No abstract

Partial Fischer esterification of poly(acrylic acid) allows tailoring of the hydrophobicity and charge density of multilayered films containing poly(allylamine hydrochloride) (PAH) and derivatized poly(acrylic acid) (d-PAA). As hydrophobicity and charge density strongly affect film permeability, control over these properties is vital for possible applications of PAH/d-PAA films as ion-separation membranes and sensors. The hydrophobicity of these films depends on both the extent of esterification and the nature of the derivatizing alcohol. Even though PAH/d-PAA films are composed of polyelectrolytes, the presence of hydrophobic ester groups results in advancing water contact angles as high as 101°. The hydrophobicity of these coatings allows them to effectively passivate underlying electrodes as shown by minimal peak currents in cyclic voltammograms (CVs) of Ru(NH3)6 3+ and Fe(CN)6 3-. Cross-linking of hydrophobic PAH/ d-PAA films via heat-induced amidation stabilizes coatings over a wide pH range but does not significantly decrease the already low film permeability to Ru(NH3)6 3+ and Fe(CN)6 3-. Stabilization due to cross-linking does, however, allow base-promoted hydrolysis of the ester groups of PAH/d-PAA coatings. After hydrolysis, films are extremely hydrophilic and selectively permeable to Ru(NH3)6 3+ over Fe(CN)6 3due to the high density of newly formed -COOgroups. In the case of some hydrolyzed films, the presence of small concentrations of Ca 2+ results in dramatic current decreases in CVs of Ru(NH3)6 3+ , suggesting possible use of these films in sensing applications.

Benzyl phenyl sulfide has been used to investigate the photocleavage mechanism for benzyl-sulfur bonds. Four experiments have shown that the reaction goes through a radical intermediate. First, the photoproducts observed can all be justified by radical mechanisms. Second, the radical intermediate was trapped with a five hexenyl tether. Third, UV analysis of analogs for the 4-NO(2) derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between sigma values and the quatum yields for loss of substituted benzyl phenyl sulide. The effect of oxygen on quantum yields is best observed after samples are thoroughly outgassed with consecutive freeze-pump-thaw cycles. It is shown that oxygen diminishes the substituent effect. Upon photolysis of the outgassed samples, the meta-substituted derivatives showed more significant variances than the para derivatives. The meta derivatives are most efficiently cleaved in the following order: 3-CN > 3-NO(2) > 3-CF(3) > 3-CH(3) > 3-OCH(3). These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH(3) derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.

The photocycloaddition of cyclic enones to C 60 is a general reaction. The addition was initially explored using a crown ether-tagged C 60 molecule which allows convenient monitoring of the reaction course by electrospray ionization mass spectroscopy (ESI-MS) after complexation with metal ions, usually K + . A Hanovia 450-W lamp was used to excite a benzene solution of fullerene and enone, with the latter in large excess. A series of experiments in which the concentration of reagents and reaction time were systematically varied established that adduct formation occurs only upon light absorption by the enones. Thus, excitation at 532 nm or in the UV range using low enone: C 60 ratios did not lead to adduct formation. ESI-MS analysis showed that monoaddition was favored at short irradiation times, and that up to seven enone units could be added to a single fullerene molecule upon longer exposure to light. Competitive experiments using 12 different enones indicated that the best product yields were obtained, in decreasing order, with 3-methyl-2-cyclohexen-1-one, isophorone, 2-cyclohepten-1-one, and 2-cyclohexen-1-one. Products isolated from reactions carried out on a larger scale were subjected to spectroscopic analysis ( 1 H NMR, 13 C NMR, IR, UV). Two monoadducts were formed from 2-cyclohexen-1-one, its 3-methyl analog, and 2-cyclohepten-1-one, which could be separated by HPLC on a Buckyclutcher column. In the first two cases, these products were unambiguously identified as cis-and trans-fused [2 + 2] cycloadducts. In the case of 2-cyclohexen-1-one, the cis-fused C 60 adduct is the major product, while in the case of 3-methyl-2-cyclohexen-1-one the trans isomer predominates. 13 C NMR spectra indicated addition occurred across [6,6] pyracyclene bonds of the fullerene to give ring-closed structures with two sp 3 fullerene carbons. These racemic [2 + 2] adducts could be enantiomerically resolved on a chiral HPLC column. Much larger optical rotations were found for the trans than for the cis isomers. Additional longwavelength bands were also found in the UV absorption and CD spectra of the trans isomers. These chiroptical effects are attributed to an additional chromophore in the trans isomers, which according to MM2 calculations is a C 2 symmetric chiral skewed fullerene moiety. 3 He-NMR analysis of the product mixture from photoaddition of 3-methyl-2-cyclohexen-1-one and 2-cyclohepten-1-one to C 60 containing a 3 He atom ( 3 He@C 60 ) led to the appearance of two new peaks at ca. -9.3 ppm relative to gaseous 3 He, consistent with formation of two monoadduct diastereomers with [6,6]-closed structures. The mechanism of the reaction presumably involves addition of enone triplet excited states to ground state fullerenes, via triplet 1,4-biradical intermediates, as in typical enone-alkene photocycloadditions. The fact that enone triplets are not quenched by triplet energy transfer to the fullerenes, which would be highly favorable energetically, is rationalized by poor coupling of the chromophores and Marcus theory.