Figure 1. Structure of OPEs with dendrimer “shell”.
on OPE’s electronic properties. It has been observed that
the conduction of the same OPE derivative could change by
several orders of magnitude when the local environment was
changed.7 Such a big influence is mainly attributed to the
fact that strong intermolecular interactions between neighbor
molecules affect the electrostatic potential distribution across
the molecular junctions and the local heat conduction.
To control the environment surrounding OPE molecules,
Tour and his collaborators have inserted OPE molecules into
the defects of alkanethiol SAMs.1c,d,8 However, this approach
produces a heterogeneous surface with little control over the
position and amount of the inserted OPEs. Meanwhile,
keeping OPE molecular wires at uniform distances is
important for addressing each individual molecule in future
molecular electronic circuits. Therefore, a strategy that could
produce a uniform monolayer while having precise control
over the local environment of OPE moieties is strongly
desired in research on molecular electronics.
between OPE moieties can be minimized while the molecules
can form a molecular scale homogeneous monolayer. Herein,
we report on the synthesis of two series of “core-shell”
OPEs (1a,b,c and 1d,e,f) for the first time (Figure 1). These
molecular wires have an OPE “core” with different dendrimer
“shells” and acetyl-protected thiol end-groups. The thiol
groups, which could be easily generated from the thioacetyl
groups, are employed as an anchoring group. Dendrimers
are chosen as “shells” mainly due to their highly branched
geometry. The outer layers of the dendrimers containing tert-
butyl groups facilitate the solubility of OPEs in organic
solvents. In fact, it was found that the series of OPEs with
two tert-butyl groups on each outer-layer benzene ring are
much more soluble in most organic solvents than those with
one tert-butyl group. Three other OPEs with relatively small
side groups, e.g., benzyl, isoamyl, and methoxyl methyl
(MOM), have also been synthesized for comparison pur-
poses.
In principle, “core-shell” OPE derivatives could be
synthesized using two strategies. One strategy starts from
constructing the “core” and then attaches the dendrimer
“shells” by SN2 substitution reaction. The alternative strategy
starts from diiodobenzene with the dendrimer “shells” already
in place. The OPE “core” is then constructed by Sonogashira
cross-coupling reaction. Considering that the first strategy
involves a basic condition in which the thioacetyl group can
easily be hydrolyzed, we adopted the second strategy in this
work, though the synthesis might suffer steric hindrance
when the “core” was constructed.
The synthesis of OPEs proceeded according to Scheme
1. First, 4-ethynyl-1-thioacetylbenzene (7) was synthesized
from 4-iodobenzenesulfonyl chloride according to the lit-
erature methods.9 Also, the first- and second-generation
dendrimers were prepared as materials;10 the details are
In this work, a pseudo “core-shell” design of molecular
wires is proposed. By surrounding a conductive core with
an insulating shell, the undesired intermolecular interaction
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