Complex metal ions, fluorescent chromophores

Chromoionophores of crown 4 and spherand cyclophane-type 5 with 2,4-dinitrophenylazophenol groups as a chromophore were also designed [7 a]. They show marked color changes from yellow into blue on complexation with cations in ethanol and chloroform solutions. This chapter describes selective complexation and coloration of azophenol hosts or fluorescent emission of benzothiazolyl crowns with alkaline metal ions and alkyl amines. 

The first concept to be considered is cation displacement of a complexed quencher of fluorescence. Scheme I illustrates the concq>t. To be operable the scheme requires a quencher that is complexed by a crown ether, a metal ion of interest that is not a quencher but is complexed effectively, and a crown ether ring that orients the complexed quencher so it will effectively quench the chromophore fluorescence. Scheme II shows a relatively simple crown ether system that we hoped would fulfill these requirements(22,2i). The l,5-naphtho-22-crown-6 compound was selected because of the ability of its crown ether band to hold a quencher against the face of the pi system of the naphthalene chromophore. The heavy atom ion Cs+ was selected as a quencher based on its propensity to increase inter-system crossing from the fluorescent Sj state to the nonfluorescent T state(i,2). It was likely, based on previous results(2), that potassium ion would be complexed by the crown, but not quench naphthalene fluorescence appreciably. 

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