Effects of Different Counter Anions on Solid-State Electron Transfer in Viologen Compounds: Modulation of Color and Piezo- and Photochromic Properties

Charge transfer (CT) and electron transfer (ET) underlie the various applications of viologen compounds. However, the CT and ET in solids are still insufficiently understood and difficult to control, which is challenging but fundamental to the design of multifunctional materials. Our strategy is to attempt a delicate control of CT and ET by subjecting model compounds to pressure. In this work, a series of viologens of the same cation with different anions show anion-dependent color and piezo-/photochromism due to ground-state CT and stimuli-induced ET; the ease of solid-state ET can be tuned in the order of Cl– > Br– > I–/BF4–/PF6–/ClO4–. With in situ high pressure UV–vis experiments, we also observed unexpectedly the pressure-induced solid-state ET in [H2bcpV]·I2 besides the pressure modulation of the CT absorption. It proves pressure is a more powerful stimulus than light in tuning solid-state CT and ET.