Optical constants of highly oriented oligothiophene films and nanoparticles

Abstract The optical constants of vapor-deposited oligothiophene films in the wavelength range from 240 to 800 nm have been determined by measuring the angular resolved polarized transmission and reflection spectra and fitting the data to the Fresnel equations for uniaxial systems. In the region of the main absorption band the absorption coefficient normal to the surface reaches kn≈1.5, whereas hardly any absorption is found for the direction parallel to the surface. The component of the refractive index along the surface normal varies between nn ≈ 3 on the low energy absorption edge and nn ≈ 0.7 on the high energy side of the absorption band, while the component parallel to the surface shows only little dispersion. This anisotropy causes a blue-shift of the absorption maximum against solution by several thousands of wavenumbers. Very similar optical constants are found for colloidal oligothiophene nanoparticles by fitting the results of light scattering experiments to Mie-theory of anisotropic particles.