Polarizability Density and Orientation Order of Molecules (Monomers) in Uniaxial Molecular (Polymer) Film

Abstract
In order to gain an adequate understanding about the nature of spectral and optical properties of uniaxial molecular (polymer) films, the data about the energy structure of molecules (monomers) and the long-range orientation order of dipole moments m_q of electronic/vibrational transitions are needed. This order is characterized by the order parameters U_q of moments m_q with respect to the optical axis n of the film. Until now, components n_j(ω), k_j(ω) of refractive indices N_j(ω) = n_j(ω) – ik_j(ω) or components ε_(1,2)j(ω) of dielectric constants ε_j(ω) = ε_1j(ω) – iε_2j(ω) of the film for the light-wave polarizations along (j = ||) and across (j = ⊥) the axis n had been used as indirect sources of such data. The direct information about the energy structure of molecules (monomers) and parameters U_q is contained in the components γ_(1,2)j(ω) of ensemble-averaged polarizabilities γ_j(ω) = γ_1j(ω) – iγ_2j(ω) of molecules (monomers). In this work, components P_(1,2)j(ω) of polarizabilities densities P_j(ω) = [ε_j(ω) – 1]/f_j(ω) = 4pNγ_j(ω) = P_1j(ω) – iP_2j(ω) are used for receiving such information. Here, f_j(ω) = 1 + L_j[ε_j(ω) – 1] are the local-field tensor components for the light wave in the film; the Lorentz-tensor components L_j are obtained using the dependences n_j(ω) in the transparency region of the film; N is the number of molecules (monomers) per unit volume of the film. Methods for determining parameters U_q for molecular transitions were developed using the dependences P_(1,2)j(ω) within the isolated absorption bands corresponding to the transitions. The methods were confirmed for the uniaxial films of the conjugated polymer F8BT with the preferred orientation of macromolecules in the film plane XY with the optical axis n||Z and the known dependences n_j(ω), k_j(ω) in the transparency and low-frequency electronic absorption ranges. The spectral-invariant correlations connecting functions P_(1,2)j(ω) and ε_(1,2)j(ω) were established.

Keywords: molecular films, polymer films, conjugated polymers, F8BT, orientation order, polarizability density, spectral invariants