Double Lamellar Morphologies and Odd-Even Effects in Two- and Three-Dimensional N,N′-bis(n-alkyl)-naphthalenediimide Materials

The fabrication of highly ordered nanostructured surfaces is desirable in supramolecular chemistry and envisaged to bolster advances in heterogeneous catalysis and microelectronic applications. Here, we report on a novel set of alkylated double N,N'-bis(n-alkyl)-naphthalenediimides (NDIs) for the functionalization of highly oriented pyrolytic graphite (HOPG) with precise double lamellar morphologies. A detailed analysis of the two-dimensional (2D) self-assembled monolayers by scanning tunneling microscopy (STM) reveals that the structural repeating unit of the double lamellae is tuned precisely by the length of the alkyl chain that is connecting the NDI units. However, the expected odd-even effect is disturbed within the monolayers of a series of homologues. In contrast, a clear odd-even effect is observed for the melting temperatures of the respective bulk materials. Small-angle X-ray scattering reveals that these bulk materials exhibit nanophase-separated lamellar phases with domain spacings that are slightly larger than the repeating units of the double lamellar structures formed on the HOPG surface. The discrepancy is assigned to a partial desorption of the alkyl spacer from the HOPG surface, which becomes more pronounced when increasing its length. Our findings suggest that this lengthening increases the conformational freedom of the molecules on the surface while retaining a double lamellar morphology.

Automated summary

The study presents a novel class of alkylated NDIs for the precise functionalization of HOPG with double lamellar morphologies, showing the unexpected disturbance of odd-even effect in the self-assembled monolayers. The results suggest that increasing the length of the alkyl spacer enhances the conformational freedom of molecules while maintaining a double lamellar morphology.