Dibromorhodamine-based photoredox catalysis under visible light for the colorimetric detection of Hg(II) ion

We demonstrated a colorimetric readout of Hg(II) ion through visible-light-induced polymerization, initiated by an Hg(II)-responsive photoredox initiator/catalyst under aerobic conditions. Thiosemicarbazide-functionalized 4′,5′-dibromorhodamine was synthesized as a stimuli-responsive photoredox catalyst. The rhodamine derivatives bearing thiosemicarbazide have been known as fluorogenic probes of Hg(II) ion with excellent sensitivity and selectivity. In this work, we incorporated a heavy halogen atom (i.e., Br) onto the xanthene core ring of the rhodamine derivative to improve the photoredox catalytic performance of rhodamine 6G, which exhibits a low catalytic performance, presumably due to the poor quantum yield of intersystem crossing. Thus, this photoredox catalyst led to the formation of a hydrogel with Hg(II) ion in a concentration of 10 μM, as the catalyst initiated free-radical polymerization of poly(ethylene glycol) diacrylate and N-vinylpyrrolidone with triethanolamine under visible light for 1 min.