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 PtL(n)SR 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(1)SR where HL(1) = 1,3-bis(2-pyridyl)benzene, and those of PtL(2)SR, where HL(2) = 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 PtL(n)Cl 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(n)Cl. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of PtL(n)Cl 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*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(n)Cl 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.