Syntheses, Structures, and Fluorescent Properties of 2-(1H-Imidazol-2-yl)phenols and Their Neutral Zn(II) Complexes

A series of 2-(imidazole-2-yl)phenol ligands L1-L6 with the general composition 4-R-4-5-R-3-6-R-2-2-(4,5-R-1,R-1-1H-imidazole-2-yl)phenol (L1: R-1 = C2H5, R-2 = R-3 = R-4 = H; L2: R-1 = C6H5, R-2 = R-3 = R-4 = H; L3: R-1 = C6H5, R-3 = OCH3, R-2 = R-4 = H; L4: R-1 = C6H5, R-4 = OCH3, R-2 = R-3 = H; L5: R-1 = C6H5, R-3 = H, R-2 = R-4 = CH3; L-6: R-1 = C6H5, R-3 = H, R-2 = R-4 = t-Bu) and L7 (2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol) and their neutral Zn(II) complexes (Z1-Z7) were synthesized and characterized by spectroscopic and elemental analyses. Molecular structures of L1, L5, Z1, and Z2 were confirmed by single-crystal X-ray diffraction. L1 crystallized in the monoclinic Cc space group, while L5, Z1, and Z2 all crystallized in the triclinic P (1) over bar space group. One-dimensional arrays based on continuous pi-pi stacking interactions and hydrogen bonding were observed for L1 and Z1, while L5 existed as discrete dimeric stack units. Z2 formed hydrogen-bonded 1D network structures but was completely devoid of pi-pi stacking interactions. Emission processes were found to be more dependent on the substituents on phenol as well as condensed media. In contrast to general conclusions on closely related systems in the literature, significant photorelaxation from the excited enol state was observed in the cases of L1 in methanol and L4 in both THF and methanol. Therefore, there exists a certain unusual hindering factor to keto-enol phototautomerism in the ligand-solvent systems. The sensing property of zinc(II) complexes was explored regarding the effects of substituents in their ligands. It was observed that coordination to the zinc(II) ion led to emission quenching for L1 and L2 while causing an enhancement of fluorescent intensity for L3, L4, L5, and L6. A linear relationship was observed between the emission intensity and the concentration of the zinc ion at the 10(-8) M level. Compared to other zinc compounds in this work, fluorescence enhancement in Z3 and Z4 showed that the methoxyl substituent is favorable for fluorescent enhancement.