Four Zn(II)-tetra(carboxyphenyl)porphyrins in solution and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle films were studied to determine the effect of the spacer length and anchoring group position (para or meta) on their binding geometry and photoelectrochemical and photophysical properties. The properties of three types of anchoring groups (COOH and COONHEt3) for four Zn(II)-porphyrins (Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (m-ZnTCP2P), and Zn(II)-5,10,15,20-tetra(3-ethynyl(4-carboxyphenyl)phenyl)porphyrin (m-ZnTC(PEP)P)) were compared. In m-ZnTCPP, m-ZnTCP2P, and m-ZnTC(PEP)P the four anchoring groups are in the meta position on the meso-phenyl rings of the porphyrin macrocycle, thus favoring a planar binding mode to the metal oxide surfaces. The three meta-substituted porphyrin salts have rigid spacer units of increasing length (phenyl (P), biphenyl (P2), and diphenylethynyl (PEP)) between the porphyrin ring and the carboxy anchoring groups, thus raising the macrocycle from the metal oxide surface. All porphyrins studied here, when bound to TiO2 and ZnO, exhibited quenching of the fluorescence emission, consistent with electron injection into the conduction band of the semiconductor. Steady-state UV-vis and fluorescence studies of p-ZnTCPP on insulating ZrO2 showed evidence of aggregation and exciton coupling. This was not observed in any of the meta-substituted porphyrins. The photoelectrochemical properties (IPCE, Voc, and Isc) of the porphyrins bound to TiO2 films in solar cells have been measured and rationalized with respect to the sensitizer binding geometry and distance from the surface.

Two bifunctional linkers, a rigid-rod p-ethynyl-isophthalic acid capped with a Ru(II)-polypyridyl complex and 3-mercaptopropionic acid, were covalently bound to ZnO nanotip films grown by metal-organic chemical vapor deposition (MOCVD) technology. This highly vertically aligned, crystalline form of ZnO had not been functionalized before. The binding was studied by Fourier transform (FT) IR and UV spectroscopies and probed, in the case of the Ru complex, by static and dynamic fluorescence quenching. The molecules did bind through the carboxylic acid groups, and the FT-IR attenuated total reflectance spectra are indicative of a bidentate carboxylate binding mode. Other molecules (heptanoic acid, isophthalic acid, and trimethoxy(2-phenylethyl)silane) were also bound to the ZnO nanotips. A comparison was made with epitaxial ZnO films grown by MOCVD and ZnO mesoporous films prepared from colloidal solutions to investigate the effect of the ZnO morphology. The ZnO nanotips were excellent binding substrates, particularly for the rigid-rod linker. Since ZnO films are etched at low pH (< 4), novel nanotip films made of ternary MgxZn1-xO, which is formed by alloying ZnO with MgO and is more resistant to acids, were developed. The MgxZn1-xO nanotip films were employed to use linkers with acidic groups and to study the effect of pH pretreatment of the surface on the binding.

The ring-chain conformations of three 7-cis isomers of vitamin A and several related hindered trienes have been examined by 'H NMR methods (long-range coupling constant, NOE, and DNMR). The dihedral angles are shown to be significantly higher than those of the related 7-trans isomers. The barriers of rotation of the 6,7 single bond, as measured by AG*,, fall in the range of 11-14 kcal/mol. For a much more hindered 7-cis diene, the corresponding barrier was found to be 19.7 i 0.3 kcal/mol.The complexation of guanidinium perchlorate with crown ethers of different ring size (18-33 ring atoms) and with different subunits, e.g., catechol and 1,3-xylyl moieties, has been studied by using two-phase extraction experiments. The results demonstrate that the 18-crown ethers are able to form perching complexes, whereas crown ethers with 327 ring atoms have a suitable ring size to form encapsulated complexes with guanidinium perchlorate. Aromatic, catechol, and especially 1,3-xylyl moieties have a destabilizing effect on the complex formation. The crystal and the molecular structures of the 1:l complexes of guanidinium perchlorate with benzo-27-crown-9 (7), dibenzo-27-crown-9 (8), and dibenzo-30-crown-10 (1 1) have been determined by X-ray crystallography. In these encapsulated complexes aJl hydrogen atoms of the guanidinium cation are used in hydrogen bonds to the macrocyclic host. The 27-crown ethers show an optimal fit between cation and the macrocyclic host with a complementary binding scheme. Dynamic 500-MHz 'H NMR spectroscopy gave the kinetic stabilities of these complexes with AGd* values of 11.5, 11.2, and 12.0 kcal mo1-l for the complexes with benzo-27-crown-9, dibenzo-27-crown-9, and dibenzo-30-crown-10, respectively.

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