Electrostatically Driven Guest Binding in Self-Assembled Molecular Network of Hexagonal Pyridine Macrocycle at the Liquid/Solid Interface: Symmetry Breaking Induced by Coadsorbed Solvent Molecules

Abstract

We present here the construction of a self-assembled two-dimensional network at the liquid/solid interface using a hexagonal pyridine macrocycle which binds an organic cation in its intrinsic porous space by electrostatic interactions. For this purpose, a hexagonal pyridinylene-butadiynylene macrocycle (PyBM) having six octyloxymethyl groups, PyBM-C8, was synthesized. As guests, tropylium (Tr) tetrafluoroborate and trioxatriangulenium (TOTA) hexafluorophosphate were used. In this study, we focused on (i) the network patterns of PyBM-C8 which change in response to its concentration and (ii) the position of the guest immobilized in the porous space of the macrocycle. Scanning tunneling microscopy (STM) observations at the interface of 1,2,4-trichlorobenzene (TCB) and highly oriented pyrolytic graphite (HOPG) revealed that PyBM-C8 formed four different polymorphs, oblique, loose hexagonal, linear, and rectangular, depending on the solute concentration and annealing treatment. Solvent TCB molecules are likely co-adsorbed not only the intrinsically porous space of PyBM-C8 (internal TCB) but also the space outside of the macrocycle between its alkyl chains (external TCB) in most of the cases. Upon adding the guest cation, whereas small Tr was not visualized in the pore due to size mismatching, larger TOTA was clearly observed in each pore. In addition, based on high resolution STM images of the rhombus packing pattern of PyBM-C8, we 3 revealed experimentally that TOTA was placed at an off-center position of the deformed hexagonal macrocyclic core in the rhombus pattern. Based on the molecular mechanics calculations, we hypothesize that the off-center location of TOTA is due to deformation of the hexagonal macrocycle through interaction with two external TCB molecules located at opposite edges of the macrocyclic core. Symmetry-breaking of the macrocyclic host framework induced by co-adsorbed surrounding solvent molecules thus play a significant role in host-guest complexation at the liquid/solid interface.

Self-Assembly of Triphenylamine Macrocycles and Co-assembly with Guest Molecules at the Liquid–Solid Interface Studied by STM: Influence of Different Side Chains on Host–Guest Interaction

“…Generally, co-adsorbed solvent molecules that actually contributed to pattern stabilization in the network have been revealed in previous studies. ,, On the basis of the experimental results, we have considered co-adsorption of solvent molecules in our system, and further DFT calculations were performed to investigate the assemblies. Panels b and d of Figure show the large-scale optimized models of the linear structure of COTPA _stru 1 and the hexagonal structure of COTPA _stru 2.…”

“…It is interesting to fabricate 2D host–guest supramolecular nanopatterns on HOPG. Sometimes, guest molecules can induce a change in the morphology of 2D self-assembly at the liquid–solid interface . Considering that the sizes of the guest and the cavity need to be matched with each other, first, we chose C60 as the guest molecules in the host–guest assembly.…”

“…Sometimes, guest molecules can induce a change in the morphology of 2D selfassembly at the liquid−solid interface. 25 Considering that the sizes of the guest and the cavity need to be matched with each other, first, we chose C60 as the guest molecules in the host− guest assembly. In previous studies, surface complexation with C60 was successfully achieved by using macrocycles and the pore sizes were ∼1.2 nm.…”

“…Tobe et al constructed a self-assembled molecular network of hexagonal pyridine macrocycles at the liquid−solid interface via electrostatic interactions, and the solvent molecules play a key role in the symmetry breaking of the macrocycle host framework in the host−guest complexation at the liquid−solid interface. 25 These STM studies have revealed the role of the molecular structure of the conjugated macrocycle, especially the significance of the skeleton on its assembled structure. Generally, π-conjugated macrocyclic molecules are modified with alkyl chains, which facilitate their solubility in solvents and the self-assembly process to form an ordered assembly sequence on the substrate.…”

“…In fact, solvent molecules in the surface assembly of macrocycle molecules cannot be ignored. Tobe et al constructed a self-assembled molecular network of hexagonal pyridine macrocycles at the liquid–solid interface via electrostatic interactions, and the solvent molecules play a key role in the symmetry breaking of the macrocycle host framework in the host–guest complexation at the liquid–solid interface . These STM studies have revealed the role of the molecular structure of the conjugated macrocycle, especially the significance of the skeleton on its assembled structure.…”