A light-harvesting array composed of porphyrins and rigid backbones

Article abstract of DOI:10.1021/ol801662q

(Chemical Equation Presented) A light-harvesting array containing rigid backbones, peripherally positioned Zn-porphyrin terminals, and a free-base (Fb) porphyrin core was prepared by a convergent method where the Sonogashira coupling reaction was used in the key steps. Effective intramolecular singlet-energy transfer from the peripheral Zn-porphyrin units to the Fb porphyrin core was observed. The efficiency of the energy transfer was compared with those of reference compounds.

Full text of DOI:10.1021/ol801662q

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4477-4480
A Light-Harvesting Array Composed of
Porphyrins and Rigid Backbones
Masatoshi Kozaki,* Atsuhiro Uetomo, Shuichi Suzuki, and Keiji Okada*
Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan
kozaki@sci.osaka-cu.ac.jp; okadak@sci.osaka-cu.ac.jp
Received July 22, 2008
ABSTRACT
A light-harvesting array containing rigid backbones, peripherally positioned Zn-porphyrin terminals, and a free-base (Fb) porphyrin core was
prepared by a convergent method where the Sonogashira coupling reaction was used in the key steps. Effective intramolecular singlet-energy
transfer from the peripheral Zn-porphyrin units to the Fb porphyrin core was observed. The efficiency of the energy transfer was compared
with those of reference compounds.
Many cyclic porphyrin assemblies have been reported as
artificial light-harvesting antennae.1 However, there are few
such systems that can be applied to charge-separation to
mimic the natural photosynthetic apparatus,2 probably be-
cause of the synthetic difficulty of introducing antennae and
charge separation moieties selectively in a system. Recently,
we reported a unique convergent method for the synthesis
of dendrimers having rigid backbones.3 This architecture is
useful for the construction of a well-defined charge-separating
system inside a dendritic structure. The rigid backbone serves
not only as a scaffold for the construction of a well-defined
functional array but also as a conductive path in electron
(2) (a) Kodis, G. Y.; Liddell, P. A.; de la Garza, L.; Clausen, P. C.;
Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. A
2002, 106, 2036. (b) Choi, M. S.; Aida, T.; Luo, H.; Araki, Y.; Ito, O.
Angew. Chem., Int. Ed. 2003, 42, 4060. (c) Nakano, A.; Osuka, A.;
Yamazaki, T; Itaya, A.; Murakami, M.; Miyasaka, H. Chem. Eur. J. 2007,
7, 3134.
(1) (a) del Rosario Benites, M.; Johnson, T. E.; Weghorn, S.; Yu, L.;
Rao, P. D.; Diers, J. R.; Yang, S. I.; Kirmaier, C.; Bocian, D. F.; Holten,
D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 65. (b) Choi, M.-S.; Yamazaki,
T.; Yamazaki, I.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 150. (c) Kobuke,
Y. Eur. J. Inorg. Chem. 2006, 2333. (d) Nakamura, Y.; Aratani, N.; Osuka,
A. Chem. Soc. ReV. 2007, 36, 831.
(3) (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.;
Suzuki, S.; Okada, K. Org. Lett. 2007, 9, 3315.
10.1021/ol801662q CCC: $40.75
Published on Web 09/23/2008
2008 American Chemical Society

Figure 1. Chemical structures of a light-harvesting array 1 and reference compounds 2-4.
transfer. Moreover, we have shown that these dendritic
molecules with the rigid backbone are valuable building
blocks for higher-ordered nanoscaled dendrimer assemblies.4
Here, we applied this method for the preparation of a light-
harvesting array 1 containing peripherally positioned Zn-
porphyrin terminals, a free-base (Fb) porphyrin core, and
rigid backbones. The abovementioned advantages of this
architecture make 1 a highly attractive light-harvesting
antenna. For elucidation of the effect of conjugated chains,
the efficiency of the energy transfer in 1 was compared with
those of reference compounds 2-4 (Figure 1).
Scheme 1. Preparation of the Conjugated Chain
A conjugated rigid chain 5 with a Zn-porphyrin unit
on one terminal was synthesized from AB3 type Fb
porphyrin 6, as shown in Scheme 1. Deprotection of the
acetylenic unit in 6 under basic conditions followed by
complexation with Zn metal afforded Zn porphyrin 85 in
88% yield (2 steps). The extension of the conjugated chain
was achieved by repetitively carrying out the Sonogashira
coupling reaction of 8 to produce 5 in 58% yield (2 steps).
A flexible chain 11 with a pinacol borate unit on one
terminal and a Zn-porphyrin unit on the other terminal
was obtained from 1,4-bis(bromomethyl)benzene (12), as
(4) Kozaki, M.; Tujimura, H.; Suzuki, S.; Okada, K. Tetrahedron Lett.
2008, 49, 2931.
shown in Scheme 2. The stepwise condensation of 12 first
with pinacol arylborate 13 and then with Zn porphyrin
156 afforded the flexible chain 11 in 51% yield (2 steps).
The Suzuki-Miyaura coupling reaction of the main chain
5 with the flexible chain 11 was carried out using cesium
carbonate as a base to afford 16in 82% yield.7 The triazene
(5) (a) Ajamaa, F.; Albrecht-Gary, A.-M.; Delgado de la Cruz, J. L.;
Elhabiri, M.; Nierengarten, J.-F.; Solladie, N.; Trabolsi, A.; Urbani, M. J.
Porphyrins Phthalocyanines 2006, 10, 1337. (b) Nakano, A.; Shimizu, H.;
Osuka, A. Tetrahedron Lett. 1998, 39, 9489.
(6) (a) Yamada, H.; Imahori, H.; Fukuzumi, S. J. Mater. Chem. 2002,
12, 2034. (b) Maes, W.; Vanderhaeghen, J.; Smeets, S.; Asokan, C. V.;
Van Renterghem, L. M.; Prez, F. E. D.; Smet, M.; Dehaen, M. J. Org.
Chem. 2006, 71, 2987.
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Org. Lett., Vol. 10, No. 20, 2008

Scheme 2
.
Preparation of the Flexible Chain
Scheme 3. Preparation of the Light-Harvesting Array
group in 16 was converted to an iodide group by heating
with iodine in iodomethane (Scheme 3). Finally, a copper-
free Sonogashira coupling reaction8 of 17 with 189 at 40 °C
for 15 h, followed by a repeated purification of the crude
product by recycling gel permeation chromatography (GPC)
afforded 1 as a purple solid in 23% yield. The light-
harvesting array 1 was soluble in various organic solvents
such as hexane, dichloromethane, THF, and toluene and
could be unambiguously characterized by NMR and MALDI-
TOF mass spectroscopy.
The UV-vis absorption spectrum of 1 in THF showed
characteristic bands for both the Fb and Zn-porphyrin units
(Soret band: λmax ) 426 nm (log ε ) 6.90); Q-bands: λmax
) 518, 557, 596, and 650 nm) along with absorption bands
due to the conjugated chains (λmax ) 346 nm) (Figure 2).
The absorption bands of 1 are essentially superimposed on
the spectra of components 17 (Soret band: λmax ) 425 nm;
Q-bands: λmax ) 595 and 557 nm) and 194 (Soret band: λmax
) 426 nm; Q-bands: λmax ) 518, 554, 595, and 652 nm). In
fact, the observed spectrum (molar absorptivity (ε) scale) of
1 can be effectively simulated by the absorption of the
components, 4 × ε17 + ε19. These results indicate that there
is a weak electronic interaction between the Fb porphyrin
core and the peripheral Zn-porphyrin units in the ground
state.
To evaluate the efficiency of the singlet-energy transfer
from the peripheral Zn-porphyrin unit to the Fb porphyrin
core, the fluorescence of 1 was measured in degassed THF
(Figure 3). The quantum yield of fluorescence (λem
)
max
(7) Aratani, N.; Osuka, A. Org. Lett. 2001, 3, 4213–4216.
(8) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467. (b) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J. S. Chem.
Mater. 1999, 11, 2974.
Figure 2. Absorption spectra of 1, 17, and 19 measured in THF and
simulated spectrum for 1 obtained from the absorption spectra of 17
and 19 (εsim ) 4 × ε17 + ε19).
(9) Onitsuka, K.; Kitajima, H.; Fujimoto, M.; Iuchi, A.; Takei, F.;
Takahashi, S. Chem. Commun. 2002, 2576.
Org. Lett., Vol. 10, No. 20, 2008
4479

by using a quenching efficiency value of R ) 43. The
small value of the quenching efficiency (R ) 43) as
compared to 57% quenching, estimated from the intensity
of the Zn-porphyrin chromophore, may be ascribed to the
partial deactivation of the excited state of the Zn-porphyrin
chromophore before energy transfer to Fb porphyrin within
1.
The efficiency of the energy transfer in 1 was compared
to those in reference compounds 2-4. The fluorescence
spectra of 2-4 were measured in THF, and the quantum
yields of Zn-porphyrin emission were determined as de-
scribed above: φf2Zn ) 0.016, φf3Zn ) 0.011, and φf4Zn
)
0013. The degree of quenching of Zn-porphyrin fluorescence
was estimated as 53% (2), 41% (3), and 45% (4) by
comparison with the fluorescence quantum yields of ap-
propriate reference compounds (see Supporting Information).
Compound 2, which is a quarter of the cyclic assembly, has
a quenching efficiency comparable to that of 1, indicating
that each wedge unit plays an independent role in the energy
transfer. The wedge 4 showed a quenching efficiency
modestly higher than that of 3, indicating the limited effect
of the conjugated chain on the energy transfer efficiency,
probably due to a small electronic coupling between the
bridging chain and the Zn-porphyrin terminal. Interestingly,
the light-harvesting array 1 as well as the wedge 2 have
quenching efficiencies higher than those of wedge 3 and 4.
These results indicate a certain level of cooperative effect
of Zn-porphyrin units in 1 and 2 that is advantageous for
effective light harvesting.
In conclusion, we prepared a light-harvesting array con-
taining the Fb porphyrin core, Zn-porphyrin terminals, and
the rigid backbones. Effective singlet-energy transfer was
observed from the peripheral Zn-porphyrin units to the Fb
porphyrin core. Thus, this array was shown to be an
important building block for the construction of a biomim-
icking light-harvesting system. The construction of such a
light-harvesting system is currently under progress in our
laboratory.
Figure 3. Fluorescence spectra of 1, 17, and 19 measured in THF
and simulated spectrum for 1 obtained from the normalized spectra of
17 and 19 (Isim ) 0.38 × I17 + 0.41 × I19 with an area ratio of S19:S17
) 2.73:1).
604, 656, and 717 nm) was determined to be φf ) 0.077
at an excitation wavelength of 557 nm, where the light
was mainly absorbed by Zn-porphyrin moieties with the
absorption (I) ratio, IZn-porphyrin moiety:IFb porphyrin moiety ) 16.2:
1, as determined from the molar absorptivity ratio of ε17:
ε19 ) 4.05:1 at 557 nm. The observed fluorescence can
be effectively simulated as a linear combination of the
normalized spectra of 17 (λem ) 604 and 653 nm, φf17
max
) 0.050) and 19 (λemmax ) 656 and 720 nm, φf19 ) 0.121),
indicating that the fluorescence of 1 can be divided into
two components with an emission area ratio of 2.73 (Fb
porphyrin):1 (Zn-porphyrin) (Figure 3). Thus, the emission
quantum yields were determined to be φf1Fb ) 0.077 ×
2.73/3.73 ) 0.056 and φf1Zn ) 0.077 × 1.0/3.73 ) 0.021
for emissions from the Fb porphyrin and Zn-porphyrin
chromophores in 1, respectively. These values can be
compared to the calculated quantum yields without energy
transfer, φf1Fb(0%ET)calc and φf1Zn(0%ET)calc estimated
from φf17, φf19, and the absorption fraction ratio (16.2:1)
at 557 nm in 1; φf1Fb(0%ET)calc ) φf19(0.121) × 1.0/17.2
) 0.0070 and φf1Zn(0%ET)calc ) φf17(0.050) × 16.2/17.2
)
0.047. The comparison of φf1Zn (0.021) with
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Science Research (19655016) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
φf1Zn(0%ET)calc (0.047) suggests 55% quenching due to
the singlet-energy transfer from Zn-porphyrin to Fb
porphyrin chromophores in 1. The degree of amplification
of the Fb porphyrin emission is estimated as follows.
Assuming that R% of the excited Zn-porphyrin is con-
Supporting Information Available: Detailed synthetic
procedure and spectral data. This material is available free
of charge via the Internet at http://pubs.acs.org.
verted to the excited Fb porphyrin, the quantum yield φf1Fb
-
(R%ET)calc is estimated as φf19(0.121) × [1 + 16.2 × (R/
100)]/17.2. The observed φf1Fb (0.056) can be reproduced
OL801662Q
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Org. Lett., Vol. 10, No. 20, 2008