Strongly photoluminescent Eu(III) tetrazolate ternary complexes with phosphine oxides as powerful sensitizers

Article abstract of DOI:10.1080/00958972.2014.987765

The Eu(III) cation forms electrically neutral photoluminescent complex with 5-(2-pyridyl-1-oxide)tetrazolate (PTO) anion. Although the photoluminescence properties of such tertiary Eu(III) and Tb(III) complexes were not as high (13 and 31% photoluminescence quantum yield, respectively) as reported for other diketonate lanthanide complexes probably because of high number of nitrogen atoms involved in PTO which leads to attachment of water molecules, reducing the luminescence quantum yield with vibrational and rotational quenching. Here, we report the removal of quencher molecules from the coordination sphere of tris-europium tetrazolate oxide complex by replacing them with various phosphine oxides which leads to improved photoluminescence quantum yield for the complexes by acting as auxiliary co-ligands with that of the main antenna 5-(2-pyridyl-1-oxide)tetrazolate. The coordination sphere in these complexes can be complemented by aromatic phosphine oxides to provide highly photoluminescent Eu(III) complexes. The highest quantum yield was 38% in 3 [Eu(PTO)₃·DPEPO](H₂O)₅ containing bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) as compared to tris-europium complex with 5-(2-pyridyl-1-oxide)tetrazolate.

Full text of DOI:10.1080/00958972.2014.987765

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Strongly photoluminescent Eu(III)
tetrazolate ternary complexes
with phosphine oxides as powerful
sensitizers
Suraj Mala, Marek Pietraszkiewicza & Oksana Pietraszkiewicza
a Institute of Physical Chemistry, Polish Academy of Sciences,
Warsaw, Poland
Accepted author version posted online: 17 Nov 2014.Published
online: 11 Dec 2014.
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To cite this article: Suraj Mal, Marek Pietraszkiewicz & Oksana Pietraszkiewicz (2015) Strongly
photoluminescent Eu(III) tetrazolate ternary complexes with phosphine oxides as powerful
sensitizers, Journal of Coordination Chemistry, 68:2, 367-377, DOI: 10.1080/00958972.2014.987765
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Journal of Coordination Chemistry, 2015
Vol. 68, No. 2, 367–377, http://dx.doi.org/10.1080/00958972.2014.987765
Strongly photoluminescent Eu(III) tetrazolate ternary
complexes with phosphine oxides as powerful sensitizers
SURAJ MAL1*, MAREK PIETRASZKIEWICZ and OKSANA PIETRASZKIEWICZ
Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland
(Received 13 June 2014; accepted 16 October 2014)
The Eu(III) cation forms electrically neutral photoluminescent complex with 5-(2-pyridyl-1-oxide)
tetrazolate (PTO) anion. Although the photoluminescence properties of such tertiary Eu(III) and Tb
(III) complexes were not as high (13 and 31% photoluminescence quantum yield, respectively) as
reported for other diketonate lanthanide complexes probably because of high number of nitrogen
atoms involved in PTO which leads to attachment of water molecules, reducing the luminescence
quantum yield with vibrational and rotational quenching. Here, we report the removal of quencher
molecules from the coordination sphere of tris-europium tetrazolate oxide complex by replacing
them with various phosphine oxides which leads to improved photoluminescence quantum yield for
the complexes by acting as auxiliary co-ligands with that of the main antenna 5-(2-pyridyl-1-oxide)
tetrazolate. The coordination sphere in these complexes can be complemented by aromatic phos-
phine oxides to provide highly photoluminescent Eu(III) complexes. The highest quantum yield was
38% in 3 [Eu(PTO)3·DPEPO](H2O)5 containing bis(2-(diphenylphosphino)phenyl) ether oxide
(DPEPO) as compared to tris–europium complex with 5-(2-pyridyl-1-oxide)tetrazolate.
The Eu(III) cation forms electrically neutral photoluminescent complex with 5-(2-pyridyl-1-oxide)
tetrazolate (PTO) anion. Although the photoluminescence properties of such tertiary Eu(III) and Tb
(III) complexes were not as high (13 and 31% photoluminescence quantum yield, respectively) as
reported for other diketonate lanthanide complexes probably because of high number of nitrogen
atoms involved in PTO which leads to attachment of water molecules, reducing the luminescence
quantum yield with vibrational and rotational quenching. Here, we report the removal of quencher
molecules from the coordination sphere of tris–europium tetrazolate oxide complex by replacing
*Corresponding author. Email: Suraj.Mal@irb.hr
1Present address: Division of Materials Physics, Ruder Boskovic Institute, Bijenicka Cesta 54, 10000 Zagreb, Croatia.
© 2014 Taylor & Francis

368
S. Mal et al.
them with various phosphine oxides which leads to improved photoluminescence quantum yield for
the complexes by acting as auxiliary co-ligands with that of the main antenna 5-(2-pyridyl-1-oxide)
tetrazolate. The coordination sphere in these complexes can be complemented by aromatic phos-
phine oxides to provide highly photoluminescent Eu(III) complexes. The highest quantum yield was
38% in 3 [Eu(PTO)3·DPEPO](H2O)5 containing bis(2-(diphenylphosphino)phenyl) ether oxide
(DPEPO) as compared to tris–europium complex with 5-(2-pyridyl-1-oxide)tetrazolate.
Keywords: Europium; Tetrazoles; Phosphine oxides; Photoluminescence; Quantum yields
1. Introduction
Lanthanide complexes give sharp and intense emission lines upon ultraviolet irradiation
because of the effective intramolecular energy transfer (ET) from the coordinated ligands to
the luminescent central lanthanide ion, which, in turn, undergoes the corresponding radia-
tive process (the so-called antenna effect) [1]. Therefore, they are increasingly used as
highly efficient electroluminescent components for light-emitting diodes [2, 3], luminescent
probes for analytes [4], labels for proteins and amino acids [5], and molecular recognition
and chirality sensing of biological substrates [6]. The major drawback of lanthanide com-
plexes is the luminescent quenching by matrix vibrations such as hydroxyl groups via non-
radiative pathways, which decreases their luminescence intensity and limits the application
for organic light-emitting devices [7]. Because of low thermal stability of some lanthanide
complexes, they are suitable only in an appropriate temperature range for applications. A
way to circumvent these difficulties is to use ligands bearing suitable chromophores capable
of forming thermally stable complexes with lanthanide ions. These ligands could play the
antenna role, absorbing light and transferring excitation energy to the emitting ion. So far,
tetrazoles have not been particularly popular as sensitizing ligands for lanthanides emitting
in a visible range. Tetrazoles are as acidic as carboxylic acids, but four nitrogens provide
multitude of options for coordination bond formation with lanthanide cations. To avoid OH
vibronic quenching from the coordination sphere of Eu(III), additional auxiliary ligands
such as phosphine oxides or nitrogen containing molecules may complement the coordina-
tion sphere. The new complexes with increased coordination number have the effect of pro-
tecting metal ions from vibration coupling and increasing their light absorption cross
section by the antenna effect [8]. Phosphine oxide ligands in the europium(III) tris(tetrazo-
late) complex can produce a square antiprismatic structure that promotes faster radiation
5
rates and an increased quantum yield because of an increase in the D0–7F2 emissions (elec-
tric dipole transitions), related to odd parity [9]. Furthermore, increased quantum efficien-
cies of the Eu(III) complexes can be expected due to extra sensitization for lanthanide
cations via phosphine oxide triplet states. The novel class of heterocyclic molecules such as
5-(2-pyridyl-1-oxide)tetrazole (HPTO) with other auxiliary co-ligands has not been utilized
much for the preparation of luminescent materials. Interest in tetrazole and its derivatives
has been growing due to success achieved in preparation of a number of highly efficient
drugs based on tetrazole [10–15]. They also play exceptional part in the synthetic chemistry
for preparation of compounds belonging to various classes [16, 17]. Metal-organic coordi-
nation complexes with tetrazolate- or carboxylate-based [18] ligands have been the subject
of intense research for decades, owing to their interesting structural topologies and wide
physical properties such as molecular magnetism, molecular absorption and recognition,
catalysis, non-linear optics and luminescence [19].

Eu(III) tetrazolates
369
Recently, we published tris complexes of pyridineoxidetetrazolate of Eu(III) and Tb(III)
complexes showing 13 and 31% photoluminescence quantum yield, respectively [20],
without any additional co-ligands in the framework. Herein, we report the synthesis and
photophysical properties of new europium(III) complexes of 5-(2-pyridyl-1-oxide)tetrazole
(HPTO) with phosphine oxide co-ligands, tri-n-octylphosphine oxide (TOPO), triphenyl-
phosphine oxide (TPPO), and bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO)
which not only enhance the photoluminescence quantum yields of Eu(III) as compared to
previously reported tris tetrazolate Eu(III) complex (13%) but also lifetimes as well. Addi-
tionally, when the Eu(III) cation is coordinatively unsaturated by the original ligands, an
additional neutral ligand (respective phosphine oxide) coordinates to the lanthanide center
to form a highly coordinated complex, thereby excluding coordination of solvent molecules
and leads to improved volatility and thermal stability of the complexes.
2. Experimental
2.1. Materials
The following commercially available chemicals were used without purification: europium
(III) nitrate hexahydrate, 99.9% (Aldrich); gadolinium(III) nitrate hexahydrate, 99.9%
(Aldrich); and terbium(III) nitrate hexahydrate, 99.9% (Aldrich). All other chemicals used
were of analytical reagent grade.
2.2. Instrumentation
Elemental analyses were performed with a Vario EL III Heraeus instrument. MS spectra
were run on an Applied Biosystems 4000 Q TRAP instrument with photospray ionization
mode and a Waters Micromass GCT Premier mass spectrometer with field desorption/field
ionization–time of flight method. IR spectra were recorded using a Perkin-Elmer 2000
1
spectrometer. H NMR spectra were recorded in CDCl3 with a Bruker DRX 200 MHz
spectrometer using TMS as internal standard. For 31P NMR, Bruker Avance 400 was used.
UV–Vis spectra were recorded with a Shimadzu UV-3100 spectrophotometer, and corrected
luminescence spectra with a Fluorolog 3 spectrofluorimeter, using quinine sulfate as a
reference.
2.3. Photoluminescence quantum yield determination
Photoluminescence quantum yields were determined using quinine sulfate as a standard
(Φ = 0.51). A methodology described by Williams et al. [21] and Eaton [22] was taken into
account to measure and to calculate the photoluminescence quantum yields of lanthanide
complexes in solution. It involves the use of well-characterized standard samples with
known ΦST values. Essentially, solutions of the standard and test samples with identical
absorbance at the same excitation wavelength can be assumed to be absorbing the same
number of photons. Hence, a simple ratio of the integrated fluorescence intensities of the
two solutions (recorded under identical conditions) will yield the ratio of the quantum yield
values. Since ΦST for the standard sample is known, it is trivial to calculate the ΦX for the
test sample. ΦX can be calculated using equation (1) as shown below,

370
S. Mal et al.
UX ¼ USTðIX=ISTÞðg2X=gS2TÞðFST=FXÞ
(1)
where ΦX and ΦST represent photoluminescence quantum yield of lanthanide complexes
(sample) and standard quinine sulfate, respectively, while ηX and ηST represent the refractive
index of solvents of lanthanide complexes (sample) and standard quinine sulfate, respec-
tively. The terms IX and IST represent the integrated emission intensity of the complex and
standard under identical conditions, respectively. FST and FX mean fractions of light
absorbed by standard and sample. Furthermore radiative and non-radiative constants (ARAD
and ANR, respectively) are calculated by equations (2) and (3) where τ represents the photo-
luminescence decay lifetime.
AARD ¼ U=s
(2)
(3)
1=s ¼ ARAD þ ANR
3. Synthetic methodology
3.1. Synthesis of ligands
The ligands HPTO and 5-(2-pyridyloxide)tetrazole were synthesized according to figure 1
by two step as reported in our previous publication [20].
3.1.1. Synthesis of 5-(2-pyridyl)tetrazole. Yield = 85%, m.p. = 220 °C. 1H NMR
(250 MHz, CDCl3, TMS) ppm: 7.59–8.76 m (4H arom), MS: m/z = 147.
3.1.2. Synthesis of 5-(2-pyridyl-1-oxide)tetrazole (HPTO). Yield = 85%, m.p. = 245 °C.
1H NMR (250 MHz, CDCl3, TMS) ppm: 7.59–8.76 m (4H arom), CHN anal. Calcd for
C6H5N5O: C, 44.17; H, 3.08; N, 42.92. Found: C, 44.07; H, 3.23; N, 42.96%. IR (KBr)
ν
max: 3100, 1496, 1245, 1100, 750, 510 cm−1.
3.2. Synthesis of the Ln(III) complexes
Complexes 4 and 5 were synthesized by the same procedure as described in our previous
publication as shown in figure 2 [20]. These were used for further analyses and photophysi-
cal studies.
3.2.1. Eu(PTO)3(H2O)3.5 (4). m.p. > 300 °C Elemental analysis (%): Calcd for
C18H19EuN15O6.5 (701): C, 30.81; H, 2.71; N, 29.95. Found: C, 30.61; H, 2.47; N, 29.67.
IR (KBr) νmax: 3300,1600, 1490, 1248, 1030, 755, 510 cm−1.
3.2.2. Gd(PTO)3(H2O)8 (5). Elemental analysis (%): Calcd for C18H28GdN15O11
(787.76): C, 26.08; H, 3.65; N, 26.65. Found: C, 25.77; H, 3.82; N, 26.40. IR (KBr) νmax
:
3310, 1608, 1498, 1248, 998, 755 cm−1.

Eu(III) tetrazolates
371
3.3. Synthesis of [Eu(PTO)3·(TPPO)2](H2O)3(CH3OH)2 (1)
Complexes 1 and 2 with corresponding phosphine oxides were prepared by stirring 1 : 2
ratio of europium 1 : 3 complex [Eu(PTO)3·(H2O)3.5] with corresponding phosphine oxide
and 3 by equimolar solutions of europium 1 : 3 complex [Eu(PTO)3·(H2O)3.5] with DPEPO
in methanol for 24 h at 70 °C. The solid product was filtered, washed with ethanol, water,
and then ethanol (figure 3), dried and stored in a desiccator over P2O5. 1H NMR
(250 MHz, DMSO-d6) ppm: 7.21–7.69 m broad (all aromatic); El. Anal. Calcd for C56H56
EuN15P2O10 (1313.05): C, 51.22; H, 4.30. Found: C, 51.27; H, 4.53. IR (KBr) νmax: 3300,
1700, 1490, 1260, 1148, 1090, 750, 450, 400 cm−1.
3.4. [Eu(PTO)3·(TOPO)2](H2O)4 (2)
1H NMR (250 MHz, DMSO-d6) ppm: 7.21, broad m (4H arom), 1.23–1.53 (t, –CH2, 36H),
1.04–1.14 (h, –CH2 near to CH3, 6H), 0.84 (t, CH3, 9H), El. Anal. (%): Calcd for
C66H122EuP2N15O9 (1483.68): C, 53.42; H, 8.28. Found: C, 53.17; H, 8.29. IR (KBr) νmax
:
3300, 1690, 1488, 1255, 1205, 1094, 755, 450, 400 cm−1.
3.5. [Eu(PTO)3·DPEPO](H2O)5 (3)
El. Anal. (%): Calcd for C54H50EuP2N15O11 (1298.98): C, 53.42; H, 8.28. Found: C, 53.17;
H, 8.29. IR (KBr) νmax: 3290, 1710, 1500, 1260 1165, 1105, 746, 450, 400 cm−1.
4. Results and discussion
4.1. Synthesis and IR spectroscopy
The ligand and the complexes were synthesized according to figures 1–3. Microanalysis of
each complex shows the combination ratio of metal, PTO, and corresponding phosphine
oxide ligand in 1 : 3 : 2, 1 : 3 : 2, and 1 : 3 : 1, respectively, for 1, 2, and 3. The presence
of water or solvent molecules was confirmed by IR spectroscopy which shows a broad band
between 3000 and 3500 cm−1. The presence of water or solvent molecules in the lattice is
reported for such tetrazolate complexes [23–25]. Shifts in the P=O stretching frequencies of
phosphine oxide molecules in 1 (from 1143 to 1148 cm−1), 2 (from 1182 to 1205 cm−1),
and 3 (from 1158 to 1165 cm−1) show the involvement of phosphoryl oxygen in complex
formation with Eu(III) ion, which is absent in [Eu(PTO)3·(H2O)3.5]. The disappearance of
N–H peak at 1640 cm−1 for all synthesized complexes confirms the coordination of N with
N
N
N
O
Toluene/Reflux
12h
MCPBA
NaN3
Et3N.HCl
+
+
MeOH/RT/48h
CN
N
NH
N
NH
N
N
N
N
Figure 1. Synthesis of 5-(2-pyridyloxide)tetrazole.

372
S. Mal et al.
N
N
N
O
O
Et4N.OH/EtOH
Reflux/14h
M*(H2O)x
M(NO3)3.5H2O
+
N
NH
N
N
N
N
N
M = Eu(III), Gd(III)
Figure 2. Synthesis of Eu(III) and Gd(III) complexes with HPTO.
N
O
N
N
O
(RP=O)n, MeOH
Eu.(H2O)3.5
Eu(OPR)n
N
N
N
N
N
N
N
3
3
(RP=O) = TPPO, TOPO, DPEPO
Figure 3. Synthesis of ternary europium complexes with TPPO, TOPO, and DPEPO.
Eu(III). The shifted band at 1260 cm−1 of N–O from 1360 cm−1 (in free HPTO) also
indicates the environment of N–O in the complex formation with Eu(III) (supporting
information figure S1, see online supplemental material at http://dx.doi.org/10.1080/
00958972.2014.987765). Far-IR spectrum reveals the coordination of Eu(III) ion with N of
tetrazole and O of pyridine oxide with sharp bands at 450 and 400 cm−1, respectively
(supporting information figure S2).
4.2. NMR spectroscopy
Due to less solubility of the synthesized ternary europium complexes (1–3) in organic solvents,
1H and 31P NMR spectrum of all ternary complexes are made in DMSO-d6. The 1H NMR spec-
tra of all complexes bears the respective peaks for HPTO and corresponding phosphine oxide.
Complex 1 clearly shows the multiplet mixed peaks for all aromatic hydrogens of HPTO and
TPPO. In 2, the respective band of all alkyl protons of TOPO phosphine oxides is little shifted
as compared to free TOPO phosphine oxide (0.86–1.58 to 0.84–1.53 ppm) due to complexation
with Eu(III). A broad peak at 7.21 ppm, which is similar to that of tris–(PTO) europium com-
plex 1, displays the pyridine oxide protons. As DMSO-d6 also has coordination ability with the
lanthanide cations, there will be competition between the large quantity of DMSO-d6 solvent
and small quantity of respective phosphine oxides (compared to solvent) in the synthesized ter-
nary complexes. In the 31P NMR spectrum, we observed one sharp peak at 27 ppm for 1 and
poorly resolved peak at 35 ppm for 2, due to its poor solubility in DMSO-d6. For the respective
triphenylphosphine oxide (TPPO) used to synthesize ternary tetrazolate Eu(III) complex, the
chemical shift for the 31P NMR signal in DMSO-d6 solvent is 39 ppm [26]. Therefore, it
indicates that at least 1 is stable in DMSO-d6 solvent (supporting information figures S3–S6).

Eu(III) tetrazolates
373
Figure 4. Optimized molecular structure of 4 and 3 by MOPAC/AM1 modeling.
4.3. Molecular structure modeling of 1–3 by the MOPAC/AM1 model
We optimized the molecular structures of all synthesized europium(III) complexes using the
MOPAC/AM1 model with parameters for europium [27] (figure 4). In 4, the central Eu(III)
is coordinated with three nitrogens of tetrazole, three oxygens of pyridine oxide, and three
water molecules giving nine-coordinate lanthanide complex with approximately tricapped
trigonal-prismatic geometry. In 1–3 Eu(III) is eight-coordinate and the structure can be
assigned as antisymmetrical square antiprism. Lanthanide complexes with seven, eight, or
nine coordination are well documented [28, 29]. The coordinated water molecules in 4 are
replaced by phosphine oxide TPPO, TOPO, and DPEPO in 1, 2, and 3, respectively. The
closer proximity of the phosphine oxide bond with Eu(III) (2.304, 2.312 Å and 2.301,
2.311 Å, respectively) observed in 1–3 as compared to the coordinated water for 4
(2.496 Å) suggests the strong interaction of phosphine oxides with Eu(III) [30]. The tor-
sional angle strain produced between HPTO and phosphine oxide (DPEPO) gives indication
of distortion of planarity around Eu(III) by the introduced phosphine oxide which leads to
non-planar geometry of the complexes. The distortion is greater in 3 with DPEPO generat-
ing more asymmetry around Eu(III).
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Absorption
Excitation
Absorption
Excitation
Absorption
Excitation
250
300
350
400
450
250
300
350
400
450
250
300
350
400
450
Wavelength(nm)
Wavelength(nm)
Wavelength(nm)
Complex 1
Complex 2
Complex 3
Figure 5. UV absorption and normalized excitation spectra of 1–3 in acetonitrile at room temperature.

374
S. Mal et al.
4.4. UV absorbance and photoluminescence studies
The room temperature absorption and normalized excitation spectra of 1–3 are shown in
figure 5. The excitation spectra were obtained by monitoring the emission wavelength of
Eu(III) at 612 nm (5D0–7F2 transition). The excitation spectra of the complexes exhibit a
broad excitation band between 250 and 325 nm, which can be assigned to π–π* transitions
of the ligands.
From the photophysical point of view, there are two distinct problems to overcome to
achieve intense and long-lived luminescence. The problems are: (i) to overcome the low
value of ε of lanthanides by introducing suitable antenna chromophores and (ii) to prevent
the rapid deactivation of the excited state by vibrations of the ligands and/or solvent mole-
cules. The introduction of HPTO and neutral auxiliary co-ligands (TPPO, TOPO, and
DPEPO) increased the molar absorption coefficient (table 1) and replaced the quenchers
(water or solvent molecules) from the inner coordination sphere of Eu(III) in 4, thus
enhancing the photoluminescence.
The room temperature-normalized emission spectra of 1, 2 and 3 (in acetonitrile solution)
under the excitation wavelength (284, 270, and 271 nm, respectively, for 1–3) that maxi-
mizes the Eu(III) emission intensity are shown in figure 6. The emission spectra of the com-
5
plexes display characteristic sharp peaks from 575 to 725-nm associated with the D0–7FJ
transitions of Eu(III). The five expected peaks of the 5D0–7F0–4 transitions are well
5
resolved, and the hypersensitive D0–7F2 transition is very intense, pointing to a highly
polarizable chemical environment around Eu(III), responsible for the brilliant red emission
of these complexes.
To describe ET process, the phosphorescence spectrum of Gd(PTO)3(H2O)x was mea-
6
sured at 77 K to calculate the triplet state of PTO ligand. As P7/2 of Gd(III) is too high in
energy to accept energy from ligand, the data obtained from phosphorescence spectrum
revealed the emissive triplet state of PTO (supporting information figure S7). Singlet state
estimation of PTO was done by referencing its absorption edge which is 29,100 cm−1. The
triplet energy (3ππ*) level of HPTO was calculated as 21,312 cm−1 (supporting information
5
figure S8). The observed single sharp emission band of D0–7F0 for ternary europium(III)
complex 1 (half band width 1 nm for 1) proves the existence of a single chemical environ-
5
ment around Eu(III). Because the intensity of D0–7F0 transition is generally low, it is diffi-
cult to conclude from insets for these transitions in case of 2 and 3, whether Eu(III) is in a
single chemical environment, or there is electronic noise, when magnifying this transition.
5
The lifetime values (δobs) of the D0 level determined from the luminescence decay pro-
files for 1–3 at room temperature by fitting with a monoexponential curve are depicted in
table 2. As from our previous measurements, 1 : 3 complex of europium(III) with PTO has
PL lifetime of 0.350 0.0105 ms, but with co-ligands TPPO, TOPO, and DPEPO it is
Table 1. Absorption wavelength maxima (λ) in nm and molar absorption coefficient
(ε) in cm−1 M−1 for 1, 2, and 3.
Complex
Wavelength (nm)
Molar absorption coefficient (cm−1 M−1
)
1
272
253
278
241
289
241
27,400
43,000
38,130
99,900
47,260
90,700
2
3

Eu(III) tetrazolates
375
0,04
5D
0
7F0
1.0
0.8
0.6
0.4
0.2
0.0
5D0
7F2
0,03
0,02
0,01
0,00
Complex 1
Complex 2
Complex 3
Complex 1
Complex 2
Complex 3
570
572
574
576
578
580
582
584
Wavelength(nm)
7F4
7F1
7F0
7F3
500
600
700
Wavelength(nm)
Figure 6. Normalized emission spectra of 1–3 in acetonitrile at room temperature.
Table 2. Experimental photoluminescence parameters. Excitation wavelength (λexc nm). 5D0 Lifetime (τobs) (error:
3%), intrinsic quantum yields (Φ in %), ARAD and ANR represent the radiative and non-radiative decay constants,
respectively, non-radiative for 1–3 in acetonitrile solution at 303 K.
Complex
λexc (nm)
Φ (%)
τ
obs (ms)
ARAD (ms−1
)
ANR (ms−1
)
1
2
3
284
270
271
15
19
38
0.660 0.0198
0.590 0.0177
0.60 0.0180
0.212
0.254
0.533
1.303
1.441
1.133
increased to 0.660 0.0198 ms. A recent study on measurement of photoluminescence
decay time reveals that it also depends on excitation wavelength of the complex [31]. We
observed maximum photoluminescence decay time at the particular excitation wavelength
mentioned in table 2. The shorter decay lifetime observed for tris–pyridinetetrazolate Eu
(III) complex 4 can be attributed to the vibronic oscillations shown by coordinated water
which promotes the faster non-radiative decay.
The complexes were stable at room temperature as none of them show photobleaching
when they are continuously excited (1 h) and emission was measured at different intervals
5
of time (supporting information figure S9 for 3, D0–7F2 transition). In order to validate
1–3, the photoluminescence quantum yield (Φ) has been determined upon ligand excitation
(table 2). The low photoluminescence quantum yield obtained for Eu(PTO)3 (13% in solu-
tion) is enhanced three times for 3 (38% in solution), which is the maximum reported for
such tetrazolate lanthanide complexes. Although there are other lanthanide complexes
reported in the literature with greater PLQY such as diketonates, dicarboxylates, and other
ternary complexes, in the field of tetrazolates, this work is novel which can replace carbox-
ylates as ligands. Further study to enhance the PLQY is still under investigation with other
co-ligands (phosphine oxides and N-containing molecules).

376
S. Mal et al.
Figure 7. Proposed ET mechanism for ternary europium complexes.
The ET mechanism diagrams of corresponding complexes are shown in figure 7. The S1
and T1 energy levels of the ligands are indicated as well as the respective emissive levels of
5
5
5
Eu(III), D2, D1, and D0. On the basis of energy levels of triplet and singlet states of
ligands (calculated from Gd complex) and higher energy levels of metal, we propose the
ET diagram for ternary Eu(III) tetrazolate complexes, in which the energy is transferred not
only via PTO triplet state to the emissive level of Eu(III), but also via triplet state of TPPO,
5
5
TOPO to mainly D1 and partially D0 states of Eu(III). In the case of DPEPO, the ET is
much more effective, having energy gap between S (DPEPO)-1S (PTO) > 3000 cm−1 and
1
1T (DPEPO)-1T (PTO) > 4000 cm−1 which can transfer energy not only via triplet state but
1
1
5
also through singlet level from S (DPEPO) to S (PTO) and further to the DJ (J = 0, 1) of
Eu(III) ions via T state of PTO. Due to more efficient ET, complexes of Eu(PTO)3 with
1
phosphine oxides such as TPPO, TOPO, and DPEPO show high photoluminescence quan-
tum yield ranging from 15 to 38% in acetonitrile solution.
5. Conclusion
Tetrazolate ligands effectively sensitize the visible emission of selected lanthanide ions.
These complexes are not susceptible to photobleaching and air oxidation, as for 1,3-diketo-
nates. The use of phosphine oxides proved to have a beneficial effect on enhancement of
the photoluminescent properties. Phosphine oxides form strong complexes with lanthanide
cations. It seems that there is still unexplored field to tailor phosphine oxide auxiliary
ligands with the triplet states optimized to have a profound impact on the photolumines-
cence quantum yields of ternary Eu(III) complexes. Work in this direction is in progress.
Funding
This work was supported by the Project FP7 Marie Curie Initial Training Network, Contract PITN-GA-2008-
215399 “Cavity-confined Luminophores for Advanced Photonic Materials: A Training Action for Young
Researchers” (FINELUMEN).

Eu(III) tetrazolates
377
Supplementary data
The supplementary data are related to mid-IR and far-IR spectra of 1–3, luminescence measurement of PTO, and
1H and 31P NMR spectra of these complexes. Supplemental data for this article can be accessed http://dx.doi.org/
10.1080/00958972.2014.987765.
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