Effect of Aliphatic Chain Length in the Ligand on Photophysical Properties and Thin Films Morphology of the Europium Complexes

Article abstract of DOI:10.1134/S0036023618020092

Chelating ligands based on polydentate diamides of 2,2'-bipyridyl-6,6'-dicarboxylic acid with a high affinity for lanthanide ions have been synthesized. The effect of the size of lipophilic aliphatic substituents in the ligand on the photophysical charact

Full text of DOI:10.1134/S0036023618020092

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2018, Vol. 63, No. 2, pp. 219–228. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © A.V. Kharcheva, N.E. Borisova, A.V. Ivanov, M.D. Reshetova, T.P. Kaminskaya, V.V. Popov, V.I. Yuzhakov, S.V. Patsaeva, 2018, published in Zhurnal
Neorganicheskoi Khimii, 2018, Vol. 63, No. 2, pp. 207–216.
PHYSICAL METHODS
OF INVESTIGATION
Effect of Aliphatic Chain Length in the Ligand on Photophysical
Properties and Thin Films Morphology of the Europium Complexes
A. V. Kharchevaa, b, *, N. E. Borisovaa, **, A. V. Ivanova, M. D. Reshetovaa,
T. P. Kaminskayab, V. V. Popovb, V. I. Yuzhakovb, and S. V. Patsaevab
aFaculty of Chemistry, Moscow State University, Moscow, 119991 Russia
bFaculty of Physics, Moscow State University, Moscow, 119991 Russia
*e-mail: harcheva.anastasiya@physics.msu.ru
**e-mail: borisova.nataliya@gmail.com
Received December 22, 2016
Abstract⎯Chelating ligands based on polydentate diamides of 2,2'-bipyridyl-6,6'-dicarboxylic acid with a
high affinity for lanthanide ions have been synthesized. The effect of the size of lipophilic aliphatic substitu-
ents in the ligand on the photophysical characteristics of europium complexes in acetonitrile solutions and in
the solid state, as well as on the morphology of thin films obtained by the spin-coating method, was studied.
The external and internal luminescence quantum yields have been measured, the luminescence lifetimes at
300 and 77 K were determined, and the sensitization efficiency values for europium complexes were calcu-
lated. The phosphorescence of gadolinium(III) complexes was used to determine the energy difference
between the triplet level of the ligand and the resonance level of europium.
DOI: 10.1134/S0036023618020092
Complexes of rare-earth ions with organic ligands sary for understanding the processes of sensitized
are widely used in optoelectronics, bioimaging, and luminescence, as well as energy transfer in biological
molecular sensing. They are perfect model systems to systems. The energy transfer from excited atoms, ions,
study electronic excitation energy transfer.
or molecules is a widespread phenomenon, especially
if the concentration of interacting species and the life-
time of the excited states are rather large.
Electronic excitation energy transfer in liquids and
solids is one of the fundamental problems of modern
spectroscopy and condensed matter physics [1, 2].
The most promising systems for studying energy
Since energy transfer is an intermediate stage between transfer are rare earth complexes with organic ligands,
the primary excitation of electrons and the processes as well as crystals activated by rare earth ions [3–11].
in which their energy is consumed, the study of mech- Materials doped with rare-earth ions, due to the high
anisms electron energy transfer, in particular, their luminescence intensity, rather long lifetimes, and the
efficiency, is essential for understanding the effect of great resistance to photobleaching, are widely used as
different types of radiation on matter, especially in promising laser emitters, wavelength converters for
those cases where not only the fact of radiation energy lighting and radiation detectors, phosphors, and lumi-
absorption, but also the subsequent phenomena aris- nescence standards and in sensors [12–15], as well as
ing in the absorbing medium, is important. Lumines- for biomedical applications, data storage systems, and
cence, radiation physics and radiochemistry, photo- solar technology [16–26].
synthesis, biochemistry and bioenergetics represent an
incomplete list of the areas of modern science in which
the electronic excitation energy transfer is of funda-
mental importance.
The luminescence of the rare earth complexes is
characteristic and is related to f–f transitions in the
metal ion [5]. Exchange of the central ion can change
the luminescence emission wavelength throughout the
In recent years, along with the study of solutions, entire electromagnetic spectrum from UV to IR [8,
studies of excitation energy transfer in crystals have 20]. However, since f–f transitions are theoretically
been extensively developed. This is explained not only forbidden, low molar extinction coefficients do not
by the progress in experimental and theoretical solid provide an effective luminescence yield upon direct
state physics, but also by numerous applications of excitation of rare earth ions. To solve this problem,
light-emitting diodes, scintillators, phosphors, and coordination compounds of lanthanides are used,
other materials used in electronics. Studying migra- which enables indirect excitation of the emission of a
tion processes of electronic excitation energy is neces- rare earth ion. Such complexes consist of two parts: a
219

220
KHARCHEVA et al.
H2C CH3 H2C CH2
H3C CH2
H2C CH2
CH2
H2C
CH2
N
N
N
N
N
O
N
Et
N
N
H3C
Et
O
O
N
O
N
Et
O
O
N
O
N
Et
Eu
Eu
O
O
O
O
O
O
O
O
O
O
N
O
O
O
O
N
O
Bipy · 4Et · Eu
Bipy · 4Hex · Eu
Fig. 1. Structural formulas of the synthesized complexes Bipy ⋅ 4Hex ⋅ Eu and Bipy ⋅ 4Et ⋅ Eu.
rare earth ion, which determines the spectral range of Bipy ⋅ 4Hex ⋅ Eu) differ in the length of aliphatic chains
at the amide group of carboxamides.
the emitted light, and an organic ligand, which
absorbs light in the UV range and, then, transfers
energy to the rare earth ion. Since the efficiency of
photoexcitation depends on the nature of the ligand
and ion type, a purposeful change in the composition
and structure of the complexes allows one to modify
their optical properties and achieve high quantum
yields of luminescence. Nevertheless, to date, no com-
pound has been found that simultaneously would pos-
sess high stability, good solubility, intense lumines-
cence with a high quantum yield, and long lumines-
cence in the millisecond range.
The compounds are at the same time readily sol-
uble in organic solvents and highly luminescent. Eu
complexes with diamides of 2,2'-bipyridin-6,6'-
dicarboxylic acid can be readily soluble in organic
solvents and exhibit photoluminescence both in
solutions and as the films deposited on the solid sup-
port. First results of investigation of these complexes
were described in [28]. In this study we performed
the examination of spectroscopic characteristics for
complexes in solutions, thin films, as well as in
microcrystal powders. We measured luminescence
quantum yield and luminescence lifetime for Eu
complexes at room temperature and at 77 K, the
internal quantum yield and constant of luminescence
sensitization. Phosphorescence spectra of Gd(III)
complexes with the same chelating ligands measured
at 77 K were used to determine the position of the
triplet level of the ligands and to calculate the energy
difference between the triplet state of the ligand and
the resonance level of Eu. We discuss the transfer of
electronic excitation energy in studied Eu complexes
in connection with the nature and lipophilicity of
chelating ligands.
The use of rare earth compounds often imposes
particular constrains on the properties of the com-
plexes. Namely, high smoothness of films is required
for optoelectronic applications [27]. Such thin films
should be produced from volatile or highly soluble rare
earth complexes in two ways: by sputtering and spin
coating. The second method requires highly soluble
compounds capable of forming smooth thin films on
silica (or ITO) substrates. For the better solubility of
compounds, the organic molecule must contain a
larger amount of lipophilic moieties. However, the
long aliphatic chains commonly used to increase the
solubility of molecules in organic solvents can affect
the photophysical properties of the rare earth com-
plexes. Therefore, this study focuses on the effect of
the aliphatic chain length in an organic ligand (an Et
group, containing two carbon atoms, and an Hex
group, containing six carbon atoms) on the photo-
physical properties of europium(III) complexes. A
comparative study of two europium complexes with
organic ligands containing ethyl or hexyl groups—Bipy ⋅
4Et ⋅ Eu and Bipy ⋅ 4Hex ⋅ Eu (Fig. 1)—have been per-
formed.
The synthesis of new phosphors with improved
characteristics is important for the development of
nanosized conjugates for biomedical research,
improvement of nanoparticle imaging methods, pro-
duction of efficient and sustainable dyes, design of
organic light-emitting diodes, and other scientific and
technical fields.
EXPERIMENTAL
Synthesis of Gadolinium and Europium Complexes
The synthesis of N,N'-diethyl- and N,N'-di(4-hex-
ylphenyl)amides of 2,2'-bipyridyl-6,6'-dicarboxylic
acid has been described elsewhere [29].
To improve the coordination properties and photo-
physical characteristics of complexes, chelating
ligands based on 2,2'-bipyridyl with two carboxamide
side groups for better binding to the metal ion and
their complexes with europium and gadolinium were
Synthesis of 2,2'-bipyridyl-6,6'-dicarboxylic acid
N,N'-diethyl-N,N'-di(4-ethylphenyl)amide. 2,2'-Bipyr-
idyl-6,6'-dicarboxylic acid (1 g, 4.1 mmol) and 0.5 mL of
synthesized. Europium complexes (Bipy ⋅ 4Et ⋅ Eu and dimethylformamide were heated under reflux in 5 mL
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EFFECT OF ALIPHATIC CHAIN LENGTH
221
Bipy ⋅ 4Et ⋅ Gd: Yield, 62%. IR (KBr, ν, cm–1): 1581
(ν(С=О)), 1610 (ν(С=N)). MALDI-TOF: m/z 788
(100%) [M-NO3]+; 761 (8%) [M-2NO3 + OH +
of thionyl chloride for 2.5 h. The excess of thionyl
chloride was distilled off, and the remaining acid chlo-
ride was dried in a vacuum of a water jet pump and dis-
solved in 10 mL of dry tetrahydrofuran. The resulting
solution was dropwise added to a mixture of N-ethyl-
4-ethylaniline (1.28 g, 8.6 mmol) and 5 mL of triethyl-
amine in 15 mL of dry tetrahydrofuran. The resulting
mixture was stirred for 4–5 h at 40–50°C and kept
stirred overnight. An equal volume of water was added
to the reaction mixture and the organic layer was sep-
arated. The aqueous fraction was extracted twice with
diethyl ether. The combined organic fractions were
washed with water and dried over anhydrous sodium
sulfate. Then, the solvent was removed in vacuo, and
the residue was treated with hexane. The resulting pale
brownish precipitate was filtered off and air dried to
give 1.58 g (76%) of the target diamide.
1Н NMR (CDCl3): 7.59 (m, 6H), 6.97 (m, 8H),
3,97 (q, 4H, J = 6.50), 2.48 (q, 4H, J = 7.25), 1.23 (t,
6H, J = 6.50), 1.06 (t, 6H, J = 7.25). 13С NMR
(CDCl3): 170.48, 156.86, 145.51, 139.55, 131.00,
130.40, 126.97, 124.30, 48.41, 31.07, 18.04, 15.73.
MALDI-TOF: m/z 507 (100% [M]+). For
С32Н34N4О2 anal. calcd. (%): C, 75.86; H, 6.76; N,
11.06. Found (%): C, 75.64; H, 6.47; N, 10.79.
H2O]+.
For С32Н34GdN7O11 anal. calcd. (%): C, 45.22; H,
4.03; N, 11.54. Found (%): C, 45.69; H, 3.88; N,
11.02.
Spectral measurements. Spectral measurements
were carried out for solutions of the Bipy ⋅ 4Et ⋅ Eu and
Bipy ⋅ 4Hex ⋅ Eu complexes in acetonitrile with con-
centrations from 1 × 10–6 to 1 × 10–5 mol/L. The films
of europium complexes were obtained from solutions
by the spin coating method at various rotation speeds
(up to 2000 rpm) using transparent quartz plates and
poly(ethylene terephthalate) plates coated with a thin
layer of indium-tin oxide (ITO) with a specific resis-
tance of 60 Ω/cm2.
Absorption spectra of the complexes in solutions
and films deposited on transparent substrates were
recorded on a Hitachi U-1900 spectrophotometer.
The luminescence emission and excitation spectra
were measured on a Hitachi F-7000 spectrometer in
90°-geometry for solutions of the complexes and in
reflection geometry for thin films on solid plates.
The absolute external quantum yields of lumines-
cence of europium complexes in acetonitrile solutions
were calculated by the reference dye method [31] from
the absorbance and the integrated luminescence
intensity. As a reference, a solution of rhodamine 6G
was selected.
The phosphorescence spectra of gadolinium
complexes with the same chelating ligands were
measured at 77 K. From these spectra, the energy of
the triplet level of the ligands was determined and the
energy difference between the triplet level of the
ligand and the resonance level of the europium ion
Synthesis of europium and gadolinium complexes
with dicarboxylic acid diamides (general procedure)
[30]. A mixture of equimolar amounts of the ligand
and gadolinium or europium trinitrate hexahydrate in
dry acetonitrile (25 mL per 100 mg of the ligand) was
heated under reflux for 7–8 h and slowly cooled to
room temperature (the mixture was left overnight for
complete precipitation of the reaction products).
A white or pale brown precipitate was filtered off
under vacuum, washed with a small amount of cold
acetonitrile, and air dried.
Bipy ⋅ 4Hex ⋅ Eu. Yield, 69%. IR (KBr, ν, cm–1): was calculated.
1588 (ν(С=О)), 1615 (ν(С=N)). MALDI-TOF: m/z
AFM measurements. The morphology of the films
was investigated by atomic force microscopy (AFM)
using a SMENA-A scanning probe microscope, the
Solver platform (NT-MDT company, Russia). Scan-
ning was carried out with DCP-11 silicon cantilevers
with a resonance frequency of 178.6 kHz by the tap-
ping method [32, 33]. AFM images were processed
using NOVA software for 2D and 3D AFM images.
1
816 (100%) [M-NO3]+. Н NMR (CD3CN): 7.14 (d,
2Н), 7.02 (br s, 2Н), 5.67 (t, 1Н); 4.40 (br s, 1Н), 3.63
(d, 1Н), 2.60 (br s,2Н), 2.49 (t, 2Н), 1.49 (tt, 2Н), 1.23
(br s, 10Н), 0.84 (t, 3Н), –0.28 (br s, 3Н).
For С40Н50EuN7O11 anal. calcd. (%): C, 50.21; H,
5.27; N, 10.25. Found (%): C, 50.95; H, 5.91; N,
10.11.
Bipy ⋅ 4Et ⋅ Eu. Yield, 76%. IR (KBr, ν, cm–1): 1584
(ν(С=О)), 1612 (ν(С=N)). MALDI-TOF: m/z 783
(100%) [M-NO3]+.
For С32Н34EuN7O11 anal. calcd. (%): C, 45.50; H,
4.06; N, 11.61. Found (%): C, 45.97; H, 4.76; N, 11.23.
Bipy ⋅ 4Hex ⋅ Gd. Yield, 73%. IR (KBr, ν, cm–1):
1582 (ν(С=О)), 1610 (ν(С=N)). MALDI-TOF: m/z
898 (100%) [M-NO3]+.
RESULTS AND DISCUSSION
Synthesis of Rare Earth Complexes
Ligands containing ethyl and hexyl groups in the
amide moiety were obtained by acylation of the corre-
sponding secondary amines with 2,2'-bipyridyl-6,6'-
dicarboxylic acid dichloride [29] (Fig. 2).
The interaction of synthesized ligands with euro-
pium and gadolinium nitrates in boiling acetonitrile
For С40Н50GdN7O11 anal. calcd. (%): C, 49.93; H,
5.24; N, 10.19. Found (%): C, 49.84; H, 5.57; N, 9.79. leads to the formation of the corresponding Bipy ⋅ Ln
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222
KHARCHEVA et al.
R
Et
H
N
O
N
THF, Et3N
Et
O
N
O
N
+
Cl
N
N
O
R
Cl
N
Et
R
R = CH2CH3 or (CH2)5CH3
Fig. 2. Scheme of the synthesis of the ligands via acylation of secondary amines with 2,2'-bipyridyl-6,6'-dicarboxylic acid dichlo-
ride.
R
R
R
Et
N
O
.
Ln(NO3)3 6H2O
CH3CN, Δ
N
N
N
N
N
Et
N
N
O
Ln = Eu, Gd
Et
R
O
O
N
Ln
O
N
Et
O
O
O
O
O
N
O
O
R
Ln
O
(CH2)5CH3
CH2CH3
(CH2)5CH3
CH2CH3
Bipy · 4Hex · Eu
Bipy · 4Et · Eu
Bipy · 4Hex · Cd
Bipy · 4Et · Cd
Eu
Eu
Gd
Gd
Fig. 3. Synthesis of lanthanide complexes with the ligands.
Absorption and Luminescence Spectra
0.4
0.3
0.2
0.1
0
Bipy · 4Et · Eu
Bipy · 4Hex · Eu
of Europium Complexes in Acetonitrile Solutions
In the absorption spectra of solutions of europium
complexes in acetonitrile, absorption is observed in
the UV range with a maximum at λ = 324 nm, charac-
teristic of the ligand (Fig. 4). The absorbance values
for solutions with different concentrations obey the
Bouguer–Lambert–Beer law.
The luminescence emission spectra of the com-
plexes show europium lines corresponding to the
5D0 → 7FJ transitions (J = 0, 1, 2, 3, 4) in the red region
of the spectrum (Fig. 5). The main europium peak is
located at λ = 615–618 nm, and a typical lifetime τobs
was ~ 1 ms (Table 1). The ratios of the integrated
intensities of different peaks (calculated as the area
under the fluorescence emission curve) differ for com-
plexes with ligands of different lipophilicity. The ratio
of the total intensity to the intensity of the line at 592 nm,
as well as the line intensities at 618 and 592 nm, are
given in Table. 1. The absolute quantum yield of lumi-
nescence of acetonitrile solutions of the europium
complexes was 18% for Bipy ⋅ 4Et ⋅ Eu and 27% for
Bipy ⋅ 4Hex ⋅ Eu.
200
250
300
Wavelength, nm
350
400
Fig. 4. Absorption spectra of europium complex solutions
in acetonitrile with a concentration of 1×10 mol/L.
–5
complexes with a yield of 50–90% depending on the
structure of the ligand and the metal ion, as we
reported for related compounds [30] (Fig. 3).
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EFFECT OF ALIPHATIC CHAIN LENGTH
223
50
40
30
20
10
0
100
Bipy · 4Et · Eu
Bipy · 4Hex · Eu
1 Bipy · 4Hex · Eu (C = 10–5 mol/L)
2 Bipy · 4Hex · Eu (C = 5 × 10–5 mol/L)
3 Bipy · 4Et · Eu (C = 10–5 mol/L)
λex = 270 nm
λem = 618 nm
4 Bipy · 4Et · Eu (C = 5 × 10–5 mol/L)
75
1
50
2
3
25
4
0
250
300
350
400
450
550
600
650
700
Wavelength, nm
Wavelength, nm
Fig. 6. Luminescence excitation spectra (λ = 618 nm) of
em
Fig. 5. Luminescence emission spectra (λ = 270 nm) of
acetonitrile solutions of europium complexes with concen-
ex
–5
–5
the europium complexes in acetonitrile with a concentra-
trations of 1 × 10 mol/L (solid line) and 5 × 10 mol/L
(dashed line).
–5
tion of 1 × 10 mol/L.
The excitation spectra of the europium complexes
at low concentrations correspond to the main absorp-
tion band of the chelating ligand; at higher concentra-
tions, changes due to reabsorption in the excitation
spectra are observed (Fig. 6).
europium complex films deposited on a quartz sub-
strate is 2 nm red shifted compared with that in the
spectra of solutions, which occurs at λ = 326 nm. The
position of the maximum of the excitation spectrum at
λ = 618 nm depends on the method of film prepara-
tion. For example, for films deposited without spin-
ning, the maximum in the excitation spectrum is
observed in a shorter wavelength region of the spec-
trum (322 nm for Bipy ⋅ 4Hex ⋅ Eu and 329 nm for Bipy ⋅
4Et ⋅ Eu) than for films deposited by the spin coating
method (333 nm for both ligands, regardless of the
rotation speed).
Absorption and Luminescence Spectra
of Europium Complexes in the Solid State
(Thin Films and Powders)
The studied europium complexes have a high lumi-
nescence intensity both in solutions and in thin films
deposited on different substrates. The films of euro-
pium complexes deposited on plates give rise to lumi- cence excitation spectrum is explained by the decrease
nescence analogous to that for solutions with a main in the film thickness and in the size of aggregates of the
emission peak at λ = 618 nm and a lifetime of about complexes with an increase in the spin-coating rota-
1.5–1.7 ms (Table 1). The UV absorption band of tion speed. This effect is more pronounced for the
The difference in the peak position in the lumines-
Table 1. Luminescence characteristics of europium complexes in acetonitrile solutions, thin films deposited onto a solid
substrate, and as powders
Solutions
(300 K)
Thin films
(300 K)
Powders
(300 K)
Powders
(77 K)
Ligand
I618/I592
Bipy ⋅ 4Et Bipy ⋅ 4Hex Bipy ⋅ 4Et Bipy ⋅ 4Hex Bipy ⋅ 4Et Bipy ⋅ 4Hex Bipy ⋅ 4Et Bipy ⋅ 4Hex
6.6
10.0
18
6.1
9.7
27
6.8
9.6
n/a
7.9
10.6
n/a
5.6
8.2
n/a
6.8
9.4
n/a
6.2
9.6
n/a
7.3
10.7
n/a
Itot/I592
External quantum
yield, ФL, %
τ
obs, ms
0.98
2.79
35.1
1.19
2.87
41.4
1.70
1.91
41.4
1.46
1.91
88.8
1.52
2.25
67.6
1.57
1.94
80.8
1.70
1.90
89.3
1.46
1.71
85.2
τR, ms
Internal quantum
yield, ФEu, %
ηsens, %
51
65
n/a
n/a
n/a
n/a
n/a
n/a
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KHARCHEVA et al.
1000
4000
3000
2000
1000
0
Bipy · 4Et · Eu
Bipy · 4Hex · Eu
Bipy · 4Et · Eu
λex = 270 nm
λex = 270 nm
Bipy · 4Hex · Eu
800
600
400
200
0
550
600
650
700
550
600
650
700
Wavelength, nm
Wavelength, nm
Fig. 8. Luminescence emission spectra (λ = 270 nm) of
Fig. 7. Luminescence emission spectra (λ = 270 nm) of
ex
ex
europium complexes in the solid state at 77 K.
europium complexes in the solid state at 300 K.
2000
1 Bipy · 4Hex · Eu (300 K)
2 Bipy · 4Et · Eu (300 K)
3 Bipy · 4Hex · Eu (77 K)
4 Bipy · 4Et · Eu (77 K)
1
1.00
Bipy · 4Et · Gd
Bipy · 4Hex · Gd
1500
1000
500
0
0.75
λem = 618 nm
2
0.50
0.25
0
3
4
250 300 350 400 450 500 550 600
Wavelength, nm
350
400
450
500
550
600
Wavelength, nm
Fig. 9. Luminescence excitation spectra (λ = 618 nm) of
em
europium complexes in the solid state at 300 K (solid line)
and 77 K (dashed line).
Fig. 10. Normalized phosphorescence spectra of gadolin-
ium complexes in the solid state at 77 K.
Bipy ⋅ 4Hex ⋅ Eu complex, which contains a more lipo-
philic hexyl substituent.
AFM Images and Morphology of Thin Films
of Europium Complexes
For the powders of the complexes, the lumines-
cence emission and excitation spectra measured at
temperatures of 300 and 77 K, are shown in Figs. 7–9.
The results of studying the microstructure of the
films showed that the film morphology of the euro-
pium complexes with dicarboxylic acid diamides on a
substrate depends both on the type of substituent in
the organic ligand and on the rotation speed of the
spin coater. Films of the compounds with ethyl (Bipy ⋅
Phosphorescence of Gd(III) Complexes at 77 K
The phosphorescence spectra of the Gd(III) com- 4Hex ⋅ Eu) or hexyl (Bipy ⋅ 4Hex ⋅ Eu) substituents,
plexes with the chelating ligands measured at 77 K and
normalized to the maximum intensity shown in Fig. 10.
obtained by a dripping method without rotating the
substrate, have a structure in the form of long and
thick interwoven filaments (Fig. 11). The deposition of
the same compounds onto the plate surface by spin-coat-
ing at different rotation speeds (1000 and 2000 rpm) gives
a continuous film with a honeycomb-type structure in
The positions of long-wavelength maxima deter-
mining the triplet level energy of the ligand were found
in the deconvoluted spectra, and the energy differ-
ences between the triplet level of the ligand and the
resonance level of europium were calculated (Table 2). the case of Bipy ⋅ 4Et ⋅ Eu (Fig. 12) or a structure with
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EFFECT OF ALIPHATIC CHAIN LENGTH
225
Table 2. Long-wavelength components of phosphorescence
spectra, triplet level energies of the ligands E(Tr), and the dif-
ference in energy between the triplet level of the ligand and the
resonance level of the europium ion ΔE(Tr – 5D0)
chaotically located rod-shaped particles in the case of
Bipy ⋅ 4Hex ⋅ Eu (Fig. 13).
The pore diameter and particle size at the film
surface depend on the spin-coater rotation speed
during the production of the film. Bipy ⋅ 4Et ⋅ Eu
films deposited onto the substrate at a rotation speed
of 1000 rpm take the form of a mesh grid with regu-
larly located pores of about 0.5 μm in diameter and of
5 nm in depth. When the rotation speed is increased
to 2000 rpm, more uniform films with sparse “craters”
of the same size as the pores in the mesh grid were
obtained. For Bipy ⋅ 4Hex ⋅ Eu films obtained at 1000 rpm,
the length of the rods is approximately 0.8 μm. When
the rotation speed is increased to 2000 rpm, the rods
are 0.3 μm in length and 2–3 nm in height.
Figures 11 and 12 show that the smoothness of
films formed by the Bipy ⋅ 4Et ⋅ Eu and Bipy ⋅ 4Hex ⋅
Eu complexes is radically different. The Et-containing
compound forms a less structured, but more ragged
layer, while the long-chain Bipy ⋅ 4Hex ⋅ Eu com-
pound forms a more ordered film with smaller changes
in height.
ΔE(Tr – 5D0),
E(Tr),
cm–1
Ligand
λ, nm
cm–1
Bipy ⋅ 4Et
Bipy ⋅ 4Hex
530
513
18868
19493
1668
2293
intramolecular energy transfer depends on the differ-
3
ence in energy between the triplet level T* of the
ligand and the resonance level of the lanthanide ion.
Since the position of the triplet level depends on the
nature of the organic ligand, the luminescent proper-
ties of the complex vary with the chemical structure of
the ligand. For effective energy transfer to the Eu(III)
ion, the energy difference ΔE between the triplet level
of the ligand and the resonance level of europium 5D0
should lie in the range from 2500 to 3500 cm–1.
The emissions of the Eu(III) complexes are due to
5
energy transitions from the D0 level. The strongest
Luminescence Characteristics
of the Europium Complexes
peaks are observed for transitions to the 7F1 (593 nm)
7
5
7
and F2 (618 nm) levels, and the D0 → F4 emission
In lanthanide coordination compounds, lumines-
cence is observed as a result of light absorption by an
organic ligand and emission of narrow spectral lines of
the rare earth ion. The process proceeds in three
stages: light absorption by the ligand, energy transfer
to the metal ion through the ligand triplet state, and
emission of characteristic radiation by the lanthanide
ion [3]. A simplified scheme of energy transfer in rare
earth coordination compounds can be represented as
5
7
often has a lower intensity [17]. The D0 → F0 emis-
sion, as a rule, is relatively weak; its intensity is very
sensitive to the environment of the ligand. Emission
from the D1 and D2 levels with smaller excitation
wavelengths of 523 and 465 nm, respectively, is also
possible [16]. The number, energy difference, and rel-
ative intensities of these lines serve as a source of
detailed information on the symmetry and structure of
5
5
3
1S*(ligand) → T*(ligand) → Ln*. The efficiency of
6
2
8
7
5
0
5
4
3
2
1
1
0
6
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
–7
–5
–10
0
1
2
3
4
5
6
μm
0
1
2
3
4
5
6
7
8
9
μm
Fig. 12. AFM image of the Bipy ⋅ 4Hex ⋅ Eu complex film
deposited onto a substrate with spinning of a spin-coater at
a speed of 1000 rpm.
Fig. 11. AFM image of the Bipy ⋅ 4Hex ⋅ Eu complex film
deposited onto a substrate without spinning.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 2 2018

226
KHARCHEVA et al.
9
8
7
6
5
4
3
2
1
0
30
20
10
0
–10
–20
–30
0
1
2
3
4
5
6
7
8
9
μm
Fig. 13. AFM image of the Bipy ⋅ 4Hex ⋅ Eu complex film deposited onto a substrate with spinning of a spin-coater at a speed of
2000 rpm.
the coordination center Eu(III). The luminescence ligand-to-metal energy transfer efficiency. Although a
lifetimes of the emitting state 5D0 of solutions of Eu(III)
number of sensitized lanthanide complexes have been
reported, the energy transfer pathways for the sensiti-
zation of lanthanide ions have not yet been fully inves-
tigated.
complexes are, as a rule, in the range 0.1–1.0 ms.
The compounds studied in this work have a lumi-
nescence signal characteristic of europium complexes,
and the luminescence excitation spectra correspond to
the ligand absorption bands. A typical luminescence
spectrum of europium complexes has two major max-
To understand the process of photoexcitation of
the europium emission, triplet levels of ligands were
studied using the phosphorescence spectra of the Gd
complexes at 77 K and their positions were compared
with the resonance level of the europium ion. Table 2
shows the wavelengths of long-wavelength compo-
nents in the phosphorescence spectra, the energy lev-
els of the triplet state of the ligands E(Tr), and the dif-
ferences in energy between the triplet level of the
ligands and the resonance level of europium. The
5
7
ima: at λ = 592 nm, corresponding to the D0 → F1
transition, and at λ = 618 nm corresponding to the
7
5D0 → F2 transition. The second maximum is the
strongest. The luminescence intensity depends on the
type of ligand. Table 1 summarizes the measured and
calculated spectral characteristics of the Bipy ⋅ 4Et ⋅ Eu
and Bipy ⋅ 4Hex ⋅ Eu complexes. The luminescence
lifetimes (τobs) for the complexes in the form of solu- ΔE(Tr – 5D0) value was about 1700 cm–1 for the Bipy ⋅
4Et ligand and 2300 cm–1 for the more lipophilic
ligand Bipy ⋅ 4Hex ligand with a longer aliphatic chain.
tions, films, and powders, as well as the external quan-
tum yield of luminescence ФL of solutions of the com-
plexes on excitation at λ = 270 nm were measured
experimentally. For thin films and powders, it was dif-
ficult to obtain the value of the external quantum yield
of luminescence. The ratios of the integrated intensi-
ties of the bands at 618 and 592 nm (I618/I592) for the
complexes in acetonitrile solutions, films, and pow-
ders at 300 and 77 K were determined from the lumi-
nescence spectra (Table 1).
The overall quantum yield of luminescence of the
rare earth complex upon excitation of the chromo-
phore is determined by the sensitization efficiency
(ηsens) and the quantum yield of lanthanide lumines-
cence (ФLn) [5]. Nonradiative processes affect the
observed luminescence lifetime τobs and the overall
quantum yield of the complex ФL.
The quantum yield of the luminescence step ФLn,
or internal quantum yield, indicates the efficiency of
radiative processes (characterized by the rate constant kr)
in comparison with nonradiative processes (with the
rate constant knr) [5].
Energy Transfer in Europium Complexes
Luminescent rare earth complexes contain a chro-
mophore absorbing light, which is an antenna for sen-
sitizing the luminescence of a rare earth ion [5]. The
observed luminescence quantum yield of a rare earth
complex (ФL) is equal to the sum of the quantum yield
of the indirect luminescence excitation and the
kr
.
ΦLn
=
kr + knr
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EFFECT OF ALIPHATIC CHAIN LENGTH
227
Though the ηsens and ФLn are usually not easily energy transfer during photoexcitation of the Eu
luminescence. The difference in energy between the
accessible, their values are used to optimize the lumi-
nescence of lanthanide complexes, since they deter-
ligand triplet level and the europium resonance level
5
mine the efficiency of individual stages of photosensi- ΔE(Tr – D0) estimated from phosphorescence spectra
tization of luminescence in rare earth complexes.
of Gd complexes using its longwave spectral component
is 1700 cm–1 for the Bipy ⋅ 4Et ligand and 2300 cm–1 for
the more lipophilic Bipy ⋅ 4Hex ligand with a longer
aliphatic chain. The internal quantum yield of lumi-
nescence was 35–40% for solutions and about 80% for
powders of the complexes. The radiative lifetime for
complex with the ligand Bipy · 4Hex was found to be
slightly longer then for complex with the Bipy · 4Et
ligand (2.87 ms against 2.79 ms in acetonitrile). Radi-
ative lifetimes in solutions are approximately 30% lon-
ger than in the solid state. The constant of lumines-
cence sensitization efficiency was obtained for the Eu
complexes: ηsens = 51% (Bipy · 4Et · Eu) and 65%
(Bipy · 4Hex · Eu).
⎛ Itot
⎞
= AMD,0n3
,
1
τR
⎜
⎟
⎠
I
⎝
MD
where n is the refractive index of a medium; AMD,0 is
the spontaneous emission probability for the 5D0 → 7F1
transition in vacuum; Itot/IMD is the ratio of the total
area of the europium emission spectrum to the area of
5
7
the D0 → F1 band. From the calculated dipole
strength, it was established that AMD,0 = 14.65 s–1 [5].
Using the equality for 1/τR and measuring the inten-
sity ratio Itot/I592, the radiative lifetimes τR for the com-
pounds were calculated (Table 1). For the complex with
the Bipy ⋅ 4Hex ligand, this value is slightly larger than
for the complex with the Bipy ⋅ 4Et ligand (2.87 and
2.79 ms, respectively for solutions in acetonitrile). The
radiative lifetimes of the complexes in solutions (n =
1.346) are about 30% longer than in the solid state
(n = 1.55).
The above europium complexes exhibit intense
luminescence with lifetimes in the millisecond range
and are readily dissolved in organic solvents, which
can facilitate their use as a light-emitting layer in
OLEDs produced using inkjet printing technologies.
The development of new nanoscale conjugates with
high quantum yield and color purity for biomedical
research and for improving the methods of nanoparti-
cle imaging is an important task of modern coordina-
tion chemistry and optics.
Knowing the radiative lifetime τR, it is possible to
calculate ΦLn, using the observed luminescence life-
time τobs [5]:
τobs
τR
ΦLn
=
.
ACKNOWLEDGMENTS
The internal quantum yield of luminescence of
europium ФEu for the complexes in solutions, thin
films, and powders (at room temperature and 77 K,
Table 1) were determined, substituting the measured
This work was supported by the Agreement with
the Ministry of Education and Science (project
no. 14.604.21.0082 RFMEFI60414X0082).
τobs and calculated τR values into this equation.
From comparison of the external and internal
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2018