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 1Lb 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 1Lb state and to shift to longer wavelengths upon addition of lithium bromide.