Stabilization of the charge-separated states of covalently linked zinc porphyrin-triphenylamine-[60]fullerene

Article abstract of DOI:10.1002/cphc.200900885

Spectroscopic, redox, computational, and electron transfer reactions of the covalently linked zinc porphyrin-triphenyl-amine-fulleropyrrolidine system are investigated in solvents of varying polarity. An appreciable interaction between triphenylamine and the porphyrin π system is revealed by steady-state absorption and emission, redox, and computational studies. Free-energy calculations suggest that the light-induced processes via the singlet-excited porphyrin are exothermic in benzonitrile, dichlorobenzene, toluene, and benzene. The occurrence of fast and efficient charge-separation processes (≈ 10¹² s ¹) via the singlet-excited porphyrin is confirmed by femtosecond transient absorption measurements in solvents with dielectric constants ranging from 25.2 (benzonitrile) to 2.2 (benzene). The rates of the charge separation processes are much less solvent-dependent, which suggests that the charge-separation processes occur at the top region of the Marcus parabola. The lifetimes of the singlet radical-ion pair (70-3000 ps at room temperature) decrease substantially in more polar solvents, which suggests that the charge-recombination process is occurring in the Marcus inverted region. Interestingly, by utilizing the nanosecond transient absorption spectral technique we can obtain clear evidence about the existence of triplet radical-ion pairs with relatively long lifetimes of 0.71 μs (in benzonitrile) and 2.2 μs (in o-dichlorobenzene), but not in toluene and benzene due to energetic considerations. From the point of view of mechanistic information, the synthesized zinc porphyrin-triphenylamine-fulleropyrrolidine system has the advantage that both the lifetimes of the singlet and triplet radical-ion pair can be determined.

Full text of DOI:10.1002/cphc.200900885

DOI: 10.1002/cphc.200900885
Stabilization of the Charge-Separated States of Covalently
Linked Zinc Porphyrin–Triphenylamine–[60]Fullerene**
Mohamed E. El-Khouly,[a] Ki-Jong Han,[b] Kwang-Yol Kay,*[b] and Shunichi Fukuzumi*[a, c]
Spectroscopic, redox, computational, and electron transfer re-
actions of the covalently linked zinc porphyrin–triphenyl-
amine–fulleropyrrolidine system are investigated in solvents of
varying polarity. An appreciable interaction between triphenyl-
amine and the porphyrin p system is revealed by steady-state
absorption and emission, redox, and computational studies.
Free-energy calculations suggest that the light-induced pro-
cesses via the singlet-excited porphyrin are exothermic in ben-
zonitrile, dichlorobenzene, toluene, and benzene. The occur-
rence of fast and efficient charge-separation processes
separation processes occur at the top region of the Marcus pa-
rabola. The lifetimes of the singlet radical-ion pair (70–3000 ps
at room temperature) decrease substantially in more polar sol-
vents, which suggests that the charge-recombination process
is occurring in the Marcus inverted region. Interestingly, by uti-
lizing the nanosecond transient absorption spectral technique
we can obtain clear evidence about the existence of triplet
radical-ion pairs with relatively long lifetimes of 0.71 ms (in ben-
zonitrile) and 2.2 ms (in o-dichlorobenzene), but not in toluene
and benzene due to energetic considerations. From the point
of view of mechanistic information, the synthesized zinc por-
phyrin–triphenylamine–fulleropyrrolidine system has the ad-
vantage that both the lifetimes of the singlet and triplet radi-
cal-ion pair can be determined.
(ꢀ1012
s
ꢁ1) via the singlet-excited porphyrin is confirmed by
femtosecond transient absorption measurements in solvents
with dielectric constants ranging from 25.2 (benzonitrile) to 2.2
(benzene). The rates of the charge separation processes are
much less solvent-dependent, which suggests that the charge-
Introduction
Studies on light-induced electron transfer in covalently linked
donor–acceptor systems have witnessed enormous growth in
recent years, mainly to address the mechanistic details of elec-
tron transfer in chemistry and biology, to develop artificial pho-
tosynthetic systems for light energy harvesting, and also to de-
velop molecular electronic devices.[1–9] Toward constructing
such donor–acceptor systems, fullerenes and porphyrins have
been utilized as important constituents owing to their rich
redox, optical, and photochemical properties.[10–13]
ceptor because of its three-dimensional structure, delocalized
p-electrons within the spherical carbon framework, low reduc-
tion potential, and absorption spectra extending over most of
the visible region. The small reorganization energy of C60 pre-
dicts that the charge separation (CS) moves upward along the
normal region into the top region of the Marcus curve, and
charge recombination lies to be significantly downward into
the inverted region.[15] In other words, the fullerene C60 in
donor–acceptor dyads accelerates photoinduced charge sepa-
ration and slows down charge recombination. In the last
There has been continuous interest in the synthesis and
property studies of metalloporphyrins because of their wide-
spread occurrence in nature, strong optical absorption and
emission, and redox properties applicable for electron-transfer
reactions related to solar energy harvesting. Moreover, elec-
tronic properties of the porphyrin p-electron system can readi-
ly be altered by covalent attachment of electron-donating or
accepting axial or peripheral substituents, although these ef-
fects are more pronounced for substitution of the pyrrole mac-
rocycles than the meso-phenyl rings.[14] To increase the donor
ability of zinc porphyrin (ZnP), the triphenylamine (TPA) entity
is covalently linked at the meso position of the zinc porphyrin
ring, producing ZnP-TPA (5). Because of the close proximity of
the triphenylamine entity to the porphyrin p-ring in the conju-
gate, delocalization of the porphyrin p-system to the triphenyl-
amine entity is expected. The ZnP-TPA with its low oxidation
potential appears to be a superior photo-inducible electron
donor relative to other zinc porphyrin derivatives. On the other
hand, fullerene (C60) is particularly appealing as electron ac-
[a] Dr. M. E. El-Khouly, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering, Osaka University
SORST (Japan) Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
Fax: (+81)06-6879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[b] Dr. K.-J. Han, Prof. Dr. K.-Y. Kay
Department of Molecular Science and Technology
Ajou University, Suwon 443-749 (South Korea)
Fax: (+82)31-219-1615
E-mail: kykay@ajou.ac.kr
[c] Prof. Dr. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University
Seoul 120-750 (South Korea)
[**] Spectroscopic, Electrochemical, Computational and Electron-Transfer
Studies
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.200900885.
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Charge-Separated States of Zinc Porphyrin–Triphenylamine–[60]Fullerene
decade, elegantly designed porphyrin-C60 dyads have been
synthesized to probe the effect of molecular topology and dis-
tance and orientation effects of the donor and acceptor enti-
ties on photoinduced charge-separation and charge-recombi-
nation processes.[16–19]
to be lower than the excited singlet porphyrin in both polar
and nonpolar solvents; 2) the electron transfer via the triplet
porphyrin is energetically favorable in both polar and less
polar solvents, but disfavorable in the nonpolar solvents;
3) since the energy of the radical-ion pair is found to be lower
than the triplet porphyrin and fullerene in the polar solvents,
the undesired charge recombination of the radical-ion pair to
the triplet state porphyrin and fullerene is excluded; 4) as dem-
onstrated here, 1 is anticipated to slow down the charge-re-
combination process after charge separation, thus generating
relatively long-lived charge-separated species.
Taking these properties into consideration, we report herein
the photoinduced electron transfer of
a donor–acceptor
system composed of triphenylamine-substituted zinc porphyrin
ZnP-TPA that is covalently linked with fullerene C60 to form
ZnP-TPA-C60 (1) (Figure 1). For this system, the electron-transfer
reactions are observed in the polar and nonpolar solvents. Sev-
eral considerations led to the design of 1; these can be sum-
marized as: 1) the studied compound with well-adjusted ener-
gies of the porphyrin, triphenylamine, and fullerene entities
possesses large negative free energies for charge-separation
process, since the energy of the radical-ion pair (RIP) is found
Results and Discussion
Synthesis and Characterization
Zinc porphyrin–triphenylamine–fulleropyrrolidine (1) was pre-
pared as depicted in Scheme 1. Every step of the reaction se-
quence proceeded smoothly and efficiently to give a good or
moderate yield of the product. Metal-free asymmetrical por-
phyrin 2 was prepared according to a typical method for the
porphyrin synthesis.[20] Subsequently, Vilsmeier formylation[21]
was carried out to produce aldehyde 3, and then fulleropyrroli-
dine formation was achieved by 1,3-dipolar cycloaddition reac-
tion between aldehyde 3 and C60 in the presence of excess N-
methylglycine(sarcosine)[22] under the condition described by
Prato[23,24] to give 4. Finally, metallation was performed using
zinc acetate to afford 1. Reference compounds 5 and 7 were
Figure 1. Molecular structure of the synthesized ZnP-TPA-C60 (1).
Scheme 1. Synthesis of 1, 5 and 7. a) (i) BF3OEt2, CH2Cl2 (ii) DDQ, NEt3. b) POCl3, DMF. c) C60, N-methylglycine, toluene. d) Zn(OAc)2, CH2Cl2/MeOH.
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K.-Y. Kay, S. Fukuzumi et al.
also synthesized in a similar way. 1 is very soluble in the
as compared to 7. On the other hand, the first reduction po-
tential (Ered) of the C60 moiety was recorded at ꢁ950 V versus
Ag/AgNO3 (which corresponds to ꢁ640 mV vs. SCE). The driv-
common organic solvents. The structure and purity of the new
1
compounds were confirmed by H- and 13C NMR spectroscop-
ies and elemental analysis (see Experimental Section). The
1H NMR spectra of 1 in CDCl3 are consistent with the proposed
structures, showing the expected features with the correct in-
tegration ratios. The MALDI-TOF mass spectra (see Figure 1 of
the Supporting Information) provided a direct evidence for the
structure of 1, showing a singly charged molecular ion peak
that matches the calculated value for the molecular weight.
Steady-state Absorption Measurements
The absorption spectrum of 7 in toluene shows the Soret ab-
sorption band at 423 nm (Figure 2). The triphenylamine moiety
has an absorption peak at about 320 nm due to an n–p* elec-
tronic transition. The fulleropyrrolidine entity in the UV/Vis
region is extremely weak compared with the huge absorption
of zinc porphyrin. In the case of 5, the Soret band is 5 nm red-
shifted as compared to 7, suggesting ground-state interactions
Figure 3. a) Differential pulse voltammogram and b) cyclic voltammogram of
1 in deaerated benzonitrile containing TBAPF6 (0.1m) at 298 K. Sweep rate:
0.1 Vsꢁ1
.
ing forces for the charge recombination (-DGCR) processes of
the radical-ion pair formed via the singlet excited porphyrin
were calculated as 1.24, 1.40, 1.80, and 1.82 eV in benzonitrile,
dichlorobenzene, toluene, and benzene, respectively. Based on
the -DGCR and the energy of the singlet porphyrin (2.00 eV),
the driving forces for the charge-separation (-DGCS) processes
of 1 were calculated as 0.76, 0.60, 0.20, and 0.18 eV in benzoni-
trile, dichlorobenzene, toluene, and benzene, respectively.[25]
These -DGCS values suggest exothermic charge-separation pro-
cess via the singlet excited porphyrin to form the radical-ion
pairs in both polar and nonpolar solvents. On the other hand,
the calculated -DGCS values via the triplet excited porphyrin
suggest an exothermic charge-separation process in polar ben-
zonitrile (-DGCS =0.31 eV) and o-dichlorobenzene (-DGCS =
0.15 eV) and an endothermic one in toluene and benzene.
Figure 2. Steady-state absorption spectra of 1, 5 and 7 in toluene (TN) and
benzonitrile (PhCN). The concentrations are kept at 2.2ꢂ10ꢁ7 m.
between the porphyrin p system and the triphenylamine
entity. When turning to 1, the Soret band is further red-shifted
by 1 nm, suggesting weak interaction between the ZnP-TPA
and C60 entities. When changing the solvent from toluene to
benzonitrile, the absorption spectrum of 1 is further red-shift-
ed by about 5 nm.
Computational Studies
The molecular geometries and electronic structures of 1 were
investigated on the basis of molecular orbital calculation using
the density functional theory (DFT) method at the B3LYP/6-
31G levels. As shown in Figure 4, the center-to-center distance
(dCC) between the porphyrin and the C60 was estimated as 16 ꢁ
with no significant interactions between them. In the opti-
mized structure, the electron distribution of the highest occu-
pied molecular orbital (HOMO) was delocalized over the por-
phyrin macrocycle and the triphenylamine entity that is help-
ing to stabilize the radical cation and consequently slowing
down the charge recombination process during the light-in-
duced electron-transfer process. The lowest unoccupied molec-
ular orbital (LUMO) was fully localized over the C60 entity.
These results suggest that the ZnP-TPA moiety acts as an elec-
Redox Measurements
Redox measurements were carried out via cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) techniques in
deaerated benzonitrile to estimate the driving forces for the
electron-transfer processes (see Figure 3). The oxidation poten-
tials of 5 were located at 303 and 602 mV versus Ag/AgNO3
(which corresponds to 593 and 892 mV vs. the saturated calo-
mel electrode, SCE) and were cathodically shifted over 300 mV
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Charge-Separated States of Zinc Porphyrin–Triphenylamine–[60]Fullerene
that of the singlet C60. However, such C60 fluorescence was not
observed in polar benzonitrile and o-dichlorobenzene due to
the highly efficient charge separation.
To complement the emission studies, the fluorescence life-
time measurements track the above consideration in a more
quantitative way. In the case of 5, the fluorescence-decay-time
profile of the singlet porphyrin exhibits a mono-exponential
decay with a lifetime (tf0) of 1800 ps. However, in the case of 1,
the quick decay of the fluorescence-time profiles of the singlet
porphyrin at 580–680 nm could be fitted satisfactorily with a
mono-exponential decay function (Figure 6a), from which
short lifetimes (tf) were evaluated as a few picoseconds
(<10 ps) in toluene, o-dichlorobenzene, and benzonitrile. The
rates of the quenching process, which is related to the elec-
Figure 4. Optimized, HOMO and LUMO structures of 1 calculated by the ab
tron-transfer
process,
were
found
to
be
fast
initio molecular orbital method at B3LYP/6-31G level.
(>1010 ꢁ1). A more accurate determination of the electron-
s
transfer rates is described later in the femtosecond laser flash
photolysis measurements.
tron donor, while the C60 moiety acts as an electron acceptor,
·ꢁ
to afford (ZnP-TPA)·+-C60
.
The time-resolved fluorescence spectrum of 1 at 1.0 ns ex-
hibited a new fluorescence peak at 720 nm, due to the popula-
tion of the singlet excited C60 (Figure 6b) in toluene, but not
observed in the polar solvents. This observation suggests that
Emission Studies
The photophysical behavior was investigated by steady-state
fluorescence using 420 nm excitation light, which selectively
excited the zinc porphyrin moiety (Figure 5). The fluorescence
spectrum of 5 in toluene is red-shifted by about 6 nm com-
pared to 7 (Figure 5), suggesting an appreciable interaction be-
tween the porphyrin and triphenylamine entities. When turn-
ing to 1, an efficient quenching of the singlet excited porphy-
rin by the attached C60 was clearly observed in the studied sol-
vents through the electron-transfer process forming (ZnP-
TPA)·+-C60·ꢁ. In nonpolar toluene, one could clearly notice the
appearance of a C60 emission that may arise from: 1) energy
transfer from the singlet excited porphyrin to the attached C60
to populate ZnP-TPA-1C60*, and/or 2) charge recombination of
·ꢁ
(ZnP-TPA)·+-C60 to populate ZnP-TPA-1C60* taking into consid-
eration that the energy level of (ZnP-TPA)·+-C60 is higher than
·ꢁ
Figure 6. a) Fluorescence time profiles obtained by 400 nm laser excitation
of 5 and 1 in the studied solvents. b) Time-resolved spectra of ZnP-TPA-1C60*
1
Figure 5. Steady-state fluorescence spectra of 7, 5 and 1 in toluene. Inset:
Fluorescence spectra of 1 in the studied solvents; lex =420 nm.
in toluene at 0.1 and 1.0 ns. Inset: fluorescence time profile of C60* at
720 nm.
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K.-Y. Kay, S. Fukuzumi et al.
the emission of C60 emerges with the decay of the singlet por-
phyrin in toluene. Since the energy level of the charge-separat-
ed state in toluene is lower than that of the singlet excited
porphyrin and higher than that of the singlet C60, it is likely
that the population of the singlet C60 moiety arises from the
rabola.[26] The rates of the charge-recombination processes (kCR)
and lifetimes of the radical-ion pair (tRIP =1/kCR) were evaluated
to be 1.35ꢂ1010 sꢁ1 (74 ps), 7.70ꢂ109 sꢁ1 (130 ps), 2.50ꢂ109 sꢁ1
(400 ps), and 2.80ꢂ108 sꢁ1 (3.6 ns) in benzonitrile, dichloroben-
zene, toluene and benzene, respectively.
·ꢁ
charge recombination of (ZnP-TPA)·+-C60 in the nonpolar sol-
The fast charge-recombination processes of 1 suggest that
the radical-ion pair keeps the singlet-spin character, because of
the charge separation via the singlet-excited porphyrin. The
observation that the kCR values increase with increasing solvent
polarities (i.e., the lifetimes of the charge-separated states are
greater in nonpolar solvents compared to those in polar sol-
vents) can be understood, if charge recombination is occurring
in the Marcus inverted region.[26] The kCS/kCR ratios via the sin-
glet excited porphyrin, which are a measure of the excellence
in the photoinduced electron-transfer systems, were deter-
mined to be 73, 130, 400 and 3600 ps in benzonitrile, o-di-
chlorobezene, toluene, and benzene, respectively, suggesting
the usefulness of the studied compound as light-harvesting
systems, especially in nonpolar solvents.
vents. The fluorescence-decay-time profile of ZnP-TPA-1C60* in
toluene exhibited a mono-exponential decay with a lifetime
(tf0) of 1200 ps, which is close to that of the fulleropyrrolidine
reference, suggesting the absence of a charge-separation pro-
cess via the singlet excited C60 in toluene.
Femtosecond Transient Absorption Studies
Spectroscopic evidence for the radical-ion pair formation of 1
was obtained from femtosecond transient absorption measure-
ments performed using 430 nm laser light, where the porphy-
rin moiety is selectively excited (see Figure 7, and Figures S5
and S6 of the Supporting Information). The time-resolved spec-
trum in polar benzonitrile at 1 ps shows an absorption band at
470 nm, which is assigned to the singlet excited porphyrin,
while the spectrum at around 30 ps shows sharp absorption
peaks of the visible (600–800 nm) and NIR (at 1000 nm) re-
gions, corresponding to the formation of the radical ion pair
(ZnP-TPA)·+-C60·ꢁ. The transient spectra in moderate o-dichloro-
Nanosecond Transient Absorption Studies
The nanosecond transient absorption spectrum of 5 showed
absorption bands at 480 and 840 nm, corresponding to the
triplet porphyrin (see Figure S7 of the Supporting Information).
The transient absorption spectra
of 1 in benzonitrile (Figure 8)
and dichlorobenzene (Figure S9
of the Supporting Information)
exhibited characteristic peaks of
the fulleropyrrolidine anion radi-
cal (C60·ꢁ) at 1000 nm and the
(ZnP-TPA)·+ radical at 600–
800 nm, which suggests the gen-
eration of the triplet radical ion
pair (ZnP-TPA)·+-C60·ꢁ via the trip-
let excited porphyrum. The char-
Figure 7. Femtosecond spectra of 1 in dearerated toluene (left) and benzonitrile (right); lex =430 nm.
benzene (es =11.00), as well as in nonpolar toluene (es =2.38)
and benzene (es =2.28), exhibited similar features as in benzo-
nitrile (es =25.70). An explanation for this analogy is based on
the corresponding energy levels of (ZnP-TPA)·+-C60 relative to
·ꢁ
the singlet excited porphyrin, guaranteeing positive driving
forces for the associated charge-separation and charge-recom-
bination processes in both polar and nonpolar solvents. The
·ꢁ
absorption band of C60 at 1000 nm is a clear diagnostic
marker band that is used to examine kinetics of the charge-
separation and charge-recombination processes. The time pro-
·ꢁ
files of C60 at 1000 nm (Figure 7, inset) showed an extremely
fast rise of (ZnP-TPA)·+-C60·ꢁ, from which the rate constants of
the charge-separation processes (kCSS) via ZnP* were calculat-
1
ed as 2.5ꢂ1012, 1.2ꢂ1012, and 1.0ꢂ1012 sꢁ1 in benzonitrile, o-di-
S
chlorobenzene, toluene, respectively. Since the kCS values are
nearly solvent-independent from solvent polarities, the CS pro-
cess may be occurring near the top region of the Marcus Pa-
Figure 8. Nanosecond transient spectra of 1 (0.1mm) in deaerated benzoni-
trile; lex =532 nm.
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Charge-Separated States of Zinc Porphyrin–Triphenylamine–[60]Fullerene
·ꢁ
acteristic transient absorption band of the C60 moiety in the
charge-separation processes (ꢀ1012 sꢁ1) via the singlet-excited
porphyrin was observed by detecting the radical-ion species in
solvents with dielectric constants ranging from 25.2 (benzoni-
trile) to 2.2 (benzene). The lifetimes of the singlet (ZnP-TPA)·+-
NIR region is a clear marker band that provides a powerful tool
for the assignment of the observed transient species and for
examining the kinetics of the charge-recombination processes.
The rates of charge recombination (kCR) were found to be
1.40ꢂ106 and 4.46ꢂ105 sꢁ1 in benzonitrile and o-dichloroben-
zene, respectively. Based on the kCR values, the lifetimes of the
radical-ion pair (tRIP) were evaluated as 0.71 and 2.2 ms in ben-
zonitrile and o-dichlorobenzene, respectively. The quantum
yield of the triplet radical-ion pair was evaluated as 40%. The
observed long triplet RIP lifetime in fact implies that intersys-
tem crossing within the RIP state (singlet RIP$triplet RIP) is
slow because otherwise triplet RIP could decay via a singlet re-
combination channel.[27]
·ꢁ
C60 (74–3000 ps at room temperature) decrease substantially
in more polar media, while the rates of charge separation are
much less solvent-dependent. This is qualitatively in line with
the fact that charge separation occurs near the Marcus top
region, while charge recombination occurs in the Marcus in-
verted region. By utilizing the nanosecond transient technique,
relatively long-lived charge-separated states of 1 on the order
of few microseconds were observed in benzonitrile (0.71 ms)
and o-dichlorobenzene (2.2 ms). Such a large ratio of charge-
separation (picoseconds) to charge-recombination (microsec-
onds) is a desirable property of molecular systems designed
for photoinduced charge separation.
The energy-level diagram constructed by utilizing the spec-
tral, electrochemical, and photochemical data of 1 is illustrated
in Figure 9. In all the studied solvents, charge separation takes
place via the singlet excited porphyrin, generating the singlet
Experimental Section
(ZnP-TPA)·+-C60 species. The charge-separated decayed to
·ꢁ
populate the singlet excited C60 in case of benzene and tolu-
ene, while it is decayed to the ground state in case of benzoni-
trile and dichlorobenzene. On the other hand, the CS process
via the triplet excited porphyrin was recorded in benzonitrile
and dichlorobenzene, but not in nonpolar solvents, which in
good agreement with the finding that the energy levels of
Materials and Instruments: Reagents and solvents were pur-
chased as reagent grade and used without further purification. All
reactions were performed using dry glassware under nitrogen at-
mosphere. Analytical thin-layer chromatography (TLC) was carried
out on a Merck 60 F254 silica gel plate and column chromatogra-
phy was performed on Merck 60 silica gel (230–400 mesh). Melting
points were determined on an Electrothemal IA 9000 series melt-
ing point apparatus and are uncorrected. NMR spectra were re-
corded on a Varian Mercury-400 (400 MHz) spectrometer with TMS
peak as reference. MALDI-TOF mass spectra were obtained in a re-
flection mode using a Kratos Compact MALDI 1 (Shimadzu), with
dithranol as matrix. UV/Vis spectra were recorded on a Jasco V-550
spectrometer. MALDI-TOF MS spectra were recorded with an Ap-
plied Biosystems Voyager-DE-STR. Elemental analyses were per-
formed with a Perkin–Elmer 2400 analyzer, and column chromato-
graphic purifications of all the new compounds were repeated sev-
eral times until correct analytical data were obtained.
·ꢁ
(ZnP-TPA)·+-C60 are found to be lower than those of the trip-
let excited porphyrin in both benzonitrile and o-dichloroben-
zene.
Conclusion
We have reported the synthesis as well as spectroscopic,
redox, computational, and electron-transfer studies of cova-
lently linked zinc porphyrin–triphenylamine–fulleropyrrolidine
in solvents of varying polarity. By combining emission data
with complementary data from femtosecond time-resolved ab-
sorption measurements, the occurrence of fast and efficient
Porphyrin-triphenylamine (2); 5-{4-(N,N-diphenylamino)phenyl}-
10,15,20-tris(4-pentylphenyl)-21H, 23H-porphine: Freshly distilled
pyrrole (187 mg, 2.79 mmol), 4-diphenylaminobenzaldehyde
Figure 9. Energy-level diagrams showing the photochemical events of 1 in benzonitrile, o-dichlorobenzene, toluene, and benzene.
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K.-Y. Kay, S. Fukuzumi et al.
(224 mg,
0.82 mmol),
4-n-pentylbenzaldehyde
(347 mg,
96.0%) as a dark brown powder. M.p.>3208C (dec.); 1H NMR
(400 MHz, CDCl3): d=8.84–8.98 (m, 8H), 7.92–8.08 (m, 8H), 7.44–
7.56 (m, 8H), 7.18–7.36 (m, 8H), 7.08 (t, J=7 Hz, 1H), 4.59 (d, J=
9 Hz, 2H), 3.84 (d, J=9 Hz, 1H), 2.85–2.95 (m, 6H), 2.73 (s, 3H),
1.80–1.95 (m, 6H), 1.40–1.60 (m, 12H), 0.90–1.05 (m, 9H); 13C NMR
(400 MHz, CDCl3): d=155.3, 152.8, 150.2, 147.1, 147.0, 146.6, 146.2,
146.1, 146.0, 145.9, 145.8, 145.4, 145.3, 145.2, 145.1, 144.9, 144.8,
144.2, 144.1, 143.6, 143.5, 143.4, 143.3, 143.1, 142.8, 142.6, 142.5,
142.4, 142.0, 141.9, 141.8, 139.3, 137.0, 135.8, 134.3, 132.0, 131.9,
129.4, 124.3, 123.1, 121.3, 121.2, 69.5, 68.4, 39.9, 36.0, 31.8, 31.4,
22.6, 14.2; UV/Vis (toluene): lmax =429, 553, 594 nm; MS (MALDI-
TOF); m/z 1828.71 (C134H72N6Zn requires 1828.51). Anal. Calcd: C,
87.90%; H, 3.96%; N, 4.59%. Found: C, 87.84%; H, 3.98%; N,
4.58%.
1.97 mmol), dry n-propanol (1.8 mL), and dry dichloromethane
(280 mL) were stirred for 10 min. BF3OEt2 (0.14 mL, 0.92 mmol) was
added via septum, and the reaction mixture was protected from
light and stirred for 80 min. 2,3-Dicyano-5,6-dichloro-1,4-benzoqui-
none (DDQ) (633 mg, 2.79 mmol) was added and stirred for further
90 min before the addition of triethylamine (0.39 mL). The solvent
was evaporated, and the dark black residue was purified by chro-
matography on silica gel, eluting with CHCl3/hexane (2:1) to give
compound 2 (76 mg, 11.0%) as a dark green powder. M.p.=82–
1
858C; H NMR (400 MHz, CDCl3): d=8.97 (d, J=5 Hz, 2H), 8.89 (d,
J=5 Hz, 2H), 8.85 (s, 4H), 8.03–8.14 (m, 8H), 7.48–7.56 (m, 6H),
7.34–7.46 (m, 10H), 7.08–7.14 (2H), 2.90–2.97 (m, 6H), 1.85–1.95
(m, 6H), 1.45–1.60 (m, 12H), 0.95–1.05 (m, 9H), ꢁ2.72 (s, 2H); Anal.
Calcd for C71H69N5: C, 85.93%; H, 7.01%; N, 7.06%. Found: C,
85.91%; H, 7.05%; N, 7.04%.
Zinc porphyrin-triphenylamine reference (5); [5-(4-diphenylamino-
phenyl)-10,15,20-tris(4-pentylphenyl)-21H,23H-porphinato]zinc:
Compound 2 (19.8 mg, 0.02 mmol) was dissolved in CH2Cl2/CH3OH
(3:1, 8 mL) and zinc acetate (37 mg, 0.2 mmol) was added. The re-
action mixture was protected from light and stirred at room tem-
perature for 90 min. The solvent was removed under reduced pres-
sure, and the product was purified by column chromatography on
silica gel, eluting with CHCl3/hexane (2:1) to give compound 5
(20.7 mg, 98.0%) as a dark green powder. M.p.=1088C; 1H NMR
(400 MHz, CDCl3): d=9.07 (d, J=5 Hz, 2H), 8.99 (d, J=5 Hz, 2H),
8.96 (s, 4H), 8.04–8.14 (m, 8H), 7.52–7.56 (m, 6H), 7.36–7.46 (m,
10H), 7.08–7.14 (2H), 2.90–2.94 (m, 6H), 1.78–1.94 (m, 6H), 1.45–
1.60 (m, 12H), 0.95–1.06 (m, 9H). Anal. Calcd for C71H67N5Zn: C,
80.78%; H, 6.40%; N, 6.63%. Found: C, 80.77%; H, 6.43%; N,
6.61%.
Porphyrin-triphenylamine-aldehyde (3); 5-[4-{N-(4-formylphenyl)-N-
phenylamino}phenyl]-10,15,20-tris(4-pentylphenyl)-21H, 23H-por-
phine: Compound 2 (59.5 mg, 0.06 mmol) and POCl3 (20 mg,
0.13 mmol) were dissolved in DMF (2 mL) and the mixture was
heated to 808C for 3 h. After cooling, distilled water (10 mL) was
added to the mixture and neutralized with 10% Na2CO3 solution.
The crude product was extracted with chloroform (15 mL) and the
organic layer was dried over MgSO4. The solvent evaporated and
the product was column chromatographed on silica gel with
CHCl3/hexane (2:1) to give compound 3 (38 mg, 62.0%) in a dark
1
green powder. M.p.=1458C; H NMR (400 MHz, CDCl3): d=9.86 (s,
1H), 8.83 (s, 4H), 8.87 (s, 4H), 8.85 (s, 4H), 8.06–8.16 (m, 8H), 7.80
(d, J=8 Hz, 2H), 7.40–7.56 (m, 12H), 7.24–7.34 (m, 3H), 2.85–2.97
(m, 6H), 1.85–1.95 (m, 6H), 1.40–1.60 (m, 12H), 0.95–1.05 (m, 9H),
ꢁ2.72 (s, 2H); Anal. Calcd for C72H69N5O: C, 84.75%; H, 6.82%; N,
6.86%. Found: C, 84.73%; H, 6.84%; N, 6.83%.
Porphyrin (6); 5,10,15,20-tetrakis(4-pentylphenyl)-21H,23H-porphi-
ne:[20b] Freshly distilled pyrrole (187 mg, 2.79 mmol), 4-n-pentylben-
zaldehyde (491 mg, 2.79 mmol), dry n-propanol (1.8 mL), and dry
dichloromethane (280 mL) were stirred for 10 min. BF3OEt2
(0.14 mL, 0.92 mmol) was added via septum, and the reaction mix-
ture was protected from light and stirred for 80 min. 2,3-Dicyano-
5,6-dichloro-1,4-benzoquinone (633 mg, 2.79 mmol) was added
and the reaction mixture was stirred for a further 90 min before
the addition of triethylamine (0.39 mL). The solvent was evaporat-
ed, and the product was purified by chromatography on silica gel,
eluting with CHCl3/hexane (2:1) to give compound 6 (81 mg,
Porphyrin-triphenylamine-C60 (4); 5-[4-{N-(4-(1-methyl-3,4-fullero-
2,3,4,5-tetrahydropyrrol-2-yl))phenyl-N-phenylamino}phenyl]-
10,15,20-tris(4-pentylphenyl)-21H, 23H-porphine: Compound
3
(30.6 mg, 0.03 mmol), C60 (44 mg, 0.06 mmol), and N-methylglycine
(60 mg, 0.67 mmol) were dissolved in toluene (30 mL) and stirred
for 10 min. Then, the mixture was heated to reflux for 12 h. After
cooling, the solvent was removed under reduced pressure and pu-
rified by column chromatography on silica gel, eluting with CHCl3/
hexane (1:1) to give compound 4 (39 mg, 73.0%) as a dark brown
powder. M.p.=3208C (dec.); 1H NMR (400 MHz, CDCl3): d= 8.74–
8.92 (m, 8H), 7.92–8.12 (m, 8H), 7.42–7.56 (m, 7H), 7.16–7.36 (m,
9H), 7.08 (t, J=7 Hz, 1H), 4.59 (d, J=9 Hz, 2H), 3.84 (d, J=9 Hz,
1H), 2.85–2.95 (m, 6H), 2.75 (s, 3H), 1.80–1.95 (m, 6H), 1.40–1.60
(m, 12H), 0.90–1.05 (m, 9H), ꢁ2.79 (s, 2H); 13C NMR (400 MHz,
CDCl3); d=155.2, 152.8, 150.2, 150.1, 147.1, 147.0, 146.6, 146.2,
146.1, 146.0, 145.8, 145.4, 145.3, 145.2, 145.1, 144.9, 144.8, 144.2,
144.1, 143.6, 143.5, 143.4, 143.3, 143.1, 142.8, 142.6, 142.5, 142.4,
142.0, 141.9, 141.8, 139.3, 139.2, 137.0, 135.8, 135.3, 134.3, 132.0,
131.9, 129.4, 124.3, 123.1, 121.3, 121.2, 69.5, 68.4, 39.9, 36.0, 31.8,
31.4, 22.6, 14.2; Anal. Calcd for C134H74N6: C, 91.03%; H, 4.22%; N,
4.75%. Found: C, 91.02%; H, 4.24%; N, 4.74%.
13.0%) as
a
dark green powder. M.p.=312–3168C; 1H NMR
(400 MHz, CDCl3): d=8.86 (s, 8H), 8.11 (d, J=8 Hz, 8H), 7.54 (d, J=
8 Hz, 8H), 2.95 (t, J=8 Hz, 8H), 1.90–1.95 (m, 8H), 1.45–1.60 (m,
16H), 1.02 (t, J=7 Hz, 12H), ꢁ2.76 (s, 2H); Anal. Calcd for C64H70N4:
C, 85.86%; H, 7.88%; N, 6.26%. Found: C, 85.83%; H, 7.90%; N,
6.27%.
Zinc porphyrin reference (7); [5,10,15,20-tetrakis(4-pentylphenyl)-
21H,23H-porphinato]zinc:[15b] Compound 6 (26.9 mg, 0.03 mmol)
was dissolved in CH2Cl2/CH3OH (3:1, 12 mL) and zinc acetate
(55 mg, 0.3 mmol) was added. The reaction mixture was protected
from light and stirred at room temperature for 90 min. The solvent
was removed under reduced pressure, and the product was puri-
fied by column chromatography on silica gel, eluting with CHCl3/
hexane (2:1) to give compound 7 (28.2 mg, 98.0%) as a bright red
powder. M.p.=309–3128C. 1H NMR (400 MHz, CDCl3): d=8.97 (s,
8H), 8.12 (d, J=8 Hz, 8H), 7.54 (d, J=8 Hz, 8H), 2.95 (t, J=8 Hz,
8H), 1.89–1.97 (m, 8H), 1.45–1.60 (m, 16H), 1.02 (t, J=7 Hz, 12H);
Anal. Calcd for C64H68N4Zn: C, 80.18%; H, 7.15%; N, 5.84%. Found:
C, 80.16%; H, 7.17%; N, 5.81%.
Zinc porphyrin-triphenylamine-C60 (1); [5-[4-{N-(4-(1-methyl-3,4-full-
ero-2,3,4,5-tetrahydropyrrol-2-yl))phenyl-N-phenylamino}phenyl]-
10,15,20-tris(4-pentylphenyl)-21H,23H-porphinato]zinc: Compound
4 (36.6 mg, 0.02 mmol) was dissolved in CH2Cl2/CH3OH (3:1, 12 mL)
and zinc acetate (37 mg, 0.2 mmol) was added. The reaction mix-
ture was protected from light and stirred at room temperature for
90 min. The solvent was removed under reduced pressure, and the
product was purified by column chromatography on silica gel,
eluting with CHCl3/hexane (2:1) to give compound 1 (35.2 mg,
Cyclic Voltammetry: The redox values were measured by means
of differential pulse voltammetry (DPV) using a BAS CV-50W Vol-
1732
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ChemPhysChem 2010, 11, 1726 – 1734

Charge-Separated States of Zinc Porphyrin–Triphenylamine–[60]Fullerene
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ing electrode, while a platinum wire served as counter electrode.
An Ag/AgCl electrode was used as reference electrode. All meas-
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(nBu)4NClO4) as supporting electrolyte at a scan rate of 0.1 Vsꢁ1
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a
digitizing oscilloscope (Tektronix, TDS3032,
300 MHz). Femtosecond transient absorption spectroscopy experi-
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tronix Corp.), an optical parametric amplifier: TOPAS (Light Conver-
sion Ltd.) and a commercially available optical detection system:
Helios provided by Ultrafast Systems LLC. The sources for the
pump and probe pulses were derived from the fundamental
output of Integra-C (780 nm, 2 mJ per pulse and fwhm=130 fs) at
a repetition rate of 1 kHz. Seventy five percent of the fundamental
output of the laser was introduced into TOPAS which has optical
frequency mixers resulting in tunable range from 285 to 1660 nm,
while the rest of the output was used for white-light generation.
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obtain the transient spectrum at a set delay time. Kinetic traces at
appropriate wavelengths were assembled from the time-resolved
spectral data. All measurements were conducted at 298 K. The
transient spectra were recorded using fresh solutions in each laser
excitation.
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Acknowledgements
Kwang-Yol Kay acknowledges the Brain Korea 21 Program of the
Ministry of Education for financial support. This work was partial-
ly supported by KOSEF/MEST through WCU project (R31–2008–
000–10010–0) and the Global COE (center of excellence) program
“Global Education and Research Center for Bio-Environmental
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Keywords: electron transfer · fullerenes · laser photolysis ·
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[25] The driving forces for DGCR and DGCS were calculated using the equa-
tions: ꢁDGCR =e(EoxꢁEred)+DGS and ꢁDGCS =DE00ꢁ(ꢁDGCR), where
DE00 is the energy of the 0–0 transition [2.00 eV for 1(ZnP-TPA)*]. DGS
refers to the static energy which was calculated by ꢁDGS =
ꢁ(e2/(4pe0))[(1/(2R+)+1/(2R )ꢁ(1/RCC)/eSꢁ(1/(2R+)+1/(2R ))/eR)], where
ꢁ
ꢁ
R+ and R are radii of the radical cation (4.2 ꢁ) and radical anion
ꢁ
(3.7 ꢁ), respectively; RCC is the center–center distance between C60 and
ZnP (16 ꢁ), which were evaluated from optimized structure. The param-
eters eR and eS refer to the solvent dielectric constants for electrochem-
istry and photophysical measurements, respectively.
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Received: November 11, 2009
Revised: December 23, 2009
Published online on February 28, 2010
1734
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