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Induction of optical activity in a nematic mesophase by l-menthol. Optical properties of liquid crystal mixtures
Volume 16, number 1
INDUCTION
CHEVICXL
15 September 1972
PHYSICS UTTERS
OF OPTICAL ACTIVITY IN A NEMATIC MESOPHASE BY I-MENTHOL. OPTiCAL PROPERTIES OF LIQUID CRYSTAL MIXTURES”’
Receiwd
22
June 1972
The heiical twistin:: powe: of I-menthol in n ncmntic solvent (RIBBA) has been measured bg mcnns of kduced optica! activity and circular dichroism. The optical rotatory dispersion curve is estremcly broadened corresponding to a widened region of total reflection. The induced t\vist sharply increases with concentration resultin_r in a maximum
vniuc prior to the breakdown of liquid crystal suucturc. of the molecular theory of Goossens.
Recently,
we reported
the induction
of optical
ac-
crystals by adding small amounts ofchiralic compounds which do not form a mesophase by themselves [l-3]. In the case of strong interaction between the chiralic solute and the nematic molecules a strong optical rotatory power has been observed as well as t,:mperature dependent circular dichroitic (CD) bands of small spectral width which give rise to brilliam reflection &ours [2, 31. These facts led to the conclusion that a helix structure which is characteristic of a cholestcric mesophase has been induced. Solving l-menthol in p-methoxybenzylidelle-p’-,!-butyl~iline (MBBA) no reflection col. ours could be observed though the strong optical rotatory power measured indicates the induction of a cholesteric structure [I] . To explain this we suggest that due to a weak interaction between the solute and the solvent the totally reflecting wavelength range is so broad that no colours could be seen by eye. In this paper we give some results which support this propoin nematic
liquid
SA.
* Parts I-iii:
38
agrcemcnt
with the predictions
?. Experimental
1. Introduction
tivity
These results xc in escellent
see refs. [l-3].
Optical rotatory power has been measured using a Zeiss Kreispoinrimeter 0.01” with a special thermostated cell which avoids twisting of the sample [4], optical pathlength 8 to 30 .um (hlylar spacers). Replacing the original light sources by a Jarrell-Ash 0.25 m
Ebert monochrcmator ivith a tungsten lamp optical rotatory dispersion (ORD) could be measured. Using a similar optical cell the CD curves had been recorded in a Cary 14 spectrophotometer inserting circular polarizers (Polaroid HNCP 37) into the beams. In all experiments the temperature was held constant within a range of 0.1” by a Haake Ultrathermostat FT.
3. Results
and discussion
3. I. Optical rotatory dispenion The ORD curve Q(X) of I-menthol in MBBA (mole fraction X= 0.018) at 25’C is given in fig. 1. The shape is analogous to that of a positive Cotton effect but the large broadening is obvious: the wavelength difference between peak and minimum amounts to Ah = 150 nm. Because of the well icnown relation between ORD curves and CD bands of cholesteric meso-
phases [5] which results from the de Vries theory [6]
3.2 +lOO +50 t
Qlmm
-100
only slightly wavelength dependent and no maximum
kinml
Fig. 1. Optiul
Circular dichroisnz
The question arose whether solutions of Z-menthol in MBBA would be circular dlchroitic at all. Within the wavelength region of the circular polarizers used (420-720 nm) a small but obvious difference of the absorptivities of left and right circularly polarized light has been found (AE = EI - E, > 0) but I.%!?is
I
500
15 September 1972
CHEMICAL PHYSICS LETTERS
Volume 16, number 1
550
600
650
700
rotatory dispersion Q,(h) of I-menthol x= 0.018, 35.1o”c.
in hIBBA,
and is similar to the Kronig-Kramers transforms in the case of chimlic chromophores in random distribution a broad (narrow) ORD curve causes a large (small) CD bandwidth and vice versa. This has been confirmed in soiutions of a strongly interacting chiralic diazacyclooctatetraene derivative in MBBA [2,3] which shows very narrow ORD curves with .U = 30 nm whereas the spectral width of the corresponding CD band is AA,,, = 25 nm [3]. Thus, from the broad ORD cuwe of the weakly interacting f-menthol a spectral width of the CD band of about AX1i2 = 100 nm may be estimated, large enough to prevent the impression of reflection colours by eye. The inversion wavelength X0 of the ORD curve at which Cpchanges sign is temperature dependent. Fig. 2 shows that A0 is red-shifted on decreasing the temperature. That is what one may expect because h, equals the maximum h,, of the CD band which is always red-shifted upon cooling in a single-component cholesteric phase [7] .
occurs. The values of AE depend on pathlength and concentration of I-menthol as well as on temperature. In fig. 3 the temperature dependence of AE at constant wavelength X, = 546 nm is given. For comparison the temperature. dependence of the rotatory power @(;r3 at A, = 546 nm is shown in fig. 3. The shape of the curve 9(7’) has been explained in previous papers [l-3] : as X, dpends on temperature (see above) the rotatory power Ial = 0 at that temperature To where X0 = X,. Because of the coincidence of X0 may be given for CD. and X,,, a similar consideration As the waveiengt!l maximum X,,, of the (largely broadened) CD band depends on temperature in the same way as X, the CD will attain a maximum at that temperature where h,, = X,. Fig. 3 shows that AE has a maximum at exactly the same temperature To where /@I = 0. In principle, a small value of dE in the visible may be explained by a CD band situated in the 3 A,). But this assumption is not in infrared (h,,,
*loo,
I t
I
I
I
_il , ITo‘;FzAfrA 3 5.0
::;,
)--+ 30
Fig. 2. Inversion
35 wavelength ho versus temperatu= thol in MBBA, X = 0.018.
35,5
36.0
36.5
37,0
0.2
-7,
‘;
37.5,
j
38.0
, LO of I-men-
Fig. 3. Circular dichroism of I-menthol in hlBBA, X = O.f_iI versus temperature. in& = El - E,: difference in absorptivitics of Ieft and right circularly polarized light, respectively. X, = 546 nm. a: optical rotatory power versus temperature at the same wavelength. 39
Volume 16. number 1
agreement with the maximum value of AE at T,. Thus, our result indicates that there must be a Gleetive reflection of circulxly polarized light though a CD band could not be measured because of its enlarged spectral width.
r
I t
600 I-menthol/MB64
3.3. Concentratio~t dependence of helical twisting po rver From the mclecular field theory of cholesterics proposed by Goossens [S] it follows that the energy of the internal field Vi of a molecule i within a cholcsteric phase splits into a “symmetric” and an “asymmetric” part the first of‘ which is determined only by electric dipole-dipole clispersion interaction (pp) whereas the second one is due to a combination of dipole-dipole and dipole-quadrupole interaction (pq or qp, respectively): -
Ifi=
cvy+ i
C(v;q+v;p). i
Inducing a helical structure in a nematic mesophase by adding optically active molecules there are two different possibilities. (I) If the solute forms a mesophase by itself (e.g., esters of cholesterol) the molecular polarizability is considerably anisotropic. Therefore, it contributes to the dipole-dipole term v$P as well as the nematic solvent and favours the alignment of molecular axes. The “asymmetric” dipole-quadrupole term VI?, however, is only determined by the small number of optically active molecules. Thus, the induced tivist only slightly increases with concentration. (2) If optically active non-mesomorphic molecules are added because of the absence of high_sisotropy they do not contribute to the term P’y but rather disturb the nematic order of the mesophase. By this, the “asymmetric” term T will become much more effective compared with V{p, Conclusively, the induced twist should increase more rapidly on increasing colt_centration than in case (1). On the other hand, as $I: solute disturbs the alignment of the molecular axes a m;tuimum twist will be expected prior to the breakdown of the liquid crystal structure. These predictions havl: been confirmed completely by the present work The helical twisting power can be given in terms of the agle 8 between the mean di40 -. :
IS September 1972
CHEMICAL PHYSICS LETTERS
_
700
Fig. 4. Helical twisting power versus concentration of mixtures ofbmenthol in hIBBA, 35.1O”C. ---Twist@ power of cholcstcryl propionate (CP) in a nemstic pliase Krsus con-
centration, taken from [9].
rections cf the long ases within two successive planes of the induced helix structure. 6’ is inversely proportional to the pitchp. As X, = lip [6] where Z is the mean refractive index the relation 8 a ho1 holds. In Eg. 4 the helical twisting power in terms of G1 versus mole fraction X of l-menthol in MBBA is given. Obviously, the helical twisting power of I-menthol is so large that small amounts of it produce a helix structure of so small a pitch that ho is shifted into the visible. At X= 0.02 the twist mavimizes as expected from the theory. In previous papers [ 1,3] we have shown that also the rotatory power versus concentration ma,,imizes at about the same mole fraction. For comparison the curve h,-‘(X) of a nematic-cholesteric mixture [9] given in fig. 4 shows the much lower twisting power efficiency of an anisotropic opticaliy active solute in good agreement with the theoretical considerations.
Acknowledgement The authors gratefully acknowledge this work by the VDEh and the Fonds Industrie.
the support of der Chemischen
Volume
16. number
1
CHEMICAL PHYSICS LETTERS
References [: ] H. Stegemeyer and K.-J. Mninusch, Chem. Phys. Letters 6 (1970) 5. [2] H. Stegemeyer and K.-J. blainusch, Chem. Phys. Letters 8 (197 1) 425. [ 31 H. Stegemeyer and K.-J. Blainusch, Naturwisscnschoften 58 (1971) 599.
IS September
1972
[4] [S] [6] [7J [SI
K.-J. hfainusch. Diplomarbeit. Berlin (1970). I.C. Chistyakov. Soviet Phys. Uspekhi 9 (1967) 551. H. de Vries, Act3 Cryst. 4 (1951) 219. H. Baesslcr, Fcstktirperproblcme 11 (1971) 99. W.J.A. Goossens, Mol. Cryst. Liquid Cryst. 12 (1971) 237. f9] T. Nakgiri, H. Kodama and K.K. Kobayashi, Phys. Rev. titters 27 (1971) 564.
41
INDUCTION
CHEVICXL
15 September 1972
PHYSICS UTTERS
OF OPTICAL ACTIVITY IN A NEMATIC MESOPHASE BY I-MENTHOL. OPTiCAL PROPERTIES OF LIQUID CRYSTAL MIXTURES”’
Receiwd
22
June 1972
The heiical twistin:: powe: of I-menthol in n ncmntic solvent (RIBBA) has been measured bg mcnns of kduced optica! activity and circular dichroism. The optical rotatory dispersion curve is estremcly broadened corresponding to a widened region of total reflection. The induced t\vist sharply increases with concentration resultin_r in a maximum
vniuc prior to the breakdown of liquid crystal suucturc. of the molecular theory of Goossens.
Recently,
we reported
the induction
of optical
ac-
crystals by adding small amounts ofchiralic compounds which do not form a mesophase by themselves [l-3]. In the case of strong interaction between the chiralic solute and the nematic molecules a strong optical rotatory power has been observed as well as t,:mperature dependent circular dichroitic (CD) bands of small spectral width which give rise to brilliam reflection &ours [2, 31. These facts led to the conclusion that a helix structure which is characteristic of a cholestcric mesophase has been induced. Solving l-menthol in p-methoxybenzylidelle-p’-,!-butyl~iline (MBBA) no reflection col. ours could be observed though the strong optical rotatory power measured indicates the induction of a cholesteric structure [I] . To explain this we suggest that due to a weak interaction between the solute and the solvent the totally reflecting wavelength range is so broad that no colours could be seen by eye. In this paper we give some results which support this propoin nematic
liquid
SA.
* Parts I-iii:
38
agrcemcnt
with the predictions
?. Experimental
1. Introduction
tivity
These results xc in escellent
see refs. [l-3].
Optical rotatory power has been measured using a Zeiss Kreispoinrimeter 0.01” with a special thermostated cell which avoids twisting of the sample [4], optical pathlength 8 to 30 .um (hlylar spacers). Replacing the original light sources by a Jarrell-Ash 0.25 m
Ebert monochrcmator ivith a tungsten lamp optical rotatory dispersion (ORD) could be measured. Using a similar optical cell the CD curves had been recorded in a Cary 14 spectrophotometer inserting circular polarizers (Polaroid HNCP 37) into the beams. In all experiments the temperature was held constant within a range of 0.1” by a Haake Ultrathermostat FT.
3. Results
and discussion
3. I. Optical rotatory dispenion The ORD curve Q(X) of I-menthol in MBBA (mole fraction X= 0.018) at 25’C is given in fig. 1. The shape is analogous to that of a positive Cotton effect but the large broadening is obvious: the wavelength difference between peak and minimum amounts to Ah = 150 nm. Because of the well icnown relation between ORD curves and CD bands of cholesteric meso-
phases [5] which results from the de Vries theory [6]
3.2 +lOO +50 t
Qlmm
-100
only slightly wavelength dependent and no maximum
kinml
Fig. 1. Optiul
Circular dichroisnz
The question arose whether solutions of Z-menthol in MBBA would be circular dlchroitic at all. Within the wavelength region of the circular polarizers used (420-720 nm) a small but obvious difference of the absorptivities of left and right circularly polarized light has been found (AE = EI - E, > 0) but I.%!?is
I
500
15 September 1972
CHEMICAL PHYSICS LETTERS
Volume 16, number 1
550
600
650
700
rotatory dispersion Q,(h) of I-menthol x= 0.018, 35.1o”c.
in hIBBA,
and is similar to the Kronig-Kramers transforms in the case of chimlic chromophores in random distribution a broad (narrow) ORD curve causes a large (small) CD bandwidth and vice versa. This has been confirmed in soiutions of a strongly interacting chiralic diazacyclooctatetraene derivative in MBBA [2,3] which shows very narrow ORD curves with .U = 30 nm whereas the spectral width of the corresponding CD band is AA,,, = 25 nm [3]. Thus, from the broad ORD cuwe of the weakly interacting f-menthol a spectral width of the CD band of about AX1i2 = 100 nm may be estimated, large enough to prevent the impression of reflection colours by eye. The inversion wavelength X0 of the ORD curve at which Cpchanges sign is temperature dependent. Fig. 2 shows that A0 is red-shifted on decreasing the temperature. That is what one may expect because h, equals the maximum h,, of the CD band which is always red-shifted upon cooling in a single-component cholesteric phase [7] .
occurs. The values of AE depend on pathlength and concentration of I-menthol as well as on temperature. In fig. 3 the temperature dependence of AE at constant wavelength X, = 546 nm is given. For comparison the temperature. dependence of the rotatory power @(;r3 at A, = 546 nm is shown in fig. 3. The shape of the curve 9(7’) has been explained in previous papers [l-3] : as X, dpends on temperature (see above) the rotatory power Ial = 0 at that temperature To where X0 = X,. Because of the coincidence of X0 may be given for CD. and X,,, a similar consideration As the waveiengt!l maximum X,,, of the (largely broadened) CD band depends on temperature in the same way as X, the CD will attain a maximum at that temperature where h,, = X,. Fig. 3 shows that AE has a maximum at exactly the same temperature To where /@I = 0. In principle, a small value of dE in the visible may be explained by a CD band situated in the 3 A,). But this assumption is not in infrared (h,,,
*loo,
I t
I
I
I
_il , ITo‘;FzAfrA 3 5.0
::;,
)--+ 30
Fig. 2. Inversion
35 wavelength ho versus temperatu= thol in MBBA, X = 0.018.
35,5
36.0
36.5
37,0
0.2
-7,
‘;
37.5,
j
38.0
, LO of I-men-
Fig. 3. Circular dichroism of I-menthol in hlBBA, X = O.f_iI versus temperature. in& = El - E,: difference in absorptivitics of Ieft and right circularly polarized light, respectively. X, = 546 nm. a: optical rotatory power versus temperature at the same wavelength. 39
Volume 16. number 1
agreement with the maximum value of AE at T,. Thus, our result indicates that there must be a Gleetive reflection of circulxly polarized light though a CD band could not be measured because of its enlarged spectral width.
r
I t
600 I-menthol/MB64
3.3. Concentratio~t dependence of helical twisting po rver From the mclecular field theory of cholesterics proposed by Goossens [S] it follows that the energy of the internal field Vi of a molecule i within a cholcsteric phase splits into a “symmetric” and an “asymmetric” part the first of‘ which is determined only by electric dipole-dipole clispersion interaction (pp) whereas the second one is due to a combination of dipole-dipole and dipole-quadrupole interaction (pq or qp, respectively): -
Ifi=
cvy+ i
C(v;q+v;p). i
Inducing a helical structure in a nematic mesophase by adding optically active molecules there are two different possibilities. (I) If the solute forms a mesophase by itself (e.g., esters of cholesterol) the molecular polarizability is considerably anisotropic. Therefore, it contributes to the dipole-dipole term v$P as well as the nematic solvent and favours the alignment of molecular axes. The “asymmetric” dipole-quadrupole term VI?, however, is only determined by the small number of optically active molecules. Thus, the induced tivist only slightly increases with concentration. (2) If optically active non-mesomorphic molecules are added because of the absence of high_sisotropy they do not contribute to the term P’y but rather disturb the nematic order of the mesophase. By this, the “asymmetric” term T will become much more effective compared with V{p, Conclusively, the induced twist should increase more rapidly on increasing colt_centration than in case (1). On the other hand, as $I: solute disturbs the alignment of the molecular axes a m;tuimum twist will be expected prior to the breakdown of the liquid crystal structure. These predictions havl: been confirmed completely by the present work The helical twisting power can be given in terms of the agle 8 between the mean di40 -. :
IS September 1972
CHEMICAL PHYSICS LETTERS
_
700
Fig. 4. Helical twisting power versus concentration of mixtures ofbmenthol in hIBBA, 35.1O”C. ---Twist@ power of cholcstcryl propionate (CP) in a nemstic pliase Krsus con-
centration, taken from [9].
rections cf the long ases within two successive planes of the induced helix structure. 6’ is inversely proportional to the pitchp. As X, = lip [6] where Z is the mean refractive index the relation 8 a ho1 holds. In Eg. 4 the helical twisting power in terms of G1 versus mole fraction X of l-menthol in MBBA is given. Obviously, the helical twisting power of I-menthol is so large that small amounts of it produce a helix structure of so small a pitch that ho is shifted into the visible. At X= 0.02 the twist mavimizes as expected from the theory. In previous papers [ 1,3] we have shown that also the rotatory power versus concentration ma,,imizes at about the same mole fraction. For comparison the curve h,-‘(X) of a nematic-cholesteric mixture [9] given in fig. 4 shows the much lower twisting power efficiency of an anisotropic opticaliy active solute in good agreement with the theoretical considerations.
Acknowledgement The authors gratefully acknowledge this work by the VDEh and the Fonds Industrie.
the support of der Chemischen
Volume
16. number
1
CHEMICAL PHYSICS LETTERS
References [: ] H. Stegemeyer and K.-J. Mninusch, Chem. Phys. Letters 6 (1970) 5. [2] H. Stegemeyer and K.-J. blainusch, Chem. Phys. Letters 8 (197 1) 425. [ 31 H. Stegemeyer and K.-J. Blainusch, Naturwisscnschoften 58 (1971) 599.
IS September
1972
[4] [S] [6] [7J [SI
K.-J. hfainusch. Diplomarbeit. Berlin (1970). I.C. Chistyakov. Soviet Phys. Uspekhi 9 (1967) 551. H. de Vries, Act3 Cryst. 4 (1951) 219. H. Baesslcr, Fcstktirperproblcme 11 (1971) 99. W.J.A. Goossens, Mol. Cryst. Liquid Cryst. 12 (1971) 237. f9] T. Nakgiri, H. Kodama and K.K. Kobayashi, Phys. Rev. titters 27 (1971) 564.
41