Single molecule conductance of porphyrin wires with ultralow attenuation

Article abstract of DOI:10.1021/ja802281c

A series of thioacetate-terminated butadiyne-linked porphyrin oligomers have been synthesized with one to three porphyrin repeat units. Single molecule electrical scanning tunneling microscopy measurements using the I(s) and I(t) methods were used to determine the molecule conductances for this series of oligomers. The molecular conductance shows an exponential falloff with sulfur-sulfur distance with a remarkably low attenuation factor of β = (0.04 ± 0.006) A^-1. Copyright

Full text of DOI:10.1021/ja802281c

Published on Web 06/17/2008
Single Molecule Conductance of Porphyrin Wires with Ultralow Attenuation
Gita Sedghi,† Katsutoshi Sawada,‡ Louisa J. Esdaile,‡ Markus Hoffmann,‡ Harry L. Anderson,*,‡
Donald Bethell,† Wolfgang Haiss,† Simon J. Higgins,† and Richard J. Nichols*,†
The Chemistry Department, UniVersity of LiVerpool, LiVerpool L69 7ZD, U.K., and the Department of Chemistry,
Chemistry Research Laboratory, UniVersity of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.
Received March 28, 2008; E-mail: harry.anderson@chem.ox.ac.uk; nichols@liv.ac.uk
Charge transport through organic molecules over long distances is
important in many biological systems, in organic photovoltaics, and
in molecular electronics. One of the goals of molecular electronics is
the design of robust molecular wires which can transport charge
efficiently over extended distances, thereby enabling molecules to act
as device interconnects. An important parameter in defining charge
transport along molecular bridges is the decrease in transmission as a
function of distance. For long-range charge transport it is important
that this factor is low and that there is strong electronic coupling
between the bridge and the terminal contacts. The decrease in
transmission as a function of bridge length has commonly been
determined through photophysical measurements of charge transfer
kinetics between donor and acceptor moieties at the bridge termini,1–4
by measurement of rates of electrochemical electron transfer across
organic monolayers on electrodes,2,5 or by measuring current through
organic monolayers between metal electrodes.6 Recently it has been
possible to directly measure the conductance of single molecules
betweenpairsofgoldelectrodes,7–9includingπ-conjugatedoligomers.10,11
The decrease in transmission as a function of bridge length is
commonly observed to follow an exponential distance dependence in
single molecule measurements, which is taken to indicate a superex-
change (or tunneling) mechanism:
Figure 1. (a) Porphyrin oligomers 1-3 and reference compound 4; (b) I(s)
scans recorded by STM for a low coverage of 1 (adsorption from a 50 µM
solution in CHCl3 + 1% pyridine) on a Au(111) film under ambient conditions.
Curves in the absence (black) and presence (red) of molecular wire formation;
I0 ) 8 nA, Vbias ) 600 mV. The distance s is the distance between the two
contacts, calibrated as described in the Supporting Information; (c) Histogram
of Iw values for 1 from 145 I(s) scans.
surface and gold STM tip, without the need to first form a metallic
junction,9 as is the case with the in situ break junction technique.8
Current plateau values (I(s) technique) or current-jumps (I(t) technique)
are then analyzed statistically to determine the single molecule
conductance.9
Figure 1b shows an I(s) scan with evidence for the formation of a
molecular bridge of porphyrin 1 between the gold STM tip and gold
surface (red curve), together with an I(s) trace in which no molecular
bridge is formed (black curve). As described in the literature, the
plateau seen in the trace is assigned to molecular bridge formation.
The current then drops rapidly at the end of this plateau (red curve),
at the distance marked as sbreak-off, due to cleavage of the molecular
junction either at the Au-S bond or Au-Au metallic bonds in the
contacts. The current plateau values, Iw, measured from I(s) curves
such as Figure 1b are then collected in current-histogram plots (Figure
1c), with the major current peak in the histogram (IM) being used to
calculate the conductance at the selected bias voltage.
The absolute distance between the STM tip and gold surface was
obtained through calibration (see Supporting Information). This enables
the correlation between computed S · · ·S distance for 1-4 and the
experimentally observed break-off points (sbreak-off in Figure 1b) to be
analyzed (Figure 2a). The spread in individual break-off points for
1-3 illustrates the stochastic nature of the junction breaking process.
However it is noteworthy that for each compound the maximum
measured break-off point is close to the computed S · · ·S separation.
These break-off distances reassure us that we are indeed measuring
porphyrin oligomer molecules bridging the junction between the two
gold contacts.
The single molecule conductance value for the monomer 1
determined by the I(s) technique is 2.13 ( 0.28 nS and that for the
I(t) technique is 2.17 ( 0.45 nS, demonstrating that there is very good
agreement between the two techniques. Similar measurements have
been made for 2-4 and the data are summarized in Figure 2b (see
also Supporting Information). Single molecule conductances for
molecules 1-4 are plotted logarithmically against the calculated S · · ·S
σM e-ꢀR
(1)
where, the conductance (σΜ) exponentially decreases with the bridge
length (R), as quantified by the attenuation factor, ꢀ. All the major
techniques for determining attenuation factors concur that π-con-
jugated bridges give significantly lower attenuation factors than
saturated ones. Ultralow attenuation factors (<0.1 Å-1 4,12,13
have
)
been reported for highly conjugated, low bandgap systems. In this
respect, conjugated porphyrin oligomers are attractive candidates
because of their strong electronic coupling.14,15 Indeed, photoin-
duced electron transfer measurements have indicated that porphyrin
oligomers can transfer charge efficiently over long distances,3,16
and other measurements indicate that these molecules have wire-
like behavior.15–17 The question that we address here is whether
porphyrin oligomers exhibit low attenuation factors when “wired”
between metal electrodes at the single molecule level. For this
purpose we have synthesized (see Supporting Information) the series
of porphyrin oligomers 1-3 (Figure 1a), bearing terminal thioac-
etates to promote attachment to gold.
The I(s) (current-distance) and I(t) methods previously developed
by Haiss et al., using scanning tunneling microscopy (STM), were
deployed for the measurement of single-molecule conductance.9 In
the I(s) technique current is monitored as molecular wires are stretched
in the STM gap, while in the I(t) technique the stochastic formation
of molecular bridges is followed by monitoring current jumps over
time. Single molecular bridges are formed between the gold substrate
† University of Liverpool.
‡ University of Oxford.
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10.1021/ja802281c CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
dependence of Figure 2b was not observed.3 This points to a
pronounced difference between the direct electrical charge transport
and intramolecular charge transfer, arising from differences in electronic
coupling between the donor/acceptor and the bridge states in the
donor-bridge-acceptor systems and the electrode Fermi levels and
the bridge states in metal-molecule-metal junctions, and illustrates
that ꢀ cannot be treated as simply a bridge-specific parameter.13 Given
the ultralow attenuation factors for porphyrin oligomers demonstrated
here, this issue of electronic coupling between the contacts or terminal
groups and the bridge is crucial for the overall junction transmission.
The field of single molecular electronics will benefit from further
studies in which direct electrical and photophysical measurements are
compared for the same homologous group of molecules.21
Acknowledgment. We thank EPSRC for support, Arjen Cnossen
for preparation of porphyrin intermediates and the EPSRC Mass
Spectrometry Service (Swansea) for mass spectra.
Supporting Information Available: Complete ref 21b; synthesis and
characterization of compounds 1-4, further details of I(s) and I(t) methods
and results, histograms of conductance values, analysis of gap separation
behavior, analysis of hopping mechanisms, and full author list for ref 21b.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 2. (a) The break-off distance, sbreak-off, versus the computed sulfur· · ·sulfur
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revealed by photophysical experiments, but in that case the exponential
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