Fluorescence quenching of a poly(para-phenylene ethynylenes) by C60 fullerenes

Complex formation between fullerenes and poly(para-phenylene ethynylene) (PPE) is studied through fluorescence quenching of PPE by C60. The influence of side chain chemistry, particularly with respect to electron donating capability, and molecular weight on the complex strength is examined in detail. Fluorescence quenching measurements indicate complex formation on the order of 103–104 dm3/mol, six orders of magnitude larger than previously studied C60–small organic molecule complexes and an order of magnitude larger than C60–cyclic polymer systems. No significant difference in complex strength was observed among PPEs with different side chain chemistry, indicating that C60 interaction occurs largely with the conjugated PPE backbone. At the lower PPE molecular weights investigated, the association constant for complex formation remains constant; however, an increase is observed at the highest molecular weight studied. We attribute this increase to the C60 molecule quenching more than one PPE unit at once, resulting in enhanced quenching at higher molecular weights and larger association constant. Based on the strength of interaction and lack of side group contribution to the interaction, we conclude that π–π interactions between the C60 cage and PPE backbone are responsible for the comparatively strong interactions observed.