Synthesis and characterization of novel fluorescent compounds derived from 1,4-diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline

Article abstract of DOI:10.1002/jhet.422

(Chemical Equation Presented) 1,4-Diethyl-1,2,3,4-tetrahydro-6- nitroquinoxaline 4 was synthesized by alkylative reduction of 6-nitroquinoxaline. Catalytic reduction of 4 followed by cyclocondensation with heterocyclic malondialdehydes afforded novel 8-(heteroaryl)-1,4-diethyl-1,2,3,4- tetrahydropyrido[2,3-g]quinoxalines. The solutions of these novel compounds having 1,4-diethyl-1,2,3,4-tetrahydroquinoxaline framework as an electron releasing system showed absorption in the range of 424-426 nm in the visible region and exhibited brilliant bluish-green fluorescence. The thermogravimetric curve obtained by thermogravimetric analysis displayed that these fluorophores possess excellent thermal stability with one-step thermal decomposition.

Full text of DOI:10.1002/jhet.422

1066
Synthesis and Characterization of Novel Fluorescent Compounds
Derived from 1,4-Diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline
Vol 47
Vijay S. Satam,a Rajkumar N. Rajule,a Amit R. Jagtap,a Samir R. Bendre,a
Hari N. Pati,b and Vinod R. Kanetkara*
aDepartment of Technology of Dyestuff and Intermediates, Institute of Chemical Technology
(ICT), Matunga, Mumbai-400019, Maharashtra, India
bDepartment of Chemistry, Sambalpur University, Jyoti Vihar-768019, Orissa, India
*E-mail: vr.kanetkar@ictmumbai.edu.in
Received October 6, 2009
DOI 10.1002/jhet.422
Published online 13 July 2010 in Wiley Online Library (wileyonlinelibrary.com).
1,4-Diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline 4 was synthesized by alkylative reduction of 6-nitro-
quinoxaline. Catalytic reduction of 4 followed by cyclocondensation with heterocyclic malondialdehydes
afforded novel 8-(heteroaryl)-1,4-diethyl-1,2,3,4-tetrahydropyrido[2,3-g]quinoxalines. The solutions of
these novel compounds having 1,4-diethyl-1,2,3,4-tetrahydroquinoxaline framework as an electron
releasing system showed absorption in the range of 424–426 nm in the visible region and exhibited bril-
liant bluish-green fluorescence. The thermogravimetric curve obtained by thermogravimetric analysis
displayed that these fluorophores possess excellent thermal stability with one-step thermal
decomposition.
J. Heterocyclic Chem., 47, 1066 (2010).
INTRODUCTION
[10,11]. Its derivatives are used in the development of
novel organic dyes and organic semiconductors [12].
Fluorescent styryl dyes based on fused quinoxaline sys-
tem are reported in the literature. In earlier work from
our laboratory, the versatility of quinoxaline has been
demonstrated [13–16]. Quinoxalines can be easily
reduced to 1,2,3,4-tetrahydroquinoxalines by reducing
agents such as lithium aluminium hydride [17] and so-
dium borohydride [18] in excellent yields. Sequential
reduction and alkylation of N-heterocycles such as
indole to N-alkylated indoline and quinoline to 1,2,3,4-
tetrahydroquinoline by sodium borohydride and tri-
fluoroacetic acid is well known [19–22]. Quinoxalines
can also be sequentially reduced and dialkylated using
sodium borohydride and carboxylic acids. 6-Nitroqui-
noxaline has been subjected to similar reductive alkyla-
tion using sodium borohydride and glacial acetic acid to
obtain 1,4-diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline
The structural diversity, stability, and biological im-
portance of heterocycles have made them attractive syn-
thetic targets over many years, and they have found
potential applications in various fields of science and
technology. Especially in the field of colorants, hetero-
cycles have gained extreme importance because of their
planar and rigid p-conjugation system. Many hetero-
cycles based on rigid ring systems such as coumarins
[1], thiazoles [2], benzimidazoles [3], pyrazines [4],
naphthalimides [5], and oxadiazoles [6] are well-estab-
lished fluorescent dye chromophores. Heterocyclic fluo-
rescent compounds have been extensively investigated
for various potential applications including tunable dye
lasers [7], molecular probes for biochemical research
[8], and traditional textile and polymer fields [9].
Quinoxaline is one of the interesting heterocyclic sys-
tems. Many quinoxaline scaffolds are found as a core
unit in a number of biologically active compounds
[23].
framework is rigid and highly electron rich. We have
The
1,4-diethyl-1,2,3,4-tetrahydroquinoxaline
C
V
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September 2010 Synthesis and Characterization of Novel Fluorescent Compounds Derived from
1,4-Diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline
1067
Scheme 1. Synthetic pathway of compounds 7a–7b.
reported mono- and bis-styryl dyes derived from 1,4-
diethyl-1,2,3,4-tetrahydro-6-methoxyquinoxaline [24].
of these highly fluorescent compounds were analyzed by
UV-vis absorption spectroscopy and fluorescence emis-
sion spectroscopy. The fluorescent compounds 7a and
7b were also evaluated for thermal stability by thermog-
ravimetric analysis. The spectroscopic properties of
these pyrido[2,3-g]quinoxaline derivatives were com-
pared with the closely related coumarin analogs 8a–8b
and styryl derivatives 9a–9b.
These dyes having orange to violet hue displayed pro-
nounced bathochromicity and good thermal stability. A
series of highly fluorescent coumarin derivatives based
on 1,4-diethyl-1,2,3,4-tetrahydroquinoxaline framework,
synthesized by us, exhibited excellent bathochromicity
[25]. These results encouraged us to envisage that, the
molecular structures possessing a strong electron donat-
ing and rigid 1,4-diethyl-1,2,3,4-tetrahydroquinoxaline
unit in conjugation with heterocyclic p-conjugated sys-
tem should exhibit brilliant fluorescence and display
absorption maxima in the yellow region of the electro-
magnetic spectrum.
RESULTS AND DISCUSSION
Synthesis of compounds 7a–7b. Substituted 1,4-
diethyl-1,2,3,4-tetrahydropyrido[2,3-g]quinoxalines 7a–
7b were synthesized by cyclocondensation of 6-amino-
1,4-diethyl-1,2,3,4-tetrahydroquinoxaline 5 and suitable
malondialdehyde derivatives 6a–6b as depicted in
Scheme 1. 4-Nitro-1,2-phenylenediamine 1 was con-
densed with glyoxal 2 in dry acetonitrile to obtain 6-
In this communication, we report the synthesis and
spectroscopic properties of novel pyrido[2,3-g]quinoxa-
line derivatives having 1,4-diethyl-1,2,3,4-tetrahydroqui-
noxaline framework as an electron releasing unit in con-
jugation with electron accepting heterocycles like benz-
imidazole and benzothiazole. The electronic properties
nitroquinoxaline
3
in excellent yield. Reductive
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V. S. Satam, R. N. Rajule, A. R. Jagtap, S. R. Bendre, H. N. Pati, and V. R. Kanetkar
Vol 47
alkylation of 6-nitroquinoxaline with sodium borohy-
dride and glacial acetic acid in dry toluene yielded 1,4-
diethyl-6-nitro-1,2,3,4-tetrahydroquinoxaline
4
as
a
bright red solid, which was then hydrogenated over 10%
palladium charcoal in glacial acetic acid to afford
6-amino-1,4-diethyl-1,2,3,4-tetrahydroquinoxaline 5. The
catalyst was filtered under nitrogen atmosphere and fil-
trate was immediately used for further reactions as the
amino compound 5 was unstable and rapidly oxidized.
The reaction of 6-amino-1,4-diethyl-1,2,3,4-tetrahydro-
quinoxaline 5 with appropriate malondialdehyde deriva-
tives 6a–6b took place readily in glacial acetic acid in
the presence of equivalent amount of p-toluenesulphonic
acid to yield 8-(heteroaryl)-1,4-diethyl-1,2,3,4-tetrahy-
dropyrido[2,3-g]quinoxalines 7a–7b. This cyclocon-
densation reaction proceeded in a facile manner owing
to the presence of electron releasing 1,4-diethyl-1,2,3,4-
tetrahydroquinoxaline framework. The structures of the
compounds were confirmed by FT-IR, 1H NMR spec-
troscopy, mass spectrometry, and elemental analysis.
The results are summarized in the experimental section.
Spectral characteristics of compounds 7a–7b. Basic
spectral characteristics of the chromophores such as
absorption maxima (kmax), emission maxima (kem), and
extinction coefficient (e) were measured in different sol-
vents and are presented in Tables 1 and 2. The elec-
tronic absorption spectra of the compounds 7a–7b dis-
played absorption maxima in the visible region from
412 to 427 nm. Compound 7a showed absorption max-
ima at 412 nm in hexane, lowest among two derivatives,
whereas compound 7b showed well-pronounced absorp-
tion maxima at 427 nm in DMF which is the highest
between the two derivatives. To investigate the influence
of solvents on the absorption maxima of compounds 7a
and 7b, their absorption spectra were measured in differ-
ent solvents such as toluene, chloroform, ethyl acetate,
hexane, methanol, DMF, acetonitrile, and ethyl acetate.
The solvents differ considerably in polarity and ability
to form H-bonds. From the presented values in Tables 1
and 2, it is apparent that practically no solvatochromism
was observed. Only in the case of 7a in hexane, signifi-
cant hypsochromic shift in the absorption maxima was
noticed. Figure 1 displays absorption maxima of the
compounds 7a–7b in methanol.
These 8-(heteroaryl)-1,4-diethyl-1,2,3,4-tetrahydropyr-
ido[2,3-g]quinoxalines were expected to be strongly flu-
orescent in view of the conjugation of rigid and electron
rich 1,4-diethyl-1,2,3,4-tetrahydropyrido[2,3-g]quinoxa-
line ring with electron accepting heterocycles. To our
delight, both compounds exhibited strong bluish-green
fluorescence with large Stokes shift values (Tables 1
and 2). Especially, fluorophore 7b containing benzothia-
zole ring showed remarkably high Stokes shift value of
107 in methanol. Figure 2 displays emission maxima of
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September 2010 Synthesis and Characterization of Novel Fluorescent Compounds Derived from
1,4-Diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline
1069
Figure 1. Absorption maxima of compounds 7a–7b in methanol.
the compounds 7a–7b in methanol. Figure 3 shows
photographs of the fluorophores 7a and 7b in UV light
(366 nm).
As stated earlier, the fluorophores 7a and 7b have
rigid and electron rich 1,4-diethyl-1,2,3,4-tetrahydropyr-
ido[2,3-g]quinoxaline ring in conjugation with electron
accepting heterocycles. The situation is rather similar to
the coumarin fluorophores 8a and 8b (Table 3) reported
by us [25]. Also, the styryl dyes 9a and 9b (Table 3)
were derived from the same electron donor and accept-
ors [24]. In short, compounds 7a–7b are structural ana-
logs of compounds 8a–8b and 9a–9b. Hence, the spec-
tral properties of 7a and 7b in methanol were also com-
pared with the established dyes 8a–8b and 9a–9b. The
comparative data are summarized in Table 3. Com-
pounds 7a and 7b showed intense yellow hue with
absorption maxima at 424 and 426 nm, respectively,
whereas compounds 8a and 8b showed bright orange
Figure 2. Emission maxima of compounds 7a–7b in methanol.
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V. S. Satam, R. N. Rajule, A. R. Jagtap, S. R. Bendre, H. N. Pati, and V. R. Kanetkar
Vol 47
hue and absorbed at 483 and 501 nm, respectively. The
large bathochromic shift in the case of 8a and 8b is
clearly due to the presence of lactone ring. The com-
pounds 8a and 8b were highly fluorescent, as it is usual
with coumarin compounds. Stokes shift value of 8a was
almost close to that of 7a, whereas Stokes shift value of
8b was lower than that of 7b. It must be noted that the
styryl dyes 9a and 9b, having same electron donating
1,4-diethyl-1,2,3,4-tetrahydroquinoxaline skeleton in
conjugation with electron accepting benzimidazole and
benzothiazole rings, respectively, were nonfluorescent.
The absence of fluorescence was probably due to lack
of rigidity provided by pyrido ring as in the case of
compounds 7a–7b and lactone ring as in the case of
compounds 8a–8b. However, the styryl dyes 9a and 9b
showed remarkably higher bathochromic shift with
absorption maxima at 501 and 528 nm, respectively,
owing to the presence of an additional electron
Figure 3. Photographs of fluorophores 7a–7b in UV light (366 nm).
Table 3
Spectral properties of compounds 7a–7b, 8a–8b, 9a–9b, and 10a–10b in methanol.
Structure
Compd.
kmax (nm)
e (l molꢀ1 cmꢀ1
)
kem (nm)
Stokes shift
Quantum yielda (U)
7a
7b
424
426
24,054
25,469
512
533
88
107
0.1
0.14
8a
8b
483
501
24,600
33,900
574
598
91
97
0.28
0.34
9a
9b
501
528
29,218
25,514
–
–
–
–
–
–
10a
10b
435
465
52,200
54,000
480
491
45
26
0.62
0.7
a Quantum yields were measured in methanol using Rhodamine-6G (U ¼ 0.94) as standard [26,27].
Journal of Heterocyclic Chemistry
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September 2010 Synthesis and Characterization of Novel Fluorescent Compounds Derived from
1,4-Diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline
1071
function of temperature. Figure 4 displays thermograph
of the fluorophores 7a and 7b. The thermogravimetric
curves for the compounds show a clear plateau followed
by a sharp and smooth decomposition curve. The loss in
weight of the compound 7a was rapid when heated
above 250ꢁC. This fact indicates that the compound is
stable up to 250ꢁC after which it decomposes rapidly
and decomposition completes at 440ꢁC. Among the two
compounds, compound 7b in particular showed excel-
lent thermal stability up to 310ꢁC. Rapid decomposition
of 7b occurred when it was heated above 310ꢁC. The
decomposition completed at about 455ꢁC. Both the fluo-
rophores underwent one-step thermal decomposition.
Coumarin chromophores 8a–8b and styryl dyes 9a–9b
also showed thermal stability above 250ꢁC with smooth,
one step thermal decomposition curve [24,25].
Figure 4. Thermogravimetric curves of fluorophores 7a–7b.
accepting cyano group. In all the three classes, the com-
pounds 7b, 8b, and 9b having benzothiazole ring under-
went bathochromic shift relative to their respective ana-
logs having benzimidazole ring, owing to high electro-
negativity of sulfur atom in the ring.
CONCLUSION
In conclusion, novel 8-(heteroaryl)-1,4-diethyl-1,2,3,4-
tetrahydropyrido[2,3-g]quinoxalines are valuable as new
fluorescent chromophores having absorption maxima at
412–427 nm and emission maxima at 502–533 nm in
different solvents. These compounds did not show any
appreciable solvatochromism and have lower fluores-
cence quantum yields than coumarins having same elec-
tron donating and accepting groups. The compounds dis-
played good thermal stability.
To complement this study, spectral properties of fluo-
rescent compounds 7a–7b and 8a–8b were compared
with that of commonly encountered commercial fluores-
cent coumarin dyes 10a (Coumarin 535) and 10b (Cou-
marin 540) (Table 3) having same electron accepting
groups. As expected, the rigid coumarins 8a and 8b
showed significant bathochromic shift in absorption
maxima compared with the nonrigid coumarins 10a and
10b. The compounds 10a and 10b, however, have
remarkably high molar extinction coefficient values and
absorb at longer wavelength compared with quinoxaline
derivatives 7a and 7b. The Stokes shift values of com-
pounds 10a and 10b are lower than that of compounds
7a–7b and 8a–8b. The fluorescence quantum yields (U)
of the compounds were measured in methanol using
Rhodamine-6G (U ¼ 0.94) [26,27] as standard. Com-
pound 7b showed U value of 0.14 which is marginally
higher than that of 7a (U ¼ 0.1). The fluorescence quan-
tum yields of compounds 7a and 7b were found to be
lower than that of coumarin derivatives. Although, the
quinoxaline derivatives 7a–7b possess similar electron
donor and acceptor groups as coumarins 8a–8b and
10a–10b, they exhibit significant hypsochromic shift in
absorption maxima and lower fluorescence quantum
yields. The lower fluorescence in quinoxaline derivatives
is due to the absence of lactone framework in coumarins
which impart rigidity, high electronegativity, and excel-
lent planarity to the molecule.
EXPERIMENTAL
All melting points were uncorrected and are in ꢁC. IR spectra
were recorded on a Bomem Hartmann and Braun MB-Series
1
FT-IR spectrometer (KBr). H NMR spectra were recorded on
Varian 300 MHz mercury plus spectrometer, and chemical shifts
are expressed in d ppm using TMS as an internal standard. Mass
spectra were recorded on Micromass: Q-T of micro (YA-105)
mass spectrometer. Microanalysis for C, H, N, and S were per-
formed on Thermofignnin elemental analyzer. Electronic spectra
were recorded on Spectronic spectrophotometer. The fluores-
cence maxima of the compounds were recorded on Jasco FP-
1520 fluorimeter. Thermogravimetric analysis was carried out
on SDT Q600 v8.2 Build 100 model of TA instruments.
Synthesis of 6-amino-1,4-diethyl-1,2,3,4-tetrahydroqui-
noxaline (5). 1,4-Diethyl-1,2,3,4-tetrahydro-6-nitroquinoxaline
4 [23] (5.0 g, 0.021 moles) and catalytic amount of palladium
on charcoal (10%) in glacial acetic acid (100 mL) were stirred
in Parr hydrogenator at 50ꢁC under an atmosphere of hydrogen
until think layer chromatography (TLC) (eluent: 10% ethyl ac-
etate in n-hexane) of the reaction mixture showed no red col-
ored spot of reactant. The reaction mixture was then filtered
under nitrogen atmosphere to separate the catalyst. 6-Amino-
1,4-diethyl-1,2,3,4-tetrahydroquinoxaline 5 thus obtained was
not isolated and subsequently used for further reaction immedi-
ately after filtration as it was found to be unstable [23]. (The
Thermal properties of fluorophores 7a and 7b. The
fluorophores were subjected to the thermogravimetric
analysis to investigate their thermal stability. The
change in weight of the compound was measured as a
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V. S. Satam, R. N. Rajule, A. R. Jagtap, S. R. Bendre, H. N. Pati, and V. R. Kanetkar
Vol 47
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General method for the synthesis of compounds
(7a–7b). Above reaction mixture, appropriate malondialdehyde
derivative 6a or 6b [28] (0.021 moles) and p-toluenesulphonic
acid (P-TSA) (3.6 g, 0.021 moles) were heated to reflux under
nitrogen atmosphere for 4 h. The reaction mixture was then
cooled to room temperature, neutralized with dilute sodium hy-
droxide solution (10%) maintaining the temperature below
15ꢁC. Dark brown solid obtained was filtered, washed with
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8-(Benzimidazol-2-yl)-1,4-diethyl-1,2,3,4-tetrahydropyrido[2,3-
g]quinoxaline (7a). (5.17 g, 69%); mp 160–162ꢁC; ir (KBr)
mmax cmꢀ1: 3090–3015, 2900–2850, 1688, 1612, 1279; 1H
NMR: d 1.20 (t, J ¼ 7.1 Hz, 3H, CH3), d 1.26 (t, J ¼ 7.1 Hz,
3H, CH3), d 3.15–3.21 (m, 2H), d 3.29 (q, J ¼ 7.1 Hz, 2H,
CH2), d 3.39 (q, J ¼ 7.1 Hz, 2H, CH2), d 3.47–3.53 (m, 2H),
d 6.69 (s, 1H, phenyl proton), d 7.03 (s, 1H, phenyl proton), d
7.31–7.37 (m, 2H, protons on benzimidazole ring), d 7.53–
7.58 (m, 1H, proton on benzimidazole ring), d 7.72–7.77 (m,
1H, proton on benzimidazole ring), d 8.55 (d, J ¼ 1.95 Hz,
1H, proton para to N of pyrido ring), d 9.26 (d, J ¼ 1.95 Hz,
1H, proton ortho to N of pyrido ring), Anal. Calcd for
C22H23N5: C, 73.92; H, 6.49; N, 19.59. Found: C, 73.95; H,
6.48; N, 19.55; ms: m/z 358 (MþþH).
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8-(Benzthiazol-2-yl)-1,4-diethyl-1,2,3,4-tetrahydroquinoxa-
line (7b). (5.74 g, 73%); mp 174–176ꢁC; ir (KBr) mmax cmꢀ1
:
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1
3100–3010, 2911–2850, 1682, 1608, 1280; H NMR: d 1.21 (t,
J ¼ 6.9 Hz, 3H, CH3), d 1.28 (t, J ¼ 6.9 Hz, 3H, CH3), d
3.13–3.18 (m, 2H), d 3.31 (q, J ¼ 6.9 Hz, 2H, CH2), d 3.41
(q, J ¼ 6.9 Hz, 2H, CH2), d 3.49–3.54 (m, 2H), d 6.71 (s, 1H,
phenyl proton), d 7.04 (s, 1H, phenyl proton), d 7.31–7.51 (m,
2H, phenyl protons on benzthiazole ring), d 7.87–7.91 (m 1H,
phenyl proton on benzthiazole ring), d 8.03–8.07 (m, 1H, phe-
nyl proton on benzthiazole ring), d 8.42 (d, J ¼ 2.42 Hz, 1H,
proton para to N of pyrido ring), d 9.11 (d, J ¼ 2.42 Hz, 1H,
proton ortho to N of pyrido ring); Anal. Calcd for C22H22N4S:
C, 70.56; H, 5.92; N, 14.96; S, 8.56. Found: C, 70.59; H, 5.88;
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