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. ■ INTRODUCTIONGenerally, materials expand on heating along all the directions due to increasing molecular vibration, which is termed as positive thermal expansion (PTE). But in very few materials, this expansion is compensated for (a zero thermal expansion, ZTE) 1−3 or overshadowed by (a negative thermal expansion, NTE) 4−7 some other structural factors. Materials with ZTE or NTE properties have useful applications in fiber optic systems, electronic, high precision optical mirrors, cookware, packaging materials for refractive index gratings, biomedical field, etc. 8,9 There are also some materials that show large anisotropy in their thermal expansion behavior. Very strong uniaxial PTE or NTE materials could be useful in the field of thermomechanical actuators. 10−15 Although there are several reports on NTE in the case of metal oxides and zeolites, and also a few in the coordination compounds, 16−33 there have been comparatively fewer thermal expansion studies on pure organic compounds. 34−41,10−15 Generally the expansion in the bulk materials along a particular direction is estimated by analyzing the cell parameters of the crystal structures. In the case of thermal expansion (PTE or NTE), the structural changes occur smoothly over a range of temperature, and it is easy to compare the cell parameters to estimate the expansion in the materials along different directions. But in some of the systems, that undergo single-crystal to single-crystal phase transformation (SCSCPT) at a particular temperature, the structural changes are rather more drastic, and the cell alignment, symmetry, Z values, etc. are usually changed after the phase transformation. 42−50 In this work we have studied SCSCPT along with thermal expansion in the materials. There are only very few ...

A variable-temperature X-ray diffraction and theoretical study of conformational polymorphism in a complex organic molecule (DTC)

“…S5 ESI †), differing at most by 0.9% at RT and by 0.5% at 100 K. Also densities (at 100 K, r(A) ¼ 1.389 g cm À3 ; r(B) ¼ 1.383 g cm À3 ) and mean coefficients of thermal expansion in the 100 K -RT range (a(A) ¼ 1.7(3) Â 10 À4 K À1 ; a(B) ¼ 1.9(2) Â 10 À4 K À1 , computed on the change of the cell volume) are almost identical as well. A similar behaviour was reported also for other organic crystals, [60][61][62] though not being a general rule. [63][64][65] No phase transitions were identied in either sample (see Tables S1 and S2 ESI †).…”

Influence of Multiple Hydrogen Bonds on Thermal Expansion Within and Between Two-Dimensional Hydrogen-Bonded Sheets

“…For each cocrystal, expansion within the 2D hydrogen-bonded sheet ranges from zero to slight PTE, while expansion between sheets that self-assemble through long π−π interactions is much more significant. 22 We demonstrate rare 1D or 2D ZTE behavior in these cocrystals due to contributions from the hydrogen-bonded chains and sheets.…”

“…The acceptor molecules include bipyridines that are directly connected (4,4′-bipyridine, 4,4′-BIPY), contain a saturated carbon bridge (1,2-bis­(4-pyridyl)­ethane, 4,4′-BPEth), or contain an unsaturated bridge (4,4′-azopyridine, 4,4′-AP and 1,2-di­(4-pyridyl)­ethylene, 4,4′-BPE) (Scheme ). For each cocrystal, expansion within the 2D hydrogen-bonded sheet ranges from zero to slight PTE, while expansion between sheets that self-assemble through long π–π interactions is much more significant . We demonstrate rare 1D or 2D ZTE behavior in these cocrystals due to contributions from the hydrogen-bonded chains and sheets.…”

“…There are no strong interactions holding the sheets together along this direction; thus, the π•••π stacking direction is most affected by the temperature changes. 22 The term "colossal TE" has been used to describe solids that exhibit TE coefficients greater than 100 MK −1 . 26 The TE coefficients for all four cocrystals along X 3 are colossal and follow the order of TA•4,4′-BPEth > TA•4,4′-BIPY > TA•4,4′-AP ≈ TA•4,4′-BPE.…”

Combination of Negative, Positive, and Near-Zero Thermal Expansion in Bis(imidazolium) Terephthalate with a Helical Hydrogen-Bonded Network

“…This study presents an explanation of the NTE phenomenon observed for bis­(imidazolium) terephthalate. , In comparison to other salts with a hydrogen-bonded network, ,,,− the salt under investigation is unique. It reveals a combination of negative, positive, and near-zero thermal expansion, which has rarely been observed.…”