Molecular energy transfer across oxide surfaces

The decay properties of the metal-to-ligand charge transfer (MLCT) excited state(s) of [Ru(bpy)(2)(4,4 '-(PO3H2)(2-)bpy)](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 (F-Ru(II)) 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 Ru-II*-to-Ru-II migration and quenching at low-energy trap sites. On mixed surfaces containing both adsorbed Ru-II and [Os(bpy)(2)(4,4 '-(CO2H)(2)bpy)](PF6)(2) (Os-II), Ru-II*-to-Os-II energy transfer occurs with DeltaG degrees = -0.40 eV. On the basis of CW emission and lifetime measurements, the extent of quenching varies with the fractional surface coverage of Os-II, Fo(s)(II). The average rate constant for energy transfer <k(en)> is exponentially dependent on distance r according to the equation, k(en)(r) = k(en)(r(o)) exp(-beta (en)(r - ro)), consistent with a dominant role for the Dexter (exchange) energy transfer mechanism. In this equation, the rate constant at close contact, r(o), is <k(en)(r(o))> = 2.7 x 10(7) s(-1) and beta = 0.35 Angstrom (-1). By using emission spectral fitting to evaluate the barrier parameters for energy transfer, the energy transfer matrix element at close contact is <V-o> = 0.4 cm(-1). As shown by CW emission measurements, the extent of Ru-II* quenching is also dependent on the fractional coverage of Ru-II but in a complex way. A qualitative model is proposed to explain the data based on (1) Ru-II* - Os-II energy transfer, (2) cross-surface energy migration by a random walk and (3) Ru-II* - Os energy transfer following Ru-II* - Ru-II migration by percolation.