1-Methyl-1,10-phenanthrolin-2(1H)-one (L) as a ligand to Eu(III): Crystal structure and luminescent properties of [Eu(L)3(NO3) 3] Dedicated to Professor George Christou on the occasion of his 60th birthday.

Article abstract of DOI:10.1016/j.poly.2013.03.001

A new coordination compound of Eu(III) with 1-methyl-1,10-phenanthroline- 2(1H)-one (L) as a ligand has been synthesized and characterized using infrared spectroscopy, elemental analysis and single crystal X-ray diffraction. The structure contains isolated [Eu(L)₃(NO₃)₃] molecules interacting through extensive π-stacking. Photoluminescence studies demonstrate that the complex shows line-like emission characteristic of Eu(III) when excited in the ligand-centered absorption bands, with a quantum yield of 22% and a lifetime of 0.74 ms.

Full text of DOI:10.1016/j.poly.2013.03.001

Polyhedron 64 (2013) 106–109
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
1-Methyl-1,10-phenanthrolin-2(1H)-one (L) as a ligand to Eu(III): Crystal
structure and luminescent properties of [Eu(L)3(NO3)3]
Sebastiaan Akerboom a, Jos J.M.H. van den Elshout a, Ilpo Mutikainen b, Wen Tian Fu a,
Elisabeth Bouwman a,
⇑
a Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
b Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55 (A.I. Virtasenaukio 1), FI-00014 University of Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 14 March 2013
A new coordination compound of Eu(III) with 1-methyl-1,10-phenanthroline-2(1H)-one (L) as a ligand
has been synthesized and characterized using infrared spectroscopy, elemental analysis and single crystal
X-ray diffraction. The structure contains isolated [Eu(L)3(NO3)3] molecules interacting through extensive
Dedicated to Professor George Christou on
the occasion of his 60th birthday.
p
-stacking. Photoluminescence studies demonstrate that the complex shows line-like emission charac-
teristic of Eu(III) when excited in the ligand-centered absorption bands, with a quantum yield of 22%
and a lifetime of 0.74 ms.
Keywords:
Eu(III)
Ó 2013 Elsevier Ltd. All rights reserved.
Carbonyl ligand
Photoluminescence
1. Introduction
[6,7]. Currently, phosphors that exhibit efficient line-like emission
in the red region with good absorption properties in the near UV to
Ever since the discovery of the antenna effect in 1942, com-
plexes with trivalent lanthanoid ions have been widely studied
for their luminescent properties. The trivalent lanthanoid ions
show interesting photophysical properties, such as narrow line-
like emission and lifetimes in the millisecond range, as a result of
the intraconfigurational 4f–4f transitions [1,2]. Strictly, these tran-
sitions are forbidden by the Laporte selection rule and in addition
many of them are spin forbidden. As a result, the absorption by 4f–
4f transitions cannot be used to efficiently excite the trivalent lan-
thanoid ions. It was found by Weissman that suitable organic li-
gands can act as an antenna that absorbs light via allowed
transitions and subsequently transfers the acquired energy to the
lanthanoid ion [3]. This mechanism, also known as sensitization,
can greatly enhance the luminescence intensity of the lanthanoid
ions, allowing application in for example sensors [1,4] and bio-as-
says [5]. The luminescence of Eu(III) is of particular interest, as its
strongest emission line is in the visible region around 614 nm. In
principle, this means that Eu(III)-based complexes are attractive
for application as a red-emitting phosphor material in phosphor
converted white light emitting diodes (PC-WLEDs). In PC-WLEDs,
the emission of a near UV or blue-emitting LED is converted into
white light by suitable phosphor materials, thereby turning the
intrinsically monochromatic emission of an LED into white light
blue spectral region (370–470 nm) are still lacking, necessitating
research in this area [6].
Ligands that are known to efficiently sensitize Eu(III) lumines-
cence are beta-diketones [8–10], aromatic carboxylates [11–13],
and calixarenes [14]. 1,10-Phenanthrolines form another class of
widely used antenna ligands, and the resulting Eu(III) complexes
exhibit bright photoluminescence upon excitation in ligand cen-
tered absorption bands [15–17]. During our investigation of the
influence of substituents on 1,10-phenanthroline on the photolu-
minescent properties of the Eu(III) complexes, out of curiosity
one of the synthetic intermediates, 1-methyl-1,10-phenanthrolin-
2(1H)-one (L) shown in Fig. 1, was used as a ligand for Eu(III)
[18]. We were surprised by the relatively easy formation of a com-
plex with a poor ketone-type ligand. To the best of our knowledge,
this molecule has never been used before as a ligand. In this paper,
we report the synthesis, crystal structure and photoluminescent
properties of the resulting complex which is described by the for-
mula [Eu(L)3(NO3)3].
2. Experimental
2.1. General
⇑
NMR spectra were recorded on a Bruker DPX300. IR spectra
were measured with a Perkin Elmer Paragon 1000 FTIR equipped
Corresponding author. Tel.: +31 71 5274550; fax: 31 71 5274761.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.03.001

S. Akerboom et al. / Polyhedron 64 (2013) 106–109
107
Table 1
Crystallographic details for [Eu(L)3(NO3)3].
Formula
FW
C39H30EuN9O12
968.68
Crystal size (mm)
Crystal color
Crystal system
Space group
a (Å)
0.20 ꢂ 0.20 ꢂ 0.10
orange
triclinic
ꢀ
P1
Fig. 1. Schematic drawing of 1-methyl-1,10-phenanthrolin-2(1H)-one (L).
10.296(1)
12.478(1)
15.661(2)
77.45(1)
85.95(1)
69.52(1)
1839.8(4)
2
1.749
1.786
29148/8357
553
0.026/0.056
1.09
b (Å)
c (Å)
a
(°)
b (°)
with a Golden Gate ATR. Luminescence data were collected using a
Shimadzu RF-5300PC spectrofluorophotometer equipped with a
solid-state sample holder. For determination of photolumines-
cence quantum yields, an Edinburgh Instruments FLS920 spectro-
photometer equipped with an integrating sphere was used. For
determination of luminescence lifetime, this spectrometer was
used together with a pulsed laser excitation source. Phenanthro-
line was purchased from Merck and used as received.
c
(°)
V (Å3)
Z
dcalc (g/cm3)
l
(mmꢀ1
)
Reflections measured/unique
Parameters
R1 (I > 2
r
(I))/wR2 (all reflections)
S
qmin/max (e/Å3)
ꢀ0.50/0.69
2.2. Synthesis of 1-methyl-1,10-phenanthrolin-2(1H)-one (L)
data were corrected for Lorentz and polarization effects, and for
absorption. The programs COLLECT [23], SHELXS-97 [24], SHELXL-97
[24,25] were used for data reduction, structure solution and struc-
ture refinement, respectively. The non-hydrogen atoms were re-
fined anisotropically. The H atoms were introduced in calculated
positions and refined isotropically riding with fixed geometry with
respect to their carrier atoms.
The ligand was prepared following literature procedures.
Phenanthroline was oxidized to its N-oxide using dihydrogen per-
oxide following the methods reported by Engbersen and Corey
[19,20]. Subsequently, the N-oxide was converted to 1-methyl-1,
10-phenanthrolin-2(1H)-one using dimethyl sulfate, following
the procedure reported by Kolling [21]. Overall yield: 86%. 1H
NMR (300 MHz, CDCl3) d/ppm: 8.94 (dd, 1H), 8.18 (dd, 1H), 7.79
(d, 1H), 7.56 (m, 2H), 7.50 (dd, 1H), 6.91 (d, 1H), 4.49 (s, 3H). 13C
NMR (75 MHz, CDCl3) d/ppm: 147.3, 139.2, 136.2, 126.8, 122.5,
3. Results and discussion
m
/cmꢀ1:1657(vs), 1646(vs), 1602(vs),
3.1. Synthesis and characterization
122.3, 122.0, 38.0. IR
1539(s), 1505(s), 1468(s), 1418(m), 1271(m), 1196(m), 1166(w),
1128(s), 1035(m), 989(m), 911(m), 843(vs), 783(s), 732(s), 705(s),
656(s), 604(s), 577(m), 546(m), 463(vs).
The ligand was obtained in a satisfactory yield following the lit-
erature procedures [19–21]. The 1H NMR spectrum of the ligand
clearly shows the presence of the N-methyl group as a singlet at
4.49 ppm and the presence of only seven aromaticprotons. The pres-
ence of the carbonyl group is revealed in the IR by the strong band
around 1650 cmꢀ1, indicating that the ligand has formed. The com-
plex synthesis was performed in boiling ethanol; extended boiling of
the ligand is necessary to ensure complete dissolution and the com-
plex mixture was boiled to slow down precipitation of the complex.
After 15 min of stirring, formation of additional precipitate was ob-
served. Due to the large number of vibrations, full assignment of the
IR spectrum is complicated. Nevertheless, Tsaryuk et al. have per-
formed detailed analysis on the vibrational spectra of similar Eu(III)
complexes with phenanthroline-type ligands [26]. Based on this
work, the absorption bands at 708, 736, 816, 1026, 1291 and
1470 cmꢀ1 can be assigned to vibrational modes of the nitrate li-
gands. Bands resulting from the phenanthrolone ligand appear at
604, 658, 781, 848, 1638, 1496 and 1543 cmꢀ1 [27].
2.3. Synthesis of [Eu(L)3(NO3)3]
0.25 g (1.2 mmol) of L was dissolved in 15 mL of ethanol and
heated to boiling for 15 min until complete dissolution of L. To this
solution was slowly added a boiling solution of 0.17 g (0.4 mmol)
Eu(NO3)3ꢁ5H2O in 10 mL ethanol. The mixture was boiled and stir-
red for 15 more minutes and cooled down to room temperature,
and the resulting precipitate was collected by filtration, washed
with ethanol and allowed to dry in air. Yield 200 mg, 50% based
on Eu, of a light yellow powder. IR
m
/cmꢀ1: 1640(s), 1616(w),
1600(m), 1564(s), 1558(s), 1538(vs), 1496(s), 1472(vs), 1456(vs),
1436(m), 1424(s), 1404(s), 1291(br, vs), 1236(s), 1200(m),
1168(m), 1144(m), 1026(s), 989(m), 922(m), 848(vs), 831(m),
816(m), 780(s), 736(s), 708(vs), 668(m), 658(s), 604(s), 578(m),
547(m), 527(m), 478(vs), 398(w), 358(m), 336(w), 326(m),
318(m). Elemental Anal. Calc. for C39H30EuN9O12 (Eu(L)3(NO3)3):
C, 48.36; H, 3.12; N, 13.01. Found: C, 48.88; H, 3.18; N, 13.26%.
3.2. Description of the crystal structure
2.4. Structure determination
A projection of the crystal structure of [Eu(L)3(NO3)3] is given in
Fig. 2 and the crystallographic data are summarized in Table 1. The
structure contains a single Eu(III) ion in the asymmetric unit, and
has two molecules in the unit cell. The first coordination sphere
of the complex contains three ligands bonded to Eu(III) through
the carbonyl oxygen of L and three bidentate nitrate ions, resulting
in a coordination number of nine around the central ion. Interest-
ingly, all ligands are lying in approximately parallel planes. As
illustrated in Fig. 2, the geometry around the Eu(III) ion is best de-
scribed as a highly distorted monocapped square antiprism with
the top vertex and a corner of one of the squares being defined
by nitrato oxygens, while the other three corners of this square
Crystals suitable for single crystal X-ray diffraction were grown
by slow diffusion of an ethanolic solution of 0.10 g (0.48 mmol) of
ligand into an ethanolic solution of 0.067 g (0.16 mmol) of
Eu(NO3)3ꢁ5H2O in a Y-tube over a period of 3 weeks. Crystals ap-
peared as large orange blocks of sizes up to 0.5 ꢂ 0.5 ꢂ 2.0 mm3.
Crystallographic data and refinement details are given in Ta-
ble 1. A crystal was selected for the X-ray measurements and
mounted to the glass fiber using the oil drop method [22] and data
were collected at 173 K on a Nonius Kappa CCD diffractometer (Mo
Ka radiation, graphite monochromator, k = 0.71073). The intensity

108
S. Akerboom et al. / Polyhedron 64 (2013) 106–109
Fig. 3. Projection of [Eu(L)3(NO3)3], showing part of the p-stacking interactions.
sus the magnetic dipole (MD) 5D0 ? 7F1 transition at 593 nm can
be used as a probe for the local symmetry around Eu(III) [31].
For [Eu(L)3(NO3)3] the ratio (5D0 ? 7F2)/(5D0 ? 7F1) is around 11.
This clearly indicates the lack of an inversion centre at Eu(III),
which is in agreement with the crystal structure. Furthermore,
the 5D0 ? 7F2 transition is visibly split into two bands centered
at 615 and 619 nm as a result of the crystal field around Eu(III).
The intensity of the 5D0 ? 7F3 and 5D0 ? 7F4 transitions at 651
and 691 nm is negligible. The decay profile of the intensity of the
5D0 ? 7F2 transition has been fitted to a single exponential func-
tion with a lifetime sexp of 0.74 ms. The radiative lifetime can be
estimated from the emission profile, using the relative intensity
of the emission lines with respect to the MD transition. The inten-
sity of this transition is of a purely MD nature, and is nearly insen-
sitive of the environment of Eu(III). Following the procedure
outlined by Werts et al., and assuming a refractive index of 1.5
for [Eu(L)3(NO3)3], a value of 1.47 ms is obtained for srad [32].
Fig. 2. Projection of the structure of [Eu(L)3(NO3)3], showing the atomic labeling
scheme. The coordination sphere around Eu is approximately monocapped square
antiprismatic.
are occupied by the carbonyl oxygens of the ligand. The other
square face comprises four nitrato oxygen atoms. The Eu–O bond
lengths are given in Table 2. They range from 2.322(1) to
2.360(2) Å for the Eu-carbonyl oxygens of the ligands, which can
be considered as normal [28]. For the chelating nitrates, Eu–O bond
0
lengths vary from 2.472(2) to 2.550(2) ÅA. The length of the C@O
double bond varies from 1.258(3) to 1.268(4) Å, indicating a slight
elongation as compared to C(sp2)@O bonds that are around 1.20 Å
on average [29]. The Eu–O–C bond angles vary markedly within the
complex, as can be seen from Table 2. p-Stacking interactions play
an important role in the crystal structure: the plane-to-plane dis-
tances between neighboring phenanthroline rings range from
3.227(1) to 4.097(1) Å. A number of these stacking interactions is
shown in Fig. 3; the distances between the centroids in this partic-
ular stacking interaction are 3.77–4.41 Å.
The intrinsic quantum yield can be found from sexp/srad and equals
50%, indicating that radiative and non-radiative decay rates are
similar in [Eu(L)3(NO3)3]. The external photoluminescence quan-
tum yield for [Eu(L)3(NO3)3]was found to be 22%. Compared to
other Eu(III)-complexes, this indicates fairly efficient photolumi-
nescence. From this value and the intrinsic quantum efficiency,
the sensitization efficiency gsens can be calculated to be 44% [33].
These values indicate that non-radiative decay processes of the
5D0 state compete effectively with the radiative processes, and that
L acts as a moderately efficient antenna. The photophysical param-
eters of [Eu(L)3(NO3)3] are listed in Table 3. Direct comparison of
these parameters to those determined for other Eu(III)-phenan-
throline complexes seems inappropriate. The methyl group at
3.3. Photoluminescence
When illuminated with a standard laboratory UV lamp emitting
at 366 nm [Eu(L)3(NO3)3] shows intense red luminescence. The so-
lid-state photoluminescence spectra of the complex recorded at
room temperature are given in Fig. 4. The excitation spectrum fea-
tures a broad band in the near-UV region with a maximum at
275 nm as a result of ligand-centered excitation. The small spike
on the shoulder of the broad band at 395 nm can be attributed to
direct excitation of the Eu(III) ion via the (5L6, 5G2, 5L7, 5G3) 7F0
transition [30]. The emission spectrum is characteristic of Eu(III)
with most of the emission intensity in the 5D0 ? 7F2 transition at
617 nm, indicating ligand to metal energy transfer. The intensity
ratio of the hypersensitive electric dipole 5D0 ? 7F2 transition ver-
Table 2
Selected bond distances and angles for [Eu(L)3(NO3)3].
Bond distance (Å)
Bond angle (°)
Eu–O1
Eu–O2
Eu–O3
Eu–O4
Eu–O5
Eu–O6
Eu–O7
Eu–O8
Eu–O9
C–O7
2.472(2)
2.516(2)
2.522(2)
2.486(2)
2.477(2)
2.550(2)
2.360(1)
2.322(1)
2.350(2)
1.265(3)
1.258(3)
1.286(4)
O1–Eu–O2
O3–Eu–O4
O5–Eu–O6
51.01(7)
51.17(6)
50.68(6)
C–O7–Eu
C–O8–Eu
C–O9–Eu
143.8(2)
152.0(2)
134.4(2)
C–O8
C–O9
Fig. 4. Excitation (left) and emission spectrum of [Eu(L)3(NO3)3].

S. Akerboom et al. / Polyhedron 64 (2013) 106–109
109
Table 3
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.
cam.ac.uk.
Photophysical parameters for [Eu(L)3(NO3)3].
sexp (ms)
srad (ms)
Uint (%)
gsens (%)
Utot (%)
References
0.74
1.47
50
44
22
s
g
exp: experimental lifetime,
sens: sensitization efficiency, Utot: external photoluminescence quantum yield.
srad: radiative lifetime, Uint: intrinsic quantum yield,
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1-Methyl-1,10-phenanthrolin-2(1H)-one has been synthesized
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Eu(III) as a central ion. The resulting complex is described by
[Eu(L)3(NO3)3] wherein the ligand is bound to Eu(III) in a mono-
dentate fashion via its carbonyl oxygen. The complex exhibits
strong photoluminescence characteristic of Eu(III) upon excitation
in the ligand-centered band using near UV radiation. The quantum
yield of this process is 22% indicating that the ligand is acting as a
moderately effective antenna for Eu(III) luminescence, despite its
unusual monodentate mode of binding to the metal ion. Further
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Appendix A. Supplementary data
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CCDC 918685 contains the supplementary crystallographic data
for [Eu(L)3(NO3)3]. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge