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Liquid Crystals from C3-Symmetric Mesogens for Second-Order Nonlinear Optics.

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Angewandte
Chemie
Functional Materials
DOI: 10.1002/anie.200600019
Liquid Crystals from C3-Symmetric Mesogens for
Second-Order Nonlinear Optics**
Gunther Hennrich,* Ana Omenat, Inge Asselberghs,
Stijn Foerier, Koen Clays, Thierry Verbiest,* and Jos!
Luis Serrano*
promising strategy to meet the growing demand for miniaturization in electronics and photonics.[1] The controlled
supramolecular arrangement of single molecules through
noncovalent interactions can be the key to bridging the gap
between chemistry (molecule design) and physics (bulk
application).[2] In the field of nonlinear optics (NLO) organic
molecules have been promising candidates for more than two
decades. Tailormade molecular materials can be obtained in a
nearly infinite variety by organic synthesis.[3] However, the
predominant tendency toward centrosymmetric ordering of
the single molecules in the bulk phase often impedes the
preparation of self-assembled NLO-active material from
molecular building units.[4] Therefore, the number of applications of organic-molecule-based NLO devices is still limited.[5]
The synthesis of octopolar NLO chromophores, non-centrosymmetric molecules with no dipole moment, is a promising
strategy to overcome this problem on the molecular level.[6]
Amongst the plethora of organic structures, octopolar trisalkynylbenzenes have been shown recently to display extremely
high second-order NLO activity.[7] The abundance of this
structural motif in liquid crystals is also striking. Star-shaped
alkynylbenzenes are flat, rigid, extended aromatic systems
that are ideally suited to forming noncovalent interactions
such as p–p stacking and van der Waals interactions.[8]
Typically, this leads to the formation of different types of
liquid-crystalline mesophases.[9]
We describe here the first members of a new family of
discotic 1,3,5-trisalkynylbenzenes (1–4) with octopolar sym-
The construction of nanoscale photonic devices from functional molecular building blocks in a bottom-up approach is a
[*] Dr. G. Hennrich
Departamento de Qumica Org!nica
Universidad Aut&noma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax: (+ 34) 91-497-3966
E-mail: gunther.hennrich@uam.es
Dr. I. Asselberghs, S. Foerier, Prof. K. Clays, Prof. T. Verbiest
Department of Chemistry
University of Leuven
Celestijnenlaan 200D, 3001 Leuven (Belgium)
Fax: (+ 32) 16-327-982
E-mail: thierry.verbiest@fys.kuleuven.be
Dr. A. Omenat, Prof. J. L. Serrano
Departamento de Qumica Org!nica
Facultad de Ciencias-ICMA
Universidad de Zaragoza-CSIC
50009 Zaragoza (Spain)
Fax: (+ 34) 976-761-209
E-mail: joseluis@unizar.es
[**] This work was supported by the MEC, Spain (BQU2000–0226 and
Ram&n y Cajal contract for G.H.), the CICYT of Spain, the FEDER
funds (EU, project MAT2003-07806-C01), and by the Diputaci&n
General de Arag&n, as well as by the University of Leuven (GOA/
2006/03), the Institute for the Promotion of Innovation through
Science and Technology in Flanders (IWT-Vlaanderen), the FWO-V
(G.0297.04), and the Belgian government (IUAP/5). I.A. acknowledges a fellowship from the Flemish Fund for Scientific Research.
(FWO-V).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 4203 –4206
metry. The donor–acceptor substitution of the arylethynylbenzene scaffold results in degenerate charge transfer within
the octopolar structure, and as a consequence, in an efficient
second-order NLO response. Long alkoxy chains are incorporated into the central benzene core to favor the formation
of different mesophases, depending on the chain lengths. The
synthesis of compounds 1–4 is based on the threefold
Sonogashira coupling of (p-nitrophenyl)acetylene with prefunctionalized 1,3,5-trialkoxy-2,4,6-triiodobenzenes in the
same manner as described recently.[10]
In solution, all octopolar compounds display a broad,
unstructured absorption band above 350 nm, indicative of
efficient charge transfer from the electron-rich benzene core
to the electron-deficient periphery. Owing to the presence of
the nitro substituents, the molecules are virtually nonfluorescent. This facilitates the accurate determination of the
quadratic hyperpolarizabilities b by means of hyper-Rayleigh
scattering.[11] A high second-order nonlinear optical response
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4203
Communications
is observed in accordance with the activities reported for
similar octopolar systems (Table 1).[7, 12] Generally, increasing
chain lengths lead to bathochromically shifted absorption
maxima, enhanced absorptivities, and enhanced b values.
Table 1: Spectroscopic data for 1–4 measured in CH2Cl2.
Cmpd. lmax(abs) [nm] emax [cm2 mol 1] bxxx,800 [10
1
2
3
4
351
358
359
359
20 070
47 916
52 092
51 612
145 2
273 4
320 20
340 20
30
esu] bxxx,0 [10
30
esu]
30 1
52 1
48 3
52 3
Since the donor capacity of the alkoxy substituents is
practically independent of the length of the alkyl chain,[13]
we attribute intramolecular effects, for example, folding of
the alkoxy chains, to be responsible for these tendencies.[14] A
similar phenomenon has been reported for related cyclic
acetylene systems.[15]
Recently, it has been reported that compounds consisting
of a rigid aromatic core substituted with polar or polarizable
groups and without flexible side chains in their structure
exhibit discotic mesophases.[16] Therefore, it is not surprising
that compounds 2–4, which incorporate a rigid aromatic core,
terminal nitro groups, and three alkyloxy side chains, show
some type of discotic mesomorphism as well. Indeed, 2–4
show liquid-crystal properties, which we studied by polarized
light optical microscopy, differential scanning calorimetry,
and X-ray diffraction. The optical properties, transition
temperatures, and enthalpies are summarized in Table 2.
Table 2: Optical and thermal properties of compounds 2–4: Phase
transition temperatures T and enthalpies DH.
Cmpd.
Phase T [8C] (DH [kJ mol 1])[a]
2
3
4
C 120.3 (20.8) ND 143.7 (0.3) I
C 40.0 (4.0) C’ 59.9 Colh 68.5 (12.0)[b] ND 111.9 (0.7) I
C 65–78 (9.7) Colh–ND–I[c]
[a] Second heating scan; C, C’: crystalline, Colh : hexagonal columnar
mesophase, ND : discotic nematic mesophase, I: isotropic liquid.
[b] Enthalpy for both transitions C–Colh -ND [c] Broad peak including
the transitions C–Colh–ND–I.
Compound 2 exhibits a discotic nematic mesophase which is
identified by its typical droplet texture under the microscope
(Figure 1). This assignment was confirmed by X-ray diffraction studies in the mesophase. The driving force to this
nematic order must be the interactions between polar groups
along with the stacking of the planar core of the molecules.
Compounds 3 and 4, which have decyloxy and dodecyloxy
chains, respectively, exhibit both a hexagonal columnar
mesophase and a discotic nematic mesophase. The characteristic mosaic textures of the columnar mesophases are shown
in Figure 2. The increase of the chain length favors a columnar
stacking of the molecules with a C3 symmetry since the
intercolumnar space is filled in an efficient manner with the
alkyl chains, which act as a lubricant between the rigid discotic
columns.
However, in both cases the X-ray diffraction data
indicates large lattice distances too great for discrete inter-
4204
www.angewandte.org
Figure 1. Nematic droplets of 2 at 143 8C upon cooling from the
isotropic liquid.
Figure 2. Hexagonal columnar mesophases. Left: In the second heating process of 3 at 62 8C; right: in the second cooling process of 4 at
69 8C.
molecular p stacking. With a value of 30.7 : obtained for 4,
we propose a unit cell that is composed of a linear dimeric
assembly, subsequently forming columnar stacks. A possible
interaction between NO2 groups of two adjacent molecules,
the supposed driving force for a supramolecular dimerization,
is expected to be weak and cannot be detected by UV/Vis
spectroscopy in either the columnar mesophase or in the
isotropic liquid.[14] The hexagonal columnar and the discotic
nematic mesophases are centrosymmetric, but compound 3
presents a positive NLO response as seen in the SHG
measurements. Therefore, molecules of 3 must arrange in a
non-centrosymmetric superstucture. This fact can be
explained by an orientation effect induced by the cell walls
leading to a non-centrosymmetric arrangement of the mesomorphic material.
For the preparation of an NLO device based on the LC
material, films of 3 100 microns thick were prepared by
heating a small amount of solid 3 between two glass slides
above the melting point, followed by rapid cooling to room
temperature. The glass surface was not subjected to any
previous treatment. The thickness of the sample was controlled by using 100-micron spacers. The sample was placed
on a heating stage for measurements of second-harmonic
intensity versus temperature. The second-harmonic generation (SHG) experiments were conducted by irradiating the
sample at a 458 angle of incidence with the fundamental beam
from a Nd:YAG laser (1064 nm, 10 Hz, 5-ns pulses) and
detecting the second-harmonic light in reflection. SHG
measurements at room temperature on a solid sample of 3
show a fairly low second-harmonic response, as would be
expected for a disoriented sample. Presumably, the small
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4203 –4206
Angewandte
Chemie
amount of SHG observed is due to a surface contribution.
Upon heating, the second-harmonic response suddenly
increased when the temperature of the sample surpassed
40 8C, which corresponds to the LC phase transition
(Figure 3). This sudden increase in SHG can only be
Figure 3. Plots of the temperature of 3 (*) and the second-harmonic
signal intensity (&, in arbitrary units) versus time t.
explained by a change in the sample symmetry. Since SHG
is only allowed in non-centrosymmetric media, breaking of
inversion symmetry must occur. In fact, the octopolar
symmetry of the molecule combined with the discotic
liquid-crystalline character of the sample at this temperature
confirms this assumption. Upon further heating until the
clearing point, the second harmonic intensity suddenly started
to decrease since the sample becomes a centrosymmetric
isotropic liquid. While these preliminary measurements
results on solid samples, which were repeated reproducibly
several times on the same sample, are only qualitative, the
important point to make here is that the sample spontaneously assembles into a non-centrosymmetric structure. In any
case, the second-order NLO activity detected is comparable
to that of conventional poled-polymer systems.[17] This
provides us with a very simple sample preparation method,
eliminating the need for tedious orientation methods such as
electric-field poling or the Langmuir–Blodgett technique.
Hence, these materials could be very useful for nonlinearoptical applications.
In summary, the properties of the different liquidcrystalline mesophases formed by the highly NLO-active
octopoles 2–4 can be adjusted easily by changing the length of
the alkoxy chains in the molecules. Molecules of 3 selforganize in a non-centrosymmetric fashion in the bulk phase
which allows for the construction of a first example of a
simple, efficient NLO device. Ongoing research is directed
towards the optimization of both the LC properties and the
device fabrication and performance (different supports, surface treatment etc.).
Experimental Section
2–4: 1,3,5-Triiodo-2,4,6-trialkoxybenzene (0.5 mmol) was stirred
together with [PdCl2(PPh3)2] (0.075 mmol, 53 mg) and CuI
(0.075 mmol, 14 mg) in Ar-degassed diisopropylamine (5 mL) for
30 min at room temperature before (p-nitrophenyl)acetylene
Angew. Chem. Int. Ed. 2006, 45, 4203 –4206
(2.5 mmol, 354 mg) was added. The mixture was heated at 80 8C for
48 h. The solvent was removed; the remaining solid was suspended in
water (50 mL) and extracted with ethyl acetate (3 E 25 mL). The
combined organic layers were dried (MgSO4) and concentrated in
vacuo to give the crude solid product which was purified by column
chromatography on silica gel (hexane/EtOAc 20:1) and final recrystallization.
1,3,5-Tris[(4-nitrophenyl)ethynyl]-2,4,6-tris(octyloxy)benzene
(2): Recrystallized from EtOH; Yield: 148 mg, 33 %. 1H NMR
(300 MHz, CDCl3): d = 8.25 (dAB, J = 8.8 Hz; 6 H), 7.65 (dAB, J =
8.8 Hz; 6 H), 4.38 (t, J = 6.5 Hz, 6 H), 1.89 (q, J = 6.5 Hz, 6 H), 1.56
(q, J = 7.0 Hz, 6 H), 1.51–1.29 (m, 24 H), 0.84 ppm (t, J = 7.0 Hz, 9 H);
13
C NMR (125 MHz, CDCl3): d = 164.4, 147.1, 131.9, 130.1, 123.8,
107.2, 95.5, 86.6, 75.3, 31.7, 30.6, 29.5, 29.3, 26.3, 22.6, 14.0 ppm.
MALDI-MS: m/z: 898 [M+]. C,H,N analysis (%) calcd for
C54H63N3O9 : C 72.16, H 7.02, N 4.68; found: C 71.80, H 6.96, N 4.74.
1,3,5-Tris[(4-nitrophenyl)ethynyl]-2,4,6-tris(decyloxy)benzene
(3): Recrystallized from iPrOH; Yield: 137 mg, 28 %. 1H NMR
(300 MHz, CDCl3): d = 8.25 (dAB, J = 8.6 Hz; 6 H), 7.65 (dAB, J =
8.6 Hz; 6 H), 4.38 (t, J = 6.5 Hz, 6 H), 1.89 (q, J = 7.0 Hz, 6 H), 1.56
(q, J = 6.5, 6 H), 1.20 (br s, 36 H), 0.86 ppm (t, J = 7.0 Hz, 9 H);
13
C NMR (125 Hz, CDCl3): d = 164.4, 147.1, 131.9, 130.1, 123.8, 107.2,
95.6, 86.7, 75.3, 31.9, 30.6, 29.7, 29.6, 29.4, 26.3, 26.1, 22.7, 14.1 ppm;
MALDI-MS: m/z: 982 [M+]. C,H,N analysis (%) calcd for
C60H75N3O9 : C 73.32, H 7.64, N, 4.28; found: C 73.08, H 7.68, N 4.32.
1,3,5-Tris[(4-nitrophenyl)ethynyl]-2,4,6-tris(dodecyloxy)benzene
(4): Precipitated from CH2Cl2/MeOH; Yield: 106 mg, 20 %. 1H NMR
(300 MHz, CDCl3): d = 8.25 (dAB, J = 8.6 Hz; 6 H), 7.65 (dAB, J =
8.6 Hz; 6 H), 4.38 (t, J = 6.5 Hz, 6 H), 1.89 (q, J = 7.0 Hz, 6 H), 1.57
(q, J = 6.5 Hz, 6 H), 1.21 (br s, 48 H), 0.87 ppm (t, J = 7.0 Hz, 9 H);
13
C NMR (125 MHz, CDCl3): d = 164.4, 147.4, 132.0, 130.4, 124.6,
107.2, 95.6, 86.8, 75.5, 32.3, 31.0, 30.8, 30.3, 30.0, 29.8, 29.6, 29.4, 26.6,
22.9, 14.1 ppm. MALDI-MS: m/z: 1066 [M+]. C,H,N analysis (%)
calcd for C66H87N3O9 : C 74.30, H 8.16, N 3.94; found: C 73.85, H 8.42,
N 3.83.
Received: January 3, 2006
Revised: March 13, 2006
Published online: May 24, 2006
.
Keywords: benzene derivatives · liquid crystals ·
materials science · nonlinear optics · self-assembly
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Communications
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