Optical properties and the photoluminescence quantum yield of organic molecular materials

Abstract

Dealing with synthetic molecules the connection with optical design in Nature resides in the way the self-ordering of molecules affects the optical properties of condensed matter. In this paper we show the different nature of optical properties as a function of the specific molecular packing in crystalline samples and vacuum sublimed films. On the other hand, investigating the optical behaviour of synthetic molecular ensembles can shed light on the understanding of natural optical designs as, conversely, the optical design in nature can help in the fabrication of molecular architectures with specific optical characteristics. The optical properties of organic conjugated materials are determined by the effective intermolecular interactions in the solid and are, therefore, highly dependent on the specific molecular packing. We show that in the model system of α-sexithienyl (T6) the origin of fluorescence is excitonic in molecularly ordered single crystals, while it is due to molecular aggregates in the case of polycrystalline thin films. Molecular aggregates are due to a modified local molecular packing, which can be induced and controlled by acting on the sublimation conditions during the film growth. The energy transport and photoluminescence quantum yield in vacuum sublimed thin films of T6 are investigated as a function of temperature in the range 30-300 K. The photoluminescence absolute quantum yield of intrinsic bulk excitons and molecular aggregate states is measured with a home-built experimental apparatus based on an integrating sphere, which allows photoluminescence quantum yield and electroluminescence quantum yield measurements in the temperature range 5-400 K. The photoluminescence quantum yield of the molecular aggregates placed below the exciton band spans from 0.1 to 5% in the temperature range 300-30 K. In the same temperature range, the quantum efficiency of the intrinsic excitons increases by a factor of two from 0.4 to 1%. Therefore, in organic thin films both the optical emission spectral properties and quantum yield depend on the relative concentration of molecular aggregates formed during the growth process.