Synthesis, structure and photophysical properties of 2-benzhydryl-4-methyl-6-(aryliminomethyl)phenol ligands and the zinc complexes thereof

The Schiff-base ligands 2-benzhydryl-4-methyl-6-((phenylimino)methyl)phenol (L1–L5), and their corresponding zinc complexes (C1–C5), were prepared and fully characterized. The UV–Vis absorption and fluorescence spectra of the ligands and their Zn(II) complexes were measured in solvents such as methanol, dichloromethane, THF, or toluene, respectively. The results showed that the fluorescence intensity of the ligands was very weak, but upon coordination with Zn2+, the fluorescence intensity was greatly enhanced, and the fluorescent quantum yield ratio of C5 (0.46) to L5 (0.0004) in methanol increased by around 1150 times. The enhanced fluorescence of the zinc complexes is due to the intramolecular charge transfer (ICT) and the stronger geometrical rigidity of the delocalized bonding planes in these molecules. Transient absorption spectrum bands with peaks at about 310 and 460 nm were attributed to triplet state absorptions and absorption dynamic curve fitting showed that lifetimes were of microsecond level scale. There was no time-resolved property observed for the bleaching spectral bands at 350 nm and at ca. 460 nm. This spectral property, like the Stokes shift, can be attributed to the typical behavior of nanosecond transient absorption of the compounds with the excited-state intramolecular proton transfer (ESIPT).