The Role of Blue-Emissive 1,8-Naphthalimidopyridine N-Oxide in Sensitizing EuIII Photoluminescence in Dimeric Hexafluoroacetylacetonate Complexes

Article abstract of DOI:10.1002/ejic.201700019

Two new fluorescence molecules incorporating coordinated pyridine N-oxide (PyNO) and the blue-emitting 1,8-naphthalimide (NI) chromophores, 4-(1,8-naphthalimido)pyridine N-oxide (4-NIPyNO) and 3-(1,8-naphthalimido)pyridine N-oxide (3-NIPyNO), are designed and used as ancillary ligands for constructing mixed-ligand tris(β-diketonate) lanthanide complexes, Ln₂(hfa)₆(4-NIPyNO)₂ and Ln₂(hfa)₆(3-NIPyNO)₃ (Ln = Eu, Gd; hfa = hexafluoroacetylacetone). Single-crystal X-ray diffraction crystallography analyses reveal that all the complexes adopt dimeric dinuclear structures; in each case, two Ln^III ions from Ln(hfa)₃ moieties are bridged by the oxygen atoms of the N-oxide groups of 4-NIPyNO and 3-NIPyNO. The luminescence quantum yield experiment reveals that the introduction of the ancillary ligands results in the Eu^III ions having luminescence enhancement about 13 times greater than that from hydrated Eu(hfa)₃(H₂O)₂. The detailed photophysical determinations are carried out to gain insight into the roles of the NIPyNO ligands in enhancing Eu^III luminescence. By investigating the ligand-centered emission, it is found that both the singlet states (S) and the triplet states (T) are responsible for the energy transfer from the NIPyNO ligands to the metal center; in particular, the energy-transfer efficiencies of the singlet states reach up to 63 % and 71 % for Eu₂(hfa)₆(4-NIPyNO)₂ and Eu₂(hfa)₆(3-NIPyNO)₃, respectively. This auxiliary-ligand designing strategy enables the use of the blue-emitting chromophore as a sensitizer of the luminescence from Eu^III ions.

Full text of DOI:10.1002/ejic.201700019

A Journal of
Accepted Article
Title: Role of Blue-emissive 1,8-Naphthalimidopyridine N-oxide
on Sensitizing Eu(III) Photoluminescence in Dimeric
Hexafluoroacetylacetonate Complexes
Authors: Hong Li, Zhao Wang, Nan-Nan Liu, Peng Chen, and Peng-Fei
Yan
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700019
Link to VoR: http://dx.doi.org/10.1002/ejic.201700019

10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
Role of Blue-emissive 1,8-Naphthalimidopyridine N-oxide on
Sensitizing Eu(III) Photoluminescence in Dimeric
Hexafluoroacetylacetonate Complexes
Zhao Wang, [a] NanNan Liu, [b] Hongfeng Li,* [a] Peng Chen [a,c] and Pengfei Yan*[a]
Abstract: Tw o novel fluorescence molecules incorporating
coordinated pyridine N-oxide (PyNO) and blue-emittting 1,8-
naphthalimide (NI) chromophore, 4-(1,8-naphthalimido) pyridine N-
oxide (4-NIPyNO) and 3-(1,8-naphthalimido) pyridine N-oxide (3-
NIPyNO) are designed and used as ancillary ligands for constructing
mixed-ligand tris(β-diketonate) lanthanide complexes Ln2(hfa)6(4-
is the premise for their practical applications. Among the reported
complexes, lanthanide β-diketonates are among the most
investigation rare earth luminescent coordination compounds
because of their high chemical stability, easy preparation and
bright emission.6 How ever, the larger radius of lanthanide ions
make the neutral tris(β-diketonate) chelates usually contain
coordinated solvent molecules, w hich w ill significantly decrease
the luminescent efficiencies of the complexes by deactivating the
excited state of Ln(III) ion through the detrimental O‒H or C‒H
oscillators.7 Thus, the introduction of the second ligand, namely
ancillary ligand that can displace the solvents, and meanw hile
provide an effective intramolecular energy transfer from the ligand
to the lanthanide ion is the one of the most effective w ay for
preparing highly luminescent lanthanide complexes. Previously,
the ancillary ligands that used for constructing mixed-ligand β-
diketonate lanthanide complexes mainly include: (i) bidentate
containing nitrogen ligands such as 2,2'-bipyridine, 1,10-
phenanthroline and their derivatives;8 (ii) monodentate and
bidentate phosphine oxide species, such as triphenylphosphine
oxide (TPPO), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene
oxide (DDXPO) and bis(2-(diphenylphosphino)phenyl) ether
oxide (DPEPO).9
In most cases, the introduction of the ancillary ligands
obviously improved the luminescent performance of the
complexes. For demonstrating the mechanism of luminescence
enhancement, one often proposes that the ancillary ligands can
offer energy transfer from their excited states to that in β-
diketones or directly to the metal centres. How ever, this
hypothesis requires that the ancillary ligand should have some
absorbance at the excitation bands. Generally, the excited states
energy levels of the ancillary ligands are obviously higher than
that in β-diketones, and the spectral overlap of their absorption
spectra are usually inefficient. For instance, the famous
lanthanide complex Eu(TTA)3Phen, the maximum absorbance of
TTA is at about 330 nm in CHCl3, w hile that of Phen is at 260
nm.10 In order to obtain the strongest emission, the maxima
absorbance of the primary ligand β-diketones are often selected
as excitation bands. In the case, the ancillary ligands w ill not
participate in the sensitization of Ln(III) ion luminescence, but only
play the roles of saturating the coordination sphere of metal ions.
Therefore, the designing of ancillary ligands that have the excited
state energy levels closing to the β-diketones w ill significantly
enhance the luminescence efficiency Ln(III) ions. Coincidently,
organic blue-emitting fluorophors, such as 1,8-naphthalimide,
coumarin, etc., generally show very similar absorbance as that in
β-diketones in the region of 320-380 nm .11 Therefore, the blue-
emitting fluorescence dyes seem to be a good candidate as
ancillary ligand to solve this issue.
NIPyNO)2 and Ln2(hfa)6(3-NIPyNO)3 (Ln
= Eu, Gd; hfa =
hexafluoroacetylacetone). Single-crystal X-ray crystallography
analyses reveal that all the complexes adopt dimeric dinuclear
structures, in each case tw o Ln(III) ions from Ln(hfa)3 moieties are
bridged by the oxygen atoms of the N-oxide groups of 4-NIPyNOand
3-NIPyNO. The luminescence quantum yield experiment reveals that
the introduction of the ancillary ligands make the Eu(III) ions have
about 13-fold luminescent enhancement compared w ith that in
hydrated Eu(hfa)3(H2O)2. The detailed photophysical determination
are carried out to insight into the roles of NIPyNOs on enhancing
Eu(III) luminescence. By investigating the ligand-centre emission, it is
found that both the singlet states (S) and the triplet states (T) are
responsible for the energy transfer from NIPyNOs to metal centre,
especially, the energy transfer efficiencies of the singlet states reach
up to 63% and 71% for Eu2(hfa)6(4-NIPyNO)2 and Eu2(hfa)6(3-
NIPyNO)3, respectively. This auxiliary ligand designing strategy
enable blue-emitting chromophore as sensitizer of Eu(III) ions
luminescence.
Introduction
In the recent tw o decades, organic lanthanide luminescent
complexes not only show extensive applications in light-
1
2
converting and optoelectronic materials but also show more
attractive prospect in bioanalysis areas such as
fluoroimmunoassay,3 analytical sensors and bioimaging.5 To
synthesize the materials w ith high luminescence quantum yields
4
[a]
Laboratory of Functional Inorganic Material Chemistry (Heilongjiang
University), Ministry of Education, School of Chemistry and Materials
Science, Heilongjiang University, Harbin 150080, PR China. E-mail:
lhf4612@163.com; yanpf@vip.sina.com.
[b]
[c]
Laboratory for Food Science and Engineering, Harbin University of
Commerce, Harbin 150076. PR China.
Supporting information for this article is given via a link at the end of
the document.
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun 130012, PR China
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
representative of blue-emitting fluorophors. Previously, 1,8-
naphthalimide as the sensitizer of Eu(III) ion luminescence has
been used to construct w hite-light material.17 The singlet state
energy level of NI locates at 27 027 cm–1, w hich is high enough to
sensitize the emissive state of Eu(III) ion (5D0, 17 500 cm–1).18
Whereas the energy of triplet state (18 500 cm–1) is too close to
the 5D0, and cannot sensitize any of the higher-lying (5D1 and
upw ards) levels, w hich w ill result in a detrimental effect on Eu(III)
ion luminescence due to back energy transfer.19 Herein, w e
employ pyridine-N-oxide as spacer designing tw o ancillary
ligands, 4-(1,8-naphthalimido) pyridine N-oxide (4-NIPyNO) and
3-(1,8-naphthalimido) pyridine N-oxide (3-NIPyNO). Firstly,
pyridine-N-oxide is a strong Lew is base w hich prefers to bind the
oxophile Ln(III) ions. Secondly, the rigid structure of PyNO
enables the substituted 1,8-naphthalimide leaving aw ay from the
Ln(III) ions and prevent the coordination of amide. Thirdly, N-aryl
substituents w ill bring the large dihedral angle betw een NI and
PyNOs, w hich make it difficult for π electron to delocalize in tw o
moieties. In addition, the electron-w ithdraw ing effects of N-oxide
and imide w ill avoid the presence of obvious intramolecular
charge transfer states (ICT).
The synthesis of the ligands is performed in tw o steps. The first
step consists in the condensation betw een 1,8-naphthalic
anhydride and 3-/4-aminopyridine in DMF. The second step is the
N-oxidation of pyridine, w hich is carried out in DMF w ith the m-
chloroperbenzoic acid as oxidizing agents. Recrystalling the
crude products from DMF give the light yellow crystals. The purity
is proven by 1H NMR spectroscopy (Figure S1 and S2) and
microanalysis. The lanthanide complexes w ere prepared by
equimolar reaction of 3-NIPyNO or 4-NIPyNO w ith Ln(hfa)3(H2O)2
in chloroform. Complexation w as evidenced by FT-IR
spectroscopy and elemental analysis. The IR spectra of four
complexes are similar, show ing strong characteristic imide C=O
and trifluoromethyl C–F vibration of hfa around 1725 cm–1 and
1270 cm–1. In addition, the absence of any broad band at 3200–
3500 cm–1 implies these complexes contain no w aters. Finally,
both the ligands and the complexes give nice single crystals
suitable for X-ray diffraction analysis. From microanalysis and
single crystals structure, it is interesting to note that the reaction
of 3/4-NIPyNO w ith Ln(hfa)3(H2O)2 in a molar ratio 1:1 afforded
tw o different types of mixed-ligand complexes, namely
Ln2(hfa)6(4-NIPyNO)2 and Ln2(hfa)6(3-NIPyNO)3. Increasing the
molar ratio up to 3:2 still results in the formation of the same types
of complexes as that obtained in 1:1 molar ratio. In other w ords,
the components of tw o complexes do not depend on the reagent
molar ratio in the reaction. The results are different from the
previously reported studies of similar reaction betw een
Ln(hfa)3(H2O)2 and 4-cyanopyridine N-oxide, w here the mixed-
ligand dimeric complexes w ith tw o and three ligands w ere
obtained w hen molar ratios of Ln(hfa)3(H2O)2 and 4-cyanopyridine
N-oxide varied from 1:1 to 1:4.20 In our case, the constant
components of tw o complexes maybe originate from their diffrent
thermodynamic stability, and/or their difference solubility in CHCl3.
Scheme 1. Synthetic routes of NIPyNOs and their Eu, Gd complexes.
In this paper, tw o blue-emitting fluorophors 4-(1,8-
naphthalimido) pyridine N-oxide (4-NIPyNO) and 3-(1,8-
naphthalimido) pyridine N-oxide (3-NIPyNO) are designed for
testing their feasibility as ancillary ligands to enhance the
luminescence of Eu(III) ions. The ligands are comprise of the
coordinated pyridine N-oxide (PyNO) and the substituted blue-
emittting 1,8-naphthalimide (NI) groups at their para- and meta-
position. As a fluorophore, 1,8-naphthalimide has been w idely
used in fluorescence sensors, biological imaging, and lighting
devices due to their high photostability and high luminescence
quantum yields.12 Pyridine-N-oxide is a strong Lew is base w hich
prefer to bind the oxophile Ln(III) ions (hard Lew is acids).13 Here,
the tw o ligands w ere successfully used to constructing mixed-
ligand lanthanide tris(β-diketonate) chelates. The factors of
introducing ancillary ligands on enhancing Eu(III) ions
luminescence are discussed in detail, and the sensitization
pathw ay(s) and the associated mechanism(s) of the energy
transfer are proposed.
Results and Discussion
Design and Synthesis of Ligands and Complexes
The design of ligands w as carried out to allow investigating the
possibility of the blue-emitting organic fluorescence molecules as
ancillary ligand on enhancing Eu(III) ion red light emission. In
lanthanide complexes, the energy level match betw een the triplet
state of ligand and the excited state of Ln(III) ion is crucial in the
development of systems that result in high quantum yields from
lanthanide-centred emission. According to Latva's empirical rule,
the optimal ligand-to-metal energy transfer process needs the
energy difference is larger than 2500 cm–1.14 Unfortunately, the
most of the blue-emitting dyes have the large singlet-triplet state
energy gaps,15 w hich result in the triplet state of ligands is very
5
close to or low er than the D0 energy level of Eu(III) ion. The low er
triplet state is ineffective on sensitizing Eu(III) ion luminescence
due to back energy transfer from metal to ligand. In contrast to the
triplet state, the singlet state is energetic enough to sensitize any
of the excited state levels (5D0 and upw ards) of Eu(III) ion.
Although it is rare, the examples of the singlet state of ligands as
primary pathw ay for sensitizing Ln(III) ions luminescence can still
be found.16 In this design, 1,8-naphthalimide (NI) is selected as a
X-ray Crystallographic Analysis
Single crystal X-ray diffraction analysis reveals that Ln2(hfa)6(4-
NIPyNO)2•CHCl3 [Ln
= Eu (1), Gd (2)] are isostructurally
crystallized in the monoclinic space group P21/c. Therefore, a full
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10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
description of 1 is given as a representative example (Figure 1).
In the asymmetric unit of 1, there are tw o crystallographically
distinct Eu2(hfa)6(4-NIPyNO)2 units (Eu1, Eu2). Eu1 ion
possesses the similar coordination environment to Eu2 ion, so the
moiety containing Eu1 is selected to describe the structural
feature. Eu1 ion is eight-coordinated by tw o oxygen atoms from
tw o crystallographically equivalent 4-NIPyNO ligands and six
oxygen atoms from three hfa ligands in a triangular dodecahedron
arrangement. The structure is symmetric due to the presence of
an inversion center, w hile the oxygen atom of 4-NIPyNO bridges
up tw o adjacent Eu1 ions. The distance of the intramolecular
Eu···Eu separations is 4.131(8) Å. The Eu–O bond lengths are in
the range of 2.352(6) to 2.456(5) Å, w hich are in good agreement
w ith the reported values.21 The Eu–O (N-oxide atoms from the
neutral 4-NIPyNO) in the range of 2.446(5) Å‒2.456(5) Å are
longer than that of the Eu–O (diketonate oxygen atoms) ranging
from 2.352(6) Å to 2.408(5) Å. The dihedral angles betw een the
naphthalimide and pyridinyl groups are 69.0°, w hich are smaller
than that of the free ligand 4-NIPyNO (74.5°). The tw o distinct
moieties comprised of Eu1 and Eu2 interact w ith each other
through the π‒π interactions, w hich could be detected among the
overlap of the naphthalimide groups of the tw o adjacent distinct
crystallographically distinct Eu(hfa)3 moieties vertically, w here
smaller intramolecular Eu···Eu distance of 3.933(5) Å is observed
in comparison w ith 1. The Eu–O bond lengths are in the range of
2.369(6)–2.565(5) Å. The Eu–O (N-oxide atoms from the neutral
3-NIPyNO) in the range of 2.462(5)–2.565(5) Å are relatively
longer as w ell. The dihedral angles betw een the naphthalimide
and pyridinyl groups are ranging from 51.4° to 58.9°, respectively,
w hich are smaller than that of the ligand 3-NIPyNO (72.0°). The
extensive π-π interactions are found among the planar
naphthalimide groups and the distances are in the range of
3.160–3.707 Å. The dihedral angles betw een tw o adjacent
naphthalimide planars are measured to be 0° and 6.93°,
respectively (Figure S4).
Electronic Spectroscopy
To insight into the electron states of the ligands and their
complexes, their UV-vis spectra and diffuse reflectance spectra
are recorded in solution and in solid state, respectively. In CHCl3,
tw o ligands show composite broad bands corresponding to π –π*
transitions w ith low est energy maxima 300–370 nm assigned to
the 1,8-naphthalimide chromophore (Figure 2). The w ell spectral
overlaps of tw o ancillary ligands in this region indicates 1,8-
naphthalimide electron transition seems not depending on the
identity of N-substituent groups. To further clear the transition
characteristic, the absorption spectrum of N-butyl-naphthalimide
is recorded and compared w ith the ligands. The very similar
spectra pattern and a slight absorption shift (2 nm) indicate that
the lack of electronic communication betw een pyridine N-oxide
and 1,8-naphthalimide. The high energy absorption bands in the
range 250–300 nm are attributed to the π–π* transitions localized
at pyridine N-oxide. The difference is that the para-substituent of
1,8-naphthalimide red-shift this absorption by 7 nm compared to
that in meta-substituent. Compared w ith tw o NIPyNOs, the mixed-
ligand complex display new bands at about 300 nm, w hich is
attribute to the π–π* transition of hexafluoroacetylacetone (hfa).
In addition, the bands of NIPyNOs in complexes only show very
slight shifts than that in free ligands, w hich indicates the
coordination of Ln(III) ions has few effects on the electron state of
1,8-naphthalimide.
moieties.
The
distances
betw een
the
naphthalimide
carbon/nitrogen atoms and the neighbor naphthalimide planar are
found in the range of 3.419–3.643 Å, and the dihedral angles
betw een tw o adjacent naphthalimide planars are found to be 2.08°
(Figure S3). Furthermore,
the tw o
crystallographically
independent moieties in the crystal structure of Eu2(hfa)6(4-
NIPyNO)2 are interconnected by the w eak intermolecular C–H···F
interactions as w ell.
Figure 1. Ball-and-stick representation of the crystal structures of 1 (a), 3 (c),
and their coordination polyhedra (b), (d).
Single crystal X-ray diffraction analysis reveals that
Ln2(hfa)6(3-NIPyNO)3•CHCl3 [Ln
isostructurally crystallized in the triclinic space group P–1.
Similarly, is given as representative example. In the
=
Eu (3), Gd (4)] are
3
a
asymmetric unit of 3, there are tw o crystallographically distinct
Eu(III) ions as w ell. Both Eu1 and Eu2 ions are nine-coordinated
to six oxygen atoms from three hfa ligands and three oxygen
atoms from three 3-NIPyNO ligands in a tricapped triangular prism
arrangement. Interestingly, three 3-NIPyNO ligands bridge up tw o
Figure 2. UV–Visible absorption spectra of the ligands and complexes in CHCl3
(top, 1 × 10‒5 M), and diffuse reflectance spectra in solid state (bottom). (3-
NIPyNO,blue line; 4-NIPyNO, red line; N-butyl-naphthalimide, purple line;
complexes 1, red dash; 3, blue dash and Eu(hfa)3(H2O)2, green dash).
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10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
The diffuse reflectance spectra of the ligands and complexes
are show n in Figure 2 (bottom). The low est energy absorbance
edges of 1,8-naphthalimide red shift from 370 nm to 400 nm
compared w ith that in solution, w hich can be attributed to the
aggregation of the ligands in solid state. From the crystal
structures, the strong π–π interaction betw een the adjacent
naphthalimide can be observed in free ligands (Figure S5). Upon
complexes formation, the low est-energy bands of the ligands are
blue-shifted by 20 nm reflecting the decrease of aggregation
extent. The result is in accordance w ith that observed in crystal
structure of the complexes, w here the π–π interaction only
presents in tw o adjacent 1,8-naphthalimide. In addition, the
diffuse reflectance spectrum of Eu(hfa)3(H2O)2 show s a large
spectral overlaps w ith tw o NIPyNOs in 250–400 nm regions. It
indicates that the excitation w indow s locating at this range w ill
simultaneously excite the β-diketones and the ancillary ligands,
and maximizing the synergetic effect of tw o ligands on sensitizing
Eu(III) ion luminescence.
HOMO - 7→LUMO and n–π* HOMO - 2→LUMO + 3 transition of
1,8-naphthalimide, respectively. Whereas, the higher energy
transitions of tw o ligands (λ = 265.56 nm, f = 0.2342 for 3-NIPyNO
and λ = 269.99 nm, f = 0.3566 for 4-NIPyNO) are attributed to π–
π* HOMO→LUMO + 2 for 3-NIPyNO and HOMO→LUMO + 1 for
4-NIPyNO transition localized in pyridine N-oxide moieties. It is
notable that the intramolecular charge transfer (ICT) transitions
are not present in tw o ligands. In N-aryl-substituent 1,8-
naphthalimide compounds, the ICT state caused by donor-to-
acceptor charge transfer from the donor-electron aryl
chromophore to 1,8-naphthalimide can usually be observed.22 In
present ligands, the absence of the ICT state are reasonable
because 1,8-naphthalimide and pyridine N-oxide both have
electron-w ithdraw ing effects. Finally, the calculated results further
support the conclusion of the absence of effective electron
communications betw een the tw o moieties.
Table 1. TD-DFT calculated singlet-singlet excitation energies (E),
corresponding wavelengths (λ), and oscillator strengths (f) for 3-NIPyNO and 4-
NIPyNO.
Ligands
E (eV)
λ (nm)
f
composition (C)
3-NIPyNO
3.3170
373.78
0.1767
72→76 (0.62)
73→76 (-0.28)
3.8655
320.75
0.0428
68→76 (0.62)
72→79 (0.26)
73→79 (-0.19)
4.6687
3.3129
265.56
374.25
0.2342
0.1834
75→78 (0.67)
4-NIPyNO
72→76 (0.66)
73→76 (-0.15)
3.8436
4.5921
322.57
269.99
0.0456
0.3566
68→76 (0.62)
72→79 (0.30)
75→77 (0.65)
Photoluminescence Properties of Complexes
To investigate the role of NIPyNOs on sensitizing Eu(III) ions
luminescence, the detailed photophysical parameters relating to
the ligands and central metals are measured, including emission
quantum yields, the emission lifetimes, the energy transfer
efficiencies, and the radiative (kr) and nonradiative rate constants
(knr). In view of the absence of energy transfer from ligand to
Gd(III) ions, tw o Gd(III) complexes are prepared for investigating
the photophysical properties of the ligands in complexes. The
excited spectra of the Eu(III) complexes in the solid state are
recorded on the monitoring the strongest emission bands of
Eu(III) ions at 612 nm, corresponding to 5D0→7F2 transition. As
show n in Figure 4, the complexes show broad excitation bands
originating from the ligands in the range of 250–450 nm, w hich
indicates the energy transfer takes place from the ligands to
Eu(III) ions. Compared w ith the excited band of Eu(hfa)3(H2O)2,
the complexes 1 and 3 show about 40 nm red shift w ith the
maxima at 363 nm and 368 nm, respectively. Combined w ith the
absorbance spectra, it can be concluded that the broad bands
should originate from the superposition of the excitation spectra
of NIPyNOs and hfa, and NIPyNOs show the more effective
sensitization on enhancing Eu(III) ions luminescence than hfa.
Figure 3. Molecular orbital diagrams of 3-NIPyNO and 4-NIPyNO.
To further insight into the excited-state of the ligands, the
corresponding electronic structure calculations using TD-DFT are
also performed using Gaussian’03 package. A summary of the
electronic structure calculation is given in Table 1 and their
corresponding molecular orbitals are show n in Figure 3.
According to the calculation, the low est energy transitions of tw o
ligands (λ = 373.78 nm, f = 0.1767 for 3-NIPyNO and λ = 374.25
nm, f = 0.1834 for 4-NIPyNO) are caused by HOMO - 3→LUMO,
HOMO - 2→LUMO transition and assigned to the π–π* and n–π*
transition localized in 1,8-naphthalimide. The absorption bands
centered at 320.75 nm for 3-NIPyNO and 322.57 nm for 4-
NIPyNO are attributed to the mixed π–π* HOMO - 3→LUMO + 3,
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10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
found to be 33% and 25%, respectively. The values are 13- and
10-times higher than hydrate Eu(hfa)3(H2O)2 (Φ = 2.6%),23 and
are comparable to that reported in 4-cyanopyridine N-oxide
mixed-ligand Eu(III) complex (Φ = 25.7%).20b In luminescent
lanthanide complexes, the luminescence quantum yield of the
complexes is mainly determined by tw o aspects: (i) the efficiency
of the energy transfer (ηsen) from ligand to metal center; (ii) and
the intrinsic quantum yield (ΦLn) of the lanthanide emission (eqn.
1)
휱풕풐풕 = 휼풔풆풏 휱푳풏
(ퟏ)
The intrinsic quantum yield (ΦLn) is the competitive result of the
radiative processes (kr) w ith the non-radiative processes (knr),
w hich can be estimated from the follow ing equations:24
Figure 4. Excitation spectra of 1, 3 and Eu(hfa)3(H2O)2.
풌풓
흉퐨퐛퐬
휱푳풏
=
ퟏ
=
(ퟐ)
풌풓 + 풌풏풓 흉풓풂풅
The emission spectra of four Eu(III) and Gd(III) complexes in
the solid state at room temperature are show n in Figure 5. Upon
excitation w ith their corresponding maxima excited bands, tw o
Gd(III) complexes display ligand-based fluorescent emission w ith
the bands in the range of 400–570 nm. Compared to Gd(III)
complexes, the emission spectra of Eu(III) complexes are mainly
dominated by the characteristic emission bands of Eu(III) ions at
580, 592, 611, 650 and 702 nm, corresponding to 5D0→7FJ (J =
0–4) transitions, respectively. Moreover, the very w eak residue of
ligand based emission can also be observed. It indicates the
energy transfer from the ligands to the Eu(III) ions is incomplete.
(See Figure 5, insert). For elucidating the beneficial effects of
ancillary ligand on the photophysical properties of Eu(III)
complexes, the photophysical data relating to the ligand- and
metal-centered emissions are recorded and summarized in Table
2.
푰풕풐풕
= 푨푴푫 ,ퟎ풏ퟑ
(ퟑ)
(
)
풌풓 =
흉풓풂풅
푰푴푫
Where kr is the radiative rate constant, knr is non-radiative rate
constant, τobs is the observed lifetime, τrad is the radiative lifetime,
AMD,0 = 14.65 s–1 is the spontaneous emission probability of the
5
magnetic dipole D0→7F1 transition, n is the refractive index of the
medium. An average index of refraction equal to 1.5 is employed.
5
I
tot
is the total integrated emission of the D0→7FJ transitions, and
IMD is the integrated emission of the 5D0→7F1 transition. With the
eqn.3, the radiative rate constant (kr) are calculated to be 856 s–1
and 862 s–1 for 1 and 3, respectively. Generally, reduction of the
geometrical symmetry of the coordination structure leads to a
larger radiative rate constant.25 From the crystal structure, the
Eu(III) ions in
triangular dodecahedron (8-TDH) coordination
1
lie in low er symmetrical eight-coordinated
geometry,
w hereas that in 3 is a higher symmetrical tricapped trigonal
prismic coordination geometry (9-TTP). Herein, this empirical law
seems not w ork w ell, because the tw o radiative rate constant (kr)
is almost the same. The observed lifetimes (τobs) are determined
by monitoring the emission decay curves w ithin the 5D0→7F2
transition at 612 nm. Typical decay profiles of complexes 1 and 3
are show n in Figure S6. The tw o decay curves are w ell-
reproduced by single-exponential functions, suggesting only one
species exists in the excited state of the complexes. The lifetimes
of the complexes 1 and 3 are found to be 703 μs and 597 μs,
respectively. With the calculated radiative lifetimes (τrad
) and
observed lifetimes, the intrinsic quantum yields (ΦLn) w ere found
to be 60% and 51% for 1 and 3. According to equation 2, w e can
deduce that the different intrinsic quantum yields should originate
from their different non-radiative rate constants (knr). The
calculated values are found to be 570 s–1and 828 s–1 for 1 and 3.
The larger value indicates that the complex 3 has the more
effective non-radiative deactivation pathw ay than that in 1. It is
w ell know n that the quenching of the lanthanide excited state by
nonradiative transition is mainly caused by the matrix vibrations.26
In view of similar dimeric binuclear structures, w e suppose that
the different quenching effects are mainly caused by their different
oscillator vibrational energy in tw o ancillary ligands NIPyNOs.
Figure 5. Emission spectra of Ln2(hfa)6(4-NIPyNO)2 (top) and Ln2(hfa)6(3-
NIPyNO)3 (bottom) in single crystal states under excitation at 380 nm (Gd, black
lines; Eu, red lines).
The luminescence quantum yields are calculated to be 42%
and 29% for 1 and 3, in w hich the metal-centered emissions are
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10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
Table 2. Radiative (kr) and nonradiative (knr) decay rates, observed luminescence lifetime of Eu3+ (τoEubs) and ligands (τLobs), intrinsic quantum yield (ΦLn), sensitization
Eu
efficiency (ηsens), quantum yield of Eu3+ (ΦEu), quantum yield of ligands (ΦL) and overall quantum yield (Φoverall). Error in τ :±0.05 ms; 10% relative error in the other
obs
values; λex = 375 nm.
Eu
obs
Complexes
kr (s‒1)
knr (s‒1)
τ
(μs)
τLobs (ns)
ΦLn (%)
ηsen (%)
ΦEu (%)
ΦL (%)
Φoverall (%)
1
856
570
703
7.1 (17.2)
60
55
33
8.5 (23.2)
41.5
2.5;7.2
3
862
828
597
51
49
25
3.6 (12.9)
28.6
(4.3;18.9)
Data in brackets represent the lifetimes and luminescence quantum yields of ligands in Gd(III) complexes.
The energy loss from the excited state of Ln(III) ions to the
vibrational energy levels of oscillators has been proposed to occur
according to the Förster energy transfer mechanism.27 The
energy transfer rate betw een the tw o centers is inversely
proportional to the distance betw een them.28 In present
complexes, only the high-energy oscillators C–H and C=O are
considered. On the base of crystal structure, the numbers of the
oscillators and their distances to metal centers w ithin the selected
C–H distance of 10.6 Å and C=O distance of 8.5 Å are calculated
and summarized in Table S2–S4. In complex 1, tw o types of
symmetrically independent centrosymmetric bimetallic molecules
exist in crystal structure. As show n in Figure S7, each molecule is
surrounded by six nearby molecules. For Eu-1, there are 45 C–H
bonds and 9 C=O bonds in the selected ranges, and 34 C–H
bonds and 10 C=O bonds are in Eu-2. The average distances are
7.928 Å for Eu···C–H and 7.78 Å for Eu···C=O. In complex 3, the
total of 128 C–H bonds and 14 C=O bonds from ten nearby
molecules are observed around the tw o luminescence centers
Eu1 and Eu2 (Figure S8). The average distances betw een the
Eu(III) and the C–H and C=O are found to be 8.024 Å and 6.79 Å.
The significant increase of the numbers of C–H bonds and C=O
bonds indicates that the quenching the exited state of Eu(III) ion
is more effective in 3 than that in 1. This result is accord w ith the
calculated non-radiative rate constant (knr).
Therefore, the emissions of the Gd complexes w ill reflect the
fluorescent properties of the ligands in the absence of any energy
transfer. The luminescence quantum yields of the ligands in
Gd(III) complexes are observed to be 23.2% and 12.9% for 2 and
4, respectively. While these values decrease to 8.5% and 3.6% in
their corresponding Eu(III) complexes. Based on these values,
the efficiency of the energy transfer from ligand to Eu(III) ion can
be estimated by
푬풖
휱
푬푻
휱푬푻 = ퟏ −
(ퟓ)
푮풅
푬푻
휱
Eu
ET
Where Φ
is the quantum yield of the residual emission of
Gd
NIPyNOs in the Eu(III) complexes, and Φ
is the emission of
ET
ligands in Gd(III) complexes in the absence of any energy transfer
to the metal. According to this equation, the efficiencies of the
energy transfer ΦET are calculated to be 63% and 72% for 1 and
3. The higher energy transfer efficiency for 3-NIPyNO mays be
due to its closer distance to the metal center than that in 4-
NIPyNO. Moreover, the luminescence lifetimes of the ligands also
show the same tendency as that observed in luminescence
efficiencies (Figure S9). The lifetimes of 4-NIPyNO decrease from
17.2 ns to 7.1 ns for their Gd(III) and Eu(III) complexes. In 3-
NIPyNO complexes, the luminescence decay of the ligand
contains tw o components w ith a shorter lifetime of 4.3 ns and a
longer-lived at 18.9 ns in Gd(III) complex, w hile it decrease to 2.5
ns and 7.2 ns in Eu(III) complex. The double-exponential decay
of the emissive state should originate from tw o kind of different
arrangement of 1,8-naphthalimide in solid state, w hich can be
clearly observed in crystal structure (Figure S4).
Apart from the intrinsic quantum yields, the sensitization
efficiency is another factor for determining the overall luminescent
quantum yields of center metal. According to equation 1, the
calculated sensitization efficiencies are found to be 55% and 49%
for 1 and 3, respectively. In present complexes, NIPyNOs and hfa
both participate in the sensitization process for Eu(III) ions
luminescence. The sensitization efficiency should therefore
The phosphorescence spectra of NIPyNOs in their Gd(III)
complexes are measured at 77 K in a CHCl3 glass w ith a delay of
100 μs after the excitation pulse. As show n in Figure S10, the
spectra are composed of tw o parts, the higher energy bands
starting from 445 nm attributes to hfa, and the low er at 528 nm
attributes to 3/4-NIPyNO, respectively. The energies of zero-
phonon transition of the triplet 3ππ* states of the hfa and 3/4-
NIPyNO w ere thus estimated to be at around 22472 (445) and
NIPyNO
sen
hfa
contain the contributions from NIPyNO (η
) and hfa (η ) as in
sen
equation 4:
휼풔풆풏 = 휼푵푰푷풚푵푶 + 휼푯푭푨 (ퟒ)
풔풆풏
풔풆풏
Herein, the similar dinuclear structures make us assume the
sensitization contributions from hfa are the same. Thus, the
differences of overall sensitization efficiency 휂푠푒푛 can be
attributed to their different contributions from tw o NIPyNOs
ligands.
The Gd(III) ions has no energy levels below 32 150 cm–1, and
thus cannot accept energy from the excited sates of the ligands.
–1
18939 cm (528 nm), respectively. The phosphorescence lifetime
of NIPyNOs in their Eu(III) and Gd(III) complexes are measured
at 77 K by monitoring the emission decay curves w ithin the
transition at 540 nm. As show n in Figure S11, the decays of the
triplet states are found in millisecond scale, and are w ell-
reproduced
by
double-exponential
functions.
The
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10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
a microsecond pulse lamp and a picosecond laser as the excitation
sources. The data were analyzed by software supplied by Edinburgh
Instruments. The quantum yieldsfor the complexeswere determined at
room temperature through an absolute method using an Edinburgh
Instruments' integrating sphere coupled to a modular EdinburghFLS920
fluorescence spectrophotometer. The valuesreported are the averageof
three independent determinationsfor each sample. Theabsolutequantum
yield wascalculated using thefollowingexpression:
phosphorescence lifetime of 4-NIPyNO in Gd(III) complexes are
found to be 10.6 ms and 44.3 ms, w hile they decrease to 2.5 ms
and 13.1 ms in Eu(III) complexes. Similar situation can also be
found in 3-NIPyNO complexes, the lifetimes decrease from 10.8
ms and 48.3 ms to 5.6 ms and 34.6 ms for their Gd(III) and Eu(III)
complexes. These decreases in phosphorescence lifetimes
indicate the presence of energy transfer from the triplet states to
the metal centres.
∫
푳풆풎풊풔풔풊풐풏
휱 =
∫
∫
푬풔풂풎풑풍풆
푬풓풆풇풆풓풆풏풄풆
−
Conclusions
Where Lemission isthe emission spectrum of the sample, collectedusing
the sphere, Esample isthe spectrum of the incident light used toexcitethe
sample, collected using the sphere, and Ereference isthe spectrumofthe
light used for excitation with only the reference in the sphere.Themethod
is accurate to within 10%.
In summary, tw o blue-emitting ancillary ligands are prepared, and
successfully used for constructing mixed-ligand lanthanide
complexes w ith hexafluoroacetylacetone. The rational design of
5
the ligands ensure their excited energy level to be higher than D0
energy level of Eu(III) ions, and effectively sensitizing center metal
luminescence. The energy transfer efficiencies from ligand to
Eu(III) ions reach up to 63% and 72% for 1 and 3. In addition, the
effective overlaps of the hfa and NIPyNOs in the absorption
spectra insure their transitions can be excited simultaneously in
the selected band, and maximize the effects of ancillary on
sensitizing center metal luminescence. Combining w ith the
contribution of replacing the solvents on eliminating harmonics of
O–H vibrations, the tw o mixed-ligand lanthanide complexes show
13- and 10-fold luminescence enhancement compared w ith the
hydrate Eu(hfa)3(H2O)2. The luminescence quantum yields are
calculated to be 41.5% and 28.6 for 1 and 3, respectively. These
results show that the family of organic blue-emitting chromophore
can be exploited as sensitizer of Eu(III) ion luminescence through
a clever design strategy.
Computational Details
DFT geometry optimizationsand TD-DFT excitation energycalculationsof
the NIPyNOs were carried out with the Gaussian 09 package.29 In both
case, the B3LYP/def2-TZVP basis set was used. The first 50
monoelectronic excitationswere calculated. All calculationsweredonein
the gas phase, and geometry optimizations were performed with no
symmetry restraints.
Synthesis of 4-(1,8-naphthalimido) pyridine N-oxide (4-NIPyNO). To a
solution of 4-aminopyridine (5.6 g, 0.06 mol) in 150 mL DMF, wasadded
potassium carbonate anhydrous (2.8 g, 0.02 mol) followed by 1,8-
naphthalic anhydride (8.2 g, 0.04 mol). Then the mixture wasstirredand
irradiated with 500 Wmicrowave powersfor 30 min at 100–140 °C. After
cooled to room temperature, the mixture waspoured into 300mLicewater.
The resultant precipitates was filtered, and successively washed with
water and ethanol for three times. The light yellow crystalsrecrystallized
from DMF were dissolved in 70 mL DMF and then 3-chloroperbenzoicacid
(6.9 g, 0.04 mol) wasadded to it. The reaction mixture wasstirred at the
room temperature for 48 h. To thissolution, 300 mL saturated potassium
carbonate wasadded, and the resulting white precipitate wasfilteredand
washed with water for three times. Recrystallization from DMF gaveyellow
Experimental Section
Materials and Instruments
crystalsof title compound (8.2 g, 70 % yield). Anal. Calcd. for C17H10N O3
2
(290.27): C, 70.34; H, 3.47; O, 16.54; N, 9.65. Found: C, 70.28; H, 3.54;
O, 16.21; N, 9.71. 1H NMR (DMSO-d6, 400 MHz): 8.55 (d, 2H), 8.52 (d,
2H), 8.43 (d, 2H), 7.95 (t, 2H), 7.52 (d, 2H). IR (KBr, cm–1): 3059, 1705,
1682, 1591, 1470, 1378, 1362, 1260, 1168, 850, 779. ESI-MSm/z313.45
[M + Na]+.
The commercially available chemicalswere analytical reagentgradeand
used without further purification. 1,8-naphthalicanhydride (99%, AR)was
purchased from Shanghai Darui Finechemical Co. (Shanghai, China).
LnCl3•6H2O was prepared according to the literature by dissolving
99.99% oxide in a slight excessof hydrochloric acid. The solution was
evaporated and the precipitate was collected from water. Elemental
analyses were performed on an Elementar Vario EL cube analyser. UV
spectra were recorded on a Perkin-Elmer Lambda 25 spectrometer, and
diffuse reflectance spectra were recorded on BaSO4 disks by using
integrating sphere coupled to the equipment. The 1H NMR spectra were
recorded on a Bruker Avance III 400 MHz spectrometer in DMSO-d6
solution. Electrospray time-of-flight (ESI-TOF) mass spectra were
recorded on Bruker maXis mass spectrometer. Crystal data of the
complexeswere collected on an Xcalibur, Eos, Gemini diffractometerwith
Mo Kαradiation (λ = 0.71073 Å). All data were collected at a temperature
of 150 K. The structureswere solved by direct methodsand refined onF2
by full-matrix least-squaresusing the SHELXTL-2014 program. TheLn(III)
ions were easily located and then non-hydrogen atoms (C, N, O and F)
were placed from the subsequent Fourier-difference maps. All non-
hydrogen atomswere refined anistropically. A summary of data collection
and refinementsis given in Table S1† Excitation and emission spectra
were measured with an Edinburgh FLS 920 fluorescence
spectrophotometer. Luminescence lifetimeswere recorded on a single
photon counting spectrometer from Edinburgh Instrument (FLS 920)with
Synthesis of 3-(1,8-naphthalimido) pyridine N-oxide (3-NIPyNO). 3-
(1,8-Naphthalimido) pyridine N-oxide wasprepared in the same manner
as used for 4-NIPyNO. (7.9 g, 68 % yield). Anal. Calcd. for C17H10N2O3
(290.27): C, 70.34; H, 3.47; O, 16.54; N, 9.65. Found: C, 70.49; H, 3.57;
O, 16.28; N, 9.73. 1H NMR (DMSO-d6, 400 MHz): 8.63 (t, 2H), 8.53 (t, 2H),
8.44 (s, 1H), 8.37 (d, 1H), 7.92 (t, 2H), 7.59 (t, 1H), 7.50 (d, 1H). IR (KBr,
cm–1): 3059, 1690, 1675, 1581, 1480, 1439, 1379, 1286,1245,1185,1017,
770. ESI-MS m/z 313.44 [M + Na]+.
Synthesis of Ln2(hfa)6(4-NIPyNO)2 [Ln = Eu, Gd]. 4-NIPyNO (0.4 g, 1.5
mmol) was added to a solution of Ln(hfa)3(H2O)2 (1.2 g, 1.5 mmol) in
chloroform (20 mL). The resulting mixture wasthen stirred and refluxedfor
4 h. Further evaporation of the solvent at –4 °C yielded colourlesscrystals,
which was filtered off anddriedin air.
Eu2(hfa)6(4-NIPyNO)2·CHCl3 (1). Yield: 82%. Anal. Calcd. for
C65H27Eu2F36Cl3N4O18 (2246.17): C, 34.76; H, 1.21; O, 12.76; N, 2.49.
Found: C, 34.83; H, 1.31; O, 12.66; N, 2.55. IR (KBr, cm–1): 3065, 1729,
1646, 1539, 1502, 1354, 1265, 1206, 1140, 796, 659.ESI-MSm/z2151.47
[M + Na]+.
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10.1002/ejic.201700019
European Journal of Inorganic Chemistry
FULL PAPER
Gd2(hfa)6(4-NIPyNO)2·CHCl3 (2). Yield: 75%. Anal. Calcd. for
C65H27Gd2F36Cl3N4O18 (2256.75): C, 34.59; H, 1.21; O, 12.76; N, 2.48.
Found: C, 34.63; H, 1.29; O, 12.61; N, 2.56. IR (KBr, cm–1): 3068, 1726,
1647, 1535, 1507, 1351, 1268, 1205, 1141, 796, 656.ESI-MSm/z2161.56
[M + Na]+.
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chloroform (20 mL). The resulting mixture wasthen stirred and refluxedfor
4 h. Further evaporation of the solvent at –4 °C yielded colourlesscrystals,
which was filtered off anddriedin air.
Eu2(hfa)6(3-NIPyNO)3·CHCl3 (3). Yield: 71%. Anal. Calcd. for.
C82H37Eu2F36Cl3N6O21 (2536.44): C, 38.83; H, 1.47; O, 13.25; N, 3.31.
Found: C, 38.90; H, 1.52; O, 13.19; N, 3.29. IR (KBr, cm–1): 3063, 1683,
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2441.42 [M + Na]+.
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1672, 1574, 1476, 1437, 1377, 1282, 1246, 1183, 1016, 767.ESI-MSm/z
2452.33 [M + Na]+.
Acknowledgements
This w ork is financially supported by the National Natural Science
Foundation of China (Nos. 51472076 & 51302068). We thank the
Key Laboratory of Functional Inorganic Material Chemistry
(Heilongjiang University), Ministry of Education and the University
of Heilongjiang for supporting this study.
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Keywords:Lanthanide • Luminescence • Blue-emitting ancillary
ligand • Singlet state
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Key Topic: Lanthanide Luminescence
Tw o blue-emitting ancillary ligands are
designed, and successfully used for
constructing mixed-ligand lanthanide
complexes w ith
Zhao Wang, NanNan Liu, Hongfeng
Li,* Peng Chen and Pengfei Yan*
Page No. – Page No.
hexafluoroacetylacetone. The high
efficient energy transfer from the
ligand to the Eu(III) ion make the
complexes show about 13-fold
luminescence enhancement than
hydrated Eu(hfa)3(H2O)2.
Title:
Role of Blue-emissive 1,8-
Naphthalimidopyridine N-oxide on
Sensitizing Eu(III) Photoluminescence
in Dimeric Hexafluoroacetylacetonate
Complexes
*one or two words that highlightthe emphasis ofthe paper or the field of the study
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