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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2017)

Anthracene derivatives can undergo photochemical reactions under light irradiation, like a [4+4] photodimerization and a Dewar isomerization. When these molecules are made into micro- and nano-sized crystals, some of them exhibit interesting photomechanical motions and deformations like bending, twisting, curling, and expanding. Both the internal and external environmental effects were found to significantly influence the photomechanical behaviors and photochemical reactions of these molecules in the solid-state. One goal of this research is to better understand the relationship between dynamics of chemical reactions and crystal photomechanical behavior they produce, and to see if these reaction rates can be enhanced by internal or external factors.

9-tert-butylanthracene (9TBA) undergoes a Dewar isomerization in solution and in polymers, but produced no photomechanical response in the crystal, even though it generates a significant volume change and a highly strained molecular structure after photoisomerization. But when this molecule is embedded in a polymer, a strong pressure dependence of the photoisomerization was observed in both forward and reverse reactions, providing us a possible way to engineer more pressure sensitive molecules in the future.

We investigated the photodimerization process of 9-methylanthracene (9MA) in polycrystalline thin films. It was found that both the photochemical reaction and nonradiative relaxation rates increase as more photoproduct is formed, which we denoted as an autocatalytic pathway. The local environment of a 9MA molecule changes as the surrounding molecules react. We derived rigorous equations and extended a current model (the Finke-Watzky Model) to describe the non-exponential reactant decay, and we were able to predict its time-dependent photoluminescence evolution over the course of the reaction, which showed agreement with the experimental results. The presence of autocatalytic reaction dynamics in a molecular crystal system may play a role in its photomechanical response. This new chemical reaction pathway is different from the single exponential process seen in photoresponsive soft polymer systems.

Besides internal factors, like intra- and intermolecular interactions, we also studied the external environmental effects on molecular crystals. Ionic surfactants like cetyltrimethylammonium bromide (CTAB) was discovered to dramatically promote both the photomechanical motion and the photochemical reaction rates of dimethyl-2-(3-anthracen-9-yl)allylidene)malonate (DMAAM) nanocrystals. These results may help illustrate how surface effects can enhance crystal reactivity. Future work will involve incorporating the photochemical processes with photomechanical behaviors and exploring more interesting mechanical motions with various shapes and sizes of molecular crystals.

    Cover page: Environmental Effects on the Solid-State Reaction Dynamics of Anthracene Derivatives
    • 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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