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5 Reversible Spin-State Switching Involving a Structural Change of Fe(NCX)2(bpp)2 2(Benzene) (X = Se, BH3) Triggered by Sorption of Benzene Molecules [30]

5 Reversible Spin-State Switching Involving a Structural Change of Fe(NCX)2(bpp)2 2(Benzene) (X = Se, BH3) Triggered by Sorption of Benzene Molecules [30]

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150



8 REVERSIBLE SPIN-STATE SWITCHING INVOLVING A STRUCTURAL CHANGE



1.00

0.99

0.98



fresh



78 K



desorbed



78 K



readsorbed



78 K



0.97



Relative transmission



0.96

1.00

0.98

0.96

0.94

0.92

0.90

1.00

0.98

0.96

0.94



FIGURE 8.8

57

€ ssbauer spectral change

Fe Mo

of fresh Fe(NCBH3)2(bpp)2 Á 2

(benzene) to desorbed sample

and then to readsorbed sample.



0.92

0.90

–4



–2



0



2



4



Velocity (mm s–1)



Fe(NCSe)2(bpp)2 Á 2(benzene), while the species disappeared when benzene was readsorbed. This reversible spin

change of iron involving the structural change between 1D and 2D interpenetrated structures was confirmed by the

magnetic measurements with SQUID.

Fe(NCBH3)2(bpp)2 shows a clear spin-crossover phenomenon between high- and low-spin states [22]. On the other

hand, Fe(NCBH3)2(bpp)2 Á 2(benzene) showed a temperature-independent high-spin state. In this case, powder X-ray

diffraction pattern also revealed a reversible structural change between 1D and 2D interpenetrated structures associated

with the desorption and adsorption of benzene molecules. SQUID revealed that the structural change of

Fe(NCBH3)2(bpp)2 Á 2(benzene) caused by desorbing benzene molecules accompanies a spin-crossover phenomenon,

while temperature-independent high-spin FeII is observed by the readsorption of benzene molecules. 57 Fe M€

ossbauer

spectra of these samples are shown in Fig. 8.8. Fresh Fe(NCBH3)2(bpp)2 Á 2(benzene) shows a high-spin FeII at 78 K. By

desorbing benzene, low-spin FeII was observed at 78 K. Readsorption of benzene caused appearance of high-spin FeII at

78 K. It is worth noting that reversible structural change and sharp “spin-state switching,” caused by the sorption and

desorption of benzene molecules, are clearly observed in the M€

ossbauer spectra.



8.6 CONCLUSIONS

The anti–gauche conformer of the bridging ligand contributes to the type of assembled structures. The gauche conformer

forms a 1D chain structure, while the anti conformer forms 2D and interpenetrated structures in Fe(NCX)2(bpa)2 (X ¼ S,

Se). 2D and interpenetrated structures enclathrate ethanol or propanol molecules. The interpenetrated structure

changes to 1D chain structure by desorbing propanol molecules.

The occurrence of spin-crossover phenomenon can be observed by enclathrating organic guest molecules such as

biphenyl in Fe(NCX)2(bpa)2 (X ¼ S, Se, BH3). The type of spin-crossover phenomenon changes, depending on the

assembled structure. By introducing one methylene group to bpa, bpp has a variety of conformer and it contributes to

the formation of a novel 2D interpenetrated structure. The benzene-enclathrated complex shows a 1D structure.

Reversible structural change caused by sorption of benzene molecules was observed in Fe(NCX)2(bpp)2 Á 2(benzene)

(X ¼ S, Se, BH3). In the case of NCSeÀ and NCBH3À complexes, reversible structural change and spin-state switching

are observed by the sorption of benzene molecules.



REFERENCES



151



It is considered that the conformer of bridging ligand changes between anti and gauche so that the complex can fit

guest molecules or avoid vacancy in the assembled structure. The coordination bond has to be dissociated to form a

new assembled structure. It is considered that the coordination bond is not strong enough to maintain the 1D structure

of Fe(NCX)2(bpp)2 Á 2(benzene) (X ¼ S, Se, BH3) when benzene molecules are desorbed.



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C H A P T E R 9



SPIN-CROSSOVER AND RELATED

PHENOMENA COUPLED WITH

SPIN, PHOTON, AND CHARGE



NORIMICHI KOJIMA AND AKIRA SUGAHARA

Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo, Japan



9.1 INTRODUCTION

When a transition metal ion with an electron configuration of dn (n ¼ 4 À 7) is octahedrally coordinated by ligands, it is

possible that the low-spin (LS) state and the high-spin (HS) state compete with each other in the ground state. If the ligand

field splitting energy (10Dq) is smaller than Coulomb interaction between electrons in the d orbitals, the electrons

occupy the eg (dx2Ày2, dz2) and t2g (dxy, dyz, dzx) orbitals with the spin alignment determined by Hund’s rule (HS state). On

the other hand, if the ligand field splitting energy is large enough, Hund’s rule is broken down because the Coulomb

interaction energy between d electrons does not overcome 10Dq, in which the spin configuration is called LS state.

Therefore, the ground state of the transition metal ion with dn (n ¼ 4 À 7) configuration is determined by the competition

between ligand field splitting energy and Coulomb interaction between d electrons. The energy diagram for dn system

called Tanabe–Sugano diagram is the most powerful method to analyze the competition between the HS and LS states as

the ground state [1]. The Tanabe–Sugano diagram is a graph that plots the energy of multiplet terms against the ratio of

ligand field (Dq) to the Racah parameter (B) representing the strength of Coulomb interaction. For the case of d5

configuration, the Tanabe–Sugano diagram indicates that the ground state changes between HS (Dq/B 2.8: 6 A1g ) and LS

(Dq/B ! 2.8: 2 T2g ) in the vicinity of Dq/B ¼ 2.8. If the energy of ground state is close to HS and LS states, the system has a

possibility to change the spin states between HS and LS by external perturbation, such as heat, applied pressure, or light

irradiation. Such HS–LS transition is called spin-crossover phenomenon.

The spin-crossover phenomenon was observed for the first time by Cambi et al. in 1930s for tris(dithiocarbamato)

iron(III) complexes, [Fe(S2CNR2)3] (R ẳ n-CnH2nỵ19, etc.) [2]. In this system, the Fe(III) atom is coordinated by six S

atoms. They reported that the magnetic susceptibility changed remarkably depending on temperature caused by the LS

ossbauer spectra exhibited a broad single quadrupole

(2 T2g , S ¼ 1/2)–HS (6 A1g , S ¼ 5/2) transition. In this system, 57 Fe M€

doublet in the temperature region of the gradual LS (S ¼ 1/2) $ HS (S ¼ 5/2) transition, which implies the rapid spin

equilibrium between the HS and LS states in the timescale of t < 10À7 s [3–5].

Thirty years later, the first Fe(II) spin-crossover complex, [Fe(phen)2X2] (phen ¼ 1,10-phenanthroline, X ¼ NCS or

NCSe), was discovered by Baker et al. [6]. In [Fe(phen)2(NCS)2], the Fe(II) atom is surrounded by six N atoms of phen

and NCS ligand molecules, and the complex exhibits a first-order phase transition associated with an abrupt LS (1 A1g ,

S ¼ 0)–HS (5 T2g , S ¼ 2) transition at 176 K, where a small thermal hysteresis attributed to the spin transition was

observed. Since the discovery of the spin-crossover phase transition for [Fe(phen)2X2] (X ¼ NCS, NCSe), various kinds

of spin-crossover complexes have been found for the electron configurations of 3dn (n ¼ 4 À 7). Most of them are Fe(II)

M€

ossbauer Spectroscopy: Applications in Chemistry, Biology, and Nanotechnology, First Edition.

Edited by Virender K. Sharma, G€

ostar Klingelh€

ofer, and Tetsuaki Nishida.

Ó 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.



152



153



9.2 PHOTOINDUCED SPIN-CROSSOVER PHENOMENA



and Fe(III) complexes [7], and to a lesser extent Co(II) complexes [8]. Relatively few examples refer to Cr(II), Mn(II), Mn

(III), and Co(III) complexes [9].

The spin-crossover phenomenon has gained renewed importance since the discovery of the photoinduced spin

transition for [Fe(ptz)6](BF4)2 (ptz ¼ 1-propyltetrazole) in 1984 [10]. This complex was known to exhibit a spin transition

at 135 K with a remarkable change of color. The color in the LS state is purple, while that in the HS state is colorless.

Decurtins et al. discovered a persistent photoinduced HS state for [Fe(ptz)6](BF4)2 when they measured the optical

absorption spectra at 8 K with white light within the spin-allowed absorption region (1 A1g ! 1 T1g transition), where the

deeply purple color was totally “bleached” and the colorless remained unchanged as long as the temperature was kept

below 50 K [10,11]. This photoinduced spin-crossover phenomenon has been named LIESST (light-induced excited spinstate trapping) by them. Being stimulated by the discovery of LIEEST for [Fe(ptz)6](BF4)2, more than 70 spin-crossover

complexes showing LIESST have been reported for Fe(II) and Fe(III) complexes [12]. If the light-induced phase transition

between nonmagnetic and magnetically ordered states is realized at room temperature, the photoinduced cooperative

phenomenon will open a large field of photonic molecular devices. From the viewpoint of this strategy, various kinds of

assembled-metal complexes whose spin state is situated in the spin-crossover region have been synthesized. Recently,

the photoinduced spin-crossover phenomenon at room temperature was reported for [Fe(pyrazine){Pt(CN)4}]

(pyrazine ¼ C4H4N2) [13].

In the case of mixed-valence systems whose spin states are situated in the spin-crossover region, it is expected that

new types of conjugated phenomena coupled with spin and charge take place between different metal ions in order to

minimize the free energy in the whole system. In fact, in (n-CnH2nỵ1)4N[FeIIFeIII(dto)3] (n ẳ 3, 4), a new type of phase

transition called charge transfer phase transition (CTPT) takes place around 120 K, where the thermally induced charge

transfer between FeII and FeIII occurs reversibly [14–16]. In the case of (n-CnH2nỵ1)4N[FeIIFeIII(mto)3] (mto ẳ C2O3S), a

rapid spin equilibrium between the HS state (6 A1g , S ¼ 5/2) and the LS state (2 T2g , S ¼ 1/2) at the FeIIIO3S3 site takes place

in a wide temperature range, which induces the valence fluctuation of the FeS3O3 and FeO6 sites through the

ferromagnetic coupling between the LS state (S ¼ 1/2) of the FeIIIS3O3 site and the HS state (S ¼ 2) of the FeIIO6

site [17].

In this chapter, we focus on the spin-crossover phenomenon and its related phenomena for Fe(II) and Fe(III)

complexes from the viewpoint of M€

ossbauer spectroscopy. In Section 9.2, the LIESST and the reverse-LIESST induced by

the d–d transition for Fe(II) and Fe(III) complexes are described. In Section 9.3, the charge transfer phase transition in

(n-CnH2nỵ1)4N[FeIIFeIII(dto)3] (n ẳ 3, 4) and the photoinduced charge transfer phase transition in (SP)[FeIIFeIII(dto)3]

(SP ¼ spiropyran) [18] are described. In Section 9.4, the rapid spin equilibrium and succeeding phenomena in

A[MIIFeIII(mto)3] (A ẳ Ph4P, (n-CnH2nỵ1)4N; M ẳ Zn, Fe) are described.



9.2 PHOTOINDUCED SPIN-CROSSOVER PHENOMENA

9.2.1 LIESST for Fe(II) Complexes

For the case of d6 configuration, the Tanabe–Sugano diagram indicates that the ground state changes between

HS (Dq/B 2.0: t2g4eg2(5 T2g )) and LS (Dq/B ! 2.0: t2g6(1 A1g )) in the vicinity of Dq/B ¼ 2.0 [1]. If the energy of ground

state is situated around Dq/B ¼ 2.0, the system has the possibility to undergo the spin-crossover phenomenon between

HS (5 T2g , S ¼ 2) and LS (1 A1g , S ¼ 0).

The spin-crossover phenomenon has gained renewed importance since the discovery of the LIESST for [Fe(ptz)6]

(BF4)2 (ptz ¼ 1-propyltetrazole) [10,11]. In this section, the LIESST and reverse-LIESST for [Fe(ptz)6](BF4)2 are described.

[Fe(ptz)6](BF4)2 has a highly symmetric rhombohedral structure (R3; Z ¼ 3) [19]. The Fe(II) ion, which is coordinated by

six N4 atoms belonging to 1-propyltetrazole (Scheme 9.1), shows the thermally induced spin transition from the LS state



4



N

N C3H7

N N 1

3

2



SCHEME 9.1

Molecular structure of 1-propyltetrazole.



154



9 SPIN-CROSSOVER AND RELATED PHENOMENA COUPLED WITH SPIN, PHOTON, AND CHARGE



FIGURE 9.1

Effective magnetic moment of

[Fe(ptz)6](BF4)2 as a function of

temperature and excitation with

Arỵ laser. (Reproduced from

Ref. 11 with permission of the

American Chemical Society.)



(1 A1g : S ¼ 0) to the HS state (5 T2g : S ¼ 2) at about 130 K, which is shown in Fig. 9.1 [11]. The spin transition from the LS

state to the paramagnetic HS state involves a crystallographic first-order phase transition with a thermal hysteresis of

about 7 K (Tc" ¼ 135 K, Tc# ¼ 128 K) and a change in Fe(II)–N bond length of !0.20ỵ [20].

According to the TanabeSugano diagram [1] for an iron with d6 electron configuration in an octahedral ligand field

(Fig. 9.2), one can expect quite different absorption spectra for the HS or LS complexes. In the HS complex, one spinallowed absorption band corresponding to the 5 T2g ! 5 E transition is observed, whereas two spin-allowed absorption

bands corresponding to the 1 A1g ! 1 T1g and 1 A1g ! 1 T2g transitions are observed for the LS complex. Therefore, the

absorption spectra of a spin-crossover complex below/above the spin transition temperature (Tc) should clearly reflect a

thermally induced spin transition. Figure 9.3 shows the optical absorption spectra for [Fe(ptz)6](BF4)2 at 273 and 8 K [11].

The one absorption band at 11,760 cmÀ1 in the HS state is assigned to the 5 T2g ! 5 E transition, and the two absorption

bands at 18,210 and 26,400 cmÀ1 in the LS state are assigned to the 1 A1g ! 1 T1g and 1 A1g ! 1 T2g transitions,

respectively. The assignment of the absorption spectra corresponding to the HS and LS states are shown in Fig. 9.2.



FIGURE 9.2

Tanabe–Sugano diagram for d6

electron configuration.

(Reproduced from Ref. 1 with

permission of the Physical Society

of Japan.)



155



9.2 PHOTOINDUCED SPIN-CROSSOVER PHENOMENA



FIGURE 9.3

Single crystal absorption spectra

(unpolarized) of [Fe(ptz)6](BF4)2

before bleaching (b.b.) and after

bleaching (a.b.) at 8 K for 2 min

with white light. (Reproduced

from Ref. 11 with permission of

the American Chemical Society.)



Taking account of the energy positions for the spin-allowed d–d transitions, the ligand field strengths for the HS and LS

states of [Fe(ptz)6](BF4)2 are estimated at 10Dq ¼ 11,760 cmÀ1 and 10Dq ¼ 20,550 cmÀ1, respectively [11]. Reflecting the

spin transition, the color of [Fe(ptz)6](BF4)2 changes drastically. [Fe(ptz)6](BF4)2 in the LS state (1 A1g : S ¼ 0) is deep

purple, whereas that in the HS state (5 T2g : S ¼ 2) is colorless. Decurtins et al. discovered the persistent photoinduced HS

state for [Fe(ptz)6](BF4)2 when they measured the optical absorption spectra at 8 K with white light or with laser light

within the singlet absorption region (1 A1g ! 1 T1g transition), where the deeply purple color is totally “bleaching” and the

colorless remains unchanged as long as the temperature is kept below 50 K [10,11]. This photoinduced spin-crossover

phenomenon has been named LIESST. Figure 9.3 shows the corresponding single crystal absorption spectrum before

and after “bleaching” at 8 K with white light, and it clearly demonstrates the light-induced and trapped spin state

corresponding to the HS state appearing above 130 K [11]. The LIESST effect is also reflected in the change of

the magnetic susceptibility, which is shown in Fig. 9.1 [11]. The excitation with the Arỵ laser light corresponding to the

1

A1g ! 1 T1g transition at T < 50 K indicates light-induced spin conversion, which results in a raise of the effective

magnetic moment from 1.0 to 6.5–7.0mB in the temperature region of 10–50 K.

ossbauer spectra of [Fe(ptz)6](BF4)2 under several conditions [10]. Figure 9.4a and b

Figure 9.4 shows the 57 Fe M€

shows the M€

ossbauer spectra before and after the irradiation of white light with 450 W xenon arc lamp at 15 K,



FIGURE 9.4

57

€ ssbauer spectra of [Fe

Fe Mo

(ptz)6](BF4)2. (a) Before bleaching

(measuring temperature TM ¼ 15

K), (b) after bleaching for 1 h at

15 K (TM ¼ 15 K), (c) after warming to 50–55 K and cooling to

TM ¼ 15 K, (d) after second

warming to 50–55 K and cooling

to TM ¼ 15 K, (e) after warming to

97 K (TM ¼ 97 K), and (f) after

warming to 148 K (TM ¼ 148 K).

(Reproduced from Ref. 10 with

permission of Elsevier.)



156



9 SPIN-CROSSOVER AND RELATED PHENOMENA COUPLED WITH SPIN, PHOTON, AND CHARGE



FIGURE 9.5

Potential wells of the 1 A1g

ground state and the thermally

accessible 5 T2g state as well as the

higher excited ligand field states

with d6 electron configuration in

the spin-crossover region. The

mechanisms of LIESST and

reverse-LIESST are indicated by

arrows. (Reproduced from Ref.

21 with permission of Elsevier.)



respectively. As shown in Fig. 9.4a and b, the spin state of [Fe(ptz)6](BF4)2 is completely converted from the LS state to

the HS state by the irradiation of white light at 15 K. The remarkable asymmetry in the quadrupole doublet

corresponding to the HS state is attributed to the texture of the thin plate crystals. The photoinduced HS state

remains as metastable state as long as the temperature is kept below 50 K, while the metastable HS state relaxes to the

ground LS state between 55 and 130 K. Figure 9.4f is the M€

ossbauer spectrum in the thermally induced HS state at 148 K.

Figure 9.5 shows the potential energy wells of the low-lying ligand field states with d6 electron configuration such as

Fe(II) in the region of spin crossover [21,22]. A mechanism for the light-induced 1 A1g ! 5 T2g conversion called LIESST

was proposed by Decurtins et al. [10,11], involving a first intersystem crossing step with DS ¼ 1 from either the initially

excited 1 T1g state to the lower lying 3 T1g state. In a second intersystem crossing step with DS ¼ 1, the system can drop



into the 5 T2g state, where at sufficiently low temperatures it remains trapped. The difference of 0.20 A in metal–ligand

bond length between the HS and LS states effectively separates the two potential wells. The trapped 5 T2g state (the

ground state for HS state) relaxes thermally back to the 1 A1g ground state above 50 K, but the system can also be optically

pumped back to the 1 A1g ground state via the lower lying 3 T1g state by light irradiation corresponding to the 5 T2g ! 5 E

transition, which is called reverse-LIESST [21,22].

Figure 9.6 shows the demonstration of LIESST and reverse-LIESST for [Fe(ptz)6](BF4)2 [21]. When the Arỵ laser

(514.5 nm), whose wavelength corresponds to that of the 1 A1g ! 1 T1g transition, is irradiated for [Fe(ptz)6](BF4)2 at

10 K, the absorption spectra corresponding to the 1 A1g ! 1 T1g and 1 A1g ! 1 T2g transitions in the LS state completely

disappear and a new absorption band corresponding to the 5 T2g ! 5 E transition in the HS state appears at 12,300 cmÀ1.

This phenomenon conclusively proves that the system after the Arỵ laser at 10 K is trapped in the metastable HS state

(5 T2g ). On the other hand, when the Krỵ laser (752.7 nm), whose wavelength corresponds to that of 5 T2g ! 5 E

transition, is irradiated for the photoinduced HS state at 10 K, the absorption spectra corresponding to the 1 A1g ! 1 T1g

and 1 A1g ! 1 T1g transitions in the LS state are completely restored, which implies the reverse-LIESST. Being stimulated

by the discovery of LIEEST for [Fe(ptz)6](BF4)2, more than 60 spin-crossover complexes showing LIESST have been

reported [9,12,23,24].

In connection with the determination of the critical temperature of LIESST, Letard et al. have proposed the following

standard condition [23,24]. The sample in the LS state is first slowly cooled down to 10 K. The sample in the LS state is then

irradiated with the light corresponding to the metal-to-ligand charge-transfer (MLCT) band or the d–d absorption band.

After irradiating the light for 1 h, the light irradiation is switched off, and the temperature is increased at the rate of

0.3 K minÀ1. The @(xMT)/@T as a function of T shows a minimum at a temperature that is defined as the critical temperature

(Tc(LIESST)). In this manner, Tc(LIESST) of [Fe(ptz)6](BF4)2 can be estimated at 60 K, which is shown in Fig. 9.7 [23].



157



9.2 PHOTOINDUCED SPIN-CROSSOVER PHENOMENA



80

1A 1T

1

1

Iºλ



60



Iλ(xo.t)



FIGURE 9.6



5T

2



xo

5



E



20



10



Near-infrared and visible

absorption spectra of [Fe(ptz)6]

(BF4)2 at 10 K: (—) before light

irradiation, (– – –) after

irradiation with l ¼ 514.5 nm

(300 mJ), and (Á Á Á Á Á Á) after

subsequent irradiation with

l ¼ 752.7 nm (%3000 mJ).

(Reproduced from Ref. 21 with

permission of Elsevier.)



x

514.5 nm



ε



0



752.7 nm



40



15



20



25



ν (× 103 cm–1)



The LIESST and reverse-LIESST observed for various kinds of Fe(II) complexes are fascinating phenomena from the

viewpoint of molecular devices. The LIESST behaves as the recording of optical information, while the reverse-LIESST

behaves as the erasing of optical memory. One of the most important properties of the optical memory is the thermal

stability of the light-induced metastable state. In the case of [Fe(ptz)6](BF4)2, however, the light-induced HS state is

persistent below 60 K. The possibility to address spin states through light irradiation will open the perspectives of optical

switches and magneto-optical storage. Up to now, in the case of Fe(II) complexes, the highest critical temperature of

LIESST is about 130 K for [Fe(L)(CN)2] Á H2O (L ¼ 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12,3,1]octadeca-1

(18),2,12,14,16-pentaene) [25]. In this case, the highest Tc(LIESST) has successfully been achieved through the hydrogen

bond between the CN ligand and H2O.

9.2.2 LIESST for Fe(III) Complexes

Figure 9.8 shows the Tanabe–Sugano diagram of d5 system [1]. In the vicinity of Dq/B ¼ 2.8, the diagram indicates that the

ground state changes between HS (Dq/B 2.8: t2g3eg2(6 A1g )) and LS (Dq/B ! 2.8: t2g5(2 T2g )). If the energy of ground state

is close to HS and LS states, the system has a possibility to undergo the spin-crossover phenomenon between HS (6 A1g ,

S ¼ 5/2) and LS (2 T2g , S ¼ 1/2) by external perturbation, such as heat, applied pressure, or light irradiation. In Fe(II) and

Fe(III) spin-crossover complexes, the most important differences are the change of bond distances between the LS and



HS states. The average change in these bond distances is 0.18 A for Fe(II) complexes, while that in these bond distances is



∂χMT/∂ T

0.0

5

–0.5



(a)



4

χmT (cm3 K mol–1)



FIGURE 9.7



(b)



–1.0



3

[Fe(ptz)6](BF4)2



2



–1.5

Tc (LIESST) = 60 K



60

T (K)



40



1

0

10



20



30



50

40

T (K)



60



70



80



90



80



(a) Temperature dependence of

molar magnetic susceptibility,

xMT, for [Fe(ptz)6](BF4)2. ^: data

recorded in the cooling mode

without light irradiation, : data

recorded with light irradiation

for 1 h at 10 K, _: data recorded in

the heating mode of 0.3 K minÀ1

without light irradiation after

the light irradiation for 1 h at

10 K. (b) Derivative curve of

@(xMT)/@T. (Reproduced from

Ref. 23 with permission of

Elsevier.)



158



9 SPIN-CROSSOVER AND RELATED PHENOMENA COUPLED WITH SPIN, PHOTON, AND CHARGE



E/B



4A



2



2A



1



70

2S



4F



4D

4P

4G



4T



1



4E

4T

2

4A 4 E

1,

2A

1

2T1



2F

2G

2H

2F

2D

4F

2I



2T



2



40

4D

4P

4G

30



2E



2A , 2

2 T1

4T



2



6



FIGURE 9.8

Tanabe–Sugano diagram for d5

electron configuration.

(Reproduced from Ref. 1 with

permission of the Physical Society

of Japan.)







A1(t23e2)

4 T (t 4 )

1 2 e



20

10

6S



6A



6S



0



1



2

1



2



3



T2 (t25)



Dq/ B



0.12 A for Fe(III) complexes. Comparing the change of bond distances between the LS and HS states for Fe(III) complexes

with that for Fe(II) complexes, it is presumed that the activation energy from the photoinduced HS state to the ground LS

state for Fe(III) complexes is much smaller than that for Fe(II) complexes. Therefore, in the case of Fe(III) complexes, it is

considered that the light-induced HS state is rapidly relaxed to the ground LS state and the persistent LIESST would not

be realized. However, beyond this speculation, various spin-crossover Fe(III) complexes exhibiting persistent LIESST

have been developed by preventing the rapid relaxation from the photoinduced HS state to the ground LS state through

the intermolecular interactions such as hydrogen bond or p–p stacking interaction between the neighboring ligands

[26,27]. Figure 9.9 shows the molecular structure and the p–p stacking between the neighboring ligands for the typical

spin-crossover Fe(III) complex exhibiting LIESST effect, [Fe(pap)2]ClO4 Á H2O (pap ¼ deprotonated bis(2-hydroxyphenyl-(2-pyridyl)-methaneimine)) [26]. This complex shows a thermally induced spin-crossover phenomenon (Tc"

¼ 180 K, Tc# ¼ 165 K) with a thermal hysteresis of 15 K, which is shown in Fig. 9.10. Moreover, this complex exhibits a

frozen-in effect in the rapid cooling process. When [Fe(pap)2]ClO4 Á H2O is rapidly cooled from room temperature to

5 K, the HS state is trapped as a metastable state without the spin-crossover transition and is stable below 105 K, which

strongly supports the first observation of LIESST for Fe(III) complex.



FIGURE 9.9

Molecular structure and the

representation of p–p stacking

between the neighboring ligands of [Fe(pap)2]ClO4 Á H2O.

(Reproduced from Ref. 26 with

permission of the American

Chemical Society.)



159



9.2 PHOTOINDUCED SPIN-CROSSOVER PHENOMENA



FIGURE 9.10

Temperature dependence of xMT

for [Fe(pap)2]ClO4 Á H2O. Sample

was cooled from 300 to 5 K (5)

and then warmed from 5 to 300 K

(4) at a rate of 2 K mmÀ1. Sample

was warmed at a rate of 2 K

mmÀ1 after it was quenched to

5 K (~). (Reproduced from

Ref. 26 with permission of the

American Chemical Society.)



Figure 9.11 shows the 57 Fe M€

ossbauer spectra of [Fe(pap)2]ClO4 Á H2O under several conditions [26]. Figure 9.11a

ossbauer spectra before and after the irradiation of white light with 400–600 nm at 13 K,

and b shows the 57 Fe M€

respectively. As shown in Fig. 9.11a and b, the spin state of [Fe(pap)2]ClO4 Á H2O is converted from the LS state to the HS

state by the irradiation at 13 K. Figure 9.11c shows the spectrum at 13 K after annealing at 130 K. The photoinduced HS

state remains as a metastable state as long as the temperature is kept below 105 K, while the metastable HS state relaxes

to the ground LS state around 130 K. In this manner, the first observation of LIESST effect with high Tc(LIESST) of 105 K

for Fe(III) complex has been achieved through the p–p stacking between neighboring ligands.



FIGURE 9.11

57

€ ssbauer spectra for [Fe

Fe Mo

(pap)2]ClO4 Á H2O at 13 K. (a)

Before light irradiation, (b) after

light irradiation, and (c) after

thermal treatment at 130 K.

(Reproduced from Ref. 26 with

permission of the American

Chemical Society.)



160



9 SPIN-CROSSOVER AND RELATED PHENOMENA COUPLED WITH SPIN, PHOTON, AND CHARGE



FIGURE 9.12

Schematic structure of

[Fe(pyrazine){Pt(CN)4}].

(Reproduced from Ref. 13 with

permission of Wiley.)



9.2.3 Recent Topics of Photoinduced Spin-Crossover Phenomena

Since the discovery of LIEEST for [Fe(ptz)6](BF4)2 in 1984 [10], more than 70 spin-crossover complexes showing LIESST

have been reported for Fe(II) and Fe(III) complexes. If the light-induced phase transition between nonmagnetic and

magnetically ordered states is realized at room temperature, the photoinduced cooperative phenomenon will open a

large field of photonic molecular devices. However, up to now, the highest critical temperature of LIESST is 130 K for

[Fe(L)(CN)2] Á H2O [25], which is a serious limitation for the development of optical switches based on spin-crossover

phenomena. One of possible approaches to overcome this problem and realize the photoinduced spin-crossover

phenomenon around room temperature is the photoirradiation within the thermal hysteresis loop of spin-crossover

transition. From the viewpoint of this strategy, recently, the photoinduced spin-crossover phenomenon at room

temperature has successfully been achieved for [Fe(pyrazine){Pt(CN)4}] (pyrazine ¼ C4H4N2) by means of photoirradiation within the thermal hysteresis loop of spin-crossover transition [13].

Figure 9.12 shows the crystal structure of [Fe(pyrazine){Pt(CN)4}] [13]. This complex shows a thermally induced

spin-crossover transition (Tc" ¼ 284 K, Tc# ¼ 308 K) with a thermal hysteresis of 24 K, which was observed by means of

magnetic susceptibility measurement and Raman spectroscopy. The spin-crossover transition has been confirmed by

57

Fe M€

ossbauer spectroscopy [13]. The M€

ossbauer spectrum at 300 K in the cooling mode consists of a single doublet

with quadrupole splitting (QS) of 1.159(5) mm sÀ1 and isomer shift (IS) of 1.047(3) mm sÀ1 whose values are typical of the

HS state (5 T2g , S ¼ 2) of Fe(II). At 80 K, a new doublet with quadrupole splitting of 0.306(4) mm sÀ1 and isomer shift of

0.439(2) mm sÀ1 whose values are typical of the LS state (1 A1g , S ¼ 0) of Fe(II). The photoinduced spin conversion

between the LS and HS states around room temperature has been confirmed by means of Raman spectroscopy within

the thermal hysteresis loop of spin-crossover transition, which is shown in Fig. 9.13 [13]. In this complex, the frequency of



FIGURE 9.13

Proportion of HS iron(II) ions

before and after a one-shot laser

pulse of irradiation on the

ascending and descending

branches of the hysteresis loop of

[Fe(pyrazine){Pt(CN)4}]. The

insets show the Raman spectra

recorded before and after light

irradiation. (Reproduced from

Ref. 13 with permission of Wiley.)



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