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

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.