Modified ene-yne compounds: A novel functional material with nonlinear optical properties

Article abstract of DOI:10.1039/c1ce06093g

The title compound, an achiral flexible molecule containing a 1,2,3-triazole structure as the acceptor subunit, crystallizes as a single enantiomorph in the space group P2₁2₁2₁. The material exhibits nonlinear optical prop

Full text of DOI:10.1039/c1ce06093g

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COMMUNICATION
Modified ene–yne compounds: a novel functional material with nonlinear
optical properties†
a
b
a
b
c
Daniel Lumpi, Berthold Stoger, Christian Hametner, Frank Kubel, Georg Reider, Hans Hagemann,
d
*
€
Alfred Karpfen and Johannes Frohlich
e
a
€
Received 24th August 2011, Accepted 29th September 2011
DOI: 10.1039/c1ce06093g
The synthesis of compound 3 is based on an organo-lithium
induced thiophene ring fragmentation (TRF) reaction,5 a reliable
methodology to selectively synthesize the Z-isomer of ene–yne species
2 in good yields (76–81%, Fig. 1). This Z-conformation of the double
bond is defined by the cyclic structure of the thiophene and could be
confirmed by NOE experiments.6
The title compound, an achiral flexible molecule containing a 1,2,3-
triazole structure as the acceptor subunit, crystallizes as a single
enantiomorph in the space group P212121. The material exhibits
nonlinear optical properties and is capable of second harmonic
generation. Thus, the developed molecular scaffold represents an
interesting novel type of NLO chromophore.
A modification of the ene–yne compound via copper(I)-catalyzed
azide–alkyne cycloaddition (CuAAC),7 one of the most popular
reactions in the click concept,8 allows a direct functionalization of the
yne-structure. In situ cleavage of the TMS group by potassium
fluoride in the course of the CuAAC reaction utilizing azidobenzene
leads to the formation of 1,4-regioisomer 3. The best results, using
conventional heating, could be achieved at 50 ꢀC and reaction times
of 18 h giving 58% isolated yield; a significant improvement could be
achieved by applying microwave (MW) irradiation9 at a temperature
Second-order nonlinear optical (NLO) materials are of critical
importance for applications in telecommunication technology and
quantum electronics, to mention just the most important.1 The
current focus on organic materials is due to their exceptionally large
second-order nonlinearity and the virtually unlimited variety of
crystal structures.2 Hence, there is a considerable interest in crystal
engineering for the design of compounds suitable for second-order
nonlinear optics.3
ꢀ
of 150 C. The MW assisted reaction, carried out in a microwave
synthesizer,10 resulted in an enhanced isolated yield of 75% and
a considerably shortened reaction time of 20 min.6 Single crystals
(Fig. 2) of 3 were obtained by crystallization from n-hexane.‡
The contribution of the Z-(alkylthio)alkenyl fragment to the
material properties is 2-fold: the thioalkyl group, which has shown to
be a good electron donor in certain aromatic systems,11 strongly
interacts with the aromatic p-system via the double bond. The high
polarizability of the weakly bound p-electrons, together with the
natural asymmetry of the donor/acceptor structure, is the prerequisite
for the high second-order hyperpolarizablity of the compound. This
delocalization also improves the stability of the target compound,
resulting in a high resistance towards moisture, oxygen and elevated
temperatures, which is also important for the processability of
the material (e.g. growth from melt). Secondly, the Z-geometry and
the interactions between alkyl substituents are crucial factors for the
enantiomorphic crystallization of 3, as discussed in detail below.
Organic molecules capable of second-order NLO applications
typically consist of aromatic p-electron systems asymmetrically
endcapped with electron donating and accepting groups to impart the
electronic bias.4 In this contribution we report on the design and
synthesis of modified ene–yne compound 3, which is examined in
detail regarding its structural and NLO properties. The presented
NLO characteristics rely on the application of a Z-(alkylthio)alkenyl
group (Fig. 1, ene-substructure) as an electron donor directly con-
nected to a heteroaromatic moiety, providing adequate acceptor
performance. Hence, the developed NLO chromophore, applying the
1,2,3-triazole core as an acceptor unit, comprises attractive molecular
building blocks towards NLO materials.
aInstitute of Applied Synthetic Chemistry, Vienna University of
Technology, Getreidemarkt 9/163, A-1060 Vienna, Austria. E-mail:
daniel.lumpi@tuwien.ac.at; Tel: +43 1 58801 163719
bInstitute of Chemical Technologies and Analytics, Vienna University of
Technology, Getreidemarkt 9/164, A-1060 Vienna, Austria
cPhotonics Institute, Vienna University of Technology, Gußhausstraße 27-
29, A-1040 Vienna, Austria
d
ꢀ
Departement de Chimie Physique, Universite de Geneve, 30, quai E.
Ansermet, 1211 Geneva 4, Switzerland
ꢀ
ꢁ
eInstitute for Theoretical Chemistry, University of Vienna, Wa€hringer
Straße 17, A-1090 Vienna, Austria
† Electronic supplementary information (ESI) available: Experimental
procedures, full analytical and spectral characterization data, and
computational parameters. CCDC reference number 835683. For ESI
and crystallographic data in CIF format see DOI: 10.1039/c1ce06093g
Fig. 1 Scheme of the synthetic approach towards compound 3; i: n-BuLi
(ꢁ80 ꢀC, Et2O), / 10 ꢀC, MeI (ꢁ50 ꢀC), ii: azidobenzene, CuSO4$5H2O,
Na ascorbate, KF, t-BuOH/H2O (1 : 1, 0.4 M), MW 150 ꢀC.
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Fig. 2 Scanning electron microscopy (SEM) image of a crystal rod of 3.
The 1,2,3-triazole moiety, previously applied as conjugative p-
linker in push–pull chromophores,12 exhibits sufficient acceptor
performance in the D–p–A fashion; no complementary attachment
of an electron-withdrawing group is necessary to achieve good NLO
efficiencies. This is, in fact, remarkable because potential applications
of this acceptor-type fragment are highly diverse due to the good
stability of the triazole as well as an exceptional compatibility of this
click reaction toward functional groups and reaction conditions.7
Further examples of the 1,2,3-triazole motive utilized in extended
p-chromophore structures are only recently appearing in the
literature.12,13
Fig. 3 Ellipsoid plots of title compound 3; viewed normal (a, top) and
parallel (b, bottom) to the molecular plane. C, N and S atoms are rep-
resented by grey, blue and yellow ellipsoids drawn at 95% probability
levels. H atoms are represented by white spheres of arbitrary radius.
The crystals show good optical transparency, exhibiting a wide
transmission window above the absorption onset at 373 nm in the
visible region.6 The crystalline material of 3 was also subjected to
thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) which both indicate a good thermal stability up to
ꢂ180 ꢀC.6 To determine whether the title compound possesses
sufficient stability for growing crystals from the melt (e.g. by the
Czochralski process), ꢂ100 mg were melted and slowly cooled to
room temperature. The solidified material was ground and subjected
to powder X-ray diffraction. The diffraction pattern was virtually
identical to the pattern obtained from a freshly synthesized sample
crystallized from n-hexane.6
The molecules are arranged around 21-axes running along the a-,
b- and c-directions. The conformation of the molecules is stabilized
by non-classical intramolecular C–H/S bonds (Fig. 4). The mole-
cules are connected via non-classical C–H/N bonds, forming two
interpenetrating three dimensional networks.6
The effect inducing the methyl group (C12) being significantly
located off the main molecular plane was further evaluated by DFT
calculations.6 Initial results pointed out that hydrogen bond type
interactions of S1 and H–C7 can be essentially neglected in order to
explain this ‘‘out of plane’’ arrangement. Torsional angle (C12–S1–
C10–C9) dependent potential calculations (Fig. 5) disclose a local
maximum (1.4–1.5 kJ) at 180ꢀ, pointing out that an ‘‘in-plane’’
configuration of the methyl group (C12) does not represent the
energetically favourable structure.
Despite being a flexible achiral molecule, the title compound crys-
tallizes in the enantiomorphic space group P212121, and, moreover, as
a single enantiomorph. Thus, the absolute structure and consequently
configuration were determined directly (Flack parameter 0.00(6)).14
The unit cell contains four crystallographically equivalent asym-
metric molecules. Since the symmetry group of the crystal is enan-
tiomorphic, all molecules are congruent. With the exception of the
S-methyl group, the molecules adopt a configuration close to planar
(Fig. 3). As expected, the phenyl and triazole rings are practically
planar, with maximum distances to the least-squares plane of
To verify this assumption, the methyl group (C11) was replaced by
hydrogen (structure 5) and the aforementioned DFT calculations
were repeated. The results indicate no local maxima, thus leading to
a minimum energy at a torsion angle of 180ꢀ. The non-symmetrical
ꢂ
ꢂ
0.0037(19) A (C3) and 0.0024(15) A (N3), respectively. They are
slightly tilted, relative to each other, by 8.87(5)ꢀ, which can be
explained by steric repulsion of the ortho-H and 5-H atoms of the
phenyl- and triazole-rings, respectively. Similar tiltings are observed
in numerous 4-phenyl-1,2,3-triazoles which are unsubstituted on the
corresponding ortho- and 5-positions.15 In other related 4-phenyl-
1,2,3-triazoles the tilting angle is vanishingly small16 or even 0, since
the molecule is located on a mirror plane.17 The preference for
a planar configuration, despite steric repulsion of ortho-H atoms, has
been explained in the case of biphenyl by p–p interaction of the
connected phenyl rings and by intermolecular interactions.18
The propenyl group in compound 3 is practically coplanar with the
phenyl ring (torsion angle 1.59(12)ꢀ). The S-methyl group is distinctly
located off the main molecular plane with a C11–C10–S1–C12
torsion angle of 29.91(14)ꢀ, thus breaking the idealized m symmetry
of the molecule.
Fig. 4 Intra- (dashed lines) and intermolecular (dotted lines) hydrogen
bonds in the crystal structure of 3. Colour codes as given in Fig. 3.
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process of frequency doubling, or second harmonic generation
(SHG), utilizing the powder technique developed by Kurtz and
Perry.20
Fig. 7 shows spectra of two different IR pulse energies (15 mJ and
30 mJ, respectively). The apparent broad line width is due to the large
(1 mm) entrance slit of the 0.25 m grating monochromator. The
feature is centred around the SH-wavelength of 532 nm ¼ 1064/2 nm.
The peak of the spectrum from 30 mJ irradiation is about 4 times
higher than that of the spectrum irradiated at 15 mJ, which is in
accordance with the quadratic intensity dependence of the SHG
process.21 No detectable signal was found for the pure glass slides at
either pulse energy.
Without any sieving, the particle size distribution was rather
inhomogeneous; inspection revealed, however, that the maximum
particle size (<1 mm) was well below the typical coherence length of
transparent organic compounds. While the measurements, therefore,
could not clarify, according to the method of Kurtz and Perry,20
whether phase matching is possible in compound 3, an approximate
measurement of the nonlinear optical coefficient is possible since the
SH efficiency scales quadratically with the nonlinear coefficient in this
particle size regime. The SH signal of our sample was compared to
that from a similarly prepared powder sample of another organic
compound22 with point group 222, which exhibits a second-order
nonlinear coefficient of 0.6 ꢃ 10ꢁ12 m Vꢁ1, somewhat higher than
potassium dihydrogen phosphate (KDP). The SH yield of 3 turned
out to be about twice as large; if phase matching can be achieved, 3
represents a highly attractive new nonlinear optical material.
In conclusion, we report on a readily accessible material of high
stability and optical quality, which exhibits an enantiomorphic
crystallization characteristic and displays second-order nonlinear
optical properties with an estimated efficiency of twice the value of
KDP. Moreover, both the implementation of the Z-(alkylthio)
alkenyl group and the 1,2,3-triazole moiety in this novel D–p–A type
chromophore express valuable impact for potential applications.
Future work will focus on understanding how modifications of the
ene-scaffold influence the molecular structure and affect the SHG
efficiencies. Furthermore, detailed research on NLO characteristics of
potential materials, also by computational chemistry, has been the
subject of recent activities.
Fig. 5 DFT based torsional angle potential calculations; filled circle:
compound 3, open circle: substrate 5 ((Z)-4-(2-(methylthio)ethen-1-yl)-1-
phenyl-1,2,3-triazole).
appearance of the single- and double-well shaped energy profiles for 3
and 5, respectively, is caused by non-zero torsion angles between the
phenyl and the triazole moiety. This evidence demonstrates that
interactions between the methyl groups are the decisive factor for the
structural characteristic; however interactions in the solid phase also
have to be taken into account to understand the crystallization.
Orientation of the crystal showed that one bond, namely the S1–
C12 bond, is parallel to a main axis (Fig. 4). The polarized Raman
spectra, shown in Fig. 6 (and in Fig. S6.1, ESI†), reveal three strongly
polarized bands in the ZZ polarization. The DFT calculation shows
that the polarized band at ca. 750 cmꢁ1 does indeed correspond to
a S1–C12 stretching mode, and the polarized band at ca. 260 cmꢁ1
also involves a motion of the C12 atom. Other bands in Fig. 6,
corresponding to multi-atom vibrational modes, also show significant
polarization behaviour.
The qualitative comparison with the calculated Raman spectrum,
also shown in Fig. 6, is very good. Due to the neglect of the inter-
molecular interactions and of anharmonicities, calculated frequencies
above 1000 cmꢁ1 (Fig. S6.1†)6 are somewhat higher than experi-
mental fundamentals.
Due to the anisotropy and the crystal point group 212121,
potentially suitable for nonlinear optical effects, the material was
investigated regarding these optical characteristics.19 For a qualita-
tive evaluation of the NLO properties of 3, we employed the
Fig. 6 Polarized Raman spectra and the theoretical (DFT) Raman
spectrum; the laboratory Z axis is parallel to the long crystal needle axis,
X is perpendicular to that axis.
Fig. 7 SHG spectra of 3 at two different fundamental pulse energies
from a Q-switched Nd:YAG laser at 1064 nm wavelength.
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Acknowledgements
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This work was supported in part by the Swiss National Science
€
Foundation. We thank H. Schmidtbauer and F. Glocklhofer for
ꢀ
contributing to the synthetic experiments. We also acknowledge F.
Plasser, P. Hans and Dr. E. Horkel for discussions regarding the
DFT calculations as well as Dr. E. Eitenberger for assistance in
preparing the SEM images. The quantum chemical calculations were
performed on the Vienna Scientific Cluster. The authors are grateful
for ample supply of computer time on this installation.
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ꢂ
ꢂ
231.3, orthorhombic, P212121, a ¼ 4.8314(2) A, b ¼ 15.4009(6) A, c ¼
3
15.6421(5) A, V ¼ 1163.90(8) A , Z ¼ 4, m ¼ 0.253 mmꢁ1, T ¼ 100 K,
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