Engineering Long-Range Order in Supramolecular Assemblies on Surfaces: The Paramount Role of Internal Double Bonds in Discrete Long-Chain Naphthalenediimides

Abstract

Achieving long-range order with surface-supported supra­molecular assemblies is one of the pressing challenges in the prospering field of non-covalent surface functionalization. Having access to defect-free on-surface molecular assemblies will pave the way for various nanotechnology applications. Here we report the synthesis of two libraries of naphthalene­diimides (NDIs) symmetrically functionalized with long aliphatic chains (C28 and C33) and their self-assembly at the 1-phenyl­octane/highly oriented pyrolytic graphite (1-PO/HOPG) interface. The two NDI libraries differ by the presence/absence of an internal double bond in each aliphatic chain (unsaturated and saturated compounds, respectively). All molecules assemble into lamellar arrangements, with the NDI cores lying flat and forming 1D rows on the surface, while the carbon chains separate the 1D rows from each other. Importantly, the presence of the unsaturation plays a dominant role in the arrangement of the aliphatic chains, as it exclusively favors interdigitation. The fully saturated tails, instead, self-assemble into a combination of either interdigitated or non-interdigitated diagonal arrangements. This difference in packing is spectacularly amplified at the whole surface level and results in almost defect-free self-assembled monolayers for the unsaturated compounds. In contrast, the monolayers of the saturated counterparts are globally disordered, even though they locally preserve the lamellar arrangements. The experimental observations are supported by computational studies and are rationalized in terms of stronger van der Waals interactions in the case of the unsaturated compounds. Our investigation reveals the paramount role played by internal double bonds on the self-assembly of discrete large molecules at the liquid/solid interface.

Combinatorial Selection Among Geometrical Isomers of Discrete Long-Carbon-Chain Naphthalenediimides Induces Local Order at the Liquid/Solid Interface

“…The discrete extension of the carbon chains in C n -NDI-C n is found to significantly decrease the obtainment of long-range order compared to the shorter analogues previously investigated. 53 We attribute this phenomenon to much stronger interchain interactions occurring at the particular chain length regime investigated. These forces locally lead to new lamellar phases in the case of unsaturated C n -NDI-C n with n ≥ 50, in which the typical flat deposition of the NDI cores on HOPG is severely modified.…”

“…Recently, we have reported on the self-assembly of long aliphatic chain-functionalized naphthalenediimides ( C n -NDI-C n design) at the 1-phenyloctane-HOPG (1-PO/HOPG) interface. 53 The synthesized NDIs featured linear carbon chains consisting of either 28 or 33 carbon atoms ( C n -NDI-C n , n ≤ 33), which only differed by the presence or absence of internal double bonds (unsaturated/saturated compounds, respectively). The scanning tunneling microscopy (STM) images revealed that all compounds self-assembled into lamellar morphologies with alternating areas of NDI cores (that lay flat and next to each other on the surface) and aliphatic chains (that fill the space in between the aromatic arrangements), in line with previous investigations.…”

“… 54 − 56 The unsaturated C n -NDI-C n consisted of nonseparable mixtures of ZZ , EZ , and EE isomers, but their simultaneous presence did not impede the obtainment of long-range ordered lamellar domains exceeding 15 000 nm 2 , which were by far superior to those of the saturated C n -NDI-C n counterparts in terms of order extent. 53 We proposed the establishment of internal unsaturations as a key structural paradigm for obtaining long-range-ordered assemblies on unconfined surfaces, which is a key challenge in the field. 1 …”

“…Building on our discovery of the paramount role of “simple” internal unsaturations in directing self-assembly, 53 we embarked on the study of a library of carbon-chain extended saturated and unsaturated C n -NDI-C n to elucidate the importance of these so far unrecognized structural parameters. We envisioned that such a study would simultaneously push the limits of self-assembly in terms of chemical structures and stability of the architectures formed, as well as unravel key aspects controlling surface-based supramolecular processes.…”

“…To reach this goal, we synthesized two libraries of C n -NDI-C n with linear carbon chains comprising from 39 to 55 carbon atoms (with 39 ≤ n ≤ 55, Chart 1 ), modifying our previous method to obtain discrete oligoethylenes. 53 The two C n -NDI-C n families differ by the presence/absence of an increasing number of internal double bonds (2 and 3 per chain). The latter, when present, are marked by the letter u , followed by the number 2 or 3 as subscript to indicate the total number of double bonds per carbon chain ( Chart 1 ).…”