A theoretical framework for the design of molecular crystal engines.

Photomechanical molecular crystals have garnered attention for their ability to transform light into mechanical work, but difficulties in characterizing the structural changes and mechanical responses experimentally have hindered the development of practical organic crystal engines. This study proposes a new computational framework for predicting the solid-state crystal-to-crystal photochemical transformations entirely from first principles, and it establishes a photomechanical engine cycle that quantifies the anisotropic mechanical performance resulting from the transformation. The approach relies on crystal structure prediction, solid-state topochemical principles, and high-quality electronic structure methods. After validating the framework on the well-studied [4 + 4] cycloadditions in 9-methyl anthracene and 9-tert-butyl anthracene ester, the experimentally-unknown solid-state transformation of 9-carboxylic acid anthracene is predicted for the first time. The results illustrate how the mechanical work is done by relaxation of the crystal lattice to accommodate the photoproduct, rather than by the photochemistry itself. The large ∼107 J m-3 work densities computed for all three systems highlight the promise of photomechanical crystal engines. This study demonstrates the importance of crystal packing in determining molecular crystal engine performance and provides tools and insights to design improved materials in silico.

Using Small Molecule Absorbers to Create a Photothermal Wax Motor.

Organic phase change materials are used in actuators like wax motors. The solid→liquid phase transition that drives expansion is commonly induced by resistive heating that requires an electrical connection. The use of light to generate a phase change provides a non-contact way to power wax motors. Here, it is demonstrated that small molecules can act as absorbers to enable a photoinduced solid→liquid melting transition in eicosane, a low molecular weight phase change material. Three different small molecule absorbers are utilized: (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), azobenzene (AZOB), and guaiazulene (GAZ). The GAZ/eicosane mixture is characterized in detail because its absorption extends out to 750 nm, opening up the possibility of using near-infrared diodes as the photon source. The GAZ/eicosane composite is incorporated into a commercial wax motor assembly and 532 nm laser light is used to lift up to 400 g. The temporal response, work and force output, and efficiency are measured, and no loss of lifting capability or degradation is observed after ten cycles of irradiation. The incorporation of small aromatic molecules with low-energy absorption features into phase change materials can provide a general way to make light powered wax motors.

Unusual concentration dependence of the photoisomerization reaction in donor-acceptor Stenhouse adducts.

Donor-acceptor Stenhouse adducts comprise a new class of reversible photochromic molecules that absorb in the visible and near-infrared spectral regions. Unimolecular photoisomerization reactions are usually assumed to be insensitive to photochrome density, at least up to millimolar concentrations. In this paper, the photoisomerization kinetics of a third-generation donor-acceptor Stenhouse adduct molecule (denoted DASA) are examined over a range of concentrations. DASA switches efficiently at micromolar concentrations in both liquid solution and in polymers, but as the photochrome concentration is increased there is a dramatic inhibition of the photoisomerization. A kinetic study of both the reactant and photoproduct decays at varying concentrations and in different hosts indicates that the forward photoisomerization and the thermal backward reaction can change by factors of 20 or more depending on DASA concentration. Femtosecond transient absorption experiments show that the initial cis → trans step of the isomerization is not affected by concentration. It is hypothesized that long-range coulombic interactions interfere with the ground state electrocyclization stage of the isomerization, which is unique to the DASA family of photochromes. The physical origin of the inhibition of photoswitching at high photochrome concentrations must be understood if the DASA class of molecules is to be used for applications that require high photochrome concentrations, including photomechanical actuation.