Efficient light-harvesting antenna with a multi-porphyrin cascade

Article abstract of DOI:10.1021/ja2050343

A light-harvesting antenna 1 comprising three varieties of porphyrins, each having a different number of ethynyl groups at its meso positions, was designed and synthesized. Antenna 1 exhibits intense absorption throughout the visible region up to 700 nm. Steady-state and time-resolved fluorescence studies showed that singlet-excited-state energy transfer occurs from the peripheral porphyrins to the central porphyrin with >90% efficiency and rate constants on the order of 10¹⁰ s^-1.

Full text of DOI:10.1021/ja2050343

COMMUNICATION
pubs.acs.org/JACS
Efficient Light-Harvesting Antenna with a Multi-Porphyrin Cascade
||
Atsuhiro Uetomo,† Masatoshi Kozaki,*,† Shuichi Suzuki,† Ken-ichi Yamanaka,*,‡ Osamu Ito,§, and
Keiji Okada*,†
†Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
‡Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan
§Fullerene Group, NIMS, 1-2-1 Sengen, Tsukuba-city, Ibaraki 305-0047, Japan
CarbonPhotoScience Laboratory, 2-1-6 Kita-Nakayama, Izumi-ku, Sendai, Miyagi 981-3215, Japan
S Supporting Information
b
arranged in a dendritic architecture in 1 in order to facilitate the
ABSTRACT: A light-harvesting antenna 1 comprising
three varieties of porphyrins, each having a different number
of ethynyl groups at its meso positions, was designed and
synthesized. Antenna 1 exhibits intense absorption through-
out the visible region up to 700 nm. Steady-state and time-
resolved fluorescence studies showed that singlet-excited-
state energy transfer occurs from the peripheral porphyrins
to the central porphyrin with >90% efficiency and rate
transfer of the excitation energy from the peripheral porphyrins
(TP-Por and DE-Por) to the central porphyrin (TE-Por). DE-Por
is located at the terminal of the phenylene ethynylene conjugated
chain, whereas TP-Por is linked through nonconjugated benzyl ether
chains. For this reason, DE-Por could be expected to function
not only as a light absorber but also as a mediator in energy trans-
fer from TP-Por* to TE-Por.
Antenna 1 was synthesized according to the previously
reported convergent method (Scheme 1).8 Pinacol borate 3 with
a TP-Por terminal was connected to DE-Por-terminated con-
jugated chain 2 using a SuzukiꢀMiyaura coupling reaction, which
afforded compound 4 in quantitative yield.9 After incorporation
of an iodo group to give 5,10 a copper-free Sonogashira coupling
reaction with 6 provided antenna 1.11 The crude product was
purified by cycling gel-permeation chromatography to afford 1 in
17% yield. Antenna 1 was highly soluble in organic solvents such
as tetrahydrofuran (THF) and dichloromethane and was unambig-
uously characterized by means of NMR and MALDIꢀTOF mass
spectrometry experiments.
Antenna 1 in THF exhibits characteristic bands due to the
component porphyrins (Figure 2). Three intense absorption
bands at λmax (log ε) = 426 nm (6.69), 453 nm (6.23), and
498 nm (5.85) were assigned to Soret bands attributed to TP-
Por, DE-Por, and TE-Por, respectively, on the basis of compar-
isons to the spectra of 3, 2, and 7. In addition, a series of weak Q
bands with maxima at λmax = 557 nm (5.28), 598 nm (5.12),
653 nm (5.56), and 697 nm (5.11) were observed as an overlap of
absorptions of TP-Por, DE-Por, and TE-Por on the basis of the
spectra of 3, 2, and 7, respectively (Figure 2 inset). Although the
observed absorption spectrum of the antenna 1 could be roughly
simulated using the spectra of the reference compounds 2, 3, and 7
[Figure S1 in the SupportingInformation (SI)], thereweremodest
differences between observed and simulated spectra. These results
suggest weak electronic interactions between the porphyrin units
in the ground state. Antenna 1 has strong absorptions across the
400ꢀ700 nm region with ε > 56 000 Mꢀ1 cmꢀ1. Furthermore,
the absorption spectral shape of 1 is roughly similar to the solar
spectrum. Because of these characteristic shapes of the absorp-
tion and emission spectra (see below), selective excitation of
TP-Por, DE-Por, and TE-Por in 1 and detection and identifica-
tion of the emitting porphyrin were possible.
constants on the order of 1010 sꢀ1
.
n essential component in an artificial photosynthetic system
is a light-harvesting antenna that strongly absorbs sunlight
A
and transfers the light energy to a reaction center with high
efficiency.1 A natural antenna system generally possesses several
types of antenna pigments, which help in collecting photons with
energies across the visible region. Moreover, these pigments are
systematically arranged to form a lowest-excited-state energy
gradient that enhances the efficiency of energy transfer.1,2 Both
the magnitude of the absorbance cross section in the visible
region and the energy-transfer efficiency are essential factors in
determining the light-harvesting ability of the antenna.3 Porphyrins
exhibit strong absorption in the visible region, making them
important chromophores for the construction of artificial light-
harvesting antennae.4 To date, a large number of porphyrin assem-
blies that act as antennae with highly efficient energy transfers
have been constructed with a combination of free-base and Zn
porphyrins as the energy acceptors and donors, respectively.5,6
However, these porphyrin antennae display the desired intense
absorption in only a limited portion of the visible region, and as a
consequence, their light-harvesting ability is restricted. Herein
we report the novel light-harvesting antenna 1 (Figure 1), which
strongly absorbs light across the 400ꢀ700 nm region with ε >
56 000 Mꢀ1 cmꢀ1 and exhibits highly efficient energy transfer
from the peripheral porphyrins to the central porphyrin.
Antenna 1 has three different Zn porphyrins: eight tetraphenyl
Zn porphyrin units (TP-Por), four Zn porphyrin units with two
meso-ethynyl and two meso-phenyl groups (DE-Por), and a Zn
porphyrin unit with four meso-ethynyl groups (TE-Por) (Figure 1).
The direct connection of ethynyl groups to the meso positions of
the porphyrin results in a considerable red shift of the absorption
bands.7 Therefore, the energy of the lowest excited state decreases in
the order TP-Por > DE-Por > TE-Por. These porphyrins are
Received: June 1, 2011
Published: July 26, 2011
r
2011 American Chemical Society
13276
dx.doi.org/10.1021/ja2050343 J. Am. Chem. Soc. 2011, 133, 13276–13279
|

Journal of the American Chemical Society
COMMUNICATION
Figure 1. Chemical structure of light-harvesting antenna 1.
Figure 2. Absorption spectra of 1 (black), 2 (green), 3 (blue), and 7
(red) measured in THF. The inset shows expanded absorption spectra
in the Q-band region.
Scheme 1
method.12 The observed fluorescence spectral shape was suc-
cessfully simulated using component fluorescence patterns from
2 and 3, showing that the observed fluorescence can be divided
into three components with TE-Por:DE-Por:TP-Por area ratios
of 91:7:1 (Figure S2). Thus, selective excitation of TP-Por gave
TE-Por* with 91% quantum efficiency for the energy transfer and
a fluorescence quantum efficiency (Φf) of 0.13 for the acceptor
chromophore TE-Por. Similarly, selective excitation of DE-Por
at 455 nm, at which wavelength I(DE-Por):I(TE-Por) = 30:1,
gave a fluorescence pattern similar to that for TE-Por with a
quantum yield of 0.13, accompanied by fluorescence quenching
of DE-Por (Figure 3b). The observed fluorescence spectrum was
reconstructed from two components with a TE-Por:DE-Por
area ratio of 94:6 (Figure S3), indicating selective singlet energy
transfer from DE-Por* to TE-Por with 94% quantum efficiency for
the energy transfer and Φf = 0.12 for the acceptor chromophore
TE-Por. The fluorescence quantum yields in these intramole-
cular energy transfers are in good agreement with the quantum
yield (0.13) for direct excitation of TE-Por in 1. Because of the
smaller loss during energy transfer and stronger absorptions of 1
in the visible region, the fluorescence from TE-Por* in 1 should
be much greater in intensity than that of 7 when the samples are
exposed to the same intensity of visible light.
The dynamic processes involved in the energy transfer were
investigated using time-resolved fluorescence spectroscopy.13 The
singlet-excited-state energy transfer in 1 involves three pro-
cesses: TP-Por* to DE-Por (EN1), TP-Por* to TE-Por (EN2),
and DE-Por* to TE-Por (EN3). First, for simpler analysis, we
measured the rate constants of energy transfer (kEN) for the model
compounds 4, 8, and 9 (Figure 4), which have the least fluorescence
overlap, to obtain approximate values for the energy-transfer
rates for EN1, EN2, and EN3, respectively. The kEN values for EN1
and EN2 are similar and on the order of 109 sꢀ1, and the value for
step EN3 is ∼10 times larger than those for EN1 and EN2 (Table 1).
These rate constants are roughly reproduced by the F€orsterequation
(Table S2 in the SI),14 although the participation of the superexchange
mechanism thorough conjugated bridges cannot be excluded.15
To evaluate the efficiency of singlet-excited-state energy
transfer from TP-Por* to TE-Por and from DE-Por* to TE-Por,
steady-state fluorescence quenching in 1 was investigated with
model compounds 2 and 3 in degassed THF. The excitation of
TP-Por in 3 and DE-Por in 2 showed characteristic fluorescence
spectra for the two porphyrin units at λEM-max = 604 and 656 nm
for 3 and 656 and 717 nm for 2 with quantum yields of 0.033 and
0.16, respectively (Figure 3). Selective excitation of TP-Por in 1
at 426 nm, at which wavelength the incident light is absorbed by
TP-Por and DE-Por with an absorption (I) ratio of I(TP-Por):
I(DE-Por) = 23:1, resulted in the appearance of an intense
emission with λEM-max = 707 and 780 nm (Figure 3a). The quantum
yield of the emission was determined to be 0.14 by the absolute
13277
dx.doi.org/10.1021/ja2050343 |J. Am. Chem. Soc. 2011, 133, 13276–13279

Journal of the American Chemical Society
COMMUNICATION
Table 1. Photophysical Data for 1, 4, 8, and 9 in THF
λex/nm
λdetect/nm
kEN/sꢀ1
energy-transfer step
1
4
8
1
9
427
427
427
456
456
610
610
610
660
660
1.2 ꢁ 1010
5.7 ꢁ 109
5.0 ꢁ 109
3.0 ꢁ 1010
3.0 ꢁ 1010
EN1 + EN2
EN1
EN2
EN3
EN3
Figure 3. Steady-state fluorescence spectra of (a) 1 (red) and 3 (blue)
(λex = 426 nm) and (b) 1 (red) and 2 (green) (λex = 453 nm) measured
in THF. Fluorescence intensities were measured after normalizing the
absorbance at the excitation wavelength.
Figure 6. Fluorescence decay curve at 710 nm of 1 excited at 427 nm in
THF (dots). The red solid line was obtained by the simulation using
eq 1. The shape decay (blue) is the laser pulse profile.
kd = 6.7 ꢁ 108 sꢀ1) for DE-Por* and using a longer decay
constant in 1 (λex = 456 nm, λdetect = 710 nm, kd = 4.0 ꢁ 108 sꢀ1
)
for TE-Por*. From these rate parameters, the following quantum
efficiencies of energy transfer were calculated: 51 and 45% from
TP-Por* to DE-Por and TE-Por, respectively (giving a total
quantum efficiency of 96%) and 98% from DE-Por* to TE-Por. The
rapid energy transfer from DE-Por* to TE-Por plays an important
role (∼50% contribution) in this cascade process by mediating the
transfer of excitation energy from TP-Por* to TE-Por.
Figure 4. Chemical structures of reference compounds 8 and 9.
Finally, we analyzed the fluorescence decay at 710 nm after
selective excitation of TP-Por (λex = 427 nm) using the deter-
mined rate parameters in Table 1. TE-Por* has a strong fluores-
cence intensity at 710 nm; however, DE-Por* and TP-Por* also
have weak fluorescence intensities at this wavelength. Obviously,
TE-Por* is generated via all three steps EN1ꢀEN3. According to
the kinetic model (see the SI), the time dependence should be
expressed by eq 1:17
ꢂ
IðtÞ ¼ a expf ꢀ ½kEN1 þ kEN2 þ kdðTP-Por Þꢃtg
Figure 5. Energy diagram and relaxation process from excited states of
TP-Por in antenna 1. * and ** denote the first and second excited states,
respectively.
ꢂ
þ b expf ꢀ ½kEN3 þ kdðDE-Por Þꢃtg
ꢂ
þ c exp½ ꢀ kdðTE-Por Þtꢃ
ð1Þ
Selective excitation of TP-Por (λex = 427 nm) in 1, which has
With the values in Figure 5, the observed decay curve at 710 nm
was successfully simulated using the parameters a = ꢀ0.32,
b = 0.10, and c = 0.58 (Figure 6). Furthermore, the time-dependent
contributions of the excited states of TP-Por*, DE-Por*, and
TE-Por* were clarified (Figure S12). These features are com-
pletely consistent with highly efficient energy transfer in 1.
In conclusion, we have synthesized a novel light-harvesting
antenna in which three kinds of porphyrins with different numbers
of ethynyl groups at the meso positions are precisely located.
Strong absorption throughout the visible region and nearly
quantitative energy transfer from the peripheral porphyrins to
the central porphyrin were observed. In addition, it should be
noted that the benzyl ether chains in the antenna 1 could easily be
replaced by dendritic structures, which would be advantageous
two possible EN routes, gave a fluorescence decay (λdetect
=
610 nm, TP-Por*) with kEN = 1.2 ꢁ 1010
s
ꢀ1. As expected, the
kEN value is very close to the sum of those obtained from the
model compounds, suggesting that the EN1 and EN2 processes
with similar rate constants occur concurrently in 1. Excitation of
DE-Por (λex = 456 nm) in 1 gave a major fluorescence decay
(λdetect = 660 nm, DE-Por*) with kEN = 3.0 ꢁ 1010 sꢀ1 for
EN3,16 which is in good agreement with the values estimated
from model compound 9. These decay parameters of TP-Por*,
DE-Por*, and TE-Por* in 1 are summarized in Figure 5. The decay
rate constants without energy transfer (kd) were determined using
model compounds 3 (λex = 427 nm, λdetect = 610 nm, kd = 5.3 ꢁ
108 sꢀ1) for TP-Por* and 2 (λex = 456 nm, λdetect = 660 nm,
13278
dx.doi.org/10.1021/ja2050343 |J. Am. Chem. Soc. 2011, 133, 13276–13279

Journal of the American Chemical Society
COMMUNICATION
for the construction of light-harvesting systems with large numbers
of chromophores. These results prove that the synthesis of
porphyrin dendrimers using snowflake architectures is a valuable
tool for the construction of artificial light-harvesting antennae.
(12) Absolute quantum yields of the fluorescence were measured
using an Otsuka Electronics QE-1100 spectrometer with an MCPD-9800
detector.
(13) Yamanaka, K.; Okada, T.; Goto, Y.; Tani, T.; Inagaki, S. Phys.
Chem. Chem. Phys. 2010, 12, 11688.
(14) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006.
’ ASSOCIATED CONTENT
(15) Pettersson, K.; Kyrychenko, A.; R€onnow, E.; Ljungdahl, T.;
Martensson, J.; Albinsson, B. J. Phys. Chem. A 2006, 110, 310.
(16) Minor decay components were observed because of the
fluorescence overlaps at 660 nm (TP-Por* and TE-Por*): τ ≈ 240 ps
(20%) and 1.9 ns (12%).
ꢄ
S
Supporting Information. Detailed synthetic procedures
b
and spectral data. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) The reverse energy-transfer processes from DE-Por* to
TP-Por and from TE-Por* to TP-Por (or DE-Por) (EN1Rꢀ3R in
Table S2) may occur because of the small energy gaps between these
excited states (Figure 5). The F€orster mechanism suggests that the
reverse energy-transfer rate constants are approximately one-tenth of
those for the forward reactions (Table S2). To avoid overparametriza-
tion and ensure their reliability for analysis, we treated the decay data
using the consecutive reaction scheme without these reverse energy
transfers.
’ AUTHOR INFORMATION
Corresponding Author
kozaki@sci.osaka-cu.ac.jp; yamanaka@mosk.tytlabs.co.jp;
okadak@sci.osaka-cu.ac.jp
’ ACKNOWLEDGMENT
This work was partially supported by Grants 23350022 and
22350066 from JSPS. We thank for Otsuka Electronics for mea-
surements of fluorescence quantum yields.
’ REFERENCES
(1) (a) Barber, J.; Andersson, B. Nature 1994, 370, 31. (b) Umena,
Y.; Kawakami, K.; Shen, J.-R.; Kamiya, N. Nature 2011, 473, 55.
(c) Blankenship, R. E. Molecular Mechanism of Photosynthesis; Blackwell
Science: Oxford, U.K., 2002.
(2) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 9635.
(3) (a) Hippius, C.; van Stokkum, I. H. M.; Gs€anger, M.; Groeneveld,
M. M.; Williams, R. E.; W€urthner, F. J. Phys. Chem. C 2008, 112, 2476.
(b) Diring, S.; Puntoriero, F.; Nastasi, F.; Campagna, S.; Ziessel, R. J. Am.
Chem. Soc. 2009, 131, 6108.
(4) (a) Harvey, P. D.; Stern, C.; Guilard, R. Bio-inspired Molecular
Devices Based on Systems Found in Photosynthetic Bacteria. In Hand-
book of Porphyrin Science; Kadash, K. M., Smith, K. M., Guilard, R., Eds.;
World Scientific: San Diego, CA, 2000; Vol. 11, Chapter 49, pp 1ꢀ179.
(b) Imahori, H. J. Phys. Chem. B 2004, 108, 6130. (c) Satake, A.; Kobuke,
Y. Tetrahedron 2005, 61, 13.
(5) (a) Aratani, N.; Kim, D.; Osuka, A. Acc. Chem. Res. 2009,
42, 1922. (b) Kozaki, M.; Uetomo, A.; Suzuki, S.; Okada, K. Org. Lett.
2008, 10, 4477. (c) Choi, M.; Aida, T.; Yamazaki, T.; Yamazaki, I. Angew.
Chem., Int. Ed. 2001, 40, 3194. (d) Li, J.; Ambroise, A.; Yang, S. I.; Diers,
J. R.; Seth, J.; Wack, C. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am.
Chem. Soc. 1999, 121, 8927.
(6) For light-harvesting antenna containing varieties of
porphyrins, see: (a) Song, H.-E.; Kirmaier, C.; Schwartz, J. K.; Hindin,
E.; Yu, L.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2006,
110, 19131. (b) Nakano, A.; Osuka, A.; Yamazaki, I.; Yamazaki, T.;
Nishimura, Y. Angew. Chem., Int. Ed. 1998, 37, 3023.
(7) (a) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994,
264, 1105. (b) Taylor, P. N.; Huuskonen, J.; Rumbles, G.; Aplin, R. T.;
Williams, E.; Anderson, H. L. Chem. Commun. 1998, 909. (c) Nakano,
A.; Osuka, A.; Yamazaki, T.; Nishimura, Y.; Akimoto, S.; Yamazaki, I.;
Itaya, A.; Murakami, M.; Miyasaka, H. Chem.—Eur. J. 2001, 7, 3134.
(8) (a) Kozaki, M.; Okada, K. Org. Lett. 2004, 6, 485. (b) Kozaki, M.;
Akita, K.; Okada, K. Org. Lett. 2007, 9, 1509. (c) Kozaki, M.; Akita, K.;
Okada, K. Org. Lett. 2007, 9, 3315.
(9) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(10) (a) Moore, J. S.; Weinstein, E. J.; Wu, Z. Tetrahedron Lett. 1991,
32, 2465. (b) Wu, Z.; Moore, J. S. Tetrahedron Lett. 1994, 35, 5539.
(11) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 16, 4467. (b) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey,
J. S. Chem. Mater. 1999, 11, 2974.
13279
dx.doi.org/10.1021/ja2050343 |J. Am. Chem. Soc. 2011, 133, 13276–13279