Fabrication And Characterization For Luminescent Organized Molecular Aggregates Based On Surfactants And Lanthanide Polyoxometalates Or Complexes

Based on their own special electronic configuration and rich 4f electron orbitals,trivalent rare earth ions possess excellent photophysical properties,such as narrow emission bands,long lifetimes(millisecond-order),large Stokes shift,wide emission range(from visible to near-infrared regions)and so on.Meanwhile,owing to the antenna effect,lanthanide polyoxometalates or complexes have significantly enhanced fluorescence properties and can be used as ideal luminophores.In recent years,the lanthanide-containing luminophores have been co-assembled into organized molecular aggregate matrix through non-covalent interactions,which can effectively improve their luminescent properties,photothermal stability and processability.Moreover,combining the rich aggregate morphology,sensitive environmental stimuli responsibility,and easily processing of organized molecular aggregates with the sensitivity of the photophysical properties of rare earth ions to their surrounding microchemical environment,the intelligent rare earth luminescent soft aggregates or ordered film materials with certain detection abilities are expected to be preparedIn addition,as a new class of non-aqueous solvent,ionic liquids(ILs)are gradually used as prominent assembly media for organized molecular aggregates;at the same time,they can also be used as ideal dispersing solvents for lanthanide-containing luminophores.Therefore,it is expected to obtain rare earth luminescent soft materials with both stability and enhanced luminescence performances by designing and constructing organized molecular aggregates based on ILs,which will expand the design scope of ordered luminescent materials.It is also of great significance for the development of functional applications for ILs and organized molecular aggregates.In this dissertation,a series of responsive organized molecular aggregates with enhanced luminescence were constructed through co-assembly of lanthanide polyoxometalates or complexes with surfactants.With IL-mediated lyotropic liquid crystals as the matrices,we also obtained improved full color tunable luminescent soft aggregate materials.The organized multicolor-emitting films were prepared by in situ photopolymerization of lyotropic liquid crystal doped with lanthanide complexes.In general,the effects of organized molecular aggregate matrix on the luminescence properties of lanthanide-containing luminophores were systematically investigated,and the ways to enhance their photophysical properties were also explored.The main outline and contents of this dissertation include the following five parts:1.A commercially available and pH-switchable zwitterionic surfactant,tetradecyldimethylamine oxide(C14DMAO),was chosen to co-assemble with a lanthanide-containing anionic POM,(Na9(EuW10O36)·32H2O,abbreviated as EuW10)in water.The much improved red-emitting luminescent nanobelts at a C14DMAO/EuW10 molar ratio(R)of 20 were obtained,which exhibited longer luminescence lifetime and higher quantum yield compared with EuW10 aqueous solution.After careful characterization of morphology and structure of nanobelts,an unusual axial lamellar aggregation arrangement mechanism was proposed.It was the partial protonation of C14DMAO at the solution pH of about 6.5 that led to positively charged micelles,being bridged by anionic EuW10 clusters to aggregate into such novel nanobelts under the synergetic effects of appropriate electrostatic,hydrogen bonding and hydrophobic interactions.The resulted pH responsive luminescent nanobelts and their aggregation model should offer attractive references for preparing smart optical supramolecular materials2.The co-assembly from EuW10 and a commercially available cationic surfactant,myristoylcholine chloride(Myr),in water could produce enhanced red-emitting luminescent aggregates,with their photophysical properties highly dependent on the molar ratio(R)between Myr and EuW10.R of 36 was finally selected owing to the displayed superior luminescence intensity and good aggregate stability.The Myr/EuW10 hybrids induced by electrostatic and hydrophobic forces presented practically as multilamellar spheres with diameters varying from 80 to 300 nm.Compared to aqueous solution of EuW10 nanocluster,a 12-fold increase in absolute luminescence quantum yield(-23.3%)was observed for hybrid spheres,which was ascribed to the efficient shielding of water molecules.An unusual aggregation arrangement mechanism and the excellent photophysical properties for these aggregates were thoroughly investigated.Both the enzyme substrate character of Myr and the sensitive coordination structure of EuW10 to surrounding environment made Myr/EuW10 aggregates exhibit multi-stimuli responsiveness to enzyme,pH,and transition metal ions,and thus providing potential applications in fluorescent sensing,target-releasing,and optoelectronics3.The surfactant-encapsulated Eu-containing POM complex(SEP)was formed via electrostatic interaction between 1-octadecyl-3-methylimidazolium bromide(OB)and EuW10 SEP was firstly self-assembled in a protic ionic liquid to prepare the soft aggregates to fundamentally avoid the fluorescence quenching by water molecules.The structures and photophysical properties of SEP or aggregates were investigated thoroughly by NMR and FTIR spectroscopy,optical and electron microscopy,small-angle X-ray scattering and fluorescence measurements.The formed gel-like aggregates were found to compose of three-dimensional networks of micro-ribbons with an interdigitated layered molecular packing of SEP,which was different from the usual inverse bilayer model of POM hybrids in common organic solvents.Compared to EuW10 solid or its aqueous solution,both SEP and its aggregates exhibited intense red luminescence with much improved lifetime and quantum efficiency.In addition,the soft aggregates exhibited an efficient energy transfer and an obviously enhanced monochromaticity,owning to the organized arrangement of EuW10 units and a confined microenvironment to isolate them from each other between adjacent layers4.The highly luminescent hexagonal(H1)LLC containing a red-emitting trisdipicolinate lanthanide complex[choline]3[Eu(DPA)3](Eu-DPA)from Pluronic 123 and 1-butyl-3-methylimidazolium hexafluorophosphate(BmimPF6)were fabricated.Then,a typical AIEgen,tetrakis(4-hydroxyphenyl)ethane(TPE-OH)showing blue light was doped into H1 matrix,accompanying with a green-emitting Tb-DPA.The emission color of such LLCs could be finely-tuned through changing the molar ratio of Eu-DPA to Tb-DPA.Remarkably,the white emitting H1 LLC with CIE coordinate of(0.328,0.315)has been prepared by accurately adjusting the relative contents of TPE-OH,Eu-DPA and Tb-DPA.Further,the emission color could be switched between bright white and blue upon tuning UV light from 254 to 365 nm repeatedly.To our best knowledge,such full color tunable luminescent LLCs with improved luminescent performances for both AIEgen and lanthanide complexes have not been reported.This facile approach is universal and various kinds of luminophores can be thus encapsulated into LLC matrices to fabricate soft materials with rich luminescent properties5.An environmentally friendly strategy to fabricate flexible multicolor-emitting films has been devloped through co-assembling red-/green-emitting trisdipicolinate lanthanide complexes Ln-DPA(Ln = Eu,Tb)into LLC matrices mainly via electrostatic interaction and further photopolymerization.The LLCs were constructed from a polymerizable surfactant,3-dodecyl-1-vinylimidazolium bromide(C12VIMBr),in aqueous solution.The maintenance of well-defined LLC nanostructures in the luminescent films was validated by small-angle X-ray scattering(SAXS)as well as scanning electron microscopy(SEM)measurements.Remarkably,the lifetime and absolute luminescence quantum efficiency of such film have been improved significantly compared with those of corresponding solid Eu-DPA complex or in aqueous solution and LLC matrices.Through tuning the molar ratio of Eu-DPA to Tb-DPA complexes,the emission color of films could be finely-tailored between red and green in the CIE chromaticity diagram.Further,the films also possessed certain mechanical strength and stability against pH,metal ions,and temperature,indicating their potential application as robust luminescent materials.