PHOTOPHYSICAL PROCESSES IN AROMATIC POLYIMIDES - STUDIES WITH MODEL COMPOUNDS

Emission mechanism in an aromatic polyimide, PI(BPDA/PDA), derived fro m biphenyltetracarboxylic dianhydride and p-phenylene diamine were stu died with ultraviolet visible absorption and fluorescence spectra of a series of the model compounds. The excitation spectrum of the intermo lecular charge-transfer (CT) fluorescence peaking around 550 nm of PI( BPDA/PDA) thin film was completely consistent with the absorption spec trum, indicating that the intermolecular CT fluorescence emission of P I (BPDA / PDA) film is not caused by direct excitation of the CT absor ption band, but by light absorption due to structural units in the pol ymer backbone. The UV-vis. absorption spectra of the model compounds c orresponding to the structural units in PI(BPDA/PDA) showed that the l ongest wavelength absorption band is due to the biphenylbisimide moiet y. The band was assigned as pi, pi transition with the polarization s pectrum of the model compound. The fluorescence spectra of the model c ompounds changed sensitively depending on the conformation around N-ph enyl bond. The lifetime measurement for the model compounds suggested that intramolecular CT process occurs very rapidly. (C) 1993 John Wile y & Sons, Inc.