Electronic energy harvesting multi BODIPY-zinc porphyrin dyads accommodating fullerene as photosynthetic composite of antenna-reaction center

Article abstract of DOI:10.1039/c002757j

Efficient electronic energy transfer (EET) in the newly synthesized dyads comprised of zinc porphyrin covalently linked to one, two or four numbers of boron dipyrrin (BDP) entities is investigated. Both steady-state and time-resolved emission as well as t

Full text of DOI:10.1039/c002757j

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Papers
Electronic energy harvesting multi BODIPY-zinc
porphyrin dyads accommodating fullerene as
photosynthetic composite of antenna-reaction
center
E. Maligaspe, T. Kumpulainen, N. K. Subbaiyan, M. E.
Zandler, H. Lemmetyinen, N. V. Tkachenko and F. D
Souza, Phys. Chem. Chem. Phys., 2010
DOI: 10.1039/c002757j
Charge transfer in hybrid organic–inorganic PbS
nanocrystal systems
Muhammad N. Nordin, Konstantinos N. Bourdakos and
Richard J. Curry, Phys. Chem. Chem. Phys., 2010
DOI: 10.1039/c003179h
Superexchange-mediated electronic energy transfer
in a model dyad
Carles Curutchet, Florian A. Feist, Bernard Van
Averbeke, Benedetta Mennucci, Josemon Jacob, Klaus
Müllen, Thomas Basché and David Beljonne, Phys.
Chem. Chem. Phys., 2010
DOI: 10.1039/c003496g

PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Electronic energy harvesting multi BODIPY-zinc porphyrin
dyads accommodating fullerene as photosynthetic composite
of antenna-reaction centerw
E. Maligaspe,a T. Kumpulainen,b N. K. Subbaiyan,a M. E. Zandler,a
H. Lemmetyinen,b N. V. Tkachenko*b and F. D’Souza*a
Received 9th February 2010, Accepted 26th April 2010
First published as an Advance Article on the web 12th June 2010
DOI: 10.1039/c002757j
Efficient electronic energy transfer (EET) in the newly synthesized dyads comprised of zinc
porphyrin covalently linked to one, two or four numbers of boron dipyrrin (BDP) entities is
investigated. Both steady-state and time-resolved emission as well as transient absorption studies
revealed occurrence of efficient singlet–singlet energy transfer from BDP to zinc porphyrin with
the time scale ranging between 28 and 48 ps. A decrease in time constants for energy transfer with
increasing the number of BDP units is observed revealing better antenna effect of dyads bearing
higher number of boron dipyrrin entities. Further, supramolecular triads to mimic the
‘antenna-reaction center’ functionality of photosynthetic reaction center have been successfully
constructed by coordinating fulleropyrrolidine appended with an imidazole ligand to the
zinc porphyrin. The structural integrity of the supramolecular triads was arrived by optical,
computational and electrochemical studies. Free energy calculations revealed possibility of
photoinduced electron transfer from singlet excited zinc porphyrin to fullerene, and the
preliminary transient absorption studies involving pump–probe technique are supportive
1
of occurrence of electron transfer from ZnP* to fullerene in the supramolecular triads.
architectures and reported light induced electronic energy and
electron transfer events.3–12
Introduction
In photosynthetic organisms, light energy harvesting antenna
(LH) complexes composed of large number of chlorophyll and
carotenoid molecules are essential for effective capturing of
solar energy and deliver that to the reaction center (RC) with
minimal loss of energy.1 The LH systems (LH1 and LH2) of
purple bacteria have been characterized at atomic resolution
and photophysical events have been probed using ultrafast
spectroscopic techniques.2 In this bacterial system, electronic
energy transfer occurs from LH2 to LH1, and finally to the
special pair in the RC where electron transfer occurs to
produce a stable charge-separated state leading to ATP
synthesis by means of transmembrane proton-motive force.1e
Mimicking the complex photochemical events of the photo-
synthetic systems using synthetic model compounds is
important not only to understand the natural systems but also
for technological advances in solar energy conversion, a
promising solution for global energy demands and environ-
mental issues. Consequently, a number of researchers have
built various synthetic models of LH, RC, and their combined
Porphyrin and other related tetrapyrroles have been widely
employed as energy capturing antenna or electron donor units
in artificial model compounds due to their structural
resemblance to natural chlorophylls.6–11 Similarly, fullerene,
C60 owing to its facile reduction potential and low-reorganization
energy in electron transfer reactions, has been widely used as
an electron acceptor.6–11 In fact, in recent years the use of
fullerene has minimized the utilization of two-dimensional
electron acceptors like quinone, methyl viologin, etc., in
donor–acceptor systems. In a few studies reporting combined
‘antenna-reaction center’ functionalities; fluorophores with
appropriate spectral and redox properties have been linked
to RC electron donor–acceptor dyads. Sequential energy
transfer followed by electron transfer in these model
compounds has been successfully demonstrated.
Boron dipyrrin, BODIPYs or BDP, is a well known
photosensitizer that has been widely used in the development
of fluorescent chemosensors for various analytes and tags to
detect biomolecules.13 When linked to appropriate electron/
energy donor/acceptor entity, it can act as an electron/energy
acceptor/donor, respectively.14–16 Interestingly, BDP can also
be used as an energy absorbing and transferring antenna
molecule in artificial photosynthetic antenna-reaction center
mimics.15a–d Few elegant systems including covalently linked
BDP-porphyrin dyads have been reported in the literature
taking advantage of BDP’s ability to act as an antenna
molecule.14–15 In the present study, we have further exploited
utilization of such dyads by synthesizing zinc porphyrin
a Department of Chemistry, Wichita State University,
1845 Fairmount, Wichita, KS 67260-0051, USA.
E-mail: Francis.DSouza@wichita.edu; Fax: +316-978-3431
b Department of Chemistry and Bioengineering, Tampere University of
Technology, P. O. Box 541, 33101 Tampere, Finland.
E-mail: nikolai.tkachenko@tut.fi
w Electronic supplementary information (ESI) available: Transient
absorption decay component spectra of BDP-ZnP:ImC60 complex in
DCB. See DOI: 10.1039/c002757j
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Chart 1 Structure of the (BDP)n-ZnP dyads (n = 1, 2, or 4), imidazole appended C60, C60Im, and the control compounds employed in the present
study to probe excitation energy and electron transfer events.
covalently linked to 1, 2 or 4 number of BDP entities to
investigate the effect of the number of antenna units on the
overall excitation energy transfer efficiency (Chart 1). Further,
zinc porphyrin in these dyads has been axially coordinated
to an electron acceptor, an imidazole functionalized
fullerene, C60Im, to form an electron donor–acceptor pair in
the supramolecular complex. Formation, and energy and
electron transfer in these novel triads have been systematically
investigated using computational, electrochemical, spectro-
scopic and time-resolved photochemical techniques.
electron transfer in supramolecular triads formed by axial
coordination of C60Im to the zinc center of (BDP)n-ZnP dyads
is discussed.
Excitation energy transfer in the multi-BDP appended zinc
porphyrin dyads
Optical absorption and fluorescence emission studies of
(BDP)n-ZnP dyads. Fig. 1 shows the optical absorption
spectra of the (BDP)n-ZnP dyads in o-dichlorobenzene
(DCB), normalized to their Soret band along with a control
compound, meso-tolyl boron dipyrromethane (BDP-control).
In the dyads, the bands at 426, 550 and 590 nm correspond to
zinc porphyrin while the band at 505 nm corresponds to the
boron dipyrrin entity.15a As expected, the 505 nm band
revealed an increase with increasing number of BDP units,
although the dependence is no strictly linear. The absorption
bands of (BDP)4-ZnP were red shifted by 1–2 nm compared to
the other two dyads. Importantly, the absorption of BDP at
505 nm in the control compound had no appreciable overlap
with the ZnP, suggesting that irradiation of the dyads at
this wavelength would selectively excite the boron dipyrrin
entity(ies) of the dyads.
Results and discussion
Synthesis and characterization
The synthesis of (BDP)n-ZnP dyads involved a multi-step
procedure as detailed in the experimental section. Briefly, for
the synthesis of BDP-ZnP, meso-(4-aminophenyl) functionalized
BDP, 1-(difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-ylidene)-
(4-aminophenyl)methyl]-3,5-dimethyl-1H-pyrrole, was reacted
with H2-5-(4-carboxyphenyl)-10,15,20-tri(p-tolyl)porphyrin
followed by metallation of free-base porphyrin to zinc
porphyrin. For (BDP)2-ZnP, two eq. of meso-aminophenyl
functionalized BDP was reacted with H2-5,15-di(p-carboxy-
phenyl)-10,20-di(p-tolyl)porphyrin followed by metallation
to zinc porphyrin. Similarly, for (BDP)4-ZnP, meso-tetra-
(4-carboxyphenyl)porphyrin was reacted with excess amount
of meso-aminophenyl functionalized BDP followed by
metallation to obtain the zinc porphyrin derivative. The newly
synthesized compounds were fully characterized by NMR,
optical and mass spectral methods.
The fluorescence emission spectrum of the BDP-zinc
porphyrin dyad, excited at 495 nm corresponding to BDP
excitation, is shown in Fig. 2 along with the emission of
BDP-control compound. In case of the dyads, a weak band
in the range of 517–525 nm corresponding to the emission of
BDP was observed in addition to the zinc porphyrin emission
bands at 602 and 650 nm.15a The BDP emission band in the
dyads was found to be quenched substantially compared to the
BDP-control lacking the covalently linked ZnP. The amount
of quenching was found to be 96%, 93% and 91%, for
BDP-ZnP, (BDP)2-ZnP and (BDP)4-ZnP dyads, respectively.
In the first part of the paper we discuss the excitation energy
transfer in the multi-BDP appended zinc porphyrin dyads and
in the second part formation and preliminary results of
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Fig.
3 Excitation spectra of equimolar concentrations of (i)
Fig. 1 Absorption spectra of (i) BDP-ZnP, (ii) (BDP)2-ZnP, (iii)
(BDP)4-ZnP and (iv) BDP in DCB, normalized to the porphyrin
Soret band.
BDP-ZnP, (ii) (BDP)2-ZnP, (iii) (BDP)4-ZnP, and (iv) BDP in DCB.
The excitation was scanned by holding the emission monochromator
at 662 nm corresponding to ZnP emission.
Computational and electrochemical studies
of (BDP)n-ZnP dyads
Since the relative orientation of the donor and acceptor
dipoles in the dyads is crucial for energy transfer efficiency,
the structure of the dyads were visualized by performing
computational studies at the B3LYP/3-21G(*) level.17,18
Fig. 4 shows the structures of the dyads energy optimized on
Born–Oppenheimer potential energy surfaces. The appended
BDP macrocycle(s) were found to be almost in the same plane
of zinc porphyrin. That is, the dihedral angle between the two
macrocycles was found to be less than 101 irrespective of the
number of BDP entities on the ZnP macrocycle. The center-to-
center (boron to zinc) and edge-to-edge (between meso carbon
atoms of the macrocycles) distances between the BDP and ZnP
macrocycles were found to be 18.8 A and 12.3 A, respectively.
That is, no steric crowding between the macrocycles was
observed. The B–B distance between the two BDP units
of (BDP)2-ZnP was found to be B37 A. In the case of
(BDP)4-ZnP, the four BDP units were arranged in an
rectangular fashion around ZnP for which the B-B distance
between the adjacent BDP entities was found to be B25 A.
The positioning of the both the BDP and ZnP macrocycles in
the same plane is expected to facilitate energy transfer owing
to better orientation factor of the dipoles.19 It may be
mentioned here that we have not further attempted to evaluate
the orientation of the dipoles of the two macrocycles due to
flexible nature and limited computational facility.
Fig.
2 Fluorescence spectra of (i) BDP-ZnP, (ii) (BDP)2-ZnP,
(iii) (BDP)4-ZnP and (iv) BDP in DCB. The concentration of BDP
was held constant by fixing the optical density of BDP at 505 nm
to 0.1. lex = 495 nm.
In a control experiment, equimolar mixture of ZnP and BDP
compounds was excited at the wavelength of the BDP
absorption band, 495 nm, and no significant emission of
ZnP was detected. The quenching of BDP emission and
occurrence of ZnP emission indicate singlet-singlet energy
transfer in the studied dyads.
The occurrence of singlet excited energy transfer from BDP
to zinc porphyrin in the dyads was also confirmed by
recording the excitation spectra of the dyads while holding
the emission monochromator at 662 nm corresponding to zinc
porphyrin emission. Such spectra shown in Fig. 3 revealed an
excitation band of not only a zinc porphyrin entity but also
that of the BDP entity of the dyad indicating energy transfer
from BDP to zinc porphyrin. Generally, the intensity of BDP
excitation band increased with increasing their numbers,
suggesting higher probability of excitation energy transfer
with increasing the number of antenna BDP units.
Further electrochemical studies were performed to evaluate
the redox potentials and establish the energy states. The
one-electron reduction of the BDP control was located at
E1/2 = À1.96 vs. Fc/Fc+ while its irreversible oxidation was
located at Epa = 0.54 V vs. Fc/Fc+. The first two reversible
oxidations of the zinc porphyrin were located at E1/2 = 0.23
and 0.57 V vs. Fc/Fc+ while the first two reversible reductions
were located at Epc = À1.95 and À2.22 V vs. Fc/Fc+,
respectively. That is, the first reduction potential of ZnP and
BDP were almost the same. The first three reductions of the
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Fig. 4 B3LYP/3-21G(*) optimized structures of (i) BDP-ZnP, (ii) (BDP)2-ZnP, and (iii) (BDP)4-ZnP dyads.
C60Im were located at E1/2 = À1.11 À1.55, and À2.09 V vs.
Fc/Fc+, respectively. The peak-to-peak separation and plots
of peak current versus square root of scan rate indicated all of
the electroreductions to be one-electron reversible processes.
For the BDP-ZnP dyad, three electrooxidations located at
0.25, 0.58, and 0.78 V vs. Fc/Fc+ were observed, the latter
being an irreversible process (Fig. 5). Two electroreductions
were also observed at À1.73 and À1.92 V, the former being the
overlapping first electroreductions of BDP and ZnP entities.
Interestingly, for the (BDP)2-ZnP and (BDP)4-ZnP dyads,
almost similar redox behavior was observed, however, the
peak currents corresponding to BDP electrooxidation and
electroreduction were higher. That is, peak currents twice as
much as ZnP in (BDP)2-ZnP and four times as much as of
ZnP in (BDP)4-ZnP were observed. The relatively difficult
oxidation of BDP and almost similar reduction of BDP in
the dyads suggest it being a poor electron donor or acceptor in
the dyads.
where DE0À0 is the energy of the lowest excited state of ZnP
(2.06 eV), DGS = Àe2/(4pe0eRRCtÀCt) and e0 and eR refer to
vacuum permittivity and dielectric constant of DCB.
The calculations revealed DGCS values to be endothermic by
0.1–0.2 eV for electron transfer from the singlet excited Zn
porphyrin to BDP suggesting less likely occurrence of such
reactions in the studied dyads.
Excited energy transfer in the (BDP)n-ZnP dyads probed by
pump–probe technique
As pointed out earlier, excitation of the BDP in the
495–505 nm range to a large extent selectively excites the
antenna unit, BDP, thus allowing us to monitor energy
transfer to the acceptor, ZnP, without it being directly getting
excited. Both steady-state emission and excitation spectra
confirm occurrence of excited energy transfer in the dyads.
Further evidence for the energy transfer and kinetics of such
process is obtained from studies involving pump–probe
technique. Here, an excitation wavelength was 500 nm,
achieved by optical parameter amplifier with the time
resolution of the instrument of 150–200 ps (FWHM of the
instrument response). Measurements were carried out in three
wavelength ranges, 440–660, 520–740 and 870–1090 nm. Since
the excitation interfered with the signal around 500 nm, in
some measurements in a small wavelength range around
500 nm was ignored. In most cases a global fit was used to
achieve coherent analysis of the data at all wavelengths.
Transient absorption spectra of BDP-ZnP in DCB are
presented in Fig. 6 for which the most informative part of
the spectrum was the visible part. At least three exponential fit
had to be used. With the excitation wavelength of 500 nm
mainly exciting the BDP chromophore, right after excitation a
bleached band at 505 nm was clearly observed. The main
lifetime for this bleached band was 48 Æ 9 ps. At longer delay
time (4100 ps), the time resolved spectrum of the dyad
corresponded to the differential spectrum of the first excited
state of ZnP. Lifetime of this excited state was too long to be
resolved with pump–probe measurements. A relatively minor
fast component with time constant of 6 ps observed for
BDP-ZnP was attributed to originate from more than one
process. That is, vibrational relaxation and solvent dynamics
of BDP could be main contributors to this component. The
ZnP internal conversion can be ruled out since the probe,
ZnP is not excited to the second excited state at the excitation
wavelength of 500 nm. Transient absorption spectra of
Using the electrochemical, computational, and excited
energy data, the free-energies of charge-separation (DGCS
)
were calculated using eqn (1) by Weller’s approach.20
ÀDGCS = DE0À0– e(Eox À Ered) + DGS
(1)
Fig. 5 Cyclic voltammograms of the investigated (a) BDP-ZnP (b)
(BDP)2-ZnP and (c) (BDP)4-ZnP dyads (B0.5 mM) in DCB containing
0.1 M (TBAP)ClO4. Scan rate = 100 mV/s. The ferrocene oxidation,
used as an internal standard, is shown in (b) (pink line).
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followed by optical absorption spectral methods. Fig. 8a
shows the spectral changes observed during the addition of
C60Im to the (BDP)2-ZnP dyad in DCB. The Soret band at
426 nm first, decreased in intensity and then increased with a
red shift to 431 nm. Isosbestic points at 422 and 429 nm were
observed indicating the occurrence of an equilibrium process
in solution (see Fig. 8a inset). It may be mentioned here that
the band at 505 nm corresponding to the boron dipyrrin
moiety revealed no peak shift during the course of the titration
indicating no significant interactions with the fullerene;
increase in the absorbance at 505 nm is attributed to the
absorption of added fulleropyrrolidine. Independent control
experiments performed in the presence of BDP control and
C60Im also revealed no spectral changes. These results suggest
that only the zinc porphyrin is involved in axial coordination
and not the boron dipyrrin of the dyad.
Fig.
6 Transient absorption decay component spectra of the
BDP-ZnP dyad in o-dichlorobenzene. The dashed line shows the
Fig. 8b shows the fluorescence spectra of the (BDP)2-ZnP
dyad in the presence of increasing amounts of C60Im at
the excitation wavelength of 495 nm corresponding to the
excitation of the BDP entity. The emission bands corresponding
to both boron dipyrrin and zinc porphyrin revealed quenching.
Similar quenching was also observed when ZnP was directly
excited at 550 nm in the triads. This quenching process in the
(BDP)2-ZnP:ImC60 triad clearly indicates the occurrence of
electron transfer from the singlet excited zinc porphyrin to the
coordinated fulleropyrrolidine.
differential absorption spectrum right after excitation (at 0 ps delay time).
(BDP)2-ZnP and (BDP)4-ZnP in o-DCB were similar to that
of the BDP-ZnP dyad (Fig. 7). The lifetime of excited state of
BDP moiety in (BDP)2-ZnP and (BDP)4-ZnP were 44 Æ 17
and 16 Æ 5 ps, respectively, though a bi-exponential fit was
used for the latter dyad. The long-lived component (c 3 ns) in
Fig. 7a corresponded to that of the excited state of porphyrin
and couldn’t be determined accurately with pump–probe
method. The fastest component can be explained similar to
the BDP-ZnP sample.
The binding constant was evaluated by constructing
Benesi–Hildebrand plot21 (Fig. 8b inset). The calculated
binding constant was found to be 3.0 Â 104, 4.4 Â 103, and
3.9 Â 103 MÀ1, respectively, for the mono, bis and tetra BDP
appended ZnP. These results indicate moderately stable
supramolecular triad formation by the axial ligation
approach. The slightly lower stability for triads with higher
number of BDP could be attributed to the steric and electronic
effects.
The results of pump–probe method suggest occurrence of
ultrafast excitation energy transfer in the studied dyads, and a
decrease in time constants for energy transfer with increasing
the number of BDP units. That is, better antenna effect for
dyads with multiple BDP units is realized not only by
increased absorption of the compound but also by a faster
energy transfer to the acceptor, porphyrin unit.
Formation and study of electron transfer of supramolecular
triads
Computational and electrochemical studies of the
(BDP)n-ZnP:ImC60 triads
First, the formation and characterization of the supramolecular
triads by axial coordination of C60Im to (BDP)n-ZnP dyads in
DCB was performed using spectroscopic and electrochemical
methods. The formation of the supramolecular triads was
The geometry of the triads was also visualized by performing
computational studies. Fig. 9 shows the optimized structures
of the triads in which coordination of imidazole of C60Im to
Zn of ZnP was evident. Minimal structural changes of the
Fig. 7 Transient absorption decay component spectra of (a) (BDP)2-ZnP and (b) (BDP)4-ZnP in o-dichlorobenzene. The dashed line shows the
differential absorption spectrum right after excitation (at 0 ps delay time).
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That is, the BDP and C60 units were separated by over 6.2 A
in these triads. The center-to-center and edge-to-edge distance
between the fullerene and ZnP macrocycles were found to be
13.0 A and 8.1 A, respectively. The frontier HOMO and
LUMO orbitals were also generated for the optimized
structures. In all these triads, the HOMO was located on the
ZnP entity while the LUMO was located on the fullerene
entity. That is, formation of ZnP+ꢁ:ImC60 as the initial
electron transfer product was visualized from these results.
The localization of HOMO and LUMO also agree well with
electrochemical results discussed earlier.
À
ꢁ
Cyclic voltammetric studies were performed to arrive at the
redox potentials of the investigated triads. Fig. 10 shows
voltammograms of BDP-ZnP dyad upon increasing additions
of C60Im. The first oxidation peak of ZnP revealed a cathodic
shift of about 60 mV upon axial coordination of C60Im.
However, no significant changes in the oxidation and
reduction potentials of BDP or C60Im were observed
Fig. 8 (a) Absorption spectral changes observed for (BDP)2-ZnP
(1.40 mM) binding to C60Im (1.13 mM each addition) in DCB. (b)
Fluorescence spectral changes observed during increasing addition of
C
60Im (1.13 mM each addition) to the solution of (BDP)2-ZnP to form
the triad in DCB. lex = 495 nm. The figure inset shows Benesi–
Hildebrand plot constructed by monitoring the emission intensity of
the at 520 nm band for calculating the binding constant. The abbre-
viations Io and I represent fluorescence intensity in the absence and
presence of fullerene, respectively.
(BDP)n-ZnP dyads upon coordination of C60Im were seen.
Importantly, no direct interaction between the fullerene and
BDP entity(ies) was observed.
Fig. 10 Cyclic voltammograms of the ZnP-BDP dyad upon increasing
addition of C60Im (0.2, 0.6, 0.8 and 1.0 eq) in o-dichlorobenzene,
0.1 M (TBA)ClO4. Scan rate = 100 mV/s.
Fig. 9 B3LYP/3-21G(*) optimized structures of (i) BDP-ZnP:ImC60, (ii) (BDP)2-ZnP:ImC60, and (iii) (BDP)4-ZnP:ImC60
.
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indicating little or no electronic perturbation of these macro-
cycles upon formation of the triads. Importantly, fullero-
pyrrolidine being better electron acceptor in terms of
reduction potential in the triads was evident from these
studies. From these electrochemical data on adding the excited
energies and distances between the chromophores (vide supra),
the free-energies of charge-separation (DGCS) and charge-
recombination (DGCR) were calculated using the Weller
equation.20 Such calculations revealed photoinduced electron
transfer from the 1ZnP* to coordinated fullerene to be
exothermic by BÀ0.80 eV for the triads while for charge
recombination, the DGCR values were BÀ1.26 eV.
The results of preliminary transient absorption studies, the
four exponential data fitting and resulting decay component
spectra, of the BDP-ZnP:ImC60 supramolecular triad in DCB
are presented in Fig. S1 (see Electronic Supplementary
Informationw). The lifetimes obtained form the fit are 8 ps,
26 ps, 62 ps and 4 ns. Same components of pure BDP-ZnP
dyad are seen for triad complex. In analogous to (BDP)-ZnP
spectrum, the fastest component observed in the triad
spectrum, 8 ps, probably originates from the solvent and
vibrational relaxation. A second component with lifetime of
26 ps can be attributed to the excitation relaxation of the BDP
chromophore, since it has spectral features similar to that of
48 ps component of BDP-ZnP dyad. Qualitatively the decrease
in the lifetime of the BDP excited state is in agreement with
steady state emission measurements, showing lower fluores-
cence intensity of BDP unit upon addition of ImC60. The
longest-lived component at 4 ns, could be attributed to the
Photoinduced electron transfer in the (BDP)n-ZnP:ImC60 triads
probed by transient spectral studies
Occurrence of sequential energy transfer followed by electron
transfer in a structurally similar boron dipyrrin-zinc porphyrin-
fulleropyrrolidine triad has recently been reported.15b The
energy level diagram (Fig. 11) constructed using the data from
the steady-state spectral, computational and electrochemical
studies suggest such a photochemical reaction mechanism is
also likely to operate in the present supramolecular triads. For
the energy level diagram, energies of the excited states of the
different entities were calculated from the fluorescence peaks
while the triplet state energy of ZnP was cited from the
literature.11b The thick arrows indicate the major photo-
chemical processes while thin arrows reveal minor processes.
Based on the distance and energetic considerations, an
electron transfer from singlet excited BDP to C60 is considered
to be an inefficient process. The preliminary results obtained
from the pump–probe transient absorption measurements
reveal that this is indeed to be the case.
À
charge separated state, ZnP+ꢁ:ImC60 ꢁ, though it is rather
similar to the longest-lived component of BDP-ZnP dyad.
However, the main difference is in the shapes of transient
absorption spectra where an increased intensity in the range of
600–700 nm for the BDP-ZnP:ImC60 triad due to the presence
of ZnP radical cation was clearly observed. The component
with lifetime of 62 ps can be attributed to the formation of
charge separated state, since it shows some increase in tran-
sient absorption in the 600–700 nm range (area of porphyrin
cation absorption).
The signal of (BDP)2-ZnP:ImC60 and (BDP)4-ZnP:ImC60
triads were much weaker due to solubility issue and as the
result lower absorption at excitation wavelength, 500 nm,
though the number of BDP units per molecule is increased
for this compounds. Signal-to-noise ratio was much poorer
and the CS state is not clearly visible from the data though
results show quenching of the lifetime of BDP chromophore.
Hence, no further studies on these triads using pump–probe
technique were performed.
Conclusions
Novel supramolecular triads to mimic the ‘antenna-reaction
center’ functionality of photosynthetic reaction center have
been successfully developed. The antenna mimic, boron
dipyrrin entities were covalently linked to the reaction center
electron donor mimic, a zinc porphyrin. Both steady-state and
time-resolved emission as well as transient absorption studies
clearly proved occurrence of efficient singlet-singlet energy
transfer from BDP to zinc porphyrin with the time scale of
28–48 ps, and a decrease in time constants for energy transfer
with increasing the number of BDP units. That is, better
antenna effect for dyads with higher numbers of BDP entities
is realized from the present study. Next, the reaction center
electron acceptor mimic, a fulleropyrrolidine appended with
an imidazole ligand was coordinated to the zinc porphyrin to
form the supramolecular triads. The structural integrity of the
supramolecular triad was arrived by optical, computational
and electrochemical studies. Energy calculations revealed
possibility of photoinduced electron transfer from singlet
excited zinc porphyrin to fullerene. Further preliminary
transient absorption studies involving pump–probe technique
Fig. 11 Energy level diagram showing the photochemical events of
the supramolecular (BDP)n-ZnP:ImC60 triads after excitation of either
the BDP or ZnP moieties. The thick arrows indicate major photo-
chemical process and thin arrow represents minor photochemical
process.
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provided evidence of electron transfer from 1ZnP* to fullerene
in the supramolecular triads.
of 2 (120 mg) in THF (30 mL) was hydrogenated over 5% Pd
on charcoal (300 mg) at room temperature and atmospheric
pressure. Then, the catalyst was removed by filtration 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
solvent yielded 44 mg of 3, (yield = 40%).
Experimental section
Chemicals
All of the reagents used in the synthesis, and o-dichlorobenzene
(in sure seal bottle under nitrogen) were from Aldrich Chemicals
(Milwaukee, WI). Tetra-n-butylammonium perchlorate,
(TBA)ClO4 was from Fluka Chemicals. Porphyrin precursor,
meso-tetra(4-carboxyphenyl)porphyrin was procured from
Frontier Scientific Inc., (Logan, UT). All the chromatographic
materials and solvents were procured from Fisher Scientific
and were used as received. All the chromatographic materials
and solvents were procured from Fisher Scientific and were
used as received. The synthesis and purification of of C60Im is
given elsewhere.22
1H NMR(CDCl3) d (ppm) 7.00 (d, 2H, phenyl H), 6.78
(d, 2H, phenyl H), 5.96 (s, 2H, pyrrole H), 3.47 (br s, 2H,
–NH2), 2.55 (s, 6H, –CH3), 1.49 (s, 6H, –CH3). ESI mass
(in CH2Cl2) calcd 339.2, found [M + 1] 340.3.
H2-5-{4-[2-(4-difluoroborondipyrrinylphenyl)amido]phenyl}-
10,15,20-tri(p-tolyl)porphyrin (4). Compound
1 (36 mg,
0.05 mmol) was taken in dry toluene (20 mL) then thionyl
chloride (80 mL, 1.1 mmol) and pyridine (80 mL) were added
and the reaction mixture was refluxed under argon for 3 h.
After cooling, the solvent was evaporated and the resulting
green compound was redissolved in dry toluene (25 mL).
Then, pyridine (260 mL) was added followed by 3 (17 mg,
0.05 mmol). The reaction mixture was allowed to stir at room
temperature under argon for 12 h. The solvent was evaporated
and the crude compound was purified by column chromato-
graphy on silica gel with hexanes:CHCl3 (60 : 40 v/v).
Evaporation of the solvent yielded the desired compound
(28 mg, yield = 80%).
1H NMR (CDCl3) d (ppm) 8.95–8.76 (m, 8H, b pyrrole H),
8.39 (d, 2H, phenyl H), 8.30 (d, 2H, phenyl H), 8.11 (d, 6H,
tolyl H), 7.98 (d, 2H, BDP phenyl H), 7.48 (d, 2H, tolyl H),
7.39 (d, 2H, BDP phenyl H), 6.04 (s, 2H, pyrrole H), 2.71
(s, 9H, tolyl CH3), 2.58 (s, 6H, pyrrole CH3), 1.58 (s, 6H,
pyrrole H), –2.78 (s, br, 2H, –NH). ESI mass (in CH2Cl2) calcd
1021.0, found [M + 1] 1022.2.
Synthesis of mono BDP-zinc porphyrin dyad
H2-5-(p-carboxyphenyl)-10,15,20-tri(p-tolyl)porphyrin (1).
4-carboxybenzaldehyde (1.0 g, 0.006 mol) was dissolved in
propaonic acid (120 mL) and pyrrole (0.027 mol, 1.85 mL) was
followed by p-tolualdehyde (2.35 mL, 0.019 mol). The mixture
was refluxed for 4 h. The propanoic acid was removed under
reduced pressure and the compound was purified by using a
basic alumina column with CHCl3:MeOH (96 : 4 v/v)
as eluent. Evaporation of the solvent yielded the desired
compound as a purple solid. Yield: 450 mg (10%).
1H NMR (CDCl3) d (ppm) 9.00–8.78 (m, 8H, b pyrrole H),
8.51 (d, 2H, phenyl H), 8.35 (d, 2H, phenyl H), 8.14 (d, 6H,
tolyl H), 7.61 (d, 6H, tolyl H), 3.76 (s, 9H, 3-CH3), À2.78
(s, br, 2H, –NH).
1-(Difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-ylidene)-
(4- nitrophenyl)methyl]-3,5-dimethyl-1H- pyrrole (2). 2,4-Di-
methylpyrrole (1.08 mL, 10.5 mmol) and 4-nitrobenzaldehyde15a
(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 0.1 M NaOH (200 mL)
and then water, dried over anhydrous Na2SO4, and filtered,
then the solvent was evaporated. Next the product was redis-
solved in methylene chloride, and p-chloranil (1.36 g, 5.5 mmol)
was added to the mixture. The resulting solution was stirred
for 10 min, 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
purified by flash column chromatography on silica gel with
hexanes:CHCl3 (60 : 40 v/v) as eluent. Evaporation of the
solvent yielded 900 mg (40%) of the desired product.
5-{4-[2-(4-difluoroborondipyrrinylphenyl)amido]phenyl}-10,15,
20-tri(p-tolyl) porphyrinato zinc(II) (5). Compound 4 (30 mg)
was dissolved in CHCl3 (15 mL) then zinc acetate in methanol
was added to the chloroform solution, and the resulting
mixture was refluxed for 2 h until the 515 nm band of
free-base porphyrin had disappeared. At the end, the organic
layer was washed with water and dried over anhydrous
Na2SO4. Chromatography on silica gel column by using
hexanes: CHCl3 (50 : 50 v/v) gave the title compound. Yield
30 mg (yield = 93%).
1H NMR (CDCl3) d (ppm) 9.00–8.82 (m, 8H, b pyrrole H),
8.39 (d, 2H, phenyl H), 8.22 (d, 2H, phenyl H), 8.11 (d, 6H,
tolyl H), 7.92 (d, 2H, BDP phenyl H), 7.55 (d, 2H, tolyl H),
7.35 (d, 2H, BDP phenyl H), 6.02 (s, 2H, pyrrole H), 2.71
(s, 9H, tolyl CH3), 2.60 (s, 6H, pyrrole CH3), 1.53 (s, 6H,
pyrrole H), ESI mass (in CH2Cl2) calcd 1085.4, found [M + 1]
1086.8. 13C NMR (CDCl3): 14.81, 17.45, 21.90, 114.52, 120.01,
121.52, 122.90, 127.41, 127.82, 128.31, 130.11, 132.05, 132.56,
133.83, 136.03, 137.52, 138.50, 139.47, 140.01, 142.23, 148.18,
149.81, 170.01.
1H NMR (CDCl3) d (ppm) 8.39 (d, 2H, phenyl H), 7.55
(d, 2H, phenyl H), 6.02 (s, 2H, pyrrole H), 2.55 (s, 6H, –CH3),
1.37 (s, 6H, –CH3). ESI mass (in CH2Cl2) calcd 369.2, found
[M + 1] 370.5.
Synthesis of (BDP)2-ZnP dyad
5-(4-Methoxycarbonylphenyl)dipyrromethane (5). This com-
pound was prepared according to the general procedure
outlined for the synthesis of dipyrromethanes by Lee and
1-(Difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-ylidene)-
(4-aminophenyl)methyl]-3,5-dimethyl-1H-pyrrole (3). A solution
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Lindsey.23 To a pyrrole (68 mL, 0.98 mol) was added methyl
4-formyl benzoate (4.0 g, 0.024 mol) and the solution was
purged with argon for 15 min. Then trifluoroacetic acid
(190 mL, 0.0024 mmol) was added and the mixture was stirred
at room temperature for 15 min. The mixture was then diluted
with CH2Cl2 and NaOH (100 mL, 0.1 M) was added, then the
organic layer was extracted over Na2SO4. 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 (yield = 30%).
1H NMR (CDCl3) d (ppm) 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,
pyrrole H), 5.54 (s, 1H, –CH–), 3.90 (s, 3H, –COOCH3).
Then, pyridine (56 mL) was added followed by 3 (28 mg,
0.08 mmol). The reaction mixture was allowed to stir at room
temperature under argon for 12 h. The solvent was evaporated
and the crude compound was purified by column chromato-
graphy on silica gel with hexanes:CHCl3 (75 : 25 v/v).
1H NMR (CDCl3) d (ppm) 8.96–8.77 (m, 8H, b pyrrole H),
8.42–8.22 (m, 8H, phenyl H), 8.12 (d, 4H, phenyl H), 8.00–7.92
(dd, 4H, phenyl H), 7.58 (d, 4H, phenyl H), 7.42–7.34 (dd, 2H,
phenyl H), 7.18 (d, 4H, phenyl H), 6.03 (s, 4H, pyrrole H), 2.54
(s, 6H, tolyl CH3), 2.52 (s, 12H, pyrrole CH3), 1.56 (s, 12H,
pyrrole H), À2.79 (s, br, 2H, –NH). ESI mass (in CH2Cl2)
calcd 1373.0, found [M + 1] 1373.8.
5,15-Di{4-[2-(4-difluoroboron dipyrrinylphenyl)amido]phenyl}-
10,20-di(p-tolyl) Porphyrinato Zinc(II) (9). The free-base
porphyrin 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 2–3 h
until the 515 nm band of free-base porphyrin had disappeared.
At the end, the reaction mixture was washed with water and
dried over anhydrous Na2SO4. Chromatography on silica gel
column by using hexanes: CHCl3 (70 : 30 v/v) gave the title
compound. Yield 25 mg (yield = 90%).
1H NMR (CDCl3) d (ppm) 9.05–8.98 (m, 8H, b pyrrole H),
8.40–8.35 (m, 8H, phenyl H), 8.11 (d, 4H, phenyl H), 8.01–7.93
(dd, 4H, phenyl H), 7.60 (d, 4H, phenyl H), 7.42–7.34 (dd, 2H,
phenyl H), 7.18 (d, 4H, phenyl H), 6.01 (s, 4H, pyrrole H), 2.78
(s, 6H, tolyl CH3), 2.61 (s, 12H, pyrrole CH3), 1.56 (s, 12H,
pyrrole H), ESI mass (in CH2Cl2) calcd 1436.4, found [M+1]
1437.3. 13C NMR (CDCl3): 14.90, 17.50, 21.90, 114.52, 119.85,
121.52, 122.90, 127.41, 127.82, 128.31, 130.11, 131.70, 132.56,
133.83, 136.04, 137.50, 138.50, 139.47, 141.01, 142.23, 148.18,
149.81, 170.20.
H2-5,15-Di(p-methoxycarbonylphenyl)-10,20-di(p-tolyl)-
porphyrin(6). This compound was prepared according to Luo
et al.24 A solution of 6 (1.6 g, 5.71 mmol) was added to
p-tolylaldehyde (675 mL, 5.71 mmol) in CHCl3, and the
reaction mixture was stirred under argon for 1 h. Then
BF3ÁO(Et)2 (724 mL, 5.71 mmol) was added, and the reaction
mixture was 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 mixture 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 concentrated. Column chromato-
graphy on silica gel with chloroform as eluent gave a mixture
of five porphyrins including the titled compound. Subsequent
column chromatography of this fraction on silica gel kwith
toluene:CHCl3 (70 : 30 v/v) as eluent yielded the titled
compound as the third band. Evaporation of the solvent
yielded the desired compound as a purple solid. Yield 208 mg
(yield = 5%).
1H NMR (CDCl3) d (ppm) 8.82 (dd, 8H, 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).
Synthesis of (BDP)4-ZnP dyad
H2-5,20,15,20-tetra{4-[2-(4-difluoroboron dipyrrinylphenyl)-
amido]phenyl} porphyrin (10). Commercially available meso-
tetra(4-carboxyphenyl)porphyrin (14 mg, 0.01 mmol) was
taken in dry toluene (20 mL) then thionyl chloride (22 mL,
0.29 mmol) and pyridine (25 mL) were added and the reaction
mixture was refluxed under argon for 3 h. After cooling, the
solvent was evaporated and the resulting green compound was
redissolved in dry toluene (20 mL). Then, pyridine (81 mL) was
added followed by 3 (22 mg, 0.05 mmol). The reaction mixture
was allowed to stir at room temperature under argon for 12 h.
The solvent was evaporated and the crude compound
was purified by column chromatography on silica gel with
hexanes:CHCl3 (40 : 60 v/v).
1H NMR (CDCl3) d (ppm) 8.95–8.80 (s, 8H, b pyrrole H),
8.42–8.25 (dd, 16H, phenyl H), 7.95 (d, 8H, phenyl H), 7.39
(d, 8H, phenyl H), 6.02 (s, 8H, pyrrole H), 2.58 (s, 24H, pyrrole
CH3), 1.55 (s, 24H, pyrrole CH3), À2.89 (s, br, 2H, –NH). ESI
mass (in CH2Cl2) calcd 2075.6, found [M + 1] 2076.9.
H2-5,15-Di(p-carboxyphenyl)-10,20-di(p-tolyl)porphyrin (7).
A suspension of 6 (200 mg, 0.264 mmol) in THF:ethanol
(1 : 1) was taken in a 250 mL round-bottomed flask and
potassium hydroxide (1.5 g, 0.027 mol) dissolved in water
(12 mL) was added to it. The reaction mixture was refluxed for
8 h. After cooling, the solvent was evaporated; the residue was
diluted with water (100 mL) and then filtered. The desired
compound was obtained as the dipotassium salt. The aqueous
suspension of porphyrin dipotassium salt was acidified with
concentrated HCl (pH 2) and then filtered. The titled
compound was obtained as a purple powder and used without
further purification. Yield 85 mg (yield = 44%).
1H NMR (DMSO-d) d (ppm) 8.8 (dd, 8H, 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), -2.95 (s, br, 2H, –NH).
H2-5,15-Di{4-[2-(4-difluoroboron dipyrrinylphenyl)amido]-
phenyl}-10,20-di(p-tolyl) porphyrin (8). Porphyrin diacid 7
(30 mg, 0.03 mmol) was taken in dry toluene (20 mL) then
thionyl chloride (56 mL, 0.76 mmol) and pyridine (56 mL) were
added and the reaction mixture was refluxed under argon for 3 h.
After cooling, the solvent was evaporated and the resulting
green compound was redissolved in dry toluene (25 mL).
5,20,15,20-tetra{4-[2-(4-difluoroboron dipyrrinylphenyl)amido]-
phenyl}porphyrinato zinc(II) (11). To a solution of 10 (20 mg)
dissolved in chloroform was added zinc acetate dissolved in
methanol, and the solution was stirred for 2 h. The solution
was washed with water and evaporated under reduced
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7442 | Phys. Chem. Chem. Phys., 2010, 12, 7434–7444

pressure. The compound was purified over silica gel column
hexanes:CHCl3 (30 : 70 v/v) as eluent. Yield 20 mg (yield = 95%).
1H NMR (CDCl3) d (ppm) 8.90 (s, 8H, b pyrrole H),
8.45–8.20 (dd, 16H, phenyl H), 7.95 (d, 8H, phenyl H), 7.39
(d, 8H, phenyl H), 6.02 (s, 8H, pyrrole H), 2.59 (s, 24H,
pyrrole CH3), 1.57 (s, 24H, pyrrole CH3) ESI mass
(in CH2Cl2) calcd 2138.9, found [M + 1] 2140.0. 13C NMR
(CDCl3): 14.95, 17.51, 114.52, 119.85, 121.52, 122.90, 127.41,
127.82, 128.82, 130.11, 131.70, 132.56, 133.83, 134.11, 136.04,
138,50, 139.47, 140.01, 142.23, 148.18, 149.81, 171.22.
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Instrumentation
The UV-visible spectral measurements were carried out with a
Shimadzu Model 1600 UV-visible spectrophotometer. The
fluorescence emission was monitored by using a Varian Eclipse
spectrometer. A right angle detection method was used. The
1H NMR studies were carried out on a Varian 400 MHz
spectrometer. Tetramethylsilane (TMS) was used as an
internal standard. Cyclic voltammograms were recorded on
a EG&G PARSTAT electrochemical analyzer using a three
electrode system. A platinum button electrode was used as the
working electrode. A platinum wire served as the counter
electrode and a Ag/AgCl electrode was used as the reference
electrode. Ferrocene/ferrocenium redox couple was used as an
internal standard. All the solutions were purged prior to
electrochemical and spectral measurements using argon gas.
The computational calculations were performed by DFT
B3LYP/3-21G(*) methods with GAUSSIAN 03 software
package17 on high speed PCs. The mass spectra were recorded
on a Varian 1200L Quadrupole MS using APCI mode in dry
CH2Cl2.
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and H. Imahori, Energy Environ. Sci., 2008, 1, 120.
Transient absorption measurements
The laser instrument used to acquire pump–probe measure-
ments as well as data analysis were described elsewhere.25 In
brief, the laser system consisted of a primary Ti:sapphire
generator (TiF-50, CDP Corp.) pumped by Nd CW laser
(Verdi-6, Coherent Inc.), femtosecond pulse amplifier
(CDP Corp.) and optical parametric amplifier (model 2017,
CDP Corp.) to produce excitation pulses at 500 nm. The time
resolved spectra were detected by a CCD detector coupled
with a monochromator. Overall time resolution of the
instrument was 150–200 fs (FWHM).
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¨
Acknowledgements
This work was financially supported by the National Science
Foundation (Grant Nos. 0804015 and EPS-0903806) and
matching support from the State of Kansas through Kansas
Technology Enterprise Corporation, and Academy of Finland
for support of this work.
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