Polyimide and Imide Compound Exhibiting Bright Red Fluorescence with Very Large Stokes Shifts via Excited-State Intramolecular Proton Transfer

A novel polyimide (PI) emitting a prominent red fluorescence was prepared based on 3,6-dihydroxy­pyromellitic dianhydride (P2HDA) and 4,4′-diaminocyclohexylmethane (DCHM). In order to investigate the fluorescence properties of the PI, an imide model compound, N,N′-dicyclohexyl-3,6-dihydroxy­pyromellitimide (P2H-Ch), corresponding to one repeating unit of the PI, was also synthesized. The UV–vis absorption and fluorescence spectra of P2H-Ch in CHCl3 and the solid PI film demonstrated intense red fluorescence at around 640 nm with a very large Stokes shift (ν) of 7655 and 8994 cm–1, respectively, via the excited-state intramolecular proton transfer (ESIPT). Moreover, the corresponding monoanion and dianion species were formed in basic conditions with an organic base (DBU) and basic salt (NaOH), which were characterized by highly visible halochromism. The introduction of −OH groups into the pyromellitic moiety of imide compounds or PIs at their para positions led to the long-wavelength fluorescent emission as well as to the high pH sensitivity.