Convenient One-Pot Synthesis of 1,2,3,4-Thiatriazoles Towards a Novel Electron Acceptor for Highly-Efficient Thermally-Activated Delayed-Fluorescence Emitters

Article abstract of DOI:10.1002/chem.201805358

A novel and unexpected convenient one-pot synthesis of 1,2,3,4-thiatriazoles has been discovered while investigating the classical tetrazine “Pinner synthesis”. The synthetic route starts from commercially-available nitrile derivatives and gives good to high yields (51–80 %) with no need to isolate any thioacylating agents. The crucial impact of the solvent on the outcome of the modified “Pinner synthesis” is moreover examined and discussed. Using this new synthetic route, a novel donor-acceptor thiatriazole derivative has been prepared, which exhibits prominent thermally-activated delayed fluorescence (TADF) in both solution and film. The photoluminescence quantum yield (PLQY) in methylcyclohexane (MCH) and Zeonex (a cyclo olefin polymer) in oxygen-free conditions were determined to be 76 and 99 %, respectively. This work provides an efficient and practical synthetic approach to functionalized 1,2,3,4-thiatriazole derivatives, and will noticeably facilitate the application of 1,2,3,4-thiatriazole as an electron acceptor in organic electronics.

Full text of DOI:10.1002/chem.201805358

DOI: 10.1002/chem.201805358
Communication
&
Fluorescence
Convenient One-Pot Synthesis of 1,2,3,4-Thiatriazoles Towards a
Novel Electron Acceptor for Highly-Efficient Thermally-Activated
Delayed-Fluorescence Emitters
Yangyang Qu,[a] Piotr Pander,[b] Audrius Bucinskas,[c] Marharyta Vasylieva,[d] Yayang Tian,[a]
Fabien Miomandre,[a] Fernando B. Dias,[b] Gilles Clavier,*[a] Przemyslaw Data,[b, d] and
Pierre Audebert*[a]
1,2,3,4-thiatriazole heterocycle was first reported in 1896 by
Abstract: A novel and unexpected convenient one-pot
synthesis of 1,2,3,4-thiatriazoles has been discovered while
Freund and co-workers.[1] Its structural assignment was firmly
established in the 1950s by Lieber and co-workers.[2] A number
investigating the classical tetrazine “Pinner synthesis”. The
of 1,2,3,4-thiatriazoles were then prepared and studied during
synthetic route starts from commercially-available nitrile
that period.[3] The chemistry of 1,2,3,4-thiatriazoles has unfortu-
derivatives and gives good to high yields (51–80%) with
nately been almost forgotten for many decades thereafter. As
no need to isolate any thioacylating agents. The crucial
a strong electron-deficient heterocycle, 1,2,3,4-thiatriazole is,
impact of the solvent on the outcome of the modified
however, potentially an excellent electron acceptor in organic
“Pinner synthesis” is moreover examined and discussed.
donor-acceptor materials. Due to charge-transfer (CT) states in-
Using this new synthetic route, a novel donor-acceptor
troduced in the donor-acceptor system, these molecules can
thiatriazole derivative has been prepared, which exhibits
exhibit interesting photophysical and electrochemical proper-
prominent thermally-activated delayed fluorescence
ties, and therefore are of interest in organic electronics,[4,5]
(TADF) in both solution and film. The photoluminescence
though strangely, no related applications have been reported
quantum yield (PLQY) in methylcyclohexane (MCH) and
so far. Thermally-activated delayed fluorescence (TADF) mole-
Zeonex (a cyclo olefin polymer) in oxygen-free conditions
cules have recently received significant attention since the first
were determined to be 76 and 99%, respectively. This
efficient TADF organic light emitting diodes (OLEDs) reported
work provides an efficient and practical synthetic ap-
in 2012 by Adachi.[6] In TADF molecules, the triplet excitons
proach to functionalized 1,2,3,4-thiatriazole derivatives,
can be converted to the emissive singlet excitons through effi-
and will noticeably facilitate the application of 1,2,3,4-thia-
cient reverse intersystem crossing (RISC) from the lowest triplet
triazole as an electron acceptor in organic electronics.
excited state (T1) to the lowest singlet excited state (S1), when
their energy difference (DEST) is small enough.[5] Most of the
TADF molecules were therefore designed by coupling various
electron donors and acceptors with a large twisting angle, to
[a] Y. Qu, Dr. Y. Tian, Prof. F. Miomandre, Dr. G. Clavier, Prof. P. Audebert
separate the spatial distributions of the highest occupied mo-
PPSM, CNRS, ENS Paris-Saclay
61 Avenue Prꢀsident Wilson
94235 Cachan (France)
lecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO). The aim is to harvest 100% of triplet exciton.[7]
The use of purely organic TADF emitters goes far beyond their
use in electroluminescent devices. They can supersede metal-
complex emitters by acting as triplet sensitizers,[8] or bio-imag-
ing dyes.[9] Molecules with efficient RISC[10] are also counted
among mechanochromic materials with switchable parameters,
such as emission color,[11] on/off TADF,[12] or interplay between
TADF and prompt fluorescence.[13] Therefore, exploration of
novel electron acceptors for TADF emitters is of clear interest
to this community.
E-mail: gilles.clavier@ens-cachan.fr
audebert@ppsm.ens-cachan.fr
[b] P. Pander, Dr. F. B. Dias, Prof. P. Data
Physics Department
Durham University
South Road
Durham DH1 3LE (UK)
[c] Dr. A. Bucinskas
Department of Polymer Chemistry and Technology
Kaunas University of Technology
ˇ
K. Barsausko St. 59
1,2,3,4-thiatriazoles have been previously prepared by diazo-
tization of thiohydrazides, or reaction of dithioates with azide
ions, in high yields (Scheme 1).[3] However, both methods re-
quire preparation of starting thioacylating agents, which are
often difficult to obtain. This increases the synthetic complexity
of 1,2,3,4-thiatriazole derivatives, especially for those with com-
plex functional groups. As a result, no 1,2,3,4-thiatriazole deriv-
atives with complex functional groups have been reported so
Kaunas LT-51423 (Lithuania)
[d] M. Vasylieva, Prof. P. Data
Faculty of Chemistry
Silesian University of Technology
M. Stzody 9
44-100 Gliwice (Poland)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201805358.
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the large availability of commercially available nitriles. Through
this approach we have designed and synthesized an example
of a donor-acceptor thiatriazole derivative using phenoxazine
as a donor, in high yield (70%). This molecule exhibits excel-
lent TADF characteristics in both solution and film. To the best
of our knowledge, this is the first example of a highly-efficient
TADF emitter using a 1,2,3,4-thiatriazole as acceptor.
Having a long experience in 1,2,4,5-tetrazine chemistry,[14,15]
we had already noticed that the solvent nature can play an im-
portant role in the tetrazine forming reaction called “Pinner
synthesis”.[16] For example, we have recently found that di-
chloromethane (DCM) can serve as a novel reagent in the syn-
thesis of 3-monosubstituted unsymmetrical 1,2,4,5-tetra-
zines.[15] In this work a comprehensive study of solvent effect
in the modified “Pinner synthesis” was performed, and we
found out that the products of the reaction are significantly in-
fluenced by the solvent employed.
The traditional “Pinner synthesis” is a two-step procedure
starting from the formal addition of hydrazine to nitrile precur-
sors in ethanol, followed by oxidation of the intermediate 1,2-
dihydrotetrazine to afford 1,2,4,5-tetrazines.[16] It has been
demonstrated later that addition of sulfur helps accelerating
the reaction.[17] Here we used 4-bromobenzonitrile as a stan-
dard nitrile precursor to survey the effect of a range of solvents
on this reaction. The results are summarized in Table 1. When
our optimized reaction conditions for tetrazine synthesis (hy-
drazine hydrate, sulfur, and heating at 908C for 1 hour in a
sealed tube with microwave irradiation, followed by oxidation),
with ethanol as a solvent in the first step, was used, the 3,6-
disubstituted symmetrical tetrazine T1 was obtained in good
yield (43%) as expected. However, when the polar protic sol-
Scheme 1. Overview of synthetic approaches to 1,2,3,4-thiatriazoles.
far. Therefore, developing practical and convenient synthetic
approaches for preparing functionalized 1,2,3,4-thiatriazole de-
rivatives is quite desirable.
In this study, we report a novel convenient one-pot synthe-
sis of 1,2,3,4-thiatriazoles directly from commercially available
nitriles in high yields (51–80%), with no need of isolating any
thioacylating agents (Scheme 1). The access to the 1,2,3,4-thia-
triazoles can therefore be significantly improved because of
Table 1. Survey of solvents and outcomes in the modified “Pinner synthesis”.[a]
Solvent
Dielectric Constant
Products[b]
polar protic solvent
non-polar solvent
ethanol
25
toluene
chloroform
THF
2.3
4.8
7.5
SN[c]
SN
SN
acetonitrile
DMF
37
38
47
polar aprotic solvent
DMSO
[a] All reactions were carried out on a 0.5 mmol scale in 2 mL solvent. MW=microwave. [b] Yields (isolated) based on the nitrile precursor. [c] SN=Starting
nitrile precursor
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vent ethanol was replaced by a non-polar (e.g., toluene,
chloroform) or a slightly polar aprotic solvent (e.g., tetrahydro-
furan (THF)), no reaction was observed and the starting nitrile
agent was fully recovered.
Table 2. Synthesis of 5-aryl-1,2,3,4-thiatriazoles TT1-TT9.[a]
When the highly polar aprotic solvent acetonitrile was used,
we isolated 16% of unsymmetrical 3-(4-bromophenyl)-6-
methyl-1,2,4,5-tetrazine T2, because acetonitrile acts as both
solvent and second nitrile precursor in the reaction. In addition
to the tetrazine, the unusual 1,2,3,4-thiatriazole TT1 was also
obtained with a 9% yield. More interestingly, when using di-
methylformamide (DMF) as a solvent, the yield of TT1 signifi-
cantly increased to 41%.[18] And remarkably, when the even
more polar solvent dimethyl sulfoxide (DMSO) was employed,
the yield was optimized to as high as 74%. These findings are
remarkably interesting because this is the first time that the
1,2,3,4-thiatriazole has been isolated using “Pinner synthesis”.
All the reactions were performed in a one-pot procedure with-
out isolating any thioacylating agents. It is worth noting that
in the case of all three polar solvents (acetonitrile, DMF and
DMSO) only a trace amount (yield below 1%) of symmetrical
tetrazine T1 could also be observed.
[a] Reactions were carried out on a 0.5 mmol scale in 2 mL DMSO. Yields
(isolated) based on the starting nitrile.
Formation of 1,2,3,4-thiatriazole in this reaction is assumed
to process through diazotization of a thiohydrazide intermedi-
ate, formed in the first step, by the nitrous acid used in the
second step. However, the detailed mechanism of formation of
these thiohydrazide intermediates is still under investigation
and it is not clear if small amounts of hydrogen sulfide are
formed or not.
to instability of the thioacylating intermediates which have
been described previously.[3]
With our convenient one-pot synthetic approach in hand,
we hypothesized that it could be used to prepare donor-ac-
ceptor type thiatriazole derivatives using an appropriate nitrile
precursor. More importantly, we also believed that 1,2,3,4-thia-
triazole could act as a novel electron acceptor for efficient
TADF emitters. Indeed one-pot or multicomponent synthesis of
functional chromophores has received considerable interest
both in academia and industry.[19] To our delight, the donor-ac-
ceptor thiatriazole derivative using phenoxazine as a donor
motif, TT9, was easily prepared in a high yield (70%) using our
approach. TT9 easily forms yellow crystals, which are stable
and could be stored at room temperature for several months
without noticeable degradation. The single-crystal structure
(Figure 1) shows a large twisting angle (678) between the
planes of donor and acceptor, which is desired to achieve
small HOMO–LUMO overlap and to realize a small DEST, and
therefore efficient TADF characteristics.[4]
The reaction with DMSO as a solvent became the most inter-
esting to us because it provides a novel, convenient pathway
to 1,2,3,4-thiatriazoles directly from commercially available ni-
trile reagents in an excellent yield. The reaction conditions
were optimized by changing the amount of sulfur and reaction
temperature. Probably acting both as an inducer and reactant
in the reaction, the amount of sulfur is crucial in this reaction.
Theoretically, one equivalent of sulfur should be enough to
obtain the thiatriazole product. In those conditions the reac-
tion already led to a reasonable yield (53%, Table S1 in Sup-
porting Information), while slight excesses (1.3 equiv) of sulfur
could still improve the yield and 1.5 equiv of sulfur gave the
best yield (74%). The reaction temperature and time were op-
timized to 908C for 1 hour, because decreasing the reaction
temperature only resulted in prolonged reaction times to
reach similar yields (Table S2). We used microwave heating
throughout, but we have verified that running the reaction in
a sealed tube with conventional heating worked fine as well,
resulting in a similar yield (72%).
TT9 was further characterized by electrochemistry (Fig-
ure S8). The cyclic voltammogram in DCM displays a reversible
oxidation wave for the phenoxazine, and an irreversible reduc-
tion one corresponding to the thiatriazole. The HOMO and
LUMO energies were determined to be 5.35 and 3.41 eV from
the onset of the respective waves.
To extend the scope of this synthetic approach, we engaged
a series of nitrile substrates in our new one-pot synthetic ap-
proach. Thus, a series of 5-aryl-1,2,3,4-thiatriazoles were suc-
cessfully prepared in good to high yields (51–80%, Table 2),
among which many were previously complicated to prepare.[3]
The reactions are simple and efficient, and the nitrile substrate
can contain different functional groups such as halogens and
hydroxyl groups. Although the reaction is versatile, unfortu-
nately, preparation of 5-alkyl-1,2,3,4-thiatriazoles using benzyl
cyanide or tert-butyl cyanide was not successful, probably due
Figure 1. Crystal structure of TT9. Hydrogen atoms are omitted for clarity.
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DFT-based calculations were also performed on TT9. The
ground state structure of the molecule shows an almost per-
pendicular twist between the phenoxazine and the thiatria-
zole-phenyl moieties and complete spatial separation of the
HOMO and LUMO (Figure 2) which is an ideal situation to
Photophysical studies demonstrated that TT9 indeed exhib-
its prominent thermally activated delayed fluorescence (TADF)
characteristics both in solution and film. Relevant photophysi-
cal properties of TT9 are displayed in Figure 3 and Table 3. Like
other donor-acceptor molecules, TT9 shows a clear solvato-
chromic effect (Figure 3a). It is highly emissive in non-polar
media such as methylcyclohexane (MCH), Toluene (Tol), and
Zeonex (a cyclo olefin polymer), whereas it is however very
Table 3. Photophysical properties of TT9.
deg
Sample
lPL
[nm][a] [%]
FPLair/FPL
tPF
[ns][d]
tDF
[ms][e]
DF/PF[f]
Figure 2. Calculated spatial distributions of the HOMO (left) and LUMO
(right) of TT9.
Zeonex film
539
65/99[b]
20.4Æ0.7 5.5Æ0.28 2.37
28.0Æ1.7
MCH solution
505
532
12/76[c]
18.6Æ0.6 22.2Æ1.2 2.65
reach a small singlet-triplet energy difference. Optimization in
the first singlet excited state gave a very similar geometry,
where the main difference is a flattening of the phenoxazine
moiety (Figure S10). The computed DEST on this excited state
optimized structure is 0.005 eV, which is excellent to get effi-
cient TADF.
[a] Emission maxima. [b] PLQY in air and N2 atmosphere. [c] PLQY in air-
equilibrated and degassed MCH solution. [d] Prompt fluorescence lifetime
determined from PL decay (Figure S4, S5). [e] Delayed fluorescence life-
time determined from PL decay (Figure S4, S5). [f] Ratio of delayed fluo-
rescence component to prompt fluorescence one.
Figure 3. (a) Normalized fluorescence spectra of TT9 in MCH, Tol and Zeonex 1% (w/w) and normalized absorption spectrum in MCH. (b) Time-resolved spec-
tra of TT9 in Zeonex. (c) PL decays of TT9 in Zeonex 1% (w/w) at various temperatures. (d) PL decay of TT9 in degassed MCH at room temperature.
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weakly emissive or non-emissive in more polar solvents (Fig-
ure S1 in Supporting Information), as is typical for strong
charge transfer (CT) excited state systems.[20] Photolumines-
cence (PL) intensity of TT9 in MCH increases 6.3-fold in de-
gassed solution relative to the air-equilibrated system (Fig-
ure S2), indicating that oxygen largely influences the PL of this
material. Comparison of PL decays of TT9 in air-equilibrated
and degassed MCH (Figure S3) shows that in the latter case, a
longer lifetime indicative of TADF, as well as a longer prompt
fluorescence lifetime than in the presence of oxygen is ob-
served. Thus oxygen apparently not only quenches the triplet
state but also the singlet one. The ratio of the delayed fluores-
cence component to the prompt fluorescence one (DF/PF) was
determined to be 2.65 from the PL decay of TT9 in degassed
MCH (Table 3). The photoluminescence quantum yield (PLQY)
of TT9 in MCH and Zeonex in air were determined to be 12%
and 65%, respectively. The PLQY in degassed MCH increased
to 76%, and the PLQY in Zeonex under nitrogen reached 99%.
This is remarkable since many CT molecules suffer from a small
radiative decay rate due to the spatial separation of HOMO
and LUMO originating from the molecular design.[4] TT9 exhib-
its prominent TADF characteristic not only in MCH but also in
Zeonex (Figure 3c,d) with a DF/PF ratio in the latter equal to
2.37. Temperature dependence of delayed fluorescence record-
ed in Zeonex (Figure 3c) is typical of TADF molecules showing
clearly the thermal activation of the process.[5] Interestingly, at
80 K the intensity of TADF is still substantial and thus phos-
phorescence is hardly distinguishable from delayed fluores-
cence (Figure 3b). This gives a clear indication for a DEST ap-
proaching zero. It is also noteworthy that the prompt fluores-
cence is completely unaffected by temperature change, which
indicates that it has almost no effect on the non-radiative
decay rate of the singlet excited state. This also contributes to
the high PLQY of the molecule. Power dependence of delayed
fluorescence of TT9 is linear in both MCH and Zeonex confirm-
ing the TADF mechanism (Figure S6, S7).
Acknowledgements
The research leading to these results has received funding
from the European Union’s Horizon 2020 research and innova-
tion programme under the Marie Skłodowska-Curie grant
agreement No 674990 (EXCILIGHT).
Conflict of interest
The authors declare no conflict of interest.
Keywords: delayed fluorescence · one-pot synthesis · Pinner
synthesis · tetrazines · thiatriazoles
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Manuscript received: October 25, 2018
Revised manuscript received: December 7, 2018
Version of record online: && &&, 0000
[16] a) A. Pinner, Ber. Dtsch. Chem. Ges. 1893, 26, 2126–2135; b) K. A. Hof-
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Chem. Eur. J. 2019, 25, 1 – 7
www.chemeurj.org
6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
One-stop shop: A novel and unexpect-
ed convenient one-pot synthesis of
1,2,3,4-thiatriazoles has been discovered
meanwhile investigating on the classical
tetrazine “Pinner synthesis”. The syn-
thetic route starts from commercially-
available nitrile derivatives and gives
good to high yields (51–80%) with no
need to isolate any thioacylating
&
Fluorescence
Y. Qu, P. Pander, A. Bucinskas,
M. Vasylieva, Y. Tian, F. Miomandre,
F. B. Dias, G. Clavier,* P. Data,
P. Audebert*
&& – &&
Convenient One-Pot Synthesis of
1,2,3,4-Thiatriazoles Towards a Novel
Electron Acceptor for Highly-Efficient
Thermally-Activated Delayed-
Fluorescence Emitters
agents. Using this new synthetic route a
novel donor-acceptor thiatriazole deriv-
ative has been prepared which exhibits
prominent thermally activated delayed
fluorescence (TADF).
Chem. Eur. J. 2019, 25, 1 – 7
www.chemeurj.org
7
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
These are not the final page numbers! ÞÞ