Optically active, three-ring calamitic liquid crystals: The occurrence of frustrated, helical and polar fluid mesophases

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Optically active, three-ring calamitic liquid

crystals: the occurrence of frustrated, helical and

polar fluid mesophases

Cite this:DOI: 10.1039/c4nj02011a

B. N. Veerabhadraswamy, D. S. Shankar Rao, S. Krishna Prasad and

C. V. Yelamaggad*

Herein, we report on the synthesis, characterization, liquid crystalline behavior and chirooptical properties

of five (R)-4-{[(4-(octan-2-yloxy)phenyl)imino]methyl}phenyl 4-(n-alkoxy)benzoates and their enantiomers,

namely, (S)-4-{[(4-(octan-2-yloxy)phenyl)imino]methyl}phenyl 4-(n-alkoxy)benzoates. These three-ring

rod-like mesogens were prepared by acid catalyzed condensation of (R)-/(S)-4-(octan-2-yloxy)anilines

with 4-formylphenyl 4-(n-alkoxy)benzoates. Thus, each pair of enantiomers comprises (R)-2-octyloxy and

(S)-2-octyloxy chiral tails. In order to understand the structure–property correlations, the length of

the paraffinic chain incorporated at the other end has been varied from n-octyloxy to n-dodecyloxy.

A detailed study carried out by means of several complimentary techniques reveals the stabilization of

liquid crystal phases that hold great promise in applied sciences especially in various device applications.

In particular, the occurrence of mesophases such as the blue phase-I/II (BPI or BPII) and chiral nematic

(N*) and chiral smectic C (SmC*) phases has been evidenced unequivocally with the help of polarizing

microscopy, differential scanning calorimetry, X-ray diffraction and electrical switching. Besides, the

occurrence of an unknown, metastable smectic (SmX) phase below the SmC* phase has been noted. This

study shows that the length of the terminal tail seems to determine the thermal range of the SmC* phase.

The enantiotropic SmC* phase exhibiting ferroelectric switching behavior occurs over 60 1C thermal

range; notably, the spontaneous polarization (P_s) value crosses over 100 nC cm^ꢀ2. The photophysical

properties and chirooptical behavior of the mesogens have been studied with the aid of UV-vis absorption

and circular dichroism (CD) spectroscopic methods, respectively; the latter technique has been especially

used to ascertain the twist sense of the N* and SmC* phases formed by a pair of enantiomers. The

reversal of the helix-sense (from right to left and vice versa) during the N*–SmC* phase transition has

been observed for the first time.

Received (in Porto Alegre, Brazil)

11th November 2014,

Accepted 6th January 2015

DOI: 10.1039/c4nj02011a

www.rsc.org/njc

(symmetry concept) or by the influence of guest chiral dopants

(which may or may not be LC) on the host achiral LC phases.

1. Introduction

Over the past several decades, thermotropic liquid crystalline These bulk aggregates, besides combining the order and mobility

organic or metallo-organic compounds (mesogens) composed on a molecular (nano scale) level, stabilize frustrated phases and

of shape-anisotropic molecules exhibiting liquid crystal (LC) helical superstructures besides some chiral layered structures.^3,4

phases (mesomorphism), upon heating, have attracted much Frustrated LC phases such as twist grain boundary (TGB) phases

attention from both researchers and technologists.¹This is and blue phases (BPs) have evoked considerable curiosity on

mainly because the LC phases, being liquid in mobility and account of their structural complexity stemming from the mole-

crystalline in their anisotropic physical properties, are highly cular self-assembly into two incompatible (layered and helicoidal)

beneficial and rewarding in the context of both basic science¹structures, which is eventually achieved by the formation of

and applied research.^1,2It is especially noteworthy in the case lattice defects. Whereas, the defect free helical superstructures

of chiral LC phases that are formed by pure mesogens posses- such as chiral nematic (N*) or chiral smectic C (SmC*) with a

sing asymmetric carbon atom(s) rendering molecular chirality spectrum of unique properties have been used as sources (media)

for device applications in various sectors.^1,3,4The SmC* phase,

for instance, exhibiting ferroelectric switching behaviour,^3,4

Centre for Nano and Soft Matter Sciences (Formerly Centre for Soft Matter Research),

wherein the spontaneous polarisation (P_s) resulting from the

Prof. U. R. Rao Road, Post Box No. 1329, Jalahalli, Bangalore 560 013, India.

E-mail: yelamaggad@cnsms.com; Fax: +91-80-28382044; Tel: +91-80-23084-233

specific structure formed by the self-assembly of chiral mesogens

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Fig. 1 Symmetry in the smectic C (a) and chiral smectic C (b) phases.

can be reoriented upon application of an electric field, has been low viscosity and a high magnitude of P_s.^4,5,7Despite the fact that

used for fabricating display devices as it oﬀers added benefits a range of ferroelectric LCs (FLCs) have been accomplished

when compared to classical LC displays (LCDs) derived from the through rational design and synthesis since 1975,^4,5,7it has been

nematic (N) phase. For example, high image quality, fast response an extremely diﬃcult assignment to find the materials exhibiting

time (in micro-seconds), optical bistability, high resolution and the optimum physical properties in pure, single component

wide-viewing angles etc. are some of the well-known advantages of systems.⁷However, multi-component systems made by doping

ferroelectric LCDs.⁴

an achiral host mixture with highly polar chiral compounds are

The smectic C (SmC) phase diﬀers from the SmC* phase in proven to be helpful in meeting the material properties

that the director of each layer is inclined at an angle to the layer required for device fabrication; thus, this means of obtaining

normal and this angle being the same for all layers. The tilt suitable FLCs is still followed to a large extent.⁸Yet, this

angle in this phase alters with temperature and it generally approach also suﬀers from technical issues as there are only

increases with decreasing temperature. As shown in Fig. 1a, it is a limited number of host SmC LCs suitable to utilize^4aalthough

characterized by a centre of symmetry, a two-fold axis of rotation a large number of eﬀorts have been explored in this regard;⁹

and a mirror plane normal to the two-fold axis; the symmetry besides, finding a chiral dopant possessing a high P_s, and a

elements correspond to the point group C_2h.¹When the SmC phase long chiral nematic pitch appears to be bothersome. Moreover,

is generated by chiral mesogens or mixed with chiral dopants, it the compatibility among host LC and the chiral dopant is a

becomes optically active where a macroscopic helical arrangement major concern; in other words, the multi-component systems

of the constituent mesogens occurs spontaneously.^3–5In fact, the should be free from phase separation problem. Besides, figuring

helix occurs as a result of a gradual change in the molecular tilt out exact molar concentrations of the guest material and host

direction (n) from layer to layer, about an axis perpendicular to the LC(s) to meet the desired properties is highly time-consuming and

layer planes. Owing to the presence of chiral molecules, the local quite tricky. Therefore, there have been continued, extensive

symmetry reduces to a two-fold axis of rotation, which creates eﬀorts into the design and synthesis of ferroelectric LCs basically

inequivalence in the dipole moment along the C₂axis normal to to understand the structure–property correlations, which may

the tilt direction (Fig. 1b), giving rise to spontaneous polarization eventually facilitate to arrive at a well-defined, one-component

(P_s) in each layer. However, on a macroscopic level there is no net chiral LC having all of the desired characteristics; such a chiral

polarization as the helix averages out the P_sto zero. Therefore, the compound has an added advantage as it can also be used as a

bulk SmC* phase is appropriately termed ‘helielectric’ instead of dopant for the host SmC phase. Thus, in continuation of our work

ferroelectric. Nonetheless, to realize a macroscopic polarization the on chiral LCs,^10–16we became interested in realizing new FLCs.

helix should be unwound by an external force such as an electric

Accordingly, we designed and prepared rather simple (over-

field or shear or by surface interaction and thereby the bulk SmC* looked) chiral calamitics comprising three-rings covalently tethered

phase becomes ferroelectric where the helical structure is to one another linearly through two polar linking groups. The

deformed. As mentioned earlier, this behavior of the SmC* present molecular design originated from the guide lines available

phase has been well exploited in developing high-resolution micro- in the literature to generate mesogens competent of stabilizing

displays that are successfully used in a range of devices viz., color the SmC*/SmC phase.^{3–5,7,17,18}The accumulated knowledge

viewfinders in digital cameras, camcorders, pico-projectors, concerning structure–property correlations suggest that a rigid,

medical imaging etc.^4,5

polarisable rod-like (long) core, having two or three-rings sub-

However, the usage of ferroelectric technology in the present stituted with relatively long paraffinic chains at both the ends,

and future advanced devices, needless to say, depends to a large favours the molecular self-assembly into the smectic phases.

degree on the availability of suitable materials capable of showing If the central linking groups possess a dipole moment normal

a wide thermal span of the requisite mesophase, preferably from to the long axis of the molecules, they provide lateral polarity

higher temperature to sub-ambient temperature,⁶possessing a to induce molecular self-assembly into layers and also assist

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molecular tilting with respect to layer normal; thereby the 2. Results and discussion

SmC phase can be stabilized. The ester (–COO–) and imine

2.1. Synthesis and molecular structural characterization

(–CHQN–) groups are well-known examples of linking units

that induce lateral polarity effectively.^4a,7,18eIndeed, in order to

reduce the local symmetry to a two-fold axis of rotation to

account for polarization in each layer, the chirality in mole-

cules, which is necessary to be introduced, can be achieved,

needless to say, by replacing one (or both) of the achiral tails

with the chiral chain. Furthermore, there is an experiential

notion that the magnitude of polarization can be improved

significantly if the stereocentre is placed nearer to the rigid-

core;¹⁷the hindered rotation of a chiral moiety with respect

to the core appears to yield a high P_svalue.^17cIt is also

perceptible from the literature that three-ring systems favour

the SmC*/SmC structures.¹⁸Thus, these aspects were consid-

ered while designing the present molecules to maximize the

likelihood of required mesophase stabilization. Here, we pre-

sent the synthesis and mesomorphic properties of five pairs of

enantiomers in which three-benzene rings are bound linearly to

each other through ester and imine linkages. The molecular

structures of mesogens along with the abbreviations used are

shown in Scheme 1; obviously, each pair of enantiomers

comprises (R)-2-octyloxy and (S)-2-octyloxy terminal tails; the

length of the achiral, paraffinic chain incorporated at the

other end has been varied from n-octyloxy to n-dodecyloxy with

the aim of studying the correlations between molecular struc-

ture and thermal property, especially in terms of stabilizing the

SmC* phase.

The target chiral rod-like Schiﬀ bases were prepared according to

Scheme 1. As can be seen, their synthesis required the preparation

of key precursors (R) and (S)-4-(octan-2-yloxy)anilines (II and IV) and

4-formylphenyl 4-(n-alkyloxy)benzoates (VIIa–e). 4-Nitrophenol was

O-alkylated with the chiral alcohols, (S)- and (R)-2-octanol, following

the Mitsunobu reaction¹⁹to obtain (R)- and (S)-4-(octan-2-yloxy)-

nitrobenzenes (I and III),²⁰which were subjected to catalytic

hydrogenation under the reaction conditions – H₂(1 atm, balloon),

Pd/C (10%), THF, to get the corresponding anilines II and IV in

excellent yields.²⁰Two-ring aldehydes VIIa–e were synthesized

starting from commercially available 4-hydroxybenzaldehyde;

it was O-alkylated with appropriate n-bromoalkane following the

Williamson ether synthesis protocol to get 4-(n-alkoxy)benz-

aldehydes (Va–e)^14–16which were oxidized with a Jones reagent to

obtain corresponding 4-(n-alkoxy)benzoic acids (VIa–e);²¹these acids

were esterified with 4-hydroxybenzaldehyde in the presence of N,N⁰-

dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine

(DMAP)²²leading to key aldehydes VIIa–e.²³The proposed meso-

gens, the three-ring Schiff bases (imines), were obtained in excellent

(82–86%) yield when anilines II & IV and aldehydes VIIa–e were

made to react with each other in refluxing ethanol in the presence of

a catalytic amount (few drops) of acetic acid. The final compounds

were characterized systematically by IR, UV-visible, ¹H and ¹³C NMR

spectroscopy and CHN analyses. While the intermediates have been

1

characterized with the help of IR and H NMR spectroscopy and

elemental microanalyses only; the data of the known intermediates

were found to be consistent with that of reported ones.

2.2. Evaluation of phase transitional behavior

The mesomorphism of the newly synthesized chiral mesogens was

examined with the aid of optical microscopic, calorimetric, X-ray

diﬀraction (XRD) and electro-optical studies. Table 1 presents the

phase sequence, transition temperatures and enthalpies of the

transitions; here, the phase transition temperatures deduced from

calorimetric measurements of the first heating–cooling cycles at a

rate of 5 1C min^ꢀ1are given. When the peaks are not seen in DSC

traces, the phase transition temperatures noted during the POM

study are taken. From the accumulated data it is apparent that all

the mesogens synthesized exhibit an identical phase transitional

behaviour; during the heating cycle they show three thermo-

dynamically stable mesophases which have been identified as

BP-I/II, N* and SmC* phases; an unusual, monotropic mesophase,

hereafter referred to as the SmX phase, is seen additionally, while

cooling the samples from their isotropic phase. Most importantly,

the SmC* phase, existing over 60 1C thermal span, shows ferro-

electric switching property with the P_svalue of about 100 nC cm^ꢀ2

.

It seems, therefore, that the present simple molecular architecture,

which has been overlooked hitherto, supports the stabilization of

the SmC* phase convincingly. The details of these experimental

investigations with analysis of the results are discussed below.

2.2.1. Microscopic and calorimetric studies. Liquid crystallinity

of all the five pairs of enantiomers was first ascertained based on the

textural observation where the characteristic optical birefringence

Scheme 1 Synthesis of chiral, three-ring rod-like LCs. Reagents & condi-

tions: (i) (S)-(+)-2-octanol, Ph₃P, DIAD, THF, rt, 18 h; (ii) (R)-(ꢀ)-2-octanol,

Ph₃P, DIAD, THF, rt, 18 h; (iii) H₂, (1 atm, ballon), 10% Pd–C, THF, rt, 4 h;

(iv) n-bromo-alkane, anhydrous K₂CO₃, DMF, 80 1C, 12 h. (v) Jones

reagent, acetone, rt, 1 h; (vi) 4-hydroxybenzaldehyde, DCC, DMAP, THF,

rt, 4 h. (vii) II & IV, EtOH, AcOH, reflux, 5 h.

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Table 1 Phase transition temperatures (1C)^aand associated enthalpies (kJ mol^ꢀ1) of transitions noted for three-ring rod-like SB(S)-n and SB(R)-n series

of compounds. ꢁ = enantiotropic phase; (ꢁ) = monotropic LC phase. Cr: crystal; SmX: unknown smectic phase; SmC*: chiral smectic C phase; N*: chiral

nematic phase; BP = blue phase-I or blue phase-II

Heating

Cooling

Heating

Cooling

Heating

Cooling

Heating

Cooling

Heating

Cooling

LCs

Cr

SmX

SmC*

N*

BP

I

SB(S)-8

ꢁ

84.5 [41.9]

53.6 [26.9]

87.2 [41.3]

54.9 [23.4]

86.1 [34.6]

54.9 [21.8]

85.9 [31.9]

55.8 [11.1]

78.0 [36.2]

31.7 [19.9]

74.6 [3.2]^c

31.0 [22.1]

66.8 [28.4]

24.7 [15.6]

68.5 [33.9]

25.3 [15.4]

63.8 [43.3]

—

(ꢁ)

—

ꢁ

122.2 [0.4]

121.3 [0.4]

122.3 [0.4]

121.4 [0.4]

124.8 [0.8]

123.7 [0.8]

124.6 [0.8]

123.8 [0.7]

127.9 [0.9]

126.9 [1.0]

127.6 [1.0]

126.8 [0.9]

128.1 [1.2]

127.1 [1.2]

128.4 [1.2]

127.3 [1.2]

130.0 [1.4]

128.9 [1.4]

130.1 [1.2]

129.1 [1.2]

ꢁ

147.9 [0.8]

147.1 [0.8]^b

148.0 [0.7]

147.0 [0.8]^b

144.2 [0.8]

143.2 [0.8]^b

143.9 [0.8]

143.1 [0.8]^b

142.8 [1.0]

141.8 [1.0]^b

142.5 [1.1]

141.7 [1.0]^b

139.0 [1.1]

138.0 [1.1]

139.3 [1.1]

138.2 [1.1]^b

138.2 [1.3]

137.1 [1.3]^b

138.3 [1.2]

137.4 [1.3]^b

ꢁ

—

147.6

—

147.3

—

143.7

—

143.4

—

142.2

—

142.0

—

138.3

—

ꢁ

59.5 [4.2]

—

59.4 [3.9]

—

58.6 [4]

—

58.7 [3.8]

—

64.8 [5.1]

—

64.9 [5.8]

—

60.2 [4.4]

—

SB(R)-8

SB(S)-9

ꢁ

(ꢁ)

ꢁ

SB(R)-9

SB(S)-10

SB(R)-10

SB(S)-11

SB(R)-11

SB(S)-12

SB(R)-12

60.0 [4.3]

—

138.6

—

137.6

—

62.6 [4.9]^d

—

63.7 [37.4]

—

62.9 [4.2]^d

137.8

a

Transition temperatures determined by both a polarizing optical microscope (POM) and peak values of the DSC traces during the first heating–

cooling cycles at 5 1C min^ꢀ1rate. The I–BP and BP–N* phase transitions were seen under POM, but they were too weak to be detected by DSC;

b

c

thus, the DH value represents the combined enthalpy for the I–BP and BP–N* transitions. An additional Cr–Cr transition has been seen at 71.9 1C

[47 kJ mol^ꢀ1]. The SmX phase freezes into a glassy state near room temperature (B30 1C) that remains unaltered till ꢀ50 1C (limitation of the

d

instrument).

coupled with spontaneous fluidity could be seen under POM;²⁴micron sized platelets, is typical of the BP-I/II phase;^3b,24in fact,

for this purpose, the sample, under investigation, held between the bright colour indicates selective reflection due to the periodic

a clean untreated glass slide and a cover slip was used. For structure. Upon cooling this phase, a transition to another meso-

confirmation of the mesophase-type assignment, both homo- phase occurs exhibiting an atypical textural pattern (Fig. 2b) which

geneously and homeotropically aligned samples were examined upon gentle shearing transformed into an oily streak pattern

thoroughly. The microphotographs of the optical textures seen occurring over a planar texture (Fig. 2c) implying the presence

for the compounds under diﬀerent experimental conditions are of the N* phase;²⁴in this planar texture, the helical axis is

presented in Fig. 2, as representative cases. Likewise, the DSC perpendicular to the glass plates, i.e., along the viewing direction.

thermograms recorded during first heating–cooling cycles of all It may be noted here that the BP–N* transition found to be too

the five pairs of enantiomers are shown in Fig. 3a. The peak weak to be detected by DSC and therefore the enthalpy (DH) value,

temperatures of the phase transitions noted from the DSC ranging between 0.8 to 1.3 kJ mol^ꢀ1(see Table 1), represents the

traces of the heating–cooling cycles were found to be in combined DH for the I–BP and BP–N* transitions.

complete agreement with those of the optical experiments. In

Upon further cooling from the unsheared N* phase, a meso-

fact, all the ten mesogens, although comprising imine linkage, phase appears displaying a texture comprising two diﬀerent

were found to be thermally stable exhibiting virtually identical patterns originating from the planar and homeotropic orientations

phase sequences with unaffected transition temperatures for any of molecules. It is apparent from the Fig. 2d that the planar region

number of heating–cooling runs of both optical and calorimetric shows the concentric circular rings as well as broken focal-conics

studies. Fig. 3b, for example, depicts the DSC traces obtained for superimposed by the equidistant (dechiralization) lines. On the

the three consecutive heating–cooling cycles of an enantiomer other hand, the homeotropic domains display a dull-grayish

where the aforesaid features can be visualized corroborating our pattern; notably, at some edges of the cell a grayish schlieren-like

remarks indubitably.

texture was also seen (Fig. 2e). These optical textures observed are

The optical textural patterns showed typical defects asso- indeed the characteristics of the SmC* phase.²⁴As can be seen in

ciated with LC phases such as BPI/II, N* and SmC* phases; the Table 1, the enthalpy of the N*–SmC* transition was found to be

SmX also exhibited textures that were not enough to assign the in the range of 0.4 to 1.4 kJ mol^ꢀ1. Upon further cooling, a sharp

structure. These samples, sandwiched between clean glass transition from the SmC* phase to the SmX phase occurs where

slides and cooled slowly from the isotropic phase, exhibit a the pseudo-isotropic pattern and distorted circular domains as

LC phase with a striking textural pattern emanating from the well as the broken focal-conic fan texture (Fig. 2f) were seen due

black background of the isotropic phase that quickly fills the to the homogeneous and homeotropic orientations of the con-

field of view (Fig. 2a) but exists for a very short thermal range. stituent mesogens, respectively. The presence of focal-conic

This texture, showing a brightly coloured pattern of individual, domains suggests a layered arrangement, while the homeotropic

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Fig. 2 Microphotographs of the optical textural patterns of the mesophases of SB(R)-10 held between two untreated slides (a–f) and treated glass

substrates (g–i): (a) the platelet texture of the BP-I/II occurring just below the isotropic phase (141.8 1C); (b) the vague texture obtained for the N* phase at

135 1C; (c) the typical oily streak texture of the N* phase formed at 130 1C when the pattern-(b) was sheared gently; (d) planarly, and homeotropically

aligned SmC* phase showing the banded concentric circles as well as the banded broken focal-conic texture, and the intense dark-greyish pattern

(115 1C), respectively; (e) the schlieren-like pattern found at the edge of the slide for the SmC* phase (115 1C); (f) the distorted circular domains as well as

the broken focal-conic fan pattern and the pseudo-isotropic texture, respectively, observed for the homogeneously and homeotropically oriented SmX

phase at 58 1C; (g) the occurrence of a classical fan-shaped texture, featuring equidistant lines on top of it, when the SmC* phase was subjected to planar

boundary conditions; (h) the textural pattern seen at 58 1C for the planarly aligned SmX phase where focal-conics, fan-shaped pattern and polygonal

domains can be seen; (i) the cloudy texture seen for the homeotropically oriented SmC* phase at 121 1C.

Fig. 3 (a) DSC traces obtained during the first heating–cooling cycles at a rate of 5 1C for all the five pairs of enantiomers; (b) DSC traces recorded for the three

successive heating (h-1, h-2 & h-3)–cooling (c-1, c-2 & c-3) cycles at a rate of 5 1C for the mesogen SB(S)-11; note that the peak positions of all the traces match.

orientation (dark-field of view) implies an orthogonal arrange- five pairs of enantiomers synthesized. To corroborate this

ment of the mesogens with respect to the layer planes. Enthalpy assignment, all of them were further examined for their optical

change measured for this transition, which is in the range of textural behaviour in glass substrates treated with polyimide

3.9 to 5.8 kJ mol^ꢀ1, was found to be much higher than those and octadecyltriethoxysilane (ODSE) that respectively ensure

obtained for the I–BP–N* and N*–SmC* transitions. Based on planar and homeotropic anchoring of mesogens. The platelet

these results it may be appropriate to point out that the SmX texture (as shown in Fig. 2a) was observed for the BP-I/II when

phase is a highly ordered smectic phase; we shall discuss about substrates treated for either planar or homeotropic geometry were

this topic later based on the results of XRD.

used. Likewise, an uncharacteristic textural pattern, as described

Thus, using ordinary (untreated) glass substrates a phase before (see Fig. 2b), was seen for the N* phase regardless of the

sequence viz., BP–N*–SmC*–SmX was established for all the surface treatment employed. When the SmC* phase is formed

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from the planar texture of the N* phase, striking fan-shaped (see, Fig. 4a and b red traces), the wider angle regime (2y E 201)

domains (batonnets) grow with the simultaneous appearance contained a weak and diﬀuse peak (viz., insets of Fig. 4a and b

of equidistant lines (stripes) on the surface of the fans. Given red traces). Indeed, XRD patterns with the single, high-angle

the fact that the SmC* phase is a layered phase formed by chiral diﬀuse scattering accompanied by a sharp low angle Bragg

molecules, it contains a modulated structure of the director reflection is typical of a smectic phase wherein the constituent

and thus, a striped microscopic texture emerges implying the mesogens, being orientationally aligned, form a layered structure

existence of a helical super structure. A transient textural pattern with inter-molecular separation within the smectic layer arising

observed while approaching the transition to the SmC* phase due to the liquid-like positional correlation in the smectic layer.

from the planarly aligned N* phase, indicates that the orientation Fig. 5 portrays the thermal variation of the layer spacing (d)

of helix axes of the N* phase perpendicular to the substrate plane derived from the low angle Bragg reflections. Table 2 provides

whereas that of the SmC* phase remains in the plane of the spacings d related to the low-angle reflections, wide-angle peak

substrate. Upon cooling the samples from the planar texture of positions and the molecular length (L) of the two mesogens

the SmC* phase, a phase transition to the SmX occurs exhibiting in their most extended form with an all-trans conformation of

a texture having focal-conics, a fan-shaped pattern and polygonal the alkoxy chains; the d/L ratio and the tilt angle (y) of the

domains, as shown in Fig. 2h. It is well-known that if the helix mesogens, estimated using the expression y = cos^ꢀ1(d/L), are

axis of the SmC* phase is along the direction of light propagation also given in the table.

(optic axis) then a grayish/cloudy textural pattern is seen; such a

It is clear from the accumulated data that the d-values,

pattern comprises a schlieren texture if the molecular tilt angle is ranging between 30.8–32.8 Å for the compound SB(R)-8 and

high.²⁴Thus, the SmC* phase of the samples held between 33.6–34.6 Å for SB(R)-12, are smaller than the respective esti-

substrates treated for homeotropic anchoring displayed a cloudy mated all-trans (theoretical) molecular length of 3.75 nm and

texture as shown in Fig. 2i. Whereas the SmX phase showed a 4.25 nm (see Fig. 6) and accordingly, the d/L ratios are less than

pseudo-isotropic pattern when investigated under the experi- one. These results obviously imply either a tilted monolayer

mental condition mentioned above.

arrangement of the molecules with respect to the layer normal

2.2.2. X-ray diﬀraction. In order to elucidate the structure direction or interdigitated organization. However, given the fact

of the smectic (SmC* and SmX) phases, X-ray measurements that d-values are shorter than the fully stretched molecular length,

were performed on unaligned (powder) samples namely, SB(R)-8

and SB(R)-12, as representative cases. The XRD experiments were

carried out with CuK_a(l = 0.15418 nm) radiation. The samples

under investigation were heated slightly above their isotropization

temperature and filled individually into two diﬀerent Lindemann

capillary (1 mm diameter) tubes and both the ends of the tubes

were flame sealed. The samples were heated and cooled from their

isotropic phase, and the profiles were recorded as a function of

temperature in the entire SmC* phase regime. Indeed, compar-

able diﬀraction patterns were obtained for both mesogens in

their SmC* phase. Fig. 4a and b (red traces) illustrate the typical

intensity versus diﬀraction (2y) profiles obtained for SB(R)-8

(at 110 1C) and SB(R)-12 (125 1C) respectively.

Specifically, the diﬀractograms obtained at diﬀerent tem-

peratures exhibited analogous characteristics; while a sharp

Fig. 5 The layer spacing (d/Å) recorded as a function of temperature in the

and intense peak was seen in the small angle region (0 o 2y o 51) SmC* phase of the mesogens SB(R)-8 and SB(R)-12, as representative cases.

Fig. 4 The low-angle and wide-angle (inset) regions of the XRD patterns obtained for the smectic phases of mesogens: (a) the intensity vs. 2y profile

obtained in the SmC* phase of SB(R)-8; (b) the intensity vs. 2y profiles of the SmC* phase (red traces) and SmX phase (blue traces) belonging to SB(R)-12.

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Table 2 XRD and related data of the SmC* belonging to compounds SB(R)-8 and SB(R)-12: the spacings (d) corresponding to the low-angle reflections,

the wide-angle peak positions, the molecular length (L) in the most extended form with an all-trans conformation of the alkoxy chains, d/L ratio and tilt

angle of the mesogens

Layer spacing – d

(low-angle peak position)/Å

Wide-angle peak

position/Å

Tilt angle (y)

y = cos^ꢀ1(d/L)

Mesogen (L/Å)

SB(R)-8 (37.5)

Temperature/1C

d/L

114

110

109

104

99

94

89

84

79

69

64

32.8

32.6

32.4

32.05

32.1

31.5

31.3

31.2

31.0

30.89

30.86

4.58

0.874

0.869

0.864

0.855

0.856

0.840

0.835

0.832

0.827

0.824

0.823

29.0

29.7

30.2

31.2

31.1

32.9

33.4

33.7

34.2

34.5

34.6

SB(R)-12 (42.5)

125

120

115

110

105

100

95

90

85

80

75

34.6

34.2

33.9

33.8

33.7

33.62

33.65

33.6

33.7

33.8

34.0

34.2

34.6

4.6

0.814

0.805

0.798

0.795

0.793

0.791

0.792

0.791

0.793

0.795

0.800

0.805

0.814

35.5

36.4

37.1

37.3

37.5

37.7

37.6

37.7

37.5

37.3

36.9

36.4

35.5

70

65

but longer than a layer spacing of the possible intercalated

Fig. 4b shows the intensity vs. 2y profiles recorded at 61 1C

structure, it is appropriate to consider the lamellar structure for the SmX phase (blue traces) of mesogen SB(R)-12; in fact,

under investigation as the SmC* phase where the constituent a virtually identical XRD pattern was obtained at 50 1C. Both the

mesogens tilt from layer to layer about an axis perpendicular diﬀractograms obtained at 61 1C and 55 1C comprised a sharp

to the layer planes yielding a helical structure; this view is and intense reflection in the low-angle region with the respective

strongly substantiated by the observation of a characteristic spacing being 44.68 Å and 44.73 Å. The occurrence of a sharp

banded fan-shaped texture as described earlier. As can be seen low-angle peak suggests a lamellar structure with the layer

in Fig. 5 (red-trace), for the mesogen SB(R)-8 the layer spacing thickness d, which is nearly equal to the theoretical molecular

decreases almost smoothly upon cooling the phase tempera- length of 4.25 nm suggesting that the long axes of mesogens

ture implying that the tilt angle of the molecules within the (director, n) lie along the layer normal, which is supported by

smectic layers increases upon lowering the temperature; while the results of a microscopic study; that is, the observation of a

for the compound SB(R)-12 the layer spacing decreases first, fan-shaped texture imply a layered structure, whereas the pseudo-

as expected, and then increases slightly upon cooling the isotropic pattern of the homeotropic alignment, imply an ortho-

SmC* phase (Fig. 5 blue-trace) which can be attributed to some gonal organization of mesogens with respect to the smectic layer

fine structural variation in the smectic layers. In essence, the planes. Interestingly, at wide-angles, both the profiles showed a

XRD study confirms the tilted organization of the mesogens diﬀuse and broad-scattering halo with d = 4.44 and 4.45 as well

within the smectic layers and thus, supports the presence of the as another intense and sharp peak with the spacings of 4.43 and

SmC* phase.

4.42 Å. The latter reflections of the two profiles are characteristic

Fig. 6 Energy-minimized ball-and-stick models of the most extended form with an all-trans conformation of the alkoxy chains (derived from MM2 of

the ChemBioDraw Ultra 12.0) of the SB(R)-8 and SB(R)-12 with respective theoretical molecular lengths of 37.5 Å and 42.5 Å.

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of liquid-like order within the smectic planes while the former change in molecular tilt direction from layer to layer, about an

peaks indicate the additional ordering of the molecules within axis vertical to the layer planes. The reduced C₂symmetry and

the layers. Seemingly, the experimental data available at this tilting of the constituent chiral mesogens result in P_sin the plane

point of time are inadequate to figure out the explicit structure of each layer, which, however, averages out to zero due to the

of the SmX phase although XRD studies point to a possible helical structure. As is well known, to obtain a macroscopic P_s

tilted hexatic phase.

the helix needs to be unwound by application of an electric field.

We now return to the temperature dependence of the layer Thus, to investigate the polar properties of the SmC* phase

spacing (tilt angle) in the SmC* phase. Both the materials show electric field experiments were carried out on mesogens SB(R)-8

large tilt angle values characteristic of systems exhibiting a direct and SB(R)-12 as representative cases. The cells fabricated using

transition from the N/N* to SmC/SmC* phase. The temperature two ITO coated glass plates pre-treated with a polyimide solution,

dependence presents an important diﬀerence between the two and rubbed unidirectionally, which enable the homogeneous

compounds. While the shorter homologue, SB(R)-8, has mono- orientation of the mesogens, were used for the study. The samples

tonically decreasing layer spacing (an increasing tilt angle) typical were filled into cells in their isotropic phase through capillary

of the SmC* phase, the longer homologue has an anomalous action and cooled slowly to the mesophase. A low frequency AC

behaviour. Upon cooling the sample, the layer spacing decreases triangular-wave voltage was applied and slowly increased. At a

up to a certain temperature and then reverses its trend (see voltage of 46.8 V_rmsand a frequency of 20 Hz, the SmC* phase of

Fig. 5). Such an unusual feature is not common in materials the sample SB(R)-8 exhibited a profile comprising a single polari-

presenting the N*–SmC* transition. Even in the more com- zation current peak for each half period indicating a ferroelectric

monly occurring SmA–SmC* transition systems only a few switching behaviour. As a representative case the current response

materials are known with such a phenomenon. The SmA phase profile obtained at 90 1C is shown Fig. 7a. The textural change

of such materials has been identified to be of the technologi- observed under POM in the field on-state and oﬀ-state as are

cally important de Vries.²⁵A possible explanation for the trend presented in Fig. 7c, d and b respectively. The macroscopic P_s

reversal of the layer spacing (or tilt angle) is that the negative value, obtained by integrating the area under the peaks, was

thermal expansion of the layer structure (owing to the stretching found to be B108 nC cm^ꢀ2. Likewise, by applying a triangular-

of the terminal alkyl chains) counterbalances the decrease in the wave electric field of about 18.5 V_rmsand a frequency of 20 Hz,

layer spacing due to the tilted nature of the phase. A second the mesophase of SB(R)-12 exhibited a single polarization current

possibility is that the reversal is a pretransition variation upon peak with P_s= 123.8 nC cm^ꢀ2; switching current response trace

approaching the low temperature SmX phase. In fact, when and optical textures obtained under these experimental condi-

the low temperature phase is a tilted hexatic such a feature is tions are shown in Fig. 7e and f, h respectively. It must be

indeed seen.²⁶

mentioned here that the switching peak of the SmC* phase

2.2.3. Electrical switching study. As discussed before, the vanishes completely when the samples are heated to their iso-

helical structure of the SmC* phase stems as a result of a gradual tropic melt implying the origin of switching from the mesophase

Fig. 7 Switching current response peaks obtained upon applying a triangular-wave filed for the SmC* phase of mesogens SB(R)-8 (90 1C; 46.8 V_rms

,

20 Hz; 09 kO; P_s= 108 nC cm^ꢀ2) (a) and SB(R)-12, (70 1C; 18.5 V_rms, 20 Hz; 09 kO; P_s= 123.8 nC cm^ꢀ2) (e). Microphotographs of the microscopic textural

changes observed under the applied electric field (c, d & g, h) and in the absence of field (b & f) (bar: 100 mm).

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technique to study the chirooptical behavior of the N* ^11,13,15,27

and SmC* ^20b,28phases. As is well-known, in a well-aligned homo-

geneous orientation (the Grandjean texture), the director of the N*

phase rotates in a helical fashion around a perpendicular axis with

a pitch p; depending upon the chirality of the constituent meso-

gens the pitch value ranges from hundreds of nanometers

(for strongly chiral mesogens) to hundreds of micrometers.

The helix-sense or handedness (left or right) of the N* phase

describes the direction in which rotation of the director occurs

from one layer to another. The CD spectral study helps in

figuring out handedness as the circularly polarised light (CPL),

being a chiral system, interacts diﬀerently with the N* phase

formed by the two enantiomers of a chiral compound. That is, the

N* phase shows CD at a given wavelength where the incident light

is resolved into its two CP components namely, left and right; at

this wavelength, depending upon the natural helical sense of the

phase formed by a particular enantiomer, the CPL of a particular

handedness is completely reflected while its counterpart is trans-

mitted. In other words the rotation of the reflected CPL agrees

with the screw sense of the N* helix while the transmitted light

possesses opposite handedness. Thus, the handedness (screw

sense) of the helical array of the N* or SmC* phase formed by

the enantiomers (mirror-image isomers) should be opposite to

each other; consequently, the enantiomeric pairs display mirror

image signals in CD spectra. In order to figure out such chiro-

optical properties, the N* and SmC* phases formed by a pair

of enantiomers, SB(R)-9 and SB(S)-9, were subjected to a CD

spectroscopic study, as representative cases.

The CD experiments were carried out using thin films of the

above mesogens contained between two quartz plates. The

samples were heated slightly (5 1C) above their isotropic melt

temperature and hard-pressed; this process was repeated a couple

of times, which ensures the spreading of the sample thinly and

evenly. In fact, such thin films were found to be crucial to work

within the upper limits (saturation) of measurement where the

stronger CD signals were found to be out of the measurable range

of the CD instrument. This means that the cells of known

thickness could not be used and hence, the present CD experi-

ments are qualitative. The samples, such as above, were cooled

slowly from their isotropic melt and mechanically sheared

repeatedly when the N* phase appears. Thereby, the N* phase

attains the Grandjean texture where the helix lies normal to the

quartz plates that reflects the incident light readily. While

cooling the samples, the CD spectra were recorded in the entire

thermal range of both N* and SmC* phases. As shown in Fig. 10,

both the phases of the samples displayed the CD signals, as

expected, where the intensity of the peaks increases with decrease

in temperature; the position, sign and intensity of the CD bands

obtained as a function of temperature for the enantiomeric pairs

are given in Table 3. It must be noted here that, the CD

phenomenon, when compared to the N* phase, was found to

be immeasurably small (nearly zero) in the spectra of the

isotropic melt implying the origin of the CD activity from the

chiral LC structure but not from the molecular chirality. Further,

the presence of CD peaks in both the LC phases of the samples

was validated by obtaining the spectra by 901 in-plane rotation of

Fig. 8 A profile showing the temperature dependence of the P_sin the

SmC* phase of compounds SB(R)-8 (red-trace) and SB(R)-12 (blue-trace).

but not from the ionic impurities. Fig. 8 illustrates the tempera-

ture dependence of P_swhere it can be seen that with increasing

temperature the P_sdecreases progressively and disappears across

the SmC*–N* transition.

Hence, the foregoing experimental results confirm the ferro-

electric nature of the chiral smectic phase. It may also be men-

tioned here that these samples were found to be very stable even

after repetitive application of the electric field.

2.2.4. UV-vis and circular dichroism spectroscopic studies.

Considering the fact that the present series of compounds are

chiral and possess light absorbing groups (chromophores), they

were investigated for their photophysical and chirooptical pro-

perties with the aid of UV-vis absorption and circular dichroism

(CD) spectroscopic techniques, respectively. The solution UV-vis

spectra (B3 ꢂ 10^ꢀ3M in CH₂Cl₂; cell length = 1 mm) of all the

mesogens were found to be virtually identical; as representative

cases, the spectra obtained for all the (S)-enantiomers are shown

in Fig. 9. Seemingly, each spectrum comprises two peaks at B275

and 340 nm which can be assigned to p–p* and n–p* transitions,

respectively. However, CD signals of the corresponding wave-

lengths were found to be non-existent (zero) in the solution CD

spectra of the samples implying the absence of chiral aggrega-

tions and thus, macroscopic helical ordering.

CD spectroscopy, wherein the circular dichroism is measured

as a function of wavelength, has been demonstrated as a powerful

Fig. 9 UV-vis spectrum recorded for the dilute (dichloromethane) solutions

of the (S)-enantiomers.

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Fig. 10 CD spectra recorded as a function of temperature in the N* (a and c) and SmC* (b and d) phases exhibited by the enantiomer SB(R)-9 and SB(S)-9.

the samples; the spectra were found to be nearly identical to However, it is interesting to note that the CD peak obtained in

those obtained in the pristine orientation. This infers that the the SmC* phase of SB(R)-9 is opposite (ꢀve) (Fig. 10b) when

CD signal is not arising from the possible linear dichroism (LD) compared to that of the +ve CD band seen for the N* phase

artifact. Further, the CD instrument used has a built in facility (Fig. 10a) of the same sample under identical experimental

to measure the LD directly. Thus, both LC phases were subjected conditions; thus, the sense of the helix in the N* phase is right-

to direct LD measurement where the activity was found to be handed while it is left-handed in the case of the SmC* phase.

zero, as contemplated.

Similarly, CD bands are positive (Fig. 10d) and negative (Fig. 10c)

As can be seen in Fig. 10a–d, the CD spectra obtained for for the SmC* and N* phases respectively formed by an enantio-

the N* and SmC* phases of enantiomers SB(R)-9 and SB(S)-9 mer SB(S)-9. These results show that the SmC* and N* structures

exhibit an excellent mirror-image relationship over their entire derived from an enantiomer possess opposite helix-sense. That

thermal range. From the Fig. 10a it can be specifically seen that is, the reversal of the helix-sense (from right to left and vice versa)

the CD spectra recorded at different temperatures for the N* occurs during the N* to SmC* phase transition. This is the first

phase of an enantiomer SB(R)-9 exhibit a broad positive peak time that such an observation has been made where the screw

with the l value ranging between 419–426 nm (Table 3) which sense of the helical array of the N* phase and the SmC* phase of

corresponds to the absorption wavelength of n–p* transition an enantiomer is opposite.

of the chiral chromophore; whereas its counterpart N* phase,

belonging to the (S)-enantiomer, SB(S)-9, displays a negative

signal (l = 418–430 nm; see Fig. 10c and Table 3), as expected.

3. Summary

It is a very widespread conception that²⁹the positive CD band

corresponds to a right-handed screw sense of a helicoidal Ten new three-ring rod-like chiral compounds, especially five

structure. Thus, the helix sense of the N* phase formed by pairs of enantiomers, have been synthesized and evaluated for

SB(R)-9, exhibiting a positive CD peak, is right-handed, whereas mesomorphism and optical properties. Each pair of enantio-

it is left-handed for the N* phase of SB(S)-9 as it shows a mers consists of (R)-2-octyloxy and (S)-2-octyloxy chains at one

negative signal. The temperature dependence of the CD spectra end while the other terminus is substituted with an n-alkoxy

recorded in the entire SmC* phase range of the enantiomers tail. Thermal investigations reveal an identical mesomorphic

SB(R)-9 and SB(S)-9 are shown in Fig. 10b and d, respectively, behavior of all the five pairs of enantiomers implying that the

where the CD peaks obtained from helical structures of opposite alteration in the length of the n-alkoxy tail has no eﬀect on the

handedness are perfect mirror images. This clearly implies the observed LC phase sequence such as BP-I/II–N*–SmC*–SmX.

mirror-image chiral structures where the handedness (screw sense) However, the transition temperatures and thus, thermal width of

of the helical array of the SmC* phase formed by the two the N* and SmC* mesophases show dependence on this varia-

enantiomers (mirror-image isomers) are opposite to each other. tion. The temperature range of the SmC* phase widens when the

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Table 3 CD spectroscopic data obtained in N* and SmC* phases as a

function of temperature for the enantiomers SB(R)-9 and SB(S)-9

mesogens showed macroscopic helices of opposite handedness, as

expected. The measurements also confirmed that the handedness

of helices of the N* and SmC* phases of an enantiomer remain

opposite. To the best of our knowledge this is the first observation

that during the phase transition from the N* phase to SmC*

phase the reversal of the helix from left (right) to right (left)

handedness occurs.

CD

Chiral

Compounds LC phase Temperature (1C)

l_max/nm CD (mdeg)

SB(R)-9

SB(S)-9

SB(R)-9

N*

126

128

130

132

134

136

426

419

426

424

423

419

+97

+142

+192

+174

+84

+58

4. Experimental

4.1. General information

N*

126

128

130

132

134

136

418

424

419

418

430

ꢀ85

ꢀ178

ꢀ34

All chemicals obtained from overseas/local companies were used as

received. All chromatographic solvents were distilled prior to use

and other solvents were purified following the standard protocols.

To monitor the progress and completion of reactions, and also to

assess the purity of the compounds thin layer chromatography

(TLC) was used; for this purpose, aluminium sheets pre-coated with

silica gel (Merck, Kieselgel60, F254) were used. Silica gel (60–120,

100–200 and 230–400 mesh), neutral aluminium oxide and basic

aluminium oxide were used as stationary phases in the column

chromatography technique. All intermediates and target com-

pounds were structurally characterized primarily with the help of

IR, NMR and FTIR spectroscopic techniques. ¹H and ¹³C NMR

spectra of the compounds in CDCl₃were recorded using a Bruker

AMX-400 (400 MHz) spectrometer. For ¹H NMR spectra, the

chemical shifts are reported in parts per million (ppm) relative to

tetramethylsilane (TMS) as an internal standard. Coupling

constants ( J) are given in Hz. IR spectra were recorded on a

FT-IR spectrometer; the spectral positions are given in the wave

number (cm^ꢀ1) unit. IR spectra were recorded on a PerkinElmer

Spectrum 1000 FT-IR spectrometer. Electronic absorption

(UV-vis) spectra were recorded with the help of Perkin Elmer’s,

Lambda 20 UV-vis spectrometer. Elemental microanalysis was

ꢀ40

ꢀ133

ꢀ18

SmC*

69

74

79

84

89

382

383

382

383

ꢀ149

ꢀ141

ꢀ131

ꢀ120

ꢀ114

ꢀ109

ꢀ103

ꢀ95

94

101

114

112

116

121

ꢀ88

ꢀ79

ꢀ65

SB(S)-9

SmC*

77

83

87

93

98

103

108

113

117

122

386

385

386

387

386

387

388

+163

+156

+148

+140

+131

+121

+110

+97

+81

+67

length of the n-alkoxy tail increases. In fact, the influence of this performed using a Eurovector E300 elemental analyzer. Mass

parameter is prominently seen in the isotropization tempera- spectra were recorded on a JEOL JMS-600H spectrometer (Tokyo,

tures, which decreases with an increase in the chain length. The Japan) in FAB+ mode using 3-nitrobenzyl alcohol as a liquid

occurrence of the technologically important SmC* phase, over the matrix. Specific rotation of the compounds was measured using

thermal width of 20–55 1C, has been established unambiguously a Rudolph AUTOPOL IV Automatic Polarimeter. CD spectra were

with the help of conventional techniques such as microscopic, recorded with the aid of a Jasco J-810 spectropolorimeter.

calorimetric and XRD. The electro-optical experiments carried out Molecular lengths of target mesogens were determined (in Å)

confirm the ferroelectric switching behavior of the SmC* phase; from the energy minimized (all-trans) structure deduced from

the calculated P_svalue was found to be over 100 nC cm^ꢀ2. Notably, ChemBio3D Ultra 12.0 programme.

these mesogens, being Schiff bases, remain intact even after

The thermal behavior of the compounds were examined

subjecting them to repetitive electric switching experiments; with the help of an optical polarizing microscope (Olympus

indeed, a similar observation was made during the repetitive BX50; Model BX50F4) equipped with a programmable hot stage

heating–cooling cycles of the microscopic and calorimetric studies. (Mettler FP90) and diﬀerential scanning calorimeter (DSC)

The solution-state UV-vis spectra (in CH₂Cl₂) of the compounds (Perkin-Elmer Diamond DSC with the PC system operating on

show two peaks at B275 and 340 nm due to p–p* and n–p* Pyris software) apriorically calibrated using pure indium as a

transitions, respectively; CD bands of the corresponding wave- standard. The phase transition temperatures and associated

lengths were found to be totally absent. However, the thin films of enthalpies were determined from DSC traces recorded at a

the samples show strong CD signals in the entire temperature scanning rate of 5 1C min^ꢀ1. X-Ray diﬀraction studies were

range of both N* and SmC* phases confirming the chromophores carried on powder samples in Lindemann capillaries with CuK_a

of the mesogens being in a macroscopic chiral environment; in (l = 0.15418 nm) radiation using the PANalytical X’Pert PRO MP

other words, the inherent helical structure of the phases is X-ray diﬀractometer consisting of a focusing elliptical mirror

evidenced formally. The N* or SmC* phase formed by enantiomeric and a fast resolution detector (PIXCEL).

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4.2. Synthesis and characterization

4.2.3. General procedure for the synthesis of 4-(n-alkoxy)-

benzaldehydes (Va–e). A mixture of 4-hydroxybenzaldehyde (1 g,

8.19 mmol, 1 eq.), n-alkylbromide (9.01 mmol, 1.1 eq.), anhydrous

K₂CO₃(9.01 mmol, 1.1 eq.) and DMF was heated in an inert

atmosphere for 12 h. After cooling, the reaction mixture was

poured into ice-cold water and the product was extracted with

dichloromethane (50 mL ꢂ 3). The combined organic layers

were washed with cold aqueous (5%) NaOH solution (25 mL ꢂ 3),

water (25 mL ꢂ 3), brine and dried over anhydrous Na₂SO₄. The

solvent was removed in vacuo and the crude product obtained was

purified by column chromatography on silica gel (100–200 mesh)

using 10% EtOAc–hexanes as an eluent.

4.2.1. General procedure for the synthesis of (R)- and (S)-1-

nitro-4-(octan-2-yloxy)benzenes (I and III). To a cooled (10–15 1C)

and magnetically well-stirred solution of 4-nitrophenol (7.2 mmol,

1 eq.), (R)-2-octanol or (S)-2-octanol, triphenylphosphine (8.6 mmol,

1.2 eq.) in dry THF, diisopropylazo dicarboxylate was added drop

wise over a period of 5 min. under an argon atmosphere. The

reaction mixture was allowed to attain room temperature (RT)

and stirred for 12 hours. The solvent was removed in vacuo and

extracted with dichloromethane (CH₂Cl₂), washed with 5% NaOH

(25 mL ꢂ 2) solution, water, brine and dried over anhydrous

Na₂SO₄. Evaporation of solvent in vacuo furnished crude oil,

which was purified by column chromatography using silica gel

(60–120 mesh). Elution with a mixture of 10% CH₂Cl₂–hexanes,

afforded the yellow oil.

Va. R_f= 0.51 (30% CH₂Cl₂–hexanes); a colorless liquid; yield:

1.54 g (80.0%); IR (KBr pellet): n_maxin cm^ꢀ12959, 2873, 1695,

1

1601, 1231, 1128 and 1113; H NMR (400 MHz, CDCl₃): d 9.88

(s, 1H, –CHO), 7.84 (d, J = 6.8 Hz, 2H, Ar), 7.01 (d, J = 6.8 Hz, 2H,

Ar), 4.05 (t, J = 6.8 Hz, 2H, –OCH₂), 1.85–1.28 (m, 12H, 6 ꢂ CH₂)

and 0.90 (t, J = 6.8 Hz, 3H, –CH₃); MS (FAB+): m/z for C₁₅H₂₃O₂

(M + 1), calculated: 235.2, found: 235.2.

(R)-1-Nitro-4-(octan-2-yloxy)benzene (I). R_f= 0.6 (20% EtOAc–

hexane); a pale yellow liquid; yield: 1.66 g (92%); IR (KBr pellet):

n

_maxin cm^ꢀ13451, 3085, 2932, 2859, 2447, 1910, 1607, 1592, 1513,

1

1466, 1380, 1340, 1296 and 1261; H NMR (400 MHz, CDCl₃): d

8.19 (d, J = 9.6 Hz, 2H, Ar), 6.92 (d, J = 9.6 Hz, 2H, Ar), 4.50 (m, 1H,

–O–CH–) and 1.80–0.86 (m, 16H, 2 ꢂ CH₃, 5 ꢂ CH₂).

Vb. R_f= 0.51 (30% CH₂Cl₂–hexanes); a colorless liquid; yield:

1.67 g (82.0%); IR (KBr pellet): n_maxin cm^ꢀ12959, 2877, 1699,

1601, 1259, 1160 and 1112; ¹H NMR (400 MHz, CDCl₃): d 9.9 (s,

1H, –CHO), 7.82 (d, J = 8.4 Hz, 2H, Ar), 6.99 (d, J = 8.8 Hz, 2H,

Ar), 4.04 (t, J = 6.4 Hz, 2H, –OCH₂), 1.88–1.31 (m, 14H, 7 ꢂ CH₂)

and 0.87 (t, J = 6.6 Hz, 3H, –CH₃); MS (FAB+): m/z for C₁₆H₂₅O₂

(M + 1), calculated: 249.2, found: 249.2.

(S)-1-Nitro-4-(octan-2-yloxy)benzene (III). R_f= 0.59 (20% EtOAc–

hexane); a pale yellow liquid; yield: 1.65 g (91%); IR (KBr pellet):

n_maxin cm^ꢀ13451, 3085, 2932, 2858, 2447, 1909, 1592, 1513,

1494, 1465, 1380, 1340, 1296 and 1261; ¹H NMR (400 MHz,

CDCl₃): d 8.19 (d, J = 9.2 Hz, 2H, Ar), 6.92 (d, J = 9.2 Hz,

2H, Ar), 4.51 (m, 1H, –O–CH–) and 1.80–0.86 (m, 16H, 2 ꢂ CH₃,

5 ꢂ CH₂).

Vc. R_f= 0.52 (30% CH₂Cl₂–hexanes); a colorless liquid; yield:

1.82 g (85.0%); IR (KBr pellet): n_maxin cm^ꢀ12926, 2855, 1670,

1215, 1160 and 1109; ¹H NMR (400 MHz, CDCl₃): d 9.88 (s, 1H,

–CHO), 7.84 (d, J = 8.8 Hz, 2H, Ar), 7.01 (d, J = 6.8 Hz, 2H, Ar),

4.05 (t, J = 6.4 Hz, 2H, –OCH₂), 1.83–1.30 (m, 16H, 8 ꢂ CH₂) and

0.90 (t, J = 6.8 Hz, 3H, –CH₃); MS (FAB+): m/z for C₁₇H₂₆O₂,

calculated: 262.2, found: 262.5.

4.2.2. General procedure for the synthesis of (R)- and (S)-4-

(octan-2-yloxy)anilines (II and IV). (R)-(I) or (S)-1-nitro-4-(octan-

2-yloxy)benzene (III) (0.1 g, 0.4 mmol, 1 eq.) was dissolved in dry

THF and 10% Pd–C (0.01 g) was added. The resultant reaction

mixture was degassed and stirred under H₂gas for 4 h at rt. The

reaction mixture was filtered, concentrated and the liquid

obtained was purified by flash column chromatography using

basic alumina as a stationary phase and 20% EtOAc–hexanes as

an eluent.

Vd. R_f= 0.52 (30% CH₂Cl₂–hexanes); a colorless liquid; yield:

1.76 g (78.0%); IR (KBr pellet): n_maxin cm^ꢀ12962, 2877, 1700,

1606, 1262, 1164 and 1113; ¹H NMR (400 MHz, CDCl₃): d 9.9 (s,

1H, –CHO), 7.82 (d, J = 8.4 Hz, 2H, Ar), 6.99 (d, J = 8.8 Hz, 2H,

Ar), 4.04 (t, J = 6.4 Hz, 2H, –OCH₂), 1.88–1.31 (m, 18H, 9 ꢂ CH₂)

and 0.87 (t, J = 6.6 Hz, 3H, –CH₃); MS (FAB+): m/z for C₁₈H₂₈O₂,

calculated: 276.2, found: 276.6.

(R)-4-(Octan-2-yloxy)aniline (II). R_f= 0.23 (10% EtOAc–hexane); a

yellow liquid; yield: 0.081 g (92%); IR (KBr pellet): n_maxin cm^ꢀ1

3358, 3222, 3036, 2957, 2929, 2857, 1624, 1509, 1463, 1376 and

1232; ¹H NMR (400 MHz, CDCl₃): d 6.74 (d, J = 8.8 Hz, 2H, Ar),

6.63 (d, J = 8.8 Hz, 2H Ar), 4.19 (m, 1H, –O–CH–), 3.42 (brs,

2H, –NH₂) and 1.73–0.86 (m, 16H, 2 ꢂ CH₃, 5 ꢂ CH₂); ¹³C NMR

(100 MHz): 151.20, 140.04, 117.84, 116.34, 75.23, 36.55, 31.78,

29.28, 25.53, 22.56, 19.83 and 14.03.

Ve. R_f= 0.53 (30% CH₂Cl₂–hexanes); a colourless liquid;

yield: 2.01 g (85.0%); IR (KBr pellet): n_maxin cm^ꢀ12927, 2855,

1

1694, 1230, 1160 and 1110; H NMR (400 MHz, CDCl₃): d 9.88

(s, 1H, –CHO), 7.83 (d, J = 6.8 Hz, 2H, Ar), 7.00 (d, J = 6.8 Hz, 2H,

Ar), 4.04 (t, J = 6.4 Hz, 2H, –OCH₂), 1.88–1.31 (m, 20H, 10 ꢂ

(S)-4-(Octan-2-yloxy)aniline (IV). R_f= 0.24 (20% EtOAc–hexane); CH₂) and 0.87 (t, J = 6.6 Hz, 3H, –CH₃); MS (FAB+): m/z for

a yellow liquid; yield: 0.08 g (91%); IR (KBr pellet): n_maxin cm^ꢀ1

C

₂₀H₃₀O₂, calculated: 290.1, found: 290.6.

3357, 3222, 2956, 2829, 2857, 1854, 1624, 1508, 1465, 1376 and

4.2.4. General procedure for the synthesis of 4-(n-alkoxy)-

1233; ¹H NMR (400 MHz, CDCl₃): d 6.74 (d, J = 8.8 Hz, 2H, Ar); benzoic acids (VIa–e). A solution of 4-(n-alkoxy)benzaldehyde

6.63 (d, J = 8.8 Hz, 2H Ar); 4.19 (m, 1H, –O–CH–), 3.40 (brs, (Va–e) (1 g) in dry acetone 20 mL was cooled to 0 1C and Jones

2H, –NH₂) and 1.74–0.86 (m, 16H, 2 ꢂ CH₃, 5 ꢂ CH₂); ¹³C NMR reagent (1.75 g of CrO₃dissolved in 5 mL water and 2 mL

(100 MHz): 150.8, 139.6, 117.53, 115.9, 74.9, 36.12, 31.35, 29.22, H₂SO₄) was added slowly maintaining the temperature below

25.1, 22.12, 19.40 and 13.58.

5 1C. The reaction mixture was stirred at RT for 1 h and poured

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into cold water. A colourless precipitate obtained was filtered,

VIIa. R_f= 0.29 (10% EtOAc–hexane); a colourless solid; yield:

washed with water thoroughly and air dried. The crude material 0.692 g (89%); m.p = 66.1 1C; IR (KBr pellet): n_maxin cm^ꢀ12933,

was recrystallized from hexanes to get the pure product.

2854, 1731, 1694, 1602, 1581, 1510, 1468, 1420, 1396 and 1306 cm^ꢀ1

,

¹H NMR (400 MHz, CDCl₃): d 10.02 (s, 1H, Ar-CHO), 8.14 (d, 2H,

J = 8.8 Hz, Ar), 7.97 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 1H,

Ar), 6.99 (d, J = 9.2 Hz, 2H, Ar), 4.06 (t, J = 6.6 Hz, 2H, –OCH₂)

and 1.86–0.91 (m, 15H, 1 ꢂ CH₃, 6 ꢂ CH₂); MS (FAB+): m/z for

VIa. R_f= 0.19 (20% EtOAc–hexanes); a colourless solid; yield:

1.27 g (84%); phase sequence: Cr 76.6 1C [60.5 J g^ꢀ1] Cr₁101.3

[44.9] SmC 107.5 [4.9] N 147.9 [7.4] I; IR (KBr pellet): n_max

in cm^ꢀ12919, 2849, 1775, 1686, 1606, 1578, 1256 and 1169;

¹H NMR (400 MHz, CDCl₃): d 8.06 (d, J = 8.8 Hz, 2H, Ar), 6.94 (d,

J = 9.2 Hz, 2H, Ar), 4.04 (t, J = 6.6 Hz, 2H, –OCH₂), 1.84–1.23

C

₂₂H₂₆O₄, calculated: 354.2, found: 355.2.

VIIb. R_f= 0.30 (10% EtOAc–hexane); a colourless solid; yield:

(m, 12H, 6 ꢂ CH₂) and 0.91 (t, J = 6.8 Hz, 3H, –CH₃₎; MS (FAB+): 0.700 g (81%); m.p = 67.4 1C; IR (KBr pellet): n_maxin cm^ꢀ12918,

m/z for C₁₅H₂₂O₃, calculated: 250.2, found: 250.3.

2851, 1737, 1698, 1603, 1580, 1510, 1471, 1422, 1397 and 1310 cm^ꢀ1

;

¹H NMR (400 MHz, CDCl₃): d 10.02 (s, 1H, Ar-CHO), 8.14 (d, J =

8.8 Hz, 2H, Ar), 7.97 (d, J = 8.4 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz,

2H, Ar), 6.99 (d, J = 8.8 Hz, 2H, Ar), 4.06 (t, J = 6.4 Hz, 2H,

–OCH₂) and 1.86–0.87 (m, 17H, 1 ꢂ CH₃, 7 ꢂ CH₂); MS (FAB+):

m/z for C₂₃H₂₈O₄, calculated: 368.2, found: 368.7.

VIb. R_f= 0.19 (20% EtOAc–hexanes); a colourless solid; yield:

1.36 g (78%); Cr 94.6 1C [124.5 J g^ꢀ1] SmC 117.2 [6.3] N 143.5

[6.4] I; IR (KBr pellet): n_maxin cm^ꢀ12918, 2851, 1774, 1686,

1

1606, 1578, 1257 and 1169; H NMR (400 MHz, CDCl₃): d 8.06

(d, J = 9.2 Hz, 2H, Ar), 6.94 (d, J = 8.8 Hz, 2H, Ar), 4.04 (t, J =

6.6 Hz, 2H, –OCH₂), 1.84–1.28 (m, 14H, 7 ꢂ CH₂) and 0.90 (t, J =

VIIc. R_f= 0.28 (10% EtOAc–hexane); a colourless solid; yield:

6.8 Hz, 3H, –CH₃); MS (FAB+): m/z for C₁₆H₂₄O₃, calculated: 0.687 g (83%); m.p = 71.8 1C; IR (KBr pellet): n_maxin cm^ꢀ12912,

264.2, found: 265.1.

2849, 1737, 1698, 1605, 1580, 1511, 1469, 1422, 1391 and 1270 cm^ꢀ1

;

¹H NMR (400 MHz, CDCl₃): d 10.02 (s, 1H, Ar-CHO), 8.15 (d, J =

8.8 Hz, 2H, Ar), 7.97 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz,

2H, Ar), 6.99 (d, J = 9.2 Hz, 2H, Ar), 4.06 (t, J = 6.6 Hz, 2H,

–OCH₂) and 1.86–0.87 (m, 19H, 1 ꢂ CH₃, 8 ꢂ CH₂); MS (FAB+):

m/z for C₂₄H₃₀O₄, calculated: 382.2, found: 382.7.

VIc. R_f= 0.19 (20% EtOAc–hexanes); a colourless solid; yield:

1.23 g (86%); Cr 86.1 1C [77.1 J g^ꢀ1] Cr₁96 [40.8] SmC 121.6 [5.1]

N 141.1 [6.6] I; IR (KBr pellet): n_maxin cm^ꢀ12918, 2851, 1774,

1

1686, 1605, 1578, 1257 and 1170; H NMR (400 MHz, CDCl₃):

d 8.06 (d, J = 8.8 Hz, 2H, Ar), 6.94 (d, J = 9.2 Hz, 2H, Ar), 4.04

(t, J = 6.6 Hz, 2H, –OCH₂), 1.84–1.28 (m, 16H, 8 ꢂ CH₂) and 0.90

VIId. R_f= 0.27 (10% EtOAc–hexane); a colourless solid; yield:

(t, J = 7 Hz, 3H, –CH₃₎; MS (FAB+): m/z for C₁₇H₂₆O₃, calculated: 0.68 g (87%); m.p = 75 1C; IR (KBr pellet): n_maxin cm^ꢀ12918,

278.2, found: 278.5.

2850, 1733, 1691, 1607, 1510, 1471 and 1257 cm^ꢀ^{1 1}H NMR

;

(400 MHz, CDCl₃): d 10.02 (s, 1H, Ar-CHO), 8.14 (d, J = 8.4 Hz,

2H, Ar), 7.97 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 2H, Ar),

6.99 (d, J = 8.8 Hz, 2H, Ar), 4.06 (t, J = 6.4 Hz, 2H, –OCH₂) and

1.84–0.86 (m, 21H, 1 ꢂ CH₃, 9 ꢂ CH₂); MS (FAB+): m/z for

VId. R_f= 0.19 (20% EtOAc–hexanes); a colourless solid; yield:

1.37 g (77%); Cr 97.2 1C [129 J g^ꢀ1] SmC 127.1 [6] N 139.2 [6.4] I;

IR (KBr pellet): n_maxin cm^ꢀ12918, 2849, 1772, 1687, 1606, 1579,

1256 and 1170; ¹H NMR (400 MHz, CDCl₃): d 8.06 (d, J = 8.8 Hz,

2H, Ar), 6.94 (d, J = 9.2 Hz, 2H, Ar), 4.04 (t, J = 6.6 Hz, 2H, –OCH₂),

1.84–1.27 (m, 18H, 9 ꢂ CH₂) and 0.90 (t, J = 7 Hz, 3H, CH₃₎; MS

(FAB+): m/z for C₁₈H₂₈O₃, calculated: 292.2, found: 293.5.

C

₂₅H₃₂O₄, calculated: 396.2, found: 396.5.

VIIe. R_f= 0.31 (10% EtOAc–hexane); a colourless solid; yield:

0.66 g (84%); m.p = 75.6 1C; IR (KBr pellet): n_maxin cm^ꢀ12918,

2851, 1737, 1698, 1603, 1580, 1510, 1471, 1422, 1397 and 1310 cm^ꢀ1

,

VIe. R_f= 0.19 (20% EtOAc–hexanes); a colourless solid; yield:

1.25 g (84%); Cr 95.1 1C [119.9 J g^ꢀ1] SmC 132.6 [7.7] N 139.1

[6.7] I IR (KBr pellet): n_maxin cm^ꢀ12919, 2851, 1774, 1686, 1605,

¹H NMR (400 MHz, CDCl₃): d 10.02 (s, 1H, Ar-CHO), 8.15 (d, J =

8.8 Hz, 2H, Ar), 7.97 (d, J = 8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 2H,

Ar), 6.99 (d, J = 8.8 Hz, 2H, Ar); 4.06 (t, J = 6.4 Hz, 2H, –OCH₂)

and 1.86–0.86 (m, 23H, 1 ꢂ CH₃, 10 ꢂ CH₂); MS (FAB+): m/z for

1

1578, 1256 and 1169; H NMR (400 MHz, CDCl₃): d 8.05 (d, J =

9.2 Hz, 2H, Ar), 6.94 (d, J = 8.8 Hz, 2H, Ar), 4.04 (t, J = 6.6 Hz, 2H,

–OCH₂), 1.84–1.27 (m, 20H, 10 ꢂ CH₂) and 0.90 (t, J = 6.8 Hz,

3H, –CH₃₎; MS (FAB+): m/z for C₁₉H₃₀O₃, calculated: 306.2,

found: 307.5.

4.2.5. General procedure for the synthesis of 4-formylphenyl

4-(n-alkoxy)benzoates VIIa–e. 4-(n-Alkoxy)benzoic acid (VIa–e)

(3.9 mmol, 1 eq.) and 4-hydroxybenzaldehyde (3.9 mmol, 1 eq.)

were dissolved in dry THF and stirred under a nitrogen atmo-

sphere for some time. After the addition of DCC (4.7 mmol,

1.2 eq.) and a catalytic amount of DMAP, the reaction mixture

was stirred at RT for 48 h. The precipitated dicyclohexylurea

was filtered off and the solvent was removed in vacuo. The

solid substance obtained was purified by column chromato-

C

₂₆H₃₄O₄, calculated: 410.3, found: 410.8.

4.2.6A. General procedure for the synthesis of (S)-4-{[(4-(octan-2-

yloxy)phenyl)imino]methyl}phenyl 4-(n-alkoxy)benzoates. A mixture

of 4-formylphenyl 4-(alkoxy)benzoate (VIIa–e) (0.4 mmol, 1 eq.) and

(S)-4-(octan-2-yloxy)aniline (IV) (0.33 mmol, 1.2 eq.), catalytic amount

of acetic acid in ethanol (10 mL) was refluxed under an inert

atmosphere for 2 h. The colourless solid separated upon cooling the

reaction mixture was collected by filtration, washed with ethanol

and air-dried. The crude product was purified by repeated recrys-

tallizations in absolute ethanol until the constant isotropization

temperature was obtained.

SB(S)-8. A colourless solid; yield: 0.117 g (86%); [a]²_D⁵ꢀ5.8

graphy on silica gel (230–400 mesh) using 10% EtOAc–hexanes (0.101%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13451, 2924, 2852,

as an eluent.

1739, 1608, 1577, 1510, 1467 and 1163; UV-vis: l_max= 339.23 nm,

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NJC

mol. conc. = 5.47 ꢂ 10^ꢀ3M, e = 0.9874 ꢂ 10²L mol^ꢀ1cm^ꢀ1

;

19.82, 14.13 and 14.09; anal. calcd for C₃₉H₅₃NO₄: C, 78.09; H,

¹H NMR (400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J = 8.91; N, 2.34. Found: C, 78.24; H, 9.01; N, 2.24.

8.8 Hz, 2H, Ar), 7.95 (d, J = 8.4 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz,

SB(S)-12. A colourless solid; yield: 0.152 g (84%); [a]²_D⁵ꢀ6.70

2H, Ar), 7.23 (d, J = 9.2 Hz, 2H, Ar), 6.99 (d, J = 9.2 Hz, 2H, Ar), 6.92

(d, J = 8.8 Hz, 2H, Ar), 4.38 (m, 1H, –O–CH–), 4.06 (t, J = 6.6 Hz, 2H,

–OCH₂), 1.84–1.71 (m, 3H, –O–CH–CH₃) and 1.60–0.87 (m, 28H,

11 ꢂ CH₂, 2 ꢂ CH₃); ¹³C NMR (100 MHz): 164.66, 163.72, 157.06,

157.04, 153.23, 144.60, 134.14, 132.37, 129.74, 122.23, 122.22,

121.32, 116.51, 114.40, 74.40, 68.40, 36.54, 31.83, 29.34, 29.32,

29.24, 29.12, 26.01, 25.55, 22.67, 22.63, 19.81, 14.11 and 14.09;

anal. calcd for C₃₆H₄₇NO₄: C, 77.52; H, 8.49; N, 2.51. Found: C,

77.61; H, 8.56; N, 2.49.

(0.101%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13451, 2925, 2857,

1736, 1608, 1508, 1466 and 1165; UV-vis: l_max= 340.94 nm, mol.

1

conc. = 5.1 ꢂ 10^ꢀ3M, e = 1.0098 ꢂ 10²L mol^ꢀ1cm^ꢀ1, H NMR

(400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J = 8.8 Hz, 2H,

Ar), 7.96 (d, J = 8.8 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz, 2H, Ar), 7.23

(d, J = 8.8 Hz, 2H, Ar), 6.99 (d, J = 9.2 Hz, 2H, Ar), 6.92 (d, J = 8.8 Hz,

2H Ar), 4.39 (m, 1H, –O–CH), 4.06 (t, J = 6.6 Hz, 2H, –OCH₂),

1.86–1.71 (m, 3H, –O–CH–CH₃) and 1.57–0.87 (m, 36H, 15 ꢂ CH₂,

2 ꢂ CH₃); ¹³C NMR (100 MHz): 164.64, 163.72, 157.07, 157.02,

SB(S)-9. A colourless solid; yield: 0.120 g (82%); [a]²_D⁵ꢀ4.22 153.23, 144.61, 134.14, 132.37, 129.73, 122.23, 122.21, 121.32,

(0.099%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13439, 2921, 2852, 116.51, 114.40, 74.39, 68.40, 36.54, 31.94, 31.83, 30.93, 29.68,

1726, 1607, 1508, 1466 and 1166; UV-vis: l_max= 340.47 nm, mol. 29.66, 29.61, 29.58, 29.37, 29.32, 29.12, 26.01, 25.56, 22.71, 22.63,

conc. = 4.72 ꢂ 10^ꢀ3M, e = 0.8257 ꢂ 10²L mol^ꢀ1cm^ꢀ1; ¹H NMR 19.82, 14.13 and 14.09; anal. calcd for C₄₀H₅₅NO₄: C, 78.26; H, 9.03;

(400 MHz, CDCl₃): d 8.48 (s, 1H, –CHQN), 8.15 (d, J = 8.8 Hz, 2H, N, 2.28. Found: C, 78.34; H, 9.30; N, 2.05.

Ar), 7.95 (d, J = 8.4 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz, 2H, Ar), 7.23 (d,

4.2.6B. General procedure for the synthesis of (R)-4-{[(4-(octan-

J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 9.2 Hz, 2H, Ar), 6.92 (d, J = 8.8 Hz, 2-yloxy)phenyl)imino]methyl}phenyl 4-(n-alkoxy)benzoates. A mixture

2H Ar), 4.37 (m, 1H, –O–CH), 4.06 (t, J = 6.6 Hz, 2H, –OCH₂), of 4-formylphenyl 4-(n-alkoxy)benzoate (VIIa–e) (0.4 mmol, 1 eq.) and

1.86–1.71 (m, 3H, –O–CH–CH₃) and 1.60–0.86 (m, 30H, 12 ꢂ CH₂, (R)-4-(octan-2-yloxy)aniline (II) (0.33 mmol, 1.2 eq.), catalytic amount

2 ꢂ CH₃); ¹³C NMR (100 MHz): 164.65, 163.72, 157.06, 157.03, of acetic acid in ethanol (10 mL) was refluxed under an inert

153.23, 144.61, 134.14, 132.37, 129.73, 122.22, 121.32, 116.51, atmosphere for 2 h. The white solid separated upon cooling

114.40, 74.39, 68.40, 36.54, 31.89, 31.83, 30.93, 29.53, 29.38, the reaction mixture was collected by filtration, washed with

29.31, 29.27, 29.12, 26.00, 25.55, 22.69, 22.62, 19.81, 14.11 and ethanol and air-dried. The crude product was purified by repeated

14.09; anal. calcd for C₃₇H₄₉NO₄: C, 77.72; H, 8.64; N, 2.45. Found: recrystallizations from absolute ethanol until the constant iso-

C, 77.52; H, 8.66; N, 2.37.

tropization temperature was noted.

SB(S)-10. A colourless solid; yield: 0.128 g (82%); [a]²_D⁵ꢀ3.4

SB(R)-8. A colourless solid; yield: 0.122 g (83%); [a]²_D⁵+6.01

(0.102%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13444, 2923, (0.098%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13446, 2925, 2856,

2851, 1734, 1607, 1507, 1467 and 1167; UV-vis: l_max= 339.41 nm, 1737, 1607, 1507,1467 and 1165; UV-vis: l_max= 340.99 nm, mol.

mol. conc. = 4.1 ꢂ 10^ꢀ3M, e = 1.61 ꢂ 10²L mol^ꢀ1cm^ꢀ1; ¹H NMR conc. = 6.99 ꢂ 10^ꢀ3M, e = 0.4147 ꢂ 10²L mol^ꢀ1cm^ꢀ1; ¹H NMR

(400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J = 9.2 Hz, 2H, (400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J = 8.4 Hz, 2H,

Ar), 7.95 (d, J = 8.4 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz, 2H, Ar), 7.23 (d, J = Ar), 7.95 (d, J = 8 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz, 2H, Ar), 7.23 (d, J =

8.8 Hz, 2H, Ar), 6.99 (d, J = 8.8 Hz, 2H, Ar), 6.92 (d, J = 8.8 Hz, 2H Ar), 8.4 Hz, 2H, Ar), 6.98 (d, J = 8.4 Hz, 2H, Ar), 6.92 (d, J = 8.4 Hz, 2H

4.39 (m, 1H, –O–CH), 4.06 (t, J = 6.6 Hz, 2H, –OCH₂), 1.86–1.71 (m, Ar), 4.38 (m, 1H, –O–CH), 4.06 (t, J = 6.4 Hz, 2H, –OCH₂), 1.86–1.72

3H, –O–CH–CH₃) and 1.60–0.87 (m, 32H, 13 ꢂ CH₂, 2 ꢂ CH₃); ¹³

C

(m, 3H, –O–CH–CH₃) and 1.60–0.89 (m, 28H, 11 ꢂ CH₂, 2 ꢂ CH₃);

NMR (100 MHz): 164.65, 163.72, 157.06, 157.04, 153.23, 144.06, ¹³C NMR (100 MHz): 164.59, 163.68, 156.96, 153.19, 144.57, 134.1,

134.14, 132.37, 129.74, 122.23, 122.22, 121.32, 116.51, 114.40, 74.40, 132.32, 129.69, 122.18, 121.29, 116.47, 114.35, 74.35, 68.36, 36.50,

68.40, 36.54, 31.92, 31.83, 29.57, 29.38, 29.32, 29.12, 26.01, 25.55, 31.78, 29.30, 29.27, 29.08, 25.97, 25.51, 22.63, 22.58, 19.77 and

22.70, 22.63, 19.82, 14.12 and 14.09; Anal. calcd for C₃₈H₅₁NO₄: C, 14.05; anal. calcd for C₃₆H₄₇NO₄: C, 77.52; H, 8.49; N, 2.51. Found:

77.91; H, 8.77; N, 2.39. Found: C, 77.71; H, 8.96; N, 2.55.

C, 77.30; H, 8.49; N, 2.42.

SB(S)-11. A colourless solid; yield: 0.142 g (85%); [a]²_D⁵ꢀ5.4

SB(R)-9. A colourless solid; yield: 0.135 g (82%); [a]²_D⁵+4.22

(0.102%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13451, 2925, 2856, (0.099%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13441, 2922,

1736, 1608, 1508, 1467 and 1165; UV-vis: l_max= 340.80 nm, mol. 2854, 1726, 1608, 1506,1465 and 1166; UV-vis: l_max= 340.79 nm,

conc. = 3.92 ꢂ 10^ꢀ3M, e = 1.0464 ꢂ 10²L mol^ꢀ1cm^ꢀ1; ¹H NMR mol. conc. = 7.87 ꢂ 10^ꢀ3M, e = 1.2198 ꢂ 10²L mol^ꢀ1cm^ꢀ1

;

(400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J = 8.8 Hz, 2H, ¹H NMR (400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J =

Ar), 7.96 (d, J = 8.8 Hz, 2H, Ar), 7.32 (d, J = 8.8 Hz, 2H, Ar), 7.23 8.4 Hz, 2H, Ar), 7.96 (d, J = 8.4 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz,

(d, J = 8.8 Hz, 2H, Ar), 6.99 (d, J = 8.8 Hz, 2H, Ar), 6.92 (d, J = 8.8 Hz, 2H, Ar), 7.23 (d, J = 8.4 Hz, 2H, Ar), 6.99 (d, J = 8.8 Hz, 2H, Ar),

2H Ar), 4.38 (m, 1H, –O–CH), 4.06 (t, J = 6.6 Hz, 2H, –OCH₂), 6.92 (d, J = 8.4 Hz, 2H, Ar), 4.38 (m, 1H, –O–CH), 4.06 (t, J =

1.86–1.71 (m, 3H, –O–CH–CH₃) and 1.62–0.87 (m, 34H, 14 ꢂ CH₂, 6.4 Hz, 2H, –OCH₂), 1.84–1.72 (m, 3H, –O–CH–CH₃) and

2 ꢂ CH₃); ¹³C NMR (100 MHz): 164.65, 163.72, 157.06, 157.04, 1.59–0.89 (m, 30H, 12 ꢂ CH₂, 2 ꢂ CH₃); ¹³C NMR (100 MHz):

153.23, 144.60, 134.14, 132.37, 129.74, 122.23, 122.22, 121.32, 164.6, 163.68, 156.98, 153.19, 144.57, 134.10, 132.33, 129.69,

116.51, 114.40, 74.39, 68.40, 36.54, 31.93, 31.83, 29.63, 29.61, 122.18, 121.28, 116.47, 114.36, 74.35, 68.36, 36.50, 31.79,

29.58, 29.38, 29.36, 29.32, 29.12, 26.00, 25.56, 22.71, 22.63, 29.35,29.28, 29.23, 29.09, 25.97, 25.52, 22.65, 22.59, 19.78 and

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14.06; anal. calcd for C₃₇H₄₉NO₄: C, 77.72; H, 8.64; N, 2.45. References

Found: C, 77.65; H, 8.62; N, 2.72.

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SB(R)-10. A colourless solid; yield: 0.158 g (84%); [a]²_D⁵+3.02

(0.099%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13445, 2924,

2853, 1734, 1607, 1506,1467 and 1166; UV-vis: l_max= 341.01 nm,

mol. conc. = 3.76 ꢂ 10^ꢀ3M, e = 1.1449 ꢂ 10²L mol^ꢀ1cm^ꢀ1

;

¹H NMR (400 MHz, CDCl₃): d 8.49 (s, 1H, –CHQN), 8.16 (d, J =

8.8 Hz, 2H, Ar), 7.95 (d, J = 8.4 Hz, 2H, Ar), 7.32 (d, J = 8.4 Hz,

2H, Ar), 7.23 (d, J = 8.8 Hz, 2H, Ar), 6.99 (d, J = 8.8 Hz, 2H, Ar),

6.92 (d, J = 8.8 Hz, 2H, Ar), 4.38 (m, 1H, –O–CH), 4.07 (t, J =

6.4 Hz, 2H, –OCH₂), 1.86–1.71 (m, 3H, –O–CH–CH₃) and

1.49–0.87 (m, 32H, 13 ꢂ CH₂, 2 ꢂ CH₃); ¹³C NMR (100 MHz):

164.60, 163.67, 156.97, 153.19, 144.57, 134.10, 132.32, 129.69,

122.17, 121.28, 116.47, 114.35, 74.35, 68.35, 36.50, 31.87, 31.78,

29.52, 29.33, 29.27, 29.07, 25.96, 25.50, 22.65, 22.58, 19.77 and

14.07; anal. calcd for C₃₈H₅₁NO₄: C, 77.91; H, 8.77; N,2.39.

Found: C, 77.51; H, 8.88; N, 2.56.

SB(R)-11. A colourless solid; yield: 0.138 g (86%); [a]²_D⁵+6.63

(0.10%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13439, 2920, 2851,

1729, 1606, 1507 and 1166; UV-vis: l_max= 340.57 nm, mol. conc. =

8.17 ꢂ 10^ꢀ3M, e = 1.5056 ꢂ 10²L mol^ꢀ1cm^ꢀ1; ¹H NMR (400 MHz,

CDCl₃): d 8.54 (s, 1H, –CHQN), 8.21 (d, J = 8.8 Hz, 2H, Ar), 8.01 (d,

J = 8.8 Hz, 2H, Ar), 7.37 (d, J = 8.4 Hz, 2H, Ar), 7.28 (d, J = 8.8 Hz,

2H, Ar), 7.04 (d, J = 9.2 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 4.38

(m, 1H, –O–CH), 4.06 (t, J = 6.6 Hz, 2H, –OCH₂), 1.86–1.71 (m, 3H,

–O–CH–CH₃) and 1.62–0.87 (m, 34H, 14 ꢂ CH₂, 2 ꢂ CH₃); ¹³C

NMR (100 MHz): 164.61, 163.69, 156.98, 153.21, 144.58, 134.12,

132.34, 129.70, 122.19, 121.30, 116.48, 114.36, 74.36, 68.36, 36.51,

31.90, 31.80, 31.58, 29.58, 29.33, 29.09, 25.98, 25.53, 22.60, 19.78

and 14.09; anal. calcd for C₃₉H₅₃NO₄: C, 78.09; H, 8.91; N, 2.34.

Found: C, 77.98; H, 8.93; N, 2.55.

SB(R)-12. A colourless solid; yield: 0.142 g (85%); [a]²_D⁵+7.24

(0.104%, CH₂Cl₂); IR (KBr pellet): n_maxin cm^ꢀ13439, 2920,

2849, 1726, 1607, 1506,1467 and 1164; UV-vis: l_max= 340.85 nm,

mol. conc. = 5.38 ꢂ 10^ꢀ3M, e = 2.232 ꢂ 10²L mol^ꢀ1cm^ꢀ1

;

¹H NMR (400 MHz, CDCl₃): d 8.48 (s, 1H, –CHQN), 8.15 (d, J =

8.8 Hz, 2H, Ar), 7.95 (d, J = 8.4 Hz, 2H, Ar), 7.32 (d, J = 8.8 Hz,

2H, Ar), 7.23 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar),

6.92 (d, J = 8.8 Hz, 2H, Ar), 4.38 (m, 1H, –O–CH), 4.06 (t, J =

6.6 Hz, 2H, –OCH₂), 1.84–1.71 (m, 3H, –O–CH–CH₃) and

1.60–0.87 (m, 36H, 15 ꢂ CH₂, 2 ꢂ CH₃); ¹³C NMR (100 MHz):

164.63, 163.71, 157.05, 157.01, 153.22, 144.59, 134.13, 132.35,

129.72, 122.22, 122.20, 121.31, 116.50, 114.38, 74.38, 68.39,

36.53, 31.93, 31.82, 30.91, 29.66, 29.64, 29.59, 29.56, 29.36,

29.11, 25.99, 25.54, 22.70, 22.61, 19.80, 14.12 and 14.08; anal.

calcd for C₄₀H₅₅NO₄: C, 78.26; H, 9.03; N, 2.28. Found: C, 78.22;

H, 9.05; N, 2.14.

Acknowledgements

CVY expresses his deep sense of gratitude to the Depart-

ment of Science and Technology (DST), New Delhi, Govt. of

India, for the financial support through SERB project No.

SR/S1//OC-04/2012.

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Article Doi

Optically active, three-ring calamitic liquid crystals: The occurrence of frustrated, helical and polar fluid mesophases

DOI: 10.1039/c4nj02011a

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Article abstract of DOI:10.1039/c4nj02011a

Full text of DOI:10.1039/c4nj02011a

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