Molecular 5,8-π-extended quinoxaline derivatives as chromophores for photoluminescence applications

Article abstract of DOI:10.1016/j.molliq.2019.111763

The 5,8-π-extended quinoxaline derivatives are widely studied due to their well-known photophysical and electrochemical properties. In order to investigate the structure-property relationship, a novel series of fluorescent calamitic liquid crystals based on the quinoxaline heterocycle was successfully synthesized and characterized. The final molecules presented calamitic mesomorphism with nematic and smectic phases. These compounds displayed intense green photoluminescence under UV light excitation in solution and in the solid state. In chloroform solution, the fluorescence quantum yields (Φ_FL = 0.54–0.62) of the quinoxaline-based derivatives were significantly higher than those previously described for similar benzothiadiazoles. The maximum emission peaks were between 511 and 520 nm with singlet excited-state lifetimes in the nanosecond timescale. The solvatochromism studies showed a significant dependence of the emission on the polarity of the solvent. Doping of the quinoxalines with fullerene C₆₀ suggests a charge transfer process, this being dependent on the π-conjugate core. The energy band gaps predicted with DFT calculations are in excellent agreement with the experimental data.

Full text of DOI:10.1016/j.molliq.2019.111763

Journal Pre-proof
Molecular 5,8-π-extended quinoxaline derivatives as chromophores for
photoluminescence applications
Leonardo de O. Aguiar, S.L. Adalberto, Ivan H. Bechtold, Sergio Fernando Curcio,
Thiago Cazati, Tiago V. Alves, André Alexandre Vieira
PII:
S0167-7322(19)33223-4
DOI:
https://doi.org/10.1016/j.molliq.2019.111763
MOLLIQ 111763
Reference:
To appear in: Journal of Molecular Liquids
Received Date: 7 June 2019
Revised Date: 23 July 2019
Accepted Date: 17 September 2019
Please cite this article as: L.d.O. Aguiar, S.L. Adalberto, I.H. Bechtold, S.F. Curcio, T. Cazati, T.V.
Alves, André.Alexandre. Vieira, Molecular 5,8-π-extended quinoxaline derivatives as chromophores
for photoluminescence applications, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/
j.molliq.2019.111763.
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Molecular 5,8-π-Extended
Quinoxaline Derivatives as
Chromophores for
Photoluminescence Applications
a
a
b
Leonardo de O. Aguiar , Adalberto S. L. Junior , Ivan H. Bechtold , Sergio
c
c
a
a
Fernando Curcio , Thiago Cazati , Tiago V. Alves and André Alexandre Vieira
a
Instituto de Química, Universidade Federal da Bahia, 40170-115.
Salvador, Bahia, Brazil.
b
Departamento de Física, Universidade Federal de Santa Catarina, 88040-900,
Florianópolis, SC, Brazil.
c
Departamento de Física, Universidade Federal de Ouro Preto, 35400-000
Ouro Preto, MG, Brazil.
Keywords: Quinoxaline, luminescence, liquid crystals, synthesis.
*
email: Vieira.andre@ufba.br

Abstract
The 5,8-π-extended quinoxaline derivatives are widely studied due to their well-
known photophysical and electrochemical properties. In order to investigate the
structure-property relationship, a novel series of fluorescent calamitic liquid
crystals based on the quinoxaline heterocycle was successfully synthesized and
characterized. The final molecules presented calamitic mesomorphism with
nematic and smectic phases. These compounds displayed intense green
photoluminescence under UV light excitation in solution and in the solid state. In
chloroform solution, the fluorescence quantum yields (Φ = 0.54–0.62) of the
FL
quinoxaline-based derivatives were significantly higher than those previously
described for similar benzothiadiazoles. The maximum emission peaks were
between 511-520 nm with singlet excited-state lifetimes in the nanosecond
timescale. The solvatochromism studies showed a significant dependence of
the emission on the polarity of the solvent. Doping of the quinoxalines with
fullerene C suggests a charge transfer process, this being dependent on the
60
π-conjugate core. The energy band gaps predicted with DFT calculations are in
excellent agreement with the experimental data.

Introduction
Organic fluorescent dyes with designed emission characteristics have
become widely used [1-3] and the areas of application are varied, including in
organic electronics[4], for instance, in organic field effect transistors (OFETs),
organic light-emitting diodes (OLEDs) and dye-sensitized solar cells (DSSCs)[5,
6
] and in bioanalytical imaging, sensing [7, 8], and diagnostics[9].
Luminescent liquid crystals (LLCs) have attracted considerable interest
because of advantages such as supramolecular self-assembly, anisotropic fluid
properties and intrinsic light-emitting ability.[10, 11] As a consequence, the
demand for novel LLC molecules presents a challenge in the area of synthetic
chemistry and, with regard to the photophysical properties, consistent structure–
property correlations need to be established. Several LLCs are described in the
literature, but the quinoxaline heterocycle has not been explored in the
preparation of calamitic liquid crystals.
Quinoxaline, also called benzopyrazine, is a heterocyclic compound
containing a ring complex made up of a benzene ring and a pyrazine ring[12,
1
3]. The quinoxalines are an important class of N-heterocycles, which have
found numerous applications in biological studies,[14, 15] fluorescent dyes,[13,
6] and as a backbone for functional materials.[17, 18] In particular, 5,8-π-
1
extended quinoxalines have been used as building blocks in photoluminescent
and electroluminescent materials because they generally show high electron
affinity and good thermal stability,[19, 20] and they can also act as electron-
transporting materials.[21-23] These derivatives have been successfully
incorporated into polymers and molecules for use as electron-transport
materials in multilayer OLEDs.[24-28] Furthermore, quinoxaline derivatives

present the advantage that they can be easily modified by attaching various
side chains, such as alkyl/alkoxy chains, aromatic rings and functional groups.
The synthesis of extended π-conjugated systems has been the key to
providing LLCs. These compounds are often based on a push-pull system,
comprised of an electron-donating group (D) and an electron withdrawing group
(A) linked through a π-conjugated spacer. In a previous study, we
systematically developed electrooptical liquid crystals based on a “D-π-A-π-D”
architecture, which contains different heterocycles as the acceptor unit, for the
design of more efficient organic systems with improved spectral response and
photo/thermal properties.
Herein, we report the design, synthesis and characterization of a series of
luminescent liquid crystals based on the 5,8-π-extended quinoxaline system.
The mesomorphic and photophysical properties of the target molecules were
investigated. Surprisingly, a high reactivity of the quinoxaline was verified via
the Sonogashira reaction and pronounced relative quantum yields were
obtained, in contrast to results previously reported in the literature [29].
Results and discussion
Synthesis
The synthetic route used to obtain the target molecules 5a-g is shown
Scheme 1 and the same procedure was adopted in our previous research[30].
It starts from the sulfur extrusion reaction of 4,7-dibromo-2,1,3-benzothiadiazole
(1) with sodium borohydride and catalytic cobalt chloride in a mixture of
ethanol/tetrahydrofuran to obtain the diamine (2) in 75% yield. The 3,6-
dibromobenzene-1,2-diamine was subjected to a condensation reaction with

glyoxal, triethylamine and ethanol at room temperature to afford the 5,8-
dibromoquinoxaline (3) in 70% yield. The aryl acetylenes 4a-g were synthetized
from the respective aryl halides via palladium catalyzed cross-coupling using 2-
methyl-3-butyn-2-ol, followed by protective group elimination. The compounds
5
a-g, with π-extended conjugation presenting the quinoxaline moiety, were
obtained via the Sonogashira coupling reaction between one equiv. of the
building block 3 and two equiv. of the corresponding aryl acetylene 4a-g, using
bis(triphenylphosphine)palladium (II) dichloride, triphenylphosphine, copper
iodide dissolved in triethylamine and tetrahydrofuran solvents. After purification,
the desired final molecules were obtained in good yields (55-84%). This
synthetic strategy is appropriate since the quinoxaline 3 proved to be a very
reactive nucleus for the Sonogashira cross coupling and the final reactions
occurred, in general, within 20 min. This result was unexpected, because the
literature describes 5,8-dibromoquinoxaline as a non-reactive system.[29]
Scheme 1. Synthesis of the target compounds 5a-g.
All of the final molecules synthesized (5a-g) were characterized by
spectroscopic methods and APPI mass spectrometry.

Liquid crystalline behavior
The thermal properties of compounds 5a-g were studied by differential
scanning calorimetry (DSC) and polarized optical microscopy (POM). The
transition temperatures and enthalpy values are summarized in Table 1. The
liquid crystalline properties of the final compounds were confirmed by X-ray
diffraction (XRD).
−
1
Table 1. Phase transition temperatures ( °C ) and enthalpies (kJ·mol ) for
compounds 5a-g.
c
b
Compound
Transition
T/ °C , heating
T/ °C , cooling
Tdec. /°C
-1
a
-1
a
(
∆H/kJ.mol )
(∆H/kJ.mol )
1
61
107
5
a
b
Cr-Iso
322
388
Cr-N
N-Iso
Cr-N
154 (18.2)
172 (0.3)
146 (21.2)
167 (2.0)
118 (4.5)
138 (24.6)
160 (7.0)
165 (broad)
54 (9.5)
133 (23.1)
167 (0.1)
132 (21.8)
166 (2.7)
117 (4.4)
130 (24.6)
159 (4.7)
162 (broad)
40 (5.5)
5
5
c
d
409
347
N-Iso
Cr(I)-Cr(II)
Cr(II)-SmC
SmC-N
5
N-Iso
Cr(I)-Cr(II)
Cr(II)-Cr(III)
Cr(III)-SmC
SmC-Iso
124 (5.6)
129 (12)
159 (3.0)
---
5
e
396
354
119 (12.2)
156 (0.7)
Cr(I)-Cr(II)
Cr(II)-SmC
SmC-Iso
Cr-SmC
SmC-Iso
68 (9.2)
124 (26.8)
159 (4.9)
160 (30.3)
63 (9.9)
115 (26.5)
158 (4.1)
138 (12.8)
225 (1.0)
5
f
3
01
5
g
2
31 (3.3)
a
b
Determined by POM and DSC measurements (10 ºC/min) on the second heating cycle.
Determined by TGA, onset of decomposition in nitrogen (10 ºC/min). Cr = Crystal, N =
c
Nematic, SmC = Smectic C, I = Isotropic liquid.

All the final compounds, with the exception of 5a, exhibited liquid crystalline
behavior, with nematic and smectic phases being present, which are typical of
calamitic-like molecules. The elongation of the aromatic moieties linked to the
5
,8-quinoxaline and the alkoxy chain lengths allow a broader mesomorphic
range and the appearance of more organized phases, such as the smectic
phase. This behavior can be attributed to an increase in the length-to-breadth
ratio of the molecule.
The mesophases were identified according to the classification reported
by Dierking and Demus.[31] The cooling of the samples 5b, 5c and 5d from the
isotropic phase showed birefringent liquid droplets, which later agglutinate to
give the schlieren texture typical of a nematic mesophase. During the slow
cooling of the nematic phases, a strong tendency to exhibit homeotropic
behavior was observed. Figure 1a shows the fully homeotropic alignment of
compound 5c at 150 °C and Figure 1b shows 5d at 161 ºC at the beginning of
the alignment process. By further cooling of 5d, the nematic phase transitions to
a SmC phase, leading to the characteristic broken fan-shaped texture. The
SmC phases were confirmed also on cooling samples 5e-g from the isotropic
state, with the growth of smectic batonnets. Selected imagens of the SmC
mesophases for 5e and 5f are shown in Figure 1.
The DSC analysis confirmed the liquid crystalline transitions observed by
POM in both the heating and cooling cycles. The mesomorphism found was
shown to be dependent on the length of the alkoxy chains attached to the
center and the aromatic π-extended moiety. Compound 5a did not show a
mesophase, which was expected due to the absence of the long alkyl chains.
Molecules 5b and 5c of the series exhibit only the nematic phase during heating

and cooling. A variation in the length of the alkoxy chains did not change the
type of mesomorphism or range presented, but there was a decrease in the
melting point, the crystal-to-nematic transition being observed at 154 ºC and
1
46 ºC for 5b and 5c, respectively. The increase in the length of the alkoxy
group to C10 (5d) led to a suppression of the nematic phase (∆T = 3) and
N
favored the smectic C (∆TSmC = 29) phase.
The compounds with longer alkoxy chains (5e and 5f) presented
exclusively the formation of smectogenic phases and the highest
mesomorphism range of this homologous series (∆TSmC = 37 and ∆TSmC = 43,
respectively). The exchange of the phenyl group with naphthyl in 5g raises the
melting temperature of quinoxaline, but also gives higher stability to the SmC
phase (∆TSmC = 87). These results show the influence of the alkoxy chain length
on the occurrence of calamitic mesomorphism.
Figure 1. Polarized optical photomicrographs of compounds (a) 5c at 150 ºC (100x),
(
b) 5d at 161 ºC (100x), (c) 5e at 140ºC (100x) and (d) 5f at 176ºC(200x) in the cooling
cycle.

High thermal stability of the final molecules was determined by
thermogravimetric analysis (TGA), with decomposition temperatures above
3
00 °C . The naphthyl derivative 5g had the lowest decomposition temperature
(Tdec = 301 °C ), while the phenylene derivatives 5a-f exhibited higher stability
(Tdec = 322-409 °C ).
X-ray diffraction studies
X-ray diffraction experiments were carried out to confirm the structure of the
SmC phases observed by POM for compounds 5d, 5e and 5f. All spectra
showed an intense peak in the low-angle region (d ), which is associated with
001
the interlayer spacing, and secondary peaks, where the ratio with d001 gives
integers, confirming the lamellar character of the smectic mesophase.[32] A
o
diffuse peak was observed at around 2θ = 20 , which corresponds to a distance
of 4.5 Å. This represents the lateral distance between the neighboring
molecules within the layers.[33] On comparing the first diffraction peak d001 with
the molecular length L (estimated by ChemBio3D Ultra software, version
1
1.0.1), one can infer that the aliphatic chains are folded or the molecules are
tilted within the layers (or both). Considering the molecules in the most
extended form (although this is not usually the case) the tilt angle (θ) of the
SmC phase was estimated using the relationship cos θ = d /L, see values in
100
Table 2. The results indicate that the tilt angle increases for longer aliphatic
chains and, if this is not the case, the longer chains must be more folded.
Figure 2 shows a representative spectrum (for compound 5d).

Table 2. Structural parameters of compounds 5d, 5e and 5f in the SmC phase.
o
Compound
L (Å)
43.1
48.1
53.2
d001 SmC (Å)
36.2
Tilt angle θ ( )
5
5
d
e
33
39
43
37.5
38.7
5f
Figure 2. X-ray pattern for compound 5d collected at 145 ºC in the SmC mesophase.
Photophysical properties
UV-vis and Fluorescence
The ultraviolet−visible absorption and fluorescence spectra for the
quinoxaline derivatives 5a-g were collected using a dilute solution (M = 1.0 ×
−5
1
0 mol/L) in chloroform, and in thin films prepared by spin coating onto quartz
substrates (Figure 3 and 5). The chloroform solvent was chosen because of the
high solubility of the molecules therein. Relevant data are summarized in
Tables 3 and 4.

Table 3. Photophysical properties of 5a–g in chloroform solution.
b
Emission
λma xb ,c
(nm)
f
f
Absorption
(nm)
Stokes
Shift
(nm)
e
k
K
r
nr
-1
d
τf
/
τ0
/
-1
Sample
λ
Φf
(s )
(s )
max
4
-1
-1 a
ns
ns
8
8
(
ε/ 10 M cm )
(x10 )
(x10 )
5
5
5
5
5
a
b
c
d
e
409 (2.8)
409 (2.4)
410 (3.5)
411 (4.0)
410 (4.9)
411 (2.1)
419 (2.0)
518
518
520
518
520
518
511
109
109
110
107
110
107
92
0.62
0.59
0.59
0.62
0.55
0.56
0.54
5.76
5.62
5.60
5.61
5.59
5.59
4.05
9.29
9.53
9.49
9.05
10.16
9.98
7.50
1.08
1.05
1.05
1.11
0.98
1.00
1.33
0.66
0.73
0.73
0.68
0.81
0.79
1.14
5
f
5g
a
−1
−1
b
−5
−1
c
Units = L mol cm ; chloroform solution (1.0 xˑ10 mol L ); excited at maximum absorption;
d
relative quantum yields in solution determined using quinine sulfate as standard (Φ = 0.546 in
Fl
e
f
1
N H SO ). Natural radiative lifetime calculated using τ =τ /Φ . k and k are the radiative and
2 4 0 f f r nr
non-radiative rate constants calculated using k = Φ / τ and k = (1-Φ ) / τ , respectively.
r
f
f
nr
f
f
Compounds 5a-f exhibited strong absorption bands in the ultraviolet region, with
maxima at around 410 nm, and molar extinction coefficients in the range of
−1
−1
2
0,000−49,000 L.mol cm . These absorption bands are assigned to π–π*
transitions. The position of the absorption maxima was not significantly altered
with different lengths of the alkoxy chains (5a-f). The highest absorption peak
for 5g was red-shifted by 9 nm compared with the average. This could be due to
an electronic effect, which lowers the HOMO-LUMO band gap due to the
presence of an elongated conjugation by the naphthyl group.[34] All of the final
compounds exhibited a similar and intense yellowish green luminescence when
excited at the maximum absorption wavelength in chloroform solution (λexc
=
4
10 nm). The maximum emission peaks were observed between 450 and 600
nm (Figure 3). The fluorescence spectra for 5a-f are similar and showed
emission maxima at 518-520 nm. The naphthyl derivative 5g presented a slight
blue-shift with emission maxima at 511 nm. The observed Stokes shifts were in
the range of 92–110 nm, with a small region of coincidence between the
absorption and emission wavelengths. All molecules 5a-g exhibited good
quantum yields in chloroform solution (Φ = 0.54-0.62), and these were
f

significantly higher than those for similar benzothiadiazoles (Φ = 0.19-0.20)[35].
f
This difference can be attributed to the nitrogen atoms in the quinoxaline π-
conjugate system, which does not suffer from the heavy atom effect[30, 36, 37].
The quantum yields are higher than those reported by Mancilha[29], probably
due to the influence of the triple bonds in compounds 5a-g, which increase the
π-conjugation length of the system[38-40].
Figure 3. Absorbance (left) and emission (right) spectra of the compounds 5a-g in
-5
chloroform solution (10 mol/L).
The singlet excited-state lifetimes for compounds 5a-g in chloroform solution were
measured at the maxima of the emission intensity after excitation at 401 nm by
time-resolved fluorescence spectroscopy and their fluorescence decay curves can
be seen in the supporting information. All compounds exhibited mono-exponential
fluorescence decay with lifetimes in the nanosecond timescale, characteristic of
monomeric emission, see values in Table 3. The singlet excited-state lifetimes (τf)
of compounds 5a-f were similar, suggesting that the lifetimes are not significantly
affected by the alkoxy chain length. Compound 5g, with a conjugation
elongated by the naphthyl group, showed the shortest lifetime. The values

obtained are similar to the lifetimes reported for other benzoheteroarene–
ethynylene systems.[41]
The natural radiative lifetimes (τ ) and radiative (k ) and non-radiative (k )
0
r
nr
fluorescence constants are also shown in Table 3. Considering τ =τ /Φ , k =Φ / τ
f
0
f
f
r
f
and k =(1-Φ )/ τ .
nr
f
f
Thin films of compounds 5b-g were investigated before and after annealing,
consisting of heating the film until it melted, followed by cooling at 10ºC/min.
Compound 5a did not form a homogeneous film, probably due to the absence of
alkyl chains. Interestingly, before annealing the 5b-g films exhibited
hypsochromic shifts of around 40 nm and 34 nm for the absorption and
emission bands, respectively, in relation to the solution.
Table 4. Summary of photophysical properties of 5b–g obtained in thin film analysis.
ꢄꢃ
ꢁꢂꢃ
b
Absorption
λmax (nm)
Natural
Absorption
λmax (nm)
Annealed
Emission
λmax (nm)
Natural
Emission
λmax (nm)
Annealed
ꢀ
(eV)
S
ample
a
a
5
5
5
5
b
c
d
e
364
365
366
368
368
421
345
356
358
359
360
354
463
484
494
485
498
517
458
459
455
459
458
478
2.84
2.89
2.90
2.86
2.88
2.79
5
f
5
g
a
b
Excited at maximum absorption; Optical bandgap calculated on the low energy edge of
ꢉꢈ
ꢆꢇꢈ
the absorption spectrum (ꢅ
= 1240/λonset).
After the annealing of the films, 5b-g showed hypsochromic shifts in the
absorbance and emission spectra in the range of 11 nm and 27 nm,
respectively.

This is probably related to H-aggregate formation in the quinoxaline films[42].
An H-aggregate is a one-dimensional array of molecules in which the transition
dipole moments of individual monomers are aligned parallel to each other but
perpendicular to the line joining their centers (face-to-face fashion), thereby
increasing the energy transition and shifting the absorption band to shorter
wavelengths, according to Kasha’s exciton model[43, 44]. The emission spectra
for compounds 5d and 5g obtained before and after annealing are shown in
Figure 4. The emission displacement is evidenced from the change in the
photoluminescence color.
Figure 4. Emission spectra for thin films of 5d (a) and 5g (c) before and after
annealing, (b) and (d) show illustrative photographs of the films.
Solvatofluorochromism properties
Compounds 5b and 5f were chosen to investigate the solvatofluorochromism of
the quinoxaline derivatives. The optical properties were measured in commonly
used organic solvents such as heptane, acetone, acetonitrile, chloroform,
dimethylformamide, tetrahydrofuran and toluene. The results indicate a
significant dependence of the emission characteristics on the solvent polarity,

which had only a slight effect on the absorbance spectra. Figure 5 shows the
emission and absorbance spectra for 5b in the different solvents (all of the data
are summarized in the Electronic Supporting Information).
Figure 5. Normalized absorbance (dashed line) and emission (solid line) spectra
for 5b in different solvents.
The compounds 5b and 5f have well-resolved absorption and emission bands.
The maximum emission bands are shifted to longer wavelengths by increasing
the solvent polarity, e.g., from 458 nm (hexane) to 537 nm (acetonitrile) in the
case of 5b. This represents a variation of 79 nm in the luminescence of the
quinoxaline derivatives. The length of the alkoxy chains has no influence on the
solvatofluorochromism of 5b and 5f in the solvents studied.
The solvent-sensitivity of the fluorophore was estimated using the Lippert-
Mataga equation.[45, 46] The plot of the Stokes shifts (∆ν) as a function of

orientation polarizability (∆f) for 5b is shown in Figure 6. The increase in ∆ν
with an increase in the solvent polarity parameter leads to a linear correlation of
0
.906 for 5b and 0.902 for 5f (see Electronic Supporting Information).
Figure 6. Changes in the Stokes shifts (∆ν) for 5b versus ∆f for the different
solvents.
Slopes of 8457 and 8344 were obtained for 5b and 5f, respectively. This
indicates that the dipole moment of quinoxalines in the excited state is higher
than in the ground state. In addition, the similarity of the slopes of the Lippert-
Mataga plots for 5b and 5f suggests that the alkoxy chains have no effective
influence on the solvatochromism of these architectures.

Molecular orbital calculations
In order to obtain further evidence of the optical properties of the molecules
investigated, we performed quantum chemical calculations with density
functional theory (DFT) using the Gaussian 09 software. In this step, the
calculations were carried out for two different compounds obtained
experimentally, 5a and 5g. To reduce the computational time, we replaced the –
OC H group with –OCH in the structure of 5g (referred to as 5g*). The
12
25
3
geometry optimizations were calculated using the Becke three-parameter set
with the Lee-Yang-Parr modification (B3LYP) together with the 6-31+G(d,p)
basis set. The vibrational harmonic frequencies were determined to confirm the
nature of the stationary points on the potential energy surface. The cartesian
coordinates of the optimized structure and the 3N-6 harmonic vibrational
frequencies are listed in Tables S.I and S.II in the Supporting Information. Figure
7
shows the optimized geometries and the molecular amplitude plots of the
HOMO and LUMO energy levels with a molecular orbital for 5a and 5g*. The
LUMO distribution of these compounds is mainly on the quinoxaline core, while
the electron density of the HOMO is mainly distributed between the substituted
phenyl/naphthyl and quinoxaline groups, indicating an electronic conjugation
between them. Although the HOMO energy levels of 5a and 5g* are identical
(−5.40 eV), a higher electron density dispersion is observed for the naphthyl
derivative, promoting a greater electronic conjugation of compound 5g*
compared to 5a. Moreover, at the B3LY/6-31+G(d,p) level of theory, the energy
band gaps for 5a and 5g* are 2.85 and 2.78 eV, respectively. The theoretical
energy gap for 5g* (2.78 eV) is in excellent agreement with the value obtained
experimentally in the thin film analysis (2.79 eV).

Figure 7. Optimized structures, molecular orbital diagrams for the HOMO and LUMO,
and band gap energies for 5a and 5g*, obtained at the DFT/B3LYP/6-31+G level of
theory.
Preliminary photoinduced charge separation
To investigate the possibility of the charge transfer process, fluorescence
quenching experiments were carried out with the quinoxalines 5e and 5g
(Figure 8). The fluorescence of the quinoxalines in toluene solution was
gradually quenched by doping with fullerene C .
60
This behavior suggests a photoinduced charge separation process involving
quinoxaline as the donor and fullerene as the acceptor.[47, 48] Moreover, the
dependence of the fluorescence intensity on the C concentration is linear, in
60
accordance with the Sterne-Volmer equation:
F0/F= 1 + Ksv [C]
where F and F represent the fluorescence intensity in the absence and
0
presence of C , respectively, Ksv is the quenching constant and [C] is the
60

3
-1
concentration of C . Thus, the Ksv values for 5e and 5g are 4.66 x 10 M and
60
3
-1
5
.84 x 10 M , respectively. This suggests charge transfer between the
quinoxaline and C [49]. In addition, the difference in the C values for 5a and
60
5
g indicates that the larger core may have a greater electron transfer capacity.
-5
Figure 8. (a) Emission spectra for 5e and 5g (1.8 × 10 M) in toluene with increasing
concentration of C : 0.0 (blank), 1.0 equiv. (1), 2.0 equiv. (2), 3.0 equiv. (3), 4.0 equiv.
60
(
4), 5.0 equiv. (5), 6.0 equiv. (6), 7.0 equiv. (7), 8.0 equiv. (8). The insets show the
Sterne-Volmer quenching plots for these compounds.
Conclusions
In summary, a series of luminescent liquid crystal derivatives obtained with the
5
,8-bis(phenylethynyl)quinoxaline core was successfully synthesized and
characterized. The relationship between the mesomorphism and the length-to-
width ratio was explored through compounds with different alkoxy chains. The
final compounds showed predominantly nematic and smectic mesophases. An
increase in the length of the alkoxy chain suppressed the nematic phase and
favored the smectic phase. These molecules exhibited strong green
fluorescence with maxima between 450 and 600 nm and pronounced quantum
yields in solution (0.54 to 0.62), values much higher than materials without the

ethynylene spacer. All compounds exhibited monomeric emission in chloroform
with excited-state lifetimes in the nanosecond timescale. The photoluminescence
of the thin solid film of 5b-g presented clear hypsochromic shifts in the
absorbance and emission wavelengths, probably related to H-aggregate
formation. Charge transfer between quinoxalines and C60 in toluene solution
was verified. Overall, the results reported herein show that it is possible to
prepare π-conjugate systems based on quinoxaline via the Sonogashira
reaction to generate new LLCs, with the potential for application in
optoelectronic devices.
Experimental
Materials
All reagents were obtained from commercial sources and used without
further purification. Sonogashira couplings were accomplished under nitrogen
atmosphere. The terminal acetylenes 1-methyloxy-4-ethynylbenzene (4a), 1-
hexyloxy-4-ethynylbenzene (4b), 1-octyloxy-4-ethynylbenzene (4c), 1-decyloxy-
4
4
-ethynylbenzene (4d), 1-dodecyloxy-4-ethynylbenzene (4e), 1-tetradecyloxy-
-ethynylbenzene (4f) and 2-(dodecyloxy)-6-ethynylnaphthalene (4g) were
prepared according to published procedures.[34] The organic solvents were of
commercial grade quality, with the exception of THF (HPLC grade), and all were
purified by traditional methods. In general, the compounds were purified by
column chromatography on silica gel (60–120 mesh) and crystallization from
analytical grade solvents. Thin layer chromatography (TLC) was performed
Macherey-Nagel, 25 mm a thickness and 5-40 µm particle diameters. For the
visualization TLC plates were either placed under ultraviolet light or stained with
iodine vapor and acidic vanillin.

Synthesis
,6-Dibromobenzene-1,2-diamine (2)
3
In a 250 mL flask were added 4,7-dibromo-2,1,3-benzothiadiazole (2.0 g, 6.8
mmol), THF (15 mL) and ethanol (25 mL). The solution was cooled down using
an ice bath and NaBH (0.76 g, 34 mmol) was added in small portions under
4
stirring. Finally, cobalt (II) chloride hexahydrate (CoCl .6H O) was added in
2
2
catalytic amount (1 mol %), making the reaction medium dark and releasing gas
H S. The mixture was stirred overnight at room temperature and then, filtered to
2
separate the black solid. The solvent was evaporated, water (50 mL) was added
and the product was extracted with dichloromethane (3×40 mL). The organic
phase was dried over MgSO and the solvent removed, resulting in a white
4
solid. Yield: 1.80 g (96%). m.p.: 87.6 – 90 °C . IR (ATR): 3397, 3367, 3327,
-1 1
2
962, 1649, 1446, 1259, 1240, 1131, 1065, 1015, 868, 791, 765 cm . H NMR
(
400 MHz, CDCl ) δ ppm: 6,84 (s, 2H); 3,90 (s, broad, 4H).
3
5
,8-Dibromoquinoxaline (3)
In a 100 mL flask was added 3,6-dibromo-1,2-benzenediamine (1.0 g, 3.76
mmol), 30 mL of absolute ethanol, 4 mL of glyoxal solution (40%) and 5 drops
of triethylamine. The reaction was stirring at room temperature and light shelter
for 12 hours. The precipitate obtained was filtered and recrystallized from
ethanol to give a yellow solid. Yield: 0.63 g (58%). IR (ATR): 3072; 3052; 1873;
1
1
759; 1676; 1584;1451; 1373; 1024; 879; 821. H NMR (400 MHz, CDCl ) δ
3
ppm: 9,01 (s, 2H); 7,99 (s, 2H). ¹³C NMR (400 MHz, CDCl ) δ ppm: 146,19;
3
1
41,76; 133,89; 124,16.

Preparation of Quinoxaline-based derivatives (5a-g)
A mixture of 5,8-dibromoquinoxaline (3) (287 mg, 1.0 mmol), Pd(PPh ) Cl (70
3
2
2
mg, 0.1 mmol), CuI (9.5 mg, 0.05 mmol) and PPh (26 mg, 0.1 mmol) in
3
triethylamine/tetrahydrofuran 1:1 (20 mL) was stirred and heating until 70ºC.
Then, the respective terminal arylacetylene (4a–g) (2.5 mmol) dissolved in
tetrahydrofuran (10mL) was added drop wise. The reaction mixture was stirred
under reflux for 20min under a nitrogen atmosphere. Cooled down to room
temperature, the solution was evaporated, and the crude product was purified
by a silica gel chromatography column using hexane/dichloromethane (70:30)
as the eluent to afford the respective final compound 5a-g.
5
,8-Bis[(4-methoxyphenyl)ethynyl]quinoxaline: Compound 5a
Yield: 74 mg (55%). IR (KBr): 2956, 2919, 2850, 2210, 1602, 1565, 1507,
-
1 1
1
2
464, 1248, 826 cm . H NMR (400 MHz, CDCl ) δ ppm: 9.00 (s, 2 H), 7.97 (s,
3
H), 7.64 (d, J = 8.0 Hz, Ar-H, 4 H), 6.93 (d, J = 8.0 Hz, Ar-H, 4 H), 3.85 (s, -
13
OCH , 6 H). C NMR (100 MHz, CDCl ) δ ppm: 160.1, 145.4, 143.1, 133.6,
3
3
1
33.4, 123.9, 115.0, 114.0, 97.8, 85.1, 55.3. APPI-MS m/z: Molecular formula
+
C H N O requires [M+H] 391.1441; found: 391.1443.
26
18
2
2
5
,8-Bis{[4-(hexyloxy)phenyl]ethynyl}quinoxaline: Compound 5b
Yield: 130 mg (71%). IR (KBr): 2954, 2928, 2859, 2206, 1604, 1566, 1509,
-
1 1
1
2
6
1
469, 1242, 832 cm . H NMR (400 MHz, CDCl ) δ ppm: 9.00 (s, 2 H), 7.96 (s,
3
H), 7.62 (d, J = 8.0 Hz, Ar-H, 4 H), 6.92 (d, J = 8.0 Hz, Ar-H, 4 H), 4.01 (t, J =
.5 Hz, -OCH , 4 H), 1.80 (m, -OCH CH , 4 H), 1.47 (m, -OCH CH CH , 4 H),
2
2
2
2
2
2
13
.36 (m, -CH , 8 H), 0.92 (t, -CH , J = 6.8 Hz, 6 H). C NMR (100 MHz, CDCl )
2
3
3

δ ppm: 159.7, 145.4, 143.1, 133.5, 133.4, 123.9, 114.7, 114.5, 98,0, 85.0, 68.1,
1.6, 29.2, 25.7, 22.6, 14.0. APPI-MS m/z: Molecular formula C H N O
3
36
38
2
2
+
requires [M+H] 531.3006; found: 530.3008.
,8-Bis{[4-(octyloxy)phenyl]ethynyl}quinoxaline: Compound 5c
Yield: 157 mg (77 %). IR (KBr): 2923, 2853, 2208, 1601, 1563, 1509, 1467,
5
-
1 1
1
7
=
243, 829 cm . H NMR (400 MHz, CDCl ) δ ppm: 8.99 (s, 2 H), 7.96 (s, 2 H),
3
.62 (d, J = 8.8 Hz, Ar-H, 4 H), 6.91 (d, J = 8.8 Hz, Ar-H, 4 H), 3.98 (t, -OCH , J
2
6.6 Hz, 4 H), 1.80 (m, -OCH CH , 4 H), 1.25 (m, -CH , 20 H), 0.89 (t, -CH , J =
2
2
2
3
1
3
6
.8 Hz, 6 H). C NMR (100 MHz, CDCl ) δ ppm: 159.7, 145.4, 143.1, 133.6, 133.4,
3
1
24.0, 114.7, 114.5, 98.0, 85.0, 68.1, 31.8, 29.7, 29.4, 29.2, 26.0, 22.7, 14.1.
+
APPI-MS m/z: Molecular formula C H N O requires [M+H] 587.3632; found:
40
46
2
2
5
87.3633.
,8-Bis{[4-(decyloxy)phenyl]ethynyl}quinoxaline: Compound 5d
5
Yield: 183 mg (82%). IR (KBr): 2954, 2919, 2851, 2203, 1604, 1568, 1509,
-
1 1
1
2
472, 1244, 835 cm . H NMR (400 MHz, CDCl ) δ ppm: 9.00 (s, 2 H), 7.97 (s,
3
H), 7.61 (d, J = 8.8 Hz, 4 H), 6.91 (d, J = 8.8 Hz, 4 H), 4.00 (t, -OCH , J = 6.6
2
Hz, 4 H), 1.83 (m, -OCH CH , 4 H), 1.48 (m, -OCH CH CH , 4 H), 1.33 (m, -
2
2
2
2
2
13
CH , 24 H), 0.89 (t, -CH , J = 6.8 Hz, 6 H). C NMR (100 MHz, CDCl ) δ ppm:
2
3
3
1
2
59.7, 145.4, 143.1, 133.5, 133.4, 124.0, 114.7, 114.5, 98.0, 85.0, 68.1, 31.9,
9.6, 29.6, 29.4, 29.3, 29.2, 26.0, 22.7, 14.1. APPI-MS m/z: Molecular formula
+
C H N O requires [M+H] 643.4258; found: 643.4256.
44
54
2
2
5
,8-Bis{[4-(dodecyloxy)phenyl]ethynyl}quinoxaline: Compound 5e
Yield: 199 mg (82%). IR (KBr): 2955, 2918, 2851, 2213, 1603, 1567, 1509,
-
1 1
1
2
464, 1245, 829 cm . H NMR (400 MHz, CDCl ) δ ppm: 9.00 (s, 2 H), 7.97 (s,
3
H), 7.61 (d, J = 8.0 Hz, 4 H), 6.90 (d, J = 8.0 Hz, 4 H), 3.99 (t, -OCH , J = 6.6
2

Hz, 4 H), 1.83 (m, -OCH CH , 4 H), 1.49 (m, -OCH CH CH , 4 H), 1.27 (m, -
2
2
2
2
2
13
CH , 32 H), 0.89 (t, -CH , J = 6.8 Hz, 6 H). C NMR (100 MHz, CDCl ) δ ppm:
2
3
3
1
2
59.7, 145.4, 143.1, 133.6, 133.4, 123.9, 114.7, 114.5, 98.1, 85.5, 68.1, 31.9,
9.7, 29.6, 29.6, 29.6, 29.4, 29.4, 29.2, 26.0, 22.7, 14.1. APPI-MS m/z:
+
Molecular formula C H N O requires [M+H] 699.4884; found: 699.4879.
48
62
2
2
5
,8-Bis{[4-(tetradecyloxy)phenyl]ethynyl}quinoxaline: Compound 5f
Yield: 190 mg (73%). IR (KBr): 2917, 2850, 2362, 1605, 1510, 1471, 1247, 835
-1 1
cm . H NMR (400 MHz, CDCl ) δ ppm: 8.99 (s, 2 H), 7.96 (s, 2 H), 7.61 (d, J =
3
8
4
.0 Hz, 4 H), 6.90 (d, J = 8.0 Hz, 4 H), 3.98 (t, -OCH , 4 H), 1.83 (m, -OCH CH ,
2 2 2
H), 1.47 (m, -OCH CH CH , 4 H), 1.26 (m, -CH , 40 H), 0.88 (t, -CH , J = 6.8
2
2
2
2
3
13
Hz, 6 H). C NMR (100 MHz, CDCl ) δ ppm: 159.7, 145.4, 143.1, 133.6, 133.4,
3
1
2
24,0, 114.7, 114.5, 98.0, 85.0, 68.1, 31.9, 29.7, 29.7, 29.6, 29.6, 29.5, 29.5,
9.4, 29.4, 29.2, 26.0, 22.7, 14.1. APPI-MS m/z: Molecular formula C H N O
2
52
70
2
+
requires [M+H] 755.5510; found: 755.5509.
5
5
,8-Bis{[6-(dodecyloxy)naphthalen-2-yl]ethynyl}quinoxaline:
g
Compound
Yield: 233 mg (84%
)
. IR (KBr): 2918, 2851, 2361, 2342, 1625, 1599, 1498,
-
1
1
1
2
467, 1257, 858 cm . H NMR (400 MHz, CDCl ) δ ppm: 9.04 (s, 2 H), 8.15 (s,
3
H), 8.04 (s, 2 H), 7.76 – 7,68 (m, 6 H), 7.18 (dd, J = 8.8 and 2.4 Hz, 2 H), 7.13
(
d, J = 2.4 Hz, 2 H), 4.09 (t, -OCH , J = 6.6 Hz, 4 H), 1.89 (m, -OCH CH , 4 H),
2 2 2
1
.54 (m, -OCH CH CH , 4 H), 1.27 (m, -CH , 32 H), 0.88 (t, -CH , J = 6.8 Hz, 6
2 2 2 2 3
13
H). C NMR (100 MHz, CDCl ) δ ppm: 158.1, 145.5, 143.3, 134.6, 133.6, 132.0,
3
1
2
29.5, 129.1, 128.4, 126.8, 124.1, 119.8, 117.6, 106.6, 98.6, 85.9, 68.2, 31.9,
9,7, 29.6, 29.6, 29.6, 29.4, 29.3, 29.2, 26.1, 22.7, 14.1. APPI-MS m/z:
+
Molecular formula C H N O requires [M+H] 799.5197; found: 799.5199.
56
66
2
2

Acknowledgments
This study was financed in part by the Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors
acknowledge the following institutions for financial support: CNPq (MCTI/CNPq
Nº 422750/2018-2-Universal), INCT/INEO, FAPEMIG, FAPESB (PIBIC) and
and PROPP/UFOP grant (#23109.003517/2018-85). The authors are also
grateful to CEBIME-UFSC for the HRMS analysis. The authors acknowledge
Prof. Hugo Gallardo for fruitful discussions and equipment facilities.
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