systems where IO2 may be involved including erythropoietic protoporphyrias2 and hematoporphyrin p h~t o c h e m o t h e r a p y .~~-~~ The relatively precise distance and angle information for 4-7 provided by N M R studies coupled with the triplet energy transfer rate constants determined by flash photolysis provide correlations among structural, dynamical, and photophysical paratneters which will be useful in developing or testing quantitative theories of energy transfer between chromophores within a single molecule.For example, it appears that a quantum mechanical calculation of orbital overlap would be necessary in order to correlate the above-mentioned electron transfer in polystyrene with structure. These molecular systems are also potentially useful for studying intramolecular electron transfer reactions; in fact, a carotenoporphyrin linked to a quinone has recently been found to form a long-lived charge-separated state upon excitation with visible light.56 (52) Mathews-Roth, M. M. -(4-hydroxyphenyl)-lO,l5,20-tris(4methylphenyl)porphyrin, 57412-08-5; 7'-apo-7'-(4-(iodomethyl)phenyl)-P-carotene, 95998-89-3; 5-(3-hydroxyphenyl)-lO,l5,20-tris(4methylphenyl)porphyrin, 57412-06-3; 5-(2-hydroxyphenyl)-lO,l5,20tris(4-methylphenyl)porphyrin, 57412-07-4; 5-(4-(3-hydroxypropoxylphenyl)-10,15,20-tris(4-methylphenyl)porphyrin, 95998-90-6; abromo-p-toluoyl chloride, 52780-16-2; 2,3-dimethoxybenzaldehyde, 86-51-1; p-tolualdehyde, 104-87-0; pyrrole, 109-97-7; propionic acid, 79-09-4; 2,6-dimethoxybenzaldehyde, 3392-97-0; ethylene oxide, 75-21-8; methyl a-bromo-p-toluate, 2417-72-3; P-apo-S'-carotenal, 1 107-26-2; oxygen, 7782-44-7. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (56) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J. C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; Nemeth, G. A.; Moore, A. L. Nature (London) 1984, 307, 630-632.Abstract: The condensation of acetaldehyde (1) to an equilibrium mixture of aldol ( 2 ) and crotonaldehyde (3) is second order in 1. An excess acidity analysis reveals that a water molecule is also involved in the rate-limiting step; the reaction is actually the base-assisted addition of vinyl alcohol to protonated 1, even in concentrated H2S04. A previous report of a kinetically first-order conversion of 1 to 3 is shown to be due to the presence of a fast-reacting oligomer of 1. The reaction of 1 in D2S04 leads to partially deuterated 3, a result ascribed to partial conversion of vinyl alcohol to deuterated 1. Hydrogen isotope exchange of 3 was also observed, but at a slower rate. The rates of enolization of 1 were studied by iodination and are consistent with previous results and the proposed mechanism. The interconversion of 2 and 3 is shown to proceed via the enol of 2; in this case the rate-limiting step is water attack on/water loss from protonated 3/2, not proton transfer at carbon.The acid-catalyzed condensation of acetaldehyde (1) to aldol

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We describe a method for the selective desorption of thiol self-assembled monolayers from gold surfaces having micrometer-scale separations on a substrate. In an electrolyte solution, the electrical resistance between the adjacent areas can be much lower than the resistance between a surface and the counter electrode. Also, both reductive and oxidative thiol desorption may occur. Therefore, the potentials of the surfaces must be independently controlled with a multichannel potentiostat and operating windows for a given thiol/electrolyte system must be established. In this study operating windows were established for 1-dodecanethiol-based SAMs in phosphate buffer, phosphate-buffered saline, and sodium hydroxide solution, and selective SAM removal was successfully performed in a four-electrode configuration.