Optical properties and the photoluminescence quantum yield of organic molecular materials

Dealing with synthetic molecules the connection with optical design in Natu re resides in the way the self-ordering of molecules affects the optical pr operties of condensed matter. In this paper we show the different nature of optical properties as a function of the specific molecular packing in crys talline samples and vacuum sublimed films. On the other hand, investigating the optical behaviour of synthetic molecular ensembles can shed light on t he understanding of natural optical designs as, conversely, the optical des ign in nature can help in the fabrication of molecular architectures with s pecific optical characteristics. The optical properties of organic conjugat ed materials are determined by the effective intermolecular interactions in the solid and are, therefore, highly dependent on the specific molecular p acking. We show that in the model system of alpha -sexithienyl (T-6) the or igin of fluorescence is excitonic in molecularly ordered single crystals, w hile 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 Tg are investigated as a function o f temperature in the range 30-300 K. The photoluminescence absolute quantum yield of intrinsic bulk excitons and molecular aggregate states is measure d with a home-built experimental apparatus based on an integrating sphere, which allows photoluminescence quantum yield and electroluminescence quantu m yield measurements in the temperature range 5-400 K. The photoluminescenc e quantum yield of the molecular aggregates placed below the exciton band s pans from 0.1 to 5% in the temperature range 300-30 K. In the same temperat ure 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 opt ical emission spectral properties and quantum yield depend on the relative concentration of molecular aggregates formed during the growth process.