Synthesis, structural and nonlinear optical properties of 2-(3-Cyano-4-{5-[1-(2-Hydroxyethyl)-3,3-dimethyl-1,3-dihydro-indol-2-ylidene] -Penta-1,3-dienyl}-5,5-dimethyl-5H-furan-2-ylidene)-malononitrile

Article abstract of DOI:10.1080/15421406.2011.556542

A chromophore for nonlinear optics with extended conjugation has been synthesized, and its structure determined from X-ray diffraction data. The compound crystallizes in the monoclinic system with the space group P2 ₁/n and Z = 4. The unit cell parameters are a = 10.7826(4) A, b = 14.5943(5) A, c = 15.3792(5) and beta = 96.213(2). A thin film containing 5% of the chromophore in amorphous polycarbonate when poled at 180C and 60 V/m yields a maximum r33 value of 206 pm/V. This is seven times higher than the value found for the archetypical inorganic material, lithium niobate. Copyright Taylor &Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.556542

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Synthesis, Structural and Nonlinear
Optical Properties of 2-(3-Cyano-4-{5-
[1-(2-Hydroxyethyl)- 3,3-Dimethyl-1,3-
Dihydro-Indol-2-ylidene]-Penta-1,3-
dienyl}-5,5-Dimethyl-5H-Furan-2-
ylidene)-Malononitrile
M. D. H. Bhuiyan a , G. J. Gainsford a , Y. Kutuvantavida a b , J. W.
Quilty a , A. J. Kay a , G. V. M. Williams a & M. R. Waterland b
a Photonics Group , Industrial Research Ltd , Lower Hutt, New
Zealand
b Institute of Fundamental Sciences , Massey University , Palmerston
North, New Zealand
Published online: 07 Oct 2011.
To cite this article: M. D. H. Bhuiyan , G. J. Gainsford , Y. Kutuvantavida , J. W. Quilty , A. J. Kay , G.
V. M. Williams & M. R. Waterland (2011) Synthesis, Structural and Nonlinear Optical Properties of 2-
(3-Cyano-4-{5-[1-(2-Hydroxyethyl)- 3,3-Dimethyl-1,3-Dihydro-Indol-2-ylidene]-Penta-1,3-dienyl}-5,5-
Dimethyl-5H-Furan-2-ylidene)-Malononitrile, Molecular Crystals and Liquid Crystals, 548:1, 272-283,
DOI: 10.1080/15421406.2011.556542
To link to this article: http://dx.doi.org/10.1080/15421406.2011.556542
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Mol. Cryst. Liq. Cryst., Vol. 548: pp. 272–283, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.556542
Synthesis, Structural and Nonlinear Optical
Properties of 2-(3-Cyano-4-{5-[1-(2-Hydroxyethyl)-
3,3-Dimethyl-1,3-Dihydro-Indol-2-ylidene]-Penta-
1,3-dienyl}-5,5-Dimethyl-5H-Furan-2-ylidene)-
Malononitrile
M. D. H. BHUIYAN,1 G. J. GAINSFORD,1
Y. KUTUVANTAVIDA,1,2 J. W. QUILTY,1 A. J. KAY,1,∗
G. V. M. WILLIAMS,1 AND M. R. WATERLAND2
1Photonics Group, Industrial Research Ltd, Lower Hutt, New Zealand
2Institute of Fundamental Sciences, Massey University, Palmerston North,
New Zealand
A chromophore for nonlinear optics with extended conjugation has been synthesized,
and its structure determined from X-ray diffraction data. The compound crystallizes in
the monoclinic system with the space group P21/n and Z = 4. The unit cell parameters
are a = 10.7826(4) Å, b = 14.5943(5) Å, c = 15.3792(5) Å and beta = 96.213(2)◦. A
thin film containing 5% of the chromophore in amorphous polycarbonate when poled at
180◦C and 60 V/µm yields a maximum r33 value of 206 pm/V. This is seven times higher
than the value found for the archetypical inorganic material, lithium niobate.
Keywords Electro-optic coefficient; nonlinear optical chromophore; X-ray crystallog-
raphy
Introduction
There is an ongoing interest in developing device quality organic nonlinear optical (NLO)
materials as the underpinning technology for a number of photonic and electro-optic (EO)
devices in industries such as telecommunications, sensing, and computing. Consequently,
recent attention has focused increasingly on new organic solid-state materials as they are
expected to offer extremely high optical nonlinearities [1, 2] and will allow EO devices to
operate efficiently at relatively low power levels [3]. This is because most of the existing
EO devices are based on inorganic compounds such as crystalline lithium niobate (LiNbO3)
[4]. However, inorganic materials such as this have moderate EO coefficients (32 pm/V)
and yield devices with a high drive voltage (c. 5V). Furthermore, inorganic substrates are
not processable and therefore are not able to be cast or spun as thin films, which limits their
ability to be fully integrated. Organic materials offer a number of significant advantages
such as low-drive voltages (<1V), higher EO coefficients (>300 pm/V), and the ability
to be dissolved and therefore processed into a number of different configurations. Indeed
∗Corresponding author. E-mail: a.kay@irl.cri.nz
272

NLO Properties of an Indoline-CTE Chromophore
273
Scheme 1. Synthetic methodology used to prepare the chromophore (Compound 5).
modern synthetic methodologies have permitted access to a vast range of organic NLO
compounds and allowed photonics researchers the opportunity to design, synthesize, test,
and continually improve these new materials at the molecular and macromolecular levels.
In order to observe a macroscopic NLO response in an organic material, it is necessary to
align the dipole moments of the embedded chromophores, and this is readily achieved by
heating a polymer film containing the material to its glass transition temperature and then
applying an electric field – a process known as poling.
In this work, we have synthesized and characterized a new NLO chromophore con-
taining an indoline donor, an acceptor based on the well-known moiety (2-(3-cyano-4,5,5-
trimethyl-5H-furan-2-ylidene)-malononitrile) – hereafter CTF – and a conjugated chain of
five carbon atoms between the donor and acceptor (Compound 5, Scheme 1). The chro-
mophore also contains a hydroxyethyl substituent on the donor nitrogen atom which will
allow for covalent attachment of the molecule to a polymer backbone, if needed, in the
future. Compound 5 is an analogue of another NLO chromophore that we have previously
studied – (Compound 1, Fig. 1) – and which was found to have a very high molecular first
hyperpolarizability, β, of approximately 1200 × 10−30 esu at 1300 nm [5]. Consequently,
derivatives of Compound 1 were deemed worthy of a more detailed study. To this end, we
report here on the structure of Compound 5 (via X-ray crystallographic studies) and pro-
vide details on the behavior of Compound 5 in solution using nuclear magnetic resonance
(NMR) and UV-Visible (UV-Vis) spectra. Furthermore, host–guest thin films containing
Figure 1. Structure of an NLO chromophore previously studied and with a molecular hyperpolariz-
ability (β) of 1200 × 10−30 esu.

274
M. D. H. Bhuiyan et al.
5% of Compound 5 in amorphous polycarbonate (APC) have been prepared and their EO
coefficients (r33 values) measured.
Experimental
Materials
Commercially available reagents were obtained from Aldrich and were used without addi-
tional purification. Compound 4 – (N-[4-(4-cyano-5-dicyanomethylene-2,2-dimethyl-2,5-
dihydro-furan-3-yl)-buta-1,3-dienyl]-N-phenyl-acetamide) – was prepared according to the
literature [6].
General Methods and Measurements
Melting points were recorded with an EZ-Melt automated melting point apparatus and are
1
uncorrected. H- and 13C-NMR spectra were recorded on a Bruker AVANCE 500 MHz
spectrometer and proton multiplicities are defined by the usual notations. Accurate mass
measurements were made on a Micromass Q-Tof Premier mass spectrometer operating
in positive-ion mode. The UV-Vis absorption spectra were recorded by a Perkin-Elmer
Lambda 900 spectrophotometer at room temperature. Column chromatography was carried
out using the gravity-feed column technique on Merck silica gel type 9385 (230–400
mesh). Crystallographic data for Compound 5 have been deposited at the Cambridge
Crystallographic Data Centre under reference 783,961. Copies of this information can be
obtained free of charge at http://www.ccdc.cam.ac.uk.
1-(2-Hydroxyethyl)-2,3,3-Trimethyl-3H-Indolium Iodide (Compound 3)
A mixture of 2,3,3-trimethyl-3H-indole (Compound 2, 5.0 g, 31.4 mmol) and 2-iodoethanol
(8.2 g, 47.7 mmol) was dissolved in 30-ml acetonitrile and the solution refluxed for 6 h. The
reaction mixture was then cooled to room temperature and 20 ml of diethyl ether was added.
The crude product precipitated and was collected by filtration and washed with 2–3 small
portions of ether to give Compound 3 as brown crystals (8.3 g, 80%) [m.p. 163◦C–165◦C;
1H-NMR (500 MHz, d6-DMSO) δ 1.57 (6H, s, 2 × CH3), 2.85 (3H, s, CH3), 3.90 (2H,
t, J 5.0 Hz, CH2), 4.62 (2H, t, J 5.0 Hz, CH2), 5.01 (1H, br s, –OH), 7.64–7.62 (2H, m,
ArH), 7.88–7.86 (1H, m, ArH), 7.99–7.89 (1H, m, ArH); 13C-NMR (125 MHz, d6-DMSO)
δ 14.5, 21.9, 50.3, 54.2, 57.7, 115.5, 123.5, 128.7, 129.2, 141.0, 141.7].
2-(3-Cyano-4-{5-[1-(2-Hydroxyethyl)-3,3-Dimethyl-1,3-Dihydro-Indol-2-ylidene]-
Penta-1,3-Dienyl}-5,5-Dimethyl-5H-Furan-2-ylidene)-Malononitrile
(Compound 5)
1-(2-hydroxyethyl)-2,3,3-trimethyl-3H-indolium iodide (Compound 3, 1 g, 3.2 mmol)
and the acceptor precursor N-[4-(4-cyano-5-dicyanomethylene-2,2-dimethyl-2,5-dihydro-
furan-3-yl)-buta-1,3-dienyl]-N-phenyl-acetamide (Compound 4, 789 mg, 2.1 mmol) were
dissolved in a mixture of methanol and isopropanol (30 ml, 1:9 ratio) and two drops of
triethylamine were added. The mixture was refluxed for 3 h, allowed to cool to ambient
temperature, and the crude product collected by filtration and washed with copious quan-
tities of hot water, followed by 2–3 small portions of cold isopropanol. The crude product

NLO Properties of an Indoline-CTE Chromophore
275
was purified by column chromatography (silica gel, ethyl acetate:dichloromethane 1:9) to
1
afford Compound 5 as a brown solid (930 mg, 78%) [m.p. 258◦C (decomp.); H-NMR
(500 MHz, d6-DMSO) δ 1.66 (6H, s, 2 × CH3), 1.70 (6H, s, 2 × CH3), 3.74 (2H, q, J
5.3, 12.0 Hz, CH2), 4.17 (2H, t, J 5.3 Hz, CH2), 5.01 (1H, t, J 5.6 Hz, –OH), 6.01 (1H,
d, J 14.4 Hz, CH), 6.36 (1H, d, J 14.4 Hz, CH), 6.44 (1H, t, J 12.5 Hz, CH), 7.21–7.29
(1H, m, ArH), 7.38 (2H, d, J 3.9 Hz, ArH), 7.58 (1H, d, J 7.5 Hz, ArH), 7.58 (1H, t, J 7.5
Hz, CH), 7.87 (1H, t, J 13.0 Hz, CH); 13C-NMR (125 MHz, d6-DMSO) δ 26.5, 27.1, 44.1,
46.5, 48.9, 58.1, 95.0, 104.4, 107.8, 111.6, 113.5, 114.5, 115.3, 122.1, 124.6, 125.4, 128.1,
141.1, 142.5, 150.6, 152.9, 168.9, 173.4, 176.1; HRMS (ESI+): For C27H27N4O2 [MH]+,
m/z calc. 439.2134. Found 439.2130, ꢀ 0.9 ppm].
Thin-Film Fabrication, Poling, and EO Coefficient Measurements
Solutions for thin-film fabrication (5% by weight) were made by mixing the chromophore
(Compound 5) (50 mg) and APC (950 mg) in cyclohexanone (5 g). The resulting solution
was warmed in a water bath at ∼40◦C for 3 h with continual stirring. The resultant solution
was then passed through a 0.45-µm syringe filter and then through a 0.22-µm syringe filter.
The thin films were made by placing 0.6 ml of the filtered host–guest polymer solution
on to a 25 mm × 25 mm ITO-coated glass slide and spinning at 1200 revolutions per
minute for 12 s. The films were then dried in a vacuum oven at 150◦C and 110 mmHg to
evaporate any residual solvent. The film thicknesses and refractive indices were measured
at 1300 nm and 1550 nm using a Metricon prism coupler. The thin films were poled under
an inert atmosphere using a parallel plate electrode poling technique, with an applied DC
field of 60V/µm of thin-film thickness (viz. 300 V across a 5-µm film). The samples were
poled for 30 min at 180◦C and then allowed to cool to room temperature while keeping the
electric field on in order to help maintain the alignment of the poled chromophores. The
observed current drops through the films were of the order of 0.12∼0.14 µA throughout
the poling process, which indicates that no dielectric breakdown occurred during the poling
process. The EO coefficients (r33 values) of the films were evaluated using the Teng–Man
modulation ellipsometry technique at 1310 nm [7].
Results and Discussion
Synthesis and Characterization
The synthesis of the chromophore (Compound 5; Scheme 1) builds on a method we have
previously used to prepare compounds containing the CTF acceptor unit and conjugated
spacers of varying length between the donor and the acceptor units [5, 6, 8, 9]. The N-2-
hydroxyethyl indolinium iodide salt (Compound 3) was synthesized by reaction of 2,3,3-
trimethyl-3H-indole with 2-iodoethanol in acetonitrile following the general procedure re-
ported by Wang et al.[10]. From our prior work, we know that for NLO chromophores with
an indoline donor, the optimal NLOresponse is found when there are five carbon atoms in the
interconnect [5], and this therefore dictates the use of N-[4-(4-cyano-5-dicyanomethylene-
2,2-dimethyl-2,5-dihydro-furan-3-yl)-buta-1,3-dienyl]-N-phenyl-acetamide, Compound 4,
as the appropriate synthon to introduce the acceptor ring and the conjugated interconnect
to the molecule. Thus, coupling of Compound 4 with the N-2-hydroxyethyl indolinium
salt (Compound 3) in alcohol containing a catalytic amount of base yielded the target
chromophore (Compound 5). As noted earlier Compound 5 is almost identical to Com-
pound 1, except that there is a hydroxyethyl group on the donor nitrogen atom rather than a

276
M. D. H. Bhuiyan et al.
1
Figure 2. Partial H-NMR spectrum of Compound 5, illustrating that a single isomer is present in
solution.
methyl substituent, and this will allow for covalent attachment of Compound 5 to a polymer
backbone. In addition, it also makes Compound 5 more soluble in low-polarity solvents
than Compound 1, and through the introduction of strong hydrogen bonding interactions,
it allowed for the growth of high-quality crystals for X-ray crystallography.
The 1H-NMR spectrum of Compound 5 shows that only one isomer is present in solu-
tion, and this is evident upon examining the resonances and splitting patterns in the olefinic
and aryl regions (Fig. 2). Our previous work has shown that zwitterionic chromophores
containing the CTF acceptor exhibit two isomers in solution, and a detailed NMR study
confirmed that these were due to the two rotational isomers that can occur around the accep-
tor ring and the terminus of the conjugated interconnect [6]. That there is only one isomer
present is most likely a reflection of the fact that Compound 5 does not have significant
Figure 3. Two possible geometrical (rotational) isomers for Compound 5.

NLO Properties of an Indoline-CTE Chromophore
277
3
Figure 4. UV-Vis spectra of Compound 5 in a range of solvents.
charge separation in its ground state. Consequently, the bond order in this compound will
be the opposite of that found in the zwitterionic analogues reported earlier. As a result,
there will be more single bond character in the conjugated polyene-acceptor ring bond,
which should make the more thermodynamically favored rotamer more accessible in solu-
tion. X-ray crystallographic studies done here (discussed later) and our earlier work [11]
suggest that for indoline-π-CTF chromophores, it is the transoid rotamer (Fig. 3) that is
energetically favored, as it is the only isomer ever observed in the crystalline state.
The UV-Vis absorption spectra of Compound 5 in a number of different solvents are
shown in Fig. 4 and the results summarized in Table 1. This compound exhibits only a
mild degree of solvatochromism, with a difference in the absorption maximum of only 8
nm across the range of solvents studied. Furthermore, with the highest energy absorption
(690 nm) observed in the lowest-polarity solvent – dioxane – and the lowest energy ab-
sorption (698 nm) seen in the highest-polarity solvent – DMF – confirmation is given that
Compound 5 exhibits positive solvatochromism and is expected to have only a moderate
degree of charge separation in the ground state.
X-Ray Crystallography
A crystal of Compound 5 was obtained by the vapor diffusion method with dichloromethane
as the solvent and ether as the precipitant. The X-ray structure is shown in Fig. 5, its unit
cell packing is shown in Fig. 6, and key crystallographic data is given in Table 2. It is found
Table 1. UV-Vis data of Compound 5 in different solvents
Solvent
ε
λmax (nm)
Log10€
Dioxane
Chloroform
Methanol
2.2
4.8
33.0
38.0
690
697
693
698
–
–
5.34
–
Dimethyl formamide

278
M. D. H. Bhuiyan et al.
Figure 5. Structure of Compound 5 with 20% probability thermal ellipsoids [14].
Figure 6. Unit cell packing of Compound 5, showing chain formation via hydrogen bonding and
molecular twisting. Attractive contacts (atoms as balls) are shown as dotted lines. Only H atoms
involved in the packing are shown for clarity. Symmetry: (i) 1/2–x, 1/2+y, 3/2–z. (ii) 1/2–x, y–1/2,
3/2–z. (iii) 1/2+x, 1/2–y, z–1/2. (iv) x–1/2, 1/2–y, z–1/2.

NLO Properties of an Indoline-CTE Chromophore
279
Table 2. Crystallographic and structure refinement data for Compound 5
Asymmetric unit
MW
C27 H26N4O2
438.52
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
135(2)
0.71073
Monoclinic
P21/n
10.7826(4)
14.5943(5)
15.3792(5)
90
96.213(2)
90
2405.93(14)
4
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
Volume (Å3)
Z
ρ (mg.m−3)
1.211
0.078
µ (mm−1
)
Crystal size (mm)
Theta range (◦)
Limiting indices
0.24 × 0.22 × 0.04
2.79–26.00
−13 ≤ h ≤13
−18 ≤ k ≤ 18
−18 ≤ l ≤ 16
28,639
Reflections collected
No. of unique data
4715
Rint
0.0637
Absorption coefficient range
Restraints
0.6087–0.7456
9
No. of parameters
321
Refinement method
P1, P2 coefficients of weighting schemea
Goodness of fit on F2
R1b,c/data number
Full-matrix least-squares method on F2
0.0338, 0.93
1.002
0.044/2872
0.110
wR2d (all data)
Largest differential peak and hole (e.A−3
)
0.165 and −0.172
2
2
2
aWeight, w = 1/[σ2(Fo ) + (P1 ∗ P)2 + P2 ∗ P], where P = (Max(Fo , 0) + 2 ∗ Fc )/3.
bIntensities two times their standard deviations (from counting statistics).
cR1 = ꢁꢀFo |– Fcꢀ/ꢁ|Fo|.
2 2
2 2 1/2
dwR2 = ꢁ[w (Fo2 – Fc ) ]/ꢁ[w (Fo ) ]
.
that the furan-2-ylidene ring (C4–C7, O1) is planar, while the component planar rings of
the indol-2-ylidene system are at 1.95(11)◦ to each other which is similar to the value of
1.94(6)◦ found for 2-(3-cyano-4-{7-[1-(2-hydroxyethyl)-3,3-dimethyl-1,3-dihydro-indol-
2-ylidene]-hepta-1,3,5-trienyl-5,5-dimethyl-5H-furan-2-ylidene)-malononitrile – Com-
pound 6 (Fig. 7) [12]. The indol-2-ylidene plane (N4, C16–C23) makes an angle
of 5.11(17)◦ to the plane running through the polymethine chain atoms (C11–C15).
This plane in turn subtends 16.51(18)◦ to the plane through the furan-2-ylidene

280
M. D. H. Bhuiyan et al.
Figure 7. Structure of related chromophores for which structural parameters have previously been
reported.
ring (C4–C7, O1). This reflects a twist along the molecules between the terminal
groups of ∼19◦, which is significantly greater than the value for both Compound 6
(∼8◦, Fig. 8) and 2-(4-{3-[1-(3-bromo-propyl)-3,3-dimethyl-1,3-dihydro-indol-2-ylidene]-
propenyl}−3-cyano-5,5-dimethyl-5H-furan-2-ylidene)-malononitrile – Compound 7 (also
∼8◦) [11].
The bond length alternation (BLA) parameter [13] for Compound 5 is 0.024 Å
(Table 3), which is similar to the values found from X-ray data for the closely related
compounds, Compound 6 (0.018Å), Compound 7 (0.012 Å) and 2-{3-cyano-5,5-dimethyl-
4-[3-(1,3,3-trimethyl-1,3-dihydro-indol-2-ylidene)-propenyl]-5H-furan-2-ylidene}-malon
onitrile – Compound 8 (0.006Å) [5]. Nonetheless, as Compound 5 has a slightly larger
BLA value, it is expected to also have the highest NLO response of these four compounds.
As noted above, this is in agreement with previous work which shows that a five-carbon
interconnect between the donor and the acceptor in indoline-π-CTF chromophores is opti-
mal [5]. It is also worth noting that Compounds 5, 6, and 8 have molecular configurations
in which the indoline dimethyl and CTF dimethyl groups are in a cis relationship with
respect to the polymethine chain, whereas the corresponding relationship in Compound 7
is trans. The unit cell packing of Compound 5 shows that the molecules are packed head
to tail, consistent with a centrosymmetric space group linked in three dimensions by
three different hydrogen bond interactions (Fig. 6, Table 4). The major hydrogen bonding
interaction (entry 1, Table 4) is between the hydroxyl proton and the cyano nitrogen,
Figure 8. Comparison of the twists from planarity of Compound 5 (lower structure, dark shading)
and 6 (upper structure, grey shading); see text.

NLO Properties of an Indoline-CTE Chromophore
281
Table 3. Selected bond distances (Å) and magnitude of BLA for CTF and Compound 5
Bond
CTF
Compound 5
C2–C6
C6–C7
C7–C4
C4–C11
C11–C12
C12–C13
C13–C14
C14–C15
C15–C16
BLA
1.358
1.445
1.343
1.472
—
—
—
—
—
1.389(3)
1.397(3)
1.413(2)
1.367(3)
1.402(3)
1.375(3)
1.391(3)
1.371(3)
1.391(3)
0.024
0.108
N2, with formation of chains of molecules along the b-axis in a C17 hydrogen bonding
motif [15]. The alkene C–H···N(cyano) interactions form edge-on dimers, motif R2 (20)
2
[15] along the a-axis direction. Finally, a weak methyl C–H···hydroxyl O interaction is
observed between approximately parallel adjacent molecules (entry 2, Table 4) in the
c-axis direction, motif R2 (28) [15]. Although unexpected, given the rigorously planar
2
nature of Compound 8, the twisting present in Compound 5 is consistent with, and indeed
can be explained by, these attractive packing interactions.
EO Coefficient
The maximum EO coefficient we were able to measure for a thin film containing 5% of
Compound 5 in APC was 206 pm/V; this measurement was obtained immediately after
poling was completed. Following this initial measurement, the sample was allowed to stand
at room temperature and under atmospheric conditions for 72 h and the r33 value regularly
re-measured over this period; the results are presented in Table 5. It was found that the
response gradually decreased to around half of its original value after 72 h (viz. 108 pm/V,
53%). Furthermore, the reduction was initially quite rapid, with a 20% loss in response
occurring after only 8 h. The rate of decay then slowed, and it took approximately 36 h for
the EO coefficient to drop by another 20%. This reduction in the observed EO coefficient
is not unexpected and is due to relaxation of the chromophore molecules, and therefore a
return to a random orientation. While presently beyond the scope of the current work, this
is a phenomenon that can be addressed by covalent attachment of the chromophore to a
Table 4. Hydrogen bond geometry Å(◦)
D–H . . . A
D–H H . . . A D . . . A D–H . . . A
Symmetrya
O2A-H2OA . . . N2 0.82(3) 2.10(3) 2.888(3)
161(4)
147
150
x, y + 1, z
C8–H8B . . . O2A 0.98
C15–H15 . . . N3 0.95
2.40
2.51
3.273(3)
3.363(3)
−x, −y + 1, −z + 2
−x + 1, −y + 1, −z + 2
aSymmetry to bring the A atom into contact.

282
M. D. H. Bhuiyan et al.
Table 5. Change in the electro-optic coefficient over 72 h for a host–guest film containing
Compound 5
% of initial
% of initial
Time (h)
r33 (pm/V)
r33 value
Time (h)
r33 (pm/V)
r33 value
0
0.5
1
2
3
4
8
9
206
192
187
184
180
169
167
163
100
93
91
89
87
82
81
79
22
25
27
28
44
46
48
72
148
141
140
139
124
123
122
108
72
69
68
67
60
60
59
53
host polymer with a high glass transition temperature (such as a polyimide) and incorporat-
ing additives that thermally cross-link around, or just below, the final poling temperature.
Nonetheless, the present work has demonstrated that a simple and readily accessible chro-
mophore such as Compound 5 can exhibit an EO coefficient almost an order of magnitude
higher than the values reported for lithium niobate (32 pm/V) [4] and potassium titanium
oxide phosphate (KTP) (36.3 pm/V) [16]. Furthermore, the macroscopic response we report
here for a host–guest film containing Compound 5 is considerably higher than for a number
of other organic systems. For example, crystals of the well-known organic salt DAST were
found to give r11 values of 53 pm/V at 1310 nm and 92 pm/V at 720 nm [17]. In addition,
host–guest thin films containing the DEMI chromophore in polysulfone were reported to
have a r33 value of 21 pm/V at 1550 nm [18], while a thin film containing 35% CLD in
APC gave an r33 value of 37 pm/V at 1310 nm [19]. While variations in chromophore
loading, the poling conditions and wavelength used to undertake the measurements all
affect the observed macroscopic nonlinearity, it is clear that Compound 5 has an excellent
EO response when compared with other materials; consequently, this is a compound wor-
thy of more detailed studies. The reason for the high r33 value found for films containing
Compound 5 is not readily obvious, but may stem from the fact that the chromophore (Com-
pound 5) has a high molecular hyperpolarizability, as deduced from the value of 1200 ×
10−30 esu found for the analogue (Compound 1). In addition, the weak hydrogen bonding
interactions that arise from the presence of the hydroxyl group (Fig. 6) appear to occur in
competition with strong dipole–dipole interactions and should make electric field poling
more efficient. Thus, overall, while Compound 5 has a very high EO coefficient and is able
to be easily prepared and processed (features not readily applicable to crystalline materials),
it is still not a viable replacement for current inorganic crystalline materials until issues
with the temporal stability of poled films have been adequately addressed.
Conclusions
For use in nonlinear optics, a simple chromophore containing an indoline donor and a
CTF acceptor has been prepared using a straightforward method. X-ray crystallographic
studies show that while the molecule has a gentle twist along its length, it has a higher
degree of BLA than its closely related counterparts and is therefore expected to provide thin
films with high macroscopic NLO responses. This was indeed the case, as the maximum

NLO Properties of an Indoline-CTE Chromophore
283
EO coefficient of a host–guest film containing the molecule was found to be some seven
times higher than the value reported for lithium niobate. Further studies to both optimize
the loading of the chromophore in a host–guest film and improve the temporal stability
of the final poled polymers using known methods are underway. These include covalent
attachment of the chromophore to a polymer with a high glass transition temperature,
the use of cross-linking agents during the poling process, and the incorporation of bulky,
arene-rich substituents to retard aggregation.
Acknowledgment
This research was funded by the New Zealand Foundation for Research, Science and
Technology (contract no. C08X0704).
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