Synthesis, Spectroscopic Characterization and DFT/TD-DFT Calculations of new Fluorescent Derivatives of Imidazo[4′,5′:3,4]Benzo[c]Isoxazole

Article abstract of DOI:10.1007/s10895-015-1736-5

An increasingly wide variety of fluorescent compounds is used in biotechnology, genomics, immunoassays, array technologies, imaging, and drug discovery. Therefore, synthesis of fluorophores with novel structural features can be interesting and useful in various fields. In this paper, four new fluorescent heterocyclic compounds with high quantum yields are introduced. These new fluorophores are synthesized in moderate to high yields via regioselective nitration of 3-alkyl-8-(4-chlorophenyl)-3 H-imidazo[4′,5′:3,4]benzo[c]isoxazoles. The latter compounds are obtained from the reaction of 1-alkyl-5-nitro-1 H-benzoimidazoles with (4-chlorophenyl)acetonitrile in basic MeOH solution. Physical spectral (UV-vis, IR, ¹HNMR, ¹³C NMR, NOESY and fluorescence) and analytical data have established the structures of synthesized compounds. The fluorescence properties of new fluorescent heterocyclic compounds are studied. The fluorescence of all compounds is very intense and fluorescence quantum yields are high (> 0.52). Density functional theory (DFT) calculations are performed to provide the optimized geometries, relevant frontier orbitals and the prediction of ¹H NMR chemical shifts for confirming the exact structure of fluorescent compounds. Calculated electronic absorption spectra were also obtained by time-dependent density functional theory (TD-DFT) method.

Full text of DOI:10.1007/s10895-015-1736-5

J Fluoresc
DOI 10.1007/s10895-015-1736-5
ORIGINAL ARTICLE
Synthesis, Spectroscopic Characterization and DFT/TD-DFT
Calculations of new Fluorescent Derivatives of Imidazo[4′,5′:3,4]
Benzo[c]Isoxazole
Shirin Ramezani1 & Mehdi Pordel1 & Safarali Beyramabadi1
Received: 29 September 2015 /Accepted: 27 November 2015
#
Springer Science+Business Media New York 2015
.
Keywords 3 H-Imidazo[4′,5′:3,4]benzo[c]isoxazole
Abstract An increasingly wide variety of fluorescent com-
pounds is used in biotechnology, genomics, immunoassays,
array technologies, imaging, and drug discovery. Therefore,
synthesis of fluorophores with novel structural features can be
interesting and useful in various fields. In this paper, four new
fluorescent heterocyclic compounds with high quantum yields
are introduced. These new fluorophores are synthesized in
moderate to high yields via regioselective nitration of 3-al-
kyl-8-(4-chlorophenyl)-3 H-imidazo[4′,5′:3,4]benzo[c]-
isoxazoles. The latter compounds are obtained from the reac-
tion of 1-alkyl-5-nitro-1 H-benzoimidazoles with (4-
chlorophenyl)acetonitrile in basic MeOH solution. Physical
spectral (UV-vis, IR, 1HNMR, 13C NMR, NOESY and fluo-
rescence) and analytical data have established the structures of
synthesized compounds. The fluorescence properties of new
fluorescent heterocyclic compounds are studied. The fluores-
cence of all compounds is very intense and fluorescence quan-
tum yields are high (> 0.52). Density functional theory (DFT)
calculations are performed to provide the optimized geome-
tries, relevant frontier orbitals and the prediction of 1H NMR
chemical shifts for confirming the exact structure of fluores-
cent compounds. Calculated electronic absorption spectra
were also obtained by time-dependent density functional the-
ory (TD-DFT) method.
.
.
.
Density functional theory NOESY Fluorescence Emission
and absorption spectra
Introduction
The recent explosion in new fluorescence applications is ac-
celerating the pace of research and development in basic and
applied life sciences, including genomics, proteomics, bioen-
gineering, medical diagnosis and industrial microbiology.
Fluorescent heterocyclic compounds are of vast interest as
useful materials in the emitters of electroluminescence devices
and in the molecular probes used for biochemical research, as
well as in the traditional textile and polymer fields [1–7]. They
are useful materials in the search for new biologically
active compounds and diagnostic methods [8].
Fluorescent chromophores are usually known to have
planar and rigid pi-conjugated systems, and many fluo-
rescent chromophores are based on rigid ring systems.
Naturally occurring substituted imidazoles, as well as
synthetic derivatives thereof, exhibit wide ranges of bi-
ological activities and optical applications such as fluo-
rescence compounds, dyes, and TPA (Two-photon
absorption) materials [9–12] making them attractive
compounds for organic chemists. Also, many commer-
cial fluorescent brighteners for application to synthetic
fibers contain an imidazole moiety [13]. On the other
hand, isoxazoles and fused isoxazoles have found con-
tinuing application in medicinal chemistry, several ex-
amples of which have advanced to general medical
practice. For example, benzo[c]isoxazole derivatives are
prescribed as antipsychotic risperidone drugs [14] and
play a key role in many organic reactions [15]. A liter-
ature survey disclosed that there are only a few
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-015-1736-5) contains supplementary material,
which is available to authorized users.
* Mehdi Pordel
mehdipordel58@yahoo.com
1
Department of Chemistry, Mashhad Branch, Islamic Azad
University, Mashhad, Iran

J Fluoresc
examples of isoxazoles or benzo[c]isoxazoles with fluo-
rescence properties. A combination of the benzo[c]-
isoxazole moiety with the imidazole nucleus may en-
hance optical and biological properties. Based on these
aspects, in current work, we synthesized some
imidazo[4′,5′:3,4]benzo[c]isoxazoles with high fluores-
cence properties as new fluorophores. Spectroscopic
characterization of these compounds were studied and
the structure of the major products was established by
NOESY (Nuclear Overhauser Effect Spectroscopy) ex-
periment and explained with DFT and TD-DFT calcula-
tions by using the B3LYP hybrid functional and the 6–
311++G(d,p) basis set.
Scheme 2 Two possible structures for the nitration of 3a–d
and the 6–311 + G(d,p) basis set. Firstly, geometry of the
compounds 4a and 4a’ were fully optimized in the chloroform
solution. The optimized geometries were confirmed to have
no imaginary frequency. Then, their optimized geometries
were used for frequency calculations.
Here, one of self-consistent reaction field methods, the so-
phisticated Polarized Continuum Model (PCM) [20] has been
used for investigation of the solvent effects. The PCM calcu-
lations have been performed in the chloroform solution and
the zero-point corrections were considered to obtain energies.
Based on the optimized geometries and using time-dependent
density functional theory (TD-DFT) [21–23] methods, the
electronic spectra of the compounds 4a and 4a’ were
predicted.
Experimental
Equipment and Materials
Melting points were measured on an Electrothermaltype-9100
melting-point apparatus. The FT-IR (as KBr discs) spectra
were obtained on a Tensor 27 spectrometer and only notewor-
thy absorptions are listed. The 13C NMR (100 MHz),
H
1
NMR (400 MHz) and NOESY spectra were recorded on a
Bruker Avance DRX-400 spectrometer. Chemical shifts are
reported in ppm downfield from TMS as internal standard;
coupling constant J is given in Hz. The mass spectra were
recorded on a Varian Mat, CH-7 at 70 eV. Elemental
analysis was performed on a Thermo Finnigan Flash EA
microanalyzer. Absorption and fluorescence spectra were
recorded on Varian 50-bio UV-Visible spectrophotometer
and Varian Cary Eclipse spectrofluorophotometer. UV–
vis and fluorescence scans were recorded from 200 to
1000 nm. All measurements were carried out at room
temperature. Compounds 1a–d [16] and 3a–d [17] were
obtained according to the published methods. Other re-
agents were commercially available.
General Procedure for the Synthesis of 4a–d from 3a–d
Nitric acid (1 ml) was added with stirring over half an hour
period to a solution of 3a–d (20 mmol) in concentrated sulfu-
ric acid (4 ml) maintained at 0–5 °C. After the addition was
completed, the mixture was allowed to warm to room temper-
ature with stirring for 30 min. After pouring this mixture into
crushed ice and water (200 ml), the solid that precipitated was
removed by filtration, was washed with water, and dried to
give crude 4a–d.
8-(4-Chlorophenyl)-3-ethyl-5-nitro-3 H-imidazo[4′,5′:3,4]
benzo[c]isoxazoles (4a). Compound 4a was passed through
silica-gel column (n-hexane-EtOAc 2:1) to give 4a as a shiny
1
yellow powder. Yield (77 %); mp 277–279 °C. H NMR
Computational Methods
(CDCl3): δ 1.69 (t, J = 7.2 Hz, 3 H), 4.43 (q, J = 7.2 Hz,
2 H), 7.62 (d, J = 8.4 Hz, 2 H), 8.20 (s, 1 H), 8.74 (s, 1 H),
8.90 (d, J = 8.4 Hz, 2 H) ppm; 13C NMR (CDCl3): δ 16.0,
41.2, 109.8, 118.2, 125.6, 126.5, 129.4, 129.5, 132.2, 137.4,
DFT calculations were performed with the Gaussian 98 soft-
ware package [18] by using the B3LYP hybrid functional [19]
Scheme 1 Synthesis of the new fluorophores 4a–d

J Fluoresc
Fig. 1 Optimized geometries of
the 4a and 4a’
140.4, 144.2, 151.0, 165.6 ppm. IR (KBr): 1358, 1542 cm−1
(NO2), MS (m/z) 344 (M++2). Anal. Calcd for C16H11ClN4O3
(342.7): C, 56.07; H, 3.23; N, 16.35. Found: C, 55.97; H,
3.20; N, 16.27.
J = 8.4 Hz, 2 H), 8.17 (s, 1 H), 8.73 (s, 1 H), 8.90 (d,
J = 8.4 Hz, 2 H) ppm; 13C NMR (CDCl3): δ 13.7, 21.3,
33.6, 54.2, 109.8, 117.9, 125.9, 126.4, 129.2, 130.4, 132.3,
138.3, 140.0, 144.7, 150.5, 165.3 ppm. IR (KBr): 1352,
1541 cm−1 (NO2), MS (m/z) 372 (M++2). Anal. Calcd for
C18H15ClN4O3 (370.8): C, 58.31; H, 4.08; N, 15.11. Found:
C, 58.21; H, 4.06; N, 14.97.
8-(4-Chlorophenyl)-3-isobutyl-5-nitro-3 H-imidazo[4′,
5′:3,4] benzo[c]isoxazoles (4d). Compound 4d was passed
through silica-gel column (n-hexane-EtOAc 2:1) to give 4d
as a shiny yellow powder. Yield (73 %); mp 269–271 °C. 1 H
NMR (CDCl3): δ 0.90 (d, J = 6.4 Hz, 6 H), 2.15–2.19 (m,
1 H), 4.32 (d, J = 7.2 Hz, 2 H), 7.62 (d, J = 8.4 Hz, 2 H), 8.16
(s, 1 H), 8.72 (s, 1 H), 8.90 (d, J = 8.4 Hz, 2 H) ppm; 13C NMR
(CDCl3): δ 20.4, 29.8, 58.9, 109.9, 117.7, 125.5, 126.0, 128.7,
130.7, 132.2, 138.6, 140.3, 144.8, 150.7, 165.0 ppm. IR
(KBr): 1355, 1541 cm−1 (NO2), MS (m/z) 372 (M++2).
Anal. Calcd for C18H15ClN4O3 (370.8): C, 58.31; H, 4.08;
N, 15.11. Found: C, 58.15; H, 4.06; N, 15.0.
8-(4-Chlorophenyl)-3-propyl-5-nitro-3 H-imidazo[4′,5′:3,
4] benzo[c]isoxazoles (4b). Compound 4b was passed
through silica-gel column (n-hexane-EtOAc 2:1) to give 4b
as a shiny yellow powder. Yield (75 %); mp 275–277 °C. 1 H
NMR (CDCl3): δ 1.07 (t, J = 7.3 Hz, 3 H), 1.90–2.02 (m, 2 H),
4.33 (t, J = 7.2 Hz, 2 H), 7.62 (d, J = 8.4 Hz, 2 H), 8.17 (s,
1 H), 8.71 (s, 1 H), 8.90 (d, J = 8.4 Hz, 2 H) ppm; 13C NMR
(CDCl3): δ 12.2, 23.5, 53.8, 109.7, 117.8, 125.9, 126.3, 129.0,
129.9, 132.1, 137.9, 140.3, 144.5, 150.6, 165.4 ppm. IR
(KBr): 1357, 1543 cm−1 (NO2), MS (m/z) 358 (M++2).
Anal. Calcd for C17H13ClN4O3 (356.8): C, 57.23; H, 3.67;
N, 15.70. Found: C, 57.11; H, 3.65; N, 15.83.
3-Butyl-8-(4-chlorophenyl)-5-nitro-3 H-imidazo[4′,5′:3,4]
benzo[c]isoxazoles (4c). Compound 4c was passed through
silica-gel column (n-hexane-EtOAc 2:1) to give 4c as a shiny
1
yellow powder. Yield (70 %); mp 272–275 °C. H NMR
(CDCl3): δ 0.93 (t, J = 7.2 Hz, 3 H), 1.40–1.48 (m, 2 H),
1.95–2.02 (m, 2 H), 4.37 (t, J = 7.2 Hz, 2 H), 7.62 (d,
Synthesis of 8-(4-Chlorobenzoyl)
-5-Ethyl-5 H-Imidazo[4′,5′:4,5]Benzo[1,2-c] [1, 2, 5]
Oxadiazole 3-Oxide (5)
Table 1 Experimental and DFT calculated 1H NMR chemical shifts of
4a and 4a’
Compound 4a (0.5 g) was heated 1 h under reflux in 10 ml of
glacial acetic acid. The solution was cooled, and an equal
volume of water was added. The precipitated solid was crys-
tallized from EtOH as yellow needles (0.33 g, 65 %)
Atom number (4a’) Chemical
Atom number (4a) Chemical
shift
shift
Calc. Exp.
Calc. Exp.
1
mp > 300 °C (decomp). H NMR (CDCl3): δ 1.54 (t,
J = 7.2 Hz, 3 H), 4.39 (q, J = 7.2 Hz, 2 H), 6.88 (s, 1 H),
8.12 (d, J = 8.7 Hz, 1 H), 8.32 (d, J = 8.7 Hz, 2 H) 8.41 (s,
1 H), ppm; 13C NMR (CDCl3): δ 15.7, 45.8, 104.6, 116.3,
128.4, 129.6, 130.0, 132.1, 132.8, 136.7, 145.5, 150.4,
159.8, 185.6 ppm. IR (KBr): 1685 cm−1 (C = O), MS (m/z)
H24, H25
H32
8.66 8.90 H28, H29
8.27 8.75 H25
9.02 8.90
8.80 8.75
8.26 8.21
7.71 7.62
H22
8.01 8.21 H26
H23, H26
7.70 7.62 H27, H30

J Fluoresc
CN
N
CN
N
Scheme 3 Some recently
synthesized fluorescent
heterocyclic compounds
containing donor sites
(endocyclic N or S) and the CN
group as an acceptor moiety
N
CN
N
N
N
N
N
N
R
N
R
H3CO
Pyrido[2',1':2,3]
3-alkyl-8-methoxy-3H-imidazo[4,5-a]
pyrido[1",2":1',2']imidazo[4',5':5,6]
pyrido[2,3-b]indole[25]
imidazo[4,5-b]quinoline-12-ylcyanides[23]
acridine-11-carbonitrile[24]
344 (M++2). Anal. Calcd for C16H11ClN4O3 (342.7): C,
56.07; H, 3.23; N, 16.35. Found: C, 55.95; H, 3.21; N, 16.49.
corresponding to the nitro group. All this evidence taken in
conjunction with molecular ion peak at m/z 344 (M++2)
strongly support the structure of compound 4a. However,
these spectral characteristics are consistent with another pos-
sible structure which is isomeric with 4a (Scheme 2).
Results and Discussion
In order to clarify the exact structure for the nitration of 3a–
d, after the compounds were passed through silica-gel column
(n-hexane-EtOAc), we made a great attempt to obtain an ap-
propriate single crystal of the products for crystallography in
different solvents, but the obtained crystals were not suitable
for X- ray crystallography. However, there are some convinc-
ing reasons that confirm that the 4a–d are the major products
for the nitration of 3a–d. For example, electron release from
the imidazole ring would make the C-5 position more suscep-
tible to electrophilic attack than C-4. Furthermore the latter is
also hindered by its proximity to the N-3 alkyl group.
Synthesis and Structures of new Compounds 4a–d
As depicted in Scheme 1, to obtain the desired new fluorescent
compounds, in the first place, the commercially available 5-
nitro-1 H-benzimidazole was alkylated with different alkyl
halides in KOH and DMF to give 1-alkyl-5-nitro-1 H-benz-
imidazoles (1a–d) at rt. [16]. Reaction of 1a–d with (4-
chlorophenyl)acetonitrile (2) in basic MeOH solution led to
the formation of 3-alkyl-8-(4-chlorophenyl)-3 H-imidazo [4′,
5′:3,4] benzo[c]isoxazoles (3a–d) in excellent yields [17].
Finally, new 3-alkyl-8-(4-chlorophenyl)-5-nitro-3 H-imidazo
[4′,5′:3,4]benzo[c]isoxazoles (4a–d) were synthesized by ni-
tration of 3a–d in moderate to high yields.
In addition, 4a–d intermediates have more resonance struc-
tures than 4’a–d intermediates (Supporting Information) and
this leads to the stability of 4a–d intermediates and thus faster
formation of 4a–d compared to 4’a–d products.
Structural assignments of new compounds 4a–d were
based upon their spectral and microanalytical (C, H and N)
Also, DFTcalculations at the level of B3LYP/6–311 + G(d,
p) can help identify the structure of the main product. For this
purpose, optimized geometries of compounds 4a and 4a’ were
obtained (Fig. 1) and then, DFT calculated chemical shifts (δ)
of compounds 4a and 4a’ were compared with the experimen-
tal values (Table 1). It can be induced from Table 1, the cal-
culated chemical shifts (δ) of compound 4a are more in agree-
ment with the experimental values, confirming validity of the
structure of compound 4a. Selected structural parameters of
4a and 4’a can be found in Supporting Information.
1
data. For example, in the aromatic region of the H NMR
spectrum of compound 4a (Supporting Information), there
are two doublet signals at δ 7.62 and 8.90 ppm attributed to
four aromatic protons of 4-chlorophenyl ring and two singlet
signals at δ 8.21 and 8.75 ppm assignable to two aromatic
protons of imidazole and phenyl ring respectively. Also, there
are 14 different carbon atoms in the 13C NMR spectrum of
compound 4a. Moreover, the FT-IR spectrum of the 4a in KBr
showed two absorption band at 1542 and 1358 cm−1
Fig. 2 Visible absorption (left) (5 × 10−5 mol L−1) and emission spectra (right)) (2 × 10−6 mol L−1) of compounds 4a–d in chloroform solvent

J Fluoresc
Table 3 Spectroscopic
data for compound 4a at
298 K in different
solvents
Investigation on the photophysical properties of the
Solvent
λ
abs (nm)
λflu (nm)
nitrated compounds can also help further to confirm the exact
structure. Recently, we have succeeded in synthesizing some
new heterocyclic compounds which had fluorescent proper-
ties [24–26]. In these compounds, there are efficient intramo-
lecular charge transfer (ICT) states from the donor sites
(endocyclic N or S) to the CN group as an acceptor moiety
(Scheme 3).
n-hexane
DCM
415
425
430
435
435
495
525
540
540
545
Acetone
Acetonitrile
DMF
In the present study, there are endocyclic N and O as
donor sites in compounds 3a–d and these compounds
are nitrated to obtain new fluorophore with an acceptor
site (NO2 group). Fortunately, new nitrated compounds
have fluorescence emission at concentration of
2 × 10−6 mol L−1 in chloroform solvent (Fig. 2). The
fluorescence excitation (λex) wavelength at 425 nm (λex/
nm) was used for all of them. Their fluorescence quan-
tum yields (ΦF) were determined via comparison
methods, using fluorescein as a standard sample in 0.1 M
NaOH and MeOH solution [27]. Visible absorption spectra
of nitrated compounds were also obtained in chloroform sol-
vent at concentration of 5 × 10−5 mol L−1 (Fig. 2).
Photophysical properties of nitrated compounds are presented
in Table 2. As is shown in the table, extinction coefficients (ε)
calculated as the slope of the plot of absorbance vs concentra-
tion are very high, and this is remarkable for optical properties
of these compounds. Further, the fluorescence spectral prop-
erties (Table 2) of compounds 4a–d are similar to each other
and fluorescence intensity in compound 4c, with a butyl
group, was the highest.
545 nm respectively, as the solvent changes from n-hexane to
DMF (Table 3).
Furthermore, TD-DFT electronic spectra calculations of 4a
and 4a’ show electronic transition bands at 420 nm (oscillator
strength: 0.3594) and 338 nm (oscillator strength: 0.4001)
respectively. When these electronic transition bands are com-
pared with the experimental value of 427 nm, structures of 4a
gets more confirmation than structures of 4a’. Electronic tran-
sition band of 4a can be corresponded to π- π* transitions
from donor endocyclic nitrogen to the acceptor nitro group.
The calculated electronic absorption spectra of 4a and 4a’ can
be found in Supporting Information.
Study on the HOMO and LUMO frontier orbitals and the
energy difference between them can provide a deeper insight
The solvatochromic properties of 4a in different solvents
were also investigated (Table 3). The fluorescence absorption
and emission spectra of 4a in polar solvents undergo a rela-
tively modest red shift. Increasing the solvent polarity stabi-
lizes the ICT excited-state molecule relative to the ground-
state molecule with the observed red shift of the absorption
maximum as the experimentally observed result (Tables 3).
For example, in the absorption and emission spectra of com-
pound 4a, λabs and λflu shift from 415 to 435 nm and 495 to
Table 2 Photophysical data for absorption (abs) and fluorescence (flu)
of 4a–d (Chloroform)
Dye
4a
4b
4c
4d
λ
abs (nm)a
427
5.6
427
5.2
423
4.8
427
4.6
ε × 10–3
[(mol L−1 −1 cm−1 b
]
)
λ
λ
ex (nm)c
flu (nm)d
425
525
425
533
425
535
425
535
Stokes shift (cm−1
)
102,040 94,339 89,285 92,592
0.52 0.55 0.57 0.55
e
ΦF
a Wavelengths of maximum absorbance; b Extinction coefficient; c Wave-
lengths of fluorescence excitation; d Wavelengths of fluorescence emis-
sion; e Fluorescence quantum yield
Fig. 3 The HOMO and LUMO frontier orbitals of 4a and 4a’

J Fluoresc
Scheme 4 NOESY spectrum of
compound 4b
into the charge transfer properties and electronic transition
bands. The HOMO and LUMO maps of the 4a and 4a’ are
shown in Fig. 3. Separation energies between the HOMO and
LUMO (Δε = εLUMO – εHOMO) in 4a and 4a’ are 2.97 and
3.11 eV (417 and 399 nm) respectively. As seen in Fig. 3, the
HOMO and LUMO of 4a are also delocalized on NO2 group
which led to the lower separation energies between the
HOMO and LUMO in 4a compared to the Δε in 4a’. It could
also be considered as another reason for the confirmation of
visible absorption in 427 nm in the nitrated compounds and
therefore the position of the nitro group in these compounds.
The data from NOESY experiment of compound 4b
showed a massive cross-peak between the H-4 proton (δH
8.71, s, CH-benzene) and the N-3 alkyl group (δH 4.33 (t,
J = 7.2 Hz, NCH2), confirming the product structure (4b) for
the nitration of 3a–d (Scheme 4). As can be seen in Scheme 4,
there are also interactions between proton of imidazole ring
(δH 8.17, s, CH-imidazole) and CH2 protons of propyl group
(δH 4.33 (t, J = 7.2 Hz, NCH2).
with interesting optical properties such as high extinction co-
efficients and high quantum yields. These compounds were
obtained from regioselective nitration of 3-alkyl-8-(4-
chlorophenyl)-3 H-imidazo [4′,5′:3,4] benzo[c]isoxazoles in
rt. Confirmation of the correct structure has been examined
by investigation on the resonance structures of intermediates
for the nitration reaction, DFT calculated chemical shifts (δ),
photophysical properties, TD-DFT electronic spectra calcula-
tions, HOMO and LUMO frontier orbitals and finally
NOESY experiment of nitrated compounds. Furthermore,
the transformation of 4a to 5 would support the site of
nitration.
This research can help to the synthesis of new fluorophores
and stereochemical determining in similar reactions. In addi-
tion, the new synthesized compounds can play a key role in
Indeed, compound 4a underwent thermal rearrangement in
high yield to its new corresponding 8-(4-chlorobenzoyl)-5-
ethyl-5 H-imidazo[4′,5′:4,5]benzo[1,2-c] [1, 2, 5]oxadiazole-
3-oxide 5 in AcOH [28] (Scheme 5). Structural assignments of
compound 5 were based on its spectral and microanalytical
data.
In conclusion, we have synthesized some new donor–ac-
ceptor fluorescent heterocyclic compounds in good yields
Scheme 5 Synthesis of the new compound 5

J Fluoresc
16. Preston, P.N. The Chemistry of Heterocyclic Compounds,
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