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 -CH ₂CO ₂H 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)H ₆, whereas BPA yielded a 1:1-cyclization product carrying three -CH ₂CO ₂H 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)H ₃. Their protonation and complexation with Gd(III) were studied by fluorescence spectra, ¹H NMR and potentiometry. The macrocyclic framework of cy(dtpabpa)H ₃ has a high rigidity; as a consequence, this ligand is incapable of forming a complex with Gd(III). In contrast, cy(bisdtpadab)H ₆, 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 β _M2L = 28.3, log β _M2LH = 31.9, log β _M2LH-1 = 21.0, logβ _M2LH-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 Gd ³⁺ ion suggests that the chelating cyclophane works potentially as fluorescent probes toward specific metal ions.