Synthesis and fluorescent properties of coumarin-benzimidazole and coumarin–phenanthroimidazole hybrids

Article abstract of DOI:10.1177/1747519819882229

Two coumarin-benzimidazole hybrids and two coumarin-phenanthroimidazole conjugates were conveniently synthesized and characterized. The UV–vis absorption, emission spectra, absolute quantum yields, and fluorescence lifetimes of these compounds in diluted dichloromethane solutions were measured.

Full text of DOI:10.1177/1747519819882229

882229CHL0010.1177/1747519819882229Journal of Chemical ResearchFan et al.
research-article2019
Research Paper
Journal of Chemical Research
1–5
© The Author(s) 2019
Article reuse guidelines:
sagepub.com/journals-permissions
Synthesis and fluorescent properties of
coumarin-benzimidazole and coumarin–
phenanthroimidazole hybrids
https://doi.org/10.1177/1747519819882229
DOI: 10.1177/1747519819882229
journals.sagepub.com/home/chl
Hongli Fan, Lingqiang Wang and Xiaolong Wang
Abstract
Two coumarin-benzimidazole hybrids and two coumarin-phenanthroimidazole conjugates were conveniently synthesized
and characterized. The UV–vis absorption, emission spectra, absolute quantum yields, and fluorescence lifetimes of these
compounds in diluted dichloromethane solutions were measured.
Keywords
absorption, benzimidazole, coumarin, fluorescence, phenanthroimidazole
Date received: 25 June 2019; accepted: 17 September 2019
Coumarins have been widely studied not only for their vari- have been investigated in many optical studies.7 However,
ous biological activities such as antibacterial, anti-HIV, and studies on the optical properties and applications of cou-
anticancer activities but also for their superior thermal sta- marin-phenanthroimidazole conjugates in the literature are
bility and outstanding optical properties including high limited.8 Recently, we have synthesized coumarin-7, cou-
quantum yields, large Stokes shifts, and superior photosta- marin-30, and their 4-substituted regioisomers and found the
bility. Thus, coumarin derivatives are extensively used as UV–vis absorption and fluorescence emission properties of
fluorescent whiteners, nonlinear optical chromophores, the isomers are different.9,10 As a part of the foregoing
fluorescent probes, laser dyes, and solar energy collectors. research in our group, we present herein the concise synthe-
Since their spectroscopic and photophysical properties can sis and fluorescent properties of regioisomeric coumarin-
be easily modified by introducing substituents on the cou- benzimidazole and coumarin-phenanthroimidazole hybrids.
marin ring, the synthesis and application of coumarin As for the reported analogs,7 these synthesized compounds
derivatives have been attracting great interest.1–3
Benzimidazole and phenanthroimidazole derivatives
have also been extensively studied because of their synthetic
utility and their variety of applications.4,5 To our knowledge,
coumarin-benzimidazole hybrids have been well studied due
to their diversified properties. They are reported to exhibit
various biological activities such as antitumor, antioxidant,
and antihepatitis.6 They are also well-known laser dyes and
The School of Chemical and Biological Engineering, Lanzhou Jiaotong
University, Lanzhou, China
Corresponding author:
Xiaolong Wang, The School of Chemical and Biological Engineering,
Lanzhou Jiaotong University, Lanzhou 730070, China.
Email: wangxl@mail.lzjtu.cn

2
Journal of Chemical Research 00(0)
Scheme 1. Synthesis of coumarin-benzimidazole and coumarin-phenanthroimidazole hybrids. Regents and conditions: (a)
Ph3P=CHCO2Et, ethanol, reflux, 5h, 72%; (b) DMF, POCl3, 60°C, 7 h, 81%; (c) ethyl acetoacetate, ZnCl2, ethanol, reflux, 5h,
82%; (d) SeO2, xylene, reflux, 10h, 58%; (e) 2-aminodiphenylamine, ammonium acetate, ethanol, reflux, 7: 54%, 8: 49%; (f)
9,10-phenanthrenequinone, aniline, ammonium acetate, acetic acid, reflux, 9: 58%, 10: 43%.
could be potentially used as laser dyes, environment-sensi- noted that compound 7 has been synthesized from 7-(diethyl-
tive fluorescent probes, or dopants for organic light-emitting amino)coumarin-3-carboxylic acid and 2-aminodiphe-
nylamine in two steps for evaluation in vitro as potent
anticancer agent.11 Moreover, intermediates 3 and 6 reacted
respectively with 9,10-phenanthrenequinone, aniline, and
ammonium acetate in acetic acid under reflux condition to
give the coumarin-phenanthroimidazole hybrids 9 and 10. To
the best of our knowledge, the synthesis of compounds 8, 9,
and 10 have not been reported in the literature. These prod-
ucts were easily purified by column chromatography on silica
gel and were characterized on the basis of spectroscopic data.
Fortunately, a single crystal of compound 7 was obtained
from petroleum ether/ethyl acetate solution for X-ray analy-
sis, whereby the molecular structure of this coumarin-benzi-
midazole hybrid was further confirmed. The compound 7
crystallized in the P212121 space group pertaining to
orthorhombic crystal system. The crystal structure of com-
pound 7 is shown in Figure 1. The crystallographic data and
experimental details are shown in Table 1. In this molecule,
the benzimidazole skeleton is not coplanar with the coumarin
ring, and the dihedral angle is 57.89°. The phenyl ring
attached to the N atom is also not coplanar with the benzimi-
dazole skeleton and the coumarin ring, and the dihedral angles
are 71.42° and 60.10°, respectively. Crystallographic data for
diodes.
Results and discussion
The synthesis of the target molecules was carried out as out-
lined in Scheme 1. As we have demonstrated previously in
the synthesis of coumarin-7 and coumarin-30,9,10 4-(diethyl-
amino)salicylaldehyde (1) underwent the Wittig reaction with
(ethoxycarbonylmethylene)triphenylphosphorane in ethanol
under reflux condition to afford 7-(diethylamino)coumarin
(2) in good yield. Subsequently, Vilsmeier–Haack reaction of
coumarin 2 with N,N-dimethylformamide (DMF) and POCl3
give rise to the intermediate 3-formylcoumarin 3 in high
yield. On the other hand, m-diethylaminophenol (4) was sub-
jected to the Pechmann reaction with ethyl acetoacetate in the
presence of ZnCl2 in ethanol under reflux condition to furnish
the 4-methylcoumarin 5 in 82% yield. Compound 5 was oxi-
dized to the 4-formylcoumarin 6 by treatment with SeO2 in
refluxing xylene in 58% yield. The intermediates 3-formyl-
coumarin 3 and 4-formylcoumarin 6 were then condensed
with 2-aminodiphenylamine in the presence of ammonium
acetate in ethanol at reflux to afford the desired coumarin-
benzimidazole hybrids 7 and 8, respectively. It should be 7 has been deposited at the Cambridge Crystallo-graphic

Fan et al.
3
Table 1 Crystallographic data of compound 7.
Parameters
7
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C26H23N3O2
409.47
183(8)
0.71073
Orthorhombic
P212121
8.1750(6)
12.8801(9)
20.3443(17)
90
β (°)
90
γ (°)
90
2142.2(3)
4
1.270
0.082
Figure 1. The molecular structure of compound 7.
Volume (Å3)
Z
ρ
calc (gcm−3)
Data Centre with the deposition number CCDC 1911906.
These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/data_request/cif.
Absorption coefficient (mm−1)
F(000)
864.0
Crystal size (mm3)
2Θ range for data collection (°)
Index ranges
0.21×0.17×0.14
6.64 to 52.04
–5⩽h⩽10
–11⩽k⩽15
–25⩽l⩽11
5223
3879 (Rint =0.0408)
3879/0/282
1.072
The UV–vis absorption and fluorescence spectra of the
synthesized compounds 7, 8, 9, and 10 in diluted dichlo-
romethane solutions are presented in Figure 2. It is shown
that the absorption maxima of these four compounds are
located at 413, 409, 409, and 402nm, respectively. The emis-
sion spectra of compound 7 exhibits blue-green emission
with a maximum peak at 473nm, and the compounds 8, 9,
and 10 show green emissions with a maximum peak at 516,
490, and 512nm, respectively. The Stoke shift values
obtained for compounds 8 and 10 are larger than those for 7
and 9. It is probable that the changes in geometry between
the ground state and the first excited singlet state of the
4-substituted hybrids are larger than those of the correspond-
ing 3-substituted analogs.12 In order to gather more informa-
tiononfluorescencepropertiesofthesynthesizedcompounds,
the absolute quantum yields and fluorescence lifetimes in
dichloromethane solutions were also determined. The quan-
tum yields determined were absolute values using an
Integrating Sphere without reference standard. The photolu-
minescence decay lifetimes of the compounds 7, 8, 9, and 10
were measured to be 2.40, 7.76, 2.65, and 5.69ns, respec-
tively, and the decay profiles are given in Figure 3. Table 2
summarizes the photophysical data of the four compounds.
In conclusion, the facile synthesis of two coumarin-ben-
zimidazole regioisomeric hybrids and two coumarin-phen-
anthroimidazole regioisomeric hybrids has been described.
The UV–vis absorption, emission spectra, absolute quan-
tum yields, and fluorescence lifetimes in diluted dichlo-
romethane solutions of these compounds were determined.
The results revealed that the photophysical properties of the
3-substituted hybrids are obviously different from those of
the corresponding 4-substituted analogs. Applicability of
these coumarin-benzimidazole and coumarin-phenanthro-
imidazole conjugates is yet to be established.
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ (I)]
R indices (all data)
R1 =0.0606, wR2 =0.1297
R1 =0.0898, wR2 =0.1591
0.18/–0.24
Largest diff. peak/hole/e (Å−3)
and 13C NMR spectra were recorded on either an Agilent
Technologies DDZ 600 MHz or a Bruker Avance III 500
MHz spectrometer. Melting points were determined using a
microscope apparatus and are uncorrected. High-resolution
mass spectra (HRMS) were determined on a Bruker Daltonics
APEX II 47e spectrometer. Single crystal X-ray diffraction
measurements were made on a Bruker X8 APEX diffractom-
eter working with graphite monochromated Mo Kα radiation.
UV–vis absorption and fluorescence spectra were recorded
on a Hitachi U-3900H spectrometer and on a Hitachi F7000
FL spectrophotometer, respectively. Fluorescent lifetimes and
absolute quantum yields were recorded on a FLS920 type
steady-state/transient fluorescence spectrometer (Edinburgh
Instruments Ltd).
7-(Diethylamino)coumarin (2), 7-(diethylamino)-3-for-
mylcoumarin (3), 7-(diethylamino)-4-methylcoumarin (5),
and 7-(diethylamino)-4-formylcoumarin (6) were synthe-
sized according to the procedures described previously.10
Synthesis of 7-(Diethylamino)-3-(1-phenyl-
1H-benzo[d]imidazol-2-yl)coumarin (7)
7-(Diethylamino)-3-formylcoumarin (0.23g, 0.94mmol),
ammonium acetate (0.45g, 5.84mmol), and N-phenyl-o-
Experimental
Reagents and solvents were all from commercial sources and phenylenediamine (0.19g, 1.03mmol) were dissolved in
used without further purification. IR spectra were performed ethanol (20mL). The reaction mixture was stirred under
1
on a Digilab FTS-3000 FTIR spectrophotometer. H NMR reflux conditions for 5h. After the completion of reaction,

4
Journal of Chemical Research 00(0)
Synthesis of 7-(Diethylamino)-4-(1-phenyl-
1H-benzo[d]imidazol-2-yl)coumarin (8)
Compound 8 was prepared from 7-(diethylamino)-4-for-
mylcoumarin (0.15g, 0.61mmol) and N-phenyl-o-
phenylenediamine (0.13g, 0.71mmol) by a method similar
to that described for 7 as yellow solid. Yield 0.12g (49%),
m.p. 218°C–220°C. IR (KBr) cm−1: 3049, 2943, 1724,
1600, 1494, 1357, 754. 1H NMR (600MHz, CDCl3): δ/ppm
7.95 (d, J=7.8Hz, 1H), 7.76 (d, J=9.0Hz, 1H), 7.49–7.30.
(m, 8H), 6.58 (dd, J=9.0 and 2.4Hz, 1H), 6.51 (d, J=2.4Hz,
1H), 5.81 (s, 1H), 3.42 (q, J=7.2Hz, 4H), 1.21 (t, J=7.2Hz,
6H). 13C NMR (150MHz, CDCl3): δ/ppm 161.0, 156.8,
150.9, 147.0, 143.5, 142.9, 136.2, 135.6, 130.0, 129.0,
128.4, 126.4, 124.5, 123.5, 120.6, 111.1, 110.9, 109.1,
107.0, 97.6, 44.8, 12.4. HRMS Calculated for C26H24N3O2
[M+H]+: 410.1863, Found: 410.1874.
Figure 2. Normalized absorption and emission spectra of
compounds 7, 8, 9, and 10 in dichloromethane (1×10–5
molL−1).
Synthesis of 7-(Diethylamino)-3-(1-phenyl-
1H-phenanthro[9,10-d]imidazol-2-yl)
coumarin (9)
7-(Diethylamino)-3-formylcoumarin (0.21g, 0.86mmol),
9, 10-phenanthrenequinone (0.22g, 1.05mmol), aniline
(0.12g, 1.30mmol), and ammonium acetate (0.20g,
2.60mmol) were dissolved in acetic acid (20mL). The
reaction mixture was refluxed under nitrogen for 12h.
After completion, the reaction mixture was cooled to room
temperature and was poured into water (100mL) and
extracted with dichloromethane (3×100 mL). The com-
bined organic phase was dried over Na2SO4. After filter-
ing, the filtrate was evaporated under reduced pressure.
The resulting residue was purified by chromatography on
silica gel using petroleum ether/ethyl acetate/dichlo-
romethane (v/v/v=3:1:1) as the eluent to afford yellow
solid product. Yield 0.25g (58%), m.p. 261°C–263°C. IR
(KBr) cm−1: 2987, 1718, 1608, 1519, 1355, 1136, 754. 1H
NMR (500MHz, DMSO-d6): δ 8.94 (d, J=8.5Hz, 1H),
8.89 (d, J=8.0Hz, 1H), 8.64 (dd, J=8.0 and 1.5Hz, 1H),
8.24 (s, 1H), 7.79–7.34. (m, 10H), 7.16 (dd, J=8.0 and
1.5Hz, 1H), 6.73 (dd, J=8.5 and 2.5Hz, 1H), 6.51 (d,
J=2.5Hz, 1H), 3.43 (q, J=7.0Hz, 4H), 1.12 (t, J=7.0Hz,
6H). 13C NMR (125MHz, DMSO-d6): δ 159.3, 156.7,
151.4, 147.6, 147.2, 137.3, 136.4, 130.2, 129.8, 129.7,
128.7, 128.5, 127.6, 127.5, 127.2, 126.7, 125.7, 125.4,
Figure 3. Fluorescence life time measurements of compounds
7, 8, 9, and 10 in dichloromethane.
Table 2. Photophysical data of the synthesized compounds.
Compound
λabs (nm)
λ
em (nm) Stokes
shift (nm)
Φf (%) τ (ns)
7
8
9
10
413
409
409
402
473
516
490
512
60
107
81
85.4
82.9
88.0
57.2
2.40
7.76
2.65
5.69
110
the solvent was evaporated under reduced pressure and the 124.5, 123.7, 122.3, 121.9, 120.2, 109.9, 109.4, 107.0,
resulting mixture was dissolved in ethyl acetate (50mL). 96.3, 44.2, 12.3. HRMS Calculated for C34H28N3O2
The organic phase was washed with water (3×50mL), [M+H]+: 510.2176, Found: 510.2189.
dried over Na2SO4, and evaporated under vacuum. The
resulting residue was purified by column chromatography
Synthesis of 7-(Diethylamino)-4-(1-phenyl-
over silica gel using petroleum ether/acetone (v/v=6:1) as
1H-phenanthro[9,10-d]imidazol-2-yl)
eluent to afford yellow solid product. Yield 0.21g (54%),
coumarin (10)
m.p. 220°C–221°C (lit.11 m.p. 198°C–200°C). IR (KBr)
cm−1: 3059, 2978, 1710, 1525, 1452, 1384, 729. 1H NMR Compound 10 was prepared from 7-(diethylamino)-4-for-
(600 MHz, CDCl3): δ/ppm 8.14 (s, 1H), 7.86 (d, J = mylcoumarin (0.26g, 1.06mmol), 9,10-phenanthrenequi-
7.8Hz, 1H), 7.48–7.27 (m, 9H), 6.59 (dd, J=9.0 and none (0.27g, 1.28mmol), aniline (0.15g, 1.61mmol), and
2.4Hz, 1H), 6.41 (d, J=2.4Hz, 1H), 3.41 (q, J=7.2Hz, ammonium acetate (0.24g, 3.12mmol) by a method simi-
4H), 1.20 (t, J=7.2Hz, 6H). 13C NMR (150MHz, CDCl3): lar to that described for 9 as yellow solid. Yield 0.23g
δ/ppm 159.1, 157.3, 151.5, 149.1, 146.2, 142.7, 136.9, (43%), m.p. 240°C–242°C. IR (KBr) cm−1: 2974, 1708,
1
136.6, 129.8, 129.4, 128.2, 126.3, 123.4, 122.8, 119.7, 1622, 1407, 1355, 1276, 1141, 758. H NMR (500MHz,
111.1, 110.4, 109.1, 108.1, 97.1, 44.9, 12.4.
DMSO-d6): δ 8.96 (d, J=8.5Hz, 1H), 8.91 (d, J=8.5Hz,

Fan et al.
5
1H), 8.66 (dd, J=8.0 and 1.5Hz, 1H), 7.81–7.36 (m, 10H), ORCID iD
7.13 (dd, J=8.5 and 1.5Hz, 1H), 6.69 (dd, J=9.5 and
2.5Hz, 1H), 6.55 (d, J=2.5Hz, 1H), 6.05 (s, 1H), 3.43 (q,
J=7.0Hz, 4H), 1.12 (t, J=7.0Hz, 6H). 13C NMR
Xiaolong Wang
https://orcid.org/0000-0003-1916-5780
References
(125MHz, DMSO-d6): δ 159.9, 156.1, 150.7, 145.5, 143.8,
1. Paul K, Bindal S and Luxami V. Bioorg Med Chem Lett
137.0, 136.6, 130.4, 130.1, 128.8, 128.7, 128.6, 127.8,
127.7, 127.6, 126.8, 126.6, 126.1, 125.8, 124.6, 123.8,
122.2, 122.0, 120.4, 110.4, 109.1, 106.8, 96.8, 44.0, 12.3.
HRMS Calculated for C34H28N3O2 [M+H]+: 510.2176,
Found: 510.2185.
2013; 23: 3667.
2. Jagtap AR, Satam VS, Rajule RN, et al. Dyes Pigm 2011; 91:
20.
3. Matta A, Bahadur V, Taniike T, et al. Dyes Pigm 2017; 140:
250.
4. Kim HJ, Heo CH and Kim HM. J Am Chem Soc 2013; 135:
17969.
5. KothavaleS,BhalekarSandSekarN.DyesPigm2018;159:209.
6. Yoon YK, Ali MA, Wei AC, et al. Bioorg Med Chem 2014;
22: 703.
Acknowledgements
We thank the Instrument Analysis Center of Lanzhou Jiaotong
University.
7. Barooah N, Mohantyab J and Bhasikuttan AC. Chem
Commun 2015; 51: 13225.
Declaration of conflicting interests
8. Zhao B, Liu T, Fang Y, et al. Tetrahedron Lett 2016; 57:
4417.
9. Wang XL, Yang F, Xue ZY, et al. J Chem Res 2015; 39: 213.
10. Wang XL, Zhou JJ, Yang F, et al. J Chem Res 2016; 40: 604.
11. Wang YB, Liu HT, Lu P, et al. Chem Biol Drug Des 2015;
86: 637.
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Funding
The author(s) received no financial support for the research, 12. Xue JQ, Zhao YX, Wu FP, et al. J Phys Chem A 2010; 114:
authorship, and/or publication of this article.
5171.