Singlet Fission in a Flexible Bichromophore with Structural and Dynamic Control

Article abstract of DOI:10.1021/jacs.0c12384

Singlet fission (SF), i.e., the splitting of a high-energy exciton into two lower-energy triplet excitons, has the potential to increase the efficiency for harvesting spectrally broad light. The path from the photopopulated singlet state to free triplets is complicated by competing processes that decrease the overall SF efficiency. A detailed understanding of the whole cascade and the nature of the photoexcited singlet state is still a major challenge. Here, we introduce a pentacene dimer with a flexible crown ether spacer enabling a control of the interchromophore coupling upon solvent-induced self-aggregation as well as cation binding. The systematic change of solvent polarity and viscosity and excitation wavelength, as well as the available conformational phase space, allows us to draw a coherent picture of the whole SF cascade from the femtosecond to microsecond time scales. High coupling leads to ultrafast SF (<2 ps), independent of the solvent polarity, and to highly coupled correlated triplet pairs. The absence of a polarity effect indicates that the solvent coordinate does not play a significant role and that SF is driven by intramolecular modes. Low coupling results in much slower SF (μ500 ps), which depends on viscosity, and leads to weakly coupled correlated triplet pairs. These two triplet pairs could be spectrally distinguished and their contribution to the overall SF efficiency, i.e., to the population of free triplets, could be determined. Our results reveal how the overall SF efficiency can be increased by conformational restrictions and control of the structural fluctuation dynamics.

Full text of DOI:10.1021/jacs.0c12384

pubs.acs.org/JACS
Article
Singlet Fission in a Flexible Bichromophore with Structural and
Dynamic Control
§
§
̂
Alexander Aster, Francesco Zinna, Christopher Rumble, Jérome Lacour,* and Eric Vauthey*
Cite This: J. Am. Chem. Soc. 2021, 143, 2361−2371
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Singlet fission (SF), i.e., the splitting of a high-energy
exciton into two lower-energy triplet excitons, has the potential to
increase the efficiency for harvesting spectrally broad light. The path from
the photopopulated singlet state to free triplets is complicated by
competing processes that decrease the overall SF efficiency. A detailed
understanding of the whole cascade and the nature of the photoexcited
singlet state is still a major challenge. Here, we introduce a pentacene
dimer with a flexible crown ether spacer enabling a control of the
interchromophore coupling upon solvent-induced self-aggregation as well
as cation binding. The systematic change of solvent polarity and viscosity
and excitation wavelength, as well as the available conformational phase
space, allows us to draw a coherent picture of the whole SF cascade from
the femtosecond to microsecond time scales. High coupling leads to ultrafast SF (<2 ps), independent of the solvent polarity, and to
highly coupled correlated triplet pairs. The absence of a polarity effect indicates that the solvent coordinate does not play a
significant role and that SF is driven by intramolecular modes. Low coupling results in much slower SF (∼500 ps), which depends on
viscosity, and leads to weakly coupled correlated triplet pairs. These two triplet pairs could be spectrally distinguished and their
contribution to the overall SF efficiency, i.e., to the population of free triplets, could be determined. Our results reveal how the
overall SF efficiency can be increased by conformational restrictions and control of the structural fluctuation dynamics.
These pairs can undergo spin conversion or dephasing to the
uncorrelated free triplet pair, T1+T1, depending on the
exchange interaction and, thus, on the energy spacing between
the 1,3,5(TT) states.34−37 However, the dynamics toward
T1+T1 are complicated by the existence of various competing
channels that are detrimental to the overall SF efficiency.
Depending on the spin character of the correlated triplet pair,
two spin-allowed decay channels can be operative:1,38
INTRODUCTION
■
Singlet fission (SF), i.e. the splitting of a singlet exciton into
two triplet excitons,1,2 offers promising perspectives to
overcome the intrinsic efficiency cap for converting spectrally
broad light into electricity,3 by limiting the thermal losses.4
The climate crisis and the need for efficient solar energy
harvesting have stimulated a spectacular revival of interest in
this process, which was first reported in 1965.5 Studies on
crystalline solids,6−12 polymers.13−16 and molecular
dimers17−28 resulted in an overall picture of SF that can be
described as a three-step process:29−32
1(TT) → S0S0
singlet channel:
loss of ≤100% of T signal
3(TT) → TS0
triplet channel:
S S F m(TT) F mT ··· T)] F T + T
1
(
(1)
[
1
0
1
1
loss of ≤50% of T signal
(2)
A key factor for efficient and rapid SF is the conservation of
angular momentum during the transition from the photo-
populated singlet excited state, S1S0, to the spin-correlated
To unravel these complex mechanistic aspects, it is
advantageous to consider the simplest multichromophoric
material, a covalently bound dimer.26 Such dimers simplify the
1
triplet pair (TT) with an overall singlet character. The latter
can evolve to 1(T···T) by spatial separation and thus a decrease
Received: November 27, 2020
Published: January 29, 2021
in electronic coupling and exchange interaction. Since
1
1
electronic coupling varies continuously, (TT) and (T···T)
should be considered as limiting cases and are often merged
into a single species, TT.30,32,33 However, different spectral
features and dynamics justify the distinction between these two
triplet pairs.30,32
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371
© 2021 American Chemical Society
2361

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
study of SF by eliminating the possibility of energy transfer to
nearby chromophores and allow for a systematic variation of
the interchromophore geometry and solvent environment.
Pentacene-based dimers are probably the most studied SF
systems for which the interchromophore distance has been
systematically tuned over a wide range.17,20−23,36,39−43 These
investigations revealed a fast appearance of triplet spectral
features in strongly coupled pentacene dimers, followed by a
decay on a picosecond to nanosecond time scale: i.e., much
faster than expected for a T1 state.17,26,39 In contrast, the
buildup of the triplet features in weakly coupled dimers was
found to be slower and their decay bimodal, with the slowest
component matching the decay of the T1 state measured in
sensitization studies.21,22,39 Time-resolved electron-spin reso-
nance experiments evidenced a strong influence of the
coupling on the spin conversion dynamics of the correlated
triplet pairs.36 Two distinct assignments of the slow
component of the bimodal triplet decay were proposed: (i)
the decay of the ”lone triplet” T1S0,36 and (ii) the decay of free
triplet pairs T1+T1.20−23,39−44 These two interpretations
presuppose significantly different SF mechanisms.
the addition of Ba2+ to form a host−guest complex with the
macrocycle. In contrast to stiff backbones, the geometrical
flexibility of the crown ether allows for structural fluctuations,
which can decrease the exchange interaction and allow for spin
conversion. However, this comes at the cost of multiple
structural subpopulations. To unravel the complicated excited-
state dynamics, we performed a systematic comparative study
involving (i) excitation wavelength (λex) dependence allowing
for the photoselection of different subpopulations, (ii) solvent
viscosity (η) dependence revealing the effect of structural
fluctuations, and (iii) change of the conformational phase
space by solvent-dependent interchromophore aggregation and
Ba2+ binding. This spectroscopic study is complemented by
molecular dynamics (MD) simulations to estimate the
structure of the various subpopulations present under different
experimental conditions.
RESULTS
■
Structural Control. Solvent-Dependent Aggregation.
The absorption spectra change significantly upon going from
the reference monomer Ref to 18C6 (Figure 1A, see section
2.3 in the Supporting Information for the photophysics of
Ref). These changes, which report on the coupling between
the chromophores, depend markedly on the solvent. In HEX
and ACN, the absorption spectrum of 18C6 is broader and
red-shifted, while the intensity ratio between the 0−0 and first
vibronic bands, I0−0/I0−1, is smaller. In contrast, in the
medium-polarity solvent tetrahydrofuran (THF), the position,
width, and I0−0/I0−1 are nearly the same for 18C6 and Ref. The
differences observed in HEX and ACN point to a close
proximity of the pentacene subunits in these solvents and to an
arrangement consistent with the spectral signature of a H-type
dimer.49−52 In THF, the similarity between the monomer and
the dimer spectra suggests a weak interchromophore coupling
as well as a negligible effect of the crown ether on the S1 ← S0
transition. These spectral changes in HEX and ACN and not in
THF can be assigned to self-aggregation. Indeed, the
propensity of aromatic molecules to aggregate has been
shown to be the weakest in medium-polarity solvents and
the strongest in high- and low-polarity solvents, as it depends
on the interplay between chromophore−solvent and chromo-
phore−chromophore interactions.53−56 Since SF involving
charge-transfer interactions has been reported to be sensitive
to solvent polarity,43 this nonlinear dependence of aggregation
on the solvent polarity allows structural effects to be
disentangled from mechanistic effects.
Here we focus on three important aspects of the SF cascade:
(i) the effect of the coupling between the chromophores on
the nature of the photopopulated S1S0 state and its dynamics,
(ii) the contributions of solvent and intramolecular modes to
the SF coordinate, and (iii) the origin of the bimodal triplet
decay. For this, we synthesized a bichromophore, named
18C6, consisting of two TIPS-pentacene subunits linked via
amide bridges to a crown ether (Scheme 1); its preparation
a
Scheme 1. Preparation and Structures of Ref and 18C6
Insight into the conformational space available to 18C6 in a
given solvent was obtained from the potentials of mean force
(PMFs) between the two pentacene subunits, w(r),
determined by MD simulations. These PMFs, illustrated in
Figure 1B, reflect the free energy of 18C6 as a function of the
center of mass distance between the two chromophores, r.
They reveal that many interconverting conformations coexist
at room temperature. These structures cannot all be differ-
entiated from their absorption spectrum, as a sufficiently large
coupling is needed to result in detectable spectral changes.
Quantum-mechanical calculations on perylene predict that the
absorption spectrum departs from that of the monomer at a
center to center distance r of ≲0.7 nm.57 We can thus assume
that the stationary absorption spectra can only distinguish
strongly coupled dimers, whose spectra differ significantly from
that of Ref, from weakly coupled dimers, which comprise all
conformers with r ≳ 0.7 nm. Therefore, red-edge illumination
a
Conditions: (i) acetyl chloride, Et3N in CH2Cl2; (ii) macrocycle 9
(see the Supporting Information), tBuOK in THF. See the Supporting
Information for full synthetic details.
involves a straightforward synthetic protocol with simple
starting materials (stable diazo reagents, 1,4-dioxane) and high
concentration conditions for the macrocyclization and amide
formation steps (see the Supporting Information for full
preparation details).45−48 As will be shown, the interchromo-
phore distance can be controlled by the solvent as well as by
2362
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Figure 1. (A) Comparison of the absorption and emission spectra of the reference monomer Ref (blue) and 18C6 (red) in the low-, intermediate-,
and high-polarity solvents n-hexane (HEX), tetrahydrofuran (THF), and acetonitrile (ACN), respectively. The formation of 18C6⊂Ba2+ (green)
upon addition of Ba(ClO4)2 in ACN can be followed by changes in the absorption and emission spectra. (B) Potentials of mean force (PMFs)
describing the free energy as a function of the interchromophore center of mass distance. The free energy of the absolute minimum of each PMF is
arbitrarily set to 0. (C) Representative snapshots from molecular dynamics simulation at local minima of the PMFs. The TIPS acetylene groups are
transparent for the sake of clarity.
in HEX and ACN should predominantly excite strongly
coupled dimers, whereas irradiation at shorter wavelength
should not allow any photoselection.
the photopopulated state of strongly coupled conformers. We
will use the nomenclature applied by Scholes and co-workers
for amorphous nanoparticle suspensions and designate the
strongly coupled, H-aggregate-like conformers as H-dimers
and the weakly coupled, monomer-like conformers as M-
dimers.30 By analogy, their first singlet excited state will be
designated as (S1S0)H and (S1S0)M, respectively. (S1S0)H
consists of multiple states depending on the nature of the
excitonic coupling, whereas (S1S0)M is doubly degenerate.
The emission spectra of 18C6 and Ref are almost identical,
suggesting that the fluorescence of 18C6 originates from the
M-dimers and that, if an excimer is formed, its lifetime is too
short and/or its radiative rate constant too small to contribute
to the fluorescence spectrum. In contrast to Ref, the
fluorescence decay of 18C6 measured by time-correlated
single-photon counting is multiexponential in all solvents, in
agreement with the presence of various subpopulations with
different interchromophore couplings (Figure S9).
Figure 1C illustrates a few representative snapshots from the
MD simulations. According to the PMFs, the strongly coupled
dimers should have a structure similar to that of the r = 0.38
nm conformer. The local minimum associated with this
conformer is deepest in ACN, intermediate in HEX, and least
favorable in THF. The PMFs therefore suggest that the relative
population of strongly coupled conformers decreases from
ACN to HEX and THF. We will show later that the results are
fully consistent with the TA spectra in different solvents. In
THF, 18C6 is predicted to exist mainly with the pentacene
subunits at r ≈ 1 nm and separated by solvent molecules. Such
weak coupling agrees well with the stationary absorption
spectrum.
Additional information on the interchromophore coupling
was obtained from the effective transition dipole moment,
|μ |, associated with the lowest energy absorption band. In the
case of Ref, |μabs| is the same in all three solvents (see section
⃗
Host−Guest Complex. In order to change the conforma-
tional phase space even more drastically, we exploited the
tendency of the unsaturated crown ether to strongly bind
divalent cations. As illustrated in Figure 1A, the formation of
the 18C6⊂Ba2+ host−guest complex upon addition of
Ba(ClO4)2 is accompanied by an increase and a blue shift of
the absorption band as well as an increase in I0−0/I0−1. MD
simulations suggest that complexation involves the rotation of
the amide groups, with the oxygen atoms participating in the
binding of the cation. As the bound cation can also interact
with the solvent, long-distance conformers are favored by a
larger solvation energy (Figure 1B,C). As shown previously,59
the electron-donating character of the amide is decreased upon
⃗
abs
2.5.5 in the Supporting Information), whereas for 18C6, it is
significantly smaller in ACN and HEX. This suggests that the
nature of the excited state of 18C6 is different in THF, where
weakly coupled dimers predominate, than in ACN and HEX,
where strongly coupled dimers are present. Such a decrease in
|μ | upon self-aggregation cannot be described by only
⃗
abs
considering the two local excited states, |LE⟩, and their
dipole−dipole interactions as in the Kasha excitonic model.58
⃗
The decreased |μabs| is attributed to the contribution of dark
excited states such as the charge transfer states, |CT⟩, and/or
the correlated triplet pair states, |TT⟩, to the wave function of
2363
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
cation binding, explaining the difference between 18C6⊂Ba2+
and Ref. The higher emission intensity upon excitation at the
isosbestic points furthermore to a significant increase in the
fluorescence quantum yield upon complexation.
Singlet-Fission Dynamics. Now that we have a better
idea of the relevant conformations of the bichromophore in the
ground state, we focus on their sub-nanosecond excited-state
dynamics, dominated by SF, as measured by combined UV−
vis (335−740 nm) and near-IR (670−1650 nm) ultrafast
transient absorption (TA) spectroscopy.
at 498 nm and a lifetime on the microsecond time scale
(Figure S6).
18C6 Dimer and 18C6⊂Ba2+ in ACN. The TA spectrum
recorded with 18C6 in ACN 200 fs after 650 nm excitation
resembles that of Ref, but is, however, not identical (Figure
2B). During the first 2 ps, where solvent relaxation is observed
with Ref, the intensity of the singlet ESA band decays partially
and new bands, which can be attributed to the triplet state,30
appear at 498, 875, and 993 nm. As these triplet ESA features
are absent in the early TA spectra of Ref, this process is
attributed to the population of the overall singlet, correlated
Reference Monomer. Excitation of Ref in HEX (Figure S4)
and ACN (Figure 2A) at 650 nm (0−0 band) leads to the
1
triplet pair (TT) via SF. The intensity of the GSB does not
increase during this process, in contrast to what could be
expected for a transition from a state with one subunit excited,
(S1S0), to a state with two subunits excited, 1(TT). This can be
explained by the large coupling in the H-dimers and by a
delocalization of the excitation over both subunits in the
(S1S0)H state. In contrast to the T1 state of Ref, a significant
part of the triplet population decays to the ground state on the
hundreds of picoseconds time scale, visible as a concurrent
decay of the GSB and triplet bands.
SF is considerably slowed down upon an increase in the
interchromophore distance via cation binding (Figure 2C).
The early TA spectra recorded with 18C6⊂Ba2+ in ACN show
only the occurrence of solvent relaxation as with Ref. SF takes
place on the hundreds of picoseconds time scale, as indicated
by the NIR triplet bands at 875 and 993 nm as well as the
visible ESA band which shifts toward 498 nm. In comparison
to 18C6, not only is SF slower but also the decay of the triplet
pair population is slowed down, as shown by a nearly constant
GSB intensity within the 0−2 ns time window of these
measurements.
Excitation-Wavelength Dependence. The above results
were obtained upon 650 nm excitation, where both H- and M-
dimers absorb. TA measurements in ACN were repeated with
pump pulses at 675 and 690 nm. A marked excitation
wavelength, λex, dependence of the early excited-state dynamics
of 18C6 can be observed, as illustrated by the TA spectra
recorded at 2 ps (Figure 3A). The contribution of the triplet
ESA around 500 nm increases significantly with λex, suggesting
that SF is faster or more efficient upon red-edge illumination:
i.e., upon selective excitation of H-dimers. Because of the
presence of many different conformers, each kinetic process
cannot be quantified by a single rate constant. As a
consequence, global analysis of the TA data with a target
model, which would allow obtaining the spectra of the
individual species/states involved as well as the rate constants
associated with the different transitions,61,62 is not possible.
Because of this and the strong overlap of the singlet and triplet
ESA bands, the effect of λex on the SF dynamics was visualized
by looking at the ratio of the intensity-normalized time profiles
around 498 nm (triplet excited state, Figure 3D) and around
535 nm (singlet excited state, Figure 3C): RT/S = ΔAT/ΔAS
(Figure 3E). The increase in RT/S(t) reflects the transition
from the photopopulated singlet state, (S1S0), to the correlated
triplet pair, 1(TT). In the case of 18C6, the increase in RT/S(t)
is a good proxy for the SF dynamics, which is however not
generally true and depends on the spectral characteristics of
the two transient species (see section 2.6 in the Supporting
Information).
Figure 2. Femtosecond transient absorption spectra measured with
Ref (A), 18C6 (B), and 18C6⊂Ba2+ (C) in ACN upon 650 nm
excitation. The inset highlights the wavelength region in the near-
infrared region where triplet absorption can be observed. The
stationary absorption and stimulated emission spectra are shown in
(A) to indicate the contribution of ground-state bleach (red) and
stimulated emission (blue).
prompt appearance of excited-state absorption (ESA) features
at around 470, 550, and 1500 nm, which together with the
ground-state bleach (GSB) and the stimulated emission (SE)
can be attributed to the population of the S1 state. In HEX,
these bands decay on the nanosecond time scale, in agreement
with the fluorescence lifetime (Figure S2). Afterward, a weak
ESA band, which can be assigned to the T1 state, is visible at
500 nm. In ACN, a shift of the ESA and SE bands occurring
during the first few picoseconds, absent in HEX, can be
attributed to solvent relaxation.60 The subsequent decay of the
S1 state population is significantly faster than in HEX, due to
the smaller S1 ← S0 gap (see section 2.3 in the Supporting
Information), and consequently the T1 band is not visible.
However, the spectrum and lifetime of the T1 state in ACN
could be obtained by triplet energy transfer from naphthale-
nediimide. The T1 state is characterized by a prominent band
Increasing the excitation wavelength results in a higher RT/S
value already at 2 ps, as shown in Figure 3A,E. A similar
wavelength dependence can be observed in HEX, whereas no
2364
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Figure 3. Excitation wavelength (λex) dependence of transient absorption (TA) spectra of 18C6 in HEX (top), THF (middle), and ACN
(bottom). (A) Comparison of visible TA spectra at 2 ps with the wavelengths used for the time profiles. (B) Near-infrared TA spectra at different
time delays upon excitation at 675 nm. (C, D) Time profiles of the singlet (S) and triplet (T) features. (E) Ratio of triplet and singlet time profiles,
RT/S(t), tracking the conversion via SF as well as the decay via the singlet annihilation channel.
difference can be detected in THF on this time scale (Figure
3A,E). This result is fully consistent with the coexistence of H-
and M-dimers in ACN and HEX and with the predominance
of the latter in THF, as inferred from the stationary absorption
spectra and the MD simulations.
depend on the solvent polarity and that the observed similarity
of time scales for polar solvation and SF in ACN is most likely
a coincidence. Here, the solvent only determines the
probability to excite H-dimers, which is higher in ACN than
in HEX. The identical SF dynamics in these solvents indicate
that the possible larger admixture of the |CT⟩ state to the
photoexcited state in ACN, as suggested by the smaller
transition dipole moment, does not play a decisive role in the
SF dynamics of these H-dimers. Similarly, SB-CS, triggered by
asymmetric solvation of the two chromophores, is not
involved. This result is consistent with our recent investigation
on a bichromophore similar to 18C6 but with perylene instead
of pentacene. We found that SB-CS in strongly coupled dimers
is overruled by relaxation along an interchromophore
coordinate toward the excimer.59
This independence on solvent polarity points to intra-
molecular modes as the main constituents of the SF
coordinate. It has has been suggested that SF in crystalline
pentacenes occurs upon relaxation of the photopopulated
singlet state toward the excimer. During relaxation, the singlet
excited state becomes quasi-degenerate with the triplet pair
state allowing for a nonadiabatic transition in ∼70 fs.66
According to this hypothesis, SF in the 18C6 H-dimers could
take place upon structural relaxation of the photopopulated
singlet state, (S1S0)H. The absence of an excimer emission can
be explained by SF occurring before this state is significantly
populated, the rate-limiting step being relaxation rather than
the SF step itself.
Intensity normalization of the RT/S(t) profiles in ACN
reveals that the λex dependence observed during the first few
picoseconds is not due to different rise times but only to
different amplitudes of this rise (Figure S13). This supports
our above assumption about two main subpopulations, namely
the strongly (H) and the weakly (M) coupled dimers.
Therefore, red-edge illumination in ACN and HEX increases
the probability to excite H-dimers, which undergo fast SF (<2
ps). In contrast, a higher fraction of M-dimers, with slower SF
dynamics (∼500 ps), is excited at shorter wavelengths.
SF in H-Dimers: Role of the Solvent and Intramolecular
Modes. The SF dynamics and efficiency in various covalently
linked pentacene dimers was reported to be sensitive to the
solvent polarity. This was attributed to changes of the relative
energies of the |LE⟩ and |CT⟩ states.17,19,40,43,63,64 Depending
on the molecular system, either a two-step process via a
distinct symmetry-broken charge-separated (SB-CS) inter-
mediate19,63,64 or a one-step SF involving significant |CT⟩
admixing to the (S1S0)H state is invoked.19,64,65
Here, the solvent has a strong influence on the conformation
of the bichromophore and hence on the coupling and on SF. In
order to disentangle the direct polarity effect of the solvent on
the SF dynamics from this indirect conformational effect, we
now only consider the H-dimers present in ACN and HEX.
Figure S11 reveals that the rise of RT/S(t) in ACN matches
closely the early ESA peak shift at 470 nm measured with Ref
and assigned to solvent relaxation. This suggests that, in ACN,
SF in H-dimers occurs on the same time scale as diffusive
solvent motion. However, an intensity normalization of RT/S(t)
in ACN and in HEX shows that, although RT/S(t) reaches
significantly larger values in ACN than in HEX independently
of λex, the rise dynamics are the same in both solvents. One can
thus conclude that the SF dynamics in H-dimers does not
SF in M-Dimers: Role of Structural Fluctuations. In THF,
T/S(t) does not exhibit the initial ∼2 ps rise, in agreement
R
with a predominance of M-dimers in this solvent. Here, SF is
not ultrafast but occurs on the hundreds of picoseconds time
scale. The slow-rising component of RT/S(t) is also present, but
with a smaller amplitude, in ACN and HEX. It can be
associated with “static” SF in dimers with a correspondingly
small coupling and/or with “dynamic” SF occurring after
conformational fluctuations of dimers with an initially smaller
coupling. This modulation of the coupling involves a relatively
2365
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
large amplitude motion of the pentacenes and can thus be
expected to depend on the solvent viscosity, η. To confirm this,
TA measurements were repeated in dodecane (N10, η = 0.9
cP) and tetradecane (N14, η = 2.1 cP). As these longer alkanes
have the same polarity as HEX (η = 0.3 cP), the distribution of
populations is expected to be similar but the interconversion
by structural fluctuations slower.
is required to achieve appropriate coupling. For the
18C6⊂Ba2+ complex, close contact between the pentacene
subunits is energetically unfavorable and only the (S1S0)M state
is photopopulated, resulting in slow SF (∼500 ps).
Nature and Dynamics of the Triplet Pairs. Different
Types of Triplet Pairs and Triplet Annihilation via the
Singlet Channel. The NIR triplet band at 993 nm measured 2
ps after excitation of 18C6 in ACN and HEX is characterized
by a shoulder on its blue side (black arrow in Figure 3B),
which disappears on the hundreds of picoseconds time scale,
together with a partial decrease in the GSB. Similar spectral
features were reported previously30 and attributed to two main
types of correlated triplet pairs. The shoulder was assigned to
strongly coupled correlated triplet pairs, 1(TT), populated
from the (S1S0)H state. In contrast, the 1(T···T) pairs
populated from the (S1S0)M state are weakly coupled and
longer lived. This hypothesis is supported here by the presence
of the shoulder in ACN and HEX, where the (S1S0)H state is
populated, but not in THF. Recent quantum-chemical studies
predict absorption of the correlated triplet pair in the 700−900
nm region with an oscillator strength increasing with the
interchromophore coupling.67
As illustrated in Figure 4, RT/S(t) at early time is nearly the
same in all three alkanes, pointing to a similar distribution of
The normalized TA profiles at the triplet maximum (Figure
3D) illustrate the influence of the interchromophore coupling
on the decay of the triplet population. In ACN and HEX, the
triplet band decays faster when H-dimers are photoselected by
red-edge excitation. Mechanistically, the sub-nanosecond decay
of the triplet signal is assigned to the internal conversion of the
1(TT) pairs via the singlet channel (eq 2), in line with previous
studies that point to a faster decay with increased coupling.26,39
This decay in ACN is even faster than that of the S1 state of the
parent Ref (Figure S15). This can be accounted for by a
smaller energy gap, since the 1(TT) state of 18C6 is lower than
the S1 state of Ref, as well as by a higher density of vibrational
states in the bichromophore.68
Figure 4. (left) Viscosity dependence of the femtosecond−pico-
second transient absorption dynamics of 18C6 in HEX, N10, and
N14 upon 650 nm excitation: triplet−singlet ratio (RT/S(t), top),
singlet (S, middle) and triplet absorption (T, bottom). (right)
Relative population of the singlet and triplet excited states of 18C6 in
HEX, N10, and N14 extracted from picosecond−microsecond
transient absorption upon excitation at 532 nm. The concentrations
are normalized to the total concentration at early times (gray).
Even though the triplet signal is decaying after about 100 ps
in ACN, RT/S(t) is still increasing, independently of λex. This
rise is due to the buildup of 1(T···T) via the slow SF occurring
in the M-dimers. Since the latter is controlled by large-
conformers. The small difference at 2 ps (inset) is most likely
due to a shift of the absorption band associated with a variation
of the refractive index and leading to the photoselection of
slightly different subpopulations (Figure S8). Normalization of
1
amplitude structural fluctuations, the decay of (TT) and the
rise of 1(T···T) can be distinguished by increasing the viscosity.
As illustrated in Figure 4, the decay of ΔAS on the hundreds of
picoseconds time scale slows down with increasing viscosity
RT/S(t) shows that the fast-rising component is insensitive to η
(Figure S14). Consequently, SF in H-dimers is independent
not only of polarity but also of viscosity. If the structural
relaxation toward the excimer invoked above is the rate-
limiting step, it does not involve large-amplitude motion.
The slower-rising component of RT/S(t) associated with SF
in M-dimers slows down further upon increasing η. One can
thus conclude that SF on this time scale occurs upon large-
amplitude structural fluctuations toward sufficiently coupled
conformations. The λex dependence of RT/S(t) in THF is
absent at early times but becomes visible on the time scale at
which the viscosity dependence appears. This suggests that,
although no H-dimers are present in THF, some photo-
selection is still possible with the less coupled dimers.
To sum up, SF in 18C6 can be discussed in terms of two
subpopulations, strongly coupled H-dimers and weakly
coupled M-dimers. The fraction of these subpopulations
probed in TA depends on λex as well as on the solvent.
Excitation of the H-dimers to the delocalized (S1S0)H state
leads to ultrafast SF (<2 ps) driven by nondiffusive structural
motion. In M-dimers, excitation is rather localized on one
chromophore and sampling of the conformational phase space
1
due to a slower (S1S0)M → (T···T) SF, whereas the early
1
decay of ΔAS due to (S1S0)H → (TT) is not affected.
Separation, Decorrelation, and Decay of the Triplet Pairs.
1
Most of the (TT) population decays through the singlet
channel, which is detrimental to a large free triplet yield.
1
Spatial separation of the nascent triplet pairs, mainly (T···T),
allows escaping this process and lengthens their lifetime
sufficiently to enable spin conversion toward free triplets,
T1+T1. Since the pentacenes are covalently linked, they cannot
separate completely but sample the available conformational
space before returning to a closer contact conformation at a
later time (secondary encounter). The duration of these
excursions out of the contact distance depends on the solvent
viscosity and on the free-energy surface. The fate of the triplet
pair upon a secondary encounter depends on its underlying
1
spin dynamics, which determines if T1+T1 rephases to (TT),
34,35,69,70
5
3(TT), or (TT).
Although all of these different triplet pairs cannot be
distinguished spectrally with TA, some insight into the
underlying spin dynamics can be obtained by looking at the
2366
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
influence of the accessible conformational space and the
solvent viscosity on the time evolution of the triplet signal. We
thus turned to picosecond−microsecond TA upon 532 nm
the T1 state of Ref populated by triplet sensitization (Figure
S16).
The TA spectra can be reproduced as a linear combination
of three components: (i) the singlet excited-state spectrum, (ii)
the early triplet spectrum related to the fast-decay component,
and (iii) the late triplet spectrum associated with the plateau
and the slow decay (Figure 5). In all cases, the late triplet
spectrum (green) is slightly red shifted in comparison to the
early triplet spectrum (blue). This trend was already observed
with triplet pairs, which can separate spatially by triplet energy
hopping and therefore no longer couple.23
1
excitation to investigate the fate of the (T···T) pairs. Figures
S18−S28 show that, in all solvents, the singlet ESA features
around 470 and 550 nm decay entirely within a few tens of
nanoseconds. Afterward, the TA spectra exhibit only the triplet
ESA bands and the GSB. A model-free analysis of these
spectra, based on spectral decomposition,62,71 was applied to
extract the time evolution of the singlet and triplet excited-state
populations (see section 2.7 in the Supporting Information).
The decay of the triplet population is bimodal for
18C6⊂Ba2+ in ACN as well as for 18C6, irrespective of the
polarity and viscosity of the solvent (Figures 4 and 5 and
Interpretation of the Bimodal Decay. As discussed in the
Introduction, the origin of the bimodal decay is debated, with
the two main hypotheses being compared in Figure 6A.
Hypothesis 1 (H1) attributes the plateau and the subsequent
slow-decay component to the lone triplet, S0T1, populated via
the triplet channel after secondary encounters, whereas
hypothesis 2 (H2) assigns them to T1+T1 after the loss of
1
1
spin correlation of the (TT) and (T···T) pairs.
Two of our experimental observations are in favor of H1
over H2. First, according to H1, Rpl, defined as the late triplet
population relative to maximum triplet population, cannot
exceed 0.5, whereas it can be as high as 1 with H2. Here, even
with 18C6 in THF and especially with 18C6⊂Ba2+, where
only M-dimers are present, Rpl, determined from the height of
the plateau relative to the maximum triplet signal, is less than
0.5 (Table 1). It is furthermore smaller for 18C6⊂Ba2+ than
for 18C6 in THF, although the weaker coupling in
18C6⊂Ba2+ should favor decoherence rather than decay via
the singlet channel.
Second, as shown in Figure S16, the decay of the late triplet
population occurs on the same time scale as that of the T1 state
of Ref and does not show any correlation with solvent viscosity
(Figure 4 and Figure S17). This observation is not compatible
with the late triplet signal being due to T1+T1 (H2). Indeed,
the conformational space in alkanes allows for close contact.
Consequently, the T1+T1 pairs can undergo secondary
encounters, enabling their decay via the singlet or triplet
channels, thus leading to a decrease in the overall triplet
Figure 5. Picosecond−microsecond transient absorption measure-
ments with 18C6 in THF (top) and 18C6⊂Ba2+ in ACN (bottom).
(left) basis spectra (red, singlet excited state; blue, early triplet; green,
late triplet) used to reproduce the TA spectra. (right) Time evolution
of the overall excited-state population (gray) and of its individual
constituents: singlet (red), early triplet (blue), and late triplet (green)
excited-state populations.
Figure S15). The two decay components are separated by a
plateau, and the slow-decay component matches the decay of
Figure 6. (A) Comparison of the singlet fission cascade based on the two hypotheses suggested in the literature. Species which cannot be spectrally
differentiated are represented with the same color. The present study supports hypothesis 1. (B) Schematic representation of the effect of
conformational space and solvent viscosity on the interchromophore coupling, which governs the singlet fission yield. Coupling increases from left
to right. The orange dots stand for weakly coupled dimers whose excited-state dynamics is strongly entangled with structural fluctuations
(represented as dashed lines); the red dots designate highly coupled dimers, whose excited-state dynamics is faster than that associated with
diffusive structural changes.
2367
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
quantitatively via the singlet channel even in the least viscous
solvent. Second, higher viscosity also slows down structural
fluctuations of M-dimers that lead to conformations with
increased coupling. Given that the triplet pairs can be
populated over a wide range of distances, diffusion-controlled
fluctuations of (S1S0)M occur until the coupling between the
TIPS-pentacenes is large enough for SF to take place without
further diffusion. Therefore, the interchromophore distance
and thus the survival probability of the triplet pair are on
average higher in viscous than in nonviscous solvents (Figure
6B). A similar effect is known for bimolecular photoinduced
electron transfer, where high viscosity favors ‘remote’ electron
transfer and thus produces longer-lived ion pairs.71 As
illustrated in Figure 4 and Figure S17, the rise of the triplet
signal slows down with increasing viscosity, whereas ΦSF
becomes larger. Therefore, as with 18C6⊂Ba2+ in ACN, the
effect of a higher survival probability, here due to triplet
formation at a geometry with low coupling, outweighs the
effect of a slower triplet population.
Table 1. Comparison of the Triplet Concentration at the
Plateau Relative to the Maximum Triplet Concentration,
Rpl, and to the Initial Concentration of the Photo-Populated
a
Singlet State, ΦS T
0
1
Rpl
Φ
S0T1
ΦSF
ACN
∼0.1
>0.68
>0.34
>0.36
>0.48
>0.54
2
ACN + Ba2+
THF
0.41
0.49
0.30
0.35
0.36
0.34
0.17
0.18
0.24
0.27
HEX
N10
N14
quantitative
≤0.5
≤1
a
Doubling the latter gives a lower limit of the singlet fission quantum
yield, ΦSF, on the basis of eq 2. Values for quantitative SF based on
hypothesis 1 are given for comparison.
population. Due to these possible reencounters, the late triplet
decay should not only be faster than that of the T1 state of Ref
but should also be significantly faster in HEX than in N14, in
contrast with the observation.
CONCLUSION
■
By using a flexible crownether-like backbone, we could study
SF in a TIPS-pentacene dimer over a wide range of
interchromophore distances. This was achieved by exploiting
the influence of the solvent and the formation of a host−guest
complex on the conformational phase space. MD simulations
as well as stationary spectroscopic measurements point to a
distribution of conformers that can be sorted into strongly
coupled H-dimers and weakly coupled M-dimers. Their
slightly different absorption spectra allow for photoselection
in λex-dependent experiments. We could spectrally and
SF Yield As a Function of Distance and Viscosity.
According to H1, the fraction of photopopulated molecules
which decay via the triplet channel, ΦS T , allows estimating a
0
1
lower limit of the singlet fission quantum yield, ΦSF, defined as
the triplet yield associated with the T1+T1 pairs.36 If we assume
a quantitative decay of T1+T1 via the triplet channel, ΦSF is
equal to twice ΦS T , which is reflected by the height of the
0
1
plateau in comparison to the concentration of the photo-
populated singlet state (Table 1). A detailed description on
how ΦSF is extracted is given in section 2.8 in the Supporting
Information.
1
kinetically differentiate two types of triplet pairs, (TT), and
1(T···T), differing in their coupling. SF in H-dimers populates
the 1(TT) state on the sub-picosecond time scale with solvent-
polarity- and viscosity-independent dynamics. The absence of a
polarity effect reveals that the solvation coordinate does not
play a significant role and that SF in these strongly coupled
dimers is driven by intramolecular modes. SF in M-dimers
occurs on the hundreds of picoseconds time scale and
populates the 1(T···T) state. This process is governed by
large-amplitude structural changes that modulate the coupling
and thus depend on viscosity.
Φ
SF is governed by a tradeoff between the rate of triplet pair
formation and the triplet pair survival, both functions of the
interchromophore distance and coupling (Figure 6B). For
18C6 in ACN, where H-dimers are predominant, triplet
formation is fast, but the survival probability of the resulting
1(TT) pairs is low due to fast annihilation along the singlet
channel (red dots in Figure 6B). Consequently, ΦSF is
estimated to be only around 0.1, much lower than the
theoretical limit of 2.
In THF, where mostly M-dimers are present, ΦSF is
The 1(TT) pairs decay mainly via internal conversion to the
ground state with only a minor fraction (∼10%) evolving to
T1+T1 via conformational changes and spin conversion. The
significantly enhanced due to the increased survival probability
1
of (T···T) formed at larger distances (orange dots in Figure
1
6B). However, large-amplitude conformational fluctuations are
required for SF to take place in M-dimers. As the latter occur
on the same time scale as the intrinsic decay of the S1 state,
part of the excited-state population is lost, explaining the
deviation from the maximum yield of 2. The highest ΦSF is
observed with 18C6⊂Ba2+ in ACN, where close-contact
conformers are structurally excluded and the triplet formation
is further slowed down. This higher yield for 18C6⊂Ba2+ in
ACN in comparison to that in THF shows that, in this case,
the effect of the longer triplet survival surpasses that of the
slower SF and the ensuing loss via the intrinsic decay of S1.
In alkanes, where both H- and M-dimers are excited, the SF
yield increases with solvent viscosity. A higher viscosity can
have two opposite effects with different impacts on H- and M-
latter channel is predominant for the (T···T) pairs. However,
the ensuing T1+T1 pairs have a limited lifetime due to the
occurrence of structural fluctuations enabling secondary
encounters and annihilation via the singlet or triplet channel
giving S0T1. This mechanism, proposed by Campos and Sfeir
et al. to account for the bimodal decay of the triplet
population,36 is confirmed here by our viscosity-dependent
measurements.
The structure−property relationships acquired from the
systematic study of this flexible crown ether based dimer can
be summarized as follows. A close-contact π-stacked H-dimer
should be prevented, and the center to center distance should
be kept slightly below about 1 nm to achieve an optimal
balance between the rate of triplet formation and the triplet
pair survival. In addition to the structural restriction, we could
show that the solvent viscosity can be used to dynamically
control the average coupling of the ensuing triplet pair and
increase ΦSF. Further fine tuning toward an optimal coupling,
1
conformers. First, it can slow down the separation of (TT)
pairs generated from (S1S0)H, hence decreasing the triplet
survival probability and ΦSF. However, this trend is not
1
experimentally observed, probably since (TT) decays nearly
2368
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(5) Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider,
W. G. Laser Generation of Excitons and Fluorescence in Anthracene
Crystals. J. Chem. Phys. 1965, 42, 330−342.
(6) Marciniak, H.; Pugliesi, I.; Nickel, B.; Lochbrunner, S. Ultrafast
Singlet and Triplet Dynamics in Microcrystalline Pentacene Films.
Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 235318.
(7) Wilson, M. W. B.; Rao, A.; Clark, J.; Kumar, R. S. S.; Brida, D.;
Cerullo, G.; Friend, R. H. Ultrafast Dynamics of Exciton Fission in
Polycrystalline Pentacene. J. Am. Chem. Soc. 2011, 133, 11830−
11833.
(8) Burdett, J. J.; Gosztola, D.; Bardeen, C. J. The Dependence of
Singlet Exciton Relaxation on Excitation Density and Temperature in
Polycrystalline Tetracene Thin Films: Kinetic Evidence for a Dark
Intermediate State and Implications for Singlet Fission. J. Chem. Phys.
2011, 135, 214508.
(9) Burdett, J. J.; Bardeen, C. J. Quantum Beats in Crystalline
Tetracene Delayed Fluorescence Due to Triplet Pair Coherences
Produced by Direct Singlet Fission. J. Am. Chem. Soc. 2012, 134,
8597−8607.
(10) Ma, L.; Zhang, K.; Kloc, C.; Sun, H.; Michel-Beyerle, M. E.;
Gurzadyan, G. G. Singlet Fission in Rubrene Single Crystal: Direct
Observation by Femtosecond Pump− Probe Spectroscopy. Phys.
Chem. Chem. Phys. 2012, 14, 8307−8312.
(11) Burdett, J. J.; Bardeen, C. J. The Dynamics of Singlet Fission in
Crystalline Tetracene and Covalent Analogs. Acc. Chem. Res. 2013, 46,
1312−1320.
(12) Piland, G. B.; Bardeen, C. J. How Morphology Affects Singlet
Fission in Crystalline Tetracene. J. Phys. Chem. Lett. 2015, 6, 1841−
1846.
(13) Musser, A. J.; Al-Hashimi, M.; Maiuri, M.; Brida, D.; Heeney,
M.; Cerullo, G.; Friend, R. H.; Clark, J. Activated Singlet Exciton
Fission in a Semiconducting Polymer. J. Am. Chem. Soc. 2013, 135,
12747−12754.
(14) Kasai, Y.; Tamai, Y.; Ohkita, H.; Benten, H.; Ito, S. Ultrafast
Singlet Fission in a Push−Pull Low-Bandgap Polymer Film. J. Am.
Chem. Soc. 2015, 137, 15980−15983.
(15) Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.;
Zhu, X.-Y.; Campos, L. M.; Sfeir, M. Y. A Design Strategy for
Intramolecular Singlet Fission Mediated by Charge-Transfer States in
Donor− Acceptor Organic Materials. Nat. Mater. 2015, 14, 426−433.
(16) Hu, J.; Xu, K.; Shen, L.; Wu, Q.; He, G.; Wang, J.-Y.; Pei, J.;
Xia, J.; Sfeir, M. Y. New Insights into the Design of Conjugated
Polymers for Intramolecular Singlet Fission. Nat. Commun. 2018, 9,
2999.
and thus higher SF yield, should be feasible thanks to a
judicious selection of solvent according to polarity and
viscosity. Additionally, the band gap of pentacene should not
be reduced by functionalization, in order to avoid a shortening
of the singlet lifetime as well as a potential acceleration of the
singlet and triplet decay channels, which is detrimental to the
overall ΦSF.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12384.
■
sı
Synthesis and characterization of Ref and 18C6,
experimental details, details of the molecular dynamics
simulations, photophysical properties of Ref, additional
spectroscopic data on 18C6, analysis of the picosecond−
microsecond transient absorption spectra, and estima-
tion of the SF yield (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
̂
Jérome Lacour − Department of Organic Chemistry,
University of Geneva, CH-1211 Geneva, Switzerland;
orcid.org/0000-0001-6247-8059;
Email: jerome.lacour@unige.ch
Eric Vauthey − Department of Physical Chemistry, University
of Geneva, CH-1211 Geneva, Switzerland; orcid.org/
0000-0002-9580-9683; Email: eric.vauthey@unige.ch
Authors
Alexander Aster − Department of Physical Chemistry,
University of Geneva, CH-1211 Geneva, Switzerland
Francesco Zinna − Department of Organic Chemistry,
University of Geneva, CH-1211 Geneva, Switzerland
Christopher Rumble − Department of Physical Chemistry,
University of Geneva, CH-1211 Geneva, Switzerland;
orcid.org/0000-0003-1757-8960
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12384
(17) Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.;
Casillas, R.; Basel, B. S.; Thoss, M.; Tykwinski, R. R.; Guldi, D. M.
Singlet Fission in Pentacene Dimers. Proc. Natl. Acad. Sci. U. S. A.
2015, 112, 5325−5330.
(18) 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.; et al. A Direct Mechanism of Ultrafast Intramolecular
Singlet Fission in Pentacene Dimers. ACS Cent. Sci. 2016, 2, 316−
324.
(19) 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. Tuning
the Role of Charge-Transfer States in Intramolecular Singlet Exciton
Fission through Side-Group Engineering. Nat. Commun. 2016, 7,
13622.
(20) Sakuma, T.; Sakai, H.; Araki, Y.; Mori, T.; Wada, T.;
Tkachenko, N. V.; Hasobe, T. Long-Lived Triplet Excited States of
Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission. J.
Phys. Chem. A 2016, 120, 1867−1875.
Author Contributions
§A.A. and F.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the “Fonds National Suisse de la Recherche
Scientifique” (200020-184843 (J.L.) and 200020-184607
(E.V.)) and the University of Geneva for their financial
support. Data can be downloaded from 10.26037/
yareta:7dwzkrfo6zd4domn3tlbuvrloe.
REFERENCES
■
(1) Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110,
6891−6936.
(2) Smith, M. B.; Michl, J. Recent Advances in Singlet Fission. Annu.
Rev. Phys. Chem. 2013, 64, 361−386.
(21) 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.; et al. Unified Model for Singlet Fission within a Non-Conjugated
Covalent Pentacene Dimer. Nat. Commun. 2017, 8, 15171.
(22) Tayebjee, M. J. Y.; Sanders, S. N.; Kumarasamy, E.; Campos, L.
M.; Sfeir, M. Y.; McCamey, D. R. Quintet Multiexciton Dynamics in
Singlet Fission. Nat. Phys. 2017, 13, 182−188.
(3) Shockley, W.; Queisser, H. J. Detailed Balance Limit of
Efficiency of P-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510−
519.
(4) Tayebjee, M. J. Y.; McCamey, D. R.; Schmidt, T. W. Beyond
Shockley−Queisser: Molecular Approaches to High-Efficiency Photo-
voltaics. J. Phys. Chem. Lett. 2015, 6, 2367−2378.
2369
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(23) Hetzer, C.; Guldi, D. M.; Tykwinski, R. R. Pentacene Dimers as
a Critical Tool for the Investigation of Intramolecular Singlet Fission.
Chem. - Eur. J. 2018, 24, 8245−8257.
Yablon, L. M.; Low, J. Z.; Paley, D. W.; et al. Tuning Singlet Fission in
π-Bridge-π Chromophores. J. Am. Chem. Soc. 2017, 139, 12488−
12494.
(43) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Reddy, S. R.; Phelan, B.
T.; Krzyaniak, M. D.; Volland, M. K.; Coto, P. B.; Young, R. M.;
Clark, T.; et al. Evidence for Charge-Transfer Mediation in the
Primary Events of Singlet Fission in a Weakly Coupled Pentacene
Dimer. Chem. 2018, 4, 1092−1111.
(44) Papadopoulos, I.; Zirzlmeier, J.; Hetzer, C.; Bae, Y. J.;
Krzyaniak, M. D.; Wasielewski, M. R.; Clark, T.; Tykwinski, R. R.;
Guldi, D. M. Varying the Interpentacene Electronic Coupling to Tune
Singlet Fission. J. Am. Chem. Soc. 2019, 141, 6191−6203.
(45) Homberg, A.; Lacour, J. From Reactive Carbenes to Chiral
Polyether Macrocycles in Two Steps − Synthesis and Applications
Made Easy? Chem. Sci. 2020, 11, 6362−6369.
(46) Poggiali, D.; Homberg, A.; Lathion, T.; Piguet, C.; Lacour, J.
Kinetics of Rh(II)-Catalyzed α-Diazo-β-Ketoester Decomposition
and Application to the [3 + 6+3 + 6] Synthesis of Macrocycles on a
Large Scale and at Low Catalyst Loadings. ACS Catal. 2016, 6, 4877−
4881.
(24) Yablon, L. M.; Sanders, S. N.; Li, H.; Parenti, K. R.;
Kumarasamy, E.; Fallon, K. J.; Hore, M. J. A.; Cacciuto, A.; Sfeir,
M. Y.; Campos, L. M. Persistent Multiexcitons from Polymers with
Pendent Pentacenes. J. Am. Chem. Soc. 2019, 141, 9564−9569.
(25) Basel, B. S.; Hetzer, C.; Zirzlmeier, J.; Thiel, D.; Guldi, R.;
Hampel, F.; Kahnt, A.; Clark, T.; Guldi, D. M.; Tykwinski, R. R.
Davydov Splitting and Singlet Fission in Excitonically Coupled
Pentacene Dimers. Chem. Sci. 2019, 10, 3854−3863.
(26) Korovina, N. V.; Pompetti, N. F.; Johnson, J. C. Lessons from
Intramolecular Singlet Fission with Covalently Bound Chromophores.
J. Chem. Phys. 2020, 152, 040904.
(27) Ribson, R. D.; Choi, G.; Hadt, R. G.; Agapie, T. Controlling
Singlet Fission with Coordination Chemistry-Induced Assembly of
Dipyridyl Pyrrole Bipentacenes. ACS Cent. Sci. 2020, 6, 2088−2096.
(28) Bergman, H. M.; Kiel, G. R.; Witzke, R. J.; Nenon, D. P.;
Schwartzberg, A. M.; Liu, Y.; Tilley, T. D. Shape-Selective Synthesis
of Pentacene Macrocycles and the Effect of Geometry on Singlet
Fission. J. Am. Chem. Soc. 2020, 142, 19850−19855.
(29) Pensack, R. D.; Ostroumov, E. E.; Tilley, A. J.; Mazza, S.;
Grieco, C.; Thorley, K. J.; Asbury, J. B.; Seferos, D. S.; Anthony, J. E.;
Scholes, G. D. Observation of Two Triplet-Pair Intermediates in
Singlet Exciton Fission. J. Phys. Chem. Lett. 2016, 7, 2370−2375.
(30) Pensack, R. D.; Tilley, A. J.; Grieco, C.; Purdum, G. E.;
Ostroumov, E. E.; Granger, D. B.; Oblinsky, D. G.; Dean, J. C.;
Doucette, G. S.; Asbury, J. B.; et al. Striking the Right Balance of
Intermolecular Coupling for High-Efficiency Singlet Fission. Chem.
Sci. 2018, 9, 6240−6259.
(31) Sandoval-Salinas, M. E.; Carreras, A.; Casado, J.; Casanova, D.
Singlet Fission in Spiroconjugated Dimers. J. Chem. Phys. 2019, 150,
204306.
(32) Miyata, K.; Conrad-Burton, F. S.; Geyer, F. L.; Zhu, X.-Y.
Triplet Pair States in Singlet Fission. Chem. Rev. 2019, 119, 4261−
4292.
(33) Sanders, S. N.; Pun, A. B.; Parenti, K. R.; Kumarasamy, E.;
Yablon, L. M.; Sfeir, M. Y.; Campos, L. M. Understanding the Bound
Triplet-Pair State in Singlet Fission. Chem. 2019, 5, 1988−2005.
(34) Bayliss, S. L.; Chepelianskii, A. D.; Sepe, A.; Walker, B. J.;
Ehrler, B.; Bruzek, M. J.; Anthony, J. E.; Greenham, N. C. Geminate
and Nongeminate Recombination of Triplet Excitons Formed by
Singlet Fission. Phys. Rev. Lett. 2014, 112, 238701.
(35) Scholes, G. D. Correlated Pair States Formed by Singlet Fission
and Exciton−Exciton Annihilation. J. Phys. Chem. A 2015, 119,
12699−12705.
(36) Pun, A. B.; Asadpoordarvish, A.; Kumarasamy, E.; Tayebjee, M.
J. Y.; Niesner, D.; McCamey, D. R.; Sanders, S. N.; Campos, L. M.;
Sfeir, M. Y. Ultra-Fast Intramolecular Singlet Fission to Persistent
Multiexcitons by Molecular Design. Nat. Chem. 2019, 11, 821−828.
(37) Taffet, E. J.; Beljonne, D.; Scholes, G. D. Overlap-Driven
Splitting of Triplet Pairs in Singlet Fission. J. Am. Chem. Soc. 2020,
142, 20040−20047.
(38) Merrifield, R. E. Magnetic Effects on Triplet Exciton
Interactions. Pure Appl. Chem. 1971, 27, 481−498.
(39) 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.; et al.
Quantitative Intramolecular Singlet Fission in Bipentacenes. J. Am.
Chem. Soc. 2015, 137, 8965−8972.
(40) 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. Solution-Based Intramolecular Singlet Fission in Cross-
Conjugated Pentacene Dimers. Nanoscale 2016, 8, 10113−10123.
(41) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Appavoo, K.;
Steigerwald, M. L.; Campos, L. M.; Sfeir, M. Y. Exciton Correlations
in Intramolecular Singlet Fission. J. Am. Chem. Soc. 2016, 138, 7289−
7297.
(47) Vishe, M.; Hrdina, R.; Poblador-Bahamonde, A. I.; Besnard, C.;
Guénée, L.; Burgi, T.; Lacour, J. Remote Stereoselective Deconjuga-
tion of α,β-Unsaturated Esters by Simple Amidation Reactions. Chem.
Sci. 2015, 6, 4923−4928.
̈
(48) Zeghida, W.; Besnard, C.; Lacour, J. Rhodium(II)-Catalyzed
One-Pot Four-Component Synthesis of Functionalized Polyether
Macrocycles at High Concentration. Angew. Chem., Int. Ed. 2010, 49,
7253−7256.
(49) Spano, F. C. The Spectral Signatures of Frenkel Polarons in H-
and J-Aggregates. Acc. Chem. Res. 2010, 43, 429−439.
(50) Beljonne, D.; Yamagata, H.; Brédas, J. L.; Spano, F. C.; Olivier,
Y. Charge-Transfer Excitations Steer the Davydov Splitting and
Mediate Singlet Exciton Fission in Pentacene. Phys. Rev. Lett. 2013,
110, 226402.
(51) Hestand, N. J.; Spano, F. C. Molecular Aggregate Photophysics
beyond the Kasha Model: Novel Design Principles for Organic
Materials. Acc. Chem. Res. 2017, 50, 341−350.
(52) Hestand, N. J.; Spano, F. C. Expanded Theory of H- and J-
Molecular Aggregates: The Effects of Vibronic Coupling and
Intermolecular Charge Transfer. Chem. Rev. 2018, 118, 7069−7163.
(53) Hunter, C. A. Meldola Lecture. The Role of Aromatic
Interactions in Molecular Recognition. Chem. Soc. Rev. 1994, 23,
101−109.
(54) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. Aromatic
Interactions. J. Chem. Soc., Perkin Trans. 2 2001, 651−669.
(55) Martinez, C. R.; Iverson, B. L. Rethinking the Term “Pi-
Stacking. Chem. Sci. 2012, 3, 2191−2201.
(56) Chen, Z.; Fimmel, B.; Wurthner, F. Solvent and Substituent
̈
Effects on Aggregation Constants of Perylene Bisimide π-Stacks − a
Linear Free Energy Relationship Analysis. Org. Biomol. Chem. 2012,
10, 5845−5855.
(57) Casanova, D. Theoretical Investigations of the Perylene
Electronic Structure: Monomer, Dimers, and Excimers. Int. J.
Quantum Chem. 2015, 115, 442−452.
(58) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. The Exciton Model
in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371−392.
(59) Aster, A.; Licari, G.; Zinna, F.; Brun, E.; Kumpulainen, T.;
Tajkhorshid, E.; Lacour, J.; Vauthey, E. Tuning Symmetry Breaking
Charge Separation in Perylene Bichromophores by Conformational
Control. Chem. Sci. 2019, 10, 10629−10639.
(60) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M.
Subpicosecond Measurements of Polar Solvation Dynamics:
Coumarin 153 Revisited. J. Phys. Chem. 1995, 99, 17311−17337.
(61) van Stokkum, I. H. M.; Larsen, D. S.; van Grondelle, R. Global
and Target Analysis of Time-Resolved Spectra. Biochim. Biophys. Acta,
Bioenerg. 2004, 1657, 82−104.
(62) Beckwith, J. S.; Rumble, C. A.; Vauthey, E. Data Analysis in
Transient Electronic Spectroscopy − an Experimentalist’s View. Int.
Rev. Phys. Chem. 2020, 39, 135−216.
(42) Kumarasamy, E.; Sanders, S. N.; Tayebjee, M. J. Y.;
Asadpoordarvish, A.; Hele, T. J. H.; Fuemmeler, E. G.; Pun, A. B.;
2370
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(63) Lukman, S.; Musser, A. J.; Chen, K.; Athanasopoulos, S.; Yong,
C. K.; Zeng, Z.; Ye, Q.; Chi, C.; Hodgkiss, J. M.; Wu, J.; et al.
Tuneable Singlet Exciton Fission and Triplet−Triplet Annihilation in
an Orthogonal Pentacene Dimer. Adv. Funct. Mater. 2015, 25, 5452−
5461.
(64) Alvertis, A. M.; Lukman, S.; Hele, T. J. H.; Fuemmeler, E. G.;
Feng, J.; Wu, J.; Greenham, N. C.; Chin, A. W.; Musser, A. J.
Switching between Coherent and Incoherent Singlet Fission via
Solvent-Induced Symmetry Breaking. J. Am. Chem. Soc. 2019, 141,
17558−17570.
(65) Margulies, E. A.; Miller, C. E.; Wu, Y.; Ma, L.; Schatz, G. C.;
Young, R. M.; Wasielewski, M. R. Enabling Singlet Fission by
Controlling Intramolecular Charge Transfer in π -Stacked Covalent
Terrylenediimide Dimers. Nat. Chem. 2016, 8, 1120−1125.
(66) Zimmerman, P. M.; Bell, F.; Casanova, D.; Head-Gordon, M.
Mechanism for Singlet Fission in Pentacene and Tetracene: From
Single Exciton to Two Triplets. J. Am. Chem. Soc. 2011, 133, 19944−
19952.
(67) Khan, S.; Mazumdar, S. Theory of Transient Excited State
Absorptions in Pentacene and Derivatives: Triplet−Triplet Biexciton
versus Free Triplets. J. Phys. Chem. Lett. 2017, 8, 5943−5948.
(68) Englman, R.; Jortner, J. The Energy Gap Law for Radiationless
Transitions in Large Molecules. Mol. Phys. 1970, 18, 145−164.
(69) Groff, R. P.; Merrifield, R. E.; Avakian, P. Singlet and Triplet
Channels for Triplet-Exciton Fusion in Anthracene Crystals. Chem.
Phys. Lett. 1970, 5, 168−170.
(70) Miyata, K.; Conrad-Burton, F. S.; Geyer, F. L.; Zhu, X.-Y.
Triplet Pair States in Singlet Fission. Chem. Rev. 2019, 119, 4261−
4292.
(71) Angulo, G.; Rosspeintner, A.; Lang, B.; Vauthey, E. Optical
Transient Absorption Experiments Reveal the Failure of Formal
Kinetics in Diffusion Assisted Electron Transfer Reactions. Phys.
Chem. Chem. Phys. 2018, 20, 25531−25546.
2371
https://dx.doi.org/10.1021/jacs.0c12384
J. Am. Chem. Soc. 2021, 143, 2361−2371