Alkane-Shape-Selective Vapochromic Behavior Based on Crystal-State Host–Guest Complexation of Pillar[5]arene Containing One Benzoquinone Unit

Colored crystals
of pillar­[5]­arene containing one benzo­quinone
unit were found to exhibit alkane-shape-selective vapo­chromic
behavior. Activated pillar­[5]­arene crystals, prepared by removing
solvated methanol from pillar­[5]­arene crystals, changed color
from dark-brown to light-red after exposure to linear alkane vapors;
however, no color changes were observed on exposure to branched or
cyclic alkanes. Uptake of methanol vapor by the activated crystals
induced a different color change, from dark-brown to black. This multi-vapo­chromism
results from the different intermolecular π-stacking interactions
between the benzo­quinone and 1,4-diethoxy­benzene units
in the alkane- and methanol-containing crystals. Unlike most known
vapo­chromic materials, these pillar­[5]­arene-based materials
were highly stable; after uptake of n-alkanes or
methanol the color of the crystals did not change after storage in
air for 3 weeks. This is because the included guests were stabilized
in the cavity by multiple CH/π interactions.