Interplay between H-bonding and alkyl-chain ordering in self-assembly of monodendritic L-alanine derivatives

Article abstract of DOI:10.1002/cphc.201100739

This paper reports on the synthesis and self-organizing properties of monodendrons consisting of L-alanine at the focal point and alkyl chains with different length at the periphery. The structures of thin films and monolayers are studied by temperature-resolved grazing-incidence X-ray diffraction and scanning force microscopy. The interplay between H-bonding and ordering of the alkyl chains results in a rich temperature-dependent phase behavior. The monodendrons form H-bonded stabilized clusters with the number of molecules depending on the length of the aliphatic chains and temperature. The clusters play the role of constitutive units in the subsequent self-assembly. Short alkyl chains allow the material to form thermodynamically stable crystalline phases. The molecules with longer side groups exhibit additional transitions from the crystalline phase to thermotropic columnar hexagonal or columnar rectangular liquid-crystalline phases. In monolayers deposited on highly ordered pyrolytic graphite, the materials show ordering similar to thin films. However, for the compound bearing hexadecyl chains the affinity of the alkyl groups to graphite dominates the self-assembly and thereby allows epitaxial growth of a 2D lattice with flat-on oriented molecules. Structural evolution of monodendrons based on L-alanine on heating is investigated by grazing-incidence X-ray diffraction and scanning force microscopy, which revealed the effect of alkyloxy chain length on the formation of different 2D and 3D phases in thin films and monolayers (see picture).

Full text of DOI:10.1002/cphc.201100739

DOI: 10.1002/cphc.201100739
Interplay between H-Bonding and Alkyl-Chain Ordering in
Self-Assembly of Monodendritic l-Alanine Derivatives
Denis V. Anokhin,[a, b] Janis Lejnieks,[c] Ahmed Mourran,*[c] Xiaomin Zhu,[c] Helmut Keul,[c]
¯
Martin Mçller,[c] Oleg Konovalov,[d] Natalia Erina,[e] and Dimitri A. Ivanov*[a]
This paper reports on the synthesis and self-organizing proper-
ties of monodendrons consisting of l-alanine at the focal point
and alkyl chains with different length at the periphery. The
structures of thin films and monolayers are studied by temper-
ature-resolved grazing-incidence X-ray diffraction and scanning
force microscopy. The interplay between H-bonding and order-
ing of the alkyl chains results in a rich temperature-dependent
phase behavior. The monodendrons form H-bonded stabilized
clusters with the number of molecules depending on the
length of the aliphatic chains and temperature. The clusters
play the role of constitutive units in the subsequent self-as-
sembly. Short alkyl chains allow the material to form thermody-
namically stable crystalline phases. The molecules with longer
side groups exhibit additional transitions from the crystalline
phase to thermotropic columnar hexagonal or columnar rec-
tangular liquid-crystalline phases. In monolayers deposited on
highly ordered pyrolytic graphite, the materials show ordering
similar to thin films. However, for the compound bearing hexa-
decyl chains the affinity of the alkyl groups to graphite domi-
nates the self-assembly and thereby allows epitaxial growth of
a 2D lattice with flat-on oriented molecules.
1. Introduction
Self-assembly of functional molecular building blocks through
noncovalent interactions has attracted considerable interest,
not only for fundamental research, but also for possible appli-
cations as a bottom-up approach for construction of molecu-
lar-scale devices or biologically active materials.[1–3] Well-de-
fined architectures can be fabricated by using the principle of
molecular recognition exploited by nature. In particular, this
approach is convenient for building regular H-bond networks
selectively formed between specially designed centers in solu-
tion or in the solid state. Self-assembly through H-bonded net-
works is advantageous due to the thermal reversibility of these
bonds, which enables formation of a system prone to undergo
repeated formation or breakdown of connectivity. One of the
prospective classes of H-bonding molecules capable of self-or-
ganization consists of proteinogenic amino acid based amphi-
phililes, for example, l-alanine derivatives. These chiral organic
molecules can self-assemble into a variety of supramolecular
architectures such as vesicles,[4–6] helical ribbons,[7] and ropes,
as well as fibrous and tubular structures.[6] In addition, FTIR
studies on N-octadecanoyl l-alanine monolayers at the air/
water interface indicated that formation of H-bonding net-
works by the amino acid resulted in enhanced interaction of
hydrocarbon chains.[8] At the same time, monodendritic meso-
gens derived from gallic acid were shown to be appropriate
building blocks for fabrication of complex thermotropic or lyo-
tropic mesophases of different symmetry.[9–11] Gin et al. report-
ed the synthesis of 3,4,5-tris(11’-acryloxyundecyloxy)benzoyl l-
alanine, a monodendritic amphiphile which self-assembles into
an inverted hexagonal lyotropic liquid-crystalline phase.[12,13]
The majority of the applications of amphiphilic molecules
rely on controlled structure formation at interfaces and on sur-
faces.[14] In thin films, the behavior of these molecules differs
from that in the bulk, since the self-assembled structure is sen-
sitive to the interfacial energies (i.e. affinity of the polar molec-
ular core and nonpolar periphery with respect to the substrate)
and to the geometrical constraints. For discotic and calamitic
molecules two- and three-dimensional structure formation on
patterned substrates such as polytetrafluoroethylene,[13,15–17]
highly-oriented pyrolitic graphite (HOPG),[18–22] and inorganic
salts[23–25] was addressed. The orientation and epitaxial growth
of the crystalline domains is believed to be determined by
commensurate configuration of the conjugated core with the
corresponding substrate crystal face. However, the role of alkyl
[a] Dr. D. V. Anokhin, Dr. D. A. Ivanov
Institut de Sciences des Matꢀriaux de Mulhouse (CNRS IS2M)
15 rue Jean Starcky, 68057 Mulhouse (France)
Fax: (+33)389608799
E-mail: dimitri.ivanov@uha.fr
[b] Dr. D. V. Anokhin
Moscow State University
Faculty of Fundamental Physical and Chemical Engineering
119991, Moscow, GSP-1, 1-51 Leninskie Gory (Russia)
[c] Dr. J. Lejnieks, Dr. A. Mourran, Dr. X. Zhu, Dr. H. Keul, Prof. M. Mçller
DWI an der RWTH Aachen e.V. und
Institut fꢁr Technische und Makromolekulare Chemie der RWTH Aachen
Pauwelsstr. 8, 52056 Aachen (Germany)
Fax: (+49)241-8023301
E-mail: mourran@dwi.rwth-aachen.de
[d] Dr. O. Konovalov
European Synchrotron Radiation Facility (ESRF)
6 rue Jules Horowitz, 38043 Grenoble (France)
[e] Dr. N. Erina
Bruker Nano-Surface Inc. (formerly Veeco Metrology Instruments)
112 Robin Hill Road, Santa-Barbara, CA 93110 (USA)
1470
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 1470 – 1478

Self-Assembly of l-Alanine Derivatives
side groups in the self-assembly process is probably underesti-
mated. In our previous works, we have shown the effect of
side-chain ordering on structure development for bulk sam-
ples, thin films, and monolayers.[26–30] The tendency of alkanes
to form oriented textures on HOPG was shown by tempera-
ture-dependent measurements on single crystals and monolay-
ers of polyethylene[31] and long alkanes.[32] When prepared by
solution casting on HOPG, these materials form single crystals
with crystalline stems oriented almost normal to the substrate.
However, after annealing at high temperature the crystal mor-
phology evolves to ribbons with chains parallel to the sub-
strate. The chains are accommodated along graphite crystallo-
graphic directions with epitaxial relation (110)PE//(0001)
HOPG.[31] The ordering of linear alkyl chains can thus play an
essential role in self-assembly of monodendrons at surfaces.[33]
Herein we report on the synthesis and self-assembling be-
havior of a novel family of monodendrons, namely, N-[3,4,5-
tris(alkyloxy)benzoyl] l-alanine bearing alkyl chains of different
lengths (Scheme 1). The effect of the chain length on the struc-
tural evolution in the bulk and in thin films was studied by dif-
ferential scanning calorimetry (DSC) and X-ray grazing-inci-
dence diffraction (GID), respectively. In addition, thin-film mor-
phology and molecular alignment in monolayers on the HOPG
substrate were addressed by scanning force microscopy (SFM).
Scheme 2. Synthetic route for the preparation of N-[3,4,5-tris-
(alkyloxy)]benzoyl l-alanine derivatives 3a–c. Conditions: i) HCl·AlaOMe,
NEt3, CHCl3, 08C!RT, overnight; ii) KOH, EtOH, H2O, reflux for 12 h then HCl
(conc.)
ester amides 2a–c, which were directly subjected to the hy-
drolysis reaction. After acidification of the reaction mixture, 3b
and 3c crystallized from the reaction mixture in high purity
and were directly used for further characterization without ad-
ditional purification, while 3a was recrystallized from ethyl ace-
tate.
2.2. Thermal Analysis and X-ray Diffraction Studies on 3a–c
The influence of the alkyl chain length on the thermal transi-
tions and resulting supramolecular self-assembly was investi-
gated by means of DSC and variable-temperature GID. The
DSC heating ramps of compound 3a shown in Figure 1 (top)
exhibit one endothermic peak with a maximum at 131.28C.
The transition was found to be reversible; it occurs at a large
supercooling (62.18C), which is typical of crystalline materials
formed by H-bonding. This conclusion was further confirmed
by variable-temperature polarized optical microscopy (POM)
measurements, which revealed formation of spherulitic struc-
ture characteristic of crystalline materials (Figure 1, bottom).
Figure 2 (left) displays a typical GID pattern of 3a recorded
at room temperature. The observed peaks can be indexed as
hk0 reflections. The d spacings and angular positions of the re-
flections allow the a and b parameters of the unit cell, as well
as the crystallographic angle g, to be determined. Other pa-
rameters cannot be identified directly due to the absence of
mixed hkl peaks. However, from the position of the 002 peak
on the equator of the pattern at ꢀ4.85 ꢁ (not shown here), the
projection of the c parameter on the {001} direction was com-
puted to be c1 =9.7 ꢁ. Based on density considerations, it was
concluded that the unit cell of 3a contains eight molecules or-
ganized in four dimers connected by hydrogen bonds
Scheme 1. Chemical structure of wedge-shaped N-[3,4,5-tris-
(alkyloxy)]benzoyl l-alanine bearing alkyl chains of different length 3a–c on
the periphery and N-benzoyl alanine M at the focal point.
2. Results and Discussion
2.1. Synthesis
The monodendrons were synthesized from commercially avail-
able l-alanine methyl ester hydrochloride and gallic acid ethyl
ester. A simple synthetic route to target monodendrons 3a–
c was developed (Scheme 2). 3,4,5-trialkyloxybenzoyl chlorides
1a–c were treated with l-alanine methyl ester hydrochloride in
chloroform in the presence of triethylamine as base.[34] After
stirring at room temperature overnight, the reaction was com-
plete and subsequent washing with water led to the pure
ChemPhysChem 2012, 13, 1470 – 1478
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
1471

D. A. Ivanov, A. Mourran et al.
lized by H-bonds.[35] Therefore, it is likely that for 3a the crystal
symmetry is mainly determined by aggregation of the alanine
cores.
Upon increasing the alkyl chain length the thermal behavior
and morphology of the sample change. The first DSC heating
ramp shows two transitions with almost equal enthalpy
changes (Figure 3, left). The order–order transition at low tem-
perature is attributed to the aliphatic moieties, and the order–
Figure 1. DSC curves of 3a recorded during a cooling/heating cycle with
dT/dt=10.08Cminꢀ1 (top). The POM image (bottom) shows the presence of
spherulites at 858C, which developed during crystallization from the melt.
Figure 3. DSC curves measured for 3b during a cooling/heating/cycle (left).
The first heating ramp (dT/dt=1.08Cminꢀ1) is shown by a dotted line, and
the first cooling and second heating (dT/dt=10.08C minꢀ1) ramps are
shown as solid lines. The optical micrographs (right) show liquid-crystalline
textures of the compound formed during cooling from the isotropic state to
90 and to 508C.
disorder transition likely corresponds to dissociation of the H-
bonded phase. A significant decrease of the first-peak intensity
during the second heating ramp indicates that annealing at
room temperature leads to modification of the film morpholo-
gy. This issue is discussed in more detail below. The PO micro-
graphs recorded at different temperatures reveal fanlike tex-
tures typical of liquid-crystalline (LC) ordering (Figure 3, right).
A slight change in the birefringence of the material can proba-
bly be accounted for by a local rearrangement of the LC phase
at high temperatures. The structural evolution of thin films at
high temperatures was monitored by GID measurements.
Figure 4 (left) displays the GID patterns of 3b annealed for
several days at room temperature. The patterns were recorded
at room temperature and at 608C. Below about 538C the dif-
Figure 2. GID pattern of 3a recorded at room temperature revealing ortho-
rhombic symmetry of the unit cell (left). Schematics showing the structure
of the unit cell of 3a containing eight molecules organized in four H-
bonded dimers (right). The molecules with the heads directed upward are
darker and those with the downward orientation are lighter.
(Figure 2, right). Taking into account the observed extinction
rules such as h+k=2n for hk0 and h=2n, k=2n, l=2n for
h00, 0k0, and 00l, respectively, as well as possible molecular
packing of dimers on the lattice, only P21/c symmetry can be
retained. Hence, a pseudo-orthorhombic crystal for 3a with
P21/c symmetry was selected with the following lattice param-
eters: a=39.1, b=16.4, c=9.7 ꢁ, a=b=g=908 (cf. Figure 2,
left). Every second molecular layer parallel to the ab plane is
shifted by a half-period in the a direction. It is noteworthy that
the molecules in the dimer are not planar, because of different
orientations of alanine heads with respect to the plane of the
alkyl chains, similar to the conformation found for the crystal
structure of 3,4,5-tridodecyloxycinnamic acid.[28] In thin films,
the hexyl side chains are oriented normal to the substrate, that
is, in the a direction, as shown in Figure 2 (right). No structural
changes were detected during heating of the film to the melt-
ing point at about 1108C. Importantly, N-benzoyl alanine with-
out side groups forms a crystal structure built of dimers stabi-
Figure 4. GID patterns of 3b (left) recorded at room temperature and at
608C. Schematic of the ab projection of the unit cell of 3b stable at elevated
temperatures (right). The Colh structure is composed of disklike supramolec-
ular hexamers.
1472
www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 1470 – 1478

Self-Assembly of l-Alanine Derivatives
fraction pattern reveals a “smectic-like” phase characterized by
an interlayer spacing of 43.0 ꢁ. This distance is comparable to
the length of the molecular dimer. In thin films, the layers are
parallel to the substrate, similar to sample 3a. However, at
608C (i.e. above the first endothermic peak) additional off-
meridional reflections appear on the pattern (Figure 4, bottom
left). The high-temperature modification was identified as
a hexagonal columnar phase Colh. It is noteworthy that the lat-
tice parameter of the Colh phase (43.6 ꢁ) is close to that of the
smectic phase.
Figure 6. GID patterns of 3c recorded at room temperature (left) and at
608C (right). The dashed arrows show selected reciprocal space vectors.
Figure 4 (right) shows a sketch of the Colh structure formed
at elevated temperatures. The columns presumably consist of
disklike clusters, each of which is formed by six molecules sta-
bilized by H-bonds. The columnar axis is parallel to the sub-
strate plane, which is typical of spin-coated films of discotics
and main-chain liquid-crystalline polymers forming columnar
phases.[26,27,36,37] Interestingly, after cooling to room tempera-
ture the hexagonal columnar phase is preserved. It is metasta-
ble at room temperature and slowly transforms into the smec-
tic structure, and this results in the increase in the enthalpy of
the “smectic–columnar” transition found by DSC.
The DSC curve (1st heating) of 3c exhibits two endothermic
maxima in the region 50–748C corresponding to a phase tran-
sition and a melting peak at 1108C (Figure 5). These thermal
events are reproducible on cooling. However, the second heat-
ing ramp shows multiple exo- and endothermic events that
probably indicate the monotropic nature of these liquid-crys-
talline phases. The low-temperature transitions were assigned
to reorganization of the alkyl moieties, whereas the intense
melting peak likely corresponds to dissociation of the H-
bonded cores.
Figure 7. Schematic representation of the ab projection of the rectangular
unit cell of 3c at room temperature. The ellipses indicate the molecules per-
tinent to one column.
45.5, b=83.2 ꢁ; g=908. The value of the c parameter cannot
be determined from the GID patterns alone, as angle b is not
known. The unit cell of 3c contains two columns with six mol-
ecules in each (Figure 7). Analysis of the observed extinction
rules reveals the presence of a glide plane parallel to the
b axis. Consequently, the molecules in the two columns should
be inclined in opposite directions with respect to the b axis (cf.
Figure 7). Such a complex molecular arrangement leads to spe-
cific orientation of the 3c molecules on the silicon substrate.
Thus, according to our pattern indexation, in thin films the
(120) plane with the corresponding repeat distance of 21.7 ꢁ is
parallel to the substrate. In this case, the intense double peak
corresponds to 020 and 100 reflections located at angles of
42.4 and 47.68 with respect to the film surface (cf. Figure 6,
left). In contrast to 3a and 3b, for this texture the 3c mole-
cules have two alternating orientations on the substrate.
Figure 5. DSC curves of 3c recorded during a cooling/heating cycle. The first
heating ramp (dT/dt=1.08Cminꢀ1) is indicated by a dotted line, and the first
cooling and second heating ramps (dT/dt=10.08Cminꢀ1) are indicated by
solid lines.
Similarly to the thermal behavior of 3b, compound 3c
shows a polymorphic transition at elevated temperature. Thus,
at 608C the 2D GID pattern reveals a second polymorph char-
acterized by a monoclinic lattice with a=43.0, b=16.8 ꢁ, and
g=82.38 (Figure 6, right). In contrast to the columnar phase,
this polymorph contains molecular dimers similar to the struc-
ture of 3a (Figure 2, right). It is noteworthy that at this temper-
ature the meridional hk0 peaks exhibit the azimuthal intensity
distribution typical of textures with so-called double-axis aver-
aging, which we previously observed in fibers of a C60-substi-
tuted porphyrin derivative.[38] In contrast to the conventional
uniaxial textures, the structure with double-axis averaging
allows 2D lattice rotation in the smectic layer plane. Conse-
quently, the high-temperature phase of 3c loses the well-de-
At room temperature the 2D GID pattern of 3c shows a com-
plex appearance (Figure 6, left). The particular shape of the
most intense off-meridional peak can be explained by the
presence of two reflections with close radial and angular posi-
tions. The detailed analysis of the pattern and its comparison
with patterns measured on bulk samples allows us to propose
for 3c a model of rectangular columnar phase with both a and
b axes inclined with respect to the substrate (Figure 7). The pa-
rameters of the rectangular 2D lattice are the following: a=
ChemPhysChem 2012, 13, 1470 – 1478
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
1473

D. A. Ivanov, A. Mourran et al.
fined orientation of the a and b axes with respect to the sub-
strate. Moreover, at elevated temperature the misorientation of
the layers is probably caused by the onset of film dewetting,
which was detected optically. After cooling to room tempera-
ture the film structure does not show any further evolution.
Table 1 summarizes the studied molecular structures, includ-
ing the phase transition temperatures Ttr and enthalpy changes
DHtr. The X-ray diffraction data measured on bulk samples
fore, the ordered structure is mainly stabilized by the H-bond
network. The enthalpy of isotropization increases with increas-
ing length of the alkyl chains and reaches a maximum value
for 3b, which forms a columnar structure (Figure 8). Probably,
the LC phase stability at high temperature is related to the
number of monodendrons associated within one aggregate. In
the Colh phase of 3b the disklike aggregate is composed of six
molecules, whereas for 3a and 3c this number is two.
Table 1. Thermal characteristics of the studied compounds determined by DSC.[a]
2.3. Ultrathin Film Morphology
Compound fp [%] Ttr [8C]
DHtr [kJmolꢀ1
]
Cooling
Morphological analysis of spin-coated films (thickness
of several tens of nanometers) deposited on silicon
oxide substrates revealed a strong effect of side-
Heating
3a (C6)
3b (C12)
3c (C16)
61.5
74.5
79.2
Crort131.2(18.5)I
I62.1(ꢀ12.1)Cr
Cr49.2(4.7)Colh115(52.3)I
Cr116.7(12.4)Cr242.9 Crmon83.2-
(6.7)Crmon112.9(38.6)I
I88.3(ꢀ47.9)Colh43.1(ꢀ5.7)Cr
I70.3(ꢀ39.5)Cr239.7(ꢀ8.4)Cr1
chain length on the molecular orientation. In some
instances, specific interactions with the substrate can
result in epitaxially driven growth of a 2D struc-
ture.[28,37] Consequently, it is important to address the
self-assembly of monolayers of compounds 3a–c on
well-defined substrates such as HOPG.
[a] Cr=crystalline phase with unknown symmetry, Crort =orthorhombic crystalline
phase, Crmon =monoclinic crystalline phase, Colh =hexagonal phase, I=isotropic
phase. The phases were assigned on the basis of X-ray diffraction data. fp is the
volume fraction of the alkyloxy chains (CnH2n+1O): fp =3MP/MW, where MP is the mo-
lecular weight of one alkyloxy (CnH2n+1O) chain (n=6, 12, 16), and MW is the molecular
weight of the compound.
Compound 3a with short alkyl chains exhibits in-
teresting structures. The SFM images measured on
the as-prepared film deposited on silicon oxide sub-
show structures close to those of the studied films. Thus, the
DSC results corresponding to the bulk materials can correlate
with the structural evolution occurring in thin films. The phase
diagrams of the bulk compounds will be discussed in a forth-
coming publication.[39] The DSC and X-ray diffraction data
show that compounds 3a, 3b and 3c exhibit various crystal-
line and liquid-crystalline structures. Long alkyl chains favor
formation of thermotropic liquid-crystalline states. To under-
stand the interplay between the formation of H-bonds and or-
ganization of the alkyl chains, the order–disorder transition en-
thalpy was plotted as function of alkyl chain length (Figure 8).
The DSC and X-ray diffraction data consistently show that the
isotropization temperature of the studied compounds is
around 1158C, regardless of the aliphatic chain length. There-
strate show formation of columns (Figure 9, top left). To moni-
tor structure formation and evolution in real time, the novel
peak force tapping (PFT) mode was used. Operation in PFT
Figure 9. SFM images (2ꢂ2 mm) of 3a measured on silicon oxide and HOPG
substrates. Peak force tapping height images on silicon oxide (top): as-pre-
pared film (left) and the same film area after annealing for 150 min at room
temperature (right). Tapping-mode height images on HOPG (bottom): as-
prepared film (left) and the same film after 1 h at room temperature (right).
The white arrows indicate the metastable helical ribbons. The green and red
triangles correspond to the same points on the images and on the height
profiles.
Figure 8. Enthalpy variation associated with the order–disorder transition as
a function of aliphatic chain length. The full and empty circles stand for
melting and recrystallization, respectively. (The data for the homologous C8
compound will be discussed in a forthcoming paper[39].)
1474
www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 1470 – 1478

Self-Assembly of l-Alanine Derivatives
mode allows the maximum interaction force between the SFM
tip and the sample surface to be controlled, and thereby
damage to such soft and fragile samples to be reduced. By
performing repeated PFT measurements on the same surface
area, it was found that, upon annealing for 150 min at room
temperature, the columnar structure transforms into a lamellar
structure, visible as high terraces in Figure 9 (top right). The
thickness of a single lamellar layer of 2.0 nm corresponds to
half of the a parameter of the crystal phase, as found from GID
(Figure 2). The SFM measurements performed on the as-pre-
pared film deposited on HOPG substrate show columnar fea-
tures similar to those found on silicon oxide (Figure 9, bottom).
However, for the columns, a helixlike superstructure with
a height of 2.6 nm and a helical pitch of 13.2 nm can be ob-
served in higher resolution images (indicated by a white arrow
in Figure 9, bottom left). The random orientation of the helical
ribbons excludes their growth by an epitaxial mechanism. The
origin of the helical features remains unclear. It is possible that
formation of this morphology is induced by chirality of the l-
alanine head. Like the film prepared on silicon oxide, this mor-
phology is found to be metastable and transforms to the la-
mellar structure after annealing for about 1 h at room temper-
ature. The lamellar thickness is close to corresponding value
for the lamellae grown on silicon oxide. The structure of the
stable phase is thus independent of the nature of the sub-
strate.
The molecules of 3b also self-assemble on HOPG by forming
well-defined columnar structures (Figure 10). The SFM image
shows that the columns are characterized by an in-plane perio-
dicity of 4.6 nm. This repeating length is close to the unit-cell
parameter of the hexagonal phase determined by GID at 608C
(Figure 4). However, this texture was found to be stable and
does not evolve with time at room temperature. The columnar
phase in monolayers is preserved at room temperature, proba-
bly due to spatial constraints preventing transformation of the
film into the stable lamellar structure. The columns oriented
parallel to the HOPG surface are regularly packed, but their in-
plane orientation is random, which excludes epitaxial growth
mechanisms of 3b on the graphite. Probably, in this case the
adlayer promotes the planar orientation of the mesostructure
on HOPG surface. This is known to occur when the structure of
the adlayer differs from or is incompatible with the organiza-
tion of the molecules in thin film.[37] Importantly, a fully identi-
cal structure of 3b was also observed on SiO2 surface (not
shown here), which means that the observations are not spe-
cific to HOPG.
Figure 10. SFM image of a thin film of 3b on HOPG, which shows a columnar
structure (top). The power spectral density and a scheme of the structure of
the adsorbed layer (bottom).
Figure 11. SFM height image of 3c on HOPG indicating different textures:
flat-on molecules visible as dark regions and edge-on molecules appearing
brighter (left). High-magnification image of the dark region boxed in the left
panel with the fine structure of the monolayer (right). The colored arrows in-
dicate three orientations of the a axis of the 2D lattice on HOPG. The inset
shows the FFT of the image. The color of the parallelograms shown on the
FFT pattern is identical to the color of the arrow positioned on the corre-
sponding single-crystal-like domain.
Compound 3c forms a layer with an average thickness of
3.3 nm (high terraces in Figure 11, left). The layer thickness is
close to 120 peak d spacings of the corresponding 3D lattice.
Thus, on HOPG 3c forms a texture with edge-on oriented mol-
ecules similar to that on silicon oxide. In addition, thinner
areas can be identified, which are visible as low terraces in the
images of Figure 11. A detailed analysis of the high-magnifica-
tion image recorded on such a low terrace reveals in-plane or-
ganization of the molecules in a 2D monoclinic unit cell
(Figure 11, right). The calculated FFT function shows a 12-spot
pattern corresponding to three orientations of the 2D lattice
with the following parameters: a=6.2, b=5.9 nm, and g=828
(cf. the inset of Figure 11, right). The area of the 2D unit cell of
36.1 nm2 is close to the ab projection of the corresponding 3D
unit cell (37.8 nm2) measured in thin film at room temperature.
Thus, the molecular arrangement in the 2D monoclinic lattice
is close to that in the (001) plane of the crystalline phase
ChemPhysChem 2012, 13, 1470 – 1478
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
1475

D. A. Ivanov, A. Mourran et al.
found in thin films on silicon oxide (Figure 7). A small deforma-
tion of the unit cell can probably be accounted for by the in-
teraction with the substrate. In our previous work, we showed
that epitaxy to graphite can significantly modify the 2D struc-
ture of wedge-shaped molecules (3,4,5-tridodecyloxycinnamic
acid) due to the tendency of alkyl chains to extend along the
[100] direction of HOPG.[28] The angle of 608 between the
a axes of the crystals shows the epitaxial character of monolay-
er growth on the HOPG surface.
at room temperature which is similar to that of 3a. However,
at higher temperatures this phase transforms into a liquid-crys-
talline hexagonal columnar phase Colh with columns parallel to
the substrate. Further increase of the side-chain length to C16
(3c) leads to crystallization in the columnar rectangular lattice
with alternating orientation of alkyl chains parallel to the sub-
strate. Heating of sample 3c results in formation of a layerlike
liquid-crystalline phase. All samples exhibit close isotropization
temperatures indicating the formation of H-bonds.
Taking into account similar molecular shapes of 3,4,5-trido-
decyloxycinnamic acid studied previously and 3c, one can sug-
gest that the hexadecyloxy side groups are also oriented paral-
lel to the principal axes of HOPG. However, due to the alternat-
ing orientation of the alkyl chains in the bulk structure of 3c,
one can suppose that on the surface the side groups are ori-
ented along both [100] and [010] directions of HOPG
(Figure 12). The observed 2D structure can be considered as an
adlayer on which a thicker film is formed. However, no epitax-
ial relations between the 2D lattice and the lattice of the thick
film structure addressed by GID were found.
The SFM measurements on monolayers of 3a–c on HOPG
surface show that the monodendrons with C6 and C12 side
chains self-assemble into 2D lattices with parameters close to
those found for thick films. In contrast, the sample with the
longest alkyl groups self-organizes in a monoclinic 2D structure
formed by flat-on oriented molecules. The molecular packing
in the monolayer is in this case similar to the ab plane of the
corresponding 3D crystalline phase formed in thick films but is
oriented at 908 with respect to the thick-film structure. The
structure modification in the monolayer is governed by the ep-
itaxial growth mode of the 2D lattice due to the strong interac-
tion of the long alkyl chains with the graphite substrate.
Variation of the ratio between the polar core and aliphatic
periphery allows the role of each constitutive unit in structure
formation to be understood. The alanine molecular tip pro-
vides for formation of the molecular clusters which play the
role of building blocks for the 2D and 3D self-assembly. The
length of linear alkyl groups is a very sensitive parameter pro-
viding control of the film texture and polymorphic behavior.
Thus, the choice of the alkyl chain length can be used to
induce the formation of molecular aggregates of particular
shape and also to design desired film textures.
Figure 12. Schematic representation of a 2D lattice of 3c on the HOPG sur-
face. The orientation of HOPG with respect to the lattice of 3c is indicated.
Experimental Section
Materials: 1-Bromododecane (97%), thionyl chloride (99+%), ethyl
3,4,5-trihydroxybenzoate (98%), l-alanine methyl ester hydrochlo-
ride, anhydrous potassium carbonate, and potassium hydroxide
were obtained from Aldrich and used as received. Dimethylforma-
mide (DMF) was dried over calcium hydride for 12 h and distilled
under vacuum. Concentrated hydrochloric acid (37%), triethyl-
amine, ethanol, dichloromethane, and chloroform were purchased
from Fluka. l-Alanine hydrochloride was pursued from Aldrich.
3,4,5-Tris(alkyloxy)benzoyl chlorides 1a–c were prepared according
to a literature procedure.[34]
Generally, for discotic molecules on silicon oxide[40] and
HOPG[19] substrates the planar film texture is considered to be
metastable due to higher surface energy of the molecular
cores. For molecules 3a and 3b the high surface energy of ala-
nine competes with the tendency of the alkyl chains to lie par-
allel to HOPG. For the molecule with the longest side groups,
that is, 3c, the role of the alkyl chains in structure formation
becomes dominant. In contrast, the alkyl chains of 3a and 3b
do not exhibit the epitaxial growth mode of the corresponding
2D self-assemblies.
N-[3,4,5-Tris(hexyloxy)]benzoyl l-Alanine Methyl Ester (2a): Freshly
prepared acid chloride 1a (0.64 g, 1.45 mmol) was dissolved in
chloroform (40 mL), and l-alanine methyl ester hydrochloride
(0.18 g, 1.45 mmol) was added. The resulting mixture was cooled
to 08C, and then a solution of triethylamine (0.3 g, 2.90 mmol) in
chloroform (5 mL) was added dropwise while the temperature was
kept below 58C. The reaction mixture was stirred at room tempera-
ture overnight. Afterwards it was diluted with chloroform (150 mL),
and the solution washed three times with distilled water (50 mL).
The organic phase was separated, the solvent evaporated, and the
residue dried in vacuo. A slightly yellow solid was obtained. Yield:
3. Conclusions
Structural studies on N-[3,4,5-tris(alkyloxy)benzoyl] l-alanines
3a–c demonstrate that the length of the alkyl side chains (C6,
C12, C16) dramatically affects the morphology and thermotropic
behavior of the compounds in thin films. Compound 3a (C6)
exhibits a layerlike orthorhombic crystalline structure, which
transforms into a monoclinic phase during heating. Compound
3b with longer alkyl groups forms a layerlike crystal structure
1
85%. H NMR (CDCl3/TMS/300 MHz): d=1.23 (t, 9H), 1.33 (overlap-
ping, m, 18H), 1.81 (m, 6H), 3.79 (s, 3H), 4.77 (q, 1H), 6.67 (d, 1H),
7.00 ppm (s, 2H); 13C NMR (CDCl3/TMS/75 MHz): d=14.0, 14.1, 18.6,
1476
www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 1470 – 1478

Self-Assembly of l-Alanine Derivatives
22.6, 22.7, 25.7, 29.3, 25.7, 29.3, 30.2, 31.5, 45.4, 48.5, 52.6, 53.4,
69.3, 73.4, 105.8, 128.8, 141.4, 153.1, 166.7, 173.8 ppm.
Grazing incidence X-ray diffraction (GID) experiments were con-
ducted on the ID10B beamline of the ESRF at a wavelength of
1.55 ꢁ and incidence angle of 0.28. The samples for GID were pre-
pared by spin coating of 10 mgmLꢀ1 solution in chloroform on
cleaned silicon wafers. The scattering was recorded with a MAR
CCD camera.
N-[3,4,5-Tris(dodecyloxy)]benzoyl l-Alanine Methyl Ester (2b): The
synthesis was carried out according to the same procedure as for
2a. A slightly yellow solid was obtained. Yield: 88%. 1H NMR
(CDCl3/TMS/300 MHz): d=0.88 (t, 9H), 1.27 (m, 54H), 1.50 (d, 3H),
1.81 (m, 6H), 3.79 (s, 3H), 4.00 (m, 6H), 4.77 (m, 1H), 6.69 (d, 1H),
7.00 (s, 2H); 13C NMR (CDCl3/TMS/75 MHz): d=14.1, 18.6, 22.7, 26.1,
29.4, 29.6, 29.6, 29.7, 30.3, 31.9, 48.5, 52.5, 69.4, 73.5, 105.8, 128.8,
141.4, 153.1, 166.7, 173.8 ppm.
Scanning force microscopy (SFM) was use to investigate the sur-
face morphology of the films at room temperature with a Nano-
scope V controller. The imaging was done in tapping mode with
standard silicon cantilevers: Nanoworld Pointprobe NCH 330 or
75 kHz and super-sharp tips from m-Masch. The films were pre-
pared by spin coating with rotation speed of 2000 rpm of approxi-
mately 40 mL of chloroform solutions with concentrations of 1 wt%
onto freshly cleaved HOPG substrates or precleaned SiO2 surface.
During operation in the recently developed peak force tapping
(PFT) mode,[41] a feedback maintains a constant peak force, which
can be as low as several piconewtons. This approach is based on
acquisition of force curves recorded at each moment of the tip–
sample interaction, which enables the nanomechanical properties
to be quantitatively assessed if the cantilever spring constant and
tip shape are calibrated. The cantilevers used for the measure-
ments in PFT mode were SNL-B with spring constant of about
0.3 Nmꢀ1 (Bruker Probes). The peak force set point was fixed at
1 nN.
N-[3,4,5-Tris(hexadecyloxy)]benzoyl l-Alanine Methyl Ester (2c): The
synthesis was carried out according to the same procedure as for
2a. A slightly yellow solid was obtained. Yield: 80%. 1H NMR
(CDCl3/TMS/300 MHz): d=0.88 (t, 9H), 1.26 (overlapping, m, 81H),
1.81 (m, 6H), 3.79 (s, 3H), 4.00 (t, 6H), 4.80 (q, 1H), 6.66 (d, 1H),
6.99 ppm (s, 2H).
N-[3,4,5-Tris(hexyloxy)]benzoyl l-Alanine (3a): Compound 2a (1 g,
1.97 mmol) and KOH (0.44 g 7.88 mmol) were dissolved in a mixture
of ethanol (30 mL) and water (5 mL). The solution was heated to
reflux for 12 h and then cooled to about 408C. Afterwards, it was
acidified with concentrated hydrochloric acid (37%) to pH 1. The
resulting precipitate was filtered off and dried in vacuo. After re-
crystallization from ethyl acetate a white solid was obtained. Yield:
Acknowledgements
1
93%. H NMR (CDCl3/TMS/300 MHz): d=1.33 (t, 9H), 1.32 (overlap-
ping, m, 21H), 1.80 (m, 6H), 4.02 (m, 6H), 4.77 (q, 1H), 6.79 (d, 2H),
7.00 ppm (s, 2H); 13C NMR (CDCl3/TMS/75 MHz): d=14.0, 14.1, 18.2,
22.6, 22.7, 25.8, 29.3, 30.3, 31.6, 31.7, 48.8, 69.4, 73.6, 76.6, 77.1,
77.5, 105.9, 128.2, 141.6, 153.2, 167.6, 176.7 ppm.
The authors acknowledge the French Agence Nationale de la Re-
cherche for financial support in the frame of SPIRWIND project
(HABISOL program) and T2T project (Blanc International pro-
gram). This work was supported by the European Commission
within FP7 Marie Curie Initial Training Network “HIERARCHY”
(Project Reference: 215851). The authors are grateful to the Russi-
an Ministry of Science and Education (project for financial sup-
port of leading scientists No. G34.31.0055 from 19.10.2011) for fi-
nancial support.
N-[3,4,5-Tris(dodecyloxy)]benzoyl l-Alanine (3b): The synthesis was
carried out by the same procedure as for 3a. Pure compound 3b
1
was obtained without recrystallization. Yield: 87%. H NMR (CDCl3/
TMS/300 MHz): d=0.88 (t, 9H), 1.27 (m, 54H), 1.57 (d, 3H) 1.77 (m,
6H), 3.99 (m, 6H), 4.76 (m, 1H), 6.80 (d, 1H), 6.99 ppm (s, 2H);
13C NMR (CDCl3/TMS/75 MHz): d=14.1, 18.2, 22.7, 26.1, 29.4, 29.5,
29.7, 29.7, 30.3, 32.0, 48.9, 69.4, 73.6, 105.9, 128.2, 141.7, 153.2,
167.7, 176.5 ppm.
Keywords: hydrogen bonds · liquid crystals · monodendrons ·
self-assembly · thin films
N-[3,4,5-Tris(hexadecyloxy)]benzoyl l-Alanine (3c): The synthesis
was carried out by the same procedure as for 3a. Yield: 91%.
1H NMR (CDCl3/TMS/300 MHz): d=0. (t, 9H), 1.26 (overlapping, m,
81H), 1.76 (m, 6H), 4.00 (t, 6H), 4.80 (q, 1H), 6.77(d, 1H), 6.99 ppm
(s, 2H); 13C NMR (CDCl3/TMS/75 MHz): d=14.1, 18.2, 22.7, 26.1,
29.4, 29.5, 29.7, 29.7, 30.3, 32.0, 48.9, 69.4, 73.6, 105.9, 128.2, 141.7,
153.2, 167.7, 176.5 ppm.
[1] J. W. Steed, J. L. Atwood, Supramolecular Chemistry, Wiley, New York,
2000.
[2] L. C. Palmer, S. Stupp, Acc. Chem. Res. 2008, 41, 1674–1684.
[3] J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH,
Weinheim, 1995.
[4] A. Ghosh, J. Dey, Langmuir 2008, 24, 6018–6026.
[5] A. Mohanty, J. Dey, Langmuir 2007, 23, 1033–1040.
[6] A. Mohanty, J. Dey, Langmuir 2004, 20, 8452–8459.
[7] S. J. Lee, E. Kim, M. L. Seo, Y. Do, Y. Lee, S. S. Lee, J. H. Jung, M. Kogiso, T.
Shimizu, Tetrahedron 2008, 64, 1301–1308.
1
Characterization: H NMR (300 MHz) and 13C NMR (75 MHz) spectra
were recorded on a Bruker DPX-300 spectrometer in CDCl3 with
TMS as internal standard at 208C.
[8] X. Du, B. Shi, Y. Liang, Langmuir 1998, 14, 3631–3638.
[9] V. Percec, G. Johansson, J. Heck, G. Ungar, S. V. Batty, J. Chem. Soc.
Perkin Trans. 1 1993, 1411–1420.
[10] U. Beginn, Prog. Polym. Sci. 2003, 28, 1049–1105.
[11] X. Zhu, B. Tartsch, U. Beginn, M. Mçller, Chem. Eur. J. 2004, 10, 3871–
3878.
Thermal polarizing optical microscopy analysis was performed on
A Zeiss Axioplan 2 polarizing microscope, equipped with METTLER
FP 90 hot stage. Images were taken from a digital Zeiss AxioCam
MRC4 camera in combination with the Zeiss AxioVision software.
[12] W. Zhou, W. Gu, Y. Xu, C. S. Pecinovsky, D. L. Gin, Langmuir 2003, 19,
6346–6348.
[13] K. R. Amundson, B. J. Sapjeta, A. J. Lovinger, Z. Bao, Thin Solid Films
2002, 414, 143–149.
[14] D. Myers, Surfactant Science and Technology, 3rd ed., Wiley, Hoboken,
2006.
Differential scanning calorimetry (DSC) measurements were per-
formed by using a Netzsch DSC 204 unit. Samples (typical weight:
5 mg) were enclosed in standard Netzsch aluminum crucibles.
Indium and palmitic acid standards were used for calibration. The
heating and cooling rates were 108Cminꢀ1
.
ChemPhysChem 2012, 13, 1470 – 1478
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
1477

D. A. Ivanov, A. Mourran et al.
[15] S. Zimmermann, J. H. Wendoff, C. Weder, Chem. Mater. 2002, 14, 2218–
2224.
[16] A. M. van de Craats, N. Stutzmann, O. Bunk, M. M. Nielsen, M. Watson, K.
Mullen, H. D. Chanzy, H. Sirringhaus, R. H. Friend, Adv. Mater. 2003, 15,
495–499.
[17] O. Bunk, M. M. Nielsen, T. I. Solling, A. M. van de Craats, N. Stutzmann, J.
Am. Chem. Soc. 2003, 125, 2252–2258.
[18] N. Kumaran, P. A. Veneman, B. A. Minch, A. Mudalige, J. E. Pemberton,
D. F. O’Brien, N. R. Armstrong, Chem. Mater. 2010, 22, 2491–2501.
[19] A. Ai, M. Cristadoro, H. J. Ralder, J. P. Rabe, K. Muellen, J. Phys. Chem. C
2008, 112, 5563–5566.
[29] J. Lejnieks, X. Zhu, J. Wang, A. Mourran, H. Keul, M. Mçller, D. V. Ano-
khin, D. A. Ivanov, ChemPhysChem 2010, 11, 3638–3644.
[30] A. Mourran, U. Ziener, M. Mçller, M. Suarez, J.-M. Lehn, Langmuir 2006,
22, 7579–7586.
[31] J. Nakamura, M. Tsuji, A. Nakayama, A. Kawaguchi, Macromolecules
2008, 41, 1358–1363.
[32] S. N. Magonov, N. A. Yerina, G. Ungar, D. H. Reneker, D. A. Ivanov, Macro-
molecules 2003, 36, 5637–5649.
[33] S.-A. Prokhorova, S. S. Sheiko, A. Mourran, R. Azumi, U. Beginn, G. Zipp,
C.-H. Ahn, M. N. Holerca, V. Percec, M. Mçller, Langmuir 2000, 16, 6862–
6867.
[20] F. Jaeckel, M. D. Watson, K. Muellen, J. P. Rabe, Phys. Rev. B 2006, 73,
045423/1.
[21] S. Hçger, K. Bonrad, A. Mourran, U. Beginn, M. Mçller, J. Am. Chem. Soc.
2001, 123, 5651–5659.
[34] J. MalthÞte, A.-M. Levelut, N. Huu Tinh, J. Phys. Lett. 1985, 46, 875–880.
[35] A. Y. Fu, D. Q. Wang, Acta Crystallogr. Sect. E 2005, 61, o2336.
[36] M. Defaux, L. Vidal, M. Moeller, R. I. Gearba, E. Dimasi, D. A. Ivanov, Mac-
romolecules 2009, 42, 3500–3509.
[22] A. Mourran, U. Ziener, M. Mçller, E. Breuning, M. Ohkita, J.-M. Lehn, Eur.
J. Inorg. Chem. 2005, 2641–2647.
[23] J. A. Last, A. C. Hillier, D. E. Hooks, J. B. Maxson, M. D. Ward, Chem.
Mater. 1998, 10, 422–437.
[37] A. Mourran, U. Beginn, G. Zipp, M. Mçller, Langmuir 2004, 20, 673–679.
[38] V. Luchnikov, D. V. Anokhin, S. Z. D. Cheng, C.-L. Wang, G. Bar, D. A.
Ivanov, J. Appl. Crystallogr. 2011, 44, 540–544.
[39] D. V. Anokhin, unpublished results.
[24] R. R. Lunt, J. B. Benziger, S. R. Forrest, Adv. Mater. 2007, 19, 4229–4233.
[25] E. Gomar-Nadal, J. Puigmarti-Luis, D. B. Amabilino, Chem. Soc. Rev. 2008,
37, 490–504.
[26] R. I. Gearba, A. I. Bondar, M. Lehmann, B. Goderis, W. Bras, M. H. J. Koch,
D. A. Ivanov, Adv. Mater. 2005, 17, 671–676.
[40] E. Grelet, H. Bock, Europhys. Lett. 2006, 73, 712–718.
[41] B. Pittenger, N. Erina, C.Su, Quantitative mechanical mapping at the
nanoscale with PeakForce QNM, Application Note AN128, 2010, Veeco
Instruments Inc., www.bruker-nano.com.
[27] R. I. Gearba, A. I. Bondar, B. Goderis, W. Bras, D. A. Ivanov, Chem. Mater.
2005, 17, 2825–2832.
[28] X. Zhu, A. Mourran, M. Moeller, U. Beginn, D. V. Anokhin, D. A. Ivanov,
Phys. Chem. Chem. Phys. 2010, 12, 1444–1452.
Received: September 22, 2011
Revised: February 1, 2012
Published online on March 7, 2012
1478
www.chemphyschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2012, 13, 1470 – 1478