Charge Transport in Sequence-Defined Conjugated Oligomers

A major challenge in synthetic polymers
lies in understanding how
primary monomer sequence affects materials properties. In this work,
we show that charge transport in single molecule junctions of conjugated
oligomers critically depends on the primary sequence of monomers.
A series of sequence-defined oligomers ranging from two to seven units
was synthesized by an iterative approach based on the van Leusen reaction,
providing conjugated oligomers with backbones consisting of para-linked
phenylenes connected to oxazole, imidazole, or nitro-substituted pyrrole.
The charge transport properties of these materials were characterized
using a scanning tunneling microscope-break junction (STM-BJ) technique,
thereby enabling direct measurement of molecular conductance for sequence-defined
dimers, trimers, pentamers, and a heptamer. Our results show that
oligomers with specific monomer sequences exhibit unexpected and distinct
charge transport pathways that enhance molecular conductance more
than 10-fold. A systematic analysis using monomer substitution patterns
established that sequence-defined pentamers containing imidazole or
pyrrole groups in specific locations provide molecular attachment
points on the backbone to the gold electrodes, thereby giving rise
to multiple conductance pathways. These findings reveal the subtle
but important role of molecular structure including steric hindrance
and directionality of heterocycles in determining charge transport
in these molecular junctions. This work brings new understanding for
designing molecular electronic components.