Description

Transition metal complexes are known to have very interesting, tuneable electronic properties which are important for developing technologies for applications in photocatalysis, for hydrogen generation and CO2 reduction. These are particularly important for addressing global issues surrounding sustainable energy and pollution reduction. Effective structures for such applications have access to long-lived excited state lifetimes. However, these tend to involve non-abundant, toxic, heavy transition metals like ruthenium, iridium and rhenium, which are not ideal for large-scale use. Among the first-row transition metals, however, Cu(I) possesses a closed shell 3d10 orbital which avoids the undesired d-d transitions. Moreover, challenges with the formation of stable copper complexes because of rapid ligand exchange in solution have been overcome by the “HETPHEN” strategy. Cu(I) diimine complexes possess many interesting characteristics. Firstly, Cu(I) complexes strongly absorb in the UV-visible region and photoinduced charge transfers can occur like metal-to-ligand charge transfers (MLCT) at around 450 nm. Cu(I) adopts a tetrahedral geometry when coordinated with four ligands. Oxidation from Cu(I) to Cu(II) induces a structural geometry change which flattens into a square planar geometry. Typically, this flattening exposes the Cu(II) centre which provides an alternative photophysical pathway, effectively quenching it and reducing the excited state lifetime. So, hindering the flattening of Cu(I) complex by using bulky ligands has been found to improve excited state lifetime. In addition, the change from closed shell d10 to open shell d9 can allow pseudo Jahn-Teller distortion to occur. The extent of which has been found to affect the energy gap between the ground and lowest MLCT excited state.

This work aims to extend the structural and electronic understanding of copper(I) diimine mesityl complexes by exploring the backbone flexibility in the presence of a bulky, electron donating ligand like triphenylamine (TPA). This was done by preparing a series of copper(I) mesityl systems with different combinations of flexible (e.g., bipyridine) and rigid (e.g., phenanthroline) backbones with TPA. Structural and electronic properties of the series were analysed using resonance Raman spectroscopy, absorption spectroscopy, x-ray crystallography, density functional theory, and alongside other analytical tools.