Synthesis and properties of NLO chromophores with fine-tuned gradient electronic structures

Article abstract of DOI:10.1039/b817789a

A novel series of heterocycle-based NLO chromophores based on different combinations of auxiliary donor (i.e., benzene, thiophene and pyrrole) and auxiliary acceptor (i.e., thiazole with different regiochemistries) were designed and synthesized. Due to the different electron-rich and poor nature of the auxiliary donors and acceptors, respectively, the resulting NLO chromophores have systematically varied ground-state electronic structures, as evidenced by the ¹H NMR, CV and UV-vis investigations. The nonlinear optical properties of the resulting NLO chromophores were studied by UV-vis spectroscopy, Hyper-Rayleigh scattering (HRS), and semi-empirical computations. All the chromophores have very large molecular hyperpolarizabilities (β_{1000 nm}) in the range of 704-1500 × 10^-30 esu (or β₀, 318-768 × 10^-30 esu), which showed a great sensitivity to the gradient electronic structures. Upon increasing the electron density from benzene to thiophene and to pyrrole, substantial increases in β₀ were observed; significantly larger β₀ values were also observed for NLO chromophores based on "matched" thiazole (C2 is connected to the acceptor) than those based on "un-matched" thiazole (C5 is connected to the acceptor). TGA investigations showed good thermal stability for the resulting NLO chromophores. However, with the increase of electron density of the auxiliary donor, a decrease in thermal and photochemical stability was observed. It is interesting to note that NLO chromophores based on triarylamine as the donor and thiazole as the auxiliary acceptor exhibited not only high thermal stability but also very large β₀ values.

Full text of DOI:10.1039/b817789a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and properties of NLO chromophores with fine-tuned gradient
electronic structures†
a
Xiaohua Ma, Fei Ma, Zhenhua Zhao, Naiheng Song and Jianping Zhang
b
a
a
b
*
*
Received 9th October 2008, Accepted 28th January 2009
First published as an Advance Article on the web 3rd March 2009
DOI: 10.1039/b817789a
A novel series of heterocycle-based NLO chromophores based on different combinations of auxiliary
donor (i.e., benzene, thiophene and pyrrole) and auxiliary acceptor (i.e., thiazole with different
regiochemistries) were designed and synthesized. Due to the different electron-rich and poor nature of
the auxiliary donors and acceptors, respectively, the resulting NLO chromophores have systematically
varied ground-state electronic structures, as evidenced by the 1H NMR, CV and UV-vis investigations.
The nonlinear optical properties of the resulting NLO chromophores were studied by UV-vis
spectroscopy, Hyper-Rayleigh scattering (HRS), and semi-empirical computations. All the
chromophores have very large molecular hyperpolarizabilities (b1000 nm) in the range of 704–1500 ꢀ
10ꢁ30 esu (or b0, 318–768 ꢀ 10ꢁ30 esu), which showed a great sensitivity to the gradient electronic
structures. Upon increasing the electron density from benzene to thiophene and to pyrrole, substantial
increases in b0 were observed; significantly larger b0 values were also observed for NLO chromophores
based on ‘‘matched’’ thiazole (C2 is connected to the acceptor) than those based on ‘‘un-matched’’
thiazole (C5 is connected to the acceptor). TGA investigations showed good thermal stability for the
resulting NLO chromophores. However, with the increase of electron density of the auxiliary donor,
a decrease in thermal and photochemical stability was observed. It is interesting to note that NLO
chromophores based on triarylamine as the donor and thiazole as the auxiliary acceptor exhibited not
only high thermal stability but also very large b0 values.
based on advanced understanding of the structure-property
Introduction
relationships are highly desired.
Organic second-order nonlinear optical (NLO) polymers with
large optical nonlinearities have potential applications in ultra-
fast electro-optic (e-o) devices.1 Recently, breakthroughs in
materials development have been made to afford NLO polymers
with extremely large e-o coefficients (e.g., r33 > 100 pm/V).2–4
However, for practical e-o applications, NLO polymers with
good comprehensive materials properties are still to be devel-
oped. One critical challenge towards highly efficient NLO
polymers has been to develop NLO chromophores with simul-
taneously very large molecular first hyperpolarizability (b), good
optical transparency, and excellent thermal and photochemical
stabilities. Despite the large number of NLO chromophores that
have been developed in the past two decades, only a few of them
exhibit promising properties for e-o device applications.2–6 Thus,
to further enhance the NLO properties and to explore the full
potential of NLO chromophores, rational structural designs
According to the bond-length alternation model,5 maximal
b of D–p–A NLO chromophores can be realized for certain
degrees of ground-state polarization. Therefore, much effort has
been devoted to the design and synthesis of NLO chromophores
with different combinations of the electron donor, the p-bridge,
and the electron acceptor, aiming at the optimization of the
ground-state electronic structure. Currently, the state-of-the-art
NLO chromophores are the so-called ‘‘FTC’’ and ‘‘CLD’’ type
of chromophores developed by Dalton and co-workers, which
coupled dialkylamino donor with a strong electron acceptor (i.e.,
2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran,
TCF) to yield very large molecular optical nonlinearities (mb >
15 000 ꢀ 10ꢁ48 esu).6 However, maximal b values for these
chromophores remain largely unrealized, especially when
considering the astonishingly large mb values (e.g., mb ¼
ꢁ488 000 ꢀ 10ꢁ48 esu) achieved for zwitterionic chromophores
by Marks and co-workers.4,7
To achieve even larger b values by using stronger electron
donor and acceptor represents a challenging task due to the
limited accessibility to such groups at the current stage. A
strategy to alleviate such problem could be through the optimi-
zation of the p-bridge.8–10 Theoretical and experimental studies
have shown that, upon introduction of electron-rich heterocycle
to the donor end of the p-bridge as auxiliary donor and electron-
poor heterocycle to the acceptor end as auxiliary acceptor, the
donor and acceptor strength will be enhanced.8–13 Substantial
increases in b values can also be expected for chromophores with
such gradient electronic structures.
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of
Polymer Chemistry and Physics of Ministry of Education, Department of
Polymer Science and Engineering, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, China. E-mail: nsong@
pku.edu.cn; Fax: +86-10-62755644; Tel: +86-10-62755644
bDepartment of Chemistry, Renmin University of China, Beijing 100872,
People’s Republic of China. E-mail: jpzhang@chem.ruc.edu.cn; Tel: +86-
10-62516604
† Electronic supplementary information (ESI) available: Experimental
details, solubility of the NLO chromophores, UV-vis absorption of the
NLO chromophores in different solvents, UV-vis absorption spectra of
the NLO chromophores and UV-vis absorption spectra of
chromophore solutions. See DOI: 10.1039/b817789a
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Among the many aromatic heterocycles, pyrrole is known to
have very high electron density and low aromaticity and can serve
as an efficient auxiliary donor to yield large molecular optical
nonlinearities.8 However, experimental studies on NLO chro-
mophores containing pyrole as the auxiliary donor were not
known until our very recent report on the first synthesis of
piperidin-1-ylpyrrole and methyl 4-methoxyphenylpyrrole-based
NLO chromophores.14 Later on, Davies et al. reported the
synthesis and properties of a diarylaminopyrrole-based NLO
chromophore.15 Although these chromophores showed promising
properties, the potential of pyrrole in enhancing the donor
strength remains largely unexplored. On the other hand, the
electron-poor five-membered heterocycle thiazole has recently
drawn great attention due to its good potential in increasing the b
valueswhen usedas the auxiliaryacceptor.10–13 Due tothe inherent
dipolar nature, thiazole can be incorporated into NLO chromo-
phores in either ‘‘matched’’ (i.e., the electron-poor C2 is connected
to the acceptor) or ‘‘un-matched’’ (i.e., the electron-rich C5 is
connected to the acceptor) configurations. Although both
connectionsare expectedtogive enhancedbvalues, the formercan
be more efficient, as indicatedby the theoretical computations.10,11
Up to now, experimental investigations on thiazole-based NLO
chromophores are still limited; those containing ‘‘matched’’
thiazole at the acceptor end of the p-bridge are not yet known.
Towards an advanced understanding of structure-property
relationship and to achieve highly efficient NLO chromophores,
we have herein designed and prepared a novel series of hetero-
cycle-based NLO chromophores with fine-tuned gradient
electronic structures. These chromophores have the same
tricyanovinyl electron acceptor and the same length of
p-conjugation, but contain different aromatic rings with varying
electron densities (i.e., pyrrole > thiophene > benzene)8,10 as the
auxiliary donor and 1,3-heteroaromatic thiazole with different
regiochemistries as the auxiliary acceptor. Examination of NLO
chromophores with such a complete combination of auxiliary
donor and acceptor should allow us to gain more insight on the
potential contribution of p-bridge to b values and offer guide-
lines for rational structure design toward highly efficient NLO
chromophores. In this study, modifications on the amino donor
part were also made by using different substitutions (e.g., dialkyl,
aryl alkyl, and diaryl substitutions) to fine tune the donor ability.
Triarylamine donor15,16 that have recently been successfully used
in NLO chromophores to afford large b values and high thermal
stability was for the first time coupled with thiazole as auxiliary
acceptor in this study to yield novel NLO chromophores with
good combination of properties.
Tianyuan Welfare Chemical Factory), potassium t-butyloxide
(t-BuOK) (Alfa Aesar Corp.), tetrabutylammonium perchlorate
(Bu4NClO4, 99%) (Fluka), 2,20-bipyridine (Sinopharm Chemcial
Reagent Co. Ltd) and thiazol-2-amine (Beijing Ouhe Technology
Co., Ltd) were purchased and used as received. Phosphorous
oxide trichloride (POCl3), trimethylsilane chloride (TMSCl),
tetrachloromethane (CCl4), thionyl chloride, sulfuryl dichloride
and aniline were purchased from Beijing Chemical Reagent Ltd
and distilled before use. N,N-Dimethylformamide (DMF) and
tetrahydrofuran were distilled over calcium hydride and metal
˚
sodium, respectively, and stored over molecular sieves (3 A)
under nitrogen. All other chemicals were purchased from Beijing
Chemical Reagent Ltd and used as received.
Instruments
1H and 13C NMR spectra were recorded on a Varian 300 MHz or
a Bruker ARX400 spectrometer using deuterated chloroform
(CDCl3) or dimethylsulfoxide (DMSO-d6) as the solvent.
Chemical shifts were reported in ppm scale with tetramethylsi-
lane as the internal standard. Infrared (IR) spectroscopies were
measured on a Nicolet Magna 750 Fourier transform infrared
spectrometer. UV-Vis absorption spectra were recorded with
a DU 640 UV-vis spectrometer. Thermogravimetric analysis
(TGA) was performed in nitrogen on a TA TGA-DSC Q600
thermogravimetric analyzer with a heating rate of 20 ꢂC/min.
Melting points were measured with a SGW-X-4 microscopic
melting point instrument. High and low resolution mass spec-
troscopy was recorded on a ZAB-HS mass spectrometer.
Elemental analysis was performed on an Elementar Vario
EL instrument (Elementar Analysensysteme GmbH). Cyclic
voltammetry (CV) was performed on a CHI 600C cyclic volta-
metric analyzer in a solution of tetrabutylammonium perchlorate
(0.1 M) in acetonitrile at a scanning rate of 200 mV/s. Before each
measurement, the cell was deoxygenated with argon for 20 min.
The working electrode was a glassy carbon disk (2 mm diameter,
freshly polished) for voltammetry. A platinum stick (1 mm thick)
was used as the counter-electrode and AgCl/Ag the reference
electrode. All potentials are reported versus the AgCl/Ag
electrode. Geometric optimization and computation of static
molecular first hyperpolarizabilities were carried out with the
MOPAC program (2007) in AM1 method.
Hyper-Rayleigh scattering (HRS) measurements
HRS measurements of the NLO chromophores were performed
at the excitation wavelength of 1000 nm. A third harmonic wave
at 355 nm with a pulse duartion of 7 ns (repetition rate, 10 Hz)
was supplied by a Q-switched Nd3+:YAG laser, and was used to
drive an optical parametric osciallator (MOPO-SL/MOPO-PO,
Spectra-Physics) to deliver the light pulses at 1000 nm. The laser
beam was passed through a Pelin–Broca prism and other steering
optics, and focused into the sample cell with a lens (f ¼ 200 mm).
The pulse energy measured in front of the sample cell was 2 mJ/
pulse. The HRS signal collected with a camera lens (f/1.4) was
further focused via a lens (f ¼ 150 mm) onto the entrance slit of
a triplet spectrometer (Trivista, SP2500i, Princeton Instruments/
Acton), and was detected by an intensified CCD detector (ICCD
PI-MAX, Princeton Instruments/Acton) operated with a gate
Experimentals
Materials
1-Methyl-5-(piperidin-1-yl)pyrrole-2-carbaldehyde,14 5-(piper-
idin-1-yl)
thiophene-2-carbaldehyde,14
2-chloromethyl
thiophene,17 2-(chloromethyl) thiazole,18 2-chloro-5-(chlor-
omethyl)thiazole,19 thiazole-5-carbaldehyde,20 4-methoxy-N-(4-
methoxyphenyl)-N-phenylbenzenamine,21
4-(bis(4-methoxy
phenyl)amino)benzaldehyde22 and tetracyanoethene23 were
prepared referring to literature methods. n-Butyl lithium
(n-BuLi) (Acros Organics), 1-iodo-4-methoxybenzene (Linzi
2976 | J. Mater. Chem., 2009, 19, 2975–2985
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1
width of 10 ns. For each HRS measurement the sample was
exposed to 1000 laser shots, and the accumulated harmonic
signal at 500 nm was extracted, via spectral deconvolution, from
the background fluorescence induced by two-photon absorption.
The intensity of HRS signal was calibrated with the Beer–
Lambert law for the correction of the chromophore self-
absorption. To determine the b value of a chromophore,
p-nitroaniline (p-NA) in dimethylsulfoxide (DMSO) was used as
an external standard, whose b value at 1000 nm (34.5 ꢀ 10ꢁ30 esu)
was calculated based on the known b value at 1064 nm (28.8 ꢀ
10ꢁ30 esu)24 within the framework of the two-level model. The
concentration of p-NA in DMSO was in the range of 1 ꢀ 10ꢁ3–5
ꢀ 10ꢁ1 mol/L. All of the sample solutions in chloroform were
prepared in the concentration range of 1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ4 mol/L.
75% yield): H NMR (300 MHz, CDCl3): d 8.75 (1H, s), 7.74
(1H, s), 4.90 (2H, d, J ¼ 4.8 Hz), 3.1 (1H, t, J ¼ 5.4 Hz).
5-(Chloromethyl)thiazole
A mixture solution of thiazol-5-ylmethanol (11.5 g, 100 mmol)
and thionyl chloride (2.36 g, 200 mmol) in chloroform was
refluxed at 80 ꢂC for 2 h. It was then poured into water (150 mL)
and extracted with ethyl acetate (3 ꢀ 50 mL). The organic
extracts were combined, washed with saturated aqueous sodium
carbonate solution (30 mL), and dried over anhydrous magne-
sium sulfate. After removal of the solvent by rotary-evaporation,
a yellow liquid product was obtained, which was used for the next
step of synthesis without further purification (3.5 g, 31% yield): 1H
NMR (300 MHz, CDCl3): d 8.80 (1H, s), 7.80 (1H, s), 4.91 (2H, s).
Preparation of intermediates
Diethyl thiazol-5-ylmethylphosphonate
Diethyl thiophen-2-ylmethyl phosphonate
A mixture of 5-(chloromethyl)thiazole (2.00 g, 15.1 mmol) in
triethyl phosphite (10.0 mL) was refluxed at 130 ꢂC for 36 h. The
excessive amount of triethyl phosphite was removed under high
vacuum (ꢃ0.5 mmHg) to give a light-yellow viscous liquid
A solution of 2-chloromethyl thiophene (3.00 g, 22.7 mmol) in
triethyl phosphite (7.50 g, 45.2 mmol) was heated to 130 ꢂC under
nitrogen and stirred for 36 h. The excess amount of triethyl
phosphite was removed under vacuum (ꢃ0.5 mmHg) to leave
a sticky yellow liquid product (5.0 g, 94% yield): 1H NMR
(300 MHz, CDCl3): d 7.17–7.20 (1H, m), 6.95–7.01 (2H, m),
3.95–4.10 (4H, m), 3.3 (1H, d, J ¼ 22 Hz), 1.28 (6H, t, J ¼ 7.2 Hz,
7.2 Hz); 13C NMR (75 MHz, CDCl3): d 132.4, 132.2, 127.3, 127.2,
126.9, 126.8, 124.6, 124.5, 62.3, 62.4, 28.8, 26.9, 16.3, 16.2.
1
product (2.0 g, 57% yield): H NMR (300 MHz, CDCl3): d 8.75
(1H, d, J ¼ 1.2 Hz), 7.55 (1H, d, J ¼ 1.2 Hz), 3.83 (4H, d, J ¼ 7.2
Hz), 3.36 (2H, d, J ¼ 21 Hz), 1.28 (6H, t, J ¼ 7.2 Hz).
Preparation of NLO chromophores
(E)-2-{2-[1-methyl-5-(piperidin-1-yl)-1H-pyrrol-2-yl] vinyl}
thiazole (P-Ti2)
Diethyl thiazol-2-ylmethylphosphonate
A solution of 1-methyl-5-(piperidin-1-yl)-1H-pyrrole-2-carbal-
dehyde (1.93 g, 10.0 mmol) and diethyl thiazol-2-ylmethyl-
phosphonate (4.70 g, 20.0 mmol) in anhydrous THF (60.0 mL)
was prepared, purged with nitrogen, and cooled with a dry ice/
acetone mixture to ꢁ78 ꢂC. To the solution was added dropwise
a solution of potassium t-butyloxide (2.24 g, 20.0 mmol) in
anhydrous tetrahydrofuran (60.0 mL). After the addition, the
reaction solution was warmed gradually up to room temperature
in 3 h and then poured into distilled water (150 mL). The mixture
solution was extracted in a separatory funnel with ethyl acetate (3
ꢀ 30 mL). The organic extracts were combined, washed with
distilled water (2 ꢀ 30 mL), and dried over anhydrous magnesium
sulfate. After removal of the solvent, the solid residue was purified
by silica-gel column chromatography (eluent: ethyl acetate/ligroin
¼ 1/4, v/v, Rf ¼ 0.5) and then recrystallized from ligroin to give
yellow sheet crystals(2.32 g, 85%yield): mp. 107–108 ꢂC; 1H NMR
(300 MHz, CDCl3): d 7.70 (1H, d, J ¼ 3.3 Hz), 7.30 (1H, d, J ¼ 16.2
Hz), 7.13 (1H, d, J ¼ 3.3 Hz), 6.97 (1H, d, J ¼ 15.9 Hz), 6.51 (2H, d,
J ¼ 3.9 Hz), 5.71 (2H, d, J ¼ 3.9 Hz), 3.56 (3H, s), 2.83 (4H, t,
J ¼ 5.1 Hz, 5.4 Hz), 1.53–1.70 (6H, m); 13C NMR (75 MHz,
CDCl3): d 168.3, 146.1, 143.0, 126.1, 123.4, 116.3, 114.5, 108.9,
96.3, 53.8, 29.8, 26.0, 24.0; anal. calcd for C15H19N3S: C, 65.90; H,
7.00; N, 15.37; found: C, 65.90; H, 6.93; N, 15.43.
A mixture of 2-(chloromethyl) thiazole (4.00 g, 30.0 mmol) and
triethyl phosphite (9.96 g, 60.0 mmol) was heated at 130 ꢂC under
nitrogen for 48 h. The excessive amount of triethyl phosphite was
removed under vacuum (ꢃ0.5 mmHg) to give a yellow viscous
liquid product (6.80 g, 96% yield): 1H NMR (300 MHz, CDCl3):
d 7.73 (1H, d, J ¼ 3.4 Hz), 7.29–7.32 (1H, m), 4.05–4.20 (4H, m),
3.70 (2H, d, J ¼ 21.2 Hz), 1.30 (6H, t, J ¼ 7.0 Hz, 7.0Hz).
[(2-Chlorothiazol-5-yl)methyl]triphenylphosphonium chloride
A
solution of 2-chloro-5-(chloromethyl)thiazole (3.00 g,
17.9 mmol) and triphenylphosphine (11.7 g, 44.6 mmol) in
chloroform (30 mL) was stirred at refluxing temperature for 48 h
before filtration to collect the solid product, which was washed
with acetone (10 mL) twice to give a white powdered product
(6.20 g, 90% yield): 1H NMR (300 MHz, CDCl3): d 7.86–
7.94 (6H, m), 7.76–7.82 (3H, m), 7.63–7.68 (6H, m), 7.25 (1H, d,
J ¼ 4.2 Hz), 5.70 (2H, d, J ¼ 14.1 Hz).
Thiazol-5-ylmethanol
To a solution of thiazole-5-carbaldehyde (0.226 g, 2.00 mmol) in
methanol (10 mL), was added powdered sodium borohydride
(0.08 g, 1.00 mmol) in portions. The resulting solution was stirred
at room temperature for 1 h. After removal of the solvent by
rotary-evaporation, the liquid residue was purified by silica-gel
column chromatography (eluent: ethanol/ethyl acetate/ligroin ¼
1/1/4, v/v/v, Rf ¼ 0.31) to give a colorless oily product (0.17 g,
(E)-2-(2-{2-[1-methyl-5-(piperidin-1-yl)-1H-pyrrol-2-yl]-
vinyl}thiazol-5-yl) ethene-1,1,2-tricarbonitrile (P-Ti2-TCV)
To a solution of P-Ti2 (0.273 g, 1.00 mmol) in anhydrous tetra-
hydrofuran (3 mL) at ꢁ78 ꢂC under nitrogen, was added n-butyl
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lithium (1.6 M in hexane, 0.94 mL, 1.5 mmol). The resulting
solution was allowed to warm gradually up to ꢁ10 ꢂC in 2 h and
then cooled again to ꢁ78 ꢂC. A solution of tetracyanoethene
(0.192 g, 1.50 mmol) in tetrahydrofuran (5 mL) was added
dropwise to the solution. After the reaction solution was stirred
at ꢁ78 ꢂC for 10 min, hydrochloric acid (1 M, 1 mL) was dropped
in. The solution was warmed up to room temperature and
poured into distilled water (30 mL). The mixture was extracted in
a separatory funnel with dichloromethane (5 ꢀ 30 mL). The
organic extracts were combined, washed with distilled water
(50 mL) twice, dried over anhydrous magnesium sulfate, and
rotary-evaporated to give a dark green solid residue, which was
purified by silica-gel column chromatography (eluent: ethyl
acetate/ligroin, 1/1, v/v, Rf ¼ 0.6) to yield a green solid product
(0.060 g, 16% yield); 1H NMR (400 MHz, CDCl3): d 8.43 (1H, s),
7.69 (1H, d, J ¼ 14.8 Hz), 6.96 (1H, d, J ¼ 4.4 Hz), 6.83 (1H, d,
J ¼ 14.8 Hz), 5.92 (1H, d, J ¼ 4.4 Hz), 3.60 (3H, s), 3.01 (4H, t,
J ¼ 4.8 Hz), 1.71–1.75 (4H, m), 1.63–1.64 (4H, m); 13C NMR (75
MHz, CDCl3): 178.2, 157.9, 154.5, 130.1, 128.6, 127.8, 119.7,
113.4, 113.0, 112.6, 110.2, 101.9, 52.8, 31.2, 25.7, 23.9; FT-IR
(KBr, cmꢁ1): 2935, 2855, 2192, 1570, 1508, 1439, 1409, 1352,
1226, 1127, 1100, 1026; HRMS for C20H18N6S (m/z): 374.1314
(calcd), 374.1311 (found).
resulting reaction solution was stirred at ꢁ78 ꢂC for additional 10
min, hydrochloric acid (1 M, 1 mL, 1.00 mmol) was dropped in.
The solution was warmed up to room temperature, poured into
distilled water (30 mL), and extracted with dichloromethane (3 ꢀ
30 mL). The extracts were combined, washed with distilled water,
and dried over anhydrous magnesium sulfate. After rotary-
evaporation to remove the solvent, the residue was run through
a silica-gel column chromatography (eluent: ethyl acetate/ligroin
¼ 1/1, v/v, Rf ¼ 0.55) to give the pure green solid product (0. 10 g,
1
8.5% yield); H NMR (400 MHz, CDCl3): d 8.44 (1H, s), 7.80
(1H, d, J ¼ 15.2 Hz), 7.08 (1H, d, J ¼ 4.4 Hz), 6.81–6.89 (5H, m),
6.16 (1H, d, J ¼ 4.4 Hz), 3.80 (3H, s), 3.34 (3H, s), 3.33 (3H, s);
13C NMR (75 MHz, CDCl3): d; 117.9, 157.7, 155.3, 148.6, 141.5,
129.6, 128.2, 128.1, 127.5, 120.2, 118.3, 114.9, 113.1, 112.8, 112.5,
111.1, 104.9, 55.6, 42.1, 31.1. FT-IR (KBr, cmꢁ1): 2948, 2208,
1595, 1505, 1462, 1396, 1337, 1233, 1172, 1037; HRMS for
C23H18N6OS (m/z): 426.1263 (calcd), 426.1276 (found).
(E)-2-{2-[5-(piperidin-1-yl)thiophen-2-yl]vinyl}thiazole (T-Ti2)
To a solution of 5-(piperidin-1-yl)thiophene-2-carbaldehyde
(1.95 g, 10.0 mmol) and diethyl thiazol-2-ylmethylphosphonate
(4.70 g, 20.0 mmol) in anhydrous tetrahydrofuran (80.0 mL) at
ꢁ78 ꢂC under nitrogen, was added dropwise a solution of
potassium t-butyloxide (2.24 g, 20.0 mmol) in anhydous tetra-
hydrofuran (60.0 mL). After the addition, the solution was
gradually warmed up to room temperature over 1 h and then
poured into distilled water (150 mL). The mixture was extracted
with ethyl acetate (3 ꢀ 50 mL). The organic extracts were
combined, washed with distilled water (2 ꢀ 30 mL), and dried
over magnesium sulfate. After removal of the solvent by rotary-
evaporation, the solid residue was purified by a silica-gel chro-
matography (eluent: ethyl acetate/ligroin ¼ 1/4, v/v, Rf ¼ 0.7)
and recrystallized from ligroin to give yellow needle-like crystals
(2.07 g, 75% yield): mp. 111–112 ꢂC; 1H NMR (300 MHz,
CDCl3): d 7.70 (1H, d, J ¼ 3.3 Hz), 7.42 (1H, d, J ¼ 15.6 Hz), 7.11
(1H, d, J ¼ 3.3 Hz), 6.86 (2H, d, J ¼ 3.9 Hz), 6.75 (1H, d, J ¼ 15.6
Hz), 5.94 (2H, d, J ¼ 3.9 Hz), 3.19 (4H, m), 1.67–1.74 (4H, m),
1.58–1.63 (2H, m); 13C NMR (75 MHz, CDCl3): d 167.8, 160.8,
143.0, 130.4, 128.4, 126.0, 116.5, 114.9, 103.7, 51.5, 25.0, 24.0;
Anal. Calcd. for C14H16N2S2: C, 60.83; H, 5.83, N, 10.13; Found:
C, 60.73; H, 5.81; N, 10.10.
(E)-N-(4-Methoxyphenyl)-N,1-dimethyl-5-[2-(thiazol-2-yl)-
vinyl]-1H-pyrrol- 2-amine (MP-Ti2)
To a solution of 5-(N-4-methoxyphenyl N-methyl amino)-1-
methyl-1H-pyrrole-2-carbaldehyde (1.22 g, 5.00 mmol) and
diethyl thiazol-2-ylmethylphosphonate (2.56 g, 10.0 mmo) in
anhydrous tetrahydrofuran (60.0 mL) at ꢁ78 ꢂC under nitrogen,
was added dropwise a solution of potassium t-butyloxide (1.12 g,
10.0 mmol) in anhydrous tetrahydrofuran (30.0 mL). After the
addition, the solution was allowed to gradually warm up to room
temperature and then poured into distilled water (100 mL).
The mixture was extracted with ethyl acetate (3 ꢀ 30 mL). The
organic extracts were combined, washed with distilled water
(30 mL) twice, and dried over anhydrous magnesium sulfate.
After removal of the solvent, the liquid residue was purified by
a silica-gel column chromatography (eluent: ethyl acetate/ligroin
¼ 1/8, v/v, Rf ¼ 0.3) to give a sticky liquid product (0.15 g, 93%
yield): 1H NMR (300 MHz, CDCl3): d 7.76 (1H, d, J ¼ 3.3 Hz),
7.39 (1H, d, J ¼ 15.9 Hz), 7.18 (1H, d, J ¼ 3.3 Hz), 7.03 (1H, d,
J ¼ 15.9 Hz), 6.82 (2H, d, J ¼ 9.0 Hz), 6.58–6.63 (3H, m), 5.96
(1H, d, J ¼ 4.2 Hz), 3.76 (3H, s), 3.41(3H, s), 3.23 (3H, s); 13C
NMR (75 MHz, CDCl3): d 167.9, 152.9, 143.4, 143.0, 140.9,
126.9, 123.1, 116.8, 115.6, 114.6, 108.6, 103.0, 55.7, 40.8, 29.8.
(E)-2-(2-{2-[5-(piperidin-1-yl)thiophen-2-yl]vinyl}thiazol-5-yl)-
ethene-1,1,2- tricarbonitrile (T-Ti2-TCV)
To a stirred solution of T-Ti2 (0.273 g, 0.989 mmol) in anhydrous
tetrahydrofuran (3.00 mL) at ꢁ78 ꢂC under nitrogen, n-butyl
lithium (1.6 M in hexane, 0.940 mL, 1.50 mmol) was added in
drops. After the addition, the solution was allowed to warm up to
ꢁ10 ꢂC over 2 h and then cooled again to ꢁ78 ꢂC, to which
a solution of tetracyanoethene (0.192 g, 1.50 mmol) in anhydrous
tetrahydrofuran (5 mL) was added dropwise. After stirring for
5 min, distilled water (3.00 mL) was added and the resulting
solution was warmed up to room temperature. After removal of
the solvent under vacuum, the residue was washed thoroughly
with diethyl ether (2 ꢀ 30 mL) and dichloromethane until
the washings were colorless. The dichlormethane washings
were combined, dried over anhydrous magnesium sulfate, and
(E)-2-[2-(2-{5-[(4-methoxyphenyl)(methyl)amino]-1-methyl-1H-
pyrrol-2-yl}vinyl)thiazol-5-yl]ethene-1,1,2-tricarbonitrile (MP-
Ti2-TCV)
To a solution of MP-Ti2 (0. 900 g, 2.77 mmol) in anhydrous
ꢂ
tetrahydrofuran (20 mL) at ꢁ78 C under nitrogen, was added
n-butyl lithium (1.6 M in hexane, 2.08 mL, 3.33 mmol). The
ꢂ
resulting solution was allowed to warm gradually up to 0 C in
ꢂ
0.5 h and then cooled again to ꢁ78 C, to which a solution of
tetracyanoethene (0.531 g, 4.15 mmol) in anhydrous tetrahy-
drofuran (10.0 mL) was added by syringe over 5 min. After the
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rotary-evaporated to give a solid residue that was recrystallized
from chloroform to yield golden colored crystals (0.150 g, 40%
yield): 1H NMR (300 MHz, CDCl3): d 8.43 (1H, s), 7.89 (1H, d,
J ¼ 15.4 Hz), 7.27 (1H, d, J ¼ 4.5 Hz), 6.56 (1H, d, J ¼ 14.4 Hz),
6.14 (1H, d, J ¼ 4.5 Hz), 3.44 (4H, t, J ¼ 4.8 Hz), 1.73–1.70 (6H,
m); FT-IR (KBr, cmꢁ1):2942, 2859, 2209, 1581, 1496, 1445, 1411,
1382, 1324, 1234, 1172, 1070, 1011, 943; HRMS for C19H15N5S2
(m/z): 377.0769 (calcd), 377.0769 (found).
(E)-N,N-Dimethyl-4-[2-(thiazol-2-yl)vinyl]benzenamine (B-Ti2)
To a solution of 4-(dimethylamino)benzaldehyde (1.49 g,
10.0 mmol) and diethyl thiazol-2-ylmethylphosphonate (4.70ꢂg,
20.0 mmol) in anhydrous tetrahydrofuran (100 mL) at ꢁ78 C
under nitrogen, was added dropwise potassium t-butyloxide
(2.24 g, 20.0 mmol) in anhydrous tetrahydrofuran. The solution
was allowed to warm up to room temperature in 0.5 h and then
poured into distilled water (100 mL). The resulting mixture was
extracted with ethyl acetate (3 ꢀ 30 mL). The organic extracts
were combined, washed with distilled water, dried over anhy-
drous magnesium sulfate, and rotary-evaporated to remove most
of the solvent. After a flash column chromatography on silica-gel
(eluent: ethyl acetate/ligroin ¼ 1/4, v/v, Rf ¼ 0.5), the resulting
yellow solid was further purified by recrystallization from ligroin
to give yellow sheet-like crystals (2.07 g, 90% yield): mp. 123–
124 ꢂC; 1H NMR (300 MHz, CDCl3): d 7.74 (1H, d, J ¼ 3.3 Hz),
7.43 (2H, d, J ¼ 6.9Hz), 7.36 (2H, d, J ¼ 16.2 Hz), 7.10–7.16 (2H,
m), 6.69 (2H, d, J ¼ 6.9Hz), 2.99 (3H, s); 13C NMR (75 MHz,
CDCl3): d 168.2, 150.7, 143.0, 134.8, 128.3, 123.6, 116.8, 116.7,
112.0, 40.2.
(E)-2-chloro-5-{2-[5-(piperidin-1-yl)thiophen-2-yl]vinyl} thiazole
(T-Ti5)
To a solution of (2-chlorothiazol-5-yl)methyl triphenyl phos-
phonium chloride (3.56 g, 8.90 mmol) in toluene (160 mL), was
added potassium t-butyloxide (1.79 g, 17.8 mmol). The solution
was stirred at room temperature for 2.5 h, followed by the
addition of 5-(piperidin-1-yl) thiophene-2-carbaldehyde (1.56 g,
8.00 mmol) in toluene (20 mL) in drops. The reaction solution
was stirred at room temperature for another 6 h before being
poured into distilled water (200 mL). The mixture was extracted
in a separatory funnel with ethyl acetate (3 ꢀ 30 mL). The
organic extracts were combined, washed with distilled water,
dried over anhydrous magnesium sulfate, and rotary-evaporated
to give a yellow solid residue that was purified by silica-gel
column chromatography (eluent: ethyl acetate/ligroin ¼ 1/10,
v/v, Rf ¼ 0.33) and recrystallized from ligroin to yield yellow
needle-like crystals (1.30 g, 54% yield): mp: 113–114 ꢂC; 1H
NMR (300 MHz, CDCl3): d 7.32 (1H, s), 6.78 (1H, d, J ¼ 15.6
Hz), 6.76 (1H, d, J ¼ 3.9 Hz), 6.50 (1H, d, J ¼ 15.6 Hz), 5.91 (1H,
d, J ¼ 3.9 Hz), 3.18 (3H, t, J ¼ 5.4 Hz), 1.56–1.74 (6H, m); 13C
NMR (75 MHz, CDCl3): d 160.0, 152.7, 147.7, 140.9, 137.6,
128.8, 126.4, 111.4, 103.7, 51.6, 25.1, 23.6. Anal. calcd for
C14H15ClN2S2: C, 54.09; H, 4.86; N, 9.01; found: C, 54.06; H,
4.90; N, 8.96.
(E)-2-{2-[4-(Dimethylamino)styryl]thiazol-5-yl}ethene-1,1,2-
tricarbonitrile (B-Ti2-TCV)
To a solution of B-Ti2 (0.230 g, 1.00 mmol) in anhydrous tetra-
hydrofuran (3.00 mL) at ꢁ78 ꢂC under nitrogen, was added
n-buthyl lithium (1.6 M in hexane, 0.94 mL, 1.50 mmol). The
ꢂ
resulting solution was stirred at ꢁ78 C for 1 h before tetracya-
noethene (0.192 g, 1.50 mmol) in anhydrous tetrahydrofurꢂan
(4 mL) was added. The reaction solution was stirred at ꢁ78 C
for another 10 min and then poured into distilled water (3 mL).
The mixture was dried under vacuum with a cold trap and the
solid residue was washed with diethyl ether (30 mL) twice and
extracted with dichloromethane (30 mL) three times. The
dichloromethane extracts were combined, dried over anhydrous
magnesium sulfate, and rotary-evaporated to give a dark green
residue that was purified by recrystallization from chloroform to
yield the dark green solid product (0.371 g, 75% yield): 1H NMR
(300 MHz, CDCl3): d 8.55 (1H, s), 7.80 (1H, d, J ¼ 15.6 Hz), 7.54
(2H, d, J ¼ 9.0 Hz), 7.13 (1H, d, J ¼ 15.6 Hz), 6.73 (2H, d, J ¼ 9.0
Hz), 3.11 (6H, s); FT-IR (KBr, cmꢁ1): 2899, 2212 (–CN), 1595,
1510, 1414, 1366, 1232, 1223, 1157, 941, 832; HRMS for
C18H13N5S (m/z): 331.0892 (calcd), 331.0896 (found); anal. calcd
for C18H13N5: C, 65.24; H, 3.95; N, 21.13; found: C, 64.15; H,
4.85; N,22.44.
(E)-2-(5-{2-[5-(Piperidin-1-yl)thiophen-2-yl]vinyl}thiazol-2-yl)-
ethene-1,1,2–tricarbonitrile (T-Ti5-TCV)
To a solution of T-Ti5 (0.100 g, 0.323 mmol) in anhydrous
tetrahydrofuran (10.0 mL) at ꢁ78 ꢂC under nitrogen, was added
dropwise a solution of t-butyl lithium (1.5 M in pentane, 0.25
mL, 0.375 mmol). The reaction solution was stirred at ꢁ78 ꢂC for
1 h before tetracyanoethene (0.042 g, 0.328 mmol) in anhydrous
tetrahydrofuran (10.0 mL) was added in drops. The color of the
solution turned to dark green right away. The solution was
stirred at ꢁ78 ꢂC for another 5 min and then poured into distilled
water (30.0 mL). Hydrochloric acid (2 M) was added to the
mixture solution to adjust the pH value to 5, followed by
extraction with dichloromethane (3 ꢀ 20 mL). The organic
extracts were combined, washed with distilled water, dried over
anhydrous magnesium sulfate, and rotary-evaporated to give
a dark green solid that was purified by recrystallization from
(E)-N,N-dimethyl-4-[2-(thiophen-2-yl)vinyl]benzenamine: (B-T)
To a solution of 4-(dimethylamino)benzaldehyde (1.49 g,
10.0 mmol) and diethyl thiophen-2-ylmethylphosphonate (4.68
g, 20.0 mmol) in anhydrous tetrahydrofuran (60 mL) at ꢁ78 ꢂC
under nitrogen, was added dropwise potassium t-butyloxide
(2.24 g, 20.0 mmol) in anhydrous tetrahydrofuran (60 mL). After
the addition, the solution was gradually warmed up to room
temperature and then poured into distilled water (100 mL). The
resulting mixture was extracted with ethyl acetate (3 ꢀ 50 mL).
The organic extracts were combined, washed with distilled water
(2 ꢀ 30 mL), and dried over magnesium sulfate. After removal of
the solvent by rotary-evaporation, the solid residue was purified
1
ethyl acetate (0.03 g, 27% yield): H NMR (300 MHz, CDCl3):
d 7.99 (1H, s), 7.32 (1H, d, J ¼ 15 Hz), 7.13 (1H, d, J ¼ 4.5 Hz),
6.54 (1H, d, J ¼ 15 Hz), 6.09 (1H, d, J ¼ 4.5 Hz), 3.38–3.41 (4H,
m), 1.54–1.72 (6H, m); FT-IR (KBr, cmꢁ1): 2944, 2856, 2210,
1584, 1499, 1446, 1386, 1347, 1247, 1609, 1012, 919; HRMS for
C19H15N5S2 (m/z): 377.0769 (calcd), 377.0770 (found).
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by a silica-gel column chromatography (eluent: ethyl acetate/
to room temperature, and then poured into distilled water
(60.0 mL). The mixture was extracted with ethyl acetate (3 ꢀ
30 mL). The organic extracts were combined, washed with
distilled water, dried over anhydrous magnesium sulfate, and
rotary-evaporated to remove the solvent. The solid residue was
purified by a silica-gel column chromatography (eluent: ethyl
acetate/ligroin ¼ 1/1, v/v, Rf ¼ 0.6) to give green crystalline
ligroin ¼ 1/4, v/v, Rf ¼ 0.7) and recrystallized from ligroin to give
1
yellow sheet crystals (1.90 g, 85% yield): mp. 137–139 ꢂC; H
NMR (300 MHz, CDCl3): d 7.36 (2H, d, J ¼ 8.7Hz), 7.11–12
(1H, m), 7.05 (1H, d, J ¼15.9 Hz), 6.96–7.01 (2H, m), 6.87 (1H, d,
J ¼ 15.9 Hz), 6.71 (2H, d, J ¼ 8.7 Hz); 13C NMR (100 MHz,
CDCl3): d 150.0, 143.9, 128.6, 127.4, 127.36, 127.32, 124.5, 122.9,
117.6, 112.4, 40.4.
ꢂ
1
products (0.368 g, 86% yield): mp. 235–237 C; H NMR (400
MHz, CDCl3): d 8.56 (1H, s), 7.76 (1H, d, J ¼ 15.6 Hz), 7.41 (2H,
d, J ¼ 8.7 Hz), 7.11–7.14 (5H, m), 6.86–6.92 (6H, m), 3.83 (6H, s);
13C NMR (75 MHz, CDCl3): d 178.3, 157.2, 157.0, 152.0, 143.0,
138.8, 130.3, 128.1, 127.8, 125.8, 118.0, 115.3, 115.0, 112.2, 112.1,
111.8, 55.5; FT-IR (KBr, cmꢁ1): 3038, 2833, 2219, 1581, 15022,
1435, 1391, 1327, 1285, 1240, 1161, 1032, 968, 824; HRMS for
C30H21N5O2S (m/z): 515.1416 (calcd), 515.1416 (found).
(E)-2-{5-[4-(dimethylamino)styryl]thiophen-2-yl}ethene-1,1,2-
tricarbonitrile (B-T-TCV)
The synthesis referred to the literature method25: to a solution of
B-T (0.460 g, 20.0 mmol) in anhydrous DMF (15 mL), tetra-
cyanoethene (0.312 g, 24.0 mmol) in anhydrous DMF (10 mL)
was added dropwise. The solution was stirred for 24 h before
being poured into distilled water (100 mL). The solid product
was collected by filtration and recrystallized from ethyl acetate to
(E)-4-methoxy-N-(4-methoxyphenyl)-N-{4-[2-(thiazol-5-yl)-
vinyl]phenyl}benzenamine (TPA-Ti5)
1
give a dark green crystalline product (0.396 g, 60% yield): H
NMR (200 MHz, CDCl3): d 7.94 (1H, d, J ¼ 4.4 Hz), 7.48 (2H, d,
J ¼ 9.0 Hz), 7.29 (1H, d, J ¼ 15.8 Hz), 7.18 (1H, d, J ¼ 4.2Hz),
7.08 (1H, d, J ¼ 15.8 Hz), 6.82 (2H, d, J ¼ 8.4 Hz), 3.10 (6H, s);
FT-IR (KBr, cmꢁ1): 3086, 2804, 2207, 1583, 1513, 1411, 1367,
1292, 1163, 1070, 995, 942, 857, 717; HRMS for C19H14N4S
(m/z): 330.0939 (calcd), 330.0935 (found).
To a solution of 4-[bis(4-methoxyphenyl)amino]benzaldehyde
(0.333 g, 1.00 mmol) and diethyl thiazol-5-ylmethyl phosphonate
(0.52ꢂ3 g, 1.20 mmol) in anhydrous tetrahydrofuran (20 mL) at
ꢁ78 C under nitrogen, was added dropwise potassium t-butyl-
oxide (1.34 g, 1.20 mmol) in anhydrous tetrahydrofuran (20 mL).
After the addition, the solution was warmed gradually up to
room temperature and then poured into distilled water (100 mL).
The mixture was extracted with ethyl acetate (3 ꢀ 50 mL). The
organic extracts were combined, washed with distilled water (2 ꢀ
30 mL), dried over anhydrous magnesium sulfate, and rotary-
evaporated to remove the solvent. The solid residue was purified
by a silica-gel column chromatography (eluent: ethyl acetate/
ligroin ¼ 1/4, v/v, Rf ¼ 0.5) to give a yellow solid product
(0.362 g, 88% yield): 1H NMR (300 MHz, CDCl3): d 8.61 (1H, s),
7.79 (1H, s), 7.26 (2H, d, J ¼ 8.7Hz), 7.07–7.10 (5H, m), 6.84–
6.90 (7H, m), 3.80 (6H, s); 13C NMR (75 MHz, CDCl3): d 156.1,
150.5, 148.8, 140.8, 140.4, 131.9, 128.2, 127.2, 126.8, 119.9,
1115.8, 115.0, 114.7, 55.4.
(E)-4-methoxy-N-(4-methoxyphenyl)-N-{4-[2-(thiazol-2-yl)-
vinyl]phenyl} benzenamine (TPA-Ti2)
To a solution of 4-[bis(4-methoxyphenyl)amino]benzaldehyde
(1.00 g, 3.00 mmol) and diethyl thiazol-2-ylmethylphosphonate
(1.06ꢂg, 4.50 mmol) in anhydrous tetrahydrofuran (30 mL) at
ꢁ78 C under nitrogen, was added dropwise potassium t-butyl-
oxide (0.672 g, 6.00 mmol) in anhydrous tetrahydrofuran. After
the addition, the solution was gradually warmed up to room
temperature and then poured into distilled water (100 mL).
The mixture was extracted with ethyl acetate (3 ꢀ 30 mL). The
organic extracts were combined, washed with distilled water (2 ꢀ
30 mL), dried over anhydrous magnesium sulfate, and rotary-
evaporated to remove the solvent. The solid residue was purified
by a silica-gel column chromatography (eluent: ethyl acetate/
ligroin ¼ 1/4, v/v, Rf ¼ 0.5) to give a light yellow solid product
(1.19 g, 96% yield): mp. 88–89 ꢂC; 1H NMR (300 MHz, CDCl3):
d 7.76 (1H, d, J ¼ 3.3 Hz), 7.32–7.37 (3H, m), 7.07–7.13 (4H, m),
3.85 (6H, s); 13C NMR (75 MHz, CDCl3): 167.8, 156.2, 149.4,
143.2, 140.1, 134.2, 127.9, 127.3, 127.0, 119.4, 118.3, 117.2,
114.72, 55.4; FT-IR (KBr, cmꢁ1): 2954, 1595, 1505, 1327, 1296,
1245, 1178, 1134, 1104, 1031, 968, 832, 719, 573, 533.
(E)-2-(5-{4-[bis(4-methoxyphenyl)amino]styryl}thiazol-2-yl)-
ethene-1,1,2-tricarbonitrile (TPA-Ti5-TCV)
To a nitrogen-purged, stirring solution of TPA-Ti5 (0.200 g,
0.480 mmol) in anhydrous tetrahydrofuran (10.0 mL) at ꢁ78 ꢂC,
was added n-butyl lithium (1.6 M in hexane, 0.350 mL, 0.560
mmol) through a syringe. The solution was stirred at ꢁ78 ꢂC for
2 h before tetracyanoethene (0.094 g, 0.730 mmol) in anhydrous
tetrahydrofuran (5 mL) was added dropwise. The solution was
stirred at ꢁ78 ꢂC for another 5 min and then poured into distilled
water (100 mL). The mixture was extracted with ethyl acetate (50
mL) three times. The organic extracts were combined, washed
with distilled water (2 ꢀ 20 mL), dried over anhydrous magne-
sium sulfate, and rotary-evaporated to concentrate the solution
to about 20 mL. The resulting golden crystals were collected by
filtration and washed with small amount of ethyl acetate (0.05 g,
20% yield): mp. 228–239 ꢂC; 1H NMR (400 MHz, CDCl3): d 8.13
(1H, s), 7.33 (2H, d, J ¼ 8.8 Hz), 7.04–7.23 (6H, m), 6.84–6.89
(6H, m), 3.82 (6H, s); 13C NMR (100 MHz, CDCl3): 157.0, 152.1,
151.1, 145.6, 145.4, 139.7, 139.1, 130.8, 129.1, 127.6, 120.1, 118.4,
114.9, 112.4, 112.3, 111.9, 111.7, 55.4; FT-IR (KBr, cmꢁ1): 3069,
(E)-2-(2-{4-[bis(4-methoxyphenyl)amino]styryl}thiazol-5-yl)-
ethene-1,1,2-tricarbonitrile (TPA-Ti2-TCV)
To a stirring solution of TPA-Ti2 (0.345 g, 0.833 mmol) in
ꢂ
anhydrous tetrahydrofuran (10 mL) at ꢁ78 C under nitrogen,
was added n-butyl lithium (1.6 M in hexane, 0.573 ml, 0.917
mmol). The solution was allowed to warmed to room tempera-
ture in 3 h and then cooled again down to ꢁ78 ꢂC. Tetracya-
noethane (0.160 g, 1.30 mmol) in anhydrous tetrahydrofuran
(5 mL) was added dropwise to the solution. The reaction solution
was stirred for another 5 min in cold bath, warmed gradually up
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2834, 2217, 1583, 1502, 1458, 1384, 1285, 1242, 1152, 1034, 958;
HRMS-ESI for C30H21N5O2S (m/z): 515.1416 (Calcd.), 515.1420
(Found).
dialkyl, aryl alkyl, and diaryl substituted amines) were also
employed in the study to tune the donor strength. Although the
diethyl analogues of B-Ti2-TCV and B-T-TCV are known in the
literature,12 these chromophores were prepared and character-
ized for a systematic comparison between the structures and
optical nonlinearities.
Results and discussion
The syntheses of key pyrrole intermediates, e.g., 1-methyl-5-
(piperidin-1-yl)-1H-pyrrole-2-carbaldehyde and 5-(N-4-methoxy-
phenyl N-methyl amino)-1-methyl-1H-pyrrole-2-carbaldehyde,
were previously reported.14 Other intermediates, i.e., the
dialkylamino- and bis(4-methoxyphenyl)amino-substituted aryl
aldehydes and the thiazole-based Horner–Wadsworth–Emmons
(HWE) reagents, were prepared referring to the literature
methods.17–24 The coupling reactions between the aldehydes
and HWE reagents (Scheme 1) were carried out at ꢁ78 ꢂC using
Synthesis
The heterocycle-based gradient NLO chromophores were
synthesized according to Scheme 1. All the chromophores have
the same tricyanovinyl electron acceptor and the same length of
p-conjugation, but contain different aromatic rings (i.e., pyrrole,
thiophene and benzene) as the auxiliary electron donor and 1,3-
heteroaromatic thiazole with different regiochemistries as the
auxiliary electron acceptor.10,11 Different amino donors (i.e.,
Scheme 1 Synthesis of NLO chromophores with different gradient electronic structures.
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potassium t-butyloxide as a base to afford the chromophore
precursors with exclusively the (E)-alkene configuration at high
yields (e.g., 75–96%). Lithiation of the chromophore precursors
at the 2- or 5-position of thiazole with n- or t-butyl lithium,
followed by a nucleophilic substitution reaction with tetracya-
noethene, yielded the desired NLO chromophores. The purity
and the chemical structures of the precursors and the chromo-
phores were demonstrated by NMR spectroscopy, elemental
analysis and high-resolution mass spectroscopy.
Electronic structure
The different electronic structures of the chromophores were first
1
analyzed by H NMR spectroscopy and cyclic voltammetry. As
shown in Table 1, the protons (i.e., H-1 and H-2, Scheme 1) on
the auxiliary donors appear at the highest fields for pyrrole (i.e.,
P-Ti2-TCV), followed by thiophene (i.e., T-Ti2-TCV) and
benzene (i.e., B-Ti2-TCV) in sequence, indicating the varying
electron density in the order of pyrrole > thiophene > benzene,
which agrees with the calculated results.8,10 Substitution of the
alicyclic donor (e.g., piperidine for P-Ti2-TCV) with a semi-
aromatic amine (e.g., methyl 4-methoxyphenyl amine) leads to
a decreased electron density of the pyrrole in MP-Ti2-TCV, as
evidenced by the pronouced shifts of H-1 and H-2 to lower fields.
Similarly, the use of diarylamine as the donor (e.g., TPA-Ti2-
TCV) results in a decrease in electron density on the auxiliary-
donor benzene in relative to that of B-Ti2-TCV. The
regiochemistry of thiazole, i.e., either C2 (‘‘matched’’ dipole
vectors between thiazole and the chromophore) or C5 (‘‘un-
matched’’) of thiazole is connected to the acceptor, affects only
slightly the electron density of the auxiliary donors (e.g., T-Ti2-
TCV vs. T-Ti5-TCV, and TPA-Ti2-TCV vs. TPA-Ti5-TCV).
Table 1 displays the redox potentials of the NLO chromo-
phores measured in Bu4NClO4/MeCN (0.1 M) by cyclic
voltammetry. As expected, the increase of electron density of
auxiliary donor from benzene to thiophene and to pyrrole led to
a decrease in oxidation potential (Eox) in the same order, e.g.
from 0.83 V for B-Ti2-TCV to 0.59 V for T-Ti2-TCV and to
0.42 V for P-Ti2-TCV. Since oxidation potential is directly
related to the ionization potential or the tendency of losing
electron, it is expected that chromophores with electron-rich
auxiliary donors would have an efficient intramolecular charge
Fig. 1 Cyclic voltammetry of TPA-Ti2-TCV and TPA-Ti5-TCV in
acetonitrile containing tetrabutylammonium perchlorate (0.1 M) as the
supporting electrolyte.
tranfer (CT) from the donor to the acceptor upon excitation. It is
worth noting that, of all the chromophores, only triarylamine
(TPA)-based ones (i.e., TPA-Ti2-TCV and TPA-Ti5-TCV),
exhibited a fully reversible oxidation process, as shown in Fig. 1.
Such a good reversibility in oxidation may contribute to a good
thermal and photochemical stability. Recently, thermally stable
TPA-based NLO chromophores have drawn some attention.15,16
In contrast to the oxidation, all the chromophores exhibited
a reversible reduction process at low potentials of ꢁ0.23–
ꢁ0.43 V, indicating a high electron affinity of the chromophores.
The regiochemistry of thiazole affect greatly on Ered,1/2. Chro-
mophores with ‘‘matched’’ thiazole (i.e., T-Ti5-TCV and TPA-
Ti5-TCV) showed much lower Ered,1/2 than their ‘‘un-matched’’
isomers (i.e., T-Ti2-TCV and TPA-Ti2-TCV, respectively),
suggesting a stronger electron-accepting ability of the former.
Optical properties
The resulting NLO chromophores showed moderate to good
solubility in common organic solvents such as choroform,
tetrahydrofuran and DMF. Good solubility in non-polar
solvents such as dioxane and toluene was also observed for
pyrrole- and TPA-based chromophores. The UV-vis absorption
maxima (lmax) and the extinction coefficients (3) of the NLO
chromophores in chloroform are summarized Table 2. All the
chromophores exhibited broad and intense CT absorptions in
the red spectral region (lmax ¼ 668–840 nm) with onset absorp-
tions in the range of 820–1030 nm (Fig. 2). A strong sensitivity of
the linear spectra to both the auxiliary donor and the auxiliary
acceptor was revealed. The significant bathochromic shift of lmax
to lower energy observed from B-Ti2-TCV to TPA-Ti2-TCV to
P-Ti2-TCV and to T-Ti2-TCV suggests an increasing ground-
state polarization for these chromophores and a donor strength
sequence in the order of Me2NPh < (MeOPh)2NPh < 2-piper-
idin-1-ylpyrrole < piperidin-1-ylthiophene. On the other hand,
the introduction of ‘‘matched’’ thiazole as the auxiliary acceptor
red-shifted significantly the lmax (e.g., 66 nm for TPA-Ti5-TCV
and 90 nm for T-Ti5-TCV) in relative to the ‘‘un-matched’’
isomers, suggesting a further shifted ground-state toward charge
separation. The observed low optical bandgaps of ‘‘matched’’
Table 1 1H NMR and cyclic voltammetry of the NLO chromophores
d/ppm a
NLO chromophores H-1
H-2
Eox/Vb Ered/Vc DE(CV)/eVd
P-Ti2-TCV
MP-Ti2-TCV
T-Ti2-TCV
T-Ti5-TCV
B-Ti2-TCV
B-T-TCV
5.94 6.97 0.42
6.16 7.08 0.49
6.15 7.27 0.59
6.09 7.13 0.51
6.74 7.52 0.83
6.85 7.50 0.91
6.87 7.42 0.73c
6.86 7.27 0.70c
ꢁ0.42
ꢁ0.36
ꢁ0.43
ꢁ0.27
ꢁ0.37
ꢁ0.33
ꢁ0.36
ꢁ0.23
0.64
0.64
0.85
0.55
0.98
0.88
0.93
0.76
TPA-Ti2-TCV
TPA-Ti5-TCV
a
Chemical shifts of protons (H1 and H2, Scheme 1) on auxiliary donor.
Anodic peak potential vs. Ag/Ag+ reference electrode for irreversible
b
c
oxidation; scan rate 0.2 V sꢁ1
.
Half potential for reversible redox
processes. d DE ¼ |EHOMO ꢁ ELUMO| ¼ e(Eox, onset ꢁ Ered, onset).
2982 | J. Mater. Chem., 2009, 19, 2975–2985
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Table 2 Optical and thermal properties of the NLO chromophores
a
l /nm
b
d
e
f
g
NLO chromophores
log3
Dldioxane-CHCl3c/nm
b1000 /10ꢁ30 esu
b0, HRS /10ꢁ30 esu
b0, Calcd /10ꢁ30 esu
Td /ꢂC
P-Ti2-TCV
MP-Ti2-TCV
T-Ti2-TCV
T-Ti5-TCV
B-Ti2-TCV
B-T-TCV
731
752
750
840
668
678
684
750
4.73
4.68
4.85
4.90
4.95
4.71
4.75
5.02
71
74
70
117
57
54
55
976
923
888
1073
882
704
1501
1406
516
486
485
576
383
318
698
768
189.5
129.1
182.2
202.7
176.7
144.6
189.9
222.3
218
182
272
224
263
288
257
274
TPA-Ti2-TCV
TPA-Ti5-TCV
79
a
b
Maximal CT absorption wavelength measured in chloroform. Logarithm of extinction coefficient in chloroform. Differential CT wavelength
c
d
between dioxane and chloroform. Molecular hyperpolarizability (b) measured by Hyper-Rayleigh scattering experiment at 1000 nm with
e
p-nitroaniline as an external standard. Intensity of second harmonic light was calibrated with the Beer–Lambert law. Static b values calculated
f
within the two-level model by using the formula b ¼ b0 ꢀ (1/lmax)4/{[(1/lmax ) ꢁ (4/l2)] ꢀ [(1/lmax2) ꢁ (1/l2)]}. Static b values calculated with the
2
MOPAC program in AM1 method. g On-set temperature for weight loss determined by TGA in nitrogen at a heating rate of 20 ꢂC/min.
Fig. 2 UV-vis absorption spectra of (a) B-Ti2-TCV, (b) TPA-Ti2-TCV,
Fig. 3 Hyper-Rayleigh scattering spectra of P-Ti2-TCV in chloroform.
From bottom to top, the concentrations are 0.1 ꢀ 10ꢁ4 M, 0.2 ꢀ 10ꢁ4 M,
(c) P-Ti2-TCV, (d) T-Ti2-TCV, (e) TPA-Ti5-TCV, and (f) T-Ti5-TCV in
chloroform.
0.8 ꢀ 10ꢁ4 M, 1 ꢀ 10ꢁ4 M, 2 ꢀ 10ꢁ4 M, and 3 ꢀ 10ꢁ4 M.
chromophores agrees with the electrochemical measurement
results (Table 1).
All the NLO chromophores exhibited large positive sol-
vatochromism (Dl ¼ 54–117 nm) from dioxane to chloroform,
suggesting the large optical nonlinearities of these chromophores
(Table 1).26 In comparison, the benzene-based chromophores
have a smaller Dl (54–79 nm) than their pyrrole and thiophene
analogues (Dl ¼ 70–117 nm). Meanwhile, the ‘‘matched’’
chromophores (i.e., T-Ti5-TCV and TPA-Ti5-TCV) showed
significantly larger Dl than their ‘‘un-matched’’ isomers (i.e.,
T-Ti2-TCV and TPA-Ti2-TCV), suggesting larger b values of the
‘‘matched’’ chromophores.
The molecular first hyperpolarizabilities (b) of the NLO
chromophores were measured by Hyper-Rayleigh scattering
(HRS) experiments at a fundamental wavelength of 1000 nm
(pulse width, 7 ns), using p-nitroaniline (p-NA, b1064 nm ¼ 28.8 ꢀ
Fig. 4 The typical linear dependence of HRS signal intensity (corrected
10ꢁ30 esu24) as an external standard.27 Since all the NLO chro-
for absorption) of P-Ti2-TCV on solution concentration. The inset shows
mophores showed very weak absorption at the second harmonic
the plot of HRS signal intensity of p-NA versus the solution concentration.
frequency (i.e. 500 nm), as shown in Fig. 2, low two-photon
resonance enhancement in b can be expected. Fig. 3 shows the
typical HRS spectra of P-Ti2-TCV in chloroform. Strong two-
photon fluorescence was observed for all the NLO chromo-
phores. In the study, the second harmonic signals at 500 nm were
extracted from the background fluorescence by spectral decon-
volution. The signal intensities were calibrated with Beer–
Lambert law to correct the absorptional effect. A linear plot of
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the signal intensities was drawn against the concentrations of the
chromophore solutions (Fig. 4). By comparing between the
Thermal and photochemical stabilities
The on-set thermal decomposition temperatures (Td) of the NLO
chromophores were recorded by TGA in nitrogen (Table 2).
slopes of the chromophores and the p-NA reference and using
1/2
the equation of b ¼ (kchromophore/kp-NA
)
b
p-NA
,
27 b values of the
ꢂ
Most of them exhibited good thermal stability over 215 C. In
NLO chromophores at 1000 nm were obtained. Table 2
summarizes the b values of the NLO chromophores at 1000 nm.
For direct comparison between the optical nonlinearities of the
chromophores, static b (b0) values were calculated from the
b values at 1000 nm within the two-level model (Table 2).29
Correlation study of the structures and the b0 values of the NLO
chromophores allows us to systematically examine the impor-
tance of the gradient electronic structures. Comparison between
B-Ti2-TCV, T-Ti2-TCV and P-Ti2-TCV shows a sensitivity of the
b0 value to the electron density of the auxiliary donors. P-Ti2-
TCV that has pyrrole, the most electron-rich five-membered
aromatic heterocycle, as the auxiliary donor exhibited a much
larger b0 of 516 ꢀ 10ꢁ30 esu than its thiophene (i.e., T-Ti2-TCV,
b0 ¼ 485 ꢀ 10ꢁ30 esu) and benzene (i.e., B-Ti2-TCV, b0 ¼ 383 ꢀ
10ꢁ30 esu) analogues. This agrees with the theoretical expecta-
tions by Albert et al.8 and Rao et al.9 In their studies, the
introduction of electron-rich pyrrole or thiophene units to the
donor end of the p–bridge as the auxiliary donor led to an
increased optical nonlinearity of the chromophores. The amino
donor also plays a role in the optical nonlinearity. Replacement
of piperidin-1-yl donor with methyl 4-methoxylphenyl amine led
to a drop in b value (i.e., MP-Ti2-TCV vs. P-Ti2-TCV), due
probably to the decrease of donor strength. However, it is
interesting to note that TPA-Ti2-TCV that have diarylamine as
the donor and electron-inefficient benzene as the auxiliary donor
exhibited a very large b0 value of 698 ꢀ 10ꢁ30 esu, which is even
larger than the pyrrole-based P-Ti2-TCV. Marder and
co-workers30 reported that bis(4-methoxyphenyl)aminophenyl
group had a high donor strength comparable to that of dime-
thylaminophenyl group. This can be confirmed in our study by
comparing the 1H NMR, CV and UV-vis spectra between B-Ti2-
TCV and TPA-Ti2-TCV. However, this does not justify the
observed much larger optical nonlinearity of TPA-Ti2-TCV than
B-Ti2-TCV and P-Ti2-TCV. Recently, Davies et al.15 reported
TPA-based NLO chromophores exhibiting very large b values
comparable to those of diarylaminothiophene- and diary-
laminopyrrole-based chromophores, although the latter were
expected to have a stronger p-donor strength.30 In comparison
of NLO chromophores with ‘‘matched’’ (i.e., Ti-Ti5-TCV and
TPA-Ti5-TCV) and ‘‘un-matched’’ thiazole (i.e., Ti-Ti2-TCV and
TPA-Ti2-TCV), the former exhibited much larger b0 values than
the latter, demonstrating the theoretical predictions by Breitung
et al.10 and by others.11
comparison, however, the pyrrole-based NLO chromophores
(i.e., P-Ti2-TCV and MP-Ti2-TCV) exhibited poorer stability
than the thiophene and benzene analogues; and ‘‘un-matched’’
NLO chromophores (i.e., Ti-Ti2-TCV and TPA-Ti2-TCV)
showed higher thermal stability than their ‘‘matched’’ counter-
parts. The photochemical stability of the NLO chromophores
was studied by monitoring the UV-vis spectral change of the
chromophore solutions in chloroform at room temperature
ꢂ
(ꢃ25 C) with and without the indoor light. It is assumed that
any decompositon of the conjugated structure would lead to
a decrease in absorbance at lmax and appearance of new
absorption peak at high energy band. In the study, no substantial
spectral change was observed when the sample solutions were
stored in dark for one week. However, under indoor light, NLO
chromophores based on electron-rich pyrrole and thiophene
showed a decrease in absorbance after 24 h and significant
bleaching after one week, suggesting a relatively poor photo-
chemical stability. In comparison, all benzene-based NLO
chromophores showed good photochemical stability under the
indoor light.
Conclusions
A new series of heterocycle-based NLO chromophores with
systematically tuned gradient electronic structures were
successfully prepared. In the series, the most electron-rich
pyrrole was for the first time coupled with the electron-poor
thiazole in NLO chromophores, which allows for a systematic
variation of the auxiliary donor from benzene to thiophene and
to pyrrole. NLO chromophores with ‘‘matched’’ and ‘‘un-
matched’’ thiazole at the acceptor end of the p-bridge were also
for the first time prepared and compared for their nonlinear
optical properties. In the study, a great sensitivity of b0 values to
the electron density of auxiliary donor and to the regiochemistry
of thiazole as the auxiliary acceptor was demonstrated. With the
increase of electron density of the auxiliary donor from benzene
to thiophene and to pyrrole, a significant increase in b value was
observed; NLO chromphores with ‘‘matched’’ thiazole as the
auxiliary acceptor exhibited much larger b0 values than their
‘‘un-matched’’ isomers. However, a tradeoff between the large
optical nonlinearity and high thermal stability was also revealed.
TGA investigations showed that pyrrole-based and ‘‘matched’’
thiazole-based NLO chromophores tended to have a decreased
thermal stability. However, exceptions were found for TPA-
based NLO chromophores (e.g., TPA-Ti5-TCV) that exhibited
both very large b0 values and good thermal stability.
Static b0 values of the NLO chromphores were also computed
with MOPAC program on geometric structures optimized by
AM1 method (Table 2). Although smaller in magnitude, the
calculated b0 values show essentially the same trends with
the experimental results in terms of the effects of the electron
density of the auxiliary donor and the regiochemistry of thiazole.
Similar to the HRS measurement results, a large calculated b0
value was found for TPA-Ti2-TCV. This indicates that the very
large experimental b0 values of TPA-based chromophores are
not due to the resonance enhancement. However, at this stage,
a rational explanation for the uniquely large b0 values of
TPA-based chromophores is yet to be come up with.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos.: 200504003 and
20433010) and the SRF for ROCS, SEM. The authors are very
grateful for the fruitful discussions with Dr M. Sun of the
Institute of Physics, Chinese Academy of Sciences.
2984 | J. Mater. Chem., 2009, 19, 2975–2985
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