Structural Variations on Self-Assembly and Macroscopic Properties of 1,4,5,8-Naphthalene-diimide Chromophores

Supramolecular assembly and macroscopic properties of a series of bis-(trialkoxybenzamide)-functionalized naphthalene-tetracarboxylicacid-diimide (NDO chromophores have been studied. The number of methylene units (0, 2, 3, 4; NDI-0, NDI-2, NDI-3, NDI-4, respectively) in between the NDI chromophore and the amide functionalities have been systematically varied to understand the effect of this simple structural variations on the self-assembly. UV-visible spectroscopic studies revealed facile self-assembly in nonpolar medium by synergistic effect of pi-stacking, hydrogen-bonding, and hydrophobic interactions. The propensity toward self-assembly was found to follow the order NDI-0 >> NDI-2 approximate to NDI-3 > NDI-4,which could be attributed to the difference in strength of the H-bonding interaction. Atomic force microscopy (AFM) studies revealed unique morphology for the self-assembled structure in each case such as nanostructure with short-range order (NDI-0), elongated nanowires (NDI-2), relatively flat nanoribbons (NDI-3), and discontinuous nanofibers (NDI-4). Effect of such diverse morphologies was found to be highly relevant in correlating the macroscopic properties such as gelation of the individual chromophore. Most strongly aggregating NDI-0 did not show gelation in any of the tested solvents but formed lyotropic mesophases (chromonic N and M- phase) which could be related to their crystal like morphology as revealed by AFM images. Contrary to this, NDI-2 and NDI-3 showed most promising gelation ability in many common organic solvents with very low critical gelation concentrations (in some cases even <0.1 wt %) and NDI-4 formed gel only in few solvents which can be attributed to the strength of the self-assembly. The viscoelastic properties of the gels were studied by rheological measurements which revealed remarkable dependence on the morphology of the self-assembled structure. For example, although the self-assembly propensity and gelation ability were almost similar between NDI-2 and NDI-3, the yield-stress of the former gel was estimated to be astonishingly high (similar to 20 times) compared to that of NDI-3 gel owing to the large differences in the aspect ratio of the respective 1D-nanostructures. Further the thermodynamic parameters such as Delta H-m (enthalpy of melting) of the gel-to-sol transition were determined for various gels and correlated with the molecular structure and self-assembly propensity in solution.