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45966-B6
Dependence of Rates of Interfacial Electron Transfer on Anchoring Group Structure and Dye MLCT State Energy

Debra L. Mohler, James Madison University

Because there has been no systematic study of the effects on ET rate of the identity/properties of the anchoring group or of the electronic coupling between the adsorbate and the semiconductor (through matching of the energy levels of the dye to that of the semiconductor conduction band), the proposed research entails the systematic design, synthesis, and photoinduced interfacial ET studies of a series of chlorotricarbonylrhenium bipyridine complexes substituted on the bipyridine ligand with a variety of anchoring groups (compounds 1 – 9) or substituents that modulate the energy levels of the ligand p-molecular orbitals (molecules 10 – 13).

After submission of this proposal, we prepared, and in collaboration with Professors Tianquan Lian and Keiji Morokuma at Emory University, we have published studies with 1, 3, and 9:  (1) to characterize contact by measuring electron transfer rate and their dependence on anchoring group and electrode, and  (2) to model these contacts by modern computation chemistry. Our findings reveal that: 

• Electron transfer rate through contacts of molecules and semiconductors and metals nanoparticles can be measured and these rates depend on the nature of the contact (anchoring group and nanoparticles).

• Computational modeling of small cluster of Ti with three molecules with different anchoring groups (PO₃H₂, SH and COOH) reveals that the electronic coupling strength depends critically on the anchoring group.

• The calculated coupling strength correlates with electron transfer rate, in that higher coupling values correspond to faster rates, as we predicted.

In addition, we have also completed the synthesis of Re complex 8 and of the bipyridine ligands in compounds 4 and 5. We have also prepared a number of intermediates in the synthesis of the ligands in 2, 6, 7, and 10–13.

In related studies, we and the Lian group have also published the study of the exciton dissociation dynamics of CdSe quantum dots adsorbed with Re(CO)₃Cl(dcbpy) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), in which excitons in CdSe dissociate by electron transfer to the rhenium complex. The rate of this electron transfer was determined by the size of the quantum dots, and dissociation half-time of approximately 2.3 ps was achieved, suggesting the possibility of separating multiple electron-hole pairs.

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