Exploration of Thiazolo[5,4-d]thiazole Linkages in Conjugated Porous Organic Polymers for Chemoselective Molecular Sieving

Article abstract of DOI:10.1002/chem.201802631

Porous organic polymers (POPs) have attracted significant attention towards molecular adsorption in recent years due to their high porosity, diverse functionality and excellent chemical stability. In this work, we present a systematic case study on the fo

Full text of DOI:10.1002/chem.201802631

A Journal of
Accepted Article
Title: Exploration of Thiazolo[5,4-d]thiazole Linkages in Conjugated
Porous Organic Polymers for Chemoselective Molecular Sieving
Authors: Bishnu Prasad Biswal, Daniel Becker, Naisa Chandrasekhar,
Jensheer Shamsudeen Seenath, Silvia Paasch, Susanne
Machill, Felix Hennersdorf, Eike Brunner, Jan J. Weigand,
Reinhard Berger, and Xinliang Feng
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Exploration of Thiazolo[5,4-d]thiazole Linkages in Conjugated
Porous Organic Polymers for Chemoselective Molecular Sieving**
Bishnu P. Biswal,[a],‡ Daniel Becker,[a],‡ Naisa Chandrasekhar, Jensheer Shamsudeen Seenath,
[a]
[a]
[b]
[b]
[c]
[b]
[c]
Silvia Paasch, Susanne Machill, Felix Hennersdorf, Eike Brunner, Jan J. Weigand, Reinhard
[a]
[a]
Berger,* Xinliang Feng*
Abstract: Porous Organic Polymers (POPs) have attracted significant attention towards molecular adsorption in recent years due to their high
porosity, diverse functionality and excellent chemical stability. In this work, we present a systematic case study on the formation of
thiazolo[5,4-d]thiazole (TzTz) linkages through model compounds and its integration to synthesize a set of three novel, thermo-chemically
stable TzTz-linked POPs, namely TzTz-POP-3, TzTz-POP-4, and TzTz-POP-5 with triphenylbenzene, tetraphenylpyrene and
tetra(hydroxyphenyl)methane cores respectively. Interestingly, the integrated TzTz moiety of the represented TzTz-POP-3 renders
chemoselective removal of organic dye Fluorescein (FL) from a mixture with Parafuchsin (FU) in aqueous solution. The TzTz-POP-3 offered
excellent chemoselectivity of ~1: 7 (FL: FU), compared to alike porous materials demonstrated for similar applications due to the presence of
multiple active anchoring sites coupled with permanent porosity and appropriate pore window.
network is highly challenging as it involves expensive transition-
metal-based catalysts and extra purification efforts, which restrict
Introduction
the up-scalable synthesis, although several linear TzTz-linked
Synthesis of conjugated porous organic networks[1-3] through
polymers are known.[29-31] In 2015, an one-pot and catalyst-free
protocol for the synthesis of two TzTz-linked porous polymers
covalent linkages is of particular interest in current materials
research owing to their wide range of applications in the area of
was demonstrated and used for the selective CO
2
adsorption over
4]
[5]
[6]
molecular
separation,[
adsorption,[
optoelectronics,
catalysis,
N
2
. Besides, this class of porous materials so far remain less
7-9]
sensing,[10] and energy conversion/storage.
[11]
In
explored due to the poor understanding of their formation and
reaction pathways. Markedly, the integrated heterocyclic
networks would be interesting for many applications especially
adsorption of heavy metals, toxic gasses and organic molecules
due to the availability of multiple anchoring sites in close proximity
coupled with porosity.[
this context, porous organic polymers (POPs)[12-18] and covalent
organic frameworks (COFs)[19-24] are the two front line members
distinguished by structural orderings. In general, COFs are
crystalline while POPs are amorphous in nature, although both
have adequate porosity.[25] Typically, POPs are formed by using
irreversible organic reactions such as Suzuki cross-coupling
reaction, Yamamoto reaction, cyclotrimerization, etc. Thus, the
resulting framework possess exceptional chemical stability due to
the formation of strong covalent aryl-aryl bonds.[6,26-28] Since
POPs feature high chemical stability, functional groups and
scalable protocols starting from a broad range of monomers, they
are preferred for many applications.[1,2,25] Despite the prompt
advancement in the field, only few limited reactions have been
employed so far, which could introduce heterocyclic units in the
porous networks. Especially, introduction of fused heterocyclic
units e.g. thiazolo[5,4-d]thiazole (TzTz) into a porous polymeric
32,33]
In this work, at first to understand the formation of TzTz
linkage pathway, we performed a series of reactions of model
compound with varying substituents and investigated their
structural outcome by means of nuclear magnetic resonance
(
NMR) spectroscopy and X-ray single crystal analysis. As a result
of substituent tailoring effect, we isolated a stable intermediate
E)-2-(5-((2-hydroxybenzylidene)amino)-4-mercaptothiazol-2-
yl)phenol-dimer (abbreviated as TzTz-M3-I) in the reaction
towards 2,5-di(2-hydroxyphenyl)thiazolo[5,4-d]thiazole
abbreviated as TzTz-M3). The structure of the intermediate
TzTz-M3-I) revealed an alternative pathway to the one that is
(
(
(
[
33-36]
currently discussed in literature,
which could be interesting
and useful for the de novo design of TzTz-linked compounds and
materials.
[a]
Dr. B. P. Biswal, D. Becker, Dr. N. Chandrasekhar, J. S. Seenath,
Dr. R. Berger and Prof. Dr. X. Feng
Faculty of Chemistry and Food Chemistry, Center for Advancing
Electronics Dresden, Technische Universitat Dresden, 01062,
Dresden, Germany.
Subsequently, a set of three novel thiazolo[5,4-d]thiazole
(
TzTz)-linked POPs namely, TzTz-POP-3, TzTz-POP-4, and
TzTz-POP-5, have been synthesized with a set of building units
such as triphenylbenzene, tetraphenylpyrene and
E-mail: xinliang.feng@tu-dresden.de;
reinhard.berger@tu-dresden.de
tetra(hydroxyphenyl)methane respectively. Due to the presence
of elongated and functionalized building units in the resulting
conjugated TzTz-POPs, they exhibit high porosities and thermo-
[
b]
Dr. S. Paasch, Dr. S. Machill and Prof. Dr. E. Brunner
Chair of Bioanalytical Chemistry, Technische Universität Dresden,
01062, Dresden, Germany.
chemical stabilities compared to the previously reported porous
polymers with TzTz linkages.[33] By taking advantages of multiple
[
c]
Dr. F. Hennersdorf and Prof. Dr. J. J. Weigand
Chair of Inorganic Molecular Chemistry, Technische Universität
Dresden, 01062 Dresden, Germany.
anchoring sites in close proximity coupled with porosity of TzTz-
POPs, we have utilized the integrated network for the
chemoselective removal of Fluorescein (FL) dye in a mixture with
Parafuchsin (FU) in aqueous solution. We have chosen TzTz-
POP-3 as a model system for the detailed dye adsorption study
[‡]
These authors contributed equally to this work.
Supporting Information and the ORCID number(s) for the author(s)
of this article can be found under https://doi.org/xxxxxxxxxxxxxx.
1
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Scheme 1. a) Schematic illustration of the synthesis of model compounds based on thiazolo[5,4-d]thiazoles (TzTz) linkage and intermediate; b) Synthesis route of
TzTz linked Porous Organic Polymers (POPs) from different symmetrical arylaldehydes and dithiooxamide (DTO, spacefilling model of optimized fragments of TzTz-
POP-3, -4 and -5 created using Materials Studio version 2017, presented at the bottom to represent the local structure of TzTz-POPs).
as it possesses appropriate pore size (~13 Å, suitable for both the
dyes) and without any additional functional groups. Notably,
TzTz-POP-3 exhibits excellent selectivity of ~1:7 compared to
alike porous materials demonstrated for similar applications. The
selective adsorption can be attributed to the preferred
intermolecular H-bonding interactions of the —OH groups of FL
spectroscopy and high-resolution electrospray ionisation mass
spectrometry (HR-ESI MS) (Figure 1a and 1b; Section S3, Figure
+
S3 and S5 in the Supporting Information) studies, we identified,
the dimeric, partially cyclised (thiazole) intermediate TzTz-M3-I
(Scheme 1a and Figure 1a). From 2D-NMR, it has been
confirmed that the two identical molecular parts are bridged via S-
+
compared to the —NH
2
groups presented in FU dye with the TzTz
S linkages. HR-ESI mass also give a clear signal of m/z 655.059,
moieties.
which supports the proposed dimeric structure of the intermediate.
Moreover, the existence of S-S linkage can be validated using
Fourier Transform Infrared and Raman spectroscopy (Figure 1c
and 1d; Figure S7 in the Supporting Information). In the FT-IR
spectra of intermediate, TzTz-M3-I a band at 447 cm-1 and two
bands 650 and 692 cm-1 corresponds well with S-S and C-S
Result and Discussion
Structural analysis of model compounds
bonds. The Raman spectrum displace the S-S stretching vibration
band centred at 446 cm-1 which is not observed in the case of
At the beginning, we expected to achieve the simple Schiff base
condensation products[36] of ortho-substituted arylaldehydes and
DTO (Figure 1a), which could be further cyclised into the stable
TzTz moiety. Assuming that, our strategy would lead to the
formation of crystalline polymers due to the dynamic nature of
Schiff base when extended with symmetrical linkers. Keeping this
in mind, we have synthesized two model compounds in refluxing
ethanol and triethylamine (TEA) by reacting DTO with
benzaldehyde to obtain 2,5-diphenylthiazolo[5,4-d]thiazole (TzTz-
M1),[37] and 2-pyridinecarboxaldehyde to obtain 2,5-di(pyridin-2-
yl)thiazolo[5,4-d]thiazole (TzTz-M2),[38] respectively (see Scheme
final TzTz product i.e. TzTz-M3. The intermediate TzTz-M3-I was
stable in the solid state for several months but it can be
transformed into the fully cyclised product (TzTz-M3; 26% yield)
under sonication for five minutes in solution (THF). Thus, we
consider that the formation pathway of TzTz in solution goes
through sequential formation of one side thiazole ring (Scheme
1
a and Figure S10 in the Supporting Information). During the
process, the intermediate (E)-2-(5-((2-
hydroxybenzylidene)amino)-4-mercaptothiazol-2-yl)phenol is
stabilized, which could be possible due to the intramolecular H-
bonding between additional hydroxyl group and the nitrogen atom
in the imine bond (C=N), resulting into the dimeric structure after
oxidative disulfide bond formation. We propose a plausible
pathway for the TzTz formation as given in Figure S10 in the
Supporting Information. Thus, according to the outcome of model
compounds synthesis, a fast and irreversible TzTz formation for
TzTz-M1 and -M2; and a stepwise formation for TzTz-M3 can be
concluded.
1
a). In both cases, only doubly cyclised TzTz products of TzTz-
M1 and TzTz-M2 were isolated and characterised in full. However,
when attempted the synthesis of 2,5-di(2-
hydroxyphenyl)thiazolo[5,4-d]thiazole (TzTz-M3) under the
[
39]
same conditions, we found that the collected precipitate (63%
yield) was neither the expected final doubly cyclised TzTz product
nor the two-sided Schiff base product as a result of coupling
between the DTO and salicyladehyde. Using two-dimensional
heteronuclear single quantum coherence (2D-HSQC) NMR
2
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Figure 1. a) 2D (HSQC) NMR spectra measured in (CD
compound (TzTz-M3-I).
3
)
2
SO; b) High resolution ESI+ mass spectrometry; c) FT-IR and d) Raman spectrum of the intermediate
Synthesis and characterization of TzTz-POPs
form the TzTz linkages. All three POPs [TzTz-POP-3, -4, and -5]
displayed similar FT-IR spectra with only a little variation in the
Afterwards, three new porous polymers namely, TzTz-POP-3,
TzTz-POP-4, and TzTz-POP-5 using DTO and symmetrical
arylaldehydes such as 1,3,5-tris(4-formylphenyl)benzene, 1,3,6,8-
peak position with regards to the —C=N and —C-S bonds of the
TzTz ring (Figure 12 in the Supporting Information). The structure
of TzTz-POPs was determined by 13C cross-polarization magic-
angle spinning (CP-MAS) NMR spectroscopy (Figure 2a). The
experimental spectra agree well with predicted 13C NMR data
calculated with the program ACD/Labs (Figure S13 in the
Supporting Information).[40] All signals can be assigned to the
respective carbon atoms. The spectra of all three compounds
show the presence of aromatic groups (chemical shift region of ca.
tetra(4-formylphenyl)pyrene,
and
tetra(4-fromyl-3-
hydroxyphenyl)methane respectively under the catalyst-free
solvothermal process have been prepared. All these POPs were
isolated in very good yields (~80%) and are insoluble in almost all
common organic solvents. All three POPs were obtained as
amorphous solids according to our powder X-ray diffraction
investigations (Figure S11 in the Supporting Information). To get
a better insight into the functional groups, bond formation and the
local mode of binding in the POPs, we have collected the FT-IR
spectra from which we can identify the newly formed C=N bonds
120-150 ppm) as well as at higher chemical shifts of carbons of
the heterocyclic TzTz ring corresponding to the building units. For
TzTz-POP-3 and -4 these signals appear at  = ~152 and ~168
ppm, whereas TzTz-POP-5 shows the same signals at  = 155.5
and 170.4 ppm respectively, due to the influence of hydroxyl
-1
(
ranging from 1580-1620 cm ). The absence of the C=O
stretching vibration of the carbonyl group that are expected in the
-1
range of 1650-1692 cm , indicates an efficient condensation to
3
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groups. Compared to TzTz-POP-3 and -4 for TzTz-POP-5 an
additional signal for an aliphatic carbon is expected caused by the
Figure 2. a) 13C cross-polarization magic-angle spinning (CP-MAS) solid state NMR spectra, *denotes spinning sidebands; b) N
adsorption isotherms; c)
2
Thermogravimetric analysis (TGA) profile; d) Scanning electron microscopy (SEM) images (Scale bar: 200 nm) and e) Transmission electron microscopy (TEM)
images (Scale bar: 50 nm) of TzTz-POP-3, -4, and -5 respectively.
tetrahedral centre. This signal is clearly detected at  = 62.8 ppm
Here, we would like to note that predicting the particle size and
shape of any organic porous materials synthesized via
solvothermal method is difficult and hard to explain. However, we
assume that due to relatively less pi-pi stacking in the case of
TzTz-POP-4 and -5 compared to TzTz-POP-3 is the reason for
the formation of smaller size particles even below 50 nm.
1
3
(
see Figure 2a). In summary, the C CP NMR spectra can
visualize the structural differences of investigated TzTz-POPs
and confirm the expected structures of these new compounds.
In order to evaluate the thermal stability of all POPs [TzTz-
POP-3, -4 and -5], we have performed thermogravimetric analysis
Nitrogen
adsorption-desorption
experiments
were
(
2
TGA) under N . From the TGA profiles, it can be derived that all
performed to examine the architectural rigidity, permanent
porosity of all three POPs at 77 K (Figure 2b). The POPs were
solvent (methanol/dichloromethane) exchanged and activated
prior to analysis to make the pores guest free at 170 ºC for 16 h
under vacuum. The fully reversible type-II isotherms show rapid
uptake at low relative pressures indicating that all these polymers
are mainly microporous in nature. The increase in the nitrogen
adsorption at a high relative pressure (~0.8) may arise due to
interparticulate porosity of the samples and voids. The Brunauer-
Emmet-Teller (BET) surface areas of these POPs were found to
be 604 m2g-1 (TzTz-POP-3), 755 m2g-1 (TzTz-POP-4) and 491
POP’s pores are guest free and have almost identical thermal
stability up to ~450 ºC (Figure 2c). However, TzTz-POP-5 has a
lower thermal stability and degradation starting from 300 ºC. After
decomposition temperature, a gradual weight loss of ~40-50% for
TzTz-POP-3 and TzTz-POP-4 and ~53% for TzTz-POP-5, were
observed until a temperature of 1000 °C. From the TGA profiles,
also it can be seen that all TzTz-POPs loss a weight of ~2% at
100 ºC, which could be attributed to moisture. However, for TzTz-
POP-4, an additional weight loss of ~7% around 200 ºC, and this
could corresponds to the trapped high boiling solvent (DMF) and
oligomeric fragments.
m2g-1 (TzTz-POP-5) [Calculated over a relative pressure (P/P
)
0
External morphologies and permanent porosity
range from 0.05 to 0.2] (Figure S14 and S15 in the Supporting
Information). Notably, the surface area values of the TzTz-POPs
The external morphologies of the TzTz-POP materials were
inspected by scanning electron microscopy (SEM). From the
SEM images it is inferred that small semi-spherical particles
are higher than the previously reported 299 m2g-1 and 488 m2g-1
for
TzTz-POP-1
and
-2
employing
phenyl
and
[
33]
tetraphenylmethane building units, respectively. In addition, the
pore size distribution (PSD) analysis has been derived from the
(
~100 nm) agglomerates to construct a colony for TzTz-POP-3.
However, for TzTz-POP-4 and -5 the particles are even smaller
and of size less than 50 nm (Figure 2d and Figure S17 in the
Supporting Information). The similar trend and morphology has
been also observed in transmission electron microscopy (TEM)
images (Figure 2e and Figure S18 in the Supporting Information).
N
2
isotherm data for all TzTz-POPs using nonlocal density
functional theory (NLDFT), which suggests primary narrow
micropores of the polymeric frameworks (Figure S14 in the
Supporting Information).
4
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Figure 3. a) and b) spacefilling models of FL and FU generated using Materials studio version 2016; c) and d) Comparison of UV-vis absorption spectra of aqueous
solutions of FL and FU after treatment with TzTz-POP-3 at different intervals (Inset: real photographs of FL and FU before and after treatment with TzTz-POP-3 at
different intervals); e) UV-vis absorption spectra of aqueous solutions of FL before and after treatment with different amount of TzTz-POP-3 for 5 mins. (Inset: real
photographs of FL before and after treatment with TzTz-POP-3 at different doses); f) Change in concentration of dyes (FL and FU) over time after treatment with
TzTz-POP-3, determined by change in absorbance relative to initial absorbance (C/C0); g) Sections of chromatogram measured by HPLC-MS of 1:1 (molar ratio)
mixture of FU and FL, before and after treatment with TzTz-POP-3 [Solid line: blank (1:1 mixture); green dotted line: after 1 min.; red dotted line: after 5 mins.; with
peaks 7.7 and 9.75 mins. retention times for FU and FL respectively (extracted ion plots of the protonated molecules are presented); h) Change in concentration of
dyes (FU and FL) over time after treatment with TzTz-POP-3, determined from the peak areas of HPLC measurements (1:1 mixture of FU and FL is considered as
100%). The ratio of FU to FL intensity counts (in %) is presented as blue font text.
Optical properties
respectively and studied the effect of functionality towards
adsorption.[42] As depicted in Figure 3, the molecular dimension
of FU (10.03 Å x 10.05 Å) and FL (10.45 Å x 9.02 Å) was
calculated based on the optimized structure using Material
Studio 2016. Both the dye molecules have almost same
dimensional size but different functionality as mentioned above
and suitable for the selective adsorption study. We have used
UV-vis spectroscopy to monitor the change of concentration of
the individual dyes at a fixed time interval upon treatment with
TzTz-POP-3. We have chosen TzTz-POP-3 for the detailed dye
adsorption study as it possesses good porosity with suitable
pore size (~13 Å, based on PSD, Figure S14 in the Supporting
Information) and does not contain any additional functional
groups except the TzTz linkage. To our delight, TzTz-POP-3 has
the capability to quickly adsorb and remove organic dye (FL)
from water as compared to FU (Figure 3c and 3d). As shown in
Figure 3c and 3d, the dye FL was almost removed within 5 min.,
showing a removal efficiency of ~99.6%, while the FU, which
Further, to understand the optoelectronic properties of the
synthesized POPs, we performed UV-vis absorption in
integrating
sphere
in
the
dispersion
state
and
photoluminescence measurements. TzTz-POP-3, -4 and -5
showed broad optical absorption centred at wavelengths around
400 nm, 422 nm and 414 nm respectively. These are in
agreement with a similar absorption maximum observed for the
model compound (see Figure S8 in the Supporting Information)
with a maximum absorption at 382 nm and 402 nm. For all three
TzTz-POPs a tailing absorption and emission can be found
above 550 nm, which is attributed to the increased conjugation
of TzTz linkages (Figure S16 in the Supporting Information).
Chemoselective dye adsorption
Taking the advantages of high porosity, excellent chemical
stability and multiple anchoring sites of fused heterocyclic moiety
of TzTz-POPs, we were motivated to investigate the dye
adsorption[41] of practical significance from aqueous solution. For
our purpose, we have intentionally selected two different organic
2
contains —NH groups, exhibited only ~29% removal efficiency
at the same time confirmed from UV-vis spectroscopy.
In addition, we checked the effect of dose of adsorbent on
dye adsorption by performing individual batch experiments with
2
dyes, namely, FU and FL containing —NH and —OH groups
5
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1
mg, 3 mg and 5 mg of TzTz-POP-3 in 5 ml of FL stock solution
studies would inspire and pave a pathway for the further
development of novel porous adsorbent materials based on
heterocyclic linkages. As a prospect, we would like to note that
various metal ions could also be anchored to TzTz-POPs for
their application in photo/electrochemical catalysis such as
Oxygen Reduction Reaction (ORR) and Hydrogen Evolution
Reaction (HER) in the future.
in water (Figure 3e). From absorption measurement, we found
that 5 mg of TzTz-POP-3 was sufficiently enough to adsorb FL
completely (~99.5%) within 5 min. from aqueous solution. We
consider that, the basic TzTz moiety of the TzTz-POP-3 helps to
discriminate functional groups and selectively interact with –OH,
2
over –NH groups under neutral condition. Similar phenomenon
has been recently reported by Ning et al. wherein basic —N-H
functionality selectively interacts with the —OH groups over —
NH
2
to
facilitate
chemoselective
separation
in Experimental Section
[43]
salicylideneanilines-based COFs. We further investigated the
chemoselective ability of TzTz-POP-3 in the mixture as it has
higher and faster uptake capacity towards —OH containing (FL)
2
Synthesis of TzTz-POP-3: A Pyrex tube (o.d. × i.d. = 1 × 0.8 cm and
length 18 cm) was charged with 1,3,5-tris(4-formylphenyl)benzene (30
mg, 76.84 μmol, 1 eq.), dithiooximide (DTO) (13.8 mg, 115.26 μmol, 1.5
eq.) and 2 ml of nitrobenzene. This mixture was sonicated for 10 min. in
order to get a homogenous dispersion. The tube was then flash frozen at
dye over —NH
powder was suspended in aqueous solution of both FL and FU
~50 μmol each; 1: 1 ratio by Vol.) and collected the samples at
2
based (FU) Dye. Towards this end, TzTz-POP-3
(
2
77 K (liquid N bath) and degassed by three freeze-pump-thaw cycles.
different time intervals such as 30 sec, 1 min., 3 min., 5 min.,
and 15 min., respectively. We performed HPLC and noticed that
the concentration of FL was largely decreased to ~8%
The tube was sealed under vacuum and then heated at 150 ºC for 4 days.
A brownish yellow colored precipitate was collected by filtration and
washed with ethanol, N,N’-dimethylacetamide (DMAc) and acetone
several times until the filtrate became colorless, dried at 150 ºC under
vacuum for 12 h to obtain the title porous polymer TzTz-POP-3 (36 mg)
in 82% isolated yield.
(
considered FL peak from the 1: 1 mixture of FL: FU as 100%),
whereas FU decreased to 62% within 5 min., resulting into an
excellent selectivity with a ratio of 1: 7.7 for FL: FU in the filtrate
(
Figure 3g, 3h and Figure S20 in the Supporting Information).
Synthesis of TzTz-POP-4: A Pyrex tube (o.d. × i.d. = 1 × 0.8 cm2 and
length 18 cm) was charged with 1,3,6,8-tetra(4-formylphenyl)pyrene (30
mg, 48.33 μmol, 1 eq.), DTO (11.6 mg, 96.67 μmol, 2 eq.) and 2 ml of
N,N’-dimethylformamide (DMF). This mixture was sonicated for 10 min.
in order to get a homogenous dispersion. The tube was then flash frozen
Finally, to showcase the reusability of TzTz-POP-3 towards
dye adsorption, we have washed the colored TzTz-POP-3 using
N,N-dimethylacetamide (DMAc) under stirring overnight,
followed with ethanol and acetone. The decoloured TzTz-POP-3
was subjected to SEM imaging and did not found any noticeable
change in the morphology (Figure S21 in the Supporting
2
at 77 K (liquid N bath) and degassed by three freeze-pump-thaw cycles.
The tube was sealed under vacuum and heated at 150 ºC for 4 days.
After cooling the precipitate was collected by filtration and washed with
ethanol, dichloromethane (DCM), DMAc and acetone several times until
the filtrate became colorless, dried at 150 ºC under vacuum for 12h to
provide targeted TzTz-POP-4 (32 mg) in 76% isolated yield as yellow
solid.
Information). Similarly, from N
BET value of 466 m g . The decrease in surface area compared
2
adsorption analysis, we got the
2
-1
2
-1
to the pristine TzTz-POP-3 (604 m g ) could be attributed to
some trapped dye molecules remaining inside the pores (Figure
S22 in the Supporting Information). Nevertheless, upon
repeating the dye adsorption cycle it has been found that the
adsorption capacity of TzTz-POP-3 towards FL was maintained
with a slower rate and with a value of ~73% after 5 min. and
further reached to 98.9% in 15 min. (Figure S23 in the
Supporting Information). This result indicates that the reusability
of TzTz-POP-3 for multiple dye adsorption cycles could be
possible.
Synthesis of TzTz-POP-5: A Pyrex tube (o.d. × i.d. = 1 × 0.8 cm2 and
length 18 cm) was charged with tetra(4-fromyl-3-hydroxyphenyl)methane
(30.2 mg, 41.60 μmol, 1 eq.), DTO (10 mg, 83.22 μmol, 2 eq.) and 2 ml
of N,N’-dimethylformamide. This mixture was sonicated for 10 min. in
order to get a homogenous dispersion. The tube was then flash frozen at
77 K (liquid N bath) and degassed by three freeze-pump-thaw cycles.
2
The tube was sealed under vacuum and then heated at 150 ºC for four
days. A black-brown jelly material formed was collected and washed with
ethanol, dichloromethane, DMAc and acetone several times until the
filtrate became colorless, dried at 150 ºC under vacuum for 12h to
provide targeted TzTz-POP-5 (31 mg) as solid material in 77% isolated
yield.
Conclusions
In summary, the effects of tailoring substituents on TzTz
formation have been studied using model compounds. On the
way, an unprecedented stable intermediate compound (TzTz-
M3-I) was isolated and characterized, which could be interesting
for the understanding of TzTz formation to design TzTz-linked
compounds and materials. Besides, we have also synthesized
three novel, thermo-chemically stable TzTz-linked POPs with a
variety of functional building units employing our one-pot
catalyst-free strategy. All three synthesized TzTz-POPs possess
very good chemical and thermal stability with excellent porosities.
Interestingly, due to presence of multiple active anchoring sites
coupled with permanent porosity, TzTz-POP-3 showed a great
potential for effective chemoselective removal of dye
Fluorescein (FL) from a mixture with Parafuchsin (FU) in
aqueous solution with an excellent selectivity of 1: 7. Overall, we
believe that the presented understanding on TzTz formation,
their integration to POPs synthesis and selective adsorption
Chemoselective dye adsorption study: All the dye adsorption
experiments were conducted at room temperature. Initial dye
concentrations [FL and FU] were fixed to be ~50 μmol in Milli-Q water.
Typically, 5 mg of TzTz-POP-3 was added into 5 ml of dye solution, and
the mixture was stirred with a magnetic stir bar at room temperature at a
stirring rate of 500 r.p.m. At appropriate time intervals (see the result
section), the mixture was filtered over Whatman PTFE 0.45 μm
membrane filters. The concentration of dye in the filtrate was detected
using an Agilent Cary 5000 UV-vis-NIR spectrophotometer. The removal
efficiency of dye was calculated according to the following equation (1):
Removal efficiency (%) = (C − C) / C × 100 (1)
0
0
where, C
the filtrate, respectively. The dye adsorption experiments were performed
times to ensure the reproducibility and the best result has been used.
0
and C are the concentrations of dyes at initial condition and in
3
The chemoselective dye adsorption study was performed using TzTz-
6
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Chemistry - A European Journal
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POP-3 as an adsorbent exactly as described above but with 1: 1 dye
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8
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FULL PAPER
Three novel, thermo-chemically stable
TzTz-linked POPs, namely TzTz-
POP-3, TzTz-POP-4, and TzTz-POP-
Bishnu P. Biswal, Daniel Becker, Naisa
Chandrasekhar, Jensheer Shamsudeen
Seenath, Silvia Paasch, Susanne
Machill, Felix Hennersdorf, Eike Brunner,
Jan J. Weigand, Reinhard Berger,*
Xinliang Feng*
5
have been synthesized.
Interestingly, the integrated TzTz
moiety of the represented TzTz-POP-
3
renders chemoselective removal of
Page No. 1 – Page No. 8
organic dye Fluorescein (FL) from a
mixture with Parafuchsin (FU) in
aqueous solution. The TzTz-POP-3
offered excellent chemoselectivity of
Title
Exploration of Thiazolo[5,4-d]thiazole
Linkages in Conjugated Porous
Organic Polymers for Chemoselective
Molecular Sieving
~
1: 7 (FL: FU), compared to alike
porous materials demonstrated for
similar applications due to the
presence of multiple active anchoring
sites coupled with permanent porosity
and appropriate pore window.
9
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