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.
The addition of research-focused experiences to undergraduate chemistry laboratory courses has been shown to bolster student learning, enhance student retention in STEM, and improve student self-identity as scientists. In the area of synthetic organic chemistry, the preparation of libraries of compounds with novel optical and electronic properties can provide a natural motivational goal for research-focused exercises that can be undertaken by individual students or collectively as a class. However, integrating such experiences into a community college teaching laboratory setting can face challenges imposed by the cost of supplies, limited laboratory space, and access to characterization facilities. To address these challenges, we have devised a sequence of inquiry-driven, research-focused laboratory exercises that can be readily integrated into an organic chemistry laboratory course with minimal cost. This sequence consists of a multistep synthesis of perylenediimide dyes that introduces students to advanced synthetic techniques, such as organometallic coupling reactions, column purification, and reactions performed under inert atmosphere. This high-yield, three-part synthesis can be easily varied by individual students or small groups within a class to form a broad library of compounds with potential utility for applications in light harvesting, molecular electronics, catalysis, and medicine. We describe the design of low-cost workstations for chemical synthesis under inert atmosphere and provide auxiliary lesson plans that can be used to expand the scope of a laboratory course beyond synthetic organic chemistry by introducing students to concepts in molecular spectroscopy.
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