Ultrafast excitation transfer and charge stabilization in a newly assembled photosynthetic antenna-reaction center mimic composed of boron dipyrrin, zinc porphyrin and fullerene

Article abstract of DOI:10.1039/c1cp90147h

A self-assembled supramolecular triad as a model to mimic the light-induced events of the photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron transfer ultimately generating a long-lived charge-separated stat

Full text of DOI:10.1039/c1cp90147h

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PAPER
Ultrafast excitation transfer and charge stabilization in a newly
assembled photosynthetic antenna-reaction center mimic composed of
boron dipyrrin, zinc porphyrin and fullerene
Francis D’Souza,*ab Channa A. Wijesinghe,b Mohamed E. El-Khouly,c
Jessica Hudson,b Marja Niemi,d Helge Lemmetyinen,d Nikolai V. Tkachenko,*d
Melvin E. Zandlerb and Shunichi Fukuzumi*ce
Received 24th March 2011, Accepted 9th May 2011
DOI: 10.1039/c1cp90147h
A self-assembled supramolecular triad as a model to mimic the light-induced events of the
photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron
transfer ultimately generating a long-lived charge-separated state, has been accomplished. Boron
dipyrrin (BDP), zinc porphyrin (ZnP) and fullerene (C60), respectively, constitute the energy
donor, electron donor and electron acceptor segments of the antenna-reaction center imitation.
Unlike in the previous models, the BDP entity was placed between the electron donor, ZnP and
electron acceptor, C60 entities. For the construction, benzo-18-crown-6 functionalized BDP was
synthesized and subsequently reacted with 3,4-dihydroxyphenyl functionalized ZnP through the
central boron atom to form the crown-BDP-ZnP dyad. Next, an alkyl ammonium functionalized
fullerene was used to self-assemble the crown ether entity of the dyad via ion–dipole interactions.
The newly formed supramolecular triad was fully characterized by spectroscopic, computational
and electrochemical methods. Steady-state fluorescence and excitation studies revealed the
occurrence of energy transfer upon selective excitation of the BDP in the dyad. Further studies
1
involving the pump–probe technique revealed excitation transfer from the BDP* to ZnP to occur
in about 7 ps, much faster than that reported for other systems in this series of triads, as a
consequence of shorter distance between the entities. Upon forming the supramolecular triad by
1
self-assembling fullerene, the ZnP* produced by direct excitation or by energy transfer
mechanism resulted in an initial electron transfer to the BDP entity. The charge recombination
resulted in the population of the triplet excited state of C60, from where additional electron
transfer occurred to produce C60ꢀÀ:crown-BDP-ZnPꢀ+ ion pair as the final charge-separated
species. Nanosecond transient absorption studies revealed the lifetime of the charge-separated
state to be B100 ms, the longest ever reported for this type of antenna-reaction center mimics,
indicating better charge stabilization as a result of the different disposition of the entities of the
supramolecular triad.
to the reaction center to accomplish a long lived charge-
Introduction
separated state as a consequence of sequential electron transfer
In natural photosynthesis, photon energy is efficiently
occurring within the energetically well-aligned entities of the
funneled with the help of light harvesting antenna pigments
reaction center.1 Inspired by this, there have been a number
a Department of Chemistry, 1155 Union Circle, #305070, Denton,
TX 76203, USA
of artificial photosynthetic models in the literature capable
of mimicking the energy and electron transfer processes of
natural photosynthesis.2–16 Both covalent bonding as well as
self-assembled supramolecular bonding approaches have been
utilized in the construction of these model systems. However,
construction of artificial antenna-reaction center models
revealing both ultrafast energy transfer and formation of
long-lived charge-separated states have remained a challenge
due to the associated synthetic challenges in addition to match-
ing and positioning of the energy states of the different entities.
b Department of Chemistry, Wichita State University, Wichita,
KS 67260-0051, USA. E-mail: Francis.DSouza@UNT.edu;
Tel: +940-369-8832
c Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan. E-mail: Fukuzumi@chem.eng.osaka.u.ac.jp;
Tel: +81-6-6879-7368
d Department Chemistry and Bioengineering, Tampere University of
Technology, P.O. Box 541, 33101, Tampere, Finland.
E-mail: nikolai.tkachenko@tut.fi
e Department of Bioinspired Science, Ewha Womens’ University,
Seoul, 120–750, Korea
c
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Supramolecular assembly of boron dipyrrin (BDP or
BODIPYs) as antenna, zinc porphyrin (ZnP) as primary
electron donor, and fullerene (C60) as primary electron acceptor
has been one of the successful approaches to build antenna-
reaction center models.17–19 In these model compounds
excitation of BDP results in efficient excitation transfer to
the ZnP entity from where an electron transfer originates to
verified in the present study by synthesizing and assembling
supramolecular triad, 4.
Results and discussion
Syntheses
form the ZnPꢀ+–C60 radical ion-pair. Using this strategy,
ꢀÀ
The synthesis of crown-BDP-ZnP dyad involved a multi-step
procedure as outlined in Scheme 2 while the details are given
in the experimental section. Briefly, 5-(3,4-dihydroxyphenyl)-
10,15,20-triphenylporphyrin 4a was prepared by the reaction
of pyrrole and 3,4-dimethoxybenzaldehyde followed by
chromatographic separation. The dimethoxyphenyl derivative
was converted into dihydroxyphenyl by reacting with BBr3 in
CH2Cl2. The BDP derivative, 4b, was prepared by the reaction
of 2,4-dimethylpyrrole and 3-formyl-benzo-18-crown-6 in the
presence of trifluoroacetic acid and p-chloranil in CH2Cl2
followed by boronation. Finally, the crown-BDP-ZnP dyad
was prepared by reacting equimolar amounts of 4a and 4b
in CH2Cl2 in the presence of AlCl3 followed by chromato-
graphic separation. Metallation using zinc acetate followed by
chromatographic purification yielded 4c. The newly synthe-
sized compound was stored in dark prior performing spectral
and photochemical studies.
to-date, we have built model compounds of the type 1–3.17–19
Models 1 and 2 utilized metal–ligand axial coordination
approach to link imidazole functionalized fullerene to ZnP
of the covalently linked BDP-ZnP dyads having up to
four BDP entities.17,18 In model 3, crown ether-alkyl cation
binding approach was utilized to assemble fullerene with the
crown appended ZnP-BDP dyad.19 In these models the donor
and acceptor entities were placed adjacent to each other to
facilitate sequential energy and electron-transfer processes.
Although charge stabilization to some extent was observed,
a long-lived charge stabilized state was difficult to attain. In
the present investigation, we have intentionally placed the
antenna unit between the electron donor zinc porphyrin and
electron acceptor fullerene to increase the distance between
them (model 4 in Scheme 1). Based on photo and redox
considerations, such a combination would make the BDP
entity not only an energy donor but also a primary electron
acceptor from the excited ZnP (vide infra). The one-electron
reduced BDP thus formed can subsequently undergo
electron migration to the fullerene entity or result in the
formation of excited triplet state fullerene from where additional
electron transfer can take place. The ultimate result of this
Optical absorption and emission studies of the dyad, 4c and
triad, 4
The absorption spectrum of crown-BDP-ZnP dyad exhibited
bands at 433, 510, 562 and 604 nm revealing the presence of
both BDP and ZnP chromophores (Fig. 1a). The BDP band at
510 nm was found to be red shifted by B6 nm compared to the
control, crown-BDP lacking ZnP while the ZnP Soret was
found also to be red shifted by B3 nm compared to control,
would be distantly separating the final radical cation (ZnPꢀ+
)
and radical anion (C60ꢀÀ) species, a requirement for generating
long-lived charge-separated species. This novel concept has been
Scheme 1 Supramolecular antenna-reaction center models built using boron dipyrrin, zinc porphyrin and fullerene entities (1–4) in
our laboratories. The crown-BDP and ZnP are control compounds used to probe energy transfer and electron transfer in the newly
designed crown-BDP-ZnP dyad binding to alkyl ammonium functionalized C60 (model 4) antenna-reaction center mimic in the present
study.
c
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dyad at 605 and 653 nm were found to be quenched by
ca. 96% as compared with the emission intensity of ZnP. The
1
quenching of ZnP* by the attached BDP may involve energy
transfer and/or electron transfer processes. Since the energy
transfer from the 1ZnP* (2.04 eV) to BDP (2.43 eV) is thermo-
1
dynamically not feasible, the electron transfer from ZnP* to
BDP is expected to dominate based on these measurements.
The excitation spectrum of the crown-BDP-ZnP was recorded
by fixing the emission monochromator to 648 nm and scanning
the excitation wavelength. Such spectrum shown in Fig. 2
revealed bands not only of ZnP but also that of BDP suggesting
excitation transfer from singlet excited BDP to ZnP in the
dyad.20 In order to estimate the efficiency of energy transfer,
the intensity ratio of the 526 nm band of BDP to 562 nm band
of ZnP was calculated and compared with the ratio of the
bands from optical absorption spectrum in Fig. 1a. This ratio
from the excitation spectrum was found to be about 25%
compared to that obtained from the absorption spectral data.
These results suggest occurrence of fairly efficient energy
1
transfer from the BDP* to ZnP in the dyad.
The self-assembly of the crown-BDP-ZnP dyad and alkyl
ammonium functionalized fullerene was monitored spectro-
scopically using optical absorption and emission studies.
Fig. 3a shows absorption spectral changes observed during
increasing addition of fullerene to the dyad solution in benzo-
nitrile. The bands corresponding to both BDP and ZnP
revealed an increase without any appreciable spectral shifts.
The fulleropyrrolidine peak appeared at 328 nm. The final
spectrum of the triad was a straightforward sum of the spectra
of the dyad and fullerene, suggesting a lack of intermolecular
interaction between the fullerene and BDP or ZnP entities.
Interestingly, the fluorescence spectrum of the dyad revealed
additional quenching corresponding to both BDP and ZnP
entities (Fig. 3b). Such quenching was negligent when the dyad
was titrated with 2-phenyl fulleropyrrolidine lacking the alkyl
ammonium cation. These studies suggest alkyl ammonium
binding to the crown ether void and subsequent intrasupra-
molecular quenching.11a The quenching data were analyzed
using the Benesi–Hildebrand method21 as shown in Fig. 3b
inset. A binding constant of 1.9 Â 103 MÀ1 was obtained
suggesting moderately stable complex formation of the alkyl
ammonium cation and the crown ether.
Scheme 2 Synthetic procedure adopted for the crown-BDP-ZnP
dyad host system in the present study.
zinc tetratolylporphyrin, ZnP. These observations suggest a fair
amount of electronic interactions between the two chromo-
phores perhaps due to the linkage through the BDP core
involving the central boron atom.16g,h The emission spectrum
of crown-BDP-ZnP when excited at 510 nm corresponding to
BDP revealed a weak emission at 526 nm corresponding to BDP
along with additional weak emission bands at 607 and 648 nm
corresponding to the appended ZnP (Fig. 1b). A comparison
of BDP emission intensity in the crown-BDP-ZnP dyad to
that of crown-BDP control revealed quenching of the former
by over 90%. These results suggest the occurrence of energy or
electron transfer from singlet excited BDP to ZnP in the dyad. By
using 430 nm excitation light, which selectively excited the ZnP
entity (Fig. 1c), the ZnP emission bands of crown-BDP-ZnP
Computational studies
Further DFT studies at the B3LYP/3-21G(*) level22,23 were
performed to visualize the geometry and electronic structure of
Fig. 1 (a) Absorption spectra of (i) crown-BDP-ZnP, (ii) ZnP, and (iii) crown-BDP in benzonitrile. (b) Fluorescence spectra of (i) crown BDP-
ZnP and (ii) ZnP in benzonitrile; lex = 510 nm. (c) Fluorescence spectra of (i) ZnP and (ii) crown BDP-ZnP in benzonitrile; lex = 430 nm.
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Electrochemical studies and energy levels
Cyclic voltammetric studies were performed to arrive at
the redox potentials of the newly assembled supramolecular
triad, 4. Fig. 5 shows cyclic voltammograms of 4 along with
the control compounds during both negative and positive
scanning of the potential. The first two reductions of the C60
alkyl ammonium cation were located at E1/2 = À1.03 and
À1.45 V vs. Fc/Fc+, respectively (Fig. 5a). The peak-to-peak
separation and plots of peak current versus square root of scan
rate indicated both reductions to be one-electron reversible
processes. The crown ether-BDP-ZnP revealed three oxidations
located at E1/2 = 0.25 and 0.58 and 0.78 V vs. Fc/Fc+. Control
experiments confirmed that the first two processes involve
oxidation of the ZnP macrocycle while the third involves
the BDP macrocycle. During reduction, the first reversible
reduction located at E1/2 = À1.64 V vs. Fc/Fc+, involved
BDP entity while the reduction located at À1.90 involved
reduction of the ZnP macrocycle (Fig. 5b). The assembled
fullerene:crown-BDP-ZnP triad, 4, revealed redox waves corres-
ponding to the presence of all three entities without significant
changes in their potentials compared to the values prior to the
assembly formation. As shown in Fig. 5c for 4, the first oxidation
of ZnP was located at 0.25 V while the first reduction of C60 was
located at À1.03 V vs. Fc/Fc+, respectively, resulting in an
electrochemically measured HOMO–LUMO gap of 1.28 V.
Using the electrochemical data, excited energies and
distances between the chromophores, the free-energies of
Fig. 2 (i) Absorption spectrum of crown ether-BDP-ZnP dyad, and
(ii) excitation spectrum of the crown-BDP-ZnP dyad obtained by
fixing the emission monochromator wavelength to 648 nm and scan-
ning the excitation wavelength in benzonitrile.
the supramolecular triad. Fig. 4a shows the optimized struc-
ture on a Born–Oppenheimer potential energy surface in
which the fullerene was far from both the BDP and ZnP
entities to cause any intermolecular interactions, an observa-
tion that readily agrees with the optical absorption data shown
in Fig. 3a. In the optimized structure, the BDP and ZnP were
coplanar with an orthogonal orientation of the bridging
phenyl entity. The crown ether bound fullerene was almost
perpendicular to the macrocycle planes. The center-to-center
distance between Zn and C60 was found to be 23.6 A while this
distance between zinc and boron atoms of the crown-BDP-
ZnP dyad was 9.5 A. The frontier HOMO and LUMO are
shown in Fig. 4b and c, respectively. The HOMO was spread
all over the porphyrin p-system with some contribution over
the phenyl ring connecting the BDP entity while the LUMO
was found to be fully localized on the fullerene entity. The
B3LYP/3-21G(*) estimated gas phase HOMO–LUMO gap
was found to be 0.72 eV. These results suggest ZnP to be the
primary electron donor and fullerene being the terminal
electron acceptor in spite of arranging them in a different
fashion as shown in Fig. 4.
charge-separation (DGCS) and charge-recombination (DGCR
)
were calculated using eqn (1) and 2 by Weller’s approach.24
ÀDGCR = e(Eox À Ered) + DGS
(1)
where DGS = Àe2/(4pe0eRRCt–Ct) and e0 and eR refer to
vacuum permittivity and dielectric constant of benzonitrile.
ÀDGCS = DE0–0 À (ÀDGCR
)
(2)
where DE0–0 is the energy of the lowest excited state of ZnP
(2.04 eV).
Such calculations revealed a DGCS value of À0.76 eV for
electron transfer from the singlet excited ZnP to fullerene, and
Fig. 3 (a) Absorption and (b) emission spectral changes observed for the crown-BDP-ZnP (6 Â 10À6 M) on increasing addition of alkyl
ammonium functionalized fullerene (0.2 eq. each addition). lex = 502 nm. The Figure inset shows Benesi–Hildebrand plot constructed to
obtain the binding constant. I0 and I represent the fluorescence intensity of the BDP emission in the absence and presence of added fullerene,
respectively.
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Fig. 4 (a) B3LYP/3-21G(*) optimized structure of the supramolecular triad comprised of alkyl ammonium functionalized fullerene bound to the
crown-BDP-ZnP dyad. The HOMO and LUMO are shown in Figures (b) and (c) respectively.
PhCN at 430 nm are shown in Fig. 6. Transient features
corresponding to both ZnPꢀ+ in the 600–700 nm region and
BDPꢀÀ in the 450–500 nm region were observed indicating the
formation of crown-BDPꢀÀ-ZnPꢀ+ as radical ion-pair species
from the initial energy transfer product, crown-BDP-1ZnP*.17
From the rise and decay of the transient band of ZnPꢀ+ at
632 nm, the rate of the charge-separation (kCS) process was
found to be 1.2 Â 1012 sÀ1 while the rate of charge recombi-
nation, kCR was evaluated to be 1.17 Â 1010
s
À1. From the
kCR, the lifetime of charge-separated state (tCS) of crown-
BDPꢀÀ-ZnPꢀ+ via the singlet-excited state of ZnP was found
to be 85 ps.
Fig. 5 Cyclic voltammograms of (a) C60 alkyl ammonium cation,
(b) crown-BDP-zinc porphyrin, and (c) fullerene:crown-BDP-ZnP triad
in benzonitrile containing 0.1 (TBA)ClO4. Scan rate = 0.1 V sÀ1
.
The * in panel a represents redox process corresponding to ferrocene
oxidation used as an internal standard.
DGCR value of À1.28 eV for the charge recombination process.
For comparison purpose, the DGCS and DGCR values for
electron transfer within the crown-BDP-ZnP dyad were also
calculated. In the case of ZnP-BDP dyad, the DGCS and DGCR
values originated from the 1ZnP* were found to be À0.15 and
À1.89 eV, respectively. These studies, in addition to the
thermodynamic feasibility of the occurrence of electron transfer,
also indicate fullerene being the superior electron acceptor in
the supramolecular triad, 4, as predicted by the computational
studies (location of the frontier HOMO and LUMO).
Fig. 6 Femtosecond transient absorption spectra of crown-BDP-ZnP
dyad in PhCN recorded at 5 and 120 ps after excitation pulse at
wavelength of 430 nm. The inset presents a time profile of the transient
absorption at 460 nm.
Photochemical studies
Femtosecond transient absorption spectra recorded at different
delay times after excitation for the crown-BDP-ZnP dyad in
c
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430 nm includes two sequential reactions: internal conversion,
crown-BDP-2ZnP* - crown-BDP-1ZnP*, and the CS, which
means that the rate constant for the CS reaction alone is
o 1.2 Â 1012
s
À1. Therefore the rate constant of CS is likely
À1, and the rate constant for the energy transfer
s
À1, according to the measurements with excita-
to be 3 Â 1012
s
is 1.4 Â 1011
tion at 510 nm. Based on this assumption, the initial fast
formation of the CS state is due to partial direct excitation
of the ZnP moieties, and the following slower growth of the
CS state population is the sequence of two reactions, energy
transfer and CS, from which the energy transfer is the time
limiting step.
Addition of fulleropyrrolidine cation to form the fullerene:
crown-BDP-ZnP triad, 4 revealed spectral features not much
different from that of the dyad with very weak absorption
band of the C60 radical anion at 1000 nm (Fig. 8). The kCR
determined by monitoring the decay of BDPꢀÀ at 470 nm
and ZnPꢀ+ at 637 nm were found to be comparable to those
obtained for the crown-BDP-ZnP dyad, being, 1.6 Â 1010 sÀ1
Fig. 7 Transient absorption decay component spectra (symbols with
solid lines, the component lifetimes are indicated in the plot) and time
resolved spectra (dashed and dotted lines at 0 and 2 ps delay time,
respectively) of crown-BDP-ZnP dyad in PhCN at the excitation
wavelength of 510 nm. Inset shows the transient absorption decay
profile at 630 nm.
and 1.19 Â 1010
s
À1, respectively. However, there was
some residual featureless absorption left after the charge
recombination, and a slow formation of the triplet-excited
state of C60 (3C60*) at 700 nm with a rate of 3.60 Â 108 sÀ1 was
observed with the decay of the CS state. These results are
suggestive of a lack of efficient formation of C60ꢀÀ-crown
BDP-ZnPꢀ+ as charge-separated species. It seems that the
C60-crown BDPꢀÀ-ZnPꢀ+ undergoes charge recombination to
Excitation at 430 nm results in selective generation of the
second singlet excited state of the porphyrin chromophore,
which decays rapidly to the first singlet excited state. In this
case the BDP moiety acts as the electron acceptor only.
To investigate the intermolecular energy transfer, the BDP
chromophore was directly excited at 510 nm. The results of the
transient absorption measurements with excitation at 510 nm
are presented in Fig. 7. The decay profiles are more complex
with this excitation wavelength since the electron transfer is
complemented by the intramolecular energy transfer. The
decays were fitted globally with three-exponential model and
the decay component spectra are shown in Fig. 7 together with
the time resolved spectra right after excitation (at 0 ps) and
at 2 ps delay time. As can be expected, the spectrum of the
long-lived component is essentially the same as the transient
absorption spectrum of the sample excited at 430 nm, and can
1
*
populate the energetically favorable C60 which decays to
3
populate the C60*. In order to confirm occurrence of such
photochemical process and additional electron transfer from
the triplet-excited state of C60 (3C60*) in the triad, subsequent
nanosecond transient spectral measurements were performed.
Fig. 9 shows the nanosecond transient spectra recorded at
different time intervals for the triad 4 at the excitation
wavelength of 430 nm. Transient peaks corresponding to
3C60* in the 650–700 nm region, ZnPꢀ+ in the 600 nm region,
and C60ꢀÀ in the 1000 nm region were clearly evident suggesting
3
be attributed to the charge separated state, crown-BDPꢀÀ
-
occurrence of electron transfer via the C60* in the triad.7–9
ZnPꢀ+, due to characteristic absorption of the porphyrin
cation in the 600–700 nm range. Right after the excitation
(at 0 ps delay) the transient absorption is rather weak in the
red part of the spectrum (600–700 nm), as can be expected for
BDP excited singlet state. However, the formation of the
charge-separated state is at least bi-exponential with time
constants of 0.3 and 7 ps. In reality, one can expect three
reactions to take place in this time domain: (i) energy transfer
from BDP to porphyrin chromophore, crown-1BDP*-ZnP -
crown-BDP-1ZnP*, (ii) electron transfer starting from the
singlet excited BDP chromophore, crown-1BDP*-ZnP -
crown-BDPꢀÀ-ZnPꢀ+, and (iii) electron transfer starting from
the singlet excited ZnP chromophore, crown-BDP-1ZnP* -
crown-BDPꢀÀ-ZnPꢀ+. If reaction (ii) was the dominating one,
i.e. its rate constant was greater than that of the energy
transfer, reaction (i), the formation of the CS state would be
essentially mono-exponential, which is not the case. On this
basis one can expect a two step process, crown-1BDP*-ZnP -
crown-BDP-1ZnP* - crown-BDPꢀÀ-ZnPꢀ+, to be the main
pathway for the charge-separation. The rate constant of
the CS estimated from the measurements with excitation at
The rates of charge-separation (kCST) and charge-recombination
(kCRT) processes via the C60* were evaluated by monitoring
3
the rise and decay of C60ꢀÀ, respectively, as shown in Fig. 9
T
T
inset. The kCS and kCR thus calculated were found to be
1.50 Â 105 sÀ1 and 1.03 Â 104 sÀ1, respectively. The lifetime
Fig. 8 Femtosecond transient absorption spectra of C60:crown-BDP-
ZnP triad in PhCN at the excitation wavelength of 430 nm.
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the ZnP to the triplet-excited state of C60 within the triad
resulting into the formation of C60ꢀÀ:crown-BDP-ZnPꢀ+
as the final charge-separated state with lifetimes of the order
of 100 ms.
A comparison of the photochemical data of the present
dyad and triad to the previously reported dyads and triads17–19
deserves special mention, that is, better figures of merit
relevant to artificial photosynthesis were obtained for the
present systems compared to the previously built BDP-ZnP
dyads and BDP-ZnP-C60 triads (1–3). As demonstrated
by pump–probe technique, energy transfer from the 1BDP*
to ZnP is much faster for the present crown-BDP-ZnP dyad
(7 ps) as compared to the rate of excitation transfer in the
BDP-ZnP dyads shown in 1–3, as a consequence of shorter
distance between the units. In the case of triad 1, where the
BDP and ZnP entities were held by a flexible ethylene oxide
linkage, upon selective excitation of BDP, efficient singlet-
Fig. 9 Nanosecond transient absorption spectra of the triad 4
at different time intervals in PhCN at the excitation wavelength of
ꢀÀ
430 nm. The inset shows decay of C60 at 1000 nm.
S
singlet energy transfer (kENT = 9.2 Â 109 sÀ1) was observed
to populate 1ZnP*.17 Subsequent Àe1lectron transfer to the
of the radical ion pair, tCS from kCRT was found to be 100 ms,
highest value observed for the triads constructed using BDP,
ZnP and fullerene as components (models 1–4 in Scheme 1)
to-date.
coordinated C60 (kCSS = 4.7 Â 109 s ) and fairly rapid charge
recombination (kCR = 2.0 Â 108
s
À1) were subsequently
observed.17 In the case of triad of the type 2 with varying
number of BDP units (1, 2 and 4), the singlet-singlet energy
transfer was rather quick and occurred in the time scale
ranging between 28 and 48 ps.18 Additionally, a decrease in
time constants with increasing the number of BDP units was
observed revealing better antenna effect of the dyads having
higher number of BDP entities. For the triad 3, having the
same antenna, electron donor and electron acceptor but
different spatial arrangement of the entities, efficient singlet-
singlet energy transfer occurred from the singlet excited state
of BDP (10–60 ps time constant depending upon the con-
former), a case similar to that observed for the present dyad.19
However, nanosecond transient absorption spectrum of the
triad 3 yielded a lifetime for the radical ion pair of about 23 ms,
a number that is significantly smaller than the 100 ms lifetime
of the radical ion pair observed for the present system 4. These
results demonstrate that by careful arrangement of the different
entities, it is possible to achieve ultrafast energy transfer and
Fig. 10 summarizes the photochemical events in the newly
assembled supramolecular triad in the present study. The
values of different energy levels were calculated from spectral
and electrochemical data while the kinetics of different photo-
chemical events leading redox species were arrived from the
data using transient spectroscopy both in the femto and nano-
second time scales. The excitation spectrum recorded for the
crown-BDP-ZnP dyad revealed energy transfer from 1BDP* to
ZnP to populate the 1ZnP*, which in turn donated its electron
to the attached BDP forming the crown-BDPꢀÀ-ZnPꢀ+. In the
supramolecular triad, direct excitation of ZnP resulted in the
formation of C60:crown-BDPꢀÀ-ZnPꢀ+ as the initial charge-
separation product. Although subsequent electron migration
to form C60ꢀÀ:crown-BDP-ZnPꢀ+ seems possible, charge
1
recombination occurs to populate the C60* which upon
3
intersystem crossing populates the C60*. A relatively slow
photoinduced electron transfer subsequently occurs from
Fig. 10 Energy level diagram showing the different photochemical events in the investigated supramolecular antenna-reaction center mimic,
60:crown-BDP-ZnP, 4.
C
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long-lived charge-separated state in model compounds
mimicking both antenna and reaction center functionalities.
Compact MALDI (Shimadzu) for the metal complex in PhCN
with dithranol used as a matrix. The computational calcula-
tions were performed by DFT B3LYP/3-21(*) method with
GAUSSIAN 03 software package22 on high speed PCs. The
HOMO and LUMO were generated using the GaussView
program.
Summary
A
novel photosynthetic ‘antenna-reaction center’ mimic
capable of ultrafast excitation transfer followed by electron
transfer to generate long-lived charge separated state has
been successfully built and studied. For the construction, the
antenna mimic, boron dipyrrin, was placed between the
electron donor, ZnP and electron acceptor, C60 to modulate
the kinetics of the photochemical events. Transient absorption
studies using pump–probe technique revealed that singlet
energy transfer from the 1BDP* to the ZnP in the crown-
BDP-ZnP dyad occurred within 7 ps. Upon forming the
supramolecular triad by complexing fullerene to the crown
Time resolved spectroscopy
The studied compounds were excited by a Panther OPO
pumped by Nd:YAG laser (Continuum, SLII-10, 4–6 ns
fwhm) with the powers of 1.5 and 3.0 mJ per pulse. The
transient absorption measurements were performed using a
continuous xenon lamp (150 W) and an InGaAs-PIN photo-
diode (Hamamatsu 2949) as a probe light and a detector,
respectively. The output from the photodiodes and a photo-
multiplier tube was recorded with a digitizing oscilloscope
(Tektronix, TDS3032, 300 MHz). Femtosecond transient ab-
sorption spectroscopy experiments were conducted using an
ultrafast source: Integra-C (Quantronix Corp.), an optical
parametric amplifier: TOPAS (Light Conversion Ltd.) and a
commercially available optical detection system: Helios pro-
vided by Ultrafast Systems LLC. The source for the pump and
probe pulses were derived from the fundamental output of
Integra-C (780 nm, 2 mJ/pulse and fwhm = 130 fs) at a
repetition rate of 1 kHz. 75% of the fundamental output of the
laser was introduced into TOPAS which has optical frequency
mixers resulting in tunable range from 285 nm to 1660 nm,
while the rest of the output was used for white light genera-
tion. Typically, 2500 excitation pulses were averaged for 5 s to
obtain the transient spectrum at a set delay time. Kinetic traces
at appropriate wavelengths were assembled from the time-
resolved spectral data.
1
ether void, the ZnP* produced either by direct excitation or
by energy transfer mechanism resulted in electron transfer
to the BDP entity. The charge recombination eventually
populated the triplet excited state of C60 from where addi-
tional electron transfer occurred to produce C60ꢀÀ:crown-
BDP-ZnPꢀ+ ion pair as the final electron-transfer product.
Nanosecond transient absorption studies revealed the lifetime
of the charge-separated state to be B100 ms, the longest ever
reported for these type of antenna-reaction center mimics,
indicating better charge stabilization as a result of different
arrangement of the antenna, donor and acceptor entities of the
supramolecular triad.
Experimental
Chemicals
The pump–probe instrument used to study transient absorp-
tion of the samples with excitation at 510 nm was described
elsewhere.18 In brief, the laser system consisted of a primary
Ti:sapphire generator (TiF-50, CDP Corp.) pumped by a Nd
CW laser (Verdi-6, Coherent Inc.), femtosecond pulse ampli-
fier and optical parametric amplifier (model 2017, CDP
Corp.). The time resolved spectra were measured by a CCD
detector coupled with a monochromator. Excitation wave-
length was 510 nm and overall time resolution of the instru-
ment was B200 fs (FWHM).
All of the reagents used in the syntheses, and benzonitrile (in
sure seal bottle under nitrogen) were obtained from Aldrich
Chemicals (Milwaukee, WI). Tetra-n-butylammonium per-
chlorate, (TBA)ClO4 was obtained from Fluka Chemicals.
All the chromatographic materials and solvents were procured
from Fisher Scientific and were used as received. The synthesis
of alkyl ammonium functionalized fulleropyrrolidine is given
elsewhere.25
Instruments
The nanosecond transient absorption measurements in
the near-IR region were measured by laser-flash photolysis;
355 nm light from a Nd:YAG laser (Spectra-Physics and
Quanta-Ray GCR-130, 6 ns fwhm) was used as an excitation
source. The monitoring lights from a pulsed Xe-lamp were
detected via Ge-avalanche photodiode module. The samples
were held in a quartz cell (1 Â 1 cm) and were deaerated by
bubbling argon gas through the solution for 20 min. All
measurements were conducted at 298 K. The transient spectra
were recorded using fresh solutions in each laser excitation.
The UV-visible spectral measurements were carried out with a
Shimadzu Model 2550 double monochromator spectrophoto-
meter. The fluorescence emission was monitored by using a
Varian Eclipse spectrometer. A right angle detection method
1
was used. The H NMR studies were carried out on a Varian
400 MHz spectrometer. Tetramethylsilane (TMS) was used
as an internal standard. Cyclic voltammograms were recorded
on an EG&G 263A potentiostat/galvanostat using a three
electrode system. A platinum button electrode was used as the
working electrode. A platinum wire served as the counter
electrode and an Ag/AgCl electrode was used as the reference
electrode. A ferrocene/ferrocenium redox couple was used as
an internal standard. All the solutions were purged prior to
electrochemical and spectral measurements using argon gas.
Matrix-assisted laser desorption/ionization time-of-flight mass
spectra (MALDI-TOF-MS) were measured on a Kratos
Syntheses
5-[(3,4-Dimethoxyphenyl)]-10,15,20-tritolylporphyrin. This
compound was synthesized by reacting 3,4-dimethoxybenzal-
dehyde (12 mmol), tolylaldehyde (37 mmol), and pyrrole
(49 mmol) in refluxing propionic acid.26 The crude product
c
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1
was purified on a basic alumina column. H NMR in CDCl3,
CDCl3, dH (ppm) 9.05–8.80 (m, 8H, b-pyrrole-H), 8.18–8.0
(m, 6H, ortho-phenyl-H), 7.65–7.61 (m, 2H, substituted
phenyl-H), 7.58–7.42 (m, 6H, meta-phenyl-H and phenyl),
7.12 (m, 1H, substituted phenyl-H), 6.9 (m, 1H, aryl-H),
6.8–6.7 (m, 2H, aryl-H), 5.99 (s, 2H, pyrrole-H), 4.2–3.6
(m, 20H, crown-H), 2.70 (s, 3H, CH3-H), 2.55 (s, 6H,-CH3-H),
1.41 (s, 6H, CH3-H), À2.69 (s (br), 2H, imino-H); MALDI-MS
calcd. 1207.22, found 1207.46.
dH (ppm) 8.94–8.86 (m, 8H, b-pyrrole-H), 8.14–8.08 (m, 6H,
ortho-phenyl-H), 7.81–7.80 (m, 1H, substituted phenyl-H),
7.79–7.74 (m, 1H, substituted phenyl-H), 7.58–7.52 (m, 6H,
meta-phenyl-H and phenyl), 7.24–7.20 (m, 1H, substituted
phenyl-H), 4.20 (s, 3H, 3-OCH3), 4.0 (s, 3H, 4- OCH3),
À2.69 (s (br), 2H, imino-H); ESI mass (in CHCl3) calcd.
716.87, found 717.1.
5-[(3,4-Dihydroxyphenyl)]-10,15,20-tritolylporphyrin, 4a. This
compound was synthesized according to the reported procedure
with few modifications.27 A 9 mL of BBr3 (1M in CH2Cl2) was
drop wise added to a solution of 5-[(3,4-dimethoxyphenyl)]-
10,15,20-tritolylporphyrin (1.0 mmol) in CH2Cl2 at À78 1C.
The solution was maintained at this temperature until the
addition was completed and stirred at room temperature for
12 h. Then the mixture was brought to below 5 1C and 100 mL
of cold water was added followed by addition of saturated
sodium bicarbonate. After stirring 1 h at room temperature the
organic layer was separated using CH2Cl2 and dried over
anhydrous Na2SO4. The solvent was evaporated and the crude
product was purified on silica column. 1H NMR in CDCl3,
dH (ppm) 8.94–8.86 (m, 8H, b-pyrrole-H), 8.14–8.08 (m, 6H,
ortho-phenyl-H), 7.81–7.80 (m, 1H, substituted phenyl-H),
7.79–7.74 (m, 1H, substituted phenyl-H), 7.58–7.52 (m, 6H,
meta-phenyl-H and phenyl), 7.24–7.20 (m, 1H, substituted
phenyl-H), 2.70 (s, 9H, CH3-H), À2.69 (s (br), 2H, imino-H).
ESI mass (in CHCl3) calcd. 688.81, found 689.04.
Crown-BDP-ZnP dyad, 4c. A 0.0125 mmol of crown-BDP-
free base porphyrin was dissolved in 30 mL of CHCl3, and
an excess of zinc acetate (50 equiv.) in methanol was added.
The course of the reaction was monitored spectroscopically.
At the end of the reaction (1 h), the solvent was evaporated
1
and the product was purified on silica gel column. H NMR
in CDCl3, dH (ppm) 9.05–8.80 (m, 8H, b-pyrrole-H), 8.18–8.0
(m, 6H, ortho-phenyl-H), 7.65–7.61 (m, 2H, substituted
phenyl-H), 7.58–7.42 (m, 6H, meta-phenyl-H and phenyl),
7.12 (m, 1H, substituted phenyl-H), 6.9 (m, 1H, aryl-H),
6.8–6.7 (m, 2H, aryl-H), 5.99 (s, 2H, pyrrole-H), 4.2–3.6
(m, 20H, crownethylene- H), 2.70 (s, 9H, CH3-H), 2.55
(s, 6H,–CH3-H), 1.41 (s, 6H, CH3-H); MALDI-MS calcd.
1270.62, found 1270.85.
Acknowledgements
This work was supported by National Science Foundation
(Grant Nos. 0804015 and EPS-0903806) and matching sup-
port from the State of Kansas through Kansas Technology
Enterprise Corporation, 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, KOSEF/MEST
through WCU project (R31-2008-000-10010-0) from Korea, and
Academy of Finland.
meso-(Benzo-18-crown-6)difluoroboron dipyrrin, 4b. This
compound was synthesized according to the procedure of
Imahori and coworkers.28 To a mixture of 4-formyl-benzo-
18-crown-6 (12.4 mmol) and 2,4-dimethylpyrrole (2.16 mL,
21.1 mmol) in 800 mL of CH2Cl2, trifluoroacetic acid
(0.19 mL, 2.47 mmol) was added. The reaction mixture was
stirred at room temperature under argon. After 1.5 h the
resulting solution was washed with 0.1M NaOH (200 mL)
and then water (200 mL). The organic layer was dried over
anhydrous Na2SO4 and evaporated under reduced pressure.
The residue was dissolved in toluene (50 mL) and p-chloranil
(2.73g, 11.1 mmol) was added. After about 10 min Et3N
(8 mL) was added followed by BF3ÁEt2O (7 mL). The mixture
was stirred for 1.5 h and then poured into water. The organic
layer was extracted and dried over anhydrous Na2SO4 and
evaporated under reduced pressure. The crude product was
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