ArticlePDF Available

Relationship between the Coordination Geometry and Spin Dynamics of Dysprosium(III) Heteroleptic Triple-Decker Complexes

Authors:

Abstract and Figures

When using single molecule magnets (SMMs) in spintronics devices, controlling the quantum tunneling of the magnetization (QTM) and spin-lattice interactions is important. To improve the functionality of SMMs, researchers have explored the effects of changing the coordination geometry of SMMs and the magnetic interactions between them. Here, we report on the effects of the octa-coordination geometry on the magnetic relaxation processes of dinuclear dysprosium(III) complexes in the low-temperature region. Mixed ligand dinuclear Dy3+ triple-decker complexes [(TPP)Dy(Pc)Dy(TPP)] (1), which have crystallographically equivalent Dy3+ ions, and [(Pc)Dy(Pc)Dy(TPP)] (2), which have non-equivalent Dy3+ ions, (Pc2– = phthalocyaninato; TPP2– = tetraphenylporphyrinato), undergo dual magnetic relaxation processes. This is due to the differences in the ground states due to the twist angle (φ) between the ligands. The relationship between the off-diagonal terms and the dual magnetic relaxation processes that appears due to a deviation from D4h symmetry is discussed.
Content may be subject to copyright.
Magnetochemistry 2019, 5, 65; doi:10.3390/magnetochemistry5040065 www.mdpi.com/journal/magnetochemistry
Article
Relationship between the Coordination Geometry
and Spin Dynamics of Dysprosium(III) Heteroleptic
Triple-Decker Complexes
Tetsu Sato
1
, Satoshi Matsuzawa
2
, Keiichi Katoh
1,
*, Brian K. Breedlove
1
and Masahiro Yamashita
1,3,4,
*
1
Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba,
Aoba-ku, Sendai 980-8578, Japan; tetsu.sato.r1@dc.tohoku.ac.jp (T.S.); breedlove@m.tohoku.ac.jp (B.K.B.)
2
Institute for Materials Research, Tohoku University, 2–1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan;
matsuzawa@imr.tohoku.ac.jp
3
Advanced Institute for Materials Research, Tohoku University, 2–1-1 Katahira, Aoba-ku,
Sendai 980-8577, Japan
4
School of Materials Science and Engineering, Nankai University, Tianjin 300350, China
* Correspondence: keiichi.katoh.b3@tohoku.ac.jp (K.K.); yamasita@agnus.chem.tohoku.ac.jp (M.Y.);
Tel.: +81-22-795-6547 (K.K.); +81-22-795-6544 (M.Y.)
Received: 10 August 2019; Accepted: 22 November 2019; Published: 26 November 2019
Abstract: When using single molecule magnets (SMMs) in spintronics devices, controlling the
quantum tunneling of the magnetization (QTM) and spin-lattice interactions is important. To
improve the functionality of SMMs, researchers have explored the effects of changing the
coordination geometry of SMMs and the magnetic interactions between them. Here, we report on
the effects of the octa-coordination geometry on the magnetic relaxation processes of dinuclear
dysprosium(III) complexes in the low-temperature region. Mixed ligand dinuclear Dy
3+
triple-decker complexes [(TPP)Dy(Pc)Dy(TPP)] (1), which have crystallographically equivalent
Dy
3+
ions, and [(Pc)Dy(Pc)Dy(TPP)] (2), which have non-equivalent Dy
3+
ions, (Pc
2–
=
phthalocyaninato; TPP
2–
= tetraphenylporphyrinato), undergo dual magnetic relaxation processes.
This is due to the differences in the ground states due to the twist angle (φ) between the ligands.
The relationship between the off-diagonal terms and the dual magnetic relaxation processes that
appears due to a deviation from D
4h
symmetry is discussed.
Keywords: Dy
3+
ion; triple-decker; spin dynamics; octa-coordination geometry
1. Introduction
Rational design and synthesis of single molecular magnets (SMMs) and molecular
nanomagnets (MNMs) suitable for quantum information processing (QIP) in quantum computers
(QCs) remains difficult [1–7]. Over the past two decades, a wide range of SMMs with controlled spin
relaxation processes and high performance have been reported. The charge density distribution of
oblate-type lanthanoid ions, (like terbium(III) and dysprosium(III)), strongly improves axial
coordination properties of square antiprism (D
4d
), pentagonal bipyramidal (D
5h
), and “vent
metallocene” type complexes (SMM characteristics have been confirmed for C
1
symmetry) [8–10]. In
the D
4d
ligand field system, the SMM characteristics can be controlled by manipulating the ground
state via rotation of the ligands by protonation/deprotonation [11,12], coupling of
Tb
3+
-bisphthalocyaninato (Pc
2–
) complex with cadmium ions, etc. [13]. Recently, several groups have
shown the relationship between octa-coordination environments and the magnetic relaxation
processes of Tb
3+
-Pc
2–
multiple-decker SMMs along the C
4
rotation axis. From these reports,
Magnetochemistry 2019, 5, 65 2 of 13
correlations between the twist angle (φ), ligand field (LF) parameters, and the lowest ground state of
Pc2– have been clarified [14]. However, the influence of the coordination environment on the spin
relaxation phenomena is not completely understood. In the case of Dy3+ ions, the magnetic relaxation
mechanism strongly depends on the electronic structure and can sometimes be complicated. In 2003,
Ishikawa and coworkers determined that the lowest ground states (Jz) of the DyPc2 complex to be
|Jz| = 13/2 by using magnetic measurements when φ is 45° and |Jz| = 11/2 when φ is 32° [15,16]. In
our previous work, we have shown that for some complexes with two crystallographically
equivalent Dy3+ ions, the ground state is split by the Zeeman effect only if the angle is 45°, resulting
in dual relaxation processes. There have been other attempts to elucidate the identities of the dual
processes by using theoretical and experimental approaches. Thus, it is important to carefully design
the coordination environment around the Dy3+ ion in order to investigate the correlation between the
magnetic relaxation process and the ground state in detail. In other words, due to the sensitivity
toward the coordination environment, Dy3+ complexes are easier to control than Tb3+ complexes. In
this paper, in addition to the dinuclear Dy3+ triple-decker complexes we have reported so far, we
discuss the relationship between coordination environment and magnetic properties for
(TPP)Dy(Pc)Dy(TPP)] (1) and [(Pc)Dy(Pc)Dy(TPP)] (2) (Pc2– = phthalocyaninato and TPP2– =
tetraphenylporphyrinato) with C4 symmetry.
2. Results and Discussion
2.1. Synthesis and Crystal Strucures
We synthesized the compounds following reported procedures [17]. Single crystal X-ray
diffraction analyses on triple-decker complexes 1 and 2 were performed to investigate the
coordination environment around the Dy3+ ions (Table S1). For 1, which crystallized in the tetragonal
space group I4/m, the twist angle (φ) between the outer TPP2– ligand and the inner Pc ligand was
determined to be 4°. Therefore, the coordination environment around the Dy3+ ions has a slightly
distorted square-prism (SP) structure. Since the inner Pc2– ligand acts as a mirror plane in the
molecule, the two Dy3+ ions are equivalent, and the distance between them was determined to be
3.71 Å. This is a relatively long value among the dinuclear Dy3+ triple-decker complexes reported so
far. Compound 2 crystallized in the monoclinic space group C21/c, and the Dy3+ ions are
inequivalent. φ between the outer and the inner Pc2– ligands was determined to be 39°, meaning that
it has a square-antiprism (SAP) structure. φ between the inner Pc2– ligand and the TPP2– ligand was
14°, which is indicative of a highly distorted SAP structure.
Besides the φ between the ligands in the multi-decker type SMM [18], the distance between the
coordination isoindole nitrogen atom (Niso) and the metal ions (d), and the angles between the C4 axis
and the direction of Dy3+–Niso (α) affect the properties of these type of SMMs [19]. From a
comparison of 1 and 2 with previously studied Dy3+ triple-decker complexes 16 (Table 1) [20,21],
site A of 1 and 2 involving a TPP2– ligand has a small φ. Moreover, distances d2 and d3 between the
Niso of the Pc2– ligand and the metal ions, as indicated in Figure 1, are greater than those in other
complexes. 6 has a large d2 at the small φ site, and φ decreases with increases in d2 and d3. In other
words, the TPP2– ligands strongly push the Dy3+ ions to the outside of the complex.
The ground states of the Dy3+ ions can be estimated from the structure, and theoretical
calculations of electrostatic potentials distributed over α for the 4f-shells indicate that the ground
state |JZ| is highly dependent on α and d [22]. The ground states of 1 and 2 were estimated from the
data in Table 1, and |JZ| = 11/2 and 13/2 are the lowest ground states, respectively. However, the
|JZ| levels of these dinuclear Dy3+ triple-decker complexes are known to mix [19], and intermediate
|JZ| levels have the lowest basal order since α is near the magic angle of 54.7° [20,21]. Therefore, φ
determines the ground states of the Dy3+ ions.
Crystal packing diagrams for 1 and 2 are shown in Figure 2. The intermolecular Dy3+-Dy3+
distances along the c axis in 1 and 2 were determined to be 13.61 Å and 11.63 Å, respectively. The
molecules of 1 and 2 are rather well-separated from neighboring molecules due to the tetraphenyl
groups of the TPP2– ligands and chloroform molecules as crystal solvents (Table S2). Furthermore,
Magnetochemistry 2019, 5, 65 3 of 13
PXRD patterns for 1 and 2 at 293 K are slightly different from those simulated from X-ray single
crystallographic data for 1 at 120 K because of partial elimination of crystal solvents (Figure S7).
From elemental analysis, some of the crystal solvent desorbed. However, the magnetic properties of
the same sample remained unchanged after several months, meaning that solvent desorption has
little effect on the magnetic properties. The crystal system is the same regardless of whether a
solvent is present or absent, meaning that it does not affect the magnetic measurements.
(a)
(b)
Figure 1. Top and side views of (a) [(TPP)Dy(Pc)Dy(TPP)] (1) and (b) [(Pc)Dy(Pc)Dy(TPP)] (2). The
bottom of (a) and (b) are enlargements of the coordination spheres around the Dy
3+
ions. H atoms
and crystal solvents are omitted for clarity. Dy
3+
: light green, C: grey, and N: light blue.
(a)
(b)
Magnetochemistry 2019, 5, 65 4 of 13
Figure 2. Packing diagrams for (a) [(TPP)Dy(Pc)Dy(TPP)] (1) and (b) [(Pc)Dy(Pc)Dy(TPP)] (2).
Intermolecular Dy
3+
distances indicated by black dotted arrows. H atoms and crystal solvents are
omitted for clarity. Dy
3+
: light green, C: grey, and N: light blue.
Table 1. Selected crystallographic data for Dy
3+
-Pc triple-decker complexes.
Complex α
1
[°] α
2
[°] α
3
[°] α
4
[°] d
1
[Å] d
2
[Å] d
3
[Å] d
4
[Å] φ
A
[°] φ
B
[°]
[(TPP)Dy(Pc)Dy(TPP)] 1 59 46 46 59 2.39 2.6
7
2.6
7
2.39 4 4
[(Pc)Dy(Pc)Dy(TPP)] 2 57 49 45 60 2.35 2.5
7
2.72 2.3
7
39 14
[(Pc)Dy(ooPc)Dy(Pc)] 3
a
57 48 48 57 2.35 2.59 2.59 2.35 45 45
[(ohPc)Dy(ohPc)Dy(ohPc)]
4
b
57 47 47 57 2.35 2.60 2.60 2.35 27 27
[(obPc)Dy(obPc)Dy(obPc)] 5
c
57 48 48 57 2.35 2.60 2.60 2.35 32 32
[Dy
4
] 6
d
57 46 49 57 2.33 2.65 2.53 2.3
7
23 45
a
ooPc = 2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato;
b
ohPc
2–
= 2,3,9,10,16,17,23,24-octakis(hexyloxy)
phthalocyaninato;
c
obPc
2–
= 2,3,9,10,16,17,23,24-octakis(butoxy)phthalocyaninato;
d
[Dy(obPc)
2
]Dy(Fused-Pc)Dy[Dy(obPc)
2
] (Fused-Pc
4
= bis{72,82,122,132,172,182-hexabutoxytribenzo[g, l, q]-5,
10, 15, 20-tetraazaporphirino}[b, e]benzenato).
2.2. Static Magnetic Properties
The static magnetic susceptibility of 1 and 2 were measured in the T range of 1.8–300 K using a
superconducting quantum interference device (SQUID) magnetometer. The χ
M
T value at 300 K is
consistent with the value expected for the two Dy
3+
ions (
6
H
15/2
, S = 5/2, L = 5, g = 4/3). Curie–Weiss
plots for 1 and 2 are shown in Figure 3. Linear approximation was performed over the entire T range
to obtain values of the Curie (C) (28.50 cm
3
K mol
1
(1) and 28.20 cm
3
K mol
1
(2)) and Weiss constants
(θ) (–2.33 K (1) and –1.97 K (2)) (Figures S8 and S9). In χ
M
T versus T plots, the values for 1 decreased
with a decrease in T, reaching a minimum of 19.9 cm
3
K mol
–1
at 1.8 K, which indicates that magnetic
properties of the Dy
3+
complexes mainly originate from LF effects, such as thermal depopulation of
the Stark sublevels [23–25]. As for 2, there was a slight increase in the χ
M
T values when T < 4 K. Since
the intermolecular metal distance is sufficiently long [26], the increase is thought to be due to the
magnetic dipole interactions between the Dy
3+
ions in the molecule. All Dy
3+
-Pc complexes so far
reported exhibit similar ferromagnetic behavior. However, the behavior of the χ
M
T values for 1 is
different from the other Dy
3+
triple-decker complexes, and an increase in the χ
M
T values was not
observed in the low T region. Although the fact that the intermolecular distance is similar for each
complex, the difference in LF parameters of the Dy
3+
ions has a dramatic effect. In other words, the
increase in the off-diagonal terms and the change in the LF splitting energy along with the change in
symmetry are important factors affecting the magnetic behavior. Fitting of the data was performed
using the PHI program with reported LF parameters [27]. However, we could not obtain consistent
results for 1 and 2 because their ground states are complicated due to mixing of the off-diagonal
terms.
Magnetochemistry 2019, 5, 65 5 of 13
(a) (b)
Figure 3. T dependence of χ
M
T measured on powder samples of (a) 1 and (b) 2 in the T range of
1.8–300 K in an H
dc
of 0.5 kOe. The inset is a magnified plot of 1.8–30 K.
In order to confirm the magnetic anisotropy of the molecule, T dependence of MH were
performed (Figure 4). For both complexes, the magnetization did not saturate up to 70 kOe.
However, splitting of the M versus HT
1
plot occurred, indicating that not only depopulation
occurred but also both complexes had large magnetic anisotropies. In addition, butterfly-type
hysteresis was not observed during the MH measurements at 1.8 K. When uniaxial anisotropy is
strong, the saturation magnetization value (M
s
) is expressed as M
s
= 1/2 × g*(z) × 𝑆
where 𝑆
= 1/2
[28]. So, the M
s
values of 1 and 2 were M
s
= 8.6 μ
B
and M
s
= 7.4 μ
B
, calculated using |J
z
| = 13/2 (g*(x) =
g*(y) = 0, g*(z) = 17.3) and 11/2 (g*(x) = g*(y) = 0, g*(z) = 14.7), respectively. Although the measured
values are larger than the calculated values (1: 13.6 μ
B
, 2: 13.7 μ
B
), they are smaller than the effective
magnetic moments (μ
eff
) of two Dy
3+
ions (μ
eff
= 21 μ
B
).
(a)
(b)
Figure 4. Magnetic field (H) dependence of the magnetization (M) for powder samples of (a) 1 and
(b) 2 in the T range of 1.8–20 K.
2.3. Dynamic Magnetic Properties
In order to investigate the magnetic relaxation processes, alternating current (ac) magnetic
susceptibility measurements were performed on powder samples of 1 and 2. For 2, the χ
M
" values
were frequency (ν) dependent in the range of 0.1–1000 Hz in a zero magnetic field (Figures S10–13),
whereas for 1, they were not. To clarify these differences, ac magnetic susceptibility measurements
were performed in different magnetic fields at 1.8 K. When H was in the range of 0–5 kOe, the
magnetization of both complexes underwent dual relaxation processes (Figure 5). As shown in
Figure 6, the magnetic relaxation times (τ) calculated from the χ
M
" versus ν plot on the low ν side
increased monotonically with an increase in H, and the τ values calculated from the high ν side
reached a local maximum in specific applied H
dc
. These results indicate that the maximum H
dc
suppresses QTM and promotes a direct process [29], and the results can be reproduced using a
mixture of QTM, direct and Raman processes with Equation (1) (Table S3):
𝜏

𝐴𝐻
𝑇 𝐵
1𝐵
𝐻
𝐷. (1)
In previous work, we have reported that compound 3 exhibited dual magnetic relaxation
processes when H
dc
was larger than 2.5 kOe [14]. To understand the details of the dual magnetic
relaxation dynamics of 1 and 2, we analyzed the ratio of relaxation time ρ (= τ
2
/ τ
1
). The ρ values of 1
Magnetochemistry 2019, 5, 65 6 of 13
correspond to the occurrence of a single relaxation process in the Dy
3+
SMM system. If the value is
large enough (>1000), dual magnetic relaxation processes are observed separately [30]. Since the
splitting of relaxation is observed at 0.25 kOe for 1 and 2, it can be said that it responds more
sensitively to H
dc
. This is another indication that the ground states of Dy
3+
ions are more complicated.
(a) (b)
Figure 5.
ν
dependence of the real (χ
M
') and imaginary (χ
M
") parts of the ac susceptibilities of (a) 1
and (b) 2 in H
dc
of 0–5 kOe at 1.8 K. Black solid lines were fitted by using an extended Debye model to
obtain τ. Argand plots are in the supporting information (Figures S16 and S17)
(a) (b)
Figure 6. (a) τ versus H for 1 at 1.8 K obtained from the least-squares fitting using an extended Debye
model. τ from the high frequency region have the maxima each other. (b) τ versus H for 2. The data
Magnetochemistry 2019, 5, 65 7 of 13
were fitted using a mixture of as the quantum tunneling of the magnetization (QTM), direct and
Raman processes with the parameters listed in the SI.
ν dependences of the χ
M
"
values of 1 and 2 were measured in the range of 1–1000 Hz in various
H
dc
, and a split in the χ
M
"
values was observed below 8 and 20 K, respectively. In a χ
M
’’ versus T plot,
a peak top was observed in the T range below 2 K, indicating that the magnetic moment was not
frozen or that a different relaxation processes, like QTM, was dominate. We used the
Kramers–Kronig equation [31–35], which infers the pre-exponential factor τ
0
and the activation
barrier U
eff
from the χ
M
’’/χ
M
’ versus T (2.5–4 K) plot, to fit the data:
χ
M
’’/χ
M
’ = ωτ,
χ
M
’’/χ
M
’ = ωτ
0
+ exp (U
eff
/T),
ln (χ
M
’’/χ
M
’) = ln (ωτ
0
) + U
eff
/T.
(2)
(3)
(4)
From a χ
M
’’/χ
M
’ plot for 1, U
eff
was determined to be about 8.1 cm
–1
, and τ
0
10
–7
s. For 2, U
eff
was
determined to be about 2.7 cm
–1
, and τ
0
10
–6
s (Figures S14 and S15). The small τ
0
indicates, that the
contribution from an Orbach process becomes small. Figure 7 shows the relationship between U
eff
and φ [15,16]. From this figure, as φ decreases from 45°, the activation energy tends to decrease. This
is because a contribution from the off-diagonal LF terms promotes QTM, and during the conversion
from SAP to SP geometries. Thus, 𝐵
becomes smaller, and off-diagonal terms 𝐵
and 𝐵
become
larger, resulting in a narrower U
eff
. It is thought that a small φ has a negative effect on the activation
barrier. In other words, the small φ of 1 and 2 cause a decrease in the activation barrier.
Figure 7. U
eff
versus φ plots for related Dy
3+
-Pc single molecule magnets (SMMs). Blue dotted lines
are guides only. Dotted circles indicate the values for 1 and 2, and the color dots indicate 36 in H
dc
.
Since 2 and 6 have two different Dy
3+
sites, the distribution of which could not be separated
distribution, both φ values are displayed.
To investigate the magnetic relaxation properties of 1 and 2 at low T, ν dependence
measurements were performed in an applied H
dc
in the T range of 1.8–4.5 K. An Argand plot for both
complexes showed that a dual magnetic relaxation process occurred. τ for each component was
calculated by fitting the imaginary component of the ac magnetic susceptibility with the extended
Debye model (Equations S2 and S3, Figure 8), and using those values, an Arrhenius plot was
obtained (Figure 9). From a fitting with Equation (5) on the values for 1, τ obtained from the low ν
side indicates that a QTM process independent of T occurs, and that obtained from the high ν side is
proportional to T
–9
, meaning that it is a Raman process (Table S4). However, the fitting of the data for
1 is not accurate due to large deviation in the τ values. In particular, when τ values at T > 2.5 K, the
spin dynamics of 1 could not be determined. On the other hand, for 2, τ obtained from the low ν side
is proportional to T
–1.7
, and that obtained from the high ν side indicates that a direct process occurs.
For both complexes, the lowest ground state estimated from the crystal structure is expected to
Magnetochemistry 2019, 5, 65 8 of 13
contain a large amount of mixing. Therefore, even when a complex has crystallographically
equivalent Dy
3+
ions, dual relaxation processes occur between mixed states in an applied magnetic
field. In the case of 1, the D
4h
symmetry around the Dy
3+
ion has a large effect on the off-diagonal
term, causing QTM to be dominant. In addition, T is proportional to n 2 which could be
reproduced using a PB process to fit the data. However, since Raman processes can involve
acoustic-optical phonons (n = 1–6) [18], it is difficult to separate each contribution due to the
complicated ground state of 2.
(a) (b)
Figure 8. ν dependence of the out of phase (χ
M
’’) parts of the ac magnetic susceptibilities of 1 (a) and 2 (b)
measured in the T range of 1.8–4.5 K in H
dc
. Black solid lines were fitted by using an extended Debye
model to obtain τ. Argand plots are located in the supporting information (Figures S18 and S19)
(a) (b)
Magnetochemistry 2019, 5, 65 9 of 13
Figure 9. An Arrhenius plot for (a) 1 and (b) 2, for which the τ values were obtained from χM’’ versus
ν plots in Hdc of 1.3 and 2 kOe, respectively, in the T range of 1.8–4.5 K. The blue circles indicate the
τ
values from the low ν region and red circles indicate those from the high ν region. Black solid lines
were fitted by using Equation (5):
𝜏 =𝐴𝐻𝑇+𝐶𝑇+𝜏
 . (5)
3. Materials and Methods
3.1. Synthesis
Solvents were used without further purification. Dy(acac)3·4H2O and the free ligand were
purchased from TCI Tokyo Chemical Industry Co., LTD, Tokyo, Japan. Dy(acac)3·4H2O (180 mg, 0.40
mmol) and H2TTP (tetraphenylporphyrin) (150 mg, 0.25 mmol) were added to dry
1,2,4-trichlorobenzene (40 mL). The solution was refluxed under nitrogen for 4 h. After cooling, Li2Pc
(158 mg, 0.60 mmol) was added to the mixture. Then, the solution was refluxed for 12 h. After
cooling, the reaction mixture was added to n-hexane (500 mL). The obtained solid was purified by
using column chromatography on silica gel with chloroform as the eluent. [(TTP)Dy(Pc)Dy(TTP)] (1)
was obtained from a deep brownish red fraction, which was the first fraction, by removing the
solvent (16%), and [(Pc)Dy(Pc)Dy(TTP)] (2) was obtained from the dark green second fraction (34%).
Spectroscopic data used for characterization are described in the SI (Figures S5 and S6). Column
chromatography (C-200 silica gel, Wako and Sephadex G-10, Pharmacia Biotech) was used to
remove the remaining impurities. Dark red block crystals of 1 were obtained from
chloroform/n-hexane (27 mg). ESI-MS: m/z (%): 2062.47242 (100) [M–1+] (Figures S1 and S2);
elemental analysis calcd (%) for C120H72N16Dy2·4CHCl3: C 58.62, H 3.02, N 8.82; found: C 60.03, H
3.21, N 8.89. Black fine needle crystals of 2 were obtained from chloroform/n-hexane (83 mg).
ESI-MS: m/z (%): 1962.38956 (100) [M+] (Figure S3 and S4); elemental analysis calcd (%) for
C108H60N20Dy2·CHCl3: C 60.01, H 2.84, N 12.72; found: C 62.91, H 3.28, N 13.36. The results of the
elemental analysis for 2 indicates desorption of some of the CHCl3 molecules compared with the
number of CHCl3 molecules calculated from the crystal structure.
3.2. Physical Measurements
Elemental analyses were conducted on a PerkinElmer 240C elemental analyzer (PerkinElmer,
Waltham, MA, USA) at the Research and Analytical Centre for Giant Molecules, Tohoku University.
UV-Vis-NIR spectra were acquired using CHCl3 solution on a Shimadzu UV-3100pc (Shimadzu,
Kyoto, Japan). IR spectroscopy was performed on ATR method on FT/IR-4200 spectrometer at 298 K.
Magnetic susceptibility measurements were conducted on a Quantum Design SQUID magnetometer
MPMS-XL and MPMS-3 (Quantum Design, San Diego, CA, USA) in the T and dc field ranges of
1.8–300 K and ±50 kOe, respectively. AC measurements were performed in the frequency range of
0.1–1000 Hz with an ac field amplitude of 3 Oe. A polycrystalline sample suspended in n-eicosane
was used for the measurements. Crystallographic data for 1 and 2 were collected at 120 K on a
Rigaku Saturn724+ CCD Diffractometer (Rigaku, Tokyo, Japan) with graphite-monochromated Mo
Kα radiation (
λ
= 0.71075 Å) produced using a VariMax microfocus X-ray rotating anode source.
Single crystals with dimensions of 0.10 × 0.07 × 0.01 mm3 (1) and 0.15 × 0.20 × 0.04 mm3 (2) were used.
Data processing was conducted using the Crystal Clear crystallographic software package [36]. The
structures were solved by using direct methods using SIR-92 [37]. Refinement was carried out using
the Yadokari-XG package [38] and SHELXT. The non-Hydrogen atoms were refined anisotropically
using weighted full-matrix least squares on F. Hydrogen atoms attached to the carbon atoms were
fixed using idealized geometries and refined using a riding model. CCDC 1940003 for 1 and 1940004
for 2. Powder X-ray diffraction was conducted on a Bruker AXS D2 phaser (Bruker Corporation
Billerica, MA, USA).
Magnetochemistry 2019, 5, 65 10 of 13
4. Conclusions
Complexes 1 and 2 were synthesized similar to previously reported Dy3+-Pc complexes with a
TPP2– ligand. The TPP2– ligands induce a smaller twist angle (φ) between the ligands than those in
the previous complexes due to the effects of steric repulsion from the phenyl group. Although 1 has
D4h symmetry due to the two TPP2– ligands, 2 has lower symmetry due to having only one TPP2–.
From dc magnetic measurements on both complexes, the χMT values decreased due to depopulation.
Measurement of M-HT–1 indicated uniaxial anisotropy, but it is smaller than the expected values for
a pseudospin model. No hysteresis opening at 1.8 K was observed, suggesting a mixture of ground
states, which is consistent with the estimation of the ground state using the value of α obtained from
the crystal structure data. In addition, dual slow magnetization relaxation was observed for both
complexes from ac magnetic susceptibility measurements in an applied Hdc. Ueff calculated by using
the Kramers–Kronig equation is very small and corresponded to the tendency of previous
triple-decker compounds to decrease with decrease of φ. From above the results, 1 and 2 are
field-induced SMMs. For 1, QTM and Raman processes occur due to the symmetry of D4h, whereas
for 2, mixed relaxation (Raman, PB) and QTM processes occur. The contributions of the Raman and
PB processes must be clarified. The magnetic processes involve spin relaxation in mixed ground
states, and the off-diagonal term is dominant. This is different from conventional dinuclear Dy3+
complexes. Multiple relaxation processes could be turned on by adjusting φ to 4° (1) and 14° (2), and
this can be used to prepare functional SMMs whose characteristics can be switched on and off by
changing φ.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1.
Figure S1: ESI-MS spectrum of 1 in CHCl3. The peak at 2062.47242 corresponds to [M-1+].
Figure S2: Experimental (top) and simulated (bottom) ESI-MS spectra of 1 in CHCl3. The peak at 2062.47242
corresponds to [M-1+].
Figure S3: ESI-MS spectrum of 2 in CHCl3. The peak at 1962.38956 corresponds to [M+].
Figure S4: Experimental (top) and simulated (bottom) ESI-MS spectra of 2 in CHCl3. The peak at 1962.38956
corresponds to [M+].
Figure S5: IR spectrum for 1 (top) and 2 (bottom) by using an ATR method at 298 K.
Figure S6: UV-vis-NIR spectra for 1 (top) and 2 (bottom) in CHCl3 (5.1×103 (1), and 4.7×103 (2)) at 298 K.
Figure S7: PXRD patterns for 1 (top) and 2 (bottom).
Figure S8: Curie–Weiss plot for 1. Linear approximation is performed over the entire T range, from which the
values of Curie constant (C) (28.50 cm3 K mol1) and Weiss constant (θ) (–2.33 K) were obtained.
Figure S9: Curie–Weiss plot for 2. Linear approximation is performed over the entire T range, from which the
values of Curie constant (C) (28.20 cm3 K mol1) and Weiss constant (θ) (–1.97 K) were obtained.
Figure S10: Frequency (
ν
) and temperature (T) dependences of the (a) in-phase (χM') and (b) out-of-phase (χM")
ac magnetic susceptibilities of 1 in 0 kOe.
Figure S11: Frequency (
ν
) and temperature (T) dependences of the (a) in-phase (χM') and (b) out-of-phase (χM")
ac magnetic susceptibilities of 2 in 0 kOe.
Figure S12: Frequency (
ν
) and temperature (T) dependences of the (a) in-phase (χM') and (b) out-of-phase (χM")
ac magnetic susceptibilities of 1 in 1.3 kOe.
Magnetochemistry 2019, 5, 65 11 of 13
Figure S13: Frequency (
ν
) and temperature (T) dependences of the (a) in-phase (χM') and (b) out-of-phase (χM")
ac magnetic susceptibilities of 2 in 2 kOe.
Figure S14:
χ
M’’/
χ
M’ versus T (2.5–4 K) plot for 1.
Figure S15:
χ
M’’/
χ
M’ versus T (2.5–4 K) plot for 2.
Figure S16: Argand plots (χM" versus χM') for 1 at 1.8 K in several dc magnetic fields (0-5 kOe). Black solid lines
were guides for eye.
Figure S17: Argand plots (
χ
M" versus
χ
M') for 2 at 1.8 K in several dc magnetic fields in the range of 0–5 kOe.
Black solid lines were guides for eye.
Figure S18: Argand plots (
χ
M" versus
χ
M') for 1 in 1.3 kOe in the T range of 1.8–4.5 K. Black solid lines are guides
for the eye.
Figure S19: Argand plots (
χ
M" versus
χ
M') for 1 in 2 kOe field in the T range of 1.8–4.5 K. Black solid lines are
guides for the eye.
Table S1: Selected crystallographic data for 1 and 2.
Table S2: Selected crystallographic data for 1 and 2.
Table S3: Parameters for fitting the τ verses H plots.
Table S4: Parameters of fitting for τ verses T plot.
Author Contributions: Conceptualization, K.K.; Data curation, K.K.; Formal analysis, T.S. and S.M.; Funding
acquisition, K.K. and M.Y.; Investigation, T.S. and S.M.; Methodology, K.K.; Project administration, K.K. and
M.Y.; Supervision, K.K.; Validation, K.K.; Visualization, B.K.B.; Writing – original draft, T.S.; Writing – review &
editing, K.K., B.K.B and M.Y..
Funding: This work was supported by a Grant-in-Aid for Scientific Research (S) (grant no. 20225003),
Grant-in-Aid for Young Scientists (B) (grant no. 24750119), Grant-in-Aid for Scientific Research (C) (grant no.
15K05467) from the Ministry of Education, Culture, Sports, Science, Technology, Japan (MEXT), CREST
(JPMJCR12L3) from JST, a Grant-in-aid for JSPS fellows from the Japan Society for the Promotion of Science
(JSPS) (25·2441), and Tohoku University Division for International Advanced Research and Education (DIARE).
M.Y. is grateful for the support from the 111 project (B18030) from China. S.M. acknowledges the support by
MD-program of Tohoku Univ.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Sessoli, R.; Gatteschi, D.; Caneschi, A. Magnetic bistability in a metal-ion cluster. Nature 1993, 363, 141–143.
2. Wernsdorfer, W.; Sessoli, R.Quantum Phase Interference and Parity Effects in Magnetic Molecular
Clusters. Science 1999, 284, 133–136.
3. Choi, K.Y.; Wang, Z.; Nojiri, H.; Van Tol, J.; Kumar, P.; Lemmens, P.; Bassil, B.S.; Kortz, U.; Dalal, N.S.
Coherent manipulation of electron spins in the (Cu3) spin triangle complex impregnated in nanoporous
silicon. Phys. Rev. Lett. 2012, 108, 1–5.
4. Vincent, R.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Balestro, F. Electronic read-out of a single nuclear
spin using a molecular spin transistor. Nature 2012, 488, 357–360.
5. Collauto, A.; Mannini, M.; Sorace, L.; Barbon, A.; Brustolon, M.; Gatteschi, D. A slow relaxing species for
molecular spin devices: EPR characterization of static and dynamic magnetic properties of a nitronyl
nitroxide radical. J. Mater. Chem. 2012, 22, 22272–22281.
6. Wedge, C.J.; Timco, G.A.; Spielberg, E.T.; George, R.E.; Tuna, F.; Rigby, S.; McInnes, E.J.L.; Winpenny,
R.E.P.; Blundell, S.J.; Ardavan, A. Chemical engineering of molecular qubits. Phys. Rev. Lett. 2012, 108, 1–5.
Magnetochemistry 2019, 5, 65 12 of 13
7. Aguilà, D.; Barrios, L.A.; Velasco, V.; Roubeau, O.; Repollés, A.; Alonso, P.J.; Sesé, J.; Teat, S.J.; Luis, F.;
Aromí, G. Heterodimetallic [LnLn’] lanthanide complexes: Toward a chemical design of two-qubit
molecular spin quantum gates. J. Am. Chem. Soc. 2014, 136, 14215–14222.
8. Long, J.; Selikhov, A.N.; Mamontova, E.; Lyssenko, K.A.; Guari, Y.; Larionova, J.; Trifonov, A.A.
Single-molecule magnet behaviour in a Dy(iii) pentagonal bipyramidal complex with a quasi-linear
Cl-Dy-Cl sequence. Dalt. Trans. 2019, 48, 35–39.
9. Guo, F.S.; Day, B.M.; Chen, Y.C.; Tong, M.L.; Mansikkamäki, A.; Layfield, R.A. A Dysprosium Metallocene
Single-Molecule Magnet Functioning at the Axial Limit. Angew. Chem. Int. Ed. 2017, 56, 11445–11449.
10. Day, B.M.; Guo, F.S.; Layfield, R.A. Cyclopentadienyl Ligands in Lanthanide Single-Molecule Magnets:
One Ring to Rule Them All? Acc. Chem. Res. 2018, 51, 1880–1889.
11. Tanaka, D.; Inose, T.; Tanaka, H.; Lee, S.; Ishikawa, N.; Ogawa, T. Proton-induced switching of the single
molecule magnetic properties of a porphyrin based TbIII double-decker complex. Chem. Commun. 2012, 48,
7796–7798.
12. Horii, Y.; Horie, Y.; Katoh, K.; Breedlove, B.K.; Yamashita, M. Changing Single-Molecule Magnet
Properties of a Windmill-Like Distorted Terbium(III) α-Butoxy-Substituted Phthalocyaninato
Double-Decker Complex by Protonation/Deprotonation. Inorg. Chem. 2018, 57, 565–574.
13. Horii, Y.; Katoh, K.; Yasuda, N.; Breedlove, B.K.; Yamashita, M. Effects of f-f interactions on the
single-molecule magnet properties of terbium(iii)-phthalocyaninato quintuple-decker complexes. Inorg.
Chem. 2015, 54, 3297–3305.
14. Katoh, K.; Aizawa, Y.; Morita, T.; Breedlove, B.K.; Yamashita, M. Elucidation of Dual Magnetic Relaxation
Processes in Dinuclear Dysprosium(III) Phthalocyaninato Triple-Decker Single-Molecule Magnets
Depending on the Octacoordination Geometry. Chem. A Eur. J. 2017, 23, 15377–15386.
15. Ishikawa, N.; Sugita, M.; Okubo, T.; Tanaka, N.; Iino, T.; Kaizu, Y. Determination of ligand-field
parameters and f-electronic structures of double-decker bis(phthalocyaninato)lanthanide complexes.
Inorg. Chem. 2003, 42, 2440–2446.
16. Ishikawa, N.; Iino, T.; Kaizu, Y. Interaction between f-electronic systems in dinuclear lanthanide
complexes with phthalocyanines. J. Am. Chem. Soc. 2002, 124, 11440–11447.
17. Tran-Thi, T.H.; Mattioli, T.A.; Chabach, D.; Cian, A.D.; Weiss, R. Hole localization or delocalization? An
optical, raman, and redox study of lanthanide porphyrin-phthalocyanine sandwich-type
heterocomplexes. J. Phys. Chem. 1994, 98, 8279–8288.
18. Zhang, P.; Guo, Y.N.; Tang, J. Recent advances in dysprosium-based single molecule magnets: Structural
overview and synthetic strategies. Coord. Chem. Rev. 2013, 257, 1728–1763.
19. Sorace, L.; Benelli, C.; Gatteschi, D. Lanthanides in molecular magnetism: Old tools in a new field. Chem.
Soc. Rev. 2011, 40, 3092–3104.
20. Katoh, K.; Morita, T.; Yasuda, N.; Wernsdorfer, W.; Kitagawa, Y.; Breedlove, B.K.; Yamashita, M.
Tetranuclear Dysprosium(III) Quintuple-Decker Single-Molecule Magnet Prepared Using a π-Extended
Phthalocyaninato Ligand with Two Coordination Sites. Chem. A Eur. J. 2018, 24, 15522–15528.
21. Katoh, K.; Horii, Y.; Yasuda, N.; Wernsdorfer, W.; Toriumi, K.; Breedlove, B.K.; Yamashita, M.
Multiple-decker phthalocyaninato dinuclear lanthanoid(iii) single-molecule magnets with dual-magnetic
relaxation processes. Dalt. Trans. 2012, 41, 13582–13600.
22. Liu, J.L.; Chen, Y.C.; Tong, M.L. Symmetry strategies for high performance lanthanide-based
single-molecule magnets. Chem. Soc. Rev. 2018, 47, 2431–2453.
23. Bi, Y.; Guo, Y.N.; Zhao, L.; Guo, Y.; Lin, S.Y.; Jiang, S.D.; Tang, J.; Wang, B.W.; Gao, S. Capping ligand
perturbed slow magnetic relaxation in dysprosium single-ion magnets. Chem. A Eur. J. 2011, 17,
12476–12481.
24. Damjanovic, M.; Katoh, K.; Yamashita, M.; Enders, M. Combined NMR analysis of huge residual dipolar
couplings and pseudocontact shifts in terbium(III)-phthalocyaninato single molecule magnets. J. Am.
Chem. Soc. 2013, 135, 14349–14358.
25. Zou, L.; Zhao, L.; Chen, P.; Guo, Y.N.; Guo, Y.; Li, Y.H.; Tang, J. Phenoxido and alkoxido-bridged
dinuclear dysprosium complexes showing single-molecule magnet behaviour. Dalt. Trans. 2012, 41,
2966–2971.
26. Katoh, K.; Breedlove, B.K.; Yamashita, M. Symmetry of octa-coordination environment has a substantial
influence on dinuclear TbIII triple-decker single-molecule magnets. Chem. Sci. 2016, 7, 4329–4340.
Magnetochemistry 2019, 5, 65 13 of 13
27. Chilton, N.F.; Anderson, R.P.; Turner, L.D.; Soncini, A.; Murray, K.S. PHI: A powerful new program for
the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and f-block complexes. J.
Comput. Chem. 2013, 34, 1164–1175.
28. Moreno Pineda, E.; Chilton, N.F.; Marx, R.; Dörfel, M.; Sells, D.O.; Neugebauer, P.; Jiang, S.D.; Collison, D.;
Van Slageren, J.; McInnes, E.J.L.; et al. Direct measurement of dysprosium(III)dysprosium(III)
interactions in a single-molecule magnet. Nat. Commun. 2014, 5, 1–7.
29. Ding, Y.S.; Yu, K.X.; Reta, D.; Ortu, F.; Winpenny, R.E.P.; Zheng, Y.Z.; Chilton, N.F. Field- and
temperature-dependent quantum tunnelling of the magnetisation in a large barrier single-molecule
magnet. Nat. Commun. 2018, 9, 1–10.
30. Guo, Y.N.; Xu, G.F.; Guo, Y.; Tang, J. Relaxation dynamics of dysprosium(iii) single molecule magnets.
Dalt. Trans. 2011, 40, 9953–9963.
31. Gass, I.A.; Moubaraki, B.; Langley, S.K.; Batten, S.R.; Murray, K.S. A π-π 3D network of tetranuclear
μ2/μ3- carbonato Dy(iii) bis-pyrazolylpyridine clusters showing single molecule magnetism features.
Chem. Commun. 2012, 48, 2089–2091.
32. Ferrando-Soria, J.; Cangussu, D.; Eslava, M.; Journaux, Y.; Lescouëzec, R.; Julve, M.; Lloret, F.; Pasán, J.;
Ruiz-Pérez, C.; Lhotel, E.; et al. Rational enantioselective design of chiral heterobimetallic single-chain
magnets: Synthesis, crystal structures and magnetic properties of oxamato-bridged MIICuII chains (M =
Mn, Co). Chem. A Eur. J. 2011, 17, 12482–12494.
33. Bartolomé, J.; Filoti, G.; Kuncser, V.; Schinteie, G.; Mereacre, V.; Anson, C.E.; Powell, A.K.; Prodius, D.;
Turta, C. Magnetostructural correlations in the tetranuclear series of {Fe3 LnO2} butterfly core clusters:
Magnetic and Mössbauer spectroscopic study. Phys. Rev. B Condens. Matter Mater. Phys. 2009, 80, 1–16.
34. Luis, F.; Bartolomé, J.; Fernández, J.; Tejada, J.; Hernández, J.; Zhang, X. Thermally activated and
field-tuned tunneling inAc studied by ac magnetic susceptibility. Phys. Rev. B Condens. Matter Mater. Phys.
1997, 55, 11448–11456.
35. Nakanishi, R.; Yatoo, M.A.; Katoh, K.; Breedlove, B.K.; Yamashita, M. Dysprosium Acetylacetonato
Single-molecule magnet encapsulated in Carbon Nanotubes. Materials (Basel) 2017, 10, 7.
36. Rigaku Corporation. Crystal Clear-SM, 1.4.0 SP1; Rigaku Corporation: Tokyo, Japan, 17 April 2008.
37. Altomare, A.; Burla, M.C.; Camalli, M.; Cascarano, G.L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A.G.G.;
Polidori, G.; Spagna, R. SIR97: A new tool for crystal structure determination and refinement. J. Appl.
Crystallogr. 1999, 32, 115–119.
38. Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E.J.; Wakita, K. Yadokari-XG, Software for Crystal Structure
Analyses, 2001; Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses. Cryst. Soc. Jpn.
2010, 51, 218–224.
© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... The d value can be described as a separation between N 4 centroids of tetrapyrrolic ligands, and it dictates the ligand field strength which in turn determines the height of magnetization relaxation barrier [8]. The θ value defies the symmetry of lanthanide ion coordination surrounding and affects the magnetization relaxation mechanisms [9,10]. While the distance d is clearly governed mainly by the size of the bridging metal center [11] and the overall redox-state of the sandwich complex [12], the skew angle θ reveals trickier dependence on structural [13], electronic [14] and supramolecular factors [15]. ...
... The main outcome of our work is the establishment and interpretation of spectroscopic signatures that can be used to determine the conformational states of trisphthalocyaninates in solution in visible and near-IR ranges. The influence of skew angles of phthalocyanine ligands in such complexes on their physical-chemical properties, including single-molecule magnetism [9,10] or nonlinear optical behaviour [70], justifies the value of these correlations as solutions are commonly used to produce Pc-bases materials. On the other hand, our work highlights the need to critically evaluate the results of quantum chemical calculations of nonrigid molecules, where combining spectroscopic data and appropriate theoretical models is particularly important. ...
Article
Full-text available
Double- and triple-decker lanthanide phthalocyaninates exhibit unique physical-chemical properties, particularly single-molecule magnetism. Among other factors, the magnetic properties of these sandwiches depend on their conformational state, which is determined via the skew angle of the phthalocyanine ligands. Thus, in the present work we report the comprehensive conformational study of substituted terbium(III) and yttrium(III) trisphthalocyaninates in solution depending on the substituents at the periphery of molecules, redox-states and nature of solvents. Conjunction of UV-vis-NIR spectroscopy and quantum-chemical calculations within simplified time-dependent DFT in Tamm–Dancoff approximation provided the spectroscopic signatures of staggered and gauche conformations of trisphthalocyaninates. Altogether, it allowed us to demonstrate that the butoxy-substituted complex behaves as a molecular switcher with controllable conformational state, while the crown-substituted triple-decker complex maintains a staggered conformation regardless of external factors. The analysis of noncovalent interactions within the reduced density gradient approach allowed to shed light on the nature of factors stabilizing certain conformers.
... Gao et al. prepared sandwich-like complexes based on phthalocyanine molecules and a closed-macrocyclic Schiff base of the form [(Pc)2Ln3(L)(OAc)(OCH3)2] (with Ln 3+ = Dy 3+ or Er 3+ , H2Pc = phthalocyanine and H2L = closed-macrocyclic Schiff base molecules), and found a single-molecule magnetic behavior in the complex containing dysprosium [88]. In these sandwich structures, the strong coupling between the lanthanide ions resulted in an important role for the rare earth atom in the SMM properties [89], making these multi-decker structures highly interesting for diverse applications [90][91][92]. ...
... Gao et al. prepared sandwich-like complexes based on phthalocyanine molecules and a closed-macrocyclic Schiff base of the form [(Pc) 2 Ln 3 (L)(OAc)(OCH 3 ) 2 ] (with Ln 3+ = Dy 3+ or Er 3+ , H 2 Pc = phthalocyanine and H 2 L = closed-macrocyclic Schiff base molecules), and found a single-molecule magnetic behavior in the complex containing dysprosium [88]. In these sandwich structures, the strong coupling between the lanthanide ions resulted in an important role for the rare earth atom in the SMM properties [89], making these multi-decker structures highly interesting for diverse applications [90][91][92]. ...
Article
Full-text available
Molecular magnets are a relatively new class of purely organic or metallo-organic materials, showing magnetism even without an external magnetic field. This interdisciplinary field between chemistry and physics has been gaining increased interest since the 1990s. While bulk molecular magnets are usually hard to build because of their molecular structures, low-dimensional molecular magnets are often easier to construct, down to dot-like (zero-dimensional) structures, which are investigated by different scanning probe technologies. On these scales, new effects such as superparamagnetic behavior or coherent switching during magnetization reversal can be recognized. Here, we give an overview of the recent advances in molecular nanomagnets, starting with single-molecule magnets (0D), typically based on Mn12, Fe8, or Mn4, going further to single-chain magnets (1D) and finally higher-dimensional molecular nanomagnets. This review does not aim to give a comprehensive overview of all research fields dealing with molecular nanomagnets, but instead aims at pointing out diverse possible materials and effects in order to stimulate new research in this broad field of nanomagnetism.
... properties. Keiichi Katoh, Masahiro Yamashita, and co-workers investigated the relationship between the coordination geometry and magnetic relaxation phenomena for dinuclear Dy complexes [9]. These results demonstrate that precise control of the coordination environment enables control of the magnetic relaxation properties. ...
Article
Full-text available
Research on molecule-based magnetic materials was systematized in the 1980s and expanded rapidly [...]
Article
Full-text available
The unique properties of natural tetrapyrrolic compounds have inspired the rapid growth of research interest in the design and synthesis of artificial porphyrinoids and their metal complexes as a basis of modern functional materials. A special role in the design of such materials is played by sandwich complexes formed by tetrapyrrolic macrocycles with rare earth elements, especially lanthanides. The development of synthetic approaches to the functionalization of tetrapyrrolic compounds and their rare earth complexes has facilitated the intensive development of new applications over the last decade. As a way of expanding the functionalities of rare earth complexes, sophisticated examples have been obtained, including mixed-ligand complexes, π-extended analogues, covalently linked and fused sandwiches, complexes with less-common tetrapyrrols, sandwiches with non-tetrapyrrolic macrocycles and even complexes containing up to six stacked ligands. This review intends to offer a general overview of the preparation of such sophisticated REE tetrapyrrolic sandwiches over the last decade as well as emphasizes the current challenges and perspectives of their application in areas such as single-molecule magnetism (SMM), organic field-effect transistors (OFET), conductive materials and nonlinear optics (NLO).
Article
Full-text available
Understanding quantum tunnelling of the magnetisation (QTM) in single-molecule magnets (SMMs) is crucial for improving performance and achieving molecule-based information storage above liquid nitrogen temperatures. Here, through a field- and temperature-dependent study of the magnetisation dynamics of [Dy(tBuO)Cl(THF)5][BPh4]·2THF, we elucidate the different relaxation processes: field-independent Orbach and Raman mechanisms dominate at high temperatures, a single-phonon direct process dominates at low temperatures and fields >1 kOe, and a field- and temperature-dependent QTM process operates near zero field. Accounting for the exponential temperature dependence of the phonon collision rate in the QTM process, we model the magnetisation dynamics over 11 orders of magnitude and find a QTM tunnelling gap on the order of 10-4 to 10-5 cm-1. We show that removal of Dy nuclear spins does not suppress QTM, and argue that while internal dipolar fields and hyperfine coupling support QTM, it is the dynamic crystal field that drives efficient QTM.
Article
Full-text available
Toward promising candidates of quantum information processing, the rapid development of lanthanide-based single-molecule magnets (Ln-SMMs) highlights design strategies in consideration of the local symmetry of lanthanide ions. In this review, crystal-field theory is employed to demonstrate the electronic structures according to the semiquantitative electrostatic model. Then, specific symmetry elements are analysed for the elimination of transverse crystal fields and quantum tunnelling of magnetization (QTM). In this way, high-performance Ln-SMMs can be designed to enable extremely slow relaxation of magnetization, namely magnetic blocking; however, their practical magnetic characterization becomes increasingly challenging. Therefore, we will attempt to interpret the experimental behaviours and clarify some issues in detail. Finally, representative Ln-SMMs with specific local symmetries are summarized in combination with the discussion on the symmetry strategies, and some of the underlying questions are put forward.
Article
Full-text available
Structures and single-molecule magnet (SMM) properties of protonated/deprotonated terbium(III) phthalocyaninato double-decker complexes were investigated. Structural analysis revealed that protonated double-decker complex (1H) adopts unsymmetrical structure, whereas deprotonated one (1⁻) shows symmetrical windmill-like distortion. Detailed magnetic analysis revealed that SMM properties of 1⁻ are superior as compared to those of 1H. These results demonstrated the manipulation of SMM properties through protonation/deprotonation.
Article
Full-text available
Dy single-molecule magnets (SMMs), which have several potential uses in a variety of applications, such as quantum computing, were encapsulated in multi-walled carbon nanotubes (MWCNTs) by using a capillary method. Encapsulation was confirmed by using transmission electron microscopy (TEM). In alternating current magnetic measurements, the magnetic susceptibilities of the Dy acetylacetonato complexes showed clear frequency dependence even inside the MWCNTs, meaning that this hybrid can be used as magnetic materials in devices.
Article
We report the synthesis and magnetic investigations of a dysprosium pentagonal bipyramidal complex [Dy(THF)5Cl2][BPh4] (1) exhibiting a linear Cl-Dy-Cl sequence suitable to provide a coordination environment allowing a zero-field slow...
Article
We report the magnetic properties and spin relaxation processes of a tetranuclear dysprosium(III) fused phthalocyaninato quintuple‐decker single‐molecule magnet (1) with non‐equivalent octacoordination geometries. The structure of 1 is regarded as a dimer of Dy3+‐Pc triple‐decker SMMs with different magnetic relaxation characteristics corresponding to the octacoordination geometry sites Dy1 with C4 symmetry (φ1 = 23°) and Dy2 with D4d symmetry (φ2 = 45°). In an Hdc of 1750 Oe and T range of 1.8–3.75 K, the quantum tunnelling of the magnetization was suppressed, and the direct process was enhanced. We examine the effects of the coordination geometry on the spin relaxation phenomena.
Article
The discovery of materials capable of storing magnetic information at the level of single molecules and even single atoms has fueled renewed interest in the slow magnetic relaxation properties of single-molecule magnets (SMMs). The lanthanide elements, especially dysprosium, continue to play a pivotal role in the development of potential nanoscale applications of SMMs, including, for example, in molecular spintronics and quantum computing. Aside from their fundamentally fascinating physics, the realization of functional materials based on SMMs requires significant scientific and technical challenges to be overcome. In particular, extremely low temperatures are needed to observe slow magnetic relaxation, and while many SMMs possess a measurable energy barrier to reversal of the magnetization (Ueff), very few such materials display the important properties of magnetic hysteresis with remanence and coercivity.
Article
The magnetic relaxation processes of dinuclear DyIII-Pc triple-decker complexes (1–3) with the same distance between the two DyIII ions are quite different. Therefore, we clarified the relationship among the coordination environment, the ground state, and the magnetic relaxation processes. In the Zeeman diagram for 1 with D4d symmetry, levels intersect in an Hdc of 2500 Oe, and dual magnetic relaxation processes occur. However, in the case of the dinuclear DyIII-Pc systems (2 and 3) with C4 geometry, which have a twist angle (ϕ) of less than 45°, dual magnetic relaxation processes do not occur. More information can be found in the Full Paper by K. Katoh et al. (DOI: 10.1002/chem.201703945).
Article
When applying single-molecule magnets (SMMs) to spintronic devices, control of the quantum tunneling of the magnetization (QTM) as well as a spin-lattice interactions are important. Attempts have been made to use not only coordination geometry but also magnetic interactions between SMMs as an exchange bias. In this manuscript, we report that dinuclear dysprosium(III) (DyIII) SMMs with the same octacoordination geometry undergo dual magnetic relaxation processes at low temperature. In the dinuclear DyIII phthalocyaninato (Pc2−) triple-decker type complex [(Pc)Dy(ooPc)Dy(Pc)] (1) (ooPc2− = 2,3,9,10,16,17,23,24-octakis(octyloxy)-19H,31H-phthalocyaninato) with a square-antiprismatic (SAP) geometry, the ground state is divided by the Zeeman effect, and level intersection occurs when a magnetic field is applied. Due to the ground state properties of 1, since the Zeeman diagram where the levels intersect in an Hdc of 2500 Oe, two kinds of QTM and direct processes occur. However, dinuclear DyIII-Pc systems with C4 geometry, which has a twist angle (φ) of less than 45°do not undergo dual magnetic relaxation processes. From magnetic field and temperature dependences, the dual magnetic relaxation processes were clarified.
Article
Abstraction of a chloride ligand from the dysprosium metallocene [(Cpttt)2DyCl] (1Dy Cpttt = 1,2,4-tri(tert-butyl)cyclopentadienide) by the triethylsilylium cation produces the first base-free rare-earth metallocenium cation [(Cpttt)2Dy]+ (2Dy) as a salt of the non-coordinating [B(C6F5)4]- anion. Magnetic measurements reveal that [2Dy][B(C6F5)4] is an SMM with a record anisotropy barrier up to 1277 cm-1 (1837 K) in zero field and a record magnetic blocking temperature of 60 K, including hysteresis with coercivity. The exceptional magnetic axiality of 2Dy is further highlighted by computational studies, which reveal this system to be the first lanthanide SMM in which all low-lying Kramers doublets correspond to a well-defined MJ value, with no significant mixing even in the higher doublets.