Electron hopping in π-stacked covalent and self-assembled perylene diimides observed by ENDOR spectroscopy

Article abstract of DOI:10.1021/ja057031k

We have carried out room-temperature, solution-phase electron paramagnetic resonance and electron-nuclear double resonance studies on a series of radical anions based upon perylene-3,4:9,10-bis(dicarboximide) (PDI). The following systems were studied: two

Full text of DOI:10.1021/ja057031k

Published on Web 01/25/2006
Electron Hopping in π-Stacked Covalent and Self-Assembled Perylene
Diimides Observed by ENDOR Spectroscopy
Michael J. Tauber, Richard F. Kelley, Jovan M. Giaimo, Boris Rybtchinski, and
Michael R. Wasielewski*
Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly, Northwestern UniVersity,
EVanston, Illinois 60208-3113
Received October 16, 2005; E-mail: m-wasielewski@northwestern.edu
The delocalization or rapid hopping of charge among neighboring
molecules is a requirement for organic electronics including
photovoltaics.1,2 Pioneering studies of charge delocalization in both
photosynthetic proteins and aromatic molecules in solution have
used electron paramagnetic resonance (EPR) and electron-nuclear
double resonance (ENDOR) spectroscopy to reveal charge sharing
in dimers, indicated by a halving of the electron-nuclear hyperfine
coupling constants (hfcc).3-9 Dimers formed from π-π interactions
between radical cations and neutral aromatic molecules in solution
Figure 1. UV-Vis spectra in CH2Cl2-3% TEA. (A) 1 and 1•-. These
are well-known;6,10-12 however, the corresponding dimers of radical
anions with neutral aromatics have been reported only once in
solution,13 even though examples in cyclophanes are known.14
Recently, we and others have shown that photoinduced charge
separation occurs readily in both covalent and self-assembled
systems based on robust perylene-3,4:9,10-bis(dicarboximide) (PDI)
molecules.15-17 To address the question of whether unpaired
electrons are delocalized or hopping among the PDI derivatives in
self-assembled materials, we have carried out room-temperature,
solution-phase EPR and ENDOR studies of the radical anions of a
PDI monomer (1), a covalent, cofacial PDI dimer (3), and a covalent
trefoil-PDI3 molecule (4) that self-assembles into π-stacked dimers
(4)2.
spectra correspond closely to those of 2 and 2•-, with peak shifts e 2 nm.
(B) 4 and 4•-. These spectra correspond closely to those of 3 and 3•-, except
maxima of 3 are red-shifted 3-4 nm relative to those of 4.
PDI.19 Furthermore, small-angle X-ray scattering (SAXS) data
strongly support a dimeric, cofacial structure for (4)2 and exclude
formation of larger aggregates even in toluene, a solvent that more
strongly favors aggregation than CH2Cl2 (Figures S5-S9).
A polar solvent, CH2Cl2, was selected for our ENDOR studies
to diminish the effects of counterions that are known to influence
charge sharing among aromatic systems in less polar solvents such
as ethers.7 Solutions of 1-4 in dry CH2Cl2 with 3% triethylamine
(TEA) (w/w) were prepared, loaded into quartz tubes, and sealed
in an N2-filled glovebox. Radical anions were generated by
photoexcitation of PDI with an Ar+ laser at 457-488 nm (7-40
mW). Electron transfer from TEA to 1*PDI, followed by proton
loss from TEA•+, leads to formation of a stable cation TEA-H+.20
Upon photoreduction of 1 or 2, near-infrared (NIR) bands due to
PDI•- appear at ∼730 and ∼898 nm (Figure 1).21 Similarly, as a
solution of 3 or (4)2 is reduced, anion bands at ∼735 and ∼910
nm appear, and the exciton-enhanced band diminishes. The reduc-
tions of 3 and (4)2 were halted before 50 and 15% completion,
respectively, to ensure maximal production of the singly reduced
•-
species 3•- and (4)2
.
EPR and ENDOR spectra were acquired with a Bruker E-580
spectrometer, fitted with an EN801 resonator, and RF power
amplifier (ENI A-500). Temperatures were maintained at or near
•-
290 K. The EPR spectra of 1•- to (4)2 are either line-broadened
or featureless spectra with g ) 2.0028-20029 (Figure S10). The
proton ENDOR spectrum22 of 1•- in CH2Cl2 exhibits four line pairs,
with isotropic hyperfine splittings of 4.81, 2.20, 1.61, and 0.36 MHz
(Figure 2a). DFT calculations are used to assign the three largest
hfcc’s to the perylene protons at the (6,12), (5,11), and (2,8)
positions, respectively. The smallest hfcc is assigned to the protons
of 2-ethylhexyl that are closest to the imide nitrogens. Simulation
of the EPR spectrum indicates that the nitrogen hfcc of 1•- is 1.74
MHz, in agreement with the DFT calculation.23 The hfcc’s obtained
here for 1•- are consistent with more highly resolved EPR/ENDOR
spectra acquired in our lab and elsewhere on PDI with an
unsubstituted perylene core.24-26 The ENDOR spectrum of 2•- in
CH2Cl2 shows that each of the perylene proton lines splits into two
The synthesis and characterization of 1 and 4 are reported in
the Supporting Information, while that of 2 and 3 were reported
earlier.18 Monomers 1 and 2 absorb strongly across the visible
spectrum with peaks at or near 548 nm (Figure 1), whereas the
peak maxima of 3 and 4 are significantly blue-shifted to ∼513 nm.
These absorption changes are characteristic of exciton coupling in
an H-aggregate geometry and reveal that 4 forms π-stacked
aggregates in CH2Cl2.17,18 Gel permeation chromatography (GPC)
on 4 in CH2Cl2 (Figure S1) and NMR in CD2Cl2 (Figure S4)
indicate the formation of dimers at the 10-3 to 10-4 M concentra-
tions of the ENDOR experiments, which is consistent with the
previous observation of dimers formed by a closely related trefoil-
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10.1021/ja057031k CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
mophoric pairs in the self-assembled dimer provides a useful
comparison to electron transport within more extended organic
molecular materials, including other systems that self-assemble in
solution.28 Our results contrast with the full delocalization (bandlike
charge transport) found recently in highly crystalline molecular
materials.1,29 The alternative localized hopping mechanisms that
describe charge transport within some PDI-based materials are likely
more closely related to the results found here.1,30
Acknowledgment. This work was supported by the Division
of Chemical Sciences, Office of Basic Energy Sciences, U.S.
Department of Energy under Grant No. DE-FG02-99ER14999. We
thank Dr. David Tiede at the Argonne National Laboratory for
assistance with SAXS experiments. M.J.T. acknowledges the donors
of the American Chemical Society Petroleum Research Fund for
partial support of this research. The Bruker E-580 spectrometer
was purchased with partial support from NSF Grant No. CHE-
0131048.
Supporting Information Available: Synthesis and characterization
of 1, 4, and 5. Spectroscopic, SAXS, and GPC data as well as DFT
calculations. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. Proton ENDOR spectra in CH2Cl2-3% TEA solution of: (a)
•-
1•-, (b) 2•-, (c) 3•- at 290 K, and (d) self-assembled (4)2 at 297 K.
Microwave powers were 4-20 mW, RF power 300-800 W, with frequency
modulation depth 50 or 100 kHz.
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•-
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The ENDOR data clearly show that rapid electron hopping occurs
in both covalent and self-assembled π-stacked PDI molecules. The
observation of a >107 Hz electron hopping rate between chro-
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