Tuning Singlet Fission in π-Bridge-π Chromophores

Article abstract of DOI:10.1021/jacs.7b05204

We have designed a series of pentacene dimers separated by homoconjugated or nonconjugated bridges that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling between the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in these molecules, we find that homoconjugated dimers display desirable excited-state dynamics, with significantly reduced recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores, since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the nonconjugated dimer, where the iSF occurs with a time constant >10 ns, comparable to the fluorescence lifetime, we used electron spin resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet excitons through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.

Full text of DOI:10.1021/jacs.7b05204

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Article
Tuning Singlet Fission in Pi-Bridge-Pi Chromophores
Elango Kumarasamy, Samuel N Sanders, Murad J. Y. Tayebjee, Amir Asadpoordarvish,
Timothy J. H. Hele, Eric G Fuemmeler, Andrew B. Pun, Lauren M. Yablon, Jonathan
Z Low, Daniel W. Paley, Jacob C Dean, Bonnie Choi, Gregory D. Scholes, Michael L.
Steigerwald, Nandini Ananth, Dane R. McCamey, Matthew Y. Sfeir, and Luis M. Campos
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05204 • Publication Date (Web): 11 Aug 2017
Downloaded from http://pubs.acs.org on August 11, 2017
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Tuning Singlet Fission in Pi-Bridge-Pi Chromophores
Elango Kumarasamy,1, ‡ Samuel N. Sanders,1, ‡ Murad J. Y. Tayebjee,2,3 Amir Asadpoordarvish,4
Timothy J. H. Hele,5,6 Eric G. Fuemmeler,5 Andrew B. Pun,1 Lauren M. Yablon,1 Jonathan Z. Low,1
Daniel W. Paley,1,7 Jacob C. Dean,8 Bonnie Choi,1 Gregory D. Scholes,8 Michael L. Steigerwald,1
Nandini Ananth,5 Dane R. McCamey,*4 Matthew Y. Sfeir,*9 and Luis M. Campos*1
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1Department of Chemistry, Columbia University, New York, New York 10027, USA
2School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney NSW 2052,
Australia
3Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
4ARC Centre of Excellence in Exciton Science, School of Physics, University of New South Wales, Sydney, NSW
2052, Australia
5Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY, 14850, USA
6On intermission from Jesus College, Cambridge University, UK
7Columbia Nano Initiative, Columbia University, New York, NY, 10027, USA
8Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
9Center for Functional Nanomaterials, Brookhaven National, Laboratory, Upton, New York 11973, USA
KEYWORDS: Singlet Fission; Homoconjugated Dimers; Non-Conjugated, Organic Electronics, Pentacene Dimers,
Electron Spin Resonance.
ABSTRACT: We have designed a series of pentacene dimers separated by homoconjugated or non-conjugated bridges
that exhibit fast and efficient intramolecular singlet exciton fission (iSF). These materials are distinctive among reported
iSF compounds because they exist in the unexplored regime of close spatial proximity but weak electronic coupling be-
tween the singlet exciton and triplet pair states. Using transient absorption spectroscopy to investigate photophysics in
these molecules, we find that homoconjugated dimers display desirable excited state dynamics, with significantly reduced
recombination rates as compared to conjugated dimers with similar singlet fission rates. In addition, unlike conjugated
dimers, the time constants for singlet fission are relatively insensitive to the interplanar angle between chromophores,
since rotation about σ bonds negligibly affects the orbital overlap within the π-bonding network. In the non-conjugated
dimer, where the iSF occurs with a time constant > 10 ns, comparable to the fluorescence lifetime, we used electron spin
resonance spectroscopy to unequivocally establish the formation of triplet-triplet multiexcitons and uncoupled triplet
exciton through singlet fission. Together, these studies enable us to articulate the role of the conjugation motif in iSF.
Introduction: Understanding the fundamental dy-
namics of singlet fission (SF) chromophores with ideal-
ized properties for next-generation optoelectronic de-
vices fuels the development of families of new materi-
als.1-6 This includes the important discovery of intramo-
lecular singlet fission polymers and oligomers, where
chromophore-chromophore interactions occur primarily
through covalent bonds.7-23 Several different bonding
connectivity schemes have been demonstrated to acti-
vate singlet fission, where the triplet generation and
decay kinetics have been shown to be highly sensitive to
the manner in which neighboring chromophores are
linked. The interaction of chromophores in iSF com-
pounds differs significantly from the through-space in-
teractions primarily found in molecular crystals, where
the through-space coupling between the chromophore
coupling is greatly influenced by the morphology.24-26,28-
30 For instance, multiexponential singlet fission rates are
observed in disordered systems when compared to mo-
noexponentional kinetics in iSF systems.31,32
Given the sensitivity of SF to structure, it remains
unclear how through-bond interactions promote fast
and efficient singlet fission, especially focusing on the
most basic pentacene dimer model. To date, several
groups have reported that connecting two pentacenes
using conjugated bridges can be used to promote effi-
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cient singlet fission, even when the proximity between avoided crossing, when resonance between the singlet
the pentacenes is significantly decreased.9,10,13,14 For ex-
ample, when the bridge shown in Figure 1 is varied from
one to three phenylene units, we have found that the
rates of singlet fission and triplet pair recombination in
pentacene dimers are drastically affected. The longer the
conjugated bridge, the slower the rate of iSF and triplet
pair recombination.11 Similarly, Zirzlmeier et al. reported
pentacene dimers that were connected through 6-
position by o-, m-, p-diethynylbenzene spacers, which
revealed that through-space and through-bond interac-
tions play crucial role in singlet fission and triplet re-
combination dynamics. They also found that faster sin-
glet fission was accompanied by faster triplet recombi-
nation.9 Further, orthogonally connected dimers report-
ed by Lukman et al. resulted in ultrafast singlet fission
and were particularly sensitive to the polarity of the me-
dium.12,33 Recently, Liu et al. designed tetracene trimer
through linear oligomerization which resulted in greatly
enhanced iSF yield (96%) relative to a similar dimer.
This SF enhancement was attributed to singlet exciton
delocalization.34 These studies all suggest that conjuga-
tion plays a significant role in driving singlet fission and
triplet recombination. Such conclusion is further sup-
ported by the fact that singlet fission is slower in twisted
dimers that lack bridging units, where conjugation is
decreased due to reduced overlap of the pi orbitals.35
exciton and triplet pair states is reached through a vi-
brational mode. However, the ultimate limits of this
hypothesis have not yet been tested, i.e., it is unknown
what happens in the excited state when both through-
space and through-bond interactions are extremely
weak.
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In this manuscript, we investigate how homoconju-
gated and non-conjugated bridging units affect the ex-
cited state dynamics of pentacene dimers. We particu-
larly focus on understating how singlet fission and tri-
plet pair recombination behave in the limit of weakly
coupled pentacene dimers (Figure 1). In homoconjugat-
ed dimers, the two pentacene chromophores are sepa-
rated by a saturated sp3 carbon, thus the pentacene-
pentacene coupling and/or electron delocalization is
expected to be weaker than in conjugated systems (such
as BP1, Figure 1).11 We postulate that this π-sigma-π
bonding scheme will make the excited state dynamics
much less sensitive to subtle variations in the geometry
of the bridge as compared to analogous conjugated di-
mers.36 Additionally, these systems are suited to intro-
duce more than one sp3 carbon in the bridge to yield
non-conjugated dimers, thus allowing us to probe the
limits of weak coupling interactions.
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Results and Discussion:
Materials Design: The chromophores shown in Fig-
ure 1 were designed as follows: In both the ethanoben-
zo[b]decacence derivative (EBD)20-22 and the spiro-
bi[cyclopenta[b]pentacene] derivative (Spi) the penta-
cenes are locked in a rigid fashion. The pentacenes in
EBD are more planar than in Spi, where the two chro-
mophores are nearly orthogonal to each other. In the
bistrifluoromethyl derivative (TFM), the two pentacene
units are connected by single saturated carbon, giving
the pentacenes some freedom to rotate relative to one
another. Finally, we use a bicyclooctane spacer (BCO) to
mimic the distance imposed by the conjugated phe-
nylene spacer in BP1, which has been previously report-
ed.11 Complete details of their synthesis and characteri-
zation are given in the supporting information.
Figure 1. The pentacene-bridge-pentacene model showing
the comparison between different bridging units. In the
bottom representations, the pentacenes are omitted to
highlight the nature of the bridging units.
Steady-State Optical Properties.
The UV-visible absorption spectra of the dimers are
shown in Figure 2, and are plotted alongside the spec-
trum of TIPS-pentacene for comparison. In all cases the
dimer spectra are qualitatively similar to the monomer
spectra, allowing us to assign dimer transitions from the
known spectrum of pentacene.37 The absorption around
650 nm corresponds to an intra-monomer HOMO to
LUMO excitation polarized along the short-axis of the
monomer, and is slightly red-shifted in Spi (by ~12 nm),
possibly due to a small interaction between pi systems
of the monomers.
Interestingly, many of aforementioned studies offer
hints that conjugation may not be strictly necessary for
singlet fission in pentacene-bridge-pentacene chromo-
phores. A recent computational study suggests that sin-
glet fission occurs by a direct mechanism in bipenta-
cene, in contrast to the charge transfer mediated (step-
wise) mechanism widely perceived to be dominant in
intermolecular singlet fission of crystalline pentacene.35
One of the key predictions from this study was that very
weak chromophore-chromophore coupling could permit
ultrafast singlet fission. The process is viable through an
The intense absorption in the UV around 310 nm
corresponds to a long-axis polarized transition and the
weak absorption around 440 nm to an almost-forbidden
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lated structures using density functional theory. Hydrogens
long-axis transition. Both the 650 nm and 440 nm tran-
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and TIPS substituents at the 6,13-position of pentacene are
omitted for clarity.
sitions are accompanied by vibrational stretching pro-
gressions commonly seen in acene spectra. Some dimers
exhibit a splitting of the 310 nm absorption, whereby the
dipole moments along the long-axis of each monomer
can combine in-phase or out-of-phase, giving two dis-
tinct absorptions. This is clearest when the long-axes of
the monomers are approximately 90° apart (as for Spi)
and absent if the long-axes are in the same (as for BP1).
The redshift in the high-energy features in BP1 is poten-
tially due to greater interactions between the chromo-
phores through a conjugated linker.
Although changes in the absorption spectrum
(where present) can indicate the extent of chromophore
interaction, the inter-chromophore coupling responsible
for SF is not available from UV-vis spectra, since the
relevant CT and TT states (or adiabatic states with that
character, see SI) are generally dark and the UV-vis
spectrum probes the adiabatic electronic states, not lo-
calized/diabatic states from whose coupling SF rates can
be determined.35,38
Singlet Fission in Homoconjugated Dimers.
We use broadband transient absorption spectrosco-
py (TAS) to study the excited state dynamics of these
molecules in dilute solution. The measurements are
carried out in a standard nearly collinear transmission
geometry. A 100 fs pump pulse is tuned to excite a vi-
bronic feature associated with the lowest energy optical
transition (~ 600 nm) and a supercontinuum white light
was used as probe (additional details in the SI). Both
femto- (mechanical delay) and nanosecond (electronic
delay) broadband probes are employed in conjunction
with this same pump pulse to extend the dynamic range
of measurement from 100 fs – 100 ꢀs. Figure 3 shows the
resulting 2D color plots produced by photoexcitation of
the pentacene dimers with a fluence of ~25 ꢀJ/cm2 in
chloroform.
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We find that efficient singlet fission occurs in all of
the homoconjugated pentacene dimers (EBD, TFM,
Spi). Moreover, despite the significantly different geom-
etries, singlet fission rates are quite similar among the
homoconjugated dimers as EBD, TFM and Spi undergo
iSF with time constants of 10 ps, 50 ps and 55 ps, respec-
tively. Singlet fission is assigned following the widely
accepted criteria.7,9-12,14,19,23,33-35,39-43 Briefly, the singlet
decay is assigned by correlating the time constants asso-
ciated with the decay of prompt fluorescence (using
photoluminescence upconversion techniques, SI) to
features in the transient absorption spectra. From this,
we determine that the photoexcited singlet exciton is
associated with photoinduced absorption bands near
~460 nm and 520 nm, and find that the singlet decays
on <100 ps timescales.
View along C2 (z) axis
View perpendicular to C2 (z) axis
Spi
EBD
TFM
BCO
BP1
Figure 2. Top: UV-Visible absorption spectra with TIPS-
pentacene and BP1 included for reference. Bottom: Calcu-
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Figure 3. Transient absorption spectroscopy in dilute chloroform solution with 600 nm excitation (~25 ꢀJ/cm2) reveals evolution
of the photoexcited singlet into triplets with singlet fission and triplet pair recombination rates that depend strongly dependent
on the properties of the bridge. In the structures of the bridges, red color indicates the connectivity of pentacene units, and the
color scales have been normalized to facilitate comparison and are therefore reported in arbitrary units (a.u.). Data prior to ~2.7
ns is collected using a mechanical delay, while the same pump pulse and an electronically controlled probe were used to gener-
ate data after ~2.7 ns.
The triplet pair state also shows similar dynamics
within the set of homoconjugated dimers and is as-
signed by comparing the products of singlet fission to
triplet sensitization studies (SI). From this comparison,
we find that the primary triplet feature in the transient
absorption spectra is a prominent photoinduced absorp-
tion near ~510 nm. This feature can be used to monitor
the triplet decay dynamics. Similar to other dimer sys-
tems, we find that the rise of the triplet pair is concomi-
tant to the decay of the singlet, indicating that no para-
sitic processes or other intermediates are present, as has
been widely suggested in the literature.10-12,14,16,44 This
allows us to quantify the yields using kinetic arguments,
since the only significant competing relaxation process
is a 12.3 ns radiative decay. Using these arguments, we
calculate that the singlet fission yields exceed 198%. As
will be discussed later, using electron spin resonance
measurements, we also show that the triplet pairs that
are formed from singlet fission remain spin coupled
across the homoconjugated bridge. Similar to other di-
mers with long lived triplet pairs, this leads to a biexpo-
nential recombination process, with a dominant short
lifetime component (< 1 ꢀs) in ESR and TA representing
the spin coupled state and a weaker component with a
time constant of ~20 µs lifetime that is assigned to a
minority species of free pentacene triplets formed via
dissociation of the triplet pair. For EBD, TFM and Spi
the spin coupled triplet pair lifetimes are extremely
long, 174 ns, 531 ns, and 705 ns, respectively.
ing evidence: (i) the prohibitively high-lying energy of
CT states with large center-center distances which we
calculate using electronic structure theory methods, (see
SI for details) compared to molecular crystals, where
additional electronic bandwidth also help to bring sin-
glet exciton and CT states into resonance (ii) the discov-
ery of a viable direct mechanism based on vibrationally
induced degeneracy between S1 and TT, despite weak
coupling,35 and (iii) a weak and non-monotonic depend-
ence of SF rates on solvent polarity or polarizability
(SI).11,35,41,45
Here, the effect of changing the conjugation motif is
highlighted by comparison to a previously reported con-
jugated dimer (BP1, Figure 4, Table 1). BP1, which con-
stitutes two pentacenes connected by a p-phenylene
bridge (Figure 1), is the conjugated dimer with the clos-
est singlet fission time constant (20 ps) to the homocon-
jugated dimers investigated here.11 From inspection, we
clearly see that the electronic coupling between the
chromophores is significantly affected by through-bond
interactions; singlet fission in BP1, where pentacenes are
connected through four sp2 hybridized carbons, has a
time constant similar to homoconjugated dimers, where
pentacene chromophores are separated by just one sp3
hybridized carbon.
Despite the faster time constants for triplet pair
generation as compared to conjugated BP1, triplet pair
recombination is > 10 times slower in EBD. Similarly,
while singlet fission is slower by a factor of approximate-
ly 2.5, the triplet pair recombination is slower by a factor
of > 30 in TFM and > 40 in Spi. These results indicate
that singlet fission and triplet pair recombination are
not governed by the exact same pentacene-pentacene
Akin to conjugated pentacene dimers, we suggest
that iSF proceeds through a direct mechanism in homo-
conjugated dimers, without significant mediation from
charge transfer (CT) states. This is based on the follow-
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and assign the relevant rate constants. It is remarkable
coupling relationship. This is an important observation
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because breaking the relationship between the rate of
triplet generation (fast singlet fission) and decay rates
(slow recombination) can play a fundamental role in
optoelectronic devices, where maximizing the overall
triplet pair lifetime is beneficial for harvesting that ener-
gy. While it has been observed in twisted dimers that
reducing the coupling between pentacene chromo-
phores preferentially extends the triplet pair lifetime,35
this is the first family of materials with fast rates of iSF,
and drastically different rates of triplet pair recombina-
tion. While it is worth noting that solid-state dynamics
are relevant to devices, solution studies isolate through-
bond singlet fission. In the solid-state, through-bond
singlet fission can be complemented by through-space
singlet fission, adding an additional SF channel. Though
it must be noted that characterization of the through-
bond SF in solution is informative to the properties of
the new materials and their potential utility.
that singlet fission can proceed even in the limit of ex-
tremely weak electronic coupling, as evidenced by the
long singlet lifetime (7.6 ns). Despite the similar inter-
chromophore separation as conjugated BP1, the rate
constant for singlet fission in BCO slower by a factor of
nearly 1000. Again, this allows us to deduce the im-
portant role of conjugation in facilitating fast and effi-
cient singlet fission.
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Identification of Triplet-Triplet Pairs.
We establish that singlet fission is operative, pro-
ducing triplet pairs as opposed to free triplet generation
by intersystem crossing, by correlating transient absorp-
tion and electron spin resonance studies (Figure 5). Like
other iSF dimers, transient absorption studies show that
the triplet population decays biexponentially, indicating
the presence of triplet pairs (TT) with an enhanced re-
combination rate (Figure 6) and a minority population
of free triplets decay with the expected rate for an indi-
vidual triplet. These biexponential decay dynamics have
also been observed in other systems11,14,39,40 and do not
change as a function of concentration or other experi-
mental parameters.
Table 1: Time constants for: rate of iSF (τiSF), triplet
pair lifetimes (τ[TT]), and individual triplet decay
(τ[T], obtained from slower triplet decay compo-
nent).
Compound
BP111
τ
iSF (ps)
20
τ
(TT) (ns)
16.5
τ
(T) (ns)
The biexponential dynamics in the TA experiments
were further probed by transient electronic spin reso-
nance (tr-ESR) measurements. tr-ESR spectroscopy has
previously been used to identify coupled triplet-triplet
pairs, (TT), and uncoupled triplets, T, in bipentacenes.39
Figure 5 shows the 80K pulsed laser, continuous micro-
wave tr-ESR spectra of Spi and BCO in toluene, using
microwave frequencies of 9.681092 and 9.615778 GHz,
respectively (additional details in the SI). The spectra
are plotted as a function of g-factor to highlight the
identical location of resonances, which arise from the g-
factor and zero-field splitting parameters of individual
pentacene triplets (g~2.0023, D~1100MHz and
E~20MHz, in accordance with previous ESR studies of
the pentacene chromophore39,46,47). Transition resonance
peaks for magnetic fields applied along the x/y molecu-
lar axes are marked on the plot.
---
10
174
24,300
23,000
19,600
18,000
EBD
TFM
50
531
Spi
55
705
BCO
~20,000
1,800
In both Spi and BCO the 5(TT)0 quintet state is gen-
erated faster than the time resolution of the experiment.
The
absorptive/emissive
resonances
for
the
5(TT)0ꢀ5(TT)± transitions are separated by (D-3E)/3, as
is expected for strongly coupled triplets (J>D,E). After
several hundred nanoseconds uncoupled triplets are
generated, probably via a geometric relaxation that re-
duces the inter-triplet exchange coupling, J. This gives
rise to absorptive/emissive T0ꢀT± transitions separated
by D-3E. The absorption/emission structure is indicative
of the selective population of T0, as is expected for tri-
plets generated by fission.48
Figure 4. Normalized kinetics monitored at the maximum
of the triplet photoinduced absorption of TFM, EBD, Spi,
and BCO as dilute solutions in chloroform following 600
nm excitation (~25 µJ/cm2).
Singlet Fission in a Non-Conjugated Dimer.
We confirm the assigned spin multiplicities using
pulsed laser, pulsed microwave measurements (addi-
Surprisingly, we find that BCO, a non-conjugated
pentacene dimer separated by 4 sp3 hybridized carbons,
is capable of singlet fission (Figure 3). A similar method-
ology to other dimers was used to identify singlet fission
5
tional details in the SI) of the (TT)0ꢀ5(TT)+ and T0ꢀT+
transitions, which yielded a Rabi nutation frequency
ratio of 1.64±0.05, in agreement with the expected value
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of √3=1.73 for strongly coupled triplet pairs.39,49 The
compare the triplet signal at a given fluence to that pro-
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strong coupling regime in ESR measurements of these
materials refers to exchange energies that are only ≳ 20
GHz. It is likely that the triplet-triplet pair coupling in
BCO is actually weak on an electronic energy scale and
this gives rise to the relatively large values of τiSF and
duced by transfer of a known number of triplets from an
external, well-characterized sensitizer, finds a yield of
63% (Supporting Information). If any other parasitic
decay processes were occurring, the sensitization meth-
ods would yield a significantly lower value for the singlet
fission yield. This compound shows that fully incoherent
singlet fission can occur even in the limit of extremely
weak electronic coupling, as long as the excited state
lifetimes permit reasonable kinetic competition with
ground state repopulation.
τ(TT)
.
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Conclusions: Through the evaluation of a family of
materials, we have found that the conjugation motif of
the interpentacene bridge is instrumental in mediating
singlet fission in pentacene dimers. The concept of in-
trachromophore coupling interactions in pi-bridge-pi
molecules was tested using homoconjugated and non-
conjugated bridging moieties. In these systems, homo-
conjugated bridges can yield singlet fission rates that are
faster than a conjugated bridge, while maintaining slow-
er triplet pair recombination. We also found that SF can
occur in the case of a non-conjugated bridge, although
the process is much slower than SF in a similar sized
conjugated bridge. We further characterized the for-
mation of triplet pairs through singlet fission using ESR
measurements. This study demonstrates the importance
of the bridge design in such compounds and emphasizes
that bridge effects must be play a key role in under-
standing SF and triplet pair recombination.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website.
Experimental methods, including details of the transient
absorption spectroscopy and photosensitization experi-
ments, synthetic details, electron spin resonance experi-
ments and characterization of compounds used in this
study.
AUTHOR INFORMATION
Corresponding Author
Figure 5: Top: Transient absorption kinetics near the tri-
plet absorption maximum, with arrows indicating times
selected for ESR spectra. Bottom: Transient ESR spectra of
Spi and BCO in toluene at given time delays after laser
excitation at 599 nm, ~70 µJ/pulse. Dashed lines mark loca-
tions of (TT)0 and T0 transition resonances.
*dane.mccamey@unsw.edu.au
*msfeir@bnl.gov
*lcampos@columbia.edu
Author Contributions
‡ These authors contributed equally.
These measurements allow us to confirm that no
other parasitic decay channels besides radiative recom-
bination are present in these dimers. Unlike in homo-
conjugated dimers, where the yield is nearly quantita-
tive, radiative recombination in BCO is a significant loss
channel. We estimate the singlet fission yield to be
~76% based on kinetic competition of SF with the typi-
cal ~12.3 ns radiative lifetime of TIPS-pentacene.50 Im-
portantly, the similarity between the triplet pair and free
triplet transient spectra allows us to directly determined
the SF yield using triplet sensitizations methods.11 In-
deed, a cross-sectional yield determination, where we
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
L.M.C. acknowledges support from the Office of Naval Re-
search Young Investigator Program (Award N00014-15-1-
2532) and Cottrell Scholar Award. S.N.S. and A.B.P. thank
the NSF for GRFP (DGE 11-44155). This research used re-
sources of the Center for Functional Nanomaterials, which
is a U.S. DOE Office of Science Facility, at Brookhaven Na-
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tional Laboratory under Contract No. DE-SC0012704. This
(16) Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Rong, R.; Miller,
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work used the Extreme Science and Engineering Discovery
Environment (XSEDE), which is supported by National
Science Foundation grant number ACI-1548562. MJYT
acknowledges receipt of an ARENA Postdoctoral Fellowship
and a Marie Curie Individual Fellowship. DRM acknowl-
edges support from an Australian Research Council Future
Fellowship (FT130100214) and through the ARC Centre of
Excellence in Exciton Science (CE170100026). Single crystal
X-ray diffraction was performed at the Shared Materials
Characterization Laboratory at Columbia University. Use of
the SMCL was made possible by funding from Columbia
University. N. A. acknowledges support from the NSF
CAREER (Award No. CHE-1555205), NSF EAGER (Award
No. CHE-1546607) and a Sloan Foundation Research Fel-
lowship. JCD and GDS acknowledge funding through the
Division of Chemical Sciences, Geosciences, and Bioscienc-
es, Office of Basic Energy Sciences of the U.S. Department
of Energy (Award No DE-SC0015429). The authors thank
Dr. Ryan Pensack for helpful discussions.
J. R.; Zhu, X.-Y.; Campos, L. M.; Sfeir, M. Y. Nat.
Mater. 2014, 14, 426.
(17) Varnavski, O.; Abeyasinghe, N.; Aragó, J.; Serrano-
Pérez, J. J.; Ortí, E.; López Navarrete, J. T.; Takimiya,
K.; Casanova, D.; Casado, J.; Goodson, T. J. Phys.
Chem. Lett. 2015, 6, 1375.
(18) Kasai, Y.; Tamai, Y.; Ohkita, H.; Benten, H.; Ito, S. J.
Am. Chem. Soc. 2015, 137, 15980.
(19) Xia, J.; Sanders, S. N.; Cheng, W.; Low, J. Z.; Liu, J.;
Campos, L. M.; Sun, T. Adv. Mater. 2016, 1601652.
(20) Alguire, E. C.; Subotnik, J. E.; Damrauer, N. H. J. Phys.
Chem. A 2015, 119, 299.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(21) Cook, J. D.; Carey, T. J.; Damrauer, N. H. J. Phys.
Chem. A 2016, 120, 4473.
(22) Vallett, P. J.; Snyder, J. L.; Damrauer, N. H. J. Phys.
Chem. A. 2013, 117, 10824.
(23) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Phelan, B. T.;
Krzyaniak, M. D.; Reddy, S. R.; Coto, P. B.; Horwitz,
N. E.; Young, R. M.; White, F. J.; Hampel, F.; Clark, T.;
Thoss, M.; Tykwinski, R. R.; Wasielewski, M. R.;
Guldi, D. M. Nat. Commun. 2017, 8, 15171.
(24) Yost, S. R.; Lee, J.; Wilson, M. W. B.; Wu, T.;
McMahon, D. P.; Parkhurst, R. R.; Thompson, N. J.;
Congreve, D. N.; Rao, A.; Johnson, K.; Sfeir, M. Y.;
Bawendi, M. G.; Swager, T. M.; Friend, R. H.; Baldo,
M. A.; Van Voorhis, T. Nat. Chem. 2014, 6, 492.
(25) Mauck, C. M.; Hartnett, P. E.; Margulies, E. A.; Ma, L.;
Miller, C. E.; Schatz, G. C.; Marks, T. J.; Wasielewski,
M. R. J. Am. Chem. Soc. 2016, 138, 11749.
(26) Eaton, S. W.; Miller, S. A.; Margulies, E. A.; Shoer, L.
E.; Schaller, R. D.; Wasielewski, M. R. J. Phys. Chem. A
2015, 119, 4151.
(27) Ryerson, J. L.; Schrauben, J. N.; Ferguson, A. J.; Sahoo,
S. C.; Naumov, P.; Havlas, Z.; Michl, J.; Nozik, A. J.;
Johnson, J. C. J. Phys. Chem. C 2014, 118, 12121.
(28) Dillon, R. J.; Piland, G. B.; Bardeen, C. J. J. Am. Chem.
Soc. 2013, 135, 17278.
(29) Kolata, K.; Breuer, T.; Witte, G.; Chatterjee, S. ACS
Nano 2014, 8, 7377.
(30) Wang, L.; Olivier, Y.; Prezhdo, O. V.; Beljonne, D. J.
Phys. Chem. Lett. 2014, 5, 3345.
(31) Mastron, J. N.; Roberts, S. T.; McAnally, R. E.;
Thompson, M. E.; Bradforth, S. E. J. Phys. Chem. B
2013, 117, 15519.
(32) Roberts, S. T.; McAnally, R. E.; Mastron, J. N.;
Webber, D. H.; Whited, M. T.; Brutchey, R. L.;
Thompson, M. E.; Bradforth, S. E. J. Am. Chem. Soc.
2012, 134, 6388.
REFERENCES
(1) Smith, M. B.; Michl, J. Annu. Rev. Phys. Chem. 2013,
64, 361.
(2) Smith, M. B.; Michl, J. Chem. Rev. 2010, 110, 6891.
(3) Hanna, M. C.; Nozik, A. J. J. Appl. Phys. 2006, 100,
074510.
(4) Tayebjee, M. J. Y.; Gray-Weale, A. A.; Schmidt, T. W.
J. Phys. Chem. Lett. 2012, 3, 2749.
(5) Tayebjee, M. J. Y.; Hirst, L. C.; Ekins-Daukes, N. J.;
Schmidt, T. W. J. Appl. Phys. 2010, 108, 124506.
(6) Tayebjee, M. J. Y.; McCamey, D. R.; Schmidt, T. W. J.
Phys. Chem. Lett. 2015, 6, 2367.
(7) Low, J. Z.; Sanders, S. N.; Campos, L. M. Chem. Mater.
2015, 27, 5453.
(8) Stern, H. L.; Musser, A. J.; Friend, R. H. In
Photochemistry; Singlet exciton fission in solution;
The Royal Society of Chemistry: Cambridge, UK, 2016;
Vol. 43, p 270.
(9) Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E.
T.; Casillas, R.; Basel, B. S.; Thoss, M.; Tykwinski, R.
R.; Guldi, D. M. Proc. Natl Acad. Sci. 2015, 112, 5325.
(10) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.;
Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. Angew.
Chem. Int. Ed. 2016, 55, 3373.
(11) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M.
T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.;
Roy, X.; Zhu, X. Y.; Steigerwald, M. L.; Sfeir, M. Y.;
Campos, L. M. J. Am. Chem. Soc. 2015, 137, 8965.
(12) Lukman, S.; Musser, A. J.; Chen, K.; Athanasopoulos,
S.; Yong, C. K.; Zeng, Z.; Ye, Q.; Chi, C.; Hodgkiss, J.
M.; Wu, J.; Friend, R. H.; Greenham, N. C. Adv. Funct.
Mater. 2015, 25, 5452.
(33) Lukman, S.; Chen, K.; Hodgkiss, J. M.; Turban, D. H.
P.; Hine, N. D. M.; Dong, S.; Wu, J.; Greenham, N. C.;
Musser, A. J. Nat. Commun. 2016, 7, 13622.
(34) Liu, H.; Wang, R.; Shen, L.; Xu, Y.; Xiao, M.; Zhang,
C.; Li, X. Org. Lett. 2017, 19, 580.
(35) Fuemmeler, E. G.; Sanders, S. N.; Pun, A. B.;
Kumarasamy, E.; Zeng, T.; Miyata, K.; Steigerwald, M.
L.; Zhu, X. Y.; Sfeir, M. Y.; Campos, L. M.; Ananth, N.
ACS Cent. Sci. 2016, 2, 316.
(36) Su, T. A.; Li, H.; Klausen, R. S.; Widawsky, J. R.; Batra,
A.; Steigerwald, M. L.; Venkataraman, L.; Nuckolls, C.
J. Am. Chem. Soc. 2016, 138, 7791.
(13) Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.;
Haiges, R.; Krylov, A. I.; Bradforth, S. E.; Thompson,
M. E. J. Am. Chem. Soc. 2016, 138, 617.
(14) Sakuma, T.; Sakai, H.; Araki, Y.; Mori, T.; Wada, T.;
Tkachenko, N. V.; Hasobe, T. J. Phys. Chem. A 2016,
120, 1867.
(15) Busby, E.; Xia, J.; Low, J. Z.; Wu, Q.; Hoy, J.; Campos,
L. M.; Sfeir, M. Y. J. Phys. Chem. B 2015, 119, 7644.
(37) Halasinski, T. M.; Hudgins, D. M.; Salama, F.;
Allamandola, L. J.; Bally, T. J. Phys. Chem. A 2000, 104,
7484.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 9
(38) Berkelbach, T. C.; Hybertsen, M. S.; Reichman, D. R. J.
1
Chem. Phys. 2013, 138, 114103.
2
3
4
(39) Tayebjee, M. J. Y.; Sanders, S. N.; Kumarasamy, E.;
Campos, L. M.; Sfeir, M. Y.; McCamey, D. R. Nat.
Phys. 2017, 13, 182.
(40) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Appavoo,
K.; Steigerwald, M. L.; Campos, L. M.; Sfeir, M. Y. J.
Am. Chem. Soc. 2016, 138, 7289.
(41) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.;
Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. Chem
2016, 1, 505.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(42) Margulies, E. A.; Logsdon, J. L.; Miller, C. E.; Ma, L.;
Simonoff, E.; Young, R. M.; Schatz, G. C.;
Wasielewski, M. R. J. Am. Chem. Soc. 2017, 139, 663.
(43) Zirzlmeier, J.; Casillas, R.; Reddy, S. R.; Coto, P. B.;
Lehnherr, D.; Chernick, E. T.; Papadopoulos, I.;
Thoss, M.; Tykwinski, R. R.; Guldi, D. M. Nanoscale
2016, 8, 10113.
(44) Trinh, M. T.; Pinkard, A.; Pun, A. B.; Sanders, S. N.;
Kumarasamy, E.; Sfeir, M. Y.; Campos, L. M.; Roy, X.;
Zhu, X.-Y. Sci. Adv. 2017, 3, e1700241.
(45) Kumarasamy, E.; Sanders, S. N.; Pun, A. B.;
Vaselabadi, S. A.; Low, J. Z.; Sfeir, M. Y.; Steigerwald,
M. L.; Stein, G. E.; Campos, L. M. Macromolecules
2016, 49, 1279.
(46) Bayliss, S. L.; Thorley, K. J.; Anthony, J. E.; Bouchiat,
H.; Greenham, N. C.; Chepelianskii, A. D. Phys. Rev. B
2015, 92, 115432.
(47) Van Strien, A. J.; Schmidt, J. Chem. Phys. Lett. 1980,
70, 513.
(48) Swenberg, C. E.; van Metter, R.; Ratner, M. Chem.
Phys. Lett. 1972, 16, 482.
(49) Weiss, L. R.; Bayliss, S. L.; Kraffert, F.; Thorley, K. J.;
Anthony, J. E.; Bittl, R.; Friend, R. H.; Rao, A.;
Greenham, N. C.; Behrends, J. Nat. Phys. 2017, 13, 176.
(50) Walker, B. J.; Musser, A. J.; Beljonne, D.; Friend, R. H.
Nat. Chem. 2013, 5, 1019.
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