An aqueous ratiometric fluorescence probe for Zn(II)

We report herein the first example of the use of metal promoted intramolecular excimer formation using a system derived from a simple linear polyamine. The system demonstrates that considerably less preorganization is necessary than that used in cyclic systems. The water solubility results in the first aqueous operative ratiometric fluorosensor for Zn2+ ions. Bis-9-anthrylpolyamines (e.g. 2) were prepared by the 4 Angstrom molecular sieve promoted condensation of the polyamine with a stoichiometrically appropriate amount of 9-anthraldehyde in CHCl2. Compound 2 (25 mu M) was titrated with Zn(ClO4)(2) in CAPS buffer (0.05 M) at pH 10.5. The most notable feature of this titration is the nearly 4-fold increase in fluorescence intensity at about 495 nm. In fact, after the addition of 1 equivalent of Zn+2 the normal anthracene fluorescence profile is no longer the dominant feature of the spectrum. Subtracting the spectrum with no Zn+2 from that with 1 equivalent reveals the dominant excimer spectrum. The mono substituted control shows only the expected chelation enhanced fluorescence based upon the complexation state of the benzylic amine nitrogen.