ENERGY-TRANSFER FROM LUMINESCENT POROUS SILICON TO ADSORBED OSMIUM(II) AND RUTHENIUM(II) POLYPYRIDYL COMPLEXES

Surface adsorption of transition-metal polypyridyl complexes on highly luminescent, anodized porous silicon (p-Si) wafers leads to dramatic quenching of the emission. Facile energy transfer occurs from the surf ace to adsorbed complexes of Ru-II and Os-II. Photoluminescence (PL) f rom underivatized p-Si is excitation- and monitoring-wavelength-depend ent. A phenomenological model is proposed to explain these observation s in which intercluster energy transfer occurs by an energy-transfer c ascade, and there is weak kinetic coupling to even lower-energy surfac e emitters. Adsorption of [Zn(dmb)(3)](PF6)(2)(dmb is 4,4'-dimethyl-2, 2'-bipyridine) or fac-[Re(bpy)(CO)(3)(4-Etpy)](PF6)(2) (bpy is 2,2'-bi pyridine; 4-Etpy is 4-ethylpyridine), possibly by ion-exchange, result s in quenching of the low-energy surface emitters. For adsorbed [M-II( bpy)(2)(4-CO2H-4'Mebpy)]-(PF6)(2) (M = Ru, Os; 4-CO(2)H4'Mebpy is 4-ca rboxylic acid-4'-methyl-2,2'-bipyridine), p-Si --> M-II energy transf er occurs to low-lying metal-to-ligand charge-transfer (MLCT) excited states on the complexes as revealed by excitation and emission measure ments. Energy transfer is efficient (>90%) and more rapid for Os-II (k (en) approximate to 6 x 10(6) s(-1)) than for Ru-II by a factor of sim ilar to 10, consistent with a decrease in driving force for p-Si --> M-II energy transfer.