Artificial Light-Harvesting Systems Based on AIEgen-branched Rotaxane Dendrimers for Efficient Photocatalysis

Article abstract of DOI:10.1002/anie.202106035

Aiming at the construction of novel platform for efficient light harvesting, the precise synthesis of a new family of AIEgen-branched rotaxane dendrimers was successful realized from an AIEgen-functionalized [2]rotaxane through a controllable divergent ap

Full text of DOI:10.1002/anie.202106035

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: Artificial Light-Harvesting Systems Based on AIEgen-branched
Rotaxane Dendrimers for Efficient Photocatalysis
Authors: Hai-Bo Yang, Wei-Jian Li, Xu-Qing Wang, Dan-Yang Zhang,
Yi-Xiong Hu, Wei-Tao Xu, Lin Xu, and Wei Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202106035
Link to VoR: https://doi.org/10.1002/anie.202106035

Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
Artificial Light-Harvesting Systems Based on AIEgen-branched
Rotaxane Dendrimers for Efficient Photocatalysis
Wei-Jian Li, Xu-Qing Wang*, Dan-Yang Zhang, Yi-Xiong Hu, Wei-Tao Xu, Lin Xu, Wei Wang* and
Hai-Bo Yang*
Dr. W.-J. Li, Dr. X.-Q. Wang, D.-Y. Zhang, Dr. Y.-X. Hu, W.-T. Xu, Prof. Dr. L. Xu, Prof. Dr. W. Wang, Prof. Dr. H.-B. Yang
Shanghai Key Laboratory of Green Chemistry and Chemical Processes & Chang-Kung Chuang Institute, School of Chemistry and Molecular Engineering,
East China Normal University
3663 N. Zhongshan Road, Shanghai 200062, P. R. China
E-mail: xqwang@chem.ecnu.edu.cn (X.-Q. W.); wwang@chem.ecnu.edu.cn (W. W.); hbyang@chem.ecnu.edu.cn (H.-B. Y.)
Abstract: Aiming at the construction of novel platform for efficient
light harvesting, the precise synthesis of a new family of AIEgen-
branched rotaxane dendrimers was successful realized from an
AIEgen-functionalized [2]rotaxane through a controllable divergent
approach. In the resultant AIE macromolecules, up to twenty-one
AIEgens located at the tails of each branches, thus making them the
first successful example of AIEgen-branched dendrimers. Attributed
to the solvent-induced switching feature of the rotaxane branches,
the integrated rotaxane dendrimers displayed interesting dynamic
feature upon the aggregation-induced emission (AIE) process.
Moreover, novel artificial light-harvesting systems were further
constructed based on these AIEgen-branched rotaxane dendrimers,
which revealed impressive generation-dependent photocatalytic
performances for both photooxidation reaction and aerobic cross-
dehydrogenative coupling (CDC) reaction.
shaped feature as well as well-tuned structures for well-defined
arrangements of chromophores.7 During the past few decades,
pioneered by Fréchet, Aida, Vögtle, and Balzani et al., diverse
light-harvesting dendrimers have been successfully constructed,
which have shown great potentials for practical applications
beyond just solar energy conversion.8
Considering the aforementioned attractive photophysical
feature of AIEgens and unique structural feature of dendrimers,
the integrated AIEgen-based dendrimers are expected to be
ideal and promising platforms for the construction of novel
artificial LHSs. However, to our surprise, compared with the
widely-investigated AIEgen-based polymers,9 AIEgen-based
dendrimers have been rarely explored and the research in their
applications for light-harvesting have never been conducted. In
particular, possibly due to the lack of suitable AIEgen units with
specific substituent functional groups for dendrimer growth,
currently no reports on dendrimers with AIEgens at either the
branches or the branching points, which could be defined as
AIEgen-branched dendrimers, have been demonstrated. Notably,
compared with AIEgen-cored or AIEgen-terminated dendrimers
Introduction
Inspired by the natural photosynthesis process, great efforts
have been paid on the construction of novel artificial light
harvesting systems (LHSs) aiming at not only the indepth
understanding of some key aspects of the natural photosynthetic
process such as photoinduced electron transfer and energy
transfer but also developing clean and sustainable energy to
combat global energy issues.1 In particular, to achieve
satisfactory energy collection efficiency, in addition to the
relatively high donor to acceptor ratio, the other prime factor is
the densely packed arrangements of donors.2 However, such
tight and compact arrangements usually lead to the quenching
of the fluorescence of the donors due to the aggregation-caused
quenching (ACQ) effect, which might result in reduced energy
collection efficiencies.3 Over the past few years, to suppress
such ACQ effect, a growing interest in the construction of novel
artificial LHSs based on luminogens with aggregation-induced
emission (AIE) characteristics (i.e. AIEgens) has been
10
11
with the AIEgens units located at either cores or peripheries
of the dendrimers (Figure 1a-b), respectively, in AIEgen-
branched dendrimers (Figure 1c-d), the AIEgen units are
precisely arranged with the skeleton of dendrimers in
a
monodispersed manner. Such well-defined skeleton of AIEgen-
branched dendrimers thus results in a higher density of AIEgens,
which is crucial for light harvesting. Moreover, based on such
high density of AIEgens, the dendrimer skeleton should
inherently impose more pronounced steric hindrance effect on
the free motion of the embedded AIEgen units, which might lead
to an enhanced AIE effect and better AIE performance.
Attributed to these attractive features, the design and synthesis
of AIEgen-branched dendrimers and the further exploration of
their applications in light harvesting will be of great importance.
Herein, based on our on-going interests in mechanically
(MIMs),12
interlocked
molecules
especially
rotaxane
13
dendrimers, a new family of interlocked AIEgen-branched
dendrimers, i.e. rotaxane dendrimers with multiple AIEgens at
the branches (Figure 1e), were designed and synthesized for the
first time. In particular, the unique structural and dynamic
features of the rotaxane dendrimers endowed the resultant AIE-
active macromolecules with interesting AIE behaviors. Moreover,
these AIE macromolecules were further employed for the
construction of novel artificial LHSs, which were proven to be
highly efficient photocatalysts for both photooxidation reaction
and aerobic cross-dehydrogenative coupling (CDC) reaction.
4
witnessed. In contrast to traditional fluorescent molecules with
ACQ characteristics, AIEgens reveal high emission in
aggregated states because of the restricted intramolecular
motion (RIM), thereby making them excellent candidates as
donors for the construction of novel artificial LHSs.5 Moreover,
besides the choices of suitable donors, the accurate positioning
of these donors, which will directly influence the key energy
transfer process, are also crucial for efficient light harvesting.6
To achieve this goal, dendrimers have evolved to be privileged
and most frequently used platforms for the constructions of
artificial LHSs due to their unique highly branched and star-
1
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Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
(
a)
(b)
With the obtained TPE-functionalized [2]rotaxane TPE-R in
hand, the targeted AIEgen-branched rotaxane dendrimers were
then prepared through a controllable divergent approach. As
outlined in Figure 2, the reaction of TPE-R with 1, 3, 5-
triethynylbenzene as the core module using CuI as catalyst
resulted in the first-generation TPE-branched rotaxane
dendrimer TG1 in 83% yield. The subsequent deprotection of
4 2
TG1 with tetrabutylammonium fluoride trihydrate (Bu NF·3H O)
gave rise to the intermediate TG1-YNE with six alkyne groups,
which was further employed as the starting material for the
synthesis of the second-generation TPE-branched rotaxane
dendrimer TG2 with nine TPE moieties in 70% yield through the
reaction with TPE-R. By repeating such deprotection-coupling
process, the synthesis of the third-generation TPE-branched
rotaxane dendrimer TG3 with twenty-one TPE moieties was then
successfully achieved.
AIEgen-cored dendrimers
c)
AIEgen-terminated dendrimers
(
(d)
The resultant TPE-branched rotaxane dendrimers TG1-TG3
was investigated by extensive characterizations including 1-D
1
13
31
multinuclear ( H, C, and P) and MALDI-TOF-MS analysis
Figures S14-S26). For instance, for TG1 and TG2, by the
(
means of mass analysis, the successful synthesis of the
targeted rotaxane dendrimers was confirmed (Figures S25-S26).
However, in the case of TG3, continuous attempts for MS
analysis has been failed possibly due to its large molecular
mass (M = 48917 Da) and low ionization efficiency. Thus GPC
experiments were then performed to evaluate the
monodispersity of these TPE-branched rotaxane dendrimers. To
our delight, as shown in the GPC curves, for each rotaxane
dendrimer, only a single peak with narrow distribution for the
AIEgen-branched dendrimers I
e)
AIEgen-branched dendrimers II
Excitation
(
Emission
Aggregate
n
number-averaged molecular weight (M ) was observed (Figures
S27-S29). Notably, the polydispersity index (PDI) values of all
rotaxane dendrimers were around 1.02, suggesting the
successful formation of monodisperse TPE-branched rotaxane
dendrimers TG1-TG3.
AIEgen-branched
rotaxane dendrimers
This work)
AIEgens as energy donors
Energy acceptors
(
Figure 1. Cartoon representation of AIEgen-based dendrimers and the design
strategy of this work. AIEgen-cored dendrimers (a), AIEgen-terminated
dendrimers (b), AIEgen-branched dendrimers (c and d), and AIEgen-branched
rotaxane dendrimers presented in this work for the construction of novel
artificial light harvesting systems (e).
Moreover,
two-dimensional
(2-D)
diffusion-ordered
spectroscopy (DOSY) analysis provided additional indirect
supports for the successful synthesis of the targeted rotaxane
dendrimers (Figures S30-S32). Only one set of signals
corresponding to the existence of sole specie was founded in all
DOSY spectra of TG1-TG3. More importantly, as revealed by
the significant decrease of the diffusion coefficients (D) along
with the increase of generations of these rotaxane dendrimers:
Results and Discussion
−
10
2
−1
for TG1, (5.76 ± 0.12) × 10 m s ; for TG2, (2.69 ± 0.13) ×
1
Synthesis and Characterization of the TPE-branched
Rotaxane Dendrimers. In this study, a new TPE-functionalized
2]rotaxane14 TPE-R was designed as the key building block
0−10 m2 s−1; for TG3, (1.58 ± 0.10) × 10−10 m2 s−1, the
monodispersity and progressive size increase of the obtained
rotaxane dendrimers were clearly confirmed. Such size changes
were further determined by atomic force microscopy (AFM)
analysis. According to the AFM images shown in Figure S33, all
rotaxane dendrimers TG1-TG3 revealed a uniform sphere-like
morphology. Moreover, the gradual increase of the average
heights from 4.0 ± 0.1 nm (TG1) to 8.1 ± 0.2 nm (TG2) and 12.5
[
(
Figure 2). In the [2]rotaxane, the pillar[5]arene macrocycle S3
functionalized with two TIPS-protected alkynes was employed
as the wheel components.15 It should be noted that the TIPS-
protected alkynes could be gently converted to the alkyne group
that further serves as reactive sites for dendrimer growth. In
addition, the bulky TPE moiety, one of the most typical AIEgens,
was chosen as one stopper in the axle component, and the
monosubstituted platinum-acetylide moiety was selected as not
only another stopper but also the dendrimer growth sites.16 More
importantly, to endow the targeted AIEgen-branched rotaxane
dendrimers with dynamic features, the urea moiety17 was
introduced as the stimuli-responsive site into the axle
component. According to such design strategy, the targeted
±
0.3 nm (TG3) was observed. According to all these results
described above, the successful synthesis of the targeted TPE-
branched rotaxane dendrimers TG1-TG3 was confirmed.
AIE Behaviors of the TPE-branched Rotaxane Dendrimers.
With the targeted TPE-branched rotaxane dendrimers in hand,
2 2
their AIE behaviors were then evaluated. In this study, CH Cl
was selected as the good solvent and MeOH was chosen as the
poor solvent, respectively. Due to the existence of the urea
moiety in each branch of these rotaxane dendrimers as the
stimuli-responsive site, the addition of MeOH was supposed to
[
2]rotaxane TPE-R was successfully prepared in 80% yield on a
gram scale through the threading-followed-by-stoppering
strategy (Scheme S1).
2
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
(a)
CuI, DCM/Et2NH
r.t., overnight, 83%
[2]rotaxane TPE-R
TPE-branched rotaxane dendrimer TG1
(b)
I
II
TG1
TG2
TG3
Figure 2. (a) Synthesis of TPE-branched rotaxane dendrimer TG1 by a CuI-catalyzed coupling reaction of TPE-R and 1, 3, 5-triethynylbenzene. (b) Schematic
representation of a controllable divergent strategy for the synthesis of TPE-branched rotaxane dendrimers TG2 and TG3. Reaction conditions: (I) (a) Bu NF·3H O,
4
2
THF, rt, 2 h; (b) TPE-R, CuI, DCM/Et
2
NH, rt, overnight, 70%; (II) (a) Bu
4
NF·3H
2 2
O, THF, rt, 2 h; (b) TPE-R, CuI, DCM/Et NH, rt, overnight, 38%.
play dual roles. On one hand, the increase amount of MeOH
could significantly change the polarity of the solvent, which might
further induce the movement of the pillar[5]arene rings away
from the urea moiety to the alkyl chain moiety through the
interaction between the urea moiety with the MeOH molecules.18
On the other hand, such solvent-induced motions of the wheel
component would further lead to the modulation of the AIE
behaviors of the integrated TPE-branched rotaxane dendrimers,
making them attractive as smart AIE-active macromolecules.
To test such hypothesis, the stimuli-responsive behavior of
addition of equivoluminal CD
3
OD to the CD
2
Cl
2
solution of TPE-
1
RC, almost no changes of the H NMR spectra were observed,
indicating the urea moiety did act as the stimuli-responsive site
(Figure S39).
Similar with [2]rotaxane TPE-R, as revealed by the 1H NMR
study, the resultant TPE-branched rotaxane dendrimers TG1-
TG3 displayed similar solvent-induced switching motions of the
pillar[5]arene rings located at both the branches and branching
points (Figures S41-S46). More importantly, along with such
switching process, the arrangements as well as the density of
the TPE moieties within the integrated rotaxane dendrimers
were further modulated, which might influence the AIE behaviors.
To evaluate such effect, the AIE behaviors of the resultant TPE-
branched rotaxane dendrimers were then investigated in detail.
As revealed by the fluorescence spectra, for all rotaxane
dendrimers, they only displayed very weak fluorescence in pure
[
2
2]rotaxane TPE-R was firstly evaluated. With the assistance of
-D H- H COSY analysis, the signals of the methylene protons
1
1
of the alkyl chain were clearly identified. By changing the volume
ratios of CD
2
Cl
2
/CD
3
OD that could adjust the solvent polarity, a
1
series of H NMR spectra were recorded. As expected, upon the
3 2 2
increase of the fractions of CD OD in CD Cl , obvious downfield
shifts of the signals of the methylene protons (H8–12) nearby the
2 2
CH Cl solution. Upon the gradual increase of the fraction of
urea moiety were observed. Meanwhile, the signals of the
MeOH (f, vol%) from 30% to 65%, only slight increase of the
emission intensity was revealed. By further increasing the f to
65% and finally to 98%, the remarkable enhancement in the
emission intensity was observed, thus indicating that all TPE-
branched rotaxane dendrimers TG1-TG3 are typical AIEgens.
Notably, as the generation of the rotaxane dendrimers increased,
the extent of the enhancement in the fluorescence intensity
when f increased from 65% to 98% decreased progressively
from 31-fold for TG1 and 14-fold for TG2 to 9-fold for TG3
(Figure 3b, d, f). Such trend might be attributed to the more
methylene protons near the platinum-acetylide moiety (H3–7
)
underwent the obvious upfield shifts (Figure 3a). Such
interesting solvent-induced motions of the wheel component of
the [2]rotaxane was possibly due to the breaking of the
hydrogen bonding between the pillar[5]arene ring and the urea
moiety that led to the translational motion of the pillar[5]arene
ring, which was in accord with our previous reports on solvent-
induced switching of similar rotaxanes.13b,13d To confirm such
hypothesis, the model [2]rotaxane TPE-RC without the urea
moiety was synthesized (Scheme S9). As expected, upon the
3
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Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
1
00000
Excited 345 nm
0%
30%
50%
(
a)
480 nm
(c)
(
b)
50
98
8
6
4
2
0000
0000
0000
0000
0
65%
70%
75%
80%
MeOH
85%
90%
SwitchingandAggregating
9
9
5%
8%
4
00
450
500
550
600
650
Wavelength (nm)
TG1: 88.0 ± 5.0 nm
8
6
0000
0
30%
%
(
d)
50 98
(e)
50%
65%
70%
75%
0000
CD
2
Cl
2
/MeOD
40000
80%
85%
90%
95%
98%
8-11
3+12
6
5
1
:2
47
2
0000
CD
2
Cl
2
/MeOD
1:1
0
4
00
450
500
550
600
650
CD
2
Cl
2
/MeOD
Wavelength (nm)
TG2: 106.0 ± 6.0 nm
5
:4
8
0000
CD
2
Cl
2
/MeOD
0%
30%
50%
(
f)
50 98
(g)
10:3
6
4
2
0000
0000
0000
0
65%
70%
75%
80%
85%
90%
CD
2
Cl
2
/MeOD
10:1
95%
98%
CD
2
Cl
2
5+6
4
14
3
13
9
10 11
400
450
500
550
600
650
8
7+12
Wavelength (nm)
TG3: 133.0 ± 4.0 nm
Figure 3. AIE behaviors of TPE-branched rotaxane dendrimers TG1-TG3. (a) Cartoon representation of the AIE behaviors of TG3 during which both the
arrangements and the density of the TPE moieties within the integrated rotaxane dendrimers were modulated through the solvent-induced switching motions of
the pillar[5]arene rings (top) and 1H NMR spectra (CD
spectra of TG1 (b), TG2 (d), and TG3 (f) in CH Cl /MeOH with various MeOH fractions ([TPE units] = 30 μM, λex = 345 nm), inset: photographs of TGn (n = 1, 2,
) in CH Cl /MeOH with 50% (left) and 98% (right) MeOH fractions upon UV lamp excitation (λex = 365 nm); TEM images of TG1 (c), TG2 (e), and TG3 (g) in
2 2
Cl , 298 K, 400 MHz) of solvent-induced switching behavior of [2]rotaxane TPE-R (bottom); Fluorescence
2
2
3
2
2
aggregate states.
pronounced depression of free motions of the incorporated TPE
units due to the more remarkable steric hindrance effect of the
higher-generation dendrimer skeleton, which thus led to a
relatively reduced AIE efficiency upon the further aggregation.
To provide additional supports for such AIE phenomenon
described above, transmission electron microscopy (TEM) and
dynamic light scattering (DLS) analysis were further performed
to investigate the aggregation of TG1-TG3. As revealed by the
TEM images, the formation of homogenous spherical particles in
the aggregated state for all TPE-branched rotaxane dendrimers
TG1-TG3 was observed (Figures 3c, e, g). In addition, the DLS
results further suggested the formation of nanoscale aggregates
of TG1-TG3 in the aggregated state (Figures S73-S75). Notably,
the values of the average diameters of the spherical particles in
TEM analysis fitted very well with that of the average
solubility in MeOH compared with TG1 (Figure S50). While in
the case of C2, attributed to the absence of urea moieties, 1
NMR study suggested that it did not show stimuli-
responsiveness upon the addition of equivoluminal CD OD
(Figure S40). It also behaved typical AIE feature similar with
TG1 (Figure S51). However, regarding to the fluorescence
H
3
F
quantum yield (Φ ), both C1 (0.28%) and C2 (1.56%) revealed
smaller values than that of TG1 (4.29%) (Table S1), thus
highlighting that not only the important role of the dendritic
rotaxane skeleton but also the solvent-switching feature of the
TPE-branched rotaxane dendrimers, both of which could lead to
the restriction of intramolecular rotations (RIR) of the TPE units
that contributed to the improvement of the AIE performances.
Construction of Artificial Light-Harvesting Systems Based
on the TPE-branched Rotaxane Dendrimers. To further
explore the potential applications of these novel AIE
h
hydrodynamic diameters (D ) determined by the DLS analysis
1
9
(
for TG1: 88.0 ± 5.0 nm from TEM analysis and 81.2 ± 4.3 nm
macromolecules, the construction of artificial LHSs based on
TPE-branched rotaxane dendrimers TG1-TG3 was then
investigated. In this study, the fluorescent dye eosin Y (ESY)
was chosen as the fluorescence energy acceptor (Figure 4a).
First of all, the successful formation of the integrated aggregate
of TGn (n = 1, 2, 3) and ESY was confirmed by DLS, TEM,
elemental mapping analysis, and fluorescence imaging (Figures
S77-S78). For instance, upon mixing ESY and TGn (n = 1, 2, 3)
(ESY and the TPE units in a 1:3 ratio) in the aggregate state,
remarkable increase of the average hydrodynamic diameters
from DLS analysis; for TG2: 106.0 ± 6.0 nm from TEM analysis
and 104.6 ± 8.8 nm from DLS analysis; for TG3: 133.0 ± 4.0 nm
from TEM analysis and 126.5 ± 4.9 nm from DLS analysis).
Notably, the DLS analysis also indicated an interesting two-step
aggregation behaviors of TPE-branched rotaxane dendrimers
(
Figure S76). The existence of low fraction of MeOH only led to
the contraction of the rotaxane dendrimers by driving the
translational motion of the pillar[5]arene ring in each branches.
With the further increase of the MeOH fractions, the resultant
contracted rotaxane dendrimers started to aggregate, thus
leading to the formation of larger aggregates.
To evaluate the unique role that the structural and dynamic
feature of the rotaxane dendrimers played in their AIE behaviors,
two model first-generation dendrimers, i.e. C1 without the
pillar[5]arene rings and C2 without the urea moieties, were
synthesized (Scheme S8 and S10). For C1, it displayed AIE
behavior only if the f were above 90%, possibly due to its better
h
(D ) was determined by DLS, respectively (Figures S73-S75).
Similar trend was also observed from the TEM analysis, from
which homogenous spherical particles with increased average
diameters were observed (Figure 4b, e, h).
To check the possibility of efficient energy transfer from the
rotaxane dendrimers TGn (n = 1, 2, 3) to ESY, the adsorption
spectrum of ESY and the emission spectra of the rotaxane
dendrimer TG1 were recorded. As show in Figure S53, these
4
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Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
(
a)
Excited
345 nm
555 nm
Aggregate
AIEgen-Branched
Rotaxane Dendrimers
Artificial Light‐Harvesting Systems
5
4
3
2
1
00000
00000
00000
00000
00000
0
10000
TG1
90-1
IRF
TG1
TG1-ESY
(
b)
(d)
(
c)
6
3
0-1
0-1
1000
15-1
9-1
6-1
3-1
100
D-A
10
1
4
00
450
500
550
600
650
0
10
20
30
40
50
Wavelength (nm)
Time (ns)
TG1-ESY: 138.0 ± 8.0 nm
6
5
4
3
2
1
00000
00000
00000
00000
00000
00000
0
10000
TG2
180-1
IRF
TG2
TG2-ESY
(
e)
(f)
(g)
9
4
2
1
9
3
0-1
5-1
7-1
8-1
-1
1
000
100
-1
D-A
1
0
1
0
10
20
30
40
50
4
00
450
500
550
600
650
Wavelength (nm)
Time (ns)
TG2-ESY: 171.0 ± 9.0 nm
8
6
4
2
00000
00000
00000
00000
0
10000
1000
100
10
TG3
315-1
210-1
IRF
TG3
TG3-ESY
(
h)
(i)
(j)
1
6
05-1
3-1
42-1
2
3
1-1
-1
D-A
1
4
00
450
500
550
600
650
0
10
20
30
40
50
Wavelength (nm)
Time (ns)
TG3-ESY: 213.0 ± 5.0 nm
Figure 4. The construction of artificial LHSs based on TPE-branched rotaxane dendrimers TGn (donor, n = 1, 2, 3) and ESY (acceptor). Cartoon representation of
LHSs based on TPE-branched rotaxane dendrimers TG3 and ESY, ET: energy transfer (a); TEM images of TG1-ESY (b), TG2-ESY (e) and TG3-ESY (h) in
aggregate states; Fluorescence spectra of TG1 (c), TG2 (f) and TG3 (i) with different ratios of TPE units in TGn-ESY ([TPE units] = 30 μM, λex =345 nm); inset:
photograph of TGn-ESY in CH
2 2
Cl /MeOH with 98% MeOH fractions upon UV lamp excitation (λex = 365 nm); Time-resolved fluorescence decay curves of TG1
and TG1-ESY (d), TG2 and TG2-ESY (g), and TG3 and TG3-ESY (j) in the aggregate states (375 nm excitation and 480 nm detection).
two spectra overlap well, suggesting the feasibility for the
construction of targeted LHSs. To confirm this hypothesis, the
gradual increase of the ratios of ESY in the integrated systems
TGn-ESY (n = 1, 2, 3) were performed. To our delight, for all
TPE-branched rotaxane dendrimers TG1-TG3, along with such
process, the emission intensity at around 480 nm originated
from the TPE units in rotaxane dendrimers TGn (n = 1, 2, 3) was
g, j). For instance, when the molar ratio of TPE/ESY was kept at
3:1, the fluorescence lifetime of TG1 was reduced from 1.77 ns
to 1.36 ns for TG1-ESY. And for TG2 and TG3, the values were
from 1.68 ns to 0.91 ns and 1.72 ns to 0.91 ns, respectively.
More importantly, according to the ratios of fluorescence
intensities of TPE units for rotaxane dendrimers, the energy-
transfer efficiencies (ΦET) were calculated to be 42.5% for TG1-
found to be decreased as indicated by their fluorescence spectra, ESY, 68.2% for TG2-ESY, and 71.6% for TG3-ESY, respectively
while a new emission band at around 555 nm derived from ESY
appeared and increased upon the excitation at 345 nm (Figure
(Figures S57-S59), which are relatively high compared with the
previously-reported supramolecular light-harvesting systems.1
Such high energy-transfer efficiencies of these TGn-ESY (n =
1, 2, 3) systems encouraged us to further explore their light-
harvesting properties. As expected, the fluorescence intensity of
ESY excited at 345 nm (absorption wavelength for TPE) was
stronger than that obtained by direct excitation at 480 nm
(absorption wavelength for ESY), which was in accord with
typical light harvesting antenna effect. More importantly, the
antenna effects of these TGn-ESY (n = 1, 2, 3) systems were
calculated to be 1.1 (TG1-ESY), 2.3 (TG2-ESY), and 4.1 (TG3-
ESY), respectively, which became larger along with the increase
9b-f
4c, f, i). According to these phenomena, efficient energy transfer
(
ET) from the rotaxane dendrimers TGn (n = 1, 2, 3) to ESY was
suggested. Moreover, as revealed by the fluorescence spectra,
slight hypochromic shifts and bathochromic shifts were observed
for TPE units and ESY, respectively, indicating the interactions
between the rotaxane dendrimers and ESY in the resultant
aggregate state.
In addition, such process was further evidenced by the
decrease of the fluorescence lifetimes (τ) of the rotaxane
dendrimers upon the mixing with ESY as determined by time-
correlated single-photon counting (TCSPC) technique (Figure 4d, of the generation of the TPE-branched rotaxane dendrimer
5
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Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
(
Figures S62-S64). Such interesting generation effect might be
as the catalyst, full conversion from sulfide to sulfoxide was
achieved after 10.0 h of photoirradiation. Moreover, for TG2-
ESY and TG3-ESY, the reaction times reduced to 8.0 h and 4.0
h, respectively, to reach the full conversion, thus indicating an
impressive generation effect of the TPE-branched rotaxane
dendrimers on photocatalysis. Notably, additional artificial LHSs
based on the corresponding model dendrimers C1 and C2 were
also constructed and tested as photocatalysts, for both of which
originated from the increasing density of TPE moiety within the
skeletons of rotaxane dendrimers TGn (n = 1, 2, 3), thus
highlighting the unique structural feature of AIEgen-branched
dendrimers.
(
a) Excited
S1
ET
S1
1O
2
T1
ET
around 15.0 h of photoirradiation was needed to trigger the full
3
.60 eV
2
.37 eV
O2
1
conversion due to their reduced O
2
generation efficiencies and
1.25 eV
S0
S0
the lower antenna effects, highlighting the superiority of the
TPE-branched rotaxane dendrimers (Figure 5b). All
photocatalytic processes were quantified by their kinetic
equations as listed in Table S3, from which the kinetic rate
constants k were calculated. Remarkably, the k values of
555 nm
Donor
Acceptor
1
1
20
5
4
3
2
00 (b)
(c)
8
6
4
2
0
0
0
0
0
-1
artificial LHSs catalysts TG2-ESY (0.336 h ) and TG3-ESY
TG1-ESY
TG2-ESY
TG3-ESY
C1-ESY
TG1-ESY
TG2-ESY
TG3-ESY
C1-ESY
-1
(0.685 h ) were 2.2-fold and 4.4-fold of that of TG1-ESY (0.154
h-1) (Figure 5c, d). In addition, the normalized 1
O
2
generation
efficiency (Φ
o
) values of artificial LHSs catalysts TG2-ESY and
C2-ESY
C2-ESY
-
20
-1
0
1
2
3
4
TG3-ESY were 2.3-fold and 4.2-fold of that of TG1-ESY, which
was almost identical with the enhancement of the k values
-
2
0
2
4
6
8
10 12 14 16 18
Reaction time (h)
Time (h)
(
Figure 5e). According to these values, when keeping the
6
5
4
3
2
1
0
6
5
4
3
2
1
0
(
d)
k
(e)
Φo
amounts of the TPE units the same, artificial LHSs based on
higher generation TPE-branched rotaxane dendrimers were
proven to display boosted photocatalytic performance, offering a
new platform for the construction of efficient photocatalysts.
Table 1. Catalyst comparison data using artificial LHSs as
photocatalysts for the aerobic CDC reaction of N-phenyl-1, 2, 3,
C1-ESY C2-ESY TG1-ESY TG2-ESY TG3-ESY
C1-ESY C2-ESY TG1-ESYTG2-ESYTG3-ESY
4-tetrahydroisoquinoline with indole.
Figure 5. Artificial LHSs based on the TPE-branched rotaxane dendrimers as
efficient photocatalysts for photooxidation reaction. (a) The artificial LHSs
TGn-ESY (n = 1, 2, 3) serve as photosensitizers that can produce 1
2
O for
photooxidation reaction of 2-(ethylsulfanyl)ethanol to 2-(ethylsulfinyl)ethan-1-ol.
The singlet energy was determined by the absorption onset on the lower
energy side (345 and 525 nm for the TPE donor and ESY acceptor,
respectively), the triplet energy of ESY was copied from a recent report (Anal.
Bioanal. Chem. 2020, 412, 7595.). ET: energy transfer, ISC: intersystem
crossing; Kinetic plot for photooxidation of sulfides under UV irradiation (b),
fitting of the pseudo-first order kinetic rate constant (c) and the normalized
kinetic rate constants (d) catalyzed by TGn-ESY (n = 1, 2, 3) and model ones
Entry
Catalyst
TG3-ESY
TG2-ESY
TG1-ESY
TG1
Time
4 h
Yield (%)[a]
89
1
2
3
4
5
6
8 h
90
C1-ESY and C2-ESY. (e) The normalized 1
, 2, 3) and C1-ESY and C2-ESY.
O
quantum yield of TGn-ESY (n =
11 h
24 h
24 h
24 h
95
2
1
trace
trace
none
Generation Effect of the TPE-branched Rotaxane
ESY
Dendrimers on Photocatalysis. Towards the mimicking of the
nature LHSs as the entry gate of photosynthesis, the resultant
TPE-branched rotaxane-dendrimer-based LHSs TGn-ESY (n =
No cat.
[a] Isolated yields of products after column chromatography.
1
, 2, 3) were further utilized as photocatalysis for chemical
In the case of aerobic CDC reaction,21 the typical cross-
transformations. The photocatalytic processes were monitored
by H NMR spectra. Herein, to evaluate the scope of the artificial
LHSs as photocatalysts, both photooxidation reaction and
aerobic CDC reaction were chosen as the reactions.
1
coupling of N-phenyl-1, 2, 3, 4-tetrahydroisoquinoline with indole
was chosen as the model reaction. To our delight, by using only
1.0 mol % artificial LHSs (based on the TPE units in the
rotaxane dendrimers) as catalysts, full conversion was achieved
within 11.0 h for TG1-ESY, 8.0 h for TG2-ESY, and 4.0 h for
TG3-ESY with the isolated yields of 95%, 90%, and 89%,
respectively, again suggesting an attractive generation effect.
Control experiments using either sole TG1 or ESY as catalyst
under the same reaction condition only resulted in a trace
conversion, indicating that the integrated artificial LHSs did
serve as the catalysts (Table 1). Encouraged by such high
catalytic activity of the resultant LHSs, the scope of the LHS-
Based on the energy transfer process, the artificial LHSs TGn-
ESY (n = 1, 2, 3) were supposed to be capable of producing
singlet oxygen ( O
1
2
), making them promising photosensitizers for
diverse types of reactions especially photooxidation reactions
Figure 5a). Thus the photooxidation of 2-(ethylsulfanyl)ethanol
to 2-(ethylsulfinyl)ethan-1-ol was selected as the model
(
2
0
reaction. Only 3.0 mol % artificial LHSs (based on the TPE
units in the rotaxane dendrimers) were employed as
photocatalysts. As monitored by H NMR, when using TG1-ESY
1
6
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Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
catalyzed coupling reaction was further examined by using TG1-
ESY as the photocatalyst. According to the results shown in
Figure S88, for indoles with both electron-withdrawing and
electron-donating groups, the desired coupling products could
be obtained in good to excellent yields. In addition, other
nucleophilic substrates such as nitromethane and dimethyl
malonate were also applicative for such photocatalyzed aerobic
CDC reaction, making the current TPE-branched rotaxane
dendrimer-based LHSs excellent candidates as efficient
photocatalysts for diverse chemical transformations.
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2020T130198) for support.
Keywords: light harvesting • aggregation-induced emission •
mechanically interlocked molecules • rotaxane dendrimers •
photocatalysis
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Angewandte Chemie International Edition
10.1002/anie.202106035
RESEARCH ARTICLE
Entry for the Table of Contents
Excited
or
AIEgen-Branched
Rotaxane Dendrimers
Artificial Light‐Harvesting Systems
A novel artificial light-harvesting system based on AIEgen-branched rotaxane dendrimers has been successfully constructed which
displayed impressive generation-dependent photocatalytic performances for both photooxidation reaction and aerobic cross-
dehydrogenative coupling reaction.
9
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