Using Spectator Ligands to Enhance Nanocrystal-to-Molecule Electron Transfer

Abstract

Semiconductor nanocrystals (NCs) have emerged as promising photocatalysts. However, NCs are often functionalized with complex ligand shells that contain not only charge acceptors but also other “spectator ligands” that control NC solubility and affinity for target reactants. Here, we show that spectator ligands are not passive observers of photoinduced charge transfer but rather play an active role in this process. We find the rate of electron transfer from quantum-confined PbS NCs to perylenediimide acceptors can be varied by over a factor of 4 simply by coordinating cinnamate ligands with distinct dipole moments to NC surfaces. Theoretical calculations indicate this rate variation stems from both ligand-induced changes in the free energy for charge transfer and electrostatic interactions that alter perylenediimide electron acceptor orientation on NC surfaces. Our work shows NC-to-molecule charge transfer can be fine-tuned through ligand shell design, giving researchers an additional handle for enhancing NC photocatalysis.

Aggregation of Charge Acceptors on Nanocrystal Surfaces Alters Rates of Photoinduced Electron Transfer

“…Figure A shows TA spectra of PbS NCs functionalized with a high surface concentration of C2-PDI (⟨ N PDI ⟩ = 33.6) recorded following photoexcitation of PbS at 800 nm. At a delay time of 0.25 ps, a broad induced absorption appears across the visible spectrum (most clearly seen from 575 to 650 nm) that has previously been assigned to interband transitions of PbS. ,, As time advances, we observe the growth of a negative band between 450 and 550 nm concomitant with strengthening of an induced absorption band peaked near 720 nm. The negative feature matches the inverse of the absorption spectrum of surface-bound C2-PDI ligands (Figure A), indicating that over time, these molecules are depopulated from their ground state.…”

“…PDI ligands and PbS NCs natively passivated with oleate were synthesized using previously reported methods. − Additional synthetic details are described in the Supporting Information. We employ asymmetric PDI ligands (Scheme ) that contain a swallowtail for enhanced solubility, a carboxylate group that allows attachment to a PbS NC surface and a linker consisting of an ethyl (C2-PDI), pentyl (C5-PDI), or benzyl (Bn-PDI) group.…”

“…To compare timescales for photoinduced electron transfer between PbS NCs that bind C2-PDI, C5-PDI, and Bn-PDI ligands, we make use of a fitting approach described by Raulerson et al that employs a kinetic model detailed by Morris-Cohen et al. , In this model, PDI ligands are taken to be dispersed among the NCs within an ensemble according to Poisson statistics. Following photoexcitation of a PbS NC, each PDI bound to its surface is assumed to be able to accept an electron with a rate, k ET 0 .…”

“…Prior reports have shown that monomeric electron acceptors anchored to NCs by short alkyl chains can tilt toward the surface, thereby reducing the tunneling gap for electron transfer. 40,60 However, the formation of cofacially stacked PDI aggregates may impact how PDIs orient with respect to the PbS surface, affecting k ET 0 . To untangle these potential structural and energetic effects, we have employed electronic structure calculations in concert with MD simulations.…”

“…Second, aggregation can restrict the ability of electron acceptors to approach the PbS surface, thereby reducing their electronic coupling. Prior reports have shown that monomeric electron acceptors anchored to NCs by short alkyl chains can tilt toward the surface, thereby reducing the tunneling gap for electron transfer. , However, the formation of cofacially stacked PDI aggregates may impact how PDIs orient with respect to the PbS surface, affecting k ET 0 .…”