The Study On The Fluorescence Of Uranyl-Organic Frameworks Materials

Since the 21st century,human society has entered a period of rapid development,which is associated with increasing demand for energy.In contrast,the diminishing reduction of non-renewable energy sources such as fossil energy,as well as the use of fossil energy will bring irreparable damage to the environment.Thus,finding a clean,efficient and sustainable alternative energy is imminent.As a relatively clean,effient and sustainable energy,nuclear energy is an ideal alternative energy.Uranium is well known as a common nuclear fuel,meanwhile,the radioactivity of uranium has been criticized.Depleted uranium has been regarded as a waste during a long period because it cannot be used as nuclear fuel,which results in relatively little research on the properties of uranium-containing compounds.In order to break this traditional opinion and promoting the synthesis of uranium-containing compounds,our group has long been committed to the design and synthesis of uranium-containing compounds and actively explored its potential applications in various fields.A series of preliminary results have been achieved.In recent years,other researchers have also been working hard to explore new applications of depleted uranium.It has been found that uranium-containing compounds have some unique inherent advantages in many fields,such as in fluorescent materials,single-molecule magnet materials,superconductor materials,catalytic materials and environmental decontamination materialsUnder solvethermal condition,with the flexible H3BCPBA as a ligand,a two-dimensional uranyl-organic framework material(compound 1)was successfully synthesized by adjusting the appropriate reaction conditions,which shows characteristic fluorescence peaks of uranyl ions.Water can quickly trigger a phase transition in a uranyl-organic framework(1)that results in the second phase 2.The SC-SC transformation process could be probed by PXRD when crystals of 1 were exposed to humid air.The change in luminescence intensity of 1 in air occurs in two steps:initial quenching followed by augmentation.During the first step,the emission intensity of 1 gradually decreased during the first hour when left under ambient atmosphere.The reasons for the initial quenching can be the uptake of water that absorbs energy from UV irradiation then releases this energy through a non-radiative pathway.It is intriguing,however,that the intensity of emission was enhanced upon further saturation of 1 with water,and began to increase dramatically in response to the phase transition into 2.As a result,the intensity of the final spectrum shows an increase over four-folds compared to the initial spectrum of 1,and over ten-folds throughout the whole process.In order to study the intrinsic mechanism of fluorescence enhancement,we selected two ligands in the same part of the two structers for DFT calculation.The calculation results show that the layer spacing of the layered uranyl framework is shrinked due to the phase transition,which leads to the enhancement of electron polarization.The interactions between the ligands increase,which results in the limition of vibration and rotation of chemical bonds inside the crystal.Thus,this part of energy was released in the form of fluorescent emission,finally leading to significant fluorescence enhancement.This is the first case of AIE effect that occurs in the uranyl-organic framework,and also the first case of AIE effect that occurs in the solid-state with an extended structure.