9-tert-Butylanthracene undergoes a photochemical reaction to form its strained Dewar isomer, which thermally back-reacts to reform the original molecule. When 9-tert-butylanthracene is dissolved in a polymer host, we find that both the forward and reverse isomerization rates are pressure-dependent. The forward photoreaction rate, which reflects the sum of contributions from photoperoxidation and Dewar isomerization, decreases by a factor of 1000 at high pressure (1.5 GPa). The back-reaction rate, on the other hand, increases by a factor of ∼3 at high pressure. Despite being highly strained and higher volume, the back-reaction reaction rate of the Dewar isomer is at least 100× less sensitive to pressure than that of the bi(anthracene-9,10-dimethylene) photodimer studied previously by our group. These results suggest that the high pressure sensitivity of the bi(anthracene-9,10-dimethylene) photodimer reaction is not just due to the presence of strained four-membered rings but instead relies on the unique molecular geometry of this molecule.

A temperature dependent (100–296 K) X-ray diffraction study of the structural changes associated with the order–disorder enantiotropic phase transition of crystalline para-terphenyl (PTP) is presented. Reflections were collected every degree through the phase transition region 183–196 K. The temperature dependent X-ray results characterize a transition from a disordered monoclinic phase (P21/a, non-standard monoclinic space group) at room temperature to an ordered triclinic phase (C-1, non-standard space group) with reticular, pseudo-merohedral twinning below 193 K. The present study confirms general conclusions of prior temperature dependent neutron scattering and low temperature X-ray studies, and it makes the following new contributions: (a) quantitative assessment of the temperature dependent residual order present in the high-temperature disordered-monoclinic phase (e.g. weak temperature dependent superlattice reflections in the long-range disordered-monoclinic phase up to 296 K), (b) quantitative measure of the temperature dependent disorder present in the low-temperature ordered-triclinic phase (e.g. down to 188 K), (c) identification of the twin symmetry law (i.e. twin components related to each other by a mirror-plane perpendicular to the b-axis or a twofold rotation parallel to the b-axis of the triclinic cell), (d) the minor/major twin components are found to be temperature dependent in the low temperature ordered-triclinic phase, (e) a revised low temperature triclinic structure (C-1 non-standard space group) with reticular, pseudo-merohedral twinning is reported, with an improved R1 value of 0.039 at 100 K. The low temperature structure retains four unique PTP enantiomers, with improved torsional angles.

The pressure- and temperature-induced polymorphic crystal phase transitions of p-terphenyl (PTP) have been modeled using a modified PCFF interaction force field. Modifications of the interaction potential were necessary to simultaneously model both the temperature-induced phase transition at ambient pressure and the pressure-induced phase transition at low temperature. Although the high-temperature and high-pressure phases are both characterized by flattening of the PTP molecule, the mechanisms of the temperature- and pressure-induced phase transitions are different. At high temperature thermal energy exceeds the torsional barrier, resulting in a bimodal phenyl ring twist angle distribution that averages to zero. In contrast, compression of PTP at high pressure results in a static planar structure. At high pressure the compression of the unit cell is also characterized by large compression of the a lattice parameter and weak compression of c, but some expansion of the b lattice parameter. The expansion of the b lattice parameter is likely associated with pressure-induced soft mode behavior of some lattice vibrations as well as soft mode behavior of pseudolocal phonons associated with impurities in PTP. The crystallographic angles α, β, and γ also indicate a triclinic crystal phase above the critical phase transition pressure of Pc ∼ 0.5 GPa at low temperature, suggesting a distinct phase separate from the monoclinic high-pressure phase at high temperature.

High-resolution infrared (IR) spectroscopy has been used to investigate the pressure-induced (0–11 kbar) polymorphic phase transition of crystalline para-terphenyl at low temperature (25 K). A number of doublet bands observed in low-pressure triclinic p-terphenyl were observed to coalesce in the high-pressure monoclinic phase. The coalescing of doublet bands was attributed to changes in factor group (Davydov) splittings associated with the transition from a low-pressure triclinic phase to a high-pressure monoclinic phase. The bands that ‘disappear’ also do not correlate with frequency changes associated with changes in molecular symmetry. Molecular dynamics (MD) simulations at low temperature (20 K) yield a non-planar average molecular structure for the high-pressure monoclinic phase, in contrast to the high-temperature monoclinic phase. The MD simulations also reveal a broadening of the distribution of ring torsion angles near the triclinic–monoclinic phase transition pressure.