The photolytic decomposition in air of a polyimide film based on a dianhydride and a diary lamine with hexafluoroisopropylidene 6F bridging groups is extremely rapid, leading to efficient chain cleavage and subsequent photooxidative decomposition. Only relatively short photolysis times with an unfiltered mediumpressure mercury lamp are required to give a clean photoablation of the 6F-6F polyimide films in air. IR difference spectroscopy shows the appearance of anhydride groups on photolysis in air, indicating a photooxidation process via decomposition of the arylimide linkage. A photophysical and photochemical analysis of several model N-arylphthalimides indicates that both solvent polarity and electron-withdrawing/ donating substituents can greatly alter the efficiency of the photolysis process. The formation of the triplet state is also dependent on the substituents and solvent polarity. In general, an increase in the CT character of the excited state by either substituent or solvent effects leads to a reduction in both the photolysis and intersystem crossing quantum yields.
Polyimides such as 6F‐6F and 6F‐ODA and model N‐arylphthalimides are stabilized against photooxidative degradation by their electron donor (D) – acceptor (A) character. We have investigated the precise origin(s) of this effect using D and A substituents on the N‐aryl groups of these compounds. The lowest excited singlet state (S1) of N‐arylphthalimides is an intramolecular charge transfer (ICT) state. A nominally twisted compound, N‐(2‐t‐butylphenyl)phthalimide, shows greatly diminished CT absorption and blue‐shifted fluorescence with reduced quantum yield when compared to the 4‐t‐butyl isomer with an identical N‐aryl donor group. It therefore seems unnecessary to claim that the ICT state of phthalimides is a so‐called TICT state. Quantum yield and fluorescence lifetime measurements lead to the conclusion that enhanced internal conversion from the ICT state (S1) to the ground state makes a significant contribution to photostabilization of these compounds by suppressing formation of the reactive triplet state. Further stabilization of polymer films may be afforded by triplet state self‐quenching which is enhanced for 6F‐ODA in increasingly poor solvents. N‐alkylarylphthalimides in which the aryl and phthalimide groups are not formally conjugated but, rather, joined by flexible methylene ‘spacers’, exhibit a different kind of fluorescent intramolecular CT singlet state whose formation can also stabilize these compounds by suppressing triplet state formation.
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