Alkylene-chain effect on microwire growth and crystal packing of π-moieties

Article abstract of DOI:10.1021/cm300747v

We developed a facile approach to modulate molecular arrangement through dimerizing of π-moieties and tuning alkylene-bridge length. Dimers of fluoranthene-fused imide (DFAI-Cns) with various lengths of alkylene chains were synthesized by a Diels-Alder re

Full text of DOI:10.1021/cm300747v

Article
pubs.acs.org/cm
Alkylene-Chain Effect on Microwire Growth and Crystal Packing of
π‑Moieties
Lin Ding,† Hai-Bin Li,‡ Ting Lei,† Han-Ze Ying,† Rui-Bo Wang,† Yan Zhou,† Zhong-Min Su,*,‡
and Jian Pei*,†
†Beijing National Laboratory for Molecular Science (BNLMS), the Key Laboratory of Bioorganic Chemistry and Molecular
Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡College of Chemistry, Northeast Normal University, Changchun 130024, China
S
* Supporting Information
ABSTRACT: We developed a facile approach to modulate molecular arrangement through
dimerizing of π-moieties and tuning alkylene-bridge length. Dimers of fluoranthene-fused imide
(DFAI-Cns) with various lengths of alkylene chains were synthesized by a Diels−Alder reaction
followed by decarbonylation. DFAI-C3 and DFAI-C5 with odd-carbon alkylene chains display
stronger one-dimensional growing tendency and better crystallinity than those with even-carbon
alkylene chains. Microwires of dimers with odd-carbon alkylene chains were successfully
obtained, and their molecular packing was analyzed by transmission electron microscopy (TEM)
and selected-area electron diffraction (SAED). Systematic investigation of their single crystal
packing showed that alkylene chains with different lengths produced two kinds of molecular
configurations. The V-shaped molecular configuration was observed from DFAI-C3 and DFAI-
C5 with odd-carbon alkylene chains; however, the Z-shaped one was observed from DFAI-C4,
DFAI-C6, and DFAI-C12 with even-carbon alkylene chains. Accordingly, we attributed the
diverse microstructures and crystallinity of the dimers to their distinct molecular configurations
in single crystals. In addition, a computational method was employed to demonstrate the weak intermolecular interactions in
these dimers. Our investigation indicates that the introduction of bridging alkylene chains is an effective approach to modulate
microwire growth and crystal packing of π-systems in solid state.
KEYWORDS: alkylene-chain effect, organic microwire, crystal packing, fluoranthene-fused imide
structure, enhancing both charge mobility and OPV perform-
ance.11a Yamaguchi introduced alkylene-chain linkers as
macrocyclic restriction, achieving a wide range of fluorescence
INTRODUCTION
■
Intensive attention has been paid to one-dimensional nano/
microstructures based on π-molecules for their potential
applications in integrated nano/micro-optoelectronics,1 includ-
ing organic field-effect transistors (OFETs),2 explosive
detectors,3 and photoswitches.4 The morphology and opto-
electronic property of materials have a close relationship with
molecular arrangement in solid state, which is determined by
their single crystal structures.1 Various approaches were
employed to modulate molecular arrangement in solid state,
such as π−π stacking,5 hydrogen-bonding,6 donor−acceptor
tuning by changing lengths of the linkers.13
In our previous contribution, a series of fluoranthene-fused
imides (FAIs) were developed and their lowest unoccupied
molecular orbital (LUMO) levels were effectively tuned from
−3.2 to −3.8 eV through molecular engineering.14 Herein, we
choose FAI derivatives as a model to study the alkylene-chain
effect on molecular packing in solid state. Alkylene chains
bridged two FAI moieties to construct a series of dimers of
fluoranthene-fused imides (DFAI-Cns). They exhibit a differ-
ent tendency of microstructure growth and crystal packing
structures by tuning the length of alkylene chains. Density
functional theory (DFT) studies are performed to obtain
transfer integrals and understand intermolecular interactions in
their crystal packing.
interaction,7 and hydrophobic interaction.8 Wurthner and Bao
̈
reported a twisted octachloroperylene diimide with high OFET
performance by proper crystal engineering.9 Anthony devel-
oped a series of silylethynyl-substituted pentacene derivatives,
and the crystal structure of pentacene unit was significantly
changed from herringbone to brickwork style.10
Over the past few years, alkyl chains become more important
in molecular engineering and attract more consideration during
molecular design for organic materials.11 Different alkyl chains
and their substituted positions in molecular skeletons affect
crystal packing and thereby lead to distinct optoelectronic
properties.12 The investigation of Facchetti revealed that N-
alkenyl substituents afforded a more compact solid-state
Received: March 7, 2012
Revised: April 23, 2012
Published: April 23, 2012
© 2012 American Chemical Society
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RESULTS AND DISCUSSION
■
Synthesis and Photophysical Properties. Scheme 1
illustrates the synthetic route to FAI-C4 and DFAI-Cns.
Scheme 1. Synthetic Route to FAI-C4 and DFAI-Cns
Figure 1. Absorption spectra (a) in dilute CH2Cl2 solutions (1 × 10−5
M) and (b) in thin films.
Cyclopentadienone 3 was prepared by an aldol condensation
reaction with 1,3-bis(4-methoxyphenyl)propan-2-one 2 and
acenaphthequinone 1 in a yield of 88%.14 An efficient Diels−
Alder reaction followed by decarbonylation between 3 and 5−
915 at 180 °C afforded dimers DFAI-C3, DFAI-C4, DFAI-C5,
DFAI-C6, and DFAI-C12. The yields ranged from 47 to 60%.
For comparison, model compound FAI-C4 was also prepared
following the same procedure using 3 and 1-butyl-1H-pyrrole-
2,5-dione 4.15
Compared with FAI-C4, dimers DFAI-Cns exhibit distinct
properties, including solubility and thermal stability. Although
the solubility of DFAI-Cns is dramatically decreased in
common solvents (CHCl3 etc.), DFAI-C3 and DFAI-C5
with odd-carbon alkylene chains showed better solubility than
DFAI-C4, DFAI-C6, and DFAI-C12 with even-carbon alkylene
chains. DFAI-Cns also have higher thermal stability (Td > 380
°C) than FAI-C4 (Td = 330 °C).
The photophysical properties of FAI-C4 and DFAI-Cns
were investigated both in dilute CH2Cl2 solutions and in thin
films. As illustrated in Figure 1, all dimers show identical
absorption features to model FAI-C4, with three absorption
peaks at 310, 340, and 404 nm. Absorption spectra in thin films
were obtained by spin-casting from their CH2Cl2 solution (0.1
mg/mL). The maximum peaks of three absorption bands in
thin films show slightly red shift about 10 nm relative to those
in dilute solutions. This result reveals that these compounds
exhibit weak intramolecular and intermolecular π−π inter-
actions in solid state.
Figure 2. SEM images of microstructures of FAI-C4 and DFAI-Cns
obtained by slow evaporation of their chloroform/ethanol mixed
solvent. (a) FAI-C4, (b) DFAI-C3, and (c) DFAI-C5 with a strong
tendency of one-dimensional growth; (d) DFAI-C4, (e) DFAI-C6,
and (f) DFAI-C12 without oriented growing tendency.
slowly evaporating their solutions (1 mg/mL in chloroform/
ethanol mixture). FAI-C4 forms decent one-dimensional (1D)
microwire. DFAI-C3 and DFAI-C5 with odd-carbon alkylene
chains preserve the same tendency of 1D growth as FAI-C4
and form well dispersed microwires. In contrast, for dimers
with even-carbon alkylene chains, they do not form good 1D
assemblies. For DFAI-C4, it assembles into thin sheet-like
Microstructure Analysis. Figure 2 illustrates scanning
electron microscopy (SEM) images of microstructures formed
by these compounds. The microstructures were all obtained by
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microstructure, which is quite bright under the microscope
because of electron accumulation on its nonconductive surface.
DFAI-C6 generates brickwork-like assemblies, and the
assemblies of DFAI-C12 display a disordered morphology.
In order to understand the relationship between crystal
packing and microstructures, single-crystal X-ray diffraction
analysis of these compounds was performed. Employing
different crystal growing conditions, we successfully cultivated
single crystals of all the compounds. Transmission electron
microscopy (TEM) and selected-area electron diffraction
(SAED) analysis were used to determine the crystallinity of
the microstructures (Figure 3 and Figure S2). FAI-C4, DFAI-
not well crystallized. SAED patterns of DFAI-C6 and DFAI-
C12 were not obtained due to their poor crystallinity.
Figure 4 shows molecular configurations in single crystals of
these compounds. FAI-C4 has two p-methoxyphenyl groups
nearly perpendicular to its fluoranthene-fused core. The torsion
angle is measured to be 69.5°. DFAI-C3 (Figure 4b and 4c)
and DFAI-C5 (Figure 4d and 4e) with odd-carbon alkylene
chains exhibit a V-shaped molecular configuration; whereas
DFAI-C4 (Figure 4f), DFAI-C6 (Figure 4g), and DFAI-C12
(Figure 4h) with even-carbon alkylene chains exhibit a Z-
shaped molecular configuration. With a short and odd-carbon
bridging alkylene chain, DFAI-C3 looks like a clip. The
dihedral angle of two side walls is 27.0° and the torsion angle is
38.4°. Due to a longer alkylene chain, for DFAI-C5, the
dihedral angle increases to 64.4° and the torsion angle reduces
to 17.7°. This configuration exposes a larger π-moiety to
achieve better orbital overlap. DFAI-C4 has a symmetric
molecular configuration with a vertical distance of 2.79 Å
between two side walls. The Z-shape of DFAI-C6 has a little
deformation with two nonparallel side walls. Due to a longer
bridging alkylene chain, the vertical distance increases to 6.28
Å. However, when the alkylene chain is too long, a gauche
conformation is observed in DFAI-C12. As a result, the vertical
distance of DFAI-C12 (0.57 Å) between two side walls
becomes even shorter than DFAI-C4’s. Obviously, alkylene
chains in these compounds adopt the optimal zigzag
conformation except in DFAI-C12. The variation in molecular
configurations of DFAI-Cns caused by alkylene-chain effect
tremendously changes the whole single crystal packing. The
existence of methoxyphenyl groups weakens the π−π
interactions in DFAI-Cns, and hence alkylene chains play
dominant roles in crystal engineering.
Crystal Packing and Theoretical Calculations. Figure 5
illustrates the single crystal packing diagrams and DFT
calculation results of FAI-C4, DFAI-C3, DFAI-C4, DFAI-C5,
and DFAI-C6. Because the single crystal quality of DFAI-C12
is not very good, its calculation and analysis are not discussed
here (see Figure S3 for its packing diagram). Crystal packing
structures were calculated by software Platon16 to provide the
information of distances of π−π stacking and exact moieties
participating in π−π interactions. DFT calculations were
performed to obtain the electron transfer integral, which is an
important factor to determine the carrier mobility of organic
materials.17 It was evaluated on the basis of fragment orbital
approach18 together with basis set orthogonalization proce-
Figure 3. TEM images and SAED patterns of the microwires (the
same batch as used in SEM images) formed by DFAI-C3 (a, b) and
DFAI-C5 (c, d). As indicated by the white arrows, both microwires are
growing along the a axis.
C3, and DFAI-C5 exhibit a set of single crystal diffraction
patterns, revealing that the microwires are single crystals. In
addition, these patterns are indexed according to their single
crystals. This result indicates the microwires have identical
crystal packing with their single crystals. Both DFAI-C3 and
DFAI-C5 show good 1D growing tendency along (100)
direction (a axis). However, SAED patterns of DFAI-C4 show
multiple sets of diffraction spots, indicating its microstructure is
Figure 4. Molecular configurations in single crystals of FAI-C4 (a), DFAI-C3 (b, c), DFAI-C5 (d, e), DFAI-C4 (f), DFAI-C6 (g), and DFAI-C12
(h).
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Figure 5. Single crystal packing diagrams and corresponding transfer integrals of (a) FAI-C4, (b) DFAI-C3, (c) DFAI-C5, (d) DFAI-C4, and (e)
DFAI-C6.
dure19 at the SAOP/TZ2P20 level of theory using the ADF
(Amsterdam density functional) package.21
a good lamellar packing with a π−π stacking distance of 3.84 Å
and a transfer integral value of 22 meV (Figure 5d). The
rotational angle between layers is 48.2°. The uniform lamellar
structure and continuous π−π interactions between layers
produce the sheet-like microstructure of DFAI-C4. DFAI-C6
shows π−π distances of 3.70 and 3.73 Å between side walls of
adjacent molecules (Figure 5e). The only transfer integral value
of 14 meV is attributed to the π−π interactions with the
distance of 3.73 Å. Another distance of 3.70 Å has no
contribution to the π−π orbital interactions because of a small
overlap area between molecules. DFAI-C12 has even more
discrete π−π interactions as shown in Figure S3. Lack of
efficient molecular interactions in DFAI-C6 and DFAI-C12
finally generates poorly crystallized microstructures. As shown
in crystal packing and theoretical calculation data, FAI and
DFAI-Cns exhibit weak π−π interactions owing to the
introduction of the methoxyphenyl groups. This result is
coincident with its absorption and emission spectra variation
from solutions to thin films.
Model FAI-C4 adopts a lamellar fashion packing structure.
Molecules are stacked in pairs through weak π−π interactions
of the imide-rings with a rotation angle of 27.4° between every
two layers. The π−π stacking distance is 3.70 Å, and the
corresponding transfer integral value is 34 meV. The value of
15 meV originates from the overlap between methoxyphenyl
group and fluoranthene-fused imide core. Because of large
transfer integrals, the microstructure has strong tendency of 1D
growth along the π−π stacking direction (see SAED patterns of
FAI-C4 in Figure S2).
DFAI-C3 adopts a sandwich-fashion packing structure, with
both intramolecular and intermolecular π−π interactions.
Three adjacent molecules are stacked head to head and tail
to tail (head: alkylene-chain segment; tail: side-wall segment).
The main π−π interactions derive from intermolecular
interactions between side walls of adjacent molecules with
distances of 3.57 Å and 3.61 Å. They contribute to the transfer
integral values of 10 and 17 meV, respectively. However, the
intramolecular π−π interactions are quite weak with a long
distance of 3.85 Å without any contribution to transfer
integrals. With major π−π interactions along the a axis,
DFAI-C3 exhibits strong tendency of 1D growth along this
direction, which is consistent with its TEM and SAED analysis.
DFAI-C5 adopts a brickwork fashion packing structure,
providing slipped face-to-face π−π interactions. The crystal
packing diagram is quite similar to that of rubrene.22 No short-
axis displacement is observed, and a suitable long-axis
displacement is predicted. There exists a close slipped π−π
overlap between side walls with a distance of 3.62 Å, leading to
a large transfer integral value of 66 meV. In this crystal
structure, sufficient and successive π−π orbital overlap is
achieved. With better π−π interactions along the a axis than the
b axis, DFAI-C5 exhibits a strong tendency of 1D growth along
this direction, which has been proved by its TEM and SAED
analysis.
CONCLUSIONS
■
In conclusion, we have developed a series of dimers DFAI-Cns
through the efficient Diels−Alder reaction followed by
decarbonylation. Different lengths of alkylene chains are
employed in the molecular design as bridging groups to
connect two FAI units, providing a good understanding of
alkylene-chain effect on the molecular arrangement in solid
state. Microwires are obtained by dimers with odd-carbon
alkylene chains, whereas dimers with even-carbon alkylene
chains do not show strong tendency of 1D growth. Single
crystal analysis is performed to explain the exact crystal packing
in these compounds. Because of weak π−π interactions in this
system, the zigzag conformation of alkylene chains is inherited
into the molecular geometry of the dimers. For compounds
with odd-carbon alkylene chains, they produce a successive and
oriented π−π overlap and hence a stronger tendency of 1D
growth. However, for the even ones, side walls are away from
each other, resulting in discrete and nonoriented molecular
interactions. The introduction of methoxyphenyl groups in π-
moieties weakens π−π stacking interactions and makes the
alkylene chain play dominant roles in crystal packing. DFT
For compounds with even-carbon alkylene chains, their
crystal packing diagrams are quite different due to the Z-shaped
molecular geometry. They all arrange in a lamellar fashion with
even weaker and more discrete π−π interactions. DFAI-C4 has
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alkylene chains well controls the geometry in such a weak π−π
interaction system. It provides us a new way to produce
desirable organic microwire materials and the concept of careful
consideration of alkyl chains during molecular design in weak
π-systems.
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ASSOCIATED CONTENT
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6320−6323.
■
S
* Supporting Information
Experimental procedure, single crystal data, H and 13C NMR
1
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A.-M.; Bao, Z.; Wurthner, F. Angew. Chem., Int. Ed. 2010, 49, 740−
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spectra of all the compounds. Figure S1 shows emission spectra
in solutions and in thin films. Figure S2 illustrates the TEM
images and SAED patterns of FAI-C4 and DFAI-C4. Figure S3
displays the crystal packing structure of DFAI-C12. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jianpei@pku.edu.cn.
■
(11) (a) Bagnis, D.; Beverina, L.; Huang, H.; Silvestri, F.; Yao, Y.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Weiss, H. C.; Blaser, D. Angew. Chem., Int. Ed. 1999, 38, 988−992.
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We thank Mr. Hai Fu for his efforts in single crystal structure
analysis. This work was supported by the Major State Basic
Research Development Program from the Ministry of Science
and Technology (No. 2009CB623601) and National Natural
Science Foundation of China.
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