TiO 2 nanoparticles when subjected to UV irradiation exhibit blue coloration as electrons are stored within the particles. Upon contact with gold nanoparticles, a partial disappearance of the blue color is seen as the stored electrons are transferred from TiO 2 to Au nanoparticles. The charge distribution between the semiconductor and metal nanoparticles causes the Fermi level to shift to more negative potentials. By employing C 60 /C 60 •as a probe−redox couple, we were able to estimate the apparent Fermi levels of TiO 2 and TiO 2 /Au nanoparticles. A Fermi level shift of −22 mV observed for the Au−TiO 2 nanocomposite is indicative of improved charge separation in semiconductor−metal systems and demonstrates its usefulness for improving the efficiency of photocatalytic reactions.

Photoinduced intramolecular electron-transfer (ET) and energy-transfer (EnT) processes in two rotaxanes, one containing both zinc porphyrin and C(60) fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)(2)-Cu(I)(phen)(2)-C(60)] and a second complex lacking the fullerene [(ZnP)(2)-Cu(I)(phen)(2)], were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) combined with a selective photoexcitation of the rotaxane moieties. The experiments were carried out in isotropic toluene and ethanol and in anisotropic nematic liquid-crystal (E-7) media over a wide range of temperatures corresponding to the different states of the solvents. The TREPR results are compared with those obtained previously by optical methods in dichloromethane at room temperature. It is demonstrated that the efficiencies and pathways of the light-driven ET and EnT processes in both rotaxanes strongly depend on the properties of their microenvironment, resulting in the formation of distinct charge-separated states under different experimental conditions. The complementary results revealed by the optical and TREPR techniques are attributed to the relatively high conformational mobility of the mechanically interlocked rotaxane systems. Because of the solute-solvent interactions, the rotaxanes are able to change conformation in different microenvironments, which affects the parameters of the photoinduced processes occurring in these systems.

Light-driven intramolecular electron transfer (ET) and energy transfer (EnT) processes in two rotaxanes, the first containing two free base porphyrins and C60 fullerene moieties incorporated around a Cu(I)bisphenanthroline core ((H2P)2-Cu(I)(phen)2-C60) and a second rotaxane lacking the fullerene moiety ((H2P)2-Cu(I)(phen)2) were studied by X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The experiments were performed in a frozen toluene and ethanol, and different phases of the nematic liquid crystal (E-7). It is demonstrated that the ET and EnT processes in the (H2P)2-Cu(I)(phen)2-C60 rotaxane in different media result in formation of the same charge separated state, namely (H2P)2•+-Cu(I)(phen)2•−-C60, while photoexcitation of the (H2P)2-Cu(I)(phen)2 rotaxane does not induce noticeable transfer processes in these matrices. The results are discussed in terms of the high conformational mobility of the rotaxanes, which enables changes in the molecular topography and resultant modification of the rates and routes of photoinduced processes occurring in these systems. The parameters of the transfer processes are compared with those obtained in our previous study of (ZnP)2-Cu(I)(phen)2-C60 and (ZnP)2-Cu(I)(phen) rotaxanes under the same experimental conditions.

No abstract