Double-decker luminescent ytterbium and erbium SMMs with symmetric and asymmetric Schiff base ligands

Article abstract of DOI:10.1039/c7nj01842h

Multifunctional molecules that respond both to magnetic fields and light are subject of study due to possible applications in fields as diverse as imaging or information processing and storage. In this paper we report visible and NIR emitting single-molec

Full text of DOI:10.1039/c7nj01842h

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Double decker luminescent Ytterbium and Erbium SMMs with
symmetric and asymmetric Schiff base ligands
Received 00th January 20xx,
Accepted 00th January 20xx
b
b
a,c,
Samira Gholizadeh Dogaheh, Hamid Khanmohammadi, E. Carolina Sañudo
*
DOI: 10.1039/x0xx00000x
Multifunctional molecules, that respond both to magnetic fields and light are object of study due to possible applications
in fields as diverse as imaging or information processing and storage. In this paper we report visible and NIR emitting
single-molecule magnets (SMMs) of Yb(III) and Er(III) with symmetric (SYML) and asymmetric (AZOL) Schiff base ligands.
The complexes prepared SYML-Ln2, AZOL-Ln3 (Ln = Er, Yb) are SMMs, with the exception of AZOL-Yb3, with a relaxation
behavior dominated by quantum tunneling of the magnetization. The multi-level double-decker structure of the
complexes is ideal for surface deposition in carbon-based materials.
www.rsc.org/
the anisotropy of the resulting complexes. A breakthrough was
1
the report of the first Single Ion Magnets (SIMs) by Ishikawa.
2
Introduction
These complexes were Tb and Dy double decker complexes of
the well-known phthalocyanine ligand. After this publication,
many others came reporting even higher temperatures for the
peaks in the out-of-phase ac magnetic susceptibility and
Molecular magnetism has been growing exponentially since
the 1990's. The discovery of single molecule magnets (SMMs)
1
by Christou, Gatteschi and Sessoli among others was a key
1
3,14
record energy barriers.
However large the energy barrier it
milestone in the development of molecular magnetism as we
know it today. SMMs are molecules that can retain the
magnetization upon removal of an applied field: this property
makes them ideal candidates for technological applications in
information storage or the new proposals of molecular
is still unusual to observe hysteresis of the magnetization
1
5
above 4 K due to relaxation enhancement by QTM. Low
operational temperatures are not the only challenges that
using SMMs for applications entail. It is understood that
researchers must finds ways to switch on/off a desired
property (magnetic interaction, QTM, spin crossover, etc...) in
order to use molecular systems in devices. Additionally, one
must be able to address these molecules while on a surface or
2
–8
spintronic devices. Additionally, the discovery of SMMs was
key in the realization of quantum properties in bulk materials:
with SMMs quantum effects like quantum tunneling of the
magnetization (QTM) could be studied and thus, new
applications like the use of SMMs and other molecular
nanomagnets (MNMs) in quantum computing were
1
6–19
integrated in a device.
These are not easy challenges and
many research efforts are now thrown at tackling these
problems. In this context, there is a great interest in preparing
multifunctional compounds that combine magnetic and optical
properties with the ultimate goal of having synergy between
them or ideally, the possibility of controlling QTM and
magnetic properties with light. In the last year, two
3
–5,9
proposed.
Classical SMMs are high nuclearity transition
metal complexes. The limits in effective working temperatures
were clear: hysteresis of the magnetization was only observed
at very low temperatures, usually below 4 K or even below the
limit of most commercial Helium cryostats of 2 K. In order to
2
0
1
0
publications have called attention to this topic. Roubeau and
co-workers report the photo switch of SMM properties on a
overcome these difficulties 3d-4f complexes and lanthanide
1
1
complexes
were proposed to improve the previously
2
1
spin crossover complex while Gao, Wang and co-workers
reported the thermostability and photoluminescence of Dy(III)
observed SMM properties. The main assumption was that the
spin-orbit coupling of lanthanide ions could help in improving
2
2
single-molecule magnets under a magnetic field. THe R
groups of the iminie ligand, naphtol and a diphenyl-azo
derivative were chosen for the stability they provide to the
Schiff base ligand and the photoluminescent properties they
add to the complex. The diphenyl-azo moiety is extremely well
studied and ubiquitous in industry. Azo-dyes have many
practical applications. In the last few years, azobenzenes have
been studied for potential applications in areas of nonlinear
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3 3
optics, optical storage media, chemosensors, liquid crystals, precipitates obtained from the reaction of AZOL with Ln(NO )
2
3
photochemical molecular switches or molecular shuttles.
(Ln = Er, Yb) in MeCN showed peaks corresponding to the
+ +
+
] and [Ln
In this paper we explore the lanthanide chemistry of an azo- species [Ln
3
(AZOL)
4
3
(AZOL)
4
(H
2
O)
2
(NO
3
)+H ] for Ln =
3
(AZOL) +
substituted Schiff-base ligand and its symmetric analogue.
Er, Yb, and other fragmentation peaks such as [Ln
+
2
H] . The lower M/z peaks were due to fragmentation of the
molecular ion. In order to further support this conclusion,
reactions were prepared with stoichiometry 3:2 AZOL:Ln and
Results and discussion
4
:3 AZOL:Ln. Maldi-MS analyses of reaction mixture aliquots of
Scheme 1 shows a representation of SYML and AZOL. The
synthesis of Schiff base ligands can be easily achieved by
condensation of an aldehyde and an amine. For the synthesis
of the symmetric Schiff base ligand SYML the condensation
reaction of a diamine o-phenylendiamine with two equivalents
of 2-hydroxy-1-naphthaldehyde was performed in ethanol
under reflux. SYML was obtained in good yield and was used
without further purification. In order to obtain asymmetric
Schiff base ligands the reaction conditions must be carefully
controlled. The two-step condensation reaction of o-
phenylendiamine with two different aldehydes was achieved
by controlling the water content of the solvent in a two-step
condensation reaction. In this case, ethanol was distilled over
molecular sieves and Mg shavings. After the first condensation
reaction with 2-hydroxy-1-naphthaldehyde the crystals of the
mono-Schiff base ligand were separated by filtration and used
in the second condensation. For the asymmetric ligand AZOL
the second condensation was performed with and azo-
substituted aromatic aldehyde. AZOL was obtained following
this two-step procedure in good yield and purity.
each of the trials showed M/z for the 4:3 AZOL:Ln species and
its fragmentation products in all cases, so AZOL-Ln3 is the
preferred product of the reaction between AZOL and Ln(III)
salts. Elemental analyses of the precipitates, as well as the
TGA, IR and SQUID magnetic susceptibility data (vide infra) of
the complexes AZOL-Ln3 (Ln = Er, Yb) confirmed that there is
an extra lanthanide ion, in the form of Ln(NO
3
)
3
. The complete
3 3
(NO )(Ln(NO ) ]
formula for AZOL-Ln is then [Ln (AZOL) (H O)
3
4
2
2
3
and the proposed structure is that shown in Scheme 2 as an
schematic representation. TGA (see Supplementary
o
Information Figure S09) up to 500 C shows the loss of non-
coordinated solvent and coordinated nitrates. This type of
structure has been previously observed with similar Schiff base
ligand complexes of the lanthanides by Li and co-workers
where they report complexes of general formula
2
4–27
[Ln (L) (solvent)
2
3
n
Ln(NO
3
)
3
] for Yb, Ho, Dy and Tb.
The
3 3
Ln(NO ) fragment easily dissociates under MALDI conditions
for AZOL-Er3 and AZOL-Yb3.
Scheme 1. The SYML and AZOL ligands.
The reactions of AZOL and SYML with the lanthanide ions were
performed in acetonitrile and using triethylamine to
deprotonate the two OH groups of the ligand. In all cases,
when AZOL was used a precipitate was obtained and it was not
possible to recrystallize it or to obtain crystals directly from the
reaction. This fact precluded the structural characterization of
the product by single-crystal X-ray diffraction. The cis-trans
isomerism of the azo group can easily happen with UV-visible
light in solution and it prevents the crystallization of the
coordination complex. Mass spectroscopy analysis of the
Scheme 2. A proposed structure for AZOL-Ln3 complexes.
The impossibility of obtaining single-crystals of AZOL-Ln3 was
one of the reasons to prepare the symmetric version of the
ligand, SYML, which should a priori react in a similar way with
the lanthanide ions and produce complexes that could be
easily crystallized. In fact, an aliquot of the acetonitrile
reaction mixture with stoichiometry 1SYML:1Ln was studied by
MALDI-TOF mass spectrometry and peaks corresponding to
2
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+
+2H] , [Ln
+
the species [Ln(SYML)
2
2
(SYML)
3
+H]
and found in the Supplementary Material. The asymmetric unit
+
] were found in the spectra for Ln = Yb, Er (see consists of one full complex and a molecule of solvation water.
[Ln
3
(SYML)
4
Supplementary Information for the MALDI-TOF spectra for the The complex can be described as a double-double decker
SYML-Er reaction system). This indicated either a SYML:Ln complex or double sandwich. The Schiff base ligands actively
product with ratio 3:2 and its fragmentation products or the use the two oxygen donors to bridge two lanthanides. The
co-existence in the reaction mixture of several species with coordination spheres of the two lanthanides in the complex
different SYML:Ln ratio. If the latter were true, in principle the are not equal. As shown in Figure 1, one lanthanide ion, Er1
different species could be isolated in crystalline form by ion in SYML-Er2 and Yb1 in SYML-Yb2, is located in a O
controlling the crystallization conditions. When the MeCN coordination pocket of two completely deprotonated SYML
reaction mixture prepared in the stoichiometric ratio for ligands. The other lanthanide ion, Yb2 and Er2, is in a O
Ln (SYML) (H O)] is left undisturbed to slowly evaporate coordination pocket with two nitrogen and four oxygen atoms
orange crystals of the neutral species [Ln (SYML) (H O)] (Ln = of SYML ligands and one coordinated water molecule. The Er1
4 4
N
5 2
N
[
2
3
2
2
3
2
Er and Yb) were obtained after a few days. Different cations and Yb1 atoms are octacoordinated with a distorted square
and anions were added to MeCN reaction mixtures with antiprism (SAP) coordination polyhedron whereas Er2 and Yb2
stoichiometry 2SYML:1Ln and 4SYML:3Ln in order to isolate are heptacoordinated with a distorted capped trigonal prism
the other two species. It was not possible to isolate the coordination polyhedron. The Ln-O and Ln-N bond lengths
mononuclear species. We succeeded in obtaining yellow have normal values for Ln(III) complexes of Er and Yb. There is
crystals of (NH py)[Ln (SYML) (NO ) ] (SYML-Ln3, Ln = Er, and a water of crystallization, this water is hydrogen bonding the
3
3
4
3 2
Yb) using 2-aminopyridine in the crystallization. The yields coordinated water molecule, with O-H··O distance of 2.723
were always very poor, below 5% and the crystals had to be
Å.
hand-picked from a mixture of orange crystals of SYML-Yb2 or
SYML-Er2 and the new yellow complex SYML-Ln3 (Ln = Er, Yb).
This fact has precluded the study of the properties of the
SYML-Ln3 products. Without the addition of 2-aminopyrdine
only orange crystals of the most insoluble of the products, the
neutral SYML-Ln2, were obtained. MALDI-TOF MS analysis of
the orange crystals showed the expected peak for [Ln (SYML)
2
3
+
H] (see Supplementary Material).
+
Description of crystal structures
Table 1. Crystallographic and data collection parameters for SYML-Er2, SYML-Yb2,
SYML-Er3 and SYML-Yb3 complexes.
SYML-Er2
SYML-Yb2
SYML-Er3
SYML-Yb3
Figure 1. Crystal structure of the SYML-Er2 anion. Carbon: grey, Hydrogen: light
grey, Oxygen: red, Nitrogen: blue, Erbium: pink.
T/K
Crystal
system
Colour
Space group
a/Å
100(2)
100(2)
100(2)
100(2)
Triclinic
Triclinic
Monoclinic
Monoclinic
Orange
Pī
Orange
Pī
Yellow
P 21/n
13.5774(17)
25.425(3)
30.838(4)
90
Yellow
P 21/c
10.5031(8)
18.0711(13)
18.3906(13)
66.800(3)
73.533(4)
85.947(4)
3073.4(4)
2
10.4575(4)
18.0686(6)
18.4181(6)
66.67(2)
73.605(2)
85.41(2)
3063.17(19)
2
13.6388(8)
25.3478(13)
31.3489(16)
90
b/Å
c/Å
α/°
β/°
102.553(8)
90
101.535(3)
90
γ/°
3
V/Å
10391(2)
4
10618.8(10)
4
Z
Radiation
MoKα
MoKα
MoKα
MoKα
2
Gof on F
1.039
0.953
1.081
1.021
Final R
R
1
= 0.0329,
= 0.0769
R
1
= 0.0451,
= 0.0867
R
1
= 0.074,
= 0.1723
R
1
= 0.0614,
Figure 2. Crystal structure of the SYML-Yb3 anion. Carbon: grey, Hydrogen: light
grey, Oxygen: red, Nitrogen: blue, Ytterbium: green.
[
I>=2σ (I)]
wR
2
wR
2
wR
2
wR = 0.1673
2
Crystallographic data and data collection details for SYML-Ln3
3+ 3+
Ln = Er and Yb ) are presented in Table 1. Figure 2 shows
Single crystals of SYML-Er2 and SYML-Yb2 suitable for X-ray
diffraction analysis were obtained by slow evaporation of
(
the crystal structure of SYML-Yb3. SYML-Yb3 and SYML-Er3 are
isostructural and only SYML-Yb3 is shown here. Similar figures
for SYML-Er3 can be found in the Supplementary Material. In
the presence of 2-aminopyridine the reaction mixtures of
SYML and the lanthanide nitrate of Er and Yb lead to the
formation of the SYML-Er3 and SYML-Yb3 metal complexes.
3
CH CN. Crystallographic data and data collection details for
3
+
3+
Er and Yb complexes are presented in Table 1. SYML-Er2
and SYML-Yb2 crystallize in the triclinic space group ī. The
P
two complexes are isostructural and only SYML-Er2 is shown
here. Figures for packing of SYML-Er2 and SYML-Yb2 can be
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These crystals are yellow plates, and they appeared as a minor N=N chromophore.
Journal Name
3
5,36
A much weaker band in the visible
-1
-1
product, while orange blocks of SYML-Ln2 also formed. SYML- region (λmax: 480 nm, ε: 8500 L mol cm ) is related to the
Yb3 crystallizes in the monoclinic 21/c space group. The phenol and imine group, and the diphenylazo moiety of the
P
asymmetric unit consists of one full complex, protonated 2- ligand. Lanthanide coordination to the AZOL ligand causes a
aminopyridine as counter ion and solvent molecules. The deprotonation of the phenol group of AZOL. The formation of
central lanthanide ion coordination sphere is unique, Yb1 or the AZOL-Er3 and AZOL-Yb3 complexes results in a red-shift of
Er1 respectively in SYML-Yb3 and SYML-Er3, while the other this absorption band and an increase of the absorption
two lanthanides have the same coordination sphere. As shown coefficient. The absorption bands appear at 430 nm (ε: 34750
-1
-1
-1
-1
in Figure 2, the Yb2 and Yb3 or Er2 and Er3 ions in the complex L mol cm ) and 420 nm (ε: 35025 L mol cm ) respectively
are located in the O coordination pockets with two nitrogen for AZOL-Er3 and AZOL-Yb3.
and four oxygen atoms of the SYML ligand and one
coordinated nitrate. Yb1 (and Er1) is located in a N
6 2
N
4
O
4
coordination sphere of two completely deprotonated SYML
ligands with a distorted square antiprism coordination
polyhedron. The Yb2 and Yb3 atoms (or Er2 and Er3 in SYML-
Er3) are octacoordinated with a distorted bi-capped trigonal
prism coordination polyhedron.
The single crystal characterization of the orange species SYML-
Yb2 and SYML-Er2 showed that these are sandwich triple
decker type complexes, similar to those prepared with the
2
8
phthalocyanine ligands and the lanthanide ions and to
reported complexes with other Schiff base ligands. Homoleptic
phthalocycanine complexes and heteroleptic complexes with
Schiff base ligands in combination with phthalocyanines have
2
9–32
similar triple decker structures.
Homoleptic, heteroleptic
and polymeric complexes with Schiff base and other ligands
such as acetate that display the basic double-decker unit seen
in SYML-Ln2 (Ln = Er, Yb) were reported by Li and
Figure 3. Electronic and emission spectra of CH
temperature of AZOL and AZOL-Ln3 complexes (Ln
wavelength 440 nm).
3
CN solutions at room
Er, Yb) (excitation
=
2
4–27,33
coworkers.
Some of the complexes reported by Li also
have a capping Ln(NO ) metalloligand, something we have not
3
3
observed for the SYML complexes of the lanthanide ions.
Furthermore, we have isolated the new species SYML-Yb3 and
SYML-Er3, in which a new SYML-Ln layer is added to obtain a
quadruple decker complex with three metals and four SYML
ligands. The double decker type of structures, with two nearly
planar ligands above and below a Er(III) or Yb(III) ion, should
3
4
display a relatively well isolated M ground state based on
J
the angular dependence of the M substates. This is particularly
J
important for Yb(III) and it should affect the photophysical and
magnetic properties of the complexes, as we show in the
following sections.
Electronic spectra and emission properties
UV–Vis absorption spectra and luminescent properties of the
free ligands AZOL and SYML, as well of their lanthanide
complexes, were studied in solution. The electronic spectra are
shown in Figures 3 and 4. Emission properties in the near IR of
the AZOL-Ln3 and SYML-Ln2 complexes were also studied in
the solid state. The AZOL absorption spectrum of AZOL-Ln3
complexes and AZOL have strong absorption bands between
Figure 4. Electronic and emission spectra of CH
temperature of SYML and SYML-Ln2 complexes (Ln
wavelength 400 nm.
3
CN solutions at room
Er, Yb). Excitation
=
The absorption changes of the azo-containing species AZOL,
AZOL-Er3 and AZOL-Yb3 upon photo-irradiation were
examined in order to observe the cis↔trans interconversion
of the azo group. In principle, this process can be controlled by
220 and 300 nm with a sharp maximum at 238 nm related to
the aromatic groups and π - π* transition of the nitro
substituent. For the AZOL there is a broad absorption band in
the UV-Visible region, with maxima at 320 nm and 381 nm
irradiating at the necessary wavelength with an intense
23,37
monochromatic source.
In the complexes and ligand
-
1
-1
(λmax: 320 nm, ε: 2365 L mol cm , λmax: 381 nm, ε: 31725 L
-1
reported here, AZOL and AZOL-Ln3, cis↔trans isomerization
could not be clearly established from the electronic spectra or
observed with the naked eye. The spectra after irradiation are
-1
mol cm ), related to the symmetry allowed π - π* transition
of the aromatic systems that overlaps with the band from the
4
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shown in the supplementary information Figure S05 as dashed coordinated water is particularly efficient quenching the
4
0
lines, the changes observed were subtle: a decrease of emission of the lanthanide. The situation of AZOL-Ln3 (Ln =
intensity of the visible absorption band and an increase in the Er, Yb) is similar: there are two distinct environments, the
intensity of the UV absorption band plus a small blue-shift.
UV–Vis spectra for SYML, SYML-Yb2 and SYML-Er2 complexes other three lanthanide ions are bound to terminal ligands that
in CH CN solution at room temperature are shown in Figure 4. can be easily exchanged in solution. NIR photoluminescence
The SYML absorption spectrum consists of four bands. The experiments on SYML-Ln2 and AZOL-Ln3 were performed at
central lanthanide is coordinated to two ligands while the
3
-1
-1
3
strong bands (λmax: 319 nm, ε: 22750 L mol cm and λmax: 370 room temperature in the solid state and in CH CN solution.
-1
-1
nm, ε: 21345 L mol cm ) arise from the symmetry allowed π - The free ligands AZOL and SYML did not display NIR
π* transitions. The much weaker bands in the visible region luminescence under similar conditions. Upon excitation in the
-1
-1
(λmax: 454 nm, ε: 12000 L mol cm and λmax: 476 nm, ε: 10300 ligand-centered transitions, from 300 to 500 nm, characteristic
-1
-1
L mol cm ) are related to the chromophores that coordinate emission bands of Yb(III) were observed in the NIR emission
to the metal, phenol and imine. The deprotonation of the OH spectra of SYML-Yb2 and AZOL-Yb3, in addition to the weak
groups and the coordination of the lanthanide ion to the imine emission from the ligand. When excited at 420 nm, a narrow
2
2
and phenoxide groups is clearly seen in the UV-Vis spectra. The signal at 976 nm that corresponds to the F5/2→ F7/2 electronic
electronic spectra SYML-Er2 and SYML-Yb2 shows the transition was revealed for both complexes AZOL-Yb3 and
coalescence of the visible bands, which are blue-shifted with SYML-Yb2 in solid state, as shown in Figure 5. Several broad
-1
respect to the free SYML spectrum and the red-shift of the 370 bands spanned approximately 700 cm in the energy spectrum
nm absorption of free SYML. The bands observed are λmax: 331 accompanied this band. A similar splitting has already
-
1
-1
-1
-
50–52
nm, ε: 30050 L mol cm and λmax: 395nm, ε: 29450 L mol cm observed in Yb(III) chelate compounds.
The lower energy
splitting of the excited and
fundamental Yb(III) states due to the low symmetry crystal
1
-1
-1
for SYML-Er2 and λmax: 331 nm, ε: 23900 L mol cm and bands are consequence of the M
-1
max: 388nm, ε: 24900 L mol cm for SYML-Yb2.
J
-1
λ
4
3
All emission spectra were collected at room temperature in field. Pope, Colacio and co-workers report the NIR emission
acetonitrile solutions of the ligand or complexes and are of a dinuclear Yb(III) complex and they assign the band at the
shown in Figure 3. Emission spectra for AZOL were collected lower energy (circa. 1050 nm) to either vibronic coupling or to
-
5
on a 2×10 M solution in CH
strong emission band at 520 nm was observed. Upon
coordination to Yb and Er, the emission spectra broadened and directly related to the crystal field splitting of the
lost definition and intensity. As shown in Figure 3 the emission multiplet observed for our compounds is similar to that
3
CN. Upon irradiation at 440 nm a the Yb(III)-Yb(III) coupling effect on the M
J
level splitting of the
2
53
F7/2 multiplet. The energy span of the NIR bands, that is
2
F
7/2
5
4
band practically disappeared: when the AZOL ligand is observed for other mononuclear and dinuclear complexes
5
coordinated to Er(III) and Yb(III) the fluorescence is heavily with similar O-N donor ligands. NIR luminescence was also
3
-5
3
quenched. This has been reported earlier for complexes of observed for AZOL-Yb3 and SYML-Yb2 in 2x10 M CH CN
paramagnetic metals and fluorescent ligands and is referred to solution at room temperature. For SYML-Er2 and AZOL-Er3
as chelation enhancement of quenching of emission (CHEQ) excitation in the ligand centered region 300-500 nm did not
2
0,38
4
The quantum yield of emission for AZOL-Ln3 was lead to the expected NIR-emission attributed to the I13/2 →
effect.
4
3+
55
below 0.1%. A fluorescent emission band from the SYML at
I
15/2 transition of the Er ion. Irradiation at 590 nm did result
10 nm was observed upon irradiation at 440 nm of 2×10 M in a very broad NIR emission peak centered at 1475 nm for
solutions of SYML in CH CN (supplementary Information Figure AZOL-Er3 and 1490 nm for SYML-Er2 that is characteristic of
S06). Figure 4 shows emission spectra in solution for SYML and Er(III) (see Supplementary Information Figure S07). Similar NIR
-7
5
3
-5
SYML-Ln2 upon irradiation at 400 nm. In the emission spectra emission spectra were observed for 2x10 M solutions in
3
+
3+
of the Er and Yb complexes of SYML, SYML-Er2 and SYML- MeCN. In all cases, NIR emission was weak. Thus in the
Yb2 the quenching of the fluorescence is clear, as was complexes reported here the sensitization process via the
observed for the emission spectra of AZOL-Ln3 complexes. electronic structure of the ligands was not very efficient.
Upon excitation at 400 nm, the quantum yield for the emission The crystal field splitting of the M
J
sublevels observed for
band was 0.8 % for the free ligand SYML and 0.3 % and 0.3 % SYML-Yb2 and AZOL-Yb3 is similar to that calculated by
4
2
43
for SYML-Er2 and SYML-Yb2 respectively.
Ishikawa
and Bünzli
for Yb(III) complexes with
The combination of lanthanide ions with fluorescent ligands phthalocyanine and other N-donor ligands, and by Pointillart
4
4
results in materials that will absorb light in the UV-Vis region of and co-workers for a redox active Yb(III) SMM. For
the spectrum and will emit both in the UV-Vis due to the ligand mononuclear Yb(III) complexes it is possible to extract crystal
and the visible and/or near IR due to the lanthanide ion. The field parameters from the NIR spectra, in particular if NIR is
organic ligand is said to act as an antenna, harvesting the light measured at low temperature and to use these parameters to
3
9,40
and transferring it to the lanthanide ion.
low molar absorptivity of the lanthanide 4f levels can be gives access to direct information on splitting of the
In this way the model the observed magnetic data. The NIR emission of Yb(III)
2
F
7/2
4
1
overcome. The complexes SYML-Ln2 (Ln = Er, Yb) reported multiplet by the ligand field. Pointillart and coworkers, among
here have two lanthanide ions that have different coordination others, do so for several mononuclear luminescent
4
5,44,46–48
spheres. One is coordinated by two ligands while the other is SMMs.
Tong et al report a half-sandwich distorted
also coordinated to a molecule of water. It is known that Yb(III) mononuclear SMM and they extract crystal field splitting
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ARTICLE
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4
9
data from the NIR emission. A detailed analysis as that
performed on mononuclear species by Pointillart et al cannot
be done in polynuclear species with inequivalent Yb(III) sites as
SYML-Yb2, however the energy span of the peaks observed in
NIR can be compared to the splitting obtained from analyzing
magnetic data.
In this Hamiltonian
σ
Bkqi are the crystal field parameters,
i
are the orbital reduction parameters
θk are the operator
equivalent factors and Okqi are operator equivalents. To
describe the crystal field of lanthanide ions second, fourth and
sixth rank operators are required. The ideal symmetry of the
Ln(III) ions in SYML-Ln2 is low and the crystal field parameters
were taken to be the same for the two lanthanide ions in
SYML-Ln2 (Ln = Yb, Er), even though the two lanthanides are
not equivalent. One lanthanide is octacoordinated in an ideal
C2v N4O4 coordination sphere and the other is in a low
symmetry Cs O5N2 coordination sphere. This a crude
approximation necessary in order to avoid the
overparametrization of the procedure and is usually accepted
when using PHI to fit magnetic data for lanthanide complexes,
4
4,53,57,58
with results similar to those reported by us here.
ꢂ
The
ꢂ
ꢂ
ꢃ
ꢀ
, ꢀ ꢄꢀ ꢄandꢄꢀ crystal field parameters were used. The Ln-
ꢁ
ꢃ
ꢅ
ꢅ
O-Ln angles (where O is the phenoxo bridging oxygen) are 110
o
o
and 108 in the crystallographically characterized SYML-Ln2
complexes. These values should result in very weak coupling
Figure 5. Solid state NIR emission of AZOL-Yb3 and SYML-Yb2 upon excitation by
4
(ferro or antiferromagnetic) between the lanthanide ions.
80 nm light at room temperature.
Susceptibility and magnetization were fitted at the same time
and the best set of parameters modelled both data sets. The
Magnetic properties
Magnetic susceptibility data were collected for SYM-Er2, SYM-
Yb2, AZOL-Er3 and AZOL-Yb3 complexes at two different
applied dc fields in the 2-300 K temperature range. It is worth
to remind the reader that for the AZOL-Ln3 complexes there
are actually four lanthanide ions per complex, one of them as a
-1
exchange was surveyed with values between 1 cm and -1 cm
-
1
.
For SYML-YB2 the effects on magnetization and susceptibility
were very subtle so J = 0 was chosen for the reported fitting.
For SYML-Yb2 with g = 8/7 the crystal field parameters were
ꢂ
ꢂ
ꢂ
ꢃ
ꢀ
0
ꢆ ꢇ ꢈ0.77, ꢀꢄ ꢆ ꢇ 0.002, ꢄꢀ ꢆ ꢇ ꢄ0.021ꢄandꢄꢄꢀ ꢆ ꢇ
ꢃ ꢅ ꢅ
ꢁ ꢃ ꢅ ꢅ
-1
ꢁ
Ln(NO
Figures 6 and 7. The
good agreement with the expected values for the proposed
3
)
3
metalloligand. Data are shown as
χ
T vs. T plots in
.059 cm . For SYML-Er2 the best fitting was obtained for g =
/5, J = -0.24 cm and the crystal field parameters ꢀ ꢆ ꢇ
5.88, ꢄꢀ ꢆ ꢇ 0.1846, ꢄꢀ ꢆ ꢇ ꢄ0.003ꢄandꢄꢀ ꢆ ꢇ ꢈ0.010
ꢃ ꢅ ꢅ
ꢃ ꢅ ꢅ
-1
cm . The fittings are shown in Figure 6 as solid lines.
χ
T product at 300 K for all samples is in
-1
ꢂ
6
ꢁ
ꢁ
ꢂ
ꢂ
ꢃ
1
1
1
4
complex formulae (Er(III): 4f , I15/2, S = 3/2, gJ = 6/5; Yb(III)
1
3 2
4
f , F7/2, S = ½, gJ = 8/7). For all the examined complexes, the
T product is non-field dependent. As temperature goes down,
for SYML-Er2 and AZOL-Er3 the T product slowly decreases
and shows a sharper decrease below 50 K. The variation of the
T product with temperature is smoother for the Yb(III)
complexes in all the temperature range. The changes in
χ
χ
χ
χ
T
product with temperature are as expected due to the
Boltzman depopulation of M sublevels and weak magnetic
J
coupling. In particular for SYML-Yb2 and AZOL-Yb3, the smooth
decrease in χT product is in agreement with the splitting of the
MJ levels observed in the emission properties of the
complexes. The susceptibility data for SYML-Yb2 and SYML-Er2
5
6
were fitted using the software PHI. The program was
designed for the treatment of systems containing orbitally
degenerate and strongly anisotropic ions, through the
inclusion of Spin-Orbit (SO) coupling and Crystal-Field (CF)
effects using Steven's Operators and the following crystal field
Hamiltonian:
Figure 6. DC magnetic susceptibility plots shown as χT vs T plots for SYML-Er2
and SYML-Yb2 at applied fields of 5000 and 300 Oe. The solid lines are the bets
fitting to the experimental data. See text for fitting parameters.
6
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ARTICLE
Figures 8, 9 and 10. SYML-Yb2 shows no out-of-phase peak
without an applied dc field, but upon application of a dc field
the tail of an out-of-phase can be observed. The optimal field
of 2000 Oe was chosen and ac data collected as a function of
frequency for temperatures between 1.8 and 5 K, Figure 8. The
out-of-phase ac susceptibility was fitted using the Debye
model with
α = 0.01(5 K) to 0.34(1.8 K). The large values for α
at low temperatures indicate a very broad distribution of
relaxation times. The fitted out-of-phase ac susceptibility data
are shown in Figure 9 as solid lines. The relaxation times
extracted were fitted using the Arrhenius equation between
1
.8 and 2.3 K to extract and effective barrier for the relaxation
of the magnetization of 5.5 for SYML-Yb2 (see
supplementary material for the full fitting parameters).
K
Figure 7. DC magnetic susceptibility plots shown as χT vs T plots for AZOL-Er3
and AZOL-Yb3 at applied fields of 3000 and 300 Oe.
Magnetization vs. field data at 2 K are shown in Supplementary
information Figure S08, the fitting is shown as a solid line. In all
cases the population of a magnetic state is observed. The
magnetization values at 5 T and 2 K for SYML-Ln2 (Ln = Er, Yb)
and AZOL-Ln3 (Ln = Er, Yb) are smaller than the expected
values for two (SYML-Ln2) and four (AZOL-Ln3 with Ln(NO ) )
3
3
2
lanthanide ions due to crystal field splitting of the F term for
7
/2
4
the Yb(III) complexes and the I15/2 term for the Er(III)
complexes. The crystal field parameters obtained from PHI can
be used to obtain the energy splitting of the M sublevels. For
J
SYML-Yb2 the lowest lying is MJ1 = MJ2 = 1/2, with MJ1 = 1/2 Figure 8. Out-of-phase AC magnetic susceptibility vs. frequency plot for SYML-
Yb2 (DC field = 2000 Oe) between 1.8 and 5 K. The solid lines are fittings to a
Debye model.
and M = 5/2 and M = M = 1/2 very close in energy (approx.
J2
J1
J2
1
0 and 20 cm-1 above the ground state). The total splitting of
-1
the M sublevels is 793 cm , in good agreement with the
J
splitting observed in the NIR emission of the Yb(III) ions in
SYML-Yb2 at room temperature. For SYML-Er2 the crystal field
parameters obtained from PHI result in a M = 1/2 and M = -
J1
J2
-
1
/2 ground state level with a total splitting of approx. 590 cm
1
.
Unfortunately, we have not been able to model the data for
AZOL-Er3 and AZOL-Yb3, with four lanthanide ions each. The
susceptibility and magnetization data suggest similar ground
M
J
state as those found for SYML-Ln2. For Ishikawa's
TBA[Yb(Pc) ] and TBA[Er(Pc) ] the lowest M were reported as
/2 and 1/2 respectively.
2
2
J
5
9
5
Given the structural characteristics of these complexes the
dynamic magnetic properties of SYML-Ln2 and AZOL-Ln3 were
studied in the absence of DC applied fields. Previously
reported salen double-decker type complexes by Li et al with
Yb(III) were found to be field induced single molecule magnets
with fast quantum tunnelling dominated relaxation of the
Figure 9. Argand plot for SYML-Yb2 at an applied dc field of 0.2 T. The solid lines
are the best fitting to a Debye model. with parameters as shown in Table S01.
3
3
magnetization. In 2012 the first polynuclear field induced
6
The tail of out-of-phase peaks is observed for SYML-Er2 in the
absence of an applied dc field. At the optimal dc field of 2000
Oe there is a separation of out-of-phase peaks at temperatures
below 3 K for slow relaxation processes and a tail assigned to a
faster relaxation or QTM, as shown in Figure S11. The ac data
for SYML-Er2 cannot be fitted to a simple Debye model or to a
0
Yb(III) was reported by Murugesu and coworkers. Most Yb(III)
complexes reported as SMMs are mononuclear field-induced
5
7,61,62
SMMs.
AC magnetic susceptibility data were collected for the SYML-
Ln2 and AZOL-Ln3 complexes for Ln = Er, Yb and are shown in
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63
modified Debye model for two processes. It is clear that the 817(s), 747(s). The precursor (0.524 g, 2 mmol) was placed in
dynamics of relaxation for these species, with two lanthanide EtOH (20 mL). This slurry was heated to reflux until a solution
ions in different crystal fields are rather complicated.
was obtained. To this solution was added 1-(3-Formyl-4-
For AZOL-Ln3 neither the Er(III) or Yb(III) analogues display hydroxyphenylazo)-4-nitrobenzene (0.542 g, 2 mmol) in small
out-of-phase ac signals down to 1.8 K without the application portions. When the addition was complete, the reaction
of a dc field. AZOL-Er3 in applied dc fields larger than 1000 Oe, mixture was stirred overnight. The mixture was filtered and
displays the tail of an out-of-phase AC magnetic susceptibility the obtained solid was washed with hot ethanol (three times)
peak when ac is plotted against temperature. Thus the new and then with diethyl ether. The yield was 61%. m.p. 245–246
1
complexes SYML-Ln2 (Ln = Er, Yb) and AZOL-Er3 complexes are °C. AZOL was characterized by IR, H-NMR and ESI-mass
1
new examples of lanthanide single-molecule magnets. The spectrometry. H NMR (d
6
-dmf): 7.0 (d); 7.2 (d); 7.35 (t); 7.45
SYML ligand provides an appropriate crystal field for a field (t); 7.55 (m); 7.65 (d); 7.85 (d); 7.95 (d); 8.15 (m); 8.5 (d); 8.6
1
3
3
induced SMM in SYML-Yb2 and SYML-Er2 but AZOL fails to do (d); 8.7 (d); 9.4 (s); 9.9 (d). C NMR (101 MHz, CDCl ) δ (ppm):
so for AZOL-Yb3. The main difference between the ligands is 170.44, 165.35, 163.17, 155.83, 155.03, 148.33, 145.50,
the presence of a substituted diphenylazo group in AZOL or a 140.97, 139.85, 137.01, 133.16, 129.68, 129.42, 128.57,
naphtol in SYML, thus preparing the symmetric and 128.14, 127.83, 127.39, 127.24, 124.74, 123.69, 123.16,
asymmetric ligands we can effectively tune the crystal field.
122.39, 119.65, 119.07, 119.00, 118.85, 118.80, 109.26. IR
-1
(
KBr, cm ): 1621(s), 1582(s), 1517(s), 1482(w), 1339(s),
1
6
C
291(w), 1173(w), 1143(w), 1104(s), 852(s), 830(w), 743(s),
+
88(w). ESI-MS: MW = 515.52 g/mol. Ms/z (M+1H ) = 516.16.
Conclusions
28 20 2 2
H N O (SYML)
Symmetric and asymmetric Schiff-base ligands with naphtyl
groups, SYML and AZOL, have been prepared. The
coordination complexes with Yb(III) and Er(III) have been
prepared and characterized, SYML-Ln2 and AZOL-Er3 are new
examples of SMMs. The crystal field was finely tuned from
SYML to AZOL and this is reflected in the properties of the
species. In summary, the complexes reported here are
especially interesting due to three facts: i) the two lanthanide
ions in SYML-Ln2 are not equivalent, this opens up possibilities
of using the complex as two qubit quantum gate with two
weakly coupled qubits; ii) the presence of the naphtyl and azo
groups in the Schiff base ligands make these species
interesting for surface deposition on metals and carbon based
materials, and iii) multifunctionality has been achieved by
combining the magnetic properties of 4f complexes with the
dual emission properties: in the UV-vis of symmetric (SYML)
and asymmetric (AZOL) Schiff base ligands and in the NIR of
the lanthanides. A more detailed study of the emission
properties of the complexes would be necessary in order to
ascertain the efficiency of the sensitization process. Azo
groups could be exploited for example to generate mechanical
SYML (N,N’-bis(1-naphthaldiamine)-
O-phenylenediamine) was
3
6
synthesized according to literature methods.
o-
Phenylendiamine (0.54 g, 5 mmol) is dissolved in 20 mL of
ethanol. To this solution, 1.72 g (10 mmol) of 2-hydroxy-1-
naphthaldehyde in 20 ml ethanol was added. The resulting
solution was refluxed for 2 h. The obtained orange precipitate
was filtered off and then dried with anhydrous diethyl ether.
1
Yield: 74%. M.p. 210. H NMR (CDCl
3
): 7.1 (d); 7.3 (t); 7.4 (m);
-1
7
.5 (t); 7.7 (d); 7.8 (d); 8.1 (d); 9.4 (d). IR (KBr, cm ): 1621 (s),
565 (s), 1469 (s), 1417 (w), 13218 (s), 1243 (w), 1173 (s), 969
1
1
3
(
w), 878 (w), 821 (s), 734 (s). C NMR (101 MHz, CDCl
ppm): 168.91, 156.19, 139.74, 136.58, 133.17, 129.35, 128.02,
27.48, 127.35, 123.57, 121.99, 119.15, 118.99, 109.37. ESI-
3
), δ
(
1
+
MS: (C28
Er (AZOL)
A stirred solution of ligand (0.1 g, 0.193mmol), Er(NO
0.057 g, 0.128 mmol) in CH CN (20 mL) was heated under
reflux. After added of Et N (81 µl, 0.868 mmol) color changed
H
20
N
2
O
2
) MW = 416.47 g/mol. Ms/z (M+1H ) = 417.16.
[
3
4
(H
2
O) (NO )(Er(NO )] (AZOL-Er3)
2
3
3 3
)
3 3 2
) ·xH O
(
3
3
from brown to dark red and a precipitate formed. The
resulting solution was stirred and heated at reflux for 6 h. The
precipitate was filtered, washed with hot acetonitrile, and
dried with anhydrous diethyl ether. Yield: 71.42% (0.08 g).
AZOL-Er3 was characterized by IR, MALDI-TOF-mass
6
4
movement on a surface.
-1
Experimental
spectrometry and EA. IR (KBr, cm ): 1608(s), 1573(s), 1517(s),
1
1
473(w), 1382(s), 13308s), 1286(w), 1178(w), 1147(w),
108(s), 986(w), 856(s), 739(s), 686(w). MALDI-TOF-MS:
All chemicals were purchased from commercial sources and
used as received. 1-(3-formyl-4-hydroxyphenylazo)-4-
nitrobenzene was synthesized according to literature
[(C30
H
19
N
5
O
4
)
4
-
Er
3
(NO
-Er(NO
Analysis
(NO )(H O)
.20; N, 10.67. Experimental: C 48.99; H 3.02; N 10.15 %.
Yb (AZOL) (H O) (NO )(Yb(NO )] (AZOL-Yb3)
A stirred solution of ligand (0.1 g, 0.193 mmol), Yb(NO
0.058 g, 0.128 mmol) in CH CN (20 mL) was heated under
reflux. After addition of Et N (81 µl, 0.868 mmol) color
3
)(H
) = 2555; M/z (M-Er(NO
calculated
2
]·2(Et O): C, 48.78; H,
2 2 3 3
O) Er(NO ) ] MW = 2982 g/mol; Ms/z
+
35
(M-2H
2
O-NO
3
3
)
3
3
)
3
+H ) = 2654.
for
method.
Elemental
C
30
H
21
N
5
O
4
(AZOL)
A mixture of 2-hydroxy-1-naphtaldehyde (1.72 g, 10 mmol)
and -phenylendiamine (1.08 g, 10 mmol) in anhydrous EtOH
80 mL) was heated to reflux for 6 h. After standing overnight,
[(C30
H
19
N
5
O
4
)
4
Er
3
3
2
2
(Er(NO
3
)
3
3
o
[
3
4
2
2
3
3 3
)
(
3 3 2
) ·5H O
brown crystals formed. The product was filtered, washed with
hexane, and dried in air. The yield was 53% (1.39 g). M.p. 171–
(
3
-
1
3
1
72 °C. IR (KBr, cm ): 3473(s), 3373(s), 1613(s), 1556(s),
491(s), 1469(w), 1317(s), 1182(s), 1156(s), 1134(w), 962(w),
changed from brown to dark red and precipitate formed. The
resulting solution was stirred and heated at reflux for 6 h. The
1
8
| J. Name., 2012, 00, 1-3
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ARTICLE
precipitate was filtered, washed with hot acetonitrile, and SHELXT-2014) and refined on F2 (SHELX-97). Cif files can be
dried with anhydrous diethyl ether. Yield: 95%. The complex obtained free of charge from the Cambridge Crystallographic
was characterized by IR, MALDI-TOF-mass spectrometry. IR Data Centre (CCDC https://summary.ccdc.cam.ac.uk/structure-
-
1
(
KBr, cm ): 1608(s), 1578(s), 1517(s), 1469(w), 1378(s), summary-form, deposit codes: 1500647-1500650). Hydrogen
1
343(s), 1286(w), 1182(w), 1139(w), 1100(s), 991(w), 860(s), atoms were included on calculated positions, riding on their
21(w), 743(s), 686(w). MALDI-TOF-MS: carrier atoms. Infra-Red spectra were performed on a Thermo
Yb (NO )(H Yb(NO ] MW = 3014.29 g/mol; scientific AVATAR 330 FT-IR; Fluorescence measures were
O-NO -Yb(NO ) = 2572; M/z (M-Yb(NO
Elemental analyses calculated
Yb (NO (Yb(NO ]·4(H O)·Et O: C, 47.08; calculate quantum yields; UV-Vis spectra were acquired in a
8
[(C30
H
19
N
5
O
4
)
4
3
3
2
O)
2
3 3
)
-
+
Ms/z (M-2H
671.
2
3
3
)
3
3
)
3
+H ) = taken in
a
NanoLogTM-Horiba JobinYvon iHR320
2
for spectrophotometer and fluorescein was used as standard to
[(C30
H
19
N
5
O
4
)
4
3
3
)(H
2
O)
2
3
)
3
2
2
H, 3.06; N, 10.63. Experimental: C 46.45; H 2.80; N 10.19 %.
Er O] (SYML-Er2)
A stirred solution of SYML (0.1 g, 0.2 mmol), Er(NO
3
0.07 g, 0.15 mmol) and Et N (100 µL, 0.6 mmol) in CH CN (20 data were collected at the Unitat d'Espectrometria de Masses
Cary 100 Scan from Varian. Irradiation of the samples was
done with an ASAHI MAX 303 Xenon lamp. Elemental analyses
were carried out at the CCiT-UB and CSIC. Mass spectrometry
[
(C28
H
18
N
2
O
2
)
3
2 2
H
3 3 2
) .xH O
(
3
mL) was heated under reflux for 6 h. After 5-6 days, orange (CCiTUB). Magnetic measurements on crushed polycrystalline,
crystals of SYML-Er2 were obtained by slow evaporation of vacuum dried samples (SQUID magnetometer equipped with a
-1
solution. Yield: 93 %. IR (KBr, cm ): 1617(s), 1608(s), 1569(s), 5T magnet, diamagnetic correction applied using Pascal's
1
534(s), 1456(s), 1382(s), 1182(s), 982(w), 821(s), 726(s), constants) were done at the Servei de Mesures Magnètiques
1
4
73(w). ESI-MS: [(C28
H
18
N
2
O
2
)
3
Er
1578. Elemental Anal. Calc. for were performed at the NMR service of CCiT-UB.
O)]·9H O·3MeCN: C, 57.52; H, 4.45; N,
2 2
H O] MW = 1592.9 g/mol. of CCiT-UB. H-NMR (Varian Unity 300 MHz on manual mode)
+
Ms/z (M+1H -H
2
O) =
[(C28
H
18
N
2
O
2
)
3
Er (H
2
2
2
6.71. Experimental: C 57.28; H 4.16; N 6.51 %.
Acknowledgements
[
(C28
H
18
N
2
O
2
)
3
Yb
2
H
2
O] (SYML-Yb2)
A stirred solution of SYML (0.1 g, 0.2mmol), Yb(NO .5H
3
)
3
2
O
ECS acknowledges financial support from the Spanish
Government Ministerio de Economía y Competitividad via
FEDER Funding (CTQ 2012-32247, CTQ2015-68370-P) and the
Catalan Government agency AGAUR (project SGR-2014-129).
SG and HK acknowledge support from Iranian Ministry of
Science Research and Technology (MSRT) for funding for a stay
at GMMF-UB.
(
0.058 g, 0.128mmol) and Et N (100 µl, 0.6mmol) in CH CN (20
3
3
mL) was heated under reflux for 6 h. After a day orange
crystals of SYML-Yb2 were obtained by slow evaporation of
-1
solution. Yield: 76 %. IR (KBr, cm ): 1617(s), 1604(s), 1573(s),
1
9
1
543(s), 1456(s), 1400(s), 1360(s), 1247(w), 1173(s), 1160(w),
82(s), 826(s), 730(s). ESI-MS: [(C28 Yb (H O)] MW =
O) = 1590. Elemental Anal.
O)]·4H O:C, 59.98; H, 3.83; N,
.00. Experimental: C 60.05 ; H 3.45; N 4.97 %.
NH py][(C28 Yb (NO )] (SYML-Yb3)
A stirred solution of SYML (0.1 g, 0.2 mmol), Yb(NO
0.058 g, 0.128mmol) and Et N (100 µl, 0.6mmol) in CH
mL) was heated under reflux for 6 h. 2-Aminopyridine (NH
H
18
N
2
O
2
)
3
2
2
+
607.49 g/mol. Ms/z (M+1H -H
Yb (H
2
Calc. for [(C28
H
18
N
2
O
2
)
3
2
2
2
5
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3
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18
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2
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2
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4
3
3
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[
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