Modulation of Thermal Expansion by Guests and Polymorphism in a Hydrogen Bonded Host

Abstract

Thermal expansions of the three isostructural solvates of an “X”-shaped host molecule, 1,1,4,4-tetrakis(4-hydroxyphenyl)cyclohexane (1), with phenol (1-ph), p-cresol (1-pc), and m-cresol (1-mc), two polymorphic solvates of 1 with o-cresol (1-oc-1D and 1-oc-2D), and a guest-free form of 1 (1-gf) have been studied. This work demonstrates that isostructural 1-ph and 1-pc show uniaxial negative thermal expansion (NTE), but along different directions and only the 1-ph system undergoes single-crystal to single-crystal phase transformation. On the other hand, the isostructural 1-mc host lattice experiences a normal positive thermal expansion (PTE). The one-dimensional hydrogen bonded triclinic form of the o-cresol solvate (1-oc-1D) exhibits a biaxial NTE, whereas the two-dimensional hydrogen bonded monoclinic form (1-oc-2D) exhibits a normal PTE. The 1-gf form experiences only a weak uniaxial NTE and smaller volumetric expansion compared to all the solvates. The uniaxial NTE that occurred in some of these materials is caused by sliding of the layers, made of hydrogen bonded tapes of the host molecules. The guest molecules experience stronger thermal vibration than the host molecules. Systems with a higher guest to the host ratio experience larger thermal expansions.