Improved Photochromic Properties on Viologen-Based Inorganic–Organic Hybrids by Using π‑Conjugated Substituents as Electron Donors and Stabilizers

A series of inorganic–organic hybrid compounds
L2(Bi2Cl10) (L = HMV2+ = N-proton-N′-methyl-4,4′-bipyridinium
for 1, L = HBzV2+ = N-proton-N′-benzyl-4,4′-bipyridinium for 2, and L = HPeV2+ = N-proton-N′-phenethyl-4,4′-bipyridinium for 3) have
been successfully synthesized by an in situ solvothermal reaction.
Compounds 1–3, with the same metal
halide as anions but different asymmetric viologen molecules as cations,
are ideal model compounds for investigating the detailed effect of
different photochromically active molecules on the photochromic properties
of the hybrids. Compound 1 shows no photochromic behavior,
but compounds 2 and 3 possess photochromism
and show a faster photoresponse rate than other reported viologen
metal halide hybrids. Studies on the relationship between the structure
and photochromic behavior clearly reveal that π-conjugated substituents
could be used to improve the photoresponsibility and enrich the developed
color efficiently and that the π···π interaction
among organic components may not only be a powerful factor to stabilize
the viologen monocation radical but also act as the second path of
electron transfer from the π-conjugated substituent to the viologen
cation for the photochromic process, which significantly influences
the photochromic properties.