Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

Article abstract of DOI:10.1002/cphc.201900603

The odd–even effect in luminescent [Eu₂(L)₃(H₂O)_x]?y(H₂O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2–6 and y=0–4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu³⁺ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand–metal coordination via the mixed mode of bridge–chelate coordination, except for the Eu³⁺-oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd–even effect on the 4 f–4 f luminescence intensity parameters (Ω₂ and Ω₄) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities (a*) were simulated by a ghost atom located at the middle of each ligand chain. The values of a* were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield (Q_Ln ^Ln) of the Eu³⁺ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω₂ and Ω₄ intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.

Full text of DOI:10.1002/cphc.201900603

Accepted Article
Title: Odd–Even Effect on Luminescence Properties of Europium
Aliphatic Dicarboxylate Complexes
Authors: Israel Assunção, Albano Carneiro, Renaldo Moura Jr., Cássio
Pedroso, Ivan Guide Nunes Silva, Maria Felinto, Ercules
Teotonio, Oscar Malta, and Hermi Brito
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ChemPhysChem
10.1002/cphc.201900603
FULL PAPER
Odd–Even Effect on Luminescence Properties of Europium
Aliphatic Dicarboxylate Complexes
[
a]
[b][c]
[d]
[a]
Israel P. Assunção, Albano N. Carneiro Neto,
Renaldo T. Moura Jr., Cássio C. S. Pedroso,
[
a]
[e]
[f]
[b]
[a]
Ivan G. N. Silva, Maria C.F.C. Felinto, Ercules E.S. Teotonio, Oscar L. Malta,* Hermi F. Brito *
2 3 2 x 2
Abstract: The odd–even effect in luminescent [Eu (L) (H O) ]·y(H O)
Introduction
complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC,
GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x = 2–6 and
y = 0–4), prepared by the precipitation method, was observed for the
first time in lanthanide compounds. The final dehydration
The
aliphatic
α,ω,-alkanedicarboxylate
anions
(

2
OOC(CH )n-2COO , 2 ≤ n ≤ 12) (Figure 1) have very
interesting features such as their versatile coordination modes, for
example monodentate, chelated bidentate, bridging or mixed
modes.[1] The coordination modes can lead to different
multidimensional structures depending mostly on the length and
flexibility of the chain.[
3+
temperatures of the Eu complexes show a zigzag pattern as a
function of the carbon chain size of the dicarboxylate ligands, leading
to the so-called odd–even effect. The FTIR data confirmed the ligand-
metal coordination via the mixed mode of bridge-chelate coordination,
1,2]
3+
In addition, there are other intrinsic
except for the Eu –oxalate complex. XRD results point out to highly
crystalline materials belonging to the monoclinic system. The odd–
physico-chemical properties of the α,ω,-alkanedicarboxylic acids
due to the odd–even effect, which is quite well established in the
even effect on the 4f-4f luminescence intensity parameters (
2
and
literature.[
3–5]
This effect plays an important role in the regular

4
) was explained by using an extension of the Dynamic Coupling
variations of different properties of compounds: melting
points,[3,5,6] solubility, nanocalorimetry, electrochemical,
emulsifying ability of some surfactants or structural self-
assembling.[8,10–12] It is noteworthy that the odd–even effect arises
from a difference in the packing mode between odd–odd or even–
even carbon members of a given homologous series.[
mechanism, herein named the Ghost-Atom model. In this method, the
[3]
[7]
[8,9]
∗
the
long-range polarizabilities ( 훼 ) were simulated by a Ghost-Atom
[9]
∗
located at the middle of each ligand chain. The values of 훼 were
estimated using the localized molecular orbital approach. The
Ln
3+
emission intrinsic quantum yield (푄Ln ) of the Eu complexes also
presented an the odd–even effect, successfully explained in terms of
4–6,13]
the zigzag behavior showed by the 
Luminescence quenching due to water molecules in the first
coordination sphere was discussed and rationalized.
2
and 
4
intensity parameters.
According to the “parallelogram-trapezoid model” proposed
by Thalladi et al.,[3] the even carbon members are arranged in
such a way that the repulsive interactions between chains are
decreased, leading to a more stable packing pattern. On the other
hand, the odd carbon members possess some intrinsic higher
energy that may be released in a process, for exemple dissolution
or melting. This can explain why the odd carbon chains have
higher solubilities and lower melting points than their even carbon
[
[
a]
b]
Dr. Israel P. Assunção, Dr. Cássio C. S. Pedroso, Dr. Ivan G. N.
Silva, Prof. Hermi F. Brito
Department of Fundamental Chemistry
Institute of Chemistry, University of São Paulo
Av. Prof. Lineu Prestes, São Paulo - SP, 05508-000, Brazil.
members. Besides,
a
nanoindentation study on α,ω-
alkanedicarboxylic acids has demonstrated that the elastic
modulus presents an odd–even conformation similar to the
melting temperature.[5]
Dr. Albano N. Carneiro Neto, Prof. Oscar L. Malta
Department of Fundamental Chemistry.
Federal University of Pernambuco
Av. Prof. Moraes Rego, Recife - PE, 50670-901, Brazil.
E-mail: oscar.malta@ufpe.br
[
[
c]
d]
Dr. Albano N. Carneiro Neto
Physics Department and CICECO – Aveiro Institute of Materials
University of Aveiro
Campus Universitário de Santiago, Aveiro, 3810-193, Portugal.
Prof. Renaldo T. Moura Jr.
Department of Chemistry and Physics
Federal University of Paraꢀba, Campus II
Cidade Universitária, Areia - PB, 58397-000 Brazil.
[
e]
f]
Prof. Maria C.F.C. Felinto
Nuclear and Energy Research Institute - IPEN/CNEN
Av. Prof. Lineu Prestes, São Paulo - SP, 05508-000, Brazil.
Figure 1. Structural formulas of eleven different aliphatic α,ω,-
alkanedicarboxylate anions OOC(CH2)n-2COO, in which 2 ≤ n ≤ 12.
[
Prof. Ercules E.S. Teotonio
Department of Chemistry
Federal University of Paraíba
Jardim Universitário, - João Pessoa - PB, 58051-970, Brazil.
The odd–even effect has been used in studies of self-
assembled monolayers (SAMs) and fully controls the ability of
molecules to form highly ordered structures such as the effect in
molecular self-assembled packing.[8,11,12,14] This zigzag
Supporting information for this article is given via a link at the end of
the document.
1
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ChemPhysChem
10.1002/cphc.201900603
FULL PAPER
conformation[2] plays a key role in the structural organization of
SAMs based on carboxylic acid structures,[4,15] which present a
potential for molecular electronic applications.
analogs La3+ and Gd3+ complexes were prepared with the
2-
3+
objective of obtaining information on O (2p)  Eu (4f) Ligand-
to-Metal Charge Transfer (LMCT) states and triplet states (T ) of
1
The last decades have seen, a significant increase in
the dicarboxylate ligands, respectively.
3
+
interest in trivalent lanthanides (Ln ) coordination compounds
[
16–18]
with organic ligands
due to the great potential for a variety of
applications such as gas storage,[19] catalyses,[20] medical uses,[21]
and optical markers.[22] The luminescence properties of the Ln3+
ions are mostly dependent on their unique energy level structures.
The 4f orbitals are effectively shielded from the chemical
environment by the filled 5s and 5p sub-shells,[23] leading to
narrow absorption and emission lines, maintaining more or less
their atomic character. The 4f–4f transitions are parity forbidden
by Laporte’s rule, yielding very low absorptivity coefficients. This
spectroscopic disadvantage is overcome by using organic ligand
as an efficient luminescence sensitizer to absorb and efficiently
energy transfer to the Ln3+ ions.[24]
Experimental Section
Synthesis of the [Ln
The dicarboxylic acids (H
ADP, H PIM, H SUB, H AZL, H
were purchased from Sigma-Aldrich and used without previous
treatment. The lanthanides chlorides (LnCl ∙6H O) were prepared
by a reaction between their corresponding lanthanides oxides,
2
(L)
3
(H
2
O)
OXA, H
SEB, H
x
]·y(H
2
O) Complexes
MAL, H SUC, H
UND, and H
2
2
2
2
GLU,
2
DOD)
H
2
2
2
2
2
2
3
2
3+
3+
3+
2 3
Ln O (99.999%) (Ln: La , Eu , and Gd ) from CSTARM and
concentrated hydrochloric acid (37%, Merck).
The synthetic procedure of the Ln3+ aliphatic α,ω,-
3
+
The great advantage of using the Eu ion as a luminescent
probe is mainly due to its spectroscopic features: i) the principal
alkanedicarboxylate complexes reported in the literature have
been obtained predominantly by the hydrothermal method,
while there are a few works reporting on the precipitation
method.[34] In the present work, some modifications in the
precipitation method were performed, such as the Ln solutions
[1,32,33]
emitting 5D
level is non-degenerate and presents a long
luminescence decay time (milliseconds) and ii) the radiative rate
of the 5  7
transition, allowed by the magnetic dipole
0
D
0
F
1
3+
mechanism, is essentially insensitive to the ligand field
addition to a heated solution at ~80 ºC of the previously
deprotonated ligand (pH ~ 7) in order to increase the solubility of
the ligands in water (yields 90%). The general reaction can be
represented by Eq. (1).
environment and can be used as a reference transition to
calculate spectroscopic parameters from the 5D

7F2,4,6
0
transitions.[24,25] From the emission and excitation spectra of Eu3+
complexes, unique information on the ligand field splitting,
intramolecular energy transfer processes, intrinsic emission
quantum yields, and overall luminescence quantum yields can be
obtained.
2[LnCl
3
(H
2
O)
6
](aq) + 3Na
2
L(aq) 
(1)
+
-
[
Ln
2
(L) (H O)
3
2
x
]∙y(H O)(s) + 6Na (aq) + 6Cl (aq)
2
Among the Ln3+ carboxylate compounds, the so-called
Metal-Organic Frameworks (MOFs) have been receiving special
attention in the fields of crystal engineering and molecular
topology owing to the facility of synthesis and the possibility of
modeling materials with desired properties. Most studies reported
in the literature have focused on the synthesis and luminescent or
magnetic properties of the Ln3+ coordination compounds.[26–28]
Systematic studies on the correlation between the structural
characteristics of the aliphatic dicarboxylate ligands and the
luminescent properties of these complexes are still scarce.
In this work, we described the elucidation of the odd–even
effect on the photoluminescence features of Eu3+ dicarboxylate
Characterization Techniques
The elemental analyses were performed in a Perkin-Elmer
CHN 2400 instruments. The infrared absorption spectra of the
compounds were measured using KBr pellets on a Bomem
−1
MB100 FTIR spectrometer from 400 to 4000 cm with a spectral
-1
resolution of 4.0 cm . Thermogravimetric curves from 25 to 800
ºC were performed on a 2950 TGA HR V5.4A under dynamic
3
-1
synthetic air atmosphere of 50 cm min with a constant heating
-1
rate of 3 ºCmin . The X-ray diffraction patterns were obtained
with a Miniflex Rigaku diffractometer using CuKα1 radiation (30 kV
and 15 mA) in the (2θ) 3 to 60º range and of 0.05 s of pass time.
The diffuse reflectance data were collected with a Shimadzu
UV-2600 equipment spectrophotometer containing an integrating
complexes.
Accordingly,
we
present
the
synthesis,
characterization as well as the experimental and theoretical
photoluminescence studies of a class of compounds with the
sphere. BaSO
4
was used as the diffuse reflectance standard.
general formula [Eu
2
(L)
3
(H
2
O)
x
]·y(H
2
O), where
L
refers to
Steady-state excitation and emission spectra of the Eu3
+


dicarboxylate ligands OOC(CH
2
)
n-2COO , in which 2 ≤ n ≤ 12,
complexes and the time-resolved phosphorescence spectra of
the Gd3+ complexes in solid state at room (~300 K) and liquid
nitrogen (77 K) temperatures were recorded at an angle of 22.5º
with x = 2–6 and y = 0–4, depending on ligand. As far as we know,
this is the first time that the odd–even effect is observed and
rationalized in the luminescence features of lanthanide
compounds. The theoretical treatment is based on previous
approaches on 4f-4f intensities.[29–31] The zigzag patterns were
successfully explained by the proposal of an extension of the
Dynamic Coupling mechanism to describe the intensities of 4f-4f
transitions. A model, herein called the “Ghost-Atom” model, was
used to simulate the long-range dispersion effects of the ligand
chain on the first coordination sphere of the trivalent europium
complexes.
(
front face) with a spectrofluorimeter (SPEX-Fluorolog 2) with
double grating between 0.5 and 2.0 mm monochromator
SPEX1680), and a 450 W Xenon lamp as excitation source. All
(
spectra were recorded using a detector mode correction. The
luminescence decay curves of the emitting levels of the
[
Eu
2
(L)
3
(H
2
O)
x
]·y(H
2
O) were measured at room temperature,
3+
while the analogous Gd complexes were measured at low
temperature (77 K), using a phosphorimeter SPEX 1934D
accessory coupled to the spectrofluorometer.
The coordination compounds were characterized by
elemental analysis, X-ray powder diffraction, thermogravimetric
analysis, and UV−visible luminescence. It is noteworthy that the
2
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FULL PAPER
Experimental Intensity Parameters
The experimental intensity parameters (Ω
calculated according to Eq (2),[35,36]
with ꢋ (t = 1, 3, 5, and 7) and ꢑ being the ranks and components,
respectively, that define the spherical harmonics 푌 푝푡 ∗ in Eqs. (8
2
and Ω
4
) can be
(2)
푡
푝
푡
푝
and 9). The magnitudes of the 훾 and Г terms are dependent on
the chemical bond, structure, and nature of the chemical
environment of the lanthanide ion:
ꢀ
3
ℏ푐 퐴
0
→휆
(휆)
Ω =
λ
2
2
ꢀ
7
5
4
푒 휔 휒⟨ 퐹 ‖푈 ‖ 퐷 ⟩
휆
0
ꢈ⁄2
푡∗
4
휋
푡ꢌꢈ 푌푝 ꢂ휃푗, 휑푗ꢄ
푡
2
훾 = 푒 ꢇ
ꢉ
∑ 휌 푔 ꢂꢃ훽 ꢄ
(8)
(9)
푝
푗
푗
푗
푅 푗푡 ꢌꢈ
ꢃꢋ + 1
2
2
푟ꢁ푓
푗
where  = 푛푟ꢁ푓ꢂ푛 + ꢃꢄ ⁄9 is the Lorentz local field correction
and 푛푟ꢁ푓 is the linear refractive index of the medium (assumed 1.5
ꢈ⁄2
푡∗
4
휋
푡ꢌꢈ
푌푝 ꢂ휃푗, 휑푗ꢄ
푡
′
Г = ꢇ
ꢉ
∑ ꢒꢂꢃ훽푗ꢄ
푗
훼푂푃ꢓ + 훼 ꢔ
푗
for these complexes). 휔 is the angular frequency of the transition.
푝
푅 푗푡 ꢌꢈ
ꢃꢋ + 1
2
7
(휆)
5
The ⟨ 퐹 ꢅ푈 ꢅ 퐷 ⟩ are the square reduced matrix elements
휆
0
and the values for these quantities are equal to 0.0032 and 0.0023
for ꢆ = 2 and 4, respectively.[37] The 퐴0→퐽 are spontaneous
where 휌 are the overlap integrals between valence orbitals of the
푗
5
→ 7F
emission coefficients assigned to the D
0
J
transitions. The
Ln and ligating atoms (or ions), 푔 are charge factors, 푅 are the
푗
푗
5D
( F
7
transition is governed practically 100% by the Magnetic
3+
0
1
equilibrium distances between Ln ion and the ligating atoms or
ꢕꢈ
Dipole mechanism, thus its spontaneous emission coefficient is
ions, and 훽 = ꢂ1 + 휌 ꢄ . Eqs. (8 and 9) are derived from the
푗
푗
formally insensitive to the chemical environment (,-0→ 1. ~ 50–80
[
29,30]
Simple Overlap Model (SOM)
for the Ligand Field and Bond
-1
s ). Therefore, it is taken as an internal standard to determine the
Overlap Model (BOM)[
31,43]
for the Dynamic Coupling mechanism,
퐴0→퐽 values for Eu3+ complexes using Eq. (3),[24,36]
respectively. The polarizability is partitioned into two terms: the
polarizability 훼푂푃ꢓ associated with the overlap in the chemical
푆0→퐽
′
푗
bond and 훼 associated to the moieties of the ligand polarizability
퐴0→퐽 = ꢇ
ꢉ 퐴0→ꢈ
(3)
푆0→ꢈ
locally influencing the Ln3+ ion.
The 훼푂푃ꢓ values can be calculated by Eq. (10) as a function
[
44]
where 푆0→ꢈ and 푆0→퐽 correspond to the areas under of the
of the 푅 , the first excitation energy ∆휀 , and 휌 . Details on these
푗 푗 푗
emission curves (proportional to the number of the emitted
quantities in Eqs. (4-10) are reported in the work of Moura Jr. et
photons) of the 5D

 F
7F
and 5D

7F2,4,6 transitions,
transition is frequently not observed
al.[31]
0
1
0
5
7
respectively. The D
experimentally.[36]
0
6
2
2
2
푗
푒 휌 푅
푗
훼푂푃ꢓ
=
(10)
ꢃ∆휀푗
Theoretical Section
It is conceived that the charge distribution localized in the
chemical bond and on the organic ligands makes significant
contributions to the ligand field in Eu3+ compounds.[45] The
contributions to the molecular polarizability of Localized Molecular
Orbitals (LMOs) can be used to estimate the isolated ligand
Theoretical Intensity Parameters
λ
The theoretical intensity parameters Ω (λ=2, 4, and 6) of 4f-
4
f transitions (from the Judd-Ofelt theory[38,39]) depend on the
chemical environment and on the lanthanide ion. The two main
contributions to the description of spontaneous emission
coefficients are the Forced Electric Dipole (FED)[38,39] and the
′
effective polarizability 훼푗. The localization of canonical molecular
orbitals can be performed by different procedures, being the
polarizability
dependent
Dynamic
Coupling
’s are given by Eqs.
(DC)[40–42]
Pipek–Mezey approach quite successful in providing LMOs for
[46]
the ligands of interest.[31]
mechanisms. The theoretical parameters Ω
λ
(
4–7),
The Odd–Even Effect in the Intensity Parameters
3
+
2
The odd–even effect of the Eu –dicarboxylate complexes
can be explained in terms of an approach named the “Ghost–
Atom” model, as mentioned above. This model presumes that a
species located at the middle of the ligand chain has the purpose
|
퐵λ푡푝
|
Ω = (ꢃꢊ + 1) ∑
(4)
λ
(
ꢃꢋ + 1)
푡,푝
of simulating the effect of the whole ligand on the first coordination
sphere of the Eu3+ complex. It is important to emphasize that this
where
approach also takes into account the zigzag alternation in the
aliphatic dicarboxylate chain, being an extension of the Bond
Overlap Model.[31]
퐵λ푡푝 = 퐵 λF 푡E 푝D + 퐵DC
λ푡푝
(5)
(6)
ꢃ
FED
λ푡푝
〈r푡ꢌꢈ〉Θ(t, ꢊ)γ푡
푝
퐵
=
Δ퐸
The Ghost–Atom Model
′
ꢈ
The way the effective polarizability 훼 has been calculated
does not consider the presence of the effective charge of the Eu
ion. Clearly, the 훼′ takes only into account the short-range
(
ꢆ + 1)(ꢃꢆ + 3) 2
3+
ꢍ퐶
휆
(휆)
푡
푝
(7)
퐵
= − [
] 〈ꢎ 〉⟨ꢏ||ꢐ ||ꢏ⟩훤 훿
휆푡푝
푡,휆ꢌꢈ
(ꢃꢆ + 1)
coordination polyhedron polarization in the COO– carboxylate
3
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ChemPhysChem
10.1002/cphc.201900603
FULL PAPER
The GA average distances (푅 ) to the Eu3 ion (Figure S1c)
+
group. The long-range ligand contributions should be included in
the DC mechanism by the aliphatic chain LMOs polarizabilities.
푣
are proportional to the carbon chain length, making Eq. (12) less
∗
푣
The long-range interactions of the aliphatic carbon chain
dependent on the 훼 values for long chains.
∗
푣
LMOs polarizabilities 훼 , given by Eq. (11), are accounted as a
representation of an atom, labeled as the Ghost–Atom (GA),
located at the middle of the dicarboxylate ligand chain. It is
important to highlight that the GA makes long-range contributions
to the first coordination sphere:
Results and Discussion
Characterization
The elemental analysis data of the Ln3 coordination
+
compounds (Ln: Eu3+ and Gd ) are in agreement with the formula
3+
훼∗ = ∑ 훼퐿푀푂푒ꢕ푑ꢖ
푣
ℎ
(11)
[
[
Ln
Ln
2
(OXA)
(SUC)
3
3
(H
(H
2
O)
O)
6
]·4(H
2
O), [Ln
2
(L)
3
(H
(H
2
O)
O)
6
] (L: MAL and GLU),
4
]·H O, [Ln (L) (H O) ]
ℎ
2
2
2
]·H O, [Ln
2
2
(ADP)
3
2
4
2
2
3
2
(
L: PIM, SUB, AZL, SEB, UND, DOD) (Table S1).
The thermogravimetric (TG) curves (Figure S2a-S2k) of the
ꢗ is the distance between the GA and the ꢘ-th LMO polarizability
ℎ
퐿푀푂
∗
훼
. The GA polarizability (훼 ) is calculated as the sum of
Eu3+ and Gd3+ dicarboxylate complexes recorded at the
temperature interval from 25 to 800 °C corroborate with the
elemental analysis data. All complexes with the same ligands and
different Ln3+ ion present similar thermal decomposition profiles,
indicating the same stoichiometry. The Ln3+ complexes with OXA,
SUC, and ADP ligands present both coordinated and hydration
water molecules in their structures, while the other remaining
complexes presented only coordinated water molecules. In
ℎ
polarizabilities multiplied by an exponential decay factor over the
distance (ꢗ ) of the GA to the center of each LMO (Figures S1a
ℎ
and S1b).
The inclusion of 훼∗ in the DC mechanism can be done
푣
adding a term in Eq. (9) in the following form:
ꢈ⁄2
푡∗
4
휋
푡ꢌꢈ
푌푝 ꢂ휃푗, 휑푗ꢄ
푡
′
general, the Eu3 dicarboxylate complexes showed relative lower
decomposition temperatures compared with those Gd3+
+
Г = ꢇ
ꢉ
ꢙ∑ ꢒꢂꢃ훽푗ꢄ
푗
훼푂푃ꢓ + 훼 ꢔ
푗
푝
푅 푗푡 ꢌꢈ
ꢃꢋ + 1
(
12)
complexes, reflecting the weaker interaction between the
europium ion and the ligands.
TG curves show that increasing the carbon chain of the
dicarboxylate ligands, the dehydration temperature (Figure 3)
decreases due to increasing hydrophobic character of the
complex. Interestingly, Figure 3 shows the zigzag pattern from the
푡∗
푌 (휃 , 휑 )
푝
푣
푣
+
∑ 훼∗
ꢚ
푣
푡ꢌꢈ
푣
푅
푣
in which ꢛ represents the GA. It must be highlighted that the GA
represents induced point dipole moments, consequently without
direct covalency effects in the first coordination sphere.
2
final temperatures of released H O molecules as a function of the
number of carbon atoms (2 ≤ n ≤ 12) in the dicarboxylate ligands.
An illustration of the aliphatic dicarboxylate ligands with an
odd (a) and even (b) carbon members of the chain is depicted in
Figure 2. The symmetry of the dicarboxylate ligands imposes the
condition that the vectors illustrated by the same numerical labels
(
in the same ligand) have the same magnitudes and chemical
environments. It can be perceived that the odd and even carbon
members have permanent dipole moments 휇⃗ ≠ ꢜ and 휇⃗ ≈ ꢜ,
푃
푃
respectively (Figures 2a and 2b).
Figure 3. Zigzag pattern obtained from final temperature (°C) of released H
2
O
molecules as a function of the number of carbon atoms in the dicarboxylate
–
–
ligands in [Eu
2 3 2 x 2 2
(L) (H O) ]·y(H O) complexes (L: OOC(CH )n-2COO , 2 ≤ n ≤ 12,
x = 2–6, and y = 0–4).
3
+
FTIR spectra of the Eu –dicarboxylate complexes (Figure
Figure 2. Vector contribution of partial dipole moments of the dicarboxylate
ligands. (a) For odd members (n=9) the resultant permanent dipole moment is
휇⃗ 푃 ≠ ꢜ and (b) for even members (n=8) 휇⃗ 푃 ≈ ꢜ . The dashed line (in the
carboxylate groups) represents the delocalized charge.
-1
S3) show a broad absorption band among 3000–3800 cm
-1
centered at around 3400 cm assigned to the O−H stretching of
2
the H O molecules, confirming their hydrated character. In
addition, some of the complexes show narrow and broad
-1
absorption bands at 3600 and 3300 cm , suggesting the
2
presence of either coordination or hydration H O molecules,
respectively.[47] Moreover, the absorption peaks observed in the
4
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ChemPhysChem
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range of 600 to 1000 cm-1 are assigned to the vibration of C−C
bonds. Except for the OXA complexes, all the complexes present
peaks around 2800–3000 cm-1 attributed to the C−H vibration and
their relative absorption intensity increases with the number of
carbon atoms in the chain.
bridge-chelate Ln −L coordination mode as discussed in the
FTIR data, even for the MAL ligand that forms chelate rings with
4 and 6 members. It is noteworthy that the XRD patterns obtained
from the PIM, AZL, UND, and DOD complexes are reported in this
work.
3+
The coordination modes of the carboxylate groups for a
given metal ion can be inferred by the difference between the
COO asymmetric and symmetric modes (Δν = νas. - νsym.)[48] when
3
+
LMCT and Triplet State of Ln Complexes
–
3+
The use of the La ion is useful to establish LMCT when
3
+
3+
compared
Eu (L) (H
with
]·y(H
the
respective
Na
2
L
salt.
For
compared with those analogues Eu –complexes. La –
complexes are used for obtaining the ligand-to-metal charge
transfer state (LMCT) due to the fact that no 4f-4f transitions can
[
2
3
2
O)
x
2
O) complexes, two distinct absorption peaks
–
were assigned to the νas.(COO ), suggesting two different
coordination modes. The Δν value at around 97 cm-1 is associated
with a bridge coordination mode, while the Δν value at 130 cm-1
is assigned to the chelate coordination mode. The results indicate
that the carboxylate groups are coordinated to the Eu3+ ions by a
mixed bridge-chelate coordination mode, except for the complex
with OXA ligand.
3+
be observed. In this sense, the La –complexes reflectance
3+
spectra can be compared to those analogues of Eu –
complexes.[ Moreover, the La ion presents an ionic radius
53]
3+
(1.216 Å) similar to the ionic radius of the Eu ion (1.120 Å).[54]
3+
2 3 2 x 2
Diffuse reflectance spectra of the [Ln (L) (H O) ]·y(H O)
complexes (Ln: La3+ and Eu ) were recorded in the UV–Vis range
3+
The XRD (Powder) patterns of [Ln
2
(L)
3
(H
2
O)
x
]·y(H
2
O)
4
(220–600 nm), using BaSO as a reflecting standard (Figure S5).
When the absorption spectra of the La and Eu complexes are
complexes (L: –OOC(CH
)n-2COO , 2 ≤ n ≤ 12) present highly
–
3+
3+
2
intense and narrow diffraction peaks predominantly at lower
angles (Figure S4), suggesting highly crystalline compounds and
large crystallite sizes. The diffractogram profiles of Eu3+ and Gd3+
complexes are similar, suggesting an isomorphic form when
containing the same ligand as reported in literature.[32,49–52]
compared in the range of 220–300 nm, only the broad absorption
bands for the Eu3 complexes assigned to LMCT transitions can
+
[53,55]
be observed.
Also, the absorption spectral profiles of different
Eu3+ complexes are quite similar, although the energy of the
LMCT bands varies as a function of the chemical environment
around the metal ion. Besides this, the diffuse reflectance spectra
2 3 2 6 2
The similarity of [Ln (OXA) (H O) ]·4(H O) diffractograms
and the standard crystal structure data (CCDC: 816524) confirms
a nine-coordinated site with a tricapped trigonal prism geometry[32]
of [Eu
2
(L)
3
(H
2
O)
x
]y(H
2
O) complexes exhibit narrow absorption
3+
peaks assigned to the intraconfigurational transitions of the Eu
ion (in cm ): F  H (31446), F  D (27624), 7F  G
-1
7
5
7
5
5
(
Figure S4). The diffractograms of the complexes with L = MAL,
0
3
0
4
0
5
(26316), F  L (25316), 7F  D (24038), 7F  D (21522),
7
5
5
5
ADP, SUB, and SEB are comparable to their respective standard
crystal structure data, CCDC: 181893,[50] 721892,[49] 700675,[51]
and 745638,[52] respectively. These results indicate a mixed
0
6
1
3
0
2
7
5
(19028, 18674), and 7F
5
F
0,1 D
1
1
0
 D (16909)
–
Figure 4. Excitation (a) and emission (b) spectra of the [Eu
x = 2–6 and y = 0–4) measured at 77 K. The excitation spectra were recorded monitoring at the 5
recorded under excitation at the 7  5
transition (~394 nm).
2
(L)
3
(H
2
O)
x
]·y(H
2
O) complexes with aliphatic dicarboxylate ligands (L: –OOC(CH
2
)n-2COO , 2 ≤ n ≤ 12,
D
0
 7
F
2
transition (~ 614 nm). The emission spectra were
F
0
L
6
5
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ChemPhysChem
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The Gd3+ ion presents a very large energy gap (32000 cm
-
1)
between the S7/2 ground state and the first P7/2 excited state
8
6
and can be used to estimate the triplet state energies (T
1
) of the
carboxylate ligands. Thus, Gd3+ complexes are useful to
understand the energy level structure of the coordinated ligands,
3
+
1
especially on the triplet (T ) energy levels. The Gd ion can act
simulating the chemical environment of the Eu3+ owing to the
similarity of their ionic radii (ꢎGdꢝꢞ = 1.107 Å).[36,54]
The phosphoresce spectra of [Gd
complexes exhibit broad emission bands with barycenters at
around 22900 cm-1 assigned to the T
S transitions from
carboxylate ligands (Figure S6). The higher energy values of the
2 3 2 x 2
(L) (H O) ]·y(H O)
1
0
T
1
states could be assigned to the absence of  conjugated
bonds.[
56]
Luminescence Properties of the Eu3+ Complexes
The excitation (Figure 4a) and emission (Figure 4b) spectra
of the [Eu
2
(L)
3
(H
2
O)
x
]·y(H
2
O) complexes were recorded at 77 K.
(L) (H O) ]·y(H O)
Figure 5. Odd–even effect on the experimental intensity parameters 
squares) and  (blue circles) of the [Eu (L) (H O) ]·y(H O) complexes as a
function of the number of carbon atoms (n) in the dicarboxylate ligand chain (L:
2
(red
The excitation spectra of the [Eu
complexes (Figure 4a) recorded in the range of 225–550 nm
2
3
2
x
2
4
2
3
2
x
2
5
7F
monitoring the emission of the D
0
2
transition (~614 nm) show
–OOC(CH
–
2
)n-2COO , 2 ≤ n ≤ 12, x = 2–6 and y = 0–4).
broad absorption bands at ~280 nm assigned to the S
intraligand and LMCT states. Moreover, there are narrow
absorption bands arising from intraconfigurational transitions of
0
S
1
Theoretical Intensity Parameters

For calculation of the theoretical intensity parameters ( ),
the experimental crystallographic data of the coordination
compounds were used to generate reduced unit cells that
simulate the total system, according to Figure 6. The
crystallographic unit cell was replicated in a size enough to extract
the Eu3+ ion (cm ): F
-1
7
5I
(35088), I
5
(34130), F
5
(33557), F
(26667), G
(19048), and
5
0
5
4
6
2
4
5
5
5
5
(
(
33112), H
7
(30675), L10 (28571), D
(25316), D
4
(27778), L
7
4
25907), 5L
5
(24331), D
5
(21552), D
5
6
3
2
1
5
(17241).[25] The excitation spectra recorded at 300 K also
D
0
2 3 2 x 2
the total structure of the [Eu (L) (H O) ]·y(H O) complexes
exhibit absorption bands arising from the 7
F
1
and
7
F
2
excited
(
Figures 6a and 6b). These structures, as a function of the number
energy levels (Figure S7) due to energy gap between these
–
of carbon atoms (n) in the dicarboxylate ligand chain (L:
7
energy levels and the F
1
0
[25]
ground level, that are close to 380 and
–
OOC(CH
2
)
n-2COO , 2 ≤ n ≤ 12), were isolated to build up proper
-1
040 cm , respectively.
thermal dependent population of the excited levels. In general, the
f-4f transitions arising from the 7F and 7
levels are not
observed for spectra recorded at a low temperature.
The emission spectra of [Eu (L) (H O) ]·y(H O) complexes
These spectral results suggest a
unit cells (Figure 6b), that were used to perform all geometry
optimizations using Periodic Boundary Conditions (PBC). From
the optimized structures, the ligand geometries were taken out
4
1
F
2
′
(
isolated) to calculate the ligand effective polarizabilities (α ).
2
3
2
x
2
The theoretical PBC equilibrium geometries were calculated
recorded from 500 to 725 nm (Figure 4b), under excitation at the
for all the Eu3+ complexes and compared with the experimental
crystallographic data, as available in literature. All calculated
geometries are quite similar to the crystallographic ones with root-
mean-square deviation ~0.475 Å (Table S2 and Figure S8). The
transition 7F
assigned to the 5
5
0
 L
6
(~394 nm) show narrow emission bands
7F transitions of the Eu3+ ion (in cm-1):
D
5
0
J
5D
15331), and D
only one emission peak attributed to the 5
suggesting a unique symmetry site around the Eu ion (Cnv, C
or C
).[36] It is important to emphasize that the excitation is
7F
(17227), D
7F
(14358). These emission spectra present
7F
(~578 nm),
(16848), D
5
7F
(16184), D
5
7F
0
0
0
1
0
2
0
3
5
7
(
0
 F
4
coordination polyhedron is formed by a Eu3 ion bridged to a
+
D
0
0
3+
2
second Eu ion by two ligands, each one with a  -oxygen
3
+
n
carboxylate shared by the dimer (Figure 6d), with exception of the
s
3+
Eu –OXA complex.
5
of the Eu3+ ion, which is far from the ligand
monitored on the L
singlet state.
6
Experimental Intensity Parameters
The odd–even effect due to the dicarboxylate chemical
environment around the Eu3+ ion was observed in the
experimental intensity parameters 
. Since it is not possible to distinguish the contributions of the
forced electric dipole and dynamic coupling mechanisms in the
experimental  and  intensity parameters, it is extremely
2
4
and  , as shown in Figure
5
2
4
important to have a theoretical detailed description of these
parameters,[ as it will be discussed in the following section.
57]
Figure 6. Schematic replication of crystallographic unit cell (a) used to build up
the reduced unit cell (b) for the PBC calculations (c). The coordination
polyhedron (d) was extracted to carry out the theoretical 
λ
intensity parameters
calculations.
6
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The coordination mode of the ligands plays an important
′
role in the determination of effective polarizability calculations (α ).
The ligand structures and the superposition of LMOs of the
coordination carboxylate group are depicted in Figure 7.
The dicarboxylate ligand with n = 2 (OXA) acts as a
bidentate chelating ligand with an oxygen atom from each
carboxylate group. The malonate ligand n = 3 (MAL) also acts as
a bidentate chelating ligand. However, coordination can occur
either with one oxygen atom from each carboxylate group or with
the two oxygen atoms from one of the carboxylate groups (Figure
7
b). For the organic ligands with n ≥ 4, the coordination occurs
with the two oxygen atoms from the same carboxylate group
Figure 7c).
(
Figure 8. Calculated permanent dipole moment μꢢ ⃗P (red squares) and the ghost-
atom polarizability 훼∗ (blue triangles) for each ligand. These values are in
function of the number of carbons in the chain (2 ≤ n ≤ 12).
Table 1. Molecular isotropic polarizabilities (α푚표푙 ), the carboxylate isotropic
polarizabilities (αCO ꢠꢟ ), ligand effective polarizabilities (αꢣ), and Ghost–Atom
∗
3
polarizabilities (훼 ). The polarizabilities are in Å .
n
훼푚표푙
훼COꢟ
ꢠ
훼ꢣ (a)
훼∗
2
3
4
5
6
7
8
9
1
1
1
9.443
11.852
13.354
15.272
17.045
18.973
20.869
22.812
24.741
26.694
28.637
2.506
2.272
2.258
2.142
2.164
2.176
2.200
2.206
2.234
2.234
2.216
1.253
1.136
1.129
1.071
1.082
1.088
1.100
1.103
1.117
1.117
1.108
3.067
2.286
2.640
2.237
2.460
2.141
2.625
2.124
2.450
2.044
2.441
0
1
2
Figure 7. Ligand structures and the superposition of LMOs in the coordination
carboxylate groups in different types of coordination.
(a)
α′ = α̅ CO
⁄
ꢃ
ꢟ
ꢠ
The molecular isotropic polarizability (α푚표푙) for all the ligand
structures was calculated, as well as their decompositions in the
LMOs contributions (Table 1). Using the decomposed α푚표푙 it is
possible to extract the coordinated carboxylate polarizability
As can be seen in Figure S9 and Table 1, αꢣ values do not
follow an odd–even effect, whereas 훼∗ values have variations
between odd and even ligands. These variations in the
polarization of the ligands, represented by the Ghost–Atom (as
(
αCOꢟ
ꢠ
), which contributes with two ligating atoms. Therefore, the
5
 7F
illustrated in Figure 9), reflects in the changes of the D
transitions by the DC mechanism.
0
J
ligand effective polarizability (one ligating atom contribution) is
′
⁄ꢃ . The Ghost-Atom polarizability α∗ is
COꢟ
taken as α = α
ꢠ
Table S5 and Figure 10 show that the Ωλ theoretical
intensities parameter values are in a good agreement with the
experimental ones. The FED mechanism participation at around
4% for Ω2 and 12% for Ω4 is less representative than the DC
obtained from Eq. (11).
The odd–even effect can be observed in Figure 8, where the
permanent dipoles (red squares) and the GA polarizabilities (blue
triangles) have opposite trends. In the cases where the values of
∗
mechanism, where calculated αꢣ and estimated α are used
ꢡ μꢢ ⃗ ꢡ ≠ ꢜ (n = odd), the ligands have a net charge separation.
(Approach A). The inclusion of the Ghost–Atom in the calculations
decreases the average relative errors with respect to the
experimental values, from 18% (Approach B) to 7% (Approach A).
P
Therefore, the odd carbon members have a more rigid electron
∗
density and smaller values of 휶 than in the cases of the even
carbon members.
7
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ChemPhysChem
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FULL PAPER
Figure 9. Representation of the long-range interaction accounted for by the Ghost-Atoms included in each ligand, emphasizing the dipole effectively induced by
the ligand long chains.
Figure 10. Odd–even effect on the theoretical intensity parameters 
2
(a) and 
4
(b) of the [Eu
2
(L)
3
(H
2
O)
x
]·y(H
2
O) complexes as a function of the number of
(A) were obtained with the inclusion of the
carbon atoms (n) in the dicarboxylate ligands (L: –OOC(CH
n-2COO , 2 ≤ n ≤ 12). In both cases, the theoretical 
–
2
)
λ
λ
Ghost–Atom polarizability, while the  (B) were obtained without this effect.
Intrinsic Quantum Yield
molecule number and multiphonon relaxation by coupling with the
O–H oscillators.
The lifetime (휏 ) values of the 5D
emitting level of the
O) complexes were obtained from the
0
[
Eu
2
(L)
(H
3 2
O)
x
]·y(H
2
Furthermore, the lifetime data are useful for determining the
number of water molecules in the first coordination sphere around
luminescence decay curves registered at room temperature
5
7
the Eu3+ ion, i.e. the hydration degree or hydration number
(
(
(
Figure S10), monitoring the emission of the D
~615 nm) with an excitation wavelength at the 7F
0
 F
2
transition
transition
5
[25]
(qH O).
In order to obtain these values, Eq. (13) was used,
0
 L
6
ꢟ
~394 nm). The lifetime values for all complexes range from 0.36
(OXA) (H O) ]·4(H O) and
] complexes that present the lowest 휏 values,
.31 and 0.30 ms, respectively (Table S5). These lower values
which leads to values with estimated errors of about ± 0.5 water
molecule per Eu3+ ion.[25,58,59]
to 0.64 ms, except for the [Eu
Eu (MAL) (H O)
2
3
2
6
2
[
0
2
3
2
6
1
qHꢟO = 1.ꢜꢤ − ꢜ.ꢥꢜ
τ
(13)
are attributed to the presence of the highest number of water
molecules in the first coordination sphere (three water molecules
3
+
per Eu ion). The [Eu
2 3 2 2 2
(SUC) (H O) ]·H O complex presents the
The number of coordinated H
2
O molecules to the Eu3+ ion
highest value of lifetime (0.639 ms), due to the lowest water
calculated by Eq. (13) usually present fractional values, lower
than those obtained experimentally. This fact can be due to
8
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ChemPhysChem
10.1002/cphc.201900603
FULL PAPER
different factors, such as the presence of further oscillators Conclusions
different from the OH mode, the CH oscillators and the
influence of the second coordination sphere that also can lead to
In this work, an odd–even effect systematic study based on
5
3+
a lowering of the lifetime values of the D
0
emitting level. In this
photoluminescent properties of
dicarboxylate complexes [Eu (L) (H
eleven Eu
aliphatic
way, the number of coordinated H
2
O molecules (Table S5) agrees
2
3
2
O)
x
]·y(H O) (L: OXA, MAL,
2
with the experimental ones obtained by the elemental and
SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x
= 2–6 and y = 0–4) was experimentally and theoretically
discussed. TG-DTG data indicate a zigzag pattern in the
thermogravimetric analyses.
퐿ꢦ
퐿ꢦ
The emission intrinsic quantum yield 푄 (using Bünzli’s
notation[22]) of the D
5
emitting level of the Eu3+ ion is the ratio
2
dehydration process in which the H O molecules are released at
0
lower temperatures for the odd carbon members of the series than
the even ones, showing the odd–even effect.
Besides, a “Ghost-Atom” model was introduced to simulate
the long-range dispersion effects of the ligand chain on the first
between the radiative ꢂ퐴푟푎푑 = ꢧ 퐴0→퐽ꢄ and the non-radiative
(
퐴ꢦ푟푎푑) + radiative decay processes. Based on the experimental
5
lifetimes of the D
0
emitting level in Eq. (14), and 퐴푟푎푑, calculated
from Eq. (3), it is possible to determine the non-radiative 퐴ꢦ푟푎푑
rates and the emission intrinsic quantum yield as given by Eq.
coordination sphere of the Eu3+ dicarboxylate complexes. The 
2
and  theoretical and experimental intensity parameters values
4
(
15).
and zigzag patterns (odd–even effect) were in almost complete
agreement, the theoretical values being efficiently described by
1
휏
퐴푡표푡
=
=
= 퐴푟푎푑 + 퐴ꢦ푟푎푑
(14)
(15)
the Ghost–Atom model. The intrinsic quantum yield (푄 ) of the
퐿ꢦ
퐿ꢦ
3
+
Eu complexes also presents the odd–even effect due to the
퐴푟푎푑
퐴푟푎푑 + 퐴ꢦ푟푎푑
radiative rate 퐴푟푎푑 that depend straightly on the  intensity
퐿ꢦ
퐿ꢦ
λ
푄
3
+
parameters, indicating that the Eu ion acts as a powerful
luminescent probe for chemical environment around lanthanide
ion.
It is noteworthy that for all the Eu3+ complexes containing a
Finally, the zigzag patterns from the photoluminescence
features of Eu3+ dicarboxylate complexes were experimentally
observed and successfully explained based on theoretical
approaches of the 4f-4f transition intensities, confirming the odd–
even effect.
퐿ꢦ
ligand with a number of carbon atoms n ≥ 5, the 푄퐿ꢦ values
Figure 11) present the zigzag behavior with respect to their
number of carbons in the chain. The presence of odd-even effect
come from the radiative rate contribution (퐴푟푎푑), while the non-
radiative rate (퐴ꢦ푟푎푑) is insensitive to the zigzag behavior (Figure
S11).
(
Acknowledgements
The authors thank the funding agencies for financial support:
Conselho Nacional de Desenvolvimento Científico e Tecnológico
(
CNPq), Fundação de Amparo à Pesquisa do Estado de São
Paulo (FAPESP).
Keywords: The odd–even effect • europium • dicarboxylate
complexes • photoluminescence intensity parameters
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1
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[
(
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Entry for the Table of Contents
FULL PAPER
Israel P. Assunção, Albano N. Carneiro
Neto, Renaldo T. Moura Jr., Cássio C.
S. Pedroso, Ivan G. N. Silva, Maria
C.F.C. Felinto, Ercules E.S. Teotonio,
Oscar L. Malta* and Hermi F. Brito*
Pages 1 – 10.
Odd–Even Effect on Luminescence
Properties of Europium Aliphatic
Dicarboxylate Complexes
Abstract
01
01
02
02
02
03
03
03
03
03
04
04
05
06
06
06
08
09
09
09
Introduction
Experimental Section
Synthesis of the [Ln2(L)3(H2O)x]•y(H2O) Complexes
Characterization Techniques
Experimental Intensity Parameters
Theoretical Section
Theoretical Intensity Parameters
The Odd–Even Effect in the Intensity Parameters
The Ghost–Atom Model
Results and Discussion
Characterization
LMCT and Triplet State of Ln3+ Complexes
Luminescence Properties of the Eu3+ Complexes
Experimental Intensity Parameters
Theoretical Intensity Parameters
Intrinsic Quantum Yield
Conclusions
Acknowledgements
References
1
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