Impact of Lattice Water on Solid-State Electron Transfer in Viologen Pseudopolymorphs: Modulation of Photo- and Piezochromism

Stimuli-induced
solid-state electron transfer (ET) underlies the
use of viologen compounds as responsive materials, but unequivocal
structure–property correlations for solid-state ET are still
lacking. With different pseudopolymorphic solids derived from N,N′-bis­(4-carboxylphenyl)­viologen
([H2bcpV]2+), here we report a systematic study
on photo- and piezochromic properties associated with ET. We show
that the higher the water content in the lattice, the less sensitively
the compounds respond to light and pressure. It is proposed that the
lattice water does not act as an electron donor but serves to change
the ET energetics through its unique polarity and hydrogen bonding
capability. The impedimental impact of water on solid-state ET of
viologen compounds has not yet been recognized and elucidated prior
to this work. The study also suggests that pressure is more powerful
than light in inducing ET.