Alkyl Chain Length-Selective Vapor-Induced Fluorochromism of Pillar[5]arene-Based Nonporous Adaptive Crystals

Developing new vapochromic fluorescent
sensing materials with selectivity
is in great demand due to serious air pollution caused by volatile
organic compounds. Here we report the first nonporous adaptive crystals
with vapor-induced fluorochromism. The emission of anthracene conjugated
pillar[5]­arene P5en crystals blue shifts from bright
yellow to green upon exposure to vapors of linear alkyl ketones with
alkyl chain length selectivity. Powder and single crystal X-ray diffraction
analyses reveal the relationship between vapor-induced crystal structural
transformation and fluorochromism. After exposure to 2-butanone, 2-pentanone,
and 2-hexanone, the anthracene groups in P5en exist as
monomers in the solid state, while after exposure to heptanone and
octanone, the anthracene groups instead aggregate as excimers in the
solid state. By solid-state NMR and thermogravimetric analysis, we
further find that the size fit between ketone molecules and interspaces
in P5en crystals lead to the alkyl chain length selectivity.
This work provides a new strategy to design vapochromic fluorescent
crystalline materials and crystalline transformation systems.