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.