Synthesis of montmorillonite-viologen intercalation compounds and their photochromic behaviour

The photochemical properties of four different viologen di-cations (V²⁺,1,1′-dipropyl-, 1,1′-diheptyl-, 1,1′-diphenylmethyl- and 1,1′-diphenylethyl-4,4′-bipyridinium di-cations) intercalated into the interlayer space of montmorillonite have been studied. The colour of the intercalation compounds which were co-intercalated by poly(vinyl pyrrolidone) (PVP) changed to blue when irradiated by a mercury lamp, showing the formation of the radical cations. Their characteristic absorptions appeared at ca. 610 and 400 nm in their visible spectra. Since the viologens were present in their cationic forms in the interlayer space it was thought that PVP acted as an electron donor for the photochemical reduction of the viologens. The intensity decay of the radical cations was very slow in comparison with the species in PVP matrices, indicating their remarkable stability.