Design And Synthesis Of Environmentally Sensitive Naphthalimide Derivatives And Their Application In Dynamic Interface Monitoring

Surfactants have been playing important roles in many areas of industry and research,such as separation,crystallization,purification,detergency,stabilization,foaming,emulsification,etc.Especially,anionic surfactants are used in greater volume than any other surfactants owing to their great capability of detergency and low cost of manufacture.As it is well known,the critical micelle concentration(CMC)of a surfactant is a crucial watershed dividing the physical properties of surfactant solutions.For example,surface tension,electrical conductivity,turbidity,viscosity of the solutions change dramatically below and above the critical concentration.Therefore,various methods have been developed to determine the CMCs of surfactants,which include but not limited to conductometry,spectrophotometry,refractometry,light scattering,potentiometry,fluorescence spectroscopy,capillary electrophoresis,and so on.Moreover,the methods used for the determination of CMCs could be also employed to acquire additional information about the structures of the aggregates of surfactants and even their interactions with other solutes.Among the methods as mentioned above,fluorescence spectroscopy is unique not only because of its high sensitivity,excellent temporal-spatial resolution,fairly simple technical implementation,better selectivity,quick response time and real-time detection,but also because it provides additional information about the systems under examination.Unlike others,the specific performance of this kind of methods is highly dependent upon the probes employed.Owing to the reason,great attention has been paid to the development or search of the fluorescent probes during the last few decades.In fact,as an ideal fluorescent probe,it needs to satisfy several requirements:(1)longer wavelength absorption and longer wavelength emission to facilitate the use,such as naked eye observation or using less expensive instruments,(2)larger molar extinction co-efficient and higher fluorescent quantum yield to allow the probing conducted at lower concentrations,and(3)greater intrinsic anisotropy to provide additional choice to conduct the measurements.1,8-Naphthalimide as a common fluorescent unit,has been widely applied in biological analysis,environmental monitoring,biochemical imaging and other fields,because of its easy structure modification,rich optical properties,absorption and emission wavelengths in the UV-Vis region,and the lack of electronic properties.However,the autofluorescence of naphthalimides has a low quantum yield,a small absorption cross-section,poor solubility,and a slight change in the microenvironment in which it is located,which limits its application to a large extent.Owing to its specific structure,1,8-naphthalimide could be modified into a 'donor-acceptor'structure,in which electrons are susceptible to be excited resulting in intramolecular charge transfer(ICT)state,which may bring polarity sensitivity to the final fluorophores.Moreover,the wavelengths of the absorption and emission of the fluorophores as produced can be tuned by varying the structure of the donating moiety,which is usually grafted on the 4-position of the unit.In this dissertation,azetidine as a strong electron donating,which not only improves the fluorescence efficiency of a variety of fluorophores,but also enhances the photochemical stability of them,was introduced into 1,8-naphthalimide.The resulting compound was taken as a core structure to design and derive two environmentally sensitive naphthalimide derivatives(NA and NPA)via the introduction of different functional groups.Specifically,the research work mainly includes the following two sections.In the first section,a new fluorescent derivative of 1,8-naphthalimide,NA,was designed and synthesized,of which azetidine was utilized as an electron-donating unit and a hydrophilic structure,2-(2-aminoethylamino)ethanol,was bonded onto the'N-imide site' to improve the water-solubility of the compound.Spectroscopy studies revealed that the fluorophore NA,as prepared shows excellent optical properties,which is characterized by a long wavelength absorption(>430 nm),high fluorescent quantum yield,a large molar absorption coefficient,good solubility in water and shows a higher fluorescent quantum yield of-20%and lifetime of-3.7 ns.Moreover,the fluorescence emission and anisotropy of the fluorophore as produced are both dependent upon the viscosity and polarity of the medium.Further studies demonstrated that NA can be used as a selective probe to visually monitor the aggregation of anionic surfactants owing to its accumulation onto the anionic surfaces of the aggregates as formed.Inspired by the discovery,NA was successfully applied for detection of cell membranes and E.Coli via monitoring their negatively charged surfaces,which is important for fast checking of biological contamination of water.In the second section,a pyrenyl unit(Py)with excellent photophysical properties as a donor was chemically connected to a naphthalimide moiety(NA)modified with azetidine as energy acceptor via a long and flexible linker,4,7,10-trioxa-1,13-tridecanediamine(TOA),resulting in a new fluorescent derivative of 1,8-naphthalimide,NPA.UV-Vis absorption and fluorescence studies revealed that the long alkoxy chain of TOA has long spacings between Py and NA,which can effectively avoid strong dipole-dipole interaction and ensure weak dipole interaction to maintain the two fluorescent units of NPA behave their respective optical properties for promoting the occurrence of Fluorescence Resonance Energy Transfer(FRET).Further study indicated that the excited state energy of Py can be efficiently transferred to the naphthalene diamide unit(NA),and the energy transfer efficiency has no obvious dependence on the solvent.In addition,the fluorescence behavior of NPA also shows a certain solvatochromic effect because it contains the environmentally sensitive unit NA.