| Polyoxometalates (POMs) are a kind of inorganic polynuclear clusters, which hasexperienced nearly200years since their discovery. POMs have prominent features in thechemical composition diversity, determined structures, redox reversibility, photochromic orelectrochromic phenomenon and luminescence for rare earth containing POMs emittingred light with long lifetime, good monochromatic and large stokes shift. So, POMs haveattracted increasing attention in photochemistry, electrochemistry, catalysis chemistry,material chemistry and other fields, which greatly promoted the development of POMs inoptical materials field.The fabrication of film is the basis to realize the luminescent switching devices.Because of the flexibility of assembly process and extensive material sources,layer-by-layer (LBL) technique is a most common method for the film fabrication, whichis proved to be reliable and controllable on molecular level. The multilayer film usuallydisplays reversible photo-and electrochromic properties, and the film changes to blueunder photo irradiation and electrochemical reduction. And POMs at reduced state are socalled―heteropoly blue‖with a broad absorption in the visible light region correspondingto the intravalence charge transfer (IVCT), which can be used as the acceptor in energytransfer process. We can design luminescent switches by combining the electrochromicPOMs with fluorescence property. Luminescent switching molecules usually exhibitreversible light emitting and quenching under external stimuli such as light irradiation,electrochemical redox and thermal stimulation. Electrochemical modulation has uniqueadvantages such as quick response, good controllability, good reversibility andenvironment friendliness thus attractting more and more research interest due to theirpotential application in the information storage, optical display devices and biosensors.In this paper, we designed a series of luminescent switches by combining theelectrochromic POMs with fluorescence property of luminescent components by thesynergies or energy transfer between the two components. We fabricated the luminescent film by LBL self-assemble method, and then realized the luminescent switching byelectrochemical stimulation. The relationship between the energy transfer efficiency andthe number of electrons transferred was studied in detail. Next, we realized the applicationin biosensor or chem-sensors of luminescent switch based on luminescent POMs. Finally,we designed luminescent switch by combining the electrochromic POMs with greenfluorescence property of luminescent components by the synergies or energy transferbetween the two components. And we realized the luminescent switching of the mixedsolution. The research work includes three parts as following:Firstly, we realized the electrochemical modulation luminescent switching byintermolecular energy transfer based on the principle of functional complementary. Wechose H3PW12O4022H2O (PW12), K6P2W18O6219H2O (P2W18),K12.5Na1.5[NaP5W30O110]15H2O (P5W30) and K28Li5[H7P8W48O184]92H2O (P8W48). Theypossess different negative charge and the number of electrons reduced. The reduced POMsserved as the energy acceptor and the Ru(phen) was choose to serve as the energy donor.Poly(ethyleneimine)(PEI) and poly(styrenesulfonate)(PSS) were empolyed as thecrosslinking agent and spacer element, respectively. We fabricated four kinds of mutilayerfilms by LBL method:[PEI/PW/Ru(phen)/PSS]n. The film was characterized by XPS andAFM. XPS showed that POMs and Ru(phen) were successfully assembled on the film.AFM images indicated that the films were relatively uniform and smooth. UV-Vis spectrapointed out that the deposition process was linear and highly reproducible. We studied theelectrochemical stability of films under double potential steps between0.7V and0.7V,the UV-Vis absorption at650nm and the current experienced no obvious change aftermany cycles, which demonstrated the stability of these films. And the film wascharacterized by UV-Vis, PL spectrum and CVs for the optical-electron activity. The filmchanged from transparent to blue with the appearance of a wide absorption in the visiblelight region when applied a reduction potential0.78V. At the same time, theluminescence quenched obviously. In the case of an oxidation potential0.78V, theabsorbance decreased and the blue color gradually disappeared with the disappearance ofthe absorption in the visible region. Meanwhile the fluorescence recovered. And thefluorescence integrated areas were almost identical at both―on‖and―off‖states after manycycles. Finally, we investigated the relationship between the energy transfer efficiency andthe number of electrons reduced. It was found that, for four kinds of thin film materials in the experiment, the more electrons reduced of POMs, the higher the efficiency of energytransfer was. And the E/R (E is energy transfer efficiency, R is surface coverage ratiobetween POMs and Ru(phen)) of PW12, P2W18, P5W30and P8W48were0.14,0.51,0.97and1.78, respectively.Secondly，we designed a chem-sensor for the detection of ascorbic acid by combiningintramolecular energy transfer and redox based on K13Eu(SiMoW10O39)2. TheK13Eu(SiMoW10O39)2solution exhibited a red light emitting at open circuit state; whenapplied reduction potential0.2V, the color of the solution changed from colorless to bluewith the appearance of a wide absorption in the visible light region. At the same time thefluorescence of the solution quenched. Ascorbic acid is a kind of reducing reagent andPOMs can be reduced by ascorbic acid. Comparing with electrochemical methods, thereduction time was shorter. The optimal concentration of ascorbic acid sensor, the linearrange, reaction balance time and other experimental conditions were determined bydynamics test of UV-Vis spectroscopy. The experimental results suggested that the UV-Visabsorbance at518nm and logarithm of the fluorescence intensity at618nm increasedlinearly with the concentration of ascorbic acid in the linear range of0to1.2mM. Thiswork provided a new route for the potential application in chem or bio-sensors based onPOM luminescent switch.Thirdly, we realized the electrochemical controlled green luminescent switching byintermolecular energy transfer based on the principle of functional complementary. Wechose K6P2W18O6219H2O (P2W18) with more reductive electron numbers and lessresponsive time. The reduced POM served as the energy acceptor and the8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) acted as the energy donor.PEI and PSS were used as a crosslinking agent and a spacer element, respectively. Wefabricated mutilayer film by LBL method:[PEI/P2W18/PEI/GO@HPTS]n. UV-Visspectroscopy suggested that the deposition process was linear and highly reproduciblefrom layer to layer, indicating that P2W18and GO@HPTS were successfully assembled onthe film. And the film was characterized by UV-Vis, PL spectrum and CVs for theoptical-electron activity. The mixed solution changed from light green to blue with theappearance of a wide absorption in the visible light region when applied a reductionpotential0.75V. At the same time, the luminescent quenched. In the case of an oxidationpotential0.75V, the blue color gradually disappeared with the disappearance of the absorption in the visible region, meanwhile the fluorescence recovered. And thefluorescence quenching of the mixed solution is92.8%in30min, indicating that theproposed idea is feasible. The research provided the experimental basis for regulating thegreen fluorescence switching for the next step. |
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