Synthesis of Tetraarylethene Luminogens by C?H Vinylation of Aromatic Compounds with Triazenes

Article abstract of DOI:10.1002/anie.201908755

Tetraarylethenes are obtained by acid-induced coupling of vinyl triazenes with aromatic compounds. This new C?H activation route for the synthesis of aggregation-induced emission luminogens is simple, fast, and versatile. It allows the direct grafting of triarylethenyl groups onto a variety of aromatic compounds, including heterocycles, supramolecular hosts, biologically relevant molecules, and commercial polymers.

Full text of DOI:10.1002/anie.201908755

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201908755
German Edition: DOI: 10.1002/ange.201908755
Synthetic Methods
À
Synthesis of Tetraarylethene Luminogens by C H Vinylation of
Aromatic Compounds with Triazenes
Abdusalom A. Suleymanov, Martin Doll, Albert Ruggi, Rosario Scopelliti, Farzaneh Fadaei-
Tirani, and Kay Severin*
Abstract: Tetraarylethenes are obtained by acid-induced
coupling of vinyl triazenes with aromatic compounds. This
À
new C H activation route for the synthesis of aggregation-
induced emission luminogens is simple, fast, and versatile. It
allows the direct grafting of triarylethenyl groups onto a variety
of aromatic compounds, including heterocycles, supramolec-
ular hosts, biologically relevant molecules, and commercial
polymers.
Compounds showing aggregation-induced emission (AIE)
properties have found numerous applications in analytical
chemistry, imaging, materials sciences, and biology.[1] Tetraar-
ylethenes are particularly popular in this context.[2] These
AIE luminogens are easy to functionalize, and the optical
properties can be modulated by steric and electronic effects.
Symmetric tetraarylethenes are conveniently obtained by
McMurry couplings.[3] However, unsymmetrical tetraaryle-
thenes with different aryl groups are needed for many
applications. Unsymmetrical tetraarylethenes such as aryltri-
phenylethenes can also be obtained by McMurry couplings of
benzophenone derivatives (Scheme 1a), but competing
homocoupling reactions compromise the yield and require
chromatographic purification.[4] The most popular route for
the synthesis of unsymmetrical aryltriphenylethenes is
depicted in Scheme 1b. Coupling of benzophenone deriva-
tives with lithiated diphenylmethane gives tertiary alcohols,
which can be dehydrated to give the desired olefins (Sche-
me 1b). This method was developed by Rathore and co-
workers,[5] and it is widely used for the preparation of AIE
luminogens. Its substrate scope is limited by the utilization of
a highly reactive organolithium reagent. Functionalized
aryltriphenylethenes can be obtained by Pd-catalyzed cross-
coupling reactions between triphenylethenyl bromide and
arylboronic acids (Scheme 1c). There are several other
methods for the synthesis of asymmetric and symmetric
tetraarylethenes,[6] but these procedures are rarely used in the
context of AIE studies.
Scheme 1. a–d) Different routes for the synthesis of aryltriphenyle-
thenes. e) Synthesis of triazene TPE-Tr.
We show that aryltriphenylethenes and other unsym-
metrical tetraarylethenes are accessible by acid-induced
vinylation of aromatic and heteroaromatic compounds with
À
triazenes (Scheme 1d). This new C H activation route is
facile and versatile. Importantly, it can be used for the direct
attachment of AIE luminogens to supramolecular hosts, to
biologically relevant molecules, and to polymers.
Recently, we started to explore the chemistry of alkynyl
and vinyl triazenes.[7] During these investigations, we found
that cyclobutenyl triazenes can be coupled under acidic
conditions to arenes, such as mesitylene and thiophene.[7e] The
À
C C coupling reactions are triggered by acid-induced cleav-
age of the triazene function. This finding prompted us to
explore if vinyl triazenes can be used more widely for the
electrophilic vinylation of aromatic compounds. In particular,
we were interested in seeing if we could use triazenes for the
synthesis of tetraarylethenes.
[*] A. A. Suleymanov, M. Doll, Dr. R. Scopelliti, Dr. F. Fadaei-Tirani,
Prof. K. Severin
Institut des Sciences et Ingꢀnierie Chimiques, Ecole Polytechnique
Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
The triphenylethenyl triazene TPE-Tr is easily accessible
by reaction of triphenylethenyl magnesium bromide with
phenyl azide (Scheme 1e).[8,9] In 1967, Jones and Miller
reported that the addition of oxo acids (HOR) to TPE-Tr
results in substitution of the triazene group by OR groups.[8]
These results demonstrated that TPE-Tr can be used as an
electrophilic vinylation reagent, but there were only few
subsequent studies, all of which focused on reactions with oxo
acids.[10]
E-mail: kay.severin@epfl.ch
Dr. A. Ruggi
Dꢀpartement de Chimie, Universitꢀ de Fribourg
1700 Fribourg (Switzerland)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201908755.
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To test if TPE-Tr was suited for the vinylation of aromatic
compounds, we investigated the reaction of TPE-Tr with
benzene, which was used as solvent. Different Brønsted and
Lewis acids were used to induce cleavage of the triazene
function (for details, see the Supporting Information). Uti-
lization of triflic acid (HOTf, 5 equiv) gave tetraphenylethene
(1) in almost quantitative yield. Clean and fast (< 10 min)
formation of 1 was also observed when the reaction was
performed in DCM (0.2m) with 20 equivalents of benzene.
For reactions of TPE-Tr with oxoacids, it was proposed
that that acid-induced cleavage of the triazene gives a vinyl
cation, which then reacts with the oxo acid.[8,10] An analogous
À
mechanism is conceivable for the C H vinylation of ben-
zene.[11] It is worth noting that the triazene function is
apparently cleaved in a highly regiospecific fashion, because
we were only able to detect vinylation and not phenylation
+
products (or compounds derived from PhN2 ).
Next, we investigated the scope of the reaction using
different aromatic compounds (Scheme 2). Monosubstituted
benzenes gave the desired tetraarylethenes 2–7 in yields
between 53 and 99%. Reactions with anisole and diphenyl
ether afforded the products 3 and 4, respectively, in the form
of a single isomer, whereas mixtures of para and ortho isomers
were observed for the other substrates. It is worth noting that
halobenzenes are competent coupling partners, even though
the yields are slightly lower (5–7). Unsuccessful coupling
reactions were observed for electron-deficient arenes such as
nitrobenzene and methyl benzoate. Di-, tri-, and tetrasub-
stituted arenes gave the corresponding products as single
isomers in high yields (10–16). Even though methyl benzoate
was not a suitable reaction partner, 3,5-dimethoxy-substituted
methyl benzoate could be converted into the corresponding
product 17 in good yield.
Heterocyclic AIE emitters have found many applications
because of their unique electronic and structural properties.[12]
The coupling of TPE-Tr with standard five-membered-ring
heterocycles using HOTf as a cleavage agent gave complex
mixtures of products. However, utilization of the weaker
trifluoroacetic acid (TFA) instead of HOTf allowed the
preparation of the desired TPE derivatives 18–22 (Scheme 2).
For the reactions with furan, thiophene, and dimethylthio-
phene, triphenylethenyl trifluoroacetate was observed as
a side product, compromising the yields of the products 18–
20. The compound 18 was additionally characterized by
single-crystal X-ray diffraction. Electron-rich N-methyl pyr-
role gave the TPE derivative 21 in quantitative yield as
a mixture of positional isomers (a/b = 70:30). To our surprise,
Scheme 2. Synthesis of aryltriphenylethenes by coupling of TPE-Tr with
different arenes. [a] para/ortho isomers. [b] a/b isomers.
The replacement of one or more phenyl groups in
tetraphenylethene by polycyclic aromatic hydrocarbons is of
interest because the resulting compounds can display superior
electroluminescence properties.[14] Our new method is suited
to prepare such compounds, as evidenced by the synthesis of
the naphthalene derivative 23 and the fluorene-containing 24
(Scheme 2). The latter was isolated as a single regioisomer.
The functionalization of supramolecular receptors with
AIE-active groups is attractive because the resulting con-
jugates can potentially be used for the optical detection of
guest molecules.[15] An advantage of our new synthetic
methodology is the possibility to convert existing receptors
in one step into AIE luminophores, given that the receptors
contain appropriate aromatic groups. To demonstrate this
point, we performed coupling reactions between TPE-Tr and
five commercially available benzo-crown ethers (Scheme 2).
The corresponding TPE derivatives 25–27 were obtained in
À
unprotected pyrrole gave also the desired C C coupling
product, 22, which was synthesized on a 1 mmol scale and
isolated in 78% yield with a high a/b ratio. Notably, 22 is
described in the literature as a precursor for the synthesis of
AIE-active BODIPY fluorophores with near-infrared emis-
sion.[13] The previous synthesis for 22 is based on a two-step
procedure involving a Pd-catalyzed cross-coupling between
triphenylethenyl bromide and N-Boc-pyrrole-2-boronic acid,
followed by Boc removal, whereas our procedure allows
direct vinylation of free pyrrole without a transition-metal
catalyst.
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moderate to good yields as single isomers (selective coupling
at the meta position). Twofold coupling reactions are also
possible, as evidenced by the syntheses of 28 and 29.
Ferrocene was found to be compatible with our vinylation
protocol, and triphenylethenylferrocene (30) was isolated in
57% yield. The compound 30 is nonfluorescent in the solid
state, presumably because the redox-active ferrocenyl groups
acts as an intramolecular quencher.
Conjugates between biomolecules and AIE luminophores
are of interest for medicinal and analytical applications.[1,16]
Following our procedure, we were able to install the TPE
moiety on a b-estradiol derivative (Scheme 2). The corre-
sponding product, 31, was isolated in good yield as a single
isomer.
The scope of the coupling reaction was extended to
different aryl-substituted vinyl triazene reagents (X-TPE-Tr)
using 1,2-dimethoxybenzene as the coupling partner (32–34,
Scheme 3). To demonstrate that our approach allows building
complex TPE derivatives in a few steps, we synthesized the
chlorinated benzo-crown ether 35, which was further func-
tionalized using a Suzuki cross-coupling reaction with tolyl
boronic acid to give the compound 36.
Scheme 4. Covalent grafting of TPE groups to commercial polymers.
tion and extensive washing of the polymer gave a highly
fluorescent TPE-PS conjugate. The latter showed a fluores-
cence quantum yield (QY) of 24% in the solid state with an
emission maximum at 500 nm (lex = 320 nm). The QY is
comparable to that of tetraphenylethene (QY= 18%, lem
=
460 nm, lex = 320 nm) and clearly different from the starting
crosslinked polystyrene (QY= 10%, lem = 360 nm, lex =
320 nm). Furthermore, we modified parylene C, which is
used as protective coating for microelectronic and medical
devices. A film of parylene C was dipped in a DCM solution
of TPE-Tr (0.2m), followed by addition of TfOH. The
resulting TPE-functionalized parylene C film (TPE-PC)
showed strong blue fluorescence, whereas the starting poly-
mer is almost nonemissive under UV-365 nm.[19] Polyvinyl-
carbazole (PVK) has numerous applications in materials
science as a conducting polymer.[20] In a similar fashion as
described above, we transformed commercially available
PVK into TPE-PVK, which showed strong fluorescence in
the solid state.
The swelling behavior of crosslinked polystyrene beads is
strongly dependent on the nature of the solvent. We
hypothesized that the degree of swelling would affect the
conformational freedom of the TPE groups, and thus the
fluorescence intensity. Indeed, suspensions of TPE-PS in
different organic solvents showed vastly different emission
intensities (Figure 1). Nonswelling solvents such as methanol
or n-hexane gave rise to a bright emission, whereas highly
swelling solvents such as toluene gave poor emission. TPE-PS
therefore allows easy optical differentiation of solvents.
To conclude, we have developed a simple, fast, and
versatile protocol for the preparation of asymmetric tetraar-
ylethene luminogens. The new methodology relies on the
acid-induced conversion of vinyl triazenes into highly electro-
philic vinylation reagents. A key feature of our procedure is
Scheme 3. Scope of vinyl triazenes used for the synthesis of the
tetraarylethenes 32–36 and post-functionalization reactions.
Finally, we applied our vinylation procedure to the
functionalization of polymers.[17] Polymers containing AIE
groups have been investigated extensively over the last years,
and numerous applications have emerged.[18] Typically, multi-
step syntheses are required to prepare polymer-AIEgen
conjugates. Our method allows the direct grafting of TPE
groups to polymers containing arenes (side or main chain;
Scheme 4). When triflic acid was added to a suspension of
crosslinked polystyrene beads (1% divinylbenzene) in a solu-
tion of TPE-Tr (10 mol%) in DCM, an instantaneous
reaction was observed (evidenced by gas evolution). Filtra-
Figure 1. The polymer TPE-PS in different organic solvents under UV
366 nm (20 mg of polymer in 0.5 mL of solvent).
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Acknowledgements
This work was supported by funding from the Ecole
Polytechnique Fꢀdꢀrale de Lausanne (EPFL), and by the
Swiss National Science Foundation. We thank Dr. Euro Solari
for elemental analyses and Mr. Miguel Marmelo for samples
of parylene C.
Conflict of interest
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The authors declare no conflict of interest.
Keywords: aggregation · tetraralyethenes · fluorescence ·
synthetic methods · vinylation
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Manuscript received: July 14, 2019
Revised manuscript received: September 5, 2019
Accepted manuscript online: September 8, 2019
Version of record online: && &&, &&&&
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Communications
Synthetic Methods
A. A. Suleymanov, M. Doll, A. Ruggi,
R. Scopelliti, F. Fadaei-Tirani,
K. Severin*
&&&& — &&&&
À
Synthesis of Tetraarylethene Luminogens
Glowing couple: A simple and versatile
coupling method to make tetraaryle-
thenes, which display active aggregation-
induced emission, from aromatic com-
pounds and vinyl triazenes is described.
The novel C H vinylation approach
allows installation of the triarylethenyl
group onto simple unactivated arenes,
heteroarenes, supramolecular hosts, bio-
active molecules, and polymers.
À
by C H Vinylation of Aromatic
Compounds with Triazenes
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
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