Spectral-Luminescent Properties of 2-Aryl-1,3,4-oxadiazoles

Article abstract of DOI:10.1134/S1070363218030349

Refluxing equimolar amounts of acylhydrazides with triethyl orthoformate in o-xylene yielded 2-aryl-1,3,4-oxadiazoles luminescing with high quantum yields in polar and nonpolar solvents (λm fl ax 299–349 nm, φ 0.20–0.62). The only exception was 2-(1,3,4-o

Full text of DOI:10.1134/S1070363218030349

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 3, pp. 602–604. © 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. 3, pp. 512–514.
LETTERS
TO THE EDITOR
Spectral-Luminescent Properties of 2-Aryl-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 October 26, 2017
Abstract—Refluxing equimolar amounts of acylhydrazides with triethyl orthoformate in o-xylene yielded
fl
m ax
2-aryl-1,3,4-oxadiazoles luminescing with high quantum yields in polar and nonpolar solvents (λ
299–
349 nm, φ 0.20–0.62). The only exception was 2-(1,3,4-oxadiazol-2-yl)phenol emitting intensely only in highly
fl
polar aprotic DMSO (λ 350 nm, φ 0.19).
m ax
Keywords: 2-aryl-1,3,4-oxadiazoles, luminescence, quantum luminescence yield, organic phosphors
DOI: 10.1134/S1070363218030349
1,3,4-Oxadiazoles and their derivatives exhibit high
and diverse biological activity [1], good electron-
conducting properties, intense luminescence in the
short-wave region of visible spectrum [2], and high
thermal and chemical stability, which makes them
widely applicable in various areas of agro- [3] and
medical chemistry [4], as well as in production of
highly effective organic and metal complex phosphors
[5–7]. For expanding the range of such compounds of
this class and for studying their spectral-luminescent
properties we employed herein refluxing equimolar
amounts of acylhydrazides 1a–1e with triethyl
orthoformate 2 in o-xylene to obtain 2-aryl-1,3,4-
oxadiazoles 3a–3e (Scheme 1).
luminescence was associated with emission of a short-
lived phototautomer resulted from an excited-state
intramolecular proton transfer (ESIPT) from the
phenolic OH group to the nearest heterocyclict
nitrogen atom [8, 9]. A strong intramolecular
hydrogen bond O–H···N between the ortho-phenolic
hydroxy group and the oxadiazole ring favors
photoinitiated proton transfer in oxadiazole 3b, as
1
confirmed by IR [ν(OH) 3203 cm–1] and H NMR
[δ(OH) 10.03 ppm] spectroscopic data A low total
luminescence quantum yield exhibited by oxadiazole
3b (φ 0.003, 0.043) is due to nonradiative deactivation
of its excited state by the ESIPT mechanism [10].
However, in highly polar DMSO the spectrum of
oxadiazole 3b contains only one short-wave high-
In the absorption spectra of oxadiazoles 3a–e, the
maximum of the long-wave band due to S0→S1
electronic π→π* transitions in conjugated 1,3,4-oxa-
diazole and aryl moieties occurs in the 246–308 nm
region. The luminescence spectra of oxadiazole 3b in
fl
intensity band (λ
350 nm, φ 0.19) with a normal
m ax
Stokes shift (4324 cm–1), assigned to the initial
benzenoid structure of 3b based on the fluorescence
Scheme 1.
isooctane and acetonitrile contain two bands: a short-
fl
wave band (λ
335, 357 nm, φ 0.001, 0.04) with
HC(OEt)3
m ax
O
fl
normal (3044, 4457 cm–1) and a long-wave band (λ
N
O
2
N
m ax
_3EtOH
470, 491 nm, φ 0.002, 0.003) with anomalously high
Ar
NHNH2
1a_1e
(11618, 12191 cm–1) Stokes shift.
Ar
3a_3e
Based on the fluorescence excitation spectra, the
short-wave luminescence band was assigned to the
initial benzenoid structure of 3b, and the long-wave
Ar = Ph (a), 2-HOC6H4 (b), 2-MeOC6H4 (c), 3,4,5-
(MeO)3C6H2 (d), 3,4,5-(EtO)3C6H2 (e).
602

603
SPECTRAL-LUMINESCENT PROPERTIES OF 2-ARYL-1,3,4-OXADIAZOLES
fl
excitation spectra. This is explained by strong inter-
molecular hydrogen bonding between the phenolic OH
group of oxadiazole 3b and highly polar aprotic
DMSO, inhibiting the ESIPT process. A similar
spectral behavior was previously observed in a struc-
turally close compound, 2-(5-methyl-1,3,4-oxadiazol-2-
yl)phenol [11]. Oxadiazoles 3a, 3c–3e, in which the
ESIPT is impossible because of the absence of a
mobile proton, emit intensely in a closely adjacent
spectral region (λmflax 299–349 nm, φ 0.20–0.62). Their
emission spectra exhibited a bathochromic shift of the
luminescence maximum with increasing polarity of the
solvent (by 2–20 nm) and also with accumulation of
electron-donating substituents in the aryl ring (by 8–42 nm).
(0.82 ), 318 (0.71), λ
357 (φ 0.001), 491 (φ 0.002);
m ax
acetonitrile, 212 (1.53), 249 (0.92), 260 (0.64), 304
fl
(0.39), λ
(0.87), λ
335 (φ 0.04), 470 (φ 0.003); DMSO, 304
350 (φ 0.19).
m ax
fl
m ax
2-(2-Methoxyphenyl)-1,3,4-oxadiazole (3c) was
prepared in a similar manner from o-methoxybenzoic
acid hydrazide 1c. Yield 1.28 g (79%), light yellow
crystals, mp 56–58°C (propan-2-ol) (mp 50–52°C
[14]). The IR and H and C NMR spectroscopic data
for 3c were consistent with those reported in [14]. UV
spectrum, λmax, nm (ε×10–4 L mol–1 cm–1, λexc 245 nm):
1
13
isooctane, 211 (2.14), 246 (1.12), 296 (0.62), 308
fl
m ax
(0.34), λ
326 φ 0.52); acetonitrile, 210 (2.63), 245
fl
m ax
(1.28), 296 (0.58), λ
336 (φ 0.42); DMSO, 298
fl
(0.38), 311 (0.25), λ 341 (φ 0.40).
m ax
Thus, 2-aryl-1,3,4-oxadiazoles 3a, 3c–3e exhibit
intense luminescence in the short-wavelength region of
the visible spectrum, which allows including these
compounds among widely demanded violet-emitting
organic phosphors. At the same time, oxadiazole 3b
containing a 2-hydroxyphenyl substituent in position 2
of the oxadiazole ring exhibits intense luminescence
only in highly polar aprotic DMSO, while in other
solvents its total luminescence quantum yield is low
due to intramolecular proton transfer (ESIPT).
2-(3,4,5-Trimethoxyphenyl)-1,3,4-oxadiazole (3d)
was prepared in a similar manner from 3,4,5-tri-
methoxybenzoic acid hydrazide 1d [15]. Yield 1.70 g
(75%), colorless crystals, mp 137–138°C (propan-2-ol)
(mp 138–139°C [16]). IR spectrum, ν, cm–1: 768, 837,
855, 966, 998, 1073, 1103, 1122, 1172, 1247 (C–O–C),
1339, 1368, 1509, 1558 (C=C), 1590 (C=N), 3075. 1H
NMR spectrum (CDCl3), δ, ppm: 3.90 s (3H, 4-OCH3),
3.92 s [6H, 3,5-(OCH3)2], 7.30 s (2H, HAr), 8.42 s (1H,
HHet). 13C NMR spectrum (CDCl3), δC, ppm: 56.59
(2OCH3), 60.68 (OCH3), 105.76 (2CAr), 126.18 (2CAr),
141.65 (CAr), 153.32 (CHet), 153.94 (CAr), 165.54
(CHet). UV spectrum, λmax, nm (ε×10–4 L mol–1 cm–1,
2-Phenyl-1,3,4-oxadiazole (3a). To a solution of
1.36 g (0.01 mol) of benzohydrazide 1a in 50 mL of
anhydrous o-xylene, 1.7 mL (0.01 mol) of triethyl
orthoformate 2 in 10 mL of anhydrous o-xylene was
added in small portions at room temperature with
stirring. The reaction mixture was refluxed for 4–8 h
(TLC control). The solvent was removed in a vacuum,
and the product was isolated by column chromato-
graphy on silica gel (0.063–0.200 mm, eluent ethyl
acetate-petroleum ether, 1 : 10), with the Rf 0.85
fraction being collected. Yield 1.02 g (70%), colorless
λexc 245 nm): isooctane, 211 (1.54), 275 (0.63), 293
fl
m ax
(0.34), λ
329 (φ 0.36); acetonitrile, 216 (3.54), 270
fl
m ax
(1.26), 291 (0.63), λ
343 (φ 0.35); DMSO, 276
fl
(1.03), 297 (0.54), λ 349 (φ 0.21).
m ax
2-(3,4,5-Triethoxyphenyl)-1,3,4-oxadiazole (3e)
was prepared in a similar manner from 3,4,5-
triethoxybenzoic acid hydrazide 1e [17]. Yield 1.95 g
(70%), colorless crystals, mp 133–135°C (propan-2-
ol). IR spectrum, ν, cm–1: 743, 764, 787, 831, 852, 889,
904, 956, 968, 1037, 1108, 1125, 1222, 1242 (C–O–C),
1284, 1294, 1354, 1496, 1513, 1554 (C=C), 1591
1
liquid (cf. [12]). The IR and H and 13C NMR
spectroscopic data for 3a were consistent with those
fl
reported in [12]. UV spectrum, λ
, nm (ε×
m ax
10–4 L mol–1 cm–1, λexc 245 nm): isooctane, 201 (0.92),
fl
246 (0.68), λ
246 [0.56], λ
299 (φ 0.20); acetonitrile, 201 [0.65],
301 (φ 0.25).
m ax
1
fl
m ax
(C=N), 3089. H NMR spectrum (CDCl3), δ, ppm:
1.30 t (3H, 4-CH3, J = 7.5 Hz), 1.38 t [6H, 3,5-(CH3)2,
J = 7.5 Hz], 4.07 q [6H, 3.4.5-(OCH2)3, J = 7.5 Hz],
7.19 s (2H, HAr), 8.46 s (1H, HHet). 13C NMR spectrum
(CDCl3), δC, ppm: 14.74 (2CH3), 15.52 (CH3), 64.77
(2OCH2), 68.97 (OCH2), 105.44 (2CAr), 118.14 (2CAr),
141.02 (CAr), 152.99 (CAr), 153.31 (CHet), 164.65
2-(1,3,4-Oxadiazol-2-yl)phenol (3b) was prepared
in a similar manner from salicylic acid hydrazide 1b.
Yield 1.08 g (67%), colorless crystals, mp 135–137°C
1
(propan-2-ol) (mp 136–138°C [13]). The IR and H
and 13C NMR spectroscopic data for 3b were
consistent with those reported in [13]. UV spectrum,
(CHet). UV spectrum, λmax, nm (ε×10–4 L mol–1 cm–1,
fl
λ
max, nm (ε×10–4 L mol–1 cm–1, λexc 245 nm): isooctane,
λexc 245 nm): isooctane, 221 (2.71), 274 (1.27), λ
331 (φ 0.37); dioxane, 276 (1.62), λ
m ax
fl
m ax
245 (1.57), 250 (1.84), 255 (1.60), 261 (1.37), 307
339 (φ 0.55);
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 88 No. 3 2018

604
MIKHAILOV et al.
fl
Russ. J. Gen. Chem., 2016, vol. 86, no. 2, p. 406. doi
10.1134/S1070363216020341
acetonitrile, 221 (3.82), 275 (1.73), 296 (0.58), λ
m ax
336 (φ 0.62). Found, %: C 60.58; H 6.47; N 10.53.
С14H18N2O4. Calculated, %: C 60.42; H 6.52; N 10.66.
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IR spectra were recorded on a Varian Excalibur
3100 FT-IR spectrometer in Nujol. H (250.13 MHz)
1
and 13C (62.90 MHz) NMR spectra were obtained on a
Bruker DPX-250 instrument. Absorption and fluore-
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ACKNOWLEDGMENTS
This study was financially supported by the Russian
Foundation for Basic Research (project no. 16-03-
00095a).
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