Analysis of reaction kinetics in the photomechanical molecular crystal 9-methylanthracene using an extended Finke-Watzky model

Cooperative Photochemical Reaction Kinetics in Organic Molecular Crystals

Photoreactive molecular crystals have been intensively investigated as next-generation functional materials. Changes in physicochemical properties are usually interpreted in terms of static pre- and post-reaction molecular structures and packings determined by X-ray structure analysis. However, to elucidate the dynamic properties, it is necessary to understand the dynamic nature of photochemical kinetics in crystals. Reaction dynamics in the crystal phase can be dramatically different from those in dilute solution because the local molecular environment evolves as the surrounding reactant molecules are transformed into products. In this Review article, we summarize multiple examples of photochemical reactions in the crystalline phase that do not follow classical kinetic behavior. We also discuss different theoretical methods that can be used to describe this behavior. This Review article should help provide a foundation for future workers to understand and analyze photochemical reaction kinetics in crystals.

Edge-to-Center Propagation of Photochemical Reaction during Single-Crystal-to-Single-Crystal Photomechanical Transformation of 2,5-Distyrylpyrazine Crystals

Photomechanical molecular crystals are promising materials for photon-powered artificial actuators. To interpret the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be an important role in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine show a smooth single-crystal-to-single-crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. We revealed that the surface effect (special reactivity at the crystal surface) in addition to the cooperative effect (the reaction is facilitated by neighboring molecules) is essential for the edge-to-center propagation of the photochemical reaction. Our results would provide a foundation for future studies of the photochemical reaction dynamics in photomechanical molecular crystals.

Photomechanical crystalline materials: new developments, property tuning and applications

This highlight gives an overview of the mechanism development, property tuning and application exploration of photomechanical crystalline materials.

Pseudoelementary Steps: A Key Concept and Tool for Studying the Kinetics and Mechanisms of Complex Chemical Systems

The concept of a pseudoelementary step (PEStep) is reviewed, a key concept for approaching the analysis of kinetics data and associated, underlying mechanisms of complex chemical systems. Following a brief Introduction, a definition of a PEStep is given: a PEStep is an initial building block for more complex reactions, that is a starting point for the initial analysis of the observed kinetics and then constructing initial, deliberately minimalistic mechanistic models for complex reactions. PESteps are, therefore and typically, composites of underlying elementary step reactions and can be very useful if not required for the inverse problem of discovering mechanisms from experimental observables for complex reactions. It is the use of PESteps in the inverse problem of mechanism determination that is a primary focus of this review. After a section detailing the results of a literature search of "pseudoelementary step" and related terms such as "pseudoelementary process", pedagogically illustrative examples are given of the use of the PEStep concept in approaching and elucidating the mechanisms of complex reactions. This review shows how the underlying elementary steps of a catalytic cycle were successfully uncovered via a PEStep approach, addresses the classic case of the use of PESteps in determining the mechanisms of oscillating reactions, and examines a well-studied case of an Ir(0)_n nanoparticle formation reaction. This latter example is illustrative in that the Ir(0)_n nanoparticle formation reaction consisting of thousands of underlying elementary steps that, however, can be treated initially kinetically as just two PESteps, a reduction in complexity of 3 orders of magnitude. Known weaknesses and caveats of the PEStep approach are also summarized and discussed. A short summary of Horituti's "Stoichiometric Number" concept is provided, a concept that would appear to merit further investigation and use in the study of complex reactions. Finally, a section is provided that lists a few, selected areas where the PEStep concept and methodology are expected to prove especially important in the future, and a Conclusions section is provided that lists 11 bullet points. The latter serves as a summary of this first review of the PEStep concept and its importance in dealing with the kinetics and in elucidating the mechanisms of more complex, multistep reactions.

Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9-Methylanthracene Single Crystals

Photomechanical molecular crystals that expand under illumination could potentially be used as photon-powered actuators. In this study, we find that the use of high-quality single crystals of 9-methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c-axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well-defined linear expansions during 100 % photoconversion, suggesting that this class of solid-state phase change material could be used for actuation.

Photochemical-induced phase transitions in photoactive semicrystalline polymers

The emergent photoactive materials obtained through photochemistry make it possible to directly convert photon energy to mechanical work. There has been much recent work in developing appropriate materials, and a promising system is semicrystalline polymers of the photoactive molecule azobenzene. We develop a phase field model with two order parameters for the crystal-melt transition and the trans-cis photoisomerization to understand such materials, and the model describes the rich phenomenology. We find that the photoreaction rate depends sensitively on temperature: At temperatures below the crystal-melt transition temperature, photoreaction is collective, requires a critical light intensity, and shows an abrupt first-order phase transition manifesting nucleation and growth; at temperatures above the transition temperature, photoreaction is independent and follows first-order kinetics. Further, the phase transition depends significantly on the exact forms of spontaneous strain during the crystal-melt and trans-cis transitions. A nonmonotonic change of photopersistent cis ratio with increasing temperature is observed accompanied by a reentrant crystallization of trans below the melting temperature. A pseudo phase diagram is subsequently presented with varying temperature and light intensity along with the resulting actuation strain. These insights can assist the further development of these materials.

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

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.

Improving photosensitivity without changing thermal reactivity in photochromic diarylbenzenes based on accurate prediction by DFT calculations

1,2-Diarylbenzenes (DABs) have been developed as a new family of fast T-type photochromic switches. However, the molecular design strategy for DABs with desired optical and thermal properties is not established. In this work, we explored the best functional in quantum chemical calculations to predict the properties of DABs. Furthermore, we newly designed and synthesized DABs based on the calculation using the best functional, resulting in the improvement of the photosensitivity in the UV-A region (i.e. a shift of absorption to lower energies and an increase in the absorption coefficient) without changing the thermal back-reaction rate.

Photomechanically Induced Magnetic Field Response by Controlling Molecular Orientation in 9-Methylanthracene Microcrystals

A surfactant-assisted seeded-growth method is used to form single-crystal platelets composed of 9-methylanthracene with two different internal molecular orientations. The more stable form exhibits a photoinduced twisting, as observed previously for 9-methylanthracene microribbons grown by the floating drop method. However, the newly discovered elongated hexagonal platelets undergo a photoinduced rolling-up and unrolling. The ability of the rolled-up cylindrical shape to trap superparamagnetic nanoparticles enables it to be carried along in a magnetic field gradient. The new photoinduced shape change, made possible by a novel surfactant-assisted crystal growth method, opens up the possibility of using light to modulate the crystal translational motion.

Gold Nanoparticle Formation Kinetics and Mechanism: A Critical Analysis of the "Redox Crystallization" Mechanism

A 2013 paper proposed a "redox crystallization" (R-C) mechanism for the formation of Au⁰ _n nanoparticles from the reduction of a AuCl₄ ^- precursor. That study used an unconventional analysis of the valuable, expertly obtained kinetics data reported, and came up with multiple claims and insights collected under the putatively new R-C mechanism. If confirmed, those claims and the R-C mechanism provide a valuable addition to the knowledge of gold nanoparticle formation kinetics and mechanisms. On the other hand, if the methodology used to support the R-C mechanism is flawed so that its resultant conclusions are incorrect, then the R-C mechanism needs to be discarded until compelling evidence for it can be gathered, evidence that would have to include the disproof of the other dominant mechanism(s) of nanoparticle formation. The present work provides a critical analysis of the evidence previously offered for the R-C mechanism, efforts that are of interest to the areas of Au⁰ _n nanoparticles, the kinetics and mechanisms of nanoparticle formation and, as it turns out, more generally to those interested in kinetic and mechanistic studies.

Photomechanical motion of diarylethene molecular crystal nanowires

Crystalline nanowires composed of the photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) are prepared and characterized. 200 nanometer diameter wires with a length of 60 microns are grown by slow solvent annealing in a porous anodic aluminum oxide template. The nanowires are oriented crystals, as determined by X-ray diffraction measurements, and can be liberated by dissolving the template in acid. They exhibit pronounced bending when exposed to ultraviolet light that can be reversed by visible light irradiation. The bending-unbending sequence can be repeated for more than 10 cycles without fatigue. This robustness results from the ability of the nanowires to maintain their crystallinity during the forward and reverse reactions. The small diameter of these nanowires allows them to achieve curvatures that are at least 40 times greater (200 mm^-1versus 5 mm^-1) than those observed for micron-thick diarylethene needles. This first demonstration of photomechanical nanostructures based on diarylethene photochromism opens up the possibility of making more complicated structures composed of this high-performance photochrome.

Surfactant-Enhanced Photoisomerization and Photomechanical Response in Molecular Crystal Nanowires

Dimethyl-2(3-anthracen-9-yl)allylidene)malonate (DMAAM) is a divinylanthracene derivative that photoisomerizes between its (E) and (Z) conformations. Crystalline nanowires composed of this molecule undergo a rapid coiling motion when exposed to visible light. In this paper, a variety of experimental techniques are used to investigate the mechanism of this transformation, including powder X-ray diffraction, polarized light microscopy, ¹H NMR, and absorption spectroscopy. The results show that the presence of a surfactant like cetyltrimethylammonium bromide (CTAB) accelerates the photochemical reaction rate by at least a factor of 10 within the nanowire and is required to observe the photoinduced coiling. The accelerated reaction facilitates the transition to an amorphous phase composed of reactant and photoproduct, which leads to the rapid, large-scale shape changes that the nanowires undergo. Disruption of the highly packed crystal structure by photoisomerization also enhances the dissolution rate by a factor of about 30. The fact that the nanowires have a nominal diameter of 200 nm suggests that the presence of surface species can influence the reaction dynamics deep inside the crystal. These results show that the reaction dynamics and photomechanical motions of nanoscale molecular crystals can be extremely sensitive to surface species.

Covalent switching, involving divinylbenzene ligands within 3D coordination polymers, indicated by changes in fluorescence

Reversible single-crystal-to-single-crystal transformations of two photo-responsive three-dimensional coordination polymers exhibit green/blue and green/blue-green fluorescence switching behaviors.