Multi-stimuli Control over Assembly and Guest Binding in Metallo-supramolecular Hosts Based on Dithienylethene Photoswitches

Abstract

It is difficult to assemble multi-component metallo-supramolecular architectures in a non-statistical fashion, which limits their development toward functional materials. Herein, we report a system of interconverting bowls and cages that are able to respond to various selective stimuli (light, ligands, anions), based on the self-assembly of a photochromic dithienylethene (DTE) ligand, L ^a , with Pd ^II cations. By combining the concept of “coordination sphere engineering”, relying on bulky quinoline donors, with reversible photoswitching between the ligand’s open ( o- L ^a ) and closed ( c- L ^a ) forms, a [Pd ₂( o- L ^a ) ₄] cage ( o- C) and a [Pd ₂( c- L ^a ) ₃] bowl ( c- B) were obtained, respectively. This structural rearrangement modulates the system’s guest uptake capabilities. Among three bis-sulfonate guests ( G1, G2, and G3), the cage can encapsulate only the smallest ( G1), while the bowl binds all of them. Bowl c- B was further used to synthesize a series of heteroleptic cages, [Pd ₂L ^A ₃L ^B], representing a motif never reported before. Additional ligands ( L ^c-f ), with short or long arms, tune the cavity size, thus enabling or preventing guest uptake. Addition of Br ^–/Ag ⁺ makes it possible to change the overall charge, again triggering guest uptake and release, as well as fourth ligand de-/recomplexation. In combination, site-selective introduction of functionality and application of external stimuli lead to an intricate system of hosts with different guest preferences. A high degree of complexity is achieved through cooperativity between only a few components.