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

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.