Fluorescent polymeric hydrogels especially multicolor fluorescent polymeric hydrogels (MFPHs) have important applications in information storage, encryption, and encoding. MFPHs are generally prepared by incorporating multiple traditional fluorescent materials into hydrogels. In recent years, nontraditional luminescent polymers without any traditional π-conjugated chromophores have received increasing attention. Here, we report a novel type of nontraditional MFPHs prepared by in situ polymerization of acrylamide (AAm) in the presence of poly(itaconic acid) (PITAc). The hydrogen-bonded mechanically strong PAAm/PITAc hydrogels show strong intrinsic fluorescence, and the fluorescence emission is excitation-dependent and metal cationresponsive. More impressively, the hydrogels treated with metal cations also possess excitation-dependent fluorescence. We developed a multi-ion inkjet printing (MIIP) technique to print texts or designed patterns onto the hydrogel surface using different metal cation solutions as inks, and then variable texts or patterns appear under the irradiation of UV, violet, and blue lights. Patterns can be further changed by selective printing, erasing, or reprinting on some regions. Therefore, multidimensional information encryption is achieved. This work provides a new strategy for preparing MFPHs for wide applications.

Nontraditional organic/polymeric luminogens, which do not contain any conventional chromophores like large 𝝅-conjugated benzene rings and/or heterocycles, have attracted rapidly growing attention owing to their importance in the fundamental understanding of photoluminescence mechanisms and potential practical applications. However, compared to traditional luminogens, most nontraditional luminogens (NTLs) emit fluorescence in the blue region, and only very limited NTLs with green, yellow, and red emissions have been reported. It is of great scientific and practical importance to develop NTLs with red-shifted emissions and understand their mechanisms. This review first provides a brief overview of the types of NTLs based on heteroatoms in them, the luminescence mechanism and luminescent characteristics of NTLs, then summarizes recent progress in NTLs with red-shifted emissions and the main strategies employed, i.e., introducing multiple nonconventional chromophores, introducing intra-/intermolecular interactions to rigidify clusters, and introducing electron-giving/withdrawing groups into molecules to lower the gap between the HOMO-LUMO energy levels. This review also provides the perspectives and outlook on future development of NTLs with red-shifted emissions.