New class of hyperpolarizable push-pull organic chromophores by applying a novel and convenient synthetic strategy

Article abstract of DOI:10.1016/j.dyepig.2015.12.013

A new class of push-pull chromophore with an azomethine unit at a key position of the chromophore in order to increase the hyperpolarizability is reported. Additionally, a novel and convenient synthesis method is introduced which is particularly advantageous compared with known procedures because it offers high atom economy, the use of low cost reagents, and mild reaction conditions together with a reduced number of reaction steps.

Full text of DOI:10.1016/j.dyepig.2015.12.013

Dyes and Pigments 127 (2016) 73e77
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New class of hyperpolarizable pushepull organic chromophores by
applying a novel and convenient synthetic strategy
,
, c, 1
S. Farsadpour a *, L. Taghizadeh Ghoochany b, Ch. Kaiser c, G. von Freymann a
a Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, D-67663 Kaiserslautern, Germany
b Department of Chemistry, University of Kaiserslautern, D-67663 Kaiserslautern, Germany
c Fraunhofer Institute for Physical Measurement Techniques IPM, Materials Characterization and Testing, Fraunhofer Platz 1, D-67663 Kaiserslautern,
Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of pushepull chromophore with an azomethine unit at a key position of the chromophore in
order to increase the hyperpolarizability is reported. Additionally, a novel and convenient synthesis
method is introduced which is particularly advantageous compared with known procedures because it
offers high atom economy, the use of low cost reagents, and mild reaction conditions together with a
reduced number of reaction steps.
Received 10 August 2015
Accepted 16 December 2015
Available online 23 December 2015
Keywords:
© 2015 Elsevier Ltd. All rights reserved.
Pushepull chromophore
Hyperpolarizability
Synthesis
NLO
Inverted solvatochromic behavior
DFT calculations
1. Introduction
explored the properties of conjugated organic systems, e.g., high
NLO susceptibility [4], low dielectric constants [5], possibilities of
Over the past two decades the rising demand for telecommu-
nication services and high speed data transmission technologies
has been the driving force for intense research on electro optic (EO)
materials. Among all EO materials a new family of EO chromo-
phores e conjugated organic molecules that are used to manipulate
light by an electric field e is pointing the way to a revolution in
telecommunications and information processing [1]. Organic non-
linear optical chromophores generally have one electron-
withdrawing group at one end and one electron-donating group
at the other, linked together by extended conjugation.
structural modification [1], and ease of processability [6] which
suited the FTC and CLD type chromophores for their applications in
photonic devices [1,4e6]. Moreover, other researches in the field of
OEO materials have been directed towards optimization of the ma-
terial parameters at the molecular level. This includes shape and
electronic optimization. Shape optimization has beenperformed not
only in linear chromophores (e.g., using a conformationally locked
polyene bridge for reduced dipolar interaction) but also on various
other types of chromophores, such as spherical shape [7], X-shape
[8] U-shape [9], H-shape [10] and Y-shape [11]. In parallel to the
shape optimization, electronic optimization has also been examined
by many research groups [12]. So far, the conventional synthetic
strategy for preparing all type of pushepull organic non-linear op-
tical chromophores has been to build the chromophore step by step
from the electron-donating group towards the electron-
withdrawing group. This synthetic scheme generally involves four
steps. In the case of tertiary aromatic amines containing hydroxyl
groups the protection of the hydroxyl groups is the first step. The
second step involves Wittig reaction of donating groups bearing
To design and synthesize organic EO (OEO) materials Seth
Marder's group [2] introduced a unified description of linear and
nonlinear polarizability by showing correlation of the polarizabil-
ities with a
p-bond order alternation in a linear polymethine dye
system. The introduction of furan-thiophene chromophore (FTC)
and ring-locked, phenyltetraene-based chromophore (CLD) by Larry
Dalton's group [3] turned out to be one of the major advances in OEO
materials during the last few years. Larry Dalton's group also
aldehyde with phosphonium salt containing a
p-bridge. The third
step, formylation of the bridge is performed through either Vils-
meier formylation using DMF/POCl3 or by lithiation with, e.g., n-
* Corresponding author. Tel.: þ49 631 2055229; fax: þ49 631 2055227.
E-mail address: s.farsadpour@gmail.com (S. Farsadpour).
Tel.: þ49 631 2055229; fax: þ49 631 2055227.
1
http://dx.doi.org/10.1016/j.dyepig.2015.12.013
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

74
S. Farsadpour et al. / Dyes and Pigments 127 (2016) 73e77
butyllithium followed by treatment with DMF. In the final step the
chromophore is obtained via the Knoevenagel condensation reac-
tion of the prepared intermediates with acceptor groups, e.g., tri-
cyanofuran acceptor (TCF) in basic condition. As an example the
synthesis of the benchmark chromophore FTC is shown in Scheme 1.
Hundreds of pushepull non-linear optical chromophores have
been prepared using this conventional synthetic strategy with its
numerous steps, leading to lower overall yields and higher overall
costs. Hence the synthesis of the chromophores with high-yield,
cost-effective chemical pathways as well as operational stability
is still a challenge. In this communication we report a novel class of
chromophores and its facile synthesis strategy.
refluxed overnight. After concentration in vacuo, the residue was
washed with diluted aqueous HCI. The crude precipitate was
filtered and recrystallized from ethanol and gave the desired
product, yield: 314 g (95%, yellow solid). 2.37 g (78%). C11H9N3O
(199.209): calcd. C 66.32, H 4.55, N 21.09; found C 65.96, H 4.54, N
20.95.1H NMR (CDCl3, 400.1 MHz, 20 ꢀC): jj ¼ 1.63 (s, 6H), 2.36 (s,
3H), ppm. 13C NMR (CDCl3, 100.62 MHz, 20 ꢀC): jj ¼ 14.30, 24.49,
58.65, 99.84, 104.94, 109.06, 110.48, 111.12, 175.28, 182.61 ppm.
2.2.2. Synthesis of key intermediate (KI) (E)-2-(3-cyano-4-(2-(5-
formylthiophen-2-yl)vinyl)-5,5-dimethylfuran-2(5H)-ylidene)
malononitrile
NaOH (0.009 g, 0.225 mmol) were added to a solution of 2,5-
Thiophenedicarboxaldehyde (1.32 g, 9.32 mmol) and 2-(3-cyano-
4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) (1.83 g,
9.19 mmol) in mixture of dry ethanol (60 mL) and dichloromethane
(10 mL). The reaction mixture was refluxed for 6 h and the product
precipitated during the reaction which was filtrated and washed
with diethyl ether and dried in vacuum, yield: 2.51 g (85%, green
solid) and it used without any purification in the next step. 1H NMR
(CDCl3, 400.13 MHz, 20 ꢀC): jj ¼ 1.78 (s, 6H, H-9), 6.87 (d, J ¼ 16.2 Hz,
1H, H-10), 7.54 (d, J ¼ 4.0 Hz,1H, H-12), 7.79 (d, J ¼ 4.0 Hz,1H, H-13),
7.85 (d, J ¼ 16.2 Hz, 1H, H-17), 9.98 (s, 1H, H-15) ppm. 13C NMR
(CDCl3, 100.61 MHz, 20 ꢀC): jj ¼ 25.38, 26.36, 97.85, 99.12, 101.19,
117.11, 133.20, 133.66, 136.44, 136.53, 137.95, 146.83, 147.34, 171.96,
182.62, 182.85 ppm. HRMS-EI: m/z calcd for C17H11N3O2S: 321.0572
[M]þ; found: 321.0567.
2. Experimental
2.1. Materials and general methods
The precursors were purchased from Sigma Aldrich and ACROS
used without further purification. All solvents were degassed ac-
cording to standard techniques before use. 1H and 13C and NMR
spectra were recorded on a Bruker Spectrospin Avance 400 device
using dimethylsulfoxide-d6 (DMSO-d6) or CDCl3 as solvent. The
chemical shifts are given in parts per million (ppm) relative to in-
ternal TMS (0 ppm) and coupling constant in Hz. Splitting patterns
were described as singlet (s), doublet (d), triplet (t), multiplet (m)
and broad (br.). The high resolution mass spectrum (HRMS) was
measured on a Waters GTC Premier. The MS spectra were obtained
using MALDI-TOF (Matrix Assisted Laser Desorption/Ionization of
Flight) on a BIFLEXIII (Bruker Inc.) CHN-Elemental analyses were
performed with a PerkineElmer Elemental Analyzer EA 2400 CHN.
Thermogravimetric and differentional thermogravimetric (TG-
DTG) analyses were carried out on a SETRAM Setsys 16/MS in-
strument using an alumina pan in flow of Nitrogen from room
temperature to 550 ꢀC with a heating rate of 5 ꢀC/min. UVeVis UV/
vis absorption spectra were recorded using a Thermo Fisher Evo-
lution 220 spectrophotometer.
2.2.3. Synthesis of the chromophore 2-(3-cyano-4-((E)-2-(5-((E)-
((4-(diethylamino)phenyl)imino)methyl)thiophen-2-yl)vinyl)-5,5-
dimethylfuran-2(5H)-ylidene)malononitrile (BL)
A mixture of key intermediate (0.5 g, 1.56 mmol), N,N-diethyl-p-
phenylenediamine (0.29 g, 1.71 mmol) and Na2SO4 (1.5 g,
1.71 mmol) was stirred for 24 h at room temperature in CH2Cl2
(10 mL) whereby its color changed to green. It was then concen-
trated and the product was precipitated by adding pentane (10 mL).
The product was collected by filtration, washed with pentane twice
and dried in vacuo as a green solid (0.62 g, 85%). C27H25N5OS
(467.59): calcd. C, 69.35; H, 5.39; N, 14.98, S, 6.86; found C, 69.57; H,
5.42; N, 15.03; S, 6.90. 1H NMR (CDCl3, 400.13 MHz, 20 ꢀC): jj ¼ 1.20
(t, J ¼ 7.1 Hz, 6H, H-21), 1.74 (s, 6H, H-9), 3.41 (m, 4H, H-20),
6.65e6.73 (m, 3H, H10, H18), 7.32 (d, J ¼ 9.1 Hz, 2H, H-17), 7.37 (d,
J ¼ 4.0 Hz, 1H, H-12), 7.44 (d, J ¼ 4.0 Hz, 1H, H-13), 7.88 (d,
J ¼ 15.9 Hz, 1H, H-7), 8.62 (s, 1H, H-15) ppm. 13C NMR (CDCl3,
100.61 MHz, 20 ꢀC): jj ¼ 12.81, 26.42, 44.75, 97.53, 97.93, 110.83,
111.21, 111.89, 112.00, 114.05, 123.83, 130.82, 136.04, 137.69, 139.52,
141.59, 144.18, 148.15, 151.80, 172.81, 175.62 ppm. MS (MALDI-TOF):
m/z calcd for C27H25N5OS: 467.18 [MþH]þ; found: 468.60.
2.2. Experimental details including characterization data
2.2.1. Synthesis of acceptor 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-
ylidene)malononitrile (TCF)
Acceptor 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malo-
nonitrile (TCF) was synthesized according to a method published in
the literature [13] with slight modification. In a 100 mL round-
bottomed flask, a solution of sodium ethoxide was prepared by
adding sodium (0.09 g, 3.91 mmol) to ethanol (60 mL). To this so-
lution 3- hydroxy-3-methyl-2-butanone (3.32 mL, 30 mmol) and
malononitrile (4.00 g, 60 mmol) were added. The resulting mixture
Scheme 1. Synthesis of the chromophore FTC.

S. Farsadpour et al. / Dyes and Pigments 127 (2016) 73e77
75
anhydrous Na2SO4 in dichloromethane at room temperature. This
step can also be done in hot toluene without adding Na2SO4
resulting in approx. 75% yield.
3.2. Thermal stability and solvatochromic behavior
Chromophore BL showed good solubility in common polar and
non-polar organic solvents. For proof-of-principle, key intermedi-
ate (KI) and product have been characterized successfully. More-
over, UVeVis as well as thermogravimetric analysis have been
examined as an important factor in electro-optic evaluation. Ther-
mal stability of the chromophores BL was evaluated by simulta-
neous thermogravimetric analysis. Since the decomposition
temperature is defined as temperature in which 5% of weight is lost
(Td, 5% lost) [16], chromophore BL has a remarkable stability up to
300 ꢀC as illustrated in the Supporting Information. This shows that
the aforementioned chromophore displays enough thermal sta-
bility for further processing, e.g., electric field poling. In order to
reveal the distinctions of the electronic structures of the chromo-
phores, UVeVis absorption spectra were measured in a series of
aprotic solvents with different polarities so that the solvatochromic
behavior of chromophore could be investigated in a wide range of
dielectric environments. The UVeVis absorption spectra of the
chromophores in five different solvents are presented in Fig. 1, and
the data are summarized in Table 1.
Solvatochromic behavior is classified as negative, positive, or
reversed behavior. In negative behavior, an increase of the solvent
polarity leads to a hypsochromic shift (blue shift) of its sol-
vatochromic band. Positive behavior occurs when a bathochromic
shift (red shift) is observed. Finally, reversed or inverted behavior
occurs when, at a given solvent polarity value, a change from a
negative to a positive behavior is observed. Chromophore BL
showed the inverted solvatochromic effect; an initial bathochromic
shift of the absorption maxima which later reverses to a hyp-
sochromic shift for more polar solvents like acetone and acetoni-
trile. This effect is limited to a series of highly polarizable
chromophores reported so far [17].
Scheme 2. Examined dye with {eWoD} pattern along the bridge.
3. Result and discussion
3.1. Synthesis route
In design of the novel chromophore, quantum mechanical cal-
culations have provided much guidance [14]. Using Density func-
tional theory (DFT) calculations, in 2008, Chafin and Lindsay [15]
have highlighted the patterns which can increase the first hyper-
polarizability. This pattern called {eWoD} in which the electron-
withdrawing (W) groups are substituted on even-numbered posi-
tions (e) of methine, and the electron-donating (D) groups on odd-
numbered positions (o) of methines (Scheme 2). This pattern po-
larizes the
p bonds along the polyene in the opposite direction that
the termini of the dye polarize the dye. Placing cyano and fluorine
groups in the {eWoD} pattern along the polyene, a sixfold higher
first hyperpolarizability was predicted compared to placing them in
the opposite substitution pattern ({oWeD}).
According to the {eWoD} pattern, they could also show that the
azomethine (also called imine) as a relatively electron withdrawing
group (compared to carbon methine) is advantageous if placed at
the even numbered positions. Hence, the other positions are called
“unfavourable”. Thus, following aforementioned approach, our
group came up with a novel class of chromophores in which the
unfavourable methine group in FTC-type chromophores (Scheme
1) is replaced with azomethine at the effective position of the
chromophore to increase the hyperpolarizability. Additionally, a
novel and convenient synthesis method has been designed which is
particularly advantageous compared with known procedures
because this method offers high atom economy, uses low-cost re-
agents, works under mild reaction conditions and requires less
reaction steps. Starting from the acceptor TCF, the chromophore
could be synthesized in just two steps (see Scheme 3). Reacting
1:1 M ratio of acceptor TCF with 2,5-thiophenedicarboxaldehyde
resulted in the key intermediate (KI) in approx. 90%, no further
purification was needed. The target chromophore BL was obtained
in good yields (>85%) by reacting 1:1 M ratio of the key interme-
diate with the N,N-diethyl-p-phenylenediamine in presence of one
3.3. Theoretical calculations
Density functional theory (DFT) was used to calculate the
HOMOeLUMO energy gap and first-order hyperpolarizability (b) of
the new chromophores in comparison with FTC as benchmark
chromophore. The DFT calculations were carried out at the hybrid
B3LYP level by employing the split valence 6e31 þ g basis set based
on the B3LYP/6-31g(d,p) optimized geometries. The data obtained
Scheme 3. Synthesis of the BL chromophore.

76
S. Farsadpour et al. / Dyes and Pigments 127 (2016) 73e77
chromophores containing azomethine displays enough thermal
stability for further processing. These chromophores also show a
strong inverted solvatochromic effect which has been previously
reported for highly polarizable chromophores. Theoretical calcu-
lations suggest that the novel chromophore BL containing azome-
thine has a lower energy gap between the HOMO and LUMO and a
larger first hyperpolarizability than those of the chromophore FTC.
Currently, we are performing Hyper-Rayleigh Scattering (HRS) ex-
periments as well as Attenuated Total Reflection (ATR) measure-
ments of the hyperpolarizability of the chromophores in solutions
and EO responses (r33 values) of guestehost polymers containing
chromophores.
Acknowledgments
We acknowledge financial support by the European-Union
through project HELOPSI, funded in the EFRE-program (Euro-
€
paischer Fonds für Regionale Entwicklung)). We acknowledge the
Fig. 1. UVeVis absorption spectra of BL in five different solvents (2 ꢂ 10ꢁ5 M) of
varying dielectric constants (1,4-dixoane: 2.25; toluene: 2.38; chloroform: 4.81;
dichloromethane: 8.93; acetone: 20.7; acetonitrile: 37.5) at room temperature.
high performance computing centre of the University of Kai-
serslautern and thank Prof. Werner Thiel for fruitful discussions
and his support.
Table 1
Appendix A. Supplementary data
UVeVis absorptions for chromophore BL in five different solvents.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.12.013.
Chromophore
la
lb
lc
ld
le
Dlf
BL
649
593
590
20.7
615
588
37.5
61
εg
4.81
2.25
2.38
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The maximum absorption wavelength (nm) of chromophores measured in chlo-
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(max) and (min): the maximum and minimum absorption wavelength of five
solvents. g Dielectric constants of the corresponding solvents.
¼
l
(max) ꢁ
l(min).
b
c
d
e
f
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