Spectral luminescent properties of 2-(2-hydroxyphenyl)-5-methyl-1,3,4-oxadiazole and its acetyl(benzoyl)oxy derivatives

Full text of DOI:10.1134/S1070428016110270

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 11, pp. 1700–1703. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © I.E. Mikhailov, Yu.M. Artyushkina, G.A. Dushenko, О.I. Mikhailova, Yu.V. Revinskii, О.N. Burov, V.I. Minkin, 2016, published in
Zhurnal Organicheskoi Khimii, 2016, Vol. 52, No. 11, pp. 1705–1708.
SHORT
COMMUNICATIONS
Spectral Luminescent Properties
of 2-(2-Hydroxyphenyl)-5-methyl-1,3,4-oxadiazole
and Its Acetyl(benzoyl)oxy Derivatives
a,b
a,c
a
a,b
I. E. Mikhailov ,* Yu. M. Artyushkina , G. A. Dushenko , О. I. Mikhailova ,
a
c
b
Yu. V. Revinskii , О. N. Burov , and V. I. Minkin
a
Southern Scientific Center of the Russian Academy of Sciences, pr. Chehova 41, Rostov-on-Don, 344006 Russia
e-mail: mikhail@ipoc.sfedu.ru
*
b
Research Institute of Physical and Organic Chemistry at the Southern Federal University, Rostov-on-Don, Russia
c
Southern Federal University, Rostov-on-Don, Russia
Received June 14, 2016
DOI: 10.1134/S1070428016110270
1
,3,4-Oxadiazoles possess high and various
π→π* electron transitions is observed in the interval
pharmaceutical and biological activity [1], high inten-
sive luminescence in the shortwave region of visual
spectrum and good electron-conducting properties [2],
they are thermally and chemically stable compounds
that facilitates their wide application in different fields
of agricultural [3] and medicinal chemistry [4], at ob-
taining highly effective organic and metal complex fluo-
rescence dyes [5, 6], and also in production of modern
electrooptical devices [7]. In this connection extending
the range of such compounds and investigation of their
spectral luminescent properties is an actual issue.
273–318 nm that indicates the existence in solutions of
benzoid structures 3–5. IR spectra and Н NMR
1
spectra of oxadiazoles 3–5 correlate with their benzoid
structures, and in case of compound 3 indicate also the
presence of a strong intramolecular hydrogen bond
between the ortho-phenol hydroxyl and the nearest
nitrogen atom of the azole ring (O–H···N). Thus in its
–
1
IR spectrum a wide band of OH group at 3187 cm
typical for intramolecular hydrogen bonds is present
that disappears at the formation of compounds 4 and 5
as a result of substitution in initial oxadiazole of the
proton of phenol hydroxyl for acetyl or benzoyl group,
and instead of the band of OH group strong bands of
At boiling equimolar amounts of salicyl acid hydra-
zide 1 and triethyl orthoacetate 2 in о-xylene 2-(2-hydro-
xyphenyl)-5-methyl-1,3,4-oxadiazole 3 was obtained,
its acylation with acetyl chloride or benzoyl chloride in the
presence of triethylamine as a base resulted in the cor-
responding acetoxy- and benzoyloxy derivatives 4 and 5.
–1
carbonyl (С=О) absorption appear: 1762 and 1735 cm
correspondingly. More than that, on the presence of the
intramolecular hydrogen bond O–H···N in oxadiazole
3
points also the shift of the stretching vibrations of its
2
3
–1
C =N bond (1605 cm ) to the low frequencies by 30–
41 cm compared to similar absorption in oxadiazoles
4 and 5 (1635, 1646 cm ) where this interaction is not
–
1
In UV absorption spectra of oxadiazoles 3–5 the
maximum of the longwave band corresponding to the
–
1
O
N
O
N
OH
O
OH
N
R
O
N
CH3
CH3
CH C(OEt)3
3
O
NHNH2
2
RCOCl, Et N
3
_
_
_
+
3
EtOH
Et HN Cl
3
1
3
4, 5
R = Me (4), Ph (5).
1
700

SPECTRAL LUMINESCENT PROPERTIES
1701
1
present. In Н NMR spectrum of oxadiazole 3 in
the stronger shift of proton signal of hydroxy group
into weak field (10.22 ppm) in this solvent than in
CDCl3.
CDCl the signal of proton of hydroxy group suffers a
3
downfield shift (9.80 ppm) that evidences the presence
of a strong intramolecular hydrogen bond between
hydrogen atom of phenol hydroxyl and nearest
nitrogen atom of azole ring. Such strong intramo-
lecular hydrogen bond in oxadiazole 3 must aid the
photoinitiated intramolecular excited proton transfer
ESIPT-process) [8, 9] from oxygen atom of phenol
group to the nearest nitrogen atom of oxadiazole that
may result in the formation of a quinoide structure.
In acyloxyoxadiazoles 4 and 5 ESIPT-process is
impossible, because there the mobile proton of phenol
group is substituted; as a result in their radiation
f l
max
spectra only one shortwave luminescence band (λ
2
96–349 nm) is present. In addition quantum yield of
(
luminescence of acetoxyoxadiazole 4 (φ 0.06–0.15) is
significantly higher than that of compound 5 (φ 0.001–
0
.003). Low quantum yield of luminescence of
Possibility of realization of ESIPT-process in
oxadiazole 3 is caused by a significant increase in the
acidity of the phenol hydroxyl and the basicity of
imine nitrogen atom in the ortho-hydroxyphenyloxa-
diazole system at its transition in the excited state, and
it is proved by its presence in oxadiazoles structurally
close to compound 3 [10, 11], and also in
styrylquinolines [12, 13]. The formation of quinoid
structure 6 as a result of ESIPT-process was detected
in the luminescence spectra of oxadiazol 3, where two
benzoyloxyoxadiazole 5 is evidently due to the lower
position of singlet and triplet n-π*-levels of benzoyl
substituent comparing to the corresponding π-π*-states
of oxadiazole fragment, as it is also observed for
benzoyl esters of о-hydroxyphenyloxadiazoles [11, 14,
15] and 8-hydroxyquinoline [16]. Consequently the
probability grows of intercombinational singlet-triplet
transfers between π-π*- and n-π*-levels of
nonconjugated oxadiazole and benzoyl fragments,
resulting in nonradiative deactivation of excited state,
which lowers the quantum yield of luminescence of
compound 5 compared to oxadiazole 4 that has higher
placed n-π*-levels of acetyl group [17].
bands were present: short-wave [isooctane (aceto-
f l
nitrile), λ
(
339 (332) nm] and longwave [isooctane
max
f l
acetonitrile), λ
490 (475) nm]. According to
max
fluorescence excitation spectra the shortwave lumines-
–
1
Hence, in highly polar aprotic DMSO 2-(2-
hydroxyphenyl)-5-methyl-1,3,4-oxadiazole 3 exhibits
an intensive luminescence in the violet region of the
visible spectrum, and in isooctane or acetonitrile due to
the ESIPT-process the intensity of this radiation
decreases significantly and an additional longwave
luminescence band appears of low intensity,
corresponding to the quinoid structure 6. The
replacement of benzoyloxy for acetoxy group in the
corresponding derivatives of oxadiazole 3 is followed
by a significant increase of luminescence quantum
yield that may be used to obtain new highly effective
organic fluorescence dyes.
cence with normal Stokes shift (2147–2242 cm )
corresponded to the benzoid structure 3, and the
longwave one with abnomally large Stokes shift (9091–
–
1
1
1309 cm ), to the quinoid form 6. The low quantum
yield of luminescence of oxadiazole 3 in mentioned
solvents (φ 0.001–0.007) is due, evidently, to the
nonradiative deactivation of its excited state by ESIPT-
mechanism [10]. However in the emission spectrum of
oxadiazole 3 in DMSO, unlike 2-(2-hydroxyphenyl)-5-
aryl-1,3,4-oxadiazoles [6] and 2-(2-hydroxyphenyl)-5-
methyl-1,2,4-oxadiazole [11], only one shortwave
f l
highly intensive band is present (λ 346 nm, φ 0.15)
max
–
1
with normal Stokes shift (2644 cm ), assigned by
fluorescence excitation spectra to benzoid structure 3.
It is explained by inhibiting the ESIPT-process due to
the formation of a strong intramolecular hydrogen
bond between phenol hydroxyl of oxadiazole 3 and
highly polar molecule of DMSO that is confirmed by
2
-(5-Methyl-1,3,4-oxadiazol-2-yl)phenol (3). To a
solution of 1.5 g (0.01 mol) of salicyl acid hydrazide 1
in 50 mL of anhydrous о-xylene while stirring was
added in small portions 1.8 mL (0.01 mol) of triethyl
orthoacetate 2. The reaction mixture was boiled for
7
h. The solvent was removed in a vacuum, the residue
was recrystallized from 2-propanol (2 × 20 mL). Yield
.40 g (81%), colorless crystals, mp 74–75°С (73–75°С
H
N
O
N
O
OH
N
O
N
1
CH3
CH3
–
1
[18]). IR spectrum (KBr), ν, cm : 3187 (OH), 1628,
1
1
605 (C=N); 1591, 1549 (С=С); 1491, 1437, 1239,
235, 1155, 1035, 833, 752, 707. Н NMR spectrum,
1
δ, ppm (CDCl ): 2.54 s (3H, CH ), 6.88 d.d (1Н, Harom,
3
6
3
3
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 11 2016

1
702
MIKHAILOV et al.
J 7.9, J 7.3 Hz), 6.99 d.d (1Н , J 8.3, J 1.0 Hz),
Сquat), 162.22 (Cheter, Сquat), 163.45 (Cheter, Сquat),
1
2
arom
1
2
–4
7
7
.32 d.d (1Нarom, J 8.3, J 7.3 Hz), 7.58 d.d (1Нarom, J1
165.19 (C=O, Сquat). UV spectrum, λmax, nm (ε × 10 ,
1
2
–
1
–1
.9, J 1.0 Hz), 9.80 s (1H, OH); (DMSO-d ): 2.57 s
L mol cm ), λ
290 nm, isooctane: 232 (4.06), 264
2
6
excit
f l
(
3H, CH ), 6.98 d.d (1Нarom, J 7.8, J 7.5 Hz), 7.06 d.d
(1.43) sh, 273 (0.95) sh; λ
296 (φ 0.003);
3
1
2
max
(
7
1Нarom, J 8.3, J 0.5 Hz), 7.43 d.d (1Н , J 8.3, J
acetonitrile: 233 (4.39), 263 (1.91) sh, 277 (0.62) sh;
1
2
arom
1
2
f l
f l
.5 Hz), 7.72 d.d (1Н , J 7.8, J 1.6 Hz), 10.22 s
λ
302 (φ 0.001); DMSO: 287 (0.17); λ
306 (φ
arom
1
2
max
max
1
3
(
(
(
1H, OH). C NMR spectrum (CDCl ), δ, ppm: 10.82
0.001). Found, %: С 68.59; 4.29;
N
17.21.
3
CH ), 108.03 (Carom, Сquat), 117.32 (Carom), 119.78
С H N O . Calculated, %: С 68.56; Н 4.32; N 17.13.
16 12 2 3
3
Carom), 126.37 (Carom), 133.38 (Carom), 157.44 (Carom
,
IR spectra were recorded on a spectrometer Varian
Сquat), 162.44 (Cheter, Сquat), 164.42 (C , Сquat). UV
spectrum, λmax, nm (ε·10 , L·mol ·cm ), λ
nm, isooctane: 247 (2.39), 252 (2.82), 263 (2.07), 306
(
0
3
1
13
heter
–1
Excalibur 3100 FT-IR. Н (250.13 МHz) and
С
–4
–1
290
excit
(
62.90 МHz) NMR spectra were recorded on a Bruker
DPX-250 instrument. As internal standard were used
signals of residual protons of the solvent. Elemental
analysis was carried out on CHN-analyzer KOVO.
Melting points were determined on a Boёtius heating
block.
f l
max
1.32), 316 (1.19) sh; λ
339 (φ 0.006), 490 (φ
.004); acetonitrile: 252 (2.07), 263 (1.46), 304 (0.89),
f l
09 (0.86) sh; λmax 332 (φ 0.007), 475 (φ 0.001);
f l
max
DMSO: 263 (1.46) sh, 303 (0.56); λ 346 (φ 0.15).
2
-(5-Methyl-1,3,4-oxadiazol-2-yl)phenyl acetate
The study was performed under a financial support
of the Russian Foundation for Basic Research (project
(
4). To a solution of 1.0 g (6 mmol) of phenol 3 in 20 mL
of anhydrous benzene while stirring was added in
succession 4 mL of triethylamine and 0.4 mL
1
6-03-00095a) and Presidential Grants Council (grant
NSh-8201.2016.3).
(
6 mmol) of acetyl chloride in 15 mL of anhydrous
benzene. The mixture was stirred at room temperature
for 2 h, then the solvent was distilled off on a water
bath. Into the flask with oily residue 20 g of crushed
ice was charged, the formed precipitate was filtered
off, washed with cold water (2 × 30 mL), dried in air,
and recrystallized from 2-propanol (2 × 30 mL). Yield
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(
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f l
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1
1
206, 1065, 765, 705. Н NMR spectrum (CDCl ), δ,
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3
arom
1
3
(
1Harom, J 5.0, J 2.5 Hz), 8.21–8.24 m (2Harom).
C
1
2
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3
3
(
(
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 11 2016