  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.
