Nonlinear optical chromophores containing a novel pyrrole-based bridge: Optimization of electro-optic activity and thermal stability by modifying the bridge

Article abstract of DOI:10.1039/c4tc00900b

Three novel second order nonlinear optical chromophores based on julolidinyl donors and tricyanovinyldihydrofuran (TCF) acceptors linked together via modified pyrrole π-conjugation (chromophores A and B) or thiophene moieties (chromophore C) as the bridges have been synthesized and systematically characterized. In particular, the pyrrole moiety bridge has been modified with the electron withdrawing group (-Br, -NO₂) substituted benzene ring. The introduction of side phenyl groups to chromophores A and B can increase the thermal and chemical stability and reduce dipole-dipole interactions so as to translate their hyperpolarizability (β) values into bulk EO performance more effectively than chromophore C. Moreover, DFT calculations suggested that the additional electron withdrawing groups in chromophores A and B could increase the β value compared to that of chromophore D without substituted phenyl groups, and they showed different influences on the solvatochromic behavior, thermal stability, and electro-optic activity of the chromophores. EO responses (r₃₃ values) of guest-host polymers containing pyrrole-bridged chromophores were reported. Incorporation of chromophores A and B into APC provided large electro-optic coefficients of 86 and 128 pm V^-1 at 1310 nm with a high loading of 30 wt%. Film-C/APC containing 25 wt% of chromophore C provides an EO coefficient of 98 pm V^-1.

Full text of DOI:10.1039/c4tc00900b

Journal of
Materials Chemistry C
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PAPER
Nonlinear optical chromophores containing a
novel pyrrole-based bridge: optimization of
electro-optic activity and thermal stability by
modifying the bridge†
Cite this: J. Mater. Chem. C, 2014, 2,
7785
Fenggang Liu,ab Haoran Wang,ab Yuhui Yang,ab Huajun Xu,ab Maolin Zhang,ab
a
a
a
a
Airui Zhang,ab Shuhui Bo, Zhen Zhen, Xinhou Liu and Ling Qiu
*
*
Three novel second order nonlinear optical chromophores based on julolidinyl donors and
tricyanovinyldihydrofuran (TCF) acceptors linked together via modified pyrrole p-conjugation
(chromophores A and B) or thiophene moieties (chromophore C) as the bridges have been synthesized
and systematically characterized. In particular, the pyrrole moiety bridge has been modified with the
electron withdrawing group (–Br, –NO2) substituted benzene ring. The introduction of side phenyl
groups to chromophores A and B can increase the thermal and chemical stability and reduce dipole–
dipole interactions so as to translate their hyperpolarizability (b) values into bulk EO performance more
effectively than chromophore C. Moreover, DFT calculations suggested that the additional electron
withdrawing groups in chromophores A and B could increase the b value compared to that of
chromophore D without substituted phenyl groups, and they showed different influences on the
solvatochromic behavior, thermal stability, and electro-optic activity of the chromophores. EO responses
(r33 values) of guest–host polymers containing pyrrole-bridged chromophores were reported.
Incorporation of chromophores A and B into APC provided large electro-optic coefficients of 86 and
128 pm Vꢀ1 at 1310 nm with a high loading of 30 wt%. Film-C/APC containing 25 wt% of chromophore
Received 2nd May 2014
Accepted 16th July 2014
DOI: 10.1039/c4tc00900b
www.rsc.org/MaterialsC
C provides an EO coefficient of 98 pm Vꢀ1
.
(donor) and an electron-withdrawing group (acceptor) coupled
through a p-conjugated bridge.8,9 Among the many types of NLO
chromophores developed so far, strong electron donor and
acceptor groups connected by chemically and thermally stable
conjugated p-bridges have been widely employed.10 Until now,
NLO chromophores containing thiophene,11,12 locked phenyl-
tetraene,7 azo13 and phenyltetraene-based14 bridges have been
reported. Among which thiophene and phenyltetraene bridges
were widely used. Further modication was widely studied to
achieve excellent and better performance.14,15 However, the
pyrrole-based bridge,16 the analogue of furan and thiophene
moieties, has seldom been noticed, especially for chromo-
phores with a strong donor and a strong acceptor possibly due
to its unstability. In fact, to the best of the authors' knowledge,
few studies have reported on the macroscopic EO response
(r33 value) of poled lms containing pyrrole-bridged chromo-
phores. Practical applications of second-order NLO materials
require thermally and chemically robust materials with a high
electro-optic (EO) coefficient (r33) value.
1. Introduction
In recent years, organic electro-optic (OEO) materials have
attracted great attention owing to their potential applications in
optical switches, optical sensors, information processors, tele-
communications, etc.1–3 Compared with inorganic materials,
OEO materials have potential advantages such as lower cost,
ease of processing, larger EO coefficients and so on.4–6 To meet
the stringent requirements for the use of devices, OEO materials
should be developed through the rational design of nonlinear
optical (NLO) chromophores to optimize their rst hyper-
polarizability (b), and effectively translate these large b values
into bulk EO activities together with improvement of other
auxiliary properties like high thermal and chemical stability.7
The second-order NLO chromophores are based on a push–
pull system, which consists of an electron-donating group
aKey Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR
China. E-mail: boshuhui@mail.ipc.ac.cn; qiuling@mail.ipc.ac.cn; Fax: +86-01-
2543529; Tel: +86-01-82543529
bUniversity of Chinese Academy of Sciences, Beijing 100043, PR China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00900b
The above points prompted us to design and synthesize a
series of pyrrole-bridged NLO chromophores with enhanced
stability. Moreover, the b values of these chromophores can be
effectively translated into high EO activities in poled polymers
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by suitable shape engineering. Modications of the pyrrole-
bridge have been explored for high electro-optic coefficients
along with improved chemical and thermal stability to with-
stand high temperatures encountered in electric eld poling
and subsequent processing of chromophore/polymer materials.
So that it can be applied to electro-optic materials.
2. Results and discussion
2.1 Synthesis and characterization of chromophores
Scheme 1 shows the synthetic approach to the new chromo-
phores A, B, and C. Starting from the amine donor aldehyde
intermediate compound 3, chromophores A, B, and C were
synthesized in good overall yields through simple four main
step reactions. The free hydroxyl group on the donors was
protected by the alkyl group to improve the solubility through
Williamson ether synthesis, aer introduction of the bridge by
Wittig condensation; compounds 5a–c were prepared in a high
yield. Treatment of compounds 5a and b with POCl3 and DMF
gave aldehydes 6a and b. Treatment of compound 5c with
n-BuLi and DMF gave an aldehyde 6c. And the nal condensa-
tions with the TCF acceptor give chromophores A, B, and C all as
green solids. All of the chromophores were fully characterized
by 1H-NMR, 13C-NMR, elemental analysis and mass spectros-
copy. These chromophores possess good solubility in common
organic solvents, such as dichloromethane, chloromethane and
acetone.
We used a strong julolidinyl-based donor17,18 and tricyano-
vinyldihydrofuran (TCF) acceptor to construct our chromo-
phores. In our syntheses, we found that when used as a bridge,
the abundant electron density of the pyrrole makes it unstable.
So, the labile nitrogen proton was replaced with aromatic blocks
to improve the stability. Moreover, a new method was proposed
to introduce some electron-withdrawing units, which acted as
additional electron acceptors, to form special types of organic
chromophores. The additional electron acceptors (–Br, –NO2)
were substituted on the benzene ring so that the properties (like
electron density) of the chromophore can be changed. The roles
of different auxiliary acceptor groups connected to the conju-
gated spacers of push–pull chromophores have been investi-
gated and evaluated by theoretical methods. These introduced
acceptor groups could not only increase the b values, but also
reduce dipole–dipole interactions of these chromophores to
improve the EO activity.
2.2 Thermal stability
The NLO chromophores must be thermally stable in order to
withstand the poling process and subsequent processing for
use. The thermal stability of the chromophores was investigated
using thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) as shown in Fig. 1. Chromophores B and
C appeared as highly crystalline compounds with a melting
In this study, we had designed and synthesized chromo-
phores A and B based on modied pyrrole bridges. Chromo-
phore C containing a thiophene bridge had been synthesized
for comparison (Chart 1). The UV-Vis, solvatochromic, redox
properties, density functional theory (DFT) quantum mechan-
ical calculations, thermal stabilities and EO activities of these
chromophores were systematically studied and compared to
illustrate the inuences of the modied bridges on rational
NLO chromophore designs. Previous studies showed that the
DFT method is considered reliable in predicting trends in
hyperpolarizability,19 and have been shown to predict experi-
ꢁ
ꢁ
point at 196 C and 198 C, respectively, while chromophore A
was obtained as an amorphous solid showing a glass transition
mental trends accurately.20 Chromophore
D containing
unmodied pyrrole bridge had not been synthesized due to its
chemical and thermal instability. However, the use of theoret-
ical methods enables us to study the effects of structural
modications on optimization of chromophore hyper-
polarizability in this series of molecules.
Scheme 1 Chemical structures and synthetic routes to chromophores
Chart 1 Chemical structures for chromophores A, B, C, and D.
7786 | J. Mater. Chem. C, 2014, 2, 7785–7795
A, B, and C.
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Fig. 1 TGA and DSC curves of chromophores A–C with a heating rate
of 10 ꢁC minꢀ1 in a nitrogen atmosphere.
temperature (Tg) at 190 ꢁC. All the chromophores exhibited
good thermal stabilitꢁies with the decomposition temperatures
ꢁ
(Td) higher than 200 C (202–251 C). Chromophore A had the
maximum decomposition temperature (Td ¼ 251 ꢁC). Chro-
mophores A and B had higher decomposition temperature than
that of chromophore C with a thiophene bridge (Td ¼ 202 C).
Fig. 2 UV-Vis absorption spectra of chromophores A, B and C in six
kinds of aprotic solvents with varying dielectric constants (3).
ꢁ
The enhanced thermal stability of chromophores A and B over
chromophore C is due to the introduction of a benzene ring into
p-bridge pyrrole. While chromophores A and B had a similar
pyrrole bridge, they show different decomposition temperatures
of 251 ꢁC and 236 ꢁC, respectively. This may be due to the
different properties of modied groups (–Br, –NO2). The data
indicate that the modied groups can inuence the thermal
stabilities of chromophores, and thus we can modify the pyrrole
bridge to improve the thermal properties.
bathochromic shi of 79 nm, from dioxane to chloroform,
displaying larger solvatochromism than that of CLD (60 nm)
and a similar single donor FTC (61 nm) chromophore,21,22 while
chromophore A showed a moderate bathochromic shi of 61
nm from dioxane to chloroform. This conrms that chromo-
phores B and C are more easily polarizable than A although
chromophore A had a similar pyrrole bridge to chromophore
B.23 It is interesting to note that on continuous increase of the
dielectric constant of the solvent beyond that of chloroform, the
NLO chromophore exhibited negative solvatochromism. For
example, compounds A and B showed hypsochromic shis of
ꢀ31 nm and ꢀ36 nm, respectively, from chloroform to acetone.
2.3 Optical properties
To explore the different charge-transfer (CT) absorption prop-
erties of each chromophore, UV-Vis absorption spectra of three
chromophores were recorded in a series of solvents with
different dielectric constants and in lms, as shown in Fig. 2.
The spectral data are summarized in Table 1. It may be seen
from Fig. 2 that chromophores A, B and C show the maximum
absorption (lmax) from 723 nm to 728 nm in chloroform.
Compared to chromophore C, chromophores A and B were
slightly blue-shied (Dl ¼ 5 nm, Dl ¼ 3 nm), and the two
compounds containing the pyrrole bridge exhibited a similar
charge-transfer band shape.
Chromophore
C
also exhibited similar solvatochromic
behavior. Such a phenomenon was reported by Davies23 and was
ascribed to the back electron transfer from the acceptor side to
the donor side in polar solvents which caused a blue-shi from
the absorption. The same explanation should also hold for the
present series of NLO chromophores.24
The photoluminescence emission spectra of A, B and C were
also measured in different organic solvents. The spectral data
are summarized in Table 1. Chromophores A, B and C showed
Stokes shis of 137 nm, 157 nm, and 228 nm in dioxane, and 130
nm, 139 nm, 223 nm in chloroform respectively. Chromophore C
Besides, the solvatochromic behavior was also an important
aspect to investigate the polarity of chromophores. It was found
that both chromophores B and C showed a very large
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Table 1 Optical property data of the chromophores
Ultraviolet spectrum
Fluorescence spectrum
Dlc (nm)
lmax (nm)
lmaxe (nm)
lmax (nm)
Dlg (nm)
d
f
Chromophores
lmaxa (nm)
lmaxb (nm)
A
B
C
662
646
649
723
725
728
61
79
79
702
706
703
799
803
877
853
864
951
54
61
74
a
d
lmax was measured in dioxane. b lmax was measured in chloroform. c Dl was the difference between a lmax and b lmax
.
lmax was measured in lms.
.
e
lmax was measured in dioxane. f lmax was measured in chloroform. g Dl was the difference between e lmax and f lmax
showed larger Stokes shis than that of chromophores A and B. of the acceptor perhaps resulting in a decreased b value of
Chromophore A showed the smallest Stokes shis.
chromophores A, B and D.30
Optimization of the molecular rst hyperpolarizability of
dipolar EO chromophores relies on tuning of the electron
density distribution through chemical modication of molec-
ular constituents.31 It may be noted that, though having a
similar pyrrole bridge, the b values of chromophores A and B
were nearly 20% higher than that of chromophore D. This can
be explained by that the abundant electron density of the
pyrrole ring would be weakened by modied benzene ring
groups which may lead to increased b values. The additional
electron acceptor incorporated into the conjugated system may
facilitate the electron transfer from the donor to the acceptor,
thus decreasing the energy barrier of the charge transfer tran-
sition which leads to an increased b value. From another
perspective, the enhanced b values of chromophores A and B
might be also ascribed to the fact that the additional electron
acceptor group added perpendicular to the conjugated back-
bone could serve as an efficient spacer to reduce the formation
of dipole couples.15,32 This may prove that the special “auxiliary
acceptor” group introduced perpendicular to the conjugated
backbone could increase the b values.
In order to determine the redox properties of chromophores
A–C, cyclic voltammetry (CV) measurements were conducted in
degassed anhydrous acetonitrile solutions containing 0.1 mol
Lꢀ1 tetrabutylammonium hexauorophosphate (TBAPF) as the
supporting electrolyte. The relative data of 1 ꢂ 10ꢀ4 mol Lꢀ1
chromophores A–C were recorded, as shown in Table 2. The
HOMO and LUMO levels of all three chromophores can be
calculated from their corresponding oxidation and reduction
potentials.33 The difference between these two values provides
the HOMO–LUMO energy difference DE (CV). The calculation
2.4 Theoretical calculations and redox properties
The DFT calculations were carried out at the hybrid B3LYP level
by employing the split valence 6-311g (d, p) basis set25,26 to
understand the ground-state polarization of the chromophores
with different bridges. When used carefully and consistently,
this method of DFT has been shown to give relatively consistent
descriptions of rst-order hyperpolarizability for a number of
similar chromophores.7,26 In order to compare the microscopic
nonlinear effect between chromophores, the b value of chro-
mophore D was also calculated. All molecules were assumed to
be in trans-congurations. The HOMO–LUMO energy gaps,
dipole moment (m), and rst hyperpolarizability (b) of the
chromophores obtained from DFT calculations are summarized
in Table 2.
For chromophores A and B, the additional electron acceptors
(–Br, –NO2) created another local dipole moment which results
in a larger total dipole moment than that of chromophore D. As
reported previously, the b value has a close relationship with the
strength of the donor and acceptor end groups and with the
nature and length of the bridge, substituents, steric hindrance
and intramolecular charge-transfer and so on.27,28 Chromo-
phore C showed the largest b value of all the four chromo-
phores. This might be due to the molecular conguration and
different combination effects among the donor, acceptor and
bridge. The location of the heterocyclic ring also plays a critical
role in determining b values, other than the nature of the ring.29
Pyrrole, being the most electron rich ve-membered hetero-
aromatic ring, may counteract the electron-withdrawing effect
Table 2 Summary of DFT and electro-chemical data
DE (DFT)a (eV)
DE (CV)b (eV)
Eox (V)
Ered (V)
me/D
btot (10ꢀ30 esu)
c
d
f
Chromophore
A
B
C
D
2.02
1.90
1.96
2.04
1.36
1.34
1.35
—
0.12
0.11
0.09
—
ꢀ1.24
ꢀ1.23
ꢀ1.26
—
20.51
21.62
20.84
18.54
672.35
686.10
809.49
571.83
a
b
DE (DFT) was calculated from DFT calculations. DE (CV) was calculated from their corresponding oxidation and reduction potentials.
c
d
e
f
Referenced to the ferrocene standard. Referenced to the ferrocene standard. m is the total dipole moment. btot is the rst-order
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
hyperpolarizability, where btot
¼
ðbxÞ þ ðbyÞ þ ðbzÞ is calculated from DFT quantum mechanical methods.
7788 | J. Mater. Chem. C, 2014, 2, 7785–7795
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results are summarized in Table 2. The HOMO levels of chro- eld, the electro-optic coefficient in the direction of the applied
mophores A–C were estimated to be ꢀ4.92, ꢀ4.91, and ꢀ4.89 eld is related to the molecular rst hyperpolarizability by37
eV, which showed a slight increase. In the meantime, the cor-
r33 ¼ |2Nf(u)bhcos3 qi/n4|
responding LUMO level of A–C only showed slight changes of
about ꢀ3.56, ꢀ3.57, and ꢀ3.54 eV, respectively. The HOMO–
where N is the chromophore number density (molecules per
LUMO gap was the lowest for chromophore B (1.34 eV) and the
cm3) in the polymer host, n is the index of refraction of the
highest for chromophore A (1.36 eV). The narrower energy gap
indicated easier charge transfer.34,35
The HOMO–LUMO energy gaps DE (DFT) were also calcu-
lated by DFT calculations as shown in Table 2. The energy gaps
chromophore-containing polymeric material, and f (u) is the
product of local-eld (Debye–Onsager) factors. cos3(q) is the
acentric order parameter. q is the angle between the permanent
dipole moment of chromophores and the applied electric eld.
between the HOMO and LUMO energy for chromophores A, B,
At low concentration, the electro-optic activity increased with
and C were 2.02, 1.90, and 1.96 eV. The electrochemical values
chromophore density, dipole moment and the strength of the
are corroborated by the DFT calculations. Trends of energy gaps
electric poling eld. However, when the concentrations of
between DE (DFT) and DE (CV) for the chromophores were
chromophores increased to a certain extent, the N and cos3(q)
found to be consistent. As is commonly observed, DE (DFT) is
signicantly overestimated when compared to the data
are no longer independent factors. Then,
obtained by CV.
hcos3(q)i ¼ (mF/5kT)[1 ꢀ L2(W/kT)]
The frontier molecular orbitals are oen used to obtain
information about the optical and electrical properties of
where k is the Boltzmann constant and T is the Kelvin (poling)
molecules.34 Fig. 3 depicts the electron density distribution of
temperature. F ¼ [f(0)EP] where Ep is the electric poling eld. L is
the HOMO and LUMO structures which indicated that the
density of the ground and excited state electron is asymmetric
along the dipolar axis of the chromophores.
the Langevin function, which is a function of W/kT, the ratio of
the intermolecular electrostatic energy (W) to the thermal
energy (kT). L is related to electrostatic interactions between
To get more information from the frontier orbitals, the
molecules.
composition of the HOMOs and LUMOs has been calculated
This relationship can be successfully applied to qualitatively
using the Multiwfn program.36 The whole chromophore mole-
predict the important trends involving electro-optic activity
cule was segmented as a donor, a p-bridge, and an acceptor as
although not the quantitative values of electro-optic coeffi-
shown in Table 3. For the chromophores A–D, the LUMO was
cients. When the intermolecular electrostatic interactions are
neglected, the electro-optic coefficient (r33) should increase
largely stabilized by the contributions from the acceptor (43.70–
47.08%) and the p-bridge (36.64–39.51%), while the HOMO was
linearly with chromophore density, dipole moment, rst
largely stabilized by the contributions from donors (56.82–
hyperpolarizability and the strength of the electric poling eld.
59.42%) and the p-bridge (24.57–25.44%). The contribution of
But chromophores with large dipole moments generate an
acceptors of chromophores A, B and D (46.81–47.08%) con-
intermolecular static electric eld dipole–dipole interaction,
taining a pyrrole-based bridge seemed to be bigger than that of
which leads to the unfavorable antiparallel packing of chro-
chromophore C (43.70%) with a thiophene-bridge for the LUMO
mophores. So the number of truly oriented chromophores (N) is
level. As for the HOMO level, the acceptors of chromophores A,
small. In molecular optimization, introducing a huge steric
B and D also contributed more than that of chromophore C.
hindrance group to isolate chromophores is the most popular
and easy way to attenuate the dipole–dipole interactions of
chromophores.38
2.5 Electro-optic performance
To achieve a high EO coefficient, the chromophore should
have facile intra-charge transfer ability. The p–p stacking
interactions and inter-molecular charge transport between
The computational approach has helped to assess the rela-
tionship between bulk and molecular level non-linear optical
(NLO) effects. For dipolar chromophores in an electric poling
Fig. 3 The frontier molecular orbitals of chromophores A–D.
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Table 3 The molecular orbital composition (%) in the ground state for chromophores A–Da
A
B
C
D
Chromophore
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
Donor
p bridge
Acceptor
56.82%
25.02%
18.16%
16.28%
36.64%
47.08%
57.82%
24.57%
17.61%
16.39%
36.80%
46.81%
59.42%
25.44%
15.14%
16.79%
39.51%
43.70%
56.77%
24.75%
18.48%
16.22%
36.95%
46.83%
a
The molecular orbital composition was calculated using the Multiwfn program with Ros-Schuit (SCPA) partition.
chromophores should be reduced in order to translate the for the difference in achievable EO performance, the order
microscopic hyperpolarizability into macroscopic EO parameter (F) was calculated by measuring UV-Vis absorption
a
response more effectively. In order to balance the steric spectra. Aer the corona poling, the dipole moments of the
hindrance and the molecular free mobility for chromophore chromophore moieties in the polymer were aligned, and the
acentric alignment in the guest–host EO polymer, the poling absorption curve decreased due to birefringence.17 The order
conditions should be carefully explored.38
parameter (F) for lms can be calculated from the absorption
In order to investigate the translation of the microscopic
hyperpolarizability into a macroscopic EO response, the poly-
mer lms doped with different concentrations of chromophores
into amorphous polycarbonate (APC) were prepared using
dibromomethane as solvent. The resulting solutions were
ltered through a 0.2 mm PTFE lter and spin-coated onto
indium tin oxide (ITO) glass substrates. Films of doped poly-
mers were baked at 80 ꢁC in a vacuum oven overnight. The
corona poling process was carried out at a temperature of 10 ꢁC
above the glass transition temperature (Tg) of the polymer. The
r33 values of poled lms were measured by the Teng–Man
simple reection method at a wavelength of 1310 nm using a
carefully selected thin ITO electrode with low reectivity and
good transparency in order to minimize the contribution from
multiple reections.39,40
To evaluate their EO activities, 10 wt% of the chromophores
A–C were doped into APC and formulated as the typical guest–
host polymer. For this series of chromophores, the poled lms
of A/APC, B/APC and C/APC afforded r33 values of 40, 53 and
60 pm Vꢀ1, respectively. To identify the underlying mechanism
Fig. 4 r33 values of NLO thin films as a function of chromophore Fig. 5 UV-Vis absorption spectra of EO polymers before and after
loading densities.
poling (10 wt%).
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changes according to the following equation: F ¼ 1 ꢀ A/A0,41 in
which A and A0 are the respective absorptions of the polymer
lms aer and before corona poling. The order parameter (F) of
poled lms was calculated. The F values of lms A/APC, B/APC,
and C/APC are 12.12%, 16.28%, 16.34%, respectively as shown
in Fig. 5. The larger order parameters of lms B and C indicate
that lms B/APC and C/APC were more easily poled than lm-
A/APC. The lm-C/APC achieved the largest r33 value probably
due to its largest b and good polarizability. With relatively low b,
the r33 value of lm-B/APC was smaller than that of lm-C/APC.
The drop in EO activity from lm-B/APC to lm-A/APC is mainly
associated with the lowest b of chromophore A in the polar
polymer matrix and the inefficient poling, because of its lower
polarizability than the other two chromophores. This corre-
sponded well with the results of the solvatochromism study,
DFT calculations and redox properties of chromophores.
The r33 values of lms A/APC, B/APC, and C/APC were
measured at different loading densities, as shown in Fig. 4. As a
result, the r33 values of lm-C/APC were gradually improved from
60 pm Vꢀ1 (10 wt%) to 98 pm Vꢀ1 (25 wt%), while the r33 values
dropped to 90 pm Vꢀ1 as the loading density increased from
25 wt% to 30 wt%. The r33 values of lm-B/APC were improved
from 53 pm Vꢀ1 (10 wt%) to 128 pm Vꢀ1 (30 wt%). A similar trend
was also observed for lm-A/APC whose r33 values increased from
40 pm Vꢀ1 (10 wt%) to 86 pm Vꢀ1 (30 wt%). When the concen-
tration of the chromophore in APC is low, lm C/APC displayed a
larger r33 value than that of the lm B/APC. As the chromophore
loading increased, this trend is reversed. This can be explained
by that, when in a low-density range, the intermolecular dipolar
interactions are relatively weak. The intermolecular dipole–
dipole interactions would become stronger and stronger,
accompanied by the increased concentration of NLO chromo-
phore moieties in the polymer which would nally lead to a
decreased NLO coefficient. The introduction of side-groups
attached to the conjugated p-system can make inter-chromo-
phore electrostatic interactions less favorable.10,42
The F values of lms A/APC, B/APC, and C/APC which
contain 25 wt% of the chromophores are 18.02%, 20.38%,
17.26%, respectively, while the F values of lms A/APC, B/APC,
and C/APC are 18.67%, 21.03%, 15.85%, respectively, with a
high loading of 30 wt% (see ESI†). The difference in the order
parameter indicates that lms A/APC and B/APC have weaker
inter-chromophore electrostatic interactions than lm-C/APC in
high density. It may be revealed by the optimized congurations
(Fig. 6) that the side benzene ring groups were perpendicular to
the direction of the dipole moment of the chromophore which
could act as the isolation group to suppress the possible
aggregation. The lm B/APC achieves the largest r33 value of
128 pm Vꢀ1, even though it has lower b. The high saturated
loading density and larger F values of lms A and B were
probably attributed to the fact that the chromophore structure
isolated the chromophores from each other more effectively,
thus improving the NLO effect at a higher chromophore loading
density level.
Fig. 6 The optimized structure of chromophores A, B and C.
to the isolation of modied benzene ring groups. However, the
different r33 values of lm-A/APC and lm B/APC prove that the
different auxiliary acceptor groups (–Br, –NO2) showed different
inuences on molecular properties. Moreover, the large r33
values of the chromophores show that the modied pyrrole
bridges were stable enough to withstand high temperatures
encountered in electric eld poling and subsequent processing
of chromophore/polymer materials.
3. Conclusion
Three organic NLO chromophores with pyrrole or thiophene
bridges have been synthesized and systematically characterized.
The new chromophores showed high thermal and chemical
stabilities, large nonlinear optical coefficient, and good solu-
bility in all common solvents. The pyrrole moiety bridge has
been substituted with modied phenyl groups. The introduc-
tion of the side phenyl groups helps to improve the chemical
and thermal stability of the unstable pyrrole moiety, so that it
could be used in electro-optic (OEO) materials. Moreover the
side groups can reduce dipole–dipole interactions so as to
translate their hyperpolarizability (b) values into bulk EO
performance more effectively than chromophore C. DFT calcu-
lations suggested that the modied electron acceptor groups
(–Br, –NO2) in chromophores A and B could increase the b value,
and they showed different inuences on the solvatochromic
behavior, thermal stability, and electro-optic activity of the
chromophores. Incorporation of chromophores A and B into
APC provided large electro-optic coefficients of 86 and 128 pm
Vꢀ1 at 1310 nm with a high loading of 30 wt%. All these might
be useful in designing other new NLO chromophores with a
modied pyrrole bridge to optimize the molecular structure for
achieving different properties. These novel modied pyrrole
bridges will show promising applications in nonlinear optical
(NLO) chromophore synthesis.
4. Experimental section
4.1 Materials and instrument
All chemicals are commercially available and are used without
Based on the above points, chromophores A and B contain- further purication unless otherwise stated. N,N-Dimethyl
ing a modied pyrrole bridge can translate their large b values formamide (DMF), phosphorusoxychloride (POCl3), tetrahy-
into bulk EO activities more effectively than chromophore C due drofuran (THF) and ether were distilled over calcium hydride
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and stored over molecular sieves (pore size 3 A). 1-(4-Bromo- as an orange oil in 83.7% yield (1.37 g, 2.51 mmol). 1H NMR (400
phenyl)-1H-pyrrole-2-carbaldehyde (1a) was prepared according MHz, acetone) d 7.60 (2H, d, J ¼ 6.6Hz, ArH), 7.38 (1H, d, J ¼ 8.7
to the literature.43 1-(4-Nitrophenyl)-1H-pyrrole-2-carbaldehyde Hz, CH), 7.23 (2H, d, J ¼ 6.6 Hz, ArH), 7.08 (1H, s, ArH), 6.87
(1b) was prepared according to the literature.44 The 2-dicyano- (1H, d, J ¼ 8.7, CH), 6.32 (1H, d, J ¼ 11.9 Hz, PrH), 6.23–6.18 (1H,
methylene-3-cyano-4-methyl-2,5-dihydrofuran (TCF) acceptor m, PrH), 6.11 (1H, d, J ¼ 11.9 Hz, PrH), 3.61 (2H, t, J ¼ 6.7 Hz,
was prepared according to the literature.45 Compounds 2a and OCH2), 3.13–3.09 (2H, m, NCH2), 3.08–3.02 (1H, m, NCH2), 1.70
2b were prepared according to the literature.46 TLC analyses (6H, m, CH2), 1.52–1.42 (2H, m, CH2), 1.36 (6H, s, CH3), 1.10
were carried out on 0.25 mm thick precoated silica plates and (6H, s, CH3), 0.95 (3H, t, J ¼ 7.3 Hz, CH3); MS (MALDI)
spots were visualized under UV light. Chromatography on silica (M+, C32H39BrN2O): calcd: 547.573; found: 547.258.
˚
gel was carried out on Kieselgel (200–300 mesh).
Compound 5b. The procedure for compound 5a was fol-
lowed to prepare 5b from 4 and 3b as a red oil in 56.7% yield
(0.87 g, 1.71 mmol). 1H NMR (400 MHz, acetone) d 8.40 (2H, d, J
¼ 9.9 Hz, ArH), 7.71 (2H, d, J ¼ 9.9 Hz, ArH), 7.17 (1H, s, PrH),
7.08 (1H, s, ArH), 7.01 (1H, d, J ¼ 16.2 Hz, CH), 6.69 (1H, d, J ¼
16.2 Hz, CH]), 6.58 (1H, d, J ¼ 3.4 Hz, PrH), 6.34 (1H, t, J ¼ 3.4
Hz, PrH), 3.83 (2H, t, J ¼ 6.6 Hz, OCH2), 3.16–3.11 (2H, m,
NCH2), 3.10–3.05 (2H, m, NCH2), 1.77 (2H, dd, J ¼ 14.5, 7.4 Hz,
CH2), 1.73–1.65 (4H, m, CH2), 1.51 (2H, dd, J ¼ 15.1, 7.4 Hz,
CH2), 1.39 (6H, s, CH3), 1.18 (6H, s, CH3), 0.96 (3H, t, J ¼ 7.4 Hz,
CH3); MS (MALDI): m/z (M+, C32H39N3O3): calcd: 513.073; found:
513.098.
Compound 5c. The procedure for compound 5a was followed
to prepare 5c from 4 and 3c as an orange oil in 86.3% yield (1.06
g, 2.59 mmol). The ratio of the Z : E isomer is 50 : 50% calcu-
lated by the integration of respective protons. 1H NMR (400
MHz, CDCl3) d 7.16 (1H, m, PrH), 7.09 (1H, d, J ¼ 5.0 Hz, PrH),
7.07 (1H, d, J ¼ 5.0 Hz, PrH), 7.04 (0.5H, d, J ¼ 3.4 Hz, CH), 6.98
(1H, s, ArH), 6.91 (1H, d, J ¼ 3.6 Hz, CH), 6.57 (0.5H, d, J ¼ 11.8
Hz, CH), 6.48 (0.5H, d, J ¼ 11.8 Hz, CH2), 3.89 (2H, m, OCH2),
3.19–3.13 (2H, m, NCH2), 3.13–3.05 (2H, m, NCH2), 1.93–1.80
(2H, m, CH2), 1.80–1.71 (4H, m, CH2), 1.58 (3H, m, CH2), 1.44
(3H, s, CH3), 1.43 (3H, s, CH3), 1.31 (3H, s, CH3), 1.17 (3H, s,
CH3), 1.01 (1.5H, t, J ¼ 7.4 Hz, CH3), 0.94 (1.5H, t, J ¼ 7.4 Hz,
4.2 Measurements and instrumentation
1HNMR spectra were recorded using an Advance Bruker 400 M
(400 MHz) NMR spectrometer (tetramethylsilane as internal
reference). The MS spectra were obtained using MALDI-TOF
(Matrix Assisted Laser Desorption/Ionization of Flight) on a
BIFLEXIII (Broker Inc.) spectrometer. The UV-Vis spectra were
recorded on a Cary 5000 photospectrometer. The TGA was
determined using a TA5000-2950TGA (TA co) with a heating rate
of 10 ꢁC minꢀ1 under the protection of nitrogen. Cyclic vol-
tammetric data were measured on a BAS CV-50W voltammetric
analyzer using a conventional three-electrode cell with Pt metal
as the working electrode, a Pt gauze as the counter-electrode,
and Ag/AgNO3 as the reference electrode at a scan rate of 50 mV
sꢀ1. 0.1 M Tetrabutylammonium hexa-uorophosphate (TBAPF)
in acetonitrile is the electrolyte. The uorescence spectra were
recorded on a Hitachi F-4500 uorescence spectrometer. The
elemental analysis was performed on a Flash EA 1112 elemental
analyzer. The melting points were obtained using a TA DSC Q10
ꢀ1
ꢁ
under N2 at a heating rate of 10 C min
.
4.3 Synthesis and characterization
Compound 4. Under a N2 atmosphere, anhydrous potassium CH3); MS (MALDI) (M+, C26H35NOS): calcd: 410.045; found:
carbonate (6.4 g, 40 mmol) was added to a solution of 410.033.
compound 3 (5.46 g, 20 mmol) and compound C4H9Br (4.11 g,
Compound 6a. DMF (0.42 g, 5.75 mmol) was added to freshly
30 mmol) in DMF (120 mL). The mixture was allowed to stir at distilled POCl3 (0.59 g, 3.84 mmol) under an atmosphere of N2
120 ꢁC for 12 h and then poured into water. The organic phase at 0 ꢁC, and the resulting solution was stirred until its complete
was extracted using AcOEt, washed with brine and dried over conversion into a glassy solid. Aer the addition of 5a (1.40 g,
MgSO4. Aer removal of the solvent under reduced pressure, 2.56 mmol) in 1,2-dichloroethane (60 mL) dropwise, the mixture
the crude product was puried by silica chromatography, was stirred at room temperature overnight, then poured into a
eluting with acetone : hexane ¼ 1 : 10 to give compound 4 as a saturation aqueous solution of sodium acetate (300 mL). Aer 2
1
yellow powder with 91.3% yield (6.02 g, 18.26 mmol). H NMR hour stirring at room temperature, the mixture extracted with
(400 MHz, acetone) d 9.90 (1H, s, CHO), 7.48 (1H, s, ArH), 3.96 chloroform (5 ꢂ 30 mL), and the organic fractions were
(2H, t, J ¼ 6.8 Hz, OCH2), 3.36–3.33 (2H, m, NCH2), 3.30–3.26 collected and dried over anhydrous MgSO4. The crude product
(2H, m, NCH2), 1.86 (2H, dd, J ¼ 14.9, 7.0 Hz, CH2), 1.76–1.66 was puried through silica gel chromatography and eluted with
(4H, m, CH2), 1.52 (2H, dt, J ¼ 14.8, 7.5 Hz, CH2), 1.41 (6H, s, acetone : hexane ¼ 1 : 5 to afford a red solid 6a in 73.2% yield
1
CH3), 1.23 (6H, s, CH3), 0.99 (3H, t, J ¼ 7.4 Hz, CH3); MS (EI) (M+, (1.08 g, 1.87 mmol). H NMR (400 MHz, acetone) d 9.38 (1H, s,
ꢁ
C
21H31NO2): calcd: 329.4837; found: 329.2485. m.p.: 223.83 C. CHO), 7.71 (2H, d, J ¼ 8.4 Hz, ArH), 7.33 (2H, d, J ¼ 8.4 Hz, ArH),
Compound 5a. Under a N2 atmosphere, to a solution of 7.26 (1H, d, J ¼ 16.3 Hz, CH), 7.14 (1H, d, J ¼ 4.2 Hz, PrH), 7.05
compound 4 (0.99 g, 3 mmol) and 3a (2.08 g, 3.6 mmol) in ether (1H, s, ArH), 6.73 (1H, d, J ¼ 4.2 Hz, PrH), 6.39 (1H, d, J ¼ 16.3
(100 mL) was added NaH (0.43 g, 18 mmol). The solution was Hz, CH), 3.78 (2H, t, J ¼ 6.7 Hz, OCH2), 3.19–3.14 (2H, m, NCH2),
allowed to stir for 48 h and then poured into water. The organic 3.13–3.07 (2H, m, NCH2), 1.76 (2H, dd, J ¼ 14.3, 7.2 Hz, CH2),
phase was extracted using AcOEt, washed with brine and dried 1.72–1.64 (4H, m, CH2), 1.52 (2H, dd, J ¼ 14.3, 7.2 Hz, CH2), 1.38
over MgSO4. Aer removal of the solvent under reduced pres- (6H, s, CH3), 1.15 (6H, s, CH3), 0.99 (3H, t, J ¼ 7.3 Hz, CH3); 13
C
sure, the crude product was puried by silica chromatography, NMR (101 MHz, acetone) d 176.77, 156.37, 143.58, 142.74,
and eluted with acetone : hexane ¼ 1 : 20 to give compound 5a 136.97, 133.39, 132.16, 130.55, 129.92, 126.37, 123.36, 122.04,
7792 | J. Mater. Chem. C, 2014, 2, 7785–7795
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Journal of Materials Chemistry C
117.58, 110.78, 106.66, 74.45, 47.09, 46.29, 40.37, 36.64, 32.60, 111.26, 109.93, 109.08, 107.86, 97.03, 92.50, 74.17, 53.50, 52.28,
32.18, 32.02, 30.70, 29.95, 19.43, 13.77; MS (MALDI) (M+, 46.44, 45.91, 39.63, 35.84, 31.96, 31.47, 29.91, 25.05, 18.57,
C
33H39BrN2O2): calcd: 575.867; found: 575.957.
Compound 6b. The procedure for compound 6a was fol- 756.686. m.p.: 189.48 C. Anal. calcd (%) for C44H46BrN5O2: C,
12.98; MS (MALDI) (M+, C44H46BrN5O2): calcd: 756.776; found:
ꢁ
lowed to prepare 6b from 5b as a red solid in 71.8% yield (0.99 g, 69.83; H, 6.13; N, 9.25; found: C, 70.03; H, 6.17; N, 9.22.
1
1.84 mmol). H NMR (400 MHz, acetone) d 9.44 (1H, s, CHO),
Chromophore B. The procedure for chromophore A was
8.43 (2H, d, J ¼ 8.9 Hz, ArH), 7.69 (2H, d, J ¼ 8.9 Hz, ArH), 7.32 followed to prepare chromophore B from 6b as a green solid in
(1H, d, J ¼ 16.2 Hz, CH), 7.24 (1H, d, J ¼ 4.2 Hz, PyH), 7.07 (1H, 33.9% yield (0.12 g, 0.17 mmol). The ratio of the Z : E isomer
s, ArH), 6.81 (1H, d, J ¼ 4.2 Hz, PyH), 6.41 (1H, d, J ¼ 16.2 Hz, was 50 : 50% calculated by the integration of respective protons.
CH), 3.79 (2H, t, J ¼ 6.7 Hz, NCH2), 3.79 (2H, t, OCH2), 3.18(2H, 1H NMR (400 MHz, acetone) d 8.57 (1H, d, J ¼ 8.9 Hz, ArH), 8.44
t, NCH2),3.13(2H, t, NCH2), 1.79–1.73 (2H, m, CH2), 1.72–1.63 (1H, d, J ¼ 6.9 Hz, ArH), 8.22 (0.5H, d, J ¼ 15.7 Hz, CH), 7.85 (2H,
(4H, m, CH2), 1.52 (2H, m, 7.6 Hz, CH2), 1.38 (6H, s, CH3), 1.14 d, J ¼ 8.9 Hz, ArH), 7.69 (0.5H, d, J ¼ 4.6 Hz, CH), 7.60 (0.5H, d, J
(6H, s, CH3), 0.99 (3H, t, CH3); 13C NMR (101 MHz, CDCl3) d ¼ 15.4 Hz, CH), 7.47 (0.5H, d, J ¼ 16.1 Hz, CH), 7.38 (0.5H, s,
176.77, 156.33, 147.21, 143.66, 143.09, 132.30, 131.18, 129.17, PrH), 7.33 (0.5H, d, J ¼ 3.4 Hz, CH), 7.14 (0.5H, s, PrH), 7.12
124.01, 122.53, 109.34, 107.36, 74.77, 47.20, 46.68, 39.87, 36.10, (0.5H, d, J ¼ 4.7 Hz, CH), 7.11 (0.5H, d, J ¼ 3.4 Hz, CH), 7.01
32.51, 32.07, 31.95, 30.77, 29.93, 19.28, 13.90; MS (MALDI) (0.5H, d, J ¼ 15.7 Hz, CH), 6.95 (1H, s, ArH), 6.63 (0.5H, d, J ¼
(M+, C33H39N3O4): calcd: 541.136; found: 541.138.
15.4 Hz, CH), 6.55 (0.5H, d, J ¼ 16.1 Hz, CH), 3.78 (2H, dd,
Compound 6c. Under a N2 atmosphere, 5c (0.41 g, 1 mmol) J ¼ 11.9, 6.6 Hz, OCH2), 3.27–3.22 (2H, m, NCH2), 3.18 (2H, dd, J
was dissolved in 150 mL of freshly distilled THF and cooled to ¼ 5.9, 4.5 Hz, NCH2), 1.81 (3H, s, CH3), 1.77–1.64 (6H, m, CH2),
ꢀ78 ꢁC. Approximately 2 equivalents of BuLi in hexane (20 mL, 5 1.64 (3H, s, CH3), 1.51 (2H, m, CH2), 1.38 (6H, s, CH3), 1.22 (3H,
mmol) was added dropwise over 20 min. Reaction continued at s, CH3), 1.14 (3H, s, CH3), 0.98 (1.5H, t, J ¼ 7.4 Hz, CH3), 0.87
ꢀ78 ꢁC for 1 h at which time DMF (0.37 g, 5 mol) was added over (1.5H, t, J ¼ 7.4 Hz, CH3); 13C NMR (101 MHz, acetone) d 176.35,
1 min. The reaction was allowed to reach RT while the solution 156.68, 146.30, 143.99, 141.73, 140.94, 134.07, 133.36, 132.59,
was stirred for 1 h. The organic phase was extracted by AcOEt, 132.16, 129.84, 125.91, 124.83, 124.54, 123.18, 122.70, 121.77,
washed with brine and dried over MgSO4. Aer removal of the 121.59, 121.57, 116.46, 111.42, 109.07, 106.94, 97.24, 96.35,
solvent under reduced pressure, the crude product was puried 74.31, 64.49, 46.46, 45.90, 39.43, 35.69, 31.94, 31.54, 31.35,
by silica chromatography and eluted with acetone : hexane ¼ 29.87, 29.61, 24.85, 18.73, 13.00; MS (MALDI) (M+, C44H46N6O4):
ꢁ
1 : 5 to give compound 6c as an orange oil in 75.6% yield (0.33 g, calcd: 722.878 found: 722.902. m.p.: 195.98 C. Anal. calcd (%)
0.76 mmol). 1H NMR (400 MHz, CDCl3) d 9.82 (1H, s, CHO), 7.64 for C44H46N6O4: C, 73.11; H, 6.41; N, 11.63; found: C, 73.20; H,
(1H, d, J ¼ 3.9 Hz, CH), 7.33 (1H, d, J ¼ 16.1 Hz, CH), 7.27 (1H, d, 6.43; N, 11.67.
J ¼ 5.9 Hz, CH), 7.05 (1H, d, J ¼ 5.9 Hz, CH), 6.96 (1H, d, J ¼ 16.1
Chromophore C. The procedure for chromophore A was
Hz, CH), 3.84 (2H, t, J ¼ 6.7 Hz, OCH2), 3.23–3.19 (2H, m, NCH2), followed to prepare chromophore C from 6c as a green solid in
1
3.16–3.12 (2H, m, CH2), 1.91–1.83 (2H, m, CH2), 1.77–1.72 (4H, 68.9% yield (0.21 g, 0.34 mmol). H NMR (400 MHz, CDCl3) d
m, CH2), 1.60–1.53 (2H, m, CH2), 1.43 (6H, s, CH3), 1.31 (6H, s, 7.73 (1H, d, J ¼ 15.5 Hz, CH), 7.31 (1H, d, J ¼ 4.0 Hz, CH), 7.26
CH3), 1.01 (3H, t, J ¼ 7.3 Hz, CH3); 13C NMR (101 MHz, CDCl3) d (1H, d, J ¼ 15.9 Hz, CH), 7.19 (1H, s, ArH), 6.96 (1H, d, J ¼ 4.0
181.23, 155.85, 153.99, 138.83, 136.30, 121.99, 123.33, 121.32, Hz, CH), 6.92 (1H, d, J ¼ 15.9 Hz, CH), 6.44 (1H, d, J ¼ 15.5 Hz,
114.32, 66.95, 46.41, 45.74, 39.11, 35.43, 31.70, 31.27, 30.14, CH), 3.78 (2H, t, J ¼ 6.7 Hz, OCH2), 3.22–3.15 (2H, m, NCH2),
29.88, 29.02 28.68, 24.50, 21.53, 18.54, 13.10; MS (MALDI) 3.14–3.07 (2H, m, NCH2), 1.67 (6H, s, CH3), 1.57–1.46 (8H, m,
(M+, C27H35NO2S): calcd: 437.347; found: 437.181.
CH2), 1.24 (6H, s, CH3), 1.19 (6H, s, CH3), 0.96 (3H, t, J ¼ 7.4 Hz,
Chromophore A. To a solution of 6a (0.29 g, 0.50 mmol) and CH3); 13C NMR (101 MHz, CDCl3) d 174.88, 171.68, 154.84,
the TCF acceptor (0.12 g, 0.60 mmol) in MeOH (60 mL) were 154.83, 154.68, 138.30, 136.77, 136.10, 130.76, 126.29, 126.27,
added several drops of triethylamine. The reaction was allowed 125.61, 122.13, 114.58, 110.25, 110.16, 110.11, 95.80, 94.39,
to stir at 78 ꢁC for 5 h. The reaction mixture was cooled and 74.74, 54.46, 46.61, 46.09, 38.73, 35.00, 31.72, 31.26, 29.90,
green crystal precipitation was facilitated. Aer removal of the 28.34, 25.53, 18.59, 13.09; MS (MALDI) (M+, C38H42N4O2S):
ꢁ
solvent under reduced pressure, the crude product was puried calcd: 618.186; found: 618.204. m.p.: 197.56 C Anal. calcd (%)
by silica chromatography and eluted with AcOEt : hexane ¼ for C38H42N4O2S: C, 73.75; H, 6.84; N, 9.05; found: C, 73.83; H,
1 : 5 to give chromophore A as a green solid in 32.3% yield (0.12 6.90; N, 9.02.
1
g, 0.16 mmol). H NMR (400 MHz, acetone) d 8.11 (1H, d, J ¼
15.6 Hz, CH), 7.62 (2H, d, J ¼ 9.5 Hz, ArH), 7.36 (2H, d, J ¼ 9.5
Acknowledgements
Hz, ArH), 7.22 (1H, s, ArH), 7.05 (1H, d, J ¼ 3.3 Hz, PyH), 6.90
(1H, d, J ¼ 3.3 Hz, PyH), 6.83 (1H, d, J ¼ 15.6 Hz, CH), 6.76 (2H, We are grateful to the National Natural Science Foundation of
s, CH), 3.62 (2H, t, J ¼ 6.6 Hz, OCH2), 3.15–3.09 (2H, m, NCH2), China (no. 11104284 and no. 61101054) for the nancial support.
3.08–3.02 (2H, m, NCH2), 1.67 (6H, s, CH3), 1.58 (6H, m, CH2),
1.53–1.44 (2H, m, CH2), 1.26 (6H, s, CH3), 1.10 (6H, s, CH3), 0.76
Notes and references
(3H, t, J ¼ 8.5 Hz, CH3); 13C NMR (101 MHz, acetone) d 175.06,
156.38, 143.69, 141.90, 140.50, 137.93, 133.55, 132.25, 126.65,
126.65, 122.63, 121.75, 121.39, 120.81, 116.84, 112.30, 111.55,
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Paper
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