Nonlinear optical chromophores based on Dewar's rules: Enhancement of electro-optic activity by introducing heteroatoms into the donor or bridge

Article abstract of DOI:10.1039/c5cp04959h

In this work, we investigated the enhancement of the electro-optic response by introducing electron-rich heteroatoms as additional donors into the donor or bridge of a conventional second-order nonlinear optical chromophore. A series of chromophores C2-C4 based on the same tricyanofuran acceptor (TCF) but with different heteroatoms in the alkylamino phenyl donor (C2 or C3) or thiophene bridge (C4) have been synthesized and systematically investigated. Density functional theory calculations suggested that chromophores C2-C4 had a smaller energy gap and larger first-order hyperpolarizability (β) than traditional chromophore C1 due to the additional heteroatoms. Single crystal structure analyses and optimized configurations indicate that the rationally introduced heteroatom group would bring larger β and weaker intermolecular interactions which were beneficial for translating molecular β into macro-electro-optic activity in electric field poled films. The electro-optic coefficient of poled films containing 25 wt% of these new chromophores doped in amorphous poly-carbonate afforded values of 83 and 91 pm V^-1 at 1310 nm for chromophores C3 and C4, respectively, which are two times higher than that of the traditional chromophore C1 (39 pm V^-1). High r₃₃ values indicated that introducing heteroatoms to the donor and bridge of a conventional molecular structure can efficiently improve the electron-donating ability, which improves the β. The long-chain on the donor or bridge part, acting as the isolation group, may reduce inter-molecular electrostatic interactions, thus enhancing the macroscopic EO activity. These results, together with good solubility and compatibility with the polymer, show the new chromophore's potential application in electro-optic devices.

Full text of DOI:10.1039/c5cp04959h

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Nonlinear optical chromophores based on
Dewar’s rules: enhancement of electro-optic
activity by introducing heteroatoms into the
donor or bridge†
Cite this:DOI: 10.1039/c5cp04959h
ab
ab
ab
bc
a
a
Huajun Xu, Dadan Yang, Fenggang Liu, Mingkai Fu, Shuhui Bo,* Xinhou Liu*
and Yuan Cao
In this work, we investigated the enhancement of the electro-optic response by introducing electron-
rich heteroatoms as additional donors into the donor or bridge of a conventional second-order
nonlinear optical chromophore. A series of chromophores C2–C4 based on the same tricyanofuran
acceptor (TCF) but with different heteroatoms in the alkylamino phenyl donor (C2 or C3) or thiophene
bridge (C4) have been synthesized and systematically investigated. Density functional theory calculations
suggested that chromophores C2–C4 had a smaller energy gap and larger first-order hyperpolarizability
(b) than traditional chromophore C1 due to the additional heteroatoms. Single crystal structure analyses
and optimized configurations indicate that the rationally introduced heteroatom group would bring
larger b and weaker intermolecular interactions which were beneficial for translating molecular b into
macro-electro-optic activity in electric field poled films. The electro-optic coefficient of poled films
containing 25 wt% of these new chromophores doped in amorphous poly-carbonate afforded values of
ꢀ
1
8
3 and 91 pm V at 1310 nm for chromophores C3 and C4, respectively, which are two times higher
ꢀ
1
than that of the traditional chromophore C1 (39 pm V ). High r33 values indicated that introducing
heteroatoms to the donor and bridge of a conventional molecular structure can efficiently improve the
electron-donating ability, which improves the b. The long-chain on the donor or bridge part, acting as
the isolation group, may reduce inter-molecular electrostatic interactions, thus enhancing the
macroscopic EO activity. These results, together with good solubility and compatibility with the polymer,
show the new chromophore’s potential application in electro-optic devices.
Received 20th August 2015,
Accepted 7th October 2015
DOI: 10.1039/c5cp04959h
www.rsc.org/pccp
of EO materials, the design and preparation of NLO chromo-
phores have been turned into a hot research spot in the area of
1
. Introduction
7
–11
During recent decades, organic and polymeric electro-optic (EO) organic EO materials.
To realize the application of devices,
materials have drawn much attention due to their attractive NLO chromophores should be required to possess several
potential applications in high speed EO devices with broad properties: large first-order hyperpolarizability (b), low optical
1
bandwidth and low driving voltage. Compared with traditional loss at operation wavelength, thermal stability and long-term
inorganic and semiconductor materials, the organic EO materials stability of polar order. Therefore, much greater effort has been
have many advantages such as larger nonlinear optical coefficients, devoted to the rational design of chromophores with not only
2
–6
simpler preparation and lower cost. As the core component high first-order hyperpolarizability but also good thermal and
photochemical stabilities, and in addition, good solubility and
1
2–15
compatibility with the polymer matrix.
a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, P. R. China. E-mail: boshuhui@mail.ipc.ac.cn, xhliu@mail.ipc.ac.cn,
xinhouliu@foxmail.com, +86-01-62554670, +86-01-82543528
It is a well-established fact that the conjugation length and
the donor/acceptor strength of these D–p–A type push–pull
chromophore molecules can have dramatic influences on their
b
c
16,17
University of Chinese Academy of Sciences, Beijing, 100043, P. R. China
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China
second order nonlinear responses.
A majority of high per-
formance NLO chromophores up to now can be divided into
three blocks: an electro-donor (typical alkyl aniline), a pi-bridge
d
College of Chemical Engineering, Sichuan University, Chengdu, 610065, P. R. China
(isophorone or heterocyclic rings) and a strong electro-acceptor.
†
CCDC 1401496. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5cp04959h
In order to achieve high EO activity, rational design and synthesis
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of dipolar NLO chromophores with high molecular first hyper-
polarizability (b) and robust thermal stability still represent
1
8–20
one of the most critical challenges.
In the past few decades,
the research on NLO chromophores has mainly focused on the
1
7,21,22
design of electron bridges and electron acceptors.
The
electron donors have received much less attention as the general
class of traditional alkyl and aryl amines were considered to be
the ideal electron donors and were constantly used. There were
some systematic studies concentrating on researching the sub- Chart 1 The structures of chromophores C2–C4.
stitutions in the p-framework from theoretical and practical
2
3–27
domains.
Jen’s group introduced an alkylthiol group into
was used as a bench chromophore. Four chromophore structures
are shown in Chart 1. For reasonable comparison, these NLO
chromophores all use 2-dicyanomethylene-3-cyano-4,5,5-dimethyl-
the electron-bridge of the CLD-type chromophore, which achieved
excellent photostability and large optical nonlinearity. Dalton’s group
synthesized a series of chromophores based on bis-(4-methoxy-
phenyl)hetero-aryl-amino type donors. The chromophores’
first-order hyperpolarizabilities and materials’ electro-optical
performance were enhanced effectively. Introducing an addi-
tional amino group into the donor moiety could effectively
modulate the conjugation of the p-electron networks and the
electronic character of the charge transfer kinetics, which was
1
13
2,5-dihydrofuran (TCF) as an electron acceptor. H and
C
NMR, MALDI-TOF and crystal structure analysis were carried
out to demonstrate the preparation of these chromophores.
Thermal stability, photophysical properties, density functional
theory (DFT) calculations and EO activities of these chromophores
were systematically studied and compared to illustrate the
influences of different heteroatom groups and sites on rational
NLO chromophore designs.
1
1,23,27–29
proved by simulated calculations.
As classical theories, Dewar’s rules have been widely
applied into chromophore structure design for increasing non-
3
0
3
1,32
linear optical activity and stability.
On the basis of this 2. Results and discussion
theory, a donor–bridge–acceptor push–pull type nonlinear optical
chromophore shows alternating electronegativities along the
charge-transfer direction (Fig. 1). It can be used to predict the
change of the molecular orbital level. For instance, the energy level
of the highest occupied molecular orbital (HOMO) will increase if
an electron-donating group is introduced into 3, 4, and 7 positions.
At the same time, the bathochromic shift of the UV-Vis absorption
spectra will be found. In order to systematically investigate the
influence of additional heteroatom groups introduced into
traditional aniline donor and bridge moieties to first-order
hyperpolarizability and macroscopic EO activities, we designed
and synthesized chromophores C2–C4, and measured their
physical and chemical properties with care. Chromophore C1
2
.1. Synthesis and characterization
Synthetic approaches for the chromophores C2–C4 are shown
in Schemes 1 and 2. All reactions were performed under nitrogen
protection. The TCF acceptor was prepared according to the
literature. Chromophores C2–C4 were synthesized in good overall
yields through simple routes which have been established for the
33
3
4,35
high yield synthesis of this kind of NLO chromophore.
All
1
the chromophores were completely characterized by H-NMR,
1
3
C-NMR, MS, and UV-Vis spectroscopic analysis and the data
obtained were in full agreement with the proposed formulations.
Fig. 1 Depiction of the substitution positions on the conjugated back-
bone for the FTC-type chromophore.
Scheme 1 Synthesis route for C2–C4.
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Fig. 2 The schematic representation of electronic transitions for the
D–p–A system as a function of dielectric media strength.
Scheme 2 Synthesis route for C4.
Fig. 3 UV-Vis spectra in different solvents.
The information of the crystal structure for chromophore C3 has
been obtained by Single-Crystal X-ray diffraction, which further Table 1 UV-Vis absorption and solvatochromic data of the chromophores
demonstrated the successful preparation of chromophores.
a
b
c
d
ꢀ1
l
max /nm
l
max /nm
l
max /nm
l
max /nm
r33/pm V
C1
C2
C3
C4
621
646
647
669
676
719
734
744
650
679
665
700
638
665
685
694
39
68
83
91
2
.2. Optical properties
The nature of the CT band and solvatochromic behavior of
C2–C4 were investigated using UV-Vis absorption measurements
in aprotic solvents with different polarities. The shape and the
position of the CT band of this type of chromophore mainly
depend on the dielectric properties of the solvents. There are
three major electronic structures which have been predicted and
a
l
l
max values were measured in dioxane, chloroform, DMF and APC.
max values were measured in dioxane, chloroform, DMF and APC.
b
c
lmax values were measured in dioxane, chloroform, DMF and APC.
l
d
max values were measured in dioxane, chloroform, DMF and APC.
illustrated in Fig. 3. Generally, the initial increase of the solvent 744 nm, respectively, which should be attributed to the enhanced
polarity will result in the red-shift of the absorption maximum electron-donation ability of additional heteroatom groups conju-
for a dye in the neutral ground state due to the strong electron gated with the p-system. At the same time, because of the fact that
polarization along the conjugated system. This change occurs to the nitrogen atom conjugated with the p-system better than the
a certain point, where the electronic structure of the chromo- oxygen atom, chromophore C3 showed a longer red-shift compared
1
6,36–39
phore reaches a cyanine-limit structure.
The blue-shift of with C2. The different sites for heteroatom groups also brought
the absorption maximum will be observed with a further different influences to the maximum absorption wavelength, which
increase of solvent polarity. As discussed in the previous section, we can observe from the spectra of C3 and C4 (lmax = 734 nm,
their modification by changing the length and chemical compo- 744 nm). The solvatochromic behavior was also explored to
sition has a significant effect on the molecular hyperpolariz- investigate the polarity of chromophores. When increasing the
ability of the molecule. The theory of bond length alternation solvent dielectric constant from dioxane to chloroform, all of
(BLA) has furthered the realization of the structure–property the chromophores showed solvatochromic red-shifts. Chromo-
relationships within polyene molecules and the use of optimal phores C2 and C4 showed similar bathochromic shifts of 73 nm
donor and acceptor strengths for a given bridge. The BLA and 75 nm when solvent was changed from dioxane to chloro-
parameter is defined as the average difference in length between form, respectively, whereas 44 nm and 40 nm blue-shifts from
adjacent carbon–carbon bonds of a molecule. BLA theory chloroform to DMF. Chromophores C2–4 exhibited larger red-
indicates that increasing the values of donor and acceptor shifts and blue-shifts in comparison to chromophores C1.
strength will improve the proportion of structure c (Fig. 2) It indicated that the p-electrons in these three chromophores
among the three resonance states within certain limits, then are easier to be polarized than chromophores C1. In three
the BLA parameter will decrease and molecular NLO activity will resonance states, structure c will contain more proportion for
be enhanced. UV-Vis spectra and solvatochromic data can reveal chromophores C2–4, which generally mean larger molecular
the change of BLA parameters to some degree. Fig. 3 displays first-order hyperpolarizabilities.
the spectra of C2–C4 in five solvents with different dielectric
constants and the spectra data are summarized in Table 1. The
2.3. Theoretical calculations
positions of lmax for the chromophores prepared in chloroform Theoretical estimation of molecular hyperpolarizability is now a
follow Dewar’s rules, which showed large red-shifts. Compared reliable and routine technique though the computational approach
with C1 (lmax = 676 nm), the lmax values of chromophores C2–C4 fails to give correct estimations of b values for structurally different
2
4,40
were shifted to the longer wavelength of 719 nm, 734 and systems and also for different reaction fields.
The goal of
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Table 2 Data from DFT calculations
can effectively make the maximum absorption wavelength red-shift,
narrow the HOMO–LUMO energy gap and improve chromophore’s
first-order hyperpolarizability.
a
b
c
ꢀ48
d
ꢀ30
m /D
DE /eV
mb /10
esu
b
zzz /10 esu
C1
C2
C3
C4
20.06
22.74
21.59
21.38
2.32
2.29
2.26
2.17
11 334
16 623
16 260
16 940
714.56
731.03
753.17
792.33
2.4. Crystal structure studies
It is an arduous task to grow single crystals for complicated
a
m was calculated using Gaussian 09 at the PBE1PBE/6-31g* level. molecules. After several attempts, we successfully got an eligible
DE = ELUMO ꢀ EHOMO was calculated using Gaussian 09 at the PBE1PBE/
b
single crystal of C3 by the evaporation of ethanol/hexane solutions
c
6
level.
-31g* level. mbzzz was calculated using Gaussian 09 at the PBE1PBE/6-31g*
d
at ambient temperature. The ORTEP diagram and the crystal data
are given in Fig. 5 and Table 3 and crystal packing diagrams are
shown in Fig. 6. C3 crystallized in the triclinic space group P1. As
Fig. 5 shows, the molecular conformation, including the hindrance
of the donor and the acceptor, effectively isolates the chromophores
and weakens the dipole–dipole interactions. This arrangement acts
to site-isolate the chromophore within the internal free volume
created by the butyl groups, which is favorable for dipole orientation
in the poling process. The crystal packing data showed ring centroid
to ring-centroid separations of 4.35 Å to 5.86 Å, which indicates
bzzz was calculated using Gaussian 09 at the PBE1PBE/6-31g* level.
this study, however, was not to obtain the exact values, but
rather to estimate trends of b. It was reinforced by theoretical
calculations and optical characterization that the b value has a
close relationship with the substituents, steric hindrance, and
intramolecular charge-transfer. In order to model the ground
state molecular geometries, the HOMO–LUMO energy gaps and
first-order hyperpolarizability (b) of the chromophores, the DFT
calculations were carried out at the PBE1PBE level by employing
the split valence 6-31g(d) basis set.
DFT calculations are summarized in Table 2.
From this, the scalar quantity of b can be computed from the
x, y, and z components according to the following equation.
44,45
better site isolation than that of 4.2 Å reported previously.
4
1–44
The data obtained from
Although it was difficult to observe the H-bonding effects on proton
1
shifts in the H NMR spectrum of C3, efficient charge transfer in
the trans conformation and the strong electron-withdrawing
effect of the TCF acceptor were shown by the crystal analytical
data, which also showed intramolecular H-bonding interactions
between C(6)–H(6)ꢁ ꢁ ꢁS(1); C(18)–H(18)ꢁ ꢁ ꢁS(1); C(18)–H(18)ꢁ ꢁ ꢁN(1)
2
2
y
2 1/2
b = (b
x
+ b
+ b
z
)
where
(Fig. 6) with distances of 3.107 Å, 3.094 Å and 2.824 Å and angles
X ꢀ
ꢁ
of 1101, 1111 and 1021. Thus, it firmly supported the conclusion
in the synthetic analysis that an intramolecular H-bonding
interaction indeed existed in C3.
1
bi ¼ biii þ 3
bijj þ bjij þ bjji
;
i; j 2 ðx; y; zÞ
iaj
The frontier molecular orbitals are often used to characterize
the chemical reactivity and kinetic stability of a molecule, and
to obtain qualitative information about the optical and electrical
2
.5. Thermal stabilities and EO performance
NLO chromophores must be thermally stable enough to with-
stand encountered high temperatures (4100 1C) in electric
field poling and subsequent processing of chromophore/polymer
24,42
properties of molecules.
Besides, the HOMO–LUMO energy
gap is also used to understand the charge transfer interaction
45,46
occurring in a chromophore molecule.
Fig. 4 depicts the
electron density distribution of the HOMO and LUMO structures.
It can be seen that the density of the ground and excited state
electron is asymmetry along the dipolar axis of the chromophores.
The HOMO–LUMO gap was the lowest for C4 (1.86 eV) and
highest for C1 (2.32 eV). It proved that the same amino groups
can show different effects on the HOMO–LUMO energy gap and
UV-Vis absorption when compared with C3. Introducing an
electron-donating group into the 4-site of the thiophene-bridge
Fig. 4 Frontier molecular orbitals of chromophores C2–C4.
Fig. 5 Thermal ellipsoid plot of C3 drawn at the 30% level.
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Table 3 Crystal data and structure refinement details for C3
Compound reference
C3
Chemical formula
Formula mass
40 51 5
C H N OS
649.93
Crystal system
Wavelength/Å
Triclinic
0.71073
a/Å
b/Å
c/Å
a (1)
b (1)
g (1)
10.387(2)
10.780(2)
17.168(4)
90.321(4)
95.987(4)
93.823(4)
1.132
1907.4(7)
153
P1%
ꢀ3
Calculated density (g cm
Unit cell volume/Å
T/K
Space group
)
3
Fig. 7 Thermogravimetric analysis of the chromophores C2–C4.
No. of formula units per unit cell, Z
F(000)
No. of measured reflections
No. of independent reflections
2
700
23 561
8813
0.035
0.0598
0.1714
1.002
films poled in contact poling to prevent the possible film
damage (in contact poling, the film thickness will be about
1.3–2.0 mm). Chromophore’s aggregation will not only diminish the
poling-induced noncentrosymmetric alignment of the chromo-
phores, but also increase optical loss. Therefore, it was important
to evaluate the molecular aggregation in guest–host films.
Chromophore’s aggregation generally will broaden the main
CT peak. Before corona poling, we measured the UV-Vis spectra
of four guest–host thin films doped with C1–4 and they are
shown in Fig. 9. Four films exhibited similar CT absorption
peaks at different wavelengths. Film-C1 was a little bit broader
than other three kinds of films, which indicated more severe
chromophore aggregation in film-C1.
R
int
Final R values (I 4 2s(I))
1
2
Final wR(F ) values (I 4 2s(I))
2
Goodness of fit on F
The EO activities (r33 values) of poled films were measured
by the Teng–Man simple reflection method at a wavelength of
1310 nm using a carefully selected thin ITO electrode with low
reflectivity and good transparency in order to minimize the
2
8,47
contribution from multiple reflections.
the poling temperature, poling time and poling voltage, the
optimum poling conditions were poled at (T + 5) 1C for 15 min
Through changing
g
under the electric field of 10–11 kV. The r33 values were measured
and are shown in Table 1 and Fig. 8. C2–C4 displayed excellent
compatibility with the polymer matrix when guest–host polymers
Fig. 6 Crystal packing diagram of chromophore C3.
materials. Fig. 7 shows the thermogravimetric analysis of C2–C4. were prepared. Film-C2, film-C3 and film-C4 showed larger EO
All of the chromophores exhibited good thermal stabilities and the
d
decomposition temperatures (T ) beyond 200 1C (C2: 246 1C, C3:
231 1C, and C4: 233 1C). The temperature in practical materials
processing was generally below 200 1C. All of the chromophores
C2–C4 are highly stable and may be widely applied in photonic
device fabrications.
In order to evaluate their EO activities, the polymer films
doped with 5–30 wt% chromophores into amorphous polycarbonate
(APC) using 1,1,2-trichloroethane (TCE) as the solvent were prepared
to investigate the translating of the microscopic hyperpolarizability
into macroscopic EO response (r33). The solution was filtered using a
0.2 mm syringe filter to remove large particulates. And then the
solution was spin-coated on indium-tin oxide (ITO) glass substrates.
The films were dried in vacuo for 12 h to remove the residual
solvent. The thickness of the films was measured using an Ambios
Technology XP-1 profilometer. The thickness of guest–host polymer
Fig. 8 EO activities of thin films as a function of chromophore loading
films was about 2.1–2.6 mm, which should be thicker than the densities.
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Fig. 10 Temporal stabilities of the poled film-C2, film-C3 and film-C4 at
8
5 1C for 300 h.
Fig. 9 UV-Vis spectra of film-C1, film-C2, film-C3 and film-C4.
experiments were performed on a Cary 5000 photo spectrometer.
activities than film-C1, which are caused by higher first-order The TGA was determined by TA5000-2950TGA (TA co) with a
ꢀ1
hyperpolarizabilities, larger steric hindrance and excellent heating rate of 10 1C min under the protection of nitrogen. The
compatibility with the polymer matrix. With the increase of single-crystal X-ray diffraction data were collected with the use of
00
chromophore loading density, the EO activity of film-C2 stopped graphite-monochromatized Mo-Ka radiation (l = 0.71073 Å) at
growth at the soonest because of the gradually increased inter- ꢀ120 1C on a Rigaku AFC10 diffractometer equipped with a Saturn
molecular interactions. Film-C3 and film-C4 showed saturated CCD detector.
density and better EO activities at 25 wt% higher than film-C2.
The long-chain on the donor or bridge part, acting as the
isolation group, may reduce inter-molecular electrostatic inter-
actions, thus the molecular first-order hyperpolarizabilities can
3
.2. Synthesis
3.2.1. Compound 6a. To a stirred solution of aminophenol
be more effectively translated into the macroscopic EO activity. (1.09 g, 10 mmol) and bromobutane (1.63 g, 15 mmol) in 10 mL
After 300 h at 75 1C, the EO activities can be retained above 75% of DMF, potassium carbonate (2.07 g, 15 mmol) and 18-crown-6
of initial data. The r33(0) values were determined after poling (0.05 g, 0.2 mmol) were added. The reaction mixture was heated
(
Fig. 10). The r33 values of films C3 and C4 are two times higher at 90 1C under nitrogen for 24 h and then poured into a 100 mL
ꢀ1 48
than those of the traditional chromophore C1 (39 pm V ),
solution of potassium carbonate. The reaction mixture was
suggesting that introducing heteroatoms to the donor and extracted by ethyl acetate (50 mL ꢂ 3), washed with brine,
bridge of conventional molecular structure can significantly and dried over MgSO . The solvent was removed in vacuo. The
4
increase their macroscopic EO activities.
resulting crude product was purified by column chromato-
graphy with hexane/ethyl acetate as the solvent and an eluent
(
VAcOEt : VHexane = 1 : 20) to give 6a as pale oil with 84% yield.
1
H NMR (400 MHz, CDCl
3
, ppm): 7.09 (t, 1H, Ar–H), 6.27 (d, 1H,
), 3.24 (t, 4H, NCH ),
), 1.49 (m, 2H, CH ), 1.34
). C NMR (101 MHz, CDCl
3
. Experimental
Ar–H), 6.19 (d, 2H, Ar–H), 3.95 (t, 2H, OCH
1
All chemicals are commercially available and are used without (m, 4H, CH
2
2
3.1. General procedures
.76 (m, 2H, CH
2
), 1.57 (m, 4H, CH
2
3
2
1
2
), 0.96 (m, 9H, CH
3
3
,
further purification unless otherwise stated. N,N-Dimethyl ppm): 189.35, 157.35, 153.00, 131.28, 106.43, 102.90, 54.95,
formamide (DMF) was distilled over calcium hydride and 50.91, 29.68, 29.52, 20.49, 20.34, 13.92. MALDI-TOF (M+,
stored over molecular sieves (pore size 3 Å). Acetone was dried C H NO): calcd: 277.24; found: 277.21.
1
8
31
with anhydrous MgSO , then distilled and stored over molecular
3.2.2. Compound 6b. The procedure for compound 6a was
4
sieves (pore size 3 Å). The 2-dicyanomethylene-3-cyano-4-methyl-2,5- followed to prepare 6b from m-phenylenediamine as colorless
1
dihydrofuran(TCF) acceptor was prepared according to the oil (91%). H NMR (400 MHz, CDCl
3
, ppm) d = 7.02 (t, J = 8.2 Hz,
2
1
literature. Compound 7c was prepared according to the 1H, Ar–H), 6.00 (dd, J = 8.2, 2.1 Hz, 2H, Ar–H), 5.91 (d, J = 2.1 Hz,
TLC analyses were carried out on 0.25 mm thick 1H, Ar–H), 3.35–3.18 (t, 8H, NCH
precoated silica plates and spots were visualized under UV (m, 8H, CH
4
9,50
literature.
2
), 1.58 (m, 8H, CH
2
), 1.48–1.25
). C NMR (101 MHz,
1
3
2
), 1.08–0.90 (m, 12H, CH
3
light. Chromatography on silica gel was carried out on Kieselgel CDCl , ppm): 150.14, 130.49, 101.02, 96.40, 51.97, 30.65, 21.33,
3
1
13
(
200–300 meshes). H and C NMR spectra were determined using 14.91. MALDI-TOF (M+, C H N ): calcd: 332.22; found: 332.31.
2
2
40 2
an Advance Bruker 400M (400 MHz) NMR spectrometer (tetra-
3.2.3. Compound 7a. A solution of 20 mL of DMF was
methylsilane as internal reference). The MS spectra were obtained cooled to 0 1C and was maintained at this temperature during the
using MALDI-TOF (Matrix Assisted Laser Desorption/Ionization dropwise addition of phosphorus oxychloride (0.64 g, 4.2 mmol).
of Flight) on a BIFLEXIII (Broker Inc.) spectrometer. The UV-Vis The solution was kept stirring at 0 1C for 2 h and the
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1
temperature was kept constant during the dropwise addition of compound 8a as an orange oil (0.32 g, 83%). H NMR (400 MHz,
a (0.55 g, 2 mmol) into 10 mL of DMF. The solution was stirred CDCl , ppm): 7.33 (d, J = 8.6 Hz, 1H, CH), 7.18 (d, J = 16.1 Hz, 1H,
6
3
for 2 h at 0 1C, then gradually warmed to room temperature and CH), 7.11 (d, J = 6.9 Hz, 1H, Ar–H), 7.07 (d, J = 4.9 Hz, 1H, CH), 6.95
stirred for 3 h at 60 1C before being poured into a 300 mL (d, J = 16.1 Hz, 1H, CH), 6.94 (s, 1H, CH), 6.24 (d, J = 6.9 Hz, 1H,
solution of potassium carbonate (10%) for quenching. The Ar–H), 6.14 (s, 1H, Ar–H), 3.99 (t, 2H, OCH ), 3.32–3.23 (m, 4H,
2
reaction mixture was extracted by ethyl acetate (50 mL ꢂ 3), NCH
washed with brine, and dried over MgSO . After removing the (m, 4H, CH
solvent, the resulting crude product was purified by column (101 MHz, CDCl
chromatography with hexane/ethyl acetate (VAcOEt : VHexane 123.53, 122.68, 121.34, 116.45, 103.77, 95.51, 67.14, 49.91, 30.55,
: 8) as an eluent to give the product as yellow oil (0.54 g, 28.67, 19.37, 18.49, 12.93, 12.88. MALDI-TOF (M+, C24 35NOS):
2
), 1.89–1.79 (m, 2H, CH
2
), 1.57 (m, 6H, CH
2
), 1.42–1.29
). C NMR
1
3
4
2
), 1.01 (t, 3H, CH
3
), 0.95 (t, 6H, CH
3
3
, ppm): 156.92, 148.10, 144.18, 126.78, 126.33,
=
1
8
H
1
9%). H NMR (400 MHz, CDCl ): 10.15 (s, 1H, CHO), 7.67 calcd: 385.24; found: 385.24.
3
(
d, J = 8.9 Hz, 1H, Ar–H), 6.22 (d, J = 8.9 Hz, 1H, Ar–H), 5.98
3.2.7. Compound 8b. The procedure for compound 8b was
: VHexane
(s, 1H, Ar–H), 4.00 (t, J = 6.3 Hz, 2H, OCH ), 3.34–3.26 (t, 4H, followed to prepare 8a from 7a as yellow solids (V
=
2
AcOEt
1
NCH
2
), 1.86–1.74 (m, 2H, CH
), 1.44–1.30 (m, 4H, CH
C NMR (101 MHz, CDCl , ppm): 185.92, 162.79, 153.32, 1H, CH), 6.99 (d, J = 8.0 Hz, 1H, Ar–H), 6.96 (s, 1H, CH), 6.93
29.02, 113.63, 103.54, 92.82, 66.87, 66.66, 49.92, 30.28, 29.79, (d, J = 4.4 Hz, 1H, CH), 6.38 (d, J = 8.0 Hz, 1H, Ar–H), 6.35 (s, 1H,
8.54, 19.28, 18.36, 17.94, 12.86, 12.75, 12.59. MALDI-TOF Ar–H), 3.30–3.24 (m, 4H, NCH ), 2.99–2.93 (m, 4H, NCH ), 1.63–
1.54 (m, 4H, CH ), 1.48 (m, 4H, CH ), 1.36 (m, 4H, CH ), 1.34–
2
), 1.68–1.54 (m, 4H, CH
2
), 1.50 1 : 20, 72%). H NMR (400 MHz, CDCl
3
, ppm) d 7.43 (d, J =
(
m, 2H, CH
2
2
), 1.01–0.83 (m, 9H, CH
3
). 16.3 Hz, 1H, CH), 7.33 (d, J = 16.3 Hz, 1H, CH), 7.08 (t, J = 4.4 Hz,
1
3
3
1
2
(
2
2
M+, C H NO ): calcd: 305.23; found: 305.22.
1
9
31
2
2
2
2
3
.2.4. Compound 7b. The procedure for compound 7b was 1.26 (m, 4H, CH ), 1.00–0.94 (t, 6H, CH ), 0.92–0.85 (t, 6H, CH ).
2 3 3
1
3
followed to prepare 7a from 6a as yellow oil (V
: 8, 90%). H NMR (400 MHz, CDCl
: VHexane
=
C NMR (101 MHz, CDCl ): 151.79, 148.67, 145.47, 142.44,
3
AcOEt
1
1
3
, ppm): 10.05 (s, 1H, 133.89, 131.48, 128.51, 125.91, 124.28, 121.55, 118.88, 107.61,
CHO), 7.70 (d, J = 8.9 Hz, Ar–H, CH), 6.32 (dd, J = 8.9, 2.3 Hz, 105.57, 104.04, 54.13, 53.30, 50.99, 29.88, 20.52, 14.01. MALDI-
1
3
1
H, Ar–H), 6.19 (d, J = 2.3 Hz, 1H, CH), 3.33–3.27 (t, 4H, NCH
2
2
), TOF (M+, C28
), 3.2.8. Compound 8c. The procedure for compound 8c was
44 2
H N S): calcd: 440.32; found: 440.37.
.12–3.06 (t, 4H, NCH
2
), 1.59 (m, 4H, CH
2
), 1.49 (m, 4H, CH
.37 (m, 4H, CH ), 1.26 (m, 4H, CH
2
2
3
), 0.96 (t, 6H, CH ), 0.87 followed to prepare 8a from 7a as yellow oil (VAcOEt : VHexane =
13
1
(t, 6H, CH3). C NMR (101 MHz, acetone) d = 205.91 (s), 188.57 (s), 1 : 20, 68%). H NMR (400 MHz, CDCl ) d = 7.25 (d, J = 8.6 Hz,
3
157.20 (s), 153.23 (s), 131.30 (s), 120.84 (s), 107.26 (s), 103.82 (s), 2H, Ar–H), 7.14–7.07 (d, J = 16.3, 1H, CH), 6.89 (d, J = 5.3 Hz, 1H,
4
9.22 (s), 45.07 (s), 12.87 (s), 12.64 (s). MALDI-TOF (M+, C H N O): CH), 6.80 (d, J = 5.3 Hz, 1H, CH), 6.67 (d, J = 16.3 Hz, 1H, CH),
23 40 2
calcd: 360.31; found: 360.31.
.2.5. Compound 7c. A solution of 20 mL of DMF and (m, 4H, CH
compound 6c (0.42 g, 2 mmol) was cooled to 0 1C and was 0.87 (t, 6H, CH
6.53 (d, J = 8.6 Hz, 2H, Ar–H), 3.22–3.16 (m, 4H, CH
2
), 2.86–2.78
),
3
), 0.78 (t, 6H, CH ). C NMR (101 MHz, CDCl3)
3
2
), 1.55–1.42 (m, 4H, CH ), 1.36–1.17 (m, 12H, CH
2
2
1
3
3
maintained at this temperature during the dropwise addition of d = 146.75, 131.97, 128.69, 126.36, 125.41, 124.32, 122.21,
phosphorus oxychloride (0.34 g, 2.2 mmol). The solution was 119.81, 115.38, 110.91, 54.68, 49.85, 29.15, 28.59, 19.45, 19.37,
stirred for 2 h at 0 1C, then gradually warmed to room 12.96. MALDI-TOF (M+, C H N S): calcd: 440.32; found: 440.37.
28 44 2
temperature and stirred overnight before being poured into a
00 mL solution of potassium carbonate (10%) for quenching. 2 mmol) in dry THF (30 mL), butyllithium (1.00 mL, 2.4 mmol,
The reaction mixture was extracted by ethyl acetate (50 mL ꢂ 3), 2.4 M in hexane) was added dropwise at ꢀ78 1C. The mixture
washed with brine, and dried over MgSO
. After removing the was stirred at ꢀ78 1C for 1 h and then allowed to slowly warm to
solvent, the resulting crude product was purified by column ꢀ20 1C for 10 min. After the mixture was cooled back down to
chromatography with hexane/ethyl acetate (VAcOEt : VHexane
3.2.9. Compound 9a. To a solution of compound 8a (0.77 g,
3
4
=
ꢀ78 1C, 1.00 mL of DMF was added dropwise. The mixture was
1
7
: 10) as an eluent to give the product as pale oil (0.35 g, then slowly warmed to room temperature and stirred for 2 h at
3%). H NMR (400 MHz, CDCl ) d = 9.70 (s, 1H, CHO), 7.48 room temperature; 30 mL of water was added to quench the
1
3
(
(
(
d, J = 5.6 Hz, 1H, CH), 6.59 (d, J = 5.6 Hz, 1H, CH), 3.41–3.34 reaction. After the solvent was evaporated, the organic layer was
m, 4H, NCH ), 1.62 (m, 4H, CH ), 1.33 (m, 4H, CH ), 0.93 extracted with ethyl ether (3 ꢂ 50 mL). The combined organic
2
2
2
1
3
t, 6H, CH
3
). C NMR (101 MHz, CDCl
3
) d = 179.55, 153.84, layers were washed with water (50 mL) and brine (50 mL) and
1
(
35.62, 119.86, 117.30, 54.29, 29.08, 19.75, 13.64. MALDI-TOF dried over anhydrous MgSO
M+, C13 21NOS): calcd: 239.13; found: 239.20. under reduced pressure, the crude product was purified by
.2.6. Compound 8a. To a solution of compound 7a (0.305 g, silica chromatography, eluting with VAcOEt : VHexane = 1 : 8 to give
4
. After the removal of the solvent
H
3
1
1
mmol) and 2-thienyl triphenylphosphonate bromide (0.48 g, compound 9a as an orange oil (0.73 g, 88%). H NMR (400 MHz,
1
.1 mmol) in ether (20 mL) was added NaH (0.24 g, 100 mmol). CDCl ) d = 9.69 (s, 1H, CHO), 7.51 (d, J = 3.7 Hz, 1H, CH),
3
The solution was allowed to stir for 24 h and then poured into 7.34 (d, J = 16.1 Hz, 1H, CH), 7.24 (s, 1H, CH), 7.03–6.96
water. The organic phase was extracted by AcOEt, washed with (d, J = 16.1 Hz, 1H, CH), 6.90 (d, J = 3.7 Hz, 1H, Ar–H), 6.19
brine and dried over MgSO
under reduced pressure, the crude product was purified by silica (t, 2H, OCH
chromatography, eluting with VAcOEt : VHexane = 1 : 20 to give 1.50 (m, 4H, CH
4
. After the removal of the solvent (d, J = 1.5 Hz, 1H, Ar–H), 6.03 (d, J = 1.5 Hz, 1H, Ar–H), 3.92
), 3.26–3.16 (t, 4H, CH ), 1.81–1.73 (m, 2H, CH ),
), 1.37–1.24 (m, 2H, CH ), 1.24–1.13 (m, 4H, CH ),
2
2
2
2
2
2
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1
3
0
.94 (t, 4H, CH
CDCl ) d = 182.06, 158.86, 155.96, 137.53, 129.55, 128.81, 16.0 Hz, 1H, CH), 6.53 (d, J = 15.5 Hz, 1H, CH), 6.39 (d, J =
24.28, 116.05, 104.93, 95.94, 50.92, 39.14, 29.70, 25.77, 23.40, 8.7 Hz, 1H, Ar–H), 6.33 (d, J = 3.7 Hz, 1H, CH), 3.39–3.24 (m, 4H,
0.39, 14.10, 13.93. MALDI-TOF (M+, C H NO S): calcd: 413.24; NCH ), 3.08–2.94 (m, 4H, NCH ), 1.75 (s, 6H, CH ), 1.60 (m, 4H,
2 3
), 0.88 (t, 6H, CH ). C NMR (101 MHz, 7.38 (d, J = 3.7 Hz, 1H, CH), 7.01 (s, 1H, Ar–H), 6.98 (d, J =
3
1
2
2
5
35
2
2
2
3
found: 413.21.
CH ), 1.54–1.45 (m, 4H, CH ), 1.43–1.24 (m, 12H, CH ), 0.97
2 2 2
3
.2.10. Compound 9b. The procedure for compound 9b (t, 6H, CH
3
), 0.89 (t, 6H, CH
3
).
1
3
was followed to prepare 9a from 8a as orange oil (VAcOEt :VHexane
: 8, 75%). H NMR (400 MHz, CDCl
=
C NMR (101 MHz, acetone) d = 178.14, 176.07, 158.00,
1
1
3
) d = 9.80 (s, 1H, CHO), 154.21, 147.13, 132.53, 118.64, 114.08, 113.34, 112.96, 109.77,
7
(
(
.63 (d, J = 6.7 Hz, 1H, CH), 7.60 (d, J = 6.7 Hz, 1H, CH), 7.45 107.53, 105.51, 97.74, 90.98, 54.69, 51.41, 26.56, 20.96, 20.66,
d, J = 16.2 Hz, 1H, CH), 7.00 (d, J = 8.8 Hz, 1H, Ar–H), 6.95 14.06. MALDI-TOF, m/z: 649.35.
d, J = 16.2 Hz, 1H, CH), 6.38 (dd, J = 8.8, 2.5 Hz, 1H, Ar–H), 6.33 HRMS (ESI) (M+, C H N OS): calcd: 649.38143; found:
4
0
51 5
(
d, J = 2.5 Hz, 1H, Ar–H), 3.32–3.25 (m, 4H, CH ), 2.99–2.92 (m, 4H, 649.38121.
2
CH ), 1.63–1.53 (m, 4H, CH ), 1.47 (m, 4H, CH ), 1.37 (m, 4H, CH ),
3.2.14. Chromophore C4. The procedure for chromophore
2
2
2
2
13
1
.28 (m, 4H, CH
MHz, CDCl ) d = 181.20, 155.11, 151.49, 148.16, 138.58, 138.58, powder (61%). H NMR (400 MHz, CDCl
36.71, 130.74, 126.50, 123.34, 118.50, 113.90, 106.37, 103.97, 53.11, 15.7 Hz, 1H, CH), 7.31 (d, J = 8.3 Hz, 2H, Ar–H), 7.21 (s, 1H, CH),
9.85, 28.52, 19.91, 12.96. MALDI-TOF (M+, C29 OS): calcd: 7.09 (d, J = 15.1 Hz, 1H, CH), 6.94 (d, J = 15.7 Hz, 1H, CH), 6.56
68.32; found: 468.22. (d, J = 8.3 Hz, 2H, Ar–H), 6.44 (d, J = 15.1 Hz, 1H, CH), 3.24
.2.11. Compound 9c. The procedure for compound 9c was (t, 4H, CH ), 2.90 (t, 4H, CH ), 1.65 (m, 4H, CH ), 1.57–1.45
2 3 3
), 0.97 (t, 6H, CH ), 0.87 (t, 6H, CH ). C NMR (101 C4 was followed to prepare chromophore C2 from 9a as dark
1
3
3
) d = 7.69 (d, J =
1
4
44 2
H N
4
3
2
2
2
followed to prepare 9a from 8a as orange oil (V
: VHexane
=
(m, 4H, CH ), 1.41–1.18 (m, 8H, CH ), 0.89 (t, 6H, CH ), 0.82
2 2 3
AcOEt
1
13
1
: 8, 73%). H NMR (400 MHz, CDCl ) d 9.63 (s, 1H, CHO), 7.45 (t, 6H, CH3). C NMR (101 MHz, CDCl ) d = 174.98, 171.61,
3 3
(
s, 1H, CH), 7.25 (d, J = 8.6 Hz, 2H, Ar–H), 7.08 (d, J = 16.2 Hz, 149.82, 147.99, 144.14, 138.28, 133.55, 132.22, 131.28, 127.79,
H, CH), 6.91 (d, J = 16.2 Hz, 1H, CH), 6.50 (d, J = 8.6 Hz, 2H, 122.72, 113.45, 110.79, 109.56, 95.78, 53.84, 49.77, 29.73, 28.81,
Ar–H), 3.23–3.11 (m, 4H, NCH2), 2.83 (t, J = 7.2 Hz, 4H, NCH ), 28.51, 25.47, 19.28, 12.87.
.51–1.40 (m, 4H, CH ), 1.36–1.27 (m, 4H, CH ), 1.27–1.14 HRMS (ESI) (M+, C40
m, 8H, CH ), 0.84 (t, 6H, CH ), 0.77 (t, 6H, CH ). C NMR 649.38138.
101 MHz, CDCl ) d = 180.77, 147.70, 147.48, 143.45, 135.95,
1
2
1
(
(
2
2
51 5
H N OS): calcd: 649.38143; found:
1
3
2
3
3
3
131.36, 130.39, 127.28, 122.92, 113.76, 110.55, 54.24, 49.76,
2
8.97, 28.52, 19.40, 19.33, 12.95. MALDI-TOF (M+, C H N OS):
2
9
44 2
4. Conclusion
calcd: 468.32; found: 468.30.
3.2.12. Chromophore 2. Compound 9a (0.41 g, 1.00 mmol) Three organic NLO chromophores modified by the heteroatom
and the TCF acceptor (0.22 g, 1.10 mmol) were dissolved in group based on Dewar’ rules have been synthesized and system-
anhydrous ethanol. The reaction mixture was allowed to stir at atically characterized by NMR, MS, UV-Vis absorption spectra,
60 1C for 1–3 h and monitored by TLC. After the removal of the and single-crystal X-ray diffraction methods. The new com-
solvents, the residue was purified by column chromatography pounds were obtained in good yields which are ensured by
eluting with hexane/ethyl acetate. The desired chromophore the simple work-up procedures. Thermogravimetric analysis
was dark blue powder. (V
: VHexane = 1 : 5, 0.32 g, yield: 54%). showed good thermal and thermoxidative stabilities of the four
AcOEt
1
H NMR d = 8.00 (d, J = 15.7 Hz, 1H, CH), 7.53 (d, J = 4.0 Hz, 1H, chromophores. The effects of bathochromic and solvatochro-
CH), 7.33 (dd, J = 12.4, 9.1 Hz, 2H, CH), 7.17 (d, J = 9.1 Hz, 1H, mic behavior on the UV-Vis absorption were also investigated to
Ar–H), 7.01 (d, J = 4.0 Hz, 1H, CH), 6.65 (d, J = 15.7 Hz, 1H, CH), compare different electron donating abilities and polarizabil-
6
4
2
.23 (dd, J = 9.1, 2.1 Hz, 1H, Ar–H), 6.14 (d, J = 2.1 Hz, 1H, Ar–H), ities. Meanwhile, DFT calculations were carried out to analyze
.00 (d, J = 6.4 Hz, 2H, CH ), 3.27 (d, J = 7.5 Hz, 4H, CH ), the effects of different heteroatom matches on chromophores’
.66 (d, J = 6.4 Hz, 2H, CH ), 1.75 (s, 6H, CH ), 1.50–1.44 first-order hyperpolarizabilities. The calculated data showed
2
2
2
3
(
(
m, 4H, CH ), 1.32–1.20 (m, 4H, CH ), 0.90 (t, 3H, CH ), 0.84 good consistency with the analysis of UV-Vis spectra and
2 2 3
t, 6H, CH ).
electro-optical measurements. All the three chromophores
3
1
3
C NMR (101 MHz, acetone) d 176.06 (s), 173.71, 158.60, showed good solubility and comparability with the polymer
1
1
6
54.75, 150.14, 139.34, 137.78, 136.79, 129.64, 128.84, 126.17, which were crucial for making high quality EO devices. The
15.38, 112.03, 111.27, 110.47, 104.42, 97.46, 95.94, 94.89, doped polymer films containing chromophores C2–4 displayed
7.17, 49.91, 30.73, 24.91, 19.46, 18.80, 12.84. MALDI-TOF, higher r33 values than film-C1 (the doping concentration of C1
m/z: 594.31.
cannot exceed 20%) at the doping concentration of 25 wt%
HRMS (ESI) (M+, C H N OS): calcd: 594.30285; found: blended in APC. Film-C4 exhibited the highest r33 value owing
4
0
51 5
5
94.30291; found: 594.30287.
.2.13. Chromophore C3. The procedure for chromophore and steric hindrances in the donor and the bridge. All these
C3 was followed to prepare chromophore C2 from 9a as dark might be useful in designing other new NLO chromophores
to efficiently enhanced microscopic optical nonlinear activity
3
1
3
powder (48%). H NMR (400 MHz, CDCl ) d = 7.77 (d, J = 15.5 Hz, with modified heteroatom groups to optimize the molecular
1H, CH), 7.62 (d, J = 16.0 Hz, 1H, CH), 7.47 (d, J = 8.7 Hz, 1H, Ar–H), structure for achieving high performance.
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Paper
Acknowledgements
20 S. J. Benight, D. H. Bale, B. C. Olbricht and L. R. Dalton,
J. Mater. Chem., 2009, 19, 7466–7475.
We are grateful to the Directional Program of the Chinese
Academy of Sciences (KJCX2.YW.H02), Innovation Fund of
Chinese Academy of Sciences (CXJJ-11-M035) and National
Natural Science Foundation of China (No. 11104284 and
2
1 M. Q. He, T. M. Leslie and J. A. Sinicropi, Chem. Mater.,
002, 14, 4662–4668.
2 J. D. Luo, M. Haller, H. Ma, S. Liu, T. D. Kim, Y. Q. Tian,
B. Q. Chen, S. H. Jang, L. R. Dalton and A. K. Y. Jen, J. Phys.
Chem. B, 2004, 108, 8523–8530.
2
2
61101054) for the financial support.
23 B. C. Rinderspacher, J. Andzelm, A. Rawlett, J. Dougherty,
D. N. Beratan and W. Yang, J. Chem. Theory Comput., 2009,
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