Thiolated Bis- <i>meta</i> -Carborane: A Molecular Rotor with Conformation-Dependent Dipole Moment

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-<i>meta</i>-carborane-thiol ( <i><b>mm</b></i> <b>-SH</b>), a rod-like molecule with two <i>meta</i>-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 <i><b>mm-SH</b></i> 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.