Photochemical charge separation in closely positioned donor-boron dipyrrin-fullerene triads

Article abstract of DOI:10.1002/chem.201002446

A series of molecular triads, composed of closely positioned boron dipyrrin-fullerene units, covalently linked to either an electron donor (donor¹-acceptor¹-acceptor²-type triads) or an energy donor (antenna-donor¹

Full text of DOI:10.1002/chem.201002446

FULL PAPER
DOI: 10.1002/chem.201002446
Photochemical Charge Separation in Closely Positioned Donor–Boron
Dipyrrin–Fullerene Triads
Channa A. Wijesinghe,[a] Mohamed E. El-Khouly,[b] Navaneetha K. Subbaiyan,[a]
Mustafa Supur,[b] Melvin E. Zandler,[a] Kei Ohkubo,[b] Shunichi Fukuzumi,*[b, c] and
Francis DꢀSouza*[a]
Abstract: A series of molecular triads,
composed of closely positioned boron
dipyrrin–fullerene units, covalently
linked to either an electron donor
(donor1–acceptor1–acceptor2-type tri-
techniques. The structures of the newly
synthesized triads were visualized by
DFT calculations, whereas the energies
of the excited states were determined
from spectral and electrochemical stud-
ies. In the case of the antenna–donor1–
acceptor1-type triads, excitation of the
antenna moiety results in efficient
energy transfer to the boron dipyrrin
entity. The singlet-excited boron dipyr-
rin thus generated, undergoes subse-
quent energy and electron transfer to
fullerene to produce a boron dipyrrin
radical cation and a fullerene radical
anion as charge-separated species. Sta-
bilization of the charge-separated state
in these molecular triads was observed
to some extent.
ACHTUNGTRENNUNGads) or an energy donor (antenna–
donor1–acceptor1-type triads) was syn-
thesized and photoinduced energy/elec-
tron transfer leading to stabilization of
the charge-separated state was demon-
strated by using femtosecond and
nanosecond transient spectroscopic
Keywords: antenna
transfer energy transfer
lerenes · photosynthesis
·
electron
·
·
ful-
Introduction
in electron-transfer reactions have led to advances in syn-
thetic electron donor–acceptor systems capable of perform-
ing fast charge separation and relatively slow charge recom-
bination.[12–14] Often, electron donors, such as porphyrin, fer-
rocene (Fc), thiophene, or carbon nanotubes, have been
linked to fullerene to create plethora of dyads.[4,5,7[ Attach-
ment of additional antenna or redox-active molecules result-
ed in polyads (triads, tetrads, etc.) capable of undergoing se-
quential energy or electron transfer, or combination of these
photochemical processes leading to long-lived charge-sepa-
rated states. In few studies, the polyads have been utilized
to build photovoltaic devices and successful conversion of
light into electricity has been demonstrated.[4] Such success
has lead to further exploration of polyads made out of dif-
ferent donor and acceptor entities capable of harvesting
solar energy.[8,15]
The ability of natural photosynthetic systems to efficiently
transform sunlight into chemical energy[1] has inspired basic
research in artificial photosynthesis[2–7] leading to the devel-
opments in the areas of photovoltaics and photocatalysis.[8–10]
The different steps in artificial photosynthesis involve the
sequence of wide-band light harvesting and funneling,
charge separation and migration, combined with slow
charge recombination.[2–7] In this regard, the delocalization
of charges within the spherical carbon structure of the rigid
aromatic p sphere of fullerenes[11] offers unique opportuni-
ties for stabilizing charge-separated states in donor–acceptor
systems.[4,5,7] The small reorganization energies of fullerenes
Recently we reported a new type of donor1–acceptor1–
acceptor2 triads in which a sensitizer, boron dipyrrin (abbre-
viated as BDP and also known as BODIPY=borondipyrro-
methene) was directly linked to an electron donor and an
electron acceptor (acceptor2).[16] The first triad, 1, was com-
posed of ferrocene–boron dipyrrin—fullerene, whereas the
second one, 2, was made out of triphenylamine–boron dipyr-
rin–fullerene (Scheme 1). Femtosecond transient absorption
studies revealed occurrence of sequential electron transfer
in the former ferrocene–boron dipyrrin–fullerene triad, ulti-
mately resulting in stabilization of the charge-separated
state. The BDP has so far been used as an antenna molecule
(energy donor) as well as an electron donor and an acceptor
in the photosynthetic antenna-reaction center mimics and
photovoltaic cells.[17–20] However, BDP has yet to be utilized
[a] C. A. Wijesinghe, N. K. Subbaiyan, Prof. M. E. Zandler,
Prof. F. DꢀSouza
Department of Chemistry, Wichita State University
1845 Fairmount, Wichita, KS 67260-0051 (USA)
Fax : (+1)316-978-3431
E-mail: Francis.DSouza@Wichita.edu
[b] Dr. M. E. El-Khouly, M. Supur, Dr. K. Ohkubo, 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
[c] Prof. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University, Seoul 120-750 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002446.
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Scheme 2. Synthetic methodology adopted for the preparation of triads
in the present study.
AHCTUNGTREGcNNUN opy, MALDI-TOF mass spectrometry (Figure S19 in the
Supporting Information), electrochemical and other spectro-
scopic methods as described in the Experimental Section.
Scheme 1. Structures of the donor1–acceptor1–acceptor2 type (1 and 2)
and the antenna–donor1–acceptor1 type (3–6) triads investigated in the
present study. Dyad 7 and compound 8 are control compounds. For other
control compounds see Figure S1 in the Supporting Information.
Optical absorption and emission studies: Figure 1 shows the
absorption spectrum of triad 3 and the relevant control com-
pounds in benzonitrile (PhCN). Absorption peaks corre-
as both, an energy acceptor and an electron donor, within a
supramolecular construct.
We report herein for the first time an extensive series of
triads in which the redox active donors in 1 and 2 are re-
placed by various antenna molecules (compounds 3–6 in
Scheme 1, control compounds 3a–6a that were used in the
present study, are given in the Supporting Information, Fig-
ure S1). Here, BDP acts as both, an energy acceptor and an
electron donor. Femtosecond and nanosecond transient ab-
sorption studies have been performed to visualize charge
stabilization at different time scales in this series of triads.
Results and Discussion
The syntheses of the triads were accomplished, first, by syn-
thesizing the difluoroboron dipyrrin dyads with the desired
donor/antenna entity in the meso position. The difluoro-
Figure 1. Normalized absorption spectra (to the 500 nm band of BDP,
except for pyrene butyric acid) of a) triad, 3, b) dyad, 7, c) pyrene butyric
acid, d) dyad 3a, and e) 8 in PhCN.
ACHTUNGTRENNUNGboron dipyrrin was subsequently converted into dioxyboron
dipyrrin derivatives by treatment with dihydroxy benzalde-
hyde in the presence of AlCl3 in dry CH2Cl2. Finally, the
electron acceptor was appended by treatment with fullerene
and N-methylglycine in toluene, according to Scheme 2 (see
the Experimental Section for details). The newly synthesized
sponding to all of the three entities of the triad are observed
and a close examination revealed the following: the pyrene
peaks located in the 300–350 nm region reveal 2–3 nm blue-
shift compared to the control compound (pyrene butyric
acid). However, an opposite effect is observed for the BDP
1
compounds were fully characterized by H NMR spectros-
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Photochemical Charge Separation in Boron Dipyrrin–Fullerene Triads
FULL PAPER
band located in the 500 nm region, that is, up to 4 nm red-
shift for the triad compared with the control compound 7,
which lacks the antenna entity, and up to 8 nm redshift com-
pared to compound 8, which lacks both pyrene and fullerene
entities. The fullerene peak maximum located around l=
328 nm is overlapped with the pyrene absorption band in
the case of triad 3. The observed spectral shift and broaden-
ing suggest occurrence of intramolecular interactions be-
tween the different entities of the triad. Similar trends in the
absorption spectra of triads 4–6 were also observed (see the
Supporting Information, Figures S2a–S4a), indicating inter-
actions between the different entities of the triads.
Weak emission of BDP, less than 6% of that observed in
the case of 3a, was observed. These observations suggest
that the emission band of BDP at l=520 nm in the case of
3a is due to efficient energy transfer[20] from pyrene to BDP.
Similar observations were also made in the case of dyads
4a–6a, where excitation of the antenna entity resulted in ef-
ficient energy transfer to the BDP entity (see Figures S2b–
S4b). Interestingly, in the case of triad 3, excitation at l=
345 nm, corresponding to the pyrene unit, resulted in no
emission of either pyrene or BDP, suggesting additional
energy- or electron-transfer processes that involve the fuller-
ene entity.
Energy transfer from the antenna unit to BDP was ob-
served in the case of dyads 3a–6a (Figure S1), which have
covalently connected antenna (pyrene, anthracene, fluorene,
or naphthalene) and BDP entities. As shown in Figure 2A,
Figure 2B shows the fluorescence emission behavior of
triad 3 and the control compounds when excited at l=
506 nm, which corresponds to the BDP entity. When com-
pounds 3a and 8 were excited at l=506 nm, emission of
BDP at l=520 nm was observed. However, in the case of
triad 3 or dyad 7, which have a fullerene as an electron ac-
ceptor, the BDP emission was found to be quenched over
99% suggesting efficient photochemical process from sin-
glet-excited BDP to the attached fullerene. This was also
the case for triads 4–6, where no emission from either the
antenna unit or the BDP unit was observed (see Figur-
es S3c–S5c). The quenching process may involve the energy
transfer and/or electron transfer from the singlet BDP to
the closely attached C60, as will discuss later in forthcoming
sections. Expanding the scale in the 700–800 nm region re-
vealed weak emission at l=720 nm corresponding to emis-
sion of fulleropyrrolidine. As discussed in the subsequent
sections, involving transient absorption studies, this is indeed
due to the occurrence of singlet–singlet energy transfers
from the BDP (2.38 eV) to C60 (1.75 eV).
Geometry optimization by using DFT calculations: The
structures of the triads were visualized by performing com-
putational calculations at the B3LYP/6-31G* level.[21] All of
the triads revealed stable structures on the Born–Oppen-
heimer potential energy surface (Figure 3). In all of the
triads the center-to-center distances between the boron
atom to the center of fullerene were found to be around
10.3 ꢂ, whereas the center-to-center distances between the
donor entity to the boron atom were found to range be-
tween 10–11 ꢂ. Additionally, the molecular size, measured
between the farthest carbon atoms of the triads, were found
to be in the range of 24–26 ꢂ, indicating smaller size for the
present triads compared to most of the triads with fullerene
as electron acceptor described in literature.[4,5,7] The frontier
HOMOs and LUMOs were also generated to visualize the
electronic structures. In the entire antenna-bearing series of
triads, that is 3–6 and the dyad 7, the LUMOs were localized
on the fullerene entity, whereas the HOMOs were located
Figure 2. Fluorescence spectra of A) pyrene butyric acid (a), 3a (b), 8
(c), 7 (d), and 3 (e) in PhCN excited at l=345 nm, which corresponds to
the pyrene unit. B) Fluorescence spectra of a) 3a, b) 3, and c) 7 in PhCN
excited at l=506 nm, which corresponds to the BDP unit. The 600–
900 nm region in B) is expanded (ꢃ100) to visualize the fullerene emis-
sion at l=720 nm.
excitation of the control compound, pyrene butyric acid, at
l=345 nm revealed emission bands in the 360–430 nm
range, corresponding to pyrene singlet emission. When dyad
3a, possessing pyrene and BDP units, was excited at this
wavelength, the emission due to pyrene was quenched over
98% with the concurrent appearance of BDP emission at
l=520 nm. In a control experiment, compound 8 having
only BDP but not pyrene unit was excited at l=345 nm.
on the BDP entity, partially extended to the pyrrolidine ni-
+
C
trogen. These results suggest formation of antenna–BDP
–
Cꢀ
C60 as charge-separated species in the photoinduced elec-
tron transfer (PET) reaction.
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S. Fukuzumi, F. DꢀSouza et al.
triads
revealed
oxidation
around 0.73 V versus Fc/Fc+,
whereas the oxidation of the
antenna entities of the triads 3–
6 occurred beyond this oxida-
tion process and was often a
quasireversible to irreversible
process. The slight changes in
the redox potentials of the enti-
ties in the triads compared to
reference 8 is consistent with
the earlier-discussed optical ab-
sorption data indicating intra-
molecular interactions. Hence,
the
probable
light-induced
charge separation is predicted
to occur between the electron-
donating BDP and the elec-
tron-accepting C60 units, sup-
porting the computational data.
Free-energy calculations for
different charge-separated (CS)
states were performed accord-
ing to Equation (1), based on
the Weller approach,[22]
Figure 3. Space-filling models of the B3LYP/6-31G*-optimized structures of the triads 1–6 and the dyad 7. The
HOMO and LUMO of triad 3 are also shown in the lower panel.
Electrochemical studies and energy states: Differential pulse
voltammetry (DPV) experiments were performed to evalu-
ate the redox potentials. The site of electron transfer corre-
sponding to different entities was determined by performing
experiments involving the control compounds, the dyads,
and the monomers. For all of the investigated triads, the
first reduction corresponding to fullerene was located at
ꢀ1.03 V versus ferrocene/ferrocenium (Fc/Fc+). Additional
reductions at ꢀ1.43 and ꢀ2.05 V were also observed and
correspond to the second and third reductions of the fuller-
ene entity. The BDP reduction in 8 was located at ꢀ1.63 V,
however, in the case of triads 3–6, having covalently linked
fullerene units, this process was anodically shifted by 50 mV
and occurred at ꢀ1.58 V versus Fc/Fc+ (Figure 4). The oxi-
dation corresponding to ferrocene in 1 was located at 0.05 V,
whereas the oxidation corresponding to triphenylamine in 2
was located at 0.61 V versus Fc/Fc+. Boron dipyrrin in the
DGCS ¼ EoxꢀEredꢀDE00þDGS
ð1Þ
where DE00 and DGS correspond to the energy of 1BDP*
and the electrostatic energy, respectively. Eox and Ered repre-
sent the oxidation potential of the donor and the reduction
potential of the acceptor, respectively. On the basis of the
first oxidation potential of BDP and the first reduction po-
tential of the C60 moiety in the triads 3–7, the driving force
for the charge-recombination (CR) process (ꢀDGCR) was
determined to be 1.71 eV in benzonitrile.[16] Considering the
1
energy level of C60* (1.75 eV), which is very close to that of
the charge-separated state in benzonitrile, the eventual
charge-separation process, to take place between BDP and
C60, is expected to be driven by only BDP*, possessing an
energy value of 2.30 eV. Accordingly, DGCS was estimated to
be 0.67 eV with respect to the ꢀDGCR value and the energy
of the singlet excited state of the BDP chromophore.
1
As reported earlier, free-energy calculations suggested oc-
currence of sequential electron transfer resulting in a distant
charge-separated state in the case of 1, and population of
3
low-lying C60* in the case of 2.[16] For the triads 3–6, bearing
an antenna unit, owing to the higher oxidation potential of
the antenna unit than that of BDP and the much lower re-
duction potential than that of fullerene, an energy level dia-
gram close to that of 2 could be constructed; as shown for
triad 3 in Figure 5. That is, excitation of the antenna units
results in the singlet excited state of the antenna unit that
would undergo energy transfer to the BDP unit, which in
turn transfers its electron to the attached C60 forming the
Figure 4. Differential pulse voltammograms of triads 3–6 and 8 in PhCN,
0.1m tetra-n-butylammonium perchlorate ((TBA)ClO4). Scan rate=
5 mVsꢀ1, pulse width=0.25 s, pulse height=0.025 V.
antenna–BDP +–C60
energy transfer from the BDP to C60 can also be consid-
.
Additionally, the singlet–singlet
C
Cꢀ
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Photochemical Charge Separation in Boron Dipyrrin–Fullerene Triads
FULL PAPER
Cꢀ
shift from BDP to the attached C60 entity and formation of
Fc+ –BDP–C60 as the final CS state. The rate constant of
C
Cꢀ
Cꢀ
the charge shift from BDP to C60 was determined to be
3.8ꢃ1011 sꢀ1. By following the decay of Fc+ –BDP–C60 in
PhCN, the CR rate constant was determined to be 2.4ꢃ
109 sꢀ1, from which a lifetime of 420 ps for the radical-ion
C
Cꢀ
pair (RIP) was derived. The finding that the tRIP of Fc+
–
C
Cꢀ
BDP–C60 is much longer than that of Fc+ –BDP (kCR
C
Cꢀ
=
5.8ꢃ1010 sꢀ1; tRIP =17 ps) revealed the effect of the electron
shift mechanism in prolonging the tRIP in triad 1. That is, as
a result of relatively distant positioning of the radical cation
and anion, stabilization of the charge-separated state is ac-
complished in triad 1 in spite of closely disposed donor and
acceptor entities.
Photoirradiation of triad 2 revealed the characteristic ab-
Cꢀ
sorption band of C60 in the near IR region with a maxi-
+
mum at lCS =1000 nm.[5] The absorption band of TPA was
C
Figure 5. Energy-level diagram depicting different photochemical pro-
cesses of triads 3–6 (X=anthracene, pyrene, 2-naphthalene, or 2-fluo-
rene; kT =decay constant).
Cꢀ
not observed suggesting that the formation of C60 results
from electron transfer from the singlet-excited BDP as an
+
C
electron donor to C60. A hole transfer from TPA to BDP
is slightly exergonic (<0.1 eV), however, the lack of a
+
ered.[23,24] The finding that the energy level of pyrene–
TPA band suggests this process to be less efficient. The
C
BDP +–C60 is comparable to that of the singlet state of C60
kCR value of TPA–BDP +–C60 was determined to be 2.0ꢃ
109 sꢀ1, from which a tRIP value of 500 ps was evaluated. At
longer time scale (1800 ps), the spectrum revealed the char-
acteristic absorption of the triplet C60 suggesting that the
radical-ion pair decays through CR to populate the triplet
C60.
C
Cꢀ
C
Cꢀ
1
suggests that electron transfer through the C60* is thermo-
dynamically unfavorable. In any case, because the energy
level of the triplet state of fullerene is the lowest, population
3
of C60* prior to the formation of the ground state species
could be rationalized from these studies. The forthcoming
section discusses these photochemical events in detail.
The tolylBDP–C60 dyad (7) revealed transient spectral
traits in which peak formation at l=1000 nm, as a finger-
print of the radical anion of C60 (Figure 6),[27,28] declares that
charge separation occurs between tolylBDP and C60. The
broadening in the absorption band in the near IR region
may suggest a contribution of an energy transfer from sin-
glet BDP to singlet C60 in the quenching process. Formation
of the charge-separated state within a few picoseconds is
probably achieved due to the close proximity of electron
donor and electron acceptor moieties. Considering this prox-
imity, charge separation prior to energy transfer between
Transient absorption studies: Femtosecond and nanosecond
transient spectral measurements were performed to identify
the electron-transfer products, the mechanistic details of
charge migration, and the kinetics of charge-separation and
charge-recombination processes. Nanosecond transient dif-
ferential absorption spectra of tolylBDP (8) excited at l=
510 nm showed weak absorbance on either side of the
bleaching signal, which were assigned to the triplet excited
state of BDP (see Figure S5 in the Supporting Informa-
tion).[25] The time profile of tolylBDP at l=400 nm confirms
a long-lived triplet state of about 45 ms. Excitation at l=
355 nm gave the same transient spectrum of the triplet state
of tolylBDP. Typically, triplet formation of BDP dyes is re-
markably inefficient.[26]
Femtosecond measurements of dyad 1a in PhCN by using
l=460 nm as excitation wavelength, which selectively ex-
cites the BDP entity, resulted in electron transfer from the
Fc to the attached BDP forming Fc+ –BDP . The nano-
second transient spectra of dyad 1a lacked absorption bands
of triplet state BDP due to the occurrence of CR to the
low-lying triplet state of the Fc entity. Interestingly, upon
photoexcitation of the BDP entity of triad 1 in deaerated
C
Cꢀ [15]
Cꢀ
PhCN, the characteristic absorption bands of BDP were
observed at early time scale (ꢁ1 ps), however, at longer
Figure 6. Femtosecond transient absorption spectra obtained by 460 nm
laser excitation of tolylBDP–C60 (7) in deaerated PhCN (black=45,
green=130, pink=360, and blue=2500 ps). The inset shows the time
Cꢀ
time scale, the band corresponding to BDP at l=590 nm
decreased in intensity with concurrent build-up of the ab-
Cꢀ
Cꢀ
sorption band of C60 at l=1000 nm, suggesting an electron
profile of C60 at l=1000 nm.
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S. Fukuzumi, F. DꢀSouza et al.
tolylBDP and C60 is also possible (Figure S6 in the Support-
ing Information). The rate constant of charge recombination
was determined from the first-order decay at lCS =1000 nm
to be 3.2ꢃ109 sꢀ1, corresponding to a lifetime (tCS) of 310 ps.
The quantum yield of charge separation[7e] was determined
from the quenching of the singlet excited state of C60 to be
0.87.
In the complementary nanosecond transient absorption
spectral measurements (Figure 7 and Figure S6 in the Sup-
Figure 8. Femtosecond transient absorption spectra obtained by 460 nm
laser excitation of pyrene–BDP–C60 (3) in deaerated PhCN (black=360.
pink=880, and blue=2900 ps). The inset shows the decay time profile of
Cꢀ
C60 at l=1000 nm.
Figure 7. Nanosecond transient absorption spectra of tolylBDP–C60 (7) in
PhCN, lex =510 nm (black=1, dark green=12, pink=20, light green=
28, purple=40, and blue=90 ms). The inset shows the time profile of
3C60* at l=700 nm.
porting Information), only the characteristic triplet-state
spectrum of C60 was observed at l=700 nm,[29,30] when excit-
ed with 510 nm laser pulses at which the tolylBDP is excited
dominantly (Figure 7). The formation of triplet C60 is most
Figure 9. Nanosecond transient absorption spectra of pyrene–BDP–C60
(3) in PhCN, lex =510 nm (black=1, yellow=5, red=15. green=30, and
3
blue=85 ms). The inset shows the time profile of C60* at l=700 nm.
likely due to the charge recombination of BDP +–C60
,
C
Cꢀ
taking into account that the energy level of triplet C60
(1.55 eV) is located lower than that of BDP +–C60 . The
triplet–triplet energy transfer from tolylBDP to C60 was
ruled out due to the lack of transient spectral features of the
triplet excited state of tolylBDP. The decay rate constant of
tolylBDP–3C60* to the ground state was found to be 4.3ꢃ
104 sꢀ1.
firm the formation of the triplet excited state of C60 with the
same decay constant (kT =4.1ꢃ104 sꢀ1, tT =24 ms at 700 nm).
In a control experiment, the nanosecond transient spectra of
dyad 3a showed weak absorption bands in the visible region
with maxima at l=430 and 630 nm that correspond to the
triplet formation of BDP (see Figure S8 in the Supporting
Information). As depicted in Figure 5 for the photoinduced
intramolecular processes of triad 3 in benzonitrile, the
345 nm laser excitation pumps up the pyrene moiety to its
singlet excited state, from which the energy transfer from
the singlet pyrene to the attached BDP takes place quite ef-
ficiently, as confirmed by steady-state emission studies.
From the time-resolved emission measurements, the charge-
separation process from the singlet BDP to the C60 was re-
corded. The charge-separated state recombined to populate
the triplet C60, but not the triplet BDP, as evidenced from
the nanosecond transient absorption measurements.
C
Cꢀ
The intramolecular photoinduced processes of 7 are com-
piled in a energy level diagram (see Figure S7 in the Sup-
porting Information). Excitation of the tolylBDP chromo-
phore results in electron transfer/energy transfer to the sin-
glet excited state of C60. The formed charge-separated state
undergoes charge recombination to produce the triplet state
of C60. Then, it decays to the ground state within 24 ms.
The femtosecond transient absorption difference spectra
of triad 3 exhibited quite the same characteristics as those
of tolylBDP–C60 except for the rate constant of charge re-
combination (kCR =1.3ꢃ109 sꢀ1), figured through the mono-
exponential decay at l=1020 nm, coinciding a CS lifetime
of 770 ps (Figure 8). Fast charge separation within a few pi-
coseconds is also favored in this triad, thereby enhancing
the formation of the triplet excited state of C60. As expected,
the nanosecond transient spectra of triad 3 (Figure 9) con-
The triads bearing anthracene, fluorene, and naphthalene
moieties, that is 4–6, revealed similar antenna effects. The
femtosecond transient absorption spectra of each triad had
the same spectral patterns expressing the formation of the
charge-separated states between the BDP moiety and the
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Photochemical Charge Separation in Boron Dipyrrin–Fullerene Triads
FULL PAPER
Instruments: The UV/Vis spectral measurements were carried out with a
Shimadzu Model 1600 UV/Vis spectrophotometer. The fluorescence
emission was monitored by using a Varian Eclipse spectrometer. A right
angle detection method was used. The 1H NMR studies were carried out
on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used
as an internal standard. Differential pulse voltammograms were recorded
on an EG&G PARSTAT electrochemical analyzer with 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 elec-
trode was used as the reference electrode. The ferrocene/ferrocenium
redox couple was used as an internal standard. All the solutions were
purged prior to electrochemical and spectral measurements with argon
gas. Matrix-assisted laser desorption/ionization time-of-flight mass spec-
tra (MALDI-TOF MS) were measured on a Kratos Compact MALDI I
(Shimadzu) for metal complex in PhCN with dithranol used as a matrix.
The computational calculations were performed by DFT B3LYP/6-31G*
methods with the Gaussian 03 software package[21] on high-speed PCs.
The HOMOs and LUMOs were generated by using the GaussView pro-
C60 unit with different lifetimes when excited at l=460 nm
(see Figure S9–S11 in the Supporting Information). Nano-
second transient absorption spectra of each triad system
confirmed formation of the triplet excited state of C60 when
laser pulses were utilized at l=510 and 355 nm (see Fig-
ure S12–S18 in the Supporting Information). The lifetimes
of the charge-separated states are compiled in Table 1. Be-
Table 1. Lifetimes of the charge-separated state in the series of triads
and dyad investigated in the present study.
Compound
kCR[a] [sꢀ1
]
tCS[a] [ps]
Fc–BDP–C60 (1)
TPA—BDP–C60 (2)
2.4ꢃ109
2.0ꢃ109
1.3ꢃ109
1.1ꢃ109
2.6ꢃ109
1.3ꢃ109
3.2ꢃ109
420
500
770
900
390
770
310
pyrene—BDP–C60 (3)
anthracene—BDP–C60 (4)
fluorene—BDP–C60 (5)
naphthalene—BDP–C60 (6)
tolylBDP–C60 (7)
gram[31]
.
Laser flash photolysis: The studied compounds were excited by a Panther
OPO pumped by Nd/YAG laser (Continuum, SLII-10, 4–6 nsfwhm) with
the powers of 1.5 and 3.0 mJ per pulse. The transient absorption meas-
urements were performed by 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 photo-
multiplier tube was recorded with a digitizing oscilloscope (Tektronix,
TDS3032, 300 MHz). Femtosecond transient absorption spectroscopy ex-
periments were conducted by using an ultrafast source: Integra-C (Quan-
tronix 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–
1660 nm, whereas the rest of the output was used for white-light genera-
tion. Typically, 2500 excitation pulses were averaged for five 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 by using fresh solutions in each laser excitation.
[a] The experimental error is ꢂ5%.
sides functioning as antenna units in triad systems, anthra-
cene, pyrene, fluorene, and naphthalene entities appear to
affect the lifetimes of the charge-separated states to some
extent, even though they are not directly involve in a charge
migration as electron donor moieties as understood from
the earlier-discussed results that were gained in electro-
chemical measurements.
Conclusion
A series of molecular triads with closely spaced different en-
tities, comprising of BDP and fullerene primary entities, and
a third entity, either a redox-active or an antenna entity, was
synthesized to examine the photodynamic properties. The
optical absorption studies revealed intramolecular interac-
tions between the entities of the triads. The geometry and
electronic structure of the triads were deduced from compu-
tational studies, whereas electrochemical studies by using
differential pulse voltammetry technique allowed establish-
ing energy states of the different triads. Fluorescence studies
revealed efficient energy transfer from the antenna entity to
General procedure for synthesis of aryl difluoroboron dipyrrin: The aryl
difluoroboron dipyrrin compounds were synthesized according to the
procedure described by Imahori et al.[18a] with some modifications. Tri-
fluoroacetic acid (0.19 mL, 2.47 mmol) was added to a mixture of aryl al-
dehyde (12.4 mmol) and 2,4-dimethylpyrrole (2.16 mL, 21.1 mmol) in
CH2Cl2 (800 mL). The reaction mixture was stirred at room temperature
under argon. After 1.5 h the resulting solution was washed with aqueous
NaOH (0.1m, 200 mL) and water (200 mL). The organic layer was dried
over anhydrous Na2SO4 and the solvent was evaporated under reduced
pressure. The residue was dissolved in toluene (50 mL) and p-chloranil
(2.73 g, 11.1 mmol) was added. After 10 min Et3N (8 mL) was added fol-
lowed by BF3·Et2O (7 mL). The mixture was stirred for 1.5 h and then
poured into water. The organic layer was extracted with CH2Cl2, dried
over anhydrous Na2SO4, and the solvent was evaporated under reduced
pressure. The crude product was purified with column chromatography
by using mixtures of CH2Cl2 and hexane as eluent.
1
the boron dipyrrin entity in triads 3–6. The BDP* transfers
its electron to the attached C60, thereby generating antenna–
BDP +–C60 charge-separated species, as revealed by femto-
second and nanosecond transient absorption techniques. The
anticipated stabilization of the charge-separated state was
observed to some extent in all investigated triads.
C
Cꢀ
1-Pyrene difluoroboron dipyrrin (3a): 1H NMR (300 MHz, CDCl3): d=
8.29–7.85 (m, 9H; pyrenyl-H), 5.99 (s, 2H; pyrrole-H), 2.6 (s, 6H; CH3),
2.41 (s, 6H; CH3), 0.85 ppm (s, 6H; CH3).
Experimental Section
9-Anthracene difluoroboron dipyrrin (4a): 1H NMR (300 MHz, CDCl3):
d=8.5 (s, 1H; aryl-H), 8.0 (d, J=8.30 Hz, 2H; aryl-H), 7.84 (d, J=
8.47 Hz, 2H; aryl-H), 7.51–7.38 (m, 4H; aryl-H), 5.85 (s, 2H; pyrrole-H),
2.6 (s, 6H;CH3), 0.8 ppm (s, 6H; CH3).
Chemicals: All of the reagents used in the syntheses, and benzonitrile (in
sure seal bottle under nitrogen) were obtained from Aldrich Chemicals
(Milwaukee). Tetra-n-butylammonium perchlorate, (TBA)ClO4, was ob-
tained from Fluka Chemicals. All the chromatographic materials and sol-
vents were procured from Fisher Scientific and were used as received.
The syntheses and purification of 1, 2, and 8 are given elsewhere.[16]
1
2-Fluorene difluoroboron dipyrrin (5a): H NMR (300 MHz, CDCl3): d=
7.92–7.82 (m, 2H; aryl-H), 7.61–7.59 (m, 1H; aryl-H), 7.47–7.28 (m, 6H;
aryl-H & dioxyaryl-H), 5.99 (s, 2H; pyrrole-H), 3.99 (s, 2H; fluorenyl-
H), 2.59 (s, 6H; CH3), 1.4 ppm (s, 6H; CH3).
Chem. Eur. J. 2011, 17, 3147 – 3156
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3153

S. Fukuzumi, F. DꢀSouza et al.
2-Napthalene difluoroboron dipyrrin (6a): 1H NMR (300 MHz, CDCl3):
d=8.02–7.75- (m, 4H; aryl-H), 7.58–7.52 (m, 3H; aryl-H), 7.38–7.34 (m,
1H; aryl-H), 6.0 (s, 2H; pyrrole-H), 2.6 (s, 6H; CH3), 1.3 ppm (s, 6H;
CH3).
p-Tolyl difluoroboron dipyrrin (7a): 1H NMR (300 MHz, CDCl3): d=
7.28 (d, J=8.54 Hz, 2H; aryl-H), 7.14 (d, J=8.19 Hz, 2H; aryl-H), 5.99
(s, 2H; pyrrole-H), 2.55 (s, 6H; CH3), 2.41 (s, 3H; CH3), 1.41 ppm (s,
6H; CH3).
2-Fluorene-dioxyboron dipyrrin–fullerene triad (5): 1H NMR (300 MHz,
CDCl3): d=7.92–7.82 (m, 2H; aryl-H), 7.61–7.59 (m, 1H; aryl-H), 7.4–
7.28 (m, 6H; aryl-H & dioxyaryl-H), 6.76 (brs, 1H; dioxyaryl-H), 5.9
(brs, 2H; pyrrole-H), 4.99 (d, J=9.42 Hz, 1H; fulleropyrrolidine-H), 4.89
(s, 1H; fulleropyrrolidine-H), 4.26 (d, J=9.56 Hz, 1H; fulleropyrrolidine-
H), 3.99 (s, 2H; fluorenyl-H), 2.91 (s, 3H; fulleropyrrolidine-H),
1.25 ppm (s, 12H; CH3); MS (MALDI-TOF): m/z calcd for
C95H34BN3O2: 1258.06; found: 1258.8.
2-Napthalene-dioxyboron dipyrrin–fullerene triad (6): 1H NMR
(300 MHz, CDCl3): d=8.02–7.79-(m, 4H; aryl-H), 7.60–7.52 (m, 3H;
aryl-H), 7.42–7.34 (m, 3H; aryl-H & dioxyaryl-H), 6.75 (brs, 1H; diox-
yaryl-H), 5.9 (brs, 2H; pyrrole-H), 4.99 (d, J=9.45 Hz, 1H; fulleropyrro-
lidine- H), 4.85 (s, 1H; fulleropyrrolidine-H), 4.22 (d, J=9.41 Hz, 1H;
fulleropyrrolidine-H), 2.85 (s, 3H; fulleropyrrolidine-H), 1.24 ppm (s,
12H; CH3); MS (MALDI-TOF): m/z calcd for C92H32BN3O2: 1220.01;
found: 1219.8.
p-Tolyl-dioxyboron dipyrrin–fullerene triad (7): 1H NMR (300 MHz,
CDCl3): d=7.40–7.38 (m, 1H; dioxyaryl-H), 7.30–7.22- (m, 1H; dioxyar-
yl-H), 7.10-7.18 (m, 4H, aryl-H), 6.75 (brs, 1H; dioxyaryl-H), 5.9 (brs,
2H; pyrrole-H), 4.99 (d, J=9.32 Hz, 1H; fulleropyrrolidine-H), 4.89 (s,
1H; fulleropyrrolidine-H), 4.26 (d, J=9.40 Hz, 1H; fulleropyrrolidine-
H), 2.91 (s, 3H; fulleropyrrolidine-H), 2.44 (s, 3H; CH3), 2.39 (s, 3H;
CH3), 1.59 (s, 3H; CH3), 1.38 (s, 3H; CH3), 1.21 ppm (s, 3H; CH3); MS
(MALDI-TOF): m/z calcd for C89H32BN3O2: 1183.98; found: 1184.3.
General procedure for synthesis of aryl-dioxyboron dipyrrin: These com-
pounds were prepared according to the literature procedure with few
modifications.[29] Aryl difluoroboron dipyrrin (0.730 mmol) was dissolved
in dry CH2Cl2 (20 mL) and stirred under argon for 10 min. Then AlCl3
(146 mg, 1.096 mmol) was added and the solution was further stirred for
15 min before addition of 3,4-dihydroxybenzaldehyde (151.4 mg,
1.096 mmol). The mixture was stirred for 20 min and the solvent was
evaporated under reduced pressure. The crude product was purified by
using a deactivated basic alumina column to give the desired compound.
1
1-Pyrene-dioxyboron dipyrrin (3b): H NMR (300 MHz, CDCl3): d=9.80
(s, 1H; aldehyde-H), 8.29–7.85 (m, 9H; pyrenyl-H), 7.4–7.3 (m, 2H; di-
oxyaryl -H), 6.88 (d, 1H; dioxyaryl-H), 5.99 (s, 2H; pyrrole-H), 2.1 (s,
6H; CH3), 0.8 ppm (s, 3H; CH3).
9-Anthracene-dioxyboron dipyrrin (4b): 1H NMR (300 MHz, CDCl3):
d=9.80 (s, 1H; aldehyde-H), 8.5 (s, 1H; aryl-H), 8.0 (d, J=8.08 Hz, 2H;
aryl-H), 7.84 (d, J=8.65 Hz, 2H; aryl-H), 7.58–7.39 (m, 6H; aryl-H & di-
oxyaryl-H), 6.95 (d, J=7.71 Hz, 1H; dioxyaryl-H), 5.9 (s, 2H; pyrrole-
H), 2.1 (s, 6H; CH3), 0.82 ppm (s, 6H; CH3).
2-Fluorene-dioxyboron dipyrrin (5b): 1H NMR (300 MHz, CDCl3): d=
9.80 (s, 1H; aldehyde-H), 7.92–7.82 (m, 2H; aryl-H), 7.61–7.59 (m, 1H;
aryl-H), 7.47–7.28 (m, 6H; aryl-H & dioxyaryl-H), 6.88 (d, J=8.22 Hz,
1H; dioxyaryl-H) 5.99 (s, 2H; pyrrole-H), 3.99 (s, 2H; fluorenyl-H), 2.0
(s, 6H; CH3), 1.39 ppm (s, 6H; CH3).
2-Napthalene-dioxyboron dipyrrin (6b): 1H NMR (300 MHz, CDCl3):
d=9.80 (s, 1H; aldehyde-H), 8.02–7.79 (m, 4H; aryl-H), 7.60–7.52 (m,
3H; aryl-H), 7.42–7.34 (m, 3H; aryl-H & dioxyaryl-H), 6.88 (d, J=
8.15 Hz, 1H; dioxyaryl-H) 5.99 (s, 2H; pyrrole-H), 2.05 (s, 6H; CH3),
1.32 ppm (s, 6H; CH3).
p-Tolyl-dioxyboron dipyrrin (7b): 1H NMR (300 MHz, CDCl3): d=9.80
(s,1H; aldehyde-H), 7.36–7.38 (dd, J=7.88, 7.75 Hz, 1H; dioxyaryl-H),
7.33–7.27 (m, 3H; dioxyaryl and aryl-H), 7.14 (d, J=8.60 Hz, 2H; aryl-
H), 6.88 (d, J=7.78 Hz, 1H; dioxyaryl-H), 5.99 (s, 2H; pyrrole-H), 2.41
(s,3H; CH3), 2.0 (s, 6H; CH3), 1.59 (s, 3H; CH3), 1.39 ppm (s, 3H; CH3).
Acknowledgements
This work was supported by the National Science Foundation (Grant
Nos. 0804015 and EPS-0903806) and matching support from the State of
Kansas through Kansas Technology Enterprise Corporation, a Grant-in-
Aid (Nos. 20108010 and 21750146) and the Global COE (center of excel-
lence) program “Global Education and Research Center for Bio-Envi-
ronmental Chemistry” of Osaka University from Ministry of Education,
Culture, Sports, Science and Technology, Japan, KOSEF/MEST through
WCU project (R31-2008-000-10010-0) from Korea.
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General procedure for synthesis of aryl-dioxyboron dipyrrin–fullerene
triads: The title compounds were synthesized according to the general
procedure described by Prato and coworkers for fulleropyrrolidines[30]
with few modifications. Fullerene C60 (147 mg, 0.204 mmol), aryl-dioxy-
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fast equilibrium.[32]
1-Pyrene-dioxyboron dipyrrin–fullerene triad (3): 1H NMR (300 MHz,
CDCl3): d=8.29–7.85 (m, 9H; pyrenyl-H), 7.4–7.3 (m, 2H; dioxyaryl-H),
6.85 (brs, 1H; dioxyaryl-H), 5.89 (brs, 2H; pyrrole-H), 4.99 (d, J=
9.50 Hz, 1H; fulleropyrrolidine- H), 4.89 (s, 1H; fulleropyrrolidine-H),
4.25 (d, J=9.70 Hz, 1H; fulleropyrrolidine-H), 2.91 (s, 3H; fulleropyrro-
lidine-H), 0.81 ppm (s, 12H; CH3); MS (MALDI-TOF): m/z calcd for
C
104H38BN3O2: 1296.11; found: 1294.4.
9-Anthracene-dioxyboron dipyrrin–fullerene triad (4): 1H NMR
(300 MHz, CDCl3): d=8.5 (s,1H; aryl-H), 8.0 (d, J=8.55 Hz, 2H; aryl-
H), 7.84 (d, J=8.55 Hz, 2H; aryl-H), 7.58–7.39 (m, 6H; aryl-H & diox-
yaryl-H), 6.75 (brs, 1H; dioxyaryl-H), 5.9 (brs, 2H; pyrrole-H), 4.99 (d,
J=9.61 Hz, 1H; fulleropyrrolidine- H), 4.89 (s, 1H; fulleropyrrolidine-
H), 4.26 (d, J=9.44 Hz, 1H; fulleropyrrolidine-H), 3.5 (s, 1H; fulleropyr-
rolidine-H), 2.91 (s, 3H; fulleropyrrolidine-H), 1.21 ppm (s, 12H; CH3);
MS (MALDI-TOF): m/z calcd for C96H34BN3O2: 1270.07; found: 1269.5.
3154
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3147 – 3156

Photochemical Charge Separation in Boron Dipyrrin–Fullerene Triads
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