The influence of different donor/acceptor matches on chromophore's nonlinear optical activity

Article abstract of DOI:10.1016/j.dyepig.2016.04.023

In this work, a series of novel second order nonlinear optical chromophores modified by heteroatom-groups in electron-donor has been synthesized and systematically investigated. The resulting nonlinear optical chromophores exhibited both good thermal stab

Full text of DOI:10.1016/j.dyepig.2016.04.023

Dyes and Pigments 131 (2016) 215e223
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The influence of different donor/acceptor matches on chromophore's
nonlinear optical activity
,
,
,
,
, b
Huajun Xu a b, Haoran Wang a b, Chaolei Hu a b, Mingkai Fu a b, Yanling He a
,
Jialei Liu a **, Xinhou Liu a
,
, *
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
a r t i c l e i n f o
a b s t r a c t s
Article history:
In this work, a series of novel second order nonlinear optical chromophores modified by heteroatom-
groups in electron-donor has been synthesized and systematically investigated. The resulting
nonlinear optical chromophores exhibited both good thermal stability and good solubility in common
organic solvents. UVeVis spectra and density functional theory calculations were carried out to inves-
tigate the influence of heteroatom-groups on the electro-optic activities. The results revealed that
increasing the electron-donor strength will not always improve the nonlinear optical activity of chro-
mophore. There is a best match for electron-donor and acceptor. As to electro-optic activities, the doped
polymer film-d3 displayed the highest electro-optic coefficient value of 37 pm/v at the doping con-
centration of 25 wt%. These results indicated that the nonlinear optical activities could be enhanced by
finding the best match for electron-donor and acceptor in chromophore design.
Received 4 March 2016
Received in revised form
7 April 2016
Accepted 9 April 2016
Available online 11 April 2016
Keywords:
NLO
Chromophore
Donor
© 2016 Published by Elsevier Ltd.
Heteroatom
Electro-optical
1. Introduction
donor/acceptor strength of these DepeA type pushepull chro-
mophore molecules can cause dramatic influences to their second
order nonlinear responses [3,5e7,13e15]. In order to search for
highly efficient NLO chromophores, finding an optimal combina-
tion of their donor/acceptor still represents one of the critical
challenges [16e18]. In the past decade, the researches on NLO
chromophores have mainly focused on the design of electron
bridges and electron acceptors [19e22]. As the important compo-
nents of NLO chromophores, however, the electron donors have
received much less attention due to that the general class of
traditional alkyl and aryl amines were considered to be the idea
electron donors and were constantly-used. These amines have been
proven to possess suitable energy levels of the nitrogen
nonbonding lone pair, which is well matched to donate electrons
During last two decades, polymeric electro-optic (EO) materials
have drawn much attention due to their attractive potential ap-
plications in high speed EO devices with broad bandwidth and low
driving voltage [1]. Compared with traditional inorganic and
semiconductor materials, the organic EO materials have many ad-
vantages such as larger nonlinear optical coefficients, simpler
preparation and lower cost [2e6]. Generally, the nonlinear optical
(NLO) chromophore molecules possessing a dipolar DepeA type
structure is the core component in such materials, which turned
the design and preparation of NLO chromophores into a hot
research spot in the area of organic EO materials [7e12]. Much
great attentions have been paid to the rational design of chromo-
phores with not only high first-order hyperpolarizability (
b) but
into the p* of the attached aromatic ring [7,23].
also good thermal and photochemical stabilities, and in addition,
have good solubility and compatibility with polymer matrix.
It is a well-established fact that the conjugation length and
Several studies have demonstrated that incorporating
heteroatom-groups into the donor moiety of chromophores could
dramatically modulate the conjugation of the
and the electronic character of the charge transfer kinetics, which
could further influence the chromophores' value [5,24e30].
p-electron networks
b
* Corresponding author.
** Corresponding author.
E-mail addresses: liujialei@mail.ipc.ac.cn (J. Liu), xhliu@mail.ipc.ac.cn,
xinhouliu@foxmail.com (X. Liu).
Meanwhile, additional heteroatoms can also provide abundant
opportunities for further modifications [5,31]. In order to investi-
gate the influence of heteroatom-group in traditional donor moiety
http://dx.doi.org/10.1016/j.dyepig.2016.04.023
0143-7208/© 2016 Published by Elsevier Ltd.

216
H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
of the NLO chromophores and to achieve better
b
value and
at this temperature during the dropwise addition of phosphorus
oxychloride (0.64 g, 4.2 mmol). The solution was kept stirring at
0
macroscopic EO activities, we designed a series of chromophores in
this article with one or two heteroatom-groups incorporated into
electron-donor part (Scheme 1). Oxygen atom and nitrogen atom
were selected as candidates due to their potential suitable electron-
donating abilities. For reasonable comparison, these NLO chromo-
phores all choose 2-dicyanomethylene-3-cyano-4,5,5-dimethyl-
2,5-dihydrofuran (TCF) as electron acceptors. 1H NMR, MS and
crystal structure analysis were carried out to demonstrated the
preparation of these chromophores. Thermal stability, photo-
physical properties, density functional theory (DFT) calculations
and EO activities of these chromophores were systematically
studied and compared to illustrate the influences of the suitable
additional heteroatoms on rational NLO chromophore designs.
ꢀC for 2 h and the temperature was kept constant during the
dropwise addition of aromatic amine (2 mmol) in 10 mL DMF. The
solution was stirred for 2 h at 0 ꢀC, then gradually warmed to room
temperature and stirred for 3 h before being poured into 300 mL
solution of potassium carbonate (10%) for quenching. The reaction
mixture was extracted by ethyl acetate (100mL ꢂ 3), washed with
brine, dried over MgSO4. The solvent was removed in vacuo. The
resulting crude product was purified by column chromatography
with hexane/ethyl acetate as solvent.
2.3.2. General procedure for the synthesis of chromophores
To a solution of aromatic aldehyde (3.0 mmol) and the TCF
acceptor (0.64 g, 3.2 mmol) in EtOH (30 mL), several drops of
triethylamine were added. The reaction was allowed to stir at 70 ꢀC
for 2 h and then cooled to room temperature. After removal of the
solvent under reduced pressure, the crude product was purified by
silica chromatography, eluting with hexane/ethyl acetate.
2. Experimental section
2.1. Materials and instrument
All chemicals are commercially available and are used without
further purification unless otherwise stated. N, N-dimethyl form-
amide (DMF) was distilled over calcium hydride and stored over
molecular sieves (pore size 3 Å). Acetone was dried with anhydrous
MgSO4, then distilled and stored over molecular sieves (pore size
2.3.3. Synthesis of compound 3
To a stirred solution of compound 2 (1.53 g, 10 mmol) and
bromobutane (2.72 g, 20 mmol) in acetonitrile (50 mL) potassium
carbonate (4.14 g, 30 mmol) and 18-crown-6 (0.1 g, 0.4 mmol) were
added. The reaction mixture was heated at reflux under nitrogen
for 24 h, filtered and evaporated to dryness. The crude product was
3
Å).
The
2-dicyanomethylene-3-cyano-4-methyl-2,5-
dihydrofuran(TCF) acceptor was prepared according to the litera-
ture [32e34]. TLC analyses were carried out on 0.25 mm thick
precoated silica plates and spots were visualized under UV light.
Chromatography on silica gel was carried out on Kieselgel
(200e300 mesh).
purified
(VAcOEt:VHexane ¼ 1:8) to give 3 as an orange powder with 82% yield
(2.17 g, 8.20 mmol). 1H NMR (400 MHz, CDCl3)
5.86e5.80 (m, 3H),
3.77 (s, 6H), 3.27e3.17 (m, 4H), 1.62e1.51 (m, 4H), 1.39e1.28 (m,
by
silica
chromatography,
eluting
with
d
4H), 0.99e0.90 (m, 6H). 13C NMR (101 MHz, CDCl3)
d 161.63, 150.03,
91.38, 87.42, 54.84, 50.75, 29.49, 20.17, 13.72. MS, m/z: 265.31 (Mþ).
2.2. Measurements and instrumentation
1-HNMR spectra were determined by an Advance Bruker 400 M
(400 MHz) NMR spectrometer (tetramethylsilane as internal-
reference). The MS spectra were obtained on MALDI-TOF (Matrix
Assisted Laser Desorption/Ionization of Flight) on BIFLEXIII (Broker
Inc.) spectrometer. The UVeVis experiments were performed on
Cary 5000 photo spectrometer. The TGA was determined by
TA5000-2950TGA (TA co) with a heating rate of 10 ꢀC minꢁ1 under
the protection of nitrogen. The single-crystal X-ray diffraction data
were collected with the use of graphite-monochromatized Mo-Ka00
2.3.4. Synthesis of compound 4
This compound was prepared from compound 3 (1.33 g,
5.0 mmol) according to the general procedure for the Vilsmeier
Reaction. After column chromatography (Vethyl acetate/Vhexane ¼ 1:4),
compound 4 was isolated with 58% yield (0.85 g, 2.9 mmol). 1H
NMR (400 MHz, CDCl3)
d 10.16 (s, 1H), 5.66 (s, 2H), 3.80 (s, 6H), 3.28
(t, 4H), 1.62e1.49 (m, 4H), 1.38e1.27 (m, 4H), 0.93 (t, 6H). 13C NMR
(101 MHz, CDCl3) d 186.27, 164.36, 154.14, 87.36, 55.65, 50.90, 29.63,
20.21, 13.80. MS, m/z: 293.22 (Mþ).
radiation (
l
¼ 0.71073 Å) at ꢁ120 ꢀC on a Rigaku AFC10 diffrac-
tometer equipped with a Saturn CCD detector.
2.3.5. Synthesis of compound 7
To a stirred solution of compound 9 [35] (2.32 g, 10 mmol) and
bromobutane (1.63 g, 12 mmol) in acetone (100 mL), anhydrous
potassium carbonate (1.66 g, 12 mmol) and 18-crown-6 (0.025 g,
0.1 mmol) were added. The reaction mixture was heated at reflux
under nitrogen for 5 h and then filtered, evaporated to dryness. The
2.3. Synthesis
2.3.1. General procedure for Vilsmeier Reaction
A solution of 20 mL DMF was cooled to 0 ꢀC and was maintained
Scheme 1. Synthetic scheme for chromophores d1ed4.

H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
217
crude product was purified by silica chromatography, eluting with
Hexane to give 10 as a white solid in 87% yield (2.50 g, 8.7 mmol). 1H
J ¼ 15.0 Hz,1H), 5.66 (s, 1H), 5.62 (s, 1H), 4.06 (t, J ¼ 6.4 Hz, 2H), 3.46
(t, J ¼ 5.9 Hz, 4H), 3.33 (s, 4H), 2.08 (d, J ¼ 5.9 Hz, 4H), 2.00 (s, 4H),
1.95e1.86 (m, 2H), 1.65 (s, 6H), 1.55 (dt, J ¼ 14.7, 7.4 Hz, 3H), 1.02 (t,
NMR (400 MHz, CDCl3)
d
5.58 (d, J ¼ 4.6 Hz, 2H), 5.40 (s, 1H), 3.96 (t,
J ¼ 6.5 Hz, 2H), 3.28 (d, J ¼ 6.1 Hz, 8H), 2.01e1.92 (m, 8H), 1.81e1.70
J ¼ 7.4 Hz, 3H). 13C NMR (101 MHz, CDCl3)
d 182.38, 175.09, 163.88,
(m, 2H), 1.49 (dt, J ¼ 14.9, 7.4 Hz, 2H), 1.03e0.93 (m, 3H). 13C NMR
153.45, 110.97, 110.31, 108.90, 104.86, 99.68, 95.71, 89.81, 68.55,
(101 MHz, CDCl3)
d 190.20 (s), 153.02 (s), 132.05 (s), 125.83 (s),
48.21, 31.18, 27.00, 25.28, 19.52, 14.15. MS, m/z: 497.25 (Mþ).
111.19 (s), 51.35 (s), 48.80 (s), 38.96 (s). MS, m/z: 311.21 (MþNaþ).
2.3.11. Preparation of the chromophores-doped polymer films
In order to evaluate their EO activities, guest-host polymeric
films using 1,1,2-trichloroethane (TCE) as the solvent were pre-
pared to investigate the translating of the microscopic hyper-
polarizability into macroscopic EO response (r33). The solution was
2.3.6. Synthesis of compound 8
This compound was prepared from compound 8 (0.42 g,
2.0 mmol) according to the general procedure for Vilsmeier Reac-
tion. After column chromatography (Vethyl acetate/Vhexane ¼ 1:5),
compound 8 was isolated with 53% yield (0.33 g, 1.06 mmol). 1H
filtered using a 0.2-mm syringe filter to remove large particulates.
NMR (400 MHz, CDCl3)
d
10.12 (s, 1H), 5.53 (s, 1H), 5.41 (s, 1H), 4.01
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 with
an Ambios Technology XP-1 profilometer. The thickness of films
(t, J ¼ 6.4 Hz, 2H), 3.36 (t, J ¼ 6.5 Hz, 4H), 3.30 (t, J ¼ 6.0 Hz, 4H),
2.03e1.97 (m, 4H), 1.94 (dd, J ¼ 12.6, 6.1 Hz, 4H), 1.80 (dt, J ¼ 14.3,
6.4 Hz, 2H), 1.51 (dq, J ¼ 14.6, 7.4 Hz, 2H), 0.97 (t, J ¼ 7.4 Hz, 3H). MS,
m/z: 339.32 (MþNaþ).
were about 2.1
mme2.6 mm, which should be thicker than the films
poled in contact poling to prevent the possible film damage (in
2.3.7. Synthesis of chromophore d1
contact poling, films thickness will be about 1.3
3. Results and discussion
mme2.0 mm).
This compound was prepared from compound 1 (0.31 g,
1.0 mmol) and TCF acceptor (0.24 g, 1.2 mmol) according to the
general procedure for the synthesis of chromophores. After column
chromatography (Vethyl acetate/Vhexane ¼ 1:5), chromophore d1 was
isolated with 83% yield (0.40 g, 0.83 mmol). 1H NMR (400 MHz,
3.1. Synthesis and characterization
CDCl3)
d
8.02 (d, J ¼ 15.8 Hz, 1H), 7.51 (d, J ¼ 9.1 Hz, 1H), 6.86 (d,
Scheme 2 showed the synthetic approach for the chromophores
d1ed4. Compounds 1, 5 and TCF acceptor were prepared according
to the literature [9,32,35]. Starting from the aromatic amine or
substituted aromatic amine, chromophores d1ed4 were synthe-
sized in good overall yields through simple 2e3 step reactions: SN2
substitution reaction with bromoethane, Vilsmeier reaction, and
the final Knoevenagel condensation with TCF acceptor. This route
has been established for the high yield synthesis of this kind of NLO
chromophores [36,37]. All the chromophores were completely
characterized by 1H NMR, 13C NMR, MS, UVeVis spectroscopic
analysis and the data obtained were in full agreement with the
proposed formulations. The information of crystal structure for d3
has been obtained by Single-Crystal X-ray diffraction, which further
demonstrated the successful preparation of chromophores.
J ¼ 15.8 Hz, 1H), 6.37 (d, J ¼ 9.0 Hz, 1H), 6.09 (s, 1H), 4.05 (t,
J ¼ 6.2 Hz, 2H), 3.44e3.35 (m, 4H), 1.94e1.83 (m, 2H), 1.73 (s, 6H),
1.69e1.65 (m, 2H), 1.56 (td, J ¼ 14.6, 7.1 Hz, 4H), 1.45e1.34 (m, 4H),
1.00 (dt, J ¼ 14.7, 7.4 Hz, 9H). 13C NMR (101 MHz, CDCl3)
d 182.65,
176.69, 175.56, 161.95, 110.41, 108.97, 104.81, 99.81, 96.47, 68.12,
31.09, 29.41, 26.91, 24.37, 20.21, 14.20. MS, m/z: 487.31 (MþHþ).
2.3.8. Synthesis of chromophore d2
This compound was prepared from compound 4 and TCF
acceptor according to the general procedure for the synthesis of
chromophores. After column chromatography (Vethyl
/
acetate
Vhexane ¼ 1:3), chromophore d2 was isolated with 85% yield. 1H
NMR (400 MHz, CDCl3)
d
8.13 (d, J ¼ 15.9 Hz, 1H), 7.30 (d,
J ¼ 15.9 Hz, 1H), 5.83 (s, 2H), 3.92 (s, 6H), 3.47e3.36 (m, 4H), 1.73 (s,
6H), 1.66 (dt, J ¼ 15.1, 7.6 Hz, 4H), 1.46e1.35 (m, 4H), 1.00 (t,
3.2. Optical properties
J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3)
d 184.90, 165.58, 159.07,
152.53, 147.02, 104.95, 94.16, 90.89, 88.62, 86.09, 64.17, 63.61, 51.96,
In order to reveal the effect of multi-heteroatoms in electron
donor moiety on the electronic structure of dipolar chromophores
d1ed4, UVeVis absorption spectra were measured in aprotic sol-
vents with different polarity so that the solvatochromic behavior of
each chromophore could be investigated in different dielectric
environments. Depending on different heteroatom-groups incor-
porated, the resulting NLO chromophores exhibited diverse charge-
transfer (CT) absorption properties. This effect was demonstrated
by comparing the maximum absorption wavelength of the syn-
thesized chromophores with the same electron acceptors used
[38,39]. Fig. 1 displays the spectra of four chromophores in dioxane
and chloroform. The spectra data are summarized in Table 1. All
47.68, 25.78, 24.88, 18.73, 14.76. MS, m/z: 474.28(Mþ).
2.3.9. Synthesis of chromophore d3
This compound was prepared from compound 5 and TCF
acceptor according to the general procedure for the synthesis of
chromophores. After column chromatography (Vethyl
/
acetate
Vhexane ¼ 1:5), chromophore d3 was isolated with 59% yield. 1H
NMR (400 MHz, CDCl3)
d
8.40 (d, J ¼ 15.8 Hz, 1H), 7.85 (d, J ¼ 9.2 Hz,
1H), 6.89 (d, J ¼ 15.8 Hz, 1H), 6.64 (d, J ¼ 9.1 Hz, 1H), 6.45 (s, 1H),
3.57e3.48 (m, 4H), 3.20 (t, J ¼ 7.3 Hz, 4H), 1.80 (s, 6H), 1.67 (dt,
J ¼ 15.1, 7.6 Hz, 4H), 1.59e1.48 (m, 4H), 1.47e1.36 (m, 4H), 1.36e1.23
(m, 5H), 0.97 (t, J ¼ 7.3 Hz, 6H), 0.86 (t, J ¼ 7.3 Hz, 8H). 13C NMR
chromophores exhibited a similar broad
pep intramolecular CT
(101 MHz, Acetone)
d
178.27, 176.12, 158.22, 154.38, 147.30, 132.71,
absorption band in the visible region with a continuous red-shift of
the maximum absorption (lmax) due to the gradually increasing
donor strength in dioxane solutions. Compared with chromophore
d1, the lmax of chromophore d3 and d4 were shifted to the longer
wavelength of 579 nm and 584 nm respectively, which may attri-
bute to the fact that the additional nitrogen atom in donor moiety
118.76, 109.94, 107.61, 105.60, 97.86, 54.81, 51.58, 26.78, 21.13, 20.84,
14.29. MS, m/z: 564.38 (MþNaþ).
2.3.10. Synthesis of chromophore d4
This compound was prepared from compound 6 and TCF
acceptor according to the general procedure for the synthesis of
conjugated with the p-system better than oxygen atom.
chromophores. After column chromatography (Vethyl
/
Besides, the solvatochromic behavior was also explored to
investigate the polarity of chromophores. When increasing the
solvent dielectric constant, all of the four chromophores showed
acetate
Vhexane ¼ 1:3), chromophore d4 was isolated with 65% yield. 1H
NMR (400 MHz, CDCl3)
d
8.19 (d, J ¼ 14.2 Hz, 1H), 6.84 (d,

218
H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
Scheme 2. The chemical structures of chromophores d1ed4.
solvatochromic red-shifts. Chromophores d1 and d3 showed
bathochromic shifts of 37 nm and 28 nm when solvent was
changed from dioxane to chloroform, respectively. However,
chromophore d4 exhibited weaker bathochromic shifts in com-
parison to others. It indicated that chromophores d1 and d3 are
more polarizable than chromophore d4 [40]. Generally, it means
substituents, steric hindrance, and intramolecular charge-transfer.
As reported earlier [45], the has been calculated at 6-31g* level.
From this, the scalar quantity of can be computed from the x, y,
and z components according following equation.
b
b
ꢀ
ꢁ
1=2
b
¼
b2x þ b2y þ b2z
the improvement of
process.
b value which is confirmed in DFT calculation
(1)
ꢀ
ꢁ
X
1
3
Where bi
¼
biii
þ
bijj
þ
bjij
þ
bjji ; i; j
3 ðx; y; zÞ
3.3. Theoretical calculations
The donor of chromophore d2 possesses two alkoxy groups
which should have stronger electron-donating ability than the
donor of d1. The bzzz value of d2 exhibits apparent decrease
compared to d1. It indicates that improving electron-donating
In order to model the ground state molecular geometries, the
HOMOeLUMO energy gaps and first-order hyperpolarizability (b)
of the chromophores with different donors, the DFT calculations
were carried out at the PBE1PBE level by employing the split
valence 6-31g* basis set [41e44]. The data obtained from DFT cal-
culations are summarized in Table 2.
ability will not always bring the enhancement of b value. There is
an optimal combination for donor and acceptor in NLO chromo-
phore design. Due to the moderate strong electron-donating ability,
chromophore d3 showed a larger
chromophores. On the contrary, chromophore d4 exhibited the
smallest value although there were two heteroatom-groups in
b value than the other three
It was reinforced by theoretical calculations and optical char-
acterizations that the
b
value has a close relationship with the
b

H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
219
torsion between acceptor ring and benzene ring of the chromo-
phore D hindered the effective
p conjugation between donor and
acceptor [46].
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 properties
of molecules [46,47]. Besides, the HOMOeLUMO energy gap is also
used to understand the charge transfer interaction occurring in a
chromophore molecule [39,40,46]. Fig. 3 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
and LUMO energy were calculated by DFT method and summarized
in Table 2. As showed in data, the HOMO energy of chromophore d3
is 0.07 eV lower than d2, while the LUMO energy of chromophore
d3 is 0.19 eV lower than that of d2. This may illustrate that the
donor structure of chromophore d3 influences the LUMO energy to
a greater extent than the HOMO energy. The HOMOeLUMO gap
was the lowest for chromophore d3 (2.98 eV), highest for chro-
mophore d2 (2.806 eV). It was well supported by the facts that the
lower is the optical gap, the greater will be the charge transfer and
higher nonlinearity [46].
To get more insight from the frontier orbitals, the composition of
the HOMOs and LUMOs has been calculated using the Multiwfn
program with Ros Schuit (SCPA) partition [48]. As shown in Table 3,
the whole chromophore molecule was segmented as heteroatom-
group, donor, and bridge þ acceptor. At the same time, the attri-
butions of the substituent groups located in donor moiety were
listed separately. For the chromophores d1ed4, the LUMO was
largely stabilized by the contributions from bridge þ acceptor
(72.05%e74.88%) and much less contributions from other seg-
ments. The heteroatom-group should contribute to HOMO more
than LUMO, which is helpful for delocalization of
p-electron. The
heteroatom-group of d3 exhibits the largest decrease in the con-
tributions to HOMO compared with LUMO. However, there are no
apparent differences in HOMO and LUMO composition contributed
Fig. 1. UVevis absorption spectra in dioxane and chloroform.
from heteroatom-group for d2 and d4. The p-electron seems to be
stagnant and cannot be effectively transferred from donor to
acceptor though the donors possess strong electron-donating
ability.
Table 1
UVeVis absorption and solvatochromic data of the chromophores.
lmaxa/(nm)
lmaxb/(nm)
Dlc/(nm)
3.4. Crystal structure studies
d1
d2
d3
d4
556
556
579
584
593
577
607
598
37
21
28
14
Single crystal suitable for X-ray diffraction was obtained for d3
by evaporation of CHCl3/hexane solutions at ambient temperature.
However, all attempts to acquire qualified crystals of other three
chromophores were failed. The ORTEP diagram and the crystal data
of the chromophore d3 are given in Fig. 4 and Table 4 and crystal
packing diagrams are shown in Fig. 5. Chromophores d3 crystal-
lized in triclinic space group P-1. Owing to the introduction of ni-
trogen atom in ortho-position, chromophores d3 exhibited two
intramolecular nonclassical hydrogen bonding interactions (d3:
C(12)eH(12)/N(4); C(14)eH(14)/N(3)), respectively.
a
lmax was measured in dioxane.
b
c
lmax was measured in chloroform.
Dl was the difference between lmax and blmax
.
a
Table 2
Data from DFT calculations.
Chromophore
EaHOMO/(eV)
EbLUMO/(eV)
D
Ec/(eV)
bdzzz/(10ꢁ30 esu)
d1
d2
d3
d4
ꢁ5.69
ꢁ5.61
ꢁ5.68
ꢁ5.54
ꢁ2.68
ꢁ2.51
ꢁ2.70
ꢁ2.50
3.01
3.10
2.98
3.04
133.09
118.32
138.13
99.11
3.5. Thermal stability and EO performance
NLO chromophores must be thermally stable enough to with-
stand encountered high temperatures (>150 ꢀC) in electric field
poling and subsequent processing of chromophore/polymer ma-
terials. All of the chromophores exhibited good thermal stabilities
and the decomposition temperatures (Td) nearby 260 ꢀC showed in
Table 5. The chromophore d3 had the minimum decomposition
temperature (Td ¼ 245 ꢀC). The temperature in practical materials
processing was generally below 200 ꢀC. Therefore, the Td of the four
chromophores was high enough to satisfy the need of application in
The values of aEHOMO
, b were calculated using gaussian09 at PBE1PBE/6-
bELUMO and c
31g* level.
cDE ¼ bELUMO
ꢁ
aEHOMO
.
donor part. It may attribute to that: 1) the mismatch for donor and
acceptor strength; 2) the acceptor ring of d4 was not coplanar with
benzene ring and showed a dihedral angle of 21.28ꢀ (Fig. 2). The

220
H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
Fig. 2. The optimized configurations of chromophores d1ed4.
Fig. 3. Frontier molecular orbitals of chromophores d1ed4.
EO device preparation [49].
with 25 wt% chromophores into amorphous polycarbonate (APC)
were prepared to investigate the translating of the microscopic
In order to evaluate their EO activities, the polymer films doped

H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
221
Table 3
Molecular orbital composition (%) in the ground state for chromophores d1ed4.
d1/%
d2/%
d3/%
d4/%
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
H-groupa
2.42
51.66
45.92
1.39
23.73
74.88
3.14
45.53
51.33
3.44
22.46
74.10
6.82
51.14
42.04
2.98
22.91
74.11
4.69
48.11
47.20
4.74
23.21
72.05
Donorb
Bridge þ acceptorc
a
Represents heteroatom-group.
Excludes the heteroatom-group.
b
c
Represents the contributions of bride (double bond) and acceptor to molecular orbital. The molecular orbital composition was calculated using Multiwfn program with
RoseSchuit (SCPA) partition.
Fig. 4. Ortep view of chromophores d3.
hyperpolarizability into macroscopic EO response (r33). The r33 of
organic material mainly depends on three critical parameters: EO
chromophore first molecular hyperpolarizability, bzzz, number
electrodes in poling process, even breakdown. According to the
experiences of our group, the film thickness should be about 2.5 mm
for corona-poling.
density, N, and the acentric order parameter, <cos3
q>.
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 (the thickness of ITO
layer: 150 Å) with low reflectivity and good transparency in order
to minimize the contribution from multiple reflections [50,51]. The
corona poling process was carried out at a temperature of 10 ꢀC
above the glass transition temperature (Tg) of the polymer. The r33
values were calculated by the following equation:
ꢀ
.
ꢁ
N
bzzzFL 3 ꢁ l2max l2
ꢀ
ꢁ
ꢀ
.
ꢁ
r33 ¼ 2
cos3
q
(2)
3
h4 1 ꢁ l2max l2
Where bzzz is the first hyperpolarizability of individual chromo-
phore,
h
is the material index of refraction and the ratio l2max l2
/
refers to the wavelength at maximum absorbance to the wave-
ꢂ
ꢃ
3=2
n2 ꢁ sin2
q
3
l m
I
1
sin2
length of the incident light field. The local field correction
ꢂ
ꢃ
r33
¼
(3)
FL ¼ ((h20
(
h2u þ 3))/(hu2 þ 2h02))((hu2 þ 2)/3)2 is the product of Lorentz
n2 ꢁ 2sin2
q
q
4pVm
and Onsager field factors for the static- and frequency-dependent
refractive indices of the material.
where r33 is the EO coefficient of the poled polymer,
l
is the optical
There is no direct relationship between film thickness and EO
activities of poled films. However, film thickness can affect the
strength of poling electric field (Ep) which has an important influ-
wavelength, h is the incidence angle, Ic is the output beam intensity,
Im is the amplitude of the modulation, Vm is the modulating
voltage, and n is the refractive indices of the polymer films.
The r33 values were measured and showed in Table 5. Because of
different structural features, the four chromophores displayed
different compatibility with polymer matrix when prepared guest-
host polymers. Due to lacking of flexible chains, a portion of
chromophore d2 was not dissolved and removed in filtering pro-
cess, which caused the chromophore concentration lower than 25%
wt. It was a similar situation for chromophore d4. The guest-host
ence on the acentric order parameter (cos3
q). In corona-poling and
contact-poling, we cannot increase the voltage applied in EO films
without a finite value. The maximum voltage depends on the poling
equipment. At a given voltage, the Ep is inversely proportional to
the film thickness. Increasing the film thickness will result in the
decrease of maximum Ep, further lower the r33 of poled film.
However, too thin EO film will lead to high current between two

222
H. Xu et al. / Dyes and Pigments 131 (2016) 215e223
Table 4
(only 13 pm/V). On the contrary, chromophore d3 obtained the
largest r33 value of 37 pm/v, which resulted from the good solubi-
Crystal data and structure refinement details for chromophore d3.
lity, favorable polarizability, and an excellent
b value.
Compound reference
d3
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C34 H47 N5
541.77
293(2) K
0.71073 Å
Monoclinic
P2(1)/c
O
4. Conclusion
A series of organic NLO chromophores with multi-heteroatoms
donors have been synthesized and systematically characterized by
NMR, MS, UVeVis absorption spectra, and single-crystal X-ray
diffraction methods. The new compounds obtained good yields
which are ensured by the simple work-up procedures. Thermog-
ravimetric analysis showed good thermal and thermooxidative
stabilities of the four chromophores. The effects of bathochromic
and solvatochromic behavior on the UVeVis absorption were also
investigated to find the optimal match of donor-acceptor. Mean-
while, DFT calculations were carried out to analyze the effects of
different heteroatom matches on chromophores' first-order
hyperpolarizabilities. Chromophore d3 with the donor derived
from m-phenylenediamine exhibits the maximum bzzz value
(138.13 ꢂ 10ꢁ30 esu) and the minimum HOMOeLUMO energy gap
(2.98 eV). In electro-optic activities, the doped polymer film con-
taining chromophore d3 displayed a maximum r33 value of 37 pm/
V at the doping concentration of 25%wt. All these might be useful in
designing other new NLO chromophores with modified hetero-
atoms groups to optimize molecular structure for achieving high
performance.
Unit cell dimensions
a ¼ 15.609(3) Å
b ¼ 15.268(3) Å
c ¼ 14.015(3) Å
3339.1(12) Å3
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta ¼ 27.48
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
4
1.078 Mg/m3
0.066 mmꢁ1
1176
0.30 ꢂ 0.20 ꢂ 0.20 mm3
1.30e27.48ꢀ
ꢁ20 ꢃ h ꢃ 20, ꢁ19 ꢃ k ꢃ 18, ꢁ18 ꢃ l ꢃ 18
13,645
7463 [R(int) ¼ 0.0429]
97.5%
Empirical
0.9869 and 0.9805
Full-matrix least-squares on F2
7463/1/362
1.016
R1 ¼ 0.0878, wR2 ¼ 0.2614
R1 ¼ 0.1567, wR2 ¼ 0.3032
0.035(4)
0.679 and ꢁ0.342 eꢁ3
Fig. 5. Crystal packing diagram of chromophore d3.
Table 5
33 values (25%wt doping content) and Td data.
Acknowledgments
r
Chromophore
d1
23
d2
13
d3
37
d4
19
We are grateful to the National Natural Science Foundation of
China (No. 11104284, No. 61101054 and No. U1533121) for the
financial support.
r33/(pm/v)
Td/ꢀC
248
269
245
251
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