Red‐shifted and Dramatically Enhanced Fluorescence Emissions from Sodium Alginate‐Polyethyleneimine Polyelectrolyte Complexes

Abstract The development of nontraditional luminogens (NTLs) with enhanced and red‐shifted emissions has attracted rapidly increasing attention. Here, a new strategy is provided for preparing NTLs with enhanced and red‐shifted emissions based on polyelectrolyte complexes (PECs) formed by an anionic polyelectrolyte sodium alginate (SA) and a cationic polyelectrolyte polyethyleneimine (PEI). The direct blending of SA and PEI leads to enhanced emissions but no red‐shift in emission wavelengths in the solution and dried films. After soaking in HCl solutions (pH≤2.0), SA‐PEI PEC films are obtained. The SA‐PEI PEC wet and dried films show maximum emissions at ≈ 370 and 470 nm as well as a significantly red‐shifted emission at 550 nm. The SA‐PEI PEC dried films maintain high quantum yields under a wide range of excitation wavelengths and the quantum yield is as high as 29.4% under 470 nm excitation. Furthermore, the films emit nearly white fluorescence under the irradiation of 290 or 370 nm UV light. The enhanced and red‐shifted emission is attributed to the formation of the compact luminescent clusters with more extended through‐space conjugation and rigidified conformations through ionic bonding between NH + groups and COO − groups and the enhanced hydrogen bonding.