Vibrational study of the methyl viologen dication MV2+ and radical cation MV˙+ in several salts and as an intercalate in some layered MPS3 compounds

Infrared and Raman spectra (1800–200 cm–1) of methyl viologen (1,1′-dimethyl-4,4′-bipyridyl dication or MV2+)([1H8] and [2H8] derivatives) have been recorded for the chloride, iodide and palladate salts and for the intercalated compounds of formula M0.84 PS3–(MV)0.16(with M = Mn and Cd), FePS3–(MV)0.15 and Zn0.71 PS3–(MV)0.29. Complete assignments have been proposed for all these species. The methyl viologen appears to be intercalated in its dicationic form and is weakly interacting with the host lattice. In the ZnPS3 system the MV2+ pyridyl rings are twisted while in MnPS3, CdPS3, and FePS3 the cation rings are coplanar and parallel to the sulphur layers. From these results, a model of packing of MV2+ in the MPS3 host lattices has been established which accounts for the different intercalation rates. Finally, the reduced product, MV˙+, has been studied in both the chloride salt and the CdPS3 intercalation compound; in the latter case the radical cation behaves as though diluted and trapped inside an inert matrix, demonstrating unexpected stability.