Phototriggered Supramolecular Polymerization

Abstract

Photo-initiated supramolecular polymerization of a naphthalenediimide (NDI-1) derivative containing an ortho-nitrobenzyl (ONB)-protected amide group is demonstrated. In a hydrocarbon solvent (methylcyclohexane), it remains as monomer. Upon photo-irradiation, deprotection of the ONB group produces NDI-2 with a free amide group, which drives supramolecular polymerization by self-complementary H-bonding between the amide groups, leading to gelation. The polymerization rate can be controlled by tuning the energy of the light source. During photopolymerization, a gradual increase in hydrodynamic radius and viscosity is noticed. More interestingly, the morphology of the supramolecular polymer of NDI-2, produced by photo-irradiation, was a spherulite, which is in sharp contrast with the fibrillar morphology of NDI-2 polymer, when assembled spontaneously without a phototrigger. This is ascribed to the ability of the ONB-caged pro-monomer (NDI-1) to act as a chain-stopper by forming a H-bonded complex with the active monomer during the growth of the supramolecular polymer under photo-irradiation.