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Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2021)

Photomechanical actuators are smart materials that transform photon energy into mechanical energy without any wired connections. Photonmechanical molecular crystals have shown superior performance on nanoscopic and microscopic scales due to their high elastic moduli, high-energy density and fast response time compared with polymer actuators. However, the development of macroscopic crystal actuators that can be used in the real world has yet to be demonstrated.

The work in this dissertation solves the problem by organizing photomechanical nanocrystals in porous inorganic templates Al2O3. It combines the best of both worlds where it is processable like polymers but it also attains high performance of organic crystals. 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, a diarylethene (DAE) derivative that undergoes a unimolecular photoisomerization between its ring-open and ring-closed isomers was used as the active organic element. The isomers are thermally stable but can be switched back and forth using UV and visible light for hundreds of cycles. Very small (sub-mg) amounts of material can move masses that are 105 times larger without being physically connected to an external power source. A light-to-work conversion efficiency for a molecular crystal photomechanical actuator has been for the first time quantitatively estimated, providing a starting point for future optimization.

Additional studies investigated how both the organic and inorganic components could affect the photomechancial response. Two alumina templates with different morphology and elastic modulus were compared. The template with a lower average elastic modulus generate about one order of magnitude more photomechanical work. Two diarylethene derivatives with different crystal deformations were studied to determine the dependence of photomechanical response on chemical and crystal structure.

The hybrid approach was then extended to porous SiO2 membranes with variable thicknesses in order to improve optical penetration and reproducibility. The photon-induced bending could be repeated for 150 cycles without loss of performance which suggests good fatigue resistance. The thickness dependence of the performance was studied, revealing thinner templates gave larger displacements. This work demonstrates that the hybrid method for making photomechanical actuators on macroscopic scales can be extended to other host materials and template geometries without sacrificing performance.

    Cover page: Hybrid Organic–Inorganic Photomechanical Actuators Based on Organized Molecular Crystals
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2020)
    4-Fluoro-9-anthracenecarboxylic acid (4F-9AC) is a thermally reversible (T-type) photomechanical molecular crystal. The photomechanical response is driven by a [4 + 4] photodimerization reaction, while the photodimer dissociation determines the reset time. In this paper, both the chemical kinetics of dimer dissociation (using a microscopic fluorescence-recovery-after-photobleaching experiment) and mechanical reset dynamics (by imaging bending microneedles) for single 4F-9AC crystals are measured. The dissociation kinetics depend strongly on the initial concentration of photodimer, slowing down and becoming nonexponential at high dimer concentrations. This dose-dependent behavior is also observed in the mechanical response of bending microneedles. A new feature in the photomechanical behavior is identified: the ability of a very weak control beam to suppress dimer dissociation after large initial dimer conversions. This phenomenon provides a way to optically control the mechanical response of this photomechanical crystal. To gain physical insight into the origin of the nonexponential recovery curves, the experimental results are analyzed in terms of a standard first-order kinetic model and a nonlinear Finke-Watzky (FW) model. The FW model can qualitatively reproduce the transition from exponential to sigmoidal recovery with larger initial conversions, but neither model can reproduce the suppression of the recovery in the presence of a weak holding beam. These results highlight the need for more sophisticated theories to describe cooperative phenomena in solid-state crystalline reactions, as well as demonstrating how this behavior could lead to new properties and/or improved performance in photomechanical materials.
      Cover page: Using light intensity to control reaction kinetics and reversibility in photomechanical crystals.
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2021)
      Crystals composed of photoreactive molecules represent a new class of photomechanical materials with the potential to generate large forces on fast timescales. An example is the photodimerization of 9-tert-butyl-anthracene ester (9TBAE) in molecular crystal nanorods that leads to an average elongation of 8%. Previous work showed that this expansion results from the formation of a metastable crystalline product. In this article, it is shown how a novel combination of ensemble oriented-crystal solid-state NMR, X-ray diffraction, and first principles computational modeling can be used to establish the absolute unit cell orientations relative to the shape change, revealing the atomic-resolution mechanism for the photomechanical response and enabling the construction of a model that predicts an elongation of 7.4%, in good agreement with the experimental value. According to this model, the nanorod expansion does not result from an overall change in the volume of the unit cell, but rather from an anisotropic rearrangement of the molecular contents. The ability to understand quantitatively how molecular-level photochemistry generates mechanical displacements allows us to predict that the expansion could be tuned from +9% to -9.5% by controlling the initial orientation of the unit cell with respect to the nanorod axis. This application of NMR-assisted crystallography provides a new tool capable of tying the atomic-level structural rearrangement of the reacting molecular species to the mechanical response of a nanostructured sample.
        Cover page: Bridging photochemistry and photomechanics with NMR crystallography: the molecular basis for the macroscopic expansion of an anthracene ester nanorod
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