Amine-Imine Tautomeric Excited State Intramolecular Proton Transfer in Metal-Organic Frameworks: Alcohol and Anion...

To study the amine-imine tautomeric alteration through excited state intramolecular proton transfer (ESIPT), three amine-functionalized metal-organic frameworks (MOFs) have been synthesized by using two different metal ions and neutral linkers along with same ESIPT-enable anionic linker through the slow diffusion process. All the MOFs have 2D structure with asymmetric units of {[Mn(2,6-dip)(2-atp)](H₂O)(CH₃OH)}_n (1), {[Mn(3,5-dip)(2-atp)](solvent)_x}_n (2) and {Zn₂(3,5-dip)₂(2-atp)(μ²-O^2-)](solvent)_x}_n (3) (where 2,6-dip=2,6-di(1H-imidazol-1-yl)pyridine, 3,5-dip=3,5-di(1H-imidazol-1-yl)pyridine, 2-atp=2-amino terephthalic acid). The 2D networks of desolvated form of compound 1, 2, and 3 show water influencing proton transfer for amine-imine tautomerism in excited state through the ionic interaction with water molecules to the frameworks. However, in case of polar small chain aliphatic alcohols like methanol, ethanol and isopropanol; the compounds do not show any dual emissive ESIPT but exhibit three different intensified single peaks for each of the compounds. This different emission intensity in presence of different alcohols are helpful to detect these alcohols selectively. In addition to that in case of all three compounds, the water assisted-ESIPT is interrupted by some strong oxidizing agents like CrO₄ ^2-, Cr₂O₇ ^2- and MnO₄ ^- ions. This phenomenon allows the method of detection for the aforesaid oxidizing ions in water by interruption of the dual emissive fluorescence.