Alternating oligo(p-phenylene vinylene)-perylene bisimide copolymers: Synthesis, photophysics, and photovoltaic properties of a new class of donor-acceptor materials

Article abstract of DOI:10.1021/ja034926t

A Suzuki polycondensation reaction has been used to synthesize two copolymers consisting of alternating oligo(p-phenylene vinylene) (OPV) donor and perylene bisimide (PERY) acceptor chromophores. The copolymers differ by the length of the saturated spacer that connects the OPV and PERY units. Photoinduced singlet energy transfer and photoinduced charge separation in these polychromophores have been studied in solution and in the solid state via photoluminescence and femtosecond pump-probe spectroscopy. In both polymers a photoinduced electron transfer occurs within a few picoseconds after excitation of the OPV or the PERY chromophore. The electron transfer from OPV excited state competes with a singlet energy transfer state to the PERY chromophore. The differences in rate constants for the electron- and energy-transfer processes are discussed on the basis of correlated quantum-chemical calculations and in terms of conformational preferences and folding of the two polymers. In solution, the lifetime of the charge-separated state is longer than in the films where geminate recombination is much faster. However, in the films some charges are able to escape from geminate recombination and diffuse away and can be collected at the electrodes when the polymers are incorporated in a photovoltaic device.

Full text of DOI:10.1021/ja034926t

Published on Web 06/18/2003
Alternating Oligo(p-phenylene vinylene)-Perylene Bisimide
Copolymers: Synthesis, Photophysics, and Photovoltaic
Properties of a New Class of Donor-Acceptor Materials
Edda E. Neuteboom,† Stefan C. J. Meskers,† Paul A. van Hal,†
Jeroen K. J. van Duren,† E. W. Meijer,† Rene´ A. J. Janssen,*,† He´le`ne Dupin,‡
Geoffrey Pourtois,‡ Je´roˆme Cornil,‡,§ Roberto Lazzaroni,‡,§ Jean-Luc Bre´das,‡,§ and
David Beljonne‡,§
Contribution from the Laboratory of Macromolecular and Organic Chemistry,
EindhoVen UniVersity of Technology, P.O. Box 513 5600 MB EindhoVen, The Netherlands
Chemistry of NoVel Materials, UniVersity of Mons-Hainaut, Place du Parc 20,
7000 Mons, Belgium, and Department of Chemistry, The UniVersity of Arizona,
Tucson, Arizona 85721
Received February 28, 2003; E-mail: r.a.j.janssen@tue.nl
Abstract: A Suzuki polycondensation reaction has been used to synthesize two copolymers consisting of
alternating oligo(p-phenylene vinylene) (OPV) donor and perylene bisimide (PERY) acceptor chromophores.
The copolymers differ by the length of the saturated spacer that connects the OPV and PERY units.
Photoinduced singlet energy transfer and photoinduced charge separation in these polychromophores have
been studied in solution and in the solid state via photoluminescence and femtosecond pump-probe
spectroscopy. In both polymers a photoinduced electron transfer occurs within a few picoseconds after
excitation of the OPV or the PERY chromophore. The electron transfer from OPV excited state competes
with a singlet energy transfer state to the PERY chromophore. The differences in rate constants for the
electron- and energy-transfer processes are discussed on the basis of correlated quantum-chemical
calculations and in terms of conformational preferences and folding of the two polymers. In solution, the
lifetime of the charge-separated state is longer than in the films where geminate recombination is much
faster. However, in the films some charges are able to escape from geminate recombination and diffuse
away and can be collected at the electrodes when the polymers are incorporated in a photovoltaic device.
Introduction
Photoinduced electron transfer between donor and acceptor
materials is also the initial step in organic and polymer solar
cells. In these cells excitations, created by the absorption of
light, must be able to diffuse to the interface between the two
materials where charge generation can occur. However, the
exciton diffusion length in organic and polymer materials is
often limited to about 10 nm. This implies that an intimate,
nanoscopic, mixing of donor and acceptor is favorable for charge
creation. Following this principle, the bulk-heterojunction cell,
in which a spontaneous phase separation of the two components
into a disordered blend is used to create a large interface, has
become one of the most promising concepts in the field of
polymer solar cells and resulted in external quantum efficiencies
of more than 50% at the absorption maximum.7-9 A further
increase of performance is expected by designing materials that
have an improved overlap of their absorption spectrum with
the terrestrial solar radiation, especially in the near-infrared
region.
Photoinduced energy and electron transfer between photo-
active and electroactive donor (D) and acceptor (A) moieties
continues to be an extremely active area of research.1-6
Numerous photophysical studies of organic D-A molecules
have been performed to elucidate and control resonant energy
transfer, charge separation, and recombination and to assess the
role of intermediates in the deactivation of the initially excited
states. The intriguing photophysical and photochemical pro-
cesses that take place in the natural photosynthetic reaction
center have stimulated efforts to design and create artificial
donor-acceptor architectures in an attempt to mimic the
conversion of light into chemical energy or to create electrical
power directly.
† Eindhoven University of Technology.
‡ University of Mons-Hainaut.
§ The University of Arizona.
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VCH: Weinheim, 1993.
A general drawback of the bulk-heterojunction is the fact that
the transport and collection of charges in a disordered nanoscale
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J. AM. CHEM. SOC. 2003, 125, 8625-8638
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A R T I C L E S
Neuteboom et al.
Figure 1. Structure of oligo(p-phenylene vinylene)-perylene bisimide alternating copolymers P1 and P2 and their model compounds M1, M2, and M3.
blend can be hindered by phase boundaries and discontinuities
such as spherical objects and cul-de-sacs. Therefore, the extent,
the characteristic dimension, and the contours of the phase
separation in polymer-polymer or polymer-molecule blends
are essential parameters of bulk-heterojunction solar cells. The
actual morphology depends, among others, on details of the
preparation procedure (e.g., solvent, temperature, drying speed,
substrate). The as-prepared morphology is likely to be kinetically
determined rather than thermodynamically stable and subject
to further reorganization in time or with temperature.
One way to overcome these drawbacks is by covalently
linking the donor and acceptor in a single polymer chain. The
covalent bond enables a predefined control over the character-
istic distance between donor and acceptor and thereby the extent
of phase separation. Two types of covalently linked donor-
acceptor polymers have recently come to the attention. The first
consists of semiconducting polymers as a donor with pendant
acceptor groups,10-15 while the second has extended donor and
acceptor units arranged in a diblock copolymer.16,17 The latter
strategy may have important advantages because the intrinsic
tendency of each segment in block copolymers to aggregate in
an individual phase provides a means to create a well-ordered
nanoscale morphology (e.g., spheres, cylinders, lamellae),
governed by the relative volume fractions. While this principle
has been utilized in various engineering materials to create
fascinating architectures, the use of block copolymers in
functional, conjugated polymers has received limited attention.18-22
Both oligo(p-phenylene vinylene) (OPV)10,16,23-26 and perylene
bisimide (PERY)27-31 chromophores have been utilized in bulk-
heterojunction-like solar cell configurations as donor and as
acceptor materials, but their combination has received little
attention. In this contribution we present the synthesis and
properties of a new class of donor-acceptor polymers, repre-
(12) Cravino, A.; Zerza, G.; Neugebauer, H.; Sariciftci, N. S.; Maggini, M.;
Bucella, S.; Svensson, M.; Andersson, M. R. Chem. Commun. 2000, 2487.
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Janssen, R. A. J.; Sheiko, S. S.; Spatz, J. P.; Mo¨ller, M.; Meijer, E. W. J.
Am. Chem. Soc. 1998, 120, 2798.
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Int. Ed. 2002, 41, 329.
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11, 15.
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Janssen, R. A. J. J. Am. Chem. Soc. 2001, 123, 6714.
(11) Zerza, G.; Cravino, A.; Neugebauer, H.; Sariciftci, N. S.; Gomez, R.; Segura,
J. L.; Martin, N.; Svensson, M.; Andersson, M. R. J. Phys. Chem. A 2001,
105, 4172.
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Yu, L. J. Am. Chem. Soc. 2000, 122, 6855.
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1998, 120, 10990.
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Chem. Eur. J. 2002, 8, 3246.
9
8626 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003

Oligo(p-phenylene vinylene)−Perylene Bisimide Copolymers
Scheme 1. Synthesis of the Perylene Bisimide Monomer 6a
A R T I C L E S
a (a) K2CO3, TBAB, acetone, reflux, 91%; (b) TosCl, pyridine, 4 °C, 75%; (c) phthalic anhydride, toluene, reflux, 94%; (d) 2, potassium tert-butoxide,
DMF, 70 °C, 44%; (e) N2H4•H2O, methanol, reflux, 95 %; (f) 3,4,9,10-perylenetetracarboxylic dianhydride, Zn(OAc)2, imidazole, 180 °C, 55%.
sented by P1 and P2 (Figure 1), that consist of alternating OPV
and PERY segments connected via saturated spacers. The
photophysics of these two alternating copolymers has been
studied in detail both experimentally and theoretically. On one
hand, fluorescence and femtosecond pump-probe spectroscopy
in solution and in the solid state have been used to elucidate
the temporal evolution of the photoexcited state and determine
the rates for charge separation and recombination; the photo-
physical properties are compared to those of a cyclic model
compound, M1 (Figure 1), in which the two groups are held in
a face-to-face orientation. On the other hand, correlated
quantum-chemical calculations have been performed to deter-
mine the different molecular parameters that are involved in
the charge separation and charge recombination rates in P2 and
M1; particular attention has been given to the role of the relative
orientations of donor and acceptor chromophores on the
electronic coupling and thermodynamic parameters for charge
transfer. The photoinduced electron-transfer reactions in the
polychromophoric alternating D-A arrays occur for P1 and P2
in the picosecond regime, both in solution and in the solid state.
To explain the influence of the spacer length, we propose that
there is a significant effect of the conformation of the polymer
chain on the rate for photoinduced energy and electron transfer.
Experimental Results and Discussion
Synthesis. The synthesis of polymers P1 and P2 and the
cyclic model compound M1 was accomplished using a pal-
ladium-catalyzed aryl-aryl coupling of appropriately function-
alized perylene bisimide and oligo(p-phenylene vinylene)
monomers 6, 10, and 13. The synthesis of the perylene bisimide
monomer 6 (Scheme 1) started with a convergent approach to
the amine 5. On one side, a Williamson etherification of
6-bromo-1-hexanol and 4-bromophenol afforded alcohol 1,
which was tosylated to give 2. On the other side, the amine of
(S)-(+)-leucinol was protected with a phthalimide group to imide
3. A second Williamson etherification of alcohol 3 with tosylate
2 gave diether 4, which was deprotected with hydrazine to the
free amine 5. Finally, reaction of 2 equiv of 5 with 3,4,9,10-
perylenetetracarboxylic dianhydride (PTCDA) resulted in perylene
bisimide 6.
(23) Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali, L.; Krasnikov, V.;
Hadziioannou, G. Chem. Commun. 1999, 617.
(24) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-G.; Echegoyen, L.;
Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G.
J. Am. Chem. Soc. 2000, 122, 7467
(25) Peeters, E.; Van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.;
Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000, 104, 10174.
(26) El-ghayoury, A.; Schenning, A. P. J. H.; Van Hal, P. A.; Van Duren, J. K.
J.; Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2001, 40, 3660-
3663.
(27) Halls, J. J. M.; Friend, R. H. Synth. Met. 1997, 85, 1307.
(28) Schmidt-Mende, L.; Fechtenkotter, A.; Mu¨llen, K.; Moons, E.; Friend, R.
H.; MacKenzie, J. D. Science 2001, 293, 1119.
The second perylene bisimide monomer 10 was obtained
following a slightly different strategy (Scheme 2). Reaction of
(S)-(+)-leucinol with di-tert-butyl dicarbonate afforded BOC-
protected amine 7. A Mitsunobu reaction of 7 with 4-bromophe-
nol was carried out to yield ether 8. Deprotection of 8 by
reaction with trifluoroacetic acid (TFA) and NaHCO3 afforded
amine 9, which was reacted with PTCDA to perylene bisimide
10.
(29) Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. AdV. Mater. 2000, 12, 1270.
(30) Dittmer, J. J.; Petritsch, K.; Marseglia, E. A.; Friend, R. H.; Rost, H.;
Holmes, A. B. Synth. Met. 1999, 102, 879.
(31) Angadi, M. A.; Gosztola, D.; Wasielewski, M. R. J. Appl. Phys.1998, 83,
6187.
The synthesis of the oligo(p-phenylene vinylene) (OPV)
monomer 13 (Scheme 3) started with treatment of (E,E)-1,4-
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A R T I C L E S
Neuteboom et al.
Scheme 2. Synthesis of the Perylene Bisimide Monomer 10a
a (g) BOC2O, THF, 97%; (h) 4-bromophenol, PPh3, DEAD, toluene, 23%; (i) 1. TFA, CH2Cl2; 2. NaHCO3, 76%; (j) 3,4,9,10-perylenetetracarboxylic
dianhydride, Zn(OAc)2, imidazole, 180 °C, 65%.
Scheme 3. Synthesis of OPV Monomer 13a
a (k) 1. n-BuLi, THF, -78 °C; 2. B(OMe)3, -78 °C; 3. H2O, 51%; (l) pinacol, CH2Cl2, reflux, 58%.
bis{4-bromo-2,5,-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-
methylbu toxy]benzene11,32 with butyllithium and subsequent
reaction with trimethyl borate and water to yield bisboronic acid
12. Condensation of bisboronic acid 12 with pinacol resulted
in the formation of the bisboralane 13.
NMR signals of the starting monomers could no longer be
detected in the polymers. The molecular weight of the polymers
determined by size-exclusion chromatography (SEC) versus
polystyrene standards is Mn ) 12.6 kg/mol for P1 and Mn )
10.8 kg/mol for P2 with polydispersities of 2.1 and 2.4,
respectively. MALDI-TOF mass spectrometry on P1 and P2
was not successful but confirmed the cyclic structure of M1 by
reproducing the exact molar mass in a high-resolution spectrum.
Absorption Spectra. UV-vis absorption spectra of polymers
P1 and P2 in toluene solution are shown in Figure 2, together
with the absorption spectra of the model compounds M2 and
M3.33 Absorption bands of the PERY chromophore appear at
460, 490, and 525 nm, while the OPV absorption peaks at ∼425
nm. As can be seen in Figure 2, the spectra of the two polymers
are almost identical and close to a linear superposition of the
spectra of M2 and M3. This demonstrates that in the two
polymers the two chromophores are indeed present in a 1:1 ratio.
The absorption spectra of P1 and P2 give no indication of a
major electronic interaction between the OPV and PERY
chromophores in the ground state. The absorption spectra further
reveal that the singlet excited state of the PERY unit lies below
that of the OPV chromophore. As a result, excitation of the
OPV chromophore may result in singlet energy transfer to the
PERY unit. Such excitation transfer has recently been described
in perylene-end-capped polyindenofluorene.34,35
1
All monomers were fully characterized using H and 13C
NMR spectroscopy, elemental analysis, and MALDI-TOF mass
spectrometry.
Polymers P1 and P2 were synthesized by a Suzuki polym-
erization using Pd(PPh3)4 as a catalyst from equimolar amounts
of bisboralane oligo(p-phenylene vinylene) monomer 13 and
the perylene bisimide monomers 6 and 10, respectively.
Polymerization of 6 with 13 afforded, apart from P1, cyclic
compound M1 as a byproduct, which could be isolated and
purified by extraction with acetone, column chromatography,
and preparative size-exclusion chromatography (SEC).
Bis(4-methoxphenyl)-OPV3, M2, was synthesized by Suzuki
coupling of 13 with 4-bromoanisol for comparison with the
polymers in the optical experiments.
Characterization of the Polymers. The polymers were
characterized by 1H and 13C NMR spectroscopy. In the 1H NMR
spectra of the polymers, the signals in the aromatic region have
shifted compared to the spectra of the monomers. For P1, the
signals of the protons of the bromophenyl ring at 7.14 and 6.46
ppm in monomer 6 shift to 7.47 and 6.81 ppm in the polymer.
1
For M1 such an electronic interaction is much more likely
because the two chromophores are in close proximity. Moreover,
the enantiomerically pure (S)-2-methylbutoxy side chains of the
In the H NMR of P2 these signals appear at 7.48 and 6.91
ppm, compared to 7.29 and 6.76 ppm for monomer 10. The 1H
(32) Jonkheijm, P.; Fransen, M.; Schenning, A. P. H. J.; Meijer, E. W. J. Chem.
Soc., Perkin Trans. 2 2000, 1280.
(33) Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225.
9
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Oligo(p-phenylene vinylene)−Perylene Bisimide Copolymers
A R T I C L E S
in M1 are in a right-handed twisted conformation.36 From these
results M1 emerges as a new chiral macrocycle consisting of
two electronically different interacting conjugated chro-
mophores. Chiral macrocycles with two identical chromophores
have been described recently.37,38
Electrochemistry. The oxidation and reduction potentials of
the oligo(p-phenylene vinylene) and perylene bisimide segments
were determined by cyclic voltammetry in dichloromethane (vs
SCE). Two reversible oxidation waves were found for M2 at
half-wave potentials of Eox,1 ) 0.80 V and Eox,2 ) 0.94 V. For
M3 the reduction potentials were determined at Ered,1 ) -0.65
V and Ered,2 ) -0.85 V.
Energy for Charge Separation. The redox potentials can
be used to estimate the Gibbs free energy of the intramolecular
charge-separated state (Gcs) of the donor-acceptor polymers
P1 and P2 and of the cyclic dyad M1 with respect to the
electronic ground state in a solvent with polarity ꢀs via the
Weller equation39
Figure 2. Absorption spectra of polymers P1 and P2 and compounds M2
and M3 dissolved in toluene. The spectra of P1, P2, and M3 have been
normalized at 527 nm. The spectrum of M2 is normalized with respect to
that of M3 to represent similar concentrations. The open circles represent
the spectrum of a 1:1 molar mixture of M2 and M3.
Gcs ) e(Eox(D) - Ered(A)) -
e2
e2
1
+
1
r-
1
1
-
+
-
(1)
(
)(
)
4πꢀ0ꢀsRcc 8πꢀ0 r
ꢀref ꢀs
To calculate GCS, the oxidation potential of the donor Eox(D)
and the reduction potential of the acceptor Ered(A) of M2 and
M3, determined in dichloromethane (ꢀref ) 8.93), were used.
The radius of the PERY anion (r- ) 4.71 Å) was estimated
from the density (F ) 1.59 g cm-3) of N,N’-dimethylperylene-
3,4:9,10-tetracarboxylic-bisimide from the X-ray crystallo-
graphic data via r- ) [3M/(4πFNA)]1/3 40
The radius of the
.
positive ion of OPV segment was estimated to be r+ ) 5.05
Å.25 The distance between the centers of donor and acceptor
moieties, Rcc, was determined from geometries of M1, P1, and
P2 optimized at the molecular mechanics level with the Dreiding
and Universal force fields; these values are 4, 31, and 22 Å for
M1, P1, and P2, respectively. Because P1 has a flexible spacer
between the OPV and PERY moieties, the distance of 31 Å for
P1 is an upper limit corresponding to the fully extended
structure. We assume that the value of 4 Å is the lower limit
for Rcc in P1 in analogy with the situation in M1. With these
parameters we find that the intramolecular charge-separated state
in M1, P1, and P2 lies at lower energy than those of the OPV
or PERY singlet excited states (Table 1, Figure 4) in toluene
(ꢀs ) 2.38). Hence, photoinduced electron transfer is expected
to be an exergonic reaction for these systems.
Fluorescence Spectra and Fluorescence Quenching. To
investigate the photophysics and especially address the pos-
sibility of photoinduced energy or electron-transfer reactions
between the donor and acceptor moieties of the polymers P1
and P2, their fluorescence was studied in toluene solution. The
fluorescence spectra of M2 and of the two polymers recorded
with (preferential) excitation of the OPV chromophore at 410
nm reveal that in P1 and P2 the OPV fluorescence at 482 nm
is quenched (Figure 5). Compared to M2, the OPV fluorescence
Figure 3. Absorption (top) and circular dichroism (bottom) spectra of
compound M1 (solid lines) in toluene. For comparison the absorption
spectrum of P1 is shown (dashed line).
OPV fragment and the two (S) stereocenters of the leucinol
group could give rise to preferential helical twisting of the OPV
and PERY chromophores in the face-to-face orientation present
in M1. The UV-vis absorption spectrum of M1 (Figure 3)
shows that, compared to P1, there is a small but distinct red
shift of the OPV band from 428 to 439 nm. The circular
dichroism (CD) spectrum of M1 (Figure 3) reveals a Cotton
effect, with a negative part at 438 nm (OPV) and a positive
part at 486 and 524 nm (PERY). Following the exciton-coupling
model,36 a bisignated Cotton effect is consistent with two
interacting chromophores in a helical orientation. Exciton theory
of chiral bichromophoric systems suggests that the positive
Cotton effect of M1 at the higher wavelengths indicates that
transition dipole moments of the OPV and PERY chromophores
(36) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy. Exciton
Coupling in Organic Stereochemistry; University Science Books, Mill
Valley, CA, 1983.
(37) Ko¨hler, B.; Enkelmann, V.; Oda, M.; Pieraccini, S.; Spada, G. P.; Scherf,
U. Chem. Eur. J. 2001, 7, 3000.
(38) Langhals, H.; Ismael, R. Eur. J. Org. Chem. 1998, 1915, 5.
(39) Weller, A. Z. Phys. Chem. Neue Folge 1982, 133, 93.
(40) Ha¨dicke, E.; Graser, F. Acta Crystallogr., Sect. C 1986, 42, 189.
(34) Herz, L. M.; Silva, C.; Friend, R. H.; Phillips, R. T.; Setayesh, S.; Becker,
S.; Marsitsky, D.; Mu¨llen, K. Phys. ReV. B 2001, 64, 195203.
(35) Beljonne, D.; Pourtois, G.; Silva, C.; Hennebicq, E.; Herz, L. M.; Friend,
R. H.; Scholes, G. D.; Setayesh, S.; Mullen, K.; Bre´das, J. L. Proc. Natl.
Acad. Sci. 2002, 99, 10982.
9
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A R T I C L E S
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Table 1. Free Energy of Intramolecular and Intermolecular
Charge-Separated States Calculated from Eq 1 in Toluene and
Relative to the Ground State and the Singlet Excited States of
OPV and PERYa
P2, respectively, relative to that of M3. For the cyclic model
compound M1, photoluminescence of the OPV chromophore
could no longer be identified (Q > 15000), indicating a very
efficient deactivation of its singlet excited state. The quenching
of the PERY emission in M1 was substantial (Q ) 270, Table
2) but less than that for P2.
These results can be rationalized by considering the diagram
shown in Figure 6. After photoexcitation of the OPV chro-
mophore, the intrinsic radiative (kr) and nonradiative decay (knr)
of the donor can be quenched by either a singlet energy transfer
reaction (ket) to the PERY chromophore or a charge separation
(kcs) to a charge-separated state (CSS) in which an electron is
transferred from the OPV to the PERY unit. Photoexcitation of
the PERY unit, however, cannot give singlet energy transfer
because it is endergonic, but charge separation can still occur,
resulting in quenching of the PERY fluorescence.
∆GCS ) GCS − E00 (eV)
RCC (Å)
GCS (eV)
S OPV
1
S PERY
1
M1
P1
P2
M2 + M3
4
0.85
2.17
2.09
2.36
-1.74
-0.42
-0.50
-0.23
-1.46
-0.15
-0.23
0.05
31b
22b
∞
a The following parameters were used: Eox(D) ) 0.80 V, Ered(A) )
-0.65 V, r+ ) 5.05 Å, r- ) 4.70 Å, ꢀs ) 2.38, ꢀref ) 8.93. For OPV(S1):
E00 ) 2.59 eV. For PERY(S1): E00 ) 2.31 eV. b Assuming a fully extended
structure of the spacer.
From the quenching of the PERY emission in P1, P2, and
M1 we conclude that in these systems electron transfer takes
place when the acceptor unit is photoexcited. To examine
whether singlet energy transfer occurs, the excitation spectra
of the PERY emission at 579 nm were recorded in toluene.41
Figure 7 shows that in the excitation spectrum of the PERY
emission of P1 and P2 both chromophores contribute to an equal
extent. This indicates that the rate for singlet energy transfer
from the OPV singlet state (ket, Figure 6) in P1 and P2 is at
least of the same order of magnitude as the rate for direct
electron transfer (kcsd, Figure 6). In contrast, the excitation
spectrum of M1 differs from its absorption spectrum in the 400-
475 nm region (Figure 7). This suggests that direct electron
transfer from the S1 state of OPV is more important here.
Photoinduced Absorption in Thin Films. To investigate the
photoinduced electron-transfer processes in more detail, we
performed photoinduced absorption (PIA) spectroscopy on long
and short time scales.
Figure 4. Energies of the singlet excited states of the OPV and PERY
chromophores; the intramolecular charge-separated states of M1, P1, and
P2 and the intermolecular charge-separated state between M2 and M3. The
energy levels of the charge-separated states were calculated from eq 1 using
the parameters described in the text and assuming toluene (ꢀs ) 2.38) as
the solvent. The range of values for P1, depending on Rcc, is given by the
arrow.
The near steady-state PIA spectrum of a thin film of P1 on
quartz, recorded with excitation at 528 nm at 80 K (Figure 8),
shows the formation of PERY-• radical anions and OPV+•
radical cations. The peaks at 1.28, 1.54, and 1.72 eV are readily
attributed to the PERY-• radical anion bands.42 The absorption
band at 0.72 eV is assigned to the OPV+• radical cations; its
position is between the absorption bands of OPV3+• and OPV4+•
radical cations.43 Comparison with spectra of OPV3+• and
OPV4+• indicates that a second absorption of the OPV+• radical
cation in P1 is expected to overlap with the PERY-• radical
anion bands in the 1.5-2.0 eV region. An almost identical PIA
spectrum was recorded for polymer P2 (Figure 8). Moreover,
the PIA spectrum does not depend on whether the sample is
irradiated at 458 nm (preferential excitation of OPV) or at 528
nm (selective excitation of PERY). This demonstrates that both
hole and electron-transfer processes take place in the solid state.
The PIA signals of both P1 and P2 were found to increase
in a nonlinear fashion with the intensity of the excitation beam
following a power-law behavior with an exponent on the order
of 0.6-0.7, indicating that bimolecular decay processes, such
as nongeminate charge recombination, contribute to the decay.
Figure 5. Fluorescence emission spectra of polymers P1 and P2 and OPV-
model M2 upon (preferential) excitation of the OPV chromophore at 410
nm and of PERY-model M3 upon excitation at 525 nm. The spectra are
corrected for the optical density at the excitation wavelength. The spectra
of the polymers are multiplied by 20 for clarity.
is reduced by a factor of Q ) 650 for P1 and Q ) 700 for P2
(Table 2). Comparison with the fluorescence spectrum of
perylene bisimide model M3, recorded with excitation at 525
nm, shows that both polymers give some emission that originates
from the PERY chromophore. When the perylene bisimide
segments of P1 and P2 were selectively excited at 525 nm, it
became evident, however, that also the PERY emission in the
polymers is strongly quenched. Quenching factors of 30 and
630 (Table 2) were found for the perylene emission of P1 and
(41) It is important to note that small concentrations of fluorescent impurities,
such as monomers, can mask the true excitation spectra of these weakly
fluorescent systems, and the data should be interpreted with caution.
(42) Salbeck, J. J. Electroanal. Chem. 1992, 340, 169-195.
(43) Van Hal, P. A.; Beckers, E. H. A.; Peeters, E.; Apperloo, J. J.; Janssen, R.
A. J. Chem. Phys. Lett. 2000, 328, 403-408.
9
8630 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003

Oligo(p-phenylene vinylene)−Perylene Bisimide Copolymers
A R T I C L E S
Table 2. Rate Constants for Energy Transfer, Direct and Indirect Charge Separation, and Charge Recombination As Obtained from
Photoluminescence Quenching and Pump-Probe PIA Spectroscopy for P1, P2, and M1 in Toluene Solution and Solid Films
a
b
PL quenching
(ket + kcsd)d (ns-1
PIA OPV
PIA PERY
c
c
QOPV
Q
)
kcsi e (ns-1
)
kcsd (ns-1
)
k
rec (ns-1
)
kcsi (ns-1
)
k )
rec (ns-1
PERY
P1
P2
M1
toluene
film
toluene
film
650
700
30
630
270
540
580
7
160
70
630
g2300
350
g2300
1700
1.9
9.5
0.6
9.5
6.9
1400
270
1.4
0.5
5.9
toluene
g15000
g12500
1200
a Excitation of the OPV chormophore at 450 nm. b Excitation of the PERY chromophore at 520 nm. c Excitation of OPV at 410 nm and PERY at 525 nm.
d From QOPV and eq 2. e From QPERY and eq 3.
Figure 6. Jablonski diagram representing the different photophysical events
that can take place in the donor-acceptor (D-A) polymers P1 and P2 and
model M1 upon excitation of the OPV chromophore (open arrow). The
singlet energy transfer (ket), the direct (kcsd) and indirect (kcsi) electron
transfer, the radiative emission (kr), and nonradiative emission (knr) are
indicated.
Figure 8. Near steady-state photoinduced absorption spectra of P1 (solid
line) and P2 (dashed line) as thin films on quartz recorded at 80 K.
Excitation wavelength is 528 nm, modulation frequency 275 Hz.
Figure 7. Normalized absorption (dashed lines) and excitation (solid lines)
spectra of polymer P1 (top), polymer P2 (middle), and model M1 recorded
in toluene (λem ) 579 nm).
All PIA bands exhibited an almost identical dependence on
changing the modulation frequency, which could be fitted to a
combined expression of a power-law dependence and bi-
molecular decay,44 giving exponents from -0.40 to -0.45 and
Figure 9. Normalized differential transmission dynamics of films of
lifetimes in the range from 1 to 2 ms.
polymers P1 (b) and P2 (O) at room temperature, recorded at 1450 nm
(low-energy absorption of OPV radical cations) after excitation at 450 nm.
Solid lines are fits to exponential growth (a) and decay (b).
Films of P1 and P2 were also investigated with femtosecond
pump-probe spectroscopy. In Figure 9 the normalized change
in transmission ∆T/T at 1450 nm (0.86 eV) for both polymers
is plotted versus the time delay after excitation with a ∼150 fs
pulse at 450 nm. At this probe wavelength a negative differential
transmission is observed, consistent with the fact that at 1450
nm we exclusively probe the formation of OPV+• radical cations
and hence the event of photoinduced charge separation. Figure
9 shows that the rate for charge separation (∼0.4 ps) is extremely
fast for both P1 and P2 in the film. The recombination process
(44) Van Hal, P. A.; Christiaans, M. P. T.; Wienk, M. M.; Kroon, J. M.; Janssen,
R. A. J. J. Phys. Chem. B 1999, 103, 4352.
9
J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003 8631

A R T I C L E S
Neuteboom et al.
Figure 10. Normalized differential transmission dynamics of polymers P1
(b) and P2 (O) in toluene solution at room temperature, recorded at 1450
nm (low-energy absorption of OPV radical cations) after excitation at 450
nm. Solid lines are fits to monoexponential growth (a) and decay (b).
Figure 11. Normalized differential transmission dynamics of M1 in toluene
solution at room temperature, recorded at 1450 nm (low-energy absorption
of OPV radical cations) after excitation at 450 nm. Solid lines are fits to
monoexponential growth (a) and decay (b).
contains several components. At short times after photoexcitation
the recombination is dominated by a process occurring with a
time constant of ∼105 ps; however, after 600 ps, ca. 20% of
the charge is still present in both polymer films. These residual
charges decay very slowly. We interpret the 105 ps decay as
geminate recombination and attributed the longer lived com-
ponents to those charges that diffuse away from their original
position and eventually may become trapped.
Photoinduced Absorption in Solution. Femtosecond pump-
probe spectroscopy was also used to study polymers P1 and
P2 in toluene solution (Figure 10). Again, the OPV chromophore
was excited at 450 nm and the change in transmission of the
OPV+• radical cation band at 1450 nm was probed in time.
While in the solid state P1 and P2 behave very similarly,
significant differences are observed in solution. In toluene,
charge separation in P1 (∼1.6 ps) is faster than that in polymer
P2 (∼2.9 ps). Both are somewhat slower than that in the solid
state. Also, the rate for charge recombination in solution is lower
than in the films. Fitting a monoexponential decay to the data
for P1 and P2 (Figure 10) provides time constants for
recombination of positive and negative charges of 530 and 1600
ps, respectively. As a result of the difference in rate constants,
∼75% of the charges are still present after 600 ps in P2 and
∼40% in P1. The rates of charge separation and charge
recombination are summarized in Table 2.
transfer upon photoexcitation of M1 in toluene is significantly
faster than that for P1 and P2 in the same solvent. Actually,
the time constant of ∼0.6 ps for the forward reaction is close
to that observed in films of P1 and P2. The back electron
transfer (recombination) in M1 is monoexponential with a time
constant of ∼144 ps. The recombination rate of M1 is therefore
higher than that of P1 and P2 in solution but close to values
observed in the polymer films. Similar to P1 and P2, photo-
excitation of the PERY chromophore at 520 nm instead of OPV
provided comparable values for the rate for charge separation.
The close correspondence in charge formation and recombina-
tion between M1 in toluene and the P1 and P2 films can be
rationalized by considering that the face-to-face orientation of
the OPV and PERY photoactive groups in M1 is also present
in the solid-state films of P1 and P2.
Photovoltaic Devices. Photovoltaic devices were prepared
by spin casting solutions of P1 and P2 in chloroform on indium
tin oxide covered with a layer of polyethylenedioxythiophene
polystyrenesulfonate (PEDOT:PSS, Bayer AG) as the transpar-
ent front electrode. A LiF/Al back electrode was deposited in a
vacuum. The I-V characteristics of the ITO/PEDOT:PSS/
polymer/LiF/Al devices were measured in the dark and under
simulated AM1.5 conditions (Figure 12). Both polymers gave
similar results. Although the open-circuit voltage in both cells
is relatively high (VOC ) 1.20 and 0.97 V, for P1 and P2,
respectively), the short-circuit current density (JSC ) 0.008 mA
cm-2 for P1 and JSC ) 0.012 mA cm-2 for P2) and the fill
factor (FF ) 0.25-0.26) are low. The low currents are attributed
to poor transport characteristics, because the initial charge
generation is efficient as inferred from fluorescence quenching
in the films.
Pump-probe experiments have also been performed with
excitation at 520 nm, where the PERY chromophore is directly
excited again with probing at 1450 nm to observe OPV+• radical
cations. The results (Table 2) are similar to those obtained with
d
i
excitation at 450 nm and demonstrate that kcs and kcs are of
the same order of magnitude.
Transient pump-probe experiments on M1 in toluene solu-
tion (Figure 11) have been performed for comparison to the
data of P1 and P2 in solution and film. The forward charge
Kinetics of Electron and Energy Transfers. Fluorescence
quenching and pump-probe experiments can be used to estimate
the rate constants for energy transfer (ket) and for direct and
9
8632 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003

Oligo(p-phenylene vinylene)−Perylene Bisimide Copolymers
A R T I C L E S
Figure 13. Cartoon representation of the conformation of alternating
donor-acceptor copolymers in solution. Donor (red) and acceptor (yellow)
units are linked by long (P1) or short (P2) spacers. Face-to-face contacts
of the two chromophores in P1 enhance the rates for photoinduced energy
and electron transfer compared to the end-to-end orientation in P2. In the
solid-state face-to-face interactions, like the one enforced in M1, are
omnipresent for both P1 and P2 via interchain interactions.
d
formation derived from pump-probe spectroscopy: kcs ) 6.3
× 1011 s-1 for P1, 3.5 × 1011 s-1 for P2, and g1.7 × 1012 s-1
for M1 (Table 2). The significantly lower value for M1 in the
latter experiment is limited by the time resolution of the pump-
probe spectroscopy (0.4-0.5 ps), and hence, the value obtained
from the OPV quenching is probably more accurate here.
Although these results cannot be used to unravel the individual
contributions of ket and kcsd in a reliable fashion, the fluorescence
excitation experiments have shown that these processes are
competitive.
Comparison of the pump-probe PIA data shows that the rate
for direct charge separation in toluene solution decreases in the
sequence M1, P1, and P2 (Table 2). The high rate for M1 is
consistent with a face-to-face orientation of the two chro-
mophores, which ensures an efficient electronic coupling in the
excited state, vide infra. The higher rate for P1 than P2 is at
first sight surprising because of the longer spacer between the
OPV and PERY groups in P1. This difference may be
rationalized by the fact that the conformation of P1 is
heterogeneous on the picosecond time scale. The origin of this
heterogeneity is the conformational flexibility of the polymer
chain, which implies that the OPV and PERY segments can be
in a range of conformations from fully extended (∼31 Å) to
face-to-face (∼4 Å). The face-to-face orientation, which likely
results in a high rate (kcsd), is not possible for P2 because the
spacer is more rigid. These differences are schematically shown
in Figure 13. The rate for charge recombination (krec) decreases
in the sequence M1, P1, P2 and, hence, follows the same trend
as that observed for charge separation. It is likely that similar
conformational differences are the cause for this result. At
present it is unknown whether migration of photogenerated
charges along the polymer chain occurs. The most likely
mechanism for such a process would certainly involve folding
to circumvent that positive (or negative) charges must reside
on or hop over acceptor (or donor) units.
The rates for indirect charge separation, i.e., originating from
the excited state of the PERY acceptor, in M1, P1, and P2 as
determined by pump-probe spectroscopy are similar to those
for direct charge transfer (Table 2). As can be seen in Table 2,
the values obtained from pump-probe spectroscopy differ
significantly from those determined from the PERY fluorescence
quenching, especially for P1 and M1. The presence of a small
(less than 0.4%) impurity containing the PERY chromophore
can explain this discrepancy for M1, but for P1 we consider
that the rather low PL quenching (QPERY ) 30) is, at least in
part, intrinsic. In a fully extended conformation of the linker
Figure 12. Current density-voltage characteristics of photovoltaic devices
(ITO/PEDOT:PSS/polymer/LiF/Al) of polymers P1 (a) and P2 (b) in the
dark (b) and under simulated AM1.5 illumination (100 mW/cm2) (O).
d
i
indirect charge separation (kcs and kcs , respectively) (Figure
d
6). The sum of ket and kcs can be related to the quenching of
the OPV fluorescence (QOPV) using the fluorescence lifetime
of the donor segment (τOPV) via
QOPV - 1
d
ket + kcs
)
(2)
τOPV
In a similar fashion it is possible to relate the quenching of the
PERY fluorescence to the rate for the indirect charge separation
QPERY - 1
i
kcs
)
(3)
τPERY
The fluorescence lifetimes of M2 and M3 in toluene solution
were determined by time-correlated single-photon counting. The
time-resolved fluorescence signal followed a monoexponential
decay with time constants of 1.2 and 4.0 ns for M2 and M3,
respectively. The use of eqs 2 and 3 is not without difficulty
because small concentrations of fluorescent impurities, such as
monomers, can easily lead to an underestimation of the actual
quenching. Pump-probe experiments generally give more
reliable numbers because they probe the bulk of the sample.
The results are collected in Table 2. The OPV fluorescence
quenching in P1 and P2 in toluene indicates that in the polymers
ket + kcsd ) 5.4-5.8 × 1011 s-1; while the rate is much higher
in the cyclic molecule M1 (ket + kcs g 1.2 × 1013 s-1). The
d
data are in satisfactory agreement with the rates for direct charge
9
J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003 8633

A R T I C L E S
Neuteboom et al.
(∼31 Å) of P1 the electronic coupling of the OPV and PERY
chromophores is negligible in the excited state and, hence, a
low rate for electron transfer, causing a decreased quenching
of the PERY emission. Because singlet energy transfer is less
dependent on the distance than electron transfer, the OPV
fluorescence in P1 is quenched to the same extent as in P2.
The conformational freedom of P1 can thus be used to
For the former process we have considered the cases where the
charge transfer is induced by excitation of either the OPV donor
(D*A f D+A- process; that is the direct transfer, corresponding
to electron transfer from D to A) or the acceptor (DA* f D+A-
process; that is the indirect transfer, corresponding to hole
transfer from A to D).
The ground-state geometries of the full chemical structures
have been optimized by means of the AM1 (Austin Model 1)
method.52 In the case of M1, the intermolecular geometric
parameters were optimized at the molecular mechanics level
using the Dreiding force field. To build the excited-state and
charge-separated geometrical structures, the equilibrium geom-
etries of the isolated donor and acceptor chromophores were
first optimized in the lowest singlet excited state and in the
singly (positively and negatively) charged ground state at the
AM1/CAS-CI (complete active space-configuration inter-
action)53 and AM1/ROHF (restricted open-shell Hartree-Fock)
level, respectively; these were then combined into the whole
structure to yield the relaxed geometries of the D*A, DA*, and
D+A- electronic states.
i
rationalize the widely different values obtained for kcs using
PL quenching and PIA spectroscopy.
d
The rate for charge separation in the films of P1 and P2, kcs
g 2.3 × 1012 s-1, is at the limit of the time resolution of the
pump-probe setup. The recombination, krec ) 9.5 × 109 s-1
,
is much slower. Both processes, however, are significantly faster
than the corresponding reactions in toluene solution. The
increased rate for charge separation can be understood by
assuming that in the films face-to-face orientations of the OPV
and PERY chromophores (like the one in M1) are present
(Figure 13). Although we have no evidence from morphological
studies, several polymer materials45-48 exist in which electron-
rich (donor) and electron-deficient (acceptor) segments form
alternating stacks in solution and in the solid state. The driving
force for such orientations is the π-stacking in combination with
electrostatic or weak charge-transfer interaction.49,50 Also, in
D-A molecular crystals the usual case is to have stacks with
alternating D-A molecules.51 A preference for face-to-face
D-A interactions, as schematically drawn in Figure 13, contrast
with the microphase separation that often occurs in block
copolymers. Apparently, the tendency for the OPV and PERY
segments to give alternating stacks in thin films of P1 and P2
is stronger than the antagonistic interactions that direct the
microscopic phase separation. One likely explanation for this
result is the limited length of the D and A segments in P1 and
P2. The proposition of the presence of alternating stacks of OPV
and PERY in films of P1 and P2 is supported by the similar
The transfer rate kCS and kCR, corresponding to the probability
for charge separation (D*A or DA* f D+A-) and charge
recombination (D+A- f DA) to occur after photoexcitation,
respectively, are evaluated using Jortner’s expression.54,55 Going
from reactant R to product P, the expression writes
1/2
2π
1
Sυ
kRP
)
VRP
e-S
×
2
∑
(
)
p
4πλskT
υ!
υ
(∆G° + λs + υp ω )2
exp -
(4)
(
)
4λskT
Here, VRP is the electronic coupling representing the tunneling
probability between the reactant and product potential-energy
surfaces (approximated as harmonics),∆G° is the variation in
Gibbs free energy during the reaction, S () {λi}/{p ωi }) is
the Huang-Rhys factor expressed in terms of the intramolecular
reorganization energy λi and the effective mode vibrational
energy p ωi , and λs is the solvent reorganization energy
including contributions from the polarization changes in the
dielectric (solvent) environment.
d
values of kcs obtained for M1 and the polymer films (Table
2). In full agreement with this view, the initial recombination
in the films is fast and the rate constant (krec ) 9.5 × 109 s-1
)
is again similar to the one observed for M1 (krec ) 6.9 × 109
s-1). The long-lived charge carriers in the films result from those
charges that escape from geminate recombination and diffuse
to different sites in the films. The relatively low number of
charges (<20%, Figure 9) that live up to 1 ns explains, at least
in part, the low currents observed in photovoltaic cells made
from P1 and P2. If the face-to-face orientations of OPV and
PERY are predominant in the films, this could be a more
important reason for the low currents, because such orientation
would severely limit charge transport.
∆G° is estimated from the enthalpies of formation, ∆Hf °,
with the solvent taken into account through the use of the
COSMO model56 implemented in the AMPAC package:57
∆G° ) ∆HP° - ∆HR° + Ecb
Theoretical Modeling of Charge Separation and Recom-
bination. Correlated semiempirical quantum-chemical calcula-
tions have been performed on the model compound M1 and a
monomer unit of P2 to assess the various molecular parameters
relevant in the charge separation and recombination processes.
with
D
A
∑∑ q q
1
i j
Ecb
)
4πꢀ0
ꢀsrij
j
i
∆HP° ) ∆Hf,D+° + ∆Hf,A-°;
(5)
(45) Lokey, S. R.; Iverson, B. L. Nature 1995, 375, 303.
(46) Zych, A. J.; Iverson, B. L. J. Am. Chem. Soc. 2000, 122, 8898.
(47) Nguyen, J. Q.; Iverson, B. L. J. Am. Chem. Soc. 1999, 121, 2639.
(48) Lokey, S. R.; Kwok, Y.; Guelev, V.; Pursell, C. J.; Hurley, L. H.; Iverson,
B. L. J. Am. Chem. Soc. 1997, 119, 7202.
(49) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc., Perkin
2 2001, 651.
∆HR° ) ∆Hf,D*° + ∆Hf,A° or ∆Hf,D° + ∆Hf,A*
°
ꢀ0 and ꢀs are the vacuum and solvent dielectric constants,
(50) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem.
ReV. 2001, 101, 3893.
(51) Wright, J. D. Molecular Crystals, 2nd ed.; Cambridge University Press:
Cambridge, 1995.
(52) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am.
Chem. Soc. 1995, 107, 3702.
(53) Typically 6-10 molecular orbitals were included in the CI active space.
9
8634 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003

Oligo(p-phenylene vinylene)−Perylene Bisimide Copolymers
A R T I C L E S
cavities surrounding the ionized donor and acceptor units. This
model assumes that the time scale of the electron-transfer
process is much shorter than that associated to the solvent
nuclear reorganization. In our approach, λs also takes into
account the spatial distribution of the positive and negative
charges by expanding the donor-acceptor interaction in the
following way
Figure 14. Schematic representation of the reactant (R) and product (P)
potential energy curves, with the definition of the reorganization energies
λi1 and λi2 (see text).
where qi, qj, and rij have the same meaning as before (note that
∑i qi ) qD for the donor and ∑j qj ) qA for the acceptor; ꢀop
and ꢀs stand for the optical and static dielectric constants of the
medium; RD and RA are effective spherical cavity radii for the
donor and acceptor, respectively). These were estimated from
the molecular volumes calculated on the basis of the van der
Waals atomic radii and the AM1 molecular geometries; the
calculated values amount to 3.91 Å for the PERY chromophore
and 4.74 Å for the OPV segment, in reasonable agreement with
the values obtained on the basis of the measured densities (4.71
and 5.05 Å, respectively). The first term in eq 7 represents the
stabilization of the isolated ions in a continuum dielectric (Born
term), while the second term describes Coulombic attraction
between the opposite charges (here developed in a multicentric
form) in the D+A- charge-separated state. Note that the solvent
reorganization energies calculated in M1 using this approach
should be considered with caution, owing to the small distance
between the positive and negative charges.60
respectively; qi and qj represent the donor and acceptor partial
atomic charges (as provided by a Mulliken population analysis
on the basis of the AM1 electronic structure of the singly
oxidized and reduced isolated molecules) that are separated by
a distance rij in the D+A- state of the whole assembly.
The reorganization energy λ includes contributions from the
vibrations of the molecules (intramolecular energy λi) and from
the polarization change in the solvent environment (solvent
reorganization energy λs): λ ) λi + λs. The intramolecular
reorganization energy (λi) corresponds to the nuclear relaxation
energy, i.e., the energy cost when going from the reactant, QR,
to the product, QP, equilibrium geometry either along the
potential energy surface of R (λi1) or along that of P (λi2) (see
Figure 14); different λi1 and λi2 values are obtained when the R
and P parabola have different curvatures, in which case λi is
58
defined as the average value of λi1 and λi2
The tunneling matrix elements for charge separation and
recombination were calculated by means of the generalized two-
state Mulliken-Hush model61 on the basis of an adiabatic
description of the reactant and product states
1
2
λ ) (λi1 + λi2)
(6)
with
ad
µRPad∆ERP
λi1 ) ED*A(QP) - ED*A(QR) ≈
VRP
)
(8)
{ED*(QP) + EA(QP)} - {ED*(QR) + EA(QR)}
(∆µ ad)2 + 4(µRP
)
ad 2
x
RP
and
This expression involves the energy difference (∆ERPad) as well
as the corresponding dipole moment difference (∆µRPad) and
transition dipole moment (µRPad) between R and P. We have
computed these parameters in the nuclear arrangement repre-
sentative of the reactants (QR) (and without taking into account
the solvent effects) by means of the semiempirical Hartree-
Fock INDO (intermediate neglect of differential overlap)62
Hamiltonian coupled to a configuration-interaction scheme
λi2 ) ED+A-(QR) - ED+A-(QP) ≈
{ED + (QR) + EA-(QR)} - {ED+(QP) + EA-(QP)}
where ED*, ED+, EA, and EA- correspond to the total energy of
the isolated donor in its lowest singlet excited and cationic states
and that of isolated acceptor in its ground and anionic states,
respectively.59 As is generally assumed, a single effective mode
at 0.2 eV (corresponding to the CdC double-bond vibration
mode) has been considered to take into account the role of
vibrations in assisting the transfer.
(59) Owing to the limited number of molecular orbitals used in the configuration
interaction expansion, the CAS-CI approach used for the geometry
optimizations provides an unbalanced treatment of electron correlation
effects in the ground state and excited states leading to overestimated
excitation energies. To overcome this problem, the energies of the electronic
states involved in the determination of λi and ∆G° were calculated by
retaining only singly excited configurations in the CI scheme, which allows
for correlation effects in a more size-consistent way and provides transition
energies in better agreement with experiment
The solvent reorganization energy (λs) is derived from the
classical dielectric continuum model of Marcus using spherical
(54) Bixon, M.; Jortner, J. In AdVances in Chemical Physics; Jortner, J., Bixon,
M., Eds.; John Wiley & Sons: New York, 1999; Vol. 106, pp 35-202.
(55) Jortner, J. J. Chem. Phys. 1976, 64, 4860
(56) Klamt, A.; Schu¨rmann, G. J. Chem. Soc., Perkin Trans. 1993, 2, 799.
(57) Ampac 6.55 ed.; Semichem: 7204 Mullen, Shawnee, KS 66216, 1997.
(58) Pourtois, G.; Beljonne, D.; Cornil, J.; Ratner, M. A.; Bre´das, J. L. J. Am.
Chem. Soc. 2002, 124, 4436.
(60) Though we stress that eq 7 is already an improvement over the standard
formulation that simply considers the interaction between point charges
located at the centers of the donor/acceptor moieties, such a simplified
model would lead to negative reorganization energies for smallsas
compared to the donor and acceptor radiisdonor-acceptor interdistances
(61) Cave, R. J.; Newton, M. D. Chem. Phys. Lett. 1996, 15, 249
(62) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111.
9
J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003 8635

A R T I C L E S
Neuteboom et al.
Table 3. Molecular Parameters Involved in the Calculation of the
Charge Separation and Charge Recombination Rates, Following
Eq 4a
overlap between the corresponding wave functions, a charac-
teristic that depends critically on symmetry and spatial confine-
ment.
process
λi (eV)
λs (eV)
VRP (cm-1
)
∆G° (eV)
kRP (ns-1
)
In that context, we note that the molecular architecture of
P2 is such that the perylene bisimides are covalently linked to
the OPV segments via their nitrogen atoms; this is far from
being optimal for photoinduced hole or electron migration since
both the HOMO and LUMO levels of the perylene derivatives
display nodes on these nitrogen atoms, thus impeding efficient
communication between the chromophores. From a detailed
analysis of the transition dipole moments that provide the
dominant contribution to the electronic coupling, we have
identified deeper lying molecular orbitals as the main contribu-
tors to the charge separation process in P2 and M1. Some of
these MOs are shown in Figure 15. For instance, for electron
transfer, the LUMO+2 level of PERY has the appropriate
symmetry and energy to interact with the LUMO level of OPV
in both the head-to-tail arrangement of P2 and, even more
strikingly, the cofacial architecture of M1. Indeed, the LUMO+2
orbital of PERY is symmetric with respect to the mirror plane
perpendicular to the molecular frame, which is also the case
for the LUMO orbital of OPV when focusing on the individual
phenylene rings (the mirror plane corresponds in this case to
the plane perpendicular to each aromatic ring and passing
through the carbon atoms in para positions). The large contribu-
tions of the PERY HOMO-4 and OPV HOMO-6 to the
electronic couplings for hole transfer can be rationalized on the
basis of similar symmetry arguments. (A detailed analysis of
these intriguing symmetry effects, which have been elegantly
described by Verhoeven,65 together with molecular engineering
strategies for the design of improved materials will be described
elsewhere66).
The thermodynamics parameters together with the resulting
charge separation and charge recombination rates, as calculated
when considering toluene as the solvent (ꢀs ) 2.38), are also
collected in Table 3. For both compounds, the relaxation
energies λi are larger for charge recombination than for charge
separation. This is expected since the donor ionized state
geometry resembles more the excited-state geometry than the
ground-state geometry. We note that λi is slightly larger for
charge separation from the excited acceptor, pointing out to a
larger geometric difference in the perylene derivative. The main
contribution to λi then arises from the reduction of the electron-
withdrawing unit. The solvent reorganization energy λs is in all
cases small due to the low dielectric constant of the solvent; in
addition, the close proximity of the positive and negative charges
in M1 further reduces the solvent-induced stabilization (the
charges are strongly bound within the molecular structure).
The driving force, ∆G°, is calculated to be negative in both
systems (confirming that the charge separation is an exergonic
process) and significantly smaller for P2 than M1, as a result
of the strong interaction between the positive and negative
charges in the cyclic structure (which stabilizes the charge-
separated state). Quantitatively, the ∆G° values are markedly
different from those obtained from the Weller equation (eq 1),
Table 1. The reason for this discrepancy lies mainly in the
second term of eq 1, i.e., the Coulomb attraction between the
M1
435 (570)
65 (71)
D*A f D+A-
DA* f D+A-
D+A- f DA
0.22
0.31
0.45
0.05
0.04
0.04
-1.1
-1.0
-1.6
451 (770)
84 (100)
43 (50)
177 (190)
P2
D*A f D+A-
DA* f D+A-
D+A- f DA
0.25
0.31
0.45
0.08
0.08
0.08
9.2
8.0
11.6
-0.6
-0.6
-2.1
10
7
0.007
a λi and λs are the internal and external reorganization energies,
respectively, and ∆G0 is the change in free Gibbs energy. The numbers
between parentheses have been obtained on the basis of the DA ground-
state geometry, see text.
involving single excitations (SCI) with respect to the Hartree-
Fock determinant.
The calculated electronic couplings, VRP, are found to depend
critically on the molecular geometry (face-to-face versus
extended structures, donor-acceptor separation, torsion angle
between donor and acceptor, relative orientations of the chro-
mophores, etc.) and on the nature of the initial excited state
(D*A versus DA*). The VRP values computed at the INDO/
SCI level using the relaxed geometries in the reactant state (e.g.,
the geometries of the donor excited state and acceptor ground
state for D*A) are listed in Table 3 for direct and indirect charge
transfer and charge recombination together with the correspond-
ing couplings estimated on the basis of the neutral ground-state
(DA) geometry. The two sets of VRP values are close, which
indicates that, in most cases, the Franck-Condon approximation
holds reasonably well, i.e., the amplitude of the electronic
tunneling matrix elements is weakly dependent on the geometric
structure used as input for the calculation.63
From Table 3 it is seen that the electronic couplings are much
larger in the case of the M1 cyclic molecule than the P2
extended structure for both charge separation and recombination.
As pointed out previously, the higher electronic couplings
obtained for M1 are consistent with stronger interactions when
the two chromophores are in a cofacial arrangement. The
molecular mechanics calculations suggest almost parallel ori-
entations for the donor and acceptor moieties in the cyclic
structure; however, the measured CD spectrum (Figure 3)
indicates a chiral arrangement of the OPV and PERY units.
We have thus repeated the electronic coupling calculations by
imposing an angle between the donor and acceptor molecular
axes from 0° to 40° (i.e., a range of angles still compatible with
the closed structure of the macrocycle) and found no significant
difference.
The relative magnitude of the electronic couplings can be
gauged qualitatively from an inspection of the frontier molecular
orbitals that participate in the description of the relevant
electronic states, Figure 15. For efficient charge transfer, the
donor and acceptor orbitals have to fulfill two criteria:64 (i) they
should lie close in energy and (ii) they should lead to significant
(63) We note, however, a significant difference between the values calculated
using the two geometries for the electron-transfer process from the donor
to the acceptor (D*A f D+A-) in M1, a feature which is due to significant
mixing between donor and acceptor excited-state wave functions when
allowing the OPV geometry to relax.
(64) Beljonne, D.; Pourtois, G.; Ratner, M. A.; Bre´das, J. L. Submitted for
publication.
(65) See, for instance: Verhoeven, J. W. In AdVances in Chemical Physics;
Jortner, J., Bixon, M., Eds.; John Wiley & Sons: New York, 1999; Vol.
106, pp 603-644.
(66) Dupin, H.; et al. Manuscript in preparation.
9
8636 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003

Oligo(p-phenylene vinylene)−Perylene Bisimide Copolymers
A R T I C L E S
Figure 15. Schematic energy diagram showing the frontier molecular orbitals of the OPV and PERY chromophores, as obtained at the INDO level. Arrows
represent the pathways that provide the main contributions to the electronic couplings (see text).
positive and negative charges. While a point-charge model is
assumed in eq 1, a more reliable estimate of this Coulomb
interaction is obtained using eq 5, which takes into account the
actual charge distributions along the donor/acceptor chro-
mophores (polarization effects, accounted for in the quantum-
chemical calculations, lead to the appearance of positive and
negative partial charges alternating along the conjugated
pathway and reduce the overall interaction between the opposite
charges, especially in M1). When correcting eq 1 for these
effects (i.e., injecting the calculated Coulomb binding energies
in eq 1 while keeping the experimental results for the other
terms), the changes in Gibbs free energy for direct charge
separation (induced by exciting the OPV segment) are ca. -0.9
and -0.4 eV for M1 and P2, respectively, in excellent
agreement with the values in Table 3 (-1.1 and -0.6 eV,
respectively).
calculated rates as a result of the high sensitivity of the rates
on the electronic couplings and mostly the various thermody-
namical parameters involved in the exponential function of eq
4.
Conclusions
Two new copolymers (P1 and P2, Figure 1) consisting of
alternating OPV donor and PERY acceptor segments have been
synthesized using a palladium-catalyzed Suzuki reaction. Ad-
ditionally, a cyclic compound M1 in which the two chro-
mophores are in a face-to-face orientation could be isolated from
the polymerization of P1. Excitation of either of the chro-
mophores in P1, P2, or M1 results in photoinduced energy-
and electron-transfer reactions. Photoluminescence quenching,
photoluminescence lifetime, and transient pump-probe spec-
troscopy have been used to assess rates for energy transfer (ket),
i
direct and indirect charge separation (kcsd and kcs ), and charge
We are now in a position to discuss the overall rates for
charge separation and charge recombination. The latter process
in P2 and, to a lesser extent, M1 involves a very large driving
force (see Table 3), corresponding to the ‘inverted’ regime in
Marcus theory (for |∆G°| . λ, the rate decreases with increasing
driving force). For charge separation, |∆G°| in P2 is close to
the total reorganization energy, while in M1 |∆G°| is signifi-
cantly greater than λ. Therefore, the activation barrier for
photoinduced charge transfer is smaller in P2 than in M1, which
partly counteracts the larger electronic tunneling matrix elements
computed for the cyclic structure. The calculated transfer rates
are consistent with this qualitative analysis, with about 1 order
of magnitude enhancement in the charge separation rates
calculated for M1 with respect to P2 and larger rates for charge
separation in comparison to recombination (especially in the
case of M1). We further note that the electron-transfer process
is predicted to be much more efficient than hole transfer in M1
while they display similar rates in P2. These trends are fully
consistent with the experimental results in Table 2; however,
there are significant deviations between the measured and
recombination (krec) of these materials in solution and in thin
films (Figure 6, Table 2). The direct forward electron transfer,
d
i.e., from OPV(S1), is extremely fast (kcs g 3.5 × 1011 s-1
)
for all systems but especially high in the case of M1. The rate
for singlet energy transfer from OPV(S1) to PERY(S1) is
competitive with the fast direct charge separation. The recom-
bination rates vary more strongly with the sample (Table 2).
The longest lifetimes (0.5-1.6 ns) are observed for P1 and P2
in toluene. Quantum-chemical calculations confirm that higher
rates are obtained in M1 with respect to P2; this mostly comes
from a larger electronic coupling due to the strong donor-
acceptor interactions in the face-to-face arrangement of the
macrocyclic compound. The calculations point to the very strong
dependence of the electronic couplings between donor and
acceptor, on molecular orientations (geometries), orbital sym-
metries and energies, and the nature of the initial excited state.
In the films, geminate recombination is much faster, although
some charges are able to diffuse away and are longer lived.
Polymers P1 and P2 have been tested in photovoltaic devices.
9
J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003 8637

A R T I C L E S
Neuteboom et al.
Hz) cw Ar-ion laser (Spectra Physics 2025) pump beam tuned to 458
or 528 nm (25 mW, beam diameter of 2 mm) and monitoring the
resulting change in transmission (∆T) of a tungsten-halogen white-
light probe beam after dispersion by a triple-grating monochromator,
using Si, InGaAs, and (cooled) InSb detectors.
The femtosecond laser system used for pump-probe experiments
consists of an amplified Ti-sapphire laser (Spectra Physics Hurricane),
providing 150 fs pulses at 800 nm with an energy of 750 µJ at 1 kHz.
Pump (450 or 520 nm, fluence 0.5 µJ/mm2) and probe (1450 nm) pulses
were created by optical parametric amplification and 2-fold frequency
doubling using two OPAs (Spectra Physics OPA-C). The pump beam
was linearly polarized at the magic angle (54.7°) with respect to the
probe beam. The temporal evolution was recorded using an InGaAs
detector and standard lock-in detection at 500 Hz.
Working cells with satisfactory open-circuit voltages (VOC
)
1-1.2 V) were prepared, but the short-circuit current densities
(JSC ) 0.008-0.012 mA cm-2) under AM1.5 conditions were
extremely low. The low currents likely result from fast geminate
recombination (more than 80% recombination in the first ns)
and poor transport characteristics due to face-to-face orientations
of OPV and PERY segments in alternating stacks in the polymer
films.
The photophysical results on P1 and P2 provide an important
guideline for design of new alternating copolymers that could
be more effective in polymer photovoltaic cells. To overcome
the intrinsic tendency of donor and acceptor segments to give
alternating stacks (Figure 13), stronger antagonistic interactions
that direct the microscopic morphology should be introduced.
This might be accomplished by using segments that differ more
strongly in size, or by utilizing the low entropy of mixing (e.g.,
in block copolymers), or by introducing anchoring points at the
polymer chains (e.g., via interchain hydrogen bonds) that secure
the relative positions of donor and acceptor. Because charge
separation is slower for an end-to-end substitution of the two
chromophores (as in P2) than for a face-to-face orientation (as
in M1), care must be taken that the rate of electron transfer
remains high enough to guarantee efficient charge generation
in these desired isolated neighboring stacks of donors and
acceptors. Current investigations are directed toward such
appealing systems.
Photovoltaic Cells. For fabrication of devices, polyethylenediox-
ythiophene polystyrenesulfonate (PEDOT:PSS, Bayer AG) was spin
coated from an aqueous dispersion under ambient conditions on
precleaned (washing, UV ozone treatment) ITO covered glass substrates,
and the layer was dried by annealing the substrate on a hot plate. The
resulting PEDOT:PSS film thickness is about 100 nm. The polymers
P1 and P2 were spin cast from chloroform to give an active layer of
∼100 nm thickness as determined with a Tencor P10 surface profiler.
Finally, a LiF(1 nm)/Al(100 nm(back electrode was deposited by
thermal evaporation under vacuum (5 × 10-6 mbar, 1 ppm O2 and <1
ppm H2O). The active area of the device is 0.1 cm2. Illumination was
performed with a Steurnagel SolarConstant 1200 solar simulator, set
to 100 mW cm-2
.
Acknowledgment. We thank Edwin H. A. Beckers for
assistance in the pump-probe experiments. The Eindhoven-
Mons collaboration is supported by the European Commission,
under the RTN network ‘LAMINATE’. The research in Eind-
hoven has been supported by the Netherlands Organization for
Chemical Research (CW), the Netherlands Organization for
Scientific Research (NWO), and the Eindhoven University of
Technology through a grant in the PIONIER program. The work
at Arizona is partly supported by the National Science Founda-
tion, the Office of Naval Research, and the IBM Shared
University Research Program. The work in Mons is partly
supported by the Belgian Federal Services for Scientific,
Technical, and Cultural Affairs (InterUniversity Attraction Pole
5/3) and the Belgian National Science Foundation (FNRS).
David Beljonne and Je´roˆme Cornil are Research Associates from
FNRS. The research of S. C. J. Meskers has been made possible
by a fellowship of the Royal Netherlands Academy of Arts and
Sciences.
Experimental Section
Synthesis. The detailed experimental procedures for the synthesis
and characterization of the compounds 1-13, M2, M3, P1, and P2
are given in the Supporting Information.
General Methods. Cyclic voltammograms were measured in 0.1
M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting
electrolyte in dichloromethane using a Potentioscan Wenking POS73
potentiostat. The working electrode 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 the reference electrode, calibrated against Fc/Fc+
(+0.43 V). UV-vis absorption spectra were recorded on a Perkin-
Elmer Lambda 900 spectrophotometer. Fluorescence spectra were
recorded on an Edinburgh Instruments FS920 double-monochromator
spectrometer and a Peltier-cooled red-sensitive photomultiplier.
Fluorescence Lifetimes. Time-correlated single-photon counting
fluorescence studies were performed using an Edinburgh Instruments
LifeSpec-PS spectrometer, consisting of a 400 nm picosecond laser
(PicoQuant PDL 800B) operated at 2.5 MHz and a Peltier-cooled
Hamamatsu microchannel plate photomultiplier (R3809U-50). Lifetimes
were determined from the data using the Edinburgh Instruments
software package.
Supporting Information Available: Experimental procedures
for the synthesis and characterization of compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org. See any current masthead page for ordering
information and Web access instructions.
Photoinduced Absorption Spectroscopy. Near steady-state pho-
toinduced absorption (PIA) spectra were recorded between 0.25 and 3
eV by exciting a thin drop-cast film on quartz in an Oxford Optistat
continuous flow cryostat with a mechanically modulated (typically 275
JA034926T
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8638 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003