Intermediate Complex-Mediated Interfacial Electron Transfer in a Radical Dianion/TiO 2 Dye-Sensitized Photocatalytic System.

We present a mechanistic study of a PTCDA 2- /TiO 2 dye-sensitized photocatalytic system, in which the stable radical dianion PTCDA 2- is formed via a two-step consecutive photoinduced electron transfer from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride). Photoexcitation of PTCDA 2- brings forth an interesting behavior known as vibrationally excited-state-selective, visible-light photocatalytic hydrogen evolution reaction (HER). In conjunction with the information gleaned from optical spectroscopy and ultrafast dynamics, we reveal that an intermediate complex (IC) state with a lifetime of ∼12 ps exists in the vicinity of a certain vibrationally excited state of PTCDA 2- . Such a unique IC state mediates the interfacial electron transfer (IET) channel from the specific excited state of PTCDA 2- to the conduction band continuum of TiO 2 . As an outcome, the effective IC-mediated IET process in this photocatalytic system leads to a remarkable HER rate that reaches ∼4660 μmol g -1 h -1 .