Mimicking photosynthetic antenna-reaction-center complexes with a (Boron Dipyrromethene)3-Porphyrin-C60 pentad

Article abstract of DOI:10.1002/chem.201002333

A highly efficient functional mimic of the photosynthetic antenna-reaction-center complexes has been designed and synthesized. The model contains a zinc(II) porphyrin (ZnP) core, which is connected to three boron dipyrromethene (BDP) units by click chemis

Full text of DOI:10.1002/chem.201002333

FULL PAPER
DOI: 10.1002/chem.201002333
Mimicking Photosynthetic Antenna-Reaction-Center Complexes with a
(Boron Dipyrromethene)3–Porphyrin–C60 Pentad
Jian-Yong Liu,[a] Mohamed E. El-Khouly,[b, c] Shunichi Fukuzumi,*[b, d] and
Dennis K. P. Ng*[a]
Abstract: A highly efficient functional
mimic of the photosynthetic antenna-
reaction-center complexes has been de-
signed and synthesized. The model
contains a zinc(II) porphyrin (ZnP)
core, which is connected to three boron
dipyrromethene (BDP) units by click
chemistry, and to a C60 moiety using
the Prato procedure. The compound
has been characterized using various
spectroscopic methods. The intramolec-
ular photoinduced processes of this
pentad have also been studied in detail
with steady-state and time-resolved ab-
sorption and emission spectroscopic
methods, both in polar benzonitrile and
nonpolar toluene. The BDP units serve
as the antennae, which upon excitation
undergo singlet–singlet energy transfer
to the porphyrin core. This is then fol-
lowed by an efficient electron transfer
to the C60 moiety, resulting in the for-
mation of the singlet charge-separated
state (BDP)3–ZnP +–C60 , which has a
lifetime of 476 and 1000 ps in benzoni-
trile and toluene, respectively. Interest-
ingly, a slow charge-recombination pro-
C
CÀ
cess (kTCR =2.6ꢀ106 sÀ1) and
a long-
lived triplet charge-separated state
(tTCS =385 ns) were detected in polar
benzonitrile by nanosecond transient
measurements.
Keywords: boron dipyrromethenes ·
electron transfer
·
fullerenes
·
photosynthesis · porphyrins
Introduction
[a] Dr. J.-Y. Liu,+ Prof. D. K. P. Ng
Department of Chemistry and
Center of Novel Functional Molecules
The Chinese University of Hong Kong
Shatin, N. T., Hong Kong (China)
Fax : (+852)2603-5057
Mimicking photosynthetic functions by using synthetic
model compounds is important for our understanding of
bioACHTNUTRGNEUNGenergetic processes. There has been considerable interest
in the development of bioinspired artificial photosynthetic
systems.[1–3] A vast number of photo- and redox-active build-
ing blocks have been selected and assembled to mimic the
primary events of natural photosynthesis, which include
light harvesting, photoinduced multistep electron transfer,
and catalysis. The studies are important not only for the
photochemical conversion of solar energy into fuels,[1a,4] but
also for the construction of various optoelectronic devices.[5]
Owing to their biological relevance and favorable proper-
ties, porphyrins have been widely used as key components
in such models, both for energy-transfer and electron-trans-
fer processes.[6–8] As another major class of functional mate-
rials, fullerene derivatives exhibit remarkable electron-ac-
cepting properties and low reorganization energies, render-
ing these compounds as ideal electron acceptors for incorpo-
ration into artificial reaction centers.[8,9] As a consequence, a
number of reaction-center mimics based on porphyrin–full-
erene conjugates have been prepared and studied.[8] The
charge-separation efficiencies of some of these systems are
comparable to those found in natural systems. In addition,
E-mail: dkpn@cuhk.edu.hk
[b] Dr. M. E. El-Khouly,+ Prof. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7370668-797-370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[c] Dr. M. E. El-Khouly+
Department of Chemistry, Faculty of Science
Kafr El-Sheikh University
Kafr El-Sheikh 33516 (Egypt)
[d] Prof. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University, Seoul 120-750 (Korea)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002333. This includes the
normalized absorption spectra of 7, 9, and 10 in benzonitrile, excita-
tion spectra of 9 and 10 in benzonitrile, nanosecond transient spectra
of 8–10 in deaerated benzonitrile, and H and 13C{1H} NMR spectra
1
of the new compounds.
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antenna functionality has also been introduced to reaction-
center mimics.[10] In these systems, singlet–singlet energy
transfer occurs, followed by charge separation, thereby
mimicking photosynthetic antenna-reaction-center com-
plexes. Both covalent[10a–e] and supramolecular[10f,g] ap-
proaches have been adopted for the assembly of these com-
ponents.
Apart from porphyrins, boron dipyrromethenes (BDPs)
are also of particular interest as light-harvesting units.[11]
Generally, they exhibit large extinction coefficients, high
fluorescence quantum yields, reasonably long excited sin-
glet-state lifetimes, and good solubility and stability in many
solvent systems.[12] More importantly, the BDP core can be
readily modified to tailor the absorption and emission prop-
erties. The major absorptions of BDPs (ꢀ500 nm) are com-
plementary to those of porphyrins (ꢀ430 and 500–600 nm).
Therefore, the resulting conjugates absorb over a broad
range in the visible region. For these conjugates, there is
also good spectral overlap between the BDP (i.e., energy
donor) emission and the porphyrin (i.e., energy acceptor)
absorption. Both of these features are desirable for efficient
intramolecular energy transfer. Hence, a combination of
BDPs, porphyrins, and fullerenes is highly desirable for en-
hancing the light-harvesting efficiency throughout the solar
spectrum, and for converting the harvested light into the
high-energy state of charge separation by photoinduced
electron transfer.[13]
Figure 1. Structure of the pentad (BDP)3–ZnP–C60 and illustration of the
photoinduced intramolecular processes involved.
4-(prop-2-ynyloxy)benzaldehyde (1) and 4-hydroxybenzalde-
hyde (in 3:1 mole ratio) with pyrrole gave the “3+1” por-
phyrin 2, which could be isolated from the reaction mixture
by column chromatography. Treatment of this compound
with ZnACHTNUTGRENUNG(OAc)2·2H2O led to metalation, giving the zinc(II)
In fact, several studies have already demonstrated the ex-
cited-state interactions between light-harvesting BDP subu-
nits and energy acceptors such as zinc(II) porphyrins,[13] per-
ylenediimides,[14] subphthalocyanines,[15] and zinc(II) and sili-
con(IV) phthalocyanines.[9c,16] A few BDP-containing supra-
molecular systems have been successfully constructed by co-
porphyrin 3. This compound was then treated successively
with 1,3-dibromopropane and 4-hydroxybenzaldehyde in the
presence of K2CO3 in N,N-dimethylformamide (DMF), to
afford the aldehyde 5 via the bromide 4. The porphyrin-con-
jugated fulleropyrrolidine 6 was prepared by the treatment
of 5 with C60 and sarcosine according to the Prato proce-
dure.[17] In the final step, click chemistry, which involves CuI-
catalyzed 1,3-dipolar cycloaddition between azides and al-
kynes,[18] was employed to link up the porphyrin–fullerene
and BDP components. The alkyne-substituted porphyrin–C60
conjugate 6 was treated with azido-BDP 7 in the presence
of sodium ascorbate and CuSO4·5H2O in a mixture of
CHCl3, EtOH, and water (12:1:1 by volume) at room tem-
perature, to give the pentad (BDP)3–ZnP–C60 (8). The re-
ordinating C60 derivatives functionalized with either
pyridine or imidazole ligand to zinc(II) porphyrins or
phthalo
cyanines.[9c,13a,b] However, to the best of our knowl-
a
ACHTUNGTRENNUNG
edge, covalently-linked BDP–porphyrin–fullerene conju-
gates so far remain extremely rare.[13c]
Herein we report the synthesis and photophysical proper-
ties of a novel pentad in which three BDP units serving as
the antennae are linked covalently to a zinc(II) porphyrin–
C60 conjugate. This pentad mimics the charge separation of
the reaction center (Figure 1). Compared with the axially
bound BDP–zinc(II)–porphyrin/phthalocyanine and C60
supramolecular systems,[9c,13a,b] this covalent pentad has ad-
vantage that the photogenerated charge-separated states
possess significant stability. The photoinduced energy- and
electron-transfer processes of this pentad have been studied
in detail by steady-state absorption and fluorescence spec-
troscopy, and using femtosecond and nanosecond laser pho-
tolysis techniques.
AHCTUNGTREGaNNUN ction was essentially completed in 24 h with a very good
isolated yield (88% after chromatographic purification).
Similarly, the reference compound (BDP)3–ZnP (10) with-
out the C60 moiety was prepared by treating 5,10,15,20-tetra-
kis[4-(prop-2-ynyloxy)phenyl]porphyrin (9) with three
equivalents of azido-BDP 7 (Scheme 2). As another refer-
ence compound, the fulleropyrrolidine 11 was also prepared
by the Prato reaction of C60
with
4-pyridinecarboxalde-
hyde.[19] All the new compounds
were characterized using vari-
ous spectroscopic methods, in-
cluding 1H and 13C{1H} NMR
spectroscopy, ESI or MALDI-
TOF mass spectrometry, and
accurate mass measurements.
Results and Discussion
Synthesis and characterization: Scheme 1 shows the synthet-
ic route used to prepare this pentad. Mixed condensation of
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Mimicking Photosynthetic Antenna-Reaction-Center Complexes
FULL PAPER
Scheme 1. Synthesis of the pentad (BDP)3–ZnP–C60 (8).
Steady-state absorption spectroscopy: To reveal the elec-
tronic interactions between the components in the pentad
(BDP)3–ZnP–C60 (8), we recorded its ground-state absorp-
tion spectrum in benzonitrile, and compared it with the
spectra of 7, 9, and 11, which served as the reference com-
pounds. As shown in Figure 2, the pentad 8 exhibits five
major absorption bands at 331, 434, 504, 566, and 607 nm,
which are shifted at most by 2 nm compared to those of the
reference compounds. This observation suggests that the
chromophoric components in 8 do not have significant
ground-state interactions. Similarly, the spectrum of the
tetrad (BDP)3–ZnP (10) in benzonitrile overlaps well with
those of 7 and 9 (Figure S1 in the Supporting Information),
indicating that the BDP and porphyrin units in 10 are elec-
tronically decoupled in the ground state.
Figure 2. Normalized absorption spectra of 7, 8, 9, and 11 in benzonitrile.
The spectra of 7 and 8 were normalized at 504 nm, those of 8 and 9 were
normalized at 434 nm, and those of 8 and 11 were normalized at 330 nm.
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S. Fukuzumi, D. K. P. Ng et al.
Scheme 2. Synthesis of the reference compound (BDP)3–ZnP (10).
Steady-state fluorescence spectroscopy: The steady-state
fluorescence spectra of compounds 7–10 in benzonitrile are
shown in Figure 3. Upon excitation at 490 nm, at which only
the BDP moieties have an absorption, the tetrad (BDP)3–
ZnP (10) gave an emission band at 615 nm due to the
zinc(II) porphyrin core. The BDP emission at ꢀ516 nm (as
seen in the spectrum of BDP 7) was greatly reduced. The
fluorescence quantum yield of 10 (Ff =0.026) was found to
be much smaller than that of BDP 7 (Ff =0.35). Excitation
of the reference compound 9 at the same position did not
give the emission band at 615 nm. These observations clear-
ly indicate the occurrence of an efficient photoinduced
energy transfer from the excited BDP units to the zinc(II)
porphyrin core in 10. The excitation spectra of 9 and 10
were also recorded by monitoring the porphyrin emission
(Figure S2 in the Supporting Information). The spectrum of
10 revealed an additional band at ꢀ500 nm, which was as-
signed to the BDP absorption. This result provided further
evidence for the energy-transfer process in 10.
Upon direct excitation at the zinc(II) porphyrin core of 10
at 550 nm, a strong emission band at 615 nm was also ob-
served, the intensity of which was close to that of the refer-
ence compound ZnP 9 (Figure 3). The results suggest that
the BDP moieties do not significantly quench the singlet ex-
cited state of porphyrin in 10 by energy- and/or electron-
transfer processes.
On the other hand, the fluorescence of the pentad
(BDP)3–ZnP–C60 (8) was strongly quenched compared with
that of the reference compound 10, both at 490 and 550 nm
excitation (Figure 3). The fluorescence quantum yield of 8
(Ff =0.006) was found to be much smaller than that of 10
(Ff =0.026). The quenching pathways may involve energy
transfer and/or electron transfer from the excited porphyrin
to the C60 moiety. Energetically, these two pathways are fea-
sible. However, as the fluorescence emission of C60 at
ꢀ720 nm was not observed,[10e] the energy-transfer process
can be excluded. Thus, photoinduced electron transfer may
take place from the singlet excited state of the porphyrin
core, which can be populated by excitation energy transfer
from the peripheral BDP substituents to the covalently
linked fullerene entity.
Electrochemistry: To estimate the driving force for the elec-
tron-transfer processes, we studied the electrochemical prop-
erties of pentad 8 by differential pulse voltammetry in de-
Figure 3. Fluorescence spectra of BDP (7), (BDP)3–ZnP–C60 (8), ZnP
(9), and (BDP)3–ZnP (10) in benzonitrile. lex =490 nm (upper) or 550 nm
(lower).
AHCTUNGTRENNUNG
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Spectroscopic evidence for the radical ion pair formation
in 8 was obtained from femtosecond transient absorption
measurements in deaerated benzonitrile by using a 430 nm
laser source, with which the porphyrin core was selectively
excited. The time-resolved spectra at >6 ps showed the
characteristic absorption bands of the radical ion pair
(BDP)3–ZnP +–C60 (Figure 6).[21,22] The characteristic ab-
C
CÀ
Figure 4. Differential pulse voltammogram of (BDP)3–ZnP–C60 (8) in
deaerated benzonitrile. Scan rate=50 mVsÀ1
.
1.20 V relative to the saturated calomel electrode (SCE);
these oxidations are due to the porphyrin and BDP units, re-
spectively. The reduction potential couples at À0.58, À0.98,
and À1.45 V are assigned to the C60 moiety, whereas the po-
tential couple at À1.18 V is due to the reduction process of
the BDP units.[13c,16] By using these electrochemical data and
the Rehm–Weller equation, we estimated the free-energy
change of charge separation (DGCS) in 8 (through changing
from (BDP)3–1ZnP*–C60 to (BDP)3–ZnP +–C60 in benzoni-
trile) to be À0.78 eV.[20] The corresponding value in toluene
was also determined (À0.43 eV) for comparison. The nega-
tive values show that this electron-transfer process is ther-
modynamically favorable, both in polar benzonitrile and
nonpolar toluene. Furthermore, the charge separation via
the triplet excited state of porphyrin (BDP)3–3ZnP*–C60 in
benzonitrile (DGTCS ꢀÀ0.26 eV) seems feasible, whereas it is
unfavorable in toluene due to energetic considerations.
C
CÀ
Figure 6. Femtosecond absorption spectra of (BDP)3–ZnP–C60 (8) in de-
AHCTUNGTREGNaNNU erated benzonitrile (lex =430 nm). The inset shows the time profile at
1000 nm.
sorption band of the C60 radical anion in the near-IR region
at ꢀ1000 nm was employed as a reliable probe to determine
the rate constants for both charge-separation and charge-re-
combination processes. From the rise profile, the rate of
charge separation (kCS S) was found to be 4.6ꢀ1011 sÀ1, indi-
cating a fast charge-separation process. The rate of charge
recombination (kCS R) was determined to be 2.1ꢀ109 sÀ1 from
the decay profile. Based on this value, the lifetime of the
charge-separated state (tSCS =1/kCS R) was calculated as 476 ps,
which is nearly twice as long as that of a related BDP–ZnP–
Femtosecond transient absorption measurements: Upon se-
lective excitation at the BDP moieties at 470 nm, the femto-
second absorption spectrum of 10 in benzonitrile at 1 ps
1
could be assigned to BDP*. However, the spectra at >10 ps
revealed an absorption band at ꢀ460 nm (Figure 5), which
1
1
suggested the formation of ZnP*. The formation of ZnP*
was clearly due to the excitation energy transfer from the
three excited BDP moieties to the porphyrin core. The time
C
60 system.[13b]
1
profile of ZnP* showed a fast rise within the initial 400 ps,
The femtosecond absorption spectra of 8 in deaerated tol-
followed by a slow decay. From the rise profile, the rate of
the singlet–singlet energy transfer was determined to be
2.7ꢀ1010 sÀ1.
uene (Figure 7) showed similar features to those recorded in
benzonitrile. kCS S was estimated to be 1.7ꢀ1011 sÀ1. From the
decay profile of the C60 radical anion, kSCR and tCS S were
found to be 1.0ꢀ109 sÀ1 and 1000 ps, respectively. The obser-
vation that the value of kSCR slightly increases along with the
solvent polarity (or the lifetime of the charge-separated
state is longer in a less polar solvent) can be understood if
the charge-recombination process occurs in the Marcus in-
verted regions.[23–25]
Photoinduced intramolecular events in microsecond time re-
gions: It is essential to follow the dynamics in microsecond
time regions by using nanosecond laser flash photolysis to
determine the contribution of the triplet excited states to
the electron-transfer processes. Upon ZnP excitation at
430 nm, the nanosecond transient spectra of the reference
compound (BDP)3–ZnP (10) in deaerated benzonitrile ex-
hibited an absorption band at ꢀ460 nm, and a number of
broad absorptions in the region of 600–900 nm (Figure 8).
Figure 5. Femtosecond absorption spectra of (BDP)3–ZnP (10) in benzo-
nitrile (lex =470 nm). The inset shows the time profile at 460 nm.
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S. Fukuzumi, D. K. P. Ng et al.
Figure 9. Nanosecond transient spectra of (BDP)3–ZnP–C60 (8) in de-
AHCTUNTGRENNGaUN erated benzonitrile (0.05 mm) (lex =490 nm). The inset shows the time
profile at 1000 nm.
Figure 7. Femtosecond absorption spectra of (BDP)3–ZnP–C60 (8) in de-
aerated toluene (lex =430 nm). The inset shows the time profile at
ACHTUNGTRENNUNG
1000 nm.
1
3
(ISC) from ZnP*. With the decay of ZnP*, the absorption
bands at 650 and 1000 nm emerged, which suggested the
generation of (BDP)3–ZnP +–C60 via 3ZnP*. From the
decay of the C60 radical-anion band at 1000 nm, the rate of
C
CÀ
charge recombination (kTCR) via ZnP* was found to be 2.6ꢀ
3
106 sÀ1. Based on this value, the lifetime of the charge-sepa-
rated state (tTCS =1/kCTR) was estimated to be 385 ns, which is
significantly longer than that of the reported BDP/zinc–
phthalocyanine/C60 supramolecular triad (39.9 ns in tolue-
ne).[9c] The quantum yield of the triplet charge-separated
state was found to be 0.27.[26] As the decay followed first-
order kinetics, this ruled out the possibility of intermolecular
electron transfer. Similar spectral features were observed
when the ZnP part of 8 was excited at 430 nm (Figure S5 in
the Supporting Information).
Figure 8. Nanosecond transient spectra of (BDP)3–ZnP (10) in deaerated
benzonitrile (0.04 mm) (lex =430 nm). The inset shows the time profile at
460 nm.
Energy diagrams: Figure 10 illustrates the energy-level dia-
grams summarizing the observed photoinduced intramolecu-
lar events of 8, in both benzonitrile and toluene. Some of
the data were taken from the model compounds, and it is as-
sumed that they are very similar to those of 8. In both sol-
3
These absorption bands are due to ZnP*, which was con-
firmed by comparison with the nanosecond transient spectra
of 9 (Figure S3 in the Supporting Information). By fitting
the decay of the absorption band at 460 nm with first-order
1
1
vents, BDP* decays to populate ZnP* through singlet–sin-
1
glet energy transfer. The ZnP* then transfers an electron to
3
kinetics, it was found that the ZnP* decayed to populate
the C60 unit to generate the charge-separated species
the ground state with a rate constant of 1.1ꢀ104 sÀ1.
By selective excitation at the BDP units of 10 using a
500 nm laser source, the nanosecond transient spectra also
(BDP)3–ZnP +–C60
,
which subsequently decays to the
C
CÀ
ground state with lifetimes of 476 and 1000 ps in benzoni-
trile and toluene, respectively. On the other hand, the
3
3
showed the characteristic absorptions of ZnP*, but not of
charge separation from ZnP* to C60 was also observed in
3BDP* (Figure S4 in the Supporting Information). This can
benzonitrile. The resulting (BDP)3–ZnP +–C60 with a trip-
let spin character has a relatively long lifetime of 385 ns.
Thus, for the pentad (BDP)3–ZnP–C60, the lifetimes of both
the singlet and triplet radical ion pairs could be determined.
C
CÀ
1
be explained by considering that the initially formed BDP*
decayed by singlet–singlet energy transfer to populate
1ZnP* (as suggested from the steady-state emission and fem-
3
tosecond spectroscopic studies), which relaxed to ZnP*. Fi-
nally, this triplet excited ZnP decayed to the ground state
with a rate constant of 1.1ꢀ104 sÀ1.
Conclusion
For the pentad (BDP)3–ZnP–C60 (8), the nanosecond tran-
sient spectra showed quite different features compared to
those of 10. As shown in Figure 9, when the BDP units of 8
were excited at 490 nm, the transient spectra of 8 in deaerat-
ed benzonitrile revealed the absorption bands of 3ZnP*,
which indicated the occurrence of intersystem crossing
We have prepared and characterized
a novel pentad
(BDP)3-ZnP-C60 (8), in which three BDP units are covalent-
ly linked to a ZnP–C60 conjugate. This pentad serves as an
excellent model for artificial photosynthetic antenna-re-
AHCTUNGTREGaNNUN ction-center complexes. It exhibits a good spectral coverage
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Mimicking Photosynthetic Antenna-Reaction-Center Complexes
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9,[29] and 11[19] were prepared as described. All other solvents and re-
agents were of reagent grade and used as received. Chromatographic pu-
rifications were performed on silica gel (Macherey–Nagel, 70–230 mesh)
and neutral alumina columns with the indicated eluents. Size-exclusion
chromatography was carried out on Bio-Rad Bio-Beads S-X1 beads
(200–400 mesh).
1H and 13C{1H} NMR spectra were recorded on a Bruker AVANCE III
400 (1H, 400; 13C, 100.6 MHz) spectrometer in CDCl3 or [D6]DMSO.
Spectra were referenced internally using the residual solvent (1H: CDCl3
(d=7.26 ppm); [D6]DMSO (d=2.50 ppm)] or solvent [13C: CDCl3 (d=
77.0 ppm); [D6]DMSO (d=39.7 ppm)) resonances relative to SiMe4.
MALDI-TOF mass spectra were taken on a Bruker Daltonics Autoflex
MALDI-TOF mass spectrometer. ESI mass spectra were measured on a
Thermo Finnigan MAT 95 XL mass spectrometer.
Differential pulse voltammograms were measured on a BAS CV-50W
voltammetric analyzer. A platinum disk electrode was used as the work-
ing electrode, and a platinum wire served as the counter electrode. SCE
was used as the reference electrode. All measurements were carried out
in deaerated benzonitrile containing 0.1m [NBu4]
ACHTUNGRTEN[NUGN ClO4] as the supporting
electrolyte, at a scan rate of 50 mVsÀ1
.
Femtosecond laser flash photolysis was conducted using a Clark-MXR
2010 laser system and an optical detection system provided by Ultrafast
Systems (Helios). The source for the pump-and-probe pulses were de-
rived from the fundamental output of the Clark laser system (775 nm,
1 mJ pulseÀ1, and fwhm=150 fs) at a repetition rate of 1 kHz. A second
harmonic generator introduced in the path of the laser beam provided
412 nm laser pulses for excitation. 95% of the fundamental output of the
laser was used to generate the second harmonic, and 5% of the deflected
output was used for white light generation. Prior to generating the probe
continuum, the laser pulse was fed to a delay line that provided an exper-
imental time window of 1.6 ns, with a maximum step resolution of 7 fs.
The pump beam was attenuated at 5 mJ pulseÀ1 with a spot size of 2 mm
diameter at the sample cell where it was merged with the white probe
pulse in a close angle (<108). The probe beam, after passing through the
2 mm sample cell, was focused on a 200 mm fiber optic cable which was
connected to a CCD spectrograph (Ocean Optics, S2000-UV-vis for visi-
ble region and Horiba, CP-140 for NIR region) for recording the time-re-
solved spectra (450–800 and 800–1400 nm). Typically, 5000 excitation
pulses were averaged to obtain the transient spectrum at a set delay time.
The kinetic traces at appropriate wavelengths were assembled from the
time-resolved spectral data.
Figure 10. Energy level diagrams showing the photoinduced intramolecu-
lar events of (BDP)3–ZnP–C60 (8) in benzonitrile (upper) and toluene
(lower).
in the visible region (300–700 nm), and undergoes a highly
efficient energy migration from the BDP-based antennae to
the ZnP core, followed by electron transfer to the C60
+
C
moiety to generate a charge-separated state (BDP)3–ZnP
–
Nanosecond time-resolved transient absorption measurements were car-
ried out using the laser system provided by UNISOKU Co., Ltd. Mea-
CÀ
C60 . From a mechanistic point of view, the advantage of
this covalently linked pentad is that the lifetimes of both the
AHCTUNGERTGsNNUN urements of nanosecond transient absorption spectra were performed
C
CÀ
singlet and triplet (BDP)3–ZnP +–C60 could be determined.
From the femtosecond transient absorption studies, the life-
according to the following procedure. A deaerated solution containing
the dyad was excited by a Panther OPO pumped by an Nd:YAG laser
(Continuum, SLII-10, 4–6 ns fwhm) at l=590 nm. The photodynamics
were monitored by continuous exposure to a xenon lamp (150 W) as a
probe light and a photomultiplier tube (Hamamatsu 2949) as a detector.
TranACTHNUGRTNEUNGsient spectra were recorded using fresh solutions in each laser excita-
tion. The solution was deoxygenated by purging with argon for 15 min
prior to measurements.
times of the singlet (BDP)3–ZnP +–C60 were determined to
be 476 and 1000 ps in benzonitrile and toluene, respectively.
Interestingly, a relatively slow charge-recombination process
C
CÀ
T
6
À1
+
C
(kCR =2.6ꢀ10 s ) and a long-lived triplet (BDP)3–ZnP
–
CÀ
T
C60 (tCS =385 ns) were detected in benzonitrile by nanosec-
ond transient measurements. The formation of a relatively
long-lived charge-separated state can be attributed to its
triplet character, which makes the charge recombination
back to the singlet ground state a forbidden process.
Porphyrin 2: Compound 1 (9.60 g, 0.06 mol) and 4-hydroxybenzaldehyde
(2.44 g, 0.02 mol) were dissolved in propionic acid (300 mL) with stirring.
The resulting solution was heated to reflux, then freshly distilled pyrrole
(5.6 mL, 0.08 mol) was added dropwise. The mixture turned rapidly to a
dark purple color. The mixture was stirred for 3 h under reflux condi-
tions, cooled briefly, then concentrated to about 100 mL under reduced
pressure. The mixture was precipitated by adding 200 mL of methanol.
The precipitate was collected by filtration and washed with methanol
(20 mLꢀ2). The purple crude product was purified by column chroma-
tography on neutral alumina using CHCl3/EtOH (v/v 10:1) as the eluent.
The product was further purified by silica gel column chromatography
using CHCl3 as the eluent (0.79 g, 5%). 1H NMR ([D6]DMSO): d=10.0
(brs, 1H; OH), 8.82–8.92 (m, 8H; pyrrole-H), 8.15 (d, J=8.4 Hz, 6H;
ArH), 8.00 (d, J=8.4 Hz, 2H; ArH), 7.43 (d, J=8.4 Hz, 6H; ArH), 7.21
(d, J=8.4 Hz, 2H; ArH), 5.11 (d, J=2.4 Hz, 6H; CH2), 3.77 (t, J=
2.4 Hz, 3H; alkyne-H), À2.91 ppm (s, 2H; NH); 13C{1H} NMR
Experimental Section
All the reactions were performed under an atmosphere of nitrogen. Pyr-
role and propionic acid were distilled prior to use. Tetrahydrofuran
(THF), toluene, and DMF were distilled from sodium benzophenone
ketyl, sodium, and barium oxide, respectively. Toluene and benzonitrile
used for photophysical measurements were of spectroscopic grade (Al-
drich), and were used without prior purification. Compounds 1,[27] 7,[28]
Chem. Eur. J. 2011, 17, 1605 – 1613
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1611

S. Fukuzumi, D. K. P. Ng et al.
([D6]DMSO): d=157.7, 157.4, 135.7, 135.5, 134.4, 132.0, 131.5 (brs),
120.6, 119.7, 119.5, 114.1, 113.5, 79.6, 78.8, 55.9 ppm; MS (ESI): m/z (%):
793 (100) [M+H]+; HRMS (ESI): m/z calcd for C53H37N4O4 [M+H]+:
793.2809; found: 793.2800.
pyrrole-H), 8.18 (d, J=8.4 Hz, 2H; ArH), 8.07 (d, J=7.6 Hz, 4H; ArH),
7.78 (d, J=8.4 Hz, 2H; ArH), 7.56 (brs, 2H; ArH), 7.41 (d, J=8.8 Hz,
2H; ArH), 7.35 (d, J=8.8 Hz, 4H; ArH), 7.15 (d, J=8.4 Hz, 2H; ArH),
7.00 (brd, J=8.0 Hz, 2H; ArH), 5.02 (d, J=2.4 Hz, 2H; CH2), 4.98 (d,
J=2.4 Hz, 4H; CH2), 4.69 (d, J=9.2 Hz, 1H; CH), 4.59 (s, 1H; CH),
4.56 (t, J=5.4 Hz, 2H; CH2), 4.28 (t, J=5.4 Hz, 2H; CH2), 3.94 (d, J=
9.2 Hz, 1H; CH), 2.72 (t, J=2.4 Hz, 1H; alkyne-H), 2.70 (t, J=2.4 Hz,
2H; alkyne-H), 2.69 (s, 3H; CH3), 2.40 ppm (quintet, J=5.4 Hz, 2H;
CH2); HRMS (MALDI-TOF): m/z calcd for C125H49N5O5Zn [M]+:
1765.3049; found: 1765.3013.
Porphyrin 3: A solution of ZnACHTNUTRGNE(NUG OAc)2·2H2O (1.10 g, 5.0 mmol) in metha-
nol (15 mL) was added dropwise to a solution of porphyrin 2 (0.40 g,
0.5 mmol) in CHCl3 (50 mL). The mixture was heated under reflux for
3 h, and then the volatiles were removed under reduced pressure. The
residue was subjected to chromatography on a silica gel column using
CHCl3/MeOH (v/v 50:1) as the eluent. The product was obtained as a
purple solid (0.42 g, 98%). 1H NMR ([D6]DMSO): d=9.95 (s, 1H; OH),
8.87 (d, J=4.8 Hz, 2H; pyrrole-H), 8.78–8.85 (m, 6H; pyrrole-H), 8.10
(d, J=8.4 Hz, 6H; ArH), 7.99 (d, J=8.4 Hz, 2H; ArH), 7.38 (d, J=
8.4 Hz, 6H; ArH), 7.20 (d, J=8.4 Hz, 2H; ArH), 5.07 (d, J=2.4 Hz, 6H;
CH2), 3.74 ppm (t, J=2.4 Hz, 3H; alkyne-H); 13C{1H} NMR
([D6]DMSO): d=157.2, 157.1, 150.1, 149.8, 149.7, 136.0, 135.6, 135.5,
133.7, 132.0, 131.8, 131.7, 121.0, 120.0, 119.9, 113.9, 113.2, 79.8, 78.8,
56.0 ppm; MS (ESI): m/z (%): 854 (100) [M]+; HRMS (ESI): m/z calcd
for C53H34N4O4Zn [M]+: 854.1866; found: 854.1873.
A
A mixture of compounds 6 (44 mg,
7
0.027 mmol), and sodium ascorbate (10.8 mg, 0.054 mmol) in a 12:1:1
mixture of CHCl3, EtOH, and water (7 mL) was stirred at room tempera-
ture for 24 h. After removing the volatiles in vacuo, the residue was puri-
fied by silica gel column chromatography using CHCl3/methanol (100:1
v/v) as the eluent, followed by size-exclusion chromatography using THF
as the eluent. The crude product was further purified by recrystallization
from a mixture of CHCl3 and hexane (66 mg, 88%). 1H NMR (CDCl3):
d=8.93 (d, J=4.4 Hz, 2H; pyrrole-H), 8.90 (d, J=4.4 Hz, 2H; pyrrole-
H), 8.82 (d, J=4.4 Hz, 2H; pyrrole-H), 8.79 (d, J=4.4 Hz, 2H; pyrrole-
H), 8.13 (d, J=8.4 Hz, 2H; ArH), 8.04 (d, J=8.0 Hz, 4H; ArH), 7.83 (s,
1H; triazole-H), 7.80 (d, J=8.4 Hz, 2H; ArH), 7.77 (s, 2H; triazole-H),
7.59 (brd, J=7.2 Hz, 2H; ArH), 7.31 (d, J=8.4 Hz, 2H; ArH), 7.20–7.28
(m, 4H; ArH), 7.13–7.16 (m, 8H; ArH), 6.96 (virtual t, J=8.8 Hz, 8H;
ArH), 5.87 (s, 6H; pyrrole-H), 5.21 (s, 2H; CH2), 5.11 (s, 4H; CH2),
4.67–4.76 (m, 7H; CH2 and CH), 4.64 (s, 1H; CH), 4.51 (t, J=5.2 Hz,
2H; CH2), 4.35–4.41 (m, 6H; CH2), 4.27 (t, J=5.2 Hz, 2H; CH2), 4.00 (d,
J=9.6 Hz, 1H; CH), 2.68 (s, 3H; CH3), 2.47 (s, 18H; CH3), 2.36–2.44 (m,
2H; CH2), 1.36 ppm (s, 18H; CH3); HRMS (MALDI-TOF): m/z calcd
for C188H115B3F6N20O8Zn [M]+: 2992.8745; found: 2992.8711.
Porphyrin 4: Anhydrous potassium carbonate (0.55 g, 4.0 mmol) was
added to a mixture of compound 3 (0.17 g, 0.2 mmol) and 1,3-dibromo-
propane (0.81 g, 4.0 mmol) in DMF (16 mL). The resulting mixture was
stirred at room temperature overnight. The volatiles were then evaporat-
ed under reduced pressure, and the residue was mixed with CHCl3
(30 mL) and water (30 mL). The aqueous layer was separated and ex-
tracted with CHCl3 (30 mLꢀ3). The combined organic fractions were
dried over anhydrous MgSO4, and then filtered. The filtrate was collected
and evaporated to dryness. The residue was purified by column chroma-
tography on neutral alumina using CHCl3 as the eluent to give compound
4 as a purple solid (0.19 g, 97%). 1H NMR (CDCl3): d=8.96–9.01 (m,
8H; pyrrole-H), 8.13 (d, J=8.4 Hz, 6H; ArH), 8.10 (d, J=8.4 Hz, 2H;
ArH), 7.29 (d, J=8.4 Hz, 6H; ArH), 7.19 (d, J=8.4 Hz, 2H; ArH), 4.88
(d, J=2.0 Hz, 6H; CH2), 4.29 (t, J=6.0 Hz, 2H; CH2), 3.74 (t, J=6.0 Hz,
2H; CH2), 2.65 (t, J=2.0 Hz, 3H; alkyne-H), 2.46 ppm (quintet, J=
6.0 Hz, 2H; CH2); 13C{1H} NMR (CDCl3): d=158.2, 157.2, 150.5, 150.4,
136.1, 135.4, 135.3, 131.9, 120.7, 120.5, 112.9, 112.6, 78.7, 75.8, 65.5, 56.1,
32.5, 30.2 ppm; MS (ESI): m/z (%): 976 (100) [M]+; HRMS (ESI): m/z
calcd for C56H39BrN4O4Zn [M]+: 976.1417; found: 976.1406.
AHCTUNGRTEG(NUNN BDP)3–ZnP tetrad 10: A mixture of compounds 7 (123 mg, 0.30 mmol)
and 9 (89 mg, 0.10 mmol), CuSO4·5H2O (7.5 mg, 0.03 mmol), and sodium
ascorbate (11.9 mg, 0.06 mmol) in a 12:1:1 mixture of CHCl3, EtOH, and
water (7 mL) was stirred at room temperature for 24 h. After removing
the volatiles in vacuo, the residue was purified by silica gel column chro-
matography using CHCl3/methanol (100:1 v/v) as the eluent, followed by
size-exclusion chromatography using THF as the eluent. The crude prod-
uct was further purified by recrystallization from a mixture of CHCl3 and
hexane (98 mg, 46%). 1H NMR ([D6]DMSO): d=8.77–8.82 (m, 8H; pyr-
role-H), 8.52 (s, 2H; triazole-H), 8.51 (s, 1H; triazole-H), 8.03–8.10 (m,
8H; ArH), 7.41–7.47 (m, 6H; ArH), 7.38 (d, J=8.8 Hz, 2H; ArH), 7.25
(d, J=8.8 Hz, 6H; ArH), 7.16 (d, J=8.8 Hz, 6H; ArH), 6.02 (s, 6H; pyr-
role-H), 5.45 (s, 4H; CH2), 5.44 (s, 2H; CH2), 5.08 (d, J=2.4 Hz, 2H;
CH2), 4.92 (t, J=4.8 Hz, 6H; CH2), 4.55 (t, J=4.8 Hz, 6H; CH2), 3.75 (t,
J=2.4 Hz, 1H; alkyne-H), 2.38 (s, 18H; CH3), 1.33 ppm (s, 18H; CH3);
13C{1H} NMR ([D6]DMSO): d=158.7, 157.8, 157.0, 154.8, 149.8, 143.1,
142.9, 142.1, 135.9, 135.5, 135.4, 131.7, 131.2, 129.4, 126.7, 125.6, 121.4,
120.1, 120.0, 115.5, 113.0, 79.7, 78.7, 66.5, 61.5, 55.9, 49.4, 14.4 ppm; MS
(ESI): m/z (%): 2145 (100) [M+Na]+; HRMS (ESI): m/z calcd for
C119H102B3F6N19NaO7Zn [M+Na]+: 2144.7643; found: 2144.7628.
Porphyrin 5: Anhydrous potassium carbonate (0.08 g, 0.6 mmol) was
added to a mixture of compound 4 (0.20 g, 0.2 mmol) and 4-hydroxy-
ACHTUNGTRENNUNGbenzaldehyde (0.07 g, 0.6 mmol) in DMF (30 mL). The resulting mixture
was stirred at 808C for 10 h. The solvent was then evaporated under re-
duced pressure, and the residue was mixed with CHCl3 (40 mL) and
water (40 mL). The aqueous layer was separated and extracted with
CHCl3 (40 mLꢀ3). The combined organic fractions were dried over an-
hydrous MgSO4, and then filtered. The filtrate was collected and evapo-
rated to dryness. The residue was subjected to silica gel column chroma-
tography using CHCl3 as the eluent, to give compound 5 as a purple solid
(0.18 g, 89%). 1H NMR (CDCl3): d=9.82 (s, 1H; CHO), 8.96 (d, J=
3.2 Hz, 8H; pyrrole-H), 8.13 (d, J=8.8 Hz, 6H; ArH), 8.11 (d, J=8.8 Hz,
2H; ArH), 7.82 (d, J=8.8 Hz, 2H; ArH), 7.34 (d, J=8.8 Hz, 6H; ArH),
7.26 (d, J=8.8 Hz, 2H; ArH), 7.05 (d, J=8.8 Hz, 2H; ArH), 4.96 (d, J=
2.0 Hz, 6H; CH2), 4.42 (t, J=6.0 Hz, 2H; CH2), 4.37 (t, J=6.0 Hz, 2H;
CH2), 2.69 (t, J=2.0 Hz, 3H; alkyne-H), 2.46 ppm (quintet, J=6.0 Hz,
2H; CH2); 13C{1H} NMR (CDCl3): d=190.7, 163.7, 158.1, 157.1, 150.3,
150.2, 136.3, 135.6, 135.4, 135.3, 131.8, 131.7, 129.7, 120.4, 120.2, 114.6,
112.8, 112.4, 78.7, 75.8, 64.8, 64.2, 56.0, 29.1 ppm; MS (ESI): m/z (%):
1016 (100) [M]+; HRMS (ESI): m/z calcd for C63H44N4O6Zn [M]+:
1016.2547; found: 1016.2521.
Acknowledgements
This work was supported financially by a strategic investments scheme
administrated by The Chinese University of Hong Kong, a Grant-in-Aid
(No. 20108010), a Global COE program “the Global Education and Re-
search Center for Bio-Environmental Chemistry” from the Japan Society
of Promotion of Science (JSPS), and KOSEF/MEST through the WCU
project (R31-2008-000-10010-0).
ZnP–C60 dyad 6: Aldehyde 5 (51 mg, 0.05 mmol) and sarcosine (69 mg,
0.78 mmol) were added to a solution of C60 (72 mg, 0.1 mmol) in toluene
(50 mL). The mixture was heated under reflux for 8 h, then allowed to
cool slowly to room temperature. The volatiles were then removed under
reduced pressure. The residue was purified by silica gel column chroma-
tography using toluene, and then toluene/CHCl3 (v/v 2:1), as the eluents.
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The product was obtained as a dark purple solid (48 mg, 54%). H NMR
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1612
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1605 – 1613

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Received: August 14, 2010
Published online: January 5, 2011
Chem. Eur. J. 2011, 17, 1605 – 1613
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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