Oxidation of a Molecule by the Biexcitonic State of a CdS Quantum Dot

Abstract

This paper describes spectroscopic evidence for the photoinduced transfer of a hole from the biexcitonic state of a CdS quantum dot (QD) to a phenothiazine (PTZ) molecular acceptor, covalently linked to the QD through phenyldithiocarbamate (PTC), with power-dependent yields of 8–21%. Visible and near-infrared transient absorption spectroscopy (TA) data suggest that the mechanisms of hole extraction include direct hole transfer from the QD’s valence band to PTZ in 2.4 ± 0.2 ps, or trapping of holes at the QD surface in ∼1 ps, followed by sequential hole transfer to PTZ. Both of these mechanisms potentially out-compete Auger recombination of biexcitonic states, which occurs within these QDs in 20 ± 1 ps. These results suggest that the PTC linkage will be useful for extracting multiple holes from a QD photosensitizer or solo photocatalyst to drive multistep oxidation reactions.

Uncovering the Role of Hole Traps in Promoting Hole Transfer from Multiexcitonic Quantum Dots to Molecular Acceptors

“…The lowest CB level (1Se state) can accommodate at most two electrons, and therefore the XB signal at the band edge will be close to the saturation level of ΔA/A0 ~1 when the CB is populated by two or more electrons. 19,37 At the early time t = 10 ps, both the QDs with and without NPTZ ligands showed nearly full saturation of the band edge XB signal as seen by the black lines in Figure 2A. For QDs without NPTZ added (top panel), the XB signal decayed to ΔA/A0 ~0.5 at longer delay times t > 100 ps, indicating that fast AR processes have brought the system from a multi-excitonic state to a single-excitonic state.…”

“…[34][35][36] Ultrafast transient absorption (TA) spectroscopy provides the time resolution required to directly track the population of holes in different states as they have established that phenothiazine derivatives can serve as a class of hole acceptors for CdS QDs or nanorods with both the high driving force and the necessary optical signature in TA to track hole transfer. 31,37,38 In these model systems, ultrafast hole trapping was observed within ~1 ps following photo-excitation, and trap-mediated hole transfer dynamics were investigated under single-excitonic or bi-excitonic conditions.…”

“…For metal chalcogenide QDs, hole trap states are prevalent on the surface due to underpassivated chalcogenide atoms. , The multitude of these relatively localized hole traps play an important role in mediating hole transfer from delocalized valence band (VB) states to surface bound acceptors. − Hole-trapping states also affect the AR lifetimes by altering the electron–hole interactions and, therefore, significantly impacting the kinetic competition between hole transfer and AR dynamics. − Ultrafast transient absorption (TA) spectroscopy provides the time resolution required to directly track the population of holes in different states as they migrate from QDs to acceptors. Previous works with TA methods by Lian et al and Weiss et al have established that phenothiazine derivatives can serve as a class of hole acceptors for CdS QDs or nanorods with both the high driving force and the necessary optical signature in TA to track hole transfer. ,, In these model systems, ultrafast hole trapping was observed within ∼1 ps following photoexcitation, and trap-mediated hole-transfer dynamics were investigated under single-excitonic or biexcitonic conditions.…”

“…31−33 Hole-trapping states also affect the AR lifetimes by altering the electron−hole interactions and, therefore, significantly impacting the kinetic competition between hole transfer and AR dynamics. 34−36 31,37,38 In these model systems, ultrafast hole trapping was observed within ∼1 ps following photoexcitation, and trap-mediated hole-transfer dynamics were investigated under single-excitonic or biexcitonic conditions.…”

Low-Dimensional Semiconductors in Artificial Photosynthesis: An Outlook for the Interactions between Particles/Quasiparticles

“…In addition to exciton–exciton annihilation, the interactions between excitons might lead to the formation of high-order quasiparticles like the biexciton. Although these quasiparticles have been deemed to be short-lived, their abundant dipole moments permit considerable energy transfer to the single-exciton (Figure d). , Traditionally, high-order interactions closely depend on the concentrations of photoinduced species, and high-density illumination would be beneficial.…”