Nonlinear optical thin film device from a chiral octopolar phenylacetylene liquid crystal

Article abstract of DOI:10.1021/jo302297j

A set of chiral discotic phenylacetylenes have been synthesized by 3-fold Sonogashira coupling between different ethynylbenzenes and triiodobenzenes. The resultant bulk materials are fully characterized by polarized optical microscopy (POM), differential

Full text of DOI:10.1021/jo302297j

Article
pubs.acs.org/joc
Nonlinear Optical Thin Film Device from a Chiral Octopolar
Phenylacetylene Liquid Crystal
Laura de Vega,† Stijn van Cleuvenbergen,‡ Griet Depotter,‡ Eva M. García-Frutos,§ Berta Gomez-Lor,§
́
Ana Omenat,∥ Rosa M. Tejedor,∥ Jose Luis Serrano,∥ Gunther Hennrich,*,† and Koen Clays*,‡
́
†Universidad Auton
́
oma de Madrid, Cantoblanco, 28049 Madrid, Spain
‡Department of Chemistry, University of Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
§Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
∥Universidad de Zaragoza, CSIC, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: A set of chiral discotic phenylacetylenes have been
synthesized by 3-fold Sonogashira coupling between different
ethynylbenzenes and triiodobenzenes. The resultant bulk materials
are fully characterized by polarized optical microscopy (POM),
differential scanning calorimetry (DSC), and X-ray diffraction. The
octopolar nature of the target compounds is studied by UV−vis
absorption spectroscopy and hyper-Raleigh scattering in solution.
Optimization of the donor−acceptor substitution yields both high
hyperpolarizability values and appreciable mesomorphic properties. A
simple thin film device for second harmonic generation has been
prepared from the nitro-substituted liquid crystalline derivative.
right choice of the terminal end groups allows for controlling
1. INTRODUCTION
the intramolecular charge-transfer character of the π-systems,
their polarizability, and thus the supramolecular order in the
bulk.10 As a further structural element, chirality as imparted by
homochiral substituents on the molecular scaffold should be
efficiently transferred to the macroscopic level and thus
contributes to the microstructuring of the final bulk material.
The use of inherently chiral mesogens results in the formation
of chiral discotic nematic LC phases as recently reviewed.11
Instead of relying on the spontaneous induction of polar order
in chiral domains,12 the introduction of chirality in the
supramolecular material leads predictably to non-centrosym-
metric order in the bulk.13 This idea has been applied recently
by Cui and co-workers in the construction of NLO-active
metal−organic frameworks based on chiral building blocks with
octopolar symmetry.14
Organic materials for second-order nonlinear optics (NLO)
and photonics remain in the focus of interest in material
science.1 Absence of centrosymmetry is a fundamental
requirement for second-order NLO effects to occur. During
the past decade a great deal of effort has been invested in the
design of chromophores that favor the non-centrosymmetric
alignment on the macroscopic scale.2 Supramolecular engineer-
ing of organic and organometallic systems with non-dipolar
symmetry has shown outstanding results in nonlinear optics.3,4
The spontaneous induction of polar order in the crystalline and
liquid crystalline phase has been recently observed in NLO
materials based on octopolar chromophores.5,6 The preparation
of bulk materials by self-assembly of molecular units into
ordered liquid crystal (LC) phases is an extremely useful
concept that has been utilized to prepare organic thin film
devices earlier on.7 The molecular shape and dimensionality of
organic octopoles favors their controlled self-assembly and thus
allows for the preparation of NLO-active thin film devices.8 The
general scheme for the design of discotic liquid crystals is
guided by the following considerations: a polarizable aromatic
nucleus surrounded by apolar lipophilic alkyl chains leads to
nanometric phase segregation. Solvophobic and π-stacking
interactions are responsible for both two-dimensional mobility
of the mesogenic molecules at the same time as 3D
macroscopic orientational order in the bulk material. Densely
functionalized low molecular weight systems such as benzene-
or triazine-based mesogens typically present low aspect ratios,
which favors their self-assembly into columnar stacks.9 The
2. RESULTS AND DISCUSSION
2.1. Synthesis. In this work we present a set of C3-
symmetric alkynylbenzenes with different substitutents at the
terminal phenyl rings and with an electron-rich central benzene
core carrying electron-donor alkoxy chains. In order to achieve
high molecular susceptibilities, i.e., β-values, the molecular
design follows the same deliberations that lead to the desired
LC properties. The electronic nature of the central benzene
core is complemented by the peripheral p-phenylethynyl
Received: October 16, 2012
Published: November 5, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Chiral Octopolar Trisalkynylbenzenes 1−4
Figure 1. Discotics 5 and 6.
fragments of variable electronic nature. The polarizability of the
π-extended disk-shaped molecules depends on the balance of
the donor−acceptor character of the lateral and central aryl
moieties.15 Therefore, the NLO and mesomorphic properties of
the organic material is affected likewise. The synthesis of the
chiral discotics 1−4 follows a similar procedure as described
previously for achiral LC analogues.16 The functionalization of
1,3,5-trifluoro-2,4,6-triiodobenzene with chiral dimethyloctano-
late is followed by a 3-fold Sonogashira coupling of the
triioderivative with the corresponing p-phenylacetylenes to give
the chiral discotic target compounds 1−4 (scheme 1). The
obtained final yields are moderate yet very reasonable for a 3-
fold coupling step.
In this work we also include a series of different ester end-
capped octopoles and study their mesomorphic properties.
Here, the nature and position of the homochiral substituents is
varied to examine the influence of the structural changes in the
molecular building blocks on the macroscopic properties of the
resulting materials. The preparation of the chiral discotics 5 and
6 follows the same synthetic procedure as applied for 1−4
(Figure 1 and Supporting Information).
LC phase, it is a promising candidate as active material in thin
film devices. Octopole 1 forms a mesophase that has been
characterized using polarized light optical microscopy (POM)
and differential scanning calorimetry (DSC). POM reveals an
“oily streaks” texture, characteristic of a chiral nematic discotic
phase between 66 and 99 °C. As common for chiral nematic
LCs,11 no conclusive data have been obtained from XRD
measurements. In the ester-terminated discotics 2, 5 and 6,
only the achiral reference 5 displays LC properties forming a
columnar hexagonal mesophase between 60 and 96 °C with a
lattice constant of a = 21.9 Å and a stacking distance h of 4.1 Å
as determined by X-ray diffraction in the mesophase (75 °C).
POM of 5 displays a characteristic mosaic texture (Table 1,
Figure 2).
The supramolecular arrangement of the octopolar mesogens
of 5 in the LC phase is clearly centrosymmentric, which makes
Table 1. LC Phase Properties of 1 and 5
a
Phase T [°C] (ΔH/kJ mol−1
)
66(16.1)
Cr HoooooooI ND HooooooI I
99(0.8)
1
5
*
2.2. LC Properties. The importance of the polarizability of
the π-system of discotic mesogens for the liquid crystallinity has
been stressed ion various occasions.16,17 While aggregation of
60(26.2)
Cr HoooooooI Colh HooooooI I
96(3.2)
the non-mesogenic 3 can be induced in suitable solvents,18
1
a
Cr, crystalline; I, isotropic liquid; ND*, chiral discotic nematic; Colh,
hexagonal columnar; 2nd heating scan.
and 5 display LC properties. Although solid 2 does not form a
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centered around 1350 cm−1 corresponding to a complex
vibration with contributions (stretching and bending) from the
central and peripheral benzene moieties that directly participate
in the intramolecular self-organization. Various negative bands
in the 1610−1400 cm−1 region, together with the characteristic
CC stretching vibration at 2212 cm−1, show that the
molecular chirality is efficiently transcribed in the LC material.
2.3. Optical and Nonlinear Optical Spectroscopy. UV−
vis absorption spectroscopy in dichloromethane solution clearly
reveals an increase of the charge-transfer character upon
increasing the electron-withdrawing nature of the terminal
phenyl substituent. Compounds 1 and 2 show a pronounced
octopolar character reflected by broad, structureless absorption
bands that are consecutively red-shifted with respect to 3
(Figure 4).
Figure 2. POM image of 1 at 82 °C and 5 at 65 °C.
this material of no use for second-order nonlinear optics,2 and 6
is an isotropic liquid at room temperature. Therefore 5 and 6
have not been the object of further spectroscopic studies.
Cicular Dichroism. In order to evaluate the supramolecular
order of 1 in the chiral discotic nematic mesophase, both
vibrational and electronic circular dichroism (CD) spectra are
recorded at different temperatures. In the absence of any
supramolecular order, the isotropic liquid (>100 °C) gives no
CD response. In the mesophase, a nonabsorptive CD signal
centered at 480 nm is measured between 65 and 95 °C. An
intense CD band appears that rapidly saturates the instrument's
detection system (90−70 °C, first cooling scan). Upon cooling
of the LC phase below its transition temperature, crystalline 1
displays a CD signal of increased intensity due to further
increasing the order in the crystalline bulk (<65 °C). The
signals obtained in the heating scans are less intense than those
measured in the corresponding cooling scans due to a poor
organization of the material.19 The signature and spectral
position of the reflection band permits an estimation of a helical
pitch of around 320 nm for a left-handed helix at the
mesophase temperature (ESI).20
Figure 4. Normalized UV−vis absorption spectra of 1−4 in CH2Cl2.
Vibrational circular dichroism (VCD) is a powerful tool to
study the supramolecular chirality that is employed addition-
ally.21 While 1 is VCD silent in its isotropic liquid state as
expected, basically all molecular features show characteristic
vibrational transitions in the LC state (Figure 3). The CH-
stretching vibrations of the peripheral phenyl units are only
weakly reflected in the VCD spectrum between 2975 and 2940
cm−1. In contrast, the most intense feature is a bisignate signal
Hyper-Rayleigh scattering (HRS) measurements in solution
confirm these findings. In absence of peripheral acceptor
groups (3), and particularly for the pentyl-substituted 4, low β-
values are measured, whereas compounds 1 and 2, substituted
with nitro or methylester acceptor groups, respectively, display
high second-order hyperpolarizabilities β (Table 2).22 Except
Figure 3. VCD (black) and IR (gray) spectrum of liquid crystalline 1 at 70 °C.
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liquid (SHG silent) of 1 enters the chiral nematic discotic
(ND*) phase. The LC phase is demarcated by the dashed gray
lines. We attribute the delayed signal onset to the time required
by the bulk material to adopt the necessary non-centrosym-
metric order. Upon further cooling, the intensity of the signal
increases due to continuous ordering of the LC phase. The
supramolecular order is maintained below 66 °C when the
sample has crystallized, reaching its maximum value around 35
°C. This behavior of the LC phase is in agreement with the
chiral memory effect observed in polymeric and LC materials.27
Table 2. Spectroscopic Properties of 1−4 Measured in
CH2Cl2 Solution
βxxx,0
λmax(abs)
[nm]
βxxx,800
[10−30
esu]
a
log ε
τfl [ns]
[10−30 esu]
σp
1
2
3
4
355
323
296
301
4.798
4.981
4.925
5.073
360 30
220 30
50 20
70 20
60
6
0.78
0.45
3.0 0.2
3.2 0.7
2.5 0.3
65 10
20 10
26 10
0.00
−0.15
a
Fluorescent lifetime (τ) derived from the HRS demodulation
fitting;26 nitro-derivative 1 is virtually nonfluorescent.
4. CONCLUSIONS
for the nitro derivative 1, the determination of the HRS
polarization ratios is hampered by strong (multi)photon
fluorescence of 2−4.23 Given the molecular structure, however,
the octopolar character of the target compounds is beyond
doubt. Table 2 correlates in a conclusive manner the
experimental findings (absorption maxima λmax(abs), extinction
coefficients ε, β-values) attributed to the octopolar character
with the predicted behavior based on the substituent constants
σp for the p-phenyl substituents.24 The supposed discrepancy
between the electronegativity of the nitro and the methylester
functions and the stated hyperpolarizabilities (β0) is the result
of a rough estimation based on a simple two-level model, which
nevertheless is within the accepted error margin. A more
complete and quantitative analysis of the absolute values of the
hyperpolarizabilities requires more elaborated dispersion
models.25
2.4. Second Harmonic Generation (SHG) Measure-
ments. For the preparation of a simple NLO device, a quartz
LC cell was filled with neat 1 in its isotropic liquid state. Upon
repetitive heating (above 100 °C) and subsequent cooling, a
homogeneous film of 1.5 μm thickness is obtained. The sample
is irradiated at 1064 nm (10 Hz, 5 ns pulses) using a Nd:YAG
laser at a 45° angle. The SHG signal at 532 nm is detected in a
reflective mode. In order to avoid a dielectric breakdown by
continuous laser irradiation and a potential alteration of the
geometry of the optics during the heating process, discrete data
points were measured in steps of 5 °C, allowing the signal at
each data point to stabilize (5 min). The SHG signal produced
by the cell itself (<2 times the background noise) is negligible.
Figure 5 shows the NLO response of the thin film device.
Starting at 110 °C, the intensity of the SHG signal increase
becomes evident below roughly 90 °C, when the isotropic
The optimization of the donor−acceptor substitution in the
octopolar discotics is beneficial for both the NLO and the LC
properties of the target compounds. The ambiguous electronic
nature of 3 and 4, an isotropic liquid at room temperature, is
the result of the neutral or electron-donor properties of the
terminal p-phenyl substituents (H or C5H11, respectively). The
acceptor-substituted octopoles 1 (NO2) and 2 (CO2CH3)
display high β-values in solution confirming a pronounced
octopolar character. The high polarizability of the π-extended
aromatic system of 1 is essential for LC behavior as well. The
mesogens form chiral discotic nematic liquid crystals and thus
act as the active layer in a simple, efficient NLO device as
probed by second-harmonic generation experiments. The
crucial non-centrosymmetric order in the bulk is guaranteed
by efficient chirality transfer from the molecular to the
supramolecular level as evidenced by circular dichroism
measurements in the LC bulk. As for the LC properties, it
becomes evident that supposedly small structural changes in the
mesogens provoke dramatic differences in the bulk properties
of the resulting material. Upon comparing the benzoic acid
ester terminated discotics, a change from the linear alkoxy
chains to homochiral alkoxy substituents at the central or
peripheral benzene rings alters drastically the behavior of the
resultant bulk material. We attribute these differences to the
difficulties in the packing of the molecular units with sterically
more demanding side chains (2 vs 5). The importance of the
right proportion of rigid and flexible units within the mesogen
is apparent when comparing liquid 6 with solid 2 or
mesomorphic 5.
In summary, the careful design of small organic molecules in
combination with liquid crystal engineering can give access to
advanced functional materials. However, this work also reveals
the difficulties of this bottom-up approach where the subtle
interplay of numerous structural and electronic parameters
needs to be adjusted to coincide in a favorable way to obtain
the final material.
5. EXPERIMENTAL SECTION
1,3,5-Triiodo-2,4,6-tri[(S)-3,7-dimethyloctyloxy]benzene
(1a). To 1,3,5-triifluoro-2,4,6-triiodobenzene (510 mg, 1.0 mmol) and
sodium(S)-3,7-dimethlyoctanolate (5.0 mmol, prepared in situ by
adding alkanol, 6.0 mmol, dropwise onto neat NaH) is added N,N-
dimethylimidazolidinone (1 mL), and the mixture is stirred at rt
overnight. The reaction mixture is purified by column chromatography
(80:1 hex−Et2O) to leave pure 1a as colorless, waxy solid. Yield: 350
1
mg, 49%. H NMR (300 MHz, CDCl3) δ 4.00 (t, J = 6.2 Hz, 6 H),
2.02−1.94 (m, 3 H), 1.80−1.70 (m, 6H), 1.61−1.48 (m, 3H), 1.42−
1.28 (m, 9H), 1.21−1.14 (m, 9H), 1.00 (d, J = 6.3 Hz, 9H), 0.88 (d, J
= 6.7 Hz, 18H); 13C NMR (125 MHz, CDCl3) δ 166.5, 163.8, 132.5,
131.2, 128.1, 107.7, 96.5, 89.5, 73.3, 52.2, 39.2, 37.5, 37.4, 29.7, 27.9,
24.7, 22.6, 19.6. MALDI-HRMS: calcd for C36H63I3O3 + Na+
947.1803, found 947.1802.
Figure 5. Plot of the SHG signal against temperature upon cooling of
1 from 110 °C.
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General Procedure for the Synthesis of 1, 2, and 4. 1,3,5-
Triiodo-2,4,6-trialkoxybenzene 1a (0.5 mmol) was stirred together
with Pd(PPh3)2Cl2 (0.075 mmol, 53 mg) and CuI (0.075 mmol, 14
mg) in degassed triethylamine (5 mL) for 30 min at room temperature
before the respective phenylethynyl compound (2.5 mmol) was added.
The mixture was heated at 70 °C for 24 h. The solvent was removed,
and the remaining solid was suspended in water (50 mL) and
extracted with ethylacetate (3 × 25 mL). The combined organic layers
were dried (MgSO4) and concentrated in vacuo to give the crude
product, which was purified by column chromatography (20:1 hex−
EtOAc).
1,3,5-Tris[(S)-3,7-dimethyloctyloxy]-2,4,6-tris(4-
nitrophenyl)ethynyl]benzene (1). Yellow, waxy solid; 13% yield
(102 mg). 1H NMR (300 MHz, CDCl3) δ 8.26 (dAB, J = 8.9 Hz, 6 H),
7.65 (dAB, J = 8.9 Hz, 6 H), 4.51−4.37 (m, 6 H), 2.07−1.96 (m, 3 H),
1.88−1.66 (m, 6 H), 1.55−1.42 (m, 3 H), 1.40−1.04 (m, 18 H), 0.93
(d, J = 6.5 Hz, 9 H), 0.81 (d, J = 6.6 Hz, 18 H); 13C NMR (125 MHz,
CDCl3) δ 164.4, 147.2, 131.9, 130.1, 123.8, 107.1, 95.6, 86.7, 73.6,
39.2, 37.5, 37.4, 29.8, 27.9, 24.7, 22.6, 22.5, 19.6. FAB-MS m/z 982
(M+). Anal. Calcd for C60H75N3O9·H2O: C, 72.04.; H, 7.76; N, 4.20.
Found: C, 72.36; H, 7.34; N, 4.40.
1,3,5-Tris[(S)-3,7-dimethyloctyloxy]-2,4,6-tris(4-
methylacetylphenyl)ethynyl)]benzene (2). Waxy, colorless solid;
14% yield (71 mg). 1H NMR (300 MHz, CDCl3) δ 8.04 (dAB, J = 8.2
Hz, 6 H), 7.57 (dAB, J = 8.9 Hz, 6 H), 4.48−4.35 (m, 6 H), 3.94 (s, 9
H), 2.03−1.92 (m, 3 H), 1.85−1.61 (m, 6 H), 1.51−1.38 (m, 3 H),
1.32−1.04 (m, 18 H), 0.92 (d, J = 6.5 Hz, 9 H), 0.81 (d, J = 6.5 Hz, 18
H); 13C NMR (125 MHz, CDCl3) δ 166.5, 163.8, 132.5, 131.2, 128.1,
107.7, 96.5, 84.6, 73.3, 52.2, 39.2, 37.5, 37.4, 29.7, 27.9, 24.7, 22.6,
22.5, 19.6. MALDI-MS m/z 1021 (M+). Anal. Calcd for
C66H84O9·H2O: C, 76.27; H, 8.34. Found: C, 76.29; H, 7.97.
The synthesis and characterization of compound 3 is reported in a
previous paper,18 although the data is erroneously assigned to the
tris[(R)-3-methyloctyl)oxy]benzene derivative.
1,3,5-Tris[4-(n-pentylphenyl)ethynyl]-2,4,6-tris[(S)-3,7-
dimethyloctyloxy]benzene (4). Orange viscous liquid, 24% yield
(129 mg). 1H NMR (300 MHz, CDCl3) δ 7.46 (dAB, J = 8.1 Hz, 6 H),
7.19 (dAB, J = 8.1 Hz, 6 H), 4.48−4.34 (m, 6 H), 2.65 (t, J = 7.7 Hz, 6
H), 2.06−1.95 (m, 3 H), 1.89−1.79 (m, 3 H), 1.75−1.59 (m, 9 H),
1.53−1.44 (m, 3 H), 1.38−1.08 (m, 30 H), 0.96−0.91 (m, 18 H), 0.85
(d, J = 6.5 Hz, 18 H); 13C NMR (125 MHz, CDCl3) δ 162.7, 143.4,
131.3, 128.5, 120.9, 108.5, 97.2, 81.1, 73.0, 39.3, 37.6, 37.5, 35.9, 31.5,
31.0, 29.8, 28.0, 24.7, 22.7, 22.6(3), 22.5(7), 19.7, 14.1; MALDI-MS
m/z 1058 (M+). Anal. Calcd for C75H108O3: C, 85.17; H, 10.29.
Found: C, 85.26; H, 10.21.
ASSOCIATED CONTENT
■
S
* Supporting Information
General methods, 1H and 13C NMR spectra of all new
compounds, and temperature-dependent CD spectroscopy of
thin films of 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gunther.hennrich@uam.es; koen.clays@fys.kuleuven.
be.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Spanish government (MEC,
CTQ2010-18813 and MAT2011-27978-C02-01) is gratefully
acknowledged.
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The ester end-capped discotics 5 and 6 are prepared in the same way
by utilizing the respective ethynylbenzenes.
From 1,3,5-triiodo-2,4,6-tridecyloxybenzene,6 pure 5 is obtained
from the remaining oil as pale colorless solid after column
chromatography (10:1 hex−EtOAc) and final recrystallization from
EtOH in 29% yield (148 mg). 1H NMR (300 MHz, CDCl3) δ 8.04 (d,
J = 8.2 Hz, 6 H), 7.57 (d, J = 8.2 Hz, 6 H), 4.37 (t, J = 6.4 Hz, 6 H),
3.94 (s, 9 H), 1.89 (q, J = 6.3 Hz, 6 H), 1.61−1.51 (m, 6 H), 1.33−
1.21 (m, 36 H), 0.87 (t, J = 6.7 Hz, 9 H); 13C NMR (125 MHz,
CDCl3) δ 166.4, 163.7, 131.1, 129.6, 128.2, 107.7, 96.5, 84.5, 75.0,
52.2, 31.9, 31.2, 30.6, 29.6(2), 29.5(7), 29.3, 26.3, 22.7, 14.1; MALDI-
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8.31. Found: C, 77.03; H, 8.34.
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1
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Hz, 6 H), 4.38 (t, J = 6.2 Hz, 6 H), 4.11−4.27 (m, 9 H), 1.93−1.82
(m, 9 H), 1.62−1.48 (m, 12 H), 1.33−1.18 (m, 12 H), 1.03 (d, J = 6.7
Hz, 9 H), 0.97 (t, J = 7.4 Hz, 9 H), 0.86 (t, J = 6.4 Hz, 9 H); 13C NMR
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