Precise Orientational Control of Electroactive Units Using a Tripodal Triptycene Scaffold to Direct Noncovalent Pairing at the Single Molecular Level

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.

断裂结（break junction）技术已被确立用于探索单分子水平的导电行为，当前研究热点已转向对通过非共价分子间作用力相互作用的双分子体系的评估。该研究方向要求对两种分子的取向进行精准调控，以使二者能够在两个电极之间形成合适的面对面排布。本文报道了一种基于三脚架型三蝶烯（tripodal triptycene）支架的制备方法，该支架可使电活性亚基以直立构型在基底表面实现精准定位。我们将给电子型四硫富瓦烯（tetrathiafulvalene）或受电子型蒽醌（anthraquinone）引入该分子支架，并证实所得分子保留了其所连接亚基特有的电子特性。我们在Au(111)基底上制备了这些分子的自组装单分子层（self-assembled monolayers, SAMs），并通过X射线光电子能谱（XPS）与扫描隧道显微镜（STM）对其进行了表征。将扫描隧道显微镜断裂结技术应用于该自组装单分子层后，观测到两种导电机制：其一为夹在两个电极之间的单分子导电，其二为通过分子间相互作用形成的同二聚体（homodimers）桥接电极所产生的导电。这一实验结果证明了使用三脚架型三蝶烯支架精准引导电活性亚基进行分子间配对的方法的有效性。我们认为本研究将为在单分子水平评估异二聚体（heterodimers）提供全新的研究路径。