Charge Transfer States in the Photophysics and Photochemistry of Polyimides and Related Phthalimides

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Polymers and High Performance Materials


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 (S-1) 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 stale 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 (S,) 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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Macromolecular Symposia



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