1,4;5,8-naphthalene-tetracarboxylic diimide derivatives as model compounds for molecular layer epitaxy

Article abstract of DOI:10.1039/b601258b

The physical properties and finite size effects observed in 1,4;5,8-naphthalene-tetracarboxylicdiimide (NTCDI)-based organic multilayers assembled by molecular layer epitaxy (MLE) are investigated by structure-property studies of low molecular weight mode

Full text of DOI:10.1039/b601258b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
1,4;5,8-naphthalene-tetracarboxylic diimide derivatives as model
compounds for molecular layer epitaxy{
Yuval Ofir, Alexander Zelichenok and Shlomo Yitzchaik*
Received 26th January 2006, Accepted 10th March 2006
First published as an Advance Article on the web 4th April 2006
DOI: 10.1039/b601258b
The physical properties and finite size effects observed in 1,4;5,8-naphthalene-
tetracarboxylicdiimide (NTCDI)-based organic multilayers assembled by molecular layer epitaxy
(MLE) are investigated by structure–property studies of low molecular weight model compounds.
These molecules, aliphatic N,N9-dihexyl-NTCDI (1), N,N9-dihexadecyl-NTCDI (2) and aromatic
N,N9-diphenyl-NTCDI (3), mimic the elemental building blocks of the solid-state
heterostructures, NTCDI-based organic superlattices. Thermal analysis combined with polarized
optical microscopy reveals that both 1 and 2 have polymorphic phases at an elevated temperature
range. All model compounds show new absorption (abs.) and photoluminescence (PL)
bathochromically shifted bands in concentrated (mM) solutions, as is also seen in MLE-derived
organic superlattices. The strong tendency to aggregate and the photophysical properties of the
model compounds are correlated with the PL and electrical field dependent electroluminescence
(EL). This clarifies the structure-tunable emission by virtue of in-plane excitons in solid-state
MLE structures. Finally, X-ray diffraction of single crystals of 1 and 3 reveal similar packing
motifs for the molecules and the resulting solid-state MLE heterostructures.
enabling monolayer-by-monolayer growth, (3) covalent
Introduction
(‘‘c-axis’’) interlayer bonding and (4) p-stacking in the x–y
plane. In our previous contribution4 we demonstrated the
applicability of the MLE approach in constructing an ultra-
thin (ca. 6 nm) OLED device.
Organic multilayered structures having a characteristic layer
thickness of ca. 1–100 nm have recently attracted significant
attention in view of their possible application in molecular
optoelectronic and electronic devices, such as organic light
emitting diodes (OLEDs) and organic field effect transistors
(OFETs).1 Novel aspects of solid-state physics in diminished
dimensions have already been recognized in organic quantum-
confined structures.2 The construction of stacked heterostruc-
tures usually utilizes ultra high vacuum deposition techniques
like organic molecular beam deposition (OMBD), and has
been demonstrated in organic superlattices built up of 1,4;5,8-
naphthalene-tetracarboxylic dianhydride (NTCDA)/3,4;9,10-
Much like atomic layer epitaxy (ALE),5 the MLE approach
utilizes the right balance between chemical adsorption and
physical desorption and thus the monolayer’s epitaxial growth
is governed by self-limiting vapor phase reactions. ALE uses
comparably small inorganic compounds to form multilayered
structures, where in-plane ordering is achieved simultaneously
with c-axis anchoring. The molecular precursors for the MLE
process are larger than those of ALE, and upon c-axis
anchoring they still have in-plane rotational freedom.
Therefore, in-plane packing can be improved by additional
face-to-face p-aggregation that in turn leads to anisotropic
physical properties of the MLE-derived heterostructures. Disc-
shaped p-conjugated organic semiconducting precursors
were chosen as the building blocks of the electronically
semiconductive layer, and flexible spacers were chosen as the
precursors for the assembly of the electronically insulating
layer. Scheme 1 shows the geometrical aspects of the model
compounds, hard disc-like molecules with two flexible arms,
capable of mimicking the MLE multilayers. In the case where
the disc-shaped unit is mesomorphic, the multilayered
structure can be viewed as an ‘‘artificial’’ smectic phase, in
which the covalent bonding forces the precursors to adopt
layered liquid-crystalline packing. Thus, the in-plane packing
and p-aggregation are the key aspects in our investigation of
model compounds determining the relationship between
structure and electronic properties.
perylene-tetracarboxylic dianhydride (PTCDA) with
characteristic layer thickness of ca. 3–4 nm.3
a
Recently we developed a new nanotechnological approach,
MLE,4 to assemble organic superlattices of monomolecular-
layer (ca. 1 nm) periodicity. This method employs a vapor-
phase self-assembly route to enhance epitaxial growth via
covalent bonding on the intermolecular level and forms highly
ordered supramolecular layered structures. The MLE
methodology comprises instrumental, synthetic and assembly
aspects which are divided into four levels: (1) a template layer
connecting the organic superlattice to the inorganic substrate
by covalent bridges, (2) self-restricted vapor-phase reactions
Inorganic and Analytical Chemistry & the Center for Nanoscience and
Nanotechnology, The Hebrew University of Jerusalem, Jerusalem,
91904, Israel. E-mail: sy@cc.huji.ac.il; Fax: +972-2-6585319;
Tel: +972-2-6586971
{ Electronic supplementary information (ESI) available: CIF data for
compounds 1 and 3, and a crystal packing diagram of compound 1.
See DOI: 10.1039/b601258b
The chemical structures of the MLE model compounds are
given in Scheme 2. We found a direct analogy between model
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photovoltaic cells,11 and as a medium for high-density
information storage.12
Furthermore, there is an independent interest in low
molecular weight naphthalene-imide based compounds,13 due
to their increased electron affinity which can be utilized as an
electron injection layer in OLED devices or high-electron
mobility n-type OFETs.14 The aromatic N,N9-diphenyl-
NTCDI has an ionization potential threshold (7.4 eV)15 that
corresponds well to the good electron-injection character. This
value is comparable to that of 7,79,8,89-tetracyanoquinodi-
methane (TCNQ), which is widely used in charge transfer
complexes. This high ionization threshold is caused by the
inductive effect of the four carbonyl electron withdrawing
units distributed symmetrically around the p-conjugated
naphthalene core.16 NTCDI was also utilized in blue
light-emitting organic electroluminescent devices along
with N-alkyl- or arenyl-4-acetylamide-1,8-naphthalimides.17
NTCDI was found to form exciplexes with diamines in the
solid-state, which resulted in a long bathochromic-shifted PL
emission.18 N,N9-dioctyl-NTCDI was also used as a doping
material in organic photovoltaic cells.19 Doping of poly[2-
methoxy-5-(29-ethylhexyloxy-p-phenylvinylene)] (MEN-PPV)
with N,N9-dioctyl-NTCDI in a Schottky-type device resulted
in an increase of current density and circuit voltage by factors
of 10 and 6 respectively. N,N9-Dihexyl-NTCDI has also
proved to be efficient in photorefractive applications.20
The possible LC-like structural transformation observed in
conjugated N,N9-bis(2-phenyl)perylene-3,4;9,10-bis(dicarbox-
imide) was used for the fabrication of optical discs.21 In
addition, the photochemical response of naphthalimides and
naphthaldiimides was investigated for versatile biological
applications.22
Scheme 1 Electronically quantum-confined structures: model com-
pound and MLE-derived organic superlattice.
Herein we provide an example of low molecular weight
model compounds that can self-assemble by p-stacking and
Van der Waals interactions. In this way the system mimics the
relationship between the structure and the electronic properties
of MLE-deposited ultra-thin films, and enables the perfection
of their assembly-performance characteristics.
Scheme 2 Chemical structures of model compounds 1–3 and
Results and discussion
NTCDA
Imidization reactions
compounds 1–3 and the NTCDI-based aromatic and aliphatic
MLE heterostructures, in the mimicking of their photophysical
properties. The strong tendency for face-to-face aggregation
observed in the model compounds explains mesophase
formation at elevated temperatures and the self-organization
observed in the MLE-derived structures. The remarkable
ability of liquid-crystalline material to self-organize sponta-
neously into highly ordered structures that, when disturbed,
simply reform the original structure is implemented in the
MLE processes. Most current liquid crystals (LCs) are
electrical insulators and are used in applications such as flat
panel displays. The recent advance in the synthesis of semi-
conducting LCs6 significantly expands the realm of potential
applications, and has encouraged many studies7 into the use of
LCs as active components in electroluminescent devices,8
molecular wires and fibers,9 photorefractive materials,10
Formation of imide bonds between diamine-containing
compounds and NTCDA, the MLE precursors, was verified
by UV-Vis and FTIR spectroscopy of model compounds 1–3.
UV-Vis lmax(e) and FTIR nCLO spectral bands of NTCDA
precursor: 369 nm (e = 2.1 6 104 L mol21 cm21) and
1779 cm21. Upon imidization in the model compounds these
bands were shifted to 381 nm (e = 2.6 6 104 L mol21 cm21
)
and 1655.9 cm21 for 1; to 380 nm (e = 2.3 6 104 L mol21 cm21
)
and 1655.5 cm21 in the case of 2; and to 382 nm (e = 2.1 6
104 L mol21 cm21) and 1655.7 cm21 in the case of 3. The
surface condensation reaction between the alkylamine and
NTCDA was studied by FTIR measurements of an alkyl-
amine monolayer self-assembled on an Au/SiO2/Si substrate.
The free alkyl-amine peak at 3200 cm21 disappeared and the
imide peak at 1655 cm21 appeared following the reaction with
NTCDA in MLE-derived films on gold. Finally, FTIR spectra
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Fig. 1 UV-Vis spectra of model compound 1 in DMF solution at 1025 M (I) and at increasing concentrations, up to 5 6 1023 M (II). UV-Vis
spectra of an MLE-deposited three-bilayer, SiO2/Pr–(NTCDI–HM)3–NH2, on a glass substrate is shown in the inset.
comparison of surface-bound NTCDI and model compounds
observed (see Figs. 1 and 2, 380 nm and 473 nm for 1 and
1–3 provides evidence for imide-bond formation.
382 nm and 518 nm for 3). UV-Vis spectra of solid-state
naphthalene-diimide based organic superlattices (see insets of
Figs. 1 and 2) exhibit a red-shifted absorption p-aggregation
band in addition to the UV band of the ‘‘isolated molecules’’
[380 nm and 515 nm for the MLE ‘‘1-like’’ structure based
on DAH and NTCDA, SiO2/Pr–(NTCDI–HM)3–NH2 (Pr =
aminopropyl template layer; HM = hexamethylene spacer),
and 381 nm and a broad band around 1100 nm for the MLE
‘‘3-like’’ structure, SiO2/Pr–(NTCDI–MDP)3–NH2, (MDP =
4,49-methylenediphenyl)].
p-Aggregation
MLE-derived organic superlattices reveal size-dependent
effects of optical, abs., PL and EL properties.4 Similarly we
observe the formation of a bathochromically shifted band
in the solutions of the model compounds (1–3). At low
concentration (1025 M, DMF) only UV bands, corresponding
to intramolecular electronic transitions, of monomers (I) are
observed, while at high concentration (1023 M, DMF) an
additional absorption of p-stacked molecular aggregates (II),
corresponding to intermolecular electronic transitions, is
MLE structures were observed to have two different PL/EL
emitting centers.4 The blue EL is attributed to a radiative
recombination process on a single NTCDI molecule, and the
Fig. 2 UV-Vis spectra of model compound 3 in DMF solution at 1025 M (I) and at increasing concentrations, up to 5 6 1023 M (II). UV-Vis
spectra of an MLE-deposited three-bilayer, SiO2/Pr–(NTCDI–MDP)3–NH2 , on a glass substrate is shown in the inset.
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and a second broad peak at around 570 nm, which were
attributed to monomer- and aggregate-centered PL respec-
tively, in contrast to the PL spectra of the diluted solution,
for which only monomer emission was observed.26 The EL
spectrum of MLE-derived NTCDI superlattice HM spacer in
the configuration SiO2/ITO/(NTCDI–HM)6/Al at 6 V is shown
in the inset of Fig. 3. The EL spectrum of MLE-derived
NTCDI superlattice with MDP spacer in the configuration
SiO2/ITO/(NTCDI–MDP)3/Al at 6 V is shown in the inset of
Fig. 4. Applying different voltages allowed tuning of the EL of
the OLED from red to blue.4a At low voltage the red emission
of the stacked NTCDI molecules (intermolecular excitons)
dominated the EL spectrum, since these centers have a lower
energy gap for excitation and charge recombination.
Increasing the applied potential led to charge recombination
at the higher energy gap (intramolecular excitons) and to blue-
shifted EL. It is clearly observed that two types of emission
bands appear. The first (I) is attributed to EL of NTCDI
‘‘isolated luminophore’’, and the second (II) to EL of NTCDI
p-aggregates. The induced aggregation in solution and the
appearance of the red-shifted absorption and emission bands
clearly modeled the red-shifted absorption and EL bands in
the solid-state and device operation, respectively. In order to
prove that the resulting MLE layers show tendency for two
dimensional aggregation, atomic force microscopy (AFM)
measurements were performed on SiO2/Pr–(NTCDI–HM)n–
NTCDI superlattices (n being the number of bilayers and
ranging from 0–2, see ref. 4 for detailed experimental on the
MLE technique). Fig. 5 shows AFM imaging of the surfaces at
various numbers of bilayers, indicating the growth in domain
size as the number of MLE layers increases, ranging from
10 nm for the first NTCDA layer to about 100–120 nm for a
three bilayer MLE superlattice structure (note that Fig. 5C is
not an artefact, and was verified on two separate samples and
at different scan sizes).
Fig. 3 PL spectra of model compound 1 in DMF solution at 1025 M,
lex(I) = 250 nm (I) and at 1023 M, lex(II) = 380 nm (II) (excitation
spectra are not given). EL of MLE-derived three-bilayer OLED,
SiO2/ITO/(NTCDI–HM)6/Al at 6 V is shown in the inset.
red-shifted EL is assigned to a radiative recombination process
between neighboring NTCDI molecules. The coexistence of
two emitting centers within the same layer is an indication of
two different excitation paths: (a) intra-molecular excitons and
(b) inter-molecular excitons,23 and is a sign of dense packing.
The trend of a new EL peak appearing due to epitaxial
ordering was also observed in a perylene-based OLED,24 and
in a single crystal of model compound 1.25 We separated these
two emission centers in the model compounds 1–3. The
emission spectra of 1 and 3 at different concentrations (1025
M
and 1023 M in dimethyl formamide (DMF)) are shown in
Figs. 3 and 4. In the case of the more concentrated solution,
which provided conditions for aggregation, two different PL
peaks were clearly observed: one in the region of 400–430 nm
Phase behavior and liquid crystalline properties
The differential scanning calorimetry (DSC) thermogram of 1
(Fig. 6) reveals two endothermic peaks upon heating, assigned
as two different polymorphs K (TK = 187 uC, DH = 29.5 J g21
)
and M (TM = 208 uC, DH = 41.2 J g21). The fact that no
exothermic transitions can be seen even though the tempera-
ture reaches 400 uC points to the very high thermal stability of
1. Two exothermic transition peaks appeared upon cooling
(with the expected hysteresis) describing the backwards phase
transitions from the isotropic melt to the K9 phase and finally
back to the crystalline phase, and proves that these are indeed
first-order phase transitions. Polarized optical images of 1
(Fig. 7A) were taken during sample cooling. Above TM
isotropic texture was observable. Around TM we observed the
formation of
a different polymorph. Finally needle-like
crystals appeared below TK. In order to understand the
thermal properties of our material we synthesized and studied
a closely related model compound 2 that has a longer aliphatic
chain (Scheme 2). As predicted, we observed the lowering
of the transition from the crystalline phase to the second
polymorph (TK =137 uC, DH = 9.2 J g21 and TM = 150 uC,
Fig. 4 PL (excitation [Ex.] and emission [Em.]) spectra of model
compound 3 in DMF solution at 1023 M. EL of MLE-derived three-
bilayer OLED, SiO2/ITO/(NTCDI–MDP)3/Al, at 6 V is shown in
the inset.
DH
=
42.3
J
g21
) upon heating and cooling, again
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Fig. 5 AFM imaging of 3-(aminopropyl)trimethoxysilane SAM surface (A) and MLE-derived bilayer superlattices in the form of SiO2/Pr–
˚
˚
˚
˚
(NTCDI–HM)n–NTCDI for n = 0 (B), n = 1 (C) and n = 2 (D). Images are 950 nm 6 950 nm; RMS roughnesses are 2.3 A, 2.2 A, 1.5 A and 7.0 A
respectively.
sign of another polymorph phase but again shows a very high
thermal stability (up to 400 uC). The molecule seems to be too
rigid for this kind of structural transformation. However,
we succeeded in growing a single crystal of this compound
enabling us to define the crystalline structure.
The growth of MLE structures with aromatic spacers was
done using MDA. The existence of a methylene unit between
the aniline groups (which are equal to the phenyl groups in
compound 3) allows enough flexibility for the buildup of an
MLE multilayer. Working with 1,4-phenylenediamine didn’t
result in ordered multilayer growth due to the high rigidity,
emphasizing the need for a flexible spacer inbetween the rigid
NTCDI units.
The phase transitions in our models, compounds 1 and 2,
point to layered polymorphic structures in MLE-derived
nanolayers exhibiting thermal stability at elevated tempera-
tures. Similarly, systems based on dioxotriazole derivatives of
perylene12 show large LC ranges. It is not yet clear what
Fig. 6 DSC thermograms of model compounds 1 (—) and 2 (---),
recorded at a heating/cooling rate of 10 uC min21
.
minimal structural features are required to produce an LC
phase. However, as for the cases of porphyrins and phthalo-
cyanines, that have aromatic cores slightly larger than NTCDI,
eight side chains are usually required to induce liquid
crystallinity. In the LC porphyrin, zinc octakis(octylethylether)
porphyrin,14 for example, the ratio of side chain atoms
(excluding hydrogens) to atoms in the aromatic core is 3.6,
whereas for 1 and 2 this ratio is 0.6 and 0.7 respectively.
Although we have substituted the NTCDI unit with a long
demonstrating the reversibility of the transitions and the very
high thermal stability of the compound. Polarized optical
images of 2 (Fig. 7B) were also taken during sample cooling.
The images revealed melting texture above TM and that of the
second polymorph around TM. Finally needle-like crystals
appear below TK without any sign of the broken conic fan-
shaped texture observed for 1. DSC of 3 does not reveal any
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Fig. 8 Crystal packing of model compound 3 deduced from single
crystal X-ray analysis.
located at inversion centers. The packing of the naphthalene-
diimide cores in model compound 129 adopts a coplanar motif,
while a herringbone motif is adopted in the case of 3. Katz et al.
reported a fluorinated-tail version of compound 1 which was
used as an n-type semiconductor,30 but the crystal structure of
this compound adopted a herringbone motif similar to that
of 3. In contrast to the herringbone structure, the coplanar
structure showed by 1 was also seen in an NTCDI-based
compound with an aromatic ether-imide tail reported by
Dingemans et al.6a
Conclusions
Fig. 7 POM images of 1 (A) and 2 (B) at 187 uC and 137 uC
respectively (magnification 6100).
Electronic transitions in organic structures are essentially con-
trolled by the p-conjugation length. Reviews of the experi-
mental data, the theoretical calculations and the relationship
between the size of the p-conjugation and absorption spectra
have been published.31 In this contribution we have described
the assembling features of a newly developed MLE method for
deposition of organic multilayered structures, which is closely
connected with p-conjugation induced molecular ordering.
Single crystal XRD, DSC, FTIR, UV-Vis, and PL measure-
ment of MLE precursor and model compounds showed a
strong tendency towards p-aggregation in the model com-
pound. Both the model compounds and the MLE-derived
superlattices show a new red-shifted intermolecular absorption
band, suggesting an electronic cross-talk between neighboring
molecules. The corresponding p-aggregation in the MLE-
derived multilayers is one of the fundamental driving forces
for ordering and serves as a basic assembling principle. This
driving force is the main methodological difference between
ALE, which was applied only to the deposition of inorganic
heterostructures, and the MLE method. UV-Vis and FTIR
spectra of precursor and model compounds point to the
formation of imide bridges, which proves the formation of
covalent bridges between adjacent layers. Thermal analysis of
model compounds 1 and 2 showed the existence of two
polymorphic phases in the thermal range of MLE’s deposition
temperature. The tendency towards aggregation in the result-
ing MLE superlattice was confirmed by AFM imaging of the
surface, while increasing the number of deposited layers led to
an increase in domain size from 10 nm to 100 nm following
alkyl tail (C16), the result does not show mesomorphic
behavior. In relation to this, it is noteworthy to look at the
work in ref. 6a that shows that an insertion of phenyl-ether
units does result in LC properties. For electronic applications,
this is highly advantageous, because the benefits from liquid
crystallinity may be utilized while keeping the volume of the
electro-inactive (insulating) side chain moiety to a minimum.
Polarized optical microscopy (POM) images of model
compounds 1 and 2 (Fig. 7A and B respectively) were taken
at 187 uC and 137 uC respectively, temperatures that are below
the melting point of each compound. Both molecules show
more than two phases by polarized microscopy in the tem-
perature range from room temperature to above the melting
point, corresponding well with the DSC data presented earlier.
X-Ray crystal analysis
In order to understand the packing motifs in the model
compounds and related MLE structures, crystals were grown.
Successful crystallization of model compounds 1 and 3 gave
the opportunity to do single crystal X-ray analysis.27,28 The
structural motifs of compound 3 are shown in Fig. 8. The
diffraction pattern in the crystalline phase of 1 could be
˚
indexed on a triclinic unit cell of dimensions a = 8.264 A; b =
˚
˚
˚
14.504 A; c = 4.888 A and a molecular length of 18.864 A, and
that of 3 could be indexed on a monoclinic unit cell of
˚
dimensions a = 5.136 A; b = 7.522 A; c = 25.623 A and a
˚
˚
˚
molecular length of 15.429 A. In both cases the molecules are
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deposition of 4 bilayers. XRD analysis of single crystals of
diffracted graphite-monochromated Mo Ka radiation (l =
˚
0.71073 A) was detected on a phosphor screen held at a
compounds 1 and 3 allowed the verification of coplanar
˚
packing of compound 1 with a core-to-core distance of 4.888 A,
distance of 6.0 cm from the crystal, operating at 243 uC. A
detector array of 512 6 512 pixels, with a pixel size of
approximately 120 mm, was employed for data collection.
A series of 30 data frames measured at 0.3u increments of v
were collected with three different 2h and Q values to assess the
overall crystal quality and to calculate a preliminary unit cell.
The measured intensities of individual reflections were plotted
at 0.3u intervals of v, and the average peak width at baseline
was less than 1.5. Reflections with I > 2s(I) were selected from
the data frames and utilized to calculate a preliminary unit cell.
For the collection of the intensity data, the detector was
positioned at a 2h value of 28u and the intensity images were
measured at 0.3u intervals of v for a duration of 20 s each. The
data frames were collected in four distinct shells which, when
combined, measured more than 1.3 hemispheres of intensity
data with a maximum 2h of 52u. For the first shell, the crystal
was positioned at Q = 0u and v = 228u and a set of 600 frames
were collected. A series of 600 frames was collected in the
second shell with a starting position of Q = 90u and v =
228.0u. The crystal was then moved to a position of Q = 180u
and v = 228.0u to measure 600 frames required for the third
shell. The last 600 frames in the fourth shell were collected
starting at Q = 270u and v = 228u. The initial 50 frames of the
first data shell were re-collected at the end of data collection to
correct for any crystal decay, but none was observed.
and a herringbone packing of compound 3 with a core-to-core
˚
distance of 5.136 A. In conclusion, we have studied the
thermal, structural and photophysical properties of model
compounds that mimic the MLE structures. We find a close
analogy in the properties of aggregated model compounds 1–3
with features of MLE-derived organic heterostructures. The
possibility of studying aggregated model compounds opens a
unique route to predicting the properties of organic quantum-
confined structures and their implementation in nanometric
devices.
Experimental
Vapor phase self-assembly by MLE
NTCDI-based organic superlattices with aromatic and alipha-
tic spacers were grown by the MLE method. An alkylamine-
containing surface, 3-(aminopropyl)trimethoxysilane, at
200–220 uC is hit with a pulse of vaporized NTCDA precursor,
forming imide linkages. Then a pulse of a vaporized diamine
spacer (aromatic 4,49-methylenedianiline (MDA) or aliphatic
DAH), is used to regenerate the amine functionality on the
surface that can again react with NTCDA. Cycling through
these steps leads to the formation of NTCDI-based organic
superlattices with covalent interplanar bonding and in-plane
p-aggregation ordering. Detailed experimental conditions of
the MLE vapor phase reactions and setup are given in ref. 4.
Immediately after collection, the raw data frames were
transferred to a second PC computer for integration by the
SAINT program package.33 The background frame informa-
tion was updated according to the equation B9 = (7B + C)/8,
where B9 is the updated pixel value, B is the background pixel
value before updating, and C is the pixel value in the current
frame. The structure was solved and refined by the SHELXTL
software package.34 Molecular packing images were processed
on a PC using Mercury 1.4 software by the Cambridge
Crystallographic Data Centre (CCDC).
Photophysical characterization
UV-Vis-NIR absorption spectra were recorded on a Shimadzu
UV-Vis-NIR 3101 PC scanning spectrophotometer; lumi-
nescence and PL were measured on a Shimadzu RF-5301 PC
spectrofluorimeter; electroluminescence of MLE-derived thin
films was observed on a homemade setup consisting of a RF-
5301 PC and a Keithly 236 source measure unit. IR spectra
were recorded using an IFS 113v Bruker FT-IR spectrometer,
with a spectral resolution of 2 cm21. The samples contained
1% NTCDA precursor and model compounds (1–3) in pressed
KBr powder pellets.
Model compound synthesis
N,N9-Dihexyl-NTCDI (1). 10 g (0.037 mol) of NTCDA and
37.4 g (0.37 mol) of n-hexylamine with a catalytic amount of
4-(dimethylamino)pyridine (DMAP) were refluxed in DMF
for 8 h. The reaction mixture was poured into water and the
precipitated solid was filtered, dried and crystallized from
toluene. Elemental analysis: Calculated for C, H, and N: 71.89;
6.91; 6.45; Found: 71.63; 7.02; 6.35. 1H NMR (300 MHz,
CDCl3) d: 0.87 (t, 6H, J = 7.0 Hz, CH3); 1.36 (m, 12H, CH2);
1.82 (m, 4H, CH2); 4.18 (t, 4H, J = 7.0 Hz, N–CH2); 8.74 (s,
4H, aromatic rings).
Structural investigations
DSC measurements were carried out on a Mettler TA 300
calorimeter at 10 K min21 heating/cooling rates. POM
observations were performed on Zeiss Universal microscope
equipped with a Mettler FP 52 heating stage. The pictures
shown here were taken with transmitted light using crossed
polarizers. AFM measurements of the MLE superlattices were
performed on a Nanoscope Dimensions 3100 scanning probe
microscope from Digital Instruments (DI) equipped with a
NanoScope IV controller, in a tapping mode using a tapping
etched silicon probe (TESP) tip, scanning at speeds of 1–2 Hz
under ambient conditions. X-Ray diffraction data was mea-
sured on a Bruker SMART APEX CCD X-ray diffractometer
system controlled by a Pentium-based PC running the SMART
software package.32 The crystal was mounted on a three-circle
goniometer with x fixed at +54.76u at room temperature. The
N,N9-Dihexadecyl-NTCDI (2). 10 g (0.037 mol) of NTCDA
and 89.9 g (0.37 mol) of 1-hexadecylamine with a catalytic
amount of DMAP were refluxed in DMF for 8 h. The reaction
mixture was poured into water and the precipitated solid
was filtered, dried and crystallized from toluene. Elemental
analysis: Calculated for C, H, and N: 77.31; 9.80; 3.92; Found:
75.86; 10.16; 3.71. 1H NMR (300 MHz, CDCl3) d: 8.75 (s, 4H,
aromatic rings); 4.20 (t, 4H); 1.25 (s, 56H); 0.87 (t, 6H).
2148 | J. Mater. Chem., 2006, 16, 2142–2149
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9 (a) C. F. van Nostrum and R. J. M. Nolte, Chem. Commun., 1996,
21, 2385; (b) E. J. Osburn, A. Schmidt, L. K. Chau, S. Y. Chen,
P. Smolenyak, N. R. Armstrong and D. F. O’Brian, Adv. Mater.,
1996, 8, 926.
10 G. P. Wiederrecht, B. A. Yoon and M. R. Wasielewski, Science,
1995, 270, 1794.
11 B. A. Gregg and Y. I. Kim, J. Phys. Chem., 1994, 98, 2412.
12 C.-y. Liu, H.-l. Pan, M. A. Fox and A. J. Bard, Chem. Mater.,
1997, 9, 1422.
13 F. Cacialli, R. H. Friend, C.-M. Bouche´, P. Le. Barny, H. Facoetti,
F. Soyer and P. Robin, J. Appl. Phys., 1998, 83, 2343.
14 (a) J. G. Laquindanum, H. E. Katz, A. Dodabalapur and A. J.
Lovinger, J. Am. Chem. Soc., 1996, 118, 11331; (b) B. Crone,
A. Dodabalapur, Y. Y. Lin, R. W. Flias, Z. Bao, A. LaDuca,
R. Sarpeshkar, H. E. Katz and W. Lin, Nature, 2000, 403, 521.
15 S. Sugiama, D. Yoshimura, T. Miyamae, T. Miyazaki, H. Ishii,
Y. Ouchi and K. Seki, J. Appl. Phys., 1998, 83, 4928.
16 H. Nakahara, K. Seki, S. Asada, K. Fukuda and H. Inikuchi,
Chem. Phys., 1987, 118, 123.
17 S. Yin, X. Liu, X. Li, W. Huang, W. Li and B. He, Thin Solid
Films, 1998, 325, 268.
18 C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 56, 799.
19 M. A. Angadi, D. Gosztola and M. R. Waseilewski, J. Appl. Phys.,
1998, 83, 6187.
N,N9-Diphenyl-NTCDI (3). 10 g (0.037 mol) of NTCDA and
6.9 g (0.075 mol) of aniline were refluxed in m-cresol for 12 h.
The reaction mixture was poured into water and the
precipitated solid was filtered, dried and triturated in boiling
toluene, filtered again and dried. Crystals were obtained from
slow crystallization in DMF. Elemental analysis: Calculated
for C, H, and N: 74.64; 3.35; 6.70; Found: 74.36; 3.54; 6.58. 1H
NMR (300 MHz, anisole-d8) d: 7.19 (broad singlet, 8H); 7.71
(broad singlet, 6H).
Acknowledgements
We thank M. Nehama and V. Burtman for their contributions
in the early stages of this project, Dr S. Cohen for single crystal
X-ray analysis, and Dr I. Popov and Dr N. Shimony-Eyal
of the Hebrew University Center for Nanoscience and
Nanotechnology for AFM characterization. S.Y. acknowl-
edges support for this research by the Israel Science
Foundation (ISF) under grant # 225/99-1 and the Israel
Academy of Science and Humanities. Y.O. thanks the Israel
Ministry of Science for an Eshkol scholarship.
20 G. P. Wiederrecht, B. A. Yoon and M. R. Wasielewski, Adv.
Mater., 1996, 6, 535.
21 J. Mizuguchi, J. Appl. Phys., 1998, 84, 4479.
22 (a) V. Guelev, S. Sorey, D. W. Hoffman and B. L. Iverson, J. Am.
Chem. Soc., 2002, 124, 2864; (b) B. M. Aveline, S. Matsugo and
R. W. Redmond, J. Am. Chem. Soc., 1997, 119, 11785.
23 Intra- and inter-molecular excitons were observed in conjugated
polymers: (a) M. Deussen, P. H. Bolivar, G. Wegmann, H. Kurz
and H. Ba¨ssler, Chem. Phys., 1996, 207, 147; (b) M. C. J. M.
Vissenberg and M. J. M. de Jong, Phys. Rev. Lett., 1996, 77, 4820;
(c) E. M. Conwell, Phys. Rev. Lett., 1997, 78, 4301; (d) M. C. J. M.
Vissenberg and M. J. M. de Jong, Phys. Rev. B., 1998, 57, 2667.
24 E. I. Haskal, Z. Shen, P. E. Burrows and S. R. Forrest, Phys. Rev.
B, 1995, 51, 4449.
25 Similar behavior was observed in a single crystal of 1 by excitation
with 390 nm radiation: blue (460 nm) and red (600 nm) PL were
observed. This behavior in the solid-state mimics the organization
in MLE-structures.
26 For simplicity, Fig. 4 shows only the two extreme luminescence
bands, although the entire range between 280–600 nm have
luminescence peaks.
27 Crystal structure of 1: C26H30N2O4, M = 434.52 g mol21, triclinic,
References
1 (a) S. R. Forrest, Nature, 2004, 428, 911; (b) Z. Chen, P. E. Burrows,
V. Bulovic, S. R. Forrest and M. E. Tompson, Science, 1997, 276,
2009; (c) Y. Roichman and N. Tessler, Appl. Phys. Lett., 2002, 80,
151; (d) K. M. Vaeth and K. F. Jensen, Adv. Mater., 1997, 9, 490;
(e) H. Sirringhaus, Adv. Mater., 2005, 17, 2411; (f) Z. Bao,
A. J. Lovinger and A. Dodabalapur, Adv. Mater., 1997, 9, 42; (g)
Y. Sun, Y. Liu and D. Zhu, J. Mater. Chem., 2005, 15, 53; (h)
C. Reese, M. Roberts, M. Ling and Z. Bao, Mater. Today, 2004, 7,
20; (i) H. E. Katz, Chem. Mater., 2004, 16, 4748; (j) M. Ling and
Z. Bao, Chem. Mater., 2004, 16, 4824.
2 (a) H. Ba¨ssler, Primary Photoexcitations in conjugated polymers:
Molecular Exciton versus Semiconducting Band Model, ed.
N. S. Sariciftci, World Scientific, Singapore, 1997, p. 51; (b)
M. Andrzejak, G. Mazur and P. Petelenz, THEOCHEM, 2000,
527, 91.
3 (a) S. R. Forrest, P. E. Burrows, E. I. Haskal and F. F. So, Phys.
Rev. B, 1994, 49, 11309; (b) Y. Imanachi, S. Hattori, A. Kakuta
and S. Numata, Phys. Rev. Lett., 1993, 71, 2098; (c) S. R. Forrest,
Chem. Rev., 1997, 97, 1793.
¯
˚
P1, a = 8.265(2) A, b = 14.504(3) A, c = 4.8883(9) A, a =
˚
˚
3
˚
98.050(3)u, b = 93.611(3)u, c = 96.27(2)u, V = 574.9(2) A ,
T = 293(1) K, Z = 1, m = 0.085 mm21, total reflections = 5933,
4 (a) V. Burtman, A. Zelichenok and S. Yitzchaik, Angew. Chem.,
Int. Ed., 1999, 38, 2041; (b) V. Burtman, A. Zelichenok and
S. Yitzchaik, ‘‘Vapor Phase Molecular Layer Epitaxy via Self-
Assembly Reactions’’, in Semiconductive Polymers, ed. B. R. Hsieh,
ACS Symp. Ser., 1999, vol. 735, ch. 25, p. 399; (c) V. Burtman,
Y. Ofir and S. Yitzchaik, Langmuir, 2001, 17, 2137; (d) Y. Ofir,
O. Shwartzglass, J. Shappir and S. Yitzchaik, ‘‘Organic Thin-Film
Electronics’’, Mater. Res. Soc. Symp. Proc., ed. A. C. Arias,
N. Tessler, L. Burgi and J. A. Emerson, vol. 871E, 2005, I3.22.
5 (a) M. Yokoyama and N. T. Chen, J. Cryst. Growth, 2001, 223,
369; (b) M. Yokoyama, N. T. Chen and H. Y. Ueng, J. Cryst.
Growth, 2000, 212, 97; (c) H. Fujiwara, H. Kiryu and I. Shimizu,
J. Appl. Phys., 1995, 77, 3927.
unique = 2243, I ¢ 2s = 1594, Rint = 0.066, R1 = 0.140, wR2
=
0.178. CCDC reference number 299524. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b601258b.
28 Crystal structure of 3: C26H14N2O4, M = 418.39 g mol21
,
monoclinic, P21/n, a = 5.1360(6) A, b = 7.5227(8) A, c =
˚
˚
3
˚
˚
V = 989.81(19) A , T = 295(1) K, Z = 2,
25.623(3) A,
m = 0.096 mm21, total reflections = 10092, unique = 1952, I ¢
2s = 1116, Rint = 0.076, R1= 0.064, wR2 = 0.125. CCDC reference
number 296236. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b601258b.
29 Due to disorder of the terminal ethylene unit of the hexyl a poor
R-factor was received in XRD analysis of compound 1; hence the
structure is not reported but the CIF file and the overall packing
motif are shown in the ESI{.
6 (a) T. J. Dingemans, S. J. Picken, N. S. Murthy, P. Mark,
T. L. StClair and E. T. Samulski, Chem. Mater., 2004, 16, 966; (b)
30 H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li,
Y.-Y. Lin and A. Dodabalapur, Nature, 2000, 404, 478–481.
31 M. Adachi, Y. Murata and S. Nakamura, J. Phys. Chem., 1995, 99,
14240.
´
E. Bia½ecka-Florjanczyk and A. Orzeszko, J. Mater. Chem., 2000,
10, 1527; (c) P. Eiselt, S. Denzinger and H. Schmidt, Liq. Cryst.,
1995, 18, 257; (d) B. A. Gregg, M. A. Fox and A. J. Bard, J. Am.
Chem. Soc., 1989, 111, 3024.
32 SMART-NT V5.6, Bruker AXS GMBH, D-76181 Karlsruhe,
Germany, 2002.
33 SAINT-NT V5.0, Bruker AXS GMBH, D-76181 Karlsruhe,
Germany, 2002.
34 SHELXTL-NT V6.1, Bruker AXS GMBH, D-76181 Karlsruhe,
Germany, 2002.
7 (a) R. A. Cormier and B. A. Gregg, Chem. Mater., 1998, 10, 1309;
(b) B. A. Gregg and R. A. Cormier, J. Am. Chem. Soc., 2001, 123,
7959.
8 Y. Geerts, U. Keller, U. Scherf, M. Schneider and K. Mu¨llen, Book
of Abstracts, 213th ACS National Meeting, San Francisco, 1997,
American Chemical Society, ORGN-045.
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