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Photoresponsive Metal-Organic Frameworks
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Metal-organic frameworks (MOFs) are a type of crystalline porous material having organic ligands connected to metal clusters. This type of material shows a terrific design adaptability due to the almost unlimited combinations of metallic salts and organic ligands. Several scientists have been attracted by this important part of reticular chemistry, probably due to the fact that the researcher’s inventiveness is the only limitation in this research field. Thus, different research groups have greatly contributed to the exponential growth of this area, using these materials in a diverse range of applications, including catalysis, water harvesting, biomedicine and sensing. The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increased parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands.

metal-organic frameworks light photoresponsive materials reticular chemistry
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    1. Introduction

    The development of organic ligands having a higher structural complexity is one of the main directions in order to prepare metal-organic frameworks (MOFs) showing advanced functionalities [1][2][3][4]. Thus, the incorporation of stimuli-responsive scaffolds inside a MOF matrix turned out to be an ideal strategy within reticular chemistry [5][6][7][8], allowing to prepare smart materials [9][10]. The application of an external stimulus induces changes in the organic ligands, leading to modifications of certain properties of the metal-organic material, such as porosity [11][12]. The selected stimulus should be able to promote modifications in the crystalline array without causing damage to its structural integrity.
    Among the available stimuli, light is particularly appealing due to: (i) the easy and remote switching control; (ii) the possibility of varying the intensity, wavelength and irradiation time; (iii) the clean and non-destructive operation [13]. Different organic ligands bearing photoactive units have been employed for the preparation of photoresponsive MOFs [14][15][16]
    Figure 1. Cartoon representing the main content.
    This short entry aims to give an insight in the preparation and application of light-responsive metal-organic frameworks by using different photoswitchable organic ligands, including azobenzenes [17], diarylethenes [18], spiropyrans [19], donor-acceptor Stenhouse adducts (DASA) [20], molecular motors [21] and interlocked compounds [22][23]. For this purpose, some recent advances in the use of the abovementioned organic ligands are highlighted [23][24][25][26][27][28][29][30][31][32][33].
    Figure 2. Selected photoactive organic ligands employed to prepare light-responsive metal-organic frameworks.

    2. Overview

    The incorporation of photoactive ligands in a MOF matrix is a suitable strategy to build smart materials. In principle, the incorporation of these photoactive ligands on a condensed solid phase should allow to take advantage of the photoswitchable properties of the ligands. Thus, changes in the structure of the ligands induced by a light input can produce changes in the material, transferring the properties of these photoactive ligands from solution to the solid state. The development of photoresponsive materials has turned out to be a topic of interest for a wide range of scientists of different research fields, including organic chemistry, inorganic chemistry, supramolecular chemistry, nanotechnology and materials science.
    Photoresponsive metal-organic frameworks are highly important because of their advantageous properties and numerous potential practical applications. Due to the intrinsic porosity that characterizes MOFs, most of the highlighted examples can be applied in selective adsorption or controlled cargo release [24][25][26][27][33]. Favorable properties of these materials herald possible applications in other fields, such as Nanorobotics, Data Storage and Optoelectronics [23][28][29][30][31][32]. Most of the currently implemented applications of these photoresponsive MOFs are at the research laboratory level, but an example employed to develop FET and LED devices is also highlighted [29].
    Table 1. General considerations of the selected photoresponsive MOFs [23][24][25][26][27][28][29][30][31][32][33].       
    Entry MOF Linker Type Principal property Practical implementations Reference
    1 U-azo Azobenzene Adsorbent CO2 adsorption [24]
    2 Th-Azo-MOF Azobenzene Adsorbent Rhodamine B adsorption [25]
    3 DTEMOF Diarylethene Reversible crystalline assembly Highly efficient molecular dispenser
    of guest molecules
    4 ECUT-30 Diarylethene +
    Adsorbent Gas capture and selective
    adsorption of guest molecules 
    5 SpiroMOFs Spiropyran Optoelectronic Development of FET and
    LED devices
    6 DUT-5(indoline)0.5–2.5(DASA) DASA Bistability Development of NVM materials [30]
    7 Moto-MOFs Molecular motor Unidirectional
    rotary motion
    Photoswitchable microfluidic pumps
    and photodriven mass transport
    8 UMUMOF-(E)-3 Interlocked
    Development of molecular dispensers
    of p-benzoquinone
    9 U-CB[8]-MPyVB Interlocked
    Development of photoactuator
    The use of different ligands, such as azobenzenes, diarylethenes, spiropyrans and DASA, has allowed the development of a wide variety of applications. The combination of reticular chemistry and molecular machinery, by incorporating light-responsive molecular motors and rotaxanes inside a MOF matrix, further expands the range of options for the development of photoresponsive materials showing improved functionalities.
    The incorporation of photoresponsive molecules within MOFs has led to the preparation of materials which change their properties in response to light irradiation. Thus, different applications, such as selective adsorption and development of optoelectronic devices can be achieved.
    In order to incorporate this photoswitchable ligands, the well-stablished synthetic protocols for obtaining this type of material have been employed, such as mixed-linker solvothermal conditions and SALE methodology [34][35]. The direct assembly of the photoactive ligands directly affords the target material, but the ligand should retain its structural integrity during the MOF formation reaction. In stark contrast, postsynthetic modifications strategies lead to the preparation of materials containing the desired units by using a premade crystalline framework. Thus, ligands that are not stable to the conditions of formation of the metal-organic material can be used in order to achieve a greater variety of functionalization. However, there are two main limitations: (i) the postsynthetic conditions must not affect the integrity of the crystalline array; (ii) the functionalization with the photoresponsive units may not be homogeneous throughout the material.
    The vast design possibility forecasts materials in which future prospects are almost limitless. Additionally, the precise modulation of the photochemical stimulus, being able to modify time, wavelength and irradiation power, leads to forecasting a promising future for this field of research.
    Despite the heartening reported results so far, this is not a fairy tale and there are still some difficulties to overcome. In addition to the issue with the photostationary equilibrium that is obtained when some of the photoresponsive ligands are used, the photochemical stimulus should fall equally on all the layers of the material. Another issue is the scale-up production of the material in order to apply these light-responsive MOFs in the industry, sometimes limited by the yield of the organic strut. Regarding industrial applications, materials having a high-performance iterability must be developed to make them cost-effective.
    Photoresponsive metal-organic frameworks have a bright outlook as adjustable scaffolds to prepare smart materials, which makes this area of study really attractive to a wide range of scientists from different research fields. This interdisciplinarity would benefit the progress of these interesting porous materials.


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    View Times: 407
    Revisions: 6 times (View History)
    Update Date: 06 Jul 2022
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