Bambusuril as a One-Electron Donor for Photoinduced Electron Transfer to Methyl Viologen in Mixed Crystals

Abstract

Methyl viologen hexafluorophosphate (MV²⁺·2PF₆^-) and dodecamethylbambus[6]uril (BU6) form crystals in which the layers of viologen dications alternate with those of a 1:2 supramolecular complex of BU6 and PF₆^-. This arrangement allows for a one-electron reduction of MV²⁺ ions upon UV irradiation to form MV^+• radical cations within the crystal structure with half-lives of several hours in air. The mechanism of this photoinduced electron transfer in the solid state and the origin of the long-lived charge-separated state were studied by steady-state and transient spectroscopies, cyclic voltammetry, and electron paramagnetic resonance spectroscopy. Our experiments are supported by quantum-chemical calculations showing that BU6 acts as a reductant. In addition, analogous photochemical behavior is also demonstrated on other MV²⁺/BU6 crystals containing either BF₄^- or Br^- counterions.