Transition-metal-induced fluorescence resonance energy transfer in a cryptand derivatized with two different fluorophores.

A laterally nonsymmetric aza cryptand has been derivatized with one 7-nitrobenz-2-oxa-1,3-diazole (fluorophore(1)) and one/two anthracenes (fluorophore(2)) to obtain 1 and 2. Their emission characteristics are probed in the presence of a number of transition metals and proton. In the case of 1, Cu(II), Zn(II), Cd(II), and proton afford a large enhancement of fluorescence, whereas Fe(II) and Ag(I) exhibit one order of magnitude less enhancement. In contrast, 2 gives a large enhancement with Cu(II), Ag(I), and proton. The enhancement is observed in the diazole moiety even when the anthracene fluorophore is excited because of substantial fluorescence resonance energy transfer from anthracene to the diazole moiety. Compounds 1 and 2 can be termed as the second-generation fluorescence signaling systems.