclear Copper(I) Complexes Combining Bis(diphenylphosphanyl)acetylene with 1,10‐Phenanthroline Ligands

“…Such compounds are also intensively used as photosensitizers for various photocatalytic reactions . While stable in the solid state, [Cu(PP)(NN)] + derivatives often undergo ligand exchange in solution and the coordination sphere around the copper(I) cation is not always easy to control . One of the major difficulties is to sufficiently increase steric hindrance without destabilizing the heteroleptic complex in order to prevent ligand distortion in the triplet metal‐to‐ligand charge transfer ( 3 MLCT) excited state , .…”

Copper(I) Phosphinooxazoline Complexes: Impact of the Ligand Substitution and Steric Demand on the Electrochemical and Photophysical Properties

“…With respect to possible applications in solar energy conversion schemes a high photostability in solution is essential. Unfortunately, heteroleptic diimine‐diphosphine Cu I complexes frequently suffer from a limited stability in solution under irradiation or catalytic conditions . Irradiation of the prototype photosensitizer [(P^P)Cu(N^N)]PF 6 (P^P=xantphos and N^N=bathocuproine) in acetonitrile with a solar light source (i.e., a 150 W Xe arc lamp) leads to ligand dissociation and the formation of the respective homoleptic complexes (Figure S14).…”