Spectral Luminescent Properties of 2-Aryl-5-(2,6-dimethoxyphenyl)-1H-1,3,4-oxadiazoles

Article abstract of DOI:10.1134/S107036321802024X

2-Aryl-5-(2,6-dimethoxyphenyl)-1H-1,3,4-oxadiazoles were synthesized by cyclization reaction of benzoylbenzohydrazides in SOCl₂, and their spectral luminescent properties were studied. The oxadiazoles containing phenyl or o-methoxyphenyl substituents in the position 2 were shown to luminesce with a high quantum yield in polar and nonpolar solvents. The replacement of these substituents by 2-hydroxyphenyl group led to oxadiazole intensely emitting only in highly polar aprotic DMSO.

Full text of DOI:10.1134/S107036321802024X

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 2, pp. 338–341. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © I.E. Mikhailov, Yu.M. Artyushkina, G.A. Dushenko, O.I. Mikhailova, Yu.V. Revinskii, V.I. Minkin, 2018, published in Zhurnal
Obshchei Khimii, 2018, Vol. 88, No. 2, pp. 342–345.
LETTERS
TO THE EDITOR
Spectral Luminescent Properties
of 2-Aryl-5-(2,6-dimethoxyphenyl)-1H-1,3,4-oxadiazoles
I. E. Mikhailova,b*, Yu. M. Artyushkinaa, G. A. Dushenkoa, O. I. Mikhailovaa,
Yu. V. Revinskiib, and V. I. Minkina
a Research Institute of Physical and Organic Chemistry, Southern Federal University,
pr. Stachki 194/2, Rostov-on-Don, 344090 Russia
*e-mail: mikhail@ipoc.sfedu.ru
b Southern Scientific Center, Federal Research Center, Russian Academy of Sciences, Rostov-on-Don, Russia
Received September 18, 2017
Abstract—2-Aryl-5-(2,6-dimethoxyphenyl)-1H-1,3,4-oxadiazoles were synthesized by cyclization reaction of
benzoylbenzohydrazides in SOCl2, and their spectral luminescent properties were studied. The oxadiazoles
containing phenyl or o-methoxyphenyl substituents in the position 2 were shown to luminesce with a high
quantum yield in polar and nonpolar solvents. The replacement of these substituents by 2-hydroxyphenyl group
led to oxadiazole intensely emitting only in highly polar aprotic DMSO.
Keywords: 2,5-diaryl-1,3,4-oxadiazoles, luminescence, luminescence quantum yield, organic phosphors
DOI: 10.1134/S107036321802024X
1,3,4-Oxadiazoles and their derivatives belong to
one of the most intensively studied classes of five-
membered nitrogen-containing heterocycles characterized
by good synthetic accessibility and high structural
variability, as well as by wide range of practical
applicability. These compounds have diverse biolo-
gical activity [1] and are extensively used in various
fields of medicinal chemistry [2] and pesticide che-
mistry [3]. Also, they possess interesting spectral
luminescent properties making them suitable for the
preparation of widely demanded organic [4–7] and
metal complex-based [8, 9] phosphors.
2,6-dimethoxybenzoic acid chloride in the presence of
triethylamine to obtain benzoylbenzohydrazides 2a–2c
whose subsequent cyclization with thionyl chloride led
to 2-aryl-5-(2,6-dimethoxyphenyl)-1H-1,3,4-oxadiazoles
3a–3c (Scheme 1).
In the absorption spectra of oxadiazoles 3a–3c the
maximum of the long-wave band due to π→π*
electron transitions is observed within the 256–316 nm
range, consistent with their benzenoid structure. The
luminescence spectra of oxadiazole 3b in isooctane,
toluene, and acetonitrile contain two bands: a short-
fl
wave band (λ
353–378 nm, φ 0.001–0.012) which
m ax
For expanding the range of compounds of this class
and for studying their spectral luminescent properties
we applied here the reaction of hydrazides 1a–1c with
has a normal Stokes shift (3019–4992 cm–1) and a long-
fl
m ax
wave band (λ
479–482 nm, φ 0.006– 0.013) which
has an anomalously large Stokes shift (10471–
Scheme 1.
R
O
OMe
R
O
O
OMe
R
N N
O
SOCl2
2,6-(MeO)2C6H3COCl
NEt3
NHNH2
NHNH
MeO
2a_2c
MeO
3a_3c
1a_1c
R = H (a), OH (b), OMe (c).
338

SPECTRAL LUMINESCENT PROPERTIES
339
10799 cm–1). Based on the fluorescence excitation
spectra the short-wave band was assigned to the initial
benzenoid structure 3, and the long-wave lumine-
scence was associated with the emission of a short-
lived phototautomer resulting from an excited-state
intramolecular proton transfer (ESIPT) from the
phenolic hydroxy group to the closest nitrogen atom of
the heterocycle [10, 11] ]. A low total luminescence
quantum yield exhibited for oxadiazole 3b (φ 0.013–
0.019) is due to nonradiative deactivation of its excited
state by the ESIPT mechanism. However, in highly
polar DMSO the spectrum of oxadiazole 3b contains
only one short-wave high-intensity band (λmflax 363 nm,
φ 0.15) with a normal Stokes shift (4098 cm–1)
assigned to the initial benzenoid structure 3b basing on
the fluorescence excitation spectra. This is explained
by strong intermolecular hydrogen bonding between
the phenolic hydroxy group of oxadiazole 3b and
highly polar aprotic DMSO inhibiting the ESIPT
process. Oxadiazoles 3a and 3c where an ESIPT
process is impossible because of the absence of a
mobile proton of the phenolic OH group emit intensely
in a closely adjacent spectral region (λmflax 352–363 nm,
φ 0.11–0.33). Similar spectral behavior was previously
observed in 1,3,4-oxadiazoles structurally similar to
compounds 3a–3c [11–15].
product was recrystallized from 2-propanol (2 × 15 mL).
Yield 2.46 g (82%), colorless crystals, mp 196–198°C.
IR spectrum, ν, cm–1: 765, 821, 1010, 1124, 1170,
1237, 1260, 1278, 1478, 1499, 1542; 1573, 1597, 1621
(C=C); 1655, 1698 (C=O); 3180 (NH). 1H NMR
spectrum (CDCl3), δ, ppm: 3.83 s (6H, 2,6-OCH3),
6.57 d (2H, HAr, J = 8.4 Hz), 7.24 t (1H, HAr, J =
8.4 Hz), 7.41–7.55 m (3H, HAr), 7.84 d.d (2H, HAr, J1 =
1.2, J2 = 7.5 Hz), 9.12 d (1H, NH, J = 3.8 Hz), 9.32 d
(1H, NH, J = 3.8 Hz). 13C NMR spectrum (DMSO-d6),
δC, ppm: 56.32 (2OCH3), 101.32 (CAr), 104.83 (2CAr),
127.92 (2CAr), 128.03 (CAr), 128.99 (2CAr), 132.32
(CAr), 133.05 (CAr), 158.13 (2CAr), 163.42 (C=O),
166.33 (C=O). Found, %: C 64.05; H 5.35; N 9.38.
C16H16N2O4. Calculated, %: C 63.99; H 5.37; N 9.33.
N'-(2-Hydroxybenzoyl)-2,6-dimethoxybenzohyd-
razide (2b) was prepared in a similar manner by the
reaction of salicylic acid hydrazide 1b with 2,6-di-
methoxybenzoic acid chloride. Yield 2.31 g (73%), color-
less crystals, mp 223–224°C (mp 222–225°C [16]). IR
1
and H and 13C NMR spectra of 2b corresponded to
those presented in [16].
N'-(2-Methoxybenzoyl)-2,6-dimethoxybenzohyd-
razide (2c) was prepared in a similar manner by the
reaction of o-methoxybenzoic acid hydrazide 1c with
2,6-dimethoxybenzoic acid chloride. Yield 2.61 g
(79%), colorless crystals, mp 150–152°C. IR spectrum,
ν, cm–1: 732, 757, 804, 1003, 1125, 1168, 1188, 1233,
1264, 1297, 1479; 1562, 1625 (C=C); 1670, 1701 (C=O);
3137 (NH). 1H NMR spectrum (CDCl3), δ, ppm: 3.82 s
(6H, 2,6-OCH3), 4.06 s (3H, OCH3), 6.55 d (2H, HAr,
J = 8.4 Hz), 6.90–7.11 m (2H, HAr), 7.29 d.d (1H, HAr,
J1 = 7.5, J2 = 7.6 Hz), 7.47 d.d (1H, HAr, J1 = 7.6, J2 =
8.2 Hz), 8.08 d.d (1H, HAr, J1 = 1.2 , J2 = 7.5 Hz), 9.65
d (1H, NH, J = 4.2 Hz), 11.17 d (1H, NH, J = 4.2 Hz).
13C NMR spectrum (CDCl3), δC, ppm: 55.93 (2OCH3),
56.01 (OCH3), 102.17 (CAr), 105.15 (2CAr), 112.47
(CAr), 115.18 (CAr), 121.63 (CAr), 128.32 (CAr), 129.17
(CAr), 132.84 (CAr), 157.93 (CAr), 159.12 (2CAr),
161.33 (C=O), 165.74 (C=O). Found, %: C 61.88; H
5.47; N 8.53. C17H18N2O5. Calculated, %: C 61.81; H
5.49; N 8.48.
Thus, the replacement of the ortho-phenolic substi-
tuent in 2-(2-hydroxyphenyl)-5-(2,6-dimethoxyphenyl)-
1H-1,3,4-oxadiazole 3b by phenyl or ortho-methoxy-
phenyl group blocks the ESIPT process in oxadiazoles
3a and 3c formed, leading to strong enhancement of
the luminescence quantum yield in nonpolar solvents
and in acetonitrile compared to oxadiazole 3b. This
allows these compounds to be included among widely
demanded oxadiazole series organic phosphors emitting
in the short-wave region of the visible spectrum.
N'-Benzoyl-2,6-dimethoxybenzohydrazide (2a).
To a solution of 1.36 g (0.01 mol) of benzoic acid hyd-
razide 1a in 20 mL of anhydrous acetonitrile, 5 mL of
triethylamine and 2.01 g (0.01 mol) of 2,6-dimethoxy-
benzoic acid chloride in 30 mL of anhydrous aceto-
nitrile were added successively with stirring. The
reaction mixture was left for 1 day at room tem-
perature and then refluxed for 3 h. The solvent was
distilled off in a vacuum to give an oily residue which
was washed with water (2 × 10 mL) and air-dried,
whereupon the resulting product was isolated by
column chromatography (eluent ethyl acetate-
petroleum ether, 1 : 2), with the Rf 0.75–0.78 fraction
being collected. After the solvent was distilled off, the
2-(2,6-Dimethoxyphenyl)-5-phenyl-1,3,4-oxadi-
azole (3a). A solution of 2.40 g (0.008 mol) of benzo-
hydrazide 2a in 50 mL of thionyl chloride was
refluxed for 6 h. The solvent was distilled off in a
vacuum, and the reaction mass was cooled to room
temperature and poured over 50 g of crushed ice. The
resulting precipitate was filtered off, and the crystalline
product was purified by column chromatography
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 2 2018

340
MIKHAILOV et al.
(eluent ethyl acetate-petroleum ether, 1: 2), with the Rf
0.75–0.80 fraction being collected. After the solvent
was distilled off the product was recrystallized from
2-propanol. Yield 1.40 g (62%), colorless crystals, mp
164–166°C. IR spectrum, ν, cm–1: 755, 787, 804, 963,
986, 1012, 1037, 1125, 1169, 1240, 1276, 1285, 1438,
1464, 1471; 1480; 1551, 1560, 1593 (C=C); 1610,
1266, 1283, 1439, 1456, 1468; 1487; 1554, 1565, 1598
(C=C); 1617, 1641 (C=N). UV spectrum, λmax, nm
[ε×10–4 L mol–1 cm–1, λexc 300 nm]: isooctane, 257
fl
[1.16], 285 [0.81], 297 [0.75], λ
(φ) 352 (0.16) ;
m ax
fl
acetonitrile, 254 [1.37], 287 [0.88], 297 [0.79], λ
m ax
(φ) 357 (0.15); DMSO, 288 [1.16], 300 [1.05], λmflax (φ)
363 (0.33). 1 H NMR spectrum (CDCl3), δ, ppm: 3.79 s
(6H, 2,6-OCH3), 3.95 s (3H, OCH3), 6.61 d (2H, HAr,
J = 8.4 Hz), 6.99–7.11 m (2H, HAr), 7.36–7.53 m (2H,
HAr), 7.95 d.d (1H, HAr, J1 = 1.8, J2 = 7.9 Hz). 13C
NMR spectrum (CDCl3), δC, ppm: 56.09 (2OCH3),
56.10 (OCH3), 103.17 (CAr), 103.90 (2CAr), 111.98
(CAr), 113.61 (CAr), 120.65 (CAr), 130.61 (CAr), 132.71
(CAr), 133.24 (CAr), 157.93 (CHet), 159.16 (CHet),
159.98 (2CAr), 163.86 (CAr). Found, %: C 65.44; H
5.13; N 9.04. C17H16N2O4. Calculated, %: C 65.38; H
5.16; N 8.97.
1632 (C=N). UV spectrum, λmax, nm [ε×10–4 L mol–1 cm–1,
fl
m ax
λ
exc 300 nm]: isooctane, 205 [2.92], 290 [0.95], λ
(φ) 358 (0.11); acetonitrile, 257 [1.36], 318 [0.83],
fl
m ax
fl
m ax
λ
(φ) 359 (0.12); DMSO, 315 [1.18], λ
(φ) 357
1
(0.23). H NMR spectrum (CDCl3), δ, ppm: 3.79 s
(6H, 2,6-OCH3), 6.62 d (2H, HAr, J = 8.4 Hz), 7.43 t
(1H, HAr, J = 8.4 Hz), 7.45–7.52 m (3H, HAr), 8.03–
8.17 m (2H, HAr). 13C NMR spectrum (CDCl3), δC,
ppm: 56.10 (2OCH3), 102.89 (CAr), 103.87 (2CAr),
124.46 (CAr), 127.00 (2CAr), 128.93 (3CAr), 131.40
(CHet), 133.56 (CHet), 159.96 (2CAr), 165.27 (CAr). Found,
%: C 68.15; H 4.99; N 9.97. C16H14N2O3. Calculated,
%: C 68.08; N 5.00; N 9.92.
IR spectra were recorded on a Varian Excalibur
1
3100 FT-IR spectrometer in Nujol. H (250.13 MHz)
and 13C (62.90 MHz) NMR spectra were obtained on a
Bruker DPX-250 instrument. The absorption and
fluorescence spectra were measured on a Cary Scan
100 spectrophotometer and a Cary Eclipse fluore-
scence spectrophotometer, respectively. The fluore-
scence quantum yields were determined relative to
anthracene in acetonitrile [17].
2-[5-(2,6-Dimethoxyphenyl)-1,3,4-oxadiazol-2-yl]-
phenol (3b) was prepared in a similar manner by the
cyclization of benzohydrazide 2b in thionyl chloride.
Yield 1.17 g (49%), colorless crystals, mp 93–95°C.
IR spectrum, ν, cm–1: 758, 831, 974, 1003, 1035, 1056,
1141, 1158, 1240, 1272, 1285, 1300, 1402, 1453,
1471, 1490; 1544, 1559 (C=C); 1598, 1629 (C=N);
3201 (OH). UV spectrum, λmax, nm [ε×10–4 L mol–1 cm–1,
ACKNOWLEDGMENTS
λexc 300 nm]: isooctane, 257 [1.85], 263 [1.77], 307
fl
This study was financially supported by the
Ministry of Education and Science of the Russian Fede-
ration in the framework of the governmental contract
(project no. 4.979.2017/PCh).
[1.25], 318 [1.12], λ
toluene, 309 [1.18], 317 [1.08], λ
482 (0.013); acetonitrile, 304 [0.74], 319 [0.62], λ
(φ) 364 (0.001), 481 (0.012);
m ax
fl
(φ) 378 (0.006),
m ax
fl
m ax
(φ) 353 (0.012), 479 (0.006); DMSO, 304 [0.74], 316
fl
m ax
[0.62], λ
(φ) 363 (0.15). 1H NMR spectrum
REFERENCES
(CDCl3), δ, ppm: 3.81 s (6H, 2,6-OCH3), 6.65 d (2H,
HAr, J = 8.4 Hz), 6.97 t (1H, HAr, J = 8.4 Hz), 7.11 d.d
(1H, HAr, J1 = 1.1, J2 = 7.6 Hz), 7.36–7.48 m (2H, HAr),
7.78 d.d (1H, HAr, J1 = 1.8, J2 = 7.9 Hz), 10.31 (1H,
OH). 13C NMR spectrum (CDCl3), δC, ppm: 56.14
(2OCH3), 103.91 (2CAr), 107.88 (CAr), 108.62 (CAr),
117.47 (CAr), 119.72 (CAr), 126.79 (CAr), 133.34 (CAr),
133.60 (CAr), 157.60 (CHet), 158.90 (CHet), 160.02
(2CAr), 164.89 (CAr). Found, %: C 64.46; H 4.75; N
9.44. C16H14N2O4. Calculated, %: C 64.42; H 4.73; N 9.39.
1. Boström, J., Hogner, A., Llinas, A., Wellner, E., and
Plowright, A.T., J. Med. Chem., 2012, vol. 55, no. 5,
p. 1817. doi 10.1021/jm2013248
2. Farshori, N.N., Banday, M.R., Ahmad, A., Khan, A.U.,
and Rauf, A., Bioorg. Med. Chem. Lett., 2010, vol. 20,
no. 6, p. 1933. doi 10.1016/j.bmcl.2010.01.126
3. Jha, K.K., Samad, A., Kumar, Ya., Shaharyar, M.,
Khosa, R.L., Jain, Ja., Kumar, V., and Singh, P., Eur. J.
Med. Chem., 2010, vol. 45, no. 11, p. 4963. doi
10.1016/j.ejmech.2010.08.003
2-(2,6-Dimethoxyphenyl)-5-(2-methoxyphenyl)-
1,3,4-oxadiazole (3c) was prepared in a similar
manner by the cyclization of benzohydrazide 2c in
thionyl chloride. Yield 1.72 g (69%), colorless crys-
tals, mp 138–140°C. IR spectrum, ν, cm–1: 753, 771,
789, 807, 965, 982, 1010, 1043, 1128, 1166, 1244,
4. Yang, H., Mu, J., Chen, X., Feng, L., Jia, J., and Wang, J.,
Dyes Pigm., 2011, vol. 91, no. 3, p. 446. doi 10.1016/
j.dyepig.2011.03.035
5. Beldovskaya, A.D., Dushenko, G.A., Vikrishchuk, N.I.,
Popov, L.D., Revinskii, Yu.V., Mikhailov, I.E., and
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 2 2018

SPECTRAL LUMINESCENT PROPERTIES
Minkin, V.I., Russ. J. Org. Chem., 2013, vol. 49, no. 12,
341
13:5<253::AID-POC238>3.0.CO;2-D
p. 1861. doi 10.1134/S1070428013120312
12. Mikhailov, I.E., Artyushkina, Yu.M., Burov, O.N.,
Dushenko, G.A., Revinskii, Yu.V., and Minkin, V.I.,
Russ. J. Gen. Chem., 2016, vol. 86, no. 2, p. 406. doi
10.1134/S1070363216020341
6. Mikhailov, I.E., Popov, L.D., Vikrishchuk, N.I.,
Beldovskaya, A.D., Revinskii, Yu.V., Dushenko, G.A.,
and Minkin, V.I., Russ. J. Gen. Chem., 2015, vol. 85,
no. 1, p. 203. doi 10.1134/S1070363215010363
7. Mikhailov, I.E., Vikrishchuk, N.I., Popov, L.D.,
Beldovskaya, A.D., Revinskii, Yu.V., Dushenko, G.A.,
and Minkin, V.I., RF Patent 2568640, Byul. Izobret.,
2015, no. 32.
8. Beldovskaya, A.D., Dushenko, G.A., Vikrishchuk, N.I.,
Popov, L.D., Revinskii, Yu.V., Mikhailov, I.E., and
Minkin, V.I., Russ. J. Gen. Chem., 2014, vol. 84, no. 1,
p. 171. doi 10.1134/S1070363214010290
13. Mikhailov, I.E., Artyushkina, Yu.M., Dushenko, G.A.,
Mikhailova, О.I., Revinskii, Yu.V., Burov, О.N., and
Minkin, V.I., Russ. J. Org. Chem., 2016, vol. 52, no. 11,
p. 1700. doi 10.1134/S1070428016110270
14. Artyushkina, Yu.M., Mikhailov, I.E., Burov, O.N.,
Dushenko, G.A., Mikhailova, O.I., Revinskii, Yu.V.,
and Minkin, V.I., Russ. J. Gen. Chem., 2016, vol. 86,
no. 12, p. 2702. doi 10.1134/S1070363216120239
15. Artyushkina, Yu.M., Mikhailov, I.E., Dushenko, G.A.,
Mikhailova, O.I., Revinskii, Yu.V., Burov, O.N., and
Kurbatov, S.V., Russ. J. Gen. Chem., 2017, vol. 53,
no. 5, p. 808. doi 10.1134/S1070428017050281
9. Mikhailov, I.E., Popov, L.D., Vikrishchuk, N.I.,
Beldovskaya, A.D., Dushenko, G.A., Revinskii, Yu.V.,
Kurbatov, S.V., and Minkin, V.I., RF Patent 2610527,
Byul. Izobret., 2017, no. 5.
16. Liu, X., Liu, W., Lee, K., Park, M., Ri, H.-Ch., Kim, G.H.,
and Lah, M.S., Dalton Trans., 2008, no. 46, p. 6579. doi
10.1039/b810711d
10. Serdyuk, O.V., Evseenko, I.V., Dushenko, G.A.,
Revinskii, Yu.V., and Mikhailov, I.E., Russ. J. Org.
Chem., 2012, vol. 48, no. 1, p. 78. doi 10.1134/
S1070428012010113
11. Doroshenko, A.O., Posokhov, E.A., Verezubova, A.A.,
and Ptyagina, L.M., J. Phys. Org. Chem., 2000, vol. 13,
no. 5, p. 253. doi 10.1002/1099-1395(200005)
17. Beldovskaya, A.D., Dushenko, G.A., Vikrishchuk, N.I.,
Popov, L.D., Revinskii, Y.V., and Mikhailov, I.E., Russ.
J. Gen. Chem., 2013, vol. 83, no. 11, p. 2075. doi
10.1134/S1070363213110200
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