Dynamic Diels–Alder reactions of maleimide–furan amphiphiles and their fluorescence ON/OFF behaviours

The occurrence of dynamic covalent reactions only requires relatively low activation energy, which allows both the forward and reverse reactions to proceed under mild conditions. Here, we report the design and synthesis of amphiphilic maleimide-furan adducts, where hydrophobic maleimide-based and hydrophilic furan-based moieties were connected by reversible dynamic covalent bonds. The Diels-Alder addition reactions of maleimide-furan adducts are simple, efficient, clean, and reversible without catalysts and side reactions, and occur under mild conditions. Single crystal X-ray diffraction revealed that the length of the dynamic covalent bonds is 1.56 Å, which is longer and weaker than for normal covalent bonds. The cleavage and reformation process of the dynamic covalent bonds was monitored by 1H NMR and fluorescence spectroscopy. 1H NMR spectroscopy revealed that the furan moieties of these new maleimide-furan amphiphiles can be exchanged in mixing systems due to dynamic Diels-Alder reactions; thus, two new maleimide-furan compounds can be transformed into each other. The maleimide-furan amphiphiles displayed reversible fluorescence ON/OFF behaviours and interesting H-bonding driven supramolecular assembly.