Issue 4, 2023

Helicene-based π-conjugated macrocycles: their synthesis, properties, chirality and self-assembly into molecular stripes on a graphite surface

Abstract

Fully aromatic helicenes are attractive building blocks for the construction of inherently chiral π-conjugated macrocyclic nanocarbons. These hitherto rare molecular architectures are envisaged to exhibit remarkable (chir)optical properties, self-assembly, charge/spin transport, induced ring current or a fascinating Möbius topology. Here the synthesis of helically chiral macrocycles that combine angular dibenzo[5]helicene units as corners and linear trans-stilbene-4,4′-diyl linkers as edges is reported. By subjecting a racemic or enantiopure divinyl derivative of dibenzo[5]helicene to olefin metathesis, which was catalysed by a 2nd generation Piers catalyst under kinetic control, a π-conjugated helicene cyclic trimer (33%) and a tetramer (22%) were obtained, which were separated by GPC. Combining racemic/asymmetric synthesis with the resolution of enantiomers/diastereomers by SFC/HPLC on a chiral column, both homochiral (+)-(M,M,M)/(−)-(P,P,P) and heterochiral (+)-(M,M,P)/(−)-(M,P,P) stereoisomers of the helicene cyclic trimer could be obtained in an enantio- and diastereomerically enriched form. The complete energy profile of their interconversion was compiled on the basis of kinetic measurements and numerical solution of the proposed kinetic model. In equilibrium, the heterochiral diastereomer predominates over the homochiral one (ca. 75 : 25 at 76 °C). π-Conjugation along a large, twisted circuit in the helicene cyclic trimer is rather disrupted, stabilising this formally antiaromatic molecule. Using an optimised PeakForce mode of ambient AFM, the self-assembly of otherwise highly mobile stereoisomers of the helicene cyclic trimer on the HOPG surface could be studied. Irrespective of the stereochemistry, strong preferences for the edge-to-edge interaction of these macrocycles were found to form very long parallel 1D molecular stripes in ordered 2D nanocrystals, a result also supported by molecular dynamics simulations. Six trityl groups, initially introduced to the macrocycle to enhance solubility, serve as a key “molecular Velcro” system in the self-assembly of macrocycles to maximise their mutual van der Waals interactions.

Graphical abstract: Helicene-based π-conjugated macrocycles: their synthesis, properties, chirality and self-assembly into molecular stripes on a graphite surface

Supplementary files

Article information

Article type
Paper
Submitted
29 Jul 2022
Accepted
21 Oct 2022
First published
28 Oct 2022

Nanoscale, 2023,15, 1542-1553

Helicene-based π-conjugated macrocycles: their synthesis, properties, chirality and self-assembly into molecular stripes on a graphite surface

V. Houska, E. Ukraintsev, J. Vacek, J. Rybáček, L. Bednárová, R. Pohl, I. G. Stará, B. Rezek and I. Starý, Nanoscale, 2023, 15, 1542 DOI: 10.1039/D2NR04209F

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