Photoinduced charge separation after excited energy transfer in snowflake-shaped Zn-porphyrin dendrimer with anthraquinone terminals: Enhancement of the electron-transfer rates by "dendrimer effect"

Article abstract of DOI:10.1246/bcsj.20100046

Photoinduced electron transfer (ET) and energy transfer (EnT) of a snowflake-shaped Zn-porphyrin dendrimer with anthraquinone terminals (dendrimer-1), which has four rigid ethynylene-phenylene conjugated chains covered by soft benzyl ether (BE) branches,

Full text of DOI:10.1246/bcsj.20100046

© 2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 10, 1223–1237 (2010) 1223
Photoinduced Charge Separation after Excited Energy Transfer in
Snowflake-Shaped Zn-Porphyrin Dendrimer with Anthraquinone
Terminals: Enhancement of the Electron-Transfer Rates
by “Dendrimer Effect”
Masatoshi Kozaki,*1 Kogen Akita,1 Keiji Okada,*1 D.-M. Shafiqul Islam,2 and Osamu Ito*2
1Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai 980-8577
Received February 19, 2010; E-mail: kozaki@sci.osaka-cu.ac.jp
Photoinduced electron transfer (ET) and energy transfer (EnT) of a snowflake-shaped Zn-porphyrin dendrimer with
anthraquinone terminals (dendrimer-1), which has four rigid ethynylene-phenylene conjugated chains covered by soft
benzyl ether (BE) branches, is investigated by means of steady-state and time-resolved fluorescence measurements.
Dendrimer-2 without anthraquinone terminals undergoes a nearly quantitative EnT from the excited singlet states of BE
branches to the Zn-porphyrin core, where the conjugated chain plays an important role as a mediator of EnT within this
antenna type dendrimer (the dendrimer effect on EnT). A comparison of the rates of charge separation via the singlet
excited state of Zn-porphyrin to the anthraquinone terminals through oligo(ethynylenephenylene)s in a series of model
compounds reveals considerable acceleration of the rate constants by the dendrimer architecture (the dendrimer effect on
ET). This acceleration is attributed to hydrophobic interactions between the flexible BE chains within dendrimer-1,
resulting in a significant conformational change into coplanar forms between the Zn-porphyrin plane and the ethynylene-
phenylene-anthraquinone plane.
Dendritic structures are quite attractive for the construction
of a well-organized assembly with a large number of functional
groups.1 One of the most attractive applications of dendritic
architectures is to construct artificial photosynthetic molecular
devices containing chromophores that are arranged for light
harvest and charge separation (CS). Many artificial light
harvesting systems have thus far been constructed with the
incorporation of a variety of chromophores, such as porphy-
rins,2 oligo(ethynylenephenylene)s,3 and naphthalimides4 into
the dendrimer architectures.5,6 Efficient light harvesting proc-
esses from dendritic branches into a core have been observed,
although the efficiency of these process is generally dependent
on the generation size in the dendrimers.3a Moreover, a nearly
100% energy transfer (EnT) can be realized by introduction of
the energy gradients into the dendritic chains.3
However, a difficulty in dendrimer architecture is often
encountered when one uses the harvested light energy as a
chemical potential through electron-transfer processes. In fact,
there have been only a few reports on dendritic systems that
function as both light harvesting and successive CS systems,6
even as CS systems alone have been incorporated in dendrimer
frameworks.4b,7 Natural photosynthetic systems employ the
strategy of integrated molecular systems, in which the reaction
center for the CS is precisely positioned inside the multiple
light harvesting antennas.8 A simpler approach may be possible
through the incorporation of a new channel of electron transfer
(ET) besides the dendritic branches into a dendrimer frame-
work. This new channel (a favorably ³-conjugated system)9
must combine the energy-harvested core (the electron donor,
for instance) with peripheral new electron acceptors. In such a
system, excitation of the energy donors embedded in dendritic
branches causes an energy migration to the core (energy
acceptor and electron donor) through-bonds and through-space,
followed by the fast ET from the excited core to the peripheral
electron acceptors through the newly added channels.
We have recently reported a convergent synthetic method-
ology for snowflake-shaped dendrimers in which conjugated
chains, oligo(ethynylenephenylene)s (abbreviated as EtPh),
were incorporated inside the benzyl ether (BE) dendritic
branches.10 The dendrimer with a 1,3,5-trisubstituted benzene
core showed highly efficient BE fluorescence quenching in
steady-state measurements, which was reasonably interpreted
by EnT from the 1BE* to the EtPh-conjugated chains. The
convergent method was successfully applied to the construction
of the Zn-porphyrin dendrimer with anthraquinone (AQ) units
at the terminal of the conjugated chains. As a preliminary
finding, we reported the quenching of the Zn-porphyrin
fluorescence by the AQ units.11 The strategy of this dendrimer
design is quite different from that of previous approaches.
The advantages of this type of dendrimer are as follows:
1) Coupling of the dendritic antenna provides a versatile
synthetic approach that is applicable to various donor-bridge-
acceptor systems, 2) the distance between the electron donor
and electron acceptors is fixed by the rigid conjugated chain,
allowing a clear argument for mechanistic points of view, and
3) because of the symmetric structure of the dendrimer, the
efficiency of the photoinduced ET process can be directly
compared to the corresponding dendron and/or their building
Published on the web October 5, 2010; doi:10.1246/bcsj.20100046

1224 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Snowflake-Shaped Zn-Porphyrin Dendrimers
O
O
t-Bu
t-Bu
O
O
O
O
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
O
O
t-Bu
O
O
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
Zn
N
N
O
N
N
N
N
Zn
O
O
O
O
O
O
O
O
t-Bu
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
t-Bu
t-Bu
O
1
2
t-Bu
t-Bu
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
O
t-Bu
t-Bu
N
N
N
O
t-Bu
t-Bu
Zn
N
N
N
O
Zn
N
N
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
t-Bu
O
t-Bu
3
4
Chart 1. Chemical structures of snowflake-shaped Zn-porphyrin dendrimers and reference compounds.
blocks, providing important information on the relationship
between the structure and efficiency of the ET. In addition, the
snowflake-shaped dendrimers are expected to serve as modular
building blocks10c for an integrated molecular system of
biomimetic photosynthesis. In order to design such integrated
multifunctional dendrimer systems, it is essential to establish
the mechanism by which the ET efficiency is influenced by the
dendrimer architecture. In this paper, we report upon the
photophysical properties of novel snowflake-shaped Zn-por-
phyrin dendrimers with the AQ and phenyl terminals, dendri-
mer-1 and dendrimer-2, respectively, and related compounds 3
and 4 (Chart 1). Significant enhancement of EnT efficiency in
dendrimer-2 (the dendrimer effect on EnT) and ET efficiency in
dendrimer-1 (the dendrimer effect on ET) was observed in
time-resolved fluorescence measurements. The origin of these
dendrimer effects is discussed in detail.
Miyaura coupling,12 iodation,13 and Sonogashira coupling
reactions.14 The anthraquinon-2-yl terminals were attached to
the dendron 5 by removal of the TBS groups in dendron 5,
followed by the Sonogashira coupling reaction with 2-
iodoanthraquinone (7)15 (Scheme 1). Dendron 8 was heated
with iodine in iodomethane to give dendron 9. The Sonogashira
coupling reaction of Zn-porphyrin core 1016 with dendron 9
yielded the desired dendrimer-1 in the form of a purple
powder. Dendrimer-2 was also successfully prepared by the
Sonogashira coupling reaction of the phenyl-terminated den-
dron 11 and porphyrin core 10 (Scheme 2). The reference
compounds 3 and 4 were obtained by employing a similar
method (Schemes 3 and 4). These compounds were soluble in
various organic solvents such as chloroform, dichloromethane,
THF, and toluene and were unambiguously characterized by
means of NMR, elemental analysis, and MALDI-TOF mass
1
spectroscopy. Interestingly, the H NMR spectra of dendrimer-
Results and Discussion
1 and dendrimer-2 showed broad signals at room temperature.
Sharp and well-resolved signals were obtained at higher
temperatures (at 135 °C for dendrimer-1 and 85 °C for dendri-
Synthesis. First, dendron 5 was prepared by the previously
reported convergent method10a,11 which employs Suzuki-

M. Kozaki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1225
t-Bu
t-Bu
R
O
O
t-Bu
t-Bu
O
O
N
N
N
N
Zn
R
R
t-Bu
t-Bu
O
O
O
O
O
O
I
10
O
R
7
R = C6H4CCH
Et2N3
R
R1
1
O
Pd(PPh3)2Cl2, CuI
i-Pr2EtN, THF
Pd(PPh3)2Cl2, CuI
i-Pr2EtN, THF
O
O
O
O
t-Bu
t-Bu
O
O
t-Bu
t-Bu
O
O
t-Bu
t-Bu
5: R = TBS
6: R = H
TBAF, THF
8: R = N3Et2
9: R = I
I2, CH3I
Scheme 1. Synthesis of dendrimer-1.
with Zn-tetraphenylporphyrin (Zn-TPP at 423 nm), suggesting
that the covalent linkage of conjugated EtPh chains slightly
expands the ³-conjugation. Importantly, dendrimer-1 with
AQ-terminals (<400 nm) exhibits Soret band that is consid-
erably broader than those for dendrimer-2 and the reference
compounds 3 (427 nm) and 4 (427 nm). The shape of the Soret
band of dendrimer-1 was independent of the concentration in
O
O
O
O
10, Pd(PPh3)2Cl2, CuI
¹7
the range between 2.5 © 10 and 2.5 © 10¹6 M. This result
2
I
i-Pr2EtN, THF
rules out the possibility of intermolecular aggregate forma-
tion.18 Further, the sharp absorption of dendrimer-2 suggests
that the broadening of dendrimer-1 is not related to structure
factors possibly caused by deformation in the core porphyrin
ring,19 but rather, to the presence of the AQ groups. Interest-
ingly, the Soret bandwidth of dendrimer-1 is strongly depend-
ent on solvent polarity. Broader absorption was observed
in more polar solvents (full width at half maximum (fwhm) =
30-37 nm in THF and 1,4-dioxane, fwhm = 43-44 nm in DMF
and benzonitrile (See SI)), indicating that there is weak
electronic communication between the Zn-porphyrin core and
the AQ-units via the EtPh-bridges in the dendrimer-1 in the
ground state accompanying the change of dipole moments.20
41%
O
O
O
O
11
Scheme 2. Synthesis of dendrimer-2.
mer-2) in tetrachloroethane-d2 (see Supporting Information
(SI)). Similar behavior was reported for the snowflake-shaped
dendrimer with a 1,3,5-substituted benzene core.10a On the
contrary, normal Fréchet type (BE type) porphyrin dendrimers
have been reported to exhibit sharp NMR signals at room
temperature.17 These results suggest that the equilibriums that
are present between various conformers are probably due to
slow rotation around the ethynylenic bonds in EtPh units that
are covered by bulky flexible BE chains in dendrimer-1 and
dendrimer-2.
Dendrimer Effect on Energy Transfer.
An energy
diagram of the local excited states in dendrimer-2 is shown in
Figure 2. The highest energy state belongs to the branched
BE-chain. Although the absorption of each BE-branch is not
high, 48 benzene rings moderately absorb the UV light to give
1
1BE*. The EnT can then proceed in the sequence of BE* ¼
1EtPh* ¼ 1Zn-porphyrin*. Alternatively, a direct EnT from
1BE* to Zn-porphyrin is possible.
Electronic Absorption Spectra. Figure 1 shows the UV-
vis absorption spectra of dendrimers-1 and -2, dendron-3, and
dyad-4 in THF. Dendrimer-1 shows the characteristic bands of
a Zn-porphyrin unit (Soret band: -max = 431 nm, Q-bands:
The absorption of conjugated EtPh-chains in dendrimer-2
observed around 370 nm sufficiently overlaps the emission of
the BE-branch around 310 nm.2b In addition, the EtPh-chains in
dendrimer-2 are expected to have emission around 440 nm,10a
the value at which the Zn-porphyrin has its intense Soret
absorption. These observations suggest that the conjugated
EtPh-chain effectively mediates the excited singlet EnT from
the BE-branches to the Zn-porphyrin core by the Förster
mechanism. To evaluate the efficiency of the EnT from the
-
max = 565, 609 nm, Table 1) along with absorption bands due
to the branched BE chains (-max = 256 nm) and the conjugated
EtPh chains (-max = 377 nm). The Soret absorption peaks of
the Zn-porphyrin core of dendrimer-1 and dendrimer-2
(433 nm) exhibit slight bathochromic shifts in comparison

1226 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Snowflake-Shaped Zn-Porphyrin Dendrimers
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
N
N
N
O
O
Zn
t-Bu
t-Bu
N
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
O
13
t-Bu
t-Bu
N
N
N
N
Pd(PPh3)2Cl2, CuI
Zn
R
TBS
R
i-Pr2EtN, THF
90%
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
t-Bu
t-Bu
O
O
t-Bu
t-Bu
96%
7, Pd(PPh3)2Cl2,
CuI, i-Pr2EtN, THF
14: R = TBS
15: R = H
TBAF, THF
5: R = N3Et2
12: R = I
60%
3: R = 2-anthraquinonyl
78%
Scheme 3. Synthesis of dendron 3.
t-Bu
t-Bu
t-Bu
t-Bu
7
O
t-Bu
t-Bu
t-Bu
N
N
N
Pd(PPh3)2Cl2, CuI
N
N
N
R
Zn
Zn
N
i-Pr2EtN, THF
O
N
3
t-Bu
3
93%
t-Bu
t-Bu
TBAF, THF 80%
4
t-Bu
t-Bu
17: R = TMS
18: R = H
Scheme 4. Synthesis of dyad 4.
Table 1. Photophysical and Electrochemical Properties of 1-4
10
8
Emissionb)
1
c)
c)
Compd -ABmaxa)/nm
Eox Ered
a)/nm
2
3
4
-
EMmax
d)
1
2
3
4
256, 377, 431, 565, 609
286, 372, 433, 560, 601
255, 284, 370, 427, 558, 598 605, 656
343, 427, 559, 598 606, 655
625
613, 658
®
®
¹0.86
®
d)
d)
6
0.74 ¹0.81
0.74 ¹0.76
a) In THF. b) In THF, excitation at Soret band. c) In 1,2-
dichloroethane, V vs. SCE, scan rate 100 mV s¹1, Pt disk was
used as a working electrode. d) Not observed due to the slow
electron-transfer rate between a redox center and the electrode.
4
ε
2
0
Under excitation of the BE-branch at 286 nm, a value at which
both the EtPh-conjugated chain and the Zn-porphyrin core
have only weak absorption, an intense fluorescence of the Zn-
porphyrin core appeared at 600-700 nm as shown in Figure 3a,
300
400
500
/nm
600
700
λ
Figure 1. UV-vis absorption spectra of 1-4 in THF.
1
suggesting efficient EnT from BE*-units to the Zn-porphyrin
core. Both direct excitation of the Zn-porphyrin core and
sensitized excitation of the BE-branch gave the same Zn-
porphyrin fluorescence with an identical quantum efficiency
(ΦF) of 0.05. Furthermore, the excitation of the EtPh-units
BE-branch to the Zn-porphyrin core via the EtPh-chain, the
steady-state fluorescence spectra of dendrimer-2 were measured
in THF by changing the excitation wavelength (Figure 3).

M. Kozaki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1227
hν
Direct EnT from
BE branch to ZnP
EnT from
BE branch
EtPh-(1BE*)-ZnP
(4.20 eV)
hν
O
O
O
O
1EtPh*-(BE)-ZnP
O
O
EnT from EtPh
EnT via EtPh
(3.10 eV)
O
O
EtPh-(BE)-1ZnP*
(2.00 eV)
O
O
O
O
O
O
O
O
O
O
O
O
N
N
N
Zn
N
O
O
Dendrimer 2 (EtPh-(BE)-ZnP)
O
O
O
O
Figure 2. Energy diagram for dendrimer-2 (EtPh-(BE)-
ZnP) (EtPh: oligo(ethynylenephenylene)s conjugated
chain, and BE: oligo(benzyl ether) branch, and ZnP: Zn-
porphyrin core, EnT: energy transfer).
O
O
O
O
O
O
EnT from BE branch
EnT from (via) EtPh
(a)
(b)
Figure 4. Schematic illustration of dendrimer effect on EnT
in dendrimer-2 showing the enhancement of fluorescence
of the Zn-porphyrin core by the excitation of dendrons (BE
branches and EtPh chains).
molecular design is quite useful for obtaining an intense
fluorescence from the core by utilizing absorptions due to a
large number of energy donor chromophores through efficient
EnT in dendrimer frameworks.4d,4e
(c)
It is interesting to compare the present results with those of
closely related Zn-porphyrin dendrimer systems without con-
jugated chains. Jiang and Aida reported a dendrimer with
similar BE-branches and a Zn-porphyrin core; the efficiency of
EnT from the BE-chains to the core Zn-porphyrin (direct EnT
in Figures 2 and 4) in a dendrimer of similar size was reported
as 54%.2b Moreover, it has been well documented that there is a
limit in the generation size for the EnT process in this type of
dendrimer framework.3a
The high efficiency of EnT observed in dendrimer-2 strongly
indicates that the EtPh-conjugated chain plays an important
role as a mediator for the EnT from the branching BE-chains to
the Zn-porphyrin core.4d,4e This mediator effect (the dendrimer
effect on EnT) is very useful to the design of a larger antenna
system.
300
400
500
/nm
600
700
λ
Figure 3. Steady-state fluorescence spectra of dendrimer-2
in THF. (a) 286-nm excitation (the intense peak at 572 nm
is the scattered overtone of excitation light). (b) 372-nm
excitation (a weak peak at 415 nm is due to the scattered
excitation light). (c) 433-nm excitation.
(372 nm) also induced Zn-porphyrin fluorescence with an
identical quantum yield (ΦF = 0.05), as shown in Figure 3b.
These results indicate that highly efficient (ca. 100%) intra-
molecular singlet EnT takes place both from the branched
BE-chains and the conjugated EtPh-chain to the Zn-porphyrin
core. Therefore, the overall fluorescence intensity from the Zn-
porphyrin core under broad-band light excitation should be
increased by the BE-excitation (286 nm) and EtPh-excitation
(372 nm), in addition to the direct excitation of Zn-porphyrin
(431, 565, and 609 nm for dendrimer-2).
Redox Potentials and Energy Diagram in Electron
Transfer.
In the previous section, we have shown that
efficient EnT can be realized by the combination of the BE
chains and EtPh conjugate chains in dendrimer-2, a process
which should be applicable to dendrimer-1 with the AQ-
terminals. The accumulated excited energy in the porphyrin
core can be utilized as a chemical potential by introducing the
AQ-electron acceptors as terminals of the conjugate chains.
Cyclic voltammograms of 1-4 were measured in 1,2-
dichloroethane (¾ = 10.36) containing tetra-n-butylammonium
perchloride as a supporting electrolyte using a Pt disk, Pt
wire, and SCE as working, counter, and reference electrodes,
Obviously, the dendrimer structure offers the great advant-
age of incorporating a large number of BE-branches and/or
EtPh-chains; from their excited states, nearly quantitative
EnT occurs, as is schematically illustrated in Figure 4. This

1228 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Snowflake-Shaped Zn-Porphyrin Dendrimers
Table 2. Steady-State Fluorescence Quenching Efficiency
and Free Energy Changes in Electron Transfer within
Dendrimer-1
rescence because of the absence of the AQ-unit. The quenching
efficiency increased in the order of dyad-4 (89%) < dendron-3
(95%) < dendrimer-1 (98% quenching) in DMF. The quench-
ing efficiency of dendrimer-1 was strongly solvent-dependent
[30% quenching (ΦF = 0.04) in 1,4-dioxane (¾ = 2.21), 69%
quenching (ΦF = 0.02) in THF (¾ = 7.58), 90% quenching
(ΦF = 0.007) in benzonitrile (¾ = 25.7), 98% (ΦF = 0.001)
quenching in DMF (¾ = 36.7)], as summarized in Table 2.
The EnT from 1Zn-porphyrin* to the AQ-terminal cannot be
considered an origin of Zn-porphyrin fluorescence quenching,
because of a highly endothermic process. On the other hand,
Solvent
¾
Quenching efficiency/%a) ¦GET/eVb)
1,4-Dioxane
THF
Benzonitrile
DMF
2.21
7.58
25.7
36.7
30 (38)
69 (73)
90 (89)
98c) (93c))
+0.36
¹0.41
¹0.63
¹0.66
a) Estimated from the fluorescence quantum yields. Values in
parentheses were calculated using the weighted average charge
separation rate constant (kCS w
)
as (kCS)w/((kCS)w + 0.63 ©
1
the ET from Zn-porphyrin* to the AQ-terminal can be an
109); see Table 4. b) Obtained using the Rehn-Weller
approximation7d,22 (Eox(Por) = 0.74 V vs. SCE, Ered(AQ) =
¹0.81 V vs. SCE, E0-0 = 2.00 eV, RDA = 29.0 ¡). Effective
ionic radii of the donor and acceptor were assumed to be
5 ¡.7d,22b c) Weighted average.
origin of Zn-porphyrin fluorescence quenching, because of an
exothermic process in polar solvents. Table 2 shows the
quenching efficiency and the free energy change in the electron
1
transfer from Zn-porphyrin* to the AQ-terminal for dendri-
mer-1, which indicate that both the quenching efficiency and
the free energy changes7d,22 are strongly dependent on the
solvent polarity; the ET-reaction is thermodynamically more
favored in more polar solvents, supporting the idea that the
electron transfer is the origin of Zn-porphyrin fluorescence
quenching. A similar trend was observed for the encapsulated
dendron-3 and the uncovered dyad-4, which exhibit a free
energy change similar to that of dendrimer-1, although the
quenching efficiencies are lower than those of dendrimer-1 in
all the solvents. A lower degree of quenching was observed in
1,4-dioxane for dendron-3 and dyad-4 because of the positive
free energy in 1,4-dioxane. The higher quenching efficiency of
dendrimer-1 as compared to that of dendrons-3 and dyad-4 is
probably not related to the free energy term, but rather, to
the kind and number of dendrons. This will be discussed in
more detail in the sections, “Modeling Study of dyad-4” and
“Dendrimer Effect and Mechanism of Electron Transfer.”
Kinetic Study of Fluorescence Quenching in Dendrimer-1.
In order to obtain a deeper insight into the fluorescence
quenching, we measured the time-profiles of the fluorescence
intensity of the Zn-porphyrin core under picosecond laser
light excitation at 410 nm using a streak-scope (Figure 6). The
fastest decay was observed for dendrimer-1. Moderately fast
decay was observed for dendron-3. The slowest fluorescence
decay was observed for dyad-4 (Figure 6a). The fluorescence
lifetime (¸F) of dendrimer-2 was almost solvent-independent,
which is consistent with no ET quenching. Therefore, dendri-
mer-2 can be used as a reference compound that provides a
standard decay rate without ET; (¸F)ref = 1.6 ns corresponding
to the intrinsic lifetime of the Zn-porphyrin core. The fluo-
rescence lifetimes are summarized in Table 3.
AQ-EtPh-(1BE*)-ZnP
(4.20 eV)
1AQ*-EtPh-(BE)-ZnP
(3.56 eV)
AQ-1EtPh*-(BE)-ZnP
AQ-EtPh-(BE)-1ZnP**
EnT
(3.10 eV)
(2.58 eV)
IC
AQ-EtPh-(BE)-1ZnP*
AQ.--EtPh-(BE)-ZnP.+
(2.00 eV)
ET
(1.59 eV)
CR
Dendrimer 1 (AQ-EtPh-(BE)-ZnP)
Figure 5. Energy diagram of dendrimer-1 (AQ-EtPh-
(BE)-ZnP) in THF, where * and ** denote the first and
the second excited states, respectively.
respectively. Dendrimer-1 and dendrimer-2 showed signifi-
cantly broad redox waves due to the slow ET rate between the
encapsulated porphyrin core and the electrode.21 On the other
hand, dendron-3 and dyad-4 showed clear reversible redox
waves at 0.74 V vs. SCE, which are attributed to the oxidation
(Eox) of the Zn-porphyrin units (Table 1). The redox waves
corresponding to the reduction (Ered) of the AQ-terminals
in 3 and 4 were observed at ¹0.81 and ¹0.76 V vs. SCE,
respectively.15 From these electrochemical data, the energy
level (¦GRIP in eV) of the radical ion pair Zn-porphyrin+/AQ
¹
within dendrimer-1, as estimated by the Rehn-Weller approx-
imation,7d,22 is 1.59 eV above the ground state in THF
(¾ = 7.58), using the center-to-center (RDA) distance between
the Zn-porphyrin core and AQ-terminal = 29.0 ¡.23 Thus, the
ET from 1Zn-porphyrin* (E0-0 = 2.00 eV) to the AQ-terminals
The decay rate of the Zn-porphyrin fluorescence in dendri-
mer-1 increases with the solvent polarity (Figure 6b). Impor-
tantly, the decay curves of dendrimer-1 are best curve-fitted as
a bi-exponential function in all solvents; the contribution of the
fast-decaying component increases with the solvent polarity.
The slower-decaying component in dendrimer-1 has a lifetime
that is comparable to that observed for dendron-3, except for
the value in 1,4-dioxane.
In order to reveal the ET kinetics in more quantitative detail,
we evaluated the rate constants for the CS (kCS) via the excited
singlet state of Zn-porphyrin using eq 1, where (¸F)ref is a
lifetime (1.6 ns) of dendrimer-2;
within dendrimer-1 in THF is an exothermic process; ¦GET
=
¹0.41 eV (Table 2). Figure 5 illustrates the energy levels
of possible radical ion pairs and the local excited states in
dendrimer-1 (AQ-EtPh-(BE)-ZnP) in THF.
Steady-State Fluorescence Quenching in Dendrimer-1.
The fluorescence from the Zn-porphyrin core (431-nm excita-
tion) was effectively quenched by the AQ-moiety (in dendri-
mer-1, dendron-3, and dyad-4) in polar solvents (benzonitrile
and DMF), whereas dendrimer-2 showed the strongest fluo-

M. Kozaki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1229
(a)
(b)
2 in THF
4 in THF
3 in THF
1 in THF
1 in 1,4-Dioxane
1 in THF
1 in PhCN
1 in DMF
Figure 6. (a) Fluorescence decays of 1-4 in the 600-660 nm range with -ex = 410 nm in THF. (b) Fluorescence decays of
dendrimer-1 in the 610-660 nm range with -ex = 410 nm in various solvents.
Table 3. Fluorescence Lifetimes (¸F) of Zn-Porphyrin Core
In a polar solvent, the kCS value increases in the order of
¸F/ns (fraction)
dyad-4 < dendron-3 < dendrimer-1 (for the fast and major
Compds
1,4-Dioxane
THF
Benzonitrile
DMF
component). The kCS value for dendrimer-1 in DMF is
almost 12 times faster than that of dyad-4, demonstrating this
advantage of the dendrimer structure (dendrimer effect on ET)
1a)
0.73 (35%) 0.32 (63%) 0.16 (88%) 0.11 (92%)
1.21 (65%) 1.20 (37%) 0.55 (12%) 0.43 (8%)
1.70 (100%) 1.60 (100%) 1.60 (100%) 1.60 (100%)
1.81 (100%) 1.10 (100%) 0.46 (100%) 0.37 (100%)
1.71 (100%) 1.60 (100%) 0.90 (100%) 0.76 (100%)
1
2
3
4
for the CS process from Zn-porphyrin* to the AQ-terminal
through the EtPh conjugate chain (Figure 7). This is the first set
of experimental data that clearly establishes how the dendrimer
architecture is relevant to the ET process of the incorporated
donor-conjugate bridge-acceptor system.
a) Double-exponential decay was observed.
Modeling Study of dyad-4. The constant D-A distance,
similar redox values, and similar excitation energy in encapsu-
lated dendron-3, uncovered dyad-4, and dendrimer-1 suggest a
minor role of the free energy term in the difference in the
electron-transfer rate in these compounds, but they also indicate
the importance of the electronic coupling matrix in the Marcus
equation. In addition, the presence of conformational isomers
(probably rotamers around ethynylenic bonds), particularly for
dendrimer-1 in polar solvents, is suggested in the following
kCS ¼ ð1=¸FÞsample ꢀ ð1=¸FÞref
ð1Þ
The kCS values are summarized in Table 4. Dendrimer-1 has
two kCS values corresponding to the two lifetimes (Table 3).
The kCS values for dendron-3 and dyad-4 in 1,4-dioxane and for
dyad-4 in THF are too small to be accurately determined.
The kCS values for dendrimer-1 are strongly solvent-polarity
dependent; the value in DMF is almost 10 times larger than that
in 1,4-dioxane in both fast and slow components. The weighted
¹1
1
average (kCS w
)
for dendrimer-1 is calculated as 0.43 © 109 s
experiments: 1) the H NMR study indicates a relatively slow
¹1
¹1
in 1,4-dioxane, 1.7 © 109 s in THF, 5.2 © 109 s in benzo-
conformational isomerism at room temperature in dendrimer-1,
2) a UV-vis spectral study reveals a CT-type broadening of the
Soret band of dendrimer-1 in polar solvents, and 3) a dynamic
fluorescence quenching study clearly shows the presence of
fast- and slow-decaying components, with the former becoming
predominant in polar solvents. These results suggest that the
fast and major isomer (rotamer) in polar solvents exhibits a
broad Soret band, and that the structure of the rotamers
definitely affects the electronic interactions in the ET process.
nitrile, and 8.1 © 109 s in DMF. Using the standard decay
rate constant of 0.63 © 109 s (for dendrimer-2), the quench-
¹1
¹1
ing efficiencies due to ET that is calculated as (kCS)w/
((kCS)w + 0.63 © 109) increase with solvent polarity; they
become 38% in 1,4-dioxane, 73% in THF, 89% in benzonitrile,
and 93% in DMF, as listed in Table 2. These quenching
efficiencies are roughly comparable to the steady-state fluo-
rescence quenching efficiencies shown in Table 2.
Table 4. Charge-Separation Rate Constants (kCS) from Excited Singlet State of Zn-Porphyrin to AQ-Terminal
in Dendrimer-1
¹1 a)
kCS/s
Compds
1,4-Dioxane
THF
Benzonitrile
DMF
1
0.782 © 109 (35%)
2.50 © 109 (63%)
0.208 © 109 (37%)
0.284 © 109
®
5.74 © 109 (88%)
1.19 © 109 (12%)
1.55 © 109
8.63 © 109 (92%)
1.70 © 109 (8%)
2.08 © 109
0.238 © 109 (65%)
®
®
3
4
0.486 © 109
0.691 © 109
a) Fluorescence lifetimes of dendrimer-2 were used as a standard (¸F)ref.

1230 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Snowflake-Shaped Zn-Porphyrin Dendrimers
Dendrimer Effect and Mechanism of Electron Transfer.
Because of the similarity of their electrochemical parameters,
the differences in the kCS values among dendrimer-1, dendron-
3, and dyad-4 can most likely be ascribed to the electronic
interaction between 1Zn-porphyrin* and AQ-moieties. Accord-
ing to the general principle of electronic interaction in
photoinduced ET, an important orbital interaction in the ET
that includes an excited electron donor and a ground state
O
O
t-Bu
t-Bu
h
ν
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
t-Bu
t-Bu
t-Bu
O
t-Bu
O
O
O
t-Bu
t-Bu
O
O
O
O
O
O
acceptor must be the interaction between the LUMOdonor and
O
31
N
N
N
N
O
the LUMOacceptor
.
Then, the differences in kCS value may be
Zn
O
attributed to the orbital interaction between the donor ³*Por
(porphyrin) orbital and the ³*AQ (AQ) acceptor orbital through
the spacer ·-orbitals or twisted ³-orbitals of the ring A. Conf 1
(Figure 8-I) has a coplanar relation between rings A, B, and C;
thus, ³*AQ extends to ring A (Figure 8-II, right-upper MO).
Although the extended ³*AQ is perpendicular to ³*Por, the
electron transfer would readily occur, as has been experimen-
tally shown in highly twisted short-linked donor*-acceptor
type dyads.32 In addition, the torsion angle between the
porphyrin ring and ring A is not exactly perpendicular, but is
65-80°,24 which allows for a direct ³-³ type molecular orbital
interaction that results in fast ET. In Conf 2, which has a
parallel orientation between the porphyrin ring and the AQ ring
separated by a twisted ring A, ³*Por interacts with the ·-bond
orbital ·ringA to give hybrid ³*Por-a·ringA (0 < a ¹ 1) orbital
(Figure 8-II, left MO), whereas ³*AQ can also interact with
O
O
O
O
t-Bu
t-Bu
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
O
O
O
O
t-Bu
t-Bu
O
O
ET from ZnP to AQ
via EtPh
Figure 7. Schematic illustration of dendrimer effect on ET
in dendrimer-1 showing ET from the Zn-porphyrin core to
AQ terminals by excitation of Zn-porphyrin.
·
to provide hybrid ³*AQ-b·ringA (0 < b ¹ 1) orbital
ringA
(Figure 8-II, right-lower MO). The ·-mixing can be visible in
the LUMOs of the model donor (Conf 2-D-LUMO) and the
model acceptor (Conf 2-A-LUMO) (Figure 8-II). Alternative-
ly, direct molecular orbital interaction is also possible between
³*Por and ³*AQ due to the loose torsion angle (65-80°)
between the porphyrin ring and the phenyl ring A, which
facilitates ET from 1Zn-porphyrin* to AQ. A similar con-
sideration is also possible for the conformers of parallel
orientation (Conf 3, 4, and 8). They have higher energies and
·-orbitals of longer sizes, however, which decrease their
contribution to the ET process.
Dendron-3 decays 3 times faster than the uncovered dyad-4
in polar solvents. The fluorescence intensities of dendron-3 and
dyad-4 decay with single exponential, and there is no evidence
for the participation of rotamers in the ET in dendron-3. The
faster rate of decay in dendron-3 may be attributed to a subtle
change in electronic perturbation caused by the introduction of
BE chains, i.e., the introduction of electron-donating BE chains
In order to gain insight into the relationship between
the structures and energies of the rotamers in the present
system, a conformational analysis of dyad-4 was carried out by
using MO calculations. The structures were optimized using
Spartan (AM1 method) with fixed dihedral angles (0 or 90°
for the phenyl rings A, B, C, and D). The dihedral angle
between Zn-porphyrin and phenyl ring A was also fixed at 90°
for the sake of simplicity, even though this dihedral angle is
typically 65-80° according to crystal structure analysis.24 The
energy of the AM1-optimized structure was recalculated using
Gaussian program (B3LYP/6-31G(d)).25 Figure 8 shows the
structures and energies (B3LYP/6-31G(d)//AM1) of the eight
possible conformers of dyad-4. The most stable Conf 1 is a
global energy minimum structure, while the other structures
(Conf 2-Conf 8) are energy maxima, from which energy
barriers of bond rotation can be estimated. The relative
¹1
energies of these rotamers (Conf 1-8) are within 16 kJ mol
,
which indicates very rapid rotation of these rings at room
temperature. Their stability generally increases in longer
conjugation, as shown in Figure 8-I. At the same conjugation
length, AQ-containing conjugations tend to provide a higher
degree of stability.
to the EtPh groups would increase the energy level of ³*AQ,
leading to a stronger orbital interaction between the ³*Por orbital
and ³*AQ orbital to give a faster ET. We assume that dendron-3
and dyad-4 undergo ET via the same rotamer, essentially the
one which has the most stable conformation, Conf 1.
The conformers of intermediate dihedral angles between
rings A-D have intermediate energies. Therefore, the energies
of these conformers may be altered by a small perturbation.
This consideration is in agreement with experimentally derived
results for the structures of widely investigated unsubstituted
and alkyl-substituted EtPh-type compounds,26-29 which show a
stability of planar forms, even though the barriers to rotation of
phenylene groups are low, which are also dependent on subtle
changes in the environments.30
On the other hand, dendrimer-1 exhibits a fluorescence
decay of double-exponential functions, and the rapidly decay-
ing components are 4-5 times faster than those of dendron-3.
The fast-decaying component is predominant in polar solvents.
Importantly, the slower component decays at a rate constant
1
that is similar to that of dendron-3. H NMR and stationary
fluorescence studies also indicate the presence of conforma-
tional isomers. We propose the structures 1-A and 1-B for
dendrimer-1 (Chart 2). The conformer 1-A has a partial

M. Kozaki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1231
(I)
Dihedral Angle
relative to ring A
Relative Energy
(kJ/mol)
Conformer
B-C-D
(II)
Figure 8. Structures and relative energies of the eight possible conformers of dyad-4 (I) and the LUMOs of the model donor and
acceptor in Conf 1 and 2 (II).
conformation similar to that of dendron-3 (Conf 1) and each
dendron has a large enough space to behave independently.
Under this condition, the kCS value would not be much different
with dendron-3. On the other hand, the conformer 1-B has a flat
conformation (Conf 2 or its analog) between the Zn-porphyrin
ring and the EtPh moiety separated by the phenyl ring A with a
torsion angle of 65-80° toward the Zn-porphyrin ring.24 This
partially overlapping orientation would facilitate the electron
transfer. The preference of flat conformation 1-B over 1-A
particularly in polar solvents, can be ascribed to the hydro-

1232 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Snowflake-Shaped Zn-Porphyrin Dendrimers
O
O
O
O
t-Bu
t-Bu
t-Bu
O
O
O
t-Bu
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
O
O
O
t-Bu
O
t-Bu
t-Bu
t-Bu
O
t-Bu
O
t-Bu
O
t-Bu
t-Bu
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
O
t-Bu
O
O
O
O
O
O
O
t-Bu
O
O
N
N
N
O
t-Bu
O
O
Zn
O
N
O
N
O
N
Zn
N
N
O
t-Bu
t-Bu
O
O
O
O
O
O
O
t-Bu
O
t-Bu
t-Bu
t-Bu
t-Bu
O
t-Bu
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
O
t-Bu
O
O
O
O
O
t-Bu
O
O
O
O
t-Bu
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
1-A
1-B
Chart 2. Proposed conformers for conformer-1-A and 1-B.
that involve many bulky BE chains. This is compatible with the
1
h
ν
fact that broader signals in the H NMR spectra were observed
h
ν
t-Bu
t-Bu
for dendrimers 1 and 2 but not for dendron-3. Further, this
partially overlapping flat conformation would permit electronic
communications between Zn-porphyrin and the four AQ
groups, even in the ground state, as observed in UV-vis
spectra in polar solvents. The larger kCS value of 1-B as
opposed to 1-A can probably be attributed to this weak
conjugation between 1ZnTPP* and the four AQ rings within the
flat conformation induced by the hydrophobic interactions of
BE chains.
t-Bu t-Bu
O
O
O
O
O
O
O
O
O
O
O
O
t-Bu
t-Bu
O
O
t-Bu
t-Bu
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
N
Zn
N
t-Bu
t-Bu
t-Bu
t-Bu
N
N
O
Conclusion
O
O
O
O
O
t-Bu
t-Bu
In summary, the ET and EnT of snowflake-shaped Zn-
porphyrin dendrimers with AQ and phenyl terminals (dendri-
mer-1 and dendrimer-2, respectively) were mainly investigated
by means of time-resolved fluorescence measurements. Highly
efficient (ca. 100%) intramolecular EnT from dendron moieties
(BE branches and/or EtPh chains) to the Zn-porphyrin core
was observed in the snowflake-shaped dendrimer-2 with phenyl
terminals. The EtPh conjugated chain was found to play an
important role as a mediator for the efficient EnT (dendrimer
effect on EnT), which facilitates the design of larger functional
antenna systems with the dendrimer architecture.
O
t-Bu
t-Bu
O
O
O
O
O
O
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
t-Bu
EnT from BE branch
EnT from (via) EtPh
ET from ZnP to AQ
via EtPh
The accumulated excited energy to the Zn-porphyrin core
can be used as a chemical potential by replacing the phenyl
groups with the AQ groups. Thus, the quenching rates of
charge separation (kCS) of dendrimer-1 and the BE-covered
dendron-3 were compared by means of dynamic fluorescence
lifetime shortening. Dendrimer-1 showed the fastest charge
separation rate. The kCS values increased in the order of
4 < 3 < 1 in all the solvents examined. The order of 4 < 3 may
be attributed to the substituent effect of BE branches on the
EtPh chain. The order 3 < 1 can be explained by a cooperative
effect of the conformational change induced by the inter-
dendron type hydrophobic interaction of BE chains and ³-
conjugation between Zn-porphyrin and the four AQ groups
(the dendrimer effect on ET). The photoinduced EnT and ET
are summarized in Figure 9.
Figure 9. Schematic illustration of photoinduced pro-
cesses obtained by the combination of EnT and ET in
dendrimer-1.
phobic interactions of dendrons within dendrimer-1. Such
a hydrophobic interaction in polar solvents has been well
documented in the literature.33 Furthermore, Aida and co-
workers have reported that van der Waals interactions between
the large dendritic wedges enforce the planar conformation on
linear conjugated chains.34 A similar attractive interaction
operates cooperatively between the dendron units in dendrimer-
1 to produce the stable planar conformer (1-B). The interchange
of the two conformations between 1-B and 1-A would be
energetically and entropically slow because of the processes

M. Kozaki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1233
These dendrimer effects establish the fact that snowflake-
type dendrimers are extremely valuable for efficient light
harvesting motifs as well as fast charge separation. Further
studies for the construction of light-emitting devices using
these dendrimer effects on EnT and artificial photosynthetic
models that employ an efficient charge separation process (the
dendrimer effect on ET) are currently in progress in our
laboratory.
corresponding triazene, iodine, and iodomethane as the solvent.
The tube was flushed with nitrogen, sealed and heated at 80 °C
overnight. The reaction mixture was cooled to room temper-
ature and diluted with dichloromethane. The mixture was
washed with sodium thiosulfate solution and brine and dried
over sodium sulfate. After filtration the solvent was evaporated.
The resulting solid was filtered through a plug of silica gel with
dichloromethane. After evaporation of the solvent in vacuo, a
pure product was obtained by recrystallization.
General Procedure for Desilylation. To the solution of
silylated alkyne (1.00 mmol) in THF (20 mL) was added TBAF
(THF solution, 1.2 mL, 1.2 mmol). The mixture was stirred for
0.5 h. After the solvent was removed in vacuo, the residue was
passed through a short column of silica gel with dichloro-
methane. The crude product was purified by recrystallization
from hexane-ethyl acetate.
Experimental
General. Melting points were taken on a Yanako MP J-3
and are uncorrected. 1H NMR spectra were recorded on a JEOL
Lambda 300, 400, and Bruker Avance 600 spectrometers. The
JEOL Lambda 400 spectrometer was used for the variable
temperature NMR measurement for dendrimers 1 and 2.
Chemical shifts were recorded in units of parts per million
downfield from tetramethylsilane as internal standard and all
coupling constants are reported in Hz. UV-vis spectra were
measured with JASCO/V-570 and Shimadzu UV-2550 spec-
trophotometers. Steady-state fluorescence spectra were mea-
sured with Shimadzu RF-5300 PC and Perkin-Elmer LS50B
spectrofluorophotometers. IR spectra were obtained on a
Shimadzu FTIR-8700 spectrometer. The mass spectra were
recorded on JEOL JMS-700T and JMX-AX500 spectrometers.
MALDI-TOF mass was measured on a Shimadzu-Kratos
AXIMA-CFR Plus spectrometer using dithranol as a matrix
reagent. Elemental analyses were obtained from the Analytical
Center in Osaka City University. Redox potentials were
measured with a voltammetric analyzer (BAS CV-50W) in a
conventional three-electrode-cell equipped with Pt-working
and counter electrodes with a SCE reference electrode at a scan
rate of 100 mV s¹1. TLC was carried out on 0.2 mm Merck
silica gel (60 F254) precoated plates. Merck silica gel 60 (0.063-
0.200 mm) was used for column chromatography. Recycling
preparative HPLC was carried out using a Japan Analytical
Industry LC-908 with JAIGEL-1H, -2H, and -2.5H GPC
columns. 2-Iodoanthraquinone (7),15 5,10,15,20-tetrakis(4-
ethynylphenyl)porphyrinatozinc(II) (10),16 and 5,15,20-
tris(3,5-di-tert-butylphenyl)-10-(4-ethynylphenyl)porphyrinato-
zinc(II) (14)35 were prepared according to the reported
methods. The synthesis of dendrons 5, 11, and 17 has already
been described in our preliminary communication as Support-
ing Information.11 Commercially available reagents and sol-
vents were purified and dried when necessary.
Dendron 6. According to the general procedure, dendron 5
(1.91 g, 1.00 mmol) in THF (20 mL) was reacted with TBAF
(1.2 mL, 1.2 mmol) to afford dendron 6 (1.56 g, 87%): yellow
1
powder; mp 145.5-146.0 °C; H NMR (400 MHz, CDCl3): ¤
7.66 (d, J = 8.8 Hz, 4H), 7.46 (d, J = 8.8 Hz, 4H), 7.44 (s, 2H),
7.40-7.30 (m, 24H), 7.04 (d, J = 8.8 Hz, 4H), 7.03 (s, 2H),
6.95 (d, J = 8.8 Hz, 4H), 6.72 (d, J = 2.2 Hz, 4H), 6.56 (t,
J = 2.2 Hz, 2H), 4.97 (s, 4H), 4.96 (s, 8H), 4.85 (s, 4H), 3.78
(q, J = 7.1 Hz, 4H), 2.97 (s, 1H), 1.30 (m, 60H); 13C NMR
(75 MHz, CDCl3): ¤ 160.27, 158.58, 158.27, 150.99, 150.92,
150.85, 145.52, 145.30, 139.44, 134.17, 133.85, 133.71,
132.73, 130.85, 130.54, 130.28, 127.60, 127.56, 125.49,
125.44, 124.19, 120.18, 117.92, 116.13, 114.09, 114.05,
106.19, 101.55, 95.06, 92.60, 84.90, 82.42, 70.00, 69.92,
69.70, 34.55 (2C), 31.32 (2C); IR (KBr): 2960.5(s), 2868.0(m),
1608.5(s), 1506.3(s), 1456.2(s), 1244.0(s), 1155.3(s),
1016.4(m), 831.3(s) cm¹1; MS (FAB) m/z 1791 (M+); Anal.
Calcd for C124H131N3O8: C, 83.14; H, 7.37; N, 2.35%. Found:
C, 83.24; H, 7.36; N, 2.41%.
Dendron 8. According to the general procedure, dendron
6 (1.25 g, 0.700 mmol), 2-iodoanthraquinone (7) (0.260 g,
0.770 mmol), bis(triphenylphosphine)palladium(II) dichloride
(44 mg, 0.070 mmol), and copper(I) iodide (27 mg, 0.14 mmol)
in ethyldiisopropylamine (10 mL) and THF (10 mL) were
reacted at 80 °C overnight. The crude product was purified by
column chromatography on silica gel (hexane-dichloromethane
1:1) and recrystallization from toluene to afford dendron 8
(1.20 g, 86%): red powder; mp 227.5-228.5 °C; 1H NMR
(400 MHz, CDCl3): ¤ 8.29-8.25 (m, 2H), 8.11 (d, J = 8.0 Hz,
1H), 7.97 (d, J = 1.7 Hz, 1H), 7.78-7.76 (m, 2H), 7.67 (d,
J = 8.6 Hz, 4H), 7.54 (d, J = 8.6 Hz, 4H), 7.45 (s, 2H), 7.40-
7.26 (m, 25H), 7.11 (s, 2H), 7.06 (d, J = 8.6 Hz, 8H), 6.73 (d,
J = 2.2 Hz, 4H), 6.57 (t, J = 2.2 Hz, 2H), 4.99 (s, 4H), 4.97 (s,
4H), 4.96 (s, 8H), 3.79 (q, J = 7.2 Hz, 4H), 1.32-1.29 (m,
60H); 13C NMR (150 MHz, CDCl3): ¤ 182.14, 181.99, 160.29,
158.93, 158.30, 150.93, 150.80, 150.68, 145.39, 144.87,
139.45, 135.47, 134.31, 133.96, 133.90, 133.85, 133.74,
133.39, 133.29, 133.04, 132.62, 131.57, 131.06, 130.80,
130.08, 129.73, 129.50, 127.72, 127.57, 127.09, 126.99,
125.46, 125.41, 124.92, 120.27, 117.95, 116.00, 114.10,
106.02, 101.53, 95.01, 94.98, 94.97, 93.04, 69.92, 69.88
(2C), 34.53 (2C), 31.35, 31.31; IR (KBr): 2960.5(m),
2868.0(w), 1676.0(m), 1593.1(s), 1508.2(s), 1456.2(m),
General Procedure for Sonogashira Coupling Reactions.
Before beginning the reactions, all solid samples were dried in
vacuo and THF, triethylamine, and ethyldiisopropylamine were
deaerated by bubbling nitrogen or argon. An oven-dried vessel
was charged with the aryl iodide, terminal alkyne, bis(triphen-
ylphosphine)palladium(II) dichloride (5 mol %), and copper(I)
iodide (10 mol %). The vessel was evacuated and backfilled
with nitrogen or argon three times. To the vessel, THF and the
amine were added in this order. The mixture was stirred at room
temperature or 60 °C. After filtration, the solvent was evapo-
rated and the resulting residue was purified by column
chromatography on silica gel. Eluents and other slight
modifications are described below for each material.
General Procedure for the Iodination of Triazenes. To a
thick-walled oven-dried screw cap tube was added the

1234 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
Snowflake-Shaped Zn-Porphyrin Dendrimers
1244.0(s), 1157.2(s), 1016.4(w), 831.3(m) cm¹1; MS (FAB)
m/z 1996 (M+); Anal. Calcd for C138H137N3O10: C, 82.97; H,
6.91; N, 2.10%. Found: C, 83.06; H, 6.89; N, 2.12%.
80 °C for 2 days. The crude product was purified by column
chromatography on silica gel (dichloromethane) and recycling
preparative HPLC (chloroform) to afford dendrimer-2 (107 mg,
1
Dendron 9. According to the general procedure, dendron
8 (999 mg, 0.500 mmol) was reacted with iodine (127 mg,
0.500 mmol) in iodomethane (10 mL). The crude product was
purified by recrystallization from toluene-hexane to afford
dendron 9 (830 mg, 82%): yellow powder; mp 239.5-240.5 °C;
1H NMR (400 MHz, CDCl3): ¤ 8.26-8.23 (m, 2H), 8.08 (d,
J = 8.0 Hz, 1H), 7.93 (d, J = 1.7 Hz, 1H), 7.77-7.74 (m, 2H),
7.71 (s, 2H), 7.58 (d, J = 8.8 Hz, 4H), 7.49 (d, J = 8.8 Hz, 4H),
7.39-7.26 (m, 25H), 7.08-7.05 (m, 10H), 6.72 (d, J = 2.2 Hz,
4H), 6.56 (t, J = 2.2 Hz, 2H), 4.99 (s, 4H), 4.97 (s, 4H), 4.95
(s, 8H), 1.31-1.29 (m, 54H); 13C NMR (150 MHz, CDCl3): ¤
181.89, 181.68, 160.31, 159.01, 158.74, 150.93, 150.78,
145.87, 144.82, 139.17, 136.90, 135.32, 133.89, 133.71,
133.23, 133.16, 132.85, 132.44, 132.39, 131.47, 131.07,
130.83, 130.15, 129.41, 129.38, 127.74, 127.56, 127.01,
126.87, 125.44, 125.39, 124.04, 119.34, 118.56, 114.33,
114.06, 105.91, 101.51, 96.11, 94.91, 94.78, 94.49, 90.95,
69.88, 69.85, 34.53 (2C), 31.35, 31.31; IR (KBr): 2962.5(s),
2868.0(m), 1676.0(m), 1593.1(s), 1508.2(s), 1456.2(m),
1282.6(s), 1155.3(s), 1016.4(m), 831.3(s) cm¹1; MS (MALDI-
TOF) m/z 2045.41 ([M + Na]+) (Calcd for C134H127IO10Na:
2045.84); Anal. Calcd for C134H127IO10: C, 79.50; H, 6.32%.
Found: C, 79.62; H, 6.30%.
41%): dark green solid; mp 300 °C; H NMR (400 MHz, tetra-
chloroethane-d2, 80 °C): ¤ 8.99 (s, 8H), 8.23 (d, J = 8.5 Hz,
8H), 7.96 (d, J = 8.5 Hz, 8H), 7.73-7.70 (m, 24H), 7.56 (d,
J = 8.8 Hz, 16H), 7.39-6.98 (m, 180H), 6.73 (d, J = 2.2 Hz,
16H), 6.57 (t, J = 2.2 Hz, 8H), 5.01 (s, 48H), 4.98 (s, 16H); IR
(KBr): 3031.9(w), 2868.0(w), 1606.6(s), 1506.3(s), 1454.2(m),
1377.1(w), 1292.2(m), 1242.1(s), 1157.2(s), 1026.1(m),
999.1(w), 827.4(m), 734.8(m), 696.3(m) cm¹1; MS (MALDI-
TOF) m/z 6492.4233; Anal. Calcd for C460H332N4O32Zn: C,
85.09; H, 5.15; N, 0.86%. Found: C, 85.14; H, 5.11; N, 0.90%.
Dendron 12.
According to the general procedure, the
reaction of dendron 5 (95.3 mg, 0.050 mmol) with iodine (12.7
mg, 0.050 mmol) in iodomethane (1.0 mL) afforded dendron
1
12 (58 mg, 60%): white solid; mp 207.0-208.0 °C; H NMR
(400 MHz, CDCl3): ¤ 7.71 (s, 2H), 7.58 (d, J = 8.8 Hz, 4H),
7.44 (d, J = 8.8 Hz, 4H), 7.40-7.27 (m, 24H), 7.03 (d, J =
8.8 Hz, 4H), 7.01 (s, 2H), 6.92 (d, J = 8.8 Hz, 4H), 6.71 (d,
J = 2.2 Hz, 4H), 6.56 (t, J = 2.2 Hz, 2H), 4.97 (s, 4H), 4.96 (s,
8H), 4.86 (s, 4H), 1.30 (s, 54H), 0.69 (s, 9H), ¹0.15 (s, 6H);
13C NMR (75 MHz, CDCl3): ¤ 160.29, 158.67, 158.51, 151.02,
150.86, 146.12, 145.02, 139.25, 136.70, 133.95, 133.69,
132.72, 132.26, 130.75, 130.59, 130.19, 127.57, 127.42,
125.51, 125.45, 122.93, 119.95, 119.71, 114.29, 114.04,
106.17, 104.38, 101.92, 101.54, 96.80, 94.46, 90.73, 69.99,
69.94, 69.72, 34.56, 34.54, 31.33, 25.93, 16.52, ¹5.06; IR
(KBr): 2960.5(m), 2868.0(w), 1608.5(s), 1508.2(s), 1456.2(m),
1245.9(m), 1159.1(m), 1016.4(m), 827.4(s) cm¹1; MS (FAB)
m/z 1931 (M+); Anal. Calcd for C126H135IO8Si: C, 78.31; H,
7.04%. Found: C, 78.43; H, 7.07%.
Dendron 14. According to the general procedure, dendron
12 (290 mg, 0.15 mmol) was reacted with porphyrin 13
(171 mg, 0.165 mmol) in the presence of bis(triphenylphosphi-
ne)palladium(II) dichloride (9.5 mg, 0.015 mmol) and copper(I)
iodide (5.7 mg, 0.03 mmol) in ethyldiisopropylamine (1.5 mL)
and THF (6.0 mL) at room temperature overnight. The crude
product was purified by column chromatography on silica gel
(hexane-dichloromethane 3:2) to afford dendron 14 (394 mg,
90%): purple solid; mp 153-154 °C; 1H NMR (400 MHz,
CDCl3): ¤ 9.01 (d, J = 4.4 Hz, 6H), 8.96 (d, J = 4.4 Hz, 2H),
8.24 (d, J = 8.0 Hz, 2H), 8.09 (m, 6H), 7.93 (d, J = 8.0 Hz, 2H),
7.79 (m, 3H), 7.71 (d, J = 8.6 Hz, 4H), 7.69 (s, 2H), 7.48 (d,
J = 8.6 Hz, 4H), 7.40-7.30 (m, 24H), 7.10 (d, J = 8.6 Hz, 4H),
7.08 (s, 2H), 6.94 (d, J = 8.6 Hz, 4H), 6.74 (d, J = 2.0 Hz, 4H),
6.58 (t, J = 2.0 Hz, 2H), 5.01 (s, 4H), 4.97 (s, 8H), 4.87 (s, 4H),
1.53 (m, 54H), 1.31 (m, 54H), 0.71 (s, 9H), ¹0.13 (s, 6H); IR
Dendrimer-1. According to the general procedure, dendron
9 (425 mg, 0.210 mmol) was reacted with porphyrin 10
(38.7 mg, 0.050 mmol) in the presence of bis(triphenylphos-
phine)palladium(II) dichloride (12.6 mg, 0.020 mmol), and
copper(I) iodide (7.6 mg, 0.040 mmol) in ethyldiisopropyl-
amine (10 mL) and THF (20 mL) at 80 °C for 4 days. After
addition of bis(triphenylphosphine)palladium(II) dichloride
(12.6 mg, 0.020 mmol), and copper(I) iodide (7.6 mg, 0.040
mmol), the mixture was stirred 80 °C for 3 days. The crude
product was purified by column chromatography on silica gel
(dichloromethane-hexane 4:1) and recycling preparative HPLC
(chloroform) to afford dendrimer-1 (150 mg, 36%). Analytical
sample was obtained by recrystallization from toluene-aceto-
1
nitrile: brown solid; mp >300 °C; H NMR (400 MHz, tetra-
chloroethane-d2, 135 °C): ¤ 8.96 (s, 8H), 8.25-8.19 (s, 16H),
8.09 (d, J = 8.0 Hz, 4H), 7.95-7.92 (m, 12H), 7.73-7.68 (m,
32H), 7.54 (d, J = 8.8 Hz, 16H), 7.35-7.27 (m, 100H), 7.14 (s,
8H), 7.11-7.06 (m, 32H), 6.72 (d, J = 2.2 Hz, 16H), 6.58 (t,
J = 2.2 Hz, 8H), 4.97 (s, 16H), 4.94 (s, 16H), 4.91 (s, 32H),
1.31-1.29 (m, 216H); IR (KBr): 3400.3(bw), 2962.5(s),
2868.0(w), 1676.0(m), 1606.6(s), 1593.1(s), 1506.3(s),
1458.2(m), 1284.5(m), 1245.9(m), 1155.3(m), 1016.4(m),
831.3(m) cm¹1; MS (MALDI-TOF) m/z 8357.04; Anal. Calcd
for C588H532N4O40Zn: C, 84.48; H, 6.41; N, 0.67%. Found: C,
84.37; H, 6.17; N, 0.72%.
Dendrimer-2. According to the general procedure, dendron
11 (265 mg, 0.17 mmol) was reacted with porphyrin 10 (31 mg,
0.040 mmol) in the presence of bis(triphenylphosphine)palla-
dium(II) dichloride (10 mg, 0.016 mmol), and copper(I) iodide
(6.0 mg, 0.032 mmol) in ethyldiisopropylamine (10 mL) and
THF (10 mL) at 80 °C for 3 days. After addition of bis(triphen-
ylphosphine)palladium(II) dichloride (10 mg, 0.016 mmol) and
copper(I) iodide (6.0 mg, 0.032 mmol), the mixture was stirred
(KBr): 2962.5(s), 2868.0(m), 1595.0(m), 1506.3(s), 1458.1(m),
¹1
1363.6(m), 1247.9(m), 1153.4(m), 1002.9(m), 827.4(m) cm
;
MS (FAB) m/z 2840 (M+); HRMS (FAB) m/z calcd for
C196H210N4O8SiZn: 2839.5209, found: 2839.5249.
Dendron 15. According to the general procedure, dendron
14 (380 mg, 0.13 mmol) in THF (20 mL) was reacted with
TBAF (0.16 mL, 0.16 mmol) for 15 min to afford dendron
15 (340 mg, 96%): purple solid; mp 163-164 °C; 1H NMR
(400 MHz, CDCl3): ¤ 9.01 (d, J = 4.4 Hz, 6H), 8.96 (d,
J = 4.4 Hz, 2H), 8.24 (d, J = 8.0 Hz, 2H), 8.09 (m, 6H), 7.93
(d, J = 8.0 Hz, 2H), 7.79 (m, 3H), 7.71 (d, J = 8.8 Hz, 4H),

M. Kozaki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1235
7.69 (s, 2H), 7.48 (d, J = 8.8 Hz, 4H), 7.40-7.32 (m, 24H),
7.10 (d, J = 8.8 Hz, 4H), 7.08 (s, 2H), 6.98 (d, J = 8.8 Hz, 4H),
6.74 (d, J = 2.2 Hz, 4H), 6.57 (t, J = 2.2 Hz, 2H), 5.01 (s, 4H),
4.97 (s, 8H), 4.87 (s, 4H), 3.01 (s, 1H), 1.52 (m, 54H), 1.31 (m,
54H); 13C NMR (100 MHz, CDCl3): ¤ 160.31, 158.69, 158.61,
151.03, 150.97, 150.53, 150.43, 150.41, 149.76, 148.57,
148.54, 145.47, 145.01, 143.47, 141.78, 139.34, 134.44,
133.86, 133.72, 133.13, 132.62, 132.47, 132.31, 132.20,
131.44, 131.07, 130.89, 130.57, 129.90, 129.66, 129.59,
129.01, 128.20, 127.62, 127.57, 125.51, 125.47, 123.60,
123.36, 122.76, 122.57, 122.17, 120.82, 120.06, 119.69,
118.62, 114.36, 114.15, 106.23, 101.60, 96.99, 91.60, 90.15,
85.28, 82.33, 70.05, 69.96, 69.75, 35.04, 34.57, 31.76, 31.35,
31.34; IR (KBr): 2962.5(s), 2868.0(m), 1595.0(s), 1506.3(s),
1458.1(m), 1363.6(m), 1247.9(m), 1151.4(m), 1002.9(m),
831.3(m) cm¹1; MS (FAB) m/z 2725 (M+); HRMS (FAB)
m/z calcd for C190H196N4O8Zn: 2725.4345, found: 2725.4314.
Dendron-3. According to the general procedure, dendron
15 (273 mg, 0.10 mmol) was reacted with 2-iodoanthraquinone
(7) (33 mg, 0.10 mmol) in the presence of bis(triphenylphos-
phine)palladium(II) dichloride (6.3 mg, 0.010 mmol), and cop-
per(I) iodide (3.8 mg, 0.020 mmol) in ethyldiisopropylamine
(1.0 mL) and THF (5.0 mL) at room temperature overnight. The
crude product was purified by column chromatography on
silica gel (hexane-dichloromethane 1:1) to afford dendron-3
crude product was purified by column chromatography on
silica gel (hexane-dichloromethane 1:1) to afford dyad-4
(108 mg, 93%): dark purple powder; mp >300 °C; H NMR
1
(400 MHz, CDCl3): ¤ 9.02 (d, J = 4.9 Hz, 6H), 8.96 (d,
J = 4.9 Hz, 2H), 8.39 (d, J = 1.7 Hz, 1H), 8.27-8.23 (m, 5H),
8.09-8.08 (m, 6H), 7.93 (d, J = 8.3 Hz, 2H), 7.90 (dd, J = 8.3,
1.7 Hz, 1H), 7.80-7.78 (m, 5H), 7.69 (d, J = 8.6 Hz, 2H), 7.62-
7.60 (m, 6H), 1.53 (s, 36H), 1.52 (s, 18H); IR (KBr): 2962.5(s),
2868.0(m), 1676.0(m), 1593.1(s), 1506.3(s), 1458.1(m),
1284.5(m), 1363.6(m), 1284.5(m), 1151.4(m), 1002.9(m),
831.3(m) cm¹1; MS (FAB) m/z 1443 (M+); Anal. Calcd for
C100H90N4O2Zn: C, 83.11; H, 6.28; N, 3.88%. Found: C, 83.24;
H, 6.30; N, 3.73%.
Conformational Analysis. Conformational analysis was
carried out for 4. The dihedral angles of ³-systems around
acetylenic bonds were assumed to be 0 or 90°, for the sake of
simplicity. Eight possible conformational isomers were gen-
erated using Spartan software23 and the structures were
optimized using the AM1 method. The molecular orbitals in
Figure 6 were generated with an isovalue of surface = 0.001 in
order to show the ·-orbital mixing in ³*Por and ³*AQ
Estimation of Energy-Transfer Efficiencies.
.
For the
investigation of light-harvesting properties of dendrimer-2,
absolute quantum yields (ΦF) of the fluorescence from the
porphyrin cope are measured using Hamamatsu photonics
C9920 spectrometer. Energy-transfer efficiencies (ΦET) are
determined as follows: ΦET = ΦF (sensitized excitation)/ΦF
(direct excitation).
Time-Resolved Fluorescence. The picosecond time-
resolved fluorescence spectra were measured using an argon-
ion pumped Ti:sapphire laser (Tsunami) and a streak scope
(Hamamatsu Photonics). Details of the setup of the experiment
are described elsewhere.36
1
(229 mg, 78%): brown solid; mp 240-241 °C; H NMR (400
MHz, CDCl3): ¤ 9.01 (d, J = 4.6 Hz, 6H), 8.96 (d, J = 4.6 Hz,
2H), 8.30-8.27 (m, 2H), 8.24 (d, J = 7.8 Hz, 2H), 8.14 (d, J =
7.8 Hz, 1H), 8.09 (m, 6H), 8.00 (m, 1H), 7.94 (d, J = 7.8 Hz,
2H), 7.80 (m, 5H), 7.73 (d, J = 8.6 Hz, 4H), 7.70 (s, 2H), 7.58
(d, J = 8.6 Hz, 4H), 7.41-7.32 (m, 25H), 7.16 (s, 2H), 7.12 (d,
J = 8.6 Hz, 4H), 7.09 (d, J = 8.6 Hz, 4H), 6.75 (d, J = 2.0 Hz,
4H), 6.58 (t, J = 2.0 Hz, 2H), 5.03 (s, 4H), 4.99 (s, 8H), 4.98
(s, 4H), 1.53 (m, 54H), 1.32 (s, 18H), 1.30 (s, 36H); IR (KBr):
2962.5(s), 2868.0(m), 1676.0(m), 1593.1(s), 1506.3(s),
1458.1(m), 1363.6(m), 1284.5(m), 1155.3(m), 1002.9(m),
831.3(m) cm¹1; MS (FAB) m/z 2931 (M+); Anal. Calcd for
C204H202N4O10Zn: C, 83.48; H, 6.94; N, 1.91%. Found: C,
83.43; H, 6.98; N, 1.78%.
This work was partially supported by a Grant-in-Aid for
Science Research (No. 19655016) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
We thank for Hamamatsu Photonics K. K. for measurements of
fluorescence quantum yields. We also thank Prof. Yoshio Aso
(Osaka University) for helpful discussion.
Dendron 18. According to the general procedure, dendron
17 (140 mg, 0.10 mmol) in THF (10 mL) was reacted with
TBAF (0.20 mL, 0.20 mmol) for 10 min. The crude product was
purified with a short column of silica gel (hexane-dichloro-
methane 8:2) to afford dendron 18 (99 mg, 80%): purple
Supporting Information
Temperature dependence of 1H NMR spectra of 1 and 2.
UV-vis absorption of dendirmer-1 and fluorescence lifetime
measurement. This material is available free of charge via the
internet: http://www.csj.jp/journals/bcsj/.
1
powder; mp >300 °C; H NMR (400 MHz, CDCl3): ¤ 9.01 (d,
J = 4.6 Hz, 6H), 8.95 (d, J = 4.6 Hz, 2H), 8.24 (d, J = 8.2 Hz,
2H), 8.09 (m, 6H), 7.92 (d, J = 8.2 Hz, 2H), 7.95 (m, 3H), 7.67
(d, J = 8.0 Hz, 2H), 7.59 (d, J = 8.0 Hz, 2H), 7.52 (dd, J =
13.0, 8.6 Hz, 4H), 3.20 (s, 1H), 1.53 (m, 54H); IR (KBr):
2962.5(s), 2868.0(m), 1593.1(s), 1517.9(m), 1363.6(m),
1247.9(m), 1001.0(s), 798.5(m), 717.5(m) cm¹1; MS (FAB)
m/z 1237 (M+); HRMS (FAB) m/z calcd for C86H84N4Zn:
1236.5989, found: 1236.5985.
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