Highly Substituted Δ3-1,2,3-Triazolines: Solid-State Emitters with Electrofluorochromic Behavior

Article abstract of DOI:10.1002/chem.201901345

Highly substituted Δ³-1,2,3-triazolines can be prepared by reaction of triarylvinyl Grignard reagents with functionalized organic azides. The heterocycles are fluorescent in the solid state, and—depending on the substituents—they can display aggregation-induced emission. Upon oxidation, the triazolines form stable radical cations with altered photophysical properties. Therefore, they represent rare examples of solid-state emitters with intrinsic electrofluorochromic behavior.

Full text of DOI:10.1002/chem.201901345

A Journal of
Accepted Article
Title: Highly-Substituted 1,2,3-Triazolines: a New Class of Solid State
Emitters with Electrofluorochromic Behavior
Authors: Abdusalom Suleymanov, Albert Ruggi, Ophélie Planes,
Anne-Sophie Chauvin, Rosario Scopelliti, Farzaneh Fadaei
Tirani, Andrzej Sienkiewicz, Alberto Fabrizio, Clémence
Corminboeuf, and Kay Severin
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To be cited as: Chem. Eur. J. 10.1002/chem.201901345
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Highly-Substituted 3-1,2,3-Triazolines: a New Class of Solid
State Emitters with Electrofluorochromic Behavior
Abdusalom A. Suleymanov,[a] Albert Ruggi,[c] Ophélie Marie Planes,[a] Anne-Sophie Chauvin,[a] Rosario
Scopelliti,[a] Farzaneh Fadaei Tirani,[a] Andrzej Sienkiewicz,[b] Alberto Fabrizio,[a] Clémence
Corminboeuf,[a] and Kay Severin*[a]
Abstract: Highly-substituted 3-1,2,3-triazolines can be prepared by
reaction of triarylvinyl Grignard reagents with functionalized organic
a)
b)
c)
azides. The heterocycles are fluorescent in solid state, and –
depending on the substituents – they can display aggregation-induced
emission. Upon oxidation, the triazolines form stable radical cations
with altered photophysical properties. Therefore, they represent rare
examples of solid state emitters with intrinsic electrofluorochromic
behavior.
Si
S
O
P
O
O
HPS
d)
e)
f)
The attachment of multiple phenyl groups to small
heterocycles can lead to molecules with interesting photophysical
properties.[1] A well-known example is hexaphenylsilole (HPS,
Figure 1a).[2] This compound is strongly luminescent in the solid
state. Efficient solid state emission is unusual, because aromatic
luminophores typically show aggregation-caused quenching
(ACQ). HPS is weakly luminescent in solution, and studying the
properties of HPS kick-started the now burgeoning field of
aggregation-induced emission (AIE).[3]
N
N
X
N
N
N
N
this work
Figure 1. Examples of polyphenylated heterocycles, which are luminescent in
the solid state.
Following the studies on HPS, numerous other
polyphenylated heterocycles were synthesized, and some of
them were found be good solid state emitters as well.[1,3]
Examples include luminophores based on thiophene dioxides,[4]
phosphole oxides,[5] pyrroles,[6] pyrazines[7] and others
(Figure 1b–e).[1] It is assumed that the propeller-shaped geometry
of these molecules helps preventing - stacking interactions in
the solid state, and thus self-quenching.[1,3]
Below, we describe a new class of solid state emitters
containing an unusual 3-1,2,3-triazoline heterocycle (Figure 1f).
A unique feature of these compounds is the possibility to convert
them into stable radical cations, which are non-emissive.
Therefore, they represent rare examples of electrofluorochromic
compounds with intrinsic redox activity.[8,9]
The fluorophores described in this work were discovered by
accident. We have recently studied the synthesis of triazenes
from organomagnesium reagents and functionalized organic
azides.[10] The reaction between G1 and 1-azido-4-iodobutane
lead to the formation of triazene T1 in high yield (Scheme 1).
Surprisingly, when the Grignard reagent G2 was used instead of
G1, we observed the formation of the triazoline 1 instead of a vinyl
triazene. We assume that the electron-withdrawing effect of the
two additional phenyl groups in G2 is responsible for the different
reactivity (see SI for mechanistic proposal).
Ph
Me
N
R = Me
R = Ph
N
N
T1
R
R
Ph
Me
N3(CH2)4I
MgBr
[a]
A. A. Suleymanov, O. M. Planes, Dr. A.-S. Chauvin, Dr. R.
Scopelliti, Dr. F. Fadaei Tirani, A. Fabrizio, Prof. Dr. C. Corminboeuf,
Prof. Dr. K. Severin
Ph
Ph
G1 (R = Me)
G2 (R = Ph)
N
1
Ph
N
Institut des Sciences et Ingénierie Chimiques
Ecole Polytechnique Fédérale de Lausanne (EPFL)
1015 Lausanne, Switzerland
N
Scheme 1. Substrate-dependent formation of a triazene (T1) or a triazoline (1).
*E-mail: kay.severin@epfl.ch
[b]
[c]
Dr. A. Sienkiewicz
Institute of Physics
École Polytechnique Fédérale de Lausanne (EPFL)
1015 Lausanne, Switzerland
Dr. A. Ruggi
Département de Chimie
Université de Fribourg
1700 Fribourg, Switzerland
In the literature, there are only few scattered reports about
2,5-dihydro-1H-1,2,3-triazoles, or 3-1,2,3-triazolines,[11,12] and
the photophysical and electrochemical properties of these
compounds are largely unexplored. We therefore decided to
investigate the scope of the new synthetic procedure.
Supporting information for this article is given via a link at the end of
the document.
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Vinyl Grignard reagents with p-tolyl or p-fluorophenyl groups
gave the corresponding triazolines 2 and 3, albeit in lower isolated
yield of 40 and 47%, respectively (Scheme 2). The azide reagent
A2 with a shorter C3H6 linker gave triazoline 4 in 44% yield. Using
an azide with a longer C5H10 linker, on the other hand, gave rise
to a complex mixture of products. Interestingly, the aromatic azide
A3 is well suited for this type of reaction, and coupling with G2
gave the corresponding triazoline 5 in 88% yield. It is worth noting
that the presence of three aryl substituents on the vinyl Grignard
reagent seems to be crucial, because other vinyl magnesium
compounds gave triazenes instead of triazolines.[10]
triazoline 1 shows an envelope-like structure for the heterocyclic
ring system (SI, Figure S8.1).
R3
I
R1
N3
N3
A1
A2
R2
or
or
R2
I
X
MgBr
N
N
R3
N3
N
A3
1_5
OTs
R1
R1
Figure 2. Cyclic voltammogram of 1 (a), synthesis of 1(NTf2) (b), EPR spectrum
of 1(NTf2) in THF (c), and structure of [1]•+ in the solid state (NTf2– and hydrogen
atoms are omitted) with thermal ellipsoids at 50% probability (d).
R2
N
N
N
N
N
N
R3
For comparison, we have also converted triazoline 5 into the
radical cation [5]•+ by reaction with Ag(NTf2). The resulting salt
5(NTf2) was characterized by X-ray crystallography and EPR
spectroscopy (for details, see SI).
The electronic structure of the radical cations [1]•+ and [5]•+
was examined at the ωB97X-D3/def2-TZVP level, including
implicit solvation in THF (SMD model). The spin density of both
radical cations is centered on the triazoline ring with the highest
amplitudes on the N1, N2, N3 and C4 atoms.
N
N
N
1 (R1= R2 = R3 = H), 73%
(R1 = R2 = F, R3 = H), 40%
(R = R2 = R3 = Me), 47%
5, 88%
4, 44%
2
1
3
Scheme 2. Synthesis of the triazolines 1–5. Reaction conditions: Grignard
reagent (1.0 equiv.), azide (1.2–1.5 equiv.), THF (0.2 M), 50 °C, 4 h, then
aqueous workup.
The new compounds 1–5 have been characterized by NMR
spectroscopy, high-resolution mass spectrometry (HRMS), UV-
Vis and fluorescence spectroscopy, cyclic voltammetry, and – in
the case of 1, 4, and 5 – by single-crystal X-ray crystallography.[13]
The electrochemical investigations (CH2Cl2, 0.1 M TBAPF6)
revealed that all triazolines undergo a reversible one-electron
oxidation at potentials between E1/2 = 0.02 and 0.16 V versus the
Fc/Fc+ redox couple (Figure 2a and Table S6). The cyclic
voltammetry data suggested that it should be possible to oxidize
the triazolines with mild oxidants. Indeed, we were able to convert
1 into the radical cation [1]•+ by reaction with Ag(NTf2) in THF. The
triazolinium salt 1(NTf2) was isolated in 80% yield as a dark red
crystalline solid. The salt was found to be stable when stored
under nitrogen.[14,15] In THF solution in the presence of oxygen,
1(NTf2) slowly decomposed with a half-life around 24 h.
Figure 3. Spin density (blue: positive spin density; red: negative spin density;
isovalue: 0.01 e-/Bohr3) of [1]•+ (a) and [5]•+ (b). Computations are at the ωB97X-
D3/def2-TZVP level.
Electron paramagnetic resonance (EPR) studies revealed the
presence of a nitrogen-centered radical of 1(NTf2) (Figure 2c; for
a simulation see the SI). The high stability of 1(NTf2) allowed us
Next, we investigated the photophysical properties of the new
heterocyclic compounds. First tests revealed minor differences
between the structurally related compounds 1, 2, and 3. For more
detailed investigations, we thus focused on the triazolines 1, 4,
and 5. All three compounds were found to be fluorescent in solid
state with comparable quantum yields of 3.0, 6.1 and 2.6%, and
decay times between 0.6 and 2.0 ns (Table 1).
to perform
a
crystallographic analysis, and
a
graphic
representation of the radical cation [1]•+ is depicted in Figure 2d.
Due to the trigonal planar geometry of the central N-atom, the
heterocyclic ring in [1]•+ is nearly planar. In contrast, the neutral
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Pronounced differences between the triazolines were
observed in solution (THF). The best solid state emitter,
compound 4, was non-emissive, whereas the quantum yield for 1
in solution was higher than in the solid state. The contrasting
behavior between 1 and 4 was surprising, given that the
structures differ by only one CH2 group. A close inspection of the
geometries of 1 and 4 in the crystal revealed a possible
explanation. For triazoline 4, the phenyl ring at C4 position is
further apart from the two geminal phenyl groups at C5 position,
allowing for a more independent rotation (for details, see the SI).
The rotational freedom of the phenyl group in 4 can lead to
increased non-radiate decay.[3]
unperturbed upon excitation, the electronic transitions of [5]•+ are
characterized by a depletion of the π-electron density on the
additional benzene ring. The contribution of the benzene electron
density to the excitation modify decisively the optical properties of
[5]•+ with respect to [1]•+.
The fluorescence of 1 and 5 is nearly fully quenched upon
oxidation. Consequently, triazolines
1 and 5 classify as
electrofluorochromic compounds. It is worth noting that most
electrofluorochromic compounds are dyads comprised of a redox-
active group, a spacer, and a fluorophore.[8] Fluorophores with
intrinsic redox-activity are much less common,[8,9] and solid state
electrofluorochromic emitters are scarce.[9e,18]
In conclusion, we have shown that highly-substituted 3-1,2,3-
triazolines can be prepared by reaction between triarylvinyl
Grignard reagents and organic azides. The new triazolines are
solid state emitters with a unique characteristic: it is possible to
convert them at low potential into non-fluorescent radical cations.
The radicals display good stability, allowing for a structural and
spectroscopic analysis. The high stability of the two redox states
suggests that these compounds could be used for preparing novel
electrofluorochromic devices. Furthermore, compounds which are
both redox-active and luminescent are of interest for applications
in imaging using a combined scanning electrochemical and
fluorescence microscope.[19]
Table 1. Photophysical properties of the triazolines 1, 4 and 5.a

em [nm] in THF
#
[%] in THF (solid)
 [ns] (solid)b
(solid)
1
4
5
510 (512)
NE (489)
500 (492)
5.0 (3.0)
NE (6.1)
0.2 (2.6)
0.7 (94%), 2.0 (6%)c
1.0
0.6 (50%), 2.0 (50%)c
a
ex = 340 nm. The solid state spectra were acquired using an integrating
b
c
sphere. NE = non-emissive. ex = 405 nm. the best fit obtained for a
biexponential decay.
Acknowledgements
Triazoline
1 displays positive solvatochromism with a
bathochromic shift of 54 nm when going from apolar hexane
(max = 484 nm) to polar MeCN (max = 538 nm). A Stokes shift of
194 nm is observed in acetonitrile (10859 cm–1), which is a very
large value for an organic fluorophore.[16]
This work was supported by funding from the Ecole Polytechnique
Fédérale de Lausanne (EPFL), and by the Swiss National
Science Foundation.
The radical cations [1]•+ and [5]•+ and their precursors 1 and 5
show markedly different optical properties. Solutions of 1 and 5 in
THF are colorless and yellow, whereas 1(NTf2) and 5(NTf2)
absorb in the visible range, and solutions appear orange/red and
green respectively (Figure 4).[17]
Keywords: electrofluorochromism • triazolines • fluorescence •
radicals • aggregation-induced emission
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Entry for the Table of Contents
COMMUNICATION
Abdusalom A. Suleymanov, Albert
Ruggi, Ophélie Marie Planes, Anne-
Sophie Chauvin, Rosario Scopelliti,
Farzaneh Fadaei Tirani, Andrzej
Sienkiewicz, Alberto Fabrizio, Clémence
Corminboeuf, and Kay Severin
Redox-switchable fluorophores based on highly-substituted 3-1,2,3-triazolines
were obtained by reaction between vinyl Grignard reagents and organic azides.
Page No. – Page No.
Highly-Substituted 3-1,2,3-
Triazolines: a New Class of Solid
State Emitters with
Electrofluorochromic Behavior
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