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1. Using light intensity to control reaction kinetics and reversibility in photomechanical crystals

   https://doi.org/10.1039/D0SC03557B  

   Easley, Connor J. ; Tong, Fei ; Dong, Xinning ; Al-Kaysi, Rabih O. ; Bardeen, Christopher J. ( September 2020 , Chemical Science)

   null (Ed.)

   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. 

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2. A theoretical framework for the design of molecular crystal engines

   https://doi.org/10.1039/D2SC05549J  

   Cook, Cameron J. ; Li, Wangxiang ; Lui, Brandon F. ; Gately, Thomas J. ; Al-Kaysi, Rabih O. ; Mueller, Leonard J. ; Bardeen, Christopher J. ; Beran, Gregory J. ( January 2023 , Chemical Science)

   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 ∼10 7 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 . 

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3. Solid state photodimerization of 9- tert -butyl anthracene ester produces an exceptionally metastable polymorph according to first-principles calculations

   https://doi.org/10.1039/C8CE01985A  

   Beran, Gregory J. ( January 2019 , CrystEngComm)

   Molecular crystal engineering seeks to tune the material properties by controlling the crystal packing. However, the range of achievable properties is constrained by the limited energy range of polymorphs which can be crystallized. Here, computational modeling highlights that a solid-state crystal-to-crystal chemical reaction in 9- tert -butyl anthracene ester (9TBAE) nanorods [Al-Kaysi et al. , J. Am. Chem. Soc. , 2006, 128 , 15938] imparts “synthetic memory” into the crystal structure that allows reproducible formation of a highly metastable, yet long-lived polymorph. Specifically, whereas the vast majority of known polymorphs exhibit lattice energy differences below 10 kJ mol −1 , the conformational polymorph formed via solid state reaction chemistry lies 14 kJ mol −1 higher in energy than the form grown from solution, according to calculations that combine a dispersion-corrected second-order Møller–Plesset perturbation theory (MP2D) treatment of the monomer and photodimer with a density functional theory treatment (B86bPBE-XDM) of the intermolecular interactions in the crystal. Moreover, the solid-state reaction environment traps a highly unstable intramolecular photodimer conformation which defies the conventional wisdom surrounding conformational polymorphs. These observations suggest that solid-state reaction chemistry represents an under-appreciated strategy for producing polymorphs that would likely be unobtainable otherwise. 

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4. Bridging photochemistry and photomechanics with NMR crystallography: the molecular basis for the macroscopic expansion of an anthracene ester nanorod

   https://doi.org/10.1039/d0sc05118g  

   Chalek, Kevin R. ; Dong, Xinning ; Tong, Fei ; Kudla, Ryan A. ; Zhu, Lingyan ; Gill, Adam D. ; Xu, Wenwen ; Yang, Chen ; Hartman, Joshua D. ; Magalhães, Alviclér ; et al ( January 2021 , Chemical Science)

   null (Ed.)

   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. 

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5. The interplay of intra- and intermolecular errors in modeling conformational polymorphs

   https://doi.org/10.1063/5.0088027  

   Beran, Gregory J. O. ; Wright, Sarah E. ; Greenwell, Chandler ; Cruz-Cabeza, Aurora J. ( March 2022 , The Journal of Chemical Physics)

   Conformational polymorphs of organic molecular crystals represent a challenging test for quantum chemistry because they require careful balancing of the intra- and intermolecular interactions. This study examines 54 molecular conformations from 20 sets of conformational polymorphs, along with the relative lattice energies and 173 dimer interactions taken from six of the polymorph sets. These systems are studied with a variety of van der Waals-inclusive density functionals theory models; dispersion-corrected spin-component-scaled second-order Møller–Plesset perturbation theory (SCS-MP2D); and domain local pair natural orbital coupled cluster singles, doubles, and perturbative triples [DLPNO-CCSD(T)]. We investigate how delocalization error in conventional density functionals impacts monomer conformational energies, systematic errors in the intermolecular interactions, and the nature of error cancellation that occurs in the overall crystal. The density functionals B86bPBE-XDM, PBE-D4, PBE-MBD, PBE0-D4, and PBE0-MBD are found to exhibit sizable one-body and two-body errors vs DLPNO-CCSD(T) benchmarks, and the level of success in predicting the relative polymorph energies relies heavily on error cancellation between different types of intermolecular interactions or between intra- and intermolecular interactions. The SCS-MP2D and, to a lesser extent, ωB97M-V models exhibit smaller errors and rely less on error cancellation. Implications for crystal structure prediction of flexible compounds are discussed. Finally, the one-body and two-body DLPNO-CCSD(T) energies taken from these conformational polymorphs establish the CP1b and CP2b benchmark datasets that could be useful for testing quantum chemistry models in challenging real-world systems with complex interplay between intra- and intermolecular interactions, a number of which are significantly impacted by delocalization error.

    

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