HAL Id: jpa-00218736
https://hal.archives-ouvertes.fr/jpa-00218736
Submitted on 1 Jan 1979
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
THE LINEAR DICHROISM OF CUBIC
MOLECULES : DISTORTION AND LOCAL FIELD CONTRIBUTIONS
L. Baraldi, G. Gottarelli, B. Samorí
To cite this version:
L. Baraldi, G. Gottarelli, B. Samorí. THE LINEAR DICHROISM OF CUBIC MOLECULES : DIS- TORTION AND LOCAL FIELD CONTRIBUTIONS. Journal de Physique Colloques, 1979, 40 (C3), pp.C3-204-C3-206. �10.1051/jphyscol:1979340�. �jpa-00218736�
JOURNAL DE PHYSIQUE Colloque C3, supplkment au no 4, Tome 40, Avril 1979, page C3-204
THE LINEAR DICHROISM OF CUBIC MOLECULES
:DISTORTION AND LOCAL FIELD CONTRIBUTIONS
L. BARALDI, G . GOTTARELLI and B.
SAM OR^
Istituto Chimica degli Intermedi, Via Risorgimento, 4, 40136 Bologna, Italy
RCsumC. - Le dichroi'sme lineaire de W(CO),, Mo(CO), et Cr(CO), dissous dans un mClange compensC de dtrivCs du cholestCrol met en Cvidence la prtsence des dCformations stkriques qui peuvent dtre la cause principale ou secondaire de I'anisotropie optique, I'autre cause Ctant l'effet des champs diklectriques locaux, I'importance relative des deux facteurs dependant de la nature des molCcules isotropes.
Abstract. - The linear dichroism of W(CO),, Mo(CO), and Cr(CO)6 dissolved in an oriented, compensated nematic mixture clearly shows the occurrence of steric deformations which can be the major or minor source of optical anisotropy, together with local dielectric field effects, depending on the nature of the isotropic molecule.
The dichroic U.V. techniques are not very useful for studying the orientation of liquid crystals owing to the very high absorption of the component mole- cules, but the use of guest molecules as internal optical probes could overcome the aforesaid pro- blem [I].
The linear dichroism (L.D.) of guest molecules oriented by a nematic matrix, obviously transparent in the investigated wavelength region, can be now recorded by a lock-in technique (i.e. a dichrograph modified by incorporating an achromatic i1/4 device before the sample [2, 31 thus making available a very high sensitivity. Differences of optical densities for the two perpendicularly polarized components
very accurate Sii elements only when the problems of the dielectric local field effects have been overcome.
The contribution of the dielectric field effects to the L.D. of an anisotropic solute arises from the different radiation fields of the two polarized compo- nents which, even though equal at incidence upon the system, become unequal in the medium on account of its linear or circular birefringence. Cubic molecules are particularly useful in studying this type of pheno- mena, being devoid of any intrinsic anisotropy.
The L.D. [7] or the L.C.I.C.D. [8] of Mo(CO), dissolved in stretched films or in a cholesteric matrix respectively have been rationalized in terms of the
are safely obtainable elements of the order
.+ 1.5
and hence one could obtain the matrix Sii of the guest molecule with the same sensitivity by following the very simple ' expression used when no mixed polarizations are present.
(ell - ~ l ) i
- = 3 Sii (1) O
E i
where
(q,
- is the linear dichroism andTi
the isotropic absorption (i-e. taken above the clearing point of the mixture) in correspondence to the transi- tion polarized along the i molecular axis [3]. .-1.5A technique in practice equivalent to the L.D. is the L.C.I.C.D. [4, 5, 61 (liquid crystal induced circular
dichroism) of an achiral guest molecule in a host 3 4 0 300 260
cholesteric matrix. FIG. 1. - The linear dichroism (A) and the isotropic absorption (B) The L.D. will be the simplest way of obtaining of W(CO), in a compensated mixture.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979340
THE LINEAR DICHROISM OF CUBIC MOLECULES C3-205
Lorentz or the Vuks model, and the problems seemed to have been solved until the L.D. and L.C.I.C.D.
spectra of W(CO),, Cr(CO), and Mo(CO), were recorded (Figs. 1, 2, 3). In the light of these experiments, other factors such as distortion from cubic symmetry must be considered.
A
W B
0.5 -1
-2
0.25
FIG. 3. - The linear dichroism (A) and the isotrobic absorption (B) of Mo(CO), in a compensated nematic mixture.
320 300 280 260
FIG. 2. - The linear dichroism (A) and the isotropic absorption (B) of Cr(CO), in a compensated nematic mixture.
a
O V
04w-.?A
a -mc3
1. Induced dichroism of W(CO),. - The L.D. '0 spectrum of W(CO), (see Fig. 1) shows a double-
humped bisignate shape in correspondence to a
u
single isotropic absorption, instead of having the same shape as the unpolarized absorption. Such spectrum cannot be explained by the Lorentz or Vuks models.
The investigated C IT,, + 'A!, transition [9] seems to be split by a sterical deformation of the octahedral symmetry and the resulting orientation seems to be responsible for the dichroism of W(CO),, which can actually be explained by this approach without taking into account any contribution from local field effects.
An octahedral molecule can undergo a tetragonal distortion to D,, symmetry orland a trigonal distor- tion to D,, symmetry, generating either elongated or compressed-shaped molecules with the molecular z or C, axis (P
>
3) preferentially parallel to the nematic director or perpendicular to, it respecti- vely (Fig. 4).The lowering of the symmetry to D,, or D,, splits the T,,(x, y, z) states into 'A,,(z) and lE,(x, y) states.
The orientation of the elongated and compressed- shaped molecules can be described by a rod-shaped
FIG. 4. -The orientations of W(CO), into the anisotropic medium (left) and the resulting uniaxial trigonal (upper) and tetra- gonal (lower) distortions toward an elongated and a compressed-
shaped molecule.
and a disc-shaped model respectively [3] and is defined by the traceless condition.
Sxx = Syy = - 2 S,,
.
(2)C3-206 L. BARALDI, G. GOTTARELLI AND B. SAM OR^
As the extinction coefficients for the i = x, y, z polarized components can be considered equal.
as in the original cubic symmetry (distortion with angle variations of only about degrees are expected [lo]), following Eq. (I), an opposite signed and equally intense L.D. or L.C.I.C.D. is predicted for the C 'TlU(z) t 'A,, and the Eu(x, y ) t 'A, transitions. Two important features of the spectral shape are thus very easily predicted. Only if the rela- tive energies of the two transitions in correspondence to the proposed distortions are known is a complete analysis of the spectra possible and can one test the consistency of the models of distortion.
This energetic analysis (which has been done [ l l ] by an exciton approach and is clearly not of great interest to a liquid crystal conference) proves that the spectra of figure 1 can be due both to the tetragonal and trigonal distortion, and unfortunately any discri- mination is impossible by these dichroic techniques.
In conclusion, the L.D. spectrum of W(CO), can be explained without taking in to account any local jeld effect. - -
2. Induced dichroism of Cr(CO),. - The double- humped shape is still present in the dichroic spectra of Cr(CO), but the intensity of the negative part seems to increase whereas the positive one to decrease.
Any possible distortion of the cubic symmetry, including deformations to a symmetry so low that the triple degeneracy is completely removed (if eq. (3) is still valid), can generate only a conservative L.D. or L.C.I.C.D. spectrum like that of W(CO),. Distortions so strong as to invalidate eq. (3), or local field effect, must be invoked in order to justify such different intensities of the two opposite-signed parts of the L.D.
spectrum of Cr(CO),.
With the Vuks model the following expression is obtainable [7] for the local field induced L.D.
This equation requires that :
1) The induced L.D. spectrum has the same shape as the unpolarized absorption, as the contribution of the solute absorption band in the U.V. region to the change in the refractive index of a solute-solvent system is very small [1 11.
2) The induced L.D. spectrum has the sign of
(rill
- n,) for a given solvent.A negative field contribution, as (rill
.-
n,) is about 0.065 for mixtures of cholesteryl chloride and choles- teryl esters similar to those used in the present work [12], is expected, and its addition to a conserva- tive S-shaped spectrum due to some distortion similar to that of W(CO), can produce something like the Cr(CO), L.D. spectrum.In conclusion, the occurrence of both distortion and local field can explain the Cr(CO), dichroic spectra.
3. Induced dichroism of Mo(CO),. - The L.D.
spectrum of Mo(CO), is opposite signed to that of Cr(CO), (Fig. 3). The positive maxima are now not very strongly shifted to the red with respect to the isotropic absorption, and here the shape of the L.D.
spectrum is much more similar to the isotropic spectrum than in the former two cases.
In the literature, local field models are available like the Lorentz one [5, 71, which can predict also a positive contribution to the L.D. (i.e. a positive L.D.
for an isotropic molecule like Mo(CO),). By adding some distortion, the red shift can also be easily pre- dicted.
In conclusion, the study of the L.D. of cubic mole- cules dissolved in oriented liquid crystals points out the existence also of distortion effects which can be the major or minor source of the L.D., depending on the nature of the molecule. Probably the nature of the matrix itself is important in this regard. N.M.R.
techniques which have already pointed out the exis- tence of distortion of highly symmetrical molecules will be used to have a better insight into the problem.
4. Experimental. - The L.D. spectra have been recorded following a procedure previously reported with a Jouan I1 dichrograph modified by incorporat- ing an achromatic 114 device [3]. The carbonyl com- pounds have been purified by sublimation.
References
[I] BLINOV, L. M., KIZEL, V. A., RUMYANTSEV, V. G. and TITOV, V. V., J. Physique CoNoq. 36 (1975) C1-69.
[2] DAVIDSSON, A. and NORDEN,-B., Chem. Scr. 9 (1976) 49.
[3] GOTTARELLI, G., SAMORI', B. and PEACOCK, R. D., J. Chent.
Soc. Perkin Trans. I1 (1977) 1208.
[4] SACKMANN, E. and VOSS, J., Chem. Phys. Lett. 14 (1972) 528.
[5] DUDLEY, R. J., MASON, S. F. and PEACOCK, R. D., J. Chem.
Soc. Faraday Trans. 11 71 (1975) 997.
161 SAEVA, F. D. and WYSOCKI, J. J., J. Am. Chem. Soc. 93 (1971) 5928.
[7] NORDEN, B., Spectrosc. Lett. 10 (1977) 455.
[8] MASON, S. F., PEACOCK, R. D., J. Chem. Soc. Chem. Commun.
(1973) 712.
[9] BEACH, N. A. and GRAY, H. B., J. Am. Chem. Soc. 90 (1968) 5713.
[lo] BAYLEY, D., BUCKINGHAM, A. D., FUJIWARE, F. and REEVES, L. W., J. Magn. Reson. 18 (1975) 344.
[ l l ] SAM OR^ , B., J. Phys. Chem. (in the press).
[12] TEUCHER, I., KO, K. and LABES, M. M., J. Chem. Phys. 56 (1972) 3308.
