SCIENTIFIC HIGHLIGHTS

Stimuli-responsive flexible materials: the role of spherical icosahedral boron clusters

This work demonstrates the role of icosahedral boron clusters to stabilize flexible Metal-Organic Frameworks (MOFs) and thus providing a new generation of porous “stimuli-responsive” or “smart” materials

The so-called “stimuli-responsive” or “smart” materials have the ability to go through conformational changes or phase transitions as a result of external chemical or physical stimuli. Such responsiveness to specific stimuli or local environment is typical for biomolecules in nature but it is particularly difficult to achieve artificially. Such materials form the corner stone of developing intelligent technologies and are at the forefront of strategies addressing a number of global challenges.

We have now developed novel 3D nano-porous materials that go through conformational changes and transform into a 2D non-porous structure as a result of an external stimuli, and then can shift to the original 3D nano-porous structure when the stimuli is reversed. This flexibility is introduced in a new Metal-Organic Frameworks (MOFs), which incorporates a flexible carborane based linker. Icosahedral carborane clusters are three-dimensional molecules with electron delocalization, highly polarizable σ-aromaticity, thermal and chemical stability and geometrical diversity. The use of spherical shaped icosahedral boron-based molecules as linkers instead of planar ones help in stabilizing the flexible structures. The spherical shape of the ligands is the key factor that enables the structures to go back to their original shape, allowing for the rearrangement of the different parts, and without collapsing the whole structure. The idea of spherically shaped linkers avoiding collapse of the structure can also be understood like this: two layers will roll over each other if separated by spheres; whereas they will collapse if non-spherical pillars are used.

As a proof of concept for potential applications, encapsulation of fullerene molecules has been achieved by trapping them during the reversible 2D to 3D transition, while the structure is being formed. The observed process constitutes a new way to encapsulate large molecules that cannot easily diffuse into the porous material.

Authors

Fangchang Tan,1 Ana López-Periago,1 Mark E. Light,2 Jordi Cirera,3 Eliseo Ruiz,3 Alejandro Borrás,1 Francesc Teixidor,1 Clara Viñas,1 Concepción Domingo,1 José Giner Planas1


  • Affiliations:

    1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
    2Department of Chemistry, University of Southampton, Highfield, UK
    3Departament de Química Inorgànica i Orgànica and Institut de Recerca de Química Teòrica i Computacional, Universitat de Barcelona, Spain. 

  • Publication

    An Unprecedented Stimuli Controlled Single-crystal Reversible Phase Transition of a Metal-Organic Framework and its Application to a Novel Method of Guest Encapsulation. 
    Advanced Materials 30, 1800726 (2018)
    DOI: 10.1002/adma.201800726

  • Figure

    3D nano-porous materials go through conformational changes and transform into a 2D non-porous structure as a result of an external stimuli.

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