Studies of molecular aggregation of a polybenzimidazole in solution by fluorescence spectroscopy

A purified, well characterized sample of polybenzimidazole (PBI) has been studied by fluorescence spectroscopy in order to obtain information concerning molecular aggregation in solution. Fluorescence and excitation spectra of PBI and a model compound have been measured in N,N-dimethylacetamide (DMA) and in formic acid (FA). The major emission band for PBI solutions is attributed an excited ¹L_b state, which consists of 0-0 and 0-1 vibrational energy bands. Concentration quenching is attributed to reabsorption of the emitted light, and self-quenching due to the formation of molecular aggregates. Therefore, the extent of molecular aggregation in solution is estimated from the self-quenching constant. In the course of the investigation, a new fluorescence band was found for the PBI-DMA solution; its intensity increased markedly due to overlapping of polymer coils. The formation of polyelectrolytes in PBI-FA solutions was confirmed by viscometry. The major fluorescence band was found to originate from the excited ¹L_b state and to shift to longer wavelengths upon addition of lithium bromide.