The Origin of Coloration in Aromatic Polyimides. An Approach from Quantum Chemistry.

Abstract

The origin of coloration in aromatic polyimides was elucidated from a viewpoint of quantum chemistry. The MOs and the difference electron density between the ground and the first excited states of the model compounds were calculated using semiempirical AM1 (MOPAC97) and INDO/S (MOS-F) methods. Two model compounds of PI(PMDA-DDE) were adopted for the MO calculations. The calculated λmax 's were around 390nm, which were in good agreement with the cut-off wavelength of the PI(PMDA-DDE) film. From the CI coefficients, the predominant electronic transition was found to be HOMO→LUMO in each model, and its contribution to the first excited state was estimated to be approximately 50%. The calculations of the MOs and the difference electron density clearly indicate that the benzene ring derived from DDE acts as an electron-donating group and the five-membered imide and the benzene rings derived from PMDA act as electron-withdrawing groups. Therefore, it is evident that a strong intramolecular CT characterizes the first excited state, and this nature of the first excited state based on the predominant HOMO→LUMO transition contributes to the coloration of aromatic polyimides.