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

Abstract

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 Ag(I), the strongest π contact former, quenches the fluorescence of cdpa, apart from paramagnetic Cu(II) and Ni(II), 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 Ag(I) is the only metal ion studied that forms π contacts with the fluorophore of cdpa in the solid state: Ag···C η(2) π contacts of 3.083 and 3.095 Å, in line with quenching of the fluorescence of the Ag(I)(cdpa) complex. In contrast, Pb(II), Zn(II), and Cd(II) 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 Pb(II) and Cd(II) 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 Cd(II) and Hg(II) 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 d(10) metal ions such as Ag(I) and Hg(II), and tetragonally distorted Cu(II) and Pd(II), prefer η(1) and η(2) π contacts, while more ionically bound metal ions such as K(I), Ba(II), and La(III), as well as d(10)s(2) metal ions such as Tl(I), Pb(II), and Bi(III), prefer η(6) contacts.

Exciplex Formation and Aggregation Induced Emission in Di‐(N‐benzyl)cyclen and Its Complexes – Selective Fluorescence with Lead(II), and as the Cadmium(II) Complex, with the Chloride Ion

“…The order of ability to induce a CHEF effect with the 289 nm peak in Figure is what one would expect if the fluorescence of the DB‐cyclen ligand and its metal ion complexes was due to the fluorescing non π contacted benzyl groups projecting out into the surrounding solution, with little influence from the cyclen moiety or any complexed metal ions it might contain. Zn II shows negligible ability to form π contacts, so that one would expect the fluorescence at 289 nm to be simply that of any non‐π contacted benzyl groups: these peaks are also observed for the Cd II , Hg II , and Cu II complexes at 289 nm, although these are much less intense, presumably due to π contact formation in solution.…”

“…The idea that the increase in fluorescence intensity was caused by weakening of fluorescence‐quenching Cd ··· C π contacts was supported by structural studies that showed that the Cd ··· C π contacts present in the Cd II adpa complex were greatly lengthened on coordination of the Cl – , Br – , SCN – , and S 2 O 3 2– as compared to the less covalently binding NO 3 – anion, or, as predicted by DFT calculation, coordinated water molecules . The more weakly π contacting coumarin fluorophore of cdpa yields Cd II complexes that do not respond with increases in fluorescence intensity on coordination of anions, since no π contacts are present to be disrupted . The suggestion that fluorescence in adpa complexes was quenched by formation of π contacts was supported by DFT calculations …”

“…Pd II ca. Hg II < Ag I , . It was also thought that the presence of more than one fluorophore might lead to turn‐on fluorescent sensors if the metal ion was not capable of simultaneously forming π contacts with more than one fluorophore.…”

“…The structures of [Pb(DB‐cyclen)(ClO 4 ) 2 ] ( 1 ), [Cu(DB‐cyclen)(ClO 4 ) 2 ] ( 2 ), [Ag(DB‐cyclen)(ClO 4 )] ( 3 ), and [Cd(DB‐cyclen)(ClO 4 )H 2 O]ClO 4 ( 4 ) were determined to aid in understanding the fluorescence and complex stabilities observed. DFT calculations were also carried out to help interpret the results obtained: these have been particularly helpful in past studies in understanding, in particular, the role of π contacts in quenching fluorescence …”

Fluorescence Chemosensory Determination of Cu2+ Using a New Rhodamine–Morpholine Conjugate

“…The fluorescence and absorbance decreased upon addition of more than 55 µM and 45 µM Cu 2+ respectively, that might be respected to the paramagnetic nature of Cu 2+ [27,36,37]. Paramagnetic square-planar or tetragonally distorted metal ions such as Co 2+ , Cr 3+ , Cu 2+ , Fe 3+ , Ni 2+ , and Pd 2+ are able to quench the fluorescence of optical chemosensors which is presumptively undesirable for analytical purposes [38]. The paramagnetic effect is basically the result of an enhanced electronic states' mixing (considered as a formally forbidden intersystem crossing) of a ligand by means of the inhomogeneous magnetic field of the paramagnetic ion [39].…”