Molecular Design,Synthesis And Properties Of Functional Schiff Base Compounds

Schiff bases is a class of organic compounds containing imine or methine imide groups,which are usually obtained by condensation reaction through aldehyde or ketone compounds and primary amine.Owing to the nitrogen atom in the C=N bond of Schiff base can provide lone pair electrons,these compounds have good coordination ability with metal ions.Therefore,most of the Schiff bases will have certain fluorescence responses to specific metals,so these compounds could be used as fluorescence probes to detect the metal ions.Currently,a large number of Schiff bases containing semicarbazones,amino acids,phenols,thiophenes,furans or acyclic polyethers units have been reported.However,the functional Schiff bases with novel structure and excellent performance is still a worthwhile issue to explore.On the other hand,the aggregation-induced emission(AIE)phenomenon of organic compounds have attracted lots of scientific and technological research interest during the past few decades because of their widespread applications,including the detection of inorganic and organic small molecules,fluorescence imaging analysis,and the preparation of solid probes.Up to now,numerous efforts have been devoted to the design and synthesis of novel AIE molecules.Although the structures of these reported AIE molecules are simple,their synthesis is usually complex,mainly due to the difficulty of the formation of different structures connected by the C-C single bond.However,through the C=N double bond,the Schiff bases with AIE effect were gained by simple condensation reaction.Meanwhile,the AIE molecules could coordinate with metal ions and form aggregate metal complexes to enhance the fluorescence emission.Therefore,this AIE molecules could be used as fluorescent probes to detect the metal ions.In this paper,we design and synthesize several Schiff bases with varied structures from 3-aminothiophene,phthalic anhydride,naphthalic anhydride and salicylaldehyde as well as its derivatives.And their structures have been characterized in detail.The optical property,pH response and AIE effect are investigated.Also,the as-prepared Schiff bases using simply as a fluorescent sensor reagent for selectively detecting metal ions are discussed.The main work of this paper are as follows:1.Synthesis and properties of three Schiff bases containing salicylaldehyde unitsThree Schiff bases SB1,SB2 and SB3 were synthesized from 3-bromothiophene and salicylaldehyde as well as its derivatives.The varied ratio of the tautomers and fluorescence behaviors resulting from the different electron-withdrawing ability and electron-releasing ability of the substituent groups in salicylaldehyde structure of the Schiff bases were gained.Nitro-substituted and none-substituted Schiff bases have strong fluorescence enhancement result from the coordinate with Zn2+ and Fe3+,respectively.While the methyl-substituted Schiff bases have no specific response to the metal ions.This is due to the electronegativity of the substituents,which effect the electron density of the C=N bond and the bond lengths of the C=N bond and C-O bond.At the same time,the introduction of electron-withdrawing groups reduced the electronic density of the C=N bond,which would reduce the pH value when the Schiff base was hydrolyzed.So the nitro-substituted Schiff base has better acid resistance and widespread applications.2.Synthesis and properties of Schiff bases with aggregation-induced emission effectsThree Schiff bases SB4,SB5 and SB6 were synthesized from phthalic anhydride,naphthalic anhydride and salicylaldehyde.The sizes of the cavities to coordinate metal ions and the selectivity to metal ions of the Schiff bases were changed by adjusting their molecular structure.The SB4 and SB5 have approximately the same size of the coordination cavities,and they selectively coordinated with Cu2+.While the SB6 possess the larger coordination cavity,so it selectively coordinated with Fe3+.The AIE properties of the three compounds were discussed and the stacking modes of Schiff' bases aggregates were explored through single crystal structure.Then the effect of the structure and the size of the stator as well as the rotor of the Schiff bases on AIE properties was investigated.These results could provide a way to further design novel molecules with AIE effects.