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Photochemically induced cholesteric-nematic transition in liquid crystals
.' ~I~OTOC~R~~~ALLY &TJCED CHOLES~ERIC-NEIMATICTRANSITION '_ INLIQU@CRYSTALS" I1_ C.&IOSKOWSKI ** , J. BOURGUIGNON, S. CANDAU*** Lnbarato& ci’Acystique
Icioi&ulaire. Universitd Louis Pasteur, 67070 Strusbourg Ctxiex, fiance
G. SOLLAIXE ***
Received 3 Fovember 1975 By doping 3 nematic liquid c;rysti with &symmetric choral compounds it has been shown that t&e twist of the resulting cholesteric structure is proportional to the solute concentration. Using as doping agent a photolabiie ctial compound a cholcsteric-nematic transition has been obtained under UV irradiation.
The pitch of cholesteric liquid $iystaIs may be changed by weak external physical perturbations [l] (temperature, pressure, electrical and magnetic fields) as well as by dissolved molecules: Fergason [2] reported color changes of chofesteric phases by traces of dissolved gases; Waas [3] observed pitch changes by photochemical decomposition of cholesterol derivatives; Sackmann 141 reported reversible color changes induced by phatochemicai cis-trans isomerisation of dissolved molecules. Cholestcric-nematic transitions induced by mixiig two components of opposite twisting power have been aiso reported [S] _ The work described in this communication reports a nematic-cholesteric transition induced by doping nematic liquid crystals with photolabile dissytimetric chiral compounds and the reverse process photochemically induced. Et is well known that on dissolving a small amount of optically active material in a nematic phase, one gets a choiesteric structtire:The main results in this field f Work suppo’rt~d by I).R.M.E. Contract no. 74/436. ** Peqanetit address: Laboratoire de Chimie Organique de l’E&le Nationale~Sup&eure disChide de Strasbourg. *** To whom corresponden& shouig be addressed. -
.
are related to chiral compounds having asymmetric carbon atoms and it was shown that the heiix pitch of the resulting cholesteric mesophase was inversely proportional to the solute concentration [6--81: p = l/2@, p being the helix pitch, c the solute concentration and 0 the microccopic twisting power, a constant dependent on both sotvent and solute. Dyssymmetric chiral compounds where the chiratity origin is confo~ational (restricted rotation about single bonds) are also able to induce a cholesteric structure f9,10]. However, the concentration dependence of the pitch has not yet been investigated for such compounds. In the frost part of this paper, we report the twisting power of a series of dissymmetric chiral compounds having a biphenyl structure (table 1) in four nematic solvents: - ~-me~hoxybenzy~dene ~~?-buty~n~ine (MBBA), - p-methoxy ~-~~-butyl~oxybe~ene (MBAB), - p-n-hexyl.p-cyanobiphenyl (rC 15), - a mixture of two Manes: 52.5% ofp-n pentylp’methoxy;biphenyhtietylene and 47.5% of p-n heptyl p*Lproproxy-biphenylacetylene. Pitches b were deter&Led
from the optical pattern
Volume
38;number
CHEMICAL PHYSICS LETTERS
3
Table-3 -Twisting pomcr of compound
6 in different
ncmatic solvents
Nematic solvent
ii31
MBAB
MBBA
Tolane
K 1s
9.3
10.2
10.4
15.2
mization barrier [ i7J (23 kcailmoie) and can be racemized either by heating in the dark (half-life of 19 minutes at 42”) or by W irradiation. Hence UV irradiation of a cholesteric structure, obtained by dissolving 1% by weight of optically active ketone 6 in nematic R 1.5, must produce a drastic change of the he1i.x pitch, the product 7 being at feast partially racemized. Actually we got a nematic mesophase as shown by the following experiments (fig. 2). Thin transparent films of liquid crystals were disposed between two quartz plates. In order to achieve a planar configuration (helical axis perpendicular to the plates and parallel to the direction of the UV beam), the cover slide was slightly dragged over the surface of the liquid crystal gl8] ; the uniformity of the orientation was checked with a polarizjng microscope. The cell, thermostated at 20”, was then exposed to
15 March 1976
UV irradiation (Philips HPW 125lamp). The quite small power density delivered by the lamp (= 1.5 W/cm’) allowed us to follow the pitch variation in a reasonable time scale. Fig. 2 shows the evolution of the optical pattern under UV exposure. Fig. 3 shows an helical axis orientation effect on the cholesteric-nematic transition rate. The s&called perpendicular orientation, with the helical axis perpendicular to the light direction, was obtained by coating with a detergent the plates of the cell. We can see that the half-transition time is about three times larger in perpendicuiar than in parallel orientation (fllZ If * 9h and $ I * 28h). This anisotropy in the half-t’ransition t&e should arise from a specific orientation of the molecule 6 with respect to the helicat axis. Depending upon whether the light ray runs parallel or perpendicular to the heIica1 axis, the chromophore will be arranged differently with respect to the light beam direction. Hence the Utr absorption will be different leading to an anisotropy of half-transition times. This orientation effect has been observed up to now only in the nematic phase 1191. A helicoidal arrangement 01 solute molecules has already been invoked to explain cholesteric induced circular dichroism /20]. The third curve in fig. 3 reports the transition kine-
Fig. 2. DropIets of a solution of compund 6 in K 15 nematic liquid crystal, between crossed polarizers. Rjngs observed Fn photographs (aI and (b) represent the arrangements of the cholestcric planes in a thin slab of the droplet. The distance between consecutive rings is equal to the helical pitch. Photograph (a) represents droplets before UV irradiation, photograph (b) after 3 hours exposure. Photograph 09 shows the droplets when irradiated for 3 dyas. The twisted arm cross is characteristic of the nematic structure (Ill. Concentric rings result from a Phase differencc between ordinary and extraordinary waves.
458
Volume 38, number
3
CHEhfICAL
PHYSICS LETTERS
15 March 1976
need by themselves a much larger amount of product. Such an application of liquid crystals is in progress. We must add that a microdiagnostic me*Jlod of optical activity using liquid crystals has been reported
[21] .
References
t11 J.L. Fergason, Appl. Opt. 7 (1968) 1729;
I 30
L
I *o
I IO
qnow.
Fig. 3. Variation of pa/p @o initial pitch) versus esposure time for a solution of compound 6 in K 15 nematic liquid crystal. Straight lines have been drawn only to visualize the orientation effect.
tic in the isotropic phase at slightly higher temperature (30”); the half-transition time (tl,, i = 50 minutes) is 10 times shorter than in parallel &ientation. This surprising result cannot be explained by a temperature effect, the half-transition times decreasing very slowly with temperature (table 4). Table 4 Temperature phase
dependence
of half-transition
times in isotropic
T(OC) r1/2
(min)
30
38.3
49
63
50
36
28
19
In conclusion, our results showed that by doping a nematic liquid crystaI with dissymmetric chiral compounds a twist proportional to the solute concentration induced. In addition an interesting apphcation of liquid crystals to organic chemistry can be proposed. we have seen that the helical pitch variation can be used to follow a photochemical reaction. This principle can be extended easily to the determination of thermal racemization barriers of chiral compounds. The main advantage of this method would be the extremely smzdl amount of optically active material required (< Smg), the kinetic parameters being determined by pitch measurements instead of polarimetric measurements which
W. Haas and J. Adams, Appl. Opt. 7 (1968) 1203; G.H. Heilmeier and J.E. Goldmacher. Appl. Phys. Letters 13 (1968) 132. I21 J.L. Fergsson, Sci. Am. 211 (1964) 76; Mol. Cryst. 1 (1966) 309. 131W. Haas, J. Adams and J. Wysocki, Mol. Cryst. Liquid Cryst. 7 (1969) 37 1. 141 E. Sackmann. J. Am. Chem. Sot. 93 (1971) 7088. ISI G. Friedel, Ann. Phys. (Paris) 18 (1922) 273; E. Sackmann, S. Mciboomand L.C. Snyder, J. Am. Chem. Sot. 89 (1967) 5981; H. Baessler and M.M. Labcs, J. Chem. Phys. 52 (1970) 631. [61 R. Cano and P. Chatelain, Compt. Rend. Acad. Sci. (Paris) 253 (1961) 1815; 259B (1964) 252; J. Adams, W. Haas and J. Wysocki, J. Liquid Crystals Ordered Fluids (1970) 463. [71 P.G. de Gennes. Physics of liquid crystals (Oxford Univ. Press, London, 1974). I81 T. Nakagiri, H. Kodama and K.K. Kobayashi, Phys. Rev. Letters 27 (197 1) 564. PI H. Stegemeyer and K.J. Mainusch, Chcm. Phys. Letters 6 (1970) 5. K.J. hlainusch and E. Steigner, Chcm. IlO1 H. Stegemcycr, Phys. Letters 8 (1971) 425. Il.11S. Can&au, P. le Roy and F. DeBeauvais, Mol. Cryst. Liquid Cryst. 23 (1973) 283. 1121 H.E. Zimmerman and D.S. Crunrine, J. Am. Chem. Sot. 94 (1972) 498. [I31 P- Newman, P. Rutkin and K. MisIow, I. Am. Chcm. Sot. 80 (1958) 465. 1141 H. Stegcmeyer and K.J. Mainusch, Chcm. Phys. Letters 16 (1972) 38. 1151 K. Mislow, M.A.W. Glass, R.E. O’Brien, P. Rutkin, D.N. Steinberg, J. Weiss and C.Djerassi, J. Am. Chem. Sot. 84 (1962) 1455. 1161 K. hlislow and A.J. Gordon, J. Am. Chem. Sot. 85 (1963) 3521. [ 171 K. Xlislow and H.B. ~opps, J. Am. Chem. SOC. 84 (1962) 3018. [ 18 1 J-E. Adams, W. Haas and J. Wysocki, J. Chem. Phys. 50 (19r;9) 2458. H-C. iuball and T. Karstens, Angew.Chem. Intern. Ed. 14 11975) 176. F.D. Saeva, P.E. Sharpe and G.R. Olin, J. Am. Chem. Sot. 95 (1973) 7656,766O. J.P. Penot, J. Jacques and J. Billard, Tetrahedron Letters (1968) 4013. 459
Icioi&ulaire. Universitd Louis Pasteur, 67070 Strusbourg Ctxiex, fiance
G. SOLLAIXE ***
Received 3 Fovember 1975 By doping 3 nematic liquid c;rysti with &symmetric choral compounds it has been shown that t&e twist of the resulting cholesteric structure is proportional to the solute concentration. Using as doping agent a photolabiie ctial compound a cholcsteric-nematic transition has been obtained under UV irradiation.
The pitch of cholesteric liquid $iystaIs may be changed by weak external physical perturbations [l] (temperature, pressure, electrical and magnetic fields) as well as by dissolved molecules: Fergason [2] reported color changes of chofesteric phases by traces of dissolved gases; Waas [3] observed pitch changes by photochemical decomposition of cholesterol derivatives; Sackmann 141 reported reversible color changes induced by phatochemicai cis-trans isomerisation of dissolved molecules. Cholestcric-nematic transitions induced by mixiig two components of opposite twisting power have been aiso reported [S] _ The work described in this communication reports a nematic-cholesteric transition induced by doping nematic liquid crystals with photolabile dissytimetric chiral compounds and the reverse process photochemically induced. Et is well known that on dissolving a small amount of optically active material in a nematic phase, one gets a choiesteric structtire:The main results in this field f Work suppo’rt~d by I).R.M.E. Contract no. 74/436. ** Peqanetit address: Laboratoire de Chimie Organique de l’E&le Nationale~Sup&eure disChide de Strasbourg. *** To whom corresponden& shouig be addressed. -
.
are related to chiral compounds having asymmetric carbon atoms and it was shown that the heiix pitch of the resulting cholesteric mesophase was inversely proportional to the solute concentration [6--81: p = l/2@, p being the helix pitch, c the solute concentration and 0 the microccopic twisting power, a constant dependent on both sotvent and solute. Dyssymmetric chiral compounds where the chiratity origin is confo~ational (restricted rotation about single bonds) are also able to induce a cholesteric structure f9,10]. However, the concentration dependence of the pitch has not yet been investigated for such compounds. In the frost part of this paper, we report the twisting power of a series of dissymmetric chiral compounds having a biphenyl structure (table 1) in four nematic solvents: - ~-me~hoxybenzy~dene ~~?-buty~n~ine (MBBA), - p-methoxy ~-~~-butyl~oxybe~ene (MBAB), - p-n-hexyl.p-cyanobiphenyl (rC 15), - a mixture of two Manes: 52.5% ofp-n pentylp’methoxy;biphenyhtietylene and 47.5% of p-n heptyl p*Lproproxy-biphenylacetylene. Pitches b were deter&Led
from the optical pattern
Volume
38;number
CHEMICAL PHYSICS LETTERS
3
Table-3 -Twisting pomcr of compound
6 in different
ncmatic solvents
Nematic solvent
ii31
MBAB
MBBA
Tolane
K 1s
9.3
10.2
10.4
15.2
mization barrier [ i7J (23 kcailmoie) and can be racemized either by heating in the dark (half-life of 19 minutes at 42”) or by W irradiation. Hence UV irradiation of a cholesteric structure, obtained by dissolving 1% by weight of optically active ketone 6 in nematic R 1.5, must produce a drastic change of the he1i.x pitch, the product 7 being at feast partially racemized. Actually we got a nematic mesophase as shown by the following experiments (fig. 2). Thin transparent films of liquid crystals were disposed between two quartz plates. In order to achieve a planar configuration (helical axis perpendicular to the plates and parallel to the direction of the UV beam), the cover slide was slightly dragged over the surface of the liquid crystal gl8] ; the uniformity of the orientation was checked with a polarizjng microscope. The cell, thermostated at 20”, was then exposed to
15 March 1976
UV irradiation (Philips HPW 125lamp). The quite small power density delivered by the lamp (= 1.5 W/cm’) allowed us to follow the pitch variation in a reasonable time scale. Fig. 2 shows the evolution of the optical pattern under UV exposure. Fig. 3 shows an helical axis orientation effect on the cholesteric-nematic transition rate. The s&called perpendicular orientation, with the helical axis perpendicular to the light direction, was obtained by coating with a detergent the plates of the cell. We can see that the half-transition time is about three times larger in perpendicuiar than in parallel orientation (fllZ If * 9h and $ I * 28h). This anisotropy in the half-t’ransition t&e should arise from a specific orientation of the molecule 6 with respect to the helicat axis. Depending upon whether the light ray runs parallel or perpendicular to the heIica1 axis, the chromophore will be arranged differently with respect to the light beam direction. Hence the Utr absorption will be different leading to an anisotropy of half-transition times. This orientation effect has been observed up to now only in the nematic phase 1191. A helicoidal arrangement 01 solute molecules has already been invoked to explain cholesteric induced circular dichroism /20]. The third curve in fig. 3 reports the transition kine-
Fig. 2. DropIets of a solution of compund 6 in K 15 nematic liquid crystal, between crossed polarizers. Rjngs observed Fn photographs (aI and (b) represent the arrangements of the cholestcric planes in a thin slab of the droplet. The distance between consecutive rings is equal to the helical pitch. Photograph (a) represents droplets before UV irradiation, photograph (b) after 3 hours exposure. Photograph 09 shows the droplets when irradiated for 3 dyas. The twisted arm cross is characteristic of the nematic structure (Ill. Concentric rings result from a Phase differencc between ordinary and extraordinary waves.
458
Volume 38, number
3
CHEhfICAL
PHYSICS LETTERS
15 March 1976
need by themselves a much larger amount of product. Such an application of liquid crystals is in progress. We must add that a microdiagnostic me*Jlod of optical activity using liquid crystals has been reported
[21] .
References
t11 J.L. Fergason, Appl. Opt. 7 (1968) 1729;
I 30
L
I *o
I IO
qnow.
Fig. 3. Variation of pa/p @o initial pitch) versus esposure time for a solution of compound 6 in K 15 nematic liquid crystal. Straight lines have been drawn only to visualize the orientation effect.
tic in the isotropic phase at slightly higher temperature (30”); the half-transition time (tl,, i = 50 minutes) is 10 times shorter than in parallel &ientation. This surprising result cannot be explained by a temperature effect, the half-transition times decreasing very slowly with temperature (table 4). Table 4 Temperature phase
dependence
of half-transition
times in isotropic
T(OC) r1/2
(min)
30
38.3
49
63
50
36
28
19
In conclusion, our results showed that by doping a nematic liquid crystaI with dissymmetric chiral compounds a twist proportional to the solute concentration induced. In addition an interesting apphcation of liquid crystals to organic chemistry can be proposed. we have seen that the helical pitch variation can be used to follow a photochemical reaction. This principle can be extended easily to the determination of thermal racemization barriers of chiral compounds. The main advantage of this method would be the extremely smzdl amount of optically active material required (< Smg), the kinetic parameters being determined by pitch measurements instead of polarimetric measurements which
W. Haas and J. Adams, Appl. Opt. 7 (1968) 1203; G.H. Heilmeier and J.E. Goldmacher. Appl. Phys. Letters 13 (1968) 132. I21 J.L. Fergsson, Sci. Am. 211 (1964) 76; Mol. Cryst. 1 (1966) 309. 131W. Haas, J. Adams and J. Wysocki, Mol. Cryst. Liquid Cryst. 7 (1969) 37 1. 141 E. Sackmann. J. Am. Chem. Sot. 93 (1971) 7088. ISI G. Friedel, Ann. Phys. (Paris) 18 (1922) 273; E. Sackmann, S. Mciboomand L.C. Snyder, J. Am. Chem. Sot. 89 (1967) 5981; H. Baessler and M.M. Labcs, J. Chem. Phys. 52 (1970) 631. [61 R. Cano and P. Chatelain, Compt. Rend. Acad. Sci. (Paris) 253 (1961) 1815; 259B (1964) 252; J. Adams, W. Haas and J. Wysocki, J. Liquid Crystals Ordered Fluids (1970) 463. [71 P.G. de Gennes. Physics of liquid crystals (Oxford Univ. Press, London, 1974). I81 T. Nakagiri, H. Kodama and K.K. Kobayashi, Phys. Rev. Letters 27 (197 1) 564. PI H. Stegemeyer and K.J. Mainusch, Chcm. Phys. Letters 6 (1970) 5. K.J. hlainusch and E. Steigner, Chcm. IlO1 H. Stegemcycr, Phys. Letters 8 (1971) 425. Il.11S. Can&au, P. le Roy and F. DeBeauvais, Mol. Cryst. Liquid Cryst. 23 (1973) 283. 1121 H.E. Zimmerman and D.S. Crunrine, J. Am. Chem. Sot. 94 (1972) 498. [I31 P- Newman, P. Rutkin and K. MisIow, I. Am. Chcm. Sot. 80 (1958) 465. 1141 H. Stegcmeyer and K.J. Mainusch, Chcm. Phys. Letters 16 (1972) 38. 1151 K. Mislow, M.A.W. Glass, R.E. O’Brien, P. Rutkin, D.N. Steinberg, J. Weiss and C.Djerassi, J. Am. Chem. Sot. 84 (1962) 1455. 1161 K. hlislow and A.J. Gordon, J. Am. Chem. Sot. 85 (1963) 3521. [ 171 K. Xlislow and H.B. ~opps, J. Am. Chem. SOC. 84 (1962) 3018. [ 18 1 J-E. Adams, W. Haas and J. Wysocki, J. Chem. Phys. 50 (19r;9) 2458. H-C. iuball and T. Karstens, Angew.Chem. Intern. Ed. 14 11975) 176. F.D. Saeva, P.E. Sharpe and G.R. Olin, J. Am. Chem. Sot. 95 (1973) 7656,766O. J.P. Penot, J. Jacques and J. Billard, Tetrahedron Letters (1968) 4013. 459