Highly efficient energy transfer in the light harvesting system composed of three kinds of boron-dipyrromethene derivatives

Article abstract of DOI:10.1021/ol702381j

A light-harvesting system containing three kinds of BODIPY fluorophores was synthesized. It exhibited very strong absorption in the region from 300 to 700 nm, and the energy transfer within it was highly efficient.

Full text of DOI:10.1021/ol702381j

ORGANIC
LETTERS
2008
Vol. 10, No. 1
29-32
Highly Efficient Energy Transfer in the
Light Harvesting System Composed of
Three Kinds of Boron
Derivatives
−Dipyrromethene
Xiaolin Zhang,† Yi Xiao,*,† and Xuhong Qian‡
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology,
Dalian 116012, China, and State Key Laboratory of Bioreactor Engineering and
Shanghai Key Laboratory of Chemical Biology, East China UniVersity of Science
and Technology, Shanghai 200237, China
xiaoyi@chem.dlut.edu.cn
Received October 17, 2007
ABSTRACT
A light-harvesting system containing three kinds of BODIPY fluorophores was synthesized. It exhibited very strong absorption in the region
from 300 to 700 nm, and the energy transfer within it was highly efficient.
The light harvesting system, which absorbs light and funnels
energy to the reaction center, plays a very important role in
natural photosynthesis.1 Many artificial light-harvesting
molecules are synthesized in succession.2 An ideal light-
harvesting system absorbs light efficiently in a relatively
broad region covering most of the sunlight spectrum. In order
to achieve this, the integration of multiple excellent chro-
mophores is necessary, and the energy transfer within the
system should be efficient. The development of highly effi-
cient light-harvesting systems remains a challenging task
as many aspects need improvment like the choice of the
chromophores with proper spectra overlaps and suited energy
levels, linker types, connection sites, and synthesis strategy,
etc.
Boron-dipyrromethene (BODIPY) dyes,3 which are widely
applied as fluorescent sensors4b and labeling reagents,3 have
remarkable properties such as high absorption coefficient,
high fluorescence yield, and excellent photostability, etc.4
The long wavelength BODIPYs, in particular, have advan-
† Dalian University of Technology.
‡ East China University of Science and Technology.
(1) Hu, X.; Damjanovic, A.; Ritz, T.; Schulten, K. Proc. Natl. Acad.
Sci. U.S.A. 1998, 95, 5935.
(2) Selected reviews: (a) Prathapan, S.; Johnson, T. E.; Lindsey, J. S. J.
Am. Chem. Soc. 1993, 115, 7519-7520. (b) Flamigni, L.; Ventura, B.; You,
C. C.; Hippius, C.; Wu¨rthner, F. J. Phys. Chem. 2007, 111, 622-630. (c)
Yilmaz, M. D.; Bozdemir, A.; Akkaya, E. U. Org. Lett. 2006, 8, 2871-
2873. (d) Hippius, C.; Schlosser, F.; Vysotsky, M. O.; Bo¨hmer, V.;
Wu¨rthner, F. J. Am. Chem. Soc. 2006, 128, 3870-3871.
(3) Hargland, R. P. Handbook of Fluorescent Probes and Research
Products, 9th ed.; Molecular Probes: Carlsbad, CA,, 2002; Chapter 1.
(4) (a) Ziessel, R.; Ulrich, G.; Harriman, A. New. J. Chem. 2007, 31,
496-501. (b) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2005, 127,
10464-10465.
10.1021/ol702381j CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/11/2007

Scheme 1. Synthetic Routes and Chemical Structures of the Compounds
tages over many other popular long wavelength dyes in these
shown in Scheme 1) containing three kinds of BODIPY dyes
(yellow-green BODIPY, pink BODIPY, and purple BODIPY
with the absorption maximum at 501, 568, and 647 nm,
respectively) to absorb the light energy in corresponding
regions. Its intense absorption from the visible to near-
infrared region covers the strong radiation scope of sunlight.
Furthermore, its good photostability, solubility, and efficient
energy transfer (99%) within it make compound 11 promis-
ing as a light-harvesting system.
aspects. For example, in the long wavelength region,
BODIPYs have a higher molar absorption coefficient than
porphyrins and better photostability than cyanine dyes, etc.
Thus, we believe that BODIPYs should be very competitive
candidates in the design of light-harvesting systems.5 How-
ever, there have been few reports on light-harvesting systems
using a long wavelength BODIPY as the energy acceptor.6
In the past, the synthetic difficulty might be the main
hindrance. But, recently, considerable progress has been
made in the efficient syntheses of BODIPYs especially for
methyl-substituted long wavelength BODIPYs,7 which en-
dows BODIPYs with a better prospect in the design of light-
harvesting systems.
Click chemistry, 1,3-dipolar cycloaddition of azides and
alkynes using catalyst copper(I), was chosen as the strategy
to connect the three BODIPYs.8 There were two consider-
ations: one was the high yield of the click reactions; the
Here, we designed and synthesized the first boron-
dipyrromethene-based light-harvesting system (compound 11,
(7) Selected reviews: (a) Rurack, K.; Kollmannsberger, M.; Daub, J.
Angew. Chem., Int. Ed. 2001, 40, 385-387. (b) Qin, W.; Baruah, M.; De
Borggraeve, W. M.; Boens, N. J. Photochem. Photobiol. A 2006, 183, 190-
197. (c) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474-
14475. (d) Coskun, A.; Deniz, E.; Akkaya, E. U. Org. Lett. 2005, 7, 5187-
5189. (e) Dost, Z.; Atilgan, S.; Akkaya, E. U. Tetrahedron Lett. 2006, 62,
8484-8488. (f) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.; Guc, D.; Akkaya,
E. U. Chem. Commun. 2006, 4398-4400.
(5) Li, F.; Yang, S.; Ciringh, Y.; Seth, J.; Martin, C.; Singh, D.; Kim,
D.; Birge, R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc.
1998, 120, 10001-10017.
(6) (a) Burghart, A.; Thoresen, L. H.; Chen, J.; Burgess, K.; Bergstro¨m,
F.; Johansson, L. B-Å. Chem. Commun. 2000, 2203-2204. (b) Harriman,
A.; Mallon, L. J.; Goeb, S.; Ziessel, R. PhysChemChemPhys. 2007, 9, 5199-
5201.
(8) Kolb, H. C.; Sharpless, K. B. Drug DiscoVery Today 2003, 8, 1128-
1137.
30
Org. Lett., Vol. 10, No. 1, 2008

other was the mild conditions which could be tolerable for
the boron complex.
The spectrum was composed of absorption bands of yellow-
green BODIPY, pink BODIPY, and purple BODIPY. Thus,
the absorption properties of compound 11 were determined
by the individual patterns of the parent chromophoric units,
and there was no interaction between the chromophores in
the ground state.2d Compared with the most widely used
light-harvesting systems based on porphyrins, the absorption
peaks that composed compound 11 had higher molar
extinction coefficient and broader absorption band. The
absorption spectrum of compound 11 spanned a very broad
region from 300 to 700 nm which overlapped the strong
radiation scope of sunlight. (The absorption spectra of other
compounds are described in the Supporting Information.)
Upon photoexcitation of compound 11 at λex ) 490 nm
(exclusive absorption of the yellow-green BODIPY unit) or
λex ) 560 nm (exclusive absorption of the pink BODIPY
unit), fluorescence emission of the central purple BODIPY
dye at 660 nm was observed (Figure 2). The emission peaks
The syntheses of the target compound 11 and reference
compounds 5-10 and 12 are depicted in Scheme 1.
Compound 1 reacted with NaN3 in DMF to produce
compound 2, which further reacted with 2,4-dimethylpyrrole
in the presence of TFA and DDQ, followed by the addition
of BF3‚Et2O, to produce compound 3. Compound 5 and 6,
synthesized through two steps according to the previous
procedure,7 reacted with compound 3 to produce the corre-
sponding compounds in the presence of sodium ascorbate
and CuSO4‚5H2O in a 1:1 mixture of ethanol and water in
the microwave reactor.
To assess the efficiency of energy transfer, the compounds
were studied by UV/vis absorption and time-resolved
fluorescence spectroscopy in CH2Cl2. The photophysical data
are summarized in Table 1. The absorption spectra of
Table 1. Photophysical Properties of Compounds in CH2Cl2
compd
ꢀ (λmax)/(M-1 cm-1) (nm)
λem/nm
Φf
τ/ns
3
5
6
7
8
9
10
11
56600 (501)
89882 (572)
82989 (644)
62010 (501), 80596 (572)
69883 (501), 92355 (644)
82176 (568), 73340 (644)
126706 (501), 87456 (646)
60436 (502), 80542 (568),
67724 (647)
511
583
656
586
659
658
660
662
0.65
0.54
0.20
0.56a
0.24a
0.18b
0.22a
0.22a
3.97
4.42
4.67
4.34
4.73
3.74
4.67
3.50
12
90100 (567)
581
0.58
4.51
b
a λex ) 490 nm. λex ) 560 nm.
Figure 2. (a) Absorption spectra of compound 11 (black, solid).
(b) Corresponding fluorescence emission spectra (blue, solid) and
(c) fluorescence excitation spectra (red, solid).
compounds 11 and reference compounds 3, 6, and 12 at equal
absorbance values are shown in Figure 1. The spectrum of
were very weak from the yellow-green BODIPY at 520 nm
and pink BODIPY at 586 nm, respectively. The quantum
yields of compound 11 were measured to be 0.22, 0.21, and
0.20 for three different excitation wavelengths (490 nm, 560
nm, 635 nm), which were also close to that of the compound
6. Obviously, the fluorescence quantum yields did not vary
significantly whether the compounds were excited at the
donor’s or acceptor’s absorbance. The absence of the
quantum yield dependence suggested almost complete energy
transfer from the donor to the acceptor.9
In the fluorescence excitation spectrum of compound 11,
all absorption bands of the parent chromophores were
observed. Moreover, the overlay of the excitation spectrum
and absorption spectrum for compound 11 showed a close
match throughout the spectrum. This indicated a high yield
of singlet-state energy transfer in the compound.10 (The
fluorescence spectra of the other compounds are described
in the Supporting Information.)
Figure 1. Absorbance spectra of compounds 3 (black, solid), 6
(green, solid), 11 (blue, solid), and 12 (red, solid) in CH2Cl2.
compound 11, as expected, was essentially equal to the sum
of the other three spectra, which showed the absorption
characteristic maxima of all three BODIPY chromophores.
(9) Wan, C. W.; Burghart, A.; Chen, J.; Bergstro¨m, F.; Johansson, L. B.
Å.; Wolford, M. F.; Kim, T. G.; Topp, M. R.; Hochstrasser, R. M.; Burgess,
K. Chem. Eur. J. 2003, 9, 4430-4441.
Org. Lett., Vol. 10, No. 1, 2008
31

donors to purple BODIPY acceptor in the harvesting system
11. Fo¨rster radii R0 of the compounds were found to be
significantly greater than the calculated donor-acceptor
distance; hence, the calculated energy transfer yields were
high. Seen from the Table 2, the energy transfer yields were
all over 99%, and the rate constants were all over 1010 s-1.
The yields of energy transfer within compounds 7 and 9 were
slightly higher than that within compound 8. The rates of
energy transfer within compound 7 and 9 were several fold
higher than that within compound 8. These differences were
ascribed to the better spectral overlap of the emission
spectrum of the donor with the absorption spectrum of the
acceptor in compounds 7 and 9 than that in compound 8.
The data of reference compounds indicated that in our
trichromophore system 11, energy transfer from any of the
donors (yellow-green BODIPY or pink BODIPY) to the
acceptor (purple BODIPY) was highly efficient. The ignor-
able energy loss can be attributable to the high quantum
yields of the donors, the high molar extinction absorption
coefficient of the acceptor, and the good spectral overlap.
In conclusion, we synthesized a novel light-harvesting
system containing three kinds of BODIPY fluorophores. It
exhibited very strong absorption in a broad region from 300
to 700 nm. Within this system, energy transfer from each
donor to the acceptor was highly efficient. This study
demonstrated the potential of the boron-dipyrromethene
derivatives in solar energy collection.
Table 2. Calculation Data of Energy Transfer for Compounds
compound (in CH2Cl2)
parameter
7
8
9
Ja/M-1 cm-1 nm4
R0b/Å
1.57 × 1015 3.60 × 1014 3.58 × 1015
49.4
17.43
99.8
38.6
17.43
99.2
55.6
17.43
99.9
Rc/Å
E (energy transfer
efficiency)/%
kT (the rate of the energy 1.30 × 1011 2.97 × 1010 2.33 × 1011
transfer)/s-1
a The overlap integral of the donor emission and the acceptor absorption
∞
spectra. J ) ∫0 ID(λ)ꢀA(λ)λ4dλ. b Calculated according to the following
equation6,11 (see the Supporting Information): k2 ) 2/3, n(CH2Cl2) )
1.4240. c Distance between the D and A groups (estimated using standard
bond lengths).
The rate and efficiency of Fo¨rster resonance energy
transfer (FRET) depend on many factors: the extent of
spectral overlap of the emission spectrum of the donor with
the absorption spectrum of the acceptor, the quantum yield
of the donor, orientation factors, and the distance between
the donor and the acceptor.12 In order to access the energy
transfer rate and yields in light harvesting system 11, we
calculated those in the reference compounds 7, 8, and 9,
respectively (see the calculation details in the Supporting
Information). Within compounds 8 and 9, the yellow-green
and pink BODIPY parts acted as donors, respectively, and
the purple BODIPY part served as the acceptor, which could
simulate the energy transfer from the yellow-green and pink
Acknowledgment. We thank the NSF of China for the
financial support (Nos. 20406004, 20572012, and 20536010).
Supporting Information Available: Experimental de-
tails, spectral data, and energy transfer calculation of the
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(10) Hsiao, J.-S.; Kureger, B. P.; Wagner, R. W.; Johnson, T. E.; Delaney,
J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.;
Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181-11193.
(11) Berberan-Santos, M. N.; Choppinet, P.; Fedorov, A.; Jullien, L.;
Valeur, B. J. Am. Chem. Soc. 2000, 122, 11876-11886.
(12) Fo¨rster, T. Ann. Phys. 1948, 55-75.
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