β-Cyclodextrin-conjugated phthalocyanines as water-soluble and recyclable sensitisers for photocatalytic applications

Article abstract of DOI:10.1039/d1cc00713k

Two zinc(ii) phthalocyanines substituted with two and four permethylated β-cyclodextrin moieties at the α positions have been synthesised and immobilised on the surface of adamantane-modified silica nanoparticles through host-guest interactions. These molecular and supramolecular systems can catalyse the photooxygenation of 1-naphthol and 2-furoic acid in organic and aqueous media with high conversion efficiency and reaction yield, and photodegradation of 2-chlorophenol in water. Having a higher photostability and recyclability, the supramolecular nanosystems are particularly promising for these photocatalytic applications.

Full text of DOI:10.1039/d1cc00713k

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
b-Cyclodextrin-conjugated phthalocyanines as
water-soluble and recyclable sensitisers for
photocatalytic applications†
Cite this: Chem. Commun., 2021,
57, 3567
Received 6th February 2021,
Accepted 2nd March 2021
ab
a
Xiao-Fei Chen
and Dennis K. P. Ng
*
DOI: 10.1039/d1cc00713k
rsc.li/chemcomm
Two zinc(II) phthalocyanines substituted with two and four permethy- through dye-sensitised photoexcitation of ground-state triplet
lated b-cyclodextrin moieties at the a positions have been synthesised oxygen. Various functional dyes, such as porphyrins, boron
and immobilised on the surface of adamantane-modified silica nano- dipyrromethenes, Rose Bengal, methylene blue, fullerenes,
particles through host–guest interactions. These molecular and supra- aggregation-induced emission dyes and ruthenium(II) and
molecular systems can catalyse the photooxygenation of 1-naphthol iridium(III) complexes have been used as singlet oxygen
and 2-furoic acid in organic and aqueous media with high conversion generators,2b,7 which can also be encapsulated in metal–organic
efficiency and reaction yield, and photodegradation of 2-chlorophenol frameworks to promote the sensitisation process and facilitate
in water. Having a higher photostability and recyclability, the supra- heterogeneous photocatalysis.8 Owing to the limited water solu-
molecular nanosystems are particularly promising for these photo- bility of these sensitisers, the photooxygenation reactions are
catalytic applications.
usually carried out in organic solvents. Transformations invol-
ving singlet oxygen as an oxidant in aqueous media are relatively
Being the lowest excited electronic state of molecular oxygen, rare and may require a carrier for the photosensitisers.9
singlet oxygen is a highly reactive species that can oxidise various
As an extension of our studies of advanced phthalocyanine-
compounds, degrade environmental pollutants and cause oxida- based photosensitisers for PDT,10 we previously examined the
tive damage to microbials and tumours.1 It is the predominant sensitising efficiency of a series of zinc(II) and silicon(IV) phtha-
reactive oxygen species in the Type II photosensitised oxidation locyanines towards the photooxygenation of 2-furoic acid,
reactions in photodynamic therapy (PDT).2 Owing to the high several alkenes and 1-naphthol in various organic solvents.11
reactivity, it can react readily with nucleic acids (preferentially at It was found that all these phthalocyanines were highly efficient
the guanine), unsaturated lipids and proteins in the cellular in promoting these reactions and could be recycled through
constituents,3 leading to cell disruption. This species is also a precipitation. We report herein an extension of this study using
highly versatile reagent for a range of chemical transformations, permethylated b-cyclodextrin substituents to increase the
such as the [4+2] cycloaddition, [2+2] cycloaddition, ene reactions hydrophilicity of the phthalocyanine core and enable anchoring
and heteroatom oxidation reactions, for example, of phenols and to silica nanoparticles through host–guest interactions that can
sulfides.1,4 Some of these reactions have been used as the key allow the photocatalytic reactions to be performed in water and
steps in the synthesis of some important natural products and facilitate the recycling of the sensitisers. To our knowledge,
drugs.5 The generation of singlet oxygen from chemical sources using water-soluble phthalocyanines as sensitisers for photo-
can occur through the oxidation of hypochlorite (ClOÀ) by H2O2, oxygenation reactions in aqueous media has rarely been
disproportionation of H2O2 catalysed by MoO42À, base-promoted reported.12 Moreover, although b-cyclodextrins have been used
disproportionation of peracids, decomposition of inorganic per- to complex with water-soluble dyes to form supramolecular
oxides (e.g. CaO2Á2H2O) and thermolysis of endoperoxides.6 Alter- sensitisers,13 b-cyclodextrin-conjugated sensitisers have not
natively, singlet oxygen can also be generated photochemically been reported for this application.
Scheme 1 shows the synthetic route used to prepare the
a Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T.,
Hong Kong, China. E-mail: dkpn@cuhk.edu.hk
conjugates ZnPc(CD)2 (1) and ZnPc(CD)4 (2), which have two and
four permethylated b-cyclodextrin moieties respectively. Treat-
ment of mono-tosylated permethylated b-cyclodextrin 314 with
3,6-dihydroxyphthalonitrile (4) and K2CO3 in N,N-dimethylfor-
mamide (DMF) gave the disubstituted product 5. Statistical con-
densation of this compound with 2 equiv. of unsubstituted
b Institute of Analysis, Guangdong Academy of Sciences (China National Analytical
Center, Guangzhou), Guangzhou 510070, China
† Electronic supplementary information (ESI) available: Experimental details,
additional spectroscopic data, and TEM images, as well as NMR and mass spectra
of all the new compounds. See DOI: 10.1039/d1cc00713k
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 3567–3570
| 3567

View Article Online
Communication
ChemComm
(0.12). The fluorescence band was slightly red-shifted with a
similar fluorescence quantum yield for both compounds in water
(Table 1).
To evaluate the photosensitising efficiency of 1 and 2 in DMF,
1,3-diphenylisobenzofuran (DPBF) was used as the singlet oxygen
scavenger and its absorbance at 415 nm was monitored during
irradiation (l 4610 nm), from which the singlet oxygen quantum
yield (FD) was determined (Table 1). It was found that both
conjugates showed a higher FD value (0.76 for 1 and 0.65 for 2)
than ZnPc (FD = 0.56).14 Interestingly, both compounds could also
sensitise the photoconversion of DPBF to the corresponding
endoperoxide in water, though the rate was slightly slower than
that of ZnPc in DMF (Fig. S3, ESI†). The results showed that both
compounds were efficient singlet oxygen generators in DMF and
water that could enable them to serve as efficient sensitisers for
photoreactions in organic and aqueous media.
To verify this hypothesis, the photocatalytic activity of 1 in
photooxygenation of 1-naphthol was first examined in CHCl3
under different reaction conditions. According to the results
summarised in Table 2, a higher oxygen content could increase
the percentage of conversion and a higher catalyst loading
could also slightly increase the reaction yield. However, increasing
the power of the light irradiation seemed not to be a favourable
condition. Under the optimised conditions, i.e. by using 0.1 mol%
of 1 and irradiation with a halogen lamp at 100 W for 1 h with
continuous bubbling of oxygen, 1-naphthol could be converted
into 1,4-naphthoquinone with 85% conversion and 92% isolated
yield (entry 6).
Scheme 1 Synthesis of b-cyclodextrin-conjugated phthalocyanines 1 and 2.
phthalonitrile (6) in the presence of lithium and anhydrous zinc(II)
acetate in n-pentanol at 150 1C led to the formation of 1 and 2,
which could be isolated by column chromatography on silica gel
followed by size-exclusion chromatography on Bio-Beads S-X1
beads in 9% and 6% yield respectively. Compound 2 is a
crosswise-functionalised or ABAB-type phthalocyanine. Its for-
mation was expected to be promoted by the bulky substituents at
the di-a-positions of the phthalonitrile 5, which would hinder the
self-condensation to form the AABB-type analogue.15 The use of
these sterically hindered and hydrophilic substituents could also
suppress the aggregation of the compounds and increase their
water solubility. The experimental details and characterisation data
are given in the ESI.†
The electronic absorption and fluorescence properties of 1
and 2 were studied in DMF and water (Fig. S1, ESI,† and
Table 1). In DMF, the di-a-substituted phthalocyanine 1 exhibited
a strong Q band at 692 nm, which was red-shifted to 717 nm for
the tetra-a-substituted conjugate 2. The Q band was unshifted for
1 in water, but was further red-shifted to 726 nm for 2 in water.
The red shift of the Q band by introducing substituents at the
a positions has been well-documented.15 The sharp and intense
Q band for these compounds indicated that they remained
essentially non-aggregated in these media, which was supported
by the linear relationship between the Q-band absorbance and the
concentration of the conjugate (see Fig. S2, ESI,† for the spectra in
water). Upon excitation at 610 nm, compound 1 showed a
fluorescence band at 696 nm with a fluorescence quantum yield
(FF) of 0.17 in DMF with reference to unsubstituted zinc(II)
phthalocyanine (ZnPc) (FF = 0.28),14 whilst the emission of 2
was red-shifted (to 722 nm) with a relatively lower value of FF
Similar results were obtained when the solvent was changed
to tetrahydrofuran (THF), CH3CN or water (with 1% CH3CN
added to facilitate the dissolution of 1-naphthol) (Table S1,
ESI†). The promising results for the aqueous reactions could be
attributed to the bulky and hydrophilic b-cyclodextrin substi-
tuents that can preserve the photosensitising property of the
phthalocyanine core in water. The photosensitising efficiency
of the tetra-a-substituted analogue 2 was also evaluated and
similar results were obtained (Table S1, ESI†). These results
showed that both 1 and 2 were indeed efficient photosensitisers
Table
2
Photooxygenation of 1-naphthol in CHCl3 using 1 as the
photosensitisera
Table 1 Photophysical properties of 1 and 2 in DMF and H2O
lmax (nm)
Light
1
Time Conv. Yield
a
b
c
Compound
Solvent
(log e)
lem (nm)
FF
FD
Entry power (W) Atmosphere (mol%) (h)
(%)
(%)
1
DMF
334 (4.73),
692 (5.30)
342 (4.78),
692 (5.23)
341 (4.62),
717 (5.26)
342 (4.66),
726 (5.16)
696
702
722
729
0.17
0.16
0.12
0.12
0.76
—
1
2
3
4
5
6
300
300
300
300
100
100
Open air
0.001
0.001
0.01
0.01
0.01
0.1
6
4
2
2
2.5
1
30
50
52
70
88
85
85
87
89
88
84
92
Bubbled air
Bubbled air
Bubbled O2
Bubbled O2
Bubbled O2
1
2
2
H2O
DMF
H2O
0.65
—
a
The mixtures were irradiated (l 4 610 nm) at ambient temperature.
The percentage of conversion and reaction yield were calculated based
a
b
Excited at 610 nm. Using ZnPc as the reference (FF = 0.28 in DMF). on the amounts of starting material recovered and isolated product
Using ZnPc as the reference (FD = 0.56 in DMF).
c
respectively.
This journal is © The Royal Society of Chemistry 2021
3568 | Chem. Commun., 2021, 57, 3567–3570

View Article Online
ChemComm
Communication
Table 3 Photooxygenation of 2-furoic acid using 1 and 2 (0.1 mol%) as
images of both nanosystems showed a uniform morphology with
a diameter of 100–200 nm (Fig. S5, ESI†). Dynamic light scattering
analysis also revealed a hydrodynamic diameter of 194 nm with a
polydispersity index of 0.04–0.05 for both nanosystems (Fig. S6,
ESI†). The loading of phthalocyanine was determined by sonicating
the nanoparticles in DMF to fully extract the phthalocyanine into
the solution, of which the concentration was quantified spectro-
the photosensitisersa
Photosensitiser
Solvent
Conv. (%)
Yield (%)
1
2
1
2
CHCl3
CHCl3
H2O
80
85
34
38
75
88
85
90
scopically. The loading was found to be 1.8 wt% (or 5.4 nmol mgÀ1
)
for 1@SiO2 and 1.9 wt% (or 5.3 nmol mgÀ1) for 2@SiO2.
H2O
These phthalocyanine-modified nanoparticles showed the
characteristic Q band of phthalocyanines in water at 692 nm
(for 1@SiO2) or 725 nm (for 2@SiO2). The position was virtually
unshifted compared with that of free 1 and 2 (Table 1), but the
absorbance and fluorescence were significantly weaker (Fig. 1a
and b). These nanoparticles could also generate singlet oxygen
a
The mixtures were bubbled with O2 and irradiated (100 W,
l 4 610 nm) at ambient temperature for 2 h. The conversion efficiency
and percentage yield were determined by 1H NMR spectroscopy using
2,5-dimethylfuran as an internal standard.
for photooxygenation of 1-naphthol both in organic and in upon excitation in water, though the efficiency was slightly
aqueous media.
lower than that of the free counterparts (Fig. S3, ESI†). Inter-
The photooxygenation reaction of 2-furoic acid to 5-hydroxy- estingly, the photostability of these nanoparticles in water was
2(5H)-furanone was also studied using 1 and 2 as the photosensi- significantly improved compared with the free phthalocya-
tisers in CHCl3 and in water (Table 3). The reactions proceeded nines. As shown in Fig. 1c, the Q-band absorbance remained
smoothly in CHCl3 for both compounds with comparable percen- almost unchanged for 1@SiO2 and 2@SiO2 upon irradiation
tage of conversion and reaction yield as those for the photooxy- with a 100 W halogen lamp for 6 h, whilst that of 1 and 2
genation of 1-naphthol, but their conversion efficiency was vanished after irradiation for 3 h.
significantly lower in water (o40%). As shown in Table S2 (ESI†),
These phthalocyanine-modified silica nanoparticles were
a longer irradiation time (to 4 h) could not improve the result. By then used to catalyse the photooxygenation of 1-naphthol and
increasing the loading of photosensitiser to 2 mol%, a moderate 2-furoic acid in water. A smaller amount of CH3CN (0.2% vs. 1%
conversion efficiency of ca. 50% could be achieved.
used in the above studies) was added in order to avoid detach-
As an important parameter of photocatalysts, the photostability ment of the phthalocyanines from the nanoparticles. As shown
of 1 and 2 was examined in different solvents by monitoring the in Table 4, both the percentage of conversion and isolated yield
change in Q-band absorbance during the irradiation time. As shown could reach ca. 90% in just 1 h for the photooxygenation of
in Fig. S4 (ESI†), the absorbance remained essentially unchanged for 1-naphthol, which were comparable with those for the free
both compounds in THF and CH3CN over 6 h. By contrast, the phthalocyanines (Table S1, ESI†). For the photooxygenation of
compounds underwent photo-bleaching in CHCl3, DMF and water, 2-furoic acid, the percentage of conversion was significantly
and the absorbance almost vanished after irradiation for 3 h. The
decrease in absorbance was moderate for CH3OH. These results
suggested that the photostability of these two compounds was
similar and followed the order: THF E CH3CN 4 MeOH 4
CHCl3 E DMF E water. It is worth noting that despite the
relatively low photostability in CHCl3 and water, as mentioned above,
both 1 and 2 could sensitise the photooxidation of 1-naphthol and
2-furoic acid effectively. This observation could also explain why a
higher power and a longer light irradiation time could not promote
the reactions in CHCl3 (Table 2) and water (Table S2, ESI†).
To address the photostability issue in water, the compounds
were immobilised on silica nanoparticles functionalised with ada-
mantane molecules on the surface via host–guest interactions.16 It
was believed that the immobilisation could enhance the photo-
stability of the compounds and enable the photooxygenation reac-
tions to be carried out heterogeneously, which can facilitate
the recycling of the photosensitisers by simple centrifugation. To
Fig. 1 (a) Electronic absorption and (b) fluorescence (lex = 610 nm)
prepare these phthalocyanine-modified nanoparticles, adaman- spectra of 1, 2, 1@SiO2 and 2@SiO2 in water. The phthalocyanine concen-
tane-functionalised silica nanoparticles17 were mixed with 1 or 2
tration was fixed at 2 mM. (c) Change in the Q-band absorbance of 1, 2,
1@SiO2 and 2@SiO2 in water upon irradiation (100 W, l 4 610 nm)
at ambient temperature. (d) Photocatalytic decomposition of 2-chloro-
(0.1 mg mLÀ1) in water. After stirring at ambient temperature for
24 h, the mixtures were centrifuged and the solid obtained was
phenol (initial concentration = 4 mM) using 1, 2, 1@SiO2 or 2@SiO2 (at
washed thoroughly with water to give 1@SiO2 and 2@SiO2 as a
1 mol% of phthalocyanine) as the photosensitiser in water (with 0.2%
greenish powder. The transmission electron microscope (TEM) CH3CN) in open air and upon irradiation with the above light source.
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 3567–3570
| 3569

View Article Online
Communication
ChemComm
Table 4 Photooxygenation of 1-naphthol and 2-furoic acid using 1@SiO2
and 2@SiO2 as the photosensitisers in water (with 0.2% CH3CN)a
of b-cyclodextrin moieties could also facilitate the immobilisation
on the surface of adamantane-modified silica nanoparticles
through host–guest interactions, giving the nanosystems 1@SiO2
and 2@SiO2. All these molecular and supramolecular systems
served as efficient sensitisers for photooxygenation of 1-naphthol
and 2-furoic acid, as well as photodegradation of 2-chlorophenol.
The nanosystems exhibited higher photostability and recyclability,
and hence are particularly promising for photocatalytic applica-
tions in aqueous media.
Photosensitiser
Substrate
Time (h)
Conv. (%)
Yield (%)
1@SiO2
2@SiO2
1@SiO2
2@SiO2
1-Naphthol
1-Naphthol
2-Furoic acid
2-Furoic acid
1
1
8
8
87
89
89
76
94
92
85
89
a
The mixtures containing the photosensitisers (at 1 mol% of phthalo-
cyanine) were bubbled with O2 and irradiated (100 W, l 4 610 nm) at
ambient temperature.
This work was supported by a grant from the Research
Grants Council of the Hong Kong Special Administrative
improved to 89% (or to 76%) after immobilising 1 (or 2) on the Region (Ref. No. 14324116).
silica nanoparticles and extending the reaction time to 8 h.
That the latter could be done was due to the enhanced photo-
stability of the nanoparticles.
Conflicts of interest
As these reactions proceeded in a heterogeneous way, the
There are no conflicts to declare.
photocatalysts could be recovered readily by centrifugation
after the reactions. The nanoparticles recovered were then
washed with water thoroughly with sonication and then used
in another run of the photooxygenation reactions. As shown in
Table S3 (ESI†), for the reaction of 1-naphthol, the yield of
the product was decreased by only 9% (for 1@SiO2) or 6%
(for 2@SiO2) after recycling 5 times (i.e. irradiation for a total of
5 h). For the reaction of 2-furoic acid, the decrease in reaction
yield was slightly higher to 16% (for 1@SiO2) or 23% (for 2@SiO2)
after 5 cycles due to the longer reaction time (i.e. irradiation for a
total of 40 h). The results showed that these nanoparticle-based
photocatalysts exhibited significantly higher photostability than
the free counterparts, enabling them to serve as improved and
recyclable photocatalysts.
Notes and references
1 I. Pibiri, S. Buscemi, A. P. Piccionello and A. Pace, ChemPhotoChem,
2018, 2, 535.
2 (a) M. S. Baptista, J. Cadet, P. Di Mascio, A. A. Ghogare, A. Greer,
M. R. Hamblin, C. Lorente, S. C. Nunez, M. S. Ribeiro, A. H. Thomas,
M. Vignoni and T. M. Yoshimura, Photochem. Photobiol., 2017,
93, 912; (b) M. Lan, S. Zhao, W. Liu, C.-S. Lee, W. Zhang and
P. Wang, Adv. Healthcare Mater., 2019, 8, 1900132.
3 P. Di Mascio, G. R. Martinez, S. Miyamoto, G. E. Ronsein,
M. H. G. Medeiros and J. Cadet, Chem. Rev., 2019, 119, 2043.
4 (a) D. Ashen-Garry and M. Selke, Photochem. Photobiol., 2014,
90, 257; (b) J. Fischer, P. Nun and V. Coeffard, Synthesis, 2020,
52, 1617.
5 A. A. Ghogare and A. Greer, Chem. Rev., 2016, 116, 9994.
6 Y. You, Org. Biomol. Chem., 2018, 16, 4044.
To extend the application of these photocatalytic systems, their
efficiency in photocatalytic decomposition of 2-chlorophenol was
examined. Chlorophenols are toxic organic pollutants in wastewater
released from the dye, herbicide and pharmaceutical industries.
Owing to their bio-resistant nature, the decomposition of this
material inevitably relies on advanced chemical methods.18 To
perform the photo-decomposition, a mixture of 2-chlorophenol
and a catalytic amount of 1, 2, 1@SiO2 or 2@SiO2 (at 1 mol%
of phthalocyanines) in water (with 0.2% CH3CN) was irradiated
(100 W) at ambient temperature, and then the concentration of
2-chlorophenol was monitored by HPLC at its absorbance at
275 nm. As shown in the degradation curves (Fig. 1d), the degrada-
tion of 2-chlorophenol was slightly faster for free 1 and 2 compared
with the nanosystems 1@SiO2 and 2@SiO2. However, all the photo-
catalysts could eventually remove virtually all the 2-chlorophenol
over a period of 4 h. Based on the trend of the curves, the two
nanosystems appeared to exhibit more sustained photocatalytic
activity as a result of their higher photostability. These nano-
7 (a) F. Hu, S. Xu and B. Liu, Adv. Mater., 2018, 30, 1801350;
(b) P. C. A. Swamy, G. Sivaraman, R. N. Priyanka, S. O. Raja,
K. Ponnuvel, J. Shanmugpriya and A. Gulyani, Coord. Chem. Rev.,
2020, 411, 213233.
8 (a) J. Hynek, M. K. Chahal, D. T. Payne, J. Labuta and J. P. Hill,
Coord. Chem. Rev., 2020, 425, 213541; (b) J. Chen, Y. Zhu and
S. Kaskel, Angew. Chem., Int. Ed., 2021, 60, 5010.
9 (a) A. Kouridaki, T. Montagnon, M. Tofi and G. Vassilikogiannakis,
Org. Lett., 2012, 14, 2374; (b) D. Kalaitzakis, M. Triantafyllakis,
I. Alexopoulou, M. Sofiadis and G. Vassilikogiannakis,
Angew. Chem., Int. Ed., 2014, 53, 13201.
10 (a) R. C. H. Wong, P.-C. Lo and D. K. P. Ng, Coord. Chem. Rev., 2019,
´
379, 30; (b) P.-C. Lo, M. S. Rodrıguez-Morgade, R. K. Pandey,
D. K. P. Ng, T. Torres and F. Dumoulin, Chem. Soc. Rev., 2020, 49, 1041.
11 H. Xu, W.-K. Chan and D. K. P. Ng, Synthesis, 2009, 1791.
¨
12 (a) G. Schneider, D. Wohrle, W. Spiller, J. Stark and G. Schulz-Ekloff,
Photochem. Photobiol., 1994, 60, 333; (b) P. D’Ambrosio, L. Tonucci,
N. d’Alessandro, A. Morvillo, S. Sortino and M. Bressan, Eur. J. Inorg.
Chem., 2011, 503.
13 (a) M. Bonchio, T. Carofiglio, M. Carraro, R. Fornasier and
U. Tonellato, Org. Lett., 2002, 4, 4635; (b) X. Leng, C.-F. Choi,
H.-B. Luo, Y.-K. Cheng and D. K. P. Ng, Org. Lett., 2007, 9, 2497.
14 J. T. F. Lau, P.-C. Lo, W.-P. Fong and D. K. P. Ng, Chem. – Eur. J.,
2011, 17, 7569.
´
systems could also be recovered and reused for the next run of 15 (a) E. Fazio, J. Jaramillo-Garcıa, G. de la Torre and T. Torres,
Org. Lett., 2014, 16, 4706; (b) W. Cao, K. Wang, I. Ledoux-Rax and
J. Jiang, Inorg. Chem. Front., 2016, 3, 1146.
photodegradation.
In summary, two zinc(II) phthalocyanines substituted with two
´
16 D. Prochowicz, A. Kornowicz and J. Lewınshi, Chem. Rev., 2017,
and four b-cyclodextrin moieties were prepared (compounds 1 and
2). With these bulky and hydrophilic substituents at the a posi-
tions, these compounds remained non-aggregated and could func-
tion as efficient photosensitisers in aqueous media. The presence
117, 13461.
17 X. Ma, S. Sreejith and Y. Zhao, ACS Appl. Mater. Interfaces, 2013,
5, 12860.
18 S. G. Poulopoulos, A. Yerkinova, G. Ulykbanova and V. J. Inglezakis,
PLoS One, 2019, 14, e0216745.
This journal is © The Royal Society of Chemistry 2021
3570
| Chem. Commun., 2021, 57, 3567–3570