Metal–ligand interactions in benzodioxotetraaza-macrocyclic metal chelates

Pyrene bichromophores composed of polyaminopolycarboxylate interlink: pH response of excimer emission

“…This pH dependence is supposed to be related to the protonation -deprotonation L. Machi et al 666 equilibrium. The NMR study of a related naphthyl -EDTA compound, 3 in Scheme 1, has shown that the first protonation occurs at amino nitrogen of the EDTA chain with a logarithmic equilibrium constant of 7.84 between M 22 and MH 2 species, and the second protonation occurs at carboxylate oxygen below pD 5.5 to form MH 2 species (17); this protonation scheme is basically identical with that reported for EDTA and its bis(amide) derivatives (19,20,22,23). The first protonation on (edtapy) 22 is, therefore, supposed to occur at amino nitrogen to form (edtapy)H 2 around pH 8.…”

“…Added proton leads to a hydrogen-bond formation between amino nitrogen N(2) and amide oxygen O(1); the interatomic distances in N(2)ZH· · ·O(1) are 1.1 and 1.8 Å , respectively. Since (edtapy) 22 consists of two chemically equivalent moieties, the acidic proton in (edtapy)H 2 is exchanged between two amino nitrogen atoms [N(2) and N(3)]; in fact, the 1 H NMR spectra of similar EDTA derivatives show a single signal for equivalent CH 2 groups over an entire pH range studied (17,19,20,22,23). Concurrently with the proton exchange, the two moieties alternate their conformations.…”

“…the equilibrium between (dtpapy) 32 and (dtpapy)H 22 , occurs around that pD value. The d j value of proton j is given by the following function of pD (20):…”

“…Here, K D is a protonation constant in D 2 O, and d j0 and d j1 are the d j values of (dtpapy) 32 associated with the first protonation process, as seen in Figure 5, in which the scales of pD and pH are displaced by one unit as an aid for comparison because the pD value observed for a weak acid is about 0.8 higher than the corresponding pH value (20,30). The largest shift upon protonation was observed for CH 2 (d) and CH 2 (e) bonded to the central amino nitrogen.…”

“…Another obvious factor is that hydrophobic derivatives should be made soluble in water by introducing hydrophilic groups into interlinking chains. These requirements are expected to be satisfied by ethylenediaminetetraacetate (EDTA), which is a typical polyaminopolycarboxylate, as reported for naphthalene-based bichromophores (17,18); the conformation of the polyamino chain may be changed with pH, and the anionic carboxylate arms may endow a pyrene derivative with water solubility (19,20). These characteristics will be more pronounced in diethylenetriaminepentaacetate (DTPA), which bears a larger number of amino and carboxyl groups.…”