Selectivity, sensitivity, and reversibility in metal ion sensing

Na⁺, K⁺, Ca²⁺, Ba²⁺, Mg²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Fe²⁺, Ag⁺, Mn²⁺, and Cu²⁺ (Figure 4.6).

The fluorescence intensities of P1-P3 were slightly altered upon addition of up to 30 equiv.

of Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Ni²⁺, Zn²⁺, Ag⁺, Co²⁺,Mn²⁺, and Cu²⁺. Whereas, Fluorescence intensities of polymers P1-P3 decreased significantly in presence of 10 equiv. of Fe²⁺. This may be due to higher coordination capabilities of imidazole receptors with Fe²⁺ ions, which can be attributed to energy and/or charge transfer due to the stronger ligand-metal interaction.^115c The selectivity towards Fe²⁺ was further proven by its interference with other background metal ions. As illustrated in Figure 4.7, the sensitivities of polymers P1-P3 towards Fe²⁺ were not significantly affected by other competitive metal ions. However, P3 showed the best quenching efficiency than P1 and P2. This could be attributed to a higher molecular weight of P3 which led to produce a better semiconducting “molecular wire effect”

than P1 and P2. The larger number of repeating units in P3 provided more binding sites and thus to cause sheer fluorescence quenching.

Figure 4.6 Fluorescence emission response profiles (I0/I) of P1, P2, and P3. The polymer concentration (1.2 × 10^-5 M), Fe²⁺ added = 10 eq (1.2 ×10^-4 M), other metals ions added = 30eq (3.6 ×10^-4 M) (Single metal system).

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Figure 4.7 Fluorescence emission response profiles (I/I0) of P1, P2, and P3. The polymer concentration (1.2 × 10^-5 M), [Fe²⁺] = 1.2 ×10^-4 M (10 eq) and in the presence of other metal ions = 1.2 ×10^-4 M (10 eq) (Dual metal system).

The PL quenching efficiencies (PLQE) and static Stern-Volmer quenching constants (Ksv) were depicted in Table 4.2. Due to the larger molecular wire effect, Ksv of P3 is ca. 5.1 and 2.5 times higher than P1 and P2, respectively. As depicted in Figure 4.8, sharp decreases of fluorescence intensities in P1-P3 were observed by the addition of Fe²⁺ up to 1 equiv. molar concentration.

Table 4.2 Photoluminescence quenching efficiency (PLQE) and Stern–Volmer Quenching constants (KSV) of polymers upon the addition of Fe²⁺

Metal Ion Polymers PLQE(%) ^a K_sv^b

Fe²⁺

P1 78.22 2.01 × 10⁶

P2 81.57 4.12 × 10⁶

P3 96.94 1.03 × 10⁷

a PLQE = A0-A/A0 ; where A0 = Area under the PL curve without metal ion. A = Area under the PL curve in presence of 10 equiv of metal ions. ^b Stern–Volmer Quenching Constants can be evaluated by the static Stern–Volmer equation I0/I = 1 + KSV [Q] where I0 is the PL

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intensity of the polymers (1.2 ×10-5 M) in absence of the quenchers, I is the PL intensity in the presence of each quencher, KSV is the Stern–Volmer quenching constant and [Q] is the quencher concentration.

Table 4.3 Photoluminescence properties of P1, P2, and P3 upon titration with Fe²⁺

Sample λem(nm)^a Φemb τ^c

P1 633 0.82 0.97

P1+Fe²⁺ 628 0.15 0.21

P2 640 0.84 1.12

P2+Fe²⁺ 632 0.11 0.18

P3 661 0.89 1.14

P3+Fe²⁺ 630 0.05 0.10

a Emission maxima of Fluorescence spectra obtained by exiting at their absorption maxima. ^b Emission quantum yields measured in THF solutions with reference to Rhodamine 6G for P1-P3, respectively. ^c The lifetimes were measured at the emission band maxima.

Fluorescence quantum yields and lifetimes of polymers P1-P3 before and after the addition of Fe²⁺ are summarized in Table 4.3. Upon the addition of Fe²⁺, the fluorescence lifetimes of the polymers decreased significantly (see Figure 4.9).¹¹⁶ In the presence of Fe²⁺, the fluorescence lifetimes of P1 and P2 were decreased almost 4.6 and 6.2 times, respectively.

The extensive decrease in the fluorescence lifetime of polymer P3 (11.4 times) further confirmed the best sensing ability of P3 due to the stronger molecular wire effect. Stern-Volmer plots for P1-P3 in the presence of Fe²⁺ indicated 1:1 stoichiometry. Fluorescence recovery tests were carried out by two suitable counter ligands disodium salt of ethylenediaminetetraacitic acid (Na₂-EDTA) and phenathroline. As illustrated in Figure 4.8, upon the addition of disodium salt of ethylenediaminetetraacitic acid (Na2-EDTA) or

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phenathroline in THF to the quenched Fe²⁺-polymer (P1-P3) solutions, the florescence intensities were almost 65-85% recovered. This indicates the reversible association of Fe²⁺

with imidazole units, via formation of stable Fe-EDTA complex. Similarly, upon the addition of phenanthroline to the quenched Fe²⁺-polymers (P1-P3) solutions, the fluorescence intensities were almost 90% recovered. This suggested that Fe²⁺ coordinates strongly via double coordinating bond with phenantroline in the presence of phenantroline, which casued the polymers to be released from Fe²⁺. Thus, for the fully quenched Fe²⁺-polymers (P1-P3) solutions, phenathroline mediated fluorescence recovery is more prominent than that of EDTA. The photographs of the quenching and recovery in florescence for all polymers (P1-P3) were also shown in the insets of Figures 4.8.

Figure 4.8 Sequential PL quenching of P1-P3 (1.2×10^-5 M) in THF/H2O (1/1) acquired by the addition of 0-10 eq of Fe²⁺ and recovery of fluorescence by the addition of Na2-EDTA and phenantroline. Upper inset PL quenching of P1 as a function of 0-10 eq [Fe²⁺]. Lower

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inset photographs of fluorescence quenching in polymer solutions upon the addition of Fe²⁺

and regain of original fluorescence upon the addition of phenanthroline or Na₂-EDTA.

Figure 4.9 Time-resolved fluorescence of polymers P1-P3, before (empty circle) and after (solid circle) the addition of Fe²⁺.

Moreover, the fluorescence on-off-on behaviour was observed for 6 successive cycles (Figure 10).Thus, a remarkable fluorescence on-off-on behaviour was achieved for the polymer solutions, during sensing of Fe²⁺ via reversible binding to imidazole receptors upon the addition of proper counter ligands (such as EDTA and phenathroline) to form more stable complexes. To the best of our knowledge, the phenanthroline ligand intervened efficient fluorescence recovery of imidazole receptors in the presence of metal ions is reported for the first time.

Figure 4.10. The switches of on-off-on fluorescent spectra of P1, P2, and P3 for seven successive cycles upon the addition of Fe²⁺ and phenanthroline.

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To further elucidate the binding mode, ¹H NMR titrations were conducted upon the addition of Fe²⁺ (in D₂O) to M1-M3 (binding probes of P1-P3, respectively) in d₈-THF (see Figure 11). In M1, the aromatic protons were gradually shifted towards up-field upon the sequential addition of Fe²⁺. The peak corresponding to dibromo thiophene unit was 0.1 ppm up-field shifted (from 7.55 to 7.45 ppm). Likewise, upon the addition of Fe²⁺, there is notable shift observed in aromatic protons corresponding to M2. The proton corresponding to dibromo thiophene was 0.13 ppm up-field shifted (from 7.58 to 7.45 ppm). Unlike M1 and M2, the proton peak corresponding to dibromo-thiophene unit in M3 was dramatically down-field shifted. The peak of dibromo thiophene in P3, originally appeared at 7.42 ppm, was 0.16 ppm down-field shifted to 7.58 ppm. This is probably due to the different electronic distribution in M3 due to the presence of anti-aromatic acenaphthene moiety (as discussed previously in the computational analysis).

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Figure 4.11 ¹H NMR titrations of monomers M1-M3 (in d8-THF) upon the sequencial addition of 0-1 equiv of Fe²⁺ (in D2O). (Arrow marks show the shifts of the peak corresponding to the dibromothophene units).