Structure and optical properties of polypyrrole oligomers prepared by solution polycondensation

A series of polypyrrole oligomers (PPOs)—MPMH, MPEH, MPTH, and MPCH—were synthesized via solution polycondensation of N-methylpyrrole and para-substituted benzaldehydes to systematically explore substituent effects on their structural, optical, and electrochemical properties. MPTH showed enhanced crystallinity via π–π stacking, while MPCH formed a honeycomb-like porous structure. Optically, MPCH exhibited red-shifted excitation and emission (394/513 nm) due to a push–pull effect. The oligomers emitted violet (MPMH, MPEH), blue (MPTH), and green (MPCH) fluorescence, respectively. Electrochemically, substituents fine-tuned HOMO levels, giving band gaps of 2.0–2.4 eV suitable for visible-light emission. This study clarifies the role of substituents in modulating optoelectronic behavior, supporting the design of functional polymers for electronics and sensing. Graphical abstract