Complete solid state photoisomerization of bis(dipyrazolylstyrylpyridine) iron(ii) to change magnetic properties

Article abstract of DOI:10.1039/c1cc11850a

Iron(ii) complexes of Z- and E-2,6-di(1H-pyrazol-1-yl)-4-styrylpyridine (Z-2 and E-2, respectively) exhibited visible light photoisomerization from Z-2 to E-2, both in solution and in solid phases. Z-2 occupied the high-spin state over the full temperatur

Full text of DOI:10.1039/c1cc11850a

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COMMUNICATION
Complete solid state photoisomerization of
bis(dipyrazolylstyrylpyridine)iron(II) to change magnetic propertiesw
Yuta Hasegawa, Kazuhiro Takahashi, Shoko Kume and Hiroshi Nishihara*
Received 1st April 2011, Accepted 18th April 2011
DOI: 10.1039/c1cc11850a
Iron(II) complexes of Z- and E-2,6-di(1H-pyrazol-1-yl)-4-
styrylpyridine (Z-2 and E-2, respectively) exhibited visible light
photoisomerization from Z-2 to E-2, both in solution and in solid
phases. Z-2 occupied the high-spin state over the full temperature
range examined, whereas E-2 displayed a spin crossover
phenomenon between 100 K and 300 K.
Scheme
1
Photoisomerization of Z-2 to E-2 by visible light
(4420 nm) irradiation. Counter anions are omitted.
Photomagnetic effects, which are magnetic effects produced by
photoinduction, have attracted attention for the development
of light-responsive single molecule devices.1 The creation of
photomagnetic systems that operate in ambient environments
requires that photomagnetic effects be accessible at room
temperature. Nevertheless, many of these materials exhibit
magnetic bistability upon illumination at cryogenic temperatures
due to the absence of an activation barrier between the
magnetic states.2 Bistability may be ameliorated by leveraging
the hysteretic properties of these materials3 or by introducing
photoisomerizable molecules that exhibit bistability at room
temperature.4
light one-way photoisomerization of [Fe(Z-1)2](BF4)2 (Z-2),
both in solid and in solution, and identified significant
photomagnetic effects in the solid state (Scheme 1). The novel
tridentate ligands Z-1 and E-1 were synthesized from
2,6-di(1H-pyrazol-1-yl)isonicotinaldehyde as the starting
material (for details, see the ESIw). The reaction of Z-1 with
Fe(BF4)2Á6H2O in acetone, Z-2 was obtained as yellow micro-
crystals, and following recrystallization from nitromethane/
diethyl ether afforded single crystals. Likewise, the reaction of
E-1 with Fe(BF4)2Á6H2O in acetone, followed by recrystallization
from propylene carbonate (PC)/ethyl acetate/diethyl ether,
afforded single crystals of [Fe(E-1)2](BF4)2Á2PC (E-2Á2PC).
The complexes were characterized by elemental analysis and
X-ray crystallography.
Iron(II) complexed with 2,6-di(1H-pyrazol-1-yl)pyridine
exhibits spin crossover (SC) between the electronic high-spin
(HS) and low-spin (LS) states.5 Coupling this iron(II) complex
to a stilbene moiety, which can act as an active photo-
isomerizable component in solution and solid states,6 may
produce photomagnetic effects under ambient conditions
as a result of the ligand-driven light-induced spin change
(LD-LISC) phenomenon, which was first proposed by
Zarembowitch et al. in 19947 and has been observed mostly
in solution8 and in thin polymer films.9 However, in spite of
current great efforts, a quantitative change in magnetic
susceptibility before and after photoirradiation in crystalline
solid state has not been reported. It should be noted that solid
state photoisomerization of the iron styrylpyridine complex
was reported by Boillot et al.,10 while the magnetic properties
have not been indicated. In the present study of iron(II)
complexes with Z- or E-2,6-di(1H-pyrazol-1-yl)-4-styrylpyridine
(denoted Z-1 or E-1, respectively), we investigated the visible
An ORTEP diagram of Z-2 is shown in Fig. 1. The torsion
angles around the alkenyl moieties, C6–C12–C13–C14 (7.551)
and C25–C31–C32–C33 (À7.731), assured the Z-configurations
of the ligands.6c At a low temperature of 90 K, the average
Fe–L bond distance (2.170 A) and the S parameter (172.421),
which is the sum of the 12 bite angles around the coordination
Department of Chemistry, School of Science,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033,
Japan. E-mail: nisihara@chem.s.u-tokyo.ac.jp;
Fax: +81 3-5841-8063; Tel: +81 3-5841-4346
w Electronic supplementary information (ESI) available. CCDC
811070–811071. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc11850a
Fig. 1 ORTEP diagram showing 50% probability and the pertinent
bonding properties of the cation molecule in Z-2 at 90 K. Hydrogen
atoms are omitted for clarity.
c
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Fig. 2 UV-vis spectral changes of Z-2 in acetone (0.106 mM) upon
irradiation with 436 nm light, and the spectrum of E-2 in acetone
(circles, 0.106 mM). Inset: time-course changes in absorbance of Z-2 at
450 nm.
Fig. 3 Time-course changes in (a) the IR spectrum, (b) colour, and
(c) transmittance at characteristic peaks of Z-2 pelletized with KBr
upon irradiation with 436 nm light.
sphere, suggested that the iron(II) center was in the HS
state.5,11 In contrast, the average Fe–L bond distance
(1.935 A) and the S parameter (84.641) for the E-2 complex
at 113 K (see the ESIw) suggested that the iron(II) center was in
the LS state.5,11 The different spin states of the Z-2 and E-2
compounds indicated that the spin state may be tuned via a
Z-to-E photoisomerization in the alkenyl moieties.
Photoisomerization of Z-2 in acetone upon photoirradiation
with 436 nm light, which corresponded to the energy of the
metal-to-ligand charge transfer (MLCT) band, was examined
by UV-vis spectroscopy (Fig. 2). In this experiment,
1.0 equivalent Z-1, which did not absorb light at this
wavelength, was added to the solution to avoid formation of
dissociated complexes (see the ESIw). The intensity of the
MLCT absorption band was observed to increase upon
photoirradiation. The spectral changes equilibrated to a
spectrum consistent with that of E-2, indicating a photo-
induced Z-to-E one-way isomerization of Z-2. On the other
hand, photoirradiation with 436 nm for E-2 in acetone caused
no photoreaction. It should be noted that the spectral changes
were absent in the dark, confirming that isomerization
occurred not by thermal activation, but by photoabsorption.
The one-way isomerization was thought to be derived from
the formal reduction of stilbene to stilbazole in the MLCT
excited state of Z-2, because electrochemically reduced
stilbene12 and some MLCT-excited ruthenium(II) complexes13
have demonstrated the same photoisomerization behaviours.
The uniqueness of the one-way photoisomerization behaviours
of the iron(II) complex was evident by comparison of its
wavelength-dependent photoisomerization with the properties
of the ligands and corresponding zinc(II) complexes (see
the ESIw).
Fig. 4 (a) Photographs and (b) temperature dependence of wMT for
Z-2 before and after visible light (4420 nm) irradiation for 115 hours.
equilibrated within 300 min to afford colour and IR spectrum
(see the ESIw) that were identical to those of E-2.
Photographs of the bulk microcrystalline compounds and
the temperature-dependence of wMT for Z-2 are shown in
Fig. 4. Bulk crystalline samples of Z-2 exhibited a wMT of
3.6 cm3 K molÀ1 at 300 K, which was typical for iron(II)
complexes in the HS state.11 This value did not change
significantly as the temperature was decreased to 30 K. Below
30 K, wMT suddenly dropped due to zero-field splitting of the
HS iron(II) complex in a distorted octahedral environment.
Thus, Z-2 occupied a HS state over the entire temperature
range measured.
In contrast, the product of Z-2 obtained after sufficient
visible light irradiation (4420 nm) for 115 hours displayed
thermal spin crossover. The irradiated sample exhibited a wMT
of 3.1 cm3 K molÀ1 at 300 K, which was smaller than the value
for Z-2 at the same temperature. wMT gradually decreased to
2.2 cm3 K molÀ1 at 100 K due to thermal spin crossover. wMT
did not change significantly upon cooling to 30 K. Below 30 K,
wMT suddenly dropped due to zero-field splitting. IR and
UV-vis spectra of the irradiated sample indicate that nearly
all of the Z-2 species in the solid state were isomerized (see the
ESIw) as well as in the solution or in a KBr pellet. Therefore
HS signatures observed in the magnetic susceptibility
measurements after photoirradiation shown in Fig. 4 were
derived from E-2.
We found that the visible light one-way photoisomerization
of Z-2 was active also in the solid phase. Irradiation of Z-2 in a
KBr pellet with 436 nm light produced gradual changes in
colour and in the IR spectrum (Fig. 3). A peak at 976 cmÀ1 in
the IR spectrum of Z-2, attributed to a C–H out-of-plane
bending mode of the alkenyl moieties,14 was shifted to 970 cmÀ1
upon photoirradiation. This energy decrease corresponded to
the difference in the peak positions of the non-substituted
Z-stilbene (966 cmÀ1) and the E-stilbene (960 cmÀ1), suggesting
the occurrence of Z-to-E photoisomerization.12 These changes
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In conclusion, we successfully demonstrated the photo-
chemical conversion of the magnetic properties of Z-2 using
visible light in both solution and solid states. Further efforts
toward identifying derivatives of Z-2 that exhibit more intense
photomagnetic conversions at room temperature are now
underway in our group.
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6848 Chem. Commun., 2011, 47, 6846–6848
This journal is The Royal Society of Chemistry 2011