Molecular Engineering of Chromophores to Enable Triplet-Triplet Annihilation Upconversion

Article abstract of DOI:10.1021/jacs.0c06386

Triplet-triplet annihilation upconversion (TTA-UC) is an unconventional photophysical process that yields high-energy photons from low-energy incident light and offers pathways for innovation across many technologies, including solar energy harvesting, photochemistry, and optogenetics. Within aromatic organic chromophores, TTA-UC is achieved through several consecutive energy conversion events that ultimately fuse two triplet excitons into a singlet exciton. In chromophores where the singlet exciton is roughly isoergic with two triplet excitons, the limiting step is the triplet-triplet annihilation pathway, where the kinetics and yield depend sensitively on the energies of the lowest singlet and triplet excited states. Herein we report up to 40-fold improvements in upconversion quantum yields using molecular engineering to selectively tailor the relative energies of the lowest singlet and triplet excited states, enhancing the yield of triplet-triplet annihilation and promoting radiative decay of the resulting singlet exciton. Using this general and effective strategy, we obtain upconversion yields with red emission that are among the highest reported, with remarkable chemical stability under ambient conditions.

Full text of DOI:10.1021/jacs.0c06386

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Article
Molecular Engineering of Chromophores to Enable Triplet−Triplet
Annihilation Upconversion
Kealan J. Fallon, Emily M. Churchill, Samuel N. Sanders, James Shee, John L. Weber, Rinat Meir,
Steffen Jockusch, David R. Reichman, Matthew Y. Sfeir,* Daniel N. Congreve,* and Luis M. Campos*
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ABSTRACT: Triplet−triplet annihilation upconversion (TTA-UC) is
an unconventional photophysical process that yields high-energy
photons from low-energy incident light and offers pathways for
innovation across many technologies, including solar energy harvest-
ing, photochemistry, and optogenetics. Within aromatic organic
chromophores, TTA-UC is achieved through several consecutive
energy conversion events that ultimately fuse two triplet excitons into a
singlet exciton. In chromophores where the singlet exciton is roughly
isoergic with two triplet excitons, the limiting step is the triplet−triplet
annihilation pathway, where the kinetics and yield depend sensitively
on the energies of the lowest singlet and triplet excited states. Herein
we report up to 40-fold improvements in upconversion quantum yields
using molecular engineering to selectively tailor the relative energies of the lowest singlet and triplet excited states, enhancing the
yield of triplet−triplet annihilation and promoting radiative decay of the resulting singlet exciton. Using this general and effective
strategy, we obtain upconversion yields with red emission that are among the highest reported, with remarkable chemical stability
under ambient conditions.
(3[An]*) energy lower than E(T1) of the sensitizer such that
triplet energy transfer (TET) from the sensitizer to the
annihilator is efficient. Finally, provided that the combined
energy of two annihilator triplet excitons, 2E(T1), is larger than
INTRODUCTION
■
Photons emitted from photoexcited organic materials are
generally lower in energy than the incident radiation used for
excitation, with the Stokes shift1 between the absorption and
the energy of the annihilator’s singlet excited state, triplet−
fluorescence dictated by rapid relaxation of the excited state.
triplet annihilation results in the generation of one singlet
Very few examples of anti-Stokes shifting systems are known,
exciton (1[An]*). This exciton can then emit a photon of
but the ability to upconvert long wavelength (low-energy)
energy higher than the incident photon used to excite the
incident light to short wavelength (high-energy) light offers
sensitizer. The exoergicity for triplet fusion can be described by
the energy gap ΔE = E(S1) − 2E(T1), and the process of TTA-
UC is favored when ΔE < 0.
attractive avenues of innovation across several technologies,2
notably those that rely on the penetration of light.3−5 For
example, photochemical reactions can be limited by light
penetration, requiring engineering of reaction vessels to
maximize surface area.6−8 However, the use of localized
apparent anti-Stokes photoluminescence can mitigate pene-
tration, allowing for efficient large-scale photochemical
reactions.9,10 Moreover, the efficiency of photovoltaic devices
could benefit from upconversion systems, harvesting trans-
mitted low-energy photons that can be converted to wave-
lengths to be absorbed in active layers.11,12
In the mechanism of TTA-UC, there are multiple events that
can impact the yield of high-energy-output photons relative to
the low-energy-input excitation. The total efficiency is a
product of four quantum yields: (1) intersystem crossing of the
sensitizer (ΦISC); (2) triplet energy transfer from the sensitizer
to the annihilator (ΦTET); (3) TTA of two annihilators in their
triplet excited state (ΦTTA); (4) photoluminescence of the
annihilator singlet state (ΦPL). Of these processes, ΦTTA is the
Triplet−triplet annihilation upconversion (TTA-UC, Figure
1A,B) is a bimolecular process that enables apparent anti-
Stokes emission. Low-energy incident light selectively excites a
ground state sensitizer (1[Sen]) molecule to its singlet excited
state (S1, 1[Sen]*). This sensitizer then undergoes intersystem
Received: June 12, 2020
3
crossing (ISC) to form a triplet exciton (T1, [Sen]*). The
second componentthe annihilator 1[An]has a triplet state
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A

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here we focus on the systematic functionalization using a single
type of functional group on the core acene motif.
Interestingly, in tetracenea well-known chromophore
involved in multiexciton processesthe energy difference
between the singlet energy and twice the energy of the triplet,
ΔE[S1−2T1], is ∼0 eV and is thus isoergic to both singlet
fission and triplet−triplet annihilation. While upconversion in
tetracene can be altered using intermolecular assemblies35,36
and chromophore connectivity,37,38 the only tetracene
derivative exhibiting high upconversion efficiency is ru-
brene.39,40 While in rubrene TTA-UC quantum yields are
high, reaching 8% (where the limit is 50%, vide infra),
photochemical processes with rubrene are very sensitive to
oxygen, leading to degradation within 30 min.39
While rubrene exhibits high upconversion efficiency, little is
known about how chemical structure modifications to the
tetracene core impact multiexciton dynamics of TTA-UC.
Therefore, we sought to test how functional group
substitutions in tetracene affect the energy of the singlet
state, such that ΔE[S1−2T1] < 0. We posit that the frontier
energy levels of tetracene (ΔE[S1−2T1] ∼ 0) can be tuned
through molecular engineering to meet the energetic require-
ment for TTA-UC,41,42 specifically using electron withdrawing
groups to lower the singlet state energy (Figure 1C). These
changes are known to have large consequences for the
optoelectronic properties of acenes. In anthracene, ΔE[S1−
2T1] is negative and triplet−triplet annihilation is exoergic.43
In pentacene, ΔE[S1−2T1] is positive, rendering it inefficient
at TTA but highly efficient at the reverse process: singlet
fission.44−46 Working with derivatives of tetracene, we find that
carbonitrile substitution (a strong electron withdrawing group)
can effectively lower E(S1), such that ΔE[S1−2T1] becomes
negative, thus leading to remarkable increases in TTA-UC
efficiencies (by up to 40-fold). Moreover, the aerobic stability
of these systems showed drastic improvements, which is
important for a variety of applications outside of an inert
atmosphere.47,48
Figure 1. Energetic landscape of TTA-UC. (A) Net process of TTA-
UC, where the notation m[Molecule]* represents the multiplicity, m,
of the systems involved and the asterisk corresponds to excited states
(no asterisk corresponds to a ground state molecule). (B) Jablonski
1
diagram of the process of TTA-UC: the sensitizer [Sen] is excited
with low-energy light; the singlet exciton 1[Sen]* undergoes
3
intersystem crossing (ISC) to a triplet exciton [Sen]*, which then
3
undergoes triplet energy transfer (TET) to the annihilator [An]*;
and, finally, the interaction between two 3[An]* yield one annihilator
1
1
in the ground state [An] and other in the singlet state [An]* by
triplet−triplet annihilation. (C) Relative energies26 E(S1) and E(T1)
of three molecules that undergo TTA, singlet fission (SF), and
competing pathways. The strategy to access the TTA channel in
tetracene derivatives by lowering E(S1) with functional groups (FG),
while maintaining E(T1) fixed, is the focus of this paper.
RESULTS AND DISCUSSION
■
Synthesis and Optical Properties. Tetracene 1 (Figure
2A) has low solubility, poor stability, and a tendency to
aggregate in solution. For these reasons triisopropylsilylacety-
lene tetracene (TIPS-tetracene, 2)along with TIPS-
anthracene43 and TIPS-pentacene44−46have become ubiq-
uitous in solution-based excitonic studies. The bulky isopropyl
groups protect the acene core sterically from degradation
reactions, while the acetylene groups stabilize the diradical
nature of the acene in its triplet state.49
value that typically deviates the most from unity, dominating
the net efficiency of TTA-UC.2,13 While a large variety of
chemical annihilators have been examined, including acenes,2
diketopyrrolopyrrole,14 diphenylisobenzofuran,15 BODIPY,16
rylenedimides,17,18 diphenyloxazole,19 and polymeric sys-
tems,20−23 overall upconversion yields consistently fall well
short of the theoretical maximum. As such, we postulate that
through molecular engineering it may be possible to tune ΦTTA
by altering the frontier energy levels by design, particularly in
known red emitters where the singlet and triplet pair energy is
nearly isoergic (e.g., Figure 1C, tetracene).24,25
In order to exploit TTA-UC in many applications, it is
imperative to develop systems that are efficient, stable, and
modular in terms of energy input and output. However, the
rational design of TTA-UC annihilators is still in its infancy.
Previous studies investigating the chemical functionalization of
anthracene have revealed interesting insights into TTA-UC
dynamics, although these reports focus solely on the
functionalization of the acetylene component using a variety
of functional groups in each study.27−30 While complexation,
self-assemblies, and dimerization have also been explored,31−34
In the tetracene derivatives studied herein, 2−8, we refer to
the short edge of the acene nearest the TIPS groups as the
“head” in tetracene, and the opposite edge we refer to as the
“tail.” To reduce the energy of the singlet state, the
chromophore was functionalized with strong electron with-
drawing carbonitrile functional groups. All carbonitrile groups
were installed on the corresponding aryl bromides using
microwave-assisted Pd-catalyzed cross-coupling with copper(I)
cyanide; additional synthetic details are available in the
Supporting Information.
The energy of the lowest singlet excited state was estimated
from the crossing point of the absorption and emission spectra
of each molecule in dilute toluene (Figures S1−S3). The
normalized absorption spectra of compounds 2−8 are shown
in Figure 2B. The functionalization of tetracene lowered E(S1)
B
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Supporting Information. Excitation with a 730 nm laser
furnished upconverted photoluminescence (Figure 3), while
the photoluminescence quantum yield (ΦPL) of each sample
was determined for each sample by excitation at 365 nm.
Intrinsic photoluminescence quantum yield (PLQY) measure-
ments of the neat annihilators are also given in Tables S1−S3.
In TTA-UC, two low-energy photons are required for the
generation of one high-energy photon, meaning that ΦTTA, and
thus ΦUC, have upper limits of 50%.53 Importantly, we focus
our discussion on two distinct upconversion quantum yields
(following the convention of ref 53): ΦUC denotes the TTA-
UC quantum yield of observed upconverted emission from the
experiment; ΦUC denotes the derived quantum yield of singlet
S
states produced in the experiment, removing optical losses
from the photoluminescence event. While the observed
emission ΦUC is desirable for applications, ΦUC better
S
quantifies the chromophore’s intrinsic protentional for TTA-
UC, and maximizing both requires optimization of both the
intrinsic photophysics and experimental conditions. We also
note that we discuss the quantum yields in terms of percent,
where the limit of both ΦUC and ΦUC is 50%, i.e.
S
ΦUC = ΦISCΦTETΦTTAΦPL × 100%
(1)
ΦUC = ΦUC/ΦPL × 100%
(2)
S
These two figures of merit are typically optimized under
different conditions: ΦUC is usually enhanced in concentrated
S
solutions where intermolecular interactions are maximized;
however, photon reabsorption and singlet energy transfer in
such systems will result in low ΦPL (and hence low ΦUC).
Likewise, in dilute solutions, photon/singlet state trapping is
minimized, typically resulting in high ΦPL (and hence high
Figure 2. (A) Structures of tetracene and its derivatives, function-
alized with TIPS-acetylene and carbonitrile groups. We refer to the
edge of tetracene nearest the TIPS-acetylene groups as the “head”
(e.g., 6) and the furthest edge the “tail” (e.g., 5). Compound 4 is a
mixture of two inseparable regioisomers. (B) Normalized steady state
absorption spectra of compounds 2−8, taken in dilute solutions in
toluene.
Φ
UC) but lower ΦUC due to nonradiative losses of transient
S
triplet excitons. As studying the upconversion characteristics
for each compound across the entire concentration space
would be very difficult, we analyzed each chromophore in two
types of solutions, (a) dilute and (b) concentrated, where the
annihilator/sensitizer quantities were (a) 1 mM/1 mol % and
(b) 9 mM/9 mol %. The upconversion quantum yields are
shown in Table 1.
on average by ∼30 meV per carbonitrile group. However, the
reduction in the three dicarbonitrile stereoisomers was
inequivalent: −70 meV for 4, −90 meV for 5, and −140
meV for 6. Notably, the reduction in singlet state energy in 6
was greater than that of the tetracarbonitrile compound 8. The
effect of the degree of carbonitrile functionalization on the
fluorescence lifetime of the functionalized tetracenes was
investigated using time-correlated single photon counting
(Figure S4). The lifetimes ranged between 14 and 18 ns
(Table S4), comparable but slightly longer than those of both
TIPS-tetracene (11 ns, Figure S5) and literature reports of
rubrene (10 ns).50
The highest ΦUC /ΦUC of the benchmark TIPS-tetracene 2
S
was 0.3/0.12%, respectively. All the carbonitrile tetracene
Triplet−Triplet Annihilation Upconversion. The reduc-
tion in E(S1) in this family of chromophores allowed us to test
how ΔE[S1−2T1] affects ΦTTA. If the carbonitrile groups do
not drastically change the energy of the triplet state, then
ΔE[S1−2T1] should become negative, thus favoring TTA to
the singlet state. The tetracene materials were paired with
palladium(II) octabutoxyphthalocyanine (PdPc),51 which
when excited at 730 nm (1.7 eV) undergoes efficient
intersystem crossing to generate a triplet exciton with E(T1)
∼1.3 eV.40 This energy is higher than E(T1) of tetracene
(∼1.15 eV), so the triplet exciton transfers to the annihilator
via Dexter energy transfer. Photon upconversion characteristics
were determined following the integrating sphere method from
de Mello et al.,52 and a brief overview is given in the
Figure 3. Absorption of PdPc (red); absorption (blue dashed) and PL
(blue solid) of 6. Upconversion experiment (purple dashed) showing
UCPL of 6 between 600 and 700 nm and reabsorption losses between
550 and 600 nm.
C
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3.0 μs (Figure S9), in good agreement with the literature.55
The lifetime of the triplet exciton in the presence of each
annihilator (τ(S+A)) ([An] = 1 × 10−3 M) was probed similarly,
and eq 3 was used to deduce ΦTET (Table 2).
Triplet energy transfer in TIPS-tetracene 2 is reasonably
efficient (ΦTET = 64%), and functionalized tetracenes show
comparable quantum yields with slightly higher values but
within range of experimental error. With the similarities in
Table 1. Triplet−Triplet Annihilation Upconversion
Characteristics of Annihilator Compounds 2−8 (An)
a
b
dilute
concentrated
c
d
c
Φ
(%)
Φ
UCS
(%)
UCS
ΦPL
ΦUC
ΦPL
ΦUC
An
(%)
(%)
(%)
(%)
2
3
4
5
6
7
8
0.14
0.21
0.18
1.6
3.5
0.65
1.4
82.4
79.1
78.5
83.8
71.4
74.4
72.4
92.2
0.12
0.17
0.15
1.3
2.5
0.5
0.30
0.12
0.38
3.5
9.0
1.8
3.6
2.7
3.3
2.3
1.8
2.5
1.4
0.01
0.005
0.01
0.1
0.15
0.05
0.02
0.07
Φ
TET, it may be inferred that the TTA-UC efficiency improves
as a result of increased exoergic triplet−triplet annihilation:
ΦTTA is improved 24-fold in 6 (5.0%) versus 2 (0.21%).
Stability of Tetracene Upconversion Chromophores.
To date, rubrene is by far the most widely employed NIR to
orange TTA-UC annihilator. However, it is unstable under
irradiation in aerobic conditions, degrading within 30 min.
Electron-deficient aromatic systems in general tend to exhibit
higher ambient stability due to a deepening of the energetic
states away from the oxidation potential, resulting in an excited
state less prone to oxidation or photodimerization.56−58
Furthermore, the addition of strongly electron withdrawing
groups to tetracene may lower its reactivity toward
endoperoxide formation with molecular oxygen. To assess
the stability of the family of tetracene materials, the steady
state absorption spectra of dilute solutions in untreated (i.e.,
not degassed) toluene were taken over a period of 14 days
(Figure 4). The samples were stored aerobically in a well-lit
area but out of direct sunlight. The absorption maximum of
TIPS-tetracene 2 after 14 days was reduced to 20% of its initial
absorbance. As hypothesized, the addition of each carbonitrile
group increases the stability of the materials, with the highest
stability seen in 8, which exhibited 85% of its original
absorbance after 14 days.
1.0
0.12
0.9
1.8
e
rubrene
0.13
3.9
a
b
Dilute: [An] = 1 × 10−3 M, [PdPc] = 1 × 10−5 M. Concentrated:
[An] = 9 × 10−3 M, [PdPc] = 8.1 × 10−4 M. Experiments were carried
out on 1 mL samples at the aforementioned concentrations in 1 cm2
cuvettes at RT. Samples were prepared in a degassed environment
(N2 glovebox) with the use of anhydrous degassed (N2) toluene.
Cuvettes were capped, sealed with Parafilm, and analyzed immediately
on the optics table. The fluence of the incident excitation was such
c
d
that the experiment was in the linear regime. QY limit is 50%. QY
limit is unity. Aldrich sublimed grade.
e
materials exhibited higher ΦUC, and all with the exception of 3
exhibited higher ΦUC . Focusing on the intrinsic ability of the
S
compounds to undergo upconversion, ΦUC shows a strong
S
correlation to the reduction in singlet state energy;
interestingly, the dicarbonitrile derivative 6 has the largest
ΔE[S1−2T1] and it yielded the highest ΦUC /ΦUC of 9.0/2.5%,
S
respectively.
Rubrene performed poorly in dilute solution but achieved
Computational Studies. Triplet−triplet annihilation can
only be more exoergic in tetracenes with lower E(S1) if E(T1)
is relatively unchanged or increased, such that ΔE[S1−2T1]
becomes more negative. Our results strongly suggest this is the
case, but they also reveal unusual characteristics, particularly in
that 6 shows the largest ΔE(S1), 30 meV larger than 8. To
understand our observations with more clarity, we investigated
the excited state energetic landscape using quantum chemical
calculations. Time-dependent density functional theory (TD-
DFT) with range-separated functionals has been shown to
provide accurate descriptions of singlet excited states of
polyacenes.59 With the use of optimized ground state
geometries (B3LYP/cc-pVTZ), vertical S1 energies were first
high ΦUC in concentrated solution. We explored the
S
concentration dependence (Figure S7) of this system and
optimized the [rubrene]/[PdPc] pair at 9 × 10−3 M/9 × 10−5
M, respectively, where we observed the highest ΦUC of 5.5%,
S
in line with concentrations used in other literature reports.54 At
the same concentration, 6 exhibits ΦUC of 7.9%, showcasing
S
the impact of functional group substitution to favor the TTA
channel.
Combining eqs 1 and 2, it is clear that ΦUC is affected by
three quantum yields, but intersystem crossing (ΦISC) can be
ignored as the sensitizer species is constant. It is therefore
important to deduce whether the observed improvements in
ΦUC result from improved (a) triplet energy transfer (ΦTET
)
S
Table 2. Quantum Yields of Triplet Energy Transfer (ΦTET
and Triplet−Triplet Annihilation (ΦTTA) of Annihilator
Compounds 2−8 (An)
)
and/or (b) triplet−triplet annihilation (ΦTTA). To distinguish
between these two quantum yields, we used transient
absorption spectroscopy to calculate the quantum yield of
triplet energy transfer from the sensitizer to the annihilator as
a
b
An
ΦTET (%)
ΦTTA (%)
ΦTET = (1 − [τ(S+A)/τS]) × 100%
2
3
4
5
6
7
8
64
68
68
70
69
62
82
0.21
0.30
0.26
2.2
5.0
1.1
(3)
Here, τS is the lifetime of the triplet exciton of the sensitizer
and τ(S+A) is the same lifetime but in the presence of an
annihilator. To measure τS, a 10−5 M solution of the sensitizer
in degassed toluene was excited with a 730 nm laser pulse.
After a delay, a broadband probe pulse examined photo-
induced absorption of the excited states in the nanosecond to
microsecond range, which can be attributed to triplet excitons
(singlet states decay before this time scale). The triplet state
photoinduced absorbance in PdPc is broad over 570−635 nm,
peaking at ∼615 nm, and decays with a time constant (τS) of
1.7
a
b
QY limit is 100%. QY limit is 50%. Experiments were performed in
degassed (bubbling with argon) toluene on samples where [PdPc] = 1
× 10−5 M and [An] = 1 × 10−3 M at RT with a cuvette having a 2 mm
path length.
D
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Figure 4. Stability of tetracene materials over a 2 week period when
stored in a well-lit (conventional fluorescent tube lighting) aerobic
environment. Plot shows λmax intensity relative to intensity on day 0.
calculated with TD-DFT (CAM-B3LYP/cc-pVTZ). To obtain
the adiabatic E(S1) values reported, geometries corresponding
to the minima of the S1 surfaces were optimized within the
Tamm−Dancoff approximation (TDA). Finally, the differences
of the total energies (computed via TD-DFT without the
TDA, using CAM-B3LYP/cc-pVTZ) between the Franck−
Condon regions and the S1 minima were added to the vertical
excitation energies in the ground state geometry.
The calculated E(S1) values (Figure 5) are consistent with
experimental observations, crucially in that the lowest energy
singlet state is expected for 6. In order to better understand the
electronic effects of the carbonitriles, we calculated difference
density plots (Figure S11) in which the electron density of the
ground state is subtracted from that of the singlet excited state.
In essence, the results reveal the character of intramolecular
charge transfer upon excitation. Figure 5A shows the difference
density plots for four of the studied tetracenes.
For the singlet excited states in 2, there is an increase of
electron density on the acene core carbons where the acetylene
groups are installed, while the π-system at the tail of the
tetracene is depleted. Compounds 3, 4, and 8 show relatively
balanced redistributions of electron density in the excited
states, and we attribute the stabilization of E(S1) in these
materials predominately to the effect of the addition of the
electron withdrawing and π-system extending carbonitriles on
the chromophore. However, both 5 and 6 exhibit considerable
increases in electron density localized at the end of the
molecule where the two carbonitrile functional groups are
installed. We postulate that the directional charge transfer is
more stabilizing in 6 than in 5, as in the former the increased
density in the adjacent acetylene units serves to extend the
conjugated π-system occupancy to a larger area.
The difference density plots of the triplet excited states show
a contrasting picture to the singlet states (Figure S11), where
all tetracenes exhibit relatively similar charge transfer character-
istics. This suggests that the chemical functionalization affects
the character of the triplet state to a much smaller degree than
the singlet states. As measuring the energy of the triplet states
in organic materials is particularly challenging, we employed a
high-level ab initio method to probe the effects of these
functionalizations on the triplet state energies. Coupled cluster
theory with single, double, and perturbative triple excitations
(CCSD(T)) is a robust and size-extensive wave function
method that has been previously employed to investigate the
electronic properties of polycyclic aromatic hydrocarbons with
high accuracy.60−62 The high computational cost of canonical
Figure 5. (A) Plots of the difference in electron density in the excited
singlet states of four tetracenes. Red regions indicate locations of
lower electron density in the excited state compared to the ground
state, while blue regions indicate higher electron density in the excited
state. (B) Experimental (blue) singlet state energies estimated from
the crossing point of absorbance and PL. The trend in the theoretical
singlet state energies (gray dash, TD-DFT) is in excellent agreement
with experimental observations. Calculated energies (DLPNO-
CCSD(T)) of the T1 states are shown with red bars. Both axis
segments show a 200 meV window to allow energy differences
between all states to be directly visually comparable.
CCSD(T) calculations has been substantially reduced via the
DLPNO approximation, which exploits the locality of
electronic correlations.63 As such, we employed DLPNO-
CCSD(T) to calculate the triplet state energies using the
optimized geometries of the singlet and triplet states (B3LYP/
cc-pVTZ). The E(T1) of 2 was calculated to be 1.23 eV, in
good agreement with reported phosphorescence of ∼1.25 eV
of TIPS-tetracene in a polystyrene matrix;64 however, we
emphasize that we focus our investigation and discussion on
trends in the calculated energies and not on the absolute values
of the energies themselves. Remarkably, at this level of theory,
the energies of the triplet states of the family of tetracenes are
relatively comparable: the largest difference in triplet energy
across the series is 14 meV, an order of magnitude less than the
largest experimental ΔE(S1) (140 meV) within the series.
These results correlate strongly with the difference density
plots in that the energy of T1 in tetracene is largely unaffected
by the addition of carbonitrile functionality.
Functionalization of Other Polyaromatic Hydrocar-
bons. In order to test the model, we turn to tuning the value
of E(S1) via carbonitrile substituents with two other common
upconverting chromophores: perylene and anthracene deriva-
tives (Figure 6).
The carbonitrile derivatives of 2,5,8,9-tetra-tert-butylpery-
lene 9 exhibited the largest changes in singlet energies, with a
lowering by 180 meV for the monocarbonitirile compound 10
E
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ΦUC (7.7%). While the higher ΦUC in functionalized
S
S
perylenes is more likely due to enhancements of the rate of
triplet−triplet annihilation, we also suggest that the higher
solubility of the functionalized perylene derivatives contributes
to increased yields in concentrated solutions.
In contrast, TIPS-anthracene 12 is known to exhibit highly
exoergic photon upconversion, reflected in its markedly high
ΦUC of 8.4% in dilute solutions. Therefore, upon the addition
of carbonitriles, the reduction of E(S1) does not have a strong
effect on its photon upconversion characteristics, as expected.
Compound 14 showed an improved ΦUC; however, it is
particularly noteworthy that the cyanated anthracenes are
considerably brighter compoundsthe ΦPL in 15 (88%) is
23% brighter than that in 12. Interestingly, it was also found
that the dilute solutions exhibit higher ΦUC than concentrated
S
Figure 6. Functionalization of perylene and anthracene with
carbonitrile groups. Compound 11 is a mixture of two inseparable
regioisomers.
solutions. Despite the concentration dependence, the observed
trend in concentrated solutions resulted in higher ΦUC yields
from the cyanated compounds, in comparison to the
benchmark compound 12. We note the higher ΦPL essentially
raised the ΦUC; e.g., in concentrated solution, compound 14 is
and an additional 70 meV for the dicyano 11. For derivatives
of TIPS-anthracene 12, the singlet state was lowered by 90
meV in 13 and a further 50 meV for both 14 and 15 (Tables
S2 and S3). In order to characterize TTA-UC, the perylene
and anthracene chromophores were paired with the sensitizer
palladium(II) meso-tetraphenyl-tetrabenzoporphyrin
(PdTPTBP),2,35,65 which was excited at 635 nm. The resulting
upconversion characteristics are shown in Table 3.
4 times more efficient than 12 due to both improved ΦUC and
S
a 3-fold increase in ΦPL. This result is of particular importance
for TTA-UC applications where the systems are constrained to
local concentration environments, such as in nanoparticles or
micelles.47
In order to test the contributions of triplet energy transfer to
the upconversion quantum yields, we turn to transient
absorption spectroscopy. For the perylene and anthracene
derivatives, we determined the decay constant of T1 of
PdTPTBP (τS) to be ∼100 μs (Figure S10), in agreement with
the literature.66 In the presence of all compounds 9−15, the
sensitizer triplet decays (τ(S+A)) before 2 μs, showcasing the
exceptional near-unity triplet energy transfer (ΦTET > 96%)
between PdTPTBP and these materials (Table 4). From these
results, it can be inferred that changes in TTA-UC character-
istics in these systems result almost exclusively from changes in
the quantum yield of triplet−triplet annihilation and photo-
luminescence, not from changes in triplet energy transfer
efficiency.
Considering the perylene annihilators, the benchmark tetra-
tert-butylperylene 9 is an efficient TTA-UC chromophore in
dilute solution (ΦUC = 5.6%, ΦUC = 3.9%) but less so in
S
concentrated media (ΦUC = 2.0%, ΦUC = 0.05%); it was found
S
that the addition of carbonitrile groups to perylene increased
the photoluminescence quantum yield (ΦPL) from 70% in 9 to
near unity (97%) in 11. The functionalized perylenes exhibited
improved ΦUC and ΦUC, with the most notable result being
S
that the ΦUC starkly improved in concentrated systems: the
ΦUC of 11 is 13 times higher than that of 9, originating from
higher ΦPL, but with notably an almost 4-fold improvement in
Table 3. Photon Upconversion Characteristics of the
Carbonitrile Functionalized Perylenes (Annihilators [An]
9−11) and Anthracenes (Annihilators [An] 12−15)
CONCLUSIONS
■
Since singlet fission is a deactivating channel for triplet−triplet
annihilation upconversion chromophores that have relatively
similar energies, E(S1) and 2E(T1), such that ΔE[S1−2T1] ≈
0, such systems generally exhibit low efficiencies for each
a
b
dilute
concentrated
c
d
c
An ΦUC (%) ΦPL (%) ΦUC (%) ΦUC (%)
Φ
PL (%)
Φ
UC (%)
S
S
9
5.6
7.5
8.4
12.8
11.6
13.9
9.7
69.9
82.8
97.1
65.4
75.8
87.5
83.0
3.9
6.2
8.2
8.4
8.8
2.0
7.5
7.7
9.6
8.0
11.3
11.4
4.9
6.8
8.4
1.7
3.8
5.4
4.7
0.05
0.51
0.65
0.15
0.30
0.61
0.55
Table 4. Quantum Yields of Triplet Energy Transfer (ΦTET
)
10
11
12
13
14
15
and Triplet−Triplet Annihilation (ΦTTA
)
a
b
An
ΦTET (%)
ΦTTA (%)
9
10
11
12
13
14
15
96
98
97
99
99
99
99
5.8
7.7
8.6
12.9
11.8
13.9
9.8
12.1
8.0
a
Dilute: [An] = 1 × 10−3 M, [PdTPTBP] = 1 × 10−5 M.
b
Concentrated: [An] = 9 × 10−3 M, [PdTPTBP] = 8.1 × 10−4 M.
Experiments were carried out on 1 mL samples at the aforementioned
concentrations in 1 cm2 cuvettes at RT. Samples were prepared in a
degassed environment (N2 glovebox) with the use of anhydrous
degassed (N2) toluene. Cuvettes were capped, sealed with Parafilm,
and analyzed immediately on the optics table. The fluence of the
incident excitation was such that the experiment was in the linear
a
b
QY limit is 100%. QY limit is 50%. Experiments were performed in
degassed (Ar) toluene on samples where [PdTPTBP] = 1 × 10−5
M
and [An] = 1 × 10−3 M at RT with a cuvette having a 2 mM path
c
d
regime. QY limit is 50%. QY limit is unity.
length.
F
https://dx.doi.org/10.1021/jacs.0c06386
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Authors
process. In order to favor the TTA channel, it was
hypothesized that with tuning the energy of the singlet state
without changing the triplet state energy, such that ΔE[S1−
2T1] < 0, the TTA channel would be favored. Functionaliza-
tion of the reported polyacenes with carbonitrile functional
groups leads to a greater lowering of E(S1) relative to E(T1),
which correlated strongly to increased quantum yields of
triplet−triplet annihilation, impacting the TTA-UC process of
three families of annihilators. For example, efficiencies of TTA-
UC showed a drastic increase in a tetracene functionalized
derivative (compound 6), relative to the parent TIPS-tetracene
(compound 2), from 0.3 to 9.0%. Moreover, such a
functionalized tetracene derivative exhibited orders of
magnitude higher aerobic stability than rubrene, the current
widely employed TTA-UC annihilator for NIR to orange light.
In dilute solution, the half-life of 8 (∼52 days) is up to 5 orders
of magnitude greater than that of rubrene (<30 min). Of
particular importance, the hypothesis was also tested with
perylene and carbonitrile-functionalized derivatives, where
upconversion efficiencies increased from 2.0% (9) up to
7.7% (11) and exhibited enhanced stability. We note that for
Kealan J. Fallon − Department of Chemistry, Columbia
University, New York, New York 10027, United States;
orcid.org/0000-0001-6241-6034
Emily M. Churchill − Department of Chemistry, Columbia
University, New York, New York 10027, United States
Samuel N. Sanders − Rowland Institute at Harvard
University, Cambridge, Massachusetts 02142, United States
James Shee − Department of Chemistry, Columbia University,
New York, New York 10027, United States; orcid.org/
0000-0001-8333-8151
John L. Weber − Department of Chemistry, Columbia
University, New York, New York 10027, United States;
orcid.org/0000-0002-4937-9651
Rinat Meir − Department of Chemistry, Columbia University,
New York, New York 10027, United States
Steffen Jockusch − Department of Chemistry, Columbia
University, New York, New York 10027, United States;
orcid.org/0000-0002-4592-5280
David R. Reichman − Department of Chemistry, Columbia
University, New York, New York 10027, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c06386
applications it is necessary to maximize both ΦUC and ΦUC for
S
any given system. The tuning of the latter can be achieved
through careful optimization of the local environment
(concentration, solvent, viscosity, etc.). As we analyzed only
two environments for each annihilator, it is highly likely that
Notes
The authors declare the following competing financial
interest(s): DNC has an ownership stake in, and is Chief
Scientific Advisor to, Quadratic 3D, Inc. SNS has an ownership
stake in, and is Chief Technology Officer of, Quadratic 3D,
Inc.
higher and/or more balanced values of ΦUC and ΦUC can be
S
achieved for these systems. Notably, it was also found that the
molecular engineering approach led to high efficiencies in
concentrated solutions. Chemical modifications that alter the
energy levels of common chromophores can result in materials
to be exploited in applications that require TTA-UC, such as
biomaterials, imaging, and solar energy harvesting systems.
ACKNOWLEDGMENTS
■
L.M.C. thanks the Camille and Henry Dreyfus Foundation and
NSF CAREER (DMR-1351293). D.N.C. and S.N.S. acknowl-
edge the support of the Rowland Fellowship at the Rowland
Institute at Harvard. K.J.F. thanks and acknowledges funding
by the U.S.−U.K. Fulbright Commission and Lloyd’s
Tercentenary Research Foundation. E.M.C. thanks the NSF
GRFP (Grant No. 1644869). S.N.S. thanks the Arnold and
Mabel Beckman Foundation for funding. R.M. thanks the
U.S.−Israel Educational Foundation and Fulbright Program for
funding. D.R.R. acknowledges the support of NSF-CHE
1954791.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c06386.
Synthesis and materials characterization, absorption and
emission spectroscopies, detailed information on nano-
second transient absorption measurements and analysis,
complete computational analysis images (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Luis M. Campos − Department of Chemistry, Columbia
University, New York, New York 10027, United States;
orcid.org/0000-0003-2110-2666; Email: lcampos@
columbia.edu
Daniel N. Congreve − Rowland Institute at Harvard
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Matthew Y. Sfeir − Department of Physics, Graduate Center
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