Transforming a Fluorochrome to an Efficient Photocatalyst for Oxidative Hydroxylation: A Supramolecular Dimerization Strategy Based on Host-Enhanced Charge Transfer

Article abstract of DOI:10.1002/anie.202100185

The development of non-covalent synthetic strategy to fabricate efficient photocatalysts is of great importance in theranostic and organic materials. Herein, a fluorochrome N,N′-dimethyl 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazolediiodide (MPT) was transformed into an efficient photocatalyst through supramolecular dimerization in the cavity of cucurbit[8]uril (CB[8]). The host-enhanced charge transfer interaction within the supramolecular dimer 2MPT-CB[8] dramatically promoted intersystem crossing to produce triplet. In addition, the staggered conformation of 2MPT-CB[8] facilitated the energy transfer and electron transfer of the triplet. As a result, 2MPT-CB[8] could serve as a high-efficiency photocatalyst for the oxidative hydroxylation of arylboronic acids. This supramolecular dimerization strategy enriches the supramolecular engineering of functional π-systems. It is anticipated that this strategy can be extended to fabricate various π-systems with tailor-made functions.

Full text of DOI:10.1002/anie.202100185

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 9384–9388
International Edition:
German Edition:
doi.org/10.1002/anie.202100185
doi.org/10.1002/ange.202100185
Supramolecular Chemistry
Transforming a Fluorochrome to an Efficient Photocatalyst for
Oxidative Hydroxylation: A Supramolecular Dimerization Strategy
Based on Host-Enhanced Charge Transfer
Bohan Tang, Weiquan Xu, Jiang-Fei Xu,* and Xi Zhang*
Abstract: The development of non-covalent synthetic strategy
to fabricate efficient photocatalysts is of great importance in
theranostic and organic materials. Herein, a fluorochrome
N,N’-dimethyl 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazole-
diiodide (MPT) was transformed into an efficient photo-
catalyst through supramolecular dimerization in the cavity of
cucurbit[8]uril (CB[8]). The host-enhanced charge transfer
interaction within the supramolecular dimer 2MPT-CB[8]
dramatically promoted intersystem crossing to produce triplet.
In addition, the staggered conformation of 2MPT-CB[8]
facilitated the energy transfer and electron transfer of the
triplet. As a result, 2MPT-CB[8] could serve as a high-
efficiency photocatalyst for the oxidative hydroxylation of
arylboronic acids. This supramolecular dimerization strategy
enriches the supramolecular engineering of functional p-
systems. It is anticipated that this strategy can be extended to
fabricate various p-systems with tailor-made functions.
fer interaction is usually not strong enough to significantly
promote ISC process in solution, especially in polar solvents
like water. Therefore, most works are performed in solid state
or molecular aggregates, such as single crystal, H-aggregate
and polymer materials.[34–40] The aggregation may cause
difficulties in characterization and sometimes hinder the
contact of triplet with other molecules, thus inhibiting the
energy transfer or electron transfer of the triplet to efficiently
conduct photochemical reactions. As a result, the radiative
transition to generate phosphorescence or non-radiative
transitions are more preferred, rather than energy transfer
or electron transfer with reactants. To promote a photochem-
ical reaction by rational designed charge transfer interaction
with well-defined supramolecular structure still remains
important but challenging.
We wondered if a supramolecular dimerization strategy
based on host-enhanced charge transfer interaction can be
employed to promote photochemical reactions. On the one
hand, the host provides a confined space and independent
microenvironment to enhance charge transfer interaction for
improved ISC efficiency. On the other hand, the host-guest
complex benefits from the well-defined structure, consider-
able solubility and adequate dynamicity to ensure the contact
of the triplet state guest with other reactants, thus facilitating
energy transfer and electron transfer.[41–54] To this end, N,Nꢀ-
dimethyl 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazole diio-
dide (MPT), an efficient fluorochrome with acceptor-donor-
acceptor structure, was designed.[55] As shown in Scheme 1, it
self-assembled into supramolecular dimer in the cavity of
cucurbit[8]uril (CB[8]) in a parallel and staggered conforma-
tion.[56] Such a conformation could strengthen the charge
transfer interaction and facilitate the contact with other
reactants. Therefore, the supramolecular dimer 2MPT-CB[8]
T
he development of efficient photosensitizers and photo-
catalysts is of great importance in theranostic and organic
materials.[1–8] To design a photosensitizer or photocatalyst,
there are two key points: 1) high-efficiency intersystem
crossing (ISC) to generate long-lived triplet state, and
2) high-efficiency energy transfer or electron transfer to
conduct photochemical reactions.[9–14] To fabricate efficient
photosensitizers and photocatalysts through non-covalent
synthesis is drawing increasing interests for its convenient
construction manners beyond molecular level.[15–25] Benefiting
from the dynamic nature of non-covalent interactions, the
fabrication of functional p-conjugates with dynamicity, stim-
uli-responsibility, adaptivity, is expected. The development of
non-covalent synthetic strategy is vital for the supramolecular
engineering of functional p-conjugates.
To promote the ISC process non-covalently, two major
methods are developed: 1) the introduction of heavy atoms to
promote spin-orbital coupling, which are usually based on the
coordination bond with heavy metals;[26–31] 2) the fabrication
of charge transfer complex, which promotes ISC through
a charge recombination mechanism.[32,33] While charge trans-
[*] B. Tang, W. Xu, Prof. J.-F. Xu, Prof. X. Zhang
Key Laboratory of Organic Optoelectronics & Molecular Engineering,
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
E-mail: xujf@mail.tsinghua.edu.cn
xi@mail.tsinghua.edu.cn
Scheme 1. a) The promotion of the intersystem crossing (ISC) and the
further energy transfer and electron transfer by the formation of
supramolecular dimer 2MPT-CB[8]. b) The excitation and fluorescent
emission of MPT. c) The structure of the host CB[8].
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100185.
9384
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would exhibit a higher ISC efficiency to generate triplet. The
transient absorption was monitored. For 2MPT-CB[8], the
followed energy transfer to generate singlet oxygen (1O2) or
the electron transfer with electron donors to generate radical
cation would be both promoted. The radical cation could
further react with oxygen to form superoxide ion (O2CÀ),
which conducted the oxidative hydroxylation of arylboronic
acids.[57–59] As a result, the fluorochrome MPT could be
transformed into an efficient photocatalyst for oxidative
hydroxylation through supramolecular dimerization with
host-enhanced charge transfer interaction.
To understand whether the ISC process of MPT could be
promoted by supramolecular dimerization, the host-enhanced
charge transfer interaction within the supramolecular dimer
2MPT-CB[8] was characterized. As shown in Figure 1a, MPT
transient species exhibited a lifetime of 60.1 ms (Figure 1c),
indicating the generation of triplet (Supporting Information,
Section 5.1). While for MPT, no long-lived species was
observed, indicating that the MPT triplet was rarely gener-
ated (Supporting Information, Figure S9). These results
indicate that the host-enhanced charge transfer interaction
within the supramolecular dimer promotes the ISC process of
MPT and increases the yield of triplet.
As the generation of triplet was promoted, the followed
energy transfer with O2 to produce 1O2 and the electron
transfer with electron donor to produce radical cation were
1
studied. The generation of O2 was studied by fluorescence
spectroscopy using 2,7-dichlorofluorescin (DCFH) as
a detecting probe. As shown in Figure 2a, 2MPT-CB[8]
Figure 2. a) The enhancement of fluorescence at 524 nm of 40 mM
DCFH and 0.50 mM MPT or 0.25 mM 2MPT-CB[8] in PBS (pH 7.4).
b) The conversion of radical cation of 0.50 mM MPT or 0.25 mM
2MPT-CB[8] with 50 mM TEOA after irradiation of 405 nm light with
different times.
Figure 1. a) The UV/Vis absorption spectra of MPT and 2MPT-CB[8].
b) The fluorescence quenching after the formation of supramolecular
dimer 2MPT-CB[8]. c) Transient absorption decay curve at 530 nm of
the deoxygenated aqueous solution of 2MPT-CB[8] at 298 K upon
excitation of 410 nm laser.
1
exhibited a higher yield of O2, which was about 35 folds
showed a broad absorption peaked at 390 nm. After encap-
sulated by CB[8], the absorption peak was narrowed and
a shoulder band at longer wavelength was observed. The
narrow down of the peak could be due to the restriction of
rotation of pyridine groups in 2MPT-CB[8]. The shoulder
band at longer wavelength clearly indicated the host-
enhanced charge transfer interaction. The promotion of ISC
process by host-enhanced charge transfer interaction was
then investigated. According to the fluorescence spectra,
a strong fluorescence quenching was observed for 2MPT-
CB[8] (Figure 1b). The Ff of MPT in aqueous solution was
measured to be 0.93, while the Ff of 2MPT-CB[8] was only
0.11. The quenching of fluorescence suggested that the ISC
process to generate triplet state could be promoted (Support-
ing Information, Section 3). To confirm the promotion of ISC
process and the generation of triplet state, laser flash
photolysis was utilized. The deoxygenated aqueous solutions
of MPT and 2MPT-CB[8] were excited by a 410 nm laser,
respectively, and a transient absorption at 530 nm was
observed in the case of 2MPT-CB[8]. The decay of the
higher than MPT (Supporting Information, Section 4). To
study the electron transfer to generate radical cation,
triethanolamine (TEOA) was used as electron donor and
UV/Vis spectroscopy was utilized to monitor the generation
of radical cation. As shown in Figure 2b, 2MPT-CB[8] showed
a significantly higher conversion and generation rate of
radical cation than MPT. Therefore, both the energy transfer
to generate O2 and the electron transfer to generate radical
cation are promoted by the host-enhanced charge transfer
interaction.
To investigate which process, either energy transfer with
O2 or electron transfer with TEOA, was more preferred, the
kinetics of these two processes was studied by laser flash
photolysis. The lifetimes of 2MPT-CB[8] triplet in the
presence of different concentrations of O2 or TEOA were
obtained, from which the quenching rates of the triplet were
calculated. The rate constants of the energy transfer with O2
and electron transfer with TEOA were obtained from the
slope of the quenching rates against the concentrations of O2
and TEOA, which was 3.1 ꢁ 107 MÀ1 sÀ1 and 1.2 ꢁ 108 MÀ1 sÀ1,
1
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Figure 4. a) The 2MPT-CB[8] catalyzed photo-induced oxidative hydrox-
ylation of CPBA. b) Time-conversion relationships of the oxidative
hydroxylation catalyzed by MPT and 2MPT-CB[8].
Figure 3. Rates of the 2MPT-CB[8] triplet quenching as a function of
a) the concentrations of O2 and b) the concentrations of TEOA. c) The
enhancement of fluorescence at 610 nm of 50 mM HE, 100 mM TEOA
with 1.0 mM MPT or 0.50 mM 2MPT-CB[8] after irradiation of 405 nm
light with different times.
results indicate that the catalytic properties of MPT are
significantly improved through supramolecular dimerization,
and 2MPT-CB[8] can serve as an efficient photocatalyst for
the oxidative hydroxylation of CPBA.
To verify the universality of 2MPT-CB[8] as photocatalyst
for the oxidative hydroxylation, various substrates were
further tested. The conversions of boronic acids were
respectively (Figure 3a,b). The quenching rate of 2MPT-
CB[8] triplet by O2 was slower than common photosensitizers,
which could be due to the steric hindrance of CB[8].[29,58,60,61]
While the rate of the electron transfer with TEOA was
comparable with common photocatalysts, which could be
owing to the dipole–dipole interaction and hydrogen bonds
between the carbonyl groups of CB[8] and TEOA.[43,62–64]
Because the rate constant of electron transfer with TEOA
exceeded energy transfer with O2, electron transfer would be
preferred in the presence of both TEOA and O2. Therefore,
1
determined by H NMR analysis of the reaction mixture, as
summarized in Table 1. For substrates with electron-with-
drawing groups (EWG), such as cyano group, aldehyde group,
nitro group, 2MPT-CB[8] led to a significantly higher
conversion approaching almost 99% (entries 1–7). Compared
with EWG locating on meta and ortho position, substrates
with EWG on para positions had a high conversion (entries 1–
6). For phenylboronic acid and 4-methylphenylboronic acid,
the conversion was lower but 2MPT-CB[8] still led to a much
higher conversion (entries 8, 9). For heterocyclic boronic
acids, 2MPT-CB[8] also had a superior conversion (entries 10
and 11). In brief, supramolecular dimerization can dramati-
cally enhance the photocatalytic efficiency of MPT, leading to
a higher conversion of the oxidative hydroxylation from
a range of arylboronic acids.
In conclusion, we have proposed a supramolecular dime-
rization strategy based on host-enhanced charge transfer
interaction to efficiently tune the photophysical and photo-
chemical processes of MPT. The supramolecular dimer
2MPT-CB[8] exhibits significantly promoted ISC process to
generate triplet, as well as high-efficiency electron transfer
process to form superoxide ion. As a result, the fluorochrome
MPT is transformed into an efficient photocatalyst to catalyze
the photo-induced oxidative hydroxylation of arylboronic
acids. The supramolecular photocatalyst may also be applied
to other reactions, such as the oxidation of alcohols, amines
and sulfides. It is inspiring that host-enhanced charge transfer
interaction exhibits significant influence on the photophysical
and photochemical properties of p-conjugates, which leads to
diverse functions. This study provides a strategy of non-
covalent synthesis for supramolecular engineering of func-
tional p-systems. It is anticipated that such a strategy can be
1
superoxide ion (O2CÀ) would be generated instead of O2 by
the reaction of radical cation and O2. To study the generation
of O2CÀ, dihydroethidium (HE), a specific fluorescence probe
towards O2CÀ, was used. As shown in Figure 3c, the fluores-
cence at 610 nm increased much faster in the presence of
2MPT-CB[8] than MPT, which indicated a higher yield of
O2CÀ. Therefore, compared with the energy transfer with O2,
2MPT-CB[8] triplet prefers electron transfer with TEOA,
leading to the increased yield of O2CÀ.
As O2CÀ can conduct the oxidative hydroxylation of
arylboronic acids, we wondered if 2MPT-CB[8] could be
used as a metal-free photocatalyst for the reaction. As shown
in Figure 4a, 4-carboxyphenylboronic acid (CPBA) was
chosen as a model substrate, 2MPT-CB[8] served as photo-
catalyst and TEOA as electron donor. The conversion of the
reaction was monitored by 1H-NMR. As shown in Figure 4b,
with only 0.5 mol% addition of 2MPT-CB[8], the conversion
grew linearly over time and reached 90% under 405 nm light
irradiation for 2 h at room temperature. On the contrary,
using MPT as photocatalyst, the conversion was just about
10% after 2-hour irradiation. In this reaction, 2MPT-CB[8]
exhibited a comparable or even better performance than
methylene blue or Ru(bpy)3Cl2, two commonly used small-
molecule photocatalysts, under irradiation of white light
(Supporting Information, Figure S26, Section 7). These
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Entry
Substrate
Conversion [%]
2MPT-CB[8]
MPT
12
1
2
ca. 99
78
7.6
3
6.6
13
ca. 99
ca. 99
ca. 89
4
5[c]
ca. 24
6
7.2
77
7
35
8.7
7.2
4.9
8.8
ca. 99
43
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Acknowledgements
This research was supported financially by the National
Natural Science Foundation of China (21821001). The authors
would like to thank Dr. Hao-Lin Wu, Dr. Xu-Bing Li
(Technical Institute of Physics and Chemistry, CAS) for
their help on laser flash photolysis and Dr. Bo Qin (Tsinghua
University) for his help on the photoreaction.
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Keywords: host–guest systems · photocatalysts ·
pi interactions · supramolecular chemistry ·
supramolecular dimers
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Manuscript received: January 5, 2021
Revised manuscript received: February 8, 2021
Accepted manuscript online: February 15, 2021
Version of record online: March 17, 2021
9388
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