The Effect of π Contacts between Metal Ions and Fluorophores on the Fluorescence of PET Sensors: Implications for Sensor Design for Cations and Anions

Abstract

The idea that M···C π contacts between diamagnetic heavy metal ions such as Pb(II), Ag(I), Pd(II), or Hg(II) and the anthracenyl fluorophore of adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine) are responsible for quenching the fluorescence of the complexes of these metal ions with adpa is explored crystallographically. The structures of [Pb(adpa)(NO3)2] (1), [Ag(adpa)NO3] (2), [Pd(adpa)NO3]NO3 (3), [Zn(adpa)(NO3)2] (4), and [Cd(adpa)Br2] (5) are reported. The π contacts with the fluorophore are for 1 are a Pb···C π contact of 3.178 Å; for 2, an Ag···C π contact of 3.016 Å; and for 3, a Pd···C π contact of 2.954 Å on the axial site of the Pd(II) ion. The Zn(II) ion in 4 has no Zn···C π contact, with the anthracenyl fluorophore rotated completely away from the Zn(II) ion. These structures confirm that in the Pb(II), Ag(I), and Pd(II) complexes of adpa, which experience strong quenching of fluorescence, there are strong M···C π contacts, as expected if it is the π contacts that quench fluorescence. In contrast, for the Zn(II) adpa complex, which forms no π contact, there is a strong increase in fluorescence intensity. The structure of 5 shows a long Cd···C π contact at 3.369 Å, in contrast to a previously reported structure with two coordinated nitrates where the Cd···C π contact is 3.097 Å. The long Cd···C π contact in [Cd(adpa)Br2] suggests how coordination of Br(-), as well as other more covalently bound ligands such as Cl(-), SCN(-), and S2O3(2-), cause an increase in fluorescence intensity, reported for the Cd(II)adpa complex in 50% CH3OH/H2O. Coordination of covalently bound ligands to the Cd(II) weakens the Cd···C π contact and so enhances fluorescence, whereas more ionically bound ligands such as SO4(2-), NO3(-), or H2O produce a strong Cd···C π contact and weakened fluorescence. Complexes of the Cd(II)/adpa type may form the basis for a new type of anion/small molecule sensor. The tendency of metal ions to form π contacts with aromatic groups is analyzed in terms of the frequency of occurrence of π contacted structures in the literature, as well as by DFT calculations on the adpa complexes.

Controlling the Fluorescence Response of PET Sensors via the Metal-Ion π-Contacting Ability of the Fluorophore: Coumarin, a Weaker π Contacter

“…Of particular interest is the presence of two π contacts between the Ag I ion and the dimethoxycoumarin fluorophore, of 3.083 and 3.095 Å. The structure of the Ag I (cdpa) complex strongly resembles that of the Ag I (adpa) complex, except that the latter has a η 1 contact with the anthracenyl fluorophore of adpa, with an Ag···C distance of 3.016 Å, 3 whereas the Ag I ion in its cdpa complex has a η 2 π contact. The presence of π contacts between the coordinated Ag I and the fluorophore of cdpa is consistent with the π-contact model of quenching of the fluorescence, in that coordination of Ag I to cdpa leads to strong quenching of the fluorescence, as also occurs with the adpa complex, where an Ag···C π contact is present.…”

“…Also shown in Table 9 for comparison are the formation constants for dpa (in aqueous solution) 42 and adpa (in 50% MeOH/H 2 O) complexes. 1,3 The fact that the log K 1 values for the dpa complexes are reported in an aqueous solution should not make a great difference because it has been found that log K 1 values for 50% MeOH/H 2 O and H 2 O are quite similar. 5 One sees that the order of stability for most metal ions with the three ligands is dpa > adpa > cdpa.…”

“…This is not an ordering of the absolute strength of π-contact formation but includes the ease of displacement of a competing solvent molecule or other ligand by the aromatic group that forms the π contact. 3 In solution, the ability to form π contacts appears to be related to low solvation energies, so that such an ability is favored by metal ions of lower charge, particularly alkali-metal ions, Tl I and Ag I . 3 Because anthracene is a strong π-contact former, one finds that all of the above metal ions cluster together to form π contacts with adpa in solution, with only Zn II and Ni II not forming a π contact and the intermediate Cd II displaying structures with and without π contacts.…”

“…2,3 In this paper, the coordination geometry of heavy d 10 metal ions and how the problem of distortion of the coordination geometry to produce long Hg−N or Ag−N bonds that lead to a PET effect and quenching of the fluorescence might be overcome are further considered.…”

Design of a novel Schiff-base fluorescent sensor for Al3+: experimental and computational studies

“…could be better exhibited by fluorescence titrations. After addition of Al 3+ , the fluorescence intensity increased significantly which was because that the chelation of the nitrogen atom of the -C=N group with Al 3+ resulted in the efficient inhibition for the PET process of the lone pair electrons on N of the -C=N-group[35][36][37][38].3.3. The sensing mechanism was .proposed base on the chelation as shown in Scheme 2.…”