Photo- and thermo-oxidative stability of aromatic spiro-linked bichromophoric cross-shaped molecules

An extensive investigation of the photostability and the thermo-oxidative stability is presented for 2,2',7,7'-tetrakis(biphenyl-4-yl)-9,9'-spirobifluorene and 2,2',4,4',7,7'-hexakis(biphenyl-4-yl)-9,9'-spirobifluorene. Both compounds are conjugated fully aromatic systems that are being discussed as active functional materials for a variety of advanced applications. The effect of atmosphere, sample thickness and preparation procedure on photo-oxidative degradation are investigated in detail by absorption and fluorescence spectroscopy. Distinct mechanisms are described in terms of relevant parameters such as the quantum yields of the photo-oxidation and the fluorescence. No oxidative degradation could be detected under nitrogen. In ambient air a strong decrease of the fluorescence performance is found due to effective quenching by defective chromophores. Chemiluminescence investigations were performed to characterise the thermo-oxidative behaviour in the temperature region between 300 and 450 K. It becomes clear that even a stable chemical structure such as the investigated aromatic system does not guarantee sufficient photostability with regard to light emitting properties.