Platinum and palladium complexes of tridentate −C^N^N (phen-ide)-pyridine-thiazol ligands – A case study involving spectroelectrochemistry, photoluminescence spectroscopy and TD-DFT calculations

Abstract

Four Pd(II) and Pt(II) complexes [M(C^N^N)Cl] (HC^N^N = 2-(6-phenylpyridin-2-yl)thiazoles) were synthesised, analysed and characterised using 1H NMR and MS in solution, as well as single crystal XRD in the solid. Cyclic voltammetry of the square planar complexes showed reversible or partially reversible reductions and irreversible oxidations. DFT calculations allowed assigning them to essentially metal-centred oxidations and ligand-centred reductions. Absorption spectra of the complexes show intense absorption bands into π-π* states in the UV to visible spectral range and long-wavelength bands which were assigned to transitions into mixed metal-to-ligand charge transfer (MLCT)/π-π* states, based on TD-DFT calculations. Comparison of Pt and Pd derivatives showed that the energy of the (MLCT)/π-π* bands are increased for Pd over Pt. This was also observed for the phosphorescence at 77 K and is attributed to the higher oxidation potential for Pd and supported by spectroelectrochemical measurements. The photoluminescence quantum yield (ΦL) drops drastically from Pt to Pd at room temperature, where only the two Pt(II) complexes are luminescent showing a broad unstructured phosphorescence from a 3MLCT state. At 77 K, the phosphorescence is blue-shifted and shows a clear vibrational progression, which is related to an enhanced ligand-centred character due to the lack of solvent stabilisation in the frozen matrix that otherwise increases the MLCT contribution. The Pd(II) complexes are not emissive at 298 K, but luminesce at 77 K. This is due to metal-centred dissociative d-d* states that facilitate radiationless deactivation, which cannot be thermally populated at low temperatures. Thus, similar ΦL are observed in frozen glassy matrices for both metals. TD-DFT calculations provided insight into the excited states and showed that the substitution pattern does not affect the emission, due to the lack of participation of the phenyl unit in the orbitals that are relevant for the description of the emissive state.