Supramolecular assemblies of metal complexes at the interlayer regions of alpha-zirconium phosphate

Date of Completion

January 1997


Chemistry, Inorganic|Chemistry, Physical




The photophysical properties of tris(2,2$\sp\prime$-bipyridine)ruthenium(II) dichloride (Ru(bpy)$\sb3\sp{2+})$ are dramatically influenced by n-butylammonium salt of alpha-Zirconium phosphate (BAZrP). The metal to ligand charge transfer (MLCT) absorption band at 452 nm is red shifted to 480 nm whereas the emission band at 610 nm is blue shifted to 580 nm when Ru(bpy)$\sb3\sp{2+}$ interacts with BAZrP. Blue shift in the emission band has been observed in ice, and glassy matrices where the rigid environment prevents the relaxation of the MLCT excited state. The interlayer region of BAZrP is rigid due to the hydrophobic butyl amine chains and the ZrP frame-work. So the blue shift in the luminescence spectrum in BAZrP indicates that Ru(bpy)$\sb3\sp{2+}$ binds in the interlayer regions. The absorbance and emission spectra of the bound metal complex are independent of the loading. This may be due to the aggregation of the molecules in the interlayer forming clusters. The excitonic interaction between Ru(bpy)$\sb3\sp{2+}$ in the aggregates causes the splitting of the $\pi$-$\pi\sp*$ band at 280 nm in the excitation spectra. The luminescence intensity increases upon binding to BAZrP compared to aqueous solution. Time-resolved emission spectra show an emission maxima at 580 nm at all delay times.^ Deconvolution of the emission data indicates the presence of two components. The major, long lived component $(\sim$1500 ns) has been assigned to the species that binds in the interlayer. The luminescence intensities and lifetimes decrease with loading. The self quenching rate constant calculated from the Stern-Volmer plots is $\rm5.8\times10\sp{10}\ m\sp2/mol.s.$ The enhanced luminescence lifetimes and the increased excited state energy were used to conduct electron transfer experiments in BAZrP using Co(bpy)$\sb3\sp{3+}$ as the quencher. Accelerated rate constants $({>}1.6\rm\times10\sp{12}\ m\sp2/mol.s)$ independent of temperature and solvent viscosity was observed. These rate constants are five orders of magnitude faster than diffusion controlled rates. Therefore, rapid migration of the excitation (antenna effect) within the pools has been proposed to explain the large self quenching rate constant and the electron transfer rates. ^