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.

Many nontraditional luminogens (NTLs) without any large π‐conjugated structures are reported to exhibit room‐temperature phosphorescence (RTP). Unfortunately, the reported NTLs mostly emit blue or green RTP. Achieving more redshifted RTP from NTLs remains a great challenge. Herein, a series of nonaromatic polymers exhibiting yellow and orange–red RTP are reported. Poly(itaconic anhydride) (PITA) does not exhibit RTP, its hydrolyzed product poly(itaconic acid) (PITAc) exhibits weak yellow RTP, but the ionized product poly(sodium itaconate) (PITANa) can emit stronger and redshifted RTP. Moreover, the ionized copolymers poly(vinyl pyrrolidone‐co‐sodium itaconate) (PIVPNa) and poly(vinyl pyrrolidone‐co‐sodium maleate) (PMVPNa), the mixture of PITANa/poly(vinyl pyrrolidone) (PVP), and the full hydrolyzed product of poly(vinyl caprolactam‐co‐sodium itaconate) (PIVCNa) all exhibit strong orange–red RTP emissions at ≈600 nm. Structural characterizations and theoretical calculations prove that hydrogen bonding and ionic bonds lead to the rigidification of polymer chain conformations, and more importantly, both intra‐ and interchain nonaromatic electron donor–acceptor (nDA) structures and hence through‐space charge transfer are formed between carboxylate and lactam groups in proper conformations, which facilitates the occurrence and redshift of RTP in NTLs. This work provides a novel strategy to design NTLs with redshifted RTP and improves the understanding of the photoluminescence mechanism of NTLs.

We summarize and propose a “gas-thermal method” for preparing non-traditional luminescent polymers (NLPs) with enhanced and red-shifted fluorescence emissions by heating existing polymers in different gas atmospheres. This study provides...

Nontraditional luminogens (NTLs) do not contain any conventional chromophores (large π-conjugated structures), but they do show intrinsic photoluminescence. To achieve photoluminescence from NTLs, it is necessary to increase the extent of through-space conjugation (TSC) and suppress nonradiative decay. Incorporating strong physical interactions such as hydrogen bonding is an effective strategy to achieve this. In this work, we carried out comparative studies on the photoluminescence behaviors of two β-enamino esters with similar chemical structures, namely methyl 3-aminocrotonate (MAC) and methyl (E)-3-(1-pyrrolidinyl)-2-butenoate (MPB). MAC crystal emits blue fluorescence under UV irradiation. The critical cluster concentration of MAC in ethanol solutions was determined by studying the relationship between the photoluminescence intensity (UV–visible absorbance) and concentration. Furthermore, MAC exhibits solvatochromism, and its emission wavelength redshifts as the solvent polarity increases. On the contrary, MPB is non-emissive in both solid state and solutions. Crystal structures and theoretical calculation prove that strong inter- and intramolecular hydrogen bonds lead to the formation of large amounts of TSC of MAC molecules in aggregated states. No hydrogen bonds and thus no effective TSC can be formed between or within MPB molecules, and this is the reason for its non-emissive nature. This work provides a deeper understanding of how hydrogen bonding contributes to the luminescence of NTLs.