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

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 ((<b>N</b>-(9-anthracenylmethyl)-<b>N</b>-(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] (<b>1</b>), [Ag­(adpa)­NO₃] (<b>2</b>), [Pd­(adpa)­NO₃]­NO₃ (<b>3</b>), [Zn­(adpa)­(NO₃)₂] (<b>4</b>), and [Cd­(adpa)­Br₂] (<b>5</b>) are reported. The π contacts with the fluorophore are for <b>1</b> are a Pb···C π contact of 3.178 Å; for <b>2</b>, an Ag···C π contact of 3.016 Å; and for <b>3</b>, a Pd···C π contact of 2.954 Å on the axial site of the Pd­(II) ion. The Zn­(II) ion in <b>4</b> 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 <b>5</b> 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