Photomechanical Anthracenophane Crystals: Theory, Experiment, and Composite Actuator Performance

The anthracene [4 + 4] photocycloaddition is a negative photochromic reaction utilized in photomechanical materials. In crystals, this reaction requires that monomeric anthracene rings adopt a face-to-face packing motif. Anthracenophane derivatives preorganize covalently attached anthracenes for an intramolecular [4 + 4] photocycloaddition reaction, decoupling the photochemistry from the crystal packing. In this work, three anthracenophanes, bi(anthracene-9,10-dimethylene) (1), (-H)monobenzoannulated [2.2](9,10)anthracenophane (2) and (-OEt)monobenzoannulated [2.2](9,10)anthracenophane (3), are assessed as potential solid-state photomechanical materials. Crystal structures of the reactant and photoproduct forms are obtained, and all crystals exhibit photosalient behavior. Although all three molecules rely on the same photochemistry, theoretical analysis predicts widely varying photomechanical work outputs among these derivatives and their polymorphs, with neat crystals of 2 predicted to exhibit a work density of up to 68 MJ/m3 due to the favorable alignment of the anthracene ring distortions. Enhanced thermal reversibility is observed for 2 and 3 due to a 60% reduction in the activation energy for photodimer dissociation, leading to recovery of the reactant within minutes at room temperature. The more soluble 3 could be incorporated into a composite ceramic-organic bending actuator that showed good reversibility, although its estimated work density of 2.6 × 103 J/m3 is several orders of magnitude less than that predicted theoretically for the neat crystal. This large discrepancy suggests that improved processing and actuator design will be required to approach the theoretical limits of these photomechanical crystals.