Glycine-glutamic-acid-based organogelators and their fluoride anion responsive properties

Novel low-molecular-weight organogelators (LMOGs) 1 and 2 derived from glycine-glutamic-acids (GGA) based dipeptide were synthesized. The difference on the chemical structures between 1 and 2 resulted in some interesting variations on their gelling behavior and fluoride anion (F-) responsive properties. Gelator 1 could gel aromatic solvents effectively, while gelator 2 possessed excellent gelation ability in protic solvents. Upon the addition of 0.5 equiv. F-, the toluene gel of 1 transformed into solution due to the disruption of the intermolecular hydrogen-bonding, whereas the alcohol gel of 2 was stable and preserved even by introducing 20 equiv. F-. Interestingly, 2 could be regarded as an efficient receptor for F- in acetonitrile (MeCN) solution. After the addition of F-, the fluorescent emission band of 2 red-shifted from 360 to 420 nm with the emission intensity enhanced drastically; the intensity of the CD signal decreased and almost disappeared after addition of 2 equiv. F-. These changes were due to the conformational change and hydrogen-bonding interaction between urea groups and F-. The results indicated that GGA-based 1 and 2 might represent potential sensor materials for the naked-eye detection of F-.