Switching Photomechanical Response by a Structural Phase Transition in a Naphthalene Diimide Derivative

A newly prepared photoresponsive material, specifically a naphthalene diimide molecule containing alkylamino groups, was found to exhibit a structural phase transition in the crystalline state. The phase transition dramatically altered the molecular arrangement from orthogonal π-π stacking to parallel π-π stacking. The photomechanical response of each crystal was also changed before and after the phase transition. While the as-prepared crystal immediately responded to light irradiation (λ=370 nm), the responsiveness of the crystal became slower after heat treatment. Upon light irradiation, radical species, such as the radical anion, radical anion dimer, and mixed valence species, were formed, which have nearly the same distribution in both phases. However, the spin system and the structure of the radical anion were quite different from each other and were found to be sensitive to the molecular arrangement. The photo-responsiveness of the crystals was corroborated by evaluating the relaxation of the radical anion.