Diels‐Alder Polymer Networks with Temperature‐Reversible Cross‐Linking‐Induced Emission Yu Jiang*, and Nikos Hadjichristidis*

A novel synthetic strategy toward reversible cross-linked polymeric materials with tunable fluorescence properties is presented. Dimaleimide-substituted tetraphenylethene (TPE-2MI), which is non-emissive due to the photo-induced electron transfer (PET) between maleimide (MI) and tetraphenylethene (TPE) groups, was used to cross-link random copolymers of methyl (MM), decyl (DM) or lauryl (LM) with furfuryl methacrylate (FM). The mixture of copolymer and TPE-2MI in DMF showed reversible fluorescence with "on/off" behavior depending on the Diels-Alder (DA)/retro-DA process, which is easily adjusted by temperature. At high temperatures, the retro-DA reaction of polymer networks is dominant, and the fluorescence is quenched by the PET mechanism. In contrast, at low temperatures, the emission recovers as the DA reaction takes over. Based on these results, a transparent PMFM/TPE-2MI film was prepared, and the emission behavior was investigated. It was found that the polymer film shows an accurate response to the external temperature and exhibited tunable fluorescent "turn on/off" behavior. These exciting results suggest the possible application of this type of reversible cross-linked materials in many areas, including information security and transmission. An example of invisible/visible writing is given.