Temperature dependence of charge separation and recombination in porphyrin oligomer-fullerene donor-acceptor systems

Article abstract of DOI:10.1021/ja2019367

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of ch

Full text of DOI:10.1021/ja2019367

ARTICLE
pubs.acs.org/JACS
Temperature Dependence of Charge Separation and Recombination
in Porphyrin OligomerꢀFullerene DonorꢀAcceptor Systems
Axel Kahnt,† Joakim K€arnbratt,† Louisa J. Esdaile,‡ Marie Hutin,‡ Katsutoshi Sawada,‡ Harry L. Anderson,‡
and Bo Albinsson†,*
†Physical Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, Kemiv€agen 3,
412 96 G€oteborg, Sweden
‡Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.
S Supporting Information
b
ABSTRACT: Electron-transfer reactions are fundamental to
many practical devices, but because of their complexity, it is often
very difficult to interpret measurements done on the complete
device. Therefore, studies of model systems are crucial. Here the
rates of charge separation and recombination in donorꢀacceptor
systems consisting of a series of butadiyne-linked porphyrin
oligomers (n = 1ꢀ4, 6) appended to C60 were investigated. At
room temperature, excitation of the porphyrin oligomer led to
fast (5ꢀ25 ps) electron transfer to C60 followed by slower (200ꢀ650 ps) recombination. The temperature dependence of the
charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge
separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed
fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the
longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of
temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer
reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the
reorganization energy (λ). For the charge-separation rate, all of the donorꢀacceptor systems could be successfully described by a
common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the
C60-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction
solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.
’ INTRODUCTION
Here we present a study of the temperature dependence of the
charge-separation and charge-recombination processes in an
electron donorꢀacceptor system based on a series of zinc
porphyrin oligomers (Pn, n = 1ꢀ4, 6) as electron donors and
an appended fullerene as the electron acceptor (Figure 1). The
butadiyne links between the porphyrin moieties in the oligomer
chain allow for almost barrierless rotation of the individual
porphyrins, giving rise to numerous different conformations that
vary with respect to their porphyrinꢀporphyrin dihedral angles.
It has previously been shown that these conformations greatly
influence the photophysical properties displayed by the system
and that the observed absorption spectrum is an average of those
for the different conformations.40 A recent study performed on
the fullerene-appended porphyrin dimer P2ꢀC60 showed how
selective excitation of twisted or planar conformations could be
used to control the rate of electron transfer.41 It was concluded
that charge separation from twisted conformers of P2ꢀC60
resulted in an electron-transfer rate that was 4 times higher than
for planar conformers. The main reason for this increase was
The production of artificial photosynthetic systems to power
solar fuel production is an interesting and promising idea, and to
accomplish it, we must rely on molecular and supramolecular
assemblies.1 In natural photosynthesis, cascades of short-range
energy-transfer and electron-transfer processes between well-
arranged chromophores occur,2ꢀ7 and a large variety of super-
and supramolecular systems have been studied in order to mimic
the natural process. In particular, those systems have been based
on chlorophylls,8,9 porphyrins,10ꢀ16 or phthalocyanines17ꢀ22
as the electron donor and single-walled carbon nanotubes
(SWCNTs),23ꢀ26 fullerenes,27ꢀ30 or graphene31,32 as the elec-
tron acceptor. A few systems employing a multiple porphyrin unit
as a photoactive electron donor have also been published.33ꢀ35 All of
these studies were performed at room temperature, but probing
the temperature dependence of electron-transfer reactions can
give additional information concerning the their mechanism.
Because of the experimental difficulties associated with perform-
ing temperature-dependent pumpꢀprobe experiments, only a
few studies of the temperature dependence of fast electron-
transfer reactions using femtosecond transient absorption spec-
troscopy have been reported to date.36ꢀ39
Received: March 3, 2011
Published: May 19, 2011
r
2011 American Chemical Society
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of the charge separation and recombination, from which reason-
able electron-transfer parameters (electronic couplings and reor-
ganization energies) were extracted.
’ EXPERIMENTAL SECTION
Materials. Unless otherwise specified, all of the measurements were
made in freshly distilled 2-methyltetrahydrofuran (2MTHF, Sigma-
Aldrich) with 1% pyridine (Sigma-Aldrich) added to avoid aggregation
of the porphyrin oligomers. The synthesis of compounds P1ꢀC60 and
P2ꢀC60 has been reported previously.44 The synthesis of compounds
P3ꢀC60, P4ꢀC60, and P6ꢀC60 is presented in the Supporting Informa-
tion (SI).
Temperature Studies. For the temperature-dependent ground-
state absorption, transient absorption, steady-state emission, and time-
resolved emission studies, a temperature-controlled cryostat (Oxford
Instruments) cooled with liquid nitrogen was used.
Steady-State Absorption and Emission Spectroscopy.
Absorption spectra were measured on a Cary 5000 UVꢀvisꢀNIR spectro-
meter. The spectra were recorded between 350 and 900 nm at 600 nm/min
with a 0.5 nm spectral bandwidth. The sample was contained in a 10 mm
quartz cuvette. Fully corrected emission spectra were recorded on a Spex
Fluorolog 3 spectrofluorometer equipped with a xenon lamp. To avoid
aggregation and inner-filter effects, the absorption was adjusted to
0.05 at the excitation wavelength.
Figure 1. Molecular structure of the porphyrin oligomer-based do-
norꢀacceptor system studied in this work. The aryl substituents (Ar) are
3,5-bis(octyloxy)phenyl groups.
Transient Absorption Spectroscopy. A pumpꢀprobe setup
was used to record transient absorption spectra. A Ti:sapphire oscillator
(Tsunami, Spectra Physics) generating pulses with a width of ∼90 fs
(fwhm) was used to seed a Ti:sapphire regenerative amplifier (Spitfire,
Spectra Physics) that was pumped by a frequency-doubled diode-
pumped Nd:YLF laser (Evolution-X, Spectra Physics) and produced
pulses of ∼110 fs duration (fwhm) at a repetition rate of 1 kHz. The
amplified laser beam (800 nm) was divided by a beamsplitter, and
the two beams were subsequently used as the pump and probe beams.
The pump beam was manipulated by an optical parametric amplifier
(OPA) (TOPAS, Light Conversion Ltd.) to yield a wavelength of
495 nm (450 nm for P1ꢀC60) and was delayed relative to the probe
pulse with an optical delay stage (range 0ꢀ1.6 ns). The probe beam was
obtained by focusing the remaining IR light on a 3 mm sapphire plate,
which generated a white-light continuum in the region 500ꢀ1000 nm.
Alternatively, to generate probe light between 980 and 1040 nm, a
TOPAS White OPA (Light Conversion Ltd.) was used. The probe light
was subsequently divided into reference and probe beams, and the latter
of these was overlapped by the pump at the sample. The probe and
reference beams were focused on the entrance slit of a spectrograph and
detected by a charge-coupled device (CCD) camera (iXon-Andor)
operating synchronously with the 1 kHz laser. The transient spectra
were obtained from the difference of the intensities of the probe light
(divided by the intensity of the reference) with and without excitation of
the sample by the pump beam. Typically 1000 spectra were averaged per
delay time.
The transient absorption decays were generally fitted by a triexpo-
nential decay function with an offset: I(t) = R1 exp(ꢀt/τ1) þ R2
exp(ꢀt/τ2) þ R3 exp(ꢀt/τ3) þ I0. For every compound at every
temperature, two time profiles were fitted: one from the visible part of
the spectrum, representing the Q-band bleaching, and one at 1015 nm,
where mostly the radical species C60•ꢀ and Pn•þ dominate the transient
absorption. In the fitting procedure, the two shortest lifetimes (τ1 and
τ2) were fixed to the corresponding lifetimes of the first singlet excited
state, which were obtained from the fluorescence lifetime measurements
(related to charge separation). The two transient absorption decays were
fitted individually with the free parameters R1, R2, R3, τ3, and I0 until a
reasonable fit was obtained. For P1ꢀC60, a slightly different procedure
was employed. Since with our laser system it was not possible to obtain
associated with a stronger electronic coupling to the C60 in the
twisted components. This might be because the excitation energy
was localized closer to the fullerene in the twisted components
than in the planar ones, where the energy is delocalized over both
porphyrins.
In the present work, we show that excitation of twisted
conformers in longer oligomers can have the opposite effect on
the electron-transfer rate, as the excitation energy can be
localized on a segment of the oligomer that is far away from
the electron acceptor. From there, it first must migrate closer to
the fullerene before charge separation can occur. Similar behavior
has been observed in natural photosynthesis and bulk-hetero-
junction solar cells. In photosynthetic organisms, excitation can
occur at any of the absorbing chromophores, from which the
excitation energy is funneled to the reaction center through
consecutive energy-transfer reactions before charge separation
can occur.42 Likewise, in bulk-heterojunction (polymer) solar
cells, excitation can occur anywhere along the conjugated poly-
mer chain, and the excitation energy migrates along the polymer
backbone until it reaches a phase boundary, where charge
separation occurs.43 The system presented here can be seen as a
simple model for these important practical charge-separation devices.
The charge separation and recombination were studied using
both steady-state and time-resolved emission as well as femto-
second transient absorption in order to probe how these processes
depend on the oligomer length. The measurements showed a
biexponential behavior of the charge-separation process for all of
the oligomers larger than the dimer. To explain this, we have
proposed a model wherein both active conformers (which
undergo rapid charge separation after excitation) and inactive
conformers are excited. The latter must first relax into the active
state before charge separation can occur, resulting in delayed charge
separation from these conformers. From this model, quantum yields
for the charge-separation process were extracted and compared to
the values obtained from the quenching of the steady-state fluores-
cence. In addition, we also explored the temperature dependence
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Table 1. Quenching Efficiencies for the PnꢀC60 Compounds
a
Relative to Pn
T/K
P1ꢀC60
P2ꢀC60
P3ꢀC60
P4ꢀC60
P6ꢀC60
140
160
180
200
220
240
260
280
300
0.993
0.995
0.994
0.993
0.993
0.993
0.992
0.992
0.990
0.965
0.973
0.972
0.973
0.975
0.976
0.976
0.975
0.970
0.858
0.818
0.842
0.868
0.898
0.905
0.909
0.910
0.858
0.884
0.834
0.688
0.729
0.767
0.799
0.819
0.835
0.840
0.782
0.770
0.438
0.480
0.519
0.555
0.585
0.602
0.610
Figure 2. Ground-state absorption spectra of Pn (black) and PnꢀC60
(red) (n = 1ꢀ4, 6) at room temperature. Each spectrum has been
normalized and offset by (n ꢀ 1) ꢁ 0.25 absorbance units.
a Values at 300 K were calculated as 1 ꢀ If(PnꢀC60)/If(Pn), where the
If’s are the integrated fluorescence intensities from samples of PnꢀC60
and Pn with equal absorbances at the excitation wavelength. At lower
temperatures, the integrated intensities were related to the intensity
recorded at 300 K.
450 nm pump light and generate probe light at ∼1015 nm in parallel
efficiently, only the recovery of the Q-band bleaching was used to
determine the lifetime of the charge-separated state. Furthermore, since
the fluorescence decay for P1ꢀC60 was monoexponential, the transient
absorption decay could be successfully modeled using a biexponential
decay function. The fitting parameters R1, R2, and τ2 were free, whereas
τ1 was fixed to the corresponding fluorescence lifetime.
Time-Resolved Fluorescence Spectroscopy. Time-resolved
emission measurements were performed using a streak camera system.
The excitation pulse was provided by a Tsunami Ti:sapphire laser
(Spectra-Physics) that was pumped by a Millennia Pro X laser
(Spectra-Physics). The Tsunami output was tuned to either 990
or 900 nm and subsequently frequency-doubled to 495 (450 nm for
P1ꢀC60). The emitted photons were passed through a spectrograph
(Acton SP2300, Princeton Instruments) and finally registered by a
streak camera (C5680, Hamamatsu) with a synchroscan unit (M5675,
Hamamatsu). Every single frame was measured and stored individually,
and the time-resolved fluorescence spectra were obtained from these
frames after jitter correction. From these fluorescence spectra, time
profiles were extracted by averaging over a selected wavelength region.
The fluorescence lifetime decays were fitted with biexponential func-
tions through deconvolution of the instrument response function.
Figure 3. Fluorescence spectra of P3ꢀC60 measured at different
temperatures.
observed for all systems. The Q-band is red-shifted from 640
to 800 nm and gains intensity as the oligomers get longer.
The influence of temperature on the absorption spectra was
investigated for all of the PnꢀC60 systems, and only minor
changes in the shape of the UVꢀvis spectra were observed over
the temperature range 300ꢀ170 K. Below 170 K, the formation
of aggregates was observed, as indicated by a sharp absorption
band that appeared in the narrow temperature range between
170 and 140 K and was red-shifted relative to the “normal”
Q-band absorption. This formation of aggregates was in parti-
cular observed for the systems containing more than two
porphyrin units (Figures S6.1ꢀS6.5 in the SI), and studies of
this phenomenon will be reported in detail elsewhere.45 In
the following, measurements at temperatures where aggregates
were expected to form (below 170 K) are presented, but all of the
analyses were restricted to temperatures above 170 K to ensure
monomolecular behavior.
Charge Separation. Steady-state fluorescence measurements
provided information on the overall quenching of the PnꢀC60
complexes. Upon excitation at 495 nm (450 nm for P1 and
P1ꢀC60), all of the studied donorꢀacceptor systems showed
efficient quenching due to electron transfer from excited Pn to
C60.41 Table 1 shows the calculated quenching efficiencies for the
whole series of molecules at temperatures between 140 and 300
K. Except for the two lowest temperatures, where aggregation
occurred, the quenching efficiencies varied systematically with
’ RESULTS AND DISCUSSION
Photophysical Studies. The electronic ground-state absorp-
tion spectra of the donorꢀacceptor complexes PnꢀC60 are
essentially identical with the sum of the corresponding Pn and
C60 reference components (Figure 2). The slight red shift in the
absorption spectra of the fullerene-terminated oligomers is due
to conjugation of the porphyrins with the p-phenylene linking
unit. All of the compounds show strong Soret-band absorptions
with maxima at 445 nm (P1ꢀC60), 460 nm (P2ꢀC60), 464 nm
(P3ꢀC60), 465 nm (P4ꢀC60), and 466 nm (P6ꢀC60) flanked by
minor shoulders at ∼495 nm for those compounds containing
more than one zinc porphyrin unit. From previous studies it is
known that for P2ꢀC60, the absorption at ∼495 nm belongs to
the planar conformer (i.e., the conformer in which the porphyrin
units are coplanar), whereas the band at 460 nm belongs to a
wide distribution of conformers.40,41 This feature allows for
nearly exclusive excitation of a planar oligomer in P2 (at
495 nm), but for the longer systems, an increasing number of
other conformers are excited at 495 nm because the oligomer
contains more subunits. In addition to the Soret-band absorp-
tion, sets of Q-bands between 550 and 850 nm (Figure 2) were
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Table 2. Temperature Dependence of the Fitted Lifetimes for
P2ꢀC60, P3ꢀC60, P4ꢀC60, and P6ꢀC60 Excited at 495 nm and
P1ꢀC60 Excited at 450 nma
P1ꢀC60
τ1/ps
P2ꢀC60
P3ꢀC60
P4ꢀC60
P6ꢀC60
T/K
τ1/ps τ2/ps τ1/ps τ2/ps τ1/ps τ2/ps τ1/ps τ2/ps
140
160
180
200
220
240
260
280
300
6.4
6.2
5.9
5.4
5.2
5.0
4.5
4.4
4.4
37
32
29
23
22
21
20
19
19
337
311
332
329
361
352
363
445
489
88
58
51
42
35
29
25
23
20
536 158 660 269 707
438 101 503 167 650
306
310
203
194
181
160
119
97 447 102 573
74 376
52 285
46 254
31 215
29 212
24 216
76 519
57 471
48 402
36 337
32 300
26 285
a The normalized pre-exponential factors are listed in Table S8.1 in the SI.
(τ1) was slightly temperature-dependent and made a larger
contribution for the shorter oligomer systems than for the longer
ones. The second, longer lifetime (τ2) also decreased with
increasing temperature for the compounds P3ꢀC60, P4ꢀC60,
and P6ꢀC60, whereas for P2ꢀC60, no clear trend was seen
(Table 2). Since the longer lifetime was significantly shorter than
the intrinsic fluorescence lifetime for the corresponding Pn
reference system (1450, 1210, 1100, 830, and 650 ps for n = 1ꢀ4
and 6, respectively) and the corresponding pre-exponential
factor was on the same order of magnitude or even larger than
that for the shorter lifetime, the longer-lived component cannot
simply be explained as the result of impurities in free Pn.
Figure 4. (top) Fluorescence decays for the PnꢀC60 compounds at
280 K (P1ꢀC60, black; P2ꢀC60, red; P3ꢀC60, green; P4ꢀC60, blue;
P6ꢀC60, cyan). (bottom) Fluorescence decays for P3ꢀC60 at different
temperatures.
both the size of the oligomer and the temperature. The tem-
perature-dependent emission spectra for P3ꢀC60 are shown in
Figure 3, and those for the other complexes are presented
in Figures S7.1ꢀS7.4 in the SI. While the shapes of the emission
spectra for P1ꢀC60 and P2ꢀC60 were unaffected by the tem-
perature change (above 170 K), the corresponding spectra for
the longer oligomers exhibited blue shifts, indicative of emission
from an increasing number of nonrelaxed conformers as the
temperature was lowered. This shows that 495 nm excitation of
the longer oligomers produces a mixture of excited conformers,
some of which are less prone to undergo charge separation. This
behavior was even more evident when the charge-separation
process was studied using time-resolved emission.
Measurements of temperature-dependent time-resolved fluor-
escence and transient absorption based on picosecond and
femtosecond laser excitation, respectively, helped provide a more
detailed understanding of the electron-transfer processes. Fluor-
escence decays give the most accurate measurement of the rate
constant for charge separation, since the signal is directly related
to the quenching process. Except in the case of P1ꢀC60, all of the
decays were biexponential at all temperatures (Figure 4 and
Figures S8.1ꢀS8.4 in the SI). In general, as shown in Figure 4, the
average fluorescence lifetime increased with the length of the
porphyrin chain and decreased with increasing temperature.
Table 2 presents the lifetimes obtained from fits to the biexpo-
nential expression I(t) = R1 exp(ꢀt/τ1) þ R2 exp(ꢀt/τ2), in
which both the pre-exponential factors (R1 and R2) and the
lifetimes (τ1 and τ2) were optimized. The normalized pre-
exponential factors were virtually temperature-independent,
and their magnitudes scaled with the oligomer length (the
average values of R1 were 0.80 for P2ꢀC60, 0.65 for P3ꢀC60,
0.51 for P4ꢀC60, and 0.27 for P6ꢀC60) The first, shorter lifetime
Model for Charge Separation. The biexponential fluorescence
decay observed for all of the PnꢀC60 compounds with n g 2
could be due to several factors, but here we will present a simple
model that successfully explains both the oligomer-length de-
pendence and the temperature dependence observed across the
whole series of molecules. The fraction of the long-lived decay is
larger for the longer oligomers and increases as temperature
is lowered (see Figure 4). We know that the porphyrin oligomers
have almost free rotation around the butadiyne bridges and that
in the ground state all of the conformations are populated even at
quite low temperatures. In the case of the longer oligomers, the
more twisted conformers dominate the mixture for statistical
reasons. In the excited state, the planar structure is stabilized, and
if solvent viscosity and/or temperature allow, the excited oligo-
mer planarizes. This process takes ∼100 ps in 2MTHF at room
temperature for a single butadiyne bridge (i.e., in P2).40 Now,
upon excitation of PnꢀC60, it is possible to localize the excitation
energy in one part of the oligomer, as was demonstrated
previously for P2.41 The degree of localization depends on the
excitation wavelength or, equivalently, different sets of confor-
mers are selected by tuning the excitation energy. In the
experiments presented here, the longest-wavelength peak of
the Soret band (495 nm) was selected for excitation. This excites
predominantly the planar conformer of P2, but for all of the
longer oligomers, this excitation wavelength gives rise to a
mixture of conformers, with longer oligomers having broader
distributions of excited conformations. Since planarization fol-
lows excitation, a thermally activated process occurs in which
excitation energy moves from being localized in one part of the
oligomer to becoming more delocalized. In the PnꢀC60 com-
pounds, where a quencher is attached to one end of the oligomer,
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Journal of the American Chemical Society
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Figure 6. Plots of ln(kCST1/2) vs 1/T for P1ꢀC60 (black), P2ꢀC60
(red), P3ꢀC60 (green), P4ꢀC60 (blue), and P6ꢀC60 (cyan). The lines
are the individual fits to the linearized form of eq 2.
Figure 5. Simplified Jablonski diagram showing excitation migration
followed by charge separation. Rate constants and symbols are explained
in the text.
Table 3. (top) Driving forces (ΔG°),a Reorganization En-
ergies (λ), and Electronic Coupling Values (V) for Charge
Separation in PnꢀC60 Obtained from Individual Fits of the
Curves in Figure 6; (bottom) Reorganization Energies and
Electronic Coupling Values for Charge Separation in PnꢀC60
Obtained by Treating the Electronic Coupling as a Global
Parameter
it is conceivable that a conformer wherein the excitation energy is
localized close to the quencher could undergo rapid electron
transfer, whereas another conformer with the excitation energy
localized far away from the quencher would first need to transfer
its energy to the “active” part of the oligomer before charge
separation could occur. In a long oligomer, this could potentially
become a very complex process, but in the suggested model
shown in Figure 5, we simply divide the molecule into two
excited-state populations, an inactive one (Pn*) that is excited
P1ꢀC60
P2ꢀC60
P3ꢀC60
P4ꢀC60
P6ꢀC60
#
“far away” from the quencher and an active one (Pn ) that is
ΔG°/eVa
λ/eV
V/cmꢀ1
ꢀ0.58
0.85
44
ꢀ0.38
0.63
21
ꢀ0.31
0.66
31
ꢀ0.28
0.73
43
ꢀ0.25
0.68
39
excited “close” to the quencher. This simplified model was
chosen because a more rigorous treatment of the data involving
additional excited-state populations was not needed to fit the
measured decays.
P1ꢀC60
P2ꢀC60
P3ꢀC60
P4ꢀC60
P6ꢀC60
The kinetic model (Figure 5) is a simple mixture of a con-
secutive first-order reaction and a first-order quenching. The
former contribution stems from the “inactive” population and
has a normalized contribution of 1 ꢀ f, where f is the fraction of
the directly excited “active” population, which undergoes first-
order (monoexponential) quenching. Mathematically, this leads
to the normalized biexponential fluorescence decay function
shown in eq 1:
λ/eV
V/cmꢀ1
0.78
0.74
0.69
0.69
0.66
34
34
34
34
34
a ΔG° values for P1ꢀC60, P2ꢀC60, and P4ꢀC60 were taken from ref 44.
The values for P3ꢀC60 and P6ꢀC60 were obtained by interpolation and
extrapolation of a plot of ΔG° vs 1/n, where n is the number of
porphyrin units.
case corresponds to a donor wherein the excitation energy is fully
delocalized over the whole chromophore.
The charge-sepaꢀra1tion rate constant is thus related to the short
lifetime (kCS = τ1 ꢀ k), and its temperature dependence is
expected to follow the Marcus equation (eq 2):
þ kÞt
IðtÞ¼ ð1 ꢀ fÞeꢀðk
1
ꢀ
ꢁ
k1
k1
þ kÞt
þ kÞt
eꢀðk
þ kÞt
1
CS
CS
þ ð1 ꢀ fÞ
eꢀðk
þ
þ feꢀðk
k1 ꢀ kCS
kCS ꢀ k1
"
#
2k1 ꢀ kCS
k1 ꢀ kCS
rffiffiffiffiffiffiffiffiffiffiffiffiffiffi
þ kÞt
eꢀðk
2
1
¼ ð1 ꢀ fÞ
π
ðΔG° þ λÞ
4λkBT
2
kCS
¼
jVj exp ꢀ
ð2Þ
p2λkBT
ð1Þ
ꢀ
ꢁ
k1
þ kÞt
þ f eꢀðk
CS
þ ð1 ꢀ fÞ
Equation 2 shows that the rate constant for electron transfer is
related to several parameters, such as the electronic coupling (V),
the reorganization energy accompanying the redistribution of
charge (λ), and the thermodynamic driving force for the
electron-transfer process (ΔG°). This equation can be rewritten
to show that a plot of ln(kCST1/2) versus 1/T should be linear;
the electronic coupling V can be obtained from the intercept of
this plot, and the slope gives the reorganization energy λ
provided that the driving force ΔG° is known. The measured
charge-separation rate constants were plotted in this way
(Figure 6), and the fits to eq 2 were excellent for all of the
oligomers. The two lowest temperatures were excluded from the
fit because of the potential for aggregation (see above), although
equally good fits were obtained even when these points were
included. The data at the lowest temperatures were excluded to
kCS ꢀ k1
where k is the rate constant for unquenched decay of the
porphyrin oligomer (assumed to be the same for the active and
inactive populations), k1 is the rate constant for transformation of
the inactive (*) excited state into the active (#) excited state, and
kCS is the charge-separation rate constant (Figure 5). As shown in
eq 1, the pre-exponential factors in a biexponential fit of the
decays are related to the excited fractions. However, it is only
when no transformation between the inactive and active excited
states occurs (i.e., when kCS . k1) that the pre-exponential
factors directly reflect the excited fractions. Conversely, in a
situation where this transformation is much faster than the
quenching (k1 . kCS) and faster than the time resolution of
the experiment, a monoexponential decay is expected. This latter
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Figure 7. Quantum yields for the formation of the charge-separated
state based on the steady-state (9) and time-resolved (b) fluorescence
measurements for P1ꢀC60 (black), P2ꢀC60 (red), P3ꢀC60 (green),
P4ꢀC60 (blue), and P6ꢀC60 (cyan) at different temperatures. The solid
lines between filled squares are guides to the eye.
ensure that all of the measurements were made on homogeneous
samples. The optimized values of the λ and V are found in
Table 3. Consistent with our model for charge separation, it was
found that the electronic coupling is on the same order of
magnitude (20ꢀ40 cmꢀ1) for all oligomer lengths. In fact, it
was possible to get a reasonable fit of the data by treating V as a
common (global) parameter in the fit to all of the oligomers
(Figure S8.5 in the SI). This yielded V = 34 cmꢀ1, which is a value
consistent with charge separation over short distances. The edge-
to-edge distance between the meso carbon of the porphyrin
closest to the appended fullerene and the covalent attachment
point of the fullerene is ∼9 Å.46
Figure 8. (top) Femtosecond transient absorption spectra of P3ꢀC60
at 200 K for a delay time of 75 ps. The NIR part of the spectrum was
collected with probe light from an OPA and the visible part with a white-
light continuum. (bottom) Transient absorption decay at 1015 nm
•þ
(O; multiplied by a factor of 5 for better visibility), where the C60•ꢀ and P3
species dominate the absorption, and at 730 nm (b), showing the
recovery of the Q-band absorption. The red solid lines show fits of the
transient absorption decays.
In the suggested charge-separation model, the long-lived
fluorescence component (τ2) is assigned to energy transfer from
the more distant, charge-separation-inactive domains of the
oligomer. As shown in Table 2, this component was also
temperature-dependent, and when the energy transfer was
treated as an Arrhenius-activated process (Figure S8.6 in the
SI), activation energies of 1ꢀ2 kcal/mol were obtained for the
longer chains containing 3, 4, and 6 porphyrin units. For
planarization of the excited perpendicular P2 in 2-MTHF, an
activation energy of 2 kcal/mol was found,40 and since these
values have similar magnitudes, it seems likely that the energy-
transfer process along the oligomer chain is in part accom-
panied by planarization. It might seem surprising that the
temperature dependence of the long-lived component in
P2ꢀC60 did not follow this trend, but since excitation at
495 nm in P2 almost exclusively populates the planar con-
former, this minor (<20% at all temperatures) fluorescence
decay component might have a different origin than for the
longer oligomers.
Quantum Yield for Charge Separation. As a consequence of
the restricted delocalization along the longer porphyrin oligo-
mers, the quantum yield for charge separation becomes less than
unity and decreases with decreasing temperature. Figure 7 shows
the charge-separation quantum yield calculated in two ways:
either directly from the quenching efficiencies (Table 1) or from
the fitted lifetimes and pre-exponential factors. From the kinetic
model (eq 1 and Figure 5), the quantum yield for charge
separation can be expressed as
and since the excited fractions (f and 1 ꢀ f) are related to the
normalized pre-exponential factors available through fitting the
decay, this can be written as
ꢂ
ꢃ
k1 ꢀ kCS kCS
2k1 ꢀ kCS
k
φCS
¼
ꢀ R
ð4Þ
2k1 þ kCS kCS þ k k1 ꢀ kCS
1k1 þ k
where R1 is the pre-exponential factor for the decay of Pn* (the
long-lived decay component). The agreement between the two
ways of calculating the charge-separation quantum yields is
reasonable and lends support to the suggested model for charge
separation.
Charge Recombination. Transient absorption measure-
ments based on femtosecond laser excitation provided evidence
for the proposed mechanism for electron transfer between the
excited oligoporphyrin and C60. As an example, Figure 8 shows
the transient absorption spectrum of P3ꢀC60 75 ps after excita-
tion, which shows characteristic features of the charge-separated
•þ
•ꢀ
•þ
state: P3 dominates the visible absorption, C60 and P3
both contribute to the NIR absorption,47,48 and Q-band bleach-
ing dominates the negative band in the 700ꢀ800 nm region. The
kinetic analyses of these bands yielded rates corresponding to the
fluorescence decay, showing the expected connection between
charge separation and decay of the singlet excited state of the
oligoporphyrin. From the decay of the radical absorption and/or
the recovery of the Q-band shown in Figure 8, the lifetime of the
charge-separated state could be determined (fitting parameters
are given in Tables S9.1ꢀS9.5b in the SI). This was done for all
kCS
k1
kCS
φCS
¼
f þ
ð1 ꢀ fÞ
k1 þ k kCS þ k
ð3Þ
kCS þ k
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Table 4. Lifetimes of the Charge-Separated States for
P1ꢀC60, P2ꢀC60, P3ꢀC60, P4ꢀC60, and P6ꢀC60 upon
Excitation at 495 nm (450 nm for P1ꢀC60) Obtained from
the Transient Absorption Decays at 1015 nm and at the
Wavelength of the Q-Band Bleaching Maxima (The Values for
P1ꢀC60 Were Found from Q-Band Bleaching at 650 nm)
τ/ps
T/K
P1ꢀC60
P2ꢀC60
P3ꢀC60
P4ꢀC60
P6ꢀC60
140
160
180
200
220
240
260
280
300
455
349
309
274
226
208
201
191
189
732
692
640
584
493
454
373
290
276
904
794
710
696
627
554
504
483
463
917
846
734
708
689
604
567
538
499
1112
1005
951
903
821
786
748
658
643
Figure 9. Plots of ln(kCRT1/2) vs 1/T for P1ꢀC60 (black), P2ꢀC60
(red), P3ꢀC60 (green), P4ꢀC60 (blue), and P6ꢀC60 (cyan). The
samples were excited at 495 nm, and the rate constants were derived
from the transient absorption decays (kCR = 1/τ). The lines are fits to the
linearized form of eq 2.
Table 5. Driving Forces (ΔG°)a, Reorganization Energies
(λ), and Electronic Coupling Values (V) for Charge Recom-
bination in PnꢀC60 Obtained from Individual Fits Assuming
Recombination to the First Excited Triplet State or Directly to
the Ground State
systems at temperatures between 140 and 300 K, and the results
are collected in Table 4.
The lifetimes of the charge-separated states have weak tem-
perature dependences for all of the oligomers, spanning from
∼200 ps for P1ꢀC60 at room temperature to 1 ns for P6ꢀC60 at
140 K. More importantly, the lifetimes of the charge-separated
states at a given temperature have the same order of magnitude
for all of the investigated systems, indicating that the positive
charge is either localized close to the fullerene or delocalized over
all of the porphyrin units and thus does not need to migrate over
a large distance to recombine. Measuring femtosecond transient
absorption decays at low temperature (in a liquid nitrogen
cryostat) is quite challenging, and the quality of the data did
not allow a thorough investigation of potential multiexponenti-
ality arising from a scenario similar to that suggested for charge
separation. However, since only almost-planar conformers or
excitations localized closest to C60 lead to charge separation, no
further structural relaxations are expected during charge recom-
bination. This should lead to a single lifetime for the charge-
separated state, and the recombination of the charge separated
state was therefore fitted using only one exponential. The whole
transient absorption decay was fitted to a triexponential decay
function, where the two shortest lifetimes were fixed to the values
obtained from the lifetimes of the fluorescence decays (cor-
responding to the charge separation). The procedure is explained
in detail in the Experimental Section.
P1ꢀC60 P2ꢀC60
180ꢀ240 K 240ꢀ300 K
Assuming Recombination to the First Triplet Excited State
P2ꢀC60 P3ꢀC60 P4ꢀC60 P6ꢀC60
parameter
ΔG°/eV þ0.12 ꢀ0.13
ꢀ0.13
0.49
ꢀ0.24 ꢀ0.30 ꢀ0.33
λ/eV
V/cmꢀ1
ꢀ
ꢀ
0.33
4.5
0.47
4.2
0.53
3.8
0.57
3.52
11.5
Assuming Recombination to the Ground State
ΔG°/eV ꢀ1.29 ꢀ1.30
ꢀ1.30
0.83
ꢀ1.31 ꢀ1.31 ꢀ1.31
λ/eV
0.95
0.96
0.98
0.99
0.99
V/cmꢀ1 8.1
5.8
13.1
5.0
4.5
4.0
a ΔG° values for P1ꢀC60, P2ꢀC60, and P4ꢀC60 were taken from refs 44
and 49. The values for P3ꢀC60 and P6ꢀC60 were obtained by
interpolation and extrapolation of a plot of ΔG° vs 1/n, where n is the
number of porphyrin units.
combination of charge recombination to the triplet and ground
states, with the former dominating at high temperatures, would
explain the change in slope. Naturally, it would be desirable to
distinguish between these two mechanisms by comparing the
residual transient absorption (or bleaching) at long times.
However, experimental difficulties combined with quite substan-
tial triplet formation in the longer systems due to the comparably
low yield for charge separation (see above) did not allow a
precise distinction between the two recombination possibilities.
Recombination through the C60 triplet state (experimentally
determined to be at 1.5 eV) is endergonic and not expected to
contribute to the observed decay.50,51 This is further supported
by the absence of the characteristic triplet C60 transient absorp-
tion that would have been expected at ∼700 nm if recombination
through this channel had been significant.51 Figure 10 sum-
marizes the recombination processes. The Marcus parameters
λ and V for recombination to either the ground state or the triplet
state as calculated from the linear fits are summarized in Table 5.
In Figure 9, the rate constants for charge recombination (kCR
)
are plotted against inverse temperature and fitted to the linear-
ized form of eq 2. Reasonable fits with similar slopes over
the whole temperature range (180ꢀ300 K) were found for all
of the systems except P2ꢀC60, which showed a change in slope
above 240 K. Such a temperature dependence might indicate a
change in mechanism for the charge recombination. As shown in
Table 5, for P2ꢀC60 and the longer systems, charge recombina-
tion to the lowest excited triplet state of the porphyrin oligomer is
exergonic. From experimental estimates of the triplet energies
and oxidation potentials, it is concluded that the driving force
increases with increasing size of the oligomer.44,49 This might
mean that recombination to the electronic ground state occurs in
P1ꢀC60 and recombination to the porphyrin-localized triplet
state occurs in P3ꢀC60, P4ꢀC60, and P6ꢀC60. In P2ꢀC60, a
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Journal of the American Chemical Society
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0.5 ps; interporphyrin torsional relaxation then leads to
delocalization of the excited state on a time scale of
∼100 ps. The results reported here provide detailed
information on the slower delocalization process. They
are not sensitive to the initial ultrafast delocalization but
demonstrate that exciton self-trapping must be much faster
than electron transfer to C60.
6. The actual charge-separation and charge-recombination
reactions are both temperature-dependent, as predicted
by the Marcus theory. Reasonable values for the reorgani-
zation energies and electronic couplings were extracted
from fits of the temperature variation of the time-resolved
fluorescence and femtosecond transient absorption decays.
7. Charge recombination occurs directly to the electronic
ground state in P1ꢀC60 and through the porphyrin-loca-
lized triplet state in P3ꢀC60, P4ꢀC60, and P6ꢀC60. In
P2ꢀC60, a combination of charge recombination to the
triplet and ground states takes place, with the former dom-
inating at high temperatures.
Figure 10. Energy diagram for the charge-recombination processes.
The energy of the charge-separated state is quite insensitive to the
oligomer length, whereas for the longer oligomers the lowest triplet state
of the oligomer is stabilized enough to become a feasible recombination
channel.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis, NMR spectra, mass
b
Because of the large driving force for recombination to the
ground state, the electron-transfer process along this pathway is
in the inverted Marcus region (|ΔG°| > λ). For recombination to
the triplet excited state, the driving forces are much smaller.
Nevertheless, the lifetimes of the charge-separated states are
quite short as a consequence of a comparatively large electronic
coupling. In comparisons of the electronic couplings for charge
separation and recombination, it is notable that these are smaller
for the recombination reaction. This might be an additional
indication that charge recombination occurs from a more deloca-
lized state than charge separation.
spectra, HPLC traces, ground-state absorption spectra, steady-
state emission spectra, time-resolved fluorescence decays and
fitting parameters, and transient absorption fitting parameters.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
balb@chalmers.se
’ ACKNOWLEDGMENT
’ CONCLUSION
This research was funded by the Swedish Research Council
(VR), the Swedish Energy Agency, the Knut and Alice Wallen-
berg Foundation, the Engineering and Physical Sciences Re-
search Council (EPSRC), and the Swiss National Science
Foundation. We thank the EPSRC Mass Spectrometry Service
(Swansea) for mass spectra.
In this study of the temperature dependence of electron-
transfer reactions in a series of porphyrin oligomer electron
donors appended with a C60 electron acceptor, the following has
been learned:
1. Charge separation from the excited donor occurs either
directly from an active part of the oligomer or through
excitation migration along the oligomer chain followed by
charge separation.
2. Direct charge separation dominates in the short oligomers,
while energy migration precedes charge separation in the
longer oligomers, particularly at high temperatures.
3. Energy migration in the oligomers is thermally activated
through dihedral conformational relaxation.
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