Effects of Metal Cation Coordination on Fluorescence Properties of a Diethylenetriamine Bearing Two End Pyrene Fragments.

Fluorescence properties of a diethylenetriamine bearing two end pyrene fragments (L) have been studied in water, where effects of adding metal cations (Zn2, Cd2, Cu2, Hg2, Ag) on the emission properties of Lhave been studied. Without metal cations, Lshows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (IM) is strong at acidic pH but decreases with a pH increase because of an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene fragment. The excimer emission is due to the static excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The excimer emission intensity (IE) is weak at acidic pH but increases with a pH increase because of the GSD stability increase associated with the deprotonation of the polyamine chain. Addition of metal cations leads to IMdecrease, where chelation-driven IMenhancement does not occur even with diamagnetic Zn2and Cd2at any pH. This is because a pyrene−metal cation -complex, formed via a donation of -electron of the pyrene fragment to the adjacent metal center, suppresses the monomer photoexcitation. IEalso decreases upon addition of metal cations because the pyrene−metal cation -complex weakens -stacking interaction of the end pyrene fragments, leading to GSD stability decrease. The emission properties of L−Zn2complexes were studied by means of time-resolved fluorescence decay measurements, and the effects of adding a less-polar organic solvent were also studied to clarify the detailed emission properties. [ABSTRACT FROM AUTHOR]