Photoinduced Electron Transfer in 2,5,8,11-Tetrakis-Donor-Substituted Perylene-3,4:9,10-bis(dicarboximides)

Article abstract of DOI:10.1021/jp511624s

A series of electron donor-acceptor compounds based on substitution of perylene-3,4:9,10-bis(dicarboximide) (PDI) with four electron donors at the 2,5,8,11-positions were synthesized and characterized using femtosecond transient absorption spectroscopy. T

Full text of DOI:10.1021/jp511624s

Article
pubs.acs.org/JPCB
Photoinduced Electron Transfer in 2,5,8,11-Tetrakis-Donor-
Substituted Perylene-3,4:9,10-bis(dicarboximides)
Leah E. Shoer, Samuel W. Eaton, Eric A. Margulies, and Michael R. Wasielewski*
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston,
Illinois 60208-3113, United States
S
* Supporting Information
ABSTRACT: A series of electron donor−acceptor compounds based on substitution of
perylene-3,4:9,10-bis(dicarboximide) (PDI) with four electron donors at the 2,5,8,11-
positions were synthesized and characterized using femtosecond transient absorption
spectroscopy. The distance between the PDI and the N,N-dimethylaniline or
phenothiazine donors was varied using one or two phenyl groups. Photoexcitation of
PDI results in rapid charge separation followed by charge recombination with time
constants ranging from tens of picoseconds to nanoseconds. The electron transfer time
constants are compared with those of the corresponding molecules in which the donor is
attached to the PDI through its imide nitrogen atom. The electron transfer reactions
through the 2,5,8,11-positions of PDI are generally much faster than those through the
imide nitrogen positions, in concert with stronger donor electronic coupling to the PDI acceptor core and in contrast to
substituents at the imide positions through which the HOMO and LUMO nodal planes pass.
molecules are π-stacked. However, as a result of the helical
nature of the assembly, the terminal donor groups are too far
apart to ensure sufficient electronic coupling for hole hopping
or delocalization to occur.
INTRODUCTION
■
Increasing worldwide energy demand has spurred research on
developing and improving renewable energy technology.
Materials based on small organic molecules have been
successfully applied in organic electronic technologies such as
organic field-effect transistors (OFETs) and organic photo-
voltaics (OPVs),1−4 but the resultant devices in the case of
OPVs have not yet attained efficiencies or stabilities that make
them commercially viable. One concern that greatly impacts the
efficiency of such small-molecule devices is intermolecular
organization and interaction within the active layer.5−7 Small
molecules that have suitable photoinitiated charge separation
behavior, the ability to self-organize into appropriate geo-
metries, and structures that mitigate unfavorable intermolecular
contact may address efficiency loss pathways such as charge
trapping.8−10
Perylene-3,4:9,10-bis(dicarboximide) (PDI) is a promising
small-molecule building block for use as an electron-accepting
chromophore in OPV active layers. PDI has a high extinction
coefficient that makes it well-suited for use in light-absorbing
applications.11,12 It also has favorable redox potentials that
make it suitable as an electron acceptor in charge separation
systems.13,14 Furthermore, because of its flat, aromatic
structure, PDI exhibits strong π-stacking behavior in single
crystals, thin films, and solution-phase aggregates.15−19
Examples of systems that take advantage of one or both of
these properties are well-known in the literature.20−23 We have
demonstrated previously that a PDI acceptor symmetrically
substituted with electron donors can undergo two-step charge
separation to generate a long-lived radical ion pair (RP).24
Dissolving this donor−acceptor system in methylcyclohexane
results in self-assembly into a helical hexamer in which the PDI
Substitution at the 1,6,7,12-positions (“bay positions”) of
PDI introduces varying degrees of PDI core distortion away
from planarity, which is frequently reflected in the structures of
its self-assembled aggregates as in the system described above.
In contrast, substitution at the 2,5,8,11-positions (“headland
positions”) does not lead to these distortions.25,26 Indeed,
structures containing phenyl substituents at the headland
positions show good slip-stacked alignment between both
adjacent PDI cores and the appended substituents.27 We have
recently shown that the slip-stacked alignment of PDI cores
significantly improves the performance of OPVs relative to
unsubstituted PDI acceptors that stack cofacially, for example,
achieving a device power conversion efficiency of 3.7% when
blended with the donor polymer PBTI3T.28 This geometrical
arrangement should be favorable for OPV active layers for two
reasons. First, it is amenable to the formation of segregated
charge transport conduits for both holes and electrons, which
should facilitate long-range charge hopping, leading to efficient
charge extraction at an electrode. Second, the acceptor moiety
is insulated from contact with other donor moieties in adjacent
molecular stacks, diminishing intermolecular charge recombi-
nation (Figure 1). Headland substituents provide an accessible
path toward such a supramolecular assembly.
Special Issue: John R. Miller and Marshall D. Newton Festschrift
Received: November 20, 2014
Revised: December 1, 2014
© XXXX American Chemical Society
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alkyl chain was used in the second imide position to ensure the
solubility of these compounds.
Spectroscopy. The steady-state absorption spectra of
compounds 1−6 (Figure 3) show the characteristic vibronic
progression of PDI with the 0−0 transition centered at 530 nm.
The 0−0, 0−1, and 0−2 vibronic bands, at 530, 495, and 460
nm, respectively, are broadened in donor-containing com-
pounds 2−4, with the biphenyl-spaced donors exhibiting less of
an effect than the phenyl-spaced donor. Features on the blue
edge of the spectra for 2−4 are attributed to donor-group
absorption. A broad charge transfer band is present in the
spectra of 2 and 3 spanning 550 to 700 nm. Both the
broadening of vibronic bands and presence of charge transfer
bands in the donor-containing compounds indicate that the
donor−PDI electronic interaction is strong at the headland
positions. The greatest effect is exhibited when the donor is
closest to the PDI core in 2. Extending the distance between
the donor and the acceptor in 3 and 4 decreases the electronic
coupling and suppresses the spectral distortions as a result.
Additionally, the initial Franck−Condon state of 4 is not in the
lowest-energy conformation; ground-state phenothiazine has a
“bowl”-shaped conformation, while the phenothiazine lowest-
excited singlet state and radical cation state are planar.33,34
Competition between nuclear reorganization and relaxation
back to the ground state diminish the appearance of charge
transfer interactions in the ground-state spectrum. Charge
transfer interactions are also limited in compounds 5 and 6
because of the presence of a node in the HOMO and LUMO of
PDI at the imide position.35
The emission spectrum of 1 shows the expected mirroring of
the absorption spectra found in PDI monomers. Unsubstituted
PDI typically has a fluorescence quantum yield approaching
unity.36,37 Thus, the low fluorescence quantum yield of 1 is
likely due to partial charge transfer,25 while the complete
fluorescence quenching in 2−6 results from complete charge
transfer, as determined using femtosecond transient absorption
(fsTA) spectroscopy to examine 1−6. The experimental
apparatus has been described previously,38,39 and the results
are summarized in Table 1. Upon photoexcitation at 525 nm, 1
exhibits ground-state bleaching (GSB) from 450 to 560 nm,
stimulated emission (SE) from 560 to 600 nm, and a broad
excited-state absorption (ESA) from 600 to 850 nm (Figure 4).
The GSB, SE, and ESA decay monoexponentially with lifetimes
of 753 5 ps in CH2Cl2 and 902 3 ps in toluene. These
lifetimes are much shorter than the ∼4 ns lifetime typically
reported for 1*PDI; the solvent dependence again indicates that
Figure 1. Schematic representation of the ideal architecture for an
OPV active layer. In this case, red = acceptor, green = primary donor,
blue = secondary donor.
Here we present the synthesis and photophysical character-
ization of a series of PDI derivatives substituted with aromatic
electron donors at the 2,5,8,11-positions (Figure 2). We varied
both the donor−acceptor distance and the electronic properties
of the donor to probe the viability of this substitution pattern
for use in photoinduced charge-separating systems. p-N,N-
dimethylaminophenyl (pDMA1), p-N,N-dimethylaminobiphen-
yl (pDMA2), and N-biphenylphenothiazine (PTZ2) donors
were chosen because of their favorable oxidation potentials and
synthetic accessibility.29,30 Furthermore, phenothiazine has
additional conjugated rings and a sulfur atom present and
thus is expected to be well-suited to stabilize an RP and extend
its charge recombination lifetime. We also present the synthesis
and characterization of pDMA1 and PTZ2 donors coupled
through the imide position of PDI for comparison with the
respective headland-substituted PDIs.
RESULTS AND DISCUSSION
■
Syntheses. The syntheses of compounds 1−4 are detailed
in Scheme 1. Headland coupling proceeded through Ru-
mediated C−H activation at the headland positions of PDI.25
Arylphenylboronic neopentyl esters are typically used to
substitute PDI in these positions because of the optimal
balance of high reactivity and low yield of side products.
However, several of the investigated donors have poor
reactivity under the conditions necessary to generate the
precursor boronic acid. The neopentyl ester substituent was
used whenever possible; however, certain donors were
synthesized using the pinacol ester derivative, which is also
reactive in this coupling reaction, although to a lesser
extent.31,32 The syntheses of the imide-linked donor−acceptor
compounds 5 and 6 are shown in Scheme 2. A longer branched
Figure 2. Structures of PDI derivatives 1−6.
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a
Scheme 1. Syntheses of PDI Derivatives 1−4
a
Reaction conditions: (i) toluene, reflux, 12 h, R = H−, Me2N−; (ii) Pd(PPh3)4, Na2CO3, 4:1 THF/H2O, 80 °C, 12 h; (iii) Pd2(dba)3, P(o-tol)3,
KOtBu, toluene, 90 °C, 12 h; (iv) Pd(dppf)Cl2, KOAc, dioxane, 110 °C, 2 h, R = Me2N−, phenothiazine−; (v) RuH2(CO)(PPh3)3, 1:1 mesitylene/
pinacolone, 140 °C, 72 h.
a
Scheme 2. Syntheses of PDI Derivatives 5 and 6
a
Reaction conditions: (i) hydrazine, Pd/C, ethanol, 90 °C, 2 days; (ii) imidazole, pyridine, reflux, 1 day, (iii) RuH2(CO)(PPh3)3, 1:1 mesitylene/
pinacolone, 140 °C, 72 h.
charge transfer interactions play a role in the faster relaxation of
the excited state.
showing that charge separation occurs on a subpicosecond time
scale.
When a strongly coupled donor is present in the system, as is
the case with 2, the GSB feature persists in the transient
spectra, but there is no SE feature, consistent with the
completely quenched fluorescence. The absorptive feature,
spanning 600−850 nm, also contains a peak at approximately
700 nm, in contrast to the broad, featureless nature of the ESA
in 1 (Figure 5). This 700 nm feature decays with lifetimes of 4.7
0.8 ps in CH2Cl2 and 26.0 1.0 ps in toluene. As this is a
much higher rate than observed for S1 decay and this system
does not exhibit SE, the peak at 700 nm is attributed to
PDI•−.40 As expected, the solvent polarity impacts the rate of
decay of the charged-separated state back to the ground state,
with more polar solvents increasing the rate.41 The PDI•−
feature appears within the instrument response function (IRF),
The results for 2 verify that charge separation is viable
through the headland positions of PDI. However, RP lifetimes
longer than 26 ps are needed to make intermolecular charge
hopping and/or delocalization within a self-assembled array of
donor−acceptor molecules competitive processes.24 One way
to extend these charge recombination lifetimes is to decrease
the degree of donor−acceptor coupling by extending the
distance between them.41 This decoupling was accomplished in
3 by the addition of a second phenyl spacer group. Once again,
PDI•− decay is rapid in CH2Cl2 (2.4 0.3 ps) and slower in
toluene (58.6
2.0 ps) (Figure 6). Furthermore, the initial
charge separation in toluene slows to 2.9 0.7 ps, well beyond
the IRF. With this slower initial charge separation, a ∼10 nm
blue shift is observed, which is attributed to the growth of
C
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Figure 3. Normalized absorption (solid) and emission (dashed) spectra of 1−6.
Table 1. Summary of fsTA Rates for Compounds 1−6
sulfur heteroatom, and large dihedral angle between the phenyl
group attached to its nitrogen atom and the phenothiazine core
all stabilize the hole generated upon charge separation in 4. As
was observed in 3, GSB is present in the spectra of 4 from 450
to 560 nm, with a small SE feature visible from 560 to 600 nm.
Once again, charge separation is sufficiently slow in toluene to
observe the growth of PDI−• at 600−850 nm (Figure 7) with a
time constant of 3.9 0.3 ps. This growth is slightly slower
than that observed for 3 but demonstrates that replacement of
pDMA donors with PTZ donors has little effect on the charge
separation rate. This spectrum decays with time constants of
6.7 0.4 ps in CH2Cl2 and 1930 240 ps in toluene; thus, the
charge recombination lifetime of 4 is 330 times longer than that
of 3, showing that the additional stabilization afforded by the
phenothiazine donor has a very large impact on this system. A
lifetime approaching 2 ns is a significant improvement toward
the regime desired for charge transport. Using a diketopyrro-
lopyrrole (DPP) electron donor covalently attached to two PDI
molecules (PDI-DPP-PDI), we have recently shown that even a
CH2Cl2
toluene
Excited-State Decay (ps)
Excited-State Decay (ps)
1
753
5
902
3
CS (ps)
<1
CR (ps)
CS (ps)
<1
CR (ps)
2
3
4
5
6
4.7 0.8
2.4 0.3
6.7 0.4
2.2 0.1
26.0 1.0
58.6 2.0
<1
2.9 0.7
3.9 0.3
<1
<1
1930 240
10.0 1.1
<1
33.0 5.8
174
2
20.5 1.9
32700 2100
PDI•−. As further evidence of the slower charge separation, SE
is observed in early spectral traces but decays at a rate very
similar to the charge separation rate. Although the charge
recombination rate in toluene decreases by more than a factor
of 2 as a result of the increased the donor−acceptor distance,
longer lifetimes are still desired.
While the oxidation potential of PTZ is similar to that of
pDMA, its extended conjugation, inclusion of a polarizable
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Figure 4. Femtosecond transient absorption spectra of 1 in CH2Cl2 (left) and toluene (right).
Figure 5. Femtosecond transient absorption spectra of 2 in CH2Cl2 (left) and toluene (right).
Figure 6. Femtosecond transient absorption spectra of 3 in CH2Cl2 (left) and toluene (right).
340 ps charge separation lifetime is sufficient to ensure a 30%
yield of free charge carriers in ordered, segregated stacks of
solid PDI-DPP-PDI.42
energy. However, the nature of this π−π interaction between
toluene and PTZ must be relatively weak because no significant
differences in the transient absorption spectra of 4 in CH2Cl2
compared to toluene are observed.
An interesting result obtained for 2−4 is the formation of
PDI triplet states in low to moderate yields (5−25%) following
charge recombination,27 which are visible in the absorptive
features at approximately 510 and 550−660 nm. As the
headland substituents are oriented at an angle of ∼60° with
respect to the PDI core,25,27 intersystem crossing is likely due
to a spin−orbit charge transfer (SOCT-ISC) mechanism. This
mechanism has been observed previously in systems where the
donor and acceptor moieties are offset by a large angle.43 In
One reason for the much larger difference in RP lifetime of
the PTZ donors in toluene versus CH2Cl2 relative to that of the
pDMA donors may be the significant internal reorganization
energy involved in phenothiazine oxidation. In its radical cation
state, phenothiazine adopts a flat conformation; while CH2Cl2
likely contributes little stabilization to either conformation, the
aromatic rings in toluene may better solvate the flat
conformation and thus increase the reorganization energy for
the charge back-transfer reaction, thus adding another solvent
reorganization energy contribution to the overall reorganization
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Figure 7. Femtosecond transient absorption spectra of 4 in CH2Cl2 (left) and toluene (right).
Figure 8. Femtosecond transient absorption spectra of 5 in CH2Cl2 (left) and toluene (right).
view of the short RP lifetimes, it is unlikely that triplet
formation occurs by the radical-pair intersystem crossing
mechanism.44−46 Triplet state formation is generally perceived
as being detrimental to device efficiency;47,48 however, the
triplet states in these systems are generated following charge
recombination and thus should not be formed competitively in
devices, where charge separation is followed by rapid charge
extraction.
demonstrates a significant difference in electronic coupling at
these positions. While we expect the rates in the imide-
substituted compounds to be uniformly lower than the rates in
the headland-substituted compounds because of the presence
of a node in the HOMO and LUMO at the imide position,49
the rate of recombination actually decreases between 2 and 5.
Although the same donor group was used, the connectivity
through the imide position significantly impacts the oxidation
potential of the pDMA1 donor, shifting it to +1.02 V (Figure
S8 in the Supporting Information). Unfortunately, the close
proximity of the donor group to the imide group makes 2 an
unsuitable model compound, and thus, we cannot use the
charge separation or recombination rates to gain reliable
information about differences in electronic coupling. However,
the addition of a second phenyl spacer group provides greater
electronic isolation of the donor from the substituent effect of
the imide, and thus, the charge separation and recombination
rates for 4 and 6 follow the expected trend. As the electron
transfer rate kET is proportional to V2, where V is the electronic
coupling matrix element for the reaction,50 we can obtain a
quantitative measure of the change in electronic coupling by
comparing the lifetimes of the processes. In doing so, we
determine that the electronic coupling through the headland
position is approximately 3.5 times greater than that through
the imide position. In cases where competitive rates are a
potential issue, as in a multistep charge transfer pathway, the
ability to choose between these two electronic coupling regimes
can be used to favor the desired process by either increasing or
decreasing the charge separation and recombination rates.
The donor−acceptor systems 5 and 6 were designed to allow
for direct comparison of the electron transfer rates through the
headland position with those through the more commonly used
imide position. Once again these systems, upon photoexcitation
at 525 nm, exhibit GSB from 440 to 560 nm and SE from 560
to 600 nm (Figures 8 and 9, respectively). This SE signal
decays rapidly, matching the charge separation dynamics.
Charge separation is further verified in toluene by the
observation of a red shift and sharpening in the maxima of
the absorptive feature from 690 to 705 nm. This charge
separation occurs in CH2Cl2 with time constants of <1 ps and
33.0 5.8 ps and in toluene with time constants of 2.9 0.7 ps
and 20.5 1.9 ps for 5 and 6, respectively. The relatively large
negative free energy for charge separation for 6 may push this
reaction into the Marcus inverted regime, as evidenced by the
slight decrease in rate in more polar solvents. Recombination to
the ground state in these systems again occurs more quickly in
CH2Cl2, with lifetimes of 2.2 0.1 ps for 5 and 174 2 ps for
6. Recombination in toluene occurs in 10.0 1.1 ps and 32.7
2.1 ns for 5 and 6, respectively.
The clear difference in RP lifetimes for compounds that have
the same donor group in different substitution patterns
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Figure 9. Femtosecond transient absorption spectra of 6 in CH2Cl2 (top left) and toluene (top right) and nanosecond transient absorption spectra
of 6 in toluene (bottom left).
CONCLUSION
ACKNOWLEDGMENTS
■
■
We have presented the synthesis and solution-phase character-
ization of donor−acceptor systems taking advantage of
substitution at the headland positions of PDI. Although the
twisting of the intervening phenyl groups between the donor
and acceptor may be expected to weaken the electronic
coupling,25 the presence of charge transfer bands in the steady-
state absorption spectra, the completely quenched fluorescence,
and the observation of rapid charge separation in the systems
presented here instead imply a much higher degree of
electronic interaction. Even the extension of the donor−
acceptor distance by addition of another phenyl group
decreases this interaction only slightly, slowing the recombina-
tion of PDI•− by a factor of 2. However, providing more
stabilization of the radical cation can bring the lifetime of the
charge-separated pair into the regime where it begins to be
competitive with intermolecular charge transport.
This work was supported by the Chemical Sciences, Geo-
sciences, and Biosciences Division, Office of Basic Energy
Sciences, U.S. Department of Energy, under Grant DE-FG02-
99ER14999.
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ASSOCIATED CONTENT
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S
* Supporting Information
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This material is available free of charge via the Internet at
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: m-wasielewski@northwestern.edu.
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