Modulation of Semiconducting Behavior and a Change in Morphology upon Gelation of a Peptide Appended Naphthalenediimide

A dipeptide-appended naphthalenediimide (NDI) is self-assembled in an aqueous phosphate buffer medium of pH 7.5 to form nanovesicular structures. Interestingly, these molecules are assembled to form hydrogels in the presence of glucono-delta-lactone (at pH 4.7) or Ru3+ solution (at pH 4.6). The gel formation is associated with a sharp morphological change from nanospheres to a nano fibrous network structure, and this nanofibrous network is responsible to trapping of many solvent molecules to form hydrogels. These gels and aggregates are well characterized by different spectroscopic and microscopic tools including UV-vis absorption and fluorescence spectroscopy, Fourier transformed infrared spectroscopy (FTIR), X-ray powder diffraction (XRPD), rheological studies, and high resolution transmission electron microscopic (HR-TEM) studies. The peptide-appended NDI was found to form J-aggregate as is evident from the UV-vis absorption studies of the aggregated species. Upon excitation at 384 nm wavelength, the glucono-delta-lactone-based hydrogel shows a greenish blue fluorescence, and similar fluorescence is observed for the aggregates of NDI-appended peptide in aqueous buffer medium at pH 7.5. The soluble aggregates and both gels (obtained from the treatment of glucono-delta-lactone and Ru3+) show wonderful current conduction and photoswitching properties. The photoconductivity and the gain in photocurrent values are enhanced in the GDL and Ru3+ mediated gels by several orders. This indicates that the morphological change from nanovesicles to ordered network of fibers has an important role in the enhancement of photocurrent gain in photoswitching experiments.