Synthesis, Characterization, and Properties of Iron(II) Spin-Crossover Molecular Photoswitches Functioning at Room Temperature

Article abstract of DOI:10.1021/acs.inorgchem.7b01952

Spin-crossover molecular switches [Fe^II(H₂B(pz)₂)₂L] (L = novel phenanthroline-based ligands featuring photochromic diarylethene units; pz = 1-pyrazolyl) were synthesized and thoroughly characterized by variable

Full text of DOI:10.1021/acs.inorgchem.7b01952

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Synthesis, Characterization, and Properties of Iron(II) Spin-Crossover
Molecular Photoswitches Functioning at Room Temperature
Max Mortel,‡ Alexander Witt,‡ Frank W. Heinemann, Sebastian Bochmann, Julien Bachmann,
̈
and Marat M. Khusniyarov*
Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nurnberg, Egerlandstrasse 1, 91058 Erlangen,
̈
Germany
S
* Supporting Information
ABSTRACT: Spin-crossover molecular switches [FeII(H2B(pz)2)2L] (L =
novel phenanthroline-based ligands featuring photochromic diarylethene
units; pz = 1-pyrazolyl) were synthesized and thoroughly characterized by
variable-temperature X-ray crystallography, Mossbauer spectroscopy, and
̈
magnetic measurements. The effect of substituents introduced into the
phenanthroline backbone (L2) and into the photochromic diarylethene unit
(L3) on photophysical properties of metal-free ligands and spin-crossover
iron(II) complexes 2 and 3, respectively, were investigated in detail. Both
ligands and complexes could be switched with light in solution at room
temperature. The photocyclization of 2 was accompanied by a high-spin to
low-spin photoconversion determined at 19%. The closed-ring isomers of
L3 and 3 reveal the lifetimes in the range of minutes, whereas those of L2 and 2 are thermally stable for days in solutions at room
temperature. The reversibility of the photoswitching can be improved by avoiding the photostationary states. Prospective
introduction of anchoring groups to the phenanthroline backbone might allow the construction of chemisorbed self-assembled
monolayers of spin-crossover species switchable with light at room temperature.
never been achieved. Alternative concepts for the photo-
switching at RT are needed.20
INTRODUCTION
■
Spin-crossover (SCO) metal complexes represent a highly
promising class of molecular switches, whose magnetic,
conductive, and other physical properties can be reversibly
altered by different physical stimuli including a change in
temperature, applied pressure, light irradiation, and applied
electric and magnetic fields.1,2 Among SCO complexes, six-
coordinate iron(II) species are the most common representa-
tives that offer a brilliant opportunity to switch reversibly
between diamagnetic low-spin (LS) and paramagnetic high-spin
(HS) electronic states. The ability for reversible switching of
diverse physical properties by an external stimulus of choice
renders SCO complexes highly promising candidates for many
future applications including molecular sensors,3 luminescent,
plasmonic,4 and display devices,5 ultrahigh-density memories,6
and building blocks for molecular electronics and spintronics.7,8
The photoswitching of SCO species is very attractive because
of the extremely fast switching (sub-picoseconds)9 and low
power dissipation. Thus, photoswitching of SCO thin films and
monolayers via a light-induced excited spin-state trapping
(LIESST) effect was achieved recently.10−15 However, very low
temperatures (usually below 50 K) are strictly mandatory here,
because the photoinduced state in the LIESST effect is
thermally unstable and relaxes to the ground state within
nanoseconds at room temperature (RT).16−19 Thus, the
photoswitching of immobilized SCO molecular complexes at
RT, which is an important goal in photomagnetic materials, has
Targeting the switching of magnetic properties at RT,21,22
very recently, we synthesized a unique iron(II) SCO molecular
switch 1 featuring a photoactive diarylethene-based ligand L1
(Scheme 1).23 Remarkably, 1 was reversibly switched between
HS and LS states by irradiation with light at RT.23,24 The
photoswitching in 1 is fundamentally different from the
previous examples, because it proceeds indirectly via the
photoreaction at the photoisomerizable ligand, that is, a ligand-
driven light-induced spin change (LD-LISC) effect.25,26 Here,
the photocyclization of the ligand triggers an SCO at the
coordinated metal ion remotely. Although such ligand-driven
photoswitching was suggested and attempted with other
systems before,25,27−31 only recently our group23,24 and the
group of Herges32 achieved efficient and reversible photo-
switching of iron(II) and nickel(II) molecular systems at RT,
respectively. Importantly, such indirect photoswitching pro-
ceeds at the single-molecule level, and the photoisomers might
be stable for years at RT.33
Currently, we successfully switched 1 with light at RT both in
solution and in the solid state.23,24 However, a question
appears, namely, is 1 a unique fine-tuned molecular switch, or is
it a member of a novel class of advanced magnetic
photoswitches? The present work provides the answer to this
Received: July 31, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01952
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water, heat absorbing (KG-5), and long-pass (λ ≥ 400 nm) filters was
used for cycloreversion with visible.
Scheme 1. Diarylethene-Based Ligands L1−L3 and
Corresponding Iron(II) Spin-Crossover Complexes 1−3
Synthesis. 5,6-Dibromo-3,8-dimethyl-1,10-phenanthroline.
Under vigorous stirring 3,8-dimethyl-1,10-phenanthroline (5.00 g,
24.0 mmol) was added to a 65% oleum (35 mL) at 0 °C to give a
brown viscous mixture. The mixture was stirred at RT for 20 min to
ensure that the reagent dissolved completely. After the reaction flask
was protected from light, Br2 (7.67 g, 48.0 mmol) was added dropwise
at 0 °C, and the reaction mixture was stirred for 20 h at RT. The dark
brown mixture was then carefully poured on ice, and the resulting
brown suspension was neutralized (pH ≈ 7) with a 25% NaOH
solution. The aqueous phase was extracted with CH2Cl2, while
insoluble residue, which hindered phase separation, was filtered off.
The organic extracts were dried over MgSO4 and filtered, and after
solvent removal a yellow crude product was dried in vacuo.
Recrystallization from CHCl3 rendered needle-shaped colorless
crystals of 5,6-dibromo-3,8-dimethyl-1,10-phenanthroline. Yield: 4.64
g, 53%. Anal. Calcd for C14H10Br2N2: C 45.94, H 2.75, N 7.65; found:
C 46.00, H 2.68, N 7.57%. High-resolution mass spectrometry
(HRMS) (electrospray ionization (ESI+)): m/z calcd. for [M + Na]+
388.9083, found 388.9086; for [M + K]+ 404.8822, found 404.8832.
1H NMR (400 MHz, CDCl3, 21.4 °C) δ: 8.98 (s, 2 H, Caryl−H), 8.42
(s, 2 H, Caryl−H), 2.63 (s, 6 H, CH3) ppm. 13C NMR (67.8 MHz,
CDCl3, 25.0 °C) δ: 152.3, 143.4, 136.4, 134.2, 127.8, 124.8 (Caryl),
18.8 (CH3) ppm.
question. Here, we report on two novel derivatives of 1. We
successfully modified the parent diarylethene-based ligand L1
by introducing methyl substituents into the phenanthroline
backbone (L2) and phenyl groups into the photoactive
diarylethene unit (L3, Scheme 1). The corresponding iron(II)
SCO molecular switches 2 and 3 were subsequently
synthesized. The physical, spectroscopic, and photophysical
properties of novel ligands and their iron(II) complexes were
investigated in detail and compared with the properties of the
parent L1/1 system.
5,6-Bis(2,5-dimethylthiophen-3-yl)-3,8-dimethyl-1,10-phenan-
throline (L2). Under inert atmosphere, degassed water (13.8 g, 764
mmol) was added to a suspension of 5,6-dibromo-3,8-dimethyl-1,10-
phenanthroline (2.00 g, 54.6 mmol), (2,5-dimethylthiophen-3-yl)-
boronic acid (5.11 g, 32.8 mmol), K3PO4 (23.2 g, 109 mmol), and
[Pd(dppf)Cl2]·CH2Cl2 (1.34 g, 16.4 mmol, dppf = 1,1′-bis-
(diphenylphosphino)ferrocene) in dry degassed 1,4-dioxane (120
mL), and the reaction mixture was stirred for 3 d at 100 °C. After
solvent removal and drying in vacuo, the remaining solids were
suspended in CH2Cl2 and water. The aqueous phase was separated
and further extracted with CH2Cl2. The combined organic extracts
were dried over MgSO4 and filtered, and the solvent was removed in
vacuo to give a crude brown product. Purification was achieved by
flash chromatography on SiO2 with CHCl3. Solvent removal and
extensive drying in vacuo rendered L2 containing residual CHCl3.
Yield: 2.10 g, 74%. Anal. Calcd for C26H24N2S2·(CHCl3)0.65: C 63.23,
H 4.91, N 5.53, S 12.67; found: C 63.41, H 5.04, N 4.95, S 11.90%.
HRMS (ESI+): m/z calcd. for [M + H]+ 429.1454, found 429.1443;
for [M + K]+ 467.1012, found 467.1000; for [2M + Na]+ 879.2654,
EXPERIMENTAL SECTION
■
Materials. All starting materials and solvents were utilized as
received without further purification unless otherwise noted. Pure
anhydrous solvents were collected from a solid-state solvent
purification system (Glass Contour; Irvine, CA). 3,8-Dimethyl-1,10-
phenanthroline,34 (2,5-dimethylthiophen-3-yl)boronic acid,35,36 bis(1-
pyrazolyl)borohydride,37 (2-methyl-5-phenylthiophen-3-yl)boronic
acid,38,39 and 5,6-dibromo-1,10-phenanthroline40 were prepared
according to literature methods with minor modifications.
1
found 879.2636; for [2M + K]+ 895.2393, found 895.2373. H NMR
Instrumentation. Elemental analyses were performed with an
EURO EA analyzer from EuroVector. Magnetic susceptibility data on
solid samples were collected with a Quantum Design MPMS 5XL
SQUID magnetometer. The direct-current (dc) susceptibility data
were collected in the temperature range of 2−300 K on powder
samples restrained within a polycarbonate gel capsule in the applied
magnetic field of 1 T at 1 K min−1 heating/cooling rate and 2 K
intervals. 57Fe Mossbauer spectra were recorded on a WissEl
Mossbauer spectrometer (MRG-500) in constant-acceleration mode.
57Co/Rh was used as the radiation source. The temperature of the
samples was controlled by an MBBC-HE0106 Mossbauer He/N2
cryostat within an accuracy of 0.3 K. Isomer shifts were determined
relative to α-iron at 298 K. The program MFIT was used for the
quantitative analysis of spectra.41 NMR spectra were recorded with
JEOL ECX 400 MHz, JEOL ECP 400 MHz, and JEOL EX 270 MHz
in rotating 5 mm o.d. tubes and processed with Delta V4.0 software
provided by JEOL Ltd. Electronic absorption spectra were recorded
with Shimadzu UV 2450 and Shimadzu UV 3600 spectrophotometers.
Powder X-ray diffraction was performed in the Bragg−Brentano
geometry on a Bruker D8 Advance equipped with a Cu Kα source and
LynxEye XE T detector.
(400 MHz, CDCl3, 22.8 °C) δ (conformer-1): 9.023 (s, 2 H,
4
C(2,9)phen−H), 7.694 (dd, J = 2.1 Hz, J = 0.9 Hz, 2 H, C(4,7)phen
−
H), 6.317 (d, J = 1.0 Hz, 2 H, Cthiophene−H), 2.505 (s, 6 H, Cphen
−
CH3), 2.376 (s, 6 H, Cthiophene−CH3), 1.976 (s, 6 H, Cthiophene−CH3)
4
ppm; δ (conformer-2): 9.019 (s, 2 H, C(2,9)phen−H), 7.751 (dd, J =
2.1 Hz, J = 0.9 Hz, 2 H, C(4,7)phen−H), 6.327 (d, J = 1.0 Hz, 2 H,
Cthiophene−H), 2.511 (s, 6 H, Cphen−CH3), 2.387 (s, 6 H, Cthiophene
−
CH3), 1.930 (s, 6 H, Cthiophene−CH3) ppm. 13C NMR (100.6 MHz,
CDCl3, 25.6 °C) δ: 151.1 (Ct), 143.1 (Cq), 135.6 (Cq), 135.5 (Cq),
134.7 (Ct), 134.5 (Ct), 134.2 (Cq), 133.8 (Cq), 133.3 (Cq), 133.2 (Cq),
132.9 (Cq), 128.3 (Ct), 128.2 (Cq), 128.1 (Cq), 127.2 (Ct), 19.01
(CH3), 15.25 (CH3), 14.10 (CH3) ppm. Crystallization from MeOH
solution rendered colorless block-shaped crystals suitable for X-ray
structure determination.
3,8-Dimethyl-5,6-bis(2-methyl-5-phenylthiophen-3-yl)-1,10-phe-
nanthroline (L3). Under inert atmosphere, dry 1,4-dioxane (16 mL)
was added to a suspension of (2-methyl-5-phenylthiophen-3-yl)-
boronic acid (0.99 g, 4.54 mmol), 5,6-dibromo-1,10-penanthroline
(516 mg, 1.53 mmol), [Pd(PPh3)4] (353 mg, 0.31 mmol), and
anhydrous sodium carbonate (971 mg, 9.2 mmol) in degassed water
(16 mL). The reaction mixture was stirred vigorously for 3 d at 95 °C.
After it cooled to RT the aqueous layer was separated and extracted
several times with CH2Cl2. The combined organic phases were washed
with water and brine, dried over MgSO4 and filtered, and the solvent
was removed in vacuo to give a crude sand brown solid. Purification
Photochemisty. All solutions for photoexperiments were prepared
under inert conditions and sealed in custom-made Quartz SUPRASIL
cells (QS). An L.O.T.-Oriel Xe(OF) 150 W arc lamp equipped with
appropriate filters was used as a light source. Water and bandpass (λ =
282 5 nm) filters were used for cyclization with UV, while a set of
B
DOI: 10.1021/acs.inorgchem.7b01952
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Scheme 2. Syntheses of Novel Diarylethene-Based Ligands L2 and L3
was achieved by washing with n-hexane followed by the flash column
chromatography on SiO2 with CHCl3. Solvent removal and drying in
vacuo yielded pure L3 as an off-white powder. Yield: 0.55 g, 69%. Anal.
Calcd (%) for C34H24N2S2: C 77.83, H 4.61, N 5.34, S 12.22; found: C
77.36, H 4.53, N 5.22, S 11.96. HRMS (ESI+): m/z calcd. for [M +
H]+ 525.1459, found 525.1448. 1H NMR (270 MHz, CDCl3, 21.4 °C)
δ (parallel): 9.25 (dd, 3J = 4.3 Hz, 4J = 1.7 Hz, 2 H, C(2,9)phen-H), 8.17
(dd, 3J = 8.3 Hz, 4J = 1.7 Hz, 2 H, C(4,7)phen-H), 7.64 (dd, 3J = 8.3 Hz,
3J = 4.3 Hz, 2 H, C(3,8)phen-H), 7.46−7.24 (m, Cphenyl-H, 10 H), 7.04
(s, 1 H, Cthiophene-H), 2.09 (s, 6 H, Cthiophene−CH3) ppm; δ
filtered off, washed with degassed water, and dried in vacuo to yield 3
as a violet powder. Yield: 202 mg, 61%. Anal. Calcd for
C46H40B2FeN10S2: C 63.18, H 4.61, N 16.02, S 7.33; found: C,
62.64; H, 4.49; N, 15.80; S, 7,43%. HRMS (ESI+): m/z calcd. for [M−
BH2(pz)2]+ 727.1574, found 727.1557; for [M−BH2(pz)2 + L3]+
1251.2962, found 1251.2933. Crystals suitable for X-ray structure
determination were obtained upon slow diffusion of hexane into a
toluene solution of 3 under inert atmosphere.
X-ray Crystallographic Data Collection and Refinement of
the Structures. Suitable crystals were embedded in protective
perfluoropolyalkyl ether oil and transferred to the cold nitrogen gas
stream of the diffractometer. Intensity data for L2 were collected at
100 K, for 2 at 100 and 283 K, and for 3 at 100 and 273 K on a Bruker
Kappa APEX2 IμS Duo diffractometer equipped with Quazar focusing
Montel optics (Mo Kα radiation, λ = 0.710 73 Å). Data were corrected
for Lorentzian and polarization effects; semi empirical absorption
corrections were applied on the basis of multiple scans by using
SADABS.42 The structures were solved by direct methods and refined
by full-matrix least-squares procedures on F2 by using SHELXTL NT
6.12.43 All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were placed in position
of optimized geometry, and their isotropic displacement parameters
were tied to those of their corresponding carrier atoms by a factor of
1.2 or 1.5. The crystallographic data, data collection, and structure
refinement details are summarized in Table S1 (Supporting
Information).
The diffraction data collected on the investigated crystal of L2
rendered the molecule in the monoclinic space group P21/n with two
molecules of CH3OH per formula unit. One of the CH3OH solvent
molecules was disordered. Two alternative orientations were refined
resulting in site occupancies of 88.9(6) and 11.1(6)% for the atoms
O1, C27 and O1A, C27A, respectively. SIMU and ISOR restraints
were applied in the refinement of the disordered atoms.
The crystals of 2 contained four molecules per unit cell with two
molecules of toluene per formula unit in the monoclinic space group
P21/n. At 100 K one of the thiophene groups was disordered, and two
alternative orientations were refined, resulting in site occupancies of
75(2) and 25(2)% for the atoms S2, C21−C26 and S2A, C21A−
C26A, respectively. The disorder in one of the thiophene rings in 2
was also found at 283 K and refined for two different orientations with
site occupancies of 61(2) and 39(2)% for the atoms S2, C21−C26 and
S2A, C21A−C26A, respectively. At 283 K one of the toluene
molecules also suffered from disorder. Two alternative orientations
were refined and resulted in site occupancies of 62(2) and 38(2)% for
the atoms C101−C107 and C111−C117, respectively. Similarity and
pseudoisotropic restraints were applied to the anisotropic displace-
ment ellipsoids of the disordered atoms.
3
4
(antiparallel): 9.25 (dd, J = 4.3 Hz, J = 1.7 Hz, 2 H, C(2,9)phen
-
H), 8.09 (dd, 3J = 8.3 Hz, 4J = 1.7 Hz, 2H, C(4,7)phen-H), 7.62 (dd, 3J
= 8.3 Hz, 3J = 4.3 Hz, 2 H, C(3,8)phen-H), 7.46−7.24 (m, Cphenyl-H, 10
H), 7.02 (s, 1 H, Cthiophene-H), 2.16 (s, 6 H, Cthiophene−CH3) ppm. 13
C
NMR (100.6 MHz, CDCl3, 21.4 °C) δ (parallel): 150.3, 145.9 (Cq),
140.6 (Cq), 136.8(Cq), 135.3 (Cq), 135.1, 134.3 (Cq), 133.2 (Cq),
129.0, 128.5 (Cq), 127.4, 126.6, 125.7, 125.3, 123.4, 14.4 (CH3) ppm;
δ (antiparallel): 150.3, 146.0 (Cq), 140.6 (Cq), 136.4(Cq), 135.2 (Cq),
135.0 (Cq), 134.2 (Cq), 133.1, 128,9, 128.7 (Cq), 127.4, 126.6, 125.7,
125.5, 123.4, 14.5 (CH3) ppm.
[FeII(H2B(pz)2)2L2] (2). A solution of potassium bis(1-pyrazolyl)-
borohydride (0.34 g, 2.0 mmol) in dry MeOH (9 mL) was added
dropwise to a stirred dry MeOH solution (5 mL) of FeSO4·7H2O
(0.25 g, 1.0 mmol) at RT, whereupon a very fine white solid
precipitated. The solid was removed by centrifugation followed by a
decantation of the colorless solution of the in situ formed
“[FeII(H2B(pz)2)2]” (pz = 1-pyrazolyl). To this solution, L2 (0.40 g,
1.0 mmol) dissolved in dry MeOH (30 mL) was added, and the
resulting violet suspension was stirred for 2.5 h at RT. The suspension
was concentrated to ∼1/3 volume and stored at −35 °C for 2 d. The
violet precipitate was filtered off, washed with degassed water, and
dried in vacuo to give 2 as a fine violet powder. Yield: 0.53 g, 76%.
Anal. Calcd for C38H40B2FeN10S2: C 58.64, H 5.18, N 17.99, S 8.24;
found: C 58.57, H 5.19, N 17.97, S 8.23%. HRMS (ESI+): m/z calcd.
for [M−BH2(pz)2]+ 631.1573, found 631.1585; for [M−BH2(pz)2 +
L2]+ 1059.2959, found 1059.2982. Crystals suitable for X-ray structure
determination were obtained upon slow diffusion of hexane into a
toluene solution of 2 under inert atmosphere.
[FeII(H2B(pz)2)2L3] (3). A solution of potassium bis(1-pyrazolyl)-
borohydride (0.141 g, 0.76 mmol) in dry, degassed MeOH (5 mL)
was added slowly to a solution of FeSO4·7H2O (0.106 g, 0.38 mmol)
in dry, degassed MeOH (3 mL) at RT under inert gas atmosphere.
After it was stirred for 10 min, the white precipitate was removed by
centrifugation. The colorless solution of in situ formed “[FeII(H2B-
(pz)2)2]” was decanted into a Schlenk tube, and a solution of L3
(0.200 g, 0.38 mmol) in dry, degassed MeOH (16 mL) was added via
cannula. The resulting violet suspension was stirred for 3 h at RT
before it was stored at −35 °C for 3 d. The fine violet precipitate was
C
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The complex 3 crystallized with a total of 1.5 molecules of toluene
per formula unit in the triclinic space group P1. Half a toluene
̅
molecule was situated on crystallographic inversion center showing a
disorder of the methyl group. The other toluene molecule was also
disordered. Two alternative orientations were refined and resulted in
site occupancies at 100 K of 62.1(4) and 37.9(4)% for the atoms
C101−C107 and C111−C117, respectively. In the structure obtained
at 273 K, those site occupancies were 37.9(6) and 62.1(6)%. Similarity
and pseudoisotropic restraints were applied to the anisotropic
displacement parameters of the disordered atoms.
RESULTS AND DISCUSSION
Synthesis and Crystal Structures. Novel photoactive
diarylethene-based ligands L2 and L3 were obtained through
■
Figure 3. Molecular structure of open-ring 3-o at 100 K showing a
parallel conformation. The H atoms are omitted for clarity; the
thermal ellipsoids are drawn at the 50% probability level.
Figure 1. Molecular structure of open-ring L2-o in the antiparallel
conformation at 100 K. Thermal ellipsoids are drawn at the 50%
probability level.
Figure 4. Variable-temperature χT products of 2 (top) and 3
(bottom) measured on powder samples at an external magnetic field of
1 T in the cooling mode (1 K min−1, 2 K intervals). See the text for the
fit parameters.
1,10-phenanthroline40 was reacted in a Suzuki cross-coupling
reaction with 2 equiv of (2-methyl-5-phenylthiophen-3-yl)-
boronic acid, which was obtained in two steps from 3,5-
dibromo-2-methylthiophene,38,39 to yield L3.
Figure 2. Molecular structure of open-ring 2-o at 100 K showing an
antiparallel conformation. The H atoms are omitted for clarity; the
thermal ellipsoids are drawn at the 50% probability level.
Generally, open-ring diarylethenes can adopt two conforma-
tions differing in mutual orientation of aryl (thiophene)
fragments: a photoactive antiparallel conformer featuring a C2
rotation axis and a photoinactive parallel conformer featuring a
mirror plane.44 Since the rotation along C(phenanthroline)−
C(thiophene) bonds in L1−L3 is sterically hindered and thus
the interconversion between the conformers is slow, both
conformers can be observed in solution at RT at the NMR time
multistep syntheses (Scheme 2). 3,8-Dimethyl-1,10-phenan-
throline34 was brominated in oleum to give a 5,6-dibromo
derivative. The latter was C−C coupled with 2 equiv of (2,5-
dimethylthiophen-3-yl)boronic acid, which was obtained from
2,5-dimethylthiophene in two steps,35,36 to give L2. A similar
route was used to synthesize L3. In the final step, 5,6-dibromo-
D
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Figure 5. Variable-temperature χT products of 2 (top) and 3
(bottom) measured in toluene solutions with the Evans NMR method
(toluene/toluene-d8/TMS = 10:2:1). van’t Hoff fit parameters for 2:
ΔH = 11.2(7) kJ mol−1, ΔS = 64(4) J K−1; for 3: ΔH = 22(1) kJ
mol−1, ΔS = 119(7) J K−1.
scale. Thus, similarly to the parent L1 ligand,23,45 two sets of
signals are observed in 1H NMR spectra of L2 and L3.
Importantly, a mixture containing both conformers at ∼1:1
ratio is usually obtained in the Suzuki reaction (vide supra).
However, because of slightly differing solubility and retention
factors, after workup, we often obtain a mixture enriched for
one or another conformer. This allows the assignment of
signals in 1H NMR spectra to different conformers (see
Experimental Section and Supporting Information). For
example, the CH3 protons of L3 resonating at 2.09 (parallel)
and 2.16 (antiparallel) ppm are clearly distinguishable.
In the context of photoswitching, samples that are enriched
for or contain exclusively a photoactive antiparallel conformer
are preferred.46,47 However, we noticed that the samples of
L1−L3 enriched significantly for one or another conformer
convert slowly and spontaneously to an ∼1:1 mixture in
solution at RT. The interconversion between the conformers
can be further accelerated by heating or ultrasonic waves. We
assume that similar effects are operative for the ligands within
the complexes 1−3. As an important consequence, the exact
ratio between the conformers in photoexperiments in solutions
(vide infra) is not known.
The crystallization from a methanolic solution rendered
colorless crystals of L2 suitable for X-ray structure determi-
nation (Figure 1). The open-ring isomer (L2-o) crystallized in
the monoclinic P21/n space group with four molecules of the
ligand molecules together with eight MeOH solvates per unit
cell. The picked crystal contained exclusively an antiparallel
conformer44 with α-methyl groups (relative to C−C thiophene-
phenanthroline bond) pointing to opposite directions. At 100
K, the thiophene rings form dihedral angles of 74.2 and 75.1°
with a nearly planar phenanthroline unit. Hence, the π-systems
Figure 6. Zero-field 57Fe Mossbauer spectra measured on 2 at 78
̈
(top), 150 (middle), and 297 K (bottom). Relative intensities: (top)
90% LS-Fe(II) in blue, 2% HS-Fe(II) in red, 8% minor HS-Fe(II) in
green; (middle) 65% LS-Fe(II) in blue, 22% HS-Fe(II) in red, 13%
minor HS-Fe(II) in green; (bottom) 85% HS-Fe(II) in red, 15%
minor HS-Fe(II), in green. See the text and Supporting Information
for the fit parameters.
of the two thiophene moieties and the phenanthroline
backbone in L2-o are essentially electronically uncoupled. A
similar orthogonality of those units was observed for a parent
L1-o ligand.45
Colorless methanolic solutions of L2 and L3 reacted
immediately with a colorless methanolic solution of “[FeII(H2B-
(pz)2)2]”, which was generated in situ from K(H2B(pz)2),37
forming deep purple precipitates of the target paramagnetic
complexes 2 and 3, respectively. While the dried powders of 2
and 3 were used for all subsequent physical and photophysical
studies, small amounts of crystalline materials were obtained
upon slow diffusion of n-hexane into toluene solutions of the
complexes. The crystals were suitable for X-ray structure
determination, which was accomplished at 100 and 283 K for 2,
and at 100 and 273 K for 3.
The complex 2 crystallized in the monoclinic P21/n space
group with four molecules of open-ring 2-o and eight molecules
of toluene per unit cell. Two (H2B(pz)2)−1 and one L2-o
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Figure 9. Multiple photoswitching of L2 (top) and L3 (bottom) in
MeCN solutions at RT (c = 6.0 × 10−5 M). The evolution of the 362
and 392 nm absorption, respectively, upon alternating photo-
cyclization with λ = 282 nm (blue) and photocycloreversion with
visible (λ ≥ 400 nm, red). The data were fitted according to the first-
order kinetics.
Figure 7. Zero-field 57Fe Mossbauer spectra measured on 3 at 77
(top) and 298 K (bottom). Relative intensities: (top) 95% LS-Fe(II)
in blue, 5% HS-Fe(II) in red; (bottom) 100% HS-Fe(II) in red. See
the text and Supporting Information for the fit parameters.
̈
ligands form a distorted N6 coordination environment around
the iron(II) central ion (Figure 2). At 100 K, 2-o shows
relatively short Fe−N bond distances in the range of 2.030(2)−
2.058(2) Å indicative of an LS-Fe(II) ion.48,49 One of the
thiophene rings is disordered showing two alternative
orientations with site occupancies of 75(2) and 25(2)%.
Interestingly, both orientations represent photoactive antipar-
allel conformers. The major orientation (75%) reveals dihedral
angles between the thiophene rings and phenanthroline
backbone of 88.1 and 89.3°, while the corresponding angles
in the alternative orientation (25%) are 88.1 and 84.9°.
Consequently, the electronic conjugation between the π-
systems of the phenanthroline and two thiophenes is negligible.
Interestingly, in the previously reported crystal structures of 1-
o,23,50 the thiophenes and phenanthroline backbone are slightly
less orthogonal showing dihedral angles in the range of 61.8−
84.0°.
At 283 K the space group is preserved. However, all Fe−N
bond distances increased significantly to 2.147(4)−2.221(4) Å,
which is typical for HS-Fe(II) complexes.13,48,49,51 Thus, similar
to a powder sample (vide infra), the crystals of 2-o show
thermally induced SCO. Similar to the low-temperature
structure, one thiophene is disordered resulting in two
alternative orientations with site occupancies of 61(2) and
39(2)% at 283 K. Both orientations are antiparallel conformers.
The thiophenes are nearly orthogonal to the phenanthroline
plane confirmed by the corresponding dihedral angles of 84.8−
89.5°.
The complex 3 crystallized in the triclinic P1 space group
̅
Figure 8. Photocyclization of L2-o (top) and L3-o (bottom) with UV
light at RT (λ = 282 nm, MeCN solutions, c = 6.0 × 10−5 M).
with two molecules of open-ring 3-o and three molecules of
toluene per unit cell. The coordination environment of 3-o
(Figure 3) is very similar to 2-o. At 100 K, 3-o is in an LS-
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Figure 11. Photocycloreversion of 1 (top), 2 (middle), and 3
(bottom) with visible light at RT (λ ≥ 400 nm, MeCN solutions, c =
1.3 × 10−5 M) starting from photocyclized solutions (λ = 282 nm).
Figure 10. Photocyclization of 1-o (top), 2-o (middle), and 3-o
(bottom) with UV light at RT (λ = 282 nm, MeCN solutions, c = 1.3
× 10−5 M).
increased compared to the low-temperature structure. The
phenyl substituents are twisted relative to the corresponding
thiophene units by 30.0 and 37.1° in the HS structure.
Fe(II) state confirmed by short Fe−N distances in the range of
1.9715(14)−2.0237(15) Å. Interestingly, the picked crystal
contained exclusively parallel conformer this time. Note that,
which conformer crystallizes out of solution usually containing
both conformers, depends on crystallization conditions. Thus,
different conformers can be crystallized from the same starting
mixed material as was demonstrated for 1-o.23,50 The dihedral
angles between the thiophene rings and phenanthroline
backbone of 57.9 and 67.7° in 3-o are smaller compared with
1-o and 2-o. The phenyl substituents are not coplanar with the
corresponding thiophene fragments but moderately turned by
29.0 and 38.6° in the crystal.
At 273 K the space group is preserved, but all Fe−N bond
distances increased significantly to 2.151(2)−2.229(2) Å
confirming a thermally induced transition to an HS-Fe(II)
state. The dihedral angles between the thiophene units and
phenanthroline backbone at 59.2 and 70.4° are slightly
Magnetic Properties. Variable-temperature magnetic
susceptibility measurements were performed on as-synthesized
powder samples of 1−3 in the temperature range of 2−300 K.
A nearly constant χT product of 3.33 cm3 mol−1 K (χ is molar
magnetic susceptibility; see Supporting Information for
effective magnetic moment) at 280−300 K measured for 2
confirms an HS-Fe(II) state (Figure 4). When slowly cooled (1
K min−1, 2 K intervals), the χT product decreases gradually to
0.17 cm3 mol−1 K at 64 K, which reflects a thermally induced
HS→LS transition. A low-temperature plateau (16−64 K) is
indicative of a nearly complete SCO with a residual HS fraction
of 5%. At temperatures below 16 K, the χT product decreases
further reaching 0.08 cm3 mol−1 K at 2 K, which is likely due to
the zero-field splitting of the residual HS fraction. When slowly
heated from 2 to 300 K, the data coincide with those obtained
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Figure 13. Multiple photoswitching of 1 (top) and 2 (bottom) in
MeCN solutions (c = 1.3 × 10−5 M) at RT. The evolution of the
absorption at 375 nm upon alternating photocyclization with λ = 282
nm (increasing absorption) and photocycloreversion with visible (λ ≥
400 nm, decreasing absorption).
0.25 cm3 mol−1 K measured at 70 K disclosing a thermally
driven SCO (Figure 4). When further cooled, the χT product
remains nearly constant followed by a drop at very low
temperatures due to the zero-field splitting of the residual HS
fraction (7%). No hysteresis was observed upon subsequent
heating. In contrast to 2, complex 3 reveals a clear one-step
SCO transition. This was readily fitted using the van’t Hoff eq 1
to give the enthalpy change ΔH = 15.9(3) kJ mol−1 and
entropy change ΔS = 74(2) J K−1 mol−1 for the LS→HS
conversion (χLT and χHT are low- and high-temperature limits
for molar magnetic susceptibility, respectively).
Figure 12. Multiple photoswitching of 1 (top), 2 (middle), and 3
(bottom) in MeCN solutions (c = 1.3 × 10−5 M) at RT. The evolution
of the absorption at 375 (1, 2) and 404 (3) nm upon alternating
photocyclization with λ = 282 nm (blue) and photocycloreversion
with visible (λ ≥ 400 nm, red). The data were fitted according to the
first-order kinetics.
χ T + χHTT exp(−ΔH + TΔS/RT)
LT
χT =
in the cooling mode, and thus no thermal hysteresis could be
detected.
1 + exp( − ΔH + TΔS/RT)
(1)
Interestingly, 2 shows a two-step SCO transition with
transition temperatures T1/2 of ∼102 and 160 K as estimated
from the first derivative d(χT)/dT (Supporting Information).
Generally, a two-step SCO might be due to the presence of two
nonequivalent complex molecules in the crystal cell or the
formation of HS/LS pairs upon transition due to short-range
interactions.52 A possible disorder or the simultaneous presence
of antiparallel and parallel conformers could be responsible for
the two-step transition as well. Note that the magnetic
measurements made on the powder sample obtained from a
MeOH solution are not directly comparable with the X-ray
structural data from few crystals obtained from a toluene/n-
hexane solution (see Supporting Information for powder XRD
data). Our attempts to crystallize 2 from MeOH were
unfortunately unsuccessful.
Similar thermodynamic parameters were previously determined
for 1 (10.2(4) kJ mol−1 and 76(3) J K−1 mol−1).23 The
transition temperature T1/2 for 3 was calculated at 215.7(5) K.
This is the highest temperature in the series of 1−3. Note that
the influence of electronic factors on T1/2 values is best
examined in solutions, where intermolecular interactions are
negligible (vide infra).
For consistency, we reinvestigated23,24,53 the magnetic
properties of 1 under similar experimental conditions (1 K
min−1, 2 K intervals, Supporting Information). The inspection
of the d(χT)/dT derivative confirms a simple one-step SCO
transition in 1. The obtained thermodynamic parameters: ΔH
= 11.8(3) kJ mol−1, ΔS = 86(2) J K−1, and the corresponding
transition temperature T1/2 = 136.9(3) K, resemble closely the
values obtained previously from the lower-quality data.23
Magnetic properties of newly synthesized complexes 2 and 3
were investigated in solution using the Evans NMR method
Complex 3 reveals a χT product of 3.38 cm3 mol−1 K at 300
K. When slowly cooled, the χT product decreases gradually to
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(Figure 5) and compared with the previously reported data on
1. The χT product of 3.26 cm3 mol−1 K measured for 2 at RT
in toluene solution is very close to the value observed in the
solid state (3.33 cm3 mol−1 K). For 3, the χT product at RT in
toluene is smaller than in the solid sample. Importantly, both 2
and 3 reveal a gradual HS→LS transition upon cooling from
RT to 188 K. The temperature-dependent data were readily
fitted with the van’t Hoff equation, and the thermodynamic
parameters for SCO were obtained (Figure 5). Note that the
calculated transition temperatures of 175.2(6) and 182.5(4) K
for solutions of 2 and 3, respectively, are higher than T1/2 of 1
determined at 164(1) K.23 Thus, the LS state is stabilized in the
series: 3 > 2 > 1.
phenanthroline framework and the thiophene moieties.45 The
spectrum of L3-o is clearly different. The spectrum features two
strong bands at 230 and 269 nm of nearly equal intensity (ε =
3.6 and 3.3 × 104 M−1 cm−1, respectively), with a shoulder at
∼300 nm becoming more pronounced.
In the UV region, the complexes 2-o and 3-o show the
absorption bands similar to the corresponding ligands
(Supporting Information). The spectrum of 2-o reveals two
strong bands at 237 (ε = 7.4 × 104 M−1 cm−1) and 281 nm (ε =
3.8 × 104 M−1 cm−1) with a shoulder, whereas 3-o features an
unresolved structure at wavelengths less than 230 nm and a
strong band at 280 nm (ε = 5.8 × 104 M−1 cm−1) with a
shoulder. Note that the low-energy UV bands of the complexes
are bathochromically shifted compared with those of the
corresponding metal-free ligands due to coordination. The
visible region is dominated by a broad relatively low-intense
metal-to-ligand charge transfer (MLCT) band at ∼550 nm (ε ≈
0.2 × 104 M−1 cm−1) for both complexes.
Photophysical Properties of Ligands. Upon UV
irradiation at λ = 282 nm, a colorless solution of L2-o became
rapidly red. Thereby the prominent absorption bands in UV
region decreased in intensity, and the new bands at 362 (with a
shoulder) and 512 nm appeared (Figure 8). These changes are
characteristic for the photocyclization of diarylethenes,44 and
very similar spectral changes were reported for the photo-
cyclization of L1-o.23 As expected, the photoreaction follows a
simple first-order kinetics (Supporting Information). The
photocyclization of L2-o was accomplished within 60 s. The
photostationary state (PSS) is characterized by the presence of
63% of open-ring and 37% of closed-ring isomers, as
determined by NMR spectroscopy (Supporting Information).
Irradiation of L2-o at λ = 254 nm yielded a very similar PSS
(Supporting Information).
The photocyclization of the colorless solution of L3-o with λ
= 282 nm gave a blue solution. Upon UV irradiation, the parent
absorption bands in UV decreased in intensity, whereas the new
bands at 392 and 584 nm emerged (Figure 8). The new bands
are bathochromically shifted compared to L2. The degree of
open-ring→closed-ring photoconversion was difficult to
determine here, because of a limited thermal stability of the
closed-ring isomer at RT (vide infra). Interestingly, the
photocyclization of L3-o with λ = 254 nm resulted in a slightly
high degree of photoconversion compared to the excitation at λ
= 282 nm, as confirmed by the electronic absorption
spectroscopy (Supporting Information).
Mossbauer Spectroscopy. Zero-field 57Fe Mossbauer
̈
̈
spectra obtained on a powder sample of 2 at different
temperatures confirm a thermally driven SCO (Figure 6).
The RT spectrum reveals a major doublet (85%) with an
isomer shift (δ) of 0.97 mm s−1 and a quadrupole splitting
(|ΔEQ|) of 1.38 mm s−1, which is assigned to a HS-Fe(II) state.
A minor doublet (15%) shows a very similar isomer shift but
much larger quadrupole splitting of 2.58 mm s−1, which is due
to a HS-Fe(II) species. When cooled to T = 105 K, the relative
intensity of the major doublet decreases to 22% with
concomitant appearance of an LS-Fe(II) species (65%)
featuring δ = 0.51 mm s−1 and |ΔEQ| = 0.44 mm s−1. Thus,
the major component in the powder sample of 2 is SCO active.
Interestingly, the relative intensity of the minor HS-Fe(II)
doublet remains nearly constant (13% at 105 K), which
confirms that the minor species is trapped in an HS state at
these temperatures. Finally, at T = 78 K, the thermally induced
SCO is nearly completed, which is confirmed by the presence
of 90% of LS-Fe(II) species, 2% of SCO HS-Fe(II) species, and
8% of a minor HS-Fe(II) species. The minor HS-Fe(II) species
showing a very similar to the major species isomer shift but
differing quadrupole splitting is likely due to slightly differing
environment of 2 in two different lattices, rather than due to
impurities.
Complex 3 shows a single quadrupole doublet with δ = 0.96
mm s−1 and |ΔEQ| = 1.43 mm s−1 at RT (Figure 7). These
parameters resemble closely those of the HS-Fe(II) state of 2
(vide supra). When cooled to 77 K, the HS doublet nearly
disappears, and a new doublet with δ = 0.51 mm s−1 and |ΔEQ|
= 0.46 mm s−1 appears (95%), which is due to the thermal
switching to the LS-Fe(II) state. Interestingly, a small amount
of an HS species (5%) present at 77 K shows distinctly different
Mossbauer parameters (δ = 1.19 mm s−1 and |ΔEQ| = 3.30 mm
For consistency, we reinvestigated the photocyclization of
L1-o under similar conditions. The photoresponse of L1-o to λ
= 282 nm (Supporting Information) is very similar to that of
L2-o and to the previously reported photocyclization with λ =
254 nm UV source.23
The photocyclization of L1−L3 was reversible. Upon
irradiation with visible light at RT, the red (L1, L2) or blue
(L3) solutions containing closed-ring ligands became colorless,
and the parent electronic absorption spectra of the open-ring
ligands were fully restored (Supporting Information). The
photocycloreversion reactions proceed very fast under our
experimental conditions: the reactions were completed within
25 and 90 s for L1/L2 and L3, respectively.
̈
s−1) compared to the HS species at RT. Although the fit
parameters of the HS species at 77 K should be considered with
care due to its very weak signal (Figure 7), we suggest that this
species is a minor impurity rather than a residual HS fraction of
the SCO complex 3. Overall, the variable-temperature
Mossbauer data obtained on 2 and 3 are in very good
̈
agreement with the magnetic measurements on the corre-
sponding solids (vide supra).
Electronic Absorption Spectroscopy. Both novel ligands
L2-o and L3-o form colorless solutions in MeCN, and their
electronic absorption spectra have no features in the visible.
The spectrum of L2-o shows two strong bands in the UV at
238 (ε = 8.7 × 104 M−1 cm−1) and 274 nm (ε = 3.4 × 104 M−1
cm−1) with a shoulder at 330 nm (Supporting Information).
These bands are bathochromically shifted relative to very
similar bands of L1-o (232 and 267 nm, respectively),23 which
were assigned to intraligand π→π* and n→π* transitions of the
An open-ring isomer is usually the ground state of a
diarylethene and thus thermodynamically stable, whereas a
higher-energy closed-ring isomer could be thermally stable for 1
× 105 years or for only several seconds at RT, depending on the
particular type of a diarylethene and its substituents.44 The half-
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life of the parent ligand L1-c was estimated at 137 h in MeCN
solution at RT.23 Here we found that the introduction of
methyl substituents into the phenanthroline backbone (L2)
and phenyl substituents into thiophene moieties (L3) both
decreased the thermal stability of the closed-ring isomers.
Whereas the half-life of L2-c decreased only moderately to
42(1) h, L3-c became much less stable showing the half-life of
only 11.99(3) min in MeCN solution at RT (Supporting
Information).
We examined the unexpected decrease of the thermal
stability of the substituted ligands by density functional theory
(DFT) calculations. The activation barrier for a thermal
cycloreversion reaction and thus the stability of a closed-ring
isomer is expected to be reversely proportional to the energy
difference between open- and closed-ring isomers.54 Thus, we
calculated open-ring/closed-ring energy differences for ligands
L1−L3 (Supporting Information). However, the energy
differences were nearly the same (25.1−26.6 kcal mol−1) for
all ligands. Moreover, L3 showed the smallest energy difference,
and thus L3-c would be expected to be the most stable in the
series, which is not confirmed experimentally.
Lehn et al. suggested that the strength of the photogenerated
C−C bond in a closed-ring isomer could be an alternative
indicator for its thermal stability.55 Indeed, the calculated C−C
bond in L3-c is slightly longer (1.537 Å) than in L1-c and L2-c
(both 1.534 Å), confirming the lower stability of L3-c.
However, this argument should be taken with care, since the
π-systems of the two thiophenes and phenanthroline are all
involved into the cyclization/reversion process. More insight
regarding the influence of substituents on thermal stability
might be obtained from transition states calculations,56 which is
beyond the scope of this work.
Photophysical Properties of Complexes. After the
reversible and multiple photoswitching of the metal-free ligands
L1−L3 at RT was confirmed (vide supra), we investigated the
photoresponse of the corresponding iron(II) complexes 1−3.
Besides reporting the photophysical properties of the newly
synthesized molecular switches 2 and 3, here we reexamined23
some properties of the parent switch 1 using a new photosetup
employed for 2 and 3.
Generally, the photoresponse of 1−3 is very similar to the
corresponding free ligands. However, the photoreactions with
the complexes proceed significantly slower, which points to
decreased quantum yields for the coordinated diarylethenes.
This is likely due to partial deactivation of diarylethene-based
electronic excited states by charge transfer and metal-based
ligand-field states. The UV-induced photocyclization in 1−3 is
accompanied by the decrease of the prominent absorption
bands in UV, and the appearance of the characteristic bands
due to the closed-ring isomers (Figure 10). The new bands of
the closed-ring 2-c (3-c) are centered at 374 and 537 nm (402
and 586 nm). These are bathochromically shifted relative to
those of the closed-ring ligands, which indicates that the
diarylethene-based ligands remain coordinated to the metal
ions.
The photocyclization of 2-o proceeds very similar to that of
1-o and follows the first-order kinetics (Supporting Informa-
tion). Although the photocyclization of 3-o could also be fitted
to the first-order kinetics, upon prolonged UV irradiation the
photocyclization rate becomes virtually constant (Supporting
Information). A similar effect was observed for the photo-
cyclization of the metal-free L3-o (vide supra). We ascribe this
to the UV-enhanced interconversion between the parallel and
antiparallel conformers of 3-o in solution.
In our previous works, we showed that the reversible
photocyclization of 1 triggers the reversible HS-to-LS transition
at the iron(II) center. This unique SCO was accomplished in
solution and in the solids state at RT, and the switching
proceeds at the molecular level.23,24 Note that the π-systems of
the phenanthroline and the two twisted thiophenes are
essentially electronically uncoupled in the open-ring L1-o. In
contrast, the closed-ring L1-c becomes nearly planar that leads
to the formation of a common π-system. Thus, we suggested
that L1-o is a weaker π-acceptor, which produces a weaker
ligand field thus stabilizing an HS state in 1-o, whereas L2-c is a
stronger π-acceptor, which produces a stronger ligand field thus
stabilizing an LS state in 1-c.23
Since the magnetic and photophysical properties of 2 are
very similar to those of 1, we expected that the photocyclization
of 2 triggers an HS-to-LS transition as well. Indeed upon bulk
photolysis of a concentrated solution of 2 in a MeCN/MeCN-
d3 mixture with UV light, the χT product decreased gradually
from 2.86 to 2.33 cm3 mol−1 K, as determined by the Evans
NMR method. Thus, ∼19% of HS species were converted into
LS species upon UV irradiation at RT. Prolonged irradiation
with UV resulted in the photodegradation of the sample, as
confirmed by electronic absorption spectroscopy. Similar
photodegradation issues in longtime bulk photolysis experi-
ments were noticed for 1, for which the photoconversion of
40% was reported.23
Further, we examined multiple photoswitching of L1−L3
ligands. The ligands were photocyclized with UV light until the
PSS was nearly reached, followed by the photocycloreversion
with visible light until another PSS was attained. Afterward the
cycle was repeated two more times, while the irradiation times
were kept constant. All three ligands could be reversibly
switched in MeCN solution at RT (Figure 9 and Supporting
Information). Interestingly, whereas L1 and L2 showed a small
degree of photodegradation, the photoswitchability of L3
improved with every cycle. Thus, the photoconversion
increased in every subsequent photocyclization step for L3.
We ascribe this effect to the conversion of the photoinactive
parallel conformer to the photoactive antiparallel conformer
under UV irradiation. This is supported by the observation that,
upon prolonged UV excitation, the rate of photocyclization
becomes nearly constant (Supporting Information). As a
consequence, the PSS for the photocyclization of L3 is difficult
to reach. Notably, the interconversion between the two
conformers in solution is very slow without UV irradiation, as
was determined by NMR spectroscopy. Therefore, we suggest
that the irradiation at λ = 282 nm could induce π→π*
electronic transitions within phenyl substituents of L3-o, thus
providing additional energy or new pathways that ultimately
facilitate the interconversion between the conformers. Usually
the conformers are tending to ∼1:1 ratio in solution, while only
the antiparallel conformer is photoactive. The disturbed
equilibrium between the conformers in the irradiated solution
and the suggested UV-enhanced interconversion between the
conformers should lead to the gradual conversion of parallel
into antiparallel conformers.
The photocyclization of 1−3 is reversible. The photo-
cycloreversion can be induced by selective irradiation into the
emerging bands of the closed-ring isomers, or by unselective
irradiation with visible light (λ ≥ 400 nm) at RT. Thus, upon
irradiation, the bands of the closed-ring complexes decrease,
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and the bands of the open-ring complexes increase in intensity
(Figure 11). Note that the spectra of the pristine open-ring 1-o
and 2-o are not fully restored, and thus the reverse reactions are
not fully complete, which is due to nonzero absorption of the
open-ring complexes in the visible.23 In contrast, the photo-
cycloreversion of 3 seems to be fully completed within 135 s,
which is very likely due to very short lifetime of 3-c (vide infra).
The thermal stability of SCO photoswitches 1−3 at RT,
which is important for potential applications, was investigated
in solutions. Similar to the free ligands, the open-ring
complexes are thermodynamically stable, whereas the UV-
generated closed-ring complexes show considerably different
stability. The half-life of 1-c was previously determined at 421 h
(17.5 d) in solution at RT.23 The thermal stability of 2-c is only
moderately lower; the respective half-life is 199(12) h (8.3(5)
d, Supporting Information). However, 3-c seems to be
considerably less stable showing the half-life of only 3.3(2)
min at RT. Thus, the stability of the closed-ring complexes, 1-c
> 2-c ≫ 3-c, follows the trend observed for the ligands: L1-c >
L2-c ≫ L3-c. Consequently, the thermal stability of closed-ring
ligands can be used as an indicator for the stability of
prospective SCO complexes.
To test the resistance of SCO photoswitches 1−3 to fatigue,
we performed three consecutive photocycles by irradiating the
complexes with alternating UV and visible light in solution at
RT. Importantly, the complexes were irradiated until the PSSs
are reached during the first photocycle. Afterward, the times for
UV and visible light irradiations were kept constant. Since the
PSS at λ = 282 nm for 3 is difficult to reach (vide supra), this
complex was photocyclized for 32 min in the first cycle, and this
time was kept constant.
The photoswitches 1 and 2 show very similar properties.
Upon every cycle, the photoresponse of these species
diminishes pointing to slow photodegradation. This can be
best followed by monitoring the prominent absorptions at 375
nm (Figure 12). For instance, the maximum of the extinction
coefficient at 375 nm, ε375, drops slightly from 2.07 × 104 M−1
cm−1 in the first cycle to 1.95 × 104 M−1 cm−1 in the third cycle
for the photocyclized solution of 2. Concomitantly, the
minimum of ε375 achieved upon photocycloreversion increases
significantly from 0.48 × 104 M−1 cm−1 after the first cycle to
0.86 × 104 M−1 cm−1 after the third cycle.
Complex 3 behaves differently. Similarly to the metal-free L3,
the photoresponse of 3 seems to increase with every cycle
(Figure 12). Although the origin of this enhancement is not
fully understood, it is likely similar to the free ligand (vide
supra). Thus, we assume that the relatively fast generation of
additional amount of antiparallel conformers in UV-irradiated
solution improves the response to UV, while the fast L3-c→L3-
o thermal relaxation produces an apparent effect that the
complex converts fully to the initial state upon irradiation with
visible. Because of these effects, the data for 3 could only be
roughly fitted according to the first-order kinetics.
CONCLUSIONS
■
The unique iron(II) spin-crossover molecular switch 1
featuring a photochromic diarylethene-based ligand has been
successfully modified. Methyl groups have been introduced into
the phenanthroline-backbone of the photochrome (2), and
phenyl groups have been introduced into the photoactive
thiophenes (3). The photoswitchability of 2 and 3 has been
investigated in solution at room temperature in detail and
compared with that of 1 redone at similar conditions. While the
modification of the phenanthroline unit had little influence on
the photophysical properties of the molecular switch, the
modification at the thiophenes decreased the thermal stability
of the closed-ring isomer significantly. Multiple photoswitching
has been readily achieved with 1 and 2 when the photosta-
tionary states were avoided. Further modifications of the
phenanthroline backbone seem to be attractive for introducing
anchoring groups thus targeting chemisorbed self-assembled
monolayers.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01952.
■
S
The details of computational, crystallographic, spectro-
scopic, and photophysical studies (PDF)
Accession Codes
CCDC 1560708−1560712 contain the supplementary crystal-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: marat.khusniyarov@fau.de.
ORCID
Frank W. Heinemann: 0000-0002-9007-8404
Julien Bachmann: 0000-0001-6480-6212
Marat M. Khusniyarov: 0000-0002-2034-421X
■
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Fonds der
Chemischen Industrie (Liebig Fellowship for M.M.K.),
Deutsche Forschungsgemeinschaft (DFG Research Grant No.
KH 279/3), and Emerging Talents Initiative (ETI) program of
During our studies, we noticed that the photodegradation of
1 and 2 increases in longtime photoexperiments and when the
solutions are irradiated longer than required to reach the PSS
(over-irradiated solutions). Thus, we repeated the experiment
on multiple photoswitching by avoiding the irradiation until the
PSSs. Remarkably, 15 photocycles were readily accomplished
with 1 and 2 (Figure 13). Although small degree of
photodegradation could be noticed in these experiments as
well, the resistance to fatigue was significantly improved by
irradiating under non-PSS conditions.
FAU Erlangen-Nurnberg. Prof. K. Meyer (FAU Erlangen-
̈
Nurnberg) is acknowledged for providing access to spectro-
̈
scopic facilities and his general support. We thank Dr. J. Sutter
for measuring Mossbauer spectra and Dr. A. Scheurer for the
̈
assistance with variable-temperature NMR spectroscopy.
REFERENCES
■
(1) Halcrow, M. A. Spin-Crossover Materials, Properties and
Applications; John Wiley & Sons, Ltd., 2013.
K
DOI: 10.1021/acs.inorgchem.7b01952
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(2) Gutlich, P.; Goodwin, H. A. Spin Crossover in Transition Metal
(20) Khusniyarov, M. M. How to Switch Spin-Crossover Metal
Complexes at Constant Room Temperature. Chem. - Eur. J. 2016, 22
(43), 15178−15191.
(21) Witt, A.; Heinemann, F. W.; Sproules, S.; Khusniyarov, M. M.
Modulation of Magnetic Properties at Room Temperature: Coordi-
nation-Induced Valence Tautomerism in a Cobalt Dioxolene Complex.
Chem. - Eur. J. 2014, 20 (35), 11149−11162.
(22) Witt, A.; Heinemann, F. W.; Khusniyarov, M. M. Bidirectional
photoswitching of magnetic properties at room temperature: ligand-
driven light-induced valence tautomerism. Chem. Sci. 2015, 6 (8),
4599−4609.
̈
Compounds I-III, Topics in Current Chemistry; Springer-Verlag: Berlin,
Germany, 2004; Vol. 233−235.
(3) Jureschi, C.-M.; Linares, J.; Boulmaali, A.; Dahoo, P.; Rotaru, A.;
Garcia, Y. Pressure and Temperature Sensors Using Two Spin
Crossover Materials. Sensors 2016, 16 (2), 187.
(4) Quintero, C. M.; Fel
́
ix, G.; Suleimanov, I.; Sanchez Costa, J.;
Molnar, G.; Salmon, L.; Nicolazzi, W.; Bousseksou, A. Hybrid spin-
́
crossover nanostructures. Beilstein J. Nanotechnol. 2014, 5, 2230−2239.
(5) Kahn, O.; Martinez, C. J. Spin-transition polymers: From
molecular materials toward memory devices. Science 1998, 279 (5347),
44−48.
(23) Milek, M.; Heinemann, F. W.; Khusniyarov, M. M. Spin
Crossover Meets Diarylethenes: Efficient Photoswitching of Magnetic
Properties in Solution at Room Temperature. Inorg. Chem. 2013, 52
(19), 11585−11592.
́
(6) Letard, J.-F.; Guionneau, P.; Goux-Capes, L. Towards spin
crossover applications. Top. Curr. Chem. 2004, 235, 221−249.
(7) Sanvito, S. Molecular spintronics. Chem. Soc. Rev. 2011, 40 (6),
3336−3355.
(24) Rosner, B.; Milek, M.; Witt, A.; Gobaut, B.; Torelli, P.; Fink, R.
̈
H.; Khusniyarov, M. M. Reversible Photoswitching of a Spin-
Crossover Molecular Complex in the Solid State at Room Temper-
ature. Angew. Chem., Int. Ed. 2015, 54 (44), 12976−12980.
(25) Boillot, M.-L.; Zarembowitch, J.; Sour, A. Ligand-Driven Light-
Induced Spin Change (LD-LISC): A Promising Photomagnetic Effect.
Top. Curr. Chem. 2004, 234, 261−276.
(8) Ruiz, E. Charge transport properties of spin crossover systems.
Phys. Chem. Chem. Phys. 2014, 16 (1), 14−22.
(9) Bertoni, R.; Lorenc, M.; Cailleau, H.; Tissot, A.; Laisney, J.;
Boillot, M.-L.; Stoleriu, L.; Stancu, A.; Enachescu, C.; Collet, E.
Elastically driven cooperative response of a molecular material
impacted by a laser pulse. Nat. Mater. 2016, 15 (6), 606−610.
(10) Bairagi, K.; Iasco, O.; Bellec, A.; Kartsev, A.; Li, D.; Lagoute, J.;
Chacon, C.; Girard, Y.; Rousset, S.; Miserque, F.; Dappe, Y. J.;
Smogunov, A.; Barreteau, C.; Boillot, M.-L.; Mallah, T.; Repain, V.
Molecular-scale dynamics of light-induced spin cross-over in a two-
dimensional layer. Nat. Commun. 2016, 7, 12212.
(26) Khusniyarov, M. M. Light-Induced Spin-Crossover. In Reference
Module in Chemistry, Molecular Sciences and Chemical Engineering;
Reedijk, J., Ed.; Elsevier, 2015.
(27) Roux, C.; Zarembowitch, J.; Gallois, B.; Granier, T.; Claude, R.
Toward Ligand-Driven Light-Induced Spin Changing. Influence of the
Configuration of 4-Styrylpyridine (stpy) on the Magnetic Properties of
FeII(stpy)4(NCS)2 Complexes. Crystal Structures of the Spin-Cross-
over Species Fe(trans-stpy)4(NCS)2 and of the High-Spin Species
Fe(cis-stpy)4(NCS)2. Inorg. Chem. 1994, 33 (10), 2273−2279.
(28) Boillot, M.-L.; Chantraine, S.; Zarembowitch, J.; Lallemand, J.
Y.; Prunet, J. First ligand-driven light-induced spin change at room
temperature in a transition-metal molecular compound. New J. Chem.
1999, 23 (2), 179−183.
(29) Hasegawa, Y.; Kume, S.; Nishihara, H. Reversible light-induced
magnetization change in an azobenzene-attached pyridylbenzimidazole
complex of iron(II) at room temperature. Dalton Trans. 2009, 2, 280−
284.
(30) Hasegawa, Y.; Takahashi, K.; Kume, S.; Nishihara, H. Complete
solid state photoisomerization of bis(dipyrazolylstyrylpyridine)iron(II)
to change magnetic properties. Chem. Commun. 2011, 47 (24), 6846−
6848.
(11) Lefter, C.; Rat, S.; Costa, J. S.; Manrique-Juar
Quintero, C. M.; Salmon, L.; Seguy, I.; Leichle, T.; Nicu, L.; Demont,
P.; Rotaru, A.; Molnar, G.; Bousseksou, A. Current Switching Coupled
́
ez, M. D.;
́
́
to Molecular Spin-States in Large-Area Junctions. Adv. Mater. 2016, 28
(34), 7508−7514.
(12) Bernien, M.; Naggert, H.; Arruda, L. M.; Kipgen, L.; Nickel, F.;
Miguel, J.; Hermanns, C. F.; Kruger, A.; Kruger, D.; Schierle, E.;
̈
̈
Weschke, E.; Tuczek, F.; Kuch, W. Highly Efficient Thermal and
Light-Induced Spin-State Switching of an Fe(II) Complex in Direct
Contact with a Solid Surface. ACS Nano 2015, 9 (9), 8960−8966.
(13) Naggert, H.; Rudnik, J.; Kipgen, L.; Bernien, M.; Nickel, F.;
Arruda, L. M.; Kuch, W.; Nather, C.; Tuczek, F. Vacuum-evaporable
̈
spin-crossover complexes: physicochemical properties in the crystalline
bulk and in thin films deposited from the gas phase. J. Mater. Chem. C
2015, 3 (30), 7870−7877.
(14) Warner, B.; Oberg, J. C.; Gill, T. G.; El Hallak, F.; Hirjibehedin,
C. F.; Serri, M.; Heutz, S.; Arrio, M.-A.; Sainctavit, P.; Mannini, M.;
Poneti, G.; Sessoli, R.; Rosa, P. Temperature- and Light-Induced Spin
Crossover Observed by X-ray Spectroscopy on Isolated Fe(II)
Complexes on Gold. J. Phys. Chem. Lett. 2013, 4 (9), 1546−1552.
(15) Naggert, H.; Bannwarth, A.; Chemnitz, S.; von Hofe, T.;
Quandt, E.; Tuczek, F. First observation of light-induced spin change
in vacuum deposited thin films of iron spin crossover complexes.
Dalton Trans. 2011, 40 (24), 6364−6366.
(31) Takahashi, K.; Hasegawa, Y.; Sakamoto, R.; Nishikawa, M.;
Kume, S.; Nishibori, E.; Nishihara, H. Solid-State Ligand-Driven Light-
Induced Spin Change at Ambient Temperatures in Bis-
(dipyrazolylstyrylpyridine)iron(II) Complexes. Inorg. Chem. 2012, 51
(9), 5188−5198.
(32) Venkataramani, S.; Jana, U.; Dommaschk, M.; Sonnichsen, F.
̈
D.; Tuczek, F.; Herges, R. Magnetic Bistability of Molecules in
Homogeneous Solution at Room Temperature. Science 2011, 331
(6016), 445−448.
(33) Feringa, B. L.; Browne, W. R. Molecular Switches; Wiley-VCH
Verlag: Weinheim, Germany, 2011; Vol. 1−2.
(34) Thiele, S.; Malmgaard-Clausen, M.; Engel-Andreasen, J.; Steen,
A.; Rummel, P. C.; Nielsen, M. C.; Gloriam, D. E.; Frimurer, T. M.;
Ulven, T.; Rosenkilde, M. M. Modulation in Selectivity and Allosteric
Properties of Small-Molecule Ligands for CC-Chemokine Receptors. J.
Med. Chem. 2012, 55 (18), 8164−8177.
(16) Gutlich, P.; Hauser, A.; Spiering, H. Thermal and Optical
̈
Switching of Iron(II) Complexes. Angew. Chem., Int. Ed. Engl. 1994, 33
(20), 2024−2054.
(17) Letard, J.-F.; Guionneau, P.; Nguyen, O.; Costa, J. S.; Marcen,
́ ́
S.; Chastanet, G.; Marchivie, M.; Goux-Capes, L. A Guideline to the
Design of Molecular-Based Materials with Long-Lived Photomagnetic
Lifetimes. Chem. - Eur. J. 2005, 11 (16), 4582−4589.
(18) McCusker, J. K.; Rheingold, A. L.; Hendrickson, D. N. Variable-
Temperature Studies of Laser-Initiated 5T2 –> 1A1 Intersystem
Crossing in Spin-Crossover Complexes: Empirical Correlations
between Activation Parameters and Ligand Structure in a Series of
Polypyridyl Ferrous Complexes. Inorg. Chem. 1996, 35 (7), 2100−
2112.
(35) Ko, C.-C.; Kwok, W.-M.; Yam, V. W.-W.; Phillips, D. L. Triplet
MLCT photosensitization of the ring-closing reaction of diarylethenes
by design and synthesis of a photochromic rhenium(I) complex of a
diarylethene-containing 1,10-phenanthroline ligand. Chem. - Eur. J.
2006, 12 (22), 5840−5848.
(36) Lim, S.-J.; An, B.-K.; Park, S. Y. Bistable Photoswitching in the
Film of Fluorescent Photochromic Polymer: Enhanced Fluorescence
Emission and Its High Contrast Switching. Macromolecules 2005, 38
(15), 6236−6239.
(19) Stock, P.; Pędzinski, T.; Spintig, N.; Grohmann, A.; Horner, G.
́
̈
High Intrinsic Barriers against Spin-State Relaxation in Iron(II)-
Complex Solutions. Chem. - Eur. J. 2013, 19 (3), 839−842.
L
DOI: 10.1021/acs.inorgchem.7b01952
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(37) Hill, A. F.; Malget, J. M.; White, A. J. P.; Williams, D. J.
Dihydrobis(pyrazolyl)borate Alkylidyne Complexes of Tungsten. Eur.
J. Inorg. Chem. 2004, 2004, 818−828.
(38) Kuhni, J.; Adamo, V.; Belser, P. Synthesis of an unsymmetrically
̈
substituted, dithienylethene-containing 1,10-phenanthroline ligand and
its ruthenium(II) complex. Synthesis 2006, 2006, 1946−1948.
(39) Walton, I. M.; Cox, J. M.; Benson, C. A.; Patel, D. G.; Chen, Y.-
S.; Benedict, J. B. The role of atropisomers on the photo-reactivity and
fatigue of diarylethene-based metal-organic frameworks. New J. Chem.
2016, 40 (1), 101−106.
(40) Chesneau, B.; Passelande, A.; Hudhomme, P. Efficient Access to
a Versatile 5,6-Dithio-1,10-phenanthroline Building Block and
Corresponding Organometallic Complexes. Org. Lett. 2009, 11 (3),
649−652.
(41) Bill, E. MFIT, version 1.1; MPI for Bioinorganic Chemistry:
Mulheim/Ruhr, Germany, 2008.
̈
(42) SADABS, 2008/1; Bruker AXS, Inc.: Madison, WI, 2009.
(43) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr.,
Sect. A: Found. Crystallogr. 2008, 64, 112−122.
(44) Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Photo-
chromism of Diarylethene Molecules and Crystals: Memories,
Switches, and Actuators. Chem. Rev. 2014, 114 (24), 12174−12277.
(45) Yam, V. W.-W.; Ko, C.-C.; Zhu, N. Photochromic and
Luminescence Switching Properties of a Versatile Diarylethene-
Containing 1,10-Phenanthroline Ligand and Its Rhenium(I) Complex.
J. Am. Chem. Soc. 2004, 126 (40), 12734−12735.
(46) Fukumoto, S.; Nakashima, T.; Kawai, T. Photon-Quantitative
Reaction of a Dithiazolylarylene in Solution. Angew. Chem., Int. Ed.
2011, 50 (7), 1565−1568.
(47) Li, W.; Jiao, C.; Li, X.; Xie, Y.; Nakatani, K.; Tian, H.; Zhu, W.
Separation of Photoactive Conformers Based on Hindered Diary-
lethenes: Efficient Modulation in Photocyclization Quantum Yields.
Angew. Chem., Int. Ed. 2014, 53 (18), 4603−4607.
(48) Thompson, A. L.; Goeta, A. E.; Real, J. A.; Galet, A.; Carmen
Munoz, M. Thermal and light induced polymorphism in iron(II) spin
̃
crossover compounds. Chem. Commun. 2004, 12, 1390−1391.
(49) Kulmaczewski, R.; Shepherd, H. J.; Cespedes, O.; Halcrow, M.
A. A Homologous Series of [Fe(H2Bpz2)2(L)] Spin-Crossover
Complexes with Annelated Bipyridyl Co-Ligands. Inorg. Chem. 2014,
53 (18), 9809−9817.
(50) Nihei, M.; Suzuki, Y.; Kimura, N.; Kera, Y.; Oshio, H.
Bidirectional Photomagnetic Conversions in a Spin-Crossover
Complex with a Diarylethene Moiety. Chem. - Eur. J. 2013, 19 (22),
6946−6949.
(51) Real, J. A.; Munoz, M. C.; Faus, J.; Solans, X. Spin Crossover in
̃
Novel Dihydrobis(1-pyrazolyl)borate [H2B(pz)2]-Containing Iron(II)
Complexes. Synthesis, X-ray Structure, and Magnetic Properties of
[FeL{H2B(pz)2}2] (L = 1,10-Phenanthroline and 2,2′-Bipyridine).
Inorg. Chem. 1997, 36 (14), 3008−3013.
(52) Gutlich, P.; Goodwin, H. A. Spin Crossover - An Overall
̈
Perspective. Top. Curr. Chem. 2004, 233, 1−47.
(53) Note that magnetic measurements on 1 were always performed
on powder samples obtained from MeOH solutions. This was not
explicitly mentioned in our previous works.
(54) Irie, M. Diarylethenes for Memories and Switches. Chem. Rev.
2000, 100 (5), 1685−1716.
(55) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Light-Triggered Molecular
Devices: Photochemical Switching Of optical and Electrochemical
Properties in Molecular Wire Type Diarylethene Species. Chem. - Eur.
J. 1995, 1 (5), 275−284.
(56) Patel, P. D.; Masunov, A. E. Theoretical Study of Photochromic
Compounds: Part 3. Prediction of Thermal Stability. J. Phys. Chem. C
2011, 115 (20), 10292−10297.
M
DOI: 10.1021/acs.inorgchem.7b01952
Inorg. Chem. XXXX, XXX, XXX−XXX