Viscochromism in a dual emitter metallopolymer containing terbium ions

Synthetic polymers containing lanthanide ions have emerged as materials with potential application in many areas, mainly due to the unique luminescent properties of these ions that complement those of their organic counterparts. The aim of this work is to strengthen the concept of chain rigidity in the emissive properties of this type of structures as previously proposed. For this purpose, the solvation effect on chain conformations was used as a probe, assuming that the higher the viscosity, the less mobile the chain will become. The higher rigidity as compared to the previous one was achieved by three ways: replacing the previous ter-pyridine sites by the less bulky di-pyridine ones; increasing the number of complexed lanthanide ions from 20 % to 80 % (molar basis) and inserting double bonds in place of single ones. The dual-emissive conjugated copolymer composed by fluorene-bipyridine units carrying complexed terbium ions, was synthesized and characterized by structural, thermal and photophysical measurements. The terbium emission occurs in dimethylsulfoxide (DMSO) and N,N–dimethylformamide (DMF) only, and its luminescence was not observed in other solvents tested. The photophysical measurements strongly suggest that the lanthanide luminescence is brought about by solvent effect in polymer’s main chain, minimizing non-radiative energy losses by restriction of intramolecular motion (RIM) due to properties of poor solvents and their high viscosity. These factors combined promoted aggregation-induced emission (AIE).