Intramolecular Singlet Fission in Oligoacene Heterodimers

Article abstract of DOI:10.1002/anie.201510632

We investigate singlet fission (SF) in heterodimers comprising a pentacene unit covalently bonded to another acene as we systematically vary the singlet and triplet pair energies. We find that these energies control the SF process, where dimers undergo SF provided that the resulting triplet pair energy is similar or lower in energy than the singlet state. In these systems the singlet energy is determined by the lower-energy chromophore, and the rate of SF is found to be relatively independent of the driving force. However, triplet pair recombination in these heterodimers follows the energy gap law. The ability to tune the energies of these materials provides a key strategy to study and design new SF materials - an important process for third-generation photovoltaics. Organic electronics: A series of oligoacene heterodimers was synthesized to test the energetic requirements for singlet fission - a photophysical process that yields two triplet excitons from a single photon. Thus a possibility is offered to tune this process.

Full text of DOI:10.1002/anie.201510632

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International Edition: DOI: 10.1002/anie.201510632
German Edition: DOI: 10.1002/ange.201510632
Singlet Fission in Oligoacenes
Intramolecular Singlet Fission in Oligoacene Heterodimers
Samuel N. Sanders+, Elango Kumarasamy+, Andrew B. Pun, Michael L. Steigerwald,
Matthew Y. Sfeir,* and Luis M. Campos*
Abstract: We investigate singlet fission (SF) in heterodimers
comprising a pentacene unit covalently bonded to another
acene as we systematically vary the singlet and triplet pair
energies. We find that these energies control the SF process,
where dimers undergo SF provided that the resulting triplet
pair energy is similar or lower in energy than the singlet state.
In these systems the singlet energy is determined by the lower-
energy chromophore, and the rate of SF is found to be
relatively independent of the driving force. However, triplet
pair recombination in these heterodimers follows the energy
gap law. The ability to tune the energies of these materials
provides a key strategy to study and design new SF materials—
an important process for third-generation photovoltaics.
Molecular dimers made from two covalently linked SF-
capable monomers have long been considered as candidates
for iSF.[5] Early work on tetracene dimers showed low iSF
yields, presumably because of the endothermicity of the iSF
process or the connectivity employed.[6] Pentacene dimers, on
the other hand, have recently been reported to undergo iSF
quantitatively.[2d,f] The potential to develop families of oli-
goacene dimers through systematic studies has motivated us
to revisit the concept of singlet fission in oligoacene
“mixtures”, which was briefly explored in the 1970s when
several groups studied crystals of one type of acene doped
with another type of acene.[7] In this vein, we explore iSF in
asymmetric systems where two different oligoacene mono-
mers are covalently linked (Scheme 1). This design feature
allows us to systematically adjust the energetics of the iSF
process, affecting both the driving force for singlet fission and
the total energy of the resulting triplet pair. In these
heterodimers, we demonstrate that the relevant singlet
energy for iSF is given by the lower singlet-state energy
monomer, and the resulting triplet pair is a sum of the
individual monomer triplets. Therefore, the fundamental
equation for energy conservation is E(S1[X]) ꢀ E(T1[X]) +
E(T1[Y]), in a dimer comprising monomer X coupled to Y.
For example, the pentacene–tetracene heterodimer is nearly
isoergic (S1[pentacene] ꢁ 1.9 eV, T1[pentacene] ꢁ 0.8 eV,
T1[tetracene] ꢁ 1.2 eV), while our previously reported bipen-
S
inglet exciton fission has attracted renewed interest in the
last decade because of its potential to enhance power
conversion efficiencies of single junction solar cells beyond
the Shockley–Queisser limit.[1] The recent discovery of an
efficient intramolecular singlet fission (iSF) process in con-
jugated polymers and small molecules has dramatically
increased the quantity and variety of materials that exhibit
this process.[2] Moreover, the mechanism of triplet pair
formation and decay may be quite different in dimers of
oligoacenes relative to their monomer counterparts in the
solid state, where singlet fission is an intermolecular process
(xSF). For example, donor–acceptor polymers reported by
our group are presumed to undergo SF through charge
transfer (CT) states, similar to the leading hypothesis for the
mechanism for solid-state SF.[2a,3] However, there is no
intrinsic CT character in molecular dimers, yet they have
been reported to undergo SF at faster rates than the donor–
acceptor polymers. There are various important aspects that
are still being actively investigated in terms of electronic
structure, excited-state energies and dynamics.[4] Thus it is
important to elucidate the mechanistic and energetic require-
ments for SF in order to optimize the design of practical SF
chromophores.
tacene
molecule
is
exoergic
(S1[pentacene] >
2xT1[pentacene]).[2d,8] Furthermore, since pentacene–anthra-
cene is significantly endoergic (E(S1[pentacene]) < E-
(T1[pentacene]) + E(T1[anthracene])), it is not expected to
undergo iSF. Here, we test this hypothesis and demonstrate
[*] S. N. Sanders,[+] Dr. E. Kumarasamy,[+] A. B. Pun,
Dr. M. L. Steigerwald, Prof. L. M. Campos
Department of Chemistry, Columbia University
3000 Broadway, MC3124, New York, NY 10027 (USA)
E-mail: lcampos@columbia.edu
Dr. M. Y. Sfeir
Center for Functional Nanomaterials
Brookhaven National Laboratory, Building 735
Upton, NY 11973 (USA)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510632.
Scheme 1. Oligoacene heterodimers and the excited-state energies of
the respective monomers.
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that asymmetric dimers undergo fast and efficient iSF,
provided that the singlet state is not significantly lower in
energy than the resulting triplet pair. We also find that
subsequent decay of the triplet pairs formed in iSF-capable
heterodimers is primarily non-radiative, and it obeys the
energy gap law for non-radiative recombination.[9]
In order to investigate singlet fission in oligoacene
heterodimers, the molecules shown in Scheme 1 were synthe-
sized by Suzuki cross-coupling reactions (see the Supporting
Information for details).[2d] The compounds are labeled as PA,
PT, PH, where P, A, Tand H refers to pentacene, anthracene,
tetracene, and hexacene, respectively. We also compare these
results to bipentacene (BP), which we recently reported, in
which two pentacenes are similarly covalently attached at the
2-position. The inclusion of tri-isopropylsilyl acetylene
(TIPS), or in the case of hexacene, tri-isobutylsilyl acetylene
groups (TIBS), renders these heterodimers soluble and
relatively stable in solution.[10]
The steady-state absorption spectra of the heterodimers
show the characteristic features of both monomers (Fig-
ure 1A). In PA, PT, and PH, we observe the prominent low-
energy singlet transition peak associated with TIPS-penta-
cene (S1[P]) at 660 nm and, respectively, its complement, with
the anthracene peak (S1[A]) at 470 nm, the tetracene peak
(S1[T]) at 550 nm, and the hexacene peak (S1[H]) at 750 nm.
A small redshift is observed in the dimers, relative to the
monomer features. When coupling pentacenes at the 2-
position, we also observe a high-energy feature in the ground
state absorption. That feature, previously reported for 2,2’
bipentacene (BP), is also observed in these compounds.[2d] It
can be seen clearly in Figure 1A for PA, but this peak in PT
and PH has been omitted for clarity (see the Supporting
Information for full spectra). This high-energy feature is
specific to directly coupled acenes at the position shown, and
does not correspond to a peak in the parent monomers.
We use broadband transient absorption spectroscopy
(TAS) to understand the exciton dynamics in these molecules.
Since we are probing the energetic requirements for iSF, the
chromophores are pumped at the lower singlet energy
selectively (P transitions for PA and PT, and H transitions
for PH) to determine if iSF occurs without significant excess
excitation energy. Figure 1B shows the resulting 2D plot of
the spectral evolution of the transient absorption spectra as
a function of time.
In the case of PA, where iSF is expected to be significantly
endothermic, we observe no significant spectral changes of
the singlet-state features. In fact, the photophysics of this
heterodimer are similar to TIPS-pentacene, with a photo-
excited singlet that decays with a 11.5 ns time constant
primarily through a radiative pathway (see the Supporting
Information). The singlet lifetime is long enough to permit
a small amount of triplet formation through intersystem
crossing (ISC). By comparing the magnitude of the ground-
state bleach in the singlet and triplet manifold, we approx-
imate a triplet yield of about 10% and an ISC time constant of
104 ns.[8c] The triplet relaxation dynamics (see the Supporting
Information) are similar to TIPS-pentacene as well, with
a decay time of 17.4 ms. This result verifies that, in the case
where energetics are not appropriate for iSF, no additional
Figure 1. A) Steady-state absorption spectra of PA, PT and PH, along
with a TIPS–anthracene derivative, TIPS–tetracene, TIPS–pentacene,
and a TIBS–hexacene derivative. Absorption spectra are taken in
chloroform and normalized at the pentacene absorption feature.
B) Transient absorption spectra of PA excited at 600 nm, PT excited at
660 nm, and PH excited at 730 nm, at fluence of 25 mJcmÀ2 in
chloroform. In each case, warmer colors represent increased absorp-
tion after excitation, and cooler colors represent decreased absorption.
decay pathways are present in these compounds beyond the
typical monomer excited-state deactivation.
However, in PT and PH, singlet fission is roughly isoergic
and exothermic, respectively. In these systems, TAS reveals
dynamics similar to those observed in BP, where the photo-
excited singlet rapidly decays into a triplet signal in dilute
solution, consistent with iSF.[2d] The triplet pair (2xT1) feature
produced by iSF is dominated by the photoinduced absorp-
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tion of the larger acene in each case, as the triplet absorption
cross-section increases with increasing acene length.[8a,c,11]
The triplet pair features ground-state bleach (GSB)
characteristics of both monomers in magnitudes correspond-
ing to the relative absorption heights in the linear spectra, as
expected for a triplet pair where both monomers are
bleached.
The time constants for singlet fission (tiSF) and triplet pair
(2xT1) decay (t2xT1) are shown in Table 1. Since there is no
Table 1: Time constants for singlet fission (tiSF) and triplet pair
recombination (t2xT1) for the pentacene–tetracene (PT) and pentacene–
hexacene (PH) heterodimers, compared to bipentacene (BP, homodi-
mer).
Compound[a]
tISC [ps]
tT1 [ns]
PA
1.0”105
1.74”104
iSF compound
tiSF (ps)
t2xT1 [ns]
PT
BP
PH
0.83
0.76
1.2
2.4
0.45
0.21
Figure 2. Comparison of triplet transient absorption spectra obtained
by photosensitization (single T1) and singlet fission (2xT1) in PT and
PH.
[a] Compound PA: S1 lifetime=11.5 ns; triplet yield of about 10%.
indication of a parasitic process that would compete with the
singlet fission process, and the rates of SF are all orders of
magnitude faster than fluorescence or internal conversion in
PT and PH, the data is consistent with a quantitative iSF
process. In other words, the rates of singlet decay and triplet
formation are directly correlated, and the yields are deter-
mined only by the kinetic competition between iSF and the
intrinsic decay processes (ca. 10 ns).[2a,d,8a,c] This is in stark
contrast to the dynamics observed in PA.
Beyond the kinetics, the heterodimers enable us to probe
the relative contributions of P, T, and H to the GSB during
iSF. Even though we qualitatively describe the absorption
spectra of the heterodimers as combinations of the absorption
features of the individual monomers, when we pump the
longer-wavelength absorption in any of the heterodimers both
ground-state absorptions are bleached impulsively in the
singlet exciton, although the longer-wavelength absorption is
bleached more thoroughly than the shorter wavelength
absorption (further details are given in the Supporting
Information). This asymmetry in bleaching is in contrast to
quantitative bleaching of both chromophores in bipentacenes,
and it arises from the greater portion of the excited singlet
wavefunction residing on the monomer unit that is associated
with the lower-energy excited state.[2d,f] Averaging over vibra-
tional and rotational degrees of freedom in the ensemble of
molecules can thus lead to some partial bleaching (not
quantitative, but non-zero) of the higher singlet-energy
chromophore in the singlet.
on either monomer. Interestingly, we do not observe any
triplet transfer, despite the inequivalent triplet energies of the
monomers. Presumably, the triplets cannot transfer in the
heterodimers reported here because of the absence of the
significant wavefunction overlap required for Dexter energy
transfer, because of the highly localized nature of acene
triplets (further details are given in the Supporting Informa-
tion).[2d,12] Given the pump energy employed in the sensitiza-
tion experiments, individual molecules contain just one triplet
exciton. Therefore, the spectra of individual triplets would
appear significantly different from the triplet pair spectra
produced by iSF. However, the ensemble contains a roughly
even number of triplets on each monomer and can therefore
be compared to iSF, which generates triplet pairs (further
details are given in the Supporting Information).
The photoinduced absorption (PIA) spectra of the T1
state resulting from sensitization and 2xT1 resulting from
singlet fission are similar, but not identical. Modest spectral
shifts of magnitude and/or wavelength of the PIA are found,
consistent with reports of directly coupled pentacene
dimers.[2d–f] These shifts result from the strong coupling of
the triplet pair when in close proximity, as demonstrated
previously.[2d] While these spectra are similar, the dynamics
are significantly different. In general, the triplet pairs
produced from iSF tend to decay on much shorter timescales
than individual triplets. In the case of PTand PH, the lifetime
of the 2xT1 is less than 3 ns, as opposed to tens of micro-
seconds for their individual T1.[8a,c,11] The distinct triplet pair
decay is apparent since both the pentacene and tetracene
GSB signals decay at the same rate.
In order to characterize the triplet pair, we compare
singlet fission studies with sensitization experiments, in which
the triplet states are populated in the heterodimers by
collisional transfer from a triplet donor (anthracene) in
excess concentration (Figure 2). In the case of the hetero-
dimers, the anthracene can collide with and populate a triplet
While energetics have a dramatic impact on whether or
not iSF will occur, the rates of iSF for PT, BP, and PH are
surprisingly insensitive to the driving force, each being ca.
1 ps. In contrast, the recombination kinetics have a clear
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GRFP (grant number DGE 11-44155). This research used
resources of the Center for Functional Nanomaterials, which
is a U.S. DOE Office of Science Facility, at Brookhaven
National Laboratory under contract number DE-SC0012704.
We are grateful to the Nuckolls lab for use of their UV/Vis
spectrophotometer.
Keywords: conjugated materials · heterodimers ·
organic electronics · photophysics · singlet fission
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3373–3377
Angew. Chem. 2016, 128, 3434–3438
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Acknowledgements
This work was funded by the Office of Naval Research Young
Investigator Program (grant number N00014-15-1-2532), ACS
Petroleum Research Fund, 3M Non-Tenured Faculty Award,
and Cottrell Scholar Award. S.N.S. and A.B.P. thank the NSF
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Received: November 16, 2015
Published online: February 2, 2016
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