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Journal ArticleDOI

Persistent Bidirectional Optical Switching in the 2D High-Spin Polymer {[Fe(bbtr)3](BF4)2}∞

TL;DR: The iron(II) centers of the covalently linked 2D coordination network shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures.
Abstract: In the covalently linked 2D coordination network {[Fe(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, the iron(II) centers stay in the high-spin (HS) state down to 10 K. They can, however, be quantitatively converted to the low-spin (LS) state by irradiating into the near-IR spin allowed 5dd band and back again by irradiating into the visible 1dd band. The compound shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures.

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Summary

  • They can, however, be quantitatively converted to the low-spin (LS) state by irradiating into the near-IR spin allowed dd band and back again by irradiating into the visible dd band.
  • The compound shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures.
  • B switching between two states of a system is of importance for applications in a large number of fields, and spin-crossover compounds of transition metal ions, in particular those of iron(II), have been considered prototypes for such switchable materials for more than a decade.
  • This is because they can be switched between the two lowest-energy spin states, namely, the low-spin (LS) and the high-spin (HS) state, thermally, by applying pressure, in pulsed magnetic fields, chemically, as well as optically.
  • The latter has been achieved for the LS → HS conversion in the so-called LIESST effect (light-induced excited spin state trapping) at low temperatures for a large number of iron(II) spin-crossover systems.
  • Invariably, the light-induced HS state is, however, a metastable state, albeit in some cases with a very long lifetime below ∼50 K.
  • Partial bidirectional switching was demonstrated in tetrazole based systems some 15 years ago, with memory effects up to ∼70 K.
  • Schematic of the relevant electronic states and processes, also known as Inset.

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Persistent Bidirectional Optical Switching in the 2D High-Spin
Polymer {[Fe(bbtr)
3
](BF
4
)
2
}
Pradip Chakraborty,
Robert Bronisz,
#
Ce
line Besnard,
Laure Gue
ne
e,
Phil Pattison,
§,
and Andreas Hauser*
,
De
partement de Chimie Physique, Universite
de Gene
ve, 30 Quai Ernest-Ansermet, CH-1211 Gene
ve 4, Switzerland
#
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, Pl-50-383 Wrocław, Poland
Laboratoire de Cristallographie, Universite
de Gene
ve, 24 Quai Ernest-Ansermet, CH-1211 Gene
ve 4, Switzerland
§
Laboratory of Crystallography, EPFL, CH-1015 Lausanne, Switzerland
SNBL (ESRF), Grenoble, France
*
S
Supporting Information
ABSTRACT: In the covalently linked 2D coordination
network {[Fe(bbtr)
3
](BF
4
)
2
}
, bbtr = 1,4-di(1,2,3-triazol-
1-yl)butane, the iron(II) centers stay in the high-spin (HS)
state down to 10 K. They can, however, be quantitatively
converted to the low-spin (LS) state by irradiating into the
near-IR spin allowed
5
dd band and back again by
irradiating into the visible
1
dd band. The compound
shows true light-induced bistability below 100 K, thus,
having the potential for persistent bidirectional optical
switching at elevated temperatures.
B
idirectional switching between two states of a system is of
importance for applications in a large number of fields,
1
and spin-crossover compounds of transition metal ions, in
particular those of iron(II), have been considered prototypes
for such switchable materials for more than a decade.
2
This is
because they can be switched between the two lowest-energy
spin states, namely, the low-spin (LS) and the high-spin (HS)
state, thermally,
3
by applying pressure,
4
in pulsed magnetic
fields,
5
chemically,
6
as well as optically.
7
The latter has been
achieved for the LS HS conversion in the so-called LIESST
effect (light-induced excited sp in sta te trapping) at low
temperatures for a large number of iron(II) spin-crossover
systems.
7,8
Invariably, the light-induced HS state is, however, a
metastable state, albeit in some cases with a very long lifetime
below 50 K.
7,8
For iron(II) complexes with no low-energy
metalligand charge transfer (MLCT) transitions, light-
induced HS LS conversion (reverse-LIESST) is likewise
possible through irradiation in the near-IR,
9
and Bousseksou et
al. have demonstrated bidirectional switching by pulsed laser
irradiation within the thermal hysteresis of a st rongly
cooperative iron(II) spin-crossover system near room temper-
ature.
10
Partial bidirectional switching was demonstrated in tetrazole
based systems some 15 years ago, with memory effects up to
70 K.
11
In the present communication, we describe the
photophysical properties of the 2D network {[Fe(bbtr)
3
]-
(BF
4
)
2
}
, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, in which, in
contrast to the ClO
4
derivative,
12
the iron(II) centers remain
in the HS state down to 10 K,
13
but can quantitatively and
reversibly be switched back and forth between the two states
using energy-selective excitation in the near-IR and the visible,
respectively. Strong cooperative effects result in a persistent
light-induced bistability below 100 K.
Figure 1 shows single crystal absorption spectra of a small
crystal ( 300 × 300 × 75 μm
3
) with the light propagating
along the c-axis at room temperature (red) and 10 K (blue)
with a cooling rate of 0.2 K/min. Both are empty in the visible
and show just the characteristic
5
T
2
5
E ligand-field transition
of the HS species at 12 050 cm
1
, as proof that the iron(II)
centers remain in the HS state down to 10 K. Prolonged
irradiation at 12 050 cm
1
(830 nm) results in almost total
bleaching of this band and replacement by a more intense band
at 18 200 cm
1
readily attributed to the spin-allowed ligand-
Received: December 21, 2011
Published: February 21, 2012
Figure 1. Single crystal absorption spectra of {[Fe(bbtr)
3
](BF
4
)
2
}
at
295 K (red), 10 K on slow cooling (blue), at 10 K after irradiation at
12 050 cm
1
(green), and on subsequent warming to 60 K (light blue),
recooling to 10 K (black), and finally on warming to 120 K (yellow).
Inset: schematic of the relevant electronic states and processes.
Communication
pubs.acs.org/JACS
© 2012 American Chemical Society 4049 dx.doi.org/10.1021/ja211897t | J. Am. Chem. Soc. 2012, 134, 40494052

field
1
A
1
1
T
1
transition of the LS species, as well as steep rise
above 25 000 cm
1
toward an intense
1
MLCT transition.
Although it is obvious that the majority of the complexes have
been transformed to the LS state, this spectrum does not yet
allow for a quantitative determination of the fraction of
photoconverted complexes.
Surprisingly, upon heating the crystal from 10 K at a rate of
0.2 K/min, the intensity of the
1
A
1
1
T
1
absorption band
starts to increase at 30 K and reaches a maximum at 60 K,
with now complete bleaching of the
5
T
2
5
E band. This
means that rather than the LS complexes returning to the HS
state, the small remaining fraction of HS complexes follow suit
and convert spontaneously to the LS state, thus, resulting in a
100% LS population. With this, the light-induced LS fraction
after irradiation at 12 050 cm
1
and 10 K can be estimated at
85% from the spectra in Figure 1. This value is in line with the
steady LS fraction also found for the light-induced return to the
LS state in related iron(II) spin-crossover systems upon
irradiation at the same energy.
9
It furthermore allows to plot
the evolution of the HS fraction on ramping the temperature
from 10 K shown in Figure 2.
The 100% LS population is maintained upon renewed
cooling to 10 K and is stable indefinitely up to 100 K. At that
temperature, there is an abrupt and quantitative return to the
HS state upon heating. The abruptness of this return is
indicative of strong cooperative effects, and the key question to
ask is whether there is a threshold value for the light-induced
LS population in order for the remaining HS complexes to
follow suit without further irradiation. That this must be the
case is borne out by the evolution of the HS fraction upon
ramping the temperature at 0.2 K/min following only a partial
light-induced LS population of 15% at 10 K. With such a small
initial LS fraction, the system indeed returns to the HS state at
60 K, as shown in Figure 2. This occurs, therefore, at a
temperature at which thermal LS HS relaxation processes set
in. This is further demonstrated by the curves showing the
evolution of the LS fraction, γ
LS
, at 65 K first as a function of
time for irradiation at 12 050 cm
1
for different total irradiation
times in order to create different initial light-induced LS
fractions, and then during the thermal relaxation at that
temperature after switching off the irradiation. Clearly, there is
a threshold or critical value of around 35% for the light-induced
LS fraction, above which the relaxation proceeds toward full LS
population and below which the system returns to full HS
population after switching off the light. The sigmoidal character
of the relaxation curves near the threshold value corroborates
the importance of cooperative effects of elastic origin
14
in this
system. Globally, these act in such a way as to stabilize the
majority of species.
15
That is, as the system is cooled down
from room temperature, the HS state is the thermodynamic
ground state down to 10 K, and the system has no need to
cross over to the LS state. Upon irradiation, the light-induced
LS population exerts an increasing internal pressure on the
remaining HS complexes, destabilizing them to such an extent
that above the critical LS population, the LS state itself
becomes the ground state of the system. This implies that the
title compound is a spin-crossover system having a very wide
thermal hysteresis with T
c
= 100 K and T
c
near or formally
even below 0 K, such that the lower branch either cannot be
reached thermodynamically or the relaxation is very slow and
thermal equilibrium is not reached within a reasonable time.
The light-induced HS LS conversion can be verified by X-
ray crystallography. At 120 K, the title compound crystallizes in
the space group P3
with one crystallographic iron(II) site and
the typical FeN bond length of the HS state of 2.19(1) Å.
12
At 70 K, the crystal still is in the same space group with the
same FeN bond length (see Table S1, Supporting
Information). Upon irradiation for 15 min at 12 050 cm
1
,
the space group has changed to P1
with a doubling of the c-axis
and the iron(II) centers occupying nonequivalent lattice sites,
very similar to the phase transition in the spin-crossover system
of the perchlorate derivative (see Figures S1S3, Supporting
Information). The average FeN bond length after irradiation
at 70 K is 1.99(2) Å, indicating that at that temperature the
Figure 2. The high-spin fraction γ
HS
as function of temperature: on
cooling from room temperature down to 10 K at a rate of 0.2 K/min
(black), after irradiation at 12 050 cm
1
at 10 K to the steady state
high-spin fraction of 15% and subsequent warming to 120 K at a rate
of 0.2 K/min (red) or stopping at 60 K and recooling to 10 K
(yellow), after a short irradiation time at 12 050 cm
1
and 10 K
resulting in an initial high-spin fraction of 85% and subsequent
warming to 120 K at a rate of 0.2 K/min (green).
Figure 3. Evolution of the LS fraction, γ
LS
, upon irradiation at 12 050
cm
1
, 10 mW/mm
2
at 65 K to different initial lig ht-induced
populations of the LS state by varying the irradiation time followed
by the thermal relaxation at that temperature.
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja211897t | J. Am. Chem. Soc. 2012, 134, 404940524050

light-induced transformation to the LS state followed by
relaxation above the critical LS fraction is indeed quantitative.
Once in the LS state at 10 K, another way to reestablish the
HS state quantitatively is by irradiating the crystal at 21 186
cm
1
(472 nm), that is, into the high-energy tail of the
1
A
1
1
T
1
transition, where absorption is weak in order to avoid
creating large concentration gradients during the light-induced
transformation. The spectrum obtained after irradiation of the
sample in the LS state at this energy and at 10 K, not explicitly
shown in Figure 1, is identical to the initial 10 K spectrum. This
is also the case at higher temperatures, for instance at 65 K.
Figure 4 demonstrates the full reversibility of the light-induced
cycle by following the evolution of the LS fraction, γ
LS
,as
function of irradiation time at that temperature first for
irradiation at 12 050 cm
1
followed by irradiation at 21 186
cm
1
(see Figure S4 in Supporting Information for spectral
evolution). The observed efficiencies are in line with previous
determinations of quantum efficiencies for both the light-
induced HS LS as well as the LS HS conversion,
respectively, the latter being about 10 times more efficient.
9
In conclusion, we have thus demonstrated fully reversible,
persistent, and wavelength-selective optical switching of the
spin state in the 2D iron(II) network compound {[Fe(bbtr)
3
]-
(BF
4
)
2
}
, with a zero-point energy difference between the two
spin states very close to zero. The persistent nature of the
switching is due to strong cooperative effects, resulting in an
extremely wide hysteresis with T
c
near or formally even below
0 K and T
c
at 100 K. The latter determines the maximum
operating temperature of the switch. In contrast to the system
discussed by Bousseksou et al.,
10
which operates with a
combination of optical switching and temperature jump effects,
the present system operates on a purely optical and wavelength
selective basis. Indeed, the only way to create the system in the
LS state at ambient pressure is through irradiation in the near
IR. The persistent nature is also different from the metastable
light-induced LS state in the dilute mixed crystal of the
perchlorate system.
16
In this case, the thermal relaxation is very
slow but not strictly zero at temperatures even below 40 K. It
is, however, related to the partial switching observed in the
compounds of some iron(II) tetrazole complexes having
multiple crystallographic sites with large residual HS fractions
at low temperatures, but for which cooperative effects are too
small and the memory is lost already at 70 K.
11
The present
system demonstrates the feasibility of persistent bidirectional
switching in HS systems at higher temperatures and thus gives
the incentive to search for systems for which T
c
is higher still
but for which the hysteresis is still as large as the 100 K of the
present system.
What remains to be done in order to arrive at an in depth
understanding of the light-induced bistability: (i) determine the
critical light-induced LS fraction for other temperatures; (ii)
investigate the role of the crystallographic phase transition in a
detailed X-ray crystallographic study; (iii) perform LIESST and
reverse-LIESST experiments on mixed crystals in order to
assess the cooperative effects quantitatively; (iv) perform
experiments under external pressure in order to move T
c
and T
c
to higher temperatures, the former possibly from below
0 to where the relaxation is no longer thermally quenched, and
the latter to establish up to which temperature bidirectional
persistent switching is feasible.
ASSOCIATED CONTENT
*
S
Supporting Information
Crystallographic data before and after irradiation at 12 050
cm
1
and at a temperature of 70 K; spectral evolution during
irradiation. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
andreas.hauser@unige.ch
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the Swiss Norwegian Beamlines for the
provision of synchrotron beamtime and thank Dmitry
Chernishov for his assistance. This work was financially
supported by the Swiss National Science Foundation (Grant
Number 200020-125175).
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upon irradiation at 12 050
cm
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, 10 mW/mm
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(red upward triangle), and subsequent irradiation
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Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja211897t | J. Am. Chem. Soc. 2012, 134, 404940524051

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Citations
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References
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Journal ArticleDOI
TL;DR: This critical review discusses recent work in the field of molecule-based spin crossover materials with a special focus on these emerging issues, including chemical synthesis, physical properties and theoretical aspects as well (223 references).
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Book ChapterDOI
TL;DR: The discovery of light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin crossover research as mentioned in this paper.
Abstract: The discovery of a light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin-crossover research. Apart from being an interesting photophysical phenomenon in its own right, it provided the means of studying the dynamics of the intersystem crossing process between the high-spin and the low-spin state in a series of compounds and over a large temperature range. It could thus be firmly established that intersystem crossing in spin-crossover compounds is a tunnelling process, with a limiting low-temperature lifetime below 50 K and a thermally activated region above 100 K. This review begins with an elucidation of the mechanism of the light-induced spin transition, followed by an in depth discussion of the chemical and physical factors, including cooperative effects, governing the lifetimes of the light-induced metastable states.

458 citations

Journal ArticleDOI
TL;DR: In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low-spin state to the high-spin (HS) state, whereas CS(2) uniquely causes the reverse HS-to-LS transition.
Abstract: The ins and outs of spin: Using the microporous coordination polymer {Fe(pz)[Pt(CN)(4)]} (1, pz=pyrazine), incorporating spin-crossover subunits, two-directional magnetic chemo-switching is achieved at room temperature. In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low-spin (LS) state to the high-spin (HS) state, whereas CS(2) uniquely causes the reverse HS-to-LS transition.

446 citations

Journal Article
TL;DR: In this article, the potential for the application of the spin crossover (SCO) phenomenon in various domains, such as molecular electronics, data storage, display devices, is reviewed, and several requirements must be fulfilled before any use in a genuine device becomes feasible.
Abstract: In this chapter we attempt to review the potential for the application of the spin crossover (SCO) phenomenon in various domains, such as molecular electronics, data storage, display devices. It is evident that SCO properties, such as room-temperature working range, chemical stability, low addressing power, short addressing time, full reversibility, are of promising value in the context of the stringent limits necessary in the future development of information technology, due to the unceasing miniaturization of the components. Of course, many requirements must be fulfilled before any use in a genuine device becomes feasible. Some of these are emphasized and discussed here. Additionally, this review reports recent progress in non-linear optics and photomagnetism of SCO materials.

445 citations

Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Persistent bidirectional optical switching in the 2d high-spin" ?

In the present communication, the authors describe the photophysical properties of the 2D network { [ Fe ( bbtr ) 3 ] ( BF4 ) 2 } ∞, bbtr = 1,4-di ( 1,2,3-triazol-1-yl ) butane, in which, in contrast to the ClO4 − derivative, the iron ( II ) centers remain in the HS state down to 10 K, but can quantitatively and reversibly be switched back and forth between the two states using energy-selective excitation in the near-IR and the visible, respectively.