Dyes and Pigments 140 (2017) 250e260
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, characterisation and photophysical studies of oxadiazolyl
coumarin: A new class of blue light emitting fluorescent dyes
Akanksha Matta a, b, c, Vijay Bahadur a, 1, Toshiaki Taniike b, Johan Van der Eycken c,
Brajendra K. Singh a, *
a
b
c
Bioorganic Laboratory, Department of Chemistry, University of Delhi, Delhi, 110 007, India
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, 923-1292, Japan
Laboratory for Organic and Bioorganic Synthesis, Department of Organic Chemistry, Ghent University, Krijgslaan 281 (S.4), B-9000, Ghent, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2016
Received in revised form
22 December 2016
Accepted 3 January 2017
Available online 21 January 2017
A library of novel 1, 2, 4-oxadiazole linked coumarin dyes have been synthesised via condensation of
corresponding acid 6 and N’-hydroxybenzimidamide 8. This new class of organic compounds were
examined for their fluorescent properties and found to emit blue light in the visible region of the
spectrum with very high Stoke's shift values. Most of these compounds demonstrated high quantum
yields and fluorescence life time in nano-second range which makes them quite lucrative to be used as
new fluorescent probes. The highest quantum yield of 0.68 was shown by compound 9j which also shows
high Stoke's shift value. The electronic structure of these coumarin-based donorepeacceptor (DepeA)type organic dyes have been examined by Density Functional Theory (DFT). TGA analysis of few of the
compounds show that they are stable up to temperature range of 0e245 C. The synthesised compounds
were characterised by NMR and mass spectrometry and the structure of two of these compounds have
been confirmed by X-ray crystallography.
© 2017 Elsevier Ltd. All rights reserved.
Keywords:
1,2,4-Oxadiazoles
Dye
Photophysical study
Thermal stability
DFT
1. Introduction
Coumarins and their analogues are not only known for their
excellent biological activities [1] but also well known for their
outstanding optical properties. Coumarin shows good spectral
response, have high photostability [2] and are also recognised as
highly efficient fluorophores with high quantum yield [3], decent
extinction coefficient [4] and large Stokes shift [5]. Moreover,
because of such exceptional optical properties, coumarin derivatives are known to find a diverse range of applications as
fluorescent dyes [6], fluorescent probes [7], emission layers in
organic light-emitting diodes (OLED) [8], optical brighteners,
nonlinear optical chromophores, fluorescent whiteners, fluorescent
indicators, optical recording and solar energy collectors [9].
The absorption and emission behaviour of coumarin vary
significantly depending upon the substituent present on the
coumarin ring [10]. The resonance contribution to the electron-
* Corresponding author.
E-mail address: [email protected] (B.K. Singh).
1
Department of Chemistry, SRM University Delhi-NCR, Sonepat, Haryana,
131029, India
http://dx.doi.org/10.1016/j.dyepig.2017.01.050
0143-7208/© 2017 Elsevier Ltd. All rights reserved.
accepting 2-pyranone moiety is responsible for this substituent
effect [11]. Coumarin itself has a very low quantum yield but with
an appropriate substitution on the coumarin moiety, the derivatives of coumarin can show strong fluorescence in the bluegreen region (400e550 nm) with a precisely good quantum yield.
This is attributed to the enhancement of intramolecular charge
transfer (ICT) upon substitution at desired position. For instance, 7amino-4-methylcoumarin (AMC) is commonly used as an important laser dye emitting in the blue region [12]. It has been reported
in several literature that presence of electron donating group at 7position of coumarin and various substitutions at 3-position modify
the fluorescent property of coumarins to a larger extent [13].
Literature guided survey revealed that, the fluorescence spectral
study of 4-substituted coumarins have not been much explored yet.
Also, it has been reported in few instances that the oxadiazole
moiety itself have been used as potential fluorescent dopants [14]
and fluorescent chemosensors [15]. Zhang et. al. reported the synthesis of bismaleimides bearing 2,5-diphenyl-1,3,4-oxadiazole
chromophores which exhibited good fluorescent properties
including excellent quantum yield [16].
In this context, as a part of the foregoing research in our group,
we have planned the synthesis of a hybrid molecule viz. 1, 2, 4-
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
oxadiazolyl coumarin with the oxadiazole ring substituted at the 4position of coumarin. Hence, a synergistic effect in the fluorescent
property of this hybrid molecule is expected by the linkage of these
two fluorescent heterocyclic moieties. Moreover, 2-(5-alkyl-1,3,4oxadiazol-2-yl)-3H-benzo[f]chromen-3-ones i.e. 1,3,4 oxadiazole
substituted at 3-position of coumarin have been reported to exhibit
excellent fluorescence properties [17].
251
bright yellow solid in 58% yield, m.p.164e168 C. IR (KBr) (cm1): n
2989, 1708, 1H NMR (DMSO-d6, 400 MHz): d (ppm) 1.36 (3 H, t,
J ¼ 6.87 Hz), 4.14 (2 H, q, J ¼ 7.63 Hz), 6.98 (1 H, s), 6.99e7.07 (2 H,
m), 8.38 (1 H, d, J ¼ 9.16 Hz), 10.09 (1 H, s); 13C NMR (DMSO-d6,
100 MHz): d (ppm) 14.59, 64.30, 101.68, 108.30, 113.38, 121.48,
127.14, 143.75, 156.13, 160.67, 162.08, 193.99; HRMS: Found:
[MþH]þ 219.0655; ‘molecular formula C12H10O4’ requires [MþH]þ
219.0579.
2. Experimental
2.1. Materials and methods
Analytical TLCs were performed on Merck silica gel 60F254
plates. All liquid column chromatographic separations were performed on column chromatography. IR spectra were recorded on a
Perkin-Elmer 2000 FT-IR spectrometer at Department of Chemistry,
University of Delhi. The 1H and 13C NMR spectra (in CDCl3 and
DMSO-d6) were recorded on a JEOL ECX-400P NMR at 400 MHz and
100 MHz, respectively at USIC, University of Delhi and Department
of Organic Chemistry, Ghent University, Belgium. TMS was used as
internal standard. The high-resolution mass spectral data was obtained using Agilent technologies 1100 series LC/MSD system at
Department of Organic Chemistry, Ghent University, Belgium. The
absorption and florescence emission spectra were recorded by
Hitachi U-2010 UVeVis spectrophotometer and Varian Cary Eclipse
Fluorescence Spectrophotometer respectively at Department of
Organic Chemistry, Ghent University, Belgium. The fluorescence
decay measurements were recorded by HORIBA Jobin Yvon Fluorohub at USIC, University of Delhi, Inda. The absorption, the fluorescence emission and fluorescence decay measurements of the
synthesised compounds were recorded at 105 M concentration in
chloroform and ethanol. Thermogravimetric analysis (TGA) was
performed on TA instruments Q50 device at Department of Organic
Chemistry, Ghent University, Belgium. Melting points were recorded on a Buchi M - 560 melting point apparatus and are uncorrected. The crystal data was collected on Oxford Xcalibur S
diffractometer (4-circle kappa goniometer, Sapphire-3 CCD detector, omega scans, graphite monochromator, and a single wavelength Enhance X-ray source with MoKa radiation). The structures
were solved by direct methods using SIR 9233 which revealed the
atomic positions, and refined using the SHELX-97 program package34 and SHELXL9735 (within the WinGX program package). All
the chemicals and reagents like substituted benzonitriles, ethylchloroformate, b-keto-esters, resorcinol etc. were purchased from
commercial sources and used as received unless otherwise
indicated.
2.2.3. Synthesis of 7-alkoxy-2-oxo-2H-chromene-4-carboxylic acid
6(a-b)
The aldehyde 5 (73.5 mmol, 1 eq.) was dissolved in DMF (60 mL).
Oxone (110 mmol, 1.5 eq.) was added in one portion and stirred at
room temperature for 3 h. After the completion of the reaction, the
reaction mixture was poured into ice-cold water. The solid obtained
was filtered and dried. The desired products 6a and 6b were obtained in 74% and 79% yield respectively and were used in the next
reaction without further purification [22]. Compound 6a was obtained as bright yellow solid in 74% yield, m.p. 217e221 C (lit. m.p.
219 C) [23]. Compound 6b was obtained as bright yellow solid in
79% yield, m.p. 163e164 C. IR (KBr) (cm1): n 3424, 2926, 1734; 1H
NMR (DMSO-d6, 400 MHz): d (ppm) 1.35 (3 H, t, J ¼ 6.87 Hz), 4.12
(2 H, q, J ¼ 6.87 Hz), 6.61 (1 H, s), 6.92e6.99 (2 H, m), 8.03e8.07
(1 H, m); 13C NMR (DMSO-d6, 100 MHz): d (ppm) 14.60, 64.31,
101.61, 109.25, 113.20, 114.30, 128.11, 144.28, 155.97, 160.22, 162.08,
165.79; HRMS: Found: [MþH]þ 235.0591; ‘molecular formula
C12H10O5’ requires [MþH]þ 235.0528.
2.2.4. Synthesis of N'-hydroxybenzimidamide 8(a-e)
N'-hydroxybenzimidamide 8(a-e) were obtained in very good
yields using literature reported procedure [24].
2.2.5. General procedure for synthesis of coumarin-linked
oxadiazole derivatives 9(a-j)
The carboxylic acid 6(a-b) (2.3 mmol, 1 eq.) was stirred in
dichloromethane for 10 min followed by addition of potassium
carbonate (3.4 mmol, 1.5 eq.). The resulting mixture was stirred for
30 min at room temperature. Ethyl chloroformate (3.4 mmol, 1.5
eq.) was added to the reaction mixture and it was stirred again for
30 min. Lastly, N'-hydroxybenzimidamide {8(a-e)} (2.3 mmol, 1 eq.)
was added and reaction mixture was refluxed for 6e8 h. After
completion of the reaction, the mixture was extracted with
dichloromethane (3 30 mL). The combined organic layer was
dried over Na2SO4, filtered and concentrated under reduced pressure. The crude product was purified by column chromatography
(5% Hexane/EtOAc) over silica gel to yield the desired products
9(aej) in moderate yields.
2.2. Synthesis
2.2.1. Synthesis of coumarin (4)
Substituted hydroxy coumarin 3 were synthesized by known
Pechmann condensation [18] and the methoxy/ethoxy coumarin
4(a,b) were obtained from hydroxy coumarin via methylation/
ethylation in presence of potassium carbonate [19].
2.2.2. Synthesis of 7-alkoxy-2-oxo-2H-chromene-4-carbaldehyde
5(a-b)
Compound 4 (98 mmol, 1eq.) was dissolved in 1,4-dioxane and
selenium dioxide (117 mmol, 1.2 eq.) was added to the solution [20].
The reaction mixture was refluxed for 48 h. After the completion of
reaction, the reaction mixture was filtered to remove the insoluble
selenium. The crude product was purified by column chromatography (50% EtOAc/CHCl3) over silica gel to yield the desired products. Compound 5a was obtained as yellow solid in 55% yield, m.p.
193e195 C (lit. m.p. 194e196 C) [21] and 5b was obtained as a
2.2.5.1. 7-Methoxy-4-(3-phenyl-1,2,4-oxadiazol-5-yl)-2H-chromen2-one (9a). It was obtained as yellow solid having m. p. 170e171 C
in 43% yield. IR (KBr) (cm1): n 2933, 1708, 1618, 1375; 1H NMR
(CDCl3, 400 MHz): d (ppm) 3.93 (3 H, s), 6.92 (1 H, d, J ¼ 2.44 Hz),
7.01 (1 H, dd, J ¼ 9.16, 2.44 Hz), 7.15 (1 H, s), 7.49e7.62 (3 H, m),
8.17e8.23 (2 H, m), 8.74 (1 H, d, J ¼ 8.54 Hz); 13C NMR (CDCl3,
100 MHz): d (ppm) 55.89 101.27, 108.48, 113.28, 115.59, 125.92,
127.61, 128.28, 129.05, 131.82, 135.61, 156.39, 159.76, 163.52, 169.30,
171.73; HRMS: Found: [MþH]þ 321.0868; ‘molecular formula
C18H12N2O4’ requires [MþH]þ 321.0797.
2.2.5.2. 4-(3-(3-bromophenyl)-1,2,4-oxadiazol-5-yl)-7-methoxy-2Hchromen-2-one (9b). It was obtained as yellow solid having m. p.
112e113 C in 41% yield. IR (KBr) nmax (cm1) ¼ 2936, 1734, 1611,
1400, 488; 1H NMR (CDCl3, 400 MHz): d (ppm) 3.94 (3 H, s) 6.93
(1 H, d, J ¼ 2.20 Hz) 7.02 (1 H, dd, J ¼ 8.79, 2.20 Hz) 7.15 (1 H, s) 7.44
(1 H, t, J ¼ 8.05 Hz) 7.72 (1 H, d, J ¼ 8.05 Hz) 8.15 (1 H, d, J ¼ 8.05 Hz)
252
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
8.36 (1 H, d, J ¼ 1.46 Hz) 8.70 (1 H, d, J ¼ 9.52 Hz); 13C NMR (CDCl3,
100 MHz): d (ppm) 64.02, 101.01, 111.40, 112.55, 115.23, 122.03,
125.34, 127.85, 129.68, 132.80, 133.30, 134.43, 152.92, 155.20, 162.27,
163.26, 163.55, 167.79, 171.65; HRMS: Found: [M - H]þ 396.9840;
‘molecular formula C18H11BrN2O4’ requires [M - H]þ 396.9902.
2.2.5.3. 4-(3-(2-fluorophenyl)-1,2,4-oxadiazol-5-yl)-7-methoxy-2Hchromen-2-one (9c). It was obtained as yellow solid having m. p.
161e162 C.in 37% yield. IR (KBr) (cm1): n 2926, 1724, 1617, 1380,
1257; 1H NMR (CDCl3, 400 MHz): d (ppm) 3.93 (3 H, s), 6.92 (1 H, d,
J ¼ 3.05 Hz), 7.00 (1 H, dd, J ¼ 9.16, 2.44 Hz), 7.16 (1 H, s), 7.28e7.38
(2 H, m), 7.55e7.61 (1 H, m), 8.19 (1 H, dd, J ¼ 7.32, 1.83 Hz), 8.70
(1 H, d, J ¼ 9.16 Hz); 13C NMR (CDCl3, 100 MHz): d (ppm) 55.90,
101.30, 108.42, 113.31, 115.74, 116.82, 124.61, 128.19, 130.74, 133.48,
135.43, 156.40, 159.68, 162.09, 163.55, 166.21, 171.41; HRMS: Found:
[MþH]þ 339.0776; ‘molecular formula C18H11FN2O4’ requires
[MþH]þ 339.0703.
2.2.5.4. 4-(3-(2-chlorophenyl)-1,2,4-oxadiazol-5-yl)-7-methoxy-2Hchromen-2-one (9d). It was obtained as yellow solid having m. p.
167e168 C in 32% yield. IR (KBr) (cm1): n 2954, 1742, 1610, 1352,
740; 1H NMR (CDCl3, 400 MHz): d (ppm) 3.93 (3 H, s), 6.92 (1 H, d,
J ¼ 1.22 Hz), 6.99 (1 H, dt, J ¼ 9.16, 1.83 Hz), 7.17 (1 H, d, J ¼ 1.22 Hz),
7.44e7.53 (2 H, m), 7.61 (1 H, d, J ¼ 7.93 Hz), 8.05 (1 H, d,
J ¼ 7.93 Hz), 8.69e8.74 (1 H, m); 13C NMR (CDCl3, 100 MHz):
d (ppm) 55.89, 101.29, 108.42, 113.31, 115.69, 125.20, 127.09, 128.24,
131.18, 131.76, 132.29, 133.72, 135.45, 156.40, 159.71, 163.55, 168.12,
171.28; HRMS: Found: [MþH]þ 355.0479; ‘molecular formula
C18H11ClN2O4’ requires [MþH]þ 355.0407.
2.2.5.5. 7-Methoxy-4-(3-(m-tolyl)-1,2,4-oxadiazol-5-yl)-2H-chromen-2-one (9e). It was obtained as yellow solid having m. p.
168e171 C in 47% yield. IR (KBr) (cm1): n 2934, 1718, 1610, 1412;
1
H NMR (CDCl3, 400 MHz): d (ppm) 2.48 (3 H, s), 3.93 (3 H, s),
6.90e6.92 (1 H, m), 7.01 (1 H, dd, J ¼ 9.16, 2.44 Hz), 7.14 (1 H, s),
7.37e7.46 (2 H, m), 7.98e8.01 (2 H, m), 8.73 (1 H, d, J ¼ 9.16 Hz); 13C
NMR (CDCl3, 100 MHz): d (ppm) 21.69, 56.20, 101.58, 108.80, 113.55,
115.85, 125.07, 126.08, 128.37, 128.59, 129.26, 132.92, 135.93, 139.22,
156.69, 160.06, 163.81, 169.70, 171.95; HRMS: Found: [MþH]þ
335.1026; ‘molecular formula C19H14N2O4’ requires [MþH]þ
335.0954.
2.2.5.6. 7-Ethoxy-4-(3-phenyl-1,2,4-oxadiazol-5-yl)-2H-chromen-2one (9f). It was obtained as yellow solid having m. p. 172e175 C in
44% yield. IR (KBr) (cm1): n 2950, 1715, 1613, 1376; 1H NMR (CDCl3,
400 MHz): d (ppm) 1.49 (3 H, t, J ¼ 7.25 Hz), 4.15 (2 H, q, J ¼ 7.12 Hz),
6.90 (1 H, d, J ¼ 2.29 Hz), 6.98 (1 H, dd, J ¼ 9.16, 2.29 Hz), 7.14 (1 H, s),
7.55e7.57 (2 H, m), 8.20e8.22 (2 H, m), 8.71 (1 H, d, J ¼ 9.16 Hz); 13C
NMR (CDCl3, 100 MHz): d (ppm) 14.51, 64.35, 101.69, 108.31, 113.60,
115.41, 125.93, 127.61, 128.20, 129.04, 131.80, 135.63, 156.38, 159.83,
162.92, 169.29, 171.76; HRMS: Found: [MþH]þ 335.1029; ‘molecular
formula C19H14N2O4’ requires [MþH]þ 335.0954.
2.2.5.7. 4-(3-(3-bromophenyl)-1,2,4-oxadiazol-5-yl)-7-ethoxy-2Hchromen-2-one (9g). It was obtained as yellow solid having m. p.
84e85 C in 31% yield. IR (KBr) (cm1): n 2976, 1729, 1603, 1395; 1H
NMR (CDCl3, 400 MHz): d (ppm) 1.51 (3 H, t, J ¼ 7.04 Hz), 4.15 (2 H,
q, J ¼ 7.15), 6.91 (1 H, s), 7.00 (1 H, d, J ¼ 6.87 Hz), 7.15 (1 H, s),
7.40e7.47 (1 H, m), 7.72 (1 H, d, J ¼ 9.16 Hz), 8.15 (1 H, d, J ¼ 7.63 Hz),
8.36 (1 H, d, J ¼ 1.53 Hz), 8.69 (1 H, d, J ¼ 9.16 Hz); 13C NMR (CDCl3,
100 MHz): d (ppm) 14.18, 64.02, 101.37, 107.89, 113.28, 115.20,
116.46, 124.30, 127.78, 130.40, 133.05, 135.06, 156.04, 159.41, 162.61,
165.71, 171.08; HRMS: Found: [M - H]þ 410.9993; ‘molecular formula C19H13BrN2O4’ requires [M - H]þ 411.0059.
2.2.5.8. 7-Ethoxy-4-(3-(2-fluorophenyl)-1,2,4-oxadiazol-5-yl)-2Hchromen-2-one (9h). It was obtained as yellow solid having m. p.
164e165 C in 45% yield. IR (KBr) (cm1): n 2970, 1729, 1603, 1392,
12621H NMR (CDCl3, 400 MHz): d (ppm) 1.49 (3 H, t, J ¼ 6.87 Hz),
4.15 (2 H, q, J ¼ 6.87 Hz), 6.90 (1 H, d, J ¼ 2.29 Hz), 6.98 (1 H, dd,
J ¼ 9.16, 2.29 Hz), 7.15 (1 H, s), 7.33e7.38 (2 H, m), 7.54e7.62 (1 H,
m), 8.15e8.22 (1 H, m), 8.68 (1 H, d, J ¼ 9.16 Hz); 13C NMR (CDCl3,
100 MHz): d (ppm) 14.77, 64.63, 102.01, 108.55, 113.91, 115.85,
124.90, 128.40, 131.04, 133.65, 135.74, 156.68, 160.01, 163.24, 166.47,
171.75; HRMS: Found: [MþH]þ 353.0934, ‘molecular formula
C19H13FN2O4’ requires [MþH]þ 353.0859.
2.2.5.9. 4-(3-(2-chlorophenyl)-1,2,4-oxadiazol-5-yl)-7-ethoxy-2Hchromen-2-one (9i). It was obtained as yellow solid having m. p.
91e93 C in 59% yield. IR (KBr) (cm1): n 2942, 1716, 1618, 1390,
783; 1H NMR (CDCl3, 400 MHz): d (ppm) 1.49 (3 H, t, J ¼ 7.25 Hz),
4.15 (2 H, q, J ¼ 6.87 Hz), 6.90 (1 H, d, J ¼ 2.29 Hz), 6.97 (1 H, dd,
J ¼ 9.16, 3.05 Hz), 7.16 (1 H, s), 7.45e7.54 (2 H, m), 7.61 (1 H, d,
J ¼ 8.39 Hz), 8.02e8.07 (1 H, m), 8.70 (1 H, d, J ¼ 9.16 Hz); 13C NMR
(CDCl3, 100 MHz): d (ppm) 14.83, 64.68, 102.04, 108.60, 113.99,
115.85, 127.41, 128.52, 131.50, 132.10, 132.62, 156.73, 160.15, 163.29,
168.46, 171.67; HRMS: Found: [M - H]þ 367.0501; ‘molecular formula C19H13ClN2O4’ requires [M - H]þ 367.0564.
2.2.5.10. 7-Ethoxy-4-(3-(m-tolyl)-1,2,4-oxadiazol-5-yl)-2H-chromen-2-one (9j). It was obtained as yellow solid having m. p.
181e182 C in 50% yield. IR (KBr) (cm1): n 2985, 1729, 1602, 1393;
1
H NMR (CDCl3, 400 MHz): d (ppm) 1.49 (3 H, t, J ¼ 7.25 Hz), 2.48
(3 H, s), 4.15 (2 H, q, J ¼ 6.87 Hz), 6.90 (1 H, d, J ¼ 2.29 Hz), 6.99 (1 H,
dd, J ¼ 9.16, 2.29 Hz), 7.14 (1 H, s), 7.36e7.47 (2 H, m), 7.99e8.01
(2 H, m), 8.71 (1 H, d, J ¼ 9.16 Hz); 13C NMR (CDCl3, 100 MHz):
d (ppm) 14.52, 21.38, 64.35, 101.71, 108.36, 113.60, 115.40, 124.79,
125.81, 128.09, 128.24 128.95, 132.62, 135.71, 138.92, 156.39, 159.86,
162.93, 169.42, 171.71; HRMS: Found: [MþH]þ 349.1184, ‘molecular
formula C20H16N2O4’ requires [MþH]þ 349.1110.
3. Results and discussion
3.1. Chemistry
The synthetic route to novel oxadiazolyl coumarin 9 has been
outlined in Scheme 1. 7-hydroxy-4-methylcoumarin (3) was prepared by Pechmann condensation reaction of resorcinol with ethylacetoacetate according to the known procedures. The hydroxyl
moiety 3 was alkylated to afford 7-alkoxy-4-methylcoumarin 4 in
good yields. Further, the aldehyde 5 was obtained by treating 4 with
selenium dioxide which was further oxidized to carboxylic acids 6
by the reaction of 5 with oxone. Lastly, the oxadiazolyl coumarin 9
were obtained by the reaction of the carboxylic acid 6 with the
appropriately substituted arylamidoximes 8 in moderate yields.
3.2. Photophysical studies
3.2.1. Fluorescence spectral studies
The fluorescence property of novel 1, 2, 4-oxadiazole linked
coumarin derivatives 9 were investigated in ethanol and chloroform. An attempt to carry out the investigation in non-polar solvents failed due to the insolubility of the compounds in these
solvents. The details have been tabulated in Table 1. The molecules
are designed in such a way to get a unique combination of an
electron releasing group i.e. methoxy or ethoxy at 7-position of the
coumarin with the oxadiazole moiety at the 4-position.
These oxadiazole linked fluorescent dyes 9a-j absorb radiation
in the UV region and emit in the blue region of visible spectrum
(450e495 nm) (Table 1). The emission spectra of all the compounds
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
253
Scheme 1. Reagents and conditions a) H2SO4, 6e8 h, rt; b) Me2SO4/Et2SO4, K2CO3, acetone, 12 h, reflux; c) SeO2, 1, 4 dioxane, 48 h, reflux; d) Oxone, DMF, 3 h, rt; e) NH2OH$HCl,
NaHCO3, ethanol, water, 20 h, rt; f) K2CO3, ethyl chloroformate, DCM, 6e8 h, reflux.
Table 1
Spectral characteristics of 4-(3-phenyl-1,2,4-oxadiazol-5-yl)-2H-chromen-2-one derivatives 9a-j in chloroform and ethanol.
Codea
R1
R2
9a
9b
9c
9d
9e
9f
9g
9h
9i
9j
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
H
3-Br
2-F
2-Cl
3-Me
H
3-Br
2-F
2-Cl
3-Me
lex (nm)
lem (nm)
Stoke's shift(nm)
Quantum yieldb(F)
450
449
453
450
450
454
452
453
446
456
105
89
114
95
47
62
49
60
52
134
0.25
0.33
0.25
0.19
0.12
0.31
0.20
0.36
0.26
0.23
Chloroform
a
b
c
345
360
339
355
403
392
403
393
394
322
lex (nm)
lem (nm)
Stoke's shift(nm)
Quantum yieldc(F)
464
466
464
464
464
468
458
465
462
465
120
207
131
110
63
80
70
120
69
120
0.62
0.41
0.57
0.39
0.52
0.60
0.52
0.64
0.52
0.68
Ethanol
344
259
333
354
401
388
388
345
393
345
Concentration of the compounds ¼ 105 M.
Quantum yield of Rhodamine 6G in chloroform is 0.70 [25].
Quantum yield of Rhodamine 6G in ethanol is 0.95 [26].
have been depicted in Figs. 1 and 2. These compounds emitted blue
light in both the tested solvents viz. chloroform and ethanol (Fig. 3).
All the synthesised compounds show a high value of Stoke's shift
but an exceptionally high stoke shift was observed in case of
compounds 9aej. These high stoke's shift values are due to effective
p-conjugation in the molecule which in turn is responsible for
charge transfer nature of emissive excited state. Fluorophores with
a high Stoke's shift value offers an advantage that the possibility of
spectral overlap between absorption and emission is eliminated
which allows the detection of fluorescence while reducing
interference. Also, the quenching of the fluorescence is eliminated
which makes the detection of the fluorescence emission very easy
[9]. The Stoke shift values obtained for the compounds 9a-j are way
too higher than 1,3,4 oxadiazole substituted at 3-position of
coumarin which ranges from 54 to 59 nm in chloroform [17].
Further, the quantum yield of the compounds was determined.
Quantum yields were calculated by relative method using equation
(1), Rhodamine 6G was used as reference fluorescent compound of
known quantum yield, i.e. 0.70 in chloroform and 0.95 in ethanol.
254
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
Fig. 1. Emission spectra of compounds 9a-j in chloroform.
Fig. 2. Emission spectra of compounds 9a-j in ethanol.
F I AF h2
Fs ¼ rg s rg2 s
Irg AFs hrg
(1)
Fs: Quantum yield of sample, Frg: Quantum yield of Rhodamine 6G,
Is: Fluorescence Intensity Area of sample, Irg: Fluorescence Intensity
Area of Rhodamine, AFrg: Absorption Factor of Rhodamine ¼
(1e10A), A ¼ absorption, AFs: Absorption Factor of sample ¼
(1e10A), A: absorption, hs:Refractive index of solvent in which
sample is prepared, hrg:Refractive index of solvent in which
Rhodamine data was taken.
The compounds exhibited varying trend of fluorescent property
in both the solvents. The quantum yield values vary from 0.12 to
0.36 in chloroform and 0.39 to 0.68 in ethanol when compared with
Rhodamine 6G. Compounds 9b, 9f and 9h displayed high quantum
yields in chloroform, the highest being for 9h at 0.36. When the
fluorescence experiments were carried out in ethanol, tremendously high stoke shift values and nominally high quantum yields
were observed. In addition, a red shift in the emission wavelength
of all the compounds was observed when the spectra are recorded
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
255
Table 2
Fluorescence life times and the radiative kr and non-radiative knr deactivation
constants.
Fig. 3. Photographs of dye 9j in a) ambient light b) UV light (366 nm) in chloroform
(left) and ethanol (right).
in more polar solvent i.e. ethanol. This corresponds to the fact that
solvent plays a crucial role in determining the optical properties of
organic compounds. In halogenated solvents fluorescence
quenching is observed which results in lowering of the fluorescence quantum yield. The high quantum yield of these new class of
compounds is attributed to the increasing conjugation length of the
chromophoric system because of the presence of a conjugated
phenyl substituted oxadiazole moiety on the 4-position of
coumarin ring.
Typically, the structure-property relationships for the fluorescence study is often unpredictable but herein, we have tried to
explain it on the basis of the calculated quantum yield values. The
occupancy of 7-position of coumarin by an electron donating group
generally increases the fluorescence property of coumarin. It was
observed that the ethoxy substituted coumarin 9f-j (Table 1)
showed a better fluorescence property than methoxy substituted
coumarin 9a-e (Table 1) in terms of quantum yield. The presence of
substituents on the phenyl ring attached to the 3-position of the
oxadiazole ring moderately affects the quantum yield. The compounds containing unsubstituted phenyl ring (9a, 9f) and the
phenyl ring substituted with electron donating methyl group (9e,
9j) shows the high quantum yield as well as high Stoke's shift value.
The compound 9j shows the highest quantum yield value of 0.68
and a very high stoke's shift of 120 nm in ethanol. On the other
hand, the presence of halogens on the phenyl ring decreases the
fluorescence quantum yield to a noticeable extent more specifically
for the chlorine and bromine containing compounds. The fluorine
containing compounds, 9c and 9h displayed a high quantum yield
values in both the solvents. This is in agreement to the fact that the
presence of fluorine atom in any chromophore resulted in increased
photostability as well as altered absorption and quantum yield [27].
Nevertheless, Stoke's shift values are less affected with the introduction of halogens on the phenyl ring.
3.2.2. Fluorescence lifetime measurements
In order to gather more information on fluorescence properties
of the synthesised dyes, fluorescence life times were measured by
exciting the sample at 370 nm. The compounds 9j, 9h, 9a, 9f with
good quantum yield in ethanol and the compounds 9h, 9b, 9i, 9a
with moderate quantum yield in chloroform were subjected to
fluorescent life time measurement in the respective solvents. The
fluorescence decays obtained in all the cases were bi-exponential.
The average fluorescence life times varied from 4.8 to
6.8 ns (Table 2). The average fluorescence life time was calculated
by using equation (2) [28].
Entry
Solvent
t1 (ns)
t2 (ns)
< t > (ns)
kr (s1)
9a
9f
9h
9j
9a
9b
9h
9i
EtOH
EtOH
EtOH
EtOH
Chloroform
Chloroform
Chloroform
Chloroform
3.3
3.7
3.2
3.2
1.5
3.3
2.1
1.7
7
7.3
7.3
7.1
5.2
7.6
5.8
6
6.4
6.7
6.6
6.5
4.8
6.8
5.1
5.5
0.96
0.89
0.97
1.03
0.52
0.48
0.70
0.47
〈t〉 ¼
X
ai t2i =ai ti
108
108
108
108
108
108
108
108
knr (s1)
0.59
0.59
0.54
0.48
1.56
0.97
1.23
1.34
108
108
108
108
108
108
108
108
(2)
<t>: average fluorescence life time, ai: normalized fluorescence
contribution, ti: fluorescence life time.
With the values of the quantum yield and life times, the radiative kr and non-radiative knr deactivation constants (Table 2) were
calculated using the classical relations kr ¼ F/t and knr ¼ (1F)/t
[29]. It is well documented in literature that the higher value of kr
indicates that the energy levels of the molecule are compatible with
high fluorescence efficiency, whereas a high value for knr suggests
that the non-radiative deactivations taking place in the molecules
such as molecular rotations, vibrations, intermolecular interactions
etc. which offers deactivation channels that compete with the
fluorescence process [30]. The values of kr was found to be higher
than knr in ethanol and the reverse order was observed in case of
chloroform i.e. knr > kr. This is in agreement with the obtained results that observed fluorescence efficiency was lower in chloroform
when compared with ethanol.
3.3. Frontier molecular orbitals
To get clearer perception of the electronic structures of the
compounds, the DFT calculations were executed. Density functional
calculations at the level of generalized gradient approximation
were performed using a program package of DMOL3 [31]. The XC
functional was selected as BP [32]. The double numerical basis set
with polarization (DNP) was employed together with a semicore
pseudopotential (DSPP) core treatment [30]. Geometry optimization was performed for 9a-j, where the COSMO solvation model
[33] was applied for chloroform and ethanol. The results shows the
calculated band gaps of the dyes 9a-j in chloroform, ethanol and
vacuum which have been summarised in Fig. 4. The calculated band
gaps of the dyes are in the range 2.23e2.29 eV in vacuum and
2.16e2.27 eV in chloroform while in ethanol, these are lowered to
2.13e2.23 eV (see Fig. 5).
The HOMO and LUMO energy level diagram gives quantitative
knowledge of electronic structure and excitation properties. The
HOMO and LUMO orbitals of compounds 9a and 9f obtained as
optimised structures are shown in Fig. 6 and those of all dyes are
given in SI (Supporting Information). In order to create an efficient
charge-separated state by photoabsorption, it is appropriate that
the HOMO is localized on the donor moiety and the LUMO on the
acceptor moiety [34]. The HOMO is delocalised mainly over the
coumarin ring while LUMO is located over both the moieties i.e. the
coumarin ring and the oxadiazole ring. These orbital contour plots
evidently signifies that by the transitions from HOMO to LUMO, the
charge-separated state is generated. In these dyes, the HOMOeLUMO excitation can be ascribed to intramolecular chargetransfer (ICT) transition from donor to acceptor. Moreover, the
well-overlapped HOMO and LUMO suggests good induction and
electronewithdrawing properties for the donor and acceptor,
256
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
Fig. 4. The HOMOeLUMO gaps (in eV) of all dyes.
Fig. 5. HOMO and LUMO frontier orbitals of dyes 9a and 9f in ethanol.
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
thermally stable compound 9j starts at 245 C and subsequent
weight loss was observed up to 330 C.
Table 3
Crystallographic data of compound 9f and 9j.
Parameters
9f
9j
Empirical formula
Formula weight
Crystal system
Space group
T (K)
l, Mo Ka (Å)
a (Å)
b (Å)
c (Å)
a ( )
b ( )
g( )
V, (Å3)
Z
r calcd (g/cm3)
m (mm1)
F(000)
Crystal size(mm3)
q range for data collections( )
Index ranges
C19H14N2O4
334.32
triclinic
P -1
293 (2)
0.71073
6.673 (4)
7.983 (6)
14.623 (9)
97.92 (5)
97.93 (5)
96.44(5)
757.41 (9)
2
1.466
0.105
348
0.25 0.23 0.21
3.15e29.24
8 h ¼ 8
10 k 10
19 l 18
3214
2841
3214/0/226
1.033
0.0465, 0.1073
0.0409, 0.1030
C20H16N2O4
348.35
triclinic
P -1
293 (2)
0.71073
7.7250 (11)
8.9012 (12)
12.6079 (16)
78.161 (11)
84.633 (11)
87.462 (11)
844.5 (2)
2
1.370
0.097
364.0
0.30 0.27 0.23
3.48e28.15
10 h ¼ 9
12 k 11
17 l 16
4673
2106
4673/0/220
1.016
0.0800, 0.1423.
0.1885, 0.2274
No. of reflections collected
No. of independent reflections(Rint)
No. of data/restrains/parameters
Goodness-of-fit on F2
Ra1, wRb2 [(I > 2s(I)]
Ra1, wRb2 (all data)
257
respectively. Therefore, this strong overlapping character in the
dyes will facilitate ICT between the donor and acceptor more efficiently which is ultimately the reason for the strong fluorescence
shown by the compounds.
3.4. Thermal stability of dyes
The dyes 9f-j with high quantum yield were subjected to thermogravimetric analysis in order to acquaint their thermal stability.
Stepwise isothermal ramping up to 800 C at 10 C/min was performed in a nitrogen atmosphere. The change in weight of the
compound was measured as a function of temperature. A compound is said to be thermally stable if at a particular temperature,
approx. 95% of the composition of the compound remains stable
[35]. Fig. 6 shows the thermogravimetric analysis curves of compounds 9f-j. It was found that all the five dyes were stable up to
nearly 200 C. The TGA curve for these dyes constitute plateaus
followed by a sharp decomposition curve. The compound 9j was
found to be most thermally stable with the thermal stability at
245 C while 9f was found to be least thermally stable with thermal
stability of 162 C. The onset of decomposition of the most
3.5. X-ray crystallography
Single crystal of compounds 9f and 9j were developed by vapour
diffusion method. Chloroform and hexane were used as the binary
solvent system for crystallisation. Both the compounds crystallized
in triclinic system with the space group P-1 and the two crystals in a
unit cell. The experimental details and crystallographic data of
compounds 9f and 9j have been described in Table 3. Hydrogen
bonding interactions of 9f and 9j have been enlisted in Tables 4 and
5 respectively. Various contacts appeared in the crystal structure
during our survey for non-bonded contacts. Non-classical hydrogen
bonding (NCHB) was observed in both the molecules. The molecular structure and H-bonding interactions of compounds 9f and 9j
have been depicted in Fig. 7 and Fig. 8 respectively. CCDC 1473881
and CCDC 1473882 contains the supplementary crystallographic
data for the compounds 9j and 9f respectively. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/data_
request/cif.
The molecular structure and H-bonding interactions of compounds 9f have been depicted in Fig. 7. The O2 (Fig. 7b, Table 4) of
the coumarin ring is involved in intermolecular hydrogen bonding
with H10B of the ethoxy group attached to 7-position of coumarin
ring. In addition to this, N2 of the oxadiazole ring shows intermolecular H-bonding with the H5 of the coumarin ring. The angles
between C(10)-H (10B)/O(2) and C(5)-H (5)$$$N(2) (Fig. 7b,
Table 4) are 116.6 and 171.8 , respectively which suggests the
former one has a weak and the latter one has a very strong intermolecular hydrogen bonding.
The molecular structure and H-bonding interactions of compounds 9j have been described in Fig. 8. The O2 (Fig. 8b, Table 5) of
the coumarin ring is involved in intermolecular hydrogen bonding
with H15 present on the benzene ring attached to the 3-position of
oxadiazole ring. In addition to this, O4 of the oxadiazole ring shows
intermolecular H-bonding with the H2 of the coumarin ring. Lastly,
O4 of the ethoxy group is involved in intermolecular H-bonding
with the H5 of the coumarin ring The angles between C(5)-H (5)$$$
O(3), C(2)-H (2)$$$O(4) and C(15)-H (15)$$$O(2) (Fig. 8b, Table 5)
are 173.50 , 164.38 and 156.86 , respectively which suggests the
former two has a strong and the latter one has a very moderate
intermolecular hydrogen bonding.
4. Conclusion
In conclusion, we have developed a method to synthesise 4oxadiazolyl coumarin and synthesised a library of novel 4-(3phenyl-1,2,4-oxadiazol-5-yl)-2H-chromen-2-one. The synthesised
Table 4
Hydrogen bonds for Compounds 9f.
D-H$$$A
D-H [Å]
H$$$A [Å]
D$$$A [Å]
:D-H$$$A [ ]
Symmetry equivalent operators
C(5)-H (5)$$$N(2)
C(10)-H (10B)/O(2)
0.930
0.970
2.500
2.652
3.423
3.205
171.77
116.56
2x,y, 2 z
1 x,y,1 z
D-H$$$A
D-H [Å]
H$$$A [Å]
D$$$A [Å]
:D-H$$$A [ ]
Symmetry equivalent operators
C(5)-H (5)$$$O(3)
C(2)-H (2)$$$O(4)
C(15)-H (15)$$$O(2)
0.930
0.930
0.930
2.526
2.577
2.557
3.451
3.481
3.432
173.50
164.38
156.86
x, y, z
x,y,2 z
x, y, 2 z
Table 5
Hydrogen bonds for Compounds 9j.
258
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
Fig. 6. Thermogravimetric analysis of dyes 9f-j.
Fig. 7. a) Crystal Structure of compound 9f (ORTEP diagram) b) A view of hydrogen bonded network.
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
259
Fig. 8. a) Crystal Structure of compound 9j (ORTEP diagram); b) A view of hydrogen bonded network.
compounds absorbs in UV region and emit in the blue region
(446e468 nm) of the visible spectrum. A bathochromic shift and
hyperchromism was observed when the optical properties were
examined in more polar solvent i.e. ethanol in contrast to chloroform. A very large stokes shift and a reasonably high quantum
yields of these fluorophores makes them more attractive for further
applications like fluorescent labelling, fluorescent probes, OLED's
and so on. This endorses that the introduction of oxadiazolyl substituent at 4-position of coumarin ring results in a fluorophore
exhibiting excellent optical properties. The synthesised dyes were
found to have moderate thermal stability and thus can find diverse
applications in photoelectronics as well.
Acknowledgement
Authors acknowledge the financial assistance from the University of Delhi RC/2015/9677 grant under the strengthening R & D
Doctoral Research Programme and DST-Indo-Belgian Project (INT/
BLG/P-4/2013). A.M. is thankful to CSIR (Council of Scientific and
Industrial Research), India for providing SRF (Senior Research
Fellowship) and NAMASTE (Erasmus Mundus Action 2) for
providing doctoral exchange scholarship.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.01.050.
References
[1] Borges F, Roleira F, Milhazes N, Santana L, Uriarte E. Simple coumarins and
analogues in medicinal chemistry: occurrence, synthesis and biological activity. Curr Med Chem 2005;12:887e916.
[2] Yu T, Zhang P, Zhao Y, Zhang H, Meng J, Yu DF, et al. Synthesis and photoluminescent properties of two novel tripodal compounds containing
coumarin moieties. Spectrochim Acta Part A 2009;73:168e73.
[3] Gordo J, Avo J, Parola AJ, Lima JC, Pereira A, Branco PS. Convenient synthesis of
3-vinyl and 3-styryl coumarins. Org Lett 2011;13:5112e5.
[4] Christie RM, Lui CH. Studies of fluorescent dyes: part 2. An investigation of the
synthesis and electronic spectral properties of substituted 3-(20 -benzimidazolyl)coumarins. Dyes Pigm 2000;47:79e89.
[5] Soumya TV, Thasnim P, Bahulayan D. Step-economic and cost effective synthesis of coumarin based blue emitting fluorescent dyes. Tetrahedron Lett
2014;55:4643e7.
[6] Jonesll G, Jackson WR, Choi C, Bergmark WR. Solvent effects on emission yield
and lifetime for coumarin laser dyes. Requirements for a rotatory decay
mechanism. J Phys Chem 1985;89:294e300.
[7] Arora HK, Aggarwal AR, Singh RP. Acid dissociation constants of electronically
excited coumarins. Ind J Chem 1982;21A:844e8.
[8] Lee MT, Yen CK, Yang WP, Chen HH, Liao CH, Tsai CH. Efficient green coumarin
dopants for organic light-emitting devices. Org Lett 2004;6:1241e4.
260
A. Matta et al. / Dyes and Pigments 140 (2017) 250e260
[9] Mahadevan MK, Harishkumar HN, Masagalli JN, Kumara MN. Synthesis and
fluorescence study of some new blue light emitting 3-(1,3-benzothiazol/
benzoxazol-2-yl)-2H-chromen-2-ones. SOP Trans Org Chem 2014;1:20e30.
[10] Ammar H, Abid S, Fery-Forgues S. Synthesis and spectroscopic study of new
biscoumarin dyes based on 7-(4-methylcoumarinyl) diesters. Dyes Pigm
2008;78:1e7.
[11] Tasior M, Kim D, Singha S, Krzeszewski M, Ahn KH, Gryko DT. p-Expanded
coumarins: synthesis, optical properties and applications. J Mater Chem C
2015;3:1421e46.
[12] Zamojc K, Wiczk W, Zaborowski B, Jacewicz D, Chmurzynski L. Fluorescence
quenching of 7-amino-4-methylcoumarin by different TEMPO derivatives.
Spectrochim Acta Part A Mol Biomol Spectrosc 2015;136:1875e80.
[13] Li H, Cai L, Chen Z. Coumarin-derived fluorescent chemosensors. Adv Chem
Sensors; 121e150.
[14] Feygelman VM, Walker JK, Katrizky AR, Deda-Szafran Z. Studies of sterically
hindered oxadiazoles as potential fluorescent dopants for polymeric scintillators. Chem Scr 1989;29:241e3.
[15] Zhang Y, Wang G, Zhang J. Study on a highly selective fluorescent chemosensor for Fe3þ based on 1,3,4-oxadiazole and phosphonic acid. Sens Actuators B 2014;200:259e68.
[16] Zhang X, Li Z. Synthesis and fluorescence behavior of 2,5-diphenyl-1,3,4oxadiazole-containing bismaleimides and bissuccinimides. Front Chem Sci
Eng 2013;7:381e7.
[17] Rajesha, Naik HSB, Harish Kumar HN, Hosamani KM, Mahadevan KM. Studies
on the synthesis and fluorescent properties of long-chained 2-(5-alkyl-1, 3, 4oxadiazol-2-yl)-3H-benzo[f]chromen-3-ones. ARKIVOC 2009;2:11e9.
[18] (a) Pechmann HV, Duisberg C. Uber die verbindungen der phenole mit acetessigather. Chem Ber 1883;16:2119e28.
(b) Parmar VS, Bisht KS, Jain R, Singh S, Sharma SK, Gupta S, et al. Synthesis,
antimicrobial and antiviral activities of novel polyphenolic compounds. Ind J
Chem 1996;35B:220e32.
(c) Wang L, Xia J, Tian H, Qian C, Ma Y. Synthesis of coumarin by Yb(OTf)3
catalyzed Pechmann reaction under the solvent-free conditions. Ind J Chem
2003;42B:2097e9.
[19] (a) Hua DH, Saha S, Roche D, Maeng J, Iguchi S, Baldwin C. Asymmetric DielsAlder reactions of carboxylic ester dienophiles promoted by chiral Lewis acids.
J Org Chem 1992;57:396e9.
(b) Maheswara M, Siddaiah V, Guri LVD, Rao YK, Rao CV. A solvent-free
synthesis of coumarins via Pechmann condensation using heterogeneous
catalyst. J Mol Catal A 2006;255:49e52.
[20] Ito K, Nakajima K. Selenium dioxide oxidation of alkylcoumarins and related
methyl-substituted heteroaromatics. J Heterocycl Chem 1988;25:511e5.
[21] Ito K, Sawanobori J. 4-Diazomethyl-7-Methoxycoumarin as a new type of
stable aryldiazomethane reagent. Syn Commun 1982;12:665e71.
[22] Travis BR, Sivakumar M, Olatunji Hollist G, borhan B. Facile oxidation of aldehydes to acids and esters with oxone. Org Lett 2003;5:1031e4.
[23] Schiavello A, Cingolani E. Benzopyrones. The synthesis and properties of some
formylcoumarins. Gazz Chim Ital 1951;81:717e24.
[24] Motta CL, Sartini S, Salerno S, Simorini F, Taliani S, Marini AM, et al. Acetic acid
aldose reductase inhibitors bearing a five-membered heterocyclic core with
potent topical activity in a visual impairment rat model. J Med Chem 2008;51:
3182e8.
[25] Alfahdawi AAA. The Spectroscopic Behaviour of Rhodamine 6G in liquid and
in solid solutions. Um-Salama Sci J 2008;5:107e14.
[26] (a) Magde D, Wong R, Seybold PG. Fluorescence quantum yields and their
relation to lifetimes of rhodamine 6G and fluorescein in nine solvents:
improved absolute standards for quantum yields. Photochem Photobiol
2002;75:327e34.
b) Brouwer AM. Standards for photoluminescence quantum yield measurements in solution. Pure Appl Chem 2011;83:2213e28.
[27] (a) Sletten EM, Swager TM. Fluorofluorophores: fluorescent fluorous chemical
tools spanning the visible spectrum. J Am Chem Soc 2014;136:13574e7.
(b) Sun H, Putta A, Kloster JP, Tottempudi UK. Unexpected photostability
improvement of aromatics in polyfluorinated solvents. Chem Commun
2012;48:12085e7.
(c) Matsui M, Joglekar B, Ishigure Y, Shibata K, Muramatsu H, Murata Y.
Synthesis
of
3-Cyano-6-hydroxy-5-[2-(perfluoroalkyl)phenylazo]-2pyridones and their application for dye diffusion thermal transfer printing.
Bull Chem Soc Jpn 1993;66:1790e904.
[28] Peng K, Visser AJWG, van Hock A, Wolfs C, Sanders JC, Hemminga MA.
Analysis of time-resolved fluorescence anisotropy in lipid-protein systems. I.
Application to the lipid probe octadecyl rhodamine B in interaction with
bacteriophage M13 coat protein incorporated in phospholipid bilayers. Eur
Biophys J 1990;18:277e84.
[29] Khemakhem K, Ammara H, Abid S, El Gharbi R, Fery-Forgues S. Spectroscopic
study of 3-aryl-7-methoxy-coumarin, iminocoumarin and bis-iminocoumarin
derivatives in solution. Dyes Pigm 2013;99:594e8.
[30] Fakhfakh M, Turki H, Fery-Forgues S, El Gharbi R. The synthesis and optical
properties of novel fluorescent iminocoumarins and bis-iminocoumarins:
investigations in the series of urea derivatives. Dyes Pigm 2010;84:108e13.
[31] Delley B. An all-electron numerical method for solving the local density
functional for polyatomic molecules. J Chem Phys 1990;92:508e17.
[32] (a) Perdew JP, Wang Y. Accurate and simple analytic representation of the
electron-gas correlation energy. Phys Rev B 1992;45:13244e9.
(b) Becke AD. A multicenter numerical integration scheme for polyatomic
molecules. J Chem Phys 1988;88:2547e53.
[33] Klamt A, Schüürmann GCOSMO. A new approach to dielectric screening in
solvents with explicit expressions for the screening energy and its gradient.
J Chem Soc Perkin Trans 1993;2:799e805.
[34] Namuangruk S, Jungsuttiwong S, Kungwan N, Promarak V, Sudyoadsuk T,
Jansang B, et al. Coumarin-based donorepeacceptor organic dyes for a dyesensitized solar cell: photophysical properties and electron injection mechanism. Theor Chem Acc 2016;135:1e13.
[35] Phadtare SB, Jarag KJ, Shankarling GS. Greener protocol for one pot synthesis
of coumarin styryl dyes. Dyes Pigm 2013;97:105e12.