Supramolecular solid-state assemblies exhibiting electrooptic effects

Full text of DOI:10.1021/ja993554e

546
J. Am. Chem. Soc. 2000, 122, 546-547
nonlinearities (Scheme 1). These supramolecular NLO self-
assemblies were built up through the spontaneous formation of
three parallel hydrogen bonds between a NLO chromophore
bearing two groups of 2,6-diacetamido-4-pyridone (D1-3) and
a monomer with a diimide group or two uracil groups (A1-3).16,17
It was found that these NLO supramolecular self-assemblies can
form amorphous films with good optical quality. Large optical
nonlinearity, electrooptic coefficient (r33) as high as 70 pm/V,
and a long-term stability (4000 h) of the second harmonic
generation (SHG) signal at room temperature were observed.
To prepare the SSA, monomers A1-3 were designed as the
hydrogen acceptors and monomers D1-3 are composed with the
NLO chromophore and two hydrogen-bonding moieties, with 2,6-
diacetamido-4-pyridone.18 The use of the hexafluoroisopropyl-
idene group is to enhance the solubility of monomers in common
organic solvents (CHCl3 and THF). A homogeneous solution were
prepared by mixing monomer A (dissolved in THF by sonication
and warming) with 1 equiv of monomer D. After THF was
removed, the resultant solid dissolved in CHCl3 and the NLO
self-assemblies were prepared from that solution as glassy solid
films with good optical quality. It is interesting to note that
compounds A1-3 are only slightly soluble in chloroform, but
the resulting assemblies are very soluble in chloroform and form
a homogeneous solution. This suggests that materials of reasonable
molecular weights are formed from the self-assembly of two
Supramolecular Solid-State Assemblies Exhibiting
Electrooptic Effects
Haythem Saadeh, Liming Wang, and Luping Yu*
Department of Chemistry and The James Franck Institute
The UniVersity of Chicago, 5735 South Ellis AVenue
Chicago, Illinois 60637
ReceiVed October 4, 1999
In the past few decades, supramolecular chemistry has grown
into a fascinating and multidisciplinary research area and numer-
ous new concepts concerning intermolecular noncovalent interac-
tion have been studied extensively.1-5 More recently, principles
of supramolecular chemistry have been introduced into the area
of materials science.5,6 These materials have the ability to undergo
spontaneous assembly into solid forms such as molecular mono-
or multilayered films, liquid crystal phases, and other solid-state
lattices.6-9Attractive examples include the work of Jean-Marie
Lehn’s school and Jean Fretchet’s group on supramolecular liquid
crystalline materials, and more recently, the work by Meijer et
al.8-10
However, the potential for combining supramolecular chemistry
with materials science has not been well explored. Few reports
exist describing supramolecular electrooptic materials formed
through noncovalent bonding.11 One possible reason is that scien-
tists are preoccupied by covalently linked polymers and their
advantageous mechanical strength.12 Supramolecular electrooptic
assemblies have a great potential to compete with traditional poly-
mers. Unlike polymers, their components are easy to purify and
the assembly is easy to prepare. For example, organic second-
order nonlinear optical materials have been studied for more than
two decades.12,13 Obstacles for the practical application of these
materials still exist, particularly in nonlinear optical performance.14
The key problem is the difficulty in incorporating new NLO
chromophores exhibiting large µâ values into a suitable polymer
backbone, because most of these chromophores are chemically
unstable and tend to decompose during the polymerization stage.15
The supramolecular self-assembly (SSA) provides an attractive
solution to this critical issue. The SSA can generate a polymer-
like solid state under mild conditions so that the NLO chro-
mophore can survive.
19-21
components D1-3 and A1-3.
The 1H NMR spectra for all of the NLO self-assemblies were
recorded in CDCl3 and revealed the formation of triple-hydrogen
bonding. A clear downfield shift in the NH signals compared to
parent monomers was observed. For example, in SS7, all NH
protons of A3 exhibited a downfield shift from δ 9.35 to 11.55,
respectively, in D1 from δ 7.56 to 8.98 (Figure 1). This char-
acteristic downfield shift is due to the formation of a triple-hydro-
gen-bonded assemblies.22,23
The molecular weight measurements of these assemblies using
GPC were not successful in THF and complete dissociation was
observed. The viscosity measurements of chloroform solutions
of SS1-7 with concentrations in a range of 0.04-0.065 g/dL,
using an Ubbelohde capillary viscometer, indicated an intrinsic
viscosity (η) around 0.06-0.09 dL/g (Table 1) for all of the SS1-
7. These values should be viewed with caution because in diluted
solutions, these systems are expected to have a small intrinsic
viscosity (η) as they exist in an equilibrium with the component
parts.
Thermal analysis by using DSC showed an exothermic transi-
tion above 120 °C, at which the hydrogen bonding is broken and
the SSA system dissociates into its components. Indeed, films of
these materials can be heated to temperatures below 120 °C for
more than 1 h without changing the optical quality of these films.
These systems exhibit a glass transition temperature (Tg) between
75 and 110 °C (Table 1). Like the corresponding E-O polymers,
In this paper we report the synthesis and characterization of
NLO supramolecular self-assemblies exhibiting large optical
(1) Lehn, J.-M. Supramolecular Chemistry; VCH: Weiheim, 1995.
(2) (a) Tsoucaris, G. Current Challenges on Large Supramolecular
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Keser, M.; Amstutez, A. Science 1997, 276, 384.
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Commun. 1994, 197.
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J. H.; Lange, R. F.; Lowe, J. K.; Meijer, E. W. Science 1997, 27, 1601.
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Famulok, M.; Rebek, J., Jr J. Am. Chem. Soc. 1991, 113, 201.
(18) (a) Detailed descriptions are described in the Supporting Information.
(b) The second harmonic generation (SHG) signals of the poled NLO
polymeric self-assembly films were measured by using a model-locked Nd:
YAG laser (Continuum-PY61C-10 with a pulse width of 25 ps and a repetition
rate of 10 Hz) as a fundamental source (1.064 µm). A quartz crystal was
used as the reference sample.
(12) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994, 94,
31.
(13) (a) Dalton, L. R.; Harper, A. W.; Ghosen, R.; Steier, W. H.; Zairi,
M.; Fetterman, H.; Shi, Y.; Mustacich, R. V.; Jen, A. K.-Y; Shea, K. J. Chem.
Mater. 1995, 7, 1060. (b) Dalton, L. R.; Harper, A. W.; Wu, B.; Ghosen, R.;
Laquindanum, J.; Laing, Z.; Hubble, A.; Xu, C. AdV. Mater. 1995, 7, 519.
(14) Saadeh, H.; Gharavi, A.; Yu, D.; Yu, L. Macromolecules 1997, 30,
5403.
(19) Jorgensen, W. L.; Pranata, J. J. Am. Chem. Soc. 1990, 112, 2008.
(20) Pranata, J.; Wierschke, S. G.; Jorgensen, W. L. J. Am. Chem. Soc.
1991, 113, 2810.
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Blanchard-Desce, M.; Perry, J. W.; Skindhoj, J. Science 1994, 263, 511. (b)
Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton, K. M.;
Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599.
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10.1021/ja993554e CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/08/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 3, 2000 547
Table 1. The Optical Properties of Self-Aassembled Polymers
Scheme 1. Synthesis of Supramolecular Self-Assemblies
SS1-7
b
H-dono H-acc viscosity
Tg
Td
r33
(pm/V)
(D)
(A)
(ηint
)
(°C) (°C) λmax
Φ
SS1
SS2
SS3
SS4
SS5
SS6
SS7
D1
D1
D2
D2
D2
D3
D3
A1
A2
A1
A2
A3
A2
A3
0.075
0.06
a
150 467 0.18
125 466 0.15
166 543 0.19
140 541 0.20
166 540 0.19
123 579 0.2
16
10
40
36
38
NA
70
a
NA
93
72
a
0.066
0.083
0.065
0.09
a
100 166 577 0.22
a The Tg was not observed. b Reproducible within experimental error
of 20%.
Scheme 2. Structures of SSA monomers A1-3 and D1-3
Figure 2. The SHG signal of SS3, SS4, and SS5 as a function of
temperature (heating rate ) 3 deg C/min). The inset shows the temporal
stability of SHG signals of SS1 and SS2 at room temperature in the air.
It was found that the poled sample showed a stable SHG signal
until the temperature gets closer to Tg of the material, wherein
the SHG signals disappear completely at Tg. Figure 2 shows the
SHG signal as a function of temperature for selected self-
assemblies (SS3, SS4, SS5). Self-assemblies made of either A1
or A3 showed better stability of the SHG signal than the ones
made of A2. At room temperature in air, all of the SS series were
stable and showed no decay in the intensity of SHG signal after
4000 h, as shown in Figure 2 (inset). These behaviors are quite
similar to corresponding polymeric materials.
The electrooptic coefficients (r33) were measured using a simple
reflection technique at 1300 nm,24 which is beyond the absorption
band of the NLO chromophore. The r33 measurements of SSA
films25 revealed a large E-O coefficient for SSA systems made
from D3 (r33 values of 50-70 pm/V). Systems prepared from
D1 showed the lowest r33 values (10-16 pm/V). Systems made
from D2 showed r33 values around 40 pm/V (Table 1). The trend
in r33 values is consistent with the µâ values of corresponding
chromophores. The highest r33 values are twice as large as
LiNBO3 and are promising for future exploration for application.
In conclusion, molecular recognition between complementary
hydrogen-bonding units could be utilized to synthesize supramo-
lecular self-assembly for electrooptic applications. The mild
preparatory conditions allow the chemically sensitive NLO
chromophores to survive. The resulting SSAs exhibit large r33
values (10-70 pm/V at 1.3 µm) and a good temporal stability at
room temperature. Various structural parameters can be changed
to fine-tune the physical properties of the SSA materials. Thus,
this approach to electrooptical materials is promising.
Acknowledgment. This work was supported by ONR and the National
Science Foundation. This work also benefited from the support of the
NSF MRSEC program at the University of Chicago.
Supporting Information Available: General experimental procedures
and 1H NMR and elemental analyses of all new compounds (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 1. 1H NMR of A3 and D3 and their supramolecular self-assembly
SS7.
JA993554E
the molecular dipoles in these SSA can be efficiently aligned along
the direction of the high electric field. Order parameters of 0.15-
0.20 (Φ ) 1- A1/A0, A0 and A1 are the absorbances of the SSA
film before and after corona poling, respectively) were observed.
(24) Teng, C. C.; Man, H. T. Appl. Phys. Lett. 1990, 56, 1734.
(25) A spin-coated film (0.4-0.60 µm) on an Indium-Tin-Oxide (ITO)
substrate was poled under a corona discharge at a temperature range from 70
to 110 °C. The poled film was coated with silver electrodes 0.1 µm thick.