Effective Na + fluorescent sensing by new podand-type receptor connecting two pyrene units and diphenyl ether

A new podand-type receptor for Na + detection has been synthesized by connecting two 1-pyrenecarbonylmethyl groups with two hydroxy groups of 2,2’-dihydroxydiphenyl ether. With the addition of Na + in the range of 20-200 µ M, the increasing monomer emission (423 nm) and the decreasing excimer emission (524 nm) of pyrene were observed. In contrast with the receptor, the fluorescence spectra of the corresponding compound obtained from 2,2’-dihydroxydiphenylmethane are only slightly affected by the addition of Na + .


Introduction
][12] There are two types of chemosensors for metal ions. 135][16][17] Moon et al. reported the Hg 2+ sensitive cyclam derivative having two pyrene units as the fluorophore. 18The other type is the chemosensor with chromophores as the monitoring sites and a non-cyclic binding site such as podands and lariat ethers. 19,20 or example, Suzuki et al. reported the alkaline earth metal ion sensitive polyoxyethylene derivatives having two pyrene units as the fluorophore. 214][25][26] We previously reported the Na + sensitive ptert-butylcalix [4]arene derivative having two pyrene units as a fluorophore, which exhibited increasing pyrene monomer and decreasing pyrene excimer emissions with the addition of Na + . 27his observation promoted us to investigate the novel podand-type fluoroionophores 1 and 2 in which the two pyrene units are connected to the hydroxy groups of 2,2'dihydroxydiphenylmethane or 2,2'-dihydroxydiphenyl ether, respectively.The binding abilities of compounds 1 and 2 toward the alkali metal ions (Li + , Na + , K + , Rb + , and Cs + ) were then investigated.

Results and Discussion
Scheme 1 shows the synthetic routes for compounds 1 and 2 having two pyrene units and compound 3 having one pyrene unit as a reference compound.The reaction of 2,2'dihydroxydiphenylmethane and 2,2'-dihydroxydiphenyl ether with two equivalents of 1-(bromoacetyl)pyrene in the presence of potassium carbonate in acetonitrile afforded compounds 1 and 2 having two pyrene units, respectively.Compound 3 having one pyrene unit was prepared by the reaction of 2,2'-dihydroxydiphenyl ether with one equivalent of 1-(bromoacetyl)pyrene in acetonitrile in the presence of sodium methoxide in order to monodeprotonate the 2,2'dihydroxydiphenyl ether.The structures of 1, 2 and 3 were determined by FAB MS and NMR spectroscopies.
The synthetic routes of 1, 2 and 3.
The UV-Vis spectra of 10.0 µM chloroform solutions of compounds 1, 2, and 3 are shown in Figure 2. Compounds 1 and 2 having two pyrene units showed an absorption band at 362.5 nm with a shoulder at 392 nm corresponding to the pyrene chromophore. 27On the other hand, the absorption spectrum of compound 3 having one pyrene unit was slightly red-shifted relative to those of compounds 1 and 2.  Figure 3 shows the fluorescence spectra of 1.0 µM chloroform solutions of compounds 1, 2, and 3 by excitation of 360 nm.Compound 3 having one pyrene unit showed only the monomer emission at 423 nm.On the other hand, both compounds 1 and 2 having two pyrenyl groups showed two emission bands at 423 and 524 nm.The emission band at 524 nm coincided with the intermolecular pyrene excimer emission band of 1-acetylpyrene observed at the concentration of 10.0 mM in chloroform by excitation of 360 nm.Therefore, the emissions at 524 nm of compounds 1 and 2 are excimer emissions based on the intramolecular interaction between the two pyrene units.It is noteworthy that the fluorescence intensity ratio (I e /I m ) for compound 2 (2.91) is greater than that of compound 1 (0.44), where I e and I m are the fluorescence intensities of the pyrene excimer (at 524 nm) and monomer (at 423 nm), respectively.These results indicate that the two pyrene units in compound 2 must be in close proximity, probably as a result of π-π stacking, while in the compound 1, they must be separated.In order to clarify the structures of compounds 1, 2, and 3 in detail, the 1 H NMR spectra were examined in DMSO-d 6 .All proton signals were unambiguously assigned by the 1 H-1 H COSY, 1 H- 13 C COSY and HMBC spectroscopies and the spectra of the pyrene unit are shown in Figure 4.In the spectrum of compound 1 having two pyrene units, the signals for H-10 (δ 8.80), H-2 (δ 8.63), H-4 (δ 8.24), and H-7 (δ 8.14) showed upfield shifts compared to the corresponding protons of compound 3 having one pyrenyl group: ∆δ{δ(3) -δ(1)} = -0.031for H-10, -0.067 for H-2, -0.027 for H-4, and -0.023 for H-7.Similar upfield shifts were observed in the spectrum of compound 2: ∆δ{δ(3) -δ(2)} = -0.077for H-10, -0.12 for H-2, -0.089 for H-4, and -0.045 for H-7.These upfield shits indicate that both pyrene units in compounds 1 and 2 would be closely located, because it is well-established that π-stacking interactions between aromatic rings result in shielding of the protons due to the anisotropy of the ring current effect. 28,29Furthermore, all pyrene protons in compound 2 were remarkably shifted to a higher magnetic field than those in compound 1.These results and fluorescence spectra described above reasonably indicate that the pyrene units in compound 2 would be located more closely than that in compound 1.To examine the metal ion binding properties of receptors 1, 2, and 3, we investigated the fluorescence changes.Figure 5 shows the fluorescence emission changes of 1, 2, and 3 (1.0 µM, excitation at 360 nm) at 423nm upon the 300 µM addition of alkali metal ions (Li + , Na + , K + , Rb + , and Cs + as thiocyanate salts) in chloroform-acetonitrile (97:3, v/v).The fluorescence changes in compounds 1 and 3 were only slightly observed with addition of the alkali metal ions.On the other hand, the fluorescence intensities of compound 2 having two pyrene units were increased by the addition of Na + and K + .Especially, Na + displayed a significant enhancement of the pyrene monomer emission of compound 2. Figure 6 shows the fluorescence changes in 2 with various Na + concentrations by excitation at 360 nm.The fluorescence intensity of the excimer emission at 524 nm of 2 gradually decreased and the intensity of the monomer emission at 423 nm of 2 dramatically increased with the increasing Na + concentrations ranging from 2 µM to 200 µM.The fluorescence emission intensity at 423 nm was saturated at 200 equiv. of Na + .These fluorescence changes indicate that the pyrene units in compound 2 would be effectively separated from each other by the Na + complex formation.In contrast, no spectral changes were observed for compound 1 having two pyrene units and 3 having one pyrene unit by the addition of Na + .This result suggests that the Na + binding ability of 2 would be attributed to the presence of the oxygen atom at the diphenyl ether component in compound 2. The stoichiometry of compound 2 was confirmed by the Job's plots (Figure 7) utilizing the fluorescent titrations of 1.0 µM chloroform-acetonitrile (97:3) solutions of 2 with the 1.0 µM chloroform-aceronitrile solutions of sodium thiocyanate.The Job's plot indicated the formation of a 1:1 complex.Figure 8 shows the plots of the fluorescence intensities against the added Na + concentration.The association constant of compound 2 for Na + was calculated to be 1.14 x 10 4 M -1 based on the Benesi-Hildebrand method by plotting the 1/(F-F 0 ) against the 1/[Na + ], where F 0 and F are the fluorescence intensities in the absence and presence of Na + shown in Figure 8.These results demonstrated that a podand-type receptor with a non-cyclic binding site can be applicable as an effective Na + fluorescence sensor.
General procedure for fluorescent study.The stock chloroform solutions of compounds 1-3 (1.03 µM) and acetonitrile solution of the alkali metal salts (LiSCN⋅2H 2 O, NaSCN, KSCN, RbSCN, and CsSCN) (10 mM) were prepared using a spectroscopic grade solvent (Wako Pure Chemical Industries, Ltd.).Test solutions were prepared by mixing the stock solution (9.7 mL) of 1-3 and the alkali metal salt stock solution in increments of 0.00 mL, 0.02 mL, 0.05 mL, 0.10 mL, 0.15 mL, 0.20 mL, 0.25 mL and 0.30 mL, followed by diluting the solution to 10.0 mL with acetonitrile.

Figure 7 .
Figure 7. Job's Plot for the binding of Na + with 2.