Ultrafast photoinduced energy and electron transfer in multi-modular donor-acceptor conjugates

Article abstract of DOI:10.1002/chem.201202265

New multi-modular donor-acceptor conjugates featuring zinc porphyrin (ZnP), catechol-chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C₆₀), or naphthalenediimide (NDI) have been newly designed and synthesized as photosynthetic

Full text of DOI:10.1002/chem.201202265

FULL PAPER
DOI: 10.1002/chem.201202265
Ultrafast Photoinduced Energy and Electron Transfer in Multi-Modular
Donor–Acceptor Conjugates
Mohamed E. El-Khouly,[a, e] Channa A. Wijesinghe,[b] Vladimir N. Nesterov,[c]
Melvin E. Zandler,[b] Shunichi Fukuzumi,*[a, d] and Francis DꢀSouza*[c]
Abstract: New multi-modular donor–
acceptor conjugates featuring zinc por-
phyrin (ZnP), catechol-chelated boron
dipyrrin (BDP), triphenylamine (TPA)
and fullerene (C60), or naphthalenedi-
and favorable orientation of the enti-
ties. Introducing either fullerene or
naphthalenediimide electron acceptors
to the TPA-BDP-ZnP triad through
metal–ligand axial coordination result-
ed in electron donor–acceptor polyads
whose structures were revealed by
spectroscopic, electrochemical and
computational studies. Excitation of
the electron donor, zinc porphyrin re-
sulted in rapid electron-transfer to co-
ordinated fullerene or naphthalenedi-
AHCTUNGTRENNiGUN mide yielding charge separated ion-
A
pair species. The measured electron
transfer rate constants from femtosec-
ond transient spectral technique in
non-polar toluene were in the range of
5.0ꢁ109–3.5ꢁ1010 sꢀ1. Stabilization of
the charge-separated state in these
multi-modular donor–acceptor polyads
is also observed to certain level.
and synthesized as photosynthetic an-
tenna and reaction-center mimics. The
X-ray structure of triphenylamine-BDP
is also reported. The wide-band captur-
ing polyad revealed ultrafast energy-
transfer (kENT =1.0ꢁ1012 sꢀ1) from the
singlet excited BDP to the covalently
linked ZnP owing to close proximity
Keywords: donor–acceptor
sys-
tems · electron transfer · energy
transfer · fullerene · artificial photo-
synthesis
Introduction
ture and funneling by the antenna entities, and charge sepa-
ration and migration resulting into long-lived charge sepa-
rated states at the reaction center.[1] A variety of donor and
acceptor molecules, covering different portions of the elec-
tromagnetic spectrum and having different redox states are
being utilized in an effort to capture most of the useful sun-
light from the visible-near IR portion, ultimately resulting
into long-lived charge separated states.[2–8] The delocaliza-
tion of charges within the spherical carbon structure of the
rigid aromatic p-sphere of fullerenes[15] has captured special
attention.[4,5,7] The small reorganization energies of fuller-
enes in electron-transfer reactions resulted in fast charge
separation and relatively slow charge recombination yielding
long-lived charge separated states.[16–19] When simple donor–
acceptor dyads were appended with additional photo- or
redox active molecules resulted in polyads (triads, tetrads,
etc.) capable of undergoing sequential energy or electron or
combination of these photochemical events. The polyads
have also been utilized to build photovoltaic devices and
successful conversion of light into electricity has been dem-
onstrated,[4,5,7] revealing the importance of building polyads
of different donor and acceptor entities for solar energy-har-
vesting applications.[8,20] The use of non-covalent interac-
tions, such as metal–ligand coordination, electrostatic effects
and hydrogen bonds, has provided a much easier means of
constructing electron donor–acceptor supramolecular en-
sembles that could mimic the function of the photosynthetic
reaction center.[6–8,19,20] However, construction of polyads by
combination of covalent and non-covalent bonding has still
remained a challenge.
The capability of photosynthetic reaction center to harvest
sunlight and convert that into chemical energy using basic
chemical ingredients[1] has inspired research on artificial
photosynthesis[2–8] that has led to the developments of pho-
tovoltaics, optoelectronics and photocatalysis.[9–14] The com-
plex events of photosynthesis include wide-band light cap-
[a] 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)6-6879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[b] Dr. C. A. Wijesinghe, Prof. M. E. Zandler
Department of Chemistry, Wichita State University
Wichita, KS 67260-0051 (USA)
[c] Dr. V. N. Nesterov, Prof. F. DꢀSouza
Department of Chemistry, University of North Texas
1155 Union Circle, 305070, Denton, TX 76203-5017 (USA)
Fax : (+1)940-565-4318
E-mail: Francis.DSouza@unt.edu
[d] Prof. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University, Seoul 120-750 (Korea)
[e] Dr. M. E. El-Khouly
Current address: Department of Chemistry
Faculty of Science, Kafr el-Sheikh University (Egypt)
Supporting information for this article (including X-ray structural
data and crystal packing diagram of the triphenylamine-boron dipyr-
rin dyad, cyclic voltammograms of the studied compounds, optical ti-
tration curves for donor–acceptor formation, MALDI-MS, and nano-
second transient data of control compounds) is available on the
WWW under http://dx.doi.org/10.1002/chem.201202265.
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In the present study, we have assembled new molecular
polyads using molecular entities combined by covalent and
non-covalent bonding revealing absorption of light at differ-
ent parts of the spectrum, and at the same time, funneling
the absorbed energy to the primary electron donor center.
Such excitation transfer, as a result of matching of energy
levels of different entities, is of paramount importance to ex-
ploit energy from a wide spectral region. The structures of
the investigated triads are shown in Figure 1a and b, where-
protein environment surrounding the photosynthetic reac-
tion center is nonpolar with a low dielectric constant.
Results and Discussion
Synthesis of TPA-BDP-ZnP triad, 1: The synthesis of TPA-
BDP-ZnP triad (1) was accomplished by using a multi-step
procedure as shown in Scheme 1. For this, first, 4-(diphenyl-
AHCTUNGTREGaNNNU mino)benzaldehyde and 2,4-dimethylpyrrole were reacted
in the presence of TFA followed by DDQ to obtain the cor-
responding dipyrromethene. This was subsequently convert-
ed into BF2-chelated dipyrromethene derivative by the reac-
tion BF3 diethyletherate. This was subsequently converted
into BF2-chelated dipyrromethane derivative by the reaction
BF3 etherate. This compound was crystallized in CH2Cl2 for
X-ray diffraction studies. In a separate experiment, a free-
base porphyrin derivative was synthesized by the reaction of
3,4-dimethoxybenzaldehyde, p-tolualdehyde, and pyrrole in
propionic acid followed by demethyllation using BBr3 at
ꢀ788C.
Metallation of porphyrin using zinc acetate followed by
chromatographic separation yielded 1b. Next, compounds
1a and 1b were reacted in the presence of AlCl3 to couple
the BDP segment to porphyrin resulting in triad 1. The phe-
nylimidazole-functionalized fulleropyrrolidine and pyridine-
functionalized naphthalenediimide were carried out accord-
ing literature methods.[24] The newly synthesized compounds
1
were fully characterized by H NMR spectroscopy, MALDI-
MS, X-ray analysis for 1a, electrochemical and other spec-
troscopic methods as described in the Experimental Section.
The X-ray structure of TPA-BDP dyad, 1a is shown in
Figure 2. The molecule crystallized in the triclinic crystal
system.[22] The distance between boron of BDP and nitrogen
of TPA is 8.71 ꢃ, whereas the connecting phenyl ring of the
two entities is positioned at 71.88. The crystal packing dia-
gram (Figure S1), atomic coordinates (Table S1), bond
lengths and angles (Tables S2–S4) are given in the Support-
ing Information.
Figure 1. Structures of the multi-modular donor–acceptor polyads featur-
ing (a) triphenylamine, BDP, ZnP and imidazole functionalized fullerene
(C60Im), and (b) triphenylamine, BDP, ZnP and pyridine appended naph-
thalenediimide (NDIpy) to probe photoinduced sequential energy- and
electron-transfer events.
Optical absorption and emission studies: Figure 3 shows the
absorption spectrum of the triad 1 in o-dichlorobenzene
(DCB). With the help of control compounds Tolyl-BDP and
dyad 1a, it has been possible to assign the absorption peaks
corresponding to different entities of the triad. The ZnP
bands are located at 427, 554; and 595 nm, whereas the
BDP had strong absorbance at 510 nm at which the ZnP ab-
sorption is negligible. The TPA entity revealed absorption at
310 nm, overlapped with the UV-bands of ZnP. The negli-
gent absorbance of ZnP in the 510 nm region has given us
an opportunity to selectivity excite the BDP entity and
probe photochemical events.
Figure 4 shows the fluorescence spectrum of 1 under dif-
ferent excitation wavelengths. When excited at 550 nm cor-
responding to ZnP visible band absorption, two emission
bands at 605 and 650 nm were observed. Excitation at
510 nm corresponding to BDP absorption revealed peaks at
as compounds labeled as Tolyl-BDP and BDP-ZnP are con-
trol compounds. The covalently linked triad features TPA,
BDP, and ZnP entities are positioned in such a fashion that
excitation of either TPA or BDP results in ultrafast transfer
of energy to the terminal energy acceptor and electron
donor, ZnP. In the presence of either one of the electron ac-
ceptor (fullerene or naphthalenediimide), which is combined
with ZnP by the coordination bond, results in photoinduced
electron-transfer generating charge-separated states. The
photochemical events in these new polyads have been inves-
tigated using the femtosecond transient absorption techni-
que in nonpolar toluene, keeping in consideration that the
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Scheme 1. Synthetic methodology adopted for the preparation of TPA-BDP-ZnP triad, 1 in the present study.
Figure 3. (i) Absorption spectrum and (ii) excitation spectrum of 1 in
DCB. For recording excitation spectrum, emission monochromator was
set to 650 nm corresponding to ZnP emission, and excitation was scan-
ned.
Figure 2. Projection diagram of the TPA-BDP dyad, 1a with 50% ther-
mal ellipsoids. Disordered part of the molecule (C22A···C27A) is omitted
for clarity.
of ZnP at the same excitation conditions. The diminished in-
tensity of BDP emission and appearance of ZnP emission
suggest occurrence excited energy transfer.[23] To secure ad-
ditional proof for energy transfer, excitation spectrum was
recorded by setting the emission monochromator to por-
520 nm corresponding to BDP emission and ZnP emission
peaks at longer wavelengths. Control experiment performed
using pristine TPA-BDP, 1a revealed BDP peak of much
higher intensity (4.5 times larger) and little or no emission
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Figure 4. Fluorescence spectra of 1 in DCB under different excitation
wavelengths; (i) 550 nm corresponding to ZnP absorption, (ii) 510 nm cor-
responding to BDP absorption and (iii) 310 nm corresponding to TPA ab-
sorbance.
phyrin emission. As shown in Figure 4, the spectrum re-
vealed peaks not only of ZnP but also of BDP confirming
the occurrence of energy transfer.[23] The absorption and ex-
citation spectra were normalized to the 550 nm peak to com-
pare the intensity of the BDP band at 510 nm. Such compar-
ison in Figure 3 gave an estimation of efficiency of energy
transfer to be about 50% because the BDP band of the ex-
citation spectrum was about half as much as that observed
in the absorption spectrum of 1. The lower efficiency of ex-
citation transfer compared with an earlier reported BDP-
ZnP dyad could be due to competing electron transfer from
singlet excited BDP to TPA entity (see below).[6f] When the
TPA entity of 1 was excited at 307 nm, broad emission in
the region of 440–550 nm with a peak at 520 nm correspond-
ing to BDP emission was observed. However, peak intensity
of ZnP at 427 or 550 nm was less. Under similar experimen-
tal conditions, the emission spectrum of the control com-
pound, TPA-BDP revealed absence of ZnP emission. This
suggests energy transfer, from excited TPA to the neighbor
BDP and not distantly located ZnP. The broad emission in
the 440 nm could be due to weak emission of TPA, and
energy transfer from singlet excited BDP (product of initial
energy transfer) to ZnP. These spectral observations reveal
sequential energy transfer, although not very efficient, from
excited TPA to BDP to ZnP when TPA was excited and
BDP to ZnP when BDP was excited, as shown in Figure 1.
Figure 5. (a) Absorption and (b) fluorescence spectral changes of
1
during increased addition of C60Im to form the tetrad. Figure 5b inset
shows Benesi–Hildebrand plot constructed to evaluate the binding con-
stant.
tion of 1. The optical absorption bands of ZnP of the triad
revealed spectral changes characteristic of axial ligation
without causing significant spectral shifts in the BDP ab-
sorption peak at 510 nm; that is, the porphyrin Soret at
427 nm revealed a red shift of 5 nm and appeared at
432 nm. An isosbestic point was also observed at 419 nm in-
dicating the presence of only one equilibrium process. The
ZnP emission was also found to be quenched during binding
of C60Im to 1 as shown in Figure 5b. A new emission band
at 713 nm was observed and independent experiments con-
firmed that this peak is due to fulleropyrrolidine at the exci-
tation maxima and not as a result of energy transfer. Using
the quenching data, the binding constant, K evaluated using
Benesi–Hildebrand method[25] was found to be 3.6ꢁ103 mꢀ1.
The magnitude of this value was nearly four times smaller
compared to C60Im binding to tetraphenylporphyrinato
zinc(II) without BDP and TPA entities, perhaps due to the
electronic effects caused by these entities. A plot of mole
ratio method confirmed 1:1 stoichiometry for the newly as-
sembled complex.
Supramolecular tetrad formation through axial coordination
of electron acceptors: Having established photoinduced se-
quential energy transfer in the TPA-BDP-ZnP triad, next
we focused on incorporating an electron acceptor moiety by
using the well-established metal–ligand axial coordination
approach.[7a,21] In the present study, we have employed two
types of electron acceptors, fullerene appended with phenyl-
imidazole, C60Im and naphthalenediimide functionalized
with a pyridine entity, NDIpy.[24] Figure 5a shows the ab-
sorption spectral changes during titration of C60Im to a solu-
Similar spectral changes were made during NDIpy bind-
ing to 1, that is, binding of pyridine entity of NDIpy to ZnP
associated absorption changes, fluorescence quenching and
1:1 molecular stoichiometry (see the Supporting Informa-
tion, Figure S3 for spectral changes). The evaluated binding
constant from optical absorption spectral studies was found
to be K=4.57ꢁ103 mꢀ1, which is comparable to that obtained
for C60Im binding to 1.
Molecular orbital calculations using B3LYP/3-21G(*)[26,27]
were performed to visualize the geometry and electronic
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structure of C60Im:1 and NDIpy:1 tetrads. Figure 6a and b
show the optimized structures of the two tetrads. In the opti-
mized structures, the BDP and ZnP macrocycles in almost
and BDP at Epa =0.81 V versus Fc/Fc+, respectively, were
observed. These potentials were slightly different from the
control compounds indicating some intramolecular interac-
tions between the entities.
Figure 7 shows CV of 1 on increasing addition of C60Im in
DCB containing 0.1m (TBA)ClO4. In addition to the redox
peaks of 1, redox processes of C60Im were also observed.
Figure 6. B3LYP/3-21G(*) optimized structures of (a) C60Im:ZnP-BDP-
TPA and (b) NDIPy:ZnP-BDP-TPA supramolecular tetrads. The frontier
HOMO and LUMO orbitals of multi-modular C60Im:ZnP-BDP-TPA su-
pramolecular tetrad are shown in (c) and (d).
Figure 7. Cyclic voltammograms of triad, 1 on increasing addition of
C60Im (0.15 equiv each addition) in DCB, 0.1m (TBA)ClO4. Scan rate=
the same plane, and the TPA and BDP macrocycles geome-
try were similar to the X-ray structure discussed in the pre-
vious section. The C60Im:ZnP segment was also close to ear-
lier reported X-ray structure[28a] with a center-to-center dis-
tance of 13.1 ꢃ. In the case of NDIpy:1 tetrad, similar struc-
ture was observed with a center-to-center distance of 9.86 ꢃ
between the ZnP and NDIpy entities.[28b] An important ob-
servation is the placement of the acceptor entity away from
the BDP macrocycle in the optimized structures, further X-
ray structure may be needed to validate such structures. As
expected, the first two HOMOs were localized on the ZnP
entity with some contributions on the catechol-boron seg-
ment; however, the first two LUMOs were fully located on
the electron acceptor entities, as shown for C60Im:1 in Fig-
ure 6c and d for the first HOMO and LUMO.
100 mVsꢀ1
.
The first reduction of C60Im was located at ꢀ1.19 V versus
Fc/Fc+, whereas the second one at ꢀ1.58 V was close to the
first reduction of BDP entity. Interestingly, the first oxida-
tion process of ZnP located at 0.20 V revealed a cathodic
shift of about 90 mV and appeared at 0.11 V due to imida-
zole entity of C60Im binding to ZnP.[29] The first reduction of
NDIpy was located at ꢀ1.17 V and upon binding to ZnP, the
potential shifts were not significantly different.[24]
Free-energy calculations for charge-recombination
(DGCR) and charge-separation (DGCS) processes were per-
formed according to Equations (1) and (2) based on the
Weller approach:[30]
Electrochemical studies and energy states: Cyclic voltamme-
try (CV) experiments were performed to evaluate the redox
potentials. The redox potentials of individual entities were
determined by performing experiments involving the control
compounds, the dyads and the monomers (see Figure S2 in
Supporting Information). Dyad, 1a revealed a reversible re-
duction located at ꢀ1.75 V versus Fc/Fc+ corresponding to
the reduction of the BDP entity. There was also a multi-
electron anodic process at Epa =0.89 V due to the oxidation
of TPA and BDP entities. Another dyad BDP-ZnP without
the TPA entity was synthesized.[32a] The CV of this dyad re-
vealed two reductions at ꢀ1.70 and ꢀ2.01 V versus Fc/Fc+
corresponding to the reductions of BDP and ZnP entities,
respectively. On the anodic side, two reversible oxidations at
0.20 and 0.56 V versus Fc/Fc+ were observed corresponding
to first and second oxidations of the ZnP entity. The CV of
the triad, 1 revealed redox processes corresponding to all
three entities; that is, BDP reduction at ꢀ1.70 V, ZnP first
reduction at ꢀ2.00 V, first two oxidations of ZnP at 0.18 and
0.49 V, and an irreversible oxidation involving both TPA
ꢀDGCR ¼ eðEoxꢀEredÞ þ DGs
ꢀDGCS ¼ DE0ꢀ0ꢀðꢀDGCR
ð1Þ
ð2Þ
Þ
1
in which DE00 and DGS correspond to the energy of ZnP*
and electrostatic energy, respectively. The Eox and Ered rep-
resent the oxidation potential of the electron donor (ZnP)
and the reduction potential of the electron acceptor (C60Im
or NDIpy), respectively. DGS refers to the static energy, cal-
culated by using the “Dielectric continuum model” accord-
ing to Equation (3):
ꢀDGS ¼ ꢀðe2=ð4pe0ÞÞ½ð1=ð2RþÞ þ 1=ð2RꢀÞꢀð1=RCCÞ=eS
ꢀð1=ð2RþÞ þ 1=ð2RꢀÞÞ=eRÞꢁ
ð3Þ
in which R+ and Rꢀ are radii of the radical cation and radi-
cal anion, respectively; RCC represents the center–center dis-
tances between ZnP and C60Im and NDIpy, which were
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S. Fukuzumi, F. DꢀSouza et al.
evaluated from the optimized structure. The symbols eR and
eS refer to solvent dielectric constants for electrochemistry
and photophysical measurements, respectively.
The DGCR is determined to be ꢀ1.40 and ꢀ1.44 eV, re-
spectively, for fullerene and naphthalenediimide acceptors
in DCB.[30] The DGCS is estimated to be ꢀ0.68 and ꢀ0.64 eV,
respectively, for fullerene and naphthalenediimide acceptors
with respect to DGCR value and energy of singlet excited
state of ZnP (2.08 eV) (see Table 1 below). It should be
Table 1. Redox potentials and free-energy changes for charge-separation
(DGCS) from the singlet excited state of the ZnP and charge-recombina-
tion (DGCR) for the investigated compounds in o-dichlorobenzene and
toluene.
[b]
[c]
Compound
1st Ox[a]
1st Red[a]
ꢀDGCS
ꢀDGCR
1a
BDP-ZnP
1
C60Im:1
NDIpy:1
0.89
0.20
0.18
0.11
0.17
ꢀ1.75
ꢀ1.70
ꢀ1.70
ꢀ1.19
ꢀ1.17
–
–
0.08[d]
0.10[d]
2.00[d]
1.98[d]
0.68[d] 0.38[e]
0.64[d] 0.34[e]
1.40[d] 1.70[e]
1.44[d] 1.74[e]
Figure 8. Femtosecond transient absorption spectra obtained by 460 nm
laser excitation of 1 in deaerated toluene. Inset: Decay time profiles of
1
1
the BDP* at 510 nm and the ZnP* at 460 nm.
[a] Additional redox processes are observed, see text for details; [b] from
Equation (2); [c] from Equation (1); [d] in o-dichlorobenzene; [e] in tol-
uene.
formation of ZnP·+ and around 1000 nm corresponding to
the formation C60Im·ꢀ were observed. No features corre-
sponding to BDP·ꢀ or TPA·+ were there suggesting that
these entities in the supramolecular assembly are involved
only in excitation transfer and not electron transfer. The
rates of charge separation, kCS and charge recombination,
kCR were evaluated by monitoring the rise and decay of the
C60Im·ꢀ as shown in the inset Figure 9. The calculated kCS
and kCR were found to be 5.00ꢁ109 sꢀ1 and 2.10ꢁ108 sꢀ1, re-
spectively, revealing fairly fast electron-transfer events.
noted, however, the ꢀDGCS and ꢀDGCR values determined
from the redox potentials in Figure 7 provide the values in
the presence of 0.10m (TBA)ClO4, which stabilizes the CS
state by the electrostatic interaction with (TBA)ClO4. How-
ever, by taking into consideration that the Weller model in
nonpolar toluene is likely to be oversimplified, it is possible
to consider the energy of the charge-separated state of
C60Im:1 (1.70 eV) and NDIpy:1 (1.74 eV) tetrads in toluene
3
as higher than that of ZnP* (1.56 eV).
Transient absorption studies: Femtosecond and nanosecond
transient spectral measurements were performed in toluene
to monitor the kinetics of energy and electron transfer
events in the studied polyads. The femtosecond transient ab-
sorption spectra of the triad 1 (Figure 8) revealed the instan-
taneous formation of the BDP singlet-excited state features.
Here, the transient absorption spectra exhibited the bleach-
ing in the visible region with a maximum at 510 nm.[31] With
time, the BDP singlet excited diminished in intensity with
concomitant increase of the ZnP absorption in the visible
region providing the direct evidence of excitation transfer
from the singlet excited BDP to ZnP.[32] The rate constant of
1
energy transfer (kENT) from BDP* to the ZnP moiety was
determined by exponential fitting of the decay of 1BDP*
1
and the rise of ZnP* to be 1.0ꢁ1012 sꢀ1 confirming occur-
1
rence of fast and efficient energy transfer from BDP* to
ZnP.
Figure 9. Femtosecond transient absorption spectra obtained by 460 nm
laser excitation of C60Im:1 in deaerated toluene. Inset: shows the rise-
decay profiles of C60ꢀ at 1000 nm.
The transient absorption spectral changes were distinctly
different when either of the electron acceptor was bound to
1, that is, spectral features corresponding to charge separa-
tion was observed. Figure 9 shows spectral changes recorded
during C60Im binding to 1 in non-polar toluene.[33,34] Clear
transient bands in the 600 nm range corresponding to the
Figure 10 shows the transient spectra at different time in-
tervals for NDIpy:1 complex in deaerated toluene. Charac-
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Figure 11. Nanosecond transient spectra of C60Im:1 in deaerated toluene;
lex =430 nm. Inset shows the decay profile of the 480 nm band.
Figure 10. Femtosecond transient absorption spectra obtained by 460 nm
laser excitation of NDIpy:1 in deaerated toluene. Figure inset shows the
time profiles of the 480 and 610 nm bands.
Conclusion
Multi-modular donor–acceptor complexes have been assem-
bled to probe sequential energy- and electron-transfer pro-
AHCTUNGTREGcNNUN esses by careful selection of energy-imparting entities cou-
pled to well-known zinc porphyrin–fullerene or zinc por-
phyrin–naphthalenediimide donor–acceptor conjugates. The
newly synthesized triad for this purpose featured triphenyl-
teristic features of NDIpy·ꢀ around 490 nm and ZnP·+
around 600 nm confirming the formation of radical pair ion
were observed.[24] By monitoring the rise and decay of these
bands (see Figure 10, inset), the kCS and kCR were evaluated
and these were found to be 3.47ꢁ1010 and 8.40ꢁ108 sꢀ1, re-
spectively. These values are slightly larger than that ob-
served for C60Im:1 that could be rationalized for the fairly
rigid two-dimensional electron acceptor naphthalenediimide
as compared to the three-dimensional electron acceptor ful-
AHCTUNGTREGaNNNU mine, boron dipyrrin and zinc porphyrin entities, and was
characterized by various spectral and electrochemical meth-
ods, including X-ray analysis of the dyad, 1a. Excitation of
the energy-imparting triphenylamine or BDP entities result-
ed excitation transfer; femtosecond transient studies re-
vealed such a process is extremely fast and occurs in the
order of sub-ps. Panchromatic energy-harvesting was ob-
served in this molecule. The structural integrity of the
donor–acceptor polyad obtained by axial coordination of
fullerene or naphthalenediimide was established from com-
putational, spectral, and electrochemical studies. The pres-
ence of an electron acceptor in the polyad promoted elec-
tron transfer, which resulted in a charge separated species; a
result fully supported by free-energy calculations. The pres-
ence of either a fullerene or naphthalenediimide entity re-
vealed that fast-forward electron-transfer and relatively slow
back electron-transfer occurs in these new polyads, which
highlights their significance as artificial antenna-reaction
center models of photosynthesis.
ACHTUNGTRENNUNGlerene. Compared with that of NDIpy:1, the slightly slower
of CS and CR processes of C60Im:1 may also be rationalized
by the longer distance-to-distance between the C60Im and
ZnP.
Further nanosecond transient spectra for the examined
compounds in deaerated toluene were measured, as shown
in Figure 11 and in the Supporting Information S4–S6. All
of the compounds, that is, dyad 1b, triad 1, and the supra-
molecular tetrads C60Im:1 and NDIpy:1 were excited by
using 430 nm, which selectively excited the ZnP entity. The
transient absorption spectra of C60Im:1 in Figure 11 exhibit-
ed the transient bands of the triplet ZnP at 480 nm, in addi-
tion to broad absorption bands in the visible region. The
characteristic absorption bands of the ZnP radical cation
and C60 radical anion were not observed in the visible and
NIR region. The observation of the ZnP triplet together
with no observation of the CS state is in a good agreement
with the finding that the CS energy is higher than the triplet
energy of ZnP (1.56 eV) (see above).[21] The ZnP triplet de-
cayed with a rate constant of 4.00ꢁ104 sꢀ1. Similar observa-
tions were made in the case of 1:NDIpy (the Supporting In-
formation, Figure S7).
Experimental Section
Chemicals: All of the reagents used in the syntheses, and o-dichloroben-
zene (in sure seal bottle under nitrogen) were obtained from Aldrich
Chemicals (Milwaukee, WI). Tetra-n-butylammonium perchlorate,
(TBA)ClO4 was obtained from Fluka Chemicals. All the chromatograph-
ic materials and solvents were procured from Fisher Scientific and were
Chem. Eur. J. 2012, 00, 0 – 0
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S. Fukuzumi, F. DꢀSouza et al.
used as received. All the chromatographic materials and solvents were
procured from Fisher Scientific and were used as received.
Crystal structure determination for compound 1a was carried out using a
Bruker SMATR APEX2 CCD-based X-ray diffractometer equipped with
Synthesis of meso-triphenylamino-difluoroboron dipyrrin, 1a:[35] To a
mixture of 4-(diphenylamino)benzaldehyde (3.39 g, 12.4 mmol) and 2,4-
dimethylpyrrole (2.16 mL, 21.1 mmol) in CH2Cl2 (800 mL), trifluoroacetic
acid (0.19 mL, 2.47 mmol) was added. The reaction mixture was stirred at
room temperature under argon. After 1.5 h the resulting solution was
washed with 0.1m NaOH (200 mL) and then water (200 mL). The organic
layer was dried over anhydrous Na2SO4 and evaporated under reduced
pressure. The residue was dissolved in toluene (50 mL) and with stirring
p-chloranil (2.73 g, 11.1 mmol) was added. After 10 min Et3N (8 mL) was
added followed by BF3·Et2O (7 mL). The mixture was stirred for 1.5 h
and then poured into water. The organic layer was extracted and dried
over anhydrous Na2SO4 and evaporated under reduced pressure. The
crude product was purified using column chromatography. 1H NMR
(300 MHz, CDCl3): d=7.21–7.39 (m, 6H), 7.0–7.12 (m, 8H), 6.0 (s, 2H),
2.59 (s, 6H), 1.6 ppm (s, 6H).
a
low temperature device and Mo-target X-ray tube (wavelength=
0.71073 ꢃ). Measurements were taken at 100(2) K.
Cyclic voltammograms were recorded on an EG&G PARSTAT electro-
chemical analyzer using a three electrode system. A platinum button
electrode was used as the working electrode. A platinum wire served as
the counter electrode and an Ag/AgCl electrode was used as the refer-
ence electrode. Ferrocene/ferrocenium redox couple was used as an inter-
nal standard. All the solutions were purged prior to electrochemical and
spectral measurements using argon gas. MALDI-TOF spectrum of the
ZnP-BDP-TPA triad was recorded in dichloromethane with an Applied
Biosystems Voyager-DE-STR using dithranol as a matrix (Supporting In-
formation, Figure S7). The computational calculations were performed
by DFT B3LYP/3–21G* methods by using the Gaussian 09 software
package[26] on high speed PCs. The frontier HOMO and LUMO were
generated by using GaussView software.
Laser flash photolysis: The studied compounds were excited by a Panther
OPO pumped by Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) with
the powers of 1.5 and 3.0 mJ per pulse. The transient absorption meas-
urements were performed using a continuous xenon lamp (150 W) and
an InGaAs-PIN photodiode (Hamamatsu 2949) as a probe light and a
detector, respectively. The output from the photodiodes and a photomul-
Synthesis of 5-[2,2’-(3,4-dimethoxyphenyl)]-10,15,20-tritolyl-porphyrin:
This compound was synthesized by treating 3,4-dimethoxybenzaldehyde
(1.55 g, 9.33 mmol), tolyaldehyde (3.31 mL, 28 mmol), and pyrrole
(2.6 mL, 37.26 mmol) in propionic acid (200 mL). The reaction mixture
was refluxed for 2 h and then the solvent was evaporated under reduced
pressure. The crude product was purified on a basic alumina column.
1H NMR (300 MHz, CDCl3): d=8.94–8.86 (m, 8H), 8.14–8.08 (m, 6H),
7.81–7.80 (m, 1H), 7.79–7.74 (m, 1H), 7.58–7.52 (m, 6H), 7.24–7.20 (m,
1H), 4.20 (s, 3H), 4.0 (s, 3H), ꢀ2.69 ppm (brs, 2H).
Synthesis of 5-[2,2’-(3,4-dihydroxyphenyl)]-10,15,20-tritolyl-porphyrin:
This compound was synthesized according to the reported procedure
with few modifications.[36] BBr3 (9 mL, 1m in CH2Cl2) was dropwise
added to a solution of 3,4-dimethoxyphneyl porphyrin (1.0 mmol) in
CH2Cl2 at ꢀ788C. The solution was maintained at this temperature until
the addition was completed and stirred at room temperature for 12 h.
Then the mixture was brought to below 58C and cold water (100 mL)
was added followed by addition of saturated sodium bicarbonate. After
stirring for 1 h at room temperature, the organic layer was separated
using CH2Cl2 and dried over anhydrous Na2SO4. The solvent was evapo-
rated and the crude product was purified on silica column. 1H NMR
(300 MHz, CDCl3): d=8.95–8.86 (m, 8H), 8.14–8.08 (m, 6H), 7.81–7.80
(m, 1H), 7.79–7.74 (m, 1H), 7.58–7.52 (m, 6H), 7.24–7.20 (m, 1H), 2.70
(s, 9H), ꢀ2.69 ppm (brs, 2H).
tiplier tube was recorded with
a digitizing oscilloscope (Tektronix,
TDS3032, 300 MHz). Femtosecond transient absorption spectroscopy ex-
periments were conducted using an ultrafast source: Integra-C (Quantro-
nix Corp.), an optical parametric amplifier: TOPAS (Light Conversion
Ltd.) and a commercially available optical detection system: Helios pro-
vided by Ultrafast Systems LLC. The source 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. 75% of
the fundamental output of the laser was introduced into TOPAS, which
has optical frequency mixers resulting in tunable range from 285 nm to
1660 nm, whereas the rest of the output was used for white light genera-
tion. Typically, 2500 excitation pulses were averaged for 5 seconds to
obtain the transient spectrum at a set delay time. Kinetic traces at appro-
priate 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.
Synthesis of 5-[2,2’-(3,4-dihydroxyphenyl)]-10,15,20-tritolyl porphyrinato-
zinc(II): An excess of zinc acetate dihydrate (50 equiv) in methanol was
added to free base porphyrin (0.0125 mmol) from the previous step dis-
solved in CHCl3 (30 mL). The course of the reaction was monitored spec-
troscopically until the reaction was complete. After the workup, the or-
ganic layer was evaporated and the crude was purified on silica gel
column. At the end of the reaction (1 h), the solvent was evaporated and
the product was purified on silica gel column. 1H NMR (300 MHz,
CDCl3): d=8.95–8.86 (m, 8H), 8.14–8.08 (m, 6H), 7.81–7.80 (m,), 7.79–
7.74 (m, 1H), 7.58–7.52 (m, 6H), 7.24–7.20 (m, 1H), 2.70 ppm (s, 9H).
Acknowledgements
This work was supported by National Science Foundation (Grant No.
CHE1110942 to F.D.), and the Global COE (center of excellence) pro-
gram “Global Education and Research Center for Bio-Environmental
Chemistry” of Osaka University from Ministry of Education, Culture,
Sports, Science and Technology, Japan, and KOSEF/MEST through the
WCU project (R31–2008–000–10010–0) from Korea.
Synthesis of TPA-BDP-ZnP triad, 1: Compound 1a (0.183 mmol) was
dissolved in dry CH2Cl2 (20 mL) and stirred under argon for 10 min.
Then, AlCl3 (36.5 mg, 0.274 mmol) was added and stirred further 15 min
before addition of 3,4-dihydroxyphenylporphyrinatozinc, 1b (206.11 mg,
0.274 mmol). The mixture was stirred for 30 min and the solvent was
evaporated under reduced pressure. The crude product was purified
using a deactivated basic alumina column to give desired compound.
1H NMR (300 MHz, CDCl3): d=9.12–8.92 (m, 8H), 8.20–8.18 (m, 6H),
7.81–7.80 (m, 1H), 7.78–7.74 (m, 1H), 7.61–7.52 (m, 6H), 7.39–7.120 (m,
15H), 6.10 (s, 2H), 2.78 (s, 9H), 2.59 (s, 6H), 1.71 ppm (s, 6H); MALDI-
MS: m/z: calcd for C78H60N7O2BZn: 1203.11 [M+]; found: 1202.79.
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[22] CCDC-888029 (1a: C31H28BF2N3) contains the supplementary crys-
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www.ccdc.cam.ac.uk/data_request/cif. 1a: C31H28BF2N3; Mw =
Chem. Eur. J. 2012, 00, 0 – 0
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491.37 gmolꢀ1; triclinic; space group Pꢀ1; unit cell dimensions: a=
9.1410(7), b=10.1869(8), c=14.7872(11) ꢃ; a=99.466(1), b=
105.950(1), g=102.092(1)8; V=1257.96(17) ꢃ3; Z=2; 1calcd =1.297
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3
Mgmꢀ ; absorption coefficient: 0.086 mmꢀ1; Crystal size: 0.35ꢁ
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Received: June 26, 2012
Published online: && &&, 2012
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www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Multi-Modular Donor–Acceptor Conjugates
FULL PAPER
Multi-modular polyads featuring both
energy-harvesting and electron-trans-
ferring entities are assembled by using
both covalent and non-covalent meth-
ods. Ultrafast energy-transfer is
observed in the triphenylamine-boron
dipyrrin-zinc porphyrin triad (see
scheme). Efficient charge separation
leading into the generation of radical
ion-pair species is observed in the
triads signifying their importance as
materials for light energy-harvesting
applications.
Artificial Photosynthesis
M. E. El-Khouly, C. A. Wijesinghe,
V. N. Nesterov, M. E. Zandler,
S. Fukuzumi,* F. DꢀSouza* &&&& — &&&&
Ultrafast Photoinduced Energy and
Electron Transfer in Multi-Modular
Donor–Acceptor Conjugates
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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ÞÞ
These are not the final page numbers!