Synthesis and luminescence properties of 2-(2-benzoyloxyphenyl)-5-aryl-1,3, 4-oxadiazoles

Full text of DOI:10.1134/S1070428013120312

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2013, Vol. 49, No. 12, pp. 1861–1863. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.D. Beldovskaya, G.A. Dushenko, N.I. Vikrishchuk, L.D. Popov, Yu.V. Revinskii, I.E. Mikhailov, V.I. Minkin, 2013, published in
Zhurnal Organicheskoi Khimii, 2013, Vol. 49, No. 12, pp. 1876–1878.
SHORT
COMMUNICATIONS
Synthesis and Luminescence Properties
of 2-(2-Benzoyloxyphenyl)-5-aryl-1,3,4-oxadiazoles
A. D. Beldovskayaa, G. A. Dushenkob, c, N. I. Vikrishchukd, L. D. Popovd, Yu. V. Revinskiib,
I. E. Mikhailovb, d, and V. I. Minkinc
a Institute of Arid Zones, Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia
b Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia
c Institute of Physical and Organic Chemistry, Southern Federal University, Rostov-on-Don, Russia
d Southern Federal University, ul. Zorge 7, Rostov-on-Don, 344090 Russia
e-mail: natvi2004@mail.ru
Received July 31, 2013
DOI: 10.1134/S1070428013120312
Bidentate ligand systems based on 2-hydroxy-
phenyloxadiazole attract strong interest due to ap-
plication of metal complexes derived therefrom in
photo- and electroluminescent devices as blue-emitting
chromophores, as well as electron-transporting materi-
als that improve charge transfer balance in organic
light-emitting diodes (OLEDs) [1, 2]. In addition,
organic luminophores based on diphenyloxadiazoles
exhibit an anomalously high Stokes shift (ASS), which
favors increase in the light yield as a result of reduced
self-absorption and minimizes liminophore photo-
degradation [3].
phenyl)-5-phenyl-1,3,4-oxadiazole (IIa) as potential
organic luminophores (Scheme 1).
The electronic absorption and luminescence spectra
of the newly synthesized compounds were compared
with those of 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-
oxadiazole (IIa) described previously [4, 5]. In the
electronic absorption spectra of oxadiazoles IIa and
IIb the long-wave absorption maximum (π–π* transi-
tion) is located at λ 313–327 nm, which indicates the
existence in solution of benzoid structures A and B
(Scheme 2). A small blue shift (by 4–6 nm) is observed
both on introduction of electron-donating methoxy
groups into the phenyl ring in position 5 and in going
to more polar solvents. The presence in solution of
structure B as the major tautomer is confirmed by the
1H NMR spectra (CDCl3) which contained a signal at
δ 10.19 (IIa) and 10.09 ppm (IIb), corresponding to
the phenolic hydroxy proton. Alternative structure C
should be characterized by NH signal in the region
δ 6–7 ppm. Furthermore, very narrow width of the OH
With a view to extend the series of such compounds
we have synthesized previously unknown 2-(2-hy-
droxyphenyl)-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadi-
azole (IIb) whose metal complexes are expected to
display not only strong luminescence but also im-
proved solubility in organic solvents due to the pres-
ence of three methoxy groups. We have also prepared
O-benzoyl derivatives of IIb and 2-(2-hydroxy-
Scheme 1.
O
N
N
OH
O
OH
O
OH
N
Ph
O
N
H
R
R
SOCl2
PhCOCl
RCOCl
NH2
N
R
O
O
N
N
H
H
O
Ia, Ib
IIa, IIb
IIIa, IIIb
R = Ph (a), 3,4,5-(MeO)3C6H2 (b).
1861

BELDOVSKAYA et al.
1862
Scheme 2.
R
N
N
OH
O
OH
N
O
HN
N
R
R
N
O
O
A
B
C
signal may be interpreted as an evidence for the pres-
ence in solution of only one rotamer B stabilized by
strong intramolecular hydrogen bond between the phe-
nolic OH group and nitrogen atom in the oxadiazole
ring. This hydrogen bond favors photoinduced intra-
molecular proton transfer (ESIPT process) [5–7] in the
transition state from the oxygen to nitrogen atom,
yielding quinoid structure C (Scheme 2). Such quinoid
structures were detected in the luminescence spectra of
induced red shift of the emission maximum by 50–
89 nm. Thus O-benzoyl derivative of 2-(2-hydroxy-
phenyl)-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole
(compound IIIb) may be regarded as an organic
luminophore with strong emission in the visible blue
region.
N′-(2-Hydroxybenzoyl)-3,4,5-trimethoxybenzo-
hydrazide (Ib). A solution of 2.3 g (0.01 mol) of
3,4,5-trimethoxybenzoyl chloride in 20 mL of toluene
was added under stirring to a suspension of 1.52 g
(0.01 mol) of salicylic acid hydrazide in 15 mL of
water, 20 mL of a saturated aqueous solution of
Na2CO3 was then added to maintain pH ~8, and the
mixture was stirred for 4 h and left overnight. The
precipitate was filtered off, washed with water (2×
20 mL), dried under reduced pressure, and recrystal-
lized from ethanol (2×30 mL). Yield 1.81 g (72%),
colorless crystals, mp 200–202°C (from ethanol). IR
spectrum, ν, cm–1: 3485 (OH), 3314 (NH), 1625, 1609,
oxadiazoles IIa and IIb (see table), which displayed
fl
two emission bands, short-wave at λ 352–367 (IIa)
max
fl
max
and 371–411 nm (IIb) and long-wave at λ 477–487
(IIa) and 465–480 nm (IIb). On the basis of the fluo-
rescence excitation spectra, the short-wave emission
band was assigned to benzoid structure B, and the
long-wave (anomalous Stokes shift 9941–10480 cm–1),
to quinoid structure C. The low luminescence quantum
yield of oxadiazoles IIa and IIb (φ = 0.011–0.064,
see table) is likely to be determined by non-radiative
decay of the excited state according to the ESIPT
mechanism [6].
1
1584, 1542 (C=C, C=N). H NMR spectrum
(DMSO-d6), δ, ppm: 3.77 s (3H, OMe), 3.91 s (6H,
OMe), 6.91 s (2H, Harom), 7.27–7.38 m (3H, Harom),
7.97 d (1H, Harom, J = 7.6 Hz), 10.53 s (1H, OH),
10.67 s (1H, NH), 11.98 s (1H, NH). Found, %:
C 53.10; H 5.30; N 8.00. C17H18N2O6. Calculated, %:
C 52.96; H 5.20; N 8.09.
Benzoyloxyphenyl-substituted oxadiazoles IIIa and
IIIb absorb at shorter wavelengths [λmax 280–305
(IIIa), 297–302 nm (IIIb)] as compared to IIa and
3
IIb; however, only one short-wave emission band was
fl
max
observed in their luminescence spectra [λ 338–346
(IIIa), 388–433 nm (IIIb)]. Furthermore, the lumines-
cence quantum yield of IIIa was fairly poor (φ =
0.032–0.087), whereas introduction of methoxy groups
into the benzene ring on C5 not only considerably
increased the quantum yield (φ 0.11–0.51) but also
2-(5-Phenyl-1,3,4-oxadiazol-2-yl)phenol (IIa).
A mixture of 2.5 g (10 mmol) of N′-benzoyl-2-
hydroxybenzohydrazide (Ia) and 7.2 mL (0.1 mol) of
thionyl chloride was heated for 4 h under reflux. The
mixture was cooled to room temperature and poured
Electronic absorption and fluorescence spectra of compounds IIa, IIb, IIIa, and IIIb
fl
Absorption, λmax, nm (ε×10–4, l mol–1 cm–1)
Fluorescence, λ , nm (φ)a
max
Comp.
no.
toluene
acetonitrile
DMSO
toluene
acetonitrile
DMSO
IIa
327 (1.59)
324 (1.89)
305 (1.93)
302 (2.21)
322 (1.78)
318 (2.67)
300 (1.79)
297 (2.40)
313b (1.83)
319 (1.92)
280b (2.05)
301 (2.11)
360, 487 (0.011)
371, 480 (0.016)
338 (0.087)
352, 486 (0.016)
411, 465 (0.018)
346 (0.032)
367, 477b (0.021)
390, 478 (0.064)
344b (0.037)
IIb
IIIa
IIIb
388 (0.51)
429 (0.16)
433 (0.11)
a
λexcit 300 nm; φ stands for fluorescence quantum yield.
In methanol.
b
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 12 2013

SYNTHESIS AND LUMINESCENCE PROPERTIES OF 2-(2-BENZOYLOXYPHENYL)-...
1863
1
onto 50 g of crushed ice. After 2 h, the precipitate was
filtered off, washed with water (2 ×15 mL), dried
under reduced pressure, and recrystallized from 50 mL
of butan-1-ol. Yield 1.7 g (70%), colorless crystals,
mp 166–168°C (from butan-1-ol); published data [4]:
mp 165°C (from ethanol). IR spectrum, ν, cm–1: 3197
(C=C, C=N). H NMR spectrum (CDCl3), δ, ppm:
3.68 s (6H, OMe), 3.85 s (3H, OMe), 7.10 s (2H,
arom), 7.34–7.64 m (6H, Harom), 8.22–8.27 m (3H,
arom). Found, %: C 66.40; H 4.50; N 6.50.
H
H
C24H20N2O6. Calculated, %: C 66.68; H 4.63; N 6.48.
The IR spectra were recorded on a Varian Excalibur
3100 FT-IR spectrometer from thin flims. The
1H NMR spectra were recorded on a Bruker DPX-250
instrument at 250.13 MHz using the residual proton
signal of the solvent as reference. The electronic
absorption spectra were measured on a Cary Scan 100
spectrophotometer. The fluorescence spectra were
obtained on a Cary Eclipse spectrofluorimeter. All
spectral measurements were carried out at room tem-
perature. The fluorescence quantum yields were deter-
mined relative to a solution of anthracene in aceto-
nitrile [6, 8]. The solvents used for spectral measure-
ments were purified and dried according to standard
procedures [9].
1
(OH), 1623, 1591, 1539 (C=C, C=N). H NMR spec-
trum (DMSO-d6), δ, ppm: 7.02–7.04 m (2H, Harom),
3
7.45–7.61 m (4H, Harom), 7.79 d (1H, Harom, J =
8.2 Hz), 8.12–8.17 m (2H, Harom), 10.15 s (1H, OH).
Found, %: C 71.20; H 4.33; N 11.57. C14H10N2O2.
Calculated, %: C 70.58; H 4.20; N 11.76.
2-[5-(3,4,5-Trimethoxyphenyl)-1,3,4-oxadiazol-2-
yl]phenol (IIb). A mixture of 3.09 g (0.01 mol) of
hydrazide Ib and 7.2 mL (0.1 mol) of thionyl chloride
was heated for 6 h under reflux. The mixture was
cooled to room temperature, poured onto 100 g of
crushed ice, and left overnight. The precipitate was
filtered off, washed with water (3×20 mL), dried
under reduced pressure, and recrystallized from 30 mL
of butan-1-ol. Yield 2 g (61%), colorless crystals,
mp 163–165°C (from butan-1-ol). IR spectrum, ν, cm–1:
This study was performed under financial support
by the Russian Foundation for Basic Research (project
nos. 11-03-00145a, 12-03-00179a), by the Chemistry
and Materials Science Department of the Russian
Academy of Sciences (program no. OKh-1), and by the
President of the Russian Federation (project no. NSh-
927.2012.3).
1
2940 (OH), 1613, 1590, 1545 (C=C, C=N). H NMR
spectrum (CDCl3), δ, ppm: 3.93 s (3H, OMe), 3.97 s
(6H, OMe), 7.01–7.03 m (1H, Harom), 7.13 d (1H,
3
H
arom, J = 7.5 Hz), 7.34 s (2H, Harom), 7.42–7.45 m
3
(1H, Harom), 7.86 d (1Harom, J = 7.3 Hz), 10.16 s (1H,
OH). Found, %: C 62.50; H 5.20; N 8.30. C17H16N2O5.
Calculated, %: C 62.21; H 4.88; N 8.54.
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1
1610, 1583 (C=C, C=N). H NMR spectrum (CDCl3),
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Calculated, %: C 73.66; H 4.09; N 8.18.
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 12 2013