Tridentate Complexes of Palladium(II) and Platinum(II) Bearing bis-Aryloxide Triazole Ligands: A Joint Experimental and Theoretical Investigation.

A novel class of palladium(II) and platinum(II) complexes bearing tridentate bis-aryloxide triazole ligands was prepared by using straightforward and high-yielding synthetic routes. The complexes were fully characterized and the molecular structures of four derivatives were unambigously determined by single-crystal X-ray diffractometric analyses. For the most promising luminescent Pt(II) derivatives, further experimental investigations were carried out to characterize their photophysical features and to ascertain the nature of the emitting excited state by means of electronic absorption, steady-state, and time-resolved emission techniques in different conditions. In degassed fluid solution the complexes displayed broad and featureless photoluminescence with λ(em) =522-585 nm, excited-state lifetime up to few microseconds and quantum yield (PLQY) up to 17%, depending on the nature of both ancillary ligand and substituent on the tridentate ligand. Computational investigation using density functional theory and time-dependent DFT were performed to gain insight into the electronic processes responsible for optical transitions and structure-photoluminescence relationship. Jointly, experimental and theoretical characterization indicated that the radiative transition arises from an excited state with admixed triplet-manifold metal-to-ligand charge transfer and ligand-centered ((3)MLCT/(3)LC) character. We elucidated the modulation of the photophysical properties upon variation of substituents for this new family of complexes.