Construction of segregated arrays of multiple donor and acceptor units using a dendritic scaffold: Remarkable dendrimer effects on photoinduced charge separation

Article abstract of DOI:10.1021/ja063081t

Dendritic molecules appended with multiple zinc porphyrin units (DP _m, m [number of zinc porphyrin units] = 6, 12, and 24) trap bipyridine compounds carrying multiple fullerene units (Py₂P _n, n [number of C₆₀ units] = 1-3), affording coordination complexes DP_m?Py₂F_n having a photoactive layer consisting of spatially segregated donor and acceptor arrays on their surface. Complexes DP_m?Py₂F_n are stable enough (K [average binding affinity] = 1.1 × 10⁶-4.4 × 10⁶ M^-1 in CHCl₃ at 25 °C) to be isolated by gel permeation chromatography. UHV-STM microscopy enables clear visualization of a petal-like structure of DP₁₂?Py₂F₃. Photoexcitation of the zinc porphyrin units in DP_m?Py ₂F_n results in a zinc porphyrin-to-fullerene electron transfer to generate a charge separation. The charge-separation rate constant (k_CS) in CH₂Cl₂ at 20 °C increases from 0.26 × 10¹⁰ to 2.3 × 10¹⁰ s^-1 upon increment of m and n, whereas the charge-recombination rate constant (k _CR) remains almost unchanged at 4.5 × 10⁶-6.7 × 10⁶ s^-1. Consequently, DP₂₄?Py ₂F₃ furnishes the largest ratio of k_CS/k _CR (3400) among the family.

Full text of DOI:10.1021/ja063081t

Published on Web 07/22/2006
Construction of Segregated Arrays of Multiple Donor and
Acceptor Units Using a Dendritic Scaffold: Remarkable
Dendrimer Effects on Photoinduced Charge Separation
Wei-Shi Li,*,† Kil Suk Kim,§ Dong-Lin Jiang,† Hiroyuki Tanaka,# Tomoji Kawai,#
Jung Ho Kwon,§ Dongho Kim,*,§ and Takuzo Aida*,†,‡
Contribution from the ERATO-SORST Nanospace Project, Japan Science and Technology
Agency (JST), National Museum of Emerging Science and InnoVation, 2-41 Aomi, Koto-ku,
Tokyo 135-0064, Japan, The Institute of Scientific and Industrial Research (ISIR), Osaka
UniVersity, 8-1, Mihogaoka, Ibaragi, Osaka 567-0047, Japan, National CreatiVe Research
InitiatiVes, Center for Ultrafast Optical Characteristics Control and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry and Biotechnology,
School of Engineering, and Center for NanoBio Integration, The UniVersity of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received May 3, 2006; E-mail: li@nanospace.miraikan.jst.go.jp; dongho@yonsei.ac.kr; aida@macro.t.u-tokyo.ac.jp
Abstract: Dendritic molecules appended with multiple zinc porphyrin units (DPm, m [number of zinc porphyrin
units] ) 6, 12, and 24) trap bipyridine compounds carrying multiple fullerene units (Py2Fn, n [number of C60
units] ) 1-3), affording coordination complexes DPm⊃Py2Fn having a photoactive layer consisting of spatially
segregated donor and acceptor arrays on their surface. Complexes DPm⊃Py2Fn are stable enough (K
[average binding affinity] ) 1.1 × 106-4.4 × 106 M-1 in CHCl3 at 25 °C) to be isolated by gel permeation
chromatography. UHV-STM microscopy enables clear visualization of a petal-like structure of DP12⊃Py2F3.
Photoexcitation of the zinc porphyrin units in DPm⊃Py2Fn results in a zinc porphyrin-to-fullerene electron
transfer to generate a charge separation. The charge-separation rate constant (kCS) in CH2Cl2 at 20 °C
increases from 0.26 × 1010 to 2.3 × 1010 s-1 upon increment of m and n, whereas the charge-recombination
rate constant (kCR) remains almost unchanged at 4.5 × 106-6.7 × 106 s-1. Consequently, DP24⊃Py2F3
furnishes the largest ratio of kCS/kCR (3400) among the family.
Introduction
in mind, we were motivated to achieve at the molecular level
such segregated arrays of multiple D and A units. Although a
In biological photosynthesis, photoinduced electron transfer
(PET) is one of the most essential events for conversion of solar
energy into chemical energy. Molecular design of artificial
photosynthetic systems involving restricted spatial arrangements
of covalently1 and noncovalently2 linked electron donor (D) and
acceptor (A) units has long been a central interest and now
attracts even greater attention for the development of optoelec-
tronic devices such as solar cells.3 For the fabrication of those
materials, charge-transfer complexations between D and A units,
leading to rapid quenching of charge-carrier transports, must
be avoided, while D and A units are required to assemble
individually to form their segregated arrays.4 Having this context
variety of D/A systems having multiple D or A units have been
reported,5 those containing large numbers of both D and A units
are unprecedented. In the present paper, we report novel
photofunctional dendrimers consisting of spatially segregated
arrays of multiple D and A units on their surface.
Our molecular design strategy (Scheme 1) made use of petal-
like dendritic structures as scaffolds6 to realize a wheel-like or
spherical arrangement of multiple zinc porphyrin (P) units (DPm;
m ) 6, 12, and 24),7 which are capable of ligating multiple
molecules of bipyridine compounds having 1-3 fullerene (F)
units (Py2Fn; n ) 1-3). Since the zinc porphyrin moieties in
DPm are located in the outermost dendritic layer, and the
† ERATO-SORST Nanospace Project, JST.
‡ The University of Tokyo.
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C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature
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§ Yonsei University; responsible for time-resolved spectroscopies.
# Osaka University; responsible for scanning tunneling microscopy.
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9
10.1021/ja063081t CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 10527-10532
10527

A R T I C L E S
Li et al.
Scheme 1. Molecular Structures of Zinc Complexes of Multiporphyrin Dendrimers DPm (m ) 6, 12, and 24), Fullerene-Appended Bipyridine
Ligands Py2Fn (n ) 1-3), and Reference Compounds P1 and Py2, and Schematic Representation of the Complexation of DP24 with Py2F3
bipyridine and fullerene units in Py2Fn are apart by roughly 2-3
nm from one another, the coordination of Py2Fn to DPm is
expected to form a photoactive layer consisting of spatially
segregated arrays of multiple donor (zinc porphyrin) and
acceptor (fullerene) units on the dendrimer surface. Molecular
design of DPm was inspired by the unique structures of light-
harvesting antenna complexes such as LH1 and LH2 in purple
bacteria,8 where multiple bacteriochlorophyll units are spatially
arranged like a wheel and ensure efficient harvesting of dilute
photons. The excitation energy captured by one of the bacte-
riochlorophyll unit migrates, without dissipation, to neighboring
chlorophyll units in the same and connecting wheel-like
chromophore arrays and is subsequently funneled to the
photosynthetic reaction center, thereby triggering an electron
9
10528 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

Segregated Arrays of Multiple Donor and Acceptor Units
A R T I C L E S
Figure 1. (a) Absorption spectral change of DP24 (1.5 × 10-7 M) upon titration with Py2F3 ([Py2F3]/[DP24] ) 0, 1.1, 2.3, 4.6, 6.8, 9.1, 11, 16, 23, 34, 52,
80, 126, 194, and 240) in CHCl3 at 25 °C. (b) Change in absorbance (A0 - A) of DP24, monitored at 415.0 nm, as a function of [Py] ()2 × [Py2F3]), and
its fitting profile. (c) Binding affinities (log K) of the pyridine units in Py2F1 (red), Py2F2 (green), and Py2F3 (blue) toward the zinc porphyrin units in P1,
DP6, DP12, and DP24 in CHCl3 at 25 °C. (d) A snapshot of gel permeation chromatography (GPC) of a mixture of DP24 and Py2F3 ([Py2F3]/[DP24] ) 25) with
CHCl3 as an eluent.
transfer. We and other groups have reported that some dendritic
multiporphyrin arrays including DPm, upon photoexcitation,
display highly efficient energy migration characteristics analo-
gous to those of the biological light-harvesting systems.6a,9
Hence, we decided to make use of zinc complexes of multi-
porphyrin dendrimers DPm with a certain structural rigidity for
an attempt to construct a concentric double layer of spatially
segregated arrays composed of multiple D and A units (Scheme
1). As an electron acceptor for DPm, we chose a fullerene such
as C60 because of its small reorganization energy and excellent
electron-accepting properties.10 Thus, we synthesized Py2Fn
having 1-3 fullerene units (n ) 1-3), where the bipyridine
(Py2) unit was expected to coordinate strongly in a bidentate
fashion to two neighboring zinc porphyrin units in DPm, thereby
allowing the formation of a fullerene array outside of the zinc
porphyrin array on the dendrimer surface (DPm⊃Py2Fn). Here,
one can change the packing densities of these D and A units in
the photoactive layer by varying the m and n values in the DPm
and Py2Fn components, respectively, and might therefore be able
to modulate the photoinduced charge-separation event.
bipyridine-terminated alcohols bearing 1-3 hydroxyl groups.11
In CHCl3, DP6 showed an absorption spectral profile typical of
5,15-diarylporphyrin zinc complexes, having a Soret absorption
band centered at 414 nm and Q-bands at 542 and 578 nm.
Compared with DP6, DP24 showed a broad Soret absorption band
centered at 415 nm, while DP12 showed a blue-shifted shoulder
at 398 nm along with a major Soret band at 411 nm. The latter
observation suggests that DP12 may adopt a planar geometry
with a H-aggregate-type arrangement of the zinc porphyrin units
along its periphery.7 On the other hand, Py2F1-Py2F3 all
displayed an electronic absorption band centered at 326 nm.11
Upon excitation at 326 nm, all Py2Fn fluoresced at 694 nm from
their fullerene units.10,11
As expected, DPm (m ) 6, 12, and 24) bound Py2Fn (n )
1-3) strongly to form stable DPm⊃Py2Fn. For example, upon
titration with Py2F3 in CHCl3 at 25 °C, DP24 (1.5 × 10-7 M)
displayed a large spectral change in the Soret and Q-bands
(Figure 1a), characteristic of the axial coordination of zinc
porphyrins, with a clear saturation profile at a molar ratio
[Py2F3]/[DP24] exceeding 12 (Figure 1b). This spectral change
profile did not give distinct isosbestic points possibly due to a
large effect of the multivalency of the complexation between
DP24 and Py2F3. However, the average binding affinity (K), as
estimated by simply assuming a one-to-one coordination
between the individual zinc porphyrin and pyridine units, was
Results and Discussion
Zinc complexes of multiporphyrin dendrimers DPm (m ) 6,
12, and 24) were synthesized according to a method analogous
to that reported previously for chiroptical sensing of chiral
bipyridine compounds.7 On the other hand, fullerene-appended
bipyridine ligands Py2Fn (n ) 1-3) were synthesized by
coupling of a fullerene-containing carboxylic acid precursor with
1.2 × 106 M-1 11
. This value is more than 2 orders of magnitude
greater than association constants reported for monodentate
coordination between zinc porphyrins and pyridine derivatives.12
Other combinations of DPm and Py2Fn for the titration all showed
analogous spectral change profiles with a marked saturation
tendency at a mole ratio [Py2Fn]/[DPm] close to m/2.11 Figure
1c shows average binding affinities K between the zinc
porphyrin and pyridine units, which are almost comparable to
one another in a range 1.1 × 106-4.4 × 106 M-1 irrespective
of the m and n values in DPm and Py2Fn, respectively. We also
found that resulting complexes DPm⊃Py2Fn are all stable under
conditions for gel permeation chromatography (GPC). For
example, a CHCl3 solution (0.5 mL) of a mixture of DP24 and
Py2F3 ([DP24] ) 1.6 × 10-5 M, [Py2F3]/[DP24] ) 25) was loaded
onto a Bio-beads S-X1 GPC column and then eluted with
CHCl3. As shown in Figure 1d, the chromatographic profile
(6) For dendritic multiporphyrin arrays: (a) Yeow, E. K. L.; Ghiggino, K. P.;
Reek, J. N. H.; Crossley, M. J.; Bosman, A. W.; Schenning, A. P. H. J.;
Meijer, E. W. J. Phys. Chem. B 2000, 104, 2596-2606. (b) Choi, M.-S.;
Aida, T.; Yamazaki, T.; Yamazaki, I. Angew. Chem., Int. Ed. 2001, 40,
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Eur. J. 2002, 8, 2667-2678. (d) Benites, M. R.; et al. J. Mater. Chem.
2002, 12, 65-80. (e) Choi, M.-S.; Yamazaki, T.; Yamazaki, I.; Aida, T.
Angew. Chem., Int. Ed. 2004, 43, 150-158. (f) Imahori, H. J. Phys. Chem.
B 2004, 108, 6130-6143. (g) Solladie´, N., Sooambar, C., Herschbach, H.,
Strub, J.-M., Leize, E., Van Dorsselaer, A., Talarico, A. M., Ventura, B.,
Flamigni, L. New J. Chem. 2005, 29, 1504-1507.
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7700-7702.
(8) (a) Mcdermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless,
A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517-
521. (b) Karrasch, S.; Bullough, P. A.; Ghosh, R. EMBO J. 1995, 14, 631-
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(9) (a) Cho, S.; Li, W.-S.; Yoon, M.-C.; Ahn, T. K.; Jiang, D.-L.; Kim, J.;
Aida, T.; Kim, D. Chem. Eur. J., in press. (b) Fujitsuka, M.; Hara, M.;
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B 2005, 109, 33-35.
(11) See Supporting Information.
(12) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.; Hawley,
J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. In Axial Coordination Chemistry
of Metalloporphyrins; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; The
Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 3, p 30.
(10) (a) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695-703. (b) Guldi,
D. M. Chem. Commun. 2000, 321-327. (c) Thomas, K. G.; George, M.
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A R T I C L E S
Li et al.
Table 1. Average Numbers of Py2Fn Bound to Single DPm
Molecule in DPm⊃Py2Fn, upon Mixing DPm with Py2Fn ([Py2Fn]/
[DPm] ) m) in CHCl3 at 25 °C, Followed by GPC Separation with
CHCl3 as an Eluent (Figure 1d)11
Py2F1
Py2F2
Py2F3
DP6
DP12
DP24
2.9
5.3
9.0
2.6
5.5
9.6
2.6
5.3
9.5
Figure 3. Cyclic voltammograms of (a) DP6 (green), DP12 (blue), and DP24
(red) alone, (b) those in the presence of Py2 ([pyridine]/[zinc porphyrin] )
1), (c) Py2F1 (red), Py2F2 (green), and Py2F3 (blue) alone, and (d) those in
the presence of DP12 ([pyridine]/[zinc porphyrin] ) 1). Conditions: 0.1 M
Bu4N+PF6- as a supporting electrolyte, scan rate: 100 mV/s, Pt electrodes,
in CH2Cl2 at 25 °C.
(Figure 2c), the complex likely lost most of the ligating Py2F1
molecules during the process for pulse injection. On the other
hand, as shown in Figure 2d, DP24⊃Py2F1 in UHV-STM
developed a rather obscure molecular image, possibly due to a
difficulty of large DP24 in flattening on the substrate surface.
Electrochemical properties of DPm, Py2Fn, and reference
complexes for DPm⊃Py2Fn were investigated by means of cyclic
voltammetry (CV). In CH2Cl2 at 25 °C, DP6, DP12, and DP24
displayed nearly identical redox properties to one another with
the first oxidation potential (E0O/x‚+) at around 0.40 V versus Fc/
Fc+ (Figure 3a). For the evaluation of the redox potentials of
the pyridine-ligating zinc porphyrin units in DPm⊃Py2Fn, we
utilized, in place of Py2Fn, reference bipyridine compound Py2
without fullerene units (Scheme 1). Upon addition of 3 equiv
of Py2 to a CH2Cl2 solution of DP6, the oxidation peak, as
expected, showed a cathodic shift down to E0O/x‚+ of 0.28 V due
to an electron donation from ligating Py2 to the zinc porphyrin
units (Figure 3b). A similar electronic effect of Py2 was observed
for the oxidation potentials of DP12 and DP24, although the extent
Figure 2. UHV-STM micrographs of (a, b) DP12 in the presence of 6
equiv of Py2F3, (c) DP6 in the presence of 3 equiv of Py2F1, and (d) DP24
in the presence of 12 equiv of Py2F1, on a Au(111) surface at a liquid
nitrogen temperature. Conditions: (a) I ) 1 pA, Vs ) +3 V; (b) I ) 1 pA,
Vs ) +2 V; (c) I ) 2 pA, Vs ) +4.5 V; (d) I ) 1 pA, Vs ) +2 V.
displayed two colored fractions. By means of absorption
spectroscopy, the first fraction was found to contain both DP24
and Py2F3 at a molar ratio [Py2F3]/[DP24] of 9.5 (Table 1),
whereas the second fraction included Py2F3 alone.11 The
observed ratio of [Py2F3]/[DP24] is fairly close to an expected
ratio of 12 for the bidentate ligation of Py2F3 to the zinc
porphyrin units in DP24. As summarized in Table 1, we likewise
isolated by GPC DPm⊃Py2Fn formed from the other combina-
tions of DPm and Py2Fn and confirmed from their compositions
that almost all the zinc porphyrin units in DPm participate in
the bidentate ligation with Py2Fn. In contrast, P1, a nondendritic
zinc porphyrin reference (Scheme 1), incapable of bidentate
ligation with Py2Fn, exhibited much smaller binding affinities
(∼2.0 × 103 M-1) toward Py2F1-Py2F3 (Figure 1c).
of such a cathodic shift was a little more explicit as the steric
0/‚+
Ox
We succeeded in visualizing some of the coordination
complexes between DPm and Py2Fn by scanning tunneling
microscopy under ultrahigh vacuum conditions (UHV-STM).
For example, when a CHCl3 solution of a mixture of DP12 and
Py2F3 ([DP12] ) 2.0 × 10-6, [Py2F3]/[DP12] ) 8.2) was
deposited on a Au(111) surface by pulse injection,13 UHV-
STM at a liquid nitrogen temperature displayed petal-like
patterns with a uniform diameter of 7 nm, assignable to DP12
adopting a planer conformation on the substrate surface (Figures
2a and 2b). UHV-STM also exhibited many bright spots at
the periphery of DP12 molecules, which are most likely fullerene
clusters of Py2F3. While DP6⊃Py2F1 showed a petal-like pattern
congestion of the dendrimer became larger: E
) 0.26
(DP12) and 0.23 V (DP24). On the other hand, Py2Fn showed
the first reduction potential (E0R/e‚d-) at -1.08 (n ) 1), -1.10 (n
) 2), and -1.10 (n ) 3) versus Fc/Fc+ in CH2Cl2 (Figure 3c),
which remained virtually intact upon complexation with DPm
such as DP12 (Figure 3d).
Photoinduced electron transfer in DPm⊃Py2Fn was confirmed
by means of steady-state emission spectroscopy and nanosecond
flash photolysis measurements. For example, excitation of a
CHCl3 solution of DP24 (1.5 × 10-7 M) at 550 nm resulted in
a fluorescence emission from the zinc porphyrin units at 591
and 635 nm (Figure 4a). Upon titration with Py2F3, the
fluorescence stepwise decreased in intensity and was quenched
almost completely in the final stage. Stern-Volmer constants
(13) (a) Tanaka, H.; Kawai, T. J. Vac. Sci. Technol. B 1997, 15, 602-604. (b)
Tanaka, H.; Kawai, T. Surf. Sci. 2003, 539, L531-L536.
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Segregated Arrays of Multiple Donor and Acceptor Units
A R T I C L E S
Figure 4. (a) Fluorescence spectral change and Stern-Volmer plot (inset)
of DP24 (1.5 × 10-7 M) upon excitation at 550 nm in the presence of Py2F3
([Py2F3]/[DP24] ) 0, 0.85, 1.7, 2.6, 3.4, 5.1, 7.7, 11, and 16 in CHCl3 at 25
°C under Ar. (b) Stern-Volmer constants of DP6, DP12, and DP24 in the
presence of Py2F1 (red), Py2F2 (green), and Py2F3 (blue).
Figure 6. (a) Charge-separation quantum yields (ΦCS), (b) charge-
separation rate constants (kCS), (c) charge-recombination rate constants (kCR),
and (d) kCS/kCR of DP6, DP12, and DP24 in the presence of Py2F1 (red),
Py2F2 (green), and Py2F3 (blue) in CH2Cl2 at 20 °C.
fullerene-appended Py2F1 was used in place of Py2, a fast-decay
fluorescing component with τf of 320 ps appeared in addition
to the slow-decay component (τf ) 1.1 ns). This fast-decay
component obviously originates from the charge-separation
process caused by the photoinduced electron transfer from DP6
to Py2F1. In the presence of Py2F2 or Py2F3 instead of Py2F1,
the lifetime of the fast-decay component of DP6 became much
shorter; τf ) 170 (Py2F2) and 150 ps (Py2F3). In contrast with
DP6⊃Py2, DP12⊃Py2 and DP24⊃Py2 exhibited a biexponential
fluorescence decay at 590 nm possibly due to a dense packing
of the zinc porphyrin units (vide ante) on the dendrimer
surface.11 Use of Py2Fn in place of Py2 to allow complexation
with DP12 or DP24 resulted in an additional decay component
with a much shorter τf value, owing to the electron transfer from
DPm to Py2Fn.11 On the basis of the fluorescence decay profiles,
the charge-separation rate constants (kCS) and quantum yields
(ΦCS) were evaluated according to the following equations by
comparison of the shortest τf components of DPm⊃Py2Fn with
average τf values of DPm⊃Py2:
Figure 5. Nanosecond transient absorption spectra at 20 °C of a CH2Cl2
solution of DP6 (1.7 × 10-5 M) containing 3 equiv of Py2F3 upon
photoexcitation at 532 nm.
(KSV, Figure 4b),11 evaluated for the combination of DPm (m )
6, 12, and 24) and Py2Fn (n ) 1-3), were roughly comparable
to their average binding affinities K (Figure 1c), indicating that
the fluorescence quenching of DPm is caused by the coordination
of electron-accepting Py2Fn. By means of transient absorption
spectroscopy, we confirmed the occurrence of an electron
transfer from the photoexcited zinc porphyrin units in DPm to
the fullerene units in ligating Py2Fn. Excitation of a CH2Cl2
solution of DP6 (1.7 × 10-5 M) at 532 nm in the presence of
3 equiv of Py2F3 resulted in a transient absorption spectrum
(Figure 5) with bands at around 680 and 1000 nm assignable
to the cation and anion radicals of the zinc porphyrin and
fullerene units, respectively. Broad absorption bands, observed
at 750-900 nm, are due to the excited triplet states of these
two units. Since the fluorescence spectral profiles of DPm⊃Py2Fn
such as DP6⊃Py2F3 under the conditions employed here did
not show any sign of energy transfer from the photoexcited zinc
porphyrin units in DPm to the fullerene units of Py2Fn,11,14 we
conclude that the quenching of the zinc porphyrin fluorescence
observed for DPm⊃Py2Fn (Figure 4) is mostly due to the intra-
complex photoinduced electron transfer between them.
1
1
kCS
)
-
( )
( )
τf
τf
DPm⊃Py2Fn
DPm⊃Py2
1
1
1
/
ΦCS
)
-
( )
( )
τ
τf
(
τf
)
[
]
f
DPm⊃Py2Fn
DPm⊃Py2
DPm⊃Py2Fn
The ΦCS values thus evaluated are all high, ranging from 0.82
to 0.94 (Figure 6a). Thus, the photoinduced electron transfer in
DPm⊃Py2Fn occurred very efficiently, irrespective of the
magnitudes of m and n. Of further interest, the kCS value
increases following the order of Py2F1 < Py2F2 < Py2F3 and
enhances, to a much greater extent, from DP6 to DP12 and then
to DP24 (Figure 6b). For example, the kCS value of 2.3 × 1010
s-1, as evaluated for DP24⊃Py2F3, is an order of magnitude
greater than that of DP6⊃Py2F1 (0.26 × 1010 s-1) and obviously
the largest among those of the DPm⊃Py2Fn family. These trends
indicate an interesting possibility that dense packing of both
the zinc porphyrin and fullerene units on the dendrimer surface
Time-resolved emission spectroscopy was employed for
investigating the charge-separation event in DPm⊃Py2Fn.11 Upon
excitation at 420 nm in CH2Cl2 at 20 °C, the fluorescence of
DP6 (1.0 × 10-6 M) in the presence of 3 equiv of fullerene-
free bipyridine displayed a monoexponential decay profile at
590 nm with a lifetime (τf) of 1.6 ns. On the other hand, when
(14) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust,
D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem. 1996,
100, 15926-15932.
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A R T I C L E S
Li et al.
plays an essential role in facilitating the charge-separation
process. As expected, nondendritic zinc porphyrin reference P1
in the presence of Py2Fn showed much smaller kCS values such
as 0.031 × 1010 (n ) 1), 0.12 × 1010 (n ) 2), and 0.13 × 1010
s-1 (n ) 3).
We also investigated the charge-recombination events in
DPm⊃Py2Fn, using the decay profiles in CH2Cl2 of the transient
absorption band at 1000 nm due to C60‚- (Figure 5).11 In every
case, the decay profile was satisfactorily fitted with two-
exponential components, where the faster decay is attributed to
the charge-recombination process, while the slower decay
originates from the excited triplet species. The charge-
recombination rate constants (kCR, Figure 6c) thus obtained are
all within a narrow range from 4.5 × 106 to 6.7 × 106 s-1, and
the lifetimes of the charge-separation state (τRIP; 150-220 ns)
are comparable to one another.11 These trends are quite
reasonable since the donor-acceptor distance in DPm⊃Py2Fn
should not be much dependent on the degrees of steric
congestion of the two ligating components but is primarily
determined by the length of the spacer unit between the
bipyridine and fullerene units in Py2Fn. The successful deter-
mination of the kCS values for DPm⊃Py2Fn allowed us to
compare the ratios of kCS/kCR, which have often been used as a
measure for the excellence of photoinduced electron-transfer
systems.15 Of interest, the values of kCS/kCR (Figure 6d) are all
large in a range from 450 (DP6⊃Py2F1) to 3400 (DP24⊃Py2F3).
In particular, the kCS/kCR ratio for DP24⊃Py2F3 is more than an
order of magnitude greater than those reported for precedent
porphyrin-fullerene supramolecular dyads and triads.15 It is
obvious that a larger number of the fullerene units in DP24⊃Py2F3
could enhance the probability of the electron transfer from the
zinc porphyrin units. However, in addition to this, one can also
presume that an efficient energy migration along the densely
packed zinc porphyrin array9 may enhance the opportunity for
this electron transfer. When the degrees of fluorescence quench-
ing (1 - I/I0) [I and I0; fluorescence intensities with and without
the quencher, respectively], observed for DP6, DP12, and DP24
upon titration with Py2F3, are plotted against the ratio of the
numbers of the fullerene and zinc porphyrin units ([fullerene]/
[zinc porphyrin]),11 it is clear that the quenching of the excited
singlet state of the zinc porphyrin units by Py2F3 is more efficient
as m in DPm is larger from 6 to 12 and then to 24 (Figure 7).
For example, the degree of fluorescence quenching of 0.6 can
be attained for DP24 only at [fullerene]/[zinc porphyrin] ) 0.19,
whereas it requires a much larger ratio of [fullerene]/[zinc
porphyrin] such as 0.64 for DP6. Since the average binding
affinities of Py2F3 toward DP6 and DP24 are not much different
Figure 7. Plots of degrees of fluorescence quenching (1 - I/I0) versus
[fullerene]/[zinc porphyrin] upon titration of DP6 (green), DP12 (blue), and
DP24 (red) ([zinc porphyrin] ) 3.6 × 10-6 M) with Py2F3 in CHCl3 at 25
°C. Concentrations of fullerene and zinc porphyrin units are given by 3 ×
[Py2F3] and m × [DPm], respectively.
from one another (Figure 1c), it is obvious that the energy
9
migration along the zinc porphyrin array in DP24 greatly
facilitates the electron transfer to the fullerene units.
Conclusions
By means of a multivalent surface ligation of dendritic
macromolecules,16 we constructed, using DPm (m ) 6, 12, and
24) and Py2Fn (n ) 1-3), a photoactive layer, consisting of
electron-donating zinc porphyrin and electron-accepting fullerene
arrays on the dendrimer surface. Upon increment of the numbers
of these donor and acceptor units, the electron transfer reaction
was remarkably facilitated, while the recombination of the
resulting charge-separated state remained virtually intact. Con-
sequently, among the DPm⊃Py2Fn family, DP24⊃Py2F3 accom-
modating 24 zinc porphyrin units and roughly 30 fullerene units
on the dendrimer surface (Table 1) achieved the largest ratio of
the charge-separation to charge-recombination rate constants
(3400), which is even 1 order of magnitude greater than those
of precedent examples. Application of this molecular design
strategy to the development of optoelectronic materials is one
of the subjects worthy of further investigations.
Acknowledgment. We are grateful to Dr. Yohei Yamamoto
for STM measurements. D.K. acknowledges the financial
support through the Star Faculty Program of the Ministry of
Education and Human Resources.
Supporting Information Available: Details for synthesis,
characterization, and spectral data (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
See any current masthead page for ordering information and
Web access instructions.
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