Bandgap Engineering of Conjugated Nanoporous Poly-benzobisthiadiazoles via Copolymerization for Enhanced Photocatalytic 1,2,3,4-Tetrahydroquinoline Synthesis under Visible Light

Article abstract of DOI:10.1002/adsc.201600125

Conveniently combining properties such as high interfacial surface area, excellent visible light absorption and semiconductor-range bandgap, conjugated nanoporous polymer networks are promising candidates as pure organic, metal-free, visible light-respons

Full text of DOI:10.1002/adsc.201600125

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DOI: 10.1002/adsc.201600125
Very Important Publication
Bandgap Engineering of Conjugated Nanoporous Poly-benzobis-
thiadiazoles via Copolymerization for Enhanced Photocatalytic
1,2,3,4-Tetrahydroquinoline Synthesis under Visible Light
Zi Jun Wang,a Saman Ghasimi,a Katharina Landfester,a and Kai A. I. Zhanga,*
a
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Fax : (+49)-6131-379-370; e-mail: kai.zhang@mpip-mainz.mpg.de
Received: January 29, 2016; Revised: April 5, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600125.
Abstract: Conveniently combining properties such
as high interfacial surface area, excellent visible
light absorption and semiconductor-range bandgap,
conjugated nanoporous polymer networks are
promising candidates as pure organic, metal-free,
visible light-responsive and heterogeneous photoca-
talysts. Here, a facile yet precise bandgap engineer-
ing strategy to achieve fine justification of the va-
lence and conduction band positions of a series of
nanoporous polymers to optimally bracket the
redox potential of the targeted individual photore-
dox reaction via copolymerization of the electron-
withdrawing benzobisthiadiazole moieties into the
polymer network backbone is presented. The en-
hanced photocatalytic activity of the nanoporous
polymer networks was demonstrated in the synthe-
sis of 1,2,3,4-tetrahydroquinoline, an important
motif in pharmaceutical compounds, via photooxi-
dative cyclization of N,N-dimethylanilines with mal-
eimides.
However, the prohibitively high cost and potential
toxicity of these rare metals often raise concerns
about their long-term sustainability. As a result, there
has been a surging interest in the field of heterogene-
ous visible light photocatalysis to pursue stable, recy-
clable, reusable and metal-free photocatalysts for or-
ganic synthesis, such as graphitic carbon nitride as
one prominent example.[4]
Conjugated nanoporous polymers (CNPs) conven-
iently combine a number of interesting properties,[5]
such as nanoporosity, high interfacial surface area and
semiconductor-range bandgap, which make them
promising candidates for heterogeneous visible light
photocatalysts. Recent studies have successfully dem-
onstrated CNPs with various functionalities and mor-
phologies to serve as effective, robust heterogeneous
visible light photocatalysts for a wide range of chemi-
cal transformations such as singlet oxygen activa-
tion,[6] selective oxidation of organic sulfides,[7] organ-
ic dye degradation,[8] oxidative hydroxylation of aryl-
boronic acids,[9] photooxidative coupling of benzyl-
Keywords: bandgap engineering; conjugated nano-
porous polymers; metal-free process; photocataly-
sis; tetrahydroquinolines
amines,[10] reductive dehalogenation reaction,[11] pho-
ACHTUNGTRENNUNG
tocatalytic reduction of Cr(VI) to Cr(III),[12] visible
light-initiated free radical and cationic polymeri-
zation[13] and light-induced hydrogen evolution.[14]
However, systematic investigations into the in-depth
understanding and further optimization of the catalyst
design for higher photocatalytic efficiencies still need
Sunlight, as a clean and inexhaustible source of re- to be undertaken.
newable energy, has inspired tremendous efforts from
To match the specific activation energies of individ-
material scientists to harvest, convert, store and uti- ual photochemical reactions, the redox potentials of
lize solar energy.[1] Indeed, the field of photocatalysis transition metal-based homogeneous organometallic
has witnessed significant progress toward the develop- complexes can be easily adjusted by modifying the
ment of visible light-responsive photocatalysts and metal center and surrounding ligands.[15] Likewise, the
the discovery of photocatalytic reactions yielding high effective valence (VB) and conduction band (CB) po-
value-added products in the past decade.[2] In the past sitions of heterogeneous metal oxide-based photoca-
two decades, various research groups have successful- talysts (e.g., TiO2 and Nb2O5) can be modified via sur-
ly investigated a plethora of organic photochemical face functionalization with heteroatoms (e.g., O, N
transformations using transition metal-based com- and S) or formation of hybrid- or hetero-structures
plexes (e.g., Ru2+ or Ir3+) or organic chromophores.[3] with selected metals.[16] A few examples using co-
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building block control for enhancing photocatalytic
efficiency were demonstrated for graphitic carbon ni-
tride-based systems.[17] Analogous to organic photo-
voltaics (OPV), an ideal conjugated polymeric photo-
catalyst should possess not only a broad absorption
range (i.e., narrow bandgap, Eg ꢀ1.6–2.0 eV) to har-
vest light from the full solar spectrum, but also a high
enough lowest unoccupied molecular orbital (LUMO)
level for efficient charge separation.[18] Motivated by
this, our group recently showed that the valence and
conduction band levels can be fine-tuned and the re-
sulting photocatalytic efficiencies of the CNP photo-
catalysts can be boosted by a facile molecular struc-
tural design strategy[10a] or the variation of the chemi-
cal composition in the electron-accepting moieties.[10b]
Nevertheless, a deeper understanding of control on
the molecular level of this class of new photocatalysts
is still of great importance for further development of
the CNPs in order to achieve enhanced photocatalytic
efficiency.
Scheme 1. Synthetic strategy and representative structures
of the conjugated nanoporous polymer series, P-BBT-0, P-
BBT-10, P-BBT-50 and P-BBT-100, where TB is 1,3,5-tri-
AHCTUNGTREGeNNNU thynylbenzene, B is 1,4-dibromobenzene and BBT is dibro-
mobenzo[1,2-c;4,5-c’]bis[1,2,5]thiadiazole. Reaction condi-
tions: Pd(0), CuI, DIPA, DMF, 80–908C, 12 h.
À
Metal-mediated dehydrogenative C C bond forma-
À
tion via C H bond cleavage has received significant
interest from organic chemists because it eliminates
the preactivation steps, such as halogenation of the
starting compounds. Specifically, direct coupling at an ed into the visible region, but also that the band posi-
sp3 carbon center adjacent to heteroatoms (e.g., N tions can be precisely aligned to bracket the targeted
and O) has been extensively investigated.[19] Tradi- photoredox reaction to yield optimal photocatalytic
tionally catalyzed by transition metal-based catalysts efficiency by the copolymerization method with opti-
(e.g., Cu2+, Ru2+ or Ir3+),[20] the oxidative direct cycli- mized composition of the electron-withdrawing co-
zation of N-methylanilines with maleimides leads in monomer (BBT).
one step to the corresponding tetrahydroquinoline
The chemical structures of the synthesized poly-
products, which are common structural motifs found mers are displayed in Scheme 1. Increasing amounts
in numerous biologically active natural products and of electron-accepting moiety BBT were incorporated
pharmacologically relevant therapeutic agents.[21] To into the conjugated network, which resulted in
avoid metal residue in the products, non-metal-con- a series of conjugated nanoporous polymers, P-BBT-
taining catalytic systems are therefore highly advanta- 0, P-BBT-10, P-BBT-50, P-BBT-100 (the numbers 0,
geous.
10, 50, 100 denote the molar feed ratios of B (phenyl
In this work, we present a bandgap engineering unit) to BBT, Scheme 1). The synthetic details and
method to fine-tune the CB and VB levels and to ach- characterization data are described in the Experimen-
ieve precise band position alignment by copolymeri- tal Section and the Supporting Information. The poly-
zation.
Certain
amounts
of
benzo[1,2-c;4,5- mers obtained are insoluble in all common organic
c’]bis[1,2,5]thiadiazole (BBT) as electron acceptor solvents tested. The Brunauer–Emmett–Teller (BET)
moiety were incorporated into a 1,3,5-cross-linked surface areas of P-BBT-0, P-BBT-10, P-BBT-50 and
benzene-based conjugated system to yield a series of P-BBT-100 were found to be 1017 m2 gÀ1, 355 m2 gÀ1,
polymer-based photocatalysts with distinctively differ- 110 m2 gÀ1 and 57 m2 gÀ1, with pore volumes of
ent VB and CB band levels and bandgaps 0.82 cm3 gÀ1, 0.32 cm3 gÀ1, 0.12 cm3 gÀ1 and 0.07 cm3 gÀ1,
(Scheme 1). Furthermore, it was found that these con- respectively. Similar average pore diameters of
jugated nanoporous polymers could serve as effective around 1.5 nm were determined for all polymers
metal-free heterogeneous photocatalysts for the visi- (Table S1 and Figures S1–S5 in the Supporting Infor-
ble light-induced direct coupling at sp3 carbon centers mation). The SEM and TEM images of the four
adjacent to heteroatoms such as N and O.[19,22] As CNPs illustrated slightly different morphologies
a pure organic catalytic example, the oxidative cycli- (Figure 1). In particular, P-BBT-0 and P-BBT-10 ex-
zation of N,N-dimethylanilines with maleimides was hibited more loosely-packed, fibrous network struc-
chosen to demonstrate the photocatalytic activity of tures whereas a flake-like morphology was observed
the well-designed conjugated nanoporous polymers. for P-BBT-50 and P-BBT-100.
We also demonstrate that not only the optical absorp-
tion of the photocatalysts can be significantly extend- typical aromatic carbon signals between 120 and
Solid state 13C/MAS NMR spectroscopy showed
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145 ppm. The signals at about 90 ppm can be assigned
ꢁ
to the C C bonds. The signal at about 153 ppm was
observed for the BBT-containing polymers P-BBT-10,
P-BBT-50, and P-BBT-100, which can be assigned to
the carbon of the C=N band in the BBT units (Fig-
AHCTUNGTREGuNNUN res S6–S9 in the Supporting Information). Thermal
gravimetric analysis (TGA) measurements showed
that the onset temperatures of thermal degradation
with less than 5% weight loss are 4918C, 3838C,
2368C and 2378C, respectively, indicating a carbonized
residue with rich nitrogen and sulfur content (Fig-
ure S10 in the Supporting Information). FT-IR spectra
identified the typical stretching mode of the aromatic
À1
ꢁ
rings between 1370 and 1500 cm ; C=C and C C
bond stretching modes could be observed at around
1600 cmÀ1 and 2250 cmÀ1, respectively. Additionally,
for polymers containing BBT units, the intensities of
the peak around 1622 cmÀ1 grew, which can be attrib-
uted to C=N bond stretching and corresponds to the
increasing BBT content (Figure S11 in the Supporting
Information).
In Figure 2a, the UV/Vis diffuse reflectance (DR)
spectra of the CNPs are displayed. P-BBT-0 showed
the narrowest absorption band, while the absorbance
spectra of P-BBT-10, P-BBT-50 and P-BBT-100 are
Figure 1. Scanning electron microscopy (SEM) images of P-
BBT-0 (a), P-BBT-10 (b), P-BBT-50 (c) and P-BBT-100 (d)
with transmission electron microscopy (TEM) images as in-
serts.
Figure 2. (a) Diffuse reflectance (DR) UV/Vis spectra, (b) Kubelka–Munk-transformed reflectance spectra and (c) conduc-
tion band (CB) and valence band (VB) positions of the polymers. (d) Proposed reaction mechanism of the 1,2,3,4-tetrahy-
droquinoline synthesis.
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Table 1. Oxidative direct cyclization of N,N-dimethylaniline
with N-phenylmaleimide catalyzed by the polymer-based
photocatalysts.[a]
bathochromically shifted towards longer wavelengths,
up to near infrared region in the latter two cases. The
Kubelka–Munk–transformed reflectance spectra of
the four polymers are shown in Figure 2b. The esti-
mated optical bandgaps are 3.12 eV, 2.20 eV, 1.92 eV
and 1.86 eV for P-BBT-0, P-BBT-10, P-BBT-50 and P-
BBT-100, respectively. The conduction band (CB)
levels of the four polymers were measured by cyclic
voltammetry (CV) (Figure 2c) and they are À1.14 V,
À0.94 V, À0.76 V and À0.75 V (versus SCE) for P-
BBT-0, P-BBT-10, P-BBT-50 and P-BBT-100, respec-
tively.
No discernible oxidative potentials of the conjugat-
ed polymers were observed by cyclic voltammetry, the
valence band (VB) levels were estimated with the
combined knowledge of the optical bandgaps and the
CB levels of the polymers and they are 1.98 V, 1.26 V,
1.16 V and 1.10 V (versus SCE) for P-BBT-0, P-BBT-
10, P-BBT-50 and P-BBT-100, respectively. It is
worth-noting that the VB and CB levels are compara-
ble to the redox potentials of established transition
metal-based photocatalysts such as ruthenium com-
plexes,[3a] which indicates the promising potential of
these conjugated nanoporous photocatalysts.
Entry Catalysts
Oxidant Light Additive Yield [%][b]
1
2
3
4
P-BBT-0
air
+
+
+
+
+
+
–
–
–
–
–
–
–
–
<5[c]
82
43
29
86
P-BBT-10 air
P-BBT-50 air
P-BBT-100 air
P-BBT-10 O2
5[d]
6[e]
7[f]
8[g]
9[h]
10[i]
12[j]
P-BBT-10
P-BBT-10 air
air
–
7
–
trace
trace
39
47
73
–
+
+
+
+
P-BBT-10 air
P-BBT-10 air
P-BBT-10 air
KI
BQ
NaN3
[a]
Standard reaction conditions: 1a (0.5 mmol), 2a
(0.25 mmol), photocatalyst (10 mg), DMF (3 mL), white
LED, room temperature, 24 h.
Isolated yield by silica chromatography unless otherwise
stated.
Yield determined by GC-MS.
O2 used as oxidant instead of air.
Under an N2 atmosphere.
In the dark.
No photocatalyst.
KI used as hole scavenger.
Benzoquinone (BQ) used as radical scavenger.
NaN3 used as singlet oxygen scavenger.
Previously, conjugated macromolecular systems
were shown to be able to mediate the electron trans-
[b]
fer
from
N,N,N,N’-tetramethylphenylenediamine
[c]
[d]
[e]
[f]
[g]
[h]
[i]
(TMPD) (Eoxi =0.12 V vs. SCE) to molecular oxygen,
which then produces a blue-colored cationic radical
and the activated oxygen species superoxide (O2CÀ)
(Ered =0.86 V vs. SCE) under light irradiation.[23] Fig-
ure S12 (Supporting Information) shows the intensity
variations in the UV/Vis absorption spectra of the
blue-colored cationic radical, which implies that P-
BBT-10 could generate the most superoxide O2CÀ
under the same reaction conditions and is likely to
[j]
have a superior photooxidative activity compared to ide anion radical, an active species in this catalytic
the rest. This finding could probably be explained by cycle (Figure S12 in the Supporting Information). It is
the second highest oxidation potential of 1.26 V and worthwhile to point out that the catalytic efficiency of
reduction potential of À0.94 V of P-BBT-10 in combi- P-BBT-10 in this reaction is absolutely comparable
nation with its optimal bandgap. Although P-BBT-0 with the metal-based catalysts such as thermally
exhibited even higher redox potentials than P-BBT- active copper chloride[20b] or the state-of-art photoca-
10, its inadequate light absorption in the visible talyst transition metal complex Ru(byy)3Cl2.[20a]
region and the large bandgap are likely to be one of Hence, the CB and CB band positions appeared to be
the major limiting factors.
of vital importance to the photocatalytic efficiencies,
We then tested oxidative cyclization reaction of we envision that other parameters such as light ab-
N,N-dimethylaniline with N-phenylmaleimides using sorption, porosity, charge separation, lifetime of excit-
the nanoporous poylmers as photocatalysts in air at ed electrons and charge mobility can also play a role
room temperature. It could be seen that not only and the observed effective catalytic efficiency de-
were the nanoporous polymer-based photocatalysts pends in fact on a combinatorial effect of all parame-
successful in yielding the desired product, but they ters.
also exhibited different photocatalytic efficiencies
To further investigate the reaction mechanism and
under the same reaction conditions (Table 1). Specifi- the individual role of the photogenerated electron-
cally, the reaction catalyzed by P-BBT-10 led to the hole pairs during the oxidative cyclization of N,N-di-
highest yield of 82% (Table 1, entry 2), which corrob- methylaniline with N-phenylmaleimide, control ex-
orates with the qualitative results showing that P- periments were conducted using P-BBT-10 as photo-
BBT-10 was the most efficient in generating superox- catalyst (Table 1). Replacing air with oxygen only led
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Table 2. The direct coupling of various N-methylanilines
with N-substituted maleimides using P-BBT-10 as photoca-
talyst.[a,b]
to negligible improvement in yield (entry 5). En-
tries 6–8 show that all three components, photocata-
lyst, light and oxygen, are indispensable for achieving
significant product yield. By adding KI as hole scav-
enger, a reduced yield of 39% was obtained (entry 9)
and similarly, a lower yield of 47% was observed
when benzoquinone (BQ) was added as superoxide
scavenger, indicating that both the photogenerated
electron and hole play a critical role in the photocata-
lytic cycle. To further investigate another possible
active oxygen species, singlet oxygen (1O2), which can
also be generated by conjugated macromolecular sys-
tems,[10,17b] we then conducted electron paramagnetic
resonance (EPR) trapping experiments performed
using 2,2,6,6-tetramethylpiperidine (TEMP) as a sin-
glet oxygen trapping agent. The 2,2,6,6-tetramethylpi-
peridinyl-1-oxyl (TEMPO-1O2) adduct could be clear-
ly determined (Figure S13 in the Supporting Informa-
tion). By using 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) as superoxide trapping agent, DMPO-O2CÀ
was also determined, indicating that P-BBT-10 was
able to generate both active oxygen species (Fig-
ure S14 in the Supporting Information). The addition
of NaN3 as singlet oxygen scavenger led to a slightly
reduced yield of the 1,2,3,4-tetrahydroquinoline prod-
uct compared to standard reaction conditions, indicat-
1
ing a minimal role played by the photogenerated O2
species in the catalytic process (entry 12). It could be
calculated that the ratio of both oxygen species was
about 2:1 for superoxide/singlet oxygen. Additionally,
changing the solvent system from DMF into toluene,
DMSO and acetonitrile was found to be detrimental
for all (not shown).
Based on our observations from the control experi-
ments, we propose a plausible mechanism similar to
the literature shown in Figure 2d.[20] Upon light irradi-
ation, a photogenerated electron-hole pair occurs.
[a]
Reaction conditions: aniline (0.5 mmol), maleimide
(0.25 mmol), P-BBT-10 (10 mg), DMF (3 mL), white
LED, room temperature, air, 24 h.
[b]
Isolated yield by silica chromatography.
The photogenerated hole oxidizes N-methylaniline to modest to good yields. It was also found that photoca-
form its cationic radical 1, which then loses a proton talyst P-BBT-10 could be reused for up to five extra
and rearranges to give the a-aminoalkyl radical 2. cycles in the model reaction (entry 2, Table 1) without
Subsequently, 2 reacts with maleimide to generate significant loss in activity (Figure S16 in the Support-
radical 3, which then undergoes cyclization to form ing Information). No morphological change was ob-
the first intermediate 4, which then is oxidized by the served as shown by the SEM image (Figure S21 in the
active oxygen species, leading to the final product Supporting Information). Additionally, by increasing
1,2,3,4-tetrahydroquinoline and the side product hy- the catalyst loading – using the double amount of P-
drogen peroxide (H2O2). The formation of H2O2 was BBT-10 in the reaction mixture – the reaction time
determined by the catalytic oxidation of N,N-diethyl- decreased to ca. 15 h to reach the same product yield.
1,4-phenylenediammonium sulfate (DPD) with horse- The non-proportional reaction time to the catalyst
radish peroxidase (POD) mixture and colorimetry by amount could be caused by the insufficient light pene-
UV-Vis spectroscopy, confirming the suggested reac- tration into the reaction medium.
tion mechanism (Figure S15 in the Supporting Infor-
mation).
In summary, we present a precise bandgap engi-
neering strategy of conjugated nanoporous poly-ben-
Furthermore, we examined the scope of this photo- zobisthiadiazole networks as metal-free, heterogene-
catalytic reaction by investigating substrates with both ous photocatalysts to allow for fine justification of the
electron-donating and electron-withdrawing function- photoredox potential and to achieve optimized cata-
al groups under the same reaction conditions using P- lytic efficiency. Via copolymerization with an optimal
BBT-10 as catalyst (Table 2). All reactions afforded composition of electron-withdrawing benzobisthiadia-
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References
zole moiety, the resulting valence and conduction
band positions of the polymers can be strategically
aligned to bracket the redox potential of targeted in-
dividual reactions. The enhanced visible light-active
photocatalytic activity of the polymers was demon-
strated in the oxidative cyclization of N,N-dimethyl-
ACHTUNGTRENNUNGanilines with maleimides to yield tetrahydroquinoline
products, which are common structural motifs found
in numerous pharmacologically relevant compounds.
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To a solution of 0.02 mmol (23 mg) of tetrakis(triphenyl-
phosphine)palladium(0), 0.02 mmol (4 mg) of CuI in 20 mL
of DMF and diisopropylamine (DIPA) (vol/vol 50/50),
0.67 mmol (100 mg) of 1,3,5-triethynylbenzene (TB), were
added 1 mmol (236 mg) of dibromobenzene (B) for P-BBT-
0; 0.9 mmol (212 mg) of B and 0.1 mmol (35.2 mg) of dibro-
mobenzo[1,2-c;4,5-c’]bis[1,2,5]thiadiazole (BBT) for P-BBT-
10; 0.5 mmol (118 mg) of B and 0.5 mmol (176 mg) of BBT
for P-BBT-50; 1 mmol (352 mg) of BBT for P-BB100. The
solution was degassed for 20 min with N2 and then was
heated to 80–908C depending on the feed ratios for 12 h.
The resulted polymers were then rinsed with Milli-Q water
several times and extracted with 50/50 dichloromethane and
methanol solution in a Soxhlet for 24 h. The insoluble solid
was then dried under vacuum overnight. Inductively coupled
plasma mass spectrometry (ICP-MS) analysis showed that
all three conjugated polymers have little or trace amount of
residual Pd (50 ppm) and Cu (30 ppm).
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In a typical procedure, N,N-dimethylaniline (0.5 mmol), mal-
eimide (0.25 mmol) and the polymer-based photocatalyst
(10 mg) were mixed in DMF (3 mL). The reaction mixture
was degassed and irradiated using a white LED lamp (1.2
W/cm2, Osa Optics) at room temperature for 24 h in air
unless otherwise noted. The product 1,2,3,4-tetrahydroqui-
noline was isolated and purified via silica chromatography.
For repeating experiments, P-BBT-10 was used as photoca-
talyst for the model reaction (entry 2, Table 1). After each
cycle, P-BBT-10 was isolated, washed with DMF and reused
for another cycle.
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Acknowledgements
The authors acknowledge the Max Planck Society for the fi-
nancial support. Z.J.W. is a recipient of a fellowship through
funding of the Excellence Initiative (DFG/GSC 266) of the
graduate school of excellence “MAINZ” (Materials Science
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8 Bandgap Engineering of Conjugated Nanoporous Poly-
benzobisthiadiazoles via Copolymerization for Enhanced
Photocatalytic 1,2,3,4-Tetrahydroquinoline Synthesis under
Visible Light
Adv. Synth. Catal. 2016, 358, 1 – 8
Zi Jun Wang, Saman Ghasimi, Katharina Landfester,
Kai A. I. Zhang*
Adv. Synth. Catal. 0000, 000, 0 – 0
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These are not the final page numbers!