Synthesis and characterization of the europium(III) pentakis(picrate) complexes with imidazolium countercations: Structural and photoluminescence study

Article abstract of DOI:10.1021/ic301776n

Six new lanthanide complexes of stoichiometric formula (C) ₂[Ln(Pic)₅]-where (C) is a imidazolium cation coming from the ionic liquids 1-butyl-3-methylimidazolium picrate (BMIm-Pic), 1-butyl-3-ethylimidazolium picrate (BEIm-Pic), and 1,3-dibutylimidazolium picrate (BBIm-Pic), and Ln is Eu(III) or Gd(III) ions-have been prepared and characterized. To the best of our knowledge, these are the first cases of Ln(III) pentakis(picrate) complexes. The crystal structures of (BEIm) ₂[Eu(Pic)₅] and (BBIm)₂[Eu(Pic)₅] compounds were determined by single-crystal X-ray diffraction. The [Eu(Pic) ₅]^2- polyhedra have nine oxygen atoms coordinated to the Eu(III) ion, four oxygen atoms from bidentate picrate, and one oxygen atom from monodentate picrate. The structures of the Eu complexes were also calculated using the sparkle model for lanthanide complexes, allowing an analysis of intramolecular energy transfer processes in the coordination compounds. The photoluminescence properties of the Eu(III) complexes were then studied experimentally and theoretically, leading to a rationalization of their emission quantum yields.

Full text of DOI:10.1021/ic301776n

Article
pubs.acs.org/IC
Synthesis and Characterization of the Europium(III) Pentakis(picrate)
Complexes with Imidazolium Countercations: Structural and
Photoluminescence Study
Alex S. Borges,†,⊥ Jose Diogo L. Dutra,‡ Ricardo O. Freire,‡ Renaldo T. Moura, Jr.,§ Jeferson G. Da Silva,†
́
Oscar L. Malta,§ Maria Helena Araujo,† and Hermi F. Brito*,∇
†Departamento de Química, Instituto de Cien
̂
cias Exatas, Universidade Federal de Minas Gerais, Belo HorizonteMG, 31270-901,
Brazil
‡Departamento de Química, Universidade Federal de Sergipe, 49100-000, Sao Cristov
́
aoSE, Brazil
̃
̃
§Departamento de Química Fundamental, Universidade Federal de Pernambuco, CCEN, 50590-470 Recife, PE, Brazil
∇Instituto de Química, Departamento de Química Fundamental, Universidade de Sao Paulo, Av. Prof. Lineu Prestes, 748, 05508-900,
̃
Sao Paulo-SP, Brazil
̃
S
* Supporting Information
ABSTRACT: Six new lanthanide complexes of stoichiometric
formula (C)2[Ln(Pic)5]where (C) is a imidazolium cation
coming from the ionic liquids 1-butyl-3-methylimidazolium
picrate (BMIm-Pic), 1-butyl-3-ethylimidazolium picrate
(BEIm-Pic), and 1,3-dibutylimidazolium picrate (BBIm-Pic),
and Ln is Eu(III) or Gd(III) ionshave been prepared and
characterized. To the best of our knowledge, these are the first
cases of Ln(III) pentakis(picrate) complexes. The crystal
structures of (BEIm)2[Eu(Pic)5] and (BBIm)2[Eu(Pic)5]
compounds were determined by single-crystal X-ray diffrac-
tion. The [Eu(Pic)5]2− polyhedra have nine oxygen atoms
coordinated to the Eu(III) ion, four oxygen atoms from bidentate picrate, and one oxygen atom from monodentate picrate. The
structures of the Eu complexes were also calculated using the sparkle model for lanthanide complexes, allowing an analysis of
intramolecular energy transfer processes in the coordination compounds. The photoluminescence properties of the Eu(III)
complexes were then studied experimentally and theoretically, leading to a rationalization of their emission quantum yields.
other solvent molecules, which have high-energy vibrational
modes, are coordinated to the metal ion.
1. INTRODUCTION
There is great interest in the study of the luminescence properties
of trivalent lanthanide complexes with organic ligands. Special
interest lies in the possibility of designing light-conversion
molecular devices (LCMDs) based on such complexes, and their
applications as optical signal amplifiers, electroluminescent
devices and luminescent probes in biological systems, etc.1−4
To obtain an efficient LCMD, it is necessary to optimize the
luminescence process, which has been achieved using different
organic chromophores. This sensitization process consists,
initially, of the absorption of light by the ligand in the UV
region, followed by a nonradiatively energy generally transferred
from the triplet state (T1) of the ligand to an excited level of the
Ln(III) ion (antenna effect),5 and, finally, the excited Ln ion
decays to the ground state via photon emission in the visible
region. The excitation in the ligands is much more efficient than
that directly in the Ln(III) ion, because absorption coefficients of
the ligands are several orders of magnitude larger than the
intrinsically low molar absorption coefficients (typically 1−10
Therefore, the design of Ln(III) complexes with efficient
photonic properties has become an important research goal,
working with many different classes of ligands, such as amide,10
sufoxides,11 β-diketones,3 picrates,12 etc. Another important
aspect in the design of highly luminescent materials is the
efficiency of the excitation energy transfer from the ligand to the
Ln(III) ion, which depends mainly on the position of the ligand’s
triplet state, relative to the emitting level of the metal ion
(resonance condition). The former depends not only on the
ligand’s nature but also on the local symmetry of the metal ion.
Moreover, in the field of Ln(III) coordination compounds, the
semiempirical Sparkle model can be used as an important tool in
the design of new LCMDs,13−15 since it allows the treatment of a
great number of lanthanide complexes in a relatively short time.
This model has been extensively applied to the calculation of
ground-state geometry of these molecules. The knowledge of the
M
−1 cm−1) of Ln ions.6−9 However, it is important to emphasize
Received: August 13, 2012
that emission by Ln(III) complexes is quenched when water or
Published: November 14, 2012
© 2012 American Chemical Society
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Article
were determined by ethylenediamine tetraacetic acid (EDTA) titration
using Xylenol Orange as an indicator. C, H, and N concentrations were
determined using a model 2400 PerkinElmer Series 2 system.
Conductivity measurements were carried out with a conductivity
meter (Model HI 8033, Hanna) using 10−3 mol L−1 solutions of
nitromethane at 25 °C. IR spectra were recorded on Perkin−Elmer
FTIR GX instrument using KBr pellets in the 400−4000 cm−1 region.
UV−vis absorption spectrum was recorded at room temperature with a
Shimadzu Model UV-2401PC device. 1H NMR spectra were recorded
on a Bruker Avance DPX200 spectrometer. Values for chemical shifts
are referenced to the residual solvent proton resonances in CDCl3, (δ
7.24 ppm) with TMS as an internal standard. Crystal diffraction data for
all compounds were collected using an Oxford-Diffraction GEMINI-
Ultra diffractometer (LabCri-UFMG) using graphite-Enhance Source
Mo Kα radiation (λ = 0.71073 Å) at 150(2) K. The data collection, cell
refinements, and data reduction were performed using the CRYSA-
LISPRO software.29 The excitation and emission spectra of the Ln(III)
complexes in solid state at room temperature (298 K) and liquid
nitrogen temperature (77 K) were recorded at an angle of 22.5° (front
face) with a spectrofluorimeter (SPEX-Fluorolog 2) with a double-
grating 0.22 m monochromator (SPEX1680), and a 450-W xenon lamp
as an excitation source. All spectra were recorded using a detector mode
correction. The luminescence decay curves of the emitting levels were
measured using a SPEX 1934D phosphorimeter accessory coupled to
the spectrofluorometer. The luminescence instruments were fully
controlled by a DM3000F spectroscopic computer program and the
spectral intensities were automatically corrected for the photomultiplier
response.
geometry is necessary to predict spectroscopic properties, singlet
and triplet energy positions, and electronic spectra of Ln(III)
complexes.16,17 With these quantities, it is possible to build rate
equations that involve the energy transfer mechanism to calculate
the quantum yields for these complexes.18
The relationship between the metal ion and the ligands in the
complex is also another important factor in designing LCMDs.
The higher ratio of ligands that efficiently transfer energy to the
Ln(III) ion leads to a greater probability of emission of light by
the ion. It has been reported in the literature that Ln(III)
complexes with a ligand-to-metal ratio of 3:1 (LnL3, tris
3,19−22
−
complex) or 4:1 (LnL4 , tetrakis complex).
In the LnL3
anhydrous complexes, the vacant coordination positions may be
occupied by additional neutral organic ligands.
−
Some LnL4 anionic complexes have been prepared
combining ionic liquids (ILs) and β-diketonate to obtain new
LCMDs.19−22 Ionic liquids (ILs) are salts with melting points of
<100 °C.23−25 The cation of an IL is usually a large organic
cation, such as imidazolium, pyridinium, or quaternary
ammonium ions, and the anion can be inorganic or organic-
like halides, BF4 , PF6 , CF3SO3 , (CF3SO2)2N−, or picrate.
Imidazolium ILs that contain the 1-alkyl-3-methylimidazolium
cation are the most popular.
−
−
−
We describe herein the preparation, characterization and
spectroscopic studies of six new Ln(III) pentakis(picrate)
complexes that, to the best of our knowledge, are the first
pentakis picrate complexes: (BMIm)2[Eu(Pic)5] (1),
(BEIm)2[Eu(Pic)5] (2), (BBIm)2[Eu(Pic)5] (3), (BMIm)2[Gd-
(Pic)5] (4), (BEIm)2[Gd(Pic)5] (5), and (BBIm)2[Gd(Pic)5]
(6). The Sparkle model was used to calculate the geometry of
complex 1. The X-ray crystal structures of compounds 2 and 3
are described. The electronic spectra of the ligands coordinated
to the Ln(III) ion were calculated using the INDO/S−CI
method.26 Furthermore, the theoretical and experimental
emission intensities of the 5D0 → 7F0−4 transitions of the Eu(III)
ion for complexes 1−3 were analyzed and discussed.
Preparation of 1-n-Butyl-3-ethylimidazolium Picrate (BEIm-
pic). BEIm-Br (10 mmol) and potassium picrate (10 mmol) were added
as solids to a mixture of deionized water (50 mL) and methanol (10
mL). The reaction mixture was stirred and refluxed for 5 h. The resultant
yellow solution was then cooled to room temperature and BEIm-pic
extracted with dichloromethane (100 mL). The dichloromethane
solution of the picrate salt was washed three times with deionized water
(100 mL each time), whereupon volatiles were removed and the product
dried under vacuum at 70 °C for 24 h. The same procedure was used to
prepare 1,3-dibutylimidazolium picrate (BBIm-pic).
BEIm-Pic (C15H19N5O7). Anal. Calcd. for C15H19N5O7 (381.34): C,
47.24; H, 5.02; N, 18.37%. Found: C, 46.91; H, 4.43; N, 18.04%. Mp:
66−70 °C. IR (KBr, cm−1): 3143 (νs(H−CC−H)), 3087 (νas(H−
CC−H)), 2964 (νas(−CH3)), 1564, 1558 (νas(−NO2)), 1485
(νas(CC)), 1366, 1337 (νs(−NO2)), 1269 (ν(C−O)). The main
The experimental analysis of the Ω2 and Ω4 5 intensity
5
parameters were made through the D0 → 7F2 and D0 → 7F4
5
transitions, using the magnetic dipole D0 → 7F1 transition as a
1
signals in H NMR (200 MHz, CDCl3, δ in ppm): δ 9.94 (1H, s,
point of reference. The theoretical analysis of these parameters
was based on the Simple Overlap Model (SOM) for the ligand
field and the average energy denominator method to treat the
forced electric dipole contribution to the 4f−4f intensities. The
dynamic coupling mechanism was also taken into account based
on the point dipole isotropic ligating atom polarizability
approximation.27
NCHN), 8,82 (2H, s, HCCNO2CH), 7.42 (1H, m, CH3NCHCHN),
7.38 (1H, m, CH3NCHCHN), 4.45 (2H, m, NCH2CH3), 4.35 (2H, m,
NCH2(CH2)2CH3), 1.90 (2H, m, NCH2CH2CH2CH3), 1.61 (3H, t, J =
7.5 Hz, N(CH2CH3), 1.37 (2H, m, N(CH2)2CH2CH3), 0.95 (3H, t, J =
7.8 Hz, N(CH2)3CH3). Yield = 74%.
BBIm-Pic (C17H23N5O7). Anal. Calcd. for C17H23N5O7 (409.39): C,
49.87; H, 5.66; N, 17.11%. Found: C, 50.09; H, 5.28; N, 17.01%. Mp:
55−59 °C. IR (KBr, cm−1): 3111 (νas(H−CC−H)), 1561
(νas(−NO2)), 1363, 1334 (νs(−NO2)), 1281 (ν(C−O)). The main
2. EXPERIMENTAL SECTION
1
signals in H NMR (200 MHz, CDCl3, δ in ppm): δ 9.90 (1H, s,
NCHN), 8.82 (2H, s, HCCNO2CH), 7.39 (2H, m, NCHCHN), 4.36
(4H, t, J = 7.4 Hz, (NCH2(CH2)2CH3)2), 1.90 (4H, m,
(NCH2CH2CH2CH3)2), 1.37 (4H, m, (N(CH2)2CH2CH3)2), 0.95
(6H, t, J = 7.3 Hz, (N(CH2)3CH3)2). Yield = 74%.
Preparation of (BMIm)2[Eu(Pic)5] (1). A ethanol solution (10 mL)
of [Eu(pic)2(H2O)6](pic)·6(H2O) (1.32 g, 1.25 mmol) was added
dropwise to a yellow acetonitrile solution (10 mL) of BMIm-Pic (1.01 g,
3.00 mmol). The mixture was stirred at room temperature for 5 h, after
which time the solvents were removed under vacuum. A yellow solid was
obtained and washed with ethanol (40 mL) and dried under vacuum.
The same procedure was used to prepare (BEIm)2[Eu(Pic)5] (2),
(BBIm)2[Eu(Pic)5] (3), (BMIm)2[Gd(Pic)5] (4), (BEIm)2[Gd(Pic)5]
(5), and (BBIm)2[Gd(Pic)5] (6).
General Methods. All manipulations were performed under N2
atmosphere, using standard Schlenk glassware. Reagents and solvents
were purchased from commercial vendors (Aldrich, Strem Chemicals),
were of the highest available purity, and were used without further
purification, unless otherwise specified. Solvents used in the syntheses
were dried by standing over appropriate drying agents and distilled
under nitrogen (dichloromethane and ethylacetate over CaH2 and
toluene over sodium wire). 1-Methylimidazole and 1-n-buthylimidazole
were distilled from KOH. 1-bromobutane and bromoethane were
washed with concentrated H2SO4 and distilled from P2O5. [Ln-
(pic)2(H2O)6](pic)·6(H2O) complex was prepared according to
literature procedures.28 [CAUTION! Although we have experienced no
problems in handling picrate compounds, these should be handled with great
caution, because of the potential for explosion.] The commom ILs
precursors (BMIm-Br, BEIm-Br, BMIm-Br, and BMIm-Pic) were
synthesized as described in the Supporting Information. The metal ions
(BMIm)2[Eu(Pic)5] (1). Anal. Calcd. for C46H40N19EuO35: C, 35.17%;
H, 2.57%; N, 16.94%; Eu, 9.67%. Found: C, 36.67%; H, 2.51%; N,
17.14%; Eu, 9.43%. IR (KBr, cm−1): 917 (ν(CN)), 1335 and 1364
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(νs(NO2)), 1542 and 1577 (νas(NO2)), 839 (δ(NO2)), 1271 (ν(CO)),
1615 (ω(CC)), 788 (γ(CH)), 3153 (νs(−HCCH−)), 3088
(νas(CH3)). Molar conductivity: 164 cm2 Ω−1 mol−1. Yield = 85%.
(BEIm)2[Eu(Pic)5] (2). Anal. Calcd. for C48H44N19EuO35: C, 36.06%;
H, 2.77%; N, 16.64%; Eu 9.50%. Found: C, 35.90%; H, 2.72%; N,
16.62%; Eu, 9.80%. IR (KBr, cm−1): 918 (ν(CN)), 1344 and 1364
(νs(NO2)), 1546 and 1578 (νas(NO2)), 844 (δ(NO2)), 1270 (ν(CO)),
1614 (ω(CC)), 788 (γ(CH)), 3148 (νs(−HCCH−)), 3102
(νas(CH3)). Molar conductivity: 158 cm2 Ω−1 mol−1. Yield = 90%.
(BBIm)2[Eu(Pic)5] (3). Anal. Calcd. for C52H52N19EuO35: C, 37.74%;
H, 3.17%; N, 16.23%; Eu, 9.18%. Found: C, 38.83%; H, 2.90%; N,
16.84%; Eu 9.62%. IR (KBr, cm−1): 920 (ν(CN)), 1346 and 1364
(νs(NO2)), 1550 and 1576 (νas(NO2)), 844 (δ(NO2)), 1266 (ν(CO)),
1614 (ω(CC)), 788 (γ(CH)), 3150 (νs(−HCCH−)), 3096
(νas(CH3)). Molar conductivity: 156 cm2 Ω−1 mol−1. Yield = 89%.
(BMIm)2[Gd(Pic)5] (4). Anal. Calcd. for C46H40N19GdO35: C, 35.05%;
H, 2.56%; N, 16.88%; Gd 9.98%; Found: C, 35.74%; H, 2.62%; N,
16.88%; Gd, 9.68%. IR (KBr, cm−1): 922 (ν(CN)), 1330 and 1364
(νs(NO2)), 1548 and 1575 (νas(NO2)), 828 (δ(NO2)), 1268 (ν(CO)),
1615 (ω(CC)), 788 (γ(CH)), 3153 (νs(−HCCH−)), 3065
(νas(CH3)). Molar conductivity: 185 cm2 Ω−1 mol−1. Yield = 85%.
(BEIm)2[Gd(Pic)5] (5). Anal. Calcd. for C48H44N19GdO35: C, 35.94%;
H, 2.76%; N, 16.59%; Eu, 9.80%. Found: C, 38.46%; H, 3.17%; N,
17.82%; Eu, 10.08%. IR (KBr, cm−1): 920 (ν(CN)), 1346 and 1364
(νs(NO2)), 1550 and 1576 (νas(NO2)), 844 (δ(NO2)), 1266 (ν(CO)),
1616 (ω(CC)), 788 (γ(CH)), 3150 (νs(−HCCH−)), 3096
(νas(CH3)). Molar conductivity: 169 cm2 Ω−1 mol−1. Yield = 90%.
(BBIm)2[Gd(Pic)5] (6). Anal. Calcd. for C52H52N19GdO35: C, 37.62%;
H, 3.16%; N, 16.03%; Gd, 9.47%. Found: C, 39.08%; H, 2.95%; N,
15.90%; Gd, 9.62%. IR (KBr, cm−1): 918 (ν(CN)), 1344 and 1364
(νs(NO2)), 1546 and 1576 (νas(NO2)), 842 (δ(NO2)), 1268 (ν(CO)),
1616 (ω(CC)), 788 (γ(CH)), 3148 (νs(−HCCH−)), 3102
(νas(CH3)). Molar conductivity: 151 cm2 Ω−1 mol−1. Yield = 89%.
Component (C) in the (C)2[Eu(Pic)5]-prepared complexes is 1-
butyl-3-methylimidazolium, (BMIm)2[Eu(Pic)5] (1); 1-butyl-3-ethyl-
imidazolium, (BEIm)2[Eu(Pic)5] (2); or 1,3-dibutylimidazolium,
(BBIm)2[Eu(Pic)5] (3) (see Scheme 1). These are the first pentakis
complexes (LnL5)2− with picrate ligand.
3. THEORETICAL DETAILS
Geometry Optimization. Initially, the crystallographic
structures of compounds 2 and 3 were used to evaluate which
computational methodology, between Sparkle/AM1, Sparkle/
PM3 and Sparkle/PM6, would be more appropriated to study
the series of Eu(III) complexes. Based on this analysis, the
ground-state geometry of compounds 1, 2, and 3 were calculated
with the Sparkle/PM3 model,14 implemented in the MOPAC
2009 package.33 The MOPAC keywords used were as follows:
SPARKLE, PRECISE, GNORM = 0.25, SCFCRT = 1.D-10 (in
order to increase the SCF convergence criterion), and XYZ (the
geometry optimizations were performed in Cartesian coordi-
nates).
The optimized geometries were used as input to calculate the
ligand singlet and triplet excited states in the Eu complexes using
configuration interaction with single excitations (CIS), based on
the intermediate neglect of the differential overlap/spectroscopic
(INDO/S) technique implemented in the ZINDO package.34,35
We have used a point charge of +3e to represent the Eu(III) ion.
The Judd−Ofelt Intensity Parameters. The forced electric
dipole contribution to the 4f−4f intensities is described by the
Judd−Ofelt theory.36,37 The intensity parameters (IP) Ωλ (λ = 2,
4, and 6) are defined by
|Bλtp|2
(2t + 1)
Ω = (2λ + 1)
∑
λ
t,p
(1)
(2)
ed
dc
λtp
Bλtp = B + B
λtp
where
2
ΔE
ed
B
=
⟨rt+1⟩θ(t, p)γpt
λtp
(3)
and
1/2
Scheme 1. Structural Representation of the Eu(III)-Picrate
Complexes with the Imidazolium Countercations: 1-Butyl-3-
methylimidazol, (BMIm)2[Eu(Pic)5] (1); 1-Butyl-3-
ethylimidazolium, (BEIm)2[Eu(Pic)5] (2); and 1,3-
Dibutylimidazolium, (BBIm)2[Eu(Pic)5] (3)
⎡
⎤
(λ + 1)(2λ + 3)
2λ + 1
dc
B
= −
⟨rλ⟩(1 − σ )
⎢
⎣
⎥
⎦
λtp
λ
× ⟨f || C(λ) || f ⟩Γtpδt,λ+1
(4)
The first and second terms in eq 2 correspond to the forced
electric dipole (“ed”) and dynamic coupling (“dc”) mechanisms,
respectively. The quantities γpt (odd-rank ligand field parameters)
and Γtp (ligating atom polarizability-dependent terms) in eqs 3
and 4, respectively, contain a sum over the ligating atoms. The
nature of the chemical environment and structural aspects in the
first coordination sphere of the Ln ion are precisely taken into
account in this sum. In eq 3, the numerical factor θ(t, λ) is a
function of the lanthanide ion; in eq 4, the quantities ⟨rλ⟩, (1 −
σλ), and ⟨f∥C(λ)∥f⟩ are a radial integral, a shielding factor, and a
one-electron reduced-matrix element, respectively.38
The experimental intensity parameters were calculated from
the coefficients of spontaneous emission, according to the
following expression:
X-ray Crystallography. The crystal structures of complexes 2 and 3
were investigated using single-crystal X-ray diffraction (XRD). Semi-
empirical from equivalents absorption correction method was applied.
The structure was solved using the SHELXS-97 program.30 Full-matrix
least-squares refinement procedure on F2 with anisotropic thermal
parameters was carried on using SHELXL-97 program.30 Positional and
anisotropic atomic displacement parameters were refined for all non-
hydrogen atoms. Hydrogen atoms were placed geometrically and the
positional parameters were refined using a riding model. The data
presented in Tables 1, 2, and 4 were compiled using WINGX and
PLATON software suites.31,32 Crystal and structure refinement data are
summarized in Table 1.
3ℏc3A0λ
4e2ω3χ⟨7Fλ U(λ) 5D0⟩2
Ωλ =
(5)
where χ is the Lorentz local field correction term, given by χ =
n(n2 + 2)2/9 and ⟨7Fλ∥U(λ)∥5D0⟩2 is a squared reduced matrix
element whose value is 0.0032 and 0.0023 for the 5D0 → 7F2 and
5D0 → 7F4 transitions, respectively.39 The index of refraction (n)
has been assumed to be equal to 1.5.
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Table 1. Crystal Data and Structure Refinement for the (BEIm)2[Eu(Pic)5] (2) and (BBIm)2[Eu(Pic)5] (3) Complexes
crystal data
2
3
empirical formula
formula weight
crystal system
space group
C48H44EuN19O35
1598.98
C52H52EuN19O35
1655.09
triclinic
triclinic
P1
̅
P1
̅
unit-cell dimensions
a (Å)
11.075(5)
12.458(5)
23.229(5)
75.927(5)
76.995(5)
88.684(5)
3027.3(19)
2
11.4030(3)
16.7616(5)
18.1245(5)
103.359(2)
92.328(2)
104.577(2)
3243.96(16)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
volume, V (Å3)
Z
density (calc.) (Mg m−3
absorption coefficient (mm−1
)
1.754
1.694
)
1.155
1.081
F(000)
1616
1680
crystal size (mm3)
0.23 × 0.11 × 0.03
2.64−26.37
−13 ≤ h ≤ 13, −15 ≤ k ≤ 15, −29 ≤ l ≤ 29
62150
0.28 × 0.14 × 0.06
2.75−26.37
−14 ≤ h ≤ 14, −20 ≤ k ≤ 20, −22 ≤ l ≤ 22
60043
θ range for data coll. (°)
index ranges
reflection collected
independent reflections
completeness (%)
12373 [R(int) = 0.0772]
99.9 (to 26.37°)
13251 [R(int) = 0.0632]
99.9 (to 26.37°)
max. and min. transmission
data/restraints/parameters
goodness of fit on F2
R indices (all data)
0.9662 and 0.7770
12373/0/928
0.9380 and 0.7516
13251/0/964
1.039
1.059
R1 = 0.0396, wR2 = 0.0786
R1 = 0.0530, wR2 = 0.0831
1.289 and −1.171
R1 = 0.0329, wR2 = 0.0600
R1 = 0.0445, wR2 = 0.0644
0.951 and −0.497
final R indices [I > 2σ(I)]
largest peak and hole (Å−3
)
mp
In eq 5, A0λ (λ = 2 and 4) represents spontaneous emission
coefficients, calculated by taking the magnetic dipole transition
where W is the transfer rate of the multipolar,
ET
2
ed
e2SL
ℏ (2J + 1)G
× ⟨ψ′J′ U(λ) ψJ⟩2
2(1 − σ1) Ωλ
⎛
⎝
⎞
⎠
7
5D0 → F1 as the reference, once this transition is practically
2π
mp
ET
⎜
⎜
⎟
⎟
W
=
F ∑
+ γ
λ
6
insensitive to the chemical environment around the Eu ion. The
following expression was used:
RL
λ
(7)
(8)
⎛
⎜
⎝
⎞⎛
0λ ⎟⎜
01 ⎠⎝
⎞
⎟
⎠
S
S
ν01
ex
ET
A0λ = A01
and W is the transfer rate of exchange,
ν
(6)
0λ
4
e2 4f|L
⎡
⎢
⎣
⎤
⎥
⎦
2
8π
3ℏ
where S01 and S0λ are the areas under the curves of the 5D0→7F1
ex
ET
(λ)
W
=
F ψ′J′ U ψJ
4
and D0→7Fλ transitions, with ν01 and ν0λ being their energy
5
(2J + 1)R
L
barycenters, respectively. The coefficient of spontaneous
emission in eq 6 (A01) is given by the expression A01 = 0.31 ×
1011(n)3(ν01)3, leading to an estimated value of ∼50 s−1.40
As mentioned above, the theoretical intensity parameters were
calculated using the spherical coordinates obtained from the
Sparkle/PM3 coordination polyhedron of the Eu(III) complexes
and eqs 1−4, with the system of coordinates centered at the
Eu(III) ion. In this procedure, the charge factors (g) that appear
in the ligand field parameters γpt and the ligating atom
polarizabilities (α) have been treated as variables within ranges
of physically acceptable values. In the present case, both g and α
were separated into two groups: one belonging to the oxygen
unidentate of the picrate ligand and the other one belonging to
the oxygen bidentate of the picrate ligand.
× ∑ |⟨φ|∑ μ (k)sm(k)|φ′⟩|2
z
m
k
At this point, it should be emphasized that shielding effects due
to the filled 5s and 5p subshells have been appropriately taken
into account in eq 7, and an effective exchange Hamiltonian41 has
been used in eq 8. The first and second terms in the parentheses
in the righthand side (rhs) of eq 7 correspond to the dipole−
dipole and dipole−2λ pole interactions. The Ωλed parameters are
the forced electric dipole contribution to the 4f−4f intensity
parameters, J is the lanthanide ion total angular momentum, ψ
specifies a given 4f spectroscopic term, SL is the dipole strength of
the ligand transition involved in the transfer process, and the
quantities ⟨ψ′J′∥U(λ)∥ψJ⟩ are reduced matrix elements of the
same type as those appearing in eq 5. In eq 8, S is the total spin
operator of the Ln ion, μz the z component of the electric dipole
operator, and sm (m = 0, 1) a spherical component of the spin
operator, both for the ligand electrons, and ⟨4f|L⟩ is the total
overlap integral between the 4f orbital and the ligand wave
functions involved in the transfer process.43 The quantities γλ and
F are, respectively, given by
Intramolecular Energy Transfer and Emission Quan-
tum Yields. The intramolecular energy transfer rates were
calculated from the approach developed by Malta et al.41,42 In
this approach, the transfer rates (WET) are expressed in terms of a
sum,
mp
ex
ET
WET = W + W
ET
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Figure 1. (A) Molecular structure of (BBIm)2[Eu(Pic)5] (3) at 50% probability level. (B) Detail of molecular structure of [Eu(Pic)5]2− complex with
atom numbering scheme in a new projection that favors the visualization of the picrate ligands. All the H atoms have been omitted for the sake of clarity.
(C) Coordination polyhedron of the Eu(III) ion in (BEIm)2[Eu(Pic)5] (2).
transfer to the 5D0 level, with the Eu(III) ion being initially in the
⟨rλ⟩2
7F0 ground state, by the exchange mechanism, is forbidden.
Besides, this energy transfer process is also forbidden by the
dipole−dipole and dipole−multipole mechanisms, as long as the
J-mixing effects, which are small, are neglected. The
interpretation of our emission quantum yield results has taken
into account these selection rules.
γλ = (λ + 1)
λ+2 2 ⟨3 C(λ) 3⟩2(1 − σ )2
λ
(RL
)
(9)
and
F =
2
⎡
⎢
⎤
⎥
⎛
⎝
⎞
⎠
1
ℏγλ
ln 2
π
Δ
⎜
⎜
⎟
⎟
exp −
ln 2
⎢
⎣
ℏγ ⎥
⎦
λ
(10)
where ⟨3∥C(λ)∥3⟩ is a reduced matrix element of the Racah
tensor operator C(λ), the σ terms are screening factors due to the
filled 5s and 5p subshells of the Eu(III) ion and ⟨rλ⟩ is the radial
expectation value of rλ for 4f electrons. Typical values of the
quantities appearing in eq 9 show that γ6 ≪ γ4 ≪ γ2.
From the above equations, the following selection rules are
derived: J + J′ ≥ λ ≥ |J − J′|, for the multipolar mechanism, and ΔJ
= 0, 1 for the exchange mechanism. In both cases, J′ = J = 0 is
excluded. From the ligand side, the selection rules can be derived
from the electric dipole strength SL and the matrix element of the
coupled operators μz and sm in eq 8.
4. RESULTS AND DISCUSSION
Characterization of the Complexes. Analytical data
indicate that the Eu(III) and Gd(III) complexes have a
stoichiometry formula of C2[Ln(pic)5], where C = BMIm,
BEIm, BBIm. All the complexes are soluble in dimethylforma-
mide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran
(THF), acetonitrile, acetone, and nitromethane, but partially
soluble in ethanol and chloroform. The complexes are also stable
in air and moisture for a long period. The molar conductance of
the picrate complexes in nitromethane indicates that all of the
complexes act as dielectrolytes,45 implying that all picrate groups
are in the coordination sphere.
The absorption band assigned to the OH out-of-plane bending
vibration of the free HPic at 1155 cm−1 disappears in the infrared
(IR) spectra (not shown) of the complexes, indicating that the H
atom of the OH group is replaced by Ln(III) ion. The vibration
ν(C−O) at 1262 cm−1 is shifted toward higher wavenumbers by
ca. 5−10 cm−1 in the complexes. This is due to the substitution of
the hydrogen atom of the OH group by the lanthanide ion upon
coordination, increasing the π-bond character in the C−O bond.
The free HPic has νas(NO2) and νs(NO2) at 1558 and 1343
cm−1, respectively, which split into two bands at ca. 1581, 1542
cm−1 and 1361, 1331 cm−1, respectively, in the complexes. These
results indicate that they are coordinated, at least in part, as
bidentate ligands through the phenolic oxygen and one oxygen of
a nitro group.46 All IR spectral profiles of the complexes are
similar.
An appropriate system of rate equations is used to describe the
kinetics of the 4f−4f luminescence in terms of the level
populations Ni.40 The general form of these equations is given by
⎛
⎝
⎞
⎠
dN
dt
l
⎜
⎜
⎟
= −
P N +
P N
lj j
∑
∑
j
il
l
⎟
i
(11)
where the sum over i and j excludes the state l, Pil stands for a
transition or transfer rate starting from state l, and Plj stands for a
transition or transfer rate ending up in this state. In the steady-
state regime, all of the dNl/dt are set equal to zero. The
theoretical emission quantum yield (qtheo) is calculated using the
steady-state populations, according to
AradN(5D0)
qtheo
=
Φ0N(S0)
(12)
where Arad is the total radiative emission rate from the 5D0 level,
N(5D0) its steady-state population, and Φ0 the absorption rate
from the ligand singlet ground state S0, with steady-state
population N(S0), to the singlet ligand excited state S1.
X-ray Crystallography and the Sparkle Model. Com-
plexes 2 and 3 crystallize in a triclinic system with space group P1.
̅
The asymmetric unit of the compounds contains one [Eu-
(Pic)5]2− anionic complex (common to both compounds and
identical with respect to the coordination form of the ligands)
and two 1-butyl-3-ethylimidazolium (BEIm) or two 1-butyl-3-
butylimidazolium (BBIm) cations for 2 and 3, respectively.
An important aspect to be emphasized is that the intra-
molecular energy transfer selection rules associated with the 5D0
and 5D1 levels of the Eu(III) ion, have been described in detail
previously in the literature.44 According to these rules, energy
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Table 2. Nonclassical Hydrogen Bonds Geometry for Complexes 2 and 3
a
D−H···A
symmetry transformations
d(D−H) (Å)
d(H···A) (Å)
d(D···A) (Å)
∠(DHA) (°)
(BEIm)2[Eu(Pic)5] (2)
C104H04B···O33
C108H08B···O24
C13H13···O25
0.99
2.54
2.37
2.47
2.59
2.48
2.57
2.59
2.49
2.42
2.43
2.37
2.36
2.58
3.473(6)
3.219(6)
3.389(5)
3265(6)
3.427(7)
3.457(10)
3.540(7)
3.279(6)
3.151(5)
3.288(5)
3.312(5)
3.221(5)
3.344(5)
157
144
163
126
159
149
163
136
134
151
170
151
138
−1 + x, 1 + y, z
1 − x, 1 − y, 1 − z
−x, −y, 1 − z
−1 + x, −1 + y, z
−1 + x, −1 + y, z
−1 + x, y, z
0.99
0.95
C109H09C···O15
C114H14A···O37
C115H15A···O23
C117H17B···O33
C118H18B···O46
C101H101···O13
C101H101···O25
C103H103···O2
C111H111···O6
C103H113···O47
0.98
0.99
0.99
0.98
0.99
0.95
1 − x, 1 − y, 1 − z
x, 1 + y, z
0.95
0.95
x, −1 + y, z
1 −x, 1 − y, z
0.95
0.95
(BBIm)2[Eu(Pic)5] (3)
C104H04A···O15
C104H04B···O46
C105H05B···O33
C107H07C···O5
C101H101···O13
C102H102···O7
C103H103...O34
C201H201···O17
C202H202···O2
C203H203···O16
1 − x, 1 − y, 1 − z
1 − x, −y, 1 − z
1 − x, −y, 1 − z
0.99
0.99
0.99
0.98
0.95
0.95
0.95
0.95
0.95
0.95
2.53
2.56
2.53
2.49
2.48
2.58
2.58
2.58
2.41
2.34
3.237(4)
3.483(4)
3.229(4)
3.318(4)
3.322(3)
3.278(4)
3.278(4)
3.110(4)
3.355(4)
3.251(4)
128
155
128
143
148
131
131
116
173
161
1 − x, −y, 1 − z
2 − x, −y −1, −z
1 −x, 1 − y, z
1 − x, −y, 1 − z
a
Symmetry transformations used to generate equivalent atoms.
Figure 2. Views of the crystal packing of compounds 2 and 3 along the x-axis. The intermolecular interactions are indicated by dashed lines.
Figure 1A shows the molecular structure of (BBIm)2[Eu(Pic)5]
(3).
capping the latter (Figure 1C). The C4V point group should
reflect the symmetry of the complex, considering the geometry
observed in the coordination polyhedron (Figure 1C).
Several geometric parameters to quantify the deformation of
the coordination polyhedra, introduced by Balic-Zunic and
Makovicky and implemented in the IVTON program are
given.47,48 These parameters are calculated, with respect to the
centroid of the coordination polyhedron, which is the point in
the coordination polyhedron where the variance of squares of
distances to ligands is minimum. These parameters are the
As can be seen in Figure 1B, the Eu(III) ion is nine-
coordinated by O atoms of five picrate anions: four bidentade
picrates via phenolic oxygen atom and one oxygen of the nitro
group in each ligand, and one monodentade picrate via phenolic
oxygen atom. The coordination geometry of the Eu(III) center
can be described as a distorted monocapped square antiprism
with the atoms O22, O31, O41, O42 on the bottom square, the
atoms O1, O11, O21, O32 on the top square, and the atom O12
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Figure 3. Calculated ground-state geometries, using the Sparkle/PM3 model, for complexes 1, 2, and 3.
central atom-centroid distance (Δ), 0.026 Å in 2 and 0.034 Å in
3, the average distance from the centroid to the ligands (r), 2.418
Å in 2 and 2.419 Å in 3, and the “sphericity” (1 − σr/r) is 0.9608
in 2 and 0.9537 in 3 (σr is the standard deviation of the distances
from the centroid to the ligands). It is noteworthy to mention
that the parameters obtained for compounds 2 and 3 indicate a
close similarity between these anionic complexes.
Intermolecular interactions involving the cationic species are
responsible for the stability of the charge in the structure, as well
as to promote the formation of hydrogen bonds with the
coordinated picrate groups (Table 2). These C−H···O
interactions play an important role in the crystal packing of the
complexes, as can be seen in Figure 2.
The semiempirical Sparkle Model was used to calculate the
structures of Eu(III) complexes 1, 2, and 3. Although the three
models Sparkle/AM1, Sparkle/PM3, and Sparkle/PM6 have
shown similar accuracy, the semiempirical AM1, PM3, and PM6
methods present differences in the prediction of the organic part
in lanthanide complexes.13−15 For this reason, we compare the
calculated structures using all versions of the Sparkle Model
predictions with the crystallographic ones. The results indicate
that the Sparkle/PM6 model is not suitable, because the
calculated structures present a coordination number different
from that given by the crystallographic structure. In the Sparkle/
PM6 structure, all five picrate ligands present a bidentate
coordination mode, whereas in the crystallographic structures,
one of these five ligands presents a monodentate coordination
mode. This problem does not occur with the Sparkle/AM1 and
Sparkle/PM3 models.
Although the Sparkle/AM1 structure reproduces the Eu−O
distances more accurately, both models present a similar
unsigned mean error (UME) for the Eu−O distances. The
UME values obtained with the Sparkle/AM1 and Sparkle/PM3
models were 0.073 and 0.096 Å, respectively. However, when all
distances of the coordination polyhedron are considered, the
Sparkle/PM3 model presents a UME value that is 21% smaller
than that of the Sparkle/AM1. This fact indicates that the O−
Eu(III)−O angles are calculated with greater accuracy by the
Sparkle/PM3 model.
Table 3. Spherical Atomic Coordinates for the Sparkle/PM3
Coordination Polyhedron, Charge Factors (g), and the
Polarizability (α) of the Coordinated Ligating Atoms for
Complexes 1, 2, and 3
Spherical Coordinates
compound
R (Å)
θ (°)
ϕ (°)
g
α (Å3)
(BMIm)2[Eu(Pic)5], 1
O (monodentate)
2.4535
2.4640
2.4697
2.4673
83.9004
20.7500
45.0451
58.3408
344.8131
291.3273
61.9672
1.66
0.07
0.07
0.07
1.42
5.90
5.90
5.90
167.6467
O (bidentate)
2.4716
2.4676
2.4758
2.4671
2.4744
82.0793
136.4844
103.2414
141.1587
139.8563
227.6074
171.5151
111.2200
36.2626
0.07
0.07
0.07
0.07
0.07
5.90
5.90
5.90
5.90
5.90
292.2950
(BEIm)2[Eu(Pic)5], 2
O (monodentate)
O (bidentate)
2.4594
85.3892
19.5094
48.0103
58.4639
342.5038
1.66
0.07
0.07
0.07
1.41
5.90
5.90
5.90
2.4638
2.4676
2.4641
298.9761
61.2130
174.1708
2.4678
2.4665
2.4727
2.4708
2.4777
83.5272
137.0276
102.0309
140.7977
139.6038
233.4437
172.0685
113.5100
37.6382
0.07
0.07
0.07
0.07
0.07
5.90
5.90
5.90
5.90
5.90
293.9408
(BBIm)2[Eu(Pic)5], 3
O (monodentate)
O (bidentate)
2.4546
87.2539
122.5652
64.2857
72.5814
124.0575
177.1665
0.66
0.07
0.07
0.07
0.07
4.78
5.90
5.90
5.90
5.90
2.4655
2.4742
2.4677
2.4744
328.6584
312.4238
29.3347
62.4199
2.4657
2.4694
2.4684
2.4715
64.5257
9.0682
105.9369
163.9353
249.6164
209.1267
0.07
0.07
0.07
0.07
5.90
5.90
5.90
5.90
100.4828
154.0715
Figure 3 shows the Sparkle/PM3 optimized molecular
structures of the three different complexes studied in the present
work. Spherical atomic coordinates for the Sparkle/PM3
coordination polyhedra are summarized in Table 3. The
crystallographic and theoretical bond distances and selected
angles around the Eu(III) ion for the compounds are shown in
Table 4.
2.46(2) Å (for 2 and 3). These values are consistent with the
average observed in the crystal structures of the nine-coordinated
EuOn compounds (2.43(9) Å).49 However, from the crystallo-
graphic data, the Eu−O distances to the nitro groups, which are
in the range of 2.295−2.361 Å, are shorter than the Eu−O
distances to the phenolato groups, which are in the range of
2.430(17)−2.636(18) Å. These values do not change when the
The crystallographic average Eu−O bond lengths are 2.42(10)
Å in 2 and 2.42(11) Å in 3, whereas the Sparkle/PM3 average is
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Table 4. Crystallographic and Theoretical Values Obtained for Selected Bonds Lengths and Angles for Compounds 2 and 3
(BEIm)2[Eu(Pic)5], 2
Sparkle/PM3
(BBIm)2[Eu(Pic)5], 3
Sparkle/PM3
crystallographic
crystallographic
Bond Lengths (Å)
2.459
Eu1−O1
2.345(2)
2.348(2)
2.555(2)
2.295(2)
2.487(2)
2.354(2)
2.484(2)
2.361(2)
2.533(2)
2.321(16)
2.323(16)
2.636(18)
2.323(17)
2.526(17)
2.354(16)
2.430(17)
2.359(16)
2.506(18)
2.455
2.466
2.469
2.468
2.474
2.466
2.474
2.468
2.472
Eu1−O11
Eu1−O12
Eu1−O21
Eu1−O22
Eu1−O31
Eu1−O32
Eu1−O41
Eu1−O42
2.471
2.478
2.467
2.473
2.464
2.468
2.464
2.468
(BEIm)2[Eu(Pic)5], 2
(BBIm)2[Eu(Pic)5], 3
crystallographic
Sparkle/PM3
crystallographic
Sparkle/PM3
Bond Angles (deg)
60.45
O11−Eu1−O12
O21−Eu1−O22
O31−Eu1−O32
O41−Eu1−O42
O1−Eu1−O11
O1−Eu1−O12
O1−Eu1−O21
O1−Eu1−O22
O1−Eu1−O31
O1−Eu1−O32
O1−Eu1−O41
O1−Eu1−O42
O11−Eu1−O21
O11−Eu1−O22
O11−Eu1−O31
O11−Eu1−O32
O11−Eu1−O41
O11−Eu1−O42
O12−Eu1−O21
O12−Eu1−O22
O12−Eu1−O31
O12−Eu1−O32
O12−Eu1−O41
O12−Eu1−O42
O21−Eu1−O31
O21−Eu1−O32
O21−Eu1−O41
O21−Eu1−O42
O22−Eu1−O31
O22−Eu1−O32
O22−Eu1−O41
O22−Eu1−O42
O31−Eu1−O41
O31−Eu1−O42
O32−Eu1−O41
O32−Eu1−O42
67.15(7)
68.60(7)
67.03(8)
65.85(7)
79.67(8)
67.07(7)
135.24(8)
139.17(7)
139.80(8)
88.12(8)
73.56(8)
73.60(7)
74.98(8)
77.00(8)
140.47(8)
131.31(7)
140.29(7)
78.86(7)
69.25(7)
130.02(7)
122.09(8)
64.68(7)
124.65(7)
131.36(7)
74.20(8)
81.93(8)
143.67(8)
134.25(7)
69.07(8)
132.15(7)
105.24(7)
69.34(7)
70.43(8)
106.36(8)
77.01(8)
141.81(7)
65.93(6)
67.45(6)
68.91(6)
65.95(6)
76.77(6)
67.74(6)
134.73(6)
136.17(6)
142.49(6)
89.20(6)
73.60(6)
74.86(6)
75.12(6)
75.41(6)
140.56(6)
130.60(6)
141.01(6)
82.32(6)
68.63(6)
127.05(5)
122.94(6)
64.91(5)
122.50(6)
135.08(5)
74.36(6)
83.05(6)
143.61(6)
134.10(6)
70.16(6)
134.49(6)
110.38(6)
68.41(5)
71.25(6)
101.85(6)
73.89(6)
139.47(6)
59.99
60.35
59.98
60.55
60.30
60.12
60.62
72.67
71.89
68.51
78.43
136.78
131.02
142.18
108.32
71.51
142.41
111.92
139.95
128.20
73.29
78.53
70.88
74.61
70.83
71.79
72.45
142.78
133.76
139.66
95.04
142.23
122.78
142.51
118.48
79.05
71.09
118.36
132.27
76.63
125.39
131.27
72.10
120.15
143.24
78.59
99.67
147.09
76.54
76.00
71.06
145.73
132.06
72.58
140.37
133.35
75.19
120.56
121.44
72.23
119.88
134.94
78.41
70.70
75.20
84.10
72.40
75.90
70.98
131.02
118.74
structures are calculated using the Sparkle/PM3 model, and the
analyses of the angle values indicate that the Sparkle/PM3 results
are in good agreement with the crystallographic ones.
Considering these bond distances and the coordination
polyhedron around the Eu(III) ions, we can suggest a lowering
of the point symmetry from C4V to C1, as indeed indicated by the
crystallographic data.
depicted in Figure 4. The experimental absorption spectra of the
three complexes show a slight difference in peak positions, with a
maximum displacement of 14 nm. These experimental spectra
display a shoulder in the region of lower energy (higher
wavelength), which suggests a ligand-to-metal charge-transfer
(LMCT) state. Both the absorption and excitation spectra (see
Figures 4 and 5) penetrate well in the visible region, which
explains the yellow color of the compounds and confirms the
absorption by picrate ligands (S0 → S* transition), and they
Absorption Spectra. Theoretical and experimental UV−vis
absorption spectra of the ligands in the Eu(III) complexes are
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Figure 4. Absorption spectra of (BEIm)2[Eu(Pic)5] (2), showing
experimental data (recorded in acetonitrile solution) and theoretical
prediction.
Figure 5. Phosphorescence spectra of complexes 4, 5, and 6 recorded at
77 K.
indicate that the energy transfer to the Eu(III) ion also is realized.
However, the theoretical absorption spectra, calculated for the
complexes, do not exhibit any significant difference.
The results suggest that the change of countercations affect
slightly the RL values (see Table 5). The RL parameter is the
Table 5. Experimental and Calculated Ligand Excited State
Energies and RL Values for Complexes 1−3
states
1
2
3
Singlet
Calc. (cm−1
)
)
25594.2
5.8327
26178
25502.0
5.1614
26709
25384.6
5.8835
25773
RL (Å)
Exp. (cm−1
)
Triplet
Calc. (cm−1
19242.3
4.3432
19512
18913.8
4.5314
19065
18449.5
3.9201
18504
RL (Å)
Exp. (cm−1
)
distance from the donor state located at the organic ligand to the
lanthanide ion. The change in the RL quantities considerably
influences the energy transfer and back-transfer rates.
Figure 6. Excitation spectra of complexes 1, 2, and 3 recorded from 250
nm to 585 nm at 77 K with excitation monitored on hypersensitive
5D0 → 7F2 transition at ∼616 nm.
Experimental and Theoretical Photoluminescence
Study. From the phosphorescence spectra of the Gd(III)
complexes, it was possible to find the positions of the first triplet
excited electronic states (T1) of the picrate ligands. The emission
spectra were recorded at 77 K, under excitation at 370 nm. The
T1 state energy was determined from the shortest wavelength
corresponding to the 0−0 phonon transition at ∼19 512, 19 065,
and 18 504 cm−1 for the complexes with countercation BMIm+
(4), BEIm+ (5), and BBIm+ (6), respectively (see Figure 5).
The excitation spectra of complexes 1, 2, and 3 recorded at 77
K are shown in Figure 6. All the excitation spectra recorded in the
range of 260−600 nm, monitored at the 5D0 → 7F2 hypersensitive
transition at 616 nm, display a large broad band ascribed to the
excited states of the ligand with three components peaking at
∼290, 370, and 470 nm, corresponding to S0 → S* transitions
and possibly an LMCT state. The energy and full width at half-
maximum (fwhm) of the broad band components are different
for each complex, showing that the countercation (C) present in
the (C)2[Eu(Pic)5] compounds influences the excitation paths of
the Eu(III) excited levels.
5
77 K. It presents narrow emission bands from the D0 → 7FJ
transitions (where J = 0−4) dominated by the hypersensitive 5D0
7
5
→ F2 transition at 616 nm. The presence of the D0 → 7F0
transition indicates that the Eu(III) ion is located in a symmetry
5
7
site of the type Cs, Cn, or Cnv. Since the D0 → F1 emission is
almost insensitive to changes in the chemical environment, it is
5
7
primarily magnetic dipole by character, while the D0 → F2
emission is essentially forced electric dipole in character, and its
intensity is very sensitive to the ligand field interaction.50
It is noteworthy that the spectra of the Eu(III) complexes
recorded at room temperature and low temperature showed
similar profiles. However, the emission spectra at room
temperature (not shown) are less resolved, compared to the
spectra obtained at liquid nitrogen temperature (77 K).
Another feature in the photoluminescence data of the pentakis
complexes is that the emission spectra of all Eu(III) complexes
do not exhibit the broad phosphorescence band of the Pic ligands
that is observed in the Gd(III) complexes (see Figure 7),
indicating energy transfer ligand-lanthanide ion.
Figure 7 shows the emission spectra of the 1, 2, and 3 Eu(III)
complexes recorded in the range of 530−720 nm, under
excitation at the broad band of the picrate ligand at 370 nm, at
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theoretical) have comparable values is an indication that the
coordination geometry around the Eu(III) ion presents a low
symmetry that originates from a distortion of a high-symmetry
coordination geometry containing a center of inversion,
according to the model developed in ref 53.
Table 6 also presents the theoretical and experimental values
of the radiative and nonradiative rates of spontaneous emission
(Arad and Anrad, respectively), emission efficiency (η), emission
quantum yield (q, theoretical), and experimental lifetimes (τ) for
the (BMIm)2[Eu(Pic)5] (1), (BEIm)2[Eu(Pic)5] (2), and
(BBIm)2[Eu(Pic)5] (3) complexes.
The lifetime of the emitting state 5D0 (τ), and the nonradiative
(Anrad) and radiative (Arad) rates, are related through
1
τ
Atot
=
= Arad + Anrad
(13)
where the Arad rate was obtained by summing over the radiative
5
rates A0λ for each D0 → 7Fλ transition (Arad = ∑λA0λ). The
definition of the emission quantum efficiency (η) of the emitting
5D0 level, used here, is
Arad
Arad + Anrad
η =
(14)
Figure 7. Emission spectra of complexes 1, 2, and 3 at 77 K recorded
The theoretical radiative and nonradiative rates, and quantum
efficiencies, are also in good agreement with the experimental
values. The value of the emission quantum efficiency, in
principle, should not be dependent on the T1 position. Thus,
the η behavior shown in Table 6 reflects vibronic couplings,
through Anrad, with the ligands, influenced by the countercations.
A detailed investigation of this point is beyond the scope of the
present work.
from 500 nm to 720 nm under excitation at 370 nm.
The experimental intensity parameters values of Ω2 and Ω4
were calculated and compared with theoretical intensity
parameters Ωλ (λ = 2, 4, and 6) obtained using the spherical
atomic coordinates from the Sparkle/PM3 coordination
polyhedron. The results are presented in Table 6. The charge
Table 6 presents also the R02 intensity parameter, which is the
Table 6. Calculated and Experimental Values of Intensity
Parameters (R02 and Ωλ), Radiative (Arad) and Nonradiative
(Anrad) Rates, Lifetimes (τ), Emission Quantum Efficiencies
(η), and Emission Quantum Yields (q) of the 5D0 Emitting
Level Determined for Complexes 1, 2, and 3
ratio between the intensities of the D0 → 7F0 and D0 → 7F2
transitions. The R02 parameter may give information on the J-
mixing effect associated with the 5D0 → 7F0 transition, as
previously described.5,54 In this case, this effect is mainly due
to the mixing between the 7F2 manifold and the 7F0 level though
the rank-two components of the ligand field. The R02 value for
compound 2 is one order of magnitude smaller than for
complexes 1 and 3, suggesting that the J-mixing effect is much
smaller in this complex, taking into consideration that, in the
three complexes, the 5D0 → 7F0 transition is allowed by
symmetry.
5
5
1
2
3
optical data
R02
exp.
calc.
exp.
calc.
15.0
exp.
calc.
0.018
0.009
12.0
10.3
0.011
9.6
Ω2 (× 10−20 cm2) 15.1
Ω4 (× 10−20 cm2) 13.9
Ω6 (× 10−20 cm2)
13.6
8.6
11.9
10.3
0.1
8.8
9.2
0.3
0.2
A
rad (s−1
nrad (s−1
)
645
591.3
851.7
691
636.5
1215.3
614
565.8
652.2
The experimental and theoretical values of the triplet states of
the 1, 2, and 3 complexes are in very satisfactory agreement. The
triplet state of the complex with the BMIm+ countercation (19
512 cm−1) is above the 5D1 (19 070 cm−1) state, and according to
the selection rules mentioned above, the energy transfer T1 →
5D1 by the exchange mechanism is much more favored than in
the cases of the complexes with the BEIm+ countercation, 2 (T1
at 19 065 cm−1) and the BBIm+ countercation, 3 (T1 at 18 504
cm−1). Particularly, in the latter case, energy back-transfer 5D1 →
T1 is expected to be more operative in the transfer process
balance. On the other hand, an opposite behavior should occur
for the less-operative T1 → 5D0 transfer process (balanced by the
back-transfer rate) by the multipolar mechanism, together with
the J-mixing effect. The transfer rates corresponding to this
process are, at the most, on the order of 106 s−1, which is the
order of magnitude of the nonradiative decay rates for 5D1 → 5D0
in coordination compounds. These facts explain both the
decrease of the emission quantum yield and the increase of the
5D0 lifetime in going from complex 1 to complex 3 (see Table 6).
A
)
797
1159
0.628
37.0
603
τ (ms)
η (%)
q (%)
0.521
45.0
0.821
51.0
41.0
38.2
34.3
27.7
46.4
10.8
factors (g) and the ligating ion polarizability (α), which are used
in the calculations of γtp and Γpt , respectively, were adjusted using a
nonlinear minimization of a four-dimensional response surface to
minimize the error between the experimental and theoretical
values of the Ω2 and Ω4 parameters. The procedure is described
in detail in the literature.51 The values of charge factors (g) and
the ligating ion polarizability (α) are given in Table 3. According
to the theory, the value of the intensity parameter Ω2 is most
influenced by small angular changes in the local geometry. This
effect, together with changes in ligating atom polarizability (α),
has been used to rationalize the fact that certain 4f−4f transitions
are hypersensitive to changes in the chemical environment.52
The fact that the Ω2 and Ω4 parameters (both experimental and
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Inorganic Chemistry
Article
From a schematic energy level diagram shown in Figure 8, the
resonance conditions may be represented. For the three Eu
complexes studied, the singlet levels are below the 5D4 europium
excited state.
C+ countercation, and this small change produces a considerable
modification in the electronic structure of the organic part of the
compounds.55,56 Since this influences the structure of the
complex, it affects the position of the triplet state and, as a
consequence, the emission quantum yield, making (C)2[Ln-
(L)5]-type compounds very interesting materials both for
theoretical studies and practical applications for designing new
LCMDs.57
5. CONCLUSIONS
In this study, we present the first (LnL5)2− pentakis complexes
with a picrate ligand, and we show that the BMIm, BEIm, and
BBIm countercations have a significant influence on their optical
properties. An additional tuning of the optical parameters of
(C)2[Eu(Pic)5]-type complexes may be achieved by an
appropriate choice of the C+ countercation. The theoretical
emission quantum yields are 38.2, 27.7, and 10.8% for the
(BMIm)2[Eu(Pic)5] (1), (BEIm)2[Eu(Pic)5] (2), and
(BBIm)2[Eu(Pic)5] (3) complexes, respectively. The triplet
state of the complex with the BMIm+ countercation (19 512
5
cm−1) is above the D1 state (19 070 cm−1), and the energy
transfer T1 → D1 by the exchange mechanism is much more
favored than in the cases of the complexes with the BEIm+
countercation (T1 at 19 065 cm−1) and the BBIm+ countercation
(T1 at 18 504 cm−1). On the other hand, energy back-transfer
increases in this same direction. These facts explain the trend
observed in the calculated emission quantum yields. A decreasing
linear behavior was observed when a plot of triplet versus lifetime
is drawn, demonstrating the influence of the countercation in the
luminescence process.
Figure 8. Energy level diagram for complexes 1, 2, and 3, showing the
most probable channel for the intramolecular energy transfer process.
From the luminescence decay curves (Figure 9), we have
5
determined the lifetime values (τ) of the emitting D0 level at
ASSOCIATED CONTENT
* Supporting Information
■
S
The deposition numbers CCDC 888983 and 888984 contain the
supplementary crystallographic data for (BEIm)2[Eu(Pic)5] (2)
and (BBIm)2[Eu(Pic)5] (3), respectively. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The
synthesis and characterization of the bromide and picrate ionic
liquids, which have been published previously, also are provided.
Absorption spectra of (BMIm)2[Eu(Pic)5] (1) and 3 complexes
were recorded in acetonitrile (experimental) and calculated in
overlay mode (theoretical). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Tel.: +55 11 30913847. Fax: +55 11 38155579. E-mail:
hefbrito@iq.usp.br.
■
Figure 9. Decay luminescence curves of complexes 1, 2, and 3 recorded
at 77 K monitored with excitation at 370 nm and emission at 616 nm.
The inset shows the relation between the lifetimes of the 5D0 level and
the T1 position.
Present Address
⊥
́
Departamento de Quımica e Biologia, Instituto Federal de
room temperature (300 K), monitored with excitation at 370 nm
and emission at 616 nm. These curves were fitted with a
monoexponential curve with lifetime values of 0.523, 0.628, and
0.821 ms for the Eu(III) complexes with BMIm+, BEIm+, and
BBIm+ countercations, respectively. A decreasing linear behavior
is observed when a triplet versus lifetime graphic is drawn (shown
in the insert of Figure 9), demonstrating the influence of the
countercation in the luminescence process.
Despite the same composition of the first coordination sphere,
the luminescence properties of these compounds are significantly
different. The existence of an additional path for fine-tuning of
the luminescence properties can be optimized by changing the
Educaca
29040-780, Brazil.
̧
o, Ciencia e Tecnologia do Espirito Santo, Vitoria-ES,
̂
́
̃
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors express sincere thanks to Laboratory of Photonic
■
Materials (UNESP-Araraquara)particularly, Dr. Sidney J. L.
́
Ribeiro and Dr. Mauricio Caiutfor some luminescence
measurements. We also thank CNPq-RENAMI, CNPq-
FAPEMIG, FAPITEC-SE, INAMI, and LabCri (X-ray diffraction
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Article
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