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

The photophysics of polyimides and several N-arylphthalimide model com pounds has been investigated as part of an effort to understand the fa ctors 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 fluorescenc e of these materials. Increasing solvent polarity causes a similar eff ect 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 fo r a given compound. The triplet states of both polymers and models hav e been characterized in solution by laser flash photolysis, and inters ystem crossing quantum yields have been determined. The triplet yields also decrease with increasing D-A character of the chromophore and wi th increasing solvent polarity. A comparison of fluorescence and phosp horescence spectra reveals a very small singlet-triplet energy gap. It seems likely that the lowering, by increased D-A character, of the yi elds of the triplet states that are intermediates in the photooxidativ e degradation enhances the photostability of aromatic-based polyimides .