Time-Dependent DFT Study of Emission Mechanism of 8-Hydroxyquinoline Derivatives as Fluorescent Chemosensors for Metal Ions



1 Introduction

Molecular recognition is a subject of considerable interest because of its implications in many fields: biology, medicine, environment, etc. In particular, the detection of metal ions involved in biological processes has received considerable attention. The biological importance of many metal ions is well established [1, 2]. However, the same and other metal ions can be toxic to life when present at certain concentrations in the environment, water supplies, food chain, and industrial chemicals and products. Consequently, an intensive effort has been devoted to develop various sensory molecular receptors capable of recognizing, sensing, and selectively transporting these positively charged substrates so that the concentrations of these metal ions of commercial value can be recovered from waste solutions, and certain toxic transition metal ions in the environment can be removed [1 - 8]. The goal of this research is the design and construction of ion-selective sensors [8]. We have developed compounds capable of selectively responding to several metal ions, and performed a theoretical investigation of the excited state properties of the compounds [9 - 11]. We now wish to report the results of our study on zinc and cadmium ions recognition by 8-hydroxyquinoline derivatives (BQOH and BQClOH). The structural formulas are shown in Figure 1. These compounds are found to have a novel character in that the fluorescence intensity of the chloro-substituted quinoline complex is stronger than that of the non-substituted quinoline. And then, the ab initio calculations were performed for a quinoline chromophore to investigate the properties of their luminescence.

Figure 1. Structural formulas of BQOH (1) and BQClOH (2).

2 Experiment

2. 1 Synthesis and measurements

BQOH was synthesized from 8-hydroxyquinolinecarbox-aldehyde and N-benzylmethylamine [12]. 8-Hydroxy-quinolinecarboxaldehyde was prepared by the oxidation of commercially available 2-methyl-8-hydroxyquinoline with selenium dioxide in dioxane [13]. BQClOH was from 5-chloro-8-hydroxyquinoline and N-benzylmethylamine [12]. The fluorescence spectra were taken on a Hitachi F-4500 fluorophotometer. The fluorescence spectra measurements were carried out in an acetonitrile solution of the BQOH and BQClOH ([BQOH or BQClOH] <10-4 mol dm-3 where no intermolecular interaction was found) at room temperature, and metal salts [Zn(ClO4)2, Cd(ClO4)2, Co(ClO4)2, Ni(ClO4)2, Cu(ClO4)2, Ca(ClO4)2, Ba(ClO4)2 and Mg(ClO4)2] were added to the solution. To prevent any nonlinearity of the fluorescence intensity, isosbestic points (253 or 256 nm) of the absorption spectra of BQOH and BQClOH were chosen as the excitation wavelength.

2. 2 Calculation procedure

All ab initio calculations were carried out by using Gaussian 98 with Linda [14] on an HPC P4L/2.2 (Linux 2.4), a PC-cluster machine with five nodes parallel. Optimized structures were obtained by B3LYP density functional theory (DFT) with 6-31G basis set for quinoline derivatives and their Zn2+ complexes; and total energies, singlet transition energies and triplet transition energies were estimated by using a time-dependent DFT (TD-DFT) method with B3LYP/6-31+G(d) at the obtained geometries.

3 Results and discussion

3. 1 Fluorescence studies for metal ion recognition

Very weak fluorescences were observed for both BQOH and BQClOH at lmax = 418 nm and 545 nm, respectively. When Co2+, Ni2+, Cu2+, Ca2+, Ba2+ and Mg2+ were added to an acetonitrile solution of BQOH, the fluorescence spectra were not or slightly changed. However, the fluorescence intensity of BQOH changed greatly with the addition of Zn2+ or Cd2+; the florescence spectra for both metal complexes were observed to be red-shifted at lmax = 550 nm. The fluorescence spectra of BQOH in the presence of several concentrations of Zn(ClO4)2 are shown in Figure 2 as a typical example. Though a similar enhancement in intensity was observed for BQClOH whose fluorescence was seen at lmax = 540-550 nm with the addition of Zn2+ or Cd2+, the fluorescence intensity of metal complexes was very different from the case of BQOH. The maximum values of the fluorescence intensity (If,max) of the metal complexes are shown in Figure 3. The order of the If,max was BQClOH-Zn2+ > BQClOH-Cd2+ >> BQOH-Cd2+ > BQOH-Zn2+.
These are very interesting results in that the fluorescence intensity of the chloro-substituted quinoline complex is stronger than that of the non-substituted one, though a substituted chlorine atom is usually thought to be a potential intramoleculer-sensitizer. It is also pointed out that the intensities of the absorption bands of BQOH-Zn2+ and BQClOH-Zn2+ were not so different from each other, in energies and intensities. In order to investigate the mechanisms of enhanced fluorescence by chlorine substitution, ab initio calculations were carried out.

Figure 2. Fluorescence spectra of BQOH and Zn2+ complexes when excited at 253 nm. [BQOH] = 1 × 10-5 mol dm-3. [Zn2+] = 0, 1 × 10-6, 5 × 10-6, 1 × 10-5, 2 × 10-5, 5 × 10-5, 1 × 10-4, 2 × 10-4, 5 × 10-4, 1 × 10-3 and 1 × 10-2 mol dm-3.

Figure 3. The maximum values of fluorescence intensity (If,max) of BQOH-M2+ and BQClOH-M2+ (M2+ = Zn2+ or Cd2+).

3. 2 The ab initio calculations

Ab initio calculations for quinoline derivatives at some conformations were carried out. And then, the zinc complexes with metal-to-ligand ratio of 1:1 were used to investigate the geometry and the electronic structure by DFT calculation. Some of the optimized structures with the energy differences (DE) from that of the most stable conformation are shown in Figure 4 and Figure 5. The obtained excited state energies for the S1, T1, and T2 states and the oscillator strength for the S0-S1 transition are summarized in Table 1; and the energy diagrams of 3a and 4a are illustrated in Figure 6.

Figure 4. Optimized molecular structures of BQOH (1) and BQClOH (2).

Figure 5. Optimized molecular structures of [ZnBQO]+ (3) and [ZnBQClO]+ (4) complexes.

Table 1. The excited state energies (E) and oscillator strengths (f).
E(S1) / cm-12978927953278822494629066
E(T1) / cm-12061811957196571929219702
E(T2) / cm-12950127641279502493528505
E(S1) / cm-1201002161519535142301425510105
E(T1) / cm-116334166581704312749128336936
E(T2) / cm-1198862111618850197032051415432

Figure 6. Energy diagrams of 3a (left) and 4a (right).

These results are very interesting in that the T2 energy of 3a is just below the energy of its S1 state, on the other hand the T2 state of 4a lies highly above its S1 state. A similar situation was also encountered for all other conformations of 3b, 3c, 4b, and 4c; the calculated results for these conformations were also shown in Table 1. According to the energy gap law, these results suggest that the inter-system crossing rate from S1 to T2 is faster in 3a than that in 4a. This result means that the chlorine-substitution on the quinoline chromophore enables the changing of its energy levels, and then it enhances the florescence intensity of the quinoline derivatives-metal complex. The larger oscillator strength in 4a may also contribute to its strong fluorescence intensity, though the absorbance of its corresponding band was weak in our experimental results.

4 Conclusion

BQOH and BQClOH hardly show fluorescence themselves, but they showed strong fluorescence with the addition of zinc or cadmium ions. The order of the If,max was BQClOH-Zn2+ > BQClOH-Cd2+ >> BQOH-Cd2+ > BQOH-Zn2+. Therefore, ab initio calculations (by Gaussian 98) using TD-DFT method with 6-31+G(d) basis set were carried out for the Zn2+ complexes of the quinoline chromophore, BQOH and BQClOH, to investigate the difference in the fluorescent intensity.
The results of the calculations showed that the T2 state of the [ZnBQO]+ was lying just below the S1 state which reduced the fluorescent intensity; while for the [ZnBQClO]+ such was not the case. In other words, when the energy levels of a compound are in the case like this, the changing of their energy levels by some chemical modification on the emissive chromophore makes possible enhancement of the florescent intensity for a chemosensor of metal ions.

This work was supported by a Gakujutsu Kokusai Shinko Kikin (Heisei 15 nendo) and an Aomori-ken Sangakukan Kyodokenkyu Suishin Gigyo (Heisei 15 nendo).


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