Persistent Room Temperature Phosphorescence For Unconjugated Natural Biomass And Its Various Applications

Distinguished from fluorescence,the emission of phosphorescence relates to the triplet state,which is susceptible to internal dissipation and external environmental quenching.Thus,phosphorescence is difficult to be observed at room temperature.Room temperature phosphorescent（RTP）materials are widely used in,bio-imaging,information encryption and anti-counterfeiting due to their ultra-long luminescence lifetime and the ability to effectively avoid background interference from excitation through time-gating techniques.Conventional organic RTP materials mainly consist of largeπ-conjugated systems based on aromatic groups and various strategies have been developed,such as host-guest doping,crystallization,space-limitation,polymer matrix and supramolecular interactions,etc.These materials mainly suffer from complex molecular design and preparation process.Recently,some unconventional RTP materials,which without aromatic groups has received a lot of attention due to the advantages of environmental friendliness and low cost.At present,the luminescence mechanism of unconventional luminescence is mainly based on clustering-triggered emission.Moreover,the research on how to effectively regulate the luminescence properties of this RTP is still in the preliminary stage,and mechanism is not systematic enough.It is still a serious challenge to developed a strategy to regulate the luminescence of unconventional RTP.Thus,the main subject of this thesis mainly using abundant natural biomass as the research object such as cellulose and chitosan,forcing on the effective regulation of luminescence properties of unconventional RTP.In addition,not only constructing a new environmentally friendly and low-cost RTP luminescent material,but also expanding the application of this material in multiple fields.The main research contents are as follows:（1）Cellulose is potentially the most abundantπ-unit-free,low-toxic and naturally renewable material.Moreover,it is an excellent material for exploring the basic theory of non-conjugated room-temperature phosphorescence.First,the photophysical properties of cellulose,a compound containing only hydroxyl groups and noπ-electrons were investigated.The room-temperature phosphorescence of cellulose was greatly enhanced by dehydration,which achieving a green afterglow visible to the naked eye compared to normal conditions.Through a series of characterizations such as in-situ variable temperature infrared and X-ray diffraction and computational analysis using DFT,it showed that the removal of crystalline water from cellulose not only could enhance the compact stacking of molecules but also stabilize the hydroxyl clusters of polysaccharide chains,leading to a significant enhancement of the phosphorescence.Moreover,this dehydration induced enhanced luminescence phenomenon is not only in purified cellulose,but paper,cotton and other plant tissues containing cellulose were observed.Based on the above phenomenon,we achieved probe-free time-resolved imaging of some plant tissues sections（e.g.,carrot,cabbage,etc.）on a smartphone-integrated-device.（2）The clustering of hydrogen bonding interactions between cellulose hydroxyl groups facilitates the enhancement of RTP emission.And the metal electrostatic interactions can be used to enhance the stacking of hydroxyl group,thus the effects of metal salts with different valence states on cellulose RTP were further investigated.The results show that the emission of cellulose RTP was further enhanced after doping metal salts(K⁺,Na²⁺,Ca²⁺,Mg²⁺,Y³⁺,etc.),which was attributable to strong electrostatic interactions by suppressing the Coulomb forces between electron-rich groups（hydroxyl groups）.The luminescence lifetime breaks through the second level.A series of characterizations combined with DFT theoretical calculations show that there have strong interactions between the metal ions and hydroxyl groups,which greatly enhance the RTP properties of cellulose.At the same time,the introduction of anions(F^-,Cl^-,Br^-)significantly increases the phosphorescence quantum yield of cellulose,which up to 5%.The RTP properties of cellulose was significantly enhanced and the regulation of unconventional RTP was achieved through synergistic of metal ions and halogens.Taking advantage of the different effects of metal ions on luminescence properties of cellulose paper,an ultra-low cost and environmentally friendly information encryption was developed by directly using metal solutions as printing inks on paper.（3）Cellulose,as the most abundant renewable resource in nature,is widely used in paper and food industries.Cellulose filter paper,as the most common material in the laboratory,also has long afterglow properties.Different metal ions have different effects on afterglow property of filter paper,which can be realized for the detection of metal ions.Thus,a time-resolved detection of trivalent chromium by the inherent afterglow in filter paper was achieved on a smartphone-integrated-device.The results show that the afterglow emission of cellulose gradually decreases with increasing of the concentration of Cr³⁺.A good linear correlation（R²=0.99744）was observed between quenched 0.2 and 1.6 nmol of Cr³⁺,with a minimum detection limit of 0.13 nmol.It may attribute to a stronger energy transfer between Cr³⁺and cellulose.This strategy not only avoid the complex and expensive large instruments,which greatly reducing the cost,but also uses cellulose filter paper as the probe which is very friendly to the environment.This strategy greatly expands the applications of unconventional luminescence materials.（4）Chitosan has a structure similar to cellulose,but contains amino acids compared to cellulose,which have a stronger attraction to metal ions.Therefore,chitosan may further stabilize the interaction with halide ions.The regulation of room temperature phosphorescent lifetime of chitosan was realized via external heavy atom effect.With the increasing of concentration of Br^-,the RTP intensity of chitosan gradually enhanced,and its phosphorescence lifetime continuously shorten in a wide range（49 to 647 ms）.Moreover,the average phosphorescence lifetime is regulated have a good linear correlation（R²=0.97733）in the molar ratio([Mg Br₂]:[C₆H₁₀NO₄])range from 0.05 to 1.A series characterization and DFT theoretical calculation were analyzed,indicating that the halide ions have significant electron transfer and easily participate in the leaping process of chitosan clusters attribute to its containing abundant outer electrons.In addition,due to the large spin-orbit coupling（SOC）of halide atoms,the radiative transition rate from the lowest triplet state to the ground state is accelerated leading to a decrease in phosphorescence lifetime.In conclusion,a wide range of luminescence lifetime regulation of unconventional RTP is achieved.This strategy provides a new insight for designing an environmentally friendly afterglow luminescence materials and expanding their application.