Intermediate Complex-Mediated Interfacial Electron Transfer in a Radical Dianion/TiO₂ Dye-Sensitized Photocatalytic System

Abstract

We present a mechanistic study of a PTCDA 2− /TiO 2 dyesensitized 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 .

Intermolecular singlet fission in a radical dianion system in solution phase

“…A proof-of-concept scrutiny is conducted on a stable radical dianion system of PTCDA 2− , which is formed in solution via a two-step consecutive photoinduced electron transfer from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride) as described in detail in our previous work. 15 Using femto/nanosecond time-resolved transient absorption (fs-/ns-TA) spectroscopy, we fully map out the cascading steps involved in the solution-phase xSF process of photoexcited PTCDA 2− , with multiple intermediate states such as excimer 1 (S 1 S 0 ), spin-correlated triplet-pair 1 (T 1 T 1 ), and spatially separated triplet-pair 1 (T 1 ⋅S 0 ⋅T 1 ) states being identified. Figure 1(a) shows the steady-state absorption spectrum (400-800 nm) recorded on the solution of PTCDA 2− in dimethyl sulfoxide (DMSO) (details of sample preparation in the supplementary material).…”

“…Figure 1(a) shows the steady-state absorption spectrum (400-800 nm) recorded on the solution of PTCDA 2− in dimethyl sulfoxide (DMSO) (details of sample preparation in the supplementary material). The prominent A band (corresponding to the lowest vibronic S 0 → S 1 transition) 15 peaks at ∼550 nm, which is selected as the photoexcitation wavelength for the following fs-/ns-TA measurements. Figure 1(b) compares the electron paramagnetic resonance (EPR) spectra recorded on PTCDA 2− (in DMSO) without and with Xenon-lamp irradiation (details in the supplementary material).…”

“…(a)Steady-state absorption spectrum (400-800 nm) recorded on PTCDA 2− (in DMSO). The bands labeled A, B, and C correspond to the vibronic excitations of S 0 → S 1 and those labeled CT to the charge-transfer excitations 15. Inset shows the molecular structure of PTCDA (i.e., the precursor of PTCDA 2− ) and the schematic of PTCDA 2− formation via a consecutive photoinduced electron transfer (PET) mechanism.…”