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

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.