Li+ Selective Podand-Type Fluoroionophore Based on a Diphenyl Sulfoxide Derivative Bearing Two Pyrene Groups

New podand-type fluoroionophores having two pyrene moieties: 2,2´-bis(1-pyrenylacetyloxy)diphenyl sulfide (3), 2,2´-bis(1-pyrenylacetyloxy)diphenyl sulfoxide (4), and 2,2´-bis(1-pyrenylacetyloxy)diphenyl sulfone (5), have been synthesized by connecting two 1-pyrenecarbonylmethyl groups with the two hydroxy groups of 2,2´-dihydroxydiphenyl sulfide, sulfoxide, and sulfone, respectively. Their complexation behavior toward alkali metal ions was examined by fluorescence spectroscopy. Among these fluoroionophores, compound 4, having a sulfinyl group, showed high selectivity toward Li+.


Introduction
Design and synthesis of fluoroionophores for metal ions is a vigorous research area [1][2][3][4][5][6]. These fluoroionophores have an ion recognition unit (such as a crown ether, calixarene or thiacalixarene) and a fluorophore unit (such as dansyl, anthracenyl, and pyrenyl groups) connected by appropriate linkers. When the ion recognition unit interacts with the target metal ions, chemical signals, such as absorbance or fluorescence intensities, are typically generated or attenuated in the fluorophore unit.

OPEN ACCESS
Among alkali metal ions a number of fluoroionophores for Na + and K + have been reported because these ions play particular roles in the regulation of many biological events. [7][8][9]. We have previously reported a p-tert-butylcalix [4]arene derivative which is connected with p-tert-butylcalix [4]arene and pyrene by a CH 2 CO linker [10]. This derivative shows an increase of pyrene monomer emission and a decrease of pyrene excimer emission with increased Na + concentration. Subsequently, we have synthesized podand-type fluoroionophores, with two pyrene units 1 and 2 using 2,2´-dihydroxydiphenylmethane as a precursor of calix [4]arene and 2,2´-dihydroxydiphenyl ether, respectively, and examined the binding abilities toward alkali metal ions [11]. Although 1 has only slight affinity for alkali metal ions, 2 has good sensitivity for Na + showing an increase of pyrene mononer emission and a decrease of pyrene excimer emission with increased Na + ion. The oxygen atom between two phenyl groups in fluoroionophore 2 was found to play a crucial role in binding Na + . This observation led us to construct novel podand-type fluoroionophores in which the oxygen atom between the two phenyl groups in 2 is replaced by different types of atoms with binding abilities for alkali metal ions other than Na + .
Fluoroionophores for Li + have been developed recently with regard to their medical and clinical applications for the treatment of manic-depressive psychosis [12][13][14]. However, relatively few studies concerning fluoroionophores for Li + have been reported compared with those for Na + and K + . Here, we report the synthesis of novel podand-type compounds 3-5 ( Figure 1) having two pyrene units as a fluorophore unit and a sulfur atom, sulfinyl group, or sulfonyl group as a binding site. Binding studies of compounds 3-5 toward alkali metal ions (Li + , Na + , K + , Rb + , Cs + ) have shown that compound 4, which has a sulfinyl group, has a good binding ability for Li + .

Results and Discussion
The fluoroionophores 3-5 were synthesized by connecting two 1-pyrenecarbonylmethyl groups with the two hydroxy groups of 2,2´-dihydroxydiphenyl sulfide, sulfoxide, or sulfone, respectively (Scheme 1). First, 2,2´-dihydroxydiphenyl sulfoxide 6, sulfide 7, and sulfone 8 were synthesized using 4-bromophenol as a starting material according to a reported procedure [15]. Then, compounds 3, 4, and 5, with two pyrene units, were synthesized by the reaction of 7, 6, and 8, respectively, with two equivalents of 1-(bromoacetyl)pyrene in the presence of potassium carbonate in acetonitrile. Furthermore, compounds 9, 10, and 11, with one pyrene unit, were synthesized as reference compounds by the reaction of 7, 6, and 8, respectively, with one equivalent of 1-(bromoacetyl)pyrene in the presence of sodium methoxide in acetonitrile. The structures of all the new fluoroionophores were determined by FAB MS and NMR spectra. Scheme 1. Synthetic routes of compounds 3, 4, and 5, with two pyrene units, and 9, 10, and 11, with one pyrene unit. COCH [16,17]. The pyrene monomer and excimer emission bands of compounds 3, 4, and 5 were shifted to longer wavelengths than the corresponding emission bands of pyrene itself. These results can presumably be attributed to the effect of the electron-withdrawing carbonyl group [18][19][20][21]. It is noteworthy that the fluorescence intensity ratio (I e /I m ) for compound 4 (1.32) is greater than that of compounds 3 (0.88) and 5 (0.69), where I e and I m are the fluorescence intensities of the pyrene excimer (at 523 nm for 3, 534 nm for 4, and 527 nm for 5) and monomer (at 419 nm for 3 and 438 nm for 4 and 5), respectively. These results indicate that the two pyrene units in compound 4 would be in closer proximity than those in compounds 3 and 5.
To examine the structures of compounds 3, 4, and 5 in detail, the 1 H-NMR spectra of 3, 4, and 5 were compared with those of the corresponding compounds 9, 10, and 11, which have one pyrene unit [22,23]. The 1 H-NMR spectra of all compounds were examined in DMSO-d 6 by 1 H-1 H COSY, 1 H-13 C COSY, and HMBC spectroscopy. The spectra of the pyrene unit are shown in Figure 4. The chemical shifts (δ) of all compounds and the induced shifts (∆δ) are listed in Table 1.  Table 1. Chemical shift values (δ) of pyrene protons in compounds 3-5 and 9-11 and the induced shifts (∆δ) between the pyrene protons a-i in compounds 3-5, with two pyrene units, and the corresponding pyrene protons in compounds 9-11, with one pyrene unit.  Table 1 shows that all the pyrene protons of 4 were shifted upfield compared to the corresponding protons of compound 10. The ∆δ (10 10 10 10-4 4 4 4) values are larger than the corresponding values of ∆δ (9-3) and ∆δ . These results suggest that both pyrene units in compound 4 are closer together than those in 3 and 5, because it is well-established that π-π stacking interactions between two aromatic rings shield the protons due to the anisotropy of the ring current effect [23,24].
To examine the alkali metal ion binding abilities of compounds 3, 4, and 5, we investigated the absorption and fluorescent spectral changes in chloroform-acetonitrile (97:3, v/v). No alkali metal ion-dependent changes in the UV-Vis spectra of 3, 4, and 5 were observed upon addition of 300 µM Li + , Na + , K + , Rb + , and Cs + (all metal ions as perchlorate salts, except for Rb + as thiocyanate salt to enhance solubility) to 1.0 µM 3, 4, and 5. On the other hand, pyrene monomer emissions of 4 (at 438 nm), and 5 (at 438 nm) were enhanced by the addition of some alkali metal ions. Figure 5 shows the relative fluorescence intensity (I-I 0 ), where I and I 0 are the fluorescence intensity in the presence of 300 equiv. of each metal ion and the fluorescence intensity in the absence of metal ions, respectively. The addition of Li + to solution 4, which has a sulfinyl group, induced a remarkable intensity change. In contrast, a moderate change in the fluorescence of compound 5, which has sulfonyl group, was observed with the addition of Li + . For 3, which has a sulfur atom, no significant changes were observed. It is noteworthy that compounds 3, 4, and 5 interact slightly with Na + compared with compound 2 [11]. The oxygen atom(s) of the SO and SO 2 groups in compounds 4 and 5, which bridge two phenyl groups, apparently have a crucial role in binding Li + , because even the compound 5 showed a moderate affinity to Li + . Figure 6 shows the change in fluorescence of compound 4 with various Li + concentrations under excitation at 360 nm. With the addition of Li + in the range of 50-300 µM, the pyrene monomer emission increased dramatically, and the pyrene excimer emission changed slightly. Similar fluorescence changes were observed for compound 5 by the addition of Li + (not shown). As shown in Figure 7, the pyrene monomer emission of 10, with one pyrene unit, was unchanged by the addition of Li + in the range of 50-300 µM. This result clearly indicates that the increases in pyrene monomer emissions of compound 4 with increasing Li + concentration were caused by interaction between the two pyrene units in 4. Namely, these interactions in 4 weaken with increasing Li + concentration.
To confirm the conformational changes in compound 4 caused by the addition of Li + , the chemical shifts of pyrene protons in compound 4 in the absence and presence of Li + were examined in CDCl 3 -CD 3 CN (97:3, v/v) by 1 H-NMR spectroscopy [7,25]. The spectra of the pyrene units in 4 in the absence (4) and presence (4-Li + ) of Li + are shown in Figure 8. The chemical shifts (δ) and induced shifts (∆δ) of pyrene protons are listed in Table 2. The signals for pyrene protons in 4-Li + showed downfield shifts relative to the corresponding protons in 4. These results indicate that the π-π stacking interaction between the two pyrene units in 4 is weakened by the addition of Li + .    The fluorophotometric and NMR spectroscopic studies described above indicate that the two pyrene units in compound 4 would be well separated in the presence of Li + compared to their location in the absence of Li + . Therefore, the pyrene monomer emission is expected to increase and excimer emission is expected to decrease with increasing Li + concentration. However, the pyrene excimer emission in 4 changed slightly with the addition of Li + , as shown in Figure 6. This is believed to be due to the small fluorescence intensity ratio (I e /I m ) for compound 4 (1.32). In general, pyrene-functionalized fluoroionophores having a large fluorescence intensity ratio (I e /I m ) before the addition of metal ions show a great decrease in pyrene excimer emission with increasing metal ion concentration. For example, two pyrene-functionalized calix [4]arenes having an (I e /I m ) ratio of about 4.16, as reported by Jin et al., showed a great decrease in pyrene excimer emission with increasing Na + concentration [26]. Even compound 2, with an (I e /I m ) ratio of 2.91, as we reported, showed a gradual decrease in pyrene excimer emission with increasing Na + concentration [10].
The stoichiometry of compound 4 and Li + was confirmed by a Job plot (

General
All melting points were determined using a Hansen & Co., Ltd. MEL-TEMP and were not corrected. The fluorescence spectra were obtained by a Hitachi F-2500 fluorescence spectrophotometer. The 1 H-and 13 C-NMR spectra (at 300 MHz and 75 MHz, respectively) were measured using a JEOL-JNM-ECP300 spectrometer. The chemical shifts were measured in ppm downfield from the tetramethylsilane as an internal standard. The HRMS (FAB) spectra were measured by a JEOL JMS-SX102A using glycerol or 3-nitrobenzyl alcohol as the matrix. All chemicals for the synthesis of 3-11 were commercially available and except for 1-(bromoacetyl)pyrene from Aldrich, were obtained from Tokyo Chemical Industry Co., Ltd. All reagents and solvents were used without further purification. Silica gel (70-230 mesh) came from Merck Ltd., Japan. Elemental analysis was performed by Mitsui Chemical Analysis & Consulting Service Inc., Japan.

General Procedure for Fluorescent Study
Stock chloroform solutions of compounds 3-5 and 10 (1.03 µM) and acetonitrile solutions of alkali metal salts LiClO 4 , NaClO 4 , KClO 4 , RbSCN, and CsClO 4 (10 mM) were prepared by using spectroscopic-grade solvent (Wako Pure Chemical Industries, Ltd., Japan). Test solutions were prepared by mixing the stock solution (9.7 mL) of 3-5 and 10 and the alkali metal salt stock solution in increments of 0.00 mL, 0.05 mL, 0.10 mL, 0.15 mL, 0.20 mL, 0.25 mL, and 0.30 mL, after which the solution was diluted to 10.0 mL with acetonitrile.

9-11
2-(1-Pyrenylacetyloxy)-2´-hydroxydiphenyl sulfide (9): A mixture of 2,2´-dihydroxydiphenyl sulfide (2.50 mmol, 0.54 g) [15] and sodium methoxide (2.91 mmol, 0.16 g) was refluxed in acetonitrile metal ions show that the difference in binding site affected metal ion selectivity and the fluoroionophore having sulfinyl group is highly selective for Li + ion over other alkali metal ions. Although a number of fluoroionophores using calixarenes and crown ethers as ion recognition unit have been reported, these results demonstrated that a podand-type fluoroionophore with a non-cyclic binding site having sulfinyl group would be applicable as an effective Li + ion fluorescence sensor. Along these lines, we are examining the synthesis of new fluoroionophore that can be able to monitor Li + ion concentrations in aqueous samples.