Microporous, tetraarylethylene-based polymer networks generated in a reductive polyolefination process

Article abstract of DOI:10.1039/c4tc02664k

Microporous Polymer Networks (MPNs) that contain aggregate-induced emission (AIE) active tetraphenylethylene (TPE) or other tetraarylethylene units have been generated in a reductive polyolefination process starting from four different tris(α,α-dichlorobenzyl)arene derivatives with dicobalt octacarbonyl or chromium(ii)acetate as reductive olefination agents. Microporosity with moderately high BET surface areas up to 500 m² g^-1 could be combined with high solid state photoluminescence quantum yields up to 25.3% for polymer network P4. This unique combination should be promising for applications as optical solid state sensors, especially for MPNs with electron-deficient triazine core units.

Full text of DOI:10.1039/c4tc02664k

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Materials Chemistry C
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Microporous, tetraarylethylene-based polymer
networks generated in a reductive polyolefination
process†
Cite this: J. Mater. Chem. C, 2015, 3,
1582
a
a
b
a
*
E. Preis, W. Dong, G. Brunklaus and U. Scherf
Microporous Polymer Networks (MPNs) that contain aggregate-induced emission (AIE) active
tetraphenylethylene (TPE) or other tetraarylethylene units have been generated in reductive
a
polyolefination process starting from four different tris(a,a-dichlorobenzyl)arene derivatives with dicobalt
octacarbonyl or chromium(II)acetate as reductive olefination agents. Microporosity with moderately high
BET surface areas up to 500 m2 gꢀ1 could be combined with high solid state photoluminescence
quantum yields up to 25.3% for polymer network P4. This unique combination should be promising for
applications as optical solid state sensors, especially for MPNs with electron-deficient triazine core units.
Received 21st November 2014
Accepted 17th December 2014
DOI: 10.1039/c4tc02664k
www.rsc.org/MaterialsC
poly-TPE networks in a reductive olenation process starting
from tris(a,a-dichlorobenzyl)arene monomers with dicobalt
octacarbonyl27,28 or chromium(II)acetate19,29 as condensing
agents. In our target networks the tetraarylethylene units
(Scheme 1) are linked together via 1,3,5-substituted benzene or
2,4,6-substituted 1,3,5-triazine cores. Jiang's MPNs, in contrast,
contain TPE units with the olenic double bonds connected via
biphenyl units.25
Introduction
Microporous Polymer Networks (MPNs) comprise a rapidly
growing area of materials science towards future mass appli-
cations in catalysis, gas storage/separation or as active porous
materials for organic electronic devices and sensors.1–7 For such
mass applications inexpensive synthesis procedures for MPN
generation without use of noble metal catalysts and under mild
reaction conditions are of particular appeal.8–18 Tetraphenyl-
ethylene (TPE) and related repeat units represent versatile
building blocks for the rational design of MPNs due to their
rigid, three-dimensional structure and interesting optical
properties, especially a distinct aggregate-induced emission
(AIE). Linear poly-TPEs (as representative example poly(1,4-
The synthesis of the monomers M1–M4 is depicted in
Scheme 2 (for detailed information see ESI†). The general
synthesis route of the polymer networks P1–P4 is outlined in
Scheme
3 for P1 as representative example. 1,3,5-Tri-
benzoylbenzene was prepared according to a reported proce-
dure.30 The until now unknown, corresponding 1,3,5-
tris(a,a-dichlorobenzyl)benzene MCl1 was generated by reac-
tion with phosphorus pentachloride (PCl5) in chlorobenzene as
solvent (Scheme 3). Aer removal of both phosphorus oxy-
chloride (POCl3) by distillation and excess PCl5 by sublimation,
respectively, the resulting oily product was used without further
purication, similar to the synthesis of bifunctional analogues
as 1,3-bis(a,a-dichlorobenzyl)benzene (made for poly-
olenation into linear coupling products).31 1H NMR spectros-
copy indicated the presence of pure 1,3,5-tris(a,a-
dichlorobenzyl)benzene MCl1 (Fig. 1). The singlet of the 1,3,5-
substituted benzene core is, thereby, shied from 8.40 ppm to
7.83 ppm.
phenylene-1,2-diphenylvinylene) DP-PPV) was rst described 30
19
¨
years ago by Horhold and co-workers. In the following
decades, a rather broad variety of TPE-based monomers, olig-
omers and (co)polymers have been extensively investigated by
Tang and co-workers thereby documenting their broad and very
promising application potential in materials science and
biology.20–24 The rst synthesis of MPNs that contain TPE
building blocks and show a pronounced AIE effect has been
reported by Jiang and co-workers.25,26 Their MPNs have been
generated in a Yamamoto-type aryl-aryl coupling starting from
tetrabromo-TPE with Ni(COD)2 as coupling reagent. In this
study, we now introduce the synthesis of structurally related
For the synthesis of monomer M2, 1,3,5-tris(4-bromophenyl)
benzene was prepared following a literature procedure.32 Subse-
quent nucleophilic substitution with copper(I) cyanide yielded
1,3,5-tris(4-cyanophenyl)benzene (Scheme 2); the rather low yield
is related to an ongoing trimerization of nitrile groups into
triazine-cored byproducts.33 Addition of phenylmagnesium
bromide, followed by hydrolysis with diluted hydrochloric acid
afforded 1,3,5-tris[4-benzoylphenyl]benzene. The corresponding
a
¨
Bergische Universitat Wuppertal, Macromolecular Chemistry Group (buwmakro) and
Institute for Polymer Technology, Gauss-Str. 20, D-42119 Wuppertal, Germany. E-mail:
scherf@uni-wuppertal.de
b
¨
¨
¨
Westfalische Wilhelms-Universitat, Institut fur Physikalische Chemie, Corrensstr. 28,
¨
D-48149 Munster, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc02664k
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Scheme 1 Idealized structures of the polymer networks P1–P4.
Scheme 2 Synthesis of the monomers M1–M4.
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Scheme 3 Synthesis of the TPE-based MPN P1 in a reductive polyolefination.
disappearance of the carbonyl signal at 196.6 ppm and formation
of an aliphatic signal at 91.7 ppm (ar2CCl2) conrmed the
complete conversion.
The resulting, moisture sensitive hexachloro monomers
prepared from the tribenzoyl derivatives M1–M4 were used
without further purication, as already mentioned. Their
reductive polyolenation into the polymer networks P1–P4 was
accomplished by using two different reducing/condensing
agents: (i) dicobalt octacarbonyl Co2(CO)8 (ref. 27 and 28) or (ii)
chromium(II)acetate Cr2ac4,19,29 respectively. The insoluble
coupling products precipitated during reaction and were iso-
lated as yellowish colored solids. The polymeric products P1, P2
and P4 show yellow photoluminescence (PL) of different
intensity (P3: no PL detected, see Table 2) and were subse-
quently extracted with several organic solvents (methanol, ethyl
acetate, 1,4-dioxane, chloroform) and nally washed with
16,18
supercritical carbon dioxide sc-CO2
characterization.
prior to further
Fig. 1 1H NMR spectra of: (A) 1,3,5-tribenzoylbenzene M1 and (B)
1,3,5-tris(a,a-dichlorobenzyl)benzene MCl1 in C2D2Cl4.
The absence of carbonyl signals at 185–200 ppm (generated
by hydrolysis of –CCl2– functions) in the solid-state 13C CPMAS
spectra of the polymer networks P1–P4 (all made with Co2(CO)8
as reducing agent, for P4 see Fig. 2, for P1–P3 see Fig. S5a–c,
ESI†) clearly evidence a successful reductive polycondensation.
In case of polymer networks that contain triazine cores (P3 and
P4), an additional signal at about 170 ppm is observed (as for
1,3,5-tris[4-(a,a-dichlorobenzyl)phenyl]benzene MCl2 interme-
diate was then generated as described for 1,3,5-tris(a,a-dichlor-
obenzyl)-benzene, also resulting in an oily product. Furthermore,
the synthesis of tris(2,4,6-benzoyl)-1,3,5-triazine M3 was accom-
plished following an already published approach.34–36 Its subse-
quent transformation into 2,4,6-tris(a,a-dichlorobenzyl)-1,3,5-
triazine MCl3 was carried out as described for MCl1 and MCl2.
1
The H NMR spectrum of the corresponding hexachloro-deriva-
tive MCl3 exhibits two aromatic signals, a doublet at 7.62 ppm
(6H) and a multiplet at 7.36–7.30 (9H) thus demonstrating a
complete transformation of carbonyl into a,a-dichloromethyl
groups. The preparation of 2,4,6-tris(4-methylphenyl)-1,3,5-
triazine and its subsequent oxidation to the corresponding
tricarboxylic acid (Scheme 2) was achieved by applying literature
methods.37,38 The reaction of the corresponding acid chloride with
benzene aer Friedel–Cras resulted in monomer M4, 2,4,6-
tris(4-benzoylphenyl)-1,3,5-triazine, that was nally converted into
the corresponding 2,4,6-tris[4-(a,a-dichlorobenzyl)phenyl]-1,3,5-
triazine MCl4 as outlined for MCl1-MCl3. In the 13C NMR spec-
trum of the hexachloro-substituted monomer MCl4 complete
Fig. 2 Solid state 13C NMR spectra of monomer M4 and polymer
network P4.
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CPMAS spectrum of another P1 sample obtained with Cr2ac4 as
condensing agent exhibits two additional carbon signals at 195
ppm for leover carbonyl groups and an aliphatic signal at 83
ppm, but, interestingly, no signal around 170 ppm. Thereby, the
presence of leover carbonyl groups suggests incomplete
conversion. For this reason only samples made with Co2(CO)8
have been considered for further characterization.
The porosity of the polymer networks P1–P4 was studied by
nitrogen gas adsorption and desorption at 77 K. The networks
P1–P4 all show type I isotherms (see Fig. 4) with a signicant
amount of nitrogen adsorbed at low pressure, followed by a
much increased adsorption at higher pressure (P/P0 > 0.9)
Fig. 3 Solid state 13C NMR spectra of polymer network P1 made with attributed to gas condensation into voids between polymer
particles.39
Co2(CO)8 (blue) or Cr2ac4 (black) as reducing agent.
As oen observed for “so” polymers networks, the desorp-
tion-branch did not connect to the adsorption-branch at low
pressure.40 P3 additionally shows a distinct hysteresis, most
probably due to capillary condensation (see Fig. 4). Moderately
high BET surface areas of 462 m2 gꢀ1 (P1), 502 m2 gꢀ1 (P2), 200
m2 gꢀ1 (P3), and 475 m2 gꢀ1 (P4) could be determined for the
relative pressure range of 0.01–0.15 P/P0 (see Table 1), while
rather high total pore volumes of 1.05–1.87 cm3 gꢀ1 have been
found for P1–P4.
Carbon dioxide and methane uptakes of the networks were
determined at 298 K, the hydrogen uptake at 77 K up to a
pressure of 1 bar. The gas uptake is signicantly increased for
the networks with additional phenylene “spacers” (P2 and P4)
for all three gases, but most pronounced for CH4 as the largest
guest molecule, thus indicating an increase of the pore diam-
Fig. 4 Nitrogen adsorption (filled symbols)/desorption (open symbols)
isotherms at 77 K for P1 (black squares), P2 (red circles), P3 (green
eter. The calculated selectivities for CO2/CH4 at 298 K are 4.7
(P1), 3.6 (P2), 4.6 (P3) and 3.6 (P4), for N2/H2 at 77 K are 24.2
(P1), 19.3 (P2), 18.0 P3 and 12.6 (P4) (for details see the ESI†).
Powder X-ray diffraction (PXRD) analysis of polymer network
P4 (Fig. S26, ESI†) only displays three broad peaks of low
intensity in the wide-angle region, thus suggesting a domi-
nantly amorphous network structure. The peaks correspond to
stars), P4 (blue triangles); the inset shows the pressure range P/P0 from
0 to 0.7.
the monomers) that is characteristic for the 1,3,5-triazine unit.
The rather weak signal at 170 ppm for the triazine-free networks
P1 and P2 may be related to carboxylic acid or ester functions
that are formed in a side reaction, probably by CO insertion. In
contrast to a P1 sample made with Co2(CO)8 (Fig. 3), the 13C
˚
distances of 6.45, 4.5 and 2.1 A, respectively, most probably
Table 1 Gas sorption properties of polymer networks P1–P4
BET surface area
Pore volume
(cc)
H2 uptake
(%)
CO2 uptake
(%)
CH4 uptake
(%)
Selectivity (CO2/CH4)
(298 K)
Selectivity (N2/H2)
(77 K)
(m2 gꢀ1
)
P1
P2
P3
P4
462
502
200
475
1.76
1.87
1.05
1.16
0.44
0.57
0.35
0.55
1.46
2.54
1.58
2.58
0.11
0.26
0.12
0.26
4.7
3.6
4.6
3.6
24.2
19.3
18.0
12.6
Table 2 Optical and thermogravimetric (TGA) data of P1–P4
First decomposition
UV/Vis (lmax, nm)
PL (lmax, nm)
PLQY (%)
step (TGA, under argon, ꢁC)
P1
P2
P3
P4
352
363
367
392
540
540
3.5
6.5
—
272 (2.5%)
386 (2.6%)
296 (7.9%)
408 (1.5%)
no PL detected
560
25.3
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540 nm) and P4 (lmax ¼ 560 nm) display a yellow photo-
luminescence; P3 is non-emissive. P1 and P2 show quite
moderate photoluminescence quantum yields (PLQY) of 3.5 and
6.5%, respectively, for an excitation at 340 nm. In contrast, P4 is
characterized by a remarkably intense PL with a PLQY of 25.3%
(excitation at 400 nm; see Fig. 5). For P3, the combination of a
comparatively low BET surface area (200 m2 gꢀ1) with the
absence of photoluminescence points for the formation of
structural defects during network formation.
The photoluminescence excitation (PLE) spectra of the
polymer networks P1, P2 and P4 are depicted in Fig. 6a with PLE
maxima at lmax ¼ 367 nm (shoulder at ca. 450 nm) for P1, lmax
¼
394 nm for P2 and lmax ¼ 410 nm (shoulder at ca. 450 nm) for P4
for a detection wavelength at 550 nm.
Finally, the microporous polymer network P4 with its elec-
tron-decient triazine cores has been tested for its interaction
with electron-rich analytes as aromatic amines (Fig. 7).
For PL quenching tests P4-pellets have been exposed to
saturated vapors of the amine analytes at atmospheric pressure
and room temperature.
Fig. 5 P4 powder (top) and P4 pellet (down) at day light (left) and
under UV irradiation (right).
For p-toluidine 41% PL quenching is observed aer exposure
for 1 min, and 73% PL quenching aer 45 min. For aniline as
analyte 39% PL quenching is observed aer 1 min exposure and
82% aer 49 min. It can be assumed that the remarkable
quenching response results from donor–acceptor interactions
between the electron-rich aromatic analytes and the electron-
poor triazine-based MPN P4. These initial results demonstrate a
remarkable potential for the solid state sensing of organic
analytes. Further experiments will be, especially, focused on the
selectivity issue and on the reversibility of the PL quenching.
reecting periodic structural motifs of P4 (as TPE–TPE and
triazine–triazine distances).
The solid state optical spectra of the polymer networks are
depicted in Fig. 6. From the onset of the absorption features
(Fig. S4, ESI†) HOMO–LUMO energy gaps of 2.90 and 2.78 eV are
estimated for the polymer networks P2 and P4, respectively. The
photoluminescence (PL) spectra of the polymer networks P1,
P2, and P4 are shown in Fig. 6b. P1 (lmax ¼ 540 nm), P2 (lmax
¼
Fig. 6 PL excitation spectra (ldet ¼ 550 nm) (a) and PL spectra (lexc
¼
400 nm) (b) of P1 (black square), P2 (red circle), P3 (blue triangle), P4 Fig. 7 PL quenching experiments with P4 pellets and p-toluidine or
(green star).
aniline as analytes.
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In conclusion, novel microporous polymer networks P1–P4 have
been successfully synthesized via a reductive polyolenation
protocol starting from tris(a,a-dichlorobenzyl)-substituted
aromatic monomers with dicobalt octacarbonyl (preferred) or
chromium(II)acetate as condensing agents. The most promising
polymer network P4 that is composed of 1,3,5-triazine cores as
linkers between aggregation induced emission (AIE)-active tet-
raphenylethylene units, reveals an intense yellow photo-
luminescence peaking at 560 nm with a remarkably high PLQY
of 25.3% in line with a moderately high BET surface area of 475
m2 gꢀ1. This combination of microporosity and high solid state
PLQY (based on the AIE effect) clearly qualies P4 with its
electron decient 1,3,5-triazine cores as an attractive target for
further investigations, as its interaction with electron rich
guests. Notably, rst, orienting PL quenching experiments have
demonstrated a distinct response to aromatic amine vapors (for
aniline and p-toluidine).
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