Perturbation of the PET Process in Fluorophore−Spacer−Receptor Systems through Structural Modification: Transition Metal Induced Fluorescence Enhancement and Selectivity

Several fluorescent signaling systems are built in the format fluorophore−spacer−receptor with ethylenediamine
or N,N-dimethylethylenediamine as the receptor, anthracene as the fluorophore, and a methylene group as the
spacer. The receptors are derivatized with different electron-withdrawing groups such as 4-nitrobenzene, 4-nitro-2-pyridine, and 2,4-dinitrobenzene, to perturb the photoinduced intramolecular electron transfer (PET) process
from the nitrogen lone-pair to the fluorophore. The photophysical properties of these supramolecular systems
and their fluorescence responses toward a number of quenching transition metal ions are reported. It is shown
that the PET is highly efficient in the absence of a metal ion. With a metal ion input, the fluorescence can
be recovered to a different extent depending on the nature of the metal and on the overall architecture of the
system as well. Despite the possibility of strong interaction between the fluorophore and the metal ion,
significant fluorescence enhancement is observed with quenching of paramagnetic transition metal ions. The
complex stability data show that the stability constants for the metal ions showing fluorescence enhancement
are of the order of 104 M-1. This study shows that structurally simple fluorescent signaling systems for
quenching transition metal ions can be built by maximizing the PET. It is also shown here that simple structural
modification can make these systems highly specific for particular transition metal ions for potential applications
in several contemporary areas of research.