Hybrid photoluminescent materials containing a benzobisthiazole core for liquid crystal and gel applications

Article abstract of DOI:10.1039/c6sm02650h

Tetra- and hexacatenar amide compounds containing a linear centrosymmetric benzobisthiazole core were synthesized with good yields. These compounds were characterized and their structures confirmed by elemental analysis, and FT-IR, Maldi mass and NMR spectroscopy. All compounds exhibited excellent thermal stability up to 330 °C. The tetracatenar series containing a double substitution in the meta positions did not show mesomorphic behaviour, whereas the hexacatenar and tetracatenar series having a double substitution in the meta and para positions showed liquid crystal properties with optical textures typical of columnar mesophases corroborated by POM analysis. The mesomorphic properties were dependent on the length, number and position of alkoxy chains attached at the end of the rigid core. XRD studies of the hexacatenar series showed the hexagonal columnar structure of the mesophases. Photoluminescence properties in solution were observed in the visible region, with good quantum yields. In the solid state, these compounds behave as blue emitters and they are able to change colour with acid or base addition. The hexacatenar benzobisthiazole compound with an alkoxy chain of 14 carbons presented properties of a supergelator in chloroform, leading to the formation of a fluorescent organogel material with fluorescence emission in the blue region.

Full text of DOI:10.1039/c6sm02650h

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Jure Dobnikar et al.
Rational design of molecularly imprinted polymers
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Hybrid photoluminescent materials containing a benzobisthiazole
core, towards liquid crystal and gel applications
E. Díaz,a E. Elgueta,b S.A. Sanchez,c J. Barberá,d J. Vergara,a M. Parra,*a and M. Dahrouch*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Tetra- and hexacatenar amide compounds containing a linear centrosymmetric benzobisthiazole core were synthesized
with good yields. These compounds were characterized and their structures confirmed by elemental analysis, FT-IR, Maldi
mass and NMR spectroscopies. All compounds exhibited excellent thermal stability up to 330°C. The tetracatenar series
containing a double substitution in the meta positions did not show mesomorphic behaviour, whereas the hexatenar and
tetracatenar series having a double substitution in the meta and para positions showed liquid crystals properties with
optical textures typical of columnar mesophases corroborated by POM analysis. The mesomorphic properties were
dependent on the length, number and position of alkoxy chains attached at the end of the rigid core. XRD studies of the
hexacatenar series showed the hexagonal columnar structure of the mesophases. Photoluminescence properties in
solution were observed in the visible region, with good quantum yields. In solid state, these compounds behave as blue
emitters and they are able to change colour with acid or base addition. The hexacatenar benzobisthiazole compound with
an alkoxy chain of 14 carbons presented properties of a super gelator in chloroform, leading to the formation of a
fluorescent organogel material with fluorescent emission in the blue region.
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Introduction
devices as organic light-emitting diodes (OLEDs), lasers or
photovoltaic cells16-20
.
Among the different mesomorphic
The scientific interest in benzobisthiazole (BBZT)
structures, the most suitable mesogens with properties in electron
transport are those presenting a columnar phase21-30. Electron
transport in this kind of structure occurs due to the presence of a
rigid core, formed by an overlapping aromatic π system, leading to
the formation of molecular pathways in one dimension. Polycatenar
compounds21,22,26 belong to a class of liquid crystals, able to
generate different mesophases (nematic, smectic, cubic and
columnar systems). Their structure corresponds to a long rod-like
core ending in two half-disc-shaped moieties with two or more
aliphatic chains. In order to achieve columnar liquid crystals (CLC),
the structure of the polycatenar compounds should incorporate
two or three paraffinic side chains at each end, leading to a tetra-
and hexacatenar system respectively22. Recently12, we reported the
synthesis and the mesomorphic study of polycatenar columnar
liquid crystals which incorporate in the core, through ester bond, a
benzobisthiazole unit. These products showed to be highly
thermally stable within the temperature range in which they exhibit
mesomorphism and, due to the presence of a benzobisthiazole unit,
they present photoluminiscent properties in the visible region with
good quantum yields.
compounds is largely linked to their potential applications in the
fabrication of nonlinear optical devices, light-emitting diodes,
transistors equipment, photo-voltaic and solar cells1-7
. The
electronic delocalization enhanced by the presence of the
heterocycle and benzene ring in BBZT compounds can explain
properties like high fluorescence, electron transport and nonlinear
optics3,7-11. One of the major inconvenient in using BBZT compounds
is the synthesis protocol which required harsh conditions. However,
recently, BBZT compounds were obtained with good yields under
mild conditions4,12-15, and investigations based on their electronic
applications could be expected. The challenge for such
investigations is the design and the synthesis of BBZT compounds
able to form materials keeping or improving the electronic
properties of BBZT systems1.
Liquid crystal materials are known for their applications in
Gel materials are usually used in the pharmaceutical,
cosmetic and food sector due to their soft properties31-35. Although
it is not always possible to predict the ability of a compound to form
a gel, structural studies allow to establish some important required
factors33,34,36,37
.
One of these factors is the intermolecular
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interactions36,38, being the more important one the hydrogen bond, min-1 in the range 25–650 oC under a steady flow of nitrogen (42 mL
usually generated from carboxylic acid, hydroxyl or amide min-1). Transition temperatures and textures of mesophases were
functional groups. Another factor is linked to the presence of some determined by a polarised optical microscopy (POM) using an
specific chemical group32,36,39: (i) an aromatic central unit allowing Olympus BX51 optical microscope equipped with an Olympus U-
the stacking effects, (ii) alkoxy long chains for their role in the TV0.5XC-3 polariser and a Linkan T95-PE heat stage. Transition
solubility or precipitation in a particular solvent, and (iii) polar temperatures and enthalpies were investigated by DSC using a TA
groups allowing to generate dipole-dipole interactions. Nowadays, Instrument Q1000 calorimeter. Samples were encapsulated in
the gel science33,37,39 especially in organic chemistry, offers aluminium pans and observed at scanning rate of 10oC min−1 on
,
promising investigations in the field of electronics. Interesting heating and cooling. The instrument was calibrated using an indium
efforts have been focused on the preparation and study of organic standard (156.6 oC, 28.44 Jg−1) under nitrogen atmosphere. XRD
gelators exhibiting
π systems, which are able to form patterns were obtained with a pinhole camera (Anton-Paar)
supramolecular organizations with nano to microscale size and operating with a point-focused Ni-filtered Cu K beam. The samples
α
could lead to potential applications in electronic devices15,31,33,37,40
.
were held in Lindemann glass capillaries (0.9 mm diameter) and
The BBZT compounds are considered as π systems, able to generate heated with a high-temperature attachment. The patterns were
dipole-dipole interactions, and therefore their incorporation in collected on flat photographic film perpendicular to the X-ray beam
adequate organic molecules could lead to very promising and spacing was obtained via Bragg´s law. Ultraviolet-visible
compounds able to self-assembly in organic solvents as gel absorbance measurements were made with UV4-200 of ATI-Unicam
materials.
spectrometer of dual beam between 200 and 600 nm.
Luminescence measurements were obtained using a Perkin-Elmer
LS50B between 340 and 700 nm. Scanning electron microscopy
(TEM) was carried out using a JEOL 1200EXII instrument with an
activation voltage of 120 kV. Solid-state emission spectra were
acquired at room temperature by a Zeiss LSM 780 NLO spectral
microscope at the CMA-Bio Bio (Centro de Microscopía Avanzada
Bio Bio) belonging to the University of Concepción. A 405 nm
wavelength excitation laser and a 20× air objective were used in all
cases. The spectral image was created from 44 images obtained in
the emission range (412– 657 nm) every 5.6 nm. The methodology
used to acquire emission spectra under a microscope was already
The aim of this article is orientated to the study of polycatenar
compounds containing a linear centrosymmetric benzobisthiazole
group, able to form liquid crystal and gel materials. Based on our
previous results, a new synthetic approach to obtain such hybrid
materials was designed. The methodology involves the
incorporation of amide groups, known for favouring hydrogen
bond. The properties of the polycatenar compounds are usually
dependent of the numbers, positions and lengths of the side
chains21. These factors are considered very determinant as space-
filling requirements in order to obtain
a molecular packing
published29
. Briefly, after excitation, the emission light is
efficiency, especially for the formation of liquid crystal materials.
Consequently, in this article, three series of compounds were
considered and synthesised: series 2a-c (tetracatenar meta, para),
series 3a-c (tetracatenar meta, meta) and series 4a-c (hexacatenar
meta, para, meta) (scheme 2). The molecular structure of these
compounds was characterized by spectroscopic analyses and their
thermal behaviour was studied by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC). The optical properties
in solution and solid state were also analysed. Finally, the liquid
crystal/gel formation and their respective characterizations are
discussed, showing the important effect of the side chains on the
polycatenar compounds.
decomposed and its spectral components are detected by a spectral
detector. Forty four emission images are taken at pre-selected
wavelength and their superposition form a spectral image. From the
spectral image, the emission spectra can be reconstructed from
each pixel. The emission spectra composed by 44 points (obtained
from 4 positions in the image) were normalized and averaged. A
solution of fluorescein at pH 9 was used as standard for instrument
calibration and comparison of the data from cuvette and
microscope. The emission spectra of the standard solution acquired
in the cuvette and in the microscope show the maximum at 510 nm.
For sample preparation, thin film were deposited and dried on an
optical glass and covered with a second cover glass.
Experimental
Synthesis and Characterization of the di-amine compound 1
4-nitrobenzoic acid (1.62 g, 9.67 mmol) was dissolved in 60 mL of
dry dichloromethane and then oxalyl dichloride (3.07 g,24.2 mmol)
and a drop of dimethyl formamide (DMF) as catalyst were added.
The mixture was stirred at room temperature for 12 h under
nitrogen atmosphere. The solution was concentrated to remove the
excess of oxalyl dichloride, and the residue was dissolved in 40 mL
of dry N-methyl-2-pyrrolidone (NMP). Then, a solution of 2,5-
diamino-1,4-benzenedithiol dihydrochloride (1.13 g, 4.60 mmol)
and 150 mL of NMP was slowly added. The resulting mixture was
stirred for 48 h at 140 °C. The mixture was then cooled to room
temperature, poured into ice-water, and neutralized with 10%
NaOH(aq). The collected brown solid was washed with water, and
then washed twice with boiling ethanol to give the intermediate
Materials
All reagents were purchased from Aldrich Chemical Co or Merck,
and were used without further purification. All solvents were dried
under standard conditions.
Characterization techniques
1H NMR and 13C NMR spectra were recorded using a Bruker Avance
400 MHz spectrometer, with CDCl3 and DMSO-d6 as solvents and
tetramethylsilane (TMS) as an internal standard. FT-IR spectra were
recorded with
a
Nicolet Magna 550 spectrometer. MALDI
Bruker Microflex.
experiments were carried out on
a
Thermogravimetric (TG) measurements were conducted with a STA
625 system in platinum pans at prescribed heating rates of 10 oC
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product I in 85% yield as a yellow solid and characterized by FT-IR calculated: C 75.48, H 9.43, N 3.91, S 4.48; found: C 75.35, H 9.31, N
spectroscopy.
3.55, S 4.37.
FT-IR (KBr) cm-1: 1518 y 1343 cm-1 (NO2). Elemental analysis data Characterization of compound 3a. 69% yield. FT-IR (KBr) cm-1: 3317
(%)for C20H10N4O4S2, calculated: C 55.29, H 2.32, N 12.90, S 14.76; (N–H), 1652 (C=O). Elemental analysis data (%)for C74H102N4O6S2,
found: C 55.22, H 2.37, N 12.94, S 14.70.
calculated: C 73.59, H 8.51, N 4.64, S 5.31; found: C 73.52, H 8.55, N
4.60, S 5.27. MALDI-TOF-MS calculated for C74H102N4O6S2 [M+H]+
1207.7, found 1207.8.
Characterization of compound 3b. 71% yield. FT-IR (KBr) cm-1:3316
(N–H), 1653 (C=O).Elemental analysis data (%) for C82H118N4O6S2,
calculated: C 74.61, H 9.01, N 4.24, S 4.86; found: C 74.60, H 8.97, N
4.26, S 4.80. MALDI-TOF-MS calculated for C82H118N4O6S2 [M+H]+
1319.9, found 1319.9.
Characterization of compound 3c. 70% yield. FT-IR (KBr) cm-1:3320
(N–H), 1654 (C=O). Elemental analysis data (%) for C90H134N4O6S2,
calculated: C 75.48, H 9.43, N 3.91, S 4.48; found: C 75.39, H 9.45, N
3.97, S 4.46. MALDI-TOF-MS calculated for C90H134N4O6S2 [M+Na]+
1454.0, found 1454.1.
The intermediate compound I (1.66 g, 3.82 mmol) was dispersed in
100 mL of glacial acetic acid at 100 °C. Subsequently to this
dispersion, SnCl2xH2O (7.93 g, 38.2 mmol) dissolved in 16 mL of
concentrated hydrochloric acid was added. The mixture was
refluxed for 15 h, then it was diluted in water (600 mL) at 60 °C and
neutralized with 10% NaOH(aq) at that temperature. The collected
yellow solid was washed with water and then washed with
methanol (50 mL) to give 1.43 g of 1 corresponding to 92% yield.
FT-IR (KBr) cm-1: 3228 y 3341 (NH2). Elemental analysis data (%)for
C20H14N4S2, calculated: C 64.15, H 3.77, N 14.96, S 17.12; found: C
64.13, H 3.79, N 14.99, S 17.09. 1H-NMR (DMSO-d6) δ (ppm): 5.94 (s,
4H, NH2, proton (a)), 6.68 (AA'BB', 4H, J = 8.0 Hz, aromatic proton
(b)), 7.77 (AA'BB', 4H, J = 8.0 Hz, aromatic proton (c)), 8.51 (s, 2H,
aromatic proton (d)). 13C-NMR (DMSO-d6) δ (ppm): 113.7 (b), 114.2
(h), 120.1 (d), 128.9 (c), 133.1 (f), 151.4 (g), 152.4 (a), 168.4 (e)
Characterization of compound 4a. 74% yield. FT-IR (KBr) cm-1:3305
(N–H), 1655 (C=O). Elemental analysis data (%) for C94H142N4O8S2,
calculated: C 74.26, H 9.41, N 3.69, S 4.22; found: C 74.20, H 9.44, N
3.73, S 4.17. MALDI-TOF-MS calculated for C94H142N4O8S2 [M+Na]+
1
1543.0, found 1543.0. H-NMR (CDCl3) δ (ppm): 0.88 (m, 18H, –CH3
proton (a)), 1.19–1.79 (m, 48H, –CH2– proton (b)), 3.98 (m, 12H,
OCH2 proton (c)), 7.06 (s, 4H, aromatic proton (d)), 7.75 (AA'BB', 4H,
aromatic proton (f)), 8.03 (AA'BB', 4H, aromatic proton (g)) 8.15 (s,
2H, N–H proton (e)), 8.38 (s, 2H, aromatic proton (h)). 13C-NMR
(CDCl3) δ (ppm): 14.3 (a); 22.8, 26.3, 29.4, 29.5, 29.6, 29.7, 29.8,
29.9, 30.0, 30.5, 32.0, 32.1(b); 69.6 (c); 73.7 (d); 106.0 (g), 115.2 (q);
120.5 (k); 123.6 (l); 129.5 (h); 129.6 (m); 134.4 (o); 140.8 (e); 141.9 8
(j), 152.2 (p); 153.4 (f); 166.0 (i); 168.2 (n).
Synthesis and Characterization of the amide compounds
corresponding to series 2, 3 and 4
All amides of series 2a-c, 3a-c and 4a-c were prepared similarly
according to the procedure described in a general method.
General method: 3,4-di-n-, 3,5-di-n-, and/or 3,4,5-tri-n-
alkoxybenzoic acid (1.68 mmol) was dissolved in 40 mL of dry
dichloromethane and then oxalyl dichloride (0.32 g, 2.52 mmol)
with a drop of DMF as catalyst were added. The mixture was stirred
at room temperature for 12 h under nitrogen atmosphere. The
solution was concentrated to remove the excess of oxalyl dichloride
and the residue was dissolved in 30 mL of dry tetrahydrofuran
(THF). A solution of compound 1 (0.311 g, 8.27x10-1 mmol), 1.0 mL
of triethylamine (TEA) and 4-(N,N-dimethyl) aminopyridine (DMAP)
as catalyst, dissolved in NMP (40 mL) was added on the acid
chloride solution. The mixture was stirred at room temperature for
48 h under nitrogen atmosphere and then was poured into a
mixture of ice-water. The solid was filtered off, washed several
times with water and purified by column chromatography by
eluting with chloroform to yield pure 2a-c to 4a-c compounds as
paled yellow solids.
Characterization of compound 4b. 75% yield. FT-IR (KBr) cm-1:3306
(N–H), 1657 (C=O). Elemental analysis data (%) for C106H166N4O8S2,
calculated: C 75.40, H 9.91, N 3.32, S 3.80; found: C 75.36, H 9.95, N
3.37, S 3.77. MALDI-TOF-MS calculated for C106H166N4O8S2 [M+Na]+
1
1711.2, found 1711.1. H-NMR (CDCl3) δ (ppm): 0.88 (m, 18H, –CH3
proton (a)), 1.16–1.79 (m, 60H, –CH2– proton (b)), 4.02 (m, 12H,
OCH2 proton (c)), 7.07 (s, 4H, aromatic proton (d)), 7.79 (AA'BB', 4H,
aromatic proton (f)), 8.00 (s, 2H, N–H proton (e)), 8.08 (AA'BB', 4H,
aromatic proton (g)), 8.45 (s, 2H, aromatic proton (h)). 13C-NMR
(CDCl3) δ (ppm): 14.2 (a); 22.9, 26.2, 29.4, 29.5, 29.6, 29.7, 29.8,
29.9, 30.5, 32.1 (b); 69.7 (c); 73.8 (d); 106.0 (g); 115.3 (q); 120.3 (k);
123.7 (l); 128.7 (h); 129.6 (m); 134.5 (o); 140.9 (e); 141.8 (j); 152.3
(p); 153.5 (f); 165.9 (i); 168.3 (n).
Characterization of compound 4c. 75% yield. FT-IR (KBr) cm-1:3308
(N–H), 1657 (C=O). Elemental analysis data (%) for C118H190N4O8S2,
Characterization of compound 2a. 63% yield. FT-IR (KBr) cm-1: 3319 calculated: C 76.32, H 10.31, N 3.02, S 3.45; found: C 76.30, H 10.35,
(N–H), 1652 (C=O). Elemental analysis data (%) for C74H102N4O6S2,
N 3.11, S 3.40. MALDI-TOF-MS calculated for C118H190N4O8S2
calculated: C 73.59, H 8.51, N 4.64, S 5.31; found: C 73.42, H 8.59, N [M+Na]+ 1879.4, found 1879.6. 1H-NMR (CDCl3) δ (ppm): 0.89 (m,
4.82, S 5.25.
18H, –CH3 proton (a)), 1.16–1.79 (m, 72H, –CH2– proton (b)), 4.03
(m, 12H, OCH2 proton (c)), 7.07 (s, 4H, aromatic proton (d)), 7.79
(AA'BB', 4H, aromatic proton (f)), 7.97 (s, 2H, N–H proton (e)), 8.09
(AA'BB', 4H, aromatic proton (g)), 8.47 (s, 2H, aromatic proton (h)).
13C-NMR (CDCl3) δ (ppm): 14.2 (a); 22.8, 26.2, 29.3, 29.4, 29.5, 29.6,
29.7, 29.8, 30.6, 32.2 (b); 69.8 (c); 73.9 (d); 106.2 (g); 115.2 (q);
120.3 (k); 123.8 (l); 128.8 (h); 129.7 (m); 134.7 (o); 140.8 (e); 141.9
(j); 152.4 (p); 153.7 (f); 165.9 (i); 168.5 (n).
Characterization of compound 2b. 65% yield. FT-IR (KBr) cm-1: 3318
(N–H), 1653 (C=O). Elemental analysis data (%) for C82H118N4O6S2,
calculated: C 74.61, H 9.01, N 4.24, S 4.86; found: C 74.46, H 9.12, N
4.11, S 4.70.
Characterization of compound 2c. 66% yield. FT-IR (KBr) cm-1: 3321
(N–H), 1651 (C=O). Elemental analysis data (%) for C90H134N4O6S2,
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Gel Formation
corresponding to the protons of the central ring can be
3a-c and 4a-c were distinguished. The 13C NMR spectrum confirmed the structure of
The gel preparations of the products 2a-c
,
carried out using solvents as chloroform and tetrahydrofuran which precursor
1 exhibiting the expected signals corresponding to the
allowed dissolving the products. Concentrations of 5, 2.5, 1.5, 1 and eight magnetically non-equivalent carbon atoms (figure 1).
0.5% were prepared in glass vials. Once the product precipitated, Moreover, two strong bands at 3228 and 3341 cm-1 attributed to
the glass vial is inverted, in order to confirm the gel formation.
the NH2 groups were identified by FT-IR spectroscopy.
Scheme 1
Results and discussion
Synthesis description and spectroscopic characterizations
The synthesis of amide compounds incorporating
centrosymmetric benzobisthiazole unit has been carried out
starting from the di-amine precursor , obtained by a condensation
a linear
1
reaction between one equivalent of 2,5-diamino-1,4-benzenedithiol
dihydrochloride12 and two equivalents of 4-nitrobenzoyl chloride,
followed by a reduction reaction of the nitro functions of the
intermediate I, as indicated in scheme 1.
The precursor
confirmed by elemental analysis and spectroscopic methods. As
expected and seen in Figure 1, the 1H NMR spectrum of precursor
dissolved in DMSO-d6 showed four signals, corresponding to the
amine and aromatic protons. clear system AA'BB' can be
1 was obtained with good yields and its structure was
1
A
observed, attributed to the side aromatic rings, and a singlet signal
Figure 1. NMR spectra of precursor 1
: a) 1HNMR and b) 13C NMR
The amide compounds of series 2a-c
benzobisthiazole unit were synthesised by a condensation reaction employing triethylamine (TEA) and 4-(N,N-dimethyl)aminopyridine
(see scheme 2) between one equivalent of the precursor and two (DMAP) as base and catalyst respectively.
, 3a-c and 4a-c including the alkoxybenzoyl chloride in NMP and dichloromethane as solvents,
1
equivalents of the corresponding 3,4-di-, 3,5-di- or 3,4,5-tri-n-
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Scheme 2
1
The structure of the amides 2a-c to 4a-c was clearly evidenced by tetrahydrofuran and, hence, they were characterized by H and 13C
the absorption bands of the amide groups observed in FT-IR NMR. Figure 2 shows the 1H and 13C NMR spectra of 4a as a
spectra, and elemental analysis. Since 2a-c showed to be insoluble representative example of the corresponding series. Both NMR
in common organic solvents, their structural characterization was spectra confirmed the expected structure, showing the central
restricted to the techniques mentioned previously. In the case of benzobisthiazole ring linked to the amide functions. The system
the 3a-c series, showing partial solubility in common organic AA'BB' corresponding to the benzobisthiazole unit was observed by
solvents, additional characterisation corresponding to the 1H NMR. The proton integrations indicate an excellent correlation
determination of the molecular weight was achieved by Maldi mass among the proton proportions of each signal. In the same way, 13
C
spectrometry. On the other side, series of amides 4a-c were highly NMR showed the presence of all the expected carbon signals
.
soluble in solvents such as dichloromethane, chloroform and
Figure 2. NMR spectra of 4a: a) 1HNMR and b) 13C NMR.
Thermogravimetric studies
occluded. The three series exhibited high thermal stability up to 330
° C. As indicated in figure 3, their thermal decomposition occurred
in one step process. It starts in a temperature range of 330-370 °C
with a 1% weight loss and ends in a temperature range of 490-500
°C with a weight loss between 65-90%.
The amide series 2a-c to 4a-c were submitted to thermogravimetric
analysis under nitrogen. No apparent weight loss between 100 °C
and ~ 330-370 °C was observed, indicating that no solvent was
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Figure 3. Thermal decomposition profiles of 2a-c, 3a-c y 4a-c.
The temperature range of thermal decomposition for the amide
c decrease slightly as the chain length increases (303 ° C for 2a, 294
series 2 explains why the mesomorphic range and the ° C for 2a and 279 ° C for 2c). The same trend was observed for the
corresponding enthalpies of this series could not be determined by series 3a-c (228 °C for 3a, 218 °C for 3b and 179 °C for 3c) and 4a-c
DSC analysis (table 1). The ester analogues, previously reported12
,
(193 °C for 4a, 175 °C for 4b and 169 °C for 4c). The melting
present also similar temperature ranges of thermal decomposition, temperatures of the similar ester compounds reported previously12
showing that the amide groups do not really affect the thermal are much lower, due the higher polarity and intermolecular
stability in such compounds.
interactions (hydrogen bonding) present in the amide functions.
The clearing temperatures and mesomorphic ranges for the series
2a-c could not be determined, since they decompose before
reaching such temperature, in accordance with the results obtained
from TGA analysis. In the case of the hexacatenar 4a-c series, it is
possible to observe that the clearing temperatures decrease upon
lengthening the alkoxy chains with values of 213 °C for 4a, 211 °C
for 4b and 208 °C for 4c, whereas mesomorphic ranges increase
from 4a to 4c, with ranges of 20 °C for 4a, 36 °C for 4b and 39 °C for
Mesomorphic properties
The mesomorphic behavior of compounds 2a-c to 4a-c was studied
by polarized light optical microscopy (POM) and thermal analysis
(differential scanning calorimetry DSC). The temperatures and
enthalpies of phase transitions are summarized in Table 1, and
figure 4 presents the textures observed through POM. The 2a-c and
4a-c series exhibit mesomorphic properties, while the 3a-c series
shows only crystalline phases. Textures observed by POM for
compounds 2a-c, in the heating process, are consistent with the
presence of columnar mesomorphism (figure 4). In the case of the
4a-c series, their textures could be seen only after a friction process
applied on the samples because they present a homeotropic
orientation: as the columns are aligned in the direction of
propagation of the polarized light, no birefringence could be
observed in these mesophases, unless they were disturbed by a
friction process. As observed in the ester series12, the mesomorphic
behaviour seems to be dependent on the number, position and
length of the flexible side chains attached at the end of the central
rigid unit. The melting temperatures of the compounds in series 2a-
4c. Compared to the hexacatenar ester series reported previously12
,
the hexacatenar amide series showed higher clearing temperatures
and a broader mesomorphic range, due to the same reasons
mentioned previously about the characteristics of the amide
function. It is important to underline that, as stated in the case of
the tetracatenar meta, meta ester compounds, the tetracatenar
meta, meta amide compounds (3a-c) did not show mesomorphic
behaviour. With this result, we can suggest that
a double
substitution in the meta positions at the end benzene rings
disfavours the formation of mesophases, due to a lower packing
efficiency and unsatisfied space-filling requirements of the alkoxy
groups.
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Figure 4. Optical textures (magnification 200 X) observed under POM in the heating cycle for the series
2 and in the
cooling cycle after friction for the series : a) Colx phase at 310 °C of 2a, b) Colx phase at 305 °C of 2b, c) Colx phase at
4
295 °C of 2c, d) Colh phase at 172 °C of 4a, e) Colh phase at 175 °C of 4b, f) Colh phase at 183 °C of 4c
.
Table 1. Phase-transition temperatures (°C), enthalpies (KJ mol-1, in
parentheses) as determined by DSC, scanning rate 10 °C min-1 and
XRD studies
Tid (°C, temperature of initial decomposition) for compounds 2a–c
to 4a–c
.
Powder samples of compounds 4a, 4b and 4c were investigated by
Compound
Phase Transitions
Tid
X-ray diffraction in various sets of conditions. The mesomorphic
2a (n= 10)
range of compounds 2a-c lies well above the upper temperature
Cr 303 (-) Colx * (-) Dec.
363
limit (250 ºC) of our X-ray apparatus. The three compounds 4a
, 4b
and 4c were studied at room temperature after thermal
a
2b (n= 12)
2c (n= 14)
3a (n= 10)
Cr 294 (106.7) Colx * (-) Dec.
Cr 279 (89.8) Colx *(-) Dec.
358
356
treatment consisting of heating up to the isotropic liquid and
cooling down to room temperature. In addition to this, 4b and 4c
were initially studied at high temperatures above the transition
from the crystalline solid to the fluid mesophase.
Cr 228 (76.6) I
I 212 (77.7) Cr
349
360
357
355
362
356
In the case of 4a, the X-ray patterns, indicated in figure 5, were
unambiguously characteristic of a hexagonal columnar mesophase.
This is revealed by the presence of a set of three sharp maxima in
the low-angle region with reciprocal spacing in the ratio 1: √3: 2,
that correspond, respectively, to the (1 0), (1 1) and (2 0) reflections
from a 2D hexagonal lattice (Table 2). The measured spacing of the
3b (n= 12)
3c (n= 14)
4a (n= 10)
4b (n= 12)
4c (n= 14)
Cr 218 (62.3) I
I 198 (61.4) Cr
Cr 179 (40.4) I
reflections yields a hexagonal lattice constant
a = 43.2 Å. The
I 163 (39.8) Cr
mesomorphic nature of the Colh phase is confirmed by the presence
in the high-angle region of the broad, diffuse halo characteristic of
the molten hydrocarbon chains.
Cr 193 (24.7) Colh 213 (7.3)I
I 192 (0.9) Colh 141 (18.1) Cr
Cr 175 (32.8) Colh 211 (8.4) I
I 210 (4.9) Colh 142 (26.5) Cr
Cr 169 (27.3) Colh 208 (8.0) I
I 203 (4.1) Colh 145 (35.3) Cr
Notes: Cr, crystal phase; Colh, hexagonal columnar phase; Colx,
unidentified columnar phase; I, isotropic phase; * undetermined
transition temperature; Dec. decomposition.
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Some structural details of the Colh mesophase for the three
compounds can be deduced from the X-ray results. A discussion of
the organization of these non-disc-like compounds in the columnar
mesophase is feasible by using some simple calculations. The
obtained parameters are not consistent with stacking of single
molecules in the columns and therefore, several molecules are
needed to generate a disc-like aggregate. The number
Z of
molecules that generate the proposed supramolecular entity can
be estimated by the following equation:
ρ
VNA
M
Z =
Figure 5. X-ray pattern of 4a recorded at room
temperature after thermal treatment.
where
ρ
is the density in g cm-3,
V
is the unit cell volume in cm3, NA
For compounds 4b and 4c, the diffractograms are more difficult to
interpret. Although the mesophases are retained at room
temperature after thermal treatment by fast cooling, some
crystallization cannot be avoided and this precludes the reliable
indexation of the maxima. Furthermore the thermal behavior of
these compounds is very complex and, even at high temperatures
in the heating process, it is impossible to get the mesophase free of
crystalline phase. Moreover, some diffraction maxima change as a
function of the precise experimental conditions. Therefore an
unambiguous interpretation of the X-ray cannot be achieved in any
of the above-mentioned conditions. However, reliable results were
finally obtained at high temperatures by heating up to the isotropic
liquid and then irradiating with the X-rays at a temperature above
the solid-to-mesophase transition (Table 2). In these conditions, the
is Avogadro’s number and
M is the molecular mass in g. Although
the exact density of these compounds is not known, its value must
be close to 1 g cm–3 for this kind of organic compound. The unit cell
volume is equal to the area
√3/2) multiplied by the mean stacking distance
considering a density of about 1 g/cm-3, it is deduced that three
molecules occupy a column “slice” of about = 4.7 Å at room
S
of the 2D hexagonal lattice (
S =
a2
h
. In the case of 4a
,
h
temperature. For the other two compounds the same calculations
lead to the conclusion that three molecules must occupy a column
“slice” of about
h = 6.1-6.3 Å. The differences in h (columns “slice”
thickness or mean stacking distance) can be assigned to the space
demand at high temperatures due to the larger conformational
disorder.
diffractograms were consistent with
a hexagonal columnar
For the three compounds the resulting aggregate must contain the
rigid regions of the molecules in the middle and a total number of
18 hydrocarbon chains surrounding the rigid cores. This structural
model is similar to that proposed for the analogous esters, the
mesophases of which were thoroughly investigated12, and thus it is
concluded that the type of packing previously proposed for the
mesophase. Indeed for 4b, two low-angle sharp maxima are
detected with reciprocal spacing in the ratio 1: √3, that correspond,
respectively, to the (1 0) and (1 1) reflections from the hexagonal
lattice. Although 4c only yielded one maximum, by analogy to its
homologous compounds, it is reasonable that the mesophase is also
hexagonal columnar. This is supported by the hexagonal lattice
esters is valid for the analogous amides 4a, 4b and 4c. This result is
constant
a
deduced considering this maximum as the (1 0)
value compares well with the value for
not unexpected considering the similar molecular structure and
identical dimensions of the two series of compounds.
reflection (Table 2). The
a
a
4b, as the observed difference is consistent with the expected
evolution in chain length taking into account the large
conformational disorder present at such high temperatures. The
The conclusion of the X-ray diffraction study is that the mesophases
of the three compounds investigated is hexagonal columnar. The
large conformational disorder, in turn, accounts for the smaller
a
liquid crystal phase is maintained at room temperature for 4a
,
values found for these two compounds compared to 4a that was
studied at room temperature and therefore must have its chains in
a more extended conformation.
whereas it partially crystallizes at both room temperature (in the
cooling process) and high temperatures (in the heating process) for
4b and 4c
.
Table 2. Structural parameters of the Colh mesophase of compounds 4a
,
4b and 4c determined by XRD.
Compound
4a
T (ºC)
dobs
dcalc
h k
Lattice
constant (Å)
Mean stacking
distance (Å)
Molecules per
aggregate
r.t.
37.2
21.7
18.8
37.4
21.6
18.7
1 0
1 1
2 0
a
= 43.2
h
= 4.7
Z
= 3
4b
4c
180
175
34.6
20.0
34.6
20.0
1 0
1 1
a
a
= 40.0
= 41.0
h
h
= 6.1
= 6.3
Z
Z
= 3
= 3
35.5
35.5
1 0
8 | J. Name., 2012, 00, 1-3
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FT-IR Study of the hydrogen bond interactions
In order to examine the presence of intermolecular hydrogen bond
interactions between NH and C=O functions in the polycatenar
amides, FT-IR measurements at variable temperature were
performed on the selected compound 4a as a representative of the
amides.
The compound was heated to the isotropic liquid and the spectra,
represented in the figure 6, were recorded at different
temperatures corresponding to different states of the cooling
process: 220 °C (liquid phase), 170 °C (columnar mesophase), 100 °
C (crystal phase). In the liquid phase, the FT-IR spectra showed NH
and C=O bands at 3306 cm-1 and 1657 cm-1 respectively. In the
mesomorphic and crystal phase, no significant shifts of these bands
were observed. This indicates that hydrogen bond interactions are
very strong with similar magnitude in the crystalline, mesomorphic
and isotropic states. However, the FT-IR spectra of the same
compound 4a, realized in solution with chloroform as solvent
without hydrogen bonding features (figure 7), allowed to see a clear
shift of these bands (NH and C=O bands found around 3430 and
1673 cm-1 respectively), indicating a disruption of these hydrogen
bond interactions in solution.
Figure 7. FT-IR spectrum of 4a in chloroform
Optical properties
Ultraviolet-visible absorption and photoluminescence
spectroscopy
Compounds incorporating benzobisthiazole units are known for
their ability to show very interesting optical properties2,4-6,12,13,30
such as high fluorescence. Considering that the length of the side
chains do not influence the optical properties, only solutions in
dichloromethane of the amides 3a and 4a as representative
compounds of the series 3 and 4, respectively, were analyzed by the
ultraviolet-visible (UV-vis) spectroscopy and their absorption-
emission spectra are depicted in figure 8. Similar experiments on
the compounds of the series
2 could not be carried out because of
their insolubility in classical organic solvents. The solution
concentration used for 3a and 4a in dichloromethane are 2.6 x 10-6
M and the photophysical data are summarized in the table 3. As
observed in the esters series12, both compounds 3a and 4a present
three absorptions with a maximum at 374 nm and two others at
358 and 391 nm. The molar extinction coefficients were determined
around 1.0x10-4 L mol-1 cm-1 for both compounds. Concerning the
emission spectra, the compounds 3a and 4a show three bands at
403, 426, 452 nm and 404, 427 and 454 nm respectively, being the
two first ones with higher emissions. The emission colors of these
compounds are in the violet-blue region. The presence of benzene
rings in the benzobisthiazole unit in the structure of 3a and 4a
explains the absorption bands through spin-allowed π-π*
transitions. Moreover, the molecular structure presents surely a
high electronic π-delocalization, as stated by the high values
observed for the molar extinction coefficients. Compared to the
esters series12, the values of the absorption or emission bands are
Figure 6. FT-IR spectra of 4a at different temperatures
higher due to
a favored planar geometry originated by the
electronic delocalization in the amide groups. The
photoluminescence quantum yield ФPL for each sample was
determined by comparison with a well-known reference compound
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(9, 10-diphenylantracene in cyclohexane, Фr=0.950). The general and Areference are the integrated areas of the emission peak of the
equation used for the determination of the relative quantum yield sample and the reference, respectively; ηsample and ηreference are the
is given as follows:
refractive indices of the sample and reference solutions in
dichloromethane (pure solvents were supposed) respectively. The
photoluminescence quantum efficiencies of 3a and 4a in
dichloromethane solution were 0.28 y 0.33 respectively, lower than
those observed for the similar ester compounds. This result can be
explained by a higher molecular interaction in the amides series,
disfavoring the luminescence property.
ηꢒꢊꢋꢌꢍꢎꢆ
ηꢒꢃꢆꢇꢆꢃꢆꢈꢉꢆ
Aꢊꢋꢌꢍꢎꢆ
Absꢃꢆꢇꢆꢃꢆꢈꢉꢆ
Absꢊꢋꢌꢍꢎꢆ
Фꢀꢁ ꢂ Фꢃꢄ ꢅ
ꢏ ꢄ ꢐ
ꢑꢄ ꢅ
ꢏ
Aꢃꢆꢇꢆꢃꢆꢈꢉꢆ
where Фr is the photoluminescence quantum yield of the reference
compound 9, 10-diphenylantracene; Absreference and Abssample are the
absorbances of the reference and the sample, respectively; Asample
Figure 8. Absorption and emission spectra of compounds 3a and 4a
.
Table 3. UV-visible absorption and emission data for compound 3a and 4a
.
c
Comp.
λ abs. max
(nm)a
λ em. max
(nm)b
ε
Stokes shift (nm)
ФPL
(L mol-1 cm-1)
1.04x104
3a
4a
358, 374, 391
358, 374, 391
403, 426, 452
404, 427, 454
45, 52, 61
46, 53, 63
0.28
0.33
1.05x104
a Measured in dichloromethane solutions (2.6 x 10-6 M)
b Excited at absorption maximum
c Quantum yield in dichloromethane solutions relative to 9,10-diphenylanthracene (Фstd = 0.95 in cyclohexane solution)
Solid-state fluorescence emission
Nowadays, a considerable interest is focused on supramolecular
Fluorescence emission spectra in solid state were obtained in a systems with fluorescent properties which can be modified by
confocal fluorescence microscope equipped with
detector and using of 405 nm laser as excitation source. Compound amide groups can be protonated in acid environment, a study on
4c was used as a representative molecule of the amide series. protonation for the compounds belonging to series and was
a
spectral external sources15,41-43 such as ions, molecules or light. Since the
3
4
Figure 9 shows the comparison of the emission spectra obtained in performed. Film samples of these compounds were exposed to
solid state and in solution for compound 4c. A bathochromic shift ultra-violet radiation of 365 nm and a green-blue emission color
was observed in the emission spectra in solid state as compared was observed (figure 10a). These materials treated with drops of
with the one in solution. This shift could be explained by the acetic acid (HAc) showed yellow emission color (figure 10b), using
intermolecular interactions present in the solid state. Indeed, an the same ultra-violet radiation. This process is completely
overlapping of molecular orbitals between molecules is generated reversible: the same acidified film, once neutralized by a base as
in the solid state, leading to a decrease in the energy gap of the ammonia solution, exhibited a green-blue color emission. Because
excited state and the shift of the emission to higher wavelengths.
of their high melting point and tendency to decomposition, the
amides of series were not submitted to this study.
2
Solid state spectra of the compounds shown in figure 10 were taken
under the microscope using a 405 nm excitation laser. The emission
spectra from neutral film corresponding to 4c (Figure 10, table 4)
showed an emission maximum at 476 nm and the acidified film of
4c-HAc (Figure 10, table 4) at 582 nm, explaining the dramatic color
change observed in the visual inspection shown in Figure 10a and
10b. This type of molecules shows areas with different electronic
density. The benzobisthiazole and amide groups show low
electronic density (denominated acceptors, A) and the lateral
benzene rings together with the alkoxy chains show high electronic
Figure 9. Emission spectra of 4c in solution and solid state.
10 | J. Name., 2012, 00, 1-3
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density (denominated acceptors, B). These types of A-B systems are
characterized by a photo-inducible fluorescence as a result of
intramolecular charge transfer (ICT) in the excited state. In these
systems, when the sample is acidified, protonation of the oxygen in
the amide groups would occur since this is the most basic area of
the molecule, generating areas with higher electron acceptor power
when compared with the neutral molecules. Even more, a system
with larger electronic delocalization is generated (Figure 11),
favoring the intramolecular charge transference and stabilization of
the excited state with the resulting change in fluorescence color44
.
Figure 11. Scheme representing the protonation of the amide
compounds and the electronic delocalization.
Gel formation and fluorescence analysis
The products from the series
potential gelators, because of their very low solubility in classical
organic solvents. Among the three compounds of series , only
product 4c, presenting the largest side chains, showed the ability to
form stable gel in organic solvents as chloroform and
2 and 3 did not show ability to be
4
a
tetrahydrofuran THF, at concentrations lower than
appreciated in figure 12 (a, b, c, d).
5 %, as
Table 5 shows the maximum solvent retention of compound 4c. As
indicated in this table, this compound was able to form a stable gel
in chloroform with a concentration of 0.8% in weight whereas in
THF the stable gel formation was observed for a concentration of
1.5% in weight (Figure 12 a and b). The large solvent retention
capacity in chloroform allows to class this compound as a super-
gelator substance, which is able to form a gel with concentrations
less than 1% in weight.
Table 5. Gel capacity of 4c versus concentration in weight
4c
5%
G
2.5%
G
1.5%
G
1%
G
0.8%
0.5%
CHCl3
THF
G
S
S
S
G
G
G
S
Figure 10. Left side: Pictures corresponding to a film of
compound 4c exposed to ultra-violet radiation of 365 nm: a)
neutral film and b) film acidified by acetic acid. Right side:
solid state fluorescence spectra of the films, determined
from a spectral image.
S: soluble, G: gel.
Since organogels with luminescent properties could be destined to
potential applications as optical sensors, the fluorescence
properties of the gel formed with 0.8% of compound 4c was
studied. The UV illumination of the gel produced blue fluorescence
(Figure 12c) confirmed by the emission spectra (Figure 12d)
obtained from the gel using a confocal microscope equipped with a
spectral detector (see methods29).
Table 4. Photophysical properties in solid state of 4a
.
Compounds
λ em. max (nm)
Emission colour
4c(neutral film)
476
582
Blue
4c-HAc(acidified film)
yellow
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Conclusions
In spite of the complex molecular structure of the polycatenar BBZT
amide compounds designed for achieving liquid crystal and gel
properties, the synthetic route proposed in this investigation can be
considered as a remarkable success from the point of view of great
yields and mild reaction conditions. The photoluminescent behavior
in solution or solid state of the BBZT unit incorporated in such large
molecules was kept and confirmed by optical studies showing good
fluorescence quantum yields in organic solvents. Even if the amide
functions did not allow to increase the optical properties because of
the hydrogen bond formation, their presence results to be an
excellent way to achieve a very attractive property concerning the
color change if acids or bases are added. Among the polycatenar
compounds synthesized in this investigation, the hexacatenar
molecules represent the series which showed the most adequate
structural characteristics for obtaining liquid crystal and organogel
properties. Indeed, as reported and stated for other liquid crystal
materials, the number, positions and lengths of the side chains in
this kind of molecules are crucial factors to target the
corresponding properties. As expected, Colh mesomorphism was
generated and determined from the analysis performed by POM,
DSC and XRD techniques, leading to very promising results for
electronic transport applications in such molecular structure
composed by benzene rings, BBZT unit and amide function. In the
case of gel materials, a remarkable result was found with the
hexacatenar BBZT compound containing alkoxy chains of 14
carbons, able to behave as a super-gelator in chloroform. By
electronic microscopy a microfiber structure is observed for the
corresponding xerogel, which is organized in an extremely
Figure 12. Formation of a fluorescent gel from compound 4c
;
(a) Gel formation at 1.5% of compound 4c in THF; (b) Gel
formation at 0.8% of compound 4c in chloroform; (c)
Photograph under illumination with a UV lamp of the gel
obtained at 0.8% of compound 4c in chloroform; (d) solid
state emission spectra for the gel obtained at 0.8% of
compound 4c in chloroform.
The morphology of the xerogels of compound 4c was studied by
scanning electron microscopy (SEM). Figure 13 shows the SEM
images of the xerogel 4c obtained after elimination of chloroform
and THF respectively. In the pictures with 500x magnification, a
lamellar morphology can be distinguished, whereas with 1500x
magnification, it is possible to observe typical fibrillar morphologies
for both gels, with a diameter of 250 nm for the gel in chloroform
and 180 nm for the gel in THF. From these images, it can be
concluded that there is a supramolecular self-organization that
follows a hierarchical order. First, gelling molecules self-organize
into microfibers which are incorporated in fibrillar networks. Finally,
these network structures are organized in lamellar systems.
intertwined network, caused by
a complex supramolecular
assembly. One would assume that the fluorescence properties of
the BBZT unit present in such sophisticated structure could be
quenched. However the optical studies showed that this property
was maintained, allowing to achieve a very attractive super-gel with
blue colour emission. This result implies that the molecular
organization in this gel material is highly ordered due to the
molecular interactions mentioned previously, avoiding a turned off
effect of the fluorescence.
The information obtained in this investigation delivers us very
important chemical keys for the design and preparation of hybrid
fluorescent materials with liquid crystal and gel-forming properties.
The very promising results encourage us to study the chemical
modifications of these polycatenar BBZT compounds changing the
side chains which are the most determinant factors implied in the
liquid crystal and gel potentialities. Since main investigation efforts
aim to the formation of solid-state blue emitters due to their
scarcity, the incorporation of BBZT unit in adequate organic systems
could be a very attractive alternative way to achieve such materials,
as investigated and described in this paper.
Finally, additional promising applications of these materials should
be oriented to the fabrication of optical chemical pH sensors.
Figure 13. SEM micrographs of xerogel 4c obtained after
elimination of chloroform (a) or THF (b), with magnification 500x
and 1500x.
Acknowledgements
The authors thank the Projects “Fondecyt N° 1140350 (J.V.) and
1140454 (S.A.S)” and the “Vice-Rectoría de Investigación” of
Concepción University (Chile) for their financial support. J. Barberá
12 | J. Name., 2012, 00, 1-3
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Kaafarani, W. Zhang, F. W. Harris and S. Z. D. Cheng,
Soft Matter, 2010, , 100.
acknowledges financial support from MINECO, Spain (project
CTQ2015-70174-P).
6
25. A. Metere, S. Sarman, T. Oppelstrup and M. Dzugutov, Soft
Matter, 2015, 11, 4606.
26. J. C. Tang, R. Huang, H. F. Gao, X. H. Cheng, M. Prehm and C.
Tschierske, Rsc Advances, 2012, 2, 2842.
27. H.-H. G. Tsai, L.-C. Chou, S.-C. Lin, H.-S. Sheu and C. K. Lai,
Tetrahedron Letters, 2009, 50, 1906.
28. R. K. Gupta, S. K. Pathak, B. Pradhan, D. S. S. Rao, S. K. Prasad
and A. S. Achalkumar, Soft Matter, 2015, 11, 3629.
29. F. A. Olate, J. A. Ulloa, J. M. Vergara, S. A. Sanchez, J. Barbera
and M. L. Parra, Liquid Crystals, 2016, 43, 811.
30. E. Y. Elgueta, M. L. Parra, J. Barberá, J. M. Vergara, M.
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