Photoinduced electron transfer from supramolecular assemblies into semiconductor nanoparticles and zinc-substituted cytochromes

Photoinduced electron transfer from S2 and S 1 states of azulene derivatives bound to TiO2 nanoparticles was investigated. Electron injection from S2 state occurred in < 100 fs and while that from S1 state occurred in < 50 fs. As in most TiO2-dye systems, charge recombination kinetics in all cases is heterogeneous implying that the process is dominated by trapping and de-trapping of excess electrons on TiO2 surfaces. Recombination resulting from electron injection from the S2 state of the directly bound dyes is up to ∼ 90% complete on a 120 ps timescale while that in tethered dyes is ∼50%. Back electron transfer resulting from S1 injection occurred on a much faster rate and is up to 70% complete on a 35 ps timescale. Surprisingly, no significant difference in the rates of recombination of species resulting from S1 injection was observed for directly bound and tethered dyes. Charge injection (140 ps) and recombination (45 +/- 5% ns) observed between azulene incarcerated within a molecular container (hemicarcerand) and TiO2 are three decades slower than those observed in the directly bound and tethered dyes. The recombination kinetics observed in encapsulated azulene is homogenous strongly suggesting that tunneling of electrons from TiO2 through the wall of the hemicarcerand is the rate limiting step in the process.; In the second part of the study, the potential use of fully encapsulating hemicarcerands in molecular transport was explored using zinc-substituted cytochrome c as a target molecule. The interaction between the encapsulated guests and the protein was probed by electron transfer quenching of the long-lived triplet phosphorescence of cytochrome c. The observed electron transfer rates (up to 1250 s-1) suggest that hemicarcerand binds in the vicinity of the exposed heme-edge of cytochrome c.; In the last part of the project, preparation and characterization of the zinc analogue of cytochrome P450cam is presented. The protein is to be used in investigations on the use of hemicarcerand as a molecular shuttle to deliver electrons or holes from small hydrophobic guests across 22 A substrate access channel of the target protein.