Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex

Article abstract of DOI:10.1021/jacs.9b07204

An important technique to realize novel electron- A nd/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex- A phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Full text of DOI:10.1021/jacs.9b07204

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Article
Observation of Proton Transfer Coupled Spin Transition and Trapping
of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex
Takumi Nakanishi, Yuta Hori, Hiroyasu Sato, Shuqi Wu, Atsushi Okazawa, Norimichi
Kojima, Takashi Yamamoto, Yasuaki Einaga, Shinya Hayami, Yusuke Horie, Hajime
Okajima, Akira Sakamoto, Yoshihito Shiota, Kazunari Yoshizawa, and Osamu Sato
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07204 • Publication Date (Web): 17 Aug 2019
Downloaded from pubs.acs.org on August 17, 2019
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Observation of Proton Transfer Coupled Spin Transition and Trap-
ping of Photoinduced Metastable Proton Transfer State in an
Fe(II) Complex.
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Takumi Nakanishi1, Yuta Hori1,2, Hiroyasu Sato3, Shuqi Wu1 Atsushi Okazawa4, Norimichi Kojima5,
Takashi Yamamoto6, Yasuaki Einaga7, Shinya Hayami7,8, Yusuke Horie9, Hajime Okajima9, Akira Sa-
kamoto9, Yoshihito Shiota1, Kazunari Yoshizawa1, and Osamu Sato1*
1Institute for Materials Chemistry and Engineering & IRCCS, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-
0395, Japan.
2Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan.
3Rigaku Corporation, 3-9-12 Matsubaracho, Akishima, Tokyo 196-8666, Japan.
4Department of Basic Science, Graduation School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-
ku, Tokyo 153-8902, Japan.
5Toyota Physical and Chemical Research Institute, Yokomichi, Nagakute, Aichi 480-1192, Japan.
6Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama,
Kanagawa 223-8522, Japan.
7Department of Chemistry, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-
ku, Kumamoto 860-8555, Japan.
8Institute of Pulsed Power Science (IPPS), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan.
9Graduate School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kana-
gawa 252-5258, Japan.
KEYWORDS (Word Style “BG_Keywords”). Proton Transfer; Spin transition;
ABSTRACT: An important technique to realize novel electron- and/or proton-based functionalities is to use a proton–electron cou-
pling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However,
although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit pro-
ton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II)
complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respec-
tively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunc-
tion with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at
low temperatures in the Fe(II) complex—a phenomenon known as the light-induced excited-spin-state trapping effect—the light-
induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer
was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light
irradiation, is useful for modulating proton-based functionalities in molecular devices.
various synergistic properties have been reported. Proton-cou-
pled electron transfer, an example of a proton–electron coupling
INTRODUCTION
Numerous functional compounds have been synthesized to
develop electronic and protonic devices 1, 2. One of the im-
portant routes to realize novel properties in solid-state materials
is to couple two or more components. Indeed, various intriguing
coupled systems, or cross-correlation systems, have been devel-
system, is well known to play an important role in biological
systems . Proton–electron coupling in solid-state materials,
6
1,5-dihalo-2,6-naphthoquinhydrones, has also been demon-
7
strated . More recently, several solid-state molecular com-
pounds exhibiting proton–electron coupling have been reported
8-10. However, although spin and charge are two important char-
acteristics of electrons, no previous study has succeeded in
demonstrating proton-transfer-coupled spin-transition (PCST)
behavior in a crystalline molecular material. The successful
coupling of proton transfer and spin transition will lead to the
realization of various coupling properties. In particular, because
oped. They include the coupling of electron and proton transfer
3, 4
and the coupling of electric and magnetic dipole moments
.
An important characteristic of coupled systems is that, when
one of the components is excited via external stimuli, the prop-
erties of the other coupled components are also modulated,
thereby demonstrating coupling and concerted effects 5. Indeed,
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Figure 1. (a) Chemical structure of Fe(II) hydrazone complexes containing two N′-(di(pyridin-2-yl)methylene) benzohydrazide (HL) lig-
ands. The HL ligands in which X is Cl and OMe are designated as HL-Cl and HL-OMe, respectively. Abbreviations of Fe(II) complexes
bearing HL-Cl ([Fe(HL-Cl)2](AsF6)2) and HL-OMe ([Fe(HL-OMe)2](ClO4)2) are [1Cl] and [1OMe], respectively. (b) Abbreviations for
the four combinations of spin state and proton position (Spin state[1Cl]Proton position) and a schematic of their structure.
a spin transition is known to be induced by light and because a
(a)
(b)
metastable excited high-spin state can be trapped at low tem-
peratures (light-induced excited-spin-state trapping, LIESST),
the compounds that demonstrate PCST behavior will undergo
photoinduced proton transfer and photoinduced excited-proton-
transfer-state trapping via the coupling mechanism.
In view of the aforementioned background information, we
attempted to synthesize an Fe(II) complex as the first example
of a crystalline material that exhibits PCST behavior. We then
aimed to achieve photocontrol of proton transfer and the trap-
ping of a photoinduced metastable proton-transfer state via the
PCST mechanism. Here, we describe thermal- and photoin-
duced PCST and the trapping of a photoinduced metastable pro-
ton-transfer state in an Fe(II) complex, [Fe(HL-Cl)2](AsF6)2
Figure 2. (a) χmT–T plot for [1Cl]. (b) Mössbauer spectra of
[1Cl]; (red line) HS Fe(II), (blue line) LS Fe(II).
([1Cl])
(HL-Cl = N′-(di(pyridin-2-yl)methylene)-2-
chlorobenzohydrazide).
N and Hyd-N–H in hydrazone can act as proton acceptor and
proton donor sites, respectively. Furthermore, the ligand field
of HL on an Fe(II) ion is within the spin-transition region.
Therefore, spin transition in Fe(II) can potentially couple to the
proton transfer between Py-N and Hyd-N; the change in spin
state in Fe(II) would influence the basicity of the proton accep-
tor and donor sites, and the proton transfer would affect the lig-
and field strength of the Fe(II) center. Furthermore, some Fe(II)
spin-transition complexes that contain hydrazone-type ligands
have been reported to exhibit the effects of LIESST 19-22. There-
fore, we synthesized Fe(II) complexes with HL derivative lig-
ands, i.e., [1Cl] and [1OMe] (Fig. 1(a)). The correlation of ab-
breviations (Py-N, Hyd-N, azomethine-N, NNHydC angle, and
CNPyC angle) frequently used in the text and the corresponding
sites in [1Cl] and [1OMe] are shown in Fig. 1(a). The correla-
The present study is organized such that the first part of the
manuscript documents PCST behavior in [1Cl] and later sec-
tions include discussions of photoinduced PCST and the trap-
ping of the photoinduced PCST. To strengthen the validity of
our interpretation of the experimental results, we synthesized an
additional compound for comparison: [Fe(HL-OMe)2](ClO4)2
([1OMe])
(HL-OMe = N′-(di(pyridin-2-yl)methylene)-2-
methoxybenzohydrazide). As opposed to [1Cl], [1OMe] exhib-
its a spin transition without proton transfer (a proton-uncoupled
spin transition). The analytical properties of the reference sam-
ple, except for the IR data, are given in the supplementary in-
formation and support the PCST behavior of [1Cl].
RESULTS AND DISCUSSION
tion of the four notations (Spin-State[1Cl]Proton-Position
:
HS[1Cl]Hyd
,
Molecular design for proton-transfer-coupled spin tran-
sition (PCST). To realize PCST and photoinduced proton
transfer, we focused on an Fe(II) complex bearing hydrazone-
type ligands that can form intramolecular hydrogen bonds. Hy-
drazone derivatives have been used to develop various func-
tional molecules in which proton dynamics such as proton relay
are observed11-14. In addition, Fe(II) complexes with NNO-type
hydrazone ligand exhibit spin transition 15-23. We designed an
Fe(II) complex with a ligand of Nʹ-(di(pyridin-2-yl)methylene)-
benzohydrazide (HL) derivative, where the HL possesses a hy-
drazone group and a pyridine ring (Fig. 1(a)). The HL can form
an intramolecular hydrogen bond between the N site in the pyr-
idine ring (Py-N) and the N site in the hydrazone moiety (Hyd-
N) even while forming a metal complex (Fig. 1(a))24-27. The Py-
HS[1Cl]Py LS[1Cl]Hyd LS[1Cl]Py) and the four molecular struc-
,
,
tures bearing the information of their spin state and proton po-
sition are shown in Fig. 1(b).
Spin-transition properties. The magnetic property meas-
urements of [1Cl] (Fig. 2(a)) show that the χmT value at room
temperature is around. 3.2 cm3 K mol−1, where χm and T are the
magnetic susceptibility and the temperature, respectively. Upon
cooling, the χmT value decreases with decreasing temperature,
approaching 0 cm3 K mol−1 at 180 K. This result suggests that
[1Cl] exhibits a gradual spin transition at T1/2 ≈ 268 K (T1/2 is
the temperature at which HS and LS fractions are equal to each
other).
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(f)
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Figure 3. (a) Single-crystal molecular structure of [1Cl] at 350 K, showing 50% probability-of-displacement ellipsoids. The AsF6− anion
and the hydrogen atoms that coordinate with carbon are omitted for clarity. (b) Schematic of the molecular structure, coordination angles,
and spin state of [1Cl] at 350 K. (c) Difference Fourier map around the intramolecular hydrogen bond in HL-Cl(A) at 350 K; the H atom
is located on the Hyd-N. The electron density at the maximum part in HL-Cl(A) and the standard deviation were 0.41 and 0.077 e Å−3,
respectively. (d) Molecular structure of [1Cl] at 180 K. (e) Schematic of the molecular structure, coordination angles, and spin state of
[1Cl] at 180 K. (f) Difference Fourier map around the intramolecular hydrogen bond in HL-Cl(A) at 180 K; the H atom is located on Py-
N. The electron density of the maximum part about HL-Cl(A) and the standard deviation were 0.59 and 0.08 e Å−3, respectively.
The Mössbauer spectrum of [1Cl] (Fig. 2(b) and Table S1) at
300 K shows quadrupole doublet peaks originating from both
the Fe(II) HS isomer (isomer shift (IS) = 0.792(7) mm s−1, quad-
rupole splitting (QS) = 2.544(14) mm s−1) and Fe(II) LS isomer
(IS = 0.10(7), QS = 0.99(15)). The estimated fractions of Fe(II)
HS and LS isomers at 300 K were 86% and 14%, respectively.
The spectrum at 60 K shows a single quadrupole doublet origi-
nating from the Fe(II) LS isomer (IS = 0.292(8), QS =
0.913(14) mm s−1). These results are consistent with the occur-
rence of a spin transition.
which implies that two kinds of intramolecular hydrogen bonds
are formed between Hyd-N and Py-N in [1Cl]. One of these
ligands forms
a shorter N···N distance (Py-N···Hyd-
N = 2.630(9) Å at 350 K) and is hereafter designated as HL-
Cl(A). The other ligand forms a longer N···N distance (Py-
N···Hyd-N = 2.693(8) Å at 350 K) and is designated as HL-
Cl(B). The positions of the protons in HL-Cl(A) and HL-Cl(B)
can be determined from the CNPyC angle and the NNHydC angle.
Upon protonation of Py-N and Hyd-N, their bond angles tend
to increase. The CNPyC angle has been reported to change from
approximately 117° to 123° upon protonation on Py-N 28. Fur-
thermore, we listed the NNHydC angles reported for the crystal
structures of hydrazone complexes (Table S4), which show that
the NNHydC angles increased from approximately 109° to 114°
upon protonation on Hyd-N.
The temperature-dependent crystal structures of [1Cl] were
measured; all of the crystallographic information and crystal
structural parameters are listed in the Supporting Information
(Tables S2 and S3). Variable-temperature single-crystal X-ray
diffraction measurements were carried out at 180 and 350 K to
investigate structural changes that occur with temperature vari-
ation (Fig. 3(a)–3(f)). The molecular packing and the labeling
scheme are summarized in the Supporting Information (Fig. S1).
The metal-to-ligand bond lengths between the Fe(II) and the az-
omethine-N at 350 K are 2.111(6) and 2.122(5) Å, which are
characteristic of an HS Fe(II) complex. By contrast, the bond
lengths at 180 K are 1.851(2) and 1.870(2) Å, which indicates
that the Fe(II) is in the LS state. These results also indicate the
occurrence of a spin transition.
At 350 K (Fig. 3(a)), the NNHydC angles in HL-Cl(A) and
HL-Cl(B) are 114.5(5)° and 115.0(5)°, respectively, which are
characteristic of protonated Hyd-N atoms (Fig. 3(b)). The
CNPyC angles in HL-Cl(A) and HL-Cl(B) are 118.0(7)°and
117.3(6)°, respectively, which are characteristic of non-proto-
nated Py-N atoms. Therefore, the protons in both HL-Cl(A) and
HL-Cl(B) were located on Hyd-N at 350 K. The maximum
electron density in the difference Fourier maps around the Hyd-
N and Py-N in the HL-Cl(A) and HL-Cl(B) ligands at 350 and
180 K presented the information of the shift of the hydrogen
atom position with temperature change. At 350 K, the maxi-
mum electron density attributable to H atoms located on Hyd-
Proton-transfer properties. Single-crystal analysis reveals
that, in the asymmetric unit of [1Cl] (Fig. 3(a)), two crystallo-
graphically independent ligands coordinate to the Fe(II) cation,
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Figure 4. (a) CNPyC angles in HL-Cl(A); experimental results
(at 180 K (LS state) and 350 K (HS state) and DFT calculation
Figure 5. (a) Temperature dependence of the IR spectra of [1Cl]
(top) and [1OMe] (bottom). Because the baselines slightly shifted
during the temperature change, offsets have been added to the
spectra of [1Cl] and [1OMe] so that the absorbances at 1700 cm−1
are the same. (b) The IR spectra calculated by DFT with a scaling
factor of 0.98: [1Cl] (top) and [1OMe] (bottom).
results () for HS[1Cl]Hyd LS[1Cl]Hyd HS[1Cl]Py, and LS[1Cl]Py . (b)
, ,
NNHydC angles in HL-Cl(A); experimental results at 180 K (LS
state) and 350 K (HS stae) and DFT calculation results for
HS[1Cl]Hyd LS[1Cl]Hyd HS[1Cl]Py, and LS[1Cl]Py.
, ,
N in both HL-Cl(A) and HL-Cl(B), suggesting that Hyd-N hosts
the H atom in both ligands (Figs. 3(c) and S1). At 180 K (Fig.
3(d)), the NNHydC and the CNPyC angles in HL-Cl(B) are
113.2(2)° and 118.0(2)°, respectively. These values indicate
that the proton in HL-Cl(B) remains on Hyd-N after the sample
is cooled from 350 to 180 K (Fig. 3(e)). In HL-Cl(A), the
NNHydC and the CNPyC angles are 108.1(2)° and 123.5(2)°, re-
spectively. The change in these angles demonstrates that the
proton was transferred from Hyd-N to Py-N upon cooling from
350 to 180 K (Fig. 3(b) and 3(e)). The difference Fourier map
around Hyd-N and Py-N in HL-Cl(A) at 180 K shows that the
H atom in HL-Cl(A) is located on Py-N (Fig. 3(f)), whereas that
around Hyd-N and Py-N in HL-Cl(B) at 180 K shows that the
H atom in HL-Cl(B) is located on Hyd-N (Fig. S1). Therefore,
proton transfer occurred in only one of the two HL ligands. This
result is consistent with the general tendency of polyprotic acids,
in which the second deprotonation is less favorable than the first
one.
(123.5°) of the LS state at 180 K is consistent with the calcu-
lated value of 124.1° for LS[1Cl]Py but differs substantially from
the value of 119.0° for LS[1Cl]Hyd. The experimental CNPyC an-
gle (118.0°) of the HS state at 350 K is consistent with the cal-
culated value (118.8°) of HS[1Cl]Hyd but not with that (124.1°)
of HS[1Cl]Py. The comparison of DFT and experimental results
supports the occurrence of PCST. Fig. 4(b) reveals that experi-
mental NNHydC angle (108.1°) of LS state at 180 K is consistent
with calculation value (109.9°) for LS[1Cl]Py but differs from
that (114.2°) of LS[1Cl]Hyd. The experimental NNHydC angle
(114.5°) of the HS state at 350 K is consistent with calculation
value (115.2°) for HS[1Cl]Hyd but not with that (110.7°) of
HS[1Cl]Py. These results also support the occurrence of PCST.
IR spectra of PCST process. Variable-temperature IR spec-
tra of [1Cl] in the fingerprint region between 1200 and
1700 cm−1 were recorded to characterize the PCST behavior
(Fig. 5(a)). The IR spectra show a clear change at the spin-tran-
sition temperature. One characteristic change observed is the
appearance of peaks at around 1389 and 1370 cm−1 upon cool-
ing, at approximately the temperature of the spin transition. To
gain insight into the spectra, we performed DFT calculations for
Proton-transfer-coupled spin transition (PCST). The
aforementioned experimental results reveal that PCST occurs in
[1Cl], where proton transfer occurs in only one of the two lig-
ands, i.e., HL-Cl(A). The PCST process is expressed as
HS[1Cl]Hyd HS[1Cl]Py, LS[1Cl]Hyd, and LS[1Cl]Py (Fig. 5(b)). The
,
LS[1Cl]Py
⇄
HS[1Cl]Hyd
simulated spectra indicate that the two peaks at 1371 and
1356 cm−1 appear upon PCST from the HS to the LS state; how-
ever, they do not appear upon the spin transition occurring with-
out proton transfer. The peaks at 1371 and 1356 cm−1, which
correspond to the experimentally observed peaks centered at
1389 and 1370 cm–1, respectively, originate mainly from the in-
To confirm the PCST, we carried out density-functional the-
ory (DFT) calculations to estimate the values of the CNPyC and
NNHydC angles (Table S5) in HS[1Cl]Hyd
,
HS[1Cl]Py, LS[1Cl]Hyd
,
C
and LS[1Cl]Py (Fig. 1(b)). The calculated CNPyC and NNHyd
angles of HL-Cl(A) are shown in Fig. 4(a) and 4(b), respec-
tively. Figure. 4(a) reveals that the experimental CNPyC angle
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Figure 6. Calculated energy diagrams of (a) [HL-Cl], (b) [1Cl] and (c) [1OMe]. The energies of [HL-Cl], [1Cl] and [1OMe] are relative
to [HL-Cl]Hyd
LS[1Cl]Py and LS[1OMe]Py, respectively. The units are in kcal mol−1. For [HL-Cl]Py, the structure with the [(Py-N)–H···(Py-
N)] hydrogen bond was obtained for optimization calculations. Any structures having [(Py-N)–H···(Hyd-N)] hydrogen bond could not be
optimized as a local minimum; the proton was always located on Hyd-N in [HL-Cl].
,
plane N–H and C–H bending vibrations in the protonated pyri-
dine ring coupled with the C–N stretching and C–N bending vi-
brations in the deprotonated hydrazone moiety 29-30. The appear-
ance of these peaks indicates that the proton at Hyd-N in HL-
Cl(A) transferred to Py-N with decreasing temperature. Thus,
the spin transition is accompanied by proton transfer. Addition-
ally, DFT calculations reveal that another characteristic peak
assigned to in-plane Py-N–H bending vibration should appear
at 1626 cm−1 after the proton-coupled spin transition from the
HS to the LS state for [1Cl] is completed. The IR spectra depict
the corresponding in-plane Py-N–H bending vibration as a
sharp peak at 1616 cm−1 for [1Cl]. These results are consistent
with the occurrence of PCST in [1Cl].
state, HS[1Cl]Hyd is more energetically stable than HS[1Cl]Py,
which is consistent with the experimental result, i.e., a proton is
on Hyd-N at the HS state. By contrast, the energies of the LS
state show that LS[1Cl]Hyd is also more stable than LS[1Cl]Py,
which is inconsistent with the experimental observations. How-
ever, the energy difference is only 0.5 kcal mol−1. Given that the
calculations are performed for isolated molecules without tak-
ing intermolecular interactions into account, the DFT results
qualitatively reflect the experimental results in the sense that the
energy of LS[1Cl]Py is greatly stabilized against LS[1Cl]Hyd (the
energy difference is merely 0.5 kcal mol−1) compared with the
HS state (the difference is 3.4 kcal mol−1).
To gain further insight into the PCST behavior, the energies
of the HL-Cl ligand that does not coordinate to Fe(II) were also
calculated (Fig. 6(a)). The energy of one of the optimized forms
of HL-Cl, wherein the proton is located on Hyd-N (hereafter
denoted as [HL-Cl]Hyd), is 0 kcal mol−1, whereas the energy of
the other form, wherein the proton is located on Py-N (hereafter
denoted as [HL-Cl]Py) is 13.3 kcal mol−1. Now, we compare the
energy difference of each state. First, we compare the energies
of [HL-Cl]Py and [HL-Cl]Hyd. The [HL-Cl]Hyd state is more sta-
ble than the [HL-Cl]Py state by 13.3 kcal mol−1 (Fig. 6(a)),
meaning that proton is located on Hyd-N. This result is con-
sistent with the reported pKa values of Hyd-N and Py-N in the
keto tautomer of hydrazone-type ligands being in the ranges
9.17–13.76 and 2.34–4.15, respectively 30-32. Next, we consider
the case where the HL-Cl ligand coordinates to HS Fe(II). When
HL-Cl coordinates to HS Fe(II), an electron pair of HL-Cl is
donated to Fe(II). The coordination strongly influences the
property of Hyd-N rather than Py-N in the remote site; the com-
To further confirm the PCST behavior, the variable-temper-
ature IR spectrum of the reference complex, [1OMe], was rec-
orded. Magnetization and Mössbauer measurements reveal that
[1OMe] exhibits one-step spin transitions (Fig. S3 and Table
S1). The changes in the CNPyC and NNHydC bond angles (Fig.
S4 and Table S3) show that [1OMe] exhibits a spin transition
without proton transfer.
LS[1OMe]Hyd
⇄
HS[1OMe]Hyd
Figure 5 shows that the IR spectrum of [1OMe] differs from
[1Cl]. Characteristic changes observed in the spectrum of [1Cl]
at around 1389, 1370 and 1616 cm-1, do not appear in the spec-
trum of [1OMe]. The experimental IR results for [1OMe] (Fig.
5(a)) is consistent with the IR spectra obtained using DFT cal-
culations (Fig. 5(b)). These results support the occurrence of
PCST in [1Cl] and the occurrence of only a spin transition with-
out proton transfer in [1OMe].
Mechanism of PCST: DFT calculation. To elucidate the
PCST behavior, we calculated energies of [1Cl] by DFT
method (Fig. 6). The DFT results show that the energies of
plexation has been reported to lead to a great increase in the
33
acidity of Hyd-N
.
Indeed, the energy difference
(3.4 kcal mol−1) between HS[1Cl]Py and HS[1Cl]Hyd (Fig. 6(b),
left) became much smaller than that (13.3 kcal mol−1) between
[HL-Cl]Py and [HL-Cl]Hyd (Fig. 6(a)) even though Hyd-N is
HS[1Cl]Py, HS[1Cl]Hyd LS[1Cl]Py, and LS[1Cl]Hyd are 4.4, 1.0, 0.0,
,
and −0.5 kcal mol−1, respectively (Fig. 6(b)). Regarding the HS
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still the stable position for the proton. Finally, we consider the
(a)
1
process by which the HS Fe(II) changes to LS Fe(II). The en-
ergy difference (0.5 kcal mol−1) between LS[1Cl]Py and
LS[1Cl]Hyd (Fig. 6(b), right) decreases compared with that
(3.4 kcal mol−1) between HS[1Cl]Py and HS[1Cl]Hyd (Fig. 6(b),
left). This result suggests that, upon spin transition from HS to
LS, the electron is slightly more donated from the HL-Cl ligand
to the Fe(II) center, thereby destabilizing the Hyd-N site and
making Py-N a more favorable site. The atomic polar tensor
(APT) charge on Fe(II) shows that the electron on the ligand
partially shifts to Fe(II) upon transition from HS to LS. The dif-
ferences in the APT charge on Fe induced by spin transition
from HS[1Cl]Hyd to LS[1Cl]Hyd and from HS[1Cl]Py to LS[1Cl]Py
are −1.39 and −1.37, respectively. The changes in the APT
charge are in agreement with the explanation that the slight
charge redistribution from the ligand to Fe(II) occurs upon tran-
sition from HS to LS. The changes in the energy levels [1OMe]
is similar to that calculated for [1Cl] (Fig. 6(c)). A distinct dif-
ference between [1OMe] and [1Cl] is that [1OMe]Hyd is much
more stable than [1OMe]Py for both the HS and LS states. This
is attributable to the electron-donating substituent OMe that re-
duces the acidity of Hyd-N. Thus, the proton should be located
on Hyd-N independent on the spin state. This is consistent with
the experimental result that spin transition in [1OMe] is not
coupled with proton transfer. The trend in the change of the en-
ergy difference shown in Fig. 6 is qualitatively consistent with
the experimental results.
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(c)
Figure 7. (a) IR difference spectrum of [1Cl] between HS[1Cl]Hyd
at 300 K and LS[1Cl]Py at 9 K. (b) Time-resolved IR spectra of
[1Cl] excited at 532 nm (ca. 100 fs, 3 J pulse−1). (c) Plots of time-
resolved IR absorption intensities of [1Cl] against the delay time
between the pump and probe pulses.
Photoinduced proton transfer. The aforementioned exper-
imental results and the DFT calculations clearly show that pro-
ton transfer and spin transition were coupled in [1Cl], which is
the first crystalline compound to demonstrate PCST. The syn-
thesis of correlation systems is important in that they can exhibit
superior functions through synergy of the coupled components.
Spin-transition behavior is known to be induced by light. Fur-
thermore, Fe(II) spin-transition complexes that contain hydra-
zone-type ligands have been reported to exhibit LIESST under
irradiation with 532-nm-wavelength light 19-22. Therefore, the
successful preparation of the PCST Fe(II) compounds poten-
tially enables the observation of the induction of PCST by light
and the trapping of the resultant excited state at low tempera-
tures. Thus, in terms of proton manipulation, proton transfer by
light and the trapping of the excited proton transfer state can be
realized via the PCST mechanism. We therefore examined the
photoeffects of [1Cl].
Time-resolved IR absorption measurements were performed
at 190 K using femtosecond laser pulses (Fig. 7). Figure 7(b)
shows that several transient absorption bands are observed at
2.5 ps after the excitation. The intensities of these transient ab-
sorption bands increase with increasing delay time within 50 ps
(Fig. 7(c)). However, the intensities and the spectral patterns do
not change after the delay time of 50 ps (Figs. 7(c) and S2). The
transient IR spectrum collected at 50 ps after the excitation is in
good agreement with the IR difference spectrum between the
HS[1Cl]Hyd state at 300 K and the LS[1Cl]Py state at 9 K (Fig.
7(a)). These results reveal that PCST is induced by light and is
completed within ca. 50 ps. Notably, the spectral pattern of the
transient IR spectrum at the delay time of 0 ps appears to differ
slightly from the patterns of the spectra after 2.5 ps (Fig. 7(b)).
Moreover, the changes in spectral patterns with isosbestic
points were observed after the delay time of 0 ps. These obser-
vations indicate that the HS state is photogenerated via one
other excited state.
Figure 8. χmT–T plots for [1Cl] before and after irradiation with
532-nm light. The inset shows the change in HS fraction with time
after irradiation at 5 K; the light-induced excited state is substan-
tially trapped. The HS fraction value is normalized as 1 at t = 0.
Trapping of photoinduced metastable proton-transfer
state. We also investigated the photoeffect at low temperature
to explore the trapping of the metastable state. First, we charac-
terized the photoeffect using a SQUID magnetometer. A pho-
toinduced increase in the χmT value was observed upon irradi-
ating LS[1Cl]Py at 5 K with 532, 660, and 780 nm light, which
corresponds to excitation of the edge or center of the metal-to-
ligand charge-transfer band (Fig. S2) 29-30, 32, 34. Figure 8 shows
the χmT–T plot before and after irradiation with 532 nm light at
5 K. The increase in the χmT value after excitation indicates that
the spin transition from LS to HS is induced and that the result-
ant excited HS state is trapped as a metastable state (LIESST
effect). The change in magnetization with time shows that the
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tween before and after irradiation corresponds with that be-
1
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4
5
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tween the HS state (300 K) and the LS state (9 K) (Fig. S2).
These results reveal that the trapped state after irradiation is ex-
pressed as HS[1Cl]Hyd. Thus, PCST is induced by light, and the
resultant excited state is trapped as a metastable state, which is
expressed as
LS[1Cl]Py
→
HS[1Cl]Hyd (excited state).
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In terms of proton position, [1Cl] undergoes light-induced pro-
ton transfer from Py-N to Hyd-N and the metastable proton-
transfer state, wherein the proton lies on Hyd-N, is trapped at
low temperatures.
The trapping of the metastable state after irradiation is also
confirmed by the single-crystal structure. Figure 8 shows that
[1Cl] exhibited LIESST with a high conversion ratio of nearly
1, which is in contrast to typical photofunctional molecules;
normally, limited moieties exhibit photoisomerization in crys-
talline states because of steric hindrance 35-38. Furthermore, the
photoinduced change occurs as a single-crystal-to-single-crys-
tal transition. Thus, structures of the photoinduced state are pre-
cisely determined by X-ray single-crystal analysis 39-40. The
crystal structure after irradiation shows only one independent
Fe(II) complex, and the coordination distances around Fe(II)
were characteristic of an HS complex (Fig. 10(a) and Table S3).
Single-crystal analysis shows that, under irradiation of [1Cl],
the NNHydC and CNPyC angles in HL-Cl(A) changed from
108.1° to 114.2° and from 123.4° to 118.4°, respectively (Fig.
10(b)); these values reveal that the protons in HL-Cl(A) trans-
ferred from Py-N to Hyd-N in conjunction with the occurrence
of the light-induced spin transition. These results demonstrate
Figure 9. The IR spectra of [1Cl] before and after irradiation with
a 532-nm light at 9 K.
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decrease in the magnetization value is less than several percent
even 24 h after irradiation, demonstrating that the light-induced
excited state is substantially trapped (Fig. 8). When the mag-
netic moment is measured after irradiation during the heating
mode from 5 K to above 100 K, the χmT value abruptly de-
creases at around 85 K, meaning that the trapped HS state re-
laxed back to the LS state.
IR spectroscopy was used to further examine the trapped state
after irradiation. As previously described, characteristic peaks
centered at 1389 and 1370 cm−1 and a sharp peak at 1616 cm−1
were observed in the IR spectrum of [1Cl] at 9 K before irradi-
ation. After light irradiation, the intensities of the peaks cen-
tered at 1389, 1370 and 1616 cm−1 decreased and the overall IR
spectrum became similar to that corresponding to the high-tem-
perature phase (Fig. 9). The IR difference spectrum of [1Cl] be-
(a)
(b)
Figure 10. Crystal structural changes with PCST. (a) Crystal structure of [1Cl] before (bottom) and after (top) irradiation with 532-nm
light at 25 K. The AsF6 anions and the hydrogen atoms coordinating with carbon atoms are omitted for clarity. (b) Schematic of the
molecular structure, coordination angles, and spin state of [1Cl] before (bottom) and after (top) irradiation with 532-nm light at 25 K. The
values of NNHydC and CNPyC angles demonstrate the photoinduced proton transfer from Py-N to Hyd-N in conjunction with the spin
transition from LS to HS under irradiation with 532-nm light.
−
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that [1Cl] undergoes light-induced PCST, wherein the proton 110). Samples were prepared by depositing ground-powdered
1
position can be switched between two sites (Fig. 10(b)).
samples onto the CaF2 plates.
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The experimental setup of picosecond time-resolved IR absorp-
tion measurements is described as follows. The fundamental
output from a Ti:sapphire regenerative amplifier (Solstice Ace,
Spectra Physics; wavelength 800 nm, power 5 W, repetition rate
1 kHz, pulse width ~100 fs) was divided into two beams. Both
were used to excite two optical parametric amplifiers (OPAs)
and generate pump and probe pulses. The probe IR pulse was
obtained by difference-frequency generation between the signal
and idler waves from one OPA (TOPAS-C, Light Conversion).
The center wavelength of the IR pulse was 7200 nm, and its
spectral bandwidth was ~170 cm−1 in FWHM. The probe beam
was divided into two paths by a ZnSe half mirror. One portion
of the beam was focused on and transmitted through the sample;
it was used as a sample beam. The other was used as a reference
beam. Both were introduced into a 19 cm spectrograph
(TRIAX190, HORIBA JOBIN YVON) with a slightly different
height offset. The dispersed two beams were simultaneously de-
tected by a 2 × 64-channel liquid-nitrogen-cooled HgCdTe de-
tector array and integrated by 128 box-car integrators (IR-12-
128, InfraRed Associates). The normalized IR signals were ob-
tained by dividing the sample intensities by the reference inten-
sities. The pump green pulse (532 nm) was obtained by sum-
frequency generation of the idler wave and the excitation pulse
(800 nm) from the other OPA (TOPAS-prime, Light Conver-
sion). The pump beam was modulated at half the repetition rate
of the probe beam (500 Hz) by a mechanical chopper. The mod-
ulated pump beam was passed through a delay stage and fo-
cused on the sample noncollinearly relative to the probe beam.
The normalized IR signals with and without the pump pulses
were separately accumulated by a computer. The pump-induced
infrared absorption was obtained by dividing the pump-on sig-
nal by the pump-off signal. The cross-correlation time between
the pump and probe pulses, which was determined by the rise
of a transient infrared absorption of photoexcited silicon due to
free carriers, was ~0.5 ps. The energy of the probe pulse at the
sample position was less than 0.5 J, and that of the pump pulse
was less than 1 J. [1Cl] was held between a grained and a plane
CaF2 plates. The sample was placed in a liquid nitrogen-cooled
cryostat (OptistatDN-V, Oxford), and kept at 190 K, at which
most of the sample was in the LS state. The total exposure time
for each time delay was 20 min.
CONCLUSIONS
To realize synergistic properties derived from the proton–
electron coupling mechanism, we attempted to synthesize an
Fe(II) complex that exhibits PCST. Through precise molecular
design, we succeeded in synthesizing an Fe(II) complex, [1Cl]
that exhibits PCST. This work represents the first demonstra-
tion of the thermally induced PCST phenomenon in a crystal-
line material. Furthermore, [1Cl] exhibits photoinduced PCST
and trapping of the photoinduced metastable state through cou-
pling of proton transfer and the light-induced spin transition ef-
fect and coupling of proton transfer and the LIESST effect, re-
spectively. Furthermore, when the PCST phenomenon is
viewed in terms of proton manipulation, photocontrol of proton
transfer and the trapping of the photoinduced metastable pro-
ton-transfer state are realized through synergistic effects. The
bistability of the light-controllable proton position is useful for
switching proton-based functionalities between two properties
in molecular devices. Notably, the correlation systems with the
PCST mechanism have the potential to exhibit various other
proton- and spin-based functionalities through cross-correlation
effects. Furthermore, we believe our study will be useful in the
development of the interdisciplinary field of molecular proton-
ics, electronics, and photonics.
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EXPERIMENTAL SECTION
Magnetic property measurement. Magnetic susceptibility
measurements for [1Cl] and [1OMe] were conducted on a
Quantum Design MPMS-5S superconducting quantum-inter-
ference device magnetometer under a 5-kOe field, with a
sweeping rate of 3 K min−1. The measurement samples were
prepared by encapsulating each crystalline compound into a
gelatin capsule except for the photomagnetic measurements.
Compound magnetic susceptibility measurements, in the light-
excited state, were performed after sample irradiation with 532
nm green light for 1.5 h. The samples subjected to irradiation
were prepared by spreading powdered samples onto colorless
tape.
Mössbauer spectroscopy. 57Fe Mössbauer spectra were rec-
orded on a constant acceleration spectrometer with a source of
57Co/Rh in the transmission mode. The measurements were per-
formed using a closed-cycle helium refrigerator (Iwatani Co.,
Ltd or ULVAC CRYOGENICS INC) and a conventional Möss-
bauer spectrometer (Wissel MVT-1000 or Topologic Systems).
All isomer shifts are given relative to -Fe at room temperature.
The Mössbauer spectra were fitted with the least-squares fitting
program MossWinn 4.0.
DFT calculation. All energy calculations for the [1X]Hyd and
[1X]Py (X = Cl and OMe) complexes in the singlet and quintet
spin states, and free ligands [HL-Cl]Hyd and [HL-Cl]Py were
carried out using restricted and unrestricted B3LYP* function-
als combined with the 6-311+G** basis set implemented in the
Gaussian 09 package 41. The B3LYP* functional, developed by
Reiher and co-workers 42,43, is a reparametrized version of the
B3LYP functional with 15% Hartree–Fock exchange instead of
20% in the original B3LYP functional44-46. The B3LYP* func-
tional was specifically developed to provide the best perfor-
mance for accurate spin-state splitting while the original
B3LYP functional results in the overestimation of HS stability.
For comparison with the experimental results, the calculated vi-
brational frequencies were rescaled by the factor of 0.98.
IR spectroscopy. The IR spectra of [1Cl] at 300 and 9 K and
[1Cl] at 9 K before and after light (532 nm) irradiation were
recorded using an FT-IR spectrophotometer (VERTEX 70,
Bruker) equipped with a closed-cycle helium refrigerator cryo-
stat (Nagase Techno-Engineering). The ground-powdered sam-
ples were held between a grained and a plane CaF2 plates. A
solid-state cw green laser (Millennia, Spectra Physics) was used
as the illumination light source. The laser wavelength was 532
nm, and the power density was 15 mW cm-2 at the sample point.
Temperature-dependent IR spectra of [1OMe] were recorded
using an FT-IR spectrophotometer (FT/IR-660 plus, Jasco)
equipped with a helium-flow-type refrigerator (Helitran LT-3–
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI:
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Journal of the American Chemical Society
(10) Mitsumi, M.; Ezaki, K.; Komatsu, Y.; Toriumi, K.; Miyatou,
Experimental conditions of synthesis, X-ray structural determina-
T.; Mizuno, M.; Azuma, N.; Miyazaki, Y.; Nakano, M.; Kitagawa, Y.;
Hanashima, T.; Kiyanagi, R.; Ohhara, T.; Nakasuji, K., Proton Order-
tion and UV-vis spectra measurement, structural parameters of the
crystal structure and DFT calculations of all compounds, UV-vis
spectra and time-resolved IR spectra of [1Cl], and the crystal struc-
ture, magnetic property, and Mössbauer spectra of [1OMe]. X-ray
crystallographic data for [1Cl] at 350, 180, and 25 K before and
after irradiation of 532 nm light and for [1OMe] at 150 and 300 K
(CIF).
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Disorder Phenomena in
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Hydrogen-Bonded Rhodium-5-
Semiquinone Complex: A Possible Dielectric Response Mechanism.
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Crystal Violet Lactone Salicylaldehyde Hydrazone Zn(II) Complex: a
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AUTHOR INFORMATION
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Corresponding Author
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* sato@cm.kyushu-u.ac.jp
ACKNOWLEDGMENT
We thank for the support by MEXT KAKENHI (Grant Numbers
JP17H01197,
JP16H00849,
JP17K05761,
JP24109014,
JP15K13710, JP17H01200, JP17H03117, JP16K05421 and
JP17H06928), by the MEXT Project of “Integrated Research Con-
sortium on Chemical Sciences”, “Elements Strategy Initiative to
Form Core Research Center”, JST-CREST “Innovative Catalysts”
JPMJCR15P5, and by the Cooperative Research Program of “Net-
work Joint Research Center for Materials and Devices”. The syn-
chrotron radiation experiments were performed at the BL02B1 of
SPring-8 with the approval of the Japan Synchrotron Radiation Re-
search Institute (JASRI) (Proposal No. 2017A1364, 2017B1285,
2018A1213 and 2018B1259). This work was partly supported by
Nanotechnology Platform Program (Molecule and Material Syn-
thesis) of MEXT, Japan. The computation was carried out using the
computer facilities at Research Institute for Information Technol-
ogy, Kyushu University.
(18) Yuan, J.; Wu, S.-Q.; Liu, M.-J.; Sato, O.; Kou, H.-Z.,
Rhodamine 6G-Labeled Pyridyl Aroylhydrazone Fe(II) Complex
Exhibiting Synergetic Spin Crossover and Fluorescence. J. Am. Chem.
Soc. 2018, 140, 9426-9433.
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Tuning size and magnetic thermal hysteresis in a new near room
temperature spin crossover compound. Dalton Trans. 2013, 42, 8205-
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