A Coumarin-Derived Fluorescence Probe Selective for Magnesium Debdas Ray and P. K. Bharadwaj* Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India Received December 10, 2007 Two different coumarin derivatives have been connected via an imine linkage to obtain a new fluorescence signaling system. This compound itself does not show any emission due to rapid isomerization around the CdN bond. However, in the presence of a Mg(II) ion, this isomerization is stopped because of bonding to the metal ion resulting in high-intensity (∼550-fold) emission. Other metal ions like Li(I), Ca(II), and Zn(II) show very little emission, while biologically relevant transition-metal ions do not show any emission. In this way, the Mg(II) ion can be detected in the presence of these ions. The design of fluorescent chemosensors is an active field of research for biological as well as analytical and environ- mental problems. 1 Selective detection of biologically im- portant cations such as Na(I), K(I), Mg(II), and Ca(II) is extremely important to ascertain their spatial concentration inside biosystems for understanding cell physiology. 2 In this respect, detection of Mg(II) in the presence of Ca(II), Na(I), and K(I) as well as biologically relevant transition-metal ions is of particular significance. Mg(II) is one of the most abundant divalent ions in the cell and plays a crucial role in cell proliferation and cell death. It also participates in the modulation of signal transduction, various transporters, and ion channels. 3–9 To understand its roll in regulating cellular processes, it is important to monitor the Mg(II) ion concen- tration in the intracellular compartments as well as its distribution throughout the body. 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