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A correlation of optical features of amorphous liquid-cholesteric liquid crystal transitions
Volume 51A, number 6
PHYSICS LETTERS
7 April 1975
A CORRELATION OF OPTICAL FEATURES OF AMORPHOUS LIQUIDCHOLESTERIC LIQUID CRYSTAL TRANSITIONS D. COATES and G.W. GRAY Department of Chemistry, The University, Hull HU6 7R1, England
Received
24 February 1975
Microscopic studies of twenty-nine compounds giving cholesteric liquid crystals have led to a correlation of optical features of their amorphous liquid-cholesteric liquid crystal transitions. The pretransitional effects for one compound have been studied by fluorine n.m.r. spectroscopy. Microscopic observations (using crossed polarisers) of the phenomena occurring at the cholesteric mesophase-amorphous isotropic liquid transition (Cli-!)
platelet texture appears at T°C,and this is superceded by a coloured cloud (seen only in transmitted light) at T1 °C.This lasts for about 0.1 °Cbefore the amor0C. On cooling, this sequence for steryl and non-steryl cholesterogens made phous liquidthe forms at T2 texture reforming just beusing reflected light [1] and transmittedhave lightbeen [2, 3, 5]. is reversed, ‘focal-conic’ In a previous article [4] we showed that the blue phase low T°C.By eye, this transition is characterised by (which we now prefer to call a blue apparition), which two different colouredbands. This type is shown by occurs for some stery~holesterogens,is visible to the (—)-2-methylbutyl 4 ‘-(4 “-nitro- and methoxy-benzyeye over the same temperature range as the platelet lideneamino)cinnamates and di-(—)-2-methylbutyl texture which can be seen microscopically in reflected terephthalylidene-bis-4 ‘-amino-benzoate and -cinnamate. light. We have now studied the Ch-I transitions of a Platelet texture. In all cases, the platelet texture lasts range of steryl and non-steryl cholesterogens, and in over about 0.5—1.0°C.When formed on cooling, the three optically different ways for non-steryl cholesteroplatelets are initially clear (usually violet and red), gens. The birefringent ‘focal-conic’ cholesteric texture, but, on further cooling, they become wrinkled and formed spontaneously on cooling the amorphous liquid, adopt a veiled appearance; the colours are then yellowprovided a convenient basis for comparing transition brown. The direction of the wrinkless is related to the temperatures on heating and cooling cycles, brightness of the platelets. Other features of the plateNon-Steryl Cholesterogens lets have already been reported [3, 4~. Type 1: On heating the ‘focal-conic’ texture, the Steryl Cholesterogens amorphous liquid forms at T°C;on cooling, the ‘focalThe striking feature of these Ch-I transitions (unlike conic’ texture reforms just below T°C.No unusual those for the non-steryl systems) is that the transitiotransitional features are seen either microscopically nal effects are difficult or impossible to see microscoor by the naked eye. This type is shown by the di-4pically in transmitted light, and reflected light must ((—)-2-methylbutyloxy)benzoate ester of 1 ,4-dthydroxy- be used. benzene. Type 1: No unusual transitional effects are seen Type 2: On heating the ‘focal-conic’ texture, a plateeither on heating or cooling; the ‘focal-conic’ texture let texture appears at T °Cand then this forms the reappears with only a little supercooling. No blue apamorphous liquid at T 1°C.On cooling, the platelet parition is visible to the naked eye. Examples from texture reappears at T1 °Cand the ‘focal-conic’ texture seven compounds which showed this behaviour are at just below T°C.By eye, this Ch-I transition is seen cholesteryl acetate and cholestanyl nonanoate. as a single coloured band both on heating and cooling. Type 2(a): On heating the ‘focal-conic’ texture, This type is shown by (—)-2-methylbutyl 4’-(4”-cyanoa blue apparition (by eye) and a platelet texture and -chloro-benzylideneamino)cinnamates. (visible microscopically in reflected light) are seen Type 3: On heating the ‘focal-conic’ texture, a at T°C,before the change to the amorphous liquid 335
Volume 51A, number 6
PHYSICS LETTERS
7 April 1975
at
T1 °C.On cooling, the platelet texture (and blue 0C, and the ‘coal-conic’ apparition) reappear at T1T C. Examples from eight texture forms Just below
Similar observations of narrow line p.m.r. signals at compositions intermediate between two amphiphilic mesophases which each give broad line signals have
compounds which showed this behaviour are cholesteryl propionate and cholesteryl butyl carbonate. Type 2(b): On heating, a direct ‘focal-conic’ texture
been interpreted in terms of intermediate cubic mesophases [7]. These wider investigations of Ch-l transitions con-
liquid transition occurs at T°C,but on cooling, a platelet texture (a blue apparition to the naked eye) occurs at T°C.The ‘focal-conic’ texture reappears at a slightly lower temperature (T 1 °C).When the platelet texture is heated it disappears at T°C. Examples from five compounds which showed this behaviour are cholestanyl heptyl carbonate and cho-
firm our earlier report [4j and clarify the situation; photomicrographs and a fuller documentation of the observations are presented in ref. [8]. We hope that the
lesteryl myristate.
will result.
to amorphous
Type 2(c): The behaviour is similar to type 2(b), but although a blue apparition is seen by eye, a platelet texture is not seen microscopically. Two compounds showed this behaviour: cholestanyl pentyl carbonate and S.cholesteryl heptyl thioate. Platelet texture. On cooling, the platelets are intitially violet and clear; they quickly (~0.1°C)become green and wrinkled, and more of them are then visible. They become red or brown, before forming the ‘focalconic’ texture. The Ch-I transitions for non-steryl and steryl cholesterogens therefore show many pretransitional similarities and the question arises whether surface forces operating in thin films are entirely responsible for the
observed effects. The optical observation by Saupe [6] that the cholesteric phase of cholesteryl p-nonylphenyl carbonate forms from the amorphous liquid via an isotropic liquid crystal phase in bulk samples, and our own broad line n.m.r. studies of cholesteryl 2-fluoroethyl carbonate make this seem unlikely. We find a sudden narrowing of the fluorine n.m.r. signal near to the I-Ch transition, indicating a sudden dramatic change in molecular orientation prior to the formation of t.he cholesteric phase proper (‘focal-conic’).
336
results stimulate further investigations of Ch-l transitions, particularly when stable cholesterogens [9] with lower transition temperatures become widely available, and that a fuller understanding of the cholesteric phase
The authors wish to thank Dr. W. Elser of the Night Vision Laboratory, Fort Belvoir, Virginia, 22060, U.S.A. for the gift of many steryl cholesterogens used in this work and also the Science Research Council, London for a maintenance grant (to D.C.).
References [11 W.
Elser, J.L.W. Pohlmann and P.F. Boyd, Mol. Cryst. and Liquid Cryst. 20 (1973) 77. [2] 0. Lehrnann, Z. Phys. Chem. 56 (1906) 750. [3] I.G.Chistyakov and L.A. Gusakova, Kirstallografiya 14 141 D. Coates and G.W. Gray, Phys. Lett. 45A, No. 2 (1973) 115. [5] D. Coates, G.W. Gray and K.J. Harrison, Mo!. Cryst. and Liquid Cryst. 22 (1973) 99. [6] Saupe, and Liquid Cryst. 7and (1969)59. [71 A. G.W. Gray Mol. and Cryst. P.A. Winsor, Mol. Cryst. Liquid Cryst.
26 (1974) 305.
[8] D. Coates, Ph.D. Thesis, Hull University, 1974. [9] D.G. McDonnell and G.W. Gray, to be published.
PHYSICS LETTERS
7 April 1975
A CORRELATION OF OPTICAL FEATURES OF AMORPHOUS LIQUIDCHOLESTERIC LIQUID CRYSTAL TRANSITIONS D. COATES and G.W. GRAY Department of Chemistry, The University, Hull HU6 7R1, England
Received
24 February 1975
Microscopic studies of twenty-nine compounds giving cholesteric liquid crystals have led to a correlation of optical features of their amorphous liquid-cholesteric liquid crystal transitions. The pretransitional effects for one compound have been studied by fluorine n.m.r. spectroscopy. Microscopic observations (using crossed polarisers) of the phenomena occurring at the cholesteric mesophase-amorphous isotropic liquid transition (Cli-!)
platelet texture appears at T°C,and this is superceded by a coloured cloud (seen only in transmitted light) at T1 °C.This lasts for about 0.1 °Cbefore the amor0C. On cooling, this sequence for steryl and non-steryl cholesterogens made phous liquidthe forms at T2 texture reforming just beusing reflected light [1] and transmittedhave lightbeen [2, 3, 5]. is reversed, ‘focal-conic’ In a previous article [4] we showed that the blue phase low T°C.By eye, this transition is characterised by (which we now prefer to call a blue apparition), which two different colouredbands. This type is shown by occurs for some stery~holesterogens,is visible to the (—)-2-methylbutyl 4 ‘-(4 “-nitro- and methoxy-benzyeye over the same temperature range as the platelet lideneamino)cinnamates and di-(—)-2-methylbutyl texture which can be seen microscopically in reflected terephthalylidene-bis-4 ‘-amino-benzoate and -cinnamate. light. We have now studied the Ch-I transitions of a Platelet texture. In all cases, the platelet texture lasts range of steryl and non-steryl cholesterogens, and in over about 0.5—1.0°C.When formed on cooling, the three optically different ways for non-steryl cholesteroplatelets are initially clear (usually violet and red), gens. The birefringent ‘focal-conic’ cholesteric texture, but, on further cooling, they become wrinkled and formed spontaneously on cooling the amorphous liquid, adopt a veiled appearance; the colours are then yellowprovided a convenient basis for comparing transition brown. The direction of the wrinkless is related to the temperatures on heating and cooling cycles, brightness of the platelets. Other features of the plateNon-Steryl Cholesterogens lets have already been reported [3, 4~. Type 1: On heating the ‘focal-conic’ texture, the Steryl Cholesterogens amorphous liquid forms at T°C;on cooling, the ‘focalThe striking feature of these Ch-I transitions (unlike conic’ texture reforms just below T°C.No unusual those for the non-steryl systems) is that the transitiotransitional features are seen either microscopically nal effects are difficult or impossible to see microscoor by the naked eye. This type is shown by the di-4pically in transmitted light, and reflected light must ((—)-2-methylbutyloxy)benzoate ester of 1 ,4-dthydroxy- be used. benzene. Type 1: No unusual transitional effects are seen Type 2: On heating the ‘focal-conic’ texture, a plateeither on heating or cooling; the ‘focal-conic’ texture let texture appears at T °Cand then this forms the reappears with only a little supercooling. No blue apamorphous liquid at T 1°C.On cooling, the platelet parition is visible to the naked eye. Examples from texture reappears at T1 °Cand the ‘focal-conic’ texture seven compounds which showed this behaviour are at just below T°C.By eye, this Ch-I transition is seen cholesteryl acetate and cholestanyl nonanoate. as a single coloured band both on heating and cooling. Type 2(a): On heating the ‘focal-conic’ texture, This type is shown by (—)-2-methylbutyl 4’-(4”-cyanoa blue apparition (by eye) and a platelet texture and -chloro-benzylideneamino)cinnamates. (visible microscopically in reflected light) are seen Type 3: On heating the ‘focal-conic’ texture, a at T°C,before the change to the amorphous liquid 335
Volume 51A, number 6
PHYSICS LETTERS
7 April 1975
at
T1 °C.On cooling, the platelet texture (and blue 0C, and the ‘coal-conic’ apparition) reappear at T1T C. Examples from eight texture forms Just below
Similar observations of narrow line p.m.r. signals at compositions intermediate between two amphiphilic mesophases which each give broad line signals have
compounds which showed this behaviour are cholesteryl propionate and cholesteryl butyl carbonate. Type 2(b): On heating, a direct ‘focal-conic’ texture
been interpreted in terms of intermediate cubic mesophases [7]. These wider investigations of Ch-l transitions con-
liquid transition occurs at T°C,but on cooling, a platelet texture (a blue apparition to the naked eye) occurs at T°C.The ‘focal-conic’ texture reappears at a slightly lower temperature (T 1 °C).When the platelet texture is heated it disappears at T°C. Examples from five compounds which showed this behaviour are cholestanyl heptyl carbonate and cho-
firm our earlier report [4j and clarify the situation; photomicrographs and a fuller documentation of the observations are presented in ref. [8]. We hope that the
lesteryl myristate.
will result.
to amorphous
Type 2(c): The behaviour is similar to type 2(b), but although a blue apparition is seen by eye, a platelet texture is not seen microscopically. Two compounds showed this behaviour: cholestanyl pentyl carbonate and S.cholesteryl heptyl thioate. Platelet texture. On cooling, the platelets are intitially violet and clear; they quickly (~0.1°C)become green and wrinkled, and more of them are then visible. They become red or brown, before forming the ‘focalconic’ texture. The Ch-I transitions for non-steryl and steryl cholesterogens therefore show many pretransitional similarities and the question arises whether surface forces operating in thin films are entirely responsible for the
observed effects. The optical observation by Saupe [6] that the cholesteric phase of cholesteryl p-nonylphenyl carbonate forms from the amorphous liquid via an isotropic liquid crystal phase in bulk samples, and our own broad line n.m.r. studies of cholesteryl 2-fluoroethyl carbonate make this seem unlikely. We find a sudden narrowing of the fluorine n.m.r. signal near to the I-Ch transition, indicating a sudden dramatic change in molecular orientation prior to the formation of t.he cholesteric phase proper (‘focal-conic’).
336
results stimulate further investigations of Ch-l transitions, particularly when stable cholesterogens [9] with lower transition temperatures become widely available, and that a fuller understanding of the cholesteric phase
The authors wish to thank Dr. W. Elser of the Night Vision Laboratory, Fort Belvoir, Virginia, 22060, U.S.A. for the gift of many steryl cholesterogens used in this work and also the Science Research Council, London for a maintenance grant (to D.C.).
References [11 W.
Elser, J.L.W. Pohlmann and P.F. Boyd, Mol. Cryst. and Liquid Cryst. 20 (1973) 77. [2] 0. Lehrnann, Z. Phys. Chem. 56 (1906) 750. [3] I.G.Chistyakov and L.A. Gusakova, Kirstallografiya 14 141 D. Coates and G.W. Gray, Phys. Lett. 45A, No. 2 (1973) 115. [5] D. Coates, G.W. Gray and K.J. Harrison, Mo!. Cryst. and Liquid Cryst. 22 (1973) 99. [6] Saupe, and Liquid Cryst. 7and (1969)59. [71 A. G.W. Gray Mol. and Cryst. P.A. Winsor, Mol. Cryst. Liquid Cryst.
26 (1974) 305.
[8] D. Coates, Ph.D. Thesis, Hull University, 1974. [9] D.G. McDonnell and G.W. Gray, to be published.