STRUCTURE and ELECTROOPTICAL PROPERTIES

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smectic structure becomes a ferroelectric medium, while the chiral anticlinic ... ferroelectric or antiferroelectric liquid crystals in the surface stabilized geometry.
Invited Paper

STRUCTURE and ELECTROOPTICAL PROPERTIES of ORTHOCONIC ANTIFERROELECTRIC LIQUID CRYSTALLINE MATERIALS R.Dąbrowski, K.Czupryński, J.Gąsowska, M.Tykarska, P.Kula, J.Dziaduszek, J.Oton+, P.Castillo+, N.Benis+ Military University of Technology, Institute of Chemistry, 00-908 Warsaw, Poland Universidad Politecnica de Madrid, ETSI Telecomunicacion, 28040 Madrid, Spain

+

ABSTRACT The optic and electrooptic properties of recently prepared orthoconic antiferroelectrics have been revived. Relation between their chemical structure and mesogenic properties, smectic layer structure and helical pitch is discussed. Keywords: orthoconic antiferroelectric, electrooptical response, phase transition, layer structure, X-ray investigation, selective reflection, pitch of helix

1. INTRODUCTION Liquid crystals may be divided into two fundamental classes: nematic liquid crystals and smectic liquid crystals. Nematic liquid crystals have only space ordering along the long molecular axis. Smectic liquid crystals show also positional order leading to a layer structure, see Fig. 1. In the layers the molecules are nearly parallel to each other and parallel to the layer normal – the smectic A, or they are tilted at θ to the layer normal, the synclinic structure - smectic C, or wherein the tilt is alternating from (-θ) to (+θ) in neighboring layers, the anticlinic structure - smectic Canti.. If molecules in the tilted phases are chiral (optically active) they are additively ordered in space in such way that they form a left or right twisted structure – helix with the pitch p. This leads simultaneously to the orientation of dipole moments of molecules in space according to the helical sense. When the helix becomes unwounded under surface interaction (p≤d, pitch shorter than cell gap), the surface stabilized ferroelectric or antiferroelectric liquid crystal, SSFLC or SSAFLC[1], the dipoles are pointed in one direction and spontaneous polarization appears in the layers (P(+) or P(-)). The chiral synclinic smectic structure becomes a ferroelectric medium, while the chiral anticlinic smectic structures becomes an antiferroelectric medium[1]. Nematic liquid crystals still dominate in display technology or different photonic applications. The use of them in plain switching effect (IPS) or vertical alignment effect (VA) gives excellent view angle and contrast[2], although these effects are still slow for very fast displaying, only a few or dozen ms is achievable. It results from the fact that molecules must be reoriented from a horizontal to vertical position, or vice-verse, by application of an electric field, Fig. 1. In the ferroelectric or antiferroelectric liquid crystals the molecules only rotate around a cone of 2θ from one fixed position with polarization P(+) to the other with polarization P(-), or from P(0) to P(+) or P(0) to P(-). The molecules do not change the tilt only its sense, therefore the time of reorientation may is much shorter, only a few or dozen µs. Displays with very high information content, very high speed and large angle of viewing can be designed using ferroelectric or antiferroelectric liquid crystals in the surface stabilized geometry.

Liquid Crystals: Optics and Applications, edited by Tomasz R. Wolinski, Marc Warenghem, Shin-Tson Wu, Proc. of SPIE Vol. 5947, 59470C, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.619805

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Surface-stabilized geometry

N

SmA*

cc c P=o

SmC*

SmC*anti

7 c7

cç ç cc 7c7

P=o

PEt

P=o

[J

ii U.. liii O-+E

P>o

2e

E-+E

E>Et

P>o

P>o

[:..J

Molecules

Molecules

reorient

tilt

Molecules rotate along the cone

Figure 1. Nematic, chiral smectic A and smectic C molecules in cells oriented by surface interactions in condition with one electric field – a) and with electric field - b).

2. OPTIC AND ELECTROOPTIC PROPERTIES OF ORTHOCONIC ANTIFERROELECTRICS In spite of the positive features mentioned SSFLCs are still only used in a limited number of applications, usually as small displays driven by active matrix on monocrystalline silicone, in photocameras for example. High cost, complicated driving systems, problems with the stable orientation of a larger surface and with the separation of the charges during the rest time currently prevent the increase of applications. SSAFLCs are tristable, which removes the charge separation problem. Also, much simpler passive matrix addressing may be used. From the other side the difficulty in obtaining well oriented layers and big pretransitional effects of different nature leads to leakage of light and decreases the contrast distinctly. Recently it was found[3,4] that the increase of the molecule tilt to 45o (cone tilt 90o) leads to the drastic change of optical properties, see Fig. 2. In this case the dielectric tensor in the antiferroelectric state is given by ε1 + ε 3 ε OAF (θ = 45 ) = o

0

2 0

ε2

0

0

0 0

ε1 + ε 3 2

and the optical indicatrix is an oblate ellipsoid with the optical axis in direction perpendicular to the glass plates, while in the ferroelectric state the indicatix is a prolate ellipsoid with the optical axis parallel to the plates and at 45o to the smectic layer normal. OAFLC becomes an optically uniaxial negative medium instead of an optically biaxial positive one. For the light coming through the layer perpendicular it behaves as isotropic medium and in the case of ε1≈ε2+ε3/2 for all incidence angle of light, Ref.[5]. Therefore defects are not seen at the surface and excellent contrast is generated. The observed contrast theoretically may be infinitively large. It is limited only by polarizer quality and the detection limit of the experimental system[3].

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Orthoconic SSAFLC

transmission of light

-

bright

- E(+)

dark

bright

Figure 2. Shape optical indicatrix an transmission light in orthoconic antiferroelectric and ferroelectric states. Dark state is generated in the antiferroelectric state and two bright states in the induced by electric field ferroelectric state (one for minus polarization and other one for plus polarization). As an example of experimental electrooptical characterization results are given for a recently prepared orthoconic mixture W-205A in Table 1, wherein it is compared with the results for regular mixture W-204D. Table 1.

Main electrooptical properties of orthoconic AFLC mixture W-205A at temperature 35oC.

Material W-205A W-204D

Selection (Voltage;Duration) (32V;1200 µs) (29V;36 µs)

Tilt angle (º) 43 22

Contrast 0.1Hz 60Hz 1380 105 35 20

Dynamic Range (V) 5.5 6.7

Response times (µs) tfall trise 90 598 24 89

Transmission(a.u)

Electrooptical studies have been performed on this orthoconic material aiming to evaluate its static and dynamic performance under passive multiplexing conditions. A number of parameters have been evaluated: static and dynamic contrast, driving scheme for passive multiplexing, rise and fall response times, dynamic range, and dynamic grey scale. The hysteresis cycle at 0.1 Hz is shown in Fig. 3.

Figure 3. Hysteresis at 0.1Hz for the orthoconic mixture W-205A. As usual for orthoconics the hysteresis is slightly shifted and the grey levels are not completely symmetric.

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Quasi-static contrast is taken from the hysteresis cycle obtained with an AC triangular waveform at low frequency, i.e. 0.1 Hz or 1 Hz. W-205A does not fully relax when driven at 35 ºC using 1 Hz signals. As usual for orthoconic AFLCs static contrast is excellent, see Table 1. Rise time is a measurement of the switching time for the voltage-induced AFLC FLC transition. It is substantially voltage-dependent, and it is usually fast enough, under required multiplexing conditions, to account for shorter slot times than the opposite FLC AFLC transition. Orthoconic AFLCs show 10%-90% rise times in the range of tens of µs. On the contrary, fall times are in principle relaxations in the absence of applied voltage. Under such conditions fall times are much longer, in the range of hundreds of ms or even seconds, because cell surface favoring the ferroelectric order[6]. These extremely long times would preclude any dynamic application of these materials unless driving schemes including forced relaxation are used. Fall times are usually the limiting factor for the dynamic response of the orthoconic material; therefore, the blanking sequence of the driving waveform becomes a crucial issue. The example shown in Fig. 4a employs a simple forced relaxation scheme based on a well pulse and a reset region.



b)

m-.

c Transmission (a.u)

a)

Waveform (V)



•S •S

- Cycle + Cycle

c)

.

I

20

I

24

28

32

Voltage (V) Figure 4. Grey levels obtained at video frequency for the orthoconic mixture W-205A driven with the addressing waveform shown on the top. More involved driving schemes, having several wells and/or AC sections have been proposed[7]. Although these schemes achieve better fall times, it must be taken into account that the number of voltage levels required for the scheme increases as well, and the electronic drivers become more complex. Therefore, the optimum waveform shall result from a trade-off between the display performance and the driving electronics complexity. Dynamic contrast is measured upon actual working conditions, using a multiplexed driving scheme at 60 Hz frame rate. The contrast is taken as the quotient between the integrated transmissions of a clear (saturated) pixel and a dark pixel

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over their whole clear and dark frames respectively. Therefore, spurious transmission arising from the blanking pulses of the driving scheme is included in both frames, resulting in a significantly lower - yet more realistic – contrast value. The dynamic response of orthoconic AFLC materials is usually much longer than the response of regular AFLCs. For example, passive multiplexing of an XGA display at 60 Hz video frequency (16.7 ms frame) requires the material to be driven with 55 µs selection slots using dual-scan mode. These narrow slots can be reached with regular AFLCs, but are usually beyond the response time of orthoconic AFLCs. Fig. 4a shows the driving scheme used to test W-205A at video frequency. It includes a selection pulse, a bias period covering most of the frame, and a blanking sequence consisting of a well pulse and a reset. The dynamic transmission corresponding to this driving scheme is shown in the Fig. 4b. Note that the slot time employed (1200 µs) for this test is much longer than the slot used in regular AFLCs. In fact, this slot would allow multiplexing less than 30 rows at 60 Hz or 60 rows at 30 Hz. In our experience, the highest number of rows that can be multiplexed at 60 Hz with current orthoconics is about 100. Above this limit, significant trailing effects become apparent. Dynamic greyscale is an important issue for two reasons. Firstly, the range is covered with the column data, whose signals are “seen” by all the rows. The lower the data voltage, the lower the crosslink between different rows. On the other hand, data drivers are intended to be made on standard electronics, effectively limiting the data voltage range. The use of high voltage data drivers, although possible, shall be avoided in practical designs. Fig. 4c shows the dynamic greyscale of W-205A Data range (10-90) is about 5.5 V, approximately the limit for conventional electronics.

3. RELATION BETWEEN CHEMICAL STRUCTURE AND MESOGENIC AND PHYSICAL PROPERTIES 3.1. Structure and phase transitions The compounds in which an antiferroelectric phase is observed are still limited mainly to the family of three or four ring esters expressed by formula I or II. H2n+1Cn X

A

Z1

B

* COOCHCrH2r R'

Z2

I

Y H2n+1Cn X

A

Z1

B

Z2 C

* COOCHCrH2r R'

II

Y

Rings A, B and C are the most often aromatic (benzene) and some sometimes heteroaromatic (pyrimidine) ones. The presence of one saturated ring is possible, although tilted phases are usually destabilized in its presence[8]. Achiral alkyl chain H2n+1Cn is bonded to the core directly or via an oxygen atom or a carboxylic group, Z1 and Z2 are single bonds or COO, COS or CH2O groups. The benzene rings may be laterally substituted, usually by one or a few fluorine atoms. A branched chain in the second position of molecules is necessary to obtain anticlinic order. It also creates chirality (optical activity). Usually 1-methylalkanol (for example 1-methylheptanol) or 1-trifluoromethylalkanol, or ω-alkoxy-1methylalkanol are used to create chirality. Many structures I and II were reported in Ref.[8-11]. They are tilted at a moderate angle, between 20-30 degree and following phase sequences are observed : Cr → SmCanti. → SmC → SmA → N → Iso; Cr → SmCanti. → SmC → SmA → Iso; Cr → SmCanti. → SmC → Iso; Cr → SmCanti. → SmA → Iso; Cr → SmCanti. → Iso The most desired compounds are those that the exhibit simultaneously a nematic and smectic A phase above the tilted phases, because a well ordered monodomain layer is then easily obtained during cooling under surface interaction. Many compounds with the synclinic smectic C phase and the smectic A and the nematic phase above it have been found, but anticlinic compounds with such phase sequence are still exotic. Recently naphthalene derivatives having simultaneously the antiferroelectric phase and the nematic phase were described[12]. We found that exchange in the compounds of formula I the alkyl unit H2n+1Cn by perfluorinated alkyl F2n+1Cn and simultaneously separating it by X=COOCmH2m or OCmH2m group from the rigid core of molecule leads to molecules having antiferroelectric properties and high tilt up to 45 degree (orthoconic antiferroelectric). Homologous series of biphenylyl benzoates – nFmB series (Z1=COO and Z2 – single bond) and phenyl biphenylates (Z1 – single bond and Z2 – COO) in a chiral, racemic and achiral forms have been prepared recently [11-13]. The relation between the chemical

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structure of the above mentioned compounds and their mesogenic properties, helical pitch and tilt with will be discussed below. The phase transitions observed in two, three and four ring esters with fixed fluorinated chain C3F7COOC6H12O as example are compared in Fig. 5. SmC*anti

Cr C3F7COOC6H1 2O

COO

SmC* SmA

COOC*H(CH3)C6H13 (S)

COO

Cr 1 69.2 Cr 76.3 SmCant i. 154.5 SmC * 215 SmA 223 Iso 14.3 24.10 0.042 1.09 3.05 C3F 7CH 2OC6H 12O

COOC*H(CH3)C6 H1 3 (S)

COO *

Cr1 44.2 Cr 62.6 SmCanti. 111.3 SmC 130 SmA 140.8 Iso 1.34 23.2 0.02 1.3 4.6 C3F 7COOC6H1 2O *

Cr 29.4 SmC 15.8

anti.

111.4 SmC 122.5 SmA 129.3 Iso 0.063 1.38 2.93

C3F 7COOC6 H1 2O *

Cr 27.6 SmC 19.3

COOC*H(CH3)C 6H 13 (S)

COO *

COO

COOC*H(CH3 )C6 H 13 (S)

91.1 SmC 118.1 SmA 118.5 Iso 0.092 1.30 3.14

anti.

C3F 7COOC6H12 O

COO

COOC*H(CH3 )C6 H13 (S)

Cr 36.1 Iso 35.2 0

40

80

120

160

200

240

o

T[ C]

Figure 5. Phase transition temperatures (oC) and enthalpies (kJ/mol) for fluorinated two, three and four ring esters. Chiral two ring esters are not mesogenic. Three ring biphenylates as well benzoates are mesogenic and show during heating the phase sequence: Cr→SmC*anti.→SmC*→SmA→Iso. The antiferroelectric phase (SmC*anti.) is observed in very broad temperature range (much broader than analogous compounds with C3H7 terminal unit have[13-15]). The ferroelectric phase (SmC*) is observed in short temperature range and orthogonal smectic A phase is observed over a very short temperature range, especially in benzoates. The ester type of the terminal chain leads to the compounds with very low melting points, near room temperature. In the homologous series nFmBi (Fig. 6a) the first member without spacer (m=0) has very high melting point and mesophases are not observed. The melting point systematically decreases and the temperature range of the SmC*anti phase is broadened as m is increased. The spacer with six methylene groups exhibits excellent mesogenic properties. The compounds with melting point near room temperature and with very low melting enthalpy and very broad temperature range of antiferroelectric phase (SmC*anti.) are created. A similar tendency is observed also in 1F1HmBi series with CF3CH2O terminal unit (Fig. 6b) instead of CnF2n+1, although for m=4 irregularity occurs, such that the member m=4 has higher melting point than members in neighborhood. The ether type of the terminal chain leads to the compounds with higher melting as well clearing points. The four ring compound has the same phases but their thermal stability is much higher, the clearing point is above 200oC. In fluorinated compounds the all transitions between mesophases are first order transitions. The enthalpies of SmC/SmA transitions are bigger (10-20 times) than the enthalpies of SmCanti/SmC transitions. Such relation was found in all compounds investigated until now. Melting points and melting enthalpy strongly depend on the length of methylene spacer placed between fluorinated terminal unit and the rigid core of the molecules, while clearing points only a little, see Fig. 7.

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C3F7COOCmH2mO

COO

COOC*H(CH 3)C6 H13

nFmBi

(S)

a)

∆Hm [kJ/mo l]

m

15,77

6

8,0; 19,66

SmC*ant i

Cr

SmC* SmA

5

16,15

4

23,35

3

7, 1; 48,07

0 0

40

CF 3CH2 OCmH2 mO

∆H m [kJ/mol]

m

27,20

5

33,05

4

34,72

3

26,15

2

80

120

COO

160

CO OC*H(CH3 )C6H13 (S)

1FH1mBi

b)

0

40

80

120

160

T[o C]

Figure 6. Diagrams of the phase transitions temperatures showing the range of phases for phenyl biphenylates: 3FmBi series – a) and 1F1HmBi series – b). F2n+1 CnCOOC6 H12O ∆Hm [kJ/mol]

COO

7

29,50

6

26,40

5

22,13

nF6Bi (S)

SmC SmA

4

15,77

3

21,30

2

17,03

1 0

F2 n+1 CnCOO C6 H 12O ∆H m [k J/mol ]

SmCanti

Cr

n

23, 05

COOC*H(CH3)C6H1 3

40

80

120

160

COOC*H(CH 3)C 6H13

COO

nF6B (S)

n

17,4 9

7

24 ,35

6

26 ,15

5

26,2 3

4

19,2 9

3

7,5; 10,7

2

33,1 8

1 0

40

80

120

160

Figure 7. Diagrams of the phase transitions temperatures for phenyl biphenylate series (nF6Bi) – a) and biphenylyl benzoates (nF6B) – b).

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The length of the fluorinated unit is also very important, its influence on the stability of all mesophases can be seen in Fig. 7a, b. In the homologous series nF6B as well in nF6Bi the phase transition temperatures SmCanti/SmC, SmC/SmA and SmA/Iso are growing with the increase of fluorinated part of the chain (when n change from 1 to 7). The same is observed in series with smaller value m[11]. The thermal stability of the SmCanti. and SmA phase is growing with increase n more than the synclinic C phase, so its temperature range becomes smaller. The SmCanti. phase is more stable and exists in over a larger temperature range in biphenylates than in benzoates (SmC*anti. nF6Bi> SmC*anti. nF6B). The same is observed for compounds with shorter spacer m. Most of the members of both homologous series nF6Bi and nF6B have small melting enthalpies and all have low melting points so they are excellent components to formulate mixtures with very low eutectic points (-30oC is easily obtained). The substitution of a benzene ring in a lateral position by a fluorine atom decreases all phase transition in both series, Fig. 8a, b. F ∆Hm [kJ/mol]

C 3F7COOC5 H10O

a)

SmC*anti

Cr

* COOCH(CH 3)C 6H13 (S)

COO

SmC* SmA

Cr 54.5 SmC *a nti. 104.0 SmC* 108.3 SmA 121.3 Iso

20,75

F C3F7 COOC5H10O 26,2

Cr 48.5 SmC *anti. 101.3 SmC * 102.7 SmA 106.4 Iso C3F7COOC5H10O

8,0; 19,7

* COOCH(CH3)C6H 13 (S)

COO

* COOCH(CH3)C 6H13 (S)

COO

Cr1 51.9 Cr 65.5 SmC

*

*

anti.

121.7 SmC 124.6 SmA 132.7 Iso 0

F COO

CF 3CH2OC5 H10 O 29,1

COO

Cr1 60.9 Cr 63.0

* SmC anti.

120

160

b)

COOCH(CH3)C6 H1 3 (S)

COOCH(CH3)C6 H1 3 (S)

*

91.0 SmC 92.5 SmA 109.1 Iso COO

CF 3CH2OC5H10O 28,0

80

Cr 51.6 SmC*an ti. 117.6 SmC* 118.3 SmA 126.3 Iso F CF 3 CH2OC5 H 10 O

7,3; 27,4

40

COOC*H(CH3 )C6 H13 (S)

Cr 77.6 SmC* anti. 126.9 SmC* 127.3 SmA 136.7 Iso 0

40

80

120

160

T[o C]

Figure 8. Influence of fluorine atom substitution on phase transition in 3F5Bi series and 1F1H5Bi series. The observed temperature transition falling is a little bigger for the compound with fluorine atom in the position 2 (more central) than in position 3 in the case of nF5Bi series. The analogous unsubstitued compounds with longer spacer (m=6) have very low melting points and the further decrease of melting points is not observed in substituted ones[12]. The exchange of the C3F7COO unit for C3F7CH2O leads to the compounds with a direct transition SmCanti-Iso (Fig. 9). ∆Hm [kJ/mol]

3,5; 19,6

F 24,7

C3 F7CH2OC3 H6O

S mCa nti

Cr

* COOCH(CH3 )C6 H13 (S)

COO

C3 F 7CH2OC3 H 6O

Sm C

Cr 51.4 SmCanti. 98.9 Iso COO

* COOCH(CH3)C6H13 (S)

Cr 1 56.0 Cr 76.6 SmCanti. 117.4 Iso 21,2

C3 F7COOC3H 6 O

COO

* COOCH(CH3 )C6H 13 (S)

Cr 69.4 SmCanti. 121.4 SmC 125.1 Iso 0

40

80

120

160

Figure 9. Influence of exchange of perfluoropropanoyl group by perfluoropropylmethyl group on phase transition.

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F2n+1 CnCOOC6H 12O

COO

COOCH(CH3)C6 H1 3

nF6Bi (S)

Sm Can ti

Cr

nF6Bi (S,R)

SmC

Sm A

∆Hm [kJ/mol]

n

∆H m [kJ/mol]

n

23,05

7

29,66

7

29,50

6

32,05

6

26,40

5

30,92

5

22,13

4

30,33

4

15,77

3

24,85

3

21,30

2

14,57

2

17,03

1

15,56

1

0

40

80

120

Cr

0

160

SmCan ti

SmC

80

120

40

SmA

160

Figure 10. Comparison of phase transition temperatures in chiral (S) and racemic homologous series nF6Bi. In the chain fluorinated compounds as was also found in hydrogenous compounds[15] optical activity and the kind of steric center has an influence on the observed phase sequence and stability of the anticlinic phase. In the racemic form of biphenylates (nF6Bi series) as well benzoates (nF6B) the phase situation is drastically different from that observed in the enantiomers, Fig. 11a, b. The anticlinic phase disappears totally in benzoates, while it is still observed in biphenylates but only in members with even number of fluorinated atoms, but also in this case the stability of the phase is much lower. The most racematic members in nF6Bi family have lower melting points than analogues (S) enantiomers and the enthalpies of SmC/SmA and SmA/Iso transitions differ but only a little in chiral and racemic forms. In achiral chain fluorinated compounds a stable enantiotropic or monotropic anticlinic phase (SmCanti) is observed in the case of compounds with 4-heptyl chain (nFmBia7 series). A longer spacer (m=6) ensures lower melting points, Fig. 11a. Swallow type 4-heptyl chain generates big steric hindrance. Shorter branched chain, isopropyl group leads to compounds with extremely high clearing points and high clearing enthalpy, see Fig. 11b. Such two ring compounds are also mesogenic. The observed phases, the anticlinic and synclinic ones are very stable but the anticlinic one appears only in the members having the odd number of methylene group in spacer and the odd number of fluorinated carbon atoms in the chain. More information about compounds with achiral terminal swallow type chain will be given in another paper[15]. a)

∆Hm [kJ /mol]

C3F7COOC6 H12 O

CO O

COOCH(C3H 7) 2

∆HC [kJ/mol]

2,01

20,76 COO

C4F9 COOC6 H12O

COOCH(C 3H7) 2

2,55

16,36 COO

C 3F 7COOC5H 11O

COOC H(C3H7) 2

28,58

1,80 0

40

80

120

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b)

∆Hm [kJ /mol]

C3F7 COOC6H1 2O

COO

∆HC [kJ /mol]

COOCH(CH 3)2

32,89

5,69 C3F7 COOC6H 12O

COO

COOCH (CH 3)2

8,03

17,57 C 3F 7COOC6 H1 2O

COO

CO OCH(CH3 )2

8,87

16,0 C4 F 9COOC5H11O

COO

COOCH(CH3)2

10,0

39,07 C3 F7COOC5 H11O

COO

COOCH(CH 3)2

9,83

19,83 0

40

80

120

160

200

240

T[oC]

Figure 11. Phases in achiral compounds with big steric hindrance – a and in achiral compounds with small steric hindrance – b.

3.2. Selective reflection and pitch

The helical pitch p that is the length of helix is an important parameter. When it is longer the easier it is for the twisted structure to be unwounded by surface interaction and thicker cells may be used. The pitch may be calculated from the measurement of selective light reflection because a simple relationship between maximum of light reflection (λmax), pitch (p) and average optical refractive index (n) is observed.

p=

λ max n



λ max 1.5

The pitch in the investigated fluorinated compounds strongly depends on temperature, see Fig. 12. Here the compounds with the same length of fluorinated unit are compared. At lower temperatures the smectic layer is more twisted and selective reflection is shifted more to the ultraviolet region. At room temperature the majority of investigated structures reflect the light in visible range 450-650 µm, what relates to 0.3-0.4 µm pitch. In the compared compounds the compound 3F3Bi has shortest pitch. It is probably only about 0.2 µm at room temperature and compounds 3F1H6Bi reflect the light in the infrared region. The following relation between the pitch and structure is seen n Fig. 12. The pitch is longer when spacer index m increases and C3F7COO terminal unit is exchanged by C3F7CH2O unit and phenyl biphenylate core (Bi) is changed for biphenylyl benzoate core (B) and benzene ring is substituted by a fluorine atom. Also the length of the fluorinated part of the chain strongly influences the length of pitch, see Fig. 12. There the pitch is compared in four homologues series: nF6Bi-a, nF6Bi-b, nF6Bi(2F)-c and nF6Bi(3F). In biphenylates nF6Bi and nF6Bi(3F) selective reflection shifts to red as n increases and for member n=7 in both series the light is reflected above 900 nm (in infrared region). In nF6Bi(2F) the maximum reflected light only has a slight dependence on n. In the benzoates series nF6B the light is reflected strongly in an irregular way, Fig 13. Also here the even member reflects light at a shorter length and the average value shows rather a tendency to decreasing with increase n.

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900 800

3F6Bi(2F) 3F1H6Bi

3F6Bi 3F6Bi(3F) 3F3Bi

λ [nm]

700 600 500

3F6B

3F3B

3F1H3B

400 0

20

40

60 T [oC]

80

100

Figure 12. Temperature dependence of selective reflection of light in different biphenylates and benzoates. nF6B

800

800

700

700

600 500 400 300

600 500 400

0

1

2

3

4

n

5

6

7

300

8

nF6Bi(2F)

900

0

800

800

700

700

600 500

2

3

4

n

5

6

7

8

6

7

8

nF6Bi(3F)

600 500 400

400 300

1

900

λ max[nm]

λmax [nm]

nF6Bi

900

λ max[nm]

λ max[nm]

900

300 0

1

2

3

4

n

5

6

7

8

0

1

2

3

4

n

5

Figure 13. Selective reflection of light in homologues series of biphenylates and benzoates upon lengths of fluorinated unit at fixed reduced temperature T-TCA=-40 deg.

3.3. X-ray diffraction pattern – the smectic layer structure Temperature dependences of the d/l ratio upon reduced temperature T-TCA are compared for chiral, racemic and achiral compounds in biphenylate (nF6Bi) and benzoate (nF6Fi) families on the example of members n=4. The temperature dependence of smectic layer spacing d was calculated from small angle X-ray diffraction and molecule length l was calculated for the most extended structure by MNDO method. The values of layer spacing d and normalized layer spacing d/l in these enantiomers and racemates are very similar, d/l drops during transition to the tilted phase, in the same way, see Fig. 14a, b. The same was observed for other members of nF6B while chiral members nF6Bi have lower d/l values than the racemic one. Ratio d/l in smectic A phase (dAmax/l) is equal about 0.86 which is much smaller than that observed in common smectic A of ca. 0.96-1.0. In achiral analogues 4F6Bia3 dAmax/l is a little above 1, such value is observed in smectic A, when dimerization is starting[17].

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1,0 5

1,00

a)

4F6Bi(S) 4F6Bi(R,S) 4F6Bia3 4F6Bia7

0,88

d /l

d /l 0,8 5

4F6B(S) 4F6B(R,S)

0,92

0,9 5 0,9 0

b)

0,96

1,0 0

0,84 0,80

0,8 0

0,76

0,7 5

0,72 0,7 0 -100

-80

-60

-40

-20

0 T-T

C-A

20

-100

-80

-60

-4 0

-20

[ C]

0 T-T

C-A

20 [ C]

Figure 14. Comparison temperature dependence of the d/l ratio upon reduced temperature for 4F6Bi(S), 4F6Bi(R,S), 4F6Bia3, 4F6Bia7 – a) and 4F6B(S), 4F6B(R,S) – b). Small values of d/l ratio in chiral and racemic nFmB and nF6Bi compounds suggest that smectic A phase must be de Vries type[18]. The molecules are tilted but freely rotate, which averages tilts to 0º. Ratio d/l depends on the length of the fluorinated chain and the kind of rigid core, Fig. 15a, b, c, dAmax/l decrease in both series with increase n. a)

1 ,00 0 ,96

nF6Bi(S)

0 ,92

n 1 2 3 4 5 6 7

d/l

0 ,88 0 ,84 0 ,80 0 ,76 0 ,72 -1 00

-80

-60

-40

-20

0

20

T-TC-A [oC]

b)

c)

n F6B n F6Bi

0 ,81

n F6B n F6Bi

0,9 2

0 ,80 0,9 0

0,8 8

A

d /l

d/l

0 ,79 0 ,78

0,8 6

0 ,77

0,8 4

0 ,76 1

2

3

4

5

6

7

1

n

2

3

4

5

6

7

n

Figure 15. Comparison of the ratio d/l upon temperature – a) and at fixed reduced temperature T-TC/A=-40 deg – b) and the maximum ratio d/l in smectic A phase (dAmax/l) – c) in homologues series nF6Bi and nF6B.

This means that the molecules in the smectic A phase become more tilted with increasing n, and they are more tilted in the nF6Bi family than in nF6B, see Fig. 15c. The values of tilt calculated from across dA/l are big and change in the range 23-33º. In the antiferroelectric phase nF6B members are more tilted than nF6Bi members, Fig. 15b. The member having 3 or 4 fluorinated carbon atoms show highest tilts, this is concordant with the measurement of tilt by optical methods[19]. Recently a combined tilted asymmetric diffuse cone model was proposed (TADC), Ref. [18]. Molecules are tilted and fluctuate freely along layer normal, so they are distributed in the space uniformly. Such model involves that the macroscopic tilt is induced but without affecting the layer spacing. Further decrease in temperature leads to additional increase of average tilt. The increase of molecules tilting in the smectic layer depends on the length of their fluorinated part, see Fig. 16a, b. It is bigger for biphenylates than benzoates and becomes smaller with the increase of n.

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1, 00

a)

0, 98

0, 92 0, 90 0, 88

0,94

b)

nF6B nF6Bi

0,93 0,92 0,91

A

d /dA

0, 94

d/d

n 1 2 3 4 5 6 7

0, 96

0,90 0,89 0,88 0,87

0, 86 -100

0,86

-80

-60

-40

-20

0

T-T

20

C-A

[oC]

1

2

3

4

5

6

7

n

Figure 16. Comparison temperature dependence of the ratio d/dAmax upon temperature in nF6Bi series – a) and at reduced temperature T-TC/A=-40 deg in homologues series nF6Bi and nF6B – b). After the transition high tilt is generated spontaneously. It then quickly increases to a saturated value, for example 44o in the case of 4F6B member[19]. This value is a little bigger than calculated from across d/l (40.36 deg.). The observed difference results from the fact that that X-rays and light do not see exactly the same part of molecules (the light rather only the rigid core of molecules, whereas X-ray the total molecules)[19].

CONCLUSION Several homologous series of three ring biphenylyl benzoates (Bi) and phenyl biphenylates (B) containing fluorinated unit in the terminal chain have been investigated and the compounds with very low melting points and very low melting enthalpy was found. They allow for the formulation of multicomponent mixtures exhibiting antiferroelectric orthoconic phase in large temperature range (from -30 above to one hundred degrees). The mixtures show excellent contrast resulting from their unusual optic properties, they are uniaxial negative in absence electric field and after applying the field became biaxial positive. Electrooptic response is asymmetric, rise time is short (dozen µs) and fall time is much longer (a few hundred µs). Probably the cell surface better favoring the anticlinic order and longer pitch may involve more symmetric response. The materials are strongly twisted (0.2-0.5 µm) and helical pitch strongly depends on the structure. It is possible to obtain compounds with a longer pitch. The high tilt is generated in two ways. Molecules are high tilted already in smectic A phase but they freely rotate along the layer normal. During the transition SmA/SmC molecules are biased in a space direction and generate rapidly a tilt, which is increasing as temperature is decreasing, to a high saturated value near the 45 deg. Acknowledgment Financial support from Polish Ministry of Sciences and Informatization 4T09A10324 and from EU projects IST “HEMIND” and TRN “SAMPA” is appreciated.

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