Unexpected remote effect in red fluorescent sensors based on extended APTRA

Herein is described the synthesis and spectroscopic characterizations of three new OFF–ON red-emitting and water-soluble sensors, CAXR (Clicked APTRA X-Rhodamine). These dyes are based on an extended APTRA (aminophenol triacetic acid) motif. Three different side chains were added by click chemistry in order to complete the coordination sphere with a chelate moiety composed of a triazolyl and an iminol. The fluorescent response (F/F0) of these probes follows the order: Cd2+>Zn2+>Pb2+>Hg2+. An important and unexpected effect of the side chain structure on the Kd was observed (up to one order of magnitude, Cadmium Kd from 252 to 21 μM). This remote effect of the side chains was studied by DFT calculations and was attributed to a twisted conformation of the CAXR-Py:Cd2+ complex.