Photochromic molecules are showing great potential as molecular components of stimulus-responsive materials. However, despite recent advances across various photoresponsive materials, the quantitative conversion in solids during photochemical reactions is hampered by the intrinsic structural flexibility of the materials. A team of Researchers from Japan have developed materials which possess intrinsic flexibility, in an otherwise crystalline frame, as a way of realizing photoresponsive crystalline materials.
Photochromic molecules show a color change upon photo-irradiation, which usually invokes a response by changing the geometric structure, electronic states or the chemical and/or physical properties within a material. However, solid-state materials lack the internal flexibility of other materials, and as a result, they are currently limited in their ability to release stress and strain during photochemical event occurrence.
Both metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) have established themselves as potential solutions by being both crystalline and flexible in nature.
The utilization of both metal ions and organic ligands in these materials allows for the creation of pores which can accommodate various functionalities and/or molecules, including molecules of a photochromic nature.
However, to even think of using these class of molecules in such a manner, key considerations had to be met, as to maximize their impact- the ligands need to show a high photo-isomerism efficiency between isotopes; the isomers need to be thermally stable; and the photochromic unit must have high fatigue resistance with high repeatability. Dithienylethene (DTE) ligands have been discovered as one such molecule that exhibits and adheres to all three considerations.
Knowing this, the Researchers designed a PCP/MOF entangled framework that possessed flexibility and provided sufficient room with the pores so that DTE could not only be up-taken, but could also accommodate structural changes in the DTE ligands (by releasing the strain across the framework) upon photo-isomerization. The formed material was a photoresponsive PCP structure with a two-fold interpenetrated framework composed of DTE-based ligands.
The Researchers used a portable LED UV (ZUV-C20H, OMRON Corporation) to irradiate the samples and used a green light (HDA-TB3, Hayashi Watch-Works Co., Ltd.) as a visible light source.
The Researchers characterized the samples using 1H nuclear magnetic resonance (NMR) spectroscopy (JEOL JNM-ECS 400), Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS, Bruker micrOTOF II), thermogravimetric analysis (TGA, Rigaku Thermo plus Evo II TG-8120), single crystal X-ray diffraction (XRD, Rigaku mercury diffractometer with a RIGAKU Saturn70 CCD detector, X-ray powder diffraction (XPRD, Rigaku UltimaIV) and volumetric adsorption isotherm measurements (BELSORP- 18PLUS; Bel Japan, Inc).
The flexible combination of PCPs and MOFs was chosen for their ability to naturally change their frameworks upon insertion of a guest species. The utilization of the framework allowed the Researchers to create a material with a high effective isomerization and cooperative structural transformations with the DTE ligands.
The Researchers have employed a strategy in which the photocrystalline materials have shown a quantitative, and reversible, response upon irradiation with ultraviolet (UV) and visible light. The incorporation of some structural flexibility in the material’s framework allowed for a two-fold interpenetration composed of DTE ligands.
The structural composition of the porous framework has enabled the Researchers to create a material that exhibits a highly efficient photochemical electrocyclization in a single-crystal-to-single-crystal manner. Using light irradiation, it was also found that the material would reversibly modulate the sorption of CO2 molecules on the porous crystal at 195 K, demonstrating and further confirming the presence of a two-fold interpenetrated framework.
The Researchers have successfully achieved reversible photomodulation of gas sorption and joins other material properties that rely on photomodulation methods, such as photomagnetization and photomechanical motions.
The Researchers have stipulated that the inherent flexibility has been derived by both local and global structural changes, and have suggested that these types of frameworks can be synthetically produced using a wide range of photoreactive species that possess an inability of undergoing isomerism/conformational change, such as azobenzene, spiropyrane and sterically hindered alkenes.
The research has opened a new avenue for porous material research and has provided a platform for future studies surrounding photochemical conversions and the photomodulation of porous properties.
“Flexible interlocked porous frameworks allow quantitative photoisomerization in a crystalline solid”- Zheng Y., et al, Nature Communications, 2017, DOI: DOI: 10.1038/s41467-017-00122-5