Charge Transfer States in the Photophysics and Photochemistry of Polyimides and Related Phthalimides
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.
Hoyle, C. E.,
Jordan, J. W.,
Pandey, C. A.,
Peeler, A. M.,
(1997). Charge Transfer States in the Photophysics and Photochemistry of Polyimides and Related Phthalimides. Macromolecular Symposia, 116, 1-13.
Available at: http://aquila.usm.edu/fac_pubs/5315