σ-Complexation as a Strategy for Designing Copper-based Light Emitting Materials
To develop highly efficient emitting materials that takes advantage of earth abundant metals, a weak coordinating σ-SiH group was introduced into a trigonal copper (I) center as a key point in a recent synthetic strategy by researchers at KAIST. By lifting the degeneracy (electrons with same energy) of relevant Cu d-based orbitals, geometry relaxation of a copper emitter in metal-to-ligand charge transfer (MLCT) excited states can be minimized. Emissive copper σ-complexes, (SiHP2)Cu(Cbz) and its derivatives have been developed and exhibit a blue/green emission band at λmax = 474–513 nm with high quantum efficiencies (0.40–0.59) and long lifetimes (15–23 μs). This emissive behavior contrasts with tetrahedral copper analogues, (PPMeP)Cu(Cbz), which do not display blue emission due to a significant Jahn-Teller distortion after the MLCT transition from nearly degenerate Cu d-based orbitals.
In Nature, blue copper proteins are a unique example showing minimal geometry variations between the Cu(I) and Cu(II) states. It is possible to minimize Jahn-Teller distortions by utilizing an unusually weak axial thioether donor in its copper center, resulting in a strong equatorial Cu-S interaction. Due to its distorted coordination geometry, the Cu d-based orbitals are further split in different energy levels; thus, a copper d9 Jahn-Teller distortion is minimized, revealing highly efficient electron transfer. Inspired by blue copper proteins, the Lee laboratory has designed and synthesized 4-coordinate copper complexes with a significantly weak axial ligand to prepare copper-based photoluminescent materials with high quantum efficiency.
As a synthetically amenable moiety to provide a weak interaction, one can introduce an σ-interaction of a Si-H bond to a copper center. The Bourissou group recently reported a σ-SiH copper complexes utilizing a diphosphine-hydrosilane ligand, [Ph2P(ο-C6H4)SiH(Me)(ο-C6H4)PPh2], in which σ-SiH copper coordination was realized. However, photophysical properties of such σ-SiH complexes of copper have not yet been investigated.
The new type of copper complex prepared by the Lee group possessess a σ-SiH-Cu moiety supported by a diphosphine-hydrosilane ligand (SiHP2, (2-iPr2PC6H4)2SiHMe) introduced as a new copper(I) synthon. A series of copper-carbazolide complexes, (SiHP2)Cu(Cbz) and its derivatives, were synthesized and fully characterized. Their photophysical properties were also studied in detail. The copper complexes exhibit bright blue/green emission at λmax = 474–513 nm with high quantum yields ( = 0.40–0.59) and long excited state lifetimes (15–23 μs). Density functional theory (DFT) computations were also performed and suggest that upon MLCT excitation, (SiHP2)Cu(Cbz) would undergo a minimal geometric relaxation; its tetrahedral analogue without a SiH bond, (PPMeP)Cu(Cbz) would involve a significant elongation of the Cu-P bond, and because of this, show no blue emission.