Visible-light photooxidation in water by1O2-generating supramolecular hydrogels

Article abstract of DOI:10.1039/c9sc06481h

We report supramolecular photocatalytic hydrogels, produced by the enzymatically driven self-assembly of low molecular weight gelators (LMWGs). These LMWG precursors are composed of the organic chromophore diketopyrrolopyrrole (DPP), which is bi-functionalized with a series of amino acid (Phe, Tyr, Leu) methyl esters.In situenzymatic hydrolysis of these photoactive precursors results in supramolecular hydrogels that provide a high density of photocatalytic sites. Under visible light irradiation these hydrophobic fibers recruit the reaction substrates and also produce¹O₂, which is used here for the photooxidation of thioanisole (aromatic substrate) and cyclohexyl methyl sulfide (aliphatic substrate), with yields as high as 100% and without over-oxidation. Finally, we demonstrate that the nature of the amino acids in the LWMGs has a central role in dictating J-/H-/mixed state aggregates, gel properties, and, hence, the efficiency of chemoselective photooxidation of thioanisole and cyclohexyl methyl sulfide inside these hydrogels.

Full text of DOI:10.1039/c9sc06481h

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DOI: 10.1039/C9SC06481H
ARTICLE
Visible-Light Photooxidation in Water by 1O2-Generating
Supramolecular Hydrogels
a,b,c,d
Sankarsan Biswas,a,b,c Mohit Kumar,a,b Andrew M. Levine,a,b,c Ian Jimenez,a,e Rein V. Ulijn,*
Received 00th January 20xx,
Accepted 00th January 20xx
Adam B. Braunschweig* a,b,c,d
DOI: 10.1039/x0xx00000x
We report supramolecular photocatalytic hydrogels, produced by the enzymatically driven self-assembly of low molecular
weight gelators (LMWGs). These LMWG precursors are composed of the organic chromophore diketopyrrolopyrrole (DPP),
which is bi-functionalized with a series of amino acid (Phe, Tyr, Leu) methyl esters. In situ enzymatic hydrolysis of these
photoactive precursors results in supramolecular hydrogels that provide a high density of photocatalytic sites. Under visible
1
light irradiation these hydrophobic fibers recruit the reaction substrates and also produce O2, which is used here for the
photooxidation of thioanisole (aromatic substrate) and cyclohexyl methyl sulfide (aliphatic substrate), with yields as high as
100% and without over-oxidation. Finally, we demonstrate that the nature of the amino acids in the LWMGs has a central
role in dictating J- / H- / Mixed state aggregates, gel properties, and, hence, the efficiency of chemoselective photooxidation
of thioanisole and cyclohexyl methyl sulfide inside these hydrogels.
reactively accessible.20 For example, the Meijer21 and Escuder22
groups have shown that a self-assembled, 1D fiber network
enhances catalytic activity compared to its non-assembled
Introduction
There is a growing need for new catalytic methods that
proceed photochemically under visible light illumination, are
compatible with aqueous solvents, and avoid the need for
expensive heavy metals, thereby reducing detrimental
environmental impacts.1-9 In this context, organic photocatalysts
that operate in aqueous solvents have been the focus of
considerable recent interest as sustainable alternatives to
counterpart.23-28 Most recent examples of hydrogel
photocatalysts29 involve co-assembling of the photocatalysts with
gelators, where catalysts and gelators are two separate
components. These hydrogel photocatalysts were used for the
oxidation of iodide,30 hydrogen production,31 polymer
33,34
crosslinking,32 and artificial photosynthesis.
There remain,
however, several drawbacks with hydrogel photocatalysts,
including the limited control of positioning and accessibility of
photoactive sites and the possibility of dissociation of non-
covalently bound photoactive sites from the gel. We propose that
by making the catalyst an integral and inseparable component of
the gelator itself, supramolecular photocatalysts could be
produced with increased number of photoactive sites and
enhanced stability.
Here, we report a new approach to aqueous photocatalysis
composed of organic chromophores that assemble into chiral
supramolecular hydrogels, where the catalytic site is itself both a
structural and functional component of the photoactive gel. The
assembly of the gels is driven by aromatic amino acid bola-
amphiphiles35 that form in a controlled manner through
biocatalytic self-assembly that occurs following hydrolysis of
methyl esters in the presence of -chymotrypsin.36,37 This
versatile and reproducible biocatalytic assembly approach
previously provided functional hydrogels by avoiding productive
kinetic aggregates, and is especially relevant to self-assembly
involving poorly soluble functional components.38,39 Here, the
precursors for these low molecular weight gelators (LMWGs) are
composed of the chromophore diketopyrrolopyrrole (DPP) with
amino acid methyl esters connected through a linker appended
to the heterocyclic N (Figure 1). The self-assembly is activated in
conventional
photocatalysts.10-14
To
date,
these
photocatalysts/photosensitizers are primarily porphyrin-based,
and their photocatalytic applications are mostly limited to water-
soluble substrates.
There remain several substantial challenges that have
precluded the broader adoption of aqueous photocatalysts,
including their inability to act upon hydrophobic molecules in
aqueous media and the lack of alternatives to porphyrin-based
photocatalytic systems that work efficiently in water. Catalytic
supramolecular hydrogels provide a promising solution to this
challenge.15-19 Hydrogel networks can address the solubility
problem because hydrophobic aromatic compounds often
associate into the nanofiber network in water and become
[a] Advanced Science Research Center, City University of New York, 85 St. Nicholas
Terrace, New York, NY 10031, USA
[b]Department of Chemistry, Hunter College, 695 Park Avenue, New York, NY 10065,
USA
[c] PhD Program in Chemistry, Graduate Center, City University of New York, 365 5th
Avenue, New York, NY 10016, USA
[d] PhD Program in Biochemistry, Graduate Center, City University of New York, 365
5th Avenue, New York, NY 10016, USA
[e] The High School for Math, Science, and Engineering, The City College of New York,
240 Convent Avenue New York, NY 10031, USA
†
Electronic Supplementary Information (ESI) available: Experimental details,
organic synthesis, NMR, MS, TEM, AFM, HPLC, UV-Vis, fluorescence emission
spectroscopy and 1O2 calculations. See DOI: 10.1039/x0xx00000x
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situ through enzymatic ester hydrolysis leading to rebalancing of with trifluoroacetic acid in 97% yield, and the final diacid was then
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DOI: 10.1039/C9SC06481H
amphiphilicity to favor unidirectional assembly. Aromatic functionalized with different amino acids via HBTU coupling in
components provide hydrophobic contacts and π…π interactions, 81% yield for DPP-(YOMe)2 (1Y), 84% yield for DPP-(FOMe)2 (1F),
and the amino acid residues provide chirality and H-bonding, and 81% yield for DPP-(LOMe)2 (1L). L-enantiomers of the amino
leading to the formation of 1D superstructures in water. The acids were incorporated to facilitate subsequent -chymotrypsin
ability to vary the amino acid side chain provides a versatile route catalysed hydrolysis. All LMWG precousors were characterized by
to explore different superstructures.40 DPP was chosen as the
chromophore as it has been used previously to generate 1O2 upon
41-45
irradiation with visible light,
agent in catalysis. 46-48
which is a powerful oxidizing
Here we demonstrate the utility of this catalytic platform by
oxidizing the sulfide thioanisole and cyclohexyl methyl sulfide to
their corresponding sulfoxide in water, which is a relevant model
system because sulfoxides are precursors in the synthesis of
several pharmaceuticals and many common preparations suffer
from overoxidation to the sulfone.46, 49 We have assessed the
catalytic activities of three such DPP-based supramolecular
hydrogels that vary in their amino acid side chains, where they
include either tyrosine (Y), phenylalanine (F) or leucine (L). The
two aromatic amino acids were chosen as they are prone to
supramolecular gelation,50 and aliphatic leucine (L) was selected
as a control because it forms fibers but is not prone to gelation.
We found that the hydrogel state plays a vital role in facilitating
catalysis, presumably by sequestering the hydrophobic
thioanisole and cyclohexyl methyl sulfide inside the fiber
networks. This study demonstrates how the incorporation of
photocatalysts in conjunction with hydrogel formation can play a
vital role in photooxidation of hydrophobic substrates in water,
towards sustainable and environmentally benign visible-light
photocatalysis.
Figure 1. Visible-light photooxidation enabled by a supramolecular hydrogel. (A)
Biocatalytic self-assembly of the LMWGs into superstructures via in situ enzymatic
hydrolysis of DPP-(XOMe)2 with -chymotrypsin. These superstructures were used
for the photooxidation of thioanisole and cyclohexyl methyl sulfide via 1O2
formation in water. (B) Photoactive precursors, DPP-(XOMe)2 and components, DPP-
(XOH)2, of the supramolecular hydrogels.
1H NMR and 13C NMR spectroscopy, and high-resolution mass
spectrometry, and all data were consistent with the proposed
structures.
Changing amino acids provided access to distinct
supramolecular structures for investigating how subtle changes
in assembly affect the photophysical properties and catalytic
performance of the hydrogels. The assembly was studied by high
Results and Discussion
The design of our LMWGs requires a chromophore that would
produce long-lived triplets in response to light, which can
generate 1O2, and to which amino acids could be readily
appended to direct subsequent aqueous supramolecular
assembly. Achieving controlled photooxidation by using dissolved
oxygen in aqueous media poses a number of significant
challenges because of the low solubility and short 1O2
lifetime.51,52 Photosensitizers with long excited state lifetimes
facilitate efficient energy transfer, converting 3O2 into 1O2,
thereby increasing catalytic efficiency. DPP was selected as the
photosensitizer because it has been shown previously to
generate 1O2 in the context of photodynamic therapy.45 It is also
known that triplet lifetimes for DPP increase from ps53 to s54
upon assembly because the excited state of DPP has the ability to
undergo singlet fission. Although, DPP has been used in the
context of organic semiconductors and solar cells,55,56 to our
knowledge it has not yet been used for aqueous catalysis or
photocatalysis.
performance
liquid
chromatography
(HPLC),
UV-Vis,
fluorescence, and circular dichroism (CD) spectroscopies,
transmission electron microscopy (TEM), atomic force
microscopy (AFM), and rheology. The DPP heterocycle is
hydrophobic and insoluble in most organic and aqueous solvents,
and, consequently, the DPP-(XOMe)2 derivatives have limited
solubility in aqueous solvents. Thus to make the hydrogels, 200
mM stock solutions of LMWGs 1Y and 1L were prepared in DMSO
and then diluted with 100 mM phosphate buffer (pH 8) containing
1 mg mL-1 -chymotrypsin (Sigma, lyophilized power, ≥ 40
units/mg protein). 1F was sparingly soluble in DMSO at room
temperature, so rather than creating a stock solution, it was
directly weighed in a vial and DMSO was added followed by
heating until solubilized to create a gel. To this hot DMSO
solution, phosphate buffer was added and the solution was
sonicated. After the solution reached room temperature, enzyme
in phosphate buffer was added to a final concentration of 1 mg
mL-1. 10 mM solutions of DPP-(YOH)2 (3Y) and DPP-(FOH)2 (3F)
formed self-supporting hydrogels after 1 h for 3Y and 2 h for 3F,
All three DPP-(XOMe)2 derivatives were synthesized in four
steps following identical procedures. First, the DPP core was
prepared by Dieckmann condensation of dimethyl succinate and
thiophene-2-carbonitrile with 87% yield. The core was then
functionalized at the heterocyclic N with tert-butyl acetate via an
SN2 substitution with 47% yield. The diester was then hydrolyzed
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Figure 2. (A) Sequental enzymatic hydrolysis of DPP-(YOMe)2 in phosphate buffer (pH 8, 100 mM, 5% DMSO) in the presence of -chymotrypsin (1 mg mL-1). (B)
Photograph of reaction vial at 0 min (left) and at 7 h (right). (C) HPLC analysis of enzymatic hydrolysis of DPP-(YOMe)2 at 10 mM. (D) CD spectra showing the formation
of homochiral superstructure during the enzymatic hydrolysis of DPP-(YOMe)2 at 10 mM. (E) TEM images of DPP-(YOMe)2 (left) and DPP-(YOH)2 (right). (F) Oscillatory
rheology of the DPP-(YOH)2 (10 mM) gel showing elastic modulus (G′) of 1.7 kPa (Angular Frequency = 10 rad/s; Frequency sweep).
while the DPP-(LOH)2 (3L) did not gel. All enzymatic hydrolysis time (Figure 2C). The intermediate 2Y was observed within the
reactions were monitored by HPLC to assess the reaction kinetics first 8 min of the reaction, after which 3Y became the dominating
and yields. The hydrolysis of 1Y took 7 h to complete (confirmed species in the reaction mixture. It was found that 92% conversion
by LC-MS, Figure 2C & S16), whereas 3L and 1F took three days into 3Y was observed within 3 h, while complete hydrolysis
for complete hydrolysis, suggesting that gelation occurs well (>96%) took 7 h. Superstructure formation during hydrolysis was
before hydrolysis is completed (Figure S17 &S18).
monitored by CD spectroscopy. After exposing 1Y to the enzyme,
Because of its faster hydrolysis and gelation, 1Y was used to aliquots of the reaction mixtures were taken periodically for CD
explore the kinetics and photophysical properties of gelation in measurements. A bisignated CD signal was observed with a
greater detail. As 3Y formed a self-supporting gel at 10 mM, the positive signal from 650 nm to 550 nm followed by a negative
characterization studies were performed at this concentration, signal from 550 nm to 450 nm (Figure 2D), and signal intensity
unless otherwise noted. We first used HPLC to monitor the increased for all peaks over time. This bisignated cotton effect is
kinetics of enzymatic hydrolysis. After exposing 1Y to the enzyme, a hallmark of dyes that are aggregated into homochiral helical
aliquots of the reaction mixtures were taken periodically and superstructures.57,58 Signal intensity approached saturation after
added to a 1:1 CH3CN:H2O mixture for HPLC-MS analysis. All 3 h, when HPLC data suggests that 92% of the hydrolysis is
species present in the enzymatic reaction were characterized by completed. Similarly, aliquots of the reaction mixtures of 3F and
high-resolution mass spectroscopy, which, together with HPLC 3L were analyzed by CD after 3 d of exposure to -chymotrypsin.
chromatograms, confirmed the presence of 1Y, MeOY-DPP-YOH When comparing CD spectra for DPP-(XOH)2, it was found that 3L
(2Y) and 3Y in the reaction mixtures. The relative peak and 3Y have similar bisignated CD signals between 650-450 nm,
integrations showed an increase of 3Y with a decay of 1Y over whereas the CD signal of 3F had a negative signal from 650 nm to
This journal is © The Royal Society of Chemistry 20xx
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560 nm followed by a positive signal from 560 nm to 525 nm moduli (G’) of 1.7 kPa (Angular Frequency = 10 rad/s) in the
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DOI: 10.1039/C9SC06481H
(Figure S20). The different CD signal for 3F, confirms that frequency sweep (Figure 2F). Rheology measurements on 3F
superstructure geometry is dependent upon F, Y, L amino acid showed the formation of a much stronger hydrogel (G′ of 21 kPa;
side chains.
The biocatalytic self-assembly of 1Y was also monitored by
Angular Frequency = 10 rad/s) compared to 3Y (Figure S26).
Next, all these DPP-(XOH)2 were then tested for their ability to
UV-Vis (Figure S21) and fluorescence (Figure S22) spectroscopies produce 1O2 upon photoirradiation. 1O2 yields for these DPP-
to observe the photophysical changes that occurred upon their (XOH)2 catalysts were determined by following a previously
transformation into 1D-superstructures. To monitor the changes reported indirect method, where methylene blue (MB) was used
of the UV-Vis spectra over time, the reaction mixture was as a standard photosensitizer and 1,3-diphenylisobenzofuran
sonicated and vortexed for 10 s, and aliquots of 1Y (10 mM in 100 (DPBF) as an indicator.63 Please note that due to high extinction
mM phosphate buffer pH 8 with 5% DMSO and 1 mg mL-1 coefficient of DPP, we were unable to use this method in water at
enzyme) were periodically analyzed. Time dependent UV-Vis the gelation concentration. All catalysts (3Y, 3F, 3L) were first
spectroscopy revealed a change in the intensity ratio of the dissolved in DMSO, then DPBF and the catalysts were mixed in a
characteristic DPP max at 512 nm and 540 nm, corresponding to 1:1 ratio and saturated with air. These mixtures were excited for
the 0-1 and 0-0 transitions, respectively (Figure S21C). It has been 2 min (ex= 534 nm for 3L, ex= 535 nm for 3Y and ex= 535 nm for
previously reported that H- and J- aggregates can be 3F) in a fluorimeter, and absorbance were taken immediately
differentiated for the DPP dyes based upon the ratio of 0-1/0-0 after excitation to monitor changes in DPBF intensity. This
vibronic peaks, where a higher 0-1 peak indicates H-aggregates, process was then repeated 5 more times. The decrease in
1
and a higher 0-0 peak is suggestive of J-aggregates.59-61 By absorbance at 418 nm from the photooxidation of DPBF by O2
monitoring the change in this ratio during hydrolysis it was found was then compared with the decrease caused by MB under
that the ratio went from more than unity (t < 40 min) to less than identical experimental conditions to provide 1O2 yields of 67% for
unity (t > 40 min), suggesting that J-aggregates form upon
biocatalytic hydrolysis and assembly. Comparative analysis of the
UV-Vis spectra (Figure S21D) of all DPP-(XOH)2 showed that 3L
has a sharper absorbance and less scattering compared to 3F and
3Y which have much broader features and more scattering in the
UV-Vis spectrum which could be related to the relative degrees
of gelation. The final 0-1/0-0 ratios of all DPP-(XOH)2 are also
dependent upon the amino acids. These data suggest that 3L
formed H-aggregates, whereas 3Y is a J-aggregate, and 3F is a
mixed state. Time dependent fluorescence for the hydrolysis of
1Y (ex = 450 nm, 10 mM) showed that the intensity at 557 nm
and 601 nm increased for the first 50 min, after which intensity
gradually decreased until 100 min, when hydrolysis is nearly
completed (Figure S22). Comparison of the fluorescence spectra
of all three DPP-(XOH)2, taken after 1 d for 3Y and 3 d for 3F and
3L, showed that 3L has the highest emission intensity among all
DPP-(XOH)2 (Figure S23), which can be explained because
aggregation of DPP leads to quenching,62 and the most weakly
assembled hydrogel has the least aggregation-induced quenching.
3F, in contrast showed the least emission intensity, suggesting
the formation of a highly aggregated species (Figure S23).
Further evidence of superstructure formation was provided
by TEM and AFM images of 1Y (Figure 2E, S10 & S12) and 3Y
(Figure 2E, S11 & S13) that were taken at 0 min and at 7 h after
exposure to the enzyme. AFM and TEM show fibers with 6.2 ± 1
nm diameters. This diameter is larger than that of the molecular
dimension of 3Y (~ 2 nm) suggesting bundle formation rather
than single molecular stacks. Microscopic evidence of fiber
formations for 3L and 3F were also obtained by TEM imaging.
Although 1Y transformed into uniform fibers after enzymatic
hydrolysis, 3L and 3F formed a distribution of both thin fibers and
bundled fibers (Figure S14 & S15). These data confirm that all
LMWGs formed 1D-superstructures upon enzymatic hydrolysis,
although only 1Y and 1F formed hydrogels. Rheology
measurements on 3Y and 3F were performed to define
viscoelastic moduli and it was found that they have an elastic
Table 1. (A) Sulfoxidation of thioanisole and cyclohexyl methyl sulfide into their
corresponding sulfoxides by photogenerated 1O2 in the presence of a supramolecular
DPP hydrogel. (B) Optimization table with varying LMWGs and solvents (D2O or H2O).
[Catalyst]
(mM)
Yield
(%)
Catalyst
Substrate
-
7
3Y
3Y
3Y
3Y
0
10
1
5
10
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
0
5
1
2
30
44
10 (1:1 D O/H O)
2
2
10 (D O)
3Y
3Y
3Y
3L
3F
3F
3F
1Y
1Y
1L
1F
3Y
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
R = Ph
86
6
5
10
14
39
67
9
2
5
17
100
2
20
30
10
5
10
10 (D O)
2
10
20
10
10
10
R = Ph
R = Ph
R = Ph
R = Ph
R = C H
6
11
R = C H
3F
3L
10
10
94
88
6
11
11
R = C H
6
Unless otherwise noted, all reactions were conducted with 1.0 mL of reaction solutions and
were used for photooxidation after enzymatic hydrolysis of corresponding DPP-(XOMe)2
without any further purification. Thioanisole and cyclohexyl methyl sulfide were added to
these solutions and stirred under positive pressure of air for 48 h. Final concentration of
the sulfides were 0.4 mM in the hydrogel. The solution was irradiated with a white halogen
light that was connected to the reaction chamber with an optical fiber and reactions were
performed at rt. All yields were calculated from HPLC.
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3Y and 71% for 3L, which is the highest 1O2 yield reported yet for 100%, 94%, and 88% for 3Y, 3F, and 3L respectively. This suggests
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DPP-based molecules,64 and 47% for 3F (Figure S24).
that 2Y is a better catalyst than 3L for non-aromatic substrates as
DOI: 10.1039/C9SC06481H
We then demonstrated the photooxidation of thioanisole and well, thus suggesting that hydrogelation, or more precisely, the
cyclohexyl methyl sulfide by these supramolecular hydrogels. The favorable fiber/solvent interactions may indeed have a positive
challenge associated with sulfoxidation in water is that the effect in catalysis, although the differences are more pronounced
precursor sulfides are hydrophobic and generally insoluble in for aromatic substrates. The overall higher yield is not surprising
water. The nanofiber network of photocatalysts surrounding as
them could increase substrate accessibility towards the enantioselective sulfoxidation using
a
similar trend has been reported previously for
series of Flavin-
a
photocatalysts. The oxidation experiments were performed in 1 cyclodextrin conjugates of both aromatic and aliphatic suflides.66
mL solutions of catalysts, those were prepared in 5 mL vials via To further confirm that this reaction is indeed photocatalyzed, we
enzymatic hydrolysis of the corresponding DPP-(XOMe)2 directly. performed control experiments both with 10 mM of 3Y in dark
Thioanisole and cyclohexyl methyl sulfide were then added into and without any catalyst. Both of these failed to produce
these systems after 7 h for 3Y and after 3 d for 3F and 3L and sulfoxide. Another control experiment for the photooxidation of
stirred under positive pressure of air for 3 days. A white light thioanisole, using agarose hydrogel (5 mg/mL), containing soluble
source (150
W Fiber Optic Dual Gooseneck Microscope DPP-acetate, 7 (see Supporting Information), was performed.
Illuminator) was used for all photooxidation reactions. To explore This experiment resulted in 5 % yield of the oxidized product,
the reaction scope and the effect of reaction conditions on yield, which is significantly lower than that of self-assembled DPP-
we varied solvent, and the amino acid substitutions chain and hydrogelator photocatalysts. We believe that this experiment
concentration of DPP-(XOH)2 (Table 1). For initial optimization of clearly suggests the advantages of hydrogelators, where the
the reaction conditions, we used thioanisole as a model photocatalyst is itself an integral and inseparable component of
substrate. For 3Y it was found that yield increases with increasing the gelator. In addition, we found that the photooxidation
concentration of supramolecular catalyst, with an increase in proceeds optimally at room temperature with 2.5 mol% catalyst
yield from 2% to 30% as [3Y] increases from 5 mM to 10 mM, and loading. Finally, the absence of any evidence of sulfone in HPLC
a similar trend was observed upon increasing [3F] from 5 mM to data showed that this approach is also chemoselective and does
10 mM giving rise to an increase from 14% to 39%. In both cases, not suffer from over oxidation.67-70
gelation was only observed at the higher concentration,
indicating that gelation and fiber formation have a positive role
in catalysis. The observed yield decreased at [3Y] > 10 mM, which
Conclusions
can be explained by the poor light penetration at higher [3Y] as a
consequence of the high extinction coefficient of the LMWGs or
because of slower diffusion of the reactants through this denser
gel. For further confirmation of the positive effect of gelation on
catalysis, 10 mM of 3L, which does not form a hydrogel, was
examined as a catalyst for the photooxidation of thioanisole.
Under the same reaction conditions, the yield at 10 mM (10%)
was much lower compared to the gel-forming samples. In
addition, a control experiment with soluble 10 mM DPP-acetate
(7) was performed which gave rise to a much reduced oxidation
yield of 3% yield compared to that observed in 3Y, 3F and 3L.
These observations strongly suggest that both fiber formation
and hydrogelation are crucial for the catalysis, presumably
because the gel networks trap hydrophobic thioanisole molecules
and improve the accessibility towards photoactive sites. All non-
assembled/randomly assembled precursors (DPP-(XOMe)2) of
these catalysts had lower yields than the assembled forms of
these catalysts. Yields were further increased to 87% in 3Y (10
mM) and 67% in 3F (10 mM) by using D2O phosphate buffer as a
A
new strategy for photooxidation that combines
supramolecular gelation and catalysis in an effort to
simultaneously achieve high yield, circumvent the need for toxic
and expensive metals, and operate in water has been studied. A
small library of supramolecular catalysts was obtained by varying
amino acids appended to the heterocyclic N of the DPP core.
Tuning the hydrophobicity of the amino acids enabled us to
access different superstructures (H- / J- / Mixed state) and
photophysical properties. In studying the conditions for the
catalytic photooxidation of thioanisole and cyclohexyl methyl
sulfide in water, it was found that the favorable solvent/fiber
interactions that cause hydrogelation, and supramolecular
aggregation of the photocatalyst play a vital role in catalytic
efficacy. The effect is more prominent in the case of aromatic
substrates. Lower yields for the oxidation of thioanisole in H2O
compared to D2O can be explained by the relative low 1O2
lifetimes in the former. This catalyst is designed following the
principles of hierarchical structure and emergent photophysics
that are common in nature, but not yet widely adopted by the
catalytic community, and this manuscript shows how at least
some of the major challenges in synthesis could be addressed by
this approach.71 Finally, the high 1O2 yield in these DPP hydrogels
and our design approach have potential further applications
towards sustainable asymmetric visible-light photocatalysis,
wastewater remediation, oxygen sensing, and photodynamic
therapy.
1
solvent. It is known that O2 has a higher lifetime in D2O (68 s)
vs H2O (4 s).65 1:1 (v:v) mixture of D2O and H2O had half the yield
(44%) compared to catalysis in 100% D2O. Photooxidation
experiments in D2O were only performed for 3Y and 3F as these
catalysts gave higher yields in H2O compared to 3L. In order to
investigate if the higher yields for 3Y and 3F are related to the
interaction of the substrate with aromatic cores of F/Y and
thioanisole, we have carried out our photooxidation reaction with
cyclohexyl methyl sulfide (aliphatic sulfide) with all three
catalysts. These experiments resulted in photooxidation yields of
Conflicts of interest
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20 N. Singh, M. P. Conte, R. V. Ulijn, J. F. Miravet and B. Escuder,
There are no conflicts to declare.
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Chem. Commun., 2015, 51, 13213-132D16O.I: 10.1039/C9SC06481H
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We thank the Air Force Office of Scientific Research (FA9550-19-
1-0220), National Science Foundation (NSF) grant number CHE-
1808143, CHE-1G10755, and CHE-2003847. National Science
Foundation CREST Center for Interface Design and Engineered
Assembly of Low Dimensional systems (IDEALS), NSF grant
number HRD-1547830. The NMR data (Bruker Avance 300 MHz)
presented herein were collected in part at the CUNY ASRC
Biomolecular NMR facility and the NMR facility of the City College
of the CUNY. Mass spectra were collected at the CUNY ASRC
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the spectral data and Dhwanit R. Dave for rheology
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Entry for the Table of Contents
An aqueous photocatalytic system has been developed by exploiting the emergent photophysical properties that arise upon the
formation of supramolecular hydrogels. Photocatalytic properties and supramolecular assembly are modulated by the amino acids
appended to an organic chromophore. This study showed that supramolecular hydrogels are a promising platform for sustainable
photocatalysis.
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