Synthesis, photophysical and nonlinear optical properties of [1,2,5]oxadiazolo[3,4-b]pyrazine-based linear push-pull systems

Article abstract of DOI:10.1016/j.jphotochem.2020.112900

A series of D-π-A chromophores based on [1,2,5]oxadiazolo[3,4-b]pyrazine electron-withdrawing group has been designed. The influence of the π-conjugated linker (1,4-phenylene and 2,5-thienylene) and the amino-electron-donating group (diphenylamino and carbazol-9-yl) was studied by cyclic voltammetry, UV-Vis and emission spectroscopy. The second order nonlinear optical properties were also studied using the electric field induced second harmonic generation (EFISH) method. The experimental results have been rationalized by theoretical DFT calculations.

Full text of DOI:10.1016/j.jphotochem.2020.112900

Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, photophysical and nonlinear optical properties of [1,2,5]
oxadiazolo[3,4-b]pyrazine-based linear push-pull systems
,
b
,
c
,
,
c
e
,b
ˇd
Egor V. Verbitskiya *, Sylvain Achelle *, Filip Bures *, Pascal le Poul , Alberto Barsella ,
,
Yuriy A. Kvashnina, Gennady L. Rusinova b, Françoise Robin-le Guenc, Oleg N. Chupakhina
,
,
b
Valery N. Charushina
a I. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, S. Kovalevskaya Str., 22, Ekaterinburg, 620108, Russia
b Ural Federal University, Mira St. 19, Ekaterinburg, 620002, Russia
c Univ Rennes, CNRS, Institut des Sciences Chimiques de Rennes-UMR6226, F35000 Rennes, France
d
´
Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studenska 573, Pardubice 53210, Czech Republic
e
´
´
Departement d’Optique ultrarapide et Nanophotonique, IPCMS, UMR CNRS 7504, Universite de Strasbourg, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of D-
π
-A chromophores based on [1,2,5]oxadiazolo[3,4-b]pyrazine electron-withdrawing group has been
nitrogen heterocycles
donor–acceptor systems
nonlinear optic
designed. The influence of the
π
-conjugated linker (1,4-phenylene and 2,5-thienylene) and the amino-electron-
donating group (diphenylamino and carbazol-9-yl) was studied by cyclic voltammetry, UV-Vis and emission
spectroscopy. The second order nonlinear optical properties were also studied using the electric field induced
second harmonic generation (EFISH) method. The experimental results have been rationalized by theoretical
DFT calculations.
fluorescence
intramolecular charge transfer
1. Introduction
planar conjugated structure. Due to the electron-deficient character of
the pyrazine, the pyrazinyl fragment can be used as electron-
Organic nonlinear optical (NLO) materials have attracted consider-
able attention in recent years because of their potential applications in
various fields of optoelectronics and photonics, including optical com-
munications, optoelectronic transfer, optical signal process and optical
data storage [1]. The main advantages of these materials are high NLO
coefficients, ultrafast optical responses, flexible molecular design and
synthesis and higher optical damage threshold, as compared to tradi-
tional inorganic solids. Generally, the NLO properties of organic mate-
rials are dependent on the molecular structures of chromophores.
Typically, in order to show a significant second-order NLO response, a
molecule must be non-centrosymmetric, with intramolecular charge
transfer (ICT) transitions at relatively low energy and characterized by a
large transition dipole moment and a large difference between the
excited state and the ground state molecular dipole moment [1e]. This
can be achieved in linear organic molecules by connecting an
withdrawing group in push-pull structures. The incorporation of pyr-
azine units in luminescent materials has been recently reviewed [3].
Moreover, it has been shown that push-pull pyrazine derivatives exhibit
interesting second-order (frequency doubling) [4] and third-order (two
photon absorption) [5] NLO properties.
It has been demonstrated that [1,2,5]oxadiazolo[3,4-b]pyrazine
(furazanopyrazine) exhibits higher electron-withdrawing character than
similar non-annulated pyrazine analogues [6]. Recently, the [1,2,5]
oxadiazolo[3,4-b]pyrazine core has been used as an acceptor part of
push-pull molecules exhibiting pronounced fluorescence in the solid
state and sensitivity towards nitroaromatic compounds [7].
This work represents further extension of our previous research
focusing on the design of novel fluorophores that incorporate diazine
scaffold. In this contribution we describe the synthesis of a new series of
D–π–A [where A equals to the 1,2,5-oxadiazolo[3,4-b]pyrazine acceptor,
electron-donor and an electron-acceptor group through a
π
-conjugated
D represents triphenylamine or carbazole donors, and π involves thie-
polarizable spacer, as it occurs in classical organic dipolar push–pull
systems [2].
nylene or phenylene groups] push-pull chromophores 7a,b and 8a,b
and systematic investigation of their photophysical and second order
Pyrazine is a nitrogen based heterocyclic compound with a rigid
NLO properties. The influences of the type of the π- linker as well as the
* Corresponding author.
E-mail address: sylvain.achelle@univ-rennes1.fr (S. Achelle).
https://doi.org/10.1016/j.jphotochem.2020.112900
Received 25 June 2020; Received in revised form 3 September 2020; Accepted 5 September 2020
Available online 10 September 2020
1010-6030/© 2020 Elsevier B.V. All rights reserved.

E.V. Verbitskiy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
nature of the electron-donating group on the photophysical properties
were thoroughly studied and structure-property relationships were
elucidated.
showed the same reduction behavior. On the other hand, the EHOMO of
diphenylamino derivatives is higher leading to a decreased band gap for
compounds 7a and 8a. These results correspond to the evolution of λmax
values obtained from the electronic spectra.
2. Results and Discussion
2.3. Photophysical properties
2.1. Synthesis
The photophysical properties of compounds 7 and 8 have been
studied in various aprotic solvents of increasing polarity and the results
are summarized in Table 2. All compounds are emissive in n-heptane.
Carbazole derivatives 7b and 8b are also emissive in 1,4-dioxane but no
emission is observed in solvents of higher polarity. For push-pull de-
rivatives, such observation is rationalized by decreased emission in-
tensity observed in polar solvents due to lowered energies of the ICT
excited state [8]. Fig. 2 shows the normalized absorption and emission
spectra in n-heptane. As expected, the replacement of the 1,4-phenylene
linker (compounds 7) by a 2,5-thienylene one (compounds 8) induces a
red shift of both absorption and emission bands as well as increases the
molar extinction coefficient due to a better charge transfer through the
heterocycle [9]. Similarly to previous observation [10], the diphenyla-
mino derivatives exhibited red-shifted absorption and emission. This
The synthesis of the linear push-pull systems required 5-(4-bromo-
phenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3) and 5-(5-bromothiophen-
2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5) as starting materials. Com-
pound 3 was prepared via a two-step procedure from commercially
available 4’-bromoacetophenone (1) in 73% overall yield (Scheme 1).
The second starting material (5) was obtained by bromination of
previously described 5-(1-benzothiophen-3-yl)pyrimidine (4) [7] with
an excess of N-bromosuccinimide in 85% yield (Scheme 2).
The target chromophores 7a-b and 8a-b were synthesized in high
yields through the Suzuki-Miyaura cross-coupling reactions shown in
Schemes 3 and 4. Furazanopyrazines 3 and 5 were coupled with the
corresponding pinacol esters of 4-(diphenylamino)phenylboronic (6a)
and 9H-carbazole-9-(4-phenyl)boronic (6b) acids under reflux in 1,4-
dioxane by using Pd(PPh3)4 as catalyst. These reactions afforded the
effect is more pronounced than changing the π-conjugated linker. The
corresponding D-
π-A dyes 7a-d and 8a-d in good yields (Schemes 3 and
absorption and emission maxima increase in the following order 7b <
8b < 7a < 8a. Compound 8b exhibited the highest emission quantum
yield in n-heptane (ΦF = 0.96). The lowest value was observed for
4).
compound 7b (Φ = 0.06), but this compound exhibited much higher
2.2. Electrochemical properties
F
quantum yield in 1,4-dioxane (ΦF = 0.29). Fig. 3 shows the color of
n-heptane solutions of compounds 7-8 under UV-irradiation.
Electrochemical behavior of compounds 7 and 8 was studied by
cyclic voltammetry (CV) in CH2Cl2 containing Bu4NPF6 electrolyte at a
scan rate of 0.1 V/s. The working electrode was a glassy carbon disk, Pt
wire was used as the counter electrode and an Ag wire as a reference
electrode. Ferrocene was used as an internal reference for potential
measurements. The first oxidation/reduction peak potentials and their
differences are listed in Table 1 and CV diagrams are shown in Fig. 1.
Compounds 7b and 8b bearing the carbazole moiety as an electron-
donating group exhibit a first irreversible oxidation process at 0.95 and
0.96 V vs. ferrocene. When the donor is the diphenylamino moiety, the
first oxidation is reversible and measured E1/2 are much lower (0.55 V
for 7a and 0.57 V for 8a; see Fig. 1) in accordance with the stronger
electron-donating effect of the diphenylamino group. For these two
compounds, a second irreversible oxidation peak can be observed at the
higher potential that modifies the reversibility of the first system, on the
reverse scan.
Whereas only moderate absorption solvatochromism was observed
for 7 and 8, which does not seem to be correlated to the solvent polarity
(the most red-shifted absorption bands are observed in CH2Cl2), a strong
positive emission solvatochromism was observed for carbazole de-
rivatives 7b and 8b as illustrated in Figs. 4 and 5. It should be noted that
emission solvatochromism was not observed for diphenylamino de-
rivatives since the emission is fully quenched in solvents of higher po-
larity than n-heptane. This phenomenon, which is due to stabilization of
the highly-polar emitting state by polar solvent, has been extensively
described for push-pull derivatives [11]. It should be noted however that
the quantum yield of compound 7b is higher in 1,4-dioxane than in
n-heptane, probably due to aggregation in this latter solvent. The slope
of the regression line of the emission maxima plotted versus the
Dimroth-Reichardt polarity parameter (ET(30)) is a way to evaluate the
ICT [12]. The slope observed for compounds 7b and 8b (see Fig. S1 in
ESI) are similar or higher as compared to pyrazine push-pull derivatives
All compounds bear the same oxadiazolopyrazine electron-
withdrawing group. For this reason, the same behavior can be
observed on reduction when scanning from 0 to ꢀ 2 V. All compounds
show a first reversible reduction around –1.1 V vs. Fc and a second quasi-
reversible system around ꢀ 2 V (scan rate = 0.1 V/s). The latter becomes
reversible when the scanning rate is increased to 1 V/s (Fig. S25). The
obtained half-wave potentials for both systems are similar regardless of
with diphenylamino substituents and more extended
π-conjugated
linker [13], indicating that the oxadiazolo part significantly enhances
the electron-withdrawing character of the pyrazine.
2.4. Second order nonlinear optical properties
the nature of the electron-donating moieties and of the
π
-conjugated
The second order NLO properties have been studied in chloroform
solution by the electric-field induced second harmonic generation
(EFISH) method at a non-resonant incident wavelength of 1907 nm. The
second harmonic at λ =953 nm is therefore well clear of the absorption
bands of the chromophores. This method provides the NLO response as
the scalar product between the permanent dipolar moment of the
linker. This trend indicates a weak interaction between the donor and
the acceptor.
The HOMO/LUMO energies were calculated from potential values
(the first oxidation and reduction processes) to evaluate the electro-
chemical band gap. The ELUMO remains unaltered as all chromophores
Scheme 1. Synthesis of 5-(4-bromophenyl)-[,2,5]oxadiazolo[3,4-b]pyrazine (3).
2

E.V. Verbitskiy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
Scheme 2. Synthesis of 5-(5-bromothiophen-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5).
Scheme 3. Synthesis of 5-[4-(heteroaryl)phenyl]-[1,2,5]oxadiazolo[3,4-b]pyrazines (7a,b).
Scheme 4. Synthesis of 5-[5-(heteroaryl)thiophen-2-yl]-[1,2,5]oxadiazolo[3,4-b]pyrazines (8a,b).
noted that positive
μ
β values are obtained indicating that both ground
Table 1
and excited states are polarized in the same direction and that the
excited state is more polarized than ground state, in accordance with the
emission solvatochromism observation (for 7b and 8b). Compounds 8
exhibit higher NLO response than analogues 7 and the diphenylamino
Electrochemical data of compounds.
ox1
[V]a
red1
[V]a
Comp.
E
E
ΔE
EHOMO
ELUMO
λmax
1/2
1/2
[V]b
[eV]c
[eV]c
[nm]d
7a
7b
8a
8b
0.55
0.95e
0.57
0.96e
–1.11
–1.10
–1.14
–1.09
1.66
2.05
1.71
2.05
–5.33
–5.73
–5.35
–5.74
–3.67
–3.69
–3.64
–3.69
746
604
727
606
derivatives a show better μβ values than carbazole analogues b. These
results are coherent with the electrochemical and photophysical obser-
vations indicating better ICT for compounds possessing 2,5-thienylene
linker and diphenylamino electron-donating group. The figure of merit
of compound 8a appears particularly interesting.
a
All potentials are given versus ferrocene.
b
c
ox1 red1
ΔE = E –E
.
1/2 1/2
EHOMO/LUMO = –(Eox1/red1+4.8).
Calculated λmax values (λ = 1241/ΔE).
Irreversible peaks Ep.
d
e
2.5. DFT calculations
Spatial and electronic properties of compounds 7a-b and 8a-b were
investigated using Gaussian®16W software [17] package at the DFT
level. Optimized geometries, energies of the frontier molecular orbitals,
→
molecule
μ
in fundamental state and the vector component of β
β values ( β0) have been
described as β// [14]. The two level corrected
μ
μ
also calculated [15]. The results are presented in Table 3. It should be
and ground state dipole moments
μ and were calculated at DFT
3

E.V. Verbitskiy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
Fig. 1. Cyclic voltammograms of 7a (green), 8a (orange) 7b (blue) and 8b (red) in CH2Cl2 solutions.
Table 2
UV/Vis and PL data in various solvents for compounds 7 and 8.
a
Compds
Solvent
λabs
(ε)
λem
[nm]
ΦF
Stokes shift [cm-
1
[nm] [(mM-1 cm-
1])
]
n-heptane
1,4-
473 (18.4)
466 (12.4)
570
0.54
3598
b
b
b
-
-
-
7a
7b
8a
dioxane
CH2Cl2
MeCN
b
b
b
485 (11.8)
457 (16.4)
414 (12.3)
-
-
-
b
b
b
-
-
-
n-heptane
1,4-
521
0.06
4961
401 (8.7)
619
0.29
8783
dioxane
CH2Cl2
MeCN
b
b
b
403 (8.9)
389 (15.1)
514 (33.2)
-
-
-
b
b
b
-
-
-
n-heptane
1,4-
607
b
0.39
b
2980
b
511 (22.7)
-
-
-
dioxane
CH2Cl2
MeCN
b
b
b
544 (22.7)
518 (20.7)
452 (25.4)
-
-
-
Fig. 2. Normalized UV/Vis (solid lines) and emission spectra (dashed lines) of
compounds 7 and 8 in n-heptane.
b
b
b
-
-
-
n-heptane
1,4-
548
627
0.96
0.35
3875
6273
450 (22.5)
8b
dioxane
CH2Cl2
MeCN
B3LYP/6-311+G(2df,p) level in CHCl3 as shown in Table 4. The elec-
tronic absorption spectra and the first-order hyperpolarizabilities β were
calculated by TD-DFT method using CAM-B3LYP/6-311+G(2df,p) in
CHCl3.
b
b
b
463 (20.3)
447 (22.6)
-
-
-
b
b
b
-
-
-
a
Fluorescence quantum yield (±10%) determined relative to 9,10-bis(phe-
nylethynyl)antracene in cyclohexane (ΦF = 1.00).
The calculated energies of the HOMO and the LUMO range from
–5.83 to –5.45 and –3.61 and –3.46 eV (Fig. 6), respectively. Both
b
No emission detected.
4

E.V. Verbitskiy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
Table 3
Results for EFISH measurements for compounds 7 and 8.
7a
7b
8a
8b
μ
μ
μ
β (10-48 esu)a
β0 (10-48 esu)b
β/MWc
700
485
1.91
70
1440
892
330
237
0.89
53
0.19
3.88
a
μ
β(2ω) at 1907 nm in CHCl3. Molecular concentrations used for the mea-
surements were in the range of 10-3 to 10-2 M,
μ
β ± 10%.
b
c
Two level corrected μβ values (μβ0) [14].
Figure of merit (molecular nonlinearity) 10-48 esu g mol-1 [16].
Table 4
DFT-calculated electronic parameters of 7 and 8.
7a
7b
8a
8b
Fig. 3. n-heptane solution under UV lamp irradiation (from left to right: 7b, 8b,
7a, 8a) c = 1-3 × 10-5 M. Photographs were taken in the dark upon irradiation
with a hand-held lamp (λem =366 nm).
E
E
HOMO (eV)a
LUMO (eV)a
–5.45
–3.53
1.92
–5.83
–3.61
2.22
–5.48
–3.46
2.02
–5.83
–3.58
2.25
ΔE (eV)a
μ
(D)a
10.7
6.5
13.1
8.8
λDmFaTx (nm/eV)b
422(2.94)
215
367(3.38)
85
479(2.59)
373
439(2.82)
149
β(-2
ω,
ω,
ω
) (10-30 esu)b,c
a
b
Calculated at DFT B3LYP/6-311++G(2df,p) level in CHCl3.
Calculated at TD-DFT (nstates = 8) CAM-B3LYP/6-311+G(2df,p) level in
CHCl3.
c
At 1907 nm in CHCl3.
Fig. 4. Normalized emission spectra of 7b in different aprotic solvents.
Fig. 6. Energy level diagram showing the electrochemical (black) and calcu-
lated (red) HOMO/LUMO energies.
deepened HOMO. The gaps are slightly lower for chromophores 7 with
1,4-phenylene π-linker.
Fig. 7 shows optimized geometries, HOMO/LUMO localizations and
Mulliken charges in target chromophores; see Fig. S3 for complete listing
of frontier molecular orbitals. All molecules are unsymmetrical and do
not belong to any group of symmetry. Whereas the HOMO is centrally
localized either on the diphenylamino or carbazole donors, the LUMO is
spread over the adjacent 1,4-phenylene bridge. The Mulliken charges
are localized accordingly, involving the donor moieties in 8 and mostly
Fig. 5. n-Heptane (left) and 1,4-dioxane (right) solution of 8b under UV lamp
c = 3 × 10-5 M. Photographs were taken in the dark upon irradiation with a
hand-held lamp (λem =366 nm).
the π-conjugated bridge between the particular donor and acceptor.
calculated and electrochemical HOMO-LUMO gaps obey the same trends
and correlates tightly (Fig. S2) and, therefore we can consider the used
DFT method as reliable. In general, the lowest HOMO–LUMO gap has
been calculated for chromophores in series a bearing N,N-diphenyla-
The calculated dipole moments reflect the geometry with the out-of-
plane orientation of both donor moieties (the torsion angle is about 50◦).
However, molecules in series a bearing more flexible diphenylamino
group possess significantly higher ground state dipole moments of
mino donor. The carbazole-terminated chromophores
b showed
5

E.V. Verbitskiy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
Fig. 7. HOMO (red) and LUMO (blue) localizations (above) and Mulliken charges (below) in chromophores 7a-b and 8a-b.
around 11 and 13 D as compared to carbazole derivatives b (Table 4).
The fundamental electronic absorption properties were investigated
by TD-DFT method; the calculated positions of the longest-wavelength
electrochemical studies of the compounds were performed with a home-
designed 3-electrodes cell (WE: glassy carbon disk, RE: Ag wire, Ce: Pt).
Ferrocene was added at the end of each experiment to determine redox
potential values. UV Vis and fluorescence spectra were recorded with a
Fluoromax-3 Jobin-Yvon Horiba spectrophotometer using standard
1 cm quartz cells. Compounds were excited at their absorption maxima
(band of lowest energy) to record the emission spectra. The ΦF values
were calculated using a well-known procedure [18] with 9,10-bis(phe-
nylethenyl)anthracene in cyclohexane (ΦF = 1.00) as standard [19].
Experimental details on EFISH measurements are described elsewhere
[20].
absorption maxima λDFT are gathered in Table 4, the spectra are pro-
max
DFT
vided in Fig. S4. The calculated λ
values correlate tightly with the
max
experimental longest-wavelength absorption maxima measured in
dichloromethane (Table 2) – see Fig. S5. The spectra consist of two
peaks; the low-energy peak corresponds to the HOMO→LUMO transi-
tion whereas the high-energy peak involves mostly the HOMO-
–2→LUMO and HOMO→LUMO+1 transitions. When comparing the
corresponding pairs of chromophores, chromophores 8a-b bearing 2,5-
thienylene linker possess bathochromically shifted longest-wavelength
absorption maxima. The same trend can be seen for chromophores a
with N,N-diphenylamino donor, when compared to carbazole de-
rivatives b.
3.2. Synthesis of 5-(4-bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine
(3)
First-order hyperpolarizabilities β were also calculated as shown in
A mixture of 4′-Bromoacetophenone 1 (1.994 g, 10 mmol) and sele-
nium dioxide (1.1 g, 10 mmol) in a solution of 1,4-dioxane (15 mL) and
water (1 mL) was refluxed for 12 h. Selenium was filtered off, washed
with 1,4-dioxane (5 mL). The solvent was evaporated at reduced pres-
sure. The residue was dissolved in a mixture of ethanol (5 mL) and acetic
acid (5 mL), 3,4-diaminofurazan (2) (1.0 g, 10 mmol) was added, and
the resulting mixture was refluxed for 1 h and cooled to room temper-
ature. A precipitate formed was filtered, washed with ethanol, and dried
in air. Compound 3 was obtained as a pale yellow solid. Yield 2.023 g
(73 %), mp 159-160 ◦C. 1H NMR (500 MHz, DMSO-d6) δ 9.80 (s, 1 H),
8.38–8.33 (m, 2 H), 7.91–7.86 (m, 2 H). 13C NMR (126 MHz, DMSO-d6)
δ 158.7, 155.6, 152.2, 151.6, 133.4, 132.4, 130.9, 127.2. Calcd. for
Table 4. The calculated β coefficients range from 85 to 373 × 10-30 esu.
Correlations of the calculated β and experimental μβ or μβ products
0
(EFISH experiment) are provided in Figs. S5 and S6. Both correlations
are very tight. In general, larger nonlinearities were calculated for
chromophores a with N,N-diphenylamino donor, the largest one belongs
to chromophore 8a with polarizable 2,5-thienylene
π-linker.
3. Experimental section
3.1. General Information
All reagents and solvents were obtained from commercial sources
and dried by using standard procedures before use. 1H and 13C NMR
spectra were recorded on a Bruker AVANCE-500 and AVANCE-600 in-
struments using Me4Si as an internal standard. IR spectra of samples
(solid powders) were recorded on a Spectrum One Fourier transform IR
spectrometer (Perkin Elmer) equipped with a diffuse reflectance
attachment (DRA) in the frequency range 4000÷400 cm-1. Spectrum
processing and band intensity determination were carried out using the
special software supplied with the spectrometer. Elemental analysis was
carried on a Eurovector EA 3000 automated analyzer. Melting points
were determined on Boetius combined heating stages and were not
corrected. The chromatographic purification of compounds was ach-
ieved with silica gel Alfa Aesar 0.040-0.063 mm (230-400 mesh), eluting
with CH2Cl2/hexane (1:2, v/v). The progress of reactions and the purity
of compounds were checked by TLC on Sorbfil plates (Russia), in which
the spots were visualized with UV light (λ 254 or 365 nm). The
C
10H5BrN4O (277.08): C, 43.35; H, 1.82; N, 20.22. Found: C, 43.37; H,
1.95; N, 20.20.
3.3. Synthesis of 5-(5-bromothiophen-2-yl)-[1,2,5]oxadiazolo[3,4-b]
pyrazine (5)
N-Bromosuccinimide (4.45 g, 25 mmol) was added to a solution of 5-
(1-benzothiophen-3-yl)pyrimidine (4) (2.04 g, 10 mmol) in DMF
(15 ml). The obtained solution was stirred overnight at room tempera-
ture. The reaction mixture was diluted with water. The formed precip-
itate was filtered off, washed with water, dried, and recrystallized from
ethanol. Yield 2.406 g (85%), orange solid, mp 213-215 ◦C. H NMR
(500 MHz, DMSO-d6) δ 9.73 (s, 1 H), 8.40 (d, J =4.1 Hz, 1 H), 7.58 (d, J
=4.1 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6) δ 154.5, 153.5, 152.0,
151.5, 142.2, 135.4, 133.4, 122.9. Calcd. for C8H3BrN4OS (283.10): C,
33.94; H, 1.07; N, 19.79. Found: C, 33.90; H, 1.02; N, 19.55.
6

E.V. Verbitskiy et al.
Journal of Photochemistry & Photobiology, A: Chemistry 404 (2021) 112900
3.4. General procedure for the synthesis of 5-[4-(heteroaryl)phenyl]-
[1,2,5]oxadiazolo[3,4-b]pyrazine (7a,b) and 5-[5-(heteroaryl)thiophen-
2-yl]-[1,2,5]oxadiazolo[3,4-b]pyrazine (8a,b)
1 H), 7.83 – 7.78 (m, 2 H), 7.54 – 7.44 (m, 4 H), 7.33 (ddd, J = 7.9, 6.9,
1.2 Hz, 2 H). 13C NMR (151 MHz, DMSO-d6) δ 155.4, 154.7, 152.7,
152.2, 152.1, 140.5, 140.3, 138.4, 136.8, 131.8, 128.4, 127.8, 127.3,
126.9, 123.5, 121.1, 120.9, 110.3.
ν
(DRA, cm-1) 3093 (br. w, C–HAr),
A mixture of 5-(4-bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine
(3) (277 mg, 1.0 mmol) [or 5-(5-bromothiophen-2-yl)-[1,2,5]oxadia-
zolo[3,4-b]pyrazine (5) (283 mg, 1.0 mmol)], corresponding arylbor-
onic acid 6a,b (1.2 mmol), Pd(PPh3)4 (115 mg, 10 mol %) and K3PO3
(530 mg, 2.5 mmol) was dissolved in 1,4-dioxane 15 mL. The reaction
mixture was degassed and refluxed for 15 h under an argon atmosphere.
After completion of the reaction (monitored by TLC), the reaction
mixture was cooled, filtered, and dissolved in a mixture of EtOAc and
water (1:1, 50 mL), and the organic layer was separated. The aqueous
layer was extracted with EtOAc (2 × 25 mL). The combined organic
extracts were dried with MgSO4 and the solvents evaporated. Purifica-
tion by silica gel column chromatography with CH2Cl2/hexane (1:2, v/
v) as an eluent to afford the title compounds (7 and 8).
3080 (br. w, C–HAr), 3060 (br. w, C–HAr), 3028 (br. w, C–HAr), 1599 (s,
C–CAr/C–NAr), 1567 (s, C–CAr/ C–NAr), 1533 (s, C–CAr/ C–NAr), 1449 (s,
C–CAr/ C–NAr), 1406 (s, C–CAr/ C–NAr), 837 (s, C–HAr), 749 (s, C–HAr),
724 (s, C–HAr). Calcd. for C26H15N5OS (445.50): C, 70.10; H, 3.39; N,
15.72. Found: C, 70.00; H, 3.54; N, 15.56.
4. Conclusion
In summary, four new push-pull chromophores bearing [1,2,5]oxa-
diazolo[3,4-b]pyrazine as electron-withdrawing part and amino group
as donor were designed. There electrochemical and photophysical
properties indicate that intense ICT occurs in these structures with
electrochemical gap below 2.1 eV and strong emission solvatochromism.
NLO responses were also measured by EFISH method. All experimental
and theoretical results indicate a significant increase of ICT when the
diphenylamino electro-donating group is used and when a 2,5-thieny-
lene bridge is replacing the 1,4-phenylene linker. Compound 8a, that
combines these two characteristics, exhibits a particularly high figure of
merit and appears as an interesting candidate for incorporation in a
polymeric matrix to obtain a material with the high electro-optic
coefficient.
3.5. ’-([1,2,5]oxadiazolo[3,4-b]pyrazin-5-yl)-N,N-diphenyl-[1,1’-
biphenyl]-4-amine (7a)
Yield 375 mg (85%), dark violet solid, mp 223-224 ◦C. 1H NMR
(600 MHz, DMSO-d6) δ 9.87 (s, 1 H), 8.54–8.47 (m, 2 H), 7.99–7.94 (m,
2 H), 7.81–7.76 (m, 2 H), 7.39–7.35 (m, 4 H), 7.14–7.10 (m, 6 H),
7.08–7.05 (m, 2 H). 13C NMR (151 MHz, DMSO-d6) δ 159.4, 156.4,
152.9, 152.1, 148.4, 147.2, 144.3, 133.0, 132.1, 130.2, 128.5, 127.2,
125.2, 124.2, 122.9.
ν
(DRA, cm-1) 3062 (w, C–HAr), 3053 (w, C–HAr),
Declaration of Competing Interest
3037(w, C–HAr), 1586 (s, C–CA/C–NAr), 1561(s, C–CAr/C–NAr), 1488 (s,
C–CAr/C–NAr), 1446 (s, C–CAr/C–NAr), 822 (s, C–HAr), 797 (s, C–HAr),
753 (s, C–HAr), 697 (s, C–HAr). Calcd. for C28H19N5O (441.49): C, 76.17;
H, 4.34; N, 15.86. Found: C, 75.95; H, 4.32; N, 16.09.
The authors report no declarations of interest.
Acknowledgments
3.6. -(4’-(9H-Carbazol-9-yl)-[1,1’-biphenyl]-4-yl)-[1,2,5]oxadiazolo
VEV is grateful to the financial support for the synthetic part from the
Russian Foundation for Basic Research (Research Project No. 18-29-
23045 mk).
[3,4-b]pyrazine (7b)
Yield 338 mg (77%), red solid, mp 288-289 ◦C. 1H NMR (600 MHz,
DMSO-d6) δ 9.91 (s, 1 H), 8.63–8.57 (m, 2 H), 8.28 (dt, J = 7.8, 1.0 Hz,
2 H), 8.20–8.11 (m, 4 H), 7.84–7.78 (m, 2 H), 7.53–7.45 (m, 4 H), 7.33
(ddd, J = 7.9, 6.7, 1.3 Hz, 2 H). 13C NMR (151 MHz, DMSO-d6) δ 159.5,
156.3, 152.9, 152.2, 143.9, 140.5, 138.1, 137.8, 134.0, 130.3, 129.3,
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2020.
112900.
128.1, 127.7, 123.5, 121.1, 120.8, 110.2.
ν
(DRA, cm-1) 3034 (w,
C–HAr), 1564 (s, C–CAr/C–NAr), 1447 (s, C–CAr/ C–NAr), 746 (s, C–HAr),
723 (s, C–HAr). Calcd. for C28H17N5O (439.48): C, 76.52; H, 3.90; N,
15.94. Found: C, 76.65; H, 3.78; N, 16.10.
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