Design, synthesis, magnetic properties of a π-radical ligand with photo-excited high-spin state and its Fe(ii) complex. the first stage of a new strategy for LIESST materials

Article abstract of DOI:10.1039/c2dt31831h

A π-radical ligand (9-[4-(6-oxo-1,5-dimethylverdazyl)phenyl]-10-[5-(2, 2′-bipyridyl)ethynyl]anthracene, L¹) with a photo-excited high-spin quartet state (S = 3/2) and its iron(ii) complex [Fe(L ¹){H₂B(Pz)₂}₂] (1) {H ₂B(Pz)₂^- = dihydrobis(1-pyrazolyl)borate} were synthesized as a candidate for a new strategy for spin-crossover compounds exhibiting light-induced excited spin state trapping (LIESST), which is via the photo-excited high-spin state of the π-conjugated aromatic system. Control compounds, ligand L² and [Fe(L²){H₂B(Pz) ₂}₂] (2), in which the verdazyl radical moiety in L ¹ was removed, were also synthesized. The photo-excited quartet state of the π-radical ligand L¹ was confirmed by the time-resolved ESR technique. Temperature dependence of the magnetic behaviors of 1 and 2 were investigated from 5 K to 350 K, showing spin-crossover transition at T _c = 222 K and at T_c = 162 K for complexes 1 and 2, respectively. The transition enthalpies and entropies were determined to be ΔH = 8.09 kJ mol^-1 and ΔS = 36.4 J K^-1 mol^-1 for 1 and to be ΔH = 22.39 kJ mol^-1 and ΔS = 138 J K^-1 mol^-1 for 2. LIESST phenomena were also observed below ca. 50 K for both complexes. The effects of the attachment of radical moiety are discussed based on the results.

Full text of DOI:10.1039/c2dt31831h

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PAPER
Design, synthesis, magnetic properties of a π-radical ligand with
photo-excited high-spin state and its Fe(II) complex. The first stage
of a new strategy for LIESST materials†
Koichi Katayama, Masakazu Hirotsu,* Isamu Kinoshita and Yoshio Teki*
Received 10th August 2012, Accepted 3rd September 2012
DOI: 10.1039/c2dt31831h
A π-radical ligand (9-[4-(6-oxo-1,5-dimethylverdazyl)phenyl]-10-[5-(2,2′-bipyridyl)ethynyl]anthracene,
L1) with a photo-excited high-spin quartet state (S = 3/2) and its iron(II) complex [Fe(L1){H2B(Pz)2}2] (1)
{H2B(Pz)2− = dihydrobis(1-pyrazolyl)borate} were synthesized as a candidate for a new strategy for spin-
crossover compounds exhibiting light-induced excited spin state trapping (LIESST), which is via the
photo-excited high-spin state of the π-conjugated aromatic system. Control compounds, ligand L2 and
[Fe(L2){H2B(Pz)2}2] (2), in which the verdazyl radical moiety in L1 was removed, were also synthesized.
The photo-excited quartet state of the π-radical ligand L1 was confirmed by the time-resolved ESR
technique. Temperature dependence of the magnetic behaviors of 1 and 2 were investigated from 5 K
to 350 K, showing spin-crossover transition at Tc = 222 K and at Tc = 162 K for complexes 1 and 2,
respectively. The transition enthalpies and entropies were determined to be ΔH = 8.09 kJ mol−1 and
ΔS = 36.4 J K−1 mol−1 for 1 and to be ΔH = 22.39 kJ mol−1 and ΔS = 138 J K−1 mol−1 for 2. LIESST
phenomena were also observed below ca. 50 K for both complexes. The effects of the attachment of
radical moiety are discussed based on the results.
significantly important in the fields of both molecular magnetism
Introduction
and molecular spintronics.16
Control of the magnetism by external stimuli is a very exciting
field in the molecular magnetism.1,2 Light-induced excited spin
state trapping (LIESST)3 and light-induced valence tautomerism
(LIVT)4,5 are most promising candidates for the molecular-based
memory devices.6,7 Although the change of the magnetic proper-
ties by irreversible photo-chemical reactions has been studied in
the early stage of the molecular magnetism,8–11 the reversible
change of the magnetic properties is recently attracting much
interests. In the electronic ground state, manipulation of the mag-
netic properties of organic radicals has been intensively investi-
gated by electron–hole doping12 and by the structure change of
the photo-chromic spin coupler by light.13 The photo-switching
between diamagnetic and paramagnetic phases was also achieved
We have studied the transient photo-induced spin alignment in
the photo-excited state using purely organic π-conjugated spin
systems which are constructed from an anthracene and stable
radicals.17–24 These are ideal model systems to study the spin
alignment on photo-excited states, because they have the follow-
ing advantages for materials science: (1) A strongly exchange-
coupled spin alignment17–24 is achieved compared to other
triplet-radical exchange coupled systems (σ-bonded systems25–27
and coordination complexes28–32); (2) The desired spin state can
be designed well by taking the topology of the π-electron
network (π-topology) into account;17–22 (3) An enhanced inter-
system crossing owing to the spin–orbit coupling via π-conju-
gation to the radical spin is available17–22 to open a new
pathway. These π-radicals with high-spin photo-excited states are
interesting because of fast photo-switching of the spin state1 and
magnetic interactions and are also the prototypes of photo-
magnetic molecular devices.33 Recently, both experimentally23,24
and theoretically,34,35 we have investigated the high-spin states
of functionalized π-radicals with unique dynamic electron polari-
zation, which are generated through quantum mixed photo-
excited states. In these systems, the unique dynamic electron
polarization is generated by a dynamic process such as energy
transfer, charge-separation and/or population transfers from the
quantum-mixed state.23,24,34,35
in
a
1,3,5-trithia-2,4,6-triazapentalenyl (TTTA).14 Recently,
negative magneto-resistance was observed in the purely organic
radical crystals at low-temperature, opening the way to the mole-
cular spintronics using organic radicals.15 In the next stage, the
spin manipulation using a dynamic process will become
Division of Molecular Material Science, Graduate School of Science,
Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka
558-8585, Japan. E-mail: mhiro@sci.osaka-cu.ac.jp,
teki@sci.osaka-cu.ac.jp; Tel: (+81) 6-6605-2559
†Electronic supplementary information (ESI) available: Solution NMR
spectra of (i), (ii), (iii), L1, and L2. Results of the DFT calculation of L1,
and complex 1. CCDC 894428. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt31831h
The main aim of this work is to explore the ability of the
π-radical ligand to display the photo-excited high-spin state
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(photo-excited high-spin π-radical ligand) to the spin-crossover
including LIESST or LIVT phenomena. As discussed in the next
section, the combination of the spin crossover complex and the
π-conjugated photo-excited high-spin systems would provide not
only novel LIESST phenomena utilizing enhanced intersystem-
crossing (EISC) of the π-radical ligand but also efficient LIESST
phenomena via antenna effect36 arising from the excitation of
π–π* transition with strong transition moment. As the first stage
to explore these effects, we have synthesized a new π-radical
ligand (L1) having photo-excited quartet high-spin state and the
Fe(II) complex, [Fe(L1){H2B(Pz)2}2] (1) and their control com-
pounds, L2 and [Fe(L2){H2B(Pz)2}2] (2) shown later. Their mag-
netic properties and LIESST phenomena were investigated. As
shown later, although the LIESST phenomena and the antenna
effect were observed, the efficiency was insufficient. In this
paper, the problems are clarified to realize the LIESST via the
excited high-spin states of the π-radical ligand as the first stage
of the new strategy for “efficient” LIESST materials utilizing the
photo-excited high-spin π-radical ligand which will open a new
channel by the enhanced intersystem crossing.
ligand by a photo-excitation of the ππ* transition,18 leading to a
total photo-excited quartet state (S = 3/2) of [Fe(II) + L]
complex. This photo-excited high-spin state is expected to be a
spin allowed transition toward the ground state in the HS phase
and forbidden to that in the LS phase. Therefore, the photo-
induced LS → HS transition is expected via the photo-excited
high-spin quartet state of L, which is generated by the EISC.17
It is well known that octahedrally coordinated Fe(II) com-
plexes show spin-crossover phenomena. The phenomena37–41 are
well investigated among transition metal complexes with d4–d7
electronic configurations, because of the following reasons: (I)
they change from the LS state (S = 0) to the HS state (S = 2) by
heat or light;40 (II) some of them show LIESST;41 (III) iron is
the most abundant element. Therefore, in this work, we have
chosen the octahedrally coordinated Fe(II) complexes as the
spin-crossover moiety and π-radical with the photo-excited high-
spin state as the ligand. In this system, the spin transition would
be induced through the fast photo-switching of the spin state of
the organic ligand. As the first stage to explore the new LIESST
materials based on the above strategy, a new π-radical ligand, L1,
was constructed by linking 2,2′-bipyridine (bpy), which is a
stable bidentate ligand, and oxo-verdazyl radical with anthracene
(Fig. 2). 2,2′-Bipyridine and its derivatives are well-utilized for
the Fe(II) spin-crossover complexes.37,38,40 Real et al. reported
Strategy of LIESST via photo-excited high-spin state of π-radical
In our previous paper,16 we proposed the possibility of a photo-
induced spin transition via a photo-excited high-spin state of
π-radical ligand as one of the potential developments of the
photo-excited high-spin π-radical toward highly functional
materials. Here, we present more specific strategy of the photo-
induced spin transition.
Our strategy for spin-crossover compounds exhibiting LIESST
via a photo-excited high-spin state of π-radical ligand is shown
in Fig. 1. We have chosen a spin-crossover complex of Fe(II) as
the source of the spin-transition, in which the spin states (S) are
0 and 2 for low-spin (LS) and high-spin (HS) states, respectively.
When we use a π-radical ligand (L) which has a doublet ground
state (S = 1/2) and a quartet lowest photo-excited state (S = 3/2),
the total ground state of [Fe(II) + L] is a doublet state (S = 1/2) in
the LS phase and nearly degenerated quartet (S = 3/2) and sextet
(S = 5/2) states in the HS phase, because the spin state of Fe(II)
with S = 2 is expected to be coupled weakly with the unpaired
spin of L with S = 1/2. As shown in Fig. 1, the EISC from the
doublet state to the quartet high-spin state occurs in the π-radical
−
that [Fe(bpy){H2B(Pz)2}2] {H2B(Pz)2 = dihydrobis(1-pyrazo-
lyl)borate} shows spin crossover phenomenon and LIESST.38–40
We used this complex as a spin crossover unit in the first model
system of our strategy. We describe synthesis of the π-radical
ligand L1, the Fe(II) complex [Fe(L1){H2B(Pz)2}2] (1), and their
electronic states of the photo-excited state as well as the ground
state, their magnetic properties including LIESST phenomena.
These results are compared with those of the control compound
with a non-radical ligand, L2 (Fig. 1), [Fe(L2){H2B(Pz)2}2] (2)
to evaluate the effects of the π-radical moiety.
Results and discussion
(I) Synthesis and molecular structure of L1
Ligands L1 and L2 were synthesized according to Scheme 1, and
characterized by IR, NMR spectroscopy and elemental analysis.
The oxo-verdazyl radical in L1 was confirmed by ESR spec-
troscopy (vide infra), and the structure of L1 was determined by
X-ray analysis.
Fig. 3 shows an ORTEP drawing of L1. Selected bond
lengths, angles and dihedral angles are listed in Table 1. The L1
molecule is located on a crystallographic C2 axis through O1,
C1, C3, C4, C7, C8, C15, C16, C17, C18, C21, C22, and C25
atoms. In the bipyridine moiety, N and C atoms adjacent to C21
and C22, and their attached H atoms are disordered over two sites
with equal occupancy factors. The planarity of the oxo-verdazyl
Fig. 1 New strategy of LIESST via photo-excited high-spin state of
π-radical.
Fig. 2 Molecular structures of (a) L1 and (b) L2.
13466 | Dalton Trans., 2012, 41, 13465–13473
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Scheme 1 Synthesis of L1 and L2.
Fig. 4 Solution ESR spectrum of L1 in CH2Cl2 with a spectral simu-
lation. (a) Observed one, (b) Simulation.
Fig. 3 ORTEP drawing of the radical ligand L1.
Table 1 Selected bond lengths (Å), angles (°) and dihedral angles (°)
of L1
Fig. 5 Time-resolved ESR spectrum of L1 at 30 K in a BuCN glass
matrix. “Abs.” and “Emi.” denote the absorption and emission of micro-
wave. (a) Observed spectrum at 0.5 μs, (b) the spectral simulation.
C(1)–O(1)
C(1)–N(1)
C(3)–C(4)
N(1)–C(1)–N(1)*
1.224(3)
1.3690(19)
1.478(3)
114.4(2)
114.64(14)
Planea
2
N(1)–N(2)
N(2)–C(3)
C(16)–C(17)
C(1)–N(1)–N(2)
N(2)–C(3)–N(2)*
1.3675(18)
1.3273(17)
1.197(3)
124.46(15)
127.41(19)
N(1)–N(2)–C(3)
Planea
1
2
3
18.77(6)
76.03(5)
0.87(6)
spectral pattern is characteristic of oxo-verdazyl radical with the
doublet ground state (S = 1/2) showing g = 2.0040 and typical
hyperfine splitting arising from four nitrogen atoms (isotropic
3
4
a Plane 1, oxo-vedazyl moiety; plane 2, phenylene moiety; plane 3,
anthracene moiety; plane 4, bipyridine moiety.
hyperfine coupling constants with aN(1) = 0.65 mT and aN(2)
=
0.53 mT) and six protons (aH = 0.53 mT) of methyl groups in
the verdazyl radical moiety. These values are the same as those
of the phenyl-verdazyl radical reported by Neugenbauer et al.42
This shows that the SOMO of L1 is localized in the verdazyl
moiety, which is consistent with the result of the density func-
tional theory (DFT) calculation shown later (Fig. 7(a) and
Fig. S8 in the ESI†).
group is consistent with the formation of radical. The dihedral
angle between the planes of anthracene and phenylene moieties
in L1 is 76.03(5)°. The molecular orbital calculation of the
ground state shows that the highest molecular orbital (HOMO) is
localized within the phenylanthracene moiety and the slight
delocalization of π-HOMO occurs in the bipyridyl group (see:
Fig. S7 in the ESI†).
To investigate the excited state of L1, we measured the time-
resolved ESR (TRESR) spectrum using the L1 diluted in a
frozen BuCN matrix. Fig. 5 shows the observed TRESR spec-
trum of L1 together with a spectral simulation, which was
observed 0.5 μs after nano-second pulsed laser excitation by
355 nm light. The spectral simulation of the TRESR was carried
out by the eigenfield43/exact diagnonalization hybrid method44
of the spin-Hamiltonian (eqn (1)) taking the dynamic electron
(II) Ground and photoexcited states of L1
The radical character of L1 in the ground state was confirmed
with a solution ESR spectrum at room temperature (Fig. 4). This
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polarization into account reported previously.17
are mainly located on the central anthracene unit and the remain-
ing one unpaired electron is located on the verdazyl radical
moiety (Fig. 7(b)). The unpaired electron of the triplet anthra-
cene moiety is slightly delocalized to the bipyridine group and
also to the radical moiety. The spin density distribution in the LS
state (S = 1/2) of 1 shows that the unpaired electron exists on the
phenyl-verdazyl radical moiety (Fig. 7(c)), which is similar to
the ground state of L1. In the high-spin state (S = 5/2) of 1, the
unpaired electrons are located on the Fe center and the phenyl-
verdazyl radical moiety (Fig. 7(d)).
H ¼ gβεH ꢀ S þ S ꢀ D ꢀ S
2
2
2
¼ gβεH ꢀ S þ D½SZ ꢁ SðS þ 1Þ=3ꢂ þ EðSX ꢁ SY
Þ
ð1Þ
From the spectral simulation, spin Hamiltonian parameters of
L1 are determined to be S = 3/2, g = 2.0040, D = 0.0214 cm−1
,
E = 0.0010 cm−1. The dynamic electron polarization occurs to
the Ms′ = 1/2 sublevels of the zero-field spin-sublevels. This
spectral pattern shows that the quartet state was only formed by
enhanced spin-orbit intersystem crossing (SO-ISC).17 Other
π-conjugated radicals having anthraquinone (iv)23 or ferrocene
(v)22 instead of 2,2′-bipyridine show TRESR spectra similar to
that of L1. Spin Hamiltonian parameters of (iv) and (v) were
S = 3/2, g = 2.0035, D = 0.0215 cm−1, E = 0.0017 cm−1 for (iv),
S = 3/2, g = 2.0037, D = 0.0240 cm−1, E = 0.0010 cm−1 for (v),
respectively.
According to the spin density of the photo-excited quartet
high-spin state of L1, a weak exchange interaction between the
Fe(II) metal and the quartet excited state of L1 is expected in the
4
excited state of 1, [1Fe(II) + L1*]. In contrast, in the meta-stable
ground state of 1, [5Fe(II) + 2L1], almost no exchange-interaction
is predicted between the S = 2 high-spin state of Fe(II) and the
S = 1/2 state of L1, giving the quantum-mixed state of S = 3/2
and S = 5/2. The electronic energy difference between the
LS and HS states of 1, EHS(B3LYP) − ELS(B3LYP), was calcu-
lated to be 8.81 kJ mol−1 from the DFT calculation, in which the
LS state has slightly lower energy. The energy difference cor-
rected for zero-point energy by the frequency calculation is
−4.051 kJ mol−1, showing the lower energy of the HS state and
the inconsistency with the magnetic data below Tc (vide infra).
However, these small energy differences are reasonable because
the experimental data contain the intermolecular interactions
and cooperative effect in the solid state, which will become
more important at low-temperature region. Gibbs free energy
correction of 1 at 298.15 K and 1 atm were obtained from har-
monic vibrational frequencies without scaling factors. After the
(III) Synthesis of iron complexes 1 and 2
Complexes 1 and 2 depicted in Fig. 6 were synthesized from L1
and L2, respectively, by modifying the procedure reported by
Real et al.40 Both complexes were characterized by UV/Vis
absorption spectroscopy, elemental analysis and magnetic
measurements.
(IV) Electronic structures by ab-initio MO calculations
To elucidate the physical properties of L1, DFT calculations were
performed at the unrestricted B3LYP/6-31G level. Structures of
1 in the low-spin state (S = 1/2) and the ferromagnetically
coupled high-spin state (S = 5/2) were optimized by the DFT
method at the unrestricted B3LYP/LANL2DZ level.
Fig. 7 shows the spin density of the ground states and the
photo-excited states of L1 and 1 obtained by DFT calculations.
The calculated total spin density distribution in the doublet
ground state shows that the unpaired electron is localized on the
phenyl-verdazyl radical moiety (Fig. 7(a)). This is consistent
with the ESR parameters of L1 being very close to those of the
phenylverdazyl radical.42 In the photo-excited quartet state of
L1, the calculated spin densities suggest that two unpaired spins
Fig. 7 Spin densities of L1 and 1. (a) the doublet (S = 1/2) ground
state of L1, (b) the quartet (S = 3/2) photo-excited state of L1. (c) the
doublet ground state of 1, (d) the sextet (S = 5/2) meta-stable ground
state of 1.
Fig. 6 Molecular structures of (a) [Fe(L1){H2B(Pz)2}2] (1) and (b)
[Fe(L2){H2B(Pz)2}2] (2).
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Fig. 8 UV/Vis absorption spectra of L1 (–), 1 (…) and 2 (—) at room
temperature in dichloromethane.
thermodynamic correction, the HS state becomes more stable
than the LS state: the energy difference is −11.55 kJ mol−1
,
which is consistent with the magnetic data at room temperature.
The details are presented in the ESI†.
(V) Optical spectra
UV/vis spectra of L1, 1 and 2 were measured at room tempera-
ture (Fig. 8). The absorption bands of L1 at 360–460 nm are
characteristic of the ππ* transitions of an anthracene moiety. The
excitation wavelength used in the TRESR shown in Fig. 5 corre-
sponds to the shoulder of these ππ* transitions. This is one of
the reasons of the weak intensity of the TRESR spectrum of the
quartet high-spin state of L1. In 1 and 2, the MLCT bands were
observed at 600 nm in addition to the ππ* transition bands. The
spectrum of [Fe(bpy){H2B(Pz)2}2] contains two MLCT band at
410 nm and 640 nm, which are assigned to the transitions from
the metal dπ orbitals into π* orbitals of the ligands.45 Therefore
the MLCT with higher energy of 1 is overlapped by the ππ*
bands of anthracene moiety. We used the wavelengths, 460 nm
(ππ* band), 500 nm (the edge of ππ* band), and 600 nm (MLCT
band) for LIESST experiments.
Fig. 9 Temperature dependence of molar magnetic susceptibilities for
(a) 1 and (b) 2. The solid curves are simulated ones using eqn (2).
The C1 term is the paramagnetic contribution arising from the
radical moiety and residual component of the high-spin state of
Fe(II) (at low-temperature). The ratio is given in the non-
interaction condition between the metal complexes by the
following equation.46
αðFeðHSÞÞ ¼ 1=ð1 þ expððΔH=RÞð1=T ꢁ 1=TcÞÞ
ð3Þ
The estimated values from the curve fitting are C0 = 3.18,
ΔH/R = 973 K (ΔH = 8.09 kJ mol−1), and Tc = 222 K. The tran-
sition entropy ΔS was estimated to be 36.4 J K−1 mol−1 from the
relation of ΔS = ΔH/Tc. C1 = 0.686 emu K mol−1, which is
slightly larger than the spin-only value of the L1 radical species
and due to residual component of the high-spin state of Fe(II)
(see Fig. 9(b); similar residual contribution was observed at the
low-temperature region of complex 2). It was unsuccessful
because the small magnitude of the exchange coupling J,
although we tried to estimate the magnitude of the exchange
interaction between the high-spin Fe(II) unit and the radical
moiety of L1 using the analytical formula:
(VI) Magnetic measurements
The temperature dependence of the χMT (χM is molar magnetic
susceptibility) for powder samples of 1 and 2 are shown in Fig. 9(a)
and 9(b), respectively. At 300 K, χMT for 1 is 3.2 emu K mol−1,
which is slightly larger than the spin-only value for high-spin
Fe(II). The value decreases gradually to 0.64 emu mol−1 K as the
temperature is lowered to 30 K, and drops suddenly below 10 K.
This may be due to the intermolecular exchange interaction
between the complexes. The value of 0.39 emu mol−1 K at 4 K
corresponds to that of the radical ligand. The solid-line curve
was obtained by the least-square fitting using the equation as
follows.
χmol ¼ ðNAμ0μB2=kBTÞ½αðFeðHSÞÞ
ꢃ ð10 þ 10expð3J=kBTÞÞ=ð1 þ 2expð3J=kBTÞÞ
þ ð1 ꢁ αðFeðHSÞÞꢂ þ ðresidual component of HSÞ
ð4Þ
χ
molT ¼ C0αðFeðHSÞÞ þ C1
ð2Þ
As shown in Fig. 9(a), the gradual spin-transition was
observed in 1. The estimated ΔH and ΔS values are smaller than
those reported in [Fe(bpy){H2B(Pz)2}2], showing that 1 has
smaller cooperativity compared with [Fe(bpy){H2B(Pz)2}2].
where α(Fe(HS)) is the mole ratio of the high-spin Fe(II) ion and
C0 is expected to be ca. 3.4 emu mol−1 K from the data of the
control compound 2 without radical species as shown in Fig. 9(b).
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On the other hand, abrupt spin conversion was observed in 2.
This behavior is similar to [Fe(bpy){H2B(Pz)2}2]. The χMT
value of 2 is 3.5 emu mol−1 K at 300 K, which is expected for
high-spin Fe(II). This value slowly decreases upon cooling. In
the temperature range (180 K–150 K) the χMT product for 2 sud-
denly decreases to 0.50 emu mol−1 K, then decreases smoothly
from 0.50 emu mol−1 K at 150 K to 0.09 emu mol−1 K at 4 K,
which corresponds to low-spin Fe(II) with S = 0. The non-zero
value at 4 K is due to the contamination of the high-spin Fe(II).
The solid-line curve was obtained by the least-square fitting
using eqn (2). The temperature dependence was well analyzed
by eqn (2). The estimated values from the curve fitting are C0 =
3.36, ΔH/R = 2693 K (ΔH = 22.39 kJ mol−1), Tc = 162 K, ΔS =
10(b) show the temperature dependence of the χMT product
recorded for 1 and 2, respectively. The magnetic moment was
immediately increased by the photo-irradiation, reaching almost
constant after 20 min irradiation. This LIESST effect was
observed below ca. 50 K. The saturated magnitude (ΔMsaturated
=
(χMT)saturated − (χMT)no-irradiation) of the increase of the χMT
value by the irradiation at 5 K were 0.5 emu mol−1 K (460 nm),
0.75 emu mol−1 K (500 nm), and 1.0 emu mol−1 K (600 nm) for
1. Their wavelengths, 460 nm, 500 nm and 600 nm, correspond
to the excitation band of ππ* transition of the anthracene moiety,
nπ transition of the verdazyl radical moiety, and MLCT band
between pyridinyl group and Fe(II) metal, respectively. Similar
LIESST phenomena were observed for the control compound 2
below ca. 50 K. The ΔMsatulated values were and 0.7 emu mol−1
138 J K−1 mol−1 and C1 = 0.10 emu K mol−1
.
K (460 nm), 2.2 emu mol−1 K (500 nm), and 2.7 emu mol−1
K
(600 nm) for 2. The temperature was increased at a rate of 0.4 K
min−1 after the light irradiation was stopped. The magnetic
moments were first slightly increased with increasing tempera-
ture and reaching the maximum at 20 K. The maximum values
(VII) LIESST phenomena
The LIESST experiments were performed on 1 and 2 by using
pulse laser (OPO + Nd : YAG ns pulse laser). A similar pro-
cedure was used for the two powdered samples of 1 and 2. The
compound was first slowly cooled from room temperature to 5 K
and the photo-irradiation was carried out at 5 K. Fig. 10(a) and
were 1.2 emu mol−1 K (600 nm) for 1 and 3.6 emu mol−1
K
(600 nm) for 2. For further heating, the magnetic moment
dropped quickly in a narrow temperature range.
Complexes 1 and 2 exhibited the LIESST phenomena in a
wide range of wavelengths of light irradiation, which includes
the ππ* band (400–500 nm) of the π-radical ligand as well as the
MLCT transition around 500–680 nm. This means that the aro-
matic rings of the organic ligand play the role of antenna.
However, the efficiency of the photo-conversion decreased in the
order of irradiation at 600 nm > 500 nm > 460 nm, which
suggests that the MLCT transition from Fe(II) to the bipyridyl
group is more effective than the ππ* transition of the anthracene
moiety in the LIESST. The LIESST for the π-radical complex 1
was suppressed compared with 2: the photo-conversion using
600 nm light was ca. 30% for 1 and almost 100% for 2. Prob-
ably, in 1 an energy transfer from the anthracene moiety to the
radical moiety occurs competitively, leading to a non-irradiative
relaxation process to the ground state through the radical moiety.
In order to investigate the photo-excited states of 1, [Fe(L1)-
{H2B(Pz)2}2] and the energy transfer to the high-spin photo-
excited state, the TRESR experiments were also carried out. We
tried to measure the high-spin photo-excited state of L1 coupled
to the Fe(II) high-spin complex. Unfortunately, TRESR was not
observed in 1, showing the low-efficiency of the intersystem
crossing toward the high-spin state, probably because of the
rapid energy relaxation to the ground state through the Fe site.
(VIII) Possible improvement to light-induced spin conversion
Based on the present results, one of the problems that was
clarified was that the energy transfer from the anthracene moiety
to the radical moiety followed by the non-irradiative relaxation
may suppress the spin conversion by LIESST. In order to make
the LIESST effect via the intersystem crossing in the π-ligand
more efficiently, an improvement of the system is required. Sup-
pression of the energy loss by the energy transfer to the radical
moiety is possible to some extent by choosing a photo-receive
antenna moiety and radical species, in which the emission
spectra of photo-receive moiety does not overlap the absorption
Fig. 10 Temperature dependence of magnetic susceptibilities and
LIESST phenomena for (a) 1 and (b) 2. The magnitude of the suscepti-
bility was corrected using the data of the bulk powder sample.
13470 | Dalton Trans., 2012, 41, 13465–13473
This journal is © The Royal Society of Chemistry 2012

View Article Online
of the radical moiety (suppression of the Förster mechanism).
A more interesting possible candidate is to use another pathway,
which leads to more efficient intersystem crossing. The pathway
via charge-separated quantum-mixed state followed by so-called
“radical-pair intersystem crossing is one of the candidates, in
which intersystem crossing is due to the de-coherence (loss of
the quantum coherence) during the charge-recombination.34,35
A combination between the photo-excited high-spin π-radical
and the valence tautomerism is also an interesting research target
as the LIVT materials.
unrestricted B3LYP density functional method was used for the
calculations. The 6-31G and LANL2DZ basis sets were used for
L1 and 1, respectively. The DFT calculations of L1 were per-
formed for the ground and photo-excited states. In complex 1 the
low-spin state (S = 1/2) and the ferromagnetically coupled high-
spin state (S = 5/2) were investigated. The optimized structures,
molecular coordinates, molecular orbitals, spin densities and cal-
culated Fe–N distances are presented in the ESI.†
(d) Materials
N,N′-Dimethylcarbonohydrazide,50
5-bromo-2,2′-bipyridine51
Experimental section
and 9-(p-formylphenyl)-10-bromoanthracene22 were prepared
according to literature procedures. Modification of the procedure
using 9-phenylanthracene instead of 9-(p-formyl-phenyl)anthra-
cene gave 9-phenyl-10-bromoanthracene. Other reagents were
used as purchased. Silica gel (Merk silica 60) or alumina (Merk
Alum. Ox. 60) was used for column chromatography.
(a) Measurements
UV/Vis spectra were recorded on a MultiSpec-1500 spectro-
photometer at room temperature. IR spectra were recorded on a
JASCO FT/IR-420 spectrophotometer in the range of
1
4000–400 cm−1. H NMR spectra were measured on a JEOL
Lambda 300 spectrometer at room temperature. ESR and
TRESR were measured on an X-band ESR spectrometer JEOL
TE300. The temperature was controlled by an Oxford ESR910
cold He gas-flow system. TRESR experiments were carried out
using BuCN glass matrix. Samples were degassed by repeated
freeze-pump-thaw cycles. Excitations of L1 and 1 were carried
out with 355 nm light from a YAG laser (Continuum Surelite
II-10). Magnetic susceptibility data were measured by a SQUID
magnetometer (Quantum Design MPMS2). LIESST experiments
were carried out with the SQUID magnetometer. The excitation
light from the pulse laser (Continuum Surelite II connected to
Continuum Surelite OPO) was introduced in the sample chamber
using the quartz fiber. In the LIESST experiments, the powder
samples thinly smeared on the tape were used and the magnitude of
the susceptibility of the thin sample was corrected using the values
of the bulk powder sample at 350 K for 1 and at 300 K for 2.
5-Ethynyl-2,2′-bipyridine (i). A solution of 5-bromo-2,2′-
bipyridine (0.24 g, 1 mmol), [Pd(PPh3)4] (58 mg), CuI (19 mg),
and trimethylsilylacetylene (0.20 g, 2 mmol) in THF (20 mL)
and Et3N (4 mL) was stirred at room temperature for 1 day under
nitrogen. Activated carbon (0.10 g) was added, and the reaction
mixture was stirred for 30 min, and filtered. The solvent of the
filtrate was removed under reduced pressure. The residue was
added to a solution of K2CO3 (0.15 g, 1.1 mmol) in MeOH
(10 mL), and filtered. The solvent of the filtrate was removed
under reduced pressure. The crude product was purified by silica
gel column chromatography (ethyl acetate–n-hexane = 1 : 1) to
give (i) in 61% yield (0.11 g). 1H NMR (CDCl3): δ 8.78 (dd, J =
2.1, 0.75 Hz, bpy, 1H), 8.69 (ddd, J = 4.8, 1.8, 1.1 Hz, bpy, 1H)
8.41 (dt, J = 7.6, 1.1 Hz, bpy, 1H), 8.40 (dd, J = 8.2, 0.75 Hz,
bpy, 1H), 7.90 (dd, J = 8.2, 2.1 Hz, bpy, 1H), 7.84 (td, J = 7.6,
1.8 Hz, bpy, 1H), 7.33 (ddd, J = 7.5, 4.8, 1.1 Hz, bpy, 1H), 3.29
(s, CH, 1H).
(b) X-ray crystallography
9-(p-Formylphenyl)-10-[5-(2,2′-bipyridyl)ethynyl]anthracene (ii).
A mixture of 5-ethynyl-2,2′-bipyridine (i) (55 mg, 0.3 mmol),
9-(p-formylphenyl)-10-bromoanthracene (0.11 g, 0.3 mmol),
[Pd(PPh3)4] (17 mg), CuI (5.7 mg) in Et3N (10 mL) and THF
(5 mL) was refluxed for 8 h under nitrogen. The solvent was
removed under reduced pressure. The crude product was purified
by silica gel column chromatography (ethyl acetate–n-hexane =
1 : 4) to give (ii) in 66% yield (91 mg). 1H NMR (CDCl3):
δ 10.2 (s, CHO, 1H), 9.08 (d, J = 2.1 Hz, bpy, 1H), 8.77–8.74
(m, an, 2H), 8.73 (d, J = 1.7 Hz, bpy, 1H), 8.54(d, J = 7.8 Hz,
bpy, 1H), 8.50 (dd, J = 8.0, 1.0 Hz, bpy, 1H), 8.20 (dd, J = 7.8,
2.1 Hz, bpy, 1H), 8.14 (d, J = 7.7 Hz, ph, 2H), 7.9 (td, J = 8.0,
1.7 Hz, bpy, 1H), 7.68–7.59 (m, ArH, 6H), 7.47–7.36 (m, ArH,
3H). Anal. Calcd for C33H20N2O·0.2CH2Cl2: C, 83.51; H, 4.31;
N, 5.87. Found: C83.46; H, 4.53; N, 5.76. IR (cm−1): 1704,
1602, 1586, 1541, 1457, 1374, 1206.
Diffraction data were collected on a Rigaku AFC-7/Mercury
CCD area-detector diffractometer with graphite monochromated
Mo-Kα radiation (λ = 0.7107 Å). The CrystalClear software was
used for the collection, processing and correction for Lorentzian
and polarization effects. Absorption corrections were applied on
comparison of multiple symmetry equivalent measurements. The
structure was solved by the SIR2004 direct method47 and
expanded using Fourier techniques and refined by full matrix
least-squares against F2 using SHELXL-97.48
Crystal data for L1: C36H25N6O, M = 557.62, monoclinic,
space group C2/c (no. 15); a = 9.454(3), b = 35.516(9),
c = 8.983(3) Å, β = 113.403(3)°, V = 2768.0(13) Å3, T = 153 K,
Z = 4, Dc = 1.338 g cm−3, μ = 0.084 mm−1; 10 630 reflections
collected and 3103 independent reflections (Rint = 0.0272), final
R indices [I > 2σ(I)] R1 = 0.0570, wR2 = 0.1394; R indices (all
data) R1 = 0.0770, wR2 = 0.1570, GOF = 1.069. CCDC 894428.
9-[4-(6-Oxo-1,5-dimethyl-1,2,4,5-tetrazinan-3-yl)phenyl]-10-
[5-(2,2′-bipyridyl)ethynyl]anthracene (iii). To a solution of car-
bonic acid bis(1-methylhydrazide) (39 mg, 0.33 mmol) in
MeOH (5 mL) and THF (5 mL), was added dropwise a solution
of (ii) (50 mg, 0.11 mmol) in MeOH (5 mL) and THF (5 mL),
and the mixture was stirred at room temperature for 3 h. The
(c) Molecular orbital calculations
Molecular orbital calculations of L1 and complex 1 were per-
formed using the Gaussian 03 program package.49 The
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reaction mixture was evaporated under reduced pressure and the
pressure. Yield: 42 mg (43%). Anal. Calcd for C44H36B2FeN10·
0.7CH2Cl2: C, 63.78; H4.48; N, 16.64. (Anal. Calcd for
C44H36B2FeN10·0.5CH2Cl2·0.5CH3OH: C, 64.28; H4.68; N,
16.66.) Found: C, 63.59; H, 4.46; N, 17.27.
crude residue was washed by MeOH to give (iii) in 57% yield
1
(35 mg) as a crude product. H NMR (CDCl3): δ 9.08 (d, J =
1.34 Hz, bpy, 1H), 8.75–8.72 (m, ArH, 3H), 8.55–8.49 (m, bpy,
2H), 8.19 (dd, J = 8.3, 1.6 Hz, bpy, 1H), 7.87 (td, J = 8.0,
1.8 Hz, bpy, 1H), 7.77 (d, J = 8.3 Hz, ph, 2H), 7.66–7.61 (m,
an, 4H), 7.48 (d, J = 8.3 Hz, ph, 2H), 7.45–7.26 (m, ArH, 3H),
5.24 (t, J = 9.9 Hz, CH, 1H), 4.55 (d, J = 9.9 Hz, NH, 2H), 3.25
(s, CH3, 6H).
Conclusions
Based on a new strategy of LIESST via the photo-excited high-
spin state of π-radical, two ligands and their iron(II) complexes
were designed, synthesized and investigated in terms of elec-
tronic states and magnetic properties. The photo-excited quartet
(S = 3/2) state of the π-radical ligand (L1) containing bipyridine,
anthracene and verdazyl radical moieties was detected by time-
resolved ESR, which is constructed by the intramolecular
ferromagnetic spin alignment between the doublet spin in the
oxo-verdazyl radical and ππ* excited triplet state of the phenyl-
anthracene. The iron(II) complexes [Fe(L1){H2B(Pz)2}2] (1) and
[Fe(L2){H2B(Pz)2}2] (2) were synthesized, where L2 has no ver-
dazyl radical moiety. Complexes 1 and 2 showed spin conversion
between LS and HS: the Tc values are 222 K and 162 K, respect-
ively. The spin transition of 1 occurred gradually compared with
that of 2, indicating the cooperative effect among the complexes
in the crystal smaller than that of 2 or [Fe(bpy){H2B(Pz)2}2]
reported in the literature.38,39 In both complexes 1 and 2,
LIESST phenomena were observed in a wide range of wave-
lengths of the excitation light. Their wavelengths, 460 nm,
500 nm and 600 nm, correspond to the excitation band of ππ*
transition of the anthracene moiety, the nπ transition of the ver-
dazyl radical moiety, and the MLCT transition from Fe(II) to
the bipyridyl group, respectively. Thus, an antenna effect by the
π-conjugated ligand was observed. The direct excitation of the
MLCT band was the most efficient in LIESST, which suggests
that an energy transfer occurs from the ππ* excited state of the
anthracene moiety or nπ state of the radical moiety to the LMCT
state. In complex 1, only 30% of low spin molecules were photo-
converted. In contrast, almost 100% photoconversion was
achieved in the control complex 2 by excitation on the MLCT
band. A possible explanation for these findings is that the rapid
energy transfer or the energy relaxation to the LS ground state
by the non-irradiative transition through radical moiety, which
overcomes the efficiency of the intersystem crossing to the
quartet high-spin state of the π-radical ligand confirmed by the
time-resolved ESR experiment. In order to make the LIESST
effect via the intersystem crossing in the π-ligand more
efficiently, an improvement of the system is required, which
leads to more efficient intersystem crossing via a charge-separ-
ated quantum-mixed state followed by so-called “radical-pair
intersystem crossing arising from the de-coherence (loss of the
quantum coherence) during the charge-recombination.34,35
9-[4-(6-Oxo-1,5-dimethylverdazyl)phenyl]-10-[5-(2,2′-bipyridyl)-
ethynyl]anthracene (L1). Solvents were purified by passage
through a column of activated alumina. A mixture of (iii)
(61 mg, 0.11 mmol) and p-benzoquinone (37 mg, 0.34 mmol) in
CH2Cl2 (20 mL) was stirred at room temperature for 1 day. The
reaction mixture was concentrated under reduced pressure. The
residue was chromatographed on alumina with CH2Cl2, and
recrystallized from n-hexane–CH2Cl2 to give L1 in 27% yield
(16 mg). Crystals of L1 appropriate for single crystal X-ray
analysis were obtained by diffusion of n-hexane into a CH2Cl2
solution of L1. Anal. Calcd for C36H25N6O: C, 77.54; H, 4.52;
N, 15.07. Found: C, 76.89; H, 4.66; N, 14.60. IR (cm−1): 1690,
1540, 1507, 1457, 1395, 1295, 1249.
9-Phenyl-10-[5-(2,2′-bipyridyl)ethynyl]anthracene (L2). A
mixture of 5-ethynyl-2,2′-bipyridine (i) (54 mg, 0.3 mmol),
9-phenyl-10-bromoanthracene (0.10 g, 0.3 mmol), [Pd(PPh3)4]
(17 mg), CuI (5.7 mg) in Et3N (10 mL) and THF (5 mL) was
refluxed for 6 h under nitrogen. The solvent was removed under
reduced pressure. The crude product was purified by silica gel
column chromatography (ethyl acetate–n-hexane = 1 : 2) to give
L2 in 53% yield (68 mg). 1H NMR (CDCl3): δ 9.81 (d, J =
2.2 Hz, bpy, 1H), 8.75–8.73 (m, an, 2H), 8.74 (d, J = 1.8 Hz,
1H), 8.52 (m, bpy, 2H), 8.19 (dd, J = 8.1, 2.2 Hz, bpy, 1H), 7.87
(td, J = 7.8, 1.8 Hz, bpy, 1H), 7.71–7.56 (m, ArH, 8H), 7.46–7.34
(m, ArH, 4H). IR (cm−1): 1651, 1587, 1541, 1458, 1432, 1396.
[Fe(L1){H2B(Pz)2}2] (1). All the manipulations were carried
out under N2 atmosphere. To a solution of KH2B(Pz)2 (20 mg,
0.11 mmol) in MeOH (3 mL) was added a solution of
Fe(ClO4)2·6H2O (20 mg, 0.054 mmol) in MeOH (2 mL). The
mixture was stirred for 10 min and filtered. The solvent of filtrate
was removed under reduced pressure. To the residue dissolved in
CH2Cl2 (10 mL) was added a solution of L1 (30 mg,
0.054 mmol) in CH2Cl2 (15 mL), and the solution was stirred
for 20 min. n-Hexane was added to the solution to give a dark
green precipitate, which was filtered, washed with n-hexane, and
dried under reduced pressure. Yield: 19 mg (38%). Anal. Calcd
for C48H41B2FeN14O·0.5CH2Cl2: C, 61.33; H, 4.46; N, 20.64.
Found: C, 61.99; H, 4.78; N, 20.36.
[Fe(L2){H2B(Pz)2}2] (2). All the manipulations were carried
out under N2 atmosphere. To a solution of KH2B(Pz)2 (46 mg,
0.25 mmol) in MeOH (10 mL) was added a solution of
Fe(ClO4)2·6H2O (45 mg, 0.12 mmol) in MeOH (5 mL). The
mixture was stirred for 10 min and filtered. To the filtrate was
added a solution of L2 (54 mg, 0.12 mmol) in CH2Cl2 (20 mL),
and the solution was stirred for 20 min. The solution was con-
centrated to remove CH2Cl2. A greenish yellow solid was preci-
pitated, filtered, washed with MeOH, and dried under reduced
Acknowledgements
This work was supported by the Grant-in-Aid for Scientific
Research (B) (No. 21350081, No. 24350076) from JSPS, Japan.
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Dalton Trans., 2012, 41, 13465–13473 | 13473