Amphiphilic π-Conjugated Poly(m-phenylene) Photosensitizer for the Eu3+ Ion: The Role of Macromolecular Chain Aggregation on the Color Tunability of Lanthanides

Abstract

Here, we report new carboxylic functionalized poly(phenylene)s and their oligomers as selective and efficient photosensitizers for Eu(3+) ions. Palladium-catalyzed Suzuki polycondensation was developed for the synthesis of carboxylic functionalized π-conjugated materials. The chemical structures of the polymer skeleton were varied using two anchoring groups consisting of ethylhexyloxy and methoxy substitution in the chain backbone. The molecular weights of the polymer samples were obtained in the range of 4000-8000 containing 20 aromatic units in the chain. Photoexcitation of the oligomer-Eu(3+) complexes resulted in magenta color emission as a result of the combination of partial blue self-emission from the chromophores along with the red color from the metal center. The ethylhexyl substituted polymer-Eu(3+) complex showed complete excitation energy transfer from the macromolecular chains to the metal center and produced bright and sharp red emission. The polymer-containing methoxy unit was found to show largely self-emission rather than photoexcitation to the metal center. Singlet and triplet energy levels of the complexes and chromophores revealed that both oligomers and polymers have almost identical energy levels for photosensitizing Eu(3+) ions. The polymers possessed typical amphiphilic structures via a rigid aromatic hydrophobic core and hydrophilic anionic periphery for self-organization in water. Both dynamic light scattering and atomic force microscope analysis confirmed the existence of the spherical shape nanometer size aggregates of the polymer chains in water. The branched ethylhexyl polymer showed the formation of loosely packed 500 nm aggregates whereas tightly packed 200 nm particles are produced by the methoxy substituted rigid polymer. These molecular aggregates behaved as templates for complexation as well as photosensitizing of the Eu(3+) ions. The loosely packed nanoaggregates (ethylhexyl polymer) contain Eu(3+) ions in the entire scaffold and showed efficient and complete energy transfer from the conjugated chain to metal ions. The tightly packed rigid-chains in methoxy polymer restricted the complete energy transfer to metal center. The molecular self-organization of the polymers played a crucial role on the efficient energy transfer from the polymer chain to metal center, more specifically Eu(3+) ion-based red emission.