Benzo[1,2-b:4,5-b']dithiophene as a weak donor component for push-pull materials displaying thermally activated delayed fluorescence or room temperature phosphorescence

Article abstract of DOI:10.1016/j.dyepig.2020.109022

In the search for high-performance donor-acceptor type organic compounds displaying thermally activated delayed fluorescence (TADF), triisopropylsilyl-protected benzo[1,2-b:4,5-b']dithiophene (BDT-TIPS) is presented as a novel donor component in combination with two known acceptors: dimethyl-9H-thioxanthenedioxide (TXO2) and dibenzo[a,c]phenazinedicarbonitrile (CNQxP). For a broader comparison, the same acceptors are also combined with the well-studied 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor. Optimized BDT-TIPS-containing structures show calculated dihedral angles of around 50° and well-separated highest occupied and lowest unoccupied molecular orbitals, although varying singlet-triplet energy gaps are observed experimentally. By changing the acceptor moiety and the resulting ordering of excited states, room temperature phosphorescence (RTP) attributed to localized BDT-TIPS emission is observed for TXO2-BDT-TIPS, whereas CNQxP-BDT-TIPS affords a combination of TADF and triplet-triplet annihilation (TTA) delayed emission. In contrast, strong and pure TADF is well-known for TXO2-DMAC, whereas CNQxP-DMAC shows a mixture of TADF and TTA at very long timescales. Overall, BDT-TIPS represents an alternative low-strength donor component for push-pull type TADF emitters that is also able to induce RTP properties.

Full text of DOI:10.1016/j.dyepig.2020.109022

Journal Pre-proof
Benzo[1,2-b:4,5-b']dithiophene as a weak donor component for push-pull
materials displaying thermally activated delayed fluorescence or room temperature
phosphorescence
Tom Cardeynaels, Simon Paredis, Andrew Danos, Dirk Vanderzande, Andrew P.
Monkman, Benoît Champagne, Wouter Maes
PII:
S0143-7208(20)31719-8
DOI:
https://doi.org/10.1016/j.dyepig.2020.109022
DYPI 109022
Reference:
To appear in: Dyes and Pigments
Received Date: 29 September 2020
Revised Date: 12 November 2020
Accepted Date: 19 November 2020
Please cite this article as: Cardeynaels T, Paredis S, Danos A, Vanderzande D, Monkman AP,
Champagne B, Maes W, Benzo[1,2-b:4,5-b']dithiophene as a weak donor component for push-pull
materials displaying thermally activated delayed fluorescence or room temperature phosphorescence,
Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.109022.
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Contributions:
T. Cardeynaels: Synthesis and structural characterization of all compounds, quantum-chemical
calculations, spectroscopic measurements and preparation of the manuscript.
S. Paredis: Spectroscopic measurements, discussion and revision of the manuscript.
A. Danos: Supervision of time-resolved spectroscopy, discussion and revision of the manuscript.
A. P. Monkman: Supervision of time-resolved spectroscopy at Durham University facilities,
discussion of the results and revision of the manuscript.
B. Champagne: Project management, supervision of the quantum-chemimcal calculations,
discussion of the results and revision of the manuscript.
D. Vanderzande and W. Maes: Project management, discussion of the results and revision of the
manuscript.

Benzo[1,2-b:4,5-b']dithiophene as a weak donor component for push-pull
materials displaying thermally activated delayed fluorescence or room
temperature phosphorescence
Tom Cardeynaels,a,b Simon Paredis,a Andrew Danos,c Dirk Vanderzande,a Andrew P. Monkman,c
Benoît Champagneb and Wouter Maesa,*
a
Hasselt University, Institute for Materials Research (IMO-IMOMEC), Design & Synthesis of Organic Semiconductors (DSOS), Agoralaan 1,
3590 Diepenbeek, Belgium and IMOMEC Division, IMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium
b
University of Namur, Laboratory of Theoretical Chemistry, Theoretical and Structural Physical Chemistry Unit, Namur Institute of
Structured Matter, Rue de Bruxelles 61, 5000 Namur, Belgium
c OEM group, Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom
* wouter.maes@uhasselt.be
Abstract
In the search for high-performance donor-acceptor type organic compounds displaying thermally activated delayed
fluorescence (TADF), triisopropylsilyl-protected benzo[1,2-b:4,5-b']dithiophene (BDT-TIPS) is presented as a novel donor
component in combination with two known acceptors: dimethyl-9H-thioxanthenedioxide (TXO2) and
dibenzo[a,c]phenazinedicarbonitrile (CNQxP). For a broader comparison, the same acceptors are also combined with the
well-studied 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor. Optimized BDT-TIPS-containing structures show calculated
dihedral angles of around 50° and well-separated highest occupied and lowest unoccupied molecular orbitals, although
varying singlet-triplet energy gaps are observed experimentally. By changing the acceptor moiety and the resulting ordering
of excited states, room temperature phosphorescence (RTP) attributed to localized BDT-TIPS emission is observed for TXO2-
BDT-TIPS, whereas CNQxP-BDT-TIPS affords a combination of TADF and triplet-triplet annihilation (TTA) delayed emission.
In contrast, strong and pure TADF is well-known for TXO2-DMAC, whereas CNQxP-DMAC shows a mixture of TADF and TTA
at very long timescales. Overall, BDT-TIPS represents an alternative low-strength donor component for push-pull type TADF
emitters that is also able to induce RTP properties.
1

Keywords
Thermally activated delayed fluorescence – Organic light-emitting diodes – Room temperature phosphorescence --
Benzo[1,2-b:4,5-b’]dithiophene – Photophysical and quantum-chemical characterizations
1. Introduction
Organic light-emitting diodes (OLEDs) have proven to be a competitive replacement for older lighting technologies such as
halogen lamps and inorganic LEDs, as well as display technologies such as liquid crystal displays (LCDs).[1-5] Nevertheless,
challenges remain toward cost-efficient, sustainable and low power-draw devices with a sufficiently long lifetime.[1, 3] On
the road to higher external quantum efficiencies (EQEs), a major step was taken by the introduction of heavy-metal-
containing organic complexes, allowing phosphorescence from triplet excitons as an efficient emission pathway.[6]
Intersystem crossing (ISC) from the singlet to the emissive triplet state allows the emitter to harvest all excitons through the
phosphorescence channel, leading to internal quantum efficiencies (IQEs) of up to 100%. Although light out-coupling is still
far from ideal and phosphorescence is an intrinsically spin-forbidden (and therefore slow) process, these phosphorescent
OLEDs (PhOLEDs) are able to afford EQEs of 30% and above.[5, 7, 8] PhOLEDs have found their way to commercial
applications such as OLED smartphone and television screens, but still suffer from two major drawbacks: a lack of stable
blue emitters due to the relatively weak ligand-metal bonds, and reduced sustainability due to the presence of heavy-metal
complexes.
Thermally activated delayed fluorescence (TADF) has the potential to overcome these issues, providing up to 100% IQE
without the use of heavy-metal complexes.[9, 10] In the TADF mechanism, the first excited singlet and triplet states have to
be close in energy. The small singlet-triplet energy splitting (ΔEST) allows reverse intersystem crossing (rISC) of excitons from
the non-emissive triplet to the singlet state.[10] In contrast to triplet emission in PhOLEDs, TADF emission therefore occurs
exclusively through the singlet state. The rISC process is crucial to obtain high efficiencies in TADF OLEDs, as excitons form
in a 25:75 singlet:triplet ratio upon electrical excitation. Typical rISC rates are on the order of 105 to 106 s-1 (compared to 102
to 104 s-1 for phosphorescence) and hence fluorescence from excitons that have undergone the rISC process has a lifetime
in the microseconds domain.[10]
Minimizing ΔEST can be achieved in TADF emitters with charge transfer (CT) excited states by imposing a spatial separation
between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). This can be
realized by using electron donating (D) and accepting (A) units which are coupled via rigid and orthogonal
spiroconjugation[11-15] or simply by ensuring a large dihedral angle between the D and A subunits through molecular
2

design to increase steric hindrance.[16-23] The latter approach has received the most attention in recent years due to the
greater design freedom in the choice and linkage patterns of the D and A moieties to form D-A or D-A-D structures. Using
relatively weak D and A components, blue and green-emitting materials have been successfully developed, whereas
stronger D and A units lead to the development of orange and red-emitting materials. If the energy of the localized excited
(LE) triplet excited states of the D or A is significantly below that of the CT triplet states (and if non-radiative decay is
negligible), emission from these LE states can sometimes be observed as room temperature phosphorescence (RTP), in
competition with or instead of TADF.[24-30] These RTP emitters typically show a relatively low triplet emission quantum
yield, rendering them inefficient for OLEDs. Nonetheless, their long-lived and often red-shifted emission can still be useful
for other applications such as bio-imaging,[27, 31, 32] sensing applications[33, 34] or glow-in-the-dark[35] and security
inks[36].
As the spin conversion processes are forbidden between orbitals of the same nature, ISC and rISC are very slow in
traditional organic emitters.[37] Phosphorescent emitters make use of the large spin-orbit coupling (SOC) induced by the
heavy-metal atoms to improve the ISC and phosphorescence emission rates. TADF emitters instead rely on the presence of
isoenergetic and vibronically coupled triplet states to allow state mixing and to create triplet states with mixed CT and LE
character.[38, 39] These coupled states with small ΔEST allow for greatly enhanced (r)ISC rates. The mixing of the triplet
states in D-A-D materials following the spin-vibronic mechanism for TADF is expected to occur via a vibrational rocking of
the D with respect to the A unit.[39-41]
A near infinite number of D-A-D materials can be prepared, with their color and TADF performance largely controlled by the
choice of D and A. Inspired by the field of organic photovoltaics, where it is widely used in push-pull type conjugated
polymers,[42, 43] benzo[1,2-b:4,5-b']dithiophene (BDT) was chosen here as a promising novel donor unit for TADF emitters.
However, coupling the BDT unit via the conventional α-positions of the thiophene subunits would lead to largely planar D-A
combinations – unsuitable for TADF. Therefore, in this work, 2,6-bis(triisopropylsilyl)benzo[1,2-b:4,5-b']dithiophene (BDT-
TIPS) was used to construct D-A-D compounds with two different acceptors, 9,9-dimethyl-9H-thioxanthene-10,10-dioxide
(TXO2)[19, 23, 44-46] and dibenzo[a,c]phenazine-11,12-dicarbonitrile (CNQxP)[47]. For comparison, the 9,9-dimethyl-9,10-
dihydroacridine (DMAC) analogues were also synthesized and investigated.
3

2. Results and discussion
2.1. Material synthesis
The novel BDT-TIPS donor building block was synthesized as a boronic ester according to literature, as shown in Scheme
1.[48] Triisopropylsilyl (TIPS) protection is necessary to prevent boroester formation on the thienyl 2- and 3-positions.
Coupling to different acceptors was achieved using Suzuki-Miyaura cross-coupling, while Buchwald-Hartwig cross-coupling
was used for the DMAC compounds. Full details of all synthetic procedures are included in the supporting information (SI).
Scheme 1: Synthesis pathways for all studied compounds: (i) n-BuLi, THF, 0 °C, 1 h; (ii) TIPSCl, THF, reflux, 16 h; (iii)
Bis(pinacolato)diboron, [Ir(OMe)(COD)]2, 4,4'-di-tert-butyl-2,2'-bipyridine, cyclohexane, reflux, 24 h; (iv) 9,9-dimethyl-10H-
acridine, Pd2(dba)3, HPtBu3BF4, NaOtBu, toluene, reflux, 21 h; (v) BDT-TIPS-pinacol, Pd(PPh3)4, K2CO3, toluene/H2O (4/1), 80
°C, 24 h; (vi) 9,9-dimethyl-10H-acridine, Pd(OAc)2, HPtBu3BF4, NaOtBu, toluene, reflux, 24 h.
2.2. Theoretical calculations
Geometry optimizations were performed on the isolated BDT-TIPS donor (Figure S8) and the 4 different D-A-D emitters
(Figure 1) using DFT (M06/6-311G(d)). The excited state properties were calculated by TDDFT using LC-BLYP with a modified
range-separating parameter (ω=0.17 a0-1) as the exchange correlation (XC) functional.[48, 49] TDDFT calculations were
performed under the Tamm-Dancoff approximation and using the polarizable continuum model (PCM) in cyclohexane to
simulate a non-polar environment in the Gaussian16 package.[50] The CT character of the involved states was investigated
by looking at the differences of ground and excited state electron densities (Figure S9 and S10). These were characterized
by the distance over which charge is transferred, calculated according to the work of Le Bahers et al.,[51] and the related
4

change in dipole moment. For the geometry optimizations, the TIPS groups were explicitly included to accurately judge
their influence on the electronic and structural properties. Although TXO2-DMAC has already been investigated before,[23,
44-46, 52] it was included in the calculations to allow straightforward comparison between the DMAC and BDT-TIPS donor
systems. The CNQxP acceptor is also known from literature and was previously used to construct D-A systems with phenyl-
spaced DMAC and diphenylamine (DPA) units.[47] However, to the best of our knowledge, direct coupling between CNQxP
and DMAC has not been reported before. A similar dibenzo[f,h]quinoxaline-2,3-dicarbonitrile entity was used to construct
D-A-D systems containing DMAC (amongst other donors), but this acceptor has a lower electron affinity, providing slightly
blue-shifted emission.[53] In these two examples, TADF was observed experimentally.
Large dihedral angles of around 90° were observed for the two DMAC-containing compounds (Table 1), leading to well-
separated HOMO and LUMO distributions (Figure 1). The two compounds with BDT-TIPS donor groups show considerably
smaller dihedral angles of around 50°. This is not unexpected, since the central six-membered ring is flanked by two smaller
five-membered rings, as opposed to the six-membered rings in DMAC, affording less steric control over the D-A dihedral
angle.[22] The consequences of these smaller dihedral angles are also apparent looking at the HOMO and LUMO
distributions (Figure 1), with the BDT-TIPS compounds showing increased overlap between the HOMO and LUMO, in
particular for TXO2, with the proximity of the two donor moieties in CNQxP-BDT-TIPS possibly leading to additional steric
interactions. The smaller calculated dihedral angles for the BDT-TIPS compounds are consistent with larger oscillator
strengths for the molecular S0->S1 transitions (having a largely dominant HOMO to LUMO character), along with their larger
singlet-triplet energy gaps (Table 1).
Table 1: TDDFT results for the first vertical singlet excitation energies and corresponding oscillator strengths, the first and
second vertical triplet excitation energies and dihedral angles (obtained from DFT geometry optimization).
∆ꢈꢉ ꢊꢉ
∆ꢈ
ꢃꢄꢊꢉ
D-A angle
(°) a
ꢆ
ꢄ
ꢄ
ꢂ
ꢃꢄ
ꢂ
ꢃꢆ
Compound
ꢀꢁ (eV)
ꢀꢅ (eV)
ꢇ (eV)
ꢁ
ꢇꢅ (eV)
(eV)
(eV)
0.02
0.93
0.02
0.00
(0.03)b
0.60
0.00
(0.01)b
0.12
TXO2-DMAC
TXO2-BDT-TIPS
CNQxP-DMAC
3.26
3.57
2.49
3.26
3.62
2.50
3.24
2.64
2.47
3.25
2.65
2.50
0.01
0.01
0.03
90
54
90
0.00
0.00
(0.01)b
0.64
(0.05)b
0.08
CNQxP-BDT-TIPS
BDT-TIPS
2.92
3.87
2.99
4.32
2.47
2.80
2.49
3.58
0.02
0.78
0.45
1.17
48
/
0.31
0.35
a Taken as the average of 4 possible torsion angles.
5

b Oscillator strengths when the D-A dihedral angle is modified by ±10°.
Figure 1: HOMO and LUMO topologies obtained using LC-BLYP(ω=0.17) for the optimized geometries of the different
emitters investigated in this work. Isocontour values of 0.02 a.u. were used for all orbitals.
TDDFT calculations were then performed to investigate excited singlet and triplet energies and their respective energy
differences (Table 1). The TXO2-based compounds have S1 excitation energies that are higher than those based on CNQxP,
indicative of the lower acceptor strength of the TXO2 core. Similarly, the singlet energies of the compounds containing the
BDT-TIPS donor are higher than for their DMAC counterparts, demonstrating the lower donor strength of the BDT-TIPS
group. While all D-A-D compounds show small energy gaps between the first and second triplet energies (∆ꢈꢉ ꢊꢉ ), only the
ꢆ
ꢄ
DMAC-containing compounds show a small calculated ΔEST. It is therefore anticipated that the BDT-TIPS donor will require
highly sophisticated molecular design strategies to afford TADF behavior, to overcome its intrinsically small D-A angles and
the associated large orbital overlap. To account for vibration-induced symmetry-breaking effects, additional TDDFT
calculations were performed with modified dihedral angles (±10°), revealing the weak intensity of the low-energy
transitions of the DMAC-based compounds, while the excitation energies and other excited state properties are only
impacted in a negligible way (Figure S1).
The CT nature of the first and second excited singlet and triplet states was also investigated (Table 2). TXO2-DMAC shows
CT character in its first excited singlet and triplet states, as shown in Figure S9. This property is associated with CT distance
(dCT) values of around 1.49 Å and relatively large change in dipole moment (Δµ) upon excitation. This strong CT nature,
combined with the small ΔEST, contributes significantly to the high EQEs (> 20%) obtained with this TADF emitter.[23, 44, 45,
6

54] TXO2-BDT-TIPS, on the other hand, shows more localized excitations for both the singlet and triplet excited states, as
shown in Figure S9 and as indicated by the much smaller dCT and Δµ values. This localized excitonic nature arises from the
shared LUMO distribution on the BDT-TIPS and acceptor units, whereas for TXO2-DMAC the LUMO resides entirely on the
TXO2 part. Despite having two triplet states in close proximity, the large ΔEST restricts the possibility for TADF to occur in
TXO2-BDT-TIPS. Like TXO2-DMAC, CNQxP-DMAC also shows good HOMO-LUMO separation and a small ΔEST. The first
singlet and triplet excited states also show strong CT character, with large values for dCT and Δµ. For CNQxP-BDT-TIPS, the
situation is less straightforward. Whereas the two first singlet excited states show strong CT character, the first and second
triplet excited states have mixed charge-transfer (CT) and localized excited (LE) state character (due to contributions of
multiple one-particle transitions) as they show intermediate dCT and Δµ values (Figure S10).
Table 2: Nature of the various transitions, charge transfer distance (dCT) and change in dipole moment (Δµ, excited – ground
state dipole) accompanying the S0->Sx and S0->Tx transitions in cyclohexane.
S1
S2
T1
T2
dCT
Δµ
(D)
dCT
Δµ
(D)
dCT
Δµ
(D)
dCT
Δµ
(D)
Compound
Nature
H->L
Nature
H-1->L
H-1->L
Nature
H->L
Nature
H-1->L
H-1->L
(Å)
(Å)
(Å)
(Å)
TXO2-DMAC
TXO2-BDT-
TIPS
1.49
8.18
1.49
8.19
1.48
8.14
1.49
8.15
H->L
0.96
4.18
3.96
2.70
0.92
4.08
3.79
2.47
24.18
16.66
H->L
0.40
4.04
2.98
0.79
0.41
4.17
2.46
0.80
24.76
5.77
CNQxP-DMAC
CNQxP-BDT-
TIPS
H->L
24.64 H-1->L
17.78 H-1->L
H->L
21.54 H-1->L
H->L
H->L
8.19
H-1->L
2.3. Photophysics
The steady-state absorption and emission spectra of the BDT-TIPS containing D-A-D materials in 1wt% doped zeonex films
show large differences in absorption and emission energy, due to the different electron-withdrawing strengths of the TXO2
and CNQxP acceptor units (Figure 2). TXO2-DMAC[23, 44-46, 52, 54] has been studied in this host before, and data are
presented here too to enable direct comparison to TXO2-BDT-TIPS. Both materials containing CNQxP have structured low
energy absorption bands, corresponding to ππ* transitions of the CNQxP unit with some D-A CT character, and are slightly
shifted with respect to each other due to the different electron-donating properties of the DMAC and BDT groups. The BDT-
TIPS and DMAC donor units also have a large influence on the D-A-D emission wavelengths, with the DMAC-based materials
showing emission maxima red-shifted by 19 nm (0.12 eV) for TXO2 and 68 nm (0.22 eV) for CNQxP compared to the BDT-
TIPS analogues. This is in line with the stronger donor character for the DMAC group, leading to a larger CT character of the
7

emission and lower energy emission. These findings are in line with the results from the TDDFT calculations, as also
evidenced by the simulated absorption spectra (Figure S1). The emission spectrum for TXO2-BDT-TIPS is quite narrow in
comparison to that of TXO2-DMAC, suggesting a more localized nature of the emission. On the other hand, the emission
spectra for both CNQxP-based compounds are broad, suggesting predominant CT character. The CNQxP-BDT-TIPS emission
is unique in that it shows some structure and its onset is very close to the absorption edge, which could be due to dual
1
1
emission of a higher energy LE and lower CT state. Indeed, separate emission bands corresponding to the shoulder and
peak of the steady-state emission are observed in time-resolved measurements (Figure S3), supporting this assignment.
Figure 2: Steady-state absorption (top) and emission (bottom) spectra at room temperature in zeonex film.
Time-dependent emission spectra and decays were then obtained using a pulsed laser and iCCD camera to investigate the
dynamic photophysics in these systems. From the contour plots of the normalized spectra and the total emission decays in
zeonex (Figure 3a,c; spectra in Figure S2), it is apparent that TXO2-BDT-TIPS has no delayed emission in the microseconds
regime (the spectra in Figure 3a in this time region are consistent with normalized CCD baseline signal). High-energy LE
emission consistent with the steady-state photoluminescence (PL) is observed at first, which relaxes over the first 50 ns to a
red-shifted CT state before emission falls below the sensitivity limit of the instrument. A longer living and further red-
shifted emission spanning into the milliseconds domain is then observed. Comparing the millisecond emission spectra at
room temperature and at 80 K (Figure 4 bottom), it is apparent that the room temperature and 80 K emission come from
the same state in TXO2-BDT-TIPS and are therefore both attributed to phosphorescence. From the onset of the prompt
8

fluorescence (PF) at room temperature and the phosphorescence at 80 K, the singlet and triplet energies and the
corresponding singlet-triplet energy gap were calculated (Table 3). With an experimental ΔEST of 0.78 eV (vs. 0.93 eV
calculated), TADF is excluded as a possible delayed fluorescence (DF) emission mechanism in TXO2-BDT-TIPS. The
occurrence of RTP is attributed to the presence of the BDT-TIPS donor, as this emission mechanism was not observed for
TXO2-DMAC (Figure 3b).
For CNQxP-BDT-TIPS, a combination of LE and CT emission seems to be present at early decay times (Figure S3), which
implies a reduced electron transfer rate to form the CT state compared to the other materials. Like in TXO2-BDT-TIPS, this is
probably due to the BDT group having moderate dihedral angles with respect to the acceptor unit and having a rather weak
electron-donating strength (as judged by comparison of the PL colour to the DMAC compounds). As the “LE” emission dies
out, the CT emission remains throughout the remainder of the decay, although at very long times (> 1 ms) the “LE” emission
seemingly reappears. The same short-wavelength emission is present at early decay times at 80 K, but does not reappear
near the end of the decay (Figure S3). Furthermore, the microsecond emission present at room temperature is absent at 80
K. At long times in the decay at 80 K, a red-shifted emission is seen, which is attributed to the true phosphorescence of the
system (Figure S3). The absence of the microsecond emission at 80 K demonstrates temperature dependency as expected
for TADF emission. The singlet (2.35 eV) and triplet (2.17 eV) emission energies from the experimental data lead to a
singlet-triplet gap of 0.18 eV, which is large but not insurmountable at room temperature (Table 3).[52] The red-shifted
microsecond emission in zeonex at room temperature has an onset of around 2.30 eV, indicating that TADF is a potential
emission pathway. The blue-shifted millisecond emission in CNQxP-BDT-TIPS is instead suggested to arise from triplet-
triplet annihilation (TTA) and subsequent emission from singlet LE states. The same LE states are also formed directly
following photoexcitation, which is why they share the same spectra at very early and very late decay times. The signal
arising from TTA is long lived but weak, and so it is only observable once the TADF emission has decayed substantially, and
is completely suppressed at low temperatures which restrict triplet migration through the film. This assignment is also
verified by the TDDFT calculations. Due to the apparent symmetry in the system, S1 and S2 are of HOMO and HOMO-1 to
LUMO character, respectively, and consist of D->A CT transitions (Figure S10). The third excited singlet state (S3) represents
the first singlet excited state with a localized character and has an energy that is only 0.12 eV (17 nm) higher than that of S1
(Table S1). Furthermore, S3 is localized on the CNQxP acceptor unit (Figure S10). With an S1 energy of 3.00 eV for the CNQxP
unit (Table S2), the position of the S3 state of CNQxP-BDT-TIPS is confirmed, suggesting S3 to be the LE state that we
observed in the experimental time-resolved decays at early and long times. Similarly, the first two excited triplet states (T1,
T2) are D->A CT transitions whereas T3 is localized on the acceptor and is 0.15 eV higher in energy than T1 (Table S1, Figure
S10). According to the spin-vibronic mechanism for TADF, these states have the potential to mix and decrease the effective
ΔEST. While the theoretical ΔES1 1 is rather large (0.45 eV), the experimental energy gap (0.18 eV) is not and considering the
T
9

precision of the TDDFT calculations and the fact that excited state relaxation is not taken into account, this mechanism is
viable.
CNQxP-DMAC has consistent emission throughout the decay with an onset of about 2.15 eV (577 nm) in the microseconds
regime. A short-wavelength contribution is present at very early decay times and reappears in the very long millisecond
emission, similar to that of CNQxP-BDT-TIPS (Figure 3e). At 80 K, the emission is slightly red-shifted and the short-
wavelength contribution at the start of the decay is less intense and does not reappear at longer times (Figure S4). Even at
80 K, delayed emission is present at all times throughout the decay. As the onset of the spectra also remains the same, this
is indicative of TADF emission with a very small ΔEST, as supported by the experimental determination of the singlet and
triplet energies (Table 3). This is unusual as the TADF seemingly persists at 80 K, whereas the low wavelength emission is
suppressed. We suggest that the very delayed high-energy emission is also generated by TTA and subsequent emission
from LE states that are initially formed by photoexcitation as the position and localization of the S3 state of CNQxP-DMAC is
similar to that of CNQxP-BDT-TIPS. Additionally, the full decay of CNQxP-DMAC (Figure 3f) shows two regions of increased
intensity at around 10-6 and 10-3 seconds, while Figure 3e shows delayed emission at a constant wavelength. Whereas the
first region can be attributed to TADF, the second region appears at earlier times with respect to the onset of TTA (> 10-3 s).
This is also unusual and may indicate the presence of two singlet excited states close in energy, as was also found using
TDDFT calculations, showing TADF with largely varying krISC values.
10

Figure 3: Contour plots for the normalized room temperature time-resolved emission spectra of TXO2-BDT-TIPS (a), TXO2-
DMAC (b), CNQxP-BDT-TIPS (d) and CNQxP-DMAC (e) in zeonex, along with the emission intensity decays (c and f).
Table 3: Experimental photophysical properties and kinetics for TXO2-DMAC, TXO2-BDT-TIPS, CNQxP-DMAC and CNQxP-
BDT-TIPS in zeonex.
f
f
Compound
ES (eV) a
3.20
ET (eV) b
3.02
ΔEST (eV) c τFp (ns)d
τFd (µs)e
67.89
kISC
krISC
TXO2-DMAC
TXO2-BDT-TIPS
CNQxP-DMAC
0.18
0.78
-0.02
12.71
4.51
5.44x107
4.27x104
g
g
3.14
2.36
2.69x104
/
/
2.07
2.09
52.66
4.67/1.80x103 1.04x107
5.17x105
11

g
g
g
g
CNQxP-BDT-TIPS
BDT-TIPS
2.35
3.24
2.17
2.48
0.18
0.76
2.12
0.78
1.86x103
4.40x104
/
/
/
/
a Taken from the onset of the prompt fluorescence (PF) emission. b Taken from the onset of the phosphorescence emission
c
f
d
e
at ms timescales at 80 K. Calculated as Es – ET. Lifetime of prompt fluorescence (Fp). Lifetime of delayed fluorescence (Fd). kISC
g
and krISC rates were determined using kinetic fitting of the PF and DF emission according to the literature.[46] Due to the
lack of microsecond emission and/or unambiguous identification of a pure TADF mechanism, kinetic fitting was not
performed.
To better understand the positioning of LE states in these materials, the BDT-TIPS donor itself was also embedded in a
zeonex matrix and its time-dependent emission was recorded at room temperature and at 80 K (Figure 4, S5). As suggested
by the calculations, the singlet and triplet emission are well separated with onsets at 3.24 and 2.48 eV, respectively (Table
3). It is remarkable how the BDT-TIPS group itself shows emission at several tens of milliseconds, even at room
temperature. Comparing the spectra at room temperature and at 80 K shows that both the prompt and millisecond
emission have the same onset at each temperature, and are thus coming from the same states in both cases. This is
attributed to the presence of the sulphur atoms in the BDT core, which enhance SOC when compared to commonly used
atoms such as carbon and nitrogen. Having two of these sulphur atoms could boost the SOC enough to enhance ISC from
the S1 to the T1 state and subsequently enable the strong RTP observed in the donor fragment.
For TXO2-BDT-TIPS, it is possible that the observed RTP is coming from localized emission corresponding to the BDT-TIPS
unit (Figure 4). When looking at the the pure BDT-TIPS and TXO2-BDT-TIPS spectra at room temperature and at 80 K, the
similarities in the peak shape and onset indicates that emission is coming from similar states in both materials. The
difference in onset could be due to the acceptor properties of TXO2, slightly lowering the BDT triplet energy level in the D-
A-D compound, or from the vibrational mode of the highest energy vibronic peak in the BDT-TIPS phosphorescence
spectrum being suppressed in the D-A-D material. Furthermore, the ground‒excited state electron density differences
predict the T1 and T2 states to be localized on the BDT-TIPS group (Figure S9). CNQxP-BDT-TIPS behaves differently, as
microsecond emission (attributed to TADF) is also observed. The large difference in the onset of the emission at several
milliseconds at room temperature and at 80 K allows us to exclude RTP for this compound. One of the reasons is that the CT
triplet of the D-A-D compound has a lower energy than the localized BDT-TIPS triplet state and coupling to the ground state
from the CT triplet state is inefficient. Alternatively, rISC could be competing with the radiative relaxation from the T1 state
due to the smaller ΔEST with respect to TXO2-BDT-TIPS and the improved SOC. The ground‒excited state electron density
differences (Figure S10) show significant CT character for the lowest excited triplet state, supporting this hypothesis.
12

Figure 4: Emission spectra obtained at a 44.7 ms delay for BDT-TIPS, CNQxP-BDT-TIPS and CNQxP-DMAC (top), and for BDT-
TIPS, TXO2-BDT-TIPS and TXO2-DMAC (bottom) at 80 K and at room temperature.
The short-wavelength emission at a few nanoseconds in the decays of CNQxP-BDT-TIPS (Figure 5) and CNQxP-DMAC (Figure
S4) is attributed to the presence of a locally excited singlet state (1LE) above the TADF-active CT singlet (vide supra). The
sterically hindered conformation in which the donor units reside likely slows down electron transfer and hence the internal
1
1
conversion of the photoexcited LE state to the lower CT state. This is especially true at 80 K, where nuclear motion is
further restricted and the prompt emission exhibits dual character over a longer time span. To explain the reoccurring
short-wavelength 1LE emission at long lifetimes, we identify TTA as the most likely mechanism. Having a much longer
lifetime than TADF, TTA typically occurs in the milliseconds domain and requires two molecules to be in close proximity. In
TTA, two triplet excitons are up-converted into one singlet exciton of higher energy, and even though the emitter is doped
at only 1 wt% in zeonex, it is plausible that weak TTA is observed at very long times (once brighter and faster-decaying
emission from TADF-active states is depleted). Because the energy of two triplet excitons is larger than the energy of the
1LE state, this state can be populated. Loss of the red emission edge at several tens of milliseconds for CNQxP-BDT-TIPS
suggests that as TADF emission dies out, the emission band becomes dominated by pure 1LE emission with significant
1
1
vibronic character. CNQxP-DMAC does not lose its red emission edge and is therefore a combination of both LE and CT
emission even at very long lifetimes - resulting from TTA and very long-lived TADF, respectively. In CNQxP-BDT-TIPS, which
also has a larger ΔEST, TTA seems to be more intense as it starts to appear at earlier decay times and becomes more
13

prominent in the longer milliseconds regime. The absence of LE emission in the milliseconds domain for both compounds at
80 K is also consistent with either a TADF or TTA mechanism, as these processes are typically both inhibited at lower
temperatures. However, as stated before, CNQxP-DMAC seemingly shows TADF emission even at 80 K.
1LE
0.8 ns
3.5 ns
1.0
0.8
0.6
0.4
0.2
0.0
10.0 ns
18.2 ns
41.6 ns
78.5 ns
156.3 ns
333.2 ns
450
450
450
500
550
600
650
700
700
700
750
Wavelength (nm)
686.4 ns
1.0
0.8
0.6
0.4
0.2
0.0
1.4
2.8
5.6
12.6
22.4
44.7
89.1
µ
µ
µ
s
s
s
µ
µ
µ
µ
1LE
s
s
s
s
500
1LE
550
600
650
750
Wavelength (nm)
158.5
354.8
707.9
1.4 ms
2.8 ms
5.6 ms
12.6 ms
28.2 ms
44.7 ms
µs
µ
1.0
0.8
0.6
0.4
0.2
0.0
s
s
µ
500
550
600
650
750
Wavelength (nm)
Figure 5: Time-resolved emission extracted from the room temperature decay of CNQxP-BDT-TIPS showing the slow
transfer from 1LE to 1CT emission at short lifetimes and the reappearance of pure 1LE emission at very long lifetimes.
2.4. Laser power experiments
To verify a TADF mechanism in the microseconds domain in both CNQxP compounds, the dependence of emission intensity
on excitation laser power was measured by attenuation of the excitation beam with reflective neutral density filters. Both
compounds were probed with a delay time after the laser pulse of 4 µs and an integration time of 12 µs. Even at very low
14

powers (0.2 µJ vs. 78.0 µJ at the start of the measurement), the slope of the log-log plot remains unity, suggesting TADF-like
behavior. In the case of TTA, we would expect this slope to increase at low excitation power (second order with the exciton
density). Similar measurements at longer delay times reveal that the slopes become slightly larger (Figure S6b,d), which
could indicate the presence of a TTA emission contribution at very long times. However as we observe a combination with
TADF at these delay times, we still do not observe a gradient of 2 that is otherwise indicative of ‘pure’ TTA emission (Figure
S7b,d).
3. Conclusions
In this work, we have introduced a triisopropyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT-TIPS) donor unit that is
directly coupled via the central phenyl moiety, giving rise to two new donor-acceptor-donor emitters. In contrast with
analogous 9,9-dimethyl-9,10-dihydroacridine (DMAC) containing compounds, TXO2-BDT-TIPS was found to show room
temperature phosphorescence as a result of a low-lying localized triplet state on the BDT-TIPS group itself. The BDT-TIPS
precursor was investigated and was found to also exhibit phosphorescence at room temperature, presumably due to the
sulphur atoms that afford increased spin-orbit coupling and thereby enhance the radiative T1->S0 relaxation. The CNQxP-
DMAC chromophore prepared in this work was found to show TADF, with a very small ΔEST value giving rise to delayed
emission even at 80 K. Its BDT-TIPS counterpart also showed delayed emission attributed to TADF. Despite a larger ΔEST
(0.18 eV) than its DMAC counterpart, the BDT-TIPS unit promotes (r)ISC by increased spin-orbit coupling in the CNQxP
1
compound, leading to long-lived orange TADF. At longer times, triplet-triplet annihilation (TTA) repopulating the LE state
gives rise to resurgent short-wavelength LE emission for both CNQxP materials.
Although RTP and very long lived TADF/TTA emission are not desirable for OLED applications, the long-lived and red-shifted
emission for TXO2-BDT-TIPS and CNQxP-BDT-TIPS may find future use in other applications such as imaging or sensing.[55-
57] Furthermore, the BDT-TIPS donor investigated here represents a valuable addition to the library of available donor
compounds for TADF, particularly suitable to generate deep-blue emission.
4. Conflicts of interest
The authors declare no competing financial interest.
5. Acknowledgements
This work is supported by the University of Namur and Hasselt University [PhD BILA scholarship T. Cardeynaels]. The
authors also thank the Research Foundation – Flanders (FWO Vlaanderen) for financial support [project G.0877.18N,
15

Hercules project GOH3816NAUHL, and SB PhD scholarship S. Paredis]. The calculations were performed on the computers
of the « Consortium des équipements de Calcul Intensif (CÉCI) » (http://www.ceci-hpc.be), including those of the « UNamur
Technological Platform of High-Performance Computing (PTCI) » (http://www.ptci.unamur.be), for which we gratefully
acknowledge the financial support from the FNRS-FRFC, the Walloon Region, and the University of Namur [Conventions No.
2.5020.11, GEQ U.G006.15, U.G018.19, 1610468, and RW/GEQ2016]. A. Danos and A.P. Monkman are supported by EU
Horizon 2020 Grant Agreement No. 732013 (HyperOLED). We thank Alastair Harrison for assistance in collecting
spectroscopic data for TXO2-DMAC.
6. Supporting information
1
Detailed information on the synthesis procedures, H and 13C NMR spectra, additional TDDFT calculations, time-resolved
photoluminescence spectra, laser power measurements, HOMO and LUMO topologies for BDT-TIPS, ground/excited state
electron density differences, and coordinates of the optimized geometries can be found in the supporting information.
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18

Graphical abstract:
19

HIGHLIGHTS
•
•
•
•
•
A benzo[1,2-b:4,5-b']dithiophene (BDT) donor is applied for TADF materials
The time-resolved photophysical properties are studied
Quantum-chemical analysis is used to estimate the charge-transfer character
Different triplet upconversion mechanisms are found (TADF vs TTA)
Room temperature phosphorescence is turned on by the BDT donor unit

Conflicts of interest
The authors declare no competing financial interest.