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A fast-response, highly selective and sensitive chemosensor, 3, for Cr 3+ detection with turn-on fluorescence behavior in the physiological pH range was designed and synthesized. The chemosensor contained a combined push-pull system in which the fluorescent phenanthro[9,10-d]oxazole moiety acts as both the electron donor and a potential binding site. An electron deficient nitrile group served as the electron acceptor. A significant enhancement of fluorescene emission intensities was observed with increasing Cr 3+ concentration upon excition at 300 nm. The emission intensity reached its maximum on adding 8 equiv. of Cr 3+ where the quantum yield of 3-Cr 3+ was found to be 0.917, ca. 7-fold larger than the chemosensor 3.The selectivity mechanism of 3 for Cr 3+ was found to be based on the combined effects of the inhibition of ICT and CHEF. Remarkably the entire process was virtually complete in only 10 seconds, with a minimum detection limit for Cr 3+ of 1.72 × 10 -8 M -1 .

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