Anisotropic optical functions of α-1,4,4-tetraphenyl-1,3-butadiene.

One of the known monoclinic polymorphs of 1,1,4,4-tetraphenyl-1,3-butadiene was grown by the floating-drop technique. This material is of interest for blue emitting devices and for fundamental studies on intermolecular interactions, excited states and vibronic coupling in the fields of chemical physics and materials science. Polarized UV-visible spectra are reported. The origin of the optical bands is resolved. The results give information on the intermolecular interactions and are compared with theoretical predictions and modeling. The full material dielectric tensor is provided, as deduced from generalized spectroscopic ellipsometry. Details on the use of this technique on strongly anisotropic and absorbing molecular materials are given. Some typical features of the optical spectra are also demonstrated to be very sensitive to the presence of misaligned domains, which, in our samples, are found to be rotated around the normal to the surface and whose amount is determined. The method allows discriminating among monocrystals, in-plane polycrystalline samples, layered samples formed by misaligned successive layers, or samples with both types of disorder. Two independent parameters giving the fraction of misaligned domains or layers are suggested for the characterization and optimization of the growth and deposition of molecular organic materials.