Thermoresponsive Organic Inclusion Compounds: Modification of Thermal Expansion Behavior by Simple Guest Replacement

It is demonstrated that guest replacement
in a series of isoskeletal
organic inclusion compounds can produce drastic changes in thermal
expansion behavior. The compounds 1, 2,
and 3 have 18-crown-6 as host molecule and nitromethane,
acetonitrile, and iodomethane, respectively, as guests. Along the
principal axis X1 the linear component of thermal expansion is negative
for 1 and 2 but positive for 3. All three compounds have varying degrees of large volumetric thermal
expansion, with coefficients of 378(22), 226(3), and 256(8) ×
10–6 K–1 for 1, 2, and 3, respectively. Crystal structure analysis
and computational methods were used to elucidate general features
of the underlying mechanism of thermal expansion for the series. The
contributions of several factors are described, including host–guest
compatibility, electrostatic effects, and steric effects. A tilting
mechanism gives rise to the negative components of thermal expansion
in 1 and 2 but is inhibited by the large
molecular volume of the guest in 3. In addition, the
thermosalient effect was observed for 2. To our knowledge
this is the first example of thermosalience reported for an inclusion
compound.