Light-mediated formation and dissociation of a two-dimensional supramolecular polymer sheet: one step closer to sustainability

Abstract

Recently, a few strategies have emerged to synthesize two-dimensional (2D) polymers comprising laterally connected areal monomeric units that extend in two orthogonal directions. Despite such progress, rational design of non-persistent 2D polymers that require neither harsh conditions nor extra chemicals to dissociate, thus imposing a potentially diminished environmental burden, remains a challenge. Here, we report a mechanically new strategy that enables formation and dissociation of a 2D supramolecular polymer sheet in response to light. The constituent monomers are cucurbit[6]uril_SP, an engineered cavitand bearing multiple photochromic spiropyran (SP) moieties at its periphery. When irradiated with UV light, the SP isomerizes into zwitterionic merocyanine (MC), affording a single-cucurbit[6]uril-thick polymer sheet in the presence of Ca²⁺via multiple in-plane inter-monomeric MC⋯Ca²⁺⋯MC bridges. The free-standing polymer is stable in the dark, but dissociates into oligomers upon exposure to visible light as the peripheral MC isomerizes back to SP. Since light is a noninvasive and environmentally benign stimulus with high spatiotemporal solution, the rationale underlying the present effort may point the way towards new 2D polymers that are non-persistent to avoid repeating the accumulation problem now haunting conventional linear polymers, thus bringing this emerging material one step closer to sustainability.