Fluorescence emissions of imide compounds and end-capped polyimides enhanced by intramolecular double hydrogen bonds

The structure and optical properties of a newly synthesized imide compound (DHNHPI) that forms intramolecular double hydrogen bonds (intra-HBs) were investigated. This compound exhibits intense absorption at 372 nm (ε = 5091 cm−1 M−1) and strong emission at 427 nm (Φ = 0.507) in CHCl3. Under basic conditions, the absorption and fluorescence peaks showed large bathochromic shifts by 70 nm and 95 nm, respectively, compared with the neutral condition due to conversion to anion form. Based on the single-crystal X-ray diffraction analysis, DHNHPI can exert intermolecular π–π interactions between adjacent molecules along the stacking direction, with resultant emission peaks exhibiting longer wavelengths (∼525 nm) in the solid state. Moreover, an end-capped polyimide having DHNHPI moieties at the termini exhibited a strong fluorescence peak at 427 nm by photo-excitation at 332 nm. The large Stokes shift of 6701 cm−1 clearly indicates the occurrence of an effective Förster-resonance energy transfer from the main chains to the fluorescent end-groups. These facts demonstrate that the incorporation of two –OH groups, which form intra-HBs, endows imide compounds and polyimides with enhanced fluorescence properties as well as high sensitivity towards pH.