Light-Controlled Reconfigurable Optical Structures Using Photomechanical Organic Crystals

In crystal form, 4-fluoro-9-anthracenecarboxylic acid (4F-9AC) undergoes a room temperature reversible intermolecular [4 + 4] photodimerization that can be harnessed for photomechanical actuation. This negative photochromic reaction enables high conversion throughout the crystal. Patterned 405 nm excitation of single 4F-9AC crystals creates localized photo-dimer regions with an expansion of up to 3% of the total crystal thickness. These photoconverted regions cause reversible surface deformations and changes in birefringence that follow the reaction progress. Temperature dependent rate measurements of the photo-dimer dissociation yield an activation energy of 97 kJ/mol, making the reversibility rate highly sensitive to temperature. By varying the crystal temperature and the light exposure level, the lifetime of the photomechanical features can be tuned from 40 milliseconds to minutes. This enables rapid updating of images and features using a commercial spatial light modulator. We use this capability to create dynamic surface relief gratings that can steer a diffracted 633 nm probe beam in arbitrary directions. The rapid mechanical response of the crystalline systems permits write-erase cycles with a duration of 1 s, which compares favorably to polymer systems.