Quenching dynamics of the photoluminescence of [Ru(bpy)3]2+-pendant PAMAM dendrimers by nitro aromatics and other materials

The Stern-Volmer quenching constants (K-SV) 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 K,, 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 K-SV values by 60%, consistent with electrostatic interactions and charge screening effects.