C O M M U N I C A T I O N S
in Figure 2. The spectra of alkyne-terminated p-OPT SAMs show
increases exponentially with molecular length L for short molecules
(OPT 1-4), as described by the nonresonant tunneling equation
a characteristic peak at 3320 cm- (t CsH stretching),
1
12,13
while
those of azide-terminated p-OPT SAMs exhibit a doublet peak
R ) R exp(ꢀL)
(1)
centered at 2120 cm-1 (N
14
0
3
asymmetric stretching). The alternate
appearance and disappearance of these two peaks in p-OPT wires
verifies the completion of click cycloaddition to all exposed reactive
end groups. In addition, the peak intensities of the triazole stretching
mode (1670 cm ) and benzene ring vibration (1610 cm ) increase
gradually upon repeated click cycloaddition, as expected. After all
the wires were end-capped, both azide and alkyne stretching peaks
disappeared in the spectra of OPT SAMs (Figure 2, right). The
thicknesses of OPT SAMs measured by ellipsometry and XPS
increase upon stepwise elongation of the molecular wires (section
where R is the effective contact resistance and ꢀ is the tunneling
0
attenuation factor. The ꢀ value obtained from a linear fit is 2.2
-1
nm , smaller than that obtained in previously reported oli-
-1
-1
1,2
goarylimine wires, suggesting more efficient electron tunneling
through the all-aromatic backbones of the OPT molecules. For long
wires (OPT 5 and longer), the resistances have a much weaker
length dependence. The linear relationship of R versus L (Figure
3B inset) for the long wires is consistent with hopping transport.
Furthermore, the clear transition in the length dependence of
resistance between 4 and 5 nm is consistent with our previous
observations in oligoarylimine wires of a change in transport
3.4, Supporting Information). Based on the ellipsometry results we
estimate that the molecules are tilted on average by ∼30° with
1
,2
respect to the surface normal.
mechanism from tunneling to hopping.
In summary, stepwise click cycloaddition represents an efficient
method to prepare long conjugated molecular wire SAMs on Au
surfaces for conduction measurements. The reaction proceeds
cleanly, and it is reasonable to expect that it can be used to introduce
a variety of functionalities and bonding architectures into the wire
backbones. The all-aromatic conjugated molecules reported here
exhibit similar electrical characteristics to other conjugated wire
molecules, with a clear transition from tunneling to hopping
transport as wire length increases. Future work will focus on
exploring the relationship between molecular architectures and
electron transport in the hopping regime.
Acknowledgment. This work is supported by the National
Science Foundation (CHE-0616427). Part of this work was carried
out in the Institute of Technology Characterization Facility,
University of Minnesota.
Supporting Information Available: Detailed experimental proce-
dures and characterization of all SAMs. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 3. Electrical measurements of molecular wires by CP-AFM. (A)
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J. AM. CHEM. SOC. 9 VOL. 132, NO. 26, 2010 8855