A single molecule stimuli-robust fluorescent hydrogel based on excited state intramolecular proton transfer

A white light-emitting hydrogel was synthesized by embedding N,N'-bis(salicylidene)-(2-(3',4'-diaminophenyl)benzothiazole) (BTS) into a polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO, F127) triblock copolymer hydrogel. BTS undergoes excited state intramolecular proton transfer (ESIPT), which leads to a broad emission spectrum where both the normal (N*) and tautomeric (T*) forms of the excited states of BTS coexist. Aggregation of BTS was prevented when it was embedded in F127 hydrogels. The BTS emission spectra are sensitive to the BTS loading in the gel, with white light being observed for some concentrations of lumiphore. In contrast, the BTS emission spectra are insensitive to pH. These experiments showed the successful application of a supramolecular strategy where the lumiphore is incorporated into a specific environment in the micelles forming the gel. The insensitivity of the emission colour to pH showed that emissive materials using a single ESIPT molecule can be achieved without the pH sensitivity normally associated with this process. These results highlight the importance of designing fluorophores with properties that enable them to be located in specific environments of supramolecular systems.