la7b02207_si_001.pdf (711.6 kB)
Influence of CH···N Interaction in the Self-Assembly of an Oligo(isoquinolyne-ethynylyne) Molecule with Distinct Conformational States
journal contribution
posted on 2017-10-02, 15:42 authored by Ajiguli Nuermaimaiti, Yanxiao Ning, Jacob L. Cramer, Katrine L. Svane, Bjørk Hammer, Kurt V. Gothelf, Trolle R. LinderothMolecular
conformational flexibility can play an important role
in supramolecular self-assembly on surfaces, affecting not least chiral
molecular assemblies. To explicitly and systematically investigate
the role of molecular conformational flexibility in surface self-assembly,
we synthesized a three-bit conformational switch where each of three
switching units on the molecules can assume one of two distinct binary
positions on the surface. The molecules are designed to promote C–H···N
type hydrogen bonds between the switching units. While supramolecular
self-assembly based on strong hydrogen-bonding interactions has been
widely explored, less is known about the role of such weaker directional
interactions for surface self-assembly. The synthesized molecules
consist of three nitrogen-containing isoquinoline (IQ) bits connected
by ethynylene spokes and terminated by tert-butyl
(tBu) groups. Using high-resolution scanning tunnelling microscopy,
we investigate the self-assembly of the IQ-tBu molecules on a Au(111)
surface under ultrahigh-vacuum conditions. The molecules form extended
domains of brick-wall structure where the molecular backbones are
packed regularly but without selection of specific molecular conformations.
However, statistical analysis of the extended network demonstrates
alignment/correlation for the orientations of the switching units
indicating specific interactions. The primary interaction motifs in
the structure are quantified from DFT calculations, showing that the
brick-wall structure is indeed stabilized by two types of weak C–H···N
bonds, involving either aromatic hydrogens on the IQ groups or nonaromatic
hydrogens on the tBu groups. Analysis of the C–H···N
interactions in the brick-wall structure explains the observed distribution
and alignment of molecular conformations as well as the overall organization
of the molecular surface structures.