A Supramolecular tetrad featuring covalently linked ferrocene-Zinc Porphyrin-BODIPY coordinated to fullerene: A charge stabilizing, photosynthetic antenna-reaction center mimic

Article abstract of DOI:10.1002/chem.201404671

A novel photosynthetic-antenna-reaction-center model compound, comprised of BF2-chelated dipyrromethene (BODIPY) as an energy-harvesting antenna, zinc porphyrin (ZnP) as the primary electron donor, ferrocene (Fc) as a hole-shifting agent, and phenylimidazole-functionalized fulleropyrrolidine (C₆₀Im) as an electron acceptor, has been synthesized and characterized. Optical absorption and emission, computational structure optimization, and cyclic voltammetry studies were systematically performed to establish the role of each entity in the multistep photochemical reactions. The energy-level diagram established from optical and redox data helped identifying different photochemical events. Selective excitation of BODIPY resulted in efficient singlet energy transfer to the ZnP entity. Ultrafast electron transfer from the 1ZnP (formed either as a result of singlet- singlet energy transfer or direct excitation) or 1C₆₀ of the coordinated fullerene resulting into the formation of the Fc-(C₆₀CIm:ZnPC+)-BODIPY radical ion pair was witnessed by femtosecond transient absorption studies. Subsequent hole migration to the ferrocene entity resulted in the Fc+-(C₆₀C+Im:ZnP)-BODIPY radical ion pair that persisted for 7-15 ms, depending upon the solvent conditions and contributions from the triplet excited states of ZnP and ImC₆₀, as revealed by the nanosecond transient spectral studies. Better utilization of light energy in generating the long-lived charge-separated state with the help of the present "antenna- reaction-center" model system has been successfully demonstrated.

Full text of DOI:10.1002/chem.201404671

DOI: 10.1002/chem.201404671
Full Paper
&
Artificial Photosynthesis
A Supramolecular Tetrad Featuring Covalently Linked Ferrocene–
Zinc Porphyrin–BODIPY Coordinated to Fullerene: A Charge
Stabilizing, Photosynthetic Antenna–Reaction Center Mimic**
Gary N. Lim,[a] Eranda Maligaspe,[b, c] Melvin E. Zandler,[b] and Francis D’Souza*[a]
1
Abstract: A novel photosynthetic-antenna–reaction-center
model compound, comprised of BF2-chelated dipyrrome-
thene (BODIPY) as an energy-harvesting antenna, zinc por-
phyrin (ZnP) as the primary electron donor, ferrocene (Fc) as
a hole-shifting agent, and phenylimidazole-functionalized
fulleropyrrolidine (C60Im) as an electron acceptor, has been
synthesized and characterized. Optical absorption and emis-
sion, computational structure optimization, and cyclic vol-
tammetry studies were systematically performed to establish
the role of each entity in the multistep photochemical reac-
tions. The energy-level diagram established from optical and
redox data helped identifying different photochemical
events. Selective excitation of BODIPY resulted in efficient
singlet energy transfer to the ZnP entity. Ultrafast electron
transfer from the ZnP* (formed either as a result of singlet–
singlet energy transfer or direct excitation) or 1C60* of the co-
ordinated fullerene resulting into the formation of the
+
Cꢀ
C
Fc–(C60 Im:ZnP )–BODIPY radical ion pair was witnessed by
femtosecond transient absorption studies. Subsequent hole
migration to the ferrocene entity resulted in the
Fc+–(C60 +Im:ZnP)–BODIPY radical ion pair that persisted for
C
7–15 ms, depending upon the solvent conditions and contri-
butions from the triplet excited states of ZnP and ImC60, as
revealed by the nanosecond transient spectral studies.
Better utilization of light energy in generating the long-lived
charge-separated state with the help of the present “anten-
na–reaction-center” model system has been successfully
demonstrated.
Introduction
forts have been devoted to develop artificial photosynthetic
reaction centers, which can mimic both the antenna and reac-
tion-center functionalities of natural photosystems.[5–19] Both
covalently linked and supramolecularly assembled donor--ac-
ceptor systems have been elegantly designed, and occurrence
of photoinduced energy and electron transfer leading into the
generation of charge-separated states have been accom-
plished. Generally, porphyrins and phthalocyanines as photo-
sensitizing electron donors[20–21] and fullerenes as electron ac-
ceptors[22–23] have been primarily employed, whereas a variety
of photo/redox-active entities have been utilized as secondary
electron donors or energy-funneling antenna molecules.[5–19]
The presence of secondary electron donors resulted in the
generation of long-lived charge-separated states by a sequen-
tial electron-transfer mechanism while the presence of energy-
funneling antenna molecules provided better utilization of
solar light (broad-band capture).
The ever increasing energy demands, and the rising concern
over environmental pollution and climate changes due to
burning of fossil fuels[1] have driven researcher to look into re-
newable forms of energy.[2–4] Towards this, development of arti-
ficial photosynthetic systems capable of producing energy by
utilizing sunlight is highly desirable, because solar energy has
the potential to fulfill the worldwide energy needs in an envi-
ronmentally benevolent manner.[1] Towards this, extensive ef-
[a] G. N. Lim, Prof. Dr. F. D’Souza
Department of Chemistry, University of North Texas
1155 Union Circle, #305070, Denton, TX 76203-5017 (USA)
E-mail: Francis.DSouza@UNT.edu
[b] Dr. E. Maligaspe, Prof. M. E. Zandler
Department of Chemistry
Wichita State University, Wichita, KS 67260-0051 (USA)
In artificial photosynthesis models, covalently linked ferro-
cene–zinc porphyrin (such as Fc–ZnP in Scheme 1) self-assem-
bled to fullerene functionalized with a ligand functionality
(such as C60Im in Scheme 1) is a well-known example for
charge stabilization through a stepwise electron-transfer–hole-
migration mechanism.[24–25] This is in contrast to simple donor–
acceptor dyads, for example C60Im:ZnP (see Scheme 1 for
structure of ZnP), where only a single-step, photoinduced,
electron transfer is witnessed upon photoexcitation of ZnP.[26]
Interestingly, when ZnP is covalently linked to BF2-chelated di-
pyrromethene (BODIPY,[27] such as ZnP–BODIPY in Scheme 1)
[c] Dr. E. Maligaspe
US Forest Products Laboratory
USDA Forest Service and University of Wisconsin - Madison
Madison WI 53726 (USA)
[**] BODIPY=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404671. The file includes figures from
the manuscript in color, the excitation spectrum of the Fc–ZnP–BODIPY
triad in DCB, fluorescence spectrum of the Fc–ZnP–BODIPY triad along with
control compounds at the porphyrin Soret band excitation in DCB and tolu-
ene, femtosecond transient spectra of the ZnP:ImC60 dyad in DCB and tolu-
ene, MALDI-mass spectra of the Fc–H2P–BODIPY and Fc–ZnP–BODIPY
triads.
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charge stabilization (through hole shift (HS), process 4 in
Figure 1). The outcome of these findings as revealed from the
spectral (optical absorbance and emission), electrochemical,
computational, and photochemical (from femtosecond and
nanosecond transient spectroscopy) studies is reported.
Results and Discussion
Synthesis of the Fc–ZnP–BODIPY triad
The synthesis of the Fc–ZnP–BODIPY triad was carried out ac-
cording to Scheme 2. The details are given in the Experimental
Section. Briefly, the precursors, 4-ferrocenylaniline (Fc-NH2) and
1-(difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-ylidene)-(4-
aminophenyl)methyl]-3,5-dimethyl-1H-pyrrole
(NH2-BODIPY)
and H2-5,15-di(p-carboxyphenyl)-10,20-di(p-tolyl) porphyrin
(HOOC-H2P-COOH) were prepared according to literature
methods[29] with a few minor modifications. Next, the acid
chloride of HOOC-H2P-COOH was obtained by treating dicar-
boxylic acid with SOCl2 followed by reacting the porphyrin
acid chloride with equimolar mixtures of Fc-NH2 and NH2-
BODIPY in the presence of pyridine as base in toluene to
obtain Fc-H2P-BODIPY, followed by chromatographic purifica-
tion. The Fc–H2P–BODIPY thus prepared was metallated using
zinc acetate to obtain the Fc–ZnP–BODIPY triad. The syntheses
of the control compounds, Fc–ZnP and ZnP–BODIPY, were car-
ried out by using a similar procedure, but with monocarboxyl-
ic-acid-functionalized free-base porphyrin, followed by zinc in-
sertion. The compounds were stored in the dark prior to spec-
tral and transient measurements.
Scheme 1. Structure of the newly synthesized Fc–ZnP–BODIPY and the con-
trol compounds, investigated in the present study.
and coordinated to C60Im, a model to visualize an antenna–re-
action-center functionality is formed.[28] That is, in the assem-
bled systems of C60Im:ZnP–BODIPY, selective excitation of
BODIPY results in efficient singlet–singlet energy transfer to
1
1
the ZnP entity to produce ZnP* within the triad. The ZnP*
thus produced promote electron transfer to produce
+
C
Cꢀ
ZnP :ImC60 charge-separated states.
In the present study, we have combined the electron-trans-
fer–hole-shift and antenna–reaction-center concepts in a single
model compound by covalently linking ferrocene (hole-shift
entity) and BODIPY (energy-transfer entity) to the opposite
sides of zinc porphyrin (see Scheme 1 for structure), and self-
assembled C60Im to the ZnP center to create a novel artificial
photosynthetic system that is capable of utilizing a broader
spectrum of light (by the antenna effect, process 2 in Figure 1),
followed by charge separation (process 3 in Figure 1), and
Optical absorption and fluorescence studies
Figure 2a shows the absorbance spectrum of the Fc–ZnP–
BODIPY triad along with the control compounds in o-dichloro-
benzene (DCB). The Soret band of ZnP at 425 nm and visible
bands at 550 and 590 nm along with a BODIPY band at
505 nm were observed. The position of these bands was not
significantly different from those of the control compounds
(shift <ꢁ2 nm). The absorbance due to the ferrocene entity
was hidden in the 320 nm band with the ZnP and BODIPY
bands in the spectral range. These results indicate little or no
intra-supramolecular interactions between the entities of the
triad. Importantly, at the wavelength of BODIPY absorption at
505 nm, no significant absorbance of ZnP or Fc was observed.
These results point out the possibility of selective excitation of
the energy-transferring antenna entity, BODIPY, in the supra-
molecular assembly.[28] Figure 2b shows the fluorescence spec-
trum of the Fc–ZnP–BODIPY triad along with the control com-
pounds in DCB. At the excitation wavelength of 500 nm, corre-
1
sponding to BODIPY absorbance, an emission from BODIPY*
Figure 1. Proposed photochemical events in the supramolecular
at 520 nm was observed. The control compounds, ZnP and Fc–
ZnP, revealed no emission in this wavelength range or at the
emission wavelengths of ZnP (<2% at 605 and 650 nm). Inter-
estingly, for the Fc–ZnP–BODIPY triad and the ZnP–BODIPY
dyad, more than 95% of the original intensity of the BODIPY
emission was found to be quenched; this was accompanied by
Fc–(ZnP:ImC60)–BODIPY tetrad featuring a zinc porphyrin (ZnP) as primary
electron donor, BODIPY as energy-transferring antenna, ferrocene (Fc) as
charge stabilizing HS agent, and fullerene (C60Im) as terminal electron ac-
ceptor. Abbreviations: EnT=excited energy transfer, ET=excited electron
transfer, HS=hole shift, and CR=charge recombination. The photochemical
events are shown as 1, 2, according to their appearance upon selective exci-
tation of the BODIPY entity of the supramolecular assembly.
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peak maxima of BODIPY, sug-
gesting the lack of interactions
between BODIPY and the added
C60Im in solution. An isosbestic
point at 419 nm was also ob-
served, indicating the existence
of an equilibrium process in so-
lution. The binding constant was
obtained by analyzing the data
with the Benesi–Hildebrand
plot,[30] as shown in inset of Fig-
ure 3a. The calculated binding
constant (K), was found to be
4.5ꢁ104 mꢀ1 not significantly dif-
ferent from C60Im binding to
ZnP (K=1.2ꢁ104 mꢀ1), Fc–ZnP
(K=2.3ꢁ104 mꢀ1)
and
ZnP–
BODIPY (K=3.0ꢁ104 mꢀ1) in
DCB.[26a,25a,28a] Further, the emis-
sion spectrum of the Fc–ZnP–
BODIPY triad was also monitored
during the course of the titra-
tion. As shown in Figure 3b,
emission bands of both BODIPY
and ZnP were found to be
quenched. In a control experi-
ment, emission of BODIPY was
monitored with increasing addi-
tion of C60Im where no signifi-
1
cant quenching of BODIPY* was
observed. On the other hand,
when pristine ZnP was titrated
with C60Im, efficient quenching
of 1ZnP* was observed both in
DCB and toluene. These results
indicate occurrence of additional
Scheme 2. Synthetic methodology adopted for the Fc–ZnP–BODIPY triad.
excited-state processes, involv-
ing the C60Im:ZnP segment of
the supramolecular tetrad as-
two news bands at 605 and 650 nm, corresponding to ZnP
emission. These results indicate occurrence of singlet–singlet
sembly.[31] Changing the solvent to toluene revealed similar
spectral features, however, with slightly improved singlet–sin-
glet energy-transfer efficiencies.
1
1
energy transfer from BODIPY* to ZnP to produce ZnP*. The
energy transfer in the ZnP–BODIPY dyad is consistent with our
previous studies.[28b] Further, an excitation spectrum recorded
by keeping the emission monochromator to 650 nm, corre-
sponding to ZnP emission, and scanning the excitation mono-
chromator, revealed peaks not only of ZnP excitation, but also
of BODIPY excitation (see Figure S1 in the Supporting Informa-
tion). These results reveal occurrence of efficient energy trans-
fer in the Fc–ZnP–BODIPY triad and the ZnP–BODIPY dyad.
Next, supramolecular assembly of the Fc–ZnP–BODIPY triad
with C60Im through metal–ligand axial coordination was stud-
ied. Figure 3a shows spectral changes during the titration of
C60Im to a solution of the Fc–ZnP–BODIPY triad in DCB. The
observed spectral changes were consistent with ZnP binding
to nitrogenous bases.[26] That is, redshifted Soret and visible
bands of ZnP were observed, however, with no changes in the
Computational and electrochemical studies
The structure of the supramolecular tetrad, Fc–(ZnP:ImC60)–
BODIPY, was optimized by using the B3LYP/6-31G* method[32]
to visualize its stable geometry. Figure 4 shows two possible
structures, although additional structures are possible due to
the flexible metal–ligand coordinate bond, which would place
the C60Im in different places with respect to the Fc–ZnP–
BODIPY geometry. In agreement with our earlier studies,[25a]
the most stable structure was found to be the one in which
fullerene is close to the ferrocene unit as shown in Figure 4a.
An alternate structure was built and optimized, in which the
C60Im unit was close to the BODIPY segment. The former struc-
ture was slightly more stable by 2.56ꢁ10ꢀ3 Hartrees. For the
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Figure 4. B3LYP/6-31G* optimized structures of the supramolecular tetrad
(Fc–(ZnP:ImC60)–BODIPY) assembled by coordinating C60Im to the Fc–ZnP–
BODIPY triad. Two structures, one in which the C60 is close to ferrocene and
another in which the C60 is close BODIPY are shown.
optimized structure shown in Figure 4a, the center-to-center
distances (RCt–Ct) between ZnP–C60, Fc–ZnP, and ZnP–BODIPY
were 13.0 16.9, and 18.7 ꢂ, respectively, whereas the corre-
sponding edge-to-edge distances were 9.6, 4.3, and 12.3 ꢂ, re-
spectively. The closest distance between ferrocene and C60 en-
tities was about 3.7 ꢂ indicating their close proximity. Such
close proximity between the final charge resting entities would
facilitate charge recombination (see below).
Figure 2. a) Absorption and b) emission spectrum (lex =500 nm) of Fc–ZnP–
BODIPY, ZnP–BODIPY, BODIPY, Fc–ZnP, and ZnP (8 mm each) in N2 saturated
DCB.
Figure 5 shows cyclic voltammograms of the Fc–ZnP–
BODIPY triad and the tetrad assembled upon coordinating
C60Im to the Fc–ZnP–BODIPY triad in DCB containing 0.1m
Figure 5. Cyclic voltammograms of the Fc–ZnP–BODIPY triad (dashed) and
the Fc–(ZnP:ImC60)–BODIPY tetrad (solid line) in DCB containing 0.1m
(TBA)ClO4. Scan rate=100 mVsꢀ1
.
(TBA)ClO4. The site of electron transfer of the individual redox
couples for this rather intricate system, involving four redox-
active entities, was deduced from careful control experiments
with pristine Fc, ZnP, BODIPY, Fc–ZnP, and ZnP–BODIPY. During
the anodic excursion of the potential of the Fc–ZnP–BODIPY
triad, an one-electron reversible oxidation of ferrocene located
at ꢀ0.03 V and a first oxidation corresponding to ZnP located
at 0.27 V versus Fc/Fc+ were observed. Two additional oxida-
tions located at 0.58 and 0.79 V versus Fc/Fc+, corresponding
to the second oxidation of ZnP and first oxidation of BODIPY,
Figure 3. a) Absorbance (inset: Benesi-Hildebrand plot to evaluate binding
constant) and b) fluorescence spectrum (lex =500 nm) of the Fc-ZnP-BODIPY
triad (8 mm) with increasing addition of C60Im (0.2 equiv each) in DCB.
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were also observed (not shown);
the latter is an irreversible pro-
cess. Similarly, during cathodic
excursion of the potential, two
reductions at ꢀ1.72 and ꢀ1.88 V
versus Fc/Fc+ were observed;
the former is an overlap of two
one-electron processes. By com-
parison with reduction potentials
of control compounds, the first
process was ascribed to the
overlap of first reductions of
BODIPY and ZnP entities, where-
as the second process is due to
the second reduction of the ZnP
entity.
For
the
Fc–(ZnP:ImC60)–
BODIPY tetrad, there was no
shift in the oxidation potential
corresponding to the Fc entity.
The first oxidation of ZnP re-
vealed a cathodic shift of 60 mV
as a result of coordination of the
imidazole entity to the Zn metal
center. On the negative side, ad-
ditional processes corresponding
to fullerene reduction were also
observed. That is, two one-elec-
tron reduction processes, at
Figure 6. Energy-level diagram showing the different photochemical events of the supramolecular
Fc–(ZnP:ImC60)–BODIPY tetrad after excitation of either the BODIPY (lex =505 nm) or ZnP (lex =Soret or visible
band) entities in DCB. Abbreviations: k stands for different kinetic processes, EM=electron migration, CS=charge
separation, ISC=intersystem crossing, and CR=charge recombination. The thick arrow represents the most prob-
able process, and the thin and dotted arrows the less likely processes. Fc–(ZnP:1ImC60*)–BODIPY is abbreviated as
1ImC60* for simplicity.
Energy-level diagram depicting excited-state events
ꢀ1.16 and ꢀ1.54 V versus Fc/Fc+ were observed. At more neg-
ative potentials, the ZnP reduction revealed a small anodic
shift.
Figure 6 shows the energy-level diagram depicting different
photochemical processes originating from the singlet excited
states of the BODIPY and ZnP entities of the tetrad. Although
intricate, due to the presence of four photo-/redox-active spe-
cies, it has been possible to identify and dissect key photo-
chemical processes. From the steady-state fluorescence, effi-
cient singlet–singlet energy transfer in the Fc–ZnP–BODIPY
triad is witnessed similar to that reported earlier for the ZnP–
BODIPY dyads.[28] The excited supramolecular tetrad, Fc–
(ZnP:ImC60)–1BODIPY*, produced by selective excitation of
BODIPY (hn’a) would undergo singlet–singlet energy transfer to
produce Fc–(1ZnP*:ImC60)–BODIPY in competition with intersys-
By using the electrochemical, computational, and excited
singlet energy data, the free energies of charge separation
(DGCS) were calculated with Equation (1) by Rehm-Weller’s ap-
proach.[33]
ꢀDGCS ¼ DE0-0ꢀðEoxꢀEredÞ þ DGS
ð1Þ
where Eox is the first one-electron oxidation potential of the
donor porphyrin, Ered is the first one-electron reduction poten-
tial of C60Im, DE0-0 is the energy of the lowest excited state of
ZnP (2.04 eV), DGS refers to the solvation energy, calculated by
using the ‘dielectric continuum model’ show in Equation (2).
tem crossing (ISC-2) to populate Fc–(ZnP:ImC60)–3BODIPY* or
+
C
a charge-separation process (CS5) to yield Fc–(ZnP :ImC60)–
1
Cꢀ
BODIPY . Fc–( ZnP*:ImC60)–BODIPY, produced either as a prod-
uct of energy transfer from the previous step or by direct exci-
tation of the ZnP Soret or visible bands (hna), could undergo
one of the following processes: 1) intersystem crossing (ISC-1)
ꢀDGs ¼ e2=4peo½ð1=2R þ 1=2R ꢀ1=RCCÞð1=esÞ
ð2Þ
þ
ꢀ
to populate Fc–(3ZnP*:ImC60)–BODIPY, 2) charge separation
Values of R+ (4.8 ꢂ), R (4.2 ꢂ), and RCC (center-to-center dis-
ꢀ
+
tance) from modeling were used. e0 and eR refer to vacuum
permittivity and the dielectric constant of DCB, respectively.
Such calculations revealed photoinduced electron transfer
C
Cꢀ
(CS1) to produce the Fc–(ZnP :ImC60 )–BODIPY radical ion
+
C
pair, 3) charge separation (CS2) to produce the Fc–(ZnP
Cꢀ
:ImC60)–BODIPY radical ion pair, or 4) charge separation (CS3)
1
+
+
C
Cꢀ
from ZnP* to coordinated fullerene to generate Fc–(ZnP
to produce the Fc –(ZnP :ImC60)–BODIPY ion pair. Based on
+
Cꢀ
C
Cꢀ
:ImC60 )–BODIPY to be exothermic by about ꢀ0.80 eV, whereas
energy considerations, formation of the Fc–(ZnP :ImC60 )–
Cꢀ
that for the formation of Fc+–(ZnP:ImC60 )–BODIPY, the free-
BODIPY radical ion pair through CS1 is the most probable pro-
+
C
Cꢀ
energy change was ꢀ0.51 eV. Energies corresponding to other
cess (shown in thicker arrows). Additionally, Fc–(ZnP :ImC60 )-
BODIPY could undergo a HS process to produce the Fc+
states are shown pictorially and discussed in the next section.
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Cꢀ
–(ZnP:ImC60 )–BODIPY ion pair. Other photochemical events
also involve direct excitation of the fullerene entity of the
tetrad to form Fc–(ZnP:1ImC60*)–BODIPY, which could undergo
+
C
Cꢀ
electron transfer to produce the Fc–(ZnP :ImC60 )–BODIPY
radical ion pair (CS4) in competition with intersystem crossing
to populate the Fc–(ZnP:3ImC60*)–BODIPY radical ion pair. Addi-
+
Cꢀ
tionally, the Fc –(ZnP :ImC60)–BODIPY ion pair produced from
CS3 would undergo electron migration (EM) to yield the Fc+
+
Cꢀ
C
Cꢀ
–(ZnP:ImC60 )–BODIPY ion pair. The Fc–(ZnP :ImC60 )–BODIPY
radical ion pair, formed either from CS1, CS4, or EM, could un-
Cꢀ
dergo HS to produce the Fc+–(ZnP:ImC60 )–BODIPY radial ion
+
Cꢀ
C
pair. Finally, the Fc+–(ZnP:ImC60 )–BODIPY and Fc–(ZnP
Cꢀ
:ImC60)–BODIPY radical ion pairs could return to the ground
state through charge recombination (CR1 and CR2). Please
note that the energy-level diagram in nonpolar toluene would
be very different, revealing high energy (150–200 mV) for the
charge-separated states due to lower energy of solvation. In
such a case, occurrence of CS3 can be ruled out as supported
by the steady-state measurements (see above). Additionally,
+
C
Cꢀ
the energy levels of both the Fc–(ZnP :ImC60)–BODIPY and
+
C
Cꢀ
the Fc–(ZnP :ImC60 )–BODIPY radical ion pairs would be
higher than those of Fc–(ZnP:3ImC60*)–BODIPY and Fc–
(3ZnP*:ImC60)–BODIPY in nonpolar toluene. In order to establish
these photochemical events in the tetrad, transient absorption
measurements with femtosecond and nanosecond pump-
probe techniques were performed in DCB and toluene, as dis-
cussed in the next section.
Figure 7. Femtosecond transient absorption spectra of the Fc–ZnP–BODIPY
triad (l=400 nm of 100 fs pulse width) at the indicated time intervals in N2-
purged a) DCB and b) toluene solutions.
1
Cꢀ
BODIPY ! Fcþ
-
ZnP
-
BODIPY
*
ð3Þ
ð4Þ
Fc
-
ZnP
ZnP
-
Femto- and nanosecond transient absorption studies
1
-
Cþ
Cꢀ
*
-
Fc
BODIPY ! Fc
-ZnP
-
BODIPY
Figure 7 shows the femtosecond transient spectral changes
observed at different time intervals for the Fc–ZnP–BODIPY
triad in DCB and toluene. At the excitation wavelength of
400 nm, due to higher absorption cross section, mainly ZnP
was excited as evidenced from the instantaneous population
(<1 ps) of the S1–Sn transition with a peak maxima in the
460 nm range; similar to that observed for pristine ZnP (Fig-
ure S2 in the Supporting Information). In the longer-wave-
length region, two depleted signals in the 550–590 nm range,
related to the ground-state absorption of the Q-bands, were
observed. A spectral signature in the 850–900 nm range, corre-
sponding to 1ZnP*, was also observed. A weak signal at
510 nm, corresponding to ground-state depletion of excited
BODIPY was also observed. Because the intensity of this band
was much lower than the transient bands of excited ZnP, no
singlet–singlet energy transfer that would occur within a few
ps was monitored. In a control experiment, as shown in Fig-
ure S3 in the Supporting Information, at the Soret excitation of
the triad, the ZnP Q(0,0) emission located in the 600 nm range
was found to be more than 55% quenched in DCB and 40% in
toluene compared with pristine ZnP. Interestingly, this quench-
ing for the Q(0,1) band located in the 650 nm range was
almost negligent for the triad in toluene. However, for ZnP–
BODIPY, this change was less than 10% in both solvents. These
results suggest occurrence of one or both of the following pro-
cesses, at least in polar DCB for the triad.
+
Cꢀ
The formation of the Fc –ZnP –BODIPY radical ion pair
would result in a new transient band in the around 660 nm,
Cꢀ [25a]
would be a difficult task due to its very low molar extinction
coefficient.[24] On the other hand, the formation of the Fc–
corresponding to ZnP ,
whereas identification of Fc+
+
·ꢀ
C
ZnP –BODIPY radical ion pair would result in a new transient
+ [26]
C
band in the 650–700 nm range, corresponding to ZnP ,
Cꢀ
whereas a positive identification of BODIPY would be difficult
due to lack of diagnostic bands of the radical anion in the
monitored spectral range, although, this process is energetical-
ly favored. Although subtle, a close examination of the transi-
ent spectra in DCB reveals a transient band around 660 nm,
Cꢀ
corresponding to the formation of ZnP , suggesting that Re-
action (3) could be the quenching mechanism in the Fc–ZnP–
BODIPY triad, at least in DCB at the excitation wavelength.
Figure 8 shows the femtosecond transient spectra of the Fc–
(ZnP:ImC60)–BODIPY tetrad in the investigated solvents. In
DCB, immediately upon excitation, bands in the 460 and
910 nm regions, corresponding to S1–Sn transitions of ZnP and
C60Im, respectively, (see Figure S5 in the Supporting Informa-
tion) were observed. The decay of the signal at 460 nm was ac-
companied by the growth of a band at 480 nm, corresponding
3
to ZnP* within the tetrad. Additionally, transient bands corre-
sponding to the formation of the initial Fc–(ZnP +:ImC60 )–
C
Cꢀ
BODIPY radical-ion-pair state was clearly witnessed. That is,
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transient bands at 1000 nm, corresponding to the formation of
Cꢀ
ImC60 , and at 650 nm, corresponding to the formation of
ZnP +, were observed. However, we could not confirm whether
C
1
1ZnP* or ImC60* or both of these species are involved in gen-
erating the charge-separated species. The time constants for
Cꢀ
the growth of the ImC60 band was found to be 16.8 ps, which
resulted into a kCS value of 5.95ꢁ1010
s
ꢀ1. Importantly, the
+
C
decay rate of ZnP was found to be slightly faster than that of
Cꢀ
ImC60 , indicating a HS to the neighboring ferrocene entity.
1
Cꢀ
Further, the decay of the ImC60 and ImC60* transient peaks
were accompanied by growth of a new weak band with
maxima at 707 nm with a shoulder band at 825 nm, character-
istic of triplet fullerene.[10d] Since the energy level of the
3
charge-separated state is lower than that of C60* in polar DCB,
3
it is likely that the population of the C60* occurs through inter-
1
system crossing of ImC60*.
Interestingly, in toluene, different spectral features were ob-
served. That is, a peak corresponding to S1–Sn transitions of
ZnP was observed at 460 nm, however, the peak for ImC60 in
the 900–950 nm region was rather bleak. The decay of the
460 nm peak was accompanied by a growing shoulder band at
485 nm, revealing formation of 3ZnP*. Importantly, evidence
for charge separation was secured by the appearance of transi-
Cꢀ
ent peaks at 1020 nm, corresponding to ImC60 , and 680 nm,
corresponding to ZnP +. The time constants for the growth of
C
Cꢀ
the ImC60 band was found to be 31.5 ps, which resulted in
a kCS value of 3.17ꢁ1010
Cꢀ
s
ꢀ1. The decay of the ImC60 band was
accompanied by a strong new band around 700 nm, corre-
3
sponding to ImC60* formation. As predicted from the energy-
level diagram discussed above; these observations suggest
3
that charge recombination results in populating C60*, which
might be in competition with a HS reaction. Importantly, the
Cꢀ
part of the signal that corresponds to ImC60 persisted beyond
3 ns, which is the time window of our instrument in both sol-
vents.
3
3
In order to spectrally identify ZnP* and C60*, and to evalu-
Cꢀ
ate persistent survival of ImC60 beyond the 3 ns as a result of
the HS process or charge separation possibly resulting from
3ZnP*, nanosecond transient absorption spectral studies were
performed. As shown in Figure 9, the triplet states of ZnP and
ImC60 were obvious with peaks in at 480 nm, corresponding to
3
3ZnP*, and at 700 nm, corresponding to C60*. In the near-IR
Cꢀ
region of 1000 nm, a peak corresponding to ImC60 was also
observed. In toluene, the decay of the peak corresponding to
Cꢀ
ImC60 was monoexponential, resulting in a kCR value of 1.44ꢁ
105 sꢀ1. However, in DCB this decay was biexponential, result-
ing in kCR values of 2.12ꢁ105 and 6.31ꢁ104 sꢀ1, respectively.
The biexponential decay in DCB could be a result of different
geometries of the tetrad (Figure 4), or independent contribu-
1
tions from the excited singlet (1ZnP* or C60*) and triplet
(3ZnP*) originated electron transfer, which was too close to re-
solve in toluene. Importantly, persistence of the radical ion pair
for 7–15 ms, depending on solvent and the nature of the excit-
ed species involved in electron transfer (singlet or triplet), were
accomplished, indicating charge stabilization as a result of HS
in the present multimodular photosynthetic-antenna–reaction-
center model compound. The kCR values are more than two
Figure 8. Femtosecond transient absorption spectra of the Fc–(ZnP:ImC60)–
BODIPY tetrad (l=400 nm of 100 fs pulse width) at the indicated time inter-
vals in N2-purged a) DCB and b) toluene. The time profile of the 1020 nm
band in c) DCB and d) toluene.
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to the primary donor–acceptor pair (zinc porphyrin–fullerene)
was synthesized and characterized, and the photochemical
events were elucidated as shown in Figure 1. Systematic spec-
tral, computational, and electrochemical studies were per-
formed to evaluate the role of each entity in the photochemi-
cal reactions. The energy-level diagram, although intricate in-
volving several competitive reactions, helped identifying domi-
nant photochemical events under the experimental conditions.
Consequently, selective excitation of BODIPY resulted in effi-
1
cient energy transfer to ZnP. ZnP* formed either as a result of
1
singlet–singlet energy transfer or direct excitation, and ImC60*
from direct excitation, depending on the solvent conditions,
revealed ultrafast electron transfer to the fullerene entity re-
+
C
Cꢀ
sulting in the Fc–(ZnP :ImC60 )–BODIPY radical ion pair. Sub-
sequent HS to the ferrocene entity resulted in the Fc+
Cꢀ
–(ZnP:ImC60 )–BODIPY radical ion pair with spatially separated
radical cation and anion species in competition with populat-
3
3
ing the ZnP* and C60* species. Nanosecond transient studies
revealed that the radical ion pair persisted for 7–15 ms, de-
pending on the solvent and the nature of the excited-state
species (singlet- or triplet-generated charge-separated state)
prior to returning to the ground state, revealing charge stabili-
zation and better utilization of solar light by the present
model compound.
Experimental Section
Chemicals: Buckminsterfullerene, C60 (+99.95%) was from SES Re-
search, (Houston, TX). All the reagents were from Aldrich Chemicals
(Milwaukee, WI), and the bulk solvents utilized in the syntheses
were from Fischer Chemicals. Tetra-n-butylammonium perchlorate,
(nBu4N)ClO4, used in electrochemical studies was from Fluka Chem-
icals. The syntheses of C60Im, Fc–ZnP and ZnP–BODIPY have been
reported previously.[26a,25a,28b]
Synthesis of Fc–ZnP–BODIPY: Scheme 2 present the general
methodology adapted for the synthesis of Fc–ZnP–BODIPY. Specific
details are given below.
Synthesis of 5-(4-methoxycarbonylphenyl)dipyrromethane (1):
This compound was synthesized according to the general proce-
dure outlined for the synthesis of dipyrromethanes by Lindsey
et al.[29a] Methyl 4-formyl benzoate (4 g, 0.024 mol) was added to
pyrrole (68 mL, 0.98 mol), and purged with argon for 15 min. Then
trifluoroacetic acid (190 mL, 0.0024 mmol) was added and the mix-
ture was stirred at room temperature for 15 min. The mixture was
diluted with CH2Cl2, and the reaction was quenched with NaOH
(100 mL, 0.1m), and the organic layer was separated and washed
with water. Excess pyrrole was distilled through vacuum at room
temperature, and the compound was purified by flash column
chromatography on silica gel with hexanes:ethyl acetate (60:40 v/
v) as eluent. Evaporation of the solvent yielded a pale-yellow solid.
Yield: 2 g (30%). 1H NMR (400 MHz, CDCl3): d=8.05–7.90 (m, 4H,
pyrrole NH and phenyl H), 7.28 (d, 2H, phenyl H), 6.75–6.65 (m,
2H, pyrrole H), 6.18–6.12 (m, 2H, pyrrole H), 5.92–5.87 (m, 2H, pyr-
role H), 5.54 (s, 1H, -CH-), 3.90 ppm (s, 3H, -COOCH3).
Figure 9. Nanosecond transient absorption spectra of the Fc–(ZnP:ImC60)–
BODIPY tetrad (l=430 nm of 7 ns pulse width) at the indicated time inter-
vals in Ar-purged a) DCB and b) toluene. The time profile of the 1020 nm
band in c) DCB and d) toluene.
orders of magnitude lower compared with those reported ear-
lier for Fc–ZnP:ImC60 triads,[26a] revealing better charge stabili-
zation.
Synthesis of H2-5,15-di(p-methoxycarbonylphenyl)-10,20-di(p-
tolyl) porphyrin (2):[29b] A solution of 1 (1.6 g, 5.71 mmol) was
added to p-tolylaldehyde (675 mL, 5.71 mmol) in CHCl3, and the re-
action mixture was stirred under argon for 1 h. Then BF3·O(Et)2
(724 mL, 5.71 mmol) was added, and the reaction mixture was
Conclusion
A novel photosynthetic model compound integrating both an
antenna (BODIPY) and a secondary electron donor (ferrocene)
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stirred in the dark for 2 h. p-Chloranil (2.11 g, 8.57 mmol) was
added to the reddish-black reaction mixture, and the resulting mix-
ture was stirred for 18 h. Triethylamine (2.4 mL, 17.25 mmol) was
added, and the reaction mixture was stirred for 1 h and then con-
centrated. Column chromatography on silica gel with chloroform
as eluent gave a mixture of five porphyrins, including the title
compound. Subsequent column chromatography of this fraction
on silica gel with toluene:CHCl3 (70:30 v/v) as eluent yielded the
title compound as the third band. Evaporation of the solvent yield-
ed the desired compound as a purple solid. Yield 208 mg (5%).
1H NMR (400 MHz, CDCl3): d=8.82 (dd, 8H, b-pyrrole H), 8.42 (d,
4H, phenyl H), 8.25 (d, 4H, phenyl H), 8.10 (d, 4H, phenyl H), 7.58
(d, 4H, phenyl H), 4.12 (s, 6H, -COOCH3), 2.7 (s, 6H, -CH3),
ꢀ2.80 ppm (brs, 2H, -NH): ESI mass (in CHCl3): m/z: calcd 758.8;
found: 759.9 [M+1].
1.49 ppm (s, 6H, -CH3); ESI mass (in CH2Cl2): m/z: calcd: 339.2;
found; 340.3 [M+1].
Synthesis of 4-ferrocenylnitrobenzene (6):[29c] Concentrated sulfu-
ric acid (25 mL) was added to ferrocene (3.80 g, 20.4 mmol) and
the resulting dark-blue solution was stirred at room temperature
for 2 h. The solution was then poured into ice-cold water (100 mL)
and warmed to room temperature. A solution of sodium nitrite
(0.91 g, 13.2 mmol) in water (5 mL) at 08C was added dropwise to
a stirred solution of 4-nitroaniline (1.66 g, 12.0 mmol) in 1:1 water/
HCl (10 mL) kept at 08C by an ice/water bath. The above mixture
was stirred for 30 min to ensure full diazotization. Copper powder
(1.0 g) was added to the ferrocenium solution, and the diazonium
solution was added dropwise with vigorous stirring. After 24 h of
stirring at room temperature, ascorbic acid (5 g) was added to
reduce any remaining ferrocenium to ferrocene. After addition of
dichloromethane, the organic layer was washed with water, dried,
and evaporated under reduced pressure. The crude product was
purified on silica gel by using CH2Cl2/hexanes (40:60 v/v). Yield:
925 mg (25%). 1H NMR (400 MHz, CDCl3): d=8.14 (d, 2H, phenyl
H), 7.56 (d, 2H, phenyl H), 4.74 (t, 2H, C5H4), 4.48 (t, 2H, C5H4),
4.05 ppm (s, 5H, C5H5); ESI mass (in CH2Cl2): m/z: calcd: 307.58;
found: 307.12.
Synthesis of H2-5,15-di(p-carboxyphenyl)-10,20-di(p-tolyl) por-
phyrin (3, HOOC–H2P–COOH):[29c] A suspension of 2 (200 mg,
0.264 mmol) in THF:ethanol (1:1) was added to a 250 mL round-
bottomed flask and potassium hydroxide (1.5 g, 0.027 mol) dis-
solved in water (12 mL) was added to it. The reaction mixture was
refluxed for 8 h. After cooling, the solvent was evaporated; the res-
idue was diluted with water (100 mL) and then filtered. The desired
compound was obtained as the dipotassium salt. The aqueous sus-
pension of porphyrin dipotassium salt was acidified with concen-
trated HCl (pH 2) and then filtered. The title compound was ob-
tained as a purple powder and used without further purification.
Synthesis of 4-ferrocenylaniline (7, Fc–NH2):[3] To a solution of 6
(162 mg, 2.28 mmol) in 1:1 HCl:ethanol, a mixture of granulated tin
(170 mg, 1.43 mmol) was added, and refluxed for 2 h. The resulting
solution was then cooled to room temperature and neutralized
with 40% aq. NaOH. Next dichloromethane was added to the reac-
tion mixture. At the end, the mixture was washed with water and
dried over anhydrous Na2SO4. After solvent evaporation, the title
compound was obtained as orange crystals upon refrigeration.
1
Yield 85 mg (44%). H NMR (400 MHz, [D6]DMSO): d=8.80 (dd, 8H,
b-pyrrole H), 8.38 (d, 8H, phenyl H), 8.18–8.10 (d, 4H, phenyl H),
7.65–7.55 (d, 4H, phenyl H), 2.55 (s, 6H, -CH3), ꢀ2.40–2.95 ppm
(brs, 2H, -NH).
1
Yield: 112 mg (78%). H NMR (400 MHz, [D6]DMSO): d=7.20 (d, 2H,
Synthesis of 1-(difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-yl-
idene)-(4-nitro-phenyl)methyl]-3,5-dimethyl-1H-pyrrole (4): 2,4-
Dimethylpyrrole (1.08 mL, 10.5 mmol) and 4-nitrobenzaldehyde
(0.93 g, 6.2 mmol) were added to distilled CH2Cl2 (600 mL) and
purged with N2, then trifluoroacetic acid (0.09 mL, 1.23 mmol) was
added and the mixture was stirred for 2 h. The resulting solution
was washed with NaOH (0.1m, 200 mL) and then water, dried over
anhydrous Na2SO4, and filtered, then the solvent was evaporated.
Next the product was redissolved in methylene chloride, and p-
chloranil (1.36 g, 5.5 mmol) was added to the mixture. The result-
ing solution was stirred for 10 min, and then triethylamine (4 mL)
and boron trifluoride etherate (3.5 mL) were added to the mixture.
After 1 h of stirring, the resulting solution was poured into water
and extracted with CH2Cl2. The organic layer was extracted with
water, and the solvent was evaporated. The compound was puri-
fied by flash column chromatography on silica gel with hexa-
nes:CHCl3 (60:40 v/v) as eluent. Evaporation of the solvent yielded
900 mg (40%) of the desired product. 1H NMR (400 MHz, CDCl3):
d=8.39 (d, 2H, phenyl H), 7.55 (d, 2H, phenyl H), 6.02 (s, 2H, pyr-
role H), 2.55 (s, 6H, -CH3), 1.37 ppm (s, 6H, -CH3); ESI mass (in
CH2Cl2): m/z: calcd: 369.2; found: 370.5 [M+1].
phenyl H), 6.52 (d, 2H, phenyl H), 5.02 (brs, 2H, -NH2), 4.58 (t, 2H,
C5H4), 4.22 (t, 2H, C5H4), 3.98 ppm (s, 5H, C5H5); ESI mass (in
CH2Cl2): m/z: calcd: 277.84; found: 277.18.
Synthesis of H2-5-{4-[2-(4-difluoroborondipyrrinylphenyl)amido]-
phenyl}-15-{4-[2-(4-ferrocenylphenyl)amido]-phenyl]10,20-di(p-
tolyl)porphyrin: Compound 3 (65.0 mg, 0.088 mmol) was dis-
solved in dry toluene (20 mL) then thionyl chloride (130 mL,
1.77 mmol) and pyridine (125 mL) was added and the reaction mix-
ture was refluxed under argon for 3 h. After cooling, the solvent
was evaporated and the resulting dark green compound was redis-
solved in dry toluene (25 mL). Then, pyridine (400 mL) was added
followed by 4-ferrocenylaniline (7; 24.5 mg, 0.088 mmol) and 5
(30.0 mg, 0.088 mmol). The reaction mixture was allowed to stir at
room temperature under argon for 12 h. The solvent was evaporat-
ed and the crude compound was purified by column chromatogra-
phy on silica gel with hexanes:CHCl3 (40:60 v/v). Evaporation of
the solvent yielded the metal-free porphyrin of compound 8
(30 mg). 1H NMR (400 MHz, CDCl3): d=8.86 (dd, 8H, b-pyrrole H),
8.33 (d, 8H, phenyl H), 8.18 (d, 4H, phenyl H), 7.98–7.06 (m, 4H,
porphyrin phenyl, 2H+2H ferrocene phenyl, 2H+2H BODIPY
phenyl), 6.02 (s, 2H, pyrrole H), 4.67 (t, 2H, C5H4), 4.33 (t, 2H, C5H4),
4.07 (s, 5H, C5H5), 2.63 (s, 6H, -CH3), 2.59 (s, 6H, -CH3), 1.25 (s, 6H,
-CH3), ꢀ2.76 (brs, 2H, -NH); MADLI-mass: m/z: calcd: 1131.12;
found: 1311.9 [M], 1310.9 [Mꢀ1], 1248.9 [MꢀCp] (see the Support-
ing Information for the spectrum).
Synthesis of 1-(difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-yl-
idene)-(4-amino-phenyl) methyl]-3,5-dimethyl-1H-pyrrole (5,
NH2–BODIPY): A solution of 4 (120 mg) in THF (30 mL) was hydro-
genated over 5% Pd on charcoal (300 mg) at room temperature
and atmospheric pressure. Then, the catalyst was removed by fil-
tration with use of Celite, and the filtrate was evaporated. The
compound was purified by flash column chromatography on silica
gel with hexane:CH2Cl2 (60:40 v/v) as eluent. Evaporation of the
Synthesis of 5-{4-[2-(4-difluoroborondipyrrinylphenyl)amido]-
phenyl}-15-{4-[2-(4-ferrocenylphenyl)amido]-phenyl]10,20-di-
(ptolyl)porphyrinato zinc(II) (8, Fc–ZnP–BODIPY): The free-base
porphyrin of compound 8 (25 mg) was dissolved in CHCl3 (10 mL)
then a saturated solution of zinc acetate in methanol was added
to the solution, and the resulting mixture was refluxed for 2 h until
the Q-band (516 nm) of free-base porphyrin had disappeared.[29c]
1
solvent yielded 44 mg of the title compound (yield 40%). H NMR
(400 MHz, CDCl3): d=7.00 (d, 2H, phenyl H), 6.78 (d, 2H, phenyl H),
5.96 (s, 2H, pyrrole H), 3.47 (brs, 2H, -NH2), 2.55 (s, 6H, -CH3),
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At the end, the reaction mixture was washed with water and dried
over anhydrous Na2SO4. Chromatography on silica gel column with
hexanes:CHCl3 (35:65 v/v) gave the title compound. Yield 27 mg.
1H NMR (400 MHz, CDCl3): d=8.95 (dd, 8H, b-pyrrole H), 8.44 (d,
formed by using the Surface Xplorer software supplied by Ultrafast
Systems.
8H, phenyl H), 8.22 (d, 4H, phenyl H), 8.13–7.2 (m, 4H, porphyrin Acknowledgements
phenyl, 2H+2H ferrocene phenyl, 2H+2H BODIPY phenyl), 6.02 (s,
2H, pyrrole H), 4.81 (t, 2H, C5H4), 4.43 (t, 2H, C5H4), 4.11 (s, 5H,
This work was supported by National Science Foundation
(Grant Nos. 1110942 and 1401188 to FD).
C5H5), 2.72 (s, 6H, -CH3), 2.69 ppm (s, 6H, -CH3), 1.27 (s, 6H, -CH3);
13C NMR (500 MHz, CDCl3): d=135, 134, 132, 131.5, 131, 130, 129,
127, 125, 68, 53, 39, 32, 31, 30.5, 29, 28, 24, 23, 14, 11 ppm; MALDI-
mass: m/z: calcd: 1372.38; found: 1373.4 [M+1], 1311.2 [MꢀZn+
H2] (see the Supporting Information for mass, 1H and 13C NMR
spectra).
Keywords: artificial photosynthesis
·
BODIPY
·
charge
stabilization · ferrocene · fullerenes · electron transfer · zinc
porphyrin
Spectral measurements: The UV/Vis spectral measurements were
carried out with a Shimadzu 2550 double monochromator UV/Vis
spectrophotometer. The fluorescence emission was monitored with
a Varian Eclipse spectrometer. A right angle detection method was
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1
used. The H NMR studies were carried out on a Varian 400 MHz
spectrometer. Tetramethylsilane (TMS) was used as an internal stan-
dard. Cyclic voltammograms were recorded on an EG&G PARSTAT
electrochemical analyzer by using a three-electrode system. A plati-
num button electrode was used as the working electrode. A plati-
num wire served as the counter electrode and an Ag/AgCl elec-
trode was used as the reference electrode. A ferrocene/ferroceni-
um redox couple was used as an internal standard. All the solu-
tions were purged with argon gas prior to electrochemical and
spectral measurements. The computational calculations were per-
formed by B3LYP/6–31G* methods with the GAUSSIAN 09 software
package.[32]
Femtosecond laser flash photolysis: Femtosecond transient ab-
sorption spectroscopy experiments were performed with an ultra-
fast femtosecond laser source (Libra) by coherent incorporation of
a diode-pumped, mode-locked Ti:Sapphire laser (Vitesse) and
a diode-pumped intracavity doubled Nd:YLF laser (Evolution) to
generate a compressed laser output of 1.45 W. For optical detec-
tion, a Helios transient absorption spectrometer coupled with
a femtosecond harmonics generator, both provided by Ultrafast
Systems LLC, were used. The source for the pump and probe
pulses were derived from the fundamental output of Libra (com-
pressed output 1.45 W, pulse width 100 fs) at a repetition rate of
1 kHz. 95% of the fundamental output of the laser was introduced
into the harmonic generator, which produces second and third
harmonics of 400 and 267 nm besides the fundamental 800 nm for
excitation, while the rest of the output was used for generation of
white light continuum. In the present study, the second harmonic
400 nm excitation pump was used in all the experiments. Kinetic
traces at appropriate wavelengths were assembled from the time-
resolved spectral data. Data analysis was performed with the Sur-
face Xplorer software supplied by Ultrafast Systems. All measure-
ments were conducted in degassed solutions at 298 K.
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Received: July 31, 2014
Published online on && &&, 0000
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FULL PAPER
One photon—two photochemical
events: Occurrence of sequential photo-
induced energy transfer (EnT) followed
by electron transfer (ET) leading to
charge stabilization in a newly assem-
bled multimodular tetrad, as a photosyn-
thetic-antenna–reaction-center mimic, is
reported (see figure; HS=hole shift).
&
Artificial Photosynthesis
G. N. Lim, E. Maligaspe, M. E. Zandler,
F. D’Souza*
&& – &&
A Supramolecular Tetrad Featuring
Covalently Linked Ferrocene–Zinc
Porphyrin–BODIPY Coordinated to
Fullerene: A Charge Stabilizing,
Photosynthetic Antenna–Reaction
Center Mimic
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