Fabrication And Performance Study Of Polyoxometalate-Based Film Materials Guided By Fluorescent Switching

Polyoxometalates,abbreviated as POMs,are a kind of inorganic metal-oxygen cluster in nanoscale.Owing to the features of rich chemical composition,inexhaustible structural diversity,and tunable properties made them have the following particular features: being highly soluble in water,having good thermal resistance,strong oxidative stability,well reversibility,fascinating electronic and protonic storage capacities,as well as flexible stepwise redox ability,being environmentally friendly,non-pollution and so on,hence,aroused the extensive research interest of the numerous researchers.Fluorescent switching refers to the appearing and disappearing of the fluorescence signal by external stimuli under the UV lamp.In recent years,POM-based fluorescent switching functional composite materials have the potential application values in the field of smart light windows,chemical sensing,optical display,fluorescence imaging,information storage,and so on,that promoting the research progress of fluorescent switching of functional composite materials.The realization of the fluorescent switching rely on that both the photoluminescence component and stimuli discoloration component containing in the functional composite materials.While POMs own the excellent photochromic and electrochromic performance,moreover,luminescent lanthanide-containing POMs themselves contain the luminescent component rare-earth ions and discoloration component POMs,therefore,they can be fabricated in the functional composite materials to study the change of the fluorescent switching when stimulated by external condition.The realization of the fluorescent switching function is through the fluorescence resonance energy transfer between the colored POM and luminescent component.The conditions of the energy transfer mainly have three points: the first one is that the energy donor and the energy acceptor are within a certain distance;the second one is that energy donor and energy acceptor ordered in proper ways;the third one is that the emission spectra of the energy donor and the absorption spectra have the overlap.Based on the above conditions,through the molecular design,we constructed a series of POMs-based fluorescent switch materials.In this paper,we integrated the electrochromism performance of POMs and the luminescent performance of the water soluble organic luminescent molecule and the rare-earth luminescent POMs,constructed the discoloration-luminescence composite film materials through the different assemble technology,and then realized the fluorescent switching of monochrome state containing red luminescence,green luminescence,orange luminescence and white luminescence,and bi-color state containing white luminescence to blue luminescence.Specific research contents are as follows:First,water-soluble green luminescence molecule 8-hydroxypyrene-1,3,6-trisulfonic aci trisodium salt（abbr: HOPTS）was chosen as the research object,through the combination of electrodeposition and Layer-by-Layer assemble（abbre: LBL）method,and obtained the hybrid GO@HOPTS by loading the HOPTS on the graphene oxide（GO）surfaces so as to assemble with POMs,and two kinds of green luminescence composite film materials were successfully fabricated.The first kind green luminescence film is firstly fabricated on conductive ITO-coated glass electrode by a facile electrodeposition method,then,the multilayer films containing different layers of P5W30 [（HOPTS）50/（PDDA/P5W30）n]（n = 10,film 1;n = 27,film 2;n = 57,film 3）were further fabricated by LBL technology on the as-prepared green luminescence film.The second kind green luminescence film {（PDDA/P5W30）5/[（PDDA/P5W30）5/PDDA/GO@HOPTS]15/（PDDA/P5W30）5} containing P5W30 and GO@HOPTS is fabricated on the quartz substrate and conductive ITO-coated glass electrode by LBL technology based on the electrostatic attraction of oppositely charged species.Electrochemical activity of the P5W30 in composite films were investigated by cyclic voltammetry（CV）,fluorescent property of HOTPS were characterized by fluorescence spectra,surface morphology of composite films were recorded by atomic force microscopy（AFM）,and constituent elements of composite film were detected by X-ray photoelectron spectroscopy（XPS）.Moveover,electrochromism performance and the fluorescent switching behaviors of composite films were investigated by in situ UV-vis and fluorescence spectro-electro-chemical system.The results showed that both kinds of film materials all display the fluorescent switching performance under the stimulus of electrochemistry,and display the good reversibility and stability.By further comparison,we can find that films fabricated by the combination strategy of electrodeposition and LBL method have a wider voltage range.Hence,this combination strategy can provide a new technology for the assembly of the functional materials.Second,we chose the orange luminescence cluster Sm PW11 containing Sm³⁺ as the research object.Sm PW11 is a molecular dyad containing the orange luminescence component Sm³⁺ and discoloration component PW11.First,the UV-vis absorption property,electrochemical activity and luminescence property of Sm PW11 were studied in different p H solutions to determine the optimal p H solution medium;and then,the orange fluorescent switching performance of Sm PW11 was investigated under the optimized p H solution medium.Subsequently,we successfully fabricated the orange luminescence composite films [PDDA/P2W18]10/[PDDA/Sm PW11]60 and [PEI/P2W18]10/[PEI/Sm PW11]60 on the ITO-coated conductive glass by choosing two kinds of the conducting polymers PEI and PDDA that having same component,different structures,different charges as molecular linkers.Orange luminescence composite films were characterized by UV-vis spectra,fluorescence spectroscopy,CV,chronoamperometry,XPS and AFM technology.Finally,we studied the fluorescent switching performance of orange luminescence composite films by in situ UV-vis and fluorescence spectroelectrochemical systems.The results indicated that the orange luminescence composite films not only display the orange luminescence but also display the reversible orange luminescence switching behaviors via the intramolecular energy transfer between the orange luminescence component Sm³⁺ and electroreduced species of PW11 under the stimulus of electrochemistry.Moreover,compared with PDDA,PEI was considered as a better molecular linker to prepare POMs-based composite film materials.Third,we chose the white luminescence cluster Dy PW11 containing Dy³⁺ as the research object.First,the UV-vis absorption spectra,electrochemical activity and luminescence spectra of Dy PW11 were studied in different p H solutions,and stable ranges of p H and the optimal p H solution medium were found finally;and then,the white fluorescent switching performance of Dy PW11 was investigated under the optimized p H solution medium.Subsequently,through the reasonable molecular design,we successfully fabricated three white luminescence composite films（PEI/Dy PW11）41,（PEI/P8W48）1/（PEI/Dy PW11）5}9 and {（PEI/P8W48）3/（PEI/Dy PW11）5}9,and studied the fluorescent switching behaviors of three white luminescence films between-0.7 V and 0.7 V by spectroelectrochemical systems.The results manifested that,on the one hand,the three white luminescence films all display good reversibility and stability;on the other hand,the film（PEI/Dy PW11）41 realized the incomplete white fluorescence quenching,the film {（PEI/P8W48）1/（PEI/Dy PW11）5}9 achieved the transfer from white to blue fluorescent switching,and the film {（PEI/P8W48）3/（PEI/Dy PW11）5}9 realized the complete white fluorescent switching.Hence,our work not only fabricated the white luminescence composite films constructed by pure inorganic white luminescence material but also provides a potential method to selectively quench the specified luminescence.Fourth,on the basis of the first part of research work,we chose the pure inorganic constituent Tb Ge W cluster containing Tb³⁺ with green luminescence as the research target.First,the UV-vis absorption spectra,electrochemical activity and luminescence spectra of Tb Ge W were studied in different p H solutions,and stable ranges of p H and the optimal p H solution medium were found finally;and then,the green fluorescent switching performance of Tb Ge W was investigated under the optimized p H solution medium.Subsequently,we successfully fabricated the green luminescence film（PEI/Tb Ge W）42 using the protonated polyelectrolyte PEI as molecular linker,however,fluorescence quenching degree of this film is only up to 24.6% upon the potential of-0.9 V.In order to promote the fluorescence quenching degree of green luminescence films in lower potential,throuth molecular design,we added the electrochromism component P2W18 in green luminescence film so as to fabricate the film（PEI/P2W18/PEI/Tb Ge W）42 and {（PEI/P2W18）3/PEI/Tb Ge W}42 with different ration of P2W18 and Tb Ge W,fluorescence quenching degree are up to 56.9% and 94.7% upon the potential of-0.7 V,respectively.The results declared that owing to the intramolecular energy transfer between the green luminescence component Tb³⁺ and discoloring component Ge W as well as the intermolecular energy transfer between green luminescence component Tb³⁺ and discoloration component P2W18,we realized the green fluorescent switching of film materials based on the pure green luminescence inorganic cluster Tb Ge W.Fifth,we chose the red luminescence cluster Eu PW11 containing Eu³⁺ as the research object.First,the UV-vis absorption spectra,electrochemical activity and luminescence spectra of Eu PW11 were studied in different p H solutions,and stable ranges of p H and the optimal p H solution medium were found finally;and then,the red fluorescent switching performance of Eu PW11 was investigated under the optimized p H solution medium.Subsequently,we successfully fabricated the film（PEI/Eu PW11）39 using the protonated polyelectrolyte PEI as the molecular linker by LBL method.Upon the potential of-0.7 V,fluorescence quenching degree of red luminescence film（PEI/Eu PW11）39 was only up to 69%.In order to realize the complete red fluorescent switching,throuth molecular design,we added the electrochromism component P5W30 in red luminescence film,and fabricated red luminescence film（PEI/P5W30/PEI/Eu PW11）39 by merging optimal electrochromism constituent.Upon the potential of-0.7 V,fluorescence quenching degree of red luminescence film（PEI/P5W30/PEI/Eu PW11）39 can up to 94%.The results proved that red luminescence can be quenched by reasonable introducing electrochromism component POMs.In all,guided by fluorescent switching,using the POMs as the building block,with the intramolecular and intermolecular energy transfer as the basic principle,we fabricated a series of film materials with different luminescence,respectively,and then realized the fluorescent switching of monochrome state containing red luminescence,green luminescence,orange luminescence and white luminescence,and bi-color state containing the transfer between white luminescence and blue luminescence.Moveover,we developed a new method to contruct film materials,namely the combination method of electrodeposition and LBL method.Hence,this work not only promote the development of the POMs in the field of fluorescent switching materials,but also provide beneficial reference values for the research of the development of the pure inorganic fluorescent switching materials.