Tetrakis-(4-thiyphenyl)methane: Origin of a reversible 3D-homopolymer

Article abstract of DOI:10.1002/adfm.201302483

The efficient syntheses of tetrakis(thiophenol)methane and of a new poly(disulfide) hyper-crosslinked polymer based on the former monomer are described. Controlled de-polymerization as well as surface post- functionalization are successfully conducted on this novel material. Direct prove of post-functionalization is obtained through solid-state fluorescence emission spectroscopy, and the number of unreacted thiol-functions on the surface of the polymeric material is indirectly quantified by de-polymerization of the post-functionalized material. The efficient generation of a hyper-crosslinked poly(disulfide) is described. After high yielding synthesis of the monomer tetrakis-(4-thiylphenyl)methane, the polymeric material is obtained in excellent yield under mild reaction conditions. Combination of controlled de-polymerization under bio-compatible conditions and postfunctionalization gives access to the number of free termini at the surface.

Full text of DOI:10.1002/adfm.201302483

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Tetrakis-(4-thiyphenyl)methane: Origin of a Reversible
3D-Homopolymer
Laure Monnereau, Martin Nieger, Thierry Muller,* and Stefan Bräse*
dendrimers[12] due to the tetrahedral
The efficient syntheses of tetrakis(thiophenol)methane and of a new
poly(disulfide) hyper-crosslinked polymer based on the former monomer are
described. Controlled de-polymerization as well as surface post-functional-
ization are successfully conducted on this novel material. Direct prove of
post-functionalization is obtained through solid-state fluorescence emission
spectroscopy, and the number of unreacted thiol-functions on the surface
of the polymeric material is indirectly quantified by de-polymerization of the
post-functionalized material.
geometry and the rigidity imposed by the
core. One can already cite triazoles,[2b,13]
boroxine rings,[14] imines,[15] or simple
carbon–carbon bonds[16] as classical junc-
tions for the generation of the deriving
polymers. The targeted TPM-SH mon-
omer 2 for the envisioned poly(disulfide)
HCP 3 has already been described in a
Japanese patent with a global yield of 10%,
obtained by sulfonylation of the original
TPM, and further reduction in the pres-
ence of LiAlH4 or NaBH4.[17] Starting
from tetrakis(bromophenyl)methane (1, TPM-Br),[10] the cor-
responding tetrakis-(thiophenol)methane (2, TPM-SH) was
prepared in two steps, including a nucleophilic substitution in
dimethylacetamide (DMAc) followed by reduction with sodium
(Scheme 1 ).[18] The synthesis occurs under milder conditions
and leads in high yield to the desired compound 2 in gram
scale. The molecular structure of 2 resulting from an X-ray dif-
fraction study is depicted in Figure 1 .[19] The crystal structure
exhibits six molecules in the unit cell showing two types of
crystallographic symmetry: S4-symmetry (1/4 molecule in the
asymmetric unit) and C2-symmetry (1/2 molecule in the asym-
metric unit), which may be attributed to weak S–S interactions
and hydrogen bonds between the thiol functions of neigh-
boring molecules (3.47–3.87 Å).
1. Introduction
Among the wide variety of macromolecular architectures that
can be generated, a sub-class comprising connecting links
deriving from sulfur-based chemistry can be distinguished.[1]
These polymers have gained interest due to the propensity of
the sulfur atom to undergo various oxidation states,[2] which
offers the possibility of further modifications, leading to an
alteration in the physical behavior of the so-generated macro-
molecular assemblies.[3] Although sulfoxyde –SO-,[2b,4] sulfonic
acid –SO3H,[5] thioester,[6] thiocarbonate, and thiourethane[7]
are already largely employed,[8] the use of the sulfide linkage
remains marginal.[9]
Herein, we report the synthesis of the first reversible three-
dimensional polymer based on a disulfide linkage and its
post-functionalization.
2.2. Generation of the 3D-Homopolymer
2. Results and Discussion
Tetrakis-thiophenol 2 was further oxidized to the corresponding
disulfide HCP 3 in ethyl acetate in presence of hydrogen per-
oxide catalyzed by sodium iodide (Scheme 2 ).[20] Considering
the insolubility of the generated polymer in all the common
solvents, a model compound study was initiated to monitor
the reaction. Commercially available p-thiocresol was oxi-
dized to the corresponding disulfide, and FTIR spectra were
recorded (see Supporting Information), showing the extinction
2.1. Preparation of the Monomer
Tetraphenylmethane (TPM)[10] has been largely used in the
generation of hyper-crosslinked polymers (HCPs)[11] and
Dr. L. Monnereau, Dr. T. Muller, Prof. S. Bräse
Institute of Organic Chemistry
KIT-Campus South, Fritz-Haber Weg 6
76131 Karlsruhe, Germany
E-mail: thierry.muller@kit.edu; braese@kit.edu
Dr. M. Nieger
Laboratory of Inorganic Chemistry
Department of Chemistry, P.O. Box 55
FI-00014 University of Helsinki, Finland
Prof. S. Bräse
Institute of Toxicology and Genetics
KIT, Campus North, Hermann-von-Helmholtz-Platz 1
76344, Eggenstein-Leopoldshafen, Germany
DOI: 10.1002/adfm.201302483
Scheme 1. 2-Step synthesis of tetrakis-(4-thiyphenyl)methane 2.
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Figure 1. Molecular structures of 29. Left: the molecule with crystallo-
graphic C2-symmetry. Right: the molecule with crystallographic S4-sym-
metry. Displacement parameters are drawn at 50% probability level (see
Supporting Information).
Figure 2. Solid-state 13C CP/MAS NMR of 2, 3, and 5a. * indicates spin-
ning sidebands.
of the thiol stretching band in parallel to the appearance of the
disulfide band (confirmed by 1H NMR).
Applying these reaction conditions to TPM-SH 2, a pre-
cipitate formed nearly instantaneously. The topology of 3 was
then investigated by FTIR, solid-state 13C cross-polarization
with magic angle spinning (13C CP/MAS NMR), differen-
tial scanning calorimetry (DSC), thermogravimetric analysis
(TGA), powder-X-ray diffraction (PXRD), and scanning elec-
tron microscopy (SEM) analysis. No stretching band of the
thiol function was detected in the FTIR spectra of 3, whereas
its solid-sate 13C CP/MAS NMR spectroscopic signature was
similar to the one of 2, with the presence of broader signals
in the case of 3, inherent to its polymeric structure (Figure 2 ).
Elemental analysis was in good accordance with the theoretical
values, discarding any hypothesis of a higher oxidation state of
the thiophenol moiety, all these analysis confirming the struc-
ture of 3. The DSC thermogram showed no glass transition
temperature (Tg) which is in accordance with a high number
of crosslinks.[4] No fusion peak was recorded either and decom-
position starts around 200 °C. TGA data confirm that 3 possess
a good thermal stability by achieving complete loss of sulfur
content at 750 °C in a nitrogen atmosphere and the PXRD pat-
terns evidenced some area of cristallinity (see Supporting Infor-
mation). The SEM analysis of 3 reveals condensed spherical
particles, with a non-uniform size distribution and diameters
ranging from 0.5 to 1.7 μm (Figure 3 ). Investigation of nitrogen
adsorption and desorption at 77.3 K indicates that 3 is a non-
porous network, exhibiting a negative value for its surface area
(Brunauer-Emmett-Teller method) whatever the reaction condi-
tions employed for its generation (see Supporting Information).
2.3. De-Polymerization
With 3 in hands, the physical and chemical properties of the
HCP were investigated by firstly exploring its controlled de-
polymerization. Disulfide bonds are usually considered as
weak covalent links as they can be easily cleaved by appropriate
reagents.[21] Two main routes were explored in depth, using
i) dithiothreitol (DTT) in slightly basic conditions,[22] or ii) the
water-soluble tris(2-carboxyethyl)phosphine (TCEP) at neutral
pH[23] (Scheme 2). Under both conditions, the monomer 2 was
recovered after 16 h and 72 h, respectively, in almost quantita-
tive yields, showing the reversible character of this 3D-polymer.
2.4. Post-Functionalization
Considering the size of the polymeric particles observed by
SEM, a non-negligible amount of unreacted termini of the
Scheme 2. Formation and reduction of the disulfide HCP 3.
Figure 3. SEM pictures of 3.
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under the conditions used to obtain 5a. After washing with
water, the intense yellow-colored insoluble material 5b showed
two carbonyl stretching bands in FTIR, whereas nor change in
the thermal behavior nor structural rearrangement compared
to 3 were observed by DSC and TGA analysis and PXRD study,
respectively. Solid-state fluorescence emission spectroscopy of
HCP 5b displayed two emissive bands located around 440 and
540 nm, respectively, when the sample was irradiated at 350 nm
(see Supporting Information). The photo-physical properties of
3 and 4b were also investigated, and no fluorescence emission
was detected in the solid-state for both of them, proving that
post-functionalization was properly performed. The fluores-
cence intensity of 5b was evaluated by a solid-state emission
quantum yield (PLQY) of 0.014, a relatively low value eventually
due to fluorescence quenching by neighboring molecules.[26]
Scheme 3. Post-functionalization of 3 with maleimides 4a and 4b.
monomer should be present at the surface of the particles.
Post-functionalization[24] was therefore explored next, taking
advantage of the reversible nature of the S–S bond to recover
the discrete molecules and thus quantifying the number of
the modified end groups. To facilitate the post-functionaliza-
tion, reaction conditions affording no by-products and high
yields were sought and firstly optimized on a discrete molecule
model. Nucleophilic addition was envisaged with maleimide 4a,
as the latter is an excellent substrate for the Thia-Michael reac-
tion. Quantitative conversion between p-thiocresol and 4a was
observed in DMAc in the presence of a small amount of acetic
acid, at room temperature or at 100 °C. Upon completion of
the reaction, both maleimide carbonyls became unequivalent,
which was confirmed in FTIR by a split of the initial carbonyl
stretching band into two distinct signals.
Reaction of HCP 3 with 4a in DMAc at 100 °C for 48 h
afforded a pale yellow insoluble polymer (Scheme 3 ), whose
FTIR spectrum showed a band relative to a carbonyl function
(see Supporting Information). At this stage, however, there was
no convincing evidence for post-functionalization of HCP 3 as
some remaining starting maleimide could have been trapped
within 3 during its swelling in DMAc. Moreover, solid-sate 13C
CP/MAS NMR did not show any signals related to a functional-
ized maleimide (Figure 2, 5a), and the DSC and TGA analysis
did not indicate any difference in the thermal behavior of the
post-functionalized polymer compared to the starting HCP 3.
In order to prove post-functionalization, de-polymerization
conditions (i) depicted in Scheme 2 were applied to HCP 5a.
The reaction sequence was conducted on gram scale in order
to detect also minor side products. After the efficient cleavage
of the polymer, only mono-functionalized TPM-SH bearing one
maleimide unit (S6, 25%) and tetrakis-(thiyphenyl)methane
(2) (75%) were isolated, which corresponds to 6% of free thio-
phenol functions present in the initial 3D-polymer 3. One
can assume that the amount of free thiol functions is actually
higher taking into account inaccessible free thiophenol func-
tions within the polymer matrix. The total ratio was, however,
still beneath the detection threshold of solid-state 13C CP/MAS
NMR (Figure 2, 5a).
3. Conclusion
We have reported the synthesis of a tetrahedral thiophenol
monomer that was further polymerized under mild condi-
tions and short reaction times to afford a poly(disulfide)
hyper-crosslinked polymer in excellent yields. Two conditions
were used to decompose this macromolecular architecture in
a controlled fashion and to recover almost quantitatively the
composing monomer. Post-functionalization of the polymeric
material was realized using the Thia-Michael reaction with
maleimide moieties. The degree of functionalization was indi-
rectly quantified taking advantage of the de-polymerization of
5a and post-functionalization was directly proven on polymer
5b through solid-state fluorescence emission spectroscopy. To
the best of our knowledge, this contribution exemplifies, for
the first time, a precise estimation of end termini, a feature
usually excluding from the characterization of such polymers
due to the quasi impossible character to access to this type of
information.
The presented, completely reversible[27] HCP, based on
disulfide bridges, combines the robustness of hyper-crossed-
linked polymer with the dynamic behavior of supramolecular
architectures held together by H- or coordination- bonding and
opens the door to innovative applications such as specific drug
transport[21b] and release or rapid access to functional sulfur tri-
pods for surface coatings. We are currently exploring some of
these options.
4. Experimental Section
Materials: NMR spectra were recorded on a Bruker AM 300 (300 MHz)
as solutions in CDCl3. Chemical shifts are expressed in parts per million
(ppm, δ) downfield from tetramethylsilane (TMS) and are referenced
to CHCl3 (7.26 ppm), as internal standard. All coupling constants (J)
are absolute values. The description of signals includes: s = singlet,
d = doublet, t = triplet, m = multiplet, and dd = doublets of doublets.
The spectra were analyzed according to first order. The signal structure
in 13C NMR was analyzed by DEPT. The solid-state NMR spectra were
measured on a Bruker Avance 400 spectrometer operating at 100.6 MHz
for 13C. The 13C CP/MAS experiments were carried out at MAS rates
of 14 kHz using densely packed powders of the compounds in 4 mm
ZrO2 rotors. The 1H Π/2 pulse was 4 μs and decoupling was used during
the acquisition. The Hartmann-Hahn condition was optimized with
Aiming at a straightforward characterization method for
monitoring the outcome of a post-functionalization reaction,
fluorescence was investigated next. Post-functionalization was
thus realized with the fluorescent dye 4b, reported to operate
as an efficient on/off probe for the detection of thiol moieties
(Scheme 3).[25] Nucleophilic addition of 3 to 4b was conducted
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adamantane at a rotational speed of 5 kHz. All spectra were measured
using a contact time of 1.5 ms and a relaxation delay of 10.0 s, and 6000
FIDs were accumulated. The microstructure of 3 was investigated by
scanning electron microscopy (SEM, Zeiss Supra S5) after metallization.
The dynamic DSC was measured on a METTLER Toledo DSC 30, in a
sealed aluminium 40 μL pan under argon atmosphere. The obtained
data was analyzed electronically with the software program STAR eSW
8.10 and plotted in a diagram. The enthalpy changing ΔH [mW] was
plotted against the temperature (−50 °C to 250 °C) of the heating rate of
10 °C min−1. Infrared spectra were recorded with a FT-IR Bruker IFS 88
spectrometer with OPUS software using the attenuated total reflection
technique (ATR). The deposit of the absorption band was given in
wave numbers ν in cm−1. The forms and intensities of the bands were
characterized as follows: vs = very strong 0–10% T, s = strong 11–40% T,
m = medium 41–70% T, w = weak 71–90% T, vw = very weak, 91–100%
T, br = broad. MS (EI) (electron impact mass spectrometry) was
performed by using a Finnigan MAT 90 (70 eV). The EA measurements
were performed on an Elementar vario MICRO device using a Sartorius
M2P precision balance. The following abbreviations were used: calc.
= calculated data, found = measured data. TGA was performed under
N2 on a Shimadzu TGA-50 thermogravimetric analyzer, with a heat rate
of 10 °C min−1. Emission and excitation spectra in the solid-state were
measured with a Horiba Scientific FluoroMax-4 spectrofluorometer using
a JX monochromator and a R928P PMT detector. For the determination
of PLQY, an absolute PL quantum yield measurement system from
Hamamatsu Photonics was used. The system consisted of a photonic
resulting material was dried at 100 °C under vacuum during 24 h
(m = 1.122 g, 93%). 13C CP/MAS NMR (400 MHz, δ): 68.1 (Cquart
,
C(Ar)4), 137.0 (Cquart,CS, CH), 148.0 (Cquart, CC) ppm; IR (ATR): ν =
3073 (vw), 3020 (vw), 2567 (S-H, vw), 1586 (vw), 1561 (vw), 1482
(w), 1398 (w), 1307 (vw), 1271 (vw), 1239 (vw), 1191 (vw), 1101 (vw),
1081 (vw), 1012 (w), 951 (vw), 912 (vw), 809 (m), 760 (vw), 737 (vw),
695 (vw), 631 (vw) cm−1 (m). Anal. calcd for C25H16S4 + Na: C 64.21,
H 3.45, S 27.42; found: C 64.48, H 3.61, S 27.18.
Conditions for Depolymerization: Scheme 2, i) To a stirred suspension of
3 (0.280 g, 0.6 mmol, 1.0 equiv.) in THF (5 mL) was added DTT (0.385 g,
2.5 mmol. 4.0 equiv.) in a buffer solution (V = 5 mL, pH 8) and the
mixture was shaken at room temperature until complete dissolution of
the solid, usually after 16 h. The mixture was then diluted in water and
CH2Cl2 and acidified with HCl 2M until pH 1 was reached. The organic
phase was separated and the aqueous phase was extracted with CH2Cl2
(2 × 20 mL). The combined organic layers were dried over Na2SO4, filtrated
and evaporated to dryness. Excess of DTT was removed by column
chromatography to afford 2 (m = 0.252 g, 90%). Scheme 2, ii) To a stirred
suspension of 3 (0.308 g, 0.7 mmol, 1.0 equiv.) in THF (5 mL) was added
TCEP (0.866 g, 3.0 mmol, 4.0 equiv.) in phosphate buffer solution (V =
5 mL, pH 7.2) and the mixture was shaken at room temperature until total
dissolution of the mixture, usually three days. The mixture was then diluted
in water and CH2Cl2 and acidified with conc. HCl (a few drops). The
organic phase was separated and the aqueous phase was extracted with
CH2Cl2 (2 × 20 mL). The combined organic layers were dried with Na2SO4,
filtrated, and evaporated to dryness to afford 2 (m = 0.273 g, 88%).
Post-Functionalization of 3 with Maleimide 4a and Generation of 5a and
S6: To a suspension of 3 (1.000 g, 2.2 mmol, 1.0 equiv.) in abs. DMAc
(20 mL) acidified by a few drops of acetic acid was added maleimide
(0.865 g, 8.9 mmol, 4.0 equiv.), and the mixture was shaken at 100 °C for
two days. After filtration and extensive washing with water, the resulting
polymer was dried at 100 °C at atmospheric pressure to afford 5a
(m = 0.805g). 13C CP/MAS NMR (400 MHz, δ): 68.3 (Cquart, C(Ar)4),
138.0 (Cquart,CS, CH), 149.7 (Cquart, CC) ppm; IR (ATR): ν = 3054 (vw),
3020 (vw), 2306 (vw), 2163 (vw), 2050 (vw), 1915 (vw), 1786 (vw),
1728 (w), 1586 (vw), 1562 (vw), 1482 (m), 1398 (w), 1339 (vw),
1307 (vw), 1270 (vw), 1219 (vw), 1191 (w), 1156 (vw), 1117 (vw),
1081 (w), 1012 (m), 951 (vw), 913 (vw), 809 (m), 738 (w), 695 (w), 631 (w),
600 (vw), 549 (w) cm−1 (m). The post-functionalized polymer 5a
(0.805 g, 1.8 mmol, 1.0 equiv.) was suspended in THF (50 mL) and DTT
(1.383g, 9.0 mmol, 4.0 equiv.) in a buffer solution (V = 50 mL, pH 8) was
added. The resulting mixture was then shaken at room temperature until
complete dissolution of the solid, usually 16 h. The mixture was then
diluted in water and CH2Cl2, and acidified with 2 M HCl. The organic
phase was separated and the aqueous phase was extracted with CH2Cl2
(2 × 20 mL). The combined organic layers were dried with Na2SO4,
filtrated and evaporated to dryness. The crude mixture was purified by
column chromatography (SiO2, CH2Cl2/cyclohexane: 50/50) to afford 2
(0.556 g, 75%) and mono-functionalized TPM S6 (0.233 g, 25%).
Post-Functionalization of 3 with the Fluorescent Dye 4b and Generation
of 5b: To a stirred suspension of 3 (0.200 g, 0.4 mmol, 1.0 equiv.) in
abs. DMAc (20 mL) acidified by a few drops of acetic acid was added
1-(8H-benzo[5,6]chromeno[4,3b]quinolin-11-yl)-1H-pyrrole-2,5-dione
(0.506 g, 1.3 mmol, 3.0 equiv.) and the mixture was shaken at 100 °C
for two days. The mixture was then filtrated and extensively washed
with water (100 mL), acetone (50 mL) and CH2Cl2 (50 mL) to afford
5b. IR (ATR): ν = 3055 (vw), 1787 (vw), 1721 (w), 1593 (vw), 1511 (vw),
1483 (w), 1437 (vw), 1396 (w), 1365 (w), 1225 (vw), 1169 (w), 1081
(vw), 1012 (w), 911 (vw), 810 (m), 749 w), 695 (vw), 630 (vw), 548 (w),
521 (w) cm−1 (m). Anal. found: C 67.48, H 3.81, S 20.21, N 1.60.
CCDC 929924 contains the supplementary crystallographic data for
this paper.[28]
multichannel analyzer PMA-12,
a model C99200–02G calibrated
integrating sphere, and monochromatic light source L9799–02
a
(150 W Xe- and Hg-Xe-lamps). Data analysis was done with the PLQY
measurement software U6039–05, provided by Hamamatsu Photonics.
Solvents, reagents and chemicals were purchased from Sigma-Aldrich,
ABCR and Acros Organics. All solvents, reagents and chemicals were
used as purchased unless stated otherwise. Tetrakis(4-bromophenyl)
methane (1)[23] and 1-(8H-benzo[5,6]chromeno[4,3b]quinolin-11-yl)-1H-
pyrrole-2,5-dione (4)[24] were obtained according to literature procedures.
Synthesis of 2: Tetrakis(bromophenyl)methane 1 (3.143 g, 4.6 mmol,
1.0 equiv.) and NaSiPr (5.388 g, 49.4 mmol, 10.0 equiv.) were suspended
under an inert atmosphere (Ar) in dimethylacetamide (DMAc) (40 mL)
and the mixture was heated for 16 h at 100 °C. Subsequently, Na (2.272 g,
98.8 mmol, 20.0 equiv.) was added under vigorous stirring and the mixture
was heated for an additional 24 hour-period. The mixture was carefully
hydrolyzed with H2O (250 mL) and tert-butyl methyl ether (MTBE) (150 mL)
was added before acidifying the mixture with conc. hydrochloric acid
(pH < 1). The layers were separated and the aqueous layer was
extracted with MTBE (2 × 75 mL). The combined organic layers were
washed with H2O (5 × 100 mL) and dried over Na2SO4. After filtration,
the crude mixture was evaporated before being subjected to column
chromatography (MTBE/cyclohexane: 50/50) to afford the title compound
as a light yellow solid (1.874 g, 90%). Rf = 0.50 (dichloromethane/
1
cyclohexane: 50/50). H NMR (300 MHz, CDCl3, δ): 3.41 (s, 4 H), 7.00
(AA’BB’, 2J = 8.4 Hz, 8 H), 7.14 (AA’BB’, 2J = 8.4 Hz, 8 H); 13C NMR
(100 MHz, CDCl3, δ): 63.5 (Cquart, C(Ar)4), 128.8 (CH), 131.4 (Cquart,CS),
131.6 (CH), 143.8 (Cquart, CC); IR (ATR): ν = 3020 (vw), 2919 (vw), 2552
(S-H, vw), 2045 (vw), 1916 (vw), 1647 (vw), 1586 (w), 1561 (vw), 1483 (m),
1400 (w), 1310 (vw), 1262 (w), 1191 (w), 1118 (vw), 1100 (m), 1013 (m),
947 (w), 900 (w), 836 (vw), 806 (m), 732 (w), 632 (w), 542 (m), 523 (m),
515 (m), 444 (vw), 402(vw) cm−1 (m); EIMS m/z (%): 448 (68) [M]+,
416 (26) [M–S]+, 339 (100) [M–C6H5S]+, 307 (36) [M–C6H5S2]+, 273 (23)
[M–C6H7S3] +. Anal. calcd for C25H20S4 + 0.3 CH2Cl2 + 0.1 C6H12: C 64.46,
H 4.55, S 26.58; found: C 64.53, H 4.55, S 26.57.
Synthesis of 3: To
a stirred suspension of tetrakis(thiophenyl)
methane 2 (1.319 g, 2.9 mmol, 1.0 equiv.) in ethyl acetate (150 mL)
were added NaI (0.004 g, 0.02 mmol, 0.01 equiv.) and aq. 30% H2O2
(2.00 mL, 29.4 mmol, 10.0 equiv.) and the mixture was shaken at room
temperature for two hours. Saturated aqueous Na2S2O3 (15 mL) was
added, and the resulting solid was filtrated and extensively washed with
water (50 mL), EtOH (50 mL), MeOH (50 mL), then with EtOAc (3 ×
50 mL), THF (50 mL), acetone (50 mL), and CH2Cl2 (50 mL). The
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
This project is part of the JointLab IP3, a joint initiative of KIT and
BASF. The authors gratefully acknowledge the help of Dr. Birgit Gordalla
and Dr. Gisela Guthausen during CP/MAS NMR measurements and
thank Dipl.-Chem. Daniel Volz for recording fluorescence spectra, Udo
Geckle for providing SEM images, and Dipl.-Chem. Mark Busch for the
illustration of this contribution.
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Received: July 24, 2013
Published online: September 17, 2013
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