Near-IR excitation transfer and electron transfer in a BF 2-chelated dipyrromethane-azadipyrromethane dyad and triad

Article abstract of DOI:10.1002/chem.201103074

A molecular dyad and triad, comprised of a known photosensitizer, BF ₂-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF₂-chelated tetraarylazadipyrromethane (ADP), have bee

Full text of DOI:10.1002/chem.201103074

FULL PAPER
DOI: 10.1002/chem.201103074
Near-IR Excitation Transfer and Electron Transfer in a BF2-Chelated
Dipyrromethane–Azadipyrromethane Dyad and Triad
Mohamed E. El-Khouly,[a] Anu N. Amin,[b] Melvin E. Zandler,[b]
Shunichi Fukuzumi,*[a, c] and Francis DꢀSouza*[b, d]
Abstract: A molecular dyad and triad,
comprised of a known photosensitizer,
BF2-chelated dipyrromethane (BDP),
covalently linked to its structural
analog and near-IR emitting sensitizer,
tial pulse voltammetry studies have re-
vealed the redox states, allowing esti-
mation of the energies of the charge-
separated states. Such calculations re-
vealed a charge separation from the
currence of energy transfer from
1BDP* to ADP (in benzonitrile and
toluene) and electron transfer from
BDP to 1ADP* (in benzonitrile, but
not in toluene). The kinetic study of
energy transfer was measured by moni-
toring the rise of the ADP emission
and revealed fast energy transfer
(ca. 1011 sꢀ1) in these molecular sys-
tems. The kinetics of electron transfer
via 1ADP*, measured by monitoring
the decay of the singlet ADP at l=
BF2-chelated tetraarylazadipyrro
N
singlet excited BDP (1BDP*) to ADP
+
C
Cꢀ
T
(BDP -ADP ) to be energetically fa-
vorable in nonpolar toluene and in
polar benzonitrile. In addition, the ex-
citation transfer from the singlet BDP
to ADP is also envisioned due to good
spectral overlap of the BDP emission
and ADP absorption spectra. Femto-
second laser flash photolysis studies
provided concrete evidence for the oc-
sized and the photoinduced energy and
electron transfer were examined by
femtosecond and nanosecond laser
flash photolysis. The structural integrity
of the newly synthesized compounds
has been established by spectroscopic,
electrochemical, and computational
methods. The DFT calculations re-
vealed a molecular-clip-type structure
for the triad, in which the BDP and
ADP entities are separated by about
14 ꢁ with a dihedral angle between the
fluorophores of around 708. Differen-
820 nm, revealed
a
relatively fast
charge-separation process from BDP to
1ADP*. These findings suggest the po-
tential of the examined ADP–BDP
molecules to be efficient photosynthet-
ic antenna and reaction center models.
Keywords: chelates · donor–accept-
or systems · electron transfer · ener-
gy transfer · near-IR emitters
Introduction
which range from solar-energy harvesting and energy con-
version to molecular logic gates and optoelectronic devi-
ces.[1–7] Tetrapyrrole macrocycles, such as porphyrins and
phthalocyanines, have widely been used for these applica-
tions due to their structural similarity to natural photosyn-
thetic pigments, established synthesis protocols, and tunable
redox and photophysical properties.[8] Interestingly, another
class of fluorophores, BF2-chelated dipyrromethanes
(BODIPY or BDP),[9] have also recently been employed to
perform the functionalities of antennas and electron donors
or acceptors in a number of molecular systems.[10–18] This is
because the BDP macrocycle can be easily modified at the
meso and b-carbon positions and exhibits attractive photo-
physical characteristics with strong absorption and emission
bands in the visible region.[9,19]
Molecular and supramolecular systems linked to two or
more fluorophores have attracted increased interest from
the scientific community due to their potential applications,
[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] A. N. Amin, Prof. M. E. Zandler, Prof. F. DꢂSouza
Department of Chemistry
Wichita State University, KS 67260-0051 (USA)
[c] Prof. S. Fukuzumi
A structural analog of BDP, BF2-chelated tetraarylazadi-
pyrromethanes (Aza-BODIPY or ADP), which have a nitro-
gen in the meso position of the macrocycle, have attracted
increased attention because of their high extinction coeffi-
cients (7–8ꢀ105 mꢀ1 cmꢀ1), large fluorescence quantum yields
above l=700 nm, and facile one-electron reduction poten-
tials.[20] Thus, they have been used in applications ranging
from photosensitizers to photodynamic therapy agents.[21]
Recently, we reported on donor–acceptor dyads and triads
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
[d] Prof. F. DꢂSouza
Department of Chemistry, University of North Texas
1155 Union Circle, #305070, Denton, TX 76203-5017 (USA)
E-mail: Francis.DSouza@UNT.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103074, and contains the
mass spectra of the new compounds, excitation spectra, and transient
absorption spectra of the compounds in toluene and benzonitrile.
Chem. Eur. J. 2012, 18, 5239 – 5247
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5239

that feature ferrocene and ADP and demonstrated efficient
photoinduced electron transfer from ferrocene to singlet ex-
cited ADP.[22a] By using transient absorption techniques, it
was possible to spectrally characterize the one-electron-re-
duced product of ADP and monitor the photoinduced elec-
tron-transfer dynamics. In a parallel study, ADP was cova-
lently linked to a known electron acceptor, fullerene, and ul-
trafast photoinduced electron transfer leading into the for-
mation of a radical ion pair (ADP +–C60 ) was successfully
C
Cꢀ
demonstrated.[22b]
Herein, we report on the application of both ADP and
BDP in excitation-transfer and electron-transfer model com-
pounds. For this purpose we have covalently linked one or
two units of BDP to ADP and formed molecular dyad and
triad systems, as shown in Scheme 1. Although structurally
similar, the BDP and ADP macrocycles differ substantially
in their optical absorption and emission properties in such
a fashion that energy transfer from singlet excited BDP to
ADP (a near-IR emitter) is achievable. This has been dem-
onstrated herein by employing both steady-state and time-
resolved studies at different time scales. In the examined
BDP-ADP and (BDP)2-ADP molecules, the BDP acts both
as antenna and electron-donor unit, whereas the ADP acts
as a promising energy and electron acceptor. Therefore, we
have successfully demonstrated the ability of singlet excited
BDP to be an energy donor and an electron donor within
a molecular dyad or triad. This is in contrast to the previous
Scheme 1. Structures of the newly synthesized BDP-ADP dyad and
(BDP)2-ADP triad and the ADP(OH)2 and tolyl-BDP control com-
pounds.
convert them to BDP entities. The structural integrity of the
newly synthesized compounds was established from
1H NMR spectroscopy, mass spectrometry (see Figure S1,
Supporting Information), and optical techniques.
1
studies in which BDP* was primarily deactivated by either
one of the two photochemical processes but not both.[10–18]
Steady-state absorption and fluorescence measurements:
The optical absorption spectra of the dyad and the triad
along with the control compounds, that is, BDP and
Results and Discussion
Syntheses of the BDP-ADP
dyad and (BDP)2-ADP triad:
The synthesis of the donor–ac-
ceptor systems involved a multi-
step approach, as shown in
Scheme 2; the details are given
in the Experimental Section.
Briefly, the BF2-chelated [5-(4-
hydroxyphenyl)-3-phenyl-1H-
pyrrol-2-yl]-[5-(4-hydroxyphen-
yl)-3-phenylpyrrol-2-ylidene]-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
was treated with 4-formylcar-
boxylic acid in the presence of
EDCl to introduce one or two
formyl groups on the ADP pe-
riphery. The formyl groups
were reacted with 2,4-dimethyl-
pyrrole to generate one or two
dipyrro
N
ACHTUNGTRENNUNG
ly, the di
N
ACHTUNGTRENNUNG
were treated with BF3OEt2 to Scheme 2. Synthetic procedure adapted for the BDP-ADP dyad and (BDP)2-ADP triad.
5240
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5239 – 5247

Dipyrromethane–Azadipyrromethane Dyads and Triads
FULL PAPER
Figure 1. Normalized absorption spectra of tolyl-BDP (–··–), ADP(OH)2
(c), BDP-ADP (-·-·-·), and (BDP)2-ADP (a) in toluene. The spectra
of tolyl-BDP, BDP-ADP, and (BDP)2-ADP were normalized at l=
505 nm.
Figure 2. Fluorescence spectra of tolyl-BDP (c) and BDP-ADP dyad
in toluene (g) and benzonitrile (a). Inset: Fluorescence spectra of
the BDP-ADP dyad and (BDP)2-ADP triad in toluene (TN) and benzo-
nitrile (PhCN) to demonstrate quenched emission of ADP in benzonitrile
compared with that in toluene. The spectra were recorded at the same
optical density at l=505 nm; lex =505 nm.
ADP(OH)2, are shown in Figure 1 in toluene and Figure S2
in the Supporting Information in benzonitrile. BDP exhibits
a visible band at l=503 nm, whereas for ADP(OH)2 this
band is located at l=670 nm, along with a less intense band
at l=462 nm. When pristine ADP with no peripheral OH
groups is used, this band is found to be located at l=
658 nm. The 12 nm red shift in the spectrum of ADP(OH)2
could be ascribed to a bathochromic effect caused by the
OH groups. For the BDP-ADP dyad, BDP and ADP bands
are located at l=505 and 676 nm, with similar peak charac-
teristics (molar extinction coefficient (e) and broadness). In-
terestingly, for the (BDP)2-ADP triad, the absorption bands
are located at l=507 and 665 nm, and the peak intensity of
the l=665 nm band corresponding to ADP is almost half of
that of the BDP band at l=507 nm.
cient excitation transfer from the singlet BDP to ADP. By
changing the solvent from toluene to benzonitrile, similar
energy transfer from the singlet BDP to the attached ADP
was clearly observed. The only difference is the considerably
higher quenching of the singlet-excited states of
BDP-1ADP* and (BDP)2-1ADP* compared to that in tolu-
ene (Figure S4, Supporting Information). These results in
1
polar benzonitrile suggest that the ADP* of the dyad and
triad, formed by excitation transfer from the singlet-excited
state of BDP, is further quenched by electron transfer from
the attached BDP.
Figure 2 shows the fluorescence emission spectra of the
investigated compounds in toluene when excited at l=
505 nm, which corresponds to the absorption maxima of the
BDP entity. Pristine BDP revealed a strong emission located
at l=517 nm (Ff =0.89 for 1BDP* and Ff =0.12 for
1ADP*). However, in the dyad and triad this band was
quantitatively quenched with the appearance of a new weak
band at l=674 nm for the dyad and l=663 nm for the
triad, respectively. The intensity of the formed BDP-1ADP*
was found to be almost the same as that obtained by the
direct excitation of BDP-ADP at l=680 nm in toluene,
which selectively excited the ADP entity (Figure S3, Sup-
porting Information). Further, excitation spectra for both
the dyad and triad were recorded by fixing the emission
monochromator to the ADP emission peak maxima. The ex-
citation spectrum revealed the absorption band of not only
ADP at l=671 nm but also BDP at l=506 nm, which pro-
vides direct proof for singlet–singlet energy transfer (Fig-
ure S4, Supporting Information). These results show energy
transfer from the singlet-excited BDP to the covalently
linked ADP in both the dyad and the triad.[23] The ratio of
peak intensities was found to be 0.44, which suggests an effi-
Electrochemistry and energy levels: To establish the energy
levels, electrochemical studies by using differential pulse
voltammetry (DPV) were performed. Figure 3 shows the
DPV of the dyad and triad along with the reference com-
pounds in o-dichlorobenzene (o-DCB), 0.10m (nBu4N)ClO4.
The one-electron reduction of ADP(OH)2 was located at
ꢀ0.99 V, whereas the one-electron reduction for tolyl-BDP
is located at ꢀ1.75 V vs. Fc/Fc+. The first one-electron re-
duction of the BDP-ADP dyad located at ꢀ0.93 V can be
ascribed to the reduction of the ADP moiety, which is anod-
ically shifted by 50 to 60 mV compared with ADP(OH)2.
The second one-electron reduction at ꢀ1.64 V can be as-
cribed to the reduction of the BDP entity, which is found to
be anodically shifted by about 106 mV compared with tolyl-
BDP. Similar shifts in the reduction potentials were ob-
served for (BDP)2-ADP triad, with the first one-electron re-
duction being at ꢀ0.85 V and the second one-electron reduc-
tion at ꢀ1.65 V, which can be attributed to reduction of the
ADP and BDP entities, respectively. The one-electron oxi-
dation potential of the tolyl-BDP is located at 0.73 V, where-
as the one-electron oxidation potential corresponding to the
BDP entity in (BDP)2-ADP and BDP-ADP are located at
Chem. Eur. J. 2012, 18, 5239 – 5247
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5241

F. DꢂSouza, S. Fukuzumi et al.
Table 1. Electrochemical redox potentials (V vs. Fc/Fc+) and driving
forces of charge recombination (DGCR) and charge separation (DGCS) for
1
the (BDP)n-ADP dyad (n=1) and triad (n=2) via 1BDP* and ADP* in
benzonitrile and toluene.
+
[a]
[a]
[b]
[b]
BDP0/
[V]
ADP0/ ꢀ DGCR
DGCS
[eV]
DGCR
[eV]
DGCS
[eV]
C
C
[V]
[eV]
tolyl-BDP 0.74
–
–
–
–
–
–
–
ADP(OH)2
(BDP)2-
ADP
(BDP)1-
ADP
–
0.78
ꢀ0.99
N
ꢀ0.93
ꢀ1.42
ꢀ0.98[c] ꢀ1.88
ꢀ0.52[c]
ꢀ0.43[d]
–
[d]
E
0.81
ꢀ0.85
ꢀ1.47
ꢀ0.93[c] ꢀ1.93
ꢀ0.38[d]
ꢀ0.47[c]
[d]
–
[a] In benzonitrile. [b] In toluene. [c] Via BDP*. [d] Via 1ADP*.
1
the energy of 1BDP* (2.40 eV) and 1ADP* (1.85 eV), the
driving forces for the charge-separation processes (ꢀDGCS)
were calculated as listed in Table 1. The negative DGCS
values of BDP-ADP and (BDP)2-ADP in benzonitrile give
the charge separation from BDP to the singlet excited ADP
in benzonitrile. The values of DGCS and DGCR suggest that
Figure 3. Differential pulse voltammograms of the indicated compounds
in o-dichlorobenzene, 0.1m (nBu4N)ClO4. Scan rate=5 mVsꢀ1
pulse
width=0.25 s, pulse height=0.025 V. The asterisk indicates the oxidation
,
1
process of ferrocene used as an internal standard.
in the event of electron transfer from BDP to ADP* the
charge separation is in the Marcus normal region, whereas
+
C
Cꢀ
the charge recombination in BDP -ADP lies in the
Marcus inverted region.[25]
0.78 and 0.81 V versus Fc/Fc+, respectively. These anodic
shifts of approximately 50 to 80 mV can be attributed to the
chemical functionalization of the BDP entity. Also, in the
case of the triad the current for the BDP oxidation was
twice as large as that of the current of the ADP reduction.
In the case of compounds with OH substituents, an addition-
al peak near 0.7 V was observed which could be ascribed to
the oxidation of phenolate substituents.
B3LYP/3-21G(*) geometry optimization studies: Figure 4a
and b show the B3LYP/3-21G(*)-optimized structures of the
dyad and triad, respectively.[26,27] The structures are fully op-
The thermodynamic driving forces for the charge-recom-
bination (ꢀDGCR) and charge-separation (ꢀDGCS) processes
were calculated by using Equations (1) and (2):[24]
Cþ
Cꢀ
ꢀDGCR ¼ E1=2ðBDP =BDPÞꢀE1=2ðADP=ADP Þ þ DGs
ð1Þ
ð2Þ
ꢀDGCS ¼ DE0ꢀ0ꢀðꢀDGCR
Þ
+
C
in which E1/2(BDP /BDP) is the first oxidation potential of
Cꢀ
BDP, E1/2(ADP/ADP ) is the first reduction potential of
ADP, DE0ꢀ0 is the energy of the 0–0 transition energy gap
between the lowest excited state and the ground state of
BDP and ADP, and DGs refers to the static Coulombic
energy, calculated by using the dielectric continuum
model[24] according to Equation (3):
Figure 4. B3LYP/3-21G(*) optimized structures of a) BDP-ADP and
b) (BDP)2-ADP. The frontier HOMO and LUMO of the (BDP)2-ADP
triad are shown in c) and d), respectively.
DGS ¼ꢀe2=4pe0½ð1=ð2RþÞ þ 1=ð2RꢀÞꢀð1=RCCÞ=eSÞ
ð3Þ
ꢀðð1=ð2RþÞ þ 1=ð2RꢀÞÞ=eRÞ
timized on a Born–Oppenheimer potential energy surface.
In the optimized structure, both the ADP and BDP macro-
cycles are found to be flat, whereas the peripheral aromatic
rings of ADP are slightly tilted. The boron–boron separation
in the BDP-ADP macrocycles is found to be 15.6 ꢁ, where-
as the distance from the closest carbon of BDP to the closet
The symbols e0, es, and eR represent vacuum permittivity
and the dielectric constants of solvent used for photochemi-
cal and electrochemical studies, respectively. RCC is the
center-to-center distance between BDP and ADP entities.
The DGCR values are summarized in Table 1. Together with
5242
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5239 – 5247

Dipyrromethane–Azadipyrromethane Dyads and Triads
FULL PAPER
Z
carbon of ADP (edge-to-edge distance) is about 12.1 ꢁ. The
two macrocycles were spatially disposed with a dihedral
angle of 54.78.
JForster
¨
¼
FDðlÞeAðlÞl4dl
ð6Þ
in which FD(l) is the fluorescence intensity of the donor
with total intensity normalized to unity, eA(l) is the molar
extinction coefficient of the acceptor expressed in units of
mꢀ1 cmꢀ1 and l in nanometers. Analysis of the data accord-
ing to Equation (6) gave a J value of 9.1ꢀ10ꢀ13 mꢀ1 cm3 for
the BDP-ADP system.
Interestingly, for the (BDP)2-ADP triad, the two coplanar-
ly positioned BDP entities linked to ADP formed a molecu-
lar-clip-type structure similar to the structure of the (Fc)2-
ADP triad reported earlier.[22a] The two BDP macrocycles
ꢀ
are separated by a distance of 15.8 ꢁ. The B B separation
and edge-to-edge distance are not significantly different
from that of dyad. The dihedral angles between the ADP
and BDP macrocycles are found to be 55.8 and 64.08 for the
two BDP entities, respectively. The frontier highest occupied
molecular orbital (HOMO) for both the dyad and triad are
found to be on the BDP entity, while the lowest unoccupied
molecular orbital (LUMO) is found to be on the ADP
entity. The location of the HOMO and LUMO are in agree-
ment with the electrochemical results, which predicted the
BDP entity to be an electron donor and the ADP entity to
be an electron acceptor.
The kFçrster value estimated by using the parameters de-
scribed in Equation (5) is found to be of the order of 4 to
8ꢀ1010 sꢀ1, depending upon solvent conditions, which reveals
fast excitation transfer. As explained below, the rate of
energy transfer measured by using the transient absorption
technique agrees well with the theoretical predictions.
Photodynamics studies: Femtosecond transient absorption
spectroscopy was used to obtain further insight into the ex-
cited state interactions in BDP-ADP dyad and (BDP)2-ADP
triad to corroborate the proposed energy-transfer and elec-
tron-transfer processes. Toward this end, the BDP-ADP
dyad and (BDP)2-ADP triad were probed with excitation at
l=490 nm to selectively excite the BDP fluorophore. The
femtosecond transient absorption spectra of tolyl-BDP in
toluene (Figure 5) revealed the instantaneous formation of
Excited energy transfer mechanism: The observed excitation
transfer between singlet-excited BDP as a donor and ADP
as an acceptor could be explained either by Dexterꢂs ex-
change mechanism[28] or Fçrsterꢂs dipole–dipole mecha-
nism.[29] The former mechanism involves a double-electron
exchange involving one electron from the LUMO of the ex-
cited donor to the empty LUMO of the acceptor with a si-
multaneous transfer of another electron from the HOMO of
the acceptor to the half-filled HOMO of the donor. The
rate constant is given by Equation (4):
kD ¼ 4p2H2JD=h
ð4Þ
in which h is Plankꢂs constant, H is the electronic exchange
parameter, and JD is the Dexter spectral overlap integral.[28]
The frontier orbitals from the DFT studies (Figure 4) in con-
junction with the spectroscopic studies reveal that such elec-
tronic interactions are almost non-existent. Therefore, the
results of the present study have been analyzed according to
Fçrsterꢂs mechanism.
According to Fçrster mechanism, the rate of excitation
energy transfer, kFçrster, is given by Equation (5):[29]
Figure 5. Differential absorption spectra obtained upon femtosecond
flash photolysis (l=480 nm) of tolyl-BDP in toluene with several time
delays between 1 and 1800 ps at room temperature. Inset: Decay profile
of the singlet BDP at l=508 nm, monitoring the intersystem crossing dy-
namics.
kForster ¼ ½8:8 ꢁ 10ꢀ25k2FDJForsterꢂ=½h4tDR6ꢂ
ð5Þ
¨
¨
in which h is the solvent refractive index, FD and tD are the
fluorescence quantum yield (0.6) and the fluorescence life-
time (1.6 ns) of the isolated donor, JFçrster is the Fçrsterꢂs
overlap integral representing the emission of the donor and
absorption of the acceptor, R is the donor–acceptor center-
to-center distance (15.6 ꢁ). In Equation (5), k2 is the orien-
tation factor and for flexible systems of the type discussed
here, a value of k2 =2/3 is generally used.[23] The JFçrster spec-
tral overlap integral representing the emission of the donor
and absorption of the acceptor is given by Equation (6):
BDP singlet-excited-state features. Here, the transient ab-
sorption spectra are dominated by pronounced bleaching
between l=450 and 600 nm, which is due to depletion of
the singlet ground state. BDP singlet-excited-state features
are long lived (i.e., 4.2 ns). A slow intersystem crossing to
the corresponding triplet state of BDP is implicit.
The spectral features in the visible region, which are seen
immediately (i.e., 1 ps) upon excitation of BDP-ADP dyad
in toluene (Figure 6), are almost the same as those recorded
Chem. Eur. J. 2012, 18, 5239 – 5247
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5243

F. DꢂSouza, S. Fukuzumi et al.
cayed with a rate constant of
1.9ꢀ109 sꢀ1, which is much
faster than that observed in tol-
uene. This observation indicates
occurrence of electron transfer
1
from BDP to ADP* in benzo-
nitrile with a rate constant of
1.40ꢀ109 sꢀ1,[30] as predicated
from the earlier steady-state
emission measurements and the
calculated negative DGCS value
in benzonitrile. It should be
noted that the anticipated ab-
sorption band of the ADP radi-
cal anion (Figure S6, Support-
ing Information) was hidden
under the strong bleaching of
1
the ADP*.
The spectral features of the
(BDP)2-ADP triad in toluene
and benzonitrile (see Figure S7
and S8, Supporting Informa-
tion) were found to be similar
to those recorded for the BDP-
ADP dyad. The kENT from
1BDP* to ADP was determined
to be 1.2ꢀ1011 sꢀ1. In benzoni-
1
trile, the ADP* of the (BDP)2-
ADP triad decayed with a rate
constant of 6.9ꢀ109 sꢀ1, which
corresponds to the kCS. The
faster kCS of the triad with a rel-
atively higher DGCS (ꢀ0.43 eV)
Figure 6. Top: Differential absorption spectra obtained upon femtosecond flash photolysis (lex =490 nm) of
BDP-ADP in toluene at 1, 5, and 35 ps. Bottom: Time–absorption profiles of the singlet states of BDP (left,
l=508 nm) and ADP (right, l=820 nm).
than
the
dyad
(DGCS =
ꢀ0.38 eV) can be understood if
for the BDP singlet-excited state (1BDP*). The absorption
of the singlet BDP at l=508 nm is greatly diminished in in-
tensity over time, with concomitant increase of the ADP
emission at l=671 and 820 nm, which provides direct evi-
dence of excitation transfer between the photoexcited BDP
unit and the singlet ground state of ADP (see Figure S3 in
the Supporting Information for femtosecond transient spec-
tra of ADP(OH)2). The kinetics of BDP to ADP energy
transfer was determined by exponential fitting of the rise
charge separation is occurring in the Marcus normal
region.[25]
Further nanosecond transient absorption spectra were re-
+
C
corded to check whether the charge-separated state (BDP
Cꢀ
-ADP ) returns to the ground state via the triplet decay
path. Upon excitation with l=490 nm laser light, the transi-
ent spectra of tolyl-BDP exhibited an extremely weak ab-
sorption in the l=400 to 450 nm range, which corresponds
to triplet BDP with a lifetime of 71 ms (Figure S9 in the Sup-
porting Information). By exciting the ADP(OH)2 control at
l=490 nm, a weak absorption of the triplet ADP was ob-
served in the l=400 to 550 nm range, with a maximum at
l=440 nm (Figure S10 in the Supporting Information). The
decay rate of triplet ADP was found to be 5.7ꢀ103 sꢀ1, from
which a lifetime of 175 ms was deduced. Because the energy
levels of the charge-separated states of the dyad and triad
are found to be higher than that of the low-lying triplet
state of ADP, the charge recombination of the radical-ion
pairs can be expected to populate the triplet ADP and the
ground state. The finding that the transient spectrum of
(BDP)2-ADP in benzonitrile exhibited rather weak signals
in the l=400 to 500 nm range, which corresponds to the for-
1
profile of the formed ADP* at 820 nm (kENT =1.1ꢀ1011 sꢀ1).
1
The decay rate of the formed ADP* was found to be 5.7ꢀ
108 sꢀ1, which is close to the decay of the singlet ADP refer-
1
ence compound, indicating that the ADP* is not quenched
by the attached BDP in toluene.
By changing the solvent from toluene to benzonitrile, sim-
ilar spectral features of the BDP-ADP dyad were observed
1
(Figure 7). From the rise profile of the formed ADP*, the
singlet BDP to ADP energy transfer in polar benzonitrile
determined as 1.2ꢀ1011 sꢀ1. The finding that the kENT value
in benzonitrile is nearly as same as the value in toluene indi-
1
cates that the energy transfer from BDP* to ADP is insen-
1
sitive to the solvent polarity. On the other hand, ADP* de-
5244
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5239 – 5247

Dipyrromethane–Azadipyrromethane Dyads and Triads
FULL PAPER
to the attached ADP. In toluene, the formed 1ADP* de-
cayed to its ground state without evidence of additional sub-
sequent energy-transfer and/or electron-transfer reactions.
1
However, an electron transfer from BDP to ADP* in ben-
+
C
Cꢀ
zonitrile was clearly observed to give (BDP -ADP ) in
both dyad and triad. It is worth noting that deactivation of
+
C
Cꢀ
the electron-transfer product, BDP -ADP leads to the
ground state without the intermediate formation of either
3BDP* or ADP*, as indicated by the photochemical meas-
3
urements.
Conclusion
The synthesis and characterization of a new type of dyad
and triad featuring BF2-chelated dipyrromethane covalently
linked to its structural analog, BF2-chelated tetraarylazadi-
pyrromethane, have been accomplished. The structural in-
tegrity of the newly synthesized dyad and triad has been es-
tablished by spectroscopic, electrochemical, and computa-
tional methods. DFT calculations revealed a molecular-clip-
type structure for the triad, in which the two coplanar BDP
entities are separated by about 16 ꢁ. The redox states were
examined by using voltammetry studies, and the free-energy
calculations suggested the possibility of charge separation
via the singlet-excited states of BDP and ADP entities.
However, selective excitation of the BDP entity in the dyad
and triad revealed quantitative quenching of the BDP fluo-
rescence with simultaneous emission from the ADP entity
in the near-IR region in both polar and nonpolar solvents.
The energy-transfer and electron-transfer products were
characterized by the femtosecond laser flash photolysis stud-
ies. The kinetics of energy transfer, measured by monitoring
either the decay of BDP emission or the rise of the ADP
emission, revealed fast energy transfer (ꢃ1011 sꢀ1) in these
molecular systems in both polar and nonpolar solvents,
which agrees well with the theoretical estimations. In con-
trast to the results obtained in toluene, further photochemi-
cal events from BDP to 1ADP* that lead to electron transfer
are observed in polar benzonitrile. The kinetics of this pro-
cess were found to be relatively fast. The present study suc-
cessfully demonstrates utilization of a near-IR emitting sen-
sitizer, BF2-chelated tetraarylazadipyrromethane, as a suit-
Figure 7. Top: Differential absorption spectra obtained upon femtosecond
flash photolysis (lex =490 nm) of BDP-ADP in benzonitrile at 5, 160, and
2500 ps. Bottom: Time–absorption profile of the singlet state of ADP at
l=820 nm, monitoring the energy-transfer and electron-transfer dynam-
ics.
mation of triplet ADP (Figure S11, Supporting Informa-
tion), suggests that the transient species of both dyad and
triad decayed mainly to populate the ground state.
Figure 8 summarizes the photochemical events in the
form of an energy-level diagram for the investigated dyad;
a similar diagram can be envisioned for the triad as well. Se-
lective excitation of the BDP entity of the dyad populates
the singlet-excited state of BDP, which transfers its energy
AHCTUNGTREGaNNNU ble candidate to build new types of donor–acceptor dyads
for excitation-transfer and electron-transfer studies.
Experimental Section
Chemicals: All the reagents were from Aldrich Chemicals (Milwaukee,
WI), whereas the bulk solvents used in the syntheses were from Fischer
Chemicals. Tetra-n-butylammonium perchlorate, (nBu4N)ClO4 used in
electrochemical studies were from Fluka Chemicals. The initial synthesis
of ADP macrocycle was performed according to the procedure reported
by OꢂShea and co-workers[20] with modifications.
Figure 8. Energy-level diagram depicting photoinduced energy-transfer
and electron-transfer processes in the BDP-ADP dyad in benzonitrile
and toluene.
The synthetic details of BF2-chelated [5-(4-hydroxyphenyl)-3-phenyl-1H-
pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine
are
provided elsewhere.[23]
Chem. Eur. J. 2012, 18, 5239 – 5247
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5245

F. DꢂSouza, S. Fukuzumi et al.
Synthesis of ADP-(ALDEHYDE)2 and ADP-(ALDEHYDE)1: 4-Car-
boxybenzaldehyde (200 mg, 1.3 mmol) was dissolved in DMF (20 cm3), to
which EDCI (255 mg, 1.3 mmol) was added at 08C under N2, followed by
the addition of BF2-chelated [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-
for metal complexes in PhCN with dithranol as a matrix. The computa-
tional calculations were performed by using DFT B3LYP/3–21G* meth-
ods with the GAUSSIAN 03 software package[27] on high-speed PCs. The
frontier HOMO and LUMO were generated by using GaussView soft-
ware.
yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine
(234.9 mg,
0.4 mmol; see Scheme 2 for structures of the abbreviated compounds),
after which the mixture was stirred for 24 h. Then the solvent was re-
moved under reduced pressure and the residue was dissolved in CH2Cl2
and washed with water. Then the organic layer was separated and dried
over Na2SO4, then the solvent was evaporated. The residue was purified
by column chromatography on silica gel with CH2Cl2/hexanes (1:1) to
give ADP-(ALDEHYDE)2 (yield 80 mg, 22%), and with CH2Cl2 to give
ADP-(ALDEHYDE)1 (yield 50 mg, 17%).
Data for ADP-(ALDEHYDE)2: 1H NMR (400 MHz, CDCl3): d=10.16
(s, 2H), 8.38 (d, J=8.14 Hz, 4H), 8.17 (d, J=8.98 Hz, 4H), 8.12–8.02 (m,
8H), 7.52–7.45 (m, 6H), 7.41 (d, J=8.96 Hz, 4H), 7.08 ppm (s, 2H)
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 mJpulseꢀ1. The transient absorption measurements were per-
formed by using a continuous xenon lamp (150 W) and an InGaAs-PIN
photodiode (Hamamatsu 2949) as a probe light and a detector, respec-
tively. The output from the photodiodes and a photomultiplier tube was
recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz).
Femtosecond transient absorption spectroscopy experiments were con-
ducted by using an Integra-C (Quantronix Corp.) as an ultrafast source,
TOPAS (Light Conversion Ltd.) as an optical parametric amplifier, and
a commercially available optical detection system (Helios, provided by
Ultrafast Systems LLC). The source for the pump and probe pulses were
derived from the fundamental output of the Integra-C (780 nm,
2 mJpulseꢀ1, 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 that result in a tunable range from l=285 to
1660 nm, whereas the rest of the output was used for white light genera-
tion. Typically, 2500 excitation pulses were averaged for 5 s to obtain the
transient spectrum at a set delay time. Kinetic traces at appropriate
wavelengths were assembled from the time-resolved spectral data. All
measurements were conducted at 298 K. The transient spectra were re-
corded using fresh solutions in each laser excitation.
Data for ADP-(ALDEHYDE)1: 1H NMR (400 MHz, CDCl3): d=10.17
(s, 1H), 8.39 (d, J=8.34 Hz, 2H), 8.14 (d, J=9.04 Hz, 2H), 8.11–8.03 (m,
10H), 7.52–7.41 (m, 8H), 7.38 (d, J=8.95 Hz, 2H), 7.11 (s, 1H), 7.01 (s,
1H), 6.96 ppm (d, J=8.88 Hz, 2H).
Synthesis of (BDP)2-ADP: Compound ADP-(ALDEHYDE)2 (63 mg,
8ꢀ10ꢀ2 mmol) and 2,4-dimethyl pyrrole (3.3ꢀ10ꢀ2 cm3, 0.32 mmol) were
dissolved in absolute methylene chloride (50 cm3) under N2. One drop of
trifluoroacetic acid was added to the reaction mixture, which was stirred
for 3 h. Then a solution of DDQ (36 mg, 0.16 mmol) in methylene chlo-
ride was added and the mixture was stirred for 1 h followed by the addi-
tion of diisopropylethylamine (0.12 cm3, 0.68 mmol) and borontrifluoride
diethyletherate (0.12 cm3, 0.97 mmol). The reaction mixture was stirred
for a further 1 h, after which it was washed with water. The organic layer
was dried over anhydrous Na2SO4 and the solvent was evaporated. The
residue was purified by column chromatography on silica gel with
CH2Cl2/hexanes (1:1) to give (BDP)2-ADP (Yield: 10 mg, 10%).
1H NMR (400 MHz, CDCl3): d=8.35 (d, J=8.28 Hz, 4H), 8.18 (d, J=
8.82 Hz, 4H), 8.09 (d, J=9.59 Hz, 4H), 7.56–7.4 (m, 14H), 7.08 (s, 2H),
6.02 (s, 2H), 2.58 (s, 12H), 1.42 ppm (s, 12H); MALDI MS: m/z calcd for
C72H56N7O4B3F6: 1229.7; found: 1236.07.
Acknowledgements
The authors are thankful to Drs. E. Maligaspe and N. K. Subbaiyan for
helpful discussions. This work was supported by the National Science
Foundation (grant nos. CHE-0804015 and CHE1110942 to F.D.),
a Grant-in-Aid (nos. 20108010 and 21750146), and the Global COE
(center of excellence) program “Global Education and Research Center
for Bio-Environmental Chemistry” of Osaka University from the Minis-
try of Education, Culture, Sports, Science, and Technology, Japan, and
KOSEF/MEST through WCU project (R31-2008-000-10010-0) from
Korea.
Synthesis of BDP-ADP: Compound ADP-(ALDEHYDE)1 (40 mg, 6ꢀ
10ꢀ2 mmol) and 2,4-dimethyl pyrrole (1.2ꢀ10ꢀ2 cm3, 0.12 mmol) were dis-
solved in (50 cm3) absolute methylene chloride under N2 atmosphere. To
the reaction mixture one drop of trifluoroacetic acid was added and was
stirred for a period of 3 h. Then a solution of DDQ (13.7 mg, 6ꢀ
10ꢀ2 mmol) in methylene chloride was added, and the stirring was contin-
ued for 1 h followed by the addition of diisopropylethylamine (0.12 cm3,
0.68 mmol) and borontrifluoride diethyletherate (0.12 cm3, 0.97 mmol).
Stirring was further continued for 1 h, after which the reaction mixture
was washed with water. The organic layer was dried over anhydrous
Na2SO4 and the solvent was evaporated. The residue was purified by
column chromatography on silica gel with CH2Cl2 to give BDP-ADP
(yield 8 mg, 15%). 1H NMR (400 MHz, CDCl3): d=8.35 (d, J=8.38 Hz,
2H), 8.13 (d, J=8.86 Hz, 2H), 8.02–8.10 (m, 8H), 7.52–7.36 (m, 8H), 7.1
(s, 1H), 7.0 (s, 1H), 6.93 (d, J=8.7, 2H), 6.02 (s, 2H), 2.58 (s, 6H),
1.42 ppm (s, 6H); MALDI MS: m/z calcd for C52H41N5O3F4B2: 880.6;
found: 884.3.
[1] a) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2009, 42,
1890–1898; b) D. Gust, T. A. Moore in The Porphyrin Handbook,
Vol. 8 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic
Press, San Diego, 2000, pp. 153–190.
[2] a) M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910–1921;
b) M. R. Wasielewski, Chem. Rev. 1992, 92, 435–461.
[3] a) G. de La Torre, P. Vazquez, F. Agullo-Lopez, T. Torres, Chem.
Rev. 2004, 104, 3723–3750; b) G. Bottari, G. de La Torre, D. M.
Guldi, T. Torres, Chem. Rev. 2010, 110, 6768–6816; c) D. M. Guldi,
G. M. A. Rahman, V. Sgobba, C. Ehli, Chem. Soc. Rev. 2006, 35,
471–487; d) S. Fukuzumi, D. M. Guldi in Electron Transfer in
Chemistry, Vol. 2 (Ed.: V. Balzani), Wiley-VCH, Weinheim, 2001,
pp. 270–337; e) L. Sꢄnchez, M. Nazario, D. M. Guldi, Angew. Chem.
2005, 117, 5508; Angew. Chem. Int. Ed. 2005, 44, 5374.
[4] a) V. Balzani, G. Bergamini, P. Ceroni, Coord. Chem. Rev. 2008, 252,
2456–2469; b) W. M. Campbell, A. K. Burrell, D. L. Officer, K. W.
Jolley, Coord. Chem. Rev. 2004, 248, 1363–1379; c) M. D. Ward,
Chem. Soc. Rev. 1997, 26, 365–375; d) V. Balzani, A. Credi, M. Ven-
turi, ChemSusChem 2008, 1, 26; e) J. L. Sessler, C. M. Lawrence, J.
Jayawickramarajah, Chem. Soc. Rev. 2007, 36, 314–325.
Spectral measurements: The UV/Vis spectral measurements were carried
out with a Shimadzu Model 2550 double monochromator UV/Vis spec-
trophotometer. 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 by using a Varian 400 MHz spec-
trometer. Tetramethylsilane (TMS) was used as an internal standard. Dif-
ferential pulse voltammograms were recorded by using an EG&G PAR-
STAT electrochemical analyzer equipped with a three electrode system.
A platinum button electrode was used as the working electrode. A plati-
num wire served as the counter electrode and an Ag/AgCl electrode 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 using argon gas. Matrix-as-
sisted laser desorption/ionization time-of-flight mass spectra (MALDI-
TOF) were measured by using a Kratos Compact MALDI I (Shimadzu)
[5] a) F. DꢂSouza, O. Ito, Coord. Chem. Rev. 2005, 249, 1410–1422; b) F.
DꢂSouza, O. Ito, Chem. Commun. 2009, 4913–4928; c) M. E. El-
Khouly, O. Ito, P. M. Smith, F. DꢂSouza, J. Photochem. Photobiol. C
2004, 5, 79–104; d) F. DꢂSouza, O. Ito, Chem. Soc. Rev. 2012, 41,
86–96; e) F. DꢂSouza, O. Ito in Multiporphyrin Arrays: Fundamen-
5246
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5239 – 5247

Dipyrromethane–Azadipyrromethane Dyads and Triads
FULL PAPER
tals and Applications (Ed.: D. Kim), Pan Stanford, Singapore, 2012,
pp. 389–437.
Seth, C. H. Martin, D. L. Singh, D. Kim, R. R. Birge, D. F. Bocian,
D. Holten, J. S. Lindsey, J. Am. Chem. Soc. 1998, 120, 10001–10017.
[19] A. Loudet, K. Burgess, Chem. Rev. 2007, 107, 4891–4932.
[20] a) A. Gorman, J. Killoran, C. OꢂShea, T. Kenna, W. M. Gallagher,
D. F. OꢂShea, J. Am. Chem. Soc. 2004, 126, 10619–10631; b) M.
Tasior, D. F. OꢂShea, Bioconjugate Chem. 2010, 21, 1130–1133.
[21] a) A. Palma, M. Tasior, D. O. Frimannsson, T. T. Vu, R. Meallet-Re-
nault, D. F. OꢂShea, Org. Lett. 2009, 11, 3638–3641; b) J. Murtagh,
D. O. Frimannsson, D. F. OꢂShea, Org. Lett. 2009, 11, 5386–5389;
c) S. O. McDonnell, M. J. Hall, L. T. Allen, A. Byrne, W. M. Gal-
lagher, D. F. OꢂShea, J. Am. Chem. Soc. 2005, 127, 16360–16361;
d) K. Flavin, K. Lawrence, J. Bartelmess, M. Tasior, C. Navio, C.
Bittencourt, D. F. OꢂShea, D. M. Guldi, S. Giordani, ACS Nano
2011, 5, 1198–1206; e) Y. Kotaoka, Y. Shibata, H. Tamiaki, Chem.
Lett. 2010, 39, 953–955.
[22] a) A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler,
M. Supur, S. Fukuzumi, F. DꢂSouza, J. Phys. Chem. A 2011, 115,
9810–9819; b) A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E.
Zandler, S. Fukuzumi, F. DꢂSouza, Chem. Commun. 2012, 48, 206–
208; c) F. DꢂSouza, A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan,
M. E. Zandler, S. Fukuzumi, J. Am. Chem. Soc. 2012, 134, 654–664.
[23] Principles of Fluorescence Spectroscopy, 3rd ed. (Ed.: J. R. Lako-
wicz), Springer, Singapore, 2006.
[6] a) S. Fukuzumi, Org. Biomol. Chem. 2003, 1, 609–620; b) S. Fukuzu-
mi, Bull. Chem. Soc. Jpn. 2006, 79, 177–195; c) S. Fukuzumi, Phys.
Chem. Chem. Phys. 2008, 10, 2283–2297; d) S. Fukuzumi, T. Kojima,
J. Mater. Chem. 2008, 18, 1427–1439; e) S. Fukuzumi, T. Honda, K.
Ohkubo, T. Kojima, Dalton Trans. 2009, 3880–3889.
[7] a) An Introduction to Molecular Electronics, (Eds.: M. C. Petty,
M. R. Bryce, D. Bloor), Oxford University Press, New York, 1995;
b) Molecular Electronics: Science and Technology (Eds.: A. Aviram,
M. Ratner), New York Academy of Sciences, New York, 1998, 852;
c) Molecular Switches (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001; d) D. Gust, T. A. Moore, A. L. Moore, Chem. Commun. 2006,
1169; e) V. Balzani, A. Credi, M. Venturi in Organic Nanostructures
(Eds.: J. L. Atwood, J. W. Steed), 2008, 1–31; f) Energy Harvesting
Materials (Ed.: D. L. Andrews), World Scientific, Singapore, 2005;
g) Molecular Electronics: Commercial Insights, Chemistry, Devices,
Architectures and Programming (Ed.: J. M. Tour), World Scientific,
River Edge, NJ, 2003.
[8] a) The Porphyrin Handbook, Vol. 1–20 (Eds.: K. M. Kadish, K. M.
Smith, R. Guilard), Academic Press, San Diego, CA, 2000; b) Phtha-
locyanine: Properties and Applications (Eds.: C. C. Leznoff, A. B. P.
Lever), Wiley-Blackwell, New York, 1993; c) C. G. Claessens, D.
Gonzꢄlez-Rodrꢅguez, T. Torres, Chem. Rev. 2002, 102, 835.
[9] a) A. Treibs, F. H. Kreuzer, Justus Liebigs Ann. Chem. 1968, 718,
208–223; b) Handbook of Fluorescent Probes and Research Chemi-
cals (Ed.: R. P. Haugland), Molecular Probes, Eugene, OR, 1996.
[10] a) D. P. Kennedy, C. M. Kormos, S. Burdette, J. Am. Chem. Soc.
2009, 131, 8578–8586; b) J. Rosenthal, S. J. Lippard, J. Am. Chem.
Soc. 2010, 132, 5536–5537.
[24] N. Mataga, H. Miyasaka in Electron Transfer, Part 2: From Isolated
Molecules to Biomolecules (Eds.: J. Jortner, M. Bixon), John Wiley
& Sons, New York, 1999, pp. 431–496.
[25] a) R. A. Marcus, J. Chem. Phys. 1965, 43, 679; b) R. A. Marcus, N.
Sutin, Biochim. Biophys. Acta 1985, 811, 265; c) R. A. Marcus,
Angew. Chem. 1993, 105, 1161; Angew. Chem. Int. Ed. Engl. 1993,
32, 1111; d) J. L. Goodman, J. E Moser, B. Armitage, S. Farid, J.
Am. Chem. Soc. 1989, 111, 3489.
[11] a) Y. C. Lee, J. T. Hupp, Langmuir 2010, 26, 3760–3765; b) J.
Godoy, G. Vives, J. M. Tour, Org. Lett. 2010, 12, 1464–1467.
[12] A. Nierth, A. Y. Kobitski, G. U. Nienhaus, A. Jꢆschke, J. Am. Chem.
Soc. 2010, 132, 2646–2654.
[26] Gaussian 03, Revision B.04, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
AHCTUNGTREGeNNUN ry, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[13] J. Karolin, L. B.-A. Johansson, L. Strandberg, T. Ny, J. Am. Chem.
Soc. 1994, 116, 7801–7806.
[14] a) H. Imahori, H. Norieda, H. Yamada, Y. Nishimura, I. Yamazaki,
Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 100–110; b) S.
Hattori, K. Ohkubo, Y. Urano, H. Sunahara, T. Nagano, Y. Wada,
N. V. Tkachenko, H. Lemmetyinen, S. Fukuzumi, J. Phys. Chem. B
2005, 109, 15368–15375.
[15] a) M. T. Whited, P. I. Djurovich, S. T. Roberts, A. C. Durrell, C. W.
Schlenker, S. E. Bradforth, M. E. Thompson, J. Am. Chem. Soc.
2011, 133, 88–96; b) T. Lazarides, T. M. McCormick, K. C. Wilson,
S. Lee, D. W. McCamant, R. Eisenberg, J. Am. Chem. Soc. 2011,
133, 350–364; c) T. H. Allik, R. E. Hermes, G. Sathyamoorthi, J. H.
Boyer, Proc. SPIE 1994, 2115, 240–248.
[16] a) F. DꢂSouza, P. M. Smith, M. E. Zandler, A. L. McCarty, M. Itou,
Y. Araki, O. Ito, J. Am. Chem. Soc. 2004, 126, 7898–7907; b) C. A.
Wijesinghe, M. E. El-Khouly, J. D. Blakemore, M. E. Zandler, S. Fu-
kuzumi, F. DꢂSouza, Chem. Commun. 2010, 46, 3301–3303; c) C. A.
Wijesinghe, M. E. El-Khouly, N. K. Subbaiyan, M. Supur, M. E.
Zandler, K. Ohkubo, S. Fukuzumi, F. DꢂSouza, Chem. Eur. J. 2011,
17, 3147–3156; d) F. DꢂSouza, C. A. Wijesinghe, M. E. El-Khouly, J.
Hudson, M. Niemi, H. Lemmetyinen, N. V. Tkachenko, M. E. Zan-
dler, S. Fukuzumi, Phys. Chem. Chem. Phys. 2011, 13, 18168–18178.
[17] a) J.-Y. Liu, M. E. El-Khouly, S. Fukuzumi, D. K. P. Ng, Chem.
Asian J. 2011, 6, 174–179; b) J.-Y. Liu, M. E. El-Khouly, S. Fukuzu-
mi, D. K. P. Ng, Chem. Eur. J. 2011, 17, 1605–1613.
[27] For a general review on DFT applications of porphyrin-fullerene
systems see: M. E. Zandler, F. DꢂSouza, C. R. Chemie 2006, 9, 960–
981.
[28] D. L. Dexter, J. Chem. Phys. 1953, 21, 836–850.
[29] T. Fçrster, Discuss. Faraday Soc. 1959, 27, 7–17.
[30] The rate constants of the electron transfer (kET) for the dyad and
the triad in benzonitrile are calculated according to kET =1/tꢀkD, in
which kET is the rate constant of electron transfer, 1/t is the experi-
mentally observed decay time of the singlet excited state, and kD is
the rate constant of the decay of the singlet excited state other than
electron transfer.
[18] a) T. Lazarides, G. Charalambidis, A. Vuillamy, M. Reglier, E.
Klontzas, G. Froudakis, S. Kuhri, D. M. Guldi, A. G. Coutsolelos,
Inorg. Chem. 2011, 50, 8926–8936; b) F. Li, S. I. Yang, T. Ciringh, J.
Received: September 30, 2011
Revised: December 8, 2011
Published online: March 13, 2012
Chem. Eur. J. 2012, 18, 5239 – 5247
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5247