Donor-acceptor Stenhouse adduct functionalised polymer microspheres

Abstract

Polymers that carry donor-acceptor Stenhouse adducts (DASAs) are a very relevant class of light-responsive materials. Capable of undergoing reversible, photoinduced isomerisations under irradiation with visible light, DASAs allow for on-demand property changes to be performed in a non-invasive fashion. Applications include photothermal actuation, wavelength-selective biocatalysis, molecular capture and lithography. Typically, such functional materials incorporate DASAs either as dopants or as pendent functional groups on linear polymer chains. By contrast, the covalent incorporation of DASAs into crosslinked polymer networks is under-explored. Herein, we report DASA-functionalised crosslinked styrene-divinylbenzene-based polymer microspheres and investigate their light-induced property changes. This presents the opportunity to expand DASA-material applications into microflow assays, polymer-supported reactions and separation science. Poly(divinylbenzene-co-4-vinylbenzyl chloride-co-styrene) microspheres were prepared by precipitation polymerisation and functionalised via post-polymerisation chemical modification reactions with 3^rd generation trifluoromethyl-pyrazolone DASAs to varying extents. The DASA content was verified via ¹⁹F solid-state NMR (ssNMR), and DASA switching timescales were probed by integrated sphere UV-Vis spectroscopy. Irradiation of DASA functionalised microspheres led to significant changes in their properties, notably improving their swelling in organic and aqueous environments, dispersibility in water and increasing mean particle size. This work sets the stage for future developments of light-responsive polymer supports in solid-phase extraction or phase transfer catalysis.

Scheme 1. (A) Reaction scheme outlining the generation of crosslinked of poly(DVB-co-VBC-co-styrene) microspheres through precipitation polymerisation followed by post-polymerisation functionalisation steps to generate the pendant DASA groups. (B) Structures and properties of DASAs in their open and closed form.

Fig. 1. SEM images of DASA polymers D1–3 (scale bar = 50 μm).

Fig. 2. Photos of DASA-functionalised polymer microspheres (D1) in toluene. (A) After being left to swell and equilibrate in the dark for 24 h. (B) After 24 h of irradiation with white light. (C) After a further 24 h in the dark at room temperature.

Fig. 3. UV-vis spectra and graphs showcasing the change in the DASA absorbance acquired in diffuse reflectance mode. DMs were suspended in toluene (0.15 mg mL⁻¹) and left in the dark overnight. Irradiation source: ThorLabs LED array red (630 nm), 1.5 mW cm⁻² at the sample. Normalised absorbance against time plots were fitted with a mono-exponential decay curve. (A–C) Selected spectra showing decrease in DASA absorbance band for D1–3 during irradiation. (D–F) Normalised absorbance against time plot for D1–3 during irradiation. Orange line shows mono-exponential fit curve. (G–I) Selected spectra showing increase in DASA absorbance band for D1–3 post-irradiation in the dark. (J–L) Normalised absorbance against time plot for D1–3 post-irradiation in the dark. Orange line shows mono-exponential fit curve.

Fig. 4. Nitrogen sorption isotherms of D2. (a) In their initial state, (b) after 4 h of white light irradiation, (c) after 24 h of white light irradiation.

Fig. 5. Photographs of aqueous dispersion of DMs as a function of DASA photoswitching. (a) Non-irradiated D2 in water. (b) D2 irradiated for 24 h with white light in water. (c) D2 irradiated for 6 h with white light in toluene, dried and dispersed in water. (d) D2 irradiated for 24 h with white light in toluene, dried and dispersed in water. (All sample concentrations were 1 mg mL⁻¹ of D2.)

Fig. 6. Partitioning of D2 in water (top)/chloroform (bottom) biphasic system. (A) Photographic stills from video: D2 without exposure to intense light source. Progression of D2 after vigorous mixing: full initial dispersion in the aqueous layer, to gradual re-entering into the chloroform layer. (B) Photographic stills from video: D2 which had been irradiated for 24 h in chloroform (1 mg mL⁻¹) with white light prior. Progression of D2 after vigorous mixing: full dispersion in the aqueous layer, partial re-entering into the chloroform layer and seemingly stable emulsion formation. The aqueous phase contains Congo Red for clarity.