Enhanced electron transfer by dendritic architecture: Energy transfer and electron transfer in snowflake-shaped Zn porphyrin dendrimers

Article abstract of DOI:10.1021/ol070245h

Equation presented Photoinduced electron transfer was observed for the snowflake-shaped dendrimer with the Zn porphyrin core and anthraquinonyl terminals. Comparison of the electron-transfer efficiency of the dendrimer with the linear analogues indicates

Full text of DOI:10.1021/ol070245h

ORGANIC
LETTERS
2007
Vol. 9, No. 8
1509-1512
Enhanced Electron Transfer by Dendritic
Architecture: Energy Transfer and
Electron Transfer in Snowflake-Shaped
Zn Porphyrin Dendrimers
Masatoshi Kozaki,* Kogen Akita, 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 January 31, 2007
ABSTRACT
Photoinduced electron transfer was observed for the snowflake-shaped dendrimer with the Zn porphyrin core and anthraquinonyl terminals.
Comparison of the electron-transfer efficiency of the dendrimer with the linear analogues indicates the advantage of the dendritic structure
for the electron-transfer process.
Dendrimers have been extensively studied as promising
candidates for a highly effective light-harvesting antenna in
artificial photosynthesis.1 Efficient exothermic energy transfer
from branches into a core has been observed in many
systems. However, fewer studies have been reported on the
electron-transfer processes using harvested photoenergy.2,3
In most cases, these studies involve dendrimers possessing
an electron acceptor at the core and large numbers of electron
donors at the peripheral ends of the branching chain or vice
versa,4 so that there are many different distances between
electron donors and acceptors that impede the analysis. For
this reason, the principle mechanism of how dendritic
architecture influences the efficiency of electron transfer has
not yet been established.
We have recently prepared a snowflake-shaped dendrimer
that has a rigid conjugated oligomer network inside the
flexible dendritic structure.5 Here, we apply this structure to
artificial photosynthesis by introducing an electron donor at
the core and electron acceptors at the terminals of conjugated
chains (Figure 1). This strategy is different from all the
(1) (a) Newcome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and
Dendrons: Concepts, Syntheses, Applications; VCH: Weinheim, Germany,
2001. (b) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and other Dendritic
Polymers; John Wiley & Sons: New York, 2002. (c) Balzani, V.; Venturi,
M.; Credi, A. Molecular DeVices and Machines -A Journey into the Nano
World; Wiley-VCH: Weinheim, Germany, 2003; p 96.
(2) Some selected papers of dendrimers with light-harvesting proper-
ties: (a) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 9635. (b) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (c) Jiang,
D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895. (d) Kimura, M.; Shiba,
T.; Yamazaki, M.; Hanabusa, K.; Shirai, H.; Kobayashi, N. J. Am. Chem.
Soc. 2001, 123, 5636. (e) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki,
I. Chem.-Eur. J. 2002, 8, 2668.
(3) Some selected papers of photoinduced electron transfer in dendrim-
ers: (a) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996,
118, 3978. (b) Rajesh, C. S.; Capitosti, G. J.; Cramer, S. J.; Moodarelli, D.
A. J. Phys. Chem. B 2001, 105, 10175. (c) Capitosti, G. J.; Guerreo, C. D.;
Binkley, D. E., Jr.; Rajesh, C. S.; Moodarelli, D. A. J. Org. Chem. 2003,
68, 247.
(4) For dendrimers with both light-harvesting and charge-separating
systems, see: (a) Guldi, D. M.; Swartz, A.; Luo, C.; Go´mez, R.; Segura, J.
L.; Mart´ın, N. J. Am. Chem. Soc. 2002, 124, 10875. (b) Qu, J.; Pschirer, N.
G.; Liu, D.; Stefan, A.; Schryver, F. C. D.; Mu¨llen, K. Chem.-Eur. J. 2004,
10, 528. (c) Thomas, K. R. J.; Thompson, A. L.; Sivakumar, A. V.; Bardeen,
C. J.; Thayumanavan, S. J. Am. Chem. Soc. 2005, 127, 373. (d) Schryver,
F. C. D.; Vosch, T.; Cotlet, M.; Van der Auwerare, M.; Mu¨llen, K.; Hofkens,
J. Acc. Chem. Res. 2005, 38, 514. (e) Nantalaksakul, A.; Dasari, R. R.;
Ahn, T.-S.; Bardeen, C. J.; Thayumanavan, S. Org. Lett. 2006, 8, 2981.
(5) Kozaki, M.; Okada, K. Org. Lett. 2004, 6, 485.
10.1021/ol070245h CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/20/2007

between the structures and the efficiency of electron-transfer
processes. We report the preparation and photophysical
properties of novel snowflake-shaped Zn porphyrin den-
drimers 1 and 2 and related compounds 3 and 4 (Figure 1).
We demonstrate enhanced electron transfer by dendritic
architectures.
Scheme 1. Synthesis of Snowflake-Shaped Zn Porphyrin
Dendrimer 1
Dendrimer 1 was synthesized according to Scheme 1. First,
dendron 5 was prepared by a method similar to that reported
previously.5 Dendron 7 was obtained in 78% yield (in two
steps) by the removal of TBDMS groups in dendron 5 with
TBAF followed by a Sonogashira coupling reaction with
2-iodoanthraquinone.6 Dendron 7 was heated with iodine in
iodomethane to give dendron 8 in 82% yield. A Sonogashira
coupling reaction of Zn porphyrin core 97 with dendron 8
afforded the desired dendrimer 1 in 36% yield as a purple
powder. Dendrimer 2 was also obtained as a purple powder
by a similar method.
Both dendrimers 1 and 2 were soluble in various organic
solvents such as chloroform, dichloromethane, THF, and
toluene and unambiguously characterized by means of NMR,
elemental analysis, and MALDI-TOF mass spectroscopy.
Interestingly, the 1H NMR spectra of 1 and 2 showed broad
signals at room temperature probably because of the slow
conformational change of the dendritic chain. Sharp, well-
resolved signals were obtained at higher temperature (at
135 °C for 1 and at 85 °C for 2) in tetrachloroethane-d2.
The UV-vis absorption spectrum of dendrimer 2 in THF
showed characteristic bands for a Zn porphyrin unit (Soret
band, λmax ) 433 nm; Q-bands: λmax ) 563, 604 nm) along
with absorption bands due to the branched benzyl ether
Figure 1. Chemical structures of snowflake-shaped Zn porphyrin
dendrimers 1 and 2, dendron 3, and dyad 4.
previous approaches; the advantages of this type of dendrimer
are that (1) coupling of the dendritic antenna provides a
versatile approach to various donor-acceptor (D-A) sys-
tems, (2) the distance between the electron donor and electron
acceptors is fixed by the rigid conjugated chain, and (3)
because of the symmetrical structure of the dendrimer the
efficiency of the electron-transfer process can be directly
compared between the dendrimers and their building blocks,
providing important information about the relationship
(6) Higgins, R. W.; Hilton, C. L.; Deodhar, S. D. J. Org. Chem. 1951,
16, 1275.
(7) Onisuka, K.; Kitajima, H.; Fujimoto, M.; Iuchi, A.; Takei, F.;
Takahashi S. Chem. Commun. 2002, 2576.
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Org. Lett., Vol. 9, No. 8, 2007

chains (λmax ) 286 nm) and the conjugated chains (λmax
372 nm) (Figure 2). The absorption of the porphyrin core
)
branched chains or conjugated chains was detected at all,
whereas closely related showflake-shaped dendrimers without
a porphyrin core showed strong emission from conjugated
chains (ΦF ) 0.50 ∼ 0.76).5
The fluorescence from the porphyrin core (431 nm
excitation) is effectively quenched in the anthraquinone
derivatives in polar solvents. Steady-state fluorescence
spectra in degassed DMF of 1-4 are shown in Figure 3,
Figure 2. Absorption spectra of dendrimers 1 and 2 and Zn-TPP
in THF.
exhibited a slight bathochromic shift compared to Zn
tetraphenylporphyrin (Zn-TPP). Of particular interest, co-
valent linkages of the terminal anthraquinone chromophores
resulted in slightly shifted and broadened bands [λmax ) 431
(Soret); 565, 609 (Q-bands) nm] compared to 2. Such
broadening was negligibly small for 3 and 4. The shape of
the Soret band was independent of the concentration in the
range between 2.5 × 10-7 and 2.5 × 10-6 mol dm-3. This
result clearly rules out the possibility of the intermolecular
aggregate formation (see Supporting Information, Figure S7).
These spectral features suggest that there is a weak electronic
interaction between the Zn porphyrin core and the four
anthraquinone chromophores in the ground state for 1.8 The
magnitude of the broadening of the absorption bands for 1
was dependent on solvents (THF (fwhm ) 30 nm for the
Soret band) ∼ 1,4-dioxane (37) < DMF (43) ≈ benzonitrile
(44)) (see Supporting Information, Figure S8), indicative of
a CT-type electronic interaction in the ground state.
To evaluate the efficiency of energy transfer from the
branches (benzyl ether chain or conjugated oligomer) to the
core (Zn porphyrin), the fluorescence of dendrimer 2 was
studied. Direct excitation of the Zn porphyrin core (433 nm)
in degassed THF gave strong fluorescence at 611 and 658
nm due to the Zn porphyrin unit. Sensitized excitation either
of the branched benzyl ether chains (286 nm excitation with
the quantum efficiency of ΦF ) 0.05) or of the conjugated
chains (372 nm with ΦF ) 0.05) resulted in the same
fluorescence patterns and efficiencies as direct excitation (ΦF
) 0.05), indicating highly efficient (∼100%) intramolecular
singlet energy transfer. These results establish that this system
has the good light-harvesting property of the snowflake-
shaped porphyrin dendrimers. No emission from either
Figure 3. Fluorescence of anthraquinone-linked dendrimer 1,
dendrimer 2, dendron 3, and dyad 4 in degassed DMF (431 nm
excitation). All the spectra were normalized to a constant absorbance
at the excitation wavelength.
where the spectra are normalized by the absorbance at the
excitation wavelength. Dendrimer 2 revealed the highest
intensity. The anthraquinone-linked dendrimer 1 showed the
lowest fluorescence intensity, and the quenching efficiency
was solvent-dependent [30% quenching (ΦF ) 0.04) in 1,4-
dioxane (ꢀ ) 2.2), 69% quenching (ΦF ) 0.02) in THF (ꢀ
) 7.6), 90% quenching (ΦF ) 0.007) in benzonitrile (ꢀ )
26), 98% (ΦF ) 0.001) quenching in DMF (ꢀ ) 37)].
Although 2-4 have fluorescence spectral shapes quite similar
to Zn-TPP, a rather broad emission band was observed for
the dendrimer 1 (see Supporting Information, Figures S11-
13). The broad emission from 1 reflects the CT-type
electronic interaction in the ground state evidenced by the
absorption spectrum.
The energy transfer from the excited singlet porphyrin to
anthraquinone is a highly endothermic process and should
not be the origin of this quenching. On the other hand, the
electron transfer from the excited singlet state of porphyrin
to the anthraquinone is an exothermic process (∆G ) -16.5
kcal mol-1).9 The fact that the quenching efficiency of 1 is
strongly dependent on the solvent polarity also indicates that
the electron-transfer mechanism is operative in the quenching
process.
Interestingly, dendron 3 has a lower quenching efficiency
than the anthraquinone-linked dendrimer 1 but a higher
(9) The ∆G value was determined using the Rehn-Weller equation. The
dendrimer 1 gave significantly broad redox waves due to the slow electron-
transfer rate between the encapsulated porphyrin core and an electrode.
(8) A similar spectral feature was reported for some porphyrins with
conjugated chains at all meso positions. (a) Yen, W.; Lo, S.; Kuo, M.; Mai,
C.; Lee, G.; Peng, S.; Yeh, C. Org. Lett. 2006, 8, 4239. (b) Li, B.; Li, J.;
Fu, Y.; Bo, Z. J. Am. Chem. Soc. 2004, 126, 3430.
Redox potentials of the dendron 3 (Ered ) -0.806 V vs SCE and Eox
)
0.744 V vs SCE) in 1,2-dichloroethane were used for the calculation.
Org. Lett., Vol. 9, No. 8, 2007
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quenching efficiency (2.2 times in DMF) than dyad 4, clearly
indicating that (1) the presence of the flexible chain covering
the conjugated chain effectively enhances the quenching and
(2) the dendrimer structure has higher quenching efficiency
(2.6 times) than the dendron structure; as a result, (3)
anthraquinone dendrimer 1 has a much higher (5.9 times)
quenching ability than the uncovered dyad 4. The quantum
yields of the fluorescence are determined as 7.4 × 10-2 for
2, 3.7 × 10-3 for 3, 8.2 × 10-3 for 4, and 1.4 × 10-3 for 1.
This is the first set of experimental data clearly establishing
how the dendrimer architecture is relevant to the electron-
transfer process. Importantly, the ratio of quenching ef-
ficiency (3:4) was found to be solvent dependent: the higher
the solvent polarity, the higher the quenching efficiency (1.0
(1,4-dioxane), 1.1 (THF), 1.7 (benzonitrile), and 2.2 (DMF)).
It is probable that the higher quenching efficiency of 1 may
be partly due to this covering effect and also partly due to
the CT-type electronic interaction in the ground state
probably reflecting the higher number (four) of anthraquinone
terminals.
It may be of interest to compare the present results with
the recently reported flexible D-A system linked by poly-
methylene chains (typically n ) 7, 8) reported by Mallouk
and co-workers. They reported that encapsulation of the
flexible chain by â-cyclodextrin molecules slows the electron
transfer from the excited ruthenium trisbipyridyl to the
viologen moiety in sharp contrast to the present study.10 In
their flexible system, the electron transfer may occur at
various D-A distances; encapsulation of the chain spacer
certainly interrupts short electron-transfer pathways, giving
a slower rate of the electron transfer. The present system
has a rigid D-A structure, and the electron transfer would
occur by a through-bond mechanism at a constant distance.
The enhanced electron transfer by covering the conjugated
chain with flexible benzyl ether chains may be related to
the conformational effect of oligo(phenylene-ethynylene)
moieties, where hydrophobic interaction between polybenzyl
ether chains favors a planar conformation.11 This conforma-
tional effect leads to different values for the through-bond
D-A coupling strength.
In summary, we prepared and characterized novel snow-
flake-shaped Zn porphyrin dendrimers, where highly efficient
(∼100%) intramolecular singlet energy transfer providing a
good light-harvesting property is attained. Steady-state
fluorescence measurement showed the highly effective
quenching for the dendrimer with anthraquinonyl terminals
by the electron-transfer mechanism. Comparison of the
electron-transfer efficiency of 1 with the linear analogues
clearly demonstrates that (1) covering of the conjugated chain
enhances the electron transfer in the dendron system and (2)
assembly of the dendrons into the dendrimer also enhances
the electron transfer partly because of the charge-transfer
interaction. The enhanced electron transfer by covering of
the conjugated chain is a new important finding that may be
general and characteristic to the molecular assembly systems.
Further studies involving the kinetics using laser flash
photolysis of the present system as well as clarification of
the origin of the covering effect for the present and other
electron-transfer systems are in progress in our laboratory.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Science Research (No. 17550044 and No.
18039033 “Super-Hierarchical Structures”) from the Ministry
of Education, Culture, Sports, Science and Technology,
Japan. M.K. acknowledges Iketani Science Technology
Foundation for financial support. We thank Hamamatsu
photonics K. K. for measurements of fluorescence quantum
yields. We also thank Prof. Yoshio Aso (Osaka University)
for helpful discussions.
Supporting Information Available: Detailed synthetic
procedure and NMR, UV-vis-NIR, and emission spectral
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL070245H
(10) Yonemoto, E. H.; Saupe, G. B.; Schmehl, R. H.; Hubig, S. M.;
Riley, R. L.; Iverson, B. L.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116,
4786.
(11) The prolonged conjugation in the covered dendrons can be supported
in the related compounds in polar solvents (see Supporting Information,
Figure S9).
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Org. Lett., Vol. 9, No. 8, 2007