Control over photoinduced energy and electron transfer in supramolecular polyads of covalently linked azaBODIPY-bisporphyrin 'Molecular Clip' hosting fullerene

Article abstract of DOI:10.1021/ja209718g

A 'molecular clip' featuring a near-IR emitting fluorophore, BF ₂-chelated tetraarylazadipyrromethane (aza-BODIPY) covalently linked to two porphyrins (MP, M = 2H or Zn) has been newly synthesized to host a three-dimensional electron acceptor fullerene via a 'two-point' metal-ligand axial coordination. Efficient singlet-singlet excitation transfer from ¹ZnP* to aza-BODIPY was witnessed in the dyad and triad in nonpolar and less polar solvents, such as toluene and o-dichlorobenzene, however, in polar solvents, additional electron transfer occurred along with energy transfer. A supramolecular tetrad was formed by assembling bis-pyridine functionalized fullerene via a 'two-point' metal-ligand axial coordination, and the resulted complex was characterized by optical absorption and emission, computational, and electrochemical methods. Electron transfer from photoexcited zinc porphyrin to C₆₀ is witnessed in the supramolecular tetrad from the femtosecond transient absorption spectral studies. Further, the supramolecular polyads (triad or tetrad) were utilized to build photoelectrochemical cells to check their ability to convert light into electricity by fabricating FTO/SnO₂/polyad electrodes. The presence of azaBODIPY and fullerene entities of the tetrad improved the overall light energy conversion efficiency. An incident photon-to-current conversion efficiency of up to 17% has been achieved for the tetrad modified electrode.

Full text of DOI:10.1021/ja209718g

Article
pubs.acs.org/JACS
Control over Photoinduced Energy and Electron Transfer in
Supramolecular Polyads of Covalently linked azaBODIPY-
Bisporphyrin ‘Molecular Clip’ Hosting Fullerene
Francis D’Souza,*,†,‡ Anu N. Amin,‡ Mohamed E. El-Khouly,§ Navaneetha K. Subbaiyan,†
Melvin E. Zandler,‡ and Shunichi Fukuzumi*,§,∥
†Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States
‡Department of Chemistry, Wichita State University, 1845, Fairmount, Wichita, Kansas 67260-0051, United States
§Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871,
Japan
∥Department of Bioinspired Science, Ewha Woman’s University, Seoul, 120-750, Korea
S
* Supporting Information
ABSTRACT: A ‘molecular clip’ featuring a near-IR emitting
fluorophore, BF2-chelated tetraarylazadipyrromethane (aza-
BODIPY) covalently linked to two porphyrins (MP, M =
2H or Zn) has been newly synthesized to host a three-
dimensional electron acceptor fullerene via a ‘two-point’
metal−ligand axial coordination. Efficient singlet−singlet
1
excitation transfer from ZnP* to aza-BODIPY was witnessed
in the dyad and triad in nonpolar and less polar solvents, such
as toluene and o-dichlorobenzene, however, in polar solvents,
additional electron transfer occurred along with energy
transfer. A supramolecular tetrad was formed by assembling
bis-pyridine functionalized fullerene via a ‘two-point’ metal−ligand axial coordination, and the resulted complex was characterized
by optical absorption and emission, computational, and electrochemical methods. Electron transfer from photoexcited zinc
porphyrin to C60 is witnessed in the supramolecular tetrad from the femtosecond transient absorption spectral studies. Further,
the supramolecular polyads (triad or tetrad) were utilized to build photoelectrochemical cells to check their ability to convert
light into electricity by fabricating FTO/SnO2/polyad electrodes. The presence of azaBODIPY and fullerene entities of the tetrad
improved the overall light energy conversion efficiency. An incident photon-to-current conversion efficiency of up to 17% has
been achieved for the tetrad modified electrode.
experimentally.10−14 Supramolecular systems involving a variety
INTRODUCTION
■
of electron donors (such as porphyrin, phthalocyanine, etc.)
and fullerene have made considerable advances in the areas of
light-induced electron-transfer chemistry and light energy
harvesting.2−9,15−19 These breakthroughs are mainly due to
the small reorganization energy of fullerenes in electron-transfer
reactions11 that would lead to ultrarapid charge separation
together with slow charge recombination leading unprecedent-
edly long-lived charge-separated states with high quantum
efficiencies.12,20 Additionally, such studies have shed light on
fundamental understanding of electron-transfer aspects, such as
distance and orientation factors related to supramolecular
organization, electronic coupling elements, reorganization
energies, electron tunneling, etc.3−9,15−20
Biomimetic artificial photosynthetic systems aimed at solar
energy conversion and fuel production must integrate modular
molecular entities to collect and funnel light energy, generate
charge-separated species, and transport the generated chemical
energy to the catalytic sites for water oxidation and CO2
reduction, similar to the natural photosynthetic systems.1
Over the years, extensive efforts have been devoted to tackle
every facet of this complex problem by molecularly engineering
the supramolecular systems; however, components that are
both efficient and robust and integrated into one working
system remain a major challenge.2−7 Novel design of light-
funneling, photoconversion, and catalytic modules capable of
self-ordering and self-assembling into an integrated functional
unit will make an efficient artificial photosynthetic system
possible.8,9
Recently, a structural analog of a well-known fluorophore,
BF2-chelated dipyrromethane (BODIPY or BDP),21 namely,
The three-dimensional electron acceptors, fullerenes, have
stimulated interest due to their extraordinary electron acceptor
properties that was predicted theoretically and confirmed
Received: October 16, 2011
Published: November 24, 2011
© 2011 American Chemical Society
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Figure 1. Structure of (a) aza(BODIPY(OH)2, (b) bispyridine functionalized fullerene, C60Py2, (c) MP-azaBODIOPY (M = 2H or Zn) dyads, (d)
(MP)2-azaBODIOPY (M = 2H or Zn) triads, and (e) supramolecular tetrad, C60Py2:(ZnP)2-azaBODIPY investigated in the present study.
BF2-chelated tetraarylazadipyrromethanes (azaBODIPY or
ADP), with a nitrogen in the meso position of the macrocycle
has attracted increasing attention, because of their high
extinction coefficients (7−8 × 105 M−1 cm−1) and large
fluorescence quantum yields beyond 700 nm.22,23 Thus, they
have been used in applications ranging from sensors and
photodynamic therapy agents.24,25 Importantly, as needed for
electron-transfer applications, the azaBODIPYs are consider-
ably easier to reduce compared to BODIPY and exhibit
diagnostic spectral bands of the one-electron reduced products
in the near-IR region, sufficiently far from the triplet absorption
bands of the partnering donor/acceptor entities. Recently, we
exploited this property of aza-BODIPY by synthesizing
covalently linked donor−acceptor dyads and triads featuring
azaBODIPY and ferrocene and demonstrated efficient photo-
induced electron transfer (PET) from ferrocene to the singlet
excited azaBODIPY using transient absorption techniques.26
Further, by replacing the ferrocene entities by BODIPY entities,
PET from the singlet excited BODIPY to azaBODIPY was
successfully demonstrated.27 The one-electron reduction
potential of azaBODIPY is more positive by about 200 mV
than that of C60.28 Thus, a combination of aza-BODIPY and
C60 as electron acceptors together with a well-known
photosensitizer electron donor, zinc porphyrin, provides an
outstanding opportunity to construct brilliant photosynthetic
reaction center models, in which a wide range of visible light
can be absorbed.
in ‘molecular clip’ hosting fullerene, as shown in Figure 1e. The
azaBODIPY is functionalized to have one and two porphyrin
entities (Figure 1c and d). The two ZnP entities of the triad are
spatially arranged in a ‘molecular clip’ structure to accom-
modate the bispyridine functionalized fullerene (Figure 1b) via
a ‘two-point’ metal−ligand axial coordination to ultimately
generate supramolecular tetrad, as shown in Figure 1e. In the
absence of fullerene, photoinduced energy transfer from the
singlet excited MP to azaBODIPY in the dyads and triads is
expected to occur due to spectral matching and energetics.
Switching of electronic processes has been realized in C60py2:
(ZnP)2-azaBODIPY supramolecular tetrad. Upon introducing
fullerene spatially close to the electron donor, electron transfer
leading to the charge-separated species is expected. That is,
noncovalent assembly of building blocks to direct the formation
of the photo/electroactive supramolecules exhibiting features
that would otherwise be difficult to attain in a single molecule is
envisioned. It should be noted that the behavior of the
examined C60py2:(ZnP)2-azaBODIPY supramolecular tetrad is
quite different from the reported bis(phenylethynyl)-
anthracene-BODIPY-ZnP:C60py2 heptad, which is acting as a
classical antenna-reaction center complex.14b Systematic photo-
chemical studies have been performed to demonstrate the
photoinduced intramolecular events of the examined com-
pounds. Furthermore, the ability of the present supramolecular
polyads (triad and tetrad) in direct conversion of light into
electricity has been tested by constructing photoelectrochem-
ical cells on FTO/SnO2 modified surfaces. Improved photo-
We report herein synthesis and photodynamics of supra-
molecular polyads of covalently linked azaBODIPY-bisporphyr-
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current efficiencies in the case of tetra modified electrode as
compared to the control triads are observed.
azaBODIPY dyad and the (ZnP)2-azaBODIOPY triad exhibited
bands corresponding to both the entities.
However, for the triad, the azaBODIOPY intensity was
roughly half of that observed for dyad owing to the presence of
two ZnP entities. It is interesting to note that the band position
of the azaBODIPY depended upon the number of free hydroxyl
groups on the macrocycle, that is, each −OH group caused
about 10 nm bathochromic shifts. The spectral features of the
free-base porphyrin derivatives were also similar (Figure S2a,
Supporting Information). Changing the solvent to more polar
benzonitrile also revealed such trends, however, with slightly
larger bathochromic shifts (Figure S2b and S2c, Supporting
Information).
Figure 2b shows the fluorescence emission spectra of the
investigated compounds in nonpolar toluene. By the Q-band
excitation of the control ZnP, two emission bands at 560 and
647 nm were observed. At this excitation wavelength,
azaBODIPY(OH)2 revealed a weak emission band at 714 nm
due to direct excitation of the fluorophore. Interestingly, for the
dyads and triad, the ZnP emission was found to be totally
quenched with the appearance of new emission bands
corresponding to azaBODIPY entity, suggestive of the
occurrence of excitation transfer in the dyad and triad.29 In a
control experiment, the emission spectra of equimolar mixture
of ZnP and azaBODIPY under similar experimental conditions
were recorded. Such spectra revealed no quenching of ZnP
(<5%) or enhanced emission of azaBODIPY, suggesting the
observed energy transfer in the dyad and triad to be an
intramolecular event. The excitation transfer for (ZnP)2-
azaBODIPY triad was found to be slightly higher than that of
ZnP-azaBODIPY dyad perhaps due to the presence of two ZnP
entities of the triad. In a slightly polar solvent, o-
dichlorobenzene, a similar spectral trend was observed although
the intensity of the energy-transfer product, azaBODIPY,
emission was slightly lower (vide infra).
RESULTS AND DISCUSSION
■
Studies of MP-azaBODIPY Dyads and (MP)2-azaBODI-
PY Triads. Syntheses of MP-azaBODIPY Dyads and (MP)2-
azaBODIPY Triads. The syntheses of the porphyrin-azaBODI-
PY systems involved a multistep approach, as detailed 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]amine was synthesized according
to our earlier reported procedure.26 Next, this compound was
treated with 5-(4′-carboxyphenyl)-10,15,20-triphenylporphyrin
in the presence of EDCl to introduce one or two free-base
porphyrin entities on the azaBODIPY periphery. The free-base
porphyrins were subsequently metalated to obtain the
respective ZnP-azaBODIPY dyad and the (ZnP)2-azaBODIPY
triad. The structural integrity of the newly synthesized
1
compounds was established from H NMR, mass (see Figure
S1, Supporting Information) and other optical techniques.
Steady-State Absorption and Fluorescence Measure-
ments. Figure 2a shows the normalized (to the Soret band
In order to confirm the occurrence of energy transfer in
nonpolar solvents, the excitation spectra of the dyad and triad
were recorded by holding the excitation monochromator to the
emission maxima of azaBODIPY, and the results are shown in
Figure S3 (Supporting Information). These spectra revealed
peaks not only of azaBODIPY but also that of ZnP for the both
the dyad and triad. Such features (ZnP peaks) were not
observed for the spectrum recorded for the equimolar mixture
of ZnP and azaBODIPY.
On the contrary, changing the solvent to polar benzonitrile
revealed quenching of the MP emission with no emission
corresponding to azaBODIPY, as shown in Figure 2c. Control
ZnP emission in benzonitrile was located at 610 and 660 nm,
while azaBODIPY(OH)2 at this excitation wavelength revealed
a band at 735 nm which is nearly 20 nm red-shifted from that
observed in toluene. In the case of ZnP-azaBODIPY dyad and
(ZnP)2-azaBODIPY triad, the ZnP emission was quenched
over 97 and 90%, respectively, with no emission of azaBODIPY
indicating additional photochemical events in polar benzonitrile
solvent.30
The dyad and triad derived from the free-base porphyrin,
H2P-azaBODIPY, and the (H2P)2-azaBODIPY also revealed
similar spectral features in spite of the red-shifted emission
bands of H2P compared to ZnP (Figure S4, Supporting
Information). Thus, energy transfer in toluene and donor H2P
emission quenching perhaps due to additional electron transfer
in benzonitrile were envisioned.
Figure 2. (a) Normalized (to the Soret band) absorption spectra in
toluene, (b) emission spectra in toluene (λex = 549 nm), and (c)
emission spectra in benzonitrile (λex = 560 nm) of ZnP (black),
azaBODIPY(OH)2 (green), ZnP-azaBODIPY dyad (blue), and the
(ZnP)2-azaBODIPY triad (red) compounds.
of porphyrin) optical absorption spectra of the ZnP-
azaBODIPY dyad and the (ZnP)2-azaBODIPY triad along
with the control compounds, viz., zinc tetraphenylporphyrin
(ZnP) and azaBODIPY(OH)2 in toluene. ZnP exhibited a
strong Soret at 422 nm and visible bands at 550 and 587 nm.
The azaBODIPY(OH)2 control (Figure 1a) had a strong
absorption at 685 nm and a weak band at 316 nm. The ZnP-
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Computational Studies of the Dyad and Triad. Figure 3a
and b shows the B3LYP/3-21G(*)31,32 optimized structures of
Figure 4. Differential pulse voltammograms of (i) azaBODIPY(OH)2,
(ii) azaBODIPY, (iii) ZnP-azaBODIPY, (iv) (ZnP)2-azaBODIPY, and
(v) (H2P)2-azaBODIPY in benzonitrile and 0.10 M of (n-Bu4N)ClO4.
Scan rate = 5 mV/s, pulse width = 0.25 s, and pulse height =0.025 V.
The asterisk in the top panel shows oxidation process of ferrocene
used as an internal standard.
Table 1. Electrochemical Redox Potentials (V vs Fc/Fc+),
Energy Levels of the Charge-Separated States (ΔGCR) and
Free-Energy Changes for Charge Separation (ΔGCS) for the
MP- azaBODIPY (M = 2H or Zn) Dyads and Triads in
Benzonitrile
MP0/•+
V
/
azaBODIPY0/•−
V
/
−ΔGCR
/
−ΔGCS
/
a
b
compound
azaBODIPY
eV
eV
Figure 3. B3LYP/3-21G(*) optimized structure of (a) ZnP-
azaBODIPY dyad and (b) the (ZnP)2-azaBODIPY triad. In the top
and bottom panels, each figure shows the frontier LUMO and HOMO
of the respective molecule.
−
−
0.33
0.33
0.60
0.60
−0.79
−0.90
−0.84
−0.77
−0.84
−0.76
−
−
1.17
1.10
1.44
1.36
−
−
0.88
0.95
0.46
0.54
azaBODIPY(OH)2
ZnP-azaBODIPY
(ZnP)2-azaBODIPY
H2P-azaBODIPY
(H2P)2-azaBODIPY
the ZnP-azaBODIPY dyad and (ZnP)2-azaBODIPY triad,
respectively. The structures are fully optimized on a Born−
Oppenheimer potential energy surface. In the optimized
structure of ZnP-azaBODIPY, both the azaBODIPY and ZnP
macrocycles are found to be flat with a slight tilt of the
peripheral aromatic rings of azaBODIPY. The boron−Zn
distance was found to be 16.0 Å, while the distance between the
closest carbon of BDP to the closet carbon of ADP (edge-to-
edge distance) was ∼12.08 Å. The two macrocycles were
spatially disposed with a dihedral angle of 77°. Interestingly, for
the (ZnP)2-azaBODIPY triad, the two coplanarly positioned
ZnP entities linked to azaBODIPY formed a molecular clip-
type structure with a Zn−B−Zn angle of 60°. The two ZnP
macrocycles are separated by a distance of 15.8 Å (Zn−Zn
distance), while the B−Zn and edge-to-edge distances were
found to be nearly the same for the two ZnP-azaBODIPY
segments of the triad. For the dyad and triad, the highest
occupied molecular orbital (HOMO) was on the ZnP entity,
while the lowest unoccupied molecular orbital (LUMO) was on
the azaBODIPY entity. The location of the HOMO and
LUMO suggests the ZnP entity to be an electron donor and the
azaBODIPY entity to be an electron acceptor.
a
ΔGCR = e(Eox − Ered). The electrostatic term was neglected due to
b
the use of a polar solvent (PhCN). −ΔGCS = ΔE0−0 − ΔGCR, where
ΔE0−0 is the energy of the lowest excited states of porphyrins being
2.05 eV for ZnP and 1.90 eV for H2P.
and oxidation processes located at −0.79 and 0.80 V vs Fc/Fc+,
respectively. The first reduction was found to be cathodically
shifted by over 200 mV compared with the well-known electron
acceptor, C60.10 Introducing the hydroxyphenyl groups on
azaBODIPY caused the reduction to be harder by about 100
mV, while the oxidation process was irreversible (from cyclic
voltammetric experiments), however, with cathodically shifted
potentials by nearly 380 mV.
For ZnP-azaBODIPY dyad and (ZnP)2-azaBODIPY triad,
the ZnP oxidation occurred at around 0.33 V vs Fc/Fc+, while
the first reduction of the porphyrin entity occurred at −1.54 V
vs Fc/Fc+. The azaBODIPY reduction in the dyad was located
at −0.84, while for the triad, it was located at −0.77 V vs Fc/
Fc+. In the case of free-base porphyrin derivatives, the first
oxidation corresponding to H2P was located at 0.60 V, while
the first reduction was located at −1.50 V vs Fc/Fc+. The peak
potentials of azaBODIPY tracked that of the ZnP-azaBODIPY
analogs. These results collectively suggest both free-base and
zinc(II) porphyrins being electron rich and azaBODIPY being
electron deficient in these molecular systems. Since there were
no appreciable changes in the oxidation potentials of the
porphyrin entities when compared to the pristine porphyrin
Electrochemistry and Energy Levels of the Dyad and
Triad. In order to establish the energy levels, electrochemical
studies using differential pulse voltammetry (DPV) were
performed. Figure 4 shows DPVs of the dyads and triads
along with the control compounds in benzonitrile containing
0.10 M of (n-Bu4N)ClO4, while the redox data are summarized
in Table 1. AzaBODIPY revealed both one electron reduction
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1
derivatives in the dyads and triads, absence of inter- or
intramolecular interactions was envisioned.
diminished ZnP* emission at 458 nm with the concomitant
1
increase of the azaBODIPY* emission at 682 and 820 nm,
The energies of the charge-separated states were calculated
using the redox, geometric, and optical data according to
Weller’s approach,33 and the calculated values are given in
Table 1. By comparing these energy levels of the charge-
separated states with the energy levels of the excited states, the
driving forces of charge separation (ΔGCS) were also evaluated.
The generation of MP•+-azaBODIPY•− (M = 2H, or Zn) for
the dyads and triads was found to be exergonic via the singlet
excited state of MP in benzonitrile.
providing direct evidence of energy transfer in the polar media
(Figure 6). From the decay profile of the ZnP* and the rise
1
Femtosecond Transient Absorption Spectral Studies of
(MP)n-azaBODIPY (M = 2H or Zn, n = 1, 2). Femtosecond
transient absorption spectroscopy was used to obtain further
insight into the excited state events of (ZnP)n-azaBODIPY and
(H2P)n-azaBODIPY (n = 1, 2) dyads and triads to corroborate
the proposed energy- and electron-transfer processes. Toward
this end, (ZnP)n-azaBODIPY and (H2P)n-azaBODIPY were
probed with 426 nm excitation to selectively excite the
porphyrin fluorophore. The femtosecond transient absorption
spectra of the ZnP reference in toluene (Figure S6, Supporting
Information) revealed the instantaneous formation of the
1ZnP* with a transient absorption maxima at 458 nm, which
Figure 6. (Left) Differential absorption spectra obtained upon
femtosecond flash photolysis (426 nm) of ZnP-azaBODIPY in
benzonitrile at the indicated time intervals. (Right) The decay profile
1
of the formed azaBODIPY* at 820 nm.
profile of the formed 1azaBODIPY*, the 1ZnP* to azaBODIPY
energy transfer in polar benzonitrile was determined to be 6.4
× 1011 s−1. Similar spectral observations were also made in the
case (ZnP)2-azaBODIPY triad wherein the kENT was found to
be 9.6 × 1011 s−1 (Figure S8a, Supporting Information), slightly
faster than that observed for the dyad.
3
decayed slowly (7.8 × 108 s−1) to populate the ZnP* with a
maximum at 480 nm.
The spectral features in the visible region, which were seen
immediately (i.e., 1 ps) upon excitation of ZnP-azaBODIPY in
toluene (Figure 5), were almost the same as those recorded for
Interestingly, in polar benzonitrile, the energy-transfer
1
product of the dyad and triad, azaBODIPY* revealed faster
decay than that observed for pristine azaBODIPY indicating
1
subsequent PET from ZnP to the azaBODIPY* to produce
(ZnP)n•+-azaBODIPY•− charge separation product, taking into
1
account that the energy transfer from azaBODIPY* to ZnP is
not feasible. Based on the decay rates of the control
1azaBODIPY* and the examined compounds, the kCS measured
for the dyad and triad was found to be 2.9 × 1010 and 4.5 ×
1010 s−1, respectively. Similar observations were also made for
the investigated H2P-azaBODIPY dyad (Figure 7) and (H2P)2-
Figure 5. (Left) Differential absorption spectra obtained upon
femtosecond flash photolysis (426 nm) of ZnP-azaBODIPY in toluene
at the indicated time intervals. (Right) The decay profile of the formed
singlet azaBODIPY at 820 nm.
the ZnP singlet excited state (1ZnP*). With time, the ZnP
singlet emission peak at 458 nm diminished in intensity with
concomitant increase of the azaBODIPY emission at 682 and
820 nm, providing direct evidence of excitation transfer
1
1
between the ZnP* and azaBODIPY. The kinetics of ZnP*
to azaBODIPY energy transfer was determined by exponential
fitting of the rise profile of the formed 1azaBODIPY* at 820 nm
(kENT = 6.6 × 1011 s−1). Further, the decay rate of the formed
1azaBODIPY* was determined as 4.6 × 108 s−1, which is close
to the decay of pristine 1azaBODIPY* indicating lack of
quenching by the attached ZnP in toluene. Similar absorption
spectra were observed in the case of (ZnP)2-azaBODIPY triad
in deaerated toluene (Figure S7, Supporting Information). The
kENT was found to be 1.0 × 1012 s−1, which was faster than that
of ZnP-azaBODIPY dyad.
Figure 7. (Left) Differential absorption spectra obtained upon
femtosecond flash photolysis (426 nm) of H2P-azaBODIPY in
benzonitrile at the indicated time intervals. (Right) The decay profile
1
of the formed azaBODIPY*at 820 nm.
azaBODIPY triad (Figure S8b, Supporting Information), where
energy transfer from 1H2P* to azaBODIPY (kENT) followed by
1
electron transfer from H2P to azaBODIPY* (kCS) was clearly
observed. The kENT and kCS measured for these processes were
found to be 2.3 × 1011 and 6.0 × 109 s−1 for the H2P-
azaBODIPY dyad and 4.0 × 1011 and 1.6 × 1010 s−1 for the
(H2P)2-azaBODIPY triad, respectively.
By changing the solvent from toluene to benzonitrile, the
transient absorption spectra of the dyad and triad exhibited
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The kinetic data for the photochemical processes of the
investigated dyads and triads are summarized in Table 2. In
Table 2. Rates of Singlet−Singlet Energy Transfer from
1MP* to azaBODIPY (in Toluene and Benzonitrile) and
1
Charge Separation from MP to azaBODIPY* (in
Benzonitrile) of the Porphyrin-azaBODIPY Dyads and
Triads
a
b
compound
solvent
toluene
kENT/s−1
kCS/s−1
ZnP-azaBODIPY
6.6 × 1011
6.4 × 1011
1.0 × 1012
9.6 × 1011
−
−
benzonitrile
toluene
2.9 × 1010
−
(ZnP)2-azaBODIPY
H2P-azaBODIPY
benzonitrile
toluene
4.5 × 1010
−
benzonitrile
toluene
2.3 × 1011
−
6.0 × 109
−
(H2P)2-azaBODIPY
benzonitrile
4.0 × 1010
1.6 × 1010
a
b
1
Energy transfer from MP* to azaBODIPY. Electron transfer from
1
MP to azaBODIPY*.
both toluene and benzonitrile, the energy-transfer process
seems to be efficient for the triads due to the presence of two
donor porphyrin entities. Between ZnP and H2P donors, kENT
values are higher for the ZnP derivatives. The kCS values also
seem to follow this trend, that is, they are higher for ZnP as
compared to H2P derived dyads and triads and exhibit
improved rates with increase in their numbers.
Supramolecular C60py2:(ZnP)2-azaBODIPY Tetrad: For-
mation and Photochemical Studies. The molecular clip-
like structure of (ZnP)2-azaBODIPY prompted us to further
use this molecule to build novel supramolecular architecture
using fullerene functionalized with two pyridine entities34
suitable for accommodating both the ZnP entities of (ZnP)2-
azaBODIPY, as shown in Figure 1d by a ‘two-point’ axial
coordination approach. Figure 8a shows the optical absorption
changes during increasing addition of C60py2 to a solution of
(ZnP)2-azaBODIPY in o-dichlorobenzene, a noncoordinating
solvent. During the titration, the ZnP bands located at 425 and
550 nm experienced diminished intensity with red shifts of the
bands to 432 and 567 nm, respectively, characteristic of axial
coordination.35 During the titration, the azaBODIPY band
located at 667 nm revealed no spectral shifts, indicating that the
added C60py2 binding to the ZnP entities and not azaBODIPY
unit. A Job’s plot mole ratio method was constructed to
evaluate the molecular stoichiometry. As shown in Figure 8b,
such plots (decrease or increase of Soret band intensity)
revealed a break at 1:1 ratio to the added fullerene to the triad,
establishing the C60py2:(ZnP)2-azaBODIPY molecular stoichi-
ometry. The binding constant was evaluated by constructing a
Benesi−Hildebrand plot,36 as shown in Figure 8c. Such plot
yielded a binding constant K of 1.85 × 105 M−1, which is nearly
two orders of magnitude higher than that reported for C60py
binding to ZnP35 as a result of two-point binding of the target
receptor.
Figure 8. UV−vis spectral changes observed on increasing addition of
C60py2 (0.05 equiv each addition) to (ZnP)2-azaBODIPY (10 μM) in
o-dichlorobenzene. (b) Plot of Jobs method of continuous variation for
C60py2:(ZnP)2-azaBODIPY formation monitored at 425 (black) and
432 (red) nm. (c) Benesi−Hildebrand plot where A is the absorbance
of the complex formed on increasing addition of C60py2 and A0 is the
absorbance of the (ZnP)2-azaBODIPY in the absence of added C60py2.
The structure of the supramolecular tetrad C60py2:(ZnP)2-
azaBODIPY was deduced from B3LYP/3-21G(*) studies. As
shown in Figure 9, the molecular clip-like structure of the
(ZnP)2-azaBODIPY accommodated the bis-pyridine function-
alized fullerene via metal-pyridine axial coordination involving
both the ZnP entities. In the optimized structure, the Zn−Zn
distance of the two ZnP entities was 12.4 Å, while that of the
Zn1−B and Zn2−B distances between the ZnP and azaBODIPY
Figure 9. B3LYP/3-21G(*) optimized structure of C60py2:(ZnP)2-
azaBODIPY and the HOMO, HOMO-1, LUMO, and LUMO+4
frontier orbitals.
were 15.7 and 15.4 Å, respectively. Distances between Zn1−C60
and Zn2−C60 (center of fullerene) were 5.2 and 5.4 Å,
respectively. The B−C60 distance was about 16.7 Å. The close
proximity of Zn−C60 compared to Zn−B was evident from
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these studies. The frontier orbitals were also generated to
visualize the HOMO and LUMO locations. Both HOMO and
HOMO-1 were found to be fully localized on the ZnP entities,
while the LUMO was on the fullerene and LUMO+4 was on
the azaBODIPY entity of the tetrad.
In order to probe the redox properties of the supramolecular
tetrad, C60py2:(ZnP)2-azaBODIPY, cyclic voltammetric studies
were performed in o-dichlorobenzene containing 0.10 M of (n-
Bu4N)ClO4 as a supporting electrolyte, and the results are
shown in Figure 10. In contrast to the computational
As discussed in the previous section, fluorescence studies of
(ZnP)2-azaBODIPY in both toluene and o-dichlorobenzene
revealed bands at 598 and 650 nm corresponding to ZnP and at
697 nm corresponding to azaBODIPY as a result of energy
transfer. Increasing addition of C60(py)2 resulted in a decrease
in these emission intensities. This suggests occurrence electron
1
transfer from ZnP* to either azaBODIPY and/or C60 (Figure
S9, Supporting Information).
In order to unravel the electron-transfer route and obtain the
kinetic information, further transient spectral studies at
femtosecond time scale were performed, as described below.
The C60(py)2:(ZnP)2-azaBODIPY was probed with 426 nm
excitation to selectively excite the porphyrin fluorophore
(Figure 11). The transient absorption spectrum measured at
Figure 10. Cyclic voltammograms of (i) (ZnP)2-azaBODIPY and (ii)
C60py2:(ZnP)2-azaBODIPY obtained by equimolar addition of C60py2
and (ZnP)2-azaBODIPY in o-dichlorobenzene containing 0.10 M of
(n-Bu4N)ClO4. Scan rate = 100 mV/s. The voltammogram
abbreviated as Fc is the one-electron redox wave for the ferrocene
internal standard.
Figure 11. Differential absorption spectra obtained upon femtosecond
flash photolysis (420 nm) of the C60py2:(ZnP)2-azaBODIPY supra-
molecular tetrad at the indicated time intervals in toluene. Inset: Rise
and decay of the C60 radical anion monitored to evaluate the charge-
separation and charge-recombination kinetics.
expectations, the first cathodic process of the tetrad
corresponding to the reduction of azaBODIPY entity was
located at −0.86 V, the second and third processes
corresponding to the reduction of C60 entity were located at
−1.12 and −1.60 V vs Fc/Fc+, and the fourth reduction
potential at −1.88 V corresponds to the reduction of ZnP
entity. The discrepancy from the computational results is
ascribed to the larger solvation of the radical anion of
azaBODIPY as compared to that of the radical anion of C60
entity. The anodic waves located at 0.30 V correspond to
oxidation of the ZnP entity. The one-electron reduction of
azaBODIPY is 250 mV easier as compared to fulleropyrrolidine.
1 ps after femtosecond laser excitation exhibited the transient
absorption of the ZnP singlet excited state (1ZnP*). With time,
1ZnP* emission peak at 458 nm is diminished in intensity with
a concomitant increase in absorbance at 1000 nm due to the
•−
formation of C60 and also absorbance at 620 nm due to
ZnP•+ formation, providing direct evidence of electron transfer
from the photoexcited ZnP unit (1ZnP*) to the ground state of
1
C60 without any indication of energy transfer from ZnP* to
1
1
azaBODIPY. The kinetics of electron transfer from ZnP* to
The energetics for PET from ZnP* to azaBODIPY and
C
60 was analyzed by exponential fitting of the rise profile of the
1ZnP* to C60py2 was evaluated using the redox and the earlier
discussed spectral data.33 Such calculations revealed that the
charge separation is possible via the both routes (ΔGCS = −0.89
eV and −0.63 eV, respectively), while charge recombination is
more exergonic involving fullerene compared to azaBODIPY
(ΔGCR = −1.16 eV involving azaBODIPY and −1.42 eV
involving C60py2). These results in conjunction with Marcus
theory of electron transfer37 suggest electron transfer involving
C60 and not azaBODIPY. Because an alternative pathway, that
is PET from 1azaBODIPY* to C60py2, reaction is slightly
endothermic; such a process is not likely to occur under the
present experimental conditions.
C60•− (at 1000 nm) or ZnP•+ (at 620 nm) (kCS = 6.2 × 1010 s−1
and kCR = 1.8 × 108 s−1). Based on the kCR value, the lifetime of
the C60(py)2•−:(ZnP)2•+-azaBODIPY was determined to be 5.5
ns, which is significantly longer as compared to the reported
porphyrin-fullerene supramolecular dyads.6
Photoelectrochemical Studies. The occurrence of electron
transfer in the present supramolecular polyads prompted us to
perform photoelectrochemical studies38 to verify their ability to
convert light into electricity. For this purpose, the donor−
acceptor entities were electrophoretically deposited on a
nanocrystalline SnO2-modified FTO surface.39 Figure 12
shows the incident photon-to-current conversion efficiency
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the other two electrodes lacking either azaBODIPY or
fullerene. It is important to note that absorption characteristics
of all three entities, viz., ZnP, C60, and azaBODIPY, were clearly
seen in the IPCE curve40 of the tetrad signifying the importance
of the multimodular supramolecular assembly and the
involvement of each of the molecular component in higher
photocurrent generation. As shown in Figure 12c, photo-
switching experiments resulted in consistent currents with a
high stability of the deposited material on the electrode surface
under the present experimental conditions.
SUMMARY
■
Control over photoinduced energy and electron transfer in the
newly designed and synthesized supramolecular polyads is
successfully demonstrated. The energy level diagram shown in
Figure 13a, where excitation of MP in the covalently linked
MP-azaBODIPY and (MP)2-azaBODIPY systems, promotes
ultrafast efficient energy transfer to generate azaBODIPY* in
nonpolar solvent. The azaBODIPY* thus formed deactivates
1
Figure 12. (a) Pictures of FTO/SnO2 modified electrodes, (b)
IPCE(%) curves, and (c) photocurrent on−off switching of (i) C60py2:
(ZnP)2-azaBODIPY tetrad, (ii) (ZnP)2-azaBODIPY triad, and (iii)
C60py2:(ZnP)2 donor−acceptor systems electrophoretically deposited
on the surface in acetonitrile solution containing (0.25 M LiI, 0.25 M
1
to the ground state directly. Changing the solvent to a more
polar benzonitrile changes the deactivation pathway of the
energy-transfer product, 1azaBODIPY* to electron transfer
involving MP to generate the charge separation product,
(MP)n•+-azaBODIPY•−. The measured rate constants of kCS
and kCR may be located near the top of Marcus parabola.37
Supramolecular introduction of a second electron acceptor,
fullerene, into the molecular assembly promotes electron
transfer from the singlet excited MP to the spatially closely
located fullerene to generate C60(py)2•−:(ZnP)2•+-azaBODIPY
charge-separated species in nonpolar solvent, as shown in
Figure 13b. That is, control over electron transfer instead of
energy transfer has been achieved. Charge stabilization to some
extent has also been observed, leading to application of such
supramolecular assemblies for direct light energy conversion.
Consequently, photoelectrochemical cells were constructed by
electrophoretic deposition of different systems on FTO/SnO2
modified surface. As predicted, higher IPCE(%) values (nearly
17% at the peak maxima) for C60py2:(ZnP)2-azaBODIPY
modified electrode compared to (ZnP)2-azaBODIPY modified
electrode (∼11%) have been observed. The present study
successfully demonstrates control over energy and electron
−
butyl methyl imidazolium iodide (BMII), 0.05 M I2) for I−/I3 redox
mediator.
(IPCE%) curves in o-dichlorobenzene along with a picture of
the modified electrodes and on−off light switching of the
photocurrent. The electrophoretic deposition procedure
resulted in good coverage of the donor−acceptor entities on
the electrode surface (Figure 12a) with optical density of the
deposited material close to 1.5. Three modified electrodes, viz.,
C60py2:(ZnP)2, (ZnP)2-azaBODIPY and C60py2:(ZnP)2-azaBO-
DIPY, were used. In the first electrode, C60py2 was complexed
to a covalently linked porphyrin dimer lacking the azaBODIPY
entity, (ZnP)234, to visualize the importance of azaBODIPY; in
the second one, (ZnP)2-azaBODIPY lacking C60py2 was
employed to visualize the importance of fullerene; and in the
third one, the supramolecular tetrad, C60py2:(ZnP)2- azaBO-
DIPY, having both fullerene and azaBODIPY were employed.
As shown in Figure 12b, the recorded IPCE curves revealed
higher currents for the tetrad modified electrode compared to
Figure 13. Energy level diagrams showing photochemical events of (a) (MP)n-azaBODIPY (M = Zn or 2H, n = 1, 2) as a function of solvent polarity
and (b) C60py2:(ZnP)2-azaBODIPY tetrad; both under the conditions of selective excitation of MP.
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electrodes were annealed in an oven for 1 h in air at 673 K. The
transfer in molecular polyads and their direct usage in light-
energy conversion application.
estimated thickness of the electrode was around 5-10 μM.42
ASSOCIATED CONTENT
■
EXPERIMENTAL SECTION
■
S
* Supporting Information
Chemicals. All the reagents were from Aldrich Chemicals
(Milwaukee, WI), while the bulk solvents utilized in the syntheses
were from Fischer Chemicals. Tetra-n-butylammonium perchlorate,
(n-Bu4N)ClO4, used in electrochemical studies was from Fluka
Chemicals. The initial synthesis of ADP macrocycle was performed
according to the procedure reported by O’Shea and co-workers22 with
necessary modifications.26
The synthetic details of BF2 chelated [5-(4-hydroxyphenyl)-3-
phenyl-1H-pyrrol-2-yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-2-
ylidene]amine,26 2-(4′-pyridyl)-5(methylene-4′-pyridyl)-3,4-fulleropyr-
rolidine (C60(Py)2),34 5-(4′-carboxyphenyl)-10,15,20-triphenylpor-
phyrin, and covalently linked zinc porphyrin dimer34 used in
photoelectrochemical studies are given elsewhere.
Additional experimental details, MALDI-mass spectra of the
investigated compounds, excitation spectrum of the dyad and
triad, transient absorption spectrum of dyad and triad in
toluene and benzonitrile, and complete details of ref 31. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Francis.DSouza@UNT.edu; fukuzumi@chem.eng.osaka-u.ac.jp
Synthesis of H2P-azaBODIPY and (H2P)2-azaBODIPY. 5-(4′-
Carboxyphenyl)-10,15,20-triphenylporphyrin (50 mg, 7.59 × 10−5
mol) was dissolved in 20 (cm3) of DMF, to which EDCI (14.54
mg, 7.59 × 10−5 mol) was added at 0 °C under N2, followed by the
addition of BF2 chelated [5-(4-hydroxyphenyl)-3-phenyl-1H-pyrrol-2-
yl]-[5-(4-hydroxyphenyl)-3-phenylpyrrol-2-ylidene]amine (13.39 mg,
2.53 × 10−5 mol), after which the mixture was stirred for 24 h. Then
the solvent was removed under reduced pressure. The residue was
dissolved in CH2Cl2, and the mixture was washed with water. Then the
organic layer was separated and dried over Na2SO4, and the solvent
was evaporated. The residue was purified by column chromatography
on silica gel with CHCl3:hexanes (1:1) to give (H2P)2-azaBODIPY:
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(grant no. 1110942 to FD), a Grant-in-Aid (nos. 20108010 and
21750146), the Global COE (center of excellence) program
“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 WCU project (R31-2008-000-10010-0) from
Korea.
1
Yield 5 mg (11%); H NMR (400 MHz, CDCl3) δ = 8.92−8.8 (m,
REFERENCES
■
16H), 8.63 (d, J = 8.39 Hz, 4H), 8.4 (d, J = 8.41 Hz, 4H), 8.29 (d, J =
8.84 Hz, 4H), 8.25−8.17 (m, 12H), 8.14 (d, J = 6.63 Hz, 4H), 7.8−
7.69 (m, 18H), 7.6 (d, J = 8.82 Hz, 4H), 7.56−7.45 (m, 6H), 7.16 (s,
2H), −2.8 (s, 4H) ppm. MALDI-mass: calculated, 1810.8; found,
1812.3. Further elution with CHCl3 gave compound H2P-azaBODIPY:
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Yield 8 mg (27%); H NMR (400 MHz, CDCl3) δ = 8.91−8.81 (m,
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(m, 8H), 8.14−8.03 (m, 6H), 7.84−7.72 (m, 9H), 7.58−7.44 (m, 8H),
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Synthesis of ZnP-azaBODIPY. To a H2P-azaBODIPY (10 mg)
dissolved in CHCl3 (10 cm3), a saturated solution of zinc acetate in
methanol (0.1 mL) was added, and the resulting mixture was stirred
for 12 h. The mixture was then washed with water, organic layer was
separated, dried over anhydrous Na2SO4, and rotary evaporated. The
residue was purified over silica gel column using CHCl3 to give
compound ZnP-azaBODIPY: Yield 9 mg (85%); 1H NMR (400 MHz,
CDCl3): δ = 9.01−8.93 (m, 8H), 8.63 (d, J = 8.23 Hz, 2H), 8.41 (d, J
= 8.1 Hz, 2H), 8.26−8.19 (m, 8H), 8.14−8.07 (m, 6H), 7.82−7.72 (m,
9H), 7.55−7.43 (m, 8H), 7.12 (s, 1H), 7.08 (s, 1H), 6.99 (d, J = 8.55
Hz, 2H) ppm. MALDI-mass: calculated, 1236.5; found, 1236.21.
Synthesis of (ZnP)2-azaBODIPY. To a (H2P)2-azaBODIPY (10
mg) dissolved in CHCl3 (10 cm3), saturated solution of zinc acetate in
methanol (0.1 mL) was added, and the resulting mixture was stirred
for 12 h. The mixture was then washed with water, organic layer was
separated, dried over anhydrous Na2SO4, and rotary evaporated. The
residue was purified over silica gel column using CHCl3:hexanes (4:6)
1
to give (ZnP)2-azaBODIPY: Yield 9 mg (84%); H NMR (400 MHz,
CDCl3): δ = 8.99−8.8 (m, 16H), 8.62 (d, J = 8.39 Hz, 4H), 8.39 (d, J
= 8.41 Hz, 4H), 8.26−8.18 (m, 16H), 8.15 (d, J = 6.63 Hz, 4H), 7.79−
7.67 (m, 18H), 7.6 (d, J = 8.82 Hz, 4H), 7.57−7.43 (m, 6H), 7.16 (s,
1H) ppm. MALDI-mass: calculated, 1937.5; found, 1943.09 (br).
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for stability. About 0.5 mL of colloidal solution was placed on an
optically transparent electrode, fluorine-doped indium tin oxide
(FTO) (Pilkington TEC-8, 6−9 ∧/square), and dried in air. The
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dx.doi.org/10.1021/ja209718g | J. Am. Chem.Soc. 2012, 134, 654−664

Journal of the American Chemical Society
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