Chapter 25
Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch025
Photophysics, Photochemistry, and Intramolecular Charge Transfer of Polyimides Masatoshi Hasegawa, Yoichi Shindo, and Tokuko Sugimura Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274, Japan
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 s . 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. 11
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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) andfilms(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 © 1995 American Chemical Society
In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
MICROELECTRONICS TECHNOLOGY
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Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch025
by subsequent coupling between the radicals formed to form a crosslinking point(il) The present study also describes comparison of the photoreactivity for various benzophenonediimides and benzophenone (BP). EXPERIMENTAL SECTION Materials. The model compounds of biphenyl- and benzophenone-type polyimides were prepared from 3,43 ' ,4',-biphenyl tetracarboxylic dianhydride (BPDA) or benzophenonetetracarboxylic dianhydride (BTDA) with the stoichiometric amount of monoamines; dianhydride recrystallized from acetic anhydride was added to the dried Ν,Ν'-dimethylacetamide solution of the monoamine with continuous stirring at room temperature for 2 h, and then the solution was refluxed for 1 h. The acylation of aromatic and aliphatic monoamines with dianhydride occurred quantitatively as well as polycondensation of polyamic acid. The precipitate was recrystallized twice from a suitable solvent The abbreviations of amines used are as follows: p-phenylenediamine (PDA), cyclohexylamine (CHA), 2-methylcyclohexylamine (MCHA), 2-methylaniline (2MA), 2-isopropylaniline (2-iPrA), 2,6-diethylaniline (2,6-DEA), 2,4-dimethylaniline (2,4-DMA), 3-ethylaniline (3-EA). The symbol of model compounds was expressed using these abbreviations of dianhydrides and monoamines. Measurements. The uv-vis absorption spectra of the model compounds were recorded on Jasco Ubest-30 spectrophotometer. The corrected luminescence spectra of PI films and the model compounds were measured at room temperature and 77 K. Fluorescence lifetimes were measured using a single photon counting system equipped with a hydrogenflashlamp (fwhm: 2 ns, frequency: 30000 Hz, excitation: 300 nm) and sharp-cut filter in front of detector. Phosphorescence lifetimes τ of the model compounds and BP in a transparent rigid glass (MTHF/ethanol=9/1 ) were measured at 77 Κ using Hitachi 850fluorescencespectrometer equipped with a phosphorescence lifetime measurement apparatus based on the sampling method. The emission quantum yields for the model compounds in solution were determined by comparing with the integralfluorescenceintensity of quinine sulfate in IN H S 0 (
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700
500 Wavelength / nm
Figure 5. Fluorescence and excitation spectra of biphenyldiimides in CH2CI2. (Reproduced with permission from ref. 4. Copyright 1994 John Wiley & Sons, Inc.)
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