Photosynthetic antenna-reaction center mimicry: Sequential energy- and electron transfer in a self-assembled supramolecular triad composed of boron dipyrrin, zinc porphyrin and fullerene

Article abstract of DOI:10.1021/jp9032194

A self-assembled supramolecular triad, a model to mimic the photochemical events of photosynthetic antenna - reaction center, viz., sequential energy and electron transfer, has been newly constructed and studied. Boron dipyrrin, zinc porphyrin, and fuller

Full text of DOI:10.1021/jp9032194

8478
J. Phys. Chem. A 2009, 113, 8478–8489
Photosynthetic Antenna-Reaction Center Mimicry: Sequential Energy- and Electron
Transfer in a Self-assembled Supramolecular Triad Composed of Boron Dipyrrin, Zinc
Porphyrin and Fullerene
Eranda Maligaspe,† Nikolai V. Tkachenko,*,‡ Navaneetha K. Subbaiyan,† Raghu Chitta,†
Melvin E. Zandler,† Helge Lemmetyinen,‡ and Francis D’Souza*,†
Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and
Tampere UniVersity of Technology, P.O. Box 541, 33101 Tampere, Finland
ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: June 8, 2009
A self-assembled supramolecular triad, a model to mimic the photochemical events of photosynthetic
antenna-reaction center, viz., sequential energy and electron transfer, has been newly constructed and studied.
Boron dipyrrin, zinc porphyrin, and fullerene respectively constitute the energy donor, electron donor, and
electron acceptor segments of the antenna-reaction center mimicry. For the construction, first, boron dipyrrin
was covalently attached to a zinc porphyrin entity bearing a benzo-18-crown-6 host segment at the opposite
end of the porphyrin ring. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the
crown ether entity via ion-dipole interactions. The newly formed supramolecular triad was fully characterized
by spectroscopic, computational, and electrochemical methods. Selective excitation of the boron dipyrrin
moiety in the dyad resulted in energy transfer over 97% efficiency creating singlet excited zinc porphyrin.
The rate of energy transfer from the decay measurements of time-correlated singlet photon counting (TCSPC)
and up-conversion techniques agreed well with that obtained by the pump-probe technique and revealed
efficient photoinduced energy transfer in the dyad (time constant in the order of 10-60 ps depending upon
the conformer). Upon forming the supramolecular triad by self-assembling fullerene, the excited zinc porphyrin
resulted in electron transfer to the coordinated fullerene yielding a charge-separated state, thus mimicking the
antenna-reaction center functionalities of photosynthesis. Nanosecond transient absorption studies yielded a
lifetime of the charge-separated state to be 23 µs indicating charge stabilization in the supramolecular triad.
The present supramolecular system represents a successful model to mimic the rather complex “combined
antenna-reaction center” events of photosynthesis.
Introduction
photocatalysts capable of producing hydrogen.2 In this regard,
several studies using synthetic models focused on mimic-
king the reaction center where photoinduced electron transfer
from the excited donor to acceptor occurs have been reported.3-11
In the majority of these studies porphyrin has been used as an
electron donor due to its close resemblance to the photosynthetic
pigment, chlorophyll, and the established synthetic manipula-
tions. The efficiency of charge separation in some of the
donor-acceptor systems has been comparable to those found
in natural reaction centers.7d,e To mimic the antenna functional-
ity, singlet-singlet energy transfer between two or more
covalently linked or self-assembled porphyrins, or other chro-
mophores, has been studied extensively.12-16 Some of these
molecular/supramolecular systems have revealed potentials for
constructing photonic, photocatalytic, and optoelectronic
devices.2,17,18
The photochemical process of natural photosynthesis involves
two major steps, namely, absorption and transportation of light
energy of appropriate wavelength by the antenna light harvesting
molecules to the reaction center and occurrence of photoinduced
electron transfer (PET) to generate charge-separated entities by
using the electronic excitation energy.1 The light energy
harvesting antenna system consists of chromophore arrays which
transport energy via a singlet-singlet energy transfer mechanism
among the chromophores. The antenna pigments, different for
different organisms, are optimized for the quality of light in a
particular environment and are effectively coupled to the reaction
center. The reaction center absorbs the excitation energy and
converts it to chemical energy in the form of transmembrane
charge separation via a multistep electron transfer reaction. The
stored electrochemical energy in the form of charge-separated
species is later converted into other forms, e.g., proton motive
force, of biological useful forms of energy.1
Although a number of studies dealing either with the reaction
center or the antenna mimicry have been reported,3-16
a
Mimicry of the photosynthetic events by using synthetic
model compounds is vital to further our understanding of the
complex processes of bioenergetics. Research in this area also
holds promise for technological advances in light energy
harvesting, building molecular optoelectronics, and to develop
molecular/supramolecular complex performing combined an-
tenna and reaction center functions has been scarce. A very few
elegantly designed molecular/supramolecular systems involving
two or more fluorophores and an electron acceptor like fullerene
have been developed and the occurrence of stepwise energy
and electron transfer events has been demonstrated.19-21 In the
present study, we report a self-assembled supramolecular
complex to perform the combined antenna-reaction center
photochemical events. The structure of the model system is
* To whom correspondence should be addressed. E-mail: francis.dsouza@
wichita.edu and nikolai.tkachenko@tut.fi.
† Wichita State University.
‡ Tampere University of Technology.
10.1021/jp9032194 CCC: $40.75  2009 American Chemical Society
Published on Web 07/06/2009

Photosynthetic Antenna-Reaction Center Mimicry
J. Phys. Chem. A, Vol. 113, No. 30, 2009 8479
SCHEME 1: Structure of the Presently Constructed Supramolecular Triad Composed of Boron Dipyrrin-Zinc
Porphyrin-Crown Ether Bound to an Alkyl Ammonium Functionalized Fulleropyrrolidine to Probe “Antenna-Reaction
Center” Functionalitiesa
a The photochemical events occurring in the triad are shown by the arrows.
shown in Scheme 1. Here, boron dipyrrin22a (also known as
BODIY, abbreviated as BDP) acts as an energy absorbing and
transferring antenna to the light energy acceptor, zinc porphyrin.
That is, when excited in the 480-515 nm wavelength region,
selective excitation of BDP occurs,21a,22 and due to good spectral
overlap between BDP emission and zinc porphyrin absorption
spectra, efficient singlet-singlet energy transfer to the zinc
porphyrin takes place. When the crown ether receptor site of
the dyad binds to an electron acceptor, an alkyl ammonium
bearing fulleropyrrolidine,23 photoinduced electron transfer from
the singlet excited porphyrin to fulleropyrrolidine subsequently
occurs. The present model compound differs from the earlier
reported one by us21a in which instead of metal-ligand axial
coordination, a crown ether-crown binding approach has been
employed. The easily oxidizable BDP unit in the present system
is also involved to some extent in stabilizing the charge-
separated state. The stronger crown ether-crown binding
approach permits us to perform the investigations in an electron
transfer favoring solvent such as polar benzonitrile.23 Systematic
spectral, computational, electrochemical, and photochemical
studies have been performed to unravel sequential energy and
electron transfer in the newly built supramolecular triad.
dipyrrinylphenyl)amido]phenyl}-10,20-di(p-tolyl)porphyrin, was
prepared by reacting equimolar amounts of the acid chlo-
ride form of 5,15-di(methoxycarbonylphenyl)-10,20-di(tolyl)-
porphyrin, 1-(difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-
ylidene)(4-aminophenyl)methyl]-3,5-dimethyl-1H-pyrrole, with
4′-aminobenzo-[18-crown-6] in dry toluene. Chromatographic
separation followed by metalation with zinc acetate gave the
desired product, 5-{4-[2-(4-benzo-[18-crown-6])amido]phenyl}-
15-{4-[2-(4-difluoroboron dipyrrinylphenyl)amido]phenyl}-
10,20-di(p-tolyl)porphyrinato zinc(II). The newly synthesized
compound was stored in the dark prior to performing the
experiments.
Ground State Properties of the Boron Dipyrrin-Zinc
Porphyrin Dyad. Figure 1a shows the optical absorption
spectrum of the dyad along with the spectra of control
compounds in benzonitrile. 1-(Difluoroboryl)-2-[(Z)-(3,5-di-
methyl-2H-pyrrol-2-ylidene)(4-tolyl)methyl]-3,5-dimethyl-1H-
pyrrole as BDP control and 5-{[4-(benzo-[18-crown-6])ami-
do]phenyl}-10,15,20-tri(p-tolyl)porphyrinatozinc(II) as zinc
porphyrin-crown ether control were used. The absorption bands
located at 431, 559, and 602 nm correspond to zinc porphyrin
while the band at 495 nm corresponds to the boron dipyrrin
entity (plot i in Figure 1a). The absorption band corresponding
to BDP revealed ∼2 nm red shift in the dyad, indicating some
ground state interactions between the two chromophores.
Importantly, the absorption of boron dipyrrin at 504 nm had no
appreciable overlap with the ZnP-crown ether control or C60
ammonium cation absorption at these wavelengths (plots ii and
iv in Figure 1b). Hence, irradiation of the dyad at the 500 nm
region is expected to a large extent to selectively excite the
boron dipyrrin moiety of the dyad.21a,22b
Results and Discussion
Syntheses. The synthesis of BDP-ZnP-crown ether in-
volved a multistep procedure as outlined in Scheme 2. First,
5-(4-methoxycarbonylphenyl)dipyrromethane was prepared by
the reaction of pyrrole and methyl 4-formyl benzoate according
to the Lindsey procedure.24 The dipyrromethane was subse-
quently reacted with p-tolylaldehyde followed by chloranil
oxidation to obtain 5,15-di(methoxycarbonylphenyl)-10,20-di-
(tolyl)porphyrin.25 The ester groups were subsequently hydro-
lyzed in THF:ethanol, using KOH to obtain H2-5,15-di(p-
carboxyphenyl)-10,20-di(p-tolyl)porphyrin.
The BDP derivative, 1-(difluoroboryl)-2-[(Z)-(3,5-dimethyl-
2H-pyrrol-2-ylidene)(4-nitrophenyl)methyl]-3,5-dimethyl-1H-
pyrrole, was prepared by the reaction of 2,4-dimethylpyrrole
and nitrobenzaldehyde in the presence of trifluoroacetic acid in
CH2Cl2.19d The nitro group was subsequently reduced to the
amino group by the hydrogenation reaction, using Pd on
charcoal catalyst followed by purification by flash chromatog-
raphy on a silica gel column.
The fluorescence emission spectrum of the BDP-zinc por-
phyrin dyad, excited at 495 nm corresponding to BDP excitation,
is shown in Figure 1b along with the emission of control
compounds. An emission band at 515 nm corresponding to the
emission of BDP was observed in addition to the zinc porphyrin
emission bands at 614 and 662 nm (Figure 1b, plot i). The area
under the 515 nm band was found to be quenched over 93%
compared to BDP control (plot ii in Figure 1b). In a control
experiment, the emission spectrum of BDP control in the
presence of an equimolar amount of zinc porphyrin-crown ether
control was recorded. Under these conditions, the presence of
zinc porphyrin had no effect on the BDP emission intensity; in
addition, no significant emission corresponding to zinc porphyrin
Finally, the BDP-porphyrin-crown ether, H2-5-{4-[2-(4-
benzo-[18-crown-6])amido]phenyl}-15-{4-[2-(4-difluoroboron

8480 J. Phys. Chem. A, Vol. 113, No. 30, 2009
Maligaspe et al.
SCHEME 2: Synthetic Procedure Adopted To Form the BDP-Zinc Porphyrin-Crown Ether Host System in the
Present Study
was observed. In separate control experiments, when ZnP-crown
ether control and C60 ammonium cation were excited at 514
nm, no significant emission at 614 and 662 nm corresponding
to ZnP in the former case or at 720 nm corresponding to
fulleropyrrolidine in the latter case were observed (Figure 1b,
plots iii and iv). These results suggest the absence of simulta-
neous excitation of the different chromophores used to build
the triad in Scheme 1 when excited at 514 nm corresponding
to BDP excitation, an important criteria to visualize sequential
energy and electron transfer in “antenna-reaction center”
mimics.
Figure 1a, and in the excitation spectrum of the dyad in Figure
2. The peak intensity ratio from the absorption spectrum was
found to be 4.1 while for the excitation spectrum this ratio was
4.05 amounting to an energy transfer efficiency of about 97%.
These observations suggest excited energy transfer as the
exclusive quenching mechanism in the BDP-zinc porphyrin
dyad.
Formation of the Supramolecular BDP-ZnP-Fullerene
Triad
The occurrence of singlet excited energy transfer from BDP
to zinc porphyrin in the dyad was also confirmed by recording
the excitation spectrum of the dyad while holding the emission
monochromator at 662 nm corresponding to zinc porphyrin
emission. Such a spectrum shown in Figure 2 revealed the
excitation band of not only the zinc porphyrin entity but also
that of the BDP entity of the dyad indicating energy transfer
from BDP to zinc porphyrin. An estimation of energy transfer
was performed by calculating the intensity ratio of the BDP
band at 504 nm to the zinc porphyrin visible band at 559 nm
(So to S1 transition) in the absorption spectrum of the dyad in
The formation of the supramolecular triad was initially
followed by optical absorption and steady-state fluorescence
methods. Figure 3 shows the spectral changes observed during
the addition of C60 alkyl ammonium cation to the BDP-ZnP
crown ether dyad in benzonitrile. The absorption bands corre-
sponding to both the zinc porphyrin and BDP entities revealed
a slight increase in their intensity without any noticeable changes
in the peak position. The increase in absorption in the 350 nm
region is due to the presence of fulleropyrrolidine in solution.
Extending the absorption wavelength well into the near-IR
region revealed no new bands corresponding to any charge

Photosynthetic Antenna-Reaction Center Mimicry
J. Phys. Chem. A, Vol. 113, No. 30, 2009 8481
Figure 3. Absorption spectral changes observed during titration of
BDP-zinc porphyrin-crown ether (3.6 × 10-6 M) binding to C60
ammonium cation (2.4 × 10-4 M each addition) in benzonitrile.
Figure 1. (a) UV-visible spectra of (i) BDP-zinc porphyrin-crown
ether dyad (0.32 µM), (ii) C60 ammonium cation (0.12 mM), (iii) p-tolyl
boron dipyrrin (BDP control, 0.32 µM), and (iv) zinc porphyrin-crown
ether (0.32 µM) in benzonitrile. (b) Fluorescence spectra of (i) BDP
control (0.32 mM), (ii) BDP-zinc porphyrin-crown ether dyad (0.32
µM), (iii) zinc porphyrin-crown ether control (0.32 µM), and (iv) C60
ammonium cation (0.12 mM) in benzonitrile. The compounds were
excited at λex ) 495 nm.
Figure 4. (a) Fluorescence spectral changes observed during increasing
addition of C60 ammonium cation (0.24 mM each addition) to a solution
of BDP-zinc porphyrin-crown ether to form the triad in benzonitrile.
λex ) 495 nm. (b) Stern-Volmer plots constructed for (i) C60
ammonium cation and (ii) 2-phenyl fulleropyrrolidine quenching
of BDP-zinc porphyrin-crown ether in benzonitrile (emission
intensity of the 614 nm band of zinc porphyrin was monitored). (c)
Benesi-Hildebrand plot constructed by monitoring the emission
intensity at 614 nm band for calculating the binding constant. The
abbreviations Io and I represent fluorescence intensity in the absence
and presence of fullerene, and ∆I is the change of fluorescence
intensity upon addition of fullerene.
Figure 2. Excitation spectrum collected at 662 nm of the boron
dipyrrin-zinc porphyrin dyad in benzonitrile.
transfer complex. These observations suggest little, if any,
interactions between the fullerene and either the zinc porphyrin
or BDP entities. That is, a nonperturbed electronic structure of
the zinc porphyrin and BDP chromophores upon formation of
the triad via crown ether-alkyl ammonium binding is borne
out from this study.
The fluorescence spectrum recorded during the formation of
the supramolecular triad revealed drastic quenching effects. As
shown in Figure 4a, increasing addition of fulleropyrrolidine
to the BDP-zinc porphyrin dyad solution quenched the
fluorescence of not only the zinc porphyrin but also that of BDP
suggesting the occurrence of excited state events. In a control
experiment, the emission of BDP-zinc porphyrin dyad was
monitored by increasing the addition of 2-phenylfulleropyrrli-
dine, that is, fulleropyrrolidine bearing no alkyl ammonium
functionality. Stern-Volmer plots of zinc porphyrin quenching
monitored at 614 nm were constructed as shown in Figure 4b;
a higher slope was obtained for C60 alkyl ammonium cation
binding to the dyad. The calculated Stern-Volmer constant,
KSV, was found to be 1.06 × 105 M-1 for C60 alkyl ammonium
cation binding to the dyad, which compared with a value of
1.9 × 103 M-1 for 2-phenylfulleropyrrolidine binding to the
dyad. The higher KSV value for the former case unanimously
proves C60 alkyl ammonium binding to the crown ether induced
intramolecular quenching. The magnitude of KSV for 2-phenyl-

8482 J. Phys. Chem. A, Vol. 113, No. 30, 2009
Maligaspe et al.
Figure 5. B3LYP/3-21G(*) optimized structures of supramolecular triad comprised of boron dipyrrin-zinc porphyrin-crown ether bound to C60
alkyl ammonium cation in the close (a and b) and extended (c and d) forms.
fulleropyrrolidine quenching is slightly larger than that predicted
for a biomolecular quenching process.26 This could be rational-
ized by the weaker porphyrin-fullerene and fullerene-crown
ether interactions reported earlier.27 To calculate the binding
constant of crown ether-alkyl ammonium cation binding, the
quenching data were analyzed by using the Benesi-Hildebrand
method.28 Such analysis yielded a straight line plot indicating a
1:1 complex formation (Figure 4c) and a binding constant, K,
of 4.6 × 104 M-1. The magnitude of the binding constant is
comparable to the K values reported earlier for other crown
ether-alkyl ammonium binding23 suggesting stable complex
formation in the studied benzonitrile solvent.
Geometry Optimization by Computational Calculations.
The structure of the supramolecular triad was visualized by
performing computational studies at the B3LYP/3-21G(*)
level.29,30 Figure 5 shows the optimized structures in two
orientations of the triad in which two forms, viz., “close” and
“extended”, were visualized. Both of the structures were energe-
tically stable as they revealed minima on a Born-Oppenheimer
potential energy surface. The “close” form was energetically
Figure 6. Cyclic voltammograms of (a) C60 alkyl ammonium cation,
favored by ∼7.6 kcal/mol, which could be attributed to the
(b) p-tolyl boron dipyrrin (BDP control) (c) zinc porphyrin-crown
spontaneous attraction between porphyrin and fullerene31 and
ether, (d) BDP-zinc porphyrin-crown ether dyad, and (e) BDP-zinc
the flexible nature of the benzo-18-crown-6 entity. However,
this energy could be an overestimation due to a large Basis Set
Superposition Errors (BSSE) as a result of the low-level B3LYP/
3-21G(*) basis set.38
porphyrin-crown ether-fullerene triad in benzonitrile containing 0.1
(TBA)ClO4. Scan rate ) 0.1 V/s.
porphyrin macrocycles were in the same plane for both forms
of the structures, which may facilitate energy transfer due to
favorable macrocycle orientation.26
In the “close” form of the triad, the distance between the
boron atom of BDP and the zinc of ZnP was 18.77 Å while the
distance between the zinc and the center of C60 was found to
be 5.59 Å suggesting closely disposed donor-acceptor entities.
Similarly, the distance between boron and the center of C60 was
found to be 20.47 Å being sufficiently far from the electron
acceptor fullerene. In the “extended” form, the distance between
the boron atom of BDP and the zinc of ZnP was 18.68 Å while
the distance between the zinc and the center of C60 was found
to be 18.59 Å suggesting distantly disposed donor-acceptor
entities. The distance between boron and the center of C60 was
found to be 35.03 Å being far from the electron acceptor
fullerene. It is interesting to note that the boron dipyrrin and
Cyclic Voltammetry Studies and Estimation of Energy
Levels. Electrochemical studies using the cyclic voltammetric
technique were performed to arrive at the redox potentials of
the newly assembled supramolecular triad. Figure 6 shows cyclic
voltammograms of the investigated compounds 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 ) -1.03 and -1.45 V
vs. Fc/Fc+, respectively (Figure 6a).23a The peak-to-peak separa-
tion and plots of peak current versus square root of scan rate

Photosynthetic Antenna-Reaction Center Mimicry
J. Phys. Chem. A, Vol. 113, No. 30, 2009 8483
indicated both electroreductions to be one-electron reversible
processes.32 The one-electron reductions of the BDP control
were located at E1/2 ) -1.56 and -1.83 V vs. Fc/Fc+ while its
irreversible oxidations were located at Epa ) 0.46 and 0.61 V
vs. Fc/Fc+ (Figure 6b), respectively. The first two reversible
oxidations of the zinc porphyrin-crown ether were located at
E1/2 ) 0.11 and 0.29 V vs. Fc/Fc+ while the first irreversible
reduction was located at Epc ) -1.85 V vs. Fc/Fc+, respectively
(Figure 6c). As shown in Figure 6d, the first two reversible
oxidations of the BDP-zinc porphyrin crown ether dyad were
located at E1/2 ) 0.17 and 0.36 V vs. Fc/Fc+ while the first two
reversible reductions were located at E1/2 ) -1.61 and -1.71
V vs. Fc/Fc+, respectively. Control experiments involving
pristine BDP and zinc porphyrin indicated that both these
oxidations involve porphyrin macrocycle and not the BDP entity.
By comparison of the redox potentials of starting control
compounds in Figure 6a-c, the first reduction of the BDP-ZnP-
crown ether dyad at -1.61 V was ascribed to the reduction of
the BDP entity while the first oxidation at 0.17 was ascribed to
the zinc porphyrin entity, respectively.
Addition of the C60 alkyl ammonium cation to the BDP-ZnP-
crown ether dyad solution to form the supramolecular triad
revealed noticeable changes in the redox potentials. That is, the
first oxidation process of the zinc porphyrin revealed a slight
positive shift and the second oxidation a slight negative shift,
located at E1/2 ) 0.20 and 0.28 V vs. Fc/Fc+, respectively
(Figure 6e). The small anodic shift of zinc porphyrin upon the
formation of the triad suggests intramolecular interactions
between the fullerene and zinc porphyrin entities, a result that
readily agrees with the predictions of the earlier discussed
computational results. As discussed earlier, our attempts to locate
a new absorption band corresponding to charge transfer interac-
tions in the near-IR region were not successful. Perhaps the
extinction coefficient for such absorption band is too small to
be detected even at relatively higher concentrations. The first
two reductions of the supramolecular triad were located at E1/2
) -1.00 and -1.44 V vs. Fc/Fc+, and by comparison with the
redox potentials of the control compounds both processes were
assigned to the one-electron reduction of the C60 alkyl am-
monium cation. These values are close to that reported for
similar fulleropyrrolidine derivatives in benzonitrile.23 The third
reduction process of the triad located at -1.62 V corresponding
to BDP reduction revealed no significant changes as compared
to the corresponding reductions of the dyad.
Figure 7. Emission decays of BDP-ZnP dyad in benzonitrile at 620
and 520 nm. The excitation wavelength was 485 nm. These measure-
ments were carried out by using the TCSPC technique. The dashed
line is the instrument response function.
values for charge separation within the BDP-zinc porphyrin,
donor-acceptor dyad were also calculated. The ∆GCS and ∆GCR
originating from the 1ZnP*-BDP were found to be -0.41 and
-1.63 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.
Time-Resolved Emission and Transient Absorption
Studies
The photodynamics of the energy and charge transfer of the
newly developed dyad and triad was studied by pico- and
femtosecond optical spectroscopy techniques. The emission
decays measured by using the time-correlated single photon
counting (TCSPC) method are shown in Figure 7. The boron
dipyrrin-zinc porphyrin dyad in benzonitrile was excited at 485
nm. At this wavelength boron dipyrrin was mainly excited. The
emission decays were monitored at 520 and 620 nm corre-
sponding to the emission wavelengths of boron dipyrrin and
zinc porphyrin fluorescence, respectively. The data could be
fitted by using a three-exponential model. The major fast
component had a lifetime of approximately 60 ps, and appeared
as fast emission decay at 520 nm and concurrent emission
growth at 620 nm, respectively. This component can be
associated with the energy transfer from the singlet excited boron
dipyrrin to zinc porphyrin. However, the lifetime determined
by this method may not be very accurate as it is approaching
the time resolution of the instrument (65 ps, fwhm). The second
component with a lifetime of 1.6 ns was only observed at 620
nm corresponding to the fluorescence lifetime of zinc porphyrin.
In addition, a very weak longest lived component with a lifetime
of 2.8 ns was seen only at 520 nm attributed to BDP.
By using the electrochemical data, excited energies and
distances between the chromophores, the free energies of charge
separation (∆GCS), and charge recombination (∆GCR) were
calculated, using eqs 1 and 2, by Weller’s approach.33
∆GCR ) e(Eox-Ered) + ∆GS
(1)
A better time resolution was achieved by using the up-
conversion method for the emission decay measurements.
Unfortunately, for this method the only available excitation
wavelength was 400 nm, at which both the BDP and ZnP
chromophores have appreciable absorption. However, the
quenching of the BDP chromophore at 520 nm was clearly seen
as presented in Figure 8. The best fit of the data was achieved
by using a three-exponential model. The two fast decay
components had almost equal relative intensities, 40% and 54%,
with lifetimes of 9 and 40 ps, respectively. The third longer-
lived component was a minor one with contribution less than
6%. The two fast decay components indicate that two conform-
ers of the BDP-ZnP dyad coexist in solution with somewhat
different mutual orientations.
where ∆GS ) -e2/(4πε0εRRCt-Ct) and ε0 and εR refer to vacuum
permittivity and dielectric constant of benzonitrile.
-GCS ) ∆E0-0 - ∆GCR
(2)
where ∆E0-0 is the energy of the lowest excited state of ZnP
(2.05 eV).
Such calculations revealed a ∆GCS value of -0.82 eV for
electron transfer from the singlet excited zinc porphyrin to
fullerene, and a ∆GCR value of -1.18 eV for the charge
recombination process, for both “close” and “extended” forms
of the structures. For comparison purposes, the ∆GCS and ∆GCR

8484 J. Phys. Chem. A, Vol. 113, No. 30, 2009
Maligaspe et al.
Figure 8. Emission decay of the BDP-ZnP dyad monitored at 520
nm by the up-conversion method in benzonitrile. The solid line presents
the data fit (see text). Excitation wavelength was 400 nm.
Figure 10. Transient absorption time profile of BDP-ZnP dyad in
benzonitrile at 530 nm (squares). The data fit is shown by the solid
line. Excitation wavelength was 500 nm.
Figure 11. Time resolved transient absorption spectra of BDP-ZnP
dyad in benzonitrile. The delay times are indicated in the plot. The
excitation wavelength was 410 nm.
Figure 9. Transient absorption component spectra of BDP-ZnP dyad
in benzonitrile (lines with symbols), and time-resolved spectrum right
after excitation (dotted line). Excitation wavelength was 500 nm.
zero delay time (right after excitation) revealed characteristic
features of both BDP and ZnP singlet excited states. A dip in
the absorbance at 500 nm due to the bleaching of the BDP
chromophore ground state was observed indicating that some
fraction of the chromophore is in the excited state. The
appearance of the 460 nm peak and the bleaching at 560 and
600 nm are typical features of the porphyrin singlet excited state.
As the delay time increased, the dip at 500 nm disappeared and
the absorption at 460 nm increased indicating the energy transfer
from BDP to ZnP chromophore.
Upon addition of C60 to the solution to form the supramo-
lecular triad, the shapes of the spectra recorded after a few
picoseconds and longer delay times changed significantly, as
presented in Figure 12. The noteworthy difference is rather
strong absorption at the red part of the spectrum (670-750 nm),
which is characteristic for the porphyrin cation radical.29 At the
same time in the near-infrared part of the spectrum, the
absorption features were broad and rather featureless at the
beginning, but at longer delays a band at 1020 nm was noticed
(the spectrum at 800 ps in Figure 11), which is indicative of
the fullerene anion radical formation. This type of behavior can
be explained by a fast formation of the porphyrin-fullerene
exciplex, which then relaxes to the charge-separated state,
The transient absorption measurements with the use of
femtosecond pump-probe methods also confirmed the occur-
rence of efficient energy transfer from BDP to ZnP, as presented
in Figure 9. The experiments were carried out with an excitation
wavelength of 500 nm to populate the BDP singlet excited state
selectively, and analyzed by using a global biexponential fitting
of the data. The initial transient absorption spectrum of the dyad
(at 0 ps delay time, shown by the dotted line in Figure 9) had
characteristic features of the bleached ground state absorption
of the BDP chromophore. With a time constant close to 40 ps,
the transient absorption spectrum turned into one with charac-
teristic features of the porphyrin singlet excited state (the longest
lived component in Figure 9). The transition time correlates well
with the emission decay lifetime at 520 nm.
By applying a three-exponential fit to the transient absorption
measurements the fit quality was slightly improved and the
calculated lifetimes, 18 and 52 ps, matched well with those
obtained from up-conversion measurements. An example of fit
is presented in Figure 10 for the transient absorption measured
at 530 nm. At this wavelength the relative intensities of the
fast component were 26% and 74%, respectively.
The absorption maximum of ZnP is located at 431 nm, which
could be used for selective excitation of the porphyrin chro-
mophore. Unfortunately this wavelength was not available for
excitation for the pump-probe instrument used. The closest
available wavelength was 410 nm, at which some fraction of
BDP chromophore was also excited. The time-resolved transient
absorption spectra obtained with the 410 nm excitation for the
BDP-ZnP dyad are presented in Figure 11. The spectrum at
ZnP+•-C60-• 34 The formation of an exciplex is conceivable due
.
to close positioning of zinc porphyrin and fullerene entities in
the supramolecular triad as shown in Figure 5a,b.
To gather quantitative information on excited state electron
transfer, a global fitting of the transient absorption data was
employed. A four-exponential model gave a reasonably small

Photosynthetic Antenna-Reaction Center Mimicry
J. Phys. Chem. A, Vol. 113, No. 30, 2009 8485
Figure 14. Transient absorption component spectra of BDP-ZnP dyad
in the presence of C60 (0.6 mM) in benzonitrile obtained by nanosecond
laser flash photolysis.
Figure 12. Time resolved transient absorption spectra of BDP-ZnP
dyad upon addition of C60 (0.6 mM) in benzonitrile. The delay times
are indicated in the plot. The excitation wavelength was 410 nm.
Figure 15. Time profiles of the transient absorption bands of the
BDP-ZnP-C60 triad (0.6 mM) in benzonitrile at two selected
wavelengths.
Figure 13. Transient absorption decay component spectra of
BDP-ZnP dyad in the presence of C60 (0.6 mM) in benzonitrile.
The component lifetimes are indicated in the plot. The excitation
wavelength was 410 nm.
high, i.e., this state is practically unobservable. Hence the
transient spectra at a time delay of a few hundred picoseconds
had virtually the same shapes at excitation wavelengths of both
410 and 500 nm.
mean square deviation value (σ ) 0.001), and it was possible
to extract three lifetimes in the time scale of the measurements,
0.6, 20, and 700 ps, and a long lifetime extended to nanosecond
time domain. The spectra of the decay components are presented
in Figure 13. The fast component, 0.6 ps, is most probably due
to the internal conversion of the second singlet excited state of
ZnP to the first singlet excited state. The second component,
20 ps, showed recovery of the BDP ground state (negative peak
at 500 nm), but at the same time it had relatively strong negative
intensity in the red part of the spectrum and around 1000 nm,
which are characteristic for the formation of the charge-separated
state. It seems that two reactions are taking place in the same
time domain, that is, the energy transfer from BDP to ZnP and
the electron transfer from ZnP to C60. The former is associated
with relatively big spectral changes, as can be seen from Figure
9, and the latter has a rather moderate effect on the transient
spectra. Although the relative contribution of the excited BDP
chromophores was smaller than that of ZnP, the electron transfer
time constant from the directly excited ZnP to fullerene was
difficult to accurately determine since it interfered strongly with
the energy transfer time constant. However, the calculated time
constant, 20 ps, was shorter than that for “pure” energy transfer
in the BDP-ZnP dyad indicating that porphyrin-fullerene
charge separation was somewhat faster than the energy transfer.
The latter makes the determination of the charge separation time
constant even more difficult when the BDP chromophore was
The spectrum of the longest lived component in Figure 13
corresponds to the charge-separated state, BDP-ZnP+•-C60
.
-•
The lifetime of this state is much longer than the maximum
delay time of the pump-probe instrument used (∼1 ns). To
gather additional information on the CS state lifetime, nano-
second flash photolysis was performed. Since the sample was
prepared in an excess of fullerene, two states can be observed
in the microsecond time domain: the triplet excited state of
fullerene and the CS state. To distinguish between them the
transient absorption decays were measured for a number of
wavelengths and fitted globally. The results of the fit are
presented in Figure 14. The component with lifetime 0.67 µs
was attributed to the fullerene triplet state (notice that the
measurements were carried out with air saturated solution, and
the reference measurements of a pure fullerene sample had
virtually the same lifetime as the fullerene triplet state), and
the component with the 23 µs lifetime was attributed to the CS
state. The decay traces at two characteristic wavelengths are
presented in Figure 15. The lifetime of the CS state for the
present triad is comparable to the supramolecular triad involving
covalently linked ferrocene-zinc porphyrin-crown ether bind-
ing to the alkyl ammonium functionalized cation supramolecular
triad (lifetime of CS state was 6.2 µs)35 suggesting charge
stabilization to some extent.
excited (at 500 nm), since in the sequential series of reactions,
BDP*-ZnP-C60 f BDP-ZnP*-C60 f BDP-ZnP+-C60
,
Energy Level Diagram. The results of the present investiga-
tion along with all of the control experiments clearly show the
-•
population of the BDP-ZnP*-C60 intermediate state is never

8486 J. Phys. Chem. A, Vol. 113, No. 30, 2009
Maligaspe et al.
electrochemical studies. Computational studies revealed two
possible structures as extreme cases of the supramolecular triad.
Energy calculations revealed the possibility of photoinduced
electron transfer from singlet excited zinc porphyrin to fullerene.
Further time-resolved emission and transient absorption studies
demonstrated the occurrence of sequential energy transfer from
1
1BDP* to ZnP followed by electron transfer from ZnP* to
fullerene in the supramolecular triad. Nanosecond transient
absorption studies revealed the existence of long-lived charge-
separated species of ZnP+•-C60-•, suggesting that the BDP is
also involved in the charge stabilization process due to its facile
oxidation.
Experimental Section
Chemicals. Buckminsterfullerene, C60 (+99.95%), was from
SES Research (Houston, TX). Benzonitrile in a sure seal bottle,
sarcosine, 2,4-dimethylpyrrole, trifluoroacetic acid (TFA), dichlo-
rodicyanobenzoquinone (DDQ), and aldehydes were from
Aldrich Chemicals (Milwaukee, WI). Tetra-n-butylammonium
perchlorate, (TBA)ClO4, was from Fluka Chemicals. All the
chromatographic materials and solvents were procured from
Fisher Scientific and were used as received. Syntheses and
purification of alkyl ammonium functionalized fulleropyrroli-
dine, zinc porphyrin, and boron dipyrrin used in control
experiments were carried out according to the literature proce-
dures.23
Figure 16. Energy level diagram showing the different photochemical
events of the supramolecular C60-ZnP-BDP triad after excitation of
either the BDP or ZnP moieties. The thick arrows indicate the major
photochemical process and the thin arrows represent the minor
photochemical process.
Synthesis of Boron Dipyrrin-Zinc Porphyrin-Crown
Ether Dyad. 5-(4-Methoxycarbonylphenyl)dipyrromethane (1).
This compound was prepared according to the general procedure
outlined for the synthesis of dipyrromethanes by Lindsey et al.24
To a pyrrole (68 mL, 0.98 mol) was added methyl 4-formyl
benzoate (4 g, 0.024 mol) and the solution was purged with
argon for 15 min. Then trifluoroacetic acid (190 µL, 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 (30%). 1H NMR (CDCl3)
δ (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).
H2-5,15-Di(p-methoxycarbonylphenyl)-10,20-di(p-tolyl)por-
phyrin (2). This compound was prepared according to Luo et
al.25 A solution of 1 (1.6 g, 5.71 mmol) was added to
p-tolylaldehyde (675 µL, 5.71 mmol) in CHCl3, and the reaction
mixture was stirred under argon for 1 h. Then BF3 ·O(Et)2 (724
µL, 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 chromatography 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 with 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 (5%). 1H NMR (CDCl3) δ (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),
occurrence of efficient energy transfer followed by electron
transfer in the studied supramolecular triad. The combination
of optical absorption, steady-state fluorescence emission, mo-
lecular modeling, electrochemistry, and time-resolved emission
and transient absorption studies has permitted the extraction of
the needed energetic and kinetic information. The different
photochemical events observed in the triad are depicted in Figure
16 in 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.36 The driving forces for the occurrence of electron
transfer from the different excited chromophores of the triad
used were as discussed earlier. It is also important to note that
the difference in free energy change of charge separation for
the “close” and “extended” forms of the triad is considered to
be negligible. In the energy level diagram, the thick arrows
indicate the major processes and the thin arrows are minor
processes. On the basis of distance and energetic considerations,
an electron transfer from singlet excited BDP to C60 is
considered to be an inefficient process.
Conclusions
A novel supramolecular triad to mimic the “antenna-reaction
center” functionality of the photosynthetic reaction center has
been successfully constructed. The antenna mimic, a boron
dipyrrin entity, was covalently linked to the reaction center
electron donor mimic, a zinc porphyrin possessing a 18-crown-6
entity, as the opposite side of the macrocycle. Both steady-state
and time-resolved emission as well as transient absorption
studies decidedly proved the occurrence of efficient singlet-singlet
energy transfer from BDP to zinc porphyrin with the time scale
of 10-50 ps. Next, the reaction center electron acceptor mimic,
a fulleropyrrolidine appended with an alkyl ammonium chain,
was self-assembled via crown-ether-alkyl ammonium binding
to form the supramolecular triad. The structural integrity of the
supramolecular triad was derived by optical, computational, and

Photosynthetic Antenna-Reaction Center Mimicry
J. Phys. Chem. A, Vol. 113, No. 30, 2009 8487
2.7 (s, 6H, -CH3), -2.80 (s, br, 2H, -NH). ESI mass (in
CHCl3) calcd 758.8, found [M + 1] 759.9.
mg). 1H NMR (CDCl3) δ (ppm) 8.95-8.75 (m, 8H, ꢀ-pyrrole
H), 8.40-8.18 (m, 8H, phenyl H), 8.10 (d, 4H, phenyl H), 7.85
(d, 2H, BDP phenyl H), 7.56 (d, 4H, phenyl H), 7.35-7.28 (m,
1H, benzo-crown phenyl H), 7.12-7.05 (m, 1H, benzo-crown
phenyl H), 6.90 (d, 1H, benzo-crown phenyl H), 6.02 (s, 2H,
pyrrole H), 4.30-4.18 (m, 4H, crownethylene H), 4.00-3.90
(m, 4H, crownethylene H), 3.82-3.70 (m, 12H, crownethylene
H), 2.70 (s, 6H, phenyl CH3), 2.60 (s, 6H, pyrrole CH3), 1.65
(s, 6H, pyrrole CH3), -2.78 (s, br, 2H, -NH). ESI mass (in
CHCl3) calcd 1400.3 (with K+), found [M + 1] (with K+)
1401.2.
H2-5,15-Di(p-carboxyphenyl)-10,20-di(p-tolyl)porphyrin (3).25
A suspension of 2 (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 title compound was obtained
as a purple powder and used without further purification. Yield
85 mg (44%). 1H NMR (DMSO-d) δ (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),
-2.40-2.95 (s, br, 2H, -NH).
H2-5-{4-[2-(4-Benzo-[18-crown-6])amido]phenyl}-15-{4-[2-
(4-difluoroboron dipyrrinylphenyl)amido]phenyl}-10,20-di(p-
tolyl)porphyrinato Zinc(II) (7). The free-base porphyrin 6 (20
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 515 nm band of
free-base porphyrin had disappeared.36 At the end, the reaction
mixture was washed with water and dried over anhydrous
Na2SO4. Chromatography on silica gel column by using hexanes:
CHCl3 (10:90 v/v) gave the title compound. Yield 20 mg (90%).
1H NMR (CDCl3) δ (ppm) 9.00-8.80 (m, 8H, ꢀ-pyrrole H),
8.43-8.21 (m, 8H, phenyl H), 8.12 (d, 4H, phenyl H), 7.88 (d,
2H, BDP phenyl H), 7.58 (d, 4H, phenyl H), 7.35-7.28 (m,
1H, benzo-crown phenyl H), 7.07-7.00 (m, 1H, benzo-crown
phenyl H), 6.85 (d, 1H, benzo-crown phenyl H), 6.01 (s, 2H,
pyrrole H), 4.28-4.15 (m, 4H, crownethylene H), 4.38-3.87
(m, 4H, crownethylene H), 3.79-3.76 (m, 12H, crownethylene
H), 2.70 (s, 6H, phenyl CH3), 2.62 (s, 6H, pyrrole CH3), 1.69
(s, 6H, pyrrole CH3). ESI mass (in CHCl3) calcd 1463.7 (with
K+), found [M + 1] (with K+) 1464.6.
1-(Difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-ylidene)(4-
nitrophenyl)methyl]-3,5-dimethyl-1H-pyrrole (4).19d 2,4-Dim-
ethylpyrrole (1.08 mL, 10.5 mmol) and 4-nitrobenzaldehyde
(0.93 g, 6.2 mmol) were added to 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 redissolved in methylene
chloride, and p-chloranil (1.36 g, 5.5 mmol) was added to the
mixture. The resulting mixture was stirred for 10 min, then
triethylamine (4 mL) and born trifluoride etherate (3.5 mL) were
added to the mixture. After 1 h of stirring, the resulting solution
was poured into water and the compound was extracted with
methylene chloride. 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 of the
desired product. 1H NMR (CDCl3) δ (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.
1-(Difluoroboryl)-2-[(Z)-(3,5-dimethyl-2H-pyrrol-2-ylidene)(4-
aminophenyl)methyl]-3,5-dimethyl-1H-pyrrole (5). A solution
of 4 (100 mg) in THF (40 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
toluene as eluent. Evaporation of the solvent yielded 36 mg of
5, 40%. 1H NMR(CDCl3) δ (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-Benzo-[18-crown-6])amido]phenyl}-15-{4-[2-
(4-difluoroboron dipyrrinylphenyl)amido]phenyl}-10,20-di(p-
tolyl)porphyrin (6). Compound 3 (40 mg, 0.05 mmol) was taken
in dry toluene (20 mL) then thionyl chloride (80 µL, 1.1 mmol)
and pyridine (80 µL) 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 (210 µL) was added
followed by 5 (18 mg, 0.05 mmol) and 4′-amino benzo-[18-
crown-6] (18 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 (10:90 v/v).
Evaporation of the solvent yielded the desired compound (24
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
1
method 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 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 with argon gas. The
computational calculations were performed by ab initio B3LYP/
3-21G(*) methods with the Gaussian 9829 software package on
high-speed PCs. The ESI-mass spectral analyses of the newly
synthesized compounds were performed by using a Fennigan
LCQ-Deca mass spectrometer. For this, the compounds (about
0.1 mM concentration) were prepared in CH2Cl2, freshly distilled
over calcium hydride.
Time-Resolved Fluorescence and Transient Absorption
Measurements.37 Fluorescence decays of the samples in the
nanosecond and subnanosecond time scales were measured by
using a time-correlated single photon counting (TCSPC) system
(PicoQuant GmBH) consisting of PicoHarp 300 controller and
PDL 800-B driver. The samples were excited with the pulsed
diode laser head LDH-P-C-485 at 485 nm and fluorescence
decays were measured at the wavelengths of emission maxima.
The signals were detected with a microchannel plate photo-
multiplier tube (Hamamatsu R2809U). The time resolution of
the TCSPC measurements was about 60-70 ps (fwhm of the
instrument response function).

8488 J. Phys. Chem. A, Vol. 113, No. 30, 2009
Maligaspe et al.
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time-resolved fluorescence was used to detect the fast processes
with a time resolution of ∼200 fs. The primary Ti:sapphire
generator (TiF-50, CDP Corp.) was pumped by Nd CW laser
(Verdi-6, Coherent Inc.), and a second harmonic (∼410 nm)
was used to excite the sample solution in a rotating cuvette.
Emission from the sample was collected to a nonlinear crystal
(NLC), where it was mixed with the so-called gate pulse, which
was the laser fundamental. The signal was measured at a sum
frequency of the gate pulse and the selected emission maximum
of the sample. The gate pulses were passed through a delay
line so that it arrived at NLC at a desired time after sample
excitation. By scanning through the delay line the emission
decay curve of the sample was detected.
Pump-probe and up-conversion techniques for time-resolved
absorption and fluorescence, respectively, were used to detect
the fast processes with a time resolution shorter than 0.2 ps.
The instrument and the data analysis procedure used have been
described earlier.37
Acknowledgment. The authors are thankful to National
Science Foundation (Grant 0804015 to FD) and the Academy
of Finland for support of this work.
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