Crystal-to-Gel Transformation Stimulated by a Solid-State E→Z Photoisomerization

Reversible Light-Triggered Stretching of Small-Molecule Photochromic Organic Nanoparticles

Functional organic nanomaterials are nowadays largely spread in the field of nanomedicine. In situ modulation of their morphology is thus expected to considerably impact their interactions with the surroundings. In this context, photoswitchable nanoparticles that are manufactured, amenable to extensive disassembling upon illumination in the visible, and reversible reshaping under UV exposure. Such reversibility turns to be strongly impaired for photochromic nanoparticles in close contact with a substrate. In situ atomic force microscopy investigations at the nanoscale actually reveal progressive disintegration of the organic nanoparticles under successive UV-vis cycles of irradiation, in the absence of intrinsic elastic forces. These results point out the dramatic interactions exerted by surfaces on the cohesion of non-covalently bonded organic nanoparticles. They invite to harness such systems, often used as biomarkers, to also serve as photoactivatable drug delivery nanocarriers.

Elastic Organic Crystals Exhibiting Amplified Spontaneous Emission Waveguides with Standard Red Chromaticity of the Rec.2020 Gamut

The use of mechanically flexible molecular crystals as optical transuding media is demonstrated for a plethora of applications; however, the spectral peaks of optical outputs located mainly in the range of 400-600 nm are insufficient for practical telecommunication and full-color display applications. Herein, two elastically bendable organic crystals are reported that show red emission of the rec.709 gamut under 365 nm UV light irradiation yet generate rec.2020 gamut red optical waveguides and amplified spontaneous emissions when irradiated by a 355 nm laser. Capitalizing on the extended π-conjugation and donor-acceptor character, as well as mechanical elasticity, these organic crystals exhibit flexible optical waveguides with Commission Internationale de L'Eclairage (CIE) coordinates of (0.70, 0.29), nearly identical to the red chromaticity of the rec.2020 gamut required for ultrahigh-definition (UHD) displays. Notably, one of the elastic crystals functions as a soft resonance cavity, resulting in amplified spontaneous emission waveguides with CIE coordinates of (0.71, 0.29) and the standard red chromaticity of the rec.2020 gamut, both in straight and bent states. This study presents a new avenue for the development of high-purity red-emissive crystalline materials to create all-organic, lightweight, and mechanically compliant optical telecommunication and UHD display devices.

Exploiting benzilic acid as a modular template: controlling photoreactivity and solid to liquid transition during photodimerization

A well-known molecule, benzilic acid, is used as a [2+2] photodimerization template by using third-generation crystal engineering principles. This template utilizes orthogonality and non-covalent interactions in an optimized way and is shown to be effective in tuning the photoreactivity of styryl pyridine derivatives. The photo-induced crystal-to-liquid transformation was observed during photodimerization. This phenomenon is explained based on slip plane and energy framework analysis.

Photoinduced Crystal-to-Liquid Transition of Acylhydrazone-Based Photoswitching Molecules

A photoinduced crystal-to-liquid transition (PCLT) behavior of new acylhydrazone derivatives (NCs) is reported. The photoswitching of the NCs was identified as a negative photochromism with a high E-to-Z conversion yield (>98%). A kinetic analysis shows a half-life of almost one month. Owing to these high photoswitching performances, we successfully isolated both E- and Z-forms, evaluated their crystal structures, and observed distinct thermal behaviors. The Z-form melts at a lower temperature than the E-form by several tens of degrees. The PCLT occurs at even lower temperatures. UV irradiation induces the E-to-Z conversion in the crystalline state, thereby inducing a eutectic melting. In addition to the PCLT, we observed a photomechanical behavior of the crystals, which suggests that the presented acylhydrazones can be new members of the photoresponsive crystalline materials.

Engineering Photomechanical Molecular Crystals to Achieve Extraordinary Expansion Based on Solid-State [2 + 2] Photocycloaddition

Photomechanical molecular crystals are promising candidates for photoactuators and can potentially be implemented as smart materials in various fields. Here, we synthesized a new molecular crystal, (E)-3-(naphthalen-1-yl)acrylaldehyde malononitrile ((E)-NAAM), that can undergo a solid-state [2 + 2] photocycloaddition reaction under visible light (≥400 nm) illumination. (E)-NAAM microcrystals containing symmetric twinned sealed cavities were prepared using a surfactant-mediated crystal seeded growth method. When exposed to light, the hollow microcrystals exhibited robust photomechanical motions, including bending and dramatic directional expansion of up to 43.1% elongation of the original crystal length before fragmentation due to the photosalient effect. The sealed cavities inside the microcrystals could store different aqueous dye solutions for approximately one month and release the solutions instantly upon light irradiation. A unique slow-fast-slow crystal elongation kinematic process was observed, suggesting significant molecular rearrangements during the illumination period, leading to an average anisotropic crystal elongation of 37.0% (±3.8%). The significant molecular structure and geometry changes accompanying the photocycloaddition reaction, which propels photochemistry to nearly 100% completion, also facilitate photomechanical crystal expansion. Our results provide a possible way to rationally design molecular structures and engineer crystal morphologies to promote more interesting photomechanical behaviors.

Structural phase transitions in flexible DUT-8(Ni) under high hydrostatic pressure

The behaviours of the open pore (op) and closed pore (cp) phases of the flexible Ni₂(ndc)₂(dabco) (ndc - 2,6-naphthalene dicarboxylate, dabco - 1,4-diazabicyclo[2.2.2]octane, DUT-8(Ni)) metal-organic framework under high hydrostatic pressures up to 10 GPa in isopropanol and silicone oil were studied by Raman spectroscopy. Ab initio simulations of vibrational spectra were performed for the open and closed pore phases, which allowed us to disclose the characteristic vibrational modes affected by the structural transitions under pressure. Analysis of theoretical and experimental Raman data suggests that the op-cp transition involves gateway vibrations at 25 and 67 cm^-1, corresponding to trampoline/rotational motions of aromatic linkers. The experiments reveal the formation of new distorted cp phases at pressures higher than 2 GPa, which are formed without amorphisation. The transition between the cp phase and the distorted cp phase is reversible. The experiments also reveal the pivotal role of the pressure transmitting medium on the phase transition behaviour.

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.

Temperature-Reliant Dynamic Properties and Elasto-Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt

Although, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid-state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two-face elasticity (within ≈1 % strain), followed by elasto-plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X-ray diffraction, μ-Raman and polarized light microscopy experiments reveal that the elasto-plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi-functional dynamic crystals.

Ultra-Low Molecular Weight Photoswitchable Hydrogelators

Two photoswitchable arylazopyrozoles form hydrogels at a concentration of 1.2 % (w/v). With a molecular weight of 258.28 g mol^-1 , these are the lowest known molecular weight hydrogelators that respond reversibly to light. Photoswitching of the E- to the Z-form by exposure to 365 nm light results in a macroscopic gel→sol transition; nearly an order of magnitude reduction in the measured elastic and loss moduli. In the case of the meta-arylazopyrozole, cryogenic transmission electron microscopy suggests that the 29±7 nm wide sheets in the E-gel state narrow to 13±2 nm upon photoswitching to the predominantly Z-solution state. Photoswitching for meta-arylazopyrozole is reversible through cycles of 365 nm and 520 nm excitation with little fatigue. The release of a rhodamine B dye encapsulated in gels formed by the arylazopyrozoles is accelerated more than 20-fold upon photoswitching with 365 nm light, demonstrating these materials are suitable for light-controlled cargo release.

Reversible Color and Shape Changes of Nanostructured Fibers of a Macrocyclic π-Extended Thiophene Hexamer Promoted by Adsorption and Desorption of Organic Vapor

A phenyl-substituted macrocyclic π-extended thiophene hexamer 1, composed of four thienylene-ethynylene and two thienylene-vinylene units, has a solid-state structure in which π-π, CH-π, and van der Waals interactions occur. Slow addition of acetone to a solution of 1 in CS₂ produces a yellow nanostructured fiber 1-A containing a 1:1.5:1 ratio of 1, acetone, and CS₂. Over a 2 min period at 25 °C, 1-A gradually changes to an orange fiber 1-B containing a 1:0.5:1 ratio of 1, acetone, and CS₂. On exposure to acetone vapor, 1-B regenerates 1-A (vapochromism), and removal of all solvents from 1-A and 1-B generates a red-orange fiber 1-C, which upon brief immersion in acetone/CS₂ produces 1-A. Furthermore, 1-C is converted to orange yellow fiber 1-D upon exposure to acetone vapor for 1 s at 25 °C. Analysis of the horizontal and vertical profiles of the X-ray diffraction (XRD) patterns shows that removal of solvent from 1-A reversibly creates 1-B in conjunction with a shape and size change along with arching.

Mechanical Actuation and Patterning of Rewritable Crystalline Monomer-Polymer Heterostructures via Topochemical Polymerization in a Dual-Responsive Photochromic Organic Material

The dark-orange monomer single crystals of 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyldidodecanoate (BIT-dodeca₂) convert to a transparent single-crystalline polymer (PBIT-dodeca₂) material via a single-crystal-to-single-crystal (SCSC) polymerization reaction under sunlight, which then undergoes reverse thermal transformation into BIT-dodeca₂ single crystals, leading to reversible photo-/thermochromism, coupled with mechanical actuation. We exploit the properties of this unique material to demonstrate the formation of monomer-polymer heterostructures in selected regions of single crystals with micrometer-scale precision using a laser. This is the first example of heterostructure patterning involving monomer-polymer domains in single crystals. We reveal that the speed of photomechanical bending induced by the polymerization reaction in this example is comparable to those of the well-known diarylethene derivatives, in which electrocyclic ring-closing-ring-opening reactions operate. Furthermore, we characterize the distinct mechanical properties of the monomer and polymer using a quantitative nanoindentation technique as well as demonstrate photopatterning on a monomer-coated paper for potential use in security devices. These crystals with several advantages, such as photomechanical bending (weight lifting) even when the crystal size is large, responsiveness to both UV and visible light, distinct solubilities (the polymer is insoluble, whereas the monomer is soluble in most organic solvents) and colors, provide unique opportunities for their use at different length scales of the sample (μm to mm) for various purposes.