Protonation and Hexagonal Stacking Synergistically Enhance Nonconventional Fluorescence of Poly(3-aminopropyl)Silsesquioxanes Hydrochloride.

The nonconventional fluorescence of amino-functionalized polysiloxanes has received extensive attention from researchers. Protonation of the amino groups has been shown to affect polysiloxane conformation and aggregation, yet its effect on the nonconventional fluorescence remains unclear. In this study, fluorescent poly(3-aminopropyl)silsesquioxane hydrochlorides with hexagonal structures are reported for the first time, prepared via the hydrolysis and condensation of 3-aminopropyltrimethoxysilane with different ratios of hydrochloric acid. Regulation of the reaction pH and the associated protonation and assembly increases the fluorescence quantum yield significantly from 0.53% to 38%. The optical properties of the solution and the solid state, along with concentration-dependent emission, confirm the fluorescence originates from clusterization-triggered emission of amino/ammonium clusters. Comprehensive structural characterization (29Si NMR, FTIR, XPS, XRD) reveals that the ratio of hydrochloric acid serves as a key parameter to regulate both protonation degree and condensation pathway. This modulation promotes strong hydrogen bonding between amino and ammonium groups, yet retains suitable chain flexibility during assembly, and leads to tighter hexagonal packing. These structural changes synergistically enable the spatial delocalization in amino/ammonium clusters and effectively suppress non-radiative transitions, thereby enhancing the quantum yield. Due to their aqueous processability and unique fluorescence, these polysiloxanes show promising potential for future applications.