A Donor-Acceptor [2]Catenane for Visible Light Photocatalysis

A Donor −Acceptor [2]Catenane for Visible Light Photocatalysis

Article

†

Yang Jiao, Luka Đ or đ evic, Haochuan Mao, Ryan M. Young, Tyler Jaynes, Hongliang Chen,

Yunyan Qiu, Kang Cai, Long Zhang, Xiao-Yang Chen, Yuanning Feng, Michael R. Wasielewski,

Samuel I. Stupp,* and J. Fraser Stoddart *

Cite This: J. Am. Chem. Soc. 2021, 143, 8000 −8010

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sı Supporting Information

ABSTRACT: Colored charge-transfer complexes can be formed by the association

between electron-rich donor and electron-deﬁcient acceptor molecules, bringing about the

narrowing of HOMO−LUMO energy gaps so that they become capable of harnessing

visible light. In an eﬀort to facilitate the use of these widespread, but nonetheless weak,

interactions for visible light photocatalysis, it is important to render the interactions strong

and robust. Herein, we employ a well-known donor−acceptor [2]catenaneformed by the

mechanical interlocking of cyclobis(paraquat-p-phenylene) and 1,5-dinaphtho[38]crown-

0in which the charge-transfer interactions between two 4,4′-bipyridinium and two 1,5-

dioxynaphthalene units are enhanced by mechanical bonding, leading to increased

absorption of visible light, even at low concentrations in solution. As a result, since this

[

2]catenane can generate persistent bipyridinium radical cations under continuous visible-

light irradiation without the need for additional photosensitizers, it can display good

catalytic activity in both photo-reductions and -oxidations, as demonstrated by hydrogen

productionin the presence of platinum nanoparticlesand aerobic oxidation of organic sulﬁdes, such as L-methionine,

respectively. This research, which highlights the usefulness of nanoconﬁnement present in mechanically interlocked molecules for

the reinforcement of weak interactions, can not only expand the potential of charge-transfer interactions in solar energy conversion

and synthetic photocatalysis but also open up new possibilities for the development of active artiﬁcial molecular shuttles, switches,

and machines.

INTRODUCTION

about (Figure 1a) the hybridization and modulation of the

■

35−37

relevant energy levels and lead to the narrowing

The past few decades have witnessed tremendous progress in

−6

HOMO−LUMO energy gaps. As a result, a colored CT

complex is formed with an absorption band red-shifted into the

visible region of the spectrum, even although both D and A

components of this complex are individually colorless.

the area of photocatalysis,

promising approach to acquire sustainable energy

solar power but also provides a powerful toolkit for the

which not only represents a

−14

from

15−17

realization of various synthetic transformations

that are

when relying solely on conventional thermal

pathways. In a photocatalytic process, light excitation is the

38−40

8−20

Moreover, photoexcitation

with visible light is more

challenging

likely to induce single-electron transfer (Figure 1b) within CT

•+

critical step in order to unlock the chemical reactivity of

molecules in their excited states. Compared with high-energy

UV irradiation, visible light

complexes and generate radical-ion pairs composed of D and

•

−

radicals. This photo-induced electron-transfer (PET)

22−24

is a cleaner and milder power

process oﬀers new opportunities for promoting catalysis on

account of its ability to produce radical species under very mild

conditions and without the need for photosensitizers. A ﬂy in

the ointment, as far as this strategy is concerned, is the weak

association between D and A components. In general, the

source with fewer safety concerns. Meanwhile, most organic

substrates are colorless and cannot be excited directly by

visible light. It follows that photocatalystsincluding tran-

sition metal complexes, inorganic nanoparticles, organic

dyes, and so onare usually required in order to harvest the

energy of visible light and trigger desirable photochemical

reactions.

binding constants associated with a single set of CT

−1

interactions are no larger than 10 M , making the

In addition to these well-recognized photocatalysts, a

fundamentally diﬀerent way to uncover more highly designed

photocatalytic systems that harness visible light has been the

Received: February 7, 2021

Published: May 24, 2021

28−31

source of some increased attention

during the past few

years. These new strategies, which take advantage of charge-

2−34

transfer (CT) interactions

between an electron-rich

donor (D) and an electron-deﬁcient acceptor (A), can bring

2021 American Chemical Society

J. Am. Chem. Soc. 2021, 143, 8000−8010

000

Article

by irradiation with visible light. The molecular nature of

DA[2]C⁴⁺comes with multiple advantages, which include (i)

the single-step synthesis of this compound, (ii) the good

−

solubility of its Cl salt in watereven if one of the

component rings, DN38C10, is only sparingly soluble in

waterfacilitating the involvement of aqueous chemistry, (iii)

enhanced CT interactions between 4,4′-bipyridinium (BIPY )

and 1,5-dioxynaphthalene (DNP) units so as to facilitate the

harnessing of visible light, (iv) the fact that the BIPY units

can serve as precursors of persistent radicals to act as

catalysts, and (v) better photostability than its analogous

supramolecular complex. Herein, after describing the synthesis

and characterization of DA[2]C following a literature

56,57,59

procedure,

we undertake an investigation of its ability

to generate radical species on irradiation with visible light and

without the need for photosensitizers. Furthermore, we have

tested the catalytic activity of DA[2]C⁴⁺in the contexts of

both photo-reduction and -oxidation by demonstrating hydro-

gen productionin conjunction with platinum nanopar-

ticlesand the aerobic oxidation of organic sulﬁdes using L-

methionine as a model substrate. In order to highlight the

eﬀect of the mechanical bonding within DA[2]C , a non-

interlocked donor−acceptor complex, DACom formed

between CBPQT⁴⁺and 1,5-bis[2-(2-(2-hydroxyethoxy)-

ethoxy)ethoxy]naphthalene (BHEEEN)is used (Figure 1c)

as a control and compared with DA[2]C⁴⁺in terms of

photocatalytic performance.

EXPERIMENTAL SECTION

■

Figure 1. (a) The charge-transfer (CT) interaction between an

electron-rich donor molecule and an electron-deﬁcient acceptor

molecule can induce the hybridization of energy levels, leading to the

narrowing of the HOMO−LUMO energy gap. (b) A new strategy for

visible-light photocatalysis using the radical species generated by the

direct excitation of a colored CT complex. (c) The structural formulas

Synthesis of DA[2]C·4Cl. The donor−acceptor [2]catenane,

DA[2]C·4Cl, was synthesized (Figure 2a and Figure S1 ) using our

previously reported single-step procedure. CBPQT·4PF (200 mg,

.18 mmol) and DN38C10 (230 mg, 0.36 mmol) were dissolved in

anhydrous MeCN (50 mL), forming a light pink solution. Thereafter,

tetrabutylammonium iodide (TBAI, 15 mg, 0.041 mmol) was added,

and the reaction mixture was stirred at 80 °C for 2 weeks. During this

time, the color of the solution turned gradually from light pink to

deep purple, indicating the enhancement of CT interactions. After

cooling to room temperature, the MeCN solution was concentrated

to 3 mL, followed by the addition of CH Cl (100 mL) to produce a

of a donor−acceptor [2]catenane, DA[2]C , and a donor−acceptor

supramolecular complex, DACom , used for visible-light photo-

catalysis in this research.

complexation far from being strong, especially in solution at

low concentrations. In order to create suﬃcient visible-light-

harvesting ability, we need to consider how to render these

purple precipitate. After ﬁltration, this crude product was puriﬁed

using reverse-phase ﬂash chromatography (C18: 0.1% TFA in H O/

.1% TFA in MeCN). Counterion exchange of the product-

weak interactions stronger and more robust through

molecular design.

−

containing fractions from TFA^t^o^P^F6 (NH PF /H O), and

subsequently from PF₆to Cl (TBACl/MeCN), generated a

−

3−45

Mechanically interlocked molecules

(MIMs) can

precipitate, which was collected by ﬁltration, washed with Me CO

enhance weak interactions by obliging their components to

several times, and dried under vacuum to a ﬀ ord DA[2]C ·4Cl as a

purple solid (146 mg, 62%). See the Supporting Information for

further details.

occupy nanoconﬁned spaces, a property which is helpful in

6−48

investigating

constructing

weak functional group interactions and

kinetically trapped systems. Theoretically,

9−53

Photo-induced Electron Transfer of DA[2]C·4Cl. Photo-

induced electron transfer of DA[2]C·4Cl was performed in order to

generate persistent BIPY^•radical cations, using triethanolamine

(TEOA) as the sacriﬁcial electron donor. A septum-capped cuvette

was charged with a stock solution of DA[2]C·4Cl (500 μL, 1.0 mM in

stoichiometric CT complexation can be achieved in donor−

acceptor MIMs regardless of concentration. Moreover, in

4,55

contrast with covalently linked donor−acceptor dyads,

the

mechanical bonding between the D and A components in

H O) and diluted with a stock solution of TEOA (500 μL, 500 mM in

MIMs fosters the dynamic nature of CT interactions to a

considerable extent, rendering it possible to use them in

adaptive catalytic processes. These features have prompted us

to exploit the use of donor−acceptor MIMs in visible-light

photocatalysis. As a proof of principle, we have employed

buﬀer, adjusted to pH = 7.0) in a N -ﬁlled glovebox. The resulting

solution was irradiated with visible light, using a home-built

photoreactor made of royal blue (450 nm) LEDs with a light

−

•+

intensity of 370 mW cm . The formation of BIPY radical cations

during the irradiation process was detected by UV/vis and EPR

spectroscopy. This home-built photoreactor was also used for the

photocatalytic production of H₂and aerobic oxidation of L-

methionine, in which vials containing the reaction mixtures were

suspended on the top of a single LED, equipped with a lens, and using

a home-built sample holder.

6,57

(

Figure 1c) a donor−acceptor [2]catenane,

namely

DA[2]C in which an electron-deﬁcient cyclophane,

cyclobis(paraquat-p-phenylene) (CBPQT ), and an elec-

tron-rich ring, 1,5-dinaphtho[38]crown-10 (DN38C10), are

mechanically interlockedto perform photocatalytic reactions

001

J. Am. Chem. Soc. 2021, 143, 8000−8010

Article

Figure 2. (a) Single-step synthesis of DA[2]C·4Cl from its component rings, cyclobis(paraquat-p-phenylene) (CBPQT·4PF ) and 1,5-

dinaphtho[38]crown-10 (DN38C10), by tetrabutylammonium iodide (TBAI)-catalyzed reversible ring opening and closure of the tetracationic

cyclophane. (b) Annotated H NMR spectrum (500 MHz, D O, 298 K) of DA[2]C·4Cl.

Photocatalytic Hydrogen Production. Photocatalytic produc-

H O) was added to a septum-capped cuvette containing an acetate

tion of H (pages S21−S28 and Figures S19−S26 in the Supporting

buﬀer (2.0 mL, 0.2 M acetic acid, 0.2 M sodium acetate, 1:1 v/v),

followed by the addition of a stock solution of TMB (40 μL, 4.2 mM

in Me SO). The cell was either purged with Ar or O , or kept in air,

Information) was performed under Ar, using DA[2]C·4Cl as the

photocatalyst, N,N,N′,N′-ethylenediamine tetrakis(methylene-

phosphonic acid) (EDTP, TCI) as the sacriﬁcial electron donor,

along with platinum nanoparticles (Pt NPs, 3 nm size and 1000 ppm

before being irradiated with 450-nm light. The photo-oxidation of

TMB was performed with stirring and monitored by UV/vis

spectroscopy, focusing on the changes in the intensity of the

absorption peaks for the oxidized TMB at around 650 and 900 nm.

dispersion in H O, Sigma-Aldrich) as the catalyst for proton reduction

and the 450-nm LED as light source. A screw-cap vial (9 mL) was

charged with 500 μL of DA[2]C·4Cl (or DACom·4Cl) stock solution

Diﬀerent ROS scavengers, including superoxide dismutase (SOD),

D-mannitol, L-tryptophan, and catalase, were added into the system,

and their inﬂuence on the photo-oxidation processes of TMB was

compared in order to identify which kind of ROS serves as the

oxidant.

(

1.0 mM in H O) and diluted with a stock solution of EDTP (500

μL, 200 mM in buﬀer, adjusted to pH 3.5). Subsequently, a 100-μL

dispersion of Pt NPs was added. The vial containing the reaction

mixture was sealed and purged for 15 min with Ar, and the pressure of

gas in the headspace was equilibrated to 1 atm. Afterward, the vial was

subjected to irradiation with 450-nm light. The gas evolved in the

headspace was analyzed by gas chromatography (GC) to determine

RESULTS AND DISCUSSION

■

the volume of H generated during the reaction. All the experiments

Synthesis and Characterization of the [2]Catenane.

were performed in duplicate to make sure the errors associated with

each data point were no more than 5%.

Photocatalytic Aerobic Oxidation of L-Methionine. Photo-

catalytic aerobic oxidation (pages S28−S39 and Figures S27−S38 in

the Supporting Information ) was performed under an atmosphere of

DA[2]C₄was synthesized from its component rings,

CBPQT ⁺and DN38C10, using our previously reported

single-step procedure conducted under thermodynamic

control. Tetrabutylammonium iodide (TBAI) was employed

as a nucleophilic catalyst to open and close reversibly the

pure O , using L-methionine (Sigma-Aldrich) as the substate,

CBPQT ring in the presence of DN38C10. The pairwise CT

DA[2]C·4Cl as the photocatalyst, and 450-nm LED as the light

source. A screw-cap vial (9 mL) was charged with 500 μL of DA[2]C·

interactions between the BIPY units in CBPQT and the

DNP units in DN38C10 drive the reaction equilibrium (Figure

S1 ) toward the formation of the thermodynamically more

Cl (or DACom·4Cl) stock solution (1.0 mM in H O) and diluted

with a stock solution of L-methionine (500 μL, 10 mM in H O). The

vial containing the solution was sealed and purged for 15 min with O₂,

and the pressure of the gas in the headspace was equilibrated to 1 atm.

Thereafter, the vial was subjected to irradiation with 450-nm light. An

aliquot (10 μL) was taken from the reaction mixture and analyzed by

high-performance liquid chromatography (HPLC) to determine the

conversion of L-methionine and obtain the yield of its sulfoxide during

the reaction. In order to record the standard HPLC trace (Figure

S29 ), L-methionine sulfoxide was prepared according to a previously

stable product, namely DA[2]C . This piece of dynamic

covalent chemistry was performed (Figure 2a) by stirring the

reaction mixture, containing CBPQT·4PF and DN38C10,

along with catalytic amounts of TBAI at 80 °C for 2 weeks,

followed by the puriﬁcation of the [2]catenane using reverse-

phase column chromatography and two counterion exchanges

to aﬀord DA[2]C·4Cl in 62% overall yield.

The structure and purity of DA[2]C⁴⁺were conﬁrmed by a

variety of NMR spectroscopic techniques (Figure 2b and

Figures S2 −S8 ) as well as by HR-MS spectrometry (Figure

reported procedure.

,3 ′,5,5 ′-Tetramethylbenzidine (TMB) was used to probe

Scheme S1) the formation of reactive oxygen species (ROS) during

the irradiation. A stock solution of DA[2]C·4Cl (100 μL, 1.9 mM in

S9 ). In the H NMR spectrum (Figure 2b) recorded at 298 K,

002

J. Am. Chem. Soc. 2021, 143, 8000−8010

Article

Figure 3. (a) UV/vis spectra of CBPQT , DA[2]C , and DACom in aqueous solutions. Inset: Magniﬁcation of the visible region (λ > 400 nm)

showing clearly the CT absorption band. (b) Concentration-dependent UV/vis spectra of DA[2]C in aqueous solution. Inset: Linear relationship

between the absorbance at 525 nm and the concentration of DA[2]C , indicating the intramolecular CT interactions present in this system. (c)

Diﬀerential pulse voltammetry (DPV) of CBPQT⁴⁺and DA[2]C in aqueous solutions, with 0.1 M NaCl as the supporting electrolyte and Ag/

AgCl as the reference electrode. (d) Calculated HOMO and LUMO energy levels showing a smaller energy gap for DA[2]C than for CBPQT .

all the protons belonging to both BIPY²⁺and DNP units are

separated into two sets of signals because both the circum-

to the two-electron reduction from CBPQT⁴⁺to CBPQT²⁽^•⁺⁾.

In contrast, the transformation from DA[2]C to DA[2]-

43,63

2(•+)

rotation and pirouetting motions

within DA[2]C are

is accomplished in a stepwise fashion with two single-

slow on the H NMR time scale in D O at room temperature.

electron reduction peaks at −530 and −630 mV versus Ag/

Compared with free CBPQT , the signals for the BIPY

AgCl. This splitting of the reduction peak arises because the

protons in DA[2]C⁴⁺display, in some cases, dramatic upﬁeld

shifts (Figure S5) as a consequence of shielding eﬀects induced

by intramolecular [π···π] stacking and CT interactions. The

DOSY NMR spectrum (Figure S8) showed only one diﬀusion

inside BIPY unit of DA[2]C is inﬂuenced by CT

interactions more signiﬁcantly than the alongside one,

leading to a larger negative shift of the reduction potential of

the former. Based on the data from spectroscopic and

electrochemical measurements, the HOMO and LUMO

energy levels of CBPQT⁴⁺were calculated (Figure 3d and

Table S1 ) to be −7.91 and −3.92 eV, respectively, with an

energy gap of 3.99 eV. In contrast, the HOMO and LUMO

energy levels of DA[2]C⁴⁺were calculated to be −5.78 and

−3.85 eV, respectively, with the energy gap being narrowed

down to 1.93 eV. The decrease in the energy gap makes it

possible to excite DA[2]C⁴⁺with visible light.

constant in D O for DA[2]C , verifying the mechanically

interlocked nature of the molecule.

When dissolved in H O, DA[2]C·4Cl exhibits a magenta

color, diﬀerent from the colorless solution of CBPQT·4Cl.

This diﬀerence in color is consistent with the UV/vis spectrum

(

Figure 3a), in which a characteristic CT absorption band

(λ_m_a_x= 525 nm) is observed for the DA[2]C·4Cl in aqueous

solution. Moreover, the CT absorption of DA[2]C·4Cl is

higher than that of DACom·4Cl recorded at the same

concentration, indicating the presence of mechanically bond-

enhanced CT association. When a series of UV/vis spectra

were recorded, they revealed a linear relationship (Figure 3b)

between the absorbance at 525 nm and the concentration of

DA[2]C·4Cl, conﬁrming the intramolecular CT interactions

present within this [2]catenane. The molar absorption

Photo-induced Radical Formation with the [2]-

Catenane. Since CT interactions involve the hybridization

of energy levels, in which the contribution to the HOMO is

mainly from the donor unit while the contribution to the

LUMO is mainly from the acceptor unit, we asked ourselves if

the excitation of DA[2]C⁴⁺with visible light might induce

single-electron transfer from a DNP unit to a BIPY unit. In

an attempt to answer this question, the photophysical

properties of DA[2]C·4Cl were investigated (Figure 4a) by

femtosecond transient absorption (fsTA) spectroscopy. Upon

excitation at 520 nm with a 100 fs pulsed laser, DA[2]C⁴

showed increases in absorbance at 399 and 645 nm, indicating

coeﬃcient of DA[2]C·4Cl was calculated to be 1.13 × 10

−

−1

cm , which is an acceptable value with no dependence

on the concentration. This property of DA[2]C enables the

adequate absorption of visible light, even at low concentrations

in solution, rendering it practical as a photocatalyst.

•+

In order to elucidate the inﬂuence of CT interactions on the

HOMO and LUMO energy levels, the reduction potentials of

DA[2]C ·4Cl and CBPQT·4Cl were measured (Figure 3c and

Figure S10 ) by cyclic voltammetry (CV) and diﬀerential pulse

voltammetry (DPV). The single reduction peak of

CBPQT·4Cl at −460 mV versus Ag/AgCl can be ascribed

the formation of the bipyridinium radical cation (BIPY ).

Moreover, the increases in absorbance at 876 and 990 nm can

possibly be assigned to the dioxynaphthalene radical cation

•+

(DNP ), since these features are not observed in the UV/vis

spectrum (Figure S13) of electrochemically reduced

CBPQT². Global analysis (Figure 4b) reveals that all the

(•+)

003

J. Am. Chem. Soc. 2021, 143, 8000−8010

(mono-reduced [2]catenane)

Article

Figure 4. (a) UV/vis/NIR femtosecond transient absorption (fsTA) spectra of DA[2]C⁴in aqueous solution at room temperature, following

pulsed excitation at 520 nm. (b) Multiple-wavelength ﬁts of the kinetics of the DA[2]C⁴excited state, showing that all the positive features at

diﬀerent wavelengths have a common lifetime of 2.8 ps. (c) Evolution of the UV/vis spectra during the continuous irradiation of DA[2]C with

(

50-nm light in the presence of an excess of a sacriﬁcial donor. Inset: Photographs of the DA[2]C aqueous solution before and after irradiation.

d) EPR spectra before and after irradiation of DA[2]C , conﬁrming the generation of persistent radical species. (e) Graphical representation of

the photo-induced electron-transfer process. The direct excitation of DA[2]C with visible light leads to the formation of the charge-separated

+(•+)

state, which, in the presence of an excess of a sacriﬁcial donor, can be transformed into DA[2]C

cation (BIPY ) that resides in the alongside position in the [2]catenane.

, bearing one long-lived bipyridinium radical

•+

positive features at diﬀerent wavelengths in the fsTA spectrum

have similar kinetics, with a common lifetime of 2.8 psthat is

Continuous light irradiation of DA[2]C⁴⁺(0.8 mM) was

performed under a N atmosphere using a large excess of

to say, a charge-separated state of DA[2]C , composed of an

TEOA (0.4 M), and the PET process was monitored by UV/

vis spectroscopy. Whereas the characteristic absorption of

BIPY^•at around 600 nm is close to the CT absorption band

at 525 nm, during the 15 min irradiation, we observed clearly

(Figure 4c) the appearance and rise of a new absorption peak

at 620 nm. This change in the UV/vis spectrum, in

combination with the color change of the solution from pink

to blue, indicates the formation of the BIPY^•radical cation.

The EPR signal (Figure 4d) observed for DA[2]C⁴in

aqueous solution after irradiation conﬁrms the existence of

persistent radicals.

•+

equal number of BIPY and DNP units, is generated by

intramolecular electron transfer upon excitation at 520 nm.

The charge-separated state of DA[2]C can, in principle,

•

produce persistent BIPY radical cations in the presence of

sacriﬁcial electron donors. The eﬃciency of this diﬀusion-

controlled electron-transfer process, however, is rather limited

because of the short lifetime of the CT excited state. In order

to overcome this problem, the sacriﬁcial donor needs to be

selected very carefully. Previous literature tells us that the

introduction of special counterions, such as (tetrakis[3,5-

bis(triﬂuoromethyl)phenyl]borate), can promote signiﬁcantly

By comparing the UV/vis spectrum of DA[2]C solution

2(•+)

after PET with the spectra of DA[2]C

(double-reduced

•

the formation of BIPY upon irradiation with visible light,

2+(•+)

[2]catenane) and DA[2]C

thanks to the interactions between BIPY and the counterions.

Although this hydrophobic tetraphenylborate anion cannot be

employed directly in our system, we took inspiration from this

example and selected TEOA as the sacriﬁcial donor, on

obtained by spectroelectrochemistry (Figures S14 and S15),

+(•+)

the PET product was assigned to be DA[2]C

, in which

•+

only one BIPY unit is reduced, and the resulting BIPY is

located alongside the DN38C10 ring so as to maximize the CT

interactions between the remaining BIPY unit and the

account of its excellent water solubility and ability to interact

with BIPY although somewhat weakly.

encircled pair of DNP units. According to the molar absorption

004

J. Am. Chem. Soc. 2021, 143, 8000−8010

Article

Figure 5. (a) Graphical representation of the proposed mechanism for photocatalytic H production using DA[2]C . First, as a result of visible-

light irradiation, DA[2]C⁴⁺is reduced by a sacriﬁcial donor to form DA[2]C

2+(•+)

. Second, this long-lived radical species can reduce protons to

produce H on the surface of platinum nanoparticles (Pt NPs), while recovering DA[2]C to sustain the catalytic cycle. (b) Stacked gas

chromatography (GC) traces to compare the amount of H evolution after 31.5 h irradiation of the photocatalytic systems containing DA[2]C or

DACom . (c) Comparison of kinetic traces for H production using either DA[2]C or DACom . The amount of H evolution was determined

from the GC traces. The experiments were performed in duplicate to make sure the error was no more than 5% for each point. (d) Comparison of

the amount of H evolution after 3.5 h irradiation of the photocatalytic systems containing diﬀerent concentrations (0, 0.05, 0.25, and 0.5 mM) of

DA[2]C .

coeﬃcient of BIPY^•⁺at 606 nm, the yield of DA[2]C²⁺⁽^•⁺⁾by

combined use of a strong sacriﬁcial electron donor, EDTP, and

PET was estimated to be around 80%, showing a high

highly active proton-reduction catalysts, Pt NPs, is expected to

eﬃciency for the electron transfer from TEOA to DA[2]C .

Thus, we have demonstrated (Figure 4e) that the direct

irradiation of DA[2]C⁴with visible light induces the

formation of a charge-separated state, which, in the presence

of an appropriate sacriﬁcial electron donor, can be transformed

facilitate the evolution of H . See the detailed discussion in the

Supporting Information . During the photocatalysis by DA[2]-

C , analysis of the sample headspace by gas chromatography

(GC) showed a continuous evolution of H (Figure 5b,c), with

a total of 18.0 μmol measured over 31.5 h. In comparison,

+(•+)

into DA[2]C

, a persistent radical species. See the detailed

although employing the complex DACom as the photo-

analysis on the PET pathways in the Supporting Information.

The process is free of the need for additional photosensitizers,

suggesting its potential in the construction of a simple

photocatalytic system.

catalyst led to H evolution, it was at a lower rate and yielded a

smaller amount (8.5 μmol after 31.5 h). The higher

photocatalytic activity of DA[2]C⁴⁺compared with that of

its non-interlocked analogue, DACom , highlights the

advantage of the mechanical bonding present in the [2]-

catenane in reinforcing the CT interactions and promoting the

Photocatalytic Hydrogen Production Using the [2]-

•

Catenane. Considering that the BIPY radical cation has

9−71

often been used

to reduce protons on the surface of

harnessing of visible light. Notably, the loading of DA[2]C in

platinum nanoparticles (Pt NPs), we attempted to use

this photocatalytic system can be lowered from 0.5 to 0.25 mM

DA[2]C·4Cl for photocatalytic production of H . In the

(Figure 5d) without loss of H -evolution activity. Furthermore,

2+(•+)

mechanism proposed in Figure 5a, DA[2]C

serves as an₊

an appreciable amount of H was still detected even at a very

electron mediator, which is formed by the PET of DA[2]C⁴

and then reduces the protons in conjunction with Pt NPs. In

the interest of proton reduction, a low pH value of the solution

low concentration (0.05 mM) of DA[2]C . Although some

metal complexes, e.g., [Ru(bpy) ] , exhibit stronger photo-

sensitizing power (Figure S24 ) than DA[2]C , they must be

used together with an electron relaysuch as CBPQT 

for eﬃcient photocatalysis. We lay claim to the fact that, by

using DA[2]C⁴⁺instead of a combination of colorless viologen

derivatives and colored photosensitizers, we have constructed a

simple photocatalytic system to produce hydrogen under

visible-light irradiation.

is favorable for the production of H because of the increased

concentration of protons. Under this condition, however,

TEOA tends to be protonated, resulting in the loss of its ability

to act as a sacriﬁcial electron donor. In order to address this

issue, N,N,N′,N′-ethylenediamine tetrakis(methylene-

phosphonic acid) (EDTP) was selected for the photocatalytic

H production, on account of its eﬀectiveness as a sacriﬁcial

Photocatalytic Aerobic Oxidation of L-Methionine

electron donor in acidic solutions.

Using the [2]Catenane. In an attempt to display the

In order to test the photocatalytic activity of DA[2]C⁴⁺for

versatility of DA[2]C , we explored its use in photocatalytic

3,74

H production, a degassed buﬀer solution (1.1 mL, pH = 3.5)

aerobic oxidation (Figure 6a) since it has been reported

•+

containing DA[2]C (0.5 mM), EDTP (100 mM), and Pt

that BIPY can produce reactive oxygen species (ROS) in the

NPs (100 ppm) was subjected to 450-nm light irradiation. The

presence of O . At the outset, we investigated the power of

005

J. Am. Chem. Soc. 2021, 143, 8000−8010

Article

Figure 6. (a) Reaction equation and standard experimental conditions for the photocatalytic aerobic oxidation of L-methionine to its sulfoxide

using DA[2]C . (b) The evolution of UV/vis spectra during the oxidation of 3,5,3′,5′-tetramethyl-benzidine (TMB), a probe molecule, conﬁrms

•

−

the generation of O by visible-light irradiation of DA[2]C in air. (c) Stacked high-performance liquid chromatography (HPLC) traces to

compare the yields of the L-methionine oxidation catalyzed by either DA[2]C or DACom . (d) Control experiments of the L-methionine

oxidation under diﬀerent conditions, showing the necessity of DA[2]C , visible light, and O , and suggesting the superoxide radical anion (O )-

•−

mediated oxidation mechanism. (e) Graphical representation of the proposed mechanism for this photocatalytic aerobic oxidation. First, visible-

2+(•+)

light-induced electron transfer between L-methionine and DA[2]C generates S-centered radical cations and DA[2]C

. Second, DA[2]C

•

−

•−

reacts with O to form O and regenerate DA[2]C to sustain the catalytic cycle. Finally, O₂oxidizes the S-centered radical cations to produce

L-methionine sulfoxide.

DA[2]C⁴to generate ROS using 3,3′,5,5′-tetramethyl-

until it reaches a plateau. Finally, L-methionine sulfoxide is

generated (Figure 6c) selectively from L -methionine in 83%

HPLC conversion. In attempts (Figures S36 and S37 ) to lower

the loading of the photocatalyst, 5 mol% DA[2]C⁴was found

to be eﬃcient in achieving a similar yield of L-methionine

sulfoxide after 20 h irradiation. Further decreasing the amount

of DA[2]C⁴⁺to 1 mol% can still promote this transformation

at a lower rate. The control experiments (Figure 6d) under

diﬀerent conditions indicate that all the factors, including

benzidine (TMB) as a probe molecule. On visible-light

irradiation of a mixture of DA[2]C and TMB in air, we

observed characteristic absorption peaks (Figure 6b) of the

oxidized TMB at around 650 and 900 nm, suggesting the

formation of ROS. The absorption became higher or lower for

similar experiments (Figure S27 ) performed with pure O or

Ar, respectively, indicating that the process is indeed oxygen-

dependent. Furthermore, in order to identify which kind(s) of

ROS was/were generated during the irradiation, diﬀerent ROS

DA[2]C , visible light, and O , play critical roles in this

•

scavengers, including D-mannitol (for OH), L-tryptophan

oxidation. Moreover, diﬀerent ROS scavengers (Figure 6d and

Figure S33 ) were introduced into the reaction mixture, and

their inﬂuence on the oxidation process was compared. The

results showed that only SOD inhibited the oxidation, while

the other scavengers did not, conﬁrming that O₂^•serves as a

key intermediate for the oxidation of L-methionine.

(

for O ), catalase (for H O ), and superoxide dismutase

2 2 2

(

SOD, for superoxide), were added (Figure S28) to the

system. In the event, SOD quenched the absorption of

oxidized TMB almost completely, while other scavengers

exhibited negligible eﬀects. These results manifested them-

−

•

−

selves in the formation of the superoxide radical anion (O

)

Based on all this data, we propose a mechanism (Figure 6e)

upon visible-light irradiation of DA[2]C⁴in the presence of

for the photocatalytic aerobic oxidation. First, photo-induced

O , in agreement with previous reports in the literature.

electron transfer occurs between DA[2]C and L-methionine,

2+(•+)

Subsequently, we tested the feasibility of DA[2]C for

photocatalytic aerobic oxidation of organic sulﬁdes using L-

methionine as a model substrate. This reaction (Figure 6a) was

performed with 10 mol% DA[2]C , 1 atm O , and 450-nm

light. Analysis of the reaction mixture by HPLC showed that

the oxidation proceeds continuously (Figure S30) for 11 h

leading to the formation of DA[2]C

and S-centered

2+(•+)

radical cations. Second, DA[2]C

can react with O to give

O₂while recovering DA[2]C to sustain the catalytic cycle.

•

−

Finally, the S-centered radical cations are likely to be

•−

oxidized by the highly reactive O₂, aﬀording L-methionine

sulfoxide as the ﬁnal product.

006

J. Am. Chem. Soc. 2021, 143, 8000−8010

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.1c01493.

Article

When employing DACom⁴⁺as the photocatalyst for this

reaction, we observed the over-oxidation (Figure 6c) of L-

methionine, resulting in the formation of its sulfone in addition

to the sulfoxide. In order to explore the reasons for the

diminished selectivity, ROS scavenger experiments (Figure

S34 ) were performed on the DACom⁴photocatalytic system,

showing that the transformation is inhibited only by L-

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

Detailed synthetic procedures and supplemental charac-

terization data (NMR, EPR, and UV/vis absorption

spectra, and electrochemical measurements as well as

GC and HPLC traces) (PDF)

•−

tryptophan. This ﬁnding means that O , rather than O

serves as the intermediate during the oxidation catalyzed by

•−

DACom , which is diﬀerent from the O -mediated pathway

AUTHOR INFORMATION

Corresponding Authors

in the case of DA[2]C . In addition, a severe bleaching

■

University, Hangzhou 310027, China; orcid.org/0000-

^F ⁱ ^g ^u ^r ^e ^S ³ ⁵ ⁾^o^f^D^A^C^o^m4 was observed after the photo-

J. Fraser Stoddart − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States; School of

Chemistry, University of New South Wales, Sydney, NSW

catalysis, whereas DA[2]C remained stable, with little change

in color. These results indicate that, compared with the

supramolecular complex, the donor−acceptor [2]catenane has

improved catalytic activity, selectivity, and stability during the

oxidation. Therefore, besides the application in photo-

reduction to produce hydrogen, we have demonstrated that

052, Australia; Department of Chemistry, Zhejiang

003-3161-3697 ; Email: stoddart@northwestern.edu

Samuel I. Stupp − Department of Chemistry, Center for Bio-

inspired Energy Science, and Department of Biomedical

Engineering, Northwestern University, Evanston, Illinois

DA[2]C can also be used for selective photo-oxidation of

organic sulﬁdes on account of its ability to harness visible light.

0208, United States; Department of Materials Science and

CONCLUSION

Engineering, Northwestern University, Evanston, Illinois

60208, United States; Department of Medicine, Northwestern

University, Chicago, Illinois 60611, United States; Simpson

Querrey Institute, Northwestern University, Chicago, Illinois

■

6−79

Although charge-transfer interactions are widely used

the templated synthesis of mechanically interlocked molecules,

the potential value of these colored compounds as photo-

catalysts has been overlooked for a long time. In this research,

we have employedwe believe for the ﬁrst timea donor−

acceptor [2]catenane in order to implement visible-light

photocatalysis without the need for additional photosensitizers.

By virtue of the intramolecular charge-transfer interactions

between bipyridinium dications and dioxynaphthalene units,

this donor−acceptor [2]catenane exhibits suﬃcient power to

harness visible light, especially in aqueous solutions of low

concentrations, facilitating the photo-induced electron transfer

to form a persistent bipyridinium radical cation. This photo-

generated radical is capable of (i) hydrogen production in

water as a result of proton reduction and (ii) aerobic oxidation

of L-methionine by generating the highly reactive superoxide

radical anion. Although this proof of concept is noteworthy as

far as the design of new photocatalysts is concerned and

highlights the beneﬁts of mechanical bonding, it carries some

of the drawbacks intrinsic to donor−acceptor systemse.g.,

the short lifetime of their charge-transfer excited states and low

molar absorption coeﬃcientsthat limit the eﬃciency of the

system. Future investigations, therefore, should aim to

0611, United States; orcid.org/0000-0002-5491-7442;

Email: s-stupp@northwestern.edu

Authors

Yang Jiao − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0002-8437-0038

Luka Đorđevic − Department of Chemistry and Center for

Bio-inspired Energy Science, Northwestern University,

Evanston, Illinois 60208, United States; orcid.org/0000-

002-8346-7110

Haochuan Mao − Department of Chemistry and Institute for

Sustainability and Energy at Northwestern, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0001-8742-089X

Ryan M. Young − Department of Chemistry and Institute for

Sustainability and Energy at Northwestern, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0002-5108-0261

Tyler Jaynes − Department of Chemistry and Center for Bio-

inspired Energy Science, Northwestern University, Evanston,

Illinois 60208, United States

Hongliang Chen − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0002-4442-7984

Yunyan Qiu − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0001-9279-4714

Kang Cai − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0002-8883-0142

Long Zhang − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0002-4631-158X

Xiao-Yang Chen − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

orcid.org/0000-0003-3449-4136

80,81

incorporate diﬀerent donor−acceptor pairs

photophysical and photoredox properties into mechanically

interlocked molecules.

with tailored

In its broadest sense, this research has demonstrated that the

2,83

nanoconﬁnement

present in mechanically interlocked

molecules is powerful in reinforcing weak interactions so that

the potentials of these forcesincluding but not limited to

charge-transfer interactionscan be exploited in order to

modulate the properties of systems and achieve desirable

outcomes. We envision that this research will not only provide

a new strategy for solar energy conversion and synthetic

photocatalysis but also open up fresh possibilities for the

design and synthesis of artiﬁcial molecular shuttles, switches,

4,84−86

and machines.

007

J. Am. Chem. Soc. 2021, 143, 8000−8010

Journal of the American Chemical Society

orcid.org/0000-0002-8832-0767

Article

Yuanning Feng − Department of Chemistry, Northwestern

University, Evanston, Illinois 60208, United States;

Michael R. Wasielewski − Department of Chemistry and

Institute for Sustainability and Energy at Northwestern,

Northwestern University, Evanston, Illinois 60208, United

States; orcid.org/0000-0003-2920-5440

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Zhu, J.; Xie, Y. Selective visible-light-driven photocatalytic CO ₂

reduction to CH mediated by atomically thin CuIn S layers. Nat.

5 8

Energy 2019, 4, 690−699.

(12) Wu, Y. A.; McNulty, I.; Liu, C.; Lau, K. C.; Liu, Q.; Paulikas, A.

P.; Sun, C.-J.; Cai, Z.; Guest, J. R.; Ren, Y.; Stamenkovic, V.; Curtiss,

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.1c01493

L. A.; Liu, Y.; Rajh, T. Facet-dependent active sites of a single Cu O

particle photocatalyst for CO reduction to methanol. Nat. Energy

Author Contributions

Y.J. and L.Đ. contributed equally to this work.

†

2019, 4, 957−968.

(13) Kazantsev, R. V.; Dannenhoffer, A. J.; Aytun, T.; Harutyunyan,

Notes

B.; Fairfield, D. J.; Bedzyk, M. J.; Stupp, S. I. Molecular control of

internal crystallization and photocatalytic function in supramolecular

nanostructures. Chem 2018, 4, 1596−1608.

The authors declare no competing ﬁnancial interest.

(14) Zuo, Q.; Feng, K.; Zhong, J.; Mai, Y.; Zhou, Y. Single-metal-

ACKNOWLEDGMENTS

■

atom polymeric unimolecular micelles for switchable photocatalytic

The authors thank Northwestern University (NU) for the

support of this research. Photocatalytic studies, carried out in

the Stupp laboratory, were supported by the Center for Bio-

Inspired Energy Science (CBES), an Energy Frontier Research

Center funded by the U.S. Department of Energy, Oﬃce of

Science, Basic Energy Sciences, under award no. DE-

SC0000989 (S.I.S.). This work was also supported by the

Department of Energy, Oﬃce of Science, Basic Energy

Sciences, under award no. DE-FG02-99ER14999 (M.R.W.).

The research made use of the Integrated Molecular Structure

and Educational Research Center (IMSERC) at NU, which

has received support from the State of Illinois and the

International Institute for Nanotechnology (IIN).

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