Switchable Fluorescent Organogels and Mesomorphic Superstructure Based on Naphthalene Derivatives

Bisurea-functionalized naphthalene organogelators via cooperative hydrogen bonding and pi-pi stacking interaction were designed and synthesized. The gelators showed excellent gelling capability in various solvents and performed switchable fluorescence in the gel state. The fluorescent emission of these compounds strongly depends on the aggregation of the fluorophore and is very sensitive to the temperature and chemical stimuli. A stronger and red-shifted emission was found in the gel state compared with the original solution. The gel-sol transition of the systems, as well as the fluorescent emission, is reversibly controlled by a change of the temperature or upon alternative addition of fluoride anions and protons. The influence of fluoride anions on the fluorescence and gel-sol processes is a result of the dissociation of intermolecular hydrogen bonds by bonding of fluoride anions with urea groups of the gelator. The obtained sol is turned to the gel state again upon addition of trifluoroacetic acid. Furthermore, polarizing optical microscopy and small-angle X-ray scattering indicated that the gelator exhibited the liquid crystalline property and displayed the column phase.