Quenching Dynamics of the Photoluminescence of [Ru(bpy)₃]²⁺-Pendant PAMAM Dendrimers by Nitro Aromatics and Other Materials

Abstract

The Stern−Volmer quenching constants (Ksv) for poly(amidoamine)- (PAMAM-) based dendrimers (generations G0−G4) modified with (4, 8, 16, 32 and 64) pendant [Ru(bpy)3]+2 (bpy is 2,2‘-bipyridine) chromophores have been measured in the presence of three nitroaromatic quenchers (2,4,6-trinitrotoluene = TNT, 2,4-dinitrotoluene = DNT, nitrotoluene = NT), two negatively charged quenchers (K3Fe(CN)6, Na4Fe(CN)6), and one energy transfer type quencher (Fe(C5H5)2). The quenching efficiencies were calculated for the dendrimers (G0−G4) in the presence of TNT and were found to peak for dend-16-Ru(bpy)3. The generation dependence of the quenching efficiency mirrors the calculated crowding factor and the variations were attributed to changes in the 3-dimensional structure with generation (size) and the associated changes in the accessibility of the [Ru(bpy)3]2+-pendant groups. Electrostatic attractions between the positively charged dendrimers and the negatively charged quenchers resulted in larger Ksv values when compared to the reference complex, [Ru(bpy)3]2+. The addition of electrolyte to an ionic strength of 0.1 M (KCl or NaCl) caused a diminution of the Ksv values by 60%, consistent with electrostatic interactions and charge screening effects.