Inorganic–Organic Hybrid Photochromic Crystalline Materials:Design,Synthesis And Properties

Smart materials are highly desirable over the recent years with the development of social technology and the change of human needs.As a new type of smart material,stimuli-sensitive materials is capable of judging and properly handling external stimuli?light,heat,electricity,machinery,chemistry,etc.?by itself,so it exhibit widespread potential applications in intelligent identification,safety protection,detection and ink-free printing.Photochromic materials as a sensitive new intelligent material have been widely concerned by scientists from all over the world.Yamase research group has carried out a detailed study on organicamine-polyoxometalates?POM?photochromic materials and proposed the photochromic mechanism of these compounds.However,challenges emerge in the development of the photochromic materials:?1?exceptional sensitive photochromic compounds are very rare which is not only used in photofluorescence switch,but also can also be quickly written and erased for memory storage objects in the computer;?2?POM has been widely used in photochromic materials,some commonly used organic ligands,such as organic amines,carboxylic acids,aromatic sulfur and spiropyrans,are used to synthesize polyoxometalate-based hybrid photochromic material.However,there are few reports on crystalline viologen-polyoxometalate hybrid photochromic materials.Only two crystalline compounds have been reported in the form of hydrogen bonds before on this research;?3?Unfortunately,the most known chromic materials only respond to monostimulus or have limited mono-application,which greatly limits the practical application of the materials.Smart materials that combine multiple functions into one photochromic materials will be a challenge for the world's leading edge materials science.In particular,the design of synthetic toxic substances?amine?emergency monitoring materials and environmental protection,low consumption of ink-free printing combined in one materials have not been reported.In response to these challenges,a series of novel coordination polymers were synthesized from electron-deficient pyridine derivatives?viologens,4-phenylpyridine,4,4'-bipyridine?as organic ligands and electron acceptors and electron-rich POM/multicarboxylic acid ligands as electron donors.By the principle of crystal engineering design,two cases of exceptional photosensitive covalent bonding POM-viologen crystalline hybrids were reported for the first time.In addition,two photochromic compounds with multiple stimulus responses were constructed and one of the compounds was studied in ultraviolet detection,amine selective detection and ink-free erasable printing.This thesis covers the following two partsPart One:Synthesis,characterization and photochromic mechanism of exceptional sensitive polyoxometalate-pyridine crystalline hybrid photochromic materialsPhotochromic materials have become one of the research hotspots because of their practical and potential applications in daily life and high-tech fields.However,exceptional sensitive photochromic compounds are very rare.POM photochromic materials are of great significance in the exploration of remote photo-controlled catalysis and photo-controlled switching.The first case of viologens-POM crystalline photochromic material was reported in 2010,and several examples of viologens-POM crystalline materials synthesized later were mostly bonded by hydrogen bonds.Compared to ionic bonds,covalent bonding will greatly shorten the distance between electron donor and electron acceptor to improve the photochromic rate.In order to design exceptional sensitive photochromic materials,for the first time,we successfully designed and synthesized two new crystalline polyoxomolybdate-viologen hybrids,compound 1-2.In addition,when studying the photochromic mechanism of these compounds,we found new and different phenomena and compared with the photochromic mechanism of POM proposed by Yamase,revealing that the photochromic process of amine—POM does not necessarily require the transfer of protons.Subsequently,compound 3-5 was synthesized for further verification.?1?As reported by Marcus,the nearer an electron donor and an electron acceptor,the faster the electron transfer.Dispersion of the salts in a solution or film may cause the separation of electron donors and electron acceptors,which does not favor the electron transfer and will weaken the photosensitivity.An applicable method to avoid this possibility is to assemble in the coordination mode.Therefore,the purpose of this work is to construct exceptional sensitive POM-based photochromic materials and study their photochromic mechanism.To fulfil this aim,we synthesized two new crystalline covalent bonding polyoxomolybdate-viologen hybrids photochromic materials for the first time by selecting two viologens ligands?one of the organic amines?with two terminal pyridyl N atoms and?NH4?6Mo7O24·4H₂O in the hydrothermal conditions.[?Bpyen?2(Mo₈O₂₆)]·2H₂O?1?[?Pbpy?2(Mo₈O₂₆)]4H₂O?2?The two compounds had exceptional sensitive photochromic properties,and the light response time was 5s and Is,respectively,which verified the Marcus mechanism By means of theoretical calculation,X-ray single crystal diffraction,X-ray photoelectron spectroscopy,single-electron paramagnetic resonance and solid diffuse reflection and so on,we not only made an in-depth study on the structure—property relationship and electron competition relationship between the two compounds,but also found a new photochromic mechanism.The results reveal that the photochromism should originate from the electron transfer between the OMo-o? Mo?VI?and viologens cation,forming viologens radicals and Mo?V?.Moreover,due to the co-existence of the two electron acceptors,Mo?VI?and viologens,the electron competition relationship is generated.The weak electron-withdrawing ability of the viologens ligand favors photoreduction of Mo?VI?to Mo?V?,while the one with strong electron-withdrawing ability is conversely.Then,we took these two compounds as a case study,and the results showed that the reversible photochromic properties did not necessarily need the transfer of protons,indicating that the proton-transfer mechanism proposed by Yamase is not suitable for the above two compounds.?2?In the previous work,we did a preliminary analysis of the two compounds showing that reversible photochromic properties can be achieved without the need for proton transfer.However,organoammonium cations in these two compounds do not contain transferable protons,so it is necessary to find a new compound model to clarify whether proton transfer is needed.In order to achieve this goal,it is necessary to find a model compound whose electron-accepting ability is highly different for protonation and deprotonation.In this work,we firstly synthesized the two new adducts 4-phenylpyridine?4pp?and carboxylic acid with unprotonated and protonated 4pp,respectively,?4pp?·?H₃tma??3;H₃tma=trimesic acid?and[H?4pp?]?H₃pma??4;H4pma=pyromellitic acid?,and studied their protonation dependent photochromic behaviour.Then,a new ammonium polyoxomolybdate[H?4pp?]4Mo₈O₂₆·2?4pp??5?was synthesized using the protonated 4pp as an indicator for proton transfer.?4pp?·?H₃tma??3?[H?4pp?]·?H₃pma??4?[H?4pp?]4Mo₈O₂₆·2?4pp??5?Electron-transfer photochromic properties of two crystalline adducts of 4pp and carboxylic acids have been studied and it has been revealed that only protonation of 4-phenylpyridine can enable photoinduced charge transfer?PCT?to achieve photochromism.That is,the protonation of 4-phenylpyridine is an electron-deficient system,and can only obtain electrons in the photochromic process producing pyridine radicals.In this case,we assume that the H atom in nitrogen should not be able to transfer to the POM.Then,through theoretical calculation,X-ray single crystal diffraction,X-ray photoelectron spectroscopy,electron paramagnetic resonance,IR,PXRD and nuclear magnetic test method we investigated the photochromism of new synthesis amine—polyoxomolybdates compounds 5,combining the control experiments revealed that,proton transfer from organoammonium did not necessarily occur in the PCT process of ammonium polyoxomolybdate.Part Two:multistimuli responsive photochromic materials:Syntheses and PropertiesThe research and development of smart testing materials has become one of the hottest issues in the development of human science and technology and social life.However,most smart materials only have a mono-application,and assembling multiple functions into one molecule will be a challenge for the world's leading materials science.?1?A novel zwitterionic metal—organic framework was synthesized by utilizing a flexible zwitterionic ligand H₃CbdcpCl?Cbdcp=N-?4-carboxybenzyl?-?3,5-dicarboxyl?pyridinium?and auxiliary ligand 4,4'-bipy.[Zn?Cbdcp??4,4'-bipy?0.5]·2H₂O?Zn-MOF??6?The compound 6 is a novel zwitterionic metal—organic framework with large holes.Through experimental data,the total and partial densities of states?DOS?and electrostatic potential surfaces?ESP?of coordination molecules,we simulated the competition mechanism between pyridine and 4,4'-bipy,The results showed that 4,4'-bipy has stronger electron-accepting ability than pyridine,and O atoms from benzene carboxylic groups are mainly electron donors.Moreover,the compound not only presented photochromic behaviors originating from the photoinduced electron transfer and the formation of bipyridinium radicals,but also exhibited fast vapochromic phenomenon for NH₃,EA,and PA.These discoveries will help the exploration of MOF-based photochromic and vapochromic sensors through choosing a suitable ligand.?2?The purpose of this work is to synthesize a novel multi-functional smart detection material.By viologens ligand?bcbpy?,pyromellitic acid,Zn2+,a novel multifunction photochromic metal-organic framework is successfully constructed integrating ultraviolet detection,amine selective detection and inkless and erasable printing.[Zn?bcbpy?0.5?pma?0.5?H₂O?]·3H₂O?7?The compound 7 exhibited an obvious colour change from colourless to blue under simulated UV light and solar UV-light radiation.The appropriate wide band gap of the material excludes the absorption of visible light.Meanwhile,the compound has a fast response to UV light with an intensity as low as 0.001 mW cm-2.Furthermore,based on the Lewis acidic site and redox ability of the bipyridinium ligand,compound showed amine-selective sensing upon contact with different amine vapors?NH₃,ethylamine,n-propylamine,n-butylamine,diethylamine and triethylamine?and was also deposited on paper for use as portable test strip.The rapidly responsive photochromic compound could also be used as an inkless and erasable print medium with low cost and environmentally friendly nature,and the resulting print was well-recognized by naked eye.The printed content was found to remain clear for 18 h in the dark,showing its good stability.Furthermore,it could be printed repeatedly for at least 5 rounds with clear color changes,which makes compound suitable for performing severally stable printing-erasing cycles with the same paper.