Platinum(II) Complexes of N^C^N‑Coordinating 1,3-Bis(2-pyridyl)benzene Ligands: Thiolate Coligands Lead to Strong Red Luminescence from Charge-Transfer States

Abstract

A new family of platinum(II) complexes of the form PtLnSR have been prepared, where Ln represents a cyclometalating, NCN-bound tridentate ligand and SR is a monodentate thiolate ligand. The complexes fall into two groups, those of PtL1SR where HL1 = 1,3-bis(2-pyridyl)benzene, and those of PtL2SR, where HL2 = methyl 3,5-bis(2-pyridyl)benzoate. Each group consists of five complexes, where R = CH3, C6H5, p-C6H4-CH3, p-C6H4-OMe, p-C6H4-NO2. These compounds, which are bright red, orange, or yellow solids, are formed readily upon treatment of PtLnCl with the corresponding potassium thiolate KSR in solution at room temperature. The replacement of the chloride by the thiolate ligand is accompanied by profound changes in the photophysical properties. A broad, structureless, low-energy band appears in the absorption spectra, not present in the spectra of PtLnCl. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of PtLnCl in the green region are replaced by a broad, structureless emission band in the red region. These new bands are assigned to a πS/dPt → π*NCN charge-transfer transition from the thiolate/platinum to the NCN ligand. This assignment is supported by electrochemical data and TD-DFT calculations and by the observation that the decreasing energies of the bands correlate with the electron-donating ability of the substituent, as do the increasing nonradiative decay rate constants, in line with the energy-gap law. However, the pair of nitro-substituted complexes do not fit the trends. Their properties, including much longer luminescence lifetimes, indicate that the lowest-energy excited state is localized predominantly on the arenethiolate ligand for these two complexes. Red-emitting thiolate adducts may be relevant to the use of PtLnCl complexes in bioimaging, as revealed by the different distributions of emission intensity within live fibroplast cells doped with the parent complex, according to the region of the spectrum examined.