Molecular Crystals With Reversible Chromic Three‐State Crystal‐to‐Crystal Transformation via the Dynamic Motion of Negatively Curved π‐Frameworks

Crystal-to-crystal transformation is an attractive aspect in the context of designing stimuli-responsive materials. Especially, multiple-state crystal-to-crystal transformations are currently gaining attention as they enable the construction of systems responsive to multiple stimuli. However, realizing such transformations is challenging in crystal engineering owing to the restricted molecular motion in the solid state. Here, we report that tetrathia[8]circulene octaoxide undergoes reversible three-state crystal-to-crystal transformations with color changes induced by the dynamic motion of negatively curved π-frameworks in response to external stimuli. DFT calculations revealed that the electronic transition with charge-transfer character dominates the color of the crystals. Conformational changes of the flexible circulene core alter the energy level of the LUMO, resulting in a change in the crystal color. The present study provides new insights into the practical applications of negatively curved π-frameworks.