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

摘要

A new family of platinum(II) complexes of the form PtL~nSR have been prepared, where L~n represents a cyclometalating, N~∧C~∧N-bound tridentate ligand and SR is a monodentate thiolate ligand. The complexes fall into two groups, those of PtL~1SR where HL~1 = 1,3-bis(2-pyridyl)- benzene, and those of PtL~2SR, where HL~2 = methyl 3,5-bis(2- pyridyl)benzoate. Each group consists of five complexes, where R = CH_3, C_6H_5, p-C_6H_4-CH_3, p-C_6H_4-OMe, p-C_6H_4-NO_2. These compounds, which are bright red, orange, or yellow solids, are formed readily upon treatment of PtL~nCl 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 PtL~nCl. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of PtL~nCl in the green region are replaced by a broad, structureless emission band in the red region. These new bands are assigned to a π_S/d_(Pt) → π_(N~∧C~∧N)~* charge-transfer transition from the thiolate/platinum to the N~∧C~∧N 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 PtL~nCl 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.