Artificial light-harvesting system based on multifunctional surface-cross-linked micelles

Article abstract of DOI:10.1002/anie.201107723

A good harvest: Two self-assembling strategies (micellization and electrostatic attraction) and covalent capture were employed to construct a robust, inexpensive, efficient artificial light-harvesting system (see picture). The synthesis was achieved by a one-pot reaction. A high density of the antenna chromophores was achieved without self-quenching and excimer formation, thus affording extremely efficient energy transfer. Copyright

Full text of DOI:10.1002/anie.201107723

.
Angewandte
Communications
DOI: 10.1002/anie.201107723
Energy Transfer
Artificial Light-Harvesting System Based on Multifunctional Surface-
Cross-Linked Micelles**
Hui-Qing Peng, Yu-Zhe Chen, Yan Zhao, Qing-Zheng Yang,* Li-Zhu Wu, Chen-Ho Tung, Li-
Ping Zhang, and Qing-Xiao Tong*
In the natural photosynthetic centers of bacteria and plants,
antenna chromophores absorb solar light and transfer the
excitation energy to the reaction center by highly efficient
singlet–singlet energy transfer.[1–6] Spatial organization of
individual chromophores is key to such efficiency: chromo-
phores need to be separated enough to minimize self-
quenching without sacrificing the dipole–dipole coupling-
mediated energy transfer.[1,2] In addition to its important role
in photosynthesis, efficient transfer of energy from multiple
chromophores to a single acceptor is of potential significance
to solar cells, photocatalysts, optical sensors and light-emit-
ting devices.[7–10] For these reasons, there has been a great deal
of interest in mimicking the natural light-harvesting pro-
cess.[11–37] A variety of scaffolds have been used including
dendrimers,[11–17] organogels,[18,19] porphyrin arrays/assem-
blies,[20–23,34] biopolymer assemblies,[24–28] and organic–inor-
ganic hybrid materials.[29–32]
Although impressive results have been obtained with the
above scaffolds, the multistep synthesis of the complex
architectures hampers their scale up and widespread applica-
tion. Nature relies on a combination of covalent and non-
covalent interactions to create the photosynthetic centers.
Covalent structures possess excellent stability and noncova-
lent self-assembled constructs provide order and synthetic
efficiency. Herein, we report a biomimetic approach to
construct artificial light-harvesting systems. We combined
two self-assembling strategies and covalent fixation to
prepare a highly efficient antenna system from readily
available building blocks. The entire synthesis was achieved
by a one-pot reaction, and the product precipitated sponta-
neously out of the reaction mixture at the end of the reaction.
The synthesis of the light-harvesting system is shown in
Scheme 1, and is based on the recently reported method to
cross-link surfactant micelles.[38,39] Our model antenna chro-
mophore is 9,10-bis(4-methylphenyl)anthracene (DPA),
a
compound with high fluorescence quantum yield
(90%).[40] Eosin Y disodium salt (EY) is the energy acceptor.
Cationic surfactant, 4-(dodecyloxy)benzyltripropargylammo-
nium bromide (1),[38] forms micelles at concentrations of
above 0.14 mm in water. Because the surface of the micelle is
covered with a dense layer of alkynyl groups, 1,4-diazidobu-
tane-2,3-diol (2)[38] could easily capture the micelle by 1,3-
dipolar cycloaddition with a CuI catalyst.[41] When 1 and 2
were used in a 1:1 ratio, the resulting surface-cross-linked
micelles (SCMs) are water-soluble nanoparticles with numer-
ous alkynes on the surface. Surface functionalization occurred
readily upon addition of a THF solution of DPA–N3 (obtained
from commercially available DPA by partial bromination and
azidation, see the Supporting Information). After 18 hours at
room temperature, the DPA-functionalized SCMs (DPA–
SCMs) precipitated spontaneously from the 2:1 THF/water
mixture, apparently as a result of the increased hydrophobic-
ity of the product. The IR spectrum of the DPA–SCMs
showed nearly complete disappearance of the alkyne peaks in
the starting SCMs (Figure S1, in the Supporting Information).
DLS (dynamic light scattering) indicated an increase in size
for the SCMs upon DPA-functionalization (Figure S2, in the
Supporting Information).
[*] H.-Q. Peng,[+] Dr. Y.-Z. Chen,[+] Prof. Q.-Z. Yang, Prof. L.-Z. Wu,
Prof. C.-H. Tung, Prof. L.-P. Zhang
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
29 Zhongguancun East Road, Beijing 100190 (China)
E-mail: qzyang@mail.ipc.ac.cn
H.-Q. Peng,[+] Prof. Q.-X. Tong
Department of Chemistry, Shantou University
Shantou, Guangdong 515063 (China)
E-mail: qxtong@stu.edu.cn
The absorption band of the DPA–SCMs is at 330–420 nm
in THF and the emission band at 390–520 nm. These spectra
match almost exactly with those of the free, monomeric DPA
in solution (Figure S3, in the Supporting Information).
Therefore, the DPA concentration ([DPA]SCMs
) in this
Prof. Y. Zhao
Department of Chemistry, Iowa State University
Ames, IA 50011-3111 (USA)
system can be determined from the absorption spectrum
and the molar coefficient extinction of DPA.
A frequent issue in light-harvesting systems with multiple
donors is the self-quenching and/or excimer formation caused
by the proximity of the chromophores. These pathways
interfere with the energy transfer and lower the overall
efficiency, and often require elaborate strategies to over-
come.[15,42] Excitingly, the fluorescence quantum yield was
0.80 and 0.90 for the micelle-bound DPA and the free
chromophore, respectively (see the Supporting Information).
Clearly, neither self-quenching nor excimer formation was
significant in the highly crowded system. We suspect there are
[+] These authors contributed equally to this work.
[**] We thank Prof. Roman Boulatov (UIUC) for his useful discussion.
We are grateful for financial support from the National Natural
Science Foundation of China (91027041, 21102155, 21072202,
20973189), the Chinese Academy of Sciences, and the Bureau of
Basic Sciences of CAS (KJCX2-EW-W09). Y.Z. thanks the U.S.
Department of Energy-Office of Basic Energy Sciences (grant DE-
SC0002142) for partial support of the research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107723.
2088
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2088 –2092

Angewandte
Chemie
light, energy migration should occur readily from
donor to donor and finally to the acceptor.
As EY was added to the DPA–SCMs, the DPA
absorption, at 330–420 nm, stayed nearly constant
while that of EY, at 450–570 nm, increased grad-
ually (Figure 1a). Importantly, the titration low-
ered the donor emission at 430 nm while enhanc-
ing that of the acceptor at 550 nm when the donor
was selectively excited at 375 nm. In the absence of
DPA–SCM, irradiation of EY at 375 nm yielded
negligible emission since EY has negligible absorp-
tion at this wavelength (Figure 2a, dotted trace).
The results indicated Fçrster energy transfer from
DPA to EY despite the low concentrations of the
two—[DPA]SCMs = 23 mm and [EY]max = 1.34 mm.
Additional evidence for the energy transfer was
obtained in a reverse titration of EY by DPA–
SCMs (Figure S4, in the Supporting Information)
and from the excitation spectrum of the EY in the
presence of DPA–SCMs (Figure 2b). The excita-
tion spectrum of the acceptor was nearly identical
to the absorption spectrum of DPA–SCM, thus
demonstrating that the donor contributed directly
to the acceptor emission.
The electrostatic assembly of the EY on the
DPA–SCM surface was confirmed by control
experiments. Energy transfer was absent under
similar conditions when free DPA was titrated by
EY (Figure 2a, and Figure S5 in the Supporting
Scheme 1. Preparation of DPA-functionalized SCMs and the construc-
tion of a light-harvesting system by introducing EY through electrostatic
interactions.
three reasons for the observed results. First, the two phenyl
groups at the 9 and 10 positions of DPA are forced out of the
anthracene plane by steric interactions, and the nonplanarity
of the chromophore may make it difficult to form stacked
structures. Second, as only short and aromatic (i.e., triazole)
linkages are used to connect DPA to the SCMs, the DPA on
the SCM surface had limited freedom. Therefore, it is possible
that the preferred orientation/distance of the self-quenching
or excimer formation for the surface-bound DPA simply
could not be achieved in our structure. Third, the positively
charged SCMs by electrostatic repulsion would reduce the
chance of intermicellar interactions of the DPA.
The Fçrster radius (R0), defined as the theoretical donor–
acceptor distance at the 50% energy-transfer efficiency, was
estimated to be 3.6 nm for DPA–EY and 2.5 nm for DPA–
DPA (see the Supporting Information).[24,43] Our previous
estimates[38,39] suggest that the average distance between the
residual alkynyl groups in SCMs is less than 2 nm. The high
density of DPA on the SCM surface thus makes it extremely
easy to shuttle energy from DPA to DPA and ultimately to the
EY acceptor. To test the hypothesis, we titrated a THF
solution of DPA–SCM with EY. The expectation was that the
negatively charged EY would assemble spontaneously on the
surface of the positively charged DPA–SCM by electrostatic
interactions. No matter which DPA chromophore absorbs
Figure 1. Absorption (a) and fluorescence (b) spectra of DPA–SCM in
THF with different concentrations of EY. [DPA]SCMs =23.0 mm. [EY] was
0.00, 0.08, 0.17, 0.33, 0.50, 0.67, 0.84, 1.00, 1.17, and 1.34 mm from
bottom to top. lex =375 nm.
Angew. Chem. Int. Ed. 2012, 51, 2088 –2092
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 2089

.
Angewandte
Communications
Figure 3. The two main pathways involved in the light-harvesting
system: 1) Direct energy transfer from donor to acceptor (path 1);
2) Energy migration from donor to donor (path 2a), then energy
transfer to acceptor (path 2b). D and A represent donor and acceptor,
respectively.
Figure 2. a) Fluorescence spectra (lex =375 nm) of EY/DPA–SCM
(1.4 mm/23 mm, solid trace) and EY/free DPA (9.9 mm/98 mm, dash-dot
trace). The fluorescence spectrum (dotted trace) of EY is shown for
comparison. b) Normalized excitation spectrum of EY/DPA–SCM
(lem =550 nm, dash-dot trace) and absorption spectrum of DPA–SCM
(solid trace). [EY]=1.4 mm. [DPA]=23.0 mm. THF was used in all
spectroscopic measurements.
Information). Moreover, when DPA–SCM was added to
a
neutral Bodipy[44] acceptor, no energy transfer was
observed, even though the DPA–Bodipy Fçrster radius was
similar to that for DPA–EY (Figure S6 in the Supporting
Information).
The majority of the DPA fluorescence was quenched by
a few percent of EY (Figure 1b). One acceptor, therefore,
must have quenched multiple donors in our system. If we
assume n DPA fluorophores could be quenched by one EY,
we could treat (DPA)n as one unit and model the quenching
by a 1:1 binding isotherm. This model combines the direct
quenching (Figure 3, path 1) and the energy-migration-
assisted energy-transfer pathway corresponding to the
antenna effect (Figure 3, path 2) and may be expressed as
Equation (1).
Figure 4. a) The nonlinear least-squares analysis of IF versus the
concentration of EY according to equation:
IF =I0 +((IlimꢁI0)/(2c0))(c0 +cEY +(1/Ka)-((c0 +cEY +(1/Ka))2ꢁ4cEYc0)
(1/2)), in which IF is the observed emission intensity of DPA–SCM, I0
the emission intensity of DPA–SCM in the absence of EY, Ilim the
emission intensity of the fully complexed DPA–SCM (assumed to be
zero in the curve fitting), c0 the concentration of (DPA)n, and cEY the
concentration of EY.[45] b) Comparison of the degree of complexation
and the degree of quenching. The degree of complexation was
calculated from Ka and c0 obtained from the curve fitting and the
degree of quenching (=1ꢁIF/I0) calculated from the observed emis-
sion intensity.
ðDPAÞn þ EY ¼ ðDPAÞn EY
ð1Þ
The quenching data indeed fit well to the 1:1 binding isotherm
(Figure 4a). Nonlinear least-squares curve fitting afforded an
association constant Ka = (4.1 ꢀ 0.2) ꢀ 106 mꢁ1 and the (DPA)n
concentration c0 = (4.8 ꢀ 0.4) ꢀ 10ꢁ7 m. Since the concentra-
tion of DPA in the solution was 2.3 ꢀ 10ꢁ5 m based on optical
density, the number of DPA chromophores in the light-
harvesting unit was calculated to be n = 48 ꢀ 4.
number of surfactant in the SCM. According to the previous
DLS study, an SCM on average contained 40–50 cross-linked
surfactants.[38] Since a 1:1 stoichiometry was employed in the
preparation of the SCMs for 1 and 2, the SCM on average
The number of the DPA chromophores per light-harvest-
ing unit was in excellent agreement with the aggregation
2090 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2088 –2092

Angewandte
Chemie
[9] D. T. McQuade, A. H. Hegedus, T. M. Swager, J. Am. Chem. Soc.
contained one residual alkyne group per surfactant. Assuming
complete functionalization of these alkyne groups by DPA–
N3, a value of n = 48 ꢀ 4 for the light-harvesting unit strongly
suggests that the light-harvesting unit was indeed the DPA–
SCM nanoparticle, on which highly efficient energy migration
took place.
Figure 4b compares the degree of complexation (i.e.,
percentage of DPA–SCMs with a bound EY) and the degree
of quenching. The degree of complexation was calculated
from Ka and c0 obtained by the nonlinear least-squares curve
fitting. The degree of quenching was obtained from the
observed emission intensity. As shown by Figure 4b, the two
sets of data overlap perfectly, thus indicating that every bound
EY completely quenched the excited DPA, regardless of its
location on the DPA–SCM.
Because the diameter of the DPA–SCM is about 20 nm
according to DLS (Figure S2), the energy transfer to EY
could not all take place through the Fçrster mechanism but,
instead, happens through both direct quenching (Figure 3,
Path 1) and energy-migration (Path 2) pathways.[24,37] The
majority of DPA chromophores on the surface of the DPA–
SCMs were not in the immediate vicinity of a bound EY.
Path 2 thus must be the dominant process responsible for the
quenching.
In summary, we have prepared a highly efficient light-
harvesting system by combining two self-assembling strat-
egies (micellization and electrostatic attraction) and covalent
capture. Inexpensive starting materials and simple chemistry
were employed. The synthesis was achieved by a one-pot
reaction at RT. The modularity of the preparation makes it
straightforward to vary the donor, the acceptor, and the ratio
between the two. A high density of the antenna chromophore
was achieved without self-quenching and excimer formation,
affording extremely efficient energy transfer. The construc-
tion of our light-harvesting system is considerably simpler
compared to most of those reported in the literature, making
it a potentially useful platform for optical, photovoltaic, or
photocatalytic devices.
2000, 122, 12389 – 12390.
[10] T. Mayr, S. M. Borisov, T. Abel, B. Enko, K. Waich, G.
Mistlberger, I. Klimant, Anal. Chem. 2009, 81, 6541 – 6545.
[11] Y. Zeng, Y. Y. Li, J. P. Chen, G. Q. Yang, Y. Li, Chem. Asian J.
2010, 5, 992 – 1005.
[12] C. Devadoss, P. Bharathi, J. S. Moore, J. Am. Chem. Soc. 1996,
118, 9635 – 9644.
[13] U. Hahn, M. Gorka, F. Vogtle, V. Vicinelli, P. Ceroni, M. Maestri,
V. Balzani, Angew. Chem. 2002, 114, 3747 – 3750; Angew. Chem.
Int. Ed. 2002, 41, 3595 – 3598.
[14] A. Adronov, S. L. Gilat, J. M. J. Frꢂchet, K. Ohta, F. V. R.
Neuwahl, G. R. Fleming, J. Am. Chem. Soc. 2000, 122, 1175 –
1185.
[15] T. Weil, E. Reuther, K. Mꢃllen, Angew. Chem. 2002, 114, 1980 –
1984; Angew. Chem. Int. Ed. 2002, 41, 1900 – 1904.
[16] G. Kodis, Y. Terazono, P. A. Liddell, J. Andreasson, V. Garg, M.
Hambourger, T. A. Moore, A. L. Moore, D. Gust, J. Am. Chem.
Soc. 2006, 128, 1818 – 1827.
[17] J. L. Wang, J. Yan, Z. M. Tang, Q. Xiao, Y. G. Ma, J. Pei, J. Am.
Chem. Soc. 2008, 130, 9952 – 9962.
[18] A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, Chem. Soc. Rev.
2008, 37, 109 – 122.
[19] A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, S. J. George,
Angew. Chem. 2007, 119, 6376 – 6381; Angew. Chem. Int. Ed.
2007, 46, 6260 – 6265.
[20] M. S. Choi, T. Aida, T. Yamazaki, I. Yamazaki, Angew. Chem.
2001, 113, 3294 – 3298; Angew. Chem. Int. Ed. 2001, 40, 3194 –
3198.
[21] N. Aratani, D. Kim, A. Osuka, Acc. Chem. Res. 2009, 42, 1922 –
1934.
[22] M. S. Choi, T. Yamazaki, I. Yamazaki, T. Aida, Angew. Chem.
2004, 116, 152 – 160; Angew. Chem. Int. Ed. 2004, 43, 150 – 158.
[23] T. S. Balaban, Acc. Chem. Res. 2005, 38, 612 – 623.
[24] R. A. Miller, A. D. Presley, M. B. Francis, J. Am. Chem. Soc.
2007, 129, 3104 – 3109.
[25] C. V. Kumar, M. R. Duff, J. Am. Chem. Soc. 2009, 131, 16024 –
16026.
[26] Y. S. Nam, T. Shin, H. Park, A. P. Magyar, K. Choi, G. Fantner,
K. A. Nelson, A. M. Belcher, J. Am. Chem. Soc. 2010, 132, 1462 –
1463.
[27] R. A. Miller, N. Stephanopoulos, J. M. McFarland, A. S. Rosko,
P. L. Geissler, M. B. Francis, J. Am. Chem. Soc. 2010, 132, 6068 –
6074.
[28] K. J. Channon, G. L. Devlin, C. E. MacPhee, J. Am. Chem. Soc.
2009, 131, 12520 – 12521.
Received: November 2, 2011
Published online: January 26, 2012
[29] H. Takeda, Y. Goto, Y. Maegawa, T. Ohsuna, T. Tani, K.
Matsumoto, T. Shimada, S. Inagaki, Chem. Commun. 2009,
6032 – 6034.
[30] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V.
Zagidullin, V. Pachenko, Y. P. Rakovich, J. F. Donegan, A. B.
Rubin, A. O. Govorov, Angew. Chem. 2010, 122, 7375 – 7379;
Angew. Chem. Int. Ed. 2010, 49, 7217 – 7221.
[31] S. Inagaki, O. Ohtani, Y. Goto, K. Okamoto, M. Ikai, K.
Yamanaka, T. Tani, T. Okada, Angew. Chem. 2009, 121, 4102 –
4106; Angew. Chem. Int. Ed. 2009, 48, 4042 – 4046.
[32] K. V. Rao, K. K. R. Datta, M. Eswaramoorthy, S. J. George,
Angew. Chem. 2011, 123, 1211 – 1216; Angew. Chem. Int. Ed.
2011, 50, 1179 – 1184.
[33] C. Curutchet, J. Kongsted, A. Munoz-Losa, H. Hossein-Nejad,
G. D. Scholes, B. Mennucci, J. Am. Chem. Soc. 2011, 133, 3078 –
3084.
Keywords: click chemistry · cross-linked micelles ·
energy transfer · light-harvesting system · self-assembly
.
[1] G. McDermott, S. M. Prince, A. A. Freer, A. M. Haw-
thornthwaitelawless, M. Z. Papiz, R. J. Cogdell, N. W. Isaacs,
Nature 1995, 374, 517 – 521.
[2] T. Pullerits, V. Sundstrom, Acc. Chem. Res. 1996, 29, 381 – 389.
[3] X. C. Hu, A. Damjanovic, T. Ritz, K. Schulten, Proc. Natl. Acad.
Sci. USA 1998, 95, 5935 – 5941.
[4] A. M. van Oijen, M. Ketelaars, J. Kohler, T. J. Aartsma, J.
Schmidt, Science 1999, 285, 400 – 402.
[5] T. Polꢁvka, H. A. Frank, Acc. Chem. Res. 2010, 43, 1125 – 1134.
[6] G. D. Scholes, G. R. Fleming, A. Olaya-Castro, R. van Gron-
delle, Nat. Chem. 2011, 3, 763 – 774.
[34] Y. Ishida, T. Shimada, D. Masui, H. Tachibana, H. Inoue, S.
Takagi, J. Am. Chem. Soc. 2011, 133, 14280 – 14286.
[35] J. H. Kim, M. Lee, J. S. Lee, C. B. Park, Angew. Chem. 2012, 124,
532 – 535; Angew. Chem. Int. Ed. 2012, 51, 517 – 520.
[7] T. Miyasaka, J. Phys. Chem. Lett. 2011, 2, 262 – 269.
[8] D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34,
40 – 48.
Angew. Chem. Int. Ed. 2012, 51, 2088 –2092
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 2091

.
Angewandte
Communications
[36] C. Y. Lee, O. K. Farha, B. J. Hong, A. A. Sarjeant, S. T. Nguyen,
J. T. Hupp, J. Am. Chem. Soc. 2011, 133, 15858 – 15861.
[37] L. Chen, Y. Honsho, S. Seki, D. L. Jiang, J. Am. Chem. Soc. 2010,
132, 6742 – 6748.
[38] S. Y. Zhang, Y. Zhao, Macromolecules 2010, 43, 4020 – 4022.
[39] S. Y. Zhang, Y. Zhao, J. Am. Chem. Soc. 2010, 132, 10642 – 10644.
[40] The fluorescence quantum yield was determined according to
a published method: S. Hamai, F. Hirayama, J. Phys. Chem. 1983,
87, 83 – 89. See the Supporting Information for details.
[41] J. E. Moses, A. D. Moorhouse, Chem. Soc. Rev. 2007, 36, 1249 –
1262.
[42] Y. Zeng, Y. Li, M. Li, G. Yang, Y. Li, J. Am. Chem. Soc. 2009,
131, 9100 – 9106.
[43] A. Montali, G. S. Harms, A. Renn, C. Weder, P. Smith, U. P.
Wild, Phys. Chem. Chem. Phys. 1999, 1, 5697 – 5702.
[44] Bodipy is the abbreviation for 8-(chloromethyl)-4,4-difluoro-
1,3,5,7 tetramethyl-2,6-diethyl-4-bora 3a,4a-diaza-s-indacene.
[45] J. Bourson, J. Pouget, B. Valeur, J. Phys. Chem. 1993, 97, 4552 –
4557.
2092 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2088 –2092