Investigating the Photochemistry of Spiropyran Metal Complexes with Online LED-NMR

Article abstract of DOI:10.1021/acs.inorgchem.9b02547

As one of the best studied photoswitches, spiropyrans (SPs) have attracted significant interest in the scientific community. Among the many stimuli to alter the isomerization into the merocyanine (MC) isomer, such as temperature, pH, solvent polarity, redox potential, or mechanical force, the ability of the MC form to act as a ligand site for metal complexation has recently raised new attention. We herein synthesize hitherto undescribed coordination compounds of 8-methoxy-1′,3′,3′-trimethyl-6-nitrospiro-[chromene-2,2′-indoline] with s-block ([Ca(MC)₄](ClO₄)₂), d-block ([Zn(MC)₂(MeCN)₂](ClO₄)₂, [Ni(MC)₂(MeCN)₂](ClO₄)₂), as well as f-block ([La(NO₃)₃(MC)₂]) metals. All complexes are structurally described by X-ray crystallography and systematically investigated in solution via Job's method of continuous variations (Job plots), as well as online NMR spectroscopic kinetic experiments with in situ irradiation of the analyte solution inside the NMR spectrometer (LED-NMR). We can unambiguously identify the photoresponsive nature of the complexes in solution, which is a crucial step toward the application of these promising molecules in material science.

Full text of DOI:10.1021/acs.inorgchem.9b02547

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Investigating the Photochemistry of Spiropyran Metal Complexes
with Online LED-NMR
Thomas J. Feuerstein,† Rouven Mu
,⊥
ller,‡,⊥ Christopher Barner-Kowollik,*,‡,∥
̈
,
†
and Peter W. Roesky*
†
Institute of Inorganic Chemistry (AOC), Karlsruhe Institute of Technology (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany
Macromolecular Architectures, Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of
‡
Technology (KIT), Engesserstrasse 18, 76128 Karlsruhe, Germany
∥
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street,
Brisbane, Queensland 4000, Australia
*
S Supporting Information
ABSTRACT: As one of the best studied photoswitches,
spiropyrans (SPs) have attracted significant interest in the
scientific community. Among the many stimuli to alter the
isomerization into the merocyanine (MC) isomer, such as
temperature, pH, solvent polarity, redox potential, or mechan-
ical force, the ability of the MC form to act as a ligand site for
metal complexation has recently raised new attention. We
herein synthesize hitherto undescribed coordination com-
pounds of 8-methoxy-1′,3′,3′-trimethyl-6-nitrospiro-[chro-
mene-2,2′-indoline] with s-block ([Ca(MC) ](ClO ) ), d-
4
4 2
block ([Zn(MC) (MeCN) ](ClO ) , [Ni(MC) (MeCN) ]-
2
2
4
2
2
2
(
ClO ) ), as well as f-block ([La(NO ) (MC) ]) metals. All
4 2 3 3 2
complexes are structurally described by X-ray crystallography
and systematically investigated in solution via Job’s method of continuous variations (Job plots), as well as online NMR
spectroscopic kinetic experiments with in situ irradiation of the analyte solution inside the NMR spectrometer (LED-NMR). We
can unambiguously identify the photoresponsive nature of the complexes in solution, which is a crucial step toward the
application of these promising molecules in material science.
INTRODUCTION
The presence of the negatively charged oxygen atom in the
MC structure results in the ability to form coordination
compounds with metal ions, which was first described by
■
Spiropyrans (SPs) are a prominent example of so-called
1
−3
photochromic molecules.
the ring-closed SP structure ring-opens into the merocyanine
MC) structure, with an accompanied change in the structural
Under the influence of UV light,
16
17
Philips and Taylor. Importantly, the formed SP−metal
18−20
complexes are also photoresponsive,
which opens many
(
application possibilities for stimuli-responsive smart materials.
In the absence of additional ligation sites except the phenolate
oxygen atom, the complex binding constants are rather weak
and electronic properties of the molecule. Being a T-type
photoswitch, the SP ⇌ MC isomerization is reversible and can
be achieved either by thermal relaxation or irradiation with
visible light. Compared to other photochromic molecules, such
and light induced ring-closure results in the expulsion of the
3,21
metal ion.
However, the stability of the complex can be
4
5
6
7
as diarylethenes, fulgides, azobenzenes, flavylium stysems,
significantly increased by using a second ligand besides MC for
complexation. Different stoichiometries depending on the
metal (M) and the SP substitution have been described, most
or others, the SP molecules are unique, as the two isomers have
vastly different properties. The MC form features a remarkable
8
−10
dipole moment compared to the SP isomer
and occupies
of them belonging either to an MC/M ratio of 2/1 or
3
22−29
more volume. In addition, the SP isomer is almost transparent
in the visible light region, whereas the MC isomer features a
broad absorption band due to the π-conjugated and almost
1/1.
Irradiation of the MC−M complexes with light was
postulated to result in the expulsion of the metal ion and
reformation of the SP structure, as well as a cis ⇌ trans
28
3
planar arrangement of the two molecule halves. Therefore, SP
molecules have been intensively studied and used for various
isomerization of the central MC double bond, while
29
maintaining the coordination toward the metal center.
11
12
applications, such as bioimaging, adaptive lenses, memory
1
3
14
15
storage, nonlinear optics, or subdiffraction lithography to
Received: August 23, 2019
name just a few.
©
XXXX American Chemical Society
A
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Despite the attractive properties of these coordination
compounds, such as applications as metal sensors, switchable
Scheme 2. Synthesis of Spiropyran Metal Complexes
[Zn(MC) (MeCN) ](ClO ) (L−Zn),
2
2
4 2
3
0
28
catalysts, prodrug delivery systems, or recyclable metal
scavengers, detailed investigations of the molecular mecha-
nisms of the MC−M complexes under irradiation are scarce.
Most literature reports made use of UV−vis spectroscopy
coupled with in situ irradiation with LED light sources to assess
the kinetics of the photoresponsive processes of the
[Ni(MC) (MeCN) ](ClO ) (L−Ni), [Ca(MC) ](ClO )
2
2
4 2
4
4 2
(L−Ca), and [La(NO ) (MC) ] (L−La)
3
3
2
2
8,29,31−34
complexes.
However, an inherent challenge of such
UV−vis studies is the missing molecular information, e.g., the
discrimination between a cis ⇌ trans isomerization in the
ligand sphere and a photodissociation of the complex itself.
Herein, we report four hitherto undescribed MC−M
complexes with metal ions from different areas of the periodic
table to characterize the distinct complex stoichiometries/
geometries, i.e., s-block (Ca), d-block (Zn, Ni), as well as rare-
earth (La) metals. A detailed crystallographic description is
followed by the photochromic behavior of the complexes in
solution. We employ not only well-established methods, such
35−37
form of L. Thereupon, the solutions darkened within 30 min to
yield dark red solutions, indicating the slow, spontaneous
formation of the open form, which is directly stabilized by the
coordination onto the metal centers. After complex formation,
the compounds were crystallized either by slow evaporation of
the solvent (L−Ni) or vapor diffusion of diethyl ether into the
acetonitrile solutions (L−Zn, L−Ca, and L−La).
as Job’s method of continuous variations,
to assess the
complex stoichiometries in solution, but also online NMR
spectroscopic kinetic experiments with in situ irradiation of the
analyte solution inside the NMR spectrometer, making use of
the recently developed LED-NMR method by Gschwind and
3
8
co-workers. We can identify and follow the precise processes
upon irradiation in real time.
X-ray Studies. Single crystals of compound [Zn(MC) -
2
3
9,40
(
MeCN) ](ClO ) (L−Zn) were obtained in the form of red
RESULTS AND DISCUSSION
2
4 2
■
needles by slow diffusion of diethyl ether in the CH CN
3
In all the described metal complexes featuring SPs in their MC
form as ligand sites, an additional binding site is necessary to
form chelating ligands. Due to the chelate effect, the metal
complex is sufficiently stabilized, which prevents a spontaneous
thermal MC to SP ring-closure and therefore a breakup of the
metal complex. Specifically, a methoxy group in ortho-position
to the oxygen atom bound to the spiro-carbon atom serves as a
readily available ligand site and was chosen for the synthesis of
mother liquor. The complex crystallizes in the triclinic space
group P1 with one molecule L−Zn and one and a half
molecules of diethyl ether in the asymmetric unit.
The coordination sphere of the Zn atom in the solid state
structure of L−Zn (refer to Figure 1) is best described as
distorted octahedral, due to the calculated distortion
42
parameters with the program Shape 2.1 (refer to Table S1).
Two molecules of L are coordinating toward the metal
center in the TTC-merocyanine form (TTC = trans−trans−
8
-methoxy-1′,3′,3′-trimethyl-6-nitrospiro-[chromene-2,2′-in-
doline] (L) according to Raston and co-workers (refer to
Scheme 1).
4
1
Scheme 1. Equilibrium between Ring-Closed SP L and the
Ring-Opened MC Form Serving as a Chelate Ligand for
Metal Complexation
Briefly, L was obtained by refluxing an equimolar solution of
,3,3-trimethyl-2-methyleneindoline and 3-methoxy-5-nitro-
1
salicylaldehyde in ethanol. Concentration of the resulting
solution and cooling affords L as green crystals in 83% yield. A
related SP structure was also successfully employed by
Giordani and co-workers for the formation of SP−metal
complexes.
Figure 1. Molecular structure of compound L−Zn in the solid state.
Hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths [Å] and angles [deg]: Zn1−O1 1.946(2),
Zn1−O2 2.265(2), Zn1−O3 1.940(2), Zn1−O4 2.346(3), Zn1−N1
28
2
.136(4), Zn1−N2 2.082(4); O1−Zn1−O3 152.71(11), O2−Zn1−
Metal Complex Synthesis. Metal complexation was
performed by addition of an acetonitrile solution of the
respective metal salt to a solution of L in acetonitrile (refer to
N2 164.21(11), O4−Zn1−N1 162.90(12), O1−Zn1−O2 75.30(9),
O2−Zn1−O3 90.09(10), O3−Zn1−N2 103.54(12), O1−Zn1−N2
9
4.99(12), O1−Zn1−N1 110.69(13), N1−Zn1−O3 90.27(13), O3−
3
9
Scheme 2). In all cases an immediate color change of the
dark blue solution of L to bright red was observed, indicating
the prompt coordination of the entirety of the merocyanine
Zn1−O4 74.12(9), O1−Zn1−O4 86.33(10), O2−Zn1−O4
102.00(10), O2−Zn1−N1 84.64(12), N1−Zn1−N2 87.32(14),
O4−Zn1−N2 89.56(11).
B
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1
cis), while the coordination sphere is completed by two
of diethyl ether in the CH CN mother liquor. The complex
3
molecules of acetonitrile.
crystallizes in the monoclinic space group P2/c with one L−
Ca, one acetonitrile, and one diethyl ether molecule in the
asymmetric unit. In contrast to the nickel and zinc complexes,
L−Ca features a spiropyran to metal stoichiometry of 4/1 and
the central calcium ion shows an 8-fold coordination (refer to
Figure 3). The coordination sphere is best described as
The Zn−O bond lengths toward the phenolate oxygen
atoms, (Zn1−O1 1.946(2) Å and Zn1−O3 1.940(2) Å) are
significantly shorter than the ones toward the methoxy oxygen
atoms (Zn1−O2 2.265(2) Å and Zn1−O4 2.346(3) Å). The
O−Zn−O bite angles of the chelating merocyanine ligands
(
O1−Zn1−O2 75.30(9)°, O3−Zn1−O4 74.12(9)°) are
notably smaller than the remaining O−Zn−O angles. The
Zn−N bond lengths are in the typical range for Zn
coordinating acetonitrile,
4
3
and the values (Zn1−N1
2
.136(4) Å and Zn1−N2 2.082(4) Å) lie in between the
Zn−O bond lengths.
Single crystals of compound [Ni(MC) (MeCN) ](ClO )
2
2
4 2
(
L−Ni) were obtained in the form of red plates after slow
removal of the solvent under reduced pressure. The complex
crystallizes in the monoclinic space group P2 /c with half a
1
molecule L−Ni and one molecule of noncoordinating
acetonitrile in the asymmetric unit. The Ni atom represents
the crystallographic inversion center. In contrast to L−Zn, the
merocyanine oxygen atoms are arranged in a square planar
coordination mode in L−Ni (refer to Figure 2). The overall
Figure 3. Molecular structure of compound L−Ca in the solid state.
Hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths [Å] and angles [deg]: Ca1−O1 2.304(3),
Ca1−O2 2.529(2), Ca1−O3 2.357(2), Ca1−O4 2.396(2), O1−
Ca1−O2 65.19(8), O1−Ca1−O3 84.69(9), O1−Ca1−O4 140.14(8),
O1−Ca1−O1′ 109.35(13), O1−Ca1−O2′ 76.54(8), O1−Ca1−O3′
1
38.70(8), O1−Ca1−O4 81.95(8), O2−Ca1−O3 74.92(8), O2−
Ca1−O4 81.68(8), O2−Ca1−O2′ 111.31(11), O2−Ca1−O3′
54.55(9), O2−Ca1−O4′ 139.21(8), O3−Ca1−O4 64.94(9), O3−
Ca1−O3′ 110.56(13), O3−Ca1−O4′ 78.77(8), O4−Ca1−O4′
14.26(12).
1
1
distorted square antiprismatic due to the determined shape
4
4,45
parameters defined by Porai−Koshits and Aslanov,
as well
as the distortion parameters calculated with the program Shape
Figure 2. Solid state structure of compound L−Ni. Hydrogen atoms
and solvent molecules are omitted for clarity. Selected bond lengths
46
2.1 (refer to Tables S3 and S4, respectively). The Ca−O
bond lengths toward the phenolate oxygen atoms are between
2.304(3) and 2.357(2) Å, and the respective bonds toward the
methoxy oxygen atoms are shortly longer within the range
[
2
1
Å] and angles [deg]: Ni1−O1 1.993(2), Ni1−O2 2.069(2), Ni1−N1
.059(3), O1−Ni1−O1′ 180.0, O2−N1−O2′ 180.0, O1−Ni1−O2
00.07(8), O1−Ni1−O2′ 79.93(8), O1−Ni1−N1 88.16(10), O2−
Ni1−N1 91.21(10).
2
.396(2)−2.529(2) Å. All Ca−O bond lengths are significantly
longer than for L−Zn and L−Ni, in agreement with the larger
van der Waals radius of calcium (2.31 Å) in comparison to
47
octahedral coordination sphere is completed by two
acetonitrile molecules, which coordinate in trans-position to
the metal center. The coordination geometry reveals slight
deviations from the ideal octahedral coordination sphere (refer
to Table S2). The structure is comparable to the Cu−
48
those of zinc (1.39 Å) and nickel (1.63 Å). Accordingly, the
O−Ca−O bite angles of the chelating merocyanines (O1−
Ca1−O2 65.19(8)° and O3−Ca1−O4 64.94(9)°) are sharper
than for L−Zn and L−Ni. Unlike L−Zn and L−Ni, the
1
28
merocyanines in L−Ca adopt the TTT-conformation. Single
merocyanine complex recently published by Giordani et al.
crystals of complex [La(NO ) (MC) ] (L−La) were obtained
The difference between the Ni−O bond lengths of the
phenolate group (Ni1−O1 1.993(2) Å) compared to the Ni−
O bond lengths of the methoxy group (Ni1−O2 2.069(2) Å)
in L−Ni is less pronounced compared to that in L−Zn, while
the O−Ni−O bite angles (O1−Ni1−O2 79.93(8)°) are
slightly widened. The N−Ni bond lengths (Ni1−N1
3 3
2
in the form of red needles from hot CH CN. L−La crystallizes
3
in the monoclinic space group P2 /c with one molecule of L−
1
La, one noncoordinating molecule of acetonitrile, and half a
molecule of diethyl ether in the asymmetric unit.
In contrast to L−Zn, L−Ni, and L−Ca, compound L−La is
a neutral complex, wherein the lanthanum ion is 10-fold
coordinated by the oxygen atoms of three chelating nitrate ions
and two merocyanine molecules, respectively (refer to Figure
4). The calculated distortion parameters indicate that the
coordination sphere is best described by a tetradecahedron or
2
.059(3) Å) are in a comparable range to the Ni−O bond
lengths. In analogy to L−Zn, the merocyanines in L−Ni adopt
1
the TTC-conformation.
Single crystals of compound [Ca(MC) ](ClO ) (L−Ca)
4
4 2
were obtained in the form of dark red plates by slow diffusion
C
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1
spiropyran L is established, and in all H NMR spectra,
resonances of the spiropyrane L of medium intensity are
1
observed. All signals in the H NMR spectra, which can be
assigned to the metal complexes (refer to Figures S5−S8),
show significant downfield shifts and signal broadening in
comparison to the spectrum of L (refer to Figure S4). Due to
the octahedral coordinated and therefore paramagnetic Ni(II)
ion, heavily shifted and broadened resonances of the metal
1
complex are observed in the H NMR spectrum of L−Ni,
while the resonances related to the metal free spiropyrane L
are sharp and exhibit well-defined coupling patterns (refer to
Figure S6). Multiple attempts to characterize the obtained
metal complexes via elemental analysis did not yield
reproducible and reliable results. Nevertheless, all metal
complexes were detected with remarkable intensity and purity
via high-resolution electrospray ionization mass spectrometry
(
HR-ESI-MS, refer to Figures S21−S24).
Figure 4. Molecular structure of compound L−La in the solid state.
Hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths [Å] and angles [deg]: La1−O1 2.446(4), La1−
O2 2.660(4), La1−O3 2.420(4), La1−O4 2.671(4), La1−O5
Job Plot Analysis. The configuration of the complexes in
solution might be different from the results obtained from X-
ray crystallography. Therefore, the complex stoichiometries in
solution were investigated via Job’s method of continuous
2
.586(4), La1−O6 2.638(4), La1−O7 2.593(5), La1−O8 2.627(5),
La1−O9 2.629(5), La1−O10 2.564, N1−O5 1.286(7), N1−O6
.262(7), N2−O7 1.286(7), N2−O8 1.278(7), N3−O9 1.277(7),
N3−O10 1.278(7), O1−La1−O2 60.96(13), O3−La1−O4
35−37,40
variations.
The absorption spectra and the corresponding Job plot of
1
L−Zn are depicted in Figures S25 and S26, respectively. The
spectrum of pure L (X = 0.0) does show the characteristic
6
0.87(13), O5−La1−O6 49.16(14), O7−La1−O8 48.97(15), O9−
Zn
La1−O10 49.4(2), O5−N1−O6 117.1(5), O7−N2−O8 116.4(5),
O9−N3−O10 116.3(5).
absorption band of the thermal equilibrium concentration of
the MC form at 580 nm. In all other spectra, the formation of
L−Zn is evident through the absorption band at 480 nm,
which is used for the Job plot analysis. The maximum of the
absorption in the Job plot is directly correlated with the
complex stoichiometry. Therefore, the maximum was
determined via interpolation of the experimental points.
Different methods have been proposed in the literature for
the interpolation equations in the Job plot analysis, ranging
from linear
coupled equilibria.
of coupled equilibria and association constants on the shape of
the corresponding Job plot and pointed out the failure of
as a slightly distorted Johnson solid J86, also called
49−51
sphenocorona (refer to Figure S16 and Table S5).
However, the coordination sphere in L−La is much more
distorted as for the previously discussed complexes. This is
most likely the result of the higher coordination number, the
strong electrostatic interactions between the ligand and the
metal atom, as well as the rigidity of the chelating ligands,
which results in the difference between the O−M−O nitrate
52,53
to sophisticated models, involving several
54−56
Recent studies investigated the impact
(
O5−La1−O6 49.16(14)°, O7−La1−O8 48.97(15)°, and
O9−La1−O10 49.4(2)°) and the merocyanine (O1−La1−
O2 60.96(13)° and O3−La1−O4 60.87(13)°) bite angles. The
La−O bonds are the shortest for the phenolate oxygen atoms
56,57
simple linear models for weakly associated complexes.
The
interpolation of the Job plots presented herein was performed
using a polynomial equation of degree 5, which is capable of
representing all experimental values in a sufficiently accurate
manner. The determined maximum of XZn = 0.37 nicely
correlates with the theoretical value of XZn = 0.33 for a
(
La1−O1 2.446(4) Å and La1−O3 2.420(4) Å), while the
La−O bond lengths for the nitrate oxygen atoms (La1−O10
.564(5) Å to La1−O6 2.638(4) Å) are slightly longer. The
2
longest La−O bonds are observed for the methoxy oxygen
atoms (La1−O2 2.660(4) Å and La1−O4 2.671(4) Å), which
agrees with the electron donating ability of the differently
substituted oxygen atoms. Comparable to the complexes L−Zn
and L−Ni and in contrast to L−Ca, the merocyanine ligands in
[
Zn(MC) ] complex with 2/1 stoichiometry, thereby confirm-
2
ing the value for L−Zn in the crystalline state. The same
results were obtained from the Job plot analysis of L−Ni, L−
Ca, and L−La (refer to Figures S27−S32) and are summarized
in Table 1, where the theoretical values from the
stoichiometries determined via X-ray crystallography are listed
next to the values determined through Job plot analysis in
1
L−La adopt the TTC-conformation. To the best of our
knowledge, L−La represents the first crystallographically
characterized rare-earth nitrate merocyanine complex.
It should be noted that, in all cases, several approaches to
vary the L to metal stoichiometry did not result in a different
complex stoichiometry; e.g., mixing a solution of L with zinc
perchlorate in a ratio of 3/1 solely reproduced crystals of
compound L−Zn in the observed 2/1 ratio.
Table 1. Comparison of the Complex Stoichiometries in the
Solid State with the Values in Solution Determined via Job
Plot Analysis
1
NMR and HR-ESI-MS Characterization. The H NMR
[M]/([M] + [L])
spectra recorded directly after dissolution of the crystals of L−
Zn, L−Ni, L−Ca, and L−La exclusively show resonances that
can be assigned to the metal complexes and additional
resonances of very low intensity, which are associated with
the metal free spiropyran L. Within 30 min, an equilibrium
between the metal complexes and the noncomplexed
complex
X-ray
Job’s plot
L−Zn
L−Ni
L−Ca
L−La
0.33
0.33
0.20
0.33
0.37
0.40
0.23
0.32
39
D
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Figure 5. (a) Equilibrium between SP−metal complexes L−Zn, L−Ca, and L−La and uncomplexed L in CD CN with highlighted proton
3
resonances used for the integration of the LED-NMR kinetic experiment. Time evolution of the L−Zn (b), L−Ca (c), L−La (d), and L
concentration vs time with the on- and off-switching of the 365 nm UV LED indicated with the violet circles.
solution. Only for L−Ni, the strong curvature of the Job plot
refer to Figure S28) hampers the precise determination of the
though the use of various chemical actinometers to determine
60
(
the light intensity inside the NMR tube.
LED-NMR spectroscopy has been employed for the
maximum and results in a slightly increased value of X = 0.40.
Ni
61,62
elucidation of photochemical reaction mechanisms
the investigation of photochromic azobenzene molecules
for a detailed description of the custom-built LED-NMR
and
LED-NMR Spectroscopy. After the elucidation of the
6
3,64
complex stoichiometries, the photochemistry of the prepared
40
(
metal complexes was investigated. For comparable SP−metal
setup employed in the current work, including the calibration
of the light source via chemical actinometry, see Supporting
Information, page S10). With the calibrated LED-NMR setup,
the SP−metal complexes are investigated directly inside the
NMR spectrometer with simultaneous irradiation of a 365 nm
LED.
complexes, literature reports describe the photoinduced
28
dissociation of SP−metal complexes, as well as photo-
58,59
triggered isomerizations in the MC ligand itself,
specifically
a cis ⇌ trans isomerization of the central MC double bond.
However, these reports solely employ UV−vis spectroscopy for
the investigation of the photoresponsiveness and therefore lack
the molecular information assessible via NMR spectroscopy,
specifically a structure−property relationship. Therefore, the
influence of light on the behavior of the prepared metal
complexes was quantitatively investigated with LED-NMR
spectroscopy employing a high-power LED at 365 nm. Later,
in 2018, Reibarkh and co-workers described methods for the
measurement of quantum yields via LED-NMR spectroscopy
The results for the kinetic LED-NMR experiments with L−
Zn, L−Ca, and L−La are shown in Figure 5b−d. For L−Zn
and L−Ca, 0.62 and 0.61 mM solutions were prepared in
CD CN by the dissolution of crystalline material. While the
3
given concentrations refer to the theoretical values for a 100%
SP−metal complex, the concentrations present at the start of
the kinetic experiments correspond to the values of the thermal
equilibrium, which were determined in quantitative NMR
E
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measurements before the kinetic run (refer to Supporting
Information, page S3). Due to the insolubility of isolated L−La
crystals, the solution for the experiment in Figure 5d was
prepared by mixing of L and [La(NO ) (H O) ] solutions in
and the free metal salt, see Figure 5a. Here, the L−La/(L +
La) ratio is changed from 11.8/1 (thermal equilibrium) to 3/1
(PSS); i.e., the concentration of L−La is reduced by 10% upon
irradiation. Like the previous two SP−metal complexes, the
photoinduced changes of the L−La ⇌ (L + La) equilibrium
are reversible in the dark.
3
3
2
6
CD CN in a molar ratio of 2/1 with an effective concentration
3
of 20.6 mM for L−La. The prepared L−La solution remained
stable at room temperature for the time of the LED-NMR
investigation; i.e., the formation of precipitate during the
measurements was not observed. Attempts to investigate L−Ni
were not successful, as the paramagnetic properties of the
nickel center resulted in line broadening of the proton
resonances, rendering the evaluation of the kinetic measure-
ments impossible. The concentrations for L−Zn and
uncomplexed L in Figure 5b of 0.52 and 0.21 mM correspond
to a L−Zn/(L + Zn) ratio of 2.4/1 in the thermal equilibrium.
Upon irradiation with the 365 nm LED, the concentration of
L−Zn decreases to 0.22 mM, while the concentration of
uncomplexed L accordingly increases to 0.65 mM, until the
photostationary state (PSS) is reached after 17.6 min. This
change corresponds to a L−Zn/(L + Zn) ratio of 1/3 in the
PSS; i.e., the concentration of L−Zn is reduced by 58% upon
irradiation. It should be noted at this stage that the equilibrium
shown in Figure 5a does only include the diligated L−Zn
complex and uncomplexed SP L. However, the presence of
monoligated L−Zn species cannot be ruled out with the
available data. The same statement does also hold true for the
equilibria of L−Ca and L−La shown in Figure 5a.
CONCLUSIONS
■
We introduce the synthesis of new SP-based metal complexes
of zinc, nickel, calcium, as well as lanthanum. SP L served as a
valuable ligand for all employed metal salts. The complex
stoichiometries obtained via X-ray crystallography were
confirmed in solution for all discussed complexes with the
help of Job’s method of continuous variations. Subsequently,
the photochemistry of SP−metal complexes was investigated
by means of LED-NMR spectroscopy. While only qualitative
results can be obtained from UV−vis measurements alone, the
versatility of LED-NMR spectroscopy was exploited for the
determination of the concentrations of the L-derived metal
complexes, as well as uncomplexed L in the thermal
equilibrium and the PSS. Specifically, the herein described
LED-NMR setup was significantly improved compared to the
setups described in the literature regarding the light intensity
reaching the analyte solution inside the NMR spectrometer.
We are confident that the gained understanding of the
photochemistry of the described SP−metal complexes will
stimulate further experiments toward the application of such
light-sensitive complexes in material science.
After turning off the UV LED, the system thermally relaxes
back to a L−Zn/(L + Zn) ratio of 3.6/1, i.e., a concentration of
0
.50 and 0.14 mM for L−Zn and L, respectively. The
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02547.
■
difference in the concentration of L compared to the start of
the kinetic run is attributed to a superimposed photo-
degradation of L during the irradiation cycle. However, the
ratio of L−Zn/(L + Zn) should not be influenced by a loss of
L and might be due to an insufficient relaxation time of the
system in the investigated time window.
*
S
Materials, characterization methods, experimental pro-
cedures, full spectroscopic data for all new compounds,
In summary, the thermal equilibrium between L−Zn and
uncomplexed L is established upon dissolution of crystalline
1
13
1
copies of H and C{ H} NMR, UV−vis, LED
emission, IR, HR-ESI-MS, NMR pulse sequence, LED-
NMR calibration, crystallographic data, and Job plots
(PDF)
L−Zn in CD CN, which corresponds to the proton resonances
3
1
for L observed in the H NMR spectrum. During irradiation
with UV light, the equilibrium concentration of L−Zn is
reduced, which is related to the increase in the integral of L in
1
Accession Codes
the H NMR spectrum, a process that is reversible in the dark.
CCDC 1948639−1948642 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Interestingly, the SP → MC transition of L is also triggered
65
with the employed UV LED, although no MC-specific
1
proton resonances could be detected in the H NMR
spectrum. Apparently, the PSS concentration of the
merocyanine form of uncomplexed L is too low to be detected
via NMR spectroscopy, which is explained by the rapid
recomplexation with zinc during the kinetic run.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: christopher.barner-kowollik@kit.edu, christopher.
barnerkowollik@qut.edu.au.
A different picture is obtained for L−Ca in Figure 5c with a
L−Ca/(L + Ca) ratio of 1/2.1 for the thermal equilibrium.
Here, the irradiation with UV light leads to an increase in the
concentration of the L−Ca complex from 0.40 to 0.48 mM of
■
2
0% in the PSS. At the same time, the concentration of
*E-mail: roesky@kit.edu.
uncomplexed L is reduced from 0.84 to 0.58 mM, according to
the stoichiometry in the equilibrium in Figure 5a. Like L−Zn,
the photoinduced shift of the equilibrium is reversible in the
dark.
ORCID
Christopher Barner-Kowollik: 0000-0002-6745-0570
Peter W. Roesky: 0000-0002-0915-3893
Author Contributions
The photochemical properties of L−La are shown in Figure
d. The results are comparable to the observations made for
⊥
T.J.F. and R.M. contributed equally.
5
L−Zn; that is, irradiation with the UV LED results in a shift of
the L−La ⇌ (L + La) equilibrium toward the uncomplexed SP
Notes
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.inorgchem.9b02547
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
19) Evans, L.; Collins, G. E.; Shaffer, R. E.; Michelet, V.; Winkler, J.
ACKNOWLEDGMENTS
■
D. Selective Metals Determination with a Photoreversible Spiroben-
C.B.-K. and P.W.R. acknowledge funding for the current
project from SFB 1176 (project B4) funded by the German
Research Council (DFG). C.B.-K. additionally acknowledges
the Australian Research Council (ARC) for a Laureate
Fellowship enabling his photochemical research program as
well as the STN program of the Helmholtz association for
continued support.
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