Structure–function relationship exploration for enhanced electro-optic activity in isophorone-based organic NLO chromophores

Article abstract of DOI:10.1016/j.dyepig.2018.04.036

Four isophorone-based chromophores FLD1-FLD4 has been synthesized and investigated based on julolidinyl donors, with modified isophorone-derived bridges and tricyanovinyldihydrofuran or phenyl-trifluoromethyl-tricyanofuran acceptors. The chromophore FLD3

Full text of DOI:10.1016/j.dyepig.2018.04.036

DyesandPigments157(2018)55–63
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Structure–function relationship exploration for enhanced electro-optic
activity in isophorone-based organic NLO chromophores
Fenggang Liua,∗, Hua Zhangb, Hongyan Xiaob, Huajun Xub, Shuhui Bob, Ling Qiub, Zhen Zhenb,
Lijie Laia, Shujie Chena, Jiahai Wanga,∗∗
a
School of Chemistry and Chemical Engineering, Guangzhou Key Laboratory for Environmentally Functional Materials and Technology, Guangzhou University, Guangzhou
510006, PR China
b
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,
PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Four isophorone-based chromophores FLD1-FLD4 has been synthesized and investigated based on julolidinyl
donors, with modified isophorone-derived bridges and tricyanovinyldihydrofuran or phenyl-trifluoromethyl-
tricyanofuran acceptors. The chromophore FLD3 was modified with a 2-((tert-butyldimethylsilyl)oxy)etha-
nethiol group covalently attached to the π-conjugate bridge to ensure its all-trans conformation and prevent
close packing of molecules, which benefit the macroscopic electro-optic efficiency. The chromophore FLD3
possesses better thermal stability than chromophore FLD1 without the isolation group on the bridge. Moreover,
density functional theory calculations suggested that the hyperpolarizability (β) value of chromophore FLD3 is
larger than that of chromophore FLD1. Polymers doped with chromophore FLD3 have been poled to afford
ultrahigh electro-optic coefficient (r33) of 238 pm/V at 1.31 μm, which is nearly 60% higher than that obtained
from chromophore FLD1 and three times higher than that of analogue chromophores FLD2 and FLD4.
Nonlinear optics
Chromophore
Isolation group
Electro-optic
Isophorone
Structure–function
1. Introduction
these large β values into bulk EO activities. Meanwhile, improvements
of other auxiliary properties like high thermal and photochemical sta-
With the development of the information technology, conventional
microelectronic materials, usually using electrons as the carriers, will
have difficulty satisfying the needs for future communication tech-
nology [1]. Compared with electronic communication, transmission of
information by photonics has many advantages such as high frequency,
wide bandwidth, high speed, and good parallelism [2,3]. The ability to
convert signals between electronic and photonic domains is central to
photonic/electronic integration. Materials and devices that effect such
transduction are referred to as “electro-optic” (EO) [4]. The organic
electro-optic (OEO) materials have attracted great attention in last
thirty years. Compared with inorganic materials, OEO materials have
potential advantages such as lower cost, ease of processing, larger EO
coefficients and so on [5,6].
bility, good transparency, as well as easy syntheses are very important.
The second-order NLO chromophores are based on a push-pull
system, which consists of an electron-donating group (Donor) and an
electron-withdrawing group (Acceptor) coupled through
a π-con-
jugated bridge which called (D–π–A) configuration [12]. The donor
moiety has largely been of the amine structure [13]. Bridge moieties
consisted of heteroaromatic (thiophene, pyrrole, furan) or polyene
(phenyltetraene) structures have reported [14–17]. The acceptor moi-
eties have developed from weaker nitro, cyano, alkoxy, isoxazolone,
tricyanovinyl, diarlythiobarbituric acid groups to strong and effective
tricyanovinyldihydrofuran (TCF) and phenyl-trifluoromethyl-tricyano-
furan (CF3-Ph-TCF) groups [1,13,18]. Among which, the CLD-type
chromophores (containing the ring-locked tetraene bridges) with TCF
or CF3-Ph-TCF acceptor were widely investigated due to their large β
and good utility for EO devices [19]. Relatively high electro-optic (EO)
coefficients r33 around 40–70 pm/V measured at the wavelengths of
1.3 μm have been achieved from poled polymeric composites using
these polyenic chromophores as dopants [20]. There remains strong
The second-order nonlinear optical (NLO) chromophores are the key
constructing blocks for OEO materials [7–11]. To meet the stringent
requirements for the using of devices, OEO materials should be devel-
oped through rational design of nonlinear optical (NLO) chromophores
to optimize their first hyperpolarizability (β), and effectively translate
∗
Corresponding author.
Corresponding author.
∗∗
E-mail addresses: liufg6@gzhu.edu.cn (F. Liu), jiahaiwang@gzhu.edu.cn (J. Wang).
https://doi.org/10.1016/j.dyepig.2018.04.036
Received 28 February 2018; Received in revised form 1 April 2018; Accepted 20 April 2018
Availableonline22April2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

F. Liu et al.
DyesandPigments157(2018)55–63
continued interest in improving the properties of these materials as well
as the synthetic strategies for their preparation.
1H NMR spectra were determined on an Advance Bruker 400 M
(400 MHz) NMR spectrometer (tetramethylsilane as internalreference).
The MS spectra were obtained on MALDI-TOF (Matrix Assisted Laser
Desorption/Ionization of Flight) on BIFLEXIII (Broker Inc.,) spectro-
meter. The UV–Vis spectra were performed on Cary 5000 photo spec-
trometer. The TGA was determined by TA5000-2950TGA (TA co) with
a heating rate of 10 °C min−1 under the protection of nitrogen.
Julolidinyl-based group as the electron donor has been reported
with better solubility, stronger electron-donating ability and larger
steric hindrance than the classical dimethylanilino moiety [21]. The
ring-fused aminophenyl structures in julolidinyl donors facilitate the
overlap of the p-orbital of the amino atom with the phenyl ring thus
providing a good mechanism to increase the electron-donating strength
[22]. Meanwhile, the steric hindrance and tension of the rigid structure
from the julolidinyl-based donor in chromophore can also suppress
aggregations. So, the electro-optic coefficient of the chromophore can
be further improved by using julolidinyl-based group as the donor.
Previous study showed that the CLD-type chromophores are often a
mixture of trans and cis isomers. The chromophore with cis and trans
configuration showed reduced μβ value than that of chromophore with
all-trans conformation. And attempts to purify these isomers through
column chromatography for higher NLO activity are proven to be very
difficult [20].
So, in this paper, we want to increase the electro-optic activity of
isophorone-based chromophore furthermore. We use julolidinyl group
as the donor of the chromophore. Meanwhile, we utilized epox-
yisophorone ring-opening chemistry to efficiently incorporate the 2-
((tert-butyldimethylsilyl)oxy)ethanethiol (STBDMS) group to the
bridge of julolidinyl-based chromophores FLD3-FLD4 (Chart 1) to en-
sure its all-trans conformation. The modification of STBDMS group on
the chromophore can prevent close packing of molecules, which in turn,
may attenuate the strong dipole-dipole electrostatic interactions bew-
teen chromophores to improve poling efficiency of EO polymers. And
compared with the nonsubstituted analogue chromophore FLD1, thio-
lated chromophore FLD3 achieves higher molecular hyperpolarizability
because of unique and moderate π-accepting ability of the substituted
group. In the meantime, we synthesized chromophore FLD4 with the
same donor and bridge but with a different CF3-Ph-TCF acceptor.
We compare the structure-property relationship between the 4 s-
order chromophores with julolidinyl-based donors and tetraene-derived
bridge, but with different position and quantity of the functionalized
connecting spacer and different acceptor. As shown in Chart 1, the
chromophores FLD1 and FLD2 have only one side-group covalently
attached to the donor. The chromophores FLD3 and FLD4 have one
side-groups covalently attached to the donor and other one side-groups
covalently attached to the π-system bridge. The four chromophores
have similar D-π-A structure, but differ in the position and quantity of
the spacer, allowing us to investigate the influence of the spacer on the
poling-induced alignment of the chromophores. The UV–Vis, solvato-
chromic behavior, DFT quantum mechanical calculations, thermal sta-
bilities and EO activities of these chromophores were systematically
studied and compared to understand their structure-property relation-
ships. An electro-optic (EO) coefficient of 238 pm/V at 1.31 μm was
obtained for film FLD3/APC, which is nearly 60% higher than that
obtained from analogue chromophore FLD1 and much higher than
chromophore FLD2 and FLD4 as well as similar reported monolithic
materials like chromophore CLD-1 [23] and chromophore YLD-124
[24].
2.2. Syntheses
2.2.1. Synthesis of 8-(2-((tert-butyldimethylsilyl)oxy)ethoxy)-1,1,7,7-
tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinoline-9-
carbaldehyde (Compound 2a)
Under a N2 atmosphere, anhydrous potassium carbonate (2.4 g,
15 mmol) was added to a solution of compound 1 (2.73 g, 10 mmol) and
compound tert-butyl(2-chloroethoxy)dimethylsilane (2.92 g, 15 mmol)
in DMF (90 mL). The mixture was allowed to stir at 90 °C for 12 h and
then poured into water. The organic phase was extracted by AcOEt,
washed with brine and dried over MgSO4. After removal of the solvent
under reduced pressure, the crude product was purified by silica
chromatography, eluting with (Acetone: Hexane = 1:10) to give com-
pound 2a as a yellow powder with 90.7% yield (3.91 g, 9.07 mmol). MS
(MALDI-TOF), m/z: 431.59 (M+). 1H NMR (300 MHz, CDCl3) δ 9.98 (s,
1H), 7.58 (s, 1H), 4.06–3.99 (m, 4H), 3.32–3.26 (m, 2H), 3.25–3.20 (m,
2H), 1.74–1.57 (m, 4H), 1.44 (s, 6H), 1.26 (s, 6H), 0.91 (s, 9H), 0.11
(m, 6H). 13C NMR (126 MHz, CDCl3) δ 187.72, 161.44, 148.29, 126.10,
125.73, 120.91, 117.04, 79.10, 62.48, 47.49, 47.45, 46.86, 46.83,
39.38, 35.69, 32.56, 32.04, 29.87, 26.03, 25.90, 18.37, −5.12. Anal.
Calcd (%) for C25H41NO3Si: C, 69.56; H, 9.57; N, 3.24; found: C, 69.57;
H, 9.55; N, 3.23;
2.2.2. Synthesis of (E)-3-(2-(8-(2-((tert-butyldimethylsilyl)oxy)ethoxy)-
1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-
yl)vinyl)-2-((2-((tert-butyldimethylsilyl)oxy)ethyl)thio)-5,5-
dimethylcyclohex-2-en-1-one (Compound 3a)
To a solution of ethanol (30 mL) was carefully added sodium metal
(0.23 g, 10.00 mmol) under N2 atmosphere. The solution was vigor-
ously stirred at room temperature until the sodium was completely
dissolved. 1-Butanethiol (0.78g, 10 mmol) was added to the above so-
lution by syringe, and the mixture was stirred for 10 min followed by
addition of isophoroneoxide (1.54 g, 10.00 mmol). The mixture became
dark shortly and was stirred at room temperature for another 1h to form
intermediate. Compound 2a (2.16 g, 5.00 mmol) was added to the
mixture which was then heated to 65 °C for 18 h. After the removal of
solvent, the residue was directly purified by column chromatography
on silica gel (hexane/ethyl acetate, v/v, 6/1) to afford a red solid 3a in
76.3% yield (2.40 g, 3.82 mmol). MS (MALDI-TOF), m/z: 628.03 (M+).
1H NMR (300 MHz, CDCl3) δ 7.88 (d, J = 16.2 Hz, 1H), 7.39 (s, 1H),
7.34 (d, J = 16.2 Hz, 1H), 4.04 (t, J = 5.1 Hz, 2H), 3.90 (t, J = 5.1 Hz,
2H), 3.56 (m, 3H), 3.27–3.21 (m, 2H), 3.20–3.12 (m, 2H), 2.82 (t,
J = 5.2 Hz, 2H), 2.65 (s, 2H), 2.44 (s, 2H), 1.75–1.69 (m, 4H), 1.43 (s,
6H), 1.29 (s, 6H), 1.07 (s, 6H), 0.91 (s, 9H), 0.13(s, 6H). 13C NMR
(126 MHz, CDCl3) δ 197.20, 161.16, 157.25, 144.72, 135.40, 126.94,
126.16, 123.62, 122.53, 122.21, 116.68, 76.33, 62.55, 60.27, 51.64,
47.41, 46.83, 41.47, 39.89, 38.78, 36.18, 32.71, 32.36, 32.22, 30.81,
30.12, 30.09, 28.45, 28.42, 26.06, 26.02, 18.48, −5.11. Anal. Calcd
(%) for C36H57NO4SSi: C, 68.85; H, 9.15; N, 2.23; found: C, 68.83; H,
9.13; N, 2.24;
2. Experimental
2.1. Materials and instrument
All chemicals are commercially available and are used without
further purification unless otherwise stated. N, N-dimethylformamide
(DMF) and tetrahydrofuran (THF) were distilled over calcium hydride
and stored over molecular sieves (pore size 3 Å). Compounds 2b-6b and
chromophores FLD1 and FLD2 were synthesized according to literature
[25]. 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 (200–300 mesh).
2.2.3. Synthesis of (E)-3-(2-(8-(2-((tert-butyldimethylsilyl)oxy)ethoxy)-
1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-
yl)vinyl)-2-((2-((tert-butyldimethylsilyl)oxy)ethyl)thio)-5,5-
dimethylcyclohex-2-en-1-one (Compound 4a)
To a solution of compound 3a (1.88 g, 3.00 mmol) and imidazole
(0.25 g, 3.60 mmol) in DMF (10 ml), tert-Butyldimethylsilyl chloride
(0.55 g, 3.60 mmol) was slowly under N2 at room temperature. After
56

F. Liu et al.
DyesandPigments157(2018)55–63
vigorously stirring for 3 h, the reaction was quenched by being poured
into 100 mL H2O. The crude product was extracted with hexane
(3 × 50 mL). The combined organic layer was washed with H2O, dried
(Na2SO4), filtered, and concentrated at reduced pressure. The crude
product was purified by column chromatography using ethyl acetate
and hexane (v/v, 1:10 to 1:5) as the eluent to afford a red solid 4a in
94.1% yield (2.10 g, 2.82 mmol). MS (MALDI-TOF), m/z: 742.23 (M+).
1H NMR (300 MHz, CDCl3) δ 7.83 (d, J = 16.2 Hz, 1H), 7.38 (s, 1H),
7.29–7.22 (d, J = 16.2 Hz, 1H), 4.02 (t, J = 5.1 Hz, 2H), 3.88 (t,
J = 5.1 Hz, 2H), 3.73–3.66 (m, 2H), 3.23–3.18 (m, 2H), 3.15–3.11 (m,
2H), 2.87 (m, 2H), 2.61 (s, 2H), 2.40 (s, 2H), 1.73–1.67 (m, 4H), 1.43
(s, 6H), 1.28 (s, 6H), 1.06 (s, 6H), 0.92 (s, 9H), 0.83 (s, 9H), 0.12 (s,
6H), 0.01 (s, 6H). 13C NMR (126 MHz, CDCl3) δ 195.67, 165.02,
156.95, 144.36, 133.83, 127.69, 126.89, 123.42, 123.19, 122.17,
117.06, 76.17, 63.31, 62.55, 52.01, 47.41, 46.83, 41.42, 39.99, 36.38,
35.82, 32.73, 32.48, 32.30, 32.22, 30.95, 30.67, 30.18, 28.44, 28.13,
26.03, 25.93, 18.52, 18.32, −5.11, −5.23. Anal. Calcd (%) for
0.94 (s, 12H), 0.84 (s, 12H). 13C NMR (126 MHz, CDCl3) δ 191.49,
156.70, 156.44, 150.45, 143.91, 131.35, 127.98, 126.86, 126.73,
124.66, 123.20, 122.26, 117.67, 75.91, 62.95, 62.56, 47.42, 46.88,
41.78, 40.12, 40.01, 37.41, 36.51, 32.75, 32.26, 31.08, 30.25, 30.07,
29.70, 28.37, 26.03, 25.89, 22.69, 18.52, 18.26, 14.13, −5.11, −5.24.
Anal. Calcd (%) for C44H73NO4SSi2: C, 68.79; H, 9.58; N, 1.82; found: C,
68.77; H, 9.59; N, 1.81;
2.2.6. Synthesis of 2-(4-((1E,3E)-3-(3-((E)-2-(8-(2-((tert-butyldimethylsilyl)
oxy)ethoxy)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]
quinolin-9-yl)vinyl)-2-((2-((tert-butyldimethylsilyl)oxy)ethyl)thio)-5,5-
dimethylcyclohex-2-en-1-ylidene)prop-1-en-1-yl)-3-cyano-5,5-dimethylfuran-
2(5H)-ylidene)malononitrile (Chromophore FLD3)
To a solution of 6a (0.31 g, 0.40 mmol) and the TCF acceptor
(0.96 g, 0.48 mmol) in MeOH (60 ml) was added several drops of trie-
thyl amine. The reaction was allowed to stir at 78 °C for 5 h. The re-
action mixture was cooled and green crystal precipitation was fa-
cilitated. After removal of the solvent under reduced pressure, the crude
product was purified by silica chromatography, eluting with (AcOEt:
Hexane = 1:5) to give chromophore FLD3 as a green solid in 43.6%
yield (0.17 g, 0.17 mmol). MS (MALDI-TOF), m/z: 949.43 (M+). 1H
NMR (400 MHz, CDCl3) δ 8.14 (dd, J = 14.5, 12.5 Hz, 1H), 7.97 (d,
J = 16.0 Hz, 1H), 7.53 (d, J = 12.2 Hz, 1H), 7.41 (s, 1H), 7.26 (s, 1H),
6.38 (d, J = 14.7 Hz, 1H), 4.04 (t, J = 5.2 Hz, 2H), 3.91 (t, J = 5.1 Hz,
2H), 3.73 (t, J = 7.2 Hz, 2H), 3.27 (t, J = 5.9 Hz, 2H), 3.23–3.16 (m,
2H), 2.74 (t, J = 7.2 Hz, 2H), 2.55 (m, 2H), 2.51 (m, 2H), 1.74 (d,
J = 5.5 Hz, 4H), 1.70 (s, 6H), 1.45 (s, 6H), 1.32 (s, 6H), 1.03 (s, 6H),
0.94 (s, 8H), 0.86 (s, 10H), 0.14 (s, 6H), 0.02 (s, 6H). 13C NMR
(101 MHz, CDCl3) δ 176.25, 173.16, 157.11, 154.98, 152.51, 144.82,
144.47, 133.01, 129.39, 127.63, 126.96, 124.29, 123.23, 122.39,
117.65, 116.20, 112.63, 112.07, 111.75, 96.75, 93.65, 62.67, 62.45,
54.90, 47.44, 46.88, 41.76, 41.20, 39.75, 38.01, 36.11, 32.66, 32.16,
30.73, 30.27, 29.99, 28.30, 26.36, 25.96, 25.85, 18.44, 18.26, 0.95,
−5.18, −5.25. Anal. Calcd (%) for C55H80N4O4SSi2: C, 69.57; H, 8.49;
N, 5.90; found: C, 69.59; H, 8.48; N, 5.91;
C
42H71NO4SSi2: C, 67.96; H, 9.64; N, 1.89; found: C, 67.97; H, 9.63; N,
1.90;
2.2.4. Synthesis of (E)-2-(3-((E)-2-(8-(2-((tert-butyldimethylsilyl)oxy)
ethoxy)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]
quinolin-9-yl)vinyl)-2-((2-((tert-butyldimethylsilyl)oxy)ethyl)thio)-5,5-
dimethylcyclohex-2-en-1-ylidene)acetonitrile (Compound 5a)
A three-necked flask was charged with NaH (0.27 g, 11.2 mmol) in
dry THF (12 mL) under N2. Diethyl(cyanomethyl)phosphonate
(1.81 mL, 1.99g, 11.2 mmol) was slowly introduced to the mixture
dropwise by syringe. After the solution became clear, compound 4a
(1.68 g, 2.25 mmol) in THF (5 mL) was added to the mixture which was
directly refluxed for 24 h. After the removal of THF in vacuo, the re-
sidue was directly purified by the column chromatography on silica gel
using ethyl acetate and hexane (v/v, 1:8 to 1:6) as the eluent to afford a
red solid 5a in 73.1% yield (1.26 g, 1.64 mmol). MS (MALDI-TOF), m/z:
765.26 (M+). 1H NMR (500 MHz, CDCl3) δ 7.83 (d, J = 16.2 Hz, 1H),
7.39 (s, 1H), 7.16 (d, J = 16.2 Hz, 1H), 6.29 (s, 1H), 4.06 (t, J = 4.8 Hz,
2H), 3.94 (t, J = 4.8 Hz, 2H), 3.73 (t, J = 5.6 Hz, 2H), 3.25 (t,
J = 4.8 Hz, 2H), 3.17 (d, J = 4.8 Hz, 2H), 2.74 (m, 2H), 2.62 (s, 2H),
2.52 (s, 2H), 1.76 (m, 4H), 1.47 (s, 6H), 1.34 (s, 6H), 1.06 (s, 6H), 0.98
(s, 9H), 0.90 (s, 9H), 0.17 (s, 6H), 0.06 (s, 6H). 13C NMR (126 MHz,
CDCl3) δ 158.58, 156.51, 149.42, 143.90, 131.51, 126.85, 125.67,
123.95, 123.17, 122.22, 119.22, 117.55, 94.30, 75.95, 62.65, 47.41,
46.81, 43.38, 41.76, 40.14, 37.69, 36.53, 32.74, 32.24, 31.08, 30.84,
30.22, 30.17, 29.71, 28.09, 26.04, 25.90, 18.50, 18.27, 14.14, 10.96,
−5.11, −5.25. Anal. Calcd (%) for C44H72N2O3SSi2: C, 69.06; H, 9.48;
N, 3.66; found: C, 69.07; H, 9.47; N, 3.65;
2.2.7. Synthesis of 2-(4-((1E,3E)-3-(3-((E)-2-(8-(2-((tert-butyldimethylsilyl)
oxy)ethoxy)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]
quinolin-9-yl)vinyl)-2-((2-((tert-butyldimethylsilyl)oxy)ethyl)thio)-5,5-
dimethylcyclohex-2-en-1-ylidene)prop-1-en-1-yl)-3-cyano-5-phenyl-5-
(trifluoromethyl)furan-2(5H)-ylidene)malononitrile (Chromophore FLD4)
Compound 6a (0.31 g, 0.40 mmol) and CF3-Ph-TCF acceptor (0.15 g,
0.48 mmol) were mixed with anhydrous ethanol (5 mL). The mixture
was allowed to stir at 65 °C for 2h. The solvent was removed under
vacuum and the residual mixture was purified by flash chromatography
on silica gel using ethyl acetate and hexane (v/v, 1: 4) to give chro-
2.2.5. Synthesis of (E)-2-(3-((E)-2-(8-(2-((tert-butyldimethylsilyl)oxy)
ethoxy)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]
quinolin-9-yl)vinyl)-2-((2-((tert-butyldimethylsilyl)oxy)ethyl)thio)-5,5-
dimethylcyclohex-2-en-1-ylidene)acetaldehyde (Compound 6a)
mophore FLD4 as
a deep green solid in 73.2% yield (0.31 g,
0.29 mmol). MS (MALDI-TOF), m/z: 1065.51 (M+). 1H NMR (300 MHz,
Acetone) δ 8.14 (d, J = 15.1 Hz, 1H), 7.99 (s, 1H), 7.76–7.70 (m, 2H),
7.62 (m, 3H), 7.59 (d, J = 7.6 Hz, 2H), 7.51–7.33 (m, 1H), 6.53 (d,
J = 15.1 Hz, 1H), 4.13 (t, J = 4.7 Hz, 2H), 3.99 (t, J = 4.7 Hz, 2H),
3.74 (t, J = 6.8 Hz, 2H), 3.51 (t, J = 5.9 Hz, 2H), 3.46–3.41 (m, 2H),
2.76 (t, J = 6.8 Hz, 2H), 2.71–2.22 (m, 4H), 1.81–1.74 (m, 4H), 1.47 (s,
6H), 1.32 (s, 6H), 0.96 (s, 15H), 0.84 (s, 9H), 0.17 (s, 6H), 0.01 (s, 6H).
13C NMR (126 MHz, Acetone) δ 175.87, 159.36, 158.05, 157.77,
156.84, 148.41, 144.35, 137.71, 131.50, 130.95, 129.98, 129.59,
128.78, 128.53, 126.78, 124.97, 124.18, 123.23, 121.65, 118.98,
116.65, 114.88, 112.84, 112.73, 112.24, 94.66, 94.42, 77.15, 62.46,
62.39, 51.83, 47.87, 47.19, 41.26, 41.08, 40.72, 38.94, 38.18, 35.15,
32.93, 32.52, 31.94, 30.41, 30.20, 29.75, 27.90, 27.19, 25.58, 25.43,
18.20, 17.98, −5.12, −5.23. Anal. Calcd (%) forC60H79F3N4O4SSi2: C,
67.63; H, 7.47; N, 5.26; found: C, 67.61; H, 7.48; N, 5.25;
The solution of compound 5a (0.77 g, 1.00 mmol) in 20.0 mL of
fresh dried toluene was cooled to −78 °C and the solution of
Diisobutylaluminum hydride in hexanes (1.5 M, 2.72 mL, 4.00 mmol)
was added dropwise. After being kept at −78 °C for 2 h, wet silica gel
(1.2 g) with 10.0 mL of H2O was added and the reaction mixture was
stirred at 0 °C for 1 h. The organic products were extracted in ethyl
acetate, washed with H2O, and the solvent evaporated in vacuum. The
residue was purified by the column chromatography on silica gel using
ethyl acetate and hexane (v/v, 1:2 to 1:1) as the eluent to afford a red
solid 6a in 71.1% yield (0.55 g, 0.71 mmol). MS (MALDI-TOF), m/z:
768.31 (M+).1H NMR (300 MHz, CDCl3) δ 10.15 (d, J = 8.1 Hz, 1H,
CHO), 7.90 (d, J = 16.2 Hz, 1H, CH), 7.37 (s, 1H, ArH), 7.13 (d,
J = 16.2 Hz, 1H), 7.01 (d, J = 8.1 Hz, 1H), 4.03 (t, J = 5.0 Hz, 2H),
3.90 (t, J = 5.0 Hz, 2H), 3.73–3.67 (m, 2H), 3.24–3.18 (m, 2H),
3.17–3.12 (m, 2H), 2.76–2.70 (m, 2H), 2.76 (s, 2H), 2.51 (s, 2H),
1.77–1.70 (m, 4H), 1.44 (s, 6H), 1.31 (s, 6H), 1.26 (s, 6H), 1.03 (s, 6H),
57

F. Liu et al.
DyesandPigments157(2018)55–63
Scheme 1. Synthetic routes for chromophores FLD1-FLD4.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of chromophores FLD1-FLD4 is depicted in Scheme 1.
Starting from the julolidine donor intermediate compounds 1a, chro-
mophores FLD1-FLD4 were synthesized in good overall yields through
simple six step reactions: the free hydroxyl group on the julolidine
donor was protected by the bulky tert-butyldimethylsilyl (TBDMS)
group to protect the hydroxyl group, improve the solubility and provide
a suitable isolation. In the presence of sodium ethoxide as the base, 2-
mercaptoethanol can be easily deprotonated to form a nucleophilic
thiolate which underwent ring-opening reaction of epoxyisophorone 1b
to selectively generate 2-mercaptoethanol -substituted isophorone 2c.
This intermediate can be reacted directly with compound 2a in one-pot
via the Knoevenagel condensation to furnish compound 3a with a high
overall yield. The hydroxyl group of compound 3a or 3b was protected
by TBDMS group to afford compound 4a or 4b. By using the Wittig-
Horner reaction, compound 4a or 4b was reacted with diethyl(cyano-
methyl)phosphonate to result in trienenitrile 5a or 5b. The reduction
with DIBAL-H followed by hydrolysis converted the nitrile group on 5a
or 5b into the corresponding aldehyde 6a or 6b. The aldehyde-con-
taining donor-bridge 6a or 6b was condensed with TCF or CF3-Ph-TCF
acceptor to furnish the chromophore FLD1-FLD4 as green solids. All of
the chromophores were fully characterized by mass spectra, 1H NMR,
13C NMR and elemental analysis. These chromophores possess good
solubility in common organic solvents, such as dichloromethane,
chloromethane and acetone.
Fig. 1. 1H NMR spectra of all-trans aldehyde 6a (B) in comparison with cis- and
trans-isomer mixtures of aldehyde 6b (A).
Compund 6b was cis- and trans-isomer mixtures as shown in Fig. 1,
so the derived chromophores FLD1 and FLD2 were also mixtures with
cis and trans conformation. The alkylation sterically rigidified the
polyenic bridge to ensure its all-trans. So the compund 6a, possess all-
trans conformation (Fig. 1). Compound 6a was condensed with the
acceptor to afford the all-trans chromophores FLD3 and FLD4. Its all-
trans conformation could provide a better conjugation and increase the
μβ value of the chromophores significantly.
Fig. 2. TGA curves of chromophores FLD1-FLD4 with a heating rate of 10 °C
min−1 in nitrogen atmosphere.
3.2. Thermal stability
decomposition temperatures (Td) higher than 200 °C (202 °C-260 °C).
Chromophore FLD3 has the highest decomposition temperature
(260 °C), followed by FLD1 (253 °C), FLD4 (219 °C) and FLD2 (202 °C).
The thermal stability of the chromophores was investigated using
thermogravimetric analysis (TGA) as shown in Fig. 2 and Table 1. All
the chromophores exhibited good thermal stabilities with the
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F. Liu et al.
DyesandPigments157(2018)55–63
Table 1
acetone and acetonitrile, chromophores FLD1–FLD4 showed re-
markably distinct optical features in solvatochromism and absorption
band shape. With the solvents varying from the dioxane to the
chloroform, a continuous shift of the absorption maxima to longer
wavelength was observed for all the four chromophores. More dramatic
effects were observed while investigating the absorption spectra of the
four chromophores in highly polar solvents, such as chloroform,
acetone and acetonitrile. Chromophores FLD1 and FLD3 showed a
bathochromic shift of its absorption maximum initially and reversed to
a hypsochromic shift for more polar solvents of acetone and acetoni-
trile, namely inverted solvatochromism [30]. However, a saturation
behavior was found for chromophores FLD2 and FLD4 in the more polar
solvents, such as acetone and acetonitrile. The absorption peak of
chromophores FLD2 and FLD4 in these solvents exhibited almost no
shift with increase of solvent polarity. It was accompanied by a no-
ticeable spectral shape change showing a dominant low-energy ab-
sorption peak and a high-energy shoulder. The chromophores FLD2 and
FLD4 showed a sharp and intense cyanine-type absorption band with a
dramatically narrowed bandwidth in highly polar solvents. These in-
teresting solvent-dependent spectral behaviors of chromophores FLD2
and FLD4 suggested that the more dipolar chromophore FLD2 and FLD4
can be polarized quite close to the cyanine limit, or even beyond that
into the zwitter-ionic regime in the most polar solvents [31].
Thermal and optical properties data of the chromophores.
a
b
Cmpd
Td (°C)
λmax (nm)
λmax (nm)
Δλc (nm)
FLD1
FLD2
FLD3
FLD4
253
202
260
219
744
975
750
961
656
771
665
753
88
204
85
208
a
b
λmax was measured in chloroform. λmax was measured in dioxane.
Δλc was the difference between λmax and λmax
.
a
b
The Td of chromophore FLD1 and FLD3 were higher than their analo-
gues chromophores FLD2 and FLD4 with CF3-Ph-TCF acceptor which is
easy to understand because the TCF acceptor displays greater thermal
stability than the analogues CF3-Ph-TCF acceptor [26,27]. The Td of
chromophore FLD3 and FLD4 which contains a solubilizing STBDMS
group at the bridge end were higher than their analogues chromophore
FLD1 and FLD2. All the thermal stabilities of four chromophores were
high enough for the application in EO device preparation [28].
3.3. Optical properties
We evaluated the UV–Vis absorption spectra of chromophores
FLD1-FLD4 in various solvents with different dielectric constants as
shown in Figs. 3 and 4. The spectral data are summarized in Table 1.
The absorption maxima (λmax) of FLD1 and FLD3 are located at 656 nm
and 665 nm in dioxane and 744 nm and 750 nm in chloroform, re-
spectively (Table 1). The λmax of chromophore FLD3 with STBDMS
substituted at the bridge are all bathochromically shifted (6–9 nm)
compared to those of the corresponding unsubstituted analogue chro-
mophore FLD1. These spectral characteristics can be rationalized by the
fact that without a huge strong end-capped acceptor to pull electron,
the neutral form will prevail over the charge separated form in the
ground state. Therefore, these intermediates show a higher degree of
BLA [29]. As a result, the STBDMS substitution in these systems simply
acts as a weak acceptor and results in decreased band gap. However,
when coupled with a strong CF3-Ph-TCF acceptor, the degree of BLA of
highly polarized push-pull chromophore FLD2 and FLD4 is reduced, in
comparison with chromophore FLD2, chromophore FLD4 exhibited
blue-shifted maximum absorptions, for example, the λmax of chromo-
phore FLD4 was 14 nm shorter than that of its analogue chromophore
FLD2 in chloroform.
3.4. Theoretical calculations
To understand the ground-state polarization and microscopic NLO
properties of the designed chromophores, the DFT calculations were
carried out at the B3LYP level by employing the split valence 6-31G
basis set using the Gaussian 09 program package [32]. The first hy-
perpolarizability (β) were carried out at the cam-B3LYP level by em-
ploying the split valence 6-31G basis set using the Gaussian 09 program
package [33]. All molecules were assumed to be in trans-configurations
[34]. The HOMO-LUMO energy gap, dipole moment (μ), β and μβ of the
chromophores obtained from DFT calculations are summarized in
Table 2.
The HOMO-LUMO energy gap was used to understand the charge
transfer interaction occurring in a chromophore molecule [35]. Fig. 5
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 energy gaps ΔE (DFT) were calculated by DFT cal-
culations as shown in Table 2. The energy gaps between the HOMO and
LUMO energy for chromophores FLD1-FLD4 were 1.933 eV, 1.859 eV,
1.923 eV and 1.863 eV, respectively. As ΔE is reduced, resulting in a
bathochromic shift of λmax within the series of compounds. These re-
sults correspond with the conclusion of UV–Vis spectra analysis.
The HOMO-LUMO energy gap is used to understand the charge
transfer interaction that occurs within a molecular, In many cases, not
only HOMO and LUMO states, the transitions related with HOMO-1, -2
and LUMO+1, +2 states are strongly contributed in such long con-
jugated materials [36]. So, the combined molecular orbital energy level
graph constituted by LUMO+2 to HOMO-2 for all the four chromo-
phores is given in Fig. 6.
Besides, the solvatochromic behavior was also an important aspect
to investigate the polarity of chromophores. As shown in Fig. 4, in non-
polar solvents such as dioxane and toluene, all four chromophores ex-
hibited a similar broad π–π* intramolecular charge-transfer (ICT) ab-
sorption band. However, in more polar solvents including chloroform,
The geometrically optimized structures of the molecules were in-
vestigated by calculating the dihedral angles of the donor moieties as
shown in Fig. 7. For example, in chromophore FLD1, the silane chain
substituent located on the oxygen atom of the donor made a large di-
hedral angle of 73.02° with average plane of the chromophore. As for
chromophore FLD3, the silane chain substituent located on the donor
and the bridge made a dihedral angle of 71.12° and 76.39° with average
plane of the chromophore, respectively. These large dihedral angles
caused large steric hindrance that suppressed aggregations among
molecules.
As reported earlier, the β value has a close relationship with the
strength of the donor and acceptor end groups and on the nature and
Fig. 3. UV–Vis absorption spectra of chromophores FLD1-FLD4 in chloroform.
59

F. Liu et al.
DyesandPigments157(2018)55–63
Fig. 4. UV–Vis absorption spectra of chromophores FLD1-FLD4 in five kinds of aprotic solvents with varying dielectric constants.
Table 2
Summary of DFT and EO coefficients of the chromophores.
Cmpd △E(DFT)a (eV) βtot (10−30esu) μc (D) μβ (10−48esu) r33 (pm/V)
b
FLD1
FLD2
FLD3
FLD4
1.933
1.859
1.923
1.863
966.6
24.86
25.36
25.02
26.12
24029.6
28854.6
26010.8
30993.9
150
58
238
82
1137.8
1039.6
1186.6
△E(DFT)a was calculated from DFT calculations.
βtotb is the first-order hyperpolarizability calculated from DFT calculations.
μc is the total dipole moment.
Fig. 6. Molecular orbital energy level diagram of chromophores FLD1-FLD4.
length of the bridge, substituents, steric hindrance and intramolecular
charge-transfer and so on [35]. The β value of chromophore FLD2 and
FLD4 were larger than that of chromophore FLD1 and FLD3 due to
narrower energy gap between HOMO and LUMO, illustrating that the
increased acceptor strength of the chromophores significantly increase
their microscopic hyperpolarizability. Chromophore FLD3 exhibited a
larger β value compared to that of chromophore FLD1, This shows that
the perturbation of STBDMS substituent does not affect the efficient
one-dimensional charge-transfer excitation which determines the
principal tensor within a two-level model [37]. Instead, an auxiliary π-
accepting STBDMS group conjugated with the amino donor and located
in the middle of the bridge may potentially generate a small off-diag-
onal component of the two-dimensional tensor [38]. The enhanced
value of chromophore FLD3 might be ascribed to the unique and
moderate π-accepting ability of the substituted group STBDMS.
Fig. 5. Frontier molecular orbitals HOMO and LUMO of chromophores FLD1-
FLD4.
60

F. Liu et al.
DyesandPigments157(2018)55–63
Fig. 8. r33 values of NLO thin films as a function of chromophore loading
densities.
chromophores with large dipole moment generate intermolecular static
electric field dipole-dipole interaction, which leads to the unfavorable
antiparallel packing of chromophores. So the number of truly oriented
chromophore (N) is small. In molecular optimization, introducing the
huge steric hindrance group to isolate chromophores is the most pop-
ular and easy way to attenuate the dipole-dipole interactions of chro-
mophores [41].
Fig. 7. Optimized structures of chromophores FLD1-FLD4.
3.5. Electro-optic performance
In order to investigate the translation of the microscopic hyperpo-
larizability into macroscopic EO response, the polymer films doped
with different doping concentration of chromophores into amorphous
polycarbonate (APC) were prepared using dibromomethane as solvent.
The resulting solutions were filtered through a 0.2-μm PTFE filter and
spin-coated onto indium tin oxide (ITO) glass substrates. Films of doped
polymers were baked at 80 °C in a vacuum oven overnight. The corona
poling process was carried out at a temperature of 10 °C above the glass
transition temperature (Tg) of the polymer. The r33 values of poled films
were measured by Teng-Man simple reflection method at a wavelength
of 1310 nm using carefully selected thin ITO electrode with low re-
flectivity and good transparency in order to minimize the contribution
from multiple reflections [39].
To evaluate their EO activities, 10 wt% of the chromophores FLD1-
FLD4 were doped into APC and formulated as the typical guest–host
polymer. By avoiding severe intermolecular electrostatic interactions at
this modest number density of chromophores, the EO performance of
thin film materials could be viewed as being more related to the in-
trinsic molecular properties for a deeper under-standing of the struc-
ture–property relationships within this series of chromophore [42].
The E-O coefficients obtained from the optimal poling conditions
are summarized in Fig. 8. For this series of chromophores, the poled
films of FLD1/APC, FLD2/APC, FLD3/APC and FLD4/APC afforded r33
values of 77, 22, 98 and 29 pm/V, respectively. The film-FLD3/APC
achieved the largest r33 value probably due to its large β and good
polarizability. With relative lower β, the r33 value of film-FLD1/APC
was smaller than that of film-FLD3/APC. The dramatic drop in EO ac-
tivity from FLD1/APC to FLD2/APC and FLD3/APC to FLD4/APC is
mainly associated with the inefficient poling due to its relatively high
conductivity under the poling conditions. The absorption maxima
(λmax) of chromophore FLD2 and chromophore FLD4 are located at
The measured r33 values depend on the chromophore number
density (N), β value, and poling efficiency, described by the cos3 (θ)
order parameter, as indicated by Ref. [40].
r33 ≈ 2Nβf cos3 θ /n4
(1)
Where the f term describes electric-field factors and n is the re-
fractive index of the film, both of which remain relatively constant for
related chromophores at similar loading densities. cos3 (θ) is the
acentric order parameter. θ is the angle between the permanent dipole
moment of chromophores and the applied electric field. At low con-
centration, the electro-optic activity increased with chromophore den-
sity, dipole moment and the strength of electric poling field. However,
when the concentrations of chromophores increased to a certain extent,
the N and cos3 (θ) are no longer independent factor. Then,
cos3(θ) = (μF/5kT)[1 − L2 (W/kT)]
(2)
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 inter-
molecular electrostatic energy (W) to the thermal energy (kT). L is re-
lated to electrostatic interactions between molecules.
This relationship succeeds in qualitatively predicting the important
trends involving electro-optic activity although not the quantitative
values of electro-optic coefficients. When the intermolecular electro-
static interactions are neglected, the electro-optic coefficient (r33
)
should increase linearly with chromophore density, dipole moment,
first hyperpolarizability and the strength of electric poling field. But
Chart 1. Chemical structure for chromophores FLD1-FLD4.
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F. Liu et al.
DyesandPigments157(2018)55–63
982 nm and 993 nm in acetone. The intramolecular electron transfer is
very strong which was related to the stronger ICT ability, made it more
sensitive to the ambient environment and thus lead to the relatively
poor photostability and chemical stability. Such a subtle difference in
the structure of the acceptor between CF3-Ph-TCF and TCF could
change the EO property of chromophores in a large degree. Chromo-
phores with stronger CF3-Ph-TCF acceptor were reported showing
larger NLO properties than chromophores with normal TCF acceptor
[43]. However, in this study, relatively weak TCF acceptor could pro-
foundly improve the macro-scopic nonlinearities for chromophore
FLD1 and FLD3 with julolidinyl-based donor and tetraene-derived
bridge. So, simply driving the chromophore absorption maxima to
lower energies is not a good strategy for enhancing r33. All these ana-
lyses illustrate the critical role of fine-tuning the electron-donating
strength of donor–acceptor polyenes in optimizing their linear and
nonlinear optical properties.
The r33 values of films containing the four chromophores were
measured at different loading densities, as shown in Fig. 8. For chro-
mophores FLD1, the r33 values of film-FLD1/APC, were gradually im-
proved from 77 pm/V (10 wt %) to 150 pm/V (25 wt %), while the r33
values dropped to 137 pm/V as the loading density increasing from
25 wt % to 30 wt %. The similar trend was also observed for film-FLD2/
APC, whose r33 values increased from 22 pm/V (10 wt %) to 58 pm/V
(25 wt %). The r33 values of film-FLD3/APC were improved from 98
pm/V (10 wt %) to 238 pm/V (30 wt %). The similar trend was also
observed for film-FLD4/APC whose r33 values increased from 29 pm/V
(10 wt %) to 82 pm/V (30 wt %). It should be mentioned, the r33 values
of FLD3/APC are within the highest one ever reported for simple guest
host EO polymers when the polymer films doped with chromophores
which end-capped with TCF acceptor [1,44,45].
Such a great increase is attributed to the purity and structural fea-
tures of chromophore FLD3. Its all-trans conformation could provide a
better conjugation and increase the μβ value of the chromophores sig-
nificantly. In addition, the central polyene bridge of chromophore FLD3
is spatially isolated by both the isophorone ring and the center-an-
chored STBDMS group. This spatial arrangement also greatly increases
the solubility of chromophore FLD3 resulting in a higher number den-
sity of chromophores that can be incorporated into the polymer matrix.
Chromophore FLD3 with a 12-membered ring donor has large three-
dimensional steric hindrances, and the STBDMS group also provides
effective site isolation to decrease the strong electrostatic interactions
among chromophores. When at a low-density range, the intermolecular
dipolar interactions are relatively weak. The intermolecular dipole-di-
pole interactions would become stronger and stronger, accompanying
with the increased concentration of NLO chromophore moieties in the
polymer which would finally lead to a decreased NLO coefficient. The
introduction of side-STBDMS group attached to the conjugated π-bridge
can make inter-chromophore electrostatic interactions less favorable, a
larger dihedral angle widens the distance between chromophores and
weakens the dipole–dipole interactions between dipolar molecules,
which benefit the macroscopic electro-optic efficiency [46].
bridge of julolidinyl based chromophores FLD3 to ensure its all-trans
conformation. The modification of STBDMS group on chromophore can
prevent close packing of molecules, and attenuate the strong dipole-
dipole electrostatic interactions bewteen chromophores to improve
poling efficiency of EO polymers. Compared with the nonsubstituted
analogue chromophore FLD1, chromophore FLD3 with more isolation
groups achieves higher molecular hyperpolarizability and enhanced E-
O coefficient. Polymers doped with 30 wt% of a spatially modified
chromophore FLD3 have been poled to afford ultra-large electrooptic
coefficients (r33) of 238 pm/V. This value is much higher than similar
reported NLO chromophore [23,24,47].
Acknowledgments
All calculations were performed on the cluster of Key Laboratory of
Theoretical and Computational Photochemistry, Ministry of Education,
China. We are grateful to the National Natural Science Foundation of
China (No. 21575078, No. 41373118, No. 61101054 and No.
21473227) and Science Foundation of Guangzhou University under
Grant No. 27000503171 for the financial support.
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