Metal-organic framework based on hinged cube tessellation as transformable mechanical metamaterial

Mechanical metamaterials exhibit unusual properties, such as negative Poisson???s ratio, which are difficult to achieve in conventional materials. Rational design of mechanical metamaterials at the microscale is becoming popular partly because of the advance in three-dimensional printing technologies. However, incorporating movable building blocks inside solids, thereby enabling us to manipulate mechanical movement at the molecular scale, has been a difficult task. Here, we report a metal-organic framework, self-assembled from a porphyrin linker and a new type of Zn-based secondary building unit, serving as a joint in a hinged cube tessellation. Detailed structural analysis and theoretical calculation show that this material is a mechanical metamaterial exhibiting auxetic behavior. This work demonstrates that the topology of the framework and flexible hinges inside the structure are intimately related to the mechanical properties of the material, providing a guideline for the rational design of mechanically responsive metal-organic frameworks

Building principles of meta-MOF (metal-organic framework)

Mechanical metamaterials, artificial materials with counter-intuitive properties such as a negative Poisson???s ratio, not exhibited by conventional materials that is found in nature. Unusual properties of metamaterials are obtained from structural geometry, not their composition. Rational design of mechanical metamaterials at the micro- and macroscale is interesting as found novel geometries. However, incorporating movable building blocks inside solids, thereby enabling us to manipulate the mechanical movement at the molecular scale, has been a difficult task. Here we report a metal-organic framework, self-assembled from a porphyrin linker and a new type of Zn-based secondary building unit, acted as a joint in hinged cube tessellation. We show that this material is indeed mechanical metamaterial, exhibiting auxetic behaviour through detailed structural analysis and theoretical calculation. This work demonstrates that the topology of the framework and flexible hinges inside the structure are intimately related to the mechanical properties of the material, providing a guideline for the rational design of mechanically responsive metal-organic frameworks

Supramolecular Coordination Polymer Formed from Artificial Light-Harvesting Dendrimer

We report the formation of supramolecular coordination polymers formed from multiporphyrin dendrimers (<b>P</b>_<b>Zn</b><b>P</b>_<b>M</b>; <b>M</b> = <b>FB</b> or <b>Cu</b>), composed of the focal freebase porphyrin (<b>P</b>_<b>FB</b>) or cupper porphyrin (<b>P</b>_<b>Cu</b>) with eight zinc porphyrin (<b>P</b>_<b>Zn</b>) wings, and multipyridyl porphyrins (<b>PyP</b>_<b>M</b>; <b>M</b> = <b>FB</b> or <b>Cu</b>), <b>P</b>_<b>FB</b> or <b>P</b>_<b>Cu</b> with eight pyridyl groups, through multiple axial coordination interactions of pyridyl groups to <b>P</b>_<b>Zn</b>s. UV–vis absorption spectra were recorded upon titration of <b>PyP</b>_<b>FB</b> to <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b>. Differential spectra, obtained by subtracting the absorption of <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b> without guest addition as well as the absorption of <b>PyP</b>_<b>FB</b>, exhibited clear isosbestic points with saturation binding at 1 equiv addition of <b>PyP</b>_<b>FB</b> to <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b>. Job’s plot analysis also indicated 1:1 stoichiometry for the saturation binding. The apparent association constant between <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b> and <b>PyP</b>_<b>FB</b> (2.91 × 10⁶ M^–1), estimated by isothermal titration calorimetry, was high enough for fibrous assemblies to form at micromolar concentrations. The formation of a fibrous assembly from <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b> and <b>PyP</b>_<b>FB</b> was visualized by atomic force microscopy and transmission electron microscopy (TEM). When a 1:1 mixture solution of <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b> and <b>PyP</b>_<b>FB</b> (20 μM) in toluene was cast onto mica, fibrous assemblies with regular height (ca. 2 nm) were observed. TEM images obtained from 1:1 mixture solution of <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b> and <b>PyP</b>_<b>FB</b> (0.1 wt %) in toluene clearly showed the formation of nanofibers with a regular diameter of ca. 6 nm. Fluorescence emission measurement of <b>P</b>_<b>Zn</b><b>P</b>_<b>M</b> indicated efficient intramolecular energy transfer from <b>P</b>_<b>Zn</b> to the focal <b>P</b>_<b>FB</b> or <b>P</b>_<b>Cu</b>. By the formation of supramolecular coordination polymers, the intramolecular energy transfer changed to intermolecular energy transfer from <b>P</b>_<b>Zn</b><b>P</b>_<b>M</b> to <b>PyP</b>_<b>M</b>. When the nonfluorescent <b>PyP</b>_<b>Cu</b> was titrated to fluorescent <b>P</b>_<b>Zn</b><b>P</b>_<b>FB</b>, fluorescence emission from the focal <b>P</b>_<b>FB</b> was gradually decreased. By the titration of fluorescent <b>PyP</b>_<b>FB</b> to nonfluorescent <b>P</b>_<b>Zn</b><b>P</b>_<b>Cu</b>, fluorescence emission from <b>P</b>_<b>FB</b> in <b>PyP</b>_<b>FB</b> was gradually increased due to the efficient energy transfer from <b>P</b>_<b>Zn</b> wings in <b>P</b>_<b>Zn</b><b>P</b>_<b>Cu</b> to <b>PyP</b><sub><b>FB.</b></sub

Enhancement of Energy Transfer Efficiency with Structural Control of Multichromophore Light-Harvesting Assembly

© 2020 The Authors. Published by Wiley-VCH GmbH. Multichromophore systems (MCSs) are envisioned as building blocks of molecular optoelectronic devices. While it is important to understand the characteristics of energy transfer in MCSs, the effect of multiple donors on energy transfer has not been understood completely, mainly due to the lack of a platform to investigate such an effect systematically. Here, a systematic study on how the number of donors (n(D)) and interchromophore distances affect the efficiency of energy transfer (eta(FRET)) is presented. Specifically,eta(FRET)is calculated for a series of model MCSs using simulations, a series of multiporphyrin dendrimers with systematic variation ofn(D)and interdonor distances is synthesized, and eta(FRET)s of those dendrimers using transient absorption spectroscopy are measured. The simulations predict eta(FRET)in the multiporphyrin dendrimers well. In particular, it is found that eta(FRET)is enhanced by donor-to-donor energy transfer only when structural heterogeneity exists in an MCS, and the relationships between the eta(FRET)enhancement and the structural parameters of the MCS are revealed11sciescopu