Angewandte
Communications
Chemie
Photochemistry Very Important Paper
Light-Harvesting Systems Based on Organic Nanocrystals To Mimic
Chlorosomes
Peng-Zhong Chen, Yu-Xiang Weng, Li-Ya Niu, Yu-Zhe Chen, Li-Zhu Wu, Chen-Ho Tung, and
Qing-Zheng Yang*
Abstract: We report the first highly efficient artificial light-
harvesting systems based on nanocrystals of difluoroboron
chromophores to mimic the chlorosomes, one of the most
efficient light-harvesting systems found in green photosynthetic
bacteria. Uniform nanocrystals with controlled donor/acceptor
ratios were prepared by simple coassembly of the donors and
acceptors in water. The light-harvesting system funneled the
excitation energy collected by a thousand donor chromophores
to a single acceptor. The well-defined spatial organization of
individual chromophores in the nanocrystals enabled an
energy transfer efficiency of 95%, even at a donor/acceptor
ratio as high as 1000:1, and a significant fluorescence of the
acceptor was observed up to donor/acceptor ratios of
200000:1.
An artificial light-harvesting with a high energy collection
efficiency should have the following two properties, namely,
1) contain multiple antenna chromophores per acceptor and
2) transfer the excitation energy with high efficiency. Artifi-
cial light-harvesting systems constructed with covalent bonds,
such as porphyrin arrays and dendrimers, contain very few
donor chromophores per acceptor because of synthetic
difficulties. Self-assembled light-harvesting systems, such as
organic gels, biopolymer assemblies, and organic–inorganic
hybrid materials, have high donor/acceptor ratios, but low
energy-transfer efficiencies probably because of the low
degree of spatial organization of the chromophores.
The chlorosomes, one of the most efficient light-harvest-
ing systems found in green photosynthetic bacteria, contains
large numbers of bacteriochlorophyll molecules organized in
stacked structures through self-assembly without any direct
involvement of proteins. Interaction between chromophores
within the chlorosomes lead to the formation of delocalized
electronic excitations, that is, excitons, which facilitate its high
energy-collection efficiency.[9] However, studies focusing on
mimicking the chlorosomes are still rare.[10] Herein, we
prepared light-harvesting systems based on organic nano-
crystals. Organic nanocrystals of chromophores containing
multiple chromophores in well-defined relative orientations
and distances may provide an ideal scaffold for artificial light-
harvesting systems. Such nanocrystals have attracted much
attention recently because of their promising applications in
electronic and photonic devices.[11] However, to the best of
our knowledge, no examples of light-harvesting antenna
systems based on nanocrystals of chromophores have been
reported. We describe two such systems that were fabricated
by coassembly of the donor and acceptor chromophores at
molar ratios ranging from 1 106:1 to 1000:1. High energy-
transfer efficiency (95%) was observed even for the nano-
crystals with a donor/acceptor ratio of 1000:1.
I
n plants and bacteria, photosynthesis usually starts with the
absorption of sunlight by antenna chromophores in light-
harvesting systems, followed by the highly efficient transfer of
the excitation energy to an acceptor of the reaction center.[1]
In bacteria, over 200 bacteriochlorophylls supply energy to
one chromophore of the reaction center with an efficiency of
over 95%.[1,2] This high efficiency is possible because of the
well-organized arrays of chromophores in the photosynthetic
membrane.[1a] Considerable effort has been devoted to mimic
this light-harvesting process, because of both its role in
photosynthesis and its potential significance to photocatalysis,
solar cells, optical sensors, and luminescent materials.[3]
Although impressive examples of artificial light-harvest-
ing systems constructed using both covalent and noncovalent
interactions have been reported,[4–8] their light-harvesting
capacity remains a fraction of that of the natural counterparts.
[*] Dr. P.-Z. Chen, Dr. L.-Y. Niu, Prof. Dr. Q.-Z. Yang
Key Laboratory of Radiopharmaceuticals
Ministry of Education, College of Chemistry
Beijing Normal University, Beijing 100875 (China)
E-mail: qzyang@bnu.edu.cn
We chose the difluoroboron b-diketonate (BF2dbk)
derivative BF2bcz as our model antenna chromophore,
because of its high fluorescence in both solution and the
solid state. BF2cna or BF2dan, analogues of BF2bcz, were used
as the energy acceptor (Figure 1). The three BF2dbk deriv-
atives were synthesized in approximately 70% yield by
Claisen condensation of the corresponding acetophenones
with benzoates, followed by treatment with BF3/Et2O (see the
Supporting Information). BF2bcz is a typical donor–acceptor–
donor (D-A-D) type fluorophore, with the difluoroboron
moiety acting as the electron acceptor and the amino group of
the carbazole acting as the electron donor. It has been
reported that a donor–acceptor dipole–dipole interaction
between two adjacent molecules can guide the preferential
Prof. Dr. Y.-X. Weng
Key Laboratory of Soft Matter physics
Institute of Physics, Chinese Academy of Sciences
Beijing 100190 (China)
Dr. P.-Z. Chen, Dr. L.-Y. Niu, Dr. Y.-Z. Chen, Prof. Dr. L.-Z. Wu,
Prof. Dr. C.-H. Tung, Prof. Dr. Q.-Z. Yang
Key Laboratory of Photochemical Conversion and
Optoelectronic Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190 (China)
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2016, 55, 2759 –2763
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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