External pressure and associated changes in lattice structures are key to realizing exotic quantum phases such as high-$T_{\mathrm{c}}$ superconductivity. While applying external pressure is a standard method to induce novel lattice structures, its impact on organic molecular crystals has been less explored. Here we report a unique structural phase transition in ${\left(\text{DMET-TTF}\right)}_2{\mathrm{AuBr}}_2$ under pressure. By combining advanced high-pressure techniques and $abinitio$ calculations, we elucidate that ${\left(\text{DMET-TTF}\right)}_2{\mathrm{AuBr}}_2$ undergoes a transition from a quasi-one-dimensional lattice to a nearly perfect rectangular lattice around 0.9 GPa. This transition leads to the realization of an antiferromagnetic Mott insulator with $T_{\mathrm{N}}=66$ K, the highest $T_{\mathrm{N}}$ in low-dimensional molecular crystal solids to date. Upon increasing the pressure, the antiferromagnetic ordering is suppressed, and a superconducting phase with $T_{\mathrm{c}}=4.8$ K emerges around 6 GPa. Our study reveals the significant impact of external pressure on lattice structures of organic molecular crystals and offers insights into how geometrical frustration relates to superconductivity. Our findings also pave the way for realizing functional organic molecular crystals through changes in lattice structures under pressure.

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• Electrical conductivity
• Electrical properties
• Fermi surface
• First-principles calculations
• Magnetic phase transitions
• Pressure effects
• Superconductivity
• Transport phenomena
• Molecular solids
• Organic compounds
• Resistivity measurements