Azido-Functionalized Aromatic Phosphonate Esters in RPOSS-Cage-Supported Lanthanide Ion (Ln = La, Nd, Dy, Er) Coordination

Article abstract of DOI:10.1021/acs.inorgchem.1c00266

Within this work, a modified preparation of diethyl 4-azidobenzylphosphonate (L1) is presented and the family of 4- or 4′-azido-substituted aromatic phosphonate esters is increased by three new ligand platforms: diisopropyl 4-azidobenzylphosphonate (L2), diisopropyl ((4′-azido-[1,1′-biphenyl]-4-yl)methyl)phosphonate (L3), and diisopropyl 4-azido-2,3,5,6-tetrafluorobenzylphosphonate (L4), which exhibit an anomalous splitting of the N3 stretching vibrations. Subsequent coordination to the in situ generated RPOSS (polyhedral oligomeric silsesquioxane)-cage-supported lanthanide precursors [(Ln{RPOSS})2(THF)m] (P1-P6) (Ln = La, Nd, Dy, Er; R = iBu, Ph; m = 0, 1) yields complexes of the general formula [Ln{RPOSS}(L1-L4)n(S1)x(THF)m] (1-30) (n = 2, 3; x = 0, 1; m = 0-2) retaining the azide unit for future semiconductor surface immobilization. Because the latter compounds are mostly oils or viscous waxes, preliminary solution-state structure elucidations via DOSY-ECC-MW estimations have been carried out which are in accordance with 1H NMR integral ratios as well as solid-state structures, where available. Moreover, the optical properties of the Nd, Dy, and Er derivatives of complexes 1-30 are examined in the visible and NIR spectral regions, where applicable.

Full text of DOI:10.1021/acs.inorgchem.1c00266

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Azido-Functionalized Aromatic Phosphonate Esters in POSS-Cage-
Supported Lanthanide Ion (Ln = La, Nd, Dy, Er) Coordination
Ingo Koehne, Miriam Gerstel, Clemens Bruhn, Johann P. Reithmaier, Mohamed Benyoucef,
and Rudolf Pietschnig*
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ABSTRACT: Within this work, a modified preparation of diethyl 4-azidobenzylphosph-
onate (L1) is presented and the family of 4- or 4′-azido-substituted aromatic phosphonate
esters is increased by three new ligand platforms: diisopropyl 4-azidobenzylphosphonate
(L2), diisopropyl ((4′-azido-[1,1′-biphenyl]-4-yl)methyl)phosphonate (L3), and diiso-
propyl 4-azido-2,3,5,6-tetrafluorobenzylphosphonate (L4), which exhibit an anomalous
splitting of the N3 stretching vibrations. Subsequent coordination to the in situ generated
RPOSS (polyhedral oligomeric silsesquioxane)-cage-supported lanthanide precursors
[(Ln{RPOSS})2(THF)m] (P1−P6) (Ln = La, Nd, Dy, Er; R = iBu, Ph; m = 0, 1) yields
complexes of the general formula [Ln{RPOSS}(L1−L4)n(S1)x(THF)m] (1−30) (n = 2,
3; x = 0, 1; m = 0−2) retaining the azide unit for future semiconductor surface
immobilization. Because the latter compounds are mostly oils or viscous waxes,
preliminary solution-state structure elucidations via DOSY-ECC-MW estimations have
1
been carried out which are in accordance with H NMR integral ratios as well as solid-
state structures, where available. Moreover, the optical properties of the Nd, Dy, and Er derivatives of complexes 1−30 are examined
in the visible and NIR spectral regions, where applicable.
cross-linkers,18 in conducting polymers,19 and for light-induced
INTRODUCTION
■
polymer surface activation.15,20,21 Very recently, organic azides
Lanthanide ions have already found widespread application in
have been used for the modification of black phosphorus in a
lighting, sensing, and display technologies, due to their
Staudinger-type reaction that is assumed to involve inter-
outstanding photoluminescence properties.1−3 Moreover,
mediate nitrenes.22 There are many approaches to aryl azide
lanthanides are promising candidates for implementation in
synthesis,14,15 and the most common way is to start from the
quantum-based information storage on an atomic and
corresponding aryl amine followed by a standard textbook
molecular level. Recent progress in this field involves reading
diazotization reaction with NaNO2 and subsequent treatment
and manipulation of distinct spins4,5 and accessing atomic
with NaN3. A more elegant variation of this procedure under
transitions for robust QuBits and magnetic data storage on
milder conditions involves tBuONO and TMSN3.23,24 The
single atoms.6,7
metal-catalyzed azidation of organic molecules, especially Cu-
In comparison to their carboxylic acid counterparts, the
mediated Ullmann-type coupling reactions of aryl halides,25,26
advantage of phosphonate esters arises from their lower
is an equally well established synthetic approach that has been
vibrational frequencies, resulting in diminished nonemissive
reviewed recently by de Bruin et al.27
excited-state quenching and improved quantum yields.8 Our
To the best of our knowledge, there are only two other 4- or
group has focused on these ligands in recent years, synthesizing
4′-azido-substituted phosphonate esters known, which are
summarized in Chart 1. Diethyl 4-azidobenzylphosphonate
(L1) was used in a novel click-chemistry approach to flame-
retardant polyurethanes,28 while the ammonium salt of diethyl
((4-azidophenyl)difluoromethyl)phosphonate (A) was eval-
several highly luminescent lanthanide-based MOFs9−11 as well
as mono- and dimeric lanthanide complexes showing
interesting splitting features in their NIR emission spectra.12
The compound class of organic azides has attracted
attention of chemists all over the world since the character-
ization of phenyl azide by Griess over 155 years ago.13 Several
members of the family are known to be labile and hence
explosive and should be handled with care in the laboratory.14
Despite this property, organic azides have attracted broad
industrial interest by proving to be versatile synthons in
organic synthesis.15−17 Owing to their higher stability, aryl
azides have found application, for example, as photoresistor
Received: January 27, 2021
Published: March 16, 2021
© 2021 The Authors. Published by
American Chemical Society
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coordinated Cu(III) species.25−27 Moreover, the formation of
the amines L2a and L3a most likely proceeds via aryl anion
radical intermediates and Cu(I) azide, as proposed in a
comprehensive FT-IR study on copper-mediated one-pot
reductive aminations of aryl halides by Monguchi et al.38
The reactions had to be carried out in two steps (see the
Experimental Section) due to the separation of elemental
copper after 3 h of reaction time accompanied by a residual
25% of unreacted S1/S2. Therefore, an additional portion of
the reagents was added again to target full turnover, and
heating was continued for 3 h. Increasing the initial amounts of
the reagents by ca. 25% also does not lead to full turnover but
rather to the formation of more Cu(0). Interestingly, in the
reaction starting from S312 the 4-protonated compound
diisopropyl 2,3,5,6-tetrafluorobenzylphosphonate (L4a) is
obtained in a yield of 66% instead of the 4-azido-substituted
species. This may be a consequence of the strongly electron
withdrawing (EWD) properties of the tetrafluorophenyl unit as
in related cases.39,40 Moreover, these reaction conditions do
not work either for the azidation of diisopropyl ((10-
bromoanthracen-9-yl)methyl)phosphonate,12 since only the
starting material is quantitatively retrieved from the reaction
mixture. Since no reaction takes place at all, it is most likely
that the increased steric demand of the anthracene backbone is
hampering an initial oxidative addition to the Cu(I) complex.
From vapor diffusion of pentanes into a saturated solution of
L3 in THF at −20 °C, only small needle-shaped crystals could
be obtained that additionally suffered from decomposition by
the X-ray irradiation after some time. X-ray data collection was
additionally hampered by a phase transition of the crystals at
100 K, which required a measurement at an elevated
temperature of 223 K. L3 crystallizes in the monoclinic
space group P21/c and does not show any π-stacking of the
aromatic rings in the solid state (Figure 1, top; structural data
are given in Table 1). Due to the poor data quality, no bond
lengths or angles are discussed in the following. Crystals of L3a
were obtained just as for L3 but in good quality suitable for a
proper SCXRD data collection. L3a crystallizes in the
monoclinic space unit group P21/n containing one molecule
in the asymmetric unit forming a three-dimensional hydrogen-
bond network between adjacent molecules (Figure 1, bottom).
In particular, the N−H···OPO distance of 2.10(2) Å and the
N−H···OO−iPr distance of 2.35(2) Å can be assigned to
moderate and weak hydrogen-bond interactions, respectively.41
Moreover, the C6 rings within the molecule are significantly
twisted by 20.6(2)° with respect to each other despite forming
a conjugated aromatic system. The phosphonate ester
diisopropyl 4-azido-2,3,5,6-tetrafluorobenzylphosphonate
(L4) is prepared in a four-step synthesis with a moderate
overall yield of 30% starting from 2,3,4,5,6-pentafluorobenzal-
dehyde (Scheme 2, right). In a first step, an azido substituent is
introduced at the para position of the starting material via a
nucleophilic aromatic substitution (SNAr), forming 4-azido-
2,3,5,6-tetrafluorobenzaldehyde (S4).42 Next, reduction with
NaBH4 yields (4-amino-2,3,5,6-tetrafluorophenyl)methanol
(S5), which is subsequently treated in an alcohol-based
Michaelis−Arbuzov43 reaction to give diisopropyl 4-amino-
2,3,5,6-tetrafluorobenzylphosphonate (S6). In the last step, a
diazotization is carried out using tBuONO followed by the
addition of TMSN3 to give the 4-azido-substituted target
compound L4. During the reaction also a minor amount (ca.
8%) of the 4-protonated species L4a is formed that could not
be removed by additional workup and column chromatog-
Chart 1. Known 4-Azido-Functionalized Phosphonate Ester
Ligands L1 and A
uated in terms of being a potent nonpeptidyl inhibitor of
protein tyrosine phosphatase 1B.29 Herein, we present a
modified procedure for the preparation of L1 as well as the
synthesis of three new 4- or 4′-azido functionalized
phosphonate ester ligands L2−L4 comprising different
aromatic backbones. In combination with our expertise in
POSS (polyhedral oligomeric silsesquioxane)-cage chemis-
try,30−32 these ligands are then applied in RPOSS-cage-
supported (R = iBu, Ph) Ln3+ ion coordination to function
as anchor groups for future immobilization of these complexes
on (In/Ga)P semiconductor surfaces and subsequent evalua-
tion of their potential as optically switchable isolated molecular
quantum bits. In this context, an azido functionalization at the
4- or 4′-position of L1−L4 is expected to be the most suitable
to avoid steric congestion between a semiconductor surface
and the rest of the POSS-cage-supported complexes. The rigid
POSS-cage structure in combination with the phosphonate
esters is assumed to enhance the excited-state lifetimes of the
metal centers, paving the way for potential future molecular
data storage and manipulation. For a comprehensive overview
on POSS-cage as well as lanthanide POSS chemistry the reader
is referred to the work of Edelmann and co-workers.33−36
RESULTS AND DISCUSSION
■
Synthesis of Ligands L1−L4. Diethyl 4-azidobenzyl-
phosphonate (L1) is prepared by starting from its 4-amino
derivative via a modified procedure by Starikova and co-
workers consisting of a standard textbook diazotization
reaction and treatment with NaN3.37 An altered workup
gives L1 as an orange oil in a quantitative yield of 98%
(Scheme 1).
Scheme 1. Synthesis of Ligand L1
Due to the lack of 4-amino derivatives, the synthesis of
diisopropyl 4-azidobenzylphosphonate (L2) and diisopropyl
((4′-azido-[1,1′-biphenyl]-4-yl)methyl)phosphonate (L3)
started from their 4-bromo derivatives S1 and S2.12 Following
́
a modified protocol by Rodriguez et al., L2 and L3 are
obtained in yields of 79% and 61%, respectively (Scheme 2,
left).28 The corresponding 4-amino-substituted side products
L2a (10%) and L3a (12%) are obtained as well in this copper-
mediated Ullmann-type coupling reaction. L2a and L3a again
can be converted to the azides via a diazotization protocol and
addition of NaN3 or TMSN3, increasing their theoretical yields
to 89% and 73%, respectively. The underlying catalytic cycle is
assumed to proceed via a CuI/CuIII pair mediated oxidative
addition−reductive elimination mechanism featuring a 4-fold-
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Scheme 2. Syntheses of Ligands L2 and L3 and Compounds L2a−L4a (Left) and of Compounds S4−S6 and L4a and Ligand
L4 (Right)
Figure 1. Molecular structures of L3 (top) and L3a (bottom) with the schematic 3D hydrogen-bond network. Anisotropic displacement
parameters are depicted at the 50% probability level. With exception of the NH2 groups, hydrogen atoms are omitted for clarity. Structural data are
given in Table 1.
a
Table 1. Selected Bond Lengths (Å) and Angles (deg) of compounds L3a, S6, P5 as [Nd{PhPOSS}(THF)2]2, 8, 31, and 32
compound
PO
P−OR
M−OPO M−Oμ ‑OSi M−Oμ ‑OSi M−Oμ ‑OH
OP−C
M−O−P
1
2
3
L3a
S6
1.467(2)
1.469(4)
1.573(2)
1.565(4)
113.1(11)
114.3(2)
P5 as [Nd{PhPOSS}(THF)2]2
2.228(6)
2.165(9)
2.315(5)
2.293(5)
2.396(6)
[Er{PhPOSS}(L1)3] (8)
1.479(9)
1.480(5)
1.471(5)
1.560(9)
1.565(6)
1.565(5)
2.330(9)
2.371(5)
2.344(5)
112.8(7)
110.3(10)
111.6(8)
152.7(7)
158.0(4)
157.5(3)
[Dy2{T7(O)3}{T6(OH)2(O)2}(μ3-OH)(L2)]2 (31)
[Er2{T7(O)3}{T6(OH)2(O)2}(μ3-OH)(L2)]2 (32)
2.295(4)
2.277(4)
2.374(4)
2.316(4)
a
Average values are given for compounds with more than one molecule in the asymmetric unit or for more than one ligand of the same kind
attached to a lanthanide ion.
raphy, leaving L4 as an orange oil in a purity of >92%. Crystals
of S6 suitable for SCXRD experiments could be obtained via
recrystallization from Et2O/pentanes (2/1) at −20 °C.
The 4-amino phosphonate ester S6 crystallizes in the
orthorhombic space group Pna21 containing four molecules
in the unit cell (Figure 2). In contrast to L3a a two-
dimensional hydrogen-bond network is exclusively formed
between the NH2 groups and neighboring PO phosphonate
moieties consisting of weak to moderate interactions with N−
H···OPO distances in a range of 2.09(2)−2.25(2) Å.
A comparison of L1−L4 and L2a−L4a shows that 31P NMR
shifts in a range from 19.6 to 26.3 ppm are recorded for the
phosphonate ester moieties (Table 2). Within the row, L1
exhibits the most deshielded and L4 the most shielded
resonance due to the highest number of EWD substituents at
the ligand backbone. In contrast, the compound ((4-
azidophenyl)difluoromethyl)phosphonate (A) shows a
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the Supporting Information). This phenomenon can be
ascribed to Fermi interactions with combination tones
including the symmetric N3 or C−N stretching mode and
other low-lying frequencies.45 Nonetheless, in comparison to
A, which exhibits a νas(N3) band at 2149 cm−1, all the
asymmetric vibrations are significantly red shifted. A similar
splitting of the symmetric N3 stretch around 1374−1391 cm−1
is observed in all four phosphonates that is most blue shifted in
L1. Contrasting trends are found for the 4-amino-substituted
species L2a and L3a. In comparison to L2a, the νas(NH2)
vibration in L3a is red-shifted while the νs(NH2) band is
slightly blue shifted.
RPOSS Cage-based Lanthanide Precursors P1−P6.
Lanthanide precursors P1−P6 with the general solution-state
structure [(Ln{RPOSS})2(THF)m] (Ln = La, Nd, Er; R = iBu,
Ph; m = 0, 1) (see the section on DOSY) are obtained in a
concerted deprotonation−metalation reaction between a
RPOSS cage (R = iBu, Ph) and the corresponding lanthanide-
(III) isopropoxides in dry THF (Scheme 3). P1−P6 are
obtained as white, blue, or pink solids in isolable yields of 86−
97%. Crystallization from different solvents afforded unique
coordination motifs in the solid state. From a solution of P3 in
dry pentanes, a tetrameric structure of the form
[Er4{iBuPOSS}2{iBuPOSSSi−OH}2(μ4-O)] is obtained at −20
°C featuring a μ4-oxo ligand and partially protonated POSS
cages (Figure 3). The cages at the Er1 ions are still trianionic,
while those at the Er2 ions are now dianionic ligands, offering
two Si−O− groups as well as a Si−OH moiety. In general, the
Er3+ ions are μ2-bridged by two Si−O− moieties of each POSS
cage, while the third donor site shows an exclusive
coordination to either Er1 (Si−O−) or Er2 (Si−OH), giving
in total a 6-fold, distorted-octahedral oxygen coordination.
Because water is formed in the synthesis of POSS cages, the
incorporated water molecule most likely originates from traces
of water in the POSS cage starting material. The tetramer en
bloc forms a tetrahedrally shaped structure with the μ4-oxo
ligand residing at the center of the Er4 plane. Owing to poor
data quality, no bond lengths or angles are considered in the
following. The present coordination motif is in close
agreement with a very similar structure by Shen and co-
workers.46 Here, a Nd3+ tetramer of the form [(Nd-
Figure 2. Molecular structure of diisopropyl 4-amino-2,3,5,6-
tetrafluorobenzylphosphonate (S6) with the schematic 2D hydro-
gen-bond network. Anisotropic displacement parameters are depicted
at the 50% probability level. With the exception of NH2 groups,
hydrogen atoms are omitted for clarity. Structural data are given in
Table 1.
Table 2. 31P NMR Shifts (ppm) and Selected IR Vibrations
(cm−1) of Ligands L1−L4 and Compounds A and L2a−L4a
31P NMR
νas(N3)
2111
2112
2122, 2092
2168, 2119
2149
νs(N3)
ν(NH2)
L1
L2
L3
L4
26.3
24.1
24.3
19.6
3.9
1391, 1381
1384, 1374
1385, 1374
1386, 1376
A29
L2a
L3a
L4a
24.6
24.7
19.7
3341 (νas), 3234 (νs)
3335 (νas), 3239 (νs)
strongly shielded signal at 3.9 ppm caused by the EWD effect
of the fluorinated methylene bridge adjacent to the
phosphorus. The 31P resonances of compounds L2a−L4a all
experience a slight deep-field shift in comparison to their 4-
azido-substituted counterparts. The asymmetric and symmetric
stretches of the N3 substituent are typically found around 2100
and 1400 cm−1, respectively.44 As expected for L1 and L2, one
intense band for the asymmetric vibration is recorded at 2111
and 2112 cm−1, respectively. In contrast, the νas(N3)
vibrational mode is anomalously split into two bands of
medium intensity at 2122 and 2092 cm−1 for L3 and of
strongly deviating intensity at 2168 and 2119 cm−1 for L4 (see
{
iBuPOSS})4NaCl] was obtained that shows a similar
coordination motif by formally hosting a NaCl molecule. In
Scheme 3. Syntheses of Lanthanide Precursors P1−P6 Showing Their Estimated Solution-State Structure
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Figure 4. Molecular structure of [Nd{PhPOSS}(THF)2]2 from vapor
diffusion of pentanes into a saturated solution of P5 in THF.
Anisotropic displacement parameters are depicted at the 50%
probability level. Hydrogens and phenyl substituents are omitted for
clarity. Structural data are given in Table 1. Symmetry code: (#1) −x
+ 1, −y + 1, −z.
Figure 3. Molecular structure of [Er4{iBuPOSS}2{iBuPOSSSi−OH}2(μ4-
O)] from a solution of P3 in dry pentanes at −20 °C. Anisotropic
displacement parameters are depicted at the 30% probability level.
The iBu substituents are omitted for clarity. A schematic drawing is
given to clarify the binding situation. Symmetry code: (#1) −x + 3/2,
y, −z + 3/2.
substituted species S1 was prepared to provide enough
phosphonate ligand for subsequent complexation. In case of
the more sterically demanding L3, reactions with addition of
only 2 equiv were performed because 1H-DOSY-MW
estimation experiments of previous reactions with 3 equiv
suggested the coordination of only two L3 ligands to the La3+
ions in solution (see section on DOSY). Moreover, no reaction
with Dy(OiPr)3 has been performed for L3 and the
corresponding iBu/PhPOSS cages. A preliminary solution-state
this complex the O2− is replaced by a Cl− anion while two Na+
cations are residing above and below the Nd4 plane that each
exhibit an occupancy of 1/2. The absence of OH stretching
modes above 3000 cm−1 in the IR spectrum of
[Er4{iBuPOSS}2{iBuPOSSSi‑OH}2(μ4-O)] (see the Supporting
Information) indicates intramolecular hydrogen bonding of
the two Si−OH groups with the silanolate units in proximity.41
For comparison, other rarely found lanthanide tetramers such
as Na6{[PhSiO2]8 Ln4(μ4-O)[O2SiPh]8}·10EtOH·8H2O (Ln =
Nd, Gd, Dy) containing a central μ4-O2− ligand have been
obtained from their anhydrous lanthanide chlorides and eight-
membered sodium phenylsiloxanolates.47 The hydrogen
positions are not fully resolved in these structures, but due
to charge compensation of the hexaanion by six sodium ions,
the presence of a O2− dianion residing at the Ln4 plane is most
likely.
From vapor diffusion of pentanes into a concentrated
solution of P5 in THF a dimeric solid-state structure of the
form [Nd{PhPOSS}(THF)2]2 is obtained at −20 °C. This
species crystallizes in the monoclinic space group P21/c
containing half of the dimer in the asymmetric unit (Figure 4).
The other half is generated by an inversion center. Each Nd3+
cation exhibits a 7-fold distorted-pentagonal-bipyramidal
oxygen coordination, but in this case only one of the three
Si−O− moieties of each POSS cage shows a μ2-bridging mode
between the metal centers. To complete the coordination
sphere, each metal ion is additionally coordinated by two THF
donor molecules.
1
structure elucidation was also carried out via H-DOSY NMR
spectroscopy for the La3+ species 1, 5, 9, 13, 17, 20, 23, and 27
(see section on DOSY). Except for the complexes [Ln-
{PhPOSS}(L3)2(THF)2] (20−22) (Ln = La, Nd, Er) that are
obtained as beige solids, brownish-orange viscous oils or sticky
waxes are obtained for compounds 1−19 and 23−30. Only in
a few cases do these oils or waxes start to solidify after some
time under argon, and only for the complex [Er{PhPOSS}-
(L1)3] (8) were crystals suitable for SCXRD experiments
obtained. 8 crystallizes in the monoclinic space group P21
containing one molecule in the asymmetric unit (Figure 5).
Despite being formally tridentate ligands, the phosphonate
esters exclusively coordinate via their PO oxygen atom to
the central Er3+ cation. In combination with the tridentate
coordination by the PhPOSS cage, a 6-fold distorted-octahedral
coordination is established.
The 31P NMR resonances of the La3+ complexes 1, 5, 9, 13,
17, 20, 23, 27 are found in a range of 19.5−25.5 ppm (Table
3). Only in the case of ligand L1 (26.3 ppm) are significant
high-field shifts of the 31P NMR signals by 0.8 ppm in 1 and
even 2.5 ppm in 5 observed upon metal ion coordination. For
phosphonate esters L2−L4, less pronounced high-field shifts of
0.1−0.2 ppm are detected upon complex formation. Again, a
splitting of the asymmetric and symmetric azide stretches is
observed for the L3- and L4-based complexes and for the
symmetric stretch in the L2-based and PhPOSS-supported
complexes. In comparison to the free ligands, the νas(N3)
vibrations generally experience a slight shift to higher
wavenumbers while the νs(N3) modes are mostly unaffected
by the lanthanide ion coordination. The 29Si NMR resonances
of the protonated POSS cages can be found with a ratio of
L1−L4-Based RPOSS-lanthanide Complexes 1−30.
The L1−L4- and RPOSS−cage-supported complexes 1−30
of the general formula [Ln{RPOSS}(L1−L4)n(S1)x(THF)m]
(Ln = La, Nd, Dy, Er; R = iBu, Ph; n = 2, 3; x = 0, 1; m = 0−2)
are obtained from in situ generated lanthanide POSS cage
complexes and subsequent addition of one of the 4-azido-
substituted phosphonate esters in yields of 73−88% (Scheme
4). In the case of L2, a 2:1 mixture of L2 and its 4-bromo-
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R
Scheme 4. Syntheses of L1−L4-Based POSS-lanthanide Complexes 1−30
Table 3. 31P NMR Shifts (ppm) and N3 Vibrations (cm−1) of
Complexes 1−30
complex
[La{iBuPOSS}(L1)3] (1)
31P
25.5
νas(N3)
2113
νs(N3)
1401
[Nd{iBuPOSS}(L1)3] (2)
[Dy{iBuPOSS}(L1)3] (3)
[Er{iBuPOSS}(L1)3] (4)
2113
2114
2113
2112
2112
2112
2111
1401
1401
1401
1393
1394
1395
1394
[La{PhPOSS}(L1)3] (5)
23.8
[Nd{PhPOSS}(L1)3] (6)
[Dy{PhPOSS}(L1)3] (7)
[Er{PhPOSS}(L1)3] (8)
[La{iBuPOSS}(L2)2(S1)] (9)
[Nd{iBuPOSS}(L2)2(S1)] (10)
[Dy{iBuPOSS}(L2)2(S1)] (11)
[Er{iBuPOSS}(L2)2(S1)] (12)
[La{PhPOSS}(L2)2(S1)] (13)
[Nd{PhPOSS}(L2)2(S1)] (14)
[Dy{PhPOSS}(L2)2(S1)] (15)
[Er{PhPOSS}(L2)2(S1)] (16)
23.9, 25.3 2114
1385
1384
1385
1385
1386, 1375
1386, 1376
1386, 1376
1386, 1376
2113
2114
2113
24.2, 25.5 2113
2113
Figure 5. Molecular structure of [Er{PhPOSS}(L1)3] (8). Anisotropic
displacement parameters are depicted at the 50% probability level.
Hydrogens and phenyl substituents are omitted for clarity. Structural
data are given in Table 1.
2113
2113
[La{iBuPOSS}(L3)2(THF)] (17) 24.1
[Nd{iBuPOSS}(L3)2(THF)] (18)
2122, 2092 1384, 1365
2124, 2095
3:1:3 at −59.1, −67.5, and −68.8 ppm and at −69.1, −77.6,
and −78.6 ppm for the iBu- and Ph-substituted derivatives,
respectively. However, the 29Si spectra of the La3+ complexes
reveal the presence of five to seven signals for the seven cage
silicon atoms within this range that partially fall close together.
In turn, this indicates a loss of symmetry within the cages of
the obtained complexes, resulting in the acquisition of an
increased number of silicon resonances. This symmetry loss
accompanied by dynamic processes is also reflected by
broadened and fanned-out signals of the iBu and Ph groups
[Er{iBuPOSS}(L3)2(THF)] (19)
2123, 2093 1385, 1376
2119, 2092 1386, 1376
2120, 2092 1386, 1376
[La{PhPOSS}(L3)2(THF)2] (20) 24.2
[Nd{PhPOSS}(L3)2(THF)2]
(21)
[Er{PhPOSS}(L3)2(THF)2] (22)
2118, 2092 1386, 1376
2170, 2119 1386, 1377
2174, 2119 1386, 1377
2168, 2119 1386, 1378
2171, 2119 1386, 1378
2173, 2119 1387, 1377
2172, 2119 1387, 1377
2171, 2119 1387, 1377
2171, 2118 1387, 1377
[La{iBuPOSS}(L4)3] (23)
19.5
19.6
[Nd{iBuPOSS}(L4)3] (24)
[Dy{iBuPOSS}(L4)3] (25)
[Er{iBuPOSS}(L4)3] (26)
[La{PhPOSS}(L4)3(THF)] (27)
[Nd{PhPOSS}(L4)3(THF)] (28)
[Dy{PhPOSS}(L4)3(THF)] (29)
[Er{PhPOSS}(L4)3(THF)] (30)
1
in their H and 13C NMR spectra.
A transfer of complexes 1−30 from Schlenk vessels to vials
under ambient conditions results in the formation of a few
crystals at the upper inside of these vials in some cases and
after some time. In the cases where crystals suitable for
SCXRD experiments were obtained, the solid-state structures
reveal their corresponding predecessors [Ln{iBuPOSS}-
(L2)2(S1)] (Ln = Dy (11), Er (12)) to have suffered from
partial T7(O−)3 cage rearrangement induced by a reaction with
airborne water with cleavage of one of the cage silicon edges
(Scheme 5). Now, the formed T6(OH)2(O−)2 cages are
dianionic tetradentate ligands offering two Si−O− as well as
two Si−OH moieties. The obtained isostructural tetrameric
lanthanide complexes of the form [Ln2{T7(O)3}-
{T6(OH)2(O)2}(μ3-OH)(L2)]2 (Ln = Dy (31), Er (32))
crystallize in the monoclinic space group P21/n (Figure 6). On
the one hand, these compounds exhibit two terminal, 6-fold-
coordinated Ln3+ cations (Ln1) where each is exclusively
coordinated by a T7(O−)3 cage, by the two T6(OH)2(O−)2
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cages, and by a μ3-bridging hydroxo ligand (O27). On the
other hand, there are two lateral Ln3+ ions (Ln2) that each
show a 7-fold coordination made up by one of the
T6(OH)2(O−)2 cages, an oxygen of a 4-azido phosphonate
ester ligand, and two μ3-hydroxo ligands. In general, the Si−O−
moieties of the T6(OH)2(O−)2 cages show a μ2-bridging motif
to the terminal and lateral metal ions while the Si−OH groups
each display a single coordination to one of the lateral
lanthanide ions. The formed intramolecular OH···O hydrogen
bond distances of 1.62(2) Å (H23) and 1.68(2) Å (H15) are
on the edge of being strong interactions, while an OH···O
distance of 2.18(2) Å for H27 is at the junction of weak
electrostatic and dispersion force interactions.41 Related
tetranuclear lanthanide complexes comprising μ3-capping
Scheme 5. Schematic Water-Induced Cleavage of a Silicon
Edge from the Trianionic T7(O−)3 Cages under Formation
of Dianionic T6(OH)2(O−)2 Cages
Figure 6. Molecular structure of [Er2{T7(O)3}{T6(OH)2(O)2}[μ3-OH](L2)]2 (32) (top) and a drawing of the first coordination sphere of the
Er3+ ions to clarify the binding situation (bottom). Anisotropic displacement parameters are depicted at the 50% probability level. Except for OH
groups, hydrogens and iBu substituents are omitted for clarity. Structural data for 31 and 32 are given in Table 1. Symm. code: #1 -x+1, -y+1, -z+1.
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1
Table 4. H-DOSY-ECC-MW Estimation of P1, P4, 1, 5, 9, 13, 17, 20, 23, and 27 in C6D6 or CDCl3 at rt
complex
MWtheo (g/mol)
MWDSE (g/mol) (MWdif (%))
MWmerge (g/mol) (MWdif (%))
[(La{iBuPOSS})2(THF)] (P1)
[(La{PhPOSS})2] (P4)
1927
2135
1735
1875
1857
1997
1746
1958
2035
2247
1858 (4)
1837 (16)
1497 (16)
1555 (21)
1365 (36)
1386 (44)
1508 (16)
1950 (0)
2305 (−16)
2276 (−6)
1822 (−5)
1898 (−1)
1667 (13)
1675 (19)
1836 (−5)
2428 (−19)
1860 (9)
[La{iBuPOSS}(L1)3] (1)
[La{PhPOSS}(L1)3] (5)
[La{iBuPOSS}(L2)2(S1)] (9)
[La{PhPOSS}(L2)2(S1)] (13)
[La{iBuPOSS}(L3)2(THF)] (17)
[La{PhPOSS}(L3)2(THF)2] (20)
[La{iBuPOSS}(L4)3] (23)
1526 (33)
1549 (45)
[La{PhPOSS}(L4)3(THF)] (27)
1903 (18)
Figure 7. Exemplary absorption spectra of complexes 2 (black), 3 (blue), and 4 (red) at rt exhibiting typically sharp Nd3+, Dy3+, and Er3+
absorption bands.
hydroxo ligands contain cubane-like [Ln4(μ3-OH)4]8+ cluster
in 32, which is in accordance with the successively decreasing
effective ionic radii of the lanthanide ions: Nd3+ (98 pm) >
Dy3+ (91 pm) > Er3+ (89 pm).49 The considered OP−C
angles experience a slight shortening from an average of
113.7(11)° in the free phosphonate esters to an average of
111.6(10)° in the complexed ligands. The shortest M−O−P
angles with an average of 152.7(7)° are found in the
monomeric complex [Er{PhPOSS}(L1)3] (8), while this
angle decreases in the tetrameric compounds from 158.0(4)°
in the Dy3+ species 31 to 157.5(3)° in the Er3+ species 32.
1H-DOSY-ECC-MW Estimation Study. A preliminary
solution-state structure elucidation of the La3+ complexes P1,
cores and comparable M−Oμ ‑OH distances.48 Moreover, a
3
comparison with the tetrameric solid-state structure of
[Er4{iBuPOSS}2{iBuPOSSSi−OH}2(μ4-O)] obtained from precur-
sor P3 indicates this structure as reflecting the intermediate
state of a reaction with water on the way to partially
decomposed POSS cages as found in compounds 31 and 32.
An evaluation of selected bond lengths shows the PO
distances of the free phosphonate esters as becoming slightly
elongated upon lanthanide ion coordination. This is in
accordance with a red shift of the PO vibration of the free
ligands L1−L3 in combination with the iBuPOSS-cage-
supported complexes. Interestingly, in combination with the
PhPOSS-cage-based lanthanides, the PO vibration in L1−L3
experiences a blue shift. Only in the case of L4 does the PO
stretch experience an expected red shift for both, the iBu-POSS-
and the PhPOSS-cage-supported lanthanide compounds. The
M−OPO distances in 8, 31 and 32 are in a range from
2.330(9) to 2.371(5) Å and decrease from Dy3+ to the smaller
1
P4, 1, 5, 9, 13, 17, 20, 23, and 27 was performed via a H-
DOSY external calibration curve (ECC) molecular weight
(MW) estimation.50−53 Previous studies showed that for most
organometallic compounds the dissipated spheres and
ellipsoids (DSE) calibration curve is most suitable for an
accurate estimation.54 Hence, only values from the DSE and,
for comparison, from the merged calibration curve are
considered (Table 4). Although this method is only strongly
reliable for molecules with a weight of up to 600 g/mol, a
previous study has shown that good results are still obtained
for aggregates up to 1000 g/mol.55 However, the obtained
results have to be considered carefully and shall rather serve as
Er3+ cation, which is also observed for the M−Oμ ‑OH bond
3
lengths. A similar trend can be derived for the corresponding
M−Oμ ‑OSi distances, which decrease from 2.396(6) Å in
2
[Nd{PhPOSS}(THF)2]2 over 2.295(4) Å in 31 to 2.277(4) Å
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Figure 8. Superimposed room-temperature μ-PL emission spectra (λexc = 355 nm) of Dy3+ complexes [Dy(NO3)3(S1)3] (black), 3 (purple), 7
(orange), 11 (red), 15 (yellow), 25 (blue), and 29 (green). The spectra are normalized to their emission band maximum.
guidance in combination with other spectroscopic methods
(for detailed DOSY data, see the Supporting Information). As
expected, with the exception of P1 and complex 20, more or
less strongly deviating DSE values are obtained for the
assumed La3+ aggregates with molecular weights of around
2000 g/mol. In contrast, good to fair values are still estimated
from the merged calibration curve for all considered complexes
showing a deviation (MWdif) from their theoretically calculated
molecular weights (MWtheo) between 1 and 19%. The
estimated MW values suggest the presence of dimeric species
for the precursors P1 and P4 and monomeric species for
complexes 1, 5, 9, 13, 17, 20, 23, and 27 in solution that most
likely carry one or two additional THF donor molecules in
increasing wavelength but still covers some of the metal-
centered absorptions.
Emission Properties of the Dy3+ Complexes. In the range
between 350 and 850 nm, Dy3+-centered emissions from the
4F9/2 level can be detected around 480 (→6H15/2), 570
(→6H13/2), 660 (→6H11/2), 750 (→6H9/2), and 840 nm
(→6H7/2), like those shown for the phosphonate ester
supported model complex [Dy(NO3)3(S1)3] (Figure 8,
black).12 However, unless the signals are amplified by e.g.
ligand to metal charge transfer (LMCT), all complexes are
expected to show no or only very weak metal-centered
emissions due to Laporte-forbidden 4f−4f transitions. To
recognize these weak emissions, spectra were recorded with a
microphotoluminescence (μ-PL) spectrometer. An excitation
of the weakly fluorescent Dy3+ complexes 3, 7, 11, 15, 25, and
29 at 355 nm results in the detection of only one broad
emission band in the region of the aforementioned energy
transitions, simultaneously excluding the presence of LMCT
processes in these systems (Figure 8, colored spectra). This
band covers the metal-centered emissions and can be ascribed
to the phosphonate ester ligands (see the Supporting
Information).
1
some cases. Thus, in accordance with and supported by H
NMR integral ratios, the solid-state structures obtained for
complex P5 and [Er{PhPOSS}(L1)3] (8), as well as the data
from elemental analysis, the molecular formulas given in Table
4 are derived for the considered La3+ complexes and their Nd,
Dy, and Er congeners. Despite the fact that the crystal
structure obtained from P5 in THF shows coordinating THF
donor molecules, it is most likely that in the PhPOSS-supported
precursors P4−P6 there are no additional THF molecules
attached to the metal ions in solution because complex- and
THF-related signals show significantly deviating diffusion
coefficients in the DOSY spectrum of P4 (see the Supporting
Information). Moreover, the signals are sharp, so that a fast
exchange between attached and detached THF molecules can
most likely be excluded.
Photoluminescence (PL) Properties. Absorption Prop-
erties of Nd3+, Dy3+, and Er3+ Complexes. The absorption
spectra of the Nd3+, Dy3+ and Er3+ complexes presented herein
basically look the same, exhibiting the anticipated metal-
centered absorption bands in a range of 300−850 nm of the
Nd3+ (λabs,max = 586 nm), Dy3+ (λabs,max = 802 nm), and Er3+
(λabs,max = 521 nm) cations, respectively. Some exemplary
spectra of complexes [Nd{iBuPOSS}(L1)3] (2) (black),
[Dy{iBuPOSS}(L1)3] (3) (blue) and [Er{iBuPOSS}(L1)3]
(4) (red) are summarized in Figure 7, showing all three
lanthanide ions sharing an absorption band at around 800 nm.
Additionally, the recorded spectra all show a strong and broad
absorption of the azido-substituted phosphonate ester ligands
in a range between 300 and 700 nm that slowly decreases with
Emission Properties of the Nd3+ and Er3+ Complexes. In
addition to the detection of ligand emission upon UV
irradiation, the metal-centered emissions were also investigated
by using excitation laser wavelengths of 750 nm (Nd3+) and
800 nm (Er3+), respectively. Due to the relatively weak
absorption band of Er3+ at around 800 nm (Figure 7), the
characteristic emission at about 1550 nm56 was not observed.
However, μ-PL spectra could be recorded for L1−L4- and
RPOSS-cage-supported Nd3+ complexes 2, 6, 10, 14, 18, 21,
24, 28 and P5, where three emission bands centered at about
900, 1060, and 1330 nm are observed that correspond to the
4
4
4
4
4
4F3/2 → I9/2, F3/2 → I11/2 and F3/2 → I13/2 transitions,
respectively (Figure 9). The μ-PL spectra are similar to those
that we have recently reported in a study on exclusively
phosphonate ester supported Nd3+ complexes.12
In comparison, the L1−L4- and RPOSS-cage-supported
Nd3+ complexes presented in this work show relatively low
emission intensities with some noise. The weak PL emission
might be explained by the rigid structure of the POSS cage
stabilizing the metal-centered excited states. As we have
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partially protonated, thus providing two anionic Si−O− and
one neutral Si−OH donor site. From a combination of in situ
generated POSS-supported lanthanide precursors with ligands
L1−L4, complexes of the general formula [Ln{RPOSS}(L1−
L4)n(S1)x(THF)m] (1−30) (Ln = La, Nd, Dy, Er; n = 2, 3; x
= 0, 1; m = 0−2) are formed. The derived complex formulas
are supported by recorded NMR data, X-ray structures,
elemental analyses, and a preliminary 1H-DOSY-ECC-MW
estimation study. The coordinated phosphonate esters show an
unusual splitting of the symmetric and/or asymmetric azide
stretching modes. Handling of compounds 1−30 under
ambient conditions results in a slow decomposition of some
of these complexes induced by airborne water. For the
compounds [Ln{iBuPOSS}(L2)2(S1)] (Ln = Dy (11), Er
(12)), solid-state structures of the general formula
[Ln2{T7(O)3}{T6(OH)2(O)2}(μ3-OH)(L2)]2 (Ln = Dy
(31), Er (32)) are obtained under ambient conditions
exhibiting partial T7(O−)3 cage decomposition with the
formation of μ3-OH bridged tetramers. Moreover, absorption
and emission spectra of the Nd3+, Dy3+, and Er3+ complexes are
recorded to explore their potential as optically switchable
isolated molecular quantum bits. In addition to ligand-centered
emission, the microphotoluminescence (μ-PL) spectra of the
Nd3+ compounds 2, 6, 10, 14, 18, 21, 24, 28, and P5 show
Figure 9. Stacked room temperature μ-PL emission spectra (λexc
=
750 nm) of Nd3+ complexes 2 (black), 6 (blue), 10 (magenta), 14
(pink), 18 (red), 21 (green), 24 (orange), 28 (cyan), and P5
(purple). There are three emission bands in the NIR, corresponding
to the 4F3/2 → 4I9/2, 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions. The
spectra are normalized to the emission band maximum of each
recorded transition.
4
4
three metal-centered emission bands ascribed to F3/2 → I9/2
4F3/2 → 4I9/2, and 4F3/2 → 4I9/2 transitions, which are similar to
those recently reported for exclusively phosphonate ester based
Nd3+ model complexes.12 The μ-PL spectra of the Dy3+
complexes 3, 7, 11, 15, 25, and 29 only exhibit broad ligand
emission in a range from 400 to 800 nm that completely covers
possibly detectable metal-centered emissions in this region.
Unexpectedly, no metal-centered emission at around 1550
nm56 could be detected for one of the present Er3+ complexes.
already reported, the exclusively phosphonate ester supported
Nd3+ complexes exhibit a splitting feature in their emission
spectra.12 For the F3/2
→
4I9/2 transition, the most
4
pronounced splitting is observed of about 22 nm. For the
4
4
4
4F3/2 → I11/2 and F3/2 → I13/2 transitions, splittings of 28
and 47 nm were recorded, respectively. The splitting was
tentatively assigned to electrostatic ligand−metal orbital
interactions which influence the electrons of the Nd3+ ion, as
was previously reported for a similar splitting feature observed
for Eu3+ clusters.57 In particular, the splitting was attributed to
an electrostatic interaction of Eu3+-centered f orbitals and O-
centered orbitals of adjacent oxo ligands.
EXPERIMENTAL SECTION
■
General Information. All manipulations involving air- and
moisture-sensitive compounds were carried out under an argon
atmosphere using Schlenk techniques or handled in an argon
glovebox. Solvents were dried over Na or K metal or Na/K alloy
and were used freshly distilled. Starting materials were purchased
commercially and were used as received, unless stated otherwise.
58
Dy(OiPr)3 and S1−S3 were prepared according to literature
CONCLUSIONS
procedures.12 Filtering of moisture-sensitive compounds was carried
out with self-made filter cannulas assembled from Whatman fiberglass
filters (GF/B, 25 mm), which were applied with Teflon tape to Teflon
cannulas. Flash chromatography was performed with an Interchim
PuriFlash XS 520Plus device using PF-30SIHP-F0020 and -F0040
columns (CV = column volumes). For TLC, precoated Macherey-
Nagel Alugram Xtra SIL G/UV254 plates were used. NMR
experiments were performed with Varian 400 and 500 MHz
spectrometers, and spectra were processed with MestReNova
■
A modified synthetic protocol of diethyl 4-azidobenzylphosph-
onate (L1) as well as the preparation of the three novel azide-
functionalized ligand platforms diisopropyl 4-azidobenzyl-
phosphonate (L2), diisopropyl ((4′-azido-[1,1′-biphenyl]-4-
yl)-methyl)phosphonate (L3) and diisopropyl 4-azido-2,3,5,6-
tetrafluorobenzylphosphonate (L4) is presented. All ligands
exhibit an anomalous splitting of their symmetric and/or
asymmetric azide stretching modes that can be ascribed to
Fermi interactions with combination tones including the
symmetric N3 or C−N stretching mode and other low-lying
(v11.0.4-18998, Mestrelab Research S.L.). H and 13C NMR spectra
1
were referenced relative to TMS using the residual solvent signals as
internal standards.59 DOSY-NMR experiments were recorded on a
Varian 400 MHz spectrometer. Sample spinning was deactivated
during the measurements, and the temperature was set and controlled
at 298 K. All DOSY experiments were performed using the Dbppste
pulse sequence.60 DOSY transformation and processing was carried
out with MestReNova (v11.0.4-18998, Mestrelab Research S.L.).
Molecular weight estimation was was carried out with the software
(v1.3) provided by Bachmann.52 IR spectra were recorded with a
Bruker diamond probe ATR IR spectrometer. Elemental analyses
were performed using a HEKAtech Euro EA-CHNS elemental
analyzer. For analyses, samples were prepared in tin cups with V2O5 as
an additive to ensure complete combustion. ESI mass spectra were
recorded on a Finnigan LCQDeca (ThermoQuest) or a MicrOTOF
frequencies.45 From reactions of POSS cages with lanthanide
R
isopropoxides lanthanide precursor complexes with the general
solution-state structure [(Ln{RPOSS})2(THF)m] (P1−P6)
(Ln = La, Nd, Er; R = iBu, Ph; m = 0, 1) are obtained that
show unique coordination motifs in the solid state depending
on their crystallization conditions in donating or nondonating
s o l v e n t s .
A
c o m p l e x o f t h e f o r m
[Er4{iBuPOSS}2{iBuPOSSSi−OH}2(μ4-O)] is obtained from a
solution of P3 in dry pentanes at −20 °C. The incorporation of
a water molecule results inthe coordination of a μ4-oxo ligand
in the Er4 plane with two of the four POSS cages being
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(Bruker Daltonics) device. GC-MS mass spectra were obtained using
a Thermo Scientific DSQ II device. Absorption spectra were acquired
with a Hamamatsu C11347 device in the range of 250−850 nm. Two
different setups were used for room-temperature μ-PL experiments.
The Dy complexes were excited at 355 nm, and the PL signal was
guided through a glass fiber to an OceanOptics USB2000+
spectrometer. For Nd complexes (λexc = 750 nm), the laser was
focused on the complexes by a microscope objective (NA = 0.7) to a
spot size of ∼1 μm. The same objective was used to collect the
emission from the complexes. The luminescence was spectrally
filtered by a 0.75 m focal length spectrometer equipped with a liquid
N2 cooled InGaAs detector. The μ-PL spectra were taken with a laser
excitation power of 50 mW.
Crystallographic Details. X-ray diffraction experiments were
performed with either a STOE IPDS 2 diffractometer with an image
plate (Ø 34 cm) using a Mo-GENIX source (λ = 0.71073 nm) or a
STOE StadiVari instrument with DECTRIS PILATUS 200 K using a
Cu-GENIX source (λ = 1.54186 nm). All structures were solved using
direct methods (SHELXT)61 and refined against F2 using the full-
matrix least-squares methods of SHELXL62 within the SHELXLE
GUI63 or with OLEX2.64 Additional programs used for structural
analysis included Mercury65 and Platon.66 CCDC 2050303−2050310
contain the supplementary crystallographic data for this paper.
https://pubs.acs.org/10.1021/acs.inorgchem.1c00266
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The federal state of Hesse, Germany, is kindly acknowledged
for financial support of the SMolBits project within the
LOEWE program.
DEDICATION
■
Dedicated to Prof. Dr. Wolfgang Kaim on the occasion of his
70th birthday.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00266.
■
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AUTHOR INFORMATION
Corresponding Author
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T.; Heinrich, A. J.; Lutz, C. P. Reading and writing single-atom
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Rudolf Pietschnig − Institute of Chemistry and Center for
Interdisciplinary Nanostructure Science and Technology
(CINSaT), University of Kassel, 34132 Kassel, Germany;
orcid.org/0000-0003-0551-3633; Email: pietschnig@
uni-kassel.de; www.uni-kassel.de/go/hym
(8) Hasegawa, Y.; Kimura, Y.; Murakoshi, K.; Wada, Y.; Kim, J.-H.;
Nakashima, N.; Yamanaka, T.; Yanagida, S. Enhanced Emission of
Deuterated Tris(hexafluoroacetylacetonato)- neodymium(III) Com-
plex in Solution by Suppression of Radiationless Transition via
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Ingo Koehne − Institute of Chemistry and Center for
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(CINSaT), University of Kassel, 34132 Kassel, Germany;
orcid.org/0000-0001-7294-5095
Miriam Gerstel − Institute of Nanostructure Technologies and
Analytics (INA) and CINSaT, University of Kassel, 34132
Kassel, Germany
Clemens Bruhn − Institute of Chemistry and Center for
Interdisciplinary Nanostructure Science and Technology
(CINSaT), University of Kassel, 34132 Kassel, Germany
Johann P. Reithmaier − Institute of Nanostructure
Technologies and Analytics (INA) and CINSaT, University
of Kassel, 34132 Kassel, Germany
Mohamed Benyoucef − Institute of Nanostructure
Technologies and Analytics (INA) and CINSaT, University
of Kassel, 34132 Kassel, Germany
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