Ultrafast Stepwise Electron Injection from Photoexcited Ru-Complex into Nanocrystalline ZnO Film via Intermediates at the Surface

Ultrafast electron injection processes from photoexcited Ru-complex, cis -bis-(4,4‘-dicarboxy-2,2‘-bipyridine)dithiocyanato ruthenium(II), [Ru(dcbpy) 2 (NCS) 2 ] (N3), into nanocrystalline ZnO film have been investigated by observing the femtosecond transient absorption in the visible and near-IR range (600−2000 nm). After photoexcitation of the metal-to-ligand charge-transfer band of the adsorbed N3 dye, a very broad transient absorption band ranging from 600 to 1500 nm having a maximum at around 1200 nm rose in less than 100 fs. Subsequently, the absorption band in the near-IR showed a multiexponential decay with time constants of ∼5 ps, ∼150 ps, and >1 ns, leading to the generation of conductive electrons which absorb at around 2000 nm. Therefore, this initial broad band is considered to be due to the intermediates during the electron transfer from excited N3 to the conduction band of ZnO. The origin of the broad absorption band around 1200 nm is discussed in terms of a charge-transfer complex between the excited N3 dye and surface states of ZnO.