IR luminescence of neodymium(III) and ytterbium(III) ions in complexes with N-alkyl-substituted 2-aminobenzoic acids

Article abstract of DOI:10.1134/S0036023611020203

The luminescence of neodymium(III) and ytterbium(III) ions in complexes with N-alkyl-substituted 2-aminobenzoic acids has been studied. The luminescence spectra of the Nd(III) complexes show two bands with maxima at 875 and 904 and 1060 nm, and the spectra of the Yb(III) complexes show one band at 980 nm. The introduction of an additional ligand or some surfactants into the Nd(III) and Yb(III) coordination sphere leads to an increase in the luminescence intensity. A correlation between the luminescence intensity of Nd(III) and Yb(III) 2(N-alkylamino)benzoates and the length of the hydrocarbon radical bound to the nitrogen atom has been studied.

Full text of DOI:10.1134/S0036023611020203

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 2, pp. 262–266. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © S.B. Meshkova, Z.M. Topilova, N.N. Devyatykh, A.N. Gusev, V.F. Shul’gin, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 2,
pp. 301–305.
PHYSICAL METHODS
OF INVESTIGATION
IR Luminescence of Neodymium(III) and Ytterbium(III) Ions
in Complexes with NꢀAlkylꢀSubstituted 2ꢀAminobenzoic Acids
S. B. Meshkovaa, Z. M. Topilovaa, N. N. Devyatykha, A. N. Gusevb, and V. F. Shul’ginb
a Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine,
Lyustdorfskaya doroga 86, Odessa, 65080 Ukraine
b Vernadsky Taurida National University, pr. Vernadskogo 4, Simferopol, 95007 Ukraine
Received March 20, 2009
Abstract—The luminescence of neodymium(III) and ytterbium(III) ions in complexes with Nꢀalkylꢀsubstiꢀ
tuted 2ꢀaminobenzoic acids has been studied. The luminescence spectra of the Nd(III) complexes show two
bands with maxima at 875 and 904 and 1060 nm, and the spectra of the Yb(III) complexes show one band at
980 nm. The introduction of an additional ligand or some surfactants into the Nd(III) and Yb(III) coordinaꢀ
tion sphere leads to an increase in the luminescence intensity. A correlation between the luminescence intenꢀ
sity of Nd(III) and Yb(III) 2(Nꢀalkylamino)benzoates and the length of the hydrocarbon radical bound to
the nitrogen atom has been studied.
DOI: 10.1134/S0036023611020203
A powerful stimulus for synthesizing and studying lanꢀ
thanide (Ln) complexes is the possibility to use them as
the lightꢀemitting component in optical diodes, organic
lightꢀemitting diodes (OLEDs) [1–4]. Lanthanide comꢀ
pounds with organic ligands exhibit narrowꢀband lumiꢀ
nescence in both the visible (Pr, Sm, Eu, Tb, Dy, Tm) and
infrared (Nd, Yb, Er) spectral ranges. In the first case,
ligands absorbing UV light are used, whereas in the secꢀ
ond case, ligands also absorbing visible light are used [5].
The luminescence of Nd(III) and Yb(III) in complexes
EXPERIMENTAL
NꢀAlkylꢀsubstituted 2ꢀaminobenzoic acids were
obtained by oxidation of corresponding 1ꢀalkylisatines
by a 30% hydrogen peroxide solution [10]. Commerꢀ
cially available Ferak, Aldrich, or Sigma
α
, 'ꢀdipyꢀ
α
ridyl (Dipy), 1,10ꢀphenanthroline (Phen), bathoꢀ
phenanthroline (bathoPhen), trioctylphosphine oxide
(TOPO), diantipyrylmethane (DAM), and its propyl
and phenyl derivatives (DAPM and DAPhM) were
used as additional ligands.
with ꢀdiketones [6, 7]; organic dyes [8], including porꢀ
β
phyrins [9]; and some other organic ligands has been
reported. To the best of our knowledge, no information is
available on the luminescence of lanthanides in comꢀ
plexes with Nꢀalkylꢀsubstituted 2ꢀaminobenzoic acids.
In this work, we studied spectralꢀluminescent properꢀ
ties of the Nd(III) and Yb(III) complexes with Nꢀalkylꢀ
substituted 2ꢀaminobenzoic acids and searched for a
relationship between the luminescence intensity and the
ligand structure.
Initial 0.01 M solutions of Nꢀalkylꢀsubstituted
2ꢀaminobenzoic acids and additional ligands were
prepared by dissolving weighed samples of correꢀ
sponding powders in ethanol. Initial 0.1 M Nd(III)
and Yb(III) chloride solutions were prepared by disꢀ
solving their oxides (99.99%) in hydrochloric acid (1 : 1)
followed by evaporation of its excess and dissolution of
the residue in twice distilled water. The complexes
were prepared by mixing a NdCl3 (YbCl3) solution
with an Nꢀalkylꢀsubstituted 2ꢀaminobenzoic acid
solution in the presence of a buffer solution maintainꢀ
ing an optimal pH value, monitored with an OPꢀ211/1
pHꢀmeter (Hungary) equipped with an ESLꢀ43ꢀ07
glass electrode and a silver/silver chloride reference
electrode. A 4% urotropine solution was used as a
buffer solution. Initial 0.01 M solutions of surfacꢀ
tants—cetyltrimethylammonium bromide (CTA),
cetylpyridinium bromide (CPB), ethonium, Triton
Xꢀ100, and Tween 80—were prepared by dissolving
weighed samples in twice distilled water.
O
OH
NH
R
R = CH3 (L1), C2H5 (L2), nꢀC3H7 (L3),
n
ꢀC4H9 (L4),
n
ꢀC5H11 (L5)
O
OH
HO
O
H
N
H
N
The neodymium complex with 2ꢀ(
amino)benzoic acid was synthesized by the following
procedure: 9 mmol of 2ꢀ( ꢀamylamino)benzoic acid
was added to a solution of 9 mmol of NaOH in 10 mL
Nꢀamylꢀ
N
L6
262

IR LUMINESCENCE OF NEODYMIUM(III) AND YTTERBIUM(III) IONS
263
3
–1
of water. Then, 3 mmol of NdСl3 · 6H2O in 10 mL of
water was added under magnetic stirring to the resultꢀ
ing solution. The reaction mixture was magnetically
stirred for 1 h under moderate heating. The resulting
precipitate was allowed to stand under the supernatant
for 24 h and then filtered, washed with water, and dried
in air. The yield of the product was 77% of the theoretꢀ
ically possible amount. According to elemental and
thermal analysis data, the composition of the complex
corresponds to the formula Nd(L5)3 · 2H2O, where HL
E × 10 , cm
S
1
24
20
16
12
8
T
1
is 2ꢀ(Nꢀamylamino)benzoic acid.
For C63H52N3NdO8 anal. calcd. (%): C, 54.13; H,
6.51; Nd, 18.05.
4
F
3/2
2
F
5/2
Found (%): C, 54.06; H, 6.22; Nd, 18.15.
IR (
cm–1): 1616 νas(СОО–), 1396 νs(СОО–).
ν
,
max
The heteroligand neodymium complex with 2ꢀ(Nꢀ
amylamino)benzoic acid and 1,10ꢀphenanthroline
was obtained by the following procedure: to a solution
of 3 mmol of neodymium 2ꢀ(Nꢀamylamino)benzoate
4
4
I
11/2
prepared by the above method, a solution of 3 mmol of
1,10ꢀphenanthroline in 10 mL of ethanol was added
under magnetic stirring. The resulting precipitate was
allowed to stand under the supernatant for 24 h and
then filtered, washed with water, and dried in air. The
yield of the product was 74%. According to elemental
and thermal analysis data, the complex has the comꢀ
2
4
S0
F
7/2
0
I
9/2
Yb
Ligands
Nd
Fig. 1. Energy level diagram for triplet states of ligands and
emitting levels of Nd(III) and Yb(IIII).
energy transfer, it is important to know which radiaꢀ
tion wavelength should be used for optimal excitation
of the ligand. As follows from the absorption spectrum
of the Nd(III) complex (Fig. 2a), the maximum light
absorption of the compounds under consideration
occurs at 300–380 nm. This is evidence that luminesꢀ
cence can be excited by the strong line of mercury at
365 nm with subsequent energy transfer from the
ligand to the Nd(III) or Yb(III) ions.
position Nd(L5)3 · Phen · 3H2O
.
For C48H62N5NdO9 anal. calcd. (%): C, 57.83; H,
6.20; N, 7.03; Nd, 14.45.
Found (%): C, 57.69; H, 5.75; N, 7.36; Nd, 14.01.
IR (
cm–1): 1615 νas(СОО–), 1425 νs(СОО–).
ν
,
max
The IR spectra of samples recorded as pellets with
KBr on a Nicolet Nexus 470 FT spectrophotometer in
the range 400–4000 cm–1. The absorption spectra
were recorded on a PerkinꢀElmer Lambdaꢀ9 UVꢀVISꢀ
NIR spectrophotometer. The luminescence spectra
were recorded on an LOMO SDLꢀ1 diffraction specꢀ
trometer with an FEUꢀ62 photomultiplier in the range
850–1090 nm for Nd(III) and 960–1000 nm for
Yb(III). Luminescence was excited by the light of a
DRShꢀ250 mercury lamp with the use of a UFSꢀ2
The reagents under consideration form with lanꢀ
thanide(III) ions complexes insoluble in water and
soluble in organic solvents; therefore, there are two
ways to observe their luminescence: in extracts and in
suspensions of precipitates in water–organic solvent
solutions [5]. The latter variant was used in this work.
The water : organic solvent ratio was 9 : 1. At compoꢀ
nent concentrations used in this study, no precipitaꢀ
tion was observed.
light cutoff filter transmitting at
λ
= 365 nm. The
Nd(III) luminescence intensity was measured for the
luminescence maxima at 875 and 1060 nm, and the
Yb(III) luminescence intensity was measured for the
peak at 980 nm.
Studying the luminescence intensity as a function
of the pH of complex solutions showed that maximum
Nd(III) and Yb(III) luminescence is observed at pH 7.
The ratio of the components in the complexes with
anthranyl acids under consideration is Ln : L = 1 : 3.
RESULTS AND DISCUSSION
The energies of the triplet Т1 levels of the ligands To objectively verify the composition of the comꢀ
(Fig. 1) measured from the phosphorescence spectra plexes, we synthesized and study neodymium homoꢀ
(77 K) of Gd(III) 2ꢀ(Nꢀalkylamino)benzoates are and heteroligand complexes with 2ꢀ(Nꢀamylꢀ
higher than the energies of the emitting 4F3/2 (Nd(III)) amino)benzoic acid and phenanthroline as an addiꢀ
2
and F5/2 (Yb(III)) levels, so that the intramolecular tional ligand. Elemental analysis and thermogravimeꢀ
energy transfer to these levels upon excitation into the try data confirm the above metal : ligand ratio (1 : 3).
ligand is in principle possible. Inasmuch as the Ln(III) The IR spectra of the complexes show easily identified
ions in complexes with organic ligands exhibit lumiꢀ bands of symmetric and asymmetric stretching vibraꢀ
nescence owing to the intramolecular ligandꢀtoꢀLn tions of the deprotonated carboxyl group. The differꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

264
MESHKOVA et al.
(а)
875 nm
(b)
980 nm
А
Nd
Yb
1.0
904 nm
1060 nm
×
4
×
4
0.5
0
400
950
200
300
850
1090
λ
, nm
1000
1
2
3
λ
, nm
Fig. 2. (a) Absorption spectra of Nd(III) ions and (b) luminescence spectra of (
1
,
2
) Nd(III) and (3) Yb(III) in complexes with
–5
–5
–4
–3
L4. (a)
c
= 1
×
10 mol/L,
c
= 3
×
10 mol/L; (b)
c
= 2
×
10 mol/L, c
= 2 × 10 mol/L, pH 7.
Nd
L4
Nd,Yb
L4
ence between the stretching vibration frequencies maxima at 875 and 904 nm and the second band with
Δν(СОО–) = νas
the carboxyl group can be assigned both the bidentate
and bidentate/bridging function.
–
νs is within 190–220 cm–1, so that
a maximum at 1060 nm (Fig. 2b). It is worth noting
that the intensity ratio of these bands differs from that in
the spectra of the Nd(III) complexes with organic dyes
Inasmuch as the Yb(III) ion has one emitting
[8] and ꢀdiketones [10] where the band at λmax = 1060 nm
β
(
2F5/2) and one ground state (2F7/2) levels, the luminesꢀ
is the strongest one and, for the first band, the luminesꢀ
cence intensity at 904 nm is higher than that at 875 nm.
This can be explained by a different structure of the
coordination polyhedron involving the nitrogen atom
cence spectrum of its complexes shows one band with
a maximum at 980 nm. The luminescence spectrum of
the Nd(III) complexes show two bands arising from
transitions from one emitting level (4F3/2) to different
ground state sublevels (4I9/2), 4I11/2): the first band with bound to a hydrocarbon radical of different length.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

IR LUMINESCENCE OF NEODYMIUM(III) AND YTTERBIUM(III) IONS
265
Table 1. Comparison of the luminescence intensities of the Nd(III) and Yb(III) ions in complexes with Nꢀalkylꢀsubstituted
2ꢀaminobenzoic acids (cNd, Yb = 1 × 10–4 mol/L, cL = 1 × 10–3 mol/L, identical recording conditions)
Ilum, arb. units, at λ, nm
Nd(III), transition
4I9/2
Yb(III), transition
4F5/2 2F7/2
980 nm
Ligand
cm–1
ET ,
1
4F3/2
875 nm
4F3/2
4I11/2
904 nm
1060 nm
L1
L2
L3
L4
L5
L6
24390
9
17
59
115
101
7
7
14
40
70
65
5
5
10
47
72
70
3
12
25
95
550
505
10
23810
24210
24100
23750
23360
Comparison of data in Table 1 demonstrates that the spectral pattern of the complexes and positions of
the Yb(III) luminescence intensity (the 2F5/2
2F7/2
luminescence maxima.
transition) is twoꢀ to fivefold higher than for Nd(III),
which has several sublevels of the groundꢀstate level
with possible radiation energy losses at them.
The introduction of 40 vol % of organic solvents
(ethanol, dioxane, acetonitrile, DMSO) into solutions
of the complexes leads to a decrease in the Nd(III) and
Yb(III) luminescence intensity, which is caused by the
destruction of complex suspensions where intermoꢀ
lecular energy losses due to thermal collision are most
pronounced.
The introduction of cationic (CTA, CPB, ethoꢀ
nium) and nonionic (Triton Xꢀ100 and Tween 80) surꢀ
factants in concentrations below the CMC into soluꢀ
tions of the complexes somewhat increases the
Nd(III) and Yb(III) luminescence intensity. The other
surfactants either have no effect on the luminescence
intensity or quench it. Such an effect of surfactants can
be caused by a rather strong hydrophobic character of
the ligands per se, which in enhanced with an increase
in the length of the hydrocarbon radical at the nitrogen
atom.
As is known, the inner coordination sphere of
Ln(III) in complexes with the vast majority of organic
ligands contains water molecules, their number
mainly depending on the hydrophobic properties of
the ligands. In particular, in Ln(III) complexes with
βꢀdiketones containing fluoroalkyl substituents (RF),
the Ln(III) luminescence intensity increases as RF
becomes longer and, hence, the ligands become more
hydrophobic [11]. It is likely that the elongation of the
hydrocarbon radical at the nitrogen atom in the comꢀ
plexes under consideration has an analogous effect on
luminescence intensity. Figure 3 shows how the
Nd(III) and Yb(III) luminescence intensity changes
as a function of the length of the hydrocarbon radical
at the nitrogen atom. A smooth increase in the lumiꢀ
nescence intensity in the series of complexes with L1–
L3 followed by a sharp increase for the complexes with
L4 and an insignificant decrease in going to L5 is most
pronounced for the Yb(III) complexes.
I
lum
1
500
400
300
200
100
To decrease the quenching effect of water moleꢀ
cules (OH oscillators) on the Ln(III) luminescence,
an additional ligand or organic solvents are introꢀ
duced. They are coordinated to the central ion, thus
displacing water molecules from the inner coordinaꢀ
tion sphere of a complex. Table 2 summarizes the
results of studying the effect of additional ligands on
the luminescence of the Nd(III) and Yb(III) comꢀ
plexes with Nꢀalkylꢀsubstituted 2ꢀaminobenzoic
acids. As is seen, the introduction of Phen, bathoꢀ
Phen, and TOPO leads to the largest luminescence
enhancement. By the degree of Nd(III) and Yb(III)
luminescence enhancement, the additional ligands are
arranged in the series Phen > TOPO > bathoPhen >
DAPM > DAM > DAPhM and Phen > bathoPhen >
DAPM > DAM > DAPhM, TOPO, respectively. The
introduction of an additional ligand does not change
2
3
1
2
3
4
5
C(R)
n
Fig. 3. Luminescence intensity of (
1
) Yb(III) (λ = 980 nm)
and (2, 3) Nd(III) ( = ( ) 875 and (3) 1060 nm) as a funcꢀ
λ
2
tion of the length of the hydrocarbon radical at the nitroꢀ
gen atom.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

266
MESHKOVA et al.
Table 2. Change in the luminescence intensity of Nd(III) and Yb(III) in complexes with L5 upon the introduction of an addiꢀ
tional ligand (cNd, Yb = 2 × 10–5 mol/L, cL5 = 2 × 10–4 mol/L, cL = 2 × 10–4 mol/L, pH 7)
I
lum, arb. units
Increase in Ilum by n times
Additional ligand
Nd(III)
Yb(III)
980
Nd(III)
1060
Yb(III)
980
875
1060
875
–
8
50
49
6
7
7
6
25
19
5
5
3
5
173
88
45
60
–
–
–
34.6
17.8
9.0
12.0
2.4
1,10ꢀPhen
bathoPhen
DAM
DAPM
DAPhM
TOPO
6.3
6.1
0.8
0.9
0.9
7.3
4.2
3.2
0.8
0.8
0.5
4.8
12
120
58
29
24
5. N. S. Poluektov, L. I. Kononenko, N. P. Efryushina,
and S. V. Bel’tyukova, Spectrophotometric and Luminesꢀ
cence Methods of Lanthanide Determination (Naukova
Dumka, Kiev, 1989) [in Russian].
6. Z. M. Topilova, S. B. Meshkova, D. V. Bol’shoi, et al.,
Zh. Neorg. Khim. 42 (1), 99 (1997) [Russ. J. Inorg.
Chem. 42 (1), 92 (1997)].
Thus, our findings demonstrate that the Yb(III)
luminescence is almost an order of magnitude stronger
than the Nd(III) luminescence. The luminescence
intensity can be increased severalꢀfold by introducing
an additional ligand, which leads to the formation of a
heteroligand complex.
7. A. O’Riordan, E. O. Connor, S. Moynihan, et al., Thin
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