Determination of the optical constants of α- and β-zinc (II)-phthalocyanine films

► The optical constants of α- and β-ZnPc were determined from UV-vis spectroscopy. ► The phase transition from α- to β-ZnPc at around 240 °C was followed progressively. ► Correct n and k are found including roughness and Kramers-Kronig-consistency. ► Numerically determined roughness fits well to AFM-derived values for both phases. ► Optical and structural data can be directly linked providing a coherent picture.