Precise Orientational Control of Electroactive Units Using a Tripodal Triptycene Scaffold to Direct Noncovalent...

A break junction technique has been established to explore conductive behavior at the single molecular level, and recent interest has shifted toward the evaluation of bimolecular systems interacting through noncovalent intermolecular forces. This requires precise control over the orientation of the two molecules so that they can adapt an appropriate face-to-face arrangement between two electrodes. Herein, we present an approach using a tripodal triptycene scaffold that allows for accurate positioning of electroactive subunits with an upright configuration on substrate surfaces. We incorporated electron-donating tetrathiafulvalene or electron-accepting anthraquinone into the molecular scaffold and confirmed that the resulting molecules retain the electronic properties particular to their attached subunits. Self-assembled monolayers (SAMs) of these molecules were prepared on Au(111) and characterized by XPS and STM. STM break junction techniques were applied to the SAMs, revealing two electrical conduction regimes; one arises from single-molecules sandwiched between two electrodes, and the second from intermolecularly interacting homodimers that bridge between electrodes. This observation demonstrates the validity of the approach of using tripodal triptycene scaffolds to precisely direct electroactive subunits to undergo intermolecular pairing. We believe that the present work will provide a new avenue for evaluating the heterodimers at the single molecular level.