Results and discussion
The transition temperatures and associated enthalpies are
reported in Table 1. As frequently observed in banana series,
the terminal chains greatly influence the occurrence of different
‘‘B’’ phases. Indeed, the dependence of the clearing points on
the length of the alkoxy terminal chains suggests at least two
regimes: the liquid crystalline–isotropic transition temperature
Tcl first decreases increasing the number n of carbons in the
terminal chains (from n~6 to 10) and then slightly increases
from n~11 to 16. The assignment of the phases based on X-ray
diffraction measurements and also on the optical observations
and electrooptical studies confirms the existence of different B
phases in the series depending on the carbon number in both
terminal chains.
Scheme 1 Reagents and conditions: a, (COCl)2, CH2Cl2; b, 4-HS-Ph-
COOH, Et3N, CH2Cl2; c, resorcinol, DCC, DMAP, CH2Cl2.
Short homologues S6 and S7
These two compounds, S6 and S7, possess two mesophases, one
smectic phase and a two dimensional one at lower temperature.
The smectic phase exhibits fan-shaped textures (Fig. 1a) like
SmA but without any homeotropic tendency. The X-ray
pattern of an oriented sample shows that this phase is a tilted
smectic without in-plane order. The tilt angle is rather small
(about 10u). For these two compounds, the layer spacing is
close to the half-length of the molecules (d~20 A for n~6 and
˚
20.8 A for n~7). These d-values suggest an intercalated
structure. So this phase probably belongs to the B6 type. It
should also be noted that this smectic phase does not show any
electrooptical switching even at high voltages.
Decreasing the temperature, a two dimensional B1 phase1
occurs. The transition is accompanied by a very low transition
enthalpy (v0.2 kJ mol21) which suggests only subtle modifica-
tions of the molecular arrangement. An evolution of the
textures towards a mosaic texture (Fig. 1b) is observed at the
B6/B1 transition. The X-ray patterns show two reflections in
the small angle region (Fig. 2) while in the wide–angle region a
broad diffuse scattering indicates the liquid like order of the
4-[(4’-Tetradecyloxybenzoyl)sulfanyl]benzoic acid: a solution
of 4-tetradecyloxybenzoyl chloride (3.7 g, 10 mmol, prepared
from the corresponding acid and oxalyl chloride) and
4-mercaptobenzoic acid (1.57 g, 10 mmol) was prepared in
dichloromethane (50 ml). To this stirred solution was then
added triethylamine (1.2 g, 12 mmol) over about five minutes.
The resulting solution was stirred overnight and then
concentrated to dryness by rotatory evaporation. The resultant
solid was hydrolysed with dilute aqueous hydrochloric acid and
extracted with ethyl acetate (300 ml). The organic solution was
dried over anhydrous Na2SO4 and the solvent was evaporated.
The solid was recrystallised from ethyl acetate. Yield: 2.6 g
˚
1
(61%); NMR H (CDCl3, ppm): 0.9 (t, 3H, CH3), 1.1–2.1 (m,
2H, 12 CH2), 4.1 (t, CH2O), 6.9–8.4 (m, 8H, Ar).
1,3-Phenylene bis{4-[(4-tetradecyloxybenzoyl)sulfanyl]ben-
zoate}: to a solution of resorcinol (110 mg, 1 mmol), DCC
(440 mg, 2.2 mmol) and DMAP (20 mg) in 20 ml of dichloro-
methane, was added 4-[(4’-tetradecyloxybenzoyl)sulfanyl]ben-
zoic acid (1.05 g, 2.2 mmol). The mixture was stirred overnight
at room temperature. The solvent was evaporated and
the solid was purified by chromatography on silica gel with
dichloromethane as solvent. The pure product was then
evaporated and finally recrystallised in an ethanol–toluene
mixture. Yield: 0.52 mg (51%); NMR 1H (CDCl3, ppm): 0.9 (t,
6H, 2 CH3), 1.3 (m, 44H, 22 CH2), 1.85 (m, 4H, 2 CH2), 4.05 (t,
4H, 2 CH2-O), 7–9 (m, 20H, Ar).
Table 1 Transition temperatures (uC) and enthalpies (italics, kJ mol21
as a function of the carbon atom number in the terminal chains (from
)
DSC runs, rate 5 uC min21
)
n
K
B1
B6
B2
I
6
.
120
14.68
.
[115]
0.18
.
160
12
.
7
.
.
.
.
.
.
.
.
.
.
120
13.9
.
.
.
.
139
0.18
.
146
14
.
.
.
.
.
.
8
120
16.1
135
16.3
Experimental
The thermal behaviour was investigated using a Perkin–Elmer
DSC7 differential calorimeter. The optical textures of the
mesophases were observed through a polarizing microscope
(Leitz Diavert) equipped with a hot stage (FP-82HT) and an
automatic controller (Mettler FP-90). Samples are observed on
regular slide glass without any surface treatment. X-Ray
diffraction experiments were carried on a 18 kW rotating anode
X-ray source (Rigaku-200) with use of Ge(111) crystal as
monochromator. The scattered radiation was collected on a
two dimensional detector (Imaging Plate system from Mar
Research, Hamburg). The samples were placed in an oven,
providing a temperature control of 0.1 K. Oriented samples of
the smectic phases were obtained by slow cooling of a drop of
the isotropic liquid. Electro-optical properties were studied
using commercial cells (from E.H.C., Japan) with a rubbed
polyimide layer (but the surface treatment is not effective to
make uniformly oriented cells). Switching current was observed
by applying a voltage-wave using a function synthetizer (HP
331 20A) and an high power amplifier (Krohn-Hite).
9
113
14.9
123
14.6
10
11
12
13
14
15
16
117
55.4
[113]
16
104
31.7
.
.
.
.
.
.
112
16.3
109
38.48
114
18.6
109
42.6
116
19.8
.
.
111
43.3
118
20.8
114
38.3
118
16.7
.
.
108
44
119
16
a[ ] Indicates a monotropic transition.
J. Mater. Chem., 2001, 11, 2946–2950
2947