Chromophore Multiplication To Enable Exciton Delocalization and Triplet Diffusion Following Singlet Fission in Tetrameric Pentacene

Article abstract of DOI:10.1002/anie.201907221

A tetrameric pentacene, PT, has been used to explore the effects of exciton delocalization on singlet fission (SF). For the first time, triplet decorrelation through intramolecular triplet diffusion was observed following SF. Transient absorption spectros

Full text of DOI:10.1002/anie.201907221

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Accepted Article
Title: Chromophore multiplication to enable exciton delocalization and
triplet diffusion following singlet fission in tetrameric pentacene
Authors: Constantin Hetzer, Bettina Basel, Sebastian M Kopp, Frank
Hampel, Fraser White, Timothy Clark, Dirk Guldi, and Rik R
Tykwinski
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907221
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A tetrameric pentacene is designed as
Constantin Hetzer,† Bettina S. Basel,†
Sebastian M. Kopp, Frank Hampel,
Fraser J. White, Timothy Clark,* Dirk M.
Guldi,* and Rik R. Tykwinski*
a
model system to explore the
influence of exciton delocalization on
singlet fission and to study subsequent
triplet decorrelation
diffusion following
via
exciton
Page No. – Page No.
intramolecular
singlet fission in dilute solution. Using
transient absorption spectroscopy, we
document the delocalization of singlet
and triplet excitons amongst the four
pentacenes.
Chromophore multiplication to enable
exciton delocalization and triplet
diffusion following singlet fission in
tetrameric pentacene
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Chromophore multiplication to enable exciton delocalization and
triplet diffusion following singlet fission in tetrameric pentacene
Constantin Hetzer,†[a] Bettina S. Basel,†[b] Sebastian M. Kopp,[c] Frank Hampel,[a] Fraser J. White,[d]
Timothy Clark,* [e] Dirk M. Guldi,*[b] and Rik R. Tykwinski*[c]
Abstract: A tetrameric pentacene PT has been used to explore
the effects of exciton delocalization on singlet fission (SF). We
document, for the first time, triplet decorrelation via intramolecular
triplet diffusion following SF. Transient absorption spectroscopy is
used to document different decorrelation mechanisms (triplet
diffusion versus structural changes) for PT and its dimeric
equivalent PD based on the rate and activation barrier of the
decorrelation step. Charge-separation experiments using
tetracyano-p-quinodimethane (TCNQ) to quench triplet excitons
formed through SF demonstrate that enhanced intersystem
crossing, i.e., spin catalysis, is a widely underestimated obstacle
to quantitative harvesting of the SF products. The importance of
spatial separation of the decorrelated triplet states is emphasized,
and independent proof that the decorrelated triplet pair state
consists of two (T1) states per molecule is provided.
tuned through control of relative distance and orientation, as well
as conjugation.[4] Despite the success of dimers in dissecting the
5]
role of, e.g., charge transfer, quintet and excimer states,[2b,
further breakthroughs in SF are currently hampered by the
intrinsic limitations outlined below.
In solids, intermolecular coupling allows diffusion of the
initial singlet excitation over several molecules.[3c] Moreover,
triplet decorrelation implies a distinct change in the coupling
6]
between triplets,[2a,
facilitated by triplet diffusion.[6-7] The
analogous diffusion is, however, impossible in molecular dimers,
and triplet-triplet annihilation of (T1T1) usually governs recovery of
(S0S0).[8] Thus, oligomers and polymers have evolved as model
systems for probing triplet diffusion.[9] Nevertheless, after SF in a
polymer, triplet diffusion from correlated triplets is hindered by
strong coupling. This ultimately results in rapid recombination,
rather than the desired formation of free triplets.
The present work documents the influence of chromophore
multiplication on iSF. Chromophore multiplication differs from
oligomerization: All chromophores remain electronically
independent because of the non-conjugated spacer in PT. The
adamantane scaffold is ideal for investigating chromophore
multiplication via tetrameric PT (Figure 1).[10] Using photophysical
analyses and comparisons to dimeric PD and monomeric PM,
supplemented with theoretical calculations, we identify
differences in excitation behavior, SF, and triplet decorrelation
that result from chromophore multiplication. This work concludes
by solution-state “harvesting” of the products of SF via charge-
separation experiments with the electron acceptor tetracyano-p-
Singlet fission (SF) depends on the interaction of a photoexcited
singlet state (S1) with
a
neighboring ground-state (S0)
chromophore, and results in the spontaneous splitting of (S1) into
a pair of correlated triplets (T1T1).[1] To harvest both triplet excitons
(T1T1) must, however, transition into a pair of uncorrelated triplets
(T1+T1). This decorrelation step is usually mediated by spin mixing
5
with a quintet spin-correlated triplet pair, (T1T1),[2] as outlined in
several excellent reviews.[3]
Successful SF requires a minimum of two chromophores,
and dimers such as PD (Figure 1) have been studied in dilute
solution to outline many of the mechanistic aspects of
intramolecular SF (iSF). The power of organic synthesis is key
and allows the electronic coupling between chromophores to be
quinodimethane
(TCNQ).
Importantly,
charge-separation
experiments provide independent evidence that the decorrelated
triplet pair state must consist of two non-interacting triplets on the
same molecule, rather than a single triplet per molecule. These
studies demonstrate that enhanced intersystem crossing (EISC)
hinders quantitative harvesting of the triplet products of SF. Thus,
[a]
[b]
[c]
Dr. C. Hetzer, Dr. F. Hampel
Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials (ICMM), Friedrich-Alexander-Universität
Erlangen-Nürnberg (FAU)
Nikolaus-Fiebiger-Straße 10, 91058 Erlangen (Germany)
B. S. Basel, Prof. Dr. D. M. Guldi
Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials (ICMM), FAU
Egerlandstrasse 3, 91058 Erlangen (Germany)
E-mail: dirk.guldi@fau.de
S. M. Kopp, Prof. Dr. R. R. Tykwinski
spatial triplet separation is now firmly established as
a
prerequisite for harvesting the two triplet excitons from SF
efficiently.
Sii-Bu3
i-Bu3Si
Sii-Bu3
Department of Chemistry, University of Alberta, Edmonton, Alberta,
Canada T6G 2G2.
PM
E-mail: rik.tykwinski@ualberta.ca
[d]
[e]
Dr. F. J. White
Rigaku Europe, Unit B6, Chaucer Business Park, Watery Lane,
Kemsing, Sevenoaks, TN15 6QY, United Kingdom.
Prof. Dr. T. Clark
Computer-Chemistry-Center, Department of Chemistry and
Pharmacy, FAU
i-Bu3Si
Sii-Bu3
i-Bu3Si
PD
Sii-Bu3
PT
Nägelsbachstrasse 25, 91052 Erlangen, Germany.
E-mail: tim.clark@fau.de
These authors contributed equally to this work.
Figure 1. Structures of PM, PT, and PD.
[†]
Supporting information for this article is given via a link at the end of
the document.
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R
1.
H
1. n-BuLi
THF
-78 °C
O
H
O
R
O
MeO
2. H2O
O
O
H
H
R
5
i-Bu3Si
R
2
CBr4
PPh3
CH2Cl2
PT
(18% from 1)
1 R = H (52% from 2)
4 R = Li
2. H2O
3. SnCl2 • 2H2O
10% aq. H2SO4
LiHMDS
THF
3
–78 °C
Scheme 1. Synthesis of PT
Tetraethynyladamantane
1
was the key building block for
Steady-state spectroscopy confirms that the pentacene moieties
of PD and PT are electronically independent in the ground state:
The intensities of the absorption bands evolve linearly as a
function of the number of chromophores (Figure S6). Closer
inspection, however, reveals that the FWHM (full width at half
maximum) decreases as a function of the number of pentacenes
(Table S1). Upon normalization, the relative intensities of the
fundamental 0-*0 transitions increase relative to those of the 0-*1,
0-*2, and 0-*3 transitions, and the increase depends on the
number of pentacene groups (Figures 3A, 3C). Thus, the Franck-
Condon overlap for the 0-*0 transition increases with the number
of chromophores. Fluorescence measurements show that the
quantum yield is significantly lower in PD and PT than in PM
(Table S2). The Stokes shifts decrease in the order of PM > PD
> PT (Figures 3B, 3D), with a concurrent sharpening of the
fluorescence spectra regardless of solvent polarity. In other words,
the polarity of the excited singlet states is largest for PM and
smallest for PT.
pentacene tetramer PT (Scheme 1). The work of Hokura and
Neamura gives, to our knowledge, the only published procedure
for 1,[11] but it has several challenging steps. Thus, an alternative
synthetic
route
was
developed.
Starting
from
1-
bromoadamantane, aldehyde 2 was formed in five steps (see SI),
and dibromoolefination gave intermediate 3. Exhaustive
elimination effected with BuLi, followed by aqueous workup, gave
1 via a Corey-Fuchs-like reaction. Lithiation of 1 with LiHMDS
gave intermediate 4, to which a solution of ketone 5 was added.
The reaction was subsequently quenched at low temperature via
the addition of H2O. Without further purification, the intermediate
was subjected to reductive aromatization.[12] Purification via
column chromatography gave tetramer PT. Tetramer PT has
reasonable solubility (ca. 4 mg mL–1) in typical organic solvents
(THF, CH2Cl2, CHCl3, toluene), but slowly decomposes over
several days in solution upon exposure to air and light. In the
absence of air (O2), solutions are stable for 3–5 days, and PT is
stable as solid under normal laboratory conditions.
X-ray crystallographic analysis of PT documents the
symmetrical structure and shows that there is no appreciable
intramolecular p-overlap between in the solid state (Figure 2, see
also SI). Each pentacene moiety shows p-stacking with that of a
neighboring molecule with an interplanar distance between acene
cores of 3.6 Å (Figure S1). p-Stacking is limited to two acenes,
however, and there is no long-range p-overlap.
Figure 3. Normalized steady-state absorption and fluorescence (excitation at
590 nm) data of PM, PD, and PT.
Figure 2. Solid-state structure of PT showing the spatial arrangement of the
pentacene units (H atoms omitted; ORTEPs at 30% probability).
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Femtosecond (fsTA) and nanosecond (nsTA) transient absorption
experiments were used to investigate the excited-state dynamics.
A change in dipole moment upon excitation is associated with
solvent relaxation during the first picoseconds after excitation,
which is typically manifested as a blue shift of singlet excited-state
features in the NIR (Figure 4B).[5a] The energy gain upon
relaxation (ΔE) decreases from PM to PD and to PT (Figure 4B),
consistent with the emission behavior. We conclude that the
polarity of the singlet-excited state decreases with the number of
pentacenes, consistent with delocalization of the singlet-excited
state over all four pentacene moieties. Calculations support this
hypothesis, showing only small splitting between the four singlet-
excited states that formally result from four locally excited
pentacene singlets (see SI). Minimal splitting, coupled with
thermal motion, leads to time-averaged delocalization (by Dexter
energy transfer) of the singlet-excited state. Note that this
observation is supported by the fact that no Davidov splitting is
observed in the experimental absorption spectra. Excited states
S1-S3, which all have significant intensity, lie within 0.02 eV. Only
S4, which is dark, is split significantly (0.14 eV) from the other
three. The same is true for the four triplet states that formally result
from localized pentacene triplet states. Thus, thermal mixing gives
way to time-averaged delocalization and reduced polarity on the
timescale of solvent reorganization.
toluene of 1.2 x 109 s–1 and 3.3 x 109 s–1 for PD and PT,
8a]
respectively, suggesting a superexchange mechanism.[5a,
Finally, (S1S0S0S0) and (S1S0) decay, and the characteristic
pentacene triplet-excited state features evolve. Concurrent with
singlet decay, ground-state bleaching intensifies by a factor of ca.
two. This suggests nearly quantitative iSF for PD and PT (Figures
4A, S7–S8), while PM produces <1% triplet (Figure S9). The
triplet-excited state decay is triexponential for PD and PT (PD/PT),
including the singlet correlated triplet pairs 1(T1T1)/1(T1T1S0S0), the
quintet correlated triplet pairs 5(T1T1)/5(T1T1S0S0), and the
decorrelated triplet pairs (T1+T1)/(T1+T1+S0+S0).[13] Triplet
5
decorrelation lifetimes stand out. The (T1T1) lifetime of PD and
5(T1T1S0S0) of PT are 91 and 70 ns, respectively. If a localized pair
of spin-correlated triplets undergoes decorrelation via structural
changes, then rates and yields of decorrelation should be
essentially equal for PT and PD because of the analogous
geometrical relationships between the pentacene chromophores.
This is not the case, however, as the triplet excitations in
5(T1T1S0S0) are free to diffuse within the tetramer, which
statistically affords more rapid decorrelation for PT. This is also
supported by the small splitting between the four relevant triplet
states of PT, as supported by theoretical calculations (vide supra).
Temperature-dependent nsTA measurements for PD and PT in
BN have been performed (Figures S13–S19) to provide the
activation energy (EA) for triplet decorrelation (Figure 5). Triplet
decorrelation arises via a change in coupling between the two
triplet states. In the solid state this occurs as two triplets diffuse
away from each other. In a dimer, changes in coupling would be
effected by, for example, bond rotations. Triplet decorrelation in
PD faces a barrier of ca. 13 meV, consistent with the geometric
rearrangement. Decorrelation in PT, however, lacks any
appreciable temperature dependence (EA = 0), and an alternative
mechanism must thus be operative: Decorrelation occurs through
diffusion of a triplet excitation to any of the pentacenes in a
ground-state configuration with a slightly different conformation
than that of the initially excited triplet.
On a longer timescale, the excited-state dynamics of PD
and PT are similar (Figures 4A, S7–S8), and singlet decay for
both is significantly faster than for PM (Table S3). In benzonitrile
(BN), the singlet excited state lifetimes are 6.2 ns (S1), 402 ps
(S1S0), and 129 ps (S1S0S0S0) in PM, PD, and PT, respectively,
and reflect the statistical likelihood of iSF. Delocalized over four
chromophores, iSF in PT offers six different pairs of pentacenes
in a favored orientation: [(T1T1S0S0), (T1S0T1S0), (T1S0S0T1),
(S0T1S0T1), (S0T1T1S0), (S0S0T1T1)]. This leads to a higher rate of
iSF for PT than PD, as well as a concurrent increase in the rate
constant of SF (kSF) by a factor of ~3, from 2.3 x 109 s–1 (PD) to
6.8 x 109 s–1 (PT). This finding is in line with results for oligomeric
pentacenes.[9b] Solvent polarity also dictates kSF with values in
Figure 4. A) Evolution associated spectra (EAS) of the TA data of PT in argon-saturated benzonitrile (BN). See Figure S7 for more details. B) EAS of the blue shift
of the fsTA NIR singlet feature of PM, PD and PT in argon-saturated BN and toluene (Tol). Ex = excitation wavelength.
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[(P•++S0)/(P•++S0+S0+S0) + TCNQ•–] (Figures 6, S35–S39), which
gives a quantum yield for ET of essentially 100% relative to
(T1+T1)/(T1+T1+S0+S0) for all data sets. Thus, only one of the two
triplets is quenched, and the other is deactivated to the ground
state due to EISC.
In conclusion, we show that the tetrameric structure of the
PT is an excellent model system for SF in solids beyond the two-
chromophore approximation of dimers. Solvent-dependent
experiments establish that the singlet-excited state of PT is
delocalized (from
a time-averaged perspective) over four
pentacenes by thermal mixing, as observed in crystalline
chromophores.[3c] Similar to (T1T1) found in PD, coupling in
(T1T1S0S0) for PT is sufficiently weak to facilitate triplet
decorrelation. The mechanisms of triplet decorrelation in PD and
PT are, however, different. In PD, temperature-dependent TA
measurements demonstrate that decorrelation is triggered by
geometric rearrangements, while in PT triplet diffusion occurs
most likely by a Dexter-type energy transfer resulting in time-
averaged delocalization. These conclusions are supported by
both experiment and theory. Experiments with TCNQ
demonstrate that, following formation of the decorrelated triplet
pair, trapping of one triplet by charge separation leads to an
accelerated decay of the second triplet to the ground state by
EISC. This establishes that efficient, spatial diffusion of the triplet
states is a crucial parameter toward harvesting both triplet
excitons.
Figure 5. Arrhenius plots of the triplet decorrelation rate kdecoor for PD and PT.
See Table S5 for details.
For SF to be implemented in solar energy conversion
schemes, it is imperative that both triplets can be harvested.
Charge-separation experiments with the acceptor tetracyano-p-
quinodimethane (TCNQ) were attempted toward achieving this
goal; an excess of TCNQ (6.3–200 equiv.) was used relative to
PD or PT to guarantee pseudo first-order reaction conditions. In
the presence of TCNQ, both PT and PD show accelerated decay
of the triplet features at 508 nm (Figure S24) and concomitant
growth of new features at 700–900 nm, which are spectroscopic
fingerprints of TCNQ•– (Figures S20–S24).[14]
Sequential analyses of the nsTA data yields good fits of the
transient kinetics using five transient species for PD/PT:
(S1S0)/(S1S0S0S0),
(T1+T1)/(T1+T1+S0+S0) and the charge-separated state (P•+
1(T1T1)/1(T1T1S0S0),
5(T1T1)/5(T1T1S0S0),
+
TCNQ•–), in which (P•+) is the one-electron oxidized form of
pentacene (Figures S26–S31). A sequential fit demonstrates that
the charge-separated state (P•+ + TCNQ•–) is predominantly
populated by a diffusion-controlled reaction of TCNQ with
(T1+T1)/(T1+T1+S0+S0). Nevertheless, a minor contribution stems
from interactions of TCNQ with the quintet state
5(T1T1)/5(T1T1S0S0).
Target analyses of the corresponding data for PT and PD
were then performed assuming that two charge separated states
(P•+
+
TCNQ•–) evolved from 5(T1T1)/5(T1T1S0S0) or
(T1+T1)/(T1+T1+S0+S0), to yield [(P•++P•+)/(P•++P•++S0+S0) + 2 ×
TCNQ•–] (Figure S33). Under this assumption, the quantum yield
for electron transfer (ET) to TCNQ was ca. 50% relative to
(T1+T1)/(T1+T1+S0+S0). In other words, one, rather than two,
charge-separated state was generated per (T1+T1)/(T1+T1+S0+S0).
The fact that (P•++T1)/(P•++T1+S0+S0) is harder to oxidize than
(T1+T1)/(T1+T1+S0+S0) is not fully compatible with our
observations. Considering that only
(T1+T1)/(T1+T1+S0+S0) is oxidized, the spectroscopic signatures
of the charge-separated state (P•+ TCNQ•–) should be
a
single triplet in
Figure 6. A) nsTA raw data for PT in Ar-saturated BN. B) nsTA raw data for PT
and TCNQ in Ar-saturated BN. C) Species associated spectra (SAS) and
corresponding population kinetics of the nsTA data shown in (B) as obtained by
target analysis. See Figure S34 for the kinetic model and Table S14 for rate
constants.
+
discernable in concert with those of the second triplet, which is
definitely not the case. Rather, the triplet decay goes hand-in-
hand with the formation of the (P•+ + TCNQ•–) features, i.e., both
triplets in (T1+T1)/(T1+T1+S0+S0) deactivate simultaneously.
Nevertheless, charge separation is only 50%. We conclude that
(P•+) accelerates deactivation of the triplet-excited state of the
neighboring pentacene, namely a rate increase of ISC induced by
a neighboring unpaired electron, i.e., enhanced intersystem
crossing (EISC) or spin catalysis.[15] Finally, a revised target
analysis considered the formation of only one charge-separated
state per 5(T1T1)/5(T1T1S0S0) or (T1+T1)/(T1+T1+S0+S0) to afford
Acknowledgements
The authors are grateful for support from the DFG Cluster of
Excellence ‘‘Engineering of Advanced Materials’’, ‘‘Solar
Technologies go Hybrid’’ (an initiative of the Bavarian State
Ministry for Science, Research and Art), the Emerging Fields
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Keywords: time-resolved spectroscopy • pentacene oligomers •
singlet fission • triplet decorrelation • charge transfer
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