Unexpected Absorbance Enhancement upon Clustering Dyes in a Polymer Matrix

PE films grafted with poly­(methyl acrylate) and labeled with pyrene groups were obtained by irradiation with γ-rays in the presence of acryloyl chloride and further reacting them with 1-pyrenebutanol or 1-pyrenemethylamine. Characterization of the polymer films benefited from the dual use of the pyrene probe as an indicator of, first, polymer chain dynamics by monitoring pyrene excimer formation by fluorescence and, second, polymer morphology by staining the pyrene-rich domains of the films with RuO₄ for scanning electron microscopy (SEM). The grafted polymers labeled with 1-pyrenemethylamine showed much stronger absorbance than those labeled with 1-pyrenebutanol despite having similar pyrene contents. The fluorescence spectra of the grafted polymers labeled with 1-pyrenebutanol exhibited monomer emission, whereas those labeled with 1-pyrenemethylamine exhibited exclusively excimer emission. These dramatic differences could be accounted for by noting that labeling of the grafted poly­(acryloyl chloride) with 1-pyrenemethylamine results in cross-linking of the polymer matrix, with an associated enhancement of the concentration of pyrene in the cross-linked domains, which was confirmed by SEM. Formation of discrete domains in the polymer film can induce multiple scattering at the domain boundaries which lengthens the path of light in the film and increases absorption of the light by the tightly packed pyrene-rich domains. Implementation of this effect for fabrication of plastic color filters should generate more efficient filters which should find numerous practical applications