Hierarchical self-assembly of naphthalene bisimides to fluorescent microspheres and fluoride sensing

Abstract

Donor acceptor π–π stacking and hydrogen bond mediated self-assembly of two synthetic naphthalene bisimides containing L-amino acids are illustrated. The solution studies exhibit a blue shift in UV/vis spectrum with increasing concentration for both phenylalanine and tyrosine-appended naphthalene bisimides. The single crystal X-ray reveals that both the Phe and Tyr side chains are in the trans position in the reported naphthalene bisimides 1 and 2. In higher order packing, the bisimide 2 molecules self-assemble through intermolecular hydrogen bonds between side chain Tyr –OH and ester –CO, and donor acceptor π–π stacking interactions between the electron deficient central naphthalene moiety and side chain tyrosine ring to generate a staircase architecture. Atomic force microscopy revealed microsphere morphology with diameters of 200–300 nm for bisimides 1 and 2. The microspheres have significant green emission upon excitation at 400 nm. Moreover these two naphthalene bisimides can be used as turn on fluoride sensor in aqueous medium.