Polysaccharide restriction on bipyridyl isomers for multicolor emissions

Abstract

Functional biomacromolecular luminophores have attracted increasing interest. Based on the concept of aggregation-induced emission (AIE), the restricted intramolecular rotation (RIR) strategy is widely used to design dyes, which generally focuses on the luminous molecular structures and their stacking modes. However, little effort has been made to reveal the interactions between the embedded fluorophores and their microenvironments. Herein, a series of polysaccharide macromolecules with multicolor emissions were obtained by crosslinking oxidized sodium alginate (OSA) with aminopyridine (Apy). Based on the para- and ortho-substituent OSA chains in the Apy units, their varying steric hindrances resulted in an enhanced fluorescence quantum yield (QY) of 0.98% for OSA-5-Apy and 15.62% for OSA-3-Apy after grafting the Apy units. With further increase in the Apy content, the rigidity of the polysaccharide matrix was improved by the covalently cross-linked bonds and hydrogen bonds, which further enhanced the fluorescence QY of OSA-5-Apy from 0.98% to 5.25%. The corresponding contributions of the restrictions to the emission intensities were elaborately examined by introducing Ca²⁺. Moreover, based on their complexation with Ca²⁺, colorful photoluminescent gels in different forms, such as Rubik's cube and fiber, were prepared using OSA-3-Apy fluorescence macromolecules, which is promising in the biomedical and engineering fields.