Mechanically tunable porous gels constructed via the dual coordination/covalent polymerization of coumarin-functionalized rhodium–organic cuboctahedra

Abstract

Polymer-based soft materials constructed from defined molecular pores, such as metal–organic polyhedra (MOPs), promise to merge the outstanding and diverse mechanical properties of conventional nonporous polymers with atomically-precise molecular recognition capabilities. Thus far, soft MOP networks have been constructed primarily using rigid, labile coordination bonds or dynamic covalent bonds, providing static networks without intrinsic mechanisms to optimize their response to mechanical stimuli. Here, we report the construction of flexible, doubly crosslinked MOP gels via mutually compatible coordination and covalent polymerization techniques. Our method employs dirhodium paddlewheel-based MOPs bearing both open metal sites, which enable their coordination-driven assembly, and photodimerizable coumarin side chains for covalent polymerization (Coumarin-RhMOPs). Incubation of Coumarin-RhMOPs with ditopic linkers enabled their coordination-driven polymerization into porous colloidal gels. Site-selective irradiation of coordination-linked Coumarin-RhMOP gels afforded doubly crosslinked gels with improved strain tolerance and higher stiffness. Selective dissociation of coordination-crosslinkers provided highly deformable covalent Coumarin-RhMOP gels. The postsynthetic addition of ditopic ligands to covalent gels enabled the reversible modulation of their mechanical properties. These findings highlight the possibility of incorporating multiple responsive crosslinks in porous MOP networks to rationally tune their responses to mechanical stress, paving the way to their practical implementation as next-generation chemical separators, catalysts, and drug delivery vehicles.