Light-controlled aggregation and gelation of viologen-based coordination polymers

TL;DR

This study explores how light irradiation induces aggregation and gelation in viologen-based coordination polymers, revealing controllable structural transitions driven by photo-induced electron transfer and supramolecular self-assembly.

Contribution

It demonstrates the light-controlled formation of aggregates and gels in viologen-based coordination polymers, highlighting the role of photo-induced electron transfer in these processes.

Findings

Aggregates are dense micron-sized spindle particles.

Gelation kinetics show arrested phase separation.

Gels have micron-scale correlation length and weak elasticity.

Abstract

Ditopic bis-(triazole-pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate pi-radicals that can self-associate to form pi-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense polydisperse micron size spindle shaped particles which grow in time. Using microscopy and time resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process.