Photoinduced interfacial electron transfer (ET) from molecular adsorbates to semiconductor nanoparticles has been a subject of intense recent interest. Unlike intramolecular ET, the existence of a quasicontinuum of electronic states in the solid leads to a dependence of ET rate on the density of accepting states in the semiconductor, which varies with the position of the adsorbate excited-state oxidation potential relative to the conduction band edge. For metal oxide semiconductors, their conduction band edge position varies with the pH of the solution, leading to pH-dependent interfacial ET rates in these materials. In this work we examine this dependence in Re(L P )(CO) 3 Cl (or ReC1P) [L P ) 2,2′-bipyridine-4,4′-bis-CH 2 PO(OH) 2 ] and Re-(L A )(CO) 3 Cl (or ReC1A) [L A ) 2,2′-bipyridine-4,4′-bis-CH 2 COOH] sensitized TiO 2 and ReC1P sensitized SnO 2 nanocrystalline thin films using femtosecond transient IR spectroscopy. ET rates are measured as a function of pH by monitoring the CO stretching modes of the adsorbates and mid-IR absorption of the injected electrons. The injection rate to TiO 2 was found to decrease by 1000-fold from pH 0-9, while it reduced by only a factor of a few to SnO 2 over a similar pH range. Comparison with the theoretical predictions based on Marcus' theory of nonadiabatic interfacial ET suggests that the observed pH-dependent ET rate can be qualitatively accounted for by considering the change of density of electron-accepting states caused by the pH-dependent conduction band edge position.

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Rigid rod-like aromatic polyimides have been widely used as a passivation or inter-dielectric layer in microelectronic devices due to their high mechanical modulus, excellent thermal stability, low thermal expansion, low solvent swelling, and low dielectric constant. [1±5] Although this outstanding property comes from the rigid phenyl and imide moieties in the polyimide backbone, the drawback of the rigid rod-like characteristics includes poor adhesion to metallic surface and large dielectric anisotropy, leading to a catastrophic device failure and signal deformation. [1,4,5] Therefore, the rigid rodlike polyimide cannot be used further for highly advanced devices with ultra large scale integration that require much more precise signal transmission.In this regard, it has been attempted to improve the weakness of rigid rod-like polyimides through a molecular composite that can be prepared by molecular level mixing of rigid rod-like and flexible coil-like polyimides because flexible coillike polyimides exhibit normally good adhesion, high elongation, and low dielectric anisotropy (see Fig. 1). [6±11] Although optically transparent polyimide composite films were successfully prepared using soluble precursors, poly(amic acid) and/or poly(amic dialkyl ester), which are well dissolved in common weak base solvents, careful studies showed that the resulting polyimide composite films were phase segregated on a scale of several nanometers, meaning a nanocomposite rather than a molecular composite, because rigid rod-like polyimides have inherently high tendency to intermolecular chain packing and preferential in-plane molecular orientation. [9,11] In particular, these previous reports focused on the physical and structural analysis in order to study the formation of molecular composite. However, the in-plane and outof-plane optical and dielectric properties have not been investigated yet, even though this is one of major causes to make the polyimide molecular composite because the dielectric constant of the packaging or inter-dielectric layers has a huge influence on the delay of signal propagation and crosstalk between other signal lines. [12] In this work, we report on the high frequency optical and dielectric anisotropies of the polyimide nanocomposite films prepared from poly(p-phenylene biphenyltetracarboxamic diethyl ester) (BPDA-PDA ES) and poly(4,4¢-oxydiphenylene biphenyltetracarboxamic diethyl ester) (BPDA-ODA ES) through the in-plane and out-of-plane refractive indices measurement. The pristine precursor films showed almost similar dielectric constant for in-plane and out-of-plane, whereas the pristine polyimide exhibited large optical and dielectric anisotropy. The dielectric anisotropy dependence of the BPDA-PDA PI/BPDA-ODA PI nanocomposite films on the composition was in good agreement with the structural anisotropy investigated by the wide angle x-ray diffraction measurement.