Molecular Energy Transfer across Oxide Surfaces

Abstract

The decay properties of the metal-to-ligand charge transfer (MLCT) excited state(s) of [Ru(bpy)2(4,4‘-(PO3H2)2bpy)](Br)2 adsorbed in nanoporous, thin ZrO2 films are complex. Decay kinetics are comparable under Ar or Ar-deaerated CH3CN suggesting that the complexes are imbedded in the open porous structures of the films. Average lifetimes are dependent on the extent of fractional coverage (FRuII) and emission maxima are time dependent. A model is invoked involving complex surface relaxation dynamics arising from a heterogeneity in adsorption sites and cross-surface RuII*-to-RuII migration and quenching at low-energy trap sites. On mixed surfaces containing both adsorbed RuII and [Os(bpy)2(4,4‘-(CO2H)2bpy)](PF6)2 (OsII), RuII*-to-OsII energy transfer occurs with ΔG° = −0.40 eV. On the basis of CW emission and lifetime measurements, the extent of quenching varies with the fractional surface coverage of OsII, FOsII. The average rate constant for energy transfer 〈ken〉 is exponentially dependent on distance r according to the equation, ken(r) = ken(ro) exp(−βen(r − rO)), consistent with a dominant role for the Dexter (exchange) energy transfer mechanism. In this equation, the rate constant at close contact, ro, is 〈ken(ro)〉 = 2.7 × 107 s-1 and β = 0.35 Å-1. By using emission spectral fitting to evaluate the barrier parameters for energy transfer, the energy transfer matrix element at close contact is 〈Vo〉 = 0.4 cm-1. As shown by CW emission measurements, the extent of RuII* quenching is also dependent on the fractional coverage of RuII but in a complex way. A qualitative model is proposed to explain the data based on (1) RuII* → OsII energy transfer, (2) cross-surface energy migration by a random walk, and (3) RuII* → Os energy transfer following RuII* → RuII migration by percolation.