Fluorescent, siderophore-based chelators. Design and synthesis of a trispyrenyl trishydroxamate ligand, an intramolecular excimer-forming sensing molecule which responds to iron(III) and gallium(III) metal cations

Two pyrene-labeled hydroxylamines, 5-Bn and 5-Bz, O-protected with the benzyl and the benzoyl group, respectively, have been prepared for the generation of siderophore-based new chelators incorporating both the pyrene chromophore and the hydroxamic acid functionality. 5-Bz formed the starting point toward the synthesis of the tripod-shaped trishydroxamate, 1. That trichromophoric ligand displays remarkable fluorescence emission properties (dual emission: ''monomer'' and excimer type) which are markedly and selectively modified by binding Fe(NO3)(3) and Ga(NO3)(3). Ferric ions induce a quasi total quenching of the pyrene fluorescence, whereas the nonquenching Ga(III) cations are observed to affect the value of the excimer-to-monomer fluorescence intensity ratio. Ethylenediaminetetraacetic acid (EDTA) competition reactions yielded an estimated value of 3.8 for log K* of the complex LFe in methanol/water (80/20 v/v), where K* = ([LFe][H+](3)/([LH(3)][Fe3+]) and L is the ligand in its totally deprotonated form. Compound 1 is the prototype of a new class of photoresponsive molecular systems which could act as sensitive probes for metal cation detection and recognition.