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 2 L A 3 L 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.

Tuning the Size and Geometry of Heteroleptic Coordination Cages by Varying the Ligand Bent Angle

“…[52][53][54][55] The latter strategy has resulted in the discovery of several mixed-ligand assemblies of the general formula [Pd n L x L' y ] 2n + (x + y = 2n). Most of these heteroleptic assemblies contain two Pd 2 + ions, [56][57][58][59][60][61][62] and there are only few examples of structurally defined mixed-ligand assemblies with n > 2. [63][64][65][66][67] We have recently reported a [Pd 6 L 6 L' 6 ] 12 + complex, which was identified in a competition experiment using a set of six different dipyridyl ligands.…”

“…Ligand L2 is more basic than L4, [76] date. [56][57][58][59][60][61][62][63][64][65][66][67] A key difficulty is the identification of a ligand combination L/L' that favors the formation of a particular heteroleptic complex over homoleptic complexes and alternative mixed-ligand assemblies. Herein, we have described an entirely new class of [Pd n L n L' n ] 2n + complexes with different sizes and geometries.…”

“…The design of structurally defined [Pd n L n L′ n ] 2 n + complexes is a challenging task, and only few examples have been reported to date [56–67] . A key difficulty is the identification of a ligand combination L/L′ that favors the formation of a particular heteroleptic complex over homoleptic complexes and alternative mixed‐ligand assemblies.…”

Increasing structural and functional complexity in self-assembled coordination cages

“…27a ). 146 Photoisomerization in this system is now able to tip the balance between formation of a [Pd 2 o- 55 4 ] cage with four open photoswitches and a [Pd 2 c- 55 3 Solv 2 ] bowl containing only three DTE bridges (as closed form) for steric reasons and two solvent molecules that complement the square-planar Pd II coordination spheres. While guest uptake behaviour already differs between the cage and the bowl, the latter one could be modulated in its host properties by applying two further stimuli ( Fig.…”

Recent advances on the construction of diarylethene-based supramolecular metallacycles and metallacages via coordination-driven self-assembly

“…This study realized the light-controlled structural conversion between different self-assembled SCCs, which played an important role in developing SCCbased molecular machines. In addition, some other diarylethene-based metallacages were also constructed by Li et al [40][41][42] and exhibited interesting properties and functions.…”