Sensing diversity in halogen-bonded multi-stimuli responsive materials: Light, pH, magnetism, and electron-rich species

The present study delves into the synthesis and characterization of multi-stimuli responsive polymers, demonstrating their sensitivity towards magnetic field, pH level, light irradiation, and electron-rich species through halogen bonding (XB). Detailed X-ray analyses of two azobenzene derivatives, each incorporating a tetrafluoroiodobenzene group, provided the compelling evidence of engagement of the building blocks in the formation of halogen bonds. Interestingly, the investigation of an ammonium analog, featuring a tetrahalogenoferrate(III) anion, not only showcased a magnetic response even upheld its ability to undergo isomerization under UV irradiation. These functional properties were subsequently harnessed in the form of multi-responsive compounds through the random quaternization of poly(2-(N,-dimethylamino) ethyl methacrylate) (PDMAEMA), and by employing varying proportions of XB-donor dye (8, 11, 15, 21, and 43 %) followed by magnetic anion exchange. Leveraging the presence of free ternary amino groups and diazobenzene moieties, the resulting polymers demonstrated pronounced sensitivity towards a variation of pH and UV light stimuli, while a careful modulation of magnetic susceptibility was found to be directly proportional to the degree of quaternization. An NMR titration, conducted using a synthetic intermediate, revealed the formation of I⋯Cl‾ interactions in the solution state, thereby underscoring the materials' sensitivity to electron-rich species. Moreover, the electron microscopic analysis displayed an alteration in morphology upon the application of a magnetic field and UV irradiation. Thus, the presented strategic framework offers an avenue for the development of multi-stimuli responsive materials for remotely controlled smart devices to act in response to a diverse set of four stimuli, and heralding a significant advancement in the realm of responsive material design.