Colorless Copolyimide Films Exhibiting Large Stokes-Shifted Photoluminescence Applicable for Spectral Conversion

Abstract

A series of semi-aliphatic polyimide (PI) copolymers (CoPIs) were prepared through the copolymerization of two dianhydrides, 1-hydroxy pyromellitic dianhydride (PHDA) and 4,4′-oxydiphthalic anhydride (ODPA), with 4,4′-diaminodicyclohexylmethane, in which the PHDA molar ratio was controlled at less than 5% to suppress aggregate formation in a solid state. Upon increasing the molar ratio of the PHDA unit, the fluorescence color of the CoPI films continuously changed from pink to yellow through an orange color owing to the enhanced emission from the anionic form of the PHDA unit, and the efficiency of the energy transfer from the locally excited S1 state to the anionic state simultaneously increased. Moreover, the CoPI film with a PHDA content of 3 mol % that formed on a silica substrate was colorless and transparent and showed bright orange fluorescence. However, this CoPI film formed on a soda-lime-silica (soda) glass substrate showed a pale-yellow color as well as yellowish fluorescence originating from the anionic form owing to the high basicity of the soda glass. To suppress the absorption and emission from the anions, a small amount of sulfuric acid was doped into the CoPI film, and a colorless and transparent film exhibiting large Stokes-shifted orange fluorescence was successfully obtained on the soda glass substrate. The wavelength-converting spectrum of the CoPI demonstrated that UV irradiation in solar light was efficiently absorbed, and its energy was converted into visible light between 500 and 700 nm with a quantum efficiency of 20%. These CoPI films exhibiting large Stokes-shifted ESIPT fluorescence on both silica and the soda glass substrates are promising materials for solar spectral conversion applications.