Excited-State Intramolecular Proton Transfer in Imide Compounds and its Application to Control the Emission Colors of Highly Fluorescent Polyimides

A new molecular design concept for colorless and multicolor light-emitting polyimides (PIs) based on excited-state proton transfer is proposed. The ultraviolet visible optical absorption spectra and fluorescence spectra of PIs containing hydroxy groups in their anhydride moieties and their corresponding imide compounds were extensively investigated. N-Cyclohexyl-3-hydroxyphthalimide (3HNHPI), which forms intramolecular hydrogen bonding between the hydroxy group and imide carbonyl group, exhibited an excited-state intramolecular proton transfer (ESIPT) emission at 534 nm with excitation at 332 nm having a large Stokes' shift of 11394 cm(-1). The highly fluorescent PIs, prepared from 4,4'-oxidiphthalic dianhydride (ODPA) and 4,4'-diaminocyclohexylmethane (DCHM) end-capped with 3-hydroxyphthalic anhydride, are colorless and exhibited two characteristic fluorescence peaks at 400 and 530 nm with excitation at 340 nm. These PIs showed gradated multicolor emission (blue, light-blue, white, and light-green) depending on the amount of fluorescent termini while maintaining colorless and transparence. It was clarified that ESIPT emission is a useful tool for controlling the absorption and fluorescence properties of PIs.