Synthesis of new derivatives of 2,4-dihydroxybenzoic acid and aldehyde and the study of the spectral-luminescent properties of their lanthanide complexes

Article abstract of DOI:10.1134/S107036321407007X

4-Acylated derivatives of 2,4-dihydroxybenzoic acid and aldehyde were synthesized. It was found that 2-hydroxy-4-[(4-decyloxy)benzoyloxy]benzaldehyde coordinates to Eu(III) ion, and when the aldehyde group is replaced by the carboxy group, a complex with Tb(III) ions is formed. The photoluminescent properties of the resulting complexes in solution were studied.

Full text of DOI:10.1134/S107036321407007X

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 7, pp. 1293–1298. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © L.G. Derkach, O.I. Teslyuk, N.S. Novikova, P.G. Doga, M.Yu. Yarkova, S.B. Meshkova, 2014, published in Zhurnal Obshchei
Khimii, 2014, Vol. 84, No. 7, pp. 1095–1101.
Synthesis of New Derivatives of 2,4-Dihydroxybenzoic Acid
and Aldehyde and the Study of the Spectral-Luminescent Properties
of Their Lanthanide Complexes
L. G. Derkach, O. I. Teslyuk, N. S. Novikova,
P. G. Doga, M. Yu. Yarkova, and S. B. Meshkova
Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine,
Lyustdorfskaya doroga 86, Odessa, 65080 Ukraine
e-mail: olgateslyuk@rambler.ru
Received November 28, 2013
Abstract—4-Acylated derivatives of 2,4-dihydroxybenzoic acid and aldehyde were synthesized. It was found
that 2-hydroxy-4-[(4-decyloxy)benzoyloxy]benzaldehyde coordinates to Eu(III) ion, and when the aldehyde
group is replaced by the carboxy group, a complex with Tb(III) ions is formed. The photoluminescent
properties of the resulting complexes in solution were studied.
Keywords: coordination compounds of lanthanides, 2,4-dihydroxybenzoic acid, benzaldehyde, luminescence
DOI: 10.1134/S107036321407007X
Coordination compounds possessing luminescent
properties attract interest for both theoretical and
practical reasons. From theoretical viewpoint, directed
synthesis of new coordination compounds that
effectively absorb light in the desired spectral region
and are capable of intense luminescence requires
establishing the relationship between the structure of
the ligands and the photoluminescent properties of
complexes thereof. As to practical reasons,
luminescent coordination compounds find application
as luminescent labels in analytical chemistry and
biology and as materials for organic electrolumine-
scent devices (organic light-emitting diodes, OLEDs) [1].
New materials are actively sought now for OLEDs.
An increasing number of relevant studies carried out
abroad demonstrate the promise of the use of electro-
luminescent devices based on lanthanide complexes.
The reason is that up to 100% power conversion
efficiencies can be achieved with triplet emitters. For
application of lanthanide complexes as emitters in
electroluminescent devices, see [6–14].
Luminescence studies on rare-earth complexes are
mostly dedicated to those with β-diketones as organic
ligands. However, a significant drawback of lanthanide
β-diketonates is their low photostability and low
thermal stability. In this respect, noticeable benefits are
offered by coordination compounds of lanthanides
with carboxylic acids, which not only possess good
UV-absorption properties and high luminescence
intensity but also outperform β-diketonates in photo-
stability and thermal stability [9, 10].
Two main classes of coordination compounds that
are applied in OLEDs are those of s, p, and d elements
exhibiting broad-band luminescence and those of
lanthanides, whose narrow-band luminescence is
associated with f–f transitions in the lanthanide ion [2].
Coordination compounds of s, p, and d metals (Mg,
Be, Al, and Zn) show resistance to moisture, oxygen,
and light and are thermally stable; also, a high degree
of purification is possible for them. Coordination
compounds of lanthanides are characterized by high
luminescence quantum yields and narrow emission
bands, and these properties make them suitable for
extensive applications in optoelectronics [3–5].
Lanthanide complexes with aldehydes have re-
ceived little research attention [15, 16].
Here, we synthesized 2-hydroxy-4-[(4-decyloxy)-
benzoyloxy]benzaldehyde (III) and 2-hydroxy-4-[(4-
decyloxy)benzoyloxy]benzoic acid (IV) (Scheme 1)
and examined how their functional groups influenced
the complexing with lanthanide ions and the photo-
luminescent properties of these complexes.
1293

1294
DERKACH et al.
Scheme 1.
O
DMF
POCl3
H
HO
OH
HO
OH
I
II
DCC, DMAP
O
H21C10
O
OH
O
6
3
5
O
H
O
OH
OH
1
12
9
Jones reagent
11
10
4
O
OH
O
2
O
7
8
H21C10O
H21C10O
III
IV
By formylation of resorcinol I according to the
known procedure from [17], 2,4-dihydroxybenzalde-
hyde II was prepared. Using the carbodiimide method
with an excess of aldehyde II, 2-hydroxy-4-[(4-decyl-
oxy)benzoyloxy]benzaldehyde III was synthesized and
further oxidized to 2-hydroxy-4-[(4-decyloxy)ben-
zoyloxy]benzoic acid IV by Jones reagent [18].
compounds III and IV as potential ligands for com-
plexing with lanthanide ions, their essential spectral
characteristics were determined. The UV spectra of
these compounds exhibit absorption bands with
maxima at λ1 = 269.2, λ2 = 319 (III), and λ = 263 nm
(IV) (Fig. 1) and high molar extinction of 42400 (III)
and 30000 L cm–1 mol–1 (IV).
The triplet-state energy level of aldehyde III
(19880 cm–1) is higher that that of the excited state of
the Eu(III) ion (17300 cm–1). The triplet state of acid
IV (20580 cm–1) is higher in energy than the excited
states of the Eu(III) (17300 cm–1) and Tb(III) ions
(20500 cm–1), and their complexing may involve an
efficient transfer of excitation energy from the ligand
to the resonance levels of the lanthanide ions.
The structure of the compounds synthesized was
proved by 1H NMR spectroscopy and mass spectrometry.
Spectral luminescent properties of 2,4-dihyd-
roxybenzoic acid derivatives and their complexes
with Ln(III) ions. To assess the complexing ability of
Aldehyde III forms an intensely luminescent
complex with Eu(III). In this case, the excitation
energy is likely to be transferred from the triplet level
5
of the ligand to the D1 energy level of the europium
ion (19000 cm–1), followed by nonradiative deactiva-
5
tion to the first excited state D0 (17300 cm–1) from
which emission occurs. The most intense band in the
luminescence spectra of europium complexes is that
5
with the maximum at 612 nm due to the D0→7F2
transition. In the case of the Tb(III) ion, the sensitized
luminescence is not observed, since the triplet state of
5
com-pound III lies in lower energy level than the D4
excited state of the Tb(III) ion (20500 cm–1), and an
energy transfer from the ligand to the resonance level
of the lanthanide is impossible.
λ, nm
Fig. 1. Absorption spectra of (1) aldehyde III and (2) acid
IV (c = 1 × 10–5 M).
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SYNTHESIS OF NEW DERIVATIVES OF 2,4-DIHYDROXYBENZOIC ACID
1295
Acid IV forms a complex with Tb(III) exhibiting
Eu–aldehyde III complex reaches 1 : 2. The Tb–acid
IV complex has the same composition.
intense luminescence with the maximum at 545 nm
5
due to the D4→7F5 transition. The triplet-state energy
Considering formation of coordinatively
unsaturated compounds in the present case, it seemed
appropriate to examine how Ilum of the complexes is
affected by surfactants and donor-active additives. Our
experiments with cationic, anionic, and nonionic
surfactants, as well as with additives containing
nitrogen and oxygen donor atoms showed that in the
presence of surfactants and additives Ilum of the Tb–
acid IV complex remains unchanged. In the case of the
complex of Eu(III) with aldehyde III, the largest
increase in Ilum is observed in the presence of organic
bases trioctylphosphine oxide and 1,10-phenanthroline
(30- and 100-fold, respectively) (Fig. 2). These
significant increases in the analytical signal are
associated with the displacement of water molecules
from the inner coordination sphere of the metal and the
formation of mixed-ligand complexes, leading to
decreased nonradiative loss of excitation energy. These
processes make it possible to increase the lumine-
scence intensity and quantum yield [12].
5
level of acid IV is higher than that of the D4 excited
state of the Tb(III) ion, so an efficient transfer of the
excitation energy to the resonance level of Tb(III) is
possible in principle. A significant energy gap between
the triplet level of the ligand and the emitting level of
the Eu(III) ion leads to a large nonradiative loss of
excitation energy, resulting in virtually zero intensity
of sensitized luminescence (Ilum).
Conditions of Eu(III) and Tb(III) complexing
with compounds III, IV. Complexing of lanthanides
with compounds III, IV is observed over a broad pH
range (4–9) with the luminescence intensity Ilum being
at a maximum in neutral medium at pH 6.9 for
compound III and 7.1 for compound IV. The medium
is brought to optimum pH by adding a 40% solution of
hexamine. At lower pH (acidic solutions) the degree of
complex formation is low, and in alkaline solutions
(pH > 9) the complexes decompose to give lanthanide
hydroxides.
The luminescence intensity Ilum of the compounds
depends on the nature of the solvent, being the highest
in water-ethanol (Eu–aldehyde III) and water-acetone
(Tb–acid IV) media containing 5 vol % of ethanol
(acetone). The introduction of organic solvents (aceto-
nitrile, dimethylformamide, and dimethyl sulfoxide)
causes a significant decrease in the analytical signal.
Due to the presence of donor oxygen (in DMSO) and
nitrogen (in DMF) atoms with which the lanthanide
ions are coordinated, these solvents exhibit ligand
properties and displace the molecules of the 2,4-
dihydroxybenzoic acid derivatives from the inner
coordination sphere of the Tb(III) ion, leading to a
decrease in Ilum.
As determined by the limiting logarithmic method,
there is one 1,10-phenanthroline (trioctylphosphine
oxide) molecule in the inner coordination sphere of the
complexes, and the component ratio in the Eu(III)–
aldehyde–1,10-phenanthroline (trioctylphosphine
oxide) mixed-ligand complexes is 1 : 2 : 1. In the
formation of mixed-ligand complexes, an additional
efficient transfer of the excitation energy from the
molecules of the organic bases to the metal ion is also
possible. The triplet-state energy level of trioctylphos-
phine oxide lies at 21980 cm–1, which enables an
5
efficient transfer of the excitation energy to the D2
energy level of europium (21500 cm–1). In the case of
1,10-phenanthroline (with the triplet energy level of
5
20650 cm–1) the energy is transferred to the D1 level
The examination of Ilum of the complexes as a func-
tion of Ln(III) and ligand concentrations showed that
the optimum concentration was 1 × 10–4 M for Ln(III)
and of 4 × 10–4 M for the ligands. The luminescence
intensity Ilum is at a maximum immediately after the
components are mixed and remains constant for 72 h.
of the Eu(III) ion (19000 cm–1), and this is followed by
5
nonradiative deactivation to the first excited state D0
(17300 cm–1).
In the presence of 1,10-phenanthroline, certain
changes in the nature of the luminescence spectrum of
the Eu(III) complex with aldehyde III were observed
Using the limiting logarithmic method, the ratio of
the components in the Ln–ligand compounds was
determined. Complexing of Eu(III) with aldehyde III
at low ligand concentrations (close to the lanthanide
concentration) leads to formation of 1 : 1 Eu–aldehyde
III complexes. With ligand concentration increasing to
the optimum level, the ratio of the components in the
5
(Fig. 2). The splitting of the band due to the D0→7F2
transition is indicative of a change in the symmetry of
the coordination polyhedron. In the case of trioctyl-
phosphine oxide this effect is not observed.
The luminescence excitation spectrum of the
complex of Eu(III) with aldehyde III exhibits a band in
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 7 2014

1296
DERKACH et al.
I
lum, arb. units
λ, nm
λ, nm
Fig. 2. Luminescence spectra of (1) Eu(III)–aldehyde III, (2) Eu(III)–aldehyde III–trioctylphosphine oxide, and (3) Eu(III)–
aldehyde III–1,10-phenanthroline complexes. λexc = 367 nm; cEu = 1 × 10–4, cIII = 4 × 10–4, and csurf = 4 × 10–4 M.
the 300–400 nm region with the maximum at 367 nm. In
the presence of trioctylphosphine oxide the nature of
the spectrum in the 200–300 nm region underwent
certain changes, with appearance of a band peaked at
λ = 292 nm, indicating the formation of mixed-ligand
complexes. In the case of 1,10-phenanthroline the
nature of the luminescence excitation spectrum
remained unchanged (Fig. 3).
320°C they are thermally stable. Above 330°C, ther-
molysis of the complexes begins. The thermal stability
of the mixed-ligand complexes of europium(III) with
1,10-phenanthroline and trioctylphosphine oxide is
determined by the temperature of the removal and
decomposition of the neutral ligand. For all complexes,
the onset of decomposition is at ~230°C and above.
Due to good luminescent characteristics combined
with high thermal stability, the compounds studied are
suitable candidates for application as emissive
materials in OLEDs.
One of the main characteristics of luminescence is
its duration which is determined by the lifetime of the
excited state (τ). The luminescence decay kinetics was
examined for the Tb(III) and Eu(III) ions in their
complexes with the 2,4-dihydroxybenzoic acid deriva-
tives (see the table). As seen from the tabulated data, in
the presence of 1,10-phenanthroline and trioctyl-
phosphine oxide, the lifetime of the excited state of the
complex of Eu(III) with aldehyde III increases 3.5 and
2 times, respectively. These data also confirm an efficient
energy transfer from the ligand to the lanthanide ion.
EXPERIMENTAL
1H NMR spectra of 5–10% solutions of the
compounds in CDCl3, DMSO-d6 were measured on a
Varian VXR-300 (operating frequency 300 MHz) and
a Bruker AVANCE DRX 500 (operating frequency
500 MHz) spectrometers with TMS as internal reference.
The mass spectra were measured on an MX-1321
spectrometer (direct admission, energy of ionizing
electrons 70 eV, ionization chamber temperature 220°C)
and on a VG 70-70EQ mass spectrometer [fast atom
bombardment, 8-keV beam of Xe atoms, m-nitro-
benzyl alcohol and poly(propylene glycol) matrix].
The IR spectra for solutions in СHCl3 were recorded
on a Specord IR-75 spectrophotometer.
To evaluate the prospects of the complexes of
Tb(III) and Eu(III) with the 2,4-dihydroxybenzoic acid
derivatives for application as OLED emissive layers,
the thermal stability ranges of these compounds were
determined. It was found that, when the temperature
increases to 100°C, the complexes undergo dehydra-
tion and with further increase in temperature to 300–
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SYNTHESIS OF NEW DERIVATIVES OF 2,4-DIHYDROXYBENZOIC ACID
lum, arb. units
1297
I
λ, nm
λ, nm
Fig. 3. Luminescence excitation spectra of (1) Eu(III)–aldehyde III, (2) Eu(III)–aldehyde III–trioctylphosphine oxide, and (3) Eu(III)–
aldehyde III–1,10-phenanthroline complexes. cEu = 1 × 10–4, cIII = 4 × 10–4, and cadd = 4 × 10–4 M.
The luminescence spectra of the Eu(III) and Tb(III)
ions and the lifetimes of the complexes thereof were
recorded in the 560–650 and 450–650 nm regions
using a Cary Eclipse Varian (Australia) spectrometer
with dual light sources (a 150-W continuous spectrum
xenon lamp and a flash xenon lamp). The lumine-
scence and excitation spectra of the ligands and com-
plexes were recorded on a Fluorolog FL 3-22
(HORIBA Jobin-Yvon, France) spectrofluorometer
with an ozone-free xenon lamp (450 W) and an
InGaAs photoresistor (DSS-IGA020L, Electro-Optical
Systems, the United States) cooled with liquid nitrogen
(for measurements in the IR region).
absorption spectra of solutions of the reactants were
recorded on a UV-2401 PC Shimadzu (Japan) spectro-
photometer. The pH measurements were carried out on
an OP-211/1 (Radelkis, Hungary) pH-meter with a
glass electrode, which was calibrated using standard
buffer solutions.
To study the complexing of the lanthanides with the
reactants synthesized, their stock solutions were
prepared. The rare-earth element chlorides were
obtained by dissolving high-purity (99.988%) oxides
in hydrochloric acid (1 : 1), whose excess was
subsequently removed by evaporation. The rare-earth
element concentration was determined by
complexometric titration. The solutions of compounds
III, IV were prepared by dissolving precisely weighed
portions of the substances in ethanol and acetone,
The triplet-state energy levels of the organic
reactants were determined from the phosphorescence
spectra of their complexes with yttrium (77 K). The
Decay kinetics and intensity of luminescence of Tb(III) and Eu(III) ions in complexes with 2,4-dihydroxybenzoic acid
derivatives
Complex
Eu–aldehyde III
τ, μs
52
I
lum, arb. units
1.2
100
30
Eu–aldehyde III–1,10-phenanthroline
Eu–aldehyde III–trioctylphosphine oxide
Tb–acid IV
181
96
639
35
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DERKACH et al.
respectively. The surfactants were purified by
recrystallization from ethanol, after which their pre-
cisely weighed portions were dissolved in bidistilled
water. A solution of 1,10-phenanthroline was prepared
by dissolving a precisely weighed portion of the
substance in bidistilled water with acidifying to pH 5
using hydrochloric acid. Solutions of trioctylphosphine
oxide and triphenylphosphine oxide for preparation of
mixed-ligand complexes Eu(III)–aldehyde III–trio-
ctylphosphine oxide (triphenylphosphine oxide) were
obtained by dissolving the appropriate weighed
portions of these substances in ethanol.
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2,4-Dihydroxybenzaldehyde II was prepared by the
procedure described in [16] from 33 g of resorcinol.
Yield: 9.6 g (23.3%).
2-Hydroxy-4-[(4-decyloxy)benzoyloxy]benzal-
dehyde III. To a solution of compound II (3.036 g,
0.022 mol) in 50 mL of anhydrous methylene chloride,
4-decyloxybenzoic acid (4.4 g, 0.011 mol) and 4-
dimethylaminopyridine (DMAP) (0.14 g, 0.0011 mol)
were added. The reaction mixture was stirred for
15 min, after which N,N'-dicyclohexylcarbodiimide
(DCC) (2.3 g, 0.011 mol) was added. The reaction
mass was stirred at room temperature for 10 h. The
resulting precipitate was filtered off and washed with
anhydrous methylene chloride, after which the solvent
was removed in a vacuum, and the residue was
recrystallized from a 10 : 1 ethanol–benzene mixture.
1
Yield 2.0 g (45.5%), mp 85°C. H NMR spectrum
(DMSO-d6), δ, ppm: 11.08 br.s (1H, OH), 10.25 s (1H,
3
3
COH), 8.07 d (2H, Н8,12, J 8.56), 7.75 d (1Н, Н6, J
3
8.30), 7.12 d (2Н, Н9,11, J 8.05), 7.04–6.77 m (2H,
3
Н3,5), 4.09 t (2H, СН2О, J 5.97), 1.86–1.63 m (2H,
СН2СН2О), 1.52–1.11 m (14H, СН2), 0.86 t (3H, СН3,
3J 6.75). Mass spectrum, m/z: 399 [M]+. Found, %: C
72.64; H 7.84. С24Н30О5. Calculated, %: C 72.36; H 7.54.
2-Hydroxy-4-[(4-decyloxy)benzoyloxy]benzoic
acid IV was obtained by the procedure described in
[17] from 0.4 g of 2-hydroxy-4-[(4-decyloxy)ben-
zoyloxy]benzaldehyde III and 2 mL of freshly
prepared Jones reagent [18]. Yield 0.302 g (72.95%),
1
mp 172.9°C. H NMR spectrum (CDCl3), δ, ppm:
11.25 br.s (1H, COOH), 10.57 s (1H, OH), 8.13 d (2Н,
3
3
Н8,12, J 7.69), 7.98 d (1H, Н6, J 7.96), 7.15–6.7 m
(4H, Н3,5,9,11), 4.05 t (2H, СН2О, 3J 5.49), 2.00–1.70 m
(2H, СН2СН2О), 1.65–1.10 m (14H, СН2), 0.89 t (3H,
3
СН3, J 6.31). Mass spectrum, m/z: 415 [M]+. Found,
%: C 69.78; H 7.44. С24Н30О6. Calculated, %: C 69.56;
H 7.25.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 7 2014