Fluorescent aza-cyclophanes derived from diethylenetriaminepentaacetic acid (DTPA), and their complexation with Gd(III)

Chelating aza-cyclophanes were synthesized by reactions of diethylenetriaminepentaacetic acid (or DTPA) dianhydride with aromatic diamines, 4,4′-diaminobibenzyl (DAB) and 4,4′-bis(1,1′-biphenyl-4, 4′-diyldioxy)dianiline (BPA). The former amine gave a 2:2-cyclization product bearing six -CH2CO2H arms, 2,12,29,39-tetraoxo-4, 7,10,31,34,37-hexakis(carboxymethylene)-1,4,7,10,13,28,31,34,37,40-decaaza-[13. 2.13.2]ethylenparacyclophane, abbreviated as cy(bisdtpadab)H6, whereas BPA yielded a 1:1-cyclization product carrying three -CH2CO2H arms, 2,12-dioxo-4,7,10-trakis(carboxymethylene)-1, 4,7,10,13-pentaaza-20,33-dioxa[13.0.1]paracyclophane, abbreviated as cy(dtpabpa)H3. Their protonation and complexation with Gd(III) were studied by fluorescence spectra,1H NMR and potentiometry. The macrocyclic framework of cy(dtpabpa)H3has a high rigidity; as a consequence, this ligand is incapable of forming a complex with Gd(III). In contrast, cy(bisdtpadab)H6, which is moderately rigid, forms a binuclear Gd(III) complex, as confirmed by a sensitive quenching of the fluorescence upon complexation. On the basis of the pH dependence of the fluorescence spectra, the logarithmic overall formation constants were determined as log βM2L= 28.3, log βM2LH= 31.9, log βM2LH-1= 21.0, logβM2LH-2= 9.8. For the chelating cyclophanes, the rigidity is one of major controlling factors for the stability of their complexes. The sharp change in the fluorescence intensity upon complexation with Gd3+ion suggests that the chelating cyclophane works potentially as fluorescent probes toward specific metal ions. © 2012 Elsevier Ltd. All rights reserved.