Characterization of a P-type photomechanical molecular crystal based on the E → Z photoisomerization of 9-divinylanthracene malonitrile

Anthracene derivatives with a 1,3-butadiene group attached at the 9-position can undergo E <-> Z (trans <-> 2 cis) photoisomerization reactions, providing a route to new photomechanical materials. In this paper, we report the properties of a new anthracene derivative, 2-(3-anthracen-9-yl-allylidene)-malononitrile (9DVAM). The structure, photophysics and photochemistry of 9DVAM are characterized in both dilute solution and in the crystal form. This molecule possesses a strong charge-transfer transition that extends its absorption into the visible region. Photoexcitation of this transition leads to a E -> Z isomerization reaction in both dilute solution and in the crystal. Rapid nonradiative relaxation and a high E -> Z photoisomerization yield appear to prevent dimerization side reactions in the crystal. Exposure to ultraviolet light can partially reverse this reaction in both liquid solution and in the crystal, allowing the molecule to be cycled back and forth between its cis and trans isomers. This switching between populations can cause reversible shape changes, like bending, in crystalline microribbons and nanowires. The divinylanthracenes provide a new class of photomechanical molecular crystals that complement existing photoreversible materials.