Sensing of Metal Ions by Hybrid Systems of a Chiral Schiff Base Zn(II) Complex and Spiropyran

We have constructed three (hybrid) systems for quantitative fluorescence sensing of metal ions by using chiral Schiff base Zn(II) complexes. System 1 is a hybrid system composed of a trans-type chiral Schiff base Zn(II) complex and merocyanine (Mc), which is open-form of spiropyran (Sp) after photoisomerization. Depending on intermolecular interactions and quenching, increase (Zn 2+ ) or decrease (Cu 2+ and Gd 3+ ) of fluorescence intensity of Zn(II) complex could be observed as functions of concentration of metal ions. System 2 is a sole component of a salen-type chiral Schiff base Zn(II) complex which can coordinate metal ions. After coordination of Zn 2+ , Cu 2+ , and Gd 3+ ions, decreasing of fluorescence intensity could be found for all the cases for system 2 under the same condition to system 1. System 3 is a hybrid system being consisted of a salen-type chiral Schiff base Zn(II) complex and Mc. Decrease or increase of fluorescence intensity is in agreement with intermolecular interactions, namely affinity for the Zn(II) complex is Zn 2+ > Cu 2+ > Gd 3+ , whereas it is reverse order for Mc. The quantitative spectroscopic nature (detected as absorption, CD, and fluorescence) of these systems is predominantly attributed to both organic ligands and Zn(II) center in a probe complex.


INTRODUCTION
We make use of finding and collection of metal ions in environment and human body, of which importance is increasing year by year [1][2][3][4][5].For this reason specific detection of metal ions in complicate systems composed of many components is needed.Moreover, a way for detection of specific metal ions controlled by external stimuli is also needed in order to avoid interference of other components.Among many metal ions as mentioned in the text of inorganic chemistry, Zn(II) ion is one of typical example of a trace essential element in human body [6], in particular important for metabolism [7] and parenteral nutrition [8].Indeed, many fluorescent sensors for Zn(II) ion used in solutions have been reported so far [9][10][11][12][13][14], and one of typical recent examples exhibit a strong fluorescence response after coordination Zn(II) ion to non-fluorescent organic ligand [15].This organic probe is a typical example of self-response fluorescent sensors to detect existence or concentration of Zn(II) ion in a pure system, which cannot control on purpose by external stimuli.Therefore, new ideas or ways for detection should be proposed to make use of external control [16][17][18][19][20][21][22][23][24].In order to realize such requirement, we will examine multi-functional compounds or hybrid functional systems composed of both fluorescent part of Zn(II) ion and external controlling parts of several mechanisms.
In the course studies on organic/inorganic hybrid functional materials composed on photochromic organic compounds and chiral metal complexes [25], the present target is aiming at application for fluorescent probes for trace metal ions.According to this concept providing benefits to realize the requirement above, we have proposed and constructed three systems, System 1, System 2, and System 3 (Figures 1-3) working well as fluorescent sensors for specific metal ions after screening tests.Coordination of metal ions to be analyzed and emission of not only chiral Schiff base Zn(II) complexes [25,26] but also photochromic spiropyran (Sp) or merocyanine (Mc) [27][28][29][30] (quenching by energy transfer [31][32][33] between these components) as well as Schiff base [34] are a key principle of the present strategy of system design.Characteristic features and benefit if any of each system are summarized below:   As depicted in Figure 1, system 1 is composed of trans-type chiral Schiff base Zn(II) complexes [25], as a fluorescence probe with blue emission, and spiropyran (Sp) or merocyanine (Mc) [27][28][29][30] As depicted in Figure 2, system 2 is composed of salen-type chiral Schiff base Zn(II) complexes [26].In order for comparison of qualitative coordination of Zn 2+ , Cu 2+ , and Gd 3+ ions, salen-type chiral Schiff base Zn(II) complexes are employed for this system.The salentype ones can act as fluorescence probes as well as direct coordination sites for metal ions to be analyzed.
Not only Zn 2+ , Cu 2+ , and Gd 3+ ions but also alkali and alkali earth ions were investigated for screening.Consequently, Ba 2+ , Ca 2+ , and Sr 2+ ions could also exhibit fairly quantitative behavior and detection of CD spectra as well as fluorescence spectra is proved to be useful for metal ion sensing.
As depicted in Figure 3, system 3 is hybrid systems composed of spiropyran (Sp) or merocyanine (Mc) and salen-type chiral Schiff base Zn(II) complexes [26].It may be straightforwardly expected specific coordination of Zn 2+ , Cu 2+ , and Gd 3+ ions, fluorescence quenching by Mc as well as CD detection of Zn(II) complexes, in addition, competition coordination of metal ions to be analyzed between two potential sites.However, the merit of this complicated system is elucidating mechanism of intermolecular interactions and spectroscopic features by means of metal coordination.

Materials
(Aldrich) and methanol (Kanto) were used as received without further purification.Trans-type [25] and salen-type [26] chiral Schiff base Zn(II) complexes were prepared according to the literature procedures.

Experimental Conditions
System 1: Screening of metal ions was carried out as follows: Hybrid system of 0.005 mM (M = mol/dm

Physical Measurements
Electronic spectra have been measured on a JASCO V-570 UV/VIS/NIR spectrophotometer in the range of 800-200 nm at 298 K. Circular Dichroism (CD) spectra have been measured on a JASCO J-820 spectropolarimeter in the range of 800-200 nm at 298 K. Fluorescence spectra have been recorded on a JASCO FP-6200 spectrophotometer at 298K.UV light irradiation (for opening Sp to form Mc) has been carried out using a Hayashi UV lamp (1.0 mW/cm 2 ).

System 1
As mentioned in METHODS, we have measured fluorescence intensity of system 1 with Zn 2+ and Gd 3+ ions after UV irradiation ( ex = 280 nm).As shown in Figure 4, the fluorescence intensity ( em = 460 nm) indicated linear correlation to the concentration of Zn
As shown Figure 4, increasing of concentration of Zn 2+ ion results in quantitatively increasing fluorescence intensity of a Zn(II) complex, while increasing of concentration of Gd 3+  Uniform decreasing of fluorescence intensity is explained that coordination of metal ions to be analyzed to the salen-type Zn(II) complex makes differences in electronic structures as binuclear metal complexes similarly, which do not indicate intense emission.Deviation from linearity for Gd 3+ is ascribed to coordination.

System 3
As for system 3, we have measured UV-vis spectra with the corresponding CD spectra (Figures 6, 8  obtained by changing excitation wavelengths.Moreover, these spectral changes before and after irradiation of UV light are in agreement with Sp (only sites of a salen-type Zn(II) complex can coordinate metal ions to be analyzed) and Mc (both sites of Mc and a salen-type Zn(II) complex can coordinate metal ions to be analyzed) to investigate competition coordination.
In Figures 7, 9, 11, obviously common CD peaks for system 3 at 260 and 280 nm or 410 nm are assigned to * band of phenyl groups or salicylaldehyde moiety, respectively.Obviously common absorption peaks for system 3 at 260 nm, 340 nm, and 530 nm are assigned to * band of phenyl groups or salicylaldehyde moiety, and Mc, respectively.Accordingly, competitive coordination and changes after UV irradiation for system 3 may be summarized Furthermore, according to UV-vis, CD and fluorescence spectra, coordination affinity to salen chiral Zn(II) complex in system 3 are found to be Zn 2+ > Cu 2+ > Gd 3+ and that to Mc is in reverse order.This result doesn't contradict that in system 1.Making use of these properties, we can remove these metals selectively in soil and water, including heavy metals and also apply those to a chromatography, an ion exchange resin and detoxification effects.

CONCLUSION
In summary, it should be noted that excitation and fluorescence wavelengths are ex = 260 nm and em = 460 nm for a trans-type Zn(II) complex, ex = 360 nm and em = 480 nm for a salen-type Zn(II) complex, and ex = 533 nm and em = 620 nm for Mc.By using the results, we have constructed and tested three (hybrid) systems for metal ion sensing for the first time.System 1 elucidates that only Zn

Figure 4 :
Figure 4: Calibration curves of fluorescence intensity ( ex = 280 nm) of a trans-type Zn(II) complex for system 1 as a function of concentration of analyzed Zn 2+ (upper) and Gd3+

Figure 5 :
Figure 5: Calibration curves of fluorescence intensity ( ex = 360 nm) of a salen-type Zn(II) complex for system 2 as a function of with concentration of analyzed Cu 2+ (upper), Zn 2+ ions before and after UV irradiation.Although these are hybrid systems, the CD spectra provide selective information of chiral species (chiral salen-type Zn(II) complexes).Because only system 3 indicated clear spectral changes detectable with CD spectra.Additionally, selective information based on fluorescence spectra of a salen-type Zn(II) complex ( em = c.a. 480 nm) and Mc ( em = 620 nm) can be

Figure 6 :
Figure 6: Comparison of CD and UV-vis spectra of system 3 with Gd 3+ ion before (upper) and after (lower) irradiation of UV light.Arrows emphasize spectral changes due to formation of Mc after UV light irradiation.

Figure 7 :
Figure 7: comparison of calibration curves of fluorescence intensity spectra of system 3 with Gd 3+ ion ( ex = 360 nm: before UV irradiation (upper) and after UV irradiation (middle) and ex = 533 nm (lower)).

Figure 8 :
Figure 8: Comparison of CD and UV-vis spectra of system 3 with Cu 2+ ion before (upper) and after (lower) irradiation of UV light.Arrows emphasize spectral changes due to formation of Mc after UV light irradiation.

Figure 9 :
Figure 9: comparison of calibration curves of fluorescence intensity spectra of system 3 with Cu 2+ ion ( ex = 360 nm: before UV irradiation (upper) and after UV irradiation (middle) and ex = 533 nm (lower)).In the case of Gd 3+ ion, after irradiation of UV light, CD peaks at 280 nm indicated definite changes, while those at 410 nm indicated little changes.Absorption peaks at 260 nm increased, while the peaks at 360 and 530 nm decreased.Fluorescence intensity increased for Zn(II) complex ( ex = 360 nm) and decrease for Mc ( ex = 533 nm) after UV irradiation.In the case of Cu 2+ ion, after irradiation of UV light, CD peaks at 260, 280, and 410 nm decreased quantitatively.UV-vis peaks at 260 nm increased, while the peaks at 360 and 530 nm decreased.The magnitude of decrease of the peak at 530 nm of Cu 2+ ion is much smaller than that of Gd 3+ ion.Fluorescence intensity decreased for Zn(II) complex ( ex = 360 nm)

Figure 10 :
Figure 10: Comparison of CD and UV-vis spectra of system 3 with Zn 2+ ion before (upper) and after (lower) irradiation of UV light.Arrows emphasize spectral changes due to formation of Mc after UV light irradiation

Figure 11 :
Figure 11: comparison of calibration curves of fluorescence intensity spectra of system 3 with Zn + ion ( ex = 360 nm: before UV irradiation (upper) and after UV irradiation (middle) and ex = 533 nm (lower)).In the case of Zn 2+ ion, quantitative spectral changes could be observed for CD bands at 260, 280, and 410 nm and little spectral changes also could be observed for UV-vis bands at 530 nm in contrast to Cu 2+ or Gd 3+ ion.However, fluorescence intensity linearly decreased for Zn(II) complex ( ex = 360 nm) and almost linearly increased for Mc ( ex = 533 nm) as a function of concentration of doped Zn 2+ ion.Partially unclear behavior of ions concentration dependence of intensity may be attributed to incomplete association of metal ions.
as follows: It seems that Gd 3+ ion transferred to from Zn(II) complex to Mc. Competitive coordination of Cu 2+ ion to Zn(II) complex (major) and Mc (minor) seem to occur.It appears Zn 2+ ion coordinates to Zn(II) complex consistently.
intensities of (not directly coordinating Mc but) trans-type Zn(II) complexes as functions of concentration of ions to be analyzed.System 2 elucidates that Zn 2+ , Cu 2+ , and Gd 3+ ions (as well as Ba 2+ , Ca 2+ , and Sr 2+ ions more fairly) exhibit quantitative decrease of fluorescence intensities of salen-type Zn(II) complexes as a function of directly coordinated metal ions.System 3 elucidates not only experimentally confirming facts of the quantitative changes of fluorescence intensities but also competitive coordination mechanism between salen-type Zn(II) complexes and Mc of Zn 2+ , Cu 2+ , and Gd 3+ ions in methanol solutions.Development of application to ion sensing with supramolecular systems can be also expected.