Enhanced electro-optic activity from the triarylaminophenyl-based chromophores by introducing heteroatoms to the donor

Article abstract of DOI:10.1039/c5tc00723b

A series of chromophores T1-T3 based on the same thiophene π-conjugation and tricyanofuran acceptor (TCF) but with different heteroatoms in the triarylaminophenyl (TAA) donors have been synthesized and systematically investigated in this study. Density functional theory (DFT) was used to calculate the HOMO-LUMO energy gaps and first-order hyperpolarizability (β) of these chromophores. Moreover, to determine the redox properties of these chromophores, cyclic voltammetry (CV) experiments were performed. After introducing the heteroatom to the benzene ring of the TAA donor, reduced energy gaps of 1.28 and 0.84 eV were obtained for chromophores T2 and T3, respectively, which was much lower than for chromophore T1 (ΔE = 1.46 eV). These chromophores showed excellent thermal stability with their decomposition temperatures all above 280 °C. Compared with results obtained from the chromophore without the heteroatom (T1), these new chromophores show better intramolecular charge-transfer (ICT) absorption. Most importantly, the high molecular hyperpolarizability (β) of these chromophores can be effectively translated into large electro-optic (EO) coefficients (r₃₃) in the poled polymers. The electro-optic coefficient of poled films containing 25 wt% of these new chromophores doped in amorphous polycarbonate (APC) afforded values of 16, 58 and 95 pm V^-1 at 1310 nm for chromophores T1-T3, respectively. High r₃₃ values indicated that introducing heteroatom to the benzene ring of the TAA donor can efficiently improve the electron-donating ability, which improves the hyperpolarizability (β). The long-chain on the benzene ring of the TAA donor, acting as the isolation group, may 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. This journal is

Full text of DOI:10.1039/c5tc00723b

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Enhanced electro-optic activity from the
triarylaminophenyl-based chromophores by
introducing heteroatoms to the donor
Cite this:DOI: 10.1039/c5tc00723b
Yuhui Yang,ab Fenggang Liu,ab Haoran Wang,ab Shuhui Bo,a Jialei Liu,a Ling Qiu,a
Zhen Zhena and Xinhou Liu*a
A series of chromophores T1–T3 based on the same thiophene p-conjugation and tricyanofuran acceptor
(TCF) but with different heteroatoms in the triarylaminophenyl (TAA) donors have been synthesized and
systematically investigated in this study. Density functional theory (DFT) was used to calculate the HOMO–
LUMO energy gaps and first-order hyperpolarizability (b) of these chromophores. Moreover, to determine the
redox properties of these chromophores, cyclic voltammetry (CV) experiments were performed. After
introducing the heteroatom to the benzene ring of the TAA donor, reduced energy gaps of 1.28 and 0.84 eV
were obtained for chromophores T2 and T3, respectively, which was much lower than for chromophore T1
(DE = 1.46 eV). These chromophores showed excellent thermal stability with their decomposition
temperatures all above 280 1C. Compared with results obtained from the chromophore without the
heteroatom (T1), 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 the poled polymers. The electro-optic
coefficient of poled films containing 25 wt% of these new chromophores doped in amorphous poly-
carbonate (APC) afforded values of 16, 58 and 95 pm Vꢀ1 at 1310 nm for chromophores T1–T3,
respectively. High r33 values indicated that introducing heteroatom to the benzene ring of the TAA
donor can efficiently improve the electron-donating ability, which improves the hyperpolarizability (b).
The long-chain on the benzene ring of the TAA donor, acting as the isolation group, may reduce inter-
molecular 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.
Received 14th March 2015,
Accepted 18th April 2015
DOI: 10.1039/c5tc00723b
www.rsc.org/MaterialsC
One of the often encountered challenges in making highly
efficient EO materials is to develop nonlinear optical (NLO)
1. Introduction
Development of highly efficient organic electro-optic (EO) materials chromophores with large hyperpolarizabilities (b) and excellent
has been the focal point of recent research among many organic thermal stabilities.10–12 When considering the design of dipolar
materials research groups. This research is driven by the attractive p-conjugated chromophores as organic second-order nonlinear
potential of improving the efficiency of nonlinear optical (NLO) optical materials, optimization of electron donor and electron
materials while lowering production costs for applications, including acceptor substituents is necessary to obtain maximum non-
electro-optic modulation (the Pockels effect), optical rectification, linearity at the molecular level. Moreover, chemical, photo-
and terahertz radiation generation and detection.1–4 Over the chemical, and thermal stability in turn depend on the nature
past decade, considerable progress has been made on developing of these groups.
organic and polymeric electro-optic (EO) materials for applica-
In general, the dipolar second-order NLO chromophore consists
tions in high-speed and broadband information technology.5–9 of a p-conjugated bridge end-capped with strong electron donor
and acceptor substituents (D–p–A). Structure–property relation-
ships that have been established indicate that large b values of
the chromophores can be achieved by the careful modification
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
of the strength of donor and acceptor moieties, as well as the
100190, P. R. China. E-mail: xinhouliu@foxmail.com, xhliu@mail.ipc.ac.cn;
nature and length of the p-conjugated spacer.13 In order to
Fax: +86-01-62554670; Tel: +86-01-82543528
b University of Chinese Academy of Sciences, Beijing 100043, P. R. China
optimize molecular hyperpolarizability, much effort has been
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focused on the design and synthesis of optimized conjugated of TAA-based NLO chromophores can be achieved compared to an
bridge and electron acceptor structures.14–19 However, in the unsubstituted TAA chromophore.
newest generation of EO chromophores, the donor units have
remained relatively unchanged. Building upon this knowledge,
2. Results and discussion
2.1 Synthesis and characterization of chromophores
we envisaged that a further enhancement of molecular hyper-
polarizability could be achieved by introducing stronger electron
donors. Unfortunately, the relative inflexibility of the structural
make-up of the currently available chromophore electron donor
units has presented hurdles in the path to achieve durable
materials with enhanced electro-optic activity, possessing a
balance of all desirable qualities.
Synthesis of T1, T2 and T3 chromophores are depicted in
Schemes 1, 2 and 3, respectively. Unmodified chromophore
T1 was synthesized according to the literature.20 The modified
chromophores T2 and T3 were prepared according to different
strategies. Chromophore T2 was obtained in six steps starting
from the commercially available iodophenol (Scheme 2). The
A class of NLO chromophores with a triarylamine (TAA) donor
and a thiophene-bridge exhibits the highest thermal stability
alkylation of 4-iodophenol with bromobutane in the presence
of all reported chromophores,4,20–22 which seems to be the best
of K2CO3 gave 1b, and subsequent conversion to 2b, according
candidate for the active EO materials in a practical device.
However, compared to their alkyl counterparts, these triaryl-
to the modified Ullmann condensation reaction,28 which was in
amine (TAA) based chromophores usually exhibit reduced hyper-
polarizabilities due to the delocalization of the nitrogen lone pair
into the two phenyl rings, which do not connect with the bridge.
The r33 values derived from aryl D–p–A chromophores remained
between 10 and 20 pm Vꢀ1 (at 1310 nm), which are much lower
than those obtained from their alkyl D–p–A counterparts.20,23,24
This may be due to the poling efficiency of these materials,
which is strongly attenuated by large dipole–dipole and p–p
stacking interactions, and the charge transport between their
triarylamino moieties under high poling electric fields.18,25–27
This encouraged us to modify TAA-based chromophores to
improve their r33 values and thus overcome their drawbacks while
maintaining high thermal stability.
In this study, we have designed and synthesized three D–p–A
chromophores (T1–T3), as shown in Chart 1, for conducting a
systematic study of their structure–property relationships. All
designed model chromophores have the same thiophene-bridge
Scheme 1 Synthetic scheme for chromophore T1.
and strong electron acceptor, TCF. They showed great solubility
in common organic solvents, good compatibility with polymers,
1
and large EO activity in the poled films. H-NMR and 13C-NMR
analysis were carried out to demonstrate the preparation of these
chromophores. Thermal stability, photophysical properties, DFT
calculations and EO activities of these chromophores were
systematically studied and compared with the unmodified TAA
chromophore (Chart 1). Our results reveal that the introduction
of the heteroatom onto the benzene ring of the TAA donor can
dramatically affect the nonlinear optical properties. In a properly
designed system, significant enhancement in the hyperpolarizability
Chart 1 The structures of chromophores T1–T3.
Scheme 2 Synthetic scheme for chromophore T2.
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good solubility in common organic solvents such as dichloro-
methane, chloromethane and acetone.
2.2 Thermal analysis
NLO chromophores must be thermally stable enough to with-
stand high temperatures (4200 1C) encountered in electric
field poling and subsequent processing of chromophore/polymer
materials. Thermal properties of these chromophores were mea-
sured by thermogravimetric analysis (TGA). The decomposition
temperature (Td) of chromophores T1–T3 is all above 280 1C. All
these chromophores showed excellent stability. It should be
pointed out that the stronger donor strength provided by adding
the butoxy groups and dibutylamine did not compromise the
thermal stability of the resulting chromophores. The Td of these
chromophores was high enough for their application in EO device
preparation.
2.3 Optical properties
Scheme 3 Synthetic scheme for chromophore T3.
In order to reveal the effect of heteroatoms on the intramolecular
charge transfer (ICT) of dipolar chromophores, UV-vis absorption
spectra of three chromophores (c = 1 ꢁ 10ꢀ5 mol Lꢀ1) were
turn converted to give 3b by the Vilsmeier reaction.29 The measured in a series of aprotic solvents with different polarity so
aldehyde 3b was condensed with 2-thienyltriphenylphosphonate that the solvatochromic behavior of these chromophores could
bromide by Wittig condensation to gain 4b. As expected, after be investigated to explore the polarity of chromophores in a wide
introduction of the thiophene bridge by Wittig condensation, range of dielectric environments (Fig. 1). The spectrum data are
treatment of compound 4b with n-BuLi and DMF gave an aldehyde summarized in Table 1. The synthesized chromophores exhibited
5b. The target chromophore T2 was obtained via a Knoevenagel a similar p - p* intramolecular charge-transfer (ICT) absorption
condensation reaction of the aldehyde 5b with acceptor TCF in the band in the visible region. T1 shows a maximum absorption
presence of a catalytic amount of triethylamine.
(lmax) of 621 nm. With two additional butoxy groups on the
Chromophore T3 was obtained by six steps in a different way donor, the lmax of T2 red-shifted by 44 nm to 665 nm. With two
(Scheme 3). The crucial reaction involves the synthesis of the additional butylamine groups on the donor, the lmax of T3
donor group of compound 3c. To obtain compound 3c, we used further red-shifted to 730 nm. This result clearly shows that the
two different methods. Starting from commercially available butoxy groups and butylamine groups provide extra donating
triphenylamine, Vielsmier reaction was used for the preparation strength to the donor part, shifting the ICT absorption band of
of triphenylamine aldehyde. Furthermore, subsequent treatment the chromophore to lower energy. As shown in Fig. 1, the peak
with potassium iodide and potassium iodate in acetic acid led to wavelength of T1, T2 and T3 showed a bathochromic shift of 44,
intermediate 1d with a yield of 91%. Compound 3c was synthe- 48 and 72 nm from dioxane to chloroform, respectively. Chromo-
sized by Ullmann reaction from triphenylamine aldehyde iodide phore T3 displayed the largest solvatochromism compared with
using the powerful catalytic system, which includes the combi- T1 and T2. The resulting spectrum data confirmed that the
nation of CuI and 2-(2,6-dimethylphenylamino)-2-oxoacetic acid chromophore with butylamine groups on the donor was more
(DMPAO). In order to improve the yields, we also found another easily polarizable than the chromophore with the butoxy groups
useful procedure. Treatment of N,N-butyl aniline with potassium on the donor. These analyses implied that the butylamine groups
iodide and potassium iodate in acetic acid led to intermediate 1c on the donor improved electron-donating capability and increased
with a yield of 88% and subsequent conversion to 2c was carried polarizability.
out according to modified Ullmann condensation reaction. The
Traditional chromophores were easy to aggregate, which
reduction with DIBAL-H followed by acid hydrolysis converts the was beneficial for self-assembly. However, it was unfavorable
nitrile group on 2c into the corresponding aldehyde 3c with 86% for the application because these chromophores were easy to
yield. The aldehyde 3c was condensed with 2-thienyltriphenyl- form the antiparallel aggregation. This phenomenon can be
phosphonate bromide by Witting condensation to gain 4c. As demonstrated by absorption intensity, wavelength and shape
expected, after introduction of the thiophene bridge by Wittig of the absorption band in UV-vis spectra. To study the effect
condensation, treatment of compound 4b with n-BuLi and DMF of heteroatoms on attenuating chromophore aggregation, the
gave an aldehyde 5c. The target chromophore T3 was obtained via absorption of the chromophore was measured in CHCl3 at
Knoevenagel condensation reaction of the aldehyde 5c with accep- different concentrations (range from 0.005 mM to 0.08 mM).
tor TCF in the presence of a catalytic amount of triethylamine.
All the chromophores were fully characterized by H-NMR, centrations. As the concentration increased, no obvious
13C-NMR, and mass spectroscopy. These chromophores possess blue-shift or red-shift was found. Even at high concentration,
Fig. 2 shows the lmax of the chromophore at different con-
1
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Fig. 2 UV-vis absorption spectra of different concentrations of chromo-
phores T1–T3 in CHCl3.
2.4 Theoretical calculations
In order to model the ground state molecular geometries, the
HOMO–LUMO energy gaps and b values of these chromophores
were calculated. DFT calculations were carried out at the hybrid
B3LYP level by employing the split valence 6-311g(d,p) basis
set.30–32 The data obtained from DFT calculations are summar-
ized in Table 2.
Fig. 1 UV-vis absorption spectra in different solvents of chromophores
T1–T3 (c = 1 ꢁ 10ꢀ5 mol Lꢀ1).
Table 1 Summary of thermal and optical properties and EO coefficients
of chromophores T1–T3
The frontier molecular orbitals are often used to character-
ize the chemical reactivity and kinetic stability of a molecule
and to obtain qualitative information about the optical and
electrical properties.32–34 Moreover, the HOMO–LUMO energy
gap is also used to understand the charge transfer interaction
occurring in a chromophore molecule.34–36 In the case of these
chromophores, Fig. 3 represents the frontier molecular orbitals
of chromophores T1, T2 and T3. According to Fig. 3, it is clear
that the electronic distribution of HOMO is delocalized over the
thiophene linkage and benzene ring, whereas LUMO is mainly
constituted by the acceptor moieties.34
Chromophore Td (1C) lmax (nm) lmax (nm) Dld (nm) r33 (pm Vꢀ1
)
a
b
c
e
T1
T2
T3
301
310
289
621
665
730
577
617
658
44
48
72
16
58
95
a
Td was determined by an onset point, and measured by TGA under
nitrogen at a heating rate of 10 1C minꢀ1
CHCl3.
were measured at the wavelength of 1310 nm.
.
lmax was measured in
b
c
d
e
l
max was measured in dioxane. Dl = lmax ꢀ lmax
. r33 values
no obvious characteristic absorption band of aggregation was
observed.
The HOMO and LUMO energies were calculated by DFT
calculations, as shown in Fig. 3 and Table 2. For chromophore
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Table 2 Data from DFT calculations
Chromophores EHOMO/eV ELUMO/eV DEa/eV DEb/eV bmaxc/10ꢀ30 esu
T1
T2
T3
ꢀ5.49
ꢀ5.23
ꢀ4.97
ꢀ3.29
ꢀ3.36
ꢀ3.29
2.20
1.87
1.68
1.46
1.28
0.84
1162
1642
2428
a
b
DE = ELUMO + EHOMO
.
Results was calculated by DFT. Results from
c
cyclic voltammetry experiment. b values were calculated using gaus-
sian03 at B3LYP/6-311g**(d,p) level and the direction of the maximum
value is directed along the charge transfer axis of the chromophores.
Fig. 4 The optimized structure of chromophores T1–T3.
This result corresponds well with the conclusion of UV-vis spectra
analysis.
2.5 Electrochemical properties
To determine the redox properties of these chromophores, cyclic
voltammetry (CV) experiments were performed 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 hexafluorophosphate
(TBAPF) in acetonitrile was used as the electrolyte. As shown
in Fig. 5, chromophores T1, T2 and T3 exhibited one quasi-
Fig. 3 The frontier molecular orbitals of chromophores T1–T3.
T3, the new donor structure narrows the energy gap between
the HOMO and LUMO with a DE value of 1.68 eV. The DE value
of T2 is 1.87 eV. By contrast, the DE value of T1 is 2.20 eV. As the
shown in the data, the LUMO energy of the three chromophores
has no obvious difference while the HOMO energy varies
widely. The HOMO energy of chromophore T3 is 0.52 eV larger
than that of T1 (ꢀ4.97 eV vs. ꢀ5.49 eV) and is 0.26 eV larger
than that of T2 (ꢀ4.97 eV vs. ꢀ5.23 eV). This may result from the
fact that the heteroatom on the donor structure influences
the HOMO energy to a greater extent than the LUMO energy.
The HOMO–LUMO gap of chromophore T3 (1.68 eV) is the
smallest and that of T1 (2.20 eV) is the largest. This result
indicates that T3 may exhibit better ICT and NLO properties
than T2 and T1. As reported34, the narrower energy gap
indicated easier charge transfer and greater nonlinearity. Chromo-
phore T3 showed a lower optical gap, which indicates that T3
may exhibit better ICT and NLO properties than the other two
chromophores.
reversible oxidative wave with a half-wave potential, E1/2
=
0.5(Eox + Ered), at about 0.75, 0.62 and 0.15 V, respectively.
Chromophore T3 showed two oxidation waves: the first one is a
reversible wave with half-wave potentials at about 0.15 V corre-
sponding to the donor group. Moreover, chromophores T1–T3
had an irreversible reduction wave corresponding to the accep-
tor moieties at Ered = ꢀ0.71, ꢀ0.66 and ꢀ0.69 V (vs. Ag/AgNO3).
It showed an energy gap (DE) value of 1.46, 1.28 and 0.84 eV for
chromophores T1, T2 and T3, respectively. The HOMO and
LUMO levels of these new chromophores were calculated from
their corresponding oxidation and reduction potentials. The
HOMO levels of T1, T2 and T3 were estimated to be ꢀ5.15,
ꢀ5.02 and ꢀ4.55 eV, respectively, while the corresponding
LUMO level of T1–T3 only showed a slight change at ꢀ3.69,
Furthermore, the theoretical microscopic zero-frequency
(static) molecular first hyperpolarizability, b, was calculated
by Gaussian 03. As reported earlier, the b values have been
calculated at the 6-311g**(d,p) level in vacuum.37 From this, the
scalar quantity of b can be computed from the x, y, and z
components according to the following equation:
2
2
2
1=2
b ¼ ðbx þ by þ bz Þ
(1)
ꢀ
ꢁ
P
1
where bi ¼ biii
þ
bijj þ bjij þ bjji
;
i; j 2 ðx; y; zÞ.
3 iaj
Due to the stronger electron-donating ability and better
conjugated system (Fig. 4), chromophore T3 showed a much
larger b value than the other two chromophores, while the
chromophore T1 exhibited the smallest b value (1162 ꢁ 10ꢀ30 esu).
Fig. 5 Cyclic voltammograms of chromophores T1–T3.
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Table 3 Summary of EO coefficients of chromophores T1–T3 at different
densities
ꢀ3.74 and ꢀ3.71 eV. This result corresponded with the conclu-
sion of UV-vis spectra analysis and the DFT results.
r33 (pm Vꢀ1
a
)
2.6 Electric field poling and EO property measurements
Wt%
T1
T2
T3
For studying EO properties derived from these chromophores, a
series of guest–host polymers were generated by formulating
the chromophores into amorphous polycarbonate (APC) using
dibromomethane as the solvent. The resulting solutions were
filtered through a 0.2 mm PTFE filter and spin-coated onto
indium tin oxide (ITO) glass substrates. Films of doped polymers
were baked in a vacuum oven at 80 1C overnight to ensure the
removal of the residual solvent. Corona poling process was
carried out at a temperature of 10 1C above the glass transition
temperature (Tg) of the polymer. r33 values were measured
using the Teng–Man simple reflection technique at the wave-
length of 1310 nm using a carefully selected thin ITO electrode
with low reflectivity and good transparency in order to minimize
the contribution from multiple reflections.38
10
15
20
25
30
7
10
14
16
—
19
30
45
58
53
28
45
69
95
92
a
r33 values were measured at the wavelength of 1310 nm.
The r33 values of films containing chromophores T1 (film-A),
T2 (film-B), T3 (film-C) were measured at different loading
densities, as shown in Table 3 and Fig. 6. For chromophore
T1, the r33 values were gradually improved from 7 pm Vꢀ1
(10 wt%) to 16 pm Vꢀ1 (25 wt%), as the concentration of
chromophore increased; moreover, a similar trend of enhance-
ment was also observed for chromophore T2 whose r33 values
increased from 19 pm Vꢀ1 (10 wt%) to 58 pm Vꢀ1 (25 wt%).
For chromophore T3, the r33 value increased from 28 pm Vꢀ1
(10 wt%) to 95 pm Vꢀ1 (25 wt%). When the loading density
increased from 25 wt% to 30 wt%, film-A with the loading
30 wt% of T1 presents an obvious phase separation and film-B
and film-C displayed downward trends.
For dipolar chromophores in an electric poling field, the
electro-optic coefficient (r33) in the direction of the applied field
is related to the molecular first hyperpolarizability and is
described as follows:
r33 = 2Nf (o)bhcos3 yi/n4
(2)
Through the outcome above, we can obviously see that film-
B containing chromophore T2 has nearly a 4 times higher r33
value and film-C containing chromophore T3 has nearly a 6
times higher r33 value than film-A containing chromophore T1,
which illustrates that the increased donor strength of the
chromophores significantly increases their macroscopic EO
activities. This result can be explained as follows: when the
concentration of chromophore in APC is low, the intermolecular
dipolar interactions are relatively weak. The film-C/APC achieved
the largest r33 value probably due to its largest b and good
polarizability. With relatively low b, the r33 value of film-B/APC
was smaller than that of film-C/APC. The drop in EO activity
from film-B/APC to film-A/APC is mainly associated with the
where r33 is the EO coefficient of the poled polymer, N repre-
sents the aligned chromophore number density and f (o)
denotes the Lorentz–Onsager local field factors. The term
hcos3 yi is the orientationally averaged acentric order parameter
characterizing the degree of the noncentrosymmetric align-
ment of the chromophore in the material and n represents
the refractive index.39 Before poling, there is no EO activity
in the EO material and the chromophores in the material
are thermally randomized. Electrical field induced poling
proceeded to induce acentric ordering of the chromophores.
Realization of large electro-optic activity for dipolar organic
chromophore-containing materials requires the simultaneous
optimization of chromophore first hyperpolarizability (b), acentric
order hcos3 yi, and number density (N). At low concentration, the
electro-optic activity increases with chromophore density, dipole
moment and the strength of the electric poling field. However,
when the concentrations of chromophores increased to a certain
extent, the N and hcos3 yi terms are no longer independent factors.
At this point
hcos3 yi = (mF/5kT)[1 ꢀ L2(W/kT)]
(3)
where k is the Boltzmann constant and T is the Kelvin (poling)
temperature. F = [f (0)EP] where Ep is the electric poling field.
L is 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 molecules. Therefore, when the intermolecular electro-
static interactions are neglected, the electro-optic coefficient
(r33) should increase linearly with chromophore density, dipole
moment, first hyperpolarizability and strength of the electric
poling field.
Fig. 6 EO coefficients of NLO thin films as a function of chromophore
loading densities.
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lowest b of chromophore A in the polar polymer matrix and the Saturated NH4Cl was added and the resulting mixture was
inefficient poling because of its lower polarizability than the extracted with EtOAc (20 ꢁ 3 mL). The combined extracts were
other two chromophores.
washed with water and dried over MgSO4. After filtration and
As the chromophore loading densities increased, the effect removal of the solvent under vacuum, the residue was purified
of inter-chromophore dipole–dipole interaction becomes stronger. by column chromatography on a silica gel (hexane/acetone, v/v,
Chromophore T2 with two butoxy groups on the aromatic amino 10/1) to obtain a yellow solid (1.22 g, 69%).
donor was designed to optimize the shape of the chromophore.
1H NMR (400 MHz, CDCl3) d 7.32 (d, J = 8.5 Hz, 2H), 7.25–
The butoxy group was also used to increase the solubility of the 7.23 (m, 4H), 7.16–7.23 (m, 5H), 7.13–7.08 (m, 5H), 7.02–7.08
chromophore. In a similar way, chromophore T3 with the butyl- (m, 2H), 6.87 (d, J = 16.2 Hz, 1H).
amide group can also control the shape of the chromophore and
13C NMR (100 MHz, CDCl3): d 120.18, 123.06, 123.52, 123.84,
increase solubility as well. As reported,40 the introduction of some 124.52, 125.48, 127.14, 127.55, 127.92, 129.25, 131.06, 143.27,
isolation groups to the chromophore moieties to further control 147.20, 147.52.
the shape of the chromophore could be an efficient approach to
minimize interactions between the chromophores. Therefore, 353.57.
MALDI-TOF: m/z calcd for C24H19NS: 353.48 [M]+; found:
when the chromophore loading increased, the introduction
3.2.2 Synthesis of compound 3a. To a solution of com-
of groups attached to the donor system can make inter- pound 2a (1.06 g, 3.00 mmol) in dry THF, (20 mL) a 2.4 M
chromophore electrostatic interactions less favorable and thus solution of n-BuLi in hexane (1.9 mL, 4.50 mmol) was added
increase the macroscopic EO activity. Thus, film-C presented dropwise at ꢀ78 1C under N2. Furthermore, this mixture was
the largest r33 values and film-A containing T1 showed the stirred for 1 h and dry DMF (0.28 mL, 3.60 mmol) was added.
smallest r33 values. This corresponds well with the results of the The resulting solution was stirred for another 1 h at ꢀ78 1C and
solvatochromism study, DFT calculations and redox properties then allowed to warm up to room temperature. The reaction
of chromophores.
was quenched with water. THF was removed by evaporation.
The residue was extracted using CH2Cl2 (3 ꢁ 30 mL). The
organic layer was dried by MgSO4 and concentrated in vacuo.
The residue was purified by column chromatography on a silica
gel (hexane/acetone, v/v, 10/1) to obtain a red solid (0.94 g, 82%).
1H NMR (400 MHz, CDCl3) d 9.82 (s, 1H), 7.63 (d, J = 3.9 Hz,
1H), 7.34 (d, J = 8.5 Hz, 2H), 7.30–7.24 (m, 4H), 7.11 (d, J = 7.9 Hz,
4H), 7.10–7.04 (m, 5H), 7.02 (d, J = 8.6 Hz, 2H).
3. Experimental
3.1 Materials and instrumentation
1H NMR spectra were obtained with an Advance Bruker 400
(400 MHZ) NMR spectrometer (tetramethylsilane as internal
reference). MS spectra were obtained on a MALDI-TOF (Matrix
Assisted Laser Desorption/Ionization Time of Flight) on BIFLEX
III (Bruker Inc.) spectrometer. UV-vis spectra were obtained
on a Cary 5000 photo spectrometer. TGA was determined by
TA5000-2950TGA (TA Co) with a heating rate of 10 1C minꢀ1
under the protection of nitrogen. Cyclic voltammetry (CV)
experiments were performed on a BAS CV-50 W voltammetric
analyzer using a conventional three-electrode cell with Pt metal
as the working electrode, Pt gauze as the counter-electrode, and
13C NMR (100 MHz, CDCl3) d 177.93, 148.86, 144.33, 142.98,
136.86, 132.86, 128.31, 125.02, 123.66, 121.56, 120.74, 119.35,
118.44, 114.58, 114.55.
MALDI-TOF: m/z calcd for C25H19NOS: 381.49 [M]+; found:
381.47.
3.2.3 Synthesis of chromophore T1. A mixture of aldehydic
bridge 5 (0.38 g, 1.00 mmol) and acceptor 6 (0.22 g, 1.10 mmol)
in chloroform (30 mL) was stirred at 62 1C for 4 h in the
presence of a catalytic amount of triethylamine. After removal
of the solvent, the residue was purified by column chromato-
graphy on a silica gel (hexane/acetone, v/v, 5 : 1) and a dark
solid was obtained (0.21 g, 38%).
Ag/AgNO3 as the reference electrode at a scan rate of 50 mV sꢀ1
.
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF) in
acetonitrile was used as the electrolyte. Ferrocene/ferrocenium
(Fc/Fc+) couple was used as an internal reference. mental
analysis was performed on a Flash EA 1112 elemental analyzer.
All chemicals, commercially available, are used without further
purification unless stated. DMF, POCl3 and THF were freshly
distilled prior to use. 2-Dicyanomethylene-3-cyano-4-methyl-
2,5-dihydrofuran (TCF) acceptor was prepared according to
the literature.41
1H NMR (400 MHz, CDCl3) d 7.78 (d, J = 15.7 Hz, 1H), 7.41–
7.33 (m, 3H), 7.29 (dd, J = 14.6, 6.8 Hz, 5H), 7.10 (dd, J = 24.2,
11.3 Hz, 8H), 7.03 (d, J = 8.0 Hz, 2H), 6.61 (d, J = 15.7 Hz, 1H),
1.76 (s, 6H).
13C NMR (100 MHz, CDCl3) d 171.24, 168.56, 148.52, 144.68,
142.78, 134.87, 133.74, 132.62, 129.32, 125.17, 123.63, 123.29,
120.95, 119.46, 117.91, 114.11, 108.13, 107.58, 106.83, 106.40,
92.72, 25.28, 22.19.
MALDI-TOF: m/z calcd for C36H26N4OS: 562.68 [M]+; found:
562.49.
3.2 Synthesis
Compound 1a was synthesized according to the literature.20
3.2.1 Synthesis of compound 2a. NaH (1.20 g, 0.05 mol)
Anal. calcd (%) for C36H26N4OS: C, 76.84; H, 4.66; N, 9.96;
was added to a mixture of compound 1a (1.37 g, 5 mmol) and found: C, 76.91; H, 4.62; N, 10.01.
2-thienyl triphenylphosphonate bromide (2.64 g, 6 mmol) in
3.2.4 Synthesis of compound 1b. K2CO3 (13.80 g, 0.10 mol)
dry THF (20 mL) at room temperature under N2. The mixture was added to a stirred mixture of 4-iodophenol (12.25 g, 0.055 mol),
turned yellow and was stirred at room temperature for 24 h. 1-bromobutane (8.22 g, 0.06 mol), and DMF (100 mL) under argon.
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The reaction mixture was refluxed for 24 h and cooled to room 6.45 (d, J = 11.9 Hz, 1H), 3.92 (m, 4H), 1.81–1.70 (m, 4H), 1.49
temperature. The resulting mixture was filtered, and most solvent (m, 4H), 0.96 (m, 8H).
of the filtrate was evaporated. The obtained oil was dissolved in
13C NMR (100 MHz, CDCl3) d 151.40, 144.19, 139.32, 136.32,
chloroform, washed with water, dried over MgSO4, evaporated, 124.83, 124.04, 123.19, 122.73, 122.39, 120.70, 119.17, 116.10,
and purified with column chromatography on a silica gel (hexane/ 114.89, 63.76, 27.18, 14.90, 9.52.
acetone, v/v, 10 : 1) to yield a colorless oil (13.05 g, 86%).
1H NMR (400 MHz, CDCl3) d 7.52 (d, J = 8.3 Hz, 2H), 6.65 (d, 497.57.
MALDI-TOF: m/z calcd for C32H35NO2S: 497.69 [M]+; found:
J = 8.4 Hz, 2H), 3.89 (t, J = 6.5 Hz, 2H), 1.81–1.67 (m, 2H), 1.47
(dd, J = 13.4, 6.2 Hz, 2H), 1.02–0.91 (m, 3H).
3.2.8 Synthesis of compound 5b. In a similar manner to
that described above, compound 5b was synthesized from 4b
13C NMR (100 MHz, CDCl3) d 158.96, 138.07, 116.88, 82.37, and obtained as a red oil (84%).
67.72, 31.15, 19.17, 13.81.
1H NMR (400 MHz, CDCl3) d 9.81 (s, 1H), 7.62 (d, J = 3.8 Hz,
MALDI-TOF: m/z calcd for C10H13OI: 276.11 [M]+; found: 1H), 7.28 (t, J = 5.6 Hz, 2H), 7.11–6.99 (m, 6H), 6.85 (dd, J = 12.8,
276.16.
8.6 Hz, 7H), 3.94 (m, 4H), 1.82–1.71 (m, 4H), 1.55–1.43 (m, 4H),
3.2.5 Synthesis of compound 2b. A mixture of 1b (8.28 g, 1.03–0.94 (m, 4H).
0.03 mol), phenylamine (1.25 g, 0.013 mmol), CuCl (0.108 g, 0.
13C NMR (100 MHz, CDCl3) d 177.87, 151.78, 149.31, 145.33,
001 mmol), phenanthroline (0.21 g, 0.001 mmol), KOH (13.2 g, 136.50, 135.78, 132.88, 128.74, 123.56, 123.20, 122.77, 121.12,
0.24 mol), and toluene (70 mL) was refluxed for 24 h. After 115.26, 113.40, 111.20, 63.76, 52.84, 27.14, 14.97, 9.50.
cooling, the resulting mixture was poured into plenty of stirred
water and extracted with chloroform. The obtained organic 525.83.
MALDI-TOF: m/z calcd for C33H35NO3S: 525.70 [M]+; found:
phase was washed several times with water, dried over magne-
3.2.9 Synthesis of compound chromophore T2. In a similar
sium sulfate, evaporated, and purified with column chromato- manner to that described above, chromophore T2 was synthe-
graphy on a silica gel (hexane/acetone, v/v, 8 : 1) to yield sized from 5b and obtained as a dark solid (48%).
yellowish oil. (3.08 g, 61%).
1H NMR (400 MHz, Acetone) d 8.15 (d, J = 15.8 Hz, 1H), 7.68
1H NMR (400 MHz, CDCl3) d 7.15 (d, J = 7.6 Hz, 2H), 7.02 (d, J = 3.6 Hz, 1H), 7.47 (d, J = 8.5 Hz, 2H), 7.36–7.18 (m, 3H),
(d, J = 8.1 Hz, 2H), 6.93 (d, J = 7.6 Hz, 2H), 6.82 (m, 5H), 3.92 7.10 (d, J = 8.6 Hz, 4H), 6.94 (d, J = 8.6 Hz, 4H), 6.83 (m, 3H),
(m, 2H), 1.81–1.70 (m, 2H), 1.55–1.43 (m, 2H), 0.97 (m, 3H).
4.00 (m, 4H), 1.89 (m, 4H), 1.73 (s, 6H), 1.50 (m, 4H), 0.98
13C NMR (100 MHz, CDCl3) d 155.36, 148.92, 141.06, 128.95, (m, 6H).
126.46, 120.91, 120.50, 67.97, 31.45, 19.36, 13.96.
13C NMR (100 MHz, Acetone) d 180.24, 172.27, 172.07,
MALDI-TOF: m/z calcd for C26H31NO2: 389.53 [M]+; found: 169.91, 152.16, 148.19, 145.58, 135.56, 135.40, 134.15, 133.15,
389.71.
128.89, 123.89, 123.47, 123.38, 123.08, 114.52, 113.61, 111.24,
3.2.6 Synthesis of compound 3b. Phosphorous oxychloride 108.44, 107.98, 107.33, 107.14, 106.63, 106.48, 105.03, 100.14,
(0.7 mL, 0.007 mol) was added dropwise to stirred DMF 96.46, 93.84, 93.34, 27.00, 21.12, 18.89, 14.74, 9.22, 8.91.
(1.2 mL, 0.014 mol) at 0 1C. The mixture was stirred for 1 h at
0 1C and additionally stirred at room temperature for 1 h. After 706.72.
MALDI-TOF: m/z calcd for C44H42N4O3S: 706.89 [M]+; found:
the addition of compound 2b (2.92 g, 0.007 mol) solution in
Anal. calcd (%) for C44H42N4O3S: C, 74.76; H, 5.99; N, 7.93;
dichloroethane (20 mL), the mixture was stirred at 90 1C for 2 h. found: C, 74.87; H, 5.94; N, 7.97.
After cooling, the solution was poured into cold water. The
3.2.10 Compound 1d was synthesized according to the
resulting mixture was neutralized to pH 7 with 2 M NaOH literature42 1H NMR (400 MHz, CDCl3) d 9.85 (s, 1H), 7.71
.
aqueous solution and extracted with chloroform. The extract (d, J = 8.6 Hz, 2H), 7.63 (d, J = 8.6 Hz, 4H), 7.05 (d, J = 8.6 Hz, 2H),
was washed with plenty of brine and water. The organic extracts 6.89 (d, J = 8.6 Hz, 4H).
were dried over MgSO4, evaporated, and purified with column
chromatography on a silica gel (hexane/acetone, v/v, 10 : 1) to 131.36, 130.36, 127.63, 120.78.
13C NMR (100 MHz, CDCl3) d 190.37, 152.21, 145.73, 138.88,
yield a yellowish oil. (2.60 g, 89%).
3.2.11 Synthesis of compound 1c. To a mixture of glacial
1H NMR (400 MHz, CDCl3) d 9.74 (s, 1H), 7.62 (d, J = 8.2 Hz, acetic acid (50 mL) and water (10 mL), concentrated sulfuric
2H), 7.11 (d, J = 8.3 Hz, 4H), 6.86 (m, 6H), 3.95 (m, 4H), 1.76 acid (1.5 mL) was added dropwise, and then N,N-dibutylaniline
(m, 4H), 1.56–1.42 (m, 4H), 0.98 (m, 6H).
13C NMR (100 MHz, CDCl3) d 190.25, 156.97, 154.16, 138.62, (2.5 g, 0.011 mol) were added. The mixture was maintained
131.43, 128.06, 127.70, 116.70, 67.96, 31.35, 19.28, 13.88. at 40 1C for 8 hours then poured into a saturated solution of
MALDI-TOF: m/z calcd for C27H31NO3: 417.54 [M]+; found: sodium bicarbonate (200 mL) and extracted with ethyl acetate
(10.3 g, 0.05 mol), iodine (5.2 g, 0.02 mol) and sodium periodate
417.56.
(100 mL); the organic layer was separated and washed 3 times
3.2.7 Synthesis of compound 4b. In a similar manner to with water, dried over anhydrous magnesium sulfate, and purified
that described above, compound 4b was synthesized from 3b as with column chromatography on a silica gel (hexane/acetone, v/v,
a yellowish oil was obtained (72%).
50 : 1) to yield a yellowish liquid. (14.3 g, 86%).
1H NMR (400 MHz, CDCl3) d 7.25 (d, J = 8.5 Hz, 2H), 7.11 (d,
1H NMR (400 MHz, CDCl3) d 7.40 (d, J = 8.0 Hz, 2H), 6.40
J = 4.7 Hz, 1H), 7.03 (d, J = 7.7 Hz, 6H), 6.99–6.95 (m, 1H), 6.87 (d, J = 8.0 Hz, 2H), 3.21 (m, 4H), 1.59–1.45 (m, 4H), 1.39–1.25
(d, J = 8.4 Hz, 3H), 6.81 (d, J = 8.5 Hz, 4H), 6.55 (d, J = 11.9 Hz, 1H), (m, 4H), 0.94 (m, 6H).
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13C NMR (100 MHz, CDCl3) d 147.73, 137.59, 114.21, 50.70,
1H NMR (400 MHz, Acetone) d 7.17–7.12 (m, 3H), 7.00
(m, 2H), 6.92 (d, J = 3.1 Hz, 2H), 6.88–6.84 (m, 2H), 6.82–6.78
29.12, 20.29, 13.95.
MALDI-TOF: m/z calcd for C14H22NI: 331.24 [M]+; found: (m, 1H), 6.55 (m, 6H), 6.43 (d, J = 11.6 Hz, 1H), 3.18 (m, 8H),
331.43.
3.2.12 synthesis of compound 2c. A mixture of 1c (9.93 g,
1.51–1.37 (m, 8H), 1.24 (m, 8H), 0.82 (m, 12H).
13C NMR (100 MHz, Acetone) d 146.36, 144.62, 141.21,
0.03 mol), 4-aminobenzonitrile (1.53 g, 0.013 mmol), CuCl 130.33, 130.13, 128.53, 128.40, 127.37, 125.87, 124.15, 121.50,
(0.108 g, 0. 001 mmol), phenanthroline (0.21 g, 0.001 mmol), 118.96, 118.11, 113.60, 51.57, 20.99, 14.39.
KOH (13.2 g, 0.24 mol), and toluene (70 mL) was refluxed for
24 h. After cooling, the resulting mixture was poured into plenty 607.76.
MALDI-TOF: m/z calcd for C40H53N3S: 607.93 [M]+; found:
of stirred water and extracted with chloroform. The organic
3.2.15 Synthesis of compound 5c. In a similar manner to
phase obtained was washed several times with water, dried over that described above, compound 4b was synthesized from 3b as
magnesium sulfate, evaporated, and purified with column a red liquid (84%).
chromatography on a silica gel (hexane/acetone, v/v, 8 : 1) to
yield a yellowish oil. (4.23 g, 62%).
1H NMR (400 MHz, Acetone) d 9.78 (s, 1H), 7.69 (d, J = 3.8 Hz,
1H), 7.15 (d, J = 3.8 Hz, 1H), 7.08–6.94 (m, 6H), 6.71–6.49
1H NMR (400 MHz, CDCl3) d 7.22 (d, J = 8.6 Hz, 2H), 6.94 (m, 8H), 3.59–2.88 (m, 8H), 1.58–1.42 (m, 8H), 1.30 (m, 8H),
(d, J = 8.4 Hz, 4H), 6.65 (d, J = 8.3 Hz, 2H), 6.51 (d, J = 8.5 Hz, 0.86 (m, 12H).
4H), 3.16 (m, 8H), 1.49 (m, 8H), 1.33–1.20 (m, 8H), 0.88 (t, J =
7.3 Hz, 12H).
13C NMR (100 MHz, CDCl3) d 188.05, 154.88, 150.79, 147.37,
141.92, 139.10, 134.15, 132.93, 117.84, 25.28, 18.70.
MALDI-TOF: m/z calcd for C41H53N3OS: 635.94 [M]+; found:
635.96.
13C NMR (100 MHz, CDCl3) d 146.31, 133.59, 132.94, 128.13,
125.22, 115.62, 112.49, 50.96, 29.49, 28.78, 20.39, 13.90.
MALDI-TOF: m/z calcd for C35H48N4: 524.78 [M]+; found:
524.49.
3.2.16 Synthesis of chromophore T3. In a similar manner
to that described above, chromophore T3 was synthesized from
4b and acceptor 5, as a dark solid (40%).
3.2.13 Synthesis of compound 3c
Approach 1. A mixture of CuI (0.38 g, 0.002 mol), DMPAO
1H NMR (400 MHz, Acetone) d 8.15 (d, J = 15.8 Hz, 1H), 7.67
(0.76 g, 0.004 mol), aryl 1d (5.27 g, 0.01 mol), K3PO4 (8.49 g, (d, J = 3.8 Hz, 1H), 7.41 (d, J = 8.4 Hz, 2H), 7.23 (m, 3H), 7.03 (d,
0.04 mol) was placed under N2. The tube was evacuated and J = 8.5 Hz, 4H), 6.82 (d, J = 15.9 Hz, 1H), 6.72 (dd, J = 16.0,
backfilled with argon, and then dibutylamine (3.88 g, 0.03 mol) 8.6 Hz, 6H), 3.37–3.26 (m, 8H), 1.87 (d, J = 14.6 Hz, 6H), 1.65–
and DMSO (40 mL) were added. The reaction mixture was stirred 1.51 (m, 8H), 1.46–1.28 (m, 10H), 0.96 (t, J = 7.3 Hz, 11H).
at 100 1C for 48 h. After aryl halide was consumed, water was
13C NMR (100 MHz, Acetone) d 176.45, 174.21, 153.17,
added and the mixture was extracted with ethyl acetate. The 150.82, 145.82, 139.66, 138.01, 137.87, 134.75, 133.71, 130.51,
organic layer was dried over MgSO4 and evaporated. The residue 128.45, 128.04, 127.76, 127.47, 125.85, 116.76, 116.68, 112.57,
was purified by column chromatography on a silica gel (hexane/ 112.36, 111.73, 110.85, 98.11, 97.22, 54.37, 50.59, 25.37,
acetone, v/v, 50: 1) and a red oil was obtained (2.26 g, 43%).
20.03, 13.34.
MALDI-TOF: m/z calcd for C52H60N6OS: 817.14 [M]+; found:
Approach 2. To a solution of 2c (2.63 g, 5 mmol) in dry 817.26.
toluene (30 mL), 1 M solution of diisobutylaluminum hydride
Anal. calcd (%) for C52H60N6OS: C, 76.43; H, 7.40; N, 10.28;
in hexane (10 mL, 10 mmol) was added with a syringe at ꢀ78 1C found: C, 76.54; H, 7.36; N, 10.33.
in a dry ice/acetone bath. The reaction was stirred at ꢀ78 1C for
2 h. After the mixture was warmed to room temperature, NH4Cl
solution was added to quench the reaction. The mixture was
stirred for 1 h to complete hydrolysis. The organic solvent was
4. Conclusion
collected and removed in vacuo. The residue was purified by In this research, a series of NLO chromophores based on a
column chromatography on a silica gel (hexane/acetone, v/v, modified TAA donor have been synthesized and systematically
10/1) to yield a red oil (2.19 g, 83%).
investigated by NMR, MS and UV-vis absorption spectroscopy.
1H NMR (400 MHz, CDCl3) d 9.62 (s, 1H), 7.50 (d, J = 8.5 Hz, The energy gap between ground state and excited state together
2H), 6.96 (d, J = 8.5 Hz, 4H), 6.72 (d, J = 8.4 Hz, 2H), 6.52 with molecular nonlinearity was studied by UV-vis absorption
(d, J = 8.7 Hz, 4H), 3.22–3.11 (m, 8H), 1.58–1.41 (m, 8H), 1.36– spectroscopy, DFT calculations and CV measurements. Theore-
1.20 (m, 8H), 0.88 (m, 12H).
tical and experimental investigations suggest that introducing
13C NMR (100 MHz, CDCl3) d 190.11, 146.31, 133.73, 131.47, the heteroatom to the benzene ring of the TAA donor can
131.36, 128.57, 128.16, 126.47, 125.82, 124.59, 117.42, 115.33, dramatically affect their nonlinear optical properties. The poling
112.40, 50.80, 29.48, 20.38, 14.02.
results of guest–host EO polymers with 25 wt% of these chromo-
MALDI-TOF: m/z calcd for C35H49N3O:527.78 [M]+; found: phores showed that polymers with chromophores T1–T3 afforded
527.79.
the large r33 values of 16, 58 and 95 pm Vꢀ1, respectively. The
3.2.14 Synthesis of compound 4c. In a similar manner to derived chromophores exhibit good thermal stability and large
that described above, compound 4c was synthesized from 3c as molecular hyperpolarizabilities, which can be effectively trans-
a red liquid (67%).
lated into very large electro-optic coefficients in poled polymers.
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19 S. Liu, M. A. Haller, H. Ma, L. R. Dalton, S. H. Jang and
A. K. Y. Jen, Adv. Mater., 2003, 15, 603–607.
We believe that these new chromophores can be used in exploring
high-performance organic EO and photorefractive materials
wherein both thermal stability and optical nonlinearity are of
equal importance.
20 Y. J. Cheng, J. D. Luo, S. Hau, D. H. Bale, T. D. Kim, Z. W.
Shi, D. B. Lao, N. M. Tucker, Y. Q. Tian, L. R. Dalton,
P. J. Reid and A. K. Y. Jen, Chem. Mater., 2007, 19, 1154–1163.
21 S. M. Budy, S. Suresh, B. K. Spraul and D. W. Smith, Jr.,
J. Phys. Chem. C, 2008, 112, 8099–8104.
Acknowledgements
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D. W. Smith, Tetrahedron Lett., 2005, 46, 3913–3916.
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We are grateful to the National Nature Science Foundation of
China (no. 61101054) for financial support.
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