Aggregation behavior in N-alkyl polybenzimidazoles: a photophysical perspective

Polybenzimidazole (PBI) is a thermally stable heteroaromatic polymer that exhibits strong inter and intramolecular interactions in solution and the solid state due to hydrogen bonding and π–π stacking between benzimidazole units. In the present study, a systematic photophysical investigation of a series of N-alkyl-substituted PBIs was carried out to understand how alkyl chain length influences aggregation behavior and molecular organization. The polymers investigated include methyl-, ethyl-, pentyl-, hexyl-, heptyl-, octyl-, decyl-, dodecyl-, tetradecyl-, and hexadecyl-substituted PBIs. UV-visible absorption and fluorescence spectroscopy were performed in both protic and aprotic solvents over a range of concentrations and excitation wavelengths. The absorption spectra reveal characteristic π–π* transitions of the benzimidazole backbone and show concentration-dependent changes in spectral behavior indicative of inter and intramolecular aggregation. Fluorescence measurements also display broad emission bands associated with excimer formation arising from the interaction between polymer chains. Further, solvent-dependent studies demonstrate that aggregation is stronger in aprotic solvents such as dimethylacetamide, whereas protic solvents partially disrupt hydrogen bonding interactions. Increasing alkyl chain length progressively reduces π–π stacking interactions but introduces hydrophobic associations between alkyl side chains, leading to aggregation of a different character. Surface morphology examined by scanning electron microscopy and atomic force microscopy reveals significant changes in nanoscale organization with increasing alkyl chain length. Smooth and compact morphologies are observed for short-chain substituted polymers, whereas long alkyl chains produce rougher surfaces and aggregated nanostructures. The combined spectroscopic and microscopic results provide detailed insight into how side-chain engineering, solvent environment, and concentration of the polymer in solution influence inter- and intramolecular interactions and photophysical properties of polybenzimidazole systems.