Linker-Dependent Singlet Fission in Tetracene Dimers

Article abstract of DOI:10.1021/jacs.8b04401

Separation of triplet excitons produced by singlet fission is crucial for efficient application of singlet fission materials. While earlier works explored the first step of singlet fission, the formation of the correlated triplet pair state, the focus of

Full text of DOI:10.1021/jacs.8b04401

Subscriber access provided by UNIV OF DURHAM
Article
Linker-Dependent Singlet Fission in Tetracene Dimers
Nadezhda V. Korovina, Jimmy Joy, Xintian Feng, Cassidy Feltenberger,
Anna I. Krylov, Stephen E. Bradforth, and Mark E Thompson
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04401 • Publication Date (Web): 17 Jul 2018
Downloaded from http://pubs.acs.org on July 17, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Linker-Dependent Singlet Fission in Tetracene Dimers
Nadezhda V. Korovina‡†, Jimmy Joy‡, Xintian Feng§, Cassidy Feltenberger, Anna I. Krylov,
Stephen E. Bradforth* and Mark E. Thompson*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
KEYWORDS Singlet fission, dimers, tetracene, femtosecond spectroscopy, cyclic voltammetry
ABSTRACT: Separation of triplet excitons produced by singlet fission is crucial for efficient application of singlet fission materials. While earlier
works explored the first step of singlet fission – the formation of the correlated triplet pair state, the focus of recent studies has been on under-
standing the second step of singlet fission – the formation of independent triplets from the correlated pair state. We present the synthesis and
excited state dynamics of meta- and para-bis(ethynyltetracenyl)benzene dimers that are analogues to the ortho-bis(ethynyltetracenyl)benzene
dimer reported by our groups previously. A comparison of the excited-state properties of these dimers allows us to investigate the effects of
electronic conjugation and coupling on singlet fission between the ethynyltetracene units within a dimer. In the para isomer, in which the two
chromophores are conjugated, the singlet exciton yields the correlated triplet pair state, from which the triplet excitons can decouple via mo-
lecular rotations. Contrastingly, the meta isomer in which the two chromophores are cross-coupled, predominantly relaxes via radiative decay.
We also report the synthesis and excited-state dynamics of two para dimers with different bridging units joining the ethynyltetracenes. The rate
of singlet fission is found to be faster in the dimer with the bridging unit that has orbitals closer in energy to that of the ethynyltetracene chro-
mophores.
INTRODUCTION
The effect of the electronic coupling on singlet fission is best demon-
strated using covalently-coupled chromophores (such as dimers and
larger arrays).11-17 Analysis of the excited-state dynamics of dimers in
solution is advantageous because it allows one to separate the effects
of coupling from the solid-state effects (such as intermolecular exci-
ton delocalization) on the rates of singlet fission. Substantial pro-
gress has been made toward the synthesis and photophysical charac-
terization of acene dimers and larger arrays, as well as in covalently
linked systems of other singlet fission chromophores.18-36 Efficient
singlet fission has been reported in dimers wherein the constituent
chromophores exhibit substantial π orbital overlap, or are linked to-
gether in a conjugated manner; conversely, dimers in which the
chromophores are cross-conjugated exhibit slower singlet fission.
Guldi et al. have reported fast and efficient singlet fission in ortho-,
meta- and para-TIPS-pentacene dimers, in which the favorable en-
ergy(S1) ≥ 2×energy(T1) alignment provides the enthalpic driving
force for fission, such that even the weakly coupled meta pentacene
dimers exhibited singlet fission on picosecond timescales.19, 37 In di-
mers of tetracene and diphenylisobenzofuran, in which the chromo-
phores are without any significant π orbital overlap or conjugation,
and are out of plane with respect to each other, singlet fission was
reported to be slow and inefficient, likely due to weak coupling and
the energy(S1) ≤ 2×energy(T1) energy alignment.38-40 Recent work
on singlet fission with pentacene has also explored the role of con-
formational dynamics in facilitating excited state processes,41 observ-
ing the correlated triplet pair state with mid-IR spectroscopy42 and
polarization resolved transient absorption microscopy,43 and using
electron paramagnetic resonance44 and femtosecond stimulated Ra-
man spectroscopy45 as probes for singlet fission. Ito et al. have theo-
retically studied how the next-nearest-neighbor interactions affect
singlet fission in ortho-, meta- and para-pentacene dimers46 while
Singlet fission is a process in which a singlet (S1) excited state of a
molecule shares energy with a neighboring chromophore, yielding a
pair of triplet (T1) excitons.1-3 Incorporation of such materials in or-
ganic solar cells could improve the power conversion efficiencies by
doubling the number of excitons generated by high energy photons,
thereby averting thermal losses. Hanna and Nozik have reported
that the Shockley-Queisser limit – the theoretical power conversion
efficiency limit of 33% for a single junction device – could be in-
creased up to 44.4% by combining an efficient singlet fission material
with a red absorber in a device4 while Tayebjee et al. take endother-
mic fission into account to derive an efficiency of 45.9%.5 In the sem-
inal work of Congreve et al., an internal quantum efficiency of 160%
was demonstrated in a pentacene/poly(3-hexylthiophene) device6;
however, the power conversion efficiencies of most singlet fission
devices are sub-optimal, which leaves room for improvement in re-
gard to material design.
An important criterion for singlet fission materials for solar cell utili-
zation is fast rate of fission, to ensure that charge injection occurs
from the triplet state, rather the singlet state; particularly, singlet fis-
sion rates faster than 100 ps are desirable.7 The rate of singlet fission
is proportional to the square of the coupling
1
̂
(
)
term: ⟨푆1푆0|퐻푒푙| 푇 푇 ⟩ between the [S1S0] state and the biexci-
1
1
ton state, often described as correlated triplet pair [1(T1T1)], and is
also dependent on the energy gap between these states.8 Early mod-
1
̂
(
)
1
els by Smith and Michl suggested that ⟨푆1푆0|퐻푒푙| 푇 푇 ⟩ is max-
1
imized in slip-stacked, π-overlapping arrangements of the chromo-
phores1 however, more sophisticated theoretical treatments3 show
that coupling can also be significant in sandwiched and even copla-
nar chromophores.3, 9, 10
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 25
1
Zeng has used quantum dynamical simulations to explore the role of
through bond coupling in singlet fission dimers.47
ethynyltetracene have bound (T1T1) states, whereas meta coupled
dimers should have un-bound 1(T1T1) states.65 The boundedness of
the 1(T1T1) state is an important factor when considering the appli-
cations of singlet fission materials. The materials in which the
1(T1T1) state is bound will not readily produce free triplet excitons,
which are essential for reaping the benefits of singlet fission in de-
vices. Therefore, it is valuable to understand the method by which
the triplets can decouple from the initially formed biexciton state.
1
2
3
4
5
6
7
8
The currently accepted mechanistic picture of singlet fission in-
volves an intermediate biexciton state48, which can be described as a
correlated triplet pair [1(T1T1)], connecting the singlet excited state
[S0S1] with the two independent triplet states [3T1].1 Previously, we
reported fast and efficient formation of the biexciton state in or-
tho-bis(ethynyltetracenyl)benzene (o-BETB), in which the two te-
tracene units are oriented in a twisted configuration with π-overlap
and appear to be strongly electronically coupled – i.e. there is good
coupling between the [S0S1] and the [1(T1T1)] states. Although the
π orbital overlap between the tetracenes provides significant
through-space coupling, ab initio calculations showed that the con-
jugating, ortho-diethynylbenzene linker also contributes to the cou-
pling, i.e., via through-bond mechanism.49 To disentangle the role of
through-bond versus through-space coupling we synthesized the
meta and the para analogs of ortho-bis(ethynyltetracenyl)benzene
(m-BETB, p-BETB-ehex and p-BETB-ohex, Figure 1). The position
of the alkyne substitution on the benzene ring heavily influences the
ground state electronic structure of molecules containing these di-
ethynylbenzene units.50 In the ortho and the para isomers, the π-sys-
tems of the two chromophores are conjugated, which leads to
through-bond contributions to the coupling, whereas in the meta
isomer the linker does not facilitate conjugation between the te-
tracene units that are pendant on the alkynes, resulting in only small
through-space coupling. The conjugation through the linkers can be
understood in terms of the cumulene-like resonance form of the or-
tho- and para-diethynylbenzenes.51, 52 Importantly, the conjugation
manifest itself in strong interactions between the chromophores
both in the ground and in the excited state. The para-diethynylben-
zene particularly facilitates efficient electronic communication be-
tween the chromophores it bridges;53-55 as a result, compounds based
on this chemical motif have been incorporated into ultrasensitive
sensors,56 energy harvesting materials,57 and molecular electronic de-
vices.58, 59 The meta-diethynylbenzene is cross-conjugating and has
been demonstrated to provide only weak coupling between the
chromophores tethered to it.60, 61 Calculations of tetracene-based di-
mers and model structures have shown that the linker contributes
significantly to the coupling, via through-bond interaction, which af-
fects the character of the states involved in singlet fission.62-64 A re-
cent theoretical work by Abraham and Mayhall provides a model for
determining the effect of the covalent linker on the boundedness of
Although the favorable energy(S1) ≥ 2×energy(T1) energy aligment
in the dimers reported by Guldi et al. ensured fast singlet fission, it
was unresolvable with the instrument response of their transient
absorption spectrometer in ortho, and para dimers and rapid
annihilation to the ground state was observed. The isoenergetic
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
energy(S1)≈2×energy(T1)
conditions
in
our
bis(ethynyltetracenyl)benzene dimers result in singlet fission rates
that are fast but still resolvable with our instrument response, and the
dynamics of the population of each of the three states involved in
singlet fission could be observed in our para dimers. In comparison
to the tetracene dimers reported by Bardeen et al., in which the
acenes were directly attached to the phenyl linker and displayed
slow singlet fission, the tetracene units in our dimers are connected
to the phenyl linker via ethynyl groups. This ensures conjugation
between the tetracenes (via the cumulene resonance structure) and
less steric hindrance within a dimer, which could result in faster
singlet fission.40 The presence of ethynyl groups in our
ethynyltetracene dimers also perturbs the S1 and T1 state energies in
a favorable way, as evidenced by faster fission rates.49 Using ultrafast
spectroscopy, we observe that the para-bis(ethynyltetracenyl)ben-
zene dimers (p-BETB-ohex: R = OC6H13 and p-BETB-ehex: R = 2-
(ethyl)hexyl in Figure 1) undergo singlet fission form independent
triplet states, whereas no significant singlet fission in the
meta-bis(ethynyltetracenyl)benzene dimer (m-BETB) is observed.
To understand the differences in the excited-state behavior of the or-
tho vs. the para dimers, we present the transient spectroscopy of
p-BETB-ehex in THF and p-BETB-ehex suspended in a rigid
polymer matrix (poly-(methylmethacrylate), PMMA) and model
the kinetics using two populations that exhibit distinct excited state
dynamics. On the basis of modeling and calculating electronic
coupling as a function of the angle between the ethynyltetracenes,
we suggest that the rotation of ethynyltetracenes to a perpendicular
orientation facilitates the formation of independent triplets (i.e.,
productive singlet fission). To study the role of the linker in
facilitating singlet fission between the tetracenes, we have also
synthesized the para-bis(ethynyltetracenyl)benzene dimers
p-BETB-ehex and p-BETB-ohex (Scheme 1) with different linker
units and observe that singlet fission is faster in p-BETB-ohex.
1
the (T1T1) state based on Ovchinnikov rule. They concluded that
the ortho- and para-benzene dimers of ethynylpentacene and
Figure 1. The chemical (top row) and space filling structures (bottom
row) of o-BETB, m-BETB, and p-BETB-ehex and p-BETB-ohex. Two
para dimers with different substituents on the linker were synthesized
(p-BETB-ohex: R = OC6H13 and p-BETB-ehex: R = 2-(ethyl)hexyl).
Scheme I: The synthetic scheme for m-, p-BETB-ohex and p-
BETB-ehex. (i) NBS, DMF, CH2Cl2, 40oC; (ii) Pd(PPh3)4, CuI,
THF, Et3N, 60oC.
2
ACS Paragon Plus Environment

Page 3 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Table 1. Electrochemical properties of the four dimers compared to the monomer, and the optical gaps of these molecules. The ferroce-
nium/ferrocene redox couple in dichloromethane was used as an internal standard for electrochemical measurements.
Molecule
PET
o-BETB
m-BETB
p-BETB-ehex
p-BETB-ohex
1st Oxidation (V) 2nd Oxidation (V)
ΔE(Ox2-Ox1) (V)
Eredox (eV)
2.38
2.32
2.30
2.38
Eoptical (eV)
2.42
Eoptical- Eredox (eV)
0.46
0.37
0.41
0.46
0.33
---
---
0.04
0.05
0.10
-0.04
0.07
0.59
0.48
0.59
0.55
0.22
0.07
0.13
0.22
2.37
2.40
2.34
2.28
2.21
SYNTHETIC AND SPECTROSCOPIC METHODS
V is the least conjugated and p-BETB-ehex is intermediate with a dif-
ference of 0.13 V between the two oxidations. The electrochemical
properties of the dimers also provide information about the energet-
ics of intramolecular charge resonance (CR) states, which are im-
portant in the singlet fission mechanism.49, 67 The approximate en-
ergy of the intramolecular CR state (Eredox, Table 1) can be roughly
estimated from the difference between the oxidation (Eox) and the
reduction (Ered) potentials and a Coulombic term.68 The values of
Eox, Ered, and the electrochemical energy gap for the dimers and the
monomer obtained by CV are given in Table S1 in the supporting
information. The values of the first excited state (Eoptical), estimated
from the absorption spectra of these dimers, are also supplied in Ta-
ble 1 for comparison. In all molecules the electrochemical gap is en-
ergetically comparable to the optical gap, suggesting that CR config-
urations lie close enough in energy to the locally excited ones to po-
tentially influence singlet fission kinetics, as suggested in the case of
the series of TIPS-pentacene dimers.37, 69
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The meta and the para dimers were synthesized as shown in Scheme
1. The alkynyl motif in these dimers allows them to be constructed
via the highly efficient Sonogashira reaction.66 m-BETB was con-
structed from 5-bromotetracene and the commercially available
meta-diethynylbenzene. The para dimers were synthesized by So-
nogashira reaction between the appropriate diethynylbenzene linker
unit and 5-bromotetracene. The synthetic details and characteriza-
tion data are available in the Supporting Information (SI). Broad-
band femtosecond and nanosecond transient absorption spectra and
time-correlated single photon counting traces were collected from
solutions of the ethynyltetracene dimers made in 1 mm path length
quartz cuvettes using freshly distilled, de-aerated THF. Briefly, the
broadband femtosecond TA setup consists of a Coherent Legend
amplifier pumping a Spectra Physics OPA to provide the pump
beam and a CaF2 crystal to provide the white light super continuum
probe beam. Nanosecond transient absorption spectroscopy was
performed using the same probe beam and pump beam from a Al-
phalas Pulselas nanosecond laser. More details on steady state and
transient absorption spectroscopy, fluorescence lifetime measure-
ments, and electrochemistry are provided in the SI.
Steady state spectra
The intensities and line shapes of the steady-state extinction spectra
are representative of the magnitude of the transition dipole mo-
ments in the molecules and can reflect the extent of the S1S0- S0S1
coupling between the chromophores within the dimers. In our pre-
vious work, we showed that the ethynyltetracene moieties in
o-BETB are aligned such that the transition dipole moments of the
lower energy transition (along the short axis of the acene) are cou-
pled in an H-aggregate-like manner, as evidenced by the enhanced
ν0-1 and diminished ν0-0 absorption compared to PET (Figure 2).49
The absorption of the ortho dimer is redistributed to longer wave-
lengths, but the integrated intensity in the visible region is less than
twice that of the monomer. The absorption spectrum of m-BETB is
nearly identical to that of PET, but with a small red-shift of 4 nm
(0.02 eV). The relative intensities of the ν0-0 and the ν0-1 vibronic fea-
tures of the PET and the m-BETB are identical. The most notable
RESULTS AND DISCUSSION
Electrochemistry
The extent to which the ethynyltetracene units are conjugated in or-
tho-, meta- and para-bis(ethynyltetracenyl)benzene dimers can be
estimated from cyclic voltametry (CV). In the dimer the first oxida-
tion should be localized on one of the tetracenes and the second ox-
idation should take place on the second tetracene. Conjugation be-
tween the ethynyltetracenes in a dimer should give rise to a signifi-
cant difference between the first and second oxidation potentials, be-
cause the two ethynyltetracene units are not acting independently.
Conversely, if the ethynyltetracene units do not interact, both units
in a dimer would oxidize at the same potential and the difference be-
tween the first and second oxidation potentials would be close to
zero. The cyclic voltamograms of the ortho-, meta-, and
para-bis(ethynyltetracenyl)benzene dimers, as well as phe-
nylethynyltetracene (PET, monomer) were collected in dichloro-
methane, with ferrocene as the internal standard (additional experi-
mental details in SI). We observed two oxidation peaks in all dimers;
however, the peaks were poorly resolved in m-BETB, due to their
proximity (see SI). The oxidation potentials and the values of the
splitting between the oxidation peaks are given in Table 1. Based on
the magnitude of the splitting of the oxidation potentials, the chro-
mophores that are the most strongly conjugated are o-BETB and
p-BETB-ohex, with ΔE (Ox2-Ox1) values of 0.22 V, m-BETB at 0.07
Figure 2. The extinction spectra of m-BETB, p-BETB-ehex, and p-
BETB-ohex compared to o-BETB and PET in THF solution.
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 25
difference between PET and m-BETB absorption properties is in monomer PET, see SI). The analogous para-bis(ethynylanthra-
1
2
3
4
5
6
7
8
their molar absorptivity spectra: the molar absorptivity of m-BETB
is twice that of PET in the visible part of the spectrum (the ratio of
integrated areas is 2.0). This suggests that tetracenes in m-BETB are
effectively independent of each other and the electronic coupling be-
tween them is weak.
cenyl)benzene dimers emit with high quantum efficiencies: 97% for
para-bis(ethynylanthracenyl)benzene,52 and 60 % for the analogous
hexyloxy substituted para-bis(ethynylanthracenyl)benzene.72 Simi-
larly to the comparison of o-BETB to its anthracene analog in our
previous work, the high quantum efficiency of emission of the an-
thracene dimers compared to the tetracene dimers suggests that a
rapid excited-state decay channel is available in the para-tetracene
dimers, which is not available in the anthracene analogs. Conversely,
m-BETB is strongly emissive in solution (Figure 3), with a quantum
yield of 62%, suggesting that the rate and yield of non-radiative ex-
cited-state decay processes are not significant in this system. The
emission decay, shown in Figure 3b, has a small delayed component
(~190 ns), analogous to those observed for weakly-coupled te-
tracene dimers by Bardeen73 and Damrauer74, and could be sugges-
tive of triplet-triplet recombination. Presuming that the delayed flu-
orescence is a result of recombination of two geminate triplets gen-
erated by singlet fission, we used a kinetic model analogous to the
one reported by Muller et al. and Cook et al. to estimate the yield of
singlet fission to be approximately 1% in m-BETB, and the rate of
the decay of the delayed fluorescence to be solvent dependent (de-
tails in SI). The photoluminescence lifetimes of all solutions of m-
BETB in THF with concentrations varying from 1.5 μM to 21 μM
(see SI for details) is the same (~170 ns), indicating the absence of
any intermolecular reactions between two dimers in solution. The
triplets on m-BETB therefore recombine via a geminate pathway.
The absorption properties of the p-BETB-ohex and p-BETB-ehex
are different from those of o-BETB, m-BETB and the monomer
PET. The molar absorptivity of p-BETB-ehex in the visible region is
more than twice that of PET, with the area under the curve greater
than twice that of PET by a factor of 2.6). The red shift and signifi-
cant difference between the integrated absorptivity of the dimer ver-
sus twice that of the monomer suggest strong exciton coupling be-
tween the two ethynyltetracene units in p-BETB-ehex.70, 71 The in-
crease in molar absorptivity of p-BETB-ohex over PET is even
greater, with p-BETB-ohex having an absorptivity more than three
times greater than that of PET in the visible region (the ratio of the
integrated areas of the absorption bands is 3.3), and is accompanied
by a substantial red-shift in the visible absorption. This further
demonstrates that the electron-donating alkoxy substituents on the
bridging benzene ring significantly alter the electronic structure of
the dimers and in this case appear to enhance exciton coupling. The
intensity of the emission (see SI) gives insight into the rates of the
excited-state decay channels. Analogously to o-BETB, the para di-
mers are very weakly emissive (Φfl < 1%), suggesting that the process
which quenches the fluorescence in these systems is orders of mag-
nitude faster than ~10 ns (viz. the excited state lifetime of the
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Transient absorption of m-BETB
The femtosecond transient absorption spectra of m-BETB in THF
following excitation at 515 nm are shown in Figure 4(a). The char-
acteristic acene Sn  S1 absorption peak is observed at 400 nm,
Figure 3. (a) Steady state absorption and emission of m-BETB in THF.
(b) Emission decay of m-BETB in THF (excitation at 405 nm, emission
at 570 nm), with noticeable delayed fluorescence on the ~200 ns time-
scale.
Figure 4. The (a) femtosecond and (b) nanosecond transient absorp-
tion of m-BETB, with the sensitized Tn  T1 absorption shown in
black. Inset to b) shows an overlay of the transient spectrum of
m-BETB at 135 ns and the sensitized TnT1 spectrum.
4
ACS Paragon Plus Environment

Page 5 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
Figure 5. The femtosecond transient absorption (a), the species associated difference spectra (b) and concentration curves (c) from the target analysis
of the TA data of p-BETB-ohex in THF.
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
along with ground state bleach at 516 nm and 480 nm – similar fea-
tures were observed in the case of the ethynyltetracene monomer
(ETTMS) in our earlier studies.49 Between 10 ps and 975 ps, there is
no significant evolution of these spectral features. The SnS1 ab-
sorption decays after 5 ns when excited at 532 nm in a nanosecond
pump broadband transient absorption experiment (Figure 4b), with
a concomitant rise of a positive feature at ~510 nm. A peak in this
spectral region has been previously observed for TnT1 absorption
in acenes like 5,12-diphenyltetracene (DPT) and ET-TMS.37, 49, 75
More importantly, the spectral shape at 135 ns is identical to the sen-
sitized Tn  T1 absorption of m-BETB (inset to Figure 4b). The rise
of the absorption feature at 510 nm is slow, suggesting that the tri-
plets could be generated in this system by intersystem crossing, with
the rates similar to other tetracene derivatives.76 The transient ab-
sorption data was fit with a two-state sequential model (see SI) with
a time constant of 15 ns for the rise of the triplet feature and a triplet
lifetime of 4.6 μs. The triplets could also arise from a channel in
which very slow (and therefore inefficient) singlet fission results in
the delayed fluorescence from m-BETB in solution (~190 ns, Figure
3b).
ground-state absorption and excited-state emission (through contri-
butions from ground-state bleaching and stimulated emission) are
superimposed on the broader excited-state absorption bands in the
overall transient absorption spectra as seen in Figures 5(b) and 6(b).
The broad absorption feature at 425 nm decays within 100 ps, giving
rise to an induced absorption peak at 555 nm in p-BETB-ohex and
540 nm in p-BETB-ehex. This feature `of the para dimers decays
much more rapidly than that of m-BETB, and most importantly the
spectral shape of the induced absorption of p-BETB-ehex and p-
BETB-ohex at ~100 ps is also different from that of o-BETB re-
ported previously.49
To get an idea of the excited-state dynamics in the p-BETB dimers,
we subjected the TA data to target analysis.78 We obtained the best
fit using a 3-state sequential model ([state 1]→[state 2]→[state 3]).
The species associated difference spectra (SADS) and correspond-
ing concentration curves obtained from target analysis are shown in
Figures 5 and 6, respectively. The shapes of the SADS of
p-BETB-ohex and p-BETB-ehex in THF are qualitatively similar
(Figure 5(b) and 6(b)). The spectral signature of state 1 in both sys-
tems resembles the typical acene SnS1 absorption49, with a sharp
peak in the 375-470 nm region with a larger (negative) amplitude
from ground-state bleach and stimulated emission. In both systems,
state 1 rapidly decays to state 2 with a broader band in the
375-470 nm region, a net absorption from 575 – 650 nm and similar
ground state bleach features to state 1. The spectral shape of state 2
of p-BETB-ohex and p-BETB-ehex in THF resembles that of the
1(T1T1) state absorption in o-BETB. The difference in the SADS
corresponding to state 2 in p-BETB-ohex and p-BETB-ehex suggests
that the spectrum originating from this state is dependent upon the
structure of the bridging linker, and therefore the coupling between
acenes. Recall that based on the splitting in the oxidation potential
peaks, the conjugation between the tetracenes is the strongest in
o-BETB and p-BETB-ohex (0.22 V), and significantly weaker
(0.13 V) in p-BETB-ehex. The rate of formation of state 2 in the para
It is reasonable to assign the negligible singlet fission and slow for-
mation of triplets in m-BETB in solution to the weak non-adiabatic
coupling, similar to the meta dimer in a series of tetracene dimers
reported by Muller et al.40 Ab initio calculations further support this
notion. The coupling parameter, ||γ||2, between the S1 and the
1(T1T1) states in m-BETB is calculated to be 4.6×10-5, a value that is
about three orders of magnitude smaller than for o-BETB (0.04).64
Additionally, the computed binding energy (Eb) of the triplets (cal-
culated as the difference between the energy of the biexciton state-
and the two separated triplets) indicates how strongly the T1 exci-
tons are coupled in the 1(T1T1) state within the dimer and is reflec-
tive of the intra-molecular chromophore coupling. The value of Eb in
m-BETB was calculated to be <0.001 eV (compared to 0.15 eV for
o-BETB), suggesting that the tetracenes are weakly coupled in this
dimer. Although singlet fission was observed to proceed on a 200 ps
timescale in the TIPS-pentacene analog of m-BETB, the rate of sin-
glet fission reported in the meta dimer in the series of the
TIPS-pentacene dimers was slower by at least three orders of mag-
nitude compared to the ortho and para dimers.37, 77
1
dimers, however, is much faster than the formation of (T1T1) in
o-BETB. The target analysis gives values of 0.1 ps (essentially on the
timescale of the instrument response) for p-BETB-ohex and 0.4 ps
1
for p-BETB-ehex (Table 2), while in o-BETB, (T1T1) was formed
in 2 ps. The faster rate of formation in the para dimers than in the
ortho dimer highlights the importance of through-bond coupling.
The result suggests that through-bond coupling in the planar
p-BETB-ehex is stronger than the coupling in o-BETB, in which the
chromophores are predominantly coupled through-space, but also
exhibit some through-bond coupling via the linker, albeit to a lesser
extent because the chromophores in o-BETB are not in plane with
Transient absorption of p-BETB-ohex and p-BETB-ehex in THF
The TA spectra of p-BETB-ohex and p-BETB-ehex in THF, excited
at 545 nm, and 530 nm, respectively, are shown in Figure 5(a) and
6(a). At early times (< 25 ps), both compounds display the charac-
teristic excited-state absorption band at 425 nm, and ground-state
bleach at 520-540 nm. The vibronic structure associated with
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. The femtosecond transient absorption (a), the species associated difference spectra (b) and concentration curves (c) from the target analysis
of the TA data of p-BETB-ehex in THF.
the linker. Unlike in o-BETB, the 1(T1T1) state in p-BETB-ohex and
p-BETB-ehex decays to a third state with an identifiably different
spectral signature, with prominent induced absorption peaks at
555 nm and 540 nm in p-BETB-ohex and p-BETB-ehex, respec-
tively. The spectral shape of state 3 in p-BETB-ohex and
p-BETB-ehex in solution bears a strong resemblance to that of the
sensitized Tn  T1 induced absorption of the monomer, ET-TMS.49
state 3 resembles the TnT1 induced absorption of the monomer
more closely than that of the dimer. We surmise that in the case of
the sensitized triplet on the dimer the TnT1 excitation maybe de-
localized over more than just the one ethynyltetracene unit on to the
phenyl linker or even the second ethynyltetracene. Therefore the
sensitized triplet absorption of the dimer differs from that of the
monomer ETTMS. Whereas when singlet fission produces two tri-
plet excitons in the dimer they likely localize to their respective
acenes; and when the acenes rotate out of plane from each other the
spectrum of the triplet excitons effectively resemble the sensitized
The absorption of state 3 could correspond to either one or two tri-
plets per dimer. There are two factors which suggest that state 3 cor-
responds to absorption of two triplets on the same dimer: (i) the rate
of formation of state 3 is fast – 19 ps in p-BETB-ohex and 48 ps in
p-BETB-ehex (Table 2). These rates of T1 formation are too fast for
intersystem crossing in acenes76, but are in the range of the measured
singlet fission rates in tetracene,8, 37, 79, 80. (ii) The rate of the decay of
state 3 is very fast – 20 ps in p-BETB-ohex and 76 ps in
p-BETB-ehex. Typically, a single triplet exciton localized on an
acene dimer decays on the timescale of microseconds.76 However,
the recent studies of pentacene dimers showed that when two triplet
excitons are located on the same dimer, the TnT1 absorption de-
cays on the 10-100 ps timescale, due to the favorable T1-T1 fusion
pathway to the ground state.77, 81 Therefore, the SADS for state 3 in
the para dimers in THF is the spectral signature of two triplets in
close proximity to each other. The most likely origin of these two tri-
plets is via singlet fission.
49
triplet spectrum of monomer ETTMS. We estimated the triplet
yield in these compounds by scaling the target analysis populations
by the extinction coefficients of the singlet and the triplet absorp-
tions (see S.I. for details) and obtained 94% for p-BETB-ehex and
70% for p-BETB-ohex. The triplet yields are low in these para dimers
because the rate of recombination of the triplets is fast relative to the
rate of their formation.
Surprisingly, the observed behavior of our ortho-,49 meta- and para-
ethynyltetracene dimers is different from that of isomorphic dimers
of TIPS-pentacene reported by Tykwinski, Guldi and coworkers.37
All TIPS-pentacene dimers undergo singlet fission, with only the
meta-TIPS-pentacene dimer undergoing fission slowly enough to be
resolved by the instrument.45, 82, 83 In our previous work on o-BETB,
the formation of only the biexciton state was observed whereas here
In Figure 7, an overlay of the SADS for state 3 of p-BETB-ehex is
made with Tn  T1 absorption spectra for a single triplet sensitized
onto p-BETB-ehex and triplet sensitized onto the model monomer
compound, ETTMS. The details regarding sensitization experi-
ments have been previously reported.49 For both the sensitized spec-
tra, a spectral shift is required to line up the peaks with the state 3
SADS. Despite small discrepancies in the intensities in the
400 - 500 nm region, the relative peak positions and relative intensi-
ties overlay well, strongly suggesting that state 3 corresponds to
TnT1 induced absorption in p-BETB-ehex. It is worth noting that
Table 2. The time constants (in ps) of the decays used to fit the TA
data of the dimers in THF.
p-BETB p-BETB
o-BETB1 m-BETB2
-ohex
0.1
19
-ehex
0.4
48
τ [S1→1(T1T1)]
τ [1(T1T1)→ 3T1]
τ [3T1]
2.0
1.5 x 104
4.6 x 106
Figure 7. The plot shows the spectral signature of state 3 from target
analysis of p-BETB-ehex in THF compared to the normalized sensi-
tized Tn  T1 absorption of ET-TMS (red-shifted, 20 nm) and sensi-
tized Tn  T1 absorption of p-BETB-ehex in THF (blue-shifted,
9 nm).
-
-
20
76
1Taken from previously published work, reference 17.
2Time constants corresponding to the intersystem crossing rate and the
decay of that triplet.
6
ACS Paragon Plus Environment

Page 7 of 25
Journal of the American Chemical Society
dependent of each other. Thus, in order to explain singlet-fission
we find evidence of predominantly radiative decay in m-BETB. We
1
2
3
4
5
6
7
8
observe that the para-ethynyltetracene dimer forms two independ-
ent triplet states via the biexciton state. Using TCSPC (with more
dynamic range), we observe that a small fraction (1%, see SI) of m-
BETB molecules gives rise to delayed fluorescence, which were not
captured in transient absorption. The delayed fluorescence in m-
BETB is solvent dependent, which is similar to the meta-TIPS-
pentacene dimer. Solvent dependence suggests involvement of
charge resonance configurations. However, in contrast to meta-
TIPS-pentacene84, we do not observe the spectral signatures of the
charge resonance states in the transient absorption spectra of m-
BETB. We explain this difference by noting that the optical and elec-
trochemical gaps are closer in meta-TIPS-pentacene than in m-
BETB (1.88 eV vs 1.87 eV in the former and 2.30 eV versus 2.40
eV in the latter), which suggests that there is a smaller contribution
of the charge resonance states in m-BETB. The analysis of the S1
trends in flexible dimers, the AM model needs to be extended to ac-
count for changes in electronic structure upon twisting. Figure 8
shows the possible molecular orientations corresponding to the cor-
related and independent triplet pair states in p-BETB-ehex. Further
insight into the effect of rotation on the excited-state energies of
p-BETB-ehex can be obtained from theory. The energies of the S1,
1(T1T1), and 5(T1T1) states were computed as a function of the dihe-
dral angle between the tetracenes in p-BETB-ehex using the re-
stricted active space configuration interaction with double spin-flip
method (RAS-2SF-CI, Table 3). The minimum of the S1 potential
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
energy surface corresponds to a planar geometry and the (T1T1),
5
and (T1T1) states are energetically downhill from the S1 state by
about 0.4 eV at all rotational angles. The coupling between the S1
and the 1(T1T1) states (||γ||2) was calculated to be largest at a planar
geometry (0.020), and is 0.004 at 60 and drops to 0.000 at 90. The
1(T1T1) surface has a minimum at 60 degrees, while the 5(T1T1) sur-
wave-function
shows
0.2%
of
charge
resonance
1
configurations in m-BETB. In the TIPS-pentacene dimers, the en-
ergy of 2 x E(T1) at 1.54 eV is far below the energy of the S1 state at
1.88 eV. This energetic driving force for singlet fission in the
TIPS-pentacene systems results in ultrafast kinetics from which
mechanistic information is not easily obtained. In the case of
ethynyltetracene dimers presented here, the E(S1) ≈ 2E(T1) isoen-
ergetic conditions make singlet fission only slightly favorable. As a
result of which the subtle differences in the effect of through-bond
and through space coupling can be investigated.
face has a minimum at 90 degrees, at which the (T1T1) and the
5(T1T1) states become degenerate (see Table 3). These values indi-
cate that the triplet excitons from the correlated triplet pair state in
p-BETB-ehex can become non-interacting in the 5(T1T1) state when
the tetracenes rotate perpendicular to each other within the dimer.
Transient absorption of p-BETB-ehex in PMMA
If rotational motion of the tetracene in para dimer in solution pro-
vides the mechanism for generating free triplets, then restricting that
motion should reduce the rate and the yield of the formation of free
triplets. To test this hypothesis, a solution of p-BETB-ehex in a rigid
polymer matrix, poly(methylmethacrylate) (PMMA) was prepared.
p-BETB-ehex in PMMA produces a substantial amount of emission
at room temperature, with a quantum efficiency of ~10 % (see SI),
which means that a significant fraction of the molecules are confined
to a geometry in which the radiative decay is the predominant relax-
ation pathway from the singlet excited state. From a comparison of
the transient absorption spectra of p-BETB-ehex in THF and in
PMMA (Figure 9), it is evident that in PMMA the excited state ab-
sorption corresponding to S1→Sn absorption centered at 420 nm de-
cays much more slowly. The characteristic TnT1 absorption peak
at 550 nm still forms in this media; however, the rate of its formation
is much slower than for p-BETB-ehex in THF. Unlike p-BETB-ehex
in THF, the transient signals observed from p-BETB-ehex in PMMA
cannot be fit using a 3-state sequential model. PMMA, which a more
rigid medium than THF, raises the barrier to rotation for the te-
tracene units in p-BETB-ehex along the diethynylbenzene linker.
This results in p-BETB-ehex dimers that are conformationally
locked in the polymer matrix in a distribution of rotational angles.
Upon excitation in this rigid medium not all the p-BETB-ehex mol-
The ortho- and the para- bis-ethynylbenzene dimers exhibit dis-
tinctly different excited-state dynamics. The S1 state in the para di-
mers yield the 1(T1T1) state, which decays to form independent tri-
1
plets, whereas the (T1T1) state in o-BETB relaxes to the ground
state without producing independent triplet excitons. According to
the model proposed by Abraham and Mayhall (AM model), both
ortho and para dimers should produce bound 1(T1T1) states that are
unlikely to form non-interacting triplet excitons.65 Although the AM
model correctly captures the important aspect of the nature of the
biexciton states in these systems (as compared against RAS-2SF-CI
calculations) and explains the observations for o-BETB, it does not
explain the observed singlet fission dynamics in our para dimers,
which we attribute to its static nature. The difference in the excited-
state processes in the ortho and para dimers and the deviation from
the AM model can be rationalized by considering rotational motions
of the chromophores in these dimers. In o-BETB the tetracenes can
rotate, they always retain some degree of π overlap, which results
in non-zero biexciton binding energy. In the para dimers, on the
other hand, the chromophore interaction is limited to through-bond
only. Upon twisting of one of the tetracenes perpendicular to the
plane of the rest of the molecule, the electronic coupling between the
chromophores is broken and Eb becomes zero (Table 3). Conse-
quently, the triplets from the correlated triplet pair state become in
Table 3. The RAS-2SF-CI computed excited state energies (in eV)
and, binding energy (in eV) and coupling for p-BETB-ehex the para
dimer as a function of the dihedral angle (in degrees) between the
tetracenes.
1
ecules which end up in the (T1T1) state will be able to rotate and
form independent triplets. The TA spectra from
Angle
0
30
60
90
E(S1)
3.84
3.91
3.86
3.87
E(1(T1T1))
3.38
Eb
||γ||2
0.020
0.020
0.004
0.000
0.04
0.04
0.01
0.00
Figure 8. The proposed decoupling of the correlated triplet state in
p-BETB-ehex (alkyl chains have been removed for clarity) to give rise
to two independent triplets.
3.45
3.37
3.39
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 11. Dynamics of excited state absorption at 425 nm of
Figure 9. The femtosecond transient absorption of p-BETB-ehex in
PMMA.
p-BETB-ehex in PMMA (black trace) overlaid with the time slices at
425 nm of transient absorption spectra obtained by combining mod-
elled transient absorption of population 1 and population 2 in different
fractions. Numbers in the legend show the fraction of population 1 in
that model. The rate constants (s-1) that gave the best fit are: kr =
5.5 × 107, knr = 2.9 × 106, k12 = 5 × 1011, k21 = 4 × 1012, k22 = 2 ×
109, k12’ = 1.6 × 1012, k23 = 1.2 × 1010, k33 = 1.2 × 109.
p-BETB-ehex in PMMA with the tetracene units locked in all dihe-
dral angles can be modeled as a sum of signals from two model pop-
ulations with extreme conformations of the para dimer: population
1 with the tetracenes coplanar with each other and population 2 with
tetracenes perpendicular (Figure 10). Population 1 represents the
dimer molecules in PMMA which cannot undergo singlet fission
completely, and the 1(T1T1) remains in equilibrium with the S1 state,
giving rise to the fluorescence signal from the PMMA sample. Pop-
ulation 2 is representative of dimer molecules that can fully undergo
singlet fission to two independent triplets, and gives rise to the triplet
band at 550 nm in the TA spectra at 957 ns. Using compartmental
models to represent the excited-state processes in the distinct popu-
lations, it is possible to obtain concentration curves that describe the
dynamics of the excited states (details in the SI).78 The transient ab-
sorption spectra corresponding to both model populations 1 and 2
can be derived by multiplying the concentration curves with the Spe-
cies Associated Difference Spectra (from Figure 6b) corresponding
realistic model, the tetracene units in p-BETB-ehex in the polymer
matrix would be locked at all dihedral angles with equal probability.
This is quite likely the case since the monomer units in PMMA are
small, and would not hold the tetracenes at any one angle preferen-
tially by non-covalent interactions. The TA spectra of p-BETB-ehex
in PMMA and its modeling using conformationally restricted popu-
lations emphasizes the importance of the rotation of the tetracene
units in allowing the 1(T1T1) state to separate to independent triplets
in p-BETB-ehex and not in o-BETB – despite the similarities in elec-
tronic coupling. A similar rotational mechanism of formation of tri-
plets from the correlated triplet pair state has been reported in bi-
pentacene dimers by Tayebjee et al.90 The phenylene bridges in
these dimers resulted in a long lived 5(T1T1) state that had a lifetime
>200 ns and allowed the authors to characterize the quintet state us-
ing time-resolved EPR. According to the authors, the coupling in the
bipentacene BP2 with the pentacenes separated by two phenylene
units is greater than in BP3 with the pentacenes separated by three
phenylene units. This difference is reflected in the fact that in BP3
the quintet state evolves in to dissociated triplets while in BP2 the
quintet state gives rise to triplets that are still weakly interacting. Alt-
hough an analogous analysis for our p-BETB systems would be in-
sightful, it is unlikely to be fruitful in the case of our dimers. Based on
the fast timescale (< 100 ps) of the conversion from 1(T1T1) to elec-
tronically independent triplets (2 x T1) in p-BETB dimers, followed
by rapid return to the ground state, we can expect that the triplet ex-
citons do not have enough time to undergo spin dephasing, and only
undergo electronic decoupling. Analogous ESR experiments to
those carried out by Tayebjee et al. would be hampered by the short
excited state lifetime observed in p-BETB dimers.
1
to the S1, (T1T1), and T1-T1 states (see SI). The fractions of the
model population 1 and population 2 that best represent
p-BETB-ehex in PMMA is obtained by comparing the time slice of
the transient absorption data at 425 nm with the time slices at the
same wavelength of the modeled transient absorption data; the latter
is obtained by combining the TA spectra of the population 1 and
population 2 in different fractions and optimizing the time constants
for the various excited state processes by repeated iterations in
MATLAB. As seen in Figure 11, the best representation of the ex-
cited state dynamics is obtained by combining the dynamics of pop-
ulations 1 and 2 in equal fractions and reproduces the 10% quantum
yield measured from the PMMA film. This suggests that in a more
Finally, of additional interest are the differences in the excited-state
kinetics of p-BETB-ohex and p-BETB-ehex. All three of the pro-
1
1
cesses: relaxation of S1 into (T1T1), conversion of (T1T1) to inde-
pendent triplets, and the decay of the resultant triplets are faster in
p-BETB-ohex compared to p-BETB-ehex (Table 2). The difference
in the coupling of the tetracene units in p-BETB-ohex and
Figure 10. The state diagrams showing (a) population 1 and (b) popu-
lation 2 used to model the TA data from p-BETB-ehex in PMMA.
8
ACS Paragon Plus Environment

Page 9 of 25
Journal of the American Chemical Society
does not form a significant number of triplets and the ortho dimer
p-BETB-ehex can be understood in terms of the relative orbital en-
1
2
3
4
5
6
7
8
ergies of the tetracenes and the two bis(ethynyl)benzene linkers. A
qualitative diagram showing the relative orbital energies of these
molecules is provided in Figure 12. In the alkyl-substituted benzene
linker, the orbitals are expected to lie energetically far from those of
the tetracenes and the coupling is not expected to be very large.
However, introducing the electron donating hexyloxy substituents
onto the benzene linker raises the energy of the HOMO, and there-
fore brings the orbital energies of the linker closer to those of the te-
tracenes, resulting in stronger coupling between the acenes and
faster singlet fission in p-BETB-ohex.
which only forms the intermediate correlated triplet pair state. We
attribute the difference in the excited-state dynamics of the ortho
and para dimers to the rotational flexibility of the acenes in the para
dimer, which breaks the coupling between the chromophores and
allows the triplets to separate from the correlated triplet pair state.
Such rotation of the tetracene units in the ortho dimer is hindered
because they are confined to a cofacial orientation. The role of te-
tracenes rotating to form separated triplets is clear when comparing
the transient absorption of the para dimer in PMMA to that in THF.
PMMA being a more rigid medium prevents the rotation and signif-
icantly hinders the formation of the two non-interacting triplets.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A comparison of the two para-bis(ethynyltetracenyl)benzene di-
mers (p-BETB-ohex vs. p-BETB-ehex) provides insight into the role
of the energetics of the linker in singlet fission in covalent dimers.
The hexyloxy-bis(ethynyl)benzene linker in p-BETB-ohex has mo-
lecular orbitals that are closer in energy to orbitals of tetracenes than
the 2-ethylhexyl-bis(ethynyl)benzene linker in p-BETB-ehex. This
results in better mixing of the linker orbitals with those of the te-
tracenes and thus better coupling in p-BETB-ohex than in
p-BETB-ehex. Better coupling between the tetracene units results in
faster formation of the correlated triplet pair state and the separated
triplets in p-BETB-ohex than in p-BETB-ehex – highlighting the role
of linker energetics in singlet fission.
Figure 12. A diagram showing the closer lying orbitals of the linker and
the tetracenes in (a) p-BETB-ehex compared to (b) p-BETB-ohex. la/b
and ha/b are the frontier molecular orbitals of tetracene.
In separate experiments reported elsewhere, a comparison of the ex-
cited state dynamics of the meta and para dimers in neat thin films91
has allowed us to separately probe intra- and inter- dimer singlet fis-
sion between tetracene units. While the meta dimer doesn’t fission
in THF solution, in film inter-dimer singlet fission is turned on facil-
itated by through space interactions between the ethynyltetracene
units on adjacent dimers. In films of the para dimer, singlet fission
was facilitated by both intra-dimer through bond coupling, and in-
ter-dimer through space coupling such that the depopulation of the
singlet state happened at least twice (within the instrument re-
sponse) as fast compared to THF solution.
CONCLUSIONS
We have synthesized the meta and para analogues of the previously
reported ortho-bis(ethynyltetracenyl)benzene to study the role of
through-bond vs. through-space coupling in facilitating singlet fis-
sion between the tetracene units. The ortho- and para- ethynylben-
zene linkers provide conjugation between the tetracene units while
the meta- ethynylbenzene linker is cross-conjugating in the ground
state. Using broadband femtosecond transient absorption spectros-
copy, we have established that the meta-(ethynyltetracenyl)benzene
predominantly undergoes radiative decay in the excited state. Sur-
prisingly, the para-bis(ethynyltetracenyl)benzene dimer undergoes
complete singlet fission, forming independent triplet states via rota-
tion of the acenes out of plane from each other. As reported previ-
ously, the ortho-bis(ethynyltetracenyl)benzene dimer only forms
the intermediate correlated triplet pair state.
ASSOCIATED CONTENT
The synthetic details, structural characterization of the materials, elec-
trochemical data, details of sample preparation for spectroscopy, and ex-
cited state kinetic models are provided in the Supporting Information.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Comparing the behavior of ortho-, meta- and para-bis(ethynylte-
tracenyl)benzene dimers provides crucial insight into the role of
conjugation and through-bond coupling in singlet fission in dimers.
The ortho and para dimers in which the tetracene units are conju-
gated through the ethynylbenzene linker exhibit singlet fission while
the cross-linked meta dimer predominantly undergoes radiative de-
cay. Furthermore the [S1]→[1(T1T1)] process is faster in
p-BETB-ohex and p-BETB-ehex than in o-BETB, suggesting that
the through-bond coupling operating in the para dimers is stronger
than the combined through-space and through-bond coupling oper-
ating in the ortho dimer. Unlike in the case of a series of pentacene
dimers, where the energetics are so favorable for singlet fission that
the ortho-, meta- and para-TIPS-pentacene dimers all displayed tri-
plet absorption signatures in picosecond timescales, in our case, only
the para dimer forms independent triplets on similar timescales.
This provides an insightful comparison with the meta dimer which
AUTHOR INFORMATION
Corresponding Author
*stephen.bradforth@usc.edu
*met@usc.edu
Author Contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
‡N.V.K and J.J. contributed equally.
Present Addresses
† nadia.korovina@nrel.gov, National Renewable Energy Laboratory,
15013 Denver West Pkwy, Golden, Colorado 80401, United States
9
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 25
Diketopyrrolopyrrole Dimer: Implications for Singlet Fission. ChemPhotoChem
2017, 2 (3), 223-233.
17. Pace, N. A.; Zhang, W.; Arias, D. H.; McCulloch, I.; Rumbles, G.; Johnson, J.
C., Controlling Long-Lived Triplet Generation from Intramolecular Singlet
Fission in the Solid State. The Journal of Physical Chemistry Letters 2017, 8 (24),
6086-6091.
18. 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. The Journal of Physical Chemistry A 2016, 120
(11), 1867-1875.
19. 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 (19), 10113-10123.
20. Sanders, Samuel N.; Kumarasamy, E.; Pun, Andrew B.; Steigerwald,
Michael L.; Sfeir, Matthew Y.; Campos, Luis M., Singlet Fission in
Polypentacene. Chem 2016, 1 (3), 505-511.
21. Cook, J. D.; Carey, T. J.; Damrauer, N. H., Solution-Phase Singlet Fission in
a Structurally Well-Defined Norbornyl-Bridged Tetracene Dimer. The Journal of
Physical Chemistry A 2016, 120 (26), 4473-4481.
22. Carey, T. J.; Snyder, J. L.; Miller, E. G.; Sammakia, T.; Damrauer, N. H.,
Synthesis of Geometrically Well-Defined Covalent Acene Dimers for
Mechanistic Exploration of Singlet Fission. The Journal of Organic Chemistry
2017, 82 (9), 4866-4874.
23. Trinh, M. T.; Pinkard, A.; Pun, A. B.; Sanders, S. N.; Kumarasamy, E.; Sfeir,
M. Y.; Campos, L. M.; Roy, X.; Zhu, X. Y., Distinct properties of the triplet pair
state from singlet fission. Science Advances 2017, 3 (7).
24. Lehnherr, D.; Alzola, J. M.; Mulzer, C. R.; Hein, S. J.; Dichtel, W. R.,
Diazatetracenes Derived from the Benzannulation of Acetylenes: Electronic
Tuning via Substituent Effects and External Stimuli. The Journal of Organic
Chemistry 2017, 82 (4), 2004-2010.
25. Liu, H.; Wang, R.; Shen, L.; Xu, Y.; Xiao, M.; Zhang, C.; Li, X., A Covalently
Linked Tetracene Trimer: Synthesis and Singlet Exciton Fission Property.
Organic Letters 2017, 19 (3), 580-583.
26. Kumarasamy, E.; Sanders, S. N.; Tayebjee, M. J. Y.; Asadpoordarvish, A.;
Hele, T. J. H.; Fuemmeler, E. G.; Pun, A. B.; Yablon, L. M.; Low, J. Z.; Paley, D.
W.; Dean, J. C.; Choi, B.; Scholes, G. D.; Steigerwald, M. L.; Ananth, N.;
McCamey, D. R.; Sfeir, M. Y.; Campos, L. M., Tuning Singlet Fission in π-Bridge-
π Chromophores. Journal of the American Chemical Society 2017, 139 (36),
12488-12494.
27. Pun Andrew, B.; Sanders Samuel, N.; Kumarasamy, E.; Sfeir Matthew, Y.;
Congreve Daniel, N.; Campos Luis, M., Triplet Harvesting from Intramolecular
Singlet Fission in Polytetracene. Advanced Materials 2017, 29 (41), 1701416.
28. Dean, J. C.; Zhang, R.; Hallani, R. K.; Pensack, R. D.; Sanders, S. N.; Oblinsky,
D. G.; Parkin, S. R.; Campos, L. M.; Anthony, J. E.; Scholes, G. D., Photophysical
characterization and time-resolved spectroscopy of a anthradithiophene dimer:
exploring the role of conformation in singlet fission. Physical Chemistry
Chemical Physics 2017, 19 (34), 23162-23175.
§xintianf@gmail.com, Department of Chemistry, University of Califor-
nia, Berkeley, California 94720; Q-Chem Inc., 6601 Owens Drive,
Suite 105 Pleasanton, California 94588
1
2
3
4
5
6
7
8
Funding Sources
M.E.T., N.V.K., S.E.B and J.J. acknowledge support from the Center for
Energy Nanoscience, an Energy Frontier Research Center funded by
the U.S., Office of Science, Office of Basic Energy Sciences (DE
SC0001013). M.E.T. and N.V.K. additionally were supported by the
Nanoflex Power Corporation. A.I.K. acknowledges support through
the DE FG02 05ER15685 grant (Department of Energy).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
REFERENCES
1. Smith, M. B.; Michl, J., Singlet Fission. Chemical Reviews 2010, 110 (11),
6891-6936.
2. Piland, G. B.; Burdett, J. J.; Dillon, R. J.; Bardeen, C. J., Singlet Fission: From
Coherences to Kinetics. The Journal of Physical Chemistry Letters 2014, 5 (13),
2312-2319.
3. Casanova, D., Theoretical Modeling of Singlet Fission. Chemical Reviews
2018.
4. Hanna, M. C.; Nozik, A. J., Solar conversion efficiency of photovoltaic and
photoelectrolysis cells with carrier multiplication absorbers. Journal of Applied
Physics 2006, 100 (7), 074510.
5. Tayebjee, M. J. Y.; Gray-Weale, A. A.; Schmidt, T. W., Thermodynamic Limit
of Exciton Fission Solar Cell Efficiency. The Journal of Physical Chemistry
Letters 2012, 3 (19), 2749-2754.
6. Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. 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 (6130), 334-337.
7. Schrauben, J. N.; Zhao, Y.; Mercado, C.; Dron, P. I.; Ryerson, J. L.; Michl, J.;
Zhu, K.; Johnson, J. C., Photocurrent Enhanced by Singlet Fission in a Dye-
Sensitized Solar Cell. ACS Applied Materials & Interfaces 2015, 7 (4), 2286-
2293.
8. Kolomeisky, A. B.; Feng, X.; Krylov, A. I., A simple kinetic model for singlet
fission: A role of electronic and entropic contributions to macroscopic rates. The
Journal of Physical Chemistry C 2014, 118 (10), 5188-5195.
9. Feng, X.; Kolomeisky, A. B.; Krylov, A. I., Dissecting the effect of morphology
on the rates of singlet fission: Insights from theory. The Journal of Physical
Chemistry C 2014, 118 (34), 19608-19617.
10. Matsika, S.; Feng, X.; Luzanov, A. V.; Krylov, A. I., What We Can Learn from
the Norms of One-Particle Density Matrices, and What We Can’t: Some Results
for Interstate Properties in Model Singlet Fission Systems. The Journal of
Physical Chemistry A 2014, 118 (51), 11943-11955.
11. 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. Nature Chemistry 2016,
8, 1120.
12. Sung, J.; Nowak-Król, A.; Schlosser, F.; Fimmel, B.; Kim, W.; Kim, D.;
Würthner, F., Direct Observation of Excimer-Mediated Intramolecular Electron
Transfer in a Cofacially-Stacked Perylene Bisimide Pair. Journal of the American
Chemical Society 2016, 138 (29), 9029-9032.
13. Schrauben, J. N.; Akdag, A.; Wen, J.; Havlas, Z.; Ryerson, J. L.; Smith, M. B.;
Michl, J.; Johnson, J. C., Excitation Localization/Delocalization Isomerism in a
Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran. The Journal of
Physical Chemistry A 2016, 120 (20), 3473-3483.
14. Freeman, D. M. E.; Musser, A. J.; Frost, J. M.; Stern, H. L.; Forster, A. K.;
Fallon, K. J.; Rapidis, A. G.; Cacialli, F.; McCulloch, I.; Clarke, T. M.; Friend, R.
H.; Bronstein, H., Synthesis and Exciton Dynamics of Donor-Orthogonal
Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap.
Journal of the American Chemical Society 2017, 139 (32), 11073-11080.
15. Wu, Y.; Wang, Y.; Chen, J.; Zhang, G.; Yao, J.; Zhang, D.; Fu, H.,
Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon.
Angewandte Chemie International Edition 2017, 56 (32), 9400-9404.
16. Mauck Catherine, M.; Bae Youn, J.; Chen, M.; Powers‐Riggs, N.; Wu, Y. L.;
29. Cook, J. D.; Carey, T. J.; Arias, D. H.; Johnson, J. C.; Damrauer, N. H.,
Solvent-Controlled Branching of Localized versus Delocalized Singlet Exciton
States and Equilibration with Charge Transfer in a Structurally Well-Defined
Tetracene Dimer. The Journal of Physical Chemistry A 2017, 121 (48), 9229-
9242.
30. Bettinger, H. F.; Einholz, R.; Gottler, A.; Junge, M.; Sattele, M.-S.; Schnepf,
A.; Schrenk, C.; Schundelmeier, S.; Speiser, B., 6,6[prime or
minute],11,11[prime
or
minute]-Tetra((triisopropylsilyl)ethynyl)-anti-
[2.2](1,4)tetracenophane: a covalently coupled tetracene dimer and its
structural, electrochemical, and photophysical characterization. Organic
Chemistry Frontiers 2017, 4 (5), 853-860.
31. Wang, X.; Wang, R.; Shen, L.; Tang, Z.; Wen, C.; Dong, B.; Liu, H.; Zhang,
C.; Li, X., Intramolecular singlet fission in a face-to-face stacked tetracene trimer.
Physical Chemistry Chemical Physics 2018, 20 (9), 6330-6336.
32. Hetzer, C.; Guldi Dirk, M.; Tykwinski Rik, R., Pentacene Dimers as a Critical
Tool for the Investigation of Intramolecular Singlet Fission. Chemistry - A
European Journal 2018, 24, 1-14.
33. Carey, T. J.; Miller, E. G.; Gilligan, A. T.; Sammakia, T.; Damrauer, N. H.,
Modular Synthesis of Rigid Polyacene Dimers for Singlet Fission. Organic Letters
2018, 20 (2), 457-460.
34. Wang, L.; Wu, Y.; Chen, J.; Wang, L.; Liu, Y.; Yu, Z.; Yao, J.; Fu, H., Absence
of Intramolecular Singlet Fission in Pentacene–Perylenediimide Heterodimers:
Wasielewski Michael, R., Charge‐Transfer Character in
a Covalent
10
ACS Paragon Plus Environment

Page 11 of 25
Journal of the American Chemical Society
The Role of Charge Transfer State. The Journal of Physical Chemistry Letters
2017, 8 (22), 5609-5615.
54. Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.;
Brutchey, R. L.; Thompson, M. E.; Bradforth, S. E., Efficient singlet fission
discovered in a disordered acene film. Journal of the American Chemical Society
2012, 134 (14), 6388-6400.
55. 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 (11), 113304.
56. Rolczynski, B. S.; Szarko, J. M.; Son, H. J.; Liang, Y.; Yu, L.; Chen, L. X.,
Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap
polymers and their implications in photovoltaic materials design. Journal of the
American Chemical Society 2012, 134 (9), 4142-4152.
57. Roberts, S. T.; Schlenker, C. W.; Barlier, V.; McAnally, R. E.; Zhang, Y.;
Mastron, J. N.; Thompson, M. E.; Bradforth, S. E., Observation of triplet exciton
formation in a platinum-sensitized organic photovoltaic device. The Journal of
Physical Chemistry Letters 2010, 2 (2), 48-54.
58. Ryerson, J. L.; Schrauben, J. N.; Ferguson, A. J.; Sahoo, S. C.; Naumov, P.;
Havlas, Z.; Michl, J.; Nozik, A. J.; Johnson, J. C., Two Thin Film Polymorphs of
the Singlet Fission Compound 1,3-Diphenylisobenzofuran. The Journal of
Physical Chemistry C 2014, 118 (23), 12121-12132.
59. Ryerson, J. L.; Schrauben, J. N.; Ferguson, A. J.; Sahoo, S. C.; Naumov, P. e.;
Havlas, Z. k.; Michl, J.; Nozik, A. J.; Johnson, J. C., Two Thin Film Polymorphs
of the Singlet Fission Compound 1, 3-Diphenylisobenzofuran. The Journal of
Physical Chemistry C 2014, 118 (23), 12121-12132.
60. Thompson, N. J.; Wilson, M. W.; Congreve, D. N.; Brown, P. R.; Scherer, J.
M.; Bischof, T. S.; Wu, M.; Geva, N.; Welborn, M.; Van Voorhis, T., Energy
harvesting of non-emissive triplet excitons in tetracene by emissive PbS
nanocrystals. Nature materials 2014, 13, 1039-1043.
61. Thorsmølle, V. K.; Averitt, R. D.; Demsar, J.; Smith, D.; Tretiak, S.; Martin,
R.; Chi, X.; Crone, B.; Ramirez, A.; Taylor, A., Morphology effectively controls
singlet-triplet exciton relaxation and charge transport in organic semiconductors.
Phys. Rev. Lett. 2009, 102 (1), 017401.
62. Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J.,
Exciton Fission and Fusion in Bis(tetracene) Molecules with Different Covalent
Linker Structures. Journal of the American Chemical Society 2007, 129 (46),
14240-14250.
63. Feng, X.; Krylov, A. I., On couplings and excimers: lessons from studies of
singlet fission in covalently linked tetracene dimers. Physical Chemistry
Chemical Physics 2016, 18 (11), 7751-7761.
64. Feng, X.; Casanova, D.; Krylov, A. I., Intra- and Intermolecular Singlet Fission
in Covalently Linked Dimers. The Journal of Physical Chemistry C 2016, 120
(34), 19070-19077.
65. Abraham, V.; Mayhall, N. J., Simple Rule To Predict Boundedness of
Multiexciton States in Covalently Linked Singlet-Fission Dimers. The Journal of
Physical Chemistry Letters 2017, 8 (22), 5472-5478.
66. Sonogashira, K., Development of Pd–Cu catalyzed cross-coupling of terminal
acetylenes with sp2-carbon halides. Journal of Organometallic Chemistry 2002,
653 (1), 46-49.
67. 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.; Swager, T. M.; Friend, R. H.; A, M.; van Voorhis, T., Nat. Chem.
2014, 6, 492.
68. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W., Structural Changes
Accompanying Intramolecular Electron Transfer:ꢀ Focus on Twisted
Intramolecular Charge-Transfer States and Structures. Chemical Reviews 2003,
103 (10), 3899-4032.
1
2
3
4
5
6
7
8
35. Liu, H.; Wang, X.; Pan, L.; Shen, L.; Wang, X.; Chen, Q.; Li, X., Synthesis and
photophysical properties of a bistetracene compound with slipped stacked
structure. Journal of Photochemistry and Photobiology A: Chemistry 2017, 340,
21-28.
36. Yamakado, T.; Takahashi, S.; Watanabe, K.; Matsumoto, Y.; Osuka, A.; Saito,
S., Conformational Planarization vs Singlet Fission: Distinct Excited‐State
Dynamics of Cyclooctatetraene‐Fused Acene Dimers. Angew. Chem. Int. Ed.
2018, 57, 5438-5448.
37. 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.
Proceedings of the National Academy of Sciences 2015, 112 (17), 5325-5330.
38. Müller, A. M.; Avlasevich, Y. S.; Müllen, K.; Bardeen, C. J., Evidence for
exciton fission and fusion in a covalently linked tetracene dimer. Chem. Phys.
Lett. 2006, 421 (4), 518-522.
39. Johnson, J. C.; Akdag, A.; Zamadar, M.; Chen, X.; Schwerin, A. F.; Paci, I.;
Smith, M. B.; Havlas, Z.; Miller, J. R.; Ratner, M. A.; Nozik, A. J.; Michl, J., Toward
Designed Singlet Fission: Solution Photophysics of Two Indirectly Coupled
Covalent Dimers of 1,3-Diphenylisobenzofuran. The Journal of Physical
Chemistry B 2013, 117 (16), 4680-4695.
40. Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J.,
Exciton fission and fusion in bis (tetracene) molecules with different covalent
linker structures. J. Am. Chem. Soc. 2007, 129 (46), 14240-14250.
41. Yamakado, T.; Takahashi, S.; Watanabe, K.; Matsumoto, Y.; Osuka, A.; Saito,
S., Conformational Planarization versus Singlet Fission: Distinct Excited‐State
Dynamics of Cyclooctatetraene‐Fused Acene Dimers. Angewandte Chemie
International Edition 2018, 57 (19), 5438-5443.
42. Grieco, C.; Kennehan, E. R.; Kim, H.; Pensack, R. D.; Brigeman, A. N.;
Rimshaw, A.; Payne, M. M.; Anthony, J. E.; Giebink, N. C.; Scholes, G. D.;
Asbury, J. B., Direct Observation of Correlated Triplet Pair Dynamics during
Singlet Fission Using Ultrafast Mid-IR Spectroscopy. The Journal of Physical
Chemistry C 2018, 122 (4), 2012-2022.
43. Folie, B. D.; Haber, J. B.; Refaely-Abramson, S.; Neaton, J. B.; Ginsberg, N. S.,
Long-Lived Correlated Triplet Pairs in a π-Stacked Crystalline Pentacene
Derivative. Journal of the American Chemical Society 2018, 140 (6), 2326-2335.
44. 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.; Thoss, M.; Tykwinski,
R. R.; Wasielewski, M. R.; Guldi, D. M., Evidence for Charge-Transfer Mediation
in the Primary Events of Singlet Fission in a Weakly Coupled Pentacene Dimer.
Chem 2018, 4 (5), 1092-1111.
45. Hart, S.; Silva, W. R.; Frontiera, R. R., Femtosecond stimulated Raman
evidence for charge-transfer character in pentacene singlet fission. Chemical
Science 2018, 9 (5), 1242-1250.
46. Ito, S.; Nagami, T.; Nakano, M., Design Principles of Electronic Couplings
for Intramolecular Singlet Fission in Covalently-Linked Systems. The Journal of
Physical Chemistry A 2016, 120 (31), 6236-6241.
47. Zeng, T., Through-Linker Intramolecular Singlet Fission: General
Mechanism and Designing Small Chromophores. The Journal of Physical
Chemistry Letters 2016, 7 (21), 4405-4412.
48. Feng, X.; Luzanov, A. V.; Krylov, A. I., Fission of entangled spins: An
electronic structure perspective. The Journal of Physical Chemistry Letters 2013,
4 (22), 3845-3852.
49. Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.;
Bradforth, S. E.; Thompson, M. E., Singlet Fission in a Covalently Linked
Cofacial Alkynyltetracene Dimer. J. Am. Chem. Soc. 2016, 138 (2), 617-627.
50. Perkampus, H.; Sandeman, I.; Timmons, C., DMS UV Atlas of Organic
Compounds, Verlag Chemie. Weinheim, Butterworths, London, England: 1966.
51. Pensack, R. D.; Tilley, A. J.; Parkin, S. R.; Lee, T. S.; Payne, M. M.; Gao, D.;
Jahnke, A. A.; Oblinsky, D.; Li, P.-F.; Anthony, J. E., Exciton Delocalization
Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives. Journal of the
American Chemical Society 2015, 137, 6790-6803.
52. Murov, S. L.; Carmichael, I.; Hug, G. L., Handbook of photochemistry. CRC
Press: 1993.
53. Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.;
Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X.; Zhu, X. Y.; Steigerwald, M. L.; Sfeir,
M. Y.; Campos, L. M., Quantitative Intramolecular Singlet Fission in
Bipentacenes. J. Am. Chem. Soc. 2015, 137 (28), 8965-8972.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
69. Berkelbach, T. C.; Hybertsen, M. S.; Reichman, D. R., Microscopic theory of
singlet exciton fission. II. Application to pentacene dimers and the role of
superexchange. The Journal of Chemical Physics 2013, 138 (11), 114103.
70. Lukman, S.; Musser, A. J.; Chen, K.; Athanasopoulos, S.; Yong, C. K.; Zeng,
Z.; Ye, Q.; Chi, C.; Hodgkiss, J. M.; Wu, J.; Friend, R. H.; Greenham, N. C.,
Tuneable Singlet Exciton Fission and Triplet–Triplet Annihilation in an
Orthogonal Pentacene Dimer. Adv. Funct. Mater. 2015, 25 (34), 5452-5461.
71. Lippitz, M.; Hübner, C. G.; Christ, T.; Eichner, H.; Bordat, P.; Herrmann, A.;
Müllen, K.; Basché, T., Coherent Electronic Coupling versus Localization in
Individual Molecular Dimers. Phys. Rev. Lett. 2004, 92 (10), 103001.
72. Nguyen, M.-H.; Nguyen, V. H.; Yip, J. H., Sequence-Specific Synthesis of
Platinum-Conjugated Trichromophoric Energy Cascades of Anthracene,
Tetracene, and Pentacene and Fluorescent “Black Chromophores”.
Organometallics 2013, 32 (24), 7283-7291.
11
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 25
73. Burdett, J. J.; Bardeen, C. J., The dynamics of singlet fission in crystalline
83. Breen, I.; Tempelaar, R.; Bizimana, L. A.; Kloss, B.; Reichman, D. R.; Turner,
tetracene and covalent analogs. Accounts of chemical research 2013, 46 (6),
1312-1320.
74. Arias, D. H.; Ryerson, J. L.; Cook, J. D.; Damrauer, N. H.; Johnson, J. C.,
Polymorphism influences singlet fission rates in tetracene thin films. Chemical
Science 2016, 7 (2), 1185-1191.
75. Marciniak, H.; Pugliesi, I.; Nickel, B.; Lochbrunner, S., Ultrafast singlet and
triplet dynamics in microcrystalline pentacene films. Physical Review B 2009, 79
(23), 235318.
76. Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T., Handbook of
Photochemistry. 3 ed.; Boca Raton, Florida, 2006.
D. B., Triplet Separation Drives Singlet Fission after Femtosecond Correlated
Triplet Pair Production in Rubrene. Journal of the American Chemical Society
2017, 139 (34), 11745-11751.
84. Hetzer, C.; Guldi Dirk, M.; Tykwinski Rik, R., Pentacene Dimers as a Critical
Tool for the Investigation of Intramolecular Singlet Fission. Chemistry � A
European Journal 2018, 0 (0).
1
2
3
4
5
6
7
8
85. Monahan, N.; Zhu, X. Y., Charge Transfer–Mediated Singlet Fission. Annual
Review of Physical Chemistry 2015, 66 (1), 601-618.
86. Zimmerman, P. M.; Bell, F.; Casanova, D.; Head-Gordon, M., Mechanism for
singlet fission in pentacene and tetracene: From single exciton to two triplets.
Journal of the American Chemical Society 2011, 133 (49), 19944-19952.
87. Aryanpour, K.; Shukla, A.; Mazumdar, S., Theory of Singlet Fission in
Polyenes, Acene Crystals, and Covalently Linked Acene Dimers. The Journal of
Physical Chemistry C 2015, 119 (13), 6966-6979.
77. McQuade, D. T.; Pullen, A. E.; Swager, T. M., Chem. Rev. 2000, 100, 2537.
78. van Stokkum, I. H. M.; Larsen, D. S.; van Grondelle, R., Global and target
analysis of time-resolved spectra. Biochimica et Biophysica Acta (BBA) -
Bioenergetics 2004, 1657 (2), 82-104.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
79. McQuade, D. T.; Kim, J.; Swager, T. M., J. Am. Chem. Soc. 2000, 122, 5885.
80. Van Dyke, D. A.; Pryor, B. A.; Smith, P. G.; Topp, M. R., Nanosecond Time-
Resolved Fluorescence Spectroscopy in the Physical Chemistry Laboratory:
Formation of the Pyrene Excimer in Solution. Journal of Chemical Education
1998, 75 (5), 615.
81. Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.;
Wasielewski, M. R., Excited singlet states of covalently bound, cofacial dimers and
trimers of perylene-3, 4: 9, 10-bis (dicarboximide) s. The Journal of Physical
Chemistry A 2008, 112 (11), 2322-2330.
82. Miller, C. E.; Wasielewski, M. R.; Schatz, G. C., Modeling Singlet Fission in
Rylene and Diketopyrrolopyrrole Derivatives: The Role of the Charge Transfer
State in Superexchange and Excimer Formation. The Journal of Physical
Chemistry C 2017, 121 (19), 10345-10350.
88. 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. Physical Review Letters 2013, 110 (22), 226402.
89. Liu, H.; Wang, Z.; Wang, X.; Shen, L.; Zhang, C.; Xiao, M.; Li, X., Singlet
exciton fission in a linear tetracene tetramer. Journal of Materials Chemistry C
2018, 6 (13), 3245-3253.
90. Tayebjee, M. J. Y.; Sanders, S. N.; Kumarasamy, E.; Campos, L. M.; Sfeir, M.
Y.; McCamey, D. R., Quintet multiexciton dynamics in singlet fission. Nature
Physics 2016, 13, 182.
91. Joy, J.; Korovina, N. V.; Thompson, M. E.; Bradforth, S. E., Decoupling Inter-
and Intra- dimer singlet fission. Proc. SPIE 10348, Physical Chemistry of
Semiconductor Materials and Interfaces XVI 2017.
S0S1

1(T1T1)  T1 - - - T1

1(T1T1)  T1 - - - T1
S0S1

1(T1T1)  T1 - - - T1
S0S1



× ×
×
12
ACS Paragon Plus Environment

Page 13 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
434x209mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
227x96mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 15 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
381x228mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 16 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
264x107mm (96 x 96 DPI)
ACS Paragon Plus Environment

Page 17 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
301x149mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 18 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
529x136mm (96 x 96 DPI)
ACS Paragon Plus Environment

Page 19 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
522x130mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
269x207mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 21 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
210x64mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
269x207mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 23 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
303x222mm (96 x 96 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 24 of 25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
271x207mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 25 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
388x249mm (96 x 96 DPI)
ACS Paragon Plus Environment