Some Hybrid Systems of Chiral Schiff Base Zn(II) Complexes and Photochromic Spiropyrans for Environmental Ion Sensing

ABSTRACT
 This is a review article including our recent results and some previous photo functional hybrid system having potential applications for environmental ion sensing. We have prepared several new and known chiral Schiff base Zn(II) complexes and measured (and also calculated) absorption and fluorescence spectra for sole complexes. After assembling hybrid systems with 1,3,3-trimethylindolino-6’-nitrobenzopyrylospiran (SP) in methanol solutions, we measured spectral changes before and after alternate irradiation of UV and visible light. Intensity of fluorescence spectra for pale yellow Zn(II) complexes (λem = 450 nm, λex = 270 and 360 nm) was quenched by colorless SP (λem = 533 nm, λex = 612 nm). After UV light irradiation to form purple merocyanine (MC), photoisomerization resulted in changes of the intensity of absorption spectra as well as fluorescence spectra. Thus the hybrid systems could successfully act as molecular logic circuit by input (excitation by light) and output (intensity of fluorescence peaks). Moreover, we investigated concentration dependence of doped Zn(II) and Cu(II) ions to confirm quenching of intensity of fluorescence peaks by Zn(II) and Cu(II) MC complexes for metal ion sensing in solutions.
 Keywords
 Zinc(II) Complexes, Spiropyran, Chirality, Fluorescence, Ion Sensing
 
 1. Introduction
 Many machines incorporate electronic components and integrate circuits for their appropriate functionality, which are essential for modern society [ http://file.scirp.org/Html/1-2200946_49119.htm#r11 http://file.scirp.org/Html/1-2200946_49119.htm#ref1] . Besides conventional miniaturization of logic circuits made of silicon semiconductors, new attention has been attracted to the molecular logic circuits using molecules and molecular assemblies as next-generation high-performance logic circuits [ http://file.scirp.org/Html/1-2200946_49119.htm#r22 http://file.scirp.org/Html/1-2200946_49119.htm#ref2] [ http://file.scirp.org/Html/1-2200946_49119.htm#r33 http://file.scirp.org/Html/1-2200946_49119.htm#ref3] . The response speed by using an optical signal is expected to exceed conventional electrical signals, which is significant point for next-generation research and design of molecular logic circuits for an optical signal. In designing logic circuits optical organic molecules [ http://file.scirp.org/Html/1-2200946_49119.htm#r44 http://file.scirp.org/Html/1-2200946_49119.htm#ref4] -[ http://file.scirp.org/Html/1-2200946_49119.htm#r77 http://file.scirp.org/Html/1-2200946_49119.htm#ref7] , biomolecules (DNA [ http://file.scirp.org/Html/1-2200946_49119.htm#r88 http://file.scirp.org/Html/1-2200946_49119.htm#ref8] [ http://file.scirp.org/Html/1-2200946_49119.htm#r99 http://file.scirp.org/Html/1-2200946_49119.htm#ref9] and proteins [ http://file.scirp.org/Html/1-2200946_49119.htm#r1010 http://file.scirp.org/Html/1-2200946_49119.htm#ref10] -[ http://file.scirp.org/Html/1-2200946_49119.htm#r1212 http://file.scirp.org/Html/1-2200946_49119.htm#ref12] ), and organic/inorganic hybrid materials [ http://file.scirp.org/Html/1-2200946_49119.htm#r1313 http://file.scirp.org/Html/1-2200946_49119.htm#ref13] , especially metal complexes [ http://file.scirp.org/Html/1-2200946_49119.htm#r1414 http://file.scirp.org/Html/1-2200946_49119.htm#ref14] have been employed. Containing photochromic dyes showing their color changes by photoisomerization and functional metal complexes may be one of the promising strategies. In this context, some ideas for constructing logic circuits based on molecular assemblies have been proposed by using metal complexes and photochromic dyes. Great attention has been paid to use weak intermolecular interactions between each component of multifunctional logic circuits of molecular assemblies.
 Previously, we have proposed “photoresponsive solvatochromism” [ http://file.scirp.org/Html/1-2200946_49119.htm#r1515 http://file.scirp.org/Html/1-2200946_49119.htm#ref15] in hybrid system consisting of azobenzene derivatives and chiral Schiff base Ni(II), Cu(II), or Zn(II) complexes showing various stereochemical changes (by thermal phase transition), paramagnetism in solutions, and potential functions as chiral catalysts. Additionally, we have reported that increasing optical anisotropy of not only azobenzene but also chiral Schiff base complexes after polarized UV (and visible) light irradiation was detected by polarized electronic and IR spectroscopy [ http://file.scirp.org/Html/1-2200946_49119.htm#r1616 http://file.scirp.org/Html/1-2200946_49119.htm#ref16] . In general, molecular orientation (long-axis) of azobenzene is gradually aligned to perpendicular to the direction of electric vector of polarized UV light accompanying with reorientation or photoisomerization to the cis-form.
 
 In this work, we have prepared three new and one known Schiff base Zn(II) complexes, Zn-secBu, Zncyclohexyl, Zn-2-naphtyl, and Zn-1-phenyl (Figure 1 http://file.scirp.org/Html/1-2200946_49119.htm#f1) and have investigated their absorption and fluorescence spectra sole components and hybrid systems with photochromic 1,3,3-trimethylindolino-6’-nitrobenzopyrylospiran (SP) [ http://file.scirp.org/Html/1-2200946_49119.htm#r1717 http://file.scirp.org/Html/1-2200946_49119.htm#ref17] -[ http://file.scirp.org/Html/1-2200946_49119.htm#r2020 http://file.scirp.org/Html/1-2200946_49119.htm#ref20] before and after UV or visible light irradiation. In addition, we have constructed threecomponent hybrid systems of Schiff base Zn(II) complexes with SP, and Zn(II) or Cu(II) ions in methanol solutions. Moreover, we have also investigated their absorption and fluorescence spectra before and after UV or visible light irradiation to discuss spectral changes of the intensity of fluorescence bands of Zn(II) complexes and/ or SP (MC) by doping Zn(II) or Cu(II) ions in order for metal ion sensing [ http://file.scirp.org/Html/1-2200946_49119.htm#r2121 http://file.scirp.org/Html/1-2200946_49119.htm#ref21] -[ http://file.scirp.org/Html/1-2200946_49119.htm#r2323 http://file.scirp.org/Html/1-2200946_49119.htm#ref23] by concentration of doped metal ions.
 

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