Synthesis and photophysical investigation of AIEgen dyes bearing quinoline and BODIPY scaffolds

Article abstract of DOI:10.1007/s10593-020-02847-6

[Figure not available: see fulltext.] Quinoline-based BODIPY AIEgen dyes were synthesized and the structures were elucidated by ¹H, ¹³C, ¹⁹F NMR, FT-IR spectroscopy and mass spectrometry methods. Their photophysical proper

Full text of DOI:10.1007/s10593-020-02847-6

DOI 10.1007/s10593-020-02847-6
Chemistry of Heterocyclic Compounds 2020, 56(12), 1542–1547
Synthesis and photophysical investigation of AIEgen dyes
bearing quinoline and BODIPY scaffolds
Yavuz Derin1*, Barış Seçkin Arslan1*, Büşra Albayrak Mısır1,
İlkay Şişman1,2, Mehmet Nebioğlu1,2, Ahmet Tutar1
1 Department of Chemistry, Faculty of Arts and Sciences, Sakarya University,
Sakarya 54050, Turkey
2 Biomedical, Magnetic and Semiconductor Materials Research Center (BIMAS-RC),
Sakarya University,
Sakarya 54050, Turkey; e-mail: yavuzderin@sakarya.edu.tr, barisseckin@gmail.com
Submitted March 22, 2020
Accepted after revision September 30, 2020
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2020, 56(12), 1542–1547
1
Quinoline-based BODIPY AIEgen dyes were synthesized and the structures were elucidated by H, 13C, 19F NMR, FT-IR spectroscopy
and mass spectrometry methods. Their photophysical properties were investigated. The dyes showed fluorescence quantum yield in the
range of 0.16–1.29% in MeOH. It was found that the presence of methoxy group and tetrazole moiety led to blue and red spectral shift,
respectively, of the UV absorption maxima of these dyes compared to their chloroquinoline analog. Stokes shifts of the dyes were in the
range of 637–955 cm–1. Aggregation-induced emission behavior of the dyes was investigated in EtOH–H2O mixture so that the dyes
exhibited 1.6- to 2.3-fold fluorescence enhancement.
Keywords: BODIPY, quinoline, aggregation-induced emission, fluorescence quantum yield.
Compounds containing quinoline ring system have been
of a particular interest to chemists due to their diverse
applications in bioorganic, medicinal, and industrial chemistry,
as well as in the field of synthetic organic chemistry.1,2
Quinolines have been mostly investigated for promising
biological activities such as antibacterial, antihypertensive,
antimalarial, antitubercular.3 Due to the extended π-electron
system, quinolines also have interesting optical properties
and potential to be utilized as optical materials and
fluorescent probes.4–6
Dipyrrin or dipyrromethene is a ligand that comprises a
methine carbon bridged between two pyrrole rings and
forms the complexes with various metals (Scheme 1).7,8
BODIPY, first reported by Treibs and Kreuzer, is boron
difluoride complex of dipyrromethene which is a well-
known representative of these complexes.9 BODIPY has
gained the popularity as a functional dye due to high
emission and absorption band at visible region, fluores-
cence quantum yield approaching 100%, being chemically
inert toward moisture, solvent, exposure to light, and, most
importantly, easily functionalizable to adjust these
features.10–12 Thanks to these properties, BODIPY has
many applications such as fluorescence probes,13,14 biolabeling
reagent, laser dyes, photosensitizers for photodynamic
therapy,15 photocatalysts,16,17 and solar cell components.18
In recent years, the dyes that are emissive in the
aggregated state have been extensively explored. This class
of dyes called AIEgen dyes or AIEgens (aggregation-
induced emission luminogens) were discovered in 2001 by
Tang et al. and are characterized by increasing emission
with the increasing polarity of the medium through
aggregate forming as a result of restriction of intra-
molecular motion.19 AIEgens have crucial role in biological
applications because most conventional dyes are hydro-
phobic and nonemissive in hydrophilic conditions due to
aggregation-caused quenching.20
In this study, three quinoline-BODIPY conjugates were
designed starting from aniline and pyrrole for the purpose
0009-3122/20/56(12)-1542©2020 Springer Science+Business Media, LLC
1542

Chemistry of Heterocyclic Compounds 2020, 56(12), 1542–1547
Scheme 1
N
N
N
OMe
O
N
HN
N
v
vi
Dipyrrin
OMe
OMe
68%
41%
3
N
N
N
B
B
NH HN
F
F
F
F
6
9
BODIPY
iii 85%
O
N
N
Cl
NH2
N
HN
Me
i
ii
v
vi
Cl
Cl
94%
84%
25%
77%
O
2
N
N
B
NH HN
1
F
F
5
iv 80%
8
N
N
N
N
N
N
N
N
N
N
N
N
v
vi
59%
28%
O
4
N
N
B
NH HN
F
F
7
10
i: Ac2O, Na2CO3, CH2Cl2, 0°C rt, 3 h. ii: 1. DMF, POCl3, 0°C. 2. , 16 h. iii: KOH, MeOH, , 2.5 h.
iv: NaN3, AcOH, EtOH, , 4 h. v: pyrrole, TFA (cat.), rt, 24 h. vi: 1. DDQ, CH2Cl2, 2 h, rt. 2. Et3N, 0°C, 15 min. 3. BF3·Et2O, rt.
of investigating their AIEgen-like properties, which could
indicate their potential therapeutic effect and ability to
serve as fluorescent bioprobes for diagnostics in medicine.
The synthesis of the dyes is shown in Scheme 1.
Acetanilide (1) formed by acetylation of the aniline was
converted to 2-chloroquinoline-3-carbaldehyde (2) by the
Vilsmeier–Haack reaction.21,22 Compound 2 reacted with
MeOH in strongly basic medium to give 2-methoxy-
quinoline-3-carbaldehyde (3) and in acidic medium with
NaN3 to yield tetrazolo[1,5-a]quinoline-4-carbaldehyde
(4).23,24 Quinoline derivatives 2–4 were condensed with
pyrrole in the presence of TFA to give dipyrromethene
derivatives 5–7, respectively. Quinoline-BODIPY dyes 8–10
were obtained by oxidation of dipyrromethanes with
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) followed
by complexation with BF3·Et2O.25 Dyes 8–10 were
characterized by 1H, 13C, and 19F NMR and FT-IR
spectroscopy and mass spectrometry.
coefficient, emission maxima, fluorescence quantum yield,
and Stokes shift (Δυ) are presented in Table 1.
Dyes 8–10 have a strong absorption peak at the range of
502–515 nm with approximately 50000 (log ε 4.70) molar
absorptivity corresponding to S0–S1 transition which can
also be described as π–π* transition and a shoulder at the
higher energy side attributed to 0–1 vibrational transition.
Besides, a band appears in the UV region (ca. 350 nm)
which corresponds to n–π* transition. As for emission
spectra, the maximum peak that can be ascribed to emission
by the BODIPY unit is located at the range of 522–538 nm
when the dyes were excited at 480 nm. These peaks are
typical for similarly substituted BODIPY dyes.25–27
It was observed that the presence of methoxy group on
quinoline ring causes a blue shift of the absorption
maximum λabs in the spectrum of a quinoline-BODIPY dye.
On the other hand, a red shift was caused by the presence
of tetrazole ring fused quinoline moiety in the dye
molecule. It is well known that BODIPY dyes bearing
The effect of chloro or methoxy substituents at the
quinoline ring or that of fused tetrazole ring on photo-
physical properties of dyes 8–10 was investigated in
different solvents by UV/vis and fluorescence spectro-
scopy. The aggregation-induced luminescence behavior
caused by restriction of intramolecular rotations was spectro-
scopically studied in gradient of EtOH–H2O mixture.
Absorption and emission spectra of dyes 8–10 in MeOH
are presented in Figure 1. The absorption and emission
spectra in various solvents for each of three compounds 8–10
are given in Figure 2 for comparison. Photophysical
parameters such as absorption maxima, molar absorption
Figure 1. Normalized absorption spectra (a) and emission spectra
(b) of compounds 8–10 in MeOH.
1543

Chemistry of Heterocyclic Compounds 2020, 56(12), 1542–1547
Table 1. Photophysical properties of dyes 8–10
Compound Solvent
λabs, nm λem, nm Δυ, cm–1 log ε φ, %
Hexane
THF
508
508
506
505
507
506
506
504
502
503
515
513
510
509
510
525
528
525
523
524
526
526
523
522
522
538
534
536
535
531
637
746
715
682
640
751
751
721
763
724
830
767
951
955
775
4.71
4.71
4.71
4.50
4.69
4.70
4.68
4.73
4.66
4.60
4.60
4.62
4.64
4.63
4.57
6.49
7.93
7.54
9.04
5.31
1.40
1.98
1.45
1.05
1.29
0.08
0.13
0.12
0.11
0.16
EtOAc
MeCN
MeOH
Hexane
THF
EtOAc
MeCN
MeOH
Hexane
THF
8
9
10
EtOAc
MeCN
MeOH
several common organic solvents, it was found that dye 10
displayed higher φ values in polar solvents than in nonpolar
solvents (Table 1).
The absorption and emission spectra were recorded in
EtOH–H2O mixture to study the aggregation-induced
emission phenomena in dyes 8–10 (Fig. 3). For many
BODIPY dyes, aggregation-caused quenching is observed.
However, quinoline-BODIPY dyes are known to be
AIEgens which fluoresce more intensely with increased
Figure 2. Absorption and emission spectra of compounds 8 (a, b),
9 (c, d), and 10 (e, f) in various solvents, respectively.
electron-donating groups have more blue-shifted spectra
than their counterparts with electron-withdrawing groups.28–30
In the case of dye 10, tetrazole ring being of a highly
electron-withdrawing nature and extending the conjugated
π-electron system causes 3 and 7 nm bathochromic shifts of
absorption (λabs) and emission (λem) maxima, respectively,
and the largest Stokes shift (Δυ), whereas the electron-
donating methoxy group in the molecule of dye 9 produces
opposite shifts of 4 and 2 nm, respectively (Table 1). The
reason for which the spectral shift values are quite low is
that these groups are not directly attached to BODIPY core.
The fluorescence quantum yields φ of the dyes 8–10 in
MeOH were found to be 5.31, 1.29, and 0.16%, respectively. It
is clearly evident that excited state of the BODIPY moiety
in dye 10 is quenched by tetrazoloquinoline group which
has strong electron-accepting power.30,31
When the φ values of dyes 8 and 9 were compared, it
was expected that dye 8 would have lower fluorescence
quantum yield than dye 9, as chlorine atom exerts heavy
atom effect which is known to diminish the fluorescence
quantum yield. However, various articles report that the
fluorescence quantum yield of the dyes increases with the
introduction of chlorine atom(s)32,33 and beside that, as an
example, chlorinated BODIPY shows higher fluorescence
quantum yield than the counterpart with a methoxy
group.34 The increase in fluorescence quantum yield of dye
8 in respect to dye 9 is due to the fact that chlorine atom
reduces the nonradiative deactivation process.32,35 When
the photophysical properties of the dyes were evaluated in
Figure 3. Absorption and emission spectra of compounds 8 (a, b),
9 (c, d), and 10 (e, f) in EtOH–H2O mixture, respectively. Inset
graph: Plot of F/F0 versus water fraction.
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Chemistry of Heterocyclic Compounds 2020, 56(12), 1542–1547
concluded that tetrazoloquinoline-BODIPY system is a
versatile AIEgen scaffold. Through synthetic modification
of quinoline or BODIPY moiety, more functional dyes will
be produced and explored in due course.
Experimental
FT-IR (ATR) spectra were recorded on a PerkinElmer
Spectrum Two instrument. The absorption and emission
spectra of the dyes were measured in quartz cuvette with
1 cm optical path using a Shimadzu UV 2600 spectro-
1
photometer and a Hitachi F7000 spectrofluorometer. H,
13C, and 19F NMR spectra were recorded on a Varian
Infinity Plus spectrometer (300, 75, and 282 MHz,
respectively) with TMS as reference. High-resolution ESI
mass spectra were recorded on a Waters SYNAPT MS
series system. The melting points were determined using a
Schorpp MPM-H1 apparatus.
All reagents and solvents used in reactions were of
reagent grade quality and were procured from commercial
suppliers. Solvents used in column chromatography were
obtained as technical grade and purified by standard
methods before use.41 Synthesis of compounds 1–4
according to the literature-described method23,24 is reported
in the Supplementary information file.
Figure 4. 19F NMR spectra recorded for dyes 8–10 in CDCl3
showing the coupling patterns and interaction of fluorine atoms
with groups at meso position.
water quantity causing the molecules to form aggregates.36
AIEgens 8, 9, and 10 show, in turn, 1.6-, 1.9-, and 2.3-fold
fluorescence enhancement F/F0 at H2O fraction
f(H2O) 100, 80, and 90%, respectively. Increment of the
water content in the dye solutions promotes aggregate
formation which results in restriction of intramolecular
rotations along with fluorescence intensity.37
2-Chloro-3-[di(1H-pyrrol-2-yl)methyl]quinoline (5).25
In a flask, pyrrole (10.0 ml, 144.1 mmol) and 3 drops of
TFA were added to 2-chloroquinoline-3-carbaldehyde (2)
(1.42 g, 7.4 mmol). The mixture was stirred at room
temperature for 24 h. After the reaction was complete, the
excess pyrrole was removed under reduced pressure. The
crude product was purified by column chromatography on
silica gel, eluent hexane–EtOAc, 10:1. Yield 1.92 g (84%),
white-off solid. IR spectrum, ν, cm–1: 3379 (N–H), 3104
(C–H Ar), 2985 (C–H aliphatic), 1615 (C=N), 1568 (C=C).
1H NMR spectrum (CDCl3), δ, ppm (J, Hz): 8.13 (2H, br. s,
NH); 8.00 (1H, d, J = 8.8, H Ar); 7.79 (1H, s, H Ar); 7.74–
7.64 (2H, m, H Ar); 7.55–7.47 (1H, m, H Ar); 6.77 (2H, td,
J = 2.7, J = 1.6, H pyrrole); 6.19 (2H, dd, J = 6.0, J = 2.7,
H pyrrole); 5.97 (1H, s, CH); 5.90–5.84 (2H, m,
H pyrrole). 13C NMR spectrum (CDCl3), δ, ppm: 151.0;
146.7; 138.1; 135.0; 130.7; 130.5; 128.0; 127.8; 127.5;
127.3; 118.1; 108.9; 108.1; 41.2.
3-[Di(1H-pyrrol-2-yl)methyl]-2-methoxyquinoline (6)
was synthesized analogously starting from 2-methoxy-
quinoline-3-carbaldehyde (3) (1.39 g, 7.4 mmol). Yield
1.52 g (68%), white-off solid. IR spectrum, ν, cm–1: 3454
(N–H), 3100 (C–H Ar), 2998 (C–H aliphatic), 1618 (C=N),
1571 (C=C), 1096 (C–O).1H NMR spectrum (CDCl3),
δ, ppm (J, Hz): 8.13 (2H, br. s, 2NH); 7.85 (1H, d, J = 8.3,
H Ar); 7.70 (1H, s, H Ar); 7.66–7.53 (2H, m, H Ar); 7.40–
7.28 (1H, m, H Ar); 6.72 (2H, td, J = 2.7, J = 1.6,
H pyrrole); 6.17 (2H, dt, J = 5.5, J = 2.7, H pyrrole); 5.93–
5.89 (2H, m, H pyrrole); 5.82 (1H, s, CH); 4.05 (3H, s,
OCH3). 13C NMR spectrum (CDCl3), δ, ppm: 160.3; 145.8;
137.2; 131.6; 129.4; 127.6; 127.2; 127.0; 125.6; 124.3;
117.5; 108.7; 107.3; 54.1; 37.0.
19F NMR spectra of many BODIPY compounds have
typical quartet signals because of coupling to 11B nucleus
1
(I 3/2, JBF = 32 Hz).38,39 However, dyes 8–10, unlike
common BODIPY dyes, exhibit doublet of quartets. It
indicates that the two fluorine atoms are chemically not
equivalent, likely because of the rotation around the C–C
bond between BODIPY and quinoline units is slow on
NMR time scale. Moreover, in such case, one of the
fluorine atoms would be closely interacting with chlorine
atom, methoxy group, or fused tetrazole ring (Fig. 4). As a
result, the geminal 19F nuclei give rise to a doublet signal.
Similar results were reported for BODIPY bearing quinone
and thiazolyl groups at meso position.39,40 It is noteworthy
that increasing electron-accepting power of the groups on
quinoline bring about an increase in the chemical shift
difference between both fluorine signals in the order of
compounds 9 < 8 < 10.
In this study, BODIPY dyes substituted by quinoline
moiety at meso position were synthesized and characterized
1
by H, 13C, and 19F NMR, FT-IR spectroscopy and mass
spectrometry methods. Doublet of quartet splitting patterns
were observed in 19F NMR spectra of the synthesized
compounds, which is not common for BODIPY dyes.
Photophysical parameters, such as fluorescence quantum
yield, molar absorption coefficient, Stokes shifts, and
absorption/emission peak maxima were evaluated in
several common organic solvents. Dye containing tetrazolo-
[1,5-a]quinoline moiety exhibited very low fluorescence
quantum yield due to highly electron-withdrawing nature
of the tetrazole group. Nevertheless, this dye showed
higher aggregation-induced fluorescence enhancement than
dyes containing methoxy- or chloroquinoline unit. It can be
4-[Di(1H-pyrrol-2-yl)methyl]tetrazolo[1,5-a]quinoline
(7) was synthesized analogously starting from tetrazolo[1,5-a]-
quinoline-4-carbaldehyde (4) (1.47 g, 7.4 mmol). Yield
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Chemistry of Heterocyclic Compounds 2020, 56(12), 1542–1547
1
1.37 g (59%), off-white solid. IR spectrum, ν, cm–1: 3326
1558 (C=C). H NMR spectrum (CDCl3), δ, ppm (J, Hz):
8.82 (1H, d, J = 8.2, H Ar); 8.18 (1H, s, H Ar); 8.12–8.05 (2H,
m, H Ar); 8.02 (2H, s, H pyrrole); 7.84 (1H, t, J = 7.1,
H Ar); 6.88 (2H, d, J = 4.2, H pyrrole); 6.55 (2H, d, J = 3.7,
H pyrrole). 13C NMR spectrum (DMSO-d6), δ, ppm: 148.3;
147.0; 139.1; 136.9; 135.4; 133.3; 133.1; 131.5; 131.1; 129.1;
124.4; 120.1; 118.5; 117.0. 19F NMR spectrum (CDCl3),
δ, ppm (J, Hz): –142.65 (dq, JFF = 88.8, JBF = 28.9);
–148.14 (dq, JFF = 81.0, JBF = 26.7). Found, m/z: 361.1207
[M+H]+. C18H12BF2N6. Calculated, m/z: 361.1179.
Photophysical studies. All absorption and emission
spectra were acquired for 10 µM dye solutions in common
organic solvents. The dyes were excited with 480 nm
wavelength to record emission spectra. Molar absorption
coefficients were calculated from Lambert–Beer equation.42
Relative fluorescence quantum yields were calculated using
a solution of rhodamine B in EtOH (φref 0.49)43 as
reference dye via the following equation.
(N–H), 3091 (C–H Ar), 2988 (C–H aliphatic), 1608 (C=N),
1567 (C=C).1H NMR spectrum (CDCl3), δ, ppm (J, Hz):
9.15 (2H, br. s, 2NH); 8.64 (1H, d, J = 8.2, H Ar); 7.91
(1H, d, J = 8.0, H Ar); 7.87–7.79 (2H, m, H Ar); 7.69 (1H,
t, J = 7.6, H Ar); 6.76 (2H, d, J = 1.5, H pyrrole); 6.13 (2H,
dd, J = 5.7, J = 2.9, H pyrrole); 6.03 (2H, s, H pyrrole);
5.99 (1H, s, CH). 13C NMR spectrum (CDCl3), δ, ppm:
147.9; 131.2; 131.1; 130.2; 129.6; 129.0; 128.4; 127.8;
124.4; 118.4; 116.9; 108.7; 107.7; 42.3.
10-(2-Chloroquinolin-3-yl)-5,5-difluoro-4λ5-5H-di-
pyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ylium-5-uide (8).
A solution of DDQ (0.812 g, 3.58 mmol) in THF (15 ml)
was added dropwise to a solution of 2-chloro-3-[di(1H-
pyrrol-2-yl)methyl]quinoline (5) (1.0 g, 3.25 mmol) in
CH2Cl2 (25 ml). After stirring the reaction mixture for 2 h,
the mixture was cooled in ice bath, and Et3N (0.75 ml) was
added cooling the flask in the ice bath. The reaction
mixture was then stirred for 15 min. Afterward, BF3·Et2O
(3 ml, 24.3 mmol) was added and the reaction mixture was
brought to room temperature and stirred overnight. After
the reaction was complete, the solvent was removed. The
crude product was purified by column chromatography on
silica gel; eluent hexane–CH2Cl2, 3:1. Yield 0.287 g (25%), red-
green solid, mp 185–186°C. IR spectrum, ν, cm–1: 3091
Subscripts smp and ref stand for sample and reference,
respectively. Variables φ, I, A, and µ stand for fluorescence
quantum yield, area under the maximum emission peak,
absorbance at the excitation wavelength (480 nm), and
refractive index of solvent, respectively. Stokes shifts were
estimated from the difference between maximum emission
and absorption wavenumber.44,45 The dye solutions in
10 µM concentration were prepared in EtOH–H2O gradient
(from 0 to 100%) to investigate aggregation-induced
emission behavior of the dyes via fluorescence and UV/vis
spectroscopic methods.
1
(C–H Ar), 1620 (C=N), 1558 (C=C). H NMR spectrum
(CDCl3), δ, ppm (J, Hz): 8.25 (1H, s, H Ar); 8.15 (1H, d,
J = 8.4, H Ar); 7.99 (2H, s, H pyrrole); 7.89 (2H, t, J = 8.1,
H Ar); 7.69 (1H, t, J = 7.6, H Ar); 6.75 (2H, d, J = 4.2,
H pyrrole); 6.54 (2H, d, J = 4.1, H pyrrole). 13C NMR
spectrum (CDCl3), δ, ppm: 148.3; 145.9; 141.0; 140.3;
135.7; 132.3; 131.1 (2 signals); 128.9; 128.4; 128.1; 126.6;
126.0; 119.6. 19F NMR spectrum (CDCl3), δ, ppm (J, Hz):
–144.58 (dq, JFF = 104.2, JBF = 28.8); –146.46 (dq, JFF = 103.8,
JBF = 27.9). Found, m/z: 354.0806 [M+H]+. C18H12BClF2N3.
Calculated, m/z: 354.0775.
Supplementary information file containing 1H, 13C,
19F NMR and mass spectra as well as synthetic methods for
all compounds is available at the journal website at
http://link.springer.com/journal/10593.
5,5-Difluoro-10-(2-methoxyquinolin-3-yl)-4λ5-5H-di-
pyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ylium-5-uide (9)
was synthesized analogously starting from 3-[di(1H-pyrrol-
2-yl)methyl]-2-methoxyquinoline (6) (0.986 g, 3.25 mmol).
Yield 0.465 g (41%), red-green solid, mp 174–175°C.
IR spectrum, ν, cm–1: 3110 (C–H Ar), 2954 (C–H aliphatic),
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1
1624 (C=N), 1549 (C=C), 1108 (C–O). H NMR spectrum
(CDCl3), δ, ppm (J, Hz): 8.02 (1H, s, H Ar); 8.00–7.88
(3H, m, H Ar, H pyrrole); 7.79–7.69 (2H, m, H Ar); 7.53–
7.40 (1H, m, H Ar); 6.80 (2H, d, J = 4.1, H pyrrole); 6.49
(2H, d, J = 4.0, H pyrrole); 4.02 (3H, s, OCH3). 13C NMR
spectrum (CDCl3), δ, ppm: 159.3; 147.4; 144.8; 142.2;
140.8; 135.8; 131.4; 131.0; 128.2; 127.5; 125.3; 124.1;
118.9; 118.5; 54.2. 19F NMR spectrum (CDCl3), δ, ppm
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JFF = 105.4, JBF = 28.3). Found, m/z: 350.1264 [M+H]+.
C19H15BF2N3O. Calculated, m/z: 350.1271.
5,5-Difluoro-10-(tetrazolo[1,5-a]quinolin-4-yl)-4λ5-5H-
dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ylium-5-uide
(10) was synthesized analogously starting from 4-[di(1H-
pyrrol-2-yl)methyl]tetrazolo[1,5-a]quinoline (7) (1.022 g,
3.25 mmol). Yield 0.327 g (28%), red-green solid, mp 247–
248°C. IR spectrum, ν, cm–1: 3116 (C–H Ar), 1605 (C=N),
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