Orientation order and polarizability density of oligofluorene molecules F(Pr)5F(MB)2 in glassy nematic phase

Aver’yanov, E.M.// Liquid Crystals and their Application//

Optical properties of uniaxial molecular films are determined by components

nj(), kj() of the refractive indices Nj() = nj() – ikj() for the light-wave po-

larizations along (j = ||) and across (j = ) the optical axis n of the film. Func-

tions nj(), kj() depend on the position of absorption bands, oscillator strengths

of the transitions corresponding to the bands, and the orientation order parame-

ters Uq of the dipole moments mq of the transitions with respect to the axis n.

Methods for determining parameters Uq for the absorption bands in uniaxial ne-

matic films were developed by the author earlier using either the components

(1,2)j() of the dielectric constants j() = [Nj()]2 = 1j() – i2j() of the film

or the components P(1,2)j() of the polarizabilities densities Pj() = [j() –

1]/fj() = 4Nj() = P1j() – iP2j() of molecules within the bands. Here, fj()

= 1 + Lj[j() – 1] are the local-field tensor components for the light wave in the

film; Lj are the Lorentz-tensor components for the film (L|| + 2L = 1); N is the

number of molecules per unit volume of the film; j() = 1j() – i2j() are the

ensemble-averaged polarizabilities of molecules. In this work, the methods were

used in order to determine the parameter Uq. This parameter corresponds to low-

frequency electronic absorption band in the glassy-nematic film formed by the

molecules-heptamers of monodisperse oligofluorene F(Pr)5F(MB)2. The de-

pendences nj(), kj() for the film in the transparency and low-frequency elec-

tronic absorption ranges are known. Experimental value L = 0.487 was obtained

using the dependences nj() in the visible transparency range. Accounting for

the components Lj, the function fj() increases the value Uq = 0.965, determined

with functions 1j() or P1j(). Spectral features of the functions nj(), kj(),

(1,2)j(), P(1,2)j() and correlations between them were interpreted quantitatively

accounting the anisotropy of the components Lj