Fluorescence and aggregation behavior of a polyimide in solutions

The UV-visible absorption and fluorescence spectra of a soluble polyimide, YS-30, in several organic solvents were measured over a wide range of conce ntration. The experimental results show that there exist both intramolecula r and intermolecular electron donor acceptor interactions for YS-30 molecul es. The fluorescence behavior of YS-30 in N,N-dimethylacetamide and in chlo roform solutions is similar in general, except that its ground-state interm olecular charge transfer emission is more obvious in N,N-dimethylacetamide solution. This difference is attributed to the greater extent of disruption of the chain packing by solvent or/and the more efficient radiationless en ergy dissipation process from the excited state complexes to chloroform. Th e intensity ratio of intermolecular charge transfer emission to intramolecu lar charge transfer emission is used to characterize the state of aggregati on of YS-30 molecules in solutions. The plot of this ratio versus concentra tion indicates the existence of two critical concentrations. It is also fou nd from the same plot that the decrease of coil size is very pronounced dur ing the initial stage of shrinkage.