SPECTRAL DIFFERENCES BETWEEN ENANTIOMERIC AND RACEMIC RU(BPY)3(2+) ON LAYERED CLAYS - PROBABLE CAUSES

The preferential self-annihilation (static and dynamic) of DELTA,LAMBDA-Ru(bpy)3(2+*) over DELTA- or LAMBDA-Ru(bpy)3(2+*) is reported for aqueous dispersions of sodium hectorite lightly loaded with the Ru(II) chelate and subjected to pulsed laser excitation. By varying the loading level over a factor of ca. 60, it is also shown that racemate emission falls off sharply with increased loading whereas emission from the enantiomeric adsorbate remains more nearly constant. The decrease in luminescence yield of racemate with increased loading is mainly associated with an attenuation in the peak emission intensity, I(0), as found from time-resolved measurements. It is proposed, based on these studies, that clays offer both quenching and nonquenching sites for sorption and that DELTA,LAMBDA-Ru(bpy)3(2+) prefers the latter at low loadings, the ions being clustered within such regions. Enantiomeric Ru(bpy)3(2+), on the other hand, is more randomly distributed over the sites. The above model also permits rationalization of (i) observed changes in emission intensity with time, (ii) anomalies in the relative emission yields of Ru(bpy)3(2+*) and Ru(phen)3(2+*), and (iii) the effect of Zn(phen)3(2+) on emission. Finally, differences in binding modes of enantiomeric and racemic chelate forms also induced differences in the flocculation trends of dispersed clays, the effects being most prominent for freshly prepared ruthenium(II) montmorillonite.