Metal ion recognition by L 2

Chapter 3 Supramolecular Photochemistry of Naphthalene and Quinoline based systems

3.1.3. Metal ion recognition by L 2

Chapter-3 Supramolecular Photochemistry of Naphthalene and Quinoline based systems

Chapter-3 Supramolecular Photochemistry of Naphthalene and Quinoline based systems

Overall intermolecular non covalent interactions results in the formation of 2D hydrophilic–hydrophobic channel network (Figure 3.1.19.b) and it also form a capsule network in the solid-state along a axis (Figure 3.1.19.c).

Figure 3.1.19. (a) ORTEP plot of complex L2AgNO3. H-atoms are omitted for clarity; (b) 2D hydrophilic–

hydrophobic channel network along the c axis; (c) Formation of capsule network along a axis.

3.1.3c. Absorption spectroscopy

The UV-visible absorption spectra (Figure 3.1.20) of L2 (10^-5 M) was recorded in dry THF at 298 K. The absorption spectrum is found to be overlapping of two transitions in the 270–310 nm regions. The π→π* transitions of the naphthalene unit appear at 283 (ε = 29400 M^-1cm^-1) and 294 nm (ε = 33300 M^-1cm^-1).^3.1.4 The band at ~250 nm is assigned to the π–π interaction between the naphthalene units of the ligand.^3.1.10 When L2 is titrated against different metal salts, an absorption peak is decreases with increasing the concentration of metal salts. But in Ag(I) salts we have seen one additional peak is

Figure 3.1.20. (a) UV-visible absorption spectra of L2 and Ag complex at 298 K, L2 in UV light (left side) and L2AgNO3 in UV light right side; (b)Intermolecular π interaction in solid state ofAg(I)-L2 complex.

Chapter-3 Supramolecular Photochemistry of Naphthalene and Quinoline based systems

showing ~ 375 nm (Figure 3.1.20.a) which may be due to intermolecular π interaction of naphthalene moieties, which is reflected in solid state behavior of single crystal of the corresponding complex (Figure 3.1.20.b).^3.1.18 In addition in presence of UV light we have seen a blue color fluorescence solution of ligand L2 dry THF but incomplex it shows red color fluorescence (Figure 3.1.20.a).

3.1.3d. Fluorescence spectroscopy

The emission spectrum of L2 was recorded in dry THF at 298 K. Free L2 shows a locally excited structured monomer emission of naphthalene. The structured non-solvatochromic naphthalene emission of L2 was observed with (0,0) band centered at 323 nm (Figure 3.1.21a) along with vibrational structures at 338 and 352 nm when excited at 300 nm. The fluorescence titration of ligand L2 with various metal ions was conducted to examine the selectivity.

Figure 3.1.21. (a) Emission spectra of L2 and L2-Ag complex in dry THF solvent; (b) Schematic representation showing the change of fluorescence quantum yield (ΦF - Φq) of L2 upon addition of the different metal salts. Φ_F and Φ_q are quantum yields of L2 in absence and presence of metal salts, respectively.

When L2 is titrated against different metal salts like Na⁺, K⁺, Ca²⁺and Mg²⁺ etc there is no change in the spectral behavior. But transition metals, e.g. Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, quenches the fluorescence, similar to heavy metal like Cd²⁺,Hg²⁺, Pb²⁺. But addition of Ag(I) results in the formation of an extra peak at higher wavelength (~ 420 nm) which is strongly support the preferred interaction in solution phase (Figure 3.1.21b).^3.1.19

In the case of the solution-state study by ¹H NMR, when AgNO3 was added to the DMSO- d₆ solution of L₂, the ¹H NMR spectrum shows a shift in the position of the NCH₂ proton from δ 3.41 ppm to the higher δ value of 3.64 ppm. This shift indicates the influence of the protonated apical nitrogen on the neighboring proton. Similar changes observed in the solution phase with other anions also (Figure 3.1.22.a and 3.1.22.b).

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Figure 3.1.22. ¹H NMR spectra of (a) L2 and (b) Ag(I) complex of L2 in DMSO-d6 (aliphatic region).

3.1.3e. Summary

The development and improvement of highly selective tripodal receptors or ionophores that allow measurements of analytes in complex real-life samples will remain a main issue in the development of chemosensors either in the fluorescence or the optical mode. The interest of many organic chemists in tripodal host compounds is expected to result in the development of many new interesting receptors, but where the low partitioning of ions and the limited solubility of several tripodal receptors might restrict their applicability. Here in. the effect of adding metal cations (Na¹⁺, K¹⁺, Ca²⁺, Mg²⁺, Cu^2+, Zn^2+, Cd²⁺, Cu²⁺, Hg²⁺, Ag⁺) on the fluorescence properties of a Tris-[2-(naphthalen-2-yloxy)-ethyl]-amine (L2) bearing three end naphthalene fragments, We found that L2 behaves as the first molecular switch capable of quenching naphthalene excimer emission by the addition of all of Ag⁺ metal cations in freshly prepared THF solvent. In summary, we have developed a new fluorescent sensor for AgI with remarkably high selectivity and sensitivity. Moreover, this molecule makes it possible to detect the Ag(I) cation potentially. We look forward to witnessing and participating in the creative and innovative development of selective, sensitive, and accurate sensors for cations and anions.

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PART - II