Changes in the conformation and electronic ...

CHANGES IN THE CONFORMATION AND ELECTRONIC STRUCTURE OF MOLECULES i

UPON PHASE TRANSITIONS IN A UNIAXIAL LIQUID CRYSTAL E. M. Aver'yanov and L. I. Mineev

UDC 535.343+541.65

Changes in the electronic structure (the displacement of levels and changes in the oscillator strength of the long-wavelength transition) and the conformation of the aromatic skeleton of the molecules in a liquid crystal of 4-n-octyloxy-4'-cyanobiphenyl have been discovered experimentally upon the isotropic-liquid-nematic-smectic A phase transitions. A functional dependence of the oscillator strengths of the electronic transitions of the molecules on the macroscopic orientational and transitional order parameters of the liquid crystal has been established and confirmed experimentally.

The investigation of the conformation of mesogenic molecules in different liquid-crystalline phases is of considerable interest for conformational analysis. The combination of different types of ordering of the molecules and the possibility of the experimental measurement of the order parameters corresponding to them permit the quantitative isolation of the influences of these types of ordering on the conformation of molecules [i] and the establishment of the functional dependence of the conformational parameters on the order parameters of a system [2]. The qualitative data on the conformational changes in the aromatic skeleton of molecules in a mesophase are still sparse and have been obtained mainly by NMR (see, for example, [3-5]). The use of optical and spectroscopic methods had been complicated by the need to take into account the anisotropy of the local field and the statistical orientational properties of conformationally unstable molecules in a liquid crystal. These problems were recently solved in [6-8]. the relationship between the electronic structure and the conformation of the ~-conjugated organic fragments appearing in the composition of the aromatic skeleton of mesogenic molecules in isotropic condensed media has been! thoroughly studied both theortically and experimentally [9, i0]. Therefore, the use of the parameters of the electronic structure of molecules as an indicator of their conformational changes in liquid crystals seems promising. In the present work we investigated the changes in the electronic structure and the conformation of the moleucles in a liquid crystal of 4-n-octyloxy-4'-cyanobiphenYl (80CB).

n-H)-tCaO~CN with the following temperatures (in ~ for the crystal-smectic A-nematic-isotropic liquid phase transitions: 53.0, 65.8, and 78.8, respectively. The biphenyl skeleton of 8OCB is a classical object of conformational analysis [ii]. The presence of one actual conformational degree of freedom in the skeleton associated with relative rotation of the benzene rings through the angle ~ around the central C-C bond significantly simplifies the interpretation of the experiment. The relationship between the electronic structure of biphenyl and its derivatives and the conformation of the skeleton in the gaseous phase, isotropic solutions, and a crystal has been thoroughly studied both experimentally and theoretically [9, i0]. The parameter of the orientational order and the anisotropy of the local field in liquid-crystalline phases of 8OCB were measured in [7]. For the analysis of the polarization of the electronic transitions of 8OCB we shall define a molecular system of coordinates, in which the z axis coincides with the long axis of the biphenyl fragment, and the y axis is the bisector of the dihedral angle ~ between the benzene rings. If the orientation of the dipole moment of the transition under investigation in the molecular system is assigned by the polar angle 8 and the azimuthal angle ~, the relaL. V. Kirenskii Institute of Physics, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 27, No. 2, pp. 82-87, March-April, 1986. Original article submitted January 21, 1985.

0022-4766/86/2702-0245512.50

9 1986 Plenum Publishing Corporation

245

0,8

0~.2.... 5o

42

34 ~3flO cm" )

Fig. I. Electronic absorption spectra of 8OCB i n i s o t r o p i r ( i , Tc - T = - 2 ~ homeotropically o r i e n t e d nematic (2, AT = 0 . 2 ~ and smectic A (3, AT = 23.3~ phases. Here T c is the temperature of the nematic-isotropic-liquid transition. tionship between the experimentally measured optical densities of a homeotropically oriented uniaxial liquid crystal (D i) and an isotropic liquid (D i) and the corresponding normalized values of the oscillator strength has the form [7] Am

r

3n•b =

p(l.ss

+

A~

3nbO i

(1)

2'

H e r e Am and Ai a r e t h e o s c i l l l a t o r strengths of the transition under investigation in the mesop h a s e and i n t h e i s o t r o p i c l i q u i d ; K = c o n s t , n b i and nb i a r e t h e b a c k g r o u n d v a l u e s o f t h e indices of refraction within the absorption band under investigation [12]; b 2 - - i] ]•b = i + L• [(nm.i) (2) a r e t h e b a c k g r o u n d v a l u e s o f t h e c o m p o n e n t s o f t h e t e n s o r o f t h e l o c a l f i e l d ; L 1 i s a component of the Lorentz tensor of the liquid crystal; p and Pi a r e t h e d e n s i t i e s of the nematic and i s o t r o p i c p h a s e s ; S~ = (3 cos2~ " t)/2~ G ~ = (3 s i n ~ cos 2~)/2, The p a r a m e t e r s

of the tensorial director Z.

S = Szz and G = Syy - Sxx a r e d e f i n e d

parameter

by t h e c o m p o n e n t s

Si~ = (3 c o s ~ 0 i z - - t > / 2 ~ ~ = x, y~ % of the orientational order of the molecular

axes relative

to the

The spectra of monodomainized films of 8OCB with a thickness d E I ~ and with homeotropic orientation of the molecules in sandwich-type cells made from fused quartz were recorded on a Specord UV-VIS spectrophotometer. The error in the thermostabilization of the samples was •176 The spectral distribution of the optical densities D i and D i of 8OCB in the liquidcrystalline and isotropic phases is shown in Fig. i. The long-wavelength electronic transition of 8OCB with Ima x = 299 nm is polarized along the long axis of the biphenyl skeleton [13], as is the stretching vibration of the C~N bond previously used in [7] for the measurement of the order parameter S in the liquid crystal under consideration. For this transition S~ = I, G~a = 0, and the orientation dependence of D i is determined only by the parameter S. The intensity of the second electronic transition with Ima x = 225 nm is practically independent of the variation of the temperature and of S. Since S >> G in cyanobiphenyls [14], S~ ~ 0 and ~ = ~m = 54"7~ for this transition. Form Fig. i it is seen that the third electronic transition with Ima x = 200 nm is polarized at an angle a < ~m relative to the long axis of the skeleton. The position of the maximum of the first band is shifted slightly toward longer wavelengths as the temperature of the mesophase is lowered. In the case of the third transition, the temperature displacement of the absorption maximum is significantly larger and has the opposite sign. The position of the band of the second transition is not dependent on the phase state of the liquid crystal. According to [12], the observed positions of the absorption bands of a liquid crystal depend on the positions of the molecular levels, the static anisotropic interactions, and the 246

0 4 Te-7,~ 16 20 b _...L___..L .......... ~ - ~ F J ....... - ~ ....... L _

~ , Jlnnd30 0--~3 -1,59joo.~o4 o-~ 1,57]

O

e, O

O oo O-

o- o o

nb.~ I ~ Df]

' ~-}OO

[]

[]

0

o o o

2~5 o

-,-o

o

3)~ ..

o

o

o

o 2~4--

A./~ . { o

~

o

o

o

o o

D

m

o

0~8

/

2,0- A'/K o9 DD 9

~I 9

9

2~ 2b

2,3

D

o

2~2-

f,8 .-

[]

nb

1,55 i

A~K-

214

9

"

0~6

F

9,1

i

0,4

9

+o-r,~ ; Fig. 2

~

i--

o

0~18

0,26

0,34

0,42

S 21

Fig. 3

Fig. 2. Temperature dependence of the characteristics of 80CB in the region of the first electronic transition for the background values of the indices of refraction (nbi,i) and the components of the tensor of the local field (fbi,i), the optical densities of the sample Di,i, and the oscillator strengths calculated from (i) Am,i/K. The symbols o and u correspond to the first and second sets of the parameters C and B for the determination of nbi and nbi with the use of (3) (an explanation is given in the text). The arrows point out the temperature TNS of the nematic-smectic A transiton. Fig. 3. Dependence of the normalized oscillator strength Am/K of the first electronic transition of 80CB on the square of the orientational order parameter in the nematic (N) and smectic A (SmA) phases (the notation is the same as in Fig. 2 ) . intermolecular resonance interactions In the case of a transition with 8 = ~M, only resonance splitting of the band in the mesophase with %Imax < %• and an overall (static and resonance) shift of the center of gravity of the doublet in the mesophase to a longer wavelength relative to the is 9 liquid [12] The constancy of the position of the second band of 8OCB upon the isotropic-liquid-nematic-smectic A transitions attests to the smallness of these effects. In the case of the transitions with ~ < ~m, the static and resonance shifts of the spectrum should also produce the inequality %• > %• which is not consistent with the displacement of the intense short-wavelength band. Therefore, it is likely that the observed shifts of the first and third bands are caused by changes in the electronic structure of the molecules in the mesophase. The determination of the oscillator strengths Am,i/K of the first electronic transition with the use of (i) and (2) requires knowledge of the background values nbi,i . They were calculated with the use of the formula b (n~,~) = I + C ~ , ~ / ( Z ~ - B~,O, (3) which accurately describes the dispersion of the indices of refraction nll, ni, and n i of 80CB in the visible and near UV regions [7]. The two sets of values of C and B found on the basis of the experimental values of n i and n i [7] in the 589-735-nm (I) and 490-735-nm (II) region of the spectrum give the values of nb• and nbi presented in Fig. 2. The components fb• and fb i were determined from (2) with the use of the previously measured values of L i [7]. The variability of nbi, nbi, fb• and fb i within the absorption band investigated is small, and the values of D• Di, nb• nbi, fbi, and fb i corresponding to the maximum of the band were used for the determination of Am/K and Ai/K (see Fig. 2). The data of p and Pi were taken from [15]. From Fig. 2 it is seen that the oscillator strength A m of the first electronic transition of 8OCB increases abruptly upon the isotropic-liquid--nematic phase transition and then increases nonlinearly as the temperature of the mesophase is lowered. After the nematicsmectic A transition, A m increases, approaching saturation. The qualitative behavior of A m is the same for the two sets of values of C and B in (3); The variation of A m in the mesophase

247

is caused by the alteration of the electronic structure of the molecules of the liquid crystal, since fb I = fb i, nbi = nbl, and p > Pi and since consideration of the corresponding corrections only reduces the magnitude of the observed effect. The experimental data can be explained by taking into account the alteration of the optimal conformation of the biphenyl fragment of 8OCB in the mesophase. It is known [9, i0] that an increase in the angle ~ between the benzene rings of biphenyl when bulky substituents are introduced into the ortho positions of the benzene rings results in a hypsochromic shift of the band with lmax = 200 nm for biphenyl. The displacement of the bands is nonlinearly dependent on ~ and amounts to only ~400 cm -I for long-wavelength transition when ~ is varied from 30 to 0~ The data for 80CB point out decreases in ~ accompanying the successive isotropic-liquid-nematic-smectic A transitions. Let us proceed to the interpretation of the functional dependence of the experimentally measured parameters of the electronic structure and the conformation of molecules on the orientational and translational order parameters of liquid crystals. According to the data in [16], in the mesophase of 80CB 95% of the molecules are dimerized, and the concentration of the dimers is very weakly dependent on the temperature and the phase state. On the other hand, variation of the temperature of the isotropic phase of 8OCB does not result in appreciable changes in the value of A i. Therefore, the observed variation of A m in the mesophase of 8OCB is caused by the appearance of a macroscopic long-range orientational (unidimensional translational) order in the system, and a phenomenological interpretation of the experiment can be given on the basis of the conception of the interaction of the order parameter of the system with the intramolecular degrees of freedom [2]. The latter correspond to the generalized coordinates Qk (k = i, 2 .... ). The parameters Qk can, in particular, describe the variation of the conformation of the molecular fragments. Variation of the Qk is accompanied by variation of the molecular properties and the features of the intermolecular interaction which influence the temperatures of the phase transitions and the degree of ordering of the system [2]. Therefore, there is an interaction of the scalar parameter Qk with the tensorial orientational order parameter ~7~ 7 S(rTr8 - 6~/3) (r7 and r$ are components of the director r) and the complex parameter ~ !~i exp (i~) o~ the translational order of the liquid crystal (I~I and # are the amplitude and phase of the density wave of the smectic layers). The optimal values of Qkm in the mesophase differ from Qki in the isotropic liquid. The invariant terms of the interaction depend on the nature of the Qk, and in the case of conformationa! changes in the lowest order, they are proportional to the products Qk.Tr(S z) and Qkl~[ 2 [2]. The corresponding corrections to the density of the free energy of a liquid crystal have the form

Afsqk + AFwQh = --AQa(?l~ Ss + 72u [~[~), where AQk = Qkm - Q k i , and ~ l k and Y2k a r e c o n s t a n t s o f t h e i n t e r a c t i o n . t r i b u t i o n t o t h e e n e r g y o f t h e m e s o p h a s e i s g i v e n by t h e t e r m s AFQh = (AQh)V2Xh' where • k is the susceptibility of the molecules to change in Qk" the terms in (4) and (5) gives Qkm= Qkl + Xh (?lhSZ + ~1~[2) 9

The m o l e c u l a r

(4) con(5)

Minimization of the sum of (6)

For relative rotations of the planar fragments through the angle ~ around the single chemical bond joining them, Q = cos 2 @ [2]. It was previously shown [2] that Eq. (6) accu-ratelydescribes the experimental plots of ~ (AT) in nematic liquid crystals of 4-methoxybenzylidene-4'-butylaniline [3] and homologs of the 4-n-alkyl-4'-cyanobiphenyl (nCB) series [i]. Since only the symmetry requirements were taken into account in the derivation of (7), and the type of conformational changes was not specified, further testing of the functional dependence of Q on S [Eq. (6)] for conformational changes differing from those considered above would be of interest. In the general case, the oscillator strength of any electronic transition of a molecule A = A(Q I, Q=, ...) is a complex and not a priori known function of the Qk" We expand this function in a series in the ~Qk in the vicinity of the Qki

A(Q~,Q2 . . . . ) = A(Qlt, Q,i, ...) -t- 2A~AQh + ... (7) and restrict ourselves to the linear approximation. Then, with consideration of (7) we obtain Am = A i (t + ~1S' + • IWIs),.

248

(8)

where the signs of the coefficients ~ i and ~2 may be either positive or negative. From Fig. 3 it is seen that the dependence of the experimental values of Am/K for the transition of 8OCB investigated on S 2 has a linear character over the entire range of the nematic phase in accordance with (8). The values of Am/K extrapolated to S = 0, which are equal to 1.93 and 1.87, are in good agreement with the experimentally measured values Ai/K = 1.87 and 1.81 (see Fig. 3) for the two sets of values of C and B in (3). Systematic deviations from the plot of Am(S 2) are observed in the smetic A phase of 8OCB, where M2 > 0. The presence of individual contributions from the orientational and translational ordering of the molecules to the variation of ~, which was previously noted for other mesogenic molecules [i, 4, 5], is displayed here. The high sensitivity of the conformation of cyanobiphenyls to intermolecular interactions and the effects of close packing is also indicated by the results of the quantum-chemical and molecular-mechanical calculations in [16]. The values of Am/K in the smectic phase at TNS - T = 0.3~ fits the plot of Am(S 2) for the nematic phase, and the deviations of the measured and extrapolated values of Am(S 2) at T < TNS are small. This supports the hypothesis regarding the continuous variation of I~I at T = TNS and the small values of l~I in the smectic phase of 8OCB [17]. The parameter ~i characterizes the measure of the perturbation of the electronic structure of the molecules by their orientational ordering. The values z I = 0.6 and 0.57 for the two sets of values of C and B in (3) correlate well with respect to their absolute values with the analogous parameters for the electronic transitions in the nconjugated matrices [8]. Dependence (8) was closely fulfilled for the latter over the broad range of temperatures for the nematic phase (AT = 70~ and case of xz ~ 0 were observed for electronic transitions of differing nature. The increase in the mean value and the anisotropy of the polarizability of nCB molecules with increasing orientational ordering of the mesophase [18], which was discovered by refraction measurements and interpreted on the basis of the data in [I], attests to the increase in the oscillator strength of the long-wavelength transition of nCB in accordance with (8). The perturbations of the electronic structure of conformationally unstable molecules in a mesophase previously discovered in [8, 20] and noted in the present work attest to the alteration of the short-range intermolecular correlations upon the appearance of long-range orientational and unidimensional translational ordering in the system. In order to ascertain the character and nature of these changes, further experimental investigations are needed. LITERATURE C I T E D I. 2.

3. 4. 5. 6.

E. M. Aver'yanov, V. A. Zhuikov, and P. V. Adomenas, Pis'ma Zh. ~ks. Teor. Fiz., 33, 262 (1981); Zh. ~ks. Teor. Fiz., 81, 210 (1981). E. M. Aver'yanov, Fiz. Tverd. Tela, 24, 2839 (1982); 25, 293 (1983); in: V. F. Shabanov (editor), Nonlinear Optics and Spectroscopy of Molecular Media [in Russian], Izd-vo IF SO Akad. Nauk SSSR, Krasnoyarsk (1974), p. 59. R. G. Dong, E. Tomchuk, C. G. Waid, et al., J. Chem. Phys., 66, 4121 (1977). A. J. Dianoux and F. J. Volino, J. Phys. (Fr.), 41, 1147 (1980). S. Limmer and M. Findeisen, Z. Naturforsch., 39a, 218 (1984). E. M. Aver'yanov, K. S. Aleksandrov, and V. F. Shabanov, Dokl. Akad. Nauk SSSR, 242, 84

(1978). 7.

E. M. Aver'yanov, P. V. Adomenas, V. A. Zhuikov, et el., Zh. Eks. Teor. Fiz., 86, 2111 (1984); 87, 1686 (1984).

8.

E. M. Aver'yanov, V. M. Muratov and V. G. Rumyantsev, Zh. ~ks. Teor. F i z . , 88, 810 (1985). G. H. Beaven, in: G. Gray (editor), Steric Effects in Conjugated Systems, Butterworth, London (1958), Chap. 3, p. 22. H. Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules, Academic Press, New York-London (1967), Chap. 9, p. 163; Chap. 12, p. 262. R. L. Avoyan, Yu. T. Struchkov, and V. G. Dashevskii, Zh. Strukt. Khim., ~, No. 2, 289 (1966). E. M. Aver'yanov, Fiz. Tverd. Tela, 22, 1867 (1980). C. David and D. Baeyvens-Volant, Mol. Cryst. Liquid Cryst., 59, 181 (1980). J. W. Emsley, G. R. Luckhurst,and S. P. Stoskley, Mol. Phys., 44, 565 (1981). P. E. Cladis, D. Guillon, F. R. Bouchet, and P. L. Finn, Phys. Rev., A21, 2594 (1981). S. Charbonier, A. Proutiere, R. Visni, et el., in: Tenth International Liquid Crystal Conference, York, England (1984), Abstract A54. M. A. Anisimov, Pis'ma Zh. ~ks. Teor. Fiz., 37, ii (1983). E. M. Aver'yanov, V. Ya. Zyryanov, V. A. Zhuikov, and Yu. I. Ruolene, Zh. Strukt. Khim., 24, No. 5, I01 (1983).

9. 10. ii. 12. 13. 14. 15. 16. 17. 18.

249