Light‐Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry

Abstract

Versatile photoresponsive gels based on tripodal low molecular weight gelators (LMWGs) are reported. A cyclohexane‐1,3,5‐tricarboxamide (CTA) core provides face‐to‐face hydrogen bonding and a planar conformation, inducing the self‐assembly of supramolecular polymers. The CTA core was substituted with three arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that isomerizes reversibly under alternating UV and green light irradiation. The E isomer of AAP is planar, favoring the self‐assembly, whereas the Z isomer has a twisted structure, leading to a disassembly of the supramolecular polymers. By using tailor‐made molecular design of the tripodal gelator, light‐responsive organogels and hydrogels were obtained. Additionally, in the case of the hydrogels, AAP was coupled to the core through hydrazones, so that the hydrogelator and, hence, the photoresponsive hydrogel could also be assembled and disassembled by using dynamic covalent chemistry.

Wavelength‐Gated Adaptation of Hydrogel Properties via Photo‐Dynamic Multivalency in Associative Star Polymers

“…Force‐field calculations reveal that the E ‐AAP possesses a planar conformation, which favors π‐π stacking in aqueous media due to aromatic donor‐acceptor interactions of the electron‐rich pyrazolyl and the electron‐poor phenyl ring (Figure 1 a, Figure S1). In contrast, the Z ‐isomer of AAP features a twisted conformation and a higher dipole moment, which impede π‐π stacking in water [24c] . We verified the calculations of the aromatic donor‐acceptor interactions via 2D NOESY (Nuclear Overhauser Effect Spectroscopy) NMR experiments and confirmed that the aromatic donor‐acceptor interactions of the E ‐isomer are distinctly stronger than the ones of the Z ‐isomer (Figure S2).…”

Pyrazolylazophenyl Ether‐Based Photoswitches: Facile Synthesis, (Near‐)Quantitative Photoconversion, Long Thermal Half‐Life, Easy Functionalization, and Versatile Applications in Light‐Responsive Systems

“…Host–gest systems of conventional azobenzenes with cyclodextrins have been well‐established, but their potentials are far from fully realized associated with the photoconversion and thermal stability issues. The use of bis‐methylated azopyrazoles has led to an improved system . Here, pzAzo ether was used to develop a new host–guest system.…”

Reversible switching of arylazopyrazole within a metal–organic cage

“…Among them, arylazopyrazoles have attracted considerable interest because of their synthetic availability and desirable photochemical properties, such as the high thermal stability of the Z isomer and a large band separation between the E and Z isomers, allowing each to be addressed with a high selectivity [35–40]. Consequently, arylazopyrazoles have been employed as photoresponsive gelators [41] and adhesives [42] and for controlling antimicrobial response [43–44], cell adhesion to surfaces [45], as well as DNA [46] and microtubule [47] self-assembly using light.…”