Highly branched photomechanical crystals

Fabrication of Hyperbranched Photomechanical Crystals Composed of a Photochromic Diarylethene

We report the fabrication of hyperbranched hollow crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene on a concave surface of the spherical glass substrate by sublimation and their practical photomechanical behaviors. The number of units of the branched structure of the hollow crystals composed of this compound is proportional to the substrate curvature of the substrate. Compared with the sublimation process of the same compound on the flat glass substrate, two kinds of the thin film domains are generated separately in the center and around the edge of the spherical glass substrate. Especially under the high relative humidity condition, the boundaries between these thin film domains move gradually around the edge through the center during as long as 6 h of sublimation time so that the hyperbranched hollow crystals are densely produced on the entire surface of the substrate. These hyperbranched hollow crystals can be prepared with the highly ordered molecular packing due to the very slow formation process of the crystalline walls of the hollow structures. Furthermore, the photo-induced bending behaviors in the few- and highly-branched hollow crystals have the practical roles in moving and bending the minute objects according to their characteristics of these branched shapes.

Solid-State [4+4] Cycloaddition and Cycloreversion with Use of Unpaired Hydrogen-Bond Donors to Achieve Solvatomorphism and Stabilization

The crystal structure of a commercially available anthracene derivative, anthracene-9-thiocarboxamide, is reported here for the first time. The compound undergoes a [4+4] cycloaddition in the solid state to afford facile synthesis of the cycloadduct (CA). The cycloaddition is also reversible in the solid state using heat or mechanical force. Due to the presence of unpaired, strong hydrogen-bond donor atoms on the CA, significant solvatomorphism is achieved, and components of the solvatomorphs self-assemble into four different classes of supramolecular structures. The CA readily crystallizes with a variety of structurally-diverse solvents including those containing oxygen-, nitrogen-, or pi-acceptors. Some of the solvents the CA crystallized with include thiophene, benzene, and the three xylene isomers; thus, the CA was employed in industrially-relevant solvent separation. However, in competition studies, the CA did not exhibit selectivity. Lastly, it is demonstrated that the CA crystallizes with vinyl-containing monomers and is currently the only compound that crystallizes with both widely used monomers 4-vinylpyridine and styrene. Solid-state complexation of the CA with the monomers affords over a 50 °C increase in the monomer's thermal stabilities. The strategy of designing molecules with unused donors can be applied to achieve separations or volatile liquid stabilization.

Structural Capture of η1-OSO to η2-(OS)O Coordination Isomerism in a New Ruthenium-Based SO2-Linkage Photoisomer That Exhibits Single-Crystal Optical Actuation

Recent discoveries of a range of single-crystal optical actuators are feeding a new form of materials chemistry, given their broad range of potential applications, from light-induced molecular motors to light sensors and optical-memory media. A series of ruthenium-based coordination complexes that exhibit sulfur dioxide linkage photoisomerization is of particular interest because they exhibit single-crystal optical actuation via either optical switching or nano-optomechanical transduction processes. We report the discovery of a new complex in this series of chemicals, [Ru(SO₂)(NH₃)₄(3-fluoropyridine)]tosylate₂ (1), which forms an η¹-OSO photoisomer with 70% photoconversion upon the application of 505 nm light. The uncoordinated oxygen atom in this η¹-OSO photoisomer impinges on one of the arene rings in a neighboring tosylate counter ion of 1 just enough that incipient nano-optomechanical transduction is observed. The structure and optical properties of this actuator are characterized via in situ light-induced single-crystal X-ray diffraction (photocrystallography), single-crystal optical absorption spectroscopy and microscopy, as well as single-crystal Raman spectroscopy. These materials-characterization methods were also used to track thermally induced reverse isomerization processes in 1. One of these processes involves an η¹-OSO to η²-(OS)O transition, which was found to proceed sufficiently slowly at 110 K that its structural mechanism could be determined via a time sequence of photocrystallography experiments. The resulting data allowed us to structurally capture the transition, which was shown to occur via a form of coordination isomerism. Our newfound knowledge about this structural mechanism will aid the molecular design of new [RuSO₂] complexes with functional applications.

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.

Modeling dark- and light-induced crystal structures and single-crystal optical absorption spectra of ruthenium-based complexes that undergo SO2-linkage photoisomerization

A family of coordination complexes of the type [Ru(SO₂)(NH₃)₄X]^m+Y_n ^- (m, n = 1 or 2) exhibit optical switching capabilities in their single-crystal states. This striking effect is caused by the light-induced formation of SO₂-linkage photoisomers, which are metastable if kept at suitably cool temperatures. We modeled the dark- and light-induced states of these large crystalline complexes via plane-wave (PW)- and molecular-orbital (MO)-based density functional theory (DFT) and time-dependent DFT in order to calculate their structural and optical properties; the calculated results are compared with experimental data. We show that the PW-DFT-based periodic models replicate the structural properties of these complexes more effectively than the MO-DFT-based molecular-fragment models, observing only small deviations in key bond lengths relative to the experimentally derived crystal structures. The periodic models were also found to more effectively simulate trends seen in experimental optical absorption spectra, with optical absorbance and coverage of the visible region increasing with the formation of the photoinduced geometries. The contribution of the metastable photoisomeric species more heavily focuses on the lower-energy end of the spectra. Spectra generated from the molecular-fragment models are limited by the geometry of the fragment used and the number of excited-state roots considered in those calculations. In general, periodic models outperform the molecular-fragment models owing to their ability to better appreciate the periodic phenomena that are present in these crystalline materials as opposed to MO approaches, which are finite methods. We thus demonstrate that PW-DFT-based periodic models should be considered as a more than viable method for simulating the optical and electronic properties of these single-crystal optical switches.

Nanooptomechanical Transduction in a Single Crystal with 100% Photoconversion

Materials that exhibit nanooptomechanical transduction in their single-crystal form have prospective use in light-driven molecular machinery, nanotechnology, and quantum computing. Linkage photoisomerization is typically the source of such transduction in coordination complexes, although the isomers tend to undergo only partial photoconversion. We present a nanooptomechanical transducer, trans-[Ru(SO2)(NH3)4(3-bromopyridine)]tosylate2, whose S-bound η1-SO2 isomer fully converts into an O-bound η1-OSO photoisomer that is metastable while kept at 100 K. Its 100% photoconversion is confirmed structurally via photocrystallography, while single-crystal optical absorption and Raman spectroscopies reveal its metal-to-ligand charge-transfer and temperature-dependent characteristics. This perfect optical switching affords the material good prospects for nanooptomechanical transduction with single-photon control.

Rapid build up of nanooptomechanical transduction in single crystals of a ruthenium-based SO2 linkage photoisomer

Single-crystal nanooptomechanical transduction occurs in [Ru(SO₂)(NH₃)₄(H₂O)]chlorobenzenesulfonate₂, reaching maximal levels within 40 s at 100 K when photostimulated by 505 nm light. Its in situ light-induced crystal structure reveals the molecular origins of this optical actuation: 26.0(3)% of the η¹-SO₂ ligand photoconverts into an η¹-OSO photoisomer which, in turn, induces a 49.6(9)° arene ring rotation in its neighbouring counter ion.

Dynamic effects in crystalline coordination polymers

Dynamic macroscopic behaviour of single crystals of coordination polymers when subjected to light, heat, and mechanical force.

Anisotropic bending and twisting behaviour of a twin crystal composed of a diarylethene

The unusual photomechanical behaviour of a “twin crystal” consisting of a diarylethene derivative was investigated.

Molecular Crystal Microcapsules: Formation of Sealed Hollow Chambers via Surfactant-Mediated Growth

Hollow organic molecular cocrystals comprised of 9-methylanthracene-1,2,4,5-tetracyanobenzene (9MA-TCNB) and naphthalene-1,2,4,5-tetracyanobenzene (NAPH-TCNB) were fabricated using a surfactant-mediated co-reprecipitation method. The crystals exhibit a narrow size distribution that can be easily tuned by varying the concentration of surfactant and incubation temperature. The rectangular crystals possess symmetrical twinned cavities with an estimated storage volume on the order of 10^-10  L. An aqueous dye solution can be incorporated into the cavities during crystal growth and stored inside for up to several hours, confirming the sealed nature of the hollow chambers. Our results demonstrate that it is possible to harness non-classical crystal growth to fabricate organic molecular crystals with novel topologies.

Single-Crystal-to-Single-Crystal [2 + 2] Photocycloaddition Reaction in a Photosalient One-Dimensional Coordination Polymer of Pb(II)

In a remarkable example, we report a one-dimensional coordination polymer (CP) of Pb(II) showing photosalient (PS) properties triggered by [2 + 2] cycloaddition of olefinic ligands, which is seldom observed in CPs. Macroscopic rod-shaped crystals show various photomechanical effects such as jumping, splitting, rolling, and breaking upon UV illumination. In this rare example, we could determine the solid-state structure of the 100% dimerized product and three intermediate structures, even after the shattering of crystals into small pieces. Detailed mechanistic investigation from the single-crystal data indicates that the strain generated in the unit cell due to anisotropic expansion played a bigger role for the PS effects. Nucleated growth of the photoproduct crystal created different domains inside the single crystal, which multiplied the already developed stress leading to the photomechanical movements. This example falls in the gray area of a clean single-crystal-to-single-crystal (SCSC) transformation and violent PS effect. Such photochemical behavior has never been reported before.

Cracking under Internal Pressure: Photodynamic Behavior of Vinyl Azide Crystals through N2 Release

When exposed to UV light, single crystals of the vinyl azides 3-azido-1-phenylpropenone (1a), 3-azido-1-(4-methoxyphenyl)propenone (1b), and 3-azido-1-(4-chlorophenyl)propenone (1c) exhibit dramatic mechanical effects by cracking or bending with the release of N₂. Mechanistic studies using laser flash photolysis, supported by quantum mechanical calculations, show that each of the vinyl azides degrades through a vinylnitrene intermediate. However, despite having very similar crystal packing motifs, the three compounds exhibit distinct photomechanical responses in bulk crystals. While the crystals of 1a delaminate and release gaseous N₂ indiscriminately under paraffin oil, the crystals of 1b and 1c visibly expand, bend, and fracture, mainly along specific crystallographic faces, before releasing N₂. The photochemical analysis suggests that the observed expansion is due to internal pressure exerted by the gaseous product in the crystal lattices of these materials. Lattice energy calculations, supported by nanoindentation experiments, show significant differences in the respective lattice energies. The calculations identify critical features in the crystal structures of 1b and 1c where elastic energy accumulates during gas release, which correspond to the direction of the observed cracks. This study highlights the hitherto untapped potential of photochemical gas release to elicit a photomechanical response and motility of photoreactive molecular crystals.

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.

Straight, bendable and bent organic crystals

trans-4-Phenylazobenzoic acid (pab) crystallized in three different morphologies: long rod-like crystals, bendable long thin crystals, and bent crystals. Of them, the bent crystals were obtained by recrystallizing after subjecting pab to UV-irradiation in solution. A small amount of cis-form in the bent crystals is responsible for the bent nature, while the elastic bending of thin platy crystals can be understood from the crystal packing.

Photoinduced peeling of molecular crystals

Block-like microcrystals composed of cis-dimethyl-2(3-(anthracen-9-yl)allylidene)malonate are grown from aqueous surfactant solutions. A pulse of 405 nm light converts a fraction of molecules to the trans isomer, creating an amorphous mixed layer that peels off the parent crystal. This photoinduced delamination can be repeated multiple times on the same block.

Generating Photonastic Work from Irradiated Dyes in Electrospun Nanofibrous Polymer Mats

For solar-driven macroscopic motions, we assert that there is a local heating that facilitates large-scale deformations in anisotropic morphologic materials caused by thermal gradients. This report specifically identifies the fate of heat generation in photonastic materials and demonstrates how heat can perform work following excitation of a nonisomerizing dye. Utilizing the electrospinning technique, we have created a series of anisotropic nanofibrous polymer mats that comprise nonisomerizing dyes. Polymers are chosen because of their relative glass transition temperatures, elastic moduli, and melting temperatures. Light irradiation of these polymer mats with an excitation wavelength matching the absorption characteristics of the dye leads to macroscopic deformation of the mat. Analysis of still images extracted from digital videos provides plots of angular displacement vs power. The data were analyzed in terms of a photothermal model. Analyses of scanning electron microscopy micrographs for all samples are consistent to local melting in low T_g polymers and softening in high T_g polymers. Dynamic mechanical analysis allowed for quantification of the modulus change under a given light fluence. We employ these data to calculate a energy conversion efficiency. These efficiencies for the polymer mats are compared to other nonmuscular systems, including a few natural, biological samples.

Control of Photomechanical Crystal Twisting by Illumination Direction

Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twisting of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) crystals depends on illumination direction. The ribbon-like crystal of 1a could be successfully prepared by a sublimation method. The ribbon crystal exhibited reversible photomechanical crystal twisting upon alternating irradiation with ultraviolet (UV) and visible light. Moreover, changing the UV illumination direction with respect to the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. Control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.

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.