The synthesis and optical properties of fluorescent quinoxalines and of electrospun fibers containing fluorescent quinoxaline

Article abstract of DOI:10.1166/jnn.2011.3245

Heterocyclic fluorophores are useful materials in the search for new biologically active compounds and diagnostic methods. We have been interested in the chemistry of nitrogen-containing heterocyclic molecules for many years. Quinoxaline is a representative fluorophore. We have reported on several quinoxalines in recent years. Quinoxaline can easily change its absorption and emission wavelength by oxidation with the proton base in the nitrogen of the quinoxaline ring. In this study, we designed and synthesized several 2,3-distyrylquinoxaline and thieno[3,4-b]quinoxaline derivatives, Each with different electron-donating capabilities. The designed quinoxalines were substituted for the dodecyloxy groups on the benzene ring and stillbene groups were attached by knoevenagel reaction or Hornor-Wadsworth-Emmons (HWE) reaction on the 2,3-positions of the pyrazine ring. They amplified the electron donating capability of the quinoxaline structure. Thus, the weak base property of nitrogen in the heterocyclic ring was increased, especially in a protonic condition. The property in an acidic condition was revealed by fluorescence quenching. However, fluorescent spectral change was observed, especially when the N,N-dimethylamino group was attached to the stillbene group. These properties were also observed in electrospun fibers containing those synthesized compounds. Electrospun fibers contained quinoxaline colorants are expected to have various applications in chemosensors. Copyright

Full text of DOI:10.1166/jnn.2011.3245

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 494–498, 2011
The Synthesis and Optical Properties of
Fluorescent Quinoxalines and of Electrospun
Fibers Containing Fluorescent Quinoxaline
Chun Keun Jang and Jae Yun Jaung∗
Department of Fiber and Polymer Engineering, Hanyang University, 17 Haengdang-dong, Seongding-gu, Seoul, 133791, Korea
Heterocyclic fluorophores are useful materials in the search for new biologically active compounds
and diagnostic methods. We have been interested in the chemistry of nitrogen-containing hetero-
cyclic molecules for many years. Quinoxaline is a representative fluorophore. We have reported on
several quinoxalines in recent years. Quinoxaline can easily change its absorption and emission
wavelength by oxidation with the proton base in the nitrogen of the quinoxaline ring. In this study,
we designed and synthesized several 2,3-distyrylquinoxaline and thieno[3,4-b]quinoxaline deriva-
tives, Each with different electron-donating capabilities. The designed quinoxalines were substituted
for the dodecyloxy groups on the benzene ring and stillbene groups were attached by knoeve-
nagel reaction or Hornor-Wadsworth-Emmons (HWE) reaction on the 2,3-positions of the pyrazine
ring. They amplified the electron donating capability of the quinoxaline structure. Thus, the weak
base property of nitrogen in the heterocyclic ring was increased, especially in a protonic condition.
The property in an acidic condition was revealed by fluorescence quenching. However, fluorescent
Delivered by Ingenta to: Rice University
spectral change was observed, especially when the N,N-dimethylamino group was attached to the
IP: 46.161.62.11 On: Wed, 15 Jun 2016 17:14:50
stillbene group. These properties were also observed in electrospun fibers containing those synthe-
Copyright: American Scientific Publishers
sized compounds. Electrospun fibers contained quinoxaline colorants are expected to have various
applications in chemosensors.
Keywords: Quinoxaline, Chemical Sensor, Fluorescence.
1. INTRODUCTION
application to chemosensors. Quinoxaline itself has a weak
base, and it can easily change its absorption or emission
wavelength by oxidation with the proton analyte in the
nitrogen of the quinoxaline ring when electron-donating
substituents are attached in its heterocyclic ring. In addi-
tion, it has been proven that many types of metal chelat-
ing ligand substituents, such as dipyrrole, terpyridine, and
crown-ether rings, can control electronic levels of the
fluorephores.6–7
In this study, we designed and synthesized several 2,3-
distyrylquinoxalines and thieno[3,4-b]quinoxaline deriva-
tives, each with different electron-donating substituents.
The chromophoric systems of these compounds were stud-
ied, along with the substituent affects on their absorption
and emission spectra in solution. And recently, there are
approaches for electrospun fibers containing various lumi-
nescent materials with substrate polymer.8–9 Thus we fab-
ricated to electrospun fibers contained these synthesized
colorants and analyzed structure and optical properties for
chemical sensor.
There has been much research done into fluorescent hete-
rocyclic compounds for application to emitters in electro-
luminescent devices and to the molecular probes used in
biochemical research, as well as in the traditional textile
and polymer fields.1–4 Heterocyclic fluorophores are useful
materials in the search for new biologically active com-
pounds and diagnostic methods. Fluorescent chromophores
are generally known to have planar and rigid ꢀ-conjugation
systems, and many fluorescent chromophores are based on
rigid ring systems such as stilbene, coumarin, naphthalim-
ide, perylene and rhodamine. Our research group has been
interested in the chemistry of nitrogen-containing hetero-
cyclic molecules for many years. Quinoxaline is a represen-
tative fluorophore. We have reported on several quinoxaline
derivative compounds in recent years.5
The fluorescent optical change due to the circumstance
of fluorophores should be apparent for them to have an
∗Author to whom correspondence should be addressed.
494
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 1
1533-4880/2011/11/494/005
doi:10.1166/jnn.2011.3245

Jang and Jaung
The Synthesis and Optical Properties of Quinoxalines and of Its Electrospun Fibers
2. EXPERIMENTAL DETAILS
(MEK), and chloroform were purchased from Samchun
Pure chemical Co. and used without further purification.
Flash chromatography was performed with Merck-EM
Type 60 (230–400 mesh) silica gel (flash). 1H-NMR spectra
were recorded using a VARIAN UnityInova 300 MHz
FT-NMR Spectrometer. The UV-visible and fluorescence
spectra were measured using SCINCO S-4100 and SHI-
MADZU RF-5301PC spectrophotometers, respectively.
The reagents and solvents used for the syntheses were all
synthetic grade, used as received. The chemicals used for
the spectroscopic analysis were all analytical reagent grade.
In a typical procedure, the solution was loaded into a
plastic syringe equipped with a 30 gauge needle made
of stainless steel. The needle was connected to a high
voltage power supply capable of generating DC voltage up
to 30 kV. The solution was continuously supplied using a
syringe pump (KDS-200, Stoeling, Wood Dale, IL) at a
rate of 0.005 ml/h–0.01 ml/h. A positive voltage of 15 and
20 kV was applied to the solution and the distance between
the needle tip and the collector (TCD) was between 15 and
30 cm. The needle, electrode, and grounded target were
all enclosed in order to reduce the effect of air currents
on the trajectory of the electrospun jet. The collected fiber
was dried at 80 ꢀC for one day to evaporate any remaining
solvents.
2.1. Typical Procedure to Synthesize 3a
The mixture of 6,7-bis(dodecyloxy)-1,3-dihydrothieno
[3,4-b]quinoxaline (2, 500 mg, 0.90 mmol),
N-dimethylaminobenzaldehyde 1.34 g (9.0 mmol) in
25 ml of diethyl ether was refluxed in the presence of a
catalystic amount of potassium t-butoxide. After 18 hr,
the solvent was removed under reduced pressure and the
residue was filtered off and washed with methanol. The
crude product was purified by column chromatography
(silica gel, ethyl acetate/n-hexane = 1/2 as the elu-
ant). 3a (orange crystal, 24%): m.p 118 ꢀC, 1H-NMR
(300 MHz, CDCl3) ꢁ: 7.89 (s, 2H, quinoxaline), 7.71
(d, 4H, J = 9ꢂ0 Hz, Ar-H), 7.34 (s, 2H, ethene), 6.85
(d, 4H, J = 9ꢂ0 Hz, Ar-H), 4.22 (t, 4H, J = 6ꢂ0 Hz,
3. RESULTS AND DISCUSSION
3.1. Synthesis
1,3-dihydrothieno[3,4-b]quinoxaline
derivatives
and
diphosphonates were prepared, referring to previous
literature.7ꢄ10 Long linear alkyl ether moiety was chosen
to improve the solubility of the resulting quinoxaline
fluorescent compounds in common organic solvents.
Scheme 1 shows the synthetic methods of the quinoxaline
Delivered by Ingenta to: Rice University
OCH2ꢃ, 3.04 (s, 12H, NCH3ꢃ, 1.97–1.92 (m, 4H, CH2ꢃ,
compounds.
IP: 46.161.62.11 On: Wed, 15 Jun 2016 17:14:50
1.54–1.28 (m, 36H, methylene), 0.89 (t, 6H, J = 6ꢂ0 Hz,
Copyright: American Scientific Publishers
In the case of 3, condensation occured on two
CH3ꢃ C52H74N4O2S Calcd. C, 76.24; H, 9.10; N, 6.84; O,
ꢅ-positions to the S-atom when an excess amount of
aldehydes was added to diethyl ether under basic con-
ditions. The compounds reacted with various types of
benzaldehydes using the Knoevenagel condensation reac-
tion. The special reactivity of 2 is therefore due to an
3.91; S, 3.91 Found. C, 75.38; H, 8.97; N, 6.81.
2.2. Typical Procedure to Synthesize 5a
Potassium t-butoxide (4eq.) was added to the stir-
ring mixture of diphosphonate (4, 1.18 g, 1.47 mmol),
N-dimethylaminobenzaldehyde 2.2 g (14.7 mmol) in
30 ml of THF at room temperature. The mixture was
stirred for 3 hr. The solvent was removed under reduced
pressure and the crude product was purified by column
chromatography (silica gel, ethyl acetate/n-hexane = 1/1
as the eluant). 5a (yellow crystal, 48%): m.p 107 ꢀC,
1H-NMR (300 MHz, CDCl3) ꢁ: 7.97(d, J = 15ꢂ6 Hz, 2H,
ethene), 7.87 (s, 2H, quinoxaline), 7.74 (d, 4H, J = 9ꢂ0 Hz,
Ar–H), 7.51 (d, J = 15ꢂ6 Hz, 2H, ethene), 7.02 (d, 4H, J =
9ꢂ0 Hz, Ar–H), 4.23 (t, 4H, J = 6ꢂ0 Hz, OCH2ꢃ, 3.04 (s,
6H, NCH3ꢃ, 1.97–1.92 (m, 4H, methylene) 1.54–1.28 (m,
36H, CH2), 0.88 (t, 6H, J = 6ꢂ0 Hz, CH3); C52H76N4O2
Calcd. C, 79.14; H, 9.71; N, 7.10; O, 4.05 Found. C, 78.79;
H, 9.56; N, 7.03.
R1
R1
N
Br
Br
N
1
P(OEt)3
N
Na2S
O
P
P
O
OEt
OEt
R1
R1
N
N
R1
R1
S
OEt
OEt
N
2
4
R2
R2
O
O
Diethyl ether/refux
THF/5?
t-BuOK
t-BuOK
R2
R2
R2
R2
R1
R1
N
N
R1
R1
N
N
S
2.3. Electrospinning Materials and Set-Up
3
5
R1 = –O(CH2)11CH3
R2 = (a)-N(CH3)2, (b)-CH3, (c)-OCH3, (d)-Br
Polystyrene (PS, Mn = 140,000) and Polycarbonate (PC,
Mn = 100,000) were purchased from Sigma Aldrich,
and N,N-dimethylformamide (DMF), methyl ethyl ketone
Scheme 1. Synthetic route of quinoxaline derivatives.
J. Nanosci. Nanotechnol. 11, 494–498, 2011
495

The Synthesis and Optical Properties of Quinoxalines and of Its Electrospun Fibers
Jang and Jaung
electron-withdrawing effect of the pyrazine ring condensed
to 3 and 4 positions of thiophene ring. Z,Zꢁ-configuration
was preferred for diethyl ether with solvent and potassium
tert-butoxide as a catalyst.
The Horner-Wadsworth-Emmons (HWE) reaction in
THF yielded compound 5. The reaction of compound 4
with 10 equivalents of a 4-substituted benzaldehyde, in the
presence of potassium tert-butoxide, gave mono and di-
substituted quinoxaline by controlling the specific reaction
time. These products can be easily separated by liquid col-
umn chromatography (using ethyl acetate/n-hexane = 1/1
as the eluant).
1H-NMR spectroscopy of compounds was used in
ꢀ
CDCl3 at 20 C, which provides structural information.
In the aliphatic region, a sharp triplet resonance corre-
sponding to the O-methylene proton was observed. The
ethylene protons of 5a appeared as a doublet at 7.97 and
7.51 ppm revealed a trans-configuration with coupling
constants of 15.6 Hz. In addition, the protons of terminal
N-CH3 were singlet at 3.04 respectively.
Fig. 1. Absorption and Emission change of 3a and from CHCl3 to
CHCl3:MeOH = 1:1.
quinoxalines onto which electron-donating substituents
have been introduced are protonated easily at the 1,4-
positions. In addition, the substituent in the stillbene group
also affects especially the N-dimethylamino group. It is
assumed that the amino group can be also protonated under
an acidic condition. Therefore the electronic level change
of LUMO of the amino group induced the red-shift of the
emission spectra. In contrast, the methyl group of still-
bene substituents has no electronic level change under a
protic condition. 3b thus only has one of the protonations
3.2. Absorption and Emission Properties
Synthesized compounds have different solubility as to
many organic solvents and UV-visible absorption which
changes the alkyl group of the p-position of aromatic
aldehyde. Table I shows the maximum absorption and
Delivered by Ingenta to: Rice University
emission wavelength of these synthesized compounds.
IP: 46.161.62.11 On: Wed, 15 Jun 2016 17:14:50
They had a solvatochromic effect. In particular, N,N-
of quinoxaline backbone, showing direct quenching of its
emission spectra.
Copyright: American Scientific Publishers
dimethylaminostyryl substituted for quinoxaline (3a) has a
unique absorption change by different solvents as shown in
Figure 1. It shows a bathochromic shift by increasing the
ratio of methanol, and the emission was reduced showing a
red shift. It seems to be caused by an aggregation-induced
red shift because of the different solubility between chloro-
form and methanol. Other compounds show their emission
quenched significantly.
The emission change under an acidic condition was
tested by adding p-toluenesulfonic acid (PTC) to the
CHCl3:MeOH = 9:1 solution. The solvent combination
was considered and selected for PTC to be more soluble
than CHCl3.
In the case of compound 5, there was a different emis-
sion change among each substituent as shown in Figure 3.
The original orange-yellow solution of 5a became green-
ish blue as the concentration of PTC was increased. The
absorption maximum of 5a at 417 nm decreased, while
that of a new absorption band which appeared at 597 nm
increased. No fluorescence was observed when the exci-
tation wavelength was 597 nm, and the emission showed
less intense, blue-shifted color under excitation at 417 nm.
On the other hand, other compounds including 5c show
another optical change as shown in Figure 4. They show a
bathochromic shift of absorption and emission spectra and
their intensity was slightly increased. The original yellow
color (ꢆmax = 423, Fmax = 476) was changed to an orange
color (ꢆmax = 502 nm, Fmax = 573). The protonation of
the imine nitrogen of 5 seems to change the electronic
state of the dyes and the process by which the fluores-
cence is generated. It has been reported that the protona-
tion of the nitrogen atom of heteroaromatic poly(arylene)
in acidic media leads to a bathochromic shift of the absorp-
tion peak.11–12
In the case of compound 3, 3a shows its emission spec-
tra from greenish-yellow to red (Fmax = 659 nm), but the
intensity was reduced as shown in Figure 2. In contrast to
other compounds, including 3b, the emission spectra were
directly quenched. Quinoxaline itself is a weak base, while
Table I. Absorption and emission wavelength of synthesized com-
pounds 3 and 5 in CHCl3.
ꢆmax
Fmax
ꢆmax
Fmax
3a
3b
3c
3d
438
387
394
390
538
483
474
476
5a
5b
5c
5d
417, 447
417
423
527
459
476
453
3.3. Electrospun Fiber and Its Sensor Ability
The synthesized quinoxalines have an optical change,
notably a fluorescent change under a protic condition.
416
496
J. Nanosci. Nanotechnol. 11, 494–498, 2011

Jang and Jaung
The Synthesis and Optical Properties of Quinoxalines and of Its Electrospun Fibers
Fig. 2. PL spectral change of 3a and 3b by adding PTC.
Electrospun fibers containing synthesized quinoxaline flu-
orophores also show an optical change under acidic
conditions.
Figure 5 contains microscopic pictures of electrospun
polystyrene (PS) (a) and polycarbonate (PC) (b) fibers con-
taining 3a. They show a uniform emission and have a
700∼1000 nm diameter under specific spinning conditions
as shown in Table II.
inner position of quinoxaline colorants. It is assumed that
the quinoxaline chromophores slowly moved nearer to the
surface of the fiber because PS fibers used DMF/MEK—a
relatively slower vaporized solvent than chloroform—for
The acid sensitivity of these fibers was tested under
nitric acid vapors and liquid conditions. In the case of
the polystyrene fibers, fluorescence significantly changed
under acid vapor condition for 60 seconds as shown in
Delivered by Ingenta to: Rice University
Figure 6. They changed from bright yellow to dark red,
IP: 46.161.62.11 On: Wed, 15 Jun 2016 17:14:50
with the critical time to complete the spectral change being
Copyright: American Scientific Publishers
about 60–90 sec.
However, polycarbonate fibers show less sensitivity than
polystyrene fibers. They also changed to a dark red color,
but not significantly, and the critical time to complete the
change was 10–15 min.
The relatively slow change of PC fibers is assumed to
be due to the morphology of polymer substrates and the
Fig. 4. Spectral change of 5c by adding PTC.
10 µm
10 µm
10 µm
10 µm
Fig. 5. Electrospun polystyrene (a) and polycarbonate (b) fibers con-
taining 3a.
Fig. 3. Spectral change of 5a by adding PTC.
J. Nanosci. Nanotechnol. 11, 494–498, 2011
497

The Synthesis and Optical Properties of Quinoxalines and of Its Electrospun Fibers
Jang and Jaung
Table II. Specific electrospinning condition of the resulting fibers.
spinning condition and solvents. The progress of further
researches will be reported elsewhere.
Tip to collector
distance (cm)
Flow rate
(mL/h)
Solvent
Voltage (kV)
Acknowledgments: This study was supported by a
grant from the Fundamental R&D Program (M200701004)
for Core Technology of Materials funded by the Ministry
of Commerce, Industry and Energy in Korea.
PS
PC
DMF/MEK
Chloroform
15
20
20
15
0.005
0.005
Concentration of Polymer: 15 wt%. Concentration of dye: 0.130 wt%.
References and Notes
1. K. Hunger, Industral Dyes, WILEY-VCH, Weiheim, Germany
(2003), p. 569.
2. I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
Molecules, Academic Press, New York (1971).
3. (a) K. Kodiro and Y. Inoue, J. Am. Chem. Soc. 125, 421 (2003);
(b) S. Yamaguchi, S. Akiyama, and K. Tamao, J. Am. Chem. Soc.
122, 6793 (2000).
4. M. D. Harvey, V. Bablekis, P. R. Banks, and C. D. Skinner, J. Chro-
matogr. B 754, 345 (2001).
Fig. 6. Emission change of 3a containing PS fiber (HNO3 vapor expo-
sure for 60 s).
5. J. Y. Jaung, Dyes Pigments 71, 245 (2006).
6. A. V. Tsukanov, A. D. Dubonosov, V. A. Bren, and V. I. Minkin,
Chem. Heterocyc. Comp. 44, 899 (2008).
7. (a) D. Aldakov and P. Anzenbacher, Jr., Chem. Comm. 1394 (2003);
(b) C. K. Jang and J. Y. Jaung, Dyes Pigments 80, 168 (2009).
8. H. Y. Wang, Y. Yang, Y. Wang, Y. Y. Zhao, X. Li, and C. Wang,
J. Nanosci. Nanotechnol. 9, 1522 (2009).
the solvent, and this resulting in higher sensitivity than that
of PC fibers.13
4. CONCLUSIONS
9. H. Yu, H. Song, G. Pan, R. Qin, L. Fan, H. Zhang, X. Bai, S. Li,
H. Zhao, and S. Lu, J. Nanosci. Nanotechnol. 8, 1432 (2008).
10. J. Pohmer, M. V. Lakshmikantham, and M. P. Cava, J. Org. Chem.
60, 8283 (1995).
In this study, we attempted to create quinoxaline based
compounds which contain various functional groups that
are sensitive in a protic condition. We also attempted to
confirm the possibility of applying electrospun fibers con-
taining these fluorescent materials to chemical sensors.
The proton sensor ability of these fibers will be
improved by controlling their diameters from the changing
Delivered by Ingenta to: Rice University
11. C. J. Yang and S. A. Jenekhe, Macromolecules 28, 1180 (1995).
IP: 46.161.62.11 On: Wed,1125. BJ.uLn. 2Le0e1a6nd1T7.:J1.4Y:a5m0amoto, Macromol. Chem. Phys. 200, 2396
Copyright: American Scientific Publishers
(1999).
13. C. C. Kuo, C. H. Lin, and W. C. Chen, Macromolecules 40, 6959
(2007).
Received: 26 March 2010. Accepted: 26 May 2010.
498
J. Nanosci. Nanotechnol. 11, 494–498, 2011