Helical Peptides Design for Molecular Dipoles Functionalization of Wide Band Gap Oxides

Article abstract of DOI:10.1021/jacs.9b12001

The use of helical hexapeptides to establish a surface dipole layer on a TiO₂ substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid

Full text of DOI:10.1021/jacs.9b12001

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Article
Helical peptides design for molecular dipoles
functionalization of wide band gap oxides
Yuan Chen, Jonathan Viereck, Ryan Harmer, Sylvie Rangan, Robert Allen Bartynski, and Elena Galoppini
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12001 • Publication Date (Web): 24 Jan 2020
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Helical peptides design for molecular dipoles functionalization of wide
band gap oxides.
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Yuan Chen1, Jonathan Viereck2, Ryan Harmer1, Sylvie Rangan2, Robert A. Bartynski2* and Elena
Galoppini1*
Chemistry Department, Rutgers University, 73 Warren Street, Newark NJ 07102, USA; Department of
Physics and Astronomy and Laboratory for Surface Modification, Rutgers University, 136
Frelinghuysen Road, Piscataway NJ 08854, USA.
KEYWORDS energy alignment, dipole, sensitization, semiconductor, TiO2.
ABSTRACT: The use of helical hexapeptides to establish a surface dipole layer on a TiO2 substrate, with the goal of influencing
the energy levels of a co-adsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric
acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a
carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)6-COOH or Z-(Aib)6-Ipa, respectively. Using a
combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO2 surfaces (either
nanostructured or single crystal TiO2(110)) was found to be highly dependent on the anchor group, with Ipa establishing a
monolayer much more efficiently than COOH. Furthermore, a monolayer of Z-(Aib)6-Ipa on TiO2(110) was exposed for
different binding times to a solution of a zinc tetraphenylporphyrin (ZnTPP) derivative terminated with an Ipa anchor group
(ZnTPP-P-Ipa). Photoemission spectroscopy revealed that ZnTPP-P-Ipa partly displaced Z-(Aib)6-Ipa, forming a co-adsorbed
monolayer on the oxide surface. The presence of the peptide molecular dipole shifted the HOMO levels of the ZnTPP group to
lower energy by ~ 300 meV, in accordance with a simple parallel plate capacitor model. These results suggest that a mixed-
layer approach, involving co-adsorption of a strong molecular dipole compound with a chromophore, is a versatile method to
shift the energy levels of such chromophore with respect to the band edges of the substrate.
experimental evidence, supported by theory,18-21 that
Introduction
permanent dipoles originating from the binding of ions or
polar molecules,13, 16, 22-24 or transient dipoles formed from
photoinduced charge separation25 have the ability to induce
Heterogeneous organic/inorganic materials are widely
used in photovoltaic,
1-2
organic electronic,3-4 molecular
such shifts and influence device properties.26-31
A
sensing5-6 and photocatalytic7-9 applications. For these
materials, energy alignment between the frontier orbitals of
an organic chromophore (or sensitizer) and the band edges
of a wide band gap metal oxide substrate is a key factor
influencing charge transfer efficiencies and device
properties. For this reason, there is an increasing interest
in developing strategies to shift energy alignment in a
controlled fashion, while preserving the HOMO-LUMO
properties of the chromophore. This would allow, for
instance, favorable positioning of the sensitizer’s energy
level in photocatalytic devices and solar cells, sensitizing
materials with wider bandgaps, or influencing electron
injection and recombination rates.
shortcoming of this approach, however, is that the
magnitude and even direction of the energy shift is difficult
to predict or control and is not transferrable from system to
system. An alternative approach is to employ molecules
with a modular chromophore-bridge-anchor architecture
where the bridge portion of the molecule contains a
permanent molecular dipole, as illustrated in Figure 1.32
The benefit of this approach is that one can synthesize
molecules with the same chromophore and anchor, but with
different dipole-containing bridging units. In this way, the
nature of the HOMO and LUMO levels of the chromophore,
and the binding of the molecule to the surface can remain
unchanged while the dipole magnitude and direction can be
varied by synthetic design. The first experimental proof-of-
concept of this approach was given with three zinc
tetraphenylporphyrin (ZnTPP) derivatives, 1-3 in Figure 1,
having similar linker structures and identical HOMO-LUMO
gaps. Derivatives 1 and 3 had, in the central phenyl ring of
the bridge, a built-in permanent dipole of identical size but
pointing in opposite direction. The unsubstituted
compound, 2, served as the reference. These compounds
A promising approach to tuning level alignment is to create
an interfacial dipole layer between the chromophore and
the metal oxide which, analogous to a parallel plate
capacitor, would generate an electrostatic potential
difference between them. Depending on the magnitude and
orientation of the dipole, that potential difference would
shift the chromophore’s orbitals to higher or lower energies
with respect to the band edges of the substrate. For
metals10-12 or semiconductors13-17 there is extensive
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were bound through an isophthalic acid (Ipa) anchor group
to a ZnO(11–20) single crystal substrate, forming a
molecular monolayer on the surface. 32
secondary structure variations and on the peptide length.19
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The intrinsic dipole moment of helical peptides has been
used extensively to probe the effect of dipole magnitude and
direction on charge transfer in electron donor-acceptor
systems in solution,34-37 on redox processes on molecular
monolayers on electrodes and on metal surfaces.37 Peptides
have inspired the synthesis of dipole-containing
structurally-related oligomers that are used to probe
electric field effects.38-40 Finally, dipole reversal is achieved
by switching the relative positions of the electron donor and
the acceptor side groups in the peptide sequence, or by
switching the position of the anchor group from the N-
terminus to the C-terminus. 35, 37, 41
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In a successful example of this approach, Maran and
coworkers developed 310-helical peptides 2-amino
isobutyric acid (Aib) capped with a thiol group. When
bound to metal electrodes, these formed a monolayer with
highly oriented dipoles that was used to probe the influence
of dipole size and reversal on interfacial redox processes.12,
41-43
The redox behavior was modulated by varying the
length of the (Aib)n peptides (n = 3-6), and the dipole
orientation was reversed by placing the thiol group on the
N-terminus or on the C-terminus.41 Maran’s work points to
key advantages in using Aib residues. Firstly, because of the
steric hindrance at the -carbon, they are highly
helicogenic. Therefore, a strong dipole can be formed with
relatively short peptides. For instance, the dipole for a 310-
helical (Aib)6 homopeptide was estimated to be about 17 D
36-37, 41, 43, which is consistent with our results (vide infra).
Secondly, they form rigid and robust 310-helices44 that,
being less sensitive to solvent environment than helices,
retain the helical structure and pack well on metal
surfaces.42
Figure 1. Schematic illustration of the interfacial dipole layer
experiment with ZnTPP derivatives 1 and 3 and reference
compound ZnTPP-P-Ipa 2 (no dipole). The dipole moment in
the bridge of 1 and 3 was created by an electron-donating
(NMe2) and electron-withdrawing (NO2) group opposite to
each other.32
In this paper we report the synthesis and binding studies of
new dipole linkers made of Aib oligopeptide bridges
terminated with two different anchor groups, Z-(Aib)6-
COOH (4) and Z-(Aib)6-Ipa (5), that is, identical
hexapeptides with either a COOH or an Ipa anchor group,
respectively, as shown in Figure 2(a). The influence of the
two different anchor groups on binding the linkers to TiO2,
either nanoparticle films or single crystal (110) surfaces, as
illustrated in Figure 2(b)(i), was probed by IR imaging and
photoemission spectroscopies, respectively. In addition, we
studied the effect of co-adsorbing in a peptide monolayer,
molecules with a chromophoric head group in what we
term a “mixed-layer” approach, schematically illustrated in
Figure 2(b)(ii). This approach differs from those previously
published and represented in Figure 1, in that here the
dipole layer is established by the dipole-containing linkers,
which creates a potential difference that can shift the
energy levels of co-adsorbed chromophoric molecules. The
results described here demonstrate the utility of peptides
as strong dipole-containing linkers for studying interfacial
dipole effects, the importance of anchor group selection,
and the viability of the “mixed-layer” approach.
UV photoemission spectroscopy (UPS) measurements of
monolayers of 1, 2, and 3 on ZnO(11–20) found that,
compared to reference compound 2, the ZnTPP HOMO of 1
was shifted towards the ZnO valence band maximum by
~100 meV, while that of 3 was shifted away by the same
amount as depicted in Figure 1. In both size and orientation,
these results were in accordance with the values predicted
by a simple parallel plate capacitor model. Interfacial
photoinduced charge transfer studies of 1, 2, and 3 bound
to TiO2 nanoparticle films (chosen for practical reasons),
aimed at probing any influence on electron injection rates,
exhibited complex kinetics and energy transfer to the
bridge which may have obscured any dipole effects.33 This
suggested that future charge transfer studies probing the
dipole effect would require an overall improved molecular
design and a larger dipole moment, preferably oriented
along the linker’s axis.
A promising class of linker molecules, which contains large
dipoles along their axis and should not complicate
photoexcitation of a chromophoric head group, are peptides
with a secondary helical structure. The helical structure,
typically a 310- or -helix, generates a dipole moment
oriented along the main molecular axis and directed from
the N-terminus (the amino end) to the C-terminus (the
COOH end), as illustrated for the peptide in Figure 2(a). The
molecular dipole size increases with each amino acid
residue, although the precise value of the molecular dipole
depends heavily on individual residue contributions, on the
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O
Z
OH
A =
1
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(a)
(b)
O
O
O
C
H
N
H
N
H
N
Z-(Aib) -COOH
6
4
)
(
CH2
O
N
H
N
H
N
H
A
COOH
COOH
O
O
O
O
A =
N
H
CH2
Aib =
N
H
O
Z-(Aib)6-Ipa
5
)
(
(i)
(iii)
(ii)
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13
14
TiO2(110)
Figure 2. (a) Molecular structure of dipole linker peptides 4
and 5; (b) Illustration of three binding concepts: (i) peptide
linkers bound as a monolayer; (ii) “Mixed-layer” approach
with ZnTPP-P-Ipa, 2, co-bound in a monolayer of 4 or 5; (iii)
Chromophore-peptide compounds bound to substrate.
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Results and Discussion
Synthesis and helical structure of the Aib-peptides
The synthesis of hexapeptide Z-(Aib)6-COOtBu (see SI)
followed classic solution-phase peptide synthesis.42 The
amino group on the N-terminus was protected with the
carboxybenzyl group (Z). The hexapeptide was prepared in
a segment condensation step from Z-(Aib)3-COOtBu, which
was prepared by peptide couplings of individual Aib
residues. Step-by-step couplings involved activation of the
carboxylic group with 1-(3-dimethyl-aminopropyl)-3-
ethylcarbodiimide hydrochloride (EDC·HCl) and 1-
Figure 3. (a) Synthesis of Z-(Aib)6-Ipa from Z-(Aib)6-COOH.
The (i  i+3) 310-helical structure and H-bonding pattern
(blue, dashed line) in Z-(Aib)6-Ipa, labeling and calculated
dimensions. (b) Geometry calculated for Z-(Aib)6-Ipa and
resulting calculated dipole. (c) X-ray crystal structure of Z-
(Aib)6-COOtBu (AcOEt and pentane not shown). The crystal
exhibited a 9513 Å3 unit cell volume, and a P2(1)/c space
group.
hydroxy-7-azabenzotriazole
(HOAT)18
in
N-
methylmorpholine (NMM). In the segment condensation of
larger fragments, however, activation of the carboxylic
group required conversion of Z-(Aib)3-COOH into an
oxazolone, followed by coupling with the N-deprotected
tripeptide, H-(Aib)3-COOtBu. The ester group in Z-(Aib)6-
COOtBu was hydrolyzed by treatment with diluted
trifluoroacetic acid (TFA) to obtain Z-(Aib)6-COOH, with the
COOH group already available for binding to surfaces,
Figure 3(a). Z-(Aib)6-Ipa, with the isophthalic acid anchor
group, was prepared by coupling Z-(Aib)6-COOH with di-t-
butyl 5-(aminomethyl)isophthalate 6, which was
synthesized from 5-methylisophthalic acid (See SI),
followed by deprotection of both t-butyl ester groups with
TFA, Figure 3(a).
In this work it was important to determine whether the
(Aib)6 peptides synthesized did have a secondary helical
structure, which is the prerequisite for the molecular dipole
formation. This was determined by X-ray diffraction, IR and
NMR of Z-(Aib)6-COOtBu.
Hexapeptide Z-(Aib)6-COOtBu was crystallized from
pentane/ethyl acetate and the crystal structure, shown in
Figure 3(c) on top and side view, indicate the expected 310-
helical structure, and is consistent with the crystal structure
of the same identical compound reported by others. 45
The helical structure was persistent in organic solvents, as
shown by the intramolecular H-bonding interactions in
solution FTIR spectra of Z-(Aib)n-COOtBu with n = 2, 3 and
6. H-bonding interactions are expected to increase with the
number of residues.42 The solutions were diluted (5 mM in
CHCl3, a solvent that does not disrupt the secondary
structure) to minimize any intermolecular hydrogen-
bonding. Figure 4 shows the FTIR spectra of Z-(Aib)n-
COOtBu in the region of the amide-A bands, corresponding
to N-H stretching (~ 3400 and 3350 cm-1). Other regions,
such as those of amide-I (C=O amide stretching ~ 1650 cm-
1) and amide-II (N-H bending () coupled with C-N
The characteristic folding of such a peptide into a 310
geometry is the result of the hydrogen bonds established
between an Aib unit (i) and the (i+3) unit. This pattern and
calculated molecular dimensions for Z-(Aib)6-Ipa, are
represented in Figure 3(a). The optimized calculated
geometry of 5 is shown in Figure 3(b). The helix is
characterized by a triangular cross section, when viewed
along the molecular axis, as seen in the figure’s inset, and
the overall molecular dipole is oriented parallel to the
molecular axis, pointing toward the Ipa anchor group, with
a magnitude of 18.6 D.
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stretching () ~1550 cm-1 and 1490 cm-1) are shown in
Figure S3 and discussed in SI.
molecule. A similarly good agreement was found for XPS
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measurement of neat Z-(Aib)6-COOH films. In general, the
line shapes of valence band photoemission spectra are
typically good fingerprints for organic compounds as they
reflect the characteristic hybridization of valence levels.
However, interpreting the spectral features usually
requires
a
comparison to the calculated electronic
very favorable
structures. Figure 5(c) illustrates
a
comparison between the measured XPS valence band
spectrum from a neat Z-(Aib)6-Ipa to the calculated DOS of
the molecule, weighted by the photoemission cross section
of each atomic orbital. The calculation was performed for
the molecule in a 310 optimized helical geometry, as shown
in Figure 3(c). A similar level of agreement between the
calculated weighted DOS and the XPS VB spectrum was
found for neat Z-(Aib)6-COOH (not shown).
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Figure 4. Selected spectral regions of the FTIR spectra of Z-
(Aib)n-COOtBu (n = 2, 3 and 6) solutions (5 mM, CHCl3) in the
3550-3200 cm-1 region.
In the 3550-3200 cm-1 region presented in Figure 4, the
band at lower energy is assigned to the NH) of H-bonded
amides, and the higher energy band to free amide
groups.46,47 As the peptide chain length increases from
(Aib)2 to (Aib)6, the intensity of the free (NH) band at 3426
cm-1 does not exhibit significant changes, whereas the H-
bonded band sharply increases in intensity and shifts from
~3400 cm-1 in Z-(Aib)2-COOtBu to 3339 cm-1 in Z-(Aib)6-
COOtBu, both spectral changes consistent with increased
intramolecular H-bonding.42
Figure 5. (a) XPS survey scan measured on a neat film of Z-
(Aib)6-Ipa with main core levels indicated. (b) The measured
stoichiometry for the sample is in good agreement with the
expected one. (c) XPS VB spectrum compared to the XPS-
weighted DOS calculated for Z-(Aib)6-Ipa.
Additional information on the secondary structure of Z-
1
(Aib)6-COOtBu in organic solutions was obtained from H
NMR spectra (see SI), including NOESY, HMBC, and solvent
titration carried out to probe which NH groups are not H-
bonded. Although overall indicative of a highly folded
structure, these experiments in (Aib)n peptides are
complicated by the absence of a H in the -position to the
carbonyl group, and were not conclusive.
Anchor group dependence of binding properties
The binding properties to TiO2 of the (Aib)6 compounds 4
and 5 studied here were found to be highly dependent on
the anchor group: either -COOH or -Ipa, respectively.
Similar binding behaviors were found for the surfaces of
both single crystal TiO2(110) and nanostructured TiO2
films. In general, carboxylic acid group forms strong
covalent bonds to metal oxide surfaces, typically through a
combination of bidentate/monodentate binding modes,49
but this was not the case for the -COOH of Z-(Aib)6-COOH.
Possible explanations for this diminished binding ability
may involve the secondary structure of the peptide. In
contrast, the Ipa anchor group in Z-(Aib)6-Ipa, which was
also the anchor group for the previously studied ZnTPP
compounds 1, 2 and 3, exhibited superior binding, which
may be because it is not expected be involved in the
secondary structure of the peptide.
In summary, although the type of helix depends on a
delicate balance between intramolecular interactions and
48
the environment,
our results indicate that the Aib6
peptides studied here are helical, in agreement with data
reported by others for (Aib)n peptides.42-43
The peptides were also studied using XPS and UPS. XPS core
level spectra provide information on the elements present
in the compound and their chemical environment, and UPS
valence band spectra are very sensitive to local
hybridization and electronic structure. Figure 5(a) shows
an XPS survey spectrum measured on a neat Z-(Aib)6-Ipa
thick film (~7 nm) on a copper surface. The XPS survey
spectrum only contains core level features associated with
the C 1s, N 1s and O 1s levels, as well as a small contribution
from Cu 2p levels originating from the underlying substrate.
The relative intensities of the core levels, normalized by
their cross sections, were used to determine the atomic
percent of each element in the measured compounds. As
indicated by the table in Figure 5(b), there was good
agreement between the film composition obtained from
XPS and the expected elemental composition for the
Binding to nanostructured TiO2 films. The binding to TiO2,
and any difference in surface coverage between Z-(Aib)6-
COOH and Z-(Aib)6-Ipa onto nanostructured TiO2 anatase
thin films, was probed using FTIR microscopic imaging, a
method that we previously employed to characterize metal
oxide films functionalization with m-scale spatial
resolution.50 The imaging results are shown in Figure S4 of
the SI file. Representative single pixel FTIR spectra of the
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peptidic Amide-I band (ν(C=O)) in the 1700-1650 cm-1, and
shoulder at ~ -533 eV. The former peak was associated with
Amide-II band (γ(N-H)) in the 1530-1450 cm-1 regions for Z-
(Aib)6-COOH and Z-(Aib)6-Ipa, are shown in Figure 6 (a) and
(b), respectively. The dashed lines on these curves illustrate
the typical spectral changes that take place upon binding
(the full spectra are in shown in Figure S2). The intensity of
the carbonyl stretching bands (νas(C=O)) of the free
carboxylic acid for Z-(Aib)6-COOH at 1741 cm-1 and for Z-
(Aib)6-Ipa at 1726 cm-1 are considerably weakened. In
addition, upon binding the spectra for both compounds
emission from oxygen of the TiO2 substrate while the latter
resulted from the bound molecules. The molecular O 1s
core level signal arose primarily from the peptidic (amide)
C=O groups and the terminal COOH, and occurred at a
binding energy consistent with that chemical environment.
The N 1s core level spectra originated from the NH groups,
consistent with a binding energy of -401 eV, while the line
shape of the C 1s core level spectra was more complicated
as a result of the complex carbon environments in both
compounds. Moreover, as each Aib unit contributes to the
large dipole across the peptide helix, different sites should
be subjected to a different local electrostatic potential
depending on their position on the helix, resulting in core
level broadening in addition to chemical shifts. Finally,
1
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5
6
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9
-
exhibit a new band at 1548 cm-1, attributed to a ν(CO2 )
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stretch and compatible with
a chelating bidentate
adsorption mode. Thus, as expected, both Z-(Aib)6-COOH
and Z-(Aib)6-Ipa bind to nanostructured TiO2 via their -
COOH and-Ipa anchor groups.
having identified the
O 1s feature from the bound
In addition to these spectral changes, the FTIR images of Z-
(Aib)6-COOH/TiO2) and Z-(Aib)6-Ipa/TiO2 films in Figures
S4(a) and S4(b), respectively, reflect the relative binding
strengths of the two molecules. These figures, plotted with
the same intensity scale, clearly show that the Z-(Aib)6-
Ipa/TiO2 films have a much higher surface coverage than
that of Z-(Aib)6-COOH/TiO2 films, indicating that Z-(Aib)6-
COOH binds much more weakly than Z-(Aib)6-Ipa.
molecules, it was straightforward to determine the
stoichiometry of the peptide monolayers.
Figure 6. (a): FTIR-ATR spectrum of neat Z-(Aib)6-COOH (black
line) and representative single pixel FTIR spectrum of Z-(Aib)6-
COOH/TiO2 (red line); (b): FTIR-ATR spectrum of neat Z-
(Aib)6-Ipa (black line) and representative single pixel FTIR
spectrum of Z-(Aib)6-Ipa/TiO2 (red line). Spectra are not
normalized
Binding to single crystal TiO2(110) surfaces. To
investigate whether the substantially weaker binding of Z-
(Aib)6-COOH compared to Z-(Aib)6-Ipa on nanostructured
TiO2 films was dependent on the morphology and
crystallinity of the substrate, binding of 4 and 5 to single
crystal rutile TiO2(110) surfaces was investigated by
photoemission spectroscopy. XPS spectra, including a
survey spectrum and detailed core levels spectra, measured
from TiO2(110) single crystal surfaces after solution
sensitization with each peptide are shown in Figure 7(a).
The spectra are normalized to the intensity of the Ti 2p core
level feature of the substrate. After exposure to the peptide
solutions, only Ti, C, N, and O related photoemission peaks
were visible, and it can be immediately seen that the
amounts of C, N and O on the crystal surface are much
smaller for Z-(Aib)6-COOH compared to Z-(Aib)6-Ipa,
indicating that there is a lower coverage of the former as
compared to the latter.
Figure 7. (a) XPS survey spectra of a TiO2(110) single crystal
after solution exposure to Z-(Aib)6-COOH or Z-(Aib)6-Ipa.
Details of the corresponding O 1s, N 1s and C 1s core levels are
shown as insets. (b)Atomic percentage measured using XPS for
core levels relevant to the adsorbates.
The measured elemental composition for the two
compounds is reported in Figure 7(b). As the TiO2(110)
surface was briefly exposed to air before immersion in the
peptide solutions and again before introduction to UHV,
additional atmospheric contaminants were expected.
Therefore, the measured composition is likely to deviate
slightly from the expected composition by the presence of
additional carbon.
A good metric for the surface
composition is the C/N ratio. In the case of Z-(Aib)6-COOH,
a C/N ratio of 10.4 was measured, twice the expected C/N
ratio of 5.3. For Z-(Aib)6-Ipa, a C/N ratio of 7.5 was
measured, much closer to the expected value of 5.8. Thus,
the surface sensitized with Z-(Aib)6-COOH contained fewer
More detailed information was obtained from the core level
spectra analysis. The O 1s core level spectra contained two
features, a dominant peak at ~ -531 eV, and a secondary
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peptides than the surface sensitized with Z-(Aib)6-Ipa, between the head of the molecule and the underlying
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indicating that the COOH group is less efficient at binding
than the Ipa group, resulting in more adsorbed
contaminants. Additionally, UV valence band spectra
measured on the two sensitized surfaces are reported in
Figure S1. These spectra were compared to the weighted
PDOS calculated for each compound. Although the VB
measured for Z-(Aib)6-Ipa were in good agreement with the
calculated electronic structure. However, owing to a higher
level of surface contamination, this was not the case for the
VB measured for Z-(Aib)6-COOH. Overall, these
observations indicated that, consistent with the
nanoparticle film studies, the peptide with the Ipa anchor
bound much more strongly to the TiO2(110) surface than
the peptide with the COOH anchor group.
substrate. This was demonstrated previously for
monolayers of the ZnTPP-P-Ipa derivatives 1, 2, and 3,
where the orientation of molecular dipole in the bridge unit,
pointing either up or down, shifted the energy of the
frontier orbitals as illustrated in Figure 1.32 In principle, a
similar (and perhaps larger) shift should occur for the
orbitals of the terminal phenyl group of the Z unit of a
monolayer of Z-(Aib)6-Ipa bound on the TiO2(110) surface.
However, the HOMO of the phenyl is well within the valence
levels of the peptide compound making possible energy
shifts difficult to observe. Moreover, the peptides have no
dipole-free analog to serve as a reference. However, if 2
were co-adsorbed on the surface in a matrix of Z-(Aib)6-Ipa,
the ZnTPP head group of 2 should be at the same
electrostatic potential as the terminal phenyl group on the
peptide, and therefore the ZnTPP HOMO should be shifted
in energy by the presence of the peptide dipole.
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Mixed layer adsorption on TiO2 surfaces
As illustrated in Figure 1, an ordered array of dipole-
containing molecules oriented with their dipole moments
(or at least a significant component of it) perpendicular to
the surface will create an electrostatic potential difference
To test this hypothesis, we co-adsorbed, through sequential
sensitization, Z-(Aib)6-Ipa and ZnTPP-P-Ipa (2) to form a
mixed layer as illustrated schematically in Figure 2(b).
Figure 8. Peptide-covered TiO2(110) surface exposed to a solution of ZnTPP-P-Ipa (2). (a) UPS VB and (b) N 1s core level spectra
evolution as a function of exposure time to 2 compared to those of a surface exposed only to 2. (c) Comparison of an experimental
N1s core level spectrum measured on a mixed layer to an N1s core level spectrum modeled from a linear combination of N 1s spectra
of a monolayer of peptide 5 and a monolayer of 2. (d) Relative 5 to 2 coverage estimated from N1s core level spectra. (e)
Representation of a mixed layer where the dipole region is indicated in green as well as the measured dipole pointing toward the
surface. ML = monolayer
UPS valence band spectra and XPS core level spectra from
such a mixed monolayer, obtained for different relative
coverage of Z-(Aib)6-Ipa and of 2 on TiO2(110), are shown
in Figure 8. In each plot, the green curves indicate spectra
from a pure Z-(Aib)6-Ipa monolayer (ML) and the purple
curves are from a pure 2 /TiO2(110) ML. To form the mixed
layers, the surface was first sensitized with a ML of Z-(Aib)6-
Ipa, and then placed in a solution of ZnTPP-P-Ipa (2) of
equal concentration for either 10 or 20 min (indicated in
orange and blue, respectively, in Figure 8).
Figure 8(a) shows UPS VB spectra measured from the 10
min and 20 min exposure surfaces and from the pure
ZnTPP-P-Ipa and Z-(Aib)6-Ipa monolayer acting as the
references. The ZnTPP-P-Ipa spectrum exhibits
a
pronounced peak at -1.6 eV attributed to the ZnTPP HOMO.
Note that here a ML refers to the coverage obtained at
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Journal of the American Chemical Society
saturation for each molecule. However, the surface
molecular density at saturation for each molecule is
different because of differences in the molecular footprint.
∆푉
, for ½
strength of 18.6 D, the potential difference
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8
monolayer of Z-(Aib)6-Ipa, between the surface and the top
∆푉 = 푛 푝
=
300
⊥
part of the dipole, was estimated to be:
휀 =
휀0
After 10 min exposure to ZnTPP-P-Ipa,
a signature
meV, where
8.85×10−12 퐶 푉−1 푚−1. Thus assuming an
0
corresponding to the ZnTPP HOMO was observed at about -
2 eV. The intensity of this feature increased after 20 min
exposure indicating additional ZnTPP-P-Ipa adsorption,
and the HOMO is found at -1.9 eV. This trend was confirmed
by the evolution of the XPS N 1s core level spectra shown in
Figure 8(b). Upon increasing binding times in the ZnTPP-P-
Ipa solution, the intensity of the N1s core level centered at
~ -401 eV, attributed to the NH groups of Z-(Aib)6-Ipa,
progressively decreased. At the same time, a new feature
centered at ~ -399 eV, characteristic of pyrrole nitrogens of
ZnTPP-P-Ipa (purple curve), increased in intensity and
reaching a binding energy of -399.3 eV. The N 1s core levels
therefore indicate a progressive displacement of Z-(Aib.)6-
Ipa by ZnTPP-P-Ipa. Further analysis of the N 1s spectrum
gave an estimate of this molecular exchange. The blue curve
of Figure 8(c) is the N 1s spectrum from the sample after a
20 min sensitization in the ZnTPP-P-Ipa solution. The
dashed black curve in the figure, which gives an excellent
account spectrum from the mixed layer, is a linear sum that
is 50% the pure Z-(Aib)6-Ipa spectrum (green curve of
Figure 8(b)) and 50% the pure ZnTPP-P-Ipa spectrum
(purple curve of Figure 8(b) with the latter shifted by -300
meV).
homogeneous mixed-layer where ½ monolayer of a Z-
(Aib)6-Ipa reference monolayer remains on the surface, and
where the rest of the TiO2 surface is occupied by ZnTPP-P-
Ipa, we can expect a negative energy shift, i.e., a shift to
deeper binding energy, of the ZnTPP HOMO and LUMO of
300 meV. The inset of Figure 8(a) shows a close-up of the
HOMO region of the VB spectra measured for a pure
monolayer of 2 (purple curve), the mixed surface produced
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after
a pure peptide layer experiences a 20 min
sensitization in 2. Clearly, the HOMO of the porphyrin unit
in the mixed layer shifted to deeper binding energy by 300
meV (from -1.6 eV to -1.9 eV), in good agreement with the
simple parallel plate capacitor model. It is useful to note
that photoemission does not provide direct information
about the spatial distribution of Z-(Aib)6-Ipa and ZnTPP-P-
Ipa within the mixed-layer. As such, islanding and
segregation of the two species could not be excluded.
However, the large value of the measured chromophore
energy shift could indicate that each chromophore was
homogeneously surrounded by peptides. These results
verify the electrostatic potential difference introduced by
different amounts of peptide in the monolayer, suggesting
that this “mixed-layer” approach is a versatile, and less
synthetically demanding, approach to take advantage of the
intrinsic molecular dipole of the peptide to produce
While the number density of a pure monolayer of each
molecule may be different, the N 1s peak of each, when
normalized to the substrate Ti 2p core level, had
comparable intensities. Therefore, the spectrum of Figure
8(c) indicates the surface has an approximately equal
fraction of a monolayer of each molecule. This proves
conclusively that sequential sensitization produced a co-
adsorbed monolayer at the surface and that the relative
coverage can be varied by changing the sensitization
conditions, such as binding time, as indicated in the table of
Figure 8(d).
electrostatic shifts in energy levels of
a co-bound
chromophore, even when the dipole-containing compound
itself is not terminated with a chromophore.
Conclusions
In this work the binding of (Aib) helical peptides terminated
with either COOH or Ipa anchor groups to nanoparticle
anatase TiO2 films and single crystal rutile TiO2 surfaces
was explored. In both cases, molecules with the Ipa anchor
group exhibited stronger binding. In addition, when a
ZnTPP-derivative and the Ipa-terminated peptide are co-
bound on the TiO2(110) surface, the strong molecular
dipole of the peptide causes the HOMO level of the ZnTPP
unit to shift by 300 meV to higher binding energies. The
direction and magnitude of the shift was very close to that
calculated from a parallel plate capacitor model based on
the surface density of molecules and the calculated peptide
dipole. These results show that using a dipole layer is a
viable method to shift, by a predictable amount and
direction, molecular energy levels with respect to the band
edges of the substrate. This approach may be applied in
fields where it is important to control the energy alignment
of chromophores on semiconductors. Although using a
mixed layer limits the number of adsorbed chromophores,
as the available area is shared with the molecules producing
the dipole layer, this approach is synthetically practical and
versatile. Also, the dipole layer method can, in principle, be
extended to chromophores with a built-in dipole unit to
produce larger energy shifts, and to more useful and
technologically-relevant nanostructured substrates.
It should be noted that handedness of the peptide helices
could influence the monolayer properties. Since the amino
acid Aib is achiral, Z-(Aib)6-Ipa is a mixture of both left- and
right-handed helices, and the Z-(Aib)6-Ipa/TiO2 monolayer
is stereo-mixed. Although the influence of helix handedness
has not been probed in this work, it has been reported that
stereo-mixed peptide SAMs exhibit different properties
than enantiopure SAMs, and that shape complementarity of
opposite-handed helices may lead to stronger
association.51-53 A second effect that may influence packing,
in our case, is the forced parallel alignment of the bound
helix dipoles following binding.
Figure 8(e) schematically illustrates a co-adsorbed layer
with the molecules drawn to scale. The peptide molecules
should establish a dipole layer (indicated by the green
region) that will shift the energy of the frontier orbitals of
the ZnTPP macrocycle, residing above this region. The
energy shift was estimated using a parallel plate capacitor
model, in which the charge density (n) is related to the
molecular dipole density at the surface, and the component
푝
of molecular dipole normal to the surface ( ⊥ ). Using a
molecular footprint slightly larger than the actual
dimensions of Z-(Aib)6-Ipa to account for inter-molecular
spacing of ~ (0.7+0.4)2 nm2 and the calculated dipole
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6H), 1.36 (s, 6H), 1.34 (s, 6H), 1.32 (s, 6H). 13C NMR (151
Experimental section
Synthesis
For the synthesis and spectral characterization of Z-(Aib)6-
COOtBu and 6 see Supporting Information.
1
2
3
4
5
6
7
8
MHz, CD3OD) δ 178.10, 177.86, 177.40, 177.22, 177.04,
176.99, 168.77, 157.93, 141.58, 138.71, 135.26, 134.23,
133.61, 132.60, 132.36, 130.58, 129.59, 129.05, 128.60,
67.64, 58.40, 58.30, 58.17, 58.15, 58.05, 57.83, 57.81, 57.78,
57.63, 57.53, 57.02, 43.63, 26.01, 25.53, 25.22. HRMS (ESI):
calculated for C41H57O12N7 + Na, 862.3957 [M+ Na]+; found
962.4023 [M+ Na]+
Z-(Aib)6-COOH (4): To a stirring solution of Z-(Aib)6-
COOtBu (1.4 g, 2.0 mmol) in anhydrous CH2Cl2 (20 mL), was
added trifluoroacetic acid (TFA) (20 mL) and the reaction
mixture was stirred under nitrogen at r.t. for 1.5 h. The
solvent was removed in vacuo at r.t. and remaining traces of
TFA were removed by adding anhydrous diethyl ether (~10
mL) and removing the solvent in vacuo a few times. The
product was precipitated from diethyl ether to give Z-
Electron spectroscopic characterization of neat molecular
films and sensitized TiO2(110) single crystal surfaces.
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The x-ray photoelectron spectroscopy (XPS) and ultraviolet
photoelectron spectroscopy (UPS) spectra of Z-(Aib)6-
COOH (4) or Z-(Aib)6-Ipa (5) were measured either from
neat molecular films or after solution sensitization of TiO2
single crystals. Neat molecular films were prepared by
placing several drops of an acetonitrile solution of 4 or 5
1
(Aib)6-COOH as a white solid in 89% yield (1.2 g). H NMR
(DMSO)  8.25 (brs, 1H), 7.83 (brs, 1H), 7.67 (brs, 1H), 7.55
(brs, 1H), 7.39-7.29 (m, 6H), 7.24 (brs, 1H), 5.09 (s, 2H),
1.33-1.32 (m, 24H), 1.27 (s, 6H), 1.26 (s, 6H).13C NMR
(DMSO) δ 175.57, 175.14, 175.07, 174.90, 173.44, 155.81,
137.15, 128.35, 127.74, 127.27, 65.56, 55.99, 55.93, 55.87,
55.76, 55.69, 54.58, 25.03, 24.89, 24.69, 24.68, 24.38. ESI-
MS: [M+Na]+ calculated: 685.3531; found: 685.3511.
onto
a polycrystalline copper sample holder and
evaporating the solvent. The thickness of these film was
typically ~ 7 nm, which was thick enough so that the spectra
were intrinsic to the target molecule (i.e., not affected by
interaction with the underlying Cu surface) but thin enough
to minimize sample charging. To measure XPS and UPS
spectra from 4 or 5 bound to the TiO2(110) surface, single
crystals were first cleaned in ultrahigh vacuum (UHV),
removed from the vacuum chamber (briefly exposed to the
ambient) and dipped into an acetonitrile solution
containing the target molecule for eight to ten hours. The
sample was then rinsed with acetonitrile in order to remove
physisorbed molecules before re-introduction to the
photoemission chamber. This method of sensitization was
Z-(Aib)6-IpatBu:
A flame dried, two neck r.b. flask
equipped with magnetic stirrer was charged under nitrogen
with Z-Aib6-COOH (145 mg, 0.218 mmol), EDC·HCl (63 mg,
0.33 mmol), HOAT (45 mg, 0.33 mmol), and 4-methyl
morpholine (36 μL, 0.33 mmol) in a N2 filled glove box,
sealed, then transferred to a Schlenk line. Anhydrous DCM
(5 mL) was added and the mixture was stirred for 20 min at
r.t. Di-tert-butyl 5-(aminomethyl)isophthalate (6) (67 mg,
0.22 mmol) was dissolved in CH2Cl2 (1 mL x 3) and slowly
injected into reaction flask, and the mixture was stirred for
3.5 days under N2. The mixture was concentrated in vacuo,
diluted with EtOAc, then washed sequentially with sat.
NH4Cl aq. (~10 mL), sat. NaHCO3 aq. (~10 mL x 2),
deionized H2O, and brine. The organic layer was dried with
Na2SO4, filtered and concentrated in vacuo. Column
chromatography (silica gel, hexane: ethyl acetate = 20:1)
afforded the product as a white solid (151 mg, 73%). 1H
NMR (600 MHz, Chloroform-d) δ 8.42 (s, 1H), 8.07 – 8.05
(m, 3H), 7.62 (s, 1H), 7.45 (s, 2H), 7.44 (s, 1H), 7.38 – 7.34
(m, 5H), 6.35 (s, 1H), 5.29 (s, 1H), 5.11 (s, 2H), 4.58 (s, 1H),
1.64 (s, 6H), 1.57 (s, 18H), 1.47 (s, 6H), 1.45 (s, 6H), 1.41 (s,
6H), 1.30 (s, 6H), 1.26 (s, 6H). HRMS (ESI): calculated for
C49H73O12N7 + Na, 974.5209 [M+ Na]+ ; found 974.5299 [M+
Na]+
tested successfully with other compounds32, 54-55
Electron spectroscopic measurements were performed in
an ESCALAB 250xi, housing XPS and UPS.
.
A
monochromatized Al-Kα source (1486.6 eV) was used to
measure core levels and VB spectra with an experimental
resolution of 0.6 eV. A He plasma source (He I (21.2 eV) and
He II (40.8 eV)) was used to measure the VB spectra and the
secondary electron cutoffs with a resolution of 0.1 eV.
Spectra are referenced with respect to the Fermi level of a
metallic surface in contact with the measured samples.
Electronic structure calculations were performed to assist
in understanding the origin of the spectral features
observed in XPS and UPS. The calculations employed the
GAMESS(US) software package56 using Becke3-Lee-Yang-
Par (B3LYP) three parameter DFT theory57-58. Geometries
of local minima on the potential energy surface were
calculated with a 6-31G(d,p) basis set59. In order to take into
account both the atomic origin of different orbitals and their
anticipated contribution to the photoemission spectra, the
partial density of states (PDOS) was calculated using a
Löwdin60 population analysis scheme, and was
subsequently weighted by appropriate photoemission
Z-(Aib)6-Ipa (5): A flame dried, two neck r.b. flask
equipped with magnetic stirrer was charged under nitrogen
with a solution of CH2Cl2 (2 mL) and Z-(Aib)6-IpatBu (148
mg, 0.155 mmol) under N2. To the mixture was added TFA
(2 mL). After 1.5 h the mixture was concentrated in vacuo,
and the remaining traces of TFA were removed by adding
portions (~10 mL) of anhydrous diethyl ether and stirring
5 min, followed by concentration in vacuo. This process was
repeated until only dry residue is left (in our case ~ 6
times). Finally, the residue was dissolved in diethyl ether
(10 mL) and stored at -20 ºC overnight. The supernatant
ether was removed with a pipette, and the remaining solid
was dried in vacuo to afford 5 as a white powder (123 mg,
94%). 1H NMR (600 MHz, Methanol-d4) δ 8.51 (s, 1H), 8.22
(s, 2H), 8.02 (s, 1H), 7.90 (s, 1H), 7.85 (s, 1H), 7.71 (s, 2H),
7.41 – 7.37 (m, 2H), 7.37 – 7.33 (m, 2H), 7.32 – 7.28 (m, 1H),
5.13 (s, 2H), 4.50 (s, 2H), 1.54 (s, 6H), 1.45 (s, 6H), 1.40 (s,
cross sections taken from the work of Yeh and Lindau61-62
.
The weighted PDOS spectra were obtained by summing the
individual weighted electronic states convoluted with a
Gaussian function having a full width at half-maxima
corresponding to the experimental resolution.
FTIR Microscopy Measurements of Solution Sensitized
TiO2 Nanostructured films
To obtain FTIR microscopy spectra and images with a
sufficiently high signal to noise ratio, measurements were
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Journal of the American Chemical Society
performed on sintered TiO2 nanoparticle films that were
ABBREVIATIONS
1
2
3
4
5
6
7
8
sensitized with a solution containing the target molecule.
The films were produced using a commercially available
TiO2 nanoparticle paste (Ti-Nanoxide, T/SP, ~18 wt%,
Solaronix, 15~20 nm, anatase). A double-polished C-
sapphire (c-Al2O3(0001) Saint-Gobain Crystals, 430 m
thick) substrate was coated with the nanoparticle paste
using the doctor blade tape casting technique and annealed
at 450 °C for 30 minutes. To sensitize the nanoparticle films,
a small volume (5 drops) of a 2 mM CH3CN solution of Z-
(Aib)6-COOH or Z-(Aib)6-Ipa was deposited onto the TiO2–
coated substrate placed flat in a glass Petri dish and then
sealed to prevent evaporation. After 1 day, the film was
removed from the Petri dish, thoroughly rinsed with neat
solvent to remove weakly bound or physisorbed molecules,
and dried under a gentle nitrogen flow. FTIR microscopy
images of the sensitized TiO2 films were collected using a
Perkin-Elmer Spotlight 300 system (Perkin Elmer Life and
Analytical Science, Inc., Waltham, MA) in the transmission
mode with an essentially linear array (16×1) of mercury-
cadmium-telluride (MCT) detector elements. Imaging size
was 100×100 μm2. Images were collected with a 6.25×6.25
m2 pixel size and 32 scans at a spectral resolution of 8 cm-
1, corresponding to 16×16 pixel images. FTIR microscopic
images were generated from FTIR spectral data using ISys
3.1 software (Malvern Instruments, UK). Image planes of
integrated band area were produced after linear baselines
were applied in spectral regions of interest. The scale bar is
color coded and selected to display the corresponding
integrated area (STD) above the detection limit.
Aib,
2-amino
isobutyric
acid;
ZnTPP,
zinc
tetraphenylporphyrin; Ipa, isophthalic acid; Z, carboxybenzyl;
DOS, density of states; PDOS partial density of states; VB,
valence band; UPS, UV photoemission spectroscopy; UHV,
ultrahigh vacuum; X-ray photoemission spectroscopy; ML,
monolayer; SAM, self-assembled monolayer
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ASSOCIATED CONTENT
Supporting Information. Synthetic procedures for Z-(Aib)6-
COOtBu and di-tert-butyl 5-(aminomethyl)isophthalate 6,
copies of spectral characterization (1H and 13C NMR, IR and MS)
of each product, UPS VB spectra of Z-(Aib)6-COOH/ TiO2(110)
and Z-(Aib)6-Ipa/ TiO2(110), NOESY of Z-(Aib)6-COOtBu,
9.
Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.;
Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.;
Meyer, T. J., Finding the Way to Solar Fuels with Dye-Sensitized
Photoelectrosynthesis Cells. J Am Chem Soc 2016, 138 (40), 13085-
13102.
1
solvent effect on the NH chemical shifts in the H NMR of Z-
10.
Uji, H.; Morita, T.; Kimura, S., Molecular direction
(Aib)6-COOtBu. This material is available free of charge via the
Internet at http://pubs.acs.org.
dependence of single-molecule conductance of a helical peptide in
molecular junction. Physical Chemistry Chemical Physics 2013, 15 (3),
757-760.
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Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.;
AUTHOR INFORMATION
Salaneck, W. R.; Brédas, J.-L., Characterization of the Interface Dipole at
Organic/ Metal Interfaces. J Am Chem Soc 2002, 124 (27), 8131-8141.
Corresponding Author
1*galoppin@newark.rutgers.edu, 2*bart@physics.rutgers.edu.
12.
Becucci, L.; Guryanov, I.; Maran, F.; Scaletti, F.; Guidelli, R., A
procedure for estimating the surface dipole potential of monolayers
adsorbed on electrodes. Soft Matter 2012, 8 (33), 8601-8607.
ACKNOWLEDGMENT
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Surface Dipoles and Electron Transfer at the Metal Oxide–Metal
Interface: 2PPE Study of Size-Selected Metal Oxide Clusters
Yang, Y.; Zhou, J.; Nakayama, M.; Nie, L.; Liu, P.; White, M. G.,
This research was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Award
DE-FG02-01ER15256. EG and YC thank Dr. Flavio Maran and
Dr. Federico Polo (University of Padova) and Dr. Laura Fabris
(Rutgers University) for discussions regarding dipole effect in
and synthesis of Aib peptides, and Dr. Richard Mendelsohn and
Dr. Carol Flach (Rutgers University) for access to FTIR imaging
instrumentation, and Dr. Thomas Emge and Ms. Katherine
Lloyd (Rutgers University) for the crystal structure of Z-(Aib)6-
COOtBu. The authors acknowledge the Office of Advanced
Research Computing (OARC) at Rutgers, The State University
of New Jersey for providing access to the Amarel cluster and
associated research computing resources that have
contributed to the results reported here.
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