Chiral Main Chain Polymers as OFF-ON Chemosensor for the Fluorometric Detection of Zn 2+
4.3 Present Study
Scheme 7
Figure 1. Fluorometric titration: (a) P1 (λex = 360 nm) and (b) P2 (λex = 360 nm) (1 x 10-5 M in THF) with different metal nitrate salts (3 x 10-4 M in water, 1 equiv with respect to the monomeric unit).
Figure 2. The polymer P2 (1 x 10-5 M in THF): (a) fluorescence image without Zn2+ and (b) fluorescence image with 1 equiv Zn2+ (3 x10-4 M in water) (both are excited using a commercially available UV lamp (λ = 360 nm). (c) The selectivity of P2 (1 x 10-5M in THF) with metal ions (3 x10-4 M in water).
Figure 3. Fluorescence spectra of 7 and P2 (1 x 10-5 M in THF) Without/With 1 equiv of Zn2+
(3 x 10-4M in water, λex = 360 nm).
The polymer P2 with Zn2+emits a strong blue fluorescence, due to the formation of Zn(II)- polymer complex that can reduce the non-radiative decay of the excited state of the Zn(II)- polymer complex as well as the quenching contributed from the photoinduced-electron- transfer (PET) of the lone pair of the electrons of nitrogen.22 Furthermore, a blue shift (~50 nm) in the emitted light was observed that could be caused by reduction in the HOMO energy level of the polymer upon the formation of the Zn(II)-polymer complex.23 The polymer P2 that gave the best fluorescence enhancement was further studied with varied Zn2+
concentrations (Figure 4). The absorption band at 290 nm slightly shifted to 293 nm with a gradual decrease of intensity. The isosbestic points at 296 and 366 nm indicate the formation of Zn(II)-polymer complex.
\Figure 4. UV-vis titration of P2 (1 x 10-5 M in THF) with 0.1-1.1 equiv of Zn2+ ion (3 x 10-4 M in water with respect to the monomeric unit of P2).
Figure 5. (a) Fluorometric titration of P2 (1 x 10-5 M in THF, λex = 360 nm) with 0.1-1.1 equiv of Zn2+ ion (3 x 10-4 M in water, with respect to the monomeric unit of P2). (b) Hill plot of polymer P2 (1 x 10-5 M in THF) with 0.1-1 equiv of Zn2+ (I = fluorescence intensity of Zn(II)-polymer complex at a given concentration of Zn2+, Imin = fluorescence intensity of polymer, Imax = saturated fluorescence intensity of Zn(II)-polymer complex).
Next, the sensing selectivity of the polymer P2 with different Zn2+concentrations was pursued (Figure 5a). The weakly emissive P2 (THF, 1.0 x 10-5 M) at ca. 519 nm, upon addition of Zn2+ (3 x 10-4 M), exhibited a blue-shifted emission at 471 nm. The enhancement in the intensity of the emission band was proportional to the Zn2+ concentration.When the Zn2+concentration was reached to 1.1 equiv with respect to the monomer unit of the polymer P2, the fluorescence intensity increased to maximum with a 22-fold enhancement. Further enhancement in the addition of Zn2+ concentration led to no change in the fluorescence intensity. By applying a Hill plot to this titration data (Figure 5b), the stability constant of Zn(II)- polymer P2 complex was calculated as 1.62 x 108 M-1.24
Figure 6. Metal ion interference with Zn(II)-polymer complex: I = fluorescence intensity of the Zn(II)-polymer complex, derived from P2 and Zn2+, in the presence of other metal salts (3 x 10-4 M, 1 equiv) and Io = fluorescence intensity of the Zn(II)-polymer complex, derived from P2 and Zn2+ (1 x10-5 M in THF).
The interference effect of other metal ions to the fluorescence of Zn(II)-P2 complex was also investigated. The addition of metal ions such as Na+, Ag+ and Cd2+ did not affect the
fluorescence intensity of the Zn(II)-polymer complex (Figure 6). In contrast, the addition of metal ions such as Ni2+, Co2+ and Pb2+, and Cu2+, Al3+, Ce3+ and Fe3+ ions to the solutions of the Zn(II)-polymer complexes led to quenching of the fluorescence of the Zn(II) complex.
Next, the metal ions (Na+, Ag+ and Cd2+) and their mixture (1:1:1) concentrations were increased from 1 to 25 equiv with respect to the Zn(II)-polymer complex whose fluorescence was measured (Figure 7). However, no change in the fluorescence intensity of the Zn(II)- polymer complex was observed.
Figure 7. Metal ion interference with Zn(II)-polymer complex: I = fluorescence intensity of the Zn(II)-polymer complex, derived from P2 and Zn2+, in the presence of Na+, Ag+, Cd2+ (3 x 10-2 M, 25 equiv) and Io = fluorescence intensity of the Zn(II)-polymer complex, derived from P2 and Zn2+ (1 x10-5 M in THF).
In conclusion, the non-linear polymers P1-P2 could serve as highly selective fluorescent chemosensor toward Zn2+. The polymer P2 having (R,R)-1,2-diphenylethylenediamine with Zn2+ has exhibited the best fluorescence enhancement as an OFF-ON chemosensor. The association constant of polymer P2 with Zn2+ was found to be 1.62 x 108 M-1.
Experimental Section
General. Ce(NO3)3·6H2O (≥98%), Cd(NO3)2·4H2O (≥99%) and Zn(NO3)2·6H2O (98%) were purchased from Aldrich. Co(NO3)2·6H2O (97%), Cu(NO3)2·3H2O (≥99%), Ni(NO3)2·6H2O (98%), Al(NO3)3·9H2O (≥95%) and NaNO3(>99%) were purchased from Merck.
Fe(NO3)3·9H2O(98%), Pb(NO3)2(98%) and AgNO3(99%) were purchased from Rankem and used as received without further purification. Column chromatography was carried out with Rankem 60-120 mesh silica gel. Analytical TLC was performed with Rankem silica gel G and GF 254 plates. NMR spectra (400 MHz for 1H and 100 MHz 13C) were recorded on DRX-400 Varian spectrometer using CDCl3 and DMSO-d6 as solvent and Me4Si as an internal standard.
Chemical shifts (δ) are reported in ppm and spin-spin coupling constants (J) are given in Hz.
Melting points were determined using Buchi B-540 and are uncorrected. IR spectra were recorded using PerkinElmer spectrum one spectrometer. UV-vis spectra were recorded on PerkinElmer Lambda 25 UV/vis spectrometer. Fluorescence spectra were recorded on Varian Carey Eclipse fluorescence spectrophotometer. Elemental analysis was carried out using PerkinElmer 2400 CHNS analyzer.
Preparation of (R,R)-Salen 7. Salicylaldehyde (61 mg, 0.50 mmol) and (1R,2R)-diphenylethylenediamine (53 mg, 0.25 mmol) were stirred for 16 h in MeOH (1 mL) at ambient temperature. The solvent was evaporated and the residue was purified on basic silica gel column chromatography using hexane and EtOAc as eluent to provide compound 7 as yellow solid.
Mp: 154-155 °C
1H NMR (400 MHz, CDCl3): δ 8.30 (s, 2H), 7.26-7.12 (m, 14H), 6.96 (d, J = 8 Hz, 2H), 6.79 (t, J = 7.2 Hz, 2H), 4.73 (s, 2H).
13C NMR (100 MHz, CDCl3): δ 166.3, 161.0, 139.5, 132.7, 131.8, 128.5, 128.0, 127.8, 118.9, 118.7, 117.0, 80.3.
FT-IR (KBr): 3446, 3061, 3027, 2923, 2879, 1630, 1582, 1496, 1450, 1410, 1276, 1208, 1058, 1028, 982, 901, 869, 755, 718 cm-1.
Anal. Calcd for C28H24N2O2: C, 79.98; H, 5.75; N, 6.66. Found: C, 79.95; H, 5.73; N, 6.69.
Metal Ion Titration. The solutions of the polymers P1-P2 (3 mL, 1 x 10-5 M in THF with respect to the monomeric unit) and metal nitrate salts (100 μL, 3 x 10-4 M in water, 1 equiv) were thoroughly mixed at ambient temperature to produce the corresponding metal-polymer complexes (Figure 1). After 5 min, the fluorescent properties of the solution having the in situ generated metal-polymer complexes were measured.
UV-vis and Fluorescence Titration of Zn2+ with Polymer P2. The solution of the polymer P2 (3 mL, 1 x 10-5 M in THF with respect to the monomeric unit) and Zn(NO3)2 (10-120 μL, 3 x 10-4 M in water, 0.1-1.2 equiv) were thoroughly mixed at ambient temperature to produce the corresponding metal-polymer complexes (Figure 4 and 5a). The UV-vis and fluorescence of the respective solutions were measured.
Fluorescence Titration of Zn(II)-P2 with Other Metal Ions. To the solutions of the Zn(II)- polymer complex (1 x 10-5 M in THF with respect to the monomeric unit), derived from Zn(NO3)2 (3 x 10-4 M in water, 1 equiv) and P2 (1 x 10-5 M in THF with respect to the monomeric unit) for 1 h, was added the solutions of various metal salts (3 x 10-4 M in water, 1-25 equiv). The resultant solutions were thoroughly mixed. After 5 min, fluorescence of the respective solutions was measured (Figure 6-7).