Photophysics, Photochemistry, and Intramolecular Charge Transfer of Polyimides

Abstract

Photophysical properties of a biphenyl-type polyimide (PI) were modeled by measuring the ultraviolet-visible absorption, emission spectrum and lifetime of its model compounds in dilute solution at room temperature and 77 K. On the basis of a photophysical model, the rate constant of the intramolecular charge-transfer (CT) process in the excited state of the biphenyldiimide model compound was estimated to be more than 5x10 11 s -1 . The relationship between the intramolecular CT and photoreaction, i.e., the photo-induced hydrogen abstraction for a benzophenone-containing PI, was discussed on the basis of the photochemical kinetic parameters for its model compounds, determined by the Stern-Volmer analysis. A relation between the intramolecular CT character of the model compounds and the kinetic parameters led to the postulation of a photophysical mechanism in which intersystem crossing followed by the hydrogen abstraction competes with the intramolecular CT process followed by effective deactivation.Polyimide precursor, polyamic acids (PAA) are generally polymerized from aromatic dianhydride and aromatic diamine. The reaction proceeds through CT interaction between dianhydride as an electron acceptor and diamine as a donor.(i) It has been believed that yellow color of model compound crystals(2) and films(3) of aromatic Pis such as KAPTON results from the CT interaction. However, these systems include necessarily contribution of both intra-and intermolecular interactions. The aim of this work is to elucidate the presence of CT interactions in aromatic Pis and to examine relationship between the intramolecular CT and photophysical(4) and photochemical(5-8) properties of Pis. We focus on the model compound systems in dilute solution which precludes intermolecular interactions.A biphenyl-type PI was selected for photophysical studies because the films of polypyromellitimides are relatively nonfluorescent.(9,10) For photoreaction studies, a benzophenone-type PI was chosen. This PI is known to be useful as a thermally stable photoresist material. Upon uv-irradiation the triplet state of the benzophenonediimide unit in the PI main chain, formed by intersystem crossing, abstracts intermolecularly hydrogen atom from the adjacent alkyl groups, followed 0097-6156/95/0614-0379$12.00/0

Soluble Polyimides with Polyalicyclic Structure. Part 9. A Polyalicyclic Polyimide Bearing Spiro Structure.

“…The origin of the coloration is known to be caused by their highiy conjugated aromatic structures and/or the charge-transfer(CT). [11][12][13] Consequently, they cannot be used in areas where colorlessness and transparency are important require- heptane structure and aromatic diamines, although it was essential that a mixture of triphenylphosphite and pyridine (TPP/Py) was added during the course of the polymerization process. [27] The obtained polyimide film showed a cutoff at around 280 nm and was transparent and colorless.…”