Probing the efficiency of electron transfer through porphyrin-based molecular wires

Article abstract of DOI:10.1021/ja067447d

Electron transfer over long distances is important for many future applications in molecular electronics and solar energy harvesting. In these contexts, it is of great interest to find molecular systems that are able to efficiently mediate electrons in a

Full text of DOI:10.1021/ja067447d

Published on Web 03/16/2007
Probing the Efficiency of Electron Transfer through
Porphyrin-Based Molecular Wires
Mikael U. Winters,† Emma Dahlstedt,‡ Holly E. Blades,‡ Craig J. Wilson,‡
Michael J. Frampton,‡ Harry L. Anderson,*,‡ and Bo Albinsson*,†
Contribution from the Department of Chemical and Biological Engineering, Physical Chemistry,
KemiVa¨gen 3, SE - 412 96 Go¨teborg, Sweden, and the Department of Chemistry, UniVersity of
Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA, United Kingdom
Received October 18, 2006; E-mail: balb@chalmers.se; harry.anderson@chem.ox.ac.uk
Abstract: Electron transfer over long distances is important for many future applications in molecular
electronics and solar energy harvesting. In these contexts, it is of great interest to find molecular systems
that are able to efficiently mediate electrons in a controlled manner over nanometer distances, that is,
structures that function as molecular wires. Here we investigate a series of butadiyne-linked porphyrin
oligomers with ferrocene and fullerene (C60) terminals separated by one, two, or four porphyrin units (Pn,
n ) 1, 2, or 4). When the porphyrin oligomer bridges are photoexcited, long-range charge separated states
are formed through a series of electron-transfer steps and the rates of photoinduced charge separation
and charge recombination in these systems were elucidated using time-resolved absorption and emission
measurements. The rates of long-range charge recombination, through these conjugated porphyrin
oligomers, are remarkably fast (kCR2 ) 15 - 1.3 × 108 s-1) and exhibit very weak distance dependence,
particularly comparing the systems with n ) 2 and n ) 4. The observation that the porphyrin tetramer
mediates fast long-range charge transfer, over 65 Å, is significant for the application of these structures as
molecular wires.
k ) k0e-âR
(1)
Introduction
The rate of electron transfer between molecules is a sensitive
function not only of parameters relating to the electron donor
and acceptor but also of the medium connecting them. The
driving force (∆G°) and reorganization energy (λ) are such
donor and acceptor parameters, while the electronic coupling
(V) is governed by the bridging structure as well as by the
donor-bridge energy gap.1 Many different molecular bridges
have been studied,2-9 and the rate of electron transfer as a
function of donor-acceptor separation (R) is often described
by the exponential expression in eq 1.
The ability of the bridge to mediate electron transfer is
quantified by the attenuation factor â, which varies with the
degree of conjugation in the bridge and also with the donor-
bridge energy gap (i.e., the height of the tunneling barrier). In
theory, values of â should range from 0 for a metallic bridge to
3-5 Å-1 for electron transfer through a vacuum.9 Most organic
π-conjugated bridges give attenuation factors in the range 0.2-
0.6 Å-1 2-5
but recently several systems have been reported with
,
extraordinarily lower values (â ) 0.01-0.06 Å-1),7,8 which
could open up the possibility of using molecular wires for taking
charge over longer distances.
† Chalmers University of Technology.
‡ University of Oxford.
Conjugated porphyrin oligomers feature long strongly coupled
π-systems and small π-π* gaps, so they are expected to be
good mediators for long-range electron transfer.10,11 For example
Osuka and co-workers have shown that edge-fused porphyrin
oligomers with lengths of up to 70 Å conduct charge efficiently
across break-junctions.12 Recently Therien et al. have reported
(1) Eng, M. P.; Albinsson, B. Angew. Chem., Int. Ed. 2006, 45, 5626-5629.
(2) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992, 114, 6227-
6238.
(3) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton,
M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119,
10563-10564.
(4) (a) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam,
E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am.
Chem. Soc. 1999, 121, 1059-1064. (b) Wiberg, J.; Guo, L.; Pettersson,
K.; Nilsson, D.; Ljungdahl, T.; Mårtensson, J.; Albinsson, B, J. Am. Chem.
Soc. 2007, 129, 155-163.
(5) Pettersson, K.; Wiberg, J.; Ljungdahl, T.; Mårtensson, J.; Albinsson, B. J.
Phys. Chem. A 2006, 110, 319-326. Eng, M. P.; Ljungdahl, T.; Mårtensson,
J.; Albinsson, B. J. Phys. Chem. B 2006, 110, 6483-6491.
(6) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607-2617. Guldi,
D. M.; Imahori, H.; Tamaki, K.; Kashiwagi, Y.; Yamada, H.; Sakata, Y.;
Fukuzumi, S. J. Phys. Chem. A 2004, 108, 541-548.
(7) Giacalone, F.; Segura, J. L.; Mart´ın, N.; Ramey, J.; Guldi, D. M. Chem.
Eur. J. 2005, 11, 4819-4834. de la Torre, G.; Giacalone, F.; Segura, J. L.;
Mart´ın, N.; Guldi, D. M. Chem.sEur. J. 2005, 11, 1267-1280.
(8) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen, L.;
Tome´, J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.sEur. J. 2005, 11,
3375-3388.
(9) Gray, H. B.; Winkler, J. R. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3534.
Paddon-Row, M. N. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH, Weinheim, 2001; Vol. 3, pp 201-215.
(10) Anderson, H. L. Chem. Commun. 1999, 2323-2330.
(11) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735-745.
(12) Yoon, D. H.; Lee, S. B.; Yoo, K.-H.; Kim, J.; Lim, J. K.; Aratani, N.;
Tsuda, A.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2003, 125, 11062-
11064. Kang, B. K.; Aratani, N.; Lim, J. K.; Kim, D.; Osuka, A.; Yoo,
K.-H. Chem. Phys. Lett. 2005, 412, 303-306.
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J. AM. CHEM. SOC. 2007, 129, 4291-4297
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A R T I C L E S
Winters et al.
involving the ferrocene moiety as a secondary donor that
repopulates the hole on the porphyrin oligomer and, thus,
recreates the bridge in the ground state. The fully charge
separated state, Fc+-Pn-C60-, recombines to the ground state
with a rate constant kCR2. In this recombination process the
electron transfer occurs from C60- (which now acts as a donor)
to Fc+ (now the acceptor) and the electronic coupling is
mediated by the porphyrin oligomer in its electronic ground
state. The purpose of this study is to relate the magnitude of
kCR2 to the length and electronic structure of the porphyrin
oligomer bridges. Ferrocene and C60 were chosen as the donor
and acceptor because their redox potentials make both the
charge-separation and charge-shift steps in Figure 1 thermody-
namically favorable for any length of porphyrin oligomer bridge.
Results and Discussion
Synthesis. The triads Fc-Pn-C60, the silicon terminated
reference dyads Fc-Pn-Si, and Si-Pn-C60, and reference
4-butylphenyl-terminated bridges Bu-Pn-Bu were all synthe-
sized (n ) 1, 2, and 4) from the silicon-terminated oligomers
Si-Pn-Si using Sonogashira coupling methodology, as shown
in Scheme 1 (see Supporting Information for yields, experi-
mental details, and characterization data). The triads Fc-Pn-
Figure 1. General structure of the triads and schematic energy level diagram
showing the charge separation (CS), charge shift (CSh), and charge
recombination (CR1 and CR2) reactions. (The aryl substituents, Ar, are
3,5-di(tert-butyl)phenyl for n ) 1 and 3,5-di(octyloxy)phenyl for n ) 2
and 4.)
C
60 were synthesized via both routes A and B; in general route
EPR measurements indicating that ethyne-linked porphyrin
oligomers can mediate essentially barrierless hole transport over
distances of up to 75 Å.13 In this study we have tested the
efficiency of electron transfer through porphyrin-based molec-
ular wires by measuring the rate of charge recombination in a
series of donor-bridge-acceptor molecules, Fc+-Pn-C60-, in
which the electron donor (C60-) is connected to the electron
acceptor (Fc+) via a set of butadiyne-linked porphyrin oligomers
(Pn, n ) 1, 2, or 4). Surprisingly, the rate of electron transfer
through the porphyrin tetramer bridge (P4, length 65 Å) is almost
the same as that through the dimer (P2, length 38 Å); just
comparing these two bridges gives an apparent attenuation factor
of â ) 0.003 Å-1. Many ferrocene-porphyrin-fullerene triads
have been investigated previously by Fukuzumi and co-workers,6
with the aim of preparing long-lived charge separated states.
The aim of this study is completely different: we are not
primarily interested in the lifetimes of the charge separated states
but rather to investigate whether the intervening bridge acts as
an efficient molecular wire.
B is more convenient because the presence of the aldehyde
substituent in Si-Pn-CHO facilitates its chromatographic
purification, compared with Fc-Pn-Si.
Absorption Spectra, Emission Spectra, and Electrochem-
istry. The absorption and emission spectra of these porphyrin
oligomers show a progressive shift to longer wavelengths with
increasing oligomer length, as discussed previously.10,14 The
ferrocene and fullerene substituents show little ground state
interaction with the porphyrin oligomer bridges, and the
absorption spectra of the Fc-Pn-C60 triads are essentially
superimposable with those of the corresponding Bu-Pn-Bu
reference compounds (see Supporting Information). In addition
to characterizing the ground state absorption and emission
spectra of these Fc-Pn-C60 triads and reference systems, the
spectra of the transient species involved in the electron-transfer
steps were recorded after electrochemical generation. These
spectra are important for identification of charge-separated
species in transient absorption experiments. In the Supporting
Information, spectra of the radical cations of the appropriately
substituted porphyrin oligomer references, radical anion of C60,
and the radical cation of ferrocene are shown. The intensities
and band maxima of the lowest transition in Si-Pn+-Si are
strongly dependent on the size of the oligomers. Si-P1+-Si
shows distinct peaks at 630 and 716 nm and minor peaks at
800 and 900 nm, whereas Si-P2+-Si exhibits an intense band
at 964 nm, and Si-P4+-Si, at 1010 nm.15 The radical anion,
C60-, shows moderately strong bands at 920 and 1070 nm in
the near IR, and the ferrocene radical cation, a weak band at
750 nm. The electrochemistry of these species was investigated
in detail, so that we can determine the energies of the charge
transfer states. For example the cyclic and square wave
voltammograms of the Bu-Pn-Bu bridges in Figure 2 show
that the first oxidation and reduction waves are split progres-
sively in the dimer and tetramer. The voltammograms of Fc-
Electron-transfer reactions can be very fast, and thus it is often
beneficial to study photoinduced electron transfer in which either
the donor or acceptor is excited to prepare an initial state with
a driving force for electron transfer. Our conjugated porphyrin
oligomers have very low-lying singlet excited states,14 so it is
difficult to find a donor or acceptor that could be excited without
transferring energy to the bridge. Therefore a slightly different
design was chosen, in which the porphyrin oligomers were
appended with a primary electron acceptor (C60) and a secondary
electron donor (ferrocene, Fc). Figure 1 shows the anticipated
processes that occur in the triads Fc-Pn-C60, after excitation
of the porphyrin oligomer. Initially, a primary charge separation
involving the transfer of an electron from the excited porphyrin
oligomer to C60 occurs. This is followed by a charge shift
(13) Susumu, K.; Frail, P. R.; Angiolillo, P. J.; Therien, M. J. J. Am. Chem.
Soc. 2006, 128, 8380-8381.
(14) Taylor, P. N.; Huuskonen, J.; Rumbles, G.; Aplin, R. T.; Williams, E.;
Anderson, H. L. Chem. Commun. 1998, 909-910.
(15) Arnold, D. P.; Hartnell, R. D.; Heath, G. A.; Newby, L.; Webster, R. D.
Chem. Commun. 2002, 754-755.
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Electron Transfer through Porphyrin Wires
A R T I C L E S
Scheme 1 a
a (a) TBAF, CH2Cl2; (b) BuC6H4I, Pd2(dba)3, CuI, PPh3, NEt3, PhMe, 40 °C; (c) TBAF, 1:1 CH2Cl2, CHCl3; (d) separation; (e) CuCl, TMEDA, CH2Cl2,
O2 (n f 2n); (f) FcC6H4I, Pd2(dba)3, CuI, PPh3, NEt3, PhMe, 40 °C; (g) IC6H4CHO, Pd2(dba)3, CuI, PPh3, NEt3, PhMe, 40 °C; (h) C60, CH3NHCH2CO2H,
PhMe, reflux.
P1-C60 illustrated in Figure 3 show that the Fc/Fc+ and C60/
C60- redox waves are readily identified; these redox potentials
do not change as the length of the intervening porphyrin bridge
is increased. The energies of the triplet states of the bridges
Bu-3Pn*-Bu were estimated from low-temperature phospho-
rescence measurements and agree well with calculated values
(Supporting Information). These experiments enabled us to
estimate the energies of the states plotted in Figure 4.
Pn-Si. In the moderately polar solvent used (THF), there was
no sign of the formation of C60*, whereas the anticipated radical
ion pairs were readily detected, showing that charge separation
is very efficient in the Si-Pn-C60 dyads and, indeed, the
estimated quantum yields for charge separation approach 100%.
In Figure 5, the transient absorption of the Pn-C60 systems
probed at 1000 nm are displayed, and it is evident that the rise
times, as well as the decay times, are becoming progressively
longer as n increases. The rate of the initial charge separation,
Time-Resolved Photophysical Measurements. The charge
separation processes in the Fc-Pn-C60 triads were followed
by femtosecond transient absorption and time-resolved fluores-
cence. The cascade of electron-transfer reactions leading to the
fully charge separated state is moderately complex, and therefore
the components Si-Pn-Si, Si-Pn-C60, and Fc-Pn-Si were
also investigated. All measurements were fitted to sums of
exponential decay functions, and the first-order rate constants
were extracted with reference to the model in Figure 1. In Table
1, the decay lifetimes for the triads, dyads, and oligomers are
collected, while Table 2 comprises the extracted rate constants,
quantum yields, and driving forces for the different processes.
kCS, is calculated from eq 2:
kCS ) 1/τ21 - 1/τ11
(2)
where τ21 and τ11 are the lifetimes of the lowest porphyrin singlet
excited state of Si-Pn-C60 and Si-Pn-Si, respectively (Table
1). A tentative analysis using the Marcus equation16 showed
that the extracted rates are consistent with a reorganization
energy between 0.49 and 0.64 eV for n ) 1-4.17 This agrees
well with the typically low values found for porphyrin-fullerene
couples, as well as with estimates obtained from the Born
1
The fluorescence from Pn* is almost completely quenched
(16) It does not significantly benefit the analysis to introduce more advanced
treatments, such as that of Jortner and co-workers: Ulstrup, J.; Jortner, J.
J. Chem. Phys. 1975, 63, 4358-4368. Jortner, J; Bixon, M. J. J. Chem.
Phys. 1988, 88, 167-170.
(17) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-322.
Marcus, R. A. J. Chem. Phys. 1956, 24, 966-978.
when the fullerene acceptor is attached and the lifetime of the
singlet excited state, as judged from transient absorption (Si-
P1-C60, Si-P2-C60, and Si-P4-C60) or time-resolved fluo-
rescence (Si-P4-C60), is significantly reduced compared to Si-
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A R T I C L E S
Winters et al.
Figure 3. Cyclic and square-wave voltammograms of monomer triad Fc-
P-C60 in THF containing Bu4NBF4 (0.1 M). Potentials are given relative
to ferrocene (Fc/Fc+) but referenced to internal decamethylferrocene (Fc*/
Fc*+). Scan parameters same as those for Figure 2.
Figure 2. Cyclic and square-wave voltammograms of reference porphyrin
oligomers Bu-P-Bu monomer (a), Bu-P2-Bu dimer (b), and Bu-P4-
Bu tetramer (c), all in THF containing Bu4NBF4 (0.1 M), referenced to
internal ferrocene (Fc/Fc+). Arrows indicate the redox potentials for the
first reduction and oxidation of Bu-P-Bu, the first and second reductions
and oxidations of Bu-P2-Bu, and the first and third/fourth reductions and
oxidations of Bu-P4-Bu. Cyclic voltammetry: scan rate 0.25 V s-1; square
wave voltammetry: step potential 2 mV, step amplitude 20 mV, square
wave frequency 8 Hz; glassy carbon working electrode, Pt counter electrode,
Ag/AgNO3 reference electrode.
Figure 4. State energies estimated from electrochemical and spectroscopic
data (Supporting Information). Red lines are for n ) 1; green lines, for
n ) 2; and blue lines, for n ) 4.
Table 1. Lifetimes Extracted from Global Fitting of Transient
dielectric continuum model.18 For the sake of this analysis, the
Absorption Data to Sums of Exponential Decay Functionsa
porphyrin-fullerene electronic coupling was assumed to be
monomer
dimer
tetramer
approximately the same for all Si-Pn-C60, V ) 22.5 ( 3 cm-1
.
lifetime
n
)
1
n
)
2
n ) 4
When the corresponding measurements were made on the
triads (Fc-Pn-C60), even larger quenching of the singlet
excited-state lifetimes were observed (cf. Table 1). Although
surprising at first, the explanation was simply that the presence
of ferrocene also quenched the porphyrin excited state by a
parallel process. Measurements on the dyads, Fc-Pn-Si, gave
the rate constants for the ferrocene quenching, kQ. Having this
information the rate constants for the initial charge separation
in the triads could also be calculated from eq 3:
Si-Pn-Si
τ
11 (ps)
21 (ps)
22 (ps)
31 (ps)
32 (ps)
33 (ps)
1450b
6.1
130
2.7
30
640
1260b
7.2
157
5.9
41
7100
830b
29b
293
18
175
7800
Si-Pn-C60
τ
τ
τ
τ
τ
Fc-Pn-C60
a Four to six different wavelengths, chosen at relevant peaks in
the spectra, were used for global analysis of transient absorption data.
Errors in the lifetimes are 10% or less. b Also from time-resolved
fluorescence.
kCS ) 1/τ31 - 1/τ11 - kQ
(3)
(37-81%) due to the competing quenching by ferrocene. We
have not investigated the ferrocene quenching in any detail.
However, no new absorption features were formed and no long-
lived transient species (τ > 100 ps) could be observed in
measurements on Fc-Pn-Si, which suggests that the fluores-
cence quenching in Fc-Pn-Si is caused by energy transfer to
low lying ferrocene excited states18 followed by rapid deactiva-
tion.
where τ31 is the singlet excited-state lifetimes of the porphyrin
oligomers in Fc-Pn-C60. As expected, the rate for initial charge
separation is similar in the dyads and triads, but the correspond-
ing quantum yield (Table 2) is significantly lower in the triads
(18) Marcus, R. A. Can. J. Chem. 1959, 37, 155-163. Marcus, R. A. J. Chem.
Phys. 1965, 43, 679-701.
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Electron Transfer through Porphyrin Wires
A R T I C L E S
Table 2. First-Order Rate Constants (k) and Driving Forces (∆G°)
as Defined by Figure 1 and Quantum Yields (Φ) for the
Charge-Transfer Reactions
process
∆G
°
[eV]a
k
Φ
[%]b,c
Fc-P*-C60 f Fc-P+-C60
-0.58
-0.33
-1.29
-0.96
-0.38
-0.31
-1.30
-0.99
-0.28
-0.30
-1.31
-1.01
(6.1 ps)-1
(39 ps)-1
37b
29c
-
Fc-P+-C60- f Fc+-P-C60
Fc-P+-C60- f Fc-P-C60
Fc+-P-C60- f Fc-P-C60
-
-
(130 ps)-1
(640 ps)-1
(7.2 ps)-1
(55 ps)-1
Fc-P2*-C60 f Fc-P2+-C60
Fc-P2+-C60- f Fc+-P2-C60
Fc-P2+-C60- f Fc-P2-C60
Fc+-P2-C60- f Fc-P2-C60
81b
61c
-
-
(160 ps)-1
(7100 ps)-1
(30 ps)-1
Fc-P4*-C60 f Fc-P4+-C60
65b
26c
-
Fc-P4+-C60- f Fc+-P4-C60
Fc-P4+-C60- f Fc-P4-C60
Fc+-P4-C60- f Fc-P4-C60
(440 ps)-1
(290 ps)-1
(7800 ps)-1
a Driving forces were estimated from spectroscopic and electrochemical
measurements (Supporting Information). b Quantum yield for the charge
-1
c
separated state (ΦCS ) kCS/(τ11 + kCS + kQ)). Overall quantum yield
Figure 6. Transient absorption decays measured at 1000 nm (top), shown
in logarithmic scale to emphasize the fact that they are single-exponential
decays at long delay times. The bottom panel shows the ground-state
recovery of the oligomers (660, 732, and 756 nm). Fc-P1-C60 is shown
by red squares; Fc-P2-C60, by green triangles; and Fc-P4-C60, by blue
circles. Note the different scales of the abscissa in the two panels.
for the fully charge separated state (Φ ) ΦCS × kCSh/(kCR1 + kCSh)).
+
difference in decay rates of Pn in the triads (τ32-1) and Si-
Pn-C60 dyads (eq 4).
kCSh ) 1/τ32 - kCR1
(4)
The introduction of the ferrocene secondary donor decreases
the lifetime of the initial charge separated state to between 30
and 175 ps (Table 1). In all three triads significant amounts of
the fully charge separated state are formed (overall quantum
yields varying between 26% and 60%), and in the following
the fate of this state will be discussed.
-
In the fully charge separated state, the distances between C60
and Fc+ are approximately 24, 38, and 65 Å in Fc-P1-C60,
Fc-P2-C60, and Fc-P4-C60, respectively, and the bridging
porphyrin oligomers are in their electronic ground states. Figure
6 shows the transient absorption decays at wavelengths where,
Figure 5. Normalized transient absorption decays and fitted curves for
Si-P1-C60 (excited at 450 nm, red squares), Si-P2-C60 (excited at 490
nm, green triangles), and Si-P4-C60 (excited at 490 nm, blue circles)
probed at 1000 nm.
-
after an initial induction time, a clean single exponential C60
Once the initial charge separation has occurred, there are two
possible pathways: First, a return to the ground state by charge
recombination (kCR1 ) τ22-1) or, second, the formation of the
fully charge separated state by hole transfer between the
porphyrin oligomer and ferrocene (charge shift reaction, kCSh).
The lifetime of the first charge separated state in Si-Pn-C60
was measured from the decay of the porphyrin radical cation
(Si-Pn+-Si) and fullerene radical anion (C60-) to be between
130 and 300 ps (Table 1, Figure 5). When the rate constants,
decay dominates the signal. The decay of the Fc-P1-C60
system is evidently faster (0.64 ns) than those of both Fc-P2-
C60 and Fc-P4-C60 (7.1 and 7.8 ns, respectively). In the lower
panel of Figure 6, the porphyrin ground state recoveries of the
corresponding systems are displayed. As expected from the
model presented in Figure 1, the porphyrin ground state
absorption recovers long before the charge separated states have
decayed. The ground state recovery times agree well with the
estimated decays of the porphyrin oligomers radical cations,
kCR1, were tested with the Marcus equation, reasonable fits were
further supporting the proposed model. The rate constants, kCR2
,
for decay of the Fc+-Pn-C60 ion pairs are all too fast for a
significant contribution from a dynamic quenching process due
to intermolecular electron transfer under the very dilute condi-
tions of these experiments (concentration ≈ 10 µM).
-
found for the same electronic coupling as for the charge
separation process (V ) 22.5 cm-1). The reorganization energies
found were, however, slightly larger at approximately 0.77-
0.80 eV.
These lifetimes provide the rate constant for charge recom-
bination of the initial charge separated state, kCR1 (Table 2). In
the charge shift reaction, the porphyrin oligomer radical
cation decays and reforms the ground state while the fullerene
radical anion remains unaffected. At the same time the
ferrocene radical cation is formed, but its transient absorption
is weak and overlaps with the stronger absorption of other
species, which makes it difficult to use quantitatively. Thus,
the rate constant for charge shift is estimated from the
Discussion of Electron-Transfer Rates. Compared to those
of similar systems, the recombination rates, kCR2, are remarkably
fast,5-7 which must be a consequence of the bridge-mediated
electronic coupling. The distance dependence of kCR2 is also
remarkable. If the logarithm of the rate constant for an electron-
transfer reaction is plotted against the Fc‚‚‚C60 distance, a
straight line with slope -â is expected from eq 1. In Figure 7,
the logarithm of kCR2 is plotted against the fullerene-ferrocene
distance and it is evident that the data do not describe a straight
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A R T I C L E S
Winters et al.
This requires a favorable energy difference between the states
involved, and if they are resonant, or nearly so, a very weak
distance dependence is generally found. In the present case
repopulating the bridge would be endergonic (Figure 4) and
thermal charge injection is unlikely to be a major contributor
to the recombination process (|∆G°| ≈ 10 kBT). Moreover, the
energy gaps are very similar for all three systems, and therefore
recombination by electron hopping is probably not the explana-
tion for the nonexponential behavior in Figure 7, as it would
have a comparable effect in all three cases. The other possibility
is that the charges recombine by forming a bridge triplet state.
For the tetramer system the oligomer triplet state and the charge
shifted state are almost isoenergetic (Figure 4), suggesting that
recombination via a triplet state might be viable. This process
would result in long-lived ground state bleaching, and this is
observed to a small extent for the tetramer system, but the
amount of bleaching does not increase on the time scale of the
decay of the Fc+-Pn-C60- state, and thus there is no evidence
to indicate that the charge shifted state recombines to a bridge
triplet state. It seems, therefore, that neither hopping nor triplet
recombination makes significant contributions to the charge
recombination process and that the reason for the nonexponential
distance dependence lies elsewhere.
Figure 7. Semilogarithmic plot of the recombination rate versus the
Fc‚‚‚C60 distance in Fc-P1-C60, Fc-P2-C60, and Fc-P4-C60.
line. The slope of a line connecting the first two data points
corresponds to â ) 0.18 Å-1, which is a value in the lower end
of those typically found for conjugated bridge structures,2-5
indicating a weak distance dependence, whereas connecting the
second and third data points results in a line with practically
no distance dependence (â ) 0.003 Å-1). Another way of
quantifying the effectiveness of these bridges is to estimate the
electronic coupling matrix element, V. The Marcus equation for
nonadiabatic electron transfer, assuming a typical fullerene-
ferrocene reorganization energy (λ ) 1.2 eV),6 gives V ) 3.3,
0.92, and 0.85 cm-1 for recombination of Fc+-P1-C60-, Fc+-
P2-C60-, and Fc+-P4-C60-, respectively. The coupling terms
for the dimer and tetramer are similar to those reported in a 43
Å long phenylenethynylene bridge by Creager and co-workers
In the McConnell model, the bridge is treated as consisting
of a number of pairwise interacting subunits, and between each
unit there is a well-defined electronic coupling (ν). Moreover,
it can be shown that the overall bridge electronic coupling is
exponentially decreasing if ν is small compared to the tunneling
barrier (approximately equal to -∆GCSh for the CR2 reaction).
In the present case, there is strong conjugation between the
porphyrin subunits and, further, a low tunneling barrier, and
thus, the McConnell model might not be well-suited to describe
the electron-transfer distance dependence of these systems.27
From quantum mechanical time-dependent density functional
theory calculations we have shown that the distance dependence
of the donor-acceptor electronic coupling can be nonexponen-
tial for π-conjugated bridges that have strongly distance
dependent state energies.1 Possibly, each porphyrin oligomer
bridge should instead be regarded as a single continuous energy
barrier. The evolution of the excitation energies of the Pn
oligomers is linear with the reciprocal oligomer length,14 and
thus the electronic structure of P2 is more similar to that of P4
than P1. For example the third-order susceptibility, two-photon
cross section, and excited-state polarizability all increase
abruptly by more than an order of magnitude on going from P1
to P2 but increase only about 2-fold from P2 to P4.28 The strong
electronic coupling that gives rise to this nonlinear optical
behavior also accounts for the pronounced difference in electron-
transfer properties between the monomer and the oligomers.
(0.7 cm-1 4a
by Wiberg et al. (1 cm-1 4b
)
and a 40 Å phenylenethynylene bridge reported
but much stronger than that reported
)
for an analogous nonconjugated Fc-porphyrin-porphyrin-C60
system (5.6 × 10-5 cm-1).6 It is remarkable that the couplings
through P2 and P4 are essentially the same.
Three mechanisms could contribute to charge recombination
from the fully charge-separated states (Fc+-Pn-C60-): (i)
through-bond electron tunneling, (ii) electron or hole hopping,
or (iii) recombination via a porphyrin oligomer triplet state.19-21
Bridge-mediated through-bond electron transfer, typically de-
scribed by the superexchange model,19 is a mechanism by which
bridge states are not populated during charge transfer. In the
superexchange description, the rate for electron transfer de-
creases exponentially with donor-acceptor separation, and
therefore this model does not adequately explain the data in
Figure 7. Electron hopping is a mechanism by which bridge
radical states are populated as kinetic intermediates, so that
electron transfer takes place in a sequence of short steps.22-26
-
-
The observation that Fc+-P2-C60 and Fc+-P4-C60 give
essentially the same charge recombination rate, despite their
very different lengths, implies that the tunneling barrier in these
systems is mainly associated with the junctions to the electron
donor and acceptor components and that there is very little
(19) Ishimura, K.; Hada, M.; Nakatsuji, H. J. Chem. Phys. 2002, 117, 6533-
6537. Rohmer, M.-M.; Veillard, A.; Wood, M. H. Chem. Phys. Lett. 1974,
29, 466-468. Armstrong, A. T.; Smith, F.; Elder, E.; McGlynn, S. P. J.
Chem. Phys. 1967, 46, 4321-4328.
(20) McConnell, H. J. Chem. Phys. 1961, 35, 508-515.
(21) Harriman, A.; Khatyr, A.; Ziessel, R.; Benniston, A. C. Angew. Chem.,
Int. Ed. 2000, 39, 4287-4290. Harriman, A.; Rostron, S. A.; Khatyr, A.;
Ziessel, R. Faraday Discuss. 2006, 377-391.
(26) Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2005, 127, 11842-11850.
(27) Reimers, J. R.; Hush, N. S. Nanotechnology 1996, 7, 417-423.
(28) Drobizhev, M.; Stepanenko, Y.; Rebane, A.; Wilson, C. J.; Screen, T. E.
O.; Anderson, H. L. J. Am. Chem. Soc. 2006, 128, 12432-12433. Thorne,
J. R. G.; Kuebler, S. M.; Denning, R. G.; Blake, I. M.; Taylor, P. N.;
Anderson, H. L. Chem. Phys. 1999, 248, 181-193. Piet, J. J.; Taylor, P.
N.; Wegewijs, B. R.; Anderson, H. L.; Osuka, A.; Warman, J. M. J. Phys.
Chem. B, 2001, 105, 97-104.
(22) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature
1998, 396, 60-63.
(23) Winters, M. U.; Pettersson, K.; Martensson, J.; Albinsson, B. Chem.sEur.
J. 2005, 11, 562-573.
(24) Lambert, C.; No¨ll, G.; Schelter, J. Nat. Mater. 2002, 1, 69-73.
(25) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 5577-5584.
9
4296 J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007

Electron Transfer through Porphyrin Wires
A R T I C L E S
barrier to tunneling through the conjugated porphyrin oligomer.
It further implies that it is insufficient to condense the distance
dependence into a single parameter, such as â.
recombination.29 The complexity of this question demands a
detailed study that is beyond the scope of the present paper.
Although the exact mechanism of electron transfer in these
systems remains to be elucidated, the observation that the
porphyrin tetramer mediates fast long-range charge transfer, over
65 Å, is significant for the application of these structures as
molecular wires. A key question for future research is whether
the very small distance dependence in the electron-transfer rates
in P2 and P4 extends to longer oligomers such as P8 and P16,
giving access to effective molecular wires with lengths in excess
of 10 nm.
Conclusions
-
The rate of charge recombination in Fc+-Pn-C60 is
remarkably fast and exhibits very weak distance dependence,
particularly when comparing the systems with n ) 2 and n )
4. This long-range charge recombination process appears to take
place via electron tunneling rather than via hopping or triplet
recombination. Future work will be aimed at further clarifying
the charge recombination mechanism by also investigating the
temperature dependence of this process. The expected difference
in activation energy between electron tunneling and hopping
should make it possible to distinguish these mechanisms from
Acknowledgment. This work was funded by the Swedish
Research Council (VR), the Knut and Alice Wallenberg
Foundation, EPSRC, the Wenner-Gren Foundations, the Hans
Werthe´n Foundation, the Foundation P E Lindahl, and the
Foundation BLANCEFLOR Boncompagni-Ludovisi. We thank
the EPSRC Mass Spectrometry Service (Swansea) for mass
spectra.
the temperature dependence of the recombination rates, kCR2
.
This requires, however, that the electronic couplings of the
systems do not vary significantly with temperature. The descrip-
tion of the charge recombination is complicated by the fact that
it is mediated by the oligomers in the ground state, in which
they are essentially free to rotate around the butadiyne axis.
The conformational distribution of the oligomers is broad at
room temperature but becomes more biased toward, on average,
more planar conformations at lower temperature. This augments
the electronic coupling, which is a strong function of the
porphyrin-porphyrin dihedral angle, and it is therefore not clear
whether a temperature study will provide definite discrimination
between hopping and tunneling. A parallel study addresses the
question of conformational heterogeneity and considers the
conformational dependence of both charge separation and charge
Supporting Information Available: Ground state absorption
spectra, spectroelectrochemistry, redox potentials, geometry
calculations, calculation of state energies, time-resolved fluo-
rescence and transient absorption, synthetic procedures, and
characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA067447D
(29) Winters, M. U.; Ka¨rnbratt, J.; Blades, H. E.; Wilson, C. J.; Frampton, M.
J.; Anderson, H. L.; Albinsson, B. Manuscript in preparation.
9
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