Crystals can organize photochromic molecules and coordinate their motions, providing a potential path to photomechanical actuators.  Photochemistry in the ordered solid-state can lead to highly nonlinear phenomena as opposed to the first-order kinetics usually observed in disordered polymers and liquids.  At the nanoscopic level, conversion of a single molecule in a crystal lattice can affect the local environment of its neighbors and modify their reaction rates.  Time-resolved microscopy experiments are used to look at reversible photochemical reactions in single microcrystals.  We find evidence for cooperative behavior, ranging from auto-catalysis to inhibition, depending on the initial concentration of reactant and product species.  We also demonstrate the existence of photochemical martensitic phase transition in 9-anthracene carboxylic acid crystals, with a phase front velocity that depends on light intensity.  At larger lengthscales, mechanical motion of the entire crystal can modify its light absorption and reaction rate, providing another source of nonlinearity.  For example, when a photoreversible molecule is irradiated with different wavelengths simultaneously, oscillatory photochemical and mechanical dynamics can be induced.  This phenomenon is demonstrated using single microwire crystals composed of (<i>Z</i>)-2-(3-(anthracen-9-yl)allylidene)malononitrile. When these microwires are exposed to a combination of ultraviolet and visible light, a continuous oscillation induces bending, rotation and wave-like translational motion. A simple model that relates the torque due to twisting to population differences due to reversible photoisomerization can qualitatively reproduce the behavior as a function of light intensity, wavelength, and polarization.  The complex oscillatory motions generated by incoherent photon sources mimic the motion of biological flagella.

Nonlinear photochemical dynamics in molecular crystals

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ACS Fall 2022

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