Solid-phase reactivity of pure methyl viologen upon thermolysis and photolysis: a vibrational and electronic study

Abstract

Two reactions are induced form pure solid methyl viologen dichloride, MV²⁺(Cl^–)₂, upon heating under vacuum: warming over 50 °C yields Cl^–MV⁺˙Cl˙via an intracomplex electron-transfer process; warming to 180 °C yields 4,4′-bipyridine and CH₃Cl, probably via a second intracomplex charge-transfer process. Both these reactions are also shown to occur upon visible excitation of dehydrated MV²⁺(Cl^–)₂. In this case, the MV²⁺→MV⁺˙ reduction does not arise from direct photochemistry but rather results from thermal activation. Similar treatment of the bromide and iodide salts, MV²⁺(Br^–)₂ and MV²⁺(I^–)₂, lead only to the dealkylation products. These reactions have been investigated and followed using i.r., Raman, u.v.-visible, and e.s.r. spectroscopy and their mechanisms are discussed.