Effect of Fluorination on the Polymorphism and Photomechanical Properties of Cinnamalmalononitrile Crystals

Cinnamalmalononitrile (CM) derivatives have been shown to exhibit a strong photomechanical response in the crystal form. In this paper, the effects of fluorine substitution on the molecular properties, crystal packing, and solid-state photochemical reactivity on this family of photochromes are explored. The addition of fluorines shifts the molecular S0 – S1 gap to a higher energy up to 0.4 eV. Fluorination also enables polymorphism in some of the derivatives that effectively controls whether or not they can undergo the [2 + 2] photodimerization. Depending on the substitution pattern, either the head-to-tail (HT, unreactive) or head-to-head (HH, reactive) crystal forms could be obtained. For some derivatives, both polymorphs could be grown depending on the solvent. Theoretical calculations on a subset of these molecules clarify how the fluorination of the CM framework modifies the polymorph landscape and shifts the energetics of the different packing motifs. The CMs appear to support a rich polymorph landscape where HH and HT structures coexist within a few kJ/mol of each other, allowing the simple exchange of an aromatic H atom for an F atom to cause a complete loss of photomechanical activity due to changes in crystal packing. The experimental and computational results highlight how even minor modifications to the molecular structure can alter the resulting crystal structures and photomechanical behavior.