Luminescent 2,1,3-benzothiadiazole-based liquid crystalline compounds

Article abstract of DOI:10.1016/j.molstruc.2007.05.006

A series of fluorescent 2,1,3-benzothiadiazole-based liquid crystals bearing a π-extended aromatic portion through C{triple bond, long}C triple bond was synthesized and characterized. The X-ray structure of the rigid core was performed. The mesomorphism a

Full text of DOI:10.1016/j.molstruc.2007.05.006

Journal of Molecular Structure 875 (2008) 364–371
www.elsevier.com/locate/molstruc
Luminescent 2,1,3-benzothiadiazole-based liquid crystalline compounds
*
´
Andre A. Vieira, Rodrigo Cristiano, Adailton J. Bortoluzzi, Hugo Gallardo
Departamento de Quı´mica, Universidade Federal de Santa Catarina – UFSC, 88040-900 Floriano´polis, SC, Brazil
Received 4 April 2007; received in revised form 2 May 2007; accepted 7 May 2007
Available online 13 May 2007
Abstract
A series of fluorescent 2,1,3-benzothiadiazole-based liquid crystals bearing a p-extended aromatic portion through C„C triple bond
was synthesized and characterized. The X-ray structure of the rigid core was performed. The mesomorphism and optical profile were
evaluated. These compounds showed preferentially smectic C and nematic phases whose stability increase with the elongation of the
p-extended aromatic portion. UV–vis spectra in solution displayed similar absorption patterns peaking around 410 and 470 nm
(e ꢀ 6.0–9.0 · 104 molꢁ1 cmꢁ1). Compounds exhibited fluorescence from green to red in solution ðkmax ¼ 500–610 nmÞ, and show large
em
Stoke’s shifts (82–141 nm) and moderate fluorescence quantum yields (UF = 0.23–0.37). Only compound containing phenylpiperazine
units displayed very low quantum yield (UF = 0.10).
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: 2,1,3-Benzothidiazoles; Luminescent liquid crystals; Functional materials
1. Introduction
transporting and electroluminescence [9]. Molecular
p-extended 2,1,3-benzothiadiazole materials also showed
good fluorescence and adequate band gap values for use
in electroluminescent devices [10]. 2,1,3-Benzothiadiazole-
based compounds, with geometric anisotropy, have been
studied as highly dichroic fluorescent dyes dispersed in a
commercial nematic liquid crystal (host–guest) [11]. How-
ever, 2,1,3-benzothiadiazole derivatives exhibiting meso-
phases have been poorly reported, only reactive mesogens
[12] and polymer based liquid crystals [13] have been pre-
pared. In this context, we have considered the synthesis
and characterization of new molecular fluorescent liquid
crystals based on 4,7-disubstituted 2,1,3-benzothiadiazole.
In order to contribute to an understanding of the underly-
ing structure–property relationships, different aromatic
units were used, linked to the benzothiadiazole core
through a C„C triple bond: phenyl, phenyl with a lateral
nitro group, biphenyl, naphthyl, phenylpiperazine, and
phenylbenzoate. Triple C„C bonds were used as linking
groups in order to produce the highly p-polarizable and
conjugated moiety. Liquid crystals containing the C„C
triple bonds are very interesting materials for reasons such
as their large birefringence [14].
Liquid crystalline compounds have found widespread
use on a vast field of technological applications, from their
more common use in flat-panel displays [1] to their use as
soft materials through supramolecular assembly [2]. Liquid
crystals containing heterocycles such as 1,3,4-oxadiazole
[3], 1,2,3-triazole [4], benzoxazole [5], etc., have recently
been explored in the design and synthesis of novel molecular
functional materials, where liquid crystalline phases, geom-
etry, polarity, luminescence, and others properties may be
varied by introducing heteroatoms [6]. Furthermore, lumi-
nescent liquid crystals are of great interest [7], since their
self-organizing properties can be exploited to achieve line-
arly polarized electroluminescence for emissive displays [8].
4,7-Disubstituted 2,1,3-benzothiadiazole derivatives are
efficient fluorophores that are able to afford well-ordered
crystal structures. Main chain conjugated polymers con-
taining its unit have been shown to possess excellent charge
*
Corresponding author. Tel.: +55 48 37219544.
E-mail addresses: hugo@qmc.ufsc.br, hugo.gallardo@pq.cnpq.br (H.
Gallardo).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.05.006

A.A. Vieira et al. / Journal of Molecular Structure 875 (2008) 364–371
365
2. Experimental
2.3.1. 4,7-Bis(2-(4-(decyloxy)phenyl)ethynyl)-[2,1,3]-benzo-
thiadiazole (3a)
2.1. General
Yield: 86%. IR (KBr) mmax/cmꢁ1: 2922, 2860, 1606,
1
1498, 1244, 1162, 1015, 836. H NMR (CDCl3) d: 7.74 (s,
Infrared spectra were recorded on a Perkin-Elmer
model 283 spectrometer in KBr discs or films. H NMR
2 H), 7.59 (d, 4H, J = 8.0 Hz), 6.90 (d, 4H, J = 8.0 Hz),
3.99 (t, 4H), 1.80 (t, 4 H), 1.28 (br, 28H), 0.89 (t, 6H).
13C NMR (CDCl3) d: 160.1, 154.6, 134.2, 133.8, 132.3,
117.3, 114.8, 114.5, 98.0, 84.5, 68.4, 32.1, 29.8, 29.6, 29.6,
29.4, 26.2, 22.9, 14.4. Elemental analysis for C42H52N2O2S:
Calcd. C, 77.73; H, 8.08; N, 4.32; S, 4.94. Found: C, 77.65;
H, 8.38; N, 4.17; S, 5.27%.
1
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. Ele-
mental analyses were carried out using a Perkin-Elmer
model 2400 instrument. The melting points, thermal
transitions 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. DSC measurements were carried out using
Shimadzu equipment with a DSC-50 module. An HP
UV–vis model 8453 spectrophotometer was used to
record absorption spectra. Fluorescence spectra were
recorded on a Hitachi-F-4500 and quantum yields were
obtained according to published method [15]. The
absorption and fluorescence spectra in solution were per-
formed at 20 ꢁC.
2.3.2. 4,7-Bis(2-(4-(decyloxy)-3-nitrophenyl)ethynyl)-[2,1,3]-
benzothiadiazole (3b)
Yield: 43%. IR (KBr) mmax/cmꢁ1: 2921, 2848, 2207,
1612, 1529, 1461, 1351, 1280, 1157, 1075, 1014, 886, 815.
1H NMR (CDCl3) d: 8.12 (s, 2H), 7.78 (d, 4H,
J = 4.0 Hz), 7.09 (d, 2H, J = 9.0 Hz), 4.14 (t, 4 H), 1.85
(t, 4 H), 1.48 (t, 4 H), 1.27 (br, 24H), 0.86 (t, 6H). 13C
NMR (CDCl3) d: 154.1, 152.8, 139.6, 137.2, 132.4, 129.0,
116.8, 114.4, 114.4, 95.0, 85.6, 69.9, 31.8, 29.5, 29.4, 29.2,
29.1, 28.8, 25.7, 22.6, 14.0. Elemental analysis for
C42H50N4O6S: Calcd. C, 68.27; H, 6.82; N, 7.58; S, 4.34.
Found: C, 68.46; H, 7.18; N, 7.16; S, 3.99%.
2.2. Materials
2.3.3. 4,7-Bis-(4-decyloxybiphenyl-4-ylethynyl)-[2,1,3]-
benzothiadiazole (3c)
All the reagents were obtained from commercial
sources and used without further purification. Sonogash-
ira’s couplings were accomplished under Ar atmosphere.
The intermediate 4,7-dibromobenzothiadiazole (1) was
obtained from commercially available o-phenylenedi-
amine according to a procedure reported previously
[10a]. The terminal acetylenes were prepared according
to published procedures [16]. The organic solvents were
of commercial grade quality except THF (HPLC grade)
and all were dried by traditional methods. In general,
all the compounds were purified by column chromatog-
raphy on silica gel (60–120 mesh), and crystallization
from analytical grade solvents. The purity of the sample
was checked by thin-layer chromatography (Merck Kie-
selgel 60F254).
Yield: 67%. IR (KBr) mmax/cmꢁ1: 2923, 2857, 1603,
1496, 1390, 1252, 1022, 819, 521. 1H NMR (CDCl3) d:
7.81 (s, 2H), 7.72 (d, 4H, J = 8.0 Hz), 7.60 (d, 4H,
J = 8.4 Hz), 7.56 (d, 4H, J = 8.8 Hz), 6.99 (d, 4H,
J = 8.0 Hz), 4.0 (t, 4H), 1.81 (t, 4H), 1.27 (br, 28H), 0.88
(t, 6H). Elemental analysis for C54H60N2O2S: Calcd. C,
80.96; H, 7.55; N, 3.50; S, 4.00. Found: C, 80.25; H, 7.96;
N, 3.46; S, 4.06%.
2.3.4. 4,7-Bis(2-(6-(decyloxy)naphthalen-2-yl)ethynyl)-
[2,1,3]-benzothiadiazole (3d)
Yield: 46%. IR (KBr) mmax/cmꢁ1: 2919, 2851, 2196, 1610,
1463, 1385, 1259, 1214, 1165, 1018, 887, 842, 464. 1H NMR
(CDCl3) d: 8.13 (s, 2H), 7.82 (s, 2H), 7.76 (m, 4H), 7.67 (d,
2H, J = 8.8 Hz), 7.19 (d, J = 8.8 Hz, 2H), 7.12 (s, 2H),
4.09 (t, 4H), 1.86 (t, 4H) 1.28 (br, 28H), 0.88 (t, 6H). 13C
NMR (CDCl3) d: 158.2, 134.6, 132.3, 132.1, 129.5, 129.0,
128.3, 126.9, 119.9, 117.2, 106.6, 98.4, 85.1, 68.1, 31.9, 29.6,
29.4, 29.3, 29.1, 26.1, 22.7, 14.1. Elemental analysis for
C50H56N2O2S: Calcd. C, 80.17; H, 7.54; N, 3.74; S, 4.28.
Found: C, 79.73; H, 7.67; N, 3.89; S, 4.24%.
2.3. General procedure for Sonogashira’s coupling reactions
A
mixture of 4,7-dibromobenzothiadiazole (1)
(0.294 g, 1.0 mmol), PdCl2(PPh3)2 (70 mg, 0.1 mmol),
CuI (9.5 mg, 0.05 mmol), and triphenylphosphine
(26.2 mg, 0.1 mmol) in triethylamine/THF 9:1 (50 mL)
was stirred under reflux for 45 min. The respective termi-
nal arylacetylene (2a–g) (2.1 mmol) dissolved in 5 mL of
triethylamine was added dropwise. The reaction mixture
was refluxed for a further 4 h. Cooling at room temper-
ature the product was almost entirely precipitated and it
was filtered washing with triethylamine (40 mL). The
crude product was purified by column chromatography
on silica gel (eluant: hexane/CH2Cl2), to give the respec-
tive final compound.
2.3.5. 4,7-Bis(2-(4-(4-decylpiperazin-1-yl)phenyl)ethynyl)-
[2,1,3]-benzothiadiazole (3e)
Yield: 66% of a red powder. IR (KBr) mmax/cmꢁ1: 2921, 2846,
1
2197, 1597, 1511, 1449, 1378, 1237, 1170, 1000, 919, 815. H
NMR (CDCl3) d: 7.72 (s, 2H), 7.56 (d, 4H, J = 8.4 Hz), 6.90
(d, 4H, J = 8.8 Hz), 3.31 (t, 4H), 2.60 (t, 4H), 2.39 (t, 4H),
1.30 (br, 40H), 0.89 (t, 6H). 13C NMR (CDCl3) d: 154.6,
151.6, 133.4, 132.1, 117.3, 115.0, 112.4, 98.7, 84.6, 59.1, 53.3,

366
A.A. Vieira et al. / Journal of Molecular Structure 875 (2008) 364–371
˚
C22H12N2S, M = 336.40, monoclinic, P21/c, a = 12.077(3) A,
48.2, 32.1, 29.8, 29.6, 27.8, 27.1, 22.9, 14.4. Elemental analysis
for C50H68N6S: Calcd. C, 76.48; H, 8.73; N, 10.70; S, 4.08.
Found: C, 76.21; H, 8.43; N, 10.79; S, 4.12%.
˚
˚
b = 10.657(6) A, c = 13.450(2) A, b = 97.79(1)ꢁ, V = 1715.1
3
(11) A , Z = 4, Dcalc = 1.303 Mg/m3, l = 0.194 mmꢁ1
,
˚
F(000) = 696, unique 2957 (Rint = 0.0178), refined parame-
ters = 226, Goof (F2) = 1.027, R1[I > 2r(I)] = 0.0435,
wR2(all data) = 0.1225.
2.3.6. 4,7-Bis(4-(4-decyloxybenzoyloxy)-1-ethynylphenyl)-
[2,1,3]- benzothiadiazole (3f)
Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre under
supplementary publication number CCDC 635394. Copies
of the data can be obtained, free of charge, on application
to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK [fax:
+44(0) 1223 336033, e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk)].
Yield: 78%. IR (KBr) mmax/cmꢁ1: 2920, 2856, 1733,
1
1603, 1504, 1257, 1202, 1162, 1071, 1017, 836. H NMR
(CDCl3) d: 8.14 (d, 4H, J = 8.8 Hz), 7.80 (s, 2H) 7.72 (d,
4H, J = 8.4 Hz), 7.26 (d, 4H, J = 8.4 Hz), 6.97 (d, 4H,
J = 8.8 Hz) 4.04 (t, 6H), 1.82 (m, 4H), 1.28 (br, 28H),
0.88 (t, 6H). 13C NMR (CDCl3) d: 207.3, 164.8, 163.9,
154.6, 151.8, 133.5, 132.7, 122.4, 121.3, 120.2, 117.3,
114.6, 97.1, 85.6, 68.6, 32.1, 31.2, 29.8, 29.6, 29.3, 26.2,
22.9, 14.4. Elemental analysis for C56H60N2O6S: Calcd.
C, 75.65; H, 6.80; N, 3.15; S, 3.61. Found: C, 75.28; H,
6.93; N, 3.39; S, 4.11%.
3. Results and discussion
3.1. Synthesis
2.4. X-ray crystallographic analysis
The synthesis of benzothiadiazole derivatives are out-
lined in Scheme 1. 4,7-Dibromo-2,1,3-benzothiadiazole 1
was prepared by condensation of o-phenylenediamine with
thionyl chloride and subsequent selective 4,7-position bro-
mination using molecular bromine in hydrobromic acid.
Terminal aryl acetylenes 2a–f were synthesized from
respective aryl halides via palladium catalysed cross-cou-
pling (Sonogashira’s coupling) [19] with commercially
available 2-methyl-3-butyn-2-ol followed by protective
group elimination. The final compounds were then pre-
pared by Sonogashira’s coupling between aryl dibromide
central core 1 and two equivalents of the respective termi-
nal aryl acetylene 2a–f.
Suitable crystals of 4 were obtained through recrystalliza-
tion from acetonitrile followed by filtration of the crystals
and washing with cold acetonitrile. A plate shaped orange
crystal was selected for crystallographic analysis, which
was carried out on a CAD-4 Enraf-Nonius diffractometer
using graphite monochromated Mo Ka radiation
˚
(k = 0.71073 A), at room temperature. Cell parameters were
determined from 25 centered reflections in the h range of
4.97ꢁ–20.10ꢁ. Intensities (3091) were collected using the x-
2h scan technique with a h angle ranging from 1.65ꢁ to
25.07ꢁ. The structure was solved using direct methods with
SIR-97 [17] and refined by full-matrix least-squares proce-
dures on F2 using SHELXL-97 [18]. H atoms were added
at their calculated positions and included in the structure
factor calculations, with C–H distances and Ueq taken from
the default of the refinement program. Selected crystal data:
Due to the difficulty of obtaining good single crystals
from the final compounds, a benzothiadiazole derivative
without alkoxy terminal chains (Structure 4) has also been
prepared, to become possible analysis of this rigid central
core by single crystal X-ray diffraction.
Scheme 1. Synthetic route for final compounds 3a–f. Conditions: (a) SOCl2, reflux; (b) Br2, HBr/H2O reflux; (c) 2-methyl-3-butyn-2-ol, PdCl2(PPh3)2,
CuI, TPP, Me3N reflux; (d) NaOH, toluene reflux; (e) PdCl2(PPh3)2, CuI, TPP, Me3N, reflux.

A.A. Vieira et al. / Journal of Molecular Structure 875 (2008) 364–371
367
ecule, different aromatic moieties linked to the 4,7-posi-
tion of 2,1,3-benzothiadiazole core through a C„C
triple bond were studied. It becomes clear that SmC
and N phase stability increase with the elongation of
the p-extended aromatic portion. This behavior may
be attributed to: (i) an increase in the length-to-breadth
ratio of the molecule, considering that the 2,1,3-benzo-
thiadiazole core disrupts the perfect rodlike structure,
and (ii) an increase in polarizability due to extension
of p-conjugation. Compound 3f, with the most
extended phenyl benzoate unit, possess a wider liquid
crystalline phase range by DT = 191.2 ꢁC, as the com-
pound 3a, with phenyl unit, presents short mesophase
range by DT = 32.7 ꢁC.
All compounds synthesized were characterized and iden-
tified by 1H and 13C NMR and IR spectroscopy and found
to be analytically pure by elemental analyses, including X-
ray diffraction analysis of compound 4.
3.2. X-ray diffraction study of the p-conjugated portion
Orange single crystals of 4 suitable for X-ray analysis
were obtained by recrystallization from acetonitrile. The
crystal structure and selected bond lengths and angles of
4 are presented in Fig. 1.
Dihedral angles between benzothiadiazole ring (S1–
N9) and the two phenyl rings are slightly different
(10.0(1)ꢁ and 6.1(1)ꢁ for C12–C17 and C22–C27, respec-
tively). These values near to 0ꢁ are in agreement with a
planar structure, and some p-orbital overlapping must
be occurring. This is also evidenced by the bond dis-
tances of C7–C10 and C11–C12 (taking one of the arms
Compound 3b, with lateral nitro substituents, pre-
sented the shortest mesophase range by DT = 7.8 ꢁC,
and only SmA phase was observed. The SmA phase
was confirmed from optical microscopy observations,
cooling the isotropic liquid the phase separates out in
ˆ
the form of batonnets at 147.1 ꢁC which coalesce and
build up the characteristic focal-conic fan texture
(Fig. 3). Compounds 3a, 3c–f exhibited SmC and N
phases. Of these, compound 3c, with the biphenyl unit,
had the highest melting point (Tm = 196.7 ꢁC), and the
SmC phase range was wider than the N phase range.
Compound 3a exhibited the lowest melting point
(Tm = 126.2 ꢁC), and the N phase range was wider than
the SmC phase range, as was observed for compounds
3e and 3f. Upon cooling 3a, 3c–f samples, the Schlieren
nematic phase transitates slowly to a Schlieren SmC phase
with a clearly visible fingerprint region between the two
phases, characterizing a N–SmC transition. Further cool-
ing of these samples leads the Schlieren SmC to change
slowly to a fan-broken shaped SmC texture.
˚
˚
only) 1.422(3) A and 1.436(4) A, respectively, which are
˚
shorter than typical C–C r bonds (1.53 A). Intermolecu-
lar packing of this compound is based only on van der
Waals forces.
3.3. Liquid crystalline behavior
The transition temperatures and phase assignments of
3a–f were investigated by polarizing light microscopy and
differential scanning calorimetry (DSC). Results are sum-
marized in Table 1. All the studied materials exhibited
liquid crystalline phases, particularly SmC and N phases
typical of calamitic liquid crystals.
A bar graph illustrating the mesophase ranges is
shown in the Fig. 2. In order to understand the effects
of varying the p-extended aromatic portion on the
anisometry and mesomorphic behavior of the final mol-
Although compounds 3c, 3e, and 3f showed wide SmC
and N phase ranges, thermal decomposition started when
entry in isotropic liquid at 334, 300, and 328 ꢁC, respec-
tively. DSC thermogram for first heating of compound 3f
exemplify this profile (Fig. 4). The compound melts to
SmC phase at 136.6 ꢁC and transitates to N phase at
202.1 ꢁC. Further heating leads to a slowly decomposition
Fig. 1. Molecular structure of compound 4 with atom labeling scheme. Ellipsoids are drawn at 40% probability level. Selected bond distances and angles:
S1–N9 1.608(2), S1–N2 1.609(2), N2–C3 1.346(3), C3–C4 1.428(3), C3–C8 1.430(3), C4–C5 1.370(3), C4–C20 1.429(4), C5–C6 1.411(3), C6–C7 1.377(3),
C7–C10 1.422(4), C7–C8 1.429(3), C8–N9 1.344(3), C10–C11 1.195(3), C20–C21 1.194(3), N9–S1–N2 101.69(11), C3–N2–S1 105.72(17), N2–C3–C4
125.9(2), N2–C3–C8 113.4(2), C8–N9–S1 105.92(18), C11–C10–C7 177.7(3), C10–C11–C12 178.3(3).

368
A.A. Vieira et al. / Journal of Molecular Structure 875 (2008) 364–371
Table 1
Phase transitions and enthalpies (kJ/mol) of compounds 3a–fa
90.2 (3.00)
126.2 (24.2)
119.5 (-22.1)
130.6 (broad)
128.1 (-1.57)
143.0 (23.4)
112.2 (-23.4)
303.0 b
158.9 (2.13)
156.5 (-2.17)
150.8 (2.10)
147.1 (-1.80)
333.8 b, c
3a
3b
3c
3d
3e
Cr
Cr'
SmC
Cr
N
SmA
N
I
I
84.0 (-2.54)
99.6 (15.4)
83.8 (-10.8)
138.0 (5.20)
130.5 (-4.40)
115.9 (8.14)
96.0 (-8.70)
156.1 (4.00)
196.7 (30.6)
188.9 (-29.2)
176.8 (18.3)
171.8 (-16.5)
185.5 (20.2)
175.3 (-17.3)
136.6 (35.8)
102.8 (-28.3)
Cr
Cr
Cr'
Cr'
Cr"
Cr"
Cr
SmC
SmC
SmC
Dec.
I
302.4 b
226.5 (3.10)
220.2 (-2.55)
228.3 (0.90)
221.6 (-0.40)
202.1 (2.50)
196.8 (-1.61)
251.0 (2.70)
249.4 (-1.00)
300.0 b, c
N
N
Dec.
327.8 b, c
67.0 (23.5)
45.4 (-19.2)
3f
Cr
Cr'
SmC
N
Dec.
Cr, Cr0, Cr00, crystalline phases; SmA, smectic A phase; SmC, Smectic C phase; N, nematic phase; I, isotropic liquid; Dec., start decomposition.
a
Determined by DSC for second heating and cooling cycle (scan rate 10 ꢁC/min).
Determined by optical microscopy.
Also confirmed by TGA analysis, onset of decomposition in nitrogen, 10 ꢁC/min.
b
c
to their structural similarity. UV–vis spectra displayed sim-
ilar absorption patterns between 400 and 500 nm, with
max
abs
maximum absorption peak wavelengths ðk Þ around
410 and 470 nm. These absorption bands are assigned to
p–p* transitions due to their high molar absorption coeffi-
cients (e = 6.5–8.9 · 104 molꢁ1 cmꢁ1). Final compounds
3a–f
present
intense
fluorescence
in
solution
max
ðk
¼ 500–610 nmÞ, and show large Stoke’s shifts (82–
em
141 nm). These compounds exhibited fluorescence quan-
tum yields (UF) from 0.23 to 0.37 relative to the standard
compound 4 (UF = 0.37) [10a]. Compound 3e showed the
lowest quantum yield of UF = 0.10 (relative to the standard
rhodamine B, UF = 0.65). [11] The weak fluorescence of 3e
compared to the other compounds may be attributed
manly to excitation n-p* from the piperazine ring that
quenches the p–p* of the p-extended aromatic portion.
Compounds 3a, 3c, and 3d (with phenyl, biphenyl, and
naphthyl, respectively) show maximum absorption wave-
length peaks about 440 nm. Compound 3e (with phenyl-
piperazine) display maximum absorption wavelength
peak red-shifted of 29 nm compared with the average for
3a, 3c, and 3d. This could be occurring because of elec-
tronic effect which lowers the HOMO-LUMO band gap,
due to the presence of strong electron-donating substituent
like the -NR2 group (present in 3e). On the other hand,
compounds 3b and 3f show a blue-shift in the spectra of
20 nm from 440 nm, probably due to the electron-with-
drawing nitro group and an ester linkage that breaks con-
jugation, respectively. This similar trend was also observed
in the fluorescence spectra. Compound 3e gave the highest
emission peak wavelength (611 nm) with the largest Stoke’s
shift (141 nm). This indicates a very efficient intramolecular
Fig. 2. Bar graph showing the mesomorphic behavior of compounds 3a–f.
of the material, also observed by optical microscopy with
darkening of nematic phase. The temperature of decompo-
sition for these compounds was confirmed by thermogravi-
metric measurements (TGA).
3.4. UV–vis and fluorescence spectroscopy features
The UV–vis absorption and fluorescence spectra of ben-
zothiadiazole derivatives in chloroform solution are pre-
sented in Figs. 5 and 6, respectively. The spectroscopy
data are listed in Table 2. The absorption and fluorescence
spectra of compounds 4 and 3a–f are similar in shape due

A.A. Vieira et al. / Journal of Molecular Structure 875 (2008) 364–371
369
Fig. 3. Photomicrographs of (a) focal-conic fan texture of SmA phase shown by 3b at 134 ꢁC (33·) upon cooling; (b) transition from schlieren to broken
fan-shaped texture of SmC phase shown by 3d at 220 ꢁC, just below N–SmC transition temperature; (33·) upon cooling; (c) Photomicrograph of schlieren
texture N phase shown by 3a at 141 ꢁC (33·).
Fig. 5. Absorption spectra of compounds 3a–f and 4 in CHCl3 solution at
20 ꢁC.
Fig. 4. Differential scanning thermogram as a function of temperature for
first heating of compound 3f (scan rate 10 ꢁC).
the literature. [21] The optical band gap varies from
charge transfer in the excited state, a push–pull system [20]
between a strong electron-donating group (-NR2) and the
electron deficient benzothiadiazole core.
The optical band gaps (Eg) of final compounds were
determined by their corresponding absorption in thin films
(Supplementary data), using the same method applied in
2.37 eV in 3e to 2.74 eV in 3b (Table 2), being comparable
to the values presented in literature for similar structures
[10]. All final compounds also displayed fluorescence in
the solid state (Fig. 7). The spectra were taken from amour-
phous powder dispersed in a proper cell for solid phase
fluorescence spectroscopy.

370
A.A. Vieira et al. / Journal of Molecular Structure 875 (2008) 364–371
Most of the compounds showed a red-shift in the solid
phase spectrum compared with their emission wavelengths
in chloroform solution. Only compound 3c has not shown
significant change in the spectra. Compound 3e showed a
blue-shift of 15 nm in the solid phase spectrum compared
with their emission in solution.
4. Conclusion
In summary, a series of fluorescent 2,1,3-benzothiadiaz-
ole-based liquid crystalline compounds was synthesized
and characterized. The mesomorphic and photophysical
behavior were studied. These compounds displayed prefer-
entially smectic C and nematic phases. The range of meso-
phase depends on the elongation of the p-extended aromatic
max
abs
portion. They exhibited similar absorption patterns (k
410–470 nm) and fluorescence shifting from green to red
Fig. 6. Fluorescence spectra of compounds 3a–f and 4 in CHCl3 solution
at 20 ꢁC.
max
in solution ðk
¼ 500–610 nmÞ with large Stoke’s shifts
em
(82–141 nm) and moderate fluorescence quantum yields
(UF = 0.23–0.37), with exception of compound 3e that dis-
played a lower quantum yield (UF = 0.10).
Table 2
UV–vis absorption and fluorescence spectroscopy data of benzothiadiaz-
ole derivatives
Acknowledgments
Compound k /nma (e/104)b
k
/nma,c Stokes shift/nm UF Eogpt/eV
max
abs
max
em
d
The authors thank Dr. Frank Quina from Universidade
de Sao Paulo (USP) and Dr. Haidi Fiedler and Dr. Edson
˜
4
413 (3.4)
439 (6.5)
420 (7.4)
438 (8.0)
446 (7.5)
470 (7.9)
421 (8.9)
499
536
506
530
540
611
503
86
97
86
92
94
0.37 2.73
0.23 2.48
3a
3b
3c
3d
3e
3f
a
0.35 2.74
Minatti from Universidade Federal de Santa Catarina
(UFSC) for the free access to the photophysical equip-
ments. The authors also thank Dr. Brenno A. DaSilveira
Neto for the sample of compound 1 used in the preliminary
studies. This work was supported by Conselho Nacional de
f
0.28
–
0.23 2.45
0.10e 2.37
0.37 2.63
141
82
10ꢁ5 M in chloroform at 20 ꢁC.
´
´
Desenvolvimento Cientıfico e Tecnologico (CNPq, Brazil),
Units, molꢁ1 cmꢁ1
.
b
c
max
abs
FINEP (Brazil) and Merck (Germany).
Excitation wavelength was at k
Relative quantum yield of fluorescence, compound 4 as standard
.
d
(UF = 0.37) Ref. [10a].
Appendix A. Supplementary data
e
Determined using using rhodamine B in EtOH (c = 10ꢁ6 M) as the
standard. (UF = 0.65) [11].
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2007.05.006.
f
3c did not form film suitable for UV–vis analysis.
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