Synthesis and mesomorphic properties of polymethylene-α,ω-bis [2-thio5-(4′,4″n-alcoxybenzoyloxy)phenyl]-1,3,4-oxadiazole

Article abstract of DOI:10.1515/znb-1998-0316

Three homologous series of polymethylene-α,ω-bis[2-thio-5(4′,4″n-alkoxybenzoyloxy) phenyl]-1,3,4-oxadiazole (series 5 a, b, c), are reported.The compounds have identical mesogenic units at both ends of a spacer (i.e. they are twins). These twins possess a spacer of 6, 8 and 10 carbon atoms joined directly to the oxadiazole ring through sulphur atoms. The lateral alkylic chains vary between 6 and 10 carbon atoms.The presence of enantiotropic and monotropic nematic phase is related to the length of the spacer group. The influence of molecular structure on the mesomorphic properties has been studied. Mesomorphic properties and phase transitions have been determined using polarizing hot-stage microscopy and differential scanning calorimetry.

Full text of DOI:10.1515/znb-1998-0316

Synthesis and Mesomorphic Properties of Polymethylene-a,u;-bis [2-thio-
5-(4',4"n-alcoxybenzoyIoxy)phenyl]-l,3,4-oxadiazole
C. Aguilera*, M. Parra, G. Fuentes
Departamento de Pohfneros*, Departamento de Qufmica Orgänica, Facultad de Ciencias
Qmmicas,Universidad de Concepcion, Casilla 3-C, Concepcion, Chile
Z. Naturforsch. 53 b, 367-370 (1998); received May 15, 1997
Liquid Crystal, Twin Mesogens, Oxadiazole, Mesomorphic Properties
Three homologous series of polymethylene-a,i<;-bis[2-thio-5(4’,4'’n-alkoxybenzoyloxy)-
phenyl]-l,3,4-oxadiazole (series 5 a, b, c), are reported.The compounds have identical meso-
genic units at both ends of a spacer (i.e. they are twins). These twins possess a spacer of
6, 8 and 10 carbon atoms joined directly to the oxadiazole ring through sulphur atoms. The
lateral alkylic chains vary between 6 and 10 carbon atoms.The presence of enantiotropic and
monotropic nematic phase is related to the length of the spacer group.
The influence of molecular structure on the mesomorphic properties has been studied. Me-
somorphic properties and phase transitions have been determined using polarizing hot-stage
microscopy and differential scanning calorimetry.
Introduction
The investigation of the relationship between
molecular structure and properties in liquid-
crystalline twins compounds is of great interest.
During the last decades these compounds had
obtained great attention [1 - 5]. The twins may be
considered as liquid crystal prepolymers which are
well suited for theoretical modelling. Lately twins
are considered in numerous publications as model
compounds for main chain polymers [6-7], In ad-
dition, they are used for conformational analysis by
means of 2H NMR spectroscopy in compounds with
deuteriated spacers [8-9].
Besides of the chemical structure and the geo-
metrical shape of the mesogenic units, the nature of
their linking and the length of the spacer have a re-
markable influence on the mesomorphic behaviour
[10-14].
5 (a,b,c)
5a x = 6
5b x = 8
5c x= 10
R = CnH2n+i, n = 6 -10
1,3,4-oxadiazole (series 5 a, b, c) were prepared
according to Scheme 1.
The homologous series 5 was synthesized by
using 4-hydroxyphenyl-hydrazide 1 as a precur-
sor [15]. The treatment of 1 with carbon disul-
fide in basic medium produces thiones 2, accord-
ing to the procedure reported in reference [16 -
17]. In the following sequences, the selective S-
alkylation of 2 leads to the homologous series of
the oxadiazoles 3.By esterification of 3 with p-n-
alkoxybenzoyl chlorides previously synthesized 4,
the homologous series 5(a, b. c) were obtained after
purification by recrystallization from dimethylfor-
mamide/water.
In this paper we describe the synthesis and
mesomorphic properties of three new homolo-
gous series of twins liquid crystals containing the
1,3,4-oxadiazole heterocycle with a flexible poly-
methylenic spacer joined to the mesogen by means
of sulphur atoms 5(a, b, c).
All the homologue of series 5a present a crystal-
crystal transition and a nematogenic enantiotropic
behaviour with exception of the monotropic com-
pound with n = 10.
Results and Discussion
Three homologous series of polymethylene -a,u>-
bis([ 2-thio - 5(4',4 "n-alkoxybenzoyloxy)phenyl])-
The homologue of series 5b with n = 7 and 8
* Reprint requests to Prof. Dr. C. Aguilera.
present a crystal-crystal transition, those with n = 6
0939-5075/98/0300-0367 $ 06.00
©
1998 Verlag der Zeitschrift für Naturforschung. All rights reserved.
K
Brought to you by | New York University Bobst Library Technical Services
Authenticated
Download Date | 7/24/15 3:36 PM

368
C. Aguilera et al. ■Twin Mesogens Containing 1,3,4 - Oxadiazole Groups
N—IN
1) KOH, HjO
NHNH2
2)C S 2
Br(CH2)xBr
EtOH/Py
.—, N—N
N—N _
h° - 0 ^ o ^ “ s(ch2>xsA
^
oh
>— ^ ^ - C C I (4)
RO-
R = CnH2n+1 >n = 6 -10
DMPA/Py
DMF
5 (a,b,c)
x = 6, 8 ,1 0
Scheme 1. Synthetic routes for twin meso-
gens.
R = CnH2n+1 i n = 6-10
Table I. Transition temperatures (°C), transition en-
thalpies (kJ/mol) and yields (%), data for the series 5a.
Table II. Transition temperatures (°C), transition en-
thalpies (kJ/mol) and yields (%), data for the series 5b.
n
6
K
N
I
Yields
n
6
7
K
N
I
Yields
24
K,
.
K,
_
134
130
135
119
112
174
(33,9)
152
(31,8)
155
(78,5)
167
(36,9)
162
185
(3,8)
172
(3,2)
173
(0,94)
174
(4,2)
160*
7
157
(43,2)
151
(33,7)
142
(66,0)
140
(58,4)
181
(4,7)
161
(3,6)
133*
7
35
18
36
32
.
21
108
8
8
9
.
.
.
35
25
4
131
-
9
.
.
138*
131*
-
10
.
137
.
10
.
(29,1)
(75,1)
K = crystal; Ki = crystal-crystal; N = nematic; I =
isotropic; () = Z\H (kJ/mol); * = transition monotropic.
K = crystal; Ki = crystal-crystal; N = nematic; I =
isotropic; () = .AH (kJ/mol); * = transition monotropic.
transition temperatures versus the number of carbon
atoms of the alkylic chain (n) are plotted in Fig. 1.
and 7 a nematogenic enantiotropic behaviour, and
the compounds with n= 8 - 10 are monotropic.
In the homologous series 5c the compounds with
n = 7 present a crystal-crystal transition and those
with n = 6-8 present a nematogenic monotropic be-
haviour. those with n = 9 and n = 10 do not present
liquid crystalline properties.
The homologous series 5 (a, b, c) presents a reg-
ular decrease of the transition temperature from ne-
matic to isotropic with an increase of both spacer
size and of the alkylic chains. The even-odd num-
ber effect where the mesophase of the compounds
The temperatures and transtion enthalpies are having even-numbered polymethylene units is of
shown in Tables I, II and III, respectively. The phase a higher degree order at the transition temperature
Brought to you by | New York University Bobst Library Technical Services
Authenticated
Download Date | 7/24/15 3:36 PM

C. Aguilera et al. • Twin Mesogens Containing 1,3.4 - Oxadiazole Groups
369
Table III. Transition temperatures (°C), transition en-
thalpies (kJ/mol) and yields (%), data for the series 5c.
n
K
K,
N
I
Yields
-
6
145
(77,2)
153
(81,3)
152
(81,6)
151
(96,8)
153
128*
143*
142*
6
7
8
.
.
143
144
30
33
35
28
9
1 0
.
-
(67,1)
K = crystal; Ki
=
N =
crystal-crystal;
nematic; 1=
isotropic; () = _1H (kJ/mol); * = transition monotropic.
than the one with odd-numbered, is not observed,
confirming that this behaviour is not always valid
[18].
The mesophases present a low mesomorphic
range. The nematic mesophase is favored, but it
possesses low stability. This could be related to the
deviation from linearity due to the presence of the
oxadiazole heterocycle [16].
Mesophase texture were observed through a po-
larized light microscope and compared with the lit-
erature [19]. The nematic mesophase presented a
marbled and schlieren type texture.
n
Fig. 1. The phase transition temperatures of the series 5a
(□ ,■ ); 5b (o, •); 5c (<>,♦); □, o K,-N;B, • N -I;O K i-I ;
♦ I-N (n is the number of C-atoms in the substituent R).
(1.32 g, 1.80 ml) of pyridine and 13.0 mmol of a,w-
dibromoalkanes were added drop by drop to a stirred
solution of 52.0 mmol (10.0 g) of 2 in 26 ml of absolute
ethanol. The mixture was refiuxed for 8 h, filtered and
the precipitate was crystallized from DMF/water (4:1).
Fusion temperature: 21°C, yield 24%.
The transition temperatures and enthalpies were
determined by DSC.
Experimental
The structure of the synthesized compounds were
confirmed by FT-IR spectroscopy (Nicolet, Magna-
IR™550), ’H and l3C NMR spectroscopy (Bruker AC-
250).
Spectroscopy characterization of the homologue 3b with
n = 8
1H NMR (d6 -DMSO, TMS, 250 MHz): <5(ppm) = 10.37
(2H, 20H); 7.85 (d, J = 8.7 Hz, 4H C6 H4); 7.05 (d, J =
8.7 Hz, 4H C6 H4); 3.34 (t, J = 7.2 Hz, 4H, 2SCH2); 1.82
(m, 4H, 2CH2); 1.40 (m, 8 H, 4CH2).
The liquid crystalline properties were studied with
an Ortholux Pol-BK-II polarizing microscope with Met-
tler FP 82 hot stage and Mettler FP 80 central proces-
sor.Measurements of temperatures and enthalpies of tran-
sition were carried out using a DSC (STA 625), differ-
ential scanning calorimeter with a heating and cooling
rate of 10°c/min, calibrated with Indium (99.99%) and
operated under nitrogen.
13C NMR (CDCI3 , TMS, 62.9 MHz): 6 (ppm) =
165.22; 162.62; 160.72; 128.27; 116.12; 113.82 (C
arom.); 32.90; 28.90; 28.14; 27.66 (aliphatic carbons)
Synthesis of polymethylen bis - a,u> -[2-thio-5(4',4"-n-
alkoxy benzoiloxy) phenyl]-1,3,4-oxadiazole 5(a,b,c)
Synthesis ofpolymethylene bis-a(uj-2-thio-5(p-hydroxy)-
phenyl-1,3,4-oxadiazole 3(a,b,c)
General method [ 16-17]: To a solution containing 1.06
mmol (0.5 g) of 3, 0.011 g of 4-dimethylamonopyridine
(DMPA) and 0.2 ml of dry pyridine in 10 ml of dry
General method [16]: The series 3 were synthesized
according to the procedure given in [17, 18], 13.0 mmol
Brought to you by | New York University Bobst Library Technical Services
Authenticated
Download Date | 7/24/15 3:36 PM

370
C. Aguilera et al. ■Twin Mesogens Containing 1,3.4 - Oxadiazole Groups
dimethylformamide,was added 2.34 mmol (0.56 g) of p-
n-alkoxybenzoyl chlorides (4). The mixture was stirred
for 24 h at room temperature, filtered and the white pre-
cipitate was crystallized from ethanol. The transition tem-
peratures, enthalpies and the yields are given in Tables I,
II and III.
(m, 8 H, 4CH2); 1.40 (m, 24H, 12CH2); 0.90 (t, J = 6.61,
6 H, 2CH?)
,3C NMR (CDCI3 , TMS. 62.9 MHz): 6 (ppm) = 174.01
(C=0, ester); 165.09; 164.687; 164.375; 163.79; 153.65;
132.34; 127.99; 122.61; 120.33; 114.39 (Carom.); 68.35;
32.57; 31.70; 29.08; 28.87; 28.40; 25.89; 22.56; 14.01
(alkyl chain carbons)
Spectroscopic characterization of the homologue 5b with
n = 7
Acknowledgements
'HNMR (CDCh, TMS, 250 MHz): <5(ppm) = 8.15 (d,
J = 8.85, 4H C6 H4) ; 8.05 (d, J = 8.85, 4H, C6 H4); 7.40
(d, J = 8.83,4H, ring joined to 1,3,4-oxadiazole); 7.00 (d,
J = 8.83, 4H, ring joined to 1,3,4-oxadiazole); 4.05 (t, J
= 6.53, 4H, 20CH 2); 3.30 (t, J = 7.28, 4H,2SCH2); 1.85
The authors wish to thank the FONDECYT (Grant
1950765) and the Scientific-Academic Collaboration Pro-
gram of Fundaciön Andes between Argentina, Brazil and
Chile for their financial support.
[11] W. Weissflog, D. Demus, S. Diele, P. Nitschke,
W. Wedler, Liq. Crystals 5, 111 (1989).
[12] S. Diele, A. Mädike, K. Knauft, J. Neutzler,
W. Weissflog, D. Demus, Liq. Crystals 10, 47
(1991).
[1] A. C. Griffin, T. R. Britt, J. Am. Chem. Soc. 103,
4957 (1981).
[2] C. Aguilera, L. Bernal, H. W. Schmidt, Bol. Soc.
Chil. Quim. 29(1), 189(1984).
[3] Jin Jun-Il, T. H. Oh, J. H. Park, J. Am. Chem. Soc.
Perkin Trans. 343 (1986).
[13] H. Hartung. F. Hoffman, C. Stützer, W. Weissflog,
Liq. Crystals 19, 839(1995).
[14] J. Andersch, C.Tschierske, Liq.Crystals 21, 51
(1996).
[4] J. L. Hogan, C. T. Imrie, and G. R. Luckhurst, Liq.
Crystals 3, 645 (1988).
[5] G. Galli, M. Laus, A. S. Angeloni, Makromol. Chem.
187.289(1986).
[15] P. A. Smith, Organic Reactions 3 (9), 366 (1946).
[16] D. Girdziunaite, C. Tschierske, E. Novotna,
H. Kresse, Liq. Crystals 10, 937 (1991).
[17] M. Parra, J. Belmar, H. Zunza, C. Zuniga,
G. Fuentes, R. Martfnez, J. Prakt. Chem. 37, 239
(1995).
[18] C. Aguilera, S. Ahmad. J. Bartulm, Mol. Crystal-
logr. and Liq. Crystallogr. 162B. 277 (1988).
[19] D. Richter, Textures of Liquid Crystals, Veb.
Deutscher Verlag, Leipzig (1980).
[6 ] H. Ringsdorf, B. Schlarb, J. Venzmer, Angew. Chem.
100. 117(1988).
[7] H. Finkelmann, Angew. Chem. Int. Ed. Engl. 26, 816
(1987).
[8 ] A. C. Griffin, E. T. Samulski , J. Am. Chem. Soc.
107, 2975 (1985).
[9] H. Toriumi, H. Furuya, A. Abe, J. Polym. 17, 895
(1985).
[10] J. II Jin, Y.- S. Chung, J.- S. Kang, R. W. Lenz. Mol.
Crystallogr. Liq. Crystallogr. Lett. 82. 261 (1982).
Brought to you by | New York University Bobst Library Technical Services
Authenticated
Download Date | 7/24/15 3:36 PM