Photoinduced electron transfer of dialkynyldisilane-linked zinc porphyrin-[60]fullerene dyad

Article abstract of DOI:10.1246/bcsj.79.1338

A zinc porphyrin-fullerene dyad with a disilane as a σ-conjugated linker has been newly synthesized to evaluate the electron transfer ability of the oligosilane chain. Its photoinduced processes have been studied using the time-resolved fluorescence and a

Full text of DOI:10.1246/bcsj.79.1338

1338 Bull. Chem. Soc. Jpn. Vol. 79, No. 9, 1338–1346 (2006)
ꢀ 2006 The Chemical Society of Japan
BCSJ Award Article
Photoinduced Electron Transfer of Dialkynyldisilane-Linked
Zinc Porphyrin–[60]Fullerene Dyad
y
ꢀ1;
Hayato Tsuji,
Mikio Sasaki,2 Yuki Shibano,1 Motoki Toganoh,1 Takeshi Kataoka,1
yy
Yasuyuki Araki,2 Kohei Tamao,
and Osamu Ito
ꢀ1;
ꢀ2
1International Research Center for Elements Science (IRCELS), Institute for Chemical Research,
Kyoto University, Uji, Kyoto 611-0011
2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Katahira, Aoba-ku, Sendai 980-8577
Received November 15, 2005; E-mail: tsuji@scl.kyoto-u.ac.jp
A zinc porphyrin–fullerene dyad with a disilane as a ꢀ-conjugated linker has been newly synthesized to evaluate
the electron transfer ability of the oligosilane chain. Its photoinduced processes have been studied using the time-
resolved fluorescence and absorption measurements. Photoexcitation of the dyad causes the energy and/or electron
transfer from the excited singlet state of the ZnP to C60 moiety in polar solvents. The charge separation takes place
as a final step in the excited-state process to yield the radical-ion pair with a radical cation on the zinc porphyrin and
a radical anion on the fullerene, similar to other porphyrin–fullerene dyads. Its lifetime has been estimated to be
0.43–0.52 ms on the basis of the decay rate of the fullerene radical anion in polar solvents.
A variety of donor–acceptor-linked molecules and supramo-
lecules has been prepared to study the intramolecular photoin-
duced electron transfer (ET) aimed at the construction of arti-
ficial photosynthetic systems.1 In particular, intramolecular
photoinduced ET processes in porphyrinoid–fullerene-linked
systems2 have been extensively studied during the past decade.
It has been demonstrated that the photoexcitation of the well-
designed porphyrinoid–fullerene molecules causes fast elec-
tron transfer from the porphyrinoid to the fullerene to afford
long lifetime charge-separated states,3,4 which are favorable
for the subsequent ET to other chromophores or electrodes.
Porphyrins and related compounds are quite appropriate as
electron donors and photosensitizers that can mimic the natural
photosynthetic systems, in which these expanded ꢁ-electron
systems work as efficient light-harvesting pigments as well
as the donors during the early stage of the ET processes.5 Full-
erenes, such as C60, are regarded as good electron acceptors
and photosensitizers,6–8 because they exhibit some advanta-
geous features over other acceptors as follows. (1) Fullerenes
have absorptions over a wide visible-region range. (2) A long
ET takes place to form the radical anion species (C60ꢁꢂ) in
the presence of electron donors, is generated by an efficient
ꢀ
intersystem crossing from the excited singlet state (1C60 ).
ꢁꢂ
(3) The resulting C60 exhibits a characteristic transient ab-
sorption band in the 1000–1100 nm region so that it is easy
to detect its generation and to trace the excited dynamics.
9,10
(4) The small reorganization energy ꢂ of C60
accelerates
the forward ET process and decelerates the back ET process.11
According to the Marcus theory, another important factor is
the electronic coupling V that controls the ET processes, which
depends not only on the donor–acceptor distance, but also on
the electronic properties of the linker. Thus, extensive studies
have been carried out to prepare the porphyrinoid–fullerene
hybrid molecules with a variety of linkers. For example, ꢀ-
carbon-based linkers, such as amides,6,12–15 imides,16 and nor-
bornylogous bridges,17 show a large attenuation factor for the
electronic coupling, while ꢁ-framework linkers, such as oli-
goynes18 and oligothiophenes,19 work as long-range molecular
wires. These linkers are mostly composed of a carbon-based
framework and other second row elements, while a heavier
element-based framework has not yet been studied to the best
of our knowledge.
ꢀ
lifetime excited triplet state (3C60 ), from which the efficient
y Present address: Department of Chimistry, Graduate School of
Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033
Silicon is a heavier homolog of carbon and can form caten-
ated systems called oligosilanes and polysilanes. These com-
pounds are regarded as a pseudo one-dimensional molecular
wire and have attracted much attention due to their unique
photophysical and electronic properties,20 as ascribed to the
yy Present address: RIKEN Frontier Research System, 2-1
Hirosawa, Wako, Saitama 351-0198
Published on the web September 5, 2006; doi:10.1246/bcsj.79.1338

H. Tsuji et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) 1339
Ar
Ar
N
Ar
Zn
Ar
MeMe
Si Si
MeMe
(5 mol amt.)
(6 mol amt.)
I
CHO
Me Me
Si Si
Me Me
Me Me
Si Si
Me Me
N
N
N
N
N
N
N
N
N
N
CHO
Zn
Ar
Ar
I
Zn
Ar
Ar
Pd(PPh3)4 (10 mol%)
CuI (15 mol%)
THF / Et3N
N
[PdCl2(PPh3)2] (15 mol%)
CuI (40 mol%)
Ar
THF / Et3N
29% (2 steps)
t-Bu
t-Bu
Ar
Ar =
Me
C60 (5 mol amt.)
Me Me
N
N
N
N
N-methylglycine (50 mol amt.)
N
Si Si
Zn
Ar
Ar
Me Me
toluene
77%
ZnP–Si2–C60
Scheme 1. Synthesis of the disilane-linked zinc porphyrin–fullerene dyad Zn–Si2–C60.
Ar
Me
Me Me
Si Si
N
N
N
N
N
Zn
Ar
Ar
Me Me
ZnP–Si2–C60
Ar
Zn
Ar
Me
Me Me
Si Si
Me Me
Me Me
Si Si
Me Me
N
N
N
Ar
N
N
ZnP–Si2
Si2–C60
Chart 1. Structures of ZnP–Si2–C60, ZnP–Si2, and Si2–C60.
Fig. 1. (a) MM2 optimized structure of two possible con-
formers of ZnP–Si2–C60. (b) Frontier orbitals of the
extended conformer based on B3LYP/6-31G calcula-
tions. Green = C, white = H, blue = N, orange = Si,
and pink = Zn.
ꢀ-electron delocalization over the silicon framework (ꢀ-conju-
gation).21 Construction of the porphyrin–fullerene-based intra-
molecular photoinduced ET system linked by a silicon chain is
of interest in the light of the evaluation of the intramolecular
ET ability along the ꢀ-conjugated Si–Si bonds. Since such
an ET system is unprecedented,22–25 we now report the synthe-
sis of the disilane-linked zinc porphyrin–fullerene dyad ZnP–
Si2–C60 (Chart 1) and its excited state dynamics on the basis
of time-resolved absorption and fluorescence measurement
studies.
at the ZnP moiety and the C60 moiety, respectively. Although
the HOMO orbitals delocalize over the ZnP and the alkyne
moieties, the radical cation and the radical anion in the charge-
separated state of the dyad are expected to mainly locate at the
ZnP moiety and exclusively at the C60 moiety, respectively.
The center-to-center distances between these two radical-ion
Results and Discussion
˚
centers (RCC) are evaluated to be 22.2 and 8.0 A for the extend-
Synthesis. The disilane-linked zinc porphyrin–fullerene
dyad ZnP–Si2–C60 was prepared as shown in Scheme 1.
The zinc porphyrin (ZnP) and p-formylphenyl moieties were
successively introduced into each end of the 1,2-diethynyl-
1,1,2,2-tetramethyldisilane by Sonogashira coupling26 fol-
lowed by the Prato reaction27 using N-methylglycine and C60
to afford the target product ZnP–Si2–C60. The reference com-
pound without the C60 moiety, ZnP–Si2, was prepared in a sim-
ilar manner. The structures of these compounds synthesized in
the present study were characterized by 1H, 13C, and 29Si NMR
and FAB mass spectra.
Conformer Considerations. The geometry optimization
of ZnP–Si2–C60 was performed by MM2 calculations. Two
of the likely structures (extended and folded conformers) are
shown in Fig. 1a, and the frontier molecular orbitals (MOs)
of the extended conformer calculated using 6-31G basis set
are shown in Fig. 1b. The HOMO and LUMO are localized
ed and the folded conformers, respectively.
In the 1H NMR spectrum of ZnP–Si2–C60, the three sets of
tert-butyl protons became magnetically unequivalent and are
shifted downfield by 0.6 ppm at a maximum, whereas the ꢃ
protons of the porphyrin ring are shifted upfield by 0.03–
0.15 ppm as compared to those of ZnP–Si2. The former phe-
nomenon can be understood by the deshielding effect of the
C60 moiety in close proximity.28 The latter case can also be
explained by the shielding effect of the five-membered ring
moiety of C6029 or the reduction in the ring current of the por-
phyrin ring by the attachment of an electron-withdrawing
C60.30 Based on these results, the folded conformer should
exist, but the ratio of the extended and folded conformers
could not be determined due to the fast equilibrium between
the conformers.
Electrochemistry Measurements. The oxidation potential
(Eox) and the reduction potential (Ered) of ZnP–Si2–C60 were

1340 Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) BCSJ AWARD ARTICLE
ꢀ
1
ꢀ
Table 1. Free-Energy Change for Charge Separation from 1ZnP –Si2–C60 (ꢀGCS( ZnP ), ZnP–Si2– C60
1
ꢀ
)
3
ꢀ
3
ꢀ
1
ꢀ
3
ꢀ
3
ꢀ
(ꢀGCS( C ), ZnP –Si2–C60 (ꢀGCS( ZnP ), and ZnP–Si2– C60 (ꢀGCS( C )) and for Charge Recombi-
)
)
60
60
nation (ꢂꢀGCR) to Ground State of ZnP–Si2–C60
a)
a)
a)
a)
a)
Solvent
ꢂꢀGCS(1ZnP
ꢀ
ꢂꢀGCS(1C
ꢀ
ꢂꢀGCS(3ZnP
ꢀ
ꢂꢀGCS(3C
ꢀ
)
60
ꢂꢀGCR
)
)
)
60
/eV
/eV
/eV
/eV
/eV
THF
BN
0.45
0.66
0.18
0.39
0.00
0.21
ꢂ0:07
1.57
1.36
0.14
ꢀ
a) Calculated from Eqs. 1–3 using the following values: ꢀE0{0 ¼ 2:02 eV for 1ZnP –Si2–C60;
ꢀ
ꢀ
ꢀE0{0 ¼ 1:75 eV for ZnP–Si2–1C60 ; ꢀE0{0 ¼ 1:57 eV for 3ZnP –Si2–C60;29 ꢀE0{0 ¼ 1:50 eV for ZnP–
Si2–3C60
;
E
¼ 0:34 V and Ered ¼ ꢂ1:06 V vs Fc/Fcþ of ZnP–Si2–C60 in BN; the radii of the
ꢀ
12,13,15,16
ox
˚
porphyrin (Rþ ¼ 4:8 A) and fullerene (R ¼ 4:7 A) and the center-to-center distance (RCC ¼ 22:2 A)
ꢂ
˚
˚
evaluated from Fig. 1; and permittivities of THF (7.58) and BN (25.2).
Table 2. UV–Vis–NIR Absorption and Fluorescence in BN of ZnP–Si2–C60 and ZnP–Si2 at Room
Temperature
Absorption
A,max/nm ("/104 Mꢂ1 cmꢂ1
445 (27),
535 (0.5), 577 (1.2), 625 (1.6)
706 (0.04)
442 (52),
Fluorescence
em/nm
Compound
Moiety
ꢂ
)
ꢂ
ꢄb)
ZnP–Si2–C60
ZnP
C60
632, 684 (sh)
2:0 ꢃ 10ꢂ3
—
a)
—
ZnP–Si2
ZnP
627, 681
7:3 ꢃ 10ꢂ2
535 (0.5), 577 (1.7), 625 (2.5)
a) Not observed in BN. b) Fluorescence quantum yields of ZnP moiety of the dyad (ꢄdyad) and the refer-
ence compound (ꢄref).
measured in benzonitrile (BN) using the square-wave voltam-
metry method in the presence of ferrocene as the internal
standard. Eox (þ0:34 V vs Fc/Fcþ) and Ered (ꢂ1:06 V vs Fc/
Fcþ) correspond to the first oxidation of the ZnP moiety and
the first reduction of the C60 moiety, respectively. From the
observed Eox and Ered values, the free-energy changes for
the charge recombination (ꢀGCR) and the charge separation
(ꢀGCS) can be calculated using the Weller’s equations;31
ꢂꢀGCR ¼ eðEox ꢂ EredÞ þ ꢀGS;
ð1Þ
ꢀꢁ
ꢂꢁ
ꢂ
e2
1
1
1
1
ꢀGS ¼
þ
ꢂ
4ꢁ"0 2Rþ 2Rꢂ RCC
"
S
ꢁ
ꢂꢁ ꢂꢃ
1
1
1
ꢂ
þ
;
ð2Þ
ð3Þ
2Rþ 2Rꢂ
"
r
ꢂꢀGCS ¼ ꢀE0{0 ꢂ ðꢂꢀGCRÞ;
where e and "0 are the elementary charge and vacuum permit-
tivity, respectively; Rþ and Rꢂ are the radii of the ion radicals
of the ZnP and C60 moiety, respectively; "S and "r are referred
to as the static permittivity of the solvent used for the electro-
chemical measurement and that used for the spectroscopic
measurements; ꢀE0{0 is referred to as the energy of the 0–0
transition. The ꢀGCS and ꢀGCR values for ZnP–Si2–C60 in
THF and BN are summarized in Table 1. In polar solvents,
the charge-separation (CS) processes via the excited singlet
Fig. 2. UV–vis–NIR absorption spectra of ZnP–Si2–C60
(red line), ZnP–Si2 (blue line), and Si2–C60 (green line)
in benzonitrile. Inset: Expanded spectra in the 480–780
nm region.
spectra of ZnP–Si2–C60 (red line) and ZnP–Si2 (blue line)
are shown in Fig. 2 together with that of Si2–C60 (green line)
in BN at room temperature. The representative absorption
peaks (ꢂA,max) are summarized in Table 2. The absorption
peak of the Soret band of ZnP–Si2–C60 (ꢂA,max ¼ 445 nm)
showed a red-shift by 3 nm and the extinction coefficient
was about half of that of ZnP–Si2, whereas the absorption
maximum wavelengths of the Q bands of ZnP–Si2–C60 and
ZnP–Si2 are essentially identical to each other. The spectrum
of ZnP–Si2–C60 slowly tails off up to 800 nm where the refer-
ꢀ
states of the ZnP moiety in ZnP–Si2–C60 (1ZnP –Si2–C60)
ꢀ
and the C60 moiety (ZnP–Si2–1C60 ) are exothermic and are
expected to occur. The CS process via the excited triplet states
ꢀ
of the C60 in ZnP–Si2–C60 (ZnP–Si2–3C60 ) is slightly endo-
thermic in THF and exothermic in BN. The CS would occur by
a super-exchange mechanism via the disilane linkage on the
basis of the relative orbital energy level of each component.
Steady-State Absorption.
The steady-state absorption

H. Tsuji et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) 1341
Fig. 4. Time-resolved fluorescence spectra of ZnP–Si2–
C60 in BN (a) and THF (b) at 0.1 ns (green line) and
1.0 ns (pink line) after excitation with 400 nm laser light,
intensities are normalized at the peak. Inset: Fluorescence
time-profile at the ZnP moiety.
Fig. 3. Fluorescence spectra of ZnP–Si2–C60 (solid line)
and ZnP–Si2 (dotted line) in BN excited at 553 nm;
(a) proportional to the fluorescence quantum yield and
(b) normalized at a peak wavelength.
ZnP–Si2–C60 in THF and BN recorded with a time-correlated
single-photon-counting apparatus with excitation at 400 nm.
In all solvents, the initial fluorescence spectrum measured at
0.1 ns after the laser excitation was almost identical to that
of ZnP–Si2. In the spectrum at 1.0 ns, at which the fluores-
cence of the ZnP moiety had mostly disappeared, the fluores-
cence from the C60 moiety was hardly detected in the 700–
750 nm region due to the low intensity.
ence compounds have no absorption. Similar absorption spec-
tra were observed in other solvents such as THF. This band can
be assigned to the CT absorption band of the folded conformer
having an interaction between the ZnP- and the C60 moiety in
the ground state due to the geometrical proximity of these two
chromophores, which can directly form an exciplex by photo-
excitation.32 The variable temperature (VT) absorption spectra
of ZnP–Si2–C60 in toluene (Supporting Information) indicates
increase in the interaction between ZnP and C60; that is, the ra-
tio of the folded conformer increases upon cooling even
though the ratio cannot be precisely determined.
Although the fluorescence decays of the ZnP moiety of
ZnP–Si2–C60 would consist of multicomponents as shown in
the inset of Fig. 4, we analyzed the primary part using a single
exponential fitting, which well reproduced the decay curve
more than 80% of the entire time profile. The fluorescence life-
times (ꢅF,dyad) of the ZnP moiety in BN and THF are summa-
rized in Table 3, in which ꢅF,dyad in toluene is also listed. The
relative fluorescence quantum yield of the dyad to the refer-
ence compound ZnP–Si2 based on the fluorescence lifetimes
(ꢅF,dyad=ꢅF,ref) was 0.08 in BN. This value is greater than
that estimated from the steady-state fluorescence measurement
(ꢄdyad=ꢄref ¼ 0:03). The difference between the two values
also indicates the existence of the non-fluorescent folded con-
former, and we can assign the observed emission mainly to the
fluorescence of the extended conformer. This assignment is
based on the assumption that the isomerization of the extended
conformer to the folded conformer would be slow enough
Steady-State and Time-Resolved Fluorescence and the
Fluorescence Lifetime. Figure 3 shows the steady-state fluo-
rescence spectra of ZnP–Si2–C60 and ZnP–Si2 in BN with ex-
citation at 553 nm, where the extinction coefficients of the two
samples are equal. These results are summarized in Table 2.
For the dyad, the fluorescence of the ZnP moiety (600–670
nm) was exclusively observed,8,9,33 whereas that of the C60
moiety, normally appearing around 720 nm,34 was hardly rec-
ognized. This observation is due to the spectral overlap with
the shifted ZnP fluorescence and/or the CS from ZnP–Si2–
ꢀ
1C60 in BN. The relative fluorescence quantum yield of
ꢀ
1ZnP for ZnP–Si2–C60 to ZnP–Si2, based on the integration
1
ꢀ
of the spectra from 560 to 750 nm, (ꢄdyad=ꢄref) was evaluated
to be 0.03 (Fig. 3a).
Figure 4 shows the time-resolved fluorescence spectra of
relative to the decay time scale of ZnP –Si2–C60, which is
supported by the fluorescence lifetime measurements in frozen
media (Supporting Information). The folded conformer, on the

1342 Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) BCSJ AWARD ARTICLE
Table 3. Fluorescence Lifetimes (ꢅF), Quenching Rates (kq(1ZnP ), Quenching Quantum Yields
ꢀ
ꢀ
(ꢁq(1ZnP )), and Rate Constant of Charge Recombination (kCR), and Lifetime of Radical-Ion Pair (ꢅRIP
)
ꢀ a)
ꢅF
/ns (%)
kq(1ZnP )
kCR
/sꢂ1
ꢅRIP
/ns
ꢀ b)
ꢁq(1ZnP )
Compound
Solvent
/sꢂ1
ZnP–Si2
ZnP–Si2–C60
Toluene
Toluene
THF
2.10 (100)
0.23 (94)
0.19 (81)
0.16 (88)
—
—
—
—
—
—
520
430
3:8 ꢃ 109
4:8 ꢃ 109
5:8 ꢃ 109
0.89
0.91
0.92
1:9 ꢃ 106
BN
2:3 ꢃ 106
a) kqð ZnP Þ ¼ 1=ꢅF,dyad ꢂ 1=ꢅF,reftol, where ꢅF,ref is the lifetime of ZnP–Si2 in toluene (2.10 ns).
1
ꢀ
tol
1
ꢀ
tol
b) ꢁqð ZnP Þ ¼ ð1=ꢅF,dyad ꢂ 1=ꢅF,ref Þ=ð1=ꢅF,dyadÞ.
other hand, would form the exciplex upon photoexcitation and
its excited-state dynamics should be too fast for the present
time scale. Thus, the origin of the observed quenching of
ꢀ
1ZnP –Si2–C60 in a polar solvent is based on the CS from
1
ꢀ
the ZnP moiety to the C60 moiety (see transient absorption
below). In a nonpolar solvent, fluorescence quenching is attrib-
uted to the CS via 1ZnP followed by CR to C60 , which
may be competitive to direct energy transfer to the ZnP–Si2–
ꢀ
1
ꢀ
ꢀ
14
ꢀ
In polar solvents, the rates (kCS(1ZnP )) and the
1C60
.
quantum yields (ꢁCS(1ZnP )) of the charge separation from
ꢀ
ꢀ
ꢀ
1ZnP –Si2–C60 were calculated to be equal to kq(1ZnP ) and
ꢁq(1ZnP ) on assuming the predominant CS process (Table 3)
ꢀ
according to the previous report.14
Transient Absorption Spectra.
Figure 5a shows the
nanosecond transient absorption spectra of ZnP–Si2–C60 in
BN measured at room temperature with an excitation wave-
length at 532 nm, together with the time-profiles at 1000 nm
in the Ar-saturated (red line) and O2-saturated (black line) so-
lutions in the inset. The absorption band around 1000 nm is
ꢁꢂ
characteristic of the C60 species.35 Although the absorption
time profile at 1000 nm shows an apparent rise, this is due to
the negative absorption by the scattered light. The other band
in the 680–500 nm region is attributed to the ZnPꢁþ species,36
which is partially hidden by the bleaching of the ground state-
ꢀ
ꢀ
ZnP absorption in addition to the 3ZnP and the 3C60 absorp-
tions. These spectral aspects unambiguously indicate the gen-
eration of the radical-ion pair (ZnPꢁþ–Si2–C60ꢁꢂ) by photoin-
ducꢁeꢂd CS in a polar solvent suchꢁꢂas BN. The absorption of the
C60 moiety in ZnPꢁþ–Si2–C60 quickly disappeared in the
O2-saturated BN, which implies that ZnPꢁþ–Si2–C60ꢁꢂ reaches
an equilibrium with ZnP–Si2–3C60 and ZnP –Si2–C60 (the
transient absorption spectra of these excited triplet states in tol-
uene are shown in Supporting Information), because the reac-
tion between the radical ions and O2 was reported to be slow.37
We assume that the equilibriums between the CS states and the
triplet states are very fast while the decay of the triplet states is
very slow, and therefore, the steady-state approximation can
be applicable for the lifetime analysis of the radical-ion pair.
Thus, the lifetime of the radical-ion pair (ꢅRIP) was determined
by single-exponential fitting to be 430 ns from the decay rate
ꢀ
3
ꢀ
Fig. 5. Transient absorption spectra of ZnP–Si2–C60 in BN
(a) and THF (b) at 0.1 ms (blue-filled circle) and 1.0 ms
(open circle) excited with 532 nm laser light at room
temperature. Insets: Absorption time-profiles at 1000 nm
under Ar-saturated solution (red line) and O2-saturated
solution (black line).
at 1000 and 650–700 nm are broadened as compared to those
in BN. This broadening is due to the overlap of the absorption
ꢀ
bands with those of the triplet species, 3ZnP –Si2–C60 and
of the C60 moiety (1000 nm) in ZnP–Si2–C60 as shown in
ZnP–Si2–3C60 , and indicates that the equilibrium between
ꢁꢂ
ꢀ
the inset of Fig. 5a. This single-exponential decay profile
means that there is small spin coupling between these radical
ion centers. The same decay rate was obtained by the decay
time-profile analysis at 660 nm in a similar manner.
the charge-separated state and the excited triplet states shifts
toward the latter in a less polar solvent. The lifetime of the rad-
ical ion pair ꢅRIP in THF was estimated to be 520 ns based on
the decay rate at 1000 nm since the absorption at 660 nm over-
lapped that of the triplet species. The slightly longer ꢅRIP in
THF than that in BN indicates that the charge recombination
Figure 5b shows the transient absorption spectra of ZnP–
ꢁꢂ
Si2–C60 in THF. The absorption bands of ZnPꢁþ–Si2–C60

H. Tsuji et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) 1343
tetramethylsilane (0.0 ppm) as an external standard. Mass spectra
were performed at the Mass Spectrum Division of Institute for
Chemical Research, Kyoto University. Recycle preparative gel
permeation chromatography (GPC) was performed using polysty-
rene-gel columns (JAIGEL 1H and 2H, LC-908, Japan Analytical
Industry) with toluene as an eluent. Thin-layer chromatography
(TLC) was performed on plates coated with 0.25 mm thickness
of silica gel 60F-254 (Merck). Column chromatography was per-
formed by using Kieselgel 60 (70–230 mesh, Merck) unless oth-
erwise stated. All reactions were carried out under nitrogen unless
otherwise stated. Dry THF was freshly distilled from sodium/ben-
zophenone under a nitrogen atmosphere before use.
Scheme 2. The energy diagram of ZnP–Si2–C60 on the basis
of thermodynamic data and possible processes.
Nanosecond transient absorption experiments were carried out
using the 532 nm light from SHG of a Nd:YAG laser (Spectra-
Physics, Quanta-Ray GCR-130, 6 ns fwhm) as an excitation
source. Monitoring light from a pulsed Xe-lamp was detected with
a Ge-APD detector in the near-IR region (600–1200 nm) and with
a Si-PIN photodiode in the visible region (400–1000 nm). All the
samples in a quartz cell (1 cm ꢃ 1 cm) were deaerated by bubbling
argon gas through the solution for 15 min. The time-resolved fluo-
rescence spectra were measured by a single photon counting
method using a streakscope (Hamamatsu Photonics, C4334-01)
equipped with a polychrometer and a SHG (400 nm) of a Ti:sap-
phire laser (Spectra-Physics, Tsunami 3950-L2S, 1.5 ps fwhm) as
an excitation source. Steady-state absorption spectra were mea-
sured on a spectrophotometer (Jasco V570 DS). Steady-state fluo-
rescence spectra were measured on a spectrofluorophotometer
(Shimadzu RF-5300 PC). Osteryoung square wave voltammetry
(OSWV) was carried out using a potentiostat (BAS, CV-50W)
in a conventional three-electrode cell equipped with Pt working
electrode, counter electrode, and an Ag/AgCl reference electrode
in the presence of tetrabutylammonium perchlorate as an electro-
lyte at room temperature.
Synthesis. ZnP–Si2–CHO: A solution of {5-iodo-10,15,20-
tris[3,5-di(t-butyl)phenyl]porphinato}zinc(II)39 (206 mg, 0.194
mmol) in THF (50 mL) was deoxygenized by bubbling with nitro-
gen for 30 min. To this solution were added 1,2-diethynyl-1,1,2,2-
tetramethyldisilane (191 mg, 1.15 mmol), dichlorobis(triphenyl-
phosphine)palladium(II) (21.4 mg, 0.0305 mmol), copper(I) iodide
(14.9 mg, 0.0782 mmol), and triethylamine (1.0 mL, 7.17 mmol).
After being stirred for 4 h at room temperature, the reaction mix-
ture was evaporated, suspended in CH2Cl2, and filtered through a
pad of silica gel (Wakogel 50C18). Evaporation gave 403 mg of a
crude product as a purple solid.
occurs in the Marcus inverted region; the large negative ꢀGCR
value decreases the rate of the charge recombination.
Energy Diagrams. On the basis of the electrochemistry
´
measurements, the Jabłonski diagram and the possible decay
processes for the extended conformer of ZnP–Si2–C60 were
drawn (Scheme 2) although there must be other decay paths,
such as isomerization to the folded conformer followed by
the immediate formation of the non-emissive exciplex, which
are not detectable on the present time scale. As demonstrated
by the time-resolved absorption measurements, the CS from
ꢀ
1ZnP –Si2–C60 obviously occurs in polar solvents. However,
ꢀ
clear-cut evidence of the CS from ZnP–Si2–1C60 was not
obtained from the spectroscopic studies although it is also pos-
´
sible in polar solvents based on the Jabłonski diꢁaꢂgram. The
lifetimes of the radical-ion pair ZnPꢁþ–Si2–C60
in polar
solvents are on the sub-microsecond order. The radical-ion
pairs are in equilibrium with the excited triplet species as sug-
gested by the transient absorption experiment in the O2-satu-
rated solution. The present decay sequences are qualitatively
consistent with those determined for other porphyrin–fullerene
dyads.14,18,38
Conclusion
A novel zinc porphyrin–fullerene dyad covalently connect-
ed by a disilane bridge (ZnP–Si2–C60) was synthesized and
1
fully identified by its H, 13C, and 29Si NMR and FAB mass
1
ꢀ
spectra. The quenching paths from ZnP –Si2–C60 are most
probably attributed to the charge separation in polar solvents.
The transient absorption spectra in Ar-saturated polar solvents
clearly show the formation of the radical ion pair. The dialky-
nyldisilane linkage plays an important role as a molecular
wire, but the characteristic contribution of the silicon bridge
to the photoexcited dynamics is not appreciable at the present
stage. We are now designing longer multi-Si linkages to con-
nect the ZnP and C60 units as well as investigating the contri-
bution of the conformational effect of the oligosilane linkage.
A solution of thus prepared crude product (361 mg, 0.174
mmol) in THF (50 mL) was deoxygenized by bubbling with nitro-
gen for 30 min. To this solution were added p-iodobenzaldehyde
(249 mg, 1.07 mmol), tetrakis(triphenylphosphine)palladium(0)
(22.4 mg, 0.0194 mmol), copper(I) iodide (4.6 mg, 0.0242 mmol),
and triethylamine (2.0 mL, 14.3 mmol). After being stirred for 12 h
at room temperature, the reaction mixture was evaporated and sus-
pended in CH2Cl2 and filtered through a pad of silica gel. After
the evaporation, the residue was subjected to GPC (toluene as an
eluent, tR ¼ 74 min). The collected fraction was evaporated and
subjected to silica-gel column (hexane/toluene = 1/1, then 1/2)
to give 61.2 mg (0.0507 mmol, 29% yield in 2 steps) of ZnP–Si2–
Experimental
General. 1H and 13C spectra were measured with JEOL EX-
270 (270 MHz for 1H and 67.94 MHz for 13C) spectrometer or
CHO as a purple solid. H NMR (300 MHz) ꢆ 0.74 (s, 6H), 0.86
1
1
Varian Mercury (300 MHz for H) in C6D6. Chemical shifts are
1
(s, 6H), 1.53 (s, 18H), 1.56 (s, 36H), 7.39 (d, J ¼ 8:1 Hz, 2H),
8.02 (t, J ¼ 1:8 Hz, 1H), 8.05 (t, J ¼ 1:8 Hz, 2H), 8.39–8.41 (m,
6H), 9.19–9.24 (m, 4H), 9.31–9.32 (m, 3H), 10.25 (d, J ¼ 4:5 Hz,
2H). 13C NMR (67.81 MHz) ꢆ ꢂ2:33, ꢂ2:06, 32.03, 35.29, 35.32,
97.51, 98.40, 99.53, 107.68, 112.27, 121.16, 123.59, 124.69,
reported in ꢆ ppm with reference to internal solvent peak for H
(C6HD5: 7.20 ppm) and 13C (C6D6: 128.0 ppm). 29Si NMR spectra
were recorded with JEOL EX-270 (59.62 MHz for 29Si) spectrom-
eter with the use of the proton-decoupled INEPT technique using

1344 Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) BCSJ AWARD ARTICLE
127.84, 129.14, 129.29, 130.17, 130.22, 131.19, 132.39, 132.44,
132.80, 133.43, 134.68, 135.72, 142.71, 142.88, 149.05, 149.08,
150.53, 150.61, 151.35, 153.03, 190.12. 29Si NMR (53.67 MHz)
ꢆ ꢂ36:04, ꢂ35:47. HRMS(FAB): Calcd for C77H88N4OSi2Zn:
1204.5788. Found: 1204.5822.
mL) was deoxygenized by bubbling with nitrogen for 30 min. To
this solution were added 1-ethynyl-1,1,2,2-tetramethyl-2-(phen-
yl)ethynyldisilane (180.6 mg, 0.745 mmol), dichlorobis(triphenyl-
phosphine)palladium(II) (11.4 mg, 0.0162 mmol), copper(I) iodide
(8.6 mg, 0.0452 mmol), and triethylamine (0.5 mL, 3.59 mmol).
After being stirred for 4 h at room temperature, the reaction mix-
ture was evaporated, suspended in CH2Cl2, and filtered through a
pad of silica gel. After evaporation, the residue was subjected to
silica-gel column chromatography (hexane/toluene = 2/1, then
1/1). The collected fraction was evaporated and subjected to
GPC (toluene as an eluent, tR ¼ 74 min) to give 52.2 mg (0.0443
mmol, 47% yield) of ZnP–Si2 as a purple solid. 1H NMR (300
MHz) ꢆ 0.76 (s, 6H), 0.87 (s, 6H), 1.52 (s, 18H), 1.56 (s, 36H),
6.93–6.95 (m, 3H), 7.02–7.06 (m, 2H), 7.50–7.53 (m, 2H), 7.59–
7.62 (m, 2H), 8.01 (t, J ¼ 1:8 Hz, 1H), 8.04 (t, J ¼ 1:8 Hz, 2H),
8.39–8.40 (m, 4H), 8.41–8.42 (m, 2H), 9.20–9.24 (m, 4H), 9.29
(d, J ¼ 4:8 Hz, 2H), 10.27 (d, J ¼ 4:5 Hz, 2H). 13C NMR (67.81
MHz) ꢆ ꢂ2:14, ꢂ2:10, 31.95, 31.98, 35.26, 35.29, 92.92, 99.04,
100.06, 108.96, 111.73, 121.16, 123.48, 123.97, 124.48, 127.84,
128.18, 128.56, 128.79, 129.20, 130.11, 130.16, 131.41, 132.31,
132.38, 132.71, 133.41, 142.68, 142.84, 149.08, 150.51, 151.24,
153.09. 29Si NMR (53.67 MHz) ꢆ ꢂ36:04, ꢂ35:99. HRMS(FAB):
Calcd for C76H88N4Si2Zn: 1176.5839. Found: 1176.5817.
Si2–C60: A solution of 1-(p-formylphenyl)-1,1,2,2-tetrameth-
yl-2-phenyldisilane40 (27.2 mg, 0.0911 mmol), C60 (324 mg, 0.449
mmol), and N-methylglycine (407 mg, 4.54 mmol) in toluene (400
mL) was deoxygenized by bubbling with nitrogen for 40 min and
heated under reflux. After being stirred for 14 h in the dark at this
temperature, the reaction mixture was allowed to cool to room
temperature and filtered through a pad of silica gel. After the
evaporation, the residue was subjected to silica-gel column chro-
matography (hexane/toluene = 4/1, then 1/1). The collected
fraction was evaporated and subjected to GPC (toluene as an elu-
ent, tR ¼ 100 min). After evaporation, the residue was washed
with MeOH to give 39.9 mg (0.0381 mmol, 42% yield) of Si2–
C60 as a brown solid. 1H NMR (300 MHz) ꢆ 0.24 (s, 3H), 0.26
(s, 3H), 0.32 (s, 6H), 2.54 (s, 3H), 3.83 (d, J ¼ 9:3 Hz, 1H), 4.51
(d, J ¼ 9:3 Hz, 1H), 4.72 (s, 1H), 7.30–7.34 (m, 3H), 7.47 (d, J ¼
7:8 Hz, 2H), 7.7–7.8 (br, 2H). 13C NMR (67.81 MHz) ꢆ ꢂ3:99,
ꢂ3:82, ꢂ3:77, 39.75, 69.44, 69.88, 77.80, 83.64, 128.82, 128.99,
134.15, 134.43, 136.11, 136.33, 136.87, 137.08, 137.86, 138.71,
139.67, 139.88, 140.18, 140.44, 140.49, 141.84, 141.97, 142.09,
142.28, 142.37, 142.43, 142.55, 142.61, 142.89, 143.01, 143.37,
143.50, 144.70, 144.88, 145.00, 145.44, 145.56, 145.69, 145.77,
145.82, 145.87, 146.00, 146.18, 146.36, 146.48, 146.53, 146.61,
146.76, 147.17, 147.53, 153.74, 154.02, 154.34, 156.73. 29Si NMR
(53.67 MHz) ꢆ ꢂ21:77, ꢂ21:42. HRMS(FAB): Calcd for C79H27-
NSi2: 1045.1682. Found: 1045.1709.
ZnP–Si2–C60: A solution of ZnP–Si2–CHO (30.6 mg, 0.0253
mmol), C60 (94.0 mg, 0.130 mmol), and N-methylglycine (115 mg,
1.28 mmol) in toluene (150 mL) was deoxygenized by bubbling
with nitrogen for 30 min and heated under reflux. After being stir-
red for 12 h in the dark at this temperature, the reaction mixture
was allowed to cool to room temperature and filtered through a
pad of silica gel. After the evaporation, the residue was subject-
ed to silica-gel column chromatography (hexane/toluene = 4/1,
then 2/3). The collected fraction was evaporated and subjected
to GPC (toluene as an eluent, tR ¼ 75 min) to give 38.1 mg
(0.0195 mmol, 77% yield) of ZnP–Si2–C60 as a dark purple solid.
1H NMR (300 MHz) ꢆ 0.70 (s, 3H), 0.73 (s, 3H), 0.76 (s, 6H), 1.58
(s, 18H), 1.59 (s, 18H), 1.62 (s, 18H), 2.45 (s, 3H), 3.03 (d, J ¼
9:6 Hz, 1H), 3.82 (s, 1H), 3.95 (d, J ¼ 9:0 Hz, 1H), 7.45–7.48 (m,
2H), 7.70 (d, J ¼ 8:7 Hz, 2H), 8.02 (t, J ¼ 1:8 Hz, 1H), 8.04 (t,
J ¼ 1:7 Hz, 2H), 8.32–8.33 (m, 2H), 8.39 (m, 2H), 8.45–8.46
(m, 2H), 9.05 (d, J ¼ 4:5 Hz, 2H), 9.12 (d, J ¼ 4:5 Hz, 2H), 9.26
(d, J ¼ 4:5 Hz, 2H), 10.15 (d, J ¼ 4:5 Hz, 2H). 13C NMR (67.81
MHz) ꢆ ꢂ2:31, ꢂ2:21, ꢂ2:03, 32.17, 32.36, 35.39, 39.60, 68.22,
69.14, 76.46, 82.35, 93.72, 99.12, 99.94, 109.04, 111.40, 121.14,
123.52, 123.77, 124.84, 126.40, 127.21, 127.84, 128.18, 128.77,
128.92, 129.07, 129.27, 129.53, 130.11, 130.24, 130.32, 130.52,
131.32, 132.46, 133.00, 133.32, 133.79, 134.22, 134.30, 135.49,
135.67, 137.68, 137.91, 138.14, 138.63, 139.07, 139.19, 139.77,
139.95, 140.08, 140.14, 140.57, 140.77, 140.92, 141.21, 141.26,
141.30, 141.43, 141.48, 141.71, 141.82, 141.86, 142.04, 142.65,
142.73, 143.22, 143.42, 143.57, 143.73, 143.78, 144.06, 144.14,
144.29, 144.36, 144.41, 144.47, 144.64, 145.10, 145.18, 145.29,
145.39, 145.43, 145.48, 146.58, 149.05, 149.10, 150.28, 150.46,
151.15, 152.12, 152.19, 152.57, 152.88, 155.41. 29Si NMR (53.67
MHz) ꢆ ꢂ35:74, ꢂ35:69. FAB-MS: 1953 (M þ Hþ).
1-Ethynyl-1,1,2,2-tetramethyl-2-(phenyl)ethynyldisilane: To
a solution of 1,2-dichloro-1,1,2,2-tetramethyldisilane (3.70 g, 19.8
mmol) in THF (20 mL) was added 18.5 mL (20.2 mmol) of a 1.09
M solution of phenylethynylmagnesium bromide in THF dropwise
over 15 min at 0 ꢄC. Upon completion of the addition, the reaction
mixture was allowed to warm up to room temperature. After being
stirred for 4 h, the resulting mixture was cooled to 0 ꢄC. To this
mixture was added 40 mL (20.0 mmol) of a 0.5 M solution of eth-
ynylmagnesium chloride in THF dropwise over 20 min at 0 ꢄC.
Upon completion of the addition, the reaction mixture was al-
lowed to warm to room temperature. After being stirred for 19 h,
the resulting mixture was evaporated to remove THF. To the res-
idue were added H2O (50 mL) and Et2O (50 mL). The resulting
biphasic mixture was separated and the aqueous layer was extract-
ed with Et2O (4 ꢃ 50 mL). The combined organic layer was wash-
ed with brine (100 mL) and dried over MgSO4. After filtration and
evaporation, the residue was distilled under reduced pressure (bp
130–135 ꢄC/8 mmHg) to give 3.54 g (14.6 mmol, 74% yield) of
1-ethynyl-1,1,2,2-tetramethyl-2-(phenyl)ethynyldisilane as a col-
This work was supported by Grants-in-Aid for ‘‘Kyoto
University Alliance for Chemistry,’’ and ‘‘Giant Molecular
Systems for Tohoku University.’’ This work was also support-
ed by Scientific Research on Priority Areas (No. 14078101,
‘‘Reaction Control of Dynamic Complexes’’ for HT and ‘‘Pho-
tofunctional Interface (417)’’) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
1
orless oil. H NMR (300 MHz) ꢆ 0.41 (s, 6H), 0.46 (s, 6H), 2.20
(s, 1H), 6.94–6.96 (m, 3H), 7.47–7.50 (m, 2H). 13C NMR (67.81
MHz) ꢆ ꢂ2:82, ꢂ2:74, 92.03, 96.51, 108.71, 123.71, 128.48,
128.68, 132.13, 132.20. 29Si NMR (53.67 MHz) ꢆ ꢂ36:89, ꢂ36:19.
HRMS(EI): Calcd for C14H18Si2: 242.0947. Found: 242.0943.
ZnP–Si2: A solution of {5-iodo-10,15,20-tris[3,5-di(t-butyl)-
phenyl]porphinato}zinc(II) (102 mg, 0.0953 mmol) in THF (25
Supporting Information
VT steady-state absorption spectra of ZnP–Si2–C60 in toluene,
fluorescence lifetime measurements of ZnP–Si2–C60 in frozen 2-
methyltetrahydrofuran, and the nanosecond transient absorption

H. Tsuji et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 9 (2006) 1345
spectra in toluene solutions of ZnP–Si2 and Si2–C60. This materi-
al is available free of charge on the web at http://www.csj.jp/
journals/bcsj/.
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