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

Abstract

Semiconductor nanocrystals (NCs) interfaced with molecular ligands that function as charge and energy acceptors are an emerging platform for the design of light-harvesting, photon-upconverting, and photocatalytic materials. However, NC systems explored for these applications often feature high concentrations of bound acceptor ligands, which can lead to ligand–ligand interactions that may alter each system’s ability to undergo charge and energy transfer. Here, we demonstrate that aggregation of acceptor ligands impacts the rate of photoinduced NC-to-ligand charge transfer between lead(II) sulfide (PbS) NCs and perylenediimide (PDI) electron acceptors. As the concentration of PDI acceptors is increased, we find the average electron transfer rate from PbS to PDI ligands decreases by nearly an order of magnitude. The electron transfer rate slowdown with increasing PDI concentration correlates strongly with the appearance of PDI aggregates in steady-state absorption spectra. Electronic structure calculations and molecular dynamics (MD) simulations suggest PDI aggregation slows the rate of electron transfer by reducing orbital overlap between PbS charge donors and PDI charge acceptors. While we find aggregation slows electron transfer in this system, the computational models we employ predict ligand aggregation could also be used to speed electron transfer by producing delocalized states that exhibit improved NC-molecule electronic coupling and energy alignment with NC conduction band states. Our results demonstrate that ligand aggregation can alter rates of photoinduced electron transfer between NCs and organic acceptor ligands and should be considered when designing hybrid NC:molecule systems for charge separation.

How Pendant Groups Dictate Energy and Electron Transfer in Perovskite–Rhodamine Light Harvesting Assemblies

“…Progressive addition of NCs increases the absorbance ratio to 2.41, closer to the value in neat ethanol, indicating its monomeric form when bound to CsPbBr 3 nanocrystals (see Supporting Information for further discussion). Thus, although RoseB exhibits aggregation in the nonpolar solvents that CsPbBr 3 favors, the nanocrystals provide a surface with which the RoseB molecules can disperse and assemble, an important factor in energy/electron transfer interactions …”

“…Thus, although RoseB exhibits aggregation in the nonpolar solvents that CsPbBr 3 favors, the nanocrystals provide a surface with which the RoseB molecules can disperse and assemble, an important factor in energy/electron transfer interactions. 61 Time-Resolved Photoluminescence Measurements. To investigate the dynamics of energy/electron transfer within these nanocrystal−chromophore complexes, we monitored the changes in photoluminescence (PL) lifetime of the nanocrystals in the presence of the acceptors.…”

Colloidal-ALD-Grown Metal Oxide Shells Enable the Synthesis of Photoactive Ligand/Nanocrystal Composite Materials

“…When the goal is to design and assemble structures consisting of NCs and chromophores capable of efficiently guiding the flow of energy or carriers between the two constituting units, the chromophores must be immobilized with an optimal density and in close proximity of the NC surface. ,− To this end, ligand exchanges have been used to successfully replace a portion of the native synthetic ligands with the desired chromophore in a one-to-one swap, a process that is mostly driven by mass action. ,, Despite being quite practical, these ligand exchanges present several limitations: (1) they require excess of the chromophore, which is often limited in supply; (2) excess chromophore passivation can hamper colloidal stability; (3) NCs need to be purified after the exchange, which leads to further loss of chromophore coverage and colloidal stability; (4) unpurified samples can suffer from detrimental absorptive screening by free ligands and have limited processability; (5) the introduction of multiple types of ligands results in their inherent competition for the limited available binding sites; and (6) sterically hindered ligands have a restricted capability of exchange. , Consequently, an alternative methodology to ligand exchanges which traps functional organic molecules on the NC surface and/or in its close proximity would be beneficial to extend and enhance the applicability of NCs.…”

Exploring Configurations of Nanocrystal Ligands Using Machine-Learned Force Fields

“…Additionally, relevant force fields may need to allow for the possibility of a covalent component of bonding between the semiconductor and the ligands which are mobile on a surface of the nanocrystal . Consequently, for PbS nanocrystals, for instance, force fields with very different partial-charge and Lennard-Jones parameters have been proposed and used in the literature. − Ab initio MD simulations are capable of alleviating the problems just discussed, but for the systems of interest, the high computational intensity limits them to relatively short simulation times …”

“…The ML force fields in this work were developed with the DeePMD package. − They were trained on DFT energies and forces using an iterative procedure beginning with a set of geometries obtained using an analytical force field (see Computational Methods and Section S1 for details). Two types of force fields were considered.…”