Synthesis of Green/Blue Light Emitting Quinolines by Aza-D-A Reaction Using InCl3 Catalyst

Article abstract of DOI:10.1007/s10895-020-02647-3

An efficient InCl₃-catalyzed sequential reaction of aromatic amines, aromatic aldehydes and functionalized alkynes leading to the formation of new quinoline derivatives exhibiting significant fluorescence activities is described. The photophysi

Full text of DOI:10.1007/s10895-020-02647-3

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02647-3
ORIGINAL ARTICLE
Synthesis of Green/Blue Light Emitting Quinolines by Aza-D-A
Reaction Using InCl3 Catalyst
Rajkumar Romeshkumar Singh1 & Thokchom Prasanta Singh1 & Ningthoujam Premananda Singh1
Shanta Singh Naorem2 & Okram Mukherjee Singh1
&
Received: 8 October 2020 /Accepted: 30 October 2020
#
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
An efficient InCl3-catalyzed sequential reaction of aromatic amines, aromatic aldehydes and functionalized alkynes leading to the
formation of new quinoline derivatives exhibiting significant fluorescence activities is described. The photophysical investiga-
tions of quinolines were carried out by absorption and photoluminescence measurements. One particular compound 4 h having
maximum intensity, emitting green colour (Φ = 0.78) with average life time of 6.20 ns was the best amongst the tested
compounds. The presence of the amino group at the 4-aryl substituent of the quinoline backbone played an important role in
executing the Povarov cyclization successfully and enhancing the flourescence properties of the newly synthesized quinolines.
.
.
.
.
Keywords 2-Ethynylaniline Quinolines Life time Photophysical properties Povarov reaction
Introduction
switches [12–14]. Moreover, quinoline-based dyes such as
ethyl red iodide and pinacyanol (Fig. 1) have been used since
the beginning of the nineteenth century in photographic plates
[15] The diverse applications of quinolines as functional ma-
terials is related to its excellent mechanical properties and high
quantum yields, making ideal materials in the electron trans-
port [16] and presenting essential characteristics for their sub-
sequent use in OLEDs [17]. Hence, a significant advance in
luminescence efficiency and brightness in OLEDs is observed
when conjugated organic compounds contain quinoline
moeties [18] Povarov reaction (aza Diel-Alder reaction) [19]
remains one of the most efficient methods affording highly
substituted and densely functionalized quinoline frameworks.
Povarov reaction involves [4 + 2] cycloaddition reaction of N-
aryl imines with electron-rich dienophiles via activation of a
terminal alkyne C-H bond and complexation of C-C multiple
bonds to facilitate C-N and C-C bond formation, which are
key intermediates for the construction of quinolines [20].
Recently, many metal salts like CuCl/AuCl [21] AuCl3/
CuBr [22], Yb(OTf)3 [23], Fe(OTf)3 [24], Cu(OTf)2 [25],
Zn(OTf)2 [26], AgNTf [27], NbCl5 [28] are explored as effec-
tive Lewis acid catalytic system for quinoline synthesis
though Povarov started with BF3·OEt2 in his original work
[19]. Thus, the development of simple and efficient protocols
for quinolines containing unique substituent from readily
available starting materials is of great interest to organic
chemists.
Quinolines constitute an important class of N-based heterocy-
clic aromatic compounds occurred as natural products and
synthetic complex organic molecules [1–4]. They are well
known for exhibiting broad spectrum of biological activities
like antitumor, antimalarial, antibacterial, antifungal, antipar-
asitic and insecticidal, antiviral, anti-inflammatory, antiplate-
let and other activities [5–8]. The most well-known and sig-
nificant quinoline alkaloids are chloroquinine and
hydroxychloroquine as antimalarial drugs recently associated
with the treatment of the pandemic SARS-CoV-2 [9] and
camptothecin an anticancer drug [10, 11] development respec-
tively, (Fig. 1). In addition to bioactivity, quinolines scaffolds
also show luminescent properties with potential applications
in organic solar cells (OSCs), organic light emitting diodes
(OLEDs), biomolecular markers, molecular probes and
Rajkumar Romeshkumar Singh and Thokchom Prasanta Singh
contributed equally to this work.
* Okram Mukherjee Singh
ok_mukherjee@yahoo.co.in
1
Chemistry Department, Manipur University, Canchipur 795003,
India
2
Chemistry Department, Nagaland University, Lumami 798627, India

J Fluoresc
Fig. 1 Biological and fluorescent
actives Quinolines
Indium trichloride (InCl3) has been widely used in organic
transformations for the construction of complex heterocycles
[29–31]. In continuation of our efforts on exploring the cata-
lytic potential of InCl3 for the synthesis of novel N-heterocy-
cles [32–34], we herein report the InCl3-mediated efficient
synthesis and the optical characterization of new 2,4-disubsti-
tuted quinoline derivatives having potential application as
dyes in organic electronic devices (Scheme 1).
CDCl3 as solvent and are relative to tetramethylsilane (TMS)
as the internal reference. Data are reported as follows: chem-
ical shift, multiplicity(s = singlet, d = doublet, dd = double
doublet, t = triplet, m = multiplet) and coupling constants (J)
in Hertz. The FT-IR spectra were recorded on a FT-IR spec-
trometer (KBr). Gas chromatography-electron impact mass
spectrometry (GC-EIMS) spectra were measured on a
Varian spectrometer using ionization by fast atom bombard-
ment (FAB). Melting points were determined on a “Veego”
capillary melting point apparatus and are uncorrected. All the
luminescence spectra and quantum yield were recorded using
Horiba Fluoromax 4 Spectrophotometer. Samples were dis-
solved in different solvents and 2 mL of each solution was put
in a 3 mL quartz cuvette and it was mounted on the sample
holder. All the measurements were carried out at room tem-
perature. For quantum yield measurement, the above cuvette
containing sample solutions was put inside an integrated
sphere and the measurement of both emission and excitations
were recorded in the form of emission spectra only.
Materials and Methods
Reagents and Instruments
All reagents and solvents were obtained from commercial
suppliers and used without further purification. All reagents
were weighed and handled in air at room temperature. For
1
compounds (4a-i) H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded on Bruker spectrometer
using CDCl3 whereas 1H NMR (400 MHz) spectrum for com-
pound 4b was recorded on FT-NMR spectrometer using
CDCl3. Chemical shifts δ are in parts per million (ppm) with
For the lifetime measurements, time-correlated single pho-
ton count (TCSPC) technique was used with the help of
Horiba DeltaFlex instrument. 2 mL of solution which contain
Scheme 1 Quinoline synthesis
by Povarov reaction using InCl3

J Fluoresc
sample was put in a quartz cuvette and it was mounted on the
sample holder.
CH, J: 9.2, 8.0 Hz), 13C NMR (100 MHz, CDCl3): 21.3 (CH3),
55.5 (OCH3), 103.7 (Ar. CH), 115.7 (Ar. CH), 118.5 (Ar. CH),
120.3 (Ar. CH), 122.1 (Ar. CH), 123.5 (Ar. CH), 126.6 (ar. CH),
127.1 (Ar. CH), 129.6 (Ar. CH), 130.6 (Ar. CH), 131.6 (Ar.
CH), 136.7 (Ar. CH), 139.1 (Ar. CH), 143.8 (Ar. CH), 144.9
(Ar. CH), 145.1 (Ar. CH), 155.0 (Ar. CH), 157.8 (Ar. CH), ESI-
MS: [M + H]+ m/z: 341; Chemical Formula: C23H20N2O.
General Procedure for the Preparation of 4a-f
An equimolar mixture of aromatic amines 1 (1.0 mmol) and
aromatic aldehydes 2 (1.0 mmol) in presence of 10 mol% of
InCl3 were refluxed in 5 mL of toluene. Then after refluxing
for 1 h (as indicated by TLC), 2-ethynylaniline 3 (1.0 mmol)
was added to reaction pot and refluxed the system for another
23 h. Then, the completions of the reactions were monitored
by UV-lamp (giving distinct blue/green colouration). The re-
action mixture was brought to room temperature and extracted
with ethylacetate (2 × 10 mL). The organic layer containing
the quinoline was dried over anhydrous Na2SO4 and then
evaporated. The crude residue was purified by column chro-
matography over silica gel using ethyl acetate/petroleum ether
(1:5) as eluent to afford pure product 4a-i in good yields.
3. 2-(2-(4-Chlorophenyl)-6-Methoxyquinolin-4-Yl)Aniline
(4c): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).
Brown solid substance, yield 0.26 g (74%), m.p. 174-
176 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR Ѵ:
3382 (NH2), 3035, 2809, 2859 (Aromatic = CH), 1900, 1615,
1515 (C=C), 1082, 815,600 (mono-substituted aromatic ring
1
C-Cl), H NMR (400 MHz, CHCl3-d): δ (ppm): 3.80(s, 3H,
CH3), 6.87(d, 1H, Ar. =CH, J: 5.2 Hz), 6.96–6.1(m, 2H, Ar.
=CH), 7.21–7.19 (m, 1H, Ar. =CH), 8.13–8.08 (m, 3H, Ar. =
CH), 7.33–7.30(m, 1H, Ar. =CH), 7.41–7.39(m, 1H, Ar.
=CH), 7.47(d, 2H, Ar. = CH, J: 6 Hz, 2H), 7.80(s, 1H, Ar.
=CH), 13C NMR(400 MHZ,CDCl3): 55.5 (OCH3), 103.7 (Ar.
CH), 113.6 (Ar. CH), 113.7 (Ar. CH), 118.5 (Ar. CH), 120.06
(Ar. CH), 122.46 (Ar. CH), 126.8 (Ar. CH), 128.5 (Ar. CH),
128.9 (Ar. CH), 129.7 (Ar. CH), 130.5 (Ar. CH), 131.6 (Ar.
CH), 133.2 (Ar. CH), 135.2 (Ar. CH), 137.9 (Ar. CH), 143.7
(Ar. CH), 145.4 (Ar. CH), 158.1 (Ar. CH), ESI-MS: [M + H]+
m/z 361; Chemical Formula: C22H17ClN2O.
Characterization Data of the Isolated Compounds 4a-I
1. 2-(2-Phenylquinolin-4-Yl)Aniline (4a): The crude sub-
stance purified by gravity column chromatography(ethyl
acetate/n-hexane 1:2).
White solid, yield 0.21 g (70%), m.p. 173-175 °C; Rf: 0.70
(SiO2, ethyl acetate/n-hexane 1:2), FTIR V: 3389, 3020(N-H);
2929, 2854(aromatic = C-H stretching); 1903, 1614(aromat-
ic = C-H bending), 1519(C=C); 1093, 806 (mono-substituted
1
ring C-H) cm−1, H NMR (400 MHz, CHCl3-d): δ (ppm):
4. 2-(2-(4-Bromophenyl)-6-Methoxyquinolin-4-Yl)Aniline
(4d): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).
3.59 (s, 2H, NH2), 6.93–6.85 (m, 3H. Ar. =CH), 7.04–7.00
(m, 2H, Ar. = CH), 7.21–7.19 (m, 1H, Ar. =CH), 7.0–7.04(d,
2H, Ar. =CH, J: 6.4 Hz), 8.12 (d, 3H, J: 7.2 Hz), 13C NMR
(100 MHz, CHCl3-d) δ (ppm): 103.7 (Ar. CH), 114.2 (Ar,
CH), 115.7 (Ar, CH), 118.5 (Ar. CH), 120.0 (Ar. CH),
122.1(Ar. CH), 123.6 (Ar. CH), 126.4(Ar. CH), 128.6 (Ar.
CH), 129.6 (Ar. CH), 130.6 (Ar. CH), 131.4 (Ar. CH),
132.2 (Ar. CH), 138.8 (Ar. CH), 144.8 (Ar. CH), 145.1 (Ar.
CH), 154.6 (Ar. CH), 157.8 (Ar. CH), ESI-MS: [M + H]+ m/z:
297; Chemical Formula: C21H16N2.
Brown solid substance, yield 0.30 g (75%) m.p. 108-
110 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR Ѵ:
3435 (NH2), 3318, 3205, 3063, 2916 (aromatic = C-H), 2354,
1917, 1626, 1583 (C=C), 1492, 1305, 1091, 1014, 823
(mono-substituted aromatic ring CH) cm−1,1H NMR
(400 MHz, CHCl3-d): δ (ppm): 3.81(s, 1H, OCH3), 6.99–
6.88(m, 3H, Ar. -CH), 7.35–7.19(m, 2H, Ar. =CH), 7.51–
7.41(m, 3H, Ar. =CH), 7.74(s, 1H, Ar. =CH), 8.14(d, 2H,
Ar. =CH, J: 8.4 Hz), 8.37(s,1H, Ar. =CH), 13C NMR
(100 MHz, CHCl3-d) δ (ppm): 55.3 (OCH3), 103.61 (Ar.
CH), 115.7 (Ar. CH), 118.6 (Ar. CH), 120.0 (Ar. CH),
122.5 (Ar. CH), 123.3 (Ar. CH), 123.6 (Ar. CH), 126.9 (Ar.
CH), 128.8 (Ar. CH), 129.7 (Ar. CH), 130.5 (Ar. CH), 131.6
(Ar. CH), 131.9 (Ar. CH), 143.7 (Ar. CH), 145.5 (ar. CH),
144.8 (Ar. CH), ESI-MS: [M + H]+ m/z: 405; Chemical
Formula: C22H17BrN2O.
2. 2-(6-Methoxy-2-(p-Tolyl)Quinolin-4-Yl)Aniline (4b):
The crude substance purified by gravity column
chromatography(ethyl acetate/n-hexane 1:2).
White solid, yield 0.24 g (73%), m.p. 156-158 °C; Rf: 0.70
(SiO2, ethyl acetate/n-hexane 1:2), FTIR V: 3375, 3025(N-H),
2909, 2867 (aromatic = C-H), 1903, 1617 (aromatic = C-H),
1
1511 (C=C), 1097, 810 (mono-substituted ring C-H), H
NMR (400 MHz, CHCl3-d): δ (ppm): 2.42(s, 3H,=CH3),
3.78(s, 3H, =OCH3), 6.95–6.85(m, 3H, Ar. =CH), 7.21(d, 1H,
Ar. =CH, J: 7.6 Hz), 7.30 (t, Ar. =CH, J: 8.0 Hz), 7.40–7.37(m,
2H, Ar. =CH), 7.82(s, 1H, AR. =CH), 8.14–8.04 (dd, 3H, Ar. =
5. 2-(6-Chloro-2-(Thiophen-2-Yl)Quinolin-4-Yl)Aniline
(4e): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).

J Fluoresc
Table 1 Optimization of reaction
conditions for the synthesis of
2,4-disubstituted quinoline
derivative (4a)a
Entry Catalyst
Mol (%) of catalyst Solvent
Reaction condition Time (h) Yieldb (%)
1.
InCl3
InCl3
InCl3
InCl3
InCl3
InCl3
InCl3
InCl3
InCl3
InCl3
InBr3
AlCl3
FeCl3
CuCl2
5
Toluene
Toluene
Toluene
CH3CN
DCM
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
24
24
24
24
24
24
24
48
48
24
24
24
24
24
24
24
24
24
24
24
65
2.
10
15
10
10
10
10
10
10
10
10
10
10
10
70
3.
70
4.
55
5.
50
6.
EtOH
45
7.
THF
40
8.
H2O
NR
NR
50
9.
No solvent 100 °C
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
CH3NO2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
50
42
34
45
Cu(OTf)2 10
Multi-spots
48
CAN
I2
10
10
10
10
10
45
TFA
30
p-TsOH
BF3OEt3
35
38
a All the reactions were performed with aniline 1a (1.0 mmol), benzaldehyde 2a (1.0 mmol) and 2-ethynylaniline
3 (1.0 mmol). b Isolated yields NR- No reaction.
Brown solid substance, yield 0.24 g (72%) m.p. 145-
147 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR Ѵ:
3489 (NH2), 3020, 2939, 2874 (aromatic = C-H), 1963, 1654,
1530 (C=C), 1083, 906,708 (mono-substituted C aromatic
3H, Ar. =CH, J: 4.8 Hz); 13C NMR (100 MHz, CHCl3-H) δ
(ppm): 55.4(OCH3), 114.3 (Ar. CH), 115.8 (Ar. CH), 118.6
(Ar. CH), 120.4 (Ar. CH), 122.6 (Ar. CH), 124.6 (Ar.CH),
126.2 (Ar.CH), 128.9(Ar.CH), 130.0 (Ar.CH), 130.6(Ar.CH),
130.7 (Ar.CH), 132.0 (Ar.CH), 143.7 (Ar.CH), 145.8
(Ar.CH), 147.3 (Ar.CH), 157.1 (Ar.CH), 161.1 (Ar.CH);
ESI-MS: [M + H]+ m/z 361. Chemical Formula:
C22H17ClN2O.
1
ring C-Cl) cm−1; H NMR (400 MHz, CHCl3-d) δ (ppm):
6.51(s, 1H, Ar. =CH); 7.03(t, 1H, J: 4.8 Hz), 7.46–7.19(m,
5H, Ar. =CH), 7.59 (s, 1H, Ar. =CH), 7.62 (d, 1 h, J: 7.2 Hz),
7.78 (d, 1H, J: 8.4 Hz), 7.98 (d, 1H, J: 8.4 Hz), 13C NMR
(100 MHz, CHCl3-H) δ (ppm): 98.9 (Ar. CH), 119.4 (Ar.
CH), 123.7 (Ar. CH), 125.2 (Ar. CH), 127.9 (Ar. CH),
128.2 (Ar. CH), 129.5 (Ar. CH), 129.9 (Ar. CH), 129.9 (Ar.
CH), 130.1 (Ar. CH), 138.6 (Ar. CH), 145.6 (Ar. CH), 147.4
(Ar. CH), 125.5 (Ar. CH), 152.9 (Ar. CH), MS: [M + H]+ m/z
337. Chemical Formula: C19H13ClN2S.
7. 2-(6-Methyl-2-(p-Tolyl)Quinolin-4-Yl)Aniline (4 g): The
crude substance purified by gravity column chromatogra-
phy (ethyl acetate/n-hexane 1:2).
Brown solid substance, yield 0.23 g (72%) m.p. 145-
147 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR
Ѵ: 3474 (NH2), 3013, 2899 (aromatic = C-H), 2854, 1990,
1657, 1526 (C=C), 1078, 879,731 (mono-substituted aromatic
6. 2-(6-Chloro-2-(4-Methoxyphenyl)Quinolin-4-Yl)Aniline
(4f): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).
1
ring C-H) cm−1; H NMR (400 MHz, CHCl3-d): δ (ppm):
2.38(s, 3H, CH3), 2.49(s, 3H, CH3), 3.49(s, 2H, NH2),
6.95(d, 1H, Ar. =CH, J: 8.8 Hz), 6.87–6.63(m, 3H, Ar.
=CH), 7.11(d, 1H, J: 7.6 Hz), 7.26–7.18(m, 1H, Ar. =CH),
7.33(s, 1H, Ar. =CH), 7.47(d, 1H, Ar. =CH, J: 8.4 Hz), 7.71(s,
1H, Ar. =CH), 8.07–8.01(dd, 3H, J: 8.4, 8.8), 13C NMR
(100 MHz, CHCl3-H) δ (ppm): 21.3 (CH3), 21.8 (CH3),
115.6 (Ar. CH), 118.4(Ar. CH), 120.1 (Ar. CH), 123.7 (Ar.
CH), 124.4 (Ar. CH), 125.7 (Ar. CH), 127.3(Ar. CH), 129.5
(Ar. CH), 129.8(Ar. CH), 130.6 (Ar. CH), 132.1 (Ar. CH),
Brown solid substance, yield 0.26 g (74%) m.p. 138-
140 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR Ѵ:
3484 (NH2), 3023, 2932 (aromatic = C-H), 2865, 1960, 1657,
1596 (C=C), 1068, 907,760 (mono-substituted aromatic ring
C-H) cm−1; 1H NMR (400 MHz, CHCl3-d): δ (ppm): 3.55(s,
2H, NH2), 3.86(s, 3H, OCH3), 6.94–6.85(m, 3H, Ar. =CH),
7.15 (d, 1H, Ar. =CH, J: 7.2 Hz), 7.33–7.25(m, 2H, Ar. =CH),
7.63 (d, 2H, Ar. =CH, J: 10 Hz), 7.84 (s, 1H, Ar. =CH), 8.12(t,

J Fluoresc
Table 2 Synthesis of various 2,4-disubstituted quinoline derivatives using Povarov reaction.a
a
Reactions condition: Aromatic amines (1.0 mmol), aromatic aldehydes (1.0 mmol), and 2-ethynylaniline (1.0 mmol) in the presence of 10 mol% of
InCl3 in 5 mL of toluene under reflux conditions.
136.4 (Ar. CH), 143.9 (Ar. CH), 145.8 (Ar. CH), 147.4 (Ar.
CH); ESI-MS: [M + H]+ m/z 325. Chemical Formula:
C23H20N2.
(CH3), 115.7 (Ar. CH), 118.5 (Ar. CH), 119.8 (Ar. CH), 123.4
(Ar. CH), 124.4(Ar. CH), 125.8 (Ar. CH), 128.7 (Ar. CH),
129.0 (Ar. CH), 129.6 (Ar. CH), 129.9 (Ar. CH), 130.6 (Ar.
CH), 132.3 (Ar. CH), 135.4 (Ar. CH), 136.9 (Ar. CH), 137.9
(Ar. CH), 143.8 (Ar. CH), 146.2 (Ar. CH), 147.3 (Ar. CH),
155.1 (Ar. CH); ESI-MS: [M + H]+ m/z 345. Chemical
Formula: C22H17ClN2.
8. 2-(2-(4-Chlorophenyl)-6-Methylquinolin-4-Yl)Aniline
(4 H): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).
Brown solid substance, yield 0.25 g (73%) m.p. 96-98 °C,
Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR Ѵ: 3474
(NH2), 3005, 2945 (aromatic = C-H), 2867, 1970, 1661,
1546 (C=C), 1078, 900, 872(mono-substituted aromatic ring
C-H) cm−1; 1H NMR (400 MHz, CHCl3-d): δ (ppm): 2.45(s,
3H, CH3), 3.52(s, 2H, NH2), 6.94–6.85(m, 2H, Ar. =CH),
7.18–7.16(m, 1H, Ar. =CH), 7.25(s, 1H, Ar. =CH), 7.34–
7.30(m, 2H, Ar. =CH), 7.47(t, 1H, Ar. =CH, J: 8.8 Hz),
7.58–7.55 (m, 1H, Ar. =CH), 7.8 (s, 1H, Ar. =CH), 8.11 (d,
3H, Ar. =CH, J: 8.8 Hz); 13C NMR (100 MHz, CDCl3): 20.8
9. 2-(6-Methyl-2-(4-Nitrophenyl)Quinolin-4-Yl)Aniline
(4i): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).
Brown solid substance, yield 0.26 g (75%) m.p. 167-
169 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2).
FTIR Ѵ: 3494 (NH2), 3028, 2938 (aromatic = C-H),
2877, 1950, 1650, 1536 (C=C), 1068, 877,721 (mono-
1
substituted aromatic ring C-H) cm−1; H NMR (400 MHz,
CHCl3-d): δ (ppm): 2.50(s, 3H, CH3), 3.57(s, 2H, NH2),

J Fluoresc
Scheme 2 Controlled experiment to established the effect of amino group in this Povarov reaction.
6.98–6.90(m, 2H, Ar. =CH), 7.20(d, 2H, J: 7.6), 7.26(s, 1H,
Ar. =CH), 7.37(t, 1H, Ar. =CH, J: 8.0 Hz), 7.45(s, 1H, Ar.
=CH), 7.72–7.62(m, 1H, Ar. =CH), 7.89 (s, 1H, Ar. =CH),
8.16 (d, 1 h, J: 8.4 Hz), 8.55 (d, 1H, J: 7.6 Hz), 9.07(s, 1H, Ar.
=CH); 13C NMR (100 MHz, CDCl3): 21.8 (CH3), 115.7 (Ar.
CH), 118.5 (Ar. CH), 119.8 (Ar. CH), 123.4 (Ar. CH), 124.4
(Ar. CH), 125.8 (Ar. CH), 128.7 (Ar. CH), 129.0 (Ar. CH),
129.6 (Ar. CH), 129.9 (Ar. CH), 130.6 (Ar. CH), 132.3 (Ar.
CH), 135.4 (Ar. CH), 137.0 (Ar. CH), 138.0 (Ar. CH), 143.8
(Ar. CH), 146.2 (Ar. CH), 147.3 (Ar. CH), 155.1 (Ar. CH);
ESI-MS: [M + H]+ m/z 356. Chemical Formula: C22H17N3O2.
10. Ethyl (E)-3-((4-Methoxybenzylidene)Amino)Benzoate
(2j’): The crude substance purified by gravity column
chromatography (ethyl acetate/n-hexane 1:2).
Brownish solid substance, yield 0.26 g (92%) m.p. 58-
62 °C, Rf: 0.70 (SiO2, ethyl acetate/n-hexane 1:2). FTIR Ѵ:
Scheme 3 Proposed mechanism
for the synthesis of Quinoline 4a

J Fluoresc
Table 3 Maximum absorption wavelength (λmax) for quinoline derivatives
Sl. No.
Compounds
Chloroform
Peaks (nm)
Acetonitrile
Peaks (nm)
Methanol
λmax (nm)
λmax(nm)
Peaks (nm)
λmax(nm)
1.
2.
3.
4.
5.
6.
7.
8.
9.
4a
4b
4c
4d
4e
4f
249, 319
250, 387
264, 333
262, 372
267, 337
265, 348
262, 347
260, 380
260, 343
249
250
333
262
267
265
262
260
260
250, 318
264, 383
260, 319
268, 351
268, 332
271, 342
279, 342
260, 384
261, 326
250
264
318
268
268
271
279
260
261
281, 319
263, 385
268, 322
264, 348
267, 332
270, 344
280,345
260, 379
260, 324
281
263
322
264
267
270
280
260
260
4 g
4 h
4i
3067 (aromatic = C-H), 2987, 1705, 1612(C=N), 1413, 1365,
stepwise controlled method so that aniline 1a (1.0 mmol)
and benzaldehyde 2a (1.0 mmol) were allowed to react in
refluxing toluene in presence of 5 mol% of InCl3 for 1 h
showing the characteristic formation of imine (as indicated
by TLC) and 2-ethynylaniline 3 (1.0 mmol) was added to
reaction pot and refluxed the system for another 23 h. The
product 2-(2-phenylquinolin-4-yl)aniline 4a was obtained in
65% yield (Table 1, entry 1), the structure of 4a was deduced
from its elemental analysis and spectral data (1H and 13C
NMR, IR). To our delight, the desired product 4a was obtain-
ed in 70% yield when we increased the catalytic loading to
10 mol% (entry 2). However, no increase in yield was ob-
served on further increasing the catalyst beyond 10 mol% (en-
try 3). Inspired by this result, we next screened the effect of
solvent on the reaction by using 10 mol% of InCl3 as standard
catalyst loading. For scrutinizing the suitable solvent system,
similar reactions (entries 4–8) were conducted in various sol-
vent systems such as CH3CN, DCM, EtOH, THF and
CH3NO2 under reflux conditions. It was noted that the
shortest reaction time and the best yield were obtained in
toluene (entry 2) under reflux condition. Interestingly, the
same reaction did not proceed and provide low yield when it
was carried out in water and solvent free condition even after
prolonging the reaction duration (Table 1, entries 9–10).
To examine the efficacy of InCl3 extensive comparative
studies with several catalytic systems have been investigated.
Thus, several reactions were scrutinized in the presence of
catalysts like InBr3, AlCl3, FeCl3, Cu(OTf)2, CuCl2, CAN,
I2, TFA, p-TsOH and BF3OEt3, respectively (Table 1, entries
11–20). All the tested acids gaive lower yields than that of
InCl3, however, in the case of Cu(OTf)2, a mixture of com-
pounds are observed. Lewis acids showed better activity as
compared to those of Bronsted acids and amongst the tested
Lewis acids InCl3 was the best catalyst in this Povarov reac-
tion (Table 1). Hence, the optimized reaction condition for the
formation of 4a was established with 10 mol% of InCl3 in
toluene under refluxing for 24 h using aniline 1a (1.0 mmol),
1
870 (monosubstituted aromatic ring C-H) cm−1; H NMR
(400 MHz, CHCl3-d) δ (ppm): 1.37 (t, 3H, -CH3, J: 6.8 Hz),
3.82(s, 3H, OCH3), 4.37–4.31(dd, 2H, J: 7.2, 7.2 Hz), 6.79–
6.77 (m, 2H, Ar. = CH), 6.88 (d, 1H, J: 8.8 Hz), 7.19 (s, 1H,
Ar. =CH), 7.22 (d, 1H, J: 4.0 Hz), 7.26 (t, 1H, J: 6.4 Hz), 7.33
(d, 1H, J: 5.6 Hz), 7.38 (d, 1H, J: 7.6 Hz), 8.52 (s, 1H, Ar. =
CH), 13C NMR-APT (100 MHz, CHCl3-d) δ (ppm): 14.3
(CH3), 55.3 (OCH3), 60.9 (-CH2-), 117.1 (Ar. CH), 114.1
(Ar. CH), 113.5 (Ar. CH), 114.3 (Ar. CH), 118.6 (Ar. CH),
128.0 (Ar. CH), 129.2 (Ar. CH), 130.9 (Ar. CH), 131.3 (Ar.
CH), 148.1 (Ar. CH), 158.9 (Ar. CH), 167.0 (C=N); ESI-MS:
[M + H]+ m/z 284. Chemical Formula: C17H17NO3.
Results and Discussion
Initially, a mixture of aniline 1a (1.0 mmol), benzaldehyde 2a
(1.0 mmol) and 2-ethynylaniline 3 (1.0 mmol) with InCl3
(5 mol%) were refluxed for 24 h in 5 mL of toluene, (E)-
N-(2-ethynylphenyl)-1-phenylmethanimine and traces of
unreacted aniline were isolated instead of our target com-
pound quinolines. Then, we modified the reaction in a
Fig. 2 UV-Vis absorption of quinoline derivatives (4a-i) in CH3Cl

J Fluoresc
benzaldehyde 2a (1.0 mmol) and 2-ethynylaniline 3
(1.0 mmol) (entry 2).
electron density on ortho to the ester carbon as well as steric
hindrance between the amino group and bulky acetate. Thus,
less chances for the formation of quinoline.
With this optimized reaction condition in our hand, its sub-
strate scope and generality was examined (Table 2). Amines
with electron-donating substituents such as methoxy and meth-
yl react with aldehydes having different substituents such as
methoxy, methyl, chloro, bromo and nitro in presence of 2-
ethynylaniline to give moderate yields (72–75%). Again, p-
chloroaniline react with p-methoxybenzaldehyde or thienyl-2-
carbaldehyde along with 2-ethynylaniline to give the corre-
sponding quinolines in comparable yields with other products.
However, the best yields were obtained when amines having
the electron-donating groups coupled with aldehydes having
electron-withdrawing substituents such as in 4d and 4i (75%
yield). Thus, a small library of highly functionalized
quinolines with potentials of exhibiting strong lumines-
cence properties under UV lamp was established. We
were curious to find out the role of amino group which
is ortho to enyne moiety.
In another experiment, the –NH2 of ethynylaniline was
acylated with acetic anhydride and allowed to react with iso-
lated Schiff base 2c’ using the established protocol. Again, the
reaction mixture gives multi-spot as observed by TLC, indi-
cating that the amino group must be involved in the cycload-
dition reaction between the Schiff base and 2-ethynylaniline
as shown in Scheme 2. Taking into consideration of the above
two observations, we propose the plausible reaction mecha-
nism of this Povarov reaction to get quinoline via oxidation.
Here, the Schiff base 2a’ undergo [4 + 2] Povarov cycloaddi-
tion reaction with 2-ethynylaniline initiated from amino
group. The cyclized dihydroquinoline A is obtained with the
departure of the InCl3 catalyst and restoration of the catalytic
cycle. Then dihydroquinoline A undergoes a spontaneous ox-
idation to give the final quinoline 4a (Scheme 3).
To understand the role of amino group, an experiment for
ethyl 3-aminobenzoate 1e (1.0 mmol), p-methylbenzaldehyde
2f (1.0 mmol) and 2-ethynylaniline 3 (1.0 mmol) were sub-
jected to establish optimized reaction condition (Table 1, entry
2). The intermediate Schiff base 2j’ was obtained instead of
expected quinoline, probably due to the reduction in the
Photophysical Properties
In this study, we examine the absorption, emission, life time
and fluorescence quantum yields of the quinolines. The
photophysical characteristics were investigated in CH3CN,
CHCl3 and CH3OH solutions. The data of UV-Vis absorption
Table 4 Photophysical data obtained from fluorescence emission of quinoline derivatives
Sl. No.
Compounds
λem
454
438
490
491
492
488
480
496
472
Δλst
Parameters
Average
Life-Time (τav)
Quantum
Yield Φfx
CIE
Coordinate
1.
4a
4b
4c
4d
4e
4f
90
(a) τ1 = 1.366
(b) τ2 = 5.494
(c) χ2 = 0.999
(a) τ1 = 1.681
(b) τ2 = 4.733
(c) χ2 = 0.999
(a) τ1 = 2.704
(b) τ2 = 10.742
(c) χ2 = 0.999
(a) τ1 = 3.700
(b) τ2 = 7.510
(c) χ2 = 0.999
(a) τ1 = 4.307
(b) τ2 = 7.409
(c) χ2 = 0.999
(a) τ1 = 2.304
(b) τ2 = 12.175
(c) χ2 = 0.999
(a) τ1 = 1.861
(b) τ2 = 8.341
(c) χ2 = 0.999
(a) τ1 = 2.488
(b) τ2 = 8.322
(c) χ2 = 0.999
(a) τ1 = 2.314
(b) τ2 = 10.852
(c) χ2 = 0.999
2.745 ns
1.805 ns
3.313 ns
4.177 ns
7.266 ns
3.435 ns
5.598 ns
6.204 ns
3.210 ns
0.01
0.01
0.09
0.20
0.09
0.04
0.02
0.78
0.16
X = 0.214
Y = 0.200
2.
100
94
X = 0.160
Y = 0.095
3.
X = 0.259
Y = 0.510
4.
95
X = 0.266
Y = 0.550
5.
95
X = 0.323
Y = 0.042
6.
108
99
X = 0.223
Y = 0.604
7.
4 g
4 h
4i
X = 0.125
Y = 0.112
8.
77
X = 0.245
Y = 0.525
9.
92
X = 0.165
Y = 0.181

J Fluoresc
Fig. 3 a Fluorescence emission of quinoline derivatives (4a–i) in CHCl3
(b), Emission (solid line) and absorption (dotted line) spectra of
compound 4 h in each solvent (CHCl3, CH3CN, CH3OH) (c),
Photograph taken under UV light (365 nm) in CHCl3 solution. d
Chromaticity diagram showing the CIE coordinates of the compounds
4a-i
are summarized in Table 3 in 10−3 mol. L−1 in CH3CN, CHCl3
and CH3OH solutions.
transitions. Bands located at 319–387 nm range can be
assigned to the possible intermolecular charge transfer transi-
tion (ICT) [35]. On changing the polarity to polar solvents like
CH3CN and CH3OH, most of the compounds experience red
shift and blue shift respectively for π-π* and n-π* peaks. The
The nature of the substituents were examined taking 4a as
reference compound, it was found that all the compounds
exhibit red shifts. The highest shift occurred in 2-position
thienyl substituted 4e, as expected (18 nm). The absorption
spectra of the 2,4-disubstituted quinoline derivatives in CHCl3
are characterized by strong absorption peaks centred at 249–
267 nm and 319–387 nm probably due to π-π* and n-π*
λmax changes by 31 nm from acetonitrile to methanol for 4a
(Table 3). It can be attributed to possible protonation of quin-
olines in methanol. However, the compound 4c showed blue
shift in both the transitions in CH3CN from CHCl3. In all the
Fig. 4 Fluorescence lifetime
decays of quinolines (4a–i)

J Fluoresc
compounds the change in λmax due to substituent’s nature
seems to be less pronounced. This can be attributed due to
the strong influence of amino group at o-position of the aryl
substituent at position 4, no significant alteration was ob-
served by changing the substituents on the phenyl ring in
position 2 as well as at position 6 of quinolines (Fig. 2). All
the compounds showed λmax in π-π* transition however,
compound 4c is the exception with λmax 323 nm (n-π* tran-
sition) having methoxy and chloro as substituents in all the
solvents (absorption spectra in CH3CN and CH3OH are
enclosed in SI). The absorption spectra of chloroform solu-
tions (10−3 mol.L−1) of 2,4-disubstituted quinoline derivatives
are depicted in Fig. 2.
The fluorescence and excitation spectra were measured
with a Horiba Fluoromax 4 Spectrophotometer. All measure-
ments were done repeatedly and reproducible results were
obtained. All the solutions were excited at around 338–
419 nm with an excitation and emission slit width of 2 nm.
As in absorption, the solvent polarity affects the fluorescence
properties of quinolines. The Stokes shift data, given by the
difference between the maximum peak of absorption and
emission spectra which is estimated from the intersection of
the absorption and emission spectra, is observed in Table 4 for
CHCl3 solution. Most of the compounds showed a similar
pattern in the shift around 472–496 nm whereas 4a
(454 nm) and 4b (438 nm) as expected show the shift at lower
wavelength. The larger Stoke’s shift values of 77–108 can be
attributed to the ICT transitions and electron-substituent prop-
erties that exist in these compounds. They showed green to
blue emission under UV-lamp (365 nm) except 4f with yel-
lowish colour (Fig. 3c) and Commission Internationale de
L’Eclairage (CIE) colour coordinates of the compounds are
summarized in Table 4 (also in Fig. 3d).
polarity (0.75, 0.18, and 0.15 for 4 h, 4d, and 4i). The lower
φ values (0.54, 0.05, and 0.01 for 4 h, 4d, and 4i) in CH3OH
solution is due to its higher polarity.
Average Lifetime
Average lifetime (τav) measurements for all the newly syn-
thesized compounds have been carried out. It was found that
4e (7.266 ns) has highest lifetime followed by 4 h (6.20 ns),
4 g (5.60 ns), 4d (4.18 ns), 4f (3.44 ns), 4c (3.313 ns), 4i
(3.21 ns), 4a (2.75 ns) and 4b (1.81 ns) (Fig. 4). The average
lifetime signifies that species with higher τav is likely to per-
sist for longer period in the excited state. Thus, compounds 4e,
4 h, 4 g and 4d can be explored for potential candidates in
fluorescence imaging.
Conclusion
In summary, InCl3-catalyzed [4 + 2] cycloaddition reaction
giving novel quinoline derivatives having an amino group at
the 4-aryl substituent is presented. To the best of our knowl-
edge, the use of 2-ethynylaniline in Povarov reaction is not
reported till now except our work. All the newly syn-
thesized quinolines gave intense green/blue fluorescence
with large Stokes shifts. Some of the compounds also
have good quantum yield as well as reasonable life
time. Studies expanding this novel modular approach
to enhance molecular diversity with better yields and
more detailed photophysical investigations are currently
underway for their photonic applications.
Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s10895-020-02647-3.
When the difference in fluorescence intensity was analyzed
in CHCl3, compounds 4d, 4 h and 4i presented greater inten-
sity, with 4 h being the highest one (Fig. 3a). On changing the
polarity of the solvents, the shifts pattern are drastically affect-
ed with low intensity observing in polar-protic solvent
CH3OH (S3) than polar aprotic solvent CH3CN (S4).
Whereas, compounds 4d, 4 h and 4i remain with greater in-
tensity than other in both the polar solvents like in
CHCl3. The intensity of peaks decreased significantly
owing to the hydrogen-bonding interaction between –
NH2 group of 4a-i and protic-solvent in CH3OH along
with polarity effect, but no such hydrogen-bonding with
CH3CN having only polar effect where CHCl3 has none
of the effect (Fig. 3d).
The fluorescence quantum yields (φ) of all the compounds
in CHCl3 solution are also summarized in Table 4. The higher
φ values were obtained in three compounds i.e., 0.78, 0.20,
and 0.16 for 4 h, 4d, and 4i respectively. And, for other com-
pounds φ values ranges between 0.01–0.09 in CHCl3 solu-
tion. The CH3CN solution exhibits lower values due to its
Acknowledgments Financial support from the DST, CSIR and DBT
(BT/PR25414/NER/95/ 1183/2017), Government of India, are gratefully
appreciated.
Compliance with Ethical Standards
Conflict of Interest The authors declare that they have no conflict of
interest.Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s10895-020-
02647-3.
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