Photoinduced Multistep Energy and Electron Transfer in an Oligoaniline-Oligo(p-phenylene vinylene)-Fullerene Triad

Article abstract of DOI:10.1021/jp035549+

A donor-donor-acceptor triad, OAn-OPV-C₆₀, with a redox gradient has been synthesized by covalently linking an oligoaniline (OAn), an oligo(p-phenylene vinylene) (OPV), and a fullerene (C₆₀) in a nonconjugated linear array. Photolumi

Full text of DOI:10.1021/jp035549+

J. Phys. Chem. A 2003, 107, 9269-9283
9269
Photoinduced Multistep Energy and Electron Transfer in an
Oligoaniline-Oligo(p-phenylene vinylene)-Fullerene Triad
Alicia Marcos Ramos,† Stefan C. J. Meskers,† Paul A. van Hal,† Joop Knol,‡
J. C. Hummelen,‡ and Rene´ A. J. Janssen*,†
Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology,
PO Box 513, 5600 MB EindhoVen, The Netherlands, and Stratingh Institute and MSC,
UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
ReceiVed: June 3, 2003; In Final Form: September 5, 2003
A donor-donor-acceptor triad, OAn-OPV-C60, with a redox gradient has been synthesized by covalently
linking an oligoaniline (OAn), an oligo(p-phenylene vinylene) (OPV), and a fullerene (C60) in a nonconjugated
linear array. Photoluminescence and femtosecond pump-probe spectroscopy studies reveal that photoexcitation
of any of the three chromophores of this triad in a polar solvent results in formation of the OAn-OPV+-
C60- charge-separated state, after an efficient ultrafast (<190 fs) singlet-energy transfer to the fullerene singlet-
excited state. The initial OAn-OPV+-C60- state can rearrange to the low-energy OAn+-OPV-C60- charge-
separated state via an intramolecular redox reaction. Because the competing charge recombination of the
-
OAn-OPV+-C60 state to the ground state is fast and increases with increasing polarity of the solvent, the
quantum yield for this charge shift is the highest (∼0.4) in weakly polar solvents such as chlorobenzene.
Once formed, the OAn+-OPV-C60- state has a long lifetime (>1 ns) due to weak electronic coupling between
the distant redox sites in the excited state. The stabilization gained is more than 1 order of magnitude in time.
The experimental results are found to be in qualitative agreement with Marcus theory. In thin films, the
-
OAn+-OPV-C60 state is formed at a higher rate and in higher quantum yield than in solution.
Introduction
we showed that photoexcitation of the oligo(p-phenylene
vinylene) unit of an OPV-C60 dyad (Figure 1) in a polar organic
Photoinduced energy and electron-transfer reactions are the
key steps in natural photosynthesis, and the elucidation of their
mechanism continues to attract considerable interest.1 Similar
processes occur in artificial photoactive and redoxactive mo-
lecular donor entities linked to acceptors. These systems are
considered to be promising for applications in molecular and
supramolecular electronics, light harvesting, and photocatalysis.2
Molecular donor-acceptor combinations also find application
in organic and polymer photovoltaic cells to convert sunlight
into electrical energy.3,4 One of the potentially promising
materials for photovoltaic cells is a blend of a conjugated
polymer as a donor and a methanofullerene derivative as the
acceptor.5 In these polymer:C60 bulk heterojunctions, the forward
electron transfer is extremely fast (,1 ps),6 whereas the electron
recombination extends to the millisecond time domain.7 The
large difference between the forward and backward transfer rates
ensures efficient charge generation and gives the opportunity
to transport and collect the photogenerated charges at the
electrodes, and both processes have been studied extensively
in recent years.
solvent results in an ultrafast (<190 fs) singlet-energy transfer
(1OPV*-C60 f OPV-1C60*), followed by a much slower (∼10
ps) electron-transfer reaction (OPV-1C60* f OPV+-C60-) that
produces a charge-separated state with a lifetime of 50 ps.9e
More recent studies revealed that the rate of the forward
electron-transfer reaction strongly depends on the relative
orientation of the two moieties.13 For an end-to-end substitution
of donor and acceptor, the electron transfer is much slower (∼10
ps) than for a face-to-face orientation (,1 ps), suggesting that
the latter configuration explains the fast forward reaction
observed in polymer:C60 blends. With respect to charge
recombination, however, there remains a substantial discrepancy
between the lifetimes of oligomer-C60 dyads in solution, which
are typically less than 100 ps, and the long-lived charges in
polymer:C60 films. Nature solved the problem of fast charge
recombination in photosynthesis by creating a multistep electron
transfer to increase the distance between the charges and slow
recombination.1 In mimicking natural photosynthesis, a number
of multisite covalently linked porphyrin-fullerene combinations
(triads, tetrads, and pentads) have been described,14,15 showing
that also in artificial fullerene systems multistep charge-transfer
results in an increase in lifetime of the charge-separated state.
Hence, inspired by nature, a tentative explanation for the long
lifetime in the polymer:C60 films is the diffusion of charges to
different sites in the blend.
In this context, there is a considerable interest in the study
of photoinduced charge generation in covalently linked dyads,
consisting of linear conjugated oligomers and fullerenes.8-12
Compared to the polymer:C60 blends, these covalently molecular
dyads are much more well-defined and allow the charge-transfer
reactions to be studied in different media. Using this approach,
In this contribution, we intend to connect the complementary
views that originate from mimicking natural photosynthesis and
polymer:C60 solar cells by extending the OPV-C60 dyad to
include a p-oligoaniline (OAn) moiety as an additional donor,
to create a donor-donor-acceptor triad (OAn-OPV-C60,
* Towhomcorrespondenceshouldbeaddressed.Phone: (+)31.40.2473597.
Fax: (+)31.40.2451036. E-mail: r.a.j.janssen@tue.nl.
† Eindhoven University of Technology.
‡ University of Groningen.
10.1021/jp035549+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/11/2003

9270 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
Figure 1. Structure of the OAn-OPV-C60 triad and reference compounds.
Figure 1). By using a meta substituted phenylene ring in OAn-
OPV-C60, the OAn and OPV parts are electronically decoupled
in the ground state and operate as isolated redox active segments.
In OAn-OPV-C60, the oxidation potential decreases from C60,
via OPV, to OAn, while at the same time the reduction potential
increases (vide infra). By introducing this redox gradient, we
expect that the energetically most favorable charge-separated
705 nm. Whereas each of the three chromophores contributes
to the absorption band at 327 nm, the absorption at 440 nm is
dominated by the π-π* transition of the OPV segment. The
absorption at 705 nm is characteristic for fulleropyrrolidines.9d
The absorption spectrum of OAn-OPV-C60 is a near super-
position of the absorption spectra of the different components
of the triad; only at high energies there is a slight deviation of
the linear combination (Figure 2). This is likely due to the fact
that the OAn-OPV-C60 triad has one less alkoxy substituted
phenylene group compared to combined chromophores (OAn
+ OPV + MP-C60).
Electrochemistry. The OAn-OPV-C60 triad exhibits a
reversible first reduction wave at -0.70 V, corresponding to
the fullerene moiety and three reversible oxidation waves due
to the OAn (+0.55 V and +1.03 V) and the OPV (+0.83 V)
moieties (potentials are given vs SCE, calibrated against Fc/
Fc+, recorded in dichloromethane with 0.1 M TBAPF6) (Table
1). These oxidation potentials are in close agreement with the
values established for the reference compounds (Table 1).9d The
small difference in oxidation potential between the OPV moiety
in triad OAn-OPV-C60 (+0.83 V) and the OPV chromophore
(+0.78 V)9d is attributed to the smaller number of electron
donating alkoxy substituents of the former. The reduction
potentials of OPV and OAn-OPV (-1.91 and -1.87 V) were
measured in tetrahydrofuran (THF) and are much more negative
than that of the fullerene (Table 1).
state corresponds to OAn+-OPV-C60 and that the lifetime
-
of this state is enhanced as a result of the larger distance between
the centers of positive and negative charge density. The detailed
analysis of the photophysical processes in OAn-OPV-C60 has
been performed using photoluminescence and femtosecond
pump-probe spectroscopy in solvents of different polarity and
in the solid state and by comparing the results with those of
the model compounds OAn-OPV, OPV-C60, OPV, OAn, and
MP-C60 (Figure 1) that have only one or two chromophores.
Results and Discussion
Synthesis. The synthesis of OAn,16 OPV, MP-C60, and
9d
OPV-C60 have been described elsewhere. The synthesis of
the triad OAn-OPV-C60 (7, Scheme 1) starts from OPV
aldehyde 1, which has been described previously.9d The alde-
hyde functionality of 1 was protected as a dimethyl acetal 2.
Aldehyde 4 was obtained after reacting amine 3 with 3-bro-
mobenzaldehyde in a palladium-catalyzed reaction. Aldehyde
4 was subsequently converted into the Schiff base 5 after
refluxing with aniline in ethanol. N-Phenylaldimine 5 was then
reacted with the methyl group of 2 in a Siegriest reaction17
affording, after acidic work up, aldehyde 6. A chlorobenzene
solution of aldehyde 6, N-methylglycine, and C60 was stirred
for 16 h in the dark at reflux temperature to yield a mixture of
C60, the desired monoadduct, and higher adducts. The triad 7
was isolated after extensive column chromatography in a 43%
yield. For the synthesis of the OAn-OPV dyad (10, Scheme
2), the bromine atom in 8 was exchanged for a deuterium atom
by lithiation and subsequent deuteration with D2O to yield 9.
A Siegriest reaction of 9 with N-phenylaldimine 5 afforded 10.
All compounds used in the photophysical investigations were
fully characterized using 1H and 13C NMR spectroscopy, mass
spectrometry, FT-IR, and elemental analysis.
Absorption Spectra of Redox States. UV/visible/near-IR
spectroscopy enables the electronic transitions of the donor-
acceptor arrays to be monitored during a stepwise oxidation
process. Quantitative chemical oxidation of the OAn-OPV dyad
in dichloromethane solution was achieved by the addition of
thianthrenium perchlorate18 (Figure 3). After the addition of one
equivalent of this oxidizing agent, the intensity of the absorption
band at 3.79 eV decreases and two absorption bands emerge in
the spectrum, one at 1.44 eV and the other overlapping with
the absorption band of the OPV unit at 2.83 eV. Comparison
with the electronic transitions of the N,N,N’,N’-tetraphenyl-1,4-
benzenediamine radical cation (1.44 and 3.05 eV)19 demonstrates
that the absorption bands are associated with the formation of
an OAn+ radical cation, whereas the disappearing band is that
of the neutral OAn moiety. When a second equivalent of
thianthrenium perchlorate is added, the band of the neutral OPV
unit, at 2.83 eV, decreases, and two absorption bands with
UV/Visible Absorption. The absorption spectrum of OAn-
OPV-C60 in toluene solution (Figure 2) exhibits two strong
absorption bands at 327 and 440 nm and a weak absorption at

Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9271
SCHEME 1. Synthesis of OAn-OPV-C60 (7)a
a a ) amberlite IR 120, trimethyl orthoformate, methanol, 70 °C, 2 H, 91%. b ) 3-bromobenzaldehyde, Pd2(dba)3, BINAP, Cs2CO3, toluene,
100° C, 5 days, 62%. c ) aniline, ethanol, 85 °C, 4 H, 79%. d ) 1. compound 2, t-BuOK, DMF, 80° C, 5 H, 72%; 2. HCl. e ) N-methylglycine,
C60, chlorobenzene, reflux, 18 H, 43%.
SCHEME 2. Synthesis of OAn-OPV (10)a
a a ) 1. n-BuLi, diethyl ether, -10 °C; 2. D2O, room temperature, 41%. b ) 1. compound 5, t-BuOK, DMF, 80° C, 3 H, 50%; 2. HCl.
vibronic fine structure, at 0.74 and 1.64 eV, related to an OPV+
radical cation,20 appear in the spectrum. At the same time, the
absorption of OAn+ at 1.44 eV remains and the second band
of OAn+ at 3.04 eV is now clearly observable. After two

9272 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
Figure 2. UV-visible absorption spectra of the OAn-OPV-C60 triad
(solid line) and model compounds OAn (dotted line), OPV (dashed
line), and MP-C60 (dashed-dotted line) recorded in toluene solution,
and the summation of the spectra of all three reference compounds
(squares). Inset: Magnification of the 705 nm absorptions.
TABLE 1: One-Electron Redox Potentials (E0) of OAn,
OPV, MP-C60, OAn-OPV, and OAn-OPV-C60 (vs SCE)
Calibrated with Fc/Fc+ (in Dichloromethane with 0.1 M
TBAPF6)
Figure 3. UV/visible/near-IR spectra recorded during the conversion
of OAn-OPV by stepwise oxidation using thianthrenium perchlorate18
in CH2Cl2: (a) before (solid line) and after (dashed line) adding 1 equiv.
and (b) after adding 1 equiv. (dashed line) and 2 equiv. (solid line).
compound
OAn
OPV
MP-C60
OAn-OPV
OAn-OPV-C60
E0red (V)
E0ox (V)
0.53/1.02
0.78
-1.91a
-0.70
-1.87a
-0.70
positive and negative charges in the charge separated state. The
radii of the positive and negative ions are given by r+ and r-,
and ꢀs is the relative permittivity of the solvent, -e is the
elemental charge, and ꢀ0 is the vacuum permittivity. The Rcc
distances were determined by molecular modeling, assuming
that the charges are located at the centers of the OAn, OPV,
and C60 moieties.22 The radius of the negative ion of C60 was
set to r- ) 5.6 Å and that of the ions of OPV to r+/r- ) 5.1
Å.9d The radius of the positive charge of OAn was set to r+ )
4.8 Å, as estimated from molecular modeling.
0.53
0.53/0.83/1.03
a Measured in THF.
equivalents of oxidizing agent, the absorption spectrum exhibits
the characteristics of both OAn+ and OPV+ radical cations and
therefore corresponds to that of the OAn+-OPV+ dication
diradical.
Energetic Considerations. In the two dyads (OAn-OPV,
OPV-C60) and triad (OAn-OPV-C60), numerous processes
may occur after photoexcitation. Apart from the intrinsic decay
of the singlet-excited state of the individual chromophores
(photoluminescence, intersystem crossing, and thermal decay),
energy and electron-transfer reactions involving more than one
chromophore (or redox active group) are possible. Whether these
reactions occur depends on, among others, whether these
processes are exergonic. The absorption spectra (Figure 2) reveal
that the lowest singlet-excited state is located on the fullerene
at 1.76 eV, followed by the singlet state of the OPV at 2.48
eV, whereas that of the OAn segment is positioned at ap-
proximately 3.40 eV.
The change in free energy for charge separation was
calculated for four solvents of interest with increasing polarity:
toluene (ꢀ ) 2.38), chlorobenzene (ꢀ ) 5.71), o-dichlorobenzene
(ꢀ ) 9.93), and benzonitrile (ꢀ ) 25.2) (Table 2). According to
eq 1, charge separation (CS: OAn-OPV-1C60* f OAn-
-
OPV+-C60-) and the charge shift (CSH: OAn-OPV+-C60
f OAn+-OPV-C60-) are energetically feasible in the three
polar solvents (Table 2).
The experimental and estimated energies of the various
neutral and charge-separated states of the OAn-OPV-C60 triad
are depicted in Figure 4, assuming chlorobenzene as the
medium. Figure 4 also shows the prevailing photophysical
reactions that have been identified (vide infra) in the triad or
its model compounds.
Photoluminescence. Photoluminescence (PL) experiments on
the triad, dyads, and model compounds were used to study the
energy and electron-transfer reactions that occur in these
molecules. These transfer relaxation pathways are expected to
quench the PL of the chromophores, especially when their rate
constants are higher than that of the intrinsic decay.
The PL at 499 nm of the OPV moiety (excitation at 440 nm)
of the OAn-OPV-C60 triad, dissolved in toluene, is highly
quenched (quenching factor Q > 4000) compared to the PL of
the corresponding OPV molecule. Apart from a residual
emission at 499 nm,23 photoexcitation of the OPV moiety of
OAn-OPV-C60 results in a weak PL signal at 715 nm (Figure
The energies of the charge-separated states can be estimated
by calculating the change in free energy (∆G°) for charge
separation using a continuum model:21
∆G° ) e(Eox(D) - Ered(A)) - E00
e2 e2
-
1
+
1
r-
1
1
-
+
-
(1)
(
)(
)
4πꢀ0ꢀsRcc 8πꢀ0 r
ꢀref ꢀs
In this equation, Eox(D) and Ered(A) are the oxidation and
reduction potentials of the donor and acceptor molecules or
moieties measured in a solvent with relative permittivity ꢀref,
E00 is the energy of the excited state from which the electron
transfer occurs, and Rcc is the center-to-center distance of the

Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9273
TABLE 2: Change in Free Energy (∆G0) with Reference to
the Lowest Singlet State, Reorganization Energy (λ), and
Barrier (∆Gq) for Charge Separation (CS), Charge
Recombination (CR1), Charge Shift (CSH), and Charge
Recombination (CR2) in Toluene (TOL), Chlorobenzene
(CB), o-Dichlorobenzene (ODCB), and Benzonitrile (BZN) as
Determined Using Eqs 1 and 2
For a further comparison, we have studied the PL of the
OAn-OPV dyad in solvents of different polarity. Figure 6a
shows that the emission of 1(OAn-OPV)*, dissolved in toluene,
stems exclusively from the OPV moiety, irrespective of the
excitation wavelength (330 nm (OAn) or 444 nm (OPV)). This
implies that an efficient singlet-energy transfer occurs from the
excited 1OAn* state to OPV (1OAn*-OPV f OAn-1OPV*).
In toluene, the fluorescence quantum yield of OAn-OPV is
slightly (∼10%) higher than that of OPV, but the PL is
progressively quenched with increasing solvent polarity (Figure
6a), providing quenching factors of Q ≈ 2, 9, and 22 for
chlorobenzene, o-dichlorobenzene, and benzonitrile, respec-
tively. The quenching of OAn-OPV PL after excitation at 440
nm in more polar solvents is attributed to an intramolecular
photoinduced electron-transfer reaction in the excited state to
produce the OAn+-OPV- state. The excitation spectrum of the
residual emission of OAn-OPV recorded in chlorobenzene
(Figure 6b) closely corresponds to the absorption spectrum of
∆G0
λ
∆Gq
reaction
solvent (eV) (eV) (eV)
-
OAn-OPV-1C60* f OAn-OPV+-C60 TOL
0.21 0.35 0.224
-0.22 0.75 0.093
CS
CB
ODCB -0.36 0.86 0.073
BZN
TOL
CB
-0.46 0.99 0.070
-1.97 0.35 1.905
-1.54 0.75 0.204
OAn-OPV+-C60-f OAn-OPV-C60
-
CR1
ODCB -1.40 0.86 0.088
BZN
-1.30 0.99 0.024
-0.08 0.35 0.051
-0.21 0.80 0.106
OAn-OPV+-C60-f OAn+-OPV-C60 TOL
-
CSH
CB
ODCB -0.25 0.91 0.119
BZN
TOL
CB
-0.29 1.06 0.140
-1.89 0.36 1.618
-1.32 0.89 0.053
OAn+-OPV-C60-f OAn-OPV-C60
CR2
1
OAn-OPV. This indicates that the OAn*-OPV f OAn-1-
OPV* singlet-energy transfer is significantly faster than an
electron transfer from the same state (1OAn*-OPV f OAn+-
OPV-). Hence, OAn+-OPV- is primarily formed via the OAn-
1OPV* singlet state, irrespective of the excitation wavelength.
ODCB -1.15 1.03 0.004
BZN
TOL
CB
-1.01 1.20 0.007
0.41 0.35 0.407
-0.07 0.80 0.165
OAn-1OPV* f OAn+-OPV-
CS
ODCB -0.22 0.91 0.133
BZN -0.34 1.06 0.123
Near Steady-State Photoinduced Absorption (PIA) Spec-
troscopy. Near steady-state PIA spectroscopy in the microsec-
ond and millisecond time domain is a very sensitive technique
(detection limit ∆T/T ∼ 10-6) to probe small concentrations of
long-lived photoexcitations such as triplet states and intermo-
lecular charge-separated states. The PIA spectrum of OAn-
OPV-C60 in toluene solution, recorded with excitation at 458
nm, exhibits a band at 1.78 eV with a shoulder at 1.52 eV,
characteristic of the long-lived (∼40 µs) triplet state of the
fulleropyrrolidine moiety (OAn-OPV-3C60*) formed via in-
5a), characteristic of the fluorescence emission band of
fulleropyrrolidines.9d In toluene, the quantum yield of this
emission is nearly identical to that of the reference compound
MP-C60. The same result is observed for the OPV-C60 dyad,
although in this case the PL quenching of the OPV is somewhat
less (Q ≈ 1500).23 The strong quenching of the 1OPV* singlet-
excited state (S1) in the OPV-C60 dyad has previously been
studied in detail and was found to involve an ultrafast photo-
induced intramolecular singlet-energy transfer (ET) toward the
fullerene moiety (1OPV*-C60 f OPV-1C60*), which occurs
with a time constant of less than 190 fs.9e,24 We propose that
the same ET process occurs in the OAn-OPV-C60 triad. In
accordance with this proposition, the excitation spectrum of the
fullerene fluorescence of OAn-OPV-C60 coincides with the
corresponding absorption spectrum of OAn-OPV-C60 (Figure
5b). It is interesting to note that the fullerene excitation spectra
of OPV-C60 and OAn-OPV-C60 differ appreciably at lower
wavelengths, where the OAn moiety absorbs. The excellent
agreement between the absorption spectrum of the OAn-OPV-
C60 triad and the excitation spectrum of the fullerene fluores-
cence of the triad implies that not only excitation of OPV, but
also excitation of OAn is responsible for the emission of the
fullerene. This points to an efficient sequential singlet-energy
transfer in the triad (Figure 4), which starts with the formation
9d
tersystem crossing from the fullerene singlet-excited state.
The photoinduced electron transfer in OAn-OPV-C60 in
o-dichlorobenzene and subsequent charge shift or charge
recombination will likely occur in the picosecond to nanosecond
time regime and cannot be resolved with the near-steady-state
PIA technique. However, absorptions of the charge-separated
states can be observed in mixtures of the different model
compounds of the triad in o-dichlorobenzene solution, by using
the redox activity of the corresponding triplet states. Photoex-
citation of OPV (at 458 nm) or MP-C60 (at 528 nm) will result
in the formation of the corresponding excited triplet states. These
triplet states (3OPV* and MP-3C60*) can undergo an electron
transfer to one of the other redox active chromophores present
in solution to produce an intermolecularly charge-separated state,
which is long-lived because the formed cation and anion radicals
diffuse away in solution.
1
of the OAn*-OPV-C60 singlet-excited state and ends at the
Accordingly, selective photoexcitation of MP-C60 at 528 nm
in mixtures with OPV or OAn in o-dichlorobenzene solution
OAn-OPV-1C60* fullerene singlet-excited state.
produces the charge-separated states OPV+/MP-C60 and
-
In a more polar solvent, like o-dichlorobenzene (ꢀ ) 9.93),
the PL of the OPV unit of the OAn-OPV-C60 triad is quenched
to a similar extent as in toluene (Figure 5a). In contrast, the PL
of the fullerene moiety at 715 nm is significantly quenched in
this more polar solvent as compared to in toluene (Figure 5a).
The same result has previously been observed for the OPV-
C60 dyad9d,e and gives evidence of a photoinduced charge
separation (CS) that occurs from the fulleropyrrolidine singlet-
OAn+/MP-C60-, respectively, which are characterized by the
absorptions of OPV+ (at 0.68 and 1.52 eV), OAn+ (at 1.40 eV),
-
and MP-C60 (at 1.24 eV) (Figure 7a). Likewise, photoexci-
tation of MP-C60 in a mixture with OAn-OPV in o-dichlo-
robenzene results in a band at 1.40 eV (Figure 7b) attributed to
the OAn+-OPV radical cation. Although in this mixture the
OAn-OPV+ radical cation can also be formed, it will probably
rearrange by an intramolecular redox reaction to the OAn+-
OPV state within the time scale of the experiment. In an
equimolar mixture of OAn, OPV, and MP-C60, charge transfer
from the positively charged OPV+ onto the OAn becomes
intermolecular, i.e., diffusion-limited and slower. This allows
-
excited state and produces the OPV+-C60 charge-separated
state. For the OAn-OPV-C60 triad, a photoinduced CS will
initially produce a similar state (OAn-OPV+-C60-), which may
then go through a charge shift (CSH) to generate the energeti-
cally more favorable OAn+-OPV-C60 state (Figure 4).
-

9274 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
Figure 4. Schematic energy levels (in chlorobenzene) and photoinduced processes in OAn-OPV-C60. The rate constants are collected in Table
3.
for the detection of a weak residual absorption band character-
istic of the OPV+ radical cation in the PIA spectrum at 0.68
eV (Figure 7b).
and triad increase significantly with solvent polarity from kCS
) 4.3 × 1010 s-1 in chlorobenzene to kCS ) 2.0 × 1011 s-1 in
benzonitrile. A similar increase is found for the charge
recombination. This effect of the medium on the rate constant
has often been observed in donor-acceptor dyads and is in
agreement with Marcus theory, vide infra.26
Even in a mixture where both OPV and MP-C60 are present
in 4-fold excess with respect to OAn, the PIA spectrum is still
dominated by the absorption of the OAn+ radical cation (Figure
7c). At high modulation frequency (2500 Hz) the relative
intensity of the OPV+ radical cation band at 0.68 eV increases
compared to low modulation frequency (75 Hz) (when normal-
ized at 1.24 eV), indicating that in this mixture OPV+ has a
shorter lifetime than the OAn+ radical cation, consistent with
the proposed OPV+ + OAn f OPV + OAn+ redox reaction.
Femtosecond Pump-Probe Spectroscopy in Polar Sol-
vents. The formation and decay of the transient charged species
generated after photoexcitation of the OPV moiety in OAn-
OPV-C60 and the reference compounds OPV-C60 and OAn-
OPV have been investigated with subpicosecond pump-probe
spectroscopy in solvents of different polarity.
In comparison to the dyad, the charge shift (CSH) from
OAn-OPV+-C60 to OAn+-OPV-C60 provides an extra
pathway in the triad for the decay of the OPV+ radical other
than charge recombination (CR1, Figure 4) to the ground state.
Hence, the OPV+ radical cation absorption may disappear faster
in the triad than in the dyad. The experimental data in Figure 8
show, however, that this difference is only discernible in
chlorobenzene, i.e., the least polar solvent. This is a direct
consequence of the increased rate for charge recombination in
more polar solvents (Figure 8), which implies that the probability
for the competing (slower) charge shift is strongly reduced.
Nonlinear least-squares analysis of the experimental OPV+
traces (Figure 8) does not give a clear indication of the
magnitude of the rate of charge shift; it yields values of kCR1
and (kCR1 + kCSH) that are essentially the same (see Table 3).
This may be explained in part by a difference in kCR1 for the
dyad and the triad and in part by the difficulties in the data
analysis itself. Due to the fact that the rates for formation and
decay of the OPV+ species are very similar, it is difficult to
estimate the rates, as nonlinear-least-squares-fit procedures tend
to yield parameters, which are strongly correlated.
-
-
Upon excitation at 450 nm of OPV-C60 or OAn-OPV-
C60, the transient absorption at 1450 nm (0.85 eV) that is
associated with the OPV+ radical cation, i.e., the OPV+-C60
-
and OAn-OPV+-C60 states, exhibits a rise and a decay
-
component (Figure 8). It is important to note that the charge
separation occurs after the ultrafast singlet-energy transfer:
(OAn-)1OPV*-C60 f (OAn-)OPV-1C60* (kET g 5.3 × 1012
s-1).9e Fitting of the temporal differential absorption data at 1450
nm to a biexponential function25 provides the rate constants for
forward and backward electron-transfer reactions (Table 3). As
can be seen in Figure 8 (and Table 3), the rate constants for the
charge separation (kCS) are equal in the dyad and triad within
experimental error. The rates for charge separation in the dyad
Additional information on the CSH reaction can be obtained
by measuring the photoinduced absorption at 1030 nm (1.20
eV). At this probe wavelength, the S1 states 1OPV* and 1MP-
C60*, as well as the radical ions MP-C60- and OAn+, contribute

Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9275
Figure 6. (a) PL spectra of OAn-OPVin toluene (solid squares),
chlorobenzene (open squares), o-dichlorobenzene (closed circles), and
benzonitrile (open circles) solutions with excitation at 444 and 330
nm (for toluene only, dashed line). (b) Excitation spectrum of the OPV
emission at 500 nm of OAn-OPV in chlorobenzene (dashed line) and
compared to the absorption spectrum (solid line).
Figure 5. (a) PL spectra of OPV-C60 in toluene (dashed line) and of
OAn-OPV-C60 in toluene (solid line) and o-dichlorobenzene (dotted
line). The inset shows a magnification of the fullerene emission. (b)
Excitation spectra of the 710 nm emission of OPV-C60 (dashed line)
and OAn-OPV-C60 (solid line) compared to the absorption spectrum
of OAn-OPV-C60 (dotted line), all in toluene.
a long-lived signal is observed which can be modeled taking
kCSH ) 5 × 109 s-1 and kCR1 ) 6 × 109 s-1. Taking a higher
(lower) value for kCSH results in a curve that bends downward
(upward) after 60 ps. Using kCSH ) 5 × 109 s-1 results in a
calculated yield of formation for the OAn+-OPV-C60- species
of about 0.4 per absorbed photon. This result can be compared
with the following crude estimate. At 30 ps, mainly the OAn-
to the induced absorption (Figure 7). The differential transmis-
sion at 1030 nm of OPV-C60 and OAn-OPV-C60 in chlo-
robenzene undergoes a strong rise and drop within 2 ps (Figure
9). This transient signal is associated with the formation and
1
decay of the OPV* singlet-excited state and involves the Sn
r S1 transition of the OPV unit.9e The short lifetime of the
1OPV* state is due to the ultrafast ET onto the fullerene moiety
as described above. After the ET, the 1030 nm signal for the
OAn-OPV-C60 triad remains constant over the time scale of
the experiment (1 ns), whereas for the dyad, the signal decays
to zero within 200 ps (Figure 9). We propose that the remaining
signal is due to absorption by C60- and OAn+ radical ions, and
hence characteristic of the OAn+-OPV-C60- charge separated
state. Using the extinction coefficients in brackets for these
species [7.7 × 104 M-1 cm-1 (OPV(S1), estimate), 7 × 103
M-1 cm-1 (MP-C60(S1)27), 7 × 103 M-1 cm-1 (MP-C60- 27),
and 8.2 × 103 M-1cm-1 (OAn+ 19)], we have modeled the time
profile for the absorption at 1030 nm (Figure 9) for both the
OPV-C60 dyad and the OAn-OPV-C60 triad. Rate constants
were taken from the transient absorption measurements at 1450
nm probe wavelength (Table 3) and the rate for charge shift
(kCSH) is used as an adjustable parameter.28 Further details of
the mathematical modeling are given in the Supporting Informa-
tion. The results show a large induced absorption due to the
OPV(S1) species. The duration of this transient is mainly
determined by the width of the laser pulses used. For the
modeling, a cross-correlation of 500 fs (fwhm) for the pump
and probe pulse was assumed. For the dyad, a smooth trace is
observed after the initial contribution of the excited OPV moiety.
-
-
OPV+-C60 state is present of which only the C60 makes a
major contribution to the absorption at 1030 nm. At t ) 200
ns, only the OAn+-OPV-C60- state is present and OAn+ and
C60- contribute almost equally to the transient absorption. The
experimental observation that the induced absorption at 1030
nm hardly changes between 30 and 200 ps thus indicates that
the probability of formation of OAn+-OPV-C60- out of OAn-
-
OPV+-C60 is roughly one-half, in good agreement with the
estimate above. Figure 9 also shows the induced absorption
signal at 1450 nm as observed for the triad. This latter signal
has been modeled using the same parameters as mentioned
above. The value of kCR1 ) 6 × 109 s-1 obtained for the triad
can be compared to kCR1 ) 12 × 109 s-1 obtained for the dyad
by probing the OPV+ cation absorption. At present, we cannot
fully account for this difference. The method used here to
determine kCSH has the advantage that it does not rely on rate
parameters obtained from model compounds. For the other two
solvents used (ODCB and BZN), a long-lived transient at 1030
nm has a weak intensity, and the signal-to-noise ratio does not
allow for an estimate of the rate for charge shift.
We also studied the reference compound OAn-OPV with
pump-probe spectroscopy in solution. In particular, it is of
interest to investigate whether an OAn-1OPV*-C60 f OAn+-
OPV--C60 electron-transfer reaction can compete with the
OAn-1OPV*-C60 f OAn-OPV-1C60* energy transfer.
-
This is consistent with the fact that the C60(S1) and the C60
groups have almost equal extinction coefficients. For the triad,

9276 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
Figure 8. Differential transmission dynamics of the OPV+ transition
at 1450 nm for OAn-OPV+-C60- (solid line) and OPV+-C60- (dashed
line) in (a) chlorobenzene, (b) o-dichlorobenzene, and (c) benzonitrile
after excitation at 450 nm.
Figure 7. (a) Normalized photoinduced absorption spectra of the
mixtures OPV/MP-C60 (1:1) (solid line) and OAn/MP-C60 (1:1)
(dashed-line) in o-dichlorobenzene (excitation at 528 nm with 25 mW
and modulation frequency of 275 Hz). (b) Photoinduced absorption
spectra of the mixtures OAn-OPV/MP-C60 (1:1) (solid line) or OAn/
OPV/MP-C60 (1:1:1) (dashed line) in o-dichlorobenzene (excitation
at 528 nm with 25 mW and modulation frequency of 275 Hz). (c)
Normalized photoinduced absorption spectra of the mixture OAn/OPV/
MP-C60 (1:4:4) in o-dichlorobenzene recorded at modulation frequen-
cies of 75 Hz (solid line) and 2500 Hz (dashed line) (excitation at 528
nm with 25 mW).
significantly reduced to ∼140 and ∼100 ps, compared to that
of the OPV model compound (1.36 and 1.44 ns, respectively).
This is in accordance with the proposed OAn-1OPV* f
OAn+-OPV- electron-transfer reaction, which reduces the
lifetime of the 1OPV* state. The lifetimes of 1OPV* and OAn-
1OPV* in toluene are very similar (1.20 and 1.38 ns, Figure 9)
as expected because electron transfer does not occur here.
However, and surprisingly, a significant increase of fluorescence
lifetime was observed for OAn-1OPV* in chlorobenzene (2.65
ns) compared to OPV (1.27 ns) (Figure 11). At first glance,
this increase in fluorescence lifetime seems to contradict the
observed OAn-1OPV* f OAn+-OPV- charge separation
reaction, which is approximately 20 times faster than the intrinsic
decay (Figure 10, Table 3). Instead of an increase in fluorescence
lifetime, a decrease would be expected. Note that except for a
loss in intensity (only a factor of 2), the fluorescence spectra of
OPV and OAn-OPV are identical (Figure 6) and that the
emission is thus from the OAn-1OPV* state. Taking these
experimental observations into consideration, we propose that
in chlorobenzene the singlet OAn-1OPV* state and the OAn+-
OPV- charge-separated state are nearly degenerate and that back
electron transfer from OAn+-OPV- reproduces the OAn-1-
OPV* singlet state. The estimated change in free energy for
OAn-1OPV* f OAn+-OPV- of only ∆G0 ) -0.07 eV
(Table 2) supports this suggestion.
As a result of the electron-hole symmetry in conjugated
oligomers, the optical spectra of OPV radical cations and OPV
radical anions are expected to be very similar, and hence, we
probe the formation of OPV- at 1450 nm.29 The transient
differential transmission recorded for OAn-OPV, dissolved in
solvents with increasing polarity, at 1450 nm after excitation
at 455 nm, is shown in Figure 10. In full agreement with the
PL quenching of the OAn-OPV fluorescence (Figure 6), the
OAn+-OPV- charge-separated state is only formed in the three
polar solvents (chlorobenzene, o-dichlorobenzene, and benzoni-
trile) but not in (less polar) toluene (Figure 10). The rate
constants for charge separation and charge recombination (k′
CS
and k′ , Table 3) increase with increasing polarity. Table 3
CR
reveals that the rate for the OAn-1OPV* f OAn+-OPV-
charge separation (k′ ) 1.6-4.1 × 1010 s-1) is significantly
CS
1
less than the rate of the OPV*-C60 f OPV-1C60* energy
transfer (kET g 5.3 × 1012 s-1), and as a consequence, the
OAn+-OPV--C60 state is unlikely be to formed in a significant
yield from OAn-1OPV*-C60. Hence, excitation of OAn-
OPV-C60 in solution will always provide OAn-OPV-1C60*
as an intermediate state, irrespective of the excitation wavelength
and solvent polarity.
Kinetic Considerations. The final outcome of a photoexci-
tation not only depends on the energetics of the reaction, but
also on the kinetics. Marcus theory provides an estimate for
the free energy barrier (∆Gq) for electron-transfer reactions
based on the change in free energy (∆G0) and the reorganization
energy (λ) via:
Charge recombination in OAn+-OPV- is almost an order
-
of magnitude slower than in OPV+-C60 and occurs in the
(∆G0 + λ)2
nanosecond regime. In this respect, a remarkable phenomenon
was observed when the fluorescence lifetimes of OAn-OPV
were recorded. In the most polar solvents o-dichlorobenzene
and benzonitrile, the lifetime of the OAn-OPV emission is
∆Gq )
(2)
4λ
The reorganization energy consists of an internal contribution

Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9277
TABLE 3: Rate Constants for the Intrinsic Decay of the Lowest-Energy Singlet-Excited Chromophore (kO0 PV, k0C60),
Singlet-Energy Transfer (kET), Charge Separation (kCS), Charge Recombination (kCR1), and Charge Shift (kCSH) in the Studied
Compounds in Toluene (TOL), Chlorobenzene (CB), o-Dichlorobenzene (ODCB), Benzonitrile (BZN), and in the Solid State
compound
TOL k (ns-1
)
CB k (ns-1
)
ODCB k (ns-1
)
BZN k (ns-1
)
film k (ns-1
)
OPV
C60
OPV
k0
k0
0.83
0.79
0.74
0.69
MP-C60
0.68
0.72
0.75
0.68
OPV-C60
kET
kCS
kCR1
kET
kCS
g5300
n.d.
g5300
50 ( 9
19 ( 3
g5300
71 ( 12
16 ( 3
15.6
n.d.
40 ( 2
12 ( 1
n.d.
250 ( 40
37 ( 8
n.d.
g3000
1.1
OAn-OPV-C60
OAn-OPV
g5300
48 ( 5
11 ( 1
16.4
201(54
61(19
40.8
g3000
49
kCR1+ kCSH
k’CS
k’CR
1.0
1.4
4.1
Figure 11. Time-resolved fluorescence of OPV and OAn-OPV in
Figure 9. Differential transmission dynamics of OAn-OPV-C60 (open
circles) and OPV-C60 (closed squares) in chlorobenzene monitored at
1030 nm with excitation at 450 nm. The induced absorption signal at
1450 nm as observed for the triad is shown with open squares. The
lines correspond to a numerical simulation based on the model described
in the text. For the simulation, the rate constants from Table 3 are used
and the rate for charge shift is used as an adjustable parameter. The
time delay has been shifted by 1 ps to show the signals on a logarithmic
plot.
toluene (TOL) and chlorobenzene (CB) recorded with excitation at 400
nm.
function of the energy barrier ∆Gq but also of the reorganization
energy (λ) and the electronic coupling (V) between donor and
acceptor in the excited-state according to the equation
(∆G0 + λ)2
4λkBT
1/2
4π2
k )
V2 exp -
(4)
2
(
)
[
]
h λkBT
The values of ∆G0, λ, and ∆Gq calculated on the basis of eqs
1-4 collected in Table 2 show that the initial charge separation
(kCS) is in the Marcus normal region (-∆G0 < λ). As the
polarity of the solvent increases, the OAn-OPV+-C60- charge-
separated state is stabilized with a concomitant increase of the
reorganization energy. The combination of these trends results
in reduction of the barrier for charge separation in more polar
solvents (Table 2). As a consequence, the rate for charge
separation (kCS) is expected to increase with polarity as has been
found experimentally (Figure 8, Table 3). Charge recombination
in OAn-OPV+-C60- is in the Marcus inverted region (-∆G0
> λ). The use of eq 4 in the inverted region often underestimates
the true rate constant because of nuclear tunneling30 but will
be used here for qualitative comparison. We find that the barrier
for charge recombination is strongly reduced in more polar
Figure 10. Differential transmission dynamics of OAn+-OPV- in
toluene (closed squares), chlorobenzene (closed circles), o-dichloroben-
zene (open circles), and benzonitrile (open squares) monitored at 1450
nm with excitation at 455 nm.
solvents, consistent with the higher recombination rate (kCR1
,
Table 3). Apart from recombination to the ground state, the
OAn-OPV+-C60 state may undergo a charge shift to form
-
OAn+-OPV-C60-. The charge shift occurs in the normal
region (-∆G0 < λ) and is energetically less favorable than the
recombination (Table 3). However, in contrast to the recombina-
tion, the barrier for charge shift is reduced with decreasing
polarity. Hence, the balance between charge recombination and
the competing charge shift, will move toward the latter with
decreasing polarity. As a result, the barrier for the charge shift
(λi) and a solvent term (λs), which can be approximated via the
Born-Hush approach to give after summation
e2 1 1
1
r-
1
Rcc
1
1
ꢀs
λ ) λ1 + λs ) λi +
+
-
-
2
(3)
+
( (
)
)(
)
4πꢀ0
2
r
n
The rate constants for the different processes are not only a

9278 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
TABLE 4: Relative Rate Constants for the Charge
Separation (CS), Charge Recombination (CR1), Charge Shift
(CSH), and Charge Recombination (CR2) in
o-Dichlorobenzene (ODCB) and Benzonitrile (BZN)
Compared to the Corresponding Rate Constant in
Chlorobenzene (CB), Calculated Using Eq 4
relative rate constant
CS
CR1
CSH
CR2
k(CB)
k(ODCB)
k(BZN)
1
2.00
2.16
1
89
980
1
0.58
0.23
1
6.25
5.06
Figure 13. (a) Differential transmission dynamics at 1450 nm of thin
films of OAn-OPV-C60 (open circles) and OPV-C60 (closed circles)
at 298 K with excitation at 450 nm. (b) Differential transmission
dynamics at 1030 nm of a thin film of OAn-OPV-C60 at 298 K with
excitation at 450 nm.
Figure 12. Photoinduced absorption spectra of OAn-OPV-C60 (solid
line) and OPV-C60 (dotted line) in thin films. Recorded at 80 K with
excitation at 458 nm (25 mW) and a modulation frequency of 275 Hz.
for the formation of an intramolecular (OAn-OPV+-C60-) or
intermolecular (OAn+-OPV-C60/OAn-OPV-C60-) charge-
separated state. The charge-separated state in the OPV-C60 and
OAn-OPV-C60 films, measured with this PIA technique,
extend into the millisecond time domain. The PIA band at 1.25
eV increases with the pump intensity (I) following a square-
root power law (-∆T/T ∼ I0.5). This suggests a nongeminate
bimolecular recombination of the photoinduced charges. We
propose that the long-lived (ms domain) charges observed with
near steady-state PIA in films of OPV-C60 and OAn-OPV-
in OAn-OPV+-C60- is lower than that for charge recombina-
tion in chlorobenzene but not in o-dichlorobenzene and ben-
zonitrile. This is in full agreement with the experimental result
that the charge shift was more easily observed in chlorobenzene
(Figures 8 and 9).
The energy barriers for relaxation to the ground state in
-
OAn+-OPV-C60 are small. However, besides the energy
barrier, the rate constant (eq 4) is also determined by the
electronic coupling V. This electronic coupling depends expo-
nentially on the center-to-center distance between the donor and
C60 at 80 K are associated with a small fraction of positive and
negative charges that have escaped from geminate recombination
by charge migration to other sites in the film where they became
trapped and are thus associated with different molecules in the
film (i.e., OAn+-OPV-C60 and OAn-OPV-C60-).
2
acceptor via V2 ) V0 (R0) exp(-â(Rcc - R0)), with R0 the
contact distance. Hence, V is orders of magnitude less for
OAn+-OPV-C60- (Rcc ) 30.0 Å) than for OAn-OPV+-C60
-
Femtosecond Pump-Probe Spectroscopy in the Solid
State. Femtosecond spectroscopy shows that in films of OPV-
C60 and OAn-OPV-C60 at room temperature a charge-
separated state is formed within 0.5 ps, as evidenced by the
instantaneous rise of the 1450 nm differential transmission
(Figure 13a) associated with OPV+ radical cations. Although a
lack of higher time-resolution precludes an unambiguous
conclusion, we have no evidence that a singlet-energy transfer
precedes the electron-transfer reaction in the solid state. Possibly,
the photoinduced electron transfer in the solid state is predomi-
(Rcc ) 15.4 Å).31 The reduction of the electronic coupling V,
caused by the longer distance between the centers of positive
and negative charge density in OAn+-OPV-C60-, is the origin
of the increase in lifetime for OAn+-OPV-C60- compared to
-
OAn-OPV+-C60
.
The rate constants of the various photoinduced processes in
solvents of different polarity have been calculated using eq 4
relative to the corresponding rate constant in chlorobenzene by
assuming that V is independent of the solvent and are collected
in Table 4.
nantly intermolecular. The OPV+-C60 state is much longer
-
Near Steady-State PIA Spectroscopy in the Solid State.
Near steady-state PIA spectra of thin films of OPV-C60 and
OAn-OPV-C60 were recorded at 80 K with excitation at 458
nm. The PIA spectrum of the OPV-C60 film (Figure 12)
exhibits the signals of OPV+ at 0.68 and 1.52 eV and that of
C60- at 1.24 eV, characteristic of a charge-separated state. The
PIA spectrum of a film of OAn-OPV-C60 (Figure 12) lacks
the bands at 0.68 and 1.52 eV and exhibits only one broad band
that peaks at 1.25 eV. This signal is attributed to the overlapping
transitions of the OAn+ and C60- radical ions and gives evidence
lived in the film than in solution (Table 3). This phenomenon
has also been observed in other donor-acceptor dyads and has
been attributed to the migration of opposite charges to different
sites in the film.8h,9d,32 In contrast, the OPV+ signal in the film
of the triad rapidly decays, with a time constant of 20 ps over
the first 100 ps. This is interpreted to result from an intermo-
lecular or intramolecular charge shift (CSH) from OPV+ to
OAn+. The time profile of the 1030 nm differential transmission
(Figure 13b) is consistent with the formation of an OAn+-
OPV-C60- state in the triad and explains the short lifetime of

Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9279
the OAn-OPV+-C60 state (Figure 13a). This time profile
we consider it likely that in the film the primary charge-
separated state involves two molecules, also the charge shift
-
cannot be fitted to monoexponential decay and suggests several
lifetimes. This fact can be rationalized by a combination of a
direct charge recombination and an indirect charge recombina-
tion after migration of the charges in the film.8h,9d It should be
noticed, however, that contrary to what is observed for the
OPV-C60 dyad the OAn+-OPV-C60- state seems to be longer
probably involves an intermolecular OAn-OPV+-C60
f
OAn+-OPV-C60 reaction, with the negative charge located
on the fullerene unit of a third molecule (OAn-OPV-C60-).
The lifetime of the charges formed in the film (OAn+-OPV-
C60/OAn-OPV-C60-) is somewhat less as compared to the
solution.
-
lived in solution than the OAn+-OPV-C60/OAn-OPV-C60
state in the film. As a tentative explanation for this difference,
we propose that in solution the triads are isolated from each
other such that the weak electronic coupling between the OAn
and the C60 decelerates the intramolecular charge recombination
in OAn+-OPV-C60-. In the film, the triads are in intimate
contact with each other, and intermolecular charge recombina-
The experiments on OAn-OPV-C60 demonstrate that in
solution the OAn-OPV+-C60- f OAn+-OPV-C60- charge
shift and the resulting spatial extension of the charges increase
the lifetime of the charge-separated state compared to OAn-
OPV+-C60-, because the electronic coupling between the redox
active groups is strongly reduced. This provides a rationale to
explain the long lifetime of the charge-separated state in
conjugated polymer:C60 blends in terms of charge migration.
The major differences in the kinetics of the electron-transfer
reactions observed after photoexcitation of OAn-OPV-C60 in
solution or in thin films further demonstrate that intermolecular
interactions are of crucial significance in this respect. Creating
and investigating well-defined multichromphoric supramolecular
donor-acceptor assemblies, consisting of many judiciously
positioned chromophores, will enable a more detailed under-
standing of photoinduced charge-separation processes in natural
and artificial systems.
-
tion between OAn+-OPV-C60 and OAn-OPV-C60 can
occur.
Conclusions
We have synthesized a molecular triad, OAn-OPV-C60,
with a redox gradient in a linear array. The photophysical
processes that may occur in this system are schematically
depicted in Figure 4 and have been investigated in solution and
in thin films with photoluminescence and transient absorption
spectroscopy.
In solution, photoexcitation of either chromophore of the
OAn-OPV-C60 triad results in an ultrafast (sequence of)
singlet-energy transfer (ET, Figure 4) and provides a singlet
state on the fulleropyrrolidine unit (OAn-OPV-1C60*), ir-
respective of the polarity of the solvent. The competitive process
Experimental Section
All reagents and solvents were used as received or purified
using standard procedures. C60 was purchased from Bucky,
U.S.A. NMR spectra were recorded on a Varian Unity Inova
and a Varian Unity Plus at frequencies of 500 and 125 MHz
1
of charge separation from the primary OAn*-OPV-C60 or
OAn-1OPV*-C60 states are more than 1 order of magnitude
slower and were not observed in the triad. In toluene, the singlet
OAn-OPV-1C60* state decays via intersystem crossing (ISC)
to the OAn-OPV-3C60* triplet state and via PL to the ground
state. In more polar solvents, the singlet OAn-OPV-1C60* state
gives rise to an intramolecular charge separation reaction (CS)
1
for H and 13C nuclei, a Varian Mercury Vx at frequencies of
1
400 and 100 MHz for H and 13C nuclei, or Varian Gemini
2000 at frequencies of 300 and 75 MHz for 1H and 13C nuclei,
respectively. Tetramethylsilane (TMS) was used as an internal
standard for 1H NMR, and CDCl3, CD3COCD3, or CS2 for was
used for 13C NMR. Infrared (FT-IR) spectra were recorded on
a Perkin-Elmer Spectrum One UATR FT-IR. Elemental analyses
were preformed on a Perkin-Elmer 2400 series II CHN
Analyzer. Matrix assisted laser desorption ionization time-of-
flight mass spectrometry (MALDI-TOF MS) was performed on
a Perseptive DE PRO Voyager MALDI-TOF mass spectrometer
using a dithranol matrix. All HPLC analyses were performed
on a Hewlett-Packard HP LC-Chemstation 3D (HP 1100
Series) with DAD detection using an Inertsil 5 Si column (250
× 3 mm). A Shimadzu LC-10AT system combined with a
Polymer Laboratories MIXED-D column (particle size: 5 µm;
length/i.d. (mm): 300 × 7.5) and UV detection were employed
for size exclusion chromatography (SEC), using CHCl3 as an
eluent (1 mL/min).
that generates the OAn-OPV+-C60 state. The rate for this
-
forward electron-transfer reaction increases with the polarity of
the solvent from kCS ) 4.8 × 1010 s-1 in chlorobenzene to kCS
) 2.0 × 1011 s-1 in benzonitrile, in qualitative agreement with
Marcus theory. Because the oxidation potential of the OAn
segment is below that of the OPV unit, the primary OAn-
OPV+-C60 charge-separated state may undergo an intramo-
-
lecular redox reaction, or charge shift (CSH), to form OAn+-
OPV-C60-. The charge recombination in OAn-OPV+-C60
-
(CR1), however, competes with the charge shift. Because the
charge recombination is slowed in less polar solvents, the
quantum yield for the charge shift is the highest (∼0.4) in the
least polar solvent in which electron transfer occurs, i.e.,
chlorobenzene. The charge recombination in the secondary
OAn+-OPV-C60- charge-separated state (CR2) is significantly
slower (kCR2 < 1 × 109 s-1) than that of the primary OAn-
(E,E)-4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-
2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbu-
toxy]}-5-benzaldehyde-dimethylacetal (2). Amberlite IR 120
(1.5 g), trimethyl orthoformate (20 mL), and 1 (1.2 g, 1.78
mmol) were added to 100 mL of methanol. The suspension was
stirred under an argon atmosphere at 70 °C for 2 h. The reaction
mixture was cooled to room temperature, and 1.5 g of Na2CO3
was added. The suspension was filtered, and the solvent was
removed in vacuo to yield 1.38 g (91%) of 2, which was used
without further purification.1H NMR (CDCl3, 400 MHz): δ
(ppm) 7.50 (d, 1H),7.49 (s, 2H), 7.44 (d, 1H), 7.18 (s, 1H),
7.17 (s, 1H), 7.16 (s, 1H), 7.10 (s, 1H), 7.08 (s, 1H), 6.73 (s,
1H), 3.92-3.74 (m, 12H), 3.42 (s, 6H), 2.24 (s, 3H), 1.98-
1.88 (m, 6H), 1.69-1.54 (m, 6H), 1.39-1.26 (m, 6H), 1.10-
OPV+-C60 state (kCR1 ) 1.1 × 1010 s-1). Hence, the
-
stabilization gained is more than 1 order of magnitude in time.
The observed trends in the various rate constants (kCS, kCSH
CR1, kCR2, kS′ C, and k′CR) with changing solvent polarity are in
,
k
qualitative agreement with Marcus theory when the free energies
of the charge-separated states are determined using a continuum
model (eq 1).
In thin films, charge generation on the OPV unit of OAn-
OPV-C60 is much faster (kCS g 3.0 × 1012 s-1) and likely
predominantly intermolecular. In the films, a subsequent charge-
shift occurs from the primary OPV+ radical cation to an OAn+
radical cation with a rate close to kCSH ) 5.0 × 1010 s-1. Because

9280 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
0.96 (m, 36 H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 151.69,
151.10, 150.96, 150.67, 150.45, 128.17, 127.71, 127.57, 126.98,
126.56, 125.21, 123.19, 123.06, 122.54, 121.71, 116.32, 111.80,
110.14, 109.94, 109.26, 108.39, 99.75, 74.69, 74.35, 74.28,
74.23, 73.73, 73.38, 54.34, 35.11, 35.08, 35.05, 34.96, 34.89,
26.36, 26.26, 26.21, 16.79, 16.70, 16.40, 11.46, 11.37, 11.33.
36H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 189.06, 156.43,
151.44, 151.26, 151.04, 150.64, 148.21, 147.84, 142.91, 142.70,
139.13, 135.22, 129.45, 129.19, 129.18, 128.50, 128.45, 127.45,
126.74, 126.48, 126.42, 125.46, 125.30, 123.97, 123.79, 123.74,
123.56, 123.19, 122.98, 122.56, 122.43, 122.40, 122.21, 121.90,
120.29, 110.64, 110.29, 110.17, 109.82, 109.74, 109.64, 74.39,
74.25, 74.11, 74.05, 73.91, 73.69, 35.13, 35.01, 34.93, 34.88,
34.84, 26.38, 26.34, 26.33, 26.18, 16.88, 16.86, 16.81, 16.76,
N-(4-Diphenylaminophenyl)-N-phenyl-3-aminobenzalde-
hyde (4). To a tube fitted with a magnetic stirrer was added 3
(0.8 g, 2.38 mmol), 3-bromobenzaldehyde (1.32 g, 7.14 mmol),
Pd2(dba)3 (0.022 g, 0.024 mmol), BINAP (0.044 g, 0.071 mmol),
and Cs2CO3 (1.16 g, 3.57 mmol). After purging with argon,
freshly distilled toluene (11.9 mL) was added. The reaction
mixture was heated at 100 °C under Ar atmosphere. After 48
h, Pd2(dba)3 (0.022 g, 0.024 mmol), BINAP (0.044 g, 0.071
mmol), and Cs2CO3 (1.16 g, 3.57 mmol) were added, and the
mixture was heated for another 72 h. After cooling to room
temperature, the reaction mixture was filtered over Celite 545
and concentrated in vacuo. Column chromatography (SiO2,
heptane/CH2Cl2 1:1, Rf ) 0.4) and evaporation to dryness from
heptane yielded 0.643 g (62%) of a yellow powder. IR (UATR)
ν (cm-1) 3034, 2923, 2851, 1698, 1583, 1485, 1277, 1263, 749,
690, 626. 1H NMR (CDCl3, 400 MHz): δ (ppm) 9.92 (s, 1H),
7.56 (t, 1H), 7.43(dt, 1H), 7.37 (t, 1H), 7.34-7.23 (m, 7H),
7.13-7.11 (m, 6H), 7.08-6.96 (m, 7H). 13C NMR (CDCl3, 100
MHz): δ (ppm) 192.24, 149.14, 148.00, 147.45, 144.17, 142.11,
137.91, 129.98, 129.70, 129.45, 128.18, 126.37, 125.41, 124.62,
124.20, 123.67, 123.07, 122.91, 122.66. Anal. Calcd for
C31H24N2O: C, 84.5; H, 5.5; N, 6.4. Found: C, 84.1; H, 5.1;
N, 6.1.
16.63, 11.52, 11.46, 11.38, 11.32. Anal. Calcd for C85H102
-
N2O7: C, 80.8; H, 8.1; N, 2.2. Found: C, 80.7; H, 7.6; N, 2.2.
MALDI-TOF MS (MW ) 1263.75) m/z ) 1263.71 [M]+.
N-Methyl-2- 4-[4-{4-[N′-(4-diphenylaminophenyl)-N′-(phen-
yl)]3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis-
[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy] -3,4-
fulleropyrrolidine (7). A solution of aldehyde 6 (0.2 g, 0.158
mmol), finely ground N-methylglycine (84.4 mg, 0.949 mmol),
and C60 (227 mg, 0.316 mmol) in chlorobenzene (60 mL) was
stirred and refluxed in the dark under an atmosphere of dry
nitrogen for 18 h. After cooling to room temperature, the solvent
was removed in vacuo, and the remaining residue was purified
by column chromatography on silica gel (toluene/cyclohexane:
2/1, Rf ) 0.4) to afford triad 7 as a ∼1/1 mixture of
diastereomers according to HPLC (eluent: toluene/cyclohexane
50/50 v/v; flow: 1 mL/min.; peaks at tr ) 9.1 and 10.3 min.).
Traces of impurities were effectively removed after a 2nd
chromatographic purification on silica gel (CS2/toluene: 1/0 to
7/3 Rf ) 0.3) to afford analytically pure material (assay >99.5%)
according to HPLC and GPC analysis. The product was
precipitated from a concentrated toluene solution with methanol
(100 mL), and the resulting solid was washed with methanol
(2 × 100 mL) and finally dried in vacuo at 55 °C. The triad
was obtained as a light brown powder (137 mg, 43%). IR
(UATR) ν (cm-1) 2957, 2914, 2872, 1589, 1502, 1491, 1264,
1190, 965, 750, 694, 526. 1H NMR (CS2, 500 MHz): δ (ppm)
7.61-6.96 (m, 35H), 5.62 (s, 1H), 5.05 (d, 1H), 4.41 (d, 1H),
4.15-3.70 (m, 12H), 2.92 (s, 3H), 2.16-1.26 (m, 18H), 1.02-
1.26 (m, 36H). 13C NMR (CS2, 125 MHz): δ (ppm) 156.50,
154.91, 154.84, 154.18, 154.15, 153.53, 151.61, 150.93, 150.76,
150.71, 150.67, 150.63,147.79, 147.40, 147.06, 146.60, 146.57,
146.52, 146.13, 146.07, 146.03, 146.00, 145.93, 145.89, 145.85,
145.75, 145.50, 145.47, 145.37, 145.26, 145.19, 145.07, 145.04,
145.01, 144.98, 144.91, 144.50, 144.37, 144.31, 144.15, 142.92,
142.86, 142.51, 142.46, 142.43, 142.40, 142.35, 142.12, 142.10,
142.06, 141.99, 141.98, 141.95, 141.92, 141.79, 141.63, 141.53,
141.51, 140.06, 139.98, 139.69, 139.51, 139.07, 136.35, 135.99,
135.88, 135.86, 134.49, 134.44, 129.42, 129.20, 129.17, 127.99,
127.68, 127.66, 127.40, 127.27, 126.88, 126.30, 125.24, 125.14,
124.85, 124.80, 123.79, 123.70, 123.61, 123.29, 122.69, 122.56,
122.52, 122.47, 122.29, 121.96, 120.39, 114.29, 114.16, 110.04,
109.63, 109.40, 109.22, 108.71, 108.61, 76.53, 75.43, 75.40,
73.97, 73.91, 73.69, 73.59, 73.52, 73.44, 72.95, 72.91, 69.70,
68.98, 40.02, 35.41, 35.31, 35.29, 35.25, 35.20, 35.18, 35.17,
26.86, 26.84, 26.80, 26.49, 17.14, 17.09, 17.06, 17.02, 17.00,
16.80, 12.04, 12.03, 12.00, 11.94, 11.89, 11.85. Anal. Calcd
for C147H107N3O6: C, 87.7; H, 5.4; N, 2.1. Found: C, 87.8; H,
5.0; N, 2.1. MALDI-TOF MS (MW ) 2011.44) m/z ) 2011.18
[M]+.
(E)-N,N′-(Diphenyl)-N′-(4-diphenylaminophenyl)-3-ami-
nobenzaldimine (5). To a suspension of 4 (0.55 g, 1.24 mmol)
in ethanol (50 mL) was added aniline (0.14 g, 1.50 mmol). The
reaction mixture was heated at 85 °C for 4 h. After cooling to
room temperature, the product precipitated slowly from the
ethanol. The product was obtained as 0.508 g (79%) of a yellow
powder after washing with ethanol. IR (UATR) ν (cm-1) 3034,
1628, 1589, 1486, 1267, 751, 693. 1H NMR (CDCl3, 400
MHz): δ (ppm) 8.34 (s, 1H), 7.61 (t, 1H), 7.53 (dt, 1H), 7.38-
7.31(m, 3H), 7.28-7.10 (m, 16H), 7.03-6.97 (m, 7H). 13C
NMR (CDCl3, 100 MHz): δ (ppm) 160.31, 151.99, 148.47,
147.78, 147.56, 143.29, 142.34, 137.40, 129.62, 129.33, 129.19,
129.09, 126.32, 125.91, 125.66, 125.26, 123.95, 123.88, 123.53,
122.82, 122.52, 122.44, 120.85. Anal. Calcd for C37H29N3: C,
86.2; H, 5.7; N, 8.1. Found: C, 85.8; H, 5.3; N, 8.0. MALDI-
TOF MS (MW ) 515.65) m/z ) 515.23 [M]+.
(E,E,E)-4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-
3-aminostyryl]2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-
2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]benzal-
dehyde (6). Schiff base 5 (0.450 g, 0.873 mmol) and acetal 2
(0.775 g, 0.873 mmol) were dissolved in DMF (5 mL). The
mixture was heated to 80 °C under an argon atmosphere, and
potassium tert-butoxide (0.352 g, 3.142 mmol) was added. The
reaction mixture was stirred for 3 h. After cooling to room
temperature, the reaction mixture was poured on ice, washed
with HCl 3N and brine, dried over MgSO4, and concentrated
in vacuo. Column chromatography (SiO2, toluene/cyclohexane
7:3 Rf ) 0.4, and heptane/CH2Cl2 6:2, Rf ) 0.2) and evaporation
to dryness from heptane yielded 0.795 g (72%) of an orange
powder. IR (UATR) ν (cm-1) 3062, 2960, 2917, 2873, 1675,
(E,E)-2-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-
2,5-bis[(S)-2-methylbutoxy]styryl}-1,4-bis[(S)-2-methylbutoxy]-
5-deuteriobenzene (9). n-BuLi 1.6 M (0.13 mL, 0.21 mmol)
was added dropwise to a solution of 8 (0.15 g, 0.16 mmol) in
freshly distilled diethyl ether (3 mL) at -10° C. The reaction
mixture was stirred for 5 min. After the addition of D2O (0.5
mL), the cooling bath was removed, and the reaction mixture
1
1589, 1504, 1490, 1422, 1262, 1200, 1039, 968, 744, 693. H
NMR (CDCl3, 500 MHz): δ (ppm) 10.43(s, 1H), 7.64 (d, 1H),
7.54 (d, 1H), 7.51 (d, 1H), 7.50 (d, 1H), 7.40 (d, 1H), 7.33 (s,
1H), 7.28-6.98 (m, 29H), 3.79 3.98 (m, 12H), 1.98-1.91 (m,
6H), 1.70-1.57 (m, 6H), 1.39-1.31 (m, 6H), 1.13-0.94 (m,

Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9281
was stirred for 2 h at room temperature. The mixture was
estracted with diethyl ether dried over MgSO4 and concentrated
in vacuo. Column chromatography (SiO2; heptane/CH2Cl2: 1/1,
Rf ) 0.3) and recrystallization from ethanol yielded 54 mg
(41%) of the product as yellow crystals. 1H NMR (CDCl3, 400
MHz): δ (ppm) 7.49 (d, 1H), 7.48 (s, 2H), 7.44 (d, 1H), 7.19-
(s, 1H), 7.18 (s, 1H), 7.17 (s, 1H), 7.10 (s, 1H), 6.82 (s, 1H),
6.72(s, 1H), 3.92-3.71 (m, 12H), 1.99-1.82 (m, 6H), 1.68-
1.54 (m, 6H), 1.38-1.23 (m, 6H), 1.20-0.86 (m, 36H). 13C
NMR (CDCl3, 100 MHz): δ (ppm) 151.68, 151.14, 150.94,
150.90, 150.44, 128.21, 127.61, 127.54, 127.10, 125.23, 123.24,
122.98, 122.82, 121.74, 116.31, 113.96, 111.58, 110.36, 109.85,
108.34, 74.68, 74.46, 74.30, 74.26, 73.59, 73.38, 35.11, 35.06,
35.04, 34.97, 34.80, 26.36, 26.34, 26.26, 26.19, 16.80, 16.70,
16.56, 16.40, 11.46, 11.40, 11.37, 11.31. Anal. Calcd for C53H79-
DO6: C, 77.8; H, 10.2. Found: C, 77.5; H, 9.1. MALDI-TOF
MS (MW ) 813.96) m/z ) 813.53[M]+.
(E,E,E)-2-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-
3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis-
[(S)-2-methylbutoxy]styryl]-1,4-bis[(S)-2-methylbutoxy]-5-
deuteriobenzene (10). Schiff base 5 (0.025 g, 0.049 mmol) and
9 (0.40 g, 0.049 mmol) were dissolved in DMF (5 mL). The
mixture was heated at 80 °C under an argon atmosphere, and
potassium tert-butoxide (0.020 g, 0.176 mmol) was added. The
reaction mixture was stirred for 5 h at 80 °C. After cooling to
room temperature, the reaction mixture was poured onto ice
and washed with HCl 3 N and brine. The organic layer was
dried over MgSO4 and concentrated in vacuo. Column chro-
matography (SiO2, toluene/cyclohexane 7:3, Rf ) 0.6) and
recyrstallization from hexane, heptane, and a few drops of CH2-
Cl2 yielded 30 mg of 10 (50%) as a yellow powder. IR (UATR)
ν (cm-1) 2960, 2917, 2873, 1588, 1504, 1489, 1255, 1205, 1042,
970, 744, 693. 1H NMR (CD3COCD3, 400 MHz): δ (ppm) 7.61
(s, 2H), 7.60 (d, 1H), 7.55 (d, 1H), 7.47(d, 1H), 7.35-6.95 (m,
30H), 4.01-3.76 (m, 12H), 2.09-1.83 (m, 6H), 1.74-1.55 (m,
6H), 1.45-1.26 (m, 6H), 0.95-1.15 (m, 36H), 13C NMR (100
MHz): δ (ppm) 155.25, 152.89, 152.84, 152.72, 150.10, 149.58,
149.54, 144.87, 144.58, 141.06, 131.32, 131.03, 130.98, 130.19,
129.47, 129.13, 128.96, 128.91, 128.15, 127.30, 127.04, 125.39,
125.35, 125.16, 124.93, 124.87, 124.73, 124.68, 124.37, 124.34,
124.30, 123.17, 122.16, 115.71, 113.14, 112.34, 111.95, 111.58,
111.39, 75.61, 75.56, 75.53, 75.36, 74.72, 36.76, 36.68, 36.63,
36.44, 27.85, 27.81, 27.60, 18.04, 17.98, 17.95, 17.92, 17.60,
12.63, 12.60, 12.58, 12.52, 12.39. Anal. Calcd for C85H102N2O7:
C,81.6; H, 8.2; N, 2.3. Found: C, 81.2; H, 7.7; N, 2.2. MALDI-
TOF MS (MW ) 1236.74) m/z ) 1235.62 [M]+.
su microchannel plate photomultiplier (R3809U-50). Lifetimes
were determined from the data using the Edinburgh Instruments
software package.
Near Steady-State PIA. Solutions were prepared in a
nitrogen-filled glovebox in order to exclude interference of
oxygen during measurements. The PIA spectra were recorded
between 0.5 and 3.0 eV by excitation with a mechanically
modulated cw Ar ion laser (λ ) 458 or 528 nm, 275 Hz) pump
beam and monitoring the resulting change in transmission of a
tungsten-halogen probe light through the sample (∆T) with a
phase-sensitive lock-in amplifier after dispersion by a grating
monochromator and detection, using Si, InGaAs, and cooled
InSb detectors. The pump power incident on the sample was
typically 25 mW with a beam diameter of 2 mm. The PIA
(-∆T/T ≈ ∆O.D.) was directly calculated from the change in
transmission after correction for the PL, which was recorded in
a separate experiment. PIA and PL spectra were recorded with
the pump beam in a direction almost parallel to the direction of
the probe beam. The solutions were studied in a 1 mm near-IR
grade quartz cell at room temperature. Solvents for PIA
measurements were distilled under nitrogen before use. The
solid-state measurements were performed on films, drop cast
from chloroform solution on quartz substrate, and held at 80 K
in an Oxford continuous flow cryostat.
Transient Subpicosecond Photoinduced Absorption. The
femtosecond laser system used for pump-probe experiments
consists of an amplified Ti/sapphire laser (Spectra Physics
Hurricane). The single pulses from a cw mode-locked Ti/
sapphire laser were amplified by a Nd:YLF laser using chirped
pulse amplification, providing 150 fs pulses at 800 nm with an
energy of 750 µJ and a repetition rate of 1 kHz. The pump
pulses at 450 nm were created via optical parametric amplifica-
tion (OPA) of the 800 nm pulse by a BBO crystal into infrared
pulses which were then two times frequency doubled via BBO
crystals. The probe beam was generated in a separate optical
parametric amplification setup in which 1030 and 1450 nm
pulses were created. The pump beam was focused to a spot
size of about 1 mm2 with an excitation flux of 1 mJ cm-2 per
pulse. For the 1030 and 1450 nm pulses a RG 850 nm cutoff
filter was used to avoid contributions of residual probe light
(800 nm) from the OPA. The probe beam was reduced in
intensity compared to the pump beam by using neutral density
filters of OD ) 2. The pump beam was linearly polarized at
the magic angle of 54.7° with respect to the probe, to cancel
out orientation effects in the measured dynamics. The temporal
evolution of the differential transmission was recorded using
Si or an InGaAs detector by a standard lock-in technique at
500 Hz. Solutions in the order of 2-5 × 10-4 M were excited
at 450 nm, i.e., providing primarily excitation of the OPV part
within the molecules. The solid-state measurements were
performed on films, drop cast from chloroform solution.
Electrochemistry. Cyclic voltammograms were measured in
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as
a supporting electrolyte in dichloromethane (or THF) using a
Potentioscan Wenking POS73 potentiostat. The working elec-
trode was a Pt disk (0.2 cm2), the counter electrode was a Pt
plate (0.5 cm2), and a saturated calomel electrode (SCE) was
used as reference electrode, calibrated against Fc/Fc+ (+0.43
V).
Acknowledgment. This research has been supported by the
Dutch Ministry of Economic Affairs, the Ministry of Education,
Culture and Science, and the Ministry of Housing, Spatial
Planning, and the Environment through the E.E.T. program
(EETK97115). The research in Eindhoven has also been
supported by The Netherlands Organization for Scientific
Research (NWO) and the Eindhoven University of Technology
through a grant in the PIONIER program. The research of S.
C. J. Meskers has been made possible by a fellowship of the
Royal Netherlands Academy of Arts and Sciences.
Absorption and Photoluminescence. UV/visible/near-IR
absorption spectra were recorded on a Perkin-Elmer Lambda
900 spectrophotometer. Fluorescence spectra were recorded on
an Edinburgh Instruments FS920 double-monochromator spec-
trometer and a Peltier-cooled red-sensitive photomultiplier.
Time-Correlated Single Photon Counting. Time-correlated
single photon counting fluorescence studies were performed
using an Edinburgh Instruments LifeSpec-PS spectrometer. The
LifeSpec-PS comprises a 400 nm picosecond laser (PicoQuant
PDL 800B) operated at 2.5 MHz and a Peltier-cooled Hamamat-

9282 J. Phys. Chem. A, Vol. 107, No. 44, 2003
Marcos Ramos et al.
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Supporting Information Available: Further details of the
mathematical modeling. This material is available free of charge
via the Internet at http://pubs.acs.org.
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(22) The assumption that charges are located at the centers is of course
a simplification of the actual situation in which charges are delocalized.
Especially for C60, the charges are not expected to be at the center but
rather at the outer surface.
(23) The PL lifetime of the residual emission at 510 nm contained a
contribution of long lifetime (∼1 ns) and is in part attributed to residual
PL emission by minor impurities in the sample.
(24) In toluene, the energy level of the initial charge separated state
[OAn-OPV+-C60-] is higher than that of OAn-OPV-1C60* due to the
destabilization in the nonpolar solvent. In such a case, one can also expect
that initial electron transfer from 1OPV* to C60 occurs to yield OAn-
OPV+-C60-, which subsequently recombines to generate OAn-OPV-
1C60*. The stepwise pathway would compete with the direct energy transfer
1
pathway from OPV* to C60 to produce the same state. However, in view
of the high rate of the energy transfer (190 fs), it is very unlikely that this
process occurs and the same rate of 190 fs has been found in ODCB, where
the OAn-OPV+-C60- state is below the OAn-OPV-1C60* state (ref 9e).
(25) The function f(t) ) a0(exp(-a1(t - a3)) - exp(-a2(t - a3)) + a4)
was used to fit the data, and all 5 parameters (a0 - a4) were optimized.
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Energy and ET in an OAn-OPV-C60 Triad
J. Phys. Chem. A, Vol. 107, No. 44, 2003 9283
(27) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
(28) Because the experimental data (Figure 9, open circles) show no
decay in the first nanosecond, a fit of the lifetime of the OAn+-OPV-
C60- state cannot give a reliable value. In the absence of a decay, we have
arbitrarily set kCR2 ) 0 s-1. Further details are described in the Supporting
Information.
(30) Jortner, J. J. Chem. Phys. 1976, 64, 4860.
(31) Imahori et. al. reported that the electronic coupling in a triad (0.019
cm-1) is much less compared to that in a dyad (3.9 cm-1), see ref 15o.
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