Hydrosoluble dendritic poly(ethylene oxide)s with zinc tetraphenylporphyrin branching points as photosensitizers

Abstract

Synthesis of dendrimer-like star-branched poly(ethylene oxide)s (PEOs) with two and three generations having Zn(II) tetraphenylporphyrin (ZnTPP) moieties at the core and at each branching point has been successfully achieved through an iterative convergent approach harnessing “click chemistry”. In the first step, a four-armed poly(ethylene oxide) star-shaped polymer was synthesized by the copper(I)-catalyzed azide–alkyne cycloaddition between α-azido poly(ethylene oxide) branches (N₃-PEO-OH) and 5,10,15,20-tetra(propargyloxyphenyl) Zn(II) porphyrin as the core. In the second step, a tetra-armed star PEO bearing 5-(4-phenoxy)-10,15,20-tris(propargyloxyphenyl) Zn(II) porphyrin at each branch end was prepared by divergence of the ω-hydroxyl PEO chain-ends through a Williamson-type etherification reaction. The resulting star PEO with 3 propargyl groups at each arm end was then subjected to a new “click chemistry” reaction with N₃-PEO-OH affording the dendrimer star-like PEO of second generation carrying 12 new ω-OH-PEO external branches attached to the four ZnTPP branching points. Repetition of the second step afforded the high molar mass third generation dendrimer-like PEO carrying 36 ω-OH PEO terminal branches with a uniform distribution of 17 zinc tetraphenylporphyrin moieties as internal branching points. The photophysical properties of the resulting polymers in organic or aqueous medium and their efficiency in singlet oxygen generation are reported.