Using Chemical Substitution to Engineer Photomechanical Cinnamalmalononitrile Crystals

The cinnamalmalononitrile (CM) family of molecules can undergo a [2+2] photodimerization in the solid-state and generate photomechanical response. Derivatives exhibit two different crystal packing motifs: head-to-head (HH) in which the molecules stack with the phenyl rings on the same side of the stack, and head-to-tail (HT) in which the phenyl rings of adjacent molecules are on opposite sides. The [2+2] photodimerization is only observed for HT packing motif. Attempts to identify chemical substitution patterns that favor the reactive HT packing based on simple steric and electrostatic considerations fail to reliably predict crystal packing, and fluorination generated both motifs in more-or-less random fashion. Empirically, substitution at the 3-position favors HT packing while substitution at the 4-position favors HH packing. Computational modeling suggests that the tendency for HH or HT packing arrangements stems from complex many-body interactions with the rest of the lattice. Modeling with periodic density functional theory shows that interactions with the rest of the lattice also explain why the HT motif is photochemically active while the HH motif is inert. Chemical substitution can also affect the photomechanical work output, with a significant variation in work between HT polymorphs composed of different molecules. In order to obtain a reactive HT polymorph, the best strategy appears to entail placing a strong electron-withdrawing group at the 3-position of the phenyl, and we confirm that an HT polymorph of 3-trifluoromethyl-cinnamalmalononitrile is a highly photosalient crystal, with a predicted ideal work density of 40 MJ/m3.