Efficient Light-Harvesting Systems with Tunable Emission through Controlled Precipitation in Confined Nanospace

Article abstract of DOI:10.1002/anie.201812146

Light harvesting is a key step in photosynthesis but creation of synthetic light-harvesting systems (LHSs) with high efficiencies has been challenging. When donor and acceptor dyes with aggregation-induced emission were trapped within the interior of cross-linked reverse vesicles, LHSs were obtained readily through spontaneous hydrophobically driven aggregation of the dyes in water. Aggregation in the confined nanospace was critical to the energy transfer and the light-harvesting efficiency. The efficiency of the excitation energy transfer (EET) reached 95 % at a donor/acceptor ratio of 100:1 and the energy transfer was clearly visible even at a donor/acceptor ratio of 10 000:1. Multicolor emission was achieved simply by tuning the donor/acceptor feed ratio in the preparation and the quantum yield of white light emission from the system was 0.38, the highest reported for organic materials in water to date.

Full text of DOI:10.1002/anie.201812146

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International Edition: DOI: 10.1002/anie.201812146
German Edition: DOI: 10.1002/ange.201812146
Energy Transfer
Efficient Light-Harvesting Systems with Tunable Emission through
Controlled Precipitation in Confined Nanospace
Chuanqi Li, Jing Zhang, Shiyong Zhang,* and Yan Zhao
Abstract: Light harvesting is a key step in photosynthesis but
creation of synthetic light-harvesting systems (LHSs) with high
efficiencies has been challenging. When donor and acceptor
dyes with aggregation-induced emission were trapped within
the interior of cross-linked reverse vesicles, LHSs were
obtained readily through spontaneous hydrophobically
driven aggregation of the dyes in water. Aggregation in the
confined nanospace was critical to the energy transfer and the
light-harvesting efficiency. The efficiency of the excitation
energy transfer (EET) reached 95% at a donor/acceptor ratio
of 100:1 and the energy transfer was clearly visible even at
a donor/acceptor ratio of 10000:1. Multicolor emission was
achieved simply by tuning the donor/acceptor feed ratio in the
preparation and the quantum yield of white light emission from
the system was 0.38, the highest reported for organic materials
in water to date.
The LHSs reported so far can be broadly classified into
two groups, those constructed on a covalent framework and
those obtained through self-assembly. The former has the
benefit of high stability but often requires substantial
synthetic efforts. The latter can be conveniently obtained by
mixing appropriate ingredients under suitable conditions but
tend to be unstable. The natural PSU, on the other hand, is
a membrane-based system compartmentalized with special-
ized components. A combination of covalent and noncovalent
strategies is utilized for the construction of the final hier-
archical structure. The membrane-enclosed architecture is
crucial to photosynthesis, as production, transport, and
accumulation of protons on the membrane creates proton
motive forces needed for the synthesis of ATP.[1a]
Inspired by the natural construction of PSU, we report
herein an artificial LHS entrapped in a cross-linked reverse
vesicle (cRV). RVs are unstable bilayer compartments
formed in organic solvents that coalesce quickly. Simple
cross-linking yielded cRVs with tunable amounts of organic
chromophores within their interior. Organic chromophores
tend to aggregate uncontrollably in water because of their
strong hydrophobicity. In our case, however, the compart-
mentation set the boundaries for the aggregation of hydro-
phobic chromophores with aggregation-induced emission
(AIE).[13] The fluorescence quantum yield for our donor
went from less than 0.002 in chloroform to up to 0.42 in water
within the cRV. The final LHS displayed an EETefficiency of
up to 95% at a donor/acceptor ratio of 100:1. The quenching
of donor was clearly detectable even at a donor/acceptor ratio
of 10000:1, making these systems one of the most efficient in
the literature. Furthermore, our synthetic strategy enabled us
to tune the emission color of the materials at will, simply by
adjusting the donor/acceptor ratio, thanks to their comple-
mentary emission colors. White-light-emitting organic mate-
rials are extremely useful in illumination and sensing,[14] and
our white-light-emitting cRVs had a quantum yield of 0.38,
the highest reported to date in water, to the best of our
knowledge.
T
he natural photosynthetic unit (PSU) is a highly efficient
Einstein photochemical machine for converting light energy
into chemical potential.[1] In higher plants and green algae, the
unit comprises two photosystems, I and II, that each employ
200–400 antenna chromophores to harvest photons under
ambient light. Over the last decades, the importance of the
process has motivated generations of researchers to perform
structural characterization, to study the intricacies of the
mechanisms, and to mimic key steps of the process for
potential applications in photosynthesis, photocatalysis, and
photovoltaics.[2]
To mimic the natural light-harvesting systems (LHSs),
chemists have used many scaffolds including dendrimers,[3]
organogels,[4] micelles,[5] vesicles,[6] host-guest assemblies,[7]
organic nanocrystals,[8] metal–organic frameworks,[9] polymer
nanoparticles,[10] biomacromolecule assemblies,[11] and
others.[12] Impressive progress has been made in recent
years, particularly in the spatial organization of multichro-
mophores to enhance the efficiency of excitation energy
transfer (EET) and minimize self-quenching.
The preparation of the cRVs is shown in Scheme 1. The
structure of the amphiphile 1 is the key to the process. Its
tetraethylene glycol (TEG) tails are soluble in chloroform
and serve as the solvent-exposed portion of the bilayer
membrane. Its carboxylate headgroup provides the main
driving force for the formation of the RVs. Importantly,
oligo(ethylene glycol) groups have good solubility in water,
making it possible to transfer the RVs into water after cross-
linking.
[*] C. Li, J. Zhang, Prof. S. Zhang
National Engineering Research Center for Biomaterials, and College
of Chemistry, Sichuan University
29 Wangjiang Road, Chengdu 610064 (China)
E-mail: szhang@scu.edu.cn
Prof. Y. Zhao
Department of Chemistry, Iowa State University
Ames, IA 50011-3111 (USA)
As shown in Figure 1a, RVs were readily formed by hand-
shaking the chloroform solution of 1 with a trace amount of
water and had an average size of about 285 nm by dynamic
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.201812146.
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light scattering (DLS). The vesicular structure was supported
by the entrapment of a hydrophilic dye, 8-hydroxy-1,3,6-
pyrenetrisulfonic acid trisodium salt (HPTS).[15] As shown in
Figure 1b, the HPTS-loaded RVs showed brighter emission
near the periphery than the interior, consistent with a vesic-
ular structure and dyes that adsorbed onto the membranes.[16]
The vesicular structure was verified further by small-angle X-
ray scattering (SAXS), which gave a wall thickness of 4.2 nm
(Figure 1c), approximately twice the length of 1 (Figure 1c,
insert), suggesting that the membranes were bilayers.
Like typical RVs, the resulting RVs of 1 were unstable and
coalesced within 24 hours (see Figure S1a in the Supporting
Information). However, when the RVs were treated with the
N-hydroxysuccinimide ester 2 and N,N-dimethylaminopyri-
dine (DMAP), stable cRVs were obtained readily (see the
Supporting Information for details). The cross-linking was
evident from the disappearance of the succinimide and
hydroxy proton signals in the 1H NMR spectra (see Fig-
ure S2).[17] After cross-linking, the vesicles remained similar in
size (Figure 1a) but became far more stable in both long-term
storage (see Figure S1b) and dilution tests (Figures 1d).
Transmission electron microscopy (TEM) revealed a spherical
structure for the cRVs (Figure 1e). The obvious contrast
between the interior and the periphery once again supported
membrane-enclosed vesicular assemblies.
Scheme 1. Schematic representation of construction of the light-har-
vesting system AIE@cRVs.
The cRVs could be transferred easily from chloroform to
water by removing the organic solvent through dialysis. Both
the DLS and TEM measurements confirmed the maintenance
of the vesicular structure after the transfer, except a slight
decrease in size (Figures 1a,f). The size change is not
surprising given the very different solvent environments
(chloroform versus water). In a control experiment, when
1 was added directly to water, DLS showed no formation of
nanoparticles. Since both the carboxylate and the TEG
groups are hydrophilic, the molecule is not expected to self-
assemble in any meaningful way without the cross-linking.
The dual solubility of cRVs in chloroform and water
allowed us to load them with hydrophobic dyes directly in
chloroform and use the membranes as the boundary to
control the aggregation of the dyes in water. Organic dyes
often display self-quenching and/or excimer formation upon
aggregation. In this study, we used AIE-based hydrophobic
dyes (TPE-CHO as the donor and TPE-TCF as the acceptor)
to construct our LHSs.
Figure 2a shows that the donor-loaded cRVs (TPE-
CHO@cRVs) had negligible fluorescence in chloroform but
displayed strong cyan-colored emission once the cRVs were
transferred into water. As a control, when Nile red was
entrapped, the opposite was observed, with the cRVs
exhibiting strong red emission in chloroform and little
emission in water (Figure 2a) due to aggregation-caused
quenching (ACQ). The quantum yield (QY) of TPE-
CHO@cRVs was measured to be less than 0.002 in chloro-
form. Once transferred into water, the QY increased dramat-
ically and, as designed, could be controlled readily through
the loading of the dye in the cRVs, up to 0.42 with a loading of
11.3 mol% dye (see Supporting Information for the determi-
nation of the loading).
Figure 1. a) Distribution of dynamic diameters of RVs formed by 1 in
chloroform, after cross-linking with 2 in chloroform, and after transfer
of the cross-linked material into water. b) Fluorescence image of
HPTS-loaded RVs in CHCl3. c) SAXS spectrum of RVs in CHCl3. The
insert shows the molecular model of 1 generated by the ACD Labs
software. d) Size and PDI (particle dispersion index) of RVs and cRVs
as a function of the concentration of 1 in CHCl3. The concentration-
independent size and PDI verified the stability of covalently captured
materials. e) TEM image of cRVs in CHCl3. The arrows pointed to the
wall of cRVs. f) TEM image of cRVs in water. [1]=0.5 mm.
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Figure 2. a) Fluorescence spectra of TPE-CHO@cRVs (lex =372 nm)
and Nile red@cRVs (lex =490 nm) in CHCl3 and in water. The inset
shows the corresponding photographs under a hand-held UV lamp
(365 nm). b) Comparison of QYs of TPE-CHO@cRVs in CHCl3 and in
water with different dye loadings.
As the acceptor, TPE-TCF absorption overlaps with the
emission peak of the TPE-CHO donor (see Figure S3). Since
both the donor and the acceptor are AIE-based and share
a common backbone, their intimate mixing upon aggregation
should be facile and facilitate EET. With the hydrophobic
AIE dyes trapped in the interior of the cRVs, making a LHS is
as simple as using a dye mixture in the preparation. To test the
light-harvesting property, three batches of AIE@cRVs (I–III)
with low (0.7%), medium (6.4%) and high (11.3%) loadings
of TPE-CHO were prepared. The donor/acceptor ratio was
varied from 10000:1 to 100:1.
As shown in Figures 3a–c, excitation of these AIE@cRVs
at 372 nm with increasing amounts of the acceptor, displayed
decreasing donor emission at 480 nm and increasing acceptor
emission at 570 nm, consistent with fluorescence resonance
energy transfer (FRET) from the donor to the acceptor.[18]
The energy-transfer efficiencies (FET) of the LHSs are shown
in Figure 3d and Table S1. At a 100:1 donor/acceptor ratio,
a very impressive FET was obtained, that is, 83%, 89% and
95%, for AIE@cRV I, II, and III, respectively. For AIE@cRV
III, a significant FET (13%) was observed even at a donor/
acceptor ratio of 10000:1, highlighting the efficiency of the
system.
Figure 3. Fluorescence spectra of AIE@cRVs I (a), II (b), and III (c) in
water at different donor/acceptor (D:A) ratios. d) Energy-transfer
efficiencies (FET) of AIE@cRVs I–III as a function of D:A ratios.
e) Fluorescence lifetime decay curves of AIE@cRV III at the donor’s
emission of 480 nm in water. f) Time-resolved fluorescence spectra of
AIE@cRV III after excitation at 371 nm with the donor/acceptor ratio
of 100:1. [1]=0.5 mm.
The energy transfer of AIE@cRV III was studied addi-
tionally by its fluorescence lifetime decay. The lifetime of the
donor emission at 480 nm was 1.97 ns and decreased contin-
uously, with increasing amounts of acceptor doped into the
mixture, to 0.52 ns at a 100:1 donor/acceptor ratio (Figure 3e,
see Table S2). At this ratio, the time-resolved fluorescence
spectra, excited at 371 nm, displayed a decrease of donor
emission (lem = 480 nm) within 2 ns, accompanied by a sharp
increase of the acceptor emission (lem = 570) (Figure 3 f).
The efficiency of a LHS is often measured by the average
number of donor molecules quenched by a single acceptor
and the antenna effect (AE) as described in the literature. The
former could be obtained by the linear curve-fitting of the
Stern–Volmer plot of the donor as a function of the donor/
acceptor ratio (KSV, Figure S6).[19] Our cRV-based LHSs were
found to have outstanding KSV values: 266 Æ 17 for cRV I,
534 Æ 23 for cRV II, and 689 Æ 28 for cRV III (Figure 4a). The
increasing KSV values with the higher loading of the dye in the
cRV was reasonable given the positive correlation between
the dye loading and donorꢀs QY (Figure 2b). The result
Figure 4. a) KSV and AE of the AIE@cRVs I–III. b) KSV and AE of the
mixed aggregates of AIE@SLS.
suggests that a larger amount of the hydrophobic dye in the
cRV aggregated more strongly and was helpful to the AIE.
The AE displayed a similar trend: 16 Æ 2 for cRV I (D:A =
500:1), to 27 Æ 2 for cRV II (D:A = 1000:1), and 35 Æ 3 for
cRV III (D:A = 1000:1). It is worth noting that AIE@cRV III,
with KSV of 689 and AE of 35, outperforms the vast majority
of artificial LHSs reported in the literature.[2–12,19]
White-light-emitting organic materials have gained much
attention in recent years because of their potential in
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illumination and sensing.[14] Thanks to the complementary
emission colors of the TPE-CHO and TPE-TCF (i.e., cyan
and orange), the emission of the dye-loaded cRVs can be
tuned at will by simply changing the donor/acceptor ratio. As
shown in Figure 5, the emission color of AIE@cRV III with
would lower the number of the donor molecules around the
acceptor, thus decreasing the overall efficiency of system (see
Figure S4). As a control, we prepared coaggregates of TPE-
CHO and TPE-TCF by reprecipitation from their tetrahy-
drofuran solution into an aqueous solution of sodium
laurylsulfonate (SLS). The materials obtained (AIE@SLS)
represent traditional organic nanoparticles prepared without
the membrane boundary of cRVs and thus any potential
nanoconfinement. As shown in Figures 4b, Figures S5–S7,
and Table S4, the energy transfer was significantly less
efficient and the highest KSV and AE values achieved were,
respectively 162 and 3.3, in sharp contrast to the 689 and 35
obtained for AIE@cRV III (Figure 4a).
Once again, the natural design with dyes aggregating in
a confined nanospace proves superior to those relying on
special organic frameworks to arrange the dyes. When we
created similar biomimetic LHSs as a primitive chloroplast
using cRVs, the membrane-enclosed structure clearly facili-
tated the self-assembly and the energy transfer of the
entrapped donor/acceptor dyes. This platform gives us
tremendous flexibility with regard to the type, amount, and
ratio of dyes loaded into the vesicles. Importantly, aggregation
of the dyes occurs spontaneously through hydrophobic
interactions and the intimate contact of the dyes was optimal
for AIE-based fluorophores and the EET of the system. In
addition to the potential of the cRVs in light harvesting and
emission, the tunable multicolored emission and water-
compatibility of our platform holds great promise in bio-
imaging and sensing. Applications of these materials are
currently being investigated and will be reported in due
course.
Figure 5. CIE 1931 chromaticity diagram showing the luminescence
color coordinates of AIE@cRV III with different donor/acceptor ratios
in water. The insert shows the corresponding fluorescence photo-
graphs under 365 nm UV light.
decreasing TPE-CHO/TPE-TCF ratios changed gradually
from cyan to orange. Significantly, the sample with a 300:1
donor/acceptor ratio displayed a pure white-light emission
with color coordinates of (0.31, 0.34), very close to the exact
white point (0.33, 0.33). The quantum yield of the white light
was 0.38 (see Table S3), the highest reported so far for white-
light-emitting organic materials in water.
Acknowledgements
The strong performance of our LHSs in multiple aspects
indicates that the cRV is an outstanding platform for light
harvesting and luminescence. Two key factors were probably
responsible for the results. First, the hydrophobically driven
aggregation of the dyes put the fluorophores within very close
distance. Our calculation affords a Fçrster radius of 3.3 nm for
the donor–donor energy transfer and 4.6 nm for the donor–
acceptor transfer (see Supporting Information for details).
Since aggregation of hydrophobic molecules seeks to mini-
mize the unfavorable exposure of hydrophobic surface, the
dyes will be in intimate contact with one another. Normally,
such aggregation is expected to cause quenching and/or
excimer formation for hydrophobic fluorophores but actually
enhances the QY of the AIE-based dyes, as shown in
Figure 2b. All these factors collectively made it possible for
the system to reach 95% FET for AIE@cRV III (Figure 3d).
Second, the confined nanospace of the cRV set the boundary
for the aggregation of donor and acceptor fluorophores and
forced them to coaggregate at the intended ratio. Intermo-
lecular interactions are known to increase, sometimes dra-
matically, when the molecules are confined in a nanospace.[20]
Owing to enclosing in the cRVs, the donor and acceptor
engage in an enhanced intermolecular interaction. Once
transferred into water, they might prefer to coaggregate at the
intended ratio, otherwise the phase separation of the two dyes
This work was supported by the National Natural Science
Foundation of China (51673130), NSF (CHE-1708526), the
National 111 Project of Introducing Talents of Discipline to
Universities (No. B16033), and the Applied Basic Research
Project of Sichuan Province (No. 15JC0440). We are grateful
to Prof. Qing-Zheng Yang (Beijing Normal University) for
valuable discussions and manuscript preparation. We also
thank Prof. Peng Wu (Sichuan University) for the time-
resolved fluorescence measurement, and the Center of Test-
ing and Analysis, Sichuan University, for NMR and TEM
measurements.
Conflict of interest
The authors declare no conflict of interest.
Keywords: aggregation · energy transfer · light harvesting ·
nanostructures · vesicles
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Accepted manuscript online: November 12, 2018
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Energy Transfer
C. Li, J. Zhang, S. Zhang,*
Y. Zhao
&&&& — &&&&
Efficient Light-Harvesting Systems with
Tunable Emission through Controlled
Precipitation in Confined Nanospace
Into the light: A naturally inspired light-
harvesting system (LHS) was fabricated
through entrapment of chromophores in
cross-linked reverse vesicles (cRVs).
Benefiting from the confined nanospace
of cRVs and the aggregation-induced
emission properties of the donor and
acceptor, the LHS achieved excellent
light-harvesting efficiency and lumines-
cence upon transfer from an organic
phase to a water phase.
6
www.angewandte.org
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
These are not the final page numbers!