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Photomagnetic Effects in Polycyanometallate Compounds An Intriguing Future Chemically Based Technology.

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Highlights
Photoinduced Magnetism
Photomagnetic Effects in Polycyanometallate
Compounds: An Intriguing Future Chemically
Based Technology?
Andrea Dei*
Keywords:
cyanides · electron transfer · magnetic properties ·
photochemistry · single-molecule magnets
Molecular systems undergoing a reversible and controlled change of their
physical properties following a change
in an external parameter offer an appealing perspective for the realization of
molecular-scale electronic devices.[1, 2] In
particular, molecules showing photochromism are of potential interest as
materials for optical data storage.[3]
Photochromism is defined as a lightstimulated reversible interconversion
between two isomers which have different electronic absorption spectra. In
practice, the photoinduced interconversion occurs by irradiating an absorption
band of the starting material with an
appropriate light source. The starting
material can in turn be regenerated by
irradiating an appropriate electronic
transition of the photoinduced product.
Since, in general, the two isomers have
different free energies at any given
temperature, the starting compound is
often the more stable one. Therefore the
photoinduction process yields a metastable product. A thermally induced
relaxation process to the ground state
may occur depending on the free-energy
potential barrier of the relaxation process.
A dramatic extension of the perspectives of photochromism was given
by the development of molecular mag-
[*] Prof. Dr. A. Dei
INSTM Research Unit
LAMM Dipartimento di Chimica dell’
Universit di Firenze
Via della Lastruccia 3
50019 Sesto Fiorentino, Firenze (Italy)
Fax: (+ 39) 055-457-3372
E-mail: andrea.dei@unifi.it
1160
netism,[4, 5] which lead to the discovery
that the switching of the magnetic
properties of a molecule and long-range
magnetic-order effects can be induced
by photoexcitation.[6, 7]
The number of coordination compounds showing photoinduced magnetic
properties in the solid state is still
limited to some dithienylethylene derivatives,[8, 9] tetracyanoethylene organometallic
compounds,[10]
spin-crossover,[11, 12] and valence-tautomeric complexes.[13, 14] In this framework the most
promising perspectives are given by the
so-called Prussian blue analogues, which
have the general formula A2xM’II(1.5x)[MIII(CN)6] (x = 0–1; A = alkali metal
ion; MII, MIII = transition-metal ions).
These compounds can be considered as
metal complexes formed by an hexacyanometallate [M(CN)6]n unit, which can
act as a hexamonodentate ligand towards up to six different metal ions M’
with the formation of three dimensional
MCNM’ arrays. This property has
been successfully exploited for the design of molecular-based magnets and
several compounds showing spontaneous magnetization with high critical
temperatures (Tc) have been obtained.
Examples are V[Cr(CN)6]·2.8 H2O and
[Cr2.12(CN)6]·2.8 H2O with Tc = 315 K
and 270 K, respectively.[15, 16]
Few years ago Hashimoto and coworkers discovered that long-range
magnetic ordering in the compound
K0.2Co1.4[Fe(CN)6]·6.9 H2O can be achieved by irradiation with red light at low
temperature, the original material being
restored by blue light irradiation or by
heating.[17] This result was extremely
important since it showed how the
magnetic behavior of a molecular-based
material can be controlled by light. It
should be mentioned that it followed the
discovery of the LIESST (light-induced
excited-spin-state trapping) effect described some years before.[6, 7, 11, 12]
Further studies on several compounds
of
formula
A2xCo(1.5x)[Fe(CN)6]·n H2O (A = Na, Rb, Cs) have
provided evidence for the rationalization of the observed phenomenon.[18–24]
Herein the main properties of some of
them are summarized and discussed.
In an extremely simplified approach,[18] it can be stated that the
changes in the magnetic properties are
associated with the photoinduced valence
tautomeric
interconversion
[Eq. (1); LS = low spin, HS = high spin].
The pair of bridged metal centers on
the left side of Equation (1) is diamagnetic while the pair on the right is
paramagnetic. When the whole lattice
is made up of paramagnetic pairs, the
compound orders ferrimagnetically below 20 K. This situation is observed in
CoII1.5[FeIII(CN)6]·6 H2O.[19] The magnetic properties of this compound are not
affected by the light since only paramagnetic pairs are present.
If a significant concentration of
diamagnetic FeII–CoIII pairs is introduced into the lattice, as in the lowtemperature phase Na0.4CoII0.3CoIII-
DOI: 10.1002/anie.200461413
Angew. Chem. Int. Ed. 2005, 44, 1160 –1163
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angewandte
Chemie
[FeII(CN)6]·5 H2O, then irradiation of
the FeII !CoIII charge-transfer band
(500–750 nm) at 5 K induces the valence
tautomeric interconversion [Eq. (1)]
which results in a change of the magnetism of the compound from paramagnetic to ferrimagnetic.[20] The new magnetic
state is stable for several days at 5 K.
The original paramagnetic material is
restored by heating to 130 K or by
irradiation at 1319 nm at 5 K. The same
behavior was observed in other sodium
and rubidium derivatives, but not in the
cesium ones.[19–23]
The different oxidation-states in
CoII1.5[FeIII(CN)6]·6 H2O and Na0.4CoII0.3CoIII[FeII(CN)6]·5 H2O are due to the
different reducing power of the CoII
center in the two lattices. In all the
above compounds the FeIII and FeII ions
are always bound to six carbon atoms
from six cyanide ligands, whereas the
hexacoordinate MII metal ions (in the
prussiate compounds of general formula
MII1.5[FeIII(CN)6]·n H2O) are bound to
four cyanide nitrogen atoms.[18–24] Their
coordination polyhedron is completed
by two water molecules. Under these
conditions the FeIII ion is not able to
oxidize the CoII ion. However, when
alkali-metal cations A+ are introduced
into the structure, the MII/FeIII ratio
decreases and reaches the limiting value
of 1 in the compounds of general
formula AM[Fe(CN)6]. In this case the
MII metal ion is coordinated by six
cyanide nitrogen donors and thus the
CoII ion can be oxidized by the FeIII
yielding an FeII-CN-CoIII pair. When
the introduction of the alkali-metal
cation into the structure is only partial,
the situation is critical, because only a
partial substitution of the two water
molecules in the coordination sphere of
CoII occurs, and thus the FeIII-CN-CoII
and FeII-CN-CoIII species have similar
energies and their relative stability is
controlled by several parameters. As a
result, Na0.4CoII0.3CoIII[FeII(CN)6] undergoes a thermally (entropy) induced
valence tautomeric transition to Na0.4CoII1.3[FeIII(CN)6] (Tc = 260 K).[21] This
transition is characterized by a large
thermal hysteresis loop (40 K). (In addition, it was recently shown that such
alkali-metal-doped compounds also undergo pressure-induced electron-transfer processes.[25]) These results can be
explained by the following observation:
Angew. Chem. Int. Ed. 2005, 44, 1160 –1163
The FeC bond lengths do not change
significantly (0.02 ) in the electrontransfer process since the Fe(t2g5)!
Fe(t2g6) change does not involve any
s* bonding orbitals. However, an increase of 0.19 is observed in the Co
N bond lengths when the LS-CoIII center
is reduced to HS-CoII since, in this
Co(t2g6)!Co(t2g5eg2) change, the population of the s* orbitals is altered. Thus, as
the temperature increases, the FeIII–CoII
species is favored by electronic and
vibrational entropy contributions.
At low temperature the irradiation
of the metal-to-metal charge-transfer
(MMCT) band FeII !CoIII affords the
metastable paramagnetic FeIII–CoII species. The resulting large change in the
CoN bond lengths must be adsorbed
by the surrounding lattice, which causes
a high degree of network strain. This
accommodation of the CoN bond
lengths involves the formation of a
magnetic-phase domain and the metastable state is trapped owing to the high
energy barrier associated with its relaxation to the original diamagnetic ground
state (see below). As a result the metastable FeIII-CN-CoII pair can be trapped
by fast cooling at low temperatures
(frozen-in effect).[7] It was therefore
rather
surprising
that
Cs0.97Co[Fe(CN)6]0.97·3.2 H2O, where the diamagnetic pairs are dominant, is rather insensitive to light stimulation.[23] Verdaguer and co-workers demonstrated that
this result arises from the lack of flexibility of the crystal lattice, the interconversion processes at the first step require
some structural vacancies.[24] It was also
suggested that all of the CoIII centers are
surrounded by six N donors, which have
a poor oxidizing power, whereas in the
sodium and rubidium derivatives the
same ions are coordinated, on average,
by five N donors and one O donor. The
calculated Franck–Condon excitation
energies of the two states support this
suggestion.[26]
As mentioned above, the metastable
states obtained upon irradiation are
stable at 5 K for several days and the
original ground state is regenerated by
heating. The thermal relaxation curves
cannot be fitted with the usual exponential law which characterizes the
relaxation process of an excited state
of diluted molecular systems, the curves
in fact have a sigmoidal shape which
www.angewandte.org
suggests the existence of a autocatalytic
process.[27] In addition, the metastable
states show large magnetic hysteresis
effects. All these properties are consistent with the existence of huge cooperative effects arising from strong intermolecular interactions. This situation
means that the photoexcitation of any
unit in the lattice induces the transformation of its surroundings, thus promoting the growth of a domain of
molecules which are in the metastable
state. The main consequence is that the
potential barrier may be so large that at
low temperatures the thermally activated relaxation process can be, in practice,
precluded. These findings suggest the
basis for a new systems for information
storage.[28]
As well as the cobalt prussiate complexes, the manganese analogues have
also been studied.[29] At room temperature the compound RbMn[Fe(CN)6]
contains antiferromagnetically coupled
HS-MnII and LS-FeIII ions. As the temperature is decreased, an electron-transfer process occurs according to the
valence tautomeric interconversion in
Equation (2) thus affording ferromagnetically coupled MnIII ions bridged by
the NC-FeII-CN diamagnetic moiety.
LS-FeIII -CN-HS-MnII !
LS-FeII -CN-HS-MnIII
ð2Þ
The transition involves a change of
the unit cell of the solid which is caused
by the strong distortion arising from the
formation of the MnIII species. At low
temperature the compound behaves as a
ferromagnet (Tc = 10 K). Light irradiation then induces the formation of the
FeIII–MnII pairs as the metastable state
and a photodemagnetization effect is
observed. A significant photoinduced
magnetization was detected for a Nd
prussiate derivative, although the origin
of the light effect has not yet been
elucidated.[30]
These significant results show the
possibility of inducing ordered magnetic
phases by light stimulation. It is important to stress, however, that the above
mentioned systems require quite a long
illumination time at very low temperature. Both these aspects were rather
discouraging for the possible technological application of these compounds
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1161
Highlights
until the following spectacular result
was obtained by Sato and co-workers.[31]
The sodium cobalt prussiate derivatives undergo reversible entropy-driven
valence-tautomeric transitions involving
the FeII-CN-CoIII diamagnetic pairs
(low-temperature (LT) phase) and the
FeIII-CN-CoII paramagnetic pairs (hightemperature (HT) phase). A compound
of
formula
Na0.36Co1.32[Fe(CN)6]·
5.6 H2O was found to exhibit a large
thermal hysteresis with Tc› = 230 K and
Tcfl = 197 K. Abrupt changes in the cT
values begin at 225 K in the heating
process (›) and at 205 K in the cooling
process (fl). The electronic spectra of
the two tautomers are characterized by
broad bands in the visible regions with
absorption maxima at around 680 nm
for the FeII-CN-CoIII (LT) phase and
590 nm for the FeIII-CN-CoII (HT)
phase. A single-shot irradiation at
520 nm of the HT phase with a laser
pulse (8 ns) at 205 K induced the complete transition to the LT phase. The
reverse LT!HT phase transition was
obtained by irradiating the sample with
a single shot at the same wavelength at
225 K. It is also remarkable that the
photoinduced transition occurs only if
the photon density of the laser pulse
exceeds a critical threshold value. This
means that the phase-transition occurs
when the number of excited centers is
higher than a threshold value necessary
to cause a domino effect with the
surrounding lattice. This result is extremely important since it shows that a
photoinduced magnetization change can
occur at high temperature provided that
the system exhibits a large hysteresis
loop.
It was found that in HS-FeII1.5III
[Cr (CN)6]·7.5 H2O the two paramagnetic metal ions experience a ferromagnetic interaction which is disconnected
by irradiation.[32] In the manganese(ii)
analogue the two metal ions experience
antiferromagnetic interactions and the
compound behaves as a ferrimagnet
(Tc = 67 K).[33] A mixed ferro–ferrimagnetic behavior characterizes the ternary
derivatives
(MnIIxFeII1x)1.5[CrIII(CN)6]·z H2O whose overall magnetization is determined by the ratio of
ferromagnetic (FeII–CrIII) and ferrimagnetic (MnII–CrIII) contributions. This
compound has a negative magnetization, but irradiation at 16 K in the
1162
presence of a weak magnetic field
(10 G) induces a change of the sign of
the magnetization, thus affording a
photoinduced magnetic pole inversion.[34] Thermal treatment restored the
starting material. This behavior is due to
the photochromic properties of the FeIICN-CrIII pairs.
A different strategy used for the
synthesis of photomagnetic materials
exploits light-sensitive polycyanometallate ligands. A small enhancement of the
magnetization was observed using the
[Fe(CN)5NO]2 complex,[6, 35] but more
significant results were obtained using
[WIV(CN)8]4 and [MoIV(CN)8]4 precursors. Again the photomagnetic effects
are due to valence tautomeric effects
[Eq. (3)], the MoV center is paramagnetic (S = 1=2 ).[36]
MoIV -CN-Mnþ ! MoV -CN-Mðn1Þþ
ð3Þ
Laser irradiation of the MMCT
MoIV !CuII band of CuII2[Mo(CN)8]·
8 H2O induces an enhancement of the
magnetization as a result of the lightinduced conversion into MoV-CN-CuI.[37]
Photomagnetic effects were also
found in cobalt–octacyanotungstate derivatives. CsCo[W(CN)8]·H2O undergoes a thermally induced phase transition with a large hysteresis loop (Tc› =
216, Tcfl = 167).[38] This transition is attributed to the valence-tautomeric interconversion of the diamagnetic LSCoIII-NC-WIV pair (stable at low temperatures) into the paramagnetic HSCoII-NC-WV pair, which is dominant at
high temperatures. Again the same
electron-transfer process could also be
induced by light irradiation in the
MMCT WIV !CoIII band at 5 K, resulting in a strong enhancement of the
magnetization.
These recent results by few research
groups provided the basis for potential
applications in the next generation of
information storage devices. Most of the
studies to date have concerned polymeric systems, but these results allow
molecular systems to be conceived.
Molecules containing up to six paramagnetic metal ions coordinated to a
polycyanometallate anion have been
isolated.[39] If such compounds are highly anisotropic, these molecules may
behave as photochromic single-molecule magnets (SMMs).[40] The way for
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
obtaining this kind of system has been
recently traced by Marvaud, Mathonire,
and
co-workers.[41]
[{Cu(tren)NC}6Mo(CN)2](ClO4)8·4.5 H2O
(Figure 1;
tren = tris(2-aminoethyl)amine) was isolated and structurally
Figure 1. X-ray crystal structure of [{Cu(tren)NC}6Mo(CN)2]8+ and an indication of the photoinduced changes. Hydrogen atoms, crystallization solvents, and counterions are omitted
for clarity (Mo yellow, C black, N light blue,
Cu dark blue).[41]
characterized. The magnetic properties
of this compound are consistent with a
diamagnetic MoIV center surrounded by
six weakly interacting S = 1=2 CuII ions.
Once irradiated with a blue light, the
magnetic properties change drastically,
thus affording an S = 3 species. It was
suggested that the light irradiation induces an electron transfer between the
MoIV and one CuII ion, thus yielding a
MoVCuICuII5 product in which the six
paramagnetic centers experience a ferromagnetic interaction. The original
compound can be restored by thermal
treatment at 280 K. It can be easily
anticipated that the developments of
this strategy will allow the design and
the synthesis of a family of discrete
molecular systems showing peculiar
photomagnetic properties.
Published online: January 14, 2005
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