298
N. Kojima and A. Okazawa
at about 7 K and disappears at about 25 K. On the other hand, the ZFCM curve
has two maxima (5 K and 17 K) and a hump at 22 K. The LTP and HTP of (SPMe)[Fe
II Fe
III (dto) 3 ] individually undergo the ferromagnetic phase transitions at 5 K
and 22 K, respectively.
When the UV light (350 nm with 40 mW cm
−2 ) is irradiated for 4 h at 300 K, the
FCM and RM curves change from the stepwise magnetization curves to normal
magnetization curves, and the peak of ZFCM at around 5 K disappears, being
reflected by the disappearance of LTP. On the other hand, being reflected by the
growth of HTP, the FCM and RM values between 5 and 22 K slightly increase after
the UV light irradiation. These photo-induced effects can be explained as follows.
The photo-isomerization of cationic SP-Me from CF to OF by UV irradiation leads to
the expansion of its own volume, which expands the unit cell volume and destabilizes the LTP. The photo-induced phase conversion of (SP-Me)[Fe
II Fe
III (dto) 3 ] is
schematically shown in Fig. 6.31e, where we name two phases, (A) and (B). Only in
the phase (A), the CTPT takes place. Suppose that the UV light irradiation induces
the transformation from the phase (A) to the phase (B), the LTP in the phase (A) is
forced to convert into the HTP in the phase (B) by UV light irradiation below 95 K.
In order to confirm the CTPT in [Fe
II Fe
III (dto) 3 ] induced by the photo-isomerization
of spiropyran, we performed UV light irradiation at 70 K, which corresponds to the
arrow of the left side in Fig. 6.31e. The FCM, RM, and ZFMC curves after UV light
irradiation for 5 h at 70 K are shown in Fig. 6.31d. Indeed, the photo-induced change
in the ferromagnetic property shows the same tendency as in the case of UV light
irradiation for 2 h at 300 K. Therefore, It is concluded that the destabilization of
LTP and the stabilization of HTP by the photo-isomerization of intercalated SP-Me
+
below 70 K induces the CTPT in the [Fe
II Fe
III (dto) 3 ] layer.
Moreover, in order to prove the disappearance of LTP by UV light irradiation,
we measured the
57 Fe Mössbauer spectra of (SP-Me)[Fe
II Fe
III (dto) 3 ] at 6 K before
and after the UV light irradiation (350 nm with 40 mW cm
−2 ) for 3 h at 300 K,
which is shown in Fig. 6.32 [7(b)]. As shown in Fig. 6.32a, the
57 Fe Mössbauer
spectra at 6 K before UV light irradiation shows two kinds of magnetically ordered
states. The sextet with a large hyperfine field from −8 to +8 mm s
−1 is typical of the
magnetically ordered spectrum of Fe
III
(t
3
2g e
2
g , S = 5/2) and the narrow quadrupole
doublet at around 0.3 mm s
−1 is assigned to the diamagnetic Fe
II
(t
6
2g , S = 0), which
indicates the magnetically ordered LTP. The magnetic sextets of Fe
II
(t
4
2g e
2
g , S = 2)
and Fe
III
(t
5
2g , S = 1/2) corresponding to the HTP are also observed. However, as
shown in Fig. 6.32b, after the UV light irradiation for 3 h at 300 K, the
57 Fe Mössbauer spectra corresponding to the LTP vanishes entirely at 6 K. In this way, the
remarkable change of
57 Fe Mössbauer spectra derived from the UV light irradiation
proves that the photo-isomerization of SP-Me
+ in (SP-Me)[Fe
II Fe
III (dto) 3 ] induces
the transformation from the phase (A) to the phase (B).
The photo-isomerization induced CTPT for (SP-Me)[Fe
II Fe
III (dto) 3 ] is illustrated
in Fig. 6.33. This new type of synergetic phenomenon coupled with spin, charge and
photon is triggered by the photo-isomerization of intercalated spiropyran from the
CF to the OF, which seems to have a significant similarity with the first event for
N. Kojima and A. Okazawa
at about 7 K and disappears at about 25 K. On the other hand, the ZFCM curve
has two maxima (5 K and 17 K) and a hump at 22 K. The LTP and HTP of (SPMe)[Fe
II Fe
III (dto) 3 ] individually undergo the ferromagnetic phase transitions at 5 K
and 22 K, respectively.
When the UV light (350 nm with 40 mW cm
−2 ) is irradiated for 4 h at 300 K, the
FCM and RM curves change from the stepwise magnetization curves to normal
magnetization curves, and the peak of ZFCM at around 5 K disappears, being
reflected by the disappearance of LTP. On the other hand, being reflected by the
growth of HTP, the FCM and RM values between 5 and 22 K slightly increase after
the UV light irradiation. These photo-induced effects can be explained as follows.
The photo-isomerization of cationic SP-Me from CF to OF by UV irradiation leads to
the expansion of its own volume, which expands the unit cell volume and destabilizes the LTP. The photo-induced phase conversion of (SP-Me)[Fe
II Fe
III (dto) 3 ] is
schematically shown in Fig. 6.31e, where we name two phases, (A) and (B). Only in
the phase (A), the CTPT takes place. Suppose that the UV light irradiation induces
the transformation from the phase (A) to the phase (B), the LTP in the phase (A) is
forced to convert into the HTP in the phase (B) by UV light irradiation below 95 K.
In order to confirm the CTPT in [Fe
II Fe
III (dto) 3 ] induced by the photo-isomerization
of spiropyran, we performed UV light irradiation at 70 K, which corresponds to the
arrow of the left side in Fig. 6.31e. The FCM, RM, and ZFMC curves after UV light
irradiation for 5 h at 70 K are shown in Fig. 6.31d. Indeed, the photo-induced change
in the ferromagnetic property shows the same tendency as in the case of UV light
irradiation for 2 h at 300 K. Therefore, It is concluded that the destabilization of
LTP and the stabilization of HTP by the photo-isomerization of intercalated SP-Me
+
below 70 K induces the CTPT in the [Fe
II Fe
III (dto) 3 ] layer.
Moreover, in order to prove the disappearance of LTP by UV light irradiation,
we measured the
57 Fe Mössbauer spectra of (SP-Me)[Fe
II Fe
III (dto) 3 ] at 6 K before
and after the UV light irradiation (350 nm with 40 mW cm
−2 ) for 3 h at 300 K,
which is shown in Fig. 6.32 [7(b)]. As shown in Fig. 6.32a, the
57 Fe Mössbauer
spectra at 6 K before UV light irradiation shows two kinds of magnetically ordered
states. The sextet with a large hyperfine field from −8 to +8 mm s
−1 is typical of the
magnetically ordered spectrum of Fe
III
(t
3
2g e
2
g , S = 5/2) and the narrow quadrupole
doublet at around 0.3 mm s
−1 is assigned to the diamagnetic Fe
II
(t
6
2g , S = 0), which
indicates the magnetically ordered LTP. The magnetic sextets of Fe
II
(t
4
2g e
2
g , S = 2)
and Fe
III
(t
5
2g , S = 1/2) corresponding to the HTP are also observed. However, as
shown in Fig. 6.32b, after the UV light irradiation for 3 h at 300 K, the
57 Fe Mössbauer spectra corresponding to the LTP vanishes entirely at 6 K. In this way, the
remarkable change of
57 Fe Mössbauer spectra derived from the UV light irradiation
proves that the photo-isomerization of SP-Me
+ in (SP-Me)[Fe
II Fe
III (dto) 3 ] induces
the transformation from the phase (A) to the phase (B).
The photo-isomerization induced CTPT for (SP-Me)[Fe
II Fe
III (dto) 3 ] is illustrated
in Fig. 6.33. This new type of synergetic phenomenon coupled with spin, charge and
photon is triggered by the photo-isomerization of intercalated spiropyran from the
CF to the OF, which seems to have a significant similarity with the first event for