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 ( 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 (K 3 Fe(CN) 6, Na 4 Fe(CN) 6 ), and one energy transfer type quencher (Fe(C 5 H 5 ) 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 sv 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.