Dynamic Order–Disorder Reversible Phase Transition and Anisotropic Thermal Expansions in a Single-Component Organic Molecular Crystal

Order–disorder phase transition and anisotropic thermal expansion phenomena are commonly reported for inorganic and multicomponent organic materials but rarely in single-component molecular crystals. Research in this domain leads to numerous practical applications and development in fundamental research. Herein, we report variable-temperature X-ray diffraction studies on an imidazole-based novel single-component material, 2-(3,5-bis(trifluoromethyl)phenyl)-4,5-dihydro-1H-imidazole. Our study demonstrates that the lightweight material undergoes isosymmetric dynamic order–disorder reversible phase transition after 150 K with a hysteresis loop of ∼15 K and displays diverse thermal expansions along the crystallographic axes before and after the phase transition. The material exhibits positive (PTE), near zero (ZTE), and negative (NTE) thermal expansions along a-, b-, and c-axes, respectively, leading to uniaxial NTE and biaxial PTE characteristics. The phase transition at low temperatures was supported via differential scanning calorimetry and variable-temperature powder X-ray diffraction studies. The mechanism of the disorder–order phase transition and the anisotropic thermal expansions were understood by analyzing eight sets of single-crystal X-ray diffraction data collected in the range of 298–100 K. While this study provides new insights into the unusual thermal behavior of single-component organic crystals, the eco-friendly pure organic material could be a promising candidate for device applications.