Exploiting excited-state aromaticity to design highly stable singlet fission materials

Article abstract of DOI:10.1021/jacs.9b06346

Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The predominant strategy for achieving such a trait is by increasing ground state diradical character; however, this greatly reduces ambient stability. Herein, we exploit Baird's rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of derivatives of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, while analogous films of TIPS-pentacene showed full degradation after 4 days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tunability and several thousand potential fission-capable candidates, while clearly demonstrating the relationship between triplet aromaticity and singlet-triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials.

Full text of DOI:10.1021/jacs.9b06346

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Article
Exploiting excited-state aromaticity to
design highly stable singlet fission materials
Kealan J. Fallon, Peter Budden, Enrico Salvadori, Alex M. Ganose, Christopher N. Savory,
Lissa Eyre, Simon Dowland, Qianxiang Ai, Stephen Goodlett, Chad Risko, David O. Scanlon,
Chris W. M. Kay, Akshay Rao, Richard H. Friend, Andrew J. Musser, and Hugo Bronstein
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06346 • Publication Date (Web): 05 Aug 2019
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Exploiting excited-state aromaticity to design highly stable singlet
fission materials
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Kealan J. Fallon,a‡ Peter Budden,b‡ Enrico Salvadori,c,d Alex M. Ganose,e,f,g Christopher N. Savory,e,f Lissa Eyre,b
Simon Dowland,b Qianxiang Ai,h Stephen Goodlett,h Chad Risko,h David O. Scanlon,e,f,g Christopher W. M.
Kay,d,i Akshay Rao,b Richard H. Friend,*b Andrew J. Musser,*j and Hugo Bronstein*a,b
a Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, U.K.
b Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, U.K.
c Department of Chemistry, University of Turin, Via Pietro Giuria 7, 10125 Torino, Italy
d London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH, U.K.
e Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London, WC1H 0AJ, U.K.
f Thomas Young Centre, University College London, Gower Street, London WC1E 6BT, U.K.
g Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Oxfordshire, OX11 0DE, U.K.
h Department of Chemistry and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky, USA.
i Department of Chemistry, University of Saarland, 66123 Saarbrücken, Germany
j Department of Physics and Astronomy, University of Sheffield, Hicks Buildling, Hounsfield Road, Sheffield S3 7RH, U.K.
Supporting Information Placeholder
ABSTRACT: Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for
only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The predomi-
nant strategy for achieving such trait is by increasing ground state diradical character, however this greatly reduces ambient stability.
Herein, we exploit Baird’s rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet
fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes
aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of
derivatives of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet
fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state.
Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, whilst analogous films of TIPS-
pentacene showed full degradation after four days, showcasing the excellent stability of this class of singlet fission scaffold. Exten-
sion of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tuneability and several thou-
sand potential fission-capable candidates, whilst clearly demonstrating the relationship between triplet aromaticity and singlet-
triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials.
cited state E(T1) be on the order of half the energy of the low-
est singlet excited state E(S1). Known systems include rylene
dyes9–12, polyenes13–15, donor-acceptor polymers16–18 and de-
rivatives of the linear acenes, particularly tetracene and penta-
cene, which have become the focus of the field2,19–31. The low
triplet energies of the acenes are a function of their migratory
Clar’s sextets resulting in their large localised diradical charac-
ter, which compromises their chemical stability (e.g. penta-
cene must be stored in the dark under inert gas lest it undergo
photocatalyzed dimerization32 or oxidation33). Unfortunately,
current established rationales behind synthesising organic
molecules with large singlet (S1) - triplet (T1) energy gaps –
such as inducing biradicaloid character – offer the
Introduction
Phenomena that permit the generation of multiple excitons
from the absorption of one photon have received large atten-
tion recently due to their potential in photovoltaic cell effi-
ciency enhancement. One such process is singlet fission,
whereby two low-energy triplet excited states are generated
from one high-energy singlet excited state, allowing quantum
efficiencies up to 200% and offering the potential to surpass
the single junction limit in solar cells1–8. Recent advancements
in singlet fission have been materials-limited due to the rarity
of molecules that meet the essential energetic requirement for
the process, namely that the energy of the lowest triplet ex-
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Figure 1 Optical properties and characteristics (a) The polycyclic aromatic hydrocarbon indolonaphthyridine thiophene (INDT) with la-
belled rings of interest; (b) Resonance forms of INDT showing origin of Baird [4n] ring (C5, blue). Values of NICS(1) calculations for
CN-INDT are shown for S0 and T1, where a more negative value denotes greater aromaticity; (c) Solution (chloroform) steady-state
absorbance spectra of the investigated family of materials; (d) Theoretical energies (eV, TD-DFT/TDA, B3LYP/6-311++G**) of S1 and
T1 relative to the ground state. The ratio of these energies is given inset.
synthetic chemist little assistance in identifying or developing
new chromophores for this purpose, although these rules are
of very good use to check whether a structure is likely to fulfil
the energetic criteria for singlet fission.34–36 Merely focussing
on increasing the amount of biradical character can often in-
volve cost to chemical stability as it can result in highly reactive
localised unpaired electrons, although this can potentially be
addressed with careful protection of the radical sites.37,38
There is therefore a need for a simple design approach to iden-
tify or design materials that undergo singlet fission at no ex-
pense to stability.
low-lying triplet state with E(T1)/E(S1) ~ 0.5, affirming its
potential for singlet fission. This stabilised triplet state has
been previously explained through the stabilised diradical na-
ture of the indigo core49–51. It can likewise be rationalised using
Baird aromaticity, due to the presence of [4n] contribution in
its resonance structure (Fig. 1a and Supplementary Section
S2). We have designed a family of materials (Fig. 1a) to inves-
tigate: (a) whether the indolonaphthyridine chromophore
does indeed undergo singlet fission; (b) whether the optical
properties and singlet fission ability could be tuned by chemi-
cal functionalisation; and (c) whether contributions from an
aromatic triplet state enhance chemical stability. The full syn-
thesis and characterisation of these materials is given in the
Supplementary Section S1, hereafter referred to as x-INDT
where x is the atom or functional group installed at the 6,6’–
positions of the INDT C6 ring (see Fig. 1a).
The solution steady-state absorbance spectra (Fig. 1c)
demonstrate the optical tuneability of this family. H-INDT
and F-INDT display comparable absorbance with λꢀꢁꢂ of
602 nm. Both Cl-INDT and Br-INDT exhibit bathochromi-
cally shifted but practically identical absorbance with λꢀꢁꢂ of
615 nm. The addition of strongly electron withdrawing car-
bonitrile functional groups was achieved through a
Rosenmund-von Braun reaction of Br-INDT and furnished
both the asymmetric mononitrile/monobromo compound
CNBr-INDT and dinitrile compound CN-INDT. The addi-
tion of each carbonitrile group narrows the optical band gap
of the material significantly, yielding λꢀꢁꢂ of 635 for the
mono- and λꢀꢁꢂ of 650 nm for the di-carbonitrile compound.
In the ground state, Hückel’s Rule rationalises the enhanced
stability of cyclic systems with [4n+2] electrons and this con-
cept of aromaticity is a fundamental pillar of chemistry. Lesser
known is Baird’s Rule, which states that in the triplet state, cy-
clic systems with [4n] electrons are aromatic and hence stabi-
lized.39,40 Following confirmatory modern theoretical analy-
sis41–43, the rule has been explored in atypical conjugated sys-
tems in the past few years44–47. Here, we demonstrate the syn-
thesis and characterisation of highly stable, tuneable organic
materials that undergo singlet fission through exploitation of
Baird’s aromaticity of the triplet excited state.
Results & Discussion
We focus in particular on derivatives of indolonaphthy-
ridine benzene or Cibalackrot (SI section 1),48 previously pro-
posed as a singlet fission candidate49. Quantum chemical cal-
culations (B3LYP/6-311G**) reveal Cibalackrot to possess a
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The energies of the S1 and T1 states of these materials were
diamagnetic current restricted to the periphery of the two
fused rings. Notice that there is a weak response reflected by
small current density vectors that could come from the delo-
calized σ-electrons rather than from π-system. The ACID
plots for all molecules are shown in Figure S3.4 with patterns
similar to that of F-INDT. Further, NICS-xy and HOMA cal-
culations (see SI) also support an increase in aromatic charac-
ter of the C5 ring in the triplet state relative to the ground state.
calculated using time-dependant density functional theory
(TD-DFT) on DFT-optimized structures. These calculations
reproduce the observed tuneability of the optical energy gap
and suggest that all of the molecules possess the relationship
of 2T1 < S1 required for singlet fission (Fig. 1d). Moreover, the
ratio of calculated T1/S1 energies reveals that this parameter,
and thus suitability for singlet fission, can be tuned through
chemical substitution. We do note that the use of TD-DFT for
the prediction of triplet state energies is problematic.52 How-
ever, whether the predicted values for the (T1/S1) ratio may
be over- or underestimated as a whole, the key trends – tunea-
bility of the ratio and its relation to triplet-state aromaticity –
should remain robust. Indeed, these values are best taken as a
guideline, since incorporation into the solid state inevitably al-
ters the energetic structure to some degree.
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We focus below on solid-state properties of Br-INDT and
CN-INDT, but full characterisation of all derivatives is pre-
sented in the Supplementary Section S4. In thin film (Fig. 3a),
all materials exhibit broader absorption and hypsochromically
shifted λꢀꢁꢂ over 500-550 nm. These shifts indicate signifi-
cant intermolecular interactions between chromophores, here
consistent with predominantly H-type exciton coupling53.
Large planar molecules such as these exhibit a propensity to
aggregate through strong π-π stacking. The large branched al-
kyl side chains attached to the out-of-plane thiophene moie-
ties suppress this coupling to a degree and minimise the stabi-
lisation of S1 in the solid state, which can unfavourably disrupt
the energetic balance for singlet fission49,51. Despite the broad
and variable lineshapes, we note that the thin-film absorption
onsets display similar trends to the solution measurements,
however we remind the reader that direct comparison of the-
oretical (gas phase) energetics to solid state experimental re-
sults is unwise as each molecule will experience different in-
termolecular coupling.
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We assessed the relative aromaticity of the molecular struc-
tures through a variety of computational methods. The aro-
maticity of the five-membered (C5) ring in the triplet state
was assessed using nucleus-independent chemical shift calcu-
lations 1 Å above the plane of the ring (NICS(1)). This data
is given in full in the Supplementary Information S3 and the
data for CN-INDT is shown in Fig. 1b. Importantly, the NICS
calculations demonstrate how the aromaticity in the C5 ring
is significantly increased in the triplet state in accordance with
Baird’s rules.
Figure 2 shows ACID (anisotropy of the induced current
density) plots for the F-INDT in the S0 and T1 states (see SI
for further details). For S0, the current density vectors on the
outer benzene rings at the ends of backbone show clear dia-
magnetic features. Such clear features are not present in the
other rings, where relatively weak induced currents seem to be
more paramagnetic. There is also a weak paramagnetic current
running along the periphery of the whole backbone. For T1,
the current density vectors on the aforementioned benzene
rings couple with the adjacent 5-membered rings, yielding a
Evidence for singlet fission. We investigated the ability of
INDT films to undergo singlet fission using time-resolved op-
tical and spin-resonance techniques. In transient absorption
spectroscopy, the sample is excited with a short laser pulse and
then interrogated with a broadband probe pulse at a controlla-
ble time delay (100 fs to 1 ms). Changes in the transmission of
the probe reflect the depopulation of the ground state, the ab-
sorption spectrum of photoexcited states, and stimulated
emission from allowed optical transitions. Full details of the
experimental setup are presented in the Supplementary Sec-
tion S4. Representative transient absorption spectra are
shown in Fig. 3b for Br-INDT and CN-INDT acquired using
a ns-µs system (λexc = 532 nm, ~1 ns). A broader probe spectral
range and equivalent measurements for the remaining mole-
cules are presented in the Supplementary Section S4.
As shown in Figure 3, we observe a distinct photoinduced
absorption band at ~680 nm (Br-INDT) or ~740 nm (CN-
INDT). This species, which persists out to the µs timescale
with little spectral evolution, should therefore only originate
from charges or triplet excitons. Triplet sensitisation in solu-
tion (green dashed)14 yields a similar spectral signature, after
taking into consideration the difference in underlying ground-
state bleach and a slight bathochromic shift of the Tꢃ → Tꢄ
transition in the solid state which is consistent with other or-
ganic systems14,15,54. Thus we assign the µs excited-state ab-
sorption to triplet excitons. This is further confirmed via
Figure 2 ACID plots for F-INDT in the S0 (left) and T1 (right)
states. The isovalue of the isosurface was set to 0.05. The induced
current density vectors are denoted by the arrows with the length
limited to 0.3~1.5. Larger images available in SI.
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Figure 3 Long-lived triplet excitations in INDT films. (Top, red) Br-INDT (Bottom, blue) CN-INDT; (a) Solution (dashed line, chloro-
form) and thin-film (solid line, spun from 5 mg/mL solutions in chloroform) steady-state absorption spectra. (b) Thin-film transient
absorption spectra acquired on ns-µs timescale following 1 ns excitation pulses at 532 nm. Corresponding decay kinetics and spectra
acquired over a broader probe range are presented in Supplementary Fig S4.4. Solution sensitized triplet signatures shown as green
dashed line; (c) Experimental (coloured) and simulated (black dash) TR-EPR spectra obtained at 10 K in dilute solution (left) and drop-
cast films (right). Simulated parameters for Br-INDT solution (D = 880 MHz, |E| = 185 MHz, PX:PY:PZ = 0.0:0.5:0.5), Br-INDT film
(D = 910 MHz, |E| = 185 MHz, PX:PY:PZ =0.2:0.6:0.2 ), CN-INDT solution (D = +830 MHz, |E| = 180 MHz, PX:PY:PZ =0.0:0.4:0.6 ).
The reported simulation for CN-INDT film is the sum of two components A and B where the T0 sublevel is preferentially populated in
high-field (component A: D = +830 MHz, |E| = 180 MHz; component B: D = –600 MHz, |E| = 60 MHz, see Supplementary section S4
for individual simulations). A = enhanced absorption, E = emission.
electron paramagnetic resonance (EPR), discussed below. We
detect similar evolution in Cl-INDT and CNBr-INDT, while
in F-INDT the spectral signature we attribute to triplets de-
cays faster, with a lifetime of 300 ns. In contrast, we do not find
evidence for triplets in H-INDT. The primary signature we
observe in H-INDT (Supplementary Fig S4.3) in this tem-
poral regime is a set of positive peaks that match the ground-
state absorption and can be assigned to bleaching of the
ground state. These are accompanied by a weak photo-in-
duced absorption (ΔT/T<0) from 650nm to the near-infra-
red which decays on an instrument-limited timescale and is as-
signed to the singlet. There is no triplet-like absorption in this
film, and there are evidently no significant long-lived elec-
tronic states in this film. The positive features that outlive the
photo-induced absorption are due to thermal modulation of
the absorption and reflectivity and are commonly observed in
a powerful tool for probing singlet fission materials57–61. In fro-
zen dilute solution, singlet fission – being an intermolecular
process – is inactive, and the zero-field sublevel populations of
triplet states in these conditions are attributed to intersystem
crossing. The frozen solution EPR spectra (Fig. 3c) of Br- and
CN-INDT exhibit emission/absorption polarization patterns
EEEAAA. In thin film, singlet fission becomes possible and
may out-compete intersystem crossing. In such a case, the
presence of a new channel for triplet formation would mani-
fest as changes in the triplet sublevel populations. The polari-
zation pattern of Br-INDT films is EAEAEA, which cannot be
caused by the same intersystem crossing pathway active in di-
lute solution. The absence of other potential mechanisms al-
lows us to attribute triplet formation in these films to singlet
fission. The spectrum of CN-INDT films is more complex –
at least two components with different zero-field splitting pa-
rameters are present. One component has zero-field splitting
parameters similar to those observed in solution but with
AEEAAE polarization (Supplementary Section S5). Im-
portantly, this polarization pattern, previously observed in
TIPS-tetracene and pentacene dimers,57,58 is a signature of tri-
plet pairs formed via singlet fission.57 On the basis of this di-
rect evidence of fission in Br-INDT and CN-INDT and the
similar sub-ns triplet formation observed in Figure. 3b, we
measurements in the ground-state absorption region11,37,55,56
.
These measurements provide clear evidence of sub-ns tri-
plet formation, but it is difficult to distinguish the mechanism
– intersystem crossing or singlet fission – from optical spec-
troscopy alone. We therefore characterised the sublevel popu-
lations of the triplet states using time-resolved EPR, which is
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Figure 4 Fast triplet formation in INDT films. Thin-film TA spectra acquired on fs-ps timescale following 250 fs excitation of (a) Br-INDT
and (b) CN-INDT. Corresponding decay kinetics and spectra acquired over a broader probe range are presented in Supplementary Fig
S4.4. Dashed spectra show steady-state absorption (black) and PL (purple); (c) Br-INDT excitation-density-dependent TA kinetics
(lines) tracking S1 features (860-880 nm), compared with transient grating PL62 kinetics acquired at high laser power (255 μJcm-2 per
pulse, green circles), and time-correlated single-photon counting kinetics acquired at low laser power (2 nJcm-2 per pulse, yellow trian-
gles). Note the instrument response of the sub-ns PL measurement is ~300 ps. (d) Comparison of initial TA signal, taken at the peak in
time of the S1 PIA at 850nm, with the triplet signal at 1-1.5 ns for two different powers: 180 μJcm-2 per pulse (black, left axis) and 9 μJcm-
2 per pulse (blue, right axis), demonstrating that excitation density does not affect triplet yield in CN-INDT.
conclude that singlet fission is active in all of the reported
however, it is certainly present by 2-3ps in CN-INDT, effec-
tively ruling out the formation of these triplets from ISC, and
confirming the result from EPR that triplets are originating
from singlet fission.
INDT derivatives except H-INDT.
Rapid triplet formation. For further insight into the mecha-
nism of singlet fission, we examine the initial decay of S1 in Br-
INDT and CN-INDT using fs-ps transient absorption spec-
troscopy (λexc= 530 nm, ~250 fs, 37kHz repetition rate). Rep-
resentative transient absorption spectra at a high excitation
density are shown in Fig. 4a and 4b for the same molecules
presented in Fig. 3. At early times there is a broad photoin-
duced absorption (ΔT/T<0) spectrally overlapped with the
ground state absorption (making the spectra highly congested
in this region) and extending to a peak in the near-infrared.
From comparison of the steady-state absorption and photolu-
minescence spectra, we can attribute much of the complex
structure of this spectrum to overlapping ΔT/T>0 signatures
from bleaching of the ground-state absorption and stimulated
emission from S1.
We observe that the dynamics of the loss of the initial pho-
toinduced absorption peaked at 850nm are strongly depend-
ent on excitation density. In Br-INDT, at high excitation den-
sities comparable with those required to measure ultrafast
photoluminescence62 (Fig. 4c), the decay is ~7 ps. This rapid
decay is likely due to singlet-singlet annihilation. At the lowest
excitation densities at which TA was measured, annihilation is
not significant, and the decay (biexponential: 58 ps and 990
ps) is in agreement with the intrinsic 1 ns lifetime from time-
correlated single-photon counting (Fig. 4c). This agreement
with photoluminescence measurements in both high- and
low-annihilation regimes firmly establishes the initially decay-
ing spectral features, peaked at 850nm, as signatures of S1 and
indicates that the INDTs are similarly sensitive to singlet-sin-
glet annihilation as perylenediimides11.
As the strong initial photoinduced absorption decays, the
underlying triplet signal is revealed (685 nm in Br-INDT, 745
nm in CN-INDT), corresponding well with the ns TA experi-
ment (Fig 3b). Due to the overlapping signals, it is not possible
to definitively distinguish the formation time of this signal,
Given that the S1 lifetime is so strongly affected by singlet-
singlet annihilation, we would expect that the triplet yield and
formation kinetics are also power dependent. Curiously, when
we calculate the yield of triplets at 1 ns in CN-INDT, based on
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the solution cross sections of the S1 → Sn and T1 → Tn transi- citation pulse energy and the steady state absorption, and us-
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tions, it is approximately 60% at all excitation densities (details
of calculations in Supplementary Info S4.9). Triplet yield is
not strongly affected by excitation density, despite the clear ex-
citation density dependence of the S1 lifetime. This is clearly
shown by Fig 4d, where the ratio of the initial S1 signal to 1ns
T1 signal is unchanged from 9 μJcm-2 per pulse to 180 μJcm-2
per pulse, while the initial S1 decay lifetime falls from ~200 ps
to 7 ps. Based on this evidence, we propose a branching on
very early timescales between populations capable and inca-
pable of singlet fission. This is likely due to morphology, with
a subset of ‘hot spots’ where triplets can be formed rapidly via
singlet fission24, and the resulting triplets are much less sus-
ceptible to annihilation due to their lower mobility. Im-
portantly, singlet fission at hotspots in this system is com-
pletely outcompeting the singlet annihilation events that oc-
cur from Förster transfer on a ps timescale, and singlet fission
is therefore likely to be occurring on fs timescales. Meanwhile,
a parallel population of singlets formed too far from the hot
spots cannot undergo singlet fission, but are mobile enough to
show a strong excitation density dependence due to singlet-
singlet annihilation. Indeed, preliminary measurements using
different solvents or film-forming conditions reveal consider-
able changes in photoluminescence quantum yield (PLQY),
suggesting the density of hot spots can be tuned through pro-
cessing (see SI 4.12). PLQY is measured in steady state at low
excitation density, such that the changes in yield cannot be at-
tributed simply to changes in singlet-singlet annihilation chan-
nels, but to the rate of intrinsic decay, which we attribute to
change in density of singet fission hotspots.63 Given that sin-
glet fission appears to be so fast, we see no reason morphology
could not be tuned to produce a quantitative singlet fission
yield as has been demonstrated in other systems such as rylene
diimides and DPP.9,12,64 To capture impor tant trend s bet ween
the different x-INDT in the branching of this mechanism, we
compare the magnitude of triplet signature at ~500 ns normal-
ised by the initial singlet excitation density taken from the ex-
ing the solution sensitisation data to account for differences in
triplet absorption cross-section relative to ground state bleach
(see SI 4.8). This gives relative triplet yields of
H < F < CNBr ≈ Cl < Br < CN, which broadly follows a trend
of increasing yield with increasing exothermicity of singlet fis-
sion. There is also a possible connection to the strength of in-
termolecular coupling27, as we observe that the CN-INDT
film absorption is most blue-shifted from the solution spec-
trum with a suppressed 0-0 transition (characteristic of H-ag-
gregation), while that of H-INDT is the least shifted (Fig 3a
and S4.1 and S4.2).
Stability. The ultimate goal of exploiting excited state aro-
maticity to generate singlet fission-capable materials is to ac-
cess triplet states without cost to the chemical stability. The
ambient stability of each INDT material was compared to
TIPS-pentacene. Thin films were identically prepared and
stored in individual clear Perspex boxes on a shelf, in air, in a
well-lit area, but out of direct sunlight, and the steady state ab-
sorption was measured periodically over 31 days (see SI sec-
tion S6). Under these ambient conditions TIPS-pentacene
showed total degradation after 4 days, whilst the INDT mate-
rials exhibited excellent ambient stability and CN-INDT
maintained over 80% of its original absorbance after 31 days
(Fig. 5c). The high chemical stability of the INDTs showcases
their robust ground and excited states despite fission occur-
ring readily (and hence a high concentration of triplets being
present) in the material. By exploiting triplet aromaticity, not
only is the energy of the triplet state lowered such that it can
undergo singlet fission, but the delocalised nature of the un-
paired electrons means they are less susceptible to photodeg-
radation reactions, in an analogous way that ground state aro-
maticity increases the stability of organic compounds. Triplet-
mediated photodegradation of acenes and other polyaromatic
hydrocarbons is well-documented, thus introducing some ar-
omatic character into excited states could lead to a generalised
method to enhance the photostability of organic materials.
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Figure 5 Ambient stability and excited state aromaticity of INDT compounds. (a) Calculated energies of S1 and T1 for 9920 candidates with
the corresponding C5 NICS(1) shown in colormap. Red bar shows S1 = 2.1 eV. (b) The relationship between the S-T gap and triplet
state aromaticity with the corresponding T1/S1 ratio shown in colour map. (c) Normalised intensity of the ꢅꢆꢇꢈ of the steady-state ab-
sorbance spectra of thin-films of TIPS-pentacene (TIPS-Pc), Br-INDT and CN-INDT spun from 5 mg/mL solutions in chloroform
over 31 days. Spectra of all materials are shown in the Supplementary Section S6.
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We note that the increase in triplet-state aromaticity does
come at some slight cost to ground-state aromaticity, but this
effect is negligible as the materials also possess significant
ground state aromaticity as evidenced by their high stability in
the ground state as well.
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5
Expanding the chemical space. The experimental proof of sin-
glet fission in INDT-based materials and their photochemical
stability benchmarks the essential validity of our approach: in-
creasing triplet aromaticity through chemical functionalisa-
tion results in more energetically favourable singlet fission,
into more stable triplet states. We now return to our design
principle to explore the scope of materials tuneability and
identify robust trends linking excited-state energetics and tri-
plet aromaticity. To widen our analysis, theoretical energies of
S1 and T1 and NICS(1) aromaticity for 9920 IND candidates
were calculated using TD-DFT incorporating the Tamm–
Dancoff approximation (TDA) at the B3LYP/6-311++G**
level of theory (rationale of these methods given in the Sup-
plementary Section S7). The results and codes used to gener-
ate and perform all calculations detailed in this report are pro-
vided online in a publicly available repository (see notes).
In this wider sample group (Fig. 5a), the correlation of in-
creased excited state aromaticity and reduced T1/S1 is une-
quivocally clear. It demonstrates that, through appropriate
chemical substitution to increase the aromaticity of the triplet
state, its energy can be lowered without affecting the energy of
the singlet – the red bar highlights candidates with S1 = 2.1 eV
and with T1 ranging from 1.5–0.5 eV. We can directly visualise
this tuning by plotting the calculated S1–T1 energy gap as a
function of triplet-state aromaticity (Fig. 5b). These results di-
rectly demonstrate that exploiting Baird-type aromaticity is a
powerful approach for manipulating the singlet-triplet energy
gap. Independent manipulation of the singlet and triplet ener-
gies in such a manner has never been demonstrated and has
implications across many areas of science such as light emit-
ting diodes, photovoltaics, photocatalysis, photochemistry,
spintronics and magnetism. In the context of singlet fission,
this data has allowed us to identify 2616 new singlet fission
candidates with a wide range of optical band-gaps and T/S ra-
tios. These are all based on straightforward substitution of the
IND chromophore, which is itself well known for photochem-
ical robustness and large oscillator strength – both ideal prop-
erties for applications of singlet fission.
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8
9
Figure 6 Amide resonances of diketopyrrolopyrrole (DPP) and
its similarity to pentalene which has an aromatic triplet excited
state.
structures induces aromaticity of the triplet excited state, re-
sulting in a widening of the S1-T1 energy gap such that fission
becomes energetically feasible.
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Fig. 6 shows amide bond resonances which result in a core
which has an electronic structure similar to pentalene, which
has an aromatic triplet state.42 This aromaticity of the triplet
state of DPP is confirmed by NICS calculations (each central-
ring has NICS(1) -3.96 using DFT/TDDFT B3LYP/6-
311G**). As detailed earlier, we suggest this is responsible for
its energetic stabilisation and potentially also explains the su-
perior stability towards photodegradation of this chromo-
phore (the unpaired electrons are delocalised rendering them
less reactive to photodegradation reactions).
Implementation of the proposed design rules to identify
and design new chromophores as singlet fission candidates or
merely as a tool to predict and rationalise the relative excited
state energetics is demonstrated below. Table 1 shows the
structures and calculated vertical excited state energies (using
DFT/TDDFT B3LYP/6-311G**) of two sets of molecules A
and B and their two regioisomers (5 or 6) based on the size
(and connectivity) of the lactam ring. It can clearly be seen
that within each set of isomers (i.e A5 vs A6) virtually all pa-
rameters that could affect excited state energies (e.g. torsion
angles, conjugation paths, atom connectivity) are identical
with the exception of the core ring size.
It should be noted that all calculations are performed at the
ground state geometry where the amide-containing cores are
completely planar. Similarly to INDT and DPP, in both cases
where a 5-membered cyclic amide is present (A5 and B5) it is
possible to draw resonance structures (see supporting infor-
mation Fig S2.5) that contain [4n] electrons suggesting that
the triplet excited state will have aromatic character. In both
cases where the central core is comprised of 6-membered
rings (A6 and B6) the resulting resonance structures have
[4n+2] electrons, suggesting that the excited states will have
antiaromatic character. Therefore, one can simply predict
without even the aid of theoretical calculations that the com-
pounds with 5-membered cyclic amides will have narrower
optical energy gaps, and larger S1-T1 energy gaps, making
them singlet fission candidates. Both of these predictions,
alongside confirmation of the reversal of aromaticity are com-
pletely supported by the theoretical calculations (Table 1).
Applicability to other systems. In order to demonstrate the ap-
plication and generalisability of the proposed methodology
we demonstrate here that excited state aromaticity can be used
to rationalise the energetic position of alternative non-acene
singlet fission candidates and also as a predictive tool to search
for new materials which fulfil the energetic design criteria.
Diketopyrolopyrole (DPP) has been suggested to undergo
singlet fission but thus far there has been no explanation for its
unusually large S1-T1 energy gap.64 Similarly, to our Cibalackrot
derivatives, we propose that the presence of [4n] resonance
It is important to note that although these dyes have been
previously reported,65,66 neither A5 nor B5 have been investi-
gated for singlet fission, indicating the ease with which new
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1
2
3
4
N
O
O
N
O
O
Ph
Ph
S
S
N
N
S
N
N
S
Ph
Ph
O
N
N
O
O
O
5
6
7
8
9
Molecule
S1 (eV)
A5
A6
B5
B6
2.29
1.04
0.45
1.25
–3.1
3.11
1.94
0.62
1.17
+5.2
2.20
0.73
0.33
1.47
–3.4
3.22
2.11
0.66
1.10
+8.7
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T1 (eV)
T1 / S1
S1 – T1 (eV)
Triplet NICS(1)
Table 1 Structures and theoretical excited state properties of two molecules A and B, and their two isomers 5 (bridged C5 lactam rings) and 6
(fused C6 lactam rings) calculated at the TDDFT/B3LYP 6-311G* level of theory
candidates that fulfil the energetic criteria for singlet fission
can be identified using this theory. Additionally, the ease with
which [4n] vs. [4n+2] cyclic motifs can be identified adds an
extremely powerful tool for the synthetic chemist to predict
and rationalise optical energy gaps.
the requirement of a [4n] cyclic contribution to a molecule’s
resonance structures translates into a simple design rule for
target singlet fission systems. Examples of these (beyond
INDT) are structures such as DPP (which has been proposed
to undergo singlet fission, but no rationale for its large singlet-
triplet energy gap has been suggested), indigo, isoindigo,71
pechmann dyes and fulvalenes. Ground state aromaticity is a
fundamental cornerstone of synthetic organic chemistry,
providing stability and tuneability to unsaturated systems, un-
derpinning the entire development of the field of organic elec-
tronics. The work we present here demonstrates that excited-
state aromaticity plays an equally important role in both the
energetic position and the stability of photoexcited triplet
states. More broadly, we envisage the same approach may be
used to tune the exchange energy in organic systems where tri-
plets play a critical role, to mitigate losses in photovoltaics, har-
vest additional carriers in light-emitting diodes, enhance the
stability of novel open-shell systems or broaden the materials
library for photon up-conversion and singlet fission.
The inclusion of some ground-state antiaromaticty need
not compromise stability of these chromophores as all the ma-
terials presented here have substantial overall aromatic charac-
ter, and experimentally both DPP and Cibalackrot are ex-
tremely stable chromophores. As seen in Figure 4 and Table 1,
the magnitude of the S1-T1 energy gap can be tuned and we
suggest that in order to control this value through chemical de-
sign it is necessary to investigate the relative contributions of
the excited states (both S1 and T1) to the aromatic [4n] core
as Baird’s rules formally applies to both types of excited states,
which is beyond the scope of this investigation. Additionally,
we again note that our calculations are performed at a rela-
tively low level of theory, but the predicted values of the com-
pounds presented in Table 1 are in good agreement with ex-
periment.65,66 Finally, we note that solid-state intermolecular
interactions play a very significant role in the process of the
singlet fission.12,27,51,67,68 These are not considered in our theo-
retical analysis of new core chromophores, but where the in-
termolecular coupling motifs are known it should be possible
to optimise the geometry either experimentally12,27,68 or com-
ASSOCIATED CONTENT
Supporting Information is available free of charge on the ACS
Publications website. Supporting Information includes synthetic
methods and characterization, spectroscopic data and analysis,
putationally51,67,69
.
AUTHOR INFORMATION
Corresponding Authors
Conclusions
Though our experimental exploration has been limited to a
single family of materials, the wide tuneability we have
demonstrated, our preliminary calculations on alternative
[4n] containing dyes and previous investigations of Baird-type
aromaticity41–43,70 indicate that the same principles can be
readily applied to other materials systems. In practical terms,
hab60@cam.ac.uk
rhf10@cam.ac.uk
a.musser@sheffield.ac.uk
Author Contributions
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Journal of the American Chemical Society
‡KF and PB contributed equally to this work.
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Funding Sources
No competing financial interests have been declared.
Notes
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Computational data is available in a publicly available reposi-
tory at: https://github.com/SMTG-UCL/singlet-fission-
screening
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ACKNOWLEDGMENT
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Musser, A. J.; Maiuri, M.; Brida, D.; Cerullo, G.; Friend, R. H.; Clark,
J. The Nature of Singlet Exciton Fission in Carotenoid Aggregates. J.
Am. Chem. Soc. 2015, 137 (15), 5130–5139.
Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.; Zhu, X.-Y.;
Campos, L. M.; Sfeir, M. Y. A Design Strategy for Intramolecular
Singlet Fission Mediated by Charge-Transfer States in Donor–
Acceptor Organic Materials. Nat. Mater. 2015, 14 (4), 426–433.
Kasai, Y.; Tamai, Y.; Ohkita, H.; Benten, H.; Ito, S. Ultrafast Singlet
Fission in a Push–Pull Low-Bandgap Polymer Film. J. Am. Chem. Soc.
2015, 137 (51), 15980–15983.
Huynh, U. N. V.; Basel, T. P.; Ehrenfreund, E.; Li, G.; Yang, Y.;
Mazumdar, S.; Vardeny, Z. V. Transient Magnetophotoinduced
Absorption Studies of Photoexcitations in Pi-Conjugated Donor-
Acceptor Copolymers. Phys. Rev. Lett. 2017, 119 (1), 017401.
Wilson, M. W. B.; Rao, A.; Clark, J.; Kumar, R. S. S.; Brida, D.; Cerullo,
G.; Friend, R. H. Ultrafast Dynamics of Exciton Fission in
Polycrystalline Pentacene. J. Am. Chem. Soc. 2011, 133 (31), 11830–
11833.
Miyata, K.; Kurashige, Y.; Watanabe, K.; Sugimoto, T.; Takahashi, S.;
Tanaka, S.; Takeya, J.; Yanai, T.; Matsumoto, Y. Coherent Singlet
Fission Activated by Symmetry Breaking. Nat. Chem. 2017, 9 (10),
983–989.
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.; et al. Direct Observation of Correlated Triplet Pair Dynamics
during Singlet Fission Using Ultrafast Mid-IR Spectroscopy. J. Phys.
Chem. C 2018, 122 (4), 2012–2022.
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. J. Am. Chem. Soc. 2018, 140 (6),
2326–2335.
Stern, H. L.; Cheminal, A.; Yost, S. R.; Broch, K.; Bayliss, S. L.; Chen,
K.; Tabachnyk, M.; Thorley, K.; Greenham, N.; Hodgkiss, J. M.; et al.
Vibronically Coherent Ultrafast Triplet-Pair Formation and
Subsequent Thermally Activated Dissociation Control Efficient
Endothermic Singlet Fission. Nat. Chem. 2017, 9 (12), 1205–1212.
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. J. Am. Chem.
Soc. 2012, 134 (14), 6388–6400.
Part of this work was funded by EU project 679789 – CONTREX
and EPSRC grant "Centre for Advanced Materials for Integrated
Energy Systems (CAM-IES)" EP/P007767/1. AJM was sup-
ported by the EPSRC (EP/M01083X/1 and EP/M025330/1).
This work made use of the ARCHER UK National Supercompu-
ting Service (http://www.archer.ac.uk) via our membership of
the UK’s HEC Materials Chemistry Consortium, which is
funded by EPSRC (EP/L000202), and the Thomas machine
which is funded by the Tier 2 Hub in Materials and Molecular
Modelling (EP/P020194/1). AMG acknowledges Diamond
Light Source for the co-sponsorship of a studentship on the
EPSRC Centre for Doctoral Training in Molecular Modelling
and Materials Science (EP/L015862/1). C.N.S. is grateful to the
EPSRC and the Department of Chemistry at UCL for the provi-
sion of a Doctoral Training Partnership studentship (Ref No.
1492829).
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