Unconventional Fluorescence in Aliphatic Hyperbranched Polyphosphate Esters: Role of End-Group Engineering and Mechanistic Insights into the Emission Behavior

Unconventional fluorescent polymers exhibit unique optical properties and good water solubility, yet regulating their fluorescence remains a challenge. In this work, we synthesized a series of aliphatic hyperbranched polyphosphate esters that display typical unconventional fluorescence, including excitation-dependent emission and concentration-enhanced emission. Experimental and theoretical calculation results revealed that inter- and intramolecular hydrogen bonds facilitated the formation of molecular aggregates and amplified n–π interactions between oxygen and P═O bonds. These strengthened n–π interactions promoted spatial electronic communication, resulting in bright fluorescence and a high quantum yield up to 32.2%. Notably, the end-group composition significantly influenced spatial electronic communication and emission properties. Polymers with a higher ratio of P = O(O)₃ end-groups relative to hydroxyl groups exhibited pronounced red-shifted fluorescence. Furthermore, these polymers showed potential applications in pH and ion sensing as well as in the fabrication of fluorescent hydrogels. This study provides new mechanistic insights into the emission behavior of unconventional fluorescent polymers.