Effects of proton and metal cations on the fluorescence properties of anthracene bearing macrocyclic polyether and polyamine receptors

An anthracene (AN) derivative bearing azacrown ether (1-aza-18-crown-6) and cyclen (1,4,7,10-tetraazacyclododecane) receptors (L1) has been synthesized. Effects of proton (H+) and metal cations (K+ and Zn2+) on the fluorescence properties of L1 have been studied in water and methanol. In water, H+ addition to L1 leads to a fluorescence intensity (IF) increase. Potentiometrical pH titration reveals that strong IF enhancement of L1 requires monoprotonation of both azacrown and cyclen moieties. Addition of Zn2+ and/or K+ is ineffective for IF enhancement because these cations do not coordinate with the azacrown moiety, leading to an electron transfer (ET) from the azacrown nitrogen to the photoexcited AN moiety. In methanol, the azacrown and cyclen moieties of L1 coordinate with K+ and Zn2+, respectively. Zn2+ or K+ addition to L1 shows very small or moderate IF enhancement because of ET from the uncoordinated azacrown or cyclen nitrogen to the excited AN. Addition of both K+ and Zn2+, however, still shows moderate IF enhancement, although both macrocycles coordinate with the respective metal cations. Ab initio calculation reveals that the azacrown–K+ coordination is weakened by an electrostatic repulsion between K+ and Zn2+ within the complex. This allows ET from the azacrown nitrogen to the excited AN, resulting in an insufficient IF enhancement.