Photodriven charge separation and transport in self-assembled zinc tetrabenzotetraphenylporphyrin and perylenediimide charge conduits

Article abstract of DOI:10.1002/anie.201309335

Zinc tetrabenzotetraphenyl porphyrin (ZnTBTPP) covalently attached to four perylenediimide (PDI) acceptors self-assembles into a π-stacked, segregated columnar structure, as indicated by small- and wide-angle X-ray scattering. Photoexcitation of ZnTBTPP rapidly produces a long-lived electron-hole pair having a 26 A average separation distance, which is much longer than if the pair is confined within the covalent monomer. This implies that the charges are mobile within their respective segregated ZnTBTPP and PDI charge conduits. Zinc tetrabenzotetraphenylporphyrin (ZnTBTPP) covalently attached to four perylenediimide (PDI) acceptors self-assembles into a π-stacked, segregated columnar structure. Photoexcitation of ZnTBTPP rapidly produces a long-lived electron-hole pair with a 26 A average separation distance, which is much longer than if the pair is confined within the covalent monomer. The charges are thus mobile within their respective segregated ZnTBTPP and PDI charge conduits.

Full text of DOI:10.1002/anie.201309335

Angewandte
Chemie
DOI: 10.1002/anie.201309335
Self-Assembled Charge Conduits
Photodriven Charge Separation and Transport in Self-Assembled Zinc
Tetrabenzotetraphenylporphyrin and Perylenediimide Charge
Conduits**
Vladimir V. Roznyatovskiy, Raanan Carmieli, Scott M. Dyar, Kristen E. Brown, and
Michael R. Wasielewski*
Abstract: Zinc tetrabenzotetraphenyl porphyrin (ZnTBTPP)
covalently attached to four perylenediimide (PDI) acceptors
self-assembles into a p-stacked, segregated columnar structure,
as indicated by small- and wide-angle X-ray scattering. Photo-
excitation of ZnTBTPP rapidly produces a long-lived elec-
tron–hole pair having a 26 ꢀ average separation distance,
which is much longer than if the pair is confined within the
covalent monomer. This implies that the charges are mobile
within their respective segregated ZnTBTPP and PDI charge
conduits.
have attracted great interest as visible chromophores for
energy and charge transport studies,[1c,3] especially with regard
to potential applications as visible light-absorbing electron
acceptors in organic photovoltaics.[4] Not only are PDIs
thermally and photochemically stable,[5] they also exhibit
a strong propensity to self-organize into ordered assemblies,
both in solution and in the solid state, by p–p stacking
interactions, which are often aided by hydrogen bonding and
nano-/micro-segregation.[1c,3] For example, we have examined
a symmetrically substituted PDI acceptor (A), in which two
donor groups, aminopyrene (D1) and p-diaminobenzene (D2)
were coupled to the PDI through its imide positions,
producing a covalent D2–D1–A–D1–D2 system.[6] This mole-
cule self-assembles into a helical hexameric structure in
methylcyclohexane solution, which upon photoexcitation
undergoes two-step, sequential electron transfer to form
T
he photoactive molecules used in artificial photosynthetic
systems for solar fuel production and in organic photovoltaics
(OPVs) for solar electricity generation require significant
molecular order to achieve high performance. The design and
synthesis of complex covalent molecular systems comprising
chromophores, electron donors, and electron acceptors, which
mimic both the light-harvesting and the charge separation
functions of photosynthetic proteins, have been demon-
strated.[1] However, the development of analogous self-order-
ing and self-assembling components is still in its early stages.[2]
We are currently developing covalently bound donor–
acceptor building blocks that self-assemble into p-stacked
segregated hole and electron charge conduits. These systems
are designed to undergo rapid, efficient photoinduced charge
separation leading to long-lived radical ion pairs (RPs,
electron-hole pairs) within the covalent building block. If
RP charge recombination is slow relative to hole and electron
hopping between the respective non-covalent p-stacked
segregated donors and acceptors in the assembly, then
efficient long distance charge transport to catalysts or
electrodes can be achieved.
+
a long-lived ion pair state, D2C –D1-ACꢀ–D1–D2, in which the
electron migrates rapidly through the p-stacked PDIs.[6] The
helical aggregate structure allows for p-stacking of the core
PDI acceptors, but prevents the donors from p-stacking, so
+
that the D2C cation is trapped on a single molecule.
Zinc porphyrins have been shown to p-stack, especially
when they bear substituents designed to induce discotic
columnar phases, resulting in significantly enhanced hole
mobilities.[7] We have previously described assemblies having
a zinc meso-tetraphenyl-porphyrin (ZnTPP) donor core
surrounded by four PDI acceptors.[8] Unfortunately, these
+
assemblies do not have sufficiently long ZnTPPC –PDICꢀ
lifetimes to allow a definitive determination of whether
charge hopping between p-stacked layers is competitive with
charge recombination. Compared to ZnTPP, benzannulated
porphyrins, such as zinc tetrabenzotetraphenylporphyrin
(ZnTBTPP), have a greater tendency to aggregate, lower
oxidation potentials,[9] and red-shifted absorption spectra that
provide extended solar spectral coverage.[10] Recently, ben-
zannulated porphyrins have been exploited in high perfor-
mance OPVs.[11] Herein we present molecule 1, which has
a central ZnTBTPP electron donor to which four PDI
electron acceptors are linked by xylyl–phenyl spacers. The
synthesis of 1 is detailed in the Supporting Information.
Briefly, condensation of PDI-derived aldehyde 2 and dihy-
In these studies, we have made extensive use of perylene-
3,4:9,10-bis(dicarboximide) (PDI) and its derivatives, which
[*] Dr. V. V. Roznyatovskiy, Dr. R. Carmieli, S. M. Dyar, K. E. Brown,
Prof. M. R. Wasielewski
Department of Chemistry and Argonne Northwestern Solar Energy
Research Center (ANSER), Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: m-wasielewski@northwestern.edu
[**] This work is supported by the Chemical Sciences, Geosciences, and
Biosciences Division, Office of Basic Energy Sciences, DOE under
grant no. DE-FG02-99ER14999.
droisoindole
3 to form the octahydro precursor of
1 (Scheme 1) was followed by DDQ oxidation and metalation
to furnish the ZnTBTPP core of 1.[10] Aldehyde 2 was
prepared by Suzuki coupling of 4-formylphenylboronic acid
with bromophenyl PDI 4, which in turn was obtained from
condensation of 5 and 4-bromo-2,5-dimethylaniline in molten
Supporting information for this article, including detailed informa-
tion about the sample preparation, electrochemistry, transient
absorption spectroscopy, and TREPR spectroscopy, is available on
the WWW under http://dx.doi.org/10.1002/anie.201309335.
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Scheme 1. Synthesis and structure of 1.
imidazole. Dihydroisoindole 3 was prepared in several steps
starting from 1,4-cyclohexadiene using the Barton–Zart
method.[12]
phores.[13] The oscillator strengths of the ZnTBTPP Soret and
Qy bands in the spectrum of 1 also increase upon dilution,
additionally suggesting a strong tendency of 1 to aggregate at
concentrations greater than about 10ꢀ6 m. While the spectra
indicate that p-stacking is occurring, they do not provide
more detailed structural information.
Additional evidence of aggregation comes from variable-
temperature 1H NMR spectroscopy, which reveals strong
intermolecular association of 1 in [D4]-1,2-dichlorobenzene
noticeable even at elevated temperatures, although some of
the resonances are better resolved at 1308C (Figures S1,S2).
The presence of large aggregates of 1 is also evident in its
MALDI-TOF mass spectra, indicating the formation of
higher-mass oligomers (Figure S3). The common strategy of
using pyridine as an axial ligand on Zn porphyrins to inhibit
aggregation has no effect on 1 in toluene, as indicated by its
NMR or UV/Vis steady-state spectra.
Steady-state UV/Vis absorption spectra of 1 in toluene
display strong PDI (0,0) and (0,1) vibronic absorption bands
at 530 and 492 nm, respectively, and the ZnTBTPP Soret and
Qy bands at 460 and 650 nm, respectively (Figure 1). The
wavelengths of these bands closely match those of the
individual dyes (Supporting Information, Figure S5 and
Ref. [13b]), indicating that the donor and acceptor are
electronically distinct; however, the intensities of all the
bands are concentration-dependent, as seen in the normalized
spectra (Figure S6). The inversion in the relative intensities of
the 0–0 and 0–1 transitions of PDI from lower to higher
concentrations of 1 is typical for H aggregates that exhibit
significant exciton coupling between the p-stacked chromo-
SAXS/WAXS measurements serve as a powerful method
for elucidating the solution-phase structures of non-covalent
aggregates.[14] Guinier analysis of the scattering data provides,
at a minimum, the radius of gyration of the complex, Rg, an
estimate of its molecular weight (provided appropriate stand-
ards are available), and a gauge of the polydispersity of the
aggregates.[15] In cases where the assemblies are nearly
monodisperse, further analysis of the SAXS/WAXS data
using atomic pair distance distribution functions (PDFs) and/
or simulated annealing procedures can be performed to
obtain the structure in solution with a resolution approaching
3–4 ꢀ for molecular weights up to about 50 kD.[15b] For
example, using these techniques we have previously obtained
structures of complex arylene diimide assemblies having
molecular weights up to about 28 kD.[6,14] The ability to carry
out structural studies on such assemblies in solution at
concentrations typical of time-resolved optical and EPR
Figure 1. Absorption spectrum of 1 recorded in toluene at different
concentrations at 295 K.
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Figure 3. MM+ optimized structure of 112 that yields the best fit to
the experimental SAXS/WAXS data. Two orthogonal views of the
model are shown. Alkyl chains are substituted by methyl groups for
clarity.
different p–p intermolecular distances required for optimal
PDI-PDI and ZnTBTPP-ZnTBTPP association within the
segregated stacks.[17] The SAXS/WAXS experiments show
that 112 dominates at the 10ꢀ4–10ꢀ5 m concentrations typical of
the photophysical experiments presented herein, so that their
observed behavior can be directly related to assembly
structure.
Selective photoexcitation of ZnTBTPP in 112 (2.3 ꢁ 10ꢀ5 m)
in toluene at 295 K with a 650 nm, 60 fs laser pulse results in
the transient absorption spectra shown in Figure 4a. The
spectra show instrument-limited bleaches of the ZnTBTPP
Soret and Qy bands followed by the rapid bleach of the PDI
ground state absorption and the concomitant appearance of
Figure 2. a) Scattering data in toluene (10ꢀ4 m). Inset: low-q region
with Guinier fit of 1. b) Pair distribution functions (PDFs) comparing
experimental data of 1 in toluene with theoretical data generated from
model structures.
spectroscopic measurements provides an important connec-
tion relating structure to function.
Figure 2a shows a logarithmic plot of scattering intensity
versus q, and the inset in Figure 2a demonstrates a linear
Guinier relationship of logarithmic plot of scattering intensity
versus q2 for the 0.040 < q < 0.050 ꢀꢀ1 range as expressed by
I(q) = Io exp(ꢀq2 Rg /3), where Io is the forward scattering
2
amplitude, Rg is the radius of gyration, and q = (4p/l) sinq (l
is the X-ray scattering wavelength, and 2q is the scattering
angle).[15a] The observed linear Guinier relationship is indica-
tive of the assembly having a fairly narrow size dispersity and
yields a mean radius of gyration of Rg = 23.5 ꢁ 0.1 ꢀ.[15]
Energy-minimized structural models of 1 were generated
using MM + force field calculations.[14] PDFs were calculated
for the experimental data and for the structural models; the
latter were then varied until a best fit with the experimental
PDF was found. Figure 2b compares the experimental PDF
data with those of model structures that showed the closest fit.
The experimental PDF differs markedly from those of smaller
model aggregates and specifically from monomeric 1. While
the experimental PDF is most closely modeled by a cofacial
dodecamer (112), it is important to note that the assembly/
disassembly of this supramolecular structure is a dynamic
process.[16] The 112 structure is most likely in equilibrium with
a narrow distribution of similarly sized structures. The
geometry-optimized p-stacked structure of 112 has a slight
intermolecular twist that results in overall helicity of the
aggregate, yet this helicity does not disrupt the individual
ZnTBTPP and PDI p stacks (Figure 3). Not surprisingly, such
behavior is common[8b] and can be rationalized in terms of the
Figure 4. Transient absorption spectra of 112 (2.3ꢀ10ꢀ5 m) in toluene
at 295 K. a) Following a 650 nm 60 fs laser pulse. Inset: transient
kinetics monitored at 710 nm. b) Following a 650 nm 7 ns laser pulse.
Inset: transient kinetics monitored at 710 nm.
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the 710 nm PDICꢀ absorption band[18] with tCS = 73 ꢁ 5 ps.
Given that the lifetime of 1*ZnTBTPP in toluene is tS = 291 ꢁ
7 ps (Figure S7), charge separation in 112 occurs with an 80%
yield. The free energy of the charge separation reaction is
DGCS = ꢀ0.37 eV as determined from the 1.89 eV lowest
excited singlet state energy of ZnTBTPP and the 1.52 eV
+
energy of the ZnTBTPPC –PDICꢀ RP (see the Supporting
Information). A similar charge separation time constant is
obtained from fitting the transient data following excitation of
112 at 525 nm in toluene solution at 295 K. In this case,
selective excitation of PDI is followed by a rapid singlet
1
energy transfer from *PDI to ZnTBTPP in t = 3.9 ꢁ 0.1 ps
followed by charge separation in tCS = 80 ꢁ 5 ps, as monitored
by formation of the PDICꢀ absorption band (Figure S8). The
corresponding nanosecond transient absorption spectra and
+
kinetics (Figure 4b) show that the lifetime of ZnTBTPPC –
PDICꢀ is tCR = 169 ꢁ 0.5 ns. Our previous experience has
shown that it is very difficult to produce a Zn porphyrin
with four covalently-attached PDIs that does not aggregate
even when disaggregating groups are purposely appended to
the PDIs.[8a] However, nanosecond transient absorption
measurements on 1 at 100 times lower concentration (2.2 ꢁ
10ꢀ7 m), where the equilibrium is strongly shifted toward the
monomer (Figure 1), show that the lifetime of ZnTBTPPC+–
PDICꢀ lifetime drops to tCR = 63 ꢁ 9 ns (Figure S9). The tCR
value for predominantly monomeric 1 is substantially shorter
Figure 5. a) Zeeman splitting of RP energy levels. b) Initial spin
population following S–T0 mixing. c) TREPR spectrum of 112 recorded
in toluene at 295 K and 100 ns following a 7 ns 532 nm laser pulse.
The superimposed solid trace is a simulation of the spectrum. d) MFE
data of 2.2ꢀ10ꢀ7 m 1 in toluene showing the B(2J) resonance and
a Lorenztian line fit to the data.
ꢁ 1 transitions between these mixed states and the pure j T+1i
and j Tꢀ1i states result in a spin-polarized EPR spectrum with
four equal intensity lines having a symmetric (e,a,e,a) anti-
phase pattern (where e denotes emission and a denotes
enhanced absorption, low to high field), if J > 0 and J >j d j . If
the g factors of the two spins are similar and/or are split by
hyperfine couplings, the two doublets will overlap strongly
and will often appear as a somewhat distorted (e,a) signal.[19,22]
Photoexcitation of 112 in toluene solution with a 532 nm
7 ns laser pulse at 295 K gives a spin-polarized RP signal
(Figure 5c). Simulation of this signal using the spin-correlated
radical pair model[19,22] gives J = 0.65 ꢁ 0.01 mT and d =
ꢀ0.17 mT. The average RP distance obtained from d is
26 ꢀ. The MM + computed molecular structure of 1 yields
a center-to-center ZnTBTPP-PDI distance of 17 ꢀ, which is
much shorter than the RP distance obtained from the TREPR
data on 112. This distance increase shows that charge transport
occurs through the p-stacked ZnTBTPP-PDI4 moieties in 112
with the 26 ꢀ RP distance, placing ZnTBTPPC+ and PDICꢀ on
average about six monomers apart within the stack.
When the concentration of 1 is lowered 100-fold to 2.2 ꢁ
10ꢀ7 m, no TREPR signal is observed, even after extensive
signal averaging. For molecules similar in size to monomeric
1, we have previously observed that molecular tumbling is
sufficiently rapid that the RP dipolar interaction is averaged
to zero.[6] However, as J depends exponentially on distance
and the spin-polarized EPR transition intensities depend on
1/J2,[19,22] confining the RP within the monomer (where the
RP distance is only 17 ꢀ) should result in a much larger J
value and thus no observable TREPR signal. Owing to the
spin-selective nature of RP charge recombination, large J
values can often be measured directly using the magnetic field
dependence of the resulting neutral triplet yield.[21] This
dependence exhibits a resonance as the triplet yield increases
at the crossing point of the j Si and j T0i energy levels
+
than tCR for 112, which strongly suggests that the ZnTBTPPC –
PDICꢀ distance is longer in the aggregate.
TREPR spectroscopy was used to determine the
ZnTBTPPC+–PDICꢀ distance within 112 and to probe the
competition between charge recombination and charge trans-
port. Following rapid charge separation, the initially formed
+
spin-correlated singlet RP, 1(ZnTBTPPC –PDICꢀ) may
undergo radical-pair intersystem crossing (RP-ISC)[19] to
+
produce the triplet RP, 3(ZnTBTPPC –PDICꢀ). Application
of a magnetic field results in Zeeman splitting of the
+
3(ZnTBTPPC –PDICꢀ) triplet sublevels, which are best de-
scribed by the j T+1i, j T0i, and j Tꢀ1i eigenstates that are
quantized along the applied magnetic field direction, while
+
1(ZnTBTPPC –PDICꢀ) is described by the j Si eigenstate
(Figure 5a).[19,22] The energy difference between j Si and
j T0i is the spin-spin exchange interaction 2J, which is assumed
to be isotropic. The distance dependence of J = J0 eꢀb(rꢀr )
,
0
where J0 is defined at the van der Waals contact distance r0
and r is the RP distance. In addition, for large molecules in
solution, such as 112, and for molecules in the solid state, the
anisotropic spin–spin magnetic dipolar coupling (d) is not
rotationally averaged to zero, where d = ꢀ2786 mTꢀrꢀ3 in
the point-dipole approximation.[20] The subsequent RP charge
+
recombination process is spin selective; that is, 1(ZnTBTPPC –
PDICꢀ) recombines to the singlet ground state, while
+
3(ZnTBTPPC –PDICꢀ) recombines to the neutral triplet state
3(ZnTBTPP–PDI).[21]
When the RP distances are greater than about 15 ꢀ, both
J and d are very small relative to the magnetic fields typical of
EPR spectroscopy (ca. 350 mT), so that the field invariant j Si
and j T0i states mix to produce coherent superposition states
j S’i and j T’i (Figure 5b), which both have triplet character
and are exclusively populated.[19,22] Microwave-induced Dm =
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[6] J. E. Bullock, R. Carmieli, S. M. Mickley, J. Vura-Weis, M. R.
(Figure 5a). We used nanosecond transient absorption spec-
troscopy to measure J directly for 2.2 ꢁ 10ꢀ7 m 1 by observing
a resonance at 2J = 153 mT (Figure 5d), so that J is 118 times
larger than that of the RP in 112. Using the 17 ꢀ and 26 ꢀ RP
distances in 1 and 112, respectively, the corresponding
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Received: October 25, 2013
Revised: December 10, 2013
Published online: February 19, 2014
Keywords: charge separation · charge transport · donor–
.
acceptor systems · organic photovoltaics · radical pairs
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