Spectroscopic characterization of the structural properties of quinoxalinophenanthrophenazine thin films

Abstract

Thin films of 2,11-bis(1,1-dimethylethyl)-6,7,15,16-tetramethylquinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (TQPP-Me) and its long chain alkoxy derivative 2,11-bis(1,1-dimethylethyl)-6,7,15,16-tetrakis(dodecyloxy)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (TQPP-OC₁₂) on silicon/native silica (Si/SiO₂)_native and fused silica substrates were analyzed in respect of their microstructural and anisotropic optical properties. The molecules, considered as candidates for organic electronic applications, have different solubilities, hence the fabrication of their solid layers was carried out by various methods. Anisotropy resulting from the alignment and shape of the organic molecules was induced by the choice of sedimentation method on inorganic substrates. TQPP-Me thin films were obtained by organic molecular beam deposition in ultra-high vacuum conditions while TQPP-OC₁₂ layers were produced from solution by spin coating. The alignment and microstructure of TQPP-Me and TQPP-OC₁₂ molecules in the formed films were studied by optical microscopy, atomic force microscopy, polarized infrared measurements in transmission and reflection modes, polarized UV-Vis absorption and fluorescence spectroscopy, and spectroscopic ellipsometry. Optical properties and morphology of both studied films exhibit different textures with strongly uniaxial anisotropy in both cases. The TQPP-Me vacuum-deposited molecules are arranged parallel to the surface while the cores of spin-coated TQPP-OC₁₂ are cofacially packed and tilted with respect to the surface normal. Ellipsometric measurements were fitted with an isotropic model; however, the results do not exclude uniaxial ordering of the investigated samples.