Electron injection dynamics of RuII(4,4′-dicarboxy-2,2′-bipyridine)2cis(NCS)2 adsorbed on MoS2 nanoclusters

The electron injection dynamics of Ru-II(4,4'-dicarboxy-2,2'-bipyridine)(2)cis(NCS)(2) (N3 dye) ads orbed on MoS2 nanoclusters have been studied using static and time-resolved optical spectroscopy. Static spectroscopy and the comparison with Ru-II(2,2'-bipyridine)(2)cis(NCS)2 indicate that the N3 is bound to the MoS2 nanoclusters and that the binding occurs through the carboxylates. The static absorption spectrum of adsorbed N-3/MoS2 is red shifted about 1.2 nm compared to N3 in an equivalent acetonitrile/hexanol/tridodecylmethylammonium iodide solution. The extent of the spectral shift indicates that the coupling is comparable to that in the N3/TiO2 case. The time-resolved results indicate that about 35% of the adsorbed dyes inject electrons into the MoS2 conduction band on the 250 ps time scale, and about 65% do not inject. These results indicate that electron injection is comparatively slow in these systems (250 ps compared to <100 fs in N3/TiO2) because of the low density of conduction band states in these quantum-confined nanoclusters. We speculate that the biphasic decay may be understood in terms of a mechanism in which one of the 4,4'-dicarboxy-2,2'-bipyridine Ligands is attached to the nanocluster, while the other ligand is not attached. The attached and nonattached ligands have strong and very weak electronic coupling to the nanocluster, respectively. Electrons in the MLCT state localized on the attached ligand undergo injection on the 250 ps time scale, while those localized on the nonattached ligand do not inject. in this model, attached-to-nonattached interligand electron transfer competes with electron injection and lowers the injection quantum yield to the observed value of 35%.