Ultrasound-accelerated synthesis of quinoline-based luminescent imines exhibiting large Stokes shift

Article abstract of DOI:10.1134/S1070363217110226

High yield synthesis of quinoline-based fluorescent imines, exhibiting the highest Stokes shift exceeding 380 nm (>18 000 cm^–1), is reported in the current publication. Such large difference in the excitation and emission wavelengths is quite u

Full text of DOI:10.1134/S1070363217110226

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 11, pp. 2664–2670. © Pleiades Publishing, Ltd., 2017.
Ultrasound-Accelerated Synthesis of Quinoline-Based
Luminescent Imines Exhibiting Large Stokes Shift1
A. R. Choudhurya, S. B. Paulb, and S. Choudhurya,b*
a Department of Chemistry, Assam University, Silchar, 788011 India
b Centre for Soft Matters, Department of Chemistry, Assam University, Silchar, 788011 India
*e-mail: sudip.choudhury@aus.ac.in; sudipch1@gmail.com
Received July 15, 2017
Abstract—High yield synthesis of quinoline-based fluorescent imines, exhibiting the highest Stokes shift
exceeding 380 nm (>18000 cm–1), is reported in the current publication. Such large difference in the excitation
and emission wavelengths is quite unusual. Synthesis of imines was accelerated by ultrasound. Both
experimental and theoretical studies have been performed.
Keywords: quinoline, fluorescence, Stokes shift
DOI: 10.1134/S1070363217110226
Organic photoluminescent compounds with large
Stokes shift and high photostability are of certain
importance in synthetic organic chemistry and design
of materials possessing superior biological and other
properties. Fluorescent compounds with large Stokes
shift are particularly important as UV in polymers
stabilization, sun-creams [1, 2], live cell imaging [3],
and super-resolution optical microscopy [4]. Fluore-
scent organic compounds with large Stokes shift
(typically over 80 nm) can minimize overlapping
between excitation and fluorescent emission bands
thus providing high signal-to-noise ratio [5]. In quest
of such materials, many luminescent compounds were
reported to have remarkably high Stokes shift, most
notably comprising of xanthene [6], benzoxazoles [3],
oligo-phenylenes [7], triazine [2], and some others.
The present study is devoted to the synthesis of
quinoline based luminescent imines, characterized by
largest Stokes shift up to > 380 nm (> 18000 cm–1). To
our best knowledge, the Stokes shifts observed in the
present work are among the largest reported so far
(Scheme 1).
containing multiple heterocyclic moieties and exhibit-
ing very large Stokes shift, is reported. Theoretical
studies also have been carried out to explore lumine-
scent behavior of the synthesized compounds.
RESULTS AND DISCUSSION
Synthesis. The target compounds 3a–3f were syn-
thesized in two steps process, in which the modified
Vilsmeier–Haack reaction of 2-chloro-3-formyl quino-
line was followed by formation of the products 3a–3f
(Scheme 2, Table 1).
Luminescence study. Six Schiff bases with diverse
aromatic amines have been synthesized and their
luminescence was studied to explore the effect of
π-electronic cores rather than mere effect of different
substituents. In that regard, keeping the aldehyde
group of the Schiff base to be the same, changes in the
imine containing fragment were brought about. Out of
these six compounds, two monoaromatic 3a, 3b, three
fused ring system 3c–3e, and one discrete ring skeleton
3f were considered to impose diversity in aromatic π-
electronic distribution.
Quinoline derivatives are important initial com-
pounds in creating optical materials. Aluminium tris-
chelate complex of 8-hydroxy-quinoline is already
used in development of organic light emitting diodes
(OLEDs). Herein, the ultrasound accelerated synthesis
of some quinoline based luminescent Schiff’s bases
UV-Vis (Fig. 1) and fluorescence (Fig. 2) spectra of
solutions (10–5 M) of the compounds 3a–3f were
studied.
The compounds exhibited photoluminescence with
moderate to strong intensity when irradiated with UV
radiation of suitable excitation frequency (Table 2).
Most notably, very high Stokes shift was observed in
1 The text was submitted by the authors in English.
2664

ULTRASOUND-ACCELERATED SYNTHESIS
2665
Scheme 1.
O
OH
N
N
O
Et2N
O
O2N
NO2
S
ClO4
O
O
S
H2N
NH2
O
O
Stokes shift: 187 nm
[3]
Stokes shift: 160 nm
[6]
Ph
Ph
R
R
x
y
OH
R
R
N
N
x
y
Me2N
N
NMe2
x = 1, 2; y = 0, 1; R = H, Me
Stokes shift: 203 nm
[7]
Stokes shift: 220 nm
[2]
Cl
N
R
N
3b
Higest Stokes shift (in the series): 383 nm
[Present work]
Scheme 2.
CHO
CH=NR
Cl
O
RNH2
POCl3−DMF, 0°C
80−90°C
Ethanol, )))
20 min, room temparture
N
N
Cl
N
H
2-chloro-3-formyl quinoline
3a−3f
three compounds 3b, 3d, 3f, reaching its value as high
as 383 nm (3b). The interesting feature observed here
was correlation between the Stokes shift and the
electron release/withdrawing nature of the imine
fragment of the Schiff base. Stronger electron
withdrawing moiety attached to nitrogen of the imine
group was matching with the increase in the Stokes
shift.
Heterocyclic fused ring systems attached to both
sides of the imine moiety did not change the emission
frequency appreciably but there was some change in
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 11 2017

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CHOUDHURY et al.
Table 1. The imines 3a–3f synthesized
Comp.
Amines RNH2
no.
Imines
mp, °C
180
Yield, %
78
3a
3b
3c
2-Aminophenol
N
OH
N
Cl
Cl
Cl
4-Aminobenzoic acid
210
220
78
71
N
N
N
N
COOH
6-Amino-1-methylquinolin-2(1H)-one
N
O
Cl
Cl
3d
6-Amino-1-ethylquinolin-2(1H)-one
214
73
N
N
N
N
O
3e
3f
2-Aminobenzothiazole
4-Aminoantipyrine
228
190
77
87
S
N
N
N
O
Cl
N
N
N
Table 2. Excitation, emission and Stokes shift exhibited by
the compounds 3a–3f
excitation value thereby rendering an appreciable
change in the Stokes shift.
Excitation,
nm
Emission,
nm
Stokes shift,
nm
Comp. no.
Computational study. Electronic transitions and
associated luminescence theoretical studies were
carried out using Gaussian09 package. The structures
of all six compounds were optimized under DFT at
B3LYP level theory with 6-31G** basis set [8, 9].
After optimization of the structures the energies of
HOMO and first three LUMOs were mainly
considered in the study. The energy level of all six
compounds are plotted and presented in Fig. 3. MO’s
3a
3b
3c
3d
3e
3f
377
297
368
264
371
318
438
680
437
451
438
438
61
383
69
187
67
120
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ULTRASOUND-ACCELERATED SYNTHESIS
2667
5
3
1
3
5
4
2
1
2
4
6
6
Wavelength, nm
Wavelength, nm
Fig. 1. UV-Vis spectra of the compounds (1) 3a, (2) 3b,
(3) 3c, (4) 3d, (5) 3e, and (6) 3f.
Fig. 2. Fluorescence spectra of the compounds (1) 3a,
(2) 3b, (3) 3c, (4) 3d, (5) 3e, and (6) 3f.
of the compound 3b which exhibited the highest
Stokes shift are presented in Fig. 4.
The study indicated also that there was an inverse
relationship between the Stoke shift and “closeness”
between the HOMO and higher MO’s (Fig. 5), which
was more pronounced in higher participating MO’s.
The Stokes shift of the emission from the quinoline
based Schiff bases containing fluorophores was found
to be strongly dependent on the electron releasing/
withdrawing groups connected to it and less prone to
the type of aromatic moiety connected to the quinoline
system via the imine linkage. The computational study
indicated that the HOMO’s of all these compounds
were mostly localized in certain portions of the
molecules, whereas the higher MO’s participating in
the excitation of electron in the course of absorption of
energy (like the LUMO+2 for 3b), could be distributed
more evenly throughout the whole molecular skeleton.
The HOMO, LUMO and LUMO+2 orbitals of the
compounds 3a (lowest Stokes shift) and 3b (highest
Stokes shift) are presented in Fig. 5 as the re-
presentative cases. Thus, a change in electron density
occurring at the later segment affected the Stokes shift
to a large extent.
According to the accumulated data the HOMOs of
all six compounds had similar energies, particularly
compounds 3a, 3c, 3d, which indicated similarity in
electronic distribution in the aromatic ring directly
connected with the imine nitrogen. The electron
donating effect of oxygen in 3a and heterocyclic
nitrogen in 3c, 3d seemed to be similar.
According to fluorescence spectra (Fig. 2), all
compounds exhibited luminescence when excited with
suitable excitation wavelength (Table 2). Among
those, the compound 3b exhibited very high Stokes
shift of 383 nm and fluoresces at 680 nm upon
excitation with 297 nm wavelength. The theoretically
calculated energy levels (Fig. 3) indicated that the
presence of comparatively low lying consecutive
unoccupied molecular orbitals in 3b were likely to be
responsible for such high Stokes shift. Such
suppression in the energy level could be due to the
presence of the electron withdrawing group –COOH.
abs
max
The observed λ
at 297 nm was in accord with
EXPERIMENTAL
theoretically calculated electron transition from
HOMO to LUMO+2 (due to low lying orbitals). The
electron upon shifting down to some lower MO,
underwent some intersystem crossing, releasing
comparatively lower energy photons to reach the
ground level. This seemed to be the reason for the long
wavelength fluorescence exhibited by 3b. In all other
cases (3a, 3c, 3e, 3f), the HOMO–LUMO (and other
UMOs) energy gaps were much larger and the
luminescence could originate from the excitation due
to HOMO to higher MO’s and transition followed by
spin relaxation.
All amines were purchased from Sigma-Aldrich
and used as obtained without further purification. The
solvents were procured from Qualigen India and
Ranchem India, and distilled before use.
1H NMR spectra were measured on
a
BrukerAvance II, 400 MHz spectrometer in CDCl3
using TMS as an internal standard. UV-Vis absorption
spectra were recorded in CHCl3 and DMSO on a
JASCOV-670 spectrophotometer. Fluorescence spectra
were recorded on a Hitachi F-4600 spectrophotometer.
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CHOUDHURY et al.
O
O
N
N
N
HOOC
N
N
OH
S
N
N
N
N
N
N
Cl
N
Cl
Cl
Cl
Cl
Cl
N
N
N
N
N
3a
3b
3e
3f
3c
3d
1.454
1.454
1.530
2
0
0.437
0.186
–1.000
–2.860
–4.973
–1.378
–4.835
–7.637
–1.378
–4.835
–1.294
–2
–4
–2.667
–3.277
–6.815
–4/526
–6.640
–5.618
–7.120
–6
–8
–7.637
–7.596
–8.093
HOMO
LUMO
LUMO+1
LUMO+2
–10
Fig. 3. Calculated energy levels of HOMO and first three LUMOs of the compounds 3a–3f.
2-Chloro-3-formyl quinoline (1) was synthesized
according to the developed earlier method [10].
glacial acetic acid as a catalyst, which gave similar
yield after 3–4 h of refluxing.
Synthesis of Schiff bases 3a–3f. A solution of
2-chloro-3-formyl quinoline (1 mmol) in ethanol was
added to an ethanol solution of an amine 2a–2f
(1 mmol). The mixture was sonicated for 20 min with
40 kHz ultrasound sonicator at room temperature. The
product started precipitating after 15 min of sonication,
which was continued for 5 min more. A product was
filtered off and recrystallized from absolute ethanol to
give the corresponding yellow crystalline compound
3a–3f. The test reaction was also performed with
conventional refluxing in presence of a few drops of
2-[(2-Chloroquinolin-3-yl)methyleneamino]phenol
(3a). Yield 78%, mp 180°C. 1H NMR spectrum, δ,
ppm: 6.986 t (1H), 7.074 d (1H), 7.273–7.294 m (1H),
7.402 d (1H), 7.633 t (1H), 7.817 t (1H), 7.983 d (1H),
8.069 d (1H), 8.977 s (1H), 9.214 s (1H), 10.5 s (1H).
13C NMR spectrum, δ, ppm: 115.4, 116.3, 120.4,
127.1, 127.3, 127.8, 128.5, 128.8, 130.1, 132.2, 135.1,
137.4, 148.6, 150.2, 152.3, 152.7.
4-[(2-Chloroquinolin-3-yl)methyleneamino]benzoic
acid (3b). Yield 78%, mp 210°C. 1H NMR spectrum,
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ULTRASOUND-ACCELERATED SYNTHESIS
2669
3c
3a
3e
3d
3b
3f
HOOC
Stokes shift
N
Fig. 5. Trends in molecular orbital energy and Stoke shift.
Cl
6.769 d (1H), 7.450 s (1H), 7.473 s (1H), 7.536 d (1H),
7.601 m (1H), 7.623 m (1H), 7.708 d (1H), 7.813 t
(1H), 7.961 d (1H), 8.049 d (1H), 9.076 s (1H). 13C
NMR spectrum, δ, ppm: 12.8, 37.5, 115.0, 120.9, 121.6,
122.7, 124.3, 127.2, 127.4, 127.7, 128.5, 128.9, 132.0,
137.6, 138.1, 138.7, 145.2, 148.6, 150.2, 155.3, 161.6.
N
3b
N-[(2-Chloroquinolin-3-yl)methylene]benzo[d]-
thiazol-2-amine (3e). Yield 77%, mp 228°C. 1H NMR
spectrum, δ, ppm: 7.432 t (1H), 7.562–7.581 m (2H),
7.644 s (1H), 7.911 t (1H), 7.984 d (1H), 8.004 d (1H),
8.141 d (1H), 9.213 s (1H), 9.643 d (1H). 13C NMR
spectrum, δ, ppm: 113.3, 119.6, 122.1, 123.3, 126.2,
126.9, 127.2, 127.6, 128.3, 129.4, 133.3, 136.7, 140.7,
148.0, 148.1, 166.2, 169.3.
Fig. 4. MO’s of the compound 3b.
4-[(2-Chloroquinolin-3-yl)methyleneamino]-1,5-
dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (3f). Yield
87%, mp 190°C. H NMR spectrum, δ, ppm: 2.559 s
1
δ, ppm: 6.566 d (1H), 7.753 s (1H), 7.773 d (2H),
8.297 d (2H), 8.992 s (1H), 10.392 s (1H). 13C NMR
spectrum, δ, ppm: 112.5, 116.8, 121.2, 126.3, 126.4,
127.7, 128.2, 130.2, 131.2, 133.9, 141.4, 148.5, 148.9,
153.0, 167.4, 189.4.
(3H), 3.225 s (3H), 7.539 s (1H), 7.557 m (2H), 7.560
d (2H), 7.577 t (1H), 7.721 t (1H), 7.928 d (1H), 7.997
d (1H), 8.914 s (1H), 10.215 s (1H). 13C NMR
spectrum, δ, ppm: 10.1, 35.6, 118.7, 124.7, 127.2,
127.3, 127.3, 128.4, 128.5, 129.3, 129.6, 130.9, 134.5,
135.3, 148.0, 150.8, 152.1, 152.2, 160.5.
6-[(2-Chloroquinolin-3-yl)methyleneamino]-1-
methylquinolin-2(1H)-one (3c). Yield 71%, mp 220°C.
1H NMR spectrum, δ, ppm: 3.775 s (3H), 6.792 d
(1H), 7.445 s (1H), 7.468 s (1H), 7.535 d (1H), 7.606 d
(1H), 7.751 d (1H), 7.820–8.079 m (4H), 9.085 s (1H).
13C NMR spectrum, δ, ppm: 29.7, 115.2, 120.7, 121.3,
122.7, 124.3, 127.2, 127.7, 128.5, 128.9, 132.1, 137.7,
138.7, 145.4, 148.6, 150.2, 155.5, 166.9.
CONCLUSIONS
In this study, some luminescent quinoline based
imines with very high Stokes shift were considered in
presence of electron withdrawing groups containing
aromatic moiety. The presence of aromatic ring
(carbocyclic–heterocyclic–fused ring) at the nitrogen
site of the imine group could be particularly important
for the fluorescence. The emission maxima could be
6-[(2-Chloroquinolin-3-yl)methyleneamino]-1-
ethylquinolin-2(1H)-one (3d). Yield 73%, mp 214°C.
1H NMR spectrum, δ, ppm: 1.405 t (3H), 4.409 d (2H),
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 11 2017

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CHOUDHURY et al.
3. Gao, Z., Hao, Y., Zheng, M., and Chen, Y., RSC Adv.,
tuned by attached electron donating/ withdrawing
groups at the aromatic moiety.
2017, vol. 7, no. 13, p. 7604. doi 10.1039/C6RA27547H
4. Sednev, M.V., Belov, V.N., and Hell, S.W., Methods
Appl. Fluoresc., 2015, vol. 3, no. 4, p. 42004. doi
10.1088/2050-6120/3/4/042004
ACKNOWLEDGMENTS
Authors are thankful to Sophisticated Analytical
Instrument Facility (SAIF), Cochin University, Kerala
for NMR analysis and BRAF, Govt of India for
computational facility. ARC thanks University Grants
Commission (UGC) for a fellowship. Financial support
from NRB, India is acknowledged.
5. Shcherbakova, D.M., Hink, M.A., Joosen, L., Ga-
della, T.W.J., and Verkhusha, V.V., J. Am. Chem. Soc.,
2012, vol. 134, no. 18, p. 7913. doi 10.1021/ja3018972
6. Liu, K., Shang, H., Kong, X., and Lin, W., J. Mater.
Chem. B., 2017, vol. 5, no. 21, p. 3836. doi 10.1039/
C7TB00187H
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