Spectral Characterization of 4-Mercaptopyridine (4-Mpy) Aggregates in Aqueous Solution

This study investigates the spectroscopic characteristics of 4-mercaptopyridine (4-Mpy) molecular aggregates formed in aqueous solution. Due to their low solubility in water, 4-Mpy molecules undergo solution-driven aggregation, which results in a blue shift in their absorption maximum and enhanced fluorescence compared to their monomeric form in methanol. This spectral behavior aligns with the predictions of the molecular exciton model and indicates the formation of H-type aggregates through intermolecular hydrogen bonding and π-π stacking interactions. Raman spectroscopy further confirms aggregation by revealing two newly enhanced low-frequency bands at 155 and 220 cm−1, corresponding to intermolecular vibrational modes. These observations are consistent with the Aggregation-Enhanced Raman Scattering (AERS) theory, which attributes the enhancements to increased polarizability and excitonic coupling within the aggregates. In addition to low-frequency features, high-frequency Raman bands associated with the monomeric form also show selective enhancement, attributed to Herzberg-Teller vibrational coupling. The result in our study indicates that 4-Mpy molecules in water self-assemble into H-type aggregates, possibly in the form of helical chain conformations.