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

Abstract

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.