# Thiolated Bis-meta-Carborane: A Molecular Rotor with Conformation-Dependent Dipole Moment

## TL;DR

A new molecule with a rotating structure and changing dipole moment is synthesized and studied for potential use in self-assembled materials.

## Contribution

The first cluster constituent of self-assembled monolayers capable of rotating to change its dipole moment is synthesized and characterized.

## Key findings

- The thiolated bis-meta-carborane molecule forms a highly ordered structure on silver surfaces.
- The molecule's dipole moment can change from 0.64 to 3.78 D through rotation.
- Nanomembranes with an effective thickness of 5 ± 1 Å are formed via low-energy electron irradiation of the monolayers.

## Abstract

The robust structure and tunable properties of 12-vertex
carborane
clusters make them highly attractive constituents of 2D and 3D self-assembled
materials as well as for various potential applications. These molecules
offer new possibilities related to their characteristic features such
as low conformational freedom, high thermal and chemical stability,
and relatively high inherent dipole moment. We synthesized and fully
characterized bis-meta-carborane-thiol (
mm

-SH), a rod-like molecule with two meta-carborane units connected by a single bond, acting
as a molecular rotor. In its supramolecular crystal structure, this
molecule exhibits an intriguing packing arrangement driven by the
interplay between the SH group of a particular conformation, which
affects intermolecular hydrogen interaction, and intramolecular dipole–dipole
interactions. This interesting interplay manifests itself clearly
in the single-crystal supramolecular arrangement, which we have analyzed
experimentally as well as computationally. The respective self-assembled
monolayer (SAM) of this dipole-responsive carborane constituent was
prepared on flat silver and gold substrates and investigated using
surface sensitive techniques such as X-ray photoelectron spectroscopy,
scanning tunneling microscopy (STM), low-energy electron diffraction
(LEED), and ellipsometry. The
mm-SH
molecules
form a highly ordered structure on the Ag(111) surface, which was
measured via LEED and STM with submolecular resolution. It is the
first cluster constituent of SAMs that can, by rotation, change the
orientation and magnitude of its inherent dipole moment ranging from
0.64 to 3.78 D. Furthermore, the formation of nanomembranes by low-energy
electron irradiation of the SAMs (with an effective thickness of 5
± 1 Å) is presented.