PHOTODEGRADATION OF POLYIMIDES .6. EFFECT OF DONOR-ACCEPTOR GROUPS ON THE PHOTOOXIDATIVE STABILITY OF POLYIMIDES AND MODEL COMPOUNDS

The photophysics of polyimides and several N-arylphthalimide model compounds has been investigated as part of an effort to understand the factors that control the oxidative photodegradation of these materials. Substituents that increase the donor-acceptor (D-A) character of both polymers and models cause an enhancement of the red-shifted electronic absorption and a shift to lower energy of the broad, weak fluorescence of these materials. Increasing solvent polarity causes a similar effect on the absorption and fluorescence spectra of the model compounds. Fluorescence quantum yields decrease with increasing D-A character in the series of model compounds and with increasing solvent polarity for a given compound. The triplet states of both polymers and models have been characterized in solution by laser flash photolysis, and intersystem crossing quantum yields have been determined. The triplet yields also decrease with increasing D-A character of the chromophore and with increasing solvent polarity. A comparison of fluorescence and phosphorescence spectra reveals a very small singlet-triplet energy gap. It seems likely that the lowering, by increased D-A character, of the yields of the triplet states that are intermediates in the photooxidative degradation enhances the photostability of aromatic-based polyimides.