Syntheses, Characterization And Photophysical Properties Of Salicylaldehyde Derivative

In this paper, a series of Schiff-base ligands, benzimidazole ligands andcorresponding Zn(II) complexes based on salicylaldehyde were prepared andcharacterized, and their photophysical properties in different solvents wereinvestigated. According to experiments, we have found that the fluorescence intensitywas greatly enhanced when coordination with zinc(II) and/or in protic solvent such asmethanol. The geometrical rigidity of the molecule significantly affected thefluorescence intensity, and all compound showed large Stokes shifted due toexcited-state intramolecular proton transfer (ESIPT). Five parts are included in thispaper as follows:Part I: In this section, I was mainly discussed the origin, occurrence mechanism,application, and prospect of the fluorescence, and introduced the research purpose andsignificance of the subject.Part II: A series of2-benzhydryl-4-methyl-6-((phenylimino)methyl)-phenolligands were synthesized and characterized by NMR, IR, and elementary analysis.And2-benzhydryl-6-((mesitylimino)methyl)-4-methylphenol (L4) was futherconfirmed by single-crystal X-ray diffraction. In addition, the photophysicalproperties of all ligands (L1-L5) were studied in different polarity solvent (such asmethanol, THF, dichloromethane and toluene), we have found that all ligands showedvery weak fluorescence intensity due to excited state intramolecular proton transfer(ESIPT), and all ligands exhibited large Stokes shift.Part III: A series of quadridentate (N^O) zinc(II) complexes were synthesized, allcomplexes were characterized by IR and elementary analysis, complexes C2and C5were further confirmed by single-crystal X-ray diffraction. In addition, the UV-visabsorption and fluorescence spectral properties of ligands and their Zn(II) complexeswere investigated. According to experiments about the coordination with Zn2+, wehave found that the fluorescence intensity was greatly enhanced, the enhancedfluorescence of the zinc complexes is due to the stronger geometrical rigidity of thedelocalized bonding planes in these molecules, for example, the fluorescent quantum yield ratio of C5(0.46) to L5(0.0004) in methanol increased by around1150times. Inaddition, the transient absorption spectra of C5and L5in toluene were investigated,and the bleaching band without time resolution at about460nm.Part IV: A series of6-benzhydryl-4-methyl-2-(1H-benzoimidazol-2-yl)phenolligands were synthesized, and all ligands were characterized by NMR, IR, andelementary analysis. And the photophysical properties of all ligands (L6-L11) wereinvestigated,6-Benzhydryl-4-methyl-2-(benzoimidazol-2-yl)phenol (L6) and2-benzhydryl-6-(5-chloro-benzoimidazol-2-yl)-4-methylphenol (L11) showed specialStokes-shifted due to having a N-H group.Part V: We mainly synthesized a series of benzoimidazolyl zinc complexes, andinvestigated their photophysical properties, all complexes were characterized by IRand elementary analysis, C6and C9were confirmed by single-crystal X-raydiffraction. All corresponding ligands showed moderate fluorescence intensity due tocontained electron-rich group such as benzoimidazolyl, after coordination withzinc(II), the fluorescence intensity was enhanced significantly. The fluorescenceintensities of the zinc complexes were heavily affected by the solvents used, withstronger intensities observed in methanol than in other solvents such asdichloromethane and toluene. The fluorescence decay mainly followed a singleexponential in toluene, whereas the double exponential decay was observed inmethanol and dichloromethane, the phenomena can be explained by the presence oftwo active species in these solvents.