Molecular aggregation and fluorescence spectra of aromatic polyimides

Abstract

Effects of molecular aggregation on emission behaviour have been studied, together with the u.v.-vis absorption spectra and DSC measurements, for several polyimides (PIs) prepared by the combination of biphenyltetracarboxylic dianhydride (BDA), pyromellitic dianhydride or benzophenonetetracarboxylic dianhydride with p-phenylene diamine (PPD) or 4,4′-diaminodiphenyl ether. The intensities of the fluorescence, previously attributed to the intermolecular charge-transfer (CT) interaction [1], increase sharply with annealing or with increasing imidization temperature for all the PIs studied. For PI(BDA/PPD), annealing induces not only increase of the intensity of the emission at 525 nm (CT 1) but also the appearance of a new emission at 540 nm (CT 2) which is considered to be due to a ground state intermolecular CT complex. The efficiency in energy transfer, measured by a depolarization technique, also increases on annealing. The CT emission spectra are also sensitive to change of molecular orientation. With increasing draw ratio of the films of polyamic acid PAA (BDA/PPD) which is a precursor of PI, the emission intensities of the resulting PI(BDA/PPD) increases for CT 1 but decreases for CT 2, showing the completely different natures of the two CT complexes.