Photoinitiated charge transport through π-stacked electron conduits in supramolecular ordered assemblies of donor-acceptor triads

Article abstract of DOI:10.1021/ja903903q

Photochemical electron donor-acceptor triads having an aminopyrene primary donor (APy) and a p-diaminobenzene secondary donor (DAB) attached to either one or both imide nitrogen atoms of a perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor were

Full text of DOI:10.1021/ja903903q

Published on Web 07/31/2009
Photoinitiated Charge Transport through π-Stacked Electron
Conduits in Supramolecular Ordered Assemblies of
Donor-Acceptor Triads
Joseph E. Bullock, Raanan Carmieli, Sarah M. Mickley, Josh Vura-Weis, and
Michael R. Wasielewski*
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern UniVersity, EVanston, Illinois 60208-3113
Received May 13, 2009; E-mail: m-wasielewski@northwestern.edu
Abstract: Photochemical electron donor-acceptor triads having an aminopyrene primary donor (APy) and
a p-diaminobenzene secondary donor (DAB) attached to either one or both imide nitrogen atoms of a
perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor were prepared to give DAB-APy-PDI and DAB-
APy-PDI-APy-DAB. In toluene, both triads are monomeric, but in methylcyclohexane, they self-assemble
into ordered helical heptamers and hexamers, respectively, in which the PDI molecules are π-stacked in
a columnar fashion, as evidenced by small- and wide-angle X-ray scattering. Photoexcitation of these
supramolecular assemblies results in rapid formation of DAB+•-PDI-• spin-polarized radical ion pairs having
spin-spin dipolar interactions, which show that the average distance between the two radical ions is much
larger in the assemblies (31 Å) than it is in their monomeric building blocks (23 Å). This work demonstrates
that electron hopping through the π-stacked PDI molecules is fast enough to compete effectively with charge
recombination (40 ns) in these systems, making these materials of interest as photoactive assemblies for
artificial photosynthesis and organic photovoltaics.
Introduction
allows precise control over the initial light harvesting and
electron transfer events, while self-assembly, or supramolecular
Achieving functional molecular devices for either solar fuels
or electricity production requires hierarchical organization at
the supramolecular level similar to that of natural photosyn-
thesis.1-3 The design and synthesis of complex, covalent
molecular systems comprised of chromophores, electron donors,
and electron acceptors, which mimic both the light-harvesting
and the charge separation functions of photosynthetic proteins,
has been demonstrated.4-10 As is the case in photosynthetic
reaction center proteins, multicomponent donor-acceptor arrays
that carry out multistep charge separation reactions have proven
most useful for producing long-lived charge-separated states.
Covalent synthesis of functional building blocks with well-
defined molecular geometries and donor-acceptor distances
organization provides a facile mechanism for assembling large
numbers of molecules into structures that can bridge length
scales from nanometers to macroscopic dimensions for efficient
long distance charge transport. It can also lead to synergistic
and emergent properties that are not intrinsic to the building
blocks themselves.
Soluble conjugated organic polymers11-13 and small mol-
ecules14-20 have been studied extensively as photoactive materi-
als for solar cells. Unlike most inorganic materials, organic
materials allow for solution processing, offering the potential
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J. AM. CHEM. SOC. 2009, 131, 11919–11929 11919

A R T I C L E S
Bullock et al.
toinduced charge and energy transfer processes.32-34 PDI is both
photochemically and thermally stable,35 and can be easily
modified at its imide nitrogens, and its 1, 6, 7, and 12 positions.
Modifications at these positions tune the electronic properties
of PDI resulting in derivatives that absorb light from the near-
ultraviolet to the near-infrared region of the spectrum. Moreover,
the redox potentials of PDI can be tuned by appropriate
substituents resulting in derivatives that are either good electron
acceptors36 or donors.37 PDI also demonstrates the ability to
self-assemble in solution via hydrophobic/hydrophilic interac-
tions as well as by π-π stacking.38-42 Self-assembled, π-stacked
PDI systems have been studied extensively; these systems
include 1-D assemblies of PDI with a solubilizing group
consisting of long alkyl chains at the imide position,43 tethered,
extended PDI systems,13,16,41,44-48 tetra-substituted phenoxy
PDI,49,50 bis-1,7-(3′,5′-di-t-butylphenoxy)-PDI,38,51,52 and con-
trolled assemblies of PDI dimers.53,54 Many of these systems
form H-aggregates having nearly cofacial orientations, which
promotes enhanced electronic communication between adjacent
chromophores enabling efficient energy and/or charge transport.
Figure 1. A multilayer organic solar cell in which covalent donor-acceptor
building blocks are self-assembled to facilitate charge migration to the
electrodes.
of inexpensive device fabrication. However, conjugated poly-
mers have low charge-carrier mobilities relative to those of
crystalline aromatic small molecules.17 This is attributed
primarily to poor solid-state order in polymers resulting in
charge traps.21 Since increasing the degree of order in the solid
state most often translates into improved charge carrier mobili-
ties, the use of self-assembly strategies offers the possibility of
greatly increasing mobilities in organic materials, while main-
taining processing advantages. A conceptual model for a
functional organic material for use in a solar cell having efficient
photodriven charge separation and subsequent charge transport
is depicted in Figure 1. In this model, efficient photodriven
multistep charge separation occurs within a covalently linked,
donor-acceptor building block resulting in formation of a long-
lived radical ion pair (RP, electron-hole pair). Efficient transport
of the separated charges over tens of nanometers to the
electrodes requires that the RP lifetime is sufficiently long to
permit rapid charge hopping of the weakly interacting, uncor-
related charges within self-ordered, segregated, donor and
acceptor conduits.
Recently, we and others have shown that photoinduced charge
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Photoinitiated Charge Transport
A R T I C L E S
(Fisher) were dried using a Glass Contour solvent system, while
MCH (Aldrich) was dried over 3 Å molecular sieves. Steady-state
absorption spectra were obtained at room temperature on a
Shimadzu 1601 UV/vis spectrometer. Electrochemical measure-
ments were performed using a CH Instruments Model 622
electrochemical workstation. All measurements were performed in
DCM containing 0.1 M tetra-n-butylammonium hexafluorophos-
phate (TBAPF6) electrolyte. A 1.0 mm diameter platinum disk
electrode, platinum wire counter electrode, and Ag/AgxO reference
electrode were employed. The ferrocene/ferrocenium redox couple
(Fc/Fc+, 0.46 vs SCE)61 was used as an internal reference for all
measurements. TBAPF6 was purchased from Aldrich and recrystal-
lized twice from ethyl acetate prior to use.
Transient Optical Absorption Spectroscopy. Femtosecond
transient absorption measurements were made using a regeneratively
amplified titanium sapphire laser system operating at a 2 kHz
repetition rate.53 The frequency-doubled output of the amplifier was
used to pump a two-stage optical parametric amplifier to generate
532 nm, 130 fs laser pulses. A small portion of the fundamental
was focused onto a sapphire disk to generate the white-light probe,
spanning 425-800 nm. Spectral and kinetic data were collected
with a CCD array detector and a 6 ns delay track. Samples in
toluene and MCH with an absorbance of 0.4-0.8 at the excitation
wavelength were irradiated in 2 mm quartz cuvettes with 1 µJ/
pulse focused to a 0.5 mm diameter spot. The total instrument
response function was 180 fs. Kinetic analysis was performed at
several wavelengths using a Levenberg-Marquardt nonlinear least-
squares fit to a sum of exponentials convoluted with a Gaussian
instrument response function.
to address the question of whether unpaired electrons are
delocalized or hopping among the PDI derivatives in self-
assembled PDI arrays, we have carried out room-temperature,
solution-phase EPR and ENDOR studies on the radical anion
of a covalent PDI3 trefoil molecule, which self-assembles into
π-stacked dimers.60 We have also explored a bichromophoric
electron donor-acceptor molecule composed of a zinc tetraphe-
nylporphyrin (ZnTPP) donor surrounded by four perylene-3,4:
9,10-bis(dicarboximide) (PDI) acceptors (ZnTPP-PDI4).58 In
toluene, ZnTPP-PDI4 self-assembles into nearly monodisperse
aggregates comprised of five molecules arranged in a columnar
stack, (ZnTPP-PDI4)5. Data for this self-assembled system was
compared with that for a monomeric ZnTPP-PDI reference
molecule. Femtosecond transient absorption spectroscopy shows
that laser excitation of both ZnTPP-PDI and (ZnTPP-PDI4)5
results in quantitative formation of ZnTPP+•-PDI-• in a few
picoseconds. The transient absorption spectra of (ZnTPP-PDI4)5
also show that the PDI-• radical interacts strongly with adjacent
PDI molecules and suggests that the electron migrates within
the columnar stack. While charge recombination occurs more
slowly within (ZnTPP-PDI4)5 (τCR ) 4.8 ns) than it does in the
monomeric ZnTPP-PDI reference molecule (τCR ) 3.0 ns), the
ZnTPP+•-PDI-• lifetime within the (ZnTPP-PDI4)5 assembly
is too short for charge transport over long distances through
the noncovalent π-stacked molecules to be competitive with
charge recombination. In the work described here, we demon-
strate that extending the lifetime of the charge separation using
two-step electron transfer in covalently linked donor-acceptor
triad building blocks 2 and 3 results in fully competitive charge
hopping within self-assembled hexamers and heptamers of the
triads, thereby demonstrating that the concept illustrated in
Figure 1 can be fulfilled in an ordered supramolecular material.
Samples for nanosecond transient absorption spectroscopy were
placed in a 10 mm path length quartz cuvette equipped with a
vacuum adapter and subjected to four freeze-pump-thaw degas-
sing cycles. The samples were excited with 7 ns, 2 mJ, 532 nm
laser pulses generated using the frequency-doubled output of a
Continuum Precision II 8000 Nd:YAG laser. The probe light in
the nanosecond experiment was generated using a xenon flashlamp
(EG&G Electrooptics FX-200) and detected using a photomultiplier
tube with high voltage applied to only four dynodes (Hamamatsu
R928). The total IRF is 7 ns and is determined primarily by the
laser pulse duration.
X-ray Scattering. Solutions of 2 and 3 (10-3-10-4 M) in MCH
were loaded into 2 mm quartz capillaries with a wall thickness of
0.2 mm. X-ray measurements on 2 were carried out at beamline
12-ID-C at the Advanced Photon Source (APS), Argonne National
Laboratory. The X-ray scattering instrument utilized a double-crystal
Si(111) monochromator and a two-dimensional mosaic CCD
detector. The scattering intensity is a function of the scattering
vector, q, which is related to the scattering angle 2θ by the relation
q ) (4π/λ)sin θ, where λ is the X-ray wavelength. The X-ray
wavelength was set at λ ) 1.0 Å and the sample-to-detector
distances were adjusted to record scattering across two different
regions of q: 0.006 Å-1 < q < 0.2 Å-1 and 0.29 Å-1 < q < 2.1 Å-1
.
X-ray scattering measurements on 3 were carried out using the
Dupont-Northwestern-Dow 5-ID-D beamline at the APS. Simul-
taneous small/wide-angle X-ray scattering (SAXS/WAXS) mea-
surements were made using a Roper Scientific simultaneous SAXS/
WAXS system.
Time-Resolved EPR (TREPR) Spectroscopy. Samples for
TREPR measurements were prepared by loading a 0.1 mM solution
of 2 or 3 into 3.8 mm o.d. (2.4 mm i.d.) quartz tubes, subjecting
them to several freeze-pump-thaw degassing cycles on a vacuum
line (10-4 Torr), and sealing them using a hydrogen torch. TREPR
measurements at 295 K and at 85 K were carried out using Bruker
Elexsys E580 spectrometer operating at 9.5 GHz. The temperature
was controlled by an Oxford Instruments CF935 continuous flow
cryostat using liquid nitrogen. Samples were photoexcited at the
indicated temperature inside the EPR cavity with 532 nm, 2.5 mJ/
Experimental Section
The syntheses of molecules 1-3 are detailed in the Supporting
Information.
UV-Vis Spectroscopy and Electrochemistry. Toluene (Ald-
rich) and nonstabilized HPLC grade dichloromethane (DCM)
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J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009 11921

A R T I C L E S
Bullock et al.
Scheme 1. The Synthesis of 1 Starting from Pyrene
pulse, 7 ns laser pulses at a 10 Hz repetition rate from the frequency-
doubled output of a Nd:YAG laser (QuantaRay DCR-2). The
polarization of the laser was set to 54.7° relative to the direction
of the static magnetic field to avoid magnetophotoselection effects
on the spectra. The field modulation was disabled to achieve a time
response of Q/πν ≈ 30 ns, where Q is the quality factor of the
resonator and ν is the resonant frequency, while microwave signals
in emission (e) and/or enhanced absorption (a) were detected in
both the real and the imaginary channels (quadrature detection).
Sweeping the magnetic field gave 2D spectra versus both time and
magnetic field. For each kinetic trace, the signal acquired prior to
the laser pulse was subtracted from the data. Kinetic traces recorded
at magnetic field values off-resonance were considered background
signals, whose average was subtracted from all kinetic traces. The
spectra were subsequently phased into a Lorentzian part and a
dispersive part, and the former, also known as the imaginary
magnetic susceptibility ꢀ′′, is presented. BDPA (R,γ-bisdiphenylene-
ꢁ-phenylallyl) dissolved in a polystyrene film and mounted into
0.8-mm-o.d., thin-walled quartz tubes was used as an internal
standard for phase alignment of the EPR spectra. Simulation of
the powder-pattern spectra of the spin-polarized RPs62 and the triplet
states63,64 resulting from charge recombination was performed using
a home-written MATLAB program65 following published procedures.
reaction with 1-octyne yields the dialkynylated pyrene, 4, which
is catalytically hydrogenated to give 5. Monobromination of 5
to produce 6 proceeds with high selectivity using slow addition
of a chloroform solution of NBS. Suzuki cross-coupling of 6
with 4-nitrophenylboronate pinacol ester gives 7, which is
subsequently brominated with NBS to give 8. The nitro group
of 8 is then reduced with hydrazine in the presence of graphite to
yield the corresponding aniline 9, which is condensed with 1,7-
bis(3,5-di-t-butylphenoxy)perylenetetracarboxydianhydride40 to yield
monoimide 10. Further condensation of 10 with 2-ethylhexyl-
amine yields 11. Finally, palladium-catalyzed Buchwald-
Hartwig coupling of 11 with commercially available 1-(4-
nitrophenyl)piperazine gives 1.
The synthesis of 2 and 3 proceeds from 6 by Buchwald-
Hartwig coupling with 1-(4-nitrophenyl)piperazine to give 12,
which is brominated to yield tetrasubstituted pyrene 13 (Scheme
2). Reduction of the nitro group in 13 to the amine with
hydrazine in the presence of graphite yields the p-diaminoben-
zene (DAB) moiety, 14, which is very easily oxidized (EOX
)
0.23 V vs SCE, see below), making this molecule prone to
degradation by exposure to air and light. Thus, the amine was
immediately dialkylated using 1-bromohexadecane to form a
stable oil, 15. Suzuki cross-coupling of 15 with 4-nitrophenyl-
boronate pinacol ester yields nitro compound 16. Reduction of
the nitro compound with hydrazine in the presence of graphite
yields the air-sensitive yellow amine 17. Condensation of 17
with the previously reported N-(2-ethylhexyl)-1,7-bis(3,5-di-t-
butylphenoxy)perylene(3,4:9,10)dicarboximidedicarboxyanhy-
dride40 yields 2, while condensation of 17 with 1,7-bis(3,5-di-
t-butylphenoxy)perylene(3,4:9,10)tetracarboxydianhydride yields
3.40
Results and Discussion
Synthesis. The synthesis of the asymmetrically substituted
pyrene used as the primary electron donor (APy) in 1-3 begins
with the bromination of pyrene, which produces principally the
1,6-dibromo isomer along some of the 1,8-isomer (Scheme 1).66
Separation of the dibromopyrene isomers is achieved by repeated
recrystallizations; however, it is more convenient to separate
the isomers at a later stage of the synthesis. A 2-fold Sonagashira
Electronic Absorption Spectroscopy. The electronic spectra
of 1-3 in toluene have a strong absorption band at 546 nm,
which is characteristic of the (0,0) band of PDI,67 and shows a
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(67) Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson,
T. M.; Wasielewski, M. R. J. Phys. Chem. A 2008, 112, 2322–2330.
9
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A R T I C L E S
Figure 2. Steady-state absorption for (a) 1, 2, and 3 in toluene; and (b) 2 and 3 in methylcyclohexane (MCH).
Scheme 2. Synthesis of Triads 2 and 3
normal vibronic progression at shorter wavelengths, while the
broad, featureless absorption band at 386 nm is typical of
substituted pyrenes68 (Figure 2a). The pyrene absorption in
compound 1 is somewhat more intense than in 2 most likely
because the electron withdrawing p-nitroaniline group in 1
increases the transition dipole moment through increased
conjugation with the electron rich pyrene. The DAB moiety in
compounds 2 and 3 also possesses a distinct absorption feature;
however, it occurs at shorter wavelengths in the UV region,
where it overlaps with absorption features from both APy and
PDI.69 In contrast, when 2 and 3 are dissolved in methylcyclo-
hexane (MCH), the PDI vibronic bands exhibit distinct intensity
changes with the (0,1) band becoming more intense than the
(0,0) band. The bands due to pyrene are slightly red-shifted to
380 nm and broadened (Figure 2b). The observed spectral
changes of PDI within 2 and 3 are similar to those observed in
other PDI-based systems,35,41,44,45,70 which form H-aggregates
and exhibit significant exciton coupling between the π-stacked
chromophores. While the spectra indicate that cofacial π-stack-
ing is occurring for both 2 and 3 in MCH, they do not provide
more detailed structural information. Moreover, the proton NMR
spectra of 2 and 3 in MCH are severely broadened and do not
offer further insights.
Structures of the Supramolecular Assemblies. The Advanced
Photon Source at Argonne National Laboratory was used to
determine the solution structures of the supramolecular as-
semblies of 2 and 3. It has been recently demonstrated that
small- and wide-angle X-ray scattering (SAXS/WAXS) from
monodisperse noncovalent aggregates using a synchrotron
source is a powerful tool for the elucidation of their solution-
phase structures.38,51,54,58–60,71-75 Guinier analysis of the scat-
tering data provides at a minimum the radius of gyration of the
complex, Rg and a gauge of the polydispersity of the
aggregates.76,77 In favorable cases, if the assemblies are nearly
monodisperse, further analysis of the SAXS/WAXS data using
(71) Zhang, R.; Thiyagarajan, P.; Tiede, D. M. J. Appl. Crystallogr. 2000,
33, 565–568.
(72) Tiede, D. M.; Zhang, R.; Chen, L. X.; Yu, L.; Lindsey, J. S. J. Am.
Chem. Soc. 2004, 126, 14054–14062.
(73) Kelley, R. F.; Lee, S. J.; Wilson, T. M.; Nakamura, Y.; Tiede, D. M.;
Osuka, A.; Hupp, J. T.; Wasielewski, M. R. J. Am. Chem. Soc. 2008,
130, 4277–4284.
(68) Kaletas, B. K.; Dobrawa, R.; Sautter, A.; Wuerthner, F.; Zimine,
M.; De Cola, L.; Williams, R. M. J. Phys. Chem. A 2004, 108, 1900–
1909.
(74) Kelley, R. F.; Shin, W. S.; Rybtchinski, B.; Sinks, L. E.; Wasielewski,
M. R. J. Am. Chem. Soc. 2007, 129, 3173–3181.
(75) Kelley, R. F.; Goldsmith, R. H.; Wasielewski, M. R. J. Am. Chem.
Soc. 2007, 129, 6384–6385.
(69) Rawashdeh, A. M. M.; Sotiriou-Leventis, C.; Gao, X.; Leventis, N.
Chem. Commun. (Cambridge, U.K.) 2001, 1742–1743.
(70) Hernando, J.; de Witte, P. A. J.; van Dijk, E. M. H. P.; Korterik, J.;
Nolte, R. J. M.; Rowan, A. E.; Garcia-Parajo, M. F.; van Hulst, N. F.
Angew. Chem., Int. Ed. 2004, 43, 4045–4049.
(76) Guinier, A.; Fournet, G. Small Angle Scattering; Wiley: New York,
1955.
(77) Svergun, D. I.; Koch, M. H. Rep. Prog. Phys. 2003, 66, 1735–1782.
9
J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009 11923

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Bullock et al.
Figure 3. SAXS/WAXS data obtained in MCH. Comparison of pair distribution functions (PDFs) generated from the WAXS data with structural fits to the
data for 2 (a) and 3 (b). Insets: Guinier fits using the SAXS data.
atomic pair distance distribution functions (PDFs) and simulated
annealing procedures can be performed to obtain the structure
in solution with a resolution approaching 3-4 Å for molecular
weights up to about 50 kDa.77–81 For example, using these
techniques, we have previously obtained structures of complex
arylene diimide assemblies up to heptamers having a molecular
weight of about 28 kDa.38,51,53,54 New computational approaches
allow SAXS/WAXS data to be interpreted in terms of coordinate
models71,72,82 and molecular dynamics simulations,83,84 and can
be used to refine the structures of self-assembled systems.85 The
ability to carry out structural studies on such assemblies in
solution at concentrations typical of time-resolved optical and
EPR spectroscopic measurements provides an important con-
nection relating structure to observed function.
The sizes of the assemblies derived from 2 and 3 in solution
were determined by analysis of the low q region of the scattering
data (0.006 Å-1 < q < 0.2 Å-1). A plot of the log of the scattering
intensity, I(q), versus q2 over this range of q is linear for both
2 and 3 (Figure 3, insets). According to the Guinier approxima-
tion,76,86 I(q) ) I(0) exp(- q2R2g/3), where I(0) is the forward
scattering amplitude, and the slope of the linear plot gives the
radius of gyration Rg, which is proportional to the size of the
assemblies. From the slope of the linear Guinier fits, Rg ) 18.3
and 19.8 Å for 2 and 3, respectively. Structural models of the
assemblies based on this information were constructed in
Hyperchem87 and energy minimized using the MM+ force field.
PDFs were calculated from the high q experimental scattering
data (0.29 Å-1 < q < 2.1 Å-1) and from the structural models.
The structural models were then varied until a best fit with the
experimental PDFs for 2 and 3 was found. These best fits are
shown in Figure 3. Both Guinier and PDF analyses yield Rg
values which are in excellent agreement with each other. A
selection of structural models illustrating the range of structures
that were examined are given in the Supporting Information
(Figures S1-S4).
Despite the fact that 3 has a larger molecular weight than 2,
their supramolecular assemblies are similar in size, with the data
for 2 best fit to a heptamer (27), while the best fit for 3 yields
a hexamer (36) (Figure 4). The WAXS data did not provide
sufficient contrast between the alkyl chains and the solvent, so
that they are lost in the subtraction of the solvent scattering. In
both structures, the PDI acceptors are approximately 3.4 Å apart
and twisted 30° relative to one another. This twisting relieves
some of the steric repulsion between the donors and allows the
PDI acceptors to π-stack as closely as possible. The structural
models show that the helical pitch of the assemblies places the
donor ends of the triad monomers at equal spacings along the
outer edges of the assemblies. As a result, the DAB donors are
positioned ∼20 Å apart, rendering charge hopping from DAB+•
of one triad to DAB of another triad highly unlikely. In contrast,
the PDI-• of one triad is π-stacked in van der Waals contact
with the PDIs of neighboring triads in the stacked assembly, so
that charge hopping of the unpaired electron from PDI-• to a
neighboring PDI is likely, as we have demonstrated in chemi-
cally reduced PDI assemblies.60
Charge Separation and Recombination. The energy of the
lowest excited singlet state of the monomeric PDI derivative
used in 1-3 is 2.23 eV.67 However, the absorption spectra of
the π-stacked PDI molecules within 27 and 36 in MCH (Figure
2b) exhibit an exciton splitting (2V) of approximately 0.35 eV,
so that the electronic absorption of the strongly disallowed lower
exciton state of the PDI in the aggregates should occur at about
2.1 eV. Assuming that the Stokes shift of the exciton coupled
PDI molecules following excitation is comparable to that of
the PDI monomer, the energy of the lower exciton state of the
PDI in 27 and 36 is estimated to be about 2.05 eV. The energies
of the excimer-like states of the PDI in 27 and 36 should be
even lower, since the emission maxima of covalent, cofacial
PDI dimers occur at about 695 nm or 1.79 eV.67
(78) Petoukhov, M. V.; Eady, N. A. J.; Brown, K. A.; Svergun, D. I.
Biophys. J. 2002, 83, 3113–3125.
(79) Svergun, D. I. J. Appl. Crystallogr. 1992, 25, 495–503.
(80) Svergun, D. I. Biophys. J. 1999, 76, 2879–2886.
(81) Volkov, V. V.; Svergun, D. I. J. Appl. Crystallogr. 2003, 36, 860–
864.
(82) O’Donnell, J. L.; Zuo, X.; Goshe, A. J.; Sarkisov, L.; Snurr, R. Q.;
Hupp, J. T.; Tiede, D. M. J. Am. Chem. Soc. 2007, 129, 1578–1585.
(83) Zuo, X.; Cui, G.; Mertz, K. M.; Zhang, L.; Lewis, F. D.; Tiede, D. M.
Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3534–3539.
(84) Mardis, K. L.; Sutton, H. M.; Zuo, X. B.; Lindsey, J. S.; Tiede, D. M.
J. Phys. Chem. A 2009, 113, 2516–2523.
The one-electron oxidation and reduction potentials for 1-3
are summarized in Table 1. Neither the oxidation potentials of
the DAB and APy donors nor the reduction potential of the
PDI acceptor used to construct the triads is significantly affected
by covalent linkages between the two redox active centers. The
energies of DAB-APy+•-PDI-• (∆GRP1) and DAB+•-APy-PDI-•
(85) Zuo, X.; Wang, J.; Foster, T. R.; Schwieters, C. D.; Tiede, D. M.;
Butcher, S. E.; Wang, Y.-X. J. Am. Chem. Soc. 2008, 130, 3292–
3293.
(86) Glatter, O. Neutron, X-ray and Light Scattering; Elsevier: Amsterdam,
1991.
(87) Hyperchem; Hypercube, Inc.: Gainesville, FL.
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11924 J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009

Photoinitiated Charge Transport
A R T I C L E S
Figure 4. MM+ optimized structures that yield the best fits to the experimental SAXS/WAXS data. Structure for 27 (a) top view and (b) side view. Structure
for 36 (c) top view and (d) side view. PDI is red, APy is cyan, and DAB is blue.
Table 1. Redox Potentials vs SCEa and Radical Ion Pair Energies
DAB
APy
PDI
toluene
MCH
comp.
EOX1
EOX2
EOX1
EOX2
ERED1
ERED2
∆GRP1
∆GRP2
∆GRP1
∆GRP2
1
2
3
–
0.23
0.22
–
0.81
0.80
1.01
1.03
1.03
1.25
1.23
1.23
–0.62
–0.61
–0.60
–0.85
–0.84
–0.83
1.86
1.87
1.86
–
1.24
1.23
1.94
1.95
1.94
–
1.36
1.35
a Values obtained by differential pulse voltammetry in dry CH2Cl2 with 0.1 M Bu4NPF6 as the supporting electrolyte and Fc/Fc+ as the internal
standard.
(∆GRP2) were calculated using methods described earlier88,89
ground state absorption of PDI and the appearance of a
stimulated emission feature at 600 nm as well as a broad
absorption feature near 700 nm, which are typical of 1*PDI.67
Notably, the 700 nm feature sharpens and red-shifts to 725 nm
with τCS ) 9.8 ps, which is indicative of formation of PDI-•,90,91
as a result of the reaction APy-1*PDI f APy+•-PDI-•. Subse-
from the experimentally determined redox potentials along with
the ionic radii and donor-acceptor distances (see Supporting
Information), which were estimated using the MM+ energy
minimized ground state structures of 2 and 3.87 A comparison
of the PDI excited singlet state and RP energies indicates that
the two-step electron transfer sequence: DAB-APy-1*PDI f
DAB-APy+•-PDI-• f DAB+•-APy-PDI-• should occur in mon-
omeric 2 and 3, and it should also occur from the lower exciton
state of 1*PDI in 27 and 36, but not from the excimer-like state
of PDI, which is too low in energy.
We have used transient absorption spectroscopy to character-
ize the formation of these RP states. Molecule 1 serves as the
dyad reference molecule for triads 2 and 3. Selective photoex-
citation of the PDI chromophore in 1 with 130 fs, 532 nm laser
pulses results in the transient absorption spectra shown in Figure
5a. They are characterized at early times by a bleach of the
quently, APy+•-PDI-• decays back to ground state with τCR
)
1.1 ns. In contrast, the femtosecond transient absorption spectra
of 2 and 27 (Figures S5a and S5b, respectively) and 3 and 36
(Figure 5, panels b and c, respectively) show that PDI-• appears
in a few picoseconds (Table 2) and persists for the entire 6 ns
range of available pump-probe delay times. The formation of
PDI-• in 27 and 36 is faster than the formation of the PDI
excimer-like state from its lower exciton state for the PDI
(90) Gosztola, D.; Niemczyk, M. P.; Svec, W. A.; Lukas, A. S.;
Wasielewski, M. R. J. Phys. Chem. A 2000, 104, 6545–6551.
(91) Kelley, R. F.; Tauber, M. J.; Wasielewski, M. R. J. Am. Chem. Soc.
2006, 128, 4779–4791.
(88) Weller, A. Z. Phys. Chem. 1982, 133, 93–98.
(89) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 6767–6777.
9
J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009 11925

A R T I C L E S
Bullock et al.
Figure 5. Femtosecond transient absorption spectra for (a) 1 in toluene, (b) 3 in toluene, (c) 36 in MCH following a 130 fs, 532 nm laser pulse.
Table 2. Time Constants for Charge Separation (τCS) and
Recombination (τCR
along the applied magnetic field. RP-ISC depends on both the
spin-spin exchange interaction, 2J, and the dipolar interaction,
D, between the two radicals that comprise the RP. The
magnitude of 2J depends exponentially on the distance r
between the two radicals and is assumed to be isotropic, while
that of D depends on 1/r3 and is anisotropic. For large molecules
in solution, such as supramolecular assemblies 27 and 36, and
for molecules in the solid state, D is not rotationally averaged
to zero. D is usually approximated by the point dipole model,96
)
τCS (ps)
τCR (ns)
compound
toluene
MCH
toluene
MCH
1
2
3
9.8 ( 0.4
6.7 ( 0.3
3.2 ( 0.1
–
1.1 ( 0.1
33.4 ( 1.5
34.2 ( 0.2
–
5.6 ( 0.1
1.3 ( 0.1
49.2 ( 1.2
40.0 ( 0.4
geometry found in these assemblies.92 The persistence of the
transient absorption changes over the 6 ns time window of these
experiments results from the rapid secondary electron transfer
reaction: DAB-APy+•-PDI-• f DAB+•-APy-PDI-•. The ob-
served PDI-• absorption band in MCH is somewhat broader
than that observed in toluene. The extinction coefficient of
DAB+• is about 4.5 times smaller than that of PDI-•,93 so that
the spectral changes expected for DAB+• formation are over-
whelmed by the larger changes due to the PDI ground state
bleach. Nanosecond transient absorption spectroscopy was used
to determine the lifetime of DAB+•-APy-PDI-• for 2 and 27
(Figures S6a and S6b, respectively) and for 3 and 36 (Figure 6,
panels a and b, respectively). The data show that DAB+•-APy-
PDI-• recombines on a ∼33-50 ns time scale with significantly
longer lifetimes observed for 27 and 36 relative to those of 2
and 3, respectively. At times longer than about 200 ns, the
spectra show small residual absorption changes that are at-
tributed to the formation of DAB-APy-3*PDI upon charge
recombination (Figure 6, insets). The time constants for the
initial photoinduced charge separation and final recombination
are given in Table 2.
Radical Pair TREPR Spectra and Charge Separation
Distances. TREPR measurements were used to probe the
competition between charge recombination and charge transport
within the self-assembled, π-stacked structures of 27 and 36.
Following rapid, two-step charge separation, the initially formed
singlet RP, 1(DAB+•-APy-PDI-•), undergoes radical-pair inter-
system crossing (RP-ISC)94,95 to produce the triplet RP,
3(DAB+•-APy-PDI-•). TREPR measurements were carried out
in a 350 mT magnetic field, so that the triplet sublevels of
3(DAB+•-APy-PDI-•) undergo Zeeman splitting, and are best
described by the T+1, T0, and T-1 eigenstates that are quantized
3µ0g2eꢁ2e
8πr3
D ) -
(1)
where µ0, ge, and ꢁe are the vacuum permeability, electronic
g-factor and Bohr magneton, respectively. In units of mT and
Å, D ) -2785 mT·Å/r3. When RP distances are >∼10-15 Å,
both D and 2J are small, so that the S and T0 spin states of the
RP are close in energy and mix, while the T+1 and T-1 states
are energetically far removed from T0 and do not mix with S.97,98
The two RP states that result from S-T0 mixing are preferentially
populated due to the initial population of S, so that the four
∆m ) (1 EPR transitions that occur between these two mixed
states and T+1 and T-1 display an intensity pattern characteristic
of the strong spin polarization.94,95 The TREPR spectrum
consists of two antiphase doublets, centered at the g-factors of
the individual radicals that comprise the pair, in which the
splitting of each doublet is determined by 2J and D. The electron
spin polarization pattern of the EPR signal, that is, which
transitions are in enhanced absorption (a) or emission (e), low
field to high field, is determined by the sign rule,98,99
(-)gives e/a
(+)gives a/e
Γ ) µ · sign[J - D(3 cos2( ) - 1)] )
{
(2)
where is the angle between the dipolar axis of the radical
pair and the direction of the magnetic field B0 and µ is -1 or
+1 for a singlet or triplet excited state precursor, respectively.
Since ultrafast charge separation in 2 and 3 as well as in 27 and
36 proceeds from 1*PDI and the resultant RP spectra exhibit an
(e, a) spin polarization pattern, eq 2 restricts the signs and
(92) Hariharan, M.; Zheng, Y.; Long, H.; Zeidan, T. A.; Schatz, G. C.;
Vura-Weis, J.; Wasielewski, M. R.; Zuo, X.; Tiede, D. M.; Lewis,
F. D. J. Am. Chem. Soc. 2009, 131, 5920–5929.
(93) Steigman, J.; Cronkright, W. J. Am. Chem. Soc. 1970, 92, 6736–
6743.
(96) Efimova, O.; Hore, P. J. Biophys. J. 2008, 94, 1565–1574.
(97) Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori,
H.; Fukuzumi, S.; Norris, J. R., Jr. Proc. Natl. Acad. Sci. U.S.A. 2005,
102, 10017–10022.
(98) Dance, Z. E. X.; Mi, Q. X.; McCamant, D. W.; Ahrens, M. J.; Ratner,
M. A.; Wasielewski, M. R. J. Phys. Chem. B 2006, 110, 25163–
25173.
(94) Closs, G. L.; Forbes, M. D. E.; Norris, J. R. J. Phys. Chem. 1987,
91, 3592–3599.
(95) Hore, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J. Chem. Phys.
Lett. 1987, 137, 495–500.
(99) Hore, P. J. In AdVanced EPR in Biology and Biochemistry; Hoff,
A. J., Ed.; Elsevier: Amsterdam 1989, pp 405-440.
9
11926 J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009

Photoinitiated Charge Transport
A R T I C L E S
Figure 6. Nanosecond transient absorption spectra for (a) 3 in toluene, (b) 3 in MCH following a 7 ns, 532 laser pulse. Insets: transient absorption kinetics
at 725 nm.
Figure 7. TREPR spectra of 2 (a) and 3 (b) at 85 K and at 100 ns following a 532 nm, 2.5 mJ laser pulse. The smooth curves superimposed on the
experimental spectra are computer simulations of the RP spectra using the parameters given in Table 1.
Table 3. RP Simulation Parameters for 2 and 3 at 100 ns after the Laser Pulse
comp.
solvent
T (K)
2J (mT)
D (mT)
r (Å)
kSS 106 (s-1
)
kTT 106 (s-1
)
2
2
27
2
3
3
36
3
TOL
TOL
MCH
MCH
TOL
TOL
MCH
MCH
295
85
295
85
295
85
295
85
2.50 ( 0.01
0.20 ( 0.01
0.27 ( 0.01
0.27 ( 0.01
2.76 ( 0.01
0.32 ( 0.01
0.30 ( 0.01
0.32 ( 0.01
–
23.2a
31 ( 1
3.0 ( 0.1
10 ( 1
4.0 ( 0.1
32 ( 1
5.0 ( 0.1
10 ( 1
5.9 ( 0.1
22 ( 1
6.0 ( 0.1
16 ( 1
-0.08 ( 0.01
-0.09 ( 0.01
-0.09 ( 0.01
–
-0.10 ( 0.01
-0.09 ( 0.01
-0.09 ( 0.01
32 ( 1
31 ( 1
31 ( 1
23.2a
30 ( 1
31 ( 1
31 ( 1
7.2 ( 0.1
19 ( 1
13 ( 1
16 ( 1
10 ( 1
a Distances calculated from energy-minimized ground state structures (see text).
magnitudes that 2J and D can adopt. Given that D is negative
(eq 1), if 2J > 0, eq 2 predicts an (e, a) pattern for all values of
2J and D, while if 2J < 0, eq 2 predicts an (e, a) pattern only
when |D(3 cos2( ) - 1)| > |2J|.94,95
spectra, NMR, and the SAXS/WAXS results, molecules 2 and
3 are monomeric in toluene at 295 K, but assemble into the
helical assemblies 27 and 36, respectively, in MCH. The
formation of these large assemblies in MCH is also reflected in
a comparison of the TREPR spectra of DAB+•-APy-PDI-• in 2
and 3 (toluene) and in 27 and 36 (MCH) at 295 K. The values
of the exchange interaction 2J for monomeric 2 and 3 in toluene
at 295 K are 2.50 and 2.76 mT, respectively, while those of 27
and 36 decrease to about 0.3 mT. At 85 K, 2 and 3 as well as
27 and 36 show similar RP spectra with 2J ∼ 0.2-0.3 mT, which
is most likely due to the formation of supramolecular assemblies
of sizes comparable to 27 and 36 upon freezing. The value of
2J depends exponentially on the distance between the two
radical ions, so that 2J is small when the distance r is large,
and can be described by103
Photoexcitation of 2 and 3 as well as assemblies 27 and 36
with a 532 nm, 7 ns laser pulse produces spin-polarized RP
signals having an (e, a) phase pattern at both 295 K and at 85
K (Figure 7). The spectra were simulated with the spin-
correlated radical pair (SCRP) model62,94,95,100,101 using the
102
reported hyperfine coupling constants for DAB•+ and PDI•-.60
The fits to the data are given in Figure 7, and the fitting
parameters are summarized in Table 3. On the basis of UV-vis
(100) Buckley, C. D.; Hunter, D. A.; Hore, P. J.; McLauchlan, K. A. Chem.
Phys. Lett. 1987, 135, 307–312.
(101) Norris, J. R.; Morris, A. L.; Thurnauer, M. C.; Tang, J. J. Chem.
Phys. 1990, 92, 4239–4249.
(102) Grampp, G.; Kelterer, A. M.; Landgraf, S.; Sacher, M.; Niethammer,
D.; Telo, J. P.; Dias, R. M. B.; Vieira, A. J. S. C. Monats. Chem.
2005, 136, 519–536.
(103) De Kanter, F. J. J.; Kaptein, R.; Van Santen, R. A. Chem. Phys.
Lett. 1977, 45, 575–579.
9
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A R T I C L E S
Bullock et al.
2J ) 2J0e-R(r-r )
(3)
0
where 2J0 is the spin-spin exchange interaction at van der
Waals contact, R is a constant, and r0 is the van der Waals
contact distance of about 3.5 Å. Thus, the decrease in 2J upon
assembly of 2 and 3 into supramolecular species 27 and 36
indicates that the average distance between the photogenerated
radical ions has increased significantly in 27 and 36. However,
problems arise when attempts are made to use values of 2J and
eq 3 as a molecular “ruler” to compare charge separation
distances in monomeric 2 and 3 with those in 27 and 36 because
the radical ions within 2 and 3 are separated by covalent bonds,
while those in 27 and 36 are separated by both covalent bonds
and noncovalent π-π interactions through a variety of path-
ways, which results in different effective values of 2J0.
A much better gauge of average RP distances in these systems
is obtained from the dipolar spin-spin interaction D using the
point dipole approximation, eq 1. The D values for monomeric
2 and 3 in toluene at 295 K are averaged to zero by the tumbling
motions of these molecules in solution, so that the intramolecular
RP distance in DAB+•-APy-PDI-• is estimated from the
calculated ground state geometry of DAB-APy-PDI using the
PM3 method.104 The spins within DAB+• and PDI-• are
symmetrically distributed across their individual π systems,
which results in a 23.2 Å DAB+•-APy-PDI-• distance. In
contrast, the tumbling rates of 27 and 36 in MCH at 295 K are
sufficiently slow that simulations of their TREPR spectra yield
D values, which give an average RP distance of 31 Å using eq
1. This average distance is much larger than the 23.2 Å
intramolecular DAB+•-APy-PDI-• distance and is consistent with
charge migration through the π-stacked PDI molecules (Figure
8). The values of D obtained for 2 and 3 in MCH at 85 K are
the same as those at 295 K, which suggests that charge hopping
is not adversely affected by lowering the temperature. Interest-
ingly, cooling the toluene samples of monomeric 2 and 3 to 85
K also produces RP spectra having the same 2J and D values
as do 27 and 36 in MCH. We have no structural information
regarding the size of the assemblies that form upon cooling these
systems to 85 K; however, the TREPR data suggest that they
form supramolecular assemblies in which electron transport
occurs among at least 6 or 7 π-stacked PDI molecules.
The values of D clearly show that the average distance
between DAB+• and PDI-• within 27 and 36 increase substan-
tially relative to those in monomeric 2 and 3. Yet, on a purely
statistical basis, assuming equal probabilities for photoexciting
each PDI chromophore within 27 and 36, as well as for producing
each possible DAB+•-PDI-• RP within the structure as a result
of rapid charge hopping between PDI molecules, the average
RP distance based on the structures obtained from the SAXS/
WAXS measurements should only increase to 25.2 Å in 27 and
to 24.0 Å in 36 (Table S1). The much longer observed average
RP distances within both supramolecular assemblies are due to
a distance distribution that favors the longer distances within
them, since the maximum RP distance in 27 is 35.4 Å, while
that in 36 is 32.7 Å. For these small assemblies, the data suggests
that the electrons are being preferentially trapped at PDI
acceptors that are at the ends of the PDI stack. This may be a
consequence of the abrupt change in PDI solvation environment
at the terminal PDI molecules of the π-stack relative to the
solvation environment of the internal PDI molecules within the
π-stack.
Figure 8. Inter- vs intramolecular charge separation distance for 3 in MCH.
TREPR Spectra of Triplet States Resulting from Charge
Recombination. The charge recombination process in 2, 3, 27,
and 36 is spin selective; that is, 1(DAB+•-PDI-•) recombines to
3
the singlet ground state, while (DAB+•-PDI-•) recombines to
yield the triplet 3*PDI, which acquires the non-Boltzmann spin
population of the triplet RP state.105 The spin polarization of
the EPR transitions exhibited by 3*PDI can be differentiated from
those of a triplet state formed by the ordinary spin-orbit
intersystem crossing mechanism by the polarization pattern of
its six EPR transitions at the canonical (x, y, z) orientations
relative to the applied magnetic field. A RP precursor that
undergoes the RP-ISC mechanism by S-T0 mixing followed by
charge recombination uniquely yields an (a, e, e, a, a, e) (low
field to high field) polarization pattern.105
Similar triplet TREPR spectra are observed for 27 and 36 at
both 295 and 85 K (Figure 9), with each having an overall width
of ∼80 mT and an electron spin polarization pattern (a, e, e, a,
a, e) uniquely attributable to a triplet state formed by the RP-
ISC mechanism. The narrow RP spectrum that appears at g ∼
2 was discussed above. The appearance of fully resolved
anisotropic triplet spectra for 27 and 36 at 295 K can be explained
by the slow tumbling rate of these protein-sized assemblies,
relative to the frequencies of their zero-field splittings. We
recently observed similar phenomena in large rod-like PDI-based
donor-acceptor systems.98 Even in toluene, where molecules
2 and 3 are in their monomeric form, a weak triplet spectrum
appears at 295 K indicating that their tumbling rate is slow
enough, so that the zero-field splitting tensor is not completely
averaged to zero. The weak triplet observed for 2 and 3 in
toluene at 295 K could not be accurately simulated because of
its low signal-to-noise. All spectra obtained at 85 K have very
similar zero-field splitting parameters D and E of about 40 and
7 mT, respectively (Table 4), while those obtained at 295 K
exhibit smaller D values due to a slight narrowing of the
spectrum as a result of partial averaging because of molecular
tumbling. The appearance of triplet states having transitions with
the spin polarization pattern diagnostic for the RP-ISC mech-
anism, even at room temperature, certifies that the spin-spin
interactions D and J within the photogenerated RPs are very
weak. Thus, all the observed spin dynamics within these
supramolecular assemblies are consistent with long average RP
(104) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209–220.
(105) Levanon, H.; Hasharoni, K. Prog. React. Kinet. 1995, 20, 309–346.
9
11928 J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009

Photoinitiated Charge Transport
A R T I C L E S
Figure 9. TREPR spectra of 2 (a) and 3 (b) recorded at 85 and at 295 K. The smooth curves superimposed on the experimental spectra are computer
simulations of the triplet spectra.
Table 4. Zero Field Splitting Parameters Obtained from
Simulations of Triplet State TREPR Spectra
charge transport conduits for both holes and electrons, thus,
making these materials of interest as photoactive assemblies for
comp.
solvent
T (K)
D (mT)
E (mT)
both artificial photosynthesis and organic photovoltaics. The
challenge for the future is to develop strategies to template these
materials onto electrodes in the appropriate orientation that
directs charge transport efficiently from the conduits to the
electrodes.
2
2
2
2
3
3
3
3
toluene
toluene
MCH
295
85
295
85
295
85
295
85
-
-
40.5 ( 0.1
34.0 ( 0.1
39.6 ( 0.1
-
6.5 ( 0.1
7.4 ( 0.1
6.8 ( 0.1
-
MCH
toluene
toluene
MCH
40.4 ( 0.1
36.3 ( 0.1
39.7 ( 0.1
6.6 ( 0.1
7.0 ( 0.1
7.0 ( 0.1
Acknowledgment. This work was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of Basic
Energy Sciences, DOE under grant no. DE-FG02-99ER14999.
Portions of this work were performed at the Dupont-Northwestern-
Dow Collaborative Access Team (DND-CAT) located at Sector 5
of the Advanced Photon Source (APS). DND-CAT is supported
by E.I. Dupont de Nemours & Co., The Dow Chemical Company
and the State of Illinois. Use of the APS was supported by the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357.
MCH
distances, which, in turn, are indicative of rapid charge transport
between the π-stacked PDI molecules.
Conclusions
We have shown that extending the lifetime of photoinitiated
charge separation using two-step electron transfer in covalently
linked donor-acceptor triad building blocks results in fully
competitive charge hopping within self-assembled hexamers and
heptamers of the triads, thereby demonstrating that charge
transport between noncovalent acceptors can compete effectively
with charge recombination within the covalent building blocks.
We are currently extending this approach to prepare triad
building blocks in which self-assembly will produce ordered
Supporting Information Available: Experimental details
including synthesis, additional SAXS/WAXS data, structural
models, and transient absorption data. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA903903Q
9
J. AM. CHEM. SOC. VOL. 131, NO. 33, 2009 11929