Design, synthesis and characterization of a linear hydrogen bonded homologous series exhibiting reentrant smectic C ordering

Journal of Physics and Chemistry of Solids 73 (2012) 1203–1212

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Design, synthesis and characterization of a linear hydrogen bonded homologous series exhibiting reentrant smectic C ordering C. Kavitha, M.L.N. Madhu Mohan n Liquid Crystal Research Laboratory (LCRL), Bannari Amman Institute of Technology, Sathyamangalam 638 401, India

a r t i c l e i n f o

abstract

Article history: Received 18 November 2011 Received in revised form 8 May 2012 Accepted 15 May 2012 Available online 26 May 2012

Design, synthesis and characterization of seven linear hydrogen bonded liquid crystal complexes derived from mesogenic p–n-decyloxy benzoic acid and p–n-alkyl benzoic acids designated as 10OØn (where n varied from ethyl to octyl) are reported. FTIR studies confirm the hydrogen bond formation in all these complexes. The phase transition temperatures and their corresponding enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. POM and DSC data are further utilized for the construction of 10OØn phase diagram. Two Odd–even effects have been evinced, one in enthalpy values and the other in corresponding transition temperatures across the isotropic to nematic phase transition. An interesting result is the observation of re-entrant smectic ordering, designated as smectic CR in three higher ordered mesogens. A new smectic ordering, smectic X, has been observed which is sandwiched between traditional smectic C and re-entrant smectic CR. Magnitudes of optical tilt angle in smectic C, smectic X and smectic CR are experimentally found to attain saturation with decrement of temperature in the corresponding phase. The occurrence of smectic X and smectic CR are discussed with relation to the molecular chemical structure. The optical filtering action in smectic C and re-entrant smectic CR phases have been analyzed. & 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Organic compounds B. Chemical synthesis C. Differential scanning calorimetry (DSC) C. Infrared spectroscopy D. Phase transitions

1. Introduction Liquid crystal technology has had a major effect on many areas of Science and Engineering, as well as in device technology. Applications for this special kind of material are still being discovered and continue to provide effective solutions to many different problems. Scientists and Engineers are able to use liquid crystals in variety of applications, because external perturbation can cause significant changes in the macroscopic properties of the liquid crystal system. Since the discovery of the first ferroelectric liquid crystal by Meyer [1] interest on these soft condensed materials has grown enormously. In the recent times, hydrogen bonded liquid crystals (HBLC) [2–7], are designed and synthesized from key functional materials selected on the basis of their molecular reorganization and self assembly capability. The commercial viabilities and applicational aspects [8–13] made many of the research groups to work on these soft materials. Even though HBLC materials are known since 1960s [2], much work has been done on these complexes [3–7,14–17] only in this present epoch. Hydrogen bond, a non covalent interaction, enables various mesogenic and non mesogenic compounds to form complexes exhibiting rich phase polymorphism. The reported data

n

Corresponding author. Tel.: þ91 9442437480; fax: þ91 4295 223 775. E-mail address: [email protected] (M.L.N. Madhu Mohan).

0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.05.013

[3–7,14–17] indicate the fact that if HBLC materials are to be mesogenic, it is enough either the proton donor or the acceptor molecule to exhibit mesogenic property. The chemical molecular structure [14–17] of HBLC is co-related to the physical properties exhibited by it. Theoretical approach relating to the liquid crystals also increases our interest in this field of present work. Explanations regarding odd–even effect, Intermolecular interactions in nematic and smectic ordering corresponding to mesogens go well with the reported [18–22] theoretical results. The formation of HBLC through hydrogen bonding between aromatic carboxylic acids as well as from mixtures of dissimilar molecules capable of interacting through H-bonding [6,8–16], reveals that the rigid core is made up of covalent and non covalent hydrogen bonding. Extensive work of hydrogen bonded liquid crystal by Kato and Frechet [8,9] opened a new chapter in synthesis, design and characterization of these mesogens which enables many young researchers to work in this field [23–30]. Occurrence of Re-entrant (RE) phase [31] is one of the oldest phenomena in the field of liquid crystals. In thermotropic liquid crystals, the phase sequence is altered on decreasing the temperature in specific cases, which results in disordered phases known as Re-entrant phenomena. The tilt angle in smectic C and Re-entrant phase are illustrated in Fig. 1d. A re-entrant phenomenon in LCs is first reported by Cladis [31] and this paved way for its recognition and utility [31–46]. Occurrence of Re-Entrant phase can be attributed to the rupture of order parameter, which

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a

b

c

d

Fig. 1. Molecular structure of p–n-decyloxy benzoic acid and p–n-alkyloxy benzoic acid hydrogen bonded complex. (a) Proposed molecular alignment in various phases. (b) Molecular arrangement of hydrogen bonded liquid crystal in smectic C phase. Tilt angle and bidirectional tilt angle are depicted. Transverse and longitudinal dipole moments are represented by mt and m2, respectively. (c) Molecular arrangement of hydrogen bonded liquid crystal in multiple smectic layers of Smectic X phase. The solid line superimposed over the smectic layer denotes the helicoidal structure. The arrows denote the direction of orientation of the molecule. The helicoidal pitch is denoted by l. (d) Representation of tilt angle in Smectic C and smectic CR phases, respectively. The solid arrows indicate the direction of molecular tilt.

in turn alters the molecular arrangement in various phases as depicted in Fig. 1a and b. The representation of helicoidal structure in smectic X is illustrated in Fig. 1c. The thermotropic

transition from ordered phase to disordered phase occurring second time is referred as Re-Entrant phenomena. RE-theories of LCs are described in various models viz., Frustrated Spin Gas

C. Kavitha, M.L.N. Madhu Mohan / Journal of Physics and Chemistry of Solids 73 (2012) 1203–1212

[36], Mean Field [35–37,40] Generalized Mean Field or Two State Interaction [34]. The RE-phenomena can be exploited in the Electro Optic (EO) sensor devices [33] and medical diagnostic [38] tools. As far as the applicational aspect is considered, Photoinduced RE-phenomena [42] and the associated EO switching emerge as highly significant fields. The central theme of this work is to study the physical and chemical properties of HBLC formed by alkyl and alkoxy carboxylic acids. With our previous experience [29–30,47–57] in designing, synthesizing liquid crystals and in continuation of our efforts to understand hydrogen bonded mesogens, eight such homologues series designated as mOØn (where n varied from 2 to 8 while m varied from 5 to 12, respectively) can be deduced with changing the alkyl and alkoxy carbon number. A systematic study for 12OØn and 11OØn homologues series has been carried out [58] by the authors. In the present work, design, synthesis and characterization of seven 10OØn homologues are discussed.

2. Material and methods Optical textural observations were made with a Nikon polarizing microscope equipped with Nikon digital CCD camera system with 5 mega pixels and 2560  1920 pixel resolutions. The liquid crystalline textures were processed, analyzed and stored with the aid of ACT-2U imaging software system. The temperature control of the liquid crystal cell was equipped by Instec HCS402-STC 200 temperature controller (Instec, USA) to a temperature resolution of 70.1 1C. This unit is interfaced to computer by IEEE –STC 200 to control and monitor the temperature. The liquid crystal sample is filled by capillary action in its isotropic state into a commercially available (Instec, USA) polyamide buffed cell with 4 mm spacer. Optical extinction technique [48] was used for determination of tilt angle. The transition temperatures and corresponding enthalpy values were obtained by DSC (Shimadzu DSC-60, Japan). FTIR spectra was recorded (ABB FTIR MB3000) and analyzed with the MB3000 software. Chemicals p–n-decyloxy benzoic acid (10BAO) and p–n-alkyl benzoic acids (nBA, n¼2 to 8) were supplied by Sigma Aldrich, (Germany) and all the solvents used were HPLC grade. 2.1. Synthesis of HBLC Intermolecular double hydrogen bonded mesogens are synthesized by the addition of one mole of p–n-decyloxy benzoic acid (10BAO) with one mole of p–n-alkyl benzoic acids (nBA n ¼2 to 8) in N,N-dimethyl formamide (DMF). Further they are subject to constant stirring for 14 h at ambient temperature of 30 1C till a white precipitate in a dense solution is formed. The white crystalline crude complexes so obtained by removing excess DMF are then recrystallized with dimethyl sulfoxide (DMSO) and the yield varied from 95% to 98%. The molecular structure of the present homologous series of p–n-decyloxy benzoic acids with p–n-alkyl benzoic acids is depicted in the Fig. 1, where n represents the alkyl carbon chain length.

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below 82.2 1C (Table 1). They show high thermal and chemical stability when subjected to repeated thermal scans performed during POM and DSC studies. 3.1. Phase identification The observed phase variants, transition temperatures and corresponding enthalpy values obtained by DSC in cooling and heating cycles for the 10OØn complexes are presented in Table 1. 3.2. 10OØn homologous series The mesogens of p–n-decyloxy benzoic acid (10BAO) and p–nalkyl benzoic acids (nBA) homologous series are found to exhibit characteristic textures [60], viz., nematic (threaded texture, Plate 1), Smectic C (schlieren texture, Plate 2) Smectic X (worm like texture, Plate 3) and Smectic CR (schlieren texture, Plate 4), respectively. The general phase sequence of the p–n-decyloxy benzoic acid and p–nalkyl benzoic acids is shown as: Iso"N-Sm C- Sm X" Sm CR"Crystal (10OØ8, 10OØ7) Iso"N-Sm C-Sm X-Sm CR"Crystal (10OØ6) Iso"N-Sm C"Crystal (10OØ5, 10OØ4) Iso"N"Crystal (10OØ3, 10OØ2) Monotropic and enantiotropic transitions are depicted as single and double arrows, respectively. 3.3. Infrared Spectroscopy (FTIR) IR spectra of free p–n-decyloxy benzoic acid, p–n-alkyl benzoic acids and their intermolecular hydrogen bonded complexes are recorded in the solid state (KBr) at room temperature. As a representative case, Fig. 2 illustrates the FTIR spectra of 10OØ2 in solid state (KBr) at room temperature. The solid state spectra of both the precursors viz., p–n alkyl benzoic acid and p–n decyloxy benzoic acid are reported [61,62] to have two sharp bands at 1685 cm  1and 1695 cm  1 attributed to n(C¼O) mode. Furthermore, in Fig. 2, peaks obtained at 2924 cm  1 and 1682 cm  1 correspond to the hydrogen bond formation upon complexation and dimmer formation, respectively. Carboxylic acid existing in monomeric form absorbs at about 1760 cm  1 because of the electron withdrawing effect. However, acids in concentrated solution or in solid state tend to dimerize through hydrogen bonding. It is reported [61,62] that this dimerrization weakens the CQO bond and lowers the stretching force constant K, resulting in lowering of the carbonyl frequency of saturated acids to  1710 cm  1. In the present work the carbonyl frequency shifts around 20 cm  1 in all complexes. These results concur with the reported data [61,62] on hydrogen bonded liquid crystals. The present complexes can therefore be referred as H-bonded liquid crystals. 3.4. DSC studies

3. Results and discussion The novel hydrogen bonded homologues series isolated under the present investigation are all mostly white crystalline solids and are stable at room temperature. They are insoluble in water and sparingly soluble in common organic solvents such as methanol, ethanol, and benzene and dichloro methane. However they show a high degree of solubility in coordinating solvents like dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and pyridine [59]. All these mesogens melt at specific temperatures

DSC thermograms are obtained in heating and cooling cycle. The sample is heated with a scan rate of 5 1C/min and held at its isotropic temperature for two minutes so as to attain thermal stability. The cooling run is performed with a scan rate of 5 1C/ min. The respective equilibrium transition temperatures and corresponding enthalpy values of the mesogens of the homologous series are listed separately in Table 1. The results obtained from polarizing optical microscopic studies also concur with these DSC transition temperatures. DSC thermograms of various complexes pertaining to 10OØn series are depicted as Fig. 3.

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Table 1 Transition temperatures and enthalpy values obtained by various techniques of 10BAO þ nBA homologous series. Complex

Phase variance

Techniques

Melt

N

C

X

CR

10BAO þ 8BA

NCXCR

DSC (h)

60.5 20.88

128.6 4.49 125.9 3.76 127.2 131.3 6.47 128.1 5.82 129.2 129.1 3.98 127.3 3.98 128.4 131.5 6.62 129.9 5.18 130.8 128.1 3.82 126.4 3.21 127.2 133.7 2.49 132.0 3.39 133.2 127.7 2.77 126.8 3.02 127.9

a

a

a

a

107

88.9

a

a

a

a

101

74.1

90.0 1.25 86.7 1.33 86.9 75.8 0.43 73.3 1.18 73.8

a

a

b

a

a

82.0

62.1

61.4 1.17 60.9

DSC (c)

10BAO þ 7BA

NCXCR

POM DSC (h)

58.0 26.2

DSC (c)

10BAO þ 6BA

NCXCR

POM DSC (h)

47.1 19.14

DSC (c)

10BAO þ 5BA

NC

POM DSC (h)

56.6 17.81

DSC (c)

10BAO þ 4BA

NC

POM DSC (h)

59.4 33.40

DSC (c)

10BAO þ 3BA

N

POM DSC (h)

82.2 28.41

DSC (c)

10BAO þ 2BA

N

POM DSC (h)

68.6 33.24

DSC (c) POM

Crystal

54.0 12.15 54.3

51.9 14.27 52.2

44.2 5.87 44.5

a

a

52.5

50.7 23.99 51.0

a

a

82.6

50.7 28.86 51.1

74.9 22.16 75.2

55.3 19.99 55.7

Temperatures in 1C while enthalpy in J/g. a b

not resolved.0 Monotropic transition.

Plate 1. Nematic texture of 10OØ8 complex.

Plate 2. Smectic C texture of 10OØ8 complex.

3.5. Phase diagram of pure p–n-decyloxy benzoic acid The phase diagram of pure p–n-decyloxy benzoic acid is reported [23] while the 10OØn homologous series is

constructed by taking into account the optical polarizing microscopic studies and the phase transition temperatures observed in the cooling run of the DSC thermogram. The phase

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smectic X is sand-witched between smectic C and re-entrant smectic CR phases. e) One of the interesting observation in the present series is the detection of odd–even effect at isotropic to nematic phase transition with respect to enthalpy and the corresponding transition temperatures. f) The occurrence of smectic X phase is attributed to the l/d ratio of the homologues series. The lower homologues failed to have a threshold value of l/d ratio which in turn reflects in the absence of smectic X phase. 4. Odd–even effect in 10OØn homologous series

Plate 3. Fully grown smectic X texture of 10OØ8 complex.

Plate 4. Smectic CR texture of 10OØ8 complex.

diagram of pure p–n-decyloxy benzoic acid is reported [15,23] to composed of two phases namely, Nematic and smectic C.

3.5.1. Phase diagram of 10OØn Phase diagram of the synthesized complexes of the present homologues series is depicted in Fig. 4. The following points can be elucidated from this figure. a) The 10OØn hydrogen bonded homologous series exhibit mesogenic behavior viz., nematic as orthogonal phase while smectic C, smectic X and smectic CR as tilted phases. b) Nematic phase is observed in all the complexes of the present homologous series with a wide thermal span. c) Smectic C is induced from butyl benzoic acid, its thermal span converges in pentyl benzoic acid and starts increasing gradually till octal benzoic acid quenching nematic phase. d) The smectic X phase is induced from hexyl to octyl benzoic acids quenching the thermal range of smectic C. Further

Odd–even effect is predominantly noticed in the present homologous series. A plot is constructed with the transition temperatures and enthalpy values corresponding to the isotropic to nematic phase on y-axis and the alkyl carbon number on x-axis. Fig. 5(a) and (b) depict such variation of transition temperatures and enthalpy values with carbon numbers in 10OØn homologous series. From this Fig. 5 it can be observed that the magnitudes of the enthalpy values corresponding to the even homologous carbon number (10OØ2, 10OØ4, 10OØ6 and 10OØ8) exhibit one type of behavior, while their odd counter parts (10OØ3, 10OØ5 and 10OØ7) show a different increment. In the literature, such behavior has been reported [63] and is referred as odd–even effect. In addition to the isotropic to Nematic odd–even effect, an interesting result relating to odd–even effect is observed in crystallization with respect to their transition temperatures with few deviation, however this is less pronounced compared to Isotropic–Nematic transition. This is not unusual as the earlier report [64] suggests an odd–even effect observed for isotropic to Smectic A and smectic A to crystal transition. The origin of the odd–even effect [18,63–66] can be understood from the consideration of the molecular structure (Fig. 1). In the even numbers of the series the disposition of the end group is such as to enhance the molecular anisotropy and hence molecular order, whereas in the odd number it has the opposite effect. As the chain length increases their flexibility increases and the odd– even effect becomes less pronounced. The results of the odd–even effect observed in the present series are in accordance with the quantitative calculations proposed by Marcelja [18,66]. It may be noted that the present linear alternate intermolecular hydrogen bonded liquid crystalline molecule is composed of a rigid aromatic ring and a flexible hydrogen bonding part. The rigid core length varies with increment in the alkyl benzoic acid carbon chain. The rich liquid crystalline phase polymorphism and the associated enthalpy values with increment of alkyl carbon chain length are thus attributed to this part of the chemical structure. Moreover the length (l) of the total HBLC varies with the alkyl carbon chain length while the width (d) remains constant. Thus the altering of l/d ratio triggers the phase variance in the homologous series in turn influences the phase transition temperatures and the corresponding enthalpy values. Hence, the rigid cores and l/d ratio plays a vital role in establishing the pronounced odd–even effect as evinced in the present homologous series.

5. Characterization of smectic C, smectic X and smectic CR phases Smectic X phase which has been identified in three complexes namely 10OØ8, 10OØ7 and 10OØ6 is characterized by various studies. In all the above complexes, smectic X is sandwiched between traditional smectic C and reentrant smectic CR phases.

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Fig. 2. FTIR spectra of 10OØ2 complex.

30

10BAO+nBA n=

Heat flow / Jg-1

2BA 20

3BA 4BA 5BA

10 6BA 7BA 0

8BA

K

N

Iso

K

N

Iso

K

N

Iso

K

N

Iso

K

N

Iso

K

CR

N

Iso

K

CR

N

Iso

50

100 Temperature / °C

Fig. 4. Phase diagram of 10OØn homologous series.

Fig. 3. DSC exothermic thermograms of 10OØn complexes.

5.2. Textural study of smectic C, smectic X and smectic CR phases

This serves as one of the strongest evidence to identify this new phase smectic X as smectic ordering. The three tilted smectic phases namely C, X and CR are characterized based on their texture, DSC thermogram and optical tilt angle are tabulated as shown in Table 2.

5.1. DSC study of smectic C, smectic X and smectic CR phases The nematic to smectic C transition is observed to be second order and hence could not be resolved in the DSC thermograms, similarly smectic C to smectic X is also observed to be second order transition. Further, the obtained DSC data reveal that smectic X to smectic CR transition is first order. The smectic CR phase is hence resolved in all the three mono-component higher homologues (namely 10OØ8, 10OØ7 and 10OØ6) and is depicted in Fig. 3.

The hydrogen bonded complexes 10OØ8, 10OØ7 and 10OØ6 on cooling from isotropic phase, nematic droplets are observed. The nematic droplets are sustained by the appearance of a multi color domain schileren texture of smectic C depicted as Plate 2. On further cooling, a worm like texture is observed which is designated as smectic X. The fully grown texture of this phase exhibiting a very narrow ( o1 1C) thermal range is given away as Plate 3. The striations in the worm like texture are manifestation of the presence of helicoidal structure. This worm like texture phase has been observed earlier in 12OØn and 11OØn HBLC homologous series and reported [48] by us. Smectic X phase is sandwiched between traditional smectic C and re-entrant smectic C referred as smectic CR. The molecular orientation in various phases is depicted in Fig. 1a–d. From these figures it can be seen that the flipping of molecules is taking place. The high magnitude of the helix in smectic X phase compared to its preceding and succeeding phases stands as a token of evidence for the molecular re-orientation. Thermodynamical conditions favor this smectic X

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phase with a narrow thermal span where the flipping of molecules occur. In the above cited complexes with further decrement of temperature this phase paves way for finger print schileren texture of smectic CR phase as shown in Plate 4.

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Although, order parameter appears to grow from zero value with decreasing temperature in direct C-phases (for n¼ 4,6,7,8), it is observed to attain a saturated value rather instantaneously for the cases of CR phases (in n ¼6,7,8). It is also noticeable that the b

6. Optical tilt angle measurements

15

Tilt angle / θ

Optical tilt angle measurements have been experimentally found by optical extinction method [48]. These studies are carried out in smectic C, smectic X and smectic CR phases of all the members of the present 10OØn homologous series and the values are tabulated in Table 2. Figs. 6–9 depict such variation of optical tilt angle with temperature for 10OØn (where n¼ 4,6,7,8), respectively. It is observed from Figs. 6–9, that the tilt angle increases with decreasing temperature and attains a saturation value. These larger magnitudes of the tilt angle are attributed to the direction of the soft covalent hydrogen bond interaction which spreads along molecular long axis with finite inclination [30]. Tilt angle is a primary order parameter [13] and the temperature variation is estimated by fitting the observed data of y(T) to the relation

10 Sm CR

5 10BAO+8BA

yðTÞpðT C TÞb

ð1Þ

The critical exponent b value estimated by fitting the data of y(T) to the above Eq. (1) is found to be 0.50 to agree with the Mean Field prediction [63,67]. The agreement of magnitude of b (0.5) with Mean Field value (0.5) infers the long-range interaction of transverse dipole moment for the stabilization of tilted smectic C and CR phases. 6

132

100

Temperature / °C Fig. 6. Temperature variation of tilt angle in smectic C, X and CR phases of 10OØ8 complex.

0

15

a 5 4 0

3 8

130

90

6 4 Alkyl carbon number

2

b

Tilt angle / θ

10BAO+nBA Heat flow / Jg-1

Temperature / °C

134

Sm C

Sm X

10 Sm CR

Sm X

Sm C

128

10BAO+7BA

126

5 2

4 6 Alkyl carbon number

8

Fig. 5. (a) Odd–even effect with respect to enthalpy values in 10OØn series. (b) Odd–even effect with respect to transition temperatures in 10OØn series.

70

80 90 Temperature / °C

100

Fig. 7. Temperature variation of tilt angle in smectic C, X and CR phases of 10OØ7 complex.

Table 2 Comparison of variables in smectic C, smectic X and smectic CR phases. Phase variance

POM

DSC

Optical tilt angle magnitude 10OØ8 Onset

Smectic C Smectic X Smectic CR

Schlieren texture (multi colored domains) Worm like texture schlieren texture (finger print)

Second order transition Second order transition First order transition

0

5112 101360 101420

10OØ7 Saturation 0

13112 131300 131300

Onset 0

6154 141480 101300

10OØ6 Saturation 0

14148 141480 151060

Onset 0

11142 181120 151300

Saturation 191420 191480 191480

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6.1. Optical tilt angle measurements in smectic C, X and CR phases

Tilt angle / θ

20

15 Sm CR

Sm X

Sm C

10BAO+6BA 10 60

70 Temperature / °C

80

Fig. 8. Temperature variation of tilt angle in smectic C, X and CR phases of 10OØ6 complex.

20

10BAO+nBA Smectic C

Tilt angle / θ

15

10

5

0 0.0

0.5

1.0 1.5 (T-Tc / °C)

2.0

2.5

Fig. 9. Temperature variation of tilt angle in smectic C phase of 10OØ4 complex.

value corresponding to the Re-entrant phases seems to pronounce the onset of order parameter fluctuations, such that they are just launched into the Critical Field regime. An overview of the yield of b values across direct-C and Re-entrant Smectic CR phases suggests that long range tilt fluctuations prevail in the former case, while they are weaker in either of the cases of Re-entrant Smectic CR or of Smectic C phase with large thermal stability. It is also noticed that tilt angle attains an asymptotic value with decreasing temperature in both of the direct and Re-entrant versions of Smectic C phases. However, asymptotic behavior is found to be conspicuously fast in the case of Re-entrant Smectic CR phases. The interesting aspect of the smectic CR phase is that the transit of the system in to the Re-entrant phase with decreasing temperature is invariably accompanied (Figs. 6–8) with a finite spontaneous tilt angle in contrast with the other cases of C-phase (where in y grows from zero value). Since, order parameter is frequently referred to the information conceived; y may guide a threshold-less device that switches between the low temperature CR-phase.

6.1.1. Tilt angle measurement in 10OØ8 complex Fig. 6 illustrates the variation of optical tilt angle with temperature in all the three (C, X and CR) smectic phases. The onset of the smectic C phase is observed at 107 1C in this complex. As the temperature is decreased, the optical tilt angle value increases from 51120 to 131120 and attains a saturation value at 102.1 1C as can be noticed from Fig. 6. The optical tilt angle has been fitted to power law and the magnitude of b (0.5) is observed to be in concurrence with Mean field theory predicted value. On further decrease of temperature, the smectic C phase of 10OØ8 paves way for a new phase namely smectic X at 88.9 1C whose thermal range is observed to be very narrow (  2.0 1C). The magnitude of tilt angle increases with decreasing temperature and attains a saturated value. The tilt angle magnitude varies from 101360 to 131300 in this smectic X phase and as the phase is fully grown the magnitude of the tilt angle saturates at 131300 . On further decrement of temperature to 86.9 1C the re-entrant smectic CR is observed. The magnitude of the tilt angle is 101420 at 86 1C and increase to 131300 before attaining a saturated value at 83 1C. In all these tilted phases viz., smectic X and smectic CR, the magnitude of the saturated optical tilt angle measured is approximately equal to the saturated tilt angle value observed in smectic C phase. 6.1.2. Tilt angle measurement in 10OØ7 complex Fig. 7 illustrates the variation of optical tilt angle with temperature in all the three (C, X and CR) smectic phases. In this hydrogen bonded complex, smectic C phase originates at 101 1C. The variation of tilt angle with temperature is depicted in Fig. 7. In smectic C phase with decreasing temperature the optical tilt angle value increases from 61540 and attains a saturation value of 141480 at a temperature of 94.5 1C. The critical exponent b value estimated by fitting the data of y(T) is observed to follow Mean field theory predicted value. At 74.1 1C a new phase designated as smectic X with a narrow temperature range (  0.3 1C) is observed. The magnitude of the tilt angle measured in this phase is 141480 . On further decrease of temperature, re-entrant smectic phase designated as CR is observed at 73.8 1C. The magnitude of the tilt angle is observed to increase with the decrement of temperature. As the phase is fully grown the magnitude of the tilt angle saturates to a value of 151060 at 73 1C. In the three tilted phases viz. smectic C, smectic X and smectic CR phases, the magnitude of the optical tilt angle at the saturation point is almost the same. 6.1.3. Tilt angle measurement in 10OØ6 complex Fig. 8 illustrates the variation of optical tilt angle with temperature in all the three (C, X and CR) smectic phases. The onset of smectic C phase in 10OØ6 is observed at 82 1C. As the temperature is decreased the magnitude of the tilt angle increases and attains a saturated value. The variation of temperature dependence of tilt angle is depicted in Fig. 8. The magnitude of the optical tilt angle value varied between 111420 to 191420 with decreasing temperature and attains a saturation value of 191420 at a temperature of 81 1C. The critical exponent b value estimated by fitting the data of y(T) is observed to follow Mean field theory predicted value. As the temperature is further decreased at 62.1 1C a new phase designated as smectic X with a very narrow ( 1.2 1C) thermal range have been observed. In this phase as the temperature is decreased in small steps of 0.1 1C, the magnitude of the tilt angle

C. Kavitha, M.L.N. Madhu Mohan / Journal of Physics and Chemistry of Solids 73 (2012) 1203–1212

Normalised Polarised Transmission

1.0

0.9

0.8 10BAO+6BA (Sm C) 10BAO+6BA (Sm CR)

0.7

10BAO+7BA (Sm C) 10BAO+7BA (Sm CR) 0.6

10BAO+8BA (Sm C) 10BAO+8BA (Sm CR) 500

550 Wavelength / (nm)

600

Fig. 10. Filtering action in the smectic C and CR phases of 10OØ6, 10OØ7 and 10OØ8.

increases from 181120 to 191480 and saturates as shown in Fig. 9. The saturated magnitude of the tilt angle 191480 is observed at 61.7 1C. On further decrement of temperature to 60.9 1C a re-entrant smectic phase designated as CR is observed. The magnitude of the tilt angle in this phase varies between 151300 and 191480 with temperature. The saturated value of 191480 is observed at 58.5 1C. In all the three tilted phases viz. smectic C, smectic X and smectic CR phases, the saturated magnitude of the optical tilt angle is observed to be almost in variant as can be seen from Fig. 8. 6.1.4. Tilt angle measurement in 10OØ4 complex The onset of the smectic C phase is observed at 81.4 1C. As the temperature is decreased, the optical tilt angle value increases from 61300 to 191180 and attains a saturation value. The magnitude of b (0.5) is observed to be in concurrence with Mean field theory predicted value. The variation of tilt angle with temperature is depicted in Fig. 9.

7. Filtering action It is reported [68,69] that liquid crystal optical filters are capable of transmitting light substantially at all wavelengths while reflecting light over a single, generally narrow, wavelength band. From the literature [70] it can be inferred that the unique optical properties of liquid crystal elements can be exploited to provide a wide variety of narrow band filtering functions extending over a wide wavelength range from the near ultraviolet to the far infrared. 7.1. Filtering action in smectic C and re-entrant smectic CR phases The filtering action in smectic C and re-entrant smectic CR phases have been analyzed. The polarized light from the liquid crystal placed under crossed polarizers is subjected to different filters and the intensity of this light is recorded and analyzed. Fig. 10 depicts the variation of light intensity in smectic C and reentrant smectic CR phases for 10OØ6, 10OØ7 and 10OØ8, respectively. In the 10OØ6 complex, in the entire thermal span of re-entrant smectic CR phase inhibits ultra violet light while the other

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frequencies are passed. In the case of smectic C in its total thermal span, the blocked frequencies extend up to visible range, only ultraviolet and infrared ranges are allowed. It can be further noted that the cut off value of filtering frequencies is more in smectic C compared to re entrant smectic CR phase. A combination of smectic C and re entrant smectic CR phases can be used as a band pass filter with the frequency range between 515 nm and 625 nm. In the 10OØ7 complex, a notch filter type action is noticed in the thermal span of smectic C. The visible range is allowed while the ultraviolet and infrared ranges are inhibited. In the re-entrant smectic CR, the filtering of visible region is noticed as in the case of smectic C phase of 10OØ6 complex. A combination of smectic C and re-entrant smectic CR phases can be used as a band pass filter with the frequency range between 525 nm and 575 nm. In the 10OØ8 complex in the re-entrant smectic CR, the notch filtering action is noticed in selected visible region as in the case of smectic C phase of 10OØ7 complex. In the smectic C, a broad frequency cut off of the visible region is noticed while higher frequencies above red and infrared are allowed. Thus from the above results the following points can be summarized: 1. Filtering action in smectic C and reentrant smectic CR are totally different implying a different molecular alignment as proposed in the previous sections. 2. These liquid crystals can be used as effective filters for various regions of the spectrum. Combination of these two phases can give rise to a perfect band pass filtering action. 3. Notch filtering action can be tuned for desired frequencies by choosing appropriate phases of the liquid crystalline complex. 8. Conclusion a) Seven hydrogen bonded mono-component liquid crystals are designed, synthesized and characterized. They are found to exhibit rich liquid crystalline polymorphism. b) Re-entrant phenomenon in smectic ordering has been identified in higher homologues and characterized by various techniques. c) A new smectic ordering, smectic X has been identified and characterized by optical tilt angle measurements. d) Odd–even effect in phase transition and corresponding enthalpy values are observed at isotropic to nematic transitions. e) Optical filtering action is recorded in smectic C and smectic CR phases and the results are compared.

Acknowledgement Infrastructural support rendered by Bannari Amman Institute of Technology is gratefully acknowledged. References [1] R.B. Meyer, L. Liebert, L. Strezelecki, P. Keller, J. Physique. Lett. 36 (1975) 69–71. [2] F. Gouda, K. Skarp, S.T. Lagerwall, Ferroelectrics 113 (1991) 165–206. [3] J.M. Wang, Y.J. Kim, C.J. Kim, K.S. Kim, Ferroelectrics 277 (2002) 185–195. [4] S.L. Wu, C.Y. Lin, Liq. Cryst. 30 (2003) 205–210. [5] P.A. Kumar, V.G.K.M. Pisipati, Adv. Mater. 12 (2000) 1617–1619. [6] C. Kittle, Introduction to Solid State Physics, Wiley Eastern Private Limited, New Delhi, 1974. [7] G.R. Luckhurst, G.W. Gray, The Molecular Physics of Liquid Crystal, Academic Press, New York, 1979. [8] T. Kato, J.M.J. Frechet, J. Am. Chem. Soc. 111 (1989) 8533–8534. [9] T. Kato, Hydrogen Bonded Liquid Crystals: Molecular Self-Assembly for Dynamically Functional Materials, Springer, Heidelberg, 2000, and the references therein.

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[10] H. Kihara, T. Kato, T. Uryu, S. Ujiie, U. Kumar, J.M.J. Frechet, D.W. Bruce, J.D. Price, Liq. Cryst. 21 (1996) 25–30. [11] A.G. Cook, U. Baumeister, C. Tschierske, J. Mater. Chem. 15 (2005) 1708–1721. [12] J.W. Goodby, R. Blinc, N.A. Clark, S.T. Lagerwall, S.A. Osipov, S.A. Pikin, T. Sakurai, Y. Yoshino, B. Zecks, Ferro Electric Liquid Crystal, Principles, Properties and Applications, Gorden & Breech Press, Philadalphia, 1991. [13] P.G. de Gennes, The Physics of Liquid Crystals, Oxford Press, London, 1974. [14] B. Sreedevi, P.V. Chalapathi, M. Srinivasulu, V.G.K.M. Pisipati, D.M. Potukuchi, Liq. Cryst. 31 (2004) 303–310. [15] P. Swathi, P.A. Kumar, V.G.K.M. Pisipati, A.V. Rajeswari, S. Sreehari Sastry, P. Narayana Murty, Z. Naturforsch 57a (2002) 797–802. [16] C. Noot, S.P. Perkins, H.J. Coles, Ferroelectrics 244 (2000) 331–338. [17] A.G. Cook, Baumeister, U.C. Tschierske, J. Mater. Chem. 15 (2005) 1708–1721. [18] S. Marcelja, Solid State Commun. 13 (1973) 759–762. [19] M. Mishra, M.K. Dwivedi, R. Shukla, S.N. Tiwari, Prog. Cryst. Growth Charact. Mater. 52 (2006) 114–124. [20] S.N. Tiwari, Prog. Cryst. Growth Charact. Mater. 52 (2006) 150–158. [21] Bernard Helffer, Xing-Bin Pan, J. Funct. Anal. 255 (2008) 3008–3069. [22] T. Kato, Norihiro Mizoshita, Curr. Opin. Solid State Mater. Sci. 6 (2002) 579–587. [23] M. Srinivasulu, P.V.V. Satyanarayana, P.A. Kumar, V.G.K.M. Pisipati, Z. Naturforsch., A: Phys. Sci. 56a (2002) 685–691. [24] M.L.N. Madhu Mohan, B. Arunachalam, C.Arravindh Sankar, Metall. Mater. Trans. A 39 (2008) 1192–1195. [25] M.L.N. Madhu Mohan, B. Arunachalam, Z. Naturforsch., A: Phys. Sci. 63a (2008) 1–5. [26] M.L.N. Madhu Mohan, V.G.K.M. Pisipati, Liq. Cryst. 26 (2000) 1609–1613. [27] P.A. Kumar, M. Srinivasulu, V.G.K.M. Pisipati, Liq. Cryst. 26 (1999) 1339–1343. [28] P. Rudquist, E. Korblova, D.M. Walba, R. Shao, N.A. Clark, J.E. Maclennan, Liq. Cryst. 26 (1999) 1555–1561. [29] M. Srinivasulu, P.V.V. Satyanarayana, P.A. Kumar, V.G.K.M. Pisipati, Liq. Cryst. 28 (2001) 1321–1329. [30] E.B. Barmatov, A. Bobrovsky, M.V. Barmatova, V.P. Shibaev, Liq. Cryst. 26 (1999) 581–587. [31] P.E. Cladis, Phys. Rev. Lett. 35 (1975) 48–51. [32] V.N. Vijayakumar, K. Murugadass, M.L.N. Madhu Mohan, Mol. Cryst. Liq. Cryst. 517 (2010) 41–60. [33] G. Sigaud, Y. Guichard, F. Hardouin, L.E. Benguigui, Phys. Rev. A 26 (1982) 3041–3043. [34] V.N. Vijayakumar, M.L.N. Madhu Mohan, Ferroelectrics 392 (2009) 81–97. [35] S. Somasekhara, R. Shashidhar, B.R. Ratna, Phys. Rev. A 34 (3) (1986) 2561–2563. [36] T.R. Bose, D. Ghose, M.K. Roy, M. Saha, D. Mukherjee, Mol. Cryst. Liq. Cryst. 172 (1989) 1–16. [37] M. Schlorrarek, M. Naumann, A. Meadicke, B. Krucke, F. Kustchel, H. Zaschke, Mol. Cryst., Liq. Cryst. 193 (1990) 191–197. [38] L. Redzihovsky, Eur. Phys. Lett. 36 (1996) 595–600. [39] J. Bharatam, C.R. Bowers, J. Phys. Chem. B 103 (1999) 2510–2515. [40] O. Osamu, Nippon Kagakkai Koen Yokoshu, Jpn. Liq. Soc. (2000) 17–18. [41] N.M. Patel, C. Rosenblatt, Y.K. Yu, Phys. Rev. E 68 (2003) 011703–011707.

[42] G. Czechowski, J. Jadzyn, Acta Phys. Pol. A 106 (2004) 475–485. [43] M.H. Zhu, C. Rosenblatt, J.M. Kim, M.E. Neubert, Phys. Rev. E 70 (2004) 031702–031706. [44] B.K. McCoy, Z.Q. Liu, S.T. Wang, V.P. Panov, J.K. Vij, J.W. Goodby, C.C. Huang, Phys. Rev. E 73 (2006) 041704. [45] M. Simoes, F.S. Alves, K.E. Yamaguti, P.A. Santoro, N.M. Kimura, A.J. Palangana, Liq. Cryst. 33 (2006) 99–102. [46] S.J. Ghosh, V. Rathos, R. Krishnaswamy, V.A. Raghunathan, A.K. Sood, Langmuir 25 (2009) 8497–8506. [47] T. Chitravel, M.L.N. Madhu Mohan, V. Krishnakumar, Mol. Cryst. Liq. Cryst. 493 (2008) 17–25. [48] V.N. Vijayakumar, M.L.N. Madhu Mohan, Solid State Commun. 149 (2009) 2090–2097. [49] N. Pongali Sathya Prabu,V.N. Vijayakumar, M.L.N.Madhu Mohan Physica B:406 (2011) 1106–1113. [50] V.N. Vijayakumar, K. Murugadass, M.L.N. Madhu Mohan, Mol. Cryst. Liq. Cryst. 515 (2009) 39–48. [51] V.N. Vijayakumar, M.L.N. Madhu Mohan, Braz. J. Phys. 39 (2009) 601–605. [52] N.Pongali Sathya Prabhu, V.N. Vijayakumar, M.L.N. Madhu Mohan, J. Mol. Struct. 994 (2011) 387–391. [53] V.N. Vijayakumar, M.L.N. Madhu Mohan, Solid State Sci. 4 (2009) 142–149. [54] M.L.N. Madhu Mohan, P.A. Kumar, B.V.S. Goud, V.G.K.M. Pisipati, Mater. Res. Bull. 34 (1999) 2167–2175. [55] N.Pongali Sathya Prabu, V.N. Vijayakumar, M.L.N. Madhu Mohan, Mol. Cryst. Liq. Cryst. 548 (2011) 73–85. [56] M.L.N. Madhu Mohan, D.M. Potukuchi, V.G.K.M. Pisipati, Mol. Cryst. Liq. Cryst. 325 (1998) 127–135. [57] M.L.N. Madhu Mohan, P.A. Kumar, V.G.K.M. Pisipati, Mol. Cryst. Liq. Cryst. 366 (2001) 431–455. [58] G.W. Gray, J.W.G. Goodby, Smectic Liquid Crystals: Textures and Structures, Leonard Hill, London, 1984. [59] C. Kavitha, N.Pongali Sathya Prabu, M.L.N. Madhu Mohan, Physica B 407 (2012) 859–867. [60] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Co-ordination Compounds, Interscience, New York, 1978. [61] T. Kato, T. Uryu, Liq. Cryst. 14 (5) (1993) 1311–1317. [62] D.L. Pavia, G.M. Lampman, G.S. Kriz, Introduction to Spectroscopy, third ed., Harcourt, College, Boston, 2001; N.Pongali Sathya Prabu, V.N. Vijayakumar, M.L.N. Madhu Mohan, J. Mol. Struct. 994 (2011) 387–391. [63] S. Chandrasekhar, Liquid Crystals, Cambridge University Press, New York, 1977. [64] J. Thoen, G. Cordoyiannis, C. Glorieux, Liq. Cryst. 36 (2009) 669–684. [65] S. Marcelja, J. Chem. Phys. 60 (1974) 3599–3604. [66] S. Senthil, K. Rameshbabu, S.L. Wu, J. Mol. Struct. 783 (2006) 215–220. [67] H.E. Stanley, Introduction to Phase Transition and Critical Phenomena, Clarendon Press, New York, 1971. [68] J.E..Adams, Liquid crystal optical filter system, U.S. Pat. No. 3,679,290, (1972). [69] J.E..Adams, Optical notch filters, U.S. Pat. No. 3,711,181, (1973). [70] P. Goldberg, J. Hansford, P.J. van Heerden, Polarization Light Suspens. Small Ferrite Part. Magn. Field 42 (10) (1971) 3874–3876.