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https://hdl.handle.net/20.500.14394/69

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Surface anchoring energy of cholesteric liquid crystals
(2019-01-01) Guo, Tianyi; Zheng, Xiaoyu; Palffy-Muhoray, Peter
In this paper, we propose a suitable surface energy expression for cholesteric liquid crystals. We show that there exists a symmetry allowed term for chiral nematics that doesn’t appear in the traditional Rapini-Papoular surface energy form. We discuss some consequences of this new surface anchoring term.
Norbornadienes: Robust and Scalable Building Blocks for Cascade “Click” Coupling of High Molecular Weight Polymers
(2019-01-01) St. Amant, Andre H.; Discekici, Emre H.; Bailey, Sophia J.; Zayas, Manuel S.; Song, Jung-Ah; Shankel, Shelby L.; Nguyen, Shay N.; Bates, Morgan W.; Anastasaki, Athina; Hawker, Craig J.; de Alaniz, Javier Read
Herein, we report the development of a scalable and synthetically robust building block based on norbornadiene (NBD) that can be broadly incorporated into a variety of macromolecular architectures using traditional living polymerization techniques. By taking advantage of a selective and rapid deprotection with tetrazine, highly reactive “masked” cyclopentadiene (Cp) functionalities can be introduced into synthetic polymers as chain-end groups in a quantitative and efficient manner. The orthogonality of this platform further enables a cascade “click” process where the “unmasked” Cp can rapidly react with dienophiles, such as maleimides, through a conventional Diels–Alder reaction. Coupling proceeds with quantitative conversions allowing high molecular weight star and dendritic block copolymers to be prepared in a single step under ambient conditions.
Photoinduced Deadhesion of a Polymer Film Using a Photochromic Donor–Acceptor Stenhouse Adduct
(2019-01-01) Mostafavi, Seyed Hossein; Li, Wangxiang; Clark, Kyle D.; Stricker, Friedrich; de Alaniz, Javier Read; Bardeen, Christopher J.
Photoisomerization of molecules dissolved in a polymer film can modulate its properties. In a previous paper (Mostafavi, S. H.; Macromolecules 2018, 51, 2388−2394), it was found that the ultraviolet light-induced photoisomerization of spiropyran dopants could substantially increase adhesion to a glass surface. In this work, a different photochromic reaction, the visible-light-induced cyclization of a donor–acceptor Stenhouse adduct (DASA), leads to the opposite effect: the deadhesion of a polystyrene film from a clean glass surface. Measurements of the shear and pull-off adhesion strengths before and after visible irradiation show a light-induced decrease of 20–30%. The time required for delamination in water shows an even more dramatic decrease of 90%. Changes in the water contact angle and other measurements suggest that molecular-level noncovalent interactions between the polymer and glass are weakened after photoisomerization, possibly due to the molecular contraction of the DASA that disrupts the interaction between its amine groups and the surface silanols. The ability to reduce polymer adhesion using visible light enables the controlled release of dye molecules from a glass container, where these have been stored as a dry powder, into an aqueous solution. Embedding photochromic molecules in a polymer can lead to new effects that may have practical applications in stimuli-responsive materials.
Performance of Composite Glass–Diarylethene Crystal Photomechanical Actuator Membranes
(2022-01-01) Dong, Xinning; Guo, Tianyi; Kitagawa, Daichi; Kobatake, Seiya; Palffy-Muhoray, Peter; Bardeen, Christopher J
Hybrid organic–inorganic composites based on organic photochromic crystals embedded in inorganic templates provide a new approach to photomechanical materials. Diarylethene (DAE) nanowire crystals grown in Al2O3 membranes have exhibited reversible photoinduced bending and lifting [Dong, X., Chem. Mater. 2019, 31, 1016−1022]. In this paper, the hybrid approach is extended to porous SiO2 membranes. Despite the different template material (SiO2 instead of Al2O3) and much larger channels (5 μm diameter instead of 0.2 μm diameter), similar photomechanical behavior is observed for this new class of organic–inorganic hybrid actuators. The ability to reuse individual glass templates across different DAE filling cycles allows us to show that the DAE filling step is crucial for determining the mechanical work done by the bending template. The bending curvature also depends quadratically on the template thickness, in good agreement with theory. The light-induced bending can be repeated for up to 150 cycles without loss of performance, suggesting good fatigue resistance. The results in this paper demonstrate that the hybrid organic–inorganic approach can be extended to other host materials and template geometries. They also suggest that optimizing the organic filling and template thickness could improve the work output by an order of magnitude.
Analysis of molecular photomechanical performance using a one-dimensional harmonic model
(2022-01-01) Berges, Adam J; Bardeen, Christopher J
The photochemical reaction of a molecule leads to a change in the position of its nuclei that can be harnessed to perform mechanical work. Photomechanical materials use this effect to act as light-powered actuators. In this paper, a one-dimensional model based on coupled harmonic potential energy surfaces is developed to describe the photomechanical response of a molecule. This model generates predictions that are qualitatively consistent with standard mechanochemistry models for ground state rate reactions. To analyze the photomechanical process, excited state dynamics like photon absorption and relaxation are included. The model allows us to derive analytical expressions for the work output, blocking force, and absorbed photon-to-work efficiency. The effects of nonadiabatic electronic coupling, unequal frequency potentials, and the cycling efficiency are also analyzed. If the starting state is the stable (lower energy) isomer, it is possible to attain photon-to-work efficiencies up to 55.4%. If initial state is higher in energy, for example a metastable isomer, then one-way efficiencies > 100% are possible due to the release of stored potential energy. Photomechanical materials can be competitive with photovoltaic–piezoelectric combinations in terms of efficiency, but current materials will require substantial improvement before they can approach the theoretical limits.
Light intensity as mechanical potential: a symmetry-based approach
(2019-01-01) Guo, Tianyi; Zheng, Xiaoyu; Palffy-Muhoray, Peter
Light-induced motion is of increasing importance in materials development. In many situations, modelling the detailed dynamics of light-induced phenomena is not practical, yet a description of the behaviour is needed. We attempt to show here that in some such situations, symmetry arguments can prove useful. We outline a simple scheme where symmetry arguments can be useful in modelling the dynamics, and show, in three examples, that in some instances, light intensity may be regarded as a mechanical potential. Such an approach may help in understanding photoactuations in situations where more detailed analyses are impractical.
Anticlinic order of long-range repulsive rodlike magnetic particles in two dimensions
(2022-01-01) Zheng, Xiaoyu; Addai, Obeng Appiagyei; Palffy-Muhoray, Peter
In the field of liquid crystals, it is well known that rodlike molecules interacting via long-range attractive interactions or short-range repulsive potentials can exhibit orientational order. In this work, we are interested in what would happen to systems of rodlike particles interacting via a long-range repulsive potential. In our model, each particle consists of a number of point dipoles uniformly distributed along the particle length, with all dipoles pointing along the z axis so that the rodlike particles repel each other when they lie in the x−y plane. Dipoles from different particles interact via an r−3 potential, where r is the distance between the dipoles. We have considered two model systems, each with N particles in a unit cell with periodic boundary conditions. In the first, particle centers are fixed on a square or triangular lattice but they are free to rotate. In the second, particles are free to translate as well as rotate in cells with variable shapes. Here they self-assemble to form configurations where the stress tensors are isotropic. Our numerical results show that, at low temperatures, the particles tend to form stripes with alternating orientations, resembling herringbone patterns or the anticlinic Sm-CA liquid crystal phase.
Regimes in the Response of Photomechanical Materials
(2022-01-01) Guo, Tianyi; Svanidze, Anastasiia; Zheng, Xiaoyu; Palffy-Muhoray, Peter
Photomechanical materials perform mechanical work in response to illumination. Photoisomerization-based photomechanical materials may operate in different regimes depending on the intensity of the illuminating light. We examine the photoresponse of liquid crystalline azo-acrylate networks and show that a material property, the characteristic intensity of the material, defines the boundaries between different regimes. Asymptotic analysis indicates that whereas at low relative light levels, photostress is proportional to intensity, at high levels, it is proportional to fluence. Model predictions are in good agreement with the experimental results.
Photomechanical Structures Based on Porous Alumina Templates Filled with 9-Methylanthracene Nanowires
(2022-01-01) Berges, Adam J; Li, Wangxiang; XU, Wenwen; Tong, Fei; Al-Kaysi, Rabih O; Haywarsd, Ryan C; Bardeen, Christopher J.
Patterning Submicron Photomechanical Features into Single Diarylethene Crystals Using Electron Beam Lithography.
(2022-01-01) Li, Wangxiang; Kitagawa, Daichi; Kobatake, Seiya; Bekyarova, Elena; Bardeen, Christopher J
A Semicrystalline Poly(azobenzene) Exhibiting Room Temperature Light-Induced Melting, Crystallization, and Alignment
(2022-01-01) Zhou, Hanato; Kuenstler, Alexa S.; Xu, Wenwen; Hu, Mingqiu; Hayward, Ryan C.
Photomechanical materials powered by light-induced changes in crystalline lattices offer promise for improved performance due to the high degree of coordination between the shape changes of individual molecules. While photoswitchable semicrystalline polymers present an attractive combination of molecular ordering and material processability, systems developed to date typically show high glass transition (Tg) and melting (Tm) temperatures, limiting their ability to undergo rapid and complete photoswitching under ambient conditions. Here, we prepare a semicrystalline poly(azobenzene) containing an ethylene glycol chain extender, denoted as P(EG-azo). Because of its backbone flexibility, P(EG-azo) shows values of Tg (−20 °C) and Tm (74 °C) that are substantially lower than the previously reported analogous polymer prepared with an alkyl chain extender. This decrease in Tg and Tm translates to rapid and thorough photomelting and photocrystallization at room temperature with high reversibility. Reversible photoactuation of P(EG-azo) fibers is demonstrated, with bending deformations corresponding to an estimated specific work of 0.6 kJ m–3, 30 times larger than for previous semicrystalline poly(azobenzene) photoactuators at room temperature. In addition, photoalignment of P(EG-azo) through selective melting and templated crystallization provides a convenient and energy-efficient route to rewritable orientation with an order parameter of up to S = 0.35.
Liquid Crystal Elastomer Waveguide Actuators
(2019-01-01) Kuenstler, Alexa S.; Kim, Hyunki; Hayward, Ryan C.
While most photomechanical materials developed to date have relied on free‐space illumination to drive actuation, this strategy fails when direct line‐of‐site access is precluded. In this study, waveguided light is harnessed by liquid crystal elastomer (LCE) nanocomposites to drive actuation. Using photo‐chemical reduction of gold salts to plasmonic nanoparticles, prescription of photoresponsive regions within fibers of mono‐domain LCEs is demonstrated with control over both the location along the fiber axis, as well as in the azimuthal direction. Due to localized photothermal heating provided by plasmonic absorption of waveguided light and resulting inhomogeneous thermally induced deformation of the LCE, reversible bending along multiple axes is demonstrated.
A multi-stage single photochrome system for controlled photoswitching responses
(2022-01-01) Stricker, Friedrich; Sanchez, David M.; Raucci, Umberto; Dolinski, Neil D.; Zayas, Manuel S.; Meisner, Jan; Hawker, Craig J.; Martínez, Todd J.; de Alaniz, Javier Read
The ability of molecular photoswitches to convert on/off responses into large macroscale property change is fundamental to light-responsive materials. However, moving beyond simple binary responses necessitates the introduction of new elements that control the chemistry of the photoswitching process at the molecular scale. To achieve this goal, we designed, synthesized and developed a single photochrome, based on a modified donor–acceptor Stenhouse adduct (DASA), capable of independently addressing multiple molecular states. The multi-stage photoswitch enables complex switching phenomena. To demonstrate this, we show spatial control of the transformation of a three-stage photoswitch by tuning the population of intermediates along the multi-step reaction pathway of the DASAs without interfering with either the first or final stage. This allows for a photonic three-stage logic gate where the secondary wavelength solely negates the input of the primary wavelength. These results provide a new strategy to move beyond traditional on/off binary photochromic systems and enable the design of future molecular logic systems.
Bringing chemical structures to life with augmented reality, machine learning, and quantum chemistry
(2022-01-01) Sakshuwong, Sukolsak; Weir, Hayley; Raucci, Umberto; Martínez, Todd J.
Visualizing 3D molecular structures is crucial to understanding and predicting their chemical behavior. However, static 2D hand-drawn skeletal structures remain the preferred method of chemical communication. Here, we combine cutting-edge technologies in augmented reality (AR), machine learning, and computational chemistry to develop MolAR, an open-source mobile application for visualizing molecules in AR directly from their hand-drawn chemical structures. Users can also visualize any molecule or protein directly from its name or protein data bank ID and compute chemical properties in real time via quantum chemistry cloud computing. MolAR provides an easily accessible platform for the scientific community to visualize and interact with 3D molecular structures in an immersive and engaging way.
Chiral photochemistry of achiral molecules
(2022-01-01) Raucci, Umberto; Weir, Hayley; Bannwarth, Christoph; Sanchez, David M.; Martínez, Todd J.
Chirality is a molecular property governed by the topography of the potential energy surface (PES). Thermally achiral molecules interconvert rapidly when the interconversion barrier between the two enantiomers is comparable to or lower than the thermal energy, in contrast to thermally stable chiral configurations. In principle, a change in the PES topography on the excited electronic state may diminish interconversion, leading to electronically prochiral molecules that can be converted from achiral to chiral by electronic excitation. Here we report that this is the case for two prototypical examples – cis-stilbene and cis-stiff stilbene. Both systems exhibit unidirectional photoisomerization for each enantiomer as a result of their electronic prochirality. We simulate an experiment to demonstrate this effect in cis-stilbene based on its interaction with circularly polarized light. Our results highlight the drastic change in chiral behavior upon electronic excitation, opening up the possibility for asymmetric photochemistry from an effectively nonchiral starting point.
Formation of rolls from liquid crystal elastomer bistrips
(2022-01-01) Chen, Yuzhen; Kuenstler, Alexa S.; Hayward, Ryan C.; Jin, Lihua
Formation of desired three-dimensional (3D) shapes from flat thin sheets with programmed non-uniform deformation profiles is an effective strategy to create functional 3D structures. Liquid crystal elastomers (LCEs) are of particular use in programmable shape morphing due to their ability to undergo large, reversible, and anisotropic deformation in response to a stimulus. Here we consider a rectangular monodomain LCE thin sheet divided into one high- and one low-temperature strip, which we dub a ‘bistrip’. Upon activation, a discontinuously patterned, anisotropic in-plane stretch profile is generated, and induces buckling of the bistrip into a rolled shape with a transitional bottle neck. Based on the non-Euclidean plate theory, we derive an analytical model to quantitatively capture the formation of the rolled shapes from a flat bistrip with finite thickness by minimizing the total elastic energy involving both stretching and bending energies. Using this analytical model, we identify the critical thickness at which the transition from the unbuckled to buckled configuration occurs. We further study the influence of the anisotropy of the stretch profile on the rolled shapes by first converting prescribed metric tensors with different anisotropy to a unified metric tensor embedded in a bistrip of modified geometry, and then investigating the effect of each parameter in this unified metric tensor on the rolled shapes. Our analysis sheds light on designing shape morphing of LCE thin sheets, and provides quantitative predictions on the 3D shapes that programmed LCE sheets can form upon activation for various applications.
Effect of halogen substitution on energies and dynamics of reversible photomechanical crystals based on 9-anthracenecarboxylic acid
(2021-01-01) Gately, Thomas J.; Sontising, Watit; Easley, Connor J.; Islam, Imadul; Al-Kaysi, Rabih O.; Beran, Gregory J. O.; Bardeen, Christopher J.
9-Anthracene carboxylic acid derivatives comprise a family of thermally reversible photomechanical molecular crystals. The photomechanical response relies on a [4 + 4] photodimerization followed by dissociation that occurs on timescales of seconds to minutes. A combined theoretical and experimental investigation is undertaken to better understand how chemical modification of the anthracene core influences energetics of both the isolated molecule and the crystal lattice. We use both density functional theory and dispersion-corrected Moller–Plesset perturbation theory computational methods to establish orbital energies, photodimerization reaction energies, and lattice energies for a set of substituted 9-anthracene carboxylic acid molecules. The calculations reveal that steric interactions play a dominant role in the ability to form photodimers and indicate an energetic threshold of 80–90 kJ per mole for the dimerization reaction. Examination of intermolecular bonding in a subset of fluorinated 9ACs revealed the absence of H⋯F intermolecular bond formation and energy differences that can explain observed trends in the dissociation kinetics and mechanical reset times. Fluorescence recovery after photobleaching experiments shows that the photodimer dissociation kinetics depend on the amount of initial photodimer, preventing a straightforward correlation between halogen atom substitution and dissociation rates using the Bell–Evans–Polanyi principle. The results clarify how molecular structure affects intermolecular interactions and photoreactivity in this family of molecular crystals, but the origin of the complex photodimer dissociation dynamics remains an open question.
Reversible Adhesion Switching Using Spiropyran Photoisomierization in a High Glass Transition Temperature Polymer
(2021-01-01) Gately, Thomas J.; Li, Wangxiang; Mostafavi, Seyed Hossein; Bardeen, Christopher J.
Previous work has shown that photoisomerization of dopant molecules in a polystyrene film can either enhance or suppress its adhesion to a polar glass surface (Mostafavi, S. H. Macromolecules 2018, 51, 2388−2394; Mostafavi, S. H. Macromolecules 2019, 52, 6311–6317). In this paper, a different polymer host, Zeonex (ZX), is used in conjunction with the photochrome spiropyran. Nonpolar ZX has a higher glass transition temperature that makes it resistant to nanoscale mechanical deformations, while the spiropyran (SP) → merocyanine (MC) photoisomerization is a reversible reaction with a large polarity change. Ultraviolet light isomerizes SP to the polar MC form, increasing both the shear and pull-off adhesion forces to a clean glass surface by a factor of 5. Visible irradiation switches it back to the nonpolar SP form and returns the film back to its original weak adhesion, in contrast to the previously studied polystyrene films. The ability of visible light to switch off the polymer–glass adhesion is harnessed to make a light-controlled payload release device as well as to accelerate the polymer film delamination rate in water by a factor of 100. The kinetics of the water delamination, as well as the origin of residual adhesion after switching back to the SP form, are investigated. This work demonstrates how light-controlled noncovalent adhesion can be used as a solvent-free method to remove protective coatings or to disassemble structures.
Using Small Molecule Absorbers to Create a Photothermal Wax Motor
(2021-01-01) Lui, Brandon F.; Bardeen, Christopher J.
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.
Shining Light on Cyclopentadienone-Norbornadiene Diels-Alder Adducts to Enable Photoinduced Click Chemistry with Cyclopentadiene
(2021-01-01) Bailey, Sophia J.; Stricker, Friedrich; Hopkins, Erik; Wilson, Maxwell Z.; de Alaniz, Javier Read
A new Diels–Alder (DA)-based photopatterning platform is presented, which exploits the irreversible, light-induced decarbonylation and subsequent cleavage of cyclopentadienone–norbornadiene (CPD–NBD) adducts. A series of CPD–NBD adducts have been prepared and systematically studied toward the use in a polymeric material photopatterning platform. By incorporating an optimized CPD–NBD adduct into polymer networks, it is demonstrated that cyclopentadiene may be unveiled upon 365 nm irradiation and subsequently clicked to a variety of maleimides with spatial control under mild reaction conditions and with fast kinetics. Unlike currently available photoinduced Diels–Alder reactions that rely on trapping transient, photocaged dienes, this platform introduces a persistent, yet highly reactive diene after irradiation, enabling the use of photosensitive species such as cyanine dyes to be patterned. To highlight the potential use of this platform in a variety of material applications, we demonstrate two proof-of-concepts: patterned conjugation of multiple dyes into a polyacrylate network and preprogrammed ligation of streptavidin into poly(ethylene glycol) hydrogels.

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