Design,Synthesis And Properties Of Non-traditional Aggregation-induced Luminescence Polymers

Aggregation-induced emission(AIE)materials have overcome the puzzle of the fluorescence quenching that are bound to happen when increasing the concentration of traditional luminescent materials.Exploring the materials possessing aggregation-induced emission effect has become a research hotspot in the field of luminescence.Among these materials,unconventional AIE polymers,as a starlet in AIE field,which have been attracted increasing attention because they do not contain any traditional aromatic blocks but exhibit bright fluorescence.Such polymers have the unique advantages of excellent biocompatibility,low cytotoxicity and environmental friendliness,so endowing their potential application prospects in bioimaging and fluorescent labeling and so forth.However,there are still many deficiencies in the research on such luminescent polymers,such as the less of species,the single emissio color,and insufficient understanding of the luminescence mechanism.In order to solve the above-mentioned problems,it is necessary to study the luminescence mechanism from the structure of functional groups and the nature of the aggregation state.Therefore,designing and developing non-traditional AIE polymers with different structures,and revealing their intrinsic luminescence mechanism has already become a hotspot and difficult point in this field.For this purpose,we design and synthesize a variety of novel nonconventional AIE polymers,including primary amine groups-terminated hyperbranched poly(amine ester),hyperbranched polyesters,oligomeric siloxanes,and hydroxyl groups-terminated multicolored fluorescence hyperbranched poly(amine ester)in this thesis.The fluorescence properties of different polymers as well as the relationship between their structure and fluorescence properties were systematically studied.Moreover,the intrinsic luminescence mechanism of multicolored fluorescence hyperbranched poly(amine ester)was elucidated by using the density functional theory.The main research contents of this thesis are as follows:(1)Primary amine groups-terminated hyperbranched poly(amine ester)(HPAE-NH₂)In order to study the effect of amino groups on the luminescence properties of hyperbranched polymers,two kinds of primary amine groups-terminated hyperbranched poly(amine ester)(HPAE-NH₂)were synthesized by Michael addition polymerization reaction of trimethylolpropane triacrylate with ethylenediamine and hexamethylenediamine.It is found that the synthesized HPAE-NH₂ has a good water solubility.And aqueous solution of HPAE-NH₂ can emit bright blue fluorescence when excited by a 365 nm UV lamp.Besides,the fluorescence intensity of HPAE-NH₂solution increases with increasing concentration,showing concentration-dependent emission feature,which indicates that it has the significant aggregation-induced emission(AIE)characteristics.Moreover,compared to P2 synthesized from ethylenediamine,P1 prepared from hexamethylenediamine shows much stronger fluorescence,which may be due to the fact that the longer carbon chains in the P1segment structure is conducive to aggregation of polymer chains.Further,both P1 and P2 possess a good film-forming property.The PVA films added P1 and P2 exhibit strong blue and yellow-blue fluorescence,respectively,upon a 365 nm UV light.It is noted that the maximum excitation and emission of pure P1 are at 461 nm and 563 nm,respectively.And its fluorescence lifetime and quantum yield are measured to be 6.83ns and 8.66%,respectively.The fluorescence of P1 solution reaches the maxima when adjusting its pH to 4,which is mainly attributed to the protonation of the tertiary amine groups,resulting in the more rigid structure of P1.Thus,the transition of non-radiative channels is inhibited effectively.In addition,the fluorescence of P1 solution quenches selectively for Hg²⁺and Fe³⁺when addition of various metal ions.Therefore,it can be used as a potential fluorescent probe for detection of Hg²⁺and Fe³⁺.(2)Hyperbranched polyester(HPE)To study the effect of hydroxyl groups on the luminescence properties of hyperbranched polymers,two types of hyperbranched polyesters(HPE)with different molecular weights were synthesized by esterification polycondensation reaction using the biomass compounds(glycerol,1,4-succinic acid and adipic acid)as raw materials,and p-toluenesulfonic acid as a catalyst.It is found that the fluorescence of HPE shows a significant concentration-dependent luminescence behavior,which indicates that it possesses a remarkable AIE characteristic.Besides,the fluorescence of HPE gradually increases with the increase of molecular weight.Compared with the AG-1 synthesized from adipic acid,the SG-1 synthesized from succinic acid exhibits stronger fluorescence,and has more broad emission band,which may be resulted from structural unit of SG-1 containing more chromophores of carbonyl groups.The fluorescence lifetime of pure SG-1 is 6.88 ns,and its fluorescence quantum yield measurement is up to 16.75%,which is the highest quantum yield reported in nonconventional luminescent polymers so far.In order to in-depth study the luminescence mechanism,linear polyester(LPE)was synthesized from 2-methyl-1,3-propanediol and 1,4-succinic acid.The results show the fluorescence intensity of HPE is stronger than that of LPE,which indicates that the intrinsic fluorescence of polyesters is not only related to the cluster formed by the ester groups and hydroxyl groups,but also has a very close relationship with the branched structure.In addition,the aggregation enhanced emission phenomenon of HPE in mixture solvent and its varying luminescent properties in different solvents,further indicating that the degree of aggregation of polymer molecular chains would impact the fluorescence.This main reason is that the aggregation of polymer molecular chains affects greatly the formation of clusters of hydroxyl and ester groups,so resulting in the different fluorescence.(3)Oligomer hyperbranched siloxane(S1)and linear siloxane(S2)For purpose of further studying the effect of amino and hydroxyl groups on the luminescence properties of hyperbranched polymers,oligomer hyperbranched siloxane(S1)and linear siloxane(S2)were synthesied by transesterification polycondensation reaction of 3-aminopropyl(diethoxy)methylsilane with glycerol and 1,4-butanediol.It is found that pure S1 and S2 emit intense blue fluorescence when illuminated with a365 nm UV light.Additionally,both S1 and S2 solutions display a significant concentration and excitation wavelength-dependent emission,as well as AIE feature.To deeply explore the essence of luminescence from point of view of the aggagtion state structure,density functional theory(DFT)method is used to calculation.The results show that the aggregation occurs when multiple S1 and S2 molecules put together,but the extent of aggregation is different.Meanwhile,multiple hydrogen bonds exist between the molecules,such as Si-O…H-N,Si-O…H-O,Si-O…H-C and H-O…H-C,which promotes the aggregation of molecules,resulting in forming the clusters of O,OH and NH₂.In the formed clusters,the electron clouds overlap would result in producing the extended electron delocalization,thereby lowering the energy gap of LOMO and HOMO,which is favorable for the luminescence.Moreover,organic solvents and metal ions can effectively adjust the photoluminescence properties of S1.(4)Hydroxyl groups-terminated multicolored fluorescence hyperbranched poly(amine ester)(HPAE-OH)Based on the study of above-mentioned luminescence mechanism of aggregation state,to obtain hyperbranched polymer having multicolored fluorescence,heteroatoms and functional groups,such as hydroxyl groups,carbonyl groups and N atoms are introduced into the polymer skeleton.A novel hydroxyl groups-terminated hyperbranched poly(amine ester)(HPAE-OH)was synthesized via a simple transesterification polycondensation method using triethyl citrate and diethanolamine as raw materials.Surprisingly,HPAE-OH can self-assemble in water to form different morphologies of supramolecular HPAE-OH as changing its concentration.It is found that the fluorescence of HPAE-OH not only has a unique concentration-dependent AIE property,but also shows significant excitation-dependent emission characteristics.In particular,pure HPAE-OH emits the multicolored fluorescence,i.e.bright blue,green and red when excited by different excitation wavelengths.The DFT theoretical calculations show that the multicolored fluorescence from pure HPAE-OH is related to the dense clusters of ester,NH and OH groups,and more importantly due to the formation of multiple planar space-conjugation rings,namely,"multi-rings induced multicolor fluorescence".In addition,it is also found that the degree of protonation of secondary amine significantly affects photoluminescence of HPAE-OH,and the highest fluorescence intensity is reached at pH=3.5.Moreover,the fluorescence of HPAE-OH exhibits a selective decrease towards Fe³⁺.Therefore,the water-soluble HPAE-OH is expected to act as a potential probe for Fe³⁺.