Engineered Photochromism in Crystalline Salicylidene Anilines by Facilitating Rotation to Reach the Colored trans-Keto Form

The photochromism of crystalline salicylidene anilines regularly occurs by a volume-conserving bicycle pedal motion that transposes the relative position of the two atoms of the central imine bond while leaving the original salicyladehyde and aniline rings unchanged. Considering the challenges involved in the preparation of packing structures that are conducive to the bicycle pedal process, we tested a design based on a structural strategy that is known to facilitate the rotation of phenyl rings in the solid state, allowing the molecules to be photoreactive. Three salicylidene aniline molecular rotors linked to bulky trityl, tetraphenylmethyl, or pentiptycene stators were prepared and crystallized, and their solid state photochromism was confirmed. In one case, isomorphous crystals of a deuterium labeled deoxo-salicylidene aniline model system were analyzed by solid state 2H NMR to confirm that the corresponding packing structure is conducive to fast rotation in the solid state.