Synthesis and fluorescence on/off switching of hyperbranched polymers having diarylethene at the branching point
Katsuya Shimizu, Rémi Métivier, Seiya Kobatake
Journal of Photochemistry and Photobiology A: Chemistry
We designed and synthesized hyperbranched polymers (HBPs) having diarylethene (DE) at branching points and having fluorene (FL) and DE derivatives between branching points in order to investigate the effect of the polymer structure to fluorescence on/off switching efficiency induced by the photochromic reaction of DE. First, HBP was synthesized by polymerization of an FL monomer (FL-St) in the presence of a DE monomer with reversible addition-fragmentation chain transfer unit (St-DE-RAFT) in the feed ratio of [FL-St]/[St-DE-RAFT] = 10. The fluorescence intensity of the resulting HBP decreased linearly as a function of the photocyclization conversion of DE. The change in the fluorescence intensity accompanied by the photochromic reactions indicates that one DE closed-ring isomer can quench 16.9 neighboring FL moieties. However, the switching rate and on/off contrast of fluorescence were relatively low because the number of FL in the polymer chain is much larger than that of DE. Therefore, we synthesized two types of HBPs having DE positioned not only at the branching points but also integrated in the polymer chains with different branching degrees. The fluorescence intensity of FL for such HBPs showed a non-linear dependence on the photocyclization conversion of DE, and a significant emission decrease with high on/off contrast even in low conversion was demonstrated. One DE closed-ring isomer quenched 5.4–6.4 F L moieties, whereas the fluorescence quantum yield in the “on” state was found to be around 0.15. The quenching of FL molecules is more pronounced for higher branching degrees in the polymers, indicating that DE closed-ring isomer can quench efficiently FL molecules when located within the polymer chains as well as at the branching points of the polymer.
Absorption, Optical properties, Photo stability, Polymers, Fluorescence, FRET, Quantum yield, Materials