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In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response.
Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework COF. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence.
This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications. The self-assembly of organic nanostructured materials with tunable morphology has become a fascinating topic in material science and supramolecular chemistry 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , However, supramolecular self-assemblies are formed by weak non-covalent interactions that are easily destroyed by heat or changes in pH, effectively limiting their practical utility 11 , 12 , 13 , To overcome these problems, scientists have introduced the bottom-up self-assembly process for the synthesis of 2D nanosheets 15 , hollow spheres 16 , and tubular morphologies 17 in covalent organic frameworks COFs These materials have attracted much attention due to their excellent chemical stability and strong mechanical properties Unfortunately, the preparation of COFs by the bottom-up self-assembly approach suffers from long reaction times and requires much effort to obtain product 15 , 16 , Triggering the self-assembly of materials to obtain a specific morphology with an external stimulus is another smart approach to control the self-assembly process 19 , 20 , 21 , This strategy is promising for those seeking to develop new well-defined nanoscale architectures in short time that also have potential applications in optics 23 , 24 , 25 , 26 , sensing 27 , 28 , 29 , and delivery systems 30 , 31 , Stimuli-responsive self-assembly of one- and two-dimensional supramolecular materials, including polyrotaxanes 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , nanotubes 41 , 42 , nanowires 43 , 44 , 45 , 46 , 47 , 48 , and nanosheets 43 , 49 , 50 , 51 , 52 , has already been extensively studied 46 , 47 , 52 , However, the modulation of morphology in response to light has rarely been reported despite being one of the most environmentally friendly sources of external stimuli that is commonly accessible in our daily life 19 , 20 , 54 , Compared to other stimuli, light has numerous advantages because it can be applied at a specific location and in the form of different wavelengths 56 , An attractive strategy to control morphological self-assembly in response to light is to incorporate photoactive molecular switches that undergo structural isomerization upon light irradiation into the COF network 58 , 59 , Among the numerous photoactive molecular switches described in the literature, spiropyran SP 61 is one of the best studied molecular switches that can reversibly photoisomerize between the hydrophobic ring-closed SP and the hydrophilic ring-opened merocyanine MC upon irradiation with visible or UV light 56 , 62 , Due to their excellent reversible reactivity, materials functionalized with spiropyran have been widely used for data storage 64 , 65 , chemical sensors 66 , 67 , 68 , 69 and developing smart materials 62 , 70 , Recently, we developed a photosensitive quasi-honeycomb COF by introducing dangling azobenzene units into the pore structure The material exhibited interesting reversible photoswitching behavior due to the free movement of the azobenzene groups within the pores upon irradiation.
Inspired by our previous work, we first attempted to intrinsically introduce spiropyran into a COF structure as a pendant group by following our reported synthetic protocol with the same Interestingly, unlike the azobenzene COF, the spiropyran COF did not exhibit any photoswitching properties, which could be explained by the larger size of the spiropyran moiety as compared to the azobenzene unit, which hinders mechanical motion within the pores. To avoid the crowding effect, we used a mixed linker synthesis method Fig.
