Controlling the structural and electrical properties of diacid oligo(phenylene ethynylene) langmuir-blodgett films

Article abstract of DOI:10.1002/chem.201203261

The preparation, characterization and electrical properties of Langmuir-Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4′-[1,4-phenylenebis(ethyne-2, 1-diyl)]dibenzoic acid (OPE2A), are de

Full text of DOI:10.1002/chem.201203261

DOI: 10.1002/chem.201203261
Controlling the Structural and Electrical Properties of Diacid
Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films
A
ACHTUNGTRENNUNG
Luz Marina Ballesteros,[a, b] Santiago Martꢀn,[c, d] Javier Cortꢁs,[a, b]
Santiago Marquꢁs-Gonzꢂlez,[e] Simon J. Higgins,[f] Richard J. Nichols,[f]
Paul J. Low,[e] and Pilar Cea *[a, b, d]
Abstract: The preparation, character-
ization and electrical properties of
Langmuir–Blodgett (LB) films com-
posed of a symmetrically substituted
oligomeric phenylene ethynylene deriv-
prepared from a basic subphase, which
show a hypsochromic shift of 15 nm.
This result, together with X-ray photo-
electron spectroscopic and quartz crys-
tal microbalance experiments, conclu-
sively demonstrate formation of one-
layer LB films in which OPE2A mole-
cules are chemisorbed onto gold sub-
ion. In contrast, LB films prepared
G
from a pure water subphase preserve
the protonated acid group, and lateral
H-bonds with neighbouring molecules
give rise to a supramolecular structure.
STM-based conductance studies re-
vealed that films prepared from a basic
subphase are more conductive than the
analogous films prepared from pure
water, and the electrical conductance
of the deprotonated films also coin-
cides more closely with single-molecule
conductance measurements. This result
was interpreted not only in terms of
ative,
namely,
4,4’-[1,4-phenylene-
A
acid
(OPE2A), are described. Analysis of
the surface pressure versus area per
molecule isotherms and Brewster angle
microscopy reveal that good-quality
Langmuir (L) films can be formed both
on pure water and a basic subphase.
Monolayer L films were transferred
onto solid substrates with a transfer
ratio of unity to obtain LB films. Both
L and LB films prepared on or from a
pure water subphase show a red shift
in the UV/Vis spectrum of about
14 nm, in contrast to L and LB films
ꢀ
ꢀ
strates and consequently COO Au
junctions are formed. In LB films pre-
pared on a basic subphase the other
terminal acid group is also deprotonat-
ed and associates with an Na+ counter-
ꢀ
ꢀ
better electron transmission in COO
Au molecular junctions, but also in
terms of the presence of lateral H-
bonds in the films formed from pure
water, which lead to reduced conduc-
tance of the molecular junctions.
Keywords: carboxylic acids · con-
ducting materials
·
Langmuir–
Blodgett films
ethynylene)s
microscopy
·
oligo(phenylene
scanning probe
·
Introduction
[a] Dr. L. M. Ballesteros, J. Cortꢀs, Dr. P. Cea
The development of smaller and more efficient electronic
devices has been a perennial concern for researchers and
companies in the electronics industry. Since the seminal
publication of Aviram and Ratner[1] molecular electronics
has been a much discussed future alternative to present-day
silicon-based technologies. However, a confounding number
of significant challenges need to be addressed before this
technology reaches fruition, and practical molecular elec-
tronic devices still remain a concept rather than a nascent
technology. However, the impact of molecular electronics
on understanding charge transport in molecules has been
more immediate. In particular, over the last decade it has
become clear that the contact between metal and molecule
plays a much more determining role in electronic transmis-
sion than was previously envisaged. In this regard, much at-
tention has shifted in recent years to understanding and con-
trolling metal–molecule contacts and developing new sur-
face-contacting paradigms.[2–26] For the development of new
devices based on molecular electronics,[27,28] it is of crucial
Departamento de Quꢁmica Fꢁsica, Facultad de Ciencias
Universidad de Zaragoza, Zaragoza 50009 (Spain)
E-mail: pilarcea@unizar.es
[b] Dr. L. M. Ballesteros, J. Cortꢀs, Dr. P. Cea
Instituto de Nanociencia de Aragꢂn (INA), Edificio i+d
Campus Rio Ebro, Universidad de Zaragoza
C/Mariano Esquillor, s/n, 50017 Zaragoza (Spain)
[c] Dr. S. Martꢁn
Instituto de Ciencia de Materiales de Aragꢂn (ICMA)
Universidad de Zaragoza-CSIC
Departamento de Fꢁsica de la Materia Condensada
50009 Zaragoza (Spain)
[d] Dr. S. Martꢁn, Dr. P. Cea
Laboratorio de Microscopꢁas Avanzadas (LMA)
C/Mariano Esquilor s/n Campus Rio Ebro, 50018 Zaragoza (Spain)
[e] S. Marquꢀs-Gonzꢃlez, Prof. P. J. Low
Department of Chemistry, University of Durham
Durham DH13LE (UK)
[f] Prof. S. J. Higgins, Prof. R. J. Nichols
Department of Chemistry, University of Liverpool
Crown Street, Liverpool, L697ZD (UK)
5352
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Chem. Eur. J. 2013, 19, 5352 – 5363

FULL PAPER
importance to understand the chemical nature and structural
properties of metal|molecule|metal junctions, since the
nature of the metal molecule interface strongly influences
the transport properties in molecular devices.[29] A number
of factors, including geometry of contacts,[30–32] bonding,[33–35]
molecule–electrode distance[36–38] and molecular orienta-
tion,[39,40] have also been found to affect the transport pro-
Results and Discussion
The compound OPE2A is characterized by a rigid molecular
structure with a highly conjugated p-electron system. In a
manner entirely analogous to other amphiphilic molecules
containing large polyaromatic moieties, OPE2A has a pro-
nounced tendency to aggregate due to strong p–p interac-
tions,[12, 59, 60] as well as to the facility of acids to aggregate in
organic solvents. Thus, the Lambert–Beer law is only fol-
lowed at concentrations lower than 2.5ꢅ10ꢀ5 m (Figure 2) in
ACHTUNGTRENNUNGcess. Even more, it still remains a challenge to determine ex-
perimentally the role of the molecule metal interface in the
transport process, and correlations between experimental
observation and theoretical models remain challenging
given the size of the computational problem and the varia-
bility of individual measurements.[15,21,32,41–43] In addition, the
electron-transfer process across the molecule|electrode junc-
tion is poorly understood,[45,46] and problems related to this
topic, such as the structure of the molecule–surface contact,
dynamics of electron transfer and the transfer mechanism
are topics of ongoing interest. In seeking to address some of
these issues, the study of oligo(phenylene ethynylene)
(OPE) derivatives, which have shown promising characteris-
tics for use in molecular electronics, has been proven in-
structive.[2,47–57] Thus, OPEs have been of particular interest
in molecular electronics due to their effective p conjugation
and rod-like structure.
Whilst many of these recent studies have been based on
single-molecule measurements, more closely packed self-as-
sembled monolayer (SAM) and Langmuir–Blodgett (LB)
films have provided important data concerning the electrical
properties of monomolecular films of active molecular com-
ponents more likely to find application in device architec-
tures. The two main advantages of the LB method over
SAM films are 1) the compatibility of the LB technique
with a wide variety of metaljorganic interfaces through the
large number of different polar functional groups that can
be physically or chemically adsorbed onto an equally wide
array of substrates, and 2) the fabrication of directionally
oriented films containing two different groups that can be
chemisorbed onto metal substrates.[58]
Figure 2. UV/Vis spectra of OPE2A in CHCl3/EtOH (4/1) solution at the
indicated
concentrations.
Molar
absorptivity
at
328 nm
is
42700 Lmolꢀ1 cmꢀ1
.
chloroform/ethanol (4/1), with higher concentrations leading
to deviations from linearity in the absorbance versus con-
centration plot. Consequently, highly dilute solutions are re-
quired to fabricate true monolayers at the air–water inter-
face. The UV/Vis spectrum of OPE2A in solution features
one peak at 328 nm with two shoulders at 359 and 380 nm
attributable to p–p* electronic transitions.[6,62]
A preliminary investigation of the formation of Langmuir
films of OPE2A involving both the concentration and the
volume of the spreading solution concluded that only solu-
tions of concentration 1ꢅ10ꢀ5 m or lower yield reproducible
isotherms. Figure 3 shows representative surface pressure
versus area per molecule (p–A) isotherms of OPE2A on
water (pH 5.9) and basic subphases (NaOH, pH 11.4). In
contrast to other OPE acid derivatives, for which a basic
This paper provides new data concerning the electrical
properties of metaljacid molecular junctions. For a better
understanding of the role played by ionic interactions and
the influence of the pH on the electrical properties of the
films, 4,4’-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid
(OPE2A, Figure 1) was synthesized and assembled in LB
films, and the quality of the films was compared to that of
SA films. In addition, the electrical properties of the films
prepared under different experimental conditions were de-
termined and compared with the single-molecule conduc-
tance.
Figure 1. Molecular structure of 4,4’-[1,4-phenylenebis(ethyne-2,1-diyl)]-
Figure 3. Surface pressure versus area per molecule isotherm of OPE2A
dibenzoic acid (OPE2A).
on a water subphase and an aqueous NaOH subphase at 208C.
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P. Cea et al.
Figure 4. BAM images recorded at the indicated surface pressures for a
pure water subphase (pH 5.9) and a basic subphase (pH 11.4). The field
of view along the x axes for the BAM images is 1650 mm.
subphase was necessary to avoid aggregation,[19,26,63] Lang-
muir films of OPE2A were homogeneous (see BAM images
in Figure 4) and did not show any evidence of 3D aggregates
when a water subphase was used. The p–A isotherms of
OPE2A on pure water are characterized by zero surface
pressure, that is, gas phase, until an area of
0.8 nm2 moleculeꢀ1 is reached with a transition from gas to
expanded liquid phase taking place for the monolayer fabri-
cated on a water subphase. As the pH increases, the lift-off
area per molecule decreases, with
a
value of
0.6 nm2 moleculeꢀ1 for the monolayers obtained on a basic
subphase. The lift-off in the isotherms is followed by a mo-
notonous increase of the surface pressure upon compres-
sion.
Figure 5. a) Normalized reflection spectra upon compression at the indi-
cated surface pressures for OPE2A monolayers prepared on the indicat-
ed subphases. b) Tilt angle f of OPE2A with respect to the liquid surface
in the compression process for monolayers prepared on the indicated
subphases.
Reflection spectroscopy is a useful method for in situ
characterization of the monolayer at the air–water inter-
face[64,65] that provides relevant information about orienta-
tion of the molecules in the film, formation and types of ag-
gregates, changes in the aggregation state during the com-
pression process and so on. However, it is well-known that
the normalized reflection spectra DRnorm =DRA of the films
provide more direct information about the orientation of
the molecules in the compression process, since the influ-
ence of the surface density is eliminated.[65,66] Normalized re-
flection UV/Vis spectra DRnorm recorded at different surface
pressures for OPE2A Langmuir films are shown in Fig-
ure 5a. In addition, quantitative analysis of the DRnorm spec-
tra allowed us to calculate the tilt angle f of the transition
dipole moment of the molecule with respect to the liquid
surface (Figure 5b). This angle was determined by compar-
ing the reflection spectra at the air–water interface and the
UV/Vis absorption spectrum of OPE2A in solution, as has
been comprehensively detailed elsewhere.[12,65] The tilt angle
of the OPE moieties with respect to the water subphase is
largely unchanged upon compression when the monolayers
are prepared on a water subphase, and only a small varia-
tion in DRnorm values is produced in the basic subphase. The
tilt angle of the molecules is around 608 for films on pure
water subphase and slightly higher (ca. 678) in the con-
densed phase of a monolayer on the basic subphase, which
is in agreement with the more expanded isotherm observed
in pure water (Figure 5b).
Interestingly, the reflection band is shifted with respect to
the solution depending on the subphase on which the mono-
layers were prepared. In the last few years a systematic
study in which different polar terminal groups have been
added to the OPE skeleton as well as alkyl chains of differ-
ent length or other hydrophobic terminal groups has been
carried out. In all previously studied cases,[12,14,19,20,26,58,63,67–70]
hypsochromic shifts of the main absorption band with re-
spect to the solution were observed both in L and LB films,
and this blue shift of the films was attributed to formation
of H-aggregates. To our knowledge this is the first example
of an OPE derivative which, when arranged in a Langmuir
film, shows a bathochromic shift (monolayers on pure
water) relative to the solution spectrum. However, the ob-
servation of a hypsochromic shift is maintained for monolay-
ers on a basic subphase. It is also noteworthy that monosub-
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Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films
FULL PAPER
stituted carboxyl OPE derivatives incorporated in L and LB
films showed a significant hypsochromic shift (36–60 nm)
compared to the solution spectrum,[19,26,63] independent of
the pH of the subphase, which leads to the conclusion that
the effect observed in OPE2A is a unique feature of this di-
carboxyl-substituted compound. The red shift of OPE2A in
L films prepared on water could be due to several factors:
Thus, the frequency change Df for a QCM quartz resonator
before and after the deposition process was determined
using the Sauerbrey equation (1),[80]
2f02Dm
Df ¼ ꢀ
ð1Þ
A11q=2m1q=2
where f0 is the fundamental resonant frequency of 5 MHz,
Dm the mass change [g], A the electrode area [cm2], 1q the
density of quartz (2.65 gcmꢀ3) and mq the shear modulus
(2.95ꢅ1011 dyncmꢀ2). Considering these values and the mo-
lecular weight of OPE2A (366 gmolꢀ1), the surface cover-
1) Solvatochromic effect: To understand the influence of
polarity on aggregation, OPE2A was dissolved in sol-
vents ranging from CHCl3 to EtOH/water (this com-
pound is not soluble in apolar solvents). A significant
hypsochromic shift was observed, from 325 (CHCl3) to
315 (EtOH) and 302 nm (EtOH/H2O 2/1). This indicates
that the observed red shift is not attributable to an in-
crease in the polarity of the environment, as might be ex-
pected at the air–water interface, especially at low sur-
face pressures. However, due to the insolubility of this
compound in apolar solvents it is not fully clear whether
a less polar environment, which could be achieved in
compact monolayers, might result in a red shift of the ab-
sorption profile.
AHCTUNGTRENNUNG
ages G obtained from Equation (1) are 5.60ꢅ10ꢀ10 and 6.5ꢅ
10ꢀ10 mol cmꢀ2 for the water and basic subphases, respec-
tively. These values correspond to transfer ratios of 0.96 for
the monolayer on pure water and 0.98 for the monolayer
onto a basic subphase. The slightly higher transfer ratio for
monolayers prepared onto a basic subphase may indicate
better interaction between the gold substrate and the car-
boxylate group in the monolayer as opposed to the carboxyl
groups present in the pure water subphase. This suggestion
is consistent with the presence of H-bonds between the car-
boxyl head groups in the L films prepared on the pure
water subphase; transference of these films onto gold sub-
strates requires rupture of this H-bond network prior to
chemisorption of the monolayer onto the gold substrate (see
below).
2) Conjugation length:[72] It has been experimentally ob-
served that OPE derivatives exhibit a red shift when the
number of phenylene ethynylene groups increases. As
will be demonstrated later, the OPE2A compound gener-
ates a supramolecular structure in monolayers through
lateral H-bonding interactions when monolayers are fab-
ricated on a water subphase (see below). These H-bonds
could constrain the phenylene rings to adopt more pla-
narized orientations, resulting in more extended p-elec-
tron delocalization.[73,74]
3) Formation of J-aggregates: For some compounds that
tend to form mainly H-aggregates, for example, merocya-
nines[75,76] and azo compounds,[77,78] incorporation of cer-
tain functional groups capable of forming H-bonds leads
to the formation of J-aggregates, which exhibit absorp-
tion spectrum which are red-shifted with respect to the
solution spectrum. However, taking into account the
angle of OPE2A monolayers with respect to the surface
(ca. 608 for monolayers spread onto pure water) and the
angle needed, according to the theoretical calculations,[79]
to exhibit a red shift (<548), we believe that, although
this effect could somehow contribute to the bathochro-
mic shift, it may not be the main cause of the red shift
observed for OPE2A Langmuir films.
Electrochemical electron-transfer currents at electrodes
under controlled potential provide an indirect measure of
defect densities in thin films and can be conveniently stud-
ied by cyclic voltammetry for film-coated electrodes.[81,82]
Cyclic voltammograms (CVs) obtained from aqueous solu-
tions containing 1 mm [RuACTHNUTGRNE(UNG NH3)6]Cl3 and 0.1m KCl for a
bare gold electrode (see details in the Experimental Sec-
tion) and for a gold working electrode modified by a one-
layer LB film deposited at 5, 10, 15 and 20 mNmꢀ1 from a
monolayer prepared on a pure water subphase are shown in
Figure 6a. (The same sort of sequence was obtained for
monolayers prepared on a basic subphase, but not shown
here for the sake of brevity.) The electrochemical response
of a bare gold electrode exhibits a clear voltammetric wave
characteristic of the ruthenium complex. There is a signifi-
cant decrease in current density for the voltammograms re-
corded with gold electrodes covered by LB films, and this
decrease in current density becomes more significant with
increasing surface pressure of transference. When the sur-
face pressure of transference was 20 mNmꢀ1, a drastic de-
crease of the reduction and oxidation peaks of the redox
probe for the modified electrode indicates effective blocking
of the electrode surface and therefore a low density of holes
or defects in the monolayer. In addition, LB films fabricated
from a basic solution block the electrode slightly better than
those prepared in pure water (see Figure 6b), in agreement
with the results obtained by other techniques (e.g., a less ex-
panded isotherm, higher surface coverage, higher tilt
angles).
Transfer of these Langmuir films onto solid supports gives
Langmuir–Blodgett films, which can be investigated by a
wider range of spectroscopic, microscopic and electrochemi-
cal methods to provide further insight into the arrangement
of OPE2A molecules in monolayers on different supports.
The transfer ratio calculated by the trough software
during deposition of the monolayer was approximately unity
at a surface pressure of 20 mNmꢀ1 for both pure water and
aqueous NaOH subphases. This uniform transfer was also
estimated by using a quartz crystal microbalance (QCM).
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P. Cea et al.
Optical properties of the transferred films offer additional
insight into the molecular arrangement and degree of order
within the film. Langmuir films of OPE2A were transferred
onto quartz substrates at 20 mNmꢀ1 and the UV/Vis absorp-
tion spectra were recorded. Figure 7 shows the electronic
spectra of OPE2A LB films prepared from water and basic
Figure 6. Cyclic voltammograms (CVs) of a) a one-layer LB film of
OPE2A (water subphase) deposited on a gold electrode at the indicated
transference surface pressures. b) Comparison of the blocking effect on
the gold electrode of an LB film transferred at a surface pressure of
20 mNmꢀ1 and a self-assembled film prepared by incubation of a gold
substrate in a 10ꢀ5 m solution for 48 h. CVs were recorded by immersing
Figure 7. Apparent molar absorptivity versus wavelength for a monomo-
lecular LB film of OPE2A transferred at 20 mNmꢀ1 from water and
basic subphases and comparison with the molar absorptivity of a solution
in CHCl3/EtOH (4/1).
the gold substrate in a 1 mm [RuACTHNUTRGNE(UNG NH3)6]Cl3 and 0.1m KCl aqueous solu-
tion at a scan rate of 0.1 Vsꢀ1 at 208C. An Ag|AgCl saturated reference
electrode was employed and the counterelectrode was a Pt sheet.
subphases together with the spectrum of OPE2A in solution
and at the water–liquid interface for comparison. The molar
(e) and apparent molar absorptivities (eapp, where apparent
denotes the orientational effect of the molecules in the L or
LB films) for solution and monolayer were obtained accord-
ing to Equations (2) and (3) for the solution and the air–
water interface[83], respectively, and Equation (4) for LB
films,
Self-assembly is a commonly used method to fabricate
films incorporating functionalized OPEs. It is well-known
that the LB method is not the most appropriate to assemble
doubly polar functionalized molecules in which strong com-
petition of the polar groups to be anchored at the air–water
interface could take place. However, in this particular case,
the LB technique proved to be a good method to incorpo-
rate OPE2A molecules into well-ordered monolayers. This
behaviour is probably due to the presence of a very rigid
OPE backbone that prevents bending of the molecule and
thus contact of both polar groups with the water surface. In
addition, the very hydrophobic OPE core tends to be situat-
ed away from the water surface, and this leads to surface be-
haviour similar to that of amphiphilic materials containing
just one polar group. To compare the efficiency of the LB
method with that of self-assembly in the arrangement of
OPE2A molecules in terms of surface coverage, self-assem-
bled monolayer (SAM) films of OPE2A were fabricated.
Gold substrates were immersed for 48 h in a 10ꢀ5 m solution
of OPE2A in ethanol. Poor blocking of the gold electrode
(Figure 6b) and a low coverage of OPE2A on gold surfaces
was determined by QCM experiments (surface coverage of
3.8ꢅ10ꢀ10 mol cmꢀ2 for SAMs versus 5.53ꢅ10ꢀ10 molcmꢀ2
for LB films prepared on a water subphase). The use of
more concentrated solutions of OPE2A (up to 10ꢀ3 m) to
produce SA films also did not result in better surface cover-
age.
Ab
Cl
ð2Þ
ð3Þ
e ¼
DR
pffiffiffiffiffiffi
eapp
¼
2:303 ꢁ 103G Rw
Ab
ð4Þ
eapp
¼
1000G
where G [mol cmꢀ2] is the surface density, Rw the reflectivity
of water (0.02), Ab the absorbance, C the solution concentra-
tion and l the cell width.
The spectra of LB films transferred from a water sub-
phase are again red-shifted by about 17 nm compared to the
solution spectra (see Figure 7), and they practically overlap
with the spectra of the monolayers at the air–water interface
(result not shown for clarity). In the case of films transferred
from a basic substrate the spectrum is blue-shifted by 14 nm
with respect to the solution, and again the LB film spectrum
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Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films
FULL PAPER
overlaps the reflection spectrum obtained on a basic sub-
phase. These results indicate that the molecular arrange-
ment at the air–liquid interface is retained when the films
are transferred onto the solid support. Additionally, the dif-
ference in apparent molar absorptivity of the molecules in
solution and in the films provides quantitative information
on the orientation of the dipole moment of transition and
the normal to the surface, as stated above. The results ob-
tained here clearly indicate that OPE2A molecules are in a
more vertical orientation with respect to the substrate when
they are fabricated from a basic subphase compared to
those fabricated from a water subphase. The LB films that
were prepared by using a water subphase were incubated
for 48 h in NaOH solution (pH 11.4) and then thoroughly
rinsed with water and dried. The maximum wavelength was
then shifted to 324 nm, that is, the initial red shift with re-
spect to the solution disappears. This result is consistent
with the red shift in films prepared on pure water being due
to the presence of lateral H-bonds between neighbouring
molecules that disappear after exposure of the film to a
basic medium. The fact that, after incubation of these films
in a water subphase, the peak is not shifted to 313 nm (posi-
tion of the maximum wavelength for films prepared on a
basic subphase) may be attributable to different orientations
of the molecules in the two films.
films of this compound prepared from a chloroform solution
appears at 317 nm, that is, again blue-shifted with respect to
the solution (328 nm). An LB film fabricated by using pure
water as subphase was redissolved in chloroform. A signifi-
cant blue shift (peak at 300 nm) of the solution of the redis-
solved film compared to the spectrum of the original solu-
tion was observed. After sonication of the solution of the re-
dissolved film in chloroform for 10 min the original spec-
trum of this compound in a chloroform solution was ob-
tained. This indicates that no chemical reaction has taken
place but significant aggregation effects definitely occur in
the monolayer. The initial blue shift of the spectrum after
redissolving the film prepared from a water subphase might
be due to the rupture of H-bonds between neighbouring
molecules, and the preservation of lateral p–p interactions
that may lead to H-aggregates. These interactions are lost
after sonication. The electrical properties of the LB films
also seem to point towards a different structure for films
transferred from water and a basic subphase (see below).
Carboxylic acids readily form head-to-head dimers in so-
ACHUTNGRENlNUG uACTHUNGRTENtUNG ion and solid state through mutual H-bonding, which pro-
vides an avenue through which to explore the nature of the
carboxyl groups in films of OPE2A, and to gather further
support for the notion of a supramolecular network linking
the exposed CO2H moieties in LB films of this compound.
Monolayers of OPE2A prepared on the two subphases were
transferred onto QCM substrates by withdrawal of sub-
strates that were initially immersed in the subphase. The
modified substrates were introduced into a behenic acid so-
Table 1 shows additional evidence for the formation of a
supramolecular structure in the L and LB films. While the
Table 1. Position of the main absorption peak [nm] for the indicated so-
ACHTUNGTRENNUNG ACHTUNGTRENNUNGtion and films.
ACTHNGUTRENNUG ACHTUNGRENtNUG ion (see Figure 8) and the frequency change Df of the
Cast
film
LB film
m
G
Basic
Water
subphase[a]
subphase[b]
CHTUNGTRENNUNG
328
317
313
345
ACHTUNGTRENNUNG
[a] pH 11.4. [b] pH 5.9.
absorption maxima of L and
LB films formed on an aqueous
subphase (pH 5.9), where the
carboxyl groups are expected to
be protonated, is red-shifted
with respect to the solution
spectrum in chloroform, the
peaks of L and LB films pre-
pared on
a basic subphase
(NaOH) are slightly blue-shift-
ed. In basic subphases the car-
boxyl groups are deprotonated,
and therefore no H-bonds be-
tween adjacent molecules are
formed. In addition, the red
shift observed for the LB films
on the water subphase may be
favoured by the spatial organi-
zation of the film, since the
Figure 8. Schematic of monomolecular LB films deposited onto gold substrates and transferred from a water
subphase a) and a basic subphase b) before and after incubation in a behenic acid solution according to the
maximum absorption for cast QCM experiments described in the text.
Chem. Eur. J. 2013, 19, 5352 – 5363
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5357

P. Cea et al.
A
subphase used. In contrast, the other terminal carboxyl
group remains protonated when the Langmuir film is pre-
pared on a pure water subphase and is deprotonated when a
basic subphase is used. Further confirmation of these con-
clusions was provided by angle-resolved (AR) XPS. Repre-
sentative C 1s XPS spectra measured at take-off angles of
90 and 608 with respect to the surface are shown in
Figure 10 for an OPE2A LB film prepared from pure water.
transferred from a basic subphase, a frequency change of
ꢀ27 Hz was recorded, which indicates, through application
of the Sauerbrey equation, that one molecule of behenic
acid was deposited per molecule of OPE2A in the film. This
result suggests that when OPE2A molecules are transferred
from a water subphase the carboxylic acid remains protonat-
ed and lateral H-bonds are formed, which renders the
CO2H group insensitive to further H-bonding interactions.
These H-bonds are strong enough to prevent surface binding
of behenic acid to the OPE2A monolayer through H-
bonded carboxylic acid dimers. However, when OPE2A
molecules are transferred from a basic subphase the carbox-
yl groups are deprotonACTHNUGRTENUNGated, and consequently they are free
to form face-to-face H-bonds with behenic acid molecules
(see Figure 8). Although this QCM experiment clearly
shows different states of the terminal carboxyl groups de-
pending on the subphase used, it does not provide informa-
tion about the dissociation state of the carboxyl groups di-
rectly attached to the gold substrate.
Figure 9 shows XPS spectra in the C 1s spectral region of
OPE2A powder and OPE2A LB films transferred onto gold
substrates from the two different subphases. The powder
Figure 10. Angle-resolved XPS spectra of a monomolecular LB film de-
posited onto a gold substrate from a pure water subphase at take-off
angles of 90 and 608.
From the AR-XPS spectra, it is clear that the intensity of
the peak corresponding to the protonated carboxyl group
(CCOOH) is larger, while a decrease in the take-off angle re-
sults in a more prominent CCOOꢀ peak. This result is consis-
tent with the model presented in Figure 8, in which the ad-
sorbate group contacting the gold surface is likely to be the
deprotonated carboxyl group, while the terminal carboxylic
acid remains protonated.
The LB films of OPE2A transferred from a pure water
subphase and a basic subphase showed very significant dif-
ferences in electrical behaviour that may also be explained
by the different protonation states of their carboxyl groups.
To determine the electrical characteristics of a monomolecu-
lar LB film transferred onto gold substrates at 20 mNmꢀ1 by
using pure water as subphase, I–V curves were recorded by
STM and averaged from multiple (ca. 420) scans at different
locations on the substrate and by using different samples to
ensure the reproducibility and reliability of the measure-
ments. Moreover, before recording the I–V curves, both the
thickness of the monolayer and the tip-to-substrate distance
(s) should be estimated in order to position the STM tip just
above the LB film and thus avoid penetration of the STM
tip into the film or the existence of a substantial gap be-
tween the STM tip and the monolayer. By using the attenu-
ation of the Au 4f signal of the substrate (see Experimental
Section), the thickness of the LB films on the gold electrode
was estimated to be (1.81ꢂ0.05) nm, in good agreement
with the determination of the tilt angle obtained from the
Figure 9. C 1s XPS spectra of OPE2A in powder and in LB films deposit-
ed onto gold substrates from water and a basic aqueous subphase.
spectrum shows a peak at 288.8 eV corresponding to the
carbon atom in the carboxyl moiety.[84–90] Films of OPE2A
molecules on gold substrates prepared from a basic sub-
phase show a peak attributable to the carboxylate carbon
atom at 287.1 eV.[91,92] This clearly indicates that OPE2A is
entirely deprotonated when transferred from a basic sub-
phase. In contrast, the peak at 288.8 eV is preserved in LB
films transferred from a water subphase, and a peak at
287.1 eV is also observed. This indicates that OPE2A con-
tains both carboxylate and carboxyl groups when transferred
from a water subphase, which, in combination with the data
provided by the QCM experiments described above, sug-
gests that the group attached to the gold substrate is depro-
tonated and chemisorbed as carboxylate, independent of the
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Chem. Eur. J. 2013, 19, 5352 – 5363

Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films
FULL PAPER
UV/Vis reflection spectra at the air–water interface. Once
the thickness of the LB film is known, the tip–substrate dis-
tance must be calibrated so that the STM tip can be placed
at a known distance above the LB film. This is achieved by
converting the set-point parameters of the STM (I0 ꢃ“set-
point current” and Ut ꢃ“tip bias”) to an absolute gap sepa-
ration, as has been reported previously and is described
below.[93–95] Depending on these parameters, the STM tip
can be located above the monolayer, in contact with the top
of the monolayer or embedded within the monolayer film.
In order to most accurately measure the trans-film conduc-
tance, it is necessary to first determine the set-point condi-
tions under which the tip is just touching the top of the LB
film. A quantitative estimation of the current decay d ln I=ds
in the LB film is required to evaluate this separation at
which the tip just touches the top of the film. Firstly, cur-
rent–distance scans were recorded with the tip fully embed-
ded in the film at sufficiently high set-point currents (I0 =
20 nA and Ut =0.6 V) to ensure that the tip was embedded
within the film, and only current–distance traces which dis-
played a monotonic exponential decrease of the tunnelling
current were selected for this quantification of d ln I=ds.
These d ln I=ds data were recorded at different substrate lo-
cations and at regular intervals during the measurements.
Any curves showing current plateaux synonymous with mo-
lecular-wire formation were rejected, since they are unsuited
for quantification of d ln I=ds. The monotonic exponential
decay curves were then plotted as lnI versus s. Averaging
the slope of the collected d ln I=ds plots gave d ln I=ds values
typically in the range of (6.91ꢂ1.37) nmꢀ1. This value is in
good agreement with those reported for similar highly con-
jugated compounds incorporated in molecular films[20,26,58]
and for single molecules.[21,93] With Ut =0.6 V and I0 =
0.15 nA as the set-point parameters, the initial tip-to-sub-
strate distance is estimated as 1.82 nm according to Equa-
tion (5), in which I0 and Ut are the set-point parameters of
the STM, in good agreement with the thickness of the
Figure 11. a) I–V curve of a one-layer LB film of OPE2A transferred
onto AuACHTUNGTRNEUNG
(111) at 20 mNmꢀ1 from water as subphase (solid line) and fit-
ting according to the Simmons equation (F=1.1 eV, a=0.41; dashed
line). Note that the experimental data and Simmons model nearly over-
lap, and this may interfere in visualization of the experimental curve. The
inset shows a magnification of the y axis to observe in more detail the
sigmoidal shape of the I–V plot. A representative STM image of the
monolayer is also shown in the inset. b) I–V curve of a one-layer LB film
of OPE2A transferred onto AuACHTNUGRTNEUNG
(111) at 20 mNmꢀ1 by using a basic sub-
phase (solid line), from single-molecule conductance values obtained by
using the I(s) method (circles) and fitting according to the Simmons
equation (F=0.73 eV, a=0.34; dashed line). The error bars represent
the standard deviation. Ut =0.6 V.
monoACHTUNGTRENNUNGlayer.
lnðG0Ut=I0Þ
d lnðIÞ=ds
ð5Þ
s ¼
Therefore, these set-point parameters were used to posi-
tion the tip just above the monolayer. For higher set-point
currents (e.g., 0.6 nA, s=1.63 nm) the tip would be embed-
ded within the monolayer, and for lower set-point currents,
the tip would not be in contact with the monolayer and, in
this case, the tunnelling current measured represents tunnel-
ling through both the monolayer and the gap between the
top of the monolayer and the tip. Figure 11a shows a repre-
sentative I–V curve obtained for a one-layer LB film from
water as subphase at Ut =0.6 V and I0 =0.15 nA. The profile
of the I–V curve is clearly symmetrical and exhibits an ap-
proximately sigmoidal profile over the full voltage region.
Nevertheless, the I–V curve becomes linear in the low-volt-
age region (from ꢀ0.6 to +0.6 V), that is, the ohmic region,
where the conductance is 0.26ꢅ10ꢀ5 G0. This conductance is
significantly lower than that exhibited by other OPE deriva-
tives assembled by the LB technique, even when these OPE
derivatives have different end groups.[20,26] This low conduc-
tance for the OPE2A monolayer fabricated on a water sub-
phase could be attributed to the “supramolecular structure”
promoted by the carboxyl group that is used as linker to
make contact with the STM tip. This supramolecular struc-
ture is formed through the lateral H-bonding interactions
between the carboxyl groups, which may also be synergistic
with lateral p–p stacking. The lower conductivity of OPE2A
monolayers fabricated on a pure water subphase would then
arise from the less effective contact of this H-bonded car-
boxylate group to the gold STM tip compared with the
chemisorption bond formed between the carboxylate group
and gold contacts.
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5359

P. Cea et al.
A basic subphase (pH 11.4) was used to retain the lower
contacting group in its deprotonated carboxylate state. The
thickness of the monolayer determined by using the attenua-
tion of the Au 4f signal from the substrate was (1.95ꢂ
0.05) nm. Taking into account that the average slope of the
corresponding d ln I=ds curves used to calculate the tip-to-
substrate distance was (5.48ꢂ0.89) nmꢀ1 when the set-point
parameters Ut =0.6 V and I0 =1 nA were used, the initial
tip-to-substrate distance was 1.95 nm, which is in agreement
with the thickness of the monolayer and indicates that the
tip is positioned just above the monolayer when these set-
point parameters are used. Figure 11b shows a representa-
tive I–V curve obtained for a one-layer LB film from a basic
subphase at Ut =0.6 V and I0 =1 nA. The profile of the I–V
curve is clearly symmetrical and exhibits an approximately
sigmoidal profile over the full voltage region, although the
I–V curve becomes linear in the low-voltage region (from
ꢀ0.6 to +0.6 V), the ohmic region, where the conductance
is 1.75ꢅ10ꢀ5 G0. This conductance is similar to, or even
larger than, that obtained for other OPE derivatives assem-
bled by using the LB technique[20,58] or for single mole-
cules.[21,93] In addition, Figure 11b also shows an I–V curve
constructed from single-molecule conductance (SMC)
values for OPE2A obtained by using the I(s) method at
eight different bias voltages. This I(s) method, developed by
Haiss et al.,[15,21,96] has been used to determine the SMC of
molecular junctions. The SMC curve coincides with the I–V
curve obtained for the LB film at 1 nA and 0.6 V, and this
indicates that with these parameters the STM tip is located
directly above the LB film and electronically coupled to a
single molecule. The two I–V curves show similarity, despite
the different molecular surroundings in the two cases: in the
LB film the molecules are packed together with neighbour-
ing OPE2A molecules, whereas no such neighbours exist for
the SMC determinations. The higher conductance for the
COO–Au molecular junctions than COOH–Au junctions
supports the notion that the former are more effective in
both their surface binding ability and in promoting electrical
transmission of the junctions, which is in agreement with
previous work.[38]
cordance with the isotherms shown in Figure 3 at a surface
pressure of 20 mNmꢀ1), s the width of the tunnelling barrier,
which was assumed to be the geometric distance between
the end groups in the OPE molecular wire as calculated
with a molecular modelling program (2.07 nm), F the effec-
tive barrier height of the tunnelling junction (relative to the
Fermi level of Au), a is related to the effective mass of the
tunnelling electron and m and e represent the mass and the
charge of an electron. The F and a parameters are then
used to best fit the I–V data in Figure 11. Good agreement
between the data and the model were obtained for F=
1.1 eV and a=0.41 when water is used as subphase, and for
F=0.73 eV and a=0.34 when a basic subphase is used.
Firstly, we emphasize that Equation (6), which is based on a
very simple model of non-resonant tunnelling, gives a rea-
sonable description of our experimental I–V data, and it is
therefore reasonable to assume that the mechanism of trans-
port through these metaljmoleculejmetal junctions is non-
resonant tunnelling. Secondly, the F value depends on the
subphase used. Thus, for a basic subphase, F=0.73 eV,
which is in good agreement with those obtained for similar
OPE derivatives assembled by self-assembly[44, 56] or by the
LB technique.[14,19,20,26,58] Meanwhile, when the subphase is
water, the effective barrier height is F=1.1 eV, which is
higher than those obtained for a basic subphase and for
20, 56,58]
other OPE derivatives.[15,
Therefore, these results
seem to indicate that the presence of protonated surface
groups (COOH) and consequent lateral H-bonds within the
monolayer decreases the conductance. This is attributed to a
more compromised electrical contact between the STM tip
and the carboxyl-terminated surface.
Conclusion
A symmetrical acid-terminated OPE derivative has been
synthesized and assembled into well-packed monolayer
films by means of the Langmuir–Blodgett technique, which
has proved to be a suitable method for obtaining homoge-
AHCTUNGTREGnNNNU eous films with high surface coverage, superior to those
The sigmoidal shape of both I–V curves (for water or
basic subphase) is indicative of a non-resonant tunnelling
mechanism of transport through these metal–molecule–
metal junctions. The Simmons model[97] is one of the sim-
plest tunnelling barrier models which has been widely used
for describing charge transport through metaljSAM or
metaljLB film junctions.[20,56,58,98] In this model, the current
I is defined as Equation (6),
achieved by self-assembly techniques for this material.
Langmuir films were prepared at the air–water interface by
using a pure water subphase and a basic subphase and char-
acterized by surface pressure versus area per molecule iso-
therms and Brewster angle microscopy, which revealed that
OPE2A can form true monomolecular films at the air–water
interface on both subphases, in contrast to singly acid substi-
tuted OPEs, which only form three-dimensional defect-free
monolayers
on
ꢄꢀ
ꢁ
ꢂ
ꢀ
ꢁ
ꢃ
ꢀ
ꢁ
ꢂ
ꢀ
ꢁ
ꢃꢅ
basic subphases.
These monomo-
lecular films were
transferred undis-
turbed onto solid
1=2
1=2
1=2
1=2
Ae
4p2ꢀhs2
eV
2
2ð2mÞ
ꢀh
eV
2
eV
2
2ð2mÞ
ꢀh
eV
2
I ¼
F ꢀ
exp ꢀ
a F ꢀ
s ꢀ F þ
ð6Þ
exp ꢀ
a F þ
s
in which V is the applied potential, A the contact area of
the molecule with the gold surface (0.31 nm2 and 0.25 nm2
for a water subphase or basic subphase, respectively in con-
substrates with a transfer ratio close to unity. Both L and
LB films of OPE2A fabricated on a pure water subphase
show a red shift of the main absorption band with respect to
5360
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Chem. Eur. J. 2013, 19, 5352 – 5363

Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films
FULL PAPER
white solid. Yield: 0.30 g, 0.56 mmol, 75%. 1H NMR (400 MHz, CDCl3):
d=8.04 (d, J=9 Hz, 4H, g), 7.59 (d, J=9 Hz, 4H, f), 7.54 (s, 4H, a), 4.33
(t, J=7 Hz, 2H, j), 1.82–1.72 (m, 4H, k), 1.49–1.40 (m, 4H, l), 1.39–1.29
(m, 8H, m/n), 0.92 ppm (t, J=7 Hz, 6H, o); 13C{1H} NMR (101 MHz,
CDCl3): d=166.2 (i), 131.9, 131.6 (f/g), 130.3 (h), 129.7 (a), 127.6, 123.2
(b/e), 91.9, 90.9 (c/d), 65.5 (j), 31.6 (k), 28.8 (l), 25.8 (m), 22.7 (n),
14.1 ppm (o); ASAP-MS(+): m/z (%): 451.19 [M+HꢀC6H13]+ (100),
534.28 [M]+ (53).
the solution, whilst films prepared on a basic subphase ex-
hibit a blue shift. A combination of QCM, XPS and UV/Vis
spectra experiments demonstrated that OPE2A was linked
through a deprotonated carboxyl group to the gold substrate
when the LB films were prepared from either a pure water
or a basic subphase. Monolayers fabricated on a pure water
subphase feature a supramolecular structure due to lateral
H-bonding interactions through
the terminal carboxyl groups. In
contrast, these lateral H-bonds
are not present in monolayers
fabricated on basic subphases.
Electrical characteristics of
the LB films on gold substrates
were obtained by recording I–V
curves with a gold STM tip
Preparation of 4,4’-(1,4-phenylenebis(ethyne-2,1-diyl))dibenzoic acid
(OPE2A): NBu4OH·30H2O (0.30 g, 0.38 mmol) dissolved in THF (3 mL)
was added to a solution of dihexyl 4,4’-[1,4-phenylenebis(ethyne-2,1-
diyl)]dibenzoate (0.05 g, 0.09 mmol) in THF (3 mL). The resulting brown
solution was stirred at room temperature for 30 min, taken to dryness
and redissolved in CHCl3 (2 mL). White solids precipitated upon addition
of concentrated HCl and sonication of the two phases. The precipitate
was collected by filtration and washed with water (2ꢅ5 mL), acetone
(2 mL) and Et2O (5 mL) and dried in air. Yield: 0.03 g, 0.08 mmol, 89%.
1H NMR (500 MHz, [D6]DMSO): d=13.21 (brs, 2H, j), 7.97 (d, J=8 Hz,
4H, g), 7.67 (d, J=8 Hz, 4H, f), 7.64 ppm (s, 4H, a); 13C NMR {1H}
(126 MHz, [D6]DMSO, 508C): d=166.4 (i), 131.6, 131.4 (f/g), 130.8 (h),
129.3 (a), 126.0, 122.2 (b/e), 91.1, 90.6 ppm (c/d); ESI-MS(ꢀ): m/z (%):
183.3 [Mꢀ2H]2ꢀ (100), 365.5 [MꢀH]ꢀ (34); TGA: incomplete combus-
tion (91%) at 10008C.
positioned just above the monolayer (as determined from
calibration of the tip-to-substrate distance and knowledge of
the thickness of the LB film determined from XPS measure-
ments). These I–V curves and good fits to the Simmons
model indicate that charge flow through the metaljmole-
culejmetal junction occurs by a non-resonant tunnelling
mechanism. Importantly, the conductance in films prepared
on basic subphases is quite similar to the SMC values. How-
ever, LB films fabricated on a pure water subphase exhibit
conductances around seven times lower. This result has
been attributed to the more effective electrical junctions
formed between carboxylate
groups and gold surfaces, as op-
posed to carboxyl groups, which
also form lateral H-bonding in-
teractions that decrease the
conductance. Thus, modulation
of conductance by pH and mo-
lecular structure control is ach-
ieved.
Film fabrication and characterization: The films were prepared on a
Nima Teflon trough with dimensions 720ꢅ100 mm, which was housed in
a constant temperature (20ꢂ18C) clean room. A Wilhelmy paper-plate
pressure sensor was used to measure the surface pressure p of the mono-
layers. The subphase was either pure water (Millipore Milli-Q purifica-
tion system, resistivity 18.2 MWcm) or a solution of NaOH or HCl pre-
pared with Milli-Q water as solvent and with pH as indicated in the
paper. To fabricate the Langmuir films a 1ꢅ10ꢀ5 m solution of OPE2A in
chloroform/ethanol (4/1, HPLC grade purchased from LabScan (99.8%)
and Panreac (99.5%), respectively) was spread by using a Hamilton sy-
ringe held very close to the surface and allowing the surface pressure to
return to a value close to zero between each addition. The use of ethanol
in the spreading solvent limits the formation of hydrogen-bonded carbox-
ylic acid dimers and aggregates in solution prior to deposition.[63] After
waiting about 15 min to allow the solvent to evaporate, slow compression
of the film began at a speed of 0.022 nm2 moleculeꢀ1 minꢀ1. Under these
experimental conditions the isotherms were highly reproducible. Direct
visualization of monolayer formation at the air–water interface was stud-
ied with a commercial micro-Brewster angle microscope (micro-BAM)
from KSV-NIMA having a lateral resolution better than 12 mm. A UV/
Vis reflection spectrophotometer with FiberLight DTM 6/50 light source,
an absolute wavelength accuracy of <0.3 nm and a resolution (Rayleigh
criterion) of >3 nm was used to obtain the reflection spectra of the
Langmuir films during the compression process.[65]
Experimental Section
General synthetic conditions: Syntheses were carried out under an
oxygen free nitrogen atmosphere by using standard Schlenk techniques.
All reaction vessels were flame-dried before use. Triethylamine was puri-
fied by distillation over CaSO4. Other reagents were purchased commer-
cially and used as received. Hexyl 4-ethynylbenzoate was prepared ac-
cording to literature procedures.[26] NMR spectra were recorded on solu-
tions in deuterated solvents on Bruker DRX-400 and Varian 500 spec-
trometers and referenced against solvent resonances (1H, 13C). ESI mass
spectra were recorded on a TQD mass spectrometer (Waters Ltd, UK).
Samples were 0.1 mgmLꢀ1 soloutions in analytical-grade methanol. Ther-
mal analyses were performed with a PerkinElmer Pyris thermogravimet-
ric analyser (heating rate 108C minꢀ1).
Preparation of dihexyl 4,4’-[1,4-phenylenebis(ethyne-2,1-diyl)]diben-
zoate: Hexyl-4-(ethynyl)benzoate (0.34 g, 1.5 mmol), 1,4-diiodobenzene
(0.25 g, 0.76 mmol), [PdACHTUNGTRENNUNG(PPh3)4] (0.045 g, 0.040 mmol) and CuI (0.007 g,
0.037 mmol) were added to NEt3 (15 mL), and the resulting white sus-
pension stirred at room temperature overnight. The precipitate was col-
lected by filtration and washed thoroughly with hexane. The solids were
dissolved in CH2Cl2 and the solution filtered through silica gel. Solvent
removal from the yellowish filtrate yielded the pure product as an off-
The monolayers at the air–water interface were transferred onto solid
supports at a constant surface pressure by the vertical dipping method
Chem. Eur. J. 2013, 19, 5352 – 5363
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5361

P. Cea et al.
(dipping speed 3 mmminꢀ1) onto gold or quartz substrates, which were
carefully cleaned as described previously.[61, 71] QCM measurements were
carried out by using a Stanford Research Systems instrument with AT-cut
a-quartz crystals with a resonant frequency of 5 MHz and circular gold
electrodes patterned on both sides. UV/Vis spectra of the LB films were
acquired on a Varian Cary 50 spectrophotometer and recorded at a
normal incident angle with respect to the film plane.
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Cyclic voltammetry (CV) experiments were carried out in an electro-
chemical cell containing three electrodes. The working electrode was
made of either a gold substrate or a gold substrate modified by the de-
posited LB film. Gold substrates were purchased from Arrandee, Germa-
ny. These were flame-annealed at approximately 800–10008C with a
Bunsen burner immediately prior to use. This procedure is known to
[15] C. Wang, A. S. Batsanov, M. R. Bryce, S. Martin, R. J. Nichols, S. J.
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result in atomically flat AuACHTNUTRGNE(NUG 111) terraces. The counterelectrode was a
platinum sheet, and the reference electrode was AgjAgCljsaturated
KCl.
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2009, 9, 117.
X-ray photoelectron spectra were acquired on a Kratos AXIS ultra DLD
spectrometer with a monochromatic AlKa X-ray source (1486.6 eV) by
using a pass energy of 20 eV. To provide precise energy calibration, the
XPS binding energies were referenced to the C 1s peak at 284.6 eV. The
thickness of LB films on gold substrates was estimated using the attenua-
tion of the Au 4f signal from the substrate according to
ILB film ¼ Isubstrate expðꢀd=l sin qÞ, where d is the film thickness, ILB film and
Isubstrate are the average of the intensities of the Au 4f5/2 and Au 4f7/2
peaks attenuated by the LB film and bare gold, respectively, q is the pho-
toelectron take-off angle and l is the effective attenuation length of the
photoelectron ((4.2ꢂ0.1) nm).[44]
[18] C. Ko, M. Huang, M. Fu, C. Chen, J. Am. Chem. Soc. 2010, 132, 756.
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[20] G. Pera, S. Martin, L. M. Ballesteros, A. J. Hope, P. J. Low, R. J.
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S.M. and P.C. are grateful for financial assistance from Ministerio de
Ciencia e Innovaciꢂn (Spain) and fondos FEDER in the framework of
projects CTQ2009-13024 and CTQ2012-33198. P.J.L holds an EPSRC
Leadership Fellowship. R.J.N. thanks EPSRC for funding. S.M. acknowl-
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