Ab initio Molecular Orbital Study of Emission Mechanism of 2,6-Bis(quinolinecarboxy)methylpyridine as Fluorescent Chemosensors for Zinc and Cadmium Ions

Jun KAWAKAMI, Ryo MIYAMOTO, Kimiaki KIMURA, Kazuhiro OBATA, Masahiko NAGAKI and Haruo KITAHARA


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1 Introduction

The biological importance of many metal ions is well established [1, 2]. However, some 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 by assembling a specific metal ion-receptor with a subunit capable of signaling the occurrence of a receptor-substrate interaction [8]. Many kinds of crown ether type macrocyclic compounds have been used for analytical applications, such as chemical sensors [9] and spectrophotometries [10]. In the fluorimetry application, fluorescent reagents, which have two aromatic hydrocarbon groups at both terminals of a linear polyether as an analogue of a crown ether, have also been used [11].
We have developed compounds capable of selectively responding to several metal ions [12 - 14]. We now wish to report the results of our study on zinc and cadmium ions recognition by 2,6-bis(quinolinecarboxy) methylpyridine (P2Q). The structural formula is shown in Figure 1. And Ab initio molecular orbital (MO) calculations were performed for a quinoline chromophore to investigate the properties of the luminescence.


Figure 1. Structural formula of P2Q.

2 Experiment

2. 1 Synthesis and measurements

P2Q was prepared from 2-Quinolinecarboxylic acid and 2,6-pyridinedimethanol in the presence of N,N-dicyclohexyl carbodiimide and 4-dimethylamino pyridine in dichloromethane. The fluorescence spectra were taken on a Hitachi F-4500 fluorophotometer. The fluorescence spectra measurements were carried out in an acetonitrile solution of the P2Q (<10-4 mol dm-3 where no intermolecular interaction was found) at room temperature, and metal salts [Ni(ClO4)2, Co(ClO4)2, Cu(ClO4)2, Ag(ClO4)2, Zn(ClO4)2, and Cd(ClO4)2] were added to the solution. To prevent any nonlinearity of the fluorescence intensity, 295nm was chosen as the excitation wavelength.

2. 2 Calculational procedure

All ab initio MO calculations were carried out using Gaussian 98 with Linda [15] 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 6-31+G(d) basis set for quinoline derivative and by Hartree-Fock level calculation with 6-31G basis set for Zn2+ complex; and total energies were estimated by B3LYP/6-31+G(d) for these structures. The singlet transition energies of the compounds were calculated at those geometries by a time-dependent DFT (TDDFT) method.
For investigating the transition energies of P2Q-Zn2+ complex, we have used diaqua-quinolinecarboxyethyl-zinc(II) complex (QNE-Zn2+-aq2) as a model compound, which only has a single quinoline chromophore. It is indeed that the water molecules were not necessarily in P2Q-Zn2+, but those were added to keep the coordination structure around the Zn2+ ion tetrahedral. The conformations of the QNE-Zn2+-aq2 complexes were obtained by following procedure. The initial structure of the complex with zinc ion being placed near the QNE with two water molecules led to two stable structures, both of which have a CS symmetry, by HF/6-31G method. For these two structures, transition energies were estimated by using TDDFT of B3LYP/6-31+G(d). The transition energies of a QNE itself were also calculated with B3LYP/6-31+G(d) at their optimized structure.

3 Results and discussion

3. 1 Fluorescence studies for metal ion recognition

When Ni2+, Co2+, Cu2+ and Ag2+ were added to an acetonitrile solution of P2Q, the fluorescence spectra were not changed. However, the fluorescence intensity of P2Q changed greatly with the addition of Zn2+ or Cd2+. The fluorescence spectra of P2Q in the presence of several concentrations of Zn(ClO4)2 are shown in Figure 2 as a typical example. We express the metal salt interactions in terms of the equilibrium:

Also, the association constants (K) should be represented as follows:

From Eqs. 2 and 3, the following equation could be derived:

where [P2Q]0 and [Zn2+]0 are the initial concentrations of P2Q and the zinc ion, I and I0 are the observed locally excited emission intensities of P2Q in the presence and absence of the zinc ion, respectively, and Imax is the observed locally excited emission intensity of P2Q and zinc ion complex. A self-written nonlinear curve-fitting computer program (Eq. 4) was used to fit the experimental titration curves. The association constant was determined from the emission-intensity changes at the emission maxima using the equation. The logK value of P2Q is 3.7.


Figure 2. Fluorescence spectra of P2Q and Zn2+ complexes when excited at 295nm. [P2Q] = 1 × 10-5 mol dm-3. [Zn2+] = 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.

3. 2 The ab initio molecular orbital calculations

The conformations of QNE and their zinc complexes are shown in Figure 3. Note that all of these structures are characterized as having CS symmetry. Rotation of C(carboxyl)-C(quinoline) gave two stable conformers for QNE. The conformer-b was 572 cm-1 more stable than conformer-a (see Figure 3). According to the conformers of QNE, it is interesting that two conformers of the Zn2+ complex were also obtained, where one of them has a carbonyl oxygen as a donor atom and the other has an etheric oxygen. Naturally, in the ground state, the former is calculated to be more stable than the latter (dE(S0) =5955 cm-1).


Figure 3. Conformations of QNE and QNE-Zn2+-aq2.

Attempts were made to estimate the transition energies by using a configuration interaction with single excitation (CIS) scheme and a time-dependent Hartree-Fock (TDHF) approach. However, both methods have led to a pp* as the lowest singlet excited state for QNE, which is not an acceptable result. Then the more sophisticated method, TDDFT, was used. When two water molecules of the ligand were removed, ligand-to-metal charge-transfer (LMCT) transitions were obseved in the near IR region, which were not acceptable as well. The transition energies, oscillator strengths and their assignments (or characters) are summarized in Table 1.

Table 1. Transition energies (l) and oscillator strengths (f) of the excited states of QNE and its Zn2+ complex.
Conformerab
l / nmfassignmentl / nmfassignment
state 1312.600.0011A'', np*320.700.0010A'', np*
state 2308.060.0277A', pp*307.960.0270A', pp*
state 3284.970.0643A'284.240.0654A'
state 4283.930.0001A''266.320.0001A''
state 5250.160.0001A''252.870.0000A''
state 6231.550.9354A'231.690.8606A'
Conformercc
l / nmfassignmentl / nmfassignment
state 1387.460.0195A', pp*378.810.0209A', pp*
state 2306.450.1685A', pp*321.450.0000A'', LMCT
state 3304.120.0001A'', LMCT301.720.1424A', pp*
state 4260.930.0000A'', LMCT283.420.0002A'', np*
state 5250.570.5073A', pp*276.120.0000A'', LMCT
state 6240.940.0002A'', np*245.680.4834A'

Calculated transition energies of QNE show very good agreement with the observed absorption spectra of P2Q in the UV-visible region (lmax/nm (e/cm-1M-1): 237 (79200), 270 (11000), 290 (10100), 320 (4400)). Both conformer-a and conformer-b have nearly the same transition energies and oscillator strengths, with np* being the lowest excited state where the non-bonding orbital is coming from the nitrogen of the quinoline and pp* the second lowest.
In the next step, forming a metal complex has made a dramatic change in the energy levels of the excited states, regardless of the structure of the conformer. The lowest excited state was pp* being located more than 50 nm longer from the np* band of QNE; and some new LMCT bands appeared. Furthermore, np* band was shifted to the higher energy region at 240-280 nm. Consequently, the results of the TDDFT calculation suggested that the allowed transition band exists for the QNE-Zn2+-aq2 complex at the region lower than the lowest np* state of QNE. Indeed, when adding zinc ion to the P2Q, the absorption band at ~300 nm was observed to be red-shifted and/or a new luminescent band at ~450 nm could be seen.

4 Conclusion

P2Q hardly shows fluorescence itself, but it showed strong fluorescence with the addition of zinc or cadmium ions (If,Zn > If,Cd). Therefore, Ab initio molecular orbital calculations (Gaussian 98) using the time-dependent density functional method with 6-31+G(d) basis set were carried out for a quinoline chromophore of P2Q and its metal complexes to investigate the emission mechanisms. The results of the molecular orbital calculations suggest that the lowest luminescent state has changed from the np* to the pp* by coordinating with a metal ion.@Actually, there are few Zn2+ or Cd2+-selective analytical reagents available. P2Q will become a good fluorescent chemosensor for Zn2+ and Cd2+.

This work was supported by a Grant-in-Aid for Research of Young Scientists No. 14740399 from JSPS and the Nishida Research Fund for Fundamental Organic Chemistry (Heisei 14 nendo).

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