Varying the Interpentacene Electronic Coupling to Tune Singlet Fission

Article abstract of DOI:10.1021/jacs.8b09510

We have designed and used four different spacers, denoted A-D, to connect two pentacenes and to probe the impact of intramolecular forces on the modulation of pentacene-pentacene interactions and, in turn, on the key steps in singlet fission (SF), that is

Full text of DOI:10.1021/jacs.8b09510

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Article
Varying the inter-pentacene electronic coupling to tune singlet fission
Ilias Papadopoulos, Johannes Zirzlmeier, Constantin Hetzer, Youn Jue Bae, Matthew D.
Krzyaniak, Michael R. Wasielewski, Timothy Clark, Rik R. Tykwinski, and Dirk M. Guldi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09510 • Publication Date (Web): 11 Mar 2019
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Varying the inter-pentacene electronic coupling to tune
singlet fission
Ilias Papadopoulos,† Johannes Zirzlmeier,† Constantin Hetzer,‡ Youn J. Bae,|| Matthew D. Krzyaniak,||
Michael R. Wasielewski,|| Timothy Clark,§ Rik R. Tykwinski,‡ ,\\ and Dirk M. Guldi†,*
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†Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-
Universität Erlangen Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
‡Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-
Universität Erlangen Nürnberg, Nikolaus-Fiebiger-Strasse 10, 91058 Erlangen, Germany
§Computer-Chemistry-Center, Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Nägelsbachstrasse 25, 91052 Erlangen, Germany
||Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston,
Illinois 60208-3113, United State
\\Present address: Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2, Canada
KEYWORDS acene oligomers, pentacene, excited states, singlet fission, time-resolved spectroscopy
ABSTRACT: We have designed and used four different spacers, denoted A-D, to connect two pentacenes and probed the impact of
intramolecular forces on the modulation of pentacene-pentacene interactions and, in turn, on the key steps in singlet fission (SF),
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that is, the (S1S0) to (T1T1) as well as (T1T1) to (T1T1) transition by means of transient absorption and electron paramagnetic
resonance measurements. In terms of 1(S1S0) to 1(T1T1) transition, a super-exchange mechanism, that is, coupling to a higher-lying
CT state to generate a virtual intermediate, enables rapid SF in A–D. Sizeable electronic coupling in A and B opens, on one hand,
an additional pathway, that is, population of a real intermediate, and changes, on the other hand, the mechanism to that of hopping.
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In turn, A and B feature much higher (T1T1) quantum yields than C and D with a maximum value of 162% for A. In terms of
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1(T1T1) to (T1T1) transition, the sizeable electronic coupling in A and B is counterproductive and, in turn, C and D give rise to
higher 5(T1T1) to (T1+T1) quantum yields than A and B with a maximum value of 85% for D.
INTRODUCTION
Singlet fission (SF) was first reported in 1965 in the context
of photophysical and magnetic observations in crystalline
anthracene and tetracene.[1-3] These observations were
confirmed subsequently and their interpretation refined,
leading to comprehensive reviews to the present day.[4-8]
SF is a process in which a singlet excited state is converted
into two triplet excited states; often referred to as a correlated
triplet pair.[9] The two resulting triplets possess a net spin of
zero, rendering SF a spin-allowed, fast transition, in stark
contrast to the spin-forbidden, slow intersystem crossing
11]
(ISC).[10] Such a multiple exciton generation (MEG),[9,
allows highly efficient solar cells to be engineered that
potentially exceed the Shockley-Queisser performance
Figure 1. Chemical structures of the investigated pentacene
dimers (A, B, C, and D).
limit.[12] This represents a fundamental advantage over
materials that generate only one electron-hole pair for each
absorbed photon. The relationship between the band gap and
the output voltage is crucial.[13] Any excess energy provided
There are several requirements for efficient SF: On the one
by the absorbed photons results in wasted thermal energy.[12, 13]
hand, the photoexcited chromophore must interact and share
SF can increase the performance limit to approximately 45%,
significantly higher than the Shockley-Queisser limit of
approximately 32%.[14] Several recent studies have
demonstrated remarkable progress in the construction of SF-
based solar cells.[11, 15-17]
its energy with a neighboring ground-state chromophore.
Overall, the SF efficiency is closely associated with the
electronic coupling between the chromophores, which is
governed by factors such as proximity, overlap, crystal
19]
packing, etc.[18,
Controlling the configurations of the
chromophores is essential for fine-tuning their coupling in the
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solid state.[20, 21] Early investigations of SF were carried out
Past work has focused mainly on through-bond interactions
exclusively in the solid state.[20] Friend and coworkers only
recently demonstrated SF in room-temperature solution.[10]
This required highly concentrated solutions of pentacene
derivatives in order to obtain SF, even though the
chromophores lack the close, ordered orientations found in
crystals.[10] Some thermodynamic requirements must also be
met for SF. First, the energy of the lowest-lying singlet-
excited state must exceed twice the energy of two triplet
excited states (S1 ≥ 2T1). To avoid the formation of higher
triplet states as a product of triplet-triplet annihilation, the
energy of the higher triplet states should significantly exceed
twice the energy of the lowest-lying triplet state (T2 ≥ 2T1).[20,
22] Recent investigations have shown that tetracene, pentacene,
hexacene, and their derivatives are promising molecular
building blocks for molecular SF materials, with pentacene
and its derivatives representing the prototypical SF material
that fulfills all of the criteria described above.[23, 24]
to control the electronic coupling between two pentacenes and,
in turn, the rate of SF. Simply put, the electronic coupling
drives not only the spin-allowed 1(S1S0) to 1(T1T1) SF, but also
the mixing between 1(T1T1) and 5(T1T1) and the ability to
dissociate into (T1 + T1). The two steps are intimately linked to
each other and require a fine balance: The molecular spacer
should provide sufficient electronic coupling between the
pentacenes to render SF facile, while at the same time keeping
the coupling sufficiently weak to allow spin decoherence. For
example, in rigid, π-bonded phenylene-spacers, the electronic
coupling is off balance and triplet-triplet annihilation is faster
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than
decoherence.[50]
In
rigid,
σ-bonded
adamantyl/bicyclooctane spacers, the coupling is balanced and
decoherence is faster than triplet-triplet annihilation.[35]
Notably is, however, the rotational and vibrational flexibility,
which does not affect the relative distance.[51-53] The electronic
coupling in even longer rigid, σ-bonded spacers than
adamantyl/bicyclooctane will likely be off balance and, in
turn, SF will not be competitive with the efficient intersystem
crossing/fluorescence.
The molecular mechanism of SF has been discussed
26]
controversially.[25,
The central questions concern the
electronic states, their coupling, and their effective nuclear
dynamics.[27-36] Two contrasting SF mechanisms have
emerged; a direct, one-step, and an indirect, two-step
mechanism. The former proceeds from the initially populated
singlet-excited state into a correlated triplet-pair state (1(TT)),
also referred to as a multiexcitonic state, which has overall
singlet character. The correlated triplet pair then dissociates
into two separated triplet excited states.[37] In the indirect,
two-step mechanism, SF is mediated by an intermediate
charge-transfer (CT) state. The two triplets, likewise, separate
into two individual triplet excited states.[20, 28-30] Furthermore
CT states, both as a virtual or real state, can play a decisive
role in the SF mechanism.[38-41] Overall, these discussions
demonstrate the complexity of SF and underline the necessity
of an improved mechanistic understanding of SF.
Figure 2. Schematic outline of pentacene dimers based on
flexible linkers that prevent through-bond coupling while
allowing through-space interactions.
The impact of through-space coupling remains, for the most
54,
55]
part, unexplored.[40,
Considering the distance
The electron spin-spin interactions, a component of the
electronic coupling, dictate the efficiency for ling lived triplets
through SF. The spin–spin interactions comprise the intra-
triplet zero field splitting, the inter-triplet exchange interaction
(JSQ) and dipolar coupling (D). The exchange interactions
depend exponentially on the distance between the two spin-
correlated triplet states and the nature of the intervening spacer
or medium.[35] Dipolar coupling, on the other hand, depend
inversely on the cube of the distance.[42] The energy splitting
dependencies on the electronic coupling, we expect
significantly altered outcomes for “through-bond” and for
“through-space” driven SF processes. To this end, studying
different linkers as a means to change the electronic coupling
enables to dissect the impact on both short-range JSQ and long-
range D and helps to distinguish between them. We therefore
explored the use of the four “flexible” spacers A–D (Figure 1)
to tether the two pentacenes (Figure 2). It is important that the
length of the flexible spacers renders a possible through-bond
electronic coupling negligible. Because SF in the linked
pentacene dimers is intra- rather than intermolecular,
bottlenecks such as diffusional encounter, etc. are eliminated.
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and subsequent mixing between the (T1T1) and (T1T1) states
depends on all three spin-spin interactions, though in SF
dimers the splitting tends to be dominated by JSQ;[27-36] the
stronger JSQ is, the higher is the 5(T1T1) energy relative to that
of 1(T1T1). If the two pentacenes interact too strongly, JSQ will
A central question relating to molecular structure is to
investigate intramolecular forces, by which pentacenes
organize in solution into ordered structures without an external
input. Are additional forces, such as specific, local interactions
among, for example, solubilizing head groups, needed in the
structural self-ordering of pentacenes? Ultimately, this will
affect the electronic communication and the question arises
whether the effective attractions are strong enough to enable
the through-space interactions required for the 1(S1S0) to
1(T1T1) transition and still allow the 1(T1T1) to 5(T1T1)
decoherence, which are intrinsic spin-vibronic interactions that
couple vibrations with spin and/or hyperfine interactions with
nuclear spins. Importantly, we expect the quantum yields for
the dissociation of 5(T1T1) into (T1+T1) to scale with the
pentacene-pentacene electronic coupling.
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be large such that the (T1T1) state initially formed may not
mix with 5(T1T1). The ultimate goal will be to dissect
contributions from D and JSQ in order to understand and tune
the coupling.
Motivated by recent results, we decided to use pentacene
dimers to investigate SF in solution. The role of crystal
packing for SF in crystalline films is fulfilled by the molecular
spacer. Both the structure and topology (rigidity, distance,
etc.) and the electronic coupling (interactions between the two
photoactive termini) are important in determining the
performance of the spacers. An important feature of
pentacene-spacer-pentacene conjugates is the possibility of
introducing a systematic alteration of separation, orientation,
and overlap without affecting the electronic nature of the
termini.[24, 38, 43-49]
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Scheme 1. Assembly of pentacene dimers A–D.
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SYNTHESIS
two pentacene chromophores. The splitting decreases in the
order A > B > D > C with 1420, 758, 678, and 492 cm-1,
respectively, in the most stable folded conformations found.
This order changes when the bridges are removed and replaced
by a single acetylenic hydrogen with splittings of 290, 742,
661, and 476 cm-1 for A, B, D and C, respectively, and when
the R3Si groups were also replaced by hydrogens (250, 718,
226, and 444 cm-1). Thus, compound A is unique among those
studied in that the bridge provides significant
hyperconjugative coupling due to the presence of C-C bonds
approximately perpendicular to the ring plane. This is evident
from the bridge contribution to the HOMO, shown in Figure 3.
These are compared in Figure 3.
Using a handful of readily available building blocks 1–3,
dimeric pentacenes A–D were assembled with a variety of
flexible spacers (Scheme 1). Dimers A and B were obtained
via desymmetrization of 6,13-pentacenequinone (1) followed
by stepwise nucleophilic acetylide addition[56] and SnCl2-
mediated reductive aromatization.[57] Dimers A and B were
obtained in 43% and 46% yield (from 1), respectively (see SI
for synthetic details). Toward dimer C, ketone 2 is formed
directly from 6,13-pentacenequinone (1) in a single step.[58]
A
four-step sequence of nucleophilic addition, desilylation,
homo-coupling under Hay conditions, and reductive
aromatization reactions gave C in 41% yield (from 2). Finally,
6,13-pentacenequinone can be converted to diyne 3 in two
steps, and dimer D (Scheme 1) was formed in two steps via a
sequence of Sonogashira coupling and reductive aromatization
(Supporting Information). From diyne 3, D was obtained in
50% overall yield (from 3).
Dimers A–D are nicely soluble in common organic solvents
(>6 mg/mL), including CHCl3, CH2Cl2, THF, and toluene.
Dimers B–D are stable toward air and moisture under normal
laboratory conditions, but slowly decompose over several days
when exposed to light and oxygen. In the case of dimer A,
however, decomposition can be rapid (< 1 day) when stored in
solution under normal laboratory conditions.
THEORY
Initial force-field and AM1 semiempirical molecular orbital
(MO) geometry optimizations revealed both extended (trans-)
conformations and several folded (gauche-) conformations as
minima. These minima were subsequently optimized using
density-functional theory (DFT) with the B3LYP hybrid
functional, Grimme’s D3 dispersion correction and the 6-
31G(d) basis set (details and results are given in the
Supporting Information, Table S10).
Figure 3. The HOMO of A, showing the conjugation through the
bridge, the inter-pentacene stacking in B and the contact between
the pentacenes and the trialkylsilyl groups of the adjacent
pentacene moiety in D.
The calculated splitting in B remains constant, regardless of
whether the bridge and substituents are present, suggesting
that this compound has the best through-space inter-pentacene
coupling. Of the two remaining compounds, D is interesting
because its coupling decreases strongly when the trialkylsilyl
substituents are removed, suggesting that in this case coupling
is transmitted by the bulky substituents.
Judging the coupling in such complex systems is difficult
because factors such as contact between the substituents and
hyperconjugation through the bridge affect both orbital and
state energies. Thus, the most reliable measure that we found
was the splitting between the first two singlet-excited states
calculated using AM1-CI with only four active orbitals
(HOMO and HOMO-1, LUMO and LUMO+1) in order to
reduce the model to a simple frontier-orbital picture for the
The calculations suggest that B has the inherently best
stacking geometry for through-space coupling, whereas A is
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exceptional in that the bridge aids coupling. The calculated S1-
fluorescence. The fact that only D exhibits fluorescence
S2 splittings suggest that the coupling decreases in the series A
>> B  D > C, in approximate agreement with experiment.
similar to that seen for TIPSPc is due to the weakest
electronic coupling within the series of pentacene dimers A–
D. Overall, the quantum yield trend follows the findings based
on absorption spectroscopy and modeling. The respective
fluorescence spectra of TIPSPc and dimers A–D are
illustrated in Figure 4.
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PHOTOPHYSICAL CHARACTERIZATION
Steady-state characterization; Ground, reduced,
oxidized, and excited states. To gather insight into
pentacene-pentacene interactions, we first directed our
attention to results of ground-state measurements. At first
glance, similar absorption spectra were found for all four
pentacene dimers: Features in the high-energy (350 to 500 nm)
range as well as in the low-energy (500 to 750 nm) range were
detected. Of great spectroscopic importance is the low-energy
range, in which the vibrational progression associated with the
lowest singlet excited state including the 0–*0, 0–*1, etc.
transitions evolves. Relative to what is seen for A–C, the long
wavelength, fundamental 0–*0 transition is red–shifted by
20 nm for D. It is the extended conjugation in D relative to A–
C, rather than any other electronic effects, which causes the
overall red-shift. A closer look reveals, however, significant
differences in, for example, the extinction coefficients: The
values for the long wavelength 0–*0 transition range from as
large as 48,000 M–1 cm–1 in D to as small as 11,500 M–1 cm–1
in A. We use the extinction coefficient of 24,500 M–1 cm–1,
which was determined for TIPSPc (Figure 4 – black line), as a
reference. Values of less than 49,000 M–1 cm–1, twice the
reference value, are taken as evidence for electronic
interactions between the two pentacenes.[58] In other words, the
TIPSPc values are only reached in D, suggesting minimum
electronic coupling. Overall, the extinction coefficients follow
the splitting between the first two singlet-excited states
determined in the theoretical section – vide supra. The
respective absorption spectra of TIPSPc and dimers A–D are
illustrated in Figure 4.[60]
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Figure 4. Top: Room temperature absorption spectra of TIPSPc
(black line), D (red line), C (blue line), B (orange line), and A
(green line) in benzonitrile. Bottom: Normalized fluorescence
spectra of TIPSPc (black dots), D (red dots), C (blue dots), B
(orange dots), and A (green dots) in benzonitrile solutions
following photoexcitation at 610 nm with an optical density of
0.025.
Independent of the conclusions drawn from the above
experiments, we postulate that the interactions between the
two pentacenes must be weak, even in the different gauche-
conformers. Complementary electrochemical investigations
only showed subtle changes: In, for example, D, the first one-
electron oxidation and the first one-electron reduction are
found at +0.33 and –1.39 V versus Fc/Fc+, respectively,
compared with +0.40 and 1.45 V for the corresponding one-
electron oxidation and reduction of TIPSPc, respectively. The
measured values are also close to those determined for C
(+0.29 and –1.57 V), B (+0.37 and –1.44 V), and A (+0.41 and
–1.44 V) (Supporting Information, Figure S2, Table S2).
Time-resolved characterization; Singlet- and Triplet-
excited states. In time-resolved fluorescence measurements
with an excitation wavelength of 610 nm, we found a major,
short-lived and a minor, long-lived component in the 650 to
700 nm range. The major lifetimes are 4.88  0.01 ns for D,
3.20  0.04 ns for C, 1.90  0.03 ns for B, and 1.38  0.02 ns
for A in benzonitrile and corroborate the trends described in
the steady-state measurements. The major lifetimes in toluene
are very similar: 6.11  0.01 ns for D, 3.48  0.04 ns for C,
1.97  0.03 ns for B, and 1.41  0.02 ns for A. Minor, long-
lived components of 7.8  0.06 ns in, for example, benzonitrile
reflect the presence of a trans-conformer in A-C except D, in
which the trans-conformation coexists with the corresponding
gauche-conformer. In toluene and benzonitrile, the gauche-to-
trans ratio is quantitatively identical at 50:50 for D, 83:17 for
C, 86:14 for B, and 90:10 for A.[61] All ratios remained
unchanged in methyl-cyclohexane and cyano-cyclohexane as
non-aromatic analogs of toluene and benzonitrile, respectively.
The lifetimes of the short-lived components in A and B ,
however, change with values in the range from 4 to 5 ns
(Supporting Information, Figure S1, Table S1). From the
aforementioned trend we conclude that aromatic solvents
augment the electronic coupling when through-space/through
solvent interactions are operative. Notably, only the gauche-
conformers, for which theory predicts a higher preference,
The spectroscopic evidence in the excited state is stronger.
In the steady-state fluorescence spectra following
photoexcitation at 610 nm, the short-wavelength, fundamental
*0–0 transitions are red-shifted relative to the long-
wavelength, fundamental 0–*0 transitions in the absorption
spectra. Regardless of the solvent polarity, the fluorescence
spectrum of D is fine-structured, similar to the fine-structured
fluorescence found for TIPSPc with overall fluorescence
quantum yields of 39.9%, 26.4%, and 20.0% in toluene, THF,
and benzonitrile, respectively. Also, the relatively high
fluorescence quantum yields for D of 17.7%, 13.3%, and 9.9%
in toluene, THF, and benzonitrile, respectively, are in sound
agreement with the characteristics of TIPSPc. Dimers A, B,
and C fluoresce less strongly, with quantum yields in
benzonitrile of 2.0%, 2.8%, and 3.5%, respectively. In contrast
to D, however, the fluorescence spectra of A–D lack any fine
structure. Mutual pentacene-pentacene interactions, which are
operative in A–C, govern the featureless, quenched
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seem to contribute to SF. The trans-conformers are SF-
inactive and, in turn, governed by a slow and inefficient triplet
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formation via intersystem crossing – vide infra.
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To establish the triplet excited-state features of A–D, (0 –
1.0 x 10-4 M) we turned to triplet-triplet energy-transfer
experiments using N-methylfulleropyrrolidine (N-MFP, 8.0 x
10-5 M, Figure 5) as triplet sensitizer using 480 nm
58]
photoexcitation (800 nJ).[50,
We started with N-MFP
Figure 5. Chemical structures of TIPSPc and N-
methylfulleropyrrolidine (N-MFP).
solutions of 8 x 10-5 M and added varying concentrations of
A–D in the range from 1.0 x 10-5 to 1.0 x 10-4 M. In the
absence of A–D, only the long-lived triplet excited-state
features of N-MFP are recorded with maxima at 421, 442, and
690 nm. In the presence of A–D, these maxima are, however,
replaced on the longer timescales by differential absorption
changes that include both a 510 nm maximum and 605 and
648 nm minima. The underlying features are assigned to the
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Time-resolved characterization; Singlet fission. In pump-
probe experiments on the femto-, pico-, and nanosecond
timescales, 610 nm excitation is followed by the instantaneous
formation of the singlet-excited state (S1S0). Here, maxima at
458 and 1250 nm and a minimum at 652 nm assist in the
spectroscopic assignment (Figure 6, top-left). For A–D, these
pure singlet excited-state features transform into a new
intermediate state within 20 ps. This transformation is most
evident when analyzing the feature at 1250 nm, which shows a
hypsochromic shift and broadening. In line with previous
work,[59] this intermediate state relates to solvent
reorganization, caused by differences in the dipole moments
between the ground and excited states: It is denoted as
(S1S0)SOLV and the driving forces stem from reducing the
solvation energy of the excited state.
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triplet excited-state fingerprints of (T1S0), that is, a single
triplet excited state on one of two pentacences in the pentacene
dimers. Pseudo first-order analyses confirmed that the
underlying triplet-triplet energy-transfer dynamics are
diffusion-controlled: 5.0 x 109 M-1s-1. The singlet ground state
extinction coefficients, derived from N-MFP sensitization, are
19,000, 23,000, 31,000, and 55,000 M-1 cm-1 for A, B, C, and
D, respectively, at the 450 nm maximum. The correlation of
these extinction coefficients with those determined from
steady-state spectroscopy for the corresponding ground states
is noteworthy. The triplet excited-state extinction coefficients
at the 510 nm maximum, are 85,390, 86,080, 109,010, and
116,350 M-1 cm-1 for A, B, C, and D, respectively. (Supporting
Information, Figure S3–S18).
(S1S0)SOLV is metastable, and at the end of its transformation,
the same triplet excited-state features are discernable that
evolved in triplet-triplet energy-transfer experiments for A–D
Figure 6. Top-Left: Differential femtosecond transient absorption spectra (λex = 610 nm, 400 nJ) of A (4 x 10-5 M) in benzonitrile with
time delays between 0 and 7500 ps; Top-Right: Respective time absorption profiles at the given wavelengths; Bottom-Left: Deconvoluted
femtosecond transient absorption spectra of the singlet excited (S1S0) (black), the intermediate state (S1S0)SOLV (red), the intermediate state
CT
1
1
(S1S0)(T T ) (blue), and the singlet correlated triplet state (T1T1) (green) of A as obtained by target analysis, using the proposed kinetic
1
model, in benzonitrile; Bottom-Right: Respective population kinetics.
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– vide supra. Most notable are maxima at 478, 510, 860, and
region. As a matter of fact, the population of (S1S0)(T1T1)CT is
accompanied by blue-shifted (S1S0)SOLV at around 1215 nm,
and the rise of the 865 and 980 nm characteristics, albeit of
much lower intensity, of 1(T1T1).
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980 nm and a minimum at 648 nm (Figure 6, top-left). The
overall singlet to triplet excited-state conversion is
accompanied at, for example, the 648 nm minimum by an
increase of the ground-state bleaching by roughly 15% (Figure
6: Top-right – blue dots), suggesting that 1(T1T1), in which two
triplet excited states are located on the same pentacene dimer,
is formed rather than 3(T1S0).
Please note that the pure (S1S0)SOLV lacks the features at 860
and 980 nm, while the pure 1(T1T1) state lacks the maximum at
1215 nm. To clarify the spectroscopic differences throughout
the (S1S0)SOLV-to-(S1S0)(T T )CT transition further, we inspected
1
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Next, multi-wavelength and target analyses were used to
determine the underlying lifetimes. Two different kinetic
models were applied to fit contributions from the SF active
gauche-conformers. In the case of C and D, a purely
sequential kinetic model was used, in which (S1S0) deactivates
via the solvent-stabilized (S1S0)SOLV to afford 1(T1T1) with rate
constants of 1.1 x 1011 and 2.0 x 108 s-1, respectively. Implicit
in this kinetic model is a super-exchange mechanism, in which
an energetically higher-lying CT-state serves as a virtual
intermediate between (S1S0)SOLV and 1(T1T1) – vide infra.
(CT)virtual is displayed in the fs-TAS for C and D shown in
Figures S20–S21.[62]
the decay associated spectra (DAS) of A – Figure 8. By virtue
of the fact that the 860 and 980 nm markers point
“downwards” we infer that they will be present throughout the
transition toward (S1S0)(T T )CT. The same considerations are
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applicable for the 1215 nm marker. It is only during the
transition toward pure 1(T1T1) that the 1215 nm marker
disappears. For B–D, the DAS are essentially identical –
Figure S22.[66]
CT
Independent proof for the existence of (S1S0)(T T ) came
1
1
from fs-TAS measurements in toluene. When using a solvent
of lower polarity, appreciable changes in the fs-TAS are in
CT
line with this expectation. To this end, for (S1S0)(T1T1) the
Fitting the data for A and B required a minor modification
of the kinetic model. Details regarding the kinetic models are
given in Figure 7. Initially, (S1S0) transforms into the same
(S1S0)SOLV intermediate, which is implemented in the kinetic
model for C and D – vide supra. In contrast to C and D,
(S1S0)SOLV deactivates in A and B in a parallel fashion to yield
1(T1T1). 1(T1T1) is formed, on one hand, directly (one-step), via
a super-exchange mechanism as described for C and D, and,
on the other hand, indirectly (two-step), via an observable
intermediate state.[63, 64]
1215 nm maximum, which relates to (S1S0)SOLV, is still
discernable and so are the maxima at 860 and 980 nm, with
1
both refer to (T1T1). But, unlike our analysis in benzonitrile,
the maximum at 511 nm, which relates to triplet excited states,
is sharper and more prominent in toluene (fs-TAS, DAS and
rate constants of A and B are illustrated in Figures S23–S25
and Table S6). In short, contributions from 1(T1T1) in
CT
(S1S0)(T1T1)
increase in toluene at the expense of
contributions from CT, which seemingly is playing only a
minor role.
Figure 8. Normalized decay associated spectra of A as obtained
by target analysis of the deconvoluted femtosecond transient
absorption spectra in benzonitrile. The figure legend refers to the
respective transitions. Please note that the 1(T1T1)-to-5(T1T1)
transition is not fully resolvable on the fs-TAS timescale.
Figure 7. Top: Kinetic model used to fit the transient absorption
data for C and D; Bottom: Kinetic model used to fit the transient
absorption data for A and B.
The modus operandi in A and B is likely to be hopping.
Interesting is the fact that the dominance of the direct (one-
step) over the indirect (two-step) pathway is linked to the
electronic coupling. For example, in C and D the relative
ratios for the indirect (two-step) and direct (one-step)
pathways are 0% to 100%. In A and B, the ratios are 45% to
55% and 65% to 35%, respectively, which demonstrates the
importance of the indirect pathway. In toluene, the ratios
We hypothesize that the intermediate state is
a
superimposition of (S1S0)SOLV and 1(T1T1) mixed with CT
characteristics. Recently, many different variations in terms of
relative ratios between the respective characteristics have been
CT [40, 41]
summed into a single term, (S1S0)(T1T1)
.
Figures 6 and
change to 70% to 30% for A and 80% to 20% for B, again due
S19 depict the fs-TAS, the time absorption profiles, the
deconvoluted species associated spectra (SAS), and the
population kinetics[65] from Glotaran target analysis for A and
B, respectively. SAS (Figure 6, bottom-left) reveal features of
CT
to
a .
larger contribution from 1(T1T1) in (S1S0)(T T )
1
1
Moreover, the relative ratio impacts the 1(T1T1) quantum
yields: The lowest quantum yields of 50% were found for D,
followed by 84% for C, 142% for B, and 162% for A in
1
(S1S0)SOLV and (T1T1), especially in the near-infrared (NIR)
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1
benzonitrile. In short, the electronic coupling and the (T1T1)
(45%), followed by A (40%), B (53%), and then C (67%).
quantum yields compare remarkably well.[67] (“Methods”
section and Supporting Information, Figure S43 and Table
S9).[35, 38, 39]
Overall, C seems to have the optimum electronic coupling to
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assist in the highest 5(T1T1) quantum yield. Relative to C, in A
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and B the (T1T1) state is raised in energy and, in turn, the
5
5(T1T1) quantum yields are lower. Notably, the (T1T1) energy
Time-resolved characterization; Spin decoherence.
Pump-probe experiments on the nano- and microsecond
increase relative to 1(T1T1) depends on the electronic coupling,
which is higher in A and B than in C. The low 5(T1T1)
quantum yield in D, in turn, might be correlated to its unique
mode in coupling, which is mediated by the Si-Si contacts as
described by modeling.
1
timescales shed light on the fate of (T1T1) in A–D. With the
exception of the early time delays, at which the 1(T1T1) growth
is still found, at any given time delay between 1 ns and 100 µs,
1
the differential absorption spectra resemble those of (T1T1).
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Multi-wavelength analyses suggest, however, that the (T1T1)
Finally, the 5(T1T1)-to-(T1+T1) transformation makes the
highest (T1+T1) quantum yields possible in the case of D with
a value of 85%, whereas A–B feature yields of about 45%.
Considering that we found the weakest electronic coupling
among the A–D series for D, the two pentacenes are fairly
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decays are multiphasic and feature three different components.
Here, target analyses using our recently published kinetic
model are decisive.[35] In the case of C and D, we modified the
kinetic model by adding the parallel decay of the SF-inactive
trans-conformer next to the decay of the SF-active gauche-
conformer; the relative ratios are based on the steady-state
fluorescence experiments – vide supra – Figures S27 and
S28).[68] In terms of the trans-conformer, only the slow
intersystem crossing to afford 3(T1S0) and the subsequent
recovery are discerned. In terms of the gauche-conformers,
5
decoupled. As such, the subsequent decoherence of (T1T1) to
afford the corresponding uncorrelated (T1+T1) is most strongly
favored. Likewise, we consider that the different mode in
coupling, the extended conjugation of the spacer, and the lack
of competing deactivations play a role in forming (T1+T1) in
such high quantum yields. The ns-TAS, time absorption
profiles, deconvoluted nanosecond transient absorption
spectra, and population kinetics derived from target analysis
for A are illustrated in Figure 9 and for B–D in Figures S26–
S28, respectively.
1
starting with (T1T1), the recovery of the ground state (S0S0)
proceeds via the sequential population of a correlated and an
5
uncorrelated pair of triplet excited states, (T1T1) and (T1+T1),
respectively. Dimer D gives the lowest 5(T1T1) quantum yields
Figure 9. Top-Left: Differential nanosecond transient absorption spectra (λex = 610 nm, 400 nJ) of the A (4 x 10-5 M) in benzonitrile with
time delays between 0 and 400 μs; Top-Right: Respective time absorption profiles at the given wavelengths; Bottom-Left: Deconvoluted
CT
1
1
nanosecond transient absorption spectra of the intermediate state (S1S0)(T T ) (black), the singlet correlated triplet state (T1T1) (red), the
1
quintet correlated triplet state 5(T1T1) (blue), and the uncorrelated triplet excited state (T1+T1) (green) of A as obtained by target analysis,
using the proposed kinetic model, in benzonitrile. Bottom-Right: Respective population kinetics. Please note that the NIR is not shown due
to the low intensity in ns-TAS.
Our recent investigations[35] have shown the impact of
modulating through-bond electronic communication regarding
conjugated and non-conjugated pentacene dimers constant,
vastly different JSQs were realized. Large values of JSQ disable
any of the aforementioned transformations in the earlier cases
and activate the ground-state recovery. Small values of JSQ
1
5
the (T1T1)-to-5(T1T1) and (T1T1)-to-(T1+T1) transformations.
By keeping the pentacene-pentacene distances in a series of
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enable both transformations in the latter cases and ground-
state recovery is negligible. In the current investigation, we
demonstrate that in the case of weak electronic couplings,
where JSQ is insignificant, through-space electronic
communication by means of D plays a decisive role. Spin
decoherence requires both weak JSQ and weak D. In D, the
weakest D results in the slowest 1(T1T1)-to-5(T1T1) and
5(T1T1)-to-(T1+T1) transformations with rate constants of 3.1 x
106 and 1.1 x 106 s-1, respectively.
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In A, which features the strongest electronic coupling in the
series of A–D, triplet-triplet annihilation is facilitated with rate
constants as high as 9.0 x 107 and 1.4 x 107 s-1, for the
respective transformations. The fact that the fluorescence
deactivation of A and B in non-aromatic methyl-cyclohexane
and cyano-cyclohexane is notably slower than in aromatic
toluene and benzonitrile points to the mediating effects of the
aromatic solvents at facilitating an electronic coupling
sufficient to have both JSQ and D operative.
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Figure 10. TrEPR spectra of dimer A in MeTHF at 85 K ran at X-
band.
Spin dephasing in 1(T1T1) and 5(T1T1) is fundamentally slow
and, as such, competes with triplet-triplet annihilation.
Considering, for example, D and comparing its 1(T1T1)
lifetime of 147 ns with those seen in electronically decoupled,
non-conjugated pentacene dimers with 90 ns (NC),[35] we
postulate, as mentioned earlier, that it is close to the intrinsic
limit in D. Still, efficient formation of 5(T1T1) seems to
Temperature and viscosity dependent studies. On one
hand, we lowered the temperature from 298 to 80 K and, on
the other hand, we increased the viscosity by using different
ratios of MeTHF and paraffin from 10/0 (v/v) to 1/9 (v/v). We,
again, opted for A and D. With respect to steady-state
fluorescence, both assays led to a reinstatement of the
vibrational fine structure for A, shown in Figure 11, and a
sharpening of the structure for D, shown in Figure S31.
Inspection of the time-resolved fluorescence measurements of
A and D revealed that both the short- and the long-lived
components of the gauche- and trans-conformations,
respectively, increased upon a temperature decrease and
viscosity increase, as seen in Figures 12 and S32, and Table
S7, respectively.
1
necessitate – like the formation of (T1T1)– sizeable electronic
5
spin dipole-dipole interactions. When comparing the (T1T1)
lifetimes of 835 ns for D and 175 ns for NC, a similar
conclusion is reached.[35] A closer look at the overall (T1+T1)
quantum yields reveals the benefits of using non-conjugated
spacers: The electronic coupling is significantly attenuated and
allows the population of (T1+T1). The respective rate constants
and quantum yields for A–D are summarized in Figure S43
and Tables S4–S5.
Time-resolved electron paramagnetic resonance (trEPR)
spectroscopy: 5(T1T1) state validation. To validate the
presence of 5(T1T1), time-resolved Electron Paramagnetic
Resonance (trEPR) spectroscopy was performed on A and D.
In frozen 2-methyltetrahydrofuran (MeTHF) at 85 K, both A
(Figure 10 and S44) and D (Figure S46) possess similar
features.[69] Upon freezing the solution of, for example, A a
distribution of different orientations goes hand-in-hand with a
distribution of JSQ. However, the width of the quintet and
triplet features, which show a ~12 mT broad feature centered
at g = 2 and one at 40 mT, respectively, matches well with the
previously measured pentacene dimer NC.[35]
Fitting the transient spectra required three exponentials,
where the first species relates to the formation of 5(T1T1)
5
followed by the bi-exponential decay of (T1T1) and (T1+T1).
The quintet feature is most evident in the first and second
species with sharp emissive and absorptive features at 340 and
352 mT, respectively. In contrast, the triplet features dominate
the third species. The dynamics observed in trEPR matches
well with those observed in low temperature transient
absorption measurements: 1(T1T1) lifetime is 222.2 ns and
1
5(T1T1) lifetime is 1.89 μs. In trEPR, the lifetime of (T1T1)
corresponds to the rise of the signal, which is 100 ns and that
Figure 11. Top: Normalized temperature dependent fluorescence
spectra (λex = 610 nm) for A (4 x 10-5 M) at 298 (black), 220
(red), 150 (blue), and 80 K (green) in 2-Methyltetrahydrofuran
(MeTHF). Bottom: Normalized viscosity dependent fluorescence
spectra for A in MeTHF/paraffin mixtures of 10/0 (v/v) (black),
5/5 (v/v) (red), and 1/9 (v/v) (blue).
5
of (T1T1) is 510 ns. Notable is the decay of (T1+T1), which is
beyond the time window of our trEPR experiments of 3000 ns.
In addition, simulations were performed on A to further
support the assignment of the triplet excited state, as well as,
to demonstrate the effect on the quintet spectrum as one
pentacene unit is rotated about the y axis of its zero field
tensor from 60-90 degrees (Figure S45).
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in D, which is the weakest among the A–D series, affords only
1
subtle changes upon changing temperature and viscosity.
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Changes were also observable in the fs- and ns-TAS
measurements, as Figures 13, 14, S32–S42, and Table S8
demonstrate. For example, as the temperature is lowered step-
by-step, the lifetimes and yields of 1(T1T1), 5(T1T1), and
(T1+T1) decrease. Interestingly, at temperatures below 150 K a
mechanistic change is noted for A: The direct, super-
exchange mechanism – top part of Figure 7 – takes over like in
the scenario established for C and D at higher temperature,
that is, all the way to room temperature.
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The outcome of lowering the temperature for D is different:
SF is completely deactivated. Instead
a conventional
intersystem crossing dominates the slow excited state
deactivation and, in turn, the inefficient triplet excited state
formation.[70] In a 1/9 (v/v) MeTHF/paraffin mixture, the same
mechanistic crossover from the indirect to the direct
mechanism occurs for A. Once again, the cause relates to
electronic couplings and geometric effects. For D, SF is not
completely deactivated under these conditions, but the overall
yields are less than 18%.
Figure 12. Top: Temperature dependent fluorescence time
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profiles (λex = 610 nm) of A (4.0 x 10-5 M) at 298 (black), 220
(red), 150 (blue), and 80 K (green) in argon saturated MeTHF.
Bottom: Viscosity dependent fluorescence time profiles
(excitation at 610 nm) of A (4.0 x 10-5 M) in argon saturated
MeTHF/paraffin mixtures of 10/0 (v/v) (black), 5/5 (v/v) (red),
and 1/9 (v/v) (blue).
CONCLUSION
In summary, we have designed four pentacene dimers, in
which four different spacers were introduced to modulate
intramolecular forces and, in turn, pentacene-pentacene
interactions. Emphasis was placed on probing the impact on
the 1(S1S0)-to-1(T1T1) transition, the 1(T1T1)-to-5(T1T1)
decoherence, as well as the (T1+T1) formation. All of the
aforementioned
intermediates
were
spectroscopical
Steady state and time-resolved fluorescence measurements
underpin the fact that SF is influenced by through-space
interactions and electronic coupling, which relate to the
relative geometry of the pentacenes. Small variations in
distances cannot be ruled out. In A, lower temperatures/higher
viscosities restrict the overall geometry, which leads to a
decreased electronic coupling and, in an extreme scenario, to
behavior similar to that found for D. The electronic coupling
characterized by transient absorption and/or electron
paramagnetic resonance measurements in a wide temperature
range, that is, from 298 to 77 K, in a wide solvent polarity
range, that is, from toluene to benzonitrile, and in a wide
solvent viscosity range, that is, from pure MeTHF to a 1/9
(v/v) MeTHF/paraffin mixture. Fluorescence spectroscopy,
namely steady-state and time-resolved, and molecular
modeling rounded off our mechanistic investigations.
Figure 13. Top row – left to right: Differential femtosecond transient absorption spectra(λex = 610 nm, 400 nJ) of A (4.0 x 10-5 M) in
MeTHF with time delays between 0 and 7 500 ps at 80 K; Respective time absorption profiles of the triplet excited state at 500 nm at given
temperatures; Deconvoluted femtosecond transient absorption spectra of the singlet excited (S1S0) (black) and the intermediate state
(S1S0)SOLV (red) of A, as obtained by global analysis using a sequential model, in MeTHF at 80 K; Respective population kinetics. Bottom
row – left to right: Differential femtosecond transient absorption spectra (λex = 610 nm, 400 nJ)of A (4.0 x 10-5 M) in a 1/9 (v/v)
MeTHF/paraffin mixture with time delays between 0 and 7500 ps; Respective time absorption profiles of the triplet excited state at 500 nm
at given ratios; Deconvoluted femtosecond transient absorption spectra of the singlet excited (S1S0) (black) and the intermediate state
(S1S0)SOLV (red) of A, as obtained by global analysis using a sequential model, in a 1/9 (v/v) MeTHF/paraffin mixture; Respective
population kinetics. The respective spectra for 1(T1T1) are not shown because global analysis hampers the complete deconvolution.
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Figure 14. Top row – left to right: Differential nanosecond transient absorption spectra (λex = 610 nm, 400 nJ) of A (4.0 x 10-5 M) in
MeTHF with time delays between 0 and 400 μs at 80 K; Respective time absorption profiles of the triplet excited state at 500 nm at given
1
temperatures; Deconvoluted nanosecond transient absorption spectra of the singlet correlated triplet state (T1T1) (black), the quintet
5
correlated triplet state (T1T1) (red), and the uncorrelated triplet excited state (T1+T1) (blue) of A, as obtained by global analysis using a
sequential model, in MeTHF at 80 K; Respective population kinetics. Bottom row – left to right: Differential nanosecond transient
absorption spectra (λex = 610 nm, 400 nJ) of A (4.0 x 10-5 M) in a 1/9 (v/v) MeTHF/paraffin mixture with time delays between 0 and 400
μs; Respective time absorption profiles of the triplet excited state at 500 nm at given ratios; Deconvoluted nanoseocond transient
absorption spectra of the singlet correlated triplet state 1(T1T1) (black), the quintet correlated triplet state 5(T1T1) (red), and the uncorrelated
triplet excited state (T1+T1) (blue) of A, as obtained by global analysis using a sequential model, in a 1/9 (v/v) MeTHF/paraffin mixture;
CT
Respective population kinetics. The respective spectra for (S1S0)(T1T1) are not shown because global analysis hampers the complete
deconvolution. Please note that the NIR is not shown due to the low intensity in ns-TAS
1
In the context of the (S1S0)-to-1(T1T1) transition, a super-
exchange mechanism, that is, coupling to a higher-lying CT
state to generate a virtual intermediate, enables rapid SF in A–
D. Sizeable electronic coupling in A and B opens, however, an
additional pathway, that is, population of a real intermediate,
measurements were performed using 2-methyltetrahydrofuran
(MeTHF, SIGMA-ALDRICH, anhydrous, ≥99%) and paraffin
(SIGMA-ALDRICH, for IR spectroscopy) mixtures of 10/0, 5/5,
and 1/9 (v/v). Data evaluation of the fs- and ns-TAS data has been
conducted by means of multiwavelength and Glotaran global and
target analysis. Global analysis was performed on the TA data sets
using a sequential model. Global target analysis was performed on
the TA data sets using the proposed kinetic models. The analytic
solution to the coupled differential equations that describe the
kinetic model is convoluted with a Gaussian instrument response
function. After the least-squares fitting has converged, the raw
data matrix is deconvoluted using the specific solution to the
kinetic model and parameters from the fit to obtain the species-
associated spectra and their populations as a function of time.
1
which is a superimposition of (S1S0)SOLV and (T1T1) mixed
with CT characteristics: ((S1S0)(T1T1)CT). In turn, A and B
1
feature much higher (T1T1) quantum yields than C and D,
1
which reach up to 162%. In the context of the (T1T1)-to-
5(T1T1) decoherence, sizeable electronic coupling as seen in A
and B is counterproductive and, in turn, D features the highest
(T1+T1) quantum yields with a value of 85%.
METHODS
Transient Electron Paramagnetic Resonance Spectroscopy.
Dimer A and D were dissolved in MeTHF to achieve an optical
density of 0.5 in 2 mm cuvette at 660 nm (ca. 10–4 M). To dry
quartz tubes (2.40 mm o.d. × 2.00 mm i.d.), this solution was
loaded, degassed with three freeze-pump-thaw cycles, and flame-
sealed under vacuum (<10-4 torr). Samples were placed in a split
ring resonator (Bruker ER4118X-MS3) with a grated window for
optical excitation. The samples were photoexcited with 7 ns, 3.0
mJ, 640 nm and 660 nm pulses for dimer A and D, respectively,
generated by an optical parametric oscillator (Spectra-Physics
Basi-scan), pumped with the output of a frequency-tripled
Photoluminescence spectroscopy. Steady-state fluorescence
spectra were carried out at a FluoroMax3 spectrometer from
Horiba in the visible detection range (RT). The data was recorded
using the FluorEssence software from Horiba Jobin Yvon.
Fluorescence lifetimes were determined by the time correlated
single photon counting (TCSPC) technique using a FluoroLog3
emission spectrometer (Horiba JobinYvon) equipped with an
R3809U-58 MCP (Hamamatsu) and a SuperK Extreme high-
power supercontinuum fiber laser EXW-6 (NKT) exciting at 610
nm (150 ps fwhm).
Transient Absorption Spectroscopy. Femtosecond transient
absorption (TA) experiments were carried out with an amplified
Ti:Sapphire CPA-2110 fs laser system (Clark MXR: output 775
nm, 1 kHz, 150 fs pulse width) using transient absorption
pump/probe detection systems (Helios and Eos, Ultrafast
Systems) with argon purged solutions. The 480 and 610 nm
excitation wavelengths, with energies of 800 and 400 nJ,
respectively, were generated with a noncolinear optical parametric
amplifier (NOPA, Clark MXR). Cryostat supported measurements
at 298, 220, 150, and 80 K were conducted using an Optistat DN2
cryostat from Oxford Instruments. Viscosity dependent
Nd:YAG laser (Spectra-Physics Quanta-Ray Pro 350). A
commercial X-band (ca. 9.5 GHz) Bruker Elexsys E680-X/W
EPR spectrometer was utilized for all EPR measurements. The
temperature was maintained at 85 K by an Oxford Instruments
CF935 continuous flow cryostat using liquid N2. Data was
collected in quadrature with Xepr software which stores the
nominally orthogonal x- and y-axis channels as real and
imaginary numbers, respectively.
Triplet Yield Determination. The TQY determination was
performed by using the normalization of the spectra at the ground
state bleaching using Glotaran target analysis and the ratio of
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(3) Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.;
singlet and triplet excited state molar extinction coefficients and
absorptions, as found in literature.[35, 38, 39] In short, this approach
is based on certain assumptions: (1) the singlet excited state is
delocalized over the entire molecule, thus bleaching both
pentacene units; (2) a single triplet excited state likewise bleaches
Schneider, W. G., Laser generation of excitons and fluorescence in
anthracene crystals. J. Chem. Phys. Rev. 1965, 42, 330.
(4) Johnson, R. C.; Merrifield, R. E., Effects of fields on the mutual
annihilation of triplet excitons in anthracene crystals. Phys. Rev. B
1970, 1, 896.
(5) Suna, A., Kinematics of exciton-exciton annihilation in
molecular crystals. Phys. Rev. B 1970, 1, 1716.
(6) Swenberg, C.E.; Geacintov, N. E., Exciton interactions in
organic solids; Wiley: New York, 1973.
(7) Merrifield, R. E., Magnetic effects on triplet exciton
interactions. Pure Appl. Chem. 1971, 27, 481.
(8) Rao, A.; Friend, R. H., Harnessing singlet exciton fission to
break the Shockley-Queisser limit. Nature Reviews Materials 2017, 2,
17063.
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the
entire
ground-state
singlet
transition;
(3)
no
overlying/additional features are located at the ground-state
bleaching area from roughly 600 to 700 nm, which allows one to
normalize the triplet excited state spectra relative to the singlet
excited state ones; and (4) that two triplet excited states on the
dimer lead to a bleaching twice that of a single one. By combining
these criteria and the kinetic model in a Glotaran target analysis,
one is able to determine the efficiency and rate constant of each
transition. In the context of the TQY, all transitions prior to the
formation of 1(T1T1) contribute to its overall yield. Details,
regarding the efficiencies and rate constants, are illustrated in the
Supporting Information, Figure S43 and Table S4, S5, and S9.
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6891.
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Singlet exciton fission in solution. Nat. Chem. 2013, 5, 1019.
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multilayer photodetectors based on singlet exciton fission. Appl. Phys.
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efficiency of p-n junction solar cells. J. Appl. Phys. 1961, 32, 510.
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R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.;
Baldo, M. A., External Quantum Efficiency Above 100% in a Singlet-
Exciton-Fission–Based Organic Photovoltaic Cell. Science 2013, 340,
334.
ASSOCIATED CONTENT
Supporting Information. (I) Listing of the general experimental
methods; (II) text and schemes describing the synthetic routes;
(III) figures and tables illustrating the results from time-resolved
fluorescence, electrochemical, femto/nanosecond transient
absorption, time-resolved electron paramagnetic resonance
spectroscopy, temperature and viscosity dependent measurements;
(IV) text and table depicting the calculation details; (V) details
describing time-resolved electron paramagnetic resonance
spectroscopy data; (VI) twenty-two NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) Hanna, M. C.; Nozik, A. J., Solar conversion efficiency of
photovoltaic and photoelectrolysis cells with carrier multiplication
absorbers. J. Appl. Phys. 2006, 100, 074510.
(15)
Ehrler, B.; Musselman, K. P.; Böhm, M. L.; Friend, R. H.;
AUTHOR INFORMATION
Greenham, N. C., Hybrid pentacene/asilicon solar cells utilizing
multiple carrier generation via singlet exciton fission. Appl. Phys.
Lett. 2012, 101, 153507.
(16) Reusswig, P.D.; Congreve, D. N.; Thompson, N. J.; Baldo, M.
A., Enhanced external quantum efficiency in an organic photovoltaic
cell via singlet fission exciton sensitizer. Appl. Phys. Lett. 2012, 101,
113304.
(17) Jadhav, P. J.; Brown, P. R.; Thompson, N.; Wunsch, B.;
Mohanty, A.; Yost, S. R.; Hontz, E.; Van Voorhis, T.; Bawendi, M.
G.; Bulović, V.; Baldo, M. A., Triplet exciton dissociation in singlet
exciton fission photovoltaics. Adv. Mater. 2012, 24, 6169.
(18) Johnson, J. C.; Reilly, T. H., III; Kanarr, A. C.; Van De
Lagemaat, J., The ultrafast photophysics of pentacene coupled to
surface plasmon active nanohole films. J. Phys. Chem. C 2009, 113,
6871.
(19) Marciniak, H.; Pugliesi, I.; Nickel, B.; Lochbrunner, S.,
Ultrafast singlet and triplet dynamics in microcrystalline pentacene
films. Phys. Rev. B 2009, 79, 235318.
(20) 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., A transferable model for singlet-
fission kinetics. Nat. Chem. 2014, 6, 492.
(21) 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.
(22) Smith, M.B.; Michl, J., Recent Advances in Singlet Fission.
Annu. Rev. Phys. Chem. 2013, 64, 361.
Corresponding Author
*dirk.guldi@fau.de
Author Contributions
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the
manuscript.
Funding Sources
Emerging Fields initiative (EFI)
Cluster of Excellence Engineering of Advanced Materials (EAM)
Solar Technologies go Hybrid (SolTech)
Natural Sciences and Engineering Research Council of Canada
(NSERC)
ACKNOWLEDGMENTS
Funding is gratefully acknowledged from the Emerging Fields
initiative (EFI) “Singlet Fission” supported by FAU, as well as the
Cluster of Excellence Engineering of Advanced Materials (EAM),
“Solar Technologies go Hybrid” – an initiative of the Bavarian
State Ministry for Science, Research and Art, and the Natural
Sciences and Engineering Research Council of Canada (NSERC).
This work was supported by the U.S. Department of Energy,
Office of Science, and Office of Basic Energy Sciences under
Award DE-FG02-99ER14999 (Y.J.B., M.D.K., M.R.W.,
experiments).
(23) Chan, W.L.; Ligges, M.; Zhu, X. Y., The energy barrier in
singlet fission can be overcome through coherent coupling and
entropic gain. Nat. Chem. 2012, 4, 840.
(24) 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.
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(60)
Overall, the electronic coupling is even in A and B too
weak to support appreciable Davydov splitting in the absorption
spectra.
(61) The gauche-to-trans ratios in A-D were determined by using
the respective fluorescence quantum yields and by relating these
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exclusively to the trans conformers.
Notable is use of the
fluorescence quantum yield of TIPSPc with 20% in benzonitrile. This
approach is justified by the different rates of singlet excited state
deactivation in the non-to-poorly fluorescent gauche conformers
relative to the strongly fluorescent trans conformers.
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(62) In the TAS of D a small bleach corresponding to stimulated
emission (≈750 nm) can be seen. We attribute the weak coupling and
the resulting high fluroescence quantum yield and slow (S1S0)SOLV
deactivation, compared to the remaining dimers, to be the cause. This
bleach shows no additional kintecs, etc..
(63) Please note that despite the competitive pathways, which
govern the parallel 1(T1T1) population in A and B, the overall
(S1S0)SOLV deactivation is appreciably faster than in C and D.
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(64) We have performed target analysis, without any kind of
normalization on the GSB, with three species rather with four species
for dimers
A and B; eliminating one species. A complete
deconvolution to a pure singlet and triplet excited state was, however,
not realized: (S1S0)SOLV, for example, carries triplet signature, on one
1
hand, and the (T1T1) features singlet character, on the other hand. In
addition "artifacts" (downwards pointing peaks at roughly 510 nm)
arose, further signalizing that deconvolution was not realized.
Furthermore, we performed target analysis, using three and four
species, but with a sequential and parallel deactivation pathway, in
both cases. Again, no meaningful deconvolution was obtained.
Further details are locasted in the Supporting Information in Figure
S29 and S30.
(65) The population kinetics illustrates the overall population of the
each species at a given time. Rates, efficiency, and nature (linear or
parallel fashion) for their population and depopulation are also
1
considered. This explains why the population of (T1T1) in A (Figure
6, bottom-right) and B (Supporting Information, Figure S19, bottom-
right) extends to the kinetics of (S1S0)SOLV and (S1S0)(T1T1)CT, as both
species contribute parallel to its formation.
(66) Decay associated spectra (DAS), in general, show the rise or
decay of signals, which are followed by the transition from one
species to another and, therefore, are able to illustrate the differences
at every wavelength. Features that are pointing upwards convey that
the following species has less or no features. Features pointing
downwards mean that additional features will appear in the next
species.
(67) 1(T1T1) quantum yield determination was based on the
normalization of the spectra at the ground state bleaching using
Glotaran target analysis, on one hand, and the ratio of singlet and
triplet excited state molar extinction coefficients and absorptions, on
the other hand.
(68) We have performed target analyses of the ns-TAS data, that is,
starting with about a 1 nanosecond time delay, using the gauche-to-
trans ratios determined in steady-state experiments – vide supra. This
selection is based on the fact that the intersystem
crossing/fluorescence dynamics in the trans-conformer are on the
order of tens of nanoseconds. Of great relevance is the fact that we
have obtained very good fits of the ns-TAS experiments, which are
shown in the supporting information (Figures S27 and S28), in cases
that the strongly fluorescent trans conformer is present in more than
10%. In contrast, 10% or less of the trans conformer leads to
unreasonable fitting results.
(69) The weaker electronic coupling in D led to an overall lower
quality of the spectra, hence shifting our focus on A.
(70) The possible formation of aggregates of D at low temperatures
might explain why 5(T1T1) was detected in trEPR measurements,
albeit with lower quality, while in ns-TAS measurements only a small
amount of triplet excited state via intersystem crossing is detected.
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