1,3-Dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide as ligand for the preparation of luminescent lanthanide complexes

Article abstract of DOI:10.1080/00958972.2019.1608440

Europium(III) coordination compounds having general formula [Eu(β-dike)₃L₂] (β-dike = dibenzoylmethanate, tenoyltrifluoroacetonate; L = 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide) were isolated and characterized. The comple

Full text of DOI:10.1080/00958972.2019.1608440

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
1,3-Dimethyl-2-phenyl-1,3-diazaphospholidine-2-
oxide as ligand for the preparation of luminescent
lanthanide complexes
Marco Bortoluzzi & Alberto Gobbo
To cite this article: Marco Bortoluzzi & Alberto Gobbo (2019): 1,3-Dimethyl-2-phenyl-1,3-
diazaphospholidine-2-oxide as ligand for the preparation of luminescent lanthanide complexes,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2019.1608440
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1608440
1
,3-Dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide
as ligand for the preparation of luminescent
lanthanide complexes
a,b
a
Marco Bortoluzzi and Alberto Gobbo
a
Dipartimento di Scienze Molecolari e Nanosistemi, Universit ꢀa Ca’ Foscari Venezia, Mestre (Ve), Italy;
b
Consorzio Interuniversitario Reattivit ꢀa Chimica e Catalisi (CIRCC), Bari, Italy
ABSTRACT
ARTICLE HISTORY
Europium(III) coordination compounds having general formula
Received 2 February 2019
Accepted 22 March 2019
[
Eu(b-dike) L ] (b-dike ¼dibenzoylmethanate, tenoyltrifluoroaceto-
3
2
nate;
L ¼1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide)
KEYWORDS
Europium(III);
gadolinium(III); terbium(III);
photoluminescence; diaza-
phospholidine-2-oxide
were isolated and characterized. The complexes exhibited bright
red emission associated to the D !F transitions of the metal
5
7
0
J
center upon excitation with near-UV light, with intrinsic quantum
yields around 51% and 65%, respectively, for the dibenzoylmetha-
nate and tenoyltrifluoroacetonate derivatives. More information
about the behavior of 1,3-dimethyl-2-phenyl-1,3-diazaphospholi-
dine-2-oxide as an antenna-ligand towards trivalent lanthanide
3 3
ions was obtained by its coordination to [Ln(NO ) ] (Ln ¼Eu, Gd,
Tb) metal fragments.
1. Introduction
Luminescent trivalent lanthanide complexes are of interest for applications in
advanced technologies [1–5], such as electronic devices [6, 7], electroluminescent
materials [8, 9], and biological probes [10–12]. Because of the hard Lewis acid charac-
ter of trivalent lanthanide ions [13], molecules containing the [P¼O] fragment behave
as good O-donor ligands towards these metal centers. Triphenylphosphine oxide
(tppo) is the most common example, and several visible- and NIR-emitting lanthanide
complexes with triphenylphosphine oxide and anionic ligands in the coordination
CONTACT Marco Bortoluzzi
markos@unive.it
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
M. BORTOLUZZI AND A. GOBBO
sphere are reported [14–25]. The structurally related chelating phosphine oxide bis(2-
diphenylphosphino)phenyl)ether (dpepo), once coordinated to [Eu(b-diketonate)3],
(
afforded luminescent complexes with quantitative excited-state energy transfer from
the ligands to the metal ion, leading to exceptionally high photoluminescence quan-
tum yields in solution and in poly(methyl methacrylate) [26].
Hexamethylphosphoramide (hmpa) is another simple molecule able to coordinate
trivalent lanthanide ions [27–42]. Nitrato complexes of Eu(III) with hmpa exhibit tribo-
luminescence [43–45] and the luminescence of Tb(III) and Gd(III) nitrato complexes
with hmpa and dibenzoylmethanate was studied [46]. The complexes
[
Ln(depma)(NO ) (hmpa) ] (depma ¼9-diethylphosphono-methylanthracene; Ln ¼Dy,
3
3
2
Gd) are recent examples of magneto-optic materials [47].
Despite the fact that molecules based on the 1,3-diazaphospholidine-2-oxide skel-
eton resemble phosphoramide and phosphine oxide ligands, their coordination chem-
istry towards lanthanide ions is poorly explored. To the best of our knowledge, no
Ln(III) coordination compound with this species was reported, despite the fact that
1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide and related species were pro-
posed as efficient extraction agents for group 3 and lanthanide ions [48]. As a part of
our research on new luminescent metal complexes with phosphoramide and arylphos-
phonic diamide ligands [49, 50], in this manuscript we report the synthesis and photo-
physical characterization of new Ln(III) complexes with 1,3-dimethyl-2-phenyl-1,3-
diazaphospholidine-2-oxide.
2. Experimental
2.1. Materials and methods
Commercial solvents (Aldrich) were purified as described [51]. Lanthanide oxides and
chlorides were purchased from Chempur. All the other reagents (Aldrich, Alfa Aesar)
were used as received. The syntheses of the lanthanide complexes were carried out
under inert atmosphere, working in a glove-box (MBraun Labstar with MB 10 G gas
purifier) filled with N and equipped for inorganic syntheses. [Ln(NO ) (H O) ] (Ln ¼Eu,
2
3 3
2
3
Gd, Tb) nitrato complexes were prepared following a reported procedure [52].
Elemental analyses (C, H, N) were carried out at the University of Padua using a
Fison EA1108 microanalyzer. Magnetic susceptibilities were measured on solid samples
at 298 K with a MK1 magnetic susceptibility balance (Sherwood Scientific Ltd.) and
corrected for diamagnetic contribution by means of tabulated Pascal’s constants [53].
Melting point measurements were carried out with a B u€ chi B 535 instrument. IR spec-
ꢀ
1
tra were collected from 4000–400 cm using a Perkin-Elmer Spectrum One spectro-
photometer. The air-sensitive samples were prepared in a glove-box using an air-tight
1
31
1
19
sample holder with KBr windows. H, PfHg, and F NMR spectra were recorded on
a Bruker AVANCE 400 spectrometer using CDCl and DMSO-d as solvents. The par-
3
6
tially deuterated fraction of the solvent was quoted with respect to tetramethylsilane
1
31
1
and used as an internal reference for H spectra. PfHgchemical shifts are reported
19
with respect to 85% H PO , with downfield shifts considered positive. F chemical
3
4
shifts are referred to CFCl3.

JOURNAL OF COORDINATION CHEMISTRY
3
1,3-Dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide (L) was obtained from reac-
tion of O¼PCl Ph with N,N’-dimethylethylenediamine in the presence of triethylamine
2
1
[
54] using CH Cl as solvent. Spectroscopic data of the ligand: H NMR (CDCl , 298 K),
2
2
3
d: 7.78–7.38 (m, 5H, Ph), 3.47–3.31 (m, 2H,CH ), 3.31–3.14 (m, 2H,CH ), 2.52 (d, 6H, J
2
2
ꢀ1
PH
31
1
¼
10.0 Hz, CH3). PfHgNMR (CDCl , 298 K), d: 29.65. IR (KBr), cm : 1188 s (ꢀ
).
3
1
P¼O
UV-VIS (CH Cl , 298 K), nm: < 300. H NMR data are comparable to those reported
2
2
3
1
[54], and the P NMR chemical shift is in line with that given [48].
2
.2. Synthesis of [Eu(b-dike) L ]
3 2
The complexes [Eu(b-dike) L ] (b-dike ¼dibenzoylmethanate (dbm), tenoyltrifluoroacet-
3
2
onate (tta); L ¼1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide) were prepared
by dissolving 1.5 mmol of dibenzoylmethane (0.336 g) or tenoyltrifluoroacetone
(
1
(
0.333 g) in THF (20 mL), subsequently reacted with potassium tert-butoxide (0.168 g,
.5 mmol). After 2 h, the solution thus obtained was slowly added to anhydrous EuCl3
0.129 g, 0.5 mmol) in 20 mL of THF and the reaction mixture was left under stirring at
room temperature for 4 h. A THF solution (10 mL) containing 1.0 mmol of L (0.210 g)
was then added and the mixture was kept overnight under stirring at room tempera-
ture. After filtration, the solvent was removed under reduced pressure and dichlorome-
thane (25 mL) was added. The solution was purified by centrifugation, then the
solvent was evaporated and diethylether (5 mL) was added. The yellowish solid that
separated out was collected by filtration and dried under vacuum. Yield ¼ 75%
(b-dike ¼dbm), 80% (b-dike ¼tta).
2
.3. Characterization of [Eu(dbm) L ]
3 2
ꢀ
1
Anal. Calcd for C H EuN4O P (1242.13 g mol , %): C, 62.85; H, 5.11; N, 4.51. Found
65
63
8 2
ꢁ
M
ꢀ3
(
%): C, 62.61; H, 5.53; N, 4.49. M.P. ( C) 125 (dec.). v
cm : 1596–1409 s (ꢀdbm), 1185 sh, 1161 m (ꢀ
(c.g.s.u.) 3.9 ꢂ10 . IR (KBr),
corr
1
ꢀ
1
). H NMR (DMSO-d , 298 K), d:
6
P¼O
7
¼
J
.70–7.44 (m, 10H, P-Ph), 7.20 (s, 2H, b-dike-CH), 7.19 (s, 1H, b-dike-CH), 6.82 (t, 6H, JHH
7.4 Hz, b-dike-Ph), 6.67 (dd, 12H, JHH ¼6.3 Hz, J ¼7.4 Hz, b-dike-Ph), 5.72 (d, 12H,
HH
¼6.3 Hz, b-dike-Ph), 3.30–3.23 (m, 4H, N-CH ), 3.23–3.15 (m, 4H, N-CH ), 2.36 (d,
HH
2
2
31
1
12H, J ¼10.0 Hz, N-Me). PfHgNMR (DMSO-d , 298 K), d: 28.1 (FWHM ¼30 Hz).
PH
6
2
.4. Characterization of [Eu(tta) L ]
3 2
ꢀ
1
Anal. Calcd for C H EuF N O P S (1235.92 g mol , %): C, 42.76; H, 3.43; N, 4.53.
44
42
9 4 8 2 3
ꢁ
M
ꢀ3
Found (%): C, 42.58; H, 3.45; N, 4.49. M.P. ( C) 78 (dec.). v
(c.g.s.u.) 4.2 ꢂ10 . IR
corr
ꢀ
1
1
(
KBr), cm : 1616–1302 s (ꢀ ), 1182 s, 1136 s (ꢀ
). H NMR (DMSO-d , 298 K), d:
P¼O 6
tta
7.72–7.45 (m, 10H, P-Ph), 7.43 (s, 3H, b-dike-thioph), 6.51 (s, 3H, b-dike-thioph), 6.35 (s,
3
H, b-dike-thioph), 3.79 (s, 3H, b-dike-CH), 3.29-3.22 (m, 4H, N-CH ), 3.22–3.15 (m, 4H,
2
31
1
N-CH ), 2.37 (d, 12H, J ¼10.0 Hz, N-Me). PfHgNMR (DMSO-d , 298 K), d: 28.1
2
PH
6
19
(FWHM ¼170 Hz). F NMR (DMSO-d , 298 K), d: –78.3.
6

4
M. BORTOLUZZI AND A. GOBBO
2.5. Coordination of 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide to
[
Ln(NO ) ] (Ln ¼Eu, Gd, Tb)
3
3
The reaction between the [Ln(NO ) (H O) ] precursors (1.0 mmol; Ln ¼Eu, 0.392 g;
3
3
2
3
Ln ¼Gd, 0.397 g; Ln ¼Tb, 0.399 g) and two equivalents of L (0.420 g) were carried out
in THF (25 mL) at room temperature under inert atmosphere (glove box). Stirring was
continued overnight, then the solvent was evaporated at reduced pressure and the
resulting solid was triturated with diethylether (10 mL), filtered and dried
under vacuum.
2.6. Characterization of nitrato-derivatives
ꢀ
1
as
IR (M ¼Eu, solid), cm : 1455 s, 1438 s (ꢀ
), 1305 s (ꢀ ONO), 1154 s (ꢀ
). IR
P¼O
N¼O
ꢀ
1
as
(
M ¼Gd, solid), cm : 1456 s, 1439 s (ꢀ
), 1307 s (ꢀ ONO), 1154 s (ꢀP¼O). IR (M ¼Tb,
N¼O
ꢀ
1
as
solid), cm : 1457 s, 1439 s (ꢀ
), 1309 s (ꢀ ONO), 1154 s (ꢀ
).
P¼O
N¼O
2.7. Photoluminescence measurements
Absorption spectra in solution were collected using a Perkin-Elmer Lambda 35 spectro-
photometer. Emission (PL) spectra of air-sensitive samples were recorded under inert
atmosphere (glove box) from 400–1035 nm with an OceanOptics Flame-T spectrometer
coupled with an optical fiber, a collimating lens and a longpass filter, using UV led
(280–375 nm) excitation sources. When necessary, samples were cooled at 200 K with
cold N gas coming from a liquid nitrogen evaporator. Photoluminescence emission (PL)
2
and excitation (PLE) measurements of air-stable compounds were carried out at room
temperature on solid samples by a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. A
continuous-wave xenon arc lamp was used as source, selecting the excitation wave-
length by a double Czerny–Turner monochromator. A single grating monochromator
coupled to a photomultiplier tube was used as detection system for optical emission
measurements. Excitation and emission spectra were corrected for the instrumental
functions. Time-resolved analyses were performed in multichannel scaling modality
(
MCS) by using a pulsed UV led source (k
¼377 nm). The intrinsic quantum yields
emission
Eu
(Q ) were estimated from the s values on the basis of Equation (1), where n indicates
Eu
the refractive index of the sample. The value of 1.5 is assumed for solid state samples in
this work for comparative purposes; it is worth noting that refractive index may differ
5
7
5
7
depending on the nature of the compound. I( D !F )/I( D !F ) is the ratio between
0
J
0
1
5
7
the total integrated emission from the Eu( D ) level to the F manifold and the inte-
0
J
5
7
grated intensity of the transition D !F [55].
0
1
5
7
ðD !F Þ
Eu
Eu
3
0
J
Q
¼14:65n I
(1)
5
7
IðD !F ÞsðsÞ
0
1
2.8. Computational details
The computational geometry optimizations of the [Eu(b-dike) L ] complexes were car-
3
2
ried out without symmetry constraints using the hybrid-GGA DFT functional EDF2 [56],

JOURNAL OF COORDINATION CHEMISTRY
5
Scheme 1. Synthesis of [Eu(b-dike) L ] complexes.
3
2
in combination with the split-valence polarized 6–31G(d,p) basis set for light atoms
and the scalar quasirelativistic 4f-in-core pseudopotential ECP52MWB for europium,
6
with the associated valence basis set [57–59]. Because of the inclusion of the 4f shell
of Eu(III) in the pseudopotential, the ‘‘restricted” formalism was applied. The stationary
points were characterized by IR simulations (harmonic approximation) [60].
Calculations were performed with Spartan ’16 (Wavefunction Inc.), build 2.0.3 [61, 62],
running on Intel Xeon-bases x84-64 workstations. The geometries of the first coordin-
ation spheres were compared with ideal geometries by means of the SHAPE software,
version 2.1 [63].
3. Results and discussion
b-Diketonate complexes having general formulas [Eu(b-dike) L ], where b-dike is
3
2
dibenzoylmethanate (dbm) or tenoyltrifluoroacetonate (tta), were obtained by reacting
EuCl with the b-diketone, previously deprotonated with potassium tert-butoxide, and
3
then with 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide, as depicted in
Scheme 1.
The complexes thus isolated are stable in air at room temperature. Despite the fact
that we were unable to obtain crystals suitable for X-ray diffraction, the characteriza-
tion data support the formation of the desired products. In particular, elemental analy-
ses agree with the proposed formulations and the molar magnetic susceptivities are in
line with the expected value for Eu(III) derivatives at room temperature [13]. The IR
spectra show, besides the typical vibrations of the b-diketonate ligands, the ꢀ
PO
ꢀ
1
stretch in the range 1186–1136 cm , close to the value observed for the free ligand.
1
The H NMR spectrum in DMSO-d of the dbm complex exhibits resonances of the
6
phenyl substituents at 6.8, 6.7 and 5.7 ppm, similar to those recorded for other euro-
pium complexes containing the [Eu(dbm) ] fragment in DMSO-d solution [64, 65].
3
6
These signals are replaced by three resonances around 7.4, 6.5, and 6.3 ppm in the tta
1
derivative, comparable to the thiophene-related H NMR signals of other [Eu(tta)3]

6
M. BORTOLUZZI AND A. GOBBO
complexes [66, 67]. The b-dike CH resonances fall at 7.2 ppm for the dbm derivative
and at meaningfully lower chemical shift value, around 3.8 ppm, for the tta complex,
in agreement with literature outcomes [64–67]. The presence of the –CF substituent
3
19
19
in the tta complex is confirmed by the F NMR singlet at –78.3 ppm. F NMR chem-
ical shifts around –80 ppm are commonly reported for the tta ligand coordinated to
Eu(III) [66–68]. The 1,3-diazaphospholidine-2-oxide ligand can be detected in the H
1
3
NMR spectrum by the N-Me resonance around 2.36 ppm, with J coupling constant
PH
of 10 Hz. The doublet is scarcely shifted with respect to the free ligand and the cou-
pling constant is not affected by the interaction with Eu(III). Two second-order multip-
lets are also present in the range 3.30–3.15 ppm, attributable to the [N-CH -CH -N]
2
2
fragment of the five-membered cycle. Also in this case the resonances are close to
those recorded for free 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide. The
presence of two groups of signals is associated to the lack of equivalence of the pro-
tons in E and Z position with respect to the P-bonded phenyl ring. The aromatic sub-
1
stituent is associated to a series of signals in the high frequency region of the H NMR
spectra. Integration data support the formation of eight-coordinate complexes with
three b-diketonate and two [O¼P]-donor ligands. Only one singlet is present in the
3
1
1
PfHgNMR spectra, broadened with respect to the free ligand because of paramag-
netic relaxation, suggesting that 1,3-diazaphospholidine-2-oxide interacts with the
Eu(III) center in DMSO-d solution. The FWHM (full width at half maximum) values are
6
30 and 170 Hz, respectively, for the dbm and tta derivatives, while the FWHM value for
the free ligand is 3 Hz. The higher broadening in [Eu(tta) L ] with respect to
3
2
[
Eu(dbm) L ] agrees with the lower steric bulk of tta, that allows a stronger coordin-
3 2
31
ation of L. Broad P NMR resonances are common for ligands with [O¼P] donor moi-
eties coordinated to Eu(III), as occurs for example for [Eu(tta) (O¼PPh ) ] and related
3
3 2
species [68]. The scarce chemical shift variation with respect to the free ligand sug-
gests fast L/DMSO-d exchange in the Eu(III) coordination sphere of the complexes
6
here described [69–71].
The DFT-optimized geometries of the [Eu(b-dike) L ] complexes, depicted in
3
2
Figure 1, support the formation of eight-coordinate compounds, with two 1,3-diaza-
phospholidine-2-oxide ligands coordinated to Eu(III) with comparable bond lengths. As
31
deduced from the
P NMR data, the Eu-O(L) distances are slightly shorter
(2.322–2.354 Å) in the tta derivative with respect to the dbm complex (2.395–2.420 Å)
as a consequence of the lower steric crowding. The bond lengths related to coordin-
ation of dbm and tta are in line with data reported for several other Eu(III) b-diketo-
nate complexes [72]. The first coordination spheres of both the complexes are
asymmetric (C ). The analysis carried out with the SHAPE software (Table 1) indicates
1
that the geometries more similar to the [EuO ] fragment of [Eu(dbm) L ] are the
8
3 2
square antiprism and the triangular dodecahedron, while in the case of [Eu(tta) L ] the
3
2
first coordination sphere resembles the biaugmented trigonal prism.
Absorption and photoluminescence data of the b-diketonate complexes are sum-
marized in Table 2. As observable in Figure 2, dichloromethane solutions of the com-
pounds are characterized by strong absorptions in the near-UV range. Solid-state
photoluminescence measurements showed that the excitation of the coordinated
ligands with wavelengths below 480 nm causes the emission from the Eu(III) center

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 1. DFT-optimized structures of the [Eu(b-dike) L ] complexes. Hydrogens are omitted for
3
2
clarity. Color map: Eu, green; C, grey; O, red; N, blue; F, light blue; P, orange; S, yellow. Selected
computed bond lengths for [Eu(dbm) L ], Å: Eu-O(dbm) 2.333, 2.377, 2.353, 2.373, 2.359, 2.364; Eu-
3
2
O(L) 2.395, 2.420. Selected computed bond lengths for [Eu(tta) L ], Å: Eu-O(tta) 2.342, 2.369, 2.355,
3
2
2.376, 2.324, 2.450; Eu-O(L) 2.322, 2.354.
Table 1. Output of the SHAPE software for the [Eu(b-dike) L ] complexes.
3
2
OP
HPY
HBPY
CU
SAPR
TDD
J26
J14
BTPR
J84
TT
[
Eu(dbm)
3
L
2
]
31.02
31.66
23.59
23.28
15.83
15.89
9.401
10.53
0.944
1.346
0.978
1.434
15.15
14.23
29.27
29.16
1.542
1.071
3.675
3.677
10.24
11.18
[
Eu(tta) L ]
3 2
OP D8h, octagon; HPY C7v, heptagonal pyramid; HBPY D6h, hexagonal bipyramid; CU O
prism; TDD D2d, triangular dodecahedron; J26 D2d, Johnson gyrobifastigium; J14 D3h, Johnson elongated triangular
bipyramid; BTPR C2v, biaugmented trigonal prism; J84 D2d, Snub disphenoid; TT T , triakis tetrahedron.
h
, cube; SAPR D4d, square anti-
d
Table 2. Absorption and photoluminescence data of the [Eu(b-dike) L ] complexes.
3
2
Compound
Data
[
Eu(dbm)
3
L
2
]
UV-VIS (CH
2
Cl
2
, r.t.), nm: 396, 347 max, 250. PL (solid, kexcitation ¼300 nm, r.t.), nm: 579
5
7
5
7
5
7
(
6
D
0
!F
0
, 0.4%), 588, 591, 597 ( D
0
!F
1
, 4.7%), 611, 613, 618, 623, 626 ( D
0
!F
2
, 74.2%),
5
7
5
7
49, 651, 653, 656 ( D
0
!F
3
, 5.0%), 690, 699, 703 ( D
0
!F
4
, 15.7%). PLE (solid, kemission
¼
5
7
6
14 nm, r.t.), nm: < 480, 464 ( D
2
F
0
). s (solid, kexcitation ¼377 nm, kemission ¼613 nm),
Eu
ms: 482. Q ¼51%. CIE 1931: x ¼0.666, y ¼0.334.
Eu
[
Eu(tta)
3
L
2
]
UV-VIS (CH
Cl
2 2
, r.t.), nm: 343 max, 270. PL (solid, kexcitation ¼400 nm, r.t.), nm: 579
5
7
5
7
5
7
(
(
(
D
D
0
!F
0
, 0.6%), 589, 592, 595 ( D
0
!F
1
, 4.6%), 611, 614, 617 ( D
0
!F
2
, 75.0%), 650, 654
5
7
5
7
0
!F
3
, 3.9%), 690, 701 ( D
0
!F , 15.9%). PLE (solid, kemission ¼614 nm, r.t.), nm: < 480
4
5
7
ligands excitation), 464 ( D
2
F
0
). s (solid, kexcitation ¼377 nm, kemission ¼613 nm), ms:
Eu
Eu
6
10. Q ¼65%. CIE 1931: x ¼0.665, y ¼0.334.
(
Figure 2). Direct excitation of Eu(III) is also present in the PLE spectra, corresponding
5
7
to the D
F transition centered at 464 nm [73]. The PL spectra display only the
0
2
5
7
5
7
5
7
typical D !F bands, the most intense occurring for J ¼2. The D !F / D !F
0
J
0
2
0
1
1
5
7
intensity ratio ranges from 15.8 (b-dike ¼dbm) to 19.2 (b-dike ¼tta) and the D !F
0
transition is separated into three peaks because of the Stark effect. These data suggest
low symmetry of the first coordination sphere, with 2 the maximum possible rotational
symmetry order, in agreement with the results of DFT calculations. The occurrence of
5
7
only one D !F transition supports the presence of only one Eu(III) emitting center,
0
0
even if such information is not conclusive [73].

8
M. BORTOLUZZI AND A. GOBBO
Figure 2. Normalized absorption (CH Cl solution, violet line), emission (solid, red line) and excita-
2
2
tion (solid, light blue line) spectra of the [Eu(b-dike) L ] complexes.
3
2
Emissions of all the compounds fall in the reddish orange region of the CIE 1931
chromaticity diagram with unitary color purity [74], as shown in Figure 3. The lifetimes
(s) were determined from the mono-exponential interpolation of the luminescence
decay curves, reported in Figure 3. The measured s values are 482 and 610 ms,
respectively, for solid [Eu(dbm) L ] and [Eu(tta) L ]. The lifetime of the tta derivative is
3
2
3 2
comparable to that reported for [Eu(tta) (ptso) ] (ptso ¼p-tolyl sulfoxide), 598 ms [75].
3
2

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 3. Semi-log plot of the luminescence decay curves of the [Eu(b-dike) L ] complexes.
3
2
b-dike ¼dbm, wine line; b-dike ¼tta, red line. Solid samples, k
¼377 nm, kemission ¼
excitation
6
13 nm, r.t. Inset: CIE 1931 chromaticity coordinates.
Eu
Eu
The Q values calculated on the basis of Equation (1) are 51% and 65%, respectively,
for the dibenzoylmethanate and tenoyltrifluoroacetonate derivatives.
Further information about the sensitization of Ln(III) luminescence by the 1,3-diaza-
phospholidine-2-oxide ligand was obtained by coordination to [Ln(NO ) ] (Ln ¼Eu, Gd,
3
3
Tb) metal fragments. The compounds isolated from the 1: 2 reaction between
[
Ln(NO ) (H O) ] and L rapidly decompose in the presence of moisture and their com-
3 3 2 3
plete characterization is beyond the scope of this work. IR data agree with the pres-
2
ence of nitrato ligands k -coordinated to Ln(III) [76]. Moreover, the coordination of L is
ꢀ
1
highlighted by the stretchings relative to the P¼O bonds around 1154 cm
.
The emission spectrum of the Eu(III) derivative, obtained by excitation with near-UV
5
7
light (Figure 4), shows the typical bands associated to the D !F transitions of the
0
J
5
7
metal center, the most intense corresponding to the hypersensitive D !F centered
0
2
at 619 nm. In the emission spectrum of the corresponding Gd(III) species a broad band
with maximum at 505 nm and a shoulder around 445 nm can be detected, tentatively
attributable to phosphorescence and fluorescence from the coordinated 1,3-diazaphos-
pholidine-2-oxide ligand. The relative intensity of the highest-energy transition slightly
decreases on cooling the sample (Figure 4). The triplet state (T) energy of the coordi-
ꢀ
1
nated ligands estimated on the basis of the emission spectrum is around 21,500 cm
well above the Eu(III) D level, 17250 cm , and also higher than the Tb(III) D reson-
ance level, 20,430 cm [1]. Despite the fact that the T – D energy gap is below the
,
5
ꢀ1
5
0
4
ꢀ
1
5
4
value suggested by Latva et al. [77] for an efficient energy transfer, the coordination
of 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide to Tb(III) nitrate afforded a

1
0
M. BORTOLUZZI AND A. GOBBO
Figure 4. PL spectra of the Ln(III) nitrato complexes with 1,3-dimethyl-2-phenyl-1,3-diazaphospholi-
dine-2-oxide. Solid samples, inert atmosphere, kexcitation ¼375 nm. Ln ¼Eu, red line, r.t. Ln ¼Gd,
blue line, r.t. Ln ¼Gd, cyan line, 200 K. Ln ¼Tb, green line, r.t.
Table 3. Photoluminescence data of the Ln(III) nitrato-complexes with 1,3-dimethyl-2-phenyl-1,3-
diazaphospholidine-2-oxide.
Ln
Eu
Data
5
7
5
7
5
7
PL (solid, kexcitation ¼375 nm, r.t.), nm: 580 ( D
0
!F
0
, 1.9%), 593 ( D
4
0
!F
1
, 16.3%). 619 ( D
0
!F
1
2
,
5
7
5
7
6
3.4%), 652 ( D
0
!F
3
, 3.8%), 689, 695, 702 ( D
0
!F
, 14.6%).
-
Gd
Tb
PL (solid, kexcitation ¼375 nm, 200 K), nm: 505 max ((ligands luminescence, FWHM: 4000 cm ), 445 sh
ꢀ1
(
ligands luminescence). Triplet state: 21,500 cm
.
5
7
5
7
5
7
5
7
PL (solid, kexcitation ¼375 nm, r.t.), nm: 490 ( D
4
!F
6
), 544 ( D
4
!F
5
), 587 ( D
4
!F
4
), 622 ( D
4
!F
3
),
5
7
6
52 ( D
4
!F
2
), 400–700 (ligands luminescence).
green-emitting complex upon excitation with near-UV light, and in the PL spectrum
5
7
the typical D !F transitions of the metal center can be detected, the most intense
4
J
centered at 544 nm and corresponding to J ¼5 (Figure 4). The Tb(III) bands are super-
imposed to a broad phosphorescence band between 400 and 700 nm, comparable to
5
that of the Gd(III) derivative, indicating that the T ! D energy transfer is not quanti-
4
tative. Photoluminescence data of the nitrato-derivatives are summarized in Table 3.
4. Conclusion
In this article, we report some preliminary results about the application of 1,3-diaza-
phospholidine-2-oxide ligands in the field of visible-emitting lanthanide complexes,
focusing attention on 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide. The col-
lected data indicate that the ligand here studied is a promising candidate for the
preparation of luminescent lanthanide derivatives, in particular considering Eu(III) as
metal center. The introduction of different substituents in the 1,3-diazaphospholidine-
2
-oxide skeleton allows tuning of the electronic and steric features of this class of
ligands. For this reason, the information summarized in this work could be helpful for
the design of new antenna-ligands to be applied for the preparation of highly lumi-
nescent lanthanide derivatives.

JOURNAL OF COORDINATION CHEMISTRY
11
Disclosure statement
There are no conflicts to declare.
Funding
Ca’ Foscari University of Venice is gratefully acknowledged for financial support (Bando Progetti
di Ateneo 2014, D.R. 553/2014 prot. 31352; Bando Spin 2018, D.R. 1065/2018 prot. 67416).
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