The synthesis of new double-donor chromophores with excellent electro-optic activity by introducing modified bridges

Article abstract of DOI:10.1039/c4cp05829a

A series of chromophores y1-y3 based on the same bis(N,N-diethyl)aniline donor and the tricyanofuran acceptor (TCF) linked together via the modified thiophene π-conjugation with different isolated groups have been synthesized and systematically investigated in this paper. Density functional theory (DFT) was used to calculate the HOMO-LUMO energy gaps and first-order hyperpolarizability (β) of these chromophores. Besides, to determine the redox properties of these chromophores, cyclic voltammetry (CV) experiments were performed. After introducing the isolation group into the thiophene, reduced energy gaps of 1.03 and 1.02 eV were obtained for chromophores y2 and y3, respectively, much lower compared to chromophore y1 ( Δ E = 1.13 eV). These chromophores showed better thermal stability with their decomposition temperatures all above 220 °C. Besides, compared with results obtained from the chromophore (y1) without the isolated group, these new chromophores show better intramolecular charge-transfer (ICT) absorption. Most importantly, the high molecular hyperpolarizability (b) of these chromophores can be effectively translated into large electro-optic (EO) coefficients (r33) in poled polymers. The electro-optic coefficient of poled films containing 25% wt of these new chromophores doped in amorphous polycarbonate (APC) afforded values of 149, 139 and 125 pm V-1 at 1310 nm for chromophores y1-y3, respectively. Besides, when the concentration was increased, the film containing chromophores y1 and y3 showed obvious phase separation, while the film with chromophore y2 showed the maximum r₃₃ value of 146 pm V^-1. Moreover, the electro-optic film prepared with these new chromophores showed greater stability. High r₃₃ values indicated that the double donors of the bis(N,N-diethyl)aniline unit can efficiently improve the electron-donating ability and the isolated groups on the thiophene bridge can reduce intermolecular electrostatic interactions, thus enhancing the macroscopic EO activity. These properties, together with the good solubility, suggest the potential use of these new chromophores as advanced material devices.

Full text of DOI:10.1039/c4cp05829a

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Journal Name
RSCPublishing
ARTICLE
Synthesis of New Double-donor Chromophores with
Excellent Electro-Optic Activity by Introducing
Modified Bridges
Cite this: DOI: 10.1039/x0xx00000x
Yuhui Yang ab, Haoran Wang ab, Fenggang Liu ab, Dan Yang ab, Shuhui Bo a, Ling Qiu a, Zhen Zhen a,
* a
Xinhou Liu
Received 00th December 2014,
Accepted 00th December 2014
DOI: 10.1039/x0xx00000x
A series of chromophores y1ꢀy3 based on the same bis(N,Nꢀdiethyl)aniline donor and tricyanofuran
acceptor (TCF) linked together via the modified thiophene πꢀconjugation with different isolated groups
have been synthesized and systematically investigated in this paper. Density functional theory (DFT) was
used to calculate the HOMO–LUMO energy gaps and firstꢀorder hyperpolarizability (β) of these
chromophores. Besides, to determine the redox properties of these chromophores, cyclic voltammetry
(CV) experiments were performed. After introducing the isolation group into the thiophene, reduced
energy gap of 1.03 and 1.02 eV were obtained for chromophore y2 and y3 respectively, much lower
compared than chromophore y1 (ꢁE = 1.13 eV).These chromophores showed better thermal stability with
www.rsc.org/
their decomposition temperatures all above 220
℃. Besides, compared with results obtained from the
chromophore (y1) without the isolated group, these new chromophores show better intramolecular chargeꢀ
transfer (ICT) absorption. Most importantly, the high molecular hyperpolarizability (β) of these
chromophores can be effectively translated into large electroꢀoptic (EO) coefficients (r33) in poled
polymers. The electroꢀoptic coefficient of poled films containing 25% wt of these new chromophores
doped in amorphous polycarbonate (APC) afforded values of 149, 139 and 125 pm/V at 1310 nm for
chromophores y1ꢀy3 respectively. Besides, when the concentration was increased, film containing with
chromophore y1 and y3 showed obvious phase separation, while film with chromophore y2 showed the
maximum r33 values of 146 pm/V. Moreover, the electroꢀoptic film prepared with these new chromophores
gave greater stability. High r33 values indicated that the double donors of the bis(N,Nꢀdiethyl)aniline unit
can efficiently improve the electronꢀdonating ability and the isolated groups on the thiophene bridge can
reduce intermolecular electrostatic interactions, thus enhancing the macroscopic EO activity. These
properties, together with the good solubility, suggest the potential use of these new chromophores as
advanced material devices.
electric field.4, 8, 9 Poled polymers are the most widely studied
organic NLO materials. The macroscopic NLO response of
1. Introduction
Integrated optical devices based on organic nonlinear optical
(NLO) polymers have emerged as one of the enabling elements
for a new generation of optical telecommunications, which
needs fast modulation and switching of optical signals at the
speed of 100 Gbit/s or above to accommodate anticipated
growth in data traffic.1ꢀ5 So the NLO materials have drawn
considerable attention over the past two decades and have
stimulated a research boom for materials with large EO
activities, both at molecular level (β) and as processed materials
(r33).2, 5ꢀ7 An organic NLO material is generally composed of
push–pull organic chromophores, in which a πꢀconjugated
bridge is endꢀ capped by a donor and an acceptor. In such
molecules, the donor and acceptor substituents provide the
requisite groundꢀstate charge asymmetry, whereas the πꢀ
conjugation bridge provides a pathway for the ultrafast
redistribution of electric charges under an applied external
such materials arises from the polingꢀinduced polar order of
noncentrosymmetric NLO chromophores in polymers.7
Unfortunately, in most of the poled polymers, the chromophore
moieties which have a rodꢀlike structure and the strong dipole–
dipole interactions between chromophores have led to
unfavorable antiparallel packing of the chromophores, making
the conversion from the high β values of the chromophores to
large macroscopic optical nonlinearities (r33) a continual
challenge.10ꢀ12 So the most effective and facile way to improve
the r33 values of the guestꢀhost EO materials is the optimization
of the pushꢀpull chromophores.
To achieve higher EO activity and suppress the dipole
interaction among chromophores, rational molecular designs of
dipole chromophores have been made from the conjugated
push–pull molecules. Many efforts have been carried out to
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design and synthesize novel NLO chromohores, seeking to
engineer NLO molecules both microscopically (β) and
macroscopically (r33).13ꢀ17 On one hand, structureꢀproperty
relationships that have been established indicate that large β
values of the chromophores can be achieved by careful
modification of the strength of donor and acceptor moieties, as
well as the nature of the πꢀconjugated spacer.18, 19 On the other
hand, as reported, the introduction of some isolation groups
(IG) to the chromophore moieties to further control the shape of
the chromophore should be an efficient approach to minimize
interactions between chromophores and thus enhance the poling
efficiency.20ꢀ25
2. Results and discussion.
2.1 synthesis and characterization of chromophore
s
The synthesis of chromophores y1, y2 and y3 are depicted in
Schemes 1. These NLO chromophores were designed to have
the same strong electron acceptor (TCF) and the same electron
donor bis(N,Nꢀdiethyl)aniline group, but have different
isolation groups on the thiophene πꢀconjugation. Chromophores
y1, y2 and y3 were prepared starting from the phosphonate
which was prepared using a procedure described in the
28
literature.26,
Reacting phosphonate with different aldehyde
(2aꢀ2c) using sodium hydride as base under a HornerꢀEmmons
reaction condition afforded ( )ꢀalkene 3aꢀ3c exclusively.
Lithiation the ( )ꢀalkene 3 with ꢀbutyl lithium followed by the
addition of DMF yielded the corresponding aldehyde 4aꢀ4c
E
n
Based on the famous principle of siteꢀisolation principle, as
well as our previous work on the “doubleꢀdonor
chromophore”,5, 26, 27 we designed and synthesized three new
double donor chromophores y1, y2 and y3. In our previous
E
.
The target chromophores y1 (with no substituent), y2 (with two
hexyloxy group) and y3 (with cycloethoxy group) on the bridge
thiophene ring were successfully obtained by condensing
aldehydes 4aꢀ4c with acceptor 5 (TCF) in ethanol in the
presence of a catalytic amount of piperidine.
work, we have found that the double donors with two N, N
diethylaniline can improve the electronꢀdonating ability thus
enhance the optical nonlinearity. The one of the N, N
ꢀ
ꢀ
diethylaniline unit can act as both the additional donor and the
isolation group (IG). On the other hand, The double donors
chromophores having the double benzene rings with an
appropriate angle make the special structure which is different
from the general NLO chromophores showing the rodꢀlike
structure and can decrease the intermolecular electrostatic
interactions.Thus it can enhance the macroscopic EO activity.
In this paper, a new synthetic methodology of attaching two
different substituents onto the πꢀconjugation bridge of NLO
chromophore was developed to obtain two new NLO
chromophores with adjustable shape, size and functionality on
the sideꢀchain. The motivation is to improve the EO activities
of guestꢀhost EO materials by a simple bridgeꢀmodifying
method for NLO chromophores. Bridge modification was
carried out to facilitate the chromophores with different steric
hindrance and free mobility, which was expected to achieve the
large macroscopic EO coefficients by optimizing the poling
condition in guestꢀhost EO polymers. In this paper, we
designed and synthesized three double donor chromophores y1,
y2 and y3 with different bridges. (Chart1). These chromophores
showed great solubility in common organic solvents, better
thermal stability (with their onset decomposition temperatures
Scheme 1 Synthetic scheme for chromophore
y1ꢀy3
2.2 Thermal analysis
NLO chromophores must be thermally stable enough to
withstand encountered high temperatures (>200
field poling and subsequent processing
chromophore/polymer materials. Thermal properties of these
chromophores were measured by Thermogravimetric Analysis
(TGA) with a heating rate of 10 °C·min−1 under nitrogen. Fig. 1
shows the thermogravimetric analysis of chromophores y1, y2
and y3. All of the chromophores exhibited good thermalꢀ
stabilities and their decomposition temperatures (Td) were
all above 220
℃), good compatibility with polymers, and large
EO activity in the poled films. 1HꢀNMR and 13CꢀNMR analysis
were carried out to demonstrate the preparation of these
chromophores. Thermal stability, photophysical properties and
DFT calculations of these chromophores were systematically
studied. Furthermore, the macroscopic EO activity (r33) of these
chromophores in amorphous polycarbonate were also analyzed.
℃
) in electric
of
above 220
℃
as summarized in Table 1. In particular, the
),
highest Td was observed for the chromophore y2 (259
℃
which indicated that the introduction of two hexyloxy groups
on the thiophene ring is thermally robust. The excellent thermal
stability of these chromophores makes them suitable for
practical device fabrication and EO device preparation.
Chart1 The structures of chromophore y1ꢀy3
2 | J. Name., 2014, 00, 1-3
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Traditional chromophores were easy to aggregate, which
were beneficial for selfꢀassembly. But it was unfavorable for
NLO application, because the chromophores were easy to form
the antiparallel aggregation. This phenomenon can be
demonstrated by absorption intensity, wavelength and shape of
absorption band in UVꢀvis spectra. To study the effect of these
isolation groups on preventing chromophore aggregation, the
absorption spectrum of these chromophores were measured in
CHCl3 at different concentration (from 0.005 mM to 0.07 mM).
The Fig. 3 showed, the λmax ofthe two chromophores at
different concentration. As the concentration rose, no blueꢀshift
or redꢀshift was found. Even at high concentration, no obvious
characteristic absorption band of aggregation was observed. So
we can confirm that there was no strong interaction among the
chromophore molecules in solvents.
Figure 1 TGA curves of chromophores y1–y3 with a
heating rate of 10 ℃ min_1 in a nitrogen atmosphere.
2.3 Optical properties
In order to reveal the effect of different isolated groups on the
πꢀconjugation bridge on the intramolecular charge transfer (ICT)
of dipolar chromophores, UVꢀVis absorption spectra of three
chromophores (c = 1
×
10ꢀ5 mol Lꢀ1) were measured in a series
of aprotic solvents with different polarity so that the
solvatochromic behavior of the three chromophores could be
investigated to explore the polarizability of chromophores in a
wide range of dielectric environments (Fig. 2). The spectrum
data are summarized in Table 1. The synthesized chromophores
exhibited a similar πꢀπ* intramolecular chargeꢀtransfer (ICT)
absorption band in the visible region. As shown in Fig. 2, in
both nonꢀpolar and polar solvents such as 1,4ꢀdioxane and
Figure 3 UVꢀVis absorption spectra of different concentrations of
chromophores y2 and y3 in CHCl3
CHCl3, these chromophores exhibited
a
similar πꢀπ*
2.4 Theoretical calculations
In order to model the ground state molecular geometries, the
HOMO–LUMO energy gaps and
chromophores were calculated. The DFT calculations were
carried out at the hybrid B3LYP level by employing the split
valence 6ꢀ311 g** (d, p) basis set.
DFT calculations are summarized in Table 2.
The frontier molecular orbitals are often used to characterize
the chemical reactivity and kinetic stability of a molecule and to
intramolecular chargeꢀtransfer (ICT) absorption band with a
continuous redꢀshift of the maximum absorption (λmax) from y1
to y3. The peak wavelength of chromophores y3 showed a
bathochromic shift of 66 nm from dioxane to chloroform,
displaying a little larger solvatochromism compared with the
chromophore y2 (61 nm in Table 1). The resulting spectrum
data indicated that the chromophores y3 were more easily
polarizable than chromophore y2.
β
values of these
29ꢀ31
The data obtained from
Table1
．Summary of Thermal and Optical Properties and EO Coefficients
obtain qualitative information about the optical and electrical
32, 33
properties of molecules.31,
Besides, the HOMOꢀLUMO
of Chromophores y1ꢀy3
a
Td
λb
λc
ꢁλd
re33
energy gap is also used to understand the charge transfer
interaction occurring in a chromophore molecule.
max
max
33ꢀ35
In the
Chronmophore
(
℃
)
(nm)
725
740
753
(nm)
654
679
687
(nm) (pm/V)
case of these chromophores, Fig.4 represents the frontier
molecular orbitals of chromophores y1ꢀy3. According to Fig. 4,
it is clear that the electronic distribution of the HOMO is
delocalized over the thiophene linkage and benzene ring,
whereas the LUMO is mainly constituted by the acceptor
moieties.33
y1
y2
y3
249
259
221
71
61
66
149
139
125
a Td was determined by an onset point, and measured by TGA under nitrogen at
a heating rate of 10 ℃/min.
b λmax was measured in CHCl3.
cλmax was measured in dioxane.
dꢁλ=λbmaxꢀ λcmax
Table2．Data from DFT calculations
Chromophores
EHOMO
eV
/
ELUMO
eV
/
ΔEa/
ΔEb/
eV
βmaxc/
10ꢀ30esu
713
eV
e r33 values were measured at the wavelength of 1310nm.
y1
ꢀ5.04
ꢀ5.08
ꢀ5.05
ꢀ2.99
ꢀ3.09
ꢀ3.07
2.05
1.99
1.98
1.12
1.03
1.02
y2
652
y3
636
ΔE = ELUMO ꢀ EHOMO
,
aResults was calculated by DFT
bResults was from cyclic voltammetry experiment
cβ values were calculated using gussian03at B3LYP/6ꢀ311 g** (d, p) level and the direction of the maximum value is d
Figure 2 UVꢀVis absorption spectra in different solvents of
chromophore y2 and y3 (c = 1×10_5 mol L_1)
transfer axis of the chromophores.
The HOMO and LUMO energy were calculated by DFT
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calculations as shown in Table 2. The energy gap between the corresponding LUMO level of y1ꢀy3 only showed slight
HOMO and LUMO energy for chromophores y1ꢀy3 were 2.05 changes at ꢀ3.82, ꢀ3.85 and ꢀ3.89 eV, respectively. This result
eV, 1.99 eV and 1.98 eV respectively. The HOMOꢀLUMO gap corresponded with the conclusion of UVꢀVis spectra analysis
was the lowest for chromophore y3 (1.98 eV) and highest for
chromophore y1 (2.05 eV). As reported,33 the optical gap is
lower and the chargeꢀtransfer (ICT) ability is greater, thus
improving the nonlinearity. As a result, chromophore y2 and y3
showed the lower optical gap than chromophore y1, so it may
indicate that chromophore y2 and y3 should exhibit better ICT
and NLO properties than chromophore y1. This result
corresponds with the Uvꢀvis results.
Further, the theoretical microscopic Zeroꢀfrequency (static)
molecular first hyperpolarizability (β) was calculated by
Gaussian 03. As the reference reported earlier, the β has been
calculated at the 6ꢀ311 g** (d, p) level in vacuum. 36 From this,
the scalar quantity of β can be computed from the x, y, and z
components according to the following equation:
Figure 5 Cyclic voltammograms of chromophore y1ꢀy3.
1/2
β
= (βx2
+
βy2 βz2
+ )
(1)
and the DFT results.
2.6 Electric field poling and EO property measurements
For studying EO property derived from these chromophores, a
series of guestꢀhost polymers were generated by formulating
the chromophores into amorphous polycarbonate (APC) using
dibromomethane as solvent. The resulting solutions were
filtered through a 0.2ꢂm PTFE filter and spinꢀcoated onto
indium tin oxide (ITO) glass substrates. Films of doped
1
βi
=
βiii
+
(βijj + β jij + β jji ),
∑
3
i, j ∈ (x, y, z)
i≠ j
Where
The data obtained from DFT calculations were summarized in
Table 2. There was no obvious change of the β values of these
chromophores y1ꢀy3 (636 ꢀ 713×10ꢀ30 esu), which illustrated
that the substituent group on the thiophene bridge had little
influence on their main conjugated structures under the
simulated condition. This result is corresponded with the
conclusion of UVꢀVis spectra analysis.
polymers were baked in a vacuum oven at 80
ensure the removal of the residual solvent. The corona poling
process was carried out at a temperature of 10 above the
℃ overnight to
℃
glass transition temperature (Tg) of the polymer. The r33 values
were measured using the TengꢀMan simple reflection technique
at the wavelength of 1310 nm using a carefully selected thin
ITO electrode with low reflectivity and good transparency in
order to minimize the contribution from the multiple reflections.
37
The r33 values of films containing chromophores y1 (fillmꢀA),
y2 (filmꢀB), y3 (filmꢀC) were measured in different loading
densities, as shown in table 3 and Fig.6, For chromophore y1,
the r33 values were gradually improved from 32 pm Vꢀ1 (10
wt%) to 149 pm Vꢀ1 (25 wt%), as the concentration of
chromophores increased, the similar trend of enhancement was
also observed for chromophore y2, whose r33 values increased
from 37 pm Vꢀ1 (10 wt%) to 139 pm Vꢀ1 (25 wt%).
Chromophoe y3 gained the r33 value from 41 pm Vꢀ1 (10 wt%)
to 125 pm Vꢀ1 (25 wt%).
Figure 4 The frontier molecular orbitals of chromophores y1ꢀy3
2.5 Electrochemical properties
To determine the redox properties of these chromophores,
cyclic voltammetry (CV) experiments were performed on a
CHI660D electrochemical workstation by a cyclic voltammetry
(CV) technique in CH3CN solution, using Pt disk electrode and
a platinum wire as the working and counter electrodes,
respectively, and a saturated Ag/AgCl electrode as the
Table 3 Summary of EO Coefficients of Chromophores y1ꢀy3 at different
densities
ar33 (pm/V)
reference electrode in the presence of 0.1
M
nꢀ
Wt%
10
y1
32
75
120
149
ꢀꢀ
y2
y3
41
79
115
125
ꢀꢀ
tetrabutylammoniumperchlorate as the supporting electrolyte.
As shown in Fig. 5, chromophores y1, y2 and y3 all exhibited
one quasi reversible oxidative wave with a halfꢀwave potential,
E1/2= 0.5(Eox + Ered), at about 0.55, 0.48 and 0.51 V
respectively. Meanwhile, these chromophores had an
irreversible reduction wave corresponding to the acceptor
moieties at Ered =ꢀ0.58, ꢀ0.55 and ꢀ0.51V (vs. Ag/AgCl). It
showed an energy gap (ꢁE) value of 1.13, 1.03 and 1.02 eV for
chromophores y1, y2 and y3 respectively. The HOMO and
LUMO levels of these new chromophores were calculated from
their corresponding oxidation and reduction potentials. The
HOMO levels of y1, y2 and y3 were estimated to be ꢀ4.95, ꢀ
4.88 and ꢀ4.91eV respectively. In the meantime, the
37
15
74
20
117
139
146
141
25
30
35
ꢀꢀ
ꢀꢀ
a r33 values were measured at the wavelength of 1310nm.
As reported earlier,20 the introduction of some isolation
groups into the chromophore moieties to further control the
shape of the chromophore could be an efficient approach to
minimize interactions between the chromophores. So by
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introducing the isolation groups into the πꢀbridge, it controls the when the loading increased to 35%, filmꢀB showed a downward
shape of the chromophore and decreased the interaction trend. The enhancement of the EO activity further demonstrated
between the chromophore thus can increase the r33 value. But that the isolated group on thiophene ring effectively suppressed
EO coefficient (r33), defining the efficiency of translating the dipoleꢀdipole interaction and assisted the orientation of
molecular microscopic hyperpolarizability into macroscopic chromophores during poling process. Meanwhile, as reported,
EO activities, was described as follows:
the electrooptical features of organic materials principal role is
important and begins to play the relation between the intraꢀ and
intermolecular dipoleꢀdipole interaction and interaction with the
phonon subsystem.38, 39
r33 =2N f(ω) β<cos3θ>/n4
where r33 is the EO coefficient of the poled polymer, N
represents the aligned chromophore number density and f(ω)
denotes the LorentzꢀOnsager local field factors. The term
<cos3θ> is the orientationally averaged acentric order parameter
characterizing the degree of noncentrosymmetric alignment of
the chromophore in the material and n represents the refractive
index.5 Before poling, there is no EO activity in the EO
material and the chromophores in material are thermal
randomization. Electrical field induced poling was proceeded to
induce the acentric ordering of chromophores. Realization of
large electroꢀoptic activity for dipolar organic chromophoreꢀ
containing materials requires the simultaneous optimization of
chromophore first hyperpolarizability (β), acentric order <
cos3θ>, and number density (N). But chromophores with large
ꢂβ generate intermolecular static electric field dipoleꢀdipole
interaction, which leads to the unfavorable antiparallel packing
of chromophores, so that the number of truly oriented
chromophore (N) is small. As a result, the isolation groups
should be appropriate.
Table4 Representative
Chromophores
r33(pm/V)a
Nb
r33 Values of the Chromophores.
y1
y2
y3
149
2.455
6.07
139
1.849
7.52
125
2.239
5.59
r33/Nc
a Experimental value from simple reflection at 1310 nm.
b Chromophore number density; in units of ×1020 molecules/cc.
c
r
33 normalized by chromophore number density; in units of ×10ꢀ19 pm cc/(V
molecules).
The stability of the poled EO films was also investigated.
After annealing at 85°C for 100 h, the poled film of APC/25%
chromophore y1, y2 and y3 poled APC/25% y1, y2 and y3 film
could retain 78%, 81% and 76% (100h) of the initial values at
85
enough.
Moreover, the order parameter changes of the poled EO
℃ respectively. This percentage was considerably higher
So it is not hard to explain the above r33 results. When the
concentration of chromophore in APC is low, the
intermolecular dipolar interactions are relatively weak and thus
the influence of different isolated groups on r33 value is small.
As the chromophore loading increased, the effect of interꢀ
films were studied through measuring the UVꢀVis absorption
spectra of EO films before and after poling. After the corona
poling, the chromophores in the polymer were aligned, and the
chromophore dipole–dipole is becoming stronger.b Compared absorption intensity decreased due to birefringence. The order
to chromophore y2, chromophore y3 with a better coplanar
geometry presented a stronger negative effect of interꢀ
parameter (Φ) can be described according to the following
equation: Φ = 1−A1/A0, where A0 and A1 are the absorbance of
the unꢀpoled and poled EO polymer films at normal incidence.
The Φ values of poled APC/25%y1 film ,APC/25%y2 film and
APC/25% y3 film were about 22.3% 20.6% and 15.1%
respectively (Fig. 7), which showed that the Φ values had
reached the average values and that the poling process was
chromophore dipole–dipole. So the film
C containing
chromophore y3 displayed a smaller r33 value than film B
containing chromophore y2. Chromophore y2 with the two
large isolated groups had the most effective steric hindrance. As
shown in table 4, although the resulting r33 value of
chromophore y2 was 139 pm/V, which was a little smaller than
chromophore y1. When effectively normalizes the r33 value for
the relative chromophore content, dividing the observed r33 by
typically efficient. The
chromophores' orientation
Φ
values are related to the
，
and more effective chromophore
N
yields a value of 7.52
×
10ꢀ19 pm cc/(V molecules), which is
10ꢀ19 pm cc/(V
orientation would provide larger Φ values . The difference in
the order parameter indicates that Films A/APC and B/APC
have weaker interꢀchromophore electrostatic interactions than
FillmꢀC/APC in high density. The obtained higher Φ values
indicated that y1 and y2 could effectively orientate in the poled
films. While, chromophore y3 with a better coplanar geometry
presented a stronger negative effect of interꢀchromophore
dipole–dipole and showed the lowest Φ values. The high
saturated loading density and larger r33 values of Films 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. These results have a good
larger than chromophore y1 (6.07
×
molecules)).Besides, chromophre y3 presented the smallest r33
values. Hence, film B containing chromophoe y2 showed much
higher efficiency of translating molecular microscopic
hyperpolarizability into macroscopic EO activities. Besides, to
our regret, as the loading density increasing from 25 wt % to 30
wt %, the filmꢀA with the loading 30 wt % of y1 and filmꢀC
with the chromophore y3 presents an obvious phase separation.
However, the filmꢀB displayed a larger r33 value of 146 pm/V.
agreement with the r33 values results.
These large r33 values and high stability together with our
previous work of double donors chromophores can effectively
prove that the double donor structure can really decrease the
This journal is © The Royal Society of Chemistry 2014
J. Name., 2014, 00, 1-3 | 5
Figure 6 EO coefficients of NLO thin films as a function of
chromophore loading densities.

Physical Chemistry Chemical Physics
Page 6 of 9
View Article Online
ARTICLE
Journal Name
DOI: 10.1039/C4CP05829A
interactions between the chromophores thus can increase the r33 POCl3 and THF were freshly distilled prior to its use. The 2ꢀ
value.
dicyanomethyleneꢀ3ꢀcyanoꢀ4ꢀmethylꢀ2,5ꢀdihydrofuran (TCF)
acceptor was prepared according to the literature.40
3.2. Synthesis
3.2.1 Synthesis of Chromophore y1
Chromophore y1 was synthesized according to the literature.26
3.2.2 Synthesis of compound 2b
Dry DMF (3 mL) was dropwise added to 3 mL of phosphorus
oxychloride (0.0327mol) maintained at 0 °C. After the solution
was stirred for 15 min, compound 1b (7.78 g, 0.0274 mol) was
added to the above Vilsmeier reagent, and the mixture was
heated to 90 °C for an additional 2 h. After cooling, the clear
red solution was added dropwise to ice water with stirring, and
the resulting mixture was allowed to stand for 2 h and extracted
with CH2Cl2 (20 mL×3). The combined extracts were washed
with water and dried over MgSO4. After filtration and removal
of the solvent under vacuum, the crude product was purified by
column chromatography to give a red liquid product (84%).
1H NMR (400 MHz, CDCl3) δ 9.99 (s, 1H), 6.62 (s, 1H), 4.32
(t,
J = 6.6 Hz, 2H), 3.97 (t, J = 6.6 Hz, 2H), 1.85 – 1.69 (m,
4H), 1.52 – 1.39 (m, 4H), 1.37 – 1.30 (m, 8H), 0.94 – 0.85 (m,
6H).
13C NMR (100 MHz, CDCl3) δ 181.60, 153.33, 149.58, 124.31,
106.62, 73.76, 70.63, 31.45, 31.42, 29.90, 29.02, 25.69, 25.39,
22.52, 13.91.
MS (EI): m/z calcd for C17H28O3S: 312.47; found: 312.49.
3.2.3 Synthesis of compound 3b
To a suspension of diethyl bis(4ꢀ(diethylamino) phenyl)
methylphosphonate (1.5 g, 3.36 mmol), and 2b (936 mg, 3
mmol) in 20 mL dry THF and NaH (0.14 g, 5.6 mmol) were
added and the mixture turned yellow. The mixture was stirred
at room temperature for 24 h. Saturated NH4Cl was added and
the resulting mixture was extracted with EtOAc (20 mL×3).
The combined extracts were washed with water and dried over
MgSO4. After filtration and removal of the solvent under
vacuum, the crude product was purified by column
chromatography to give a yellow product (0.81 g, 44%).
1H NMR (400 MHz, Acetone) δ 7.15 (m, 3H,overlap), 6.98 (d,
J
= 8.6 Hz, 2H), 6.77 (d,
J
J
= 8.6 Hz, 2H), 6.62 (d,
= 6.4 Hz, 2H), 3.91 (t,
J
J
= 8.9 Hz,
= 6.4 Hz,
2H), 6.04 (s, 1H), 4.07 (t,
Figure 7 UVꢀVis absorption spectra of E–O polymers before and after poling.
2H), 3.48 – 3.35 (m, 8H), 1.81 – 1.69 (m, 4H), 1.64 – 1.42 (m,
4H), 1.38 – 1.23 (m, 8H), 1.21 – 1.11 (m, 12H), 0.90 (m, 6H).
MALDIꢀTOF: m/z calcd for C38H56N2O2S: 604.93 [M]+;
found:604.87.
3. Experiments
3.1. Materials and instrumentation
3.2.4 Synthesis of compound 4b
To a solution of 3a (1.21 g, 2.0 mmol) in dry THF (20 ml) was
added a 2.4 M solution of nꢀBuLi in hexane (1.3 ml, 3.0 mmol)
1H NMR spectra were determined by an Advance Bruker
400(400 MHZ) NMR spectrometer (tetramethylsilane as
internal reference). The MS spectra were obtained on MALDIꢀ
TOFꢀ(Matrix Assisted Laser Desorption/Ionization of Flight)
on BIFLEXIII(Broker Inc.) spectrometer. The UVꢀVis spectra
were performed on Cary 5000 photo spectrometer. The TGA
was determined by TA5000ꢀ2950TGA (TA Co) with a heating
dropwise at ꢀ78
℃ under N2. After this mixture was stirred at
this temperature for 1 h, and the dry DMF (0.20 ml, 2.4 mmol)
was introduced. The resulting solution was stirred for another
1h at ꢀ78 °C and then allowed to warm up to room temperature.
The reaction was quenched by water. THF was removed by
evaporation. The residue was extracted with CH2Cl2 (3 x 30
ml). The organic layer was dried (MgSO4) and concentrated in
vacuo. The residue was purified by column chromatography on
silica gel (hexane/acetone, v/v, 10/1) to obtain a red solid (0.93
g, 74%)
rate of 10
℃/ min under the protection of nitrogen. Cyclic
voltammetry (CV) experiments were performed on a CHI660D
electrochemical workstation by a cyclic voltammetry (CV)
technique in CH3CN solution, using Pt disk electrode and a
platinum wire as the working and counter electrodes,
respectively, and a saturated Ag/AgCl electrode as the
1H NMR (400 MHz, Acetone) δ 9.70 (s, 1H), 7.08 – 7.03 (m,
2H), 7.02 (s, 1H), 6.88 – 6.83 (m, 2H), 6.68 – 6.63 (m, 2H),
6.53 – 6.48 (m, 2H), 4.19 – 4.14 (m, 2H), 3.98 – 3.90 (m, 2H),
3.29 (m, 8H), 1.72 – 1.60 (m, 4H), 1.48 – 1.31 (m,4H), 1.28 –
1.18 (m, 8H), 1.04 (m, 12H), 0.77 (m, 6H).
reference electrode in the presence of 0.1
M
nꢀ
tetrabutylammoniumperchlorate as the supporting electrolyte.
The ferrocene/ferrocenium (Fc/Fc+) couple was used as an
internal reference. All chemicals, commercially available, are
used without further purification unless stated. The DMF,
6 | J. Name., 2014, 00, 1-3
This journal is © The Royal Society of Chemistry 2014

Page 7 of 9
Journal Name
Physical Chemistry Chemical Physics
View ArticAle ROnTliInCeLE
DOI: 10.1039/C4CP05829A
13C NMR (100 MHz, Acetone) δ 179.72, 156.06, 148.25, In a similar manner described above, chromophore y3 was
147.96, 146.26, 145.75, 137.31, 131.05, 128.97, 128.72, synthesized from 4c and acceptor 5, as a dark solid (40%).
125.07, 121.23, 112.14, 111.10, 110.93, 74.29, 74.22, 44.04, 1H NMR (400 MHz, Acetone) δ 7.64 (d,
J
J
= 15.1 Hz, 1H), 7.13
= 8.7 Hz, 2H), 6.73
= 9.0 Hz, 2H), 6.18 (d, J=15.1 Hz
44.01, 31.60, 31.40, 30.03, 29.80, 25.86, 25.40, 22.55, 22.39, (d,
13.50, 13.42, 12.08, 12.00. (d,
J
J
= 9.0 Hz, 2H), 7.04 (s, 1H), 6.89 (d,
= 8.7 Hz, 2H), 6.57 (d,
J
MALDIꢀTOF: m/z calcd for C39H56N2O3S: 632.94 [M]+; 1H), 4.39 (m, 2H), 4.32 (m, 2H), 3.41 – 3.28 (m, 8H), 1.60 (s,
found:632.89.
3.2.5 Synthesis of chromophore y2
6H), 1.08 (m, 12H).
13C NMR (100 MHz, Acetone) δ 184.70, 176.72, 111.69,
A mixture of aldehydic bridge 4b (0.632 g, 1 mmol) and 111.08, 109.54, 104.58, 100.89, 97.14, 62.11, 62.04, 56.48,
acceptor 5 (0.24 g, 1.2 mmol) in ethanol (20 mL) in the 43.39, 43.34, 27.07, 25.87, 25.11, 25.03, 23.33, 16.15, 16.10,
presence of a catalytic amount of piperidine was stirred at 70 11.51.
°C for 3 h. After removal of the solvent, the residue was MALDIꢀTOF: m/z calcd for C40H41N5O3S: 671.85 [M]+;
purified by column chromatography on silica gel (hexane/ethyl found:672.12.
acetate, v/v, 5:1), a dark solid was obtained (0.27 g, 33%).
1H NMR (400 MHz, Acetone) δ 7.91 (d,
J
= 15.6 Hz, 1H), 7.30
= 8.6 Hz, 2H), 6.85 (d,
= 9.0 Hz, 2H), 6.21 (d, = 15.6 Hz,
– 7.25 (m, 2H), 7.25 (s, 1H), 7.00 (d,
= 8.6 Hz, 2H), 6.69 (d,
J
J
Conclusions
J
J
In this research, a series of NLO chromophores based on
thiophene bridge bearing on different substituted groups with
the same bis(N,Nꢀdiethylaniline) donors and electron acceptor
(TCF) have been synthesized and systematically investigated
by NMR, MS and UVꢀvis absorption spectra. The energy gap
between ground state and excited state together with molecular
nonlinearity were studied by UVꢀvis absorption spectroscopy,
DFT calculations and CV measurements. Theoretical and
experimental investigations suggest that the isolation group
play a critical role in affecting the linear and nonlinear
properties of dipolar chromophores. In general, the energy gap
of y2 and y3chromophore is 1.03 and 1.02 eV respectively,
which is much lower than y1 chromophore without isolation
group on thiophene ring. These results confirmed that
chromophores with double donors and with different isolation
groups on the thiophene bridge exhibiting good thermal
stability and large molecular hyperpolarizabilities, which can be
effectively translated into very large EO coefficients in poled
polymers. The poling results of guestꢀhost EO polymers with
25 wt% of these chromophores showed that polymers with
chromophores y1ꢀ y3 afforded the large r33 values of 149, 139
and 125 pm/V respectively. Moreover, the stability of poled
APC/25% y1, y2 and y3 film could retain 78% ,81% and 76%
1H), 4.32 (m, 2H), 4.10 (m, 2H), 3.46 (m, 8H), 1.88 – 1.76 (m,
4H), 1.73 (s, 6H), 1.54 (m, 4H), 1.42 – 1.31 (m, 8H), 1.27 –
1.13 (m, 12H), 0.91 (m, 6H).
13C NMR (100 MHz, Acetone) δ 176.22, 172.76, 154.58,
148.20, 148.04, 147.82, 146.82, 139.35, 135.77, 131.60,
130.57, 129.27, 128.79, 120.15, 112.24, 111.57, 110.60,
110.36, 107.34, 96.31, 73.95, 73.43, 43.61, 43.53, 31.16, 30.77,
29.45, 25.17, 25.11, 25.01, 22.02, 21.86, 12.85, 11.57.
MALDIꢀTOF: m/z calcd for C50H66N5O3S: 814.13 [M]+;
found:814.75.
3.2.6 Synthesis of compound 2c
In a similar manner described above, 2c was synthesized from
1c as yellow solid (92%).
1H NMR (400 MHz, Acetone) δ 9.89 (d,
J
= 0.9 Hz, 1H), 7.03
(d,
J
= 0.9 Hz, 1H), 4.45 (dd,
J
= 5.2, 3.0 Hz, 2H), 4.34 (dd, J =
5.2, 3.0 Hz, 2H).
13C NMR (100 MHz, Acetone) δ 179.92, 149.97, 143.11,
118.97, 110.96, 66.47, 65.37.
MS (EI): m/z calcd for C7H6O3S: 170.19; found: 170.21.
3.2.7 Synthesis of compound 3c
In a similar manner described above, 3c was synthesized from
2c as yellow solid (74%).
1H NMR (400 MHz, Acetone) δ 7.15 (d,
J
= 9.0 Hz, 2H), 7.05
(s, 1H), 7.00 (d,
(d,
2H), 3.44 (m, 8H), 1.19 (m, 12H).
J = 8.7 Hz, 2H), 6.80 (d, J = 8.7 Hz, 2H), 6.65
= 9.0 Hz, 2H), 6.08 (s, 1H), 4.28 (m, 2H), 4.23 – 4.18 (m,
(100h) of the initial values at 85
℃
respectively.Those
J
consequences indicate that these chromophores with double
N,Nꢀdiethylaniline donors and isolated groups on the thiophene
bridge could efficiently reduce the interchromophore
electrostatic interactions and enhance the macroscopic optical
nonlinearity. These novel chromophores showed promising
applications in NLO chromophore synthesis. We believe that
these novel chromophores can be used in exploring highꢀ
performance organic EO and photorefractive materials where
both thermal stability and optical nonlinearity are of equal
importance.
13C NMR (100 MHz, Acetone) δ 147.84, 147.21, 141.40,
139.56, 138.63, 131.42, 130.22, 127.78, 126.48, 117.45,
112.27, 111.66, 111.51, 98.11, 64.90, 64.62, 44.07, 12.13,
12.08.
MALDIꢀTOF: m/z calcd for C28H34N2O2S: 462.65 [M]+;
found:462.61.
3.2.8 Synthesis of compound 4c
In a similar manner described above, 4c was synthesized from
3c as red solid (83%).
1H NMR (400 MHz, CDCl3) δ 9.70 (s, 1H), 7.24 (d,
J
J
= 8.8 Hz,
= 8.6 Hz,
Acknowledgements
We are grateful to the National Nature Science Foundation of
China (no. 61101054) for financial support.
2H), 7.06 (s, 1H), 7.03 (d,
2H), 6.58 (d,
J = 8.6 Hz, 2H), 6.72 (d,
J
= 8.8 Hz, 2H), 4.33 (m, 2H), 4.31 (m, 2H), 3.46
– 3.32 (m, 8H), 1.19 (m, 12H).
13C NMR (100 MHz, CDCl3) δ 178.26, 147.30, 146.89, 146.73,
144.66, 137.61, 130.14, 129.37, 128.15, 127.97, 123.94,
114.25, 111.20, 110.02, 109.37, 64.41, 63.52, 43.37, 43.33,
11.62.
Notes and references
a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b University of Chinese Academy of Sciences, Beijing 100043, PR China
* Corresponding authors. Tel.: +86-01-82543528; Fax: +86-01-62554670
MALDIꢀTOF: m/z calcd for C29H34N2O3S: 490.66 [M]+;
found:491.19.
.
3.2.9 Synthesis of chromophore y3
E-mail address: xinhouliu@foxmail.com and xhliu@mail.ipc.ac.cn (X. Liu)
This journal is © The Royal Society of Chemistry 2014
J. Name., 2014, 00, 1-3 | 7

Physical Chemistry Chemical Physics
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8 | J. Name., 2014, 00, 1-3
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Page 9 of 9
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP05829A
Graphical Absrtact
New double-donor chromophores were synthesized and the large hyperpolarizability can be
effectively translated into large electro-optic coefficients.
1