Inverting the Swelling Trends in Modular Self‐Oscillating Gels Crosslinked by Redox‐Active Metal Bipyridine Complexes

Abstract

The developing field of active, stimuli-responsive materials is in need for new dynamic architectures that may offer unprecedented chemomechanical switching mechanisms. Toward this goal, syntheses of polymerizable bipyridine ligands, bis(4-vinylbenzyl)[2,2′-bipyridine]-4,4′-dicarboxylate and N4,N4′-bis(4-vinylphenyl)-2,2′-bipyridine-4,4′-dicarboxamide, and a number of redox-active Ruthenium(II) and Iron(II) complexes with them are reported. Detailed characterizations by NMR, Fourier transform infrared spectroscopy, high-resolution mass-spectrometry, X-ray, and cyclic voltammetry show that the topology of these molecules allows them to serve as both comonomers and crosslinkers in polymerization reactions. Electronic properties of the ligands are tunable by choosing carboxylate-or carboxamido-linkages between the core and the vinylaryl moieties, leading to a library of Ru and Fe complexes with the M(III)/M(II) standard redox potentials suitable for catalyzing self-oscillating Belousov-Zhabotinskii (BZ) reaction. New poly(Nisopropylacrylamide)-based redox-responsive functional gels containing hydrophilic comonomers, which have been prepared using representative Ru bpy complexes as both a crosslinker and redox-active catalyst, exhibit a unique feature: their swelling/contraction mode switches its dependence on the oxidation state of the Ru center, upon changing the ratio of comonomers in the hybrid gel network. The BZ self-oscillations of such crosslinked hydrogels have been observed and quantified for both supported film and freestanding gel samples, demonstrating their potential as chemomechanically active modules for new functional materials.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201704205.Ruthenium complexes with polypyridine ligand core, in particular, have occupied a significant place in this body of work, in big part due to the fact that they have been recognized to be useful in a wide variety of applications, which rely on their chemical stability, as well as a unique combination of photochemical, electrochemical, and photophysical properties. Among many potential applications that have emerged over the years, some recent examples include mononuclear water oxidation catalysis, [3] light-emitting electrochemical cells, [4] photoredox catalysis coupled with organocatalysis, [5] light-activated anticancer drugs, [6] and dye-sensitized solar cells. [7,8] Interest in integrating these valuable properties of polypyridine metal complexes into materials and devices gave rise to research in bipyridine-containing polymer ligands, as well as metallopolymers, coordination polymers, supramolecular polymers, and crosslinked polymer gels containing bipyridine moieties. [9] The targeted applications and research areas include, among others, electroluminescent, electrochromic, and photoluminescent materials, [9b,10,11] sensors, [9b,12] and chemomechanically active self-oscillating gels. [13][14][15][16] Interdisciplinary field of development and ...

Viewpoint: Homeostasis as Inspiration—Toward Interactive Materials

“…Notably, if the catalyst is also a crosslinker, in the presence of a hydrophilic co‐monomer, the swelling/contraction response of the hydrogel to the catalyst redox state exhibits a strong non‐monotonicity with respect to the co‐monomer concentration. Upon increase of concentration of hydrophilic co‐monomer in the polymer network, the gels revert their swelling trends from higher swelling in the reduced state (unusual) to higher swelling in the oxidized state (normal), an observation that highlights how complex the chemomechanical interactions are in these systems …”

“…In this way, bending of the fins stops the reaction, allowing the structure to cool and resurrect, leading to sustained oscillations. Stimuli‐responsive hydrogels are ideal for creating chemomechanical feedback loops, as volume changes, in turn, influence mass transport and chemical reaction rates, for example, in self‐oscillating gels and rhythmic drug‐release systems . Other types of chemomechanical feedback can include self‐shadowing, as has been demonstrated for light‐responsive liquid‐crystalline elastomer thin‐films (Figure c‐ii) .…”

Stimuli‐Responsive Ruthenium‐Containing Polymers

“…Active Ru‐bipyridine complexes containing six polymerizable double bonds have also been utilized in preparing RCPs. The octahedral complexes worked as both the cross‐linking points of RCPs and the catalysts for BZ reaction (Figure b) . Such RCPs exhibited autonomous volume swelling–deswelling behaviors but in an inverted manner.…”

Active gels: from chemical microreactor to polymeric actuator

“…Controlling strain is crucial for the design of shape changing functional elements, such as bimorphs [18][19][20]. The composition of the BZ gels [22,28,33], the architecture of the catalyst [22,26], the charge of the monomer [21,22], the porous structure of the hydrogel [22][23][24][25], the shape of the gels [27,29] and the chemical environment of the gel [30] have been reported to influence the swelling of the BZ gel. Here, we provide experimental and theoretical evidence supporting that the strain of a BZ gel can be made arbitrarly large by decreasing its bulk modulus, elucidating a system-independant mechanism for increasing the strain of active gels.…”