6 Molecular Magnetism of Metal Complexes and Light-Induced …
269
the rapid spin equilibrium at the Fe
III site, (C 6 H 5 ) 4 P[Mn
II Fe
III (mto) 3 ] undergoes the
successive magnetic phase transitions at 30 and 23 K.
In the case of (n-C n H 2n+1 ) 4 N[Fe
II Fe
III (dto) 3 ], in addition to the ferromagnetic
phase transition, a spin-entropy driven charge transfer phase transition (CTPT)
takes place, in which the electrons of Avogadro’s constant transfer between the
Fe
II and Fe
III sites [4]. At the CTPT, the Fe valence state is dynamically fluctuated, which was revealed by means of muon spin relaxation (μSR) [5]. The CTPT
and the ferromagnetic phase transition strongly depend on the honeycomb ring size
in [Fe
II Fe
III (dto) 3 ] ∞ [6]. The increase of counter-cation size expands the honeycomb ring, which stabilizes and destabilizes the high temperature phase (HTP) and
the low temperature phase (LTP), respectively. In order to control the ferromagnetic properties and the CTPT by means of light irradiation, we have synthesized
a photo-responsive organic–inorganic hybrid system, (SP-Me)[Fe
II Fe
III (dto) 3 ] (SP
= spiropyran), and discovered that the photo-isomerization of intercalated SP-Me
by UV light irradiation induces the CTPT from the LTP to the HTP in the twodimensional [Fe
II Fe
III (dto) 3 ] layer and the change of T C from 5 to 22 K, by means
of magnetization measurement and
57 Fe Mössbauer spectroscopy [7].
In the Sect. 6.4, we show various kinds of molecular magnets such as SMMs
including transition-metal clusters, low-coordinated Fe complexes, and single-chain
magnets (SCMs) with easy-plane anisotropy.
6.2 Spin Crossover Phenomena
6.2.1 Static and Dynamic Spin Crossover Phenomena
Octahedral transition-metal complexes with d
4 –d
7 configurations have a possibility
of spin crossover transition between the LS and HS states in the ground state. If
the ligand field splitting energy is smaller than the spin-pairing energy in the d
orbitals, the d electrons occupy the t 2g (d xy , d yz , d zx ) and the e g (d x 2 −y 2 , d z 2 ) orbitals
being followed by Hund’s rule, in which the spin configuration shows the maximum
spin multiplicity (HS state). On the other hand, if the ligand field splitting energy
is larger than the spin-pairing energy, Hund’s rule is broken down, in which the
spin configuration shows the minimum spin multiplicity (LS state). Therefore, the
ground state of the transition metal ion with d
n (n = 4–7) is ruled by the competition
between ligand field splitting energy and spin-pairing energy in the d electrons, which
is schematically shown in Fig. 6.1.
The energy diagram for d
n system called Tanabe–Sugano diagram is the most
effective tool to analyze the competition between the HS and LS states as the ground
state [8]. The Tanabe–Sugano diagram exhibits the energies of multiplets for 3d
n (n
= 2–8) system as a function of the ratio of ligand field (Dq) to the Racah parameter
(B) representing the strength of Coulomb interaction between 3d electrons. In the
case of 3d
n (n = 4–7), if the ground energies of HS and LS states are close to each
269
the rapid spin equilibrium at the Fe
III site, (C 6 H 5 ) 4 P[Mn
II Fe
III (mto) 3 ] undergoes the
successive magnetic phase transitions at 30 and 23 K.
In the case of (n-C n H 2n+1 ) 4 N[Fe
II Fe
III (dto) 3 ], in addition to the ferromagnetic
phase transition, a spin-entropy driven charge transfer phase transition (CTPT)
takes place, in which the electrons of Avogadro’s constant transfer between the
Fe
II and Fe
III sites [4]. At the CTPT, the Fe valence state is dynamically fluctuated, which was revealed by means of muon spin relaxation (μSR) [5]. The CTPT
and the ferromagnetic phase transition strongly depend on the honeycomb ring size
in [Fe
II Fe
III (dto) 3 ] ∞ [6]. The increase of counter-cation size expands the honeycomb ring, which stabilizes and destabilizes the high temperature phase (HTP) and
the low temperature phase (LTP), respectively. In order to control the ferromagnetic properties and the CTPT by means of light irradiation, we have synthesized
a photo-responsive organic–inorganic hybrid system, (SP-Me)[Fe
II Fe
III (dto) 3 ] (SP
= spiropyran), and discovered that the photo-isomerization of intercalated SP-Me
by UV light irradiation induces the CTPT from the LTP to the HTP in the twodimensional [Fe
II Fe
III (dto) 3 ] layer and the change of T C from 5 to 22 K, by means
of magnetization measurement and
57 Fe Mössbauer spectroscopy [7].
In the Sect. 6.4, we show various kinds of molecular magnets such as SMMs
including transition-metal clusters, low-coordinated Fe complexes, and single-chain
magnets (SCMs) with easy-plane anisotropy.
6.2 Spin Crossover Phenomena
6.2.1 Static and Dynamic Spin Crossover Phenomena
Octahedral transition-metal complexes with d
4 –d
7 configurations have a possibility
of spin crossover transition between the LS and HS states in the ground state. If
the ligand field splitting energy is smaller than the spin-pairing energy in the d
orbitals, the d electrons occupy the t 2g (d xy , d yz , d zx ) and the e g (d x 2 −y 2 , d z 2 ) orbitals
being followed by Hund’s rule, in which the spin configuration shows the maximum
spin multiplicity (HS state). On the other hand, if the ligand field splitting energy
is larger than the spin-pairing energy, Hund’s rule is broken down, in which the
spin configuration shows the minimum spin multiplicity (LS state). Therefore, the
ground state of the transition metal ion with d
n (n = 4–7) is ruled by the competition
between ligand field splitting energy and spin-pairing energy in the d electrons, which
is schematically shown in Fig. 6.1.
The energy diagram for d
n system called Tanabe–Sugano diagram is the most
effective tool to analyze the competition between the HS and LS states as the ground
state [8]. The Tanabe–Sugano diagram exhibits the energies of multiplets for 3d
n (n
= 2–8) system as a function of the ratio of ligand field (Dq) to the Racah parameter
(B) representing the strength of Coulomb interaction between 3d electrons. In the
case of 3d
n (n = 4–7), if the ground energies of HS and LS states are close to each