Abstract
Dynamic materials comprising spiropyrans have emerged as one of the most interesting and promising class of stimulus-responsive materials. Spiropyrans are often embedded in polymer matrices; their covalent attachment into porous monolithic silsesquioxane frameworks, however, is virtually unexplored. We demonstrate that a silylated spiropyran derivative can be covalently incorporated into ultralight silsesquioxane-based bulk materials by a two-step co-condensation sol–gel approach without restricting its conformational freedom and thus its stimulus-responsive properties. UV–vis measurements prove the conversion of the colorless closed-ring form of the spiropyran molecule into its highly colored purple isomer or the yellow colored protonated structure thereof. The transformation can be triggered simply by irradiation of the spiropyran-containing silsesquioxane monolith with UV or visible light or by the pH value of the chemical environment. A strong dependence of the surface polarity and water wettability on the prevalent isomer was observed. The contact angle of a water droplet on the monolithic surface can be altered from 146 to 100° by irradiation of the monolith with UV light for 3 min. Additionally, the prepared materials possess high specific surface areas, low bulk densities, and porosities of up to 84%.
Highlights
Dynamic materials in which selected properties can be “switched on” and “off” more and more attract the attention of researchers,[1] and the demand for novel stimulus-responsive materials is rising due to their broad applicability ranging from optical memory devices to holographic gratings or optical sensors.[2−4] Stimulus-responsive organic molecules such as azobenzenes,[5,6] stilbenes,[7,8] diarylethenes,[9] or fulgides[10] are well-known and have long been the focus of interest
To enable a covalent attachment of spiropyran to a polysilsesquioxane network using sol−gel chemistry, the photo-responsive molecule N-hydroxyethyl-spiropyran was modified with hydrolyzable alkoxy groups by reaction with 3(triethoxysilyl)propylisocyanate in DMF following a protocol published by Allouche et al (Figure 1).[25]
The preparation of spiropyran-embedded porous polymethylsilsesquioxanes can be successfully performed by a two-step sol−gel processing route involving co-condensation of MTMS with silylated spiropyran
Summary
Dynamic materials in which selected properties can be “switched on” and “off” more and more attract the attention of researchers,[1] and the demand for novel stimulus-responsive materials is rising due to their broad applicability ranging from optical memory devices to holographic gratings or optical sensors.[2−4] Stimulus-responsive organic molecules such as azobenzenes,[5,6] stilbenes,[7,8] diarylethenes,[9] or fulgides[10] are well-known and have long been the focus of interest. In the SP form, both parts are not in conjugation and the isomer mainly absorbs in the UV region, whereas the isomer undergoes a 6-π electrocyclic ring opening reaction by an external stimulus and forms a highly colored merocyanine (MC) product, which can be protonated to the yellow colored MCH+ isomer.[11] In recent literature studies, a rising number of publications about the incorporation of SPs in diverse materials can be found.[12−25] To name some recent examples, Sumaru et al reported the possibility to use spiropyranfunctionalized poly(N-isopropylacrylamide) (pNIPAAm) for the recognition of cyclodextrin,[26] Cao et al described the possibility to capture or release cells on or from spiropyranmodified Janus particles by UV or visible light irradiation,[27] and several publications exploited the different colorations of SP for chemical drawing purposes or mechanochemical studies (e.g., in stress/strain sensing and damage reporting).[28−32] Further examples for SP-based dynamic materials can be found in an excellent review recently given by R.
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