Photoinduced Mechanical Motions of Pseudorotaxane Crystals Composed of Azobenzene and Ferrocenyl Groups on an Axle and a Crown Ether Ring

Photosalience and Thermal Phase Transitions of Azobenzene- and Crown Ether-Based Complexes in Polymorphic Crystals

Stimuli-responsive molecular crystals have attracted considerable attention as promising smart materials with applications in various fields such as sensing, actuation, and optoelectronics. Understanding the structure-mechanical property relationships, however, remains largely unexplored when it comes to functionalizing these organic crystals. Here, we report three polymorphic crystals (Forms A, B, and C) formed by the non-threaded complexation of a dibenzo[18]crown-6 (DB18C6) ether ring and an azobenzene-based ammonium cation, each exhibiting distinct thermal phase transitions, photoinduced deformations, and mechanical behavior. Structural changes on going from Form A to Form B and from Form C to Form B during heating and cooling, respectively, are observed by single-crystal X-ray crystallography. Form A shows photoinduced reversible bending, whereas Form B exhibits isotropic expansion. Form C displays uniaxial negative expansion with a remarkable increase of 44% in thickness under photoirradiation. Force measurements and nanoindentation reveal that the soft crystals of Form A with a low elastic modulus demonstrate a significant photoresponse, attributed to the non-threaded molecular structure, which permits flexibility of the azobenzene unit. This work represents a significant advance in the understanding of the correlation between structure-thermomechanical and structure-photomechanical properties necessary for the development of multi-stimulus-responsive materials with tailored properties.

Facile Photoresponsive Actuators Based on Ferrocene-Doped Poly(butyl methacrylate)

This paper presents facile photoresponsive actuators comprising ferrocene as a guest chromophore and poly(butyl methacrylate) (PBMA) as a host matrix. The ferrocene-doped PBMA film exhibits mechanical expansion and contraction when a 445 nm laser is turned on and off, respectively. The photoresponsive film is attached by a commercially available acetylcellulose adhesive tape, which exhibits a bending motion that is controlled by turning the laser on and off. Thereafter, the double-layer film is employed to fabricate a table-shaped lifting machine (0.7 mg) that lifts a 10.5 mg object up and down by turning the laser on and off, respectively, and the mechanical force offered by the double-layer film is recorded. Additionally, the film is coated with gold and applied to an electric circuit that serves as a reversible photoresponsive switch. This film preparation technique is applied to other chromophores (e.g., Coumarin 343, Rhodamine 6G, Sudan Blue II, and Solvent Green 3) to independently control the motions of the films with 445, 520, and 655 nm lasers. The ferrocene-containing films also exhibit photoinduced healing from mechanical damage. Finally, the photoirradiation-accompanied morphological changes in the film are observed via small-angle X-ray scattering.

Light-responsive rotaxane-based materials: inducing motion in the solid state

Light-responsive rotaxane-based solid-state materials are ideal scaffolds in order to develop smart materials due to the properties provided by the mechanical bond, such as control over the dynamics of the components upon application of external stimuli. This perspective aims to highlight the relevance of these materials, by pointing out recent examples of photoresponsive materials prepared from a rotaxanated architecture in which motion of the counterparts and/or macroscopic motion of the interlocked materials are achieved. Although further development is needed, these materials are envisioned as privileged scaffolds which will be used for different advanced applications in the area of molecular machinery.

Voltage-Controlled Conversion from CDS to MDS in an Azobenzene-Based Organic Memristor for Information Storage and Logic Operations

For organic memristors, non-zero-crossing current-voltage (I-V) curves are often observed, which can be attributed to capacitive effects. If the conversion between the capacitance-dominated state (CDS) and the memristance-dominated state (MDS) can be realized in a controllable manner, more device functions can be obtained. In this work, a two-terminal memristor using a common organic dye, azobenzene (AZB), as the active layer was prepared. It is found that as the applied voltage gradually increases, the device can transition from CDS to MDS. In the low voltage range (<1 V), the device is in CDS, and the capacitance is significantly increased by ∼10⁴ compared to the theoretical value. In the high voltage range (>1 V), the device is in MDS, achieving an HRS (high resistance state)/LRS (low resistance state) resistance ratio of ∼10⁴, and the logic operations are achieved. Through the analysis of the I-V curve, energy diagram of the materials, and computer simulation results, the mechanisms of CDS, MDS, and their conversion are proposed. This work provides an in-depth understanding of the working mechanism of organic memristors and demonstrates the potential of AZB-based organic memristors for information storage and logic display applications.

Ferrocene-Containing Pseudorotaxanes in Crystals: Aromatic Interactions with Hammett Correlation

Single crystals of pseudorotaxanes, [(FcCH₂NH₂CH₂Ar)(DB24C8)][PF₆] (DB24C8 = dibenzo[24]crown-8, Fc = Fe(C₅H₄)(C₅H₅), Ar = -C₆H₃-3,4-Cl₂, -C₆H₃-3,4-F₂, -C₆H₄-4-F, -C₆H₄-4-Cl, -C₆H₄-4-Br, -C₆H₃-3-F-4-Me, -C₆H₄-4-I) and [(FcCH₂NH₂CH₂C₆H₄-4-Me)(DB24C8)][Ni(dmit)₂] (dmit = 1,3-dithiole-2,4,5-dithiolate), were obtained from solutions containing DB24C8 and ferrocenylmethyl(arylmethyl)ammonium. X-ray crystallographic analyses of the pseudorotaxanes revealed that the aryl ring of the axle moiety and the catechol ring of the macrocyclic component were at close centroid distances and parallel or tilted orientation. The structures with parallel aromatic rings showed correlation of the distances between the centroids to Hammett substituent constants of the aryl groups.

Ring rotation of ferrocene in interlocked molecules in single crystals

This work describes unique molecular motions of ferrocene-containing interlocked molecules observed by single-crystal X-ray crystallography. The rotational flexibility of ferrocene is achieved using combinations of ferrocene-tethered ammonium and 30-membered ring dibenzo-crown ether. By contrast, ferrocene was locked in the complex with an 18-membered ring dibenzo-crown ether and CH2Cl2. When the complex was heated at 358 K, CH2Cl2 was removed from the complex, which led to drastic structural changes, including a semieclipsed-to-disordered transition of ferrocene and flipping of the dibenzo-crown ether.

Further investigations of the crystal-to-crystal phase transition of a [2]pseudorotaxane composed of ferrocene-terminated dialkylammonium and dibenzo[24]crown-8-ether

A pseudorotaxane with different or mixed counter anions undergoes thermal phase transition, which is related to the interaction between the counter anion and macrocyclic component.