A new indole incorporated chemosensor exhibiting selective colorimetric and fluorescence ratiometric signaling of fluoride

We have synthesized an internal charge transfer based chemosensor, Q-1 utilizing π-rich indole as the H-donor and the cyano-quinazolinone ring as the π-acceptor. The probe is the first example of an indole incorporated receptor that exhibits both absorbance and emission red shifts in the presence of fluoride, offering dual naked eye detection (yellow to red) as well as fluorescence ratiometric capability. The 1H NMR study supports the F−-induced deprotonation of the indole N-H bond, a process that significantly perturbs both the ground as well as the excited states of the probe via the electronic charge shift. It is noteworthy that in contrast to fluoride, the commonly competing AcO− and H2PO4- as well as HSO4-, Cl−, Br−, I−, NO3- and SCN− induced none or relatively negligible optical perturbation even at significantly higher concentrations.