Comparison of structural, electronic, and fluorescent properties of Zn-terpyridine, Zn- 1,10-phenanthroline, and Zn-hydroxyquinoline metal complexes

Abstract

Terpyridine is a heteronuclear, N-N-N tridentate ligand while 1,10-Phenanthroline (1,10-Phen) and 8- hydroxyquinoline (8-HQ) are N-N and N-O bidentate ligands that are commonly used for the synthesis of metal complexes. Although the Cu2+, Ag+, and Ru2+ complexes of these ligands have been investigated, the Zn2+ complexes of these ligands have not gained much attention. One-pot Kröhnke type synthesis of 4/-(4-Nitrophenyl)-2,2/:6/,2//-terpyridine ligand (Nitrotyp) and 4/-(phenyl)-2,2/:6/,2//- terpyridine ligand (Phentyp) using 2-acetylpyridine and aryl aldehyde is followed by the characterization using UV-visible, FT-IR, and H1 NMR spectroscopy. Then Nitrotyp and Phentyp were separately coupled with Zn(II) and characterized by UV-visible and FT-IR spectroscopy. According to the UV-visible spectroscopy, after the coordination with Zn(II), both Nitrotyp and Phentyp ligands exhibit a red shift in absorption maxima (λmax) from 272 nm to 284 nm and from 277 nm to 283 nm, respectively. All these electronic transitions are ligand-centered π π* transitions. According to the FTIR spectroscopy, the C=N stretching frequencies for the Nitrotyp and Phentyp free ligands are 1593 cm- 1 and 1582 cm-1, respectively. Compared to the FT-IR spectra of the Zn complexes of the same ligand, the stretching frequency for the C=N bond is 1601 cm-1 and 1552 cm-1, respectively. The shift of the IR stretching frequency of the C=N bond compared to the relevant free ligand indicates the coordination of the ligands with Zn(II). In addition, commercially available 1,10-phen and 8-HQ ligands were combined with Zn(II) and characterized by UV-visible, and FT-IR spectroscopy. The formation of the Zinc complex in the 8-HQ ligand leads to a significant red shift of λmax from 241 nm to 259 nm. In contrast, free 1,10-Phen exhibits a blue shift from λmax= 229 nm to λmax= 225 nm upon coordination with Zn. As reported by FT-IR spectroscopy, the C=N stretching frequencies for the 1,10-Phen and 8- HQ free ligand are 1582 cm-1 and 1576 cm-1, respectively. Compared to the FT-IR spectra of the Zn complexes of these ligands the stretching frequency for the same bond is 1589 cm-1 and 1587 cm-1, respectively. The Zn(II)-to-metal ratio of all complexes was identified as 1:1 using the job’s plot method. Hence, it is concluded that the chemical formulae for the synthesized Zn(II) complexes are [Zn(Nitrotyp)Cl]+, [Zn(Phentyp)Cl]+ [Zn(1,10-Phen)Cl2], and [Zn(8-HQ)Cl2]. According to the fluorescence spectra, all the free ligands exhibit less fluorescence intensity compared to the corresponding Zn(II) complex and, [Zn(Nitrotyp)Cl]+ complexes exhibit hypsochromically (blue) shifted emission spectra, compared to the corresponding free ligand. In contrast, [Zn(Phentyp)Cl]+, [Zn(1,10-Phen)Cl2], and [Zn(8-HQ)Cl2] complexes demonstrate a bathochromically (red) shifted emission spectra. Out of the four complexes, [Zn(Phentyp)Cl]+ ((2.4 × 10-7 M) shows a higher fluorescence intensity, even at low concentrations than other complexes. Replacing one of the pyridine rings with a phenyl group triggers the diminution of the fluorescence intensity and it is indicated by the high fluorescence intensity of the [Zn(Phentyp)Cl]+ complex compared to the [Zn(1,10-Phen)Cl2] complex. These results make the [Zn(Phentyp)Cl]+ complex a more promising agent for the detection of anions in a given medium, than the other three complexes.