Spectroscopy and photoredox properties of soluble platinum(II) alkynyl complexes

Six complexes of platinum(II) with a terdentate π-acceptor ligand, 2,6-(N-(n-hexyl)benzimidazol-2′-yl)pyridine), and different ethynylbenzene ligands were synthesized and investigated by means of optical absorption, luminescence, and time-resolved emission spectroscopy. These complexes display similar photophysical and electrochemical properties as previously investigated analogs with the 2,2′:6′,2″-terpyridine ligand. The energy of the luminescence band maximum is a function of the nature of the chemical substituents attached to the ethynylbenzene ligand, luminescence intensities and lifetimes correlate with the luminescence wavelength according to the energy-gap law. The emissive excited states of some of these complexes are quenched reductively with efficiencies near the diffusion-controlled limit, even for moderate electron donors such as phenothiazine or triphenylamine. A complex with a dimethylamine substituent attached to the ethynylbenzene ligand exhibits photophysical properties that are strongly dependent on the protonation state of the amine. A dimer complex with a diethynyl-substituted xanthene bridging ligand displays absorption and emission behavior that is essentially identical to that of some of the monomeric platinum complexes investigated in this work. Short Pt(II)–Pt(II) contacts are only observed in the crystal structure of a precursor complex. A key feature of the new complexes is their good solubility in common organic solvents, thanks to the presence of two hexyl chains that are attached to the terdentate ligands.