Direct probing molecular twist and tilt in aromatic self-assembled monolayers

Article abstract of DOI:10.1021/ja0751882

Using a nitrile tailgroup as a spectroscopic marker, both twist and tilt of the aromatic backbones in several typical aromatic SAMs on Au(111) have been directly determined in a single experiment. Whereas the exact value of the twist angle depends on the molecular architecture, it was found to be quite noticeable in all SAMs (40-50°) and close to the respective value for aromatic bulk systems (32°). Copyright

Full text of DOI:10.1021/ja0751882

Published on Web 11/28/2007
Direct Probing Molecular Twist and Tilt in Aromatic Self-Assembled
Monolayers
Nirmalya Ballav,† Bjo¨rn Schu¨pbach,‡ Ole Dethloff,‡ Peter Feulner,§ Andreas Terfort,*,‡ and
Michael Zharnikov*,†
Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, 69120 Heidelberg, Germany, Department Chemie,
Philipps-UniVersita¨t Marburg, 35032 Marburg, Germany, and Physikdepartment E20, Technische UniVersita¨t
Mu¨nchen, D-85747 Garching, Germany
Received July 12, 2007; E-mail: michael.zharnikov@urz.uni-heidelberg.de
The emergence of molecular electronics1 and other new tech-
nologies ranging from sensor fabrication2 to chemical lithography3
has triggered significant interest in aromatic self-assembled mono-
layers (SAMs) with chalcogen headgroups.2,4 Both understanding
the properties of these systems and their applications rely on a
precise knowledge of their structure. This issue has been addressed
by spectroscopy,4-10 X-ray scattering,11 and scanning micro-
scopy.6,8,12-15 According to the results of the two latter techniques,
a variety of molecular arrangements is possible, depending on the
exact architecture of the molecular backbone, where aromatic units
are frequently combined with different tailgroups or/and aliphatic
linkers.8,6,14 In particular, for densely packed films, x3 × x3 and
x3 × 2x3 structures were observed, and herringbone packing was
assumed,6,13,14 similar to the respective bulk materials.16 However,
the proposed molecular arrangements mostly treated the SAM
constituents as being oriented upright without taking into account
their exact orientation, which is generally given by two angles (see
Figure 1), viz. the tilt angle of the molecular axis (â) and the twist
angle of the aromatic backbone with respect to the plane spanned
Figure 1. A schematic drawing of the orientation of the BP0CN molecule
in the respective SAMs. The angles R, â, and γ describe the molecular
by the surface normal and the molecular axis (γ). Unfortunately,
these angles could not be determined independently by spectro-
scopic experiments, most of which imply that the molecules are
noticeably tilted (â ) 16-37°)5-10 but fail to deliver both â and γ
values. Only for a few aromatic SAMs could both tilt and twist
angles be derived either from a complex analysis of Fourier
transform infrared spectra (FTIR)8 or from a combination of FTIR
and near-edge X-ray absorption fine structure (NEXAFS) data.7
The complexity of the above procedures and assumptions made in
the course of the data evaluation affect, however, the reliability of
the resulting values of â and γ.
In this communication, we present first direct measurements of
both tilt and twist angles for several typical aromatic SAMs, NC-
(C6H4)2-(CH2)n-SH (n ) 0-2) and NC-(C6H4)3-CH2-SH on
Au(111), abbreviated as BPnCN and TP1CN, respectively. For this
purpose, we applied NEXAFS spectroscopy. The presence of the
nitrile tailgroup in the target molecules is a crucial point since this
group possesses two mutually perpendicular π* orbitals (see Figure
1), which, due to the hybridization with the π* orbitals of the phenyl
rings, are oriented either perpendicular (π1*) or parallel (π3*) to
the ring plane.17 Further, the energies of these two orbitals are
different,17 so that the orientation of their transition dipole moments
(TDMs) can be independently derived.
orientation. The πph* orbitals of the biphenyl backbone are perpendicular
to the ring plane; the respective transition dipole moment TDMπ is shown
as a magenta arrow; π1* (green) and π3* (blue) orbitals of the CN group
are perpendicular and parallel to the ring plane, respectively. At γ ) 0,
TDMπ lies in the plane spanned by the z- and the 4,4′-axes.
tion about the twist angle (a typical value for aromatic bulk systems,
32°),16 the tilt angle â could then be calculated for a variety of
aromatic SAMs5,9,10 according to the relation7
cos(R) ) sin(â)cos(γ)
(1)
This simple approach relies, however, on the assumption for γ,
which makes the derived tilt angles not absolutely reliable, similar
to the FTIR and FTIR/NEXAFS procedures mentioned above.
In the case of CN substitution, no assumptions for γ are
necessary. The C K-edge and N K-edge NEXAFS spectra of BPnCN
(n ) 0-2) and TP1CN SAMs acquired at X-ray incidence angles
of 90 and 20° are presented in Figure 2a and 2b, respectively. The
C K-edge spectra of all films exhibit several π*, σ*, and R*
(Rydberg) resonances characteristic of phenyl rings;18 the most
prominent absorption feature is the π1* resonance at ∼285.0 eV.
The N K-edge spectra are dominated by the π1* and π3* resonances
related to the CN group17 at 398.7 and 399.6 eV, respectively. Both
C and N K-edge spectra show noticeable linear dichroism, that is,
a dependence of the absorption resonance intensity on the incidence
angle of the X-ray beam. This is a clear signature of orientational
order in the SAMs.18
Without the CN substitution, only the TDM orientation for the
π* orbitals for the phenyl rings, that is, angle R (see Figure 1), can
be determined. Knowing this angle and making a realistic assump-
The average tilt angles of all relevant π* orbitals, R, could be
directly obtained from a simple quantitative analysis of the entire
set of the NEXAFS spectra acquired at different angles of X-ray
† Universita¨t Heidelberg.
‡ Philipps-Universita¨t Marburg.
§ Technische Universita¨t Mu¨nchen.
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J. AM. CHEM. SOC. 2007, 129, 15416-15417
10.1021/ja0751882 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
twisted differently as in the molecular state (torsion), the derived
γ values represent the average over the respective values for the
individual rings. Note that the results are also not affected by a
presumable herringbone packing of the SAMs of this study.19
Further, the derived twist angles can be used to calculate
molecular tilt â on the basis of Rph, according to eq 1. The respective
values in Table 1 are very close to the molecular tilt values
calculated from eqs 1 and 3 on the basis of R1 and R3, which is a
further proof of the reliability of the results. The â values exhibit
all tendencies observed previously for aromatic SAMs, viz. a
decrease of molecular inclination with increasing length of the
aromatic backbone5 and an odd-even change of the inclination as
far as this backbone is combined with an aliphatic linker.7,9,10 There
is a disturbance related to the attachment of a nitrile group,20 but
its extent does not exceed 8-9° in terms of the average molecular
inclination, so that the derived twist angles can be considered to
be typical of non-substituted aromatic SAMs, as well, even though
the exact values can be slightly different.
In summary, we have shown that aromatic molecules in the
respective SAMs on Au(111) are not only tilted but also noticeably
twisted (40-50°), with the exact twist angle depending on molecular
architecture. We hope that these findings help to design exact
structural models for practically relevant aromatic SAMs on coinage
metal substrates.
Acknowledgment. We thank M. Grunze for the support of this
work, and the MAX-lab staff for the assistance. This work has been
supported by DFG (ZH 63/9-2) and the EC through the IA-SFS
project within the Sixth Framework Programme.
Figure 2. Carbon K-edge (a) and nitrogen K-edge (b) NEXAFS spectra of
BPnCN (n ) 0-2) and TP1CN SAMs on Au acquired at X-ray incidence
angles of 90° (thick line) and 20° (thin line, shadowed). The most prominent
absorption resonances are marked.
Table 1. Tilt and Twist Angles (°) of the BPnCN (n ) 0-2) and
TP1CN SAMs Derived from the NEXAFS Data (accuracy (3-5°)
Supporting Information Available: Synthesis, SAM preparation
and characterization and NEXAFS data analysis. This material is
available free of charge via the Internet at http://pubs.acs.org.
BP0CN BP1CN BP2CN TP1CN
tilt angle of the π1* orbital (phenyl), Rph 60.6
67.3
67.6
63.1
49.8
36.3
36.7
63.0
65.6
63.7
47.0
37.3
41.7
67.3
68.1
66.3
47.1
33.3
34.5
tilt angle of the π1* orbital (CN), R1
tilt angle of π3* orbital (CN), R3
twist angle (γ) from R1 and R3
molecular tilt (â) from R1 and R3
61.8
65.9
40.8
38.7
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cos(R3) ) sin(â)cos(π/2 - γ)
(3)
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