Photo-oxidation of polymers used in electroluminescent devices

Two electroluminescent polymers, poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene) (BCHA-PPV) and poly(3-octylthiophene) (P3OT), have been characterized on Au and Al substrates in terms of photo-oxidative stability after exposure to UV and visible irradiation from a Hg lamp. Thin polymer films were analyzed in situ using IR reflection absorption spectroscopy (IRRAS), and ex situ by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection (ATR) IR spectroscopy. The oxidation of BCHA-PPV is quite extensive, resulting in the formation of an ester and volatile aldehyde species. Singlet oxygen, formed by energy transfer from the BCHA-PPV triplet state, appears responsible for at least one photo-oxidation pathway. BCHA-PPV triplet exciton states will also form during electroluminescent device operation (the singlet exciton state is responsible for luminescence in the film) resulting in the formation of reactive singlet oxygen. Thus, it appears that BCHA-PPV will slowly self-degrade in the presence of oxygen when used as the active layer in electroluminescent devices. P3OT shows no detectable oxidation during similar exposure.