A liquid-crystalline single-molecule magnet with variable magnetic properties

Article abstract of DOI:10.1002/anie.200905007

A terbium double-decker phthalocyanine complex has been prepared and characterized. The mesomorphism of the liquidcrystalline complex is used as a tool to reversibly adjust its magnetic properties, thus allowing the coexistence of two different relaxation processes, which can be reversibly modified by simple thermal treatments, to be observed.

Full text of DOI:10.1002/anie.200905007

Angewandte
Chemie
DOI: 10.1002/anie.200905007
Single-Molecule Magnets
A Liquid-Crystalline Single-Molecule Magnet with Variable
Magnetic Properties**
Mathieu Gonidec, Fernando Luis, ꢀlex Vꢁlchez, Jordi Esquena, David B. Amabilino,* and
Jaume Veciana*
Dedicated to Jean-Pierre Sauvage on the occasion of his 65th birthday
Single-molecule magnets (SMMs) are attractive because they
present magnetic bistability of each isolated molecule,[1] thus
enabling the discovery of a wide variety of intriguing
phenomena and the possibility of preparing multifunctional
molecular nanosystems.[2] The so-called double-decker lan-
thanide complexes of phthalocyanines, in which a lanthanide
ion with a large total angular momentum is coordinated
between two parallel phthalocyanine derivatives, are partic-
ularly attractive because of their relatively high blocking
temperatures.[3–5]
For this reason, we prepared compound 1, a chiral
derivative of the double-decker terbium complex. It behaves
both as a liquid crystal at room temperature and as a single-
Our interest in these systems arose from the possibility of
using organic synthesis to functionalize the SMM core. In
particular, we have been engaged in a program aimed at
preparing chiral SMMs which might show interesting chirop-
tical phenomena,[6,7] such as magnetochiral dichroism
(MChD), which could be useful for data storage and
processing.[8] To facilitate processing of SMMs and related
materials, the incorporation of long alkyl chains is advanta-
geous, and we were particularly attracted by liquid-crystalline
phases, as previously carried out with a Mn12 SMM[9] and spin-
crossover iron(II) compounds.[10]
molecule magnet at low temperatures. Importantly, when the
material is cooled at different rates, the magnetic properties
vary because of different degrees of supramolecular order.
Herein we describe the preparation and characterization of
this liquid-crystalline terbium double-decker phthalocyanine
complex and how its mesomorphic properties can be used as a
tool to adjust its magnetic properties reversibly at will. The
terbium ion is used as a direct and very sensitive probe of the
structural changes occurring at low temperatures, at which
magnetic relaxation is dominated by direct tunneling tran-
sitions.
[*] M. Gonidec, Prof. D. B. Amabilino, Prof. J. Veciana
Departament de Nanociꢀncia Molecular i Materials Orgꢁnics,
Institut de Ciꢀncia de Materials de Barcelona, Consejo Superior de
Investigaciones Cientꢂficas (CSIC), Networking Research Center on
Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN)
Campus de la U.A.B., 08193 Bellaterra (Spain)
Fax: (+34)93-580-5729
E-mail: amabilino@icmab.es
vecianaj@icmab.es
Homepage: http://www.icmab.es/nmmo/
The chiral metal-free (free-base) phthalocyanine 5, bear-
ing eight identical stereocenters, was prepared in three steps
from 4,5-dibromocatechol (Scheme 1) and the chiral bro-
moalkyl compound 2 derived from methyl lactate by a known
procedure.[11]
Dr. F. Luis
Instituto de Ciencia de Materiales de Aragꢃn, CSIC-Universidad de
Zaragoza and Departamento de Fꢂsica de la Materia Condensada,
Universidad de Zaragoza
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
ꢄ. Vꢂlchez, Dr. J. Esquena
Institut de Quꢂmica Avanꢅada de Catalunya, CSIC,
Barcelona (Spain)
Compound 3 was converted into the bis(cyano) derivative
4 using zinc(II) cyanide,[12] and the purified compound was
cyclized in n-hexanol using lithium metal to give 5. The
corresponding terbium double-decker complex 1 was pre-
pared by reacting 5 with anhydrous terbium chloride and
lithium bis(trimethylsilyl)amide with minor changes from a
published method.[13] The new compound was thoroughly
purified by column chromatography, and was characterized
by MALDI-TOF mass spectrometry, IR, UV/Vis absorption
spectroscopy, circular dichroism spectroscopy, and elemental
[**] We warmly thank Amable Bernabꢆ for recording the LDI-TOF MS
and IR spectra. This work was supported by the Marie Curie EST
FuMaSSEC, EU NoE MAGMANet (515767-2), EMOCIONA
(CTQ2006-06333/BQU), Grup de Recerca de Catalunya (2009 SGR-
516) and Molecular Nanoscience (CSD2007-00010) and Molchip
(MAT2009-13977-C03) projects
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905007.
Angew. Chem. Int. Ed. 2010, 49, 1623 –1626
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1623

Communications
small angle region (0–88) at 298 K, with d spacing ratios of
pffiffi
1: 3:2, thus allowing unambiguous identification of a hex-
agonal lattice. The observed reflections, obeying (h2 + hk +
k2)1/2 = d/dhkl, were labeled (100), (110), and (200). This
observation confirms the hexagonal columnar mesophase,
which is qualitatively consistent with previously reported
mesomorphic double-decker compounds of cerium[15–17] and
various lanthanides.[18,19]
The phase behavior of the free-base phthalocyanine 5 is
found to be qualitatively similar to the behavior of 1. It also
presents a hexagonal columnar mesophase, as evidenced by
POM and SAXS (see the Supporting Information). Never-
theless, the clearing point is much lower in complex 1 than in 5
(304 K compared with 415 K), as occurs in analogous
systems.[18–20]
The properties of single-molecule magnets can be very
sensitive to the structural changes in the solid state. Differ-
ences of magnetic behavior have been seen in dodecamanga-
nese complexes and attributed to Jahn–Teller isomerism in
the coordination sphere of the manganese ion.[21] It has also
been shown that the degree of dipolar interactions and also
changes in the matrix arrangements around the molecular
core of double-decker lanthanide complexes can influence the
magnetic behavior of these compounds.[22,23] For this reason,
the mesomorphism of 1 can be seen as a tool to tune the
superstructure and probe the robustness of its magnetic
properties in a variety of discrete structural situations. As the
SMM behavior and magnetic ordering occur at low temper-
atures, the sample can be kinetically trapped in a given
structural state, and its magnetic properties measured at low
temperature. Using suitable rates of cooling, a structurally
disordered sample 1dis, an intermediate partially ordered
sample 1po, or an ordered crystalline state 1cr can be prepared
from the same specimen.
Scheme 1. Synthesis of 1 from 4,5-dibromocatechol and (S)-1-bromo-
2-dodecyloxypropane (RBr; 2).
Alternating current magnetic susceptibility measurements
of 1dis and 1cr as a function of temperature and frequency of
the oscillating magnetic field were carried out with a SQUID
magnetometer. The disordered sample 1dis was prepared by
warming up the material to the isotropic phase ex-situ at
333 K for 2 minutes and quenching at 150 K inside the
susceptometer. The ordered sample 1cr was prepared entirely
in situ by warming up the same sample again and cooling it
down slowly at a controlled rate. By doing so, any change in
composition is ruled out, and the only difference from one
sample to the other being the structure that is reached in the
frozen state.
The temperature dependence of the in-phase (cM’) and
out-of-phase (cM’’) susceptibilities measured at several fre-
quencies of the two samples (see Supplementary Informa-
tion) proved to be very similar to previously reported neutral
double-decker complexes of terbium.[24] The cM’T product of
the ordered and disordered samples converge above 54 K and
for all the studied frequencies (0.5–1488 Hz) to approximately
the same value (9.2–9.6 emumolꢀ1). Similarly, the cM’’(T)
curves of 1dis and 1cr show qualitatively the same behavior
under the above-mentioned conditions. Having a closer look
at the data however, the appearance of a second drop can be
observed in the cM’(T) curves corresponding to the disordered
sample, which becomes more evident below 15 Hz. The peak
analysis. The absorption spectrum of 1 shows the character-
istic bands of neutral double-decker complexes,[14] and the
optical activity of the compound was shown by circular
dichroism spectroscopy, with Cotton effects at 668, 610, 455,
and 368 nm (see the Supporting Information).
Polarized optical microscopy (POM) shows that com-
pound 1 is mesomorphic at room temperature, with a clearing
point at 304 K and an optical texture that suggests a columnar
(Col) mesophase (see the Supporting Information). Upon
cooling from the isotropic (I) liquid, some hexagonal den-
dritic growth could be observed in the optical texture of 1
under uncrossed polarizing filters at 298 K. The observation
of this fan-like texture suggests a homeotropic alignment on
glass of a columnar hexagonal (Colh) mesophase.[15–18]
Differential scanning calorimetry (DSC) shows only one
endothermic peak (see the Supporting Information) at 261 K
corresponding to the Cr!Colh transition with DH =
86.1 kJmolꢀ1 and DS = 340.7 JKꢀ1 molꢀ1, but does not show
any peak near 304 K corresponding to the clearing point
observed by POM, presumably because of the small enthalpy
difference between the two phases.
Small-angle X-ray scattering (SAXS) was used to confirm
the nature of the observed mesophase. Only three reflections
(see the Supporting Information) could be observed in the
1624
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 1623 –1626

Angewandte
Chemie
in cM’’(T) also appears to be shifted up by about 2–3 K and the
divergence at low temperature is notably higher for the
disordered sample 1dis with respect to the ordered one 1cr (see
the Supporting Information).
The difference between the magnetization dynamics of 1cr
and 1dis samples shows up more clearly in the frequency-
dependent plots of the magnetic susceptibility (Figure 1).
sample 1po dropped even lower than in the disordered sample
dis to a value of only 32%. The reversibility of the process was
1
checked by submitting the sample to the same thermal
treatments after several months. The new data proved to be
identical to the first set (see the Supporting Information).
A deeper insight into the nature of the two species, and
thus also on the influence of the thermal history, is provided
by the relaxation times t1 and t2, which are plotted in Figure 2
as a function of the reciprocal temperature.
*
Figure 1. cM’’(u) for the sample 1dis (&) and 1cr ( ) at 258C. The solid
lines are best fits to a Cole–Cole model.
Although, the former sample shows mainly one peak
centered at 0.3 Hz in the cM’’ versus u plot at 25 K, the
disordered sample clearly shows two peaks, which are
centered at 0.3 and 40 Hz. This result is evidence of the
coexistence of two different magnetic relaxation processes,
meaning that the sample behaves magnetically as a mixture of
two different entities. As changes in the chemical composition
can be ruled out, the nature of these slow and fast relaxing
species must be associated with different molecular confor-
mations or supramolecular arrangements. These structural
changes can modify the anisotropy parameters and the
intermolecular dipolar interactions, both of which may play
a role in determining the magnetic relaxation rates. Deciding
which of these two effects dominates requires a more detailed
and quantitative analysis, which we describe in what follows.
The cM’’(u) curves were fitted as the weighted sum of two
Cole–Cole functions [Eq. (1)][25]
Figure 2. Log(t) versus 1/T plots for a) 1dis and b) 1cr. Key: c &
log(t1), d & log(t2) obtained from cM’’(u). log(t) obtained from
c’’(T). t1 and t2 are the relaxation times of the fast and slow species,
respectively.
~
1ꢀai
X
ðwtiÞ
cosðpai=2Þ
We find that for the two samples, log(t2) is proportional to
1/T for T> 30 K, and becomes weakly dependent on T (more
specifically t2 / Tꢀ1; see the Supporting Information) at lower
temperatures. This behavior shows a crossover from a
thermally activated Orbach mechanism that is predominant
at high temperature to a direct or phonon-induced tunneling
process taking over at T< 30 K. The Arrhenius analysis gives
Dobs = 4.80 ꢀ 102 cmꢀ1 for sample 1cr and Dobs = 4.22 ꢀ 102 cmꢀ1
for sample 1dis, which is in agreement with previously reported
barrier height in neutral terbium double-deckers.[3] A crucial
observation is that the relaxation times of the two species
diverge dramatically in the low-temperature or tunneling
regime, where t1 becomes two orders of magnitude shorter
than t2. The same applies under magnetic fields (see the
c00ðwÞ ¼
ðct ꢀ cS Þxi
ð1Þ
i
i
1ꢀai
2ꢀ2ai
1 þ 2ðwtiÞ
sinðpai=2Þ þ ðwtiÞ
i¼1;2
where t1 and t2 are the relaxation times of the fast and slow
magnetic species, respectively, and x1 = 1ꢀx2 is the fraction of
the former. The results show that the percentage of slowly
relaxing species was almost doubled, changing from 41% to
73% upon switching from 1dis to 1cr, which indicates that the
more slowly relaxing fraction is characteristic of the ordered
or crystalline state. After letting the sample equilibrate at
room temperature (i.e., in the mesophase) for several weeks,
it was quenched again to 150 K using the same procedure as
that used for 1dis. The magnetic characterization reveals that
the ratio of slowly relaxing species in this partially ordered
Angew. Chem. Int. Ed. 2010, 49, 1623 –1626
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1625

Communications
[5] N. Ishikawa, Polyhedron 2007, 26, 2147 – 2153.
[6] P. Gerbier, N. Domingo, J. Gomez-Segura, D. Ruiz-Molina, D. B.
Amabilino, J. Tejada, B. E. Williamson, J. Veciana, J. Mater.
Chem. 2004, 14, 2455 – 2460.
[7] N. Domingo, P. Gerbier, J. Gomez, D. Ruiz-Molina, D. B.
Amabilino, J. Tejada, J. Veciana, Polyhedron 2003, 22, 2355 –
2358.
[8] C. Train, R. Gheorghe, V. Krstic, L.-M. Chamoreau, N. S.
Ovanesyan, G. L. J. A. Rikken, M. Gruselle, M. Verdaguer, Nat.
Mater. 2008, 7, 729 – 734.
[9] a) E. Terazzi, C. Bourgogne, R. Welter, J.-L. Gallani, D. Guillon,
G. Rogez, B. Donnio, Angew. Chem. 2008, 120, 500 – 505; Angew.
Chem. Int. Ed. 2008, 47, 490 – 495.
[10] a) M. Seredyuk, A. B. Gaspar, V. Ksenofontov, Y. Galyametdi-
nov, M. Verdaguer, F. Villain, P. Gꢁtlich, Inorg. Chem. 2008, 47,
10232 – 10245; b) A. B. Gaspar, M. Seredyuk, P. Gꢁtlich, Coord.
Chem. Rev. 2009, 253, 2399 – 2413; c) A. B. Gaspar, M. Seredyuk,
P. Gꢁtlich, J. Mol. Struct. 2009, 924–926, 9 – 19.
[11] E. Gomar-Nadal, C. Rovira, D. B. Amabilino, Tetrahedron 2006,
62, 3370 – 3379.
[12] A. G. M. Barrett, G. R. Hanson, A. J. P. White, D. J. Williams,
A. S. Micallef, Tetrahedron 2007, 63, 5244 – 5250.
[13] T. Gross, F. Chevalier, J. S. Lindsey, Inorg. Chem. 2001, 40, 4762 –
4774.
[14] A. De Cian, M. Moussavi, J. Fischer, R. Weiss, Inorg. Chem.
1985, 24, 3162 – 3167.
[15] F. Nekelson, H. Monobe, Y. Shimizu, Chem. Commun. 2006,
3874 – 3876.
[16] F. Nekelson, H. Monobe, M. Shiro, Y. Shimizu, J. Mater. Chem.
2007, 17, 2607 – 2615.
[17] F. Nekelson, H. Monobe, Y. Shimizu, Mol. Cryst. Liq. Cryst.
2007, 479, 205 – 211.
[18] K. Ban, K. Nishizawa, K. Ohta, A. M. Van de Craats, J. M.
Warman, I. Yamamoto, H. Shirai, J. Mater. Chem. 2001, 11, 321 –
331.
[19] a) K. Binnemans, J. Sleven, S. De Feyter, F. C. De Schryver, B.
Donnio, D. Guillon, Chem. Mater. 2003, 15, 3930 – 3938; b) K.
Binnemans, C. Gorller-Walrand, Chem. Rev. 2002, 102, 2303 –
2346.
Supporting Information), which should tend to suppress the
influence of dipolar interactions. The strong difference
between t1 and t2 must then be associated with changes in
the molecular conformation. Tunneling rates are determined
by the intensity and symmetry of the off-diagonal anisotropy
terms present in the spin Hamiltonian which, in its turn,
reflect the local molecular symmetry and are, for this reason,
extremely sensitive to even very weak distortions in the plane
perpendicular to the easy magnetic axis.[26,27] Based on these
observations, we assign the origin of the fast relaxing species,
characteristic of the disordered 1dis sample, to intramolecular
distortions.
Although the coexistence of two different relaxation
processes is a common feature in some SMMs,[9,21] it is the first
time that it is observed in lanthanide double-decker com-
plexes, and it is the only system in which the ratio between
these responses has been shown to be reversibly modified by
simple thermal treatments.
In conclusion, we have shown the important influence that
the structural environment of a single-molecule magnet has
on its magnetic behavior, taking advantage of the phase
behavior of the complex. A reversible change of the magnetic
properties was achieved by simple heating and cooling cycles.
The absence of compositional change was demonstrated by
the reversibility of the process, thus providing evidence for
the structural origin of the modification of the magnetic
behavior, because of variations in the arrangement of the
molecules. This discovery was made possible by the synthesis
of a room-temperature liquid-crystalline molecular material
whose phase behavior depends critically on the chiral
substituents.
Received: September 7, 2009
Revised: December 3, 2009
Published online: January 27, 2010
[20] Z. Belarbi, C. Sirlin, J. Simon, J. J. Andre, J. Phys. Chem. 1989,
93, 8105 – 8110.
[21] S. M. J. Aubin, Z. Sun, H. J. Eppley, E. M. Rumberger, I. A.
Guzei, K. Folting, P. K. Gantzel, A. L. Rheingold, G. Christou,
D. N. Hendrickson, Inorg. Chem. 2001, 40, 2127 – 2146.
[22] a) N. Ishikawa, M. Sugita, W. Wernsdorfer, Angew. Chem. 2005,
117, 2991 – 2995; Angew. Chem. Int. Ed. 2005, 44, 2931 – 2935.
[23] F. Branzoli, P. Carretta, M. Filibian, G. Zoppellaro, M. J. Graf,
J. R. Galꢂn-Mascarꢃs, O. Fuhr, S. Brink, M. Ruben, J. Am.
Chem. Soc. 2009, 131, 4387 – 4396.
Keywords: lanthanides · liquid crystals · magnetic properties ·
metallomesogens · single-molecule studies
.
[1] D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets,
Oxford University Press, Oxford, 2006.
[2] See, for example: a) D. Gatteschi, L. Bogani, A. Cornia, M.
Mannini, L. Sorace, R. Sessoli, Solid State Sci. 2008, 10, 1701 –
1709; b) M. Cavallini, C. Albonetti, F. Biscarini, Adv. Mater.
2009, 21, 1043 – 1053; c) E. Vucic, H. M. H. F. Sanders, F. Arena,
E. Terreno, S. Aime, K. Nicolay, E. Leupold, M. Dathe,
N. A. J. M. Sommerdijk, Z. A. Fayad, W. J. M. Mulder, J. Am.
Chem. Soc. 2009, 131, 406 – 407; d) I. P. Suzdalev, Russ. Chem.
Rev. 2009, 78, 249 – 282.
[24] N. Ishikawa, M. Sugita, N. Tanaka, T. Ishikawa, S.-y. Koshihara,
Y. Kaizu, Inorg. Chem. 2004, 43, 5498 – 5500.
[25] K. S. Cole, R. H. Cole, J. Chem. Phys. 1941, 9, 341 – 351.
[26] A. Cornia, R. Sessoli, L. Sorace, D. Gatteschi, A. L. Barra, C.
Daiguebonne, Phys. Rev. Lett. 2002, 89, 257201.
[27] M. Evangelisti, F. Luis, F. L. Mettes, N. Aliaga, G. Aromꢄ, J. J.
Alonso, G. Christou, L. J. de Jongh, Phys. Rev. Lett. 2004, 93,
117202.
[3] N. Ishikawa, M. Sugita, T. Ishikawa, S.-y. Koshihara, Y. Kaizu, J.
Phys. Chem. B 2004, 108, 11265 – 11271.
[4] N. Ishikawa, M. Sugita, T. Ishikawa, S.-y. Koshihara, Y. Kaizu, J.
Am. Chem. Soc. 2003, 125, 8694 – 8695.
1626
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 1623 –1626