Synthesis and Characterization\nof Eco-Friendly Water-Processable\nDual-Tone Photoresists Containing Photoreversible Coumarin Groups

In this study, we successfully synthesized and assessed\neco-friendly\nwater-soluble polymers containing photoreversible coumarin groups\nas dual-tone photoresists. The synthesized polymers do not require\nany photoinitiator or photoactive compound and can be used as both\nnegative- and positive-type photoresists by simply controlling the\nexposure conditions. The monomer, 7-(4-vinyl-benzyloxyl)-coumarin\n(VBCC), incorporating a coumarin group, was synthesized through etherification\nbetween 4-vinyl benzyl chloride and 7-hydroxycoumarin, with its chemical\nstructure confirmed by NMR and FT-IR spectroscopies. Subsequently,\nVBCC underwent polymerization with sodium <i>p</i>-styrenesulfonate\n(NaSS) to impart water solubility. The chemical structure of P(VBCC-<i>co</i>-NaSS) at various monomer ratios was also verified using\nNMR and FT-IR spectroscopies, while its photoreversible chemical reaction\nwas confirmed through UV–vis and FT-IR spectroscopic techniques.\nUnder 365 nm UV exposure, a cyclobutane ring formed via the photodimerization\nof coumarin groups, which was then cleaved and reverted to its original\ncoumarin structure under DUV exposure. The water solubility of P(VBCC-<i>co</i>-NaSS) increased with a higher NaSS amount, albeit at\nthe expense of photospeed. Considering the water solubility and photospeed,\nP(VBCC-<i>co</i>-NaSS) with a VBCC to NaSS ratio of 6:4\nwas chosen as the optimal photoresist. Well-defined negative- and\npositive-type patterns could be achieved depending on photolithographic\nprocesses. This study underscores the potential of P(VBCC-<i>co</i>-NaSS) as an environmentally friendly, water-processable,\nand photoreversible photoresist.