Spectral-Luminescent Properties of 2-(2,6-Difluorophenyl)-5-phenyl-[2-acetyl(benzoyl)oxyphenyl]-1,3,4-oxadiazoles

Article abstract of DOI:10.1134/S107036321810033X

2-(2,6-Difluorophenyl)-5-phenyl(2-hydroxyphenyl)-1,3,4-oxadiazoles were synthesized by cyclization of N-(2,6-difluorobenzoil) derivatives of benzohydazide or salicylhydrazide in thionyl chloride. When acylating 2-[5-(2,6-difluorophenyl)-1,3,4-oxadiazole-2

Full text of DOI:10.1134/S107036321810033X

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 10, pp. 2219–2222. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © I.E. Mikhailov, Yu.M. Artuyshkina, G.A. Dushenko, Yu.V. Revinskii, V.I. Minkin, 2018, published in Zhurnal Obshchei Khimii,
2018, Vol. 88, No. 10, pp. 1735–1738.
LETTERS
TO THE EDITOR
Spectral-Luminescent Properties
of 2-(2,6-Difluorophenyl)-5-phenyl-[2-acetyl(benzoyl)oxyphenyl]-
1,3,4-oxadiazoles
I. E. Mikhailova*, Yu. M. Artuyshkinaa, G. A. Dushenkoa, Yu. V. Revinskiib, and V. I. Minkina
a Southern Federal University, Research Institute of Physical and Organic Chemistry,
pr. Stachki 194/2, Rostov-on-Don, 344090 Russia
*e-mail: mikhail@ipoc.sfedu.ru
b “Southern Scientific Center of Russian Academy of Sciences” Federal Research Center, Rostov-on-Don, Russia
Received July 19, 2018
Abstract―2-(2,6-Difluorophenyl)-5-phenyl(2-hydroxyphenyl)-1,3,4-oxadiazoles were synthesized by
cyclization of N-(2,6-difluorobenzoil) derivatives of benzohydazide or salicylhydrazide in thionyl chloride.
When acylating 2-[5-(2,6-difluorophenyl)-1,3,4-oxadiazole-2-yl]phenol by acetyl or benzoyl chloride
corresponding acetate and benzoate were obtained. Spectral-luminescent properties of the synthesized
compounds were studied.
Keywords: 2,5-diaryl-1,3,4-oxadiazoles, luminescence, luminescence quantum yield, organic luminophors
DOI: 10.1134/S107036321810033X
2,5-Diaryl-1,3,4-oxadiazoles and their derivatives
are actively used in production of high-demand high-
performance organic [1–4] and metal-complex [5–7]
luminophors intensively emitting in the short-wave
region of visible spectrum, highly selective fluorescent
chemosensors [8], and also electron-transport and
emissive materials for organic light-emitting diodes
(OLEDs) [9]. The expansion of the set of such com-
pounds and the study of their physical properties is an
urgent task.
properties were also studied. In the electron absorption
spectra of compounds 3a, 4a, and 4b maxima of long-
wavelength bands caused by the S0→S1 π→π* electron
transitions in 1,3,4-oxadiazole and aryl fragments
conjugated with each other lie in the range of 268–
280 nm, which points to the existence in solutions of
compounds in benzenoid forms 3 and 4. There is no
clear effect of solvent polarity on the position of
absorption and emission bands of these compounds,
whereas luminescence quantum yields depend on the
polarity of the medium. In the spectra of compounds 3
and 4a quantum yields increase with increasing solvent
polarity, and in the spectrum of compound 4b an
inverse relationship is observed. In the UV region of
the emission spectra of compounds 3a, 4a, and 4b
there is a short-wave luminescence band (λflmax = 333–
343 nm) with Stokes shift of 6226–7676 cm–1,
assigned to benzenoid structures 3 and 4 on the basis
of the fluorescence excitation spectra. Luminescence
quantum yields of oxadiazole 3a (φ = 0.56–0.98) and
acetyloxyoxadiazole 4a (φ = 0.49–0.95) are much
higher than that of benzoyloxyoxadiazole 4b (φ =
0.009–0.027). The low quantum yield of benzoyloxy-
oxadiazole 4b seems to result from a lower position of
For this purpose the benzoylation of benzo-
hydrazide 1a or salicylhydrazide 1b by 2,6-difluoro-
benzoic acid chloride was carried out using triethyl-
amine as a base that yielded benzoylbenzohydazides
2a and 2b. Their subsequent cyclization by thionyl
chloride resulted in 2-(2,6-difluorophenyl)-5-phenyl(2-
hydroxyphenyl)-1,3,4-oxadiazoles 3a and 3b. The
acylation of oxadiazole 3b by acetyl or benzoyl
chloride acetate led to the formation of acetate 4a and
benzoate 4b (Scheme 1).
Structures of compounds 2a, 3a, 4a, and 4b were
1
established using elemental analysis, H NMR, 13C
NMR, and IR spectroscopy; their spectral-luminescent
2219

2220
MIKHAILOV et al.
Scheme 1.
F
R1
O
O
R1
H
2,6-F2C6H3COCl
N
NHNH2
N
NEt3
H
F
O
1а, 1b
2а, 2b
SOCl2
F
R2C(O)O
F
N
O
R1
N
O
N
N
R1 = OH
R2COCl
F
NEt3
F
3а, 3b
4а, 4b
R1 = H (а), OH (b); R2 = Me (a), Ph (b).
singlet and triplet n–π* levels of the benzoyl sub-
stituent compared to the corresponding π–π* states of
oxadiazole fragment not conjugated with it, as for
example is the case in benzoyl esters of 2-(oxadiazol-5-
yl)phenol [10-12] and quinolin-8-ol [13]. Because of
this the probability of intercombination singlet-triplet
transitions between π–π* and n–π* levels of non-
conjugated among themselves oxadiazole and benzoyl
fragments increases. It promotes radiationless deactiva
-tion of the excited state that reduces luminescence
quantum yield of benzoate 4b compared to acetate 4a,
in which n–π* levels of acetyl groups lie higher [14].
solvent was removed in a vacuum, and 50 g of crushed
ice was added to the oily residue. The resulting
precipitate was filtered off, washed with water (2×
10 mL), dried in air, and recrystallized from 2-propanol
(2×20 mL). Yield 1.08 g (72%), colorless crystals, mp
206–208°C. IR spectrum, ν, cm–1: 760, 812, 840, 1008,
1009, 1236, 1267, 1279, 1420, 1447, 1482, 1502,
1541, 1546, 1559; 1574, 1577, 1608, 1617, 1646
(C=C); 1670, 1685 (C=O); 3061, 3198 (NH). 1H NMR
spectrum (DMSO-d6), δ, ppm: 7.24–7.31 t (2H, HAr,
J = 7.7 Hz), 7.55–7.65 m (4H, HAr), 7.97 d (2H, HAr,
J = 6.1 Hz), 10.55 s (1H, NH), 10.75 s (1H, NH).
13C NMR spectrum (DMSO-d6), δС, ppm: 112.34,
112.66, 127.92 (2С), 127.98 (2С), 128.98 (2С), 132.40 t
(2С, JCF = 12.6 Hz), 157.63 d (JCF = 8.2 Hz), 159.58,
161.59 (JCF = 8.2 Hz), 165.70 (C=O), 166.32 (C=O).
Found, %: С 60.47; Н 3.58; F 13.95; N 10.43.
С14H10F2N2O2. Calculated, %: С 60.87; Н 3.65; F
13.76; N 10.14.
Thus, 2-(2.6-difluorophenyl)-5-phenyl-1,3,4-oxa-
diazole 3a luminescents intensively in the UV region
of the visible spectrum (λflmax = 337–340 nm, φ = 0.56–
0.98). Introduction of acetyloxy group 3a in the ortho-
position of the oxadiazole 5-phenyl ring slightly
reduces the luminescence quantum yield of acetate 4a
(λfl = 333–343 nm, φ = 0.49–0.95) and the replace-
max
ment of the acetyloxy group in oxadiazole 4a by the
2,6-Difluoro-N'-(2-hydroxybenzoyl)benzohyd-
razide (2b) was obtained similarly by the reaction of
salicylic acid hydrazide 1b with 2,6-difluorobenzoyl
chloride. Yield 1.21 g (83%), colorless crystals, mp
195–197°C (propane-2-ol), (mp 194–196°C [15]). IR,
1H NMR, and 13C NMR spectra of compound 2b are
consistent with those described in [15].
benzoyloxy group results in a significant decrease in
the luminescence quantum yield of benzoate 4b (λfl
337–341 nm, φ = 0.009–0.027).
=
max
N'-Benzoyl-2.6-difluorobenzohydrazide (2a). To
a solution of 0.68 g (0.005 mol) of benzohydrazide 1a
in 50 mL of anhydrous benzene 2 mL of triethylamine
and 0.88 g (0.005 mol) of 2,6-difluorobenzoyl chloride
in 20 mL of anhydrous benzene were successively
added at stirring. The reaction mixture was stirred for
2 h at room temperature and then boiled for 6 h. The
2-(2,6-Difluorophenyl)-5-phenyl-1,3,4-oxadiazole
(3а). A solution of 1.38 g (0.005 mol) of benzohyd-
razide 2a in 15 mL of SOCl2 was boiled for 5 h. After
distilling off thionyl chloride in a vacuum and cooling
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SPECTRAL-LUMINESCENT PROPERTIES
2221
the reaction mass to room temperature 30 g of crushed
ice was added to it. A formed precipitate was filtered
off and purified using column chromatography on
silica gel (0.063–0.200 mm, eluent ethyl acetate–
petroleum ether, 1 : 5), collecting the fraction with Rf
0.60–0.65. After distilling off the solvent the residue
was recrystallized from 2-propanol (2×15 mL). Yield
0.87 g (67%), colorless crystals, mp 206–208°C. IR
spectrum, ν, cm–1: 792, 927, 968, 995, 1014, 1083,
1240, 1268, 1283, 1363, 1450; 1485, 1548 (C=C); 1591,
1630 (C=N). UV spectrum, λmax, nm (ε×10–4, L mol–1 cm–1,
2.43 s (3H, СН3), 7.05‒7.16 m (2Н, HAr), 7.24 d.d (1H,
HAr, J1 = 7.5, J2 = 7.7 Hz), 7.40 d.d (1H, HAr, J1 = 7.7,
J2 = 8.1 Hz), 7.45‒7.63 m (2Н, HAr), 8.14 d.d (1H,
HAr, J1 = 2.5, J2 = 8.1 Hz). 13C NMR spectrum
(CDCl3), δ, ppm: 21.16, 112.27, 112.32, 117.25, 124.29,
126.60, 129.64, 133.21 t (2С, JCF = 10.8 Hz), 148.92
(2С), 156.64, 158.69 d (JCF = 5.7 Hz), 162.53, 162.83 d
(JCF = 5.7 Hz), 169.77 (C=O). Found, %: С 60.53; Н
3.04; F 11.95; N 8.74. С16H10F2N2O3. Calculated, %: С
60.76; Н 3.19; F 12.01; N 8.86.
2-[5-(2,6-Difluorophenyl)-1,3,4-oxadiazol-2-yl]-
phenyl benzoate (4b) was obtained similarly by
benzoylation of oxadiazole 3b with benzoyl chloride in
the presence of triethylamine. Yield 1.38 g (73%),
colorless crystals, mp 104–106°C (propan-2-ol). IR
spectrum, ν, cm–1: 752, 766, 798, 830, 972, 1007,
1076, 1125, 1136, 1153, 1165, 1239, 1273, 1285,
1404, 1450, 1541; 1560, 1593 (C=C), 1627 (C=N),
1729 (C=O). UV spectrum, λmax, nm (ε×10–4,
L mol–1 cm–1, λexc 270 nm): isooctane, λmax 230 (3.22),
λexc 270 nm): isooctane, λmax 273 (2.39), λfl 338, φ
max
0.81; toluene 275 (2.54), λfl
340, φ 0.56; aceto-
max
1
nitrile, λmax 271 (2.58), λfl
337, φ 0.98. Н NMR
max
spectrum (CDCl3), δ, ppm: 6.99–7.14 m (2H, HAr),
7.43–7.60 m (4H, HAr), 8.12d.d (2H, HAr, J1 = 2.5, J2 =
13
7.5 Hz). C NMR spectrum (CDCl3), δC, ppm: 112.27,
112.64, 126.95 (2C), 127.17 (2C), 128.61 (2C), 132.43 t
(2C, JCF = 10.7 Hz), 158.68 d (JCF = 7.9 Hz), 159.87,
162.89 d (JCF = 7.9 Hz), 164.61 (C=O), 165.12 (C=O).
Found, %: C 65.32; H 3.08; F 14.91; N 10.83.
C14H8F2N2O. Calculated, %: C 65.12; H 3.12; F 14.71;
N 10.85.
269 (2.07), λfl
339, φ 0.027; toluene 275 (2.11),
max
λfl
341, φ 0.018; acetonitrile, λmax 231 (3.56), 268
max
(2.59), λflmax 337, φ 0.009. 1Н NMR spectrum (CDCl3),
δ, ppm: 7.01‒7.12 m (2H, HAr), 7.37‒7.75 m (7Н,
HAr), 8.24‒8.33 m (3H, HAr). 13C NMR spectrum
(CDCl3), δ, ppm: 112.31, 112.39, 117.12, 117.82,
124.71 (2C), 127.63, 128.91 (2C), 129.66, 129.78,
130.64, 133.87 t (2С, JCF = 10.1 Hz), 147.62 (2С),
157.74, 159.37 d (JCF = 5.9 Hz), 161.54, 162.77 d
(JCF = 5.9 Hz), 165.74 (C=O). Found, %: С 66.51; Н
3.14; F 10.25; N 7.34. С21H12F2N2O3. Calculated, %: С
66.67; Н 3.20; F 10.04; N 7.40.
2-[5-(2,6-Difluorophenyl)-1,3,4-oxadiazol-2-yl]-
phenol (3b) was obtained similarly by cyclization of
benzohydazide 2b in thionyl chloride. Yield 0.63 g
(57%), light-brown crystals, mp 119–121°C (2-pro-
1
panol), (mp 118–120°C [15]). IR, UV, H NMR, and
13C NMR spectra of compounds 3b are consistent with
those described in [15].
2-[5-(2,6-Difluorophenyl)-1,3,4-oxadiazol-2-yl]-
phenyl acetate (4a). To a solution of 1.37 g (0.005 mol)
of oxadiazole 3b and 1.05 mL (0.0075 mol) of
triethylamine in 30 mL of anhydrous benzene 0.43 mL
(0.006 mol) of acetyl chloride in 15 mL of anhydrous
benzene was added dropwise within 15 min at stirring.
The reaction mixture was stirred at room temperature
for 2 h and left for a day. After removal of the solvent
in a vacuum 30 g of crushed ice was added to the oily
residue, a formed precipitate was filtered off, dried in
air, and recrystallized from 2-propan-ol (2×10 mL).
Yield 1.25 g (79%), colorless crystals, mp 104–106°C
(propan-2-ol). IR spectrum, ν, cm–1: 751, 775, 795,
827, 878, 919, 987, 1011, 1038, 1059, 1071, 1130,
1161, 1223, 1258, 1279, 1370, 1439, 1490; 1540, 1576
(C=C); 1590, 1625 (C=N), 1759 (C=O). UV spectrum,
The IR spectra were taken on a Varian Excalibur
1
3100 FT-IR spectrometer in mineral oil. The Н NMR
(250.13 MHz) and 13C (62.90 MHz) spectra were
recorded on a Bruker DPX-250 instrument in CDCl3
and DMSO-d6 solutions. Signals of residual solvent
protons were used as the internal standard. The
absorption and fluorescence spectra were measured on
a Cary Scan 100 spectrophotometer and a Cary Eclipse
spectrofluorimeter, respectively. Fluorescence quantum
yields were determined relative to a naphthalene
acetonitrile solution [16]. Melting points were
determined on a Boёtius apparatus.
ACKNOWLEDGMENTS
max, nm (ε×10–4, L mol–1 cm–1, λexc 270 nm): iso-
This work was financially supported by the
Ministry of education and science of the Russian
Federation in the framework of the State contract
(project no. 4.979.2017/PCh).
λ
octane, λmax, 270 (2.03), λflmax 333, φ 0.79; toluene 280
(2.41), λfl 343, φ 0.49; acetonitrile, λmax 269 (2.15),
max
λflmax 334, φ 0.95. 1Н NMR spectrum (CDCl3), δ, ppm:
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MIKHAILOV et al.
8. Lin, L., Wang, D., Chen, S.-H., Wang, D.-J., and
CONFLICT OF INTERESTS
Yin, G.-D., Spectrochim. Acta (A), 2017, vol. 174,
p. 272. doi 10.1016/j.saa.2016.11.053
No conflict of interests was declared by the authors.
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