Surfactant-Enhanced Photoisomerization and Photomechanical Response in Molecular Crystal Nanowires

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.

Mechanical Properties and Photomechanical Fatigue of Macro- and Nanodimensional Diarylethene Molecular Crystals

The diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene, undergoes a reversible photoisomerization between its ring-open and ring-closed forms in the solid-state and has applications as a photomechanical material. Mechanical properties of macrocrystals, nanowire single crystals, and amorphous films as a function of multiple sequential UV and visible light exposures have been quantified using atomic force microscopy nanoindentation. The isomerization reaction has no effect on the elastic modulus of each solid. But going from the macro- to the nanowire crystal results in a remarkable over 3-fold decrease in the elastic modulus. The macrocrystal and amorphous solids are highly resistant to photomechanical fatigue, while nanowire crystals show clear evidence of photomechanical fatigue attributed to a transition from crystal to amorphous forms. This study provides first experimental evidence of size-dependent photomechanical fatigue in photoreactive molecular crystalline solids and suggests crystal morphology and size must be considered for future photomechanical applications.

Reversible Phase Transition of Porous Coordination Polymers

Porous coordination polymers or metal-organic frameworks with reversible phase-transition behavior possess some attractive properties, and can respond to external stimuli, including physical and chemical stimuli, in a dynamic fashion. Their phase transitions can be triggered by adsorption/desorption of guest molecules, temperature changes, high pressure, light irradiation, and electric fields; these mainly include two types of transitions: crystal-amorphous and crystal-crystal transitions. These types of porous coordination polymers have received much attention because of their interesting properties and potential applications. Herein, reversible phase transition porous coordination polymers are summarized and classified based on different stimuli sources. Corresponding typical examples are then introduced. Finally, examples of their applications in gas separation, chemical sensors, guest molecule encapsulation, and energy storage are also presented.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

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

The molecule (E)-(5-(3-anthracen-9-yl-allylidene)-2,2-dimethyl-[1,3] dioxane-4,6-dione) (E-AYAD) undergoes E→Z photoisomerization. In the solid state, this photoisomerization process can initiate a physical transformation of the crystal that is accompanied by a large volume expansion (ca. 10 times), loss of crystallinity, and growth of large pores. This physical change requires approximately 10 % conversion of the E isomer to the Z isomer and results in a gel-like solid with decreased stiffness that still retains its mechanical integrity. The induced porosity allows the expanding gel to engulf superparamagnetic nanoparticles from the surrounding liquid. The trapped superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline solid instead of a dilute solution, provides a new route for in situ production of functional porous materials.

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.