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

The
π-contact hypothesis, that quenching of the fluorescence
of complexes of photoinduced electron transfer sensors with heavy
diamagnetic metal ions may be caused by π contacts between the
metal ion and the fluorophore of the sensor, is examined with a study
of the fluorescent properties of the sensor 4-[[bis­(2-pyridinylmethyl)­amino]­methyl]-6,7-dimethoxy-1-benzopyran-2-one
(cdpa) and the structures of its complexes with some metal ions. The
coumarin-type fluorophore of cdpa is a weaker π-contact former
than the anthracenyl fluorophore of the analogue adpa (Inorg.
Chem. 2014, 53, 9014): only
AgI, the strongest π contact former, quenches the
fluorescence of cdpa, apart from paramagnetic CuII and
NiII, which quench fluorescence by a redox mechanism not
requiring π contacts. The structures of [Ag­(cdpa)­NO3] (1), [Pb­(cdpa)­(NO3)2] (2), [Zn­(cdpa)­(NO3)2] (3), [Cd­(cdpa)­Cl2]2 (4), [Cd­(cdpa)2H2O]­(NO3)2 (5), and [Hg­(cdpa)2H2O]­(NO3)2 (6) are reported. Structure 1 shows that
AgI is the only metal ion studied that forms π contacts
with the fluorophore of cdpa in the solid state: Ag···C
η2 π contacts of 3.083 and 3.095 Å, in
line with quenching of the fluorescence of the AgI(cdpa)
complex. In contrast, PbII, ZnII, and CdII show chelation-enhanced fluorescence in their cdpa complexes,
and the structures of 2–4 show that
the fluorophore of cdpa in each case forms no π contacts. By
contrast, the adpa complexes of PbII and CdII show π contacts with its more strongly π-contacting
fluorophore (Inorg. Chem. 2014, 53, 9014). The structures of 5 and 6 show bis-complexes of cdpa: the coordination geometries of CdII and HgII are discussed in relation to the number
of covalently bound donor atoms present. The preferred hapticity of
π-contacted metal ions is evaluated from the literature structures,
suggesting that d10 metal ions such as AgI and
HgII, and tetragonally distorted CuII and PdII, prefer η1 and η2 π
contacts, while more ionically bound metal ions such as KI, BaII, and LaIII, as well as d10s2 metal ions such as TlI, PbII,
and BiIII, prefer η6 contacts.