Synthesis and nonlinear optical properties of novel conjugated small molecules based on indole donor

Article abstract of DOI:10.1016/j.molstruc.2018.03.133

Buchwald-Hartwig amination of aryl halides with solvent-free condition was used for the preparation of electron donors with high density of electron cloud. Organic second order nonlinear optical chromophore ZML-c based on this electron donor was designed and synthesized. And chromophore ZML-ref without modified groups was also prepared as the contrast. Their delocalized energy levels were estimated by UV–Vis. spectra. Their thermal properties were studied by thermogravimetric analysis (TGA). Obviously, chromophore ZML-c showed lower delocalized energy levels and lower thermal decomposition temperature (just about 160 °C). The poled films showed the largest electro-optic (EO) coefficients of 23.5 pm/V and 10.6 pm/V respectively for chromophore ZML-c and ZML-ref. These results indicate that effective modification in indole ring may be a promising way to drastically change the electro-optical features of the titled compounds and show a direction of designing new electron donor in the field of nonlinear optical materials.

Full text of DOI:10.1016/j.molstruc.2018.03.133

Accepted Manuscript
Synthesis and nonlinear optical properties of novel conjugated small molecules based
on indole donor
Jialei Liu, Maolin Zhang, Wu Gao, A.A. Fedorchuk, I.V. Kityk
PII:
S0022-2860(18)30430-7
10.1016/j.molstruc.2018.03.133
MOLSTR 25072
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 22 November 2017
Revised Date: 8 March 2018
Accepted Date: 31 March 2018
Please cite this article as: J. Liu, M. Zhang, W. Gao, A.A. Fedorchuk, I.V. Kityk, Synthesis and nonlinear
optical properties of novel conjugated small molecules based on indole donor, Journal of Molecular
Structure (2018), doi: 10.1016/j.molstruc.2018.03.133.
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ACCEPTED MANUSCRIPT
Synthesis and nonlinear optical properties of novel conjugated
small molecules based on indole donor
a*
a
b
c
d*
Jialei Liu , Maolin Zhang , Wu Gao , A.A.Fedorchuk , I.V. Kityk
a. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China;
b. Xi’an catalyst new materials Co., Ltd, Xi’an 710299, China;
c. Ivan Franko National University of Lviv, Faculty of Chemistry, Lviv, Ukraine
d. Institute of Optoelectronics and Measuring Systems, Faculty of Electrical Engineering, Armii Kajowej 17,
PL-42-201
Abstract
Buchwald-Hartwig amination of aryl halides with solvent-free condition was used for the
preparation of electron donors with high density of electron cloud. Organic second order
nonlinear optical chromophore ZML-c based on this electron donor was designed and
synthesized. And chromophore ZML-ref without modified groups was also prepared as the
contrast. Their delocalized energy levels were estimated by UV-Vis. spectra. Their thermal
properties were studied by thermogravimetric analysis (TGA). Obviously, chromophore
ZML-c showed lower delocalized energy levels and lower thermal decomposition
o
temperature (just about 160 C). The poled films showed the largest electro-optic (EO )
coefficients of 23.5 pm/V and 10.6 pm/V respectively for chromophore ZML-c and ZML-ref.
These results indicate that effective modification in indole ring may be a promising way to
drastically change the electro-optical features of the titled compounds and show a direction of
designing new electron donor in the field of nonlinear optical materials.
Keywords: Chromophore; Indole; Electron donor; Nonlinear optic; Buchwald-Hartwig
*
Corresponding author: liujialei@mail.ipc.ac.cn;iwank74@gmail.com
1
. Introduction
1

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Due to the special properties of photonics and electronics, organic conjugated small
molecules have been widely studied in the recent years. Especially they have widely
application in organic photovoltaic, organic light emitting diode, photochromism and so on.
[
1-5] Nonlinear optical materials are also prepared based on these organic conjugated small
molecules. [6-10] Nonlinear optical materials have some special requirements for these
organic conjugated small molecules, such as strong electronic mobility, non-central symmetry
of the electronic cloud, good thermal and optical stabilities and so on.[11-14] Due to its
advantages in bandwidth, response time, machinability and electro-optics coefficients, lots of
new chemical reactions and functional groups were used to design and prepare organic
conjugated small molecules for nonlinear optical materials application.[15-19] As we know,
the introduction of strong electron donors and large conjugated structures can improve the
nonlinear optical properties greatly. [20-22] And in the recent years, lots of novel
chromophores were reported. [33-36] Indole groups constructed by six-membered conjugated
group and five-membered conjugated group, which can improve the conjugated structure
greatly. But there is just a nitrogen atom used as the rich electrons atom, which is not enough.
If a second atom with rich electron can introduced to indole, the electron donating ability of
indole groups can be improved greatly. And the EO coefficients of nonlinear optical materials
based on these conjugated small molecules will become larger. [23-25]
To solve this problem, solvent-free Buchwald–Hartwig reaction is used based on the
catalytic system of Pd(OAc) /dicyclohexyl(2',6'-diisopropoxy-[1,1'-biphenyl]-2-yl)phosphine.
2
Pyrrolidine is introduced to the conjugated system of indole with high yield, wild chemical
reaction conditions. The nitrogen atom of pyrrolidine can be used as the second rich electron
centre of such donor groups and the density of electronic cloud of the indole groups is
improved greatly. By the introduction of carbon carbon double bonds as electronic bridges
and tricyanyl furan as the electronic acceptors, a novel organic conjugated small molecule
with D-π-A structure is prepared. And its linear optical property, nonlinear optical property,
thermal property and solubility are studied in details in this paper.
2
. Results and discussion
2
.1 synthesis and structure of chromophore ZML-c and ZML-ref
The structure and synthesis of chromophore ZML-c was shown in scheme 1. The
2

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introduction of pyrrolidine to indole was the most important reaction in the synthesis of
chromophore ZML-c. And the chemical reaction conditions were studied in details. The
reactive temperature, selection of protective gas and catalysts were all optimized. The
selection of catalyst influenced the yield of this reaction greatly. The most active catalytic
systems were those bearing bulky phosphine ligands. The highest catalytic activity was
exhibited by Pd(OAc) /RuPhos, which gave a virtually quantitative yield. Surprisingly, the
2
influence of air to this reaction is very poor, even suffering aerobic conditions, such reaction
with the catalyst of Pd(OAc) /RuPhos is still very high.
2
Vilsmeier reaction was used to introduce aldehyde group to indole, the control of
temperature and humidity have great influence on the yield of compound 3. So, the reactants
and
solvents
used
in
this
step
were
all
purified
and
dried.
2
-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) was prepared according
to the literatures. And the coupling between TCF and indole donor was operated by
Knoevenagel condensation reaction under the catalyst of piperdinie. The synthesis of
chromophore ZML-ref was a little simpler, just including two steps: Vilsmeier reaction and
Knoevenagel condensation reaction, which has been reported by Kim S et al in 2011. [37]
Scheme 1. The structure and synthesis of chromophores ZML-c and ZML-ref
2
.2. Linear optical features of chromophore ZML-c
3

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The linear optical features of chromophroe ZML-c and ZML-ref were shown in fighre 1.
They were measured in the solution of tetrahydrofuran(THF). Chromophore ZML-ref showed
us two strong absorption peaks respectively at 309 nm and 489 nm. The absorption peak was
stronger at 309 nm than 489 nm. These absorption peaks were attributed to the n-π* and π-π*
transition of the electrons in chromophore ZML-ref. Obviously, after the introduction of
pyrrolidine group in the donor part, the strong absorption peak at 309 nm disappeared. And
the absorption peak at 489 nm was redshifted to 559 nm. After the introduction of piperidine
group in the donor part, the n-π* transition was reduced and theπ-π* transition was enhanced.
This was a positive factor for improving the nonlinear optical property of this kind of
materials.
Else, chromophores based on traditional aniline electron donors, using the same electron
acceptor and bridge with chromophore ZML-c showed the strongest absorption peaks at
around 550 nm. As known, absorption peaks have directly relationships with the delocalized
energy of this kind of chromophores. Normally, chromophores with lower delocalized energy
could show us higher first order hyperpolarizability. The reason for this phenomenon is that
the electron mobility of chromophores under external electric fields is decided by their
delocalized energy level. As the delocalized energy level is reduced, their electron mobility
can be improved greatly. And the improvement of the electron mobility can improve their
second nonlinear optical susceptibility at the same time. Comparing the absorption spectrum
of chromophore ZML-c and ZML-ref, the advantage for the introduction of piperidine group
to the donor part is very obvious.
4

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0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
ZML-ref
ZML-c
-0.1
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. UV-Vis. absorption spectrum of chromophores ZML-c and ZML-ref in THF
3
.2. Thermal stability of chromophore ZML-1
5

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1
05
00
1
ZML-c
ZML-ref
95
90
85
80
75
70
65
60
55
0
100
200
300
400
500
o
Temperature ( C)
Fig. 2. TGA profile of chromophores ZML-c and ZML-ref
Thermal stability is an important property for organic second nonlinear optical
chromophore, which can decide its applications and test methods in the following steps. The
processing of optics-electronic materials and devices is very complicated, mostly including
sputtering, plating edible and package process. So the thermal stability can greatly influence
the processing. In this paper, the thermal stabilities of chromophore ZML-c and ZML-ref
were investigated by thermogravimetric analysis (TGA). The TGA profiles for chromophore
ZML-c and ZML-ref were shown in figure 2. The thermal decomposition temperatures were
o
o
1
60 C and 247 C respectively for chromophore ZML-c and ZML-ref. Due to the rich
electronic property, chromophore ZML-c showed us a lower thermal decomposition
o
temperature, which was just 160 C. Traditional chromophores based on aniline donors
o
usually showed us thermal decomposition temperatures around 250 C, which was between
chromophore ZML-c and ZML-ref. This result indicated that the thermal stability was
influenced by their rich electronic level greatly. Else, the decomposition of chromophore
6

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ZML-c was in one stage. But the decomposition of chromophore ZML-ref was on three
stages; in the first stage, the weight loss was about 10wt%; in the second stage the weight loss
o
was about 30wt%. And when the temperature reach 500 C, the weight loss of chromophroe
ZML-c and ZML-ref were about the same (60%wt). Such a phenomenon was hard to explain
for us.
Following the previous studies most of the processing (electronic field poling)
o
temperatures were carried out around 100-120 C. The thermal decomposition temperature
was high enough for their application as materials for electro-optic polymer light modulators,
even for chromophore ZML-c with a lower thermal decomposition temperature.
3
.3. Nonlinear optical property of chromophore ZML-c and ZML-ref
Nonlinear optical property of such kind of materials is the focus of our attention, which
can decide the drive voltage and electro-optical conversion efficiency of the devices prepared
based on these materials. The nonlinear optical property of chromophore ZML-c and
ZML-ref are studied by Teng-Man method at the laser wavelength of 1310 nm. And the
results are called electro-optics (EO) coefficients. Before the measurements, such kind of
materials should be prepared to optical films on the indium tin oxide (ITO)glass substrate by
the method of spin coating and undergo the process of electronic field poling.
Table. 1. EO coefficients of chromophores ZML-c and ZML-ref
a
chromophore
ZML-c
10wt%
15.4
9.8
15wt%
18.9
10.6
20wt%
23.5
9.3
25wt%
21.2
6.8
b
ZML-ref
a. Chromophore loading density
b. EO coefficients@1310 nm (pm/V)
The EO coefficients of chromophore ZML-c and ZML-ref were shown in table 1.
Chromophore ZML-c showed us the largest EO coefficients at the chromophore loading
density of 20wt% about 23.5 pm/V. chromophore ZML-ref showed us the largest EO
coefficients at the chromophore loading density of about 15wt% about 10.6 pm/V. the
difference between the largest EO coefficients of chromophore ZML-c and ZML-ref was
attributed to the electron donating ability of the donors. Else, the five-member cyclic
structure of piperidine in the donor of chromophroe ZML-c increased the distance between
7

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the chromophore molecules and reduced the intermolecular dipole interaction force. These
could improve the poling efficiency and EO coefficients greatly.
Else, we also comparing the EO coefficients of chromophore ZML-c and ZML-ref with
the traditional chromophore using aniline as donor, whose largest EO coefficients was about
1
6.5 pm/V with a loading density of 25 wt% in APC at wavelength 1310 nm. Obviously, the
traditional chromophore using aniline as donor showed larger EO coefficients than
chromophore ZML-ref and smaller EO coefficients than chromophore ZML-c. This was
mainly due to the electron donating ability of the donor and the intermolecular dipole
interaction forces. [26]
3
.4 Theoretical calculation of chromophores ZML-c and ZML-ref
To analyze the origin of the enhanced effect we have performed quantum chemical
calculations using Gaussian W09 package [27]. To clarify the role of the particular
chromophores, we performed additional quantum chemical simulations particularly for
hyperpolarizability at a wavelength of 1310 nm. Molecule models optimization and
calculation of first order hyperpolarizabilities such as HOMO and LUMO shape were
performed using DFT and B3LYP functional supplemented [28, 29] with the standard
6
-31G(d) basis set.
Shapes of HOMO and LUMO orbitals of ZML-c and ZML-ref molecules are presented
at Figure 3. Addition of donor pyrrolidine fragment in ZML-c leads to the greater separation
of the molecule on two parts – donor and acceptor ones in comparison with ZML-ref. At the
case of ZML-c molecule one conclude that we deal with the clear space donor-acceptor space
separation defining the principal charge transfer contrary to ZML-ref, where HOMO and
LUMO overlap for the most part of molecule. So, including the fact, that D-π-A structure is
more expressed for ZML-c one can suppose, that NLO properties, namely SHG, likely would
be better for the mentioned one. This assumption is also confirmed by the following first
order hyperpolarizability (β) calculations and the experimental measurement of both samples
properties.
8

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Fig. 3. HOMO and LUMO shape of chromophores ZML-c and ZML-ref
The hyperpolarizability (β) is a third order tensor which can be described by a 3 × 3 × 3
matrix with 27 components, which can be reduced to 10 components due to the Kleinman
symmetry [30]. Using x, y and z components hyperpolarizability (β) could be written as
follows:
The (β) components of GAUSSIAN 09 output are reported in atomic units and therefore
-33
the calculated values are converted into esu units (1 a.u = 8.6393 × 10 esu). The computed
values of the dipole moment ꢀ for the ZML-c molecule are equals to 19.7216 Debye, and
1
5.6567 Debye for ZML-ref, which also confirms more expressed dipole-acceptor structure
for the ZML-c. Both static and dynamic (at the 1310 nm) average hyperpolarizabilities (β) are
-28
-28
also greater for ZML-c molecule (1.311·10 and 7.988·10 respectively) then for the
-29
-28
ZML-ref one (5.266·10 and 1.064·10 respectively) (Table 2). Thereby, according to the
9

ACCEPTED MANUSCRIPT
calculations, ZML-c would possess more effective SHG, what is in good agreement with the
experimental measurements.
Table. 2. Teoretically calculated hyperpolarizabilities of chromophore
ZML-c and ZML-ref
ZML-c
Dynamic 1310 nm
Tensor
ZML-ref
Dynamic 1310 nm
component,
a.u.
βxxx
βxxy
βxyy
βxxz
βxyz
βxzz
βyyy
βyyz
βyzz
βzzz
Static
Static
14506.500
-2912.830
469.183
24.630
91826.900
-13668.500
-203.119
37.975
5890.480
-1520.370
30.024
0.468
-0.029
-0.510
62.841
-0.031
5.242
12118.300
-2431.500
-20.042
1.173
-0.073
-6.194
73.542
-0.029
6.026
0.001
11.801
13.610
-82.261
-10.269
3.723
-6.447
-0.851
-146.513
202.032
3.749
-6.546
-0.791
0.001
βx
βy
βz
14893.422
-2929.546
27.502
91477.268
-13473.014
40.933
5919.994
-1452.287
0.437
12092.064
-2351.932
1.145
β
15178.835
1.311·10
92464.125
7.988·10
6095.528
5.266·10
12318.668
1.064·10
-
28
-28
-29
-28
β, esu
3
. Experiments
3
.1 instruments and materials
1
H NMR spectra were determined by an Advance Bruker 400M (400MHz) 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 UV-Vis. experiments were performed on Cary 5000 photo
spectrometer. The TGA was determined by TA5000-2950TGA (TA co.) with a heating rate
-1
of 10 °C min under the protection of nitrogen. All chemicals, commercially available, are
used without further purification unless stated. 2-dicyanomethylene-3-cyano-4-methyl-2,
5
-dihydrofuran (TCF) acceptor was prepared according to the literature. [31]
3
.2 Poling and EO coefficients measurements
1
0

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Guest–host polymers were prepared by formulating chromophores ZML-c and ZML-ref
into APC using dibromomethane (CH Br ) as the solvent. The resulting solutions were
2
2
filtered through a 0.22 µm teflon membrane filter and spin-coated onto indium tin oxide (ITO)
glass substrates. Films of doped polymers were baked in a vacuum oven at 45ºC to remove
the residual solvent. The poling process was carried out at a temperature of T of the polymer.
g
The EO coefficients were measured using Teng-Man simple reflection technique at the
wavelength of 1310 nm. [32]
3
.3 Synthesis of 6-(pyrrolidin-1-yl)-1H-indole
-bromo-1H-indole (2.05 g, 10.2 mmol), pyrrolidine (0.71 g, 10 mmol), Pd(OAc)2 (0.05
6
g, 0.02 mmol), RuPhos (0.18 g, 0.04 mmol) and powdered NaOtBu (1.2 g, 1.2 mmol) were
dissolved in a screw-cap vial equipped with a magnetic stir bar. The vial was transferred to a
preheated oil bath (110 °C). After 12 h, the reaction mixture was cooled and poured into 20
mL water, and then extracted by 10 mL dichloromethane for three times. The organic phase
was separated, the solvent was evaporated in vacuo, and the product was isolated by flash
1
chromatography on a silica gel column. H NMR (400 MHz, CDCl , ppm): δ=7.82 (s, 1H), 7.46 (d, J
3
=
8.6 Hz, 1H), 6.97 (s, 1H), 6.57 (d, J = 8.6 Hz, 1H), 6.53 (s, 1H), 6.41 (s, 1H), 3.32 (s, 5H), 2.02 (s, 5H);
1
1
1
3C NMR (101 MHz, Acetone) δ 145.03 (s), 138.30 (s), 121.66 (s), 120.49 (s), 119.74 (s), 107.70 (s),
+
01.24 (s), 163.03 (s), 93.13 (s), 48.12 (s), 25.20 (s). MS (EI) (M , C H N ): calcd: 186.253; found:
1
2
14
2
86.0974.
3
.4 synthesis of chromophore ZML-c
Compound 3
-(pyrrolidin-1-yl)-1H-indole (0.56 g, 3.0 mmol) was dissolved in 15 ml DMF in an
6
oven-dried two necked-flask. Then, the mixture was cooled to 0 ∼ 5 °C and POCl (0.34 ml)
3
was added by drop-wise. After the POCl was added, the mixture was stirred at this
3
o
temperature for another 2 h and heated to 90 C for 3 h. Then, the solution was poured into
saturated NaHCO ice-water and stirred for another 1h. The organic phase was extracted with
3
CH Cl and dried over anhydrous MgSO . After removal of the solvent, the pure product was
2
2
4
obtained by a silica-gel flash column chromatography (acetone: petroleum ether=1:15, v/v) to
1
give a bright yellow solid compound 3 (0.39 g, 1.8 mmol) in 61% yield. H NMR (400 MHz,
Acetone, ppm): δ =10.65 (s, 1H), 9.91 (s, 1H), 7.98 (d, J = 8.6 Hz, 1H), 7.89 (s, 1H), 6.65 (d,
1
1

ACCEPTED MANUSCRIPT
+
J = 8.7 Hz, 1H), 6.59 (s, 1H), 3.29 (s, 4H), 2.02 (s, 4H); MS (EI) (M , C H N O ): calcd:
1
3
14
2
2
14.26; found:214.1053.
Chromophore ZML-c
Compound 3 (0.39g, 1.3 mmol) and TCF (0.27 g, 1.4 mmol) were dissolved in 20 ml
ethanol. Then, several drops of pyridine was added to the solution. Then the mixture was
heated under reflux for 3-4 h. The solution was cooled to room temperature. The resulting
product was filtered off washed with cold ethanol and dried as purple solid 5a (0.28 g, 0.6
1
mmol) in 43% yield. H NMR (400 MHz, DMSO, ppm): δ= 12.29 (s, 1H), 8.17 (d, J = 15.7
Hz, 2H), 7.72 (s, 1H), 6.83 (s, 1H), 6.73 (s, 1H), 6.60 (s, 1H), 3.30 (s, 4H), 1.99 (s, 4H), 1.80
+
(
s, 6H); MS(MALDI-TOF), m/z: 395.803(M ).
3
.5 synthesis of chromophore ZML-ref
The synthesis of chromophore ZML-ref is used the method of synthesis of chromophore
1
ZML-c. Yield 41%l. H NMR (400 MHz, CDCl ): δ 7.96 (d, J = 15.1 Hz, 1H), 7.79 (d, J =
3
8
.7 Hz, 1H), 7.58 (m, 2H), 7.27 (s, 1H), 6.56 (d, J = 8.3 Hz, 1H), 6.30 (d, J=8.3Hz, 1H), 1.78
+
(
s, 6H); MS (MALDI-TOF), m/z: 326.105 (M ). Elemental Analysis: C, 73.15; H, 4.35; N,
1
7.66; O, 4.84(found); C, 73.61; H, 4.32; N, 17.17; O, 4.90 (calculated).
4
. Conclusions
Buchwald-Hartwig amination of aryl halides with solvent-free condition was confirmed
as a efficient and green reaction for the preparation of novel electron donor. And such kind of
reaction may be widely used in the synthesis of novel organic conjugated small molecules.
Indole modified by pyrrodine groups showed us excellent electron donating ability.
Comparing with chromophroe ZML-ref, after the modification by pyrrodine group,
chromophore ZML-c showed us lower delocalized energy levels and larger EO coefficients.
These advantages indicated that the effective modification in indole ring may be a potential
direction for the design and preparation of electron donor of NLO chromophore.Though the
o
thermal decomposition temperature of chromophore ZML-c reduced to 160 C; after some
other modification for chromophore ZML-c, the thermal stability would be improved in the
future. To analyze the origin of the enhanced effect we have performed quantum chemical
calculations using Gaussian W09 package. The computed values of the dipole moment ꢀ and
the average hyperpolarizability (β) for the ZML-c molecule are equals to 19.7216 Debye,
1
2

ACCEPTED MANUSCRIPT
2
,41×10–28 esu and 15.6567 Debye, 1,89×10–28 esu for ZML-ref respectively. At the case of
ZML-c molecule one conclude that we deal with the clear space donor-acceptor space
separation defining the principal charge transfer contrary to ZML-ref, where HOMO and
LUMO overlap for the most part of molecule.
Acknowledgements
The work reported here was supported by (1) The National Natural Science Foundation
of China (No. 51503215 and 61671413), (2) The National Key Scientific Instrument and
Equipment Development Project of China (2014YQ24044508), (3) National Key Research
and Development Program of China (2016YFC0101602), (4) Shanxi Province Science
Foundation for Youths (201601D021080), (5) Research Project Supported by Shanxi
Scholarship Council of China (2016-085), (6) Science Foundation of North University of
China (XJJ2016019).
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.
Buchwald-Hartwig amination of aryl halides was used for the preparation of
electron donors
. Novel chromophore ZML-c based on this electron donor was designed and
synthesized
Chromophore ZML-c showed good thermal, optical NLO optical properties
.
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.