J. Phys. Chem. C 2008, 112, 13215–13225
13215
Experimental and Theoretical Identification of Valence Energy Levels and Interface Dipole
Trends for a Family of (Oligo)Phenylene-ethynylenethiols Adsorbed on Gold
Chad Risko,†,* Christopher D. Zangmeister,‡ Yuxing Yao,§ Tobin J. Marks,† James M. Tour,§
Mark A. Ratner,† and Roger D. van Zee‡
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, EVanston,
Illinois 60332, Chemical Science and Technology Laboratory, National Institute of Standards and Technology,
Gaithersburg, Maryland 20899, and Smalley Institute for Nanoscale Science and Technology, Department of
Chemistry, Rice UniVersity, Houston, Texas 77005
ReceiVed: March 17, 2008; ReVised Manuscript ReceiVed: May 16, 2008
Metal-molecule-metal junctions composed of organic molecular wires formed via self-assembly are of
relevance in the empirical evaluation of single-molecule electronics. Key to understanding the effects of
these monolayer structures on the transport through single molecules, however, is discerning how the molecular
electronic levels evolve under the influence of the metal substrate and intermolecular interactions. We present
a joint experimental and computational investigation of the electronic structure and electrostatic properties of
a series of self-assembled donor- and acceptor-substituted (oligo)pheneylene-ethynylenethiols (OPEs) on gold.
Photoemission spectroscopy is employed to determine the energy-level alignment for these monolayers. Isolated
molecule and small cluster calculations are performed to assess changes in geometry, electronic structure,
and charge distribution upon chemisorption. The calculated densities of electronic states allow assignment of
the higher-lying occupied states mapped by experimental photoemission data. Calculated estimates of the
surface, bond dipole, and image potential energies are used to estimate contributions to the measured work
function changes; good correlations between the experimental and theoretical values are found. Importantly,
these results point to a dependence of the dipole contributions on the orientational order of the SAM.
I. Introduction
Organic molecules bound to metal and inorganic semiconductor substrates are of technological interest with prospective
applications in single-molecule electronic devices, molecular/
polymeric thin-film electronics, dye-sensitized solar cells,
biochemical redox sensors, engineered molecular nanostructures,
etc.1–15 Essential to describing the electrical performance of
molecule-based wire systems is understanding how molecular
adsorption and coupling to the electrode affects the electronic
structure of the molecular components.8,16,17 Although molecular
orbitals are not the channels by which charge moves though
the molecular wire, the energies of the molecular states have
been shown to strongly correlate with transmission characteristics;18,19 therefore, it is important to gain direct insight into
these one-electron levels.
The relative alignment of a self-assembled monolayer’s
(SAMʼs) energy levels with respect to the electrode
Fermi energy can have a profound impact on the current-voltage
(I-V) characteristics of a device,20 with the formation of an
interfacial dipole playing a key role in the alignment.7,16,17,21–37
A number of factors can lead the presence of these interfacial
dipoles, including: native surface dipoles due to the leakage of
electron density from the electrode surface;38 charge transfer
between the molecule and substrate upon adsorption; image
charge interactions due to the redistribution of electron density
in the electrode; and any other physical phenomena that induce
charge redistributions within or between the substrate and
* E-mail: [email protected].
† Northwestern University.
‡ National Institute of Standards and Technology.
§ Smalley Institute for Nanoscale Science and Technology.
organic layer. By understanding the formation of these dipoles
and the redistribution of charge, we may be able to control the
interfacial interactions through molecular design.
Ultraviolet photoemission spectroscopic (UPS) methods
provide insight into the electronic structure and coupling to the
inorganic substrate by directly probing the occupied molecular
electronic states.7,16,21,39 In photoemission experiments, ionization energies are measured as electrons are photoemitted
from the molecular orbital structure and a radical-cation is
formed. The kinetic energy of the photoemitted electrons is slow
enough to include both the relaxation of the electronic polarization of the surrounding molecules and the structural relaxation
of the molecular ion;21 therefore, Koopmans’ theorem40 is not
directly valid.21 Obtaining appropriate and relevant theoretical
descriptions of these large structures, however, is a formidable
challenge as: (i) simply the sheer number of chemical systems
and interactions needed to be taken into account is immense,
and (ii) we are at the boundary where approximations involved
with band structures and densities of states (to deal with the
delocalization of valence electrons in metals) encounter those
approximations of molecular electronic structure theory (to
describe the discrete molecular orbital configurations and
energies of single molecules).41
Thiol-bound SAMs, especially (oligo)pheneylene-ethynylenethiol (OPE) SAMs,4–6,16,37,42–52 have undergone intense experimental and theoretical investigation and scrutiny;4–6,16,37,43,45,53–83
this class of molecules is the subject of this investigation. In
this work, we use gas-phase and extended-molecule model
calculations to explore the influence of the gold electrode on
the electronic structure of a series of donor- and acceptorsubstituted OPEs (see Figure 1) and compare these results
directly to UPS measurements of the SAMs on gold surfaces.
10.1021/jp8023183 CCC: $40.75  2008 American Chemical Society
Published on Web 08/06/2008
13216 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Figure 1. Chemical structures of substituted OPE and smaller systems.
We are especially interested in exploring electrostatic interactions between the SAMs and the gold surface, in particular, the
surface potential energy due to the intrinsic dipole moment of
the OPE systems and image dipole effects.
II. Methodology
a. Computational Methodology. Adsorption processes have
been studied theoretically using various levels of theory and a
number of different models, including dipped adcluster,84 local
space approximation,85–87 embedded cluster,41 and slab calculations with periodic boundary conditions.55,60,80–83,88 Because of
its simplicity, the extended-molecule (i.e., bare cluster) approach
has been widely used as a starting point in single-molecule electronics to understand electrode-molecule interactions.45,62,63,68,89–91
Although pertinent results have been obtained for numerous
structures and geometries of molecule-surface interactions, the
model has shortcomings, including neglect of long-range
contributions of importance in determining binding energies and
relative stabilities.92 Taking note of these assumptions and
limitations, we use the extended-molecule model herein to
understand the influence of the gold contact on the molecular
electronic structure to provide a basis for future slab calculations.
We begin by examining the single molecule properties in
various redox and deprotonated states for a series of donor and
acceptor substituted OPE-thiols; benzenethiol (B) and 4-(phenylethynyl)benzenethiol (PEB) are included in the theoretical
analysis to probe the consequences of conjugation length.
Geometry optimizations using density functional theory (DFT)
for the neutral, radical-ion, thiyl (neutral, open-shell dehydrogenated thiol), and thiolate (anionic, closed-shell deprotonated
thiol) molecular states were carried out using the B3LYP
functional93–95 and a 6-31G** basis set. We then examine an
extended-molecule picture, where three gold atoms serve as a
minimal representation for the gold electrode. The extendedmolecule model provides a static picture of the Au-molecule
interaction in one of many possible geometries; the nature of
the Au-S bond (e.g., binding site, number of atomic Au-S
interactions, etc.) is not well defined experimentally or theoretically (i.e., the nature of the bond description depends on density
functional and basis set choice) because of the relatively flat
potential energy surface55 for binding; we note that a very recent
crystallographic study of thiol-monolayer-protected gold nanoparticles suggests an interesting bonding motif for such
interactions.96,97 The present calculations were carried out under
the following assumptions: chemisorption of a deprotonated thiyl
structure, frozen Au cluster with no interactions from other
layers or bulk, predominately hollow-site sulfur position as the
starting geometry,55 and no interaction with neighboring molecules. For the extended-molecule calculations, the same
functionals and basis sets were utilized for all non-gold atoms,
Risko et al.
with the exception of sulfur to which an extra diffuse function
was added to extend the interaction with the gold cluster; the
small-core SBKJC effective core potential (ECP) basis was used
for the gold atoms to allow for possible chemistries with a larger
number of core electrons. Electronic density of states (DOS)
for the isolated thiols and extended-molecule systems were
created by convoluting the one-electron energies with Gaussian
functions characterized by a full width at half-maximum (fwhm)
of 0.5 eV. All calculations were carried out with QChem
(version 2.1).98
b. Monolayer Preparation and Chacterization. Monolayers were grown on polycrystalline gold films prepared by
evaporation of ∼0.2 µm of Au onto a ∼20 nm Cr adhesion
layer on Si(100) wafers. The substrates were cleaned prior to
monolayer growth by exposure to ultraviolet light/ozone for ∼20
min, followed by rinsing with water that was treated to remove
organic, ionic, and biological impurities (F ≈ 18.2 MΩ cm).
Finally, the substrates were dried with a stream of ultrapure
gaseous nitrogen.
OPE, OPE-F, OPE-NH2, OPE-NO2, and OPE-bisNO2 were
synthesized as previously described.46,99 Monolayers of OPE,
OPE-F, OPE-NH2, OPE-NO2, and OPE-bisNO2 were grown
from a ∼0.5 mM solution in a ratio of 2:1 (10 mL total volume)
anhydrous CH2Cl2 to anhydrous C2H5OH.16,46,100 Monolayers
of OPE-NO2 and OPE-bisNO2 were prepared from thioacetates
and were deprotected with 120 µL of sulfuric acid prior to
introduction of the substrate. Monolayers of OPE-F and OPENH2 were deprotected with NH4OH.5,16,100 Monolayer growth
was carried out in an Ar-purged dry box ([O2] < 1 µmol/mol;
[H2O] < 0.5 µmol/mol). The substrates were allowed to incubate
for about a day. After removal from solution, the substrates were
rinsed with copious amounts of CH2Cl2 in the dry box.
Reflection-absorption infrared spectroscopy, contact angle
measurements, and spectroscopic ellipsometry were acquired
on samples grown concurrently with those used in the photoemission investigation.5,16,46,100 These data were consistent with
results from previous reports of densely packed, ordered
monolayers of similar molecules. Absorption spectra in the
ultraviolet and visible range were also acquired for ∼10 µM
solutions in CH4 using a commercial spectrophotometer. The
thioacetate compounds were deprotected prior to measurement.
Preparation of these solutions involved handling very small
quantities of material in the glovebox, resulting in considerable
uncertainty in the amount of compound going into solution.
Repeated measurements suggest that the fractional standard
deviation between measurements may be as great as 50%.
c. Electronic Structure Characterization of Monolayers.
Samples were mounted on a vacuum chuck immediately after
being removed from the dry box. All measures were taken to
reduce exposure of the monolayers to the ambient prior to being
placed in vacuum. Each sample was measured after identical
growth time, ambient exposure and handling, and vacuum
conditions. The samples were placed in a load lock, pumped
down, and held at ∼1 × 10-8 Torr (1 Torr ≈ 133 Pa) overnight.
The next day, samples were moved into the analysis chamber.
The base pressure of the analysis chamber was typically ∼1 ×
10-9 Torr during the course of the experiments. Ultraviolet
photoelectron spectra were obtained using a He(I) source (21.22
eV) in a commercial spectrometer also equipped with an X-ray
source at room temperature. The light was incident at 45° and
54° from the surface normal for ultraviolet and X-ray photoemission, respectively. Photoelectrons were collected at normal
emission with a hemispherical electrostatic analyzer.
(Oligo)Phenylene-ethynylenethiols Adsorbed on Gold
Two quantities were extracted from the ultraviolet photoemission spectra: the work function and the binding energy of
the molecular highest occupied π-state (EB,π). The work function
of the monolayer covered surface (φmono) was determined by
subtracting the width [half-maximum at the Fermi edge (EF) to
the half-maximum of high-binding energy cutoff (EB,max)] of
the photoemission spectrum from the source photon energy (hυ),
that is, φmono ) hυ - (EB,max - EF). The work function of the
bare gold (φAu) was determined in the same way. Bare samples
were cleaned with ultraviolet light/ozone and were rinsed with
water before being transferred into the vacuum; the samples
were then sputtered until no contaminants could be detected
using X-ray photoemission. Typically, φAu ≈ 5.2 eV. The work
function shift is simply ∆φ ) φmono - φAu. The binding energy
of the highest occupied π-state (EB,π) is the difference between
EF and the centroid of the feature in the monolayer spectrum
corresponding to the highest occupied orbital of the isolated
molecule. The peak centroid was determined by fitting the
appropriate energy interval with a Gaussian for the peak and a
polynomial for the background. Each value of EB,π, φmono, and
φAu is an average determined from multiple spectra recorded
using two independently prepared samples.
X-ray photoemission spectra (XPS) were acquired after
collection of ultraviolet photoelectron data. Survey and highresolution spectra were acquired for each monolayer to collect
elemental information, determine film quality, and ascertain
monolayer coverage. Of particular importance was the position
of the S 2p at ∼162.2 ( 0.2 eV, typical with the formation of
a S-Au bond. No higher energy S 2p peaks were observed.
Oxygen levels were <1% for OPE, OPE-F, and OPE-NH2. The
Cl 2p intensity (from CH2Cl2) was used as a marker for the
presence of intercalated solvent in the monolayer. In all spectra,
any inclusion of solvent was below the limit of detection.
Monolayer coverage and film quality was determined using the
S 2p/Au 4f and other peak intensity ratios.5,16,46 These ratios
show intact molecular films with molecular coverages consistent
with other studies of OPEs on Au.5 We also have previously
observed that neither eliminating exposure to laboratory ambient
nor annealing nor an extended period in ultrahigh vacuum
(UHV) affected the measured spectra of OPEs on Au to within
the signal-to-noise of our experiments. Given the large number
of compounds studied here and minute quantities of available
sample, extensive repetition of those tests in this investigation
was not feasible. Instead, we assume that the lessons of the
earlier investigations also apply here.
d. UV/vis Spectroscopy. Absorption spectra in the ultraviolet
and visible range were acquired for ∼10 µM solutions in CCl4
using a commercial spectrophotometer. The thioacetate compounds were deprotected prior to measurement. Preparation of
these solutions involved handling very small quantities of
material in the glovebox, resulting in considerable uncertainty
in the amount of compound going into solution. Repeated
measurements suggest that the fractional standard deviation
between measurements may be as great as 50%.
III. Results
a. Molecular Structure. Selected geometric parameters for
the neutral, radical-ion, dehydrogenated, deprotonated, and Aucluster structures are listed in Tables S2-S9 of the Supporting
Information, with Figure S1 providing the bond numbering
scheme. The gas-phase neutral geometries show the expected
bond-length alternation (BLA)101 patterns of OPE structures,
with slight modifications due to the asymmetric thiol substitution
and donor/acceptor substitution on the central phenylene ring
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13217
of the extended conjugated systems. In general, geometric
changes due to oxidation, reduction, or dehydrogenation are
more pronounced for the smaller B and PEB structures versus
the full OPE structures. Oxidation and reduction across the
molecular series produce anticipated BLA changes in the
conjugated π-structure, moving the geometries toward more
quinoidal-like structures. The S-C bond length is more affected
by oxidation (shortening by 0.03-0.07 Å, 1 Å ≡ 10-10 m)
versus reduction (lengthening by 0.01-0.04 Å), although the
effects are lessened with the gradual increase in conjugation
length across the molecular series; there appears to be no
significant donor or acceptor effects from the substituents on
the central phenylene rings of the full OPE structures.
It is of interest to understand the nature of the dehydrogenated
(thiyl, ArS · ) and deprotonated (thiolate, ArS-) structures as
these are thought to be the competing forms to bind to the gold
surface. Dehydrogenation and deprotonation generally only
disturb the geometries nearest the thiol substituent. The S-C
bond shortens rather considerably (∼0.06-0.07 Å), whereas the
two neighboring C-C bonds lengthen (∼0.02-0.03 Å); there
are relatively minor shifts in the remaining conjugated segments
across the series. The patterns in geometric modification to the
S-C bonds, however, do differ. Dehydrogenation induces
similar changes in bond lengths across the series, with the thiyl
S-C bonds on the order of 1.72 Å. For the deprotonated
systems, the S-C bond lengths fall within a range of 1.71-1.75
Å; additionally, there appears to be a dependence on the strength
of donor and acceptor substituent and π-conjugation length;
OPE-bisNO2 and OPE-NO2 have the shortest S-C bonds
(whereas the other substituted OPEs are similar) and B has a
considerably longer S-C bond.
Calculated gas-phase ionization potential and electron affinity
energies102 are plotted in Figure 2 (energies are given in Table
S1 of Supporting Information). Trends in the ionization potential
and electron affinity energies follow both conjugation length
of the molecular structure and donor-acceptor strength of the
substituent on the central phenylene unit. The adiabatic ionization potential energies range from 7.92 eV for B to 6.22 eV for
OPE-NH2. Assuming vacuum-level alignment and taking into
account the Fermi energy (EF) for Au as 5.2 eV below the
vacuum, we would expect the barrier to transport through the
highest occupied π-state (hole-type transport) to be smallest for
OPE-NH2 when compared to the other molecular species, with
OPE-F and OPE not far behind. We note that the gas-phase
adiabatic dehydrogenation and deprotonation energies are
considerable, although, in general, deprotonation is shown to
be an energetically easier process; a number of experimental
factors, including stabilization due to solvation and/or interactions with the metal surface, could significantly change the
relative energetics of these processes.
Binding of the π-conjugated systems to the gold cluster,
interestingly, induces very little geometric change versus the
neutral thiol structure. The geometric changes are much more
substantial if one compares either the dehydrogenated or
deprotonated forms to the extended-molecule structure.55,60
Geometric parameters describing both the S-C bonds and the
π-conjugated segments are shown to change very little at this
level of theory between the thiol and gold-bound structure. These
results point to a stabilizing effect of the Au-S bond that is
similar to the H-S bond of the original thiol. For the series of
molecules, the S atoms lie approximately 2.2 Å above the plane
of the Au cluster. In addition, the S atom tends to slide toward
one of the edges, in accordance with recent theoretical estimates
13218 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Figure 2. (top) Calculated vertical and adiabatic ionization potential
(VIP and AIP, respectively) and electron affinity (VEA and AEA,
respectively) energies for the isolated molecules (see Ref. 102). (bottom)
Selected calculated valence molecular orbital energies for both the
isolated molecules (M) and extended-molecule (EM) systems (note the
limited energetic perturbation induced by the Au3 cluster; see Electronic
Structure in the Results section for further details). The dashed and
dotted lines represent the energies of the interfacial S-Au3 levels. The
trends of the molecular orbital energies qualitatively match the
calculated trends of the IP and EA energies, although a lack of
quantitative agreement suggests a breakdown of Koopmans’ theorem.
Data are provided in Table S1 of Supporting Information.
that the most energetically favorable bonding point is a bridgefcc (face-centered cubic) site for B.55
Mulliken charge distributions for the present molecular series
can be found in Tables S9-S16 of the Supporting Information.
Comparing the neutral, isolated thiol charge distributions with
the organic component of the cluster calculations with fixed
cluster geometry reveals minimal charge transfer (<0.1 e) in
these systems upon adsorption to the small cluster, a result in
agreement with previous estimates. The Mulliken charges are
shown to undergo little to no charge redistribution along the
conjugated backbones, with the most significant (although still
small) charge redistributions occurring for the sulfur and
phenylene ring closest to the cluster.
b. Electronic Structure. We wish to understand the evolution of the electronic states of the molecular systems from the
gas-phase to chemisorbed on a metal surface. Selected valence
molecular orbital energies for the isolated thiol and extendedmolecule model systems are plotted in Figure 2 (energies are
given in Table S1 of the Supporting Information), with a
representative electronic density of states (DOS) for OPE-F
Risko et al.
given in Figure 3 (the remaining DOSs are provided in the
Supporting Information) and selected valence molecular orbital
densities shown in Figure 4. For the isolated molecular clusters,
the trends in the molecular orbital energies match expectationssas
the π-conjugation is increased, the HOMO-LUMO energy gap
decreasesswith a similar trend for relative donor/acceptor
strength of the substituent on the central phenylene ring. The
trends of the HOMO and LUMO energies match well the
calculated VIP/AIP and VEA/AEA values (see Figure 2).
Using this electronic structure information, TDDFT calculations were performed and the calculated vertical transitions were
compared to experimental optical absorption spectra, see Figure
5; TDDFT-determined excitation energies, oscillator strengths,
and configurations for the first few excited states, along with
absorption peak maxima and molar absorptivies, are provided
in Table 1. As either the π-conjugation length increases or the
relative strength of the donor/acceptor substituent increases,
the calculated excitation energies red-shift to lower energies;
these results are in agreement with the HOMO-LUMO gaps
for these molecules. The similar absorption maxima of OPE
and OPE-F indicate comparable induction effects by the F atom
substituent for both the S0 and S1 states.46 Across this molecular
series, the S1 states are predominately characterized by a HOMO
f LUMO transition. The calculated absorption maxima are in
good agreement with the spectra; the red-shift of the calculated
transition energies is a known problem for TDDFT methods
due to electron self-interaction. The OPE systems with the
stronger donor/acceptor substituentssOPE-NH2, OPE-NO2, and
OPE-bisNO2spossess an additional peak blue-shifted with
respect to the principal π f π* transition, both experimentally
and theoretically; such donor substituent effects have been
shown for nonthiolated, methoxy-substituted OPEs.103 Empirically, these transitions in the blue portion of the spectrum have
greater molar absorptivities than the principal π f π* transition.
This trend is correctly determined for the OPE-bisNO2 compound (HOMO f LUMO+1), but is reversed for both the NH2
and NO2 substituents (HOMO-1 f LUMO); this disagreement
between the empirical molar absorptivities and calculated
oscillator strengths has been previously noted.103
Simulating binding to the metal surface through the extendedmolecule model shows rather consistent trends in the DOS
across the molecular series. The energies of the π-centered
HOMO and LUMO of the isolated molecular structures are
found to be minimally affected by the metal cluster. Pictorial
representations of these molecular orbitals show nominal
perturbation of the molecular orbital density by the Au cluster.
The most noticeable change is an increased density located on
the S atom in the HOMO. The DOS plots also show a peak
with no electron population in the midgap of these HOMO and
LUMO levels that is comprised of two S-Au-cluster centered
orbitals; such interface levels induced by the interaction between
organic and metal substrates are well-established for these types
of junctions and lead to nonzero DOS near EF in full band
structure simulations of aryl-SAM adsorption on metal substrates;35,36,60,104,105 these interface levels have been discussed
earlier for aromatics in junctions as well.106,107 Across the
molecular series, the predominant peak associated with these
levels is located between -3.4 and -3.5 eV. As this peak is
energetically consistent, its relative position with respect to the
molecule-centered HOMO and LUMO levels shifts with increasing conjugation and donor/acceptor substituent strength;
thus, for B, PEB, and OPE-F, the interfacial levels are closer
to the occupied levels, whereas for the remaining systems the
unoccupied states are closer; because the LUMO is so energeti-
(Oligo)Phenylene-ethynylenethiols Adsorbed on Gold
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13219
Figure 3. Electronic density of states for OPE-F and Au3-OPE-F systems.
Figure 4. Selected valence molecular orbital densities for OPE-F and Au3-OPE-F.
cally stabilized for the NO2-containing systems, there is overlap
between the interfacial levels and the unoccupied molecular
level. We also note an additional consistent metal-dominated
peak near -0.9 to -1.0 eV; for B, similar overlap between the
metal-centered level and LUMO occurs. The results for the
shape of the OPE DOS appear consistent with a recently
reported full periodic band structure calculation of OPE on a
Au(111) surface, in particular the presence of a distinct peak
near EF.52
c. Photoelectron Spectroscopy. Figure 6 shows the ultraviolet photoemission spectrum for each of the substituted OPE
compounds assembled on Au. Here the positions are relative
to the Au Fermi level. A first observation is that the spectra are
remarkably similar despite the addition of one or more substituents to the middle ring. The lowest lying occupied state is a
mix of the Au d-bands and the S p-orbitals (see Discussion
below) and is observed at 1.3 eV. Because the electron
attenuation length is only about 8-10 Å, shorter than the
molecular length for each of the three-ring molecules, it is
typically not observed in the monolayers studied here.108 We
do see a small contribution for this state in the OPE-NH2
spectrum, shown in Figure 6. From our previous studies and
the calculations described above it is known that the feature
near 2 eV is a delocalized molecular orbital, so we use this
feature as EB,π.16,37,46,100 The values of EB,π (given in Table 2)
determined from ultraviolet photoemission show that substitution
on OPE does not greatly affect the position of EB,π; we note
good agreement between the energetic range of 0.4 eV among
the measured values of EB,π to the 0.6 eV range of the calculated
HOMO energies of the OPE extended-molecule models. The
value of EB,π shifts by less than 0.6 eV across the series from
unsubstituted (R1 ) R2 ) H) OPE. The difference in the
monolayer work function from that of the clean surface captures
the rearrangement in the interfacial electrostatics. The monolayer
work function (φmono) and change to the surface work function
(∆φ) is shown in Table 2.
IV. Discussion
As mentioned above, discerning the molecular electronic
levels near the electrode Fermi energy is important both for
understanding conductance through molecular wires and in the
design of future molecular conduction candidates. PES methods
provide a direct means to empirically probe these states and
reveal how the electronic levels of the molecule and electrode
change upon adsorption. Using the calculated electronic DOS,
we can make assignments as to the how the molecular electronic
states change upon adsorption and identify the states that may
be important for conduction. To account for the neglect of
polarization effects in the extended-molecule model that are of
13220 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Risko et al.
importance for the renormalization of the molecular electronic
states upon metal-surface binding,17 we rigidly shift the
simulated DOS such that the calculated first ionization energy
aligns with the measured EB,π,109,110 see Figure 7 for OPE-NH2
and the Supporting Information for the remaining systems. The
occupied electronic structure of OPE chemisorbed on Au has
been described in depth previously.16,37,100 For each OPE system,
we obtain good agreement between the shape of PES spectra
and the Gaussian-convoluted DOS. As noted above, the lowest
lying filled state in each system resembles the HOMO of the
isolated thiol, but now with a significant contribution from the
sulfur 2pz and some density on the Au. The second bands of
the PES spectra are composed of two-to-three electronic states,
with the relative ordering of these states being dependent upon
the donor/acceptor substitution. For OPE, the two electronic
states comprising this band can best be described as being: (i)
fully delocalized across the π-backbone, sulfur, and Au3 cluster;
and (ii) localized on the sulfur and Au3 cluster. This pattern is
repeated for OPE-F, though the energetic spacing between these
electronic levels is dramatically reduced (nearly isoenergetic).
For OPE-NO2 and OPE-bisNO2, the ordering is reversed, with
the fully delocalized level stabilized energetically versus the
localized Au3 orbital; the energetic spacing between these two
level groups is larger for OPE-bisNO2. The situation is different
for OPE-NH2, where a purely molecular-based level localized
predominantly on the central phenylene ring and amine substituent is the first of three levels comprising the second PES
band; the other two levels follow the pattern for the two NO2substituted systems. We therefore observe, in addition to the
rather minimal effect of donor- and acceptor-substituent on the
initial ionization energy, a substituent dependence on the order
of the next few valence occupied electronic states.
The measured change in work function (∆φ) due to monolayer deposition, as shown in Table 2, varies from -0.27 to
-1.02 eV across the substituted OPE series, a range of ∼0.8
eV. The energetic magnitude of the change in ∆φ across the
series is consistent in magnitude with ∆φ measured for
alkylthiols/fluorinated-alkylthiols108 (∼2 eV) and calculated for
4′-donor/acceptor-substituted 4-mercaptobiphenyls (∼5 eV);80
we note that the larger magnitudes for the change in ∆φ in
refs. 80 and 108 are a reflection of the donor/acceptor substituents being at the end of the molecule, whereas the donor/
acceptor substitutions herein are in the middle of the molecular
structure (vide infra). The charge rearrangement leading to this
shift in the work function, as previously mentioned, can arise
from a number of factors. We focus here on three aspects:
change in surface potential energy due to the intrinsic dipole
moment of the molecule, dipole moments due to the formation
of the Au-S bond, and the potential energy due to the
redistribution of charge (i.e., image charge) in the metal.
Charge polarization within the monolayer causes a potential
energy shift. Treating the monolayer as a homogeneous sheet,
that shift can be estimated using the Helmholtz equation,111,112
∆Vmol )
eN(µ
bmoln̂)
ε0ε
(1)
where N is the adsorbate areal density, e is the unit of electric
charge, b
µmol is the molecular dipole moment, n̂ is a unit vector
perpendicular to the surface, ε is the dielectric constant, and ε0
is the vacuum permittivity. This relationship can provide an
estimate of the molecular component of the interface potential
energy (∆Vmol).47,48,60,82,113 Using either the calculated charge
distributions for the isolated thiol or Au-cluster molecules,
changes in the surface potential energy were calculated, see
Figure 5. Optical absorption spectra of (a) B, (b) PEB, (c) OPE, (d)
OPE-F, (e) OPE-NH2, (f) OPE-NO2, and (g) OPE-bisNO2 acquired in
10 µM CCl4 solutions. Scale bar represents 1 × 105 mol L-1 cm-1.
TABLE 1: UV/vis Absorption Maxima (Eop) and Molar
Absorptivities (ε) and TDDFT Vertical Excitation Energies
(Ev), Oscillator Strengths, and Excitation Configurations for
the Lowest Lying Allowed Singlet Transitionsa
Eop ε (mol L-1 Ev oscillator
(eV)
cm-1)
(eV) strength
B
5.2 3.9 × 10
PEB
OPE
OPE-F
OPE-NH2
3.7
3.6
3.6
3.4
4.0
3.5
3.8
4
5.51
0.20
8.5 × 104
5.5 × 104
3.8 × 104
6.3 × 104
3.95
3.33
3.29
3.15
3.76
2.73
3.39
1.13
2.05
1.98
1.36
0.60
0.61
0.10
OPE-bisNO2 3.1
3.7 4.5 × 104
2.25
3.32
0.44
1.26
OPE-NO2
a
3.0 × 104
configuration
H-1 f L (6 %);
H f L+1 (92%)
H f L (99%)
H f L (99%)
H f L (99%)
H f L (98%)
H-1 f L (92%)
H f L (97%)
H-1 f L (91%);
H f L+1 (5%)
H f L (98%)
H f L+1 (92%)
H, HOMO; L, LUMO.
Table 3. Assuming molecular densities in the SAMs of ∼4 ×
1018 m-2 and ε ) 3.0, the orientation dependence as a function
of θ and φ was investigated (see Figure 8). The results for the
series of molecular structures indicate a dependence of the
potential energies as a function of orientation; the results in
Table 3 are given for the approximated OPE orientation of θ,
φ ) 0, and (30°.5,46,114 For B, PEB, OPE, and OPE-bisNO2,
the potential energies are relatively flat and centered about a
perfectly upright orientation; the asymmetrically substituted
OPE-NH2, OPE-F, and OPE-NO2, on the other hand, show a
strong orientation dependence as the single donor/acceptor
substituent is moved with respect to the surface plane. These
results indicate, as expected, that for large-scale SAM depositions, where sections of the SAM may have differing molecular
orientations, the surface potential energy can vary. Using the
charge distributions from either the isolated thiol molecule or
the molecular component of the Au3-cluster species produces
essentially the same calculated surface potential energy, an
outcome consistent with the observed minimal charge-transfer
between the metal cluster and molecule. In general, these results
for ∆Vmol should be considered an upper bound estimate to the
change in surface potential energy, as no intermolecular polar-
(Oligo)Phenylene-ethynylenethiols Adsorbed on Gold
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13221
ignored, as has been discussed previously.88,113,114 For completeness, we have made estimates of the potential energies due to
image effects (Table 3); the reported results are for isolated (i.e.,
non-SAM) single molecules, hence the deposition areal density
(N) is not used, but rather we estimate the image interaction
only of a single molecule (eq 3). The total potential energy due
to the presence of image charges in the metal is taken as the
sum of potential energies due to (i) self-image interactions
between the surface charges and their images and (ii) crossimage attractive/repulsive interactions between all molecular and
image charges, see Figure 8, as written in eq 3,117
n
Vim_tot ) Vsi + Vci )
i)j
Figure 6. UP spectra of (a) OPE, (b) OPE-F, (c) OPE-NH2, (d) OPENO2, and (e) OPE-bisNO2 monolayers absorbed on Au.
TABLE 2: UPS Measured Binding Energy of the Molecular
Highest Occupied π-state (EB,π), Work Function of the
Monolayer Covered Surface (φmono), and Change in Metal
Work Function upon Monolayer Deposition (∆φ)
OPE
OPE-F
OPE-NH2
OPE-NO2
OPE-bisNO2
EB,π (eV)
φmono (eV)
∆φ (eV)
-2.06
-2.03
-2.21
-2.46
-2.34
4.27
4.17
4.64
4.14
4.89
-0.89
-0.99
-0.52
-1.02
-0.27
ization effects are directly computed in these model systems to
reflect the real dielectric of the molecular monolayers;60,80,81,115,116
the proper accounting of these interactions, of course, will be
theory-dependent and may rely strongly on the ability to describe
weak van der Waals interactions, a problematic area for many
commonly used DFT methods. These results are consistent with
previous single-molecule estimates of the surface potential
energy for arylthiols; estimates using molecular monolayers
provide a lower estimate, thus necessitating the introduction of
an effective dielectric constant to account for depolarization
within the monolayer.60
A similar expression can be used to determine the effects on
the potential energy of dipole formation due to the Au-S bond
(∆Vchem)
∆Vchem )
eN(µ
bAu-Sn̂)
ε0ε
(2)
where b
µAu-S is the dipole associated with the formation of the
Au-S bond (i.e., bond dipole), see Table 3. The Au-S dipole
can be estimated from the amount of charge transfer at the
interface and the Au-S distance. We determine that the potential
energy due to the formation of the chemisorptive bond to be
on the order of -0.2 to -0.5 eV, which are values on the same
order of magnitude as previous estimates.47,108
When an adsorbate is deposited on the surface of an infinite
perfect conductor, a charge redistribution within the conductor
is induced (the image charge representation) causing a change
in the ionization potential energy of the system.117 Image charge
effects in these types of SAM monolayers are sometimes
qq
n
qq
∑ 8πεεi 0jbrij +∑ 8πεεi 0jbrij
(3)
i*j
where i and j are for the surface and image charges, respectively.
The data indicate a roughly equal contribution to the total image
potential energy from the self- and cross-image interactions.
Across the series, the image potential energies range from -0.1
to 0.5 eV, a possibly important contribution to the potential felt
by the SAM monolayers. Indeed, recent results for simulations
of benzene on a graphite electrode indicate that image energies
are an important factor in the renormalization of molecular
orbital energies upon adsorption on an electrode surface.17 These
results, like those for the surface potential energy, do show
orientation dependence, especially for the asymmetrically
substituted systems. We note that these results should be viewed
as upper bounds as there is no consideration of molecule-molecule
interactions or retardation effects due to the dielectric of the
metal substrate.118
Figure 9 shows a comparison of the experimental work
function change with the calculated estimate from the above
potential energies. The calculated estimates in Figure 9 are the
average of the surface, bond dipole, and image potential energies
for θ, φ ) 0, and (30°; because there is a strong orientation
dependence for the asymmetrically substituted systems, the
average is used. As can be seen from the linear regression
analysis, there is a fairly strong correlation (R2 ) 0.92) between
the experimental and calculated values. We expect that better
representations of the three potential energies we considered
herein, through slab calculations that take into account a large
metal surface and intermolecular interactions, should improve
the theoretical estimates. Here it is also important to note the
differences in what is measured experimentally and what is
predicted theoretically. The calculations estimate the potential
shift associated with the monolayer dipole, the Au-S bond,
and the image potential. The experiment compares the potential
shift of a bare metal surface to that of the monolayer-covered
surfaces. Thus, the experimental measurements encompass the
calculated quantities, but other effects too, such as the attenuation of the intrinsic surface dipole by the monolayer. Thus, if
the experiments reflect the model and the calculations capture
the essential elements of the experiment, then it can be expected
that the measured and computed quantities will scale with each
other, although the absolute values will not be the same.
Closer examination of Figure 9 indicates an apparent lack of
correlation between the relative strength of the donor/acceptor
substitution on the OPE backbone and both the magnitude and
direction (sign) of ∆φ, a feature observed both experimentally
and theoretically. As previously shown for 4′-donor/acceptorsubstituted 4-mercaptobiphenyls by Heimel et al.,60,80,81 end
substitution (see Figure 10) with an electron donor (acceptor)
[electropositive (electronegative) group] results in a negative
(positive) ∆φ due to the positive (negative) end of the molecular
dipole pointing away from the surface, and the magnitude of
13222 J. Phys. Chem. C, Vol. 112, No. 34, 2008
Risko et al.
Figure 7. Comparison between the PES (top) and DFT-derived DOS (bottom) for OPE-NH2; the vertical bars refer to the shifted energies of the
occupied molecular orbitals. Molecular orbitals densities for the first few valence orbitals are also provided.
TABLE 3: Calculated Estimates of ∆Vmol, ∆Vchem, and
Vim-tot as Determined from the Cluster Calculations
image
B
PEB
OPE
OPE-F
OPE-NH2
OPE-NO2
OPE-bisNO2
θ, φ
∆Vmol
(eV)
∆Vchem
(eV)
Vci
(eV)
Vsi
(eV)
Vim-tot
(eV)
-30
0
30
-30
0
30
-30
0
30
-30
0
30
-30
0
30
-30
0
30
-30
0
30
0.14
0.09
0.14
-0.11
-0.23
-0.11
-0.20
-0.38
-0.23
-0.07
-0.80
-1.00
-0.21
-0.51
-0.44
0.54
-1.01
-1.88
0.34
0.29
0.34
-0.23
-0.30
-0.23
-0.20
-0.27
-0.20
-0.19
-0.26
-0.19
-0.20
-0.27
-0.20
-0.16
-0.21
-0.16
-0.26
-0.35
-0.26
-0.35
-0.47
-0.35
0.03
0.03
0.03
0.04
0.04
0.04
0.05
0.05
0.05
0.10
0.09
0.09
0.19
0.16
0.16
0.16
0.14
0.14
0.23
0.21
0.23
0.03
0.03
0.03
0.04
0.04
0.04
0.05
0.05
0.05
0.10
0.09
0.09
0.17
0.16
0.16
0.15
0.14
0.15
0.22
0.21
0.22
0.07
0.06
0.07
0.09
0.08
0.09
0.10
0.10
0.10
0.20
0.18
0.19
0.36
0.32
0.33
0.32
0.28
0.29
0.45
0.42
0.44
∆φ correlates directly with the relative strength of the donor
(acceptor), see Table 4 and Figure 11.119 The trends herein do
not follow this picture, however, as indicated by the observation
that the single substitution of either a donor or acceptor along
the OPE backbone both result in a negative ∆φ. The results
for OPE-bisNO2 differ, however, as the opposing directional
orientation of the individual NO2 substituents complicates the
overall picture. Inspection of the total molecular dipole moments
(µ) and their vector components (µx, µy, and µz, with the
z-direction taken as normal to the surface plane), see Table 4
and Figures 10 and 11, suggest that the relative placement of
the donor or acceptor substituent along the backbone plays a
significant role. The µz component of the total dipole moment
for each OPE structure is positive (i.e., the positive end of the
dipole points away from the surface), with OPE-NO2 > OPE-F
Figure 8. Pictorial representation of the self- and cross-image
interactions and orientation parameters (θ and φ) used for the potential
energy calculations.
> OPE (> OPE-bisNO2) > OPE-NH2. Using unsubstituted OPE
as the baseline, the larger (positive) values of µz for OPE-NO2
and OPE-F suggest that the acceptor substituents located in the
center of the molecular structure strongly pull electron density
from the conjugated end protruding above the central phenylene
(Oligo)Phenylene-ethynylenethiols Adsorbed on Gold
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13223
Figure 9. Experimental versus theoretical estimate of ∆φ (eV) and
least-squares fit of the data (line). Theoretical estimate includes
summation of bond dipole and average surface and image potential
energies for θ, φ ) 0, and (30°. Error bars represent one standard
deviation of the data set.
Figure 10. Pictorial representation of the donor/acceptor biphenyl (left)
and OPE (right) molecular systems, see Table 4 and Figure 11. The
colored discs represent the π-conjugated backbone (purple), thiol linker
(yellow), and placement of the donor/acceptor substituent (gray).
TABLE 4: B3LYP/6-31G**-derived x-, y-, and
z-Components (µx, µy, and µz) and Total Dipole Moments (µ)
for the OPE Structures and a Series 4′-Donor/Acceptor
Substituted 4-Mercaptobiphenylsa
π-backbone
substituent
µx
µy
OPE
biphenyl
µz
µ
bisNO2
NO2
F
H
NH2
0.94
-3.20
-0.10
0.91
2.54
0.00
0.00
0.00
0.00
0.00
0.29
1.54
1.37
0.69
0.14
0.98
3.55
1.37
1.14
2.55
NO2
CNb
F
Hb
SHb
NH2b
0.47
0.81
0.87
0.89
-0.49
0.21
0.67
0.02
0.01
0.00
0.65
0.82
-4.87
-4.58
-0.54
0.87
1.25
2.86
4.94
4.65
1.03
1.25
1.49
2.98
a
All dipole moments are in Debye. See Figure 10 for orientation
and Figure 11 for a graphical representation. b Substituents used in
ref. 80.
ring, inducing the larger positive dipole moment along the
z-direction and, therefore, larger (more negative) ∆φ, whereas
the smaller (still positive) µz for OPE-NH2 reflects the electron
donating ability of the amine substituent (although it is not strong
Figure 11. Graphical representation of the B3LYP/6-31G**-derived
x- and z-components (µx and µz) and total dipole moments (µ) for the
OPE structures (left) and series of 4′-donor/acceptor substituted
4-mercaptobiphenyls (right); all dipole moments are in Debye (see Table
4). Also depicted are the theoretical estimates of ∆φ (eV) for the OPE
structures (triangles) calculated herein and 4′-donor/acceptor substituted
4-mercaptobiphenyls (circles) as reported in ref. 80. †Substituents used
in ref. 80.
enough to change the direction of µz). These results for the singly
substituted systems suggest that there indeed does exist a
correlation between the strength of the donor/acceptor and ∆φ
through µz (Figure 11), although the direction of ∆φ with the
substitution may seem at first counterintuitive. We note that the
µx components in the OPE structures follow the relative trends
of the electron accepting (donating) strength for the singly
substituted systems, with OPE-NO2 and OPE-F having a
negative µx (with OPE-NO2 > OPE-F) and that of OPE-NH2
being positive; the positive µx values for OPE and OPE-bisNO2
are due to the asymmetric nature of the thiol end group, which
has the hydrogen atom directed along the positive x-direction.
These results also aid in addressing an apparent discrepancy
with previous UPS investigations of alkylthiols versus fluorinatedalkylthiols;108 Alloway et al.108 showed that unsubstituted
alkylthiols produce a much more significant change in ∆φ
(-1.35 eV) than their fluorinated counterparts (∆φ ∼0.06 eV
- 0.46 eV) and that the direction of ∆φ changes with
fluorination. The results herein, however, show the opposite
effect for both OPE-F and OPE-NO2 versus OPE; not only is
the magnitude of ∆φ larger for these two acceptor-substituted
systems (see Table 2 and Figure 9), but the values of ∆φ all
point in the same direction (have the same sign). From the above
analysis, however, we see that each system has a positive µz,
with the magnitude following the relative acceptor strength of
the substituent; hence, we obtain a larger ∆φ for each of these
two acceptor-substituted systems that point in the same direction
as the unsubstituted structure. Overall, these results point to a
strong orientation dependence of the donor/acceptor substituent
in the OPE series on both the relative magnitude and direction
of ∆φ. We expect that future experimental and theoretical
studies that probe these effects in a controlled manner could
provide informative insight into fully managing ∆φ through
precise adsorbate design.
V. Conclusions
We have presented here a joint experimental and computational investigation of the electronic structure and electrostatic
properties of a series of donor- and acceptor-substituted OPEs
on gold. Calculations show that the effect of chemisorption on
13224 J. Phys. Chem. C, Vol. 112, No. 34, 2008
the valence electronic structure is minimal, confirming experimental observations. The calculated densities of electronic states
allow us to identify the higher-lying occupied states that may
be of relevance for single molecule transport through these
molecular wires. Simulated spectra agree well with measured
spectra and guide assignment of the spectral features. Calculated
estimates of the surface, bond dipole, and image potential
energies correlate well with the measured change in work
function for the self-assembled monolayers. The contribution
of the charge asymmetry within the monolayer itself depends
upon substitution and orientation, whereas the polarization along
the Au-S bond contributes about -0.2 to -0.5 eV to the work
function shift for all monolayers. The image potential energy
is found to be a significant contributor to the work function
shift when a polarizable functional group is attached to the
molecule. We next must probe with more extensive slab
models17,52,55,60,80–83,88 for these compounds to take into account
molecular interactions with both a periodic metal and the molecular substrate to verify that our simple model systems are
correct. In the future, special emphasis will need to be placed
on the effects of substituent placement and molecular orientation
to provide a more quantitative picture on the effects of the
molecular contributions to the changes in the measured work
function.
Acknowledgment. The authors are appreciative to the
reviewers of this manuscript; the insight they provided greatly
added to the overall quality of the manuscript. C. R. would like
to thank Gemma Solomon and Maxim Sukharev for invaluable
discussions and Notker Rösch for a detailed description of
surface/image charge interactions. C. D. Z and R. D. v. Z would
like to acknowledge Steve Robey for insight into the photoemission measurements. The work at Northwestern was supported by the NSF through the Northwestern University MRSEC
(DMR-0520513), Chemistry Division of the NSF [CHE0719420], and by the Office of Naval Research [N00014-051-0766]. The work at Rice was supported by the Defense
Advanced Research Projects Agency and Office of Naval
Research.
Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
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