Hole Transfer Dynamics from a CdSe/CdS Quantum Rod to a Tethered Ferrocene Derivative

Abstract

Hole transfer between a CdSe/CdS core/shell semiconductor nanorod and a surface-ligated alkyl ferrocene is investigated by a combination of ab initio quantum chemistry calculations and electrochemical and time-resolved photoluminescence measurements. The calculated driving force for hole transfer corresponds well with electrochemical measurements of nanorods partially ligated by 6-ferrocenylhexanethiolate. The calculations and the experiments suggest that single step hole transfer from the valence band to ferrocene is in the Marcus inverted region. Additionally, time-resolved photoluminescence data suggest that two-step hole transfer to ferrocene mediated by a deep trap state is unlikely. However, the calculations also suggest that shallow surface states of the CdS shell could play a significant role in mediating hole transfer as long as their energies are close enough to the nanorod highest occupied molecular orbital energy. Regardless of the detailed mechanism of hole transfer, our results suggest that holes may be extracted more efficiently from well-passivated nanocrystals by reducing the energetic driving force for hole transfer, thus minimizing energetic losses.

Hole Transfer from Photoexcited Quantum Dots: The Relationship between Driving Force and Rate

“…This value is in good agreement with our previous study on CdSe/CdS core/shell rods that employed the same experimental technique corroborated by theoretical work to determine the position of the valence band edge of the CdSe core at +0.8 V vs ferrocene. 29 Slight deviations may be a result of the smaller CdSe cores used in the rod study, which would shift the valence band down by a small amount.…”

“…However, in the experimental system, the ferrocene ligand will also be surrounded by lower dielectric alkane ligands along with chloroform, thus reducing the reorganization energy. 29 We therefore used values for λ of 400 and 500 meV to generate expected rate vs driving force plots ( Figure 5), against which we can compare our experimental data. It is clear that the standard two-state Marcus model does not fit our current data since we do not observe an inverted region.…”

Efficiency of Hole Transfer from Photoexcited Quantum Dots to Covalently Linked Molecular Species

“…The hole acceptor used is ferrocene, whose oxidation potential lies approximately 850 meV above the valence band of the CdSe core based on a previous measurement. (19) The large thermodynamic driving force for photoinduced hole transfer allows this process to compete with native radiative recombination. This is reflected in the measured PL (35) and PL lifetime, (19) yet the hole transfer rate in our study is most sensitive to modulations in the electronic coupling, achieved by varying the thickness and composition of the barrier material between the CdSe core and the acceptor.…”

“…(19) The large thermodynamic driving force for photoinduced hole transfer allows this process to compete with native radiative recombination. This is reflected in the measured PL (35) and PL lifetime, (19) yet the hole transfer rate in our study is most sensitive to modulations in the electronic coupling, achieved by varying the thickness and composition of the barrier material between the CdSe core and the acceptor. We preclude the possibility of resonance energy transfer due to the lack of spectral overlap of the ferrocene absorption with QD emission.…”

“…(7) A growing body of spectroscopic work has examined charge transfer rates from QDs to molecular charge acceptors typically physisorbed onto the QD surface, exploring the parameter space in the Marcus equation. (8) Electron transfer studies (8)(9)(10)(11)(12)(13) outnumber hole studies, (14)(15)(16)(17)(18)(19) despite hole transfer being the limiting factor in the efficiencies of QD sensitized solar cells and in QD-based colloidal photocatalytic hydrogen evolving systems. (20,21) To establish a sound model for charge transfer from nanocrystals to molecular acceptors, we must address the features of this system that make the process more difficult to characterize than that of the pure molecular case.…”