Effect of heteroatom exchange (S/Se) in the mesomorphism and physical properties of benzochalcogenodiazole-based liquid crystals

Article abstract of DOI:10.1016/j.dyepig.2018.04.049

In order to investigate the effect on the structure-property relationship by the exchange of sulfur atom by selenium, a new series of fluorescent calamitic liquid crystals based on 4,7-disubstituted-2,1,3-benzoselenadiazole was successfully synthesized and characterized. The mesomorphic properties were studied by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. Thermal properties have evidenced the influence of changing sulfur by selenium atom in the formation of liquid crystalline phases and mesomorphism range. Photophysical studies were performed in chloroform solution, where these compounds displayed strong orange luminescent under UV light, with relevant quantum yields (Φ_F = 0.14–0.16) and greater solvatochromic dependence. Electrochemical studies showed oxidation and reduction patterns similar to 2,1,3-benzothiadiazoles analogues, however presenting lower band gap.

Full text of DOI:10.1016/j.dyepig.2018.04.049

Accepted Manuscript
Effect of heteroatom exchange (S/Se) in the mesomorphism and physical properties
of benzochalcogenodiazole-based liquid crystals
Elias Regis, Leonardo de O. Aguiar, Patricia Tuzimoto, Edivandro Girotto, Tiago E.
Frizon, Alexandre G. Dal Bó, Eduardo Zapp, Raphael Marra, Hugo Gallardo, André A.
Vieira
PII:
S0143-7208(18)30289-4
10.1016/j.dyepig.2018.04.049
DYPI 6710
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 6 February 2018
Revised Date: 20 April 2018
Accepted Date: 25 April 2018
Please cite this article as: Regis E, Aguiar LdO, Tuzimoto P, Girotto E, Frizon TE, Dal Bó AG, Zapp E,
Marra R, Gallardo H, Vieira AndréA, Effect of heteroatom exchange (S/Se) in the mesomorphism and
physical properties of benzochalcogenodiazole-based liquid crystals, Dyes and Pigments (2018), doi:
10.1016/j.dyepig.2018.04.049.
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ACCEPTED MANUSCRIPT
Effect of heteroatom exchange (S/Se) in the mesomorphism
and physical properties of benzochalcogenodiazole-based
liquid crystals
Abstract
In order to investigate the effect on the structure-property relationship by the
exchange of sulfur atom by selenium, a new series of fluorescent calamitic
liquid crystals based on 4,7-disubstituted-2,1,3-benzoselenadiazole was
successfully synthesized and characterized. The mesomorphic properties were
studied by differential scanning calorimetry, polarized optical microscopy and X-
ray diffraction. Thermal properties have evidenced the influence of changing
sulfur by selenium atom in the formation of liquid crystalline phases and
mesomorphism range. Photophysical studies were performed in chloroform
solution, where these compounds displayed strong orange luminescent under
UV light, with relevant quantum yields (ΦF = 0.14–0.16) and greater
solvatochromic dependence. Electrochemical studies showed oxidation and
reduction patterns similar to 2,1,3-benzothiadiazoles analogues, however
presenting lower band gap.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Effect of heteroatom exchange (S/Se) in
the mesomorphism and physical
properties of benzochalcogenodiazole-
based liquid crystals
Elias Regisa, Leonardo de O. Aguiara, Patricia Tuzimotob, Edivandro Girottoc, Tiago
E. Frizond, Alexandre G. Dal Bóe, Eduardo Zappf, Raphael Marrad, Hugo Gallardoc
and André A. Vieira*a
a Universidade Federal da Bahia,40170-115, Salvador, Bahia, Brazil.
b Instituto Federal do Paraná, Campus Pinhais, 83330-200, Pinhais, Paraná, Brazil.
c Universidade Federal de Santa Catarina, 88040-900, Florianópolis, SC, Brazil.
d Universidade Federal de Santa Catarina,88905-120,Campus Araranguá,SC, Brazil.
e Universidade do Extremo Sul Catarinense,88806-000, Criciúma, SC, Brazil.
f Universidade Federal de Santa Catarina,89065-300, Campus Blumenau,SC, Brazil.
Keywords: 2,1,3-benzoselenadiazole, heterocycles, luminescence, liquid crystals.
*email: vieira.andre@ufba.br

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1. Introduction
Organic molecules able to exhibit liquid crystalline phases are of great interest
for many areas of applied science, for instance, as organic light-emitting diodes
(OLEDs), display technology (LCD), organic field effect transistors (OFETs), organic
photovoltaic devices (OPVs), anisotropic ion transportation, and photo- and
semiconducting materials.[1–3] Among these liquid crystalline behavior, molecules
exhibiting smectic (Sm) and columnar phases are good candidates for applications
as charge carrier mobility due to their arrangement in the mesophase.[4] In this way,
the rational molecular design of new thermotropic liquid crystals for applications in
advanced functional materials involves the selection of an appropriate central core,
linking and peripherical group. Over several years, a large number of LC compounds
containing heterocyclic central units have been synthesized. Heterocycles rings such
as oxadiazoles [5–8], thiadiazoles [9,10], triazoles [11,12], pyrazoles [13,14] and
isoxazoles [15] have been much investigated. Beyond these heterocycles, our group
has experience in the synthesis and study of fluorescent thermotropic liquid crystals
π-conjugated based on the 2,1,3-benzothiadiazole (BTD).[16–18]
In recent years, comparison of the effects caused by the presence of S or Se
as heteroatom is becoming common for polymer and fluorophores[19–21], but
extremely rare for LCs work. In terms of molecular design, selenium atom has larger
size, less electronegativity, and is softer and more easily polarized than sulfur, what
could have an important influence on the thermal, optical and electrochemical
properties of selenium derivatives. [4,22]
Mindful of the potential of benzochalcogendiazole, another interesting
heterocycle is the 2,1,3-benzoselenadiazole (BSe). The BSe is similar to BTD with

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respect to strong and stable fluorescence, good thermal stability and good electron-
withdrawing ability.[23]
Herein, we describe the synthesis, characterization and study of thermotropic
liquid crystals based on the heterocycle BSe with different alkoxy chains,
rationalizing the impact of the exchange in the chemical and physical properties of
these liquid crystals when compared to published data from BTD.[24]
2. Results and discussion
2.1. Synthesis
The synthetic route followed for the preparation of the final compounds based
on 2,1,3-benzoselenadiazole 5a–d is presented in Scheme 1. The synthesis follows
the same procedure already adopted in ours previous works.[16,24] It starts from the
condensation of o-phenylenediamine with thionyl chloride and bromination of the
heterocycle to obtain the 4,7-dibromo-2,1,3-benzothiadiazole (2). The Sonogashira
coupling reaction between the aryl dibromide 2 and two equivalents of terminal aryl
acetylenes 3a–d allowed to obtain the intermediates molecules 4a–d. The 2,1,3-
benzothiadiazole molecules underwent procedures for sulfur extrusion with sodium
borohydride and catalytic cobalt chloride in mixture of ethanol/tetrahydrofuran (1:1)
to obtain the respective diamines in 62-88% yield. [25] In the literature, some works
report about the instability of similar diamines,[26,27] for this reason after isolation
and purification the last reaction was performed immediately without
characterization. The target compounds 5a-d were obtained by cyclization of the
diamines intermediates with selenium oxide in a mixture of ethanol/THF as solvent
under heating. After purification by chromatograph column, the desired final
compounds were obtained in good yields (52–88%). All final compounds 5a–d

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1
synthesized were characterized by H and 13C NMR, mass spectrometry and IR
spectroscopy.
<< Scheme 1 >>
Scheme 1. Synthesis of the final compounds 5a-d.
2.2. Liquid crystalline behavior
The mesomorphic behavior of compounds 5a-d was characterized and studied
by differential scanning calorimetry (DSC) and polarized optical microscopy (POM).
The transition temperatures and enthalpy values for the final compounds are
summarized in Table 1. The liquid crystalline properties of final compounds were
confirmed by X-ray diffraction (XRD). The investigation of the thermal stability of the
target molecules was performed by thermogravimetric analysis (TGA), which
indicated high thermal stability, with decomposition temperatures above 401 °C,
under nitrogen atmosphere. The thermal stability of the benzoselenadiazole
compounds 5a-d is clearly lower than the corresponding benzothiadiazole 4a-d.
Table 1. Phase transition temperatures (°C) and enthalpies (kJ mol−1) for
compounds 4a-d and 5a-d.
Compound
Transition
T/°C, heating
T/°C, cooling
Tdec. /°C b
(∆H/kJ mol-1) a (∆H/kJ mol-1) a
Cr-N
N-I
141 (91.8)
176 (3.5)
133 (- 90.7)
175 (- 3.6)
448
409
4a
5a
CrI-CrII
CrII-SmC
SmC-I
28 (5.3)
155 (12.5)
189 (1.8)
11 (-6.2)
146 (-14.4)
186 (-0.7)
Cr-N (Cr-SmC)d
(N-SmC)d
N-I
128 (63.2)
---
157 (2.6)
114 (-65.9)
130 (-5.0)
157 (-3.1)
468
4b

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CrI-CrII
CrII-SmC
SmC-I
57 (25.3)
147 (17.1)
200 (10.2)
39 (-20.3)
134 (-19.9)
196 (-8.4)
415
481
5b
4c
CrI-CrII
CrII-SmC
SmC-N
N-I
---
105 (-9.8)
113 (-50.4)
142 (-4.4)
151 (-4.7)
119 (65.6)
143 (4.7)
152 (3.5)
CrI-CrII
CrII-SmC
SmC-I
77 (22.8)
134 (18.4)
191 (3.5)
60 (-18.3)
119 (-15.1)
187 (-3.6)
401
445
405
5c
4d
5d
Cr-SmC
SmC-N
N-I
96 (81.6)
115 (39.6)
146 (11.8)
77 (-58.3)
108 (-30.6)
142 (-11.7)
CrI-CrII
CrII-SmC
SmC-I
88 (37.5)
133 (31.2)
190 (15.5)
63 (-28.9)
108 (-33.8)
188 (-12.7)
a Determined by POM and DSC measurements (10 ºC/min) on second heating cycle. b Determined by
TGA, onset of decomposition in nitrogen (20 ºC/min). c Cr = Crystal, N = Nematic, SmC = Smectic C, I
= Isotropic liquid. d Observed on cooling cycle.
All final compounds 5a-d exhibited liquid crystalline behavior presenting
exclusively smectic mesophases. The textures observed by POM on cooling from
the isotropic liquid were identified as smectic phase, what is consistent with the
molecular anisometry of these compounds. On the cooling cycle it is possible to
observe the growth from the isotropic melt of elongated gems or smectic bâtonnets,
revealing the transition Isotropic-smectic.[28] It is important to emphasize that all the
intermediate molecules 4a-d also present mesomorphic behavior however displaying
nematic and smetic phases.[24]
The presence of the smectic C (SmC) phase was confirmed from the
microscopic observation of the characteristic striated texture, followed by the
appearance of a broken fan-shaped texture. Two representative photomicrographs
of the natural texture for the compounds 5c and 5d are presented in Figure 1. A
characteristic schlieren texture typical of SmC phase it is observed at 186ºC for 5c
(Fig. 1a) and a broken fan-shaped texture for 5d at 158ºC (Fig. 1b). Interestingly

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different from benzothiadiazoles 4a-d, the formation of nematic mesophases for
benzoselenadiazole-based molecules has not been observed, demonstrating the
impact of the selenium atom on such systems. The exchange of sulfur by selenium
favored a different molecular dipole moment and lateral interactions, giving rise only
to phases presenting besides orientational also positional order. This could be an
indicative that N···Se interactions are possibly occurring, as observed in previous
works. [29–31]
<< Figure 1 >>
Figure 1. Polarized optical photomicrographs of compounds 5c at 186 ºC (left) and
5d at 158 ºC (right) on cooling cycle.
In order to study the effects of length of alkoxy chains on the mesomorphic
behavior of the final materials 5a-d, different alkoxy chains were connected to the
4,7-diphenylethynyl-2,1,3-benzoselenadiazole core. The final molecules showed
dependence on the length of the peripheral groups to the stability of liquid crystalline
phases, where this behavior could be attributed to an increase in the length-to-width
ratio of the molecule. Compounds 5a, 5b, 5c and 5d confirm this trend with alkyl
chains of 8, 10, 12 and 14 carbon atoms, respectively. All final molecules 5a-d
exhibited enantiotropic SmC phases associated with higher SmC-I transition
temperatures. The SmC-I temperatures transition did not show great variation with
the increase in the length of the alkoxy chain, remaining close to 189-200 ºC. For the
benzothiadiazole compounds (4a-d), a marked decrease in the clearing point was
observed with the elongation of the molecules. On the other hand, the increase in
alkoxy chain lengths resulted in lower temperatures for the SmC-Cr transitions, e.g.
the crystallization for 5a occurs at 146 °C while for 5d at 108 °C. The increased

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anisometry leads to a decrease in the crystal-mesophase transition temperature,
thus allowing a wide mesomorphic range. The compound 5d exhibited a broad-range
liquid crystalline behavior from 188°C to 108°C when the crystallization occurs.
These results show that the variation of the alkyl chains from 8 to 14 carbons can
reduce by 38°C the crystallization temperature, as well as increase the mesomorphic
range. The measured enthalpies associated with the corresponding transitions are
consistent with those reported for the analogous benzothiadiazoles. A bar graph
comparative of the mesophase ranges from the target compounds 5a-d to 4a-d is
shown in the Figure 2.
<< Figure 2 >>
Figure 2. Bar graph presenting the mesomorphic behavior of compounds 4a–d and
5a-d on cooling cycle.
XRD studies
In order to analyze and confirm the structure of the liquid crystalline behavior
of the benzoselenadiazole compounds 5a and 5b have been further analyzed via
XRD. Figure 3 shows the diffractogram patterns of compound 5b. Table 2
summarizes the complete set of parameters obtained for both compounds.
<< Figure 3 >>
Figure 3. XRD patterns of compound 5b on cooling cycle, 155 ºC.

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In the scattering pattern of the compounds, an intense diffraction peak is
observed respectively at q = 0.17 Å-1 and q = 0.16 Å-1 for 5a and 5b. Additionally, in
all cases a weaker secondary maxima was observed (q = 0.34 Å-1 and q = 0.31 Å-1).
These reflections clearly suggest a layer ordering typical of a lamellar organization
characteristic of smectic mesophases. Therefore, the evidenced peaks are assigned
to first- and second-order Bragg reflections and the corresponding layer spacings
can be calculated as:
π
q
2
(1)
=
d
The results are given in Table 2.
Table 2 Structural parameters of compounds 5a and 5b.
Compound L (Å) Mesophase q-spacing (Å-1) d-spacing (Å-1)
5a
5b
37.8
42.8
Smectic C
Smectic C
0.17
0.16
36.9
39.2
Observing the XRD diffractograms for compounds 4b (ESI)[24] and 5b (figure
3), in order to perform a comparison of the two series by taking these compounds as
examples, it is possible to notice the presence of a similar pattern on the Bragg’s
reflection peaks, that was characterized for both heteroatoms as a smectic phase.
Comparing the d-spacing with the molecular length L = 37.8 Å (estimated by
ChemBio3D Ultra Software, version 11.0.1, PerkinElmerInformatics. Inc., Waltham,
MA, USA), the ratio d-spacing/L = 0.97 for 5a and 0,91 for 5b indicates that the
aliphatic chains are slightly interdigitated or that the molecules are almost in the most
extended configuration, which is not very common for this kind of molecules. At first,

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this study was suggesting a smectic A phase, but analyzing the POM textures from
compounds 5a-d, it was noticed textures from smectic C phases, that are tilted within
the layers. In this case, d-spacing would not reflect the molecular length but the
interlayer distance from the arrangement in the mesophase.
As previously reported [29–31], 2,1,3-benzoselenadiazoles could form dimers
through Se…N interactions. Estimating the most extended configuration for the dimer
of compound 5a (by ChemBio3D Ultra Software, version 11.0.1,
PerkinElmerInformatics. Inc., Waltham, MA, USA), the length would be close to 46.6
Å. Once smectic C phase is tilted and commonly the molecules adopt some degree
of conformational disorder in the mesophase, which reduces the effective molecular
length, d-spacing should be smaller than dimer length. In our case, if the molecules
adopt the dimer configuration, it would favor the formation of smectic phases, also
corroborating with higher melting and clearing points for compounds 5a-d due to the
lateral interactions caused by the exchange of sulfur for selenium atom.
2.3. Photophysical properties
2.3.1. UV-vis and Fluorescence
The photophysical properties of the final compounds 4a-e and 5a-d were
investigated in chloroform solutions at room temperature. The UV-vis absorption and
fluorescence spectroscopy data for solutions are summarized in Table 3.

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Table 3. Summary of photophysical properties of 4a-e and 5a-e.
Absorptionb
Emissionc Stokes shift
d
Compound
ΦPL
λmax (nm) (ε)a
437 (4.4 x 104)
437 (4.4 x 104)
438 (4.3 x 104)
438 (4.3 x 104)
466 (1.7 x 104)
466 (2.4 x 104)
466 (1.7 x 104)
468 (1.9 x 104)
λmax (nm)
(nm)
4a
4b
4c
4d
5a
5b
5c
5d
541
541
541
541
576
576
576
577
104
104
103
103
110
110
110
109
0.20
0.20
0.19
0.20
0.14
0.15
0.16
0.16
a
b
c
Units = L mol-1 cm-1. in chloroform solution (1.0 xˑ10−5 mol L-1). Excited at absorption maxima
band. d Quantum yields were determined with the quinine sulfate solution as reference (ΦPL = 0.55 in
0.5 mol L-1 H2SO4).
All materials displayed similar absorption patterns with intense absorption bands with
maxima at 466-468 nm. The absorption band at 466 nm is assigned to the π–π*
transition and this is characteristic of the benzoselenadiazole heterocycle due to the
high molar absorption coefficients measured (~ 2.0 x 10-4 mol cm-1). All the
compounds showed strong orange emission in solution (λFL = 500–700 nm), with
intense emission maxima at 576 nm. The absorbance and emission values for 5a-d
showed significant red shift relative to benzothiadiazoles (4a-d), the displacement
was of 30 and 35 nm, respectively, clearly demonstrating the influence of a single
atom (S to Se) on the optical properties.[32,33] Consequently, due to the
displacement presented the values of Stokes shifts were higher for 5a-d than 4a-d,
in the range of 109–110 nm, indicating an efficient intramolecular charge transfer
(ICT) in the excited state between the alkoxy group and the BSe unit.[34]
The fluorescence quantum yields (ФF) of 5a-d were determined in CHCl3 using
quinine sulfate as standard (Фr = 0.55 in 0.5 mol/L H2SO4). The ФF values could be

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calculated using the equation[35]: ФF = Фr (Fs/Fr)(Ar/As)(ηs/ηr)2, where s and r denote
the sample and reference, respectively, F is the integrated fluorescence intensity, A
is the absorbance at the excited wavelength, and η is the refractive index of the
solvent. The values are summarized in Table 3, where it is possible to notice that all
compounds presented relevant values of quantum yields (ФF) from 0.14 to 0.16. This
means, on average, quantum yield values 25% lower than the benzothiadiazoles 4a-
d.[24] The Se atom is heavier than S atom, which might exert a heavy atom effect
and result in a fluorescence quenching by increased probability of intersystem
crossing.[31,36]
2.3.2. Solvatochromism
Further, we have performed a solvatochromic study of 5b to ascertain the role
of solvent polarity on the photoluminescent behavior. The optical properties were
measured in heptane (Hep), toluene (Tol), chloroform (CHCl3), tetrahydrofuran
(THF), acetone (Acet), dimethylformamide (DMF) and acetonitrile (CH3CN) solutions.
The UV-vis and fluorescence spectra of 5b in various solvents are presented in
Figure 4 and the data have been listed in Table 4.

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Table 4. UV-visible absorption and emission data for compound 5b in solution of
different solvents.
b
ΦPL
Absorptionb Emissionc Stokes shift
Compound
λmax (nm) (ε)a λmax (nm)
(nm)
5b (Hept)
5b (Tol)
5b (CHCl3)
5b (THF)
5b (Acet)
461 (1.1 x 104)
462 (1.8 x 104)
468 (2.4 x 104)
460 (2.2 x 104)
455 (0.9 x 104)
529
550
576
556
564
68
88
108
96
0.79
0.35
0.15
0.36
0.20
0.18
0.09
109
5b (DMF)
5b (CH3CN)
457 (2.0 x 104)
569
562
112
108
454 (4.1 x 104)
a Units = L mol-1 cm-1. b [c] = (1.5 xˑ10−5 mol L-1). c Excited at absorption maxima. d Quantum yields
were determined with the quinine sulfate solution as reference (ΦPL = 0.55 in 0.5 mol L-1 H2SO4).
<< Figure 4 >>
Figure 4. Absorbance and emission normalized spectra of 5b in different solvents
(left). Natural emission spectra of 5d in different solvents (right).
The UV-vis absorption spectra of 5b in different solutions are similar, but slightly
solvent-dependent. Increasing solvent polarity, the absorption bands showed
blueshift, indicating a small solvatochromic effect in the ground state. In the
fluorescence spectra it was observed a significative hypsochromic shift in the
maximum emission in heptane, toluene and THF (less polar solvents) compared to
CHCl3. The largest hypsochromic shifts were presented for heptane and toluene, 47
and 26 nm, respectively. This type of optical behavior for apolar organic solvents
was similar that exhibited for the benzothiadiazole compounds. Using a more polar
solvent like dimethylformamide or acetonitrile a small bathochromic shift in the

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maxima emission was registered. In general, the increase of solvent polarity
resulting in a longer wavelength emission was observed. This positive polar
solvatochromism may indicate that the excited state is more polar than the ground
state and can be attributed to intramolecular charge transfer effect (ICT). This
phenomenon might be related to the inherent ICT in the D–A molecules.
The Figure 4 presents the emission spectra of 5d, and it is possible to note a
significant difference of the intensity in different solvents. The higher emission is in
heptane and decreases with increasing solvent polarity, the lower intensity for
acetonitrile. The enormous difference in the emission intensity is reflected in the
quantum yield values, ranging from 0.79 to heptane to 0.09 for acetonitrile. The
quantum yield values determined for 5b show the crucial role of the solvent in the
light-emitting process for these benzoselenadiazole-based molecules. This indicate a
greater solvatochromic dependence of 5b than compound 4b. For instance,
irradiating the solutions of 5d at 365 nm it is possible observe a color change from
red to green (Figure 5). A large Stokes shift was also observed with the variation of
the solvent (68–112 nm).
<< Figure 5 >>
Figure 5. Color change in emission of compound 5d in different solvents.
2.4. Electrochemical characterization
The cyclic voltammogram curves for 5a-d exhibited the distinct anodic and cathodic
behavior, the oxidation and reduction peaks were directly correlated to the electron
transfer at the HOMO (ionization potential - IP) and LUMO (electron affinity - EA),
respectively, and both data have been determined using a known method (EA =

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ꢄꢅꢆꢂꢇ
ꢄꢅꢆꢂꢇ
(ꢀ
) + 4.4 eV) and IP = (ꢀ
) + 4.4 eV.[37] The well-defined reduction at
ꢁꢂꢃ
ꢄꢈꢉ
negative potentials (values listed in Table 5) can be attributed to the reduction of the
benzoselenadiazole ring. In addition, at positive potentials (values listed in Table 5)
the compounds exhibit two irreversible oxidation peaks involving multiple
deelectronation processes. [38,39] In the series 5a-d the insertion of longer carbon
chains substituent does not cause significant change in the band gap of the
compounds. A reduction of the benzoselenadiazole ring was very similar to that
obtained with benzothiadiazole ring as presented by Frizon et al.[18], these values
are in agreement with the expected, results of the small difference in the
electronegativity between the sulfur and the selenium.[39] However, the substitution
of the chalcogenic element causes a facilitation in the electrochemical reduction of
the ring, confirming the values presented in Table 5, consequently the values of EA
is also reduced, so the bands gaps values are also lower for the benzoselenadiazole
than for the benzothiadiazole.
The both benzochalcogenadiazoles are typical examples of substitution of an atom
in order to improve the properties of the molecules.[40] The replacing of sulfur by
selenium atoms, the degree of the electron-deficient characteristic of the
benzothiadiazole unit decreases, resulting in stabilization of the HOMO and
destabilization of the LUMO [41–43]. It is possible to observe that the photoelectric
properties of the conjugated compounds can be easily tuned by the simple
incorporation of different electronically elements, in this study by the substitution of
the sulfur by the selenium in the heterocycle.[40]
All compounds exhibited both oxidation and reduction peaks, suggesting the
possibility of both hole and electron transport, which is a property required in the
construction of single layer electroluminescent devices, ie, it would not be necessary

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to employ different molecules in the electron-transporting and hole-transporting
layers of organic light emitting devices.[37] It was also possible to determine for all
compounds studied the electrochemical (ꢀꢊꢂꢋꢍ ꢌ) band gaps, which presented good
agreement with the values calculated for optical (ꢀꢊꢄꢋꢌꢌ) band gaps. Moreover, the
values are in the appropriate range (between 1.5 and 5.5 eV) for application in OLED
devices.
Table 5. Optical and electrochemical properties of compounds 5a-d, where Eox is the
onset potential of oxidation, Ered is the onset potential of reduction, Ip (HOMO) is the
ionization potential, Ea (LUMO) is the electron affinity, Eg is the band gap and λonset is
the absorption onsets wavelength.
Compounds
Parameters
5a
5b
5c
5d
1.02
Eox (V)a
Ered (V)a
1.20
1.12
0.92
-1.11
-5.66
-3.33
2.30
-1.19
-5.56
-3.25
2.31
-1.12
-5.36
-3.32
2.04
-1.16
-5.54
-3.28
2.27
IP (HOMO) (eV)b
EA (LUMO) (eV)c
ꢀꢊꢂꢋꢍ ꢌ (eV)
λonset (nm)
540.97
536.13
536.11
529.92
ꢀꢊꢄꢋꢌꢌ (eV)d
2.29
2.31
2.31
2.34
b
c
aEox/red (vs. NHE) = Eox/red (vs. Ag/Ag+) + 0.266; IP = -(Eox + 4,44) eV; EA = - (Ered + 4,44)
eV; dOptical bandgap calculated on the low energetic edge of the absorption spectrum (ꢀꢊꢄꢋꢌꢌ
= 1240/λonset
)
2.5. Morphological study
Atomic force microscopy (AFM) was used to investigate the morphological aspects
of spin-coated films of LC compounds 5a-d. AFM images were obtained in dynamic

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mode using previously cleaned silicon plates as substrates. Figure 6 shows the AFM
image of compound 5b dissolved in chloroform and deposited onto silicon via the
spin-coating method at 100 rpm for 20 s. The obtained films presented good
homogeneity, as demonstrated by the AFM image of the film for compound 5b, with
a mean surface roughness of around 0.4 um. In addition, the films did not show the
formation of crystallized domains getting good thin films. This is an important
property for future technological applications.
<< Figure 6 >>
Figure 6. AFM images showing the surface morphology of solid films of compound
5b.
3. Conclusions
In summary, a series of new fluorescent liquid crystal derivative from the heterocycle
2,1,3-benzoselenadiazole was synthesized and characterized. The thermal,
photophysical and electrochemistry properties were studied. The compounds 5a-d
displayed exclusively smectic C phases with characteristic broken fan-shaped
texture. The mesophase range varied according to the length of the alkyl chains,
where for 5d was observed a broader mesomorphism range (∆TSmC = 60). All
compounds exhibited similar absorption patterns with intense absorption bands at
466-468 nm and fluorescence in the region of red in 576-577 nm with large Stoke’s
shifts (109–110 nm) and moderate fluorescence quantum yields (ΦF = 0.14–0.16).
The solvatochromism investigated for 5b displayed highly dependence of the

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emission with solvent whose variation presented was from 529 to 576 nm. The cyclic
voltammogram curves for benzoselenadiazoles was similar to the benzothiadiazoles,
but the bands gaps values are lower.
4. Experimental
4.1. General
Infrared spectra were recorded on a Bruker VERTEX model 70/70V
1
spectrometer in KBr discs or films. H NMR spectra were obtained with a Varian
Mercury Plus 400-MHz instrument using tetramethylsilane (TMS) as the internal
standard. 13C NMR spectra were recorded on a Varian Mercury Plus 100-MHz
spectrometer. The melting points and mesomorphic textures were determined using
an Olympus BX50 microscope equipped with a Mettler Toledo FP-82 hot stage and a
PM-30 exposure control unit.
Thermal transitions and enthalpies were determined by differential scanning
calorimetry (DSC) using the DSC-Q 2000 calorimeter. Thermogravimetric analysis
(TGA) was carried out using a Shimadzu analyzer with the TGA-50 module.
The SAXS experiments were performed on the SAXS1 beamline at the
Brazilian Synchrotron Light Laboratory (LNLS - Campinas, SP, Brazil). The samples
were sandwiched between two Kapton® foils and loaded to a temperature-controlled
sample holder.[44] The collimated beam crossed the samples through an evacuated
flight tube and was scattered to a 2D CCD detector with an active area of 16 cm2.
The 2D scattering patterns were collected after an exposure time of 20 s. To cover
the desired q range (from 0.10 to 0.45 Å-1), where n ~ 1 for the X-rays, the sample-
to-detector distance was set to 501.2 mm (silver behenate was used as standard).
The 2D images were found to be isotropic and they were corrected by taking into

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account the detector dark noise and normalized by the sample transmission using a
360° azimuthal scan. This procedure was performed using the FIT2D software
developed by A. Hammersley.
AFM was used to investigate the morphology of the spin-coated films of
compounds 5a-d. The AFM measurements were carried out using a Shimadzu SPM-
9700 instrument with dynamic mode scanning and were performed at UNESC.
The relative fluorescence quantum yields (ФPL) were determined according to a
published method.[35] A Varian's Cary 50 UV-vis spectrophotometer was used to
record absorption spectra. Fluorescence spectra were recorded on a Hitachi
fluorescence spectrophotometer (Model F-7000).
The electrochemical measurements were performed on a Dropsens Stat 400
BiPotenciostat/Galvanostat. The cyclic voltammetry experiments were performed
using ca. 0.1 mg/mL solutions of benzoselenadiazole-derived in 0.1 M tetra-n-butyl
ammonium hexafluorophosphate (TBAPF6) in dichloromethane as the supporting
electrolyte, and the ferrocene/ferricenium (Fc/Fc+) redox couple was employed as
the internal reference. The experiments were performed using a standard three-
electrode cell with a circular glassy carbon electrode, a Pt-wire counter electrode and
Ag/Ag+ (AgNO3 0.01 M in acetonitrile) as the reference electrode. The
measurements were performed in an electrolyte solution previously purged with
purified nitrogen gas. The UV-vis absorption spectra were performed on a Shimadzu
UV-1800 spectrophotometer.
4.2. Materials
All the reagents were obtained from commercial sources and used without
further purification. The preparation of the benzo[2,1,3]thiadiazole (1), ,7-
dibromobenzo[2,1,3]thiadiazole (2), aryl acetylenes 3a–d and benzothiadiazoles

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compounds 4a-d were prepared according to published procedures.[24,45] The
organic solvents were of commercial grade quality except THF (HPLC grade) and all
were purified by traditional methods. In general, all the compounds were purified by
column chromatography on silica gel (60–120 mesh), and crystallization from
analytical grade solvents. The purity of the sample was confirmed by thin-layer
chromatography (Merck Kieselgel 60F254).
General procedure for sulfur extrusion reaction
2,1,3-benzothiadiazoles 4a-d (0,5 mmol), tetrahydrofuran (30 mL) and ethanol (30
mL) were placed into a 125 mL flask. The solution was cooled down using an ice
bath and NaBH4 (5 mmol) was added in small portions. Finally, CoCl2.6H2O (1 mol
%) was added. The reaction is followed by the release of H2S gas. The mixture was
stirred for 8 h at room temperature and then, filtered to separate the black solid. The
solvent was evaporated, water (50 mL) was added and the organic product was
extracted with CH2Cl2 (3 x 30 mL). The combined organic extracts were dried over
MgSO4 and the solvent removed, resulting in a yellow solid. The product was purified
by column chromatography on alumina (eluent hexane/dichloromethane (1:1)) to
obtain the diamines. The diamines were used immediately after fast purification to
avoid the decomposition. Yield: 62-88%.
General procedure for 2,1,3-benzoselenadiazole cyclization
To a solution of the respective diamine (0,4 mmol) in ethanol-tetrahydrofuran (2:1)
(30 mL) at 60 °C was added a solution of selenium dioxide (66 mg, 0.8 mmol) in hot
water (2 mL). After 8 h, the suspension was evaporated and the residues were
extracted with CHCl3. The organic layer was washed with water and then brine, dried
over anhydrous MgSO4, and then evaporated in vacuum to dryness. The residues

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were purified by silica gel column chromatography with hexane/dichloromethane as
eluent to furnish compounds 5a-d as orange solids.
4,7-bis{[4-(octyloxy)phenyl]ethynyl}benzo[2,1,3]selenadiazole; Compound 5a.
Yield: 62 %. IR (KBr): 2954, 2921, 2855, 2207, 1602, 1565, 1507, 1473, 1302, 1282,
1244, 1170, 1109, 1024, 832 cm-1. 1H NMR (400 MHz, CDCl3) δ ppm: 7.65 (s, 2 H),
7.59 (d, 4H, J = 8.0 Hz), 6.90 (d, 4H, J = 8.0 Hz), 3.99 (t, 4H, OCH2-), 1.80 (m, 4 H,
OCH2CH2-), 1.46 (m, 4 H, -CH2-), 1.29 (m, 16 H, -CH2-), 0.89 (t, 6H, -CH3). 13C NMR
(100 MHz, CDCl3) δ ppm: 159.8, 159.4, 133.5, 132.3, 118.9, 114.6, 97.7, 84.8, 68.2,
31.8, 29.3, 29.2, 26.0, 22.6, 14.0. APPI-MS m/z = 641.2643 [M+H]+.
4,7-bis{[4-(decyloxy)phenyl]ethynyl}benzo[2,1,3]selenadiazole; Compound 5b.
Yield: 88 %. IR (KBr): 2978, 2927, 1435, 1391, 1354, 1223, 1162, 1139, 1025, 951,
893 cm-1. 1H NMR (400 MHz, CDCl3) δ ppm: 7.65 (s, 2 H), 7.58 (d, 4H, J = 8.0 Hz),
6.89 (d, 4H, J = 8.0 Hz), 3.98 (t, 4H, J = 6.0 Hz, OCH2-), 1.79 (t, 4 H, OCH2CH2-),
13
1.28 (m, 28H, -CH2-), 0.89 (t, 6H, -CH3). C NMR (100 MHz, CDCl3) δ ppm: 159.8,
159.4, 133.5, 132.3, 118.9, 114.6, 114.5, 97.7, 84.8, 68.1, 31.9, 29.6, 29.4, 29.3,
29.2, 26.0, 22.7, 14.1. APPI-MS m/z = 697.3275 [M+H]+.
4,7-bis{[4-(dodecyloxy)phenyl]ethynyl}benzo[2,1,3]selenadiazole; Compound 5c
Yield: 52 %. IR (KBr): 2954, 2918, 2850, 1600, 1507, 1474, 1462, 1249, 1109, 1023,
839 cm-1. 1H NMR (400 MHz, CDCl3) δ ppm: 7.65 (s, 2 H), 7.60 (d, 4H, J = 8.0 Hz),
6.91 (d, 4H, J = 8.0 Hz), 3.99 (t, 4H, J = 6.0 Hz, OCH2-), 1.81 (m, 4 H), 1.28 (m, 36H,
13
-CH2-), 0.90 (t, 6H, -CH3). C NMR (100 MHz, CDCl3) δ ppm: 159.8, 159.4, 133.5,
132.3, 119.0, 114.6, 114.5, 97.7, 84.8, 68.2, 31.9, 29.6, 29.5, 29.4, 29.3, 29.2, 26.0,
22.7, 14.1. APPI-MS m/z = 753.3901 [M+H]+.
4,7-bis{[4-(tetradecyloxy)phenyl]ethynyl}benzo[2,1,3]selenadiazole; Compound 5d

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Yield: 74 %. IR (KBr): 2955, 2917, 2850, 2207, 1603, 1507, 1474, 1462, 1281, 1244,
1170, 1022, 833 cm-1. 1H NMR (400 MHz, CDCl3) δ ppm: 7.65 (s, 2 H), 7.59 (d, 4H, J
= 8.0 Hz), 6.91 (d, 4H, J = 8.0 Hz), 4.00 (t, 4H, J = 6.0 Hz, OCH2-), 1.81 (t, 4 H,
13
OCH2CH2-), 1.47 (m, 4H, -CH2-), 1.27 (m, 40H, -CH2-), 0.89 (t, 6H, -CH3). C NMR
(100 MHz, CDCl3) δ ppm: 159.8, 159.4, 133.5, 132.3, 118.9, 114.6, 97.7, 84.7, 68.1,
31.9, 29.6, 29.3, 29.1, 26.0, 22.6, 14.1. APPI-MS m/z = 809.4519 [M+H]+.
5. Acknowledgments
We thank the following institutions for financial support of CNPq (MCTI/CNPq
Nº 14/2014 - Universal) and FAPESB (PIBIC). The authors are also thankful to
CEBIME-UFSC for the HRMS analysis. The LNLS is acknowledged for supplying the
SAXS beam time (proposal 20170066).
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