Biaxial Negative Thermal Expansion in an Organic Imidazolium Salt: Modulation of the Thermal Expansion Property by Methyl Substitution

Abstract

The thermal expansion of an organic salt, 4-methylimidazolum-4-hydroxybenzoate (4-MIMD-HBC) has been studied by variable temperature single-crystal X-ray diffraction (VT-SCXRD). The salt exhibits unusual biaxial negative thermal expansion (NTE) and uniaxial large positive thermal expansion (PTE), whereas imidazolium-4-hydroxybenzoate (IMD-HBC) shows uniaxial NTE and biaxial PTE. Although the crystal structures of IMD-HBC and 4-MIMD-HBC are almost similar, there is a significant difference of the thermal expansion property between these two salts. The scissor-like motion of the hydrogen bonded networks is responsible for uniaxial NTE and PTE in both IMD-HBC and 4-MIMD-HBC salts along two principal axes, X1 and X3. IMD-HBC shows PTE along the other principal axis X2, where 4-MIMD-HBC exhibits NTE. The difference in thermal expansion between the two salts along X2 arises due to the methyl substituent, which induces HBC moieties in 4-MIMD-HBC to come closer to each other as the temperature increases, resulting contraction along X2.

Tuning of thermal expansion properties of a mixed-ligand MOF by ligand variation

“…The modulation of thermal expansion in 1,1,4,4-tetrakis(4-hydroxyphenyl)cyclohexane by guest incorporation was studied by Saha et al 16 Controlling thermal expansion in organic co-crystals by the incorporation of molecules with similar chemical structures and by substitution of the functional groups has been reported by Hutchins et al 17 Recently, Chuskit et al reported modulation of the thermal expansion property of organic salt by methyl substitution. 18 Tuning of the thermal expansion properties in MOFs has been mostly carried out by exchanging guest molecules. Here, we report for the first time the tuning of thermal expansion of a MOF by varying the ligand without changing the overall structural framework.…”