SUPRAMOLECULAR ASSEMBLIES OF TRIS(2,2'-BIPYRIDINE) RUTHENIUM(II) BOUND TO HYDROPHOBICALLY-MODIFIED ALPHA-ZIRCONIUM PHOSPHATE - PHOTOPHYSICAL STUDIES

Interaction of tris(2,2'-bipyridine)ruthenium(II) dichloride (RU(bpy)(3)(2+)) with n-butylammonium salt of a-Zirconium phosphate (BAZrP) causes extensive red shifts of the metal-to-ligand charge-transfer (MLCT) absorption band of the metal complex (from 452 to 480 nm). In contrast, the luminescence maximum is blue shifted, from 610 nm in aqueous media, to 580 nm when the metal complex was bound to BAZrP. Such a blue shift in the emission was observed when RU(bpy)(3)(2+) was present in rigid matrices such as ice, or ethanol-methanol glasses at low temperature. Therefore, the microenvironment surrounding the metal complex in BAZrP is rigid and does not permit the relaxation of the initially produced metal-to-ligand charge-transfer (MLCT) excited state even at room temperature. Upon binding to the zirconium phosphate, the emission yield of RU(bpy)(3)(2+) is increased nearly 5-fold when compared to aqueous solutions. Such large increases in the emission were not observed with RU(bpy)(3)(2+) in other heterogeneous media. The absorption and emission spectral maxima are independent of loading of the metal complex, strongly suggesting the formation of islands or pools of these metal complexes at the interlayer regions of BAZrP. The luminescence intensities decrease with loading of the metal complex indicating the strong self quenching of the luminescence within these pools. Stern-Volmer plots were constructed from the steady-state data and the estimated K-SV value was 1.1 x 10(6) m(2)/mol. Powder X-ray diffraction patterns of the samples indicate that intercalation of RU(bpy)(3)(2+) into BAZrP increases the interlayer separation from 18.6 to 19.5 Angstrom. Time-resolved emission studies indicate the presence of two distinct species, a major long-lived component of similar to 1500 ns (>90%) and a minor short-lived component of similar to 400 ns (<10%). These excited-state lifetimes are much longer than what was observed in other heterogeneous media. The luminescence lifetimes decrease with loading. The self quenching rate constant estimated from the time-resolved emission data is 5.8 x 10(10) m(2)/(mol s). The large self quenching rate constant and extremely slow diffusion in the interlayer regions suggest rapid migration of the excitation within these pools. However, no direct evidence was observed for the energy migration in the current studies. The binding of the metal complexes to BAZrP was found to be highly sensitive to the presence of salts. For example, addition of potassium chloride/iodide released the metal complex into the aqueous phase, causing a large decrease in the luminescence intensity. A similar effect with sodium iodide or sodium chloride was not observed. Such ion-sensitive emission can be useful in the design of luminescence sensors. The longer lifetime and the increased energy of the luminescent state of Ru(bpy)(3)(2+), when bound to BAZrP, are welcome changes for the application of these assemblies in solar energy conversion and storage.