Self-assembly of 1-D organic semiconductor nanostructures Thuc-Quyen Nguyen,* a Richard Martel, b Mark Bushey, c Phaedon Avouris, d Autumn Carlsen, e Colin Nuckolls f and Louis Brus f Received 13th July 2006, Accepted 29th November 2006 First published as an Advance Article on the web 4th January 2007 DOI: 10.1039/b609956d This review focuses on the molecular design and self-assembly of a new class of crowded aromatics that form 1-D nanostructures via hydrogen bonding and p–p interactions. These molecules have a permanent dipole moment that sums as the subunits self assemble into molecular stacks. The assembly of these molecular stacks can be directed with electric fields. Depending on the nature of the side-chains, molecules can obtain the face-on or edge-on orientation upon the deposition onto a surface via spin cast technique. Site-selective steady state fluorescence, time-resolved fluorescence, and various types of scanning probe microscopy measurements detail the intermolecular interactions that drive the aromatic molecules to self- assemble in solution to form well-ordered columnar stacks. These nanostructures, formed in solution, vary in their number, size, and structure depending on the functional groups, solvent, and concentration used. Thus, the substituents/side-groups and the proper choice of the solvent can be used to tune the intermolecular interactions. The 1-D stacks and their aggregates can be easily transferred by solution casting, thus allowing a simple preparation of molecular nanostructures on different surfaces. 1. Introduction The self-organization of small molecules into larger functional nanostructures is a cornerstone of biological systems and is a powerful tool to create novel materials with emergent or amplified properties. 1 Recently, there is increasing interest in using elementary building blocks such as atoms and molecules to form molecular wires or 1-D nanostructures in a controlled and predictable fashion. 2 Discotic liquid crystals, 3 discovered in 1977 by Chandrasekar and coworkers, 4 are examples of such systems. This class of materials self-assembles to form arrays of columnar stacks. Individual stacks have been com- pared to molecular wires because the column’s interior con- sists of co-facially aromatic cores (conducting cores) while its exterior is surrounded by a hydrocarbon chain (an insulating layer). 5 This arrangement of the aromatic cores yields useful and interesting electronic and optic properties. 6 For tradi- tional discotics, the intermolecular interactions between the subunits are weak due to the poor electrostatic attraction between the electron rich p-surfaces. 7 Several approaches have been used to increase the affinity between the molecules within the columnar stack including metal–ligand interactions, 8 re- cognition of polymer strands, 9 electrostatic complimentarity between p-faces, 10 and hydrogen bonds. 11 Understanding self-assembly processes and the preferred molecular arrangement of functional aromatic compounds are prerequisites for achieving predictive models that take into consideration the composition of the molecular components. As an example, organic and polymeric semiconductors having better order and packing density usually exhibit better elec- trical conduction. 12 A systematic investigation on the local order properties of a given molecular design is therefore important for making and improving organic semiconducting assemblies, but a detailed investigation of the resulting struc- ture of ordered assemblies remains challenging experimentally. Scanning probe microscopy is now among the most powerful techniques to probe the arrangement at a surface with sub- nanometer resolution. Scanning tunneling spectroscopy (STM) has been used widely to study the self-assembled processes of aromatic molecules and alkyl thiols on metal surfaces 13 and molecules at the solid-liquid interface on highly oriented pyrolytic graphite (HOPG) substrates. 14 Atomic force microscopy (AFM) is commonly used to probe surface topography. Electrostatic force microscopy (EFM) is another form of scanning probe microscopy that allows the simulta- neous mapping of surface topography and electrostatic field gradients. EFM has been employed to study trapped charge in SiO 2 layers 15 and surface charge in semiconductors 16 and organic materials. 17 A combination of proximal probe techni- ques is however required in order to gain insights on the local structure of the assembly and on the specific interactions driving the assembly process. This review discusses the assembly characteristics in solu- tion and thin film of a new class of columnar discotic liquid crystal materials that is held together with hydrogen bonds and p–p interactions. 18 These aromatic molecules are com- posed of a 1-D stack of an aromatic core surrounded by a hydrocarbon sheet. The core is a hexa-substituted aromatic (1 and 2 in Fig. 1) consisting of three meta-disposed amides that a Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106. E-mail: [email protected]b De ´partement de Chimie, Universite ´ de Montre ´al, Montre ´al, Que ´bec, Canada c Scripps Research Institute, La Jolla, CA, USA d IBM Watson Research Center, Yorktown Heights, NY, USA e Physics Department, Albany University, Albany, NY, USA f Chemistry Department, Columbia University, New York, NY, USA This journal is c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1515 INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics
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Self-assembly of 1-D organic semiconductor nanostructures
Thuc-Quyen Nguyen,*aRichard Martel,
bMark Bushey,
cPhaedon Avouris,
d
Autumn Carlsen,eColin Nuckolls
fand Louis Brus
f
Received 13th July 2006, Accepted 29th November 2006
First published as an Advance Article on the web 4th January 2007
DOI: 10.1039/b609956d
This review focuses on the molecular design and self-assembly of a new class of crowded
aromatics that form 1-D nanostructures via hydrogen bonding and p–p interactions. These
molecules have a permanent dipole moment that sums as the subunits self assemble into
molecular stacks. The assembly of these molecular stacks can be directed with electric fields.
Depending on the nature of the side-chains, molecules can obtain the face-on or edge-on
orientation upon the deposition onto a surface via spin cast technique. Site-selective steady state
fluorescence, time-resolved fluorescence, and various types of scanning probe microscopy
measurements detail the intermolecular interactions that drive the aromatic molecules to self-
assemble in solution to form well-ordered columnar stacks. These nanostructures, formed in
solution, vary in their number, size, and structure depending on the functional groups, solvent,
and concentration used. Thus, the substituents/side-groups and the proper choice of the solvent
can be used to tune the intermolecular interactions. The 1-D stacks and their aggregates can be
easily transferred by solution casting, thus allowing a simple preparation of molecular
nanostructures on different surfaces.
1. Introduction
The self-organization of small molecules into larger functional
nanostructures is a cornerstone of biological systems and is a
powerful tool to create novel materials with emergent or
amplified properties.1 Recently, there is increasing interest in
using elementary building blocks such as atoms and molecules
to form molecular wires or 1-D nanostructures in a controlled
and predictable fashion.2 Discotic liquid crystals,3 discovered
in 1977 by Chandrasekar and coworkers,4 are examples of
such systems. This class of materials self-assembles to form
arrays of columnar stacks. Individual stacks have been com-
pared to molecular wires because the column’s interior con-
sists of co-facially aromatic cores (conducting cores) while its
exterior is surrounded by a hydrocarbon chain (an insulating
layer).5 This arrangement of the aromatic cores yields useful
and interesting electronic and optic properties.6 For tradi-
tional discotics, the intermolecular interactions between the
subunits are weak due to the poor electrostatic attraction
between the electron rich p-surfaces.7 Several approaches havebeen used to increase the affinity between the molecules within
the columnar stack including metal–ligand interactions,8 re-
cognition of polymer strands,9 electrostatic complimentarity
between p-faces,10 and hydrogen bonds.11
Understanding self-assembly processes and the preferred
molecular arrangement of functional aromatic compounds
are prerequisites for achieving predictive models that take into
consideration the composition of the molecular components.
As an example, organic and polymeric semiconductors having
better order and packing density usually exhibit better elec-
trical conduction.12 A systematic investigation on the local
order properties of a given molecular design is therefore
important for making and improving organic semiconducting
assemblies, but a detailed investigation of the resulting struc-
ture of ordered assemblies remains challenging experimentally.
Scanning probe microscopy is now among the most powerful
techniques to probe the arrangement at a surface with sub-
force microscopy (AFM) is commonly used to probe surface
topography. Electrostatic force microscopy (EFM) is another
form of scanning probe microscopy that allows the simulta-
neous mapping of surface topography and electrostatic field
gradients. EFM has been employed to study trapped charge in
SiO2 layers15 and surface charge in semiconductors16 and
organic materials.17 A combination of proximal probe techni-
ques is however required in order to gain insights on the local
structure of the assembly and on the specific interactions
driving the assembly process.
This review discusses the assembly characteristics in solu-
tion and thin film of a new class of columnar discotic liquid
crystal materials that is held together with hydrogen bonds
and p–p interactions.18 These aromatic molecules are com-
posed of a 1-D stack of an aromatic core surrounded by a
hydrocarbon sheet. The core is a hexa-substituted aromatic (1
and 2 in Fig. 1) consisting of three meta-disposed amides that
aDepartment of Chemistry and Biochemistry, University of California,Santa Barbara, CA 93106. E-mail: [email protected]
bDepartement de Chimie, Universite de Montreal, Montreal, Quebec,Canada
c Scripps Research Institute, La Jolla, CA, USAd IBM Watson Research Center, Yorktown Heights, NY, USAePhysics Department, Albany University, Albany, NY, USAfChemistry Department, Columbia University, New York, NY, USA
This journal is �c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1515
INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics
are flanked by substituents other than hydrogen at each of the
remaining positions. Unlike traditional discotics, these mole-
cules stack to form molecular fibers due to a synergy between
p-stacking and hydrogen bonding to produce a relatively
strong association (molecule-to–molecule cohesion) in the
stacking direction but a comparatively weaker interaction
between fibers (Fig. 1A).18 The substituent, R0 (R00 for 2a), is
a long alkyl chain to ensure the solubility in common organic
solvents whereas R0 contains an amide group, which provides
a dipole moment and hydrogen bonding that contributes to
enhance the molecular cohesion in the stacking direction (Fig.
1). The substituents in the side groups (R0 and R00) can be used
to tailor the orientation of molecules on a surface and the
intermolecular distance within a fiber through steric interac-
tions.18c Monitoring the assembly of these mesogens in mono-
layer films by scanning probe microscopy has yielded films
with two orientations. In one surface conformation a two-
dimensional sheet results that is macroscopically polar (face-
on orientation, Fig. 2A). In the other orientation on the
surface, 1-D p-stacks result that are only a few molecules wide
but microns in length (edge-on orientation, Fig. 2B). The
length of the fibers formed on surfaces depends on the solvent,
the concentration as well as the underlying substrate. In the
bulk, these materials form highly regular and well-organized
columnar assemblies.18g In solution, fluorescence spectroscopy
gave clear indication that the underlying self-assembly process
produces 1-D stacks. The length of the fibers and their number
can be varied dramatically using different solvent and con-
centration. In ultra-thin films, 1b, 1c, 2a, and 2b assembles into
dipolar columns that have their long axes and dipole moments
parallel to the surface19 that can be directed with the electric
field. The self-assembly of molecules 1 and 2 on graphite was
examined by AFM, EFM, and ultrahigh vacuum (UHV)-
STM. There are six sections below describing the self-assembly
characteristics of 1 and 2: (1) the molecular design of this new
class of discotic crystals, (2) controlling the self-assembly by
functional groups, (3) the self-assembly in solution, (4) the
effects of solvents on the aggregation/self-assembly, (5) the
effects of concentration and temperature on the self-assembly,
and (6) the effects of the surface type on the self-assembly.
2. Molecular design
Although there are other examples of benzene rings that are
held cofacially by hydrogen bonds,20 the highly substituted
nature of these subunits gives rise to new nanostructures,
Fig. 1 Crowded aromatics and their energy minimized molecular models. Side-chains and hydrogens have been removed to clarify the view (in the
model: the R0 and R00 have been removed and red = oxygen; blue = nitrogen; grey = carbon).
Fig. 2 Schematic drawings of the face-on top view (A) and edge-on side view (B) orientation of hexa-substituted aromatics on graphite substrates.
Reprinted with permission from ref. 18c. Copyright 2004 American Chemical Society.
1516 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 This journal is �c the Owner Societies 2007
unique polar properties, and unusual phase behavior. For the
molecules in Fig. 1, the design principle explored was how to
use the flanking alkoxy groups for 1 and alkynyl substituents
for 2 to force the amides out of the plane of the central
aromatic ring and into a conformation that is predisposed to
form three intermolecular hydrogen bonds.
Fig. 1 shows the energy-minimized dimeric models for both
1 and 2.18 The flanking alkoxyl groups for 1 and alkynl groups
for 2 force the amides out the plane of the central aromatic
ring and into a conformation that allows the formation of
three intermolecular hydrogen bonds. The size of the func-
tional groups determines the angle of twist for the amide out
of the aromatic ring plane and consequently modulates the
distance between adjacent benzene rings. From models, the
center-to-center distance between the benzene rings is ca. 3.8 A
for 1 and ca. 3.6 A for 2 reflecting the relative size of the
alkoxyl and the alkynyl groups. Additionally, each of the
subunits has a permanent dipole moment that is perpendicular
to the aromatic ring plane. The dipoles could sum as the
molecules stack yielding columns that have a macroscopic
dipole moment, similar to the moment that is seen for some
metallomesogens and conical liquid crystals.21 These polar
columns could be used as model systems to understanding
how polar properties emerge on the nanoscale as well as how
charges transport in 1-D nanostructures.
Because this class of discotic molecules was unknown before
the studies below were initiated, a large number of derivatives
were synthesized to establish the structure/property relation-
ships. The synthetic procedures were developed by the Nuck-
olls group. Bulk self-assemblies of 1 and 2 were studied by
X-ray diffraction. X-Ray diffraction studies of 1a show two
size: 1.5 � 1.5 mm), and (D) dodecane (scan size: 1 � 1 mm) spin-cast on graphite. Reprinted with permission from ref. 18g. Copyright 2004
American Chemical Society.
Fig. 14 Topographic images from methylene chloride spin-cast on graphite for: (A) compound 2b (scan size: 1.5 � 1.5 mm) and (B) compound 2a
(scan size: 2 � 2 mm). Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
This journal is �c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1527
chloride at 245 1C for 1 min and cooling down at 5 1C min�1,
only aggregates/fibers are observed as shown in Fig. 16. These
fibers are packed closely together with the same height as
observed in individual fibers before annealing.
Next, we investigate the effects of concentration on the way
the molecules pack in solution and on the film morphology.
Fig. 17 presents the PLE and PL spectra (normalized at the
monomer emission peak) of 2b in methylene chloride at
different concentrations, B10�4 M, B10�5 M, B10�6 M.
As the concentration increases, the aggregate band is red-
shifted and increases in the intensity. This peak is at about
340 nm at low concentration and shifted to 365 nm at higher
concentration, and it is related to the fibrous structures seen
with the AFM (Fig. 5C, 13, 16, and 18). In addition, the
number of fibers/aggregates increases with concentration, as
seen in the enhancement of the red-shifted emission (aggregate
emission at 450 nm). The PL spectra are also red shifted with
an increase in concentration, which is a signature of the
formation of aggregates in solution. This can also be seen in
the film morphology as shown in Fig. 18. At high concentra-
tion, the fibers formed are short and pack closely together.
They also form a multilayered film on the graphite substrate
(Fig. 18A and Fig. 15A). As the concentration of the solution
decreases, the fiber length increases and creates isolated stacks
of fibers, oriented according to the graphite lattice (Fig. 18B).
This can be explained using basic concepts of crystal growth,
but in this case, it is a 1-D growth process. There are several
factors that influence the numbers of fibers formed and their
length. These are the nucleation sites, the nucleation rate, and
the growth rate. Generally, the nucleation sites, which deter-
mines the number of fibers formed, increase with the concen-
tration, and this is the same for the nucleation rate.38 The
growth rate is controlled by the diffusion process, and hence,
at higher concentration or higher temperature, the probability
that the molecules encounter a nucleation site increases.38
Consequently, the monomer depletion rate is much higher
for concentrated solution than for dilute solution. Thus, there
are more fibers in concentrated solution, but they are much
shorter compared to the low concentration. Therefore, con-
centration can be used to control the length and number of the
fibers formed.
Fig. 15 PLE and PL of compound 2b in methylene chloride (MeCl2,
solid curve) and in methanol (MeOH, dashed curve) spin-cast on
quartz substrates. The spectra are normalized at the monomer peak.
The PLE spectra were collected at 360 nm and the PL spectra were
excited at 320 nm. Reprinted with permission from ref. 18g. Copyright
2004 American Chemical Society.
Fig. 16 AFM images of 2b (A, scan size: 3 � 3 mm) and 1b (B, scan
size: 2 � 2 mm) films annealing at 245 1C for 1 min and the cross
section of 1b. The height of the fiber is 1.558 nm.
1528 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 This journal is �c the Owner Societies 2007
7. The effects of the surface type on the
self-assembly
In this section, we examine how the surface polarity influences
the self-assembled nanostructures.
Spin-casting 1b from CH2Cl2 solution (B10�6 M) onto the
basal plane of highly ordered pyrolytic graphite (HOPG) or
onto a silicon wafer having a 200 nm thermally grown silicon
oxide layer produces very thin, elongated nanostructures. A
typical atomic force micrograph (AFM) is shown in Fig. 19A
on HOPG. The fibers are only one-molecule high, a few
molecules in width, but microns in length. On graphite, the
molecules form straight fibers in registry with the graphite
lattice. When using a cleaned silicon wafer (200 nm silicon
oxide) as the substrate, the fibers were much longer and tended
to bundle together to form ropes. This aggregation may arise
from the mismatch between the polar and hydrophilic silicon
oxide and the hydrophobic columns trying to minimize con-
tact. Nonetheless, we found conditions to form very long ropes
of columns that are about 50 nm in diameter. They are shown
in the micrograph Fig. 19B where each fiber is about 25
molecules wide. In contrast to the regular arrangement of
fibers on HOPG that arrange along the graphite lattice, on
silicon oxide these ropes orient randomly on the glassy,
amorphous silicon oxide layer.
The interaction between the molecule and the surface is
important for guiding the patterns formed by the film, such as
the orientation of the fibers and their size or number on
the surface. For example, compound 1b deposited on graphite
at low concentration (B10�6 M) gives fibers that are
spread out to form a monolayer on graphite, due to the strong
van der Waals interaction between graphite and the molecules.
The fibers are straight, packed parallel and in registry with
the graphite lattice at either 601 or 1201 angles. On Si/SiO2
(see Fig. 19B), the fibers have the tendency to form
bundles (7–50 nm in diameter) and orient randomly. This is
a result of a less favorable interaction between this hydrophilic
substrate and the hydrophobic aggregates. Thus, the fibers
bundle up together to minimize the interactions with the
surface and optimize the van der Waals interaction among
the fibers.
It is possible to use the electric field between vertical
(between two ITO-coated glass plates) and lateral electrodes
(between two gold electrodes patterned on silicon wafers) to
direct the self-assembly of these 1-D nanostructures. For the
lateral electrodes, there are several physical parameters influ-
enced the alignment of the molecules under an external electric
field: the width of the electrodes, the height/thickness of the
electrode, the gap size, the strength of the electric field, and the
number of molecules between the gap area.
Fig. 17 (A) PLE and (B) PL spectra of compound 2b in methylene
chloride at different concentration: 9.45 � 10�4 M (dot-dashed curve),
9.45 � 10�5 M (dashed curve), 4.74 � 10�5 M (dot curve), and 4.74 �10�6 M (solid curve). The PLE (collected at 420 nm) and PL (excited at
270 nm) spectra are normalized at the monomer peak. Reprinted with
permission from ref. 18g. Copyright 2004 American Chemical Society.
Fig. 18 AFM images of compound 2b in methylene chloride at different concentration spin-cast on graphite: (A) 9.45 � 10�5 M (scan size: 1.5 �1.5 mm) and (B) 9.45 � 10�6 M (scan size: 3 � 3 mm). Reprinted with permission from ref. 18g. Copyright 2004 American Chemical Society.
This journal is �c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1529
8. Summary and future work
This review details the design, synthesis and the self-assembly
in solution and in thin film of hexasubstituted aromatics that
form columnar nanostructures via hydrogen bonds and p–pinteractions. The structures of 1 and 2 are unique synthetic
targets due to the highly substituted central benzene ring. The
functional side groups can be used as a tool to control the
molecular packing, orientation, and intermolecular spacing of
overcrowded aromatics in thin films. When using compact side
groups as in molecule 1b, the molecules stack to form fiber and
these fibers bundle together to form helices. Using compact
side groups and a larger p-core as in 2b leads to the formation
of straight and isolated fibers with a reducing of the inter-
molecular spacing within a columnar stack. The assembly of
these subunits produces polar stacks in solution and can be
transferred onto surfaces. Spin casting films of 1a produces
polar monolayers (face-on orientation) whereas in 1b, 1c, 2a,
and 2b, molecules self-assemble forming long fibers that can be
visualized with AFM, EFM, and STM. The exact mechanisms
of molecular packing motifs in 1b and 1c are unclear at the
moment and appears to be quite complex. Thus, modeling is
needed to further understand these molecular packing motifs.
The processing conditions such as solvent, concentration, and
type of substrate used provide control on the size, orientation
and number of the aggregates formed. Finally, it is possible to
align 1-D organic nanostructures using DC voltage, and that
the alignment depends on the strength of the electric field, the
thickness of the electrodes, the gap size, and the number of
molecules within the gap area. Using this method with more
electroactive molecules represents an untested method to
assemble and wire 1-D organic semiconductors in devices.
By the proper tuning of the chemical functionality, these
columnar structures can be used as model systems for inves-
tigating the charge transport in 1D organic semiconducting
nanostructures.
Acknowledgements
This work was supported by the Office of Basic Energy
Sciences, US D.O.E. (#DE-FG02-01ER15264) and US
National Science Foundation CAREER award (#DMR-02-
37860). TQN thanks the UCSB setup fund for financial
support. We thank Dr Arnold Tamayo for the energy mini-
mized molecular models of compounds 1a and 1b shown
in Fig. 3.
References
1 (a) J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim,1995; (b) J.-M. Lehn, Struct. Bonding, 2000, 96, 3–29; (c) G. M.Whitesides and B. Grzybowski, Science, 2002, 295, 2418–2421; (d)D. S. Lawrence, T. Jiang and M. Levett, Chem. Rev., 1995, 95,2229–2260; (e) M. M. Conn and J. Rebek, Chem. Rev., 1997, 97,1647–1668; (f) D. Philp and J. F. Stoddart,Angew. Chem., Int. Ed.Engl., 1996, 35, 1155–1196; (g) M. Muthukumar, C. K. Ober andE. L. Thomas, Science, 1997, 277, 1225–1232; (h) L. J. Prins, D.N. Reinhoudt and P. Timmerman, Angew. Chem., Int. Ed., 2001,40, 2382–2426.
2 (a) J. Wu, A. C. Grimsdale and K. L. Muellen, J. Mater. Chem.,2005, 15, 41–52; (b) T. Sekiguchi, S. Yoshida and K. M. Itoh,Phys. Rev. Lett., 2005, 95, 106101–106104; (c) G. H. Woehrle, M.G. Warner and J. E. Hutchison, Langmuir, 2004, 20, 5982–5988;(d) A. Gesquiere, M. M. S. Abdel-Mottaleb, S. De Feyter, F. C.De Schryver, F. Schoonbeek, J. van Esch, R. M. Kellogg, B. L.Feringa, A. Calderone, R. Lazzaroni and J.-L. Bredas, Langmuir,2000, 16, 10385–10391; (e) J. V. Barth, J. Weckesser, C. Cai, P.Gunter, L. Burgi, O. Jeandupeux and K. Kern, Angew. Chem.,Int. Ed., 2000, 39, 1230–1234; (f) B. Wen, C. Liu and Y. Liu,Chem. Lett., 2005, 34, 396–397; (g) B. W. Messmore, J. F. Hulvat,E. D. Sone and S. I. Stupp, J. Am. Chem. Soc., 2004, 126,14452–14458; (h) A. Datar, R. Oitker and L. Zang, Chem.Commun., 2006, 15, 1649–1651; (i) S. Lin, M. Li, E. Dujardin,C. Girard and S. Mann, Adv. Mater., 2005, 17, 2553–2559; (j) Z.Deng, Y. Tian, S.-H. Lee, A. E. Ribbe and C. Mao, Angew.Chem., Int. Ed., 2005, 44, 3582–3585; (k) J. Bae, J.-H. Choi, Y.-S.Yoo, N.-K. Oh, B.-S. Kim and M. Lee, J. Am. Chem. Soc., 2005,127, 9668–9669.
3 (a) S. Chandrasekhar and G. S. Ranganath, Discotic liquidcrystals, Rep. Prog. Phys., 1990, 53, 57–84; (b) C. Destrade, P.Foucher, H. Gasparoux, H. T. Nguyen, A. M. Levelut and J.Malthete, Disk-like mesogen polymorphism, Mol. Cryst. Liq.Cryst., 1984, 106, 121–46; (c) S. Chandrasekhar, S. Prasad andS. Krishna, ‘‘Recent developments in discotic liquid crystals’’,Contemp. Phys., 1999, 40, 237–245.
4 (a) S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Liquidcrystals of disc-like molecules, Pramana, 1977, 9, 471–80; (b) S.Chandrasekhar, B. K. Sadashiva, K. A. Suresh, N. V. Madhu-sudana, S. Kumar, R. Shashidhar and G. Venkatesh, Disk-likemesogens, J. Phys. Colloq., 1979, 3, 120–4.
5 S. Chandrasekhar, Columnar, discotic nematic and lamellarliquid crystals their structures and physical properties, Handb.Liq. Cryst., 1998, 2B, 749–780.
Fig. 19 Topographic images of 1b on HOPG (A, scan size: 2 � 2 mm) and on a cleaned silicon wafer with 200 nm of thermally grown silicon
dioxide (B, scan size: 3 � 3 mm).
1530 | Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 This journal is �c the Owner Societies 2007
6 (a) N. Boden and B. Movaghar, Handb. Liq. Cryst. Res., 1998,2B, 781–798; (b) N. Boden, R. J. Bushby, J. Clements and B.Movaghar, J. Mater. Chem., 1999, 9, 2081–2086; (c) A. M. Van deCraats, J. M. Warman, A. FechtenkKtter, J. D. Brand, M. A.Harbison and K. Mullen, Adv. Mater., 1999, 11, 1469–1472; (d)V. Percec, M. Glodde, T. K. Bera, Y. Miura, I. Shiyanovskaya,K. D. Singer, V. S. Balagurusamy, P. A. Heiney, I. Schnell, A.Rapp, H.-W. Spiess, S. D. Hudson and H. Duan, Nature, 2002,419, 384–387; (e) V. Percec, G. Johansson, J. Heck, G. Ungar andS. V. Batty, J. Chem. Soc., Perkin Trans. 1, 1993, 1411–1420; (f) J.Simon and C. Sirlin, Pure Appl. Chem., 1989, 61, 1625–1629; (g)O. E. Sielcken, L. A. van de Kuil, W. Drenth, J. Schoonman andR. J. M. Nolte, J. Am. Chem. Soc., 1990, 112, 3086–3093; (h) T.Christ, B. Gluesen, A. Greiner, A. Kettner, R. Sander, V.Stuempflen, V. Tsukruk and J. H. Wendorff, Adv. Mater., 1997,9, 48–52; (i) D. Adam, P. Schuhmacher, J. Simmerer, L.HNnssling, K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf andD. Haarer, Nature, 1994, 371, 141–143; (j) L. Schmidt-Mende, A.FechtenkKtter, K. Mullen, E. Moons, R. H. Friend and J. D.MacKenzie, Science, 2001, 293, 1119–1122.
7 C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112,5525–5534.
8 (a) Metallomesogens, ed. J. L. Serrano, VCH, New York, 1996;(b) J. Simon, P. Bassoul, in Phthalocyanines Properties andApplications, Vol. 2, ed. C. C. Leznoff and A. B. P. Lever,VCH, New York, 1989, ch. 6; (c) A. G. Serrette, C. K. Lai andT. M. Swager, Chem. Mater., 1994, 6, 2252–2268.
9 V. Percec, Handb. Liq. Cryst. Res., 1997, 2B, 259–346.10 (a) M. Muller, C. Kubel and K. Mullen, Chem.–Eur. J., 1998, 4,
2099–2109; (b) H. Bengs, M. Ebert, O. Karthaus, B. Kohne, K.Praefcke, H. Ringsdorf, J. H. Wendorff and R. Wuestefeld, Adv.Mater., 1990, 2, 141–144; (c) M. Weck, A. R. Dunn, K. Matsu-moto, G. W. Coates, E. B. Lobkovsky and R. H. Grubbs, Angew.Chem., Int. Ed., 1999, 38, 2741–2745.
11 Hydrogen bonds used to stabilize p stacks: (a) Y. Matsunaga, N.Miyajima, Y. Nakayasu, S. Sakai and M. Yonenaga, Bull.Chem.Soc. Jpn., 1988, 61, 207–210; (b) L. Brunsveld, H. Zhang, M.Glasbeek, J. A. J. M. Vekemans and E. W. Meijer, J. Am. Chem.Soc., 2000, 122, 6175–6182, and references therein; (c) Y. Yasuda,E. Iishi, H. Inada and Y. Shirota, Chem. Lett., 1996, 7, 575–576;(d) M. P. Lightfoot, F. S. Mair, R. G. Pritchard and J. E. Warren,Chem. Commun., 1999, 19, 1945–1946; (e) E. Fan, J. Yang, S. J.Geib, T. C. Stoner, M. D. Hopkins and A. D. Hamilton, J. Chem.Soc., Chem. Commun., 1995, 12, 1251–1252; (f) D. Ranganathan,S. Kurur, R. Gilardi and I. L. Karle, Biopolymers, 2000, 54,289–295; (g) C. M. Paleos and D. Tsiourvas, Angew. Chem., Int.Ed. Engl., 1995, 34, 1696–1711, and references therein; (h) M. J.Brienne, J. Gabard, J.-M. Lehn and I. Stibor, J. Chem. Soc.,Chem. Commun., 1989, 24, 1868–1870; (i) D. Goldmann, R.Dietel, D. Janietz, C. Schmidt and J. H. Wendorff, Liq. Cryst.,1998, 24, 407–411; (j) G. Ungar, D. Abramic, V. Percec and J. A.Heck, Liq. Cryst., 1996, 21, 73–86; (k) V. Percec, C.-H. Ahn, T.K. Bera, G. Ungar and D. J. P. Yeardley, Chem.–Eur. J., 1999, 5,1070–1083; (l) J. Malthete, A. M. Levelut and L. Liebert, Adv.Mater., 1992, 4, 37–41; (m) D. Pucci, M. Veber and J. Malthete,Liq. Cryst., 1996, 21, 153–155.
12 (a) T.-Q. Nguyen, J. Wu, V. Doan, S. H. Tolbert and B. J.Schwartz, Science, 2000, 288, 652–656; (b) M. Funahashi and J.-I.Hanna, Phys. Rev. Lett., 1997, 78, 2184–2187. Polymer assembly;(c) L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons,R. H. Friend and J. D. MacKenzie, Science, 2001, 293,1119–1122; (d) H. Sirringhaus, P. J. Brown, R. H. Friend, M.M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H.Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig and D. M. deLeeuw, Nature, 1999, 401, 685–688. Discotic assembly; (e) V.Percec, M. Glodde, T. K. Bera, Y. Miura, I. Shiyanovskaya, K.D. Singer, V. S. K. Balagurusamy, P. A. Heiney, I. Schnell, A.Rapp, H. W. Spiess, S. D. Hudson and H. Duan, Nature, 2002,419, 384–387.
13 (a) L. Mueller-Meskamp, B. Luessem, S. Karthaeuser, S. Prikho-dovski, M. Homberger, U. Simon and R. Waser, Phys. StatusSolidi A, 2006, 203, 1448–1452; (b) A. Ogunrinde, K. W. Hippsand L. Scudiero, Langmuir, 2006, 22, 5697–5701; (c) H. Inoue, G.Yoshikawa and K. Saiki, Jpn. J. Appl. Phys. P. 1, 2006, 45,1794–1796; (d) B. Luessem, L. Mueller-Meskamp, S. Karthaeu-
ser, R. Waser, M. Homberger and U. Simon, Langmuir, 2006, 22,3021–3027; (e) A. A. Dameron, L. F. Charles and P. S. Weiss, J.Am. Chem. Soc., 2005, 127, 8697–8704; (f) G. M. Florio, T. L.Werblowsky, T. Mueller, B. J. Berne and G. W. Flynn, J. Phys.Chem. B, 2005, 109, 4520–4532; (g) V. Arima, E. Fabiano, R. I. R.Blyth, F. Della Sala, F. Matino, J. Thompson, R. Cingolani andR. Rinaldi, J. Am. Chem. Soc., 2004, 126, 16951–16958; (h) E.Marx, K. Walzer, R. J. Less, P. R. Raithby, K. Stokbro and N. C.Greenham,Org. Electronics, 2004, 5, 315–320; (i) A. A. Dameron,J. W. Ciszek, J. M. Tour and P. S. Weiss, J. Phys. Chem. B, 2004,108, 16761–16767; (j) G. V. Nazin, S. W. Wu and W. Ho, Proc.Natl. Acad. Sci. U. S. A., 2005, 102, 8832–8837.
14 (a) W. Mamdouh, H. Uji-i, J. S. Ladislaw, A. E. Dulcey, V.Percec, F. C. De Schryver and S. De Feyter, J. Am. Chem. Soc.,2006, 128, 317–325; (b) H. Fukumura, H. D. I-i, H. Uji-I, S.Nishio, H. Sakai and A. Ohuchi, ChemPhysChem, 2005, 6,2383–2388; (c) S. De Feyter and F. C. De Schryver, J. Phys.Chem. B, 2005, 109, 4290–4302; (d) J. A. A. W. Elemans, M. C.Lensen, J. W. Gerritsen, H. van Kempen, S. Speller, R. J. M.Nolte and A. E. Rowan, Adv. Mater., 2003, 15, 2070–2073; (e) M.D. Watson, F. Jaeckel, N. Severin, J. P. Rabe and K. Muellen, J.Am. Chem. Soc., 2004, 126, 1402–1407; (f) D. G. Yablon, H.Fang, L. C. Giancarlo and G. W. Flynn, ACS Symp. Ser., 2002,810, 205–224.
15 (a) G. H. Buh, H. J. Chung and Y. Kuk, Appl. Phys. Lett., 2001,79, 2010; (b) C. Y. Ng, T. P. Chen, H. W. Lau, Y. Liu, M. S. Tse,O. K. Tan and V. S. W. Lim, Appl. Phys. Lett., 2004, 85, 2941; (c)H. Dongmoa, J. F. Carlottia, G. Bruguierb, C. Guascha, J.Bonneta and J. Gasiot, Appl. Surf. Sci., 2003, 212–213, 607.
16 (a) T. Uchihashi, T. Okusako, T. Tsuyuguchi, Y. Sugawara, M.Igarashi, R. Kaneko and S. Morita, J. Appl. Phys. Pt. I, 1994, 33,5573–5576; (b) T. D. Krauss and L. E. Brus, Phys. Rev. Lett.,1999, 83, 4840–4843; (c) T. D. Krauss, S. O’Brien and L. E. Brus,J. Phys. Chem. B, 2001, 105, 1725; (d) O. Cherniavskaya, L. Chenand L. Brus, J. Phys. Chem. B, 2004, 108, 4946; (e) P. M. Bridger,Z. Z. Bandic, E. C. Piquette and T. C. McGill, Appl. Phys. Lett.,1999, 74, 3522.
17 (a) H. Yamada, T. Fukuma, K. Umeda, K. Kobayashi and K.Matsushige, Appl. Surf. Sci., 2002, 188, 391–398; (b) J. N. Barisci,R. Stella, G. M. Spinks and G. G. Wallace, Electrochim. Acta,2000, 46, 519; (c) H. Takano and M. D. Porter, J. Am. Chem.Soc., 2001, 123, 8412; (d) S. Howell, D. Kuila, B. Kasibhatla, C.P. Kubiak, D. Janes and R. Reifenberger, Langmuir, 2002, 18,5120; (e) J. W. Hong, Sang-il Park and Z. G. Khim, Rev. Sci.Instrum., 1999, 70, 1375.
18 (a) M. L. Bushey, A. Hwang, P. W. Stephens and C. Nuckolls, J.Am. Chem. Soc., 2001, 123, 8157–8158; (b) M. L. Bushey, A.Hwang, P. W. Stephens and C. Nuckolls, Angew. Chem., Int. Ed.,2002, 41, 2828–2831; (c) T.-Q. Nguyen, M. L. Bushey, L. E. Brusand C. Nuckolls, J. Am. Chem. Soc., 2002, 124, 15051–15054; (d)W. Zhang, D. Horoszewski, J. Decatur and C. Nuckolls, J. Am.Chem. Soc., 2003, 125, 4870–4873; (e) M. L. Bushey, T.-Q.Nguyen and C. Nuckolls, J. Am. Chem. Soc., 2003, 125,8264–8269; (f) M. L. Bushey, T.-Q. Nguyen, W. Zhang, D.Horoszewski and C. Nuckolls, Angew. Chem., Int. Ed., 2004,43, 5446–5453; (g) T.-Q. Nguyen, R. Martel, P. Avouris, M.Bushey, C. Nuckolls and L. E. Brus, J. Am. Chem. Soc., 2004,126, 5234–5242.
19 T.-Q. Nguyen, R. Martel, A. Carlsen, P. Avouris, M. Bushey andC. Nuckolls, Nano. Lett., unpublished work.
20 Hydrogen bonds used to stabilize p stacks: (a) Y. Matsunaga, N.Miyajima, Y. Nakayasu, S. Sakai and M. Yonenaga, Bull. Chem.Soc. Jpn., 1988, 61, 207–210; (b) L. Brunsveld, H. Zhang, M.Glasbeek, J. A. J. M. Vekemans and E. W. Meijer, J. Am. Chem.Soc., 2000, 122, 6175–6182, and references therein; (c) Y. Yasuda,E. Iishi, H. Inada and Y. Shirota, Chem. Lett., 1996, 7, 575–576;(d) M. P. Lightfoot, F. S. Mair, R. G. Pritchard and J. E. Warren,Chem. Commun., 1999, 19, 1945–1946; (e) E. Fan, J. Yang, S. J.Geib, T. C. Stoner, M. D. Hopkins and A. D. Hamilton, J. Chem.Soc., Chem. Commun., 1995, 12, 1251–1252; (f) D. Ranganathan,S. Kurur, R. Gilardi and I. L. Karle, Biopolymers, 2000, 54,289–295; (g) C. M. Paleos and D. Tsiourvas, Angew. Chem., Int.Ed. Engl., 1995, 34, 1696–1711, and references therein; (h) M. J.Brienne, J. Gabard, J.-M. Lehn and I. Stibor, J. Chem. Soc.,Chem. Commun., 1989, 24, 1868–1870; (i) D. Goldmann,
This journal is �c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 1515–1532 | 1531
R. Dietel, D. Janietz, C. Schmidt and J. H. Wendorff, Liq. Cryst.,1998, 24, 407–411; (j) G. Ungar, D. Abramic, V. Percec and J. A.Heck, Liq. Cryst., 1996, 21, 73–86; (k) V. Percec, C.-H. Ahn, T.K. Bera, G. Ungar and D. J. P. Yeardley, Chem.–Eur. J., 1999, 5,1070–1083; (l) J. Malthete, A. M. Levelut and L. Liebert, Adv.Mater., 1992, 4, 37–41; (m) D. Pucci, M. Veber and J. Malthete,Liq. Cryst., 1996, 21, 153–155.
21 (a) D. Kilian, D. Knawby, M. A. Athanassopoulou, S. T.Trzaska, T. M. Swager, S. Wrobel and W. Haase, Liq. Cryst.,2000, 27, 509–521; (b) H. Zimmermann, R. Poupko, Z. Luz and J.Billard, Z. Naturforsch., A: Phys. Phys. Chem. Kosmophys., 1985,40, 149–160; (c) J. Malthete and A. Collet, J. Am. Chem. Soc.,1987, 109, 7544–7545.
22 (a) A. Rochefort and R. Martel, Ph. Avouris. Nano Lett., 2002, 2,877–880; (b) E. Baillard, S. Hamel and A. Rochefort, OrganicElectronics, 2006, 7, 144–154.
23 A. M. Van de Craats and J. M. Warman, Adv. Mater., 2001, 13,130–133.
24 (a) B. D. Terris, J. E. Stern, D. Rugar and H. J. Mamin, Phys.Rev. Lett., 1989, 63, 2669; (b) T. Uchihashi, T. Okusako, T.Tsuyuguchi, Y. Sugawara, M. Igarashi, R. Kaneko and S.Morita, Jrn. J. Appl. Phys. Pt. I, 1994, 33, 5573; (c) M. P. deSanto, R. Barberi, L. M. Blinov, S. P. Palto and S. G. Yudin,Mol. Mater., 2000, 12, 329–345.
25 (a) T. D. Krauss, S. O’Brien and L. E. Brus, J. Phys. Chem. B,2001, 105, 1725; (b) J. Jiang, T. D. Krauss and L. E. Brus, J. Phys.Chem. B, 2000, 104, 11936; (c) O. Cherniavskaya, L. Chen, V.Chen and L. E. Brus, J. Phys. Chem. B, 2004, 108, 4946–4961; (d)C. H. Ben-Porat, O. Cherniavskaya, L. E. Brus, K.-S. Cho and C.B. Murray, J. Phys. Chem. A, 2004, 108, 7814–7819; (e) L. Chen,R. Ludeke, X. Cui, A. G. Schrott, C. R. Kagan and L. E. Brus, J.Phys. Chem., 2005, 109, 1834–1838; (f) L. Chen, O. Cherniavs-kaya, A. Shalek and L. E. Brus, Nano Lett., 2005, 5, 2241–2245.
26 (a) J. Kong and H. Dai, J. Chem. Phys. B, 2001, 105, 2890–2893; (b)M. Shim, A. Javey, N. W. S. Kam and H. Dai, J. Am. Chem. Soc.,2001, 123, 11512; (c) K. Bradley, M. Briman, A. Star and G.Gruner,Nano Lett., 2004, 4, 253–256; (d) J. Liu, M. J. Casavant, M.Cox, D. A.Walters, P. Boul, W. Lu, A. J. Rimberg, K. A. Smith, G.T. Colbert and R. E. Smalley, Chem. Phys. Lett., 1999, 303,125–129; (e) M. J. O’Connell, P. Boul, L. M. Ericson, C. Huffman,Y. Wang, E. Haroz, C. Kuper, J. Tour, K. D. Ausman and R. E.Smalley, Chem. Phys. Lett., 2001, 332, 461–466; (f) M. Sano, A.Kamino, J. Okamura and S. Shinkai, Nano Lett., 2002, 2, 531–533.
27 Surface poling has been observed locally in polymer films: X. Q.Chen, H. Yamada, Y. Terai, T. Horiuchi, K. Matsushige and P.S. Weiss, Thin Solid Films, 1999, 353, 259–263.
28 We measured the C–C distance of our graphite area and com-pared to the known value of C–C bond in graphite. We used the
C–C distance of 1.42 A in graphite lattice as the reference. TheC–C distance in our STM experiments was larger than for theknown value due to the piezo drift, a common problem encoun-ters in STM measurements. Often, we measure the referencesample everyday because the piezo drift changes from day today. So our calculated value has been corrected for the piezo drift.
29 (a) J. J. van Gorp, J. A. J. M. Vekemans and E. W. Meijer, J. Am.Chem. Soc., 2002, 124, 14759–14769; (b) P. Johnkheijm, F. J. M.Hoeben, R. Kleppinger, J. van Herrikhuyzen, A. P. H. J. Schen-ning and E. W. Meijer, J. Am. Chem. Soc., 2003, 125,15941–15949.
30 (a) A. Chattopadhyay, S. Mukherjee and H. Raghuraman, J.Phys. Chem. B, 2002, 106, 13002–13009; (b) A. Chattopadhyayand S. Mukherjee, Langmuir, 1999, 15, 2142–2148; (c) S. S.Rawat, S. Mukherjee and A. Chattopadhyay, J. Phys. Chem. B,1997, 101, 1922–1929.
31 Similar to what was observed for conjugated polymers, see: T.-Q.Nguyen, I. B. Martini, I. J. Liu and B. J. Schwartz, J. Phys. Chem.B, 2000, 104, 237–255.
32 S. T. Yau, D. N. Petsev, B. R. Thomas and P. G. Vekilov, J. Mol.Biol., 2000, 303, 667–678.
33 K. Shirai, M. Matsuoka and K. Fukunishi, Dyes Pigm., 1999, 42,95–101.
34 (a) R. B. Martin, Chem. Rev., 1996, 96, 3043–3064; (b) W. Wang,L.-S. Li, G. Helms, H.-H. Zhou and A. D. Q. Li, J. Am. Chem.Soc., 2003, 125, 1120–1121; (c) W. Wang, J. J. Han, L.-Q. Wang,L.-S. Li, W. J. Shaw and A. D. Q. Li, Nano Lett., 2003, 3,455–458.
35 CRC Handbook of Chemistry and Physics, 76th edn, CRC Press,Boca Raton, FL, 1995–1996, pp. 6–245, and 6–249.
36 (a) E. Y. Sheu, K. S. Liang and L. Y. Chiang, J. Phys. (Paris),1989, 50, 1279–1295; (b) O. Braitbart, R. Sasson and A. Weinreb,Mol. Cryst. Liq. Cryst., 1988, 159, 233–242; (c) D. Markovitsi, D.A. Germain, P. Millie, P. Lecuyer, L. Gallos, P. Argyrakis, H.Bengs and H. Ringsdorf, J. Phys. Chem., 1995, 99, 1005–1017; (d)K. E. S. Phillips, T. J. Katz, S. Jockusch, A. J. Lovinger and N. J.Turro, J. Am. Chem. Soc., 2001, 123, 11899–11907; (e) C. Nuck-olls, T. J. Katz, G. Katz, P. T. Collings and L. Castellanos, J. Am.Chem. Soc., 1999, 121, 79–88; (f) C. Nuckolls and T. J. Katz, J.Am. Chem. Soc., 1998, 120, 9541–9544; (g) U. Rohr, P. Schilicht-ing, A. Bohm, M. Gross, K. Meerholz, C. Brauchle andK. Mullen, Angew. Chem., Int. Ed., 1998, 37, 1434–1437; (h)Ref. 29b,c.
37 P. Attard, Mol. Phys., 1996, 89, 691–709.38 (a) O. Galkin and P. G. Vekilov, J. Am. Chem. Soc., 2000, 122,
156–163; (b) J. Lin, J. Zhu and D. Zhou, Eur. Polym. J., 1999, 36,309–314; (c) D. N. Petsev, K. Chen, O. Gliko and P. G. Vekilov,Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 792–796.
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