Compression of a Flapping Mechanophore Accompanied by Thermal Void Collapse in a Crystalline Phase

Mechanical control of the molecular energy landscape is an important issue in modern materials science. Mechano­phores play a unique role in that the mechanical responses are induced against the activation barrier for intramolecular transformation with the aid of external forces. Here we report an unprecedented activation process of a flexible flapping mechano­phore. Namely, thermal void collapse in a crystalline phase triggers mechano­phore compression in a definite proportion. Unfavored conformational planarization of the flapping mechano­phore is compulsorily induced by packing force, leading to a total energy gain in crystal packing. Fluorescence chromism indicates extended π conjugation resulting from the mechano­phore compression, giving rise to an energy transfer from the unpressed to compressed conformers.