Tunable supramolecular networks via cis-trans metal–ligand isomerization

Abstract

Using Monte Carlo simulations we studied the effect of cis-trans isomerization of 2 : 1 ligand–metal complexes on self-assembly and network formation of metallo-supramolecular polymers in a good solvent. Comparing two systems containing only cis- or trans- isomers, we found that in the oligomer-rich region the fraction of both 2 : 1 and 3 : 1 ligand–metal complexes is rather similar for both systems and as a result the average molecular weight and reversible network properties are also nearly the same. In the metal-rich region a larger fraction of cis-2 : 1 ligand–metal complexes results in a noticeably larger average molecular weight of associating polymers. Accordingly, the onset of formation of a reversible network containing cis-isomers occurs at a considerably lower oligomer number density and for a larger range of metal-to-oligomer ratios. The weight fraction of the network containing cis-isomers and its equilibrium elastic plateau modulus considerably exceeds that for the trans-isomer network, especially in the metal-rich region. We predict conditions when trans- to cis- isomerization can trigger the sol-network transition and/or result in a significant change in materials properties, such as molecular weight and especially elastic plateau modulus. We discuss the molecular mechanisms of network transformation upon cis-trans isomerization, which vary with metal-to-oligomer ratio and originate from different morphologies of the reversible network. These results demonstrate that the cis-trans isomerization of 2 : 1 ligand–metal complexes significantly affects the microscopic and macroscopic properties of the metallo-supramolecular polymers and could be used to design new types of smart materials.