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IR spectroscopic study of molecular aggregation in condensate films of polar liquid crystals
PII:SO968-5677(97)00032-l
SupmmolecularScience4 (1997)485489 0 1997BlsevierScienceLtd Printed in Great Britain. All rights reserved 0968-5677/97/$17.00
ELSEVIER
IR spectroscopic study of molecular aggregation in condensate films of polar liquid crystals T. I. Shabatina*, E. V. Vovk, T. V. Khasanova, G. N. Andrew and G. B. Sergeev Department of Chemistry, Moscow State University, 119899 Moscow, Russia (Received 6 September 1996; revised 23 December 1996)
Molecular association in polar liquid crystals influences almost all physical and chemical properties of these compounds and is of great importance for their use as materials for molecular electronics. There are only few reports available that present estimations of the degree of dimerization and enthalpy via dielectric measurements of bulk samples and of solutions in non-polar solvents. In the present work, the molecular aggregation of liquid-crystalline 4-pentyUcyanobipheny1 (XB), 4_pentyl_4’-cyanophenyl pyridine (Spy), 4-pentyl-4’cyanophenylcyclohexane (SCH), 4octyUcyanobiphenyl (8CB) and 4octyloxy_4’-cyanobiphenyl (80CB) in molecular condensate films and inert matrices has been studied by
low-temperature reflection and transmission vibrational spectroscopy in a wide temperature range from 80 to 330K. A red shift of the CN stretching vibration band was considered to be due to the formation of molecular aggregates. The factors affecting their thermodynamic Elsevier Science Ltd. All rights reserved.
stability are considered.
0 1997
(Keywordo:liquid rr);sta& amdemate Rims; molecafrr aggregatioa)
INTRODUCTION Polar liquid crystals such as alkyl/ alkoxycyanobiphenyls and their derivatives are widely used as materials for molecular electronics’. Molecular aggregation in these compounds has a considerable influence on almost all of their physical and chemical properties24. There are only few reports that have presented estimations of the degree of dimerization and enthalpy via dielectric measurements of bulk samples and of solutions in non-polar solventss7. At the same time, direct data on the formation of molecular aggregates during molecular condensation and its effect on the thermodynamic behaviour of selforganizing systems are of great interest for modem supramolecular chemistry’.‘. In the present work the molecular aggregation of the polar liquid crystals shown in Scheme I in molecular condensate films and inert matrices has been studied by means of low-temperature reflection and transmission vibrational spectroscopy in the wide temperature range from 80 to 330K.
EXPERIMENTAL The thermal behaviour of cyanobiphenyl molecular condensate films of 20-100 pm thickness was studied *To whom correspondence should be addressed
by low-temperature infra-red (IR) spectroscopy at 77-330 K in a special evacuated cryostat”, I’; a scheme of the experimental set-up is presented in Figure 1. The film samples were obtained by deposition of cyanobiphenyl molecular vapour on the polished surface of a copper cube that was cooled by liquid nitrogen (77 K) and then heated .to 300-350 K. The cyanobiphenyl deposition rate was lOI to lo’* molecules cm-’ s-‘. The residual gas pressure in the system did not exceed 5x10p4 torr at all temperatures up to 350K (we did not observe the desorption of cyanophenyl film under these experimental conditions). The IR spectra of the samples were recorded in reflection mode on a Specord IR-75 instrument for film condensate samples and in transmission mode on a Specord 80 instrument for bulk samples and solutions. Spectroscopic accuracy was up to 0.5cm-‘. The temperature of the samples was maintained within fl K at 77-270 K and within 4~0.5K at 290350 K. In some cases the molecular vapour of the cyanophenyl substance was co-deposited with a matrix of inert molecules: decane (at 80-100 K) and argon (at 5-1OK). The last experiments were made in co-operation with Professor G. Sheina and Dr A. Ivanov by using the helium refrigerating system at the Institute of Low Temperature Physics and Engineering at the Ukrainian Academy of Sciences’2.
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
485
Molecular
aggregation
of liquid crystals: T. I. Shabatina
4-pentyl-4’-cyanobiphenyl
(SCB)
4-pentyl-4’-cyanophenylpyridine
(5Py)
4-pentyl-4’-cyanophenylcyclohexane
cSh flcN 4-octyl-4’-cyanobiphenyl
et al.
(5CH)
TK_N=~O~K
TN_r=328 K
(8CB)
4-octyloxy-4’-cyanobiphenyl
(80CB)
Scheme 1
-6
Figure 1 The experimental set-up for IR study of liquid-crystalline film samples in reflection mode: I, inlets for reagent vapours; 2, polished copper cube, cooled with liquid nitrogen; 3, KBr window; 4, thermocouple; 5, feeders; 6, liquid nitrogen; 7, to vacuum pump
486
SUPRAMOLECULAR
RESULTS AND DISCUSSION Film samples of 5CB, 5Py, 5CH, 8CB and 80CB, of thickness 20-IOOpm, were obtained by molecular beam deposition from the vapour phase on a copper surface cooled by liquid nitrogen and then heated to 300-350 K. As examples, the IR spectra obtained for molecular condensate films of the liquid crystals 5CB and 8CB at 80K and at various temperatures after annealing the samples are presented in Figure 2a and Figure 26, respectively. The band corresponding to stretching vibrations of the CN groups was chosen for further study, as it is the most informative for these compounds according to reference 13. A red shift (to lower frequencies) of the CN band maxima at 2 16-270 K (Figures 2a and 2b, curves 2 and 3) was found. We consider that this transformation, corresponding to the formation of inner dimers (or higher molecular aggregates), took place in molecular condensate films of both liquid crystals. Further heating of the samples led to a reverse shift of the CN stretching
SCIENCE Volume 4 Numbers 3-4 1997
Molecular
7 ?
JV-
4 Y
f
+
WA
of liquid crystals: T. 1. Shabatina et al.
Similar results were obtained with a special technique for stabilization of monomer and dimer species in an inert hydrocarbon matrix at 80-130 K and an argon matrix at 5-10K’2. Figure 3 presents the data obtained for molecular species of 2-pcyanophenyl-5pentylpyridine (5Py), a heteroaromatic derivative of 5CB, stabilized in an argon matrix for different values of the cyanophenyl/argon ratio from l/ 1000 to l/10 and for a pure cyanophenyl film. It was shown that increasing both the cyanophenyl content of the system and the sample temperature led to a red shift of the CN band owing to the formation of inner dimers. The comprehensive study was conducted on a series of cyanophenyl solutions in non-polar hydrocarbon solvents (decane and pentane) in a wide concentration range from 0.01 to 1.5M. It was shown that in highly dilute solutions (less than 0.05 M) the maximum absorbance of the CN groups increases linearly with cyanobiphenyl concentration. For more concentrated solutions there was a negative deviation owing to the molecular association process. In this case the CN absorbance band was the sum of the absorbances of monomer and dimer (or higher aggregates). Computer treatment15, based on theoretical simulation of the experimental spectra obtained for a number of solutions of different cyanophenyl concentrations, allowed us to obtain line shapes for spectra of monomeric and associative SI ecies. In Figure 4 an example is presented of the superpositional experimental and computer
1
3
aggregation
2200
w (cm-‘)
(b)
I-
2250
2200 Y (cm-‘)
Figure 2 Thermal behaviour of IR spectra of 4-pe.ntyM’-cyanobiphenyl (5CB) (a) and 4-octyl-4’-cyanobiphenyl (8CB) (b) condensate films, 1% 50pm: 1, 80K, 2, 216K; 3,263K; 4, 322K
vibration band to higher frequencies, indicating partial dissociation of the cyanobiphenyl dimers and/or higher aggregates in the mesophase and isotropic phase (see Figures 2a and 2b, curve 4). The effect was confirmed by computer modelling of the harmonic frequencies of theoretical IR spectra for cyanobiphenyl monomeric and dimeric structures14. All calculations were carried out by using the GAMESS program suite.
Figure 3 CN stretching vibrations region of 2-p-cyanophenyl-Spentylpyridine (SpY) molecular species isolated in an inert argon matrix at SlOK for different values of cyanophenyl/argon ratio: (a) l/1000; (b) l/400; (c) l/50; (d) l/10; (e) pure film; I Z! 20pm
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
487
Molecular aggregation
l
of liquid crystals: T. 1. Shabatina et al.
1
‘E 0
600
a2
3E
500-
03
. .
2; ,sj 400 8 e 5: 4 J! 2
.
q n. 0 l
0 . 0
300-
.
.
d
200 loolY
4 I
,
2220
4 I
I
2225
I
2230
2235
I
2240
Wavenumber Figure 4 Experimental superpositional and computer modelling monomer and dimer IR spectra for 8CB solution in decane at 290 K, initial concentration of cyanobiphenyl, Cs = 0.5 M: 1, experimental spectrum; 2, computer modelling dimer spectrum; 3, computer modelling monomer spectrum
modelling spectra of monomer and molecular associative species for the case of 8CB in decane solution. The initial concentration of cyanobiphenyl is Co=05 M. It should be mentioned that the method used does not allow us to distinguish between the spectra of dimers and higher associates if their spectral characteristics do not differ significantly (lower than 0.5 cm-‘). The spectroscopic characteristics obtained for the monomers (v~, Ed) and molecular associates (vn, n) of the cyanophenyls under investigation are presented in Table 1. It is important that the spectroscopic data obtained for monomers in film samples are the same as the values obtained for highly diluted cyanophenyl solutions. The IR spectra of cyanophenyl bulk films were recorded at different temperatures within the mesophase interval. It was found that CN absorption band maximum and line shape depended on sample temperature. Varying the temperature changed the ratio of monomer to associative species. Computer modelling of the experimental spectra as the sum of two modes:
where aI and u2 are the relative amounts of monomeric and associative cyanophenyl species, calibrated by using M and &o values, allowed us to obtain u = u&al -t al)the association degree of cyanophenyl molecules at a given temperature. The association enthalpy (AH, kJmol_‘) of cyanophenyls in the mesophase was estimated from the temperature dependence of a values. The data obtained are presented in Table 1. Table 1
CONCLUSIONS In the present work we performed an IR spectroscopic study of the thermal behaviour of the mesogenic cyanobiphenyls 4-pentyl-4’-cyanobiphenyl (5CB), 4pentyl-4’-cyanophenylpyridine (5Py), 4-pentyl-4’cyanophenylcyclohexane (5CH) 4-octyl-4cyanobiphenyl (8CB) and 4-octyloxy-4-cyanobiphenyl (80CB) in molecular condensate films, the bulk phase and in inert matrices. It was found that the absorbance maxima and line shape of CN group stretching vibrations are dependent on the quantitative ratio of monomer to dimer (or higher associates), which was changed by varying the temperature of the liquid-crystalline sample. Thermodynamic data obtained for the molecular association process of the named polar liquid crystals revealed that the most stable associates are formed in the case of cyanobiphenyl molecules compared with their hydrogenated or heteroaromatic derivatives. Higher thermodynamic stability was shown for aggregates of alkoxycyanobiphenyls than for aggregates of alkylcyanobiphenyls. Increasing the length of the alkyl chain did not significantly influence the molecular association parameters.
Some spectroscopic and thermodynamic characteristics of cyanophenyls
Substance
VT (cm-‘)
vr
5CB SCH 5EY 8CB 80CB
2230 (f0.5) 2232 (f0.5) 2232 (f0.5) 2231 (f0.5) 2231 (f0.5)
2225 (f0.5) 2225 (f0.5) 2227 (f0.5) 2227.5 (f0.5) 2226.5 (ztzO.5)
488
Analysis of the data in Table 1 allowed us to compare the relative stability of the liquid-crystalline aggregates with respect to the molecular structure of the polar mesogen. The most stable molecular associates were formed in the case of cyanobiphenyls as opposed to their hydrogenated or heteroaromatic derivatives16. This is possibly due to strong parallel stacking of the two benzene rings and overlapping of the rc-aromatic systems of the two monomeric units. There is only one benzene unit in the SCH molecular structure, and this leads to a significant decrease of the association enthalpy. For 5Py molecules we obtained a AH value that is lower than for 5CH; this could be connected with the formation of a dimeric structure that is less planar. In the case of differently substituted cyanobiphenyls (Table 1), we can see that the most stable dimers were formed in the case of alkoxycyanobiphenyl, 80CB, in comparison with alkylcyanobiphenyls, 5CB and 8CB. This could be possibly due to increased conjugation with the oxygen atom. The increased length of the alkyl chain did not have a significant influence on the molecular association parameters.
SUPRAMOLECULAR
(cm-‘)
EF x JO@ (M-r cm-‘)
EmD(Ix x 1O-3 (M-’ cm-‘)
AH (kJ mol-‘)
0.23 (f0.15) 0.29 (f0.15) 0.18 (f0.13) 0.19 (f0.07) 0.21 (fO.08)
0.59 0.57 0.94 0.87 0.67
-19f5 -3fl -12f5 -18f5 -29f7
SCIENCE Volume 4 Numbers 3-4 1997
(f0.25) (f0.25) (f0.35) (f0.30) (f0.25)
Molecular
aggregation
2.
ACKNOWLEDGEMENTS
3.
We gratefully acknowledge Professor A. V. Nemukhin (Moscow State University) for help in performing the quantum chemical calculations and Professor G. G. Sheina and Dr A. I. Ivanov (Institute of Low Temperature Physics and Engineering at the Ukrainian Academy of Sciences) for their help with matrix isolation experiments. We sincerely appreciate the help of Professor M. F. Grebenkin, who stimulated this work, and thank Professor G. R. Luckhurst (Southampton University) and Professor V. A. Batyuk (Moscow Baumann State Technical University) for fruitful discussions. This work was partially supported by INTAS grant 94-4299, the OMMEL International Scientific Programme and the Federal Scientific Research Programme ‘Russian Universities’.
4. 5. 6. I. 8. 9.
IO. II.
13.
15.
I. Grebenkin. M. F. and Ivachenko, A. V., Liquid
Crystalline Materials. Khimia, Moscow, 1989, 288 pp. (in Russian).
Zeinalov, R. A.. Blinov, L. M., Grebenkin. M. F. and Ostrovskii, B. I.. Krisrallograjiya (Russ.), 1988. 33(l), 185. Toriyama, K. and Dunmur, D. A.. Mol. Crysr. Lig. Crysr.. 1986, 139, 139. Ferrarini, A., Luckhurst, Cl. R.. Nordio, P. L. and Roskilly, S. L., Liq. Crysr., 1996, 21(3), 373. Wilson, M. R. and Dunmur, D. A., Liq. Cryst.. 1989. 5, 987. Kedziora, P. and Jadzyn, J., Liq. Crysr.. 1990, 8,445. Lisetzkii, L. N. and Antonyan, T. P., J. Phys. Chem. (Russ.), 1985.59, 1813. Barbero, R. and Durand, G., Appl. Phys. Leaf., 1991, 58, 2907. Ali-Adib, Z., Hodge, P., Tredgold. R. H., Wooley, N. and Pidduck, A., Thin Solid Films. 1994, 242, 157. Sergeev, G. B., Smirnov. V. V. and Zagorskii, V. V., J. Organomet. Chem.. 1980, 201, 9. Shabatina, T. I., Khasanova. T. V. and Sergeev, G. B., Vestnik Mask. Univ.. Ser. 2, Khimia, 1995, 34(4), 369.
12.
14.
REFERENCES
of liquid crystals: T. I. Shabatina et al.
16.
SUPRAMOLECULAR
Khasanova, T. V., Ivanov, A. I., Shabatina, T. 1.. Vovk, E. V. and Sergeev, G. B., in Abstracts of 2nd International Conference on Low Temperature Chemistry, Kansas City. 4-9 August 1996, p. 191. Kirov, N. and Simova, P., Vibrational Spectroscopy of Liquid CrysfaLs. Bulgarian Academy of Sciences Sotia. 1984, p. 330. Khasanova, T. V., M.Sc.D. thesis, Lomonosov Moscow State University, 1994. Shabatina. T. I., Vovk, E. V., Andreev, G. N.. Bogomolov, A. Yu., Morosov, Yu. N. and Sergeev. G. B., Mol. Crysr. Liq. Crysf., in press. Shabatina, T. I., Khasanova, T. V., Vovk, E. V. and Sergeev, G. B., Thin Solid Films, 1996, 2l34-2g5, 573.
SCIENCE Volume 4 Numbers 3-4 1997
489
SupmmolecularScience4 (1997)485489 0 1997BlsevierScienceLtd Printed in Great Britain. All rights reserved 0968-5677/97/$17.00
ELSEVIER
IR spectroscopic study of molecular aggregation in condensate films of polar liquid crystals T. I. Shabatina*, E. V. Vovk, T. V. Khasanova, G. N. Andrew and G. B. Sergeev Department of Chemistry, Moscow State University, 119899 Moscow, Russia (Received 6 September 1996; revised 23 December 1996)
Molecular association in polar liquid crystals influences almost all physical and chemical properties of these compounds and is of great importance for their use as materials for molecular electronics. There are only few reports available that present estimations of the degree of dimerization and enthalpy via dielectric measurements of bulk samples and of solutions in non-polar solvents. In the present work, the molecular aggregation of liquid-crystalline 4-pentyUcyanobipheny1 (XB), 4_pentyl_4’-cyanophenyl pyridine (Spy), 4-pentyl-4’cyanophenylcyclohexane (SCH), 4octyUcyanobiphenyl (8CB) and 4octyloxy_4’-cyanobiphenyl (80CB) in molecular condensate films and inert matrices has been studied by
low-temperature reflection and transmission vibrational spectroscopy in a wide temperature range from 80 to 330K. A red shift of the CN stretching vibration band was considered to be due to the formation of molecular aggregates. The factors affecting their thermodynamic Elsevier Science Ltd. All rights reserved.
stability are considered.
0 1997
(Keywordo:liquid rr);sta& amdemate Rims; molecafrr aggregatioa)
INTRODUCTION Polar liquid crystals such as alkyl/ alkoxycyanobiphenyls and their derivatives are widely used as materials for molecular electronics’. Molecular aggregation in these compounds has a considerable influence on almost all of their physical and chemical properties24. There are only few reports that have presented estimations of the degree of dimerization and enthalpy via dielectric measurements of bulk samples and of solutions in non-polar solventss7. At the same time, direct data on the formation of molecular aggregates during molecular condensation and its effect on the thermodynamic behaviour of selforganizing systems are of great interest for modem supramolecular chemistry’.‘. In the present work the molecular aggregation of the polar liquid crystals shown in Scheme I in molecular condensate films and inert matrices has been studied by means of low-temperature reflection and transmission vibrational spectroscopy in the wide temperature range from 80 to 330K.
EXPERIMENTAL The thermal behaviour of cyanobiphenyl molecular condensate films of 20-100 pm thickness was studied *To whom correspondence should be addressed
by low-temperature infra-red (IR) spectroscopy at 77-330 K in a special evacuated cryostat”, I’; a scheme of the experimental set-up is presented in Figure 1. The film samples were obtained by deposition of cyanobiphenyl molecular vapour on the polished surface of a copper cube that was cooled by liquid nitrogen (77 K) and then heated .to 300-350 K. The cyanobiphenyl deposition rate was lOI to lo’* molecules cm-’ s-‘. The residual gas pressure in the system did not exceed 5x10p4 torr at all temperatures up to 350K (we did not observe the desorption of cyanophenyl film under these experimental conditions). The IR spectra of the samples were recorded in reflection mode on a Specord IR-75 instrument for film condensate samples and in transmission mode on a Specord 80 instrument for bulk samples and solutions. Spectroscopic accuracy was up to 0.5cm-‘. The temperature of the samples was maintained within fl K at 77-270 K and within 4~0.5K at 290350 K. In some cases the molecular vapour of the cyanophenyl substance was co-deposited with a matrix of inert molecules: decane (at 80-100 K) and argon (at 5-1OK). The last experiments were made in co-operation with Professor G. Sheina and Dr A. Ivanov by using the helium refrigerating system at the Institute of Low Temperature Physics and Engineering at the Ukrainian Academy of Sciences’2.
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
485
Molecular
aggregation
of liquid crystals: T. I. Shabatina
4-pentyl-4’-cyanobiphenyl
(SCB)
4-pentyl-4’-cyanophenylpyridine
(5Py)
4-pentyl-4’-cyanophenylcyclohexane
cSh flcN 4-octyl-4’-cyanobiphenyl
et al.
(5CH)
TK_N=~O~K
TN_r=328 K
(8CB)
4-octyloxy-4’-cyanobiphenyl
(80CB)
Scheme 1
-6
Figure 1 The experimental set-up for IR study of liquid-crystalline film samples in reflection mode: I, inlets for reagent vapours; 2, polished copper cube, cooled with liquid nitrogen; 3, KBr window; 4, thermocouple; 5, feeders; 6, liquid nitrogen; 7, to vacuum pump
486
SUPRAMOLECULAR
RESULTS AND DISCUSSION Film samples of 5CB, 5Py, 5CH, 8CB and 80CB, of thickness 20-IOOpm, were obtained by molecular beam deposition from the vapour phase on a copper surface cooled by liquid nitrogen and then heated to 300-350 K. As examples, the IR spectra obtained for molecular condensate films of the liquid crystals 5CB and 8CB at 80K and at various temperatures after annealing the samples are presented in Figure 2a and Figure 26, respectively. The band corresponding to stretching vibrations of the CN groups was chosen for further study, as it is the most informative for these compounds according to reference 13. A red shift (to lower frequencies) of the CN band maxima at 2 16-270 K (Figures 2a and 2b, curves 2 and 3) was found. We consider that this transformation, corresponding to the formation of inner dimers (or higher molecular aggregates), took place in molecular condensate films of both liquid crystals. Further heating of the samples led to a reverse shift of the CN stretching
SCIENCE Volume 4 Numbers 3-4 1997
Molecular
7 ?
JV-
4 Y
f
+
WA
of liquid crystals: T. 1. Shabatina et al.
Similar results were obtained with a special technique for stabilization of monomer and dimer species in an inert hydrocarbon matrix at 80-130 K and an argon matrix at 5-10K’2. Figure 3 presents the data obtained for molecular species of 2-pcyanophenyl-5pentylpyridine (5Py), a heteroaromatic derivative of 5CB, stabilized in an argon matrix for different values of the cyanophenyl/argon ratio from l/ 1000 to l/10 and for a pure cyanophenyl film. It was shown that increasing both the cyanophenyl content of the system and the sample temperature led to a red shift of the CN band owing to the formation of inner dimers. The comprehensive study was conducted on a series of cyanophenyl solutions in non-polar hydrocarbon solvents (decane and pentane) in a wide concentration range from 0.01 to 1.5M. It was shown that in highly dilute solutions (less than 0.05 M) the maximum absorbance of the CN groups increases linearly with cyanobiphenyl concentration. For more concentrated solutions there was a negative deviation owing to the molecular association process. In this case the CN absorbance band was the sum of the absorbances of monomer and dimer (or higher aggregates). Computer treatment15, based on theoretical simulation of the experimental spectra obtained for a number of solutions of different cyanophenyl concentrations, allowed us to obtain line shapes for spectra of monomeric and associative SI ecies. In Figure 4 an example is presented of the superpositional experimental and computer
1
3
aggregation
2200
w (cm-‘)
(b)
I-
2250
2200 Y (cm-‘)
Figure 2 Thermal behaviour of IR spectra of 4-pe.ntyM’-cyanobiphenyl (5CB) (a) and 4-octyl-4’-cyanobiphenyl (8CB) (b) condensate films, 1% 50pm: 1, 80K, 2, 216K; 3,263K; 4, 322K
vibration band to higher frequencies, indicating partial dissociation of the cyanobiphenyl dimers and/or higher aggregates in the mesophase and isotropic phase (see Figures 2a and 2b, curve 4). The effect was confirmed by computer modelling of the harmonic frequencies of theoretical IR spectra for cyanobiphenyl monomeric and dimeric structures14. All calculations were carried out by using the GAMESS program suite.
Figure 3 CN stretching vibrations region of 2-p-cyanophenyl-Spentylpyridine (SpY) molecular species isolated in an inert argon matrix at SlOK for different values of cyanophenyl/argon ratio: (a) l/1000; (b) l/400; (c) l/50; (d) l/10; (e) pure film; I Z! 20pm
SUPRAMOLECULAR
SCIENCE Volume 4 Numbers 3-4 1997
487
Molecular aggregation
l
of liquid crystals: T. 1. Shabatina et al.
1
‘E 0
600
a2
3E
500-
03
. .
2; ,sj 400 8 e 5: 4 J! 2
.
q n. 0 l
0 . 0
300-
.
.
d
200 loolY
4 I
,
2220
4 I
I
2225
I
2230
2235
I
2240
Wavenumber Figure 4 Experimental superpositional and computer modelling monomer and dimer IR spectra for 8CB solution in decane at 290 K, initial concentration of cyanobiphenyl, Cs = 0.5 M: 1, experimental spectrum; 2, computer modelling dimer spectrum; 3, computer modelling monomer spectrum
modelling spectra of monomer and molecular associative species for the case of 8CB in decane solution. The initial concentration of cyanobiphenyl is Co=05 M. It should be mentioned that the method used does not allow us to distinguish between the spectra of dimers and higher associates if their spectral characteristics do not differ significantly (lower than 0.5 cm-‘). The spectroscopic characteristics obtained for the monomers (v~, Ed) and molecular associates (vn, n) of the cyanophenyls under investigation are presented in Table 1. It is important that the spectroscopic data obtained for monomers in film samples are the same as the values obtained for highly diluted cyanophenyl solutions. The IR spectra of cyanophenyl bulk films were recorded at different temperatures within the mesophase interval. It was found that CN absorption band maximum and line shape depended on sample temperature. Varying the temperature changed the ratio of monomer to associative species. Computer modelling of the experimental spectra as the sum of two modes:
where aI and u2 are the relative amounts of monomeric and associative cyanophenyl species, calibrated by using M and &o values, allowed us to obtain u = u&al -t al)the association degree of cyanophenyl molecules at a given temperature. The association enthalpy (AH, kJmol_‘) of cyanophenyls in the mesophase was estimated from the temperature dependence of a values. The data obtained are presented in Table 1. Table 1
CONCLUSIONS In the present work we performed an IR spectroscopic study of the thermal behaviour of the mesogenic cyanobiphenyls 4-pentyl-4’-cyanobiphenyl (5CB), 4pentyl-4’-cyanophenylpyridine (5Py), 4-pentyl-4’cyanophenylcyclohexane (5CH) 4-octyl-4cyanobiphenyl (8CB) and 4-octyloxy-4-cyanobiphenyl (80CB) in molecular condensate films, the bulk phase and in inert matrices. It was found that the absorbance maxima and line shape of CN group stretching vibrations are dependent on the quantitative ratio of monomer to dimer (or higher associates), which was changed by varying the temperature of the liquid-crystalline sample. Thermodynamic data obtained for the molecular association process of the named polar liquid crystals revealed that the most stable associates are formed in the case of cyanobiphenyl molecules compared with their hydrogenated or heteroaromatic derivatives. Higher thermodynamic stability was shown for aggregates of alkoxycyanobiphenyls than for aggregates of alkylcyanobiphenyls. Increasing the length of the alkyl chain did not significantly influence the molecular association parameters.
Some spectroscopic and thermodynamic characteristics of cyanophenyls
Substance
VT (cm-‘)
vr
5CB SCH 5EY 8CB 80CB
2230 (f0.5) 2232 (f0.5) 2232 (f0.5) 2231 (f0.5) 2231 (f0.5)
2225 (f0.5) 2225 (f0.5) 2227 (f0.5) 2227.5 (f0.5) 2226.5 (ztzO.5)
488
Analysis of the data in Table 1 allowed us to compare the relative stability of the liquid-crystalline aggregates with respect to the molecular structure of the polar mesogen. The most stable molecular associates were formed in the case of cyanobiphenyls as opposed to their hydrogenated or heteroaromatic derivatives16. This is possibly due to strong parallel stacking of the two benzene rings and overlapping of the rc-aromatic systems of the two monomeric units. There is only one benzene unit in the SCH molecular structure, and this leads to a significant decrease of the association enthalpy. For 5Py molecules we obtained a AH value that is lower than for 5CH; this could be connected with the formation of a dimeric structure that is less planar. In the case of differently substituted cyanobiphenyls (Table 1), we can see that the most stable dimers were formed in the case of alkoxycyanobiphenyl, 80CB, in comparison with alkylcyanobiphenyls, 5CB and 8CB. This could be possibly due to increased conjugation with the oxygen atom. The increased length of the alkyl chain did not have a significant influence on the molecular association parameters.
SUPRAMOLECULAR
(cm-‘)
EF x JO@ (M-r cm-‘)
EmD(Ix x 1O-3 (M-’ cm-‘)
AH (kJ mol-‘)
0.23 (f0.15) 0.29 (f0.15) 0.18 (f0.13) 0.19 (f0.07) 0.21 (fO.08)
0.59 0.57 0.94 0.87 0.67
-19f5 -3fl -12f5 -18f5 -29f7
SCIENCE Volume 4 Numbers 3-4 1997
(f0.25) (f0.25) (f0.35) (f0.30) (f0.25)
Molecular
aggregation
2.
ACKNOWLEDGEMENTS
3.
We gratefully acknowledge Professor A. V. Nemukhin (Moscow State University) for help in performing the quantum chemical calculations and Professor G. G. Sheina and Dr A. I. Ivanov (Institute of Low Temperature Physics and Engineering at the Ukrainian Academy of Sciences) for their help with matrix isolation experiments. We sincerely appreciate the help of Professor M. F. Grebenkin, who stimulated this work, and thank Professor G. R. Luckhurst (Southampton University) and Professor V. A. Batyuk (Moscow Baumann State Technical University) for fruitful discussions. This work was partially supported by INTAS grant 94-4299, the OMMEL International Scientific Programme and the Federal Scientific Research Programme ‘Russian Universities’.
4. 5. 6. I. 8. 9.
IO. II.
13.
15.
I. Grebenkin. M. F. and Ivachenko, A. V., Liquid
Crystalline Materials. Khimia, Moscow, 1989, 288 pp. (in Russian).
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